OCS Study 

MMS 2006-045 

 

Effects of Oil and Gas Exploration 

and Development at Selected Continental 

Slope Sites in the Gulf of Mexico

Volume II: Technical Report 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 

OCS Study 

MMS 2006-045 

Effects of Oil and Gas Exploration 

and Development at Selected Continental 

Slope Sites in the Gulf of Mexico 

Volume II: Technical Report 

Author 

Continental Shelf Associates, Inc. 

Prepared under MMS Contract 

1435-01-00-CT-31034 

by 

Continental Shelf Associates, Inc. 

759 Parkway Street 

Jupiter, Florida 33477-9596 

Published by 

New Orleans

July 2006 
iii 

Disclaimer 

This report was prepared under contract between the Minerals Management Service (MMS) and 

Continental Shelf Associates, Inc. This report has been technically reviewed by the MMS and 

has been approved for publication. Approval does not signify that the contents necessarily reflect 

the views or policies of the MMS, nor does mention of trade names or commercial products 

constitute endorsement or recommendation for use. It is, however, exempt from review and 

compliance with MMS editorial standards. 

Report Availability 

Extra copies of this report may be obtained from the Public Information Office at the following 

address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, LA 70123-2394 

Telephone: (504) 736-2519 or 

1-800-200-GULF 

Citation 

Suggested citation: 

Continental Shelf Associates, Inc. 2006. Effects of Oil and Gas Exploration and Development at 

Selected Continental Slope Sites in the Gulf of Mexico. Volume II: Technical Report. 

U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, LA. OCS Study MMS 2006-045. 636 pp. 
v 

Acknowledgments 

The program was conducted by a large, multidisciplinary team under the direction of Continental 

Shelf Associates, Inc. (CSA). David Gettleson was the program manager and Alan Hart served as 

deputy program manager and data manager. Neal Phillips was the scientific editor for the final 

report. Principal investigators are acknowledged at the head of each report chapter. 

We appreciate the assistance of MMS staff including Greg Boland (Contracting Officer’s 

Technical Representative), Mary Boatman (Technical Performance Evaluation Committee), 

Debra Bridge (Contracting Officer), and Tom Ahlfeld. Jill Leale with the MMS Mapping & 

Automation Unit provided historical well data and georeferenced maps of well locations based on 

the MMS Technical Information Management System. 

Drilling discharge data were kindly provided by Vince Cottone (ChevronTexaco) and Kent 

Satterlee, Ellen Polanski, and Jane Chady (Shell). Joe Smith (ExxonMobil) sent unpublished 

reports that were helpful in drilling discharge calculations. 

The Scientific Review Board consisted of Andy Glickman (ChevronTexaco), Donald Harper 

(Texas A&M University at Galveston), and Jim Ray (Oceanic Environmental Solutions). 

Additional input in the study design was provided by Peter Arnold (independent consultant), 

Maynard Brandsma (Brandsma Engineering), and Woollcott Smith (Temple University). 

We wish to recognize the following individuals for their roles in the field surveys: 

• Jay Northcutt (C&C Technologies) – Project manager, geophysical cruises 

• Tony George (C&C Technologies) – Manager, geophysical interpretation 

• Scott Melancon – Party chief, geophysical Cruises 2A and 3A 

• Tim MacEwan – Senior operator, geophysical Cruise 2A 

• Scott McBay, David Aucoin, and Paige Melancon – Party chiefs, geophysical Cruise 1A 

• Roger Fay (TDI-Brooks International, Inc.) – Chief scientist, chemical/biological cruises 

• Lynwood Powell (CSA) – Field operations manager, chemical/biological Cruises 1B and 2B; 

navigation/post-plotting and geographic information system development 

• Frank Johnson (CSA) – Field operations manager, chemical/biological Cruise 3B 

Other cruise participants including staff from Continental Shelf Associates, Inc., C&C 

Technologies, TDI-Brooks International, Inc., Florida Institute of Technology, and Louisiana 

State University are too numerous to list individually, but their contributions are sincerely 

appreciated. We also wish to acknowledge Wayne Isphording (University of South Alabama) for 

conducting laboratory sediment grain size analyses. 

The CSA document production team included Melody Powell (document production coordinator 

and technical editor), Heidi Glick (technical editor), Suzanne Short (graphics), and Debbie Raffel, 

Karen Stokesbury, Lynanne Rockhill, and Debbie Cannon (word processing). 
vii 

Contents 

Page 

LIST OF FIGURES......................................................................................................................xv

LIST OF TABLES..................................................................................................................... xliii

LIST OF ACRONYMS AND ABBREVIATIONS..................................................................... li

1. INTRODUCTION................................................................................................................1 

 1.1 Background..................................................................................................................1 

 1.2 Study Objectives..........................................................................................................3 

 1.3 Report Organization ....................................................................................................3 

2. STUDY DESIGN AND GENERAL METHODS ..............................................................5 

 2.1 Site Selection ...............................................................................................................5 

 2.2 Far-field Sites (Reference Areas) ................................................................................7 

 2.3 Sampling Cruises.........................................................................................................8

 2.3.1 Geophysical Cruises .......................................................................................8 

 2.3.2 Chemical/Biological Cruises ........................................................................14 

 2.4 Sampling Design .......................................................................................................14 

 2.4.1 Geophysical Characterization.......................................................................15 

 2.4.2 Chemical/Biological Sampling.....................................................................15 

 2.5 Sampling Methodology .............................................................................................16 

 2.5.1 Box Cores .....................................................................................................16 

 2.5.2 Sediment Profile Imaging.............................................................................20 

 2.5.3 Seafloor Photographs....................................................................................20 

 2.5.4 Baited Traps..................................................................................................23 

 2.6 Sampling Locations and Data Collected....................................................................23 

3. DRILLING ACTIVITIES.................................................................................................31 

 3.1 Near-field Sites..........................................................................................................32 

 3.1.1 Viosca Knoll Block 916 (Exploration Site)..................................................32 

 3.1.2 Garden Banks Block 516 (Exploration/Development Site)..........................34 

 3.1.3 Garden Banks Block 602 (Post-Development Site) .....................................39 

 3.1.4 Mississippi Canyon Block 292 (Post-Development Site) ............................42 

 3.1.5 Summary ......................................................................................................45 

 3.2 Far-field Sites ............................................................................................................48 

 3.2.1 Viosca Knoll Block 916 ...............................................................................54 

 3.2.2 Garden Banks Block 516..............................................................................54 

 3.2.3 Garden Banks Block 602..............................................................................54 

 3.2.4 Mississippi Canyon Block 292.....................................................................55 

 3.2.5 Summary and Preliminary Evaluation..........................................................55 

4. GEOPHYSICAL CHARACTERIZATION OF STUDY SITES...................................57 

 4.1 Introduction and Geologic Framework......................................................................57 

 4.2 Data Collection Schedule and Instrumentation .........................................................59 

 4.3 Viosca Knoll Block 916 – Exploration Site ..............................................................62 

 4.3.1 Introduction ..................................................................................................62 

 4.3.2 Data Quality for the Pre-Drilling Survey .....................................................62 

 4.3.3 Site Characterization: Pre-Drilling ...............................................................64 
Contents 

(continued) 

Page 

viii 

 4.3.4 Data Quality for the Post-Drilling Survey....................................................64 

 4.3.5 Site Characterization: Post-Drilling..............................................................69 

 4.4 Mississippi Canyon Block 292 – Post-Development Site .........................................69 

 4.4.1 Introduction ..................................................................................................69 

 4.4.2 Data Quality..................................................................................................72 

 4.4.3 Site Characterization ....................................................................................72 

 4.5 Garden Banks Block 602 – Post-Development Site ..................................................79 

 4.5.1 Introduction ..................................................................................................79 

 4.5.2 Data Quality..................................................................................................79 

 4.5.3 Site Characterization ....................................................................................79 

 4.6 Garden Banks Block 516 – Exploration/Development Site ......................................85 

 4.6.1 Introduction ..................................................................................................85 

 4.6.2 Data Quality..................................................................................................85 

 4.6.3 Site Characterization ....................................................................................88 

 4.7 Conclusions ...............................................................................................................93 

5. SEDIMENT GRAIN SIZE................................................................................................95 

 5.1 Introduction ...............................................................................................................95 

 5.2 Laboratory Methods ..................................................................................................95 

 5.3 Results .......................................................................................................................95 

 5.3.1 Viosca Knoll Block 916 ...............................................................................97 

 5.3.2 Garden Banks Block 516............................................................................102 

 5.3.3 Garden Banks Block 602............................................................................109 

 5.3.4 Mississippi Canyon Block 292...................................................................117 

 5.4 Discussion................................................................................................................124 

6. SEDIMENT PROFILE IMAGING AT EXPLORATION AND 

EXPLORATION/DEVELOPMENT SITES .................................................................131 

 6.1 Introduction .............................................................................................................131 

 6.2 Methods ...................................................................................................................131 

 6.3 Garden Banks Block 516 – Cruise 1B.....................................................................132 

 6.3.1 Survey Design and Location ......................................................................132 

 6.3.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy................133 

 6.3.3 Depth of the Apparent Redox Potential Discontinuity...............................138 

 6.3.4 Infaunal Successional Stages......................................................................141 

 6.3.5 Organism-Sediment Index (OSI)................................................................146 

 6.4 Viosca Knoll Block 916 – Cruise 1B (Pre-Drilling) ...............................................150 

 6.4.1 Survey Design and Location ......................................................................150 

 6.4.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy................157 

 6.4.3 Depth of the Apparent Redox Potential Discontinuity...............................158 

 6.4.4 Infaunal Successional Stages......................................................................160 

 6.4.5 Organism-Sediment Index..........................................................................164 
Contents 

(continued) 

Page 

ix 

 6.5 Garden Banks Block 516 – Cruise 2B.....................................................................173 

 6.5.1 Survey Design and Location ......................................................................173 

 6.5.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy................174 

 6.5.3 Depth of the Apparent Redox Potential Discontinuity...............................177 

 6.5.4 Infaunal Successional Stage .......................................................................179 

 6.5.5 Organism-Sediment Index..........................................................................186 

 6.6 Viosca Knoll Block 916 – Cruise 3B (Post-Drilling)..............................................191 

 6.6.1 Survey Design and Location ......................................................................191 

 6.6.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy................191 

 6.6.3 Depth of the Apparent Redox Potential Discontinuity...............................193 

 6.6.4 Infaunal Successional Stages......................................................................193 

 6.6.5 Organism-Sediment Index..........................................................................196 

 6.7 Summary and Conclusions ......................................................................................200 

 6.7.1 Garden Banks Block 516............................................................................200 

 6.7.2 Viosca Knoll Block 916 .............................................................................205 

 6.8 Plates........................................................................................................................207 

7. SEDIMENT PROFILE IMAGING AT POST-DEVELOPMENT SITES.................215 

 7.1 Introduction .............................................................................................................215 

 7.2 Methods ...................................................................................................................215 

 7.2.1 Survey Design and Transect Location........................................................215 

 7.2.2 Camera Operation and Field Sampling ......................................................218 

 7.2.3 Image Analysis ...........................................................................................222 

 7.3 Results .....................................................................................................................225 

 7.3.1 Garden Banks Block 602............................................................................225 

 7.3.2 Mississippi Canyon Block 292...................................................................232 

 7.4 Discussion................................................................................................................239 

 7.4.1 Garden Banks Block 602............................................................................239 

 7.4.2 Mississippi Canyon Block 292...................................................................240 

 7.5 Conclusions .............................................................................................................240 

 7.5.1 Garden Banks Block 602............................................................................240 

 7.5.2 Mississippi Canyon Block 292...................................................................241 

8. METALS, TOTAL ORGANIC CARBON, AND REDOX CONDITIONS IN 

SEDIMENTS.....................................................................................................................243 

 8.1 Introduction .............................................................................................................243 

 8.2 Methods ...................................................................................................................245

 8.2.1 Sampling and Field Measurements.............................................................245 

 8.2.2 Interstitial Water Collection and Analysis..................................................247 

 8.2.3 Laboratory Analysis ...................................................................................247 

 8.2.4 Quality Control and Quality Assurance......................................................251 

 8.3 Viosca Knoll Block 916 – Exploration Site ............................................................252

 8.3.1 Metals and Total Organic Carbon ..............................................................252 

 8.3.2 Sediment Geochronology ...........................................................................258 

 8.3.3 Sediment Oxygen Levels and Redox Conditions .......................................258 

 8.3.4 Interstitial Water Composition ...................................................................264 
Contents 

(continued) 

Page 

x 

 8.4 Garden Banks Block 516 – Exploration/Development Site ....................................271

 8.4.1 Metals and Total Organic Carbon ..............................................................271 

 8.4.2 Sediment Geochronology ...........................................................................281 

 8.4.3 Sediment Oxygen Levels and Redox Conditions .......................................281 

 8.4.4 Interstitial Water Composition ...................................................................286 

 8.5 Garden Banks Block 602 And Mississippi Canyon Block 292 – 

Post-Development Sites...........................................................................................289

 8.5.1 Metals and Total Organic Carbon ..............................................................289 

 8.5.2 Sediment Geochronology ...........................................................................299 

 8.5.3 Sediment Oxygen Levels and Redox Conditions .......................................301 

 8.5.4 Interstitial Water Composition ...................................................................303 

 8.5.5 Metals in Isopods and Crabs.......................................................................309 

 8.6 Discussion of Sediment Trace Metals .....................................................................314 

 8.7 Conclusions .............................................................................................................325 

9. SEDIMENT AND TISSUE HYDROCARBONS ..........................................................327 

 9.1 Introduction .............................................................................................................327 

 9.2 Analytical Methods .................................................................................................329 

 9.2.1 Extraction of Sediments for Aromatic and Saturated Hydrocarbons .........329 

 9.2.2 Extraction of Tissues for Aromatic Hydrocarbons.....................................329 

 9.2.3 Determination of Aromatic Hydrocarbons .................................................329 

 9.2.4 Determination of Aliphatic Hydrocarbons .................................................330 

 9.2.5 Quality Assurance/Quality Control ............................................................330 

 9.3 Results – Sediments.................................................................................................331 

 9.3.1 Viosca Knoll Block 916 – Exploration Site ...............................................331 

 9.3.2 Garden Banks Block 516 – Exploration/Development Site .......................334 

 9.3.3 Garden Banks Block 602 – Post-Development Site...................................337 

 9.3.4 Mississippi Canyon Block 292 – Post-Development Site ..........................340 

 9.4 Results – Tissue Samples ........................................................................................343 

 9.5 Discussion................................................................................................................344 

10. SEDIMENT MICROBES................................................................................................345 

 10.1 Introduction .............................................................................................................345

 10.2 Materials and Methods ............................................................................................347

 10.2.1 ATP Assay..................................................................................................348 

 10.2.2 Microbial Respiration.................................................................................350 

 10.2.3 Sediment DNA Extraction, Quantification, and Amplification..................351 

 10.2.4 Toxicity Bioassay .......................................................................................351 

 10.3 Results .....................................................................................................................351

 10.3.1 ATP Analysis..............................................................................................351 

 10.3.2 Toxicity Bioassay .......................................................................................353 

 10.3.3 DNA Assay.................................................................................................356 

 10.3.4 Respiration..................................................................................................356 

 10.4 Discussion................................................................................................................357
Contents 

(continued) 

Page 

xi 

11. MEIOFAUNA AND MACROINFAUNA......................................................................361 

 11.1 Introduction .............................................................................................................361 

 11.2 Materials and Methods ............................................................................................361 

 11.3 Meiofaunal Results and Discussion.........................................................................362 

 11.3.1 Viosca Knoll Block 916 .............................................................................362 

 11.3.2 Garden Banks Block 516............................................................................363 

 11.3.3 Garden Banks Block 602............................................................................370 

 11.3.4 Mississippi Canyon Block 292...................................................................373 

 11.3.5 Individual Groups and Environmental Correlations...................................373 

 11.3.6 Nematode Feeding Groups and Harpacticoid Reproductive Status............376 

 11.3.7 Discussion ..................................................................................................376 

 11.4 Macroinfaunal Results and Discussion....................................................................380 

 11.4.1 Viosca Knoll Block 916 .............................................................................380 

 11.4.2 Garden Banks Block 516............................................................................385 

 11.4.3 Garden Banks Block 602............................................................................388 

 11.4.4 Mississippi Canyon Block 292...................................................................388 

 11.4.5 Individual Groups and Environmental Correlations...................................392

 11.4.6 Detailed Macroinfaunal Taxonomic Analysis............................................401 

 11.4.7 Discussion ..................................................................................................410 

 11.5 Conclusions .............................................................................................................412 

 11.5.1 Meiofauna...................................................................................................412 

 11.5.2 Macroinfauna..............................................................................................412 

12. MEGAFAUNA AND IMAGE ANALYSIS ...................................................................415 

 12.1 Introduction: Image-Based Sampling......................................................................415 

 12.2 Methods ...................................................................................................................416 

 12.2.1 Field Sampling............................................................................................416 

 12.2.2 Image Processing........................................................................................416 

 12.2.3 Color Analysis............................................................................................421 

 12.2.4 Color Models..............................................................................................421 

 12.3 Results .....................................................................................................................422 

 12.3.1 Texture Analysis.........................................................................................422 

 12.3.2 Color Analysis............................................................................................422 

 12.4 Faunal Analysis .......................................................................................................430

 12.5 Discussion................................................................................................................439 

 12.5.1 Faunal Results ............................................................................................439 

 12.5.2 Color Analysis............................................................................................439 

 12.5.3 Consideration of Size .................................................................................439 

13. GENETIC DIVERSITY OF HARPACTICOID COPEPODS ....................................441 

 13.1 Introduction .............................................................................................................441 

 13.2 Methods ...................................................................................................................443 

 13.2.1 Collection and Processing of Samples........................................................443 

 13.2.2 Genetic Methods.........................................................................................443 

 13.2.3 Data Analysis..............................................................................................444 

 13.3 Results .....................................................................................................................445 
Contents 

(continued) 

Page 

xii 

 13.3.1 Numerical Response to Disturbance by Bathycletopsyllus sp....................445 

 13.3.2 Genetic Divergence ....................................................................................445 

 13.3.3 Population Structure ...................................................................................447 

 13.3.4 Genetic Diversity........................................................................................450 

 13.4 Discussion................................................................................................................450 

 13.4.1 Numerical Response to Disturbance by Bathycletopsyllus sp....................450 

 13.4.2 Genetic Divergence ....................................................................................453 

 13.4.3 Population Structure ...................................................................................454 

 13.4.4 Genetic Diversity........................................................................................455 

 13.5 Conclusions .............................................................................................................457 

14. SEDIMENT TOXICITY .................................................................................................459 

 14.1 Introduction .............................................................................................................459 

 14.2 Materials and Methods ............................................................................................459 

 14.2.1 Sediment Sampling Locations....................................................................459 

 14.2.2 Test Organism Collection...........................................................................459 

 14.2.3 Sediment Sample Preparation.....................................................................460 

 14.2.4 Toxicity Test...............................................................................................460 

 14.3 Results .....................................................................................................................461 

 14.3.1 Mississippi Canyon Block 292...................................................................461 

 14.3.2 Garden Banks Block 602............................................................................463 

 14.4 Discussion................................................................................................................464 

 14.4.1 Mississippi Canyon Block 292...................................................................464 

 14.4.2 Garden Banks 602 ......................................................................................466 

 14.5 Conclusions .............................................................................................................469 

15. SYNTHESIS .....................................................................................................................471 

 15.1 Introduction .............................................................................................................471 

 15.1.1 Baseline Conditions....................................................................................471 

 15.1.2 Drilling Activities.......................................................................................474 

 15.2 Nature and Extent of Impacts ..................................................................................476 

 15.2.1 Physical ......................................................................................................476 

 15.2.2 Chemical.....................................................................................................489 

 15.2.3 Biological ...................................................................................................505 

 15.3 Site Comparisons.....................................................................................................522 

 15.3.1 Areal Extent of Impacts..............................................................................522 

 15.3.2 Severity of Impacts.....................................................................................523 

 15.3.3 Duration of Impacts....................................................................................526 

 15.4 Methodology Evaluation .........................................................................................531 

 15.4.1 Selection of Far-Field Sites ........................................................................531 

 15.4.2 Geophysical Surveys ..................................................................................532 

 15.4.3 Seafloor Photography .................................................................................532 

 15.4.4 Detecting Benthic Community Impacts .....................................................533 

 15.5 Conclusions .............................................................................................................533 
Contents 

(continued) 

Page 

xiii 

16. ECOLOGICAL RISK ASSESSMENT..........................................................................535 

 16.1 Introduction .............................................................................................................535 

 16.2 Problem Formulation...............................................................................................536 

 16.2.1 Deepwater Drilling in the Gulf of Mexico .................................................536 

 16.2.2 Drilling Practices and Discharge Histories.................................................537 

 16.2.3 The Benthic Environment of the Study Area .............................................539 

 16.2.4 Conceptual Model ......................................................................................542 

 16.3 Exposure Assessment ..............................................................................................549 

 16.3.1 Physical Disturbance of Sediments ............................................................549 

 16.3.2 Chemical Alteration of Sediments..............................................................551 

 16.3.3 Metals in Sediments ...................................................................................556 

 16.3.4 Polycyclic Aromatic Hydrocarbons and Metals in Tissues of 

Benthic Megafauna.....................................................................................559 

 16.3.5 Summary of Exposure ................................................................................562 

 16.4 Effects Assessment..................................................................................................563 

 16.4.1 Sediment Toxicity ......................................................................................563 

 16.4.2 Sediment Microbial Activity ......................................................................564 

 16.4.3 Meiofaunal and Macroinfaunal Communities............................................569 

 16.4.4 Demersal/Benthic Megafauna ....................................................................573 

 16.4.5 Summary of Injury .....................................................................................573 

 16.5 Risk Characterization ..............................................................................................575 

 16.5.1 Causes of Sediment Toxicity......................................................................575 

 16.5.2 Causes of Organic Enrichment...................................................................575 

 16.5.3 Changes in Benthic Communities ..............................................................576 

 16.5.4 Summary ....................................................................................................577 

17. LITERATURE CITED ...................................................................................................579
xv 

List of Figures 

Figure Page 

 1.1 Study sites ....................................................................................................................... 1 

 1.2 Number of wells drilled in the deepwater Gulf of Mexico over the past 

decade (From: Richardson et al. 2004) ........................................................................... 2 

 2.1 Location of near-field (NF) and far-field (FF) sites for Viosca Knoll 

Block 916 ........................................................................................................................ 9 

 2.2 Location of near-field (NF) and far-field (FF) sites for Garden Banks 

Block 516 ...................................................................................................................... 10 

 2.3 Location of near-field (NF) and far-field (FF) sites for Garden Banks 

Block 602 ...................................................................................................................... 11 

 2.4 Location of near-field (NF) and far-field (FF) sites for Mississippi 

Canyon Block 292 ......................................................................................................... 12 

 2.5 Schedule including sampling cruises and wells drilled during this study ..................... 13 

 2.6 Idealized sampling plans for study sites........................................................................ 17 

 2.7 Deployment of box core ................................................................................................ 18

 2.8 Schematic of box core subsampling .............................................................................. 19 

 2.9 Schematic diagram of the sediment profile camera and its operation ........................... 21 

 2.10 Configuration of dragged sled for seafloor photography .............................................. 22 

 2.11 Schematic diagram of baited traps used to obtain organisms for tissue 

analyses ......................................................................................................................... 24 

 2.12 Near-field sampling locations at Viosca Knoll Block 916 ............................................ 26 

 2.13 Near-field sampling locations at Garden Banks Block 516........................................... 27 

 2.14 Near-field sampling locations at Garden Banks Block 602........................................... 28 

 2.15 Near-field sampling locations at Mississippi Canyon Block 292.................................. 29 

 3.1 Well locations in and near the Viosca Knoll Block 916 near-field site......................... 33 

 3.2 Well locations in and near the Garden Banks Block 516 near-field site ....................... 36 

 3.3 Diagram of subsea wellheads, manifold, and flowlines at the Garden 

Banks Block 516 site..................................................................................................... 38 
List of Figures 

(continued) 

Figure Page 

xvi 

 3.4 Well locations in and near the Garden Banks Block 602 near-field site ....................... 40 

 3.5 Diagram of subsea wellheads, manifold, and flowlines at the Garden 

Banks Block 602 site..................................................................................................... 43 

 3.6 Well locations in and near the Mississippi Canyon Block 292 near-field 

site ................................................................................................................................. 44 

 3.7 Diagram of subsea wellheads, manifold, and flowlines at the 

Mississippi Canyon Block 292 site ............................................................................... 46 

 3.8 Timeline showing drilling activities at the four near-field sites.................................... 47 

 3.9 Location of blocks with previous wells in relation to Viosca Knoll 

Block 916 near-field (NF) and far-field (FF) sites ........................................................ 50 

 3.10 Location of previous wells in relation to Garden Banks Block 516 

near-field (NF) and far-field (FF) sites.......................................................................... 51 

 3.11 Location of blocks with previous wells in relation to Garden Banks 

Block 602 near-field (NF) and far-field (FF) sites ........................................................ 52 

 3.12 Location of blocks with previous wells in relation to Mississippi 

Canyon Block 292 near-field (NF) and far-field (FF) sites........................................... 53 

 4.1 A computer enhanced shaded relief map of the northern Gulf of 

Mexico continental slope............................................................................................... 58 

 4.2 Working configuration of the HUGIN 3000 autonomous underwater 

vehicle showing the “free-swimming” HUGIN collecting data from a 

predetermined height above the seafloor....................................................................... 61 

 4.3 Location map of Viosca Knoll (VK) Block 916............................................................ 63 

 4.4 Chirp sonar subbottom profile collected during Cruise 1A (pre-drilling) 

on Survey Line 1011 approximately 50 m west of the proposed wellsite 

in Viosca Knoll (VK) Block 916................................................................................... 65 

 4.5 A shaded relief image of the Viosca Knoll (VK) 916 study area 

showing the location of the wellsite for VK 916 Well No. 1 with regard 

to the distinct channel-levee feature trending roughly 

northwest-southeast through the eastern part of the block ............................................ 66 

 4.6 A sector of the side-scan sonar mosaic produced for the Viosca Knoll 

(VK) Block 916 area of interest .................................................................................... 67 
List of Figures 

(continued) 

Figure Page 

xvii 

 4.7 A section of a chirp sonar subbottom profile acquired along Line 136, 

which trends north-south across the Viosca Knoll (VK) Block 916 

wellsite........................................................................................................................... 68 

 4.8 Map of cuttings and anchor scars around the Viosca Knoll (VK) 

Block 916 site based on the Cruise 3A geophysical survey 

(August 2002)................................................................................................................ 70

 4.9 A portion of side-scan sonar Line 104 showing debris that appears to 

be pipe on the seafloor within the Viosca Knoll Block 916 study area......................... 71 

 4.10 A shaded relief image of the Mississippi Canyon (MC) Block 292 

study area showing the approximate location of the study site on the 

southeastern flank of a distinct dome-like feature produced by a salt 

mass in the shallow subsurface...................................................................................... 73 

 4.11 Chirp sonar subbottom profile (Line 319) illustrating an area of 

seafloor erosion that in plan-view trends northeast-southwest just south 

of the Mississippi Canyon Block 292 wellsites............................................................. 74 

 4.12 Image of a portion of a 120-kHz side-scan sonar survey line (Line 316) 

that trends north-south a short distance to the west of the Mississippi 

Canyon Block 292 well cluster...................................................................................... 75 

 4.13 Map of drilling mud, cuttings, and anchor scars around the Mississippi 

Canyon (MC) Block 292 site based on the Cruise 2A geophysical 

survey (June-July 2001) ................................................................................................ 77 

 4.14 A sector of the side-scan sonar mosaic for the Mississippi Canyon 

(MC) Block 292 area..................................................................................................... 78 

 4.15 A shaded relief image of the Garden Banks (GB) 602 and GB 516 

areas............................................................................................................................... 80 

 4.16 A section of side-scan sonar Line 116 showing the well cluster in 

Garden Banks (GB) Block 602 and radial pattern of high reflectance 

(dark streaks and patches, interpreted as possible cuttings from rig 

discharges)..................................................................................................................... 82 

 4.17 A section of side-scan sonar Line 112 illustrating the man-made scars 

in the hemipelagic surface sediments of the Garden Banks Block 602 

study area....................................................................................................................... 83 
List of Figures 

(continued) 

Figure Page 

xviii 

 4.18 Map of drilling mud, cuttings, and anchor scars around the Garden 

Banks (GB) 602 site based on the Cruise 2A geophysical survey 

(June-July 2001) ............................................................................................................ 84 

 4.19 This reduced version of the side-scan sonar mosaic illustrates the 

seafloor complexity to the southeast of the Garden Banks Block 516 

wellsite........................................................................................................................... 86 

 4.20 This section of chirp sonar subbottom profile (Line 212) crosses a 

prominent fault that trends east-west along the flank of the 

salt-supported ridge to the south of the drilling site in Garden Banks 

Block 516 ...................................................................................................................... 87 

 4.21 This section of side-scan sonar data from Garden Banks Block 516, 

Survey Line 227, illustrates the complexity of the seafloor in the 

transition area between the salt-supported high ground to the southeast 

of the wellsite and the basin floor where the wellsite is located ................................... 89 

 4.22 This portion of chirp sonar subbottom profile (Garden Banks Block 

516, Line 227) shows a hemipelagic sediment drape over a chaotic and 

largely acoustically opaque submarine landslide deposit.............................................. 90 

 4.23 Section of side-scan sonar record from Garden Banks Block 516, 

Line 216 showing the wellsite area with high-reflectance (dark) areas 

interpreted as possible cuttings from rig discharges and low-reflectance 

(light) areas interpreted as possible mud/cuttings from well jetting.............................. 91 

 4.24 Map of drilling mud, cuttings, and anchor scars around the Garden 

Banks Block 516 site based on the Cruise 2A geophysical survey 

(June-July 2001) ............................................................................................................ 92 

 5.1 Ternary plots summarizing grain size data for Viosca Knoll Block 916 ...................... 99 

 5.2 Percentages of sand, silt, and clay in relation to proximity to drilling at 

Viosca Knoll Block 916 .............................................................................................. 100 

 5.3 Grain size distribution at Viosca Knoll Block 916 on the post-drilling 

survey (Cruise 3B)....................................................................................................... 101 

 5.4 Percentages of sand and sediments >0.5 mm (coarse sand, very coarse 

sand, and gravel) on Cruise 3B at Viosca Knoll Block 916 in relation to 

geophysically mapped cuttings.................................................................................... 103 
List of Figures 

(continued) 

Figure Page 

xix 

 5.5 Relationships between sand percentages and concentrations of drilling 

indicators (barium and synthetic-based fluid [SBF] concentrations) at 

Viosca Knoll Block 916 on the post-drilling survey (Cruise 3B) ............................... 104 

 5.6 Depth profiles from discretionary stations (DS) at Viosca Knoll Block 

916 on the post-drilling survey (Cruise 3B) ................................................................ 105 

 5.7 Ternary plots summarizing grain size data for Garden Banks Block 516................... 107 

 5.8 Percentages of sand, silt, and clay in relation to proximity to drilling at 

Garden Banks Block 516............................................................................................. 108 

 5.9 Grain size distribution at Garden Banks Block 516 on Cruise 2B .............................. 110 

 5.10 Percentages of sand on Cruise 1B and Cruise 2B at Garden Banks 

Block 516 in relation to geophysically mapped cuttings............................................. 111 

 5.11 Relationships between sand percentages and concentrations of drilling 

indicators (barium and synthetic-based fluids [SBF] at Garden Banks 

Block 516 on Cruised 1B and 2B)............................................................................... 112 

 5.12 Depth profiles from discretionary stations (DS) at Garden Banks Block 

516 on Cruise 2B (post-development)......................................................................... 113 

 5.13 Ternary plot summarizing grain size data for Garden Banks Block 602 .................... 115 

 5.14 Percentages of sand, silt, and clay in relation to proximity to drilling at 

Garden Banks Block 602............................................................................................. 116 

 5.15 Grain size distribution at Garden Banks Block 602 on Cruise 2B .............................. 118 

 5.16 Sand percentages on Cruise 2B at Garden Banks Block 602 in relation 

to geophysically mapped cuttings ............................................................................... 119 

 5.17 Relationships between sand, silt, and clay percentages and 

concentrations of barium (drilling fluid indicator) at Garden Banks 

Block 602 on Cruise 2B .............................................................................................. 120 

 5.18 Depth profiles from discretionary stations (DS) at Garden Banks 

Block 602 on Cruise 2B (post-development) .............................................................. 121 

 5.19 Ternary plot summarizing grain size data for Mississippi Canyon 

Block 292 .................................................................................................................... 123 
List of Figures 

(continued) 

Figure Page 

xx 

 5.20 Percentages of coarse and very coarse sand (>0.5 mm), sand, silt, and 

clay in relation to proximity to drilling at Mississippi Canyon 

Block 292 .................................................................................................................... 125 

 5.21 Sand percentages on Cruise 2B at Mississippi Canyon Block 292 in 

relation to geophysically mapped cuttings .................................................................. 126 

 5.22 Relationships between sand, silt, and clay percentages and 

concentrations of barium (drilling fluid indicator) at Mississippi 

Canyon Block 292 on Cruise 2B ................................................................................. 127 

 5.23 Grain size distribution at Mississippi Canyon Block 292 on Cruise 2B ..................... 128 

 5.24 Depth profiles from discretionary stations (DS) at Mississippi Canyon 

Block 292 on Cruise 2B (post-development) .............................................................. 129 

 6.1 Location of Garden Banks Block 516 exploration/development site and 

associated reference areas where sediment profile imaging (SPI) 

transects were surveyed............................................................................................... 134 

 6.2 Garden Banks Block 516 near-field (NF) station map, showing 

positions of sediment profile imaging transects and stations sampled 

during Cruise 1B (October-November 2000).............................................................. 135 

 6.3 Garden Banks Block 516 Far-field 2 (FF2) station map, showing 

positions of sediment profile imaging transect and stations sampled 

during Cruise 1B (October-November 2000).............................................................. 135 

 6.4 Garden Banks Block 516 Far-field 4 (FF4) station map, showing 

positions of sediment profile imaging transect and stations sampled 

during Cruise 1B (October-November 2000).............................................................. 136 

 6.5 Garden Banks Block 516 Far-field 6 (FF6) station map, showing 

positions of sediment profile imaging transect and stations sampled 

during Cruise 1B (October-November 2000).............................................................. 136 

 6.6 Mapped distribution of mean apparent redox potential discontinuity 

(RPD) depths along the three near-field transects sampled at Garden 

Banks Block 516 during Cruise 1B (October-November 2000) ................................. 139 

 6.7 Redox potential discontinuity (RPD) depth-frequency distributions at 

three near-field transects in Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 140 
List of Figures 

(continued) 

Figure Page 

xxi 

 6.8 Mapped distribution of mean apparent redox potential discontinuity 

(RPD) depths along Transect FF2 at Garden Banks Block 516 during 

Cruise 1B (October-November 2000) ......................................................................... 142 

 6.9 Mapped distribution of mean apparent redox potential discontinuity 

(RPD) depths along Transect FF4 at Garden Banks Block 516 during 

Cruise 1B (October-November 2000) ......................................................................... 142 

 6.10 Mapped distribution of mean apparent redox potential discontinuity 

(RPD) depths along Transect FF6 at Garden Banks Block 516 during 

Cruise 1B (October-November 2000) ......................................................................... 143 

 6.11 Redox potential discontinuity (RPD) depth-frequency distributions at 

three far-field transects at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 144 

 6.12 Mapped distribution of infaunal successional stages along all three 

near-field transects at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 145 

 6.13 Mapped distribution of infaunal successional stages along Transect FF2 

at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 147 

 6.14 Mapped distribution of infaunal successional stages along Transect FF4 

at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 147 

 6.15 Mapped distribution of infaunal successional stages along Transect FF6 

at Garden Banks Block 516 during Cruise 1B (October-November 

2000)............................................................................................................................ 147 

 6.16 Mapped distribution of the Organism-Sediment Index (OSI) along all 

three near-field transects at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 148 

 6.17 Organism-Sediment Index (OSI) class-frequency distributions at all 

three near-field transects sampled at Garden Banks Block 516 during 

Cruise 1B (October-November 2000) ......................................................................... 149 

 6.18 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF2 at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 151 
List of Figures 

(continued) 

Figure Page 

xxii 

 6.19 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF4 at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 151 

 6.20 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF6 at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 151 

 6.21 Organism-Sediment Index (OSI) class-frequency distributions at all 

three far-field transects at Garden Banks Block 516 during Cruise 1B 

(October-November 2000) .......................................................................................... 152 

 6.22 Organism-Sediment Index (OSI) class-frequency distributions at all 

near-field and far-field transects at Garden Banks Block 516 during 

Cruise 1B (October-November 2000) ......................................................................... 153 

 6.23 Location of Viosca Knoll Block 916 exploration site and associated 

reference areas where sediment profile imaging (SPI) transects were 

surveyed on Cruise 1B................................................................................................. 154 

 6.24 Viosca Knoll Block 916 near-field (NF) station map, showing positions 

of sediment profile imaging transects and stations...................................................... 155 

 6.25 Viosca Knoll Block 916 Far-field 2 (FF2) station map, showing 

positions of sediment profile imaging transects and stations ...................................... 155 

 6.26 Viosca Knoll Block 916 Far-field 4 (FF4) station map, showing 

positions of sediment profile imaging transect and stations........................................ 156 

 6.27 Viosca Knoll Block 916 Far-field 6 (FF6) station map, showing 

positions of sediment profile imaging transect and stations........................................ 156 

 6.28 Mapped distribution of apparent redox potential discontinuity (RPD) 

depth, in centimeters, along the two near-field transects in Viosca 

Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 159 

 6.29 Redox potential discontinuity (RPD) depth-frequency distributions at 

two near-field transects in Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 160 

 6.30 Redox potential discontinuity (RPD) depth-frequency distributions 

along Transect FF2 at Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 161 
List of Figures 

(continued) 

Figure Page 

xxiii 

 6.31 Redox potential discontinuity (RPD) depth-frequency distributions at 

three far-field transects sampled at Viosca Knoll Block 916 during 

pre-drilling Cruise 1B (October-November 2000) ...................................................... 162 

 6.32 Redox potential discontinuity (RPD) depth-frequency distributions 

along Transect FF4 at Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 163 

 6.33 Redox potential discontinuity (RPD) depth-frequency distributions 

along Transect FF6 at Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 163 

 6.34 Mapped distribution of infaunal successional stages along Transects 

NF-1 and NF-2 at Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 165 

 6.35 Mapped distribution of successional stages along Transect FF2 at 

Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 166 

 6.36 Mapped distribution of successional stages along Transect FF4 at 

Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 166 

 6.37 Mapped distribution of successional stages along Transect FF6 at 

Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 167 

 6.38 Mapped distribution of the Organism-Sediment Index (OSI) along the 

two near-field transects taken at Viosca Knoll Block 916 during 

pre-drilling Cruise 1B (October-November 2000) ...................................................... 168 

 6.39 Organism-Sediment Index (OSI) class-frequency distributions at 

two near-field transects in Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 169 

 6.40 Mapped distribution of the Organism-Sediment Index (OSI) along the 

Transect FF2 at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 170 

 6.41 Organism-Sediment Index (OSI) class-frequency distributions at three 

far-field transects in Viosca Knoll Block 916 during pre-drilling 

Cruise 1B (October-November 2000) ......................................................................... 171 
List of Figures 

(continued) 

Figure Page 

xxiv 

 6.42 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF4 at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 172 

 6.43 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF6 at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000) .......................................................................................... 172 

 6.44 Organism-Sediment Index (OSI) class-frequency distributions at all 

near-field and far-field transects sampled at Viosca Knoll Block 916 

during pre-drilling Cruise 1B (October-November 2000)........................................... 173 

 6.45 Garden Banks Block 516 near-field (NF) map showing transects and 

stations sampled during Cruise 2B (July 2001)........................................................... 175 

 6.46 Garden Banks Block 516 Far-field 3 (FF3) station map, showing 

transects and stations sampled during Cruise 2B (July 2001) ..................................... 175 

 6.47 Garden Banks Block 516 Far-field 4 (FF4) station map, showing 

transects and stations sampled during Cruise 2B (July 2001) ..................................... 176 

 6.48 Garden Banks Block 516 Far-field 5 (FF5) station map, showing 

transects and stations sampled during Cruise 2B (July 2001) ..................................... 176 

 6.49 Mapped distribution of apparent redox potential discontinuity (RPD) 

depth, in centimeters, along the three near-field transects taken at 

Garden Banks Block 516 during Cruise 2B (July 2001) ............................................. 178 

 6.50 Redox potential discontinuity (RPD) depth-frequency distributions at 

three near-field transects sampled at Garden Banks Block 516 during 

Cruise 2B (July 2001).................................................................................................. 180 

 6.51 Mapped distribution of apparent redox potential discontinuity (RPD) 

depth, in centimeters, along the two transects surveyed at Far-field 3 

(FF3) during Cruise 2B (July 2001) ............................................................................ 181 

 6.52 Mapped distribution of apparent redox potential discontinuity (RPD) 

depth, in centimeters, along Transect FF4 at Garden Banks Block 516 

during Cruise 2B (July 2001) ...................................................................................... 181 

 6.53 Mapped distribution of apparent redox potential discontinuity (RPD) 

depth, in centimeters, along Transect FF5 at Garden Banks Block 516 

during Cruise 2B (July 2001) ...................................................................................... 181 
List of Figures 

(continued) 

Figure Page 

xxv 

 6.54 Redox potential discontinuity (RPD) depth-frequency distributions at 

three far-field transects sampled at Garden Banks Block 516 during 

Cruise 2B (July 2001).................................................................................................. 182 

 6.55 Mapped distribution of infaunal successional stages along all three 

near-field transects at Garden Banks Block 516 during Cruise 2B 

(July 2001)................................................................................................................... 184 

 6.56 Mapped distribution of infaunal successional stages along the two 

transects at Garden Banks Block 516, Far-field 3 (FF3) during 

Cruise 2B (July 2001).................................................................................................. 185 

 6.57 Mapped distribution of infaunal successional stages along Transect FF4 

at Garden Banks Block 516 during Cruise 2B (July 2001) ......................................... 185 

 6.58 Mapped distribution of infaunal successional stages along Transect FF5 

at Garden Banks Block 516 during Cruise 2B (July 2001) ......................................... 185 

 6.59 Mapped distribution of the Organism-Sediment Index (OSI) along all 

three near-field transects at Garden Banks Block 516 during Cruise 2B 

(July 2001)................................................................................................................... 187 

 6.60 Organism-Sediment Index (OSI) class-frequency distribution for all 

stations belonging to all three near-field transects surveyed at Garden 

Banks Block 516 during Cruise 2B (July 2001).......................................................... 188 

 6.61 Mapped distribution of the Organism-Sediment Index (OSI) along 

far-field transects at FF3 at Garden Banks Block 516 during Cruise 2B 

(July 2001)................................................................................................................... 188 

 6.62 Organism-Sediment Index (OSI) frequency distributions at the three 

far-field transects at Garden Banks Block 516 during Cruise 2B 

(July 2001)................................................................................................................... 189 

 6.63 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF4 at Garden Banks Block 516 during Cruise 2B (July 2001) ................... 190 

 6.64 Mapped distribution of the Organism-Sediment Index (OSI) along 

Transect FF5 at Garden Banks Block 516 during Cruise 2B (July 2001) ................... 190 

 6.65 Viosca Knoll Block 916 near-field station map showing positions of 

sediment profile imaging transects and stations surveyed during 

post-drilling Cruise 3B (August 2002) ........................................................................ 192 
List of Figures 

(continued) 

Figure Page 

xxvi 

 6.66 Mapped distribution of apparent redox potential discontinuity depth in 

centimeters, along the three near-field transects at Viosca Knoll Block 

916 during post-drilling Cruise 3B (August 2002)...................................................... 194

 6.67 Apparent redox potential discontinuity (RPD) depth-frequency 

distributions at the three near-field transects at Viosca Knoll Block 916 

during post-drilling Cruise 3B (August 2002)............................................................. 195 

 6.68 Mapped distribution of infaunal successional stages along all three 

near-field transects surveyed at Viosca Knoll Block 916 during 

post-drilling Cruise 3B (August 2002) ........................................................................ 197 

 6.69 Mapped distribution of the Organism-Sediment Index (OSI) along the 

three near-field transects at Viosca Knoll Block 916 during 

post-drilling Cruise 3B (August 2002) ........................................................................ 198 

 6.70 Organism-Sediment Index (OSI) class-frequency distributions at the 

three near-field transects at Viosca Knoll Block 916 during 

post-drilling Cruise 3B (August 2002) ........................................................................ 199 

 7.1 Location of Garden Banks Block 602 post-development site and 

associated reference areas where sediment profile imaging (SPI) 

transects were surveyed............................................................................................... 216 

 7.2 Location of Mississippi Canyon Block 292 post-development site and 

associated reference areas where sediment profile imaging (SPI) 

transects were surveyed............................................................................................... 217 

 7.3 Locations of sediment profile imaging (SPI) transects at Garden Banks 

Block 602 near-field site on Cruise 2B (July 2001) .................................................... 219 

 7.4 Locations of sediment profile imaging (SPI) transects at Mississippi 

Canyon Block 292 near-field site on Cruise 2B (July 2001)....................................... 219 

 7.5 Locations of sediment profile imaging (SPI) transects at Garden Banks 

Block 602 far-field sites on Cruise 2B (July 2001)..................................................... 220 

 7.6 Locations of sediment profile imaging (SPI) transects at Mississippi 

Canyon Block 292 far-field sites on Cruise 2B (July 2001)........................................ 221 

 7.7 Number and appearance of color sediment layers at stations along 

Garden Banks Block 602 near-field transects ............................................................. 226 
List of Figures 

(continued) 

Figure Page 

xxvii 

 7.8 Histogram showing the distribution of apparent color redox potential 

discontinuity (RPD) layer depth for Garden Banks Block 602 near-field 

(NF) and far-field (FF) transects ................................................................................. 227 

 7.9 Appearance of color sediment layers at stations along Garden Banks 

Block 602 far-field transects ....................................................................................... 229 

 7.10 Average thickness of color layers (cm) at Garden Banks Block 602 

sites.............................................................................................................................. 231 

 7.11 Average depth of apparent color redox potential discontinuity (RPD) 

layer (cm) at Garden Banks Block 602 far-field (FF) and near-field 

(NF) transects .............................................................................................................. 231

 7.12 Appearance of color sediment layers at stations along Mississippi 

Canyon Block 292 near-field transects........................................................................ 233 

 7.13 Histogram showing the distribution of apparent color redox potential 

discontinuity (RPD) layer depth for Mississippi Canyon Block 292 

near-field (NF) and far-field (FF) transects................................................................. 234 

 7.14 Sediment profile imaging image from Mississippi Canyon Block 292, 

Station NF-1.08, showing a large burrow structure near the center and 

extending down from sediment-water interface to bottom of image........................... 235 

 7.15 Appearance of color sediment layers at stations along Mississippi 

Canyon Block 292 far-field transects .......................................................................... 237 

 7.16 Average thickness of color layers (cm) at Mississippi Canyon 

Block 292 sites ............................................................................................................ 238 

 7.17 Fauna from Mississippi Canyon Block 292 ................................................................ 238 

 7.18 Average depth of apparent color redox potential discontinuity (RPD) 

layer (cm) at Mississippi Canyon Block 292 far-field (FF) and 

near-field (NF) transects.............................................................................................. 239 

 8.1 Photographs showing (a) oxygen probe inserted in top of sediment core 

and close-up of electrode and electronic control box, (b) redox 

potential (Eh) electrode inserted in sediment core, and (c) whole-core 

squeezer ....................................................................................................................... 246 
List of Figures 

(continued) 

Figure Page 

xxviii 

 8.2 Concentrations of barium (Ba) with markers and numbers showing 

means and lines showing standard deviations in sediment from 

near-field (NF) and far-field (FF) stations at Viosca Knoll (VK) 916 

during pre-exploration drilling Cruise 1B and post-exploration drilling 

Cruise 3B..................................................................................................................... 255 

 8.3 Concentrations of barium (Ba) at Viosca Knoll (VK) 916 near-field 

(NF) stations during post-drilling Cruise 3B............................................................... 255 

 8.4 Vertical profiles for concentrations of barium (Ba) in sediment from 

discretionary (DS) stations during post-exploration Cruise 3B to 

Viosca Knoll (VK) 916 ............................................................................................... 256 

 8.5 Concentrations of barium (Ba) versus mercury (Hg) for sediments from 

far-field (FF) and near-field (NF) stations from Viosca Knoll (VK) 916 

collected during pre-exploration Cruise 1B and post-exploration 

Cruise 3B..................................................................................................................... 257 

 8.6 Vertical profiles for concentrations of mercury (Hg) in sediment from 

discretionary (DS) stations during post-exploration Cruise 3B to Viosca 

Knoll (VK) 916 ........................................................................................................... 257 

 8.7 Concentrations of total organic carbon (TOC) with circles and numbers 

showing means and lines showing standard deviations in sediment 

from near-field (NF) and far-field (FF) stations from Viosca Knoll 

(VK) 916 during pre-exploration drilling Cruise 1B and 

post-exploration drilling Cruise 3B ............................................................................. 259 

 8.8 Concentrations of barium (Ba) versus total organic carbon (TOC) for 

sediments from far-field (FF) and near-field (NF) stations from Viosca 

Knoll (VK) 916 collected during pre-exploration Cruise 1B and 

post-exploration Cruise 3B.......................................................................................... 259 

 8.9 Vertical profiles for concentrations of total organic carbon (TOC) in 

sediment from discretionary (DS) stations for post-exploration 

Cruise 3B to Viosca Knoll (VK) 916 .......................................................................... 260 

 8.10 Vertical profiles for activities of excess 210Pb in sediment from far-field 

(FF) and near-field (NF) stations at Viosca Knoll (VK) 916 for 

Cruise 1B..................................................................................................................... 261 

 8.11 Approximate redox potential (Eh) values at which various redox 

reactions occur in water (after Drever 1997)............................................................... 262 
List of Figures 

(continued) 

Figure Page 

xxix 

 8.12 Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in 

sediment from representative near-field (NF), far-field (FF), and 

discretionary (DS) stations from Viosca Knoll (VK) 916 (note different 

depth scales for oxygen versus Eh) ............................................................................. 263 

 8.13 Integrated amounts of oxygen in the sediment column at near-field 

(NF), far-field (FF), and discretionary (DS) stations at Viosca Knoll 

(VK) 916 for pre-exploration Cruise 1B and post-exploration 

Cruise 3B..................................................................................................................... 265 

 8.14 Vertical profiles for dissolved oxygen, Eh, nitrate, sulfate, manganese, 

and iron from interstitial water from Station FF-B02, Viosca Knoll 

(VK) 916, for pre-exploration Cruise 1B .................................................................... 266 

 8.15 Vertical profiles for dissolved oxygen, Eh, nitrate, sulfate, manganese, 

and iron from interstitial water from Station NF-B02, Viosca Knoll 

(VK) 916, for pre-exploration Cruise 1B .................................................................... 267 

 8.16 Vertical profiles for dissolved oxygen, Eh, sulfate, ammonia, 

manganese, and iron from interstitial water at Station FF-B03, Viosca 

Knoll (VK) 916, for post-exploration Cruise 3B......................................................... 269 

 8.17 Vertical profiles for dissolved oxygen, Eh, sulfate, ammonia, 

manganese, and iron from interstitial water at Station DS-2, Viosca 

Knoll (VK) 916, for post-exploration Cruise 3B......................................................... 270 

 8.18 Vertical profiles for concentrations of manganese (Mn) in sediment 

from discretionary (DS) stations for post-exploration Cruise 3B to 

Viosca Knoll (VK) 916 ............................................................................................... 271 

 8.19 Concentrations of aluminum (Al) versus (a) iron (Fe) and (b) barium 

(Ba) for sediments from far-field (FF), near-field (NF), and 

discretionary (DS) stations from Garden Banks (GB) 516 collected 

during Cruises 1B and 2B............................................................................................ 273 

 8.20 Concentrations of barium (Ba) with markers and numbers showing 

means and lines showing standard deviations in sediment from 

near-field (NF) and far-field (FF) stations at Garden Banks (GB) 516 

during Cruises 1B and 2B............................................................................................ 274 

 8.21 Concentrations of barium (Ba) at Garden Banks (GB) 516 near-field 

(NF) stations during Cruises 1B and 2B...................................................................... 275 
List of Figures 

(continued) 

Figure Page 

xxx 

 8.22 Vertical profiles for concentrations of barium (Ba) and aluminum (Al) 

in sediment from discretionary (DS) stations at Garden Banks 

(GB) 516 for Cruise 2B ............................................................................................... 277 

 8.23 Concentrations of aluminum (Al) versus manganese (Mn) for 

sediments from far-field (FF), and near-field (NF), and discretionary 

(DS) stations from Garden Banks (GB) 516 collected during 

Cruises 1B and 2B....................................................................................................... 277 

 8.24 Concentrations of barium (Ba) versus (a) mercury (Hg) and (b) lead 

(Pb) for sediments from far-field (FF) and near-field (NF) stations from 

Garden Banks (GB) 516 collected during Cruises 1B and 2B .................................... 278 

 8.25 Vertical profiles for concentrations of mercury (Hg) in sediment from 

discretionary (DS) stations for Cruise 2B at Garden Banks (GB) 516........................ 279 

 8.26 Concentrations of total organic carbon (TOC) with markers and 

numbers showing means and lines showing standard deviations in 

sediment from near-field (NF) and far-field (FF) stations from Garden 

Banks (GB) 516 during Cruises 1B and 2B ................................................................ 279 

 8.27 Concentrations of barium (Ba) versus total organic carbon (TOC) for 

sediments from far-field (FF), near-field (NF), and discretionary (DS) 

stations from Garden Banks (GB) 516 collected during Cruises 1B and 

2B ................................................................................................................................ 280 

 8.28 Vertical profiles for concentrations of total organic carbon (TOC) in 

sediment from discretionary (DS) stations for Cruise 2B at Garden 

Banks (GB) 516........................................................................................................... 280 

 8.29 Vertical profiles for activities of excess 210Pb in sediment from Garden 

Banks (GB) 516 for Station FF6 during Cruise 2B and Station NF-B02 

for Cruise 2B ............................................................................................................... 282

 8.30 Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in 

sediment from representative near-field (NF) and far-field (FF) stations 

at Garden Banks (GB) 516 from Cruise 1B ................................................................ 283 

 8.31 Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in 

sediment from representative near-field (NF), far-field (FF), and 

discretionary (DS) stations from Garden Banks (GB) 516 from 

Cruise 2B..................................................................................................................... 284 
List of Figures 

(continued) 

Figure Page 

xxxi 

 8.32 Integrated amounts of oxygen in the sediment column at near-field 

(NF), far-field (FF), and discretionary (DS) stations at Garden Banks 

(GB) 516 for Cruises 1B and 2B ................................................................................. 286 

 8.33 Vertical profiles for dissolved oxygen, Eh, nitrate, sulfate, manganese, 

and iron from interstitial water from Station FF-B02 at Garden Banks 

(GB) 516 during Cruise 1B ......................................................................................... 287 

 8.34 Vertical profiles for dissolved oxygen, Eh, ammonia, sulfate, 

manganese, and iron from interstitial water from Station NF-B01 at 

Garden Banks (GB) 516 during Cruise 1B.................................................................. 288 

 8.35 Vertical profiles for dissolved oxygen, Eh, ammonia, sulfate, 

manganese, and iron from interstitial water from Station FF-B06 at 

Garden Banks (GB) 516 during Cruise 2B.................................................................. 290 

 8.36 Vertical profiles for concentrations of manganese (Mn) in sediment 

from discretionary (DS) stations at Garden Banks (GB) 516 for 

Cruise 2B..................................................................................................................... 291 

 8.37 Concentrations of barium (Ba) with markers and numbers showing 

means and lines showing standard deviations in sediment from 

near-field (NF) and far-field (FF) stations at Garden Banks (GB) 602 

and Mississippi Canyon (MC) 292 during post-development Cruise 2B.................... 293 

 8.38 Concentrations of barium (Ba) at Garden Banks (GB) 602 and 

Mississippi Canyon (MC) 292 near-field (NF) stations during 

post-development Cruise 2B ....................................................................................... 294 

 8.39 Vertical profiles for concentrations of barium (Ba) in sediment from 

discretionary (DS) stations at Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292........................................................................................................ 295 

 8.40 Concentrations of barium (Ba) versus (a) lead (Pb) and (b) mercury 

(Hg) for sediments from far-field (FF) and near-field (NF) stations 

from Garden Banks (GB) 602 and Mississippi Canyon (MC) 292 

collected during Cruise 2B .......................................................................................... 296 

 8.41 Concentrations of mercury (Hg) with markers and numbers bars 

showing means and lines showing standard deviations in sediment 

from near-field (NF) and far-field (FF) stations at Garden Banks 

(GB) 602 and Mississippi Canyon (MC) 292 during Cruise 2B ................................. 297 
List of Figures 

(continued) 

Figure Page 

xxxii 

 8.42 Vertical profiles for concentrations of mercury (Hg) in sediment from 

discretionary (DS) stations at Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292........................................................................................................ 298 

 8.43 Concentrations of total organic carbon (TOC) with markers and 

numbers showing means and lines showing standard deviations in 

sediment from near-field (NF) and far-field (FF) stations at Garden 

Banks (GB) 602 and Mississippi Canyon (MC) 292 during 

post-development Cruise 2B ....................................................................................... 298 

 8.44 Vertical profiles for concentrations of total organic carbon (TOC) in 

sediment from discretionary (DS) stations at Garden Banks (GB) 602 

and Mississippi Canyon (MC) 292.............................................................................. 299 

 8.45 Vertical profiles for activities of excess 210Pb in sediment from Garden 

Banks (GB) 602 Station FF-B05 and excess 210Pb and 137Cs from 

Mississippi Canyon (MC) 292 Station FF-B02........................................................... 300 

 8.46 Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in 

sediment from representative near-field (NF), far-field (FF), and 

discretionary (DS) stations at Garden Banks (GB) 602 .............................................. 302 

 8.47 Integrated amounts of oxygen in the sediment column at near-field 

(NF), far-field (FF), and discretionary (DS) stations at Garden Banks 

(GB) 602 and Mississippi Canyon (MC) 292.............................................................. 303 

 8.48 Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in 

sediment from representative near-field (NF), far-field (FF), and 

discretionary (DS) stations at Mississippi Canyon (MC) 292..................................... 304 

 8.49 Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, 

manganese, and sulfate from interstitial water from Station FF-B05 at 

site Garden Banks (GB) 602 during post-development Cruise 2B.............................. 305 

 8.50 Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, 

manganese, and sulfate from interstitial water from discretionary 

station DS-2 at Garden Banks (GB) 602 during post-development 

Cruise 2B..................................................................................................................... 306 

 8.51 Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, and 

sulfate from interstitial water from Station FF-B02 at Mississippi 

Canyon (MC) 292 during post-development Cruise 2B.............................................. 307 
List of Figures 

(continued) 

Figure Page 

xxxiii 

 8.52 Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, 

manganese, and sulfate from interstitial water from discretionary 

station DS-2 at Mississippi Canyon (MC) 292 during post-development 

Cruise 2B..................................................................................................................... 308 

 8.53 Vertical profiles for concentrations of manganese (Mn) in sediment 

from discretionary (DS) stations at Garden Banks (GB) 602 and 

Mississippi Canyon (MC) 292 for post-development Cruise 2B ................................ 309 

 8.54 Photograph of giant amphipod Bathynomus giganteus from the Gulf of 

Mexico......................................................................................................................... 310 

 8.55 Photograph of red crab Chaceon quinquedens from the Gulf of Mexico.................... 310 

 8.56 Water content and concentrations of arsenic (As), barium (Ba), 

cadmium (Cd), chromium (Cr), and copper (Cu) in soft tissue from the 

isopod Bathynomus giganteus from near-field (NF) and far-field (FF) 

stations from post-development sites Garden Banks (GB) 602 and 

Mississippi Canyon (MC) 292..................................................................................... 312 

 8.57 Concentrations of iron (Fe), mercury (Hg), nickel (Ni), lead (Pb), 

vanadium (V), and zinc (Zn) in soft tissue from the isopod Bathynomus 

giganteus from near-field (NF) and far-field (FF) stations from 

post-development sites Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292........................................................................................................ 313 

 8.58 Water content and concentrations of arsenic (As), barium (Ba), 

cadmium (Cd), chromium (Cr), and copper (Cu) in soft tissue from the 

crab Chaceon from near-field (NF) and far-field (FF) stations from 

post-development sites Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292........................................................................................................ 315 

 8.59 Concentrations of iron (Fe), mercury (Hg), nickel (Ni), lead (Pb), 

vanadium (V), and zinc (Zn) in soft tissue from the crab Chaceon from 

near-field (NF) and far-field (FF) stations from post-development sites 

Garden Banks (GB) 602 and Mississippi Canyon (MC) 292...................................... 316 

 8.60 Concentrations of aluminum (Al) versus iron (Fe) for sediments from 

all stations.................................................................................................................... 317 

 8.61 Concentrations of aluminum (Al) versus vanadium (V) for all 

sediments collected during the project ........................................................................ 317 
List of Figures 

(continued) 

Figure Page 

xxxiv 

 8.62 Concentrations of aluminum (Al) versus zinc (Zn) for sediments from 

(a) far-field (FF) stations and (b) all stations (FF and near-field [NF], 

including discretionary [DS] stations)......................................................................... 319 

 8.63 Concentrations of aluminum (Al) versus lead (Pb) for sediments from 

far-field (FF) and near-field (NF) stations (data for discretionary [DS] 

stations are plotted as NF samples) ............................................................................. 320 

 8.64 Concentrations of aluminum (Al) versus arsenic (As) for sediments 

from far-field (FF) and near-field (NF) stations (data for discretionary 

[DS] stations are plotted as NF samples)..................................................................... 321 

 8.65 Concentrations of (a) aluminum (Al) versus mercury (Hg) and 

(b) barium (Ba) versus Hg for sediments from far-field (FF) and 

near-field (NF) stations (data for discretionary [DS] stations are plotted 

as NF samples) ............................................................................................................ 322 

 8.66 Concentrations of aluminum (Al) versus (a) cadmium (Cd) and 

(b) copper (Cu) for sediments from far-field (FF) and near-field 

(NF) stations (data for discretionary stations are plotted as NF samples)................... 323 

 8.67 Concentrations of aluminum (Al) versus (a) nickel (Ni) and 

(b) chromium (Cr) for sediments from far-field (FF) and near-field 

(NF) stations (data for discretionary stations are plotted as NF samples)................... 324 

 9.1 Typical background hydrocarbon distribution in deep sea sediment .......................... 328 

 9.2 A sediment hydrocarbon pattern that displays a synthetic based fluid 

pattern.......................................................................................................................... 328 

 9.3 Spatial distribution of synthetic-based fluid (SBF) concentrations at 

Viosca Knoll (VK) Block 916 on Cruise 1B (pre-drilling) and 

Cruise 3B (post-drilling).............................................................................................. 333 

 9.4 Spatial distribution of synthetic-based fluid (SBF) concentrations at 

Garden Banks (GB) Block 516 on Cruise 1B and Cruise 2B...................................... 336 

 9.5 Spatial distribution of synthetic-based fluid (SBF) concentrations at 

Garden Banks (GB) Block 602 on Cruise 2B.............................................................. 339 

 9.6 Spatial distribution of synthetic-based fluid (SBF) concentrations at 

Mississippi Canyon (MC) Block 292 on Cruise 2B.................................................... 342 
List of Figures 

(continued) 

Figure Page 

xxxv 

 10.1 A comparison between the recovery of a stable internal adenosine 

triphosphate (ATP) standard to the recovery of the 14C ATP adsorption 

standard ....................................................................................................................... 349 

 10.2 The effect of sediment grain size on the recovery of the 14C ATP 

extraction standard....................................................................................................... 350 

 10.3 Microbial adenosine triphosphate (ATP) concentrations at the 

four study sites............................................................................................................. 354 

 10.4 The relative toxicity of sediment extracts for selected sampling sites at 

VK 916 before and after drilling ................................................................................. 355 

 10.5 The relative toxicity of sediment extracts for selected sampling sites at 

GB 516 before and after drilling ................................................................................. 355 

 10.6 The relative toxicity of sediment extracts for selected sampling sites at 

a) GB 602 and b) MC 292 ........................................................................................... 356 

 10.7 Respired carbon dioxide from acetate in sediments from GB 516, 

Cruise 1B..................................................................................................................... 357 

 11.1 Total meiofaunal densities at the four study sites........................................................ 365 

 11.2 Taxonomic composition of meiofauna at Viosca Knoll Block 916 ............................ 366 

 11.3 Taxonomic composition of meiofauna at Garden Banks Block 516........................... 369 

 11.4 Taxonomic composition of meiofauna at Garden Banks Block 602 and 

Mississippi Canyon Block 292.................................................................................... 372 

 11.5 Comparison of near-field and far-field abundances of meiofaunal 

nematodes, harpacticoids, and annelids in post-drilling samples ................................ 375 

 11.6 Total macroinfaunal densities at the four study sites .................................................. 382 

 11.7 Taxonomic composition of macroinfauna at Viosca Knoll Block 916 ....................... 383 

 11.8 Taxonomic composition of macroinfauna at Garden Banks Block 516...................... 387 

 11.9 Taxonomic composition of macroinfauna at Garden Banks Block 602 

and Mississippi Canyon Block 292 ............................................................................. 390 

 11.10 Comparison of near-field and far-field abundances of macroinfaunal 

annelids, gastropods, and bivalves in post-drilling samples........................................ 393 
List of Figures 

(continued) 

Figure Page 

xxxvi 

 11.11 Comparison of near-field and far-field abundances of macroinfaunal 

amphipods, aplacophorans, and ostracods in post-drilling samples............................ 394 

 11.12 Annelid (predominantly polychaete) densities vs. barium 

concentrations on post-drilling cruises........................................................................ 396 

 11.13 Annelid (predominantly polychaete) densities vs. synthetic-based fluid 

(SBF) concentrations on post-drilling cruises ............................................................. 397 

 11.14 Gastropod densities vs. sediment barium, synthetic-based fluid (SBF), 

and total organic carbon concentrations at near-field (NF), post-drilling 

stations......................................................................................................................... 398 

 11.15 Amphipod densities vs. sediment barium concentrations on 

post-drilling cruises ..................................................................................................... 399 

 11.16 Ostracod densities vs. sediment barium concentrations on post-drilling 

cruises.......................................................................................................................... 400 

 11.17 Macroinfaunal community characteristics for the detailed, 24-station 

analysis ........................................................................................................................ 405 

 11.18 Station/cruise groups resolved by multidimensional scaling of 

macroinfaunal data from the 24-station dataset........................................................... 406 

 11.19 Comparison of sediment barium, synthetic-based fluid (SBF), and total 

organic carbon concentrations at station groups identified from 

multidimensional scaling ordination ........................................................................... 408 

 12.1 Locations of camera sled transects at Viosca Knoll (VK) Block 916 

near-field (NF) site ...................................................................................................... 417 

 12.2 Locations of camera sled transects at Garden Banks (GB) Block 516 

near-field (NF) site ...................................................................................................... 418 

 12.3 Locations of camera sled transects at Garden Banks (GB) Block 602 

near-field (NF) site ...................................................................................................... 419 

 12.4 Locations of camera sled transects at Mississippi Canyon (MC) Block 

292 near-field (NF) site ............................................................................................... 420 

 12.5 Image analysis results for Viosca Knoll (VK) Block 916, Cruise 1B, 

near-field transects ...................................................................................................... 423 
List of Figures 

(continued) 

Figure Page 

xxxvii 

 12.6 Image analysis results for Viosca Knoll (VK) Block 916, Cruise 3B, 

Near-field Transect 1................................................................................................... 424 

 12.7 Image analysis results for Viosca Knoll (VK) Block 916, Cruise 3B, 

Near-field Transect 2................................................................................................... 425 

 12.8 Locations of example photographs in relation to geophysically mapped 

cuttings deposits at Viosca Knoll (VK) Block 916 on Cruise 3B ............................... 426 

 12.9 Image analysis results for Garden Banks (GB) Block 516, Cruise 1B, 

Near-field Transect 1................................................................................................... 427 

 12.10 Image analysis results for Garden Banks (GB) Block 516, Cruise 2B, 

Near-field Transect 1................................................................................................... 428 

 12.11 Image analysis results for Garden Banks (GB) Block 516, Cruise 1B, 

far-field transects......................................................................................................... 429

 12.12 Image analysis results for Garden Banks (GB) Block 602, Cruise 2B, 

Near-field Transect 3................................................................................................... 431 

 12.13 Image analysis results for Garden Banks (GB) Block 602, Cruise 2B, 

far-field transects......................................................................................................... 432

 12.14 Image analysis results for Mississippi Canyon (MC) Block 292, 

Cruise 2B, Near-field Transect 3................................................................................. 433 

 12.15 Image analysis results for Mississippi Canyon (MC) Block 292, 

Cruise 2B, far-field transects....................................................................................... 434 

 13.1 Relationship between synthetic based fluid (SBF) concentrations and 

the number of Bathycletopsyllus sp. per box core at the near-field sites..................... 446 

 13.2 Haplotype network of 15 unique cytochrome c oxidase subunit I COX I 

sequences..................................................................................................................... 448 

 13.3 Relationship between haplotype diversity and nucleotide diversity of 

COX I sequences for five species of harpacticoid copepods including 

Bathycletopsyllus sp. ................................................................................................... 451 

 13.4 Mismatch distributions for pairwise haplotype distances between 

Bathycletopsyllus sp. individuals collected at Garden Banks (GB) 

Block 602 and Mississippi Canyon (MC) Block 292.................................................. 452 

 14.1 Sediment toxicity test system ...................................................................................... 461 
List of Figures 

(continued) 

Figure Page 

xxxviii 

 14.2 Regression of barium concentration against fraction of amphipods 

surviving in a 10-day sediment toxicity test with near-field and far-field 

sediments from Mississippi Canyon Block 292 .......................................................... 465 

 14.3 Regression of synthetic based fluid (SBF) concentration against 

fraction of amphipods surviving in a 10-day sediment toxicity test with 

near-field and far-field sediments from Mississippi Canyon Block 292..................... 466 

 14.4 Regression of barium concentration against fraction of amphipods 

surviving in a 10-day sediment toxicity test in near-field and far-field 

sediments at Garden Banks Block 602........................................................................ 467 

 14.5 Regression of synthetic based fluid (SBF) concentration against 

fraction of amphipods surviving in a 10-day sediment toxicity test with 

near-field and far-field sediments from Garden Banks Block 602.............................. 468 

 14.6 Regression of polycyclic aromatic hydrocarbon (PAH) concentration 

against fraction of amphipods surviving in a 10-day sediment toxicity 

test with near-field and far-field sediments from Garden Banks 

Block 602 .................................................................................................................... 469 

 15.1 Microbial biomass and meiofaunal and macroinfaunal densities at 

far-field sites................................................................................................................ 473 

 15.2 Extent of geophysically mapped drilling mud and cuttings deposits 

around (a) Viosca Knoll 916 on Cruise 3A; (b) Garden Banks 516 on 

Cruise 2A; (c) Garden Banks 602 on Cruise 2A; and (d) Mississippi 

Canyon 292 on Cruise 2A ........................................................................................... 481 

 15.3 Side-scan sonar mosaic showing highly reflective seafloor (dark areas, 

presumed rig discharge deposits) at Viosca Knoll 916 on Cruise 3A 

(post-exploration) ........................................................................................................ 484 

 15.4 Distribution of highly reflective seafloor (dark areas, presumed rig 

discharge deposits) at Garden Banks 516 on Cruise 2A 

(post-development)...................................................................................................... 485 

 15.5 Isopach (thickness) plot of geophysically mapped drilling deposits at 

Viosca Knoll 916......................................................................................................... 486 

 15.6 Sediment percentages of sand, silt, and clay in relation to proximity to 

drilling ......................................................................................................................... 487 

 15.7 Far-field concentrations of barium and synthetic-based fluids ................................... 490 
List of Figures 

(continued) 

Figure Page 

xxxix 

 15.8 Barium and synthetic-based fluid (SBF) concentrations on Cruise 3B at 

Viosca Knoll 916 site in relation to geophysically mapped rig 

discharge deposits........................................................................................................ 492 

 15.9 Barium and synthetic-based fluid (SBF) concentrations on Cruise 2B at 

Garden Banks 516 in relation to geophysically mapped mud and 

cuttings deposits .......................................................................................................... 493 

 15.10 Barium and synthetic-based fluid (SBF) concentrations on Cruise 2B at 

Garden Banks 602 in relation to geophysically mapped mud and 

cuttings deposits.......................................................................................................... 494 

 15.11 Barium and synthetic-based fluid (SBF) concentrations on Cruise 2B at 

Mississippi Canyon 292 site in relation to geophysically mapped mud 

and cuttings deposits ................................................................................................... 495 

 15.12 Sediment concentrations of barium, synthetic-based fluids (SBF), and 

polycyclic aromatic hydrocarbons (PAHs) in relation to proximity to 

drilling......................................................................................................................... 496 

 15.13 Relationship between total organic carbon (TOC) and synthetic-based 

fluid (SBF) concentrations in near-field sediments at the four sites ........................... 500 

 15.14 Integrated amounts of oxygen in the sediment column at near-field 

(NF), far-field (FF), and discretionary (DS) stations at all four sites.......................... 501 

 15.15 Redox potential (Eh) values at 1 to 3 cm depth in the sediment column 

at Viosca Knoll 916 and Garden Banks 516 on post-exploration cruises 

in relation to geophysically mapped mud and cuttings deposits................................. 503 

 15.16 Redox potential (Eh) values at 1 to 3 cm depth in the sediment column 

at Garden Banks 516, Garden Banks 602, and Mississippi Canyon 292 

on post-development cruises in relation to geophysically mapped mud 

and cuttings deposits ................................................................................................... 504 

 15.17 Microbial adenosine triphosphate (ATP) concentrations at the four 

study sites.................................................................................................................... 508 

 15.18 Microbial adenosine triphosphate (ATP) concentrations at Viosca 

Knoll 916 and Garden Banks 516 on post-drilling cruises in relation to 

sediment synthetic-based fluid (SBF) concentrations................................................. 510 

 15.19 Microbial adenosine triphosphate (ATP) concentrations at Garden 

Banks 602 and Mississippi Canyon 292 on Cruise 2B (post-drilling) in 

relation to sediment synthetic-based fluid (SBF) concentrations................................ 511 
List of Figures 

(continued) 

Figure Page 

xl 

 15.20 Meiofaunal and macroinfaunal densities in relation to proximity to 

drilling......................................................................................................................... 513 

 15.21 Annelid (predominantly polychaete) densities vs. synthetic-based fluid 

(SBF) concentrations on post-drilling cruises............................................................. 514 

 15.22 Gastropod densities vs. sediment barium, synthetic-based fluid (SBF), 

and total organic carbon concentrations at near-field (NF), post-drilling 

stations ........................................................................................................................ 515 

 15.23 Amphipod densities vs. sediment barium concentrations on 

post-drilling cruises..................................................................................................... 517 

 15.24 Macroinfaunal community characteristics for the detailed, 24-station 

analysis........................................................................................................................ 518 

 15.25 Fish abundance along camera sled transects at each site ............................................ 519 

 15.26 Ophiuroid abundance along camera sled transects at each site................................... 520 

 15.27 Geophysically mapped areal extent of drilling discharge deposits in 

relation to (a) number of wells drilled and (b) estimated 

synthetic-based mud (SBM) cuttings discharges ........................................................ 524 

 15.28 Areal extent of cuttings deposits (number of near-field stations with 

very high barium or synthetic-based fluid [SBF] concentrations) in 

relation to estimated synthetic-based mud (SBM) and water-based mud 

(WBM) cuttings discharges......................................................................................... 525 

 15.29 Relationship between post-drilling sediment barium and 

synthetic-based fluid (SBF) concentrations within near-field sites and 

estimated volumes of SBF cuttings discharges........................................................... 527 

 15.30 Difference between mean near-field and far-field sediment oxygen 

levels in relation to sediment synthetic-based fluid (SBF) 

concentrations and estimated SBF cuttings discharges............................................... 528 

 15.31 Plots of (a) geometric mean synthetic-based fluid (SBF) concentration 

and (b) geophysically mapped areal extent of drilling discharge 

deposits in relation to estimated synthetic-based mud (SBM) cuttings 

discharges.................................................................................................................... 529 

 16.1 Facilities from which offshore wells are drilled change with water 

depth ............................................................................................................................ 537 
List of Figures 

(continued) 

Figure Page 

xli 

 16.2 Dispersion and fates of drilling muds and cuttings following discharge to 

the ocean............................................................................................................................... 547 

 16.3 Total meiofaunal densities in near-field (NF) and far-field (FF) 

sediments from the four study sites............................................................................. 570 

 16.4 Total macroinfaunal densities in near-field (NF) and far-field (FF) 

sediments from the four study sites............................................................................. 570 
xliii 

List of Tables 

Table Page 

 2.1 Coordinates of near-field and far-field sites.................................................................... 6 

 2.2 Sampling cruises.............................................................................................................. 8 

 2.3 Numbers of samples collected and transects completed during 

chemical/biological cruises........................................................................................... 25 

 2.4 Numbers of animals collected from baited trap sets during Cruise 2B ......................... 25 

 3.1 Wells drilled within 10 km of the Viosca Knoll (VK) Block 916 

near-field (NF) site........................................................................................................ 32 

 3.2 Discharge summary for Viosca Knoll Block 916 Well No. 1 

(27 November to 18 December 2001) .......................................................................... 34 

 3.3 Wells drilled within 10 km of the Garden Banks (GB) Block 516 

near-field (NF) site........................................................................................................ 35 

 3.4 Discharge summary for Garden Banks Block 516 wells within the 

near-field site ................................................................................................................ 37 

 3.5 Wells drilled within 10 km of the Garden Banks (GB) Block 602 

near-field (NF) site center............................................................................................. 39 

 3.6 Discharge summary for Garden Banks Block 602 wells within the 

near-field site ................................................................................................................ 41 

 3.7 Wells drilled within 10 km of the Mississippi Canyon (MC) Block 292 

near-field (NF) site........................................................................................................ 42 

 3.8 Discharge summary for Mississippi Canyon Block 292 wells within the 

near-field site ................................................................................................................ 45 

 3.9 Drilling summary for wells within each near-field site................................................. 48 

 3.10 Previous wells near far-field (FF) sites.......................................................................... 49 

 4.1 Geophysical surveys...................................................................................................... 60 

 5.1 Summary of baseline grain size characteristics for sites based on 

far-field data.................................................................................................................. 96 

 5.2 Analysis of variance (ANOVA) results for sediment grain size ................................... 96 

 5.3 Sediment grain size results for Viosca Knoll Block 916............................................... 98 
List of Tables 

(continued) 

Table Page 

xliv 

 5.4 Sediment grain size results for Garden Banks Block 516 ........................................... 106 

 5.5 Sediment grain size results for Garden Banks Block 602 ........................................... 114 

 5.6 Sediment grain size results for Mississippi Canyon Block 292 .................................. 122 

 6.1 Comparison between Cruise 1B and Cruise 2B sediment profile 

imaging (SPI) survey results for Garden Banks Block 516 near-field 

stations ........................................................................................................................ 201 

 6.2 Comparison between Cruise 1B (pre-drilling) and Cruise 3B 

(post-drilling) sediment profile imaging (SPI) survey results for Viosca 

Knoll Block 916 near-field stations ............................................................................ 202 

 7.1 Phi scale sizes and sediment descriptors ..................................................................... 224 

 8.1 Number of samples collected and analyzed for various parameters at 

each site ...................................................................................................................... 244 

 8.2 Summary of instrumental methods and method detection limits 

(MDLs) for metal analysis of sediment and organisms .............................................. 249 

 8.3 Data quality objectives and criteria ............................................................................. 249 

 8.4 Means ± standard deviations with ranges in parentheses for metals, and 

total organic carbon (TOC) for Viosca Knoll 916 for pre- and 

post-exploration cruises .............................................................................................. 253 

 8.5 Results of statistical comparisons (t-tests) for concentrations of metals 

among cruises (1B for pre-exploration and 3B for post-exploration) 

and zones for Viosca Knoll 916.................................................................................. 254 

 8.6 Summary data for dissolved oxygen, redox potential (Eh), and pH for 

sediment from Viosca Knoll (VK) 916 at near-field (NF), far-field 

(FF), and discretionary (DS) stations during pre-exploration Cruise 1B 

and post-exploration Cruise 3B .................................................................................. 260 

 8.7 Selected reactions showing the decomposition of organic matter by 

various oxidation agents (after Froelich et al. 1979) .................................................. 265 

 8.8 Means ± standard deviations with ranges in parentheses for metals, and 

total organic carbon (TOC) for Garden Banks 516 for Cruises 1B and 

2B................................................................................................................................ 272 
List of Tables 

(continued) 

Table Page 

xlv 

 8.9 Results of statistical comparisons (t-tests) for concentrations of metals 

among cruises (1B and 2B) and zones (NF and FF) for Garden 

Banks 516 ................................................................................................................... 274

 8.10 Summary data for dissolved oxygen, redox potential (Eh), and pH for 

sediment from Garden Banks (GB) 516 at near-field (NF), far-field 

(FF), and discretionary (DS) stations during Cruises 1B and 2B ............................... 285 

 8.11 Means, standard deviations, and ranges for metals and total organic 

carbon (TOC) for Garden Banks (GB) 602 and Mississippi Canyon 

(MC) 292 during post-development Cruise 2B .......................................................... 292 

 8.12 Results of statistical comparisons (t-tests) for post-development Garden 

Banks (GB) 602 and Mississippi Canyon (MC) 292 during Cruise 2B...................... 293 

 8.13 Summary data for dissolved oxygen, redox potential (Eh), and pH for 

sediment from Garden Banks (GB) 602 and Mississippi Canyon 

(MC) 292 at near-field (NF), far-field (FF), and discretionary (DS) 

stations during post-development Cruise 2B .............................................................. 301 

 8.14 Means ± standard deviations for samples of the giant amphipod 

Bathynomus giganteus from far-field (FF) and near-field (NF) stations 

from post-development sites Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292 ....................................................................................................... 311 

 8.15 Means ± standard deviations for samples of the red crab Chaceon 

quinquedens from far-field (FF) and near-field (NF) stations from 

post-development sites Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292 ....................................................................................................... 311 

 9.1 Sediment synthetic based fluid (SBF) and total polycyclic aromatic 

hydrocarbon (PAH) concentrations at Viosca Knoll Block 916................................. 331 

 9.2 Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) sediment concentrations in the Viosca Knoll Block 916 

discretionary box core samples from Cruise 3B (August 2002)................................. 332 

 9.3 Sediment synthetic based fluid (SBF) and total polycyclic aromatic 

hydrocarbon (PAH) concentrations at Garden Banks Block 516 ............................... 334 

 9.4 Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) sediment concentrations in the Garden Banks Block 516 

discretionary box core samples from Cruise 2B (July 2001)...................................... 335 
List of Tables 

(continued) 

Table Page 

xlvi 

 9.5 Sediment synthetic based fluid (SBF) and total polycyclic aromatic 

hydrocarbon (PAH) concentrations at Garden Banks Block 602 on 

Cruise 2B (July 2001)................................................................................................. 337 

 9.6 Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) sediment concentrations in the Garden Banks Block 602 

discretionary box core samples from Cruise 2B (July 2001)...................................... 338 

 9.7 Sediment synthetic based fluid (SBF) and total polycyclic aromatic 

hydrocarbon (PAH) concentrations at Mississippi Canyon Block 292 

on Cruise 2B (July 2001)............................................................................................ 340 

 9.8 Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) sediment concentrations in the Mississippi Canyon Block 292 

discretionary box core samples from Cruise 2B (July 2001)...................................... 341 

 9.9 Summary of polycyclic aromatic hydrocarbons (PAH) in tissue 

samples from Garden Banks Block 602 (GB 602) and Mississippi 

Canyon Block 292 (MC 292)...................................................................................... 343 

 10.1 Numbers of microbiotal subsamples collected from box cores................................... 347 

 10.2 Sediment adenosine triphosphate (ATP) concentrations (ng g-1 dry wt) 

for all near-field and far-field sites ............................................................................. 352 

 11.1 Meiofaunal community statistics for Viosca Knoll Block 916 ................................... 364 

 11.2 ANOVA results for meiofaunal groups....................................................................... 367 

 11.3 Meiofaunal community statistics for Garden Banks Block 516.................................. 368 

 11.4 Meiofaunal community statistics for Garden Banks Block 602, 

Cruise 2B (July 2001)................................................................................................. 371 

 11.5 Meiofaunal community statistics for Mississippi Canyon Block 292, 

Cruise 2B (July 2001)................................................................................................. 374 

 11.6 Rank correlation coefficients (Spearman’s rho) between meiofaunal 

densities and environmental variables within areas exposed to drilling 

discharges (near-field sites on post-drilling cruises) .................................................. 377 

 11.7 Additional meiofaunal analyses from Cruise 2B: nematode trophic 

groups and harpacticoid copepod reproductive condition .......................................... 377 

 11.8 Macroinfaunal community statistics for Viosca Knoll Block 916 .............................. 381 
List of Tables 

(continued) 

Table Page 

xlvii 

 11.9 ANOVA results for macroinfaunal groups.................................................................. 384 

 11.10 Macroinfaunal community statistics for Garden Banks Block 516............................. 386 

 11.11 Macroinfaunal community statistics for Garden Banks Block 602, 

Cruise 2B (July 2001)................................................................................................. 389 

 11.12 Macroinfaunal community statistics for Mississippi Canyon Block 292, 

Cruise 2B (July 2001)................................................................................................. 391 

 11.13 Rank correlation coefficients (Spearman’s rho) between macroinfaunal 

densities and environmental variables within areas exposed to drilling 

discharges (near-field sites on post-drilling cruises) .................................................. 395 

 11.14 Samples included in the 24-station analysis of macroinfauna..................................... 402 

 11.15 Ten most abundant macroinfaunal taxa for the 24-station analysis ............................ 403 

 11.16 Macroinfauna community statistics for the 24-station analysis .................................. 403 

 11.17 Station groups (A through D) resolved by multi-dimensional scaling 

(MDS)......................................................................................................................... 407 

 11.18 Average abundance of infaunal species accounting for at least 50% of 

the within-group similarity in station (sample) groups A through D.......................... 409 

 12.1 Results of photographic faunal survey ........................................................................ 435 

 12.2 Results of comparisons between near-field and far-field tows.................................... 438 

 13.1 Number of Bathycletopsyllus sp. collected from box cores at the 

near-field sites of Garden Banks (GB) Block 602 and Mississippi 

Canyon (MC) Block 292............................................................................................. 447 

 13.2 Distribution of Bathycletopsyllus sp. haplotypes at each sampled 

location in Garden Banks Block 602 and Mississippi Canyon 

Block 292.................................................................................................................... 449 

 13.3 Analysis of molecular variance across the populations from the Garden 

Banks 602 and Mississippi Canyon 292 sites............................................................. 449 

 14.1 Characteristics of the two development drilling sites from which 

sediments for solid phase toxicity testing were collected........................................... 459 

 14.2 Toxicity test conditions ............................................................................................... 461 
List of Tables 

(continued) 

Table Page 

xlviii 

 14.3 Stocking density for test chambers found at test completion to have 

more than 20 amphipods............................................................................................. 462 

 14.4 Amphipod survival in the toxicity tests with sediments from 

Mississippi Canyon Block 292, performed 7 to 17 August 2001 ............................... 462 

 14.5 Amphipod survival in the toxicity tests with sediments from Garden 

Banks Block 602, performed 20 to 30 August 2001................................................... 464 

 15.1 Comparison of baseline environmental characteristics based on data 

from far-field sites....................................................................................................... 472 

 15.2 Drilling summary for wells within each near-field site............................................... 475 

 15.3 Impact summary.......................................................................................................... 477 

 15.4 Geophysically estimated areal extent of drilling mud and cuttings 

deposits at each near-field site .................................................................................... 480 

 15.5 Confidence intervals for far-field barium and synthetic-based fluid 

(SBF) concentrations................................................................................................... 491 

 15.6 Organism-sediment index (OSI) and successional stage for Viosca 

Knoll 916 (VK 916) and Garden Banks 516 (GB 516)............................................... 506 

 15.7 Presence of microbial mats as recorded in sediment profile images........................... 509 

 15.8 Comparison of sites with respect to areal extent of mud and cuttings 

deposits........................................................................................................................ 522 

 15.9 Comparison of near-field sites with respect to impact severity .................................. 523 

 15.10 Timing of drilling vs. cruise dates............................................................................... 526 

 15.11 Impacts observed at various time intervals after drilling, including 

wells in blocks adjacent to the study sites................................................................... 530 

 16.1 Summary of drilling activities within 500 m of the four drillsites before 

and during this study ................................................................................................... 538 

 16.2 Names and chemical structures of synthetic chemicals used most 

frequently in the Gulf of Mexico for SBM (From Neff et al., 2000) .......................... 543 

 16.3 Concentration ranges of several metals in water based muds (WBM) 

from different sources and in typical soils and marine sediments .............................. 544 
List of Tables 

(continued) 

Table Page 

xlix 

 16.4 Concentrations of synthetic based fluid (SBF) chemical in near-field, 

far-field, and discretionary box core sediments from the four sites 

monitored in this investigation.................................................................................... 552 

 16.5 Concentrations of hydrocarbons in composite samples of water based 

mud (WBM) and drill cuttings (in parentheses) from three drilling 

depths in a well in the Point Arguello Field, California.............................................. 553 

 16.6 Concentrations of total polycyclic aromatic hydrocarbons (PAH) in 

near-field, far-field, and discretionary box core sediments from the four 

sites.............................................................................................................................. 554 

 16.7 Mean (± standard deviation) and range of concentrations of total 

organic carbon in sediments from the four study sites................................................ 555 

 16.8 Mean (± standard deviation) and range of concentrations of barium in 

sediments from the four study sites............................................................................. 556 

 16.9 Mean (± standard deviation) and range of concentrations of mercury in 

sediments from the four study sites............................................................................. 557 

 16.10 Mean (± standard deviation) and range of concentrations of lead in 

sediments from the four study sites............................................................................. 558 

 16.11 Mean (± standard deviation) and range of concentrations of zinc in 

sediments from the four study sites............................................................................. 559 

 16.12 Concentrations of total polycyclic aromatic hydrocarbons (PAH) (μg/g 

dry wt) in whole soft tissues of giant isopods Bathynomus giganteus

and red crabs Chaceon quinquedens collected in July 2001 from 

MC 292 and GB 602 ................................................................................................... 560 

 16.13 Mean (± standard deviation) concentrations of 11 metals in whole soft 

tissues of giant isopods Bathynomus giganteus and red crabs Chaceon 

quinquedens collected in July 2001 from MC 292 and GB 602 ................................. 561 

 16.14 Summary of evidence of physical disturbance to or chemical exposure 

of the near-field benthic environment at the four study sites...................................... 562 

 16.15 Survival of amphipods Leptocheirus plumulosus in 10-day sediment 

toxicity tests with 12 replicate sediment samples each from near-field 

and far-field stations at MC 292 and GB 602 ............................................................. 564 
List of Tables 

(continued) 

Table Page 

l 

 16.16 Ranges of redox potential discontinuity (RPD) depths and Eh at 1 cm 

in sediments from near-field (NF), discretionary (DS), and far-field 

(FF) zones at the four study sites ................................................................................ 567 

 16.17 Sediment adenosine triphosphate (ATP) concentrations (ng/g dry wt) 

in surface sediments collected at 12 near-field and 6 far-field stations at 

the four study sites ...................................................................................................... 568 

 16.18 Benthic macrofaunal community parameters for near-field (NF) and 

far-field (FF) sediments at the four study sites ........................................................... 571 

 16.19 Summary of evidence of biological injury to the near-field benthic 

environment at the four study sites ............................................................................. 574 
li 

List of Acronyms and Abbreviations 

AAS atomic absorption 

spectrometry 

ACL Acoustic Command Link 

ADL Acoustic Data Link 

Al aluminum 

AMOVA analysis of molecular variance 

ANOVA analysis of variance 

AODC acridine orange direct count 

API American Petroleum Institute 

As arsenic 

ASTM American Society for Testing 

and Materials 

ATP adenosine triphosphate 

AUV autonomous underwater 

vehicle 

Ba barium 

BaSO4 barite (barium sulfate) 

bbl barrel 

Bp base pair 

Cd cadmium 

COX I cytochrome c oxidase 

subunit I 

Cr chromium 

CRM certified reference material 

Cu copper 

CVAAS cold vapor atomic absorption 

spectrometry 

DGPS differential global positioning 

system 

DIW deionized water 

DNA deoxyribonucleic acid 

DOC dissolved organic carbon 

DS discretionary station 

EDTA ethylenediaminetetraacetic 

acid 

Eh redox potential 

FAAS flame atomic absorption 

spectrometry 

Fe iron 

FF far-field 

FIT Florida Institute of 

Technology 

FOV field of view 

GB 516 Garden Banks Block 516 

GB 602 Garden Banks Block 602 

GC/MS gas chromatography-mass 

spectrometry 

GC-FID gas chromatography-flame 

ionization detection 

GFAAS graphite furnace atomic 

absorption spectrometry 

GIS geographic information 

system 

GOOMEX Gulf of Mexico Offshore 

Operations Monitoring 

Experiment 

H2O2 hydrogen peroxide 

HCl hydrochloric acid 

Hg mercury 

HiPAP High Precision Acoustic 

Positioning 

HSD Honest Significant Difference 

IAPSO International Association for 

the Physical Sciences of the 

Ocean 

IC ion chromatography 

ICP-MS inductively coupled plasma – 

mass spectrometry 

ID internal diameter 

IO internal olefin 

IS internal standard 

kHz kilohertz 

LAO linear-alpha-olefin 

LPIL Lowest Practical 

Identification Level 

MAFLA Mississippi-Alabama-Florida 

MC 292 Mississippi Canyon Block 292 

MDL method detection limit 

MDS multidimensional scaling 

MMS Minerals Management 

Service 

Mn manganese 

mtDNA mitochondrial 

deoxyribonucleic acid 

NF near-field 

NH4

+ ammonium 

Ni nickel 

NIST National Institute of Standards 

and Technology 

NO3

- nitrate 

NOAA National Oceanic and 

Atmospheric Administration 

NRC National Research Council 

NTIS National Technical 

Information Service 
List of Acronyms and Abbreviations 

(continued) 

lii 

OSI organism-sediment index 

PAH polycyclic aromatic 

hydrocarbon 

PAO poly-alpha-olefin 

Pb lead 

PCR polymerase chain reaction 

PO4

3- phosphate 

REMOTS remote ecological monitoring 

of the seafloor 

RGB red, green, and blue 

RPD redox potential discontinuity 

RRF relative response factor 

rRNA ribosomal ribonucleic acid 

RSD relative standard deviation 

SBF synthetic-based fluid 

SBM synthetic-based mud 

SD standard deviation 

SDS sodium dodecyl sulfate 

SIMPER similarity percentage 

breakdown 

SO4

2- sulfate 

SOP standard operating procedure 

SPI sediment profile imaging 

ST sidetrack 

TOC total organic carbon 

UCM unresolved complex mixture 

USEPA United States Environmental 

Protection Agency 

V vanadium 

VK 916 Viosca Knoll Block 916 

WBM water-based mud 

Zn zinc 
Chapter 1 

Introduction 

Neal W. Phillips and Alan D. Hart 

Continental Shelf Associates, Inc. 

This is the final report of a field study to assess the impacts of oil and gas exploration and 

development at four sites on the Gulf of Mexico continental slope (Figure 1.1). The study was 

conducted for the Minerals Management Service (MMS) by a team of scientists under the 

management of Continental Shelf Associates, Inc. Key participants are listed in the 

Acknowledgments, and authors/principal investigators are listed at the head of each chapter. 

1.1 BACKGROUND 

The deepwater Gulf of Mexico (water depths greater than or equal to 305 m) has seen a 

remarkable increase in oil and gas exploration, development, and production in recent years 

(Cranswick and Regg 1997; Baud et al. 2000; Richardson et al. 2004; French et al. 2005). 

Favorable economics, the development of innovative technologies, the announcement of large 

deepwater discoveries, the passage of the Deep Water Royalty Relief Act, and the opportunity to 

lease new prospects have all contributed to the revitalization of exploration and development in 

the Gulf of Mexico (Ward 2000; Richardson et al. 2004). As of March 2005, more than 900 

exploration wells had been drilled in the deepwater Gulf, at least 115 deepwater discoveries had 

been announced, and there were 107 deepwater production projects (French et al. 2005). The 

number of wells drilled in the deepwater Gulf has increased dramatically in the past decade, with 

much of the activity in water depths of 450 to 1,500 m (Figure 1.2). 

LOUISIANA

MISSISSIPPI

Gulf of Mexico

0 25 50 75 100 NAUTICAL MILES

Houston

Galveston

Mobile

New

Orleans

96o 94o 92o 90o 88o

30o

28o

26o

Corpus 

Christi

N TEXAS

2000 m

1000 m

3000 m

2000 m

1000 m

3000 m

Viosca Knoll 916

Mississippi 

Canyon 

292

Garden Banks 516

Legend

= Study Site

Garden Banks 602

ALABAMA

Figure 1.1. Study sites. 

1 
0

50

100

150

200

250

199219931994

19951996

19971998

1999

2000

200120022003

Year

No. of Wells

Other deepwater wells

Water depths 450-1,500 m

Figure 1.2. Number of wells drilled in the deepwater Gulf of Mexico over the past decade 

(From: Richardson et al. 2004). 

Deepwater operations are significantly different from conventional operations on the continental 

shelf, which have been studied for decades. They are farther from shore, encounter different 

environmental conditions, are technologically more sophisticated, and may involve much higher 

production rates. These differences present many technical and regulatory challenges (MMS 

2000a, 2004a). MMS has responded by conducting environmental workshops (Carney 1998; 

MMS 2003), preparing environmental impact assessments (MMS 2000a,b), sponsoring scientific 

literature reviews (Continental Shelf Associates, Inc. 2000; Gallaway et al. 2001; Nowlin et al. 

2001), initiating new field studies of deepwater benthic communities (MacDonald 2002; Rowe 

and Kennicutt 2002), and requiring additional environmental information from deepwater 

operators (MMS 2004b). 

This study addresses one aspect of deepwater operations: the impacts of drilling activities on the 

benthic environment around wellsites. Studies on the continental shelf have documented impacts 

of water-based drilling fluids and cuttings on benthic organisms near wellsites (National Research 

Council 1983; Neff 1987; Neff et al. 1989a; Hinwood et al. 1994; Hyland et al. 1994). However, 

because of the environmental and technological differences, the applicability of this knowledge to 

deepwater benthic communities is unknown (Carney 1998, 2001). For example, the increased use 

of synthetic-based drilling muds (SBMs) may result in volumes and characteristics of discharged 

material quite different from those used for decades in shallower water (Neff et al. 2000; Boehm 

et al. 2001). However, other than opportunistic studies (Gallaway et al. 1997), there have been 

few field measurements and observations around deepwater drillsites in the Gulf of Mexico. One 

other benthic study that included some deepwater sites (the Gulf of Mexico Comprehensive 

Synthetic Based Muds Monitoring Program) was conducted by a consortium of offshore 

operators, mud companies, chemical companies, the MMS, and the Department of Energy 

(Continental Shelf Associates, Inc. 2004). 

2 
3 

1.2 STUDY OBJECTIVES 

The objectives of this study were to assess the physical, chemical, and biological impacts of oil 

and gas development at selected exploration and development wellsites on the Gulf of Mexico 

continental slope. The findings from this study will assist the MMS in conducting environmental 

analyses, as well as in developing mitigative measures and regulations specifically tailored to 

deepwater operations. 

Specific objectives were to document (1) drilling mud and cuttings accumulations; (2) physical 

modification/disturbance of the seabed due to anchors and their mooring systems; (3) debris 

accumulations; (4) physical/chemical modification of sediments; and (5) effects on benthic 

organisms. 

1.3 REPORT ORGANIZATION 

Study design and general sampling methodology are discussed in Chapter 2. This includes site 

selection, statistical design, cruise scheduling, sampling patterns at each site, and general 

methodology for collecting samples and data. The drilling history of each study site is discussed 

in Chapter 3. Subsequent chapters describe the methods and results for each individual 

discipline: 

• Chapter 4 – Geophysical Characterization 

• Chapter 5 – Sediment Grain Size 

• Chapters 6 and 7 – Sediment Profile Imaging 

• Chapter 8 – Metals, Total Organic Carbon, and Redox Conditions 

• Chapter 9 – Sediment and Tissue Hydrocarbons 

• Chapter 10 – Sediment Microbes 

• Chapter 11 – Meiofauna and Macroinfauna 

• Chapter 12 – Megafauna and Image Analysis 

• Chapter 13 – Genetic Diversity of Harpacticoid Copepods 

• Chapter 14 – Sediment Toxicity 

The report culminates in a Synthesis (Chapter 15) and an Ecological Risk Assessment 

(Chapter 16). Additional supporting data are presented in the Appendices. 
Chapter 2 

Study Design and General Methods 

Neal W. Phillips and Alan D. Hart 

Continental Shelf Associates, Inc. 

This chapter describes site selection, sampling design and general methodology, cruise 

scheduling, and numbers and locations of samples collected at each site. Detailed methodology 

for individual tasks is discussed in subsequent chapters. 

2.1 SITE SELECTION Program Overview 

• Four “near-field” study sites 

• Six “far-field” (reference) areas for each site 

(10 to 25 km away) 

• Water depths about 1,000 to 1,150 m 

• VK 916 exploration site sampled before and after 

drilling of a single exploration well 

• GB 516 exploration/development site sampled 

after exploration drilling and again after several 

development wells were drilled 

• GB 602 and MC 292 post-development sites 

sampled once after drilling of several exploration 

and development wells 

• Six cruises (three geophysical, three 

chemical/biological) 

• Geophysical mapping included bathymetry, 

side-scan sonar, and subbottom profiling 

5 

Four study sites were selected (Figure 1.1). 

Coordinates are listed in Table 2.1. The sites 

were as follows: 

• Viosca Knoll Block 916 (VK 916) was an 

exploration site sampled before and after 

drilling of a single exploration well. 

• Garden Banks Block 516 (GB 516) was an 

exploration/development site that was 

sampled once after exploration drilling and 

again after several development wells were 

drilled. 

• Garden Banks Block 602 (GB 602) and 

Mississippi Canyon Block 292 (MC 292) 

were post-development sites sampled once 

after several exploration and development 

wells had been drilled. 

• Chemical/biological surveys included box coring, 

sediment profile imaging, seafloor photographs, 

and baited traps (for tissue samples)

Site selection was based on MMS requirements and operators’ drilling schedules rather than any 

attempt to select geographically or bathymetrically representative sites. The MMS requested that 

two exploration sites and three post-development sites be selected on the northern Gulf of Mexico 

continental slope (the number of post-development sites was ultimately reduced to two). The 

general requirements were as follows: 

• Sites should be located in water depths greater than 1,000 m. 

• Sites should be located at a similar water depth and be biologically and geologically similar. 

• At least one exploration site and one post-development site must be located as far east as 

possible in the Central Planning Area (e.g., near DeSoto Canyon). Ideally, the other sites 

should be selected to represent the central Gulf and western Gulf. 

• Sites drilled with an anchored drilling unit were preferred so that physical effects of 

anchoring could be studied. 

For exploration sites, it was preferred that no previous drilling had occurred in the block. For 

post-development sites, all drilling activities were to have been completed prior to the first cruise. 
6 

Table 2.1. Coordinates of near-field and far-field sites. Each site is defined as a circle (see text); 

center coordinates are listed. 

X/Y Coordinatesa Latitude/Longitude Site Water 

Depth (m) X (ft) Y (ft) Latitude Longitude 

Viosca Knoll Block 916 

Near-Field 1,125 1,356,696.75 10,564,161.84 29° 06' 24.31"N 87° 53' 19.48"W 

Far-Field 1 1,120 1,307,060.85 10,546,095.85 29° 03' 21.38"N 88° 02' 37.34"W 

Far-Field 2 1,110 1,326,067.53 10,552,404.38 29° 04' 25.47"N 87° 59' 03.73"W 

Far-Field 3 1,100 1,386,058.05 10,578,800.84 29° 08' 51.37"N 87° 47' 49.52"W 

Far-Field 4 1,035 1,392,675.95 10,584,517.89 29° 09' 48.42"N 87° 46' 35.29"W 

Far-Field 5 1,020 1,400,341.89 10,589,614.93 29° 10' 39.39"N 87° 45' 09.17"W 

Far-Field 6 1,015 1,406,544.02 10,592,651.90 29° 11' 09.86"N 87° 43' 59.40"W 

Garden Banks Block 516 

Near-Field 1,033 1,839,531.91 9,975,977.93 27° 29' 23.81"N 92° 23' 08.26"W 

Far-Field 1 1,000 1,807,547.80 9,961,278.59 27° 26' 59.64"N 92° 29' 04.20"W 

Far-Field 2 1,025 1,874,369.31 9,983,961.58 27° 30' 41.04"N 92° 16' 40.80"W 

Far-Field 3 1,000 1,879,644.10 9,985,191.94 27° 30' 52.92"N 92° 15' 42.12"W 

Far-Field 4 1,015 1,887,623.85 9,989,128.90 27° 31' 31.44"N 92° 14' 13.20"W 

Far-Field 5 1,015 1,889,749.42 9,991,213.58 27° 31' 51.96"N 92° 13' 49.44"W 

Far-Field 6 1,020 1,895,561.98 9,989,178.54 27° 31' 31.44"N 92° 12' 45.00"W 

Garden Banks Block 602 

Near-Field 1,125 1,815,625.00 9,934,903.00 27° 22' 38.02"N 92° 27' 35.79"W 

Far-Field 1 1,125 1,767,744.00 9,934,848.00 27° 22' 39.25"N 92° 36' 27.09"W 

Far-Field 2 1,100 1,765,104.00 9,902,640.00 27° 17' 20.26"N 92° 36' 57.49"W 

Far-Field 3 1,175 1,768,272.00 9,876,768.00 27° 13' 03.85"N 92° 36' 23.26"W 

Far-Field 4 1,175 1,795,200.00 9,875,184.00 27° 12' 47.23"N 92° 31' 24.95"W 

Far-Field 5 1,100 1,813,152.00 9,891,552.00 27° 15' 28.67"N 92° 28' 05.28"W 

Far-Field 6 1,125 1,872,816.00 9,935,904.00 27° 22' 45.08"N 92° 17' 01.13"W 

Mississippi Canyon Block 292 

Near-Field 1,034 1,129,042.00 10,419,990.00 28° 42' 13.08"N 88° 35' 44.19"W 

Far-Field 1 1,120 1,062,336.00 10,421,136.00 28° 42' 15.01"N 88° 48' 13.39"W 

Far-Field 2 1,030 1,067,088.00 10,430,640.00 28° 43' 49.81"N 88° 47' 21.64"W 

Far-Field 3 1,025 1,107,216.00 10,459,152.00 28° 48' 37.85"N 88° 39' 55.42"W 

Far-Field 4 1,050 1,107,744.00 10,450,704.00 28° 47' 14.29"N 88° 39' 48.16"W 

Far-Field 5 1,025 1,139,424.00 10,455,984.00 28° 48' 10.82"N 88° 33' 52.92"W 

Far-Field 6 1,035 1,143,648.00 10,459,680.00 28° 48' 47.96"N 88° 33' 05.98"W 

a

 X/Y coordinates are for Universal Transverse Mercator, North American Datum 1927, Zone 15 (Garden 

Banks Block 516 and Garden Banks Block 602) and Zone 16 (Mississippi Canyon Block 292 and Viosca 

Knoll Block 916). 
7 

Exploration Sites. To select potential exploration sites, the team worked with the MMS and 

operators to identify Exploration Plans for wells in water depths of 1,000 to 1,500 m with a 

projected starting date in the fourth quarter of 2000. This range of water depths was selected 

because anchoring impacts were of interest and anchored rigs are infrequently used in deeper 

water. It also was assumed that the geology and biology of sites within this depth range in the 

Central and Western Planning Areas would be generally similar. This process ultimately resulted 

in the selection of VK 916, which fulfills the requirement for a site near the eastern edge of the 

Central Planning Area. No previous wells had been drilled in VK 916. 

Although a second exploration site with no previous drilling was sought, none was available 

where an operator was planning to drill within the time frame for the pre- and post-drilling 

cruises. Therefore, GB 516 in the Western Planning Area was selected as the best available site, 

even though previous exploration wells had been drilled in the block. This was essentially an 

exploration/development site that was sampled after exploration drilling and again after 

additional development wells were drilled during this study. See Chapter 3 for further discussion 

of previous drilling. 

Post-Development Sites. To select potential post-development sites, the team worked with the 

MMS and operators to identify development/production sites in water depths greater than 

1,000 m where drilling (with an anchored drilling rig) would be completed by September 2000. 

Five potential sites met these requirements: 

• Garden Banks Block 602 

• Mississippi Canyon Block 292 

• Mississippi Canyon Block 934 

• Mississippi Canyon Block 687 

• East Breaks Block 945 

The first three sites were selected due to their similar water depths (approximately 1,000 to 

1,150 m). Mississippi Canyon Block 934 was ultimately dropped from the program for budgetary 

reasons before any surveys were conducted there. The final post-development sites were GB 602 

and MC 292. The former is in the Western Planning Area, and the latter fulfills the requirement 

for a site near the eastern edge of the Central Planning Area. 

2.2 FAR-FIELD SITES (REFERENCE AREAS) 

For chemical/biological sampling, each study site was defined as a circle 500 m in radius around 

the drilling location (rationale is discussed later in this chapter). Six circular far-field sites (or 

“reference areas”) also were designated for each site. These were chosen to be at about the same 

depth as the near-field site and 10 to 25 km away. Each far-field site was a circle 204 m in 

radius, such that the total area of six far-field sites combined was equal to that of the near-field 

site, and the number of samples was allocated so that the sampling intensity was the same. 
8 

Far-field sites were located using bathymetric maps provided by the MMS showing locations of 

previous wells in the vicinity of each near-field site. These maps were used to plot numerous 

potential far-field sites that met the criteria (distance of 10 to 25 km from the corresponding 

near-field site and similar water depths). From these possibilities, six far-field sites were 

randomly chosen for each site. Although previous wellsites were avoided to the extent 

practicable, most far-field sites had at least one previous well drilled within 10 km (see 

Chapter 3). Table 2.1 lists the latitude/longitude coordinates and water depths of far-field sites. 

Locations are shown on Figures 2.1 through 2.4. 

2.3 SAMPLING CRUISES 

Figure 2.5 shows the program schedule, including the timing of the six cruises (Table 2.2) and 

drilling activities that occurred at GB 516 and VK 916 during this study. Previous drilling at the 

two post-development sites (GB 602 and MC 292) as well as at GB 516 occurred at various times 

before this study began, as discussed under drilling history in Chapter 3. 

Table 2.2. Sampling cruises. 

Cruise Site 1A 1B 2A 2B 3A 3B 

Cruise Type: Geophysical Chem/Biol Geophysical Chem/Biol Geophysical Chem/Biol 

Cruise Start: 11/10/2000 10/23/2000 6/24/2001 7/8/2001 8/7/2002 8/4/2002 

Cruise End: 1/1/2001 11/17/2000 7/7/2001 7/25/2001 8/12/2002 8/14/2002 

VK 916 pre-drilling pre-drilling -- -- post-drilling post-drilling 

GB 516 -- post-drillinga post-drilling post-drilling -- -- 

GB 602 -- -- post-drilling post-drilling -- -- 

MC 292 -- -- post-drilling post-drilling -- -- 

a

 Cruise 1B at GB 516 preceded drilling of several development wells; however, exploration wells had been drilled 

previously (see Chapter 3). 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll. 

2.3.1 Geophysical Cruises 

Cruise 1A was conducted using a high resolution deep-tow system aboard the R/V OCEAN 

SURVEYOR, a geophysical survey vessel. Cruises 2A and 3A were conducted aboard the 

M/V RIG SUPPORTER using the C&C Technologies autonomous underwater vehicle (AUV), 

the HUGIN 3000. A differential global positioning system (DGPS) was used for navigation on 

all cruises. 

• Cruise 1A was conducted from 10 November 2000 to 1 January 2001. This cruise was 

originally intended to survey all of the sites. However, due to extended weather delays, as 

well as equipment, software, and vessel problems, VK 916 was the only site surveyed. 
Figure 2.1. Location of near-field (NF) and far-field (FF) sites for Viosca Knoll Block 916.

FF1

FF2

FF3

FF4

FF5

FF6

NF

Viosca Knoll

Destin Dome

Mississippi Canyon Desoto Canyon

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

916 Lease block

Bathymetry (100-m intervals)

916

9
Figure 2.2. Location of near-field (NF) and far-field (FF) sites for Garden Banks Block 516.

FF1

FF2

FF3

FF4

FF5

FF6

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

516 Lease block

Bathymetry (100-m intervals)

NF

516

10
Figure 2.3. Location of near-field (NF) and far-field (FF) sites for Garden Banks Block 602.

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

FF1

FF2

FF3 FF4

FF5

FF6

602 Lease block

Bathymetry (100-m intervals)

NF

602

11
Figure 2.4. Location of near-field (NF) and far-field (FF) sites for Mississippi Canyon Block 292.

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

292 Lease block

Bathymetry (100-m intervals)

FF1

FF2

FF3

FF4

FF5

FF6

NF

292

12
Figure 2.5. Schedule including sampling cruises and wells drilled during this study. Previous drilling at GB 516, GB 6

MC 292 occurred at various times before this study, and dates are not shown (see Chapter 3).

Cruise 1A (geophysical)Cruise 1B (chemical/biological)Drilling at GB 516Cruise 2A (geophysical)Cruise 2B (chemical/biological)Drilling at VK 916Cruise 3A (geophysical)Cruise 3B (chemical/biological)

Exploration drilling at VK 916 (11/27 - 12/18/2001)

Development drilling at GB 516 (11/6/2000 to 2/5/2001)

Pre-drilling - VK 916 only (11/10/2000 - 1/1/2001)

Post-drilling - GB 516 (sampled first) and Pre-drilling VK 916 (10/23 - 11

Post-drilling - GB 516, GB 602, MC 292 

Post-drilling - GB 516, GB 602, MC 2

Post-drilling - VK 9

Post-drilling - VK 91

Event

2000 2001 

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Ma

LEGENDPre-drilling cruise (VK 916 only)

Post-drilling cruise

Drilling during this study

Note: For GB 516, Cruise 1B is shown as a post-drilling cruise because there was previous exploration drilling. 

However, the cruise preceded additional, development drilling at this site.

13
14 

• Cruise 2A was conducted from 24 June to 7 July 2001 using the same vessel and navigation. 

Surveys were conducted at both post-development sites (GB 602 and MC 292), and a 

post-drilling survey was conducted at GB 516 following the completion of development 

drilling there. Due to a delay in the drilling schedule at VK 916, the post-drilling cruise at 

that site was postponed. 

• Cruise 3A was conducted from 7 to 12 August 2002. This was a post-drilling survey of the 

VK 916 site following the completion of exploration drilling there. 

2.3.2 Chemical/Biological Cruises 

Chemical/Biological Cruises 1B and 2B were conducted aboard the R/V GLORITA, which was 

mobilized out of Freeport, Texas. Cruise 3B was conducted aboard the R/V J.W. POWELL, 

which was mobilized out of Houma, Louisiana. Both ships were equipped with DGPS 

navigation. 

• Cruise 1B was conducted from 23 October to 17 November 2000. During this cruise, 

samples and photographs were collected at both GB 516 and VK 916. 

• Cruise 2B was conducted from 8 to 25 July 2001. During this survey, samples and 

photographs were collected at both post-development sites (GB 602 and MC 292) and at the 

GB 516 site after the completion of development drilling there. Due to a delay in the drilling 

schedule at VK 916, the post-drilling cruise at that site was postponed. 

• Cruise 3B was conducted from 4 to 14 August 2002. This cruise was a post-drilling survey 

of the VK 916 site following the completion of exploration drilling there. 

2.4 SAMPLING DESIGN 

The VK 916 exploration site was sampled before and after drilling. Sampling at this site 

conforms to the “optimal impact study design” (Green 1979) in which pre-drilling baseline data 

are collected at near-field sites and corresponding reference areas. Hypotheses can be tested 

concerning both temporal (before and after) and spatial differences (near-field versus far-field). 

The experimental design supports using analysis of variance/covariance for statistical testing. 

The two post-development sites, MC 292 and GB 602, were sampled only after drilling of several 

previous wells. Sampling at these sites was intended to provide a “snapshot” of impacts after the 

completion of development operations, similar to the Gulf of Mexico Offshore Operations 

Monitoring Experiment (GOOMEX) study (Kennicutt et al. 1996a). At these sites, impact must 

be inferred from spatial differences alone – i.e., comparisons with reference areas. Comparisons 

of the near-field area to the far-field areas can be used to directly examine potential impacts. 

The situation at GB 516 presents the most difficulties for interpretation. Although surveys were 

conducted before and after development drilling, the site had already been exposed to previous 

exploration drilling. The two surveys at this site essentially provide two snapshots of drilling 

impacts – one after exploration drilling and the second after development drilling. As with the 

two post-development sites, impact can be inferred from spatial differences – i.e., comparisons 

with reference areas. In addition, temporal comparisons between surveys may provide some 

indication of development drilling impacts. 
15 

The two broad categories of data collection were geophysical characterization and 

chemical/biological sampling. The sampling design for each type is explained below. 

2.4.1 Geophysical Characterization 

Geophysical characterization was intended to determine (1) the areal extent and accumulation of 

muds and cuttings; (2) the physical modification or disturbance of the sea bed due to impacts 

from anchors and their mooring systems including chains or wire ropes; and (3) the accumulation 

of debris attributable to oil and gas development activity. 

To address these objectives, bathymetric, side-scan sonar, and subbottom profile data were 

collected at each site. These data were used to construct broad-scale images of the seafloor, 

which were interpreted to map possible anchor scars and accumulations of cuttings and drilling 

fluids. An area approximately 3,000 m in radius was surveyed at each site, to encompass the 

anchor patterns for typical moored drilling units in this water depth. Further information on the 

areal coverage and detailed methods for the geophysical surveys are provided in Chapter 4. 

The design of this part of the study allows a non-statistical evaluation of physical impacts through 

interpretation of spatial patterns within sites, as well as comparisons of before-and-after surveys 

at the exploration sites. Since far-field sites were not surveyed, there are no spatial comparisons 

with “unaffected” areas. Information from the operators, including wellsite and anchor locations 

and reports from previous geophysical surveys, aided in the interpretation. 

2.4.2 Chemical/Biological Sampling 

Chemical/biological sampling was intended to document (1) physical and chemical modification 

of sediments; and (2) effects on benthic organisms. Samples and data were collected with four 

types of equipment: 

• Box core sampler (to obtain samples of sediment, microbiota, meiofauna, and macroinfauna) 

• Sediment profile imaging (SPI) camera (to obtain cross-sectional images of the upper 

sediment column) 

• Still camera on a towed/dragged sled (to obtain photographs of the seafloor and megafauna) 

• Baited traps (to obtain animals for tissue analyses) 

For chemical/biological sampling, each study site was defined as a circle 500 m in radius around 

the drilling location. The 500-m radius was chosen in order to sample intensively where SBM 

cuttings are most likely to accumulate at concentrations sufficient to produce biological effects. 

Because the distribution of SBM cuttings typically is patchy (Neff et al. 2000), a higher sampling 

density near the drillsite increases the chances of sampling where the cuttings are deposited. 

Preliminary calculations and discussions with industry representatives indicated that 

(1) concentrations of synthetic based fluids (SBFs) most likely would not be detectable beyond 

500 m from the discharge point; and (2) SBF concentrations exceeding 1 ppm (an apparent 

threshold for biological effects; Neff et al. 2000) are most likely to occur within 300 m of the 

discharge point. Therefore, the project team and MMS agreed that a radius of 500 m would be 

used to define a near-field site, and that 8 of the 12 box cores at each site would be collected 

within 300 m of the wellsite. 
16 

Figure 2.6 shows the idealized sampling pattern for chemical/biological sampling. Box cores 

were to be collected at 12 randomly selected locations within the 500-m radius at each study site 

(8 samples within 300 m and 4 samples between 300 and 500 m radii). Box cores also were to be 

collected at 12 far-field locations (two locations selected randomly within each of six far-field 

sites). Sediment profile photographs were to be collected along three drift transects in the 

near-field and along one transect at each of three far-field sites, with approximately 36 images per 

transect. Still photographs of the seafloor and megafauna were collected along three transects at 

each near-field site and along one transect at each of three far-field sites (approximately 

375 photographs per transect). Baited traps were to be deployed at the two post-development 

sites to collect organisms for tissue analyses. For each site, eight traps were deployed in the 

near-field, and four traps were placed at each of two corresponding far-field sites. The actual 

number of samples and transects differed from the planned number in some cases, as summarized 

later in Tables 2.3 and 2.4. For example, during Cruise 3B, far-field SPI transects were 

attempted at VK 916 FF2 and FF4; however, due to problems with the camera, no usable data 

were collected. Also, one of the baited traps at MC 292 did not capture any animals because high 

current and inadequate weight on the ground line kept it off the bottom. 

Box core locations were determined randomly within each site on each cruise. For this purpose, 

the 500-m circle was divided into quadrants to ensure that box cores were distributed in all 

directions around the site center. Precise placement of box cores is very difficult in this water 

depth, so a couple of samples at GB 516 that were pre-plotted just inside the 500-m radius 

actually fell outside it. In addition to the regular box core samples, subsurface sediment samples 

for chemistry analyses also were collected from three non-random “discretionary” stations within 

the near-field zone during Cruise 2B (GB 516, GB 602, and MC 292) and Cruise 3B (VK 916). 

The “discretionary” stations were chosen within areas identified from geophysical surveys as 

likely mud and cuttings deposits. 

Camera sled and SPI transects were not randomized; the transects were conducted so as to drift 

across each site based on prevailing currents. The still camera was programmed to take 

photographs at regular intervals (every 5 seconds). The SPI system was repeatedly lowered to 

take as many profiles as possible during each transect (not explicitly random or regular intervals). 

As discussed in Chapter 3, three of the sites (GB 516, GB 602, and MC 292) had well-related 

structures on the bottom including subsea wellheads, manifolds, flowlines, and umbilicals. 

Sampling locations were adjusted where necessary to avoid contacting these structures. 

2.5 SAMPLING METHODOLOGY 

2.5.1 Box Cores 

Sediment samples were collected with a 0.25-m2 Gray-O’Hara box core (Figure 2.7). Procedures 

for operation of this box core were patterned after Hessler and Jumars (1974) as modified by 

Blake et al. (1985, 1987) and Blake and Grassle (1994). The box was partitioned into subcores 

for various sample types (Figure 2.8). Subcores with a total surface area of 0.1 m2

 were taken for 

macrofauna; other subcores were obtained for meiofauna, microbiota, chemistry, and other 

parameters as required. Thus, both the chemical/sediment samples and the biology samples were 

taken from each deployment. A Sonardyne positioning system was used during the box coring. 

A transponder was attached to the side of the box corer, giving its position and height off the 

bottom. Precision was 0.2% of slant range, or about 2 m for box coring in this depth. 
Figure 2.6. Idealized sampling plans for study sites.

>10 km

Far-Field Site GB 602 and MC 292

Post-Development Sites

 Legend:

= Box core (12 per site and 2 per far-field ) near-field site

= Megafauna trap (8 per site and 4 at each of two far-field s) near-field site

 Legend:

= Box core (12 per near-field site and 2 per far-field site)

= Sediment profile imagery transect (3 per site 

 And 1 at each of three far-field s; goal = 36 images

 per transect)

near-field

site

= Megafauna camera sled drift (3 per site and 1 at

 each of three far-field s; goal = 375 photos per transect)

near-field 

site

VK 916 and GB 516

Exploration and 

Exploration/Development Sites

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site Far-Field Site

Far-Field Site

Far-Field Site

Far-Field Site

>10 km

1,000-m diameter

(500-m radius)

1,000-m diameter

(500-m radius)

(sampled before and after drilling)

(sampled only after drilling)

= Sediment profile imagery transect (3 per site and

 1 at each of three far-field s; goal = 36 images

 per transect)

near-field 

site

= Megafauna camera sled drift (3 per site and 1 at

 each of three far-field s; goal = 375 photos

 per transect)

near-field 

site

17
Figure 2.7. Deployment of box core. 

After retrieval of the box core, the entire surface of the sample was examined. Sediment color, 

evidence of disturbance, and any interesting or unusual biological or physical features were 

noted. During the processing of biology, sediment, and chemistry subcores, care was taken to 

avoid disturbing or otherwise contaminating the samples before they were processed. If a 

subcore was considered to be disturbed, or exhibited evidence of leakage, it was replaced with an 

alternate subcore. 

Subsamples were removed from each box core for analysis of 

• Sediment grain size and clay mineralogy (Chapter 5) 

• Sediment metals and total organic carbon (Chapter 8) 

• Sediment redox chemistry (direct Eh and pH measurements) (Chapter 8) 

• Sediment hydrocarbons (Chapter 9) 

• Microbiota (Chapter 10) 

• Meiofauna (Chapter 11) 

• Macroinfauna (Chapter 11) 

• Harpacticoid genetic diversity (Chapter 13) 

• Sediment toxicity testing (GB 602 and MC 292 only; Chapter 14) 

In addition, sediment samples for radionuclide analysis and interstitial water for interstitial water 

chemistry were removed from selected box cores at each site (Chapter 8). 

18 
Macroinfauna

1,024 cm2

Sediment Toxicity 

196 cm2

Meiofauna

38.3 cm2

Meiofauna

38.3 cm2

Meiofauna

38.3 cm2

Hydrocarbons

81.8 cm2

Mineralogy

71.3 cm2

Redox/

Grain Size

71.3 cm2

Metals/Total 

Organic Carbon

71.3 cm2

Interstitial 

Water/

Radionuclides

38.3 cm2

Microbiology

0.5 cm2

Meiofauna

5 cm2

Figure 2.8. Schematic of box core subsampling. The box is 50 cm on a side (0.25 m2).

19
20 

2.5.2 Sediment Profile Imaging 

At each site, SPI photographs were collected along three drift transects in the near-field and at 

three of the six far-field sites (Figure 2.6). Although the plan was to take 36 images along each 

drift transect, the actual number of usable images per transect varied, as discussed in Chapters 6 

and 7. 

SPI photographs were taken with a Benthos Model 3731 sediment profile camera (Ocean Imaging 

Systems, North Falmouth, Massachusetts) (Figure 2.9). The profile camera works like an 

inverted periscope with a deep-sea 35-mm camera mechanism mounted horizontally inside a 

water-tight housing on top of a wedge-shaped prism. The camera provides images of up to 20 cm 

of the upper sediment column in profile. During each SPI camera transect, the camera array was 

regularly lowered to the bottom. A transponder was attached to the side of the SPI camera, 

giving its position and height off the bottom. As the frame reached the bottom, the sliding prism 

device would penetrate the seafloor sediment and automatically trigger the camera's shutter. The 

photograph, taken through the prism, shows a vertical swath of sediment in front of the prism’s 

glass face. When the camera was raised between photographs, a wiper blade cleaned the 

faceplate. Detailed methods are provided in Chapters 6 and 7. 

2.5.3 Seafloor Photographs 

A camera on a dragged sled was used to collect still photographs of the seafloor and benthic 

organisms. These photographs were used for visual analyses of sediment appearance and 

megafauna (Chapter 12). 

The seafloor photographs were collected using a camera and flash unit that was housed in an 

aluminum sled dragged behind the survey vessel (Figure 2.10). The camera was a Benthos 

Model 372 deep-sea 35-mm camera, which takes 800 exposures per loading. The camera system 

was deployed with a pre-programmed time delay of 90 minutes before acquiring images and a set 

time interval of 5 seconds between photographs. 

The drag sled was connected to a 272-kg weight by a 30-m length of wire rope (Figure 2.10). 

The weight was connected to the main winch cable of the survey vessel. A transponder was 

mounted on the weight to aid in determining when the sled was approaching the seafloor. 

Once deployed on the bottom, the sled was in constant contact with the seafloor. This method 

ensured a constant camera-to-seafloor distance. The drag sled was able to traverse seafloor 

changes of up to 1 m due to wide “runners” extending horizontally from the back and vertically 

from the front of the sled. 

While the drag sled was being deployed, the survey vessel maintained a slight forward motion 

into the surface current, approximately 0.25 to 0.5 knots. Once the sled was on the bottom, the 

survey vessel increased speed to 0.5 to 1 knot. Global positioning system navigational positions 

were recorded. Times corresponding to beginning of deployment, sled on bottom, sled off 

bottom, and end of recovery also were recorded. After the sled was retrieved, the still camera 

was removed, and two lengths of film (“tails”) were processed to check for proper photographic 

area and data chamber exposure settings.
INNER FRAME

OUTER FRAME

WEIGHT

WEIGHT PACK

MUD DOOR

ELASTIC STRAP

MUD DOOR

BASE (SLED)

END CAP

(HOUSING FOR 

ELECTRONICS)

FACE PLATE

PRISM

MUD DOOR

HINGE

CAMERA

PINGER

WINDOW

MIRROR

DISTILLED WATER INSIDE PRISM

SEAFLOOR CAMERA PHOTOGRAPHS 

IMAGE

IMAGE REFLECTS OFF 45o

MIRROR 1. 2.

3.

LIFTING BAR 

Figure 2.9. Schematic diagram of the sediment profile camera and its operation.

FACEPLATE OR

 “WINDOW” OF 

PRISM AGAINST

SURFACE TO BE

PHOTOGRAPHED

DEPLOYED ON THE

SEAFLOOR

Signal Rate

Doubles

“DOWN POSITION”

Transecting the 

Sediment-Water Interface

Acoustic

Signal to

the Surface

21
computer

transceiver

transponder

down weight

drag sled

Figure 2.10. Configuration of dragged sled for seafloor photography.

Still camera 

aimed at seafloor

transponder

22
23 

2.5.4 Baited Traps 

Baited traps were used during Cruise 2B to collect animals for tissue analyses at the two 

post-development sites (GB 602 and MC 292) and corresponding far-field sites. A diagrammatic 

representation of a typical trap set is shown in Figure 2.11. The traps were similar in design to 

those used in the U.S. Pacific Northwest in the commercial dungeness crab fishery. The traps are 

2 m in length by 1.5 m wide, with two openings for fish to enter. 

At each trap station, a set of four baited traps was deployed along a common, weighted line, with 

each trap separated by a distance of 46 m. One end of the ground line was connected to a 

separate line that extended to the surface and was supported there by a pair of inflatable buoys 

and a single spar buoy marker. After deployment, each trap set was allowed to “fish” for a period 

of 18 to 24 hours, after which the traps were retrieved and any organisms captured were removed. 

2.6 SAMPLING LOCATIONS AND DATA COLLECTED 

Table 2.3 summarizes numbers of samples collected and transects completed during each cruise. 

Detailed listings are provided in Appendix A1. Table 2.4 lists numbers of animals captured in the 

baited traps. 

Figures 2.12 through 2.15 show the sampling locations for each near-field site on each cruise. 

Far-field sampling locations are shown in Appendix A2. 
Figure 2.11. Schematic diagram of baited traps used to obtain organisms for tissue analyses.

GROUND LINE

TRAP

WEIGHT 

SURFACE

 LINE

SURFACE

FLOATS

SPAR 

BUOY

RETRIEVAL 

LINE & FLOAT

SCHEMATIC VIEW OF THE TRAP SHO

WEBBING ENTRANCES AND BAIT B

(TRAP FRAME WEBBING NOT SHOW

24
25 

Table 2.3. Numbers of samples collected and transects completed during chemical/biological 

cruises. 

Box Cores SPI Transects Sled Transects Trap Setsa

Cruise and Site 

NF FF NF FF NF FF NF FF 

Cruise 1B 

VK 916 12 10 2 3 3 4 -- -- 

GB 516 12 12 3 3 3 5 -- -- 

Cruise 2B 

GB 516 12+3b 12 3 3 3 3 -- -- 

GB 602 12+3b 12 3 3 3 3 2 1 

MC 292 12+3b 12 2 3 3 3 2 2 

Cruise 3B 

VK 916 12+3b 12 3 0c 3 2 -- -- 

NF = near-field; FF = far-field; SPI = sediment profile imaging. a Each trap set included four traps. 

b Three additional “discretionary” box cores samples for subsurface sediment analyses were taken at near-field sites 

during Cruises 2B and 3B. 

c During Cruise 3B, far-field transects were attempted at VK 916 FF2 and FF4; however, due to problems with the SPI 

camera, no usable data were collected. 

Table 2.4. Numbers of animals collected from baited trap sets during Cruise 2B. 

Number Captured 

Station Species Type Trap 

1 

Trap 

2 

Trap 

3 

Trap 

4 Total 

TPH 

Samples 

Metals 

Samples 

Garden Banks 602 

Bathynomus giganteus Isopod 4 4 3 7 18 4 4 NF-Trap1 Chaceon quinquedens Crab 24 24 24 21 93 4 4 

Bathynomus giganteus Isopod 12 8 10 5 35 4 4 NF-Trap2 Chaceon quinquedens Crab 11 5 6 8 30 4 4 

Bathynomus giganteus Isopod 11 17 12 21 61 8 8 FF5-Trap Chaceon quinquedens Crab 5 0 1 2 8 4 4 

Mississippi Canyon 292 

Bathynomus giganteus Isopod 0a 3 2 3 8 4 4 NF-Trap1 Chaceon quinquedens Crab 0a 1 4 1 6 3 3 

Bathynomus giganteus Isopod 1 5 0 4 10 8 8 NF-Trap2 Chaceon quinquedens Crab 15 17 1 30 63 0 0 

Bathynomus giganteus Isopod 5 7 2 2 16 8 8 

FF3-Trap Chaceon quinquedens Crab 13 15 6 21 55 8 8 

Centrophorus uyato? b Fish 0 0 0 1 1 5 5 

Bathynomus giganteus Isopod 5 2 3 6 16 8 8 

FF4-Trap Chaceon quinquedens Crab 3 9 5 16 33 8 8 

Centrophorus uyato? b Fish 0 0 0 1 1 0 0 

TPH = total petroleum hydrocarbons. 

a

 Lead (#1) trap did not rest on bottom due to high current and inadequate weight on ground line. 

b

 Single specimens were frozen and donated to Texas A&M University museum. 
Figure 2.12. Near-field sampling locations at Viosca Knoll Block 916.

VK 916

CRUISE 1B 

(pre-drilling)

VK 916

CRUISE 3B 

(post-drilling)

8

4

11

12

7

3

5

1

2

9

6

10

Sled NF2-R3

Sled NF4-R7

Sled NF1-R2

SPI NF3

SPI NF1

SPI NF2

10

12

11

Sled NF1-R6

Sled NF3-R15

SPI NF1

SPI NF2

2

4

9

5

3

6

7

1

Sled NF2-R10

8

SITE CENTER

29° 06' 24.31"N

87° 53' 19.48"W

Sediment profile imaging (SPI) transect

Camera sled transect

1 Box core sample

LEGEND

Box core - discretionary sample DS-1

Arrows indicate 

transect direction

New wellsite

(drilled during

this study)

500-m radius

500-m radius

DS-1

DS-2

DS-3

26
Figure 2.13. Near-field sampling locations at Garden Banks Block 516.

GB 516

CRUISE 2B

(post-drilling)

GB 516

CRUISE 1B 

(post-drilling)

SITE CENTER

27° 29' 23.81"N

92° 23' 08.26"W

Arrows indicate 

transect direction

Sediment profile imaging (SPI) transect

Camera sled transect

1 Box core sample

LEGEND

Box core - discretionary sample DS-1

1 2

3

4

5

6

7

8

10 9

12

11

Sled NF2-R2

Sled NF3-R3

Sled NF1-R1

SPI-NF2 SPI-NF1

SPI-NF3

Sled NF2-R4

8

10

11 12

1

2

3

4

-

6 5

DS-1

DS-2

Sled NF1-R3 SPI-NF1

DS-3

Sled NF3-R5

SPI-NF3

SPI-NF2

Previous wellsite

500-m radius

500-m radius

New wellsite

(drilled during

 this study)

7

9

27
Figure 2.14. Near-field sampling locations at Garden Banks Block 602.

GB 602 (Post-Development Site)

Cruise 2B (post-drilling)

3

4

6

8

10

11

1

5

2

7

9

DS-2

12

DS-1

DS-3

Sled NF1-R7

Sled NF2-R10 Sled NF3-R11

SPI NF1

SPI NF2

SPI NF3

SITE CENTER

27° 22' 38.02"N

92° 27' 35.79"W

Arrows indicate transect direction

Trap NF1

Trap NF2

Sediment profile imaging (SPI) transect

Camera sled transect

1 Box core sample

LEGEND

DS-1 Box core - discretionary sample

Megafauna trap set (4 traps)

Previous wellsite

500-m radius

28
Figure 2.15. Near-field sampling locations at Mississippi Canyon Block 292.

MC 292 (Post-Development Site)

Cruise 2B (post-drilling)

3

4

10

11

7 DS-2

12

DS-1

DS-3

Sled NF1-R14

Sled NF2-R15

SPI NF1

SPI NF2 6

2

5

1

8

9

Sled NF3-R18

SITE CENTER

28° 42' 13.08"N

88° 35' 44.19"W

Sediment profile imaging (SPI) transect

Camera sled transect

1 Box core sample

LEGEND

Box core - discretionary sample DS-1

Megafauna trap set (4 traps)

Arrows indicate transect direction

Trap NF2

Trap NF1

Previous wellsite

500-m radius

29
31 

Chapter 3 

Drilling Activities 

Neal W. Phillips 

Continental Shelf Associates, Inc. 

This section summarizes drilling activities at and near the study sites, including 

• Wellsite, anchor locations, and drilling discharge information for each near-field site 

• Other wells drilled within 10 km of each near-field site 

• Wells drilled within 10 km of far-field (reference) sites 

Well locations were plotted by the MMS based on data from their Technical Information 

Management System. Detailed listings of well locations within 10 km of each site are presented 

in Appendix B1. All wellsites plotted here refer to surface (seafloor) locations. Sidetracks 

(directionally drilled wells branching off an existing borehole) are listed in the MMS database as 

separate wells from the same surface location. Exploration and development plans for each of the 

four blocks were downloaded from the MMS website. Drilling discharge information was 

obtained from operators and the MMS. 

Both water-based muds (WBMs) and synthetic-based muds (SBMs)1

 were used for drilling at the 

near-field sites. The general sequence of events was similar for most wells. The initial well 

interval was drilled “open hole” using the drill bit and twisting pipe. The flow of spud mud and 

water through the drill bit jetted the cuttings out of the way and up along the pipe to the seabed 

surface, where they accumulated around the hole in a small pile. Spud muds consisting primarily 

of water and bentonite clay were used for this jetting process. At this point, the drill bit and drill 

pipe were removed. A surface casing was inserted in the hole, and cement was pumped down the 

casing pipe, where it emerged from the hole and flowed up along the outside of the casing pipe. 

Once the cement hardened, appropriate wellhead equipment was attached to the surface of the 

casing (at the seafloor). The wellhead was connected to a marine riser system that returned muds 

and cuttings to the drilling rig during the remainder of the drilling process. Typically, one or 

more well intervals were drilled using WBM, with WBM cuttings and muds discharged from the 

rig. Finally, the mud system was switched to SBM for drilling of remaining well intervals. In 

some cases, there were bulk discharges of remaining WBM prior to switching to SBM. During 

SBM intervals, SBM cuttings were discharged, but muds were retained, except for those adhering 

to cuttings (see Chapter 16 for information on effluent limitation guidelines for drilling 

discharges). In addition, mud inventory reports indicate that some quantities of SBMs were lost 

to the formation, lost during casing/cementing operations, or left in the wellbore for future 

sidetracks. 

Because they branch off an existing borehole, sidetracks do not involve the initial release of 

water-based mud and cuttings at the seafloor. In addition, sidetracks are usually shorter in 

duration than straight wells and have smaller hole sizes, resulting in lower discharge volumes per 

well. Sidetracks at these sites were drilled primarily using SBMs. 

SBMs used in the Gulf of Mexico are almost exclusively internal olefins or linear-α-olefins (Neff 

et al. 2000). Internal olefins such as Novaplus, Novadril, or Syn-Teq were used at one or more of 

 

1 The terms synthetic-based mud (SBM) and synthetic-based fluid (SBF) are often used interchangeably. However, in this report, 

SBF refers to chemical measurements of the base fluid in sediment, whereas SBM refers to the drilling muds and cuttings per se.
32 

the study sites. Petrofree LE, a linear-α-olefin, may have been used at GB 516 and GB 602. 

Petrofree ester may also have been used in one well at GB 602. 

Discharge volumes were estimated using hole sizes (diameters and lengths for well intervals) 

provided by the operators. In some cases, actual well intervals were available; in others, planned 

well intervals from Exploration Plans were used. To allow consistent comparisons among wells, 

cuttings volume was always estimated in this report as 1.5 times hole volume (operators used 

multipliers ranging from 1.05 to 4.0). Drilling fluid volume (for WBM intervals) was estimated 

as 11 times hole volume (Walk, Haydel and Associates 1989). Detailed calculations for each 

well are presented in Appendix B2. 

3.1 NEAR-FIELD SITES 

3.1.1 Viosca Knoll Block 916 (Exploration Site) 

Viosca Knoll Block 916 is located about 170 km SSE of Mobile, Alabama. Water depth at the 

wellsite is approximately 1,125 m. 

3.1.1.1 Overview 

Table 3.1 lists all wells drilled within 10 km of the VK 916 near-field site through the end of 

Cruise 3B (14 August 2002). There were no previous wells in this block. However, several wells 

had been drilled within 10 km. The nearest wells were located about 2.3 km NNW in VK 872 

and were drilled approximately 2 years before the start of this study (Figure 3.1). 

Table 3.1. Wells drilled within 10 km of the Viosca Knoll (VK) Block 916 near-field (NF) site. 

Timing Distance 

from NF 

Site Center 

Block Number 

of Wells Dates Prior to 

This Study 

During This Study 

(between Cruises 1B 

and 3B) 

0 VK 916 1 Nov-Dec 2001 -- •

2.3 km VK 872 2 Nov-Dec 1998 • -- 

3.5 km VK 873 1 Feb-Mar 1988 • -- 

4.6 -7.4 km VK 915 8 Jan 2001-June 2002 -- •

5.3 km VK 915 9 Apr 1993-Apr 1998 • -- 

6.4 km VK 871 1 May-July 1994 • -- 

7.2 km VK 917 1 Nov-Dec 2001 -- •

7.6 km VK 827 2 Sep-Oct 1998; June 2000 • -- 

During this study, Shell drilled a single exploration well in VK 916. Details are provided below. 

Other drilling that occurred within 10 km of the near-field site during the program included 

8 wells in VK 915 between January 2001 and June 2002, with distances ranging from 4.6 to 

7.4 km; and 1 well 7.2 km away in VK 917, drilled in November-December 2001 (Table 3.1). 

Due to the distances from these other wells, any effects are assumed to be negligible in 

comparison with the single well drilled in VK 916 during this study. 
Figure 3.1. Well locations in and near the Viosca Knoll Block 916 near-field site.

Legend

Previous wellsite

N

S

W E

New well (drilled during this study)

500-m radius

VK 916, Well No. 1

Nov-Dec 2001 500-m radius

VK 916, Well No. 1

Nov-Dec 2001

2.3 km

VK 872 Well No. 1 

and sidetrack

Nov-Dec 1998

Anchor radius

2,440 m

(a) Well within the

VK 916 near-field site

(b) Anchor radius and 

nearby well in VK 872

Approximate anchor location

Approximate anchor locations based 

on radius shown in Exploration Plan 

and scars evident on side-scan sonar 

survey

33
34 

3.1.1.2 Drilling Program 

A single exploration well (No. 1) was spudded on 27 November 2001 and reached total depth on 

18 December 2001, when it was plugged and abandoned. The wellsite was at the center of the 

VK 916 near-field site in a water depth of approximately 1,125 m. 

The drilling rig was the NOBLE JIM THOMPSON, a semisubmersible. Anchor patterns shown 

in the Exploration Plan indicate eight anchors would be deployed radially at distances of 2,440 m 

from the wellsite, with approximately 450 m of anchor chain on the seafloor near each anchor 

location. The anchor locations were revised after the initial Exploration Plan was submitted, and 

therefore the anchor radius is shown with presumed anchor locations based on scars from the 

side-scan sonar survey (Figure 3.1). 

Both WBM and SBM were used during drilling (Table 3.2). The first well interval was jetted 

using seawater and a seawater-based bentonite gel mud. During this first interval, all of the mud 

and cuttings were released at the seafloor. The second interval also used seawater and gel mud, 

with mud and cuttings discharged from the drilling rig. There was a bulk discharge of WBM at 

the end of the second interval. The remaining well intervals were drilled using the SBM 

Syn-Teq, an internal olefin isomer. Cuttings were discharged during these intervals, but SBM 

was recirculated and retained. The drilling mud contractor’s inventory indicates there were some 

losses of SBM to the formation and during casing and cementing operations. 

Table 3.2. Discharge summary for Viosca Knoll Block 916 Well No. 1 (27 November to 

18 December 2001). 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Well Discharges (bbl) 

Interval 

Hole 

Size 

(inches) WBM WBM 

Cuttings WBM WBM Cuttings 

SBM 

Cuttings 

SBM Name 

IA 36 3,074 419 -- -- -- 

IB 26 -- -- 12,807 a 1,746 -- -- 

II 20 -- -- -- -- 950 Syn-Teq 

III 14.75 -- -- -- -- 951 Syn-Teq 

IV 10.63 -- -- -- -- 609 Syn-Teq 

Total: 3,074 419 12,807 1,746 2,510 

SBM = synthetic-based mud; WBM = water-based mud. 

Source: Spud date and total depth date as listed in Minerals Management Service Technical Information Management 

System. Well intervals and hole sizes were provided by Shell, and discharge volumes were estimated using hole 

volume times 11 for drilling fluids, hole volume times 1.5 for cuttings. Synthetic drilling fluids were not discharged 

(other than those adhering to cuttings), but the inventory indicates SBM losses including 256 bbl lost and 141 bbl left 

behind the liner during interval II, 1,634 bbl lost during casing and cementing operations during interval III, and 

160 bbl lost to the formation during interval IV. a Water-based mud was dumped at end of this interval to prepare for SBM system.

3.1.2 Garden Banks Block 516 (Exploration/Development Site) 

Garden Banks Block 516 is located in the Western Planning Area, about 275 km SSE of 

Cameron, Louisiana. The center of the site has a water depth of 1,033 m. The prospect is named 

Serrano. 
35 

3.1.2.1 Overview 

Three exploration wells had been drilled in GB 516 prior to this study, and five development 

wells were drilled during the study. Figure 3.2 shows the well locations, and Table 3.3 lists all 

wells drilled within 10 km through the end of Cruise 2B (25 July 2001). 

Table 3.3. Wells drilled within 10 km of the Garden Banks (GB) Block 516 near-field (NF) site. 

Timing Distance 

from NF 

Site Center 

Block Number 

of Wells Dates Prior to 

This Study 

During This Study 

(between Cruises 1B 

and 2B) 

0 GB 516 2 July-Aug 1999 • -- 

0-50 m GB 516 5 Nov 2000-Feb 2001 -- •

1.15 km GB 516 1 Sep 1995-July 1996 • -- 

3.9 km GB 515 1 June 1999 • -- 

8.2 km GB 559 6 Feb 1999-Oct 2000 • •

a

7.5-9.1 km GB 426/471 40 Feb 1987-June 2000 • -- 

8.4 km GB 426 1 May 2001 -- •

a Two of these wells overlapped a few days with the start of Cruise 1B. 

Two of the previous wells (a straight well and one sidetrack) were at the center of the near-field 

site and were drilled in July-August 1999. The other was 1,150 m to the southeast and was 

drilled between September 1995 and July 1996. The five new wells included two additional 

sidetracks of the 1999 well at the center of the GB 516 near-field site, and one straight well and 

two sidetracks at a location about 50 m to the northeast of the site center. 

Several other wells had been drilled within 10 km of the GB 516 site (Table 3.3). The nearest 

was a single well 3.9 km southwest in GB 515 (drilled in June 1999). Six previous wells were 

drilled about 8.2 km to the southwest in GB 559 (February 1999 to October 2000). Two of these 

were ongoing at the end of October 2000 during the first few days of Cruise 1B. Also, 40 wells 

were drilled at locations 7.5 to 9.1 km away in the southeast corner of GB 426 and the adjacent 

corner of GB 471 between February 1987 and June 2000, with one additional well in GB 426 

during this program (May 2001). These were associated with Shell’s Auger facility in GB 426. 

Due to the distance, any effects are assumed to be negligible in comparison with the wells drilled 

within the GB 516 near-field site before and during this study. 

3.1.2.2 Drilling Program 

Exploration Well No. 1 was drilled in GB 516 between September 1995 and July 1996. 

Discharge data were not obtained for this well, which is 1,150 m southeast of the near-field site 

center. The Exploration Plan indicates that WBM and SBM (Novadril or Petrofree ester) were 

planned to be used. 

Between 13 July and 21 August 1999, Well No. SE001 and one sidetrack were drilled at the 

center of the near-field site. These two exploration wells were drilled about 14 to 15 months prior 

to Cruises 1A and 1B. 
Figure 3.2. Well locations in and near the Garden Banks Block 516 near-field site.

Legend

Previous wellsite

N

S

W E

New well (drilled during this study)

(a) Wells within the

GB 516 near-field site

(b) Anchor radius and 

nearby well in GB 516

500-m radius Well No. SE002 and

sidetracks ST01 and ST02

Nov 2000-Feb 2001

Well No. SE001 

and sidetrack ST01

Jul-Aug 1999

Well No. SE001

sidetracks ST02 

and ST03

Nov-Dec 2000

500-m radius

Wells drilled in near-field site

(SE001 and 3 sidetracks,

SE002 and 2 sidetracks)

Well No. 1

Sept 1995-Jul 1996

1.15 km

2,620-m anchor radius

Wellheads and other 

structures placed in this 

area between pre- and

post-drilling surveys

(see Fig. 3.3)

Well SE001 anchor locations (approx.)

Well No. 1 anchor locations (approx.)

Flowline and

umbilical to/from GB 426

Umbilicals to GB 559

Approximate anchor locations 

are based on Exploration Plans

36
37 

In 2000, Shell filed a development plan for GB 516, and subsequently five development wells 

were drilled between 6 November 2000 and 5 February 2001. These included two additional 

sidetracks of Well No. SE001 at the center of the GB 516 near-field site, and Well No. SE002 and 

two sidetracks at a surface location about 50 m to the northeast of the site center. At the same 

time, a flowline and an umbilical were placed on the seafloor, connecting to processing facilities 

on the Auger platform in GB 426, approximately 10 km to the north (Figure 3.3). Two 

umbilicals also connect to the nearby Oregano prospect in GB 559. In addition to the flowline 

and umbilicals, structures on the seafloor in GB 516 include two subsea wellheads, a flowline 

sled, and an umbilical distribution module. 

Both water-based muds and associated cuttings, as well as SBM cuttings, were discharged during 

drilling. Exploration and development plans included the option of using either of two SBMs, 

Novaplus (an internal olefin) or Petrofree LE (a linear-α-olefin). Novaplus was the only SBM 

used for the wells and sidetracks drilled in 2000-2001; it is not known which SBM was used on 

the 1999 wells. Table 3.4 summarizes the discharge data. 

Table 3.4. Discharge summary for Garden Banks Block 516 wells within the near-field site. 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Discharges (bbl) Well No. Dates 

WBM WBM 

Cuttings WBM WBM 

Cuttings 

SBM 

Cuttings 

SBM Name 

SE001 & 

SE001 ST 1 

7/13/99-8/15/99 

8/20/99-8/21/99 6,501a 887a 14,731a 2,009a 1,904a Novaplus or 

Petrofree LE

SE001-ST 2 11/29/00-12/15/00 -- -- -- -- 2,031 Novaplus 

SE001-ST 3 12/24/00-12/29/00 -- -- -- -- 672 Novaplus 

SE002 & 

SE002-ST 1 

11/06/00-11/21/00 

11/24/00-11/25/00 6,501 887 14,731 2,009 1,904 Novaplus 

SE002-ST 2 1/30/01-2/05/01 -- -- -- -- 802 Novaplus 

Total: 13,002 1,774 29,462 4,018 7,313 

SBM = synthetic-based mud; ST = sidetrack; WBM = water-based mud. 

Source: Spud date and total depth date as listed in Minerals Management Service Technical Information Management 

System. Well intervals and hole sizes were provided by Shell, and discharge volumes were estimated using hole 

volume times 11 for drilling fluids, hole volume times 1.5 for cuttings. 

a Actual well intervals not available; assumed identical to Well Nos. SE002 and SE002-ST 1. 

Semisubmersible drilling rigs were used for all of the wells. Well No. SE001 and Sidetrack 1 

were drilled by the TRANSOCEAN MARIANUS. The Exploration Plan indicates an anchor 

radius of 2,620 m with 550 m of anchor chain on the bottom at each location. Sidetracks 2 and 3 

of Well No. SE001, as well as Well No. SE002 and Sidetracks 1 and 2, were drilled by the 

DIAMOND OCEAN WORKER. Anchor locations are not known, but the radius is assumed to 

be similar. Approximate anchor locations are plotted on Figure 3.2. 
Figure 3.3. Diagram of subsea wellheads, manifold, and flowlines at the Garden Banks Block 516 site.

Host platform 

(Auger, GB 426)

Approx. 10 km

Oregano Wellsites

(GB 559)

Flowlines and umbilical to 

Auger platform (GB 426)

GB 

426 Auger platform

Wellsites

N

S

W E

GB 559

(Oregano)

GB 516

(Serrano)

Umbilical

Flowline

Flowline

Umbilical 

Termination

Assembly

Well 

Jumper

Oregano Well #1

Oregano Well #2

Umbilical Termination Head

Facilities placed on seafloor in 

GB 516 near-field site between 

pre- and post-drilling surveys

Umbilical Termination Head

Well Jumper

Subsea Pig Launcher/Solvent Interface

38
39 

3.1.3 Garden Banks Block 602 (Post-Development Site) 

Garden Banks Block 602 is located in a water depth of 1,125 m, about 280 km SSE of Cameron, 

Louisiana. The block is leased by Shell, and the prospect has been named Macaroni. 

3.1.3.1 Overview 

Seven wells were drilled within the GB 602 near-field site between September 1995 and 

January 2001 (Figure 3.4). All of these were prior to the only sampling at this site (Cruises 2A 

and 2B). Three other exploration/development wells were drilled in GB 602 approximately 

2.3 km NNE of the site center (Table 3.5). The only other wells within a 10-km radius were 

6 wells in GB 559 and 1 well in GB 600, the latter of which was being drilled at the time of 

Cruise 2B. 

Table 3.5. Wells drilled within 10 km of the Garden Banks (GB) Block 602 near-field (NF) site 

center. 

Distance Timing 

from NF 

Site Center 

Block Number 

of Wells Dates Prior to 

Cruise 2B

During 

Cruise 2B

Notes 

22 m GB 602 6 Sep 1995-Jan 2001 • -- Well Nos. 2, 4, and 5, 

each with a straight well 

and a single sidetrack 

247 m GB 602 1 Sep 1995 • -- Well No. 1, lost to 

shallow water flow 

1.3 km GB 602 1 Feb 1996 • -- Geotechnical borehole 

(9 days) 

2.3 km GB 602 3 Apr-Aug 1996 • -- Well No. 3 and two 

sidetracks 

6.6-6.7 km GB 559 6 Feb 1999-Oct 2000 • -- 

8.3 km GB 600 1 June-Aug 2001 -- • Drilling ongoing during 

Cruise 2B (8 to 25 July 

2001) 

In addition to the exploration and development wells, in February 1996, one geotechnical 

borehole was drilled in GB 602, about 1.3 km northeast of the near-field site center (Figure 3.4). 

The Exploration Plan indicates that a dynamically positioned survey vessel would be used, and 

therefore no anchoring was involved. Because there was no anchoring and only small quantities 

of mud and cuttings were discharged at the seafloor, any effects are assumed to be negligible in 

comparison with the wells drilled within the near-field site. 

3.1.3.2 Drilling Program 

Figure 3.4 shows a diagram of the GB 602 site. Exploration Well No. 1, located 247 m west of 

the site center, was drilled in September 1995 and permanently abandoned (“junked”) after it was 

lost to shallow water flow. Six other wells (three straight wells with corresponding sidetracks) 

were drilled at three surface locations near the site center (which is between the three wellsites 

and about 22 m from each). Exploration Well No. 2 was drilled between September 1995 and 

January 1996, and a sidetrack was drilled in February-March 1997. Development Wells No. 4 

(and sidetrack) and No. 5 (and sidetrack) were drilled between December 1998 and January 2001. 
Figure 3.4. Well locations in and near the Garden Banks Block 602 near-field site.

Legend

Previous wellsite

500-m radius

N

S

W E

Well No. 1

Sep 1995

Well No. 4

Dec 1998-Mar 1999

+ sidetrack Nov 2000-Jan 2001

Well No. 5

Dec 1998-Mar 1999

+ sidetrack Apr-May 1999

Well No. 2

Sep 1995-Jan 1996

+ sidetrack Feb-Mar 1997

Geotechnical 

borehole 

Feb 1996

1.3 km

(a) Wells within the

GB 602 near-field site

(b) Anchor radius and nearby 

wells in GB 602

3,000-m radius

Well No. 3 and two 

sidetracks

Apr-Aug 1996

500-m radius

Flowlines and 

umbilical to/from 

GB 426

Wellheads, manifold,

and other structures 

located in this area 

(see Fig. 3.5)

Anchor location for 

Well No. 3

Well 1 anchor locations

Well 3 anchor locations

Wells 2, 4, 5 anchor locations

2.3 km

Well No. 1

Sep 1995

Well Nos. 2, 4, and 5

(see [a] for dates)

40
41 

Table 3.6 summarizes estimated drilling discharges from wells within the near-field site. Well 

interval data were provided by Shell. Both WBM and SBM were used during drilling. After the 

initial jetting, during which WBM and associated cuttings were released at the seafloor, each well 

had four or five intervals of drilling with WBM, with mud and cuttings discharges from the 

drilling rig. Subsequent intervals were drilled using SBM, with discharge of SBM cuttings. 

Table 3.6. Discharge summary for Garden Banks Block 602 wells within the near-field site. 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Well No. Discharges (bbl) Dates

WBM WBM 

Cuttings WBM WBM Cuttings 

SBM 

Cuttings 

SBM Name 

1 9/13/95-9/21/95 2,770 378 30,447 4,152 5,098 Novadril or 

Petrofree ester 

2 9/25/95-1/21/96 2,770 378 14,938 2,037 5,589 Novaplus or 

Petrofree LE 

2-ST1 2/25/97-3/24/97 2,714 370 9,198 1,254 3,839 Novaplus or 

Petrofree LE 

4 12/10/98-9/27/99 4,760 649 9,055 1,235 3,079 Novaplus or 

Petrofree LE 

4-ST1 11/29/00-1/3/01 2,714a 370a 9,198a 1,254a 3,839a Novaplus or 

Petrofree LE 

5 12/11/98-3/7/99 5,504 751 9,267 1,264 3,056 Novaplus or 

Petrofree LE 

5-ST1 4/26/99-5/29/99 2,714a 370a 9,198a 1,254a 3,839a Novaplus or 

Petrofree LE 

Total: 23,946 3,266 91,301 12,450 28,339 

SBM = synthetic-based mud; ST = sidetrack; WBM = water-based mud. 

Source: Spud date and total depth date as listed in Minerals Management Service Technical Information Management 

System. Well intervals and hole sizes were provided by Shell, and discharge volumes were estimated using hole 

volume times 11 for drilling fluids, hole volume times 1.5 for cuttings. 

a Well intervals not available; assumed same as No. 2 sidetrack based on similar duration of drilling.

The Exploration Plan for Well No. 1 included the option of two SBMs, Novadril (an internal 

olefin) or Petrofree ester. The Development Plan (for Well Nos. 2, 4, and 5) included Novaplus 

(an internal olefin) and Petrofree LE (a linear-α-olefin). It is not known which SBM was actually 

used. 

Anchored semi-submersible rigs were used to drill the wells. The proposed drilling rig for Well 

Nos. 1 and 2 was the SONAT RATHER. The NOBLE PAUL ROMANO was the rig for Well 

No. 4 and a sidetrack as well as the sidetrack of Well No. 5. For Well No. 5, the rig was the 

TRANSOCEAN RICHARDSON. The Exploration Plan for Well Nos. 1 and 2 shows an anchor 

radius of 2,377 m with 914 m of anchor chain contacting the bottom at each location. The 

development plan for Well Nos. 4 and 5 shows an anchor radius of 2,540 m with approximately 

650 m of chain on the bottom at each location. The anchor radii are shown on Figure 3.4. 
42 

In addition to the drilling activities, several structures were installed on the seafloor (Figure 3.5). 

These included a four-well manifold, an umbilical distribution module, and flowlines. Typically, 

a subsea well manifold is about 5 to 10 m square and 5 m or more in height. Most of the 

structures are within a radius of about 75 m. Two insulated flowlines were connected to 

processing facilities located on the Auger platform in GB 426, approximately 19 km to the north. 

3.1.4 Mississippi Canyon Block 292 (Post-Development Site) 

Mississippi Canyon Block 292 is located in a water depth of approximately 1,034 m and is 

approximately 200 km southeast of New Orleans. The block is leased by Texaco, and the 

prospect has been named Gemini. 

3.1.4.1 Overview 

Figure 3.6 shows a diagram of wellsite locations at the MC 292 near-field site. Five wells were 

drilled at three closely-spaced locations near the center of the site between May 1995 and 

July 1999. From south to north, these are Well Nos. 1, 3 (and sidetrack), and 4 (and sidetrack). 

These are producing wells with a subsea manifold and flow lines carrying gas to VK 900, about 

44 km to the north. 

The nearest other wells are 2.5 km to the north in MC 248 (three wells, January to March 2000) 

(Table 3.7). Five other wells were drilled in MC 292 between September 1996 and March 1997 

(Well No. 2 and several sidetracks), but these were more than 3 km SSE of the near-field site 

center. A total of six additional wells had been drilled within MC 247, 248, and 291 at distances 

ranging from 2.5 to 4.0 km (Table 3.7). Due to the distances, any effects are assumed to be 

negligible in comparison with the wells drilled within the MC 292 near-field site. 

Table 3.7. Wells drilled within 10 km of the Mississippi Canyon (MC) Block 292 near-field 

(NF) site. 

Distance Timing 

from NF Site 

Center 

Block Number 

of Wells Dates Prior to 

This Study During This Study 

0 m MC 292 2 Feb-June 1999 • -- 

15 m MC 292 3 May 1995-July 1999 • -- 

2.5 km MC 248 3 Jan-Mar 2000 • -- 

2.9 km MC 291 1 Aug-Oct 1997 • -- 

3.2 km MC 292 5 Sep 1996-Mar 1997 • -- 

4.0 km MC 247 2 Nov 1997-Feb 1998 • -- 

3.1.4.2 Drilling Program 

Well No. 1 was drilled from May to September 1995 and was subsequently completed in 1999 as 

part of development operations. Development Well Nos. 3 and 4 and their respective sidetracks 

were drilled and completed from February to July 1999. The surface locations of Well Nos. 3 

and 4 were 15 m and 30 m, respectively, north of Well No. 1. The MC 292 near-field site is 

centered on the middle (Well No. 3) location. 
Figure 3.5. Diagram of subsea wellheads, manifold, and flowlines at the Garden Banks Block 602 site.

Host platform 

(Auger, GB 426)

Approx. 19 km

N

S

W E

43
Figure 3.6. Well locations in and near the Mississippi Canyon Block 292 near-field site.

Legend

Previous wellsite

N

S

EW

Anchor radius

(3,000 m)

500-m radius

(a) Wells within the

MC 292 near-field site

Well No. 1

(May-Sep 1995)

Well No. 3 and sidetrack

(Feb-Jun 1999)

Well No. 4 and sidetrack

(Feb-Jul 1999)

Wellheads, manifold,

and other structures 

located in this area 

(see Fig. 3.10)

500-m radius

3 wells in MC 248

Jan-Mar 2000

Well No. 1, No. 3 and 

sidetrack, No. 4 and 

sidetrack (May 1995-Jul 1999)

1 well in MC 291

(Aug-Oct 1997)

Flowlines and 

umbilical to/from

VK 900

(b) Anchor radius and 

nearby wells

44
45 

Well Nos. 1, 3, and 4 were drilled with WBM, which were discharged along with cuttings 

(Table 3.8). These included initial discharges at the seafloor during jetting, and subsequent 

discharges from the drilling rig. The two sidetracks (3-ST1 and 4-ST1) were drilled using 

Novaplus, an internal olefin SBM. Due to the short duration of drilling (11 days and 8 days for 

3-ST1 and 4-ST1, respectively), relatively small quantities of SBM cuttings were discharged from 

the drilling rig. 

Table 3.8. Discharge summary for Mississippi Canyon Block 292 wells within the near-field 

site. 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Discharges (bbl) Well No. Dates

WBM WBM 

Cuttings WBM WBM 

Cuttings 

SBM 

Cuttings 

SBM Name 

1 5/7/95 -9/7/95 3,351 457 35,648 4,861 -- -- 

3 2/11/99-4/15/99 3,490 476 29,156 3,976 -- -- 

3-ST1 6/10/99-6/21/99 -- -- -- -- 462 Novaplus 

4 2/4/99-3/13/99 3,490 476 29,156 3,976 -- -- 

4-ST1 7/8/99-7/16/99 -- -- -- -- 1,028 Novaplus 

Total: 10,331 1,409 93,960 12,813 1,490 

SBM = synthetic-based mud; ST = sidetrack; WBM = water-based mud. 

Source: Spud date and total depth date as listed in Minerals Management Service Technical Information Management 

System. Well intervals and hole sizes were provided by Texaco, and discharge volumes were estimated using hole 

volume times 11 for drilling fluids, hole volume times 1.5 for cuttings. 

All three wells and sidetracks were drilled using a semisubmersible rig, the OCEAN STAR. The 

Exploration Plan indicates an anchor radius of approximately 3,000 m. Exact anchor locations 

could not be plotted based on the available information. 

In addition to drilling the wells, a four-well manifold, two flowlines, and a hydraulic/electric 

control umbilical with chemical injection capability were installed (Figure 3.7). The flowlines 

extend north from the manifold to Platform “A” in VK 900. 

3.1.5 Summary 

The four near-field sites represent a range of drilling activities. Table 3.9 summarizes the 

number of wells drilled in the near-field sites before and during this study, as well as the 

discharge data. Figure 3.8 shows the timing of drilling activities at each of the sites in relation to 

sampling cruises during this study. 

VK 916 is the simplest case, with no previous drilling in the near-field site and a single 

exploration well drilled during this study. A semisubmersible drilling rig was used, and one set 

of anchors was deployed. Both WBM and SBM were used during drilling. This site was 

sampled before and after drilling. 
Figure 3.7. Diagram of subsea wellheads, manifold, and flowlines at the Mississippi Canyon Block 292 site.

Host platform (VK 900)

Approx. 44 km

N

S

W E

MC 

292

Platform A

Wellsites

Flowlines

(44

km)

VK 

900

Dual flowlines

Hydraulic/electrical control umbilical

Umbasse

4-well retrievable

manifold

Flowline jumper

Pipeline end 

termination

Well jumper

Piggingloop

46
Cruise 1B

Jan 96 Jan 97 Jan 98 Jan 99 Jan 00 Jan 01 Jan 96 Jan 97 Jan 98 Jan 99 Jan 00 Jan 01 

Wells #SE001

and #SE001-ST1

Viosca Knoll 916

Garden Banks 516

Wells #SE001-ST2, ST3

#SE002 and #SE002-ST1, 

Cruises 1A,1B 

Jan 96 Jan 97 Jan 98 Jan 99 Jan 00 Jan 01 

Garden Banks 602

Well #2-ST1

Cruises 

Wells #1 and #2

Wells #4, #5 

and #5-ST1

Cruises 

Well #4-ST1

Jan 96 Jan 97 Jan 98 Jan 99 Jan 00 Jan 01 

Mississippi Canyon 292

Well #1

Well #3, #4

#3-ST1, and #4-ST1

Cruises 

Jan 95Jan 95Jan 95Jan 95

Figure 3.8. Timeline showing drilling activities at the four near-field sites.

47
48 

Table 3.9. Drilling summary for wells within each near-field site.

Number of Wells 

Within 500-m Radius 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Discharges (bbl) Site 

Prior to 

this Study 

During 

this Study WBM WBM Cuttings WBM WBM Cuttings 

SBM 

Cuttings 

SBM Type 

VK 916 0 1 3,074 419 12,807 1,746 2,510 Syn-Teq (IO) 

GB 516 2 5 13,002 1,774 29,462 4,018 7,313 

Novaplus (IO); 

possibly Petrofree 

LE (LAO) (1 well)

GB 602 7 0 23,946 3,266 91,301 12,450 28,339 

Novadril (IO) or 

Petrofree ester 

(1 well); 

Novaplus (IO) or 

Petrofree LE (LAO) 

(all others) 

MC 292 5 0 10,331 1,409 93,960 12,813 1,490 Novaplus (IO) 

IO = internal olefin; LAO = linear-α-olefin; SBM = synthetic-based mud; WBM = water-based mud. 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll. 

GB 602 and MC 292 are development sites with several previous wells and no new drilling 

during this study. Drilling occurred at various times over several years, so there are multiple 

anchor deployments and sets of drilling discharges. An important difference is that most of the 

drilling at MC 292 was done with WBM, with SBM used only for two sidetracks. Therefore, the 

total quantity of SBM cuttings discharged at MC 292 was the smallest among the four sites. 

These sites were sampled once during Cruises 2A/2B to provide a snapshot of post-drilling status. 

GB 516 is the most complex case, with two previous wells and five new wells drilled during this 

study. Overall, activities were similar to those at the other development sites (GB 602 and 

MC 292) in that multiple wells were drilled over time, with multiple anchor deployments and 

drilling discharges; and subsea wellheads, manifolds, and flowlines were installed connecting to a 

remote platform. The difference is that two “snapshots” were obtained – once during Cruise 1B 

after the two exploration wells had been drilled and again on Cruises 2A/2B after the five 

additional development wells had been drilled. 

3.2 FAR-FIELD SITES 

For each of the four near-field sites, six far-field sites (or “reference areas”) were designated. 

These were chosen to be at about the same depth as the near-field site and 10 to 25 km away. The 

intention was also to locate these sites away from other previous wellsites, to the extent possible. 

However, due to the large number of wellsites in the Gulf of Mexico, it was difficult to find 

far-field sites at similar water depths that were not within 10 km of previous drilling. 

Table 3.10 summarizes the number of wells within 10 km of each far-field site. Well locations 

are shown on Figures 3.9 through 3.12. Each site is discussed individually below, followed by a 

summary and preliminary evaluation. 
49 

Table 3.10. Previous wells near far-field (FF) sites. 

No. of Previous 

Wellsites Site 

Within 

10 km 

Within 

3 km 

Distance to 

Nearest 

Well(s) 

Block of Nearest 

Well(s) 

Dates of 

Nearest Well(s) 

Rating of 

FF Siteb

Viosca Knoll Block 916 (exploration site) 

FF1 63 4 1.9 km VK 957 (4 wells) Jan-July 1989 Fair 

FF2 41 4 2.6 km VK 915 (4 wells) Mar-May 2002 Fair 

FF3 30 0 5.5 km VK 829 (1 well) June-Aug 1994 Good 

FF4 26 0 5.7 km VK 829 (1 well) June-Aug 1994 Good 

FF5 25 0 6.3 km VK 786 (24 wells) June 1995-June 2002 Good 

FF6 25 0 6.7 km VK 786 (24 wells) June 1995-June 2002 Good 

Garden Banks Block 516 (exploration/development site) 

FF1 19 0 4.6 km GB 559 (6 wells) Feb 1999-Oct 2000 Good 

FF2 6 0 8.6-8.9 km GB 386 (6 wells) June 1997-Mar 1999 Good 

FF3 6 0 8.5-8.6 km GB 386 (6 wells) June 1997-Mar 1999 Good 

FF4 7 0 7.8 km GB 386 (2 wells) Nov 1998-Mar 1999 Good 

FF5 12 0 7.5 km GB 386 (2 wells) Nov 1998-Mar 1999 Good 

FF6 3 0 8.4-8.9 km GB 386-387 (3 wells) Apr 1990-Mar 1999 Good 

Garden Banks Block 602 (post-development site) 

FF1 1 0 6.5 km GB 600 (1 well)a June-Aug 2001 Good 

FF2 0 0 >10 km -- -- Excellent 

FF3 0 0 >10 km -- -- Excellent 

FF4 0 0 >10 km -- -- Excellent 

FF5 0 0 >10 km -- -- Excellent 

FF6 1 0 6.3 km GB 562 (1 well) Aug-Sep 2000 Good 

Mississippi Canyon Block 292 (post-development site) 

FF1 12 2 1.1 km 

2.7 km 

MC 243 (1 well) 

MC 243 (1 well) 

Sep-Oct 1994 

Aug-Sep 1999 

Poor 

FF2 13 1 2.0 km MC 243 (1 well) Aug-Sep 1999 Fair 

FF3 2 0 7.4 km 

7.5 km 

MC 201 (1 well) 

MC 160 (1 well) 

Jan 1988 

Sep-Nov 1989 

Good 

FF4 7 0 7.1 km MC 201 (1 well) Jan 1988 Good 

FF5 9 0 4.6 km MC 160 (1 well) Sep-Nov 1989 Good 

FF6 6 0 4.8 km MC 160 (1 well) Sep-Nov 1989 Good 

a Drilling of this well was ongoing during Cruise 2B (8 to 25 July 2001). 

b Excellent – no previous wells within 10 km; Good – no previous wells within 3 km, but one or more 

between 3 km and 10 km away; Fair – previous well(s) between 1.5 km and 3 km away; and Poor – 

previous well(s) within 1.5 km.
Figure 3.9. Location of blocks with previous wells in relation to Viosca Knoll Block 916 near-field (NF) and far-field (F

FF1

FF2

FF3

FF4

FF5

FF6

NF

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

Previous well(s) (before Cruise 1A/1B)Well(s) drilled during this study (between Cruises 1A/1B and 3A/3B)

916

Platform 

(Ram-Powell) with 

multiple wells

Platform 

(Marlin) with 

multiple wells

Platform 

(Petronius) with 

multiple wells

Circle

radii aro

50
Figure 3.10. Location of previous wells in relation to Garden Banks Block 516 near-field (NF) and far-field (FF) sites.

FF1

FF2 FF3

FF4 FF5

FF6

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

Previous well(s) (before Cruises 1A/1B)Well(s) drilled during this study (between Cruises 1A/1B and 2A/2B)

516

Platform (Auger) 

with multiple wells

51
FF1

FF2

FF3 FF4

FF5

FF6

NF

602

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

Previous well(s) (before Cruises 2A/2B)Well in progress during Cruises 2A/2B

Figure 3.11. Location of blocks with previous wells in relation to Garden Banks Block 602 near-field (NF) and far-fiel

Circles are 10-km 

radii around each site

52
Figure 3.12. Location of blocks with previous wells in relation to Mississippi Canyon Block 292 near-field (NF) and f

FF1

FF2

FF3

FF4

FF5

FF6

NF

Near-field (NF) site (500-m radius)Far-field (FF) site (204-m radius)

LEGEND

Previous well(s) (before Cruises 2A/2B)

292

Platform (Matterhorn) 

with multiple wells

53
54 

3.2.1 Viosca Knoll Block 916 

All VK 916, far-field sites had previous drilling within 10 km, and two of them (FF1 and FF2) 

had previous wells in the same block (Figure 3.9). 

Site FF1 had the largest number of previous wells, 63 within 10 km and 4 wells within 3 km. The 

nearest wells were about 1.9 km to the west in the same lease block (VK 957); these four wells 

were drilled between January and July 1989. There also were 40 wells drilled between 

February 1987 and December 2001 at the Ram-Powell location in adjacent block VK 956, about 

4.7 km west of FF1. 

Site FF2 had 41 previous wells within 10 km and 4 wells within 3 km. The nearest were four 

wells approximately 2.6 km away in VK 915. These were drilled between March and May 2002, 

just before Cruises 3A and 3B. 

Sites FF3, FF4, FF5, and FF6 had no previous wells within 3 km. However, there were numerous 

previous wells between 5 and 10 km away, most of these at a single location (Petronius platform) 

in VK 786. 

3.2.2 Garden Banks Block 516 

At the time of the post-drilling survey (Cruise 2B, July 2001), no previous wells had been drilled 

in any of the blocks where GB 516 far-field sites were located. However, all of the far-field sites 

had one or more previous wells within 10 km (Figure 3.10). 

Three blocks adjacent to FF1 had previous wells. The closest was about 4.6 km ESE in GB 559, 

where six wells were drilled between February 1999 and October 2000. In June 1999, one well 

was drilled in GB 515, about 7.1 km to the ENE. Also, 11 wells were drilled between 

September 1995 and January 2001 in GB 602, approximately 7 to 8.5 km SSE of FF1. In 

addition, drilling was ongoing during Cruise 2B at a single wellsite GB 600, about 8.8 km SW of 

FF1. Drilling occurred between 24 June and 13 August 2001 (Cruise 2B was conducted from 8 to 

25 July 2001). 

For sites FF2 through FF6, there were no previous wells in adjacent blocks. However, each site 

was within 10 km of at least one previous well in GB 386, 387, or 562. The nearest wells were 

generally 7.5 to 8.9 km away and were drilled between June 1997 and March 1999. 

3.2.3 Garden Banks Block 602 

At the time of Cruise 2B (July 2001), no previous wells had been drilled in any of the blocks 

where GB 602 far-field sites were located (Figure 3.11). There were no previous wells within 

10 km of FF2, FF3, FF4, or FF5. 

There was one well being drilled within 10 km of the FF1 site at the time of Cruise 2B. This 

exploration well in GB 600 is about 6.5 km ENE of FF1. Drilling occurred between 24 June and 

13 August 2001 (Cruise 2B was conducted from 8 to 25 July 2001). 
55 

There was one previous well within 10 km of the FF6 site. This exploration well was drilled 

between 14 August and 19 September 2000 in Block 562, about 6.3 km NNE of FF6. 

3.2.4 Mississippi Canyon Block 292 

All of the MC 292 far-field sites had previous wells within 10 km (Figure 3.12). FF1 and FF2 

had the largest number of previous wells, with 12 or 13 within 10 km. The closest to FF1 was a 

single well 1.1 km away in MC 243, drilled in September-October 1994. The closest to FF2 was 

a single well 2 km away in MC 243, drilled in August-September 1999. 

For FF3 and FF4, the nearest well was over 7 km away. For FF5 and FF6, the nearest previous 

well was nearly 5 km away. The nearest wells to each of these sites were drilled in the late 

1980’s (January 1988-November 1989). 

3.2.5 Summary and Preliminary Evaluation 

To avoid impacts of previous drilling, two factors to consider are anchor radius and areal extent 

of drilling mud and cuttings deposits. In this water depth, anchoring (including dragging of 

chains and cables) can affect specific locations within about 3 km around each drilling site. 

Planned anchoring locations usually are presented in exploration plans or development plans 

submitted to MMS, but actual locations may vary. Although the chances of randomly collecting 

a sample (e.g., box core) within an anchor scar are small, it would be wise to place reference sites 

at least 3 km from any previous drillsite. 

With respect to drilling discharges, the main concern is to avoid cuttings deposits from previous 

wells. In this water depth, drilling mud discharges (except for those released at the wellbore 

during jetting) are likely to be thinly dispersed over many kilometers or tens of kilometers 

(Brandsma 1990). While it is impossible to avoid potential exposure to mud particles, 

accumulations beyond a few hundred meters from the wellsite presumably would be miniscule 

and undetectable by the most sensitive measures. 

In contrast, because of the hydrophobic nature of the base fluid, SBM cuttings tend to clump 

together and settle rapidly to the seafloor (Brandsma 1996; Neff et al. 2000). Previous 

observations indicate that most SBM cuttings are deposited within about 250 m of the drillsite, 

both on the continental shelf and upper slope (Continental Shelf Associates, Inc. 2004). During 

this study, SBF concentrations at most stations near the 500-m near-field radius were low, but one 

discretionary station approximately 1 km from the GB 602 wellsites showed elevated SBF 

concentrations (see Chapter 15). 

WBM cuttings have a wider range of settling velocities, and in theory, some particles may be 

carried tens of kilometers before settling to the bottom (Brandsma 1990). Observations during 

this study indicate geophysically mappable deposits (interpreted as cuttings) extended for several 

hundred meters to about 1 km from the site center (see Chapter 4). 

Far-field sites located at least 3 km from previous wellsites are likely to avoid previous anchoring 

impacts. With respect to avoiding cuttings and drilling mud deposits, farther is better, but 
56 

certainly a far-field site within (or close to) 1 km of any previous wellsite would be suspect. 

Based on this discussion, the far-field sites are rated as follows in Table 3.10: 

• Excellent – no previous wells within 10 km 

• Good – no previous wells within 3 km, but one or more between 3 km and 10 km away 

• Fair – previous well(s) between 1.5 km and 3 km away 

• Poor – previous well(s) within 1.5 km 

Based on this rating system, all of the far-field sites were rated as “excellent” or “good” with the 

following exceptions (Table 3.10): 

• VK 916, FF1 – rated fair; 4 wells were drilled only 1.9 km away, but the drilling occurred 

more than 10 years ago. This site also had a very large number of wells within 10 km (63). 

• VK 916, FF2 – rated fair, with 4 recent wells (March-May 2002) located 2.6 km away and a 

total of 41 wells within 10 km. 

• MC 292, FF1 – rated poor, with one well 1.1 km away (September-October 1994) and 

another 2.7 km away (August-September 1999). 

• MC 292, FF2 – rated fair, with the nearest well 2 km away (August-September 1999). 

The adequacy of far-field sites is discussed further in Chapter 15, which includes a statistical 

evaluation of barium and SBF concentrations. Briefly, the analysis indicates there are some 

differences among far-field sites, some of which may reflect the effects of previous drilling. 

However, barium and SBF concentrations at the far-field sites are orders of magnitude lower than 

most values measured in the near-field, and differences among far-field sites are small relative to 

the variations in the near-field. For this reason, all far-field sites are retained for analysis in 

subsequent chapters. 
57 

Chapter 4 

Geophysical Characterization 

of Study Sites 

Harry H. Roberts 

Louisiana State University 

4.1 INTRODUCTION AND GEOLOGIC FRAMEWORK 

This study focuses on the effects of oil and gas exploration and development at four selected sites 

on the continental slope of the northern Gulf of Mexico. The study sites are located in the 

world’s most mature deepwater hydrocarbon exploration-production province. The continental 

slope offshore of the northern Gulf states is also perhaps the most geologically complex 

continental slope in today’s oceans. The shaded relief map of Figure 4.1 reflects the complexity 

of the slope’s surface topography, a reflection of an equally complex subsurface geology. 

Our present knowledge base for both the surface and subsurface geology of the northern Gulf’s 

continental slope is largely a product of the search for and production of oil and gas. Since the 

late 1970’s, when the exploration front moved to the upper continental slope, an enormous 

database of 3D-seismic, bathymetry, high resolution geohazard-scale acoustic and sediment core 

data have been compiled for deep water. These data sets confirm the geologic complexity of this 

province and support the idea that the slope’s geologic configuration derives from the 

interdependency of terrigenous clastic sedimentation and salt tectonics (Humphries 1978). 

Research conducted in the early and mid-1990’s emphasizes that variations in sedimentation rate 

affect the growth of salt bodies. Salt deformation, in turn, exerts a strong control on the locations 

and orientations of intraslope minibasins and associated fault networks (Talbot 1993; Vendeville 

and Jackson 1993; Weimer et al. 1994; Diegel et al. 1995; Rowan 1995; Rowan et al. 1995). 

The northern Gulf of Mexico continental slope covers an area of over 193,000 mi2

 and is defined 

by the shelf edge at roughly the 200-m isobath to the upper limit of the continental rise at an 

approximate depth of 2,600 to 2,750 m (Bouma and Roberts 1990). The most complex part of the 

basin’s continental slope occurs opposite east Texas and Louisiana. In this province, regional 

topography is dominated by domes or knolls associated with salt in the shallow subsurface and 

intervening basins, as discussed above and is illustrated by Figure 4.1. Seafloor slopes 

associated with these regional-scale features range from less than 1o on the dome tops and along 

the basin floors to over 20o on the side of domes and on basin flanks. Observable on this regional 

topography of domes and basins are smaller-scale geologic features that are associated with high 

variability in sediment type and local relief. Near-surface geology and topography of the upper 

continental slope in the northern Gulf of Mexico is highly influenced by cyclic episodes of shelf 

edge progradation and sediment input to deep water. In addition to filling mini-basins and 

mobilizing salt, sediment loading during periods of lowered sea level activates faults. Faulting 

creates abrupt relief and steep slopes (some near vertical) on the modern ocean floor of the slope. 

Faults function as avenues of transport for fluids and gases to or near the modern seafloor. The 

process of fluid and gas expulsion has an important impact on the present surficial geology of the 

slope. Expulsion of large volumes of fluidized sediment results in the formation of mud 

volcanoes and mud flows (Neurauter and Roberts 1992; Kohl and Roberts 1994). Seepage and 

venting results in the development of highly populated communities of chemosynthetic organisms 

(Kennicutt et al. 1985), creation of brine pools and pockmarks (MacDonald et al. 1990), gas 

hydrate mound formation (Brooks et al. 1985), and precipitation of carbonates and other exotic 

minerals to form hardgrounds, chimneys, and mound-like buildups (Roberts and Aharon 1994). 
Figure 4.1. A computer enhanced shaded relief map of the northern Gulf of Mexico continental slope. Mult

were the primary input data for generating this view of the slope’s complex surface topography. complexity is a product of the interaction of massive volumes of sediment input to the slope durisea level and associated modification of underlying salt masses. The three general areas of data are shown.

Figure 4.1. A computer enhanced shaded relief map of the northern Gulf of Mexico continental slope. Mult

were the primary input data for generating this view of the slope’s complex surface topography. complexity is a product of the interaction of massive volumes of sediment input to the slope durisea level and associated modification of underlying salt masses. The three general areas of data are shown.

58
59 

These various seafloor responses are all products of vertical flux of hydrocarbon gases, crude oil, 

and other formation fluids migrating to the ocean floor along faults. 

The Mississippi River, which feeds a delta that has nearly prograded to the shelf edge, makes a 

comparatively small impact on modern slope sedimentation. In response to a lack of siliciclastic 

sediment input during the present highstand of sea level, a thin hemipelagic sediment unit drapes 

slope topography. These sediments are composed of numerous calcareous pelagic foraminifera 

tests in a matrix of hemipelagic clay. When studied carefully using X-ray radiography, sediments 

of the hemipelagic drape have a massive character, rarely display bedding, are highly bioturbated, 

and frequently show the effects of early diagenesis. 

On the upper continental slope, the hemipelagic drape is frequently missing from the tops of 

regional domes and knolls. Sedimentologists with experience in the Gulf of Mexico consider this 

relationship a product of dome top erosion by Loop Current intrusions directly on the slope or the 

impact of Loop Current eddies that transit from east to west along the shelf-slope break. The 

Loop Current can impact the seafloor to depths of nearly 1,000 m (Hamilton 1990). Dome tops 

on the upper continental slope within this depth range frequently exhibit seafloor erosion 

exposing older sediments and frequently deformed sediments. Both slumping due to 

oversteepening on salt dome flanks and faulting associated with salt movement frequently cause 

dome flank and dome top sediment deformation. 

In contrast to seafloor covered with a hemipelagic drape deposit, there are vast areas of slope 

impacted by fluid and gas expulsion. These areas appear much more complex both visually and 

on geophysical records. 

4.2 DATA COLLECTION SCHEDULE AND INSTRUMENTATION 

The data collection plan as originally drafted included the acquisition of geophysical data 

(side-scan sonar, subbottom profiles, and bathymetry) from four sites within the upper continental 

slope province of the northern Gulf of Mexico. As initially planned, these data sets were to be 

collected with a high resolution deep-tow system. Delays associated with weather and 

instrumentation problems mandated a rethinking of the data collection schedule and equipment to 

be focused on the data collection program. Ultimately, one exploration site, VK 916, was 

completed with the deep-tow system between November 2000 and January 2001 (Cruise 1A) and 

resurveyed with different instrumentation in May 2002 (Cruise 3A). The other three sites –

MC 292, GB 602, and GB 516 were surveyed in June-July 2001 (Cruise 2A). Table 4.1

summarizes the dates of data collection, locations, and types of data collection systems used. 

Figure 4.1 illustrates the general areas of the continental slope where specific sites were 

surveyed. 

The pre-drilling survey at VK 916 (Cruise 1A) was the only one conducted for this project with 

the deep-tow system, which involved two boats. The data collection systems in the deep-tow fish 

included (1) a dual frequency EdgeTech side-scan sonar that operated at center frequencies of 

120 kHz and 420 kHz, (2) an EdgeTech chirp subbottom profiler with a frequency bandwidth of 2 

to 10 kHz, and (3) a deepwater precision depth sensor. An electro-hydraulic winch with 

approximately 6,400 m of cable controlled the depth of the fish above the bottom. 
60 

Table 4.1. Geophysical surveys. 

Cruise Dates Location Survey Timing Equipmenta

1A 11-12 Nov. 2000 VK 916 Pre-drilling Deep-tow 

29-30 Nov. 2000 VK 916 Pre-drilling Deep-tow 

6 Dec. 2000 VK 916 Pre-drilling Deep-tow 

9-10 Dec. 2000 VK 916 Pre-drilling Deep-tow 

30 Dec. 2000 VK 916 Pre-drilling Deep-tow (fathometer only) 

1 Jan. 2001 VK 916 Pre-drilling Deep-tow 

2A 25-29 June 2001 MC 292 Post-drilling AUV 

2-4 July 2001 GB 602 Post-drilling AUV 

5-7 July 2001 GB 516 Post-drilling AUV 

3A 16-18 May 2002 VK 916 Post-drilling AUV 

a Deep-tow (two boats): EdgeTech side-scan sonar, EdgeTech chirp subbottom profiler, Odom Hydrographic Systems, 

Inc. deep-water echotrac (single-beam), and cesium magnetometer. AUV (HUGIN 3000): EdgeTech side-scan sonar, 

EdgeTech chirp subbottom profiler, and Simrad EM2000 swath bathymetry. 

AUV = autonomous underwater vehicle; GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll. 

The initial deep-tow survey at VK 916 was plagued by rough weather and sea state conditions, 

equipment failure, and persistent electronic problems with the towfish, resulting in lengthy, costly 

delays (see Appendix C1 for details). Therefore, it was decided that data quality would be better 

and data collection schedules could be met more efficiently for other sites using the C&C 

Technologies AUV, the HUGIN 3000. The post-exploration survey of VK 916 (Cruise 3A) also 

used the AUV (Table 4.1). 

Figure 4.2 illustrates the working configuration of the HUGIN 3000 AUV with the M/V RIG 

SUPPORTER. The HUGIN is an untethered survey vehicle designed to collect high resolution 

geophysical data in deep water (to about 2,700 m). Data collection systems include (1) a Sinrad 

EM 2000 swath bathymetry system, (2) an EdgeTech side-scan sonar, and (3) an EdgeTech chirp 

subbottom profiler. The swath bathymetry system collects soundings under the AUV in a 200-m 

swath composed of up to 111 individual beams. A precision depth sensor on the AUV provides 

an accurate depth of the fish above the bottom. Once the data are downloaded from the AUV 

data storage drives, bathymetric data are processed using C&C Technologies software, and maps 

are produced. The EdgeTech side-scan sonar has proved to be a reliable and high resolution 

source of seafloor data. This side-scan simultaneously transmits two linearly swept frequency 

modulated pulses centered at two frequencies, 120 kHz and 410 kHz. In this project, survey lines 

were set up so that at 120-kHz frequency, swaths would overlap approximately 20%. This 

configuration resulted in continuous coverage of the study area. Mosaics of the seafloor were 

constructed. The limited high resolution lines were processed at the 410-kHz frequency. Data 

processing and production of hard copy products can be carried out aboard the mother ship. 

Subbottom data were acquired with the EdgeTech chirp sonar subbottom profiler, which 

transmits pulses that are generated in the frequency range of 2 to 8 kHz. This system provides 

extremely high resolution data on near seafloor stratigraphy and is very reliable. For this project, 

the chirp subbottom profiler provided valuable subsurface information when used in conjunction 

with side-scan sonar data for interpretation of seafloor features and sediment distribution patterns. 
Figure 4.2. Working configuration of the HUGIN 3000 autonomous underwater vehicle 

showing the “free-swimming” HUGIN collecting data from a predetermined 

height above the seafloor. The HUGIN remains in constant communication with 

the mother ship through acoustic links.

Figure 4.2. Working configuration of the HUGIN 3000 autonomous underwater vehicle 

showing the “free-swimming” HUGIN collecting data from a predetermined 

height above the seafloor. The HUGIN remains in constant communication with 

the mother ship through acoustic links.

61
62 

Data transmission is accomplished by using three transponders: (1) HiPAP (High Precision 

Acoustic Positioning), (2) ACL (Acoustic Command Link), and (3) ADL (Acoustic Data Link). 

The AUV is powered by an aluminum-oxygen fuel cell that sustains continuous operation for 

nearly 2 days before a fuel cell change. 

The AUV is computer-controlled. Two titanium spheres house three computers, which control 

navigation, data collection/processing, and system status. Dual 50-gigabyte data storage drives 

manage the high rate of digital data collection. The mother ship is in constant contact with the 

AUV through three top-side computers. All sensors are constantly monitored, and quality control 

of data acquisition is accomplished through transmission of limited data sets by way of an 

acoustic link. A DGPS provides the mother ship positions while AUV positions are calculated 

using ultra short baseline acoustics, inertial navigation, and Doppler velocity log. Other 

instruments used to help operate the AUV are (1) a precision depth sensor, (2) an altimeter, (3) an 

acoustic Doppler log, and (4) a salinity/temperature probe to generate data for calculating sound 

velocity in the water column. The vehicle is capable of transmitting data to the mother ship for 

quality control purposes through an acoustic link. 

4.3 VIOSCA KNOLL BLOCK 916 – EXPLORATION SITE 

4.3.1 Introduction 

The Viosca Knoll lease area is located east of the modern lobe of the Mississippi River delta 

(Figure 4.1). The shelf edge forms the upper boundary of this area. Most of the Viosca Knoll 

lease blocks fall within upper continental slope water depths. However, the southeast corner of 

the lease area extends into mid-slope depths. Figure 4.3 shows the approximate location of the 

wellsite in the northeast corner of VK 916. This area of the slope is dominated by updip lobate 

protrusions at the shelf edge and distinct channel-levee systems that originate near the shelf edge 

and cross the entire slope, where they terminate on the deep Gulf of Mexico basin floor at a water 

depth of approximately 2,450 m. These channel-levee systems represent conduits for bypassing 

sediment from the shelf edge at periods of lowered sea level during the Pleistocene. The 

channel-levee systems can be linked to shelf edge deltas, which form the compensationally 

stacked protrusions that occur at the shelf-slope break east of the modern Mississippi delta to the 

head of DeSoto Canyon. 

The VK 916 site is located on the western flank of a well-defined channel-levee system. In 

addition to these sediment by-pass conduits, the Viosca Knoll lease area, as well as the adjacent 

northern part of the Mississippi Canyon lease area, are regions of the slope that display distinct 

surface topography related to the occurrence of salt masses in the shallow subsurface. 

4.3.2 Data Quality for the Pre-Drilling Survey 

As Table 4.1 documents, data collection at VK 916 started in November 2000 using a deep-tow 

system for collecting high resolution acoustic data (bathymetry, side-scan sonar, and chirp sonar). 

The pre-drilling site survey at VK 916 (Cruise 1A) was accomplished between November 2000 

and January 2001. Data products for the pre-drilling survey are given in Appendix C1. 
Figure 4.3. Location map of Viosca Knoll (VK) Block 916. Note that on the computer enhanced multibeam bat

occurs on the western flank of a channel-levee system that starts just below the shelf-slope break.

VK 916

63
64 

Weather and instrumentation problems, including loss of major instrumentation components, 

affected the deep-tow survey throughout the November 2000 to January 2001 data collection 

period. Despite substantial difficulties, useable data sets were generated. Two data sets are very 

important to the objectives and goals of this research program: side-scan sonar mosaics and high 

resolution chirp sonar subbottom profiles. The side-scan data were considered to be fair. A 

problem that manifested itself as banding appeared on the record up to 50 m from the nadir, or 

survey track line. The records were still useable, and some of the banding was removed during 

processing. 

Figure 4.4 is an example of the chirp sonar subbottom data acquired near the proposed wellsite in 

VK 916. Chirp sonar subbottom profiles were considered of good quality even though some of 

the first records collected were noisy. The noise problem was found to be in an electronic circuit 

and was eventually corrected. Another problem resulted from flying the deep-tow fish less than 

25 m over the bottom. When closer to the bottom, part of the record was eliminated. So, the fish 

was kept above this critical height. 

4.3.3 Site Characterization: Pre-Drilling 

Data products from the pre-drilling survey (Cruise 1A) are provided in Appendix C1. The 

bathymetry of the study area clearly shows the channel-like feature that trends roughly 

northwest-southeast through the eastern sector of VK 916. At the proposed wellsite in the 

northeastern part of the block, water depths are approximately 1,128 m, shallowing to the north 

and deepening to the south. The side-scan sonar data indicate no disturbances on the seafloor in 

the vicinity of the proposed wellsite, and as discussed previously, subbottom profiles near the 

wellsite and across the study area in general describe a hemipelagic sediment drape over the 

seafloor throughout the area of interest. These surficial sediments exhibit no surface irregularity 

or significant amplitude changes within the area around the proposed wellsite. Undisturbed 

seafloor is the interpretation of this region of VK 916 as determined from evaluation of the 

geophysical data sets. 

4.3.4 Data Quality for the Post-Drilling Survey 

The post-drilling survey of VK 916 (Cruise 3A) was conducted 16-18 May 2002 using the AUV. 

The data collection sensors associated with the AUV and its basic operations are discussed in a 

previous section of this chapter. All data products from the post-exploration AUV survey are 

provided in Appendix C2. 

Quality of side-scan sonar, chirp sonar subbottom profiles, and multibeam bathymetry from the 

AUV survey of the VK 916 study site was excellent. The high resolution multibeam bathymetry 

(see bathymetry map in Appendix C2) provides the necessary data to construct shaded relief 

images of the study area (Figure 4.5). Side-scan sonar data were collected over the entire study 

area (defined by a circle of 3,050-m radius from the wellsite) using the low frequency 120-kHz 

setting. A higher resolution data set was collected around the wellsite using the high frequency 

410-kHz setting. Mosaics from both frequencies are included in Appendix C2. Both the 120-kHz 

and 410-kHz side-scan sonar records were of excellent quality. Figure 4.6 illustrates a portion of 

the 410-kHz side-scan sonar mosaic centered on the wellsite. The chirp sonar subbottom profile 

records (e.g., Figure 4.7) also were of very high quality and were important in identifying areas 

of probable deposition of drilling mud and well cuttings on the modern seafloor. 
Figure 4.4. Chirp sonar subbottom profile collected during Cruise 1A (pre-drilling) on Survey Line 1011 

approximately 50 m west of the proposed wellsite in Viosca Knoll (VK) Block 916. Note 

the well-defined hemipelagic drape (acoustically opaque surface unit) overlying closely 

spaced reflectors. Also, the uniform character of the seafloor reflector suggests no 

disturbance or foreign material in the area.

Figure 4.4. Chirp sonar subbottom profile collected during Cruise 1A (pre-drilling) on Survey Line 1011 

approximately 50 m west of the proposed wellsite in Viosca Knoll (VK) Block 916. Note 

the well-defined hemipelagic drape (acoustically opaque surface unit) overlying closely 

spaced reflectors. Also, the uniform character of the seafloor reflector suggests no 

disturbance or foreign material in the area.

65
Figure 4.5. A shaded relief image of the Viosca Knoll (VK) 916 study area showing the location of 

the wellsite for VK 916 Well No. 1 with regard to the distinct channel-levee feature 

trending roughly northwest-southeast through the eastern part of the block. Note the 

previous wellsite about 2.3 km away in VK 872.

VK 872 Well No. 1

and sidetrack

Nov-Dec 1998

VK 916 Well No. 1

Nov-Dec 2001 

(during this study)

Figure 4.5. A shaded relief image of the Viosca Knoll (VK) 916 study area showing the location of 

the wellsite for VK 916 Well No. 1 with regard to the distinct channel-levee feature 

trending roughly northwest-southeast through the eastern part of the block. Note the 

previous wellsite about 2.3 km away in VK 872.

VK 872 Well No. 1

and sidetrack

Nov-Dec 1998

VK 916 Well No. 1

Nov-Dec 2001 

(during this study)

VK 872 Well No. 1

and sidetrack

Nov-Dec 1998

VK 916 Well No. 1

Nov-Dec 2001 

(during this study)

66
Figure 4.6. A sector of the side-scan sonar mosaic produced for the Viosca Knoll (VK) Block 916 area of interest. No

seafloor areas roughly radiating from the wellsite. See Figure 4.7 for a chirp subbottom profile along Line 

Well No. 1

(VK 916)

500-m radius

Side-scan sonar 

Line 136

Figure 4.6. A sector of the side-scan sonar mosaic produced for the Viosca Knoll (VK) Block 916 area of interest. No

seafloor areas roughly radiating from the wellsite. See Figure 4.7 for a chirp subbottom profile along Line 

Well No. 1

(VK 916)

500-m radius

Side-scan sonar 

Line 136

67
Figure 4.7. A section of a chirp sonar subbottom profile acquired along Line 136, which trends north-south across the Vio

wellsite. Note the high surface reflectivity around the wellsite. This high amplitude response of the seafloor rhigh density drilling mud and cuttings from seafloor releases during well jetting.

150 meters

VK 916 Well No. 1

Figure 4.7. A section of a chirp sonar subbottom profile acquired along Line 136, which trends north-south across the Vio

wellsite. Note the high surface reflectivity around the wellsite. This high amplitude response of the seafloor rhigh density drilling mud and cuttings from seafloor releases during well jetting.

150 meters

VK 916 Well No. 1

68
69 

4.3.5 Site Characterization: Post-Drilling 

On the post-drilling survey, the seafloor around the wellsite was considerably different on high 

resolution acoustic records than most of the surrounding seafloor (Figures 4.6 and 4.7). The 

chirp sonar subbottom profiles proved very important for identifying areas interpreted as having 

been impacted by drilling mud and well cuttings, both of which have a higher specific gravity 

than the surficial hemipelagic muds. Barite, a common component of drilling mud, has a specific 

gravity that ranges between 4.3 to 4.6, while calcite and aragonite, the many microfossil tests that 

comprise the hemipelagic sediment, have a specific gravity of 2.6 to 2.7. In addition, cuttings are 

derived from subsurface sediments that are much more compacted and therefore are denser than 

surface hemipelagic sediments that drape most of the northern Gulf continental slope. 

Consequently, these denser sediments, when deposited over hemipelagic sediments, create an 

acoustic impedance difference that translates into a higher amplitude reflection on subbottom 

profiler records, as can be clearly seen on Figure 4.7. When mapped from both surface 

reflectance on side-scan sonar and chirp sonar subbottom records, the zone of high amplitude 

surface response describes a northeast-southwest trending area shown in Figure 4.8. This zone is 

about 610 m in its longest dimension and averages about 250 m wide. The thickness of 

deposition was estimated to range from about 0 to 45 cm (see the Isopach Map in Appendix C2). 

The side-scan data also define two pockmarks on the seafloor near the drilling site and a number 

of drag marks related primarily to anchoring during the drilling operation. Subbottom data 

clearly define the channel of the channel-levee system discussed previously. On the Seafloor 

Investigation Map of Appendix C2, this channel-like feature is defined by mass movement and 

debris flow deposits because these types of deposits fill the channel. 

Figure 4.8 also shows that possible cuttings and anchor scars were evident from two previous 

wells in VK 872, about 8 km to the north/northeast of the VK 916 wellsite. These wells were 

drilled in November-December 1998, about 3.5 years prior to Cruise 3A. The side-scan sonar 

records also showed anchor scars associated with an older wellsite in VK 873 (drilled in March 

1988, 14 years before Cruise 3A). 

Figure 4.9 shows an area of debris on the seafloor west-northwest of the drilling site. These 

sonar targets appear elongated and are probably pipes. These targets were not present in the data 

sets collected for the pre-drilling survey. In fact, the only targets on the side-scan sonar records 

of the pre-drilling survey were the pockmarks near the drilling site. 

4.4 MISSISSIPPI CANYON BLOCK 292 – POST-DEVELOPMENT SITE 

4.4.1 Introduction 

The MC 292 site is located on the upper continental slope east of Mississippi Canyon and 

southeast of the modern lobe of the Mississippi River delta (Figure 4.1). Water depths in the 

area of interest range from about 1,280 m in the southeastern part of the area to approximately 

920 m in the northwestern sector (see bathymetry map in Appendix C3). The area around the 

wellsite has a water depth of approximately 1,034 m. 
Figure 4.8. Map of cuttings and anchor scars around the Viosca Knoll (VK) Block 916 site based on 

the Cruise 3A geophysical survey (August 2002). High-reflectance areas are interpreted 

as possible cuttings.

500-m radius VK 916

Well No. 1

Nov-Dec 2001

Limit of mosaic

VK 872 

Well No. 1 

and sidetrack

Nov-Dec 1998

Legend

= High-reflectance area

= Anchor scar

= Wellsite

70
Figure 4.9. A portion of side-scan sonar Line 104 showing debris that appears to be pipe on the seafloor within the Vio

area.

71
72 

The MC 292 site is located on the southeastern flank of a regional dome-like feature supported by 

a shallow subsurface salt mass (Figure 4.10). The slope gradient increases significantly on the 

flank and to the south of this structure. A canyon-like feature trending northwest-southeast along 

the eastern side of the dome may have functioned as a conduit (channel-levee system) for 

transporting shallow-water shelf edge sediments to deepwater depositional sites at some time in 

the past. However, subsequent salt movement appears to have blocked part of the channel, unlike 

more continuous counterparts to the east. This blockage probably resulted from the late 

Pleistocene sediment loading of the slope by updip shelf edge deltas derived from the Mobile and 

perhaps the Pascagoula rivers causing movement of shallow salt across the downslope trend of 

the channel-levee system. 

Because of the dominant east-to-west surface currents in this part of the northern Gulf of Mexico, 

suspended sediments from the Mississippi River rarely impact the area of interest. Therefore, a 

persistent unit of hemipelagic sediments is draped across the topographic features of this part of 

the continental slope. However, because the Loop Current frequently impacts the shelf edge and 

upper slope east of the modern lobe of the Mississippi River delta, this hemipelagic drape is 

sometimes eroded on the tops and flanks of salt supported domes and knolls. Figure 4.11 shows 

a high resolution chirp sonar subbottom profile segment (Line 319) south of the wellsites 

illustrating seafloor erosion along the southeastern flank of the salt-supported feature. The entire 

area of seafloor erosion mapped in MC 292 can be found on the Seafloor Investigation Map of 

Appendix C3. Trending in the same direction as the area of seafloor erosion 

(southwest-northeast) are a series of surface and near-surface faults mapped from the chirp sonar 

subbottom records. These faults are slightly to the south of the main zone of erosion and clearly 

follow the perimeter of the subsurface salt mass. No fluid and gas expulsion features could be 

found in association with the faulting. 

4.4.2 Data Quality 

The high resolution acoustic data collected by the AUV in support of the post-development 

survey of the MC 292 well area were of excellent quality. The multibeam bathymetric data 

produced a high quality map that details the dominant features of the seafloor. Combined with 

the high quality side-scan sonar images of the seafloor and the very high resolution chirp sonar 

subbottom profiles, these state-of-the-art data sets provided unmatched insight into the surface 

and near-surface geologic conditions of the study area. Map products are provided in 

Appendix C3. 

4.4.3 Site Characterization 

Both the low frequency (120 kHz) and high frequency (410 kHz) side-scan sonar images of the 

wellsites and surrounding areas show variations in surface backscatter strength and definable 

patterns of seafloor reflectance. Figure 4.12 is a 120-kHz side-scan sonar image of the wellsite 

vicinity. Note the radial pattern of high backscatter reflectance (darker areas) and the zone of no 

or minimal reflectance (white area) that occurs southeast of the well cluster. These two areas are 

very different with regard to surface reflectivity and are interpreted to represent two types of 

drilling-related deposition. 

 
Figure 4.10. A shaded relief image of the Mississippi Canyon (MC) Block 292 study area showing the approximate loc

the southeastern flank of a distinct dome-like feature produced by a salt mass in the shallow subsurface. Nfeature along the eastern side of this dome.

MC 292 MC 292

Figure 4.10. A shaded relief image of the Mississippi Canyon (MC) Block 292 study area showing the approximate loc

the southeastern flank of a distinct dome-like feature produced by a salt mass in the shallow subsurface. Nfeature along the eastern side of this dome.

MC 292 MC 292

73
Figure 4.11. Chirp sonar subbottom profile (Line 319) illustrating an area of seafloor erosion that in plan-view trends no

south of the Mississippi Canyon Block 292 wellsites.

74
Figure 4.12. Image of a portion of a 120-kHz side-scan sonar survey line (Line 316) that trends north-south a short dista

Mississippi Canyon Block 292 well cluster. Note the radial pattern of high-reflectance (dark) areas (interpfrom rig discharges) and the low-reflectance (white) area near the wells (interpreted as possible mud/cuttin

Figure 4.12. Image of a portion of a 120-kHz side-scan sonar survey line (Line 316) that trends north-south a short dista

Mississippi Canyon Block 292 well cluster. Note the radial pattern of high-reflectance (dark) areas (interpfrom rig discharges) and the low-reflectance (white) area near the wells (interpreted as possible mud/cuttin

75
76 

The combination of a smooth seafloor (little backscatter on side-scan sonar records) and a high 

amplitude response at the seafloor on high resolution subbottom profiles identifies areas of 

probable drilling mud and cuttings deposition from initial discharges during jetting of wells. 

Figure 4.13 illustrates this area around the well cluster in MC 292, which is approximately 75 m 

wide and slightly over 150 m in length. 

Outside of the low reflectance area near the wellsites, the high reflectance star-like pattern 

radiating from the wellsites is a little more difficult to interpret. The high reflectivity on 

side-scan sonar records means a higher “surface roughness” than surrounding areas of 

undisturbed seafloor. This surface roughness can develop from a variety of processes including: 

(1) deposition of well cuttings that are coarser-grained than typical hemipelagic sediments, 

(2) disturbances of the bottom (producing grooves, furrows, etc.) caused by the process of setting 

the anchors for drilling operations, (3) the dropping of sediment clumps to the seafloor from the 

anchor cables as they are pulled into place, and (4) a combination of some or all of the above. 

Even though the maps in Appendix C3 identify these areas as well cuttings, all of the 

aforementioned processes could produce the reflectance patterns observed on side-scan sonar 

records. The Seafloor Investigation Map (Appendix C3) shows that the radial high reflectance 

patterns roughly align themselves with the linear and radial anchor cable scars on the seafloor in 

the far-field away from the well cluster. 

Twenty-six side-scan sonar targets were identified within the study area. These sonar targets are 

identified in the Survey Report of Appendix C3. The wellheads and manifolds are clearly visible 

on the side-scan image shown in Figure 4.12. The subbottom profiles clearly identified a series 

of northeast-southwest trending faults in the southeastern and eastern sectors of the study area. 

The seafloor was offset by as much as 0.6 m by some of the faults. However, fluid and gas 

expulsion features were not identified. The fault trend is interpreted to mark the approximate 

edge of the salt mass in the subsurface. 

Within the MC 292 survey area, anchor scars and possible drilling mud and cuttings were evident 

from other nearby wells in MC 291, MC 292, and MC 248 (Figure 4.13). Figure 4.14 shows a 

portion of the side-scan sonar mosaic focusing on the wellsite in MC 248 where three wells were 

drilled between January and March 2000. The low reflectance areas near the wellsite are 

interpreted to be drilling mud and cuttings released at the seafloor during the initial jetting of the 

well, and the bands radiating from the wellsite are interpreted to be rig-discharged cuttings that 

settled to the seafloor. 
Figure 4.13. Map of drilling mud, cuttings, and anchor scars around the Mississippi Canyon (MC) 

Block 292 site based on the Cruise 2A geophysical survey (June-July 2001). Low-reflectance 

areas are interpreted as possible mud/cuttings from seafloor releases during well jetting. 

High-reflectance areas are interpreted as possible cuttings from rig discharges. 

Limit of survey area

5 Wells

Sep 1996-May 2001

(MC 292)

500-m radius

5 Wells

(including 

sidetracks)

May 1995-

July 1999

1 Well

Aug-Oct 1997

(MC 291)

3 Wells

Jan-Mar 2000

(MC 248)

Flowlines and umbilical 

to VK 900

Closeup (500-m radius)

Legend

= High-reflectance area

= Anchor scar

= Low-reflectance area

= Wellsite

77
Wellsite

(MC 248)

Well No. 1 and 

two sidetracks

Jan-Mar 2000

F

um

MC 

an

Anchor scars

500 meters

N

S

W E

Figure 4.14. A sector of the side-scan sonar mosaic for the Mississippi Canyon (MC) Block 292 area. This closeup sh

adjacent block (MC 248) where three wells were drilled from January to March 2000, about 1.5 years befSee Figure 4.13 for location relative to the MC 292 wellsite. Note the high-reflectance areas (possible cuwellsite and the low-reflectance area immediately around the wellsite (possible mud/cuttings from well je

78
79 

4.5 GARDEN BANKS BLOCK 602 – POST-DEVELOPMENT SITE 

4.5.1 Introduction 

The study site in GB 602 is located in an intraslope basin directly south of the chenier plain coast 

of western Louisiana. Water depths in the area of interest range from about 1,067 m in the 

northwest to 1,130 m at the center of the basin just south of the wellsite area. Figure 4.1 shows 

the general area of the continental slope in which the two Garden Banks sites are located. 

Figure 4.15 is a shaded relief map showing both the GB 602 and GB 516 locations. 

The GB 602 site is located near the center of an intraslope basin on the upper continental slope. 

This basin is downslope of a shelf edge that has been impacted by the deposition of shelf margin 

deltas during late Pleistocene periods of lowered sea level (Suter and Berryhill 1985). These 

deltas and their fluvial systems were the sources of sediment for filling intraslope basins in this 

general area. At the same time, this sedimentary loading caused adjustment of subsurface salt, 

which activated faults and caused vertical migration of fluids and gas. The basin is surrounded 

by high and irregular topographic features related to salt in the shallow subsurface. These basin 

margins are the originating points for submarine landslides, faulting, and fluid/gas expulsion. 

North of the GB 602 study site is Shell’s Auger Field. In this area, large submarine landslides 

followed by fluid and gas expulsion have impacted both the latest sedimentary fill in the basin 

and the character of the seafloor on basin-flanking domes. However, for most of the area of 

interest in GB 602, the seafloor sediments are composed of a well-developed hemipelagic drape. 

In chirp sonar subbottom profiler records, this area is represented by uniform and parallel 

subbottom reflectors. 

4.5.2 Data Quality 

The high resolution acoustic data collected by the AUV in support of the post-development 

survey of the GB 602 well area were of excellent quality. The multibeam bathymetric data 

produced a high quality map that details the dominant features of the seafloor. Combined with 

the high quality side-scan sonar images of the seafloor and the very high resolution chirp sonar 

subbottom profiles, these state-of-the-art data sets provided unmatched insight into the surface 

and near-surface geology conditions of the study area. Map products are provided in 

Appendix C3. 

4.5.3 Site Characterization 

The side-scan data from the study area describe a very uniform reflectance except for the 

man-made features and disturbances of surficial sediment. As might be expected from the 

seafloor in intraslope basins, the hemipelagic drape deposited over the slope as sea level rose 

from the latest Pleistocene glacial maximum (approximately 18 thousand years before present) is 

very uniform. Unless submarine landslides and/or fluid/gas expulsion from adjacent high relief 

areas disrupt the hemipelagic drape deposits, basin floors tend to be essentially featureless. The 

GB 602 site is no exception. On both side-scan sonar and chirp sonar subbottom profiler records, 

the basin fill is uniform and without evidence of natural disruptions. However, data from the 

AUV identified 43 sonar targets related to human activity in the area, as shown on the Seafloor 

Investigation Map, which along with bathymetry maps and side-scan sonar mosaics are presented 

in Appendix C3. 
Figure 4.15. A shaded relief image of the Garden Banks (GB) 602 and GB 516 areas. The yellow dots represent the ap

study sites within GB 602 and GB 516.

Auger 

Field

80
81 

Figure 4.16 illustrates seafloor characteristics of the well cluster area in GB 602 as imaged 

acoustically using side-scan sonar. Dominant features in this image are the areas of high 

reflectance (dark) that are arranged as linear features radiating from the wellsite zone, a low 

reflectance area around the wells, and a mound-like feature to the west of the well cluster. The 

Seafloor Investigation Map of Appendix C3, compiled from features on the side-scan sonar 

mosaic, shows that within the 3,050-m radius circle around the drill site, there are numerous 

linear scars on the seabed. An example of these scars is illustrated in Figure 4.17. These scars 

are associated with anchoring in preparation for drilling activities. The radial patterns of these 

impressions in the bottom sediments roughly align themselves with the radial high reflectance 

zones on side-scan records that radiate from the well cluster. On the map of Figure 4.18, these 

radial high reflectance patterns are interpreted as well cuttings. However, as explained in a 

previous section of this chapter, clumps of sediment dropped from the anchoring cables also can 

contribute to the high backscatter (reflectance) on side-scan sonar records since the host 

hemipelagic sediments are typically uniformly fine-grained, and except for burrowing, the surface 

is relatively smooth. 

The zone of low reflectivity around the well cluster is interpreted as drilling mud and cuttings 

released at the seafloor during the initial jetting of wells. In this case, the area is small, 

approximately 75 m wide and 213 m in its longest dimension. A small mound of possible drilling 

mud and cuttings also is interpreted to be located approximately 200 m west of the main well 

cluster. This low-relief mound is a little over 30 m in its north-south (longest) dimension and is 

located next to an earlier wellsite (well no. 1, see Chapter 3). 

Other seafloor features of the area include two flowlines and an umbilical trending north from the 

well cluster site (Figure 4.18). 

Within the GB 602 survey area, anchor scars and mud/cuttings were evident from other nearby 

wells in the same block (Figure 4.18). In particular, drilling mud and cuttings were mapped 

around a wellsite approximately 2.3 km north/northeast of the main GB 602 site; three wells were 

drilled there between April and August 1996, about 5 years prior to Cruise 2A (see Chapter 3). 

The low reflectance areas near the wellsite are interpreted to be drilling mud and cuttings from 

initial well jetting, and the bands radiating from the wellsite are interpreted to be rig-discharged 

cuttings, as explained previously. 

A geotechnical borehole was drilled in February 1996 about 1.3 km northeast of the main site, but 

no mud or cuttings deposits are apparent around this wellsite. 
Figure 4.16. A section of side-scan sonar Line 116 showing the well cluster in Garden Banks (GB) Block 602 and radi

reflectance (dark streaks and patches, interpreted as possible cuttings from rig discharges). Note the regiovicinity of the wells (interpreted as possible mud/cuttings from well jetting). Well Nos. 2, 4, and 5 (and sicluster, and Well No. 1 was drilled about 250 m to the west.

Numbers indicate side-scan sonar targets including wellheads (#36, #37, and #40)

Well No. 1

September 1995

Figure 4.16. A section of side-scan sonar Line 116 showing the well cluster in Garden Banks (GB) Block 602 and radi

reflectance (dark streaks and patches, interpreted as possible cuttings from rig discharges). Note the regiovicinity of the wells (interpreted as possible mud/cuttings from well jetting). Well Nos. 2, 4, and 5 (and sicluster, and Well No. 1 was drilled about 250 m to the west.

Numbers indicate side-scan sonar targets including wellheads (#36, #37, and #40)

Well No. 1

September 1995

82
Figure 4.17. A section of side-scan sonar Line 112 illustrating the man-made scars in the hemipelagic surface sedimen

Block 602 study area. These scars were made during anchoring for drilling activities.

83
Figure 4.18. Map of drilling mud, cuttings, and anchor scars around the Garden Banks (GB) 602 site 

based on the Cruise 2A geophysical survey (June-July 2001). Low-reflectance areas are 

interpreted as possible mud/cuttings from seafloor releases during well jetting. 

High-reflectance areas are interpreted as possible cuttings from rig discharges.

Well No. 3 and 

2 sidetracks

Apr-Aug 1996

Geotechnical

borehole

Feb 1996

500-m radius

Well No.1

Sept 1995

Well Nos. 2, 4, 5 

(and sidetracks)

Sep 1995-Jan 2001

Limit of survey area

Legend

= High-reflectance area

= Anchor scar

= Low-reflectance area

= Wellsite

Flowlines and umbilical 

to GB 426

Closeup

(500-m radius)

84
85 

4.6 GARDEN BANKS BLOCK 516 – EXPLORATION/DEVELOPMENT SITE 

4.6.1 Introduction 

The GB 516 site is located on the upper continental slope southwest of the distinct mound-shaped 

“diapiric hill” in the basin containing Shell’s Auger Field (Figure 4.15). The area of interest is 

located on the northern flank of an east-west trending salt-supported ridge that exhibits extremely 

complex seafloor topography and geology. The bathymetric map of the region provided in 

Appendix C3 illustrates the complexity of this area to the south of the wellsite. Within the 

3,050-m radius circle around the well location, water depths vary from 790 m on the ridge top to 

slightly over 1,035 m immediately north of the wellsite. Even though bathymetry of the wellsite 

area describes a rather smooth seafloor, it is clear from the side-scan sonar mosaic and chirp 

sonar subbottom profile data that large areas of the basin fill to the south and east of the wellsite 

have been impacted by faulting, sediment gravity flows, and other mass movement events; 

fluid/gas expulsion; and seafloor lithification. This site is clearly the most geologically diverse 

and complex of all the sites investigated in this research project. 

The complex seafloor area is identified on the reduced side-scan sonar mosaic of the area 

Figure 4.19. The full-scale map is included in Appendix C3. As explained above, this area of 

complicated seafloor is associated with a salt-supported ridge that rises over 244 m above the 

basin floor near the wellsite. Salt movement related to sedimentary loading during periods of 

falling sea level to early rise of sea level during the Pleistocene is largely responsible for the 

faulting that is prevalent in GB 516 south of the wellsite. In addition to offsetting the seafloor, as 

can be observed in data sets from this area, e.g., Figure 4.20, the faults function as conduits for 

vertical migration of fluids (including fluidized sediment and crude oil) and gases to the present 

day seafloor. As discussed in the introductory section of this chapter, the flux rate of 

hydrocarbon-carrying fluids and gases to the seafloor has an important impact on geologic and 

biologic response. The response can be quite variable from depositional features that form from 

expulsion of fluidized sediment to lithification of seafloor sediments. The acoustic data collected 

in the GB 516 area indicate that this region probably has all of these responses. 

4.6.2 Data Quality 

The high resolution acoustic data collected by the AUV in support of the post-development 

survey of the GB 516 well area were of excellent quality. The multibeam bathymetric data 

produced a high quality map that details the dominant features of the seafloor. Combined with 

the high quality side-scan sonar images of the seafloor and the very high resolution chirp sonar 

subbottom profiles, these state-of-the-art data sets provided unmatched insight into the surface 

and near-surface geologic conditions of the study area. Map products are provided in 

Appendix C3. 
Area of Complex 

Seafloor Features

Well Nos. SE001, SE002,

and 5 sidetracks

July-Aug 1999 and 

Nov 2000-Feb 2001

Well No. 1

Sept 1995-

July 1996

Figure 4.19. This reduced version of the side-scan sonar mosaic illustrates the seafloor complexity to the 

southeast of the Garden Banks Block 516 wellsite. The variability of backscatter 

reflectance in this area is produced by a variety of features including faults, submarine 

landslides, fluid/gas expulsion vents, and zones of seafloor lithification. 

500-m radius

Legend

= Wellsite

86
Figure 4.20. This section of chirp sonar subbottom profile (Line 212) crosses a prominent fault that trends east-west alo

salt-supported ridge to the south of the drilling site in Garden Banks Block 516. Note the numerous small expulsion feature (pockmark).

87
88 

4.6.3 Site Characterization 

High backscatter (reflectance) areas to the southeast of the wellsite (Figure 4.19) are probably 

related to zones of seafloor lithification. The numerous fluid/gas expulsion features scattered 

throughout this region of GB 516 (e.g., Figure 4.20, Appendix C3) suggest that hydrocarbon 

gases and probably crude oil are reaching the modern seafloor. Microbial utilization of 

hydrocarbons produces carbonates that bind surficial sediments to create cemented pavements, 

crusts, and mounds. These features usually occur in areas that are impacted by faulting such as 

this region of the block, which borders the salt-supported high ground. They produce a strong 

reflectance, or backscatter, on side-scan sonar records. 

In addition to the faulting and fluid and gas expulsion features, the high slopes coupled with 

periodic salt in the southern part of the area of interest promote sediment instability. As a result, 

mass movement is common, and in this area, there is abundant evidence of slope failure and 

transport of sediment northward into the adjacent basin. Figure 4.21 clearly shows that the 

surface reflects the underlying blocks and irregular depositional surface of debris from a buried 

submarine landslide. The subbottom data of Figure 4.22 illustrate the irregular top surface and 

acoustic transparency of the submarine landslide deposits. Because these deposits are draped 

with both hemipelagic deposits and stratified units beneath, the landslide probably occurred at or 

before the latest Pleistocene glacial maximum (approximately 18 thousand years before present). 

Toward the wellsite, the surface geology becomes more undisturbed, and subsurface stratigraphy 

is composed of uniform parallel reflectors. 

Figures 4.23 and 4.24 illustrate the wellsite area and the interpreted backscatter reflectance 

patterns. As in the other study sites, the dark reflectance patterns radiating from the wellsite are 

interpreted as the products of well cuttings and clumps of sediment/sediment disturbances caused 

by the process of anchoring prior to drilling operations. When these patterns are compared with 

cable scars on the bottom (Seafloor Interpretation Map, Appendix C3), there is a favorable 

alignment. The zone of low backscatter in the immediate vicinity of the drilling site is interpreted 

as an area of drilling mud and cuttings from the initial jetting of wells. The fine-grained nature of 

drilling mud and lack of bioturbation features as compared to the older surrounding seafloor 

account for the lack of reflectance. These areas typically have a high amplitude response (high 

acoustic impedance) on subbottom records because of the high specific gravity minerals used in 

drilling mud. 

In addition to the mud and cuttings around the study site, the mapping showed deposits around a 

previous wellsite about 1.2 km to the southeast (Figure 4.24). This well was drilled between 

September 1995 and July 1996, about 5 years prior to Cruise 2A. 
Figure 4.21. This section of side-scan sonar data from Garden Banks Block 516, Survey Line 227, illustrates the compl

transition area between the salt-supported high ground to the southeast of the wellsite and the basin floor wlocated. This complexity is associated with both faulting and submarine landslide deposits. Note the errat

Figure 4.21. This section of side-scan sonar data from Garden Banks Block 516, Survey Line 227, illustrates the compl

transition area between the salt-supported high ground to the southeast of the wellsite and the basin floor wlocated. This complexity is associated with both faulting and submarine landslide deposits. Note the errat

89
Figure 4.22. This portion of chirp sonar subbottom profile (Garden Banks Block 516, Line 227) shows a hemipelagic se

chaotic and largely acoustically opaque submarine landslide deposit.

90
Figure 4.23. Section of side-scan sonar record from Garden Banks Block 516, Line 216 showing the wellsite area with 

areas interpreted as possible cuttings from rig discharges and low-reflectance (light) areas interpreted as po

well jetting.

Numbers indicate side-sctargets including wellhea(#19 and #22)

Figure 4.23. Section of side-scan sonar record from Garden Banks Block 516, Line 216 showing the wellsite area with 

areas interpreted as possible cuttings from rig discharges and low-reflectance (light) areas interpreted as po

well jetting.

Numbers indicate side-sctargets including wellhea(#19 and #22)

91
3, 40'0

3, 375'

Figure 4.24. Map of drilling mud, cuttings, and anchor scars around the Garden Banks Block 516 site 

based on the Cruise 2A geophysical survey (June-July 2001). Low-reflectance areas are 

interpreted as possible mud/cuttings from seafloor releases during well jetting. 

High-reflectance areas are interpreted as possible cuttings from rig discharges.

Limit of survey area

Well No. 1

Sept 1995-

July 1996

500-m radius

Well Nos. SE001, SE002,

and 5 sidetracks

July-Aug 1999 and 

Nov 2000-Feb 2001

Closeup

(500-m radius)

Legend

= High-reflectance area

= Anchor scar

= Low-reflectance area

= Wellsite

92
93 

4.7 CONCLUSIONS 

Side-scan sonar and chirp sonar subbottom profiler records were useful, though not definitive, in 

mapping deposits of suspected drilling mud and cuttings. In the immediate vicinity of wellsites, 

some areas showed a combination of a smooth seafloor (little backscatter on side-scan sonar 

records) and a high amplitude response at the seafloor on subbottom profiles. These were 

interpreted as zones where muds and cuttings were released at the seafloor during the initial 

jetting of each well (see Chapter 3). Typically, these initial deposits create a shallow, compact 

mound around the wellbore. Generally, these areas were mapped within about 100 m of 

wellsites. Other activities including installation of seafloor equipment (see Chapter 3) may also 

have contributed to the observed low-reflectance pattern around the wellsites. 

More extensive areas where side-scan sonar showed high reflectivity extending in a radial pattern 

around the wellsites were interpreted as possible cuttings discharged from the drilling rig. These 

areas typically extended several hundred meters from wellsites, with the greatest extent (about 

1 km from the wellsite) observed at GB 602 and GB 516. Cuttings are derived from subsurface 

sediments that are much more compacted and therefore denser than surface hemipelagic 

sediments that drape most of the northern Gulf continental slope. When scattered across the 

seafloor by surface discharges, these cuttings create an acoustic impedance difference that 

translates into a higher amplitude reflection on side-scan sonar and subbottom profiler records. 

Although the maps identify these areas as cuttings, other processes could produce the reflectance 

patterns, including seafloor disturbances (grooves, furrows, etc.) caused by the process of setting 

anchors and the dropping of sediment clumps to the seafloor from the anchor cables as they are 

pulled into place. 

The geophysical surveys also detected anchor scars ranging from less than 100 m to over 3 km in 

length, extending to the limit of the surveyed area around the wellsites. VK 916 had only one set 

of anchor scars associated with the exploratory drilling at this site. The other three sites, surveyed 

post-development, had several sets of anchor scars, reflecting the use of different drilling rigs to 

drill various wells over time. 

Persistent cuttings and/or anchor scars from previous activities at nearby wellsites also were 

evident. At VK 916, possible cuttings and anchor scars were present from two previous wells 

drilled in nearby VK 872 in November to December 1998, about 3.5 years prior to Cruise 3A. 

The VK 916 survey also showed anchor scars associated with an older wellsite in VK 873 (drilled 

14 years before Cruise 3A). At the MC 292 site, suspected cuttings deposits and anchor scars 

were evident around a wellsite in the adjacent block MC 291 from drilling between August and 

October 1997. Within the GB 602 survey area, anchor scars and mud/cuttings were evident from 

three wells drilled between April and August 1996, about 5 years prior to Cruise 2A. In the 

GB 516 area, the mapping showed drilling deposits around a previous wellsite drilled between 

September 1995 and July 1996, about 5 years prior to Cruise 2A. Taken together, these 

observations suggest that physically detectable impacts of drilling activities can persist for several 

years in the deepwater environment. 
94 

Editor’s Note: In subsequent chapters, the high-reflectivity areas extending radially around 

wellsites are identified as “possible cuttings” or “rig discharge deposits” and the low-reflectance 

areas in the immediate vicinity of wellsites are identified as “well jetting deposits.” No 

ground-truthing was conducted to verify that these designations are accurate. This issue is 

discussed further in Chapter 15 where other lines of evidence including sediment chemistry can 

be brought to bear. 
95 

Chapter 5 

Sediment Grain Size 

Neal W. Phillips and Robert B. Cady 

Continental Shelf Associates, Inc. 

5.1 INTRODUCTION 

On each cruise, sediment samples were collected using a box core at random stations within 

near-field and far-field sites (see Chapter 2). From each box core, a subsample of the top 2 cm 

was obtained for grain size analysis. Additional box cores were collected from three non-random 

discretionary stations placed in geophysically mapped cuttings/mud zones within each near-field 

site during Cruises 2B and 3B. Subsamples from these box cores were sectioned in 2-cm 

intervals to a depth of 10 cm. Raw grain size data are presented in Appendices D1, D2, and D3. 

5.2 LABORATORY METHODS 

Sediment grain size was analyzed by Dr. Wayne Isphording (University of South Alabama/Tierra 

Consulting). Standard sieve and hydrometer methodologies were used, as specified in American 

Society for Testing and Materials (ASTM) D-422. Statistical measures of central tendency and 

dispersion (mean diameter, sorting, skewness, and kurtosis) were generated. Additional 

information produced by these analyses includes descriptive properties of each sample based 

upon sediment classification procedures (Folk 1974). 

The text refers to phi intervals and percentages of gravel, sand, silt, and clay. These are defined 

as follows: 

• Phi = – log2 (D) where D = particle diameter in mm 

• Gravel = particles with diameters > 2 mm (< phi –1) 

• Sand = particles with diameters between 2 mm and 62.5 μm (phi –1 to 4.0) 

• Silt = particle diameters between 62.5 μm and 3.9 μm (phi 4.0 to 8.0) 

• Clay = particle diameters < 3.9 μm (> phi 8.0) 

5.3 RESULTS 

At the far-field sites away from drilling influences, sediments were predominantly clay (~60% to 

75%) and silt (~20% to 30%) at all four locations (Table 5.1). Mean grain sizes generally were 

in the 1 to 3 μm range. Sediments at the two western sites (Garden Banks Blocks 516 and 602) 

had slightly higher sand percentages (e.g., some values greater than 10%) compared with the two 

sites closer to the Mississippi River (Viosca Knoll Block 916 and Mississippi Canyon Block 292) 

(generally less than 5% sand). Small percentages of gravel were present at only three stations 

(including both near- and far-field samples). 

Table 5.2 provides analysis of variance (ANOVA) results for mean grain size and sand, silt, and 

clay percentages at each site. Individual sites are discussed in the following sections. 
96 

Table 5.1. Summary of baseline grain size characteristics for sites based on far-field data. 

Values are means of all far-field stations, with ranges in parentheses. 

Mean Percentages Site Cruise Mean Grain 

Size (μm) Sand Silt Clay 

VK 916 1B 2.44 

(1.34 – 4.41)

3.49 

(2.09 – 5.40)

29.14 

(23.82 – 43.64)

67.25 

(50.96 – 73.22)

3B 1.75 

(1.19 – 2.34)

3.12 

(1.69 – 4.77)

21.13 

(16.08 – 27.18)

75.75 

(69.03 – 81.45)

GB 516 1B 2.95 

(1.88 – 5.63)

5.28 

(3.03 – 12.04)

31.69 

(19.76 – 60.15)

63.03 

(34.53 – 77.21)

2B 2.40 

(0.66 – 10.00)

6.16 

(4.18 – 10.38)

32.07 

(15.96 – 54.43)

61.78 

(40.09 – 76.93)

GB 602 2B 2.03 

(0.27 – 13.23)

10.28 

(5.22 – 16.50)

21.47 

(11.92 – 61.82)

68.26 

(29.16 – 78.62)

MC 292 2B 1.61 

(0.18 – 7.42)

1.93 

(0.65 – 3.07)

28.37 

(16.30 – 64.13)

69.70 

(34.25 – 81.50)

Table 5.2. Analysis of variance (ANOVA) results for sediment grain size. Statistically 

significant differences (p<0.05) are indicated by bold type and an asterisk. Where 

the overall ANOVA was not significant, probabilities for cruise, site, and interaction 

are not listed. Multiple comparison results (Tukey’s honest significant difference) 

are provided for significant ANOVAs. For testing, mean grain size in microns 

(rather than phi units) was used. Sand, silt, and clay percentages were 

arcsin-transformed. 

ANOVA Result (probability > F) Site Parameter 

Overall Cruise NF vs. FF Interaction Comparisons 

VK 916 Mean 0.0710 -- -- -- -- 

Sand 0.0224* 0.0957 0.3353 0.0127* Cr3 NF > Cr1 NF 

Silt 0.0055* 0.0015* 0.1270 0.4330 Cr1 NF > Cr3 FF 

Cr1 FF > Cr3 FF 

Clay 0.0164* 0.0093* 0.1147 0.2337 Cr3 FF > Cr1 NF 

Cr3 FF > Cr1 FF 

GB 516 Mean 0.7491 -- -- -- -- 

Sand <0.0001* <0.0001* 0.0032* 0.0113* Cr2 NF > all others 

Silt 0.3766 -- -- -- -- 

Clay 0.8643 -- -- -- -- 

GB 602 Mean 0.2151 N/A -- N/A -- 

Sand 0.4477 N/A -- N/A -- 

Silt 0.1102 N/A -- N/A -- 

Clay 0.0813 N/A -- N/A -- 

MC 292 Mean 0.8159 N/A -- -- -- 

Sand 0.0013* N/A 0.0013* N/A NF > FF 

Silt 0.8082 N/A -- N/A -- 

Clay 0.6716 N/A -- N/A -- 

Cr = cruise; NF = near-field; FF = far-field; N/A = not applicable (there was only one cruise at GB 602 and MC 292).
97 

5.3.1 Viosca Knoll Block 916 

Viosca Knoll Block 916 (VK 916) was sampled on Cruise 1B (October/November 2000) and 

Cruise 3B (August 2002). The two cruises served as pre- and post-drilling surveys for this 

exploration site. Basic statistics are summarized in Table 5.3. Ternary plots are shown in 

Figure 5.1. 

Mean grain size ranged from 1.02 to 6.40 μm, with most stations being in the 2 to 3 μm range. 

Based on the grain size statistics, sediments can be characterized as poorly or very poorly sorted 

and the size distribution was coarse-skewed. The Folk’s textural description was primarily “clay” 

or “mud” on Cruise 1B and “clay” on Cruise 3B. On Cruise 1B, sediments averaged about 67% 

clay and 30% silt at both near-field and far-field sites. Cruise 3B sediments on average had more 

clay (73%) and less silt (23%). Small percentages of gravel were present at two stations 

(FF4-B02 on Cruise 1B and NF-B01 on Cruise 3B). 

On Cruise 1B, there was considerable variation in the relative proportions of silt and clay, 

particularly at the near-field site. Clay ranged from about 41% to 84% at near-field stations as 

compared with 51% to 73% at far-field stations. The range was much narrower on Cruise 3B 

(63% to 78% near-field, 69% to 81% far-field). 

On the post-drilling survey, several near-field stations had slightly elevated sand percentages 

compared with the far-field (Figure 5.1). This pattern is more easily seen on Figure 5.2, which 

shows sand, silt, and clay percentages for post-drilling stations grouped into three categories: 

• Far-field stations (>10 km from drillsite) 

• Near-field stations 300 to 500 m from site center 

• Near-field stations within 300 m of site center 

Most stations had less than 5% sand, with the exception of a few near-field, post-drilling stations. 

Among random near-field stations, the highest post-drilling sand percentage was 7.70% at station 

NF-B01. However, one discretionary station (NF-DS3) had 17.14% sand. Near-field sand 

percentages were significantly higher on Cruise 3B than on Cruise 1B (Table 5.2). 

Significant differences were also detected for silt and clay (Table 5.2), but some of these appear 

to be temporal (cruise-to-cruise) variations unrelated to drilling. For example, Cruise 3B far-field 

sites had significantly less silt and more clay than both near-field and far-field sites sampled on 

Cruise 1B. Although Figure 5.2 suggests that post-drilling, near-field stations tended to have 

more silt and less clay than far-field stations, these differences were not significant. 

Differences in sand/silt/clay percentages are part of a larger pattern in which the post-drilling 

grain size distribution in the near-field was shifted toward coarser particles. Figure 5.3 compares 

the post-drilling grain size distribution for average near-field and far-field sediments and for 

near-field station NF-DS3. This station is used as an example because it had high barium 

(73,000 μg/g) and synthetic-based fluid (SBF) (27,319 μg/g) concentrations, indicating the 

presence of drilling fluids and cuttings (see Chapters 8 and 9). The grain size distribution was 

coarse-skewed at both near-field and far-field stations, but more so in the near-field. Elevated 

percentages in the sand and coarse silt phi intervals at Station NF-DS3 are evident on Figure 5.3. 
98 

Table 5.3. Sediment grain size results for Viosca Knoll Block 916. 

Station Gravel 

(%) 

Sand 

(%) 

Silt 

(%) 

Clay 

(%) 

Mean Grain Size

(μm) 

Folk’s Textural 

Description 

CRUISE 1B (Pre-Drilling)

Near-Field 

NF-B01 0.00 4.24 35.64 60.12 2.91 Mud 

NF-B02 0.00 2.47 27.28 70.25 2.23 Clay 

NF-B03 0.00 1.30 28.31 70.40 2.28 Clay 

NF-B04 0.00 2.48 56.40 41.11 6.40 Mud 

NF-B05 0.00 3.82 37.84 58.34 3.67 Mud 

NF-B06 0.00 2.33 19.77 77.90 1.30 Clay 

NF-B07 0.00 4.53 40.40 55.07 3.68 Mud 

NF-B08 0.00 3.80 29.96 66.23 2.46 Clay 

NF-B09 0.00 3.46 27.70 68.84 2.32 Clay 

NF-B10 0.00 2.55 26.23 71.23 1.89 Clay 

NF-B11 0.00 2.51 26.35 71.14 2.30 Clay 

NF-B12 0.00 1.23 14.59 84.18 1.02 Clay 

Far-Field 

FF2-B01 0.00 3.30 26.23 70.47 2.26 Clay 

FF2-B02 0.00 3.16 29.95 66.89 2.27 Clay 

FF3-B01 0.00 4.90 39.25 55.85 3.59 Mud 

FF3-B02 0.00 3.69 25.62 70.69 2.24 Clay 

FF4-B01 0.00 5.40 43.64 50.96 4.41 Mud 

FF4-B02 1.20 3.10 24.93 70.77 1.34 Slightly gravelly mud 

FF5-B01 0.00 2.96 23.82 73.22 1.86 Clay 

FF5-B02 0.00 3.10 25.77 71.13 2.20 Clay 

FF6-B01 0.00 3.22 24.91 71.87 2.10 Clay 

FF6-B02 0.00 2.09 27.24 70.67 2.15 Clay 

CRUISE 3B (Post-Exploration)

Near-Field 

NF-B01 0.74 7.70 28.91 62.65 2.69 Slightly gravelly mud 

NF-B02 0.00 3.41 22.39 74.21 1.86 Clay 

NF-B03 0.00 5.48 26.76 67.77 2.36 Clay 

NF-B04 0.00 2.73 24.59 72.68 2.20 Clay 

NF-B05 0.00 6.83 26.43 66.74 2.27 Clay 

NF-B06 0.00 6.54 23.23 70.23 2.09 Clay 

NF-B07 0.00 7.61 29.50 62.89 2.86 Clay 

NF-B08 0.00 3.18 28.53 68.28 2.32 Clay 

NF-B09 0.00 2.25 24.04 73.71 2.01 Clay 

NF-B10 0.00 2.16 19.94 77.91 1.35 Clay 

NF-B11 0.00 5.01 24.28 70.72 2.25 Clay 

NF-B12 0.00 3.05 25.92 71.03 2.24 Clay 

Far-Field 

FF1-B01 0.00 3.96 26.05 69.99 2.20 Clay 

FF1-B02 0.00 3.40 25.02 71.58 2.32 Clay 

FF2-B01 0.00 2.54 17.68 79.78 1.45 Clay 

FF2-B02 0.00 2.27 16.66 81.07 1.37 Clay 

FF3-B01 0.00 3.02 17.70 79.28 1.46 Clay 

FF3-B02 0.00 3.34 23.78 72.88 1.91 Clay 

FF4-B01 0.00 2.47 16.08 81.45 1.19 Clay 

FF4-B02 0.00 2.87 27.18 69.95 2.12 Clay 

FF5-B01 0.00 1.69 17.16 81.15 1.43 Clay 

FF5-B02 0.00 2.98 17.56 79.46 1.32 Clay 

FF6-B01 0.00 4.18 22.45 73.38 1.89 Clay 

FF6-B02 0.00 4.77 26.20 69.03 2.34 Clay 
Figure 5.1. Ternary plots summarizing grain size data for Viosca Knoll Block 916.

0 10 20 30 40 50 60 70 80 90 100 % Clay

% Sand

0

10

20

30

40

50

60

70

80

90

100

% Silt

0

10

20

30

40

50

60

70

80

90

100

NF

FF

VK 916 Cruise 1B (pre-drilling)

0 10 20 30 40 50 60 70 

% Sand

6

70

80

90

100

% Silt

0

10

20

30

40

50

60

70

80

90

100

NFFF

VK 916 Cruise 3B (post-explora

99
Figure 5.2. Percentages of sand, silt, and clay in relation to proximity to drilling at Viosca Knoll 

Block 916. Points are individual stations including non-random "discretionary" stations.

Only data from the post-drilling survey (Cruise 3B) are included. Note the elevated sand 

percentages at some near-field stations.

Sand

0

5

10

15

20

Percentage

Silt

0

10

20

30

40

Percentage

Clay

0

20

40

60

80

100

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

100
Figure 5.3. Grain size distribution at Viosca Knoll Block 916 on the post-drilling survey (Cruise 3B).

(a) Comparison of near-field and far-field averages. (b) Comparison of near-field station 

NF-DS3 and far-field average. Note the shift toward coarser sediments in the near-field.

(a) Near-Field vs. Far-Field Mean

(b) Near-Field Station NF-DS3 vs. Far-Field Mean

0

5

10

15

20

Phi Intervals

Percentage

-1 4 8 12

Sand Silt Clay

Gravel

NF-DS3

Far-Field Mean

0

5

10

15

20

Phi Intervals

Percentage

Near-Field Mean

Far-Field Mean

Sand Silt Clay

Gravel

-1 4 8 12

101
102 

Within the near-field site, areas geophysically mapped as cuttings (see Chapter 4) tended to have 

relatively high percentages of sand (Figure 5.4). In the near-field, sand percentage was 

positively correlated with sediment barium (r = 0.76, n = 12, p<0.01) and SBF (r = 0.68, n = 12, 

p<0.02) (calculated using random stations only) (Figure 5.5) suggesting that the presence of 

drilling discharges increased the percentage of sand. However, some near-field stations with 

elevated barium and/or SBF had sand percentages similar to those seen in the far-field. 

Subsamples from the three discretionary stations on Cruise 3B were sectioned in 2-cm intervals to 

a depth of 10 cm. Generally, sediments in the top two intervals (0 to 4 cm) had more sand and 

silt and less clay than deeper intervals (Figure 5.6). Station NF-DS3 was most strongly affected, 

with the sand percentage declining from 17.14% at the surface to less than 2% at depths greater 

than 4 cm. Only a slight trend (if any) was present at Station NF-DS1. The apparent depth of the 

mud/cuttings layer at this site (about 4 cm) is consistent with estimates presented in Chapter 8

based on barium and total organic carbon data. 

5.3.2 Garden Banks Block 516 

Garden Banks Block 516 (GB 516) was sampled on Cruise 1B (October/November 2000) and 

Cruise 2B (July 2001). The first cruise was post-exploration, as two previous wells had been 

drilled at this site in 1999 (see Chapter 3). The second cruise was post-development, following 

the drilling of five new wells. Basic grain size statistics are summarized in Table 5.4. Ternary 

plots are shown in Figure 5.7. 

Mean grain size ranged from 0.55 to 10.0 μm, with most stations being in the 2 to 4 μm range. 

Sediments can be characterized as poorly or very poorly sorted and the size distribution was 

coarse-skewed. The Folk’s textural description was primarily “clay” or “mud” on both cruises, 

but several near-field stations on Cruise 2B were classified as “sandy clay” or “sandy mud.” On 

Cruise 1B, sediments averaged about 64% clay and 31% silt at both near-field and far-field sites. 

Cruise 2B sediments on average had more sand (8%) and slightly less silt (29%) and clay (62%). 

Gravel-sized particles (>2 mm) were not present in any samples. 

On both cruises, the range of silt and clay concentrations was greater in the far-field than in the 

near-field. One station on Cruise 1B (FF4-B01) and two on Cruise 2B (FF1-B02 and FF2-B02) 

had noticeably more silt and less clay than the other far-field stations (Figure 5.7). 

On Cruise 1B, sand percentages were similar at near-field and far-field stations. However, on 

Cruise 2B, several near-field stations had elevated sand percentages (Figure 5.7). This trend is 

more easily seen on Figure 5.8, which shows sand, silt, and clay percentages for post-drilling 

stations grouped into the same three categories mentioned previously (far-field, near-field 300 to 

500 m, and near-field within 300 m). ANOVA indicated that near-field sand percentages on 

Cruise 2B were higher than far-field percentages, and also higher than both near- and far-field 

percentages from Cruise 1B (Table 5.2). 
Figure 5.4. Percentages of sand and sediments >0.5 mm (coarse sand, very coarse sand, and gravel) on 

Cruise 3B at Viosca Knoll Block 916 in relation to geophysically mapped cuttings. 

Replicate values for far-field stations are shown in a separate box.

500 m radius

1

2

3

4

5

6

7

8

9

11

12

10

DS3

DS2

DS1

1

2

3

4

5

6

7

8

9

11

12

10

Possible 

Cuttings

DS3

DS2

DS1

Sand

Sediments >0.5 mm

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

500 m radius

Legend

Wellsite

0.11 - 1.0

0.01 - 0.1

0

Percentage > 0.5 mm

>1.0

Legend

Wellsite

7.50 - 10

5.00 - 7.49

Sand Percentage

>10

0 - 4.99

Possible 

Cuttings

103
Figure 5.5. Relationships between sand percentages and concentrations of drilling indicators (barium and 

synthetic-based fluid [SBF] concentrations) at Viosca Knoll Block 916 on the post-drilling

survey (Cruise 3B). Within the near-field, sand percentage was positively correlated with 

barium and SBF concentrations.

Sand Percentage vs. Barium

0

5

10

15

20

0 50000 100000 150000 200000

Barium Concentration (μg/g)

Sand Percentage

Near-Field

Far-Field

Sand Percentage vs. SBF

0

5

10

15

20

0 10000 20000 30000 40000 50000 60000

SBF Concentration (μg/g)

Sand Percentage

Near-Field

Far-Field

104
Figure 5.6. Depth profiles from discretionary stations (DS) at Viosca Knoll Block 916 on the 

post-drilling survey (Cruise 3B).

0 5 10 15 20 25 30

Silt Percentage

Depth (cm)

DS1

DS2

DS3

0 20 40 60 80 100

Clay Percentage

Depth (cm)

DS1

DS2

DS3

8-10

6-8

4-6

2-4

0-2

0 5 10 15 20

Sand Percentage

Depth (cm)

DS1

DS2

DS3

8-10

6-8

4-6

2-4

0-2

8-10

6-8

4-6

2-4

0-2

105
106 

Table 5.4. Sediment grain size results for Garden Banks Block 516. 

Station Gravel 

(%) 

Sand 

(%) 

Silt 

(%) 

Clay 

(%) 

Mean Grain Size

(μm) 

Folk’s Textural 

Description 

CRUISE 1B (post-exploration)

Near-Field 

NF-B01 0.00 4.50 33.31 62.19 3.18 Mud 

NF-B02 0.00 6.80 37.61 55.59 3.84 Mud 

NF-B03 0.00 4.68 22.30 73.02 1.80 Clay 

NF-B04 0.00 5.11 36.17 58.71 3.52 Mud 

NF-B05 0.00 4.70 32.36 62.94 2.76 Mud 

NF-B06 0.00 5.09 30.58 64.33 3.11 Clay 

NF-B07 0.00 4.85 21.72 73.43 1.96 Clay 

NF-B08 0.00 5.37 29.10 65.52 2.43 Clay 

NF-B09 0.00 6.53 23.12 70.35 2.20 Clay 

NF-B10 0.00 4.42 33.89 61.69 2.77 Mud 

NF-B11 0.00 5.37 32.42 62.21 2.81 Mud 

NF-B12 0.00 7.25 24.77 67.99 2.35 Clay 

Far-Field 

FF1-B01 0.00 5.84 34.58 59.58 3.43 Mud 

FF1-B02 0.00 4.35 33.81 61.84 3.05 Mud 

FF2-B01 0.00 3.03 19.76 77.21 1.88 Clay 

FF2-B02 0.00 12.04 27.48 60.48 3.40 Sandy clay 

FF3-B01 0.00 6.09 32.59 61.32 2.99 Mud 

FF3-B02 0.00 4.74 27.09 68.17 2.52 Clay 

FF4-B01 0.00 5.33 60.15 34.53 5.63 Mud 

FF4-B02 0.00 6.61 27.31 66.08 2.70 Clay 

FF5-B01 0.00 3.32 27.99 68.68 2.32 Clay 

FF5-B02 0.00 4.56 30.26 65.18 2.98 Clay 

FF6-B01 0.00 4.06 30.77 65.17 2.30 Clay 

FF6-B02 0.00 3.37 28.54 68.10 2.22 Clay 

CRUISE 2B (post-development)

Near-Field 

NF-B01 0.00 5.18 23.71 71.12 2.60 Clay 

NF-B02 0.00 8.95 31.25 59.80 1.23 Mud 

NF-B03 0.00 11.25 37.16 51.58 4.95 Sandy mud 

NF-B04 0.00 9.21 31.14 59.65 1.69 Mud 

NF-B05 0.00 15.16 18.76 66.08 1.89 Sandy clay 

NF-B06 0.00 18.23 36.18 45.59 6.80 Sandy mud 

NF-B07 0.00 9.80 25.22 64.97 0.94 Clay 

NF-B08 0.00 5.83 32.08 62.09 2.41 Mud 

NF-B09 0.00 8.12 22.36 69.51 2.26 Clay 

NF-B10 0.00 12.86 23.46 63.67 2.90 Sandy clay 

NF-B11 0.00 7.98 14.77 77.25 0.55 Clay 

NF-B12 0.00 9.74 25.58 64.68 0.75 Clay 

Far-Field 

FF1-B01 0.00 6.60 31.58 61.82 1.60 Mud 

FF1-B02 0.00 6.46 46.42 47.12 4.54 Mud 

FF2-B01 0.00 5.29 29.69 65.02 2.93 Clay 

FF2-B02 0.00 5.48 54.43 40.09 7.46 Mud 

FF3-B01 0.00 5.03 32.85 62.13 1.01 Mud 

FF3-B02 0.00 10.38 25.53 64.10 2.65 Sandy clay 

FF4-B01 0.00 6.93 30.13 62.94 1.15 Clay 

FF4-B02 0.00 7.11 15.96 76.93 2.01 Clay 

FF5-B01 0.00 4.18 33.63 62.19 0.66 Mud 

FF5-B02 0.00 5.39 30.76 63.85 1.28 Clay 

FF6-B01 0.00 5.25 27.95 66.80 10.00 Clay 

FF6-B02 0.00 5.76 25.91 68.33 8.62 Clay 
Figure 5.7. Ternary plots summarizing grain size data for Garden Banks Block 516.

0 10 20 30 40 50 60 70 80 90 100 % Clay

% Sand

0

10

20

30

40

50

60

70

80

90

100

% Silt

0

10

20

30

40

50

60

70

80

90

100

NFFF

0 10 20 30 40 50 60 70 

% Sand

6

70

80

90

100

% Silt

0

10

20

30

40

50

60

70

80

90

100

NF

FF

GB 516 Cruise 1B (post-exploration) GB 516 Cruise 2B (post-develo

107
Figure 5.8. Percentages of sand, silt, and clay in relation to proximity to drilling at Garden Banks

Block 516. Points are individual stations including non-random "discretionary" stations. 

Note the elevated sand percentages at some near-field stations and elevated silt at some far-field 

stations.

Sand

0

5

10

15

20

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

Silt

0

20

40

60

80

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

Clay

0

20

40

60

80

100

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

Cruise 2B

Cruise 1B

Legend

108
109 

The grain size distribution at GB 516 was bimodal at both near-field and far-field stations, with 

peaks around phi 4.5 to 5.0 (coarse silt) and phi 10 (clay) (Figure 5.9). On average, near-field 

stations had higher percentages in the coarser phi classes (sand) and lower percentages in the 

coarse silt range when compared with the far-field stations. Figure 5.9(b) shows that one 

discretionary station (NF-DS2) had elevated percentages in several clay size categories, possibly 

due to drilling fluid particles. This station had very high concentrations of barium (198,000 μg/g) 

and SBF (117,280 μg/g), indicating the presence of drilling fluids and cuttings (see Chapters 8 

and 9). 

On Cruise 2B, most near-field stations were within geophysically mapped cuttings zones (see 

Chapter 4) and had higher sand percentages than most far-field stations (Figure 5.10). However, 

one near-field station (NF-B10) outside the cuttings zone had >10% sand. On Cruise 1B, there 

was little or no difference between sand percentages at near-field vs. far-field stations (there was 

no geophysical map for this cruise, so the location of stations in relation to cuttings zones is 

unknown). There was no consistent relationship between sand percentages and sediment barium 

and SBF concentrations on either cruise (Figure 5.11). 

Subsamples from the three discretionary stations on Cruise 2B were sectioned in 2-cm intervals to 

a depth of 10 cm. While all of the stations had higher sand percentages at the surface than at 

10 cm depth, there was also a subsurface maximum for Stations DS1 and DS2 (Figure 5.12). 

Based on sand percentages, the thickness of mud and cuttings at this site may extend to a depth of 

6 to 8 cm. Station DS2 was quite different from the other two stations, in particular having more 

silt and less clay at all depths below 2 cm. It is not known whether this represents an impact or 

natural variation. 

5.3.3 Garden Banks Block 602 

Garden Banks Block 602 (GB 602) was sampled on Cruise 2B (July 2001). This was a 

post-development survey; seven wells had been drilled within the near-field site between 

September 1995 and January 2001 (see Chapter 3). Basic grain size statistics are summarized in 

Table 5.5. Ternary plots are shown in Figure 5.13. 

Mean grain size ranged from 0.27 to 13.23 μm. Mean grain size was generally <2 μm at far-field 

stations and in the 2 to 4 μm range at near-field stations. Sediments can be characterized as very 

poorly or extremely poorly sorted and the size distribution was strongly coarse-skewed. The 

Folk’s textural description was primarily “sandy mud” or “sandy clay” in the near-field and 

“clay” or “sandy clay” in the far-field. Gravel-sized particles were not present in any samples. 

At both near-field and far-field sites, there was one station with much more silt and less clay than 

the other stations (Figure 5.13). Aside from these samples, the near-field stations generally had a 

greater range of silt and clay content than the far-field stations. The same pattern is evident on 

Figure 5.14, which shows sand, silt, and clay percentages for stations grouped into the same three 

categories mentioned previously (far-field, near-field 300 to 500 m, and near-field within 300 m). 

Generally, there is a pattern of increasing silt and decreasing clay near drilling. However, the 

ANOVAs for silt and clay are not significant (Table 5.2). 
Figure 5.9. Grain size distribution at Garden Banks Block 516 on Cruise 2B. (a) Near-field mean vs. 

far-field mean; (b) discretionary station NF-DS2 vs. far-field mean.

(a) Near-Field Mean vs. Far-Field Mean

Near-Field Mean

Far-Field Mean

Sand Silt Clay

Gravel

0

5

10

-1 4 8 12

Phi Intervals

Percentage

0

5

10

15

-1 4 8 12

Phi Intervals

Percentage

(b) Discretionary Station NF-DS2 

vs. Far-Field Mean

Station NF-DS2

Far-Field Mean

Sand Silt Clay

110
Sand

Cruise 1B

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

2

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

500 m radius

6

10 1

9

5

4

8

12

7

11

3

(There was no geophysical 

survey prior to this cruise, 

hence no mud/cuttings map.)

2 500 m radius

6

10 1

9

5

4

8

12

7

11

3

DS1

DS2

DS3

Possible

Cuttings

Legend

Previous wellsite

7.50 - 9.99

5.00 - 7.49

Sand Percentage

>10

0 - 4.99

Legend

Previous wellsite

7.50 - 9.99

5.00 - 7.49

Sand Percentage

>10

0 - 4.99

Sand

Cruise 2B

Figure 5.10. Percentages of sand on Cruise 1B and Cruise 2B at Garden Banks Block 516 in relation to

geophysically mapped cuttings. There was no geophysical map for Cruise 1B. Replicate 

values for far-field stations are shown in a separate box.

New wellsite

111
Figure 5.11. Relationships between sand percentages and concentrations of drilling indicators (barium and synthetic-ba

Garden Banks Block 516 on Cruises 1B and 2B. There were no significant correlations between sand perc

concentrations.

GB 516 - Cruise 2B

05

101520

0 100000 200000 300000 400000

Barium Concentration (μg/g)

Sand Percentage Near-Field Far-Field

GB 516 - Cruise 2B

05

101520

0 50000 100000 

SBF Concentration (μg/g)

Sand Percentage 

GB 516 - Cruise 1B

Barium Concentration (μg/g)

Sand Percentage Near-Field Far-Field

GB 516 - Cruise 1B

SBF Concentration (μg/g)

Sand Percentage 

05

101520

0 100000 200000 300000 400000

05

101520

0 50000 100000 

112
Figure 5.12. Depth profiles from discretionary stations (DS) at Garden Banks Block 516 on Cruise 2B

(post-development).

0 20 40 60 80 100

Depth (cm)

DS1

DS2

DS3

Silt Percentage

8-10

6-8

4-6

2-4

0-2

0 20 40 60 80 100

Clay Percentage

Depth (cm)

DS1

DS2

DS3

8-10

6-8

4-6

2-4

0-2

Note: Separate silt and clay values were not 

obtained for the 4-6 cm interval of DS2.

Sand Percentage

0 5 10 15

Depth (cm)

DS1

DS2

DS3

8-10

6-8

4-6

2-4

0-2

113
114 

Table 5.5. Sediment grain size results for Garden Banks Block 602. 

Station Gravel 

(%) 

Sand 

(%) 

Silt 

(%) 

Clay 

(%) 

Mean Grain 

Size (μm) 

Folk’s Textural 

Description 

Near-Field 

NF-B01 0.00 6.50 59.38 34.12 4.36 Mud 

NF-B02 0.00 10.59 15.25 74.17 2.62 Sandy clay 

NF-B03 0.00 21.12 31.70 47.18 7.10 Sandy mud 

NF-B04 0.00 7.11 20.37 72.52 2.66 Clay 

NF-B05 0.00 10.67 29.49 59.85 2.39 Sandy clay 

NF-B06 0.00 12.83 30.03 57.14 3.25 Sandy mud 

NF-B07 0.00 12.93 35.19 51.87 3.78 Sandy mud 

NF-B08 0.00 11.22 22.03 66.76 3.41 Sandy clay 

NF-B09 0.00 6.64 32.41 60.95 3.25 Mud 

NF-B10 0.00 10.02 36.41 53.57 3.86 Sandy mud 

NF-B11 0.00 14.55 21.23 64.22 2.31 Sandy clay 

NF-B12 0.00 13.48 17.55 68.98 2.16 Sandy clay 

Far-Field 

FF1-B01 0.00 7.70 20.87 71.43 1.95 Clay 

FF1-B02 0.00 10.42 20.45 69.13 0.51 Sandy clay 

FF2-B01 0.00 13.48 15.70 70.82 0.97 Sandy clay 

FF2-B02 0.00 7.40 16.92 75.68 1.10 Clay 

FF3-B01 0.00 5.22 25.29 69.50 0.67 Clay 

FF3-B02 0.00 13.84 23.44 62.72 0.56 Sandy clay 

FF4-B01 0.00 10.14 16.52 73.34 0.88 Sandy clay 

FF4-B02 0.00 11.84 11.92 76.25 1.16 Sandy clay 

FF5-B01 0.00 7.36 14.02 78.62 0.27 Clay 

FF5-B02 0.00 9.02 61.82 29.16 13.23 Silt 

FF6-B01 0.00 10.44 16.31 73.26 1.67 Sandy clay 

FF6-B02 0.00 16.50 14.33 69.17 1.35 Sandy clay 
Figure 5.13. Ternary plot summarizing grain size data for Garden Banks Block 602.

0 10 20 30 40 50 60 70 80 90 100 % Clay

% Sand

0

10

20

30

40

50

60

70

80

90

100

% Silt

0

10

20

30

40

50

60

70

80

90

100

GB 602 Cruise 2B (post-development)

NF

FF

115
Figure 5.14. Percentages of sand, silt, and clay in relation to proximity to drilling at Garden Banks

Block 602. Points are individual stations including non-random "discretionary" stations.

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

Sand

0

5

10

15

20

25

Percentage

Silt

0

20

40

60

80

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

Clay

0

20

40

60

80

100

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

116
117 

Sand percentages were similar at near-field and far-field stations with the exception of one 

near-field station with the highest value (NF-B03, 21.12%). The ANOVA for sand was not 

significant (Table 5.2). 

The grain size distribution at GB 602 was bimodal at both near-field and far-field stations, with 

peaks around phi 3.5 to 4.0 (very fine sand) and phi 9.0 (clay) (Figure 5.15a). On average, 

near-field stations had higher percentages in the coarser phi classes (phi <3.0) and lower 

percentages in the 3.5 and 4.0 phi intervals when compared with the far-field stations. 

Figure 5.15b shows the grain size distribution at Station NF-B03, which had high concentrations 

of drilling fluid indicators (barium: 111,000 μg/g and SBF: 8,446 μg/g) (see Chapters 8 and 9). 

The increase in the medium to coarse sand fraction is particularly evident at this station. 

Most of the near-field stations at GB 602 were within zones geophysically mapped as cuttings 

(see Chapter 4). There was little or no difference in sand percentage among near-field stations 

within vs. outside the cuttings zones (Figure 5.16). Generally, near-field and far-field stations 

had similar sand percentages. There was no consistent relationship between sand, silt, and clay 

percentages vs. sediment barium concentrations (Figure 5.17). 

Subsamples from the three discretionary stations were sectioned in 2-cm intervals to a depth of 

10 cm. Sand percentages show a subsurface maximum for all three stations in the 4 to 6 cm range 

(Figure 5.18). This may reflect layering from previous drilling discharges. Silt percentages for 

DS1, which had the highest barium and SBF concentration (see Chapters 8 and 9), declined from 

the surface to a depth of 6 to 8 cm. The thickness of mud and cuttings layers at this site may 

extend to a depth of 6 to 8 cm. 

5.3.4 Mississippi Canyon Block 292 

Mississippi Canyon Block 292 (MC 292) was sampled on Cruise 2B (July 2001). This was a 

post-development survey; five wells were drilled near the center of the site between May 1995 

and July 1999 (see Chapter 3). Basic grain size statistics are summarized in Table 5.6. A ternary 

plot is shown in Figure 5.19. 

Mean grain size ranged from 0.18 to 7.42 μm. Mean grain size was generally <3 μm at both 

far-field and near-field stations. Generally, sediments can be characterized as very poorly or 

extremely poorly sorted and the size distribution was strongly coarse-skewed. The Folk’s textural 

description was primarily “clay” or “mud” at both far-field and near-field stations. Gravel-sized 

particles (>2 mm) were not present in any samples. 

At the far-field site, there was one station (FF5-B02) with much more silt and less clay than the 

other stations (Figure 5.19). The silt and clay content of near-field and far-field stations did not 

differ significantly according to the ANOVA (Table 5.2). 
Figure 5.15. Grain size distribution at Garden Banks Block 602 on Cruise 2B. (a) Near-field mean vs. 

far-field mean; (b) near-field Station NF-B03 vs. far-field mean.

(b) Near-Field Station NF-B03 vs. Far-Field Mean

0

5

10

15

Phi Intervals

Percentage

(a) Near-Field Mean vs. Far-Field Mean

Near-Field Mean

Far-Field Mean

-1 4 8 12

Sand Silt Clay

Gravel

Phi Intervals

Percentage

Near-Field Mean

Far-Field Mean

Sand Silt Clay

Gravel

0

5

10

15

-1 4 8 12

118
FF1FF2FF3FF4FF5FF6

Far-Field

Legend

Previous wellsite

7.50 - 9.99

5.00 - 7.49

Sand Percentage

>10

0 - 4.99

Figure 5.16. Sand percentages on Cruise 2B at Garden Banks Block 602 in relation to geophysically mapped cuttings. R

far-field stations are shown in a separate box.

500 m radius

Possible

Cuttings

1

5

2 9

6

10

11

12

3

7

4

DS2 8

DS1

DS3

119
Figure 5.17. Relationships between sand, silt, and clay percentages and concentrations of barium

(drilling fluid indicator) at Garden Banks Block 602 on Cruise 2B. There were no 

significant correlations between barium concentrations and sand, silt, or clay percentages.

Sand Percentage vs. Barium

0

5

10

15

20

25

0 50000 100000 150000

Barium Concentration (μg/g)

Sand Percentage

Near-Field

Far-Field

Silt Percentage vs. Barium

0

20

40

60

80

0 50000 100000 150000

Barium Concentration (μg/g)

Silt Percentage

Near-Field

Far-Field

Clay Percentage vs. Barium

0

20

40

60

80

100

0 50000 100000 150000

Barium Concentration (μg/g)

Clay Percentage

Near-Field

Far-Field

120
Figure 5.18. Depth profiles from discretionary stations (DS) at Garden Banks Block 602 on Cruise 2B

(post-development).

Sand Percentage

0 5 10 15 20 25

Depth (cm)

DS1

DS2

DS3

8-10

6-8

4-6

2-4

0-2

0 10 20 30 40 50 60

Depth (cm)

DS1

DS2

DS3

Silt Percentage

8-10

6-8

4-6

2-4

0-2

0 20 40 60 80 100

Depth (cm)

DS1

DS2

DS3

Clay Percentage

8-10

6-8

4-6

2-4

0-2

121
122 

Table 5.6. Sediment grain size results for Mississippi Canyon Block 292. 

Station Gravel 

(%) 

Sand 

(%) 

Silt 

(%) 

Clay 

(%) 

Mean Grain 

Size (μm) 

Folk’s Textural 

Description 

Near-Field 

NF-B01 0.00 3.34 19.04 77.62 1.26 Clay 

NF-B02 0.00 2.78 26.28 70.94 2.08 Clay 

NF-B03 0.00 7.14 22.20 70.66 0.77 Clay 

NF-B04 0.00 4.04 30.55 65.41 0.69 Clay 

NF-B05 0.00 5.79 40.98 53.23 1.95 Mud 

NF-B06 0.00 3.04 18.27 78.69 0.99 Clay 

NF-B07 0.00 3.14 43.53 53.33 2.95 Mud 

NF-B08 0.00 8.51 35.30 56.19 0.87 Mud 

NF-B09 0.00 1.46 19.39 79.15 1.41 Clay 

NF-B10 0.00 13.83 23.02 63.14 2.07 Sandy clay 

NF-B11 0.00 3.72 24.96 71.32 0.82 Clay 

NF-B12 0.00 3.59 21.75 74.66 1.74 Clay 

Far-Field 

FF1-B01 0.00 1.79 17.70 80.51 1.17 Clay 

FF1-B02 0.00 1.89 23.29 74.82 1.75 Clay 

FF2-B01 0.00 3.07 23.62 73.31 1.93 Clay 

FF2-B02 0.00 2.50 25.16 72.34 2.20 Clay 

FF3-B01 0.00 1.15 19.80 79.06 0.31 Clay 

FF3-B02 0.00 0.65 29.99 69.36 0.41 Clay 

FF4-B01 0.00 2.78 38.53 58.70 1.02 Mud 

FF4-B02 0.00 1.75 27.86 70.40 0.59 Clay 

FF5-B01 0.00 2.19 16.30 81.50 1.06 Clay 

FF5-B02 0.00 1.62 64.13 34.25 7.42 Mud 

FF6-B01 0.00 2.65 19.82 77.53 1.24 Clay 

FF6-B02 0.00 1.14 34.24 64.62 0.18 Mud 
Figure 5.19. Ternary plot summarizing grain size data for Mississippi Canyon 292.

0 10 20 30 40 50 60 70 80 90 100 % Clay

% Sand

0

10

20

30

40

50

60

70

80

90

100

% Silt

0

10

20

30

40

50

60

70

80

90

100

MC 292 Cruise 2B (post-development)

NF

FF

123
124 

Figure 5.20 shows sand, silt, and clay percentages for stations grouped into the same three 

categories mentioned previously (far-field, near-field 300 to 500 m, and near-field within 300 m). 

Also shown is the percentage of particles > 0.5 mm in diameter (coarse and very coarse sand). 

While silt and clay percentages in the near-field were within the far-field range, sand and 

coarse/very coarse sand percentages were elevated at some near-field stations. For example, sand 

percentages at far-field stations were all less than 3.1%, but those in the near-field ranged as high 

as 13.83% (NF-B10). The ANOVA indicates that sand percentages were significantly higher in 

the near-field (Table 5.2). 

However, the pattern of elevated sand percentages did not correspond with geophysically mapped 

cuttings zones (Figure 5.21). Also, there was no consistent relationship between sand, silt, and 

clay percentages vs. sediment barium concentrations (Figure 5.22) or SBF concentrations (not 

shown). 

The grain size distribution at MC 292 was bimodal at both near-field and far-field stations, with 

peaks around phi 4.5 to 5.0 (coarse silt) and phi 9.5 to 11 (clay) (Figure 5.23a). On average, 

near-field stations had higher percentages in the coarser phi classes (sand) and slightly lower 

percentages in several silt and clay phi intervals when compared with the far-field stations. 

Figure 5.23b shows the grain size distribution at Station NF-B01, which had the highest 

concentrations of drilling fluid indicators (barium: 39,700 μg/g and SBF: 2,408 μg/g) (see 

Chapters 8 and 9). The elevated clay percentages may reflect the presence of drilling fluid 

particles. 

Subsamples from the three discretionary stations were sectioned in 2-cm intervals to a depth of 

10 cm. At all three stations, sediments were predominantly clay at all depths, with most of the 

variation in the top 4 cm (Figure 5.24). At Station DS1, the sand percentage was elevated in the 

top 6 cm. Station DS3 had slightly elevated sand and silt in the top 2 cm. Station DS2 had a 

subsurface peak in sand and silt percentages at 2 to 4 cm depth and relatively high sand 

percentages all the way to 10 cm depth. 

5.4 DISCUSSION 

Drilling discharges have been shown to affect sediment grain size and mineralogy around 

wellsites in shallow water (National Research Council 1983; Neff 1987; Continental Shelf 

Associates, Inc. 1989; Hinwood et al. 1994). The solid components of drilling fluids (muds) 

consist of clay-size particles (e.g., bentonite, barite). Cuttings are particles of crushed rock 

produced by the grinding action of the drill bit as it penetrates into subsurface formations (Neff 

1987). They can range in size from clay-sized particles to coarse gravel and have an angular 

configuration. While very coarse (gravel-sized) particles were noted near some drillsites in 

another deepwater monitoring program (Continental Shelf Associates, Inc. 2004), in the present 

study, apparently the cuttings were mostly within the size range of particles present in surficial 

sediments. Alternatively, the coarsest deposits may not have been sampled because our stations 

were not close enough to the wellsites. 
Figure 5.20. Percentages of coarse and very coarse sand (>0.5 mm), sand, silt, and clay in relation to proximity to drillin

Canyon Block 292. Points are individual stations including non-random "discretionary" stations. Note the 

sand and coarse/very coarse sand at some near-field stations.

Sand

0

5

10

15

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

Far-Field Near

Silt

0

20406080

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

Clay

Percentage

Coarse/very coarse sand (>0.5 mm)

Percentage

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

Far-Field Nea

0

20406080

100

0246

125
FF1

FF2FF3FF4FF5FF6

Far-Field

Legend

Previous wellsite

7.50 - 9.99

5.00 - 7.49

Sand Percentage

>10

0 - 4.99

Figure 5.21. Sand percentages on Cruise 2B at Mississippi Canyon Block 292 in relation to geophysically mapped cuttin

Possible Cuttings

500 m radius 10

6 2

5

1

8

9

12

DS1

DS2

DS3

4

3

7

11

126
Figure 5.22. Relationships between sand, silt, and clay percentages and concentrations of barium

(drilling fluid indicator) at Mississippi Canyon Block 292 on Cruise 2B.

Sand Percentage vs. Barium

0

5

10

15

0 10000 20000 30000 40000 50000

Barium Concentration (μg/g)

Sand Percentage

Near-Field

Far-Field

Silt Percentage vs. Barium

0

20

40

60

80

0 10000 20000 30000 40000 50000

Barium Concentration (μg/g)

Silt Percentage

Near-Field

Far-Field

Clay Percentage vs. Barium

0

20

40

60

80

100

0 10000 20000 30000 40000 50000

Barium Concentration (μg/g)

Clay Percentage

Near-Field

Far-Field

127
Silt

Figure 5.23. Grain size distribution at Mississippi Canyon Block 292 on Cruise 2B. (a) Near-field mean 

vs. far-field mean; (b) Station NF-B01 vs. far-field mean.

(b) Station NF-B01 vs. Far-Field Mean

(a) Near-Field Mean vs. Far-Field Mean

5

10

Percentage

-1 4 8 12

Near-Field Mean

Far-Field Mean

Sand Clay

Phi Intervals

Near-Field Mean

Far-Field Mean

Phi Intervals

10

15

5

-1 4 8 12

Sand Silt Clay

Percentage

128
Figure 5.24. Depth profiles from discretionary stations (DS) at Mississippi Canyon Block 292 on 

Cruise 2B (post-development).

Sand Percentage

0123456

Depth (cm)

DS1

DS2

DS3

8-10

6-8

4-6

2-4

0-2

0 5 10 15 20 25 30

Depth (cm)

DS1

DS2

DS3

Silt Percentage

8-10

6-8

4-6

2-4

0-2

0 20 40 60 80 100

Depth (cm)

DS1

DS2

DS3

Clay Percentage

8-10

6-8

4-6

2-4

0-2

129
130 

Cuttings with adhering SBMs tend to clump together when discharged and settle rapidly to the 

seafloor (Brandsma 1996; Neff et al. 2000). Once deposited, these clumps are eventually broken 

up due to microbiological activity and physical disturbance. Time intervals of 5 months to nearly 

2 years elapsed prior to post-drilling surveys, depending on the site. If loosely adhering clumps 

were present after this time, they would have been disaggregated during sampling and grain size 

analysis. 

Drilling discharges deposited within the near-field sites presumably consist mainly of (1) cuttings 

and water-based muds released at the seafloor during the early stage of drilling; and 

(2) surface-discharged cuttings with small percentages of adhering drilling fluids. The latter is 

probably the main contributor at our stations. Also, water-based drilling fluids discharged at the 

surface would likely have been widely dispersed while settling to the bottom in a water depth of 

1,000 m or more and probably would not appreciably affect grain size of bottom sediments. 

In this study, there were no major, gross changes in sediment grain size distribution around the 

wellsites due to drilling. Sediments at post-drilling, near-field stations remained predominantly 

clay and silt. Only three samples had small percentages of gravel-sized particles and these 

included both near- and far-field stations. The main impact appears to be patchy areas of elevated 

sand percentages in the near-field. Near-field, post-drilling elevations in sand percentages were 

detected statistically at VK 916, GB 516 (Cruise 2B), and MC 292, but not at GB 602 nor at 

GB 516 on Cruise 1B. Elevated sand percentages are consistent with results of another deepwater 

monitoring program (Continental Shelf Associates, Inc. 2004). 

Large variations in the relative percentages of silt and clay were evident at all four sites (e.g., see 

ternary diagrams). Some of these variations were observed at both near- and far-field sites 

(including pre-drilling samples at VK 916) and are assumed due to natural processes rather than 

drilling discharges. However, there was increased clay content in some samples with high 

concentrations of drilling fluid indicators (e.g., GB 516 Station NF-DS2, Figure 5.9; GB 602 

Station NF-B03, Figure 5.15). Also, at GB 602 (and to a lesser extent, VK 916), there was 

generally more silt and less clay near drilling. 

Grain size variations were patchy within near-field sites and the spatial pattern did not necessarily 

correspond to the cuttings or mud zones mapped in Chapter 4. Also, except for VK 916, there 

were no strong correlations between percentages of sand, silt, or clay and concentrations of 

drilling indicators such as barium and SBF. These findings suggest that grain size distribution 

per se is not a very good indicator of drilling discharge impacts. Other physical measures have 

been determined to be more definitive in identifying cuttings, such as visual analysis for angular 

or tabular particles (Continental Shelf Associates, Inc. 2004). 

Due to the relatively subtle nature of grain size changes, the vertical profiles at discretionary 

stations were not very helpful in identifying the depths of impacts. The most clearcut results 

were seen at the exploration site (VK 916) where only a single well was drilled; here, impacts 

were evident to a depth of about 4 cm. The other three sites, all of which had multiple wells 

drilled over several years, provide some evidence of possible effects at greater depths, 

approaching 10 cm. However, other variables such as barium or SBF concentration are 

undoubtedly better indicators of the depth of mud and cuttings deposits. 
131 

Chapter 6 

Sediment Profile Imaging at Exploration 

and Exploration/Development Sites 

Isabelle P. Williams, Donald C. Rhoads, James A. Blake, and Pamela L. Neubert 

ENSR International, Marine and Coastal Center 

6.1 INTRODUCTION 

ENSR International, Marine and Coastal Center was responsible for analyzing the sediment 

profile images collected at GB 516 and VK 916. Sediment profile imaging (SPI) surveys were 

conducted at both sites in October-November 2000 (Cruise 1B). A second survey was conducted 

at GB 516 in July 2001 (Cruise 2B) and at VK 916 in August 2002 (Cruise 3B). GB 516 was an 

exploration/development site that was sampled once after exploration drilling and again after 

development drilling. VK 916 was an exploration site that was sampled before and after drilling 

of a single exploration well (see Chapter 3). 

Detailed data summaries are provided in Appendix E1 (Cruise 1B), Appendix E2 (Cruise 2B), and 

Appendix E3 (Cruise 3B). Reduced versions of the images (plates) are presented at the end of the 

chapter, and larger versions are provided in Appendix E4. 

6.2 METHODS 

For the Cruise 1B surveys at GB 516 and VK 916, as well as the Cruise 2B survey at GB 516, 

sediment profile images were provided as photographic slides. These were digitized and stored 

on CD-ROM as Kodak® .pcd files by Boston Photo Imaging. All of the Cruise 1B slides were 

digitized. During Cruise 2B at GB 516, two pictures were taken at each station. From these two 

images, the second (showing deeper camera penetration) image was chosen for digitization, 

unless the camera had penetrated so deeply that no sediment-water interface was visible. For the 

VK 916 post-drilling survey (Cruise 3B), a digital camera system was used so that the sediment 

profile images were provided directly on CD-ROMs; the best image for each station was chosen 

for analysis. 

The digitized images were analyzed with ImagePro Plus 4.5 software. Each digitized image was 

analyzed for penetration depth, surface roughness, apparent redox potential discontinuity (RPD) 

depth, grain size major mode, infaunal successional stage, the presence of methane bubbles, 

anoxia of the water overlying the sediment surface, and biogenic features, such as burrows and 

tubes. Any additional observations were entered into a comment field. A specialized Microsoft 

AccessTM database program, written at ENSR International, automatically calculates the 

organism-sediment index (OSI) from the entries recorded for RPD, successional stage, 

presence/absence of methane, and presence/absence of apparent anoxia at the sediment surface 

(Rhoads and Germano 1982). The data are compiled onto separate data sheets for ease in 

re-examination, which is performed for quality control purposes. In addition, the Microsoft 

AccessTM program generates a Microsoft ExcelTM spreadsheet containing all of the recorded data. 

Edited versions of these spreadsheets are given in Appendices E1, E2, and E3. 
132 

The following is a description of each of the SPI parameters that were measured: 

1. Penetration depth is measured from the sediment-water interface to the bottom of the image 

(20-cm maximum) and is a measure of substratum softness, which depends on characteristics 

such as water content and sediment grain size. 

2. Surface roughness is the difference between the least and greatest penetration depth across 

the sediment-water interface depicted on the image (the width is 15 cm). It may be a measure 

for physical disturbance, natural or anthropogenic, or of biological activity such as 

burrowing. 

3. The apparent RPD depth is measured from the sediment-water interface to the depth in the 

sediment at which there is a change in sediment color from tan or brownish (ferric 

hydroxides) in the well-oxygenated surface layer to grayish (ferric hydroxides being reduced) 

or black (presence of sulfide, anoxic conditions) at a few millimeters to several centimeters in 

depth where there is a lack or absence of oxygen. The RPD depth depends on physical and 

biological mechanisms such as currents, organic loading, and bioturbation by infaunal 

organisms, and is commonly used as a first approximation measure of the health of a benthic 

habitat. 

4. Methane bubbles, discernable by their strong reflectance (silvery color), form only under 

severely oxygen-depleted sediment conditions as a result of anaerobic bacterial metabolism. 

5. The grain size major mode is the dominant particle size in an image, measured visually by 

comparing the slide with a photograph of phi size classes. 

6. Infaunal successional stages (seres) are derived from a paradigm describing recolonization of 

disturbed shallow water habitats. Application of this paradigm to our study sites is an 

extrapolation, as detailed knowledge about infaunal successional stages is not available for 

these sites. Stage I organisms live very close to the sediment-water interface and are 

pioneers, as they require little oxidized sediment. By their feeding and burrowing activities, 

these Stage I animals, often small annelids, deepen the RPD, preparing the sediment for 

colonization by somewhat larger animals such as amphipods (Stage II). Stage III organisms 

are large, deep-burrowing, head-down deposit feeders, usually polychaetes and echinoderms, 

which aerate the sediment to several centimeters in depth. Their presence indicates an 

equilibrium community and a healthy environment. The term “azoic” is used here to describe 

retrograde areas where there are apparently no macrofauna present (no Stage I tubes visible at 

the sediment surface and no Stage III feeding voids observed at depth). Of course, these 

areas are not truly azoic because mats of filamentous sulphate-reducing bacteria may be 

present, as well as meiofauna and possibly macroinfauna that did not leave any visible traces. 

6.3 GARDEN BANKS BLOCK 516 – CRUISE 1B 

6.3.1 Survey Design and Location 

GB 516 was an exploration/development site that was sampled once after exploration drilling and 

again after several development wells were drilled (see Chapter 3). The first SPI survey was 

conducted from 28 October until 1 November 2000 (Cruise 1B). This block is approximately 

130 nm from the Mississippi Delta, encompasses depths of 800 to 1,100 m, and contains wellsites 
133 

in the northwest quadrant. All near-field SPI images were taken in depths of about 1,000 to 

1,100 m (Figure 6.1). Most stations in the near-field were located within a 1,000-m diameter 

circle roughly centered on a wellsite where two exploratory wells were drilled in 1999 (see 

Chapter 3). Three sub-parallel near-field transects were sampled in a southeast to northwest 

direction, generally oriented normal to the local bathymetric contours. Each station was treated 

separately in this analysis; no pair of stations was considered as true replicates, as the ship was 

not repositioned between samples. 

In addition, samples were collected along three control transects in the far-field. These transects 

were located 10 to 20 km east of the near-field site and were selected to represent ambient or 

reference bottom conditions (Figure 6.1). 

6.3.1.1 Near-Field Stations 

Transect NF-1 consisted of 28 stations. Usable SPI photographs (i.e., image quality that allowed 

analysis) were obtained from 27 stations. Transect NF-2 consisted of 36 stations, and 30 of these 

yielded usable SPI images. Transect NF-3 consisted of 20 stations, of which 18 were usable. 

Most near-field stations were positioned within the near-field site. The exceptions were 

Stations NF-1.01 to NF-1.10, located on a portion of Transect NF-1 that extended beyond the 

near-field site to the southeast. Figure 6.2 shows near-field transects. 

6.3.1.2 Far-Field Stations 

The three far-field transects are shown in Figures 6.3, 6.4, and 6.5. Transect FF2 had 22 stations, 

and all images were usable. Transect FF4 consisted of 17 stations, and all images were usable. 

Transect FF6 consisted of 20 stations, with usable data from 18 stations. 

6.3.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy 

Sediment profile images provide information about small-scale vertical gradients in sediment 

structure, as recorded within the photographs. Features observed include changes in sediment 

fabric (spatial arrangement of grains or grain aggregates), texture, color, and microstratigraphy 

(laminations, graded bedding, homogeneous layers, burrow-mottled intervals, etc.). This 

information is used, in conjunction with other SPI parameters, to infer sedimentation processes. 

6.3.2.1 Near-Field Stations 

The texture (grain size) at most stations sampled at GB 516 during Cruise 1B varies with depth 

below the sediment-water interface. The upper few centimeters of sediment consist of sand-sized 

aggregates in the range of 2-1 to 3-2 phi (fine to medium sand). The component particles of these 

sand-sized aggregates consist only of silt and clay-sized particles. Deeper layers consist of very 

fine silt to clay particles (greater than 4 phi). Near-surface aggregates are interpreted as 

accumulated fecal pellets, pseudofeces, or parcels of sediment that were broken up by the process 

of bioturbation (foraging and/or burrowing). Below the depth of active bioturbation, sand-sized 

aggregates are compacted (coalesced) and therefore have lost their sand-sized appearance. 
Figure 6.1. Location of Garden Banks Block 516 exploration/development site and associated reference areas where s

(SPI) transects were surveyed.

FF1

FF2

FF3

FF4

FF5

FF6

NF

516

Near-field (NF) site

LEGEND

516 Lease block

Bathymetry (100-m intervals) Far-field (FF) site with SPI transects

FF sites with

SPI transects 

on Cruise 1B

FF sites with

SPI transects 

on Cruise 2B

No SPI 

transects

NF site with 

SPI transects 

on Cruises 

1B and 2B

Far-field (FF) site with no SPI transects

134
135 

Station Map 

GB516 NF 1.28

1.27 1.261.25

1.24

1.23

1.22

1.20

1.19

N.S.

N.S.

1.18 1.17

1.16 1.15

1.14 1.13

1.121.11 1.10

1.09

1.08

1.07 1.06

1.05 1.04

1.03

1.02

N.S. N.S.

1.01

2.36 2.35

2.34

2.33 2.32

2.31 2.30

2.29 2.28

2.27 2.26

2.25 2.24

2.23 2.22

2.21 2.20

2.19 2.18

2.17 2.16

2.15 2.14

2.13 2.12

2.11 2.10

2.09

2.07

2.08

2.06

N.S.

N.S. N.S.

N.S. N.S.

N.S.

N.S. 3.18

3.17 3.16 3.15 3.14

3.13 3.12 3.11 3.10

3.09 3.08

3.07 3.06 3.05 3.04 3.03 3.02

3.01

Site Center –

27o29.3969' N

092o23.1377' W

NF1

NF2

NF3

Figure 6.2. Garden Banks Block 516 near-field (NF) station map, showing positions of 

sediment profile imaging transects and stations sampled during Cruise 1B 

(October-November 2000). The NF site radius is 500 m. 

Station Map

GB516 FF2

S1.22

S1.21

S1.20

S1.19

S1.18

S1.17

S1.16

S1.15

S1.14

S1.13

S1.12

S1.11

S1.10

S1.09

S1.08

S1.07

S1.06 S1.05

S1.04 S1.03

S1.02 S1.01

Site Center –

27°30.6840' N

92°16.6800' W

Figure 6.3. Garden Banks Block 516 Far-field 2 (FF2) station map, showing positions of 

sediment profile imaging transect and stations sampled during Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 
Station Map

GB516 FF4

S2.01

S2.02

S2.04

S2.05

S2.06

S2.07

S2.08

S2.09

S2.10

S2.11

S2.12

S2.13

S2.14

S2.15

S2.16

S2.17

S2.18

Site Center –

27o31.5240' N

092o14.2200' W

Figure 6.4. Garden Banks Block 516 Far-field 4 (FF4) station map, showing positions of sediment 

profile imaging transect and stations sampled during Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

Station Map 

GB516 FF6

S3.01

N.S.

S3.03

S3.04

S3.05

N.S.

S3.07

S3.08

S3.09

S3.10

S3.11

S3.12

S3.13

S3.14

S3.15

S3.16

S3.17

S3.18

S3.19 S3.20

Site Center –

27o31.5240' N

092o12.7500' W

Figure 6.5. Garden Banks Block 516 Far-field 6 (FF6) station map, showing positions of 

sediment profile imaging transect and stations sampled during Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

136 
137 

Overall, sediment color, fabric, and microstratigraphy at stations sampled along Transects NF-1 

and NF-2 differ from those sampled along Transect NF-3. For example, the sediment profile 

image from Station NF-1.19 (Plate 1) on Transect NF-1 shows a granular surface layer of pink to 

reddish-tan sediment underlain by a dark gray to black sulfidic layer. Deeper in the deposit there 

are three to four gray to black layers interposed within this pink to reddish-tan sediment. The 

pink to reddish-tan color is imparted to the sediment by the presence of mineral particles coated 

with ferric hydroxide ("rust") and/or manganese carbonate. Manganese is present in very high 

concentrations in Mississippi Delta sediments (Trefry and Presley 1982), and the oxidized form, 

manganese carbonate, can be brown or pink (R. Aller, pers. comm., 2003). At some previous 

time, these particles were exposed to high oxygen tensions so that the iron present was 

precipitated as surface coatings. Once oxidized, these ferric hydroxide coatings tend to be 

metastable and remain oxidized even when surrounded by anoxic interstitial waters. The 

gray-to-black layers are reduced sediments associated with the anaerobic decay of organic matter 

and accumulation of iron sulfides. The layering of oxidized and reduced sediments strongly 

suggests a recent history of changing inputs into the upper sediment column. Although this 

stratigraphy was common to both Transects NF-1 and NF-2, it also was observed (but not as well 

developed) at the far-field stations (see below). 

Color, fabric, and stratigraphy at several stations along Transects NF-1 and NF-2 are distinctive 

in that they represent environments that are extremely enriched organically (Plates 2 to 5). 

Station NF-2.12 is representative of areas where a white microbial mat at the sediment surface 

overlies a sulfidic gray layer, which in turn overlies a mottled light gray to tan sediment (Plate 2). 

No apparent RPD is observed; i.e., there is no oxidized surface layer. This is consistent with 

measured oxygen penetration depths as small as 0.3 cm (Chapter 8). The image (Plate 3) taken 

at Station NF-2.21 also shows the presence of a white surface microbial mat (on the right). 

Filaments of these microbial colonies are seen suspended above the sediment-water interface. 

Some tan-colored sedimentary intervals are present under the mat, along with buried gray sulfidic 

sediment at the SPI camera penetration depth. Station NF-2.22 has a microbial mat and a gray 

mottled sediment column at SPI camera penetration depth (Plate 4). A microbial mat is also 

present at Station NF-2.23 (Plate 5), but the sediment column below the mat is completely gray to 

black; no ferric hydroxide (yellow to tan color) is observed. The sequence of stations going from 

NF-2.21 to NF-2.22 then to NF-2.23 is presumed to represent a spatial gradient of decreasing 

benthic habitat quality (i.e., increasing organic enrichment over time). 

Near-field Transect NF-3 does not have any microbial mats on the sediment surface, and most of 

the sediment is pink to reddish-tan. For example, the image taken at Station NF-3.17 shows 

burrow-mottled oxidized sediment with only a hint of gray layering at the penetration depth of the 

camera (Plate 6). A feeding void is present. The general fabric and color of stations on 

Transect NF-3 are more similar to the reference (ambient) far-field stations than they are to 

stations on Transects NF-1 and NF-2 (see below) and suggest that Transect NF-3 is in an area 

with less organic input than that covered by Transects NF-1 and NF-2. 

6.3.2.2 Far-Field Stations 

All of the images taken at far-field transect stations show surface sediments consisting of 

sand-sized organic-mineral aggregates (2-1 phi or medium sand). At depth, the sand-sized 

texture is lost due to compaction, and most of the deeper sediment column is silt-clay in texture 
138 

(greater than 4 phi). As reported in Chapter 5, samples of the top 2 cm indicate mean grain sizes 

of about 9 to 8 phi, finer than the sandy surface sediments but consistent with silt-clay seen at 

depth. Sediment color is predominantly pink to reddish-tan with most stations showing gray to 

black sulfidic layers at depth. For example, Station FF6-3.13 (Plate 7) has pink to tan mottled 

surface sediments underlain by a thin layer of sulfidic gray to black sediment, which, in turn, 

overlies a homogeneous light gray layer. Far-field Station FF2-1.4 (Plate 8) also shows a 

near-surface layer of mottled pink to tan sediment overlying faint gray layers at depth. This color 

and fabric is similar to that observed at stations imaged on Transect NF-3 (e.g., compare with SPI 

Station NF-3.17 [Plate 6]). 

6.3.3 Depth of the Apparent Redox Potential Discontinuity 

The near-surface zone of intensive particle mixing and interstitial water exchange is recognized in 

SPI images by a difference in color between the mixed zone (light tan or pink) and underlying 

sediment (darker red, gray, or black). With the relative absence of reduced compounds such as 

hydrogen sulfide and/or iron sulfides, the light tan color in the mixed zone results from the 

oxidation of particulate iron coatings on sediment particles (ferric hydroxide) while the light pink 

color can be imparted by manganese carbonate (Dr. Robert Aller, Stony Brook University, NY, 

pers. comm.). The zone of intensive mixing can be related to physical processes such as sediment 

resuspension or bed-load transport, but in most cases mixing is related to bioturbation by resident 

infauna. Vertical gradients in sediment color, as described above, are related to the oxidation 

state of the sediment. However, because we are inferring the oxidation state from sediment color, 

this parameter is called the “apparent” RPD. The true RPD (as measured by polarigraphic 

electrodes) is usually shallower than the apparent RPD. Deeply oxidized surface sediment (i.e., a 

deep apparent RPD) is usually indicative of a well-developed and active bioturbating infaunal 

community with oxygenated overlying water. Surface mixed zones that are shallow or absent 

usually are associated with ecologically stressed benthic systems, such as recently disturbed 

seabeds devoid of infauna or early successional stages, or bottom areas overlain by hypoxic or 

anoxic water. 

6.3.3.1 Near-Field Stations 

Figure 6.6 shows the mapped distribution of mean apparent RPD depths for the three near-field 

transects. Areas of moderate to deep surface-mixed zones (≥0.5 cm), are distinguished by 

diagonal lines. The area of thin RPD depths, even zero thickness at many stations, encompasses 

most of that part of Transect NF-1 lying within the 1-km diameter circle as well as most of 

Transect NF-2. Most stations on Transect NF-3 (82%) have apparent RPD depths greater than 

0.5 cm in thickness. For all near-field stations where apparent RPD depths could be determined 

(n = 74), the mean RPD depth was 0.83 cm. 

Apparent RPD depth-frequency histograms are shown in Figure 6.7. Apparent RPD depths are 

skewed toward the 0.0 to 0.99-cm class on all three transects, but Transect NF-2 has fewer 

stations with deeper RPD depths than either Transects NF-1 or NF-3. The four stations with 

RPD depths greater than 3 cm on Transect NF-1 are all on that portion of the transect outside of 

the target circle, and the four stations with RPD depths greater than 2 cm on Transect NF-2 are all 

clustered on the southernmost end of the line. 
N.S. N.S.

N.S.

N.S.

0.64 0.72 0.88

0.44 0.62

0.67

0.56

0.80

0.64

Indet. 1.01

1.10 0.29

3.07

0.46

2.71 3.57

4.52 N

NF2

NF3

Area of 

anomalously  low RPD’s

≥

0.00 GB516 NF

0.00 0.000.00

0.00

0.00

0.00

0.18

0.83

N.S.

N.S.

0.34 0.42

0.29` 0.38

0.47

0.52

0.81

0.49

0.85

1.60

0.75

3.02 1.42

1.35 5.23

5.88

0.90

N.S. 

0.20 0.33

0.35

1.10 0.18

0.00

0.00

0.00 0.00

0.00 0.00

0.00 0.15

0.00 0.00

0.00 0.00

0.39 0.00

N.S. 0.21

0.14 0.26

0.16 0.00

0.00 0.33

1.45 0.41 0.29

0.33

N.S.

N.S. N.S.

RPD 0.50 cm

Figure 6.6. Mapped distribution of mean apparent redox potential discontinuity (RPD) depths along the th

sampled at Garden Banks Block 516 during Cruise 1B (October-November 2000). The NF si

139
GB516 NF-2

0

5

10

15

20

25

30

0-0.99 1.00-1.99 2.00-2.99 3.00-3.99 4.00-4.99 5.00-5.99 6.00-6.99

Mean Apparent RPD Classes (cm)

GB516 NF-3

0

2

4

6

8

10

12

0-0.99 1.00-1.99 2.00-2.99 3.00-3.99 4.00-4.99 5.00-5.99 6.00-6.99

Mean Apparent RPD Classes (cm)

GB516 NF-1

0

5

10

15

20

25

0-0.99 1.00-1.99 2.00-2.99 3.00-3.99 4.00-4.99 5.00-5.99 6.00-6.99

Mean Apparent RPD Classes (cm)

N=17;x=1.34+1.28

N=30;x=0.21+0.33

N=27;x=1.20+1.84

Figure 6.7. Redox potential discontinuity (RPD) depth-frequency distributions at three near-field 

transects in Garden Banks Block 516 during Cruise 1B (October-November 2000). 

140 
141 

6.3.3.2 Far-Field Stations 

All stations but one on Transect FF2 have mean apparent RPD depths equal to or greater than 

0.5 cm (Figure 6.8). Far-field Transects FF4 (Figure 6.9) and FF6 (Figure 6.10) both show 

patchy distributions of mean apparent RPD depths with approximately 38% and 20%, 

respectively, of the RPD values equal to, or less than, 0.5 cm in thickness. No zero values were 

measured along any of the three far-field transects. For all far-field stations where the apparent 

RPD depth could be determined (n = 53), the mean RPD depth was 1.04 cm. 

The RPD depth-frequency distribution at all three far-field transects show that most values fall 

below 1.0 cm in thickness (Figure 6.11). However, Transect FF2 has the greatest proportion of 

stations (41%) with values equal to or greater than 1.0 cm in thickness. Transects FF4 and FF6 

have similar RPD depth-frequency distributions with 25% and 20%, respectively, of stations with 

RPD depths equal to or greater than 1.0 cm. 

6.3.4 Infaunal Successional Stages

Macroinfaunal colonization of soft granular substrata proceeds from the sediment-water interface 

downward. Following a physical or chemical benthic disturbance, small opportunistic or 

pioneering species tend to be first colonizers, particularly in a primary succession. These Stage I 

species usually are dominated by small polychaetes that live near the sediment-water interface, 

and many build dense fields of tubes. Over time, the deeper sediment column is invaded by later 

colonizing species, particularly head-down deposit-feeders that form distinctive feeding voids 

within the sediment column that are visible in SPI images (Stage III species). Stage I and 

Stage III species often coexist, and these associations are identified as Stage I-III seres. It is 

sometimes possible to identify transitional seres (Stage II) that involve species that “fill-in” 

between Stage I and Stage III, but this requires detailed knowledge about the successional 

dynamics at the study site. As this knowledge is lacking for the sites mapped here, no Stage II 

seres are identified. 

In this survey, we have identified three successional bottom types: 

1. Bottom sediments that are retrograde areas where there are apparently no macrofauna present 

(no Stage I tubes visible at the sediment surface and no Stage III feeding voids observed at 

depth). These heavily impacted stations are called "azoic" even though mats of filamentous 

sulphate-reducing bacteria may be present (see Plates 2-5). 

2. Bottom sediments that exhibit only Stage I pioneers (Stage I seres). 

3. Bottom sediments that show evidence of subsurface feeding voids (with, or without, Stage I 

species at the surface). These are mapped as Stage I-III or Stage III seres. This sequence of 

seres is presumed to represent a gradient from heavily impacted (azoic) benthic sediments to 

those that are least impacted (Stage III seres). 
142 

Figure 6.8. Mapped distribution of mean apparent redox potential discontinuity (RPD) depths 

along Transect FF2 at Garden Banks Block 516 during Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

Figure 6.9. Mapped distribution of mean apparent redox potential discontinuity (RPD) depths 

along Transect FF4 at Garden Banks Block 516 during Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

0.90

0.67

0.53

0.61

1.40

1.62

1.00

0.77

0.74

0.97

0.47

0.98

1.00

1.08

0.82

1.35

1.07 0.86

0.89 0.51

1.21 4.49

GB516 FF2

RPD ≥ 0.50 cm

0.59

0.37

0.31

0.55

1.47

1.72

2.56

0.37

0.19

0.55

0.82

0.23

0.68

4.01

0.75

Indet.

0.40

GB516 FF4

RPD ≥ 0.50 cm
143 

Figure 6.10. Mapped distribution of mean apparent redox potential discontinuity (RPD) depths 

along Transect FF6 at Garden Banks Block 516 during Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

6.3.4.1 Near-Field Stations 

Figure 6.12 shows the mapped distribution of successional seres as inferred from SPI data. 

Transect NF-2 has the lowest overall successional status, with almost half of the stations being 

azoic (43%) and 87% of the stations being either azoic or in a pioneering (Stage I) sere. 

Transect NF-1 has only four azoic stations (16%), and 76% are in either an azoic or pioneering 

(Stage I) status. Transect NF-3 stands in marked contrast to Transects NF-1 and NF-2 as over 

half (56%) of the stations along Transect NF-3 showed evidence (subsurface feeding voids) of 

Stage III species. No azoic stations were found along Transect NF-3 (Figure 6.12). Comparison 

of the near-field successional sere map (Figure 6.12) with the near-field map of the depth of the 

apparent RPD (Figure 6.6) shows that all of the azoic stations fall within the area of anomalously 

thin apparent RPD depths (zero or less than 0.5 cm in thickness). The location of this area 

northwest of the site center is generally consistent with the zone of relatively high barium and 

SBF concentrations (see Chapters 8 and 9). 

0.30

N.S.

0.57

0.58 Indet.

N.S.

0.62

Indet.

3.33

0.77

0.85

0.88

1.02

0.51

0.77

3.99

0.33

0.51

Indet. 0.35

GB516 FF6

RPD ≥ 0.50 cm
144 

Figure 6.11. Redox potential discontinuity (RPD) depth-frequency distributions at three far-field 

transects at Garden Banks Block 516 during Cruise 1B (October-November 2000). 

GB516 FF4

0

2

4

6

8

10

12

14

0-0.99 1.00-1.99 2.00-2.99 3.00-3.99 4.00-4.99

Mean Apparent RPD Classes (cm)

Frequency

GB516 FF6

0

2

4

6

8

10

12

14

0-0.99 1.00-1.99 2.00-2.99 3.00-3.99 4.00-4.99

Mean Apparent RPD Classes (cm)

Frequency

N=16; x=0.97+1.03

N=15; x=1.03+1.10

GB516 FF2

0

2

4

6

8

10

12

14

0-0.99 1.00-1.99 2.00-2.99 3.00-3.99 4.00-4.99

Mean Apparent RPD Classes (cm)

Frequency

N=22; x=1.09+0.81
Figure 6.12. Mapped distribution of infaunal successional stages along all three near-field transects at Garden Banks Bloc

(October-November 2000). The NF site radius is 500 m.

I

Azoic I I

Azoic

Azoic

Azoic

I

I

N.S.

N.S.

I-III I

I-III I

I-III I

I-III

I

Indet.

I

I

I Indet.

I I-III

I

I

N.S. N

I-III Azoic

Azoic

I I

I I

I-III Azoic

Azoic Azoic

Azoic Azoic

Azoic Azoic

Azoic Azoic

Azoic I

N.S. Azoic

I-III I

I I

I I

I

I

I-III

I

N.S.

N.S. N.S.

N.S. N.S.

N.S.

N.S. I-III

I-III I

I-III I-III

I I

I-III I

III I-III

I-III I I I-III

I I

I-III

NF2

NF3

I-III; III

GB516 N

Stage I seres 

Stage III sere

I

Microbial mat 

Azoic 

(No macrofau

145
146 

6.3.4.2 Far-Field Stations 

Far-field Transect FF2 shows a mixture of Stage I and III seres, with over half (59%) of the 

stations dominated by Stage I seres. These Stage I areas (Stations FF2.8 to FF2.16) are located 

predominantly in the center of this transect (Figure 6.13). Stations along Transect FF4 are 

divided evenly between Stage I and III seres, but there is no spatial gradient apparent along the 

transect (Figure 6.14). Finally, Transect FF6 also is a patchy mixture of Stage I and III seres, 

with slightly more than half of the stations (56%) dominated by Stage III species (Figure 6.15). 

The northwestern end of this transect (Stations FF6.17 through FF6.20) is dominated by Stage I 

seres. No azoic stations were observed along any of the three far-field transects. 

6.3.5 Organism-Sediment Index (OSI) 

The OSI is a parameter that describes overall benthic habitat quality (Rhoads and 

Germano 1986). The index takes into account the depth of the apparent RPD, successional stage, 

presence or absence of sediment methane bubbles, and presence or absence of reduced sediment 

near the sediment-water interface. The value of the index can range from +11 (best habitat value) 

to –10 (worst habitat value). Application of this paradigm to our study sites is an extrapolation, 

as detailed knowledge about infaunal successional stages is not available for these sites. 

However, past experience in mapping this index in a variety of shallow and deep water 

environments around the world has shown that indices of +6 or greater represent benthic habitats 

with minimal chemical or physical disturbance. Values of the OSI below +6 tend to be associated 

with disturbed benthic habitats. Negative OSI values reflect a highly disturbed benthos, such as 

bottom areas impacted by dredging, dumping, chemical contamination, or seafloor areas overlain 

by anoxic or hypoxic water. The GB 516 data set includes OSI values ranging from the highest 

value (+11) to a low of –8. 

6.3.5.1 Near-Field Stations 

The worst benthic habitat quality (negative OSI values) of the GB 516 survey was at 7 stations 

along Transect NF-1 (Stations NF-1.22-1.28) and at 19 stations (Stations NF-2.11-2.12; 

2.16-2.17; 2.19-2.28; 2.30-2.32; 2.34-2.35) along Transect NF-2 (Figure 6.16). This area of the 

seafloor appears to have experienced organic enrichment and high inventories of sedimentary 

sulfides at some earlier time. The benthic macroinfaunal community is depauperate, and 

filamentous anaerobic bacteria dominate. These lowest OSI values occur at the northwestern end 

of Transects NF-1 and NF-2 in the deeper end of the transects. The stations with the best benthic 

habitat quality (OSI equal to, or greater than +6) are found at 8 stations (1, 3, 4, 7, 12, 14, 16, and 

18) located at the southeastern end of Transect NF-1; four of these are outside of the 1-km 

diameter circle. Transect NF-3 has 11 stations with mean OSI values equal to, or greater than +6. 

Figure 6.17 shows the frequency distributions of OSI values at the three near-field transects. 

Using these distributions, transects are ranked on overall benthic habitat quality, from high to 

low, as NF-3 > NF-1 > NF-2. 
147 

Figure 6.13. Mapped 

distribution of infaunal 

successional stages along 

Transect FF2 at Garden Banks 

Block 516 during Cruise 1B 

(October-November 2000). 

The FF site radius is 204 m. 

Figure 6.14. Mapped 

distribution of infaunal 

successional stages along 

Transect FF4 at Garden Banks 

Block 516 during Cruise 1B 

(October-November 2000). 

The FF site radius is 204 m. 

Figure 6.15. Mapped 

distribution of infaunal 

successional stages along 

Transect FF6 at Garden Banks 

Block 516 during Cruise 1B 

(October-November 2000). 

The FF site radius is 204 m. 

GB516 FF2

I-III

I

I-III

I-III

I-III

I-III

I

I

I

I

I

I

I

I

I

I-III

I I-III

I I

I-III I-III

Stage I seres only

I-III Stage III seres present

I 

GB516 FF4 

I-III

I

I

I

I-III

I

I-III

I-III

I-III

I-III 

I

Indet.

I

I

I-III 

I-III

I

Stage I seres only

I-III Stage III seres present 

I

 

GB516 FF6

I-III

N.S

I-III

I-III

Indet

N.S

I-III

Indet I-III

I-III

I-III

I

I

I-III

I

I-III

I

I

I I

I-III Stage III seres present

I Stage I seres only
Figure 6.16. Mapped distribution of the Organism-Sediment Index (OSI) along all three near-field transects at Garden Ban

Cruise 1B (October-November 2000). The NF site radius is 500 m. 

GB516 NF

-3

-8 -3 -3

-8-8

-8

2

3

N.S.

N.S.

6 2

6 2

6 2

7 2 Indet. 4

2

6 Indet.

3 11

7 3

N.S. N

6 -3

-3

3 -2

-3 -3

1 -8

-8 -8

-8 -7

-8 -8

-8 -8

-7 -3

N.S. -7

2 2

2 -3

-3 2

3

2

6

2

N.S.

N.S. N.S.

N.S. N.S.

N.S.

N.S. 6

6 3

6 6

2 2

7 2

Indet. 7

7 2

6 6

5 6

11

NF2

NF3

OSI ≥ 7, Highest habitat OSI = 1 -6, Declining hab

OSI < 0, Area of poorest 

148
-8.0 to -7.1

-7.0 to -6.1

-6.0 to -5.1

-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1

-2.0 to -1.1

-1.0 to -0.1

0.0-0.91.0-1.92.0-2.93.0-3.94.0-4.95.0-5.96.0-6.97.0-7.98.0-8.99.0-9.9

10.0-10.911.0-11.9

-8.0 to -7.1-7.0 to -6.1-6.0 to -5.1-5.0 to -4.1-4.0 to -3.1

-3.0 to -2.1-2.0 to -1.1-1.0 to -0.1

0.0-0.91.0-1.92.0-2.9

3.0-3.9

4.0-4.9

5.0-5.9

6.0-6.9

7.0-7.9

8.0-8.9

9.0-9.9

10.0-10.9

11.0-11.9

-8.0 to -7.1-7.0 to -6.1-6.0 to -5.1-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1-2.0 to -1.1

-1.0 to -0.1

0.0-0.9

1.0-1.9

2.0-2.93.0-3.94.0-4.9

5.0-5.9

6.0-6.9

7.0-7.98.0-8.99.0-9.9

10.0-10.9

11.0-11.9

Figure 6.17. Organism-Sediment Index (OSI) class-frequency distributions at all three near-field 

transects sampled at Garden Banks Block 516 during Cruise 1B 

(October-November 2000). 

149 
150 

6.3.5.2 Far-Field Stations 

The spatial pattern of OSI values along the three far-field transects was patchy. Segments of each 

of the three transects contained stations with relatively high OSI values (equal to, or greater than 

+6) alternating with those having lower values. However, no far-field stations (Figures 6.18, 

6.19, and 6.20) were found with zero or negative OSI values. The frequency histograms show 

this bimodality (Figure 6.21). The overall distribution of OSI values at the three transects 

appeared to be similar. If one compares the far-field OSI distributions with all near-field stations 

falling within the reference circle, the distributions are similar for values of OSI equal to or 

higher than 1. However, the near-field sampling sites show a unique signature, with 26 stations 

falling within the poor habitat (or negative OSI) category (Figure 6.22), suggesting that habitat 

quality within the GB 516 near-field site was lower than that within far-field sites. 

6.4 VIOSCA KNOLL BLOCK 916 – CRUISE 1B (PRE-DRILLING) 

6.4.1 Survey Design and Location 

A pre-drilling SPI survey was conducted at VK 916 in October-November 2000 (Cruise 1B). 

This block lies within a depth range of about 1,000 to 1,300 m (Figure 6.23) and is 

approximately 50 nm east of the mouth of the Mississippi Delta. All of the near-field SPI images 

were taken near the 1,200-m depth contour, within a 1,000-m diameter circle, about 0.5 nm NNE 

of the block center. Two intersecting transects were sampled at the near-field site. The first 

transect (NF-1) was sampled in a northwest to southeast direction, oriented normal to the 

bathymetric contour. The second transect (NF-2) was sampled from east to west tangentially to 

the local bathymetry. One image per station was acquired in this reconnaissance mapping survey 

to rapidly delineate spatial gradients in benthic habitat quality. No attempt was made to replicate 

stations along the two VK 916 near-field transects (Figure 6.24). 

In addition, samples were collected along three control transects at far-field sites. Two of these 

sites were situated approximately 12 to 15 km northeast of the near-field site, while the third was 

located 10 km to the southwest (Figure 6.23). Far-field transects were selected with the 

assumption that they represented ambient or reference bottom conditions. This survey was 

intended to establish a pre-drilling baseline in order to evaluate potential changes in seafloor 

conditions related to subsequent drilling activities.

6.4.1.1 Near-Field Stations 

All near-field stations on the two transects were located inside the 1,000-m diameter circle within 

the VK 916 Block. Usable SPI photographs (i.e., image quality that allowed analysis) were 

obtained from all but one station along both near-field transects, for a total of 78 analyzable 

images, 45 from Transect NF-1 and 33 from Transect NF-2. Figure 6.24 shows these near-field 

transects along with the site center latitude and longitude. 

6.4.1.2 Far-Field Stations 

The three far-field transects are shown in Figures 6.25, 6.26, and 6.27. Each far-field site 

consists of a single transect lying largely within the perimeter of a 408-m diameter circle. 

Transect FF2 had 20 stations (Figure 6.25). Transect FF4 consisted of 18 stations (Figure 6.26), 

and Transect FF6 consisted of 20 stations (Figure 6.27). Analyzable images were obtained from 

55 stations out of a total of 58 stations sampled. 
 

7

2

6

6 7

8

3 3

2

3

1

3

3 3 3

7 3

7 3

27

11

GB516 FF2

151 

Figure 6.18. Mapped distribution of 

the Organism-Sediment Index (OSI) 

along Transect FF2 at Garden Banks 

Block 516 during Cruise 1B 

(October-November 2000). The FF site 

radius is 204 m. 

OSI ≥ 6

6

2

2

2

7

4

9

6

6

6

3

Indet.

2

7

6

Indet.

2

GB516 FF4 Figure 6.19. Mapped distribution of 

the Organism-Sediment Index (OSI) 

along Transect FF4 at Garden Banks 

Block 516 during Cruise 1B 

(October-November 2000). The FF site 

radius is 204 m. 

OSI ≥ 6

6

N.S.

6

6

Indet.

N.S.

6

Indet. 10 7 7

3

3

6

2

11

2

2

2 Indet.

GB516 FF6

OSI ≥ 6 Figure 6.20. Mapped distribution of 

the Organism-Sediment Index (OSI) 

along Transect FF6 at Garden 

Banks Block 516 during Cruise 1B 

(October-November 2000). The FF site 

radius is 204 m. 
GB 516 FF2

0

2

4

6

8

10

0.5-1.4 1.5-2.4 2.5-3.4 3.5-4.4 4.5-5.4 5.5-6.4 6.5-7.4 7.5-8.4 8.5-9.4 9.5-10.4 10.5-11.4

OSI Classes 

Frequency

GB 516 FF4

0

1

2

3

4

5

6

0.5-1.4 1.5-2.4 2.5-3.4 3.5-4.4 4.5-5.4 5.5-6.4 6.5-7.4 7.5-8.4 8.5-9.4 9.5-10.4 10.5-11.4

OSI Classes

Frequency

N=15; x=4.67+2.35

GB 516 FF6

0

1

2

3

4

5

6

0.5-1.4 1.5-2.4 2.5-3.4 3.5-4.4 4.5-5.4 5.5-6.4 6.5-7.4 7.5-8.4 8.5-9.4 9.5-10.4 10.5-11.4

OSI Classes

Frequency

N=15; x=5.27+2.89

N=22; x=4.55+2.60

Figure 6.21. Organism-Sediment Index (OSI) class-frequency distributions at all three far-field 

transects at Garden Banks Block 516 during Cruise 1B (October-November 2000). 

152 
GB516 - NFS - All data within 1000 m circle

0

2

4

6

8

10

12

14

16

-8.0 to -7.1-7.0 to -6.1-6.0 to -5.1-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1

-2.0 to -1.1-1.0 to -0.1

0.0-0.91.0-1.9

2.0-2.93.0-3.9

4.0-4.95.0-5.96.0-6.97.0-7.98.0-8.9

9.0-9.9

10.0-10.9

11.0-11.9

OSI Classes

Frequency

N=64; x=0.16+5.46

GB516 - FF2, FF4, FF6

0

2

4

6

8

10

12

14

-8.0 to -7.1-7.0 to -6.1

-6.0 to -5.1-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1-2.0 to -1.1

-1.0 to -0.1

0.0-0.91.0-1.9

2.0-2.93.0-3.94.0-4.95.0-5.96.0-6.97.0-7.98.0-8.99.0-9.9

10.0-10.9

11.0-11.9

OSI Classes

Frequency

N=52; x=4.79+2.58

153 

Figure 6.22. Organism-Sediment Index (OSI) class-frequency distributions at all near-field and 

far-field transects at Garden Banks Block 516 during Cruise 1B 

(October-November 2000). 
Figure 6.23. Location of Viosca Knoll Block 916 exploration site and associated reference areas where sediment profile 

were surveyed on Cruise 1B.

FF1

FF2

FF3

FF4

FF5

FF6

NF

Viosca Knoll

Destin Dome

Mississippi Canyon Desoto Canyon

916

Near-field (NF) siteFar-field (FF) site with SPI transects

LEGEND

916 Lease block

Bathymetry (100-m intervals) Far-field (FF) site with no SPI transects

154
1.45

1.44 1.43

1.42 1.41

1.40 1.39

1.38 1.37

1.36 1.35

1.34 1.33

1.32 1.31

1.30 1.29

1.28 1.27

1.26 1.25

1.24

1.23

1.22 1.21

1.20 1.19

1.18 1.17

1.16 1.15

1.14 1.13

1.12

1.11

1.10 1.09

1.08 1.07

1.06

1.04 1.03

1.02 1.01

1.05

2.012.022.032.04

2.05

2.06

2.072.08

2.092.102.11

2.12

2.132.14

2.15

2.16

2.17

2.18

2.192.20

2.212.22

2.23

2.242.252.26

2.272.28

2.292.30

2.31

2.32

2.33

2.34

Station Map

VK916 NF

Site Center –

29o06.4056' N

087o53.3246' W

NF1

NF2

Figure 6.24. Viosca Knoll Block 916 near-field (NF) station map, showing positions of sediment 

profile imaging transects and stations. The NF site radius is 500 m. 

155 

Figure 6.25. Viosca Knoll Block 916 Far-field 2 (FF2) station map, showing positions of 

sediment profile imaging transects and stations. The FF site radius is 204 m. 

S1.20

S1.19

S1.18

S1.17

S1.16

S1.15

S1.14

S1.13

S1.12

S1.11

S1.10

S1.09

S1.08

S1.07

S1.06

S1.05

S1.04

S1.03

S1.02

S1.01

Station Map

VK916 FF2

Site Center –

29o04.4244' N

087o59.0621' W
Station Map 

VK916 FF4 

S2.01

S2.03

S2.04

S2.05

S2.06 S2.07 S2.08 S2.09

S2.10

S2.11

S2.12

S2.13

S2.14

S2.15

S2.16

S2.17

S2.18

Site Center –

29o09.8071' N

087o46.5882' W

N.S.

Figure 6.26. Viosca Knoll Block 916 Far-field 4 (FF4) station map, showing positions of 

sediment profile imaging transect and stations. The FF site radius is 204 m. 

Station Map 

VK916 FF6 

S3.01

S3.02

S3.03

S3.04

S3.05

S3.06

S3.07

S3.08

S3.09

S3.10

S3.11

S3.12

S3.13

S3.14

S3.15

S3.16

S3.17

S3.18

S3.19

S3.20

Site Center –

29o11.1643' N

087o43.9900' W

Figure 6.27. Viosca Knoll Block 916 Far-field 6 (FF6) station map, showing positions of 

sediment profile imaging transect and stations. The FF site radius is 204 m. 

156 
157 

6.4.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy 

As described in Section 6.3.2, a variety of sedimentary features that can be observed in sediment 

profile images such as changes in sediment texture, color, fabric, boundary roughness, and 

microstratigraphy can help elucidate sedimentary processes in the region undergoing 

examination. 

6.4.2.1 Near-Field Stations 

All near-field stations show sediments in the seabed that consist of a near-surface interval of 

reddish-brown sediment overlying a light to medium gray clay at depth. The mean thickness of 

the reddish brown layer at NF-1 stations is 5.53 cm (range = 2.75 to 9.11 cm), and NF-2 stations 

have a mean red layer thickness of 4.75 cm (range = 2.01 to 8.61 cm). The reddish color of these 

surficial sediments is due to ferric hydroxide coatings (“rust”) on the surfaces of sediment 

particles. The oxidation state of these particles indicates that they have been exposed to dissolved 

oxygen before and/or during sedimentation. 

The origin of small-scale boundary roughness could not be determined for all stations, but at 

those stations where roughness elements were apparent, the generic source was estimated as 

being dominated either by biological activities or physical processes. Sediment-water interface 

boundary roughness along Transect NF-1 apparently is predominately (97%) due to biogenic 

features such as tube aggregations, feeding pits, etc. Transect NF-2, on the other hand, has 

boundary roughness that appears to be mainly (70%) due to surface erosion or bottom scour. 

The texture of the upper few centimeters of the reddish-brown layer consists of sand, mainly 

2-1 phi (fine sand), but may range from very fine sand (4-3 phi) to medium sand (3-2 phi). As 

reported in Chapter 5, samples of the top 2 cm indicate a mean grain size in the range of 9 to 

8 phi. The sediment fabric is burrow mottled. This sand is composed of aggregates of silt and 

clay bound into sand-sized particles. The process of aggregation is interpreted to represent the 

biological “packaging” of silt and clay into sand-sized pellets, pseudofeces, small tubes, or 

parcels of sediment that were broken up in the process of sediment bioturbation (foraging and/or 

burrowing). At depth, these aggregates tend to lose their sand-sized outlines due to compaction, 

and the texture appears to be predominantly very coarse silt and clay (>4 phi). The underlying 

gray clay appears homogeneous and is entirely composed of cohesive clay-sized particles. The 

contact between the overlying rust-colored sediment and the underlying gray clay appears to be 

very sharp but irregular. The thickness of the gray clay cannot be determined as it extends below 

the camera’s field of view. All near-field stations have similar gradients in fabric, texture, 

structure, and microstratigraphy. Examples of typical sediment profile images from 

Transects NF-1 and NF-2 are shown in Plate 9. 

The geotechnical consistency of the underlying gray clay is stiff and cohesive, and the clay tends 

to adhere to the camera’s optical widow. Many images show ribbon-like pieces of gray clay at, 

or near, the surface that were wiped off the prism window by its wiper as the optical prism 

entered the bottom. The cohesive nature of this clay suggests that it has a water content of 50% 

or less (Boswell 1961) and is therefore geotechnically considered to be “over consolidated.” It is 

likely that the contact between the overlying oxidized, bioturbated sediment and the underlying 

gray clay may represent a significant temporal interruption. Therefore, the underlying clay is 
158 

hypothesized to be much older (e.g., Pleistocene) than the rust-colored sediment accumulating 

above this cohesive unit. 

6.4.2.2 Far-Field Stations 

All of the far-field stations were similar to the near-field stations in texture, gradients in color, 

fabric, and microstratigraphy. The detailed description that appears above for near-field stations 

also applies to the far-field stations. The mean thickness (with ranges) of the reddish-brown 

oxidized surface sediment for the three far-field transects are FF2 = 5.06 cm (2.84 to 9.96 cm), 

FF4 = 5.12 cm (4.43 to 8.80 cm), and FF6 = 4.50 (2.90 to 6.48 cm). Small-scale boundary 

roughness is dominated (54% to 75%) by biological features. Typical far-field sediment profile 

images are shown in Plates 10 (FF2 and FF4) and 11 (FF6). 

6.4.3 Depth of the Apparent Redox Potential Discontinuity 

The near-surface zone of intensive particle mixing and interstitial water exchange is recognized in 

SPI images as a difference in color between the mixed zone (light tan/pink) and underlying 

sediment (darker red, gray, or black). The color difference is imparted by the oxidation state of 

particulate iron (or manganese) coatings on sediment particles (ferric hydroxide or manganese 

carbonate) and the relative absence of reduced compounds such as hydrogen sulfide and/or iron 

or manganese sulfides. Because we are inferring the oxidation state from sediment color, this 

parameter is called the “apparent” RPD. The true RPD (depth at which the Eh = 0 potential) can 

only be measured with a polarigraphic electrode, and experience has shown that the Eh = 0 depth 

is usually shallower than the apparent RPD as estimated by sediment color changes (Pearson and 

Stanley 1979; Revsbech et al. 1979). In addition, once ferric hydroxide coatings have developed 

on sediment particles, they can remain oxidized even though they subsequently may have been 

buried below the true redox (i.e., in a reducing environment). This is the primary reason that the 

apparent RPD overestimates the true RPD. Nevertheless, spatial gradients in the depth of the 

apparent RPD can prove useful in mapping organic enrichment, bioturbation depths, and the 

presence of low bottom-water dissolved oxygen tensions (Pearson and Stanley 1979; Rhoads and 

Germano 1982). 

In general, the apparent RPD depth was difficult to measure at the VK 916 near-field and far-field 

sites for two reasons. First, the biological mixing depth appears to be very shallow, and second, 

the color difference between the mixing zone and underlying oxidized sediment is difficult to 

resolve. 

6.4.3.1 Near-Field Stations 

Figure 6.28 shows the mapped distribution of mean apparent RPD depths at the near-field 

stations. The hatched area delimits those stations with mean apparent RPD depths greater than 

0.5 cm. All stations on Transect NF-2 have mean apparent RPD depths greater than 0.5 cm. 

However, 25% of the stations on Transect NF-1 have RPD depths that are less than 0.5 cm. 

These stations are distributed along Transect NF-1 in a patchy manner, although the majority 

occur in the western half of the transect. For all near-field stations where RPD depth could be 

determined (n = 74), the mean apparent RPD depth was 0.93 cm. The histograms (Figure 6.29) 

of mean apparent RPD depth classes for both near-field transects show that the mean apparent 

RPD depth for Transect NF-1 (0.78 cm) is shallower than that for Transect NF-2 (1.14 cm). 
0.86

0.38

0.85

0.95

0.31

0.44

0.67

0.76

0.25

0.84

0.36 0.

65

0.42

0.51

0.62

0.42

0.82

0.82

0.72

0.57

0.45

0.98

0.82

2.03

0.84

0.48

0.78

0.60

0.68

1.13

0.59

1.01

0.98

0.98

1.07

1.05

0.36

0.84

0.77

1.58

0.91 0.30

0.99 1.96

Indet.

1.07 0.78

Indet. 1.14

1.01

1.03

1.221.58

1.071.07Indet. 0.73

1.56

1.04

0.88

1.28

0.98

1.16

0.771.61

0.72

0.74

0.73

0.630.60

2.87

0.66

1.04

N.S.

1.10

2.15

0.83

Indet.

2.01

VK916 NF 

NF1

NF2

RPD ≥ 0.50 cm

Figure 6.28. Mapped distribution of apparent redox potential discontinuity (RPD) depth, in 

centimeters, along the two near-field transects in Viosca Knoll Block 916 during 

pre-drilling Cruise 1B (October-November 2000). The NF site radius is 500 m. 

6.4.3.2 Far-Field Stations 

The deepest mean apparent RPD depths were mapped along Transect FF2 (Figure 6.30). Most 

values are greater than 1.00 cm, with a transect mean of 1.36 cm (Figure 6.31). In comparison, 

all stations along Transect FF4 (Figure 6.32) and most stations along Transect FF6 (Figure 6.33) 

have mean apparent RPD depth values less than 1.00 cm. For all far-field stations where RPD 

depth could be determined (n = 51), the mean apparent RPD depth was 0.91 cm. RPD frequency 

histograms show that the mean apparent RPD values for both Transects FF4 and FF6 are less than 

1.00 cm (Figure 6.31). 

159 
Figure 6.29. Redox potential discontinuity (RPD) depth-frequency distributions at two 

near-field transects in Viosca Knoll Block 916 during pre-drilling Cruise 1B. 

6.4.4 Infaunal Successional Stages 

Infaunal successional stages are defined in Sections 6.2 and 6.3.4. As for the GB 516 site, 

knowledge about possible Stage II seres is lacking for the VK 916 area, and thus, Stage II seres 

cannot be identified. At both the near-field and far-field transects, all stations were occupied by 

infauna. No azoic stations were observed. In the VK 916 survey, we identified two successional 

bottom types: 

1. Bottom sediments that exhibit only Stage I pioneers (Stage I seres); and 

2. Bottom sediments that show evidence of subsurface feeding voids (with or without Stage I 

species at the surface) and that are mapped as Stage I-III or Stage III seres. 

160 
RPD ≥ 0.50 cm

0.81

1.75

Indet.

1.96

1.23

Indet.

Indet.

1.72

1.05

1.51

2.84

1.15

1.08

1.17

1.54

Indet.

0.69

1.71

1.15

0.41

VK916 FF2

Figure 6.30. Redox potential discontinuity (RPD) depth-frequency distributions along 

Transect FF2 at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

We note that both Stage I and Stage III infauna observed in profile images were small in size and 

low in density. This observation is similar to observations made during a previous benthic 

sampling program, which indicated that the deeper water soft bottoms of the Gulf of Mexico 

appear to represent oligotrophic benthic systems dominated by sparse populations of very small 

infaunal organisms dominated by polychaetes (Yingst and Rhoads 1985). 

161 
VK916 FF2

0

1

2

3

4

5

6

7

8

0.0-0.49 0.50-0.99 1.00-1.49 1.50-1.99 2.00-2.49 2.50-2.99

Mean Apparent RPD Classes (cm)

Frequency

N=16; x=1.36+0.58

VK916 FF4

0

2

4

6

8

10

12

14

16

0.0-0.49 0.50-0.99 1.00-1.49 1.50-1.99 2.00-2.49 2.50-2.99

Mean Apparent RPD Classes (cm)

Frequency

N=16; x=0.73+0.15

VK916 FF6

0

2

4

6

8

10

12

0.0-0.49 0.50-0.99 1.00-1.49 1.50-1.99 2.00-2.49 2.50-2.99

Mean Apparent RPD Classes (cm)

Frequency

N=19; x=0.69+0.30

Figure 6.31. Redox potential discontinuity (RPD) depth-frequency distributions at three far-field 

transects sampled at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). 

162 
0.59

0.91

0.74

N.S.

0.54 0.88

0.79 0.49

0.86

0.91

0.85

0.85

0.45

0.60

0.76

0.78

0.64

VK916 FF4 

RPD≥ 0.5 cm

Figure 6.32. Redox potential discontinuity (RPD) depth-frequency distributions along 

Transect FF4 at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

1.29

0.40

0.33

0.64

0.33

0.87

N.S.

0.98

0.63

0.60

0.39

0.86

1.03

0.35

0.87

1.29

0.64

0.62 0.49

0.54

RPD ≥ 0.5 cm

VK916 FF6 

Figure 6.33. Redox potential discontinuity (RPD) depth-frequency distributions along 

Transect FF6 at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). The FF site radius is 204 m. 

163 
164 

6.4.4.1 Near-Field Stations 

The distribution of successional seres in the near-field shows that Stage I-III seres predominate, 

with five local patches consisting of 3 to 6 adjacent stations dominated by Stage I seres 

(Figure 6.34). The mosaic pattern of Stage I and Stage I-III seres found at the near-field site 

shows the scale of local seafloor disturbances. 

6.4.4.2 Far-Field Stations 

Far-field Transect FF2 consists of a patchy mixture of Stage I and Stage I-III seres. These 

successional stage sere clusters are equally represented in number along this transect, and Stage I 

seres appear to be concentrated in one large and two smaller areas (Figure 6.35). Likewise, 

Transects FF4 and FF6 both show three clusters of stations dominated by pioneering seres 

(Stage I) (Figures 6.36 and 6.37). The patch size of successional seres at all the mapped sites 

suggests that the benthic community represents a mosaic of past disturbances, and the scale of 

these disturbances are on the order of tens to a hundred meters across. The actual patch size is 

difficult to estimate from a single transect. In order to estimate patch dimensions more 

effectively, successional stages would need to be mapped on a two-dimensional station matrix. 

6.4.5 Organism-Sediment Index 

The OSI, a parameter that depicts overall benthic habitat quality, is described in Sections 6.2 and 

6.3.5. The value of the index can range from +11 (highest habitat value) to –10 (negative habitat 

value. Values of the OSI below +6 tend to be associated with disturbed benthic habitats. VK 916 

and associated far-field transects have OSI values ranging from +2 to +9. No negative OSI 

values were present (see below). 

6.4.5.1 Near-Field Stations 

Near-field Transects NF-1 and NF-2 show a patchy distribution of OSI values ranging from +2 to 

+8 (Figure 6.38). The OSI frequency histograms for Transects NF-1 and NF-2 both show 

distinctive bimodal distributions (Figure 6.39). The +7 to +8 class contains the greatest number 

of stations, indicating that most of the site is representative of high-quality habitat. The 

bimodality shown in the histogram is caused by the successional stage assignment at each station 

and by the fact that physical and chemical disturbances are at a minimum. Stations with only 

Stage I seres present and with thin mean apparent RPD depths tend to yield the OSI value of +2. 

Stations with Stage I-III seres and deeper mean apparent RPD depths produce the major mode of 

+7, thereby causing the demonstrated bimodal distribution. 
I-III

I-III I

I-III I-III

I-III I-III

I-III I

I I

I-III I

I I

I-III I-III

I-III

I-III

I-III I-III

I-III

I-III

I-III I-III

I-III I-III

I-III I

I-III I

I-III I-III

I-III I

I-III I

I I

I

I I=III

I-III I-III

I

I-III I-III

I-IIII-III

I-III

I

I

I

I

I-III

I-III

I

I-III

I-III

I-III

I

I-III

I

I-III

I

I-III

I-III

I

I-III

I

I

I-III

I

N.S.

I

I

I-III

I

I

VK916 NF

NF1

NF2

 N.S. 

Stage I seres only

Stage III seres present

 No Sample

I 

I-III; III 

Figure 6.34. Mapped distribution of infaunal successional stages along Transects NF-1 and NF-2 

at Viosca Knoll Block 916 during pre-drilling Cruise 1B (October-November 

2000). The NF site radius is 500 m. 

165 
166 

I

I-III

I

I

I-III

I

I

I

I

I

I-III

I-III

I-III

I-III

I-III

I

I

I-III

I-III

I-III

VK916 FF2

Stage I seres only

Stage III seres 

I

I III

Figure 6.35. Mapped distribution of successional stages along Transect FF2 at Viosca Knoll 

Block 916 during pre-drilling Cruise 1B (October-November 2000). The FF site 

radius is 204 m. 

I

I

I-III

I-III

I-III

N.S.

I

I-III I

I-III

I-III

I

I

I

I-III

I

I-III

VK 916 FF4 

N.S. 

Stage I seres only

Stage III seres present

No Sample

I 

I-III

Figure 6.36. Mapped distribution of successional stages along Transect FF4 at Viosca Knoll 

Block 916 during pre-drilling Cruise 1B (October-November 2000). The FF site 

radius is 204 m. 
VK916 FF6 

I-III

I-III

I-III

I-III

I

I

N.S.

I

I

I-III

I-III

I-III

I-III

I-III

I

I

I

I

I

I

 N.S. 

Stage I seres only

Stage III seres present

 No sample

I 

I-III; III 

Figure 6.37. Mapped distribution of successional stages along Transect FF6 at Viosca Knoll 

Block 916 during pre-drilling Cruise 1B (October-November 2000). The FF site 

radius is 204 m. 

6.4.5.2 Far-Field Stations 

Far-field Transect FF2 also shows a patchy bimodal distribution of OSI values (Figures 6.40 and 

6.41). The OSI frequency distribution for FF2 is comparable to that described for both near-field 

transects (Figure 6.39), with the major mode falling within the +7 to +8 class. 

Organism-sediment indices at Transects FF4 and FF6 also have patchy (Figures 6.42 and 6.43) as 

well as bimodal distributions (Figure 6.41) but have most OSI values falling below +7. 

Frequency histograms for the combined far-field OSI values compared to the combined near-field 

values (Figure 6.44) show that the near-field and far-field populations are similar, indicating that, 

before drilling commenced, the VK 916 site is comparable to the far-field reference transects in 

habitat quality as measured with the OSI parameter. 

167 
168 

Figure 6.38. Mapped distribution of the Organism-Sediment Index (OSI) along the two 

near-field transects taken at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). The NF site radius is 500 m. 

I Stage I seres 

7 6 3 7 6 6 6 7 2 3 2 6 2 2 2 6 7 7 6

6

6 7 7 8 76 7 62 7

3 72 3 34

3

In

de

2 677

t

7

7

8

7 7Indet.

7

7

3

3

4

3 7

Indet.

2

877

37

3 7

4

6

N.S.

3

4

7

4

VK916 NF 

NF1

NF2

OSI ≥ 6

6 6

2

2

5

6

3

Indet.
Figure 6.39. Organism-Sediment Index (OSI) class-frequency distributions at two near-field 

transects in Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). 

169 
3

8

Indet.

4

7

Indet.

Indet.

4

3

4

9

7

7

7

8

Indet.

2

8

7

6

OSI ≥ 6

VK916 FF2

Figure 6.40. Mapped distribution of the Organism-Sediment Index (OSI) along Transect FF2 at 

Viosca Knoll Block 916 during pre-drilling Cruise 1B (October-November 2000). 

The FF site radius is 204 m. 

170 
VK916 FF2

0

1

2

3

4

5

6

7

2.0-2.9 3.0-3.9 4.0-4.9 5.0-5.9 6.0-6.9 7.0-7.9 8.0-8.9 9.0-9.9

OSI Classes

Frequency

N=16; x=5.88+2.19

VK916 FF4

0

1

2

3

4

5

6

7

2.0-2.9 3.0-3.9 4.0-4.9 5.0-5.9 6.0-6.9 7.0-7.9 8.0-8.9 9.0-9.9

OSI Classes

Frequency

N=16; x=4.56+1.93

VK916 FF6

0

1

2

3

4

5

6

7

2.0-2.9 3.0-3.9 4.0-4.9 5.0-5.9 6.0-6.9 7.0-7.9 8.0-8.9 9.0-9.9

OSI Classes

Frequency

N=19; x=4.39+2.06

Figure 6.41. Organism-Sediment Index (OSI) class-frequency distributions at three far-field 

transects in Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). 

171 
VK916 FF4 

6

7

6

N.S.

2

7 3

6

7

3

3

3

2

6

3 3

6

OSI ≥ 6

Figure 6.42. Mapped distribution of the Organism-Sediment Index (OSI) along Transect FF4 at 

Viosca Knoll Block 916 during pre-drilling Cruise 1B (October-November 2000). The 

FF site radius is 204 m. 

VK916 FF6

7

6

6

6

2

3

N.S.

3

2

6

6

7

7

6

3

3

2

2 2

2

OSI ≥ 6

Figure 6.43. Mapped distribution of the Organism-Sediment Index (OSI) along Transect FF6 at 

Viosca Knoll Block 916 during pre-drilling Cruise 1B (October-November 2000). The 

FF site radius is 204 m. 

172 
VK916 - NF-1, NF-2

0

5

10

15

20

25

30

2.0-2.9 3.0-3.9 4.0-4.9 5.0-5.9 6.0-6.9 7.0-7.9 8.0-8.9 9.0-9.9

OSI Classes

Frequency

VK916 - FF2, FF4, FF6

0

2

4

6

8

10

12

14

2.0-2.9 3.0-3.9 4.0-4.9 5.0-5.9 6.0-6.9 7.0-7.9 8.0-8.9 9.0-9.9

OSI Classes

Frequency

N=51; x=4.86+2.14

N=74; x=5.15+2.02

Figure 6.44. Organism-Sediment Index (OSI) class-frequency distributions at all near-field and 

far-field transects sampled at Viosca Knoll Block 916 during pre-drilling Cruise 1B 

(October-November 2000). 

6.5 GARDEN BANKS BLOCK 516 – CRUISE 2B 

6.5.1 Survey Design and Location 

A second sediment profile image survey of GB 516 was conducted in mid-July 2001 (Cruise 2B) 

after the completion of additional drilling there (see Chapter 3). Three transects were sampled at 

the near-field site. In addition, images were collected along three control transects in the 

far-field. The relative locations of the near-field and far-field sites have been shown previously in 

Figure 6.1. Two images per station were acquired in this mapping survey designed to rapidly 

delineate spatial gradients in benthic habitat quality. The image showing deeper penetration of 

the camera, as long as the sediment-water interface was visible, was chosen for analysis. 

173 
174 

6.5.1.1 Near-Field Stations 

Three sampling transects (NF-1, NF-2, and NF-3) were located within the 1,000-m diameter 

circle defining the area of interest within Block 516 (Figure 6.45). Each transect consisted of 

36 stations, and image data were successfully obtained at all stations except for one station on 

Transect NF-3. As spacing between stations is shortest at Transect NF-3, intermediate at 

Transect NF-1, and longest at Transect NF-2, there are differing scales of spatial resolution. 

6.5.1.2 Far-Field Stations 

Far-field stations, presumably representing ambient conditions, were imaged at three localities 

(Transects FF3, FF4, and FF5). The diameter for each circle of interest in the far-field was 

408 m. Stations at Transect FF3 were located along two adjacent sub-parallel transects 

comprising 40 stations (Figure 6.46). Transect FF4 was a single transect consisting of 

36 stations (Figure 6.47). No successful pictures were obtained from Station FF4.28 and so the 

sample size was reduced to n = 35. Transect FF5 was a single transect consisting of 36 stations, 

and images were obtained at all stations (Figure 6.48). 

6.5.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy 

As described in Section 6.3.2, sediment profile images may provide information about small-scale 

vertical gradients in the sediment structure that can be used, in conjunction with other SPI 

parameters, to infer sedimentation, geochemical, and biological processes. 

6.5.2.1 Near-Field Stations 

The bulk sediment texture at all stations on the three near-field transects is very fine-grained with 

silt and clay particles (>4 phi) comprising the majority of the sediment. However, the upper few 

millimeters of the sediment surface generally have a sandy texture, owing to the presence of 

sand-sized grain aggregates, as shown, for example, at Station NF-1.6B (Plate 12). These 

aggregates commonly are produced by meiofaunal burrowing as well as by pelletization of the 

sediment by the macrofauna. All of these sediments show a remarkable diversity of thinly 

bedded or laminated intervals within the images. Profile images show from 2 to 11 layers within 

the field of view (FOV), with most images showing between 4 and 7 layers. The layering is 

recognizable by differences in sediment color rather than from textural gradients. Red to brown 

oxidized intervals are layered within thin black to gray reduced sediments (Plate 12: NF-1.6B, 

4 layers; Plate 13A: NF-3.27B, 6 layers; and Plate 13B: NF-2.35B, 7 layers). 

Near-field Transects NF-1 and NF-2 show significant accumulations of black, presumably 

sulfidic sediment. Eleven contiguous stations along Transect NF-2 show this phenomenon as do 

10 stations at the northern end of NF-1. At some stations, these black layers are present right up 

to the sediment-water interface. These black units are bounded above and/or below by light gray 

sediment (Plates 14A and 14B: NF-1.25B, 1.28B; Plates 15A and 15B: NF-2.17A, 2.18A; 

Plates 16A and 16B: NF-2.19A, 2.20A; and Plate 17: NF-2.25A). At other stations, the black 

stratigraphic unit, bounded by the light gray sediment, is located under (ambient?) reddish brown 

sediment (Appendix E4, Plates 18A and 18B: NF-1.19B, 1.12B). The genesis of the black and 

light gray layers is unknown but may be stratigraphy that is related to drilling muds. Chemical 

analysis of samples would be required to determine origins. 
175 

Figure 6.45. Garden Banks Block 516 near-field (NF) map showing transects and stations 

sampled during Cruise 2B (July 2001). The NF site radius is 500 m.

23 

24 25 26 

27 28

29 30

07

08

09

10

11

12 13

14

15

16 17

18

19

20 21

22

01

03 02

04

05 06

31 32 33 34

35

36

37

38

39

40

Station Map 

 

GB516 FF3

 

Site Center – 

27o 30.8984 ' N 

092o 15.7079' W 

N

FF3 (1-22)

FF3 (23 - 40) 

Figure 6.46. Garden Banks Block 516 Far-field 3 (FF3) station map, showing transects and 

stations sampled during Cruise 2B (July 2001). The FF site radius is 204 m.
176 

N

02

03 04

06 05

07 08

09 10

11 12

13 14

15 16

17 18

19 20

22 21

24 23

25 26

27

29 30

31

32

34 33

35

N.S.

36

Station Map 

GB516 FF4

Site Center –

27o31.5240' N

092o14.2200' W

01

Figure 6.47. Garden Banks Block 516 Far-field 4 (FF4) station map, showing transects and 

stations sampled during Cruise 2B (July 2001). The FF site radius is 204 m. 

01

02

03

04

05

07 06

08

10 09

11 12

13 14

15

16 17

18

19

20 21

22 23

24

25

26

27

28

29

30

31

32

33

34 35

36

N

 Site Center –

27o31.8660 ́N

092o13.8240 ́W

Station Map 

GB516 FF5 

Figure 6.48. Garden Banks Block 516 Far-field 5 (FF5) station map, showing transects and 

stations sampled during Cruise 2B (July 2001). The FF site radius is 204 m. 
177 

6.5.2.2 Far-Field Stations 

The bulk sediment texture of far-field stations consists of silt-clay sized particles (>4 phi) 

overlain by surficial pelletal sands. These sediments also show subsurface color layering but lack 

the dominance of gray to black reduced zones that are so common in the near-field stations 

(Plates 14-18). Plate 19 (FF3-13B) shows a typical profile image of the ambient far-field 

seafloor. Tan to reddish brown sediments show many infaunal organisms, subsurface feeding 

voids, and homogeneously burrowed intervals. If layers are present at depth, they tend to be 

indistinct due to mixing by bioturbation. 

6.5.3 Depth of the Apparent Redox Potential Discontinuity 

The near-surface zone of intensive particle mixing and interstitial water exchange is recognized in 

SPI images by a difference in color between the mixed zone (light tan) and underlying sediment 

(dark red, gray, or black). The light tan to reddish brown color in the mixed zone is imparted by 

the oxidation of particulate iron coatings on sediment particle surfaces (ferric hydroxide) and the 

relative absence of reduced compounds such as hydrogen sulfide and/or metal sulfides. The zone 

of intensive mixing can be related to physical processes such as sediment resuspension or 

bed-load transport, but in most cases, mixing is related to bioturbation by resident infauna. 

Vertical gradients in sediment color, as described above, are related to the oxidation state of the 

sediments. However, as we are inferring the oxidation state from sediment color, this parameter 

is called the “apparent” RPD. The true RPD (as measured with polarigraphic electrodes) is 

usually shallower than the apparent RPD. Once surface sediment is oxidized, the grain coatings 

may remain in an oxidized state even through the particles may be advected downward into 

deeper anoxic sediment. Oxidized particle coatings may be metastable for long periods of time. 

The long time constant of oxidized sediment contributes to the disparity between the apparent 

RPD and the true chemical RPD. 

Deeply oxidized sediment (i.e., a deep apparent RPD) is usually indicative of a well developed 

and active bioturbating infauna in a benthic habitat overlain by oxygen-rich water (see Plate 19

for an imaged example). Surface mixed zones that are shallow or absent usually are associated 

with ecologically stressed benthic systems, such as recently disturbed seabeds devoid of infauna 

or early successional stages, or bottom areas overlain by hypoxic or anoxic bottom water. Koenig 

et al. (2001) give further information about the imaging of oxygen distributions at benthic 

interfaces. 

6.5.3.1 Near-Field Stations 

Figure 6.49 shows the mapped distribution of mean apparent RPD depths for the three near-field 

transects. All transect stations have mean apparent RPD depths less than 0.5 cm thick, with the 

exception of Transect NF-3, Station NF-3.6. A cluster of contiguous stations (n = 10) on 

Transect NF-1 either lack an apparent RPD (zero thickness) or possess an apparent RPD that is 

anomalously thin relative to other stations on the transect. Black reducing sediments dominate 

the surface stratigraphy at these stations. Similarly, Transect NF-2 has a cluster of 11 stations in 

the middle of the transect that are devoid of an apparent RPD or have anomalously thin oxidized 

surface layers and are dominated by black surface sediment (Plates 14-18). For all near-field 

stations where RPD could be determined (n = 106), the mean apparent RPD was 0.18 cm. 
Figure 6.49. Mapped distribution of apparent redox potential discontinuity (RPD) depth, in centimeters, along the three ne

at Garden Banks Block 516 during Cruise 2B (July 2001). The NF site radius is 500 m. 

0.28

0.21 0.31

0.15

0.16

0.230.220.19

0.26 0.26 0.20

0.20

0.21

0.23

0.22

0.22 0.31

0.30

0.19

0.26

0.23

0.24 0.19

0.26

0.19 0.00

0.00

0.00

0.00

0.00 0.000.05

0.08

0.44

0.000.00

0.21

0.59

0.15

0.14

0.15

0.19

0.27 0.17

0.21 0.17

0.22 0.25

0.18 0.20

0.23 0.17

0.38 0.31

0.32

N.S. 0.29

0.23 0.29

0.35 0.21

0.20 0.24

0.28 0.31

0.30 0.31

0.27

0.29

0.30

0.31

0.15

0.12

0.11

0.12

0.14

0.14 0.21 0.15 0.260.13

0.14

0.15

0.15

0.13

0.14

0.00

0.00

0.00

0.00

0.00

0.11

0.00

0.00

0.00

0.00

0.09

0.18

0.24

0.15

0.16

0.14

0.19

0.23

0.24

0.13

RPD Depth (cm)

GB516 NF Station Map GB516 NF 

RPD DEPTH (cm) 

178
179 

The mapped patterns are reflected in the bimodal RPD-frequency distributions for Transects NF-1 

and NF-2 (Figure 6.50) reflecting oxygen stressed stations versus normally aerated sediments. 

Near-field Transect NF-3 has a more normally distributed frequency distribution, as all stations 

had measurable apparent RPD depths (Figure 6.50). 

6.5.3.2 Far-Field Stations 

Figures 6.51, 6.52, and 6.53 show the mapped distributions of mean apparent RPD values for 

Transects FF3, FF4, and FF5, respectively. The two sub-parallel transects at FF3 show values 

similar to those mapped at near-field Transect NF-3, and both have the same population mean 

(0.25 cm). No values of 0.50 cm or greater were measured along Transect FF3, and the 

frequency distribution approximates a normal distribution (Figure 6.54). 

Far-field Transect FF4 has two stations with mean apparent RPD depths >0.50 cm 

(Stations FF4.33 and FF4.35). Both stations are located at the extreme southwest end of the 

transect (Figure 6.52). The resulting RPD-frequency distribution is shifted toward deeper RPD 

depths relative to Transect FF3, with a mean of 0.29 cm (Figure 6.54). Transect FF5 has 6 

stations with mean RPD depths equal to or greater than 0.50 cm, and these stations are distributed 

along the length of the entire transect (Figure 6.53). Transect FF5 had the highest mean apparent 

RPD depth (0.38 cm) (Figure 6.54). For all far-field stations where RPD depths could be 

determined (n = 110), the mean apparent RPD depth was 0.30 cm. 

6.5.4 Infaunal Successional Stage 

Infaunal successional stage designations are described in Sections 6.2 and 6.3.4. 

We identified three kinds of successional bottom types in images collected during Cruise 2B at 

GB 516: 

1. Bottom sediments that are retrograde areas where there are apparently no macrofauna present 

(no Stage I tubes visible at the sediment surface and no subsurface Stage III feeding voids 

observed). These heavily impacted stations are called “azoic” (see Plates 14-18). 

2. Bottom sediments that exhibit only Stage I pioneers (Stage I seres). 

3. Bottom sediments that show evidence of subsurface feeding voids (with, or without Stage I 

species at the surface). These are mapped as Stage I-III or Stage III seres. 

The three-stage sequence outlined above, when mapped along a spatial (or temporal) gradient, is 

used to identify zones of impact and benthic habitat quality. The interplay between degrees and 

spatial scales of disturbance and succession is discussed further by Zajac (2001). 
GB516 NF3 2001 

0

1

2

3

4

5

6

7

8

9

10

0.0-0.03

0.04-0.07

0.08-0.11

0.12-0.15

0.16-0.19

0.20-0.23

0.24-0.27

0.28-0.31

0.32-0.35

0.36-0.39

0.40-0.43

0.44-0.47

0.48-0.51

0.52-0.55

0.56-0.59

0.60-0.63

Mean Apparent RPD Classes (cm)

Frequency

GB516 NF1 2001

0

2

4

6

8

10

12

0.0-0.03

0.04-0.07

0.08-0.11

0.12-0.15

0.16-0.19

0.20-0.23

0.24-0.27

0.28-0.31

0.32-0.35

0.36-0.39

0.40-0.43

0.44-0.47

0.48-0.51

0.52-0.55

0.56-0.59

0.60-0.63

Mean Apparent RPD Classes (cm)

Frequency

GB516 NF2 2001

0

2

4

6

8

10

12

14

16

0.0-0.03

0.04-0.07

0.08-0.11

0.12-0.15

0.16-0.19

0.20-0.23

0.24-0.27

0.28-0.31

0.32-0.35

0.36-0.39

0.40-0.43

0.44-0.47

0.48-0.51

0.52-0.55

0.56-0.59

0.60-0.63

Mean Apparent RPD Clases (cm)

Frequency

N=36;x=0.17+0.11

N=36;x=0.12+0.08

N=34;x0.25+0.09

Figure 6.50. Redox potential discontinuity (RPD) depth-frequency distributions at three 

near-field transects sampled at Garden Banks Block 516 during Cruise 2B 

(July 2001). 

180 
181 

0.34

0.34

0.16

0.16 0.11

0.39 0.45

0.25 0.37

0.24

0.38

0.46

0.23

0.18

0.31

0.17 0.17

0.24

0.33

0.26

0.34

0.34

0.32

0.36

0.24

0.24

0.25

0.28

N.S.

0.35

0.24

0.52

0.53

0.34

0.18

0.25

RPD Depth (cm)

GB516 FF4 

0.26

0.26

0.33 0.34

0.52

Indet. 0.38

0.28

0.53 0.35

0.33 0.61

0.19 0.34

0.58 0.28

0.30

0.34

0.38

0.43 0.40

0.48 0.38

0.50 0.46

0.34

0.24

0.31

0.26

0.41

0.40

0.43 0.48

0.42 0.50

0.42

RPD Depth (cm)

GB516 FF5

0.32

0.29

0.39 0.23 0.17

0.21

0.23 0.19

0.25

0.26

0.26

0.22 0.31

0.15 0.27

0.22

0.24

0.23 0.20

0.21

0.21

0.34

0.18 0.32

0.17

0.20

0.19 0.25

0.29

0.17 0.30

0.24

0.26

0.31

0.23

0.26

0.24 0.21

0.40

GB516 FF3 

RPD Depth (cm)

FF3 (1-22)

FF3 (23-40)

0.41

Figure 6.51. Mapped distribution of 

apparent redox potential discontinuity 

(RPD) depth, in centimeters, along the 

two transects surveyed at Far-field 3 

(FF3) during Cruise 2B (July 2001). 

The FF site radius is 204 m.

Figure 6.52. Mapped distribution of 

apparent redox potential discontinuity 

(RPD) depth, in centimeters, along 

Transect FF4 at Garden Banks Block 516 

during Cruise 2B (July 2001). The 

FF site radius is 204 m.

Figure 6.53. Mapped distribution of 

apparent redox potential discontinuity 

(RPD) depth, in centimeters, along 

Transect FF5 at Garden Banks Block 516 

during Cruise 2B (July 2001). The 

FF radius is 204 m.

GB516 FF3 
0.0-0.03

0.04-0.07

0.08-0.11

0.12-0.15

0.16-0.19

0.20-0.23

0.24-0.27

0.28-0.310.32-0.35

0.36-0.390.40-0.43

0.44-0.47

0.48-0.51

0.52-0.55

0.56-0.59

0.60-0.63

Frequency

0.0-0.03

0.04-0.07

0.08-0.11

0.12-0.15

0.16-0.19

0.20-0.23

0.24-0.27

0.28-0.31

0.32-0.35

0.36-0.39

0.40-0.43

0.44-0.47

0.48-0.51

0.52-0.55

0.56-0.59

0.60-0.63

Frequency

0.0-0.03

0.04-0.07

0.08-0.11

0.12-0.15

0.16-0.19

0.20-0.23

0.24-0.27

0.28-0.31

0.32-0.35

0.36-0.39

0.40-0.43

0.44-0.47

0.48-0.51

0.52-0.55

0.56-0.59

0.60-0.63

Frequency

Figure 6.54. Redox potential discontinuity (RPD) depth-frequency distributions at three far-field 

transects sampled at Garden Banks Block 516 during Cruise 2B (July 2001). 

182 
183 

6.5.4.1 Near-Field Stations 

Transect NF-1 shows a patchy distribution of Stage I and Stage I-III seres over the southern two 

thirds of the transect (Figure 6.55). The northern end of the transect (Stations NF-1.27 through 

1.36) consists of five Stage I and four azoic stations. This is the same end of the transect that 

showed the accumulation of sulfidic black sediment and associated thin to zero thickness 

apparent RPDs. The successional status at the northern end of this transect is poor. Near-field 

Transect NF-1 has only 33% of the stations in a high-order successional stage (I-III). 

Transect NF-2 similarly has a cluster of mid-transect stations (n = 8) that apparently lack 

macrofauna and are mapped as being azoic. These stations include those with accumulations of 

black sulfidic sediment and thin to zero RPD depths (Plates 15-17). Of the seven southernmost 

stations on Transect NF-2, six of seven stations are dominated by Stage I seres (Figure 6.55). 

Only 34% of NF-2 stations are in a high order stage (I-III). 

Transect NF-3 is a patchy mosaic of Stage I and Stage I-III seres. Interestingly, a section of 

Transect NF-3 that is located adjacent to the degraded portion of Transect NF-1 

(Stations NF-3.01 through about 3.14), does not appear to be dominated by Stage I or azoic seres 

(Figure 6.55). This suggests that the factors that have had a negative impact on the benthic 

infauna at the northern end of Transect NF-1 have not adversely affected nearby stations on 

Transect NF-3. Half of Transect NF-3 stations are in the highest successional status (I-III). 

6.5.4.2 Far-Field Stations 

Both transects at FF3 show over 60% of the stations in a Stage I-III status. No clearly defined 

successional gradients are apparent from the mapped distributions (Figure 6.56). Transect FF4 

consists of a patchy mosaic of Stage I and Stage I-III over the length of the transect, with no clear 

large-scale gradients in successional status (Figure 6.57). Only 42% of the stations are 

represented by the highest successional seres (I-III). Similarly, Transect FF5 consists of a patchy 

mosaic of seres with no clear successional gradients and with 61% of the stations populated by 

Stage I-III seres (Figure 6.58). 

Overall, one can rank and compare the successional status of the near-field and far-field transects 

by the percentage of the stations that show the highest successional status (i.e., presence of 

Stage III seres). The lowest percentage of Stage III seres is found at Transects NF-1 (33%) and 

NF-2 (34%). Transect NF-3 had 50% of the stations showing Stage III seres. Far-field 

Transects FF3 and FF5 had more than 60% of the sampled stations showing Stage III seres (64% 

and 61%, respectively). Transect FF4 had 42% of the stations showing Stage III infauna. 
Figure 6.55. Mapped distribution of infaunal successional stages along all three near-field transects at Garden Banks Bloc

(July 2001). The NF site radius is 500 m. 

Azoic Indeterminate

Indet. 

I-III

I-III

I

I-III I-III

I

I-III Indet. I I

I-III

I

I

I-III

I

I-III

Azoic

Azoic

Azoic

Azoic Azoic

I-III

Azoic

Azoic

I

Azoic

I

I-III

I-III

I

I

I-III

I

I

I

I

I-III

I I-III

I-III

I-III

I I I

I

I-III I-III

I-III

I-III

I

I-III

I I-III

I

I I

I

I

I

I-III

I

I-III

I

Azoic

I

Azoic

Azoic

I

II I Azoic

Indet.

I-III I-III

I I I-III I

I-III

I

I-III I

III III-III I

Indet. I-III

I-III I

I-III Indet.

I I

I I-III

I I-III

I I

Indet. I I-III

I-III

GB516 NF

I-III

GB 516 NF 

 No macrofauna present

Stage III seres present

Stage I seres only

 Azoic 

I

I-III; III 

184
I

I

I-III I-III

I

I-III

I

I-III

I I

I

I-III

I-III

I

I

I

I-III

I I

I-III

I

I

I-III

I I

I-III I

N.S. I

I-III I

I

I I-III

I-III I

GB516 FF4

 N.S.

Stage I seres only

Stage III seres present

 No sample

I

I-III; III

Figure 6.56. Mapped 

distribution of infaunal 

successional stages along the 

two transects at Garden Banks 

Block 516, Far-field 3 (FF3) 

during Cruise 2B (July 2001). 

Figure 6.57. Mapped 

distribution of infaunal 

successional stages along 

Transect FF4 at Garden Banks 

Block 516 during Cruise 2B 

(July 2001). The FF site radius 

is 204 m.

Figure 6.58. Mapped 

distribution of infaunal 

successional stages along 

Transect FF5 at Garden Banks 

Block 516 during Cruise 2B 

(July 2001). The FF site radius 

is 204 m.

185
186 

6.5.5 Organism-Sediment Index 

The OSI is described in Sections 6.2 and 6.3.5. The value of the index can range from +11 

(highest habitat value) to –10 (worst habitat value). Values of the OSI below +6 tend to be 

associated with disturbed benthic habitats. Disturbance need not be related to anthropogenic 

activity. For example, disturbance may be naturally induced, such as bottom excavations by 

foraging by predators (especially large fish such as skates and rays), or natural hypoxia, brine, or 

hydrocarbon seeps. 

Negative OSI values are associated with highly disturbed benthic habitats such as impacts due to 

dredging and disposal, chemical contamination or hypoxic/brine water. The GB 516 Cruise 2B 

data set includes values ranging from –8 to +6, thus encompassing stations representing a wide 

range of habitat quality. 

6.5.5.1 Near-Field Stations 

The stations with degraded habitat as indicated by low OSI values at GB 516 during Cruise 2B 

were located at the northernmost end of Transect NF-1 (Stations NF-1.27 through 1.32 and 

NF-1.34 through 1.36) and the mid-region of Transect NF-2 (Stations NF-2.17 through 2.21 and 

NF-2.23 through 2.26) (Figure 6.59). The low OSIs at these stations are related to the presence 

of black sufidic sediment, thin (or zero) apparent RPDs and their azoic status. No methanogenic 

sediments were observed in this survey (at least no methane gas bubbles were seen in the profile 

photos). Transect NF-3 had no negative OSI values. 

The OSI frequency distribution (Figure 6.60) for stations on all near-field stations combined 

shows a polymodal distribution. The negative values form clusters around –8.0, and –4.0 to –2.0, 

and correspond to highly impacted stations. The highest OSI values, representing the stations 

with the best habitats, form a modality at +6.0 to 6.9. The major mode is located at +2.0-2.9 and 

represents an intermediate class of impacted stations. 

6.5.5.2 Far-Field Stations 

Figure 6.61 shows the spatial distribution of OSI values over the FF3 transects. No apparent 

spatial trends in OSI values are seen, and no negative values are present. Most values (64%) fall 

within the 6.0 to 6.9-frequency class (Figure 6.62). Stations with OSI values of +6 or more are 

considered to be minimally impacted relative to stations with lower OSI values. 

Transect FF4 has no apparent spatial trends in OSI values, and no negative values are present 

(Figure 6.63). The dominant OSI frequency falls within the 2.0-2.9 class (Figure 6.62). Only 

37% of the values are within the 6.0 to 6.9-frequency class. 

 

Sixty-six percent of stations on Transect FF5 have OSI values within the 6.0 to 6.9-frequency 

class (Figures 6.62 and 6.64). These higher values are interspersed with stations having OSI 

values within the 2.0 to 2.9 class. 
6 2

6

6

6

2 2 2

2 6 6

6

6

2

6 6 2

2

2

2

2

2 2 6

2

5 -3

-8

-3 -8 -8 -2

-2 2 -3 -8

Indet. (2)

6

6

2

4

6

2

6 6

6 2

Indet. 2

2 6

2 6 2 2

2 6

Indet. 2

6 6

Indet.

2

6 2 6 2

2 6

6

6

2

6 6

2

Indet. 6

2

2

2

2

6

2

2

6

6

2

-4

-8

-3

-8

6

-4 -4

-8

-8

-8

6

2

6

2 2

2 2

6

Figure 6.59. Mapped distribution of the Organism-Sediment Index (OSI) along all three near-field transects at Garden Ban

Cruise 2B (July 2001). The NF site radius is 500 m. 

OSI 

GB516 NF 

6

GB 516 NF 

OSI 

187
GB516 Combined NF Transects 2001

0

5

10

15

20

25

30

35

40

45

-8.0 to -7.1

-7.0 to -6.1

-6.0 to -5.1

-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1

-2.0 to -1.1

-1.0 to -0.1

0.0 to

0.9

1.0 to

1.9

2.0 to

2.9

3.0 to

3.9

4.0 to

4.9

5.0 to

5.9

6.0 to

6.9

OSI Classes

Frequency

N=101; 0=2.21 ± 4.19 

Figure 6.60. Organism-Sediment Index (OSI) class-frequency distribution for all stations 

belonging to all three near-field transects surveyed at Garden Banks Block 516 

during Cruise 2B (July 2001). 

6

6

2

2

6 2

2 2

6

6

2

2 6

2 6

6

6

2 17

6

Indet.

6

2

6

6

66

2

6 6

2 2 6 6

6

2

6

6

6

6

GB516 FF3 

OSI 

FF3 (1-22)

FF3 (23-40)

Figure 6.61. Mapped distribution of the Organism-Sediment Index (OSI) along far-field 

transects at FF3 at Garden Banks Block 516 during Cruise 2B (July 2001). The FF 

site radius is 204 m. 

188
189 

Figure 6.62. Organism-Sediment Index (OSI) frequency distributions at the three far-field 

transects at Garden Banks Block 516 (GB516) during Cruise 2B (July 2001). 

GB516 FF3 2001

0

4

8

1 2

1 6

2 0

2 4

2 8

-8.0 to -7.1

-7.0 to -6.1

-6.0 to -5.1

-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1

-2.0 to -1.1

-1.0 to -0.1

0.0 to

0.9

1.0 to 1.9

2.0 to

2.9

3.0 to

3.9

4.0 to

4.9

5.0 to

5.9

6.0 to

6.9

O SI Classes

Frequency

GB516 FF4 2001

0

4

8

1 2

1 6

2 0

2 4

-8.0 to -7.1

-7.0 to -6.1

-6.0 to -5.1

-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1

-2.0 to -1.1

-1.0 to -0.1

0.0 to

0.9

1.0 to 1.9

2.0 to

2.9

3.0 to

3.9

4.0 to

4.9

5.0 to

5.9

6.0 to

6.9

OSI Classes

Frequency

GB516 FF5 2001

0

4

8

12

16

20

24

-8.0 to -7.1

-7.0 to -6.1

-6.0 to -5.1

-5.0 to -4.1

-4.0 to -3.1

-3.0 to -2.1

-2.0 to -1.1

-1.0 to -0.1

0.0 to

0.9

1.0 to

1.9

2.0 to

2.9

3.0 to

3.9

4.0 to

4.9

5.0 to

5.9

6.0 to

6.9

OSI Classes

Frequency

N = 39; x = 4.56 ± 1.94

N = 35; x = 3.49 ± 1.96

N = 35; x = 4.40 ± 1.99
190 

Figure 6.63. Mapped 

distribution of the 

Organism-Sediment Index 

(OSI) along Transect FF4 at 

Garden Banks Block 516 

during Cruise 2B (July 2001). 

The FF site radius is 204 m.

2

2

6 6

6 2

6

2

6

2 2

6

2

2

2 2

2

2

2

2

2

2

6

2

2

6

N.S.

2 6

6

6

2

2

2 6

6

OSI 

GB516 FF4 

6

6

2

6

6

6 Indet.

6

6 2

6

6

2 6

2

2

2

6

6

2

6 6

6

6

2

6

2

6

6

2

2

6

2

2 6

2

OSI 

GB516 FF5 

Figure 6.64. Mapped 

distribution of the 

Organism-Sediment Index 

(OSI) along Transect FF5 at 

Garden Banks Block 516 

during Cruise 2B (July 2001). 

The FF site radius is 204 m.
191 

6.6 VIOSCA KNOLL BLOCK 916 – CRUISE 3B (POST-DRILLING) 

6.6.1 Survey Design and Location 

The post-drilling sediment profile image survey of VK 916 was conducted on 7 to 8 August 2002 

(Cruise 3B), immediately after drilling was completed. Three transects were sampled at the 

near-field site. Station locations are shown on Figure 6.65. 

6.6.1.1 Near-Field Stations 

There were 119 near-field stations located on three transects. Transects NF-1 and NF-2 each 

consisted of 40 stations, while Transect NF-3 had 39 stations. Usable SPI images were obtained 

at 112 of the 119 stations attempted (no data were collected at Stations NF-1.22, NF-2.1, NF-2.2, 

NF-2.8, NF-2.24, NF-3.1, and NF-3.33). Seventy-five percent of the usable images (n = 83) were 

located within the 1,000-m site diameter. Thirty-three stations (29 with usable SPI images) were 

located outside of the circle perimeter. 

6.6.1.2 Far-Field Stations 

No usable far-field data were obtained at VK 916 during Cruise 3B. Far-field transects were 

attempted at two sites; however, due to camera problems, no usable data were collected. 

Therefore, results presented in the following sections are limited to near-field conditions as 

defined by the three near-field transects. 

6.6.2 Sediment Fabric, Texture, Color, and Small-Scale Stratigraphy 

Images from all stations on the three near-field transects show at least three microstratigraphic 

units. First, the uppermost surface layer tends to consist of a pink to red high-water content layer 

(Plates 20-24 and 27-29). The mean thickness of the surface pink to red layer is comparable in 

all three transects (NF-1 = 3.30 cm, NF-2 = 3.44 cm, and NF-3 = 3.26 cm). The assumption of 

high-water content is based on the floccular appearance of this layer and the presence of 

dewatering pipes observed in some images (see arrows just below sediment-water interface in 

Plate 20). Dewatering structures typically are observed in rapidly deposited (or redeposited) fine 

sediments. A few stations lack this red to pink layer, especially where the surface is experiencing 

apparent anaerobic conditions (Plates 30-35). Chemical reduction imparts a gray to black color 

to the sediment due to the presence of sulfides. 
192 

Figure 6.65. Viosca Knoll Block 916 near-field station map showing positions of sediment 

profile imaging transects and stations surveyed during post-drilling Cruise 3B 

(August 2002). The NF site radius is 500 m. 
193 

The dominant major-modal grain size of the surface layer sediments is sand ranging from 3-2 phi 

(fine sand) to 2-1 phi (medium sand). These sand-sized particles are formed from 

organic-mineral aggregates that result in small mud clasts and/or pellets. The sediment surface 

layer has a chaotic fabric, that is, an apparently random fabric consisting of subrounded to 

angular gray mud clasts intermixed with oxidized red to pink sand or reduced gray to black fluid 

mud. The sand-sized floccular surface sediment occupies interstitial spaces between these larger 

clasts. The cohesive gray clasts appear to have their origin in the underlying cohesive gray 

homogeneous clay (>4 phi major mode). The boundary between the surface unit (red-pink or 

black-black sand) and underlying cohesive clay is dominated by these mud clasts, which form an 

uneven and irregular contact (Plates 20-35). Cohesive sediment (plucking) erosion appears to 

have taken place near the surface of the cohesive gray clay layer either before or during 

deposition of the surface layer, resulting in clay clasts being mixed into the overlying surface 

layer (see Plates 20-35). These clay clasts also were observed in the pre-drilling survey. Small 

scale surface roughness is dominated by physical processes (e.g., erosional relief or dewatering 

mounds) at all stations – i.e., no biological structures, such as feeding mounds or pits, were 

observed that could explain surface topography. 

6.6.3 Depth of the Apparent Redox Potential Discontinuity 

Figure 6.66 shows the mapped distribution of the apparent RPD as recognized by the surface 

mixed zone (light tan to pink or red) and underlying darker red, gray, or black sediment. This 

color transition was very difficult to detect and apparently was absent in most profile images. No 

apparent RPD was observed in sediments at any stations sampled along Transects NF-1 and 

NF-2. Transect NF-3 also was dominated by sediments showing zero RPD thickness, but 

21 stations did show the presence of a very thin mean apparent RPD layer, less than 1.5 cm thick. 

The frequency distributions of the apparent RPD classes for stations sampled within the 1,000-m 

target circle are shown in Figure 6.67. The thin or absent RPD in these sediments strongly 

suggests that surface sediments have been deposited recently and have not yet been exposed to 

surficial oxidation. 

6.6.4 Infaunal Successional Stages 

Infaunal successional stages are defined in Section 6.3.4. Because the structure of Stage II 

assemblages in the VK 916 area is unknown, our mapping effort is confined to the following 

three successional bottom types: 

1. Azoic bottom sediments that show no apparent signs of macrofaunal life at the surface of the 

bottom or at depth. However, this does not preclude the presence of microbial mats. In fact, 

most stations designated as azoic have visible evidence of bacterial aggregations or mats. 

(Examples are shown in Plates 25-26 and 30-35). 

2. Bottom sediments that show evidence of only Stage I pioneering organisms near the sediment 

surface (Stage I seres). 

3. Bottom sediments that show evidence of subsurface feeding voids (with or without Stage I 

species at the surface). These seres are mapped as Stage I-III or III. Examples are shown in 

Plates 22-25 and 28-35). 
194 

N.S. 

N.S. 0.00 

0.00 0.00 

0.00 0.00 N.S. 0.00 

0.00 0.00 

0.00 

0.00 

0.00 

0.00 0.00 

0.00 

0.00 0.00 

0.00 0.00 

0.00 0.00 N.S. 

0.00 Indet.

0.00 0.00

0.00 

0.00

0.00

0.00

0.00 

0.00 

0.00 

0.00

0.00 0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 0.00 

0.00 

0.00 

0.00 0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

N.S. 

0.00 

0.00 

0.00 

0.00 

0.00 0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 0.00 

0.00 0.00 

0.00 

N.S. 

0.63 1.12 1.41 

0.00 

0.00 0.00 

0.00 

0.36 0.32 

0.46 

0.38 

1.01 0.67 

0.51 0.58 

0.25 0.54 

0.00 0.00 

0.71 0.55 0.00 

0.23 

0.19 

0.00 

0.19 0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.20 

0.62 

0.00 

0.34 

Figure 6.66. Mapped distribution of apparent redox potential discontinuity depth in 

centimeters, along the three near-field transects at Viosca Knoll Block 916 during 

post-drilling Cruise 3B (August 2002). The NF site radius is 500 m. 
VK916 - NF3 - All data within 1000-m circle

0

5

10

15

0.0-0.09

0.10-0.190.20-0.290.30-0.390.40-0.490.50-0.590.60-0.690.70-0.790.80-0.890.90-0.991.00-1.09

1.10-1.19

1.20-1.291.30-1.39

1.40-1.49

Apparent RPD Depth (cm)

Frequency

N=31; x=0.26+0.28

VK916 - All data within 1000-m circle

0

10

20

30

40

50

60

70

0.0-0.09

0.10-0.19

0.20-0.290.30-0.39

0.40-0.490.50-0.590.60-0.690.70-0.79

0.80-0.890.90-0.991.00-1.091.10-1.19

1.20-1.291.30-1.39

1.40-1.49

Apparent RPD Depth (cm)

Frequency

NF1

195 

Figure 6.67. Apparent redox potential discontinuity (RPD) depth-frequency distributions at the 

three near-field transects at Viosca Knoll Block 916 during post-drilling Cruise 3B 

(August 2002 ). The lower box presents an expansion of the data for NF-3. 

NF2 NF3

N=82; x=0.10+0.21
196 

Figure 6.68 shows the mapped distribution of successional stages along the three near-field 

transects. Transect NF-1 has 7 stations that are apparently azoic, Transect NF-2 has 13 azoic 

stations, and Transect NF-3 has 8 azoic stations. Therefore, about 25% of the near-field stations 

with usable SPI images (n = 112) show no visual evidence of macrofaunal life on those images. 

This is a significant change from the pre-drilling survey, where no stations were classified as 

azoic. These apparently azoic stations are patchily distributed over the three transects and are 

separated by stations showing evidence of Stages I-III or III. 

Stage I seres are seen only occasionally in the SPI images from this survey. For example, no 

Stage I seres are observed in Transect NF-1 and are visible in only 22% and 14% of the stations 

with usable SPI images in Transects NF-2 and NF-3, respectively. The VK 916 post-drilling 

survey shows that the deeper Stage III seres dominate those stations where evidence of infauna is 

present in the surveyed area. The paucity of Stage I seres at the sediment surface suggests that 

recent surface disturbance (deposition, resuspension, and/or erosion) may have selectively 

compromised near surface-dwelling species relative to deeper-living infauna. A similar 

phenomenon has been noted in studies of the ecological effects of bottom trawling (Rosenberg 

et al. 2003). In contrast, Stage I seres dominated all near-field stations in the pre-drilling survey. 

6.6.5 Organism-Sediment Index 

The OSI, a parameter that provides a scale of overall benthic habitat quality, is described in 

Section 6.3.5. The value of the index can range from +11 (best habitat quality) to –10 (worst 

habitat quality). Values of OSI below +6 tend to be associated with disturbed habitats. 

Because the apparent RPD depths were very shallow (even nonexistent at many stations) and 

several stations were azoic, the overall distribution of OSI values ranges from –7 to +6 

(Figure 6.69). Those stations with zero RPD depths and an azoic status tend to map as OSI = -4. 

Stations with relatively deep RPD depths and associated Stage III seres map in the +6 to +7 

range. The OSI frequency distributions for each of the three near-field transects are shown in 

Figure 6.70. The major modal OSI class for Transects NF-1 and NF-2 falls just below +6 (class 

+5.0 to +5.9), while the major mode on Transect NF-3 is within the class +6.0 to +6.9. However, 

Transect NF-3 also shows the greatest range (-7 to +7) of values, with 8 stations having negative 

OSI values. 

Overall, the post-drilling survey indicates that the near-field benthic system is near, or just below, 

the OSI threshold of +6 (disturbed habitat criterion) and that 22% of the stations show very poor 

habitat quality as manifested by negative OSI values. In the pre-drilling survey, near-field 

stations ranged from +2 to +9, and no negative values were present. The major mode fell within 

the +7 to +8 classes (Figure 6.39), an overall indication of very good habitat quality. Using the 

OSI as a general measure of benthic habitat quality, a significant negative change in habitat 

conditions has been documented in the post-drilling survey at VK 916. 
N.S. 

N.S. III Azoic Azoic 

Azoic III 

N.S. Azoic 

III Azoic 

Azoic III 

I-III 

I-III I-III I-III 

III Azoic 

I-III I-III 

III I-III 

N.S. 

III III

III III

Azoic

III

III

I

Azoic

Azoic

Azoic

III III Azoic

Azoic

III 

III 

III 

III 

Azoic 

Azoic 

III 

Azoic

III III 

Azoic

III 

III III 

III 

III 

III 

III 

III 

Azoic

Azoic

N.S. 

III 

III 

Azoic

III 

III III 

III 

III 

III 

III 

III 

III 

III 

III 

III III 

III III 

III 

N.S. 

III III 

Azoic

Azoic

III III 

III 

III III Azoic III 

Azoic III 

Azoic III 

III III 

Azoic Azoic

I-III III III 

III 

III 

III 

III III 

I-III 

Azoic

III 

III 

III 

III 

I-III 

III 

I-III 

I-III 

Figure 6.68. Mapped distribution of infaunal successional stages along all three near-field 

transects surveyed at Viosca Knoll Block 916 during post-drilling Cruise 3B 

(August 2002). The NF site radius is 500 m. 

197 
N.S. 

N.S. 5 -4 -4 

-4 5 

N.S. -4 

5 -4 

5 

-4 

5 

5 5 5

5 -4 

1 5 

5 5 N.S. 

5 Indet.

5 5

-4 

5

5

1

-4 

-4 

-4 

5 5 -4 

-4 

5 

5 

5 

5 

-4 

-4 

5 

-4 

5 5 

-4 

5 

5 5 

5 

5 

5 

5 

5 

-4 

-4 

N.S. 

5 

5 

-4 

5 

5 5 

5 

5 

5 

5 

5 

5 

5 

5 

5 5 

5 5 

5 

N.S. 

6 7 -2 

-4 

5 5 

5 

6 2 

-3 

6 

-2 6 

-7 

6 

6 6 

-4 -4 

6 6 5 

6 

6 

5 

6 5 

5 

-4 

5 

1 

5 

1 

6 

6 

5 

6 

Figure 6.69. Mapped distribution of the Organism-Sediment Index (OSI) along the three 

near-field transects at Viosca Knoll Block 916 during post-drilling Cruise 3B 

(August 2002). The NF site radius is 500 m. 

198 
VK916 NF1 2002 - All data within 1000-m circle

0

10

20

30

-7.0 to -6.1

-6.0 to -5.1-5.0 to -4.1-4.0 to -3.1

-3.0 to -2.1-2.0 to -1.1

-1.0 to -0.1

0.0-0.91.0-1.92.0-2.93.0-3.94.0-4.95.0-5.96.0-6.97.0-7.9

OSI Classes

Frequency

N=32; x=3.88+3.02

VK916 NF2 2002 - All data within 1000-m circle

0

4

8

12

-7.0 to -6.1

-6.0 to -5.1-5.0 to -4.1-4.0 to -3.1

-3.0 to -2.1-2.0 to -1.1

-1.0 to -0.1

0.0-0.9

1.0-1.92.0-2.93.0-3.94.0-4.95.0-5.96.0-6.9

7.0-7.9

OSI Classes

Frequency

N=37; x=3.27+4.01

VK916 NF3 2002 - All data within 1000-m circle

0

4

8

12

16

-7.0 to -6.1

-6.0 to -5.1-5.0 to -4.1

-4.0 to -3.1-3.0 to -2.1

-2.0 to -1.1-1.0 to -0.1

0.0-0.91.0-1.92.0-2.9

3.0-3.9

4.0-4.9

5.0-5.9

6.0-6.97.0-7.9

OSI Classes

Frequency

N=31; x=3.35+3.96

Figure 6.70. Organism-Sediment Index (OSI) class-frequency distributions at the three 

near-field transects at Viosca Knoll Block 916 during post-drilling Cruise 3B 

(August 2002). 

199 
200 

6.7 SUMMARY AND CONCLUSIONS 

In order to systematically compare benthic conditions between the two surveys at each site, tables 

of SPI observations and parameters have been prepared for both GB 516 (Table 6.1) and VK 916 

(Table 6.2). 

6.7.1 Garden Banks Block 516 

6.7.1.1 Depth of the Mean Apparent Redox Potential Discontinuity 

The Cruise 1B (post-exploration) survey of near-field transect stations showed that mean apparent 

RPD depths were skewed toward low (less than 1 cm deep) values (Table 6.1). Stations with 

mean apparent RPD depths equal to or greater than 1.0 cm ranged from only 7% at Transect NF-2 

to 35% at Transect NF-3. Seven stations on Transect NF-1 and 14 stations on Transect NF-2 had 

mean apparent RPD depths of zero, that is, there was no measurable apparent RPD. In contrast, 

during the same cruise, all stations on Transect NF-3 and on all three far-field transects had mean 

apparent RPD depths greater than zero. Stations with mean apparent RPD depths greater than 

1.0 cm ranged from 20% along Transect FF6 to 36% at Transect FF2. 

During Cruise 2B (post-development), the survey transects were located in slightly different 

positions from those mapped on Cruise 1B and so a transect-by-transect or station-by-station 

comparison is not possible. Nevertheless, the overall frequency distribution of mean apparent 

RPD depths showed most values skewed to low values. No near-field or far-field stations had 

mean apparent RPD depths greater than 1.0 cm deep. In the Cruise 2B survey, 8 stations on 

Transect NF-1 and 9 stations on Transect NF-2 had RPD depths of zero. All far-field stations had 

shallow yet measurable mean apparent RPD depths, although, as described above, all were found 

to be less than 1.0 cm deep. 

In summary, a comparison of mean apparent RPD depths between Cruises 1B and 2B shows that 

the shallowest (i.e., zero) RPD depths exist at near-field stations in both surveys. However, a 

comparison of frequency distributions of RPD depths at all near-field stations combined between 

surveys shows that mean apparent RPD depths at near-field stations during Cruise 1B were nearly 

five times deeper than those measured during Cruise 2B. Similarly, far-field stations had mean 

apparent RPD depths more than three times greater on Cruise 1B than on Cruise 2B. 

Experience in mapping apparent RPD depths in a wide range of marine environments and depths 

indicates that the range of values measured at the near-field and far-field stations at GB 516 is 

extremely narrow compared to measurements made in shallow water environments (typically 

from 0 to 10 cm). This suggests that surficial biogenic mixing at GB 516 is shallow compared to 

that normally seen in shallow water locations. While the range in apparent RPD depths is small 

in both surveys, the most significant and objective findings are that zero apparent RPD depths are 

present at some near-field stations in both surveys. The positive, albeit qualitative, correlation of 

those stations with RPD depth of 0.00 cm, with the presence of sulphidic surface sediments and a 

negative correlation with abundant infaunal macroinfaunal life, as discussed further below, 

supports the interpretation that such bottom areas are ecologically stressed. 
Table 6.1. Comparison between Cruise 1B (post-exploration) and Cruise 2B (post-development) sediment profile ima

results for Garden Banks Block 516 near-field stations. 

SPI Parameter Cruise 1B (Oct-Nov 2000) Cruise 2B (July 2001) C

Sediment conditions Surface sediments consist of organic-mineral 

aggregate sand (3-2 to 2-1 phi) overlying >4 phi tan to gray clay. Evidence of reduced layers at depth. Reducing conditions at the sediment surface were seen at 28% of the sampled stations. 

Surface consists of sand-sized aggregates. Sediment is >4 phi at depth and consists of black to gray layers intercalated with red to brown layers. There is some black sediment at the sediment surface. 

C

Mean thickness of apparent redox potential discontinuity (RPD) 

The major modal mean RPD thickness is 0.00 to 0.99 cm. RPD depth is 0.00 cm at 21 stations. 

The major modal RPD thickness lies within 0 to 0.31 cm. RPD depth is 0.00 cm at 17 stations. 

C

Successional status 74% to 87% of NF-1 and NF-2 stations, 

respectively, are either azoic or Stage I. Half of the NF-3 stations contain Stage I-III infauna. Microbial mats are present near the sediment surface at 22 stations (29%). 

43% (NF-3) to 67% (NF-1 and NF-2) of stations are either azoic or Stage I. No stations had surficial microbial mats; however, 16 stations (15%) had thick layers of black sulfidic sediments extending to the sediment surface. 

Cir

Organism-Sediment Index (OSI) 

Transects NF-1 and NF-2 are dominated by OSI classes <+6 with negative indices at many stations (33%). At Transect NF-3, many stations (61%) have OSI values ≥+6, 

and no station has a negative OSI value. 

For all combined near-field transects, the OSI major mode was +2 to +2.9, and a minor mode of +6 to +6.9 was present. OSI values were negative at 18 near-field stations (18%). 

Bmspig

201
Table 6.2. Comparison between Cruise 1B (pre-drilling) and Cruise 3B (post-drilling) sediment profile imaging (SPI) 

Viosca Knoll Block 916 near-field stations. 

SPI Parameter Cruise 1B (Oct-Nov 2000) Cruise 3B (Aug 2002) 

Sediment conditions • Thickness of surface red to pink layer is 

4.89 to 5.59 cm. 

• Surface sediment grain size is 4-3 to 2-1 phi 

(organic mineral aggregates). Subsurface grain size is >4 phi gray clay. 

• Boundary roughness, where determinable, 

is 30% (NF-2) to 97% (NF-1) biogenic. 

• Thickness of surface red to pink 

layer is 3.26 to 3.44 cm. 

• Grain size is the same as baseline • Boundary roughness is 100% 

physical. 

• L

r

• N

g

• L

Mean thickness of apparent redox potential discontinuity (RPD) 

• The major modal mean RPD thickness is 

0.78 to 1.14 cm. 

• There are no zero values. 

• The 0.00 cm thickness class 

dominates with sulfides at the surface at several sites. 

• L

Successional status • Stage I-III dominates most stations. 

• There are no azoic stations. 

• 25% of stations are azoic. • Stage I organisms are rare and 

were seen at 0% (NF-1) to 22% (NF-2) of stations; this included stations classified as Stage I-III. 

• Stage III dominates. • Microbial mats are present at 

several stations. 

• R

c

Organism-Sediment Index (OSI) 

• Most OSI values are >+6 with a major 

mode of +7.0 to +7.9. 

• Range is +2 to +9. • No negative values were recorded. 

• The OSI has a major mode of 

+5.0 to +5.9. 

• 25% of the stations had negative 

OSI values. 

• C

hmqs

202
203 

6.7.1.2 Successional Status 

The Cruise 1B survey showed that segments of near-field Transects NF-1 and NF-2 were 

apparently devoid of any evidence of infaunal macroinfaunal life, either as imaged organisms or 

biogenic structures such as active feeding voids, tubes, or burrows. Transect NF-1 displayed 

azoic conditions at 16% of the stations on the northwest end. Forty-three percent of those stations 

located at the northwest end of Transect NF-2 also were interpreted to be azoic with respect to 

macrofauna. Microbial mats (Plates 2-5) were also widespread at the northwest ends of 

Transects NF-1 and NF-2. Those stations with no apparent macrofauna, but with microbial mats, 

are the same stations with zero mean apparent RPD depths as described above. This makes 

ecological sense, as biogenic mixing of surface sediment is nil under anoxic/sulfidic conditions. 

Some patchy microbial mats also were observed at stations located along the southeastern portion 

of Transect NF-1 and at Stations NF-2.11 and NF-2.12, but these clusters of stations showed 

evidence of Stage I worms at the sediment surface. In addition, those stations with patchy 

microbial mats and Stage I infauna at the southeastern portion of Transect NF-1 all had 

measurable apparent RPD depths. 

The Cruise 2B survey also found stations apparently devoid of macrofauna on near-field 

Transects NF-1 and NF-3 (Note: these transect numbers are not equivalent in position to 

Transects NF-1 and NF-3 in the Cruise 1B survey). Four stations (11%) at the northern end of 

Transect NF-1 were azoic, and nearby stations were in a Stage I sere. The mid-region of 

Transect NF-2 had 8 stations (23%) devoid of macrofauna. As described above, these retrograde 

successional stations are the same stations where thin or zero apparent RPD depths were mapped. 

Black to gray sulfidic sediment, at or near the sediment-water interface, also was mapped at 

near-field stations during Cruise 2B (Plates 14-18), but no well-developed microbial mats were 

observed in the sediment profile images taken during this survey. Well-developed microbial mats 

were observed only during Cruise 1B. The reason for this difference is unknown. 

The successional status at far-field stations was similar on both surveys. Macrofauna were found 

at all stations where the successional status could be determined. Stage I seres appear to be 

ubiquitous, and Stage III seres, as inferred from the presence of subsurface feeding voids, were 

also present at some of these stations. No sulfidic sediment or microbial mats were imaged at any 

far-field stations on either survey. 

6.7.1.3 Organism-Sediment Index and Overall Benthic Habitat Quality 

The OSI is a summary index that combines the mean depth of the RPD and successional status 

into a single number, useful for mapping benthic disturbance gradients in space and time (see 

methods section). Experience in mapping this parameter in a wide variety of environments has 

shown that OSI values of +6 or more represent relatively undisturbed benthic habitats, in relation 

to physical disturbance and/or chemical stress factors. At the other end of the spectrum, 

OSI values that are negative represent highly impacted and stressed benthic habitats. OSIs in the 

intermediate range, less than +6 but still positive, represent intermediate mosaics of disturbance 

or recovery. 
204 

On Cruise 1B, along GB 516 near-field transects, OSI values at 71% of the stations fell below +6 

and were negative at 36% of the stations. In contrast, considering all three far-field stations 

together, half (50%) had OSI values below +6, and no negative OSI indices were recorded. 

During Cruise 2B, the GB 516 near-field stations had slightly fewer OSI values below +6 (62%) 

than found during the earlier survey, and only 18% of stations had negative OSI values. Of 

far-field stations sampled during Cruise 2B, 46% of stations had OSI values below +6, and no 

negative values were seen. 

Based on the OSI statistics outlined above, no difference is detected in the far-field stations 

between the two surveys. The OSI values at near-field stations appear to be somewhat higher 

during Cruise 2B. The proportion of negative OSIs during Cruise 2B is less than half that of 

Cruise 1B. However, this comparison is compromised by the fact that the spatial location of the 

near-field transects was not the same in the two surveys. This brings into question whether 

spatial variation in the OSI can equal or exceed temporal differences and points out the 

importance of reoccupying the same stations and transects over time to detect habitat change. 

Overall, the most important findings of the two surveys were that black sulphidic muds with azoic 

or retrograde benthic communities are present in the near-field area. These degraded benthic 

habitats stand in marked contrast to habitats seen at far-field stations that exhibit aerobic 

boundary layer conditions with well-developed Stage I and I-III communities. 

Drilling discharges are assumed to be the main cause for the degraded benthic habitats at some 

near-field stations. Two exploration wells had been drilled at the GB 516 site about 14 months 

prior to Cruise 1B, and five development wells were drilled between Cruise 1B and Cruise 2B 

(see Chapter 3). Sediment profile images showed the presence of buried sulfidic layers between 

oxidized sediment layers at depth within the bottom (Plates 12, 13A, 13B, 16B, 18A, and 18B). 

While these may be due to drilling discharges, natural processes may also be a contributing 

factor, as buried dark layers also can be seen at some far-field stations (Plate 7). These buried 

layers may represent past benthic anoxic and/or eutrophication events having spatial scales less 

than 1,000 m (as judged by the mapped distribution of impacted stations in our surveys). These 

events may be associated with local eutrophication, deposition of reactive organic matter, and 

enhanced anaerobic microbial production. These phenomena would explain the storage of 

sulfidic sediment in the sediment column in the form of black to gray layers. As these transient 

anoxic events pass, normal pelagic sedimentation would resume and the gray/black layers would 

be covered with tan oxidized sediment containing normal benthic communities. 

6.7.1.4 Summary of Results 

Table 6.1 summarizes SPI observations and measurements for the GB 516 site for Cruise 1B 

(October-November 2000) and Cruise 2B (July 2001). Given the observed change (or no 

apparent change) in SPI parameters over the period of observation, conclusions are drawn as to 

changes in benthic habitat conditions at the site. It can be seen from Table 6.1 that SPI 

parameters range from comparable to an apparent improvement in benthic habitat conditions in 

the Cruise 2B survey compared to the Cruise 1B survey. This comparison is somewhat 

compromised by the fact that the transects and station locations between the surveys are not 

exactly the same. Nevertheless, the GB 516 site consists of a mosaic of very poor benthic 
205 

conditions alternating with areas of the bottom that represent moderately high benthic habitat 

quality. This finding is consistent for both surveys. 

6.7.2 Viosca Knoll Block 916 

The summary below includes the baseline (Cruise 1B) results from the far-field stations, but the 

pre-drilling and post-drilling comparison is limited to the near-field area as no far-field stations 

were sampled on the post-drilling survey (Cruise 3B). Due to camera problems, no usable data 

were collected in the far-field on Cruise 3B. 

6.7.2.1 Depth of the Mean Apparent Redox Potential Discontinuity 

The baseline survey (Cruise 1B) showed that the surface sediment mixing depths (apparent RPD) 

at the VK 916 near-field stations are thin, with 66% of the values falling below 1.0 cm in depth. 

Far-field stations also are shallowly mixed, with 69% of the values falling below 1.0 cm in depth. 

No zero depths were recorded. 

The post-drilling survey (Cruise 3B) was limited to the near-field site. Stations were located in 

slightly different positions from those occupied in the pre-drilling survey so that a 

station-by-station comparison is not possible. However, post-drilling images showed that the 

depth of the apparent RPD is zero (0.00 cm) at more than 80% of the stations (n = 91). 

A comparison of mean apparent RPD depths between the baseline and post-drilling survey in the 

near-field shows that the apparent RPD depth is much shallower in the post-drilling survey as 

evidenced by an increase in the percentage of stations showing no RPD (i.e., no oxidized 

sediment at the sediment surface) from 0% to 80%. 

6.7.2.2 Successional Status 

The pre-drilling survey showed that Stage I-III seres dominate all near-field and far-field transect 

stations. No azoic stations were encountered. On Cruise 1B, the VK 916 benthic environment 

was in a higher successional status than the GB 516 site (due to the impacts of previous drilling at 

GB 516). 

The post-drilling survey in the VK 916 near-field area found that 25% of the stations were azoic. 

Stage I seres alone were seen at only one station. Only 14% of stations where Stage III 

organisms were present also had Stage I organisms and could be classified as Stage I-III. The 

dominant seres found during the post-drilling survey were Stage III. Microbial mats were seen at 

several stations. 

Stage I organisms were much reduced in numbers in the post-drilling survey, indicating a 

retrograde successional condition. This reduction is interpreted to be caused by a recent 

disturbance of the sediment surface. 

6.7.2.3 Organism-Sediment Indices and Overall Benthic Habitat Quality 

The baseline VK 916 survey found that in the near-field site, only 40% of the stations have OSI 

values less than +6. This means that 60% of the stations have a high habitat quality. No negative 
206 

values were present. Far-field transects had 47% of stations with OSI values less than +6. No 

negative OSI values were present. More than half of the near-field and far-field stations are 

characterized as having high benthic habitat quality based on this index. As a comparison, the 

near-field and far-field OSIs at VK 916 are comparable to the OSI distributions for GB 516 

far-field areas as mapped in both surveys. 

The VK 916 post-drilling survey showed that 25% of the near-field stations surveyed had 

negative OSI values and that the OSI major mode was +5 to +5.9. This indicates marginal to 

poor habitat quality. There was a distinct degradation of habitat quality from good in the 

pre-drilling survey to marginal or poor seen in the post-drilling study. 

The shallow stratigraphy of the VK 916 area, as imaged in sediment profile photographs, is 

different from that mapped at GB 516. On Cruise 1B, no buried black or gray layers were 

observed at or under the surface of the seafloor in the VK 916 area. There is, instead, a thin layer 

of oxidized tan bioturbated sediment overlying a homogeneous light gray clay (Plates 9A, 9B, 

10A, 10B, and 11). The gray clay appears to be an older over-consolidated deposit, as it 

prevented deep camera prism penetration and the clay adhered to the prism window, suggesting 

that it had low water content. 

Some of the post-drilling stations sampled in the near-field showed a similar stratigraphy 

(Plates 20-24 and 27-29). Degraded areas, absent in the baseline survey, showed black sulfidic 

sediments at or near the sediment surface (Plates 25-26 and 30-35). 

6.7.2.4 Summary of Results 

Table 6.2 summarizes baseline (Cruise 1B, October-November 2000) and post-drilling 

(Cruise 3B, August 2002) SPI results at the VK 916 site with conclusions regarding change in 

habitat quality. Unlike the GB 516 results, major changes were detected in all SPI parameters at 

VK 916. Firstly, about 2 cm of red to pink surface layer apparently were lost to erosion. Also, as 

a consequence of erosion, the thin layer of oxidized surface sediment observed in the baseline 

survey was missing at most stations. All of the boundary roughness is interpreted to be due to 

physical (rather than biological) processes. Secondly, one of the most significant observations is 

the presence of reduced sulfidic surface sediments associated with anaerobic bacterial mats seen 

in the post-drilling survey. Sulfidic sediments and bacterial mats were not observed in the 

VK 916 pre-drilling survey. Thirdly, the post-drilling survey shows retrograde successional 

conditions at VK 916 relative to the baseline. Twenty-five percent of the post-drilling stations 

apparently are azoic, and Stage I seres are rarely encountered. This observation is consistent with 

intensive surface erosion/disturbance and is a phenomenon commonly encountered in SPI surveys 

of areas that have been trawled (Rosenberg et al. 2003). In the VK 916 case, trawling is less 

probable than anchor chain disturbance. However, the data are not conclusive about whether the 

erosion is related to anchoring since some evidence of erosion was observed on the pre-drilling 

survey and no far-field, post-drilling data were available for comparison. Lastly, the habitat 

summary parameter (OSI) shows a significant decline in benthic habitat quality in the 

post-drilling survey. The baseline major modal OSI determined in the pre-drilling survey was in 

the +7.0 to +7.9 class, and no negative OSI values were seen. In the post-drilling survey, the 

major modal OSI had dropped two OSI units to +5.0 to +5.9, and 22% of the stations had 

negative OSI values. 
207 

6.8 PLATES 

Reduced versions of the photographs (plates) cited in the text are shown on the following pages. 

Larger versions of the images are shown in Appendix E4. Original image width is 15 cm. 

Plate Station Description/Notes 

Garden Banks Block 516, Cruise 1B 

1 NF-1.19 Granular surface layer of pink to reddish-tan sediment underlain by a dark gray to 

black sulfidic layer. 

2 NF-2.12 White microbial mat at the sediment surface overlies a sulfidic gray layer, which in 

turn overlies a mottled light gray to tan sediment. 

3 NF-2.21 A white surface microbial mat is present on the right. 

4 NF-2.22 Shows a microbial mat and a gray mottled sediment column at sediment profile 

imaging camera penetration depth. 

5 NF-2.23 A microbial mat and the sediment column below completely gray to black; very little 

ferric hydroxide (yellow to tan color) is observed. 

6 NF-3.17 Burrow-mottled oxidized sediment with only a hint of gray layering at the camera 

penetration depth. A feeding void is present (arrow). 

7 FF6-3.13 Pink to tan mottled surface sediments underlain by a thin layer of sulfidic gray to 

black sediment, which in turn, overlies a homogeneous light gray layer. 

8 FF2-1.4 Near-surface layer of mottled pink to tan sediment overlying faint gray layers at depth.

Viosca Knoll Block 916, Cruise 1B 

9A,B A: NF-1.22 

B: NF-2.27 

Layer of reddish-brown sediment overlying a homogeneous gray clay. Feeding voids 

(arrows) are present in both pictures, and a worm is seen at depth in 9A. 

10A,B A: FF2-1.11 

B: FF4-2.7 Images typical of VK 916 far-field sites during the pre-drilling cruise. 

11 FF6-3.18 Image typical of VK 916 far-field sites during the pre-drilling cruise. 

Garden Banks Block 516, Cruise 2B 

12 NF-1.6B The upper few millimeters of the sediment consist of sand-sized grain aggregates; four 

layers of oxidized and reduced sediments are visible. 

13A,B A: NF-3.27B 

B: NF-2.35B 

13A shows six layers of oxidized and reduced sediments, and 13B has seven layers of 

oxidized and reduced sediments. 

14A,B A: NF-1.25B 

B: NF-1.28B 

Black reduced sediments extending to the sediment-water interface and bounded 

above and below (A) or only below (B) by light gray sediment. 

15A,B A: NF-2.17A 

B: NF-2.18A 

Black reduced sediments extending to the sediment-water interface and bounded 

below by light gray sediment. 

16A,B A: NF-2.19A 

B: NF-2.20A 

Black reduced sediments extending to the sediment-water interface and bounded 

below (A) or above and below (B) by light gray sediment. 

17 NF2-25A Black reduced sediments extending to the sediment-water interface and bounded 

below by light gray sediment. 

18A,B A: NF-1.19B 

B: NF-3.12B 

Black reduced sediments bounded by light gray sediment, and under ambient? 

reddish-brown sediment. 

19 FF3-13B Image typical of far-field sites during Cruise 2B, showing absence of dark black 

reduced sediments. Feeding voids (arrows) are present. 

Viosca Knoll Block 916, Cruise 3B 

20 NF-1.01 

(1808) 

Shows a layer of red to reddish-gray sediment over a region of chaotic fabric, which in 

turn, overlays a homogeneous gray clay. The surface is erosional, and no Stage I tubes 

are visible. Dewatering channels are present (3 arrows near surface) in the 

uncompacted surface region. A tiny feeding void (arrow) is present at 18-cm depth. 

21 NF-1.37 

(1919) Numerous infaunal feeding voids (arrows). 

22 NF-1.40 

(1924) 

Shows chaotic fabric, feeding voids (arrows), and small reddish patches of 

lignosulfate. 
208 

Plate Station Description/Notes 

23 NF-2.14 

(0155) 

Shows Stage I infauna on the sediment surface (white arrows) and feeding voids 

(black arrows). 

24 NF-2.21 

(0207) 

A layer of red to reddish-gray sediment over a region of chaotic fabric, which in turn, 

overlays a homogeneous gray clay. Stage I tubes (white arrows) are visible on the 

sediment surface. A tiny feeding void (black arrow) is present at 18-cm depth. 

25 NF-2.39 

(0235) 

Shows sulfidic areas within the layer of red to reddish-gray sediment. Thin strands of 

a small disturbed bacterial mat are visible in the upper layer (white arrows). A region 

of chaotic fabric is still present between the red layer and the homogeneous gray clay. 

The surface is erosional, and no Stage I tubes are visible. A tiny feeding void (black 

arrow) is present at 18-cm depth. 

26 NF-2.40 

(0236) 

Shows sulfidic areas within the layer of red to reddish-gray sediment. Thin strands of 

a small disturbed bacterial mat are visible in the upper layer (white arrows). A region 

of chaotic fabric is still present between the red layer and the homogeneous gray clay. 

A few recumbent tubes are present near the surface. No voids are present. 

27 NF-3.14 

(0449) Shows a redox potential discontinuity (light pinkish-red sediment at surface). 

28 NF-3.21 

(0502) 

Shows a redox potential discontinuity (light-brown sediment at surface). Stage I tubes 

are present on the sediment surface (white arrows). A feeding void is present at 16-cm 

depth (black arrow). An example of successional Stage I-III. 

29 NF3-29 

(0514) 

Shows a thin redox potential discontinuity (patches of light-brown sediment at 

surface). Stage I tubes are present on the sediment surface (white arrows). A feeding 

void is present at 16-cm depth (black arrow). 

30 NF-3.34 

(0522) 

Shows sulfidic areas within the layer of red to reddish-gray sediment. Thin strands of 

a small disturbed bacterial mat are visible in the upper layer (white arrows). A region 

of chaotic fabric is still present between the upper layer and the homogeneous gray 

clay. A recumbent tube is present on the surface (white dashed arrow). Feeding voids 

are visible at depth (black arrows). 

31 NF-3.35 

(0523) 

Shows sulfidic areas within the layer of red to reddish-gray sediment. Thin strands of 

a small disturbed bacterial mat are visible in the upper layer (white arrows). A region 

of chaotic fabric is still present between the upper layer and the homogeneous gray 

clay. Recumbent/buried tubes are present on the surface (white dashed arrows). 

Feeding voids are visible at depth (black arrows). 

32 NF-3.36 

(0525) 

Shows a thick sulfidic area underlying a thin redox potential discontinuity (<1 cm). A 

patchy white bacterial mat is visible in the upper layer (white arrows). Stage I tubes 

(dashed white arrows) are present on the surface. Small feeding voids are visible at 

depth (black arrows). The successional stage is I-III; Organism-Sediment Index is 6. 

33 NF-3.37 

(0527) 

Shows a sulfidic area underlying a thin redox potential discontinuity (<1 cm). Patches 

of a buried white bacterial mat are visible in the upper layer (white arrows). 

Recumbent/buried tubes (dashed white arrows) are present near the surface. A small 

feeding void is visible at depth (black arrow). 

34 NF-3.38 

(0529) 

Shows a patchy sulfidic layer and bacterial mat. The red layer is absent. Patches of a 

buried white bacterial mat are visible in the upper layer (dashed black arrows). Stage I 

tubes (white arrows) are present on the surface. A feeding void is visible at depth 

(black arrow). The successional stage is I-III; Organism-Sediment Index is 5. 

35 NF-3.39 

(0531) 

Shows a sulfidic layer underlying a thin redox potential discontinuity. The red layer is 

absent. Patches of a buried white bacterial mat are visible in the upper layer (dashed 

black arrows). A few Stage I tubes (white arrows) are present on the surface. Feeding 

voids are visible at depth (black arrows). The successional stage is I-III; 

Organism-Sediment Index is 6. 
Plate 1 Plate 2 Plate 3 

Plate 4 Plate 5 Plate 6 

Plate 7 Plate 8 

Garden Banks Block 516 

Cruise 1B 

Plates 1-6 

Near-Field 

Plates 7-8 

Far-Field 

209 
Plate 9A Plate 9B 

Plate 10A Plate 10B 

Plate 11 

Viosca Knoll Block 916 

Cruise 1B 

(pre-drilling) 

Plates 9A, 9B 

Near-Field 

Plates 10A, 10B, 11 

 Far-Field 

210 
Plate 12 Plate 13A Plate 13B 

Plate 14A Plate 14B 

Plate 15A Plate 15B 

Garden Banks Block 516 

Cruise 2B 

Plates 12, 13A, 13B 

14A, 14B, 15A, 15B 

Near-Field 

211 
Plate 16A Plate 16B Plate 17 

Plate 18A Plate 18B Plate 19 

Garden Banks Block 516 

Cruise 2B 

Plates 16A, 16B, 

17, 18A, 18B 

Near-Field 

Plate 19 

Far-Field 

212 
Plate 20 Plate 21 Plate 22 

Plate 23 Plate 24 Plate 25 

Plate 26 Plate 27 

Viosca Knoll Block 916 

Cruise 3B 

(post-drilling) 

Plates 20-27 

Near-Field 

213 
Plate 28 Plate 29 Plate 30 

Plate 31 Plate 32 Plate 33 

Plate 34 Plate 35 

Viosca Knoll Block 916 

Cruise 3B 

(post-drilling) 

Plates 28-35 

Near-Field 

214 
215 

Chapter 7 

Sediment Profile Imaging at 

Post-Development Sites 

Robert J. Diaz 

Virginia Institute of Marine Science 

7.1 INTRODUCTION 

Rhoads and Cande (1971) developed sediment profiling as a means of obtaining in situ data to 

investigate processes structuring the sediment-water interface. The technology of remote 

ecological monitoring of the seafloor (REMOTS – Rhoads and Germano 1982) or sediment 

profile imaging (SPI) has allowed for the development of a better understanding of the 

complexity of sediment dynamics, from both a biological and physical point of view (for 

examples, see Rhoads and Germano [1986], Valente et al. [1992], Diaz et al. [1994], Bonsdorff et 

al. [1996], Nilsson and Rosenberg [2000], and Rosenberg et al. [2001]). This approach to 

evaluating the environment, and potential impacts, can be easily combined with classical 

approaches to habitat and impact assessment, providing scientists and managers with a more 

holistic ecosystem view. In addition, SPI serves to provide ground-truth data for acoustic 

methods such as side-scan and multibeam sonar. 

This chapter deals only with sediment profile camera surveys at post-development sites GB 602 

and MC 292. For SPI data from the other two sites, see Chapter 6. 

7.2 METHODS 

7.2.1 Survey Design and Transect Location 

An SPI survey was conducted in July 2001 (Cruise 2B) at GB 602 and MC 292. The general 

rationale for the field sampling design can be found in Chapter 2. For the SPI component, a basic 

transect design was adopted. Along each transect, a Benthos sediment profile camera was 

deployed, generally 36 times (actual numbers of usable photographs for each transect are given in 

the Results section) 

Transects were surveyed at a near-field area, located near previous oil and gas drilling site(s), and 

three far-field areas, located at least 10 km from the near-field and presumably outside the 

influence of oil development activities (Figures 7.1 and 7.2). At GB 602, three near-field 

transects and three far-field transects were completed. At MC 292, two near-field transects and 

three far-field transects were surveyed. 
Figure 7.1. Location of Garden Banks Block 602 post-development site and associated reference areas where sedim

transects were surveyed.

Near-field (NF) siteFar-field (FF) site with SPI transects

LEGEND

FF1

FF2

FF3 FF4

FF5

FF6

602 Lease block

Bathymetry (100-m intervals)

NF

602

Far-field (FF) site with no SPI transects

216
Figure 7.2. Location of Mississippi Canyon Block 292 post-development site and associated reference areas where 

(SPI) transects were surveyed.

FF1

FF2

FF3

FF4

FF5

FF6

NF

292

Near-field (NF) siteFar-field (FF) site with SPI transects

LEGEND

292 Lease block

Bathymetry (100-m intervals)

Far-field (FF) site with no SPI transects

217
218 

All near-field SPI images were taken along transects located near or within a 500-m radius circle 

centered on previous wellsites (Figures 7.3 and 7.4). At GB 602, three near-field transects were 

sampled – two in an east-to-west direction (NF-1 and NF-2) and one north-to-south (NF-3) 

(Figure 7.3). Transect NF-1 was located north of the wellsites, the first half passing through an 

area with several fingers of possible cuttings as mapped by the side-scan sonar data. Transect 

NF-2 went within 100 m of the wellsites, with over half of the transect over bottom classified as 

possible cuttings based on side-scan sonar data. Transect NF-3, to the east of the wellsites by 

about 250 m, crossed several fingers of possible cuttings as mapped by the side-scan sonar 

(Figure 7.3). Samples were collected along three transects at far-field areas (Transects FF1, FF4, 

and FF5) intended to represent ambient or reference bottom conditions (Figure 7.5). 

At MC 292, two near-field transects were sampled, one in a southwest-to-northeast direction 

(NF-1) and the other in a northwest-to-southeast direction (NF-2) (Figure 7.4). Transect NF-1 

passed about 200 m northwest of the wellsites in an area with thin fingers of possible cuttings as 

defined by the side-scan sonar data. Transect NF-2 passed about 200 m to the southwest from 

wellsites with a little less than half of the transect over bottom classified as possible cuttings 

based on the side-scan sonar (Figure 7.4). Additional samples were collected along three 

far-field transects (FF1, FF4, and FF5) located at least 10 km from the near-field (Figure 7.6). 

7.2.2 Camera Operation and Field Sampling 

A Benthos Model 3731 sediment profile camera was used to collect images (see Chapter 2, 

Figure 2.9). The profile camera works like an inverted periscope with a deep-sea 35-mm camera 

mechanism mounted horizontally inside a water-tight housing on top of a wedge-shaped prism. 

The prism has a Plexiglas® faceplate at the front with a mirror placed at a 45° angle at the back. 

The camera lens looks down at the mirror, which is reflecting the image from the faceplate. The 

prism has an internal strobe mounted inside at the back of the wedge to provide illumination for 

the image; this chamber is filled with distilled water, so the camera always has an optically clear 

path to shoot through. This wedge assembly is mounted on a moveable carriage within a stainless 

steel frame. The frame is lowered to the seafloor on a winch wire, and the tension on the wire 

keeps the prism in its “up” position. When the frame comes to rest on the seafloor, the winch 

wire goes slack, and the camera prism descends into the sediment at a slow, controlled rate by the 

dampening action of a hydraulic piston so as not to disturb the sediment-water interface. On the 

way down, it trips a trigger that activates a time-delay circuit to allow the prism to penetrate the 

seafloor before any image is taken. The knife-sharp edge of the prism transects the sediment, and 

the prism penetrates the bottom. The strobe is discharged twice with each lowering to obtain two 

cross-sectional images of the upper 20 cm of the sediment column. After the two replicate 

images are obtained at the first location, the camera is then raised up about 2 to 3 m off the 

bottom to allow the strobe to recharge. The strobe recharges within 5 seconds, and the camera is 

ready to be lowered again for another two images. When the camera is raised between 

photographs, a wiper blade cleans the faceplate. Surveys can be accomplished rapidly by 

“pogo-sticking” the camera across an area of seafloor while recording positional fixes on the 

surface vessel. The resulting images give the viewer the same perspective as looking through the 

side of an aquarium half-filled with sediment. 
GB 602

Near-Field Site

(500 m radius)

Site Center:

27° 22' 38.02"N

92° 27' 35.79"W

NF-2

NF-1

36 stations 36 usable images

 38 stations, 36 usable images

37 stations

37 usable

images

NF-3

Possible Cuttings

(geophysical map)

Legend

Wellsite

Figure 7.3. Locations of sediment profile imaging (SPI) transects at Garden Banks Block 602 

near-field site on Cruise 2B (July 2001). Arrows indicate transect direction, and 

dots are sequentially numbered SPI stations. Geophysically mapped areas of 

possible cuttings (see Chapter 4) are also shown. 

MC 292

Near-Field Site

(500 m radius)

Site Center:

28° 42' 13.08"N

88° 35' 44.19"W

NF-1

NF-2 54 stations, 54 usable images

52 stations, 44 usable images

Possible Cuttings

(geophysical map)

Legend

Wellsite

Figure 7.4. Locations of sediment profile imaging (SPI) transects at Mississippi Canyon 

Block 292 near-field site on Cruise 2B (July 2001). Arrows indicate transect 

direction, and dots are sequentially numbered SPI stations. Geophysically mapped 

areas of possible cuttings (see Chapter 4) are also shown. 

219 
220 

Figure 7.5. Locations of sediment profile imaging (SPI) transects at Garden Banks Block 602 far-field

sites on Cruise 2B (July 2001). Arrows indicate transect direction, and dots are sequentially

numbered SPI stations.

Each far-field

site is 408 m in diameter

FF4-SPI

GB 602 Far-Field 4

Site Center:

27° 12' 47.23"N

92° 31' 24.95"W

FF5-SPI

GB 602 Far-Field 5

Site Center:

27° 15' 28.67"N

92° 28' 05.28"W

 36 stations

36 usable images

 36 stations

27 usable images

FF1-SPI

GB 602 Far-Field 1

Site Center:

27° 22' 39.25"N

92° 36' 27.09"W

 36 stations

28 usable images
Figure 7.6. Locations of sediment profile imaging (SPI) transects at Mississippi Canyon Block 292

far-field sites on Cruise 2B (July 2001). Arrows indicate transect direction, and dots are

sequentially numbered SPI stations.

Each far-field

site is 408 m in diameter

FF1-SPI

MC 292 Far-Field 1

Site Center:

28° 42' 15.01"N

88° 48' 13.39"W

FF4-SPI

MC 292 Far-Field 4

Site Center:

28° 47' 14.29"N

88° 39' 48.16"W

 36 stations

36 usable images

MC 292 Far-Field 5

Site Center:

28° 48' 10.82"N

88° 33' 52.92"W

FF5-SPI

 36 stations

36 usable images

 36 stations

36 usable images

221 
222 

Kodak Ektachrome® color slide film (ISO 200) was used throughout the survey. At the 

beginning of each survey day, the time on the camera's internal data logger was synchronized 

with the internal clock on the computerized navigation system being used to conduct the survey. 

A Benthos Model 2216 Deep Sea Pinger was attached to the camera frame and wired to the 

camera housing; when the strobe discharged, the ping rate doubled for 10 seconds. By 

monitoring the pinger signals through the use of a hydrophone on the research vessel, the scientist 

operating the camera was able to verify that a picture had been taken at each station. After the 

ship got into position at the start of a camera transect sampling line, the camera was lowered to 

the bottom and remained near the bottom until all the stations along the transect had been 

sampled. Each SPI replicate is identified by the time recorded on the film and on disk along with 

vessel position. Even though multiple images were taken at each location, each image was 

assigned a unique frame number by the data logger and cross-checked with the time stamp in the 

navigational system’s computer data file. Redundant sample logs were kept by the field crew. 

Test exposures of the Kodak® Color Separation Guide (Publication No. Q-13) were fired on deck 

at the beginning and end of each roll of film to verify that all internal electronic systems were 

working to design specifications and to provide a color standard against which final film 

emulsion can be checked for proper color balance. Charged spare batteries were carried in the 

field at all times to ensure uninterrupted sample acquisition. After deployment of the camera at 

each station, the frame counter also was checked to ensure that the requisite number of replicates 

had been taken at each location. In addition, a prism penetration depth indicator on the camera 

frame was checked to verify that the optical prism had actually penetrated the bottom to a 

sufficient depth to acquire a profile image. If images were missed (frame counter indicator), 

additional replicates were taken. 

7.2.3 Image Analysis 

Sediment profile images were analyzed visually by projecting the images and recording all 

features seen into a preformatted standardized spreadsheet file. The images were then digitized 

using a Nikon LS-2000 scanner and analyzed using Adobe® PhotoShop®, Image Pro®, and the 

public domain Image program (available through the National Technical Information Service). 

Steps in the computer analysis of each image were standardized and followed the basic 

procedures in Viles and Diaz (1991). Data from each image were sequentially saved to a 

spreadsheet file for later analysis. Details of how these data were obtained can be found in Diaz 

and Schaffner (1988) and Rhoads and Germano (1986). Two derived image parameters, 

estimated infaunal successional stage and the organism-sediment index (OSI) (see Rhoads and 

Germano 1986), were not calculated because of the uniformity of images and general lack of 

biogenic activity. A summary of major parameters measured follows. 

7.2.3.1 Prism Penetration 

This parameter provided a geotechnical estimate of sediment compaction with the profile camera 

prism acting as a dead weight penetrometer. The further the prism entered into the sediment, the 

softer the sediments, and presumably the higher the water content. Penetration was measured as 

the distance the sediment moved up the 23-cm length of the faceplate. The weight on the camera 

frame was kept at 90 kg, so prism penetration provided a means for assessing the relative 

compaction between stations. 
223 

7.2.3.2 Surface Relief 

Surface relief or boundary roughness was measured as the difference between the maximum and 

minimum distance the prism penetrated. This parameter also estimated small-scale bed 

roughness, on the order of the prism faceplate width (16.5 cm). The origin of bed roughness can 

be determined from visual analysis of the images. In physically dominated habitats, features such 

as bedforms and sediment granularity cause bed roughness. In biologically dominated habitats, 

bed roughness is a result of biogenic activity such as tube structures, defecation mounds, feeding 

pits, or epifaunal organisms such as hydroids. 

7.2.3.3 Apparent Color Redox Potential Discontinuity Layer 

This parameter is an important estimator of benthic habitat quality (Rhoads and Germano 1986; 

Diaz and Schaffner 1988; Nilsson and Rosenberg 2000), providing an estimate of the depth to 

which sediments appear to be oxidized. The term apparent is used in describing this parameter 

because no actual measurement was made of the redox potential in these samples. It is assumed 

that given the complexities of iron and sulfate reduction-oxidation chemistry, the reddish-brown 

sediment color tones (Diaz and Schaffner 1988; Rosenberg et al. 2001) indicate sediments are in 

an oxidative geochemical state, or at least are not intensely reducing. This is in accordance with 

the classical concept of RPD layer depth, which associates it with sediment color (Fenchel 1969; 

Vismann 1991). The apparent color RPD has been very useful in assessing the quality of a 

habitat for epifauna and infauna from both physical and biological points of view. Rhoads and 

Germano (1986), Bonsdorff et al. (1996), Nilsson and Rosenberg (2000), and Rosenberg et al. 

(2001) all found the depth of the RPD layer from sediment profile images to be directly correlated 

to the quality of the benthic habitat. These authors all found that deeper RPD layers were always 

associated with higher benthic habitat quality. 

7.2.3.4 Sediment Grain Size 

Grain size is an important parameter for determining the nature of the physical forces acting on a 

habitat and is a major factor in determining benthic community structure (Rhoads 1974). The 

sediment type descriptors used for image analysis follow the Wentworth classification as 

described in Folk (1974) and represent the major modal class for each image. Maximum grain 

size also was estimated. Grain size was determined by comparison of collected images with a set 

of standard images for which mean grain size had been determined in the laboratory. Table 7.1

provides phi scale sizes corresponding to sediment descriptors used in the current analysis. 
224 

Table 7.1. Phi scale sizes and sediment descriptors. 

Phi Scale Upper Limit 

Size (mm) 

Grains per cm of 

image 

SPI 

Descriptor 

Sediment Size Class 

and Subclass 

-6 to -8 256.0 <1 CB Cobble 

-2 to -6 64.0 <1 PB Pebble 

-1 to -2 4.0 2.5 GR Gravel 

1 to -1 2.0 5 CD Coarse sand 

2 to 1 0.5 20 MS Medium sand 

4 to 2 0.25 40 FS Fine sand 

4 to 3 0.12 80 VFS Very fine sand 

5 to 4 0.06 160 FSSI Fine sandy silt 

8 to 5 0.0039 >320 SI Silt 

6 to 5 0.0039 >320 SIFS Silty fine sand 

8 to 6 <0.0039 >320 CLSI Clayey silt 

>8 to 7 <0.0039 >320 SICL Silty clay 

>8 <0.0005 >2,560 CL Clay 

7.2.3.5 Surface Features 

These parameters included a wide variety of features (beds, biogenic mounds, shell, amphipod 

tubes, worm tubes). Each contributes information on the type of habitat and its quality for 

supporting benthic species. The presence of certain surface features is indicative of the overall 

nature of a habitat. For example, bedforms are always associated with physically dominated 

habitats, whereas the presence of worm tubes or feeding pits would be indicative of a more 

biologically accommodated habitat (Rhoads and Germano 1986; Diaz and Schaffner 1988). 

Surface features were visually evaluated from each image and compiled by type and frequency of 

occurrence. 

7.2.3.6 Subsurface Features 

Like surface features, these parameters included a wide variety of features and revealed a great 

deal about physical and biological processes influencing the bottom (infauna, burrows, 

water-filled voids, gas voids, sediment layering). For example, habitats with grain-size layers or 

homogeneous color layers are generally dominated by physical processes, while habitats with 

burrows, infaunal feeding voids, and/or visible infauna are generally dominated by biological 

processes (Rhoads and Germano 1986; Diaz and Schaffner 1988; Valente et al. 1992; Nilsson and 

Rosenberg 2000). Subsurface features were visually evaluated from each image and compiled by 

type and frequency of occurrence. 

7.2.3.7 Statistics 

For statistical analysis, each station on a transect was considered a replicate representing a 

sequential sampling of the given area. Analysis of variance or t-tests were used to test for 

differences between and within transects and areas. Normality was checked with the 

Shapiro-Wilk test and homogeneity of variance with Bartlett’s test. Data were square-root or log 
225 

(x + 1) transformed when necessary. Data that were not normally distributed were analyzed with 

Kruskal-Wallis test (Zar 1999). 

7.3 RESULTS 

7.3.1 Garden Banks Block 602 

All SPI data from GB 602 are contained in Appendix F1. 

7.3.1.1 GB 602 Near-field Transects 

The three near-field transects are shown in Figure 7.3. Transect NF-1 contained 36 usable (i.e., 

image quality that allowed analysis) SPI images obtained from the 38 stations sampled. All 

36 stations on Transect NF-2 had usable SPI images, and all 37 stations on Transect NF-3 had 

usable images. Stations along transects were spaced approximately at intervals of 30 to 50 m. 

7.3.1.2 GB 602 Far-field Transects 

The three far-field transects are shown in Figure 7.5. Transect FF1 had 36 stations with 

28 usable images, and all 36 stations from Transect FF4 had usable images, as did 27 of the 

36 stations from Transect FF5. 

7.3.1.3 GB 602 Near-field Sediment Conditions 

Sediment grain size at SPI stations along the GB 602 near-field transects was very uniform, with 

all stations being silty-clay (>8 to 7 phi). While there was little variation in sediment grain size, 

sediment texture did vary with depth below the sediment-water interface. Deeper sediment 

appeared to be more compacted than surface sediments, while the upper few centimeters of the 

sediment appeared to have higher porosity. Sediment fabric tended to be homogeneous within a 

layer, which was an indication that the layers have not been significantly reworked by biogenic 

processes. 

The most striking feature of the GB 602 near-field transects was the color layering that occurred 

along all three transects (Figure 7.7). Of the 109 near-field stations, only NF-1.13 and NF-2.04 

did not have color layering. At both of these stations, the prism penetration was shallow, 5.6 cm 

at NF-1.13 and 2.7 cm at NF-2.04, and no layers were detected. The sediment-water interface at 

both stations also appeared to be disturbed, which would have destroyed surface sedimentary 

layering. The average number of layers per image was significantly different between transects, 

(Kruskal-Wallis, df = 2, p = 0.013) with Transect NF-1 (mean of 3.5 layers/image, 0.25 SE) 

having more layers per image than Transects NF-2 and NF-3 (2.7±0.28 and 2.8±0.16, 

respectively). However, there did not appear to be any broad spatial pattern to the layering 

(Figure 7.7). Along Transect NF-2, the number of layers per image was lower in the area 

mapped as being possible cuttings (mean of 3.3 layers/image, 0.39 SE) relative to the rest of the 

transect (2.1±0.38), (t-test, df = 33, p = 0.039). 
Figure 7.7. Number and appearance of color sediment layers at stations along GB 602 near-field

transects. Dark sediment layers are likely due to deposition of cuttings and drilling mud. 

Arrows point to location of stations along transects. All example images are 15 cm wide.

NF1-29 NF1-22 NF1-16 NF1-32 NF1-04

NF1-38

NF1-02

NF-2

NF-1

NF-3

Number of 

Color Layers

1

7

NF2-01

NF2-12 NF2-03

NF2-16

NF2-24

NF2-36 NF2-31 Number of Color Layers

1

7

NF3-37 NF3-30 NF3-22 NF3-11 NF3-05 NF3-01

Number of 

Color Layers

1

5

NF1-10

500 m radius

Possible Cuttings 

(geophysical map)

Legend

Wellsite

226 
Sediments composing the layers were light to very dark in color. The darker gray to black colors 

could have been an indication of the presence of cuttings and drilling mud (also see Chapter 12). 

Even though the color of the layers at the near-field varied, there did not appear to be any 

variation in the sediment grain size between color layers. The coarsest sediment grains appeared 

to be silt (4 to 8 phi) and the finest clay (>8 phi). 

Many of the stations along the near-field transects had obvious signs of sediment oxygen 

depletion at the time of sampling. At the six stations from NF-2.31 to NF-2.36, there were 

indications of severe oxygen depletion, with the possible presence of bacterial mats on the 

sediment surface. In all cases, the whitish microbial mat on the sediment surface overlaid dark 

gray sediment. Under normal conditions and with an unimpacted benthic community, the depth 

of the RPD layer in these types of muddy sediments should have been >0.5 cm. However, 83% 

of the near-field stations had RPD depths of 0.5 cm or less (Figure 7.8). By comparison, on 

far-field transects, only 9% of the stations had RPD depths of 0.5 cm or less and there was no 

evidence of severe oxygen depletion. 

0

10

20

30

40

50

60

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Apparent Color RPD Depth (cm)

Number of Stations

NF FF

Figure 7.8. Histogram showing the distribution of apparent color redox potential discontinuity 

(RPD) layer depth for Garden Banks Block 602 near-field (NF) and far-field (FF) 

transects. Values for RPD are midpoint of intervals in centimeters. 

227 
228 

There were no signs of a well developed, deep dwelling, bioturbating infauna at any of the 

near-field stations. There was also no indication of the presence of epifaunal organisms. Small 

tubes, <1 mm diameter, at the sediment-water interface were the most common biogenic structure 

and were present at 51% (56 of 109) of the stations along the near-field transects. No burrow 

structures or active feeding voids were observed at any station in the near-field or far-field, but 

small infaunal worms occurred at a total of six stations (NF-2.11, NF-2.23, NF-2.29, NF-3.08, 

NF-3.11, and NF-3.15). 

Overall, sediment color, fabric, and microstratigraphy at stations taken along all three near-field 

transects (NF-1, NF-2, and NF-3) were similar, suggesting that drilling muds and cuttings were 

widespread and present at all sampled stations. 

7.3.1.4 GB 602 Far-field Sediment Conditions 

Sediment grain size at SPI stations along the GB 602 far-field transects also was very uniform, 

with all stations being silty-clay (>8 to 7 phi). Overall, the stations along the far-field transects 

appeared to have a higher clay content relative to the near-field transects. Based on the sediment 

grain-size analysis from box core samples, the average grain-size for GB 602 far-field stations 

was 9.8 phi and 8.3 phi for near-field stations (see Chapter 5). Sediment fabric of the far-field 

station was also homogeneous, with little evidence of the presence of a bioturbating infauna. 

Even though layered, surface sediments appeared to be just as compact as deeper sediments. 

Relative to the near-field, the most striking feature of the GB 602 far-field transects was the lack 

of dark color layering. Unlike the near-field transects, which had prominent dark color layering, 

sediments on the far-field transects were more uniform in color. Layers that did occur were 

various tones of reddish-brown along all three transects and not the dark gray layer that was 

interpreted as indicative of cuttings or drilling muds on the near-field transects (Figure 7.9). 

Of the 91 far-field stations, 79% (71 stations) had some evidence of color layering. For 

comparison, 98% of the near-field stations had color layering. Transect FF1, located about 15 km 

west of the near-field site, had the least number of layers, with only one layer at 19 stations. At 

the other nine stations on Transect FF1, no layers were detected. The average number of layers 

per image was significantly different between transects (Kruskal-Wallis, df = 2, p = <0.0001), 

with Transect FF1 (mean of 0.7 layers/image, 0.09 SE) having fewer layers per image than 

Transect FF5 (2.4±0.16), which in turn had fewer layers than Transect FF4 (1.3±0.21). Transect 

FF5 was about 13 km south, and Transect FF4 was about 19 km south of the near-field site. 

Within a far-field transect, there did not appear to be any spatial pattern to the layering. 
FF1.35 

FF1.06 

FF4.36 FF4.01 

FF5.01 

FF5.36 

Figure 7.9. Appearance of color sediment layers at stations along Garden Banks Block 602 

far-field transects. Transects are not in original orientation; see Figure 7.5. Arrows 

point to location of stations along transects. All example images are 15 cm wide. 

229 
230 

At the first four stations (FF4.1, FF4.6, FF4.7, and FF4.8) along the eastern end of Transect FF4, 

there appeared to be some variation in the sediment grain size between color layers. Surface 

sediment layers appeared to be siltier than deeper sediment layers, with the deeper layers having 

higher clay content. The deeper sediments also were bluer-gray in color at these four stations 

(Figure 7.9). Sediments along Transect FF1 were similar between stations, being uniformly light 

reddish-brown. Surface sediments along Transect FF5 were similar in color to the other two 

transects, but deeper sediments tended to be darker brown. There was no dark gray sediment at 

the far-field sites. 

The color layers at the far-field transects tended to be thicker than the near-field transects. Layers 

at Transects FF1 and FF5 were thicker than all other transects (Kruskal-Wallis, df = 5, 

p = <0.0001), and Transect NF-1 had significantly thinner layers than all other transects 

(Figure 7.10). The larger number of thinner layers in the near-field may be an indication of 

multiple disturbance events associated with oil and gas development activities. Thicker and less 

color differences between layers in the far-field may be more representative of natural 

sedimentary conditions. 

There were few indications of the presence of infaunal organisms and no epifauna observed at 

far-field stations. Infaunal organisms were observed at only four far-field stations (FF1.19, 

FF1.21, FF4.07, and FF4.10). Tubes at the sediment-water interface were the most common 

biogenic structure and were present at 48% (44 of 91) of the stations along the far-field transects. 

The highest number of tubes per image was 10 at FF1.19. The tubes were all small, with 

diameters of about 1 mm. There did not appear to be a well developed infaunal community along 

any of the far-field transects. 

In contrast to the near-field transects, all far-field stations appeared to have oxidized sediments at 

the sediment-water interface. Overall, near-field RPD layers were shallower than far-field 

(Figure 7.11). The shallowness of the RPD layer (<0.5 cm) at many stations indicated that 

bioturbation was not an important factor in mixing surface sediment. The RPD along Transect 

FF5 averaged 0.6 cm (0.03 SE) and averaged 0.8 cm (0.03) along both Transects FF4 and FF5. 

Even though the difference between the transects was small, Transect FF5 had thinner RPD 

layers (analysis of variance [ANOVA], df = 2, p = <0.0001) compared to Transects FF1 and FF4 

(Figure 7.11). 

Overall, sediment color, fabric, and microstratigraphy at stations taken along all three far-field 

transects (FF1, FF4, and FF5) were similar, with slightly more variation in grain size and less 

variation in layer color than near-field transects. 
Average Thickness of Color Layer (cm)

0

1

2

3

4

5

6

7

8

9

10

11

12

FF1 FF4 FF5 NF1 NF2

231 

NF3

GB602 Sites

NF-1 NF-2 NF-3 

Figure 7.10. Average thickness of color layers (cm) at Garden Banks Block 602 sites. Box is 

interquartile range, tails are range, line in box is median, and line extended from 

box is mean, and width of box is proportional to sample size. 

RPD Layer Depth (cm)

0

0.2

0.4

0.6

0.8

1

1.2

FF1 FF4 FF5 NF1 NF2 NF3

Transect

Figure 7.11. Average depth of apparent color redox potential discontinuity (RPD) layer (cm) at 

Garden Banks Block 602 far-field (FF) and near-field (NF) transects. Box is 

interquartile range, tails are range, line in box is median, line extended from box is 

mean, horizontal line is grand mean, and width of box is proportional to sample 

size. 
232 

7.3.2 Mississippi Canyon Block 292 

All SPI data from MC 292 are contained in Appendix F2. 

7.3.2.1 MC 292 Near-field Transects 

The two near-field transects are shown in Figure 7.4. Usable images were obtained from all 

54 stations on Transect NF-1 and 44 of 52 stations on Transect NF-2. Toward the end of 

Transect NF-2, the SPI camera ran out of film, so images were not obtained from the last eight 

stations. Stations along transects were spaced at intervals of approximately 30 to 50 m. 

7.3.2.2 MC 292 Far-field Transects 

The three far-field transects are shown in Figure 7.6. All three transects had 36 stations with 

36 usable images. 

7.3.2.3 MC 292 Near-field Sediment Conditions 

Sediment grain size at SPI stations along the MC 292 near-field transects was rather uniform, 

with 16 stations being silty-clay (>8 to 7 phi) and 82 being clay (>8 phi). The slightly siltier 

sediments tended to occur in patches along the transects. On Transect NF-1, the 10 stations from 

NF-1.33 to NF-1.42 were silty-clay as were the 2 stations at the eastern end of Transect NF-1. 

Along Transect NF-2, silty-clay occurred in two small patches, Stations NF-2.25 and NF-2.26, 

and NF-2.32 and NF-2.33. The texture of the silty-clay sediments was more variable with depth 

below the sediment-water interface. Deeper sediment appeared to be more compacted than 

surface sediments, while the upper few centimeters of the sediment appeared to have higher 

porosity. The texture of clay sediments was more uniform. Sediment fabric tended to be 

homogeneous for both sediment types, which was an indication that biogenic processes were not 

a major factor in reworking the sediments. 

The most striking feature of the MC 292 near-field transects was the uniformity of the color 

layering that occurred along both transects (Figure 7.12). The only variation in the color of the 

layers was associated with the stations that had silty-clay sediments. At the 16 stations that were 

silty-clay, layering tended to be darker gray. All of the other 82 near-field stations had reddish 

brown color layering. (At Station NF-2.25, the sediment-water interface was disturbed and no 

layer could be measured). 

There was little variation in the number of layers per image, with most of the near-field stations 

having one. All stations on Transect NF-2 and all but 12 stations on Transect NF-1 had one layer. 

The 12 stations with 2 layers were also those with silty-clay sediments and concentrated from the 

midpoint to the eastern end of Transect NF-1 (Figure 7.12). 

Sediments composing the layers were reddish brown and darker in hue than underlying 

sediments, which were light grayish. The darker gray to black colored sediments along 

Transect NF-1 could be an indication of the presence of cuttings and drilling mud (also see 

Chapter 12). While the clay content of sediments appeared to be very high at all stations, those 

stations with darker gray layers appeared to be slightly coarser (siltier). 
Figure 7.12. Appearance of color sediment layers at stations along Mississippi Canyon Block 292 

near-field transects. Dark sediment layers are likely due to deposition of cuttings and 

drilling mud. Arrows point to location of stations along transects. All example images 

are 15 cm wide.

500 m radius

NF-1

NF-2

NF2-01

NF2-10

NF2-21

NF2-26 NF2-

30

NF2-33 NF2-37

NF2-45

NF1-01

NF1-54

NF1-52

NF1-43

NF1-40

NF2-05 NF1-25 NF1-33

NF1-08

NF1-47

NF1-42

NF1-36

NF1-30

NF2-42

NF2-32

NF2-16

Possible Cuttings 

(geophysical map)

Legend

Wellsite

233
The distribution of the apparent color RPD layer depth suggests that sediment oxygen levels may 

have been low at some of the silty-clay stations along Transect NF-1 (Stations NF-1.34, NF-1.35, 

NF-1.36, NF-1.38, NF-1.39, and NF-1.40). None of the stations along near-field Transect NF-2 

or any of the far-field transects had obvious signs of oxygen depletion at the time of sampling. 

Under normal conditions and with an unimpacted benthic community, the depth of the RPD layer 

in these types of muddy sediments should have been >0.5 cm (Figure 7.13). The distribution of 

RPD layer depths between near-field and far-field transects was similar, except for the shallowest 

values occurring along Transect NF-1 (Figures 7.12 and 7.13). While some evidence of infaunal 

activity (infauna, burrows, tubes, and feeding voids) was observed at 82% (80 of 98) of the 

near-field stations, there did not appear to be a well developed bioturbating benthic community. 

The clayey sediments did not appear to be biogenically mixed. There were signs of deep 

dwelling infauna at 21% (21 of 98) of the near-field stations, with infauna occurring from 0.3 to 

13.2 cm below the sediment-water interface. There was no indication of the presence of 

epifaunal organisms on the sediment surface at any near-field station. Small tubes, <1 mm 

diameter, at the sediment-water interface were the most common biogenic structure and were 

present at 70% of the stations along the near-field transects. Burrow structures were observed at 

22% of near-field stations. The largest burrow at the MC 292 site was observed at 

Station NF-1.08 (Figure 7.14). Only Station NF-2.07 appeared to have an active feeding void. 

Figure 7.13. Histogram showing the distribution of apparent color redox potential discontinuity 

(RPD) layer depth for Mississippi Canyon Block 292 near-field (NF) and far-field 

(FF) transects. Values for RPD are mid-point of intervals in centimeters. 

234 
Figure 7.14. Sediment profile imaging image from Mississippi Canyon Block 292, 

Station NF-1.08, showing a large burrow structure near the center and extending 

down from sediment-water interface to bottom of image. Image is 15 cm wide. 

Overall, sediment color, fabric, and microstratigraphy at stations taken along all two near-field 

transects (NF-1 and NF-2) could be divided into two classes that may be related to the 

presence/absence of drilling muds and cuttings. The largest class of stations (80 of 98) had 

reddish-brown surficial sediments overlaying light grayish clayey sediments. The smaller class 

of stations (18 of 98) had darker gray sediments, either at or near the sediment surface, which 

may have been due to the presence of drilling muds and cuttings. The difference in these two 

classes can be seen in Figure 7.12. 

7.3.2.4 MC 292 Far-field Sediment Conditions 

Sediment grain size and texture at SPI stations along the MC 292 far-field transects was very 

uniform, with all stations being clay (>8 phi) and having a consistent texture with depth from the 

235 
236 

sediment surface. Sediment fabric was also homogeneous with depth from the sediment surface, 

indicating that the level of biogenic activity was not sufficient to significantly rework and mix the 

sediments. 

Overall, the stations along the far-field transects appeared to have a slightly higher clay content 

relative to some of the stations on near-field transects. Based on the sediment grain size analysis 

from box core samples, the average grain size for MC 292 was 9.9 phi for far-field stations and 

9.6 phi for near-field stations (see Chapter 5). Sediment fabric of the far-field stations was also 

homogeneous, with little evidence of the presence of bioturbating infauna. 

Relative to the near-field, the most striking feature of the MC 292 far-field transects was the lack 

of dark color layering. Unlike the near-field transects, which did have stations with prominent 

dark color layering, sediments on the far-field transects were uniform in color. Layering that did 

occur was various tones of reddish-brown along all three transects and not the darker gray 

layering that was interpreted as indicative of cuttings or drilling muds along the near-field 

transects (Figure 7.15). While all of the 108 far-field stations had evidence of color layering with 

all but 1 station having one layer (the exception was FF1.10, which had 2 layers), there was little 

variation in sediments between transects (Figure 7.15). 

The clayey sediment station in the near-field and far-field appeared to be similar in color, texture, 

and fabric (Figures 7.12 and 7.15). Within and between far-field transects, there did not appear 

to be any spatial pattern relative to sediment properties. Thus, sediments over the entire MC 292 

study area may be similar, with the exception of the silty-clay near-field stations, which may have 

drilling muds and cuttings present. Far-field Transect FF1 was located about 20 km west of the 

MC 292 near-field site, Transect FF4 about 11 km northwest, and Transect FF5 about 11 km to 

the north. 

The thickness of color layers at near-field and far-field transects was different for only 

Transect NF-2, which had thicker color layers than all other transects (Kruskal-Wallis, df = 4, 

p = <0.0001). Layers at far-field transects tended to have a narrower range than near-field 

transects (Figure 7.16). This difference between near- and far-field for MC 292 is reversed from 

GB 602, where far-field layers were thicker. This may be a reflection of the fact that most of the 

MC 292 near-field stations did not appear to be impacted by drilling mud and cuttings. 

There was evidence of infaunal activity (infauna, burrows, tubes, and feeding voids) observed at 

76% (82 of 108) of the far-field stations, but there did not appear to be a well developed 

bioturbating benthic community. The clayey sediments did not appear to be biogenically mixed. 

There were signs of deep dwelling infauna at 31% (33 of 108) of the far-field stations, with 

infauna occurring from 1.1 to 14.2 cm below the sediment-water interface. There was only one 

visible epifaunal organism on the sediment surface at Station FF1.16. This organism appeared to 

be a small hexacoral (Figure 7.17a). Small tubes, <1 mm diameter, at the sediment-water 

interface were the most common biogenic structure and were present at 61% of the stations along 

the far-field transects (Figure 7.17b). Burrow structures were observed at 15% of far-field 

stations. Only three stations (FF1.21, FF1.25, and FF5.25) appeared to have an active feeding 

void. The largest void was observed at Station FF5.25 (Figure 7.17b). 
Figure 7.15. Appearance of color sediment layers at stations along Mississippi Canyon 

Block 292 far-field transects. Transects are not in original orientation 

 (see Figure 7.6). Arrows point to location of stations along transects. All example 

images are 15 cm wide. 

237 
Average Thickness of Color Layer (cm)

238 

Figure 7.16. Average thickness of color layers (cm) at Mississippi Canyon Block 292 sites. Box 

is interquartile range, tails are range, line in box is median, line extended from box 

is mean, and width of box is proportional to sample size. 

Figure 7.17. Fauna from Mississippi Canyon Block 292. a: Hexacoral (center of image) at 

Station FF1.16. Image is 5 cm wide. b: Small tubes at sediment-water interface 

and large oxic feeding void at Station FF5.25. Image is 15 cm wide. 

0

2

4

6

8

10

12

14

FF1 FF4 FF5 NF-1 NF-2

Transect

(a) (b)
239 

RPD Layer Depth (cm)

In contrast to the near-field transects, all far-field stations appeared to have oxidized sediments at 

the sediment-water interface. The thickness of the RPD layers at far-field stations indicated that 

bioturbation was an important factor in mixing surface sediment (Figure 7.13). The RPD layers 

along Transect FF5 were deeper than at Transects FF1 and FF4 and averaged 1.6 cm (0.09 SE), 

(ANOVA, df = 2, p = <0.0001). Along Transect FF4, the RPD layer depth averaged 1.0 cm 

(0.10) and 0.7 cm (0.10) along Transect FF1 (Figure 7.18). RPD layer depths at the near-field 

were not significantly different than far-field transects (Figure 7.18). 

Figure 7.18. Average depth of apparent color redox potential discontinuity (RPD) layer (cm) at 

Mississippi Canyon Block 292 far-field (FF) and near-field (NF) transects. Box is 

interquartile range, tails are range, line in box is median, line extended from box is 

mean, horizontal line is grand mean, and width of box is proportional to sample 

size. 

Overall, sediment color, fabric, and microstratigraphy at stations taken along all three far-field 

transects (FF1, FF4, and FF5) were similar, with no apparent variation in grain size or in layer 

color. 

7.4 DISCUSSION 

7.4.1 Garden Banks Block 602 

The sediment type at GB 602 stations was uniform, with all near-field and far-field stations being 

silty-clay (>8 to 7 phi), but the appearance of near-field and far-field stations was different in 

terms of the color and number of sedimentary layers (Figures 7.7 and 7.8). Near-field layers 

were dark gray in color, which was consistent with the presence of drilling muds and cuttings (see 

0

0.5

1

1.5

2

2.5

3

FF1 FF4 FF5 NF-1 NF-2

Transect
240 

Chapters 4 and 8). Layers at the far-field stations were reddish-brown and appeared to be similar 

to surficial sedimentary layers in other silty-clay deep-sea habitats (Diaz et al. 1994). 

Sediment oxygen levels may have been low at 83% of the near-field stations at the time of 

sampling. The six stations from NF-2.31 to NF-2.36 appeared to be affected by severe oxygen 

depletion, with the possible presence of bacterial mats on the sediment surface. At the far-field 

transects, only 9% of the stations appeared to be hypoxic, with no evidence of severe oxygen 

depletion. 

There were no signs of a well developed, deep dwelling, bioturbating infauna at any of the 

near-field stations. Infauna and biogenic structures were more common at far-field stations but 

were still not sufficient to bioturbate sediments. Sediment fabric tended to be homogeneous 

within the color layers, which also was an indication that the layers have not been significantly 

reworked by biogenic processes. 

7.4.2 Mississippi Canyon Block 292 

The sediment type at MC 292 near-field stations varied over a narrow range of silty-clay (>8 to 

7 phi) to clay (>8 phi), with most stations being clay. All far-field stations were clay. The 

appearance of all near-field and far-field stations was similar, with the exception of 16 near-field 

silty-clay stations, which had darker gray colored layers (Figures 7.12 and 7.15). Layers at all 

other MC 292 stations were reddish-brown and appeared to be similar to surficial sedimentary 

layers in other silty-clay deep-sea habitats (Diaz et al. 1994). 

The darker gray to black colored sediments at some of the near-field stations could indicate the 

presence of cuttings and drilling mud. There was little variation in the number of layers per 

image, with most of the near-field and all of the far-field stations having one. Stations with two 

layers were also those with silty-clay sediments. Based on the apparent color RPD layer depth, 

bottom water may have been hypoxic at some of the silty-clay near-field stations. None of the 

other near-field or far-field stations had obvious signs of oxygen depletion at the time of 

sampling. 

While evidence of infaunal activity (infauna, burrows, tubes, feeding voids) was observed at 82% 

of near-field and 76% of far-field stations, there did not appear to be a well developed 

bioturbating benthic community. The clayey sediments did not appear to be biogenically mixed 

as the sediment fabric was homogeneous with depth from the sediment surface, indicating that the 

level of biogenic activity was not sufficient to significantly rework and mix the sediments. 

7.5 CONCLUSIONS 

7.5.1 Garden Banks Block 602 

All the near-field stations at GB 602 appeared to be impacted from oil and gas development 

activities. Sediment color, fabric, and microstratigraphy at near-field stations were all similar and 

consistent with the presence of drilling muds and cuttings. At far-field stations, sediment color, 

fabric, and microstratigraphy were similar, with slightly more variation in grain size and 

reddish-brown layers consistent with background sedimentary conditions. While it appeared that 
241 

oil and gas development impacts did extend outside the boundary of the GB 602 near-field site, 

impacts did not extend out to the far-field sites. 

7.5.2 Mississippi Canyon Block 292 

At MC 292, sediment color, fabric, and microstratigraphy at near-field stations could be divided 

into two classes that may be related to the presence/absence of drilling muds and cuttings. The 

largest class, 82% of stations, had reddish-brown surficial sediments overlaying light grayish 

clayey sediments. The smaller class, 18% of stations, had darker gray sediments, either at or near 

the sediment surface, which are interpreted as indicating the presence of drilling muds and 

cuttings. The difference in these two classes can be seen in Figure 7.12. At all far-field stations, 

sediment color, fabric, and microstratigraphy were similar, with no apparent variation in grain 

size or in layer color consistent with background sedimentary conditions. It appeared that oil and 

gas development impacts did not extend to the far-field sites. 
Chapter 8 

Metals, Total Organic Carbon, and 

Redox Conditions in Sediments 

John H. Trefry, Robert P. Trocine, 

Robert D. Rember and Michelle L. McElvaine 

Florida Institute of Technology 

8.1 INTRODUCTION 

The chemistry study described in this chapter was designed to help identify spatial and temporal 

trends in (1) the presence of drilling discharges and (2) redox conditions in sediments at each site. 

The supporting data set is quite diverse and includes the following: (1) data for 13 metals: 

aluminum (Al), arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), 

mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), vanadium (V), and zinc (Zn), and total 

organic carbon (TOC) in sediments, (2) data for 11 metals in biota (As, Ba, Cd, Cr, Cu, Fe, Hg, 

Ni, Pb, V, and Zn), (3) sedimentation rates, (4) vertical profiles for dissolved oxygen and redox 

potential (Eh) in sediment, and (5) data for selected redox-active chemicals in interstitial water. 

The data set is summarized by site and cruise for each sample type in Table 8.1. The complete 

data set is tabulated by cruise in Appendix G. 

Concentrations of metals and TOC in sediments provide information about sediment composition, 

redox state, and the presence of drilling discharges or other anthropogenic inputs. Aluminum is a 

good indicator of the relative abundance of clay minerals and other aluminosilicates. The Al 

content of suspended sediment introduced to the Gulf of Mexico by the Mississippi River 

averages ~8.6% (Trefry and Presley 1982). Concentrations of Ba help identify the presence of 

drilling discharges because barite (BaSO4) is a common component of drilling fluids. Elevated 

levels of Ba in sediments, in the absence of SBFs, help support the presence of water-based 

drilling mud. Concentrations of Fe generally correlate well with concentrations of Al and support 

identification of the terrigenous fraction of sediment. Manganese is a redox-sensitive metal that 

can be depleted in nearshore, reducing sediments relative to Mississippi River suspended 

sediment, and greatly enriched in slowly accumulating and oxidizing slope sediments relative to 

river suspended sediments (Trefry and Presley 1982). Concentrations of TOC vary with sediment 

accumulation rate and biological productivity in the overlying water column. The rate of 

accumulation of organic carbon plays a role in controlling the redox state of the sediment. The 

other nine metals (As, Cd, Cr, Cu, Hg, Ni, Pb, V, and Zn) are naturally occurring in sediments 

from the Gulf of Mexico, with any enrichment due to a variety of possible sources. For example, 

concentrations of Hg may be elevated in sediments around drilling sites due to inputs of BaSO4, 

in which Hg is a natural impurity (Trefry et al. 2003). Concentrations of Pb and Zn can be 

elevated from a variety of sources including pipe dope used during drilling operations. 

Both surface samples (0 to 2 cm) and sediment cores (0 to 10 cm) were collected for this study. 

The cores provide information on the thickness of layers containing Ba or SBF. A few 

representative cores were age-dated using 137Cs and excess 210Pb techniques. 

243 
Table 8.1. Number of samples collected and analyzed for various parameters at each site. 

Site Cruise 

Surface 

Sediment 

Metals, TOC 

Sediment 

Cores 

Metals, TOC 

Probe O2

and Eh 

Profiles 

Metals 

in Biota (S) or (IW) 

VK 

916 

1B 

Pre-exploration 

NF 12 

FF 10 

--- NF 12 

FF 10 

--- 2 S (NF, FF) 

2 IW (NF, FF) 

3B 

Post-exploration 

NF 12 

FF 12 

3 NF cores 

(n = 15) 

NF 13 

 FF 8 

--- 2 IW (NF, FF) 

GB 

516 

1B 

Post-exploration 

NF 12 

FF 12 

--- NF 12 

FF 11 

--- 1 S (FF) 

2 IW (NF, FF) 

2B 

Post-development 

NF 12 

FF 12 

3 NF cores 

(n = 15) 

NF 13 

FF 12 

--- 1 IW (FF) 

GB 

602 

2B 

Post-development 

NF 12 

FF 12 

3 NF cores 

(n = 15) 

NF 15 

FF 12 

NF 12 

FF 6 

1 S (FF) 

2 IW (NF, FF) 

MC 

292 

2B 

Post-development 

NF 12 

FF 12 

3 NF cores 

(n = 15) 

NF 15 

FF 12 

NF 12 

FF 12 

1 S (FF) 

2 IW (NF, FF) 

FF = far-field; GB = Garden Banks; IW = interstitial water; MC = Mississippi Canyon; NF = near-field; 

S = sedimentation rate; TOC = total organic carbon; VK = Viosca Knoll. 

Vertical profiles for concentrations of dissolved oxygen in sediment and the overall redox state 

(Eh) of the sediment column were obtained at most sites (Table 8.1). Concentrations of 

dissolved oxygen and Eh are primarily controlled by the input rate of organic matter as influenced 

by the following factors: (1) biological productivity in the surface water and benthic environment, 

(2) the rate at which terrestrial organic matter is added via rivers, and/or (3) introduction of 

organic matter from discharges of various municipal and industrial waste materials. Changes in 

redox environment at the sediment-water interface and in the upper layers of the sediment column 

can play a role in controlling the spatial distribution of benthic fauna. Synthetic-based drilling 

fluids contain hydrocarbons (olefins) that can be metabolized by sediment bacteria. The presence 

of this additional source of sediment organic matter may alter the background redox environment. 

In addition to probe measurements, more complete redox information was obtained at a few 

representative sites where interstitial water was collected and analyzed for dissolved Mn2+, Fe2+, 

ammonium (NH4

+

), nitrate (NO3

-

), sulfate (SO4

2-), phosphate (PO43-), and alkalinity. Each of 

these parameters provides additional information about the redox environment in the sediments. 

In this chapter, the results are presented by drilling site in the following order: (1) VK 916 for 

pre- and post-exploratory drilling, (2) GB 516 for exploratory/development drilling, and 

(3) GB 602 and MC 292 for the post-development phase. In each case, results for metals and 

TOC are presented first. This information helps to define the sediment composition at random 

near-field (near-field at <1,000 m from the drilling site) and random far-field (far-field at >3 km 

from drilling site) locations. In some instances, sediment cores were collected at additional, fixed 

near-field stations, which are denoted as discretionary (DS) stations. Then, results for vertical 

profiles for dissolved oxygen and Eh (redox potential) in sediment cores (and pH in some cores) 

are introduced along with the smaller data set for sediment interstitial water. The collective data 

will be used to test for significant differences in sediment composition and properties (1) between 

near-field and far-field stations, and/or (2) between pre- and post-development as appropriate. 

244 
Samples of biota were collected only from the post-development sites (GB 602 and MC 292) and 

will be discussed along with other parameters for those sites. Finally, an overview section on 

trace metals is presented at the end of the chapter to augment information introduced in each 

separate section and to provide a study-wide perspective on possible metal contamination at 

drilling sites. 

8.2 METHODS 

8.2.1 Sampling and Field Measurements 

Sediments were collected using a stainless steel box corer (50 cm x 50 cm x 50 cm) as described 

in Chapter 2. For metals, the surface 2 cm of sediment were removed using a Teflon® spatula 

and placed in a plastic vial. The vial was sealed with a layer of Parafilm® and stored frozen. At 

three discretionary stations from each site, the box core was carefully subsampled by pushing a 

10-cm diameter Teflon® tube into the sediment. The sediment was extruded from the top, and 

2-cm sections were sliced off, placed in plastic vials, and stored frozen. 

One subsample from each box core was collected for probe measurements of dissolved oxygen 

and Eh. Values for pH were determined for selected cores as time permitted. Each core was 

immediately analyzed for oxygen using a 5-cm long microprobe (Microelectrodes, Inc. MI-730 

O2 probe) lowered from the top of the core (Figure 8.1). The probe was mounted on a 

microscope stage that was fixed vertically above the core. By lowering the microscope stage in 

millimeter increments, according to the attached scale, oxygen measurements were taken to the 

depth of oxygen depletion. Next, Eh and pH were measured through holes pre-drilled at 2-cm 

intervals in the wall of the core tube (Figure 8.1). The holes were covered with tape during 

sampling, and the tape was removed immediately before inserting a probe. The Eh was measured 

first (Orion Model 96-78-00 Platinum Redox Electrode), and pH and temperature measurements 

followed (Sentron Red Line pH probe). 

All probes were calibrated prior to use for each core. The oxygen probe was calibrated using two 

beakers of deionized water: one that was equilibrated with the atmosphere using an air stone and 

one that was deoxygenated by purging with nitrogen. The oxygen meter was adjusted to 20.9% 

while the probe was immersed in the first solution and was zeroed with the probe submerged in 

deoxygenated water. A one point calibration with an Orion redox standard was performed for the 

Eh probe. The pH probe was calibrated with pH 7 and pH 10 buffers. 

245 
246 

 

 

Figure 8.1. Photographs showing (a) oxygen probe inserted in top of sediment core and close-up 

of electrode and electronic control box, (b) redox potential (Eh) electrode inserted in 

sediment core, and (c) whole-core squeezer. 

Eh electrode

(a) 

(b) 

(c) 

O2 electrode

Squeezer 
8.2.2 Interstitial Water Collection and Analysis 

To obtain interstitial water, one 7-cm diameter subcore was transferred to a whole core squeezer, 

similar to that described by Bender et al. (1987) for interstitial water extraction (Figure 8.1). By 

raising the lower piston to apply pressure to the core, 16 interstitial water samples were 

simultaneously collected from depths of 0 to 34 cm. Samples were collected directly into 

acid-washed glass syringes at 1-cm intervals in the uppermost 5 cm, 2-cm intervals from 5 to 

13 cm, and 3-cm intervals below 13 cm. At the completion of interstitial water collection, 

samples were filtered through Type A/E glass fiber filters and divided for analysis. A 1-ml 

sample was diluted five-fold and analyzed for phosphate using the ascorbic acid method (Clesceri 

et al. 1989). A 0.5-ml sample was diluted four-fold and analyzed for ammonia using the 

oxidation method (Matsunaga and Nishimura 1974). All colorimetric analyses performed aboard 

ship were carried out within 24 hours of sample collection using a Spectronic Instruments Spec 

20 Genysis spectrometer. The remaining interstitial water was divided into three portions 

(frozen, acidified, and unaltered) for further analysis at Florida Institute of Technology (FIT). 

The frozen portion (≥3 ml) was used to determine concentrations of nitrate and sulfate using a 

Dionex DX-600 ion chromatograph (IC). Before injection into the system, samples and standards 

were passed through Dionex OnGuard II® Ag cartridges to remove chloride. Standards were 

prepared from mixtures of standard seawater (from the International Association for the Physical 

Sciences of the Ocean, IAPSO) and Dionex Five Anion Standard. The IC was operated under the 

following conditions: 9 mM Na2CO3 eluent, 1.00 ml/min flow rate, Dionex IonPac® AS9-HC 

4-mm column, 100-mA suppressor current, and 35°C oven temperature. 

The acidified portion (1.5 to 2 ml) was acidified immediately upon collection with 20 μL of 6 N 

hydrochloric acid (HCl). It was later analyzed for Fe and Mn by atomic absorption spectrometry 

(AAS) using a Perkin-Elmer Model 4000 instrument. To eliminate the effects of dissolved salts, 

samples were diluted by a factor of four before analysis. Standards and blanks were prepared in 

1:4 seawater: deionized water (DIW). The concentrations of Fe and Mn measured in the 

interstitial water represent the dissolved fraction and will be referred to as Fe2+ and Mn2+

concentrations. 

The unaltered interstitial water sample was used for chlorinity and alkalinity analysis. Chlorinity 

was determined by Mohr titration whereby a 0.5-ml sample was added to 20 ml DIW and titrated 

with ~0.04 N AgNO3. Standard seawater (IAPSO) was titrated and used to standardize the 

analysis. Alkalinity was determined by titrating a 2-ml sample with 0.01 N HCl until the pH 

reached 4.00 or lower. The pH probe used to monitor the titration was first calibrated with pH 4 

and pH 7 buffers. The volume of acid and final pH were used to calculate the alkalinity using an 

equation modified from Strickland and Parsons (1972). 

8.2.3 Laboratory Analysis 

8.2.3.1 Metals in Sediments 

After being transported to FIT, sediment samples were initially brought to room temperature, and 

then each wet sediment sample was homogenized in the original 75-ml plastic vial using a 

Teflon® mixing rod. A portion (~2 g) of each sample was then transferred into a preweighed 

247 
plastic vial to determine water content. Once transferred, the wet sediment and the vial were 

reweighed. In addition, 2 to 4 g of sediment were transferred into polypropylene-copolymer 

centrifuge tubes to be used for the determination of sediment Hg content. Samples intended for 

determination of water content were frozen, freeze dried, and reweighed to determine the water 

content. The dried sediment samples were again homogenized using a Teflon® mixing rod. 

For analysis of total Al, As, Ba, Cd, Cr, Fe, Mn, Ni, Pb, V, and Zn, about 0.4 g of sediment were 

digested in Teflon® beakers with HClO4-HNO3-HF and diluted to 20 ml (Trefry and Metz 1984). 

Total digestion of the sediments is preferred as a starting point in environmental evaluations 

because then no doubt remains about the absolute amount of metal associated with a sample. In 

the digestion process, 1 ml HClO4, 1 ml HNO3, and 3 ml HF were added to the sediment in the 

Teflon® beaker and heated at 50°C with a Teflon® watch cover in place until a moist paste 

formed. The mixture was heated for another 3 hours at 80°C with an additional 2 ml HNO3 and 

3 ml HF before bringing the sample to dryness. Finally, 1 ml HNO3 and about 30 ml of DIW 

were added to the sample and heated strongly to dissolve perchlorate salts and reduce the volume. 

The completely dissolved and clear samples were diluted to 20 ml with DIW. This technique is 

100% efficient with no loss of the elements studied and has been used successfully in the 

laboratory at FIT for many years with a variety of sediment types. For samples determined to 

have very high concentrations of Ba, based on residual white solids after digestion, a smaller 

mass of sample (10 to 20 mg of sediment) was digested and reanalyzed for Ba. A certified 

reference material (CRM), MESS-2, a marine sediment from the National Research Council 

(NRC) of Canada, also was digested (in 0.4-g and 20-mg amounts) and analyzed to check the 

accuracy of the method. 

Sediment subsamples to be analyzed for Hg were digested by heating 2 to 4 g of wet sediment in 

acid-washed, polypropylene-copolymer centrifuge tubes with 4 ml HNO3 and 2 ml H2SO4. 

Sample tubes were heated for 1 hour in a 90°C water bath and allowed to cool. Each tube was 

centrifuged at 2,000 rpm, and the supernatant was decanted into a 25-ml graduated cylinder. The 

sediment pellet was rinsed twice with 5 ml DIW, centrifuged, and decanted into the graduated 

cylinder before diluting to a final volume of 20 ml with DIW. The CRM MESS-2 was again 

digested and analyzed to check the accuracy of the method. 

Sediment samples, CRMs, and procedural and reagent blanks were analyzed by flame atomic 

absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), 

cold vapor atomic absorption spectrometry (CVAAS) or inductively coupled plasma – mass 

spectrometry (ICP-MS). Mercury concentrations were measured by CVAAS using a Laboratory 

Data Control Model 1235 Mercury Monitor. Concentrations of Al, Cr, Cu, Fe, Mn, V, and Zn 

were determined by FAAS. Concentrations of As were determined by GFAAS using a 

Perkin-Elmer Model 5100 instrument. Concentrations of Ba, Cd, Ni, and Pb were determined by 

ICP-MS using a Perkin-Elmer ELAN® 5000 instrument. All analytical techniques followed 

manufacturers’ specifications and standard operating procedures (SOPs) on file at FIT. These 

methods are closely akin to the U.S. Environmental Protection Agency (USEPA) methods 

described for Series 7000 FAAS, and GFAAS, Series 7470 CVAAS, and Series 6010A ICP-MS 

(USEPA 1991). Method detection limits (MDLs) and data quality information are listed in 

Tables 8.2 and 8.3 and Appendix G. 

248 
Table 8.2. Summary of instrumental methods and method detection limits (MDLs) for metal 

analysis of sediment and organisms. 

Sediments Organisms 

Parameter 

Method 

MDLs 

(μg metal/g 

dry sediment) 

Method 

MDLs 

(μg metal/g 

tissue dry weight)

Metals 

Al – aluminum FAAS 10 -- -- 

As – arsenic GFAAS 0.2 GFAAS 0.03 

Ba – barium ICP-MS 2 ICP-MS 0.01 

Cd – cadmium ICP-MS 0.01 GFAAS 0.001 

Cr – chromium FAAS 3 GFAAS 0.003 

Cu – copper FAAS 3 FAAS 0.7 

Fe – iron FAAS 10 FAAS 2.5 

Hg – mercury CVAAS 0.001 CVAAS 0.001 

Mn – manganese FAAS 2.5 -- -- 

Ni – nickel ICP-MS 0.5 GFAAS 0.01 

Pb – lead ICP-MS 0.2 ICP-MS 0.001 

V – vanadium FAAS 4.5 GFAAS 0.007 

Zn – zinc FAAS 0.4 FAAS 0.4 

Other Parameters 

TOC – total organic 

carbon 

Shimadzu 

Carbon System 

0.06% -- -- 

CVAAS = cold vapor atomic absorption spectrometry; FAAS = flame atomic absorption spectrometry; 

GFAAS = graphite furnace atomic absorption spectrometry; ICP-MS = inductively coupled plasma-mass spectrometry; 

MDL = method detection limit. 

Table 8.3. Data quality objectives and criteria. 

Element or 

Sample Type Minimum Frequency Data Quality Objective/ Acceptance Criteria 

Initial calibration Prior to every batch of samples 3 to 5 point curve depending on the element 

and a blank. Standard curve correlation 

coefficient r ≥ 0.999 for all analytes 

Continuing 

calibration 

Must end every analytical sequence; 

for flame, repeat all standards every 

5 samples; for graphite furnace and 

ICP-MS, recheck standard after 

every 8 to 10 samples 

% RSD <15% for all analytes 

Certified and 

standard reference 

materials 

One per batch of 20 samples Values must be within 20% of accepted 

values for >85% of the certified analytes 

and within 25% for Hg 

Method blank One per batch of 20 samples No more than 2 analytes to exceed 5x MDL 

Matrix spike and 

spike method blank 

One per batch of 20 samples % RSD 80% to 120% 

Lab duplicate One per batch of 20 samples RSD <25% for 65% analytes 

ICP–MS = inductively coupled plasma-mass spectrometry; MDL = method detection limit; 

RSD = relative standard deviation. 

249 
Labware used in the digestion process was acid washed with hot, 8N HNO3 and rinsed three 

times with DIW. Two procedural blanks, two duplicate samples, and two CRMs were prepared 

with each set of 40 samples. 

8.2.3.2 Metals in Biota 

Concentrations of 11 metals (As, Ba, Cd, Cr, Cu, Fe, Hg, Ni, Pb, V, and Zn) were determined in 

samples of the giant isopod Bathynomus giganteus and the red crab Chaceon quinquedens from 

near-field and far-field stations at sites GB 602 and MC 292. In the field, the internal organs 

were removed, combined, homogenized, and freeze-dried. In the laboratory, prior to acid 

digestion, the homogenized tissue samples were thawed and remixed with a Teflon® stirring rod. 

The samples were then split into two portions, one subsample to be digested wet for Hg and the 

other to be freeze-dried and digested for determination of the remaining trace metals. The freezedried subsamples also were used to calculate the percent water content so that Hg concentrations 

could be presented on a wet-weight and dry-weight basis. 

The concentrations of all metals (except Hg) were determined using 4 to 6 g of wet weight tissue 

weighed into 100-ml glass digestion flasks. These subsamples were freeze-dried, reweighed for 

percent water content, and then digested by the sequential addition of concentrated, high-purity 

HNO3, hydrogen peroxide (H2O2), and HCl, with gentle refluxing. Tissue CRMs were digested at 

the same time as the experimental samples. Once the tissue samples and CRMs were completely 

dissolved, the clear solutions were transferred to graduated cylinders, diluted to 20 ml with DIW 

rinses of the digestion flasks, and then stored in labeled 30-ml polyethylene screw-cap bottles for 

trace metal analysis. 

Mercury determinations were carried out using 0.4 to 0.7 g of wet tissue, and dry SRMs were 

weighed into 50-ml glass digestion tubes. These subsamples were digested by the addition of 

concentrated, high-purity HNO3 and H2SO4 and refluxing at 90°C for 1 hour in the sealed tubes. 

The dissolved samples were transferred to graduated cylinders, diluted to 20 ml with DIW rinses 

of the digestion tubes, and then stored in labeled 30-ml polyethylene screw-cap bottles. 

Metal concentrations in the digested tissue samples, CRMs, and blanks were determined by 

FAAS, GFAAS, CVAAS, or ICP-MS. The method used for each element and the corresponding 

MDLs are given in Table 8.2. All analytical techniques followed manufacturers’ specifications 

and SOPs on file at FIT. These methods are based on USEPA methods described for Series 7000 

(FAAS and GFAAS), Series 7470 (CVAAS), and Series 6010A (ICP-MS) (USEPA 1991). 

8.2.3.3 Total Organic Carbon 

A 0.5 to 1 g portion of the freeze-dried sediment was placed in a 10-ml Pyrex® beaker and wetted 

with 10% H3PO4 to remove any inorganic carbon that was present. The sediment was dried at 

60°C and reweighed to determine the increase in weight due to the formation of CaHPO4 as a 

result of adding phosphoric acid. Then, approximately 200 to 400 mg of pre-treated sediment 

were weighed into ceramic boats and combusted at 900°C in a Shimadzu TOC-5050A carbon 

system with SSM-5000A solid sampling module following the manufacturer’s instructions. The 

TOC content of the sediment samples was determined using a four-point calibration curve with 

pure sucrose as the standard and MESS-2 as the CRM. The TOC concentrations were corrected 

250 
to account for increases in sediment mass following addition of H3PO4. The calibration curve 

was checked every 10 samples by analyzing pure sucrose. 

8.2.3.4 Sediment Geochronology 

Sediment geochronology was determined using 137Cs and excess 210Pb following methods 

described by Kang et al. (2000). Vials containing about 10 g of freeze-dried sediment were 

counted for 2 to 3 days until peak areas were sufficient to provide <10% counting error for total 

210Pb. The activities of 210Pb, 214Pb, 214Bi, and 137Cs were determined using a well-type, intrinsic 

germanium detector (WiGe, Princeton Gamma Tech). Detector efficiency was determined using 

the following: NBS 4350B, river sediment and NBS 4354, freshwater lake sediment from 

National Institute of Standards and Technology (NIST), and RGU-1 and RGTh-1 from the 

International Atomic Energy Agency. The specific activity (dpm/g) of each sediment sample was 

calculated from the detector efficiency, gamma intensity, geometry factor, and sample weight 

(Kang et al. 2000). All values were decay corrected to the date of coring. Errors are shown on 

the basis of 1-sigma counting statistics. 

8.2.4 Quality Control and Quality Assurance 

8.2.4.1 Sample Tracking Procedure 

All sediment samples were collected by, transported by, and stored by personnel from FIT. Biota 

samples were in the custody of Continental Shelf Associates, Inc. until transferred to FIT. 

Standard chain-of-custody procedures were followed for all samples. Upon return to or arrival at 

the laboratory, each sample was inspected to ensure that it was intact and that the identification 

number was clearly readable. All sediment and organism samples were kept frozen (-20°C) until 

processed for analysis. 

8.2.4.2 Quality Control Measurements for Analysis 

Quality control measures included balance calibration, instrument calibration (FAAS, GFAAS, 

CVAAS, ICP-MS, TOC analyzer, and in-situ instrument sensors), matrix spike analysis for each 

metal, duplicate sample analysis, analysis of CRMs and standard reference materials (SRMs), 

procedural blank analysis, and standard checks. With each batch of up to 40 samples, two 

procedural blanks, two CRMs, two duplicate samples, and two matrix-spiked samples were 

analyzed. Data quality objectives (DQOs) for these quality control measurements are provided in 

Table 8.3. 

8.2.4.3 Instrument Calibration 

Electronic balances used for weighing samples and reagents were calibrated prior to each use 

with certified NIST traceable standard weights. All pipets (electronic or manual) were calibrated 

prior to use. Each of the spectrometers used for metal analysis was initially standardized with a 

three- to five-point calibration with a linear correlation coefficient of r ≥ 0.999 required before 

experimental samples could be analyzed. Analysis of complete three- to five-point calibrations 

and/or single standard checks alternated every 5 to 10 samples until all the analyses were 

complete. The relative standard deviation (RSD) between complete calibration and standard 

251 
check was required to be <15%, or recalibration and reanalysis of the affected samples were 

performed. 

8.2.4.4 Matrix Spike Analysis 

Matrix spikes were prepared for a minimum of 5% of the total number of samples analyzed and 

included each metal to be determined. Results from matrix spike analysis using the method of 

standard additions provide information on the extent of any signal suppression or enhancement 

due to the sample matrix. If necessary (i.e., spike results outside 80% to 120% limit), spiking 

frequency was increased to 20%, and a correction was applied to the metal concentrations of the 

experimental samples. 

8.2.4.5 Duplicate Sample Analysis 

Duplicate samples from homogenized field samples (as distinct from field replicates) were 

prepared in the laboratory for a minimum of 5% of the total samples. These laboratory duplicates 

were included as part of each set of sample digestions and analyses and provide a measure of 

analytical precision. 

8.2.4.6 Procedural Blank Analysis 

Two procedural blanks were prepared with each set of 40 samples to monitor potential 

contamination resulting from laboratory reagents, glassware, and processing procedures. These 

blanks were processed using the same analytical scheme, reagents, and handling techniques as 

used for the experimental samples. 

8.2.4.7 Certified Reference Material and Standard Reference Material Analysis 

A common method used to evaluate the accuracy of environmental data is to analyze reference 

materials, samples for which consensus or "accepted" analyte concentrations exist. The following 

CRMs and SRMs were used: Marine Sediment CRM MESS-2 (NRC), Lobster Hepatopancreas 

CRM TORT-2 (NRC), Dogfish Muscle CRM DORM-2 (NRC), and Trace Elements in Water 

SRM #1643d (NIST). Metal concentrations obtained for the reference materials were required to 

be within 20% of accepted values for >85% of other certified analytes. 

Data for quality assurance/quality control measurements for each cruise are given with the 

complete data sets in Appendix G. 

8.3 VIOSCA KNOLL BLOCK 916 – EXPLORATION SITE 

8.3.1 Metals and Total Organic Carbon 

Means, standard deviations, maximums and minimums for concentrations of metals, and TOC for 

surface (0 to 2 cm) sediments from site VK 916 are summarized in Table 8.4. The data in 

Table 8.4 are for random stations in near-field and far-field zones and do not include results from 

the discretionary stations that were sampled during Cruise 3B. The complete data set is presented 

in Appendix G. Comparisons discussed here are between cruises and zones. 

252 
Table 8.4. Means ± standard deviations with ranges in parentheses for metals, and total organic 

carbon (TOC) for Viosca Knoll 916 for pre- and post-exploration cruises. 

Far-Field Near-Field 

Element Pre-Exploration 

Cruise 1B 

Post-Exploration 

Cruise 3B 

Pre-Exploration 

Cruise 1B 

Post-Exploration 

Cruise 3B 

Al (%) 7.04 ± 0.58 

(5.78-7.79)

7.11 ± 0.57 

(5.93-7.72)

7.29 ± 0.20 

(6.92-7.57)

6.82 ± 0.82 

(4.75-7.69)

As (μg/g) 12.3 ± 1.7 

(9.6-14.4)

12.4 ± 1.8 

(8.9-14.8)

12.6 ± 0.8 

(11.1-13.7)

13.8 ± 2.5 

(10.1-19.9)

Ba (%) 0.087 ± 0.017 

(0.070-0.118)

0.094 ± 0.022 

(0.067-0.139)

0.109 ± 0.016 

(0.080-0.129)

3.59 ± 5.26 

(0.105-17.1)

Cd (μg/g) 0.22 ± 0.02 

(0.17-0.24)

0.17 ± 0.02 

(0.14-0.21)

0.28 ± 0.04 

(0.18-0.33)

0.23 ± 0.06 

(0.14-0.32)

Cr (μg/g) 73.2 ± 4.4 

(65.8-78.4)

76.3 ± 3.2 

(70.3-83.2)

52.7 ± 14.2 

(31.2-68.0)

73.6 ± 6.3 

(62.7-80.2)

Cu (μg/g) 25.2 ± 1.1 

(23.1-27.0)

26.6 ± 1.1 

(24.4-28.7)

26.8 ± 1.1 

(25.1-28.5)

31.0 ± 6.7 

(25.8-48.0)

Fe (%) 3.34 ± 0.10 

(3.18-3.53)

3.41 ± 0.10 

(3.27-3.60)

3.47 ± 0.13 

(3.31-3.75)

3.15 ± 0.38 

(2.42-3.68)

Hg (ng/g) 71 ± 4 

(62-78)

77 ± 8 

(64-87)

71 ± 3 

(65-75)

90 ± 28 

(71-153)

Mn (μg/g) 6,250 ± 2,110 

(2,810-10,000)

6,760 ± 2,500 

(2,070-12,000)

8,790 ± 1,550 

(5,780-10,900)

6,360 ± 3,500 

(1,040-10,400)

Ni (μg/g) 40.3 ± 1.4 

(38.6-42.5)

39.7 ± 2.2 

(36.1-42.7)

40.7 ± 2.0 

(37.3-44.0)

36.6 ± 7.2 

(23.2-43.6)

Pb (μg/g) 28.3 ± 1.8 

(25.3-30.9)

25.4 ± 1.4 

(22.4-27.4)

30.3 ± 3.3 

(25.0-34.8)

29.9 ± 3.5 

(26.3-36.9)

V (μg/g) 133 ± 7 

(123-144)

124 ± 7 

(115-137)

137 ± 4 

(130-144)

115 ± 18 

(88-130)

Zn (μg/g) 104 ± 3 

(99-106)

107 ± 4 

(101-116)

108 ± 2 

(106-110)

106 ± 13 

(88-142)

TOC (%) 1.44 ± 0.11 

(1.32-1.58)

1.53 ± 0.13 

(1.35-1.73)

1.54 ± 0.21 

(1.30-1.90)

2.02 ± 0.77 

(1.39-3.45)

Mean concentrations of Al and Fe in surface sediments collected during pre- and post-exploration 

cruises for near-field and far-field stations varied by <10% (Table 8.4). The mean Fe/Al ratio of 

0.47 for all far-field and near-field surface sediments from site VK 916 was about 10% lower 

than the value of 0.53 for suspended sediment (4.61% Fe/8.65% Al) from the Mississippi River 

(Trefry and Presley 1982). The uniformity in the Fe/Al ratios for sediments from site VK 916 

support a similar terrigenous source for these sediments. The 13% discrepancy in Fe/Al ratios for 

sediments at site VK 916 versus suspended sediment from the Mississippi River is small enough 

to assume that the Mississippi River is the primary source of the aluminosilicates at this site with 

possible, minor alteration of Fe levels over time due to scavenging or sediment diagenesis. 

253 
Concentrations of Mn in surface sediments for far-field stations at site VK 916 varied by as much 

as a factor of six (Table 8.4) and show some spatial variability in the redox environment across 

the study area. Concentrations of Mn in suspended sediment from the Mississippi River are 

reported at ~1,300 ± 200 μg/g (Trefry and Presley 1982); therefore, sediments from far-field 

stations at site VK 916 contained 2 to 10 times more Mn than incoming river particles. These 

higher Mn values in slope sediments are consistent with an upward flux of reduced Mn2+ from 

deeper in the sediment column to the more oxygenated surface layer of sediment where the Mn2+

oxidizes and precipitates as MnO2 (Trefry and Presley 1982). The variability in Mn 

concentrations for surface sediments from far-field stations partly results from sampling a fixed 

layer of sediment (0 to 2 cm) where the thickness of the Mn-rich layer is variable and most often 

<2 cm. 

Concentrations of Mn in surface sediments at near-field stations show a similar degree of 

variability to those at far-field stations. The only three values for Mn that were <2,000 μg/g, 

considering all stations from VK 916, were found for near-field stations during the postexploration cruise. Sediment from these three stations contained 2.2% to 3.4% TOC relative to 

1.5% TOC in far-field samples. These observations support reductive dissolution of Mn and 

diffusion of Mn2+ from the sediment to the overlying water column at some near-field stations 

when reducing conditions develop where drilling discharges are found. This information and the 

results for Mn in sediment cores will be further considered in discussions regarding the redox 

state of the sediment. 

Mean concentrations of Ba were not statistically different for far-field versus near-field sediments 

during the pre-exploration cruise (Tables 8.4 and 8.5 and Figure 8.2). However, mean 

concentrations of Ba increased by ~30-fold at near-field stations following exploratory drilling 

(Figure 8.2). The maximum concentration of Ba during the post-exploration survey was 17.1% 

at Station B01 (Figure 8.3). The maximum Ba level represents a sediment content of ~32% 

barite (using a Ba level of 53% for industrial barite; Trefry et al. 2003). Sediments at nine 

stations from the center portion of the near-field zone contained Ba at levels of ~1% or greater. 

In the peripheral areas of the near-field zone, concentrations of Ba in sediments approached 

background levels. The highest Ba concentration in the near-field zone during the preexploration cruise was 0.13%. 

Table 8.5. Results of statistical comparisons (t-tests) for concentrations of metals among cruises 

(1B for pre-exploration and 3B for post-exploration) and zones for Viosca Knoll 916. 

Results for discretionary stations were not included in the statistical tests. 

Dependent 

Variable 

Independent 

Variable Probability > F Interpretation 

Log Ba zone <0.0001 NF post-expl.>NF pre-expl. 

Cd cruise 0.0026 FF pre-expl.> FF post-expl. 

Cr cruise 0.0005 NF post-expl.>NF pre-expl. 

Hg cruise 0.039 NF post-expl.>NF pre-expl. 

Mn cruise 0.018 NF post-expl.<NF pre-expl. 

TOC cruise/zone <0.0001 NF post-expl.>NF pre-expl. 

FF = far-field; NF = near-field; TOC = total organic carbon. 

254 
 

0.05

0.1

1

10

30

Ba (%)

NF FF NF FF

VK 916 (1B) VK 916 (3B)

0.087 0.098

3.32

0.108

Pre-Exploration Post-Exploration

Figure 8.2. Concentrations of barium (Ba) with markers and numbers showing means and 

lines showing standard deviations in sediment from near-field (NF) and far-field 

(FF) stations at Viosca Knoll (VK) 916 during pre-exploration drilling Cruise 1B 

and post-exploration drilling Cruise 3B. In some cases, the line for the standard 

deviation is obscured by a marker or appears skewed due to the logarithmic scale. 

VK 916 

Post-Drilling 

(Cruise 3B) Barium (%)

0.9

0.1

3.0

8.2

0.3

3.7

17.1

0.2

0.2

8.7

0.1 500-m radius

(NF)

3.3

6.6

7.3

0.4

All FF stations

(Cruises 1B & 3B)

0.09 ± 0.02%

NF Pre-Drilling 

(Cruise 1B)

0.11 ± 0.02%

LEGEND

= Box core station with Ba concentration

= Wellsite drilled during this study

Figure 8.3. Concentrations of barium (Ba) 

during post-drilling Cruise 3B. 

at Viosca Knoll (VK) 916 near-field (NF) stations 

Inset boxes show mean barium concentrations at all 

far-field (FF) stations and at NF stations during pre-drilling Cruise 1B. Data for the 

discretionary stations are underlined. The bold NF circle represents a radius of 

500 m. Shaded oval highlights area where Ba concentrations are ~1% or greater. 

255 
Vertical profiles for Ba in sediments were obtained for the three discretionary stations during the 

post-exploration cruise. The cores were subsectioned in five, 2-cm thick intervals over the top 

10 cm. Concentrations of Ba were at background levels (~0.1%) at less than 2 to 4 or less than 

4 to 6 cm in each instance (Figure 8.4). The three discretionary stations were located in the 

shaded area on Figure 8.3. Based on the sediment Ba data, the sphere of influence for drilling 

discharges is well contained within the 500-m near-field zone and extends to depths ≤ 4 cm. 

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-1 (3B)

 

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-2 (3B)

 

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-3 (3B)

Figure 8.4. Vertical profiles for concentrations of barium (Ba) in sediment from discretionary 

(DS) stations during post-exploration Cruise 3B to Viosca Knoll (VK) 916. 

Mean concentrations of Al, As, Cu, Fe, Ni, Pb, V, and Zn were not significantly different among 

far-field and near-field stations for both the pre-exploration and post-exploration cruises 

(Tables 8.4 and 8.5). However, concentrations of Mn, Cr, and Hg, like Ba, showed a near-field 

zone-wide shift in concentrations that can be related to exploratory drilling. Some isolated shifts 

in metal concentrations were identified by examining metal data for sediment from Station B01 

(post-exploration), where Ba levels were 17.1%. Concentrations of Al, Fe, Ni, and V were ~30% 

lower at this station due to dilution of the natural aluminosilicate material, with ~30% barite in 

the sediments that was associated with drilling discharges. However, concentrations of As, Cu, 

and Zn were greater in sediment from Station B01 (post-exploration) than levels in background 

sediment by about 40%, 80%, and 30%, respectively. These increases are directly related to 

increases in concentrations of Ba; however, such increases were not clearly observed in other 

samples with as much as 8% Ba, except for Cu at Station B07 (Cu = 39.1 μg/g; Ba = 8.2%). Four 

near-field samples from Cruise 3B that contained >3% Ba also contained 20% to 50% higher 

levels of Hg (Figure 8.5). Vertical profiles for Hg in sediments from discretionary stations from 

Cruise 3B show the same minor enhancement in Hg levels at the tops of the cores at 

Stations DS-2 and DS-3 (Figure 8.6). A more detailed, separate section on trace metals in 

sediments that provides an overview of all drilling sites is presented at the end of the chapter. 

256 
Total Ba (%)

0 10 20 30 40

Total Hg (ng/g)

0

100

200

VK 916 1B FF 

VK 916 1B NF 

VK 916 3B FF 

VK 916 3B NF 

Figure 8.5. Concentrations of barium (Ba) versus mercury (Hg) for sediments from far-field 

(FF) and near-field (NF) stations from Viosca Knoll (VK) 916 collected during 

pre-exploration Cruise 1B and post-exploration Cruise 3B. Most points from FF 

samples and NF samples from Cruise 1B are hidden beneath the NF data points 

from Cruise 3B at low concentrations of Ba. 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-1 (3B)

 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-2 (3B)

 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-3 (3B)

Figure 8.6. Vertical profiles for concentrations of mercury (Hg) in sediment from discretionary 

(DS) stations during post-exploration Cruise 3B to Viosca Knoll (VK) 916. Squares 

at 0 cm on each graph represent mean Hg values at far-field stations. 

257 
Concentrations of TOC in near-field surface sediments during the post-exploration cruise (3B) 

averaged about one-third greater than found in far-field samples or near-field samples from the 

pre-exploration cruise (Table 8.4 and Figure 8.7). The maximum value for TOC in far-field or 

near-field samples from Cruise 1B was 1.9%. Four near-field samples and the three discretionary 

samples from Cruise 3B contained >2% TOC, with a maximum level of 3.45%. Each of the 

seven samples with >2% TOC contained >3% Ba (Figure 8.8). Vertical profiles for TOC 

(Figure 8.9) show similar trends to those observed for Ba (Figure 8.4), with elevated levels of 

TOC restricted to the top 2 to 4 cm of the sediment column. 

8.3.2 Sediment Geochronology 

Vertical profiles for excess 210Pb at Stations FF3 and NF-3 for site VK 916 from Cruise 1B are 

remarkably similar and provide a good test of how representative a single core can be from one of 

these deepwater sites (Figure 8.10). In both cores, two different slopes for excess 210Pb versus 

depth are observed (Figure 8.10). Possible explanations for the two slopes are sediment mixing 

(biological and/or physical) in the top 5 to 7 cm or a shift in sedimentation rate. The 137Cs profile 

was not preserved in either core, and thus confirmation of mixing was not independently 

determined. Based on results to be presented for the other deepwater sites, the sedimentation rate 

of 0.06 cm/y, based on the lower portion of the excess 210Pb profile, is most likely a better 

estimate for this site. Geochronology at near-field stations is distorted in the presence of drilling 

discharges. This distortion shows the presence of drilling mud or cuttings but does not 

differentiate between the two fractions. More detail on near-field samples is given in the results 

for GB 516 where a far-field core and a near-field core were studied. 

8.3.3 Sediment Oxygen Levels and Redox Conditions 

Vertical profiles for dissolved O2 and Eh were obtained using probes at near-field, far-field, and 

discretionary stations during both the pre- and post-exploration cruises. Measurement of pH was 

carried out at as many stations as time permitted. However, similar results were obtained for 

most stations, and thus the pH measurements were not carried out if they delayed processing of 

the subsequent core. The complete data set for oxygen, Eh, and pH is presented in tabular and 

graphical forms in Appendix G. 

Values for Eh represent the sum of all oxidation/reduction reactions that are occurring in the 

sediment. An approximate Eh value can be given for the occurrence of various redox reactions 

(Figure 8.11). For example, the onset of bacterial reduction of nitrate (a replacement bacterial 

oxidizing agent for oxygen) to ammonia occurs at an Eh somewhat <200 mV. The onset of 

sulfate reduction to H2S occurs as Eh values fall below –100 mV (Figure 8.11). 

At VK 916, concentrations of dissolved oxygen in the sediment range from ~200 μM (or 

6.4 mg/L where 1 mg/L = 31.2 μM) in the bottom water collected in the box core (bottom water 

value is plotted at a depth of 0.0 cm on each graph) to <2 μM (MDL) at varying depths in the 

cores. Concentrations of dissolved oxygen in the overlying water from the box core probably do 

not provide an exact measure of O2 levels in near-bottom water; however, they provide a 

comparable and useful point of reference for each sediment core. Dissolved oxygen levels in the 

overlying water, at 1 to 2 m above bottom, are typically in the range of 200 to 250 μM. 

258 
0

1

2

3TOC (%)

NF FF NF FF

VK 916 (1B) VK 916 (3B)

1.44 1.54

1.99

1.54

Pre- PostFigure 8.7. Concentrations of total organic carbon (TOC) with circles and numbers showing 

means and lines showing standard deviations in sediment from near-field (NF) and 

far-field (FF) stations from Viosca Knoll (VK) 916 during pre-exploration drilling 

Cruise 1B and post-exploration drilling Cruise 3B. 

Total Ba (%)

0 10 20 30 40

TOC (%)

0

1

2

3

4

5

6

VK 916 1B FF

VK 916 3B FF

VK 916 1B NF

VK 916 3B NF

Figure 8.8. Concentrations of barium (Ba) versus total organic carbon (TOC) for sediments 

from far-field (FF) and near-field (NF) stations from Viosca Knoll (VK) 916 

collected during pre-exploration Cruise 1B and post-exploration Cruise 3B. Some 

points from FF samples and NF samples from Cruise 1B are hidden beneath the NF 

data points for Cruise 3B at low concentrations of Ba. 

259 
0 1 2 3 4

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-1 (3B)

 

0 1 2 3 4

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-2 (3B)

 

0 1 2 3 4

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-3 (3B)

Figure 8.9. Vertical profiles for concentrations of total organic carbon (TOC) in sediment from 

discretionary (DS) stations for post-exploration Cruise 3B to Viosca Knoll 

(VK) 916. 

Concentrations of dissolved O2 in sediment from both far-field and near-field sites during the preexploration cruise decreased to non-detectable levels (<2 μM) at sediment depths of 1.6 to 3.5 cm 

(Table 8.6 and Figure 8.12). Ranges in Eh and pH values also were similar for far-field and 

near-field sediments from Cruise 1B (Table 8.6). A typical vertical profile for dissolved oxygen 

at Station FF3 shows an exponential decrease in dissolved O2 in the sediment (Figure 8.12). At 

the same depth of ~3 cm, where oxygen concentrations approached zero at Station FF3, the Eh 

decreased sharply from +400 to +100 mV (Figure 8.12, note different depth scales for oxygen 

and Eh). This shift signals a change from oxic to suboxic conditions where nitrate and 

manganese and iron oxides, instead of oxygen, serve as electron acceptors during the oxidation of 

organic matter (Figure 8.11). The conditions summarized in Table 8.6 and shown in 

Figure 8.12 also identify background conditions in sediments at this study site. 

Table 8.6. Summary data for dissolved oxygen, redox potential (Eh), and pH for sediment from 

Viosca Knoll (VK) 916 at near-field (NF), far-field (FF), and discretionary (DS) 

stations during pre-exploration Cruise 1B and post-exploration Cruise 3B. 

Site Zone Zero O2

depth (cm) Eh1 cm range (mV) Eh10 cm range (mV) pH 

VK 916 

Pre-Exploration 

(Cruise 1B) 

NF 1.6 – 3.5 +675 +328 +331 +69 7.3 – 7.9 

FF 2.4 – 3.1 +614 +135 +175 +56 7.4 – 8.0 

VK 916 

Post-Exploration 

(Cruise 3B) 

NF 0.2 – 3.0 +529 -79 +428 +66 -- 

DS 0.1 – 0.5 +31 -65 +107 -16 -- 

FF 0 – 3.2 +514 +218 +195 +91 -- 

260 
VK 916 FF3

ln Excess 210Pb Activity (dpm/g)

-2 0 2 4 6 8

Sediment Depth (cm)

10

0

5

10

15

20

25

30

S = 0.06 cm/y

r = 0.99

VK 916 NF-3

ln Excess 210Pb Activity (dpm/g)

-2 0 2 4 6 8

Sediment Depth (cm)

10

0

5

10

15

20

25

30

S = 0.06 cm/y

r = 0.99

Figure 8.10. Vertical profiles for activities of excess 210Pb in sediment from far-field (FF) and 

near-field (NF) stations at Viosca Knoll (VK) 916 for Cruise 1B 

(S = sedimentation rate, r = correlation coefficient, calculated using only the red 

points). 

261 
Eh

(mvolts)

400

-200

0

200

Typical

Reactants/Products

NO3

-

 / N2

NO3

-

 / NH4

+

MnO2 / Mn2+

Fe2O3 / Fe2+

SO4

2- / H2S

CO2 / CH4

Figure 8.11. Approximate redox potential (Eh) values at which various redox reactions occur in 

water (after Drever 1997). 

262 
MMS Deep Gulf of Mexico Cruise 1B: VK 916 FF3-B01

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sediment Depth (cm)

6 7 8 9

pH

-200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

pH

Eh

MMS Deep Gulf of Mexico Cruise 3B: VK 916 DS-B01

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sediment Depth (cm)

-200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

MMS Deep Gulf of Mexico Cruise 3B: VK 916 NF-B02

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sediment Depth (cm)

-200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

Figure 8.12. Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in sediment 

from representative near-field (NF), far-field (FF), and discretionary (DS) stations 

from Viosca Knoll (VK) 916 (note different depth scales for oxygen versus Eh). 

263 
During the post-exploration cruise, values for dissolved oxygen and Eh at many near-field and 

discretionary stations were considerably lower than at far-field stations. For example, the lowest 

Eh levels in the top 1 cm of the sediment at near-field and discretionary stations were -79 and 

-65 mV, respectively, relative to +218 mV in far-field sediments (Table 8.6 and Figure 8.12). 

Likewise, the sediment depth to zero oxygen was generally closer to the sediment-water interface 

at near-field and discretionary stations from the post-exploration cruise relative to far-field 

stations (Table 8.6). At some near-field stations where no drilling discharges were identified 

(Figure 8.3), vertical profiles for oxygen and Eh were similar to those observed at far-field 

stations (Figure 8.12). 

To help summarize and simplify the O2 data, the results for dissolved oxygen also are presented 

as the integrated (total) amount of oxygen in the sediment column (as nmoles/cm2

). These 

oxygen inventories are presented as the average and standard deviation for near-field, far-field, 

and discretionary stations from the pre- and post-exploration cruises. The mean integrated 

amounts of oxygen (ΣO2) in the sediments at far-field stations from Cruises 1 and 2 and near-field 

stations from Cruise 1B are not statistically different at 84 to 90 nmol/cm2 (Figure 8.13). In 

contrast, values for ΣO2 in sediments collected at near-field and discretionary stations during the 

post-exploration cruise averaged about one-half and one-eighth of pre-exploration values, 

respectively (Figure 8.13). This shift is directly related to the presence of drilling discharges. 

The rapid deposition of several centimeters of more organic-rich sediment at many stations in the 

near-field zone leads to an increasing rate of bacterial activity with resulting depletion of 

dissolved oxygen. 

8.3.4 Interstitial Water Composition 

Interstitial water was collected at one far-field and one near-field station from both the pre- and 

post-exploration cruises. The interstitial water data will be used here to determine how well the 

dissolved oxygen and Eh data predict the reactants and products of bacterial decomposition of 

organic matter in sediments. The interstitial water data, along with the oxygen and Eh data, also 

will be used to further identify changes in the redox state of the sediments in the presence of 

drilling discharges. The interstitial water data are tabulated in Appendix G, and vertical profiles 

for selected chemical species in several cores are presented below. 

As background information, Table 8.7 shows some of the reactions that may occur as oxygen and 

subsequent oxidizing agents are used by bacteria to facilitate energy production from the 

oxidation of organic matter. As oxygen is depleted during oxidation of organic matter, nitrate 

(and nitrite) are produced as shown in Eq. 1 (Table 8.7). The shift from oxic decomposition of 

detrital organic matter by bacteria can occur abruptly at rather shallow depths in the sediment to 

sub-oxic (nitrate reduction) decomposition (Eq. 2, Table 8.7). Upon depletion of oxygen and 

nitrate, other less efficient oxidizing agents, such as Mn and Fe oxides and sulfate are used by 

resident bacteria (Eqs. 3, 4, and 5, Table 8.7). Levels of Mn2+ and Fe2+ in interstitial water 

increase as metal oxides are chemically reduced and solubilized. Throughout the process of 

organic mater decomposition, concentrations of various by-products, such as dissolved ammonia, 

H2S, CO2, and phosphate, increase (Table 8.7). 

264 
M

0

30

60

90

120

150

Integrated O2 (nmol/cm2

)

VK 916

Pre-Exploration

86

44

11

84

Post-Exploration

NF FF DS NF FF

86 90

Figure 8.13. Integrated amounts of oxygen in the sediment column at near-field (NF), 

far-field (FF), and discretionary (DS) stations at Viosca Knoll (VK) 916 for 

pre-exploration Cruise 1B and post-exploration Cruise 3B. Markers and numbers 

show means, and lines show standard deviations. 

Table 8.7. Selected reactions showing the decomposition of organic matter by various 

oxidation agents (after Froelich et al. 1979). 

(CH2O)106(NH3)16H3PO4 + 138 O2 = 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O Eq. 1 

(CH2O)106(NH3)16H3PO4 + 94.4 HNO3 = 106 CO2 + 55.2 N2 + H3PO4 + 177.2 H2O Eq. 2 

(CH2O)106(NH3)16H3PO4 + 212 MnO2 + 424 H+ = 106 CO2 + 16 NH3 + H3PO4 + 212 

Mn2+ +318 H2O 

Eq. 3 

(CH2O)106(NH3)16H3PO4 + 212 Fe2O3 (or 424 FeOOH) + 848 H+ = 106 CO2 + 16 NH3

+ H3PO4 + 424 Fe2+ + 530 H2O 

Eq. 4 

(CH2O)106(NH3)16H3PO4 + 53 SO4

2- + 424 H+

 = 106 CO2 + 16 NH3 + H3PO4 + 53 S2- 

+318 H2O 

Eq. 5 

Results for interstitial water from one far-field and one near-field station from the pre-exploration 

cruise (1B) for site VK 916 are very similar (Figures 8.14 and 8.15). The Eh of >+600 mV in the 

top layer of the core decreased to +200 to +100 mV by depths of 5 to 10 cm and did not decrease 

below about +100 mV deeper in both cores. These Eh values for both the far-field and near-field 

cores from site VK 916 are consistent with a transition from oxic to sub-oxic conditions over the 

top few centimeters of the core. Nitrate was detected in the top 5 cm of each core, and Mn2+ and 

Fe2+ were not detected in the top few cm of each core (Figures 8.14 and 8.15). As Eh decreased 

265 
Pore Water Results for VK 916 FF-B02 (Cruise 1B) 

Figure 8.14. Vertical profiles for dissolved oxygen, Eh, nitrate, sulfate, manganese, and iron 

from interstitial water from Station FF-B02, Viosca Knoll (VK) 916, for 

pre-exploration Cruise 1B. 

266 
Pore Water Results for VK 916 NF-B02 (Cruise 1B) 

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

6789

pH

-200 100 400 700

Eh (mV)

0

10

20

30

pH

Eh

0 10 20 30 40

Nitrate (μM)

0

10

20

30

0 10 20 30

Sulfate (mM)

0

10

20

30

0 50 100 150 200 250

Manganese (μM)

0

10

20

30

0 12 24 36 48 60

Iron (μM)

0

10

20

30

Figure 8.15. Vertical profiles for dissolved oxygen, Eh, nitrate, sulfate, manganese, and iron 

from interstitial water from Station NF-B02, Viosca Knoll (VK) 916, for 

pre-exploration Cruise 1B. 

267 
in the upper few centimeters, concentrations of Mn2+ in the interstitial water increased 

(Figures 8.14 and 8.15). Concentrations of Fe2+ did not increase until depths >15 cm, and no 

discernible sulfate reduction or detectable sulfide (i.e., <2 μM total H2S) were observed in either 

core (Figures 8.14 and 8.15). Thus, the dominant reactions in the far-field core were those 

described for Eqs. 2, 3, and 4 (Table 8.7 and Figure 8.11). The good relationship of the probe 

results for oxygen and Eh with the interstitial water data for both far-field and near-field stations 

during Cruise 1B support use of the probe data to assess the redox state of the sediments at the 

other stations. The similar data for depth to zero oxygen, Eh (Table 8.6), and integrated oxygen 

(Figure 8.13) for both far-field and near-field stations from site VK 916, coupled with agreement 

for the limited interstitial water data, show that the redox state in sediments from the far-field and 

near-field zones was similar during the pre-exploration phase of this study. 

Interstitial water was collected from Stations FF3 and DS-2 during the post-exploration cruise. 

At Station DS-2, the top two sections of the sediment core at 0 to 2 cm and 2 to 4 cm contained 

6.6% and 0.8% Ba and 2.7% and 2.0% TOC, respectively, relative to background levels of about 

0.1% Ba and 1.5% TOC at far-field stations. Oxygen levels were below detection limits (<2 μM) 

in the top millimeter of sediment at Station DS-2 to yield an integrated oxygen level of 

0 nmol/cm2

, relative to an oxygen penetration depth of 3.1 cm and an integrated oxygen level of 

88 nmol/cm2

 at Station FF3 (Figures 8.16 and 8.17). The Eh at Station FF3 decreased from 

~+500 mV in the top 2 cm to +100 mV from 8 cm to the base of the core (Figure 8.16). At 

Station DS-2, the Eh was about 0 mV in the top 2 cm and then about +100 mV through the 

remainder of the core (Figure 8.17). Concentrations of dissolved ammonia were below detection 

limits (<1 μM) in the top of the core from Station FF3, increasing to ~50 μM at 25 cm 

(Figure 8.16). Dissolved ammonia levels of 16 to 40 μM were found in the top 4 cm of the core 

from Station DS-2, with maximum levels downcore of 70 μM (Figure 8.17). A somewhat 

similar comparison is observed for Mn2+, with low Mn2+ levels of 10 μM at the top of the core 

from Station FF3 relative to 200 to 500 μM at Station DS-2 (Figures 8.16 and 8.17). No sulfate 

reduction was discernible in either core; and, the vertical profiles for Fe2+ were similar in both 

cores (Figures 8.16 and 8.17). 

The thickness of layers containing drilling discharges observed during the post-exploration cruise, 

based on concentrations of Ba and TOC, was ~4 cm in each of the three discretionary cores 

collected. The thin layer of material leads to depletion of dissolved oxygen in the sediments and 

a distinct decrease in redox potential as previously described (Figure 8.13 and Table 8.6). The 

apparent impact on the chemical composition of the interstitial water seems limited to the top of 

the core and is most obvious for concentrations of dissolved ammonia and Mn2+. Concentrations 

of sediment Mn in the top 2 cm of the cores from the three discretionary stations (Cruise 3B) 

range from 1,600 μg/g at Station DS-2 to 5,300 μg/g at Station DS-1, relative to average levels of 

~7,000 μg/g in far-field sediments (Figure 8.18). However, sediment Mn levels increased to 

>6,000 μg/g in the 2 to 4 cm section from all three discretionary cores (Figure 8.18). 

Significantly lower levels of sediment Mn in the top layers of sediment from discretionary and 

near-field stations during the post-exploration cruise result from lower Mn levels in the drilling 

discharges and a diffusive flux of Mn2+ from the sediment column at these stations. Overall, the 

shift in redox state is readily discernible, yet limited to the top 4 cm of the sediment column. 

268 
Pore Water Results for VK 916 FF-B03 (Cruise 3B) 

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

Sediment Depth (cm)

-200 100 400 700

Eh (mV)

0

10

20

30

Sediment Depth (cm)

0 10 20 30

Sulfate (mM)

0

10

20

30

Sediment Depth (cm)

0 20 40 60 80 100

Ammonia (μM)

0

10

20

30

Sediment Depth (cm)

0 100 200 300 400 500

Manganese (μM)

0

10

20

30

Sediment Depth (cm)

0 20 40 60 80 100

Iron (μM)

0

10

20

30

Sediment Depth (cm)

Figure 8.16. Vertical profiles for dissolved oxygen, Eh, sulfate, ammonia, manganese, and iron 

from interstitial water at Station FF-B03, Viosca Knoll (VK) 916, for 

post-exploration Cruise 3B. 

269 
Pore Water Results for VK 916 DS-2 (Cruise 3B) 

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

Sediment Depth (cm)

-200 100 400 700

Eh (mV)

0

10

20

30

Sediment Depth (cm)

0 10 20 30

Sulfate (mM)

0

10

20

30

Sediment Depth (cm)

0 20 40 60 80 100

Ammonia (μM)

0

10

20

30

Sediment Depth (cm)

0 100 200 300 400 500

Manganese (μM)

0

10

20

30

Sediment Depth (cm)

0 20 40 60 80 100

Iron (μM)

0

10

20

30

Sediment Depth (cm)

Figure 8.17. Vertical profiles for dissolved oxygen, Eh, sulfate, ammonia, manganese, and iron 

from interstitial water at Station DS-2, Viosca Knoll (VK) 916, for post-exploration 

Cruise 3B. 

270 
0 6000 12000 18000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-1 (3B)

0 6000 12000 18000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-2 (3B)

0 6000 12000 18000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

VK 916, DS-3 (3B)

Figure 8.18. Vertical profiles for concentrations of manganese (Mn) in sediment from 

discretionary (DS) stations for post-exploration Cruise 3B to Viosca Knoll 

(VK) 916. 

8.4 GARDEN BANKS BLOCK 516 – EXPLORATION/DEVELOPMENT SITE 

8.4.1 Metals and Total Organic Carbon 

Means, standard deviations, maximums and minimums for concentrations of metals, and TOC for 

surface (0 to 2 cm) sediments from site GB 516 are summarized in Table 8.8. The data in 

Table 8.8 are for random stations in near-field and far-field zones and do not include results from 

the fixed, discretionary stations. The complete data set is presented in Appendix G. Once again, 

comparisons discussed here are between cruises and zones. However, as discussed in Chapter 3, 

some exploratory drilling and drilling discharges occurred prior to Cruise 1B at site GB 516, and 

additional wells were drilled prior to Cruise 2B. Thus, the data from GB 516 evaluate part of a 

continuum in the exploration/development process rather than the clearer pre- and 

post-exploration sampling at VK 916. 

Mean concentrations of Al and Fe in surface sediments collected during Cruises 1B and 2B for 

near-field and far-field stations varied by <10% (Table 8.8), and the mean Fe/Al ratio of 0.47 for 

all surface sediments from GB 516 is the same as observed at VK 916. Data for Al versus Fe 

from all 213 data points in the complete study are shown in Figure 8.19a. The good linear fit of 

the data (r = 0.95) supports a uniform source (the Mississippi River) for aluminosilicates 

throughout the study area. Points with low levels of both Fe and Al are observed when the 

normal aluminosilicates are mixed with and diluted by drilling discharges. Lower levels of Fe 

and Al were observed in near-field sediments from GB 516 (Table 8.8) relative to far-field 

stations due to the influence of drilling discharges. The effect is greatest for samples from 

Station DS-2 for Cruise 2B, where all Fe and Al levels were <1% and <2%, respectively 

(Figure 8.19a). Overall, sediments with concentrations of Al <6% also contain Ba levels >1% 

(Figure 8.19b). All data points at lower levels of Al and elevated concentrations of Ba are from 

near-field or discretionary stations. As the fraction of drilling discharges increases (as 

represented by Ba concentrations), the proportion of terrigenous clay (as represented by Al 

271 
concentrations) decreases (Figure 8.19b). Data for samples from far-field stations have low 

levels of Ba (<0.2%) relative to those where drilling discharges are present and thus plot along 

the baseline at about 6% to 7% Al on Figure 8.19b. 

Table 8.8. Means ± standard deviations with ranges in parentheses for metals, and total organic 

carbon (TOC) for Garden Banks 516 for Cruises 1B and 2B. 

Far-Field Near-Field 

Element Post-Exploration 

Cruise 1B 

Post-Development 

Cruise 2B 

Post-Exploration 

Cruise 1B 

Post-Development 

Cruise 2B 

Al (%) 6.40 ± 0.18 

(6.00-6.62) 

6.03 ± 0.21 

(5.76-6.37) 

6.25 ± 0.92 

(4.59-8.17) 

5.78 ± 1.00 

(3.29-6.84) 

As (μg/g) 11.0 ± 0.7 

(9.7-12.2) 

11.1 ± 0.8 

(9.8-12.6) 

10.3 ± 2.0 

(5.2-12.3) 

11.1 ± 1.2 

(8.5-13.0) 

Ba (%) 0.128 ± 0.034 

(0.076-0.197) 

0.148 ± 0.056 

(0.079-0.277) 

2.45 ± 3.83 

(0.31-13.5) 

5.90 ± 9.57 

(0.86-35.1) 

Cd (μg/g) 0.32 ± 0.03 

(0.28-0.36) 

0.27 ± 0.02 

(0.24-0.30) 

0.31 ± 0.03 

(0.26-0.38) 

0.28 ± 0.05 

(0.23-0.41) 

Cr (μg/g) 66.2 ± 3.0 

(60.6-69.6) 

59.5 ± 7.3 

(39.5-68.0) 

51.0 ± 9.2 

(31.1-63.7) 

62.6 ± 9.9 

(41.4-74.7) 

Cu (μg/g) 27.1 ± 0.6 

(26.2-28.1) 

28.2 ± 1.0 

(26.1-29.7) 

27.0 ± 1.7 

(23.5-30.3) 

32.7 ± 5.1 

(27.0-45.8) 

Fe (%) 2.93 ± 0.11 

(2.75-3.11) 

2.90 ± 0.17 

(2.52-3.15) 

2.82 ± 0.41 

(2.20-3.64) 

2.83 ± 0.43 

(1.76-3.23) 

Hg (ng/g) 108 ± 9 

(92-118) 

116 ± 20 

(95-154) 

101 ± 15 

(79-128) 

123 ± 39 

(88-237) 

Mn (μg/g) 4,180 ± 1,240 

(3,080-7,570) 

3,650 ± 585 

(2,740-4,840) 

3,470 ± 1,870 

(658-6,970) 

2,070 ± 1,140 

(285-3,490) 

Ni (μg/g) 44.2 ± 4.2 

(36.2-49.1) 

43.2 ± 2.6 

(37.7-46.8) 

39.8 ± 6.5 

(24.9-47.9) 

34.5 ± 7.7 

(14.1-42.3) 

Pb (μg/g) 24.5 ± 1.6 

(20.9-27.3) 

24.0 ± 2.0 

(20.9-26.8) 

24.1 ± 2.5 

(18.3-28.7) 

33.6 ± 10.3 

(21.0-60.3) 

V (μg/g) 124 ± 6 

(116-137) 

129 ± 5 

(122-137) 

119 ± 10 

(89-128) 

118 ± 22 

(57-140) 

Zn (μg/g) 90.2 ± 3.3 

(83.4-95.4) 

86.8 ± 3.5 

(81.5-92.6) 

94.4 ± 14.9 

(84.1-133) 

96.2 ± 38.3 

(63.1-213) 

TOC (%) 0.82 ± 0.08 

(0.70-0.95) 

1.08 ± 0.07 

(0.97-1.27) 

1.25 ± 0.69 

(0.84-3.31) 

1.32 ± 0.51 

(0.82-2.87) 

272 
Total Al (%)

0 2 4 6 8 10 12

Total Fe (%)

0

2

4

6

Y = 0.46X + 0.11

 r = 0.95, n = 213

(a)

GB 516 1B

GB 516 2B

GB 602 2B

MC 292 2B

VK 916 1B

VK 916 3B

Total Al (%)

02468 10

Total Ba (%)

0

10

20

30

40

(b) GB 516 1B FF

GB 516 2B FF

GB 516 1B NF

GB 516 2B NF

GB 516 2B DS

Figure 8.19. Concentrations of aluminum (Al) versus (a) iron (Fe) and (b) barium (Ba) for 

sediments from far-field (FF), near-field (NF), and discretionary (DS) stations 

from Garden Banks (GB) 516 collected during Cruises 1B and 2B. 

273 
At GB 516, concentrations of Ba are statistically lower for far-field versus near-field sediments 

during both cruises (Tables 8.8 and 8.9 and Figure 8.20). Mean Ba concentrations were 

~20-fold greater at near-field than far-field stations during Cruise 1B (Table 8.8 and 

Figure 8.20). Barium levels in near-field sediments from Cruise 2B were about 40 times greater 

than at far-field stations (Table 8.8 and Figure 8.20) but not significantly greater than at 

near-field stations sampled during Cruise 1B. 

Table 8.9. Results of statistical comparisons (t-tests) for concentrations of metals among cruises 

(1B and 2B) and zones (NF and FF) for Garden Banks 516. Results for discretionary 

stations were not included in the statistical tests. 

Dependent 

Variable 

Independent 

Variable Probability > F Interpretation 

Log Ba cruise <0.0001 NF Cruise 2B>NF Cruise 1B 

Hg cruise 0.039 NF Cruise 2B>NF Cruise 1B 

Mn cruise 0.018 NF Cruise 2B<NF Cruise 1B 

TOC zone <0.0001 NF>FF (both cruises) 

FF = far-field; NF = near-field; TOC = total organic carbon. 

0.1

1

10

30

Ba (%)

NF FF NF FF

Cruise 1B Cruise 2B

0.128 0.148

2.45

5.90

GB 516

Figure 8.20. Concentrations of barium (Ba) with markers and numbers showing means and 

lines showing standard deviations in sediment from near-field (NF) and far-field 

(FF) stations at Garden Banks (GB) 516 during Cruises 1B and 2B. In some cases, 

the line for the standard deviation appears skewed due to the logarithmic scale. 

The spatial distribution of Ba in sediments for both cruises is shown in Figure 8.21. The inner 

portion of the near-field zone with sediments containing >5% Ba is similar in size for both 

sampling periods; however, the number of data points and the magnitude of the Ba concentrations 

are greater for the second cruise (Figure 8.21). The area of the near-field zone with sediment 

samples containing >1% Ba approximately doubled between the two cruises (Figure 8.21). 

274 
GB 516 

(Cruise 1B) 

Barium (%)

500-m radius

(NF)

All FF stations

(Cruise 1B)

0.13 ± 0.03%

LEGEND: 

= Box core station with Ba concentration 

= Previous wellsite

1.2 13.5

1.1

1.4

1.2

7.1

1.0 0.3

0.4 1.4

0.3

0.3

500-m radius

(NF)

All FF stations

(Cruise 2B)

0.15 ± 0.06%

1.4

0.9

2.0 3.1

0.9

1.6

35.1

1.3

-

4.3 9.9

28.1

19.8

0.5

5.0

5.5

LEGEND: 

= Box core station with Ba concentration 

= Previous wellsite 

= New wellsite drilled during this study

GB 516 

(Cruise 2B) 

Barium (%)

Figure 8.21. Concentrations of barium (Ba) at Garden Banks (GB) 516 near-field (NF) stations 

during Cruises 1B and 2B. Inset boxes show mean Ba concentrations at all 

far-field (FF) stations. Data for the discretionary stations from Cruise 2B are 

underlined. The bold NF circle represents a radius of 500 m. The darker shaded 

oval highlights area where Ba concentrations are ≥ 5%, and the lighter shaded, 

larger oval highlights area where Ba concentrations are ≥ 1%. 

275 
Vertical profiles for Ba in sediments were obtained for the three discretionary stations during 

Cruise 2B. Concentrations of Ba were above background levels (~0.1%) in surficial sediments 

from each discretionary station (Figure 8.22). The elevated Ba concentrations decreased to 

background levels at 4 to 6 cm and 6 to 8 cm for Stations DS-3 and DS-1, respectively 

(Figure 8.22). At Station DS-2, Ba concentrations were >30% at 8 to 10 cm, the deepest layer 

analyzed. Vertical profiles for Al in the sediment column mirror the profiles for Ba, with lower 

levels of Al in the presence of greater amounts of Ba (Figure 8.22). Based on the sediment Ba 

data, the sphere of influence for drilling discharges was directed toward the southwest and may 

have extended beyond the near-field zone (Figure 8.21). Drilling discharges were found at 

sediment depths ≥ 10 cm at one station. 

Concentrations of Mn in surface sediments at near-field stations show a larger range in 

concentrations relative to far-field stations (Table 8.8). All nine surface sediment samples with 

Mn levels <2,000 μg/g were collected at near-field stations, two during Cruise 1B and seven 

during Cruise 2B (Figure 8.23). These observations support reductive dissolution of Mn at some 

near-field stations when reducing conditions develop where drilling discharges are found. This 

information and the results for Mn in sediment cores will be further considered in discussions 

regarding the redox state of the sediment. 

Mean concentrations of Al, As, Cd, Fe, Ni, Pb, V, and Zn were not significantly different among 

far-field and near-field stations for both the two cruises (Tables 8.8 and 8.9). The metal that 

showed the greatest zone-wide shift in concentrations that can be related to drilling was Ba, with 

a significant change also observed for Hg (Table 8.9). Some isolated shifts in metal 

concentrations can be identified by examining metals data on a station-by-station basis. For 

example, at Station NF-B03A (Cruise 2B), Ba levels were 35.1%, and concentrations of Al, Fe, 

Ni, and V were 40% to 70% lower at this station due to dilution of the natural aluminosilicate 

material, with ~65% barite in the sediments, which was associated with drilling discharges. 

However, concentrations of Cu, Hg, and Pb were greater in sediment from Station NF-B03A 

(Cruise 2B) than levels in background sediment by about 60%, 100%, and 250%, respectively. 

These increases are directly related to increases in concentrations of Ba as shown for Hg and Pb 

in Figure 8.24 

Vertical profiles for Hg in sediments from discretionary stations from Cruise 2B show minor 

enhancement in Hg levels at the top of the core at Station DS-1, high Hg levels throughout the 

core at Station DS-2, and no enrichment at Station DS-3 (Figure 8.25). A more detailed, separate 

section on trace metals in sediments, which provides an overview of all drilling sites, is presented 

at the end of the chapter. 

276 
0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-1 (2B)

 

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-2 (2B)

 

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-3 (2B)

0 2 4 6 8

Total Al (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-1 (2B)

 

0 2 4 6 8

Total Al (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-2 (2B)

 

0 2 4 6 8

Total Al (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-3 (2B)

Figure 8.22. Vertical profiles for concentrations of barium (Ba) and aluminum (Al) in sediment 

from discretionary (DS) stations at Garden Banks (GB) 516 for Cruise 2B. 

Total Al (%)

02468 10

Total Mn (μg/g)

0

5000

10000

15000

20000

GB 516 1B FF

GB 516 2B FF

GB 516 1B NF

GB 516 2B NF 

GB 516 2B DS

Figure 8.23. Concentrations of aluminum (Al) versus manganese (Mn) for sediments from 

far-field (FF), near-field (NF), and discretionary (DS) stations from Garden Banks 

(GB) 516 collected during Cruises 1B and 2B. 

277 
Total Ba (%)

0 10 20 30 40

Total Hg (ng/g)

0

100

200

300

GB 516 1B FF 

GB 516 1B NF

GB 516 2B FF

GB 516 2B NF

(a)

Total Ba (%)

0 10 20 30 40

Total Pb (μg/g)

0

100

200

300

400

GB 516 1B FF

GB 516 2B FF

GB 516 1B NF

GB 516 2B NF

(b)

Figure 8.24. Concentrations of barium (Ba) versus (a) mercury (Hg) and (b) lead (Pb) for 

sediments from far-field (FF) and near-field (NF) stations from Garden Banks 

(GB) 516 collected during Cruises 1B and 2B. Points from FF samples are hidden 

beneath points for NF samples at Ba levels <0.2%. 

278 
0 200 400 600 800

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS 1 (2B)

0 200 400 600 800

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS 2 (2B)

0 200 400 600 800

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS 3 (2B)

Figure 8.25. Vertical profiles for concentrations of mercury (Hg) in sediment from discretionary 

(DS) stations for Cruise 2B at Garden Banks (GB) 516. Square at 0 cm on each 

graph represents mean Hg values at far-field stations. 

Concentrations of TOC in near-field surface sediments are statistically higher than at far-field 

stations during both cruises (Table 8.9 and Figure 8.26). The maximum values for TOC in nearfield samples from both cruises are about three times greater than levels at far-field stations. 

Surface sediments from two near-field stations and two discretionary stations contain >2% TOC, 

with a maximum level of 7.16%. Each of the samples with >2% TOC contains ≥ 10% Ba 

(Figure 8.27). Vertical profiles for TOC (Figure 8.28) show similar trends to those observed for 

Ba (Figure 8.22), with elevated levels of TOC at Station DS-1 restricted to the top 2 to 4 cm, 

very high levels of TOC throughout the 10-cm length of the core at Station DS-2, and no 

enrichment at Station DS-3. 

0

1

2

3

TOC (%)

NF FF NF FF

Cruise 1B Cruise 2B

0.82

1.08

1.32 1.25

GB 516

Figure 8.26. Concentrations of total organic carbon (TOC) with markers and numbers showing 

means and lines showing standard deviations in sediment from near-field (NF) and 

far-field (FF) stations from Garden Banks (GB) 516 during Cruises 1B and 2B. 

The lines for the standard deviations for the FF data are obscured by the markers. 

279 
Total Ba (%)

0 10 20 30 40

TOC (%)

0

2

4

6

8

10

GB 516 NF 1B 

GB 516 NF 2B 

GB 516 FF 2B 

GB 516 FF 1B 

GB 516 DS 2B 

Figure 8.27. Concentrations of barium (Ba) versus total organic carbon (TOC) for sediments 

from far-field (FF), near-field (NF), and discretionary (DS) stations from Garden 

Banks (GB) 516 collected during Cruises 1B and 2B. Some points from FF and 

NF samples are hidden beneath the NF and DS data points for Cruise 2B at low 

concentrations of Ba. 

Sediment Depth (cm)

Sediment Depth (cm)

Sediment Depth (cm)

Figure 8.28. Vertical profiles for concentrations of total organic carbon (TOC) in sediment from 

discretionary (DS) stations for Cruise 2B at Garden Banks (GB) 516. 

280 
8.4.2 Sediment Geochronology 

The vertical profile for excess 210Pb at Station FF6 from Cruise 2B was used to calculate a 

sedimentation rate of 0.07 cm/y (Figure 8.29). This rate is similar to the value of 0.06 cm/y 

determined for VK 916 and is consistent with expected rates on the slope. 

At Station NF-B02 from Cruise 2B, the top 2 cm of sediment have ~2 times lower activities of 

excess 210Pb than found in the next three, deeper layers (Figure 8.29). Barium concentrations in 

the 0 to 2 cm section of sediment from Station NF-B02 were 1.6% relative to background levels 

of ~0.1%. Thus, some drilling mud and cuttings are most likely distorting the expected activity 

of excess 210Pb. Below the surface anomaly, a continuous decrease in the activities of excess 

210Pb was used to calculate a sedimentation rate of 0.06 cm/y, the same as found for the 

undistorted core from Station FF6. The excess 210Pb data for Station NF-B02 identify the 

presence of drilling discharges in the top 2 cm, but do not help distinguish between drilling mud 

versus cuttings. 

8.4.3 Sediment Oxygen Levels and Redox Conditions 

Concentrations of dissolved O2 in sediment from far-field stations at GB 516 are similar for both 

sampling periods and decrease to non-detectable levels (<2 μM) at sediment depths of 2.7 to 

5.8 cm (Figures 8.30 and 8.31 and Table 8.10). Ranges in Eh and pH values also are generally 

similar for far-field sediments from both cruises (Table 8.10). Typical vertical profiles for 

dissolved oxygen for far-field stations at GB 516 during the two cruises show exponential 

decreases in dissolved O2 in the sediment (Figures 8.30 and 8.31). At the same depth of ~5 cm, 

where that oxygen concentrations approached zero at Station FF3 (Cruise 1B), the Eh decreases 

sharply from +600 to +300 mV (Figure 8.30, note different depth scales for oxygen and Eh). 

This shift signals a change from oxic to suboxic conditions, where nitrate and manganese and iron 

oxides serve as electron acceptors during the oxidation of organic matter (Figure 8.11). At 

Station FF6 during Cruise 2B, the oxygen and Eh profiles were slightly different, with a sharper 

decrease in oxygen levels with increasing depth and lower levels of Eh (Figure 8.31). These two 

profiles highlight observed differences at far-field stations as summarized in Table 8.10. For 

example, the Eh in the top 1 cm ranged from +692 mV to +104 mV for all far-field stations 

(Table 8.10). Data for near-field stations during Cruise 1B were highly variable, with Eh levels 

as low as -163 mV in the top 1 cm of sediment and oxygen penetration depths as low as 0.3 cm 

(Table 8.10 and Figure 8.30). However, results for other near-field stations were similar to those 

observed for far-field stations (e.g., NF-B10, Figure 8.30). As discussed previously, evidence for 

the presence of drilling discharges was found during Cruise 1B. 

281 
GB 516 2B FF6

ln Excess210 Pb Activity (dpm/g)

012345

Sediment Depth (cm)

0

5

10

15

20

S = 0.072 cm/y 

 r = 0.95 

Figure 8.29. Vertical profiles for activities of excess 210Pb in sediment from Garden Banks 

(GB) 516 for Station FF6 during Cruise 2B and Station NF-B02 for Cruise 2B 

(S = sedimentation rate, r = correlation coefficient, calculated using all points for 

FF6 and only the red points for NF-B02). 

GB 516 2B NF-2

ln Excess210 Pb Activity (dpm/g)

-1 0 1 2 3 4 5

Sediment Depth (cm)

0

5

10

15

20

S = 0.06 cm/y 

 r = 0.98 

282 
MMS Deep Gulf of Mexico Cruise 1: GB 516 FF3- B01

0 50 100 150 200 250

Oxygen (μM)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

700

6789

pH

- 200 100 400

Eh (mV)

0

5

10

15

20

25

30

pH

Eh

MMS Deep Gulf of Mexico Cruise 1: GB 516 NF-B02

-

6789

pH

- 200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

pH

Eh

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1 .0

1.5

2.0

2.5

MMS Deep Gulf of Mexico Cruise 1: GB 516 NFB10

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

6789

pH

- 200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

pH

Eh

Figure 8.30. Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in sediment 

from representative near-field (NF) and far-field (FF) stations at Garden Banks 

(GB) 516 from Cruise 1B (note different depth scales for oxygen versus Eh). 

283 
Sediment Depth (cm)

Sediment Depth (cm) Sediment Depth (cm)

Sediment Depth (cm) Sediment Depth (cm)

Sediment Depth (cm)

Figure 8.31. Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in sediment 

from representative near-field (NF), and far-field (FF), and discretionary (DS) 

stations from Garden Banks (GB) 516 from Cruise 2B (note different depth scales 

for oxygen versus Eh). 

284 
Table 8.10. Summary data for dissolved oxygen, redox potential (Eh), and pH for sediment from 

Garden Banks (GB) 516 at near-field (NF), far-field (FF), and discretionary (DS) 

stations during Cruises 1B and 2B. 

Site Zone Zero O2

depth (cm) Eh1 cm range (mV) Eh10 cm range (mV) pH 

NF 0.3 – 5.7 +664 -163 +528 +56 6.8 – 8.1 

GB 516 

(Cruise 1B) 

FF 3.4 – 5.8 +692 +406 +598 +303 7.3 – 8.3 

NF 0.2 – 2.5 +506 -107 +259 +29 6.9 – 7.8 

GB 516 DS 0.1 – 0.3 +196 -171 +39 -78 6.6 – 10.5 

(Cruise 2B) 

FF 2.7 – 5.7 +492 +104 +390 +101 6.9 – 7.9 

During Cruise 2B, values for dissolved oxygen and Eh at many near-field and all three 

discretionary stations were considerably lower (Table 8.10). For example, the lowest Eh level in 

the top 1 cm of the sediment at near-field and discretionary stations was -107 and -171 mV, 

respectively, relative to +104 mV in far-field sediments (Table 8.10 and Figure 8.31). Likewise, 

the sediment depth to zero oxygen was closer to the sediment-water interface at near-field and 

discretionary stations from Cruise 2B relative to far-field stations (Table 8.10). Some near-field 

stations (e.g., NF-B05, Figure 8.31), had similar oxygen and Eh profiles as observed at far-field 

stations. Overall, the sediment depth to zero oxygen and the Eh values were lower at near-field 

stations from Cruise 2B than during Cruise 1B (Table 8.10), yet results for the near-field stations 

from Cruise 1B showed lower levels of oxygen and Eh. 

The mean integrated amounts of oxygen (ΣO2) in the sediments at far-field stations from 

Cruises 1B and 2B and near-field stations from Cruise 1B were not statistically different, even 

though the mean value for the near-field stations from Cruise 1B was about 20% lower than at 

far-field stations (Figure 8.32). Absolute mean values for ΣO2 at the far-field stations for GB 516 

(153 and 165 nmol/cm2

) are significantly greater than at VK 916 (90 and 84 nmol/cm2), most 

likely a function of higher levels of TOC at VK 916 (~1.5% TOC) relative to GB 516 (~1.0% 

TOC) because sedimentation rates are similar. 

In contrast, values for ΣO2 in sediments collected at near-field and discretionary stations during 

Cruise 2B averaged about 15% and 10% of far-field values, respectively (Figure 8.32). This shift 

is directly related to the presence of drilling discharges. The rapid deposition of several 

centimeters of more organic-rich sediment at many stations in the near-field zone leads to an 

increasing rate of bacterial activity with resulting depletion of dissolved oxygen. The mean value 

for ΣO2 for near-field stations from Cruise 2B was about 20% of the value from Cruise 1B. 

Therefore, most of the change in the redox environment of the sediments that resulted from 

drilling discharges occurred between the two cruises. 

285 
0

50

100

150

200

Integrated O2 (nmol/cm2

)

GB 516

Cruise 1B

122

153

25 17

165

Cruise 2B

NF FF DS NF FF

Figure 8.32. Integrated amounts of oxygen in the sediment column at near-field (NF), far-field 

(FF), and discretionary (DS) stations at Garden Banks (GB) 516 for Cruises 1B 

and 2B. Markers and numbers show means, and lines show standard deviations. 

The line for the standard deviation for the DS data is obscured by the marker. 

8.4.4 Interstitial Water Composition 

Results for interstitial water from one far-field and one near-field station from Cruise 1B for 

GB 516 show some similarities and some distinct differences (Figures 8.33 and 8.34). Levels of 

ΣO2 were 251 nmol/cm2

 in the far-field core relative to 74 nmol/cm2 in the near-field core. The 

Eh of >+600 mV in the top layer of the far-field core decreased gradually to +200 to +100 mV by 

depths of 15 to 20 cm. In the near-field core, the Eh at the top of the core was about +200 mV 

and increased to +400 mV from 5 to 15 cm and then decreased to +200 to +100 mV at 20 cm. No 

discernible sulfate reduction or detectable sulfide (i.e., <2 μM total H2S) were observed in either 

core (Figures 8.33 and 8.34). Nitrate was detected in the top layers of both cores (Figure 8.33). 

Concentrations of Fe2+ increased at depths below 20 cm in both cores; however, Fe2+ also was 

detected in the top few centimeters of the near-field core. Levels of Mn2+ were below detection 

limits in the top centimeters of the far-field core, with a gradual increase to ~100 μM at a depth of 

~20 cm. In contrast, the Mn profile in the near-field core was marked by a sharp increase in the 

top few centimeters and a decrease to ~100 μm by 5 cm. 

The sediments from Station FF2 (Cruise 1B) show no evidence of drilling discharges (0.13% Ba), 

and levels of TOC were 0.9%. In contrast, the top layer of sediment at Station NF-B01 

(Cruise 1B) contained 1.2% Ba and 1.8% TOC. Collectively, the interstitial water results for 

far-field and near-field stations during Cruise 1B show that the upper few centimeters of sediment 

grade from oxic to sub-oxic in both cores, with the inventory of oxygen being more depleted in 

the near-field core. Once again, the surface 10 cm of the Mn2+ profiles are very different between 

the far-field and near-field stations during Cruise 1B, with much greater amounts of Mn2+ being 

produced in the near-field core, where oxygen levels are depleted within the top 2 cm of the 

sediment column. These shifts are directly related to the loading of TOC and sediment from 

drilling discharges. 

286 
Pore Water Results for GB 516 FF-B02 (Cruise 1B) 

0 10 20 30

Sulfate (mM)

0

10

20

30

Sediment Depth (cm)

Figure 8.33. Vertical profiles for dissolved oxygen, Eh, nitrate, sulfate, manganese, and iron 

from interstitial water from Station FF-B02 at Garden Banks (GB) 516 during 

Cruise 1B. 

0 50 100 150 200 250

Nitrate (μM)

0

10

20

30

Sediment Depth (cm)

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

Sediment Depth (cm)

6789

pH

-200 100 400 700

Eh (mV)

0

10

20

30

Sediment Depth (cm)

pH

Eh

0 1

2

24 36 48 60

Iron (μM)

0

10

20

30

Sediment Depth (cm)

Manganese (μM)

0 50 100 150 200 250

0

10

20

30

Sediment Depth (cm)

287 
Pore Water Results for GB 516 NF-B01 (Cruise 1B) 

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

Sediment Depth (cm)

6789

pH

-200 100 400 700

Eh (mV)

0

10

20

30

Sediment Depth (cm)

pH

Eh

0 20 40 60 80 100

Ammonia (μM)

0

10

20

30

Sediment Depth (cm)

 

0 10 20 30

Sulfate (mM)

0

10

20

30

Sediment Depth (cm)

0 100 200 300 400 500

Manganese (μM)

0

10

20

30

Sediment Depth (cm)

 

0 20 40 60 80 100

Iron (μM)

0

10

20

30

Sediment Depth (cm)

Figure 8.34. Vertical profiles for dissolved oxygen, Eh, ammonia, sulfate, manganese, and iron 

from interstitial water from Station NF-B01 at Garden Banks (GB) 516 during 

Cruise 1B. 

288 
During Cruise 2B, interstitial water was collected at Station FF6. The profiles for various 

chemical constituents in the interstitial water from Station FF6 during Cruise 2B are similar to 

those for the Cruise 1B samples in that no sulfate reduction and no sulfide are detected and the 

Mn2+ and Fe2+ profiles are similar to those at Station FF2 from Cruise 1B (Figure 8.35). 

However, the sediments from Station FF6 (Cruise 2B) are more reducing than was observed in 

the two cores previously described for GB 516. This manifests in lower levels of Eh (+100 to 

-100 mV) and higher levels of dissolved ammonia at the top of the core (Figure 8.35). However, 

this difference is most likely part of the continuum of background stations, as Ba levels (0.12%) 

and concentrations of TOC (1%) are compatible with natural levels and show no indication of the 

presence of drilling discharges. 

Sediment cores were collected during Cruise 2B at each of the discretionary stations for chemical 

analysis of sediment solids. Sediments from Stations DS-1 and DS-2 contained elevated levels of 

Ba and TOC relative to Station DS-3 (Figure 8.36). As a result of the presence of drilling 

discharges, remobilization and loss of Mn from the sediments via the interstitial water is clearly 

observed at Stations DS-1 and 2, but not Station DS-3. 

8.5 GARDEN BANKS BLOCK 602 AND MISSISSIPPI CANYON BLOCK 292 – 

POST-DEVELOPMENT SITES 

8.5.1 Metals and Total Organic Carbon 

Means, standard deviations, maximums, and minimums for concentrations of metals, and TOC 

for surface (0 to 2 cm) sediments from development sites GB 602 and MC 292 are summarized in 

Table 8.11. The data in Table 8.11 are for random stations in near-field and far-field zones and 

do not include results from fixed discretionary stations. The complete data set is presented in

Appendix G. Comparisons discussed here are between zones at each site because each site was 

sampled only once after undergoing some degree of development. 

Mean concentrations of Al and Fe in surface sediments from near-field versus far-field stations at 

each site varied by <10% (Table 8.11). The mean Fe/Al ratios of 0.52 at MC 292 and 0.49 at site 

GB 602 are close to a value of 0.53 for suspended sediment (4.61% Fe/8.65% Al) from the 

Mississippi River (Trefry and Presley 1982). The uniformity in the Fe/Al ratios for sediments 

from these two sites supports a similar terrigenous source via the Mississippi River for these 

sediments, as previously discussed. 

Mean concentrations of Ba concentrations were ~60-fold and ~12-fold greater at near-field than 

far-field stations for GB 602 and MC 292, respectively (Table 8.11 and Figure 8.37). At each 

site, Ba levels are statistically higher in the near-field zone than in the far-field zone (Table 8.12), 

with a much greater degree of enrichment in sediment Ba levels at GB 602. The spatial 

distribution of Ba in sediments for GB 602 shows two zones where concentrations were >5% 

(Figure 8.38). No Ba concentrations >5% were found in sediments from MC 292 (Figure 8.38). 

Concentrations of Ba >1%, relative to background levels of ~0.1%, are found throughout the 

~500 m radius from the center of drilling site GB 602, but only for a small portion of MC 292 

(Figure 8.38). 

289 
Pore Water Results for GB 516 FF-B06 (Cruise 2B) 

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

6789

pH

-200 100 400 700

Eh (mV)

0

10

20

30

pH

Eh

0 10 20 30

Sulfate (mM)

0

10

20

30

0 20 40 60 80 100

Ammonia (μM)

0

10

20

30

0 100 200 300 400 500

Manganese (μM)

0

10

20

30

0 20 40 60 80 100

Iron (μM)

0

10

20

30

Figure 8.35. Vertical profiles for dissolved oxygen, Eh, ammonia, sulfate, manganese, and iron 

from interstitial water from Station FF-B06 at Garden Banks (GB) 516 during 

Cruise 2B. 

290 
0 6000 12000 18000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-1 (2B)

0 6000 12000 18000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-2 (2B)

0 6000 12000 18000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 516, DS-3 (2B)

28% Ba 

2.4% TOC 

20% Ba 

Drilling Discharges 

Drilling Discharges 

Drilling Discharges 

0.5% Ba 

7.2% TOC 1.3% TOC 

Figure 8.36. Vertical profiles for concentrations of manganese (Mn) in sediment from 

discretionary (DS) stations at Garden Banks (GB) 516 for Cruise 2B. Line shows 

demarcation of the presence of drilling discharges (above the line). Data are for 

concentrations of barium (Ba) and total organic carbon (TOC) in the 0 to 2 cm 

section of the core. 

Vertical profiles for Ba in sediments were obtained for three near-field discretionary stations at 

each site during Cruise 2B (Figure 8.39). Concentrations of Ba are above background levels of 

~0.1% in the top 2 cm in all six cores (Figure 8.39). However, elevated Ba levels are found no 

deeper than 6 cm in any of the cores (Figure 8.39). As observed for the surface samples, 

concentrations of Ba in the cores from GB 602 are as much as 10 times greater than at MC 292. 

Concentrations of Mn in surface sediments at near-field stations show a larger range in 

concentrations relative to far-field stations (Table 8.11). All nine samples with values for Mn 

<2,000 μg/g at GB 602 and four of fives samples with Mn levels <2,000 μg/g at MC 292 were 

collected from the near-field zone. These observations support reductive dissolution of Mn and 

diffusion of Mn2+ from the sediments to the overlying water column at some near-field stations 

when reducing conditions develop where drilling discharges are found. This information and the 

results for Mn in sediment cores will be further considered in discussions regarding the redox 

state of the sediment. 

Mean concentrations of Al, As, Cd, Fe, and V are not significantly different between far-field and 

near-field stations for surface sediments from both sites GB 602 and MC 292 (Tables 8.11 and 

8.12). At GB 602, concentrations of Ba, Cr, Cu, Pb, and Zn are significantly greater at near-field 

stations than at far-field stations, whereas levels of Ni are significantly lower at near-field stations 

than far-field stations (Tables 8.11 and 8.12). The only metal that shows a zone-wide shift in 

concentrations that can be related to drilling at site MC 292 is Ba. 

291 
Table 8.11. Means, standard deviations, and ranges for metals and total organic carbon (TOC) 

for Garden Banks (GB) 602 and Mississippi Canyon (MC) 292 during 

post-development Cruise 2B. 

Element GB 602 

Far-field

GB 602 

Near-field

MC 292 

Far-field

MC 292 

Near-field 

Al (%) 5.61 ± 0.26 

(5.34-6.08) 

5.74± 0.48 

(4.89-6.17) 

7.85 ± 0.84 

(6.18-9.86) 

7.44 ± 0.25 

(7.01-7.91) 

As (μg/g) 10.6 ± 0.3 

(10.0-11.0) 

11.5 ± 1.3 

(9.1-13.2) 

14.5 ± 1.9 

(11.8-17.1) 

14.0 ± 2.0 

(10.1-16.5) 

Ba (%) 0.085 ± 0.030 

(0.057-0.155) 

5.08 ± 3.19 

(1.66-11.6) 

0.076 ± 0.011 

(0.063-0.092) 

0.90 ± 1.10 

(0.126-3.97) 

Cd (μg/g) 0.27 ± 0.07 

(0.22-0.46) 

0.33 ± 0.10 

(0.19-0.54) 

0.20 ± 0.03 

(0.17-0.25) 

0.21 ± 0.04 

(0.17-0.32) 

Cr (μg/g) 50.9 ± 5.6 

(39.6-57.8) 

62.2 ± 4.8 

(55.3-68.0) 

77.0 ± 9.7 

(63.2-103) 

74.5 ± 13.6 

(43.7-91.0) 

Cu (μg/g) 28.7 ± 0.6 

(27.7-29.8) 

36.0 ± 6.0 

(30.9-49.6) 

27.7 ± 3.8 

(20.5-36.0) 

28.2 ± 1.3 

(26.8-31.3) 

Fe (%) 2.71 ± 0.14 

(2.58-2.95) 

2.87 ± 0.15 

(2.58-3.07) 

4.04 ± 0.42 

(3.20-5.11) 

3.88 ± 0.17 

(3.71-4.24) 

Hg (ng/g) 98 ± 7 

(84-108) 

104 ± 27 

(70-167) 

70 ± 6 

(61-82) 

82 ± 12 

(61-109) 

Mn (μg/g) 3,170 ± 283 

(2,850-3,700) 

2,320 ± 1,990 

(417-5,990) 

3,570 ± 1,420 

(921-5,610) 

3,890 ± 2,050 

(722-6,730) 

Ni (μg/g) 41.0 ± 2.3 

(37.7-44.8) 

32.2 ± 6.1 

(23.6-43.8) 

39.1 ± 4.1 

(28.9-46.3) 

38.1 ± 2.9 

(31.7-41.7) 

Pb (μg/g) 21.6 ± 1.5 

(19.5-24.1) 

41.8 ± 10.0 

(22.1-57.0) 

31.4 ± 1.1 

(30.2-34.0) 

30.1 ± 5.0 

(24.6-44.4) 

V (μg/g) 116 ± 5 

(109-125) 

118 ± 9 

(100-130) 

157 ± 18 

(119-199) 

157 ± 8 

(147-177) 

Zn (μg/g) 77.4 ± 5.2 

(71.8-89.3) 

90.3 ± 8.9 

(76.2-101) 

113 ± 12 

(90-141) 

120 ± 58 

(50-294) 

TOC (%) 1.12 ± 0.13 

(0.91-1.32) 

1.44 ± 0.28 

(1.13-1.99) 

1.41 ± 0.08 

(1.26-1.51) 

1.35 ± 0.12 

(1.20-1.63) 

292 
0.05

0.1

1

10

30

Ba (%)

NF FF NF FF

GB 602 (2B) MC 292 (2B)

0.085 0.076

0.90

5.08

Figure 8.37. Concentrations of barium (Ba) with markers and numbers showing means and 

lines showing standard deviations in sediment from near-field (NF) and far-field 

(FF) stations at Garden Banks (GB) 602 and Mississippi Canyon (MC) 292 during 

post-development Cruise 2B. In some cases, the line for the standard deviation 

appears skewed due to the logarithmic scale. The line for standard deviation for 

the FF data at MC 292 is obscured by the marker. 

Table 8.12. Results of statistical comparisons (t-tests) for post-development Garden Banks 

(GB) 602 and Mississippi Canyon (MC) 292 during Cruise 2B. Comparisons are 

between near-field (NF) and far-field (FF) stations at each site during one sampling 

trip. Results for discretionary stations were not included in the statistical tests. 

Dependent 

Variable 

Independent 

Variable Probability > F Interpretation 

GB 602 

log Ba zone <0.0001 NF > FF 

Cr zone <0.0001 NF > FF 

Cu zone 0.003 NF > FF 

Ni zone <0.0011 FF > NF 

Pb zone <0.0001 NF > FF 

Zn zone 0.0007 NF > FF 

TOC zone 0.0033 NF > FF 

MC 292 

log Ba zone <0.0001 NF > FF 

Hg zone 0.011 NF > FF 

293 
GB 602

Post-Drilling

Figure 8.38. Concentrations of barium (Ba) at Garden Banks (GB) 602 and Mississippi Canyon 

(MC) 292 near-field (NF) stations during post-development Cruise 2B. Inset 

boxes show mean Ba concentrations at all far-field (FF) stations. Data for the 

discretionary stations are underlined. The circle represents a radius of 500 m. 

The darker shaded ovals highlight areas where Ba concentrations are ≥ 5%, and the 

lighter shaded, larger oval highlights area where Ba concentrations are ≥ 1%. 

(Cruise 2B) 

Barium (%)

500-m radius

(NF)

All FF stations

(Cruise 2B)

0.09 ± 0.03%

#

11.1

2.8

6.5

11.6

4.1

3.3

1.7

5.0

4.5

3.5

4.7

2.4

12.9

4.4

LEGEND: 

= Box core station with Ba concentration 

= Previous wellsite

500-m radius

(NF)

All FF stations

(Cruise 2B)

0.07 ± 0.01%

0.5

0.3

0.5

0.2

0.3 1.2

0.5

1.5

0.6 0.8

1.2

4.0

2.0

0.1

0.2

LEGEND: 

= Box core station with Ba concentration 

= Previous wellsite

MC 292

Post-Drilling

(Cruise 2B) 

Barium (%)

294 
0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-1 (2B)

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-2 (2B)

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-3 (2B)

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-1 (2B)

 

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-2 (2B)

0.01 0.1 1 10 40

Total Ba (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-3 (2B)

Figure 8.39. Vertical profiles for concentrations of barium (Ba) in sediment from discretionary 

(DS) stations at Garden Banks (GB) 602 and Mississippi Canyon (MC) 292. 

Some elevated levels of selected metals were observed at each site. Concentrations of Pb at 

>50 μg/g were found at five (near-field + discretionary) stations from site GB 602 and none from 

site MC 292 relative to background levels of 20 to 35 μg/g (Figure 8.40a). The elevated Pb 

levels were found for stations where sediment Ba concentrations also were elevated. Mercury 

concentrations greater than about 0.15 μg/g were found at three (near-field + discretionary) 

stations from GB 602 and none from MC 292 relative to background levels of 70 to 100 ng/g 

(Figure 8.40b). No significant difference was found for Hg concentrations in sediments from 

near-field versus far-field stations for both sites (Figure 8.41). Vertical profiles for Hg in 

sediments from discretionary stations show significantly elevated Hg levels only in one core from 

GB 602 to a depth of 2 cm (Figure 8.42). The lack of any high levels of Hg is most likely due to 

the lack of sediment samples with greatly elevated levels of Ba (>20%). The highest Zn 

concentration, of 294 μg/g, was found at one near-field station from MC 292. A more detailed 

discussion of the complete data set for metals in sediments from all sites is presented in a later 

section of this chapter. 

295 
Total Ba (%)

0 5 10 15 20

Total Pb (μg/g)

0

20

40

60

80

100

GB 602 2B FF

MC 292 2B FF

GB 602 2B NF

MC 292 2B NF

(a)

Total Ba (%)

0 10 20

Total Hg (ng/g)

0

100

200

300

400

(b)

GB 602 2B NF

GB 602 2B FF

MC 292 2B FF

MC 292 2B NF

Figure 8.40. Concentrations of barium (Ba) versus (a) lead (Pb) and (b) mercury (Hg) for 

sediments from far-field (FF) and near-field (NF) stations from Garden Banks 

(GB) 602 and Mississippi Canyon (MC) 292 collected during Cruise 2B. Points 

from FF samples are hidden beneath points for NF samples at Ba levels <0.2%. 

296 
0

50

100

150

Total Hg (ng/g)

NF FF NF FF

GB 602 (2B) MC 292 (2B)

98

70

82

104

Figure 8.41. Concentrations of mercury (Hg) with markers and numbers showing means and 

lines showing standard deviations in sediment from near-field (NF) and far-field 

(FF) stations at Garden Banks (GB) 602 and Mississippi Canyon (MC) 292 during 

Cruise 2B. The lines for standard deviation for the FF data are obscured by the 

markers. 

Concentrations of TOC in surface sediments from near-field stations are statistically higher than 

at far-field stations for GB 602 but not for MC 292 (Figure 8.43). The maximum values for TOC 

in near-field or discretionary samples from GB 602 and MC 292 were 2.07% and 1.63%, 

respectively. These maximum levels are not greatly enriched above background levels of 1.1% to 

1.4% and are considerably below maximum values of 7.2% at GB 516 and 3.4% at VK 916, even 

though average Ba concentrations for GB 602 of 5.1% are similar to mean levels of 3.6% at 

VK 916 (post-development) and 5.9% at GB 516 (post-development). Vertical profiles for TOC 

(Figure 8.44) show relatively uniform trends at close to background values with the exception of 

Station DS-1 from GB 602, where values for the top 4 cm are 70% to 140% greater than 

background values. 

297 
0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DISC 1 (2B)

 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

G

 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-1 (2B) GB 602, DS-2 (2B) B 602, DISC 2 (2B) GB 602, DISC 3 (2B) GB 602, DS-3 (2B) 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

M

 

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

M

0 50 100 150 200

Total Hg (ng/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-1 (2B) C 292, DISC 1 (2B) MC 292, DS-2 (2B) C 292, DISC 2 (2B) MC 292, DISC 3 (2B) MC 292, DS-3 (2B) 

Figure 8.42. Vertical profiles for concentrations of mercury (Hg) in sediment from discretionary 

(DS) stations at Garden Banks (GB) 602 and Mississippi Canyon (MC) 292. 

0.0

0.5

1.0

1.5

2.0

TOC (%)

NF NF

1.12

1.41 1.35 1.44

FF FF

GB 602 (2B) MC 292 (2B)

FF FF 

Figure 8.43. Concentrations of total organic carbon (TOC) with markers and numbers showing 

means and lines showing standard deviations in sediment from near-field (NF) and 

far-field (FF) stations at Garden Banks (GB) 602 and Mississippi Canyon 

(MC) 292 during post-development Cruise 2B. 

298 
0123 4

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DISC 1 (2B)

0123

TOC (%)

4

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DISC 1 (2B)

01234

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-1 (2B) GB 602, DS-2 (2B) GB 602, DISC 3 (2B) GB 602, DS-3 (2B) 

01234

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DISC 1 (2B)

 

0123 4

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DISC 2 (2B)

 

01234

TOC (%)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-1 (2B) MC 292, DS-2 (2B) MC 292, DISC 3 (2B) MC 292, DS-3 (2B) 

Figure 8.44. Vertical profiles for concentrations of total organic carbon (TOC) in sediment 

from discretionary (DS) stations at Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292. 

8.5.2 Sediment Geochronology 

The vertical profile for excess 210Pb at Station FF5 for GB 602 from Cruise 2B shows an 

exponential decrease with depth on the natural logarithm plot (Figure 8.45). The resulting 

sedimentation rate of 0.04 cm/y is similar to results obtained for VK 916 and GB 516. In 

contrast, the sedimentation rate at MC 292 is more than three times greater at 0.14 cm/y. The 

137Cs profile is well defined for MC 292 and supports a sedimentation rate of ~0.1 cm/y 

(5 cm/50 y). The higher sedimentation rate at MC 292 relative to the other three slope locations 

(VK 916, GB 516, and GB 602) is consistent with the location of MC 292 closer to the pathway 

of sediment transport from the Mississippi River. 

299 
GB 602 FF5 2B

ln Excess 210Pb Activity (dpm/g)

-2 0 2 4

Sediment Depth (cm)

0

5

10

15

20

S = 0.04 cm/y

r

2 = 0.98

MC 292 FF2 2B

ln Excess 210Pb Activity (dpm/g)

012345

Sediment Depth (cm)

0

5

10

15

20

137Cs Activity (dpm/g)

0.0 0.1 0.2 0.3

Pb-210

Cs-137 

S = 0.14 cm/y

r

2 1950 = 0.94

Figure 8.45. Vertical profiles for activities of excess 210Pb in sediment from Garden Banks 

(GB) 602 Station FF-B05 and excess 210Pb and 137Cs from Mississippi Canyon 

(MC) 292 Station FF-B02 (S = sedimentation rate, r = correlation coefficient). 

300 
8.5.3 Sediment Oxygen Levels and Redox Conditions 

At GB 602, concentrations of dissolved O2 in sediment from far-field sites decreased to 

non-detectable levels (<2 μM) at sediment depths of 2.8 to more than 6.7 cm; all of which were 

greater than maximum levels at the near-field and discretionary stations (Table 8.13 and 

Figure 8.46). Similarly, the minimum value for Eh in the top centimeter from far-field stations 

of +225 mV was much greater than the lowest values of -167 and -296 mV when considering all 

near-field and all discretionary stations, respectively (Table 8.13). The contrast is quite striking 

in Figure 8.46 for the far-field versus near-field and discretionary examples shown. Consistent 

with these trends, the mean value for ΣO2 for the far-field sites is ~10 times greater than at nearfield or discretionary stations (Figure 8.47). 

Table 8.13. Summary data for dissolved oxygen, redox potential (Eh), and pH for sediment from 

Garden Banks (GB) 602 and Mississippi Canyon (MC) 292 at near-field (NF), 

far-field (FF), and discretionary (DS) stations during post-development Cruise 2B. 

Site Zone Zero O2

depth (cm) Eh1 cm range (mV) Eh10 cm range (mV) pH 

GB 602 

(Cruise 2B) 

NF 0.1-2.8 +402 -167 +358 -85 6.8-8.0 

DS 0.2-1.7 +424 -296 +273 -165 7.2-8.0 

FF 2.8->6.1 +462 +225 +368 +93 7.0-7.5 

MC 292 

(Cruise 2B) 

NF 0.2-3.0 +350 -163 +155 -387 6.5-7.6 

DS 0.8-1.7 +321 +29 +102 +85 7.1-7.4 

FF 0.2->3.3 +449 +158 +164 -49 7.0-8.0 

At MC 292, differences between far-field and near-field stations are less striking. For example, 

the sediment depth to zero oxygen ranged from 0.2 to about 3.3 at both far-field and near-field 

stations. Thus, the profiles for dissolved oxygen (Figure 8.48) and the values for ΣO2 are more 

similar among far-field, near-field, and discretionary stations (Figure 8.47). This degree of 

similarity is partly due to lower levels of ΣO2 at far-field stations from site MC 292 (65 nmol/cm2

) 

than for far-field stations from GB 602 (227 nmol/cm2

). This difference in background 

conditions between the two sites is most likely related to the proximity of MC 292 to the 

Mississippi River delta and the higher sedimentation rate. 

301 
Figure 8.46. Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in sediment 

from representative near-field (NF), far-field (FF), and discretionary (DS) stations 

at Garden Banks (GB) 602 (note different depth scales for oxygen versus Eh). 

302 
Figure 8.47. Integrated amounts of oxygen in the sediment column at near-field (NF), far-field 

(FF), and discretionary (DS) stations at Garden Banks (GB) 602 and Mississippi 

Canyon (MC) 292. Markers and numbers show means and lines show standard 

deviation. 

8.5.4 Interstitial Water Composition 

At GB 602, the Eh was between +300 to +400 mV throughout the core from Station FF5 relative 

to 0 to -250 mV at Station DS-2 (Figures 8.49 and 8.50). A 6-cm thick layer of sediment 

containing drilling discharges was observed at Station DS-2 (>3% Ba, but only ~1.1% TOC). 

The lower Eh in the DS-2 core is consistent with the differences in the depth of oxygen 

penetration of ~6 cm at Station FF5 relative to ~0.5 cm at Station DS-2 (Figures 8.49 and 8.50). 

Strongly reducing conditions in the top 6 cm of the core from Station DS-2 lead to large 

concentrations of dissolved Mn2+ and NH4

+

 relative to the far-field core, where concentrations of 

both chemical species were near detection limits (Figures 8.49 and 8.50). Once again, the 

difference in redox conditions between far-field and near-field stations is very clear, and the Eh 

and dissolved oxygen data provide a reasonably good picture of chemical conditions in the 

interstitial water. 

At MC 292, differences in redox state between the far-field and discretionary cores, as 

demonstrated by dissolved oxygen, Eh, dissolved Mn2+, and ammonia, are minor (Figures 8.51

and 8.52). This lack of difference is partly related to the low levels of material from drilling 

discharges (<1% Ba and no difference in levels of TOC), as well as the naturally more reducing 

environment in the more rapidly accumulating sediments at MC 292. 

The distinction between the discretionary stations at GB 602 versus MC 292 is further 

demonstrated by the vertical profiles for sediment Mn (Figure 8.53). At GB 602, the surface 

layer of sediment contains very low levels of Mn (~600 μg/g), most likely the result of reduction 

of Mn oxides in the surface layer of sediment, with subsequent diffusion of Mn2+ from the 

sediments to the overlying water. In contrast, at MC 292, the surface layer of sediment contains 

~4,000 μg/g of Mn, showing much less, if any, reduction loss of Mn from the sediments. These 

observations are consistent with a lesser impact from drilling discharges at MC 292 than at 

GB 602. 

GB 602

22

227

55

38

65

NF FF NF FF

MC 292

DS DS

25

0

50

100

150

200

250

Integrated O2 (nmol/cm2

)

303 
MMS Deep Gulf of Mexico Cruise 2: MC 292 FF5-B01

0 50 100 150 200 250

Oxygen (μM)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Sediment Depth (cm)

6 7 8 9

pH

-200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

pH

Eh

MMS Deep Gulf of Mexico Cruise 2: MC 292 NF-B09

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sediment Depth (cm)

6 7 8 9

pH

-200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

pH

Eh

MMS Deep Gulf of Mexico Cruise 2: MC 292 DS-B01

0 50 100 150 200 250

Oxygen (μM)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sediment Depth (cm)

6 7 8 9

pH

-200 100 400 700

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

pH

Eh

Figure 8.48. Vertical profiles for dissolved oxygen, redox potential (Eh), and pH in sediment 

from representative near-field (NF), far-field (FF), and discretionary (DS) stations at 

Mississippi Canyon (MC) 292 (note different depth scales for oxygen versus Eh). 

304 
Pore Water Results for GB 602 FF-B05 (Cruise 2B)

Sediment Depth (cm)

Sediment Depth (cm)

Sediment Depth (cm)

Sediment Depth (cm)

Sediment Depth (cm)

Sediment Depth (cm)

Figure 8.49. Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, manganese, and 

sulfate from interstitial water from Station FF-B05 at site Garden Banks (GB) 602 

during post-development Cruise 2B. 

305 
Pore Water Results for GB 602 DS-2 (Cruise 2B) 

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

Sediment Depth (cm)

6 7 8 9

pH

-300 0 300 600

Eh (mV)

0

5

10

15

20

25

30

Sediment Depth (cm)

pH

Eh

0 10 20 30 40

Nitrate (μM)

0

10

20

30

Sediment Depth (cm)

0 20 40 60

Ammonia (μM)

0

10

20

30

Sediment Depth (cm)

0 100 200 300

Manganese (μM)

0

10

20

30

Sediment Depth (cm)

0 10 20 30

Sulfate (mM)

0

10

20

30

Sediment Depth (cm)

Figure 8.50. Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, manganese, and 

sulfate from interstitial water from discretionary station DS-2 at Garden Banks 

(GB) 602 during post-development Cruise 2B. 

306 
Pore Water Results for MC 292 FF-B02 (Cruise 2B) 

0 20 40 60

Ammonia (μM)

0

10

20

30

Sediment Depth (cm)

0 10 20 30 40

Nitrate (μM)

0

10

20

30

Sediment Depth (cm)

6789

pH

- 200 100 400 700

Eh (mV)

0

10

20

30

pH

Eh

Sediment Depth (cm)

0 10 20 30

Sulfate (mM)

0

10

20

30

Sediment Depth (cm)

0 50 100 150 200 250

Oxygen (μM)

0

10

20

30

Sediment Depth (cm)

Figure 8.51. Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, and sulfate from 

interstitial water from Station FF-B02 at Mississippi Canyon (MC) 292 during 

post-development Cruise 2B. 

307 
Pore Water Results for MC 292 DS-2 (Cruise 2B) 

Figure 8.52. Vertical profiles for dissolved oxygen, Eh, nitrate, ammonia, manganese, and 

sulfate from interstitial water from discretionary station DS-2 at Mississippi 

Canyon (MC) 292 during post-development Cruise 2B. 

308 
0 6,000 12,000 18,000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-1 (2B)

0 6,000 12,000 18,000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-2 (2B)

0 6,000 12,000 18,000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

GB 602, DS-3 (2B)

0 6,000 12,000 18,000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-1 (2B)

 

0 6,000 12,000 18,000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-2 (2B)

 

0 6,000 12,000 18,000

Total Mn (μg/g)

0

2

4

6

8

10

12

Sediment Depth (cm)

MC 292, DS-3 (2B)

Figure 8.53. Vertical profiles for concentrations of manganese (Mn) in sediment from 

discretionary (DS) stations at Garden Banks (GB) 602 and Mississippi Canyon 

(MC) 292 for post-development Cruise 2B. 

8.5.5 Metals in Isopods and Crabs 

Concentrations of 11 metals (As, Ba, Cd, Cr, Cu, Fe, Hg, Ni, Pb, V, and Zn) were determined in 

samples of the giant isopod Bathynomus giganteus (Figure 8.54) and the red crab Chaceon 

quinquedens (Figure 8.55) from near-field and far-field stations at GB 602 and MC 292. The 

concentrations are presented here on a dry weight basis; however, water content is available for 

each sample in Appendix G. In general, the water content in each species is 70% to 80% by 

weight. In many cases, the standard deviation for each metal is large, with a coefficient of 

variance of 50% or more (Tables 8.14 and 8.15). 

309 
30 cm 

Figure 8.54. Photograph of giant amphipod Bathynomus giganteus from the Gulf of Mexico. 

20 cm 

Figure 8.55. Photograph of red crab Chaceon quinquedens from the Gulf of Mexico. 

Due to the large standard deviations and small number of samples from each station, most 

differences among the various metals for near-field versus far-field were not statistically 

significant as tested by ANOVA. However, for Bathynomus giganteus, levels of Ba, Cr, and Pb 

at GB 602 were greater at near-field stations than far-field stations, and concentrations of Cd and 

Hg were greater at far-field stations than near-field stations (Table 8.14 and Figures 8.56 and 

8.57). In contrast, no significant differences were observed for metal concentrations for 

near-field versus far-field specimens of Bathynomus giganteus from MC 292 (Table 8.14 and 

Figures 8.56 and 8.57). Some isopods from GB 602 may have contained sediment that led to the 

higher levels of Ba observed. 

310 
Table 8.14. Means ± standard deviations for samples of the giant amphipod Bathynomus 

giganteus from far-field (FF) and near-field (NF) stations from post-development 

sites Garden Banks (GB) 602 and Mississippi Canyon (MC) 292. 

As 

(μg/g) 

Ba 

(μg/g) 

Cd 

(μg/g) 

Cr 

(μg/g) 

Cu 

(μg/g) 

Fe 

(μg/g) 

Hg 

(μg/g) 

Ni 

(μg/g) 

Pb 

(μg/g) 

V 

(μg/g)

Zn 

(μg/g) 

GB 602 

 FF 

(n = 3) 

61 

±33 

15 

±9 

14 

±6 

0.43 

±0.04 

95 

±86 

1,450 

±540 

8.1 

±6.8 

0.55 

±0.45 

0.26 

±0.06 

1.7 

±1.5 

327 

±94 

NF 

(n = 6)

39 

±28 

164 

±136 

7 

±4 

1.2 

±1.2 

51 

±50 

979 

±549 

3.1 

±1.3 

1.3 

±0.8 

0.61 

±0.39 

2.8 

±1.5 

344 

±272 

MC 292 

FF 

(n = 3)

28 

±23 

16 

±15 

16 

±17 

1.5 

±1.3 

73 

±59 

2,250 

±2,530 

4.3 

±4.4 

0.9 

±0.6 

0.46 

±0.38 

3.0 

±1.7 

360 

±339 

NF 

(n = 6)

39 

±21 

17 

±8 

14 

±10 

1.1 

±0.8 

75 

±86 

1,310 

±1,220 

3.3 

±2.8 

0.9 

±0.5 

0.38 

±0.27 

2.8 

±1.1 

264 

±183 

Table 8.15. Means ± standard deviations for samples of the red crab Chaceon quinquedens from 

far-field (FF) and near-field (NF) stations from post-development sites Garden 

Banks (GB) 602 and Mississippi Canyon (MC) 292. 

As 

(μg/g) 

Ba 

(μg/g) 

Cd 

(μg/g) 

Cr 

(μg/g) 

Cu 

(μg/g) 

Fe 

(μg/g) 

Hg 

(μg/g) 

Ni 

(μg/g) 

Pb 

(μg/g) 

V 

(μg/g) 

Zn 

(μg/g) 

GB 602 

FF 

(n = 3) 170 

±29 

11 

±7 

12 

±5 

0.7 

±0.2 

151 

±65 

254 

±81 

1.5 

±0.4 

9.2 

±2.4 

0.32 

±0.14 

3.3 

±1.0 

261 

±31 

NF 

(n = 6) 65 

±25 

54 

±46 

2.1 

±1.1 

1.0 

±0.6 

59 

±45 

216 

±111 

0.54 

±0.14

3.4 

±1.6 

0.26 

±0.10 

1.3 

±0.7 

148 

±61 

MC 292 

FF 

(n = 3) 131 

±46 

3.9 

±1.8 

2.9 

±2.3 

0.6 

±0.4 

199 

±70 

612 

±215 

0.8 

±0.4 

10 

±3.2 

0.25 

±0.12 

2.8 

±1.3 

226 

±27 

NF 

(n = 6) 133 

±37 

43 

±38 

101 

±13 

2.5 

±2.0 

248 

±73 

1,340 

±1,100 

1.0 

±0.4 

14 

±4.6 

0.89 

±0.70 

6.0 

±4.0 

230 

±107 

311 
50

60

70

80

90

100

Water (%)

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

30

60

90

120

As (μ/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

100

200

300

Ba (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

10

20

30

40

Cd (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

1

2

3

Cr (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

200

400

600

800

1000

Cu (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

Figure 8.56. Water content and concentrations of arsenic (As), barium (Ba), cadmium (Cd), 

chromium (Cr), and copper (Cu) in soft tissue from the isopod Bathynomus 

giganteus from near-field (NF) and far-field (FF) stations from post-development 

sites Garden Banks (GB) 602 and Mississippi Canyon (MC) 292. 

312 
0

2000

4000

Fe (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

 

0

5

10

15

Hg (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

1

2

3

Ni (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

 

0.0

0.5

1.0

Pb (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

0

1

2

3

4

5

V (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

 

0

200

400

600

800

Zn (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Bathynomus (Isopod)

Figure 8.57. Concentrations of iron (Fe), mercury (Hg), nickel (Ni), lead (Pb), vanadium (V), 

and zinc (Zn) in soft tissue from the isopod Bathynomus giganteus from near-field 

(NF) and far-field (FF) stations from post-development sites Garden Banks 

(GB) 602 and Mississippi Canyon (MC) 292. 

313 
For the red crab Chaceon quinquedens, concentrations of Ba are greater in specimens from 

near-field versus far-field stations at GB 602, whereas concentrations of As, Cd, Cu, Hg, Ni, V, 

and Zn are greater in specimens from far-field stations than near-field stations (Table 8.15 and 

Figures 8.58 and 8.59). For MC 292, concentrations of Ba, Cd, Cr, and V were greater in 

samples collected at near-field stations than at far-field stations (Table 8.15 and Figures 8.58 and 

8.59). 

8.6 DISCUSSION OF SEDIMENT TRACE METALS 

Concentrations of 13 metals (Al, As, Ba, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V, and Zn) were 

determined in 163 surface sediments (0 to 2 cm) and 50 subsurface sediments (from 12 cores) at 

all four sites (VK 916, GB 516, GB 602, and MC 292) during this study. The data set is used 

here to evaluate possible sediment contamination from trace metals at near-field versus far-field 

sites and pre- versus post-exploration/development. 

Concentrations of Fe in sediments from far-field, near-field, and discretionary stations from all 

sites follow a strong, positive linear trend versus Al (r = 0.95, Figure 8.60) as described 

previously. Concentrations of trace metals in sediments generally correlate well with 

concentrations of Al and Fe because concentrations of most metals are very low in quartz sand or 

carbonate shell material and much higher in fine-grained aluminosilicates. Aluminum and Fe are 

rarely introduced by anthropogenic processes and are present at percent levels in most sediment 

relative to parts per million (μg/g) levels for trace metals. Concentrations of Al and Fe often can 

be used to normalize concentrations of trace metals and thereby incorporate the metal-controlling 

variables of grain size, organic carbon content, and mineralogy. In the ideal case, under natural 

conditions, a good linear correlation is observed between concentrations of a trace metal and Al 

and/or Fe. Plots of trace metal concentrations versus Fe or Al have been used in various forms 

for many years to identify sediment metal contamination (Trefry and Presley 1976; Schropp et al. 

1990). 

Under background conditions, concentrations of trace metals in sediments generally follow a 

strong linear trend versus Al and/or Fe in a given depositional environment. For example, 

concentrations of V in sediments from all far-field, near-field, and discretionary stations from this 

study correlate well with Al (r = 0.91, Figure 8.61) and Fe (r = 0.96) for all surface and 

subsurface sediments collected. The broad range in V concentrations, yet good linear fit for Al 

(and Fe) versus V, is consistent with mixing of relatively uniform composition, metal-rich 

aluminosilicate phases with metal-poor quartz sand and carbonates. Aluminum was chosen for 

normalization in the present study because it is least affected by chemical weathering and 

diagenesis. Vanadium levels in background sediment from the Gulf of Mexico are predicted to 

follow the trend presented in Figure 8.61. Thus, the strong relationship for Al versus V in 

Figure 8.61 also supports a lack of anthropogenic inputs of V and no impact on V levels due to 

sediment diagenesis. The three points that plot slightly above the upper prediction interval on 

Figure 8.61 exceed that limit by <10% and are consistent with the statistical boundaries of a 

95% prediction interval. Eight samples with concentrations of Al and V at <4% and <60 μg/g, 

respectively, contain 30% to 60% industrial barite (18% to 34% Ba), which contains no 

significant amounts of Al or V and thus results in a dilution of these two metals. 

314 
0

50

100

150

200

As (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

10

20

30

Cd (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

50

60

70

80

90

100

Water (%)

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

20

40

60

80

100

Ba (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

1

2

3

4

5

Cr (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

100

200

Cu (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

Figure 8.58. Water content and concentrations of arsenic (As), barium (Ba), cadmium (Cd), 

chromium (Cr), and copper (Cu) in soft tissue from the crab Chaceon from 

near-field (NF) and far-field (FF) stations from post-development sites Garden 

Banks (GB) 602 and Mississippi Canyon (MC) 292. 

315 
0

100

200

300

400

Zn (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0.0

0.5

1.0

1.5

2.0

Pb (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0.0

0.5

1.0

1.5

2.0

Hg (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

1000

2000

3000

Fe (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

5

10

15

20

Ni (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

0

4

8

12

V (μg/g) dry weight

NF FF NF FF

GB 602 MC 292

Chaceon (Crab)

Figure 8.59. Concentrations of iron (Fe), mercury (Hg), nickel (Ni), lead (Pb), vanadium (V), 

and zinc (Zn) in soft tissue from the crab Chaceon from near-field (NF) and 

far-field (FF) stations from post-development sites Garden Banks (GB) 602 and 

Mississippi Canyon (MC) 292. 

316 
Total Al (%)

0 2 4 6 8 10 12

Total Fe (%)

0

2

4

6

Y = 0.46X + 0.11

 r = 0.95, n = 213

GB 516 1B

GB 516 2B

GB 602 2B

MC 292 2B

VK 916 1B

VK 916 3B

Figure 8.60. Concentrations of aluminum (Al) versus iron (Fe) for sediments from all stations. 

Solid line is from linear regression with equation, correlation coefficient (r), and 

number of data points (n). Dashed lines show 95% prediction interval. 

Total Al (%)

0 2 4 6 8 10 12

Total V (μg/g)

0

50

100

150

200

Y = 17.7X + 11.2

 r = 0.91, n = 213

GB 516 1B

GB 516 2B

GB 602 2B

MC 292 2B

VK 916 1B

VK 916 3B

Figure 8.61. Concentrations of aluminum (Al) versus vanadium (V) for all sediments collected 

during the project. Solid line is from linear regression for all samples with 

equation, correlation coefficient (r), and number of samples (n). Dashed lines 

show 95% prediction interval. 

317 
When a graph for Al versus Zn was constructed using all the data, at least 10 values did not fit a 

simple linear trend (Figure 8.62). Thus, a line and prediction interval were constructed for Al 

versus Zn using only the far-field data (Figure 8.62a). The resulting good correlation coefficient 

for the far-field data supports a linear relationship for Al versus Zn in background sediments 

(Figure 8.62a). When data from the near-field and discretionary stations are added to the Al 

versus Zn plot, 11 or more points plot at values of Zn that are ≥ 25% above the upper prediction 

interval (Figure 8.62b). Six of the anomalous points are for discretionary stations from GB 516 

(Cruise 2B). The other anomalous points are from selected near-field stations, all of which are 

post-exploration or post-development samples (Figure 8.62b). 

Results for Pb follow a similar trend as described for Zn with ~20 data points that plot as positive 

anomalies with respect to the x axis (Figure 8.63). Once again, the data for the far-field samples 

were used to define background conditions (Figure 8.63). Six Pb values, five from discretionary 

stations at GB 516 and one from a discretionary station at GB 602, exceed 100 μg/g. An 

additional 10 samples have Pb levels that are greater than 40 μg/g (Figure 8.63). All of the 

anomalous points are from GB 516 or GB 602. In addition to the anomalous data described 

above, 12 other samples contain Pb at concentrations that are >25% above the upper prediction 

interval on the Al versus Pb graph (Figure 8.63). In contrast with the positive anomalies, about 

15 data points for post-exploration or post-development samples show lower Pb levels than 

predicted based on Al concentrations in the sediments. These lower Pb levels may be due to the 

presence of clays in drilling mud or cuttings (source of Al), which are low in Pb. 

Nine sediment samples contain As at levels that plot above the upper prediction interval in 

Figure 8.64. Seven of the samples with elevated As concentrations are from GB 516. Most of 

the samples with Pb concentrations that plot below the lower prediction interval follow the same 

trend as for As. 

Considerable interest has been generated regarding concentrations of Hg in sediments adjacent to 

drilling sites because concentrations of total Hg near drilling sites are often 2 to 10 times higher 

than in nearby background sediments (Neff 2002a). This Hg is known to be a natural impurity in 

barite (Kramer et al. 1980; Trefry and Smith 2003). 

When concentrations of Hg from this study are normalized to concentrations of Al, about 

15 samples have Hg levels that are elevated above the trend observed in the Hg/Al ratio for farfield sediments (Figure 8.65a). Two separate background lines are developed in the Al versus 

Hg plot using far-field samples from (1) GB 516 and GB 602 and (2) MC 292 and VK 916 

because Hg levels are naturally higher at the GB sites, where sediments accumulate more slowly 

and increase the scavenging time for particles to adsorb Hg from the water column. With respect 

to Hg concentrations determined during this study, five samples from GB 516 contain Hg at 

levels >400 ng/g (Figure 8.65a). At eight additional stations, concentrations of Hg are 

>150 ng/g. 

Most of the sediments with elevated concentrations of Hg also contain elevated levels of Ba 

(Figure 8.65b). Previous work, as summarized by Trefry and Smith (2003), shows that the 

barite-bound Hg has a very low degree of bioavailability. 

318 
Total Al (%)

02468 10

Total Zn (μg/g)

0

100

200

300

Y = 14.6X - 1.3

 r = 0.93, n = 76

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF (a) 

Total Al (%)

02468 10

Total Zn (μg/g)

0

100

200

300

Y = 14.6X - 1.3

 r = 0.93, n = 76

DS

NF8

NF10

NF1

NF2

NF4

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 516 2B NF

GB 602 2B NF 

MC 292 2B NF

VK 916 1B NF

VK 916 3B NF

(b) 

Figure 8.62. Concentrations of aluminum (Al) versus zinc (Zn) for sediments from (a) far-field 

(FF) stations and (b) all stations (FF and near-field [NF], including discretionary 

[DS] stations). Solid lines on both graphs are from linear regression for FF 

samples with equations, correlation coefficient (r), and number of samples (n). 

Dashed lines above and below solid line show 95% prediction interval. 

319 
Total Al (%)

0 2 4 6 8 10

Total Pb (μg/g)

0

20

40

60

80

200

400

Y = 3.7X + 5.7

 r = 0.76, n = 76

DS

NF3

DS

NF1,2,8

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 516 2B NF

GB 602 2B NF

MC 292 2B NF

VK 916 1B NF

VK 916 3B NF

Figure 8.63. Concentrations of aluminum (Al) versus lead (Pb) for sediments from far-field (FF) 

and near-field (NF) stations (data for discretionary [DS] stations are plotted as NF 

samples). Solid line is from linear regression for FF samples with equations, 

correlation coefficient (r), and number of samples (n). Dashed lines above and 

below solid line show 95% prediction interval. 

Correlations between concentrations of Ba and total Hg can sometimes be used to help confirm 

that excess total Hg in sediment is associated with barite. Trefry et al. (2003) obtained values of 

r>0.92 for Ba versus Hg in sediments adjacent to drilling sites when sufficient data were available 

to create separate plots for each site. The Ba versus Hg relationship also can be used to calculate 

the concentration of total Hg in the barite used at a particular site. Barite has been specifically 

identified by x-ray diffraction in many near-field samples from the study area. In this study, the 

following two data sets contained high enough Ba levels to estimate the Hg content of the 

industrial barite: (1) five samples from DS-2 stations at GB 516 and (2) the remaining near-field 

samples (n = 35) at GB 516 (Figure 8.65b). If the two regression lines in Figure 8.65b are 

extrapolated to pure barite at 58.8% Ba, the concentrations of total Hg are 944 and 260 ng/g 

(obtained by substituting 58.8% Ba for x in the equations in Figure 8.65b). Typical “industrial 

barite” contains 85% to 95% barite (i.e., 50% to 56% Ba). Based on the range of Ba levels in 

industrial barite (50% to 56% Ba), the average values for total Hg in the (apparently) two 

different barites used at GB 516 are ~870 and 240 μg/g (at 53% Ba). These calculated values are 

in line with USEPA regulations, which allow a maximum Hg level of 1,000 ng/g (1 μg/g) in 

barite (USEPA 1993). 

320 
Total Al (%)

0 2 4 6 8 10

Total As (μg/g)

0

10

20

30

Y = 1.5X + 1.7

 r = 0.69, n = 76

DS

NF1

NF3A

DS

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 602 2B NF

MC 292 2B NF

VK 916 1B NF

VK 916 3B NF

GB 516 2B NF

Figure 8.64. Concentrations of aluminum (Al) versus arsenic (As) for sediments from far-field 

(FF) and near-field (NF) stations (data for discretionary [DS] stations are plotted as 

NF samples). Solid line is from linear regression for FF samples with equations, 

correlation coefficient (r), and number of samples (n). Dashed lines above and 

below solid line show 95% prediction interval. 

The graph for Al versus Cd (Figure 8.66) does not show the same goodness of fit as observed for 

V and Zn, partly because of the greater variability in Cd concentrations. All Cd concentrations 

are <0.6 μg/g; however, samples from discretionary stations at GB 516 and GB 602 plot above 

the other data in a manner similar to that observed for Pb. These samples may have a minor 

anthropogenic contribution of Cd. Concentrations of Cu (Figure 8.66) from sediments with Al 

levels of 5%-8% are similar and do not yield a clear trend. However, samples from the 

discretionary stations at site GB 516 show distinct positive anomalies relative to the other 

samples. Plots for both Ni and Cr (Figure 8.67) show no anomalously high values, yet some low 

concentrations are observed in a manner similar to that described for Pb. 

321 
Total Al (%)

02468 10

Total Hg (ng/g)

0.0

0.2

0.4

0.6

0.8

DS

NF3A

DS NF3

800

400

200

600

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF 

GB 516 2B NF

GB 602 2B NF 

MC 292 2B NF 

VK 916 1B NF

VK 916 3B NF

(a)

Total Ba (%)

0 10 20 30 40 50 60

Total Hg (ng/g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Y = 0.013X + 0.18

 r = 0.63, n = 5

Y = 0.0029X + 0.09

 r = 0.73, n = 35

Industrial Barite

Hg = 0.87 μg/g

Hg = 0.24 μg/g

1200

600

(b)

Figure 8.65. Concentrations of (a) aluminum (Al) versus mercury (Hg) and (b) barium (Ba) 

versus Hg for sediments from far-field (FF) and near-field (NF) stations (data for 

discretionary [DS] stations are plotted as NF samples). Solid lines on (a) are from 

linear regression for FF samples as described in the text. Solid lines on (b) are for 

NF samples from Garden Banks (GB) 516 with equations, correlation coefficient 

(r), and number of samples (n). Extrapolated concentrations of Hg, based on 

53% Ba in industrial barite, are shown in (b) along vertical line. 

322 
Total Al (%)

0 2 4 6 810

Total Cd (μg/g)

0.0

0.2

0.4

0.6

0.8

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 516 2B NF

GB 602 2B NF

MC 292 2B NF

VK 916 1B NF 

VK 916 3B NF (a)

Total Al (%)

02468 10

Total Cu (μg/g)

0

50

100

150

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 516 2B NF

GB 602 2B NF 

MC 292 2B NF

VK 916 1B NF

VK 916 3B NF

(b)

Figure 8.66. Concentrations of aluminum (Al) versus (a) cadmium (Cd) and (b) copper (Cu) for 

sediments from far-field (FF) and near-field (NF) stations (data for discretionary 

stations are plotted as NF samples). Solid lines are from linear regression (set 

through the origin) for FF samples. 

323 
Total Al (%)

0 2 4 6 8 10

Total Ni (μg/g)

0

50

100

GB 516 1B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 516 2B NF

GB 602 2B NF

MC 292 2B NF 

VK 916 1B NF

VK 916 3B NF 

GB 516 2B FF

(a)

Y = 5.4X + 10.1

 r = 0.64

Total Al (%)

02468 10

Total Cr (μg/g)

0

50

100

150

Y = 9.4X + 7.4

 r = 0.66

GB 516 1B FF

GB 516 2B FF

GB 602 2B FF

MC 292 2B FF

VK 916 1B FF

VK 916 3B FF

GB 516 1B NF

GB 516 2B NF 

GB 602 2B NF

MC 292 2B NF

VK 916 1B NF 

VK 916 3B NF

(b)

Figure 8.67. Concentrations of aluminum (Al) versus (a) nickel (Ni) and (b) chromium (Cr) for 

sediments from far-field (FF) and near-field (NF) stations (data for discretionary 

stations are plotted as NF samples). Solid lines are from linear regression for FF 

samples with equations, correlation coefficient (r), and number of samples (n). 

Dashed lines show 95% prediction intervals. 

324 
8.7 CONCLUSIONS 

The data for metals, TOC, and redox parameters include one set of pre-drilling samples (VK 916 

on Cruise 1B) and four sets of post-drilling samples (GB 516 on Cruise 1B and Cruise 2B; 

GB 602 and MC 292 on Cruise 2B). Overall, the following general trends were observed at sites 

where drilling discharges had occurred: 

• Concentrations of Ba were typically enriched by >10 fold at NF versus FF stations. The 

degree of enrichment was typically on the order of 20- to 40-fold. 

• Concentrations of Hg, Zn, As, and Pb were elevated in 6% to 15% of the NF samples relative 

to FF samples. These increases were directly related to increases in concentrations of Ba. 

Calculations indicate that Hg concentrations in barite deposited at these sites were in line 

with USEPA regulations. 

• Concentrations of TOC were typically about one-third greater in NF sediments relative to FF 

sediments. 

• The amount of integrated O2 was commonly 2 to 10 times lower in NF sediments relative to 

FF sediments. 

Most NF versus FF differences in tissue metals in the giant isopod Bathynomus giganteus and the 

red crab Chaceon quinquedens were not significant. This was due to the large standard 

deviations and small number of samples from the two sites (GB 602 and MC 292) where these 

animals were collected. However, for isopods at GB 602, concentrations of Ba, Cr, and Pb were 

greater at NF stations than FF stations and concentrations of Cd and Hg were greater at FF 

stations than NF stations. No significant differences were observed for isopods from MC 292. 

For red crabs at GB 602, concentrations of Ba were greater in specimens from NF versus FF 

stations whereas concentrations of As, Cd, Cu, Hg, Ni, V, and Zn were greater in specimens from 

FF stations than NF stations. For MC 292, concentrations of Ba, Cd, Cr, and V were greater in 

samples collected at NF stations than at FF stations. The most consistent finding was elevated 

Ba, which may be due to sediment particles retained in the gut. 

325 
327 

Chapter 9 

Sediment and Tissue Hydrocarbons 

Thomas J. McDonald and James M. Brooks 

TDI-Brooks International, Inc. 

9.1 INTRODUCTION 

During three cruises, sediments were collected for hydrocarbon analysis from four deepwater 

sites in the northern Gulf of Mexico: 

• VK 916 – exploration site 

• GB 516 – exploration/development site 

• GB 602 – post-development site 

• MC 292 – post-development site 

Cruise 1B (October/November 2000) was the pre-drilling survey for GB 516 and VK 916. 

Cruise 2B (July 2001) was the post-drilling survey for GB 516 and the two post-development 

sites (GB 602 and MC 292). Cruise 3B (August 2002) was the post-drilling survey at VK 916. 

Further details of surveys and general sampling methods are provided in Chapter 2. 

Sediment samples were collected at 12 near-field stations and 12 far-field stations at each site. In 

addition to these routinely collected sediments, three discretionary box-core samples were located 

in suspected cuttings depositional areas mapped during geophysical surveys. All of the sediment 

samples were analyzed for the presence of polycyclic aromatic hydrocarbons (PAH) by gas 

chromatography-mass spectrometry (GC/MS) and synthetic based fluid (SBF) by gas 

chromatography-flame ionization detection (GC-FID). Detailed results for each sample are 

provided in Appendix H. 

PAHs are widely used as an indicator of petroleum input in environmental samples. PAHs 

containing condensed rings and simple alkylations (naphthalenes, fluorenes, phenanthrenes, 

dibenzothiophenes, fluoranthenes, and chyrsenes, etc.) are generally the analytes of interest. 

PAHs are ubiquitous in the marine environments and can have multiple sources such as 

petroleum, early diagenesis, coals, combustion, immature/mature sediments, and anthropogenic 

inputs. 

Synthetic hydrocarbons used in SBMs include linear-α-olefins, poly-α-olefins, internal olefins, 

and esters. A GC-FID chromatogram displaying a typical background deep-sea sediment 

hydrocarbon distribution is shown in Figure 9.1. Figure 9.2 displays a deep-sea sediment 

hydrocarbon distribution pattern that is dominated by SBF. The types of SBMs used at the study 

sites included internal olefins, linear-α-olefins, and esters as discussed in Chapter 3. 
IS

IS

Intensity

Time

Figure 9.1. Typical background hydrocarbon distribution in deep sea sediment. The two 

large peaks are internal standards (IS). The first peak is the solvent peak. 

IS

IS

Intensity

Time

328 

Figure 9.2. A sediment hydrocarbon sample that displays a synthetic based fluid pattern. 
329 

9.2 ANALYTICAL METHODS 

9.2.1 Extraction of Sediments for Aromatic and Saturated Hydrocarbons 

An automated extraction apparatus (Dionex ASE200 Accelerated Solvent Extractor - USEPA 

Method SW-846 3545) was used to extract various organics from 2 to 20 g of pre-dried samples. 

The extractions were performed using methylene chloride solvent inside stainless steel extraction 

cells held at elevated temperature and solvent pressure. The extracts dissolved in the solvent 

were then transferred from the heated extraction cells to glass collection vials containing 

activated Cu granules to minimize matrix interference during quantitative determinations. The 

extracts were subjected to silica gel/alumina columns to remove biogenic organic material that 

can cause positive interference with this method. The extracts were then concentrated to a final 

volume of 1 mL using an evaporative solvent reduction apparatus. Final extracts were submitted 

for determination of aromatic and aliphatic hydrocarbon analytes including SBF. 

9.2.2 Extraction of Tissues for Aromatic Hydrocarbons 

An automated extraction apparatus (Dionex ASE200 Accelerated Solvent Extractor - USEPA 

Method SW-846 3545) was used to extract various organics from 2 to 15 g of pre-dried samples 

(chemically dried or freeze-dried). The extractions were performed using methylene chloride 

solvent inside stainless steel extraction cells held at elevated temperature and solvent pressure. 

The extracts dissolved in the solvent were then transferred from the heated extraction cells to 

glass collection vials. Extracts were then concentrated to a final volume of 4 mL using an 

evaporative solvent reduction apparatus. These extracts were then subjected to high performance 

liquid chromatography (HPLC) cleanup for lipid removal. After HPLC, the extracts were then 

concentrated to 0.5 mL using an evaporative solvent reduction apparatus and submitted for 

determination of aromatic hydrocarbon analytes. 

9.2.3 Determination of Aromatic Hydrocarbons 

PAHs and their alkylated homologues in extracts of sediment and biological tissue (Modified 

SW-846 8270b) were quantified by GC/MS in selected ion monitoring mode. The gas 

chromatograph was temperature-programmed and operated in splitless mode. The capillary 

column was a J&W Scientific DB-5MS® (60 m long by 0.25 mm internal diameter [ID] and 

0.25 μm film thickness) or equivalent. The data acquisition system allowed continuous 

acquisition and storage of all data during analysis and displayed ion abundance versus time or 

scan number. 

Calibration solutions were prepared at five concentrations ranging from 0.02 to 1 μg/mL by 

diluting a commercially available solution containing the analytes of interest (typically NIST 

SRM 1491). For each analyte of interest, a relative response factor (RRF) was determined for 

each calibration level. All five response factors were then averaged to produce a mean relative 

response factor for each analyte. 

An analytical set consisted of standards, samples, and quality control samples. Each extraction 

batch was analyzed as an analytical set including samples and the following quality control 

samples: method blank, matrix spike, matrix spike duplicate, and SRMs. 
330 

9.2.4 Determination of Aliphatic Hydrocarbons 

Saturated hydrocarbons in extracts of sediment (Modified USEPA Method SW-846 8100 and 

USEPA Method 1663) were quantified by high resolution, capillary gas chromatography with 

GC-FID. Normal alkanes with 10 to 34 carbons (C10 to C34), the isoprenoids pristane and 

phytane, the unresolved complex mixture (UCM), and the total resolved peaks were determined 

with this procedure. The gas chromatograph was temperature-programmed and operated in split 

mode. The capillary column is a Restek Scientific RTX-1 (30 m long by 0.25 mm ID and 

0.25 μm film thickness). Carrier flow is by electronic pressure control. The autosampler is 

capable of making 1 to 5 mL injections. Dual columns and dual FIDs were used. The data 

acquisition system is by HP ChemStation software, capable of acquiring and processing GC data. 

A calibration curve was established by analyzing each of five calibration standards (1.25, 10, 25, 

40, and 50 μg/mL) and fitting the data to a straight line using the least square technique. For each 

analyte of interest, a response factor (RF) was determined for each calibration level. All five 

response factors were then averaged to produce a mean relative response factor for each analyte. 

If an individual aliphatic hydrocarbon is not in the calibration solutions, an RF was estimated 

from the average RF of the hydrocarbon eluting immediately before the compound. The RF used 

to calculate UCM was an average of all response factors for n-alkanes contained in the calibration 

solutions. 

An analytical set consists of standards, samples, and quality control samples. Each extraction 

batch was analyzed as an analytical set, including samples and some or all of the following 

quality control samples: method blank, matrix spike, duplicate, matrix spike duplicate, and SRM. 

9.2.5 Quality Assurance/Quality Control 

The quality assurance/quality control procedure for this program included the analyses of a 

method blank, duplicate, blank spike, and blank spike duplicate per analytical batch of no more 

than 20 samples. Standard reference oil (NIST 1582) was analyzed with each extraction set. 

Standard reference sediment (NIST 1941a) was analyzed with each sediment batch, and standard 

reference tissue (NIST 1947a) was analyzed with each tissue batch. Method blanks were used to 

determine that sample preparation and analyses were free of contaminants. The duplicate sample 

was used to determine the precision of the analysis. The blank spike and blank spike duplicate 

were used to measure accuracy and precision of the analysis. Standard reference materials were 

used to evaluate accuracy. All quality assurance samples were subject to the identical preparation 

and analysis steps as samples. The quality assurance criterion for blanks specifies that target 

analytes must be less than three times the method detection limits (MDLs). The quality assurance 

criteria for spike recoveries were between 40% and 120%. The quality assurance criterion for 

valid spiked duplicates was ± 25%. The quality assurance criterion for valid duplicates was ± 25%. 

Surrogate solutions equivalent to 5 to 10 times the MDL were prepared for various hydrocarbon 

analyses. The appropriate surrogate solution was added to every sample, including quality 

control samples. The data were corrected based on surrogate recovery. The quality assurance 

criteria for surrogate recoveries were between 40% and 120%, except d12-perylene, which ranged 

from 10% to 120%. 
331 

9.3 RESULTS – SEDIMENTS 

This section summarizes data for sediment SBF and PAH concentrations. Concentrations of all 

analytes are presented in Appendix H. SBF concentrations reported here are total petroleum 

hydrocarbons in the SBF range. Non-zero values for far-field sites do not indicate that SBFs are 

present there; rather, these represent background concentrations of hydrocarbons that coelute with 

SBFs in the analytical procedure. 

9.3.1 Viosca Knoll Block 916 – Exploration Site 

Sediments from the near-field and far-field stations at the VK 916 location were sampled during 

Cruise 1B (October/November 2000) and Cruise 3B (August 2002). In addition to the box cores 

collected at the near-field and far-field locations, three discretionary box cores were located in 

suspected cuttings depositional areas during Cruise 3B. These box cores were sampled in 2-cm 

intervals from 0 cm to 10 cm. The sediment SBF and total PAH concentrations are listed in 

Tables 9.1 and 9.2. 

Table 9.1. Sediment synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) concentrations at Viosca Knoll Block 916. 

SBF (μg/g)a Total PAH (ng/g) 

Sample Cruise 1B Cruise 3B Cruise 1B Cruise 3B 

Nov 2000 Aug 2002 Nov 2000 Aug 2002 

NF-B01 6.1 10,283 241 NA 

NF-B02 5.1 228 249 437 

NF-B03 5.5 753 225 275 

NF-B04 4.8 7.5 241 244 

NF-B05 6.6 6,560 254 NA 

NF-B06 6.1 20,364 294 NA 

NF-B07 5.1 47,920 235 NA 

NF-B08 5.4 36.4 227 252 

NF-B09 3.5 17.9 194 268 

NF-B10 2.4 8.5 159 254 

NF-B11 5.0 7.8 235 254 

NF-B12 4.6 98.0 233 250 

FF2-B01 4.5 12.8 175 247 

FF2-B02 4.3 111b 222 399 

FF3-B01 5.7 11.7 270 315 

FF3-B02 6.1 13.1 299 316 

FF4-B01 4.4 2.2 236 181 

FF4-B02 4.7 10.0 246 278 

FF5-B01 7.2 10.9 246 297 

FF5-B02 4.4 5.5 209 248 

FF6-B01 4.4 11.7 237 335 

FF6-B02 5.9 13.7 388 305 

FF = far-field ; NF = near-field; NA = not available. a Far-field values represent background concentrations of hydrocarbons that coelute with SBFs. 

b This FF site may have been exposed to previous drilling discharges. Four wells were drilled between 

March and May 2002 at a wellsite 2.6 km away (see Chapter 3). 
332 

Table 9.2. Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon (PAH) 

sediment concentrations in the Viosca Knoll Block 916 discretionary box core 

samples from Cruise 3B (August 2002). 

Sample SBF (μg/g) Total PAH (ng/g) 

DS-B1 (0-2) 10,431 NA 

DS-B1 (2-4) 3,903 410 

DS-B1 (4-6) 940 2163 

DS-B1 (6-8) 283 212 

DS-B1 (8-10) 178 182 

DS-B2 (0-2) 28,138 NA 

DS-B2 (2-4) 1,673 30.2 

DS-B2 (4-6) 459 190 

DS-B2 (6-8) 159 225 

DS-B2 (8-10) 278 208 

DS-B3 (0-2) 27,319 NA 

DS-B3 (2-4) 1,425 NA 

DS-B3 (4-6) 788 34.2 

DS-B3 (6-8) 771 33.4 

DS-B3 (8-10) 585 11.8 

DS = discretionary sample; NA = not available. 

SBF concentrations in the Cruise 1B near-field sediments ranged from 2.4 to 6.6 μg/g and from 

4.3 to 7.2 μg/g in far-field sediments. SBF concentrations in Cruise 3B near-field sediments 

ranged from 7.5 to 47,920 μg/g and in far-field sediments ranged from 2.2 to 111 μg/g. Surface 

sediment SBF concentrations in the near-field and discretionary box core samples are displayed 

in Figure 9.3. Total PAH concentrations in Cruise 1B near-field sediments ranged from 159 to 

294 ng/g and in far-field sediments ranged from 175 to 388 ng/g. Total PAH concentrations in 

Cruise 3B near-field sediments ranged from 244 to 437 ng/g and in far-field sediments ranged 

from 181 to 399 ng/g. Four of the Cruise 3B near-field locations could not be analyzed for PAH 

due to the high concentrations of SBF in the sediments. 

Using the Tukey’s Honest Significant Difference (HSD) test, there was no significant difference 

among the sediment SBF concentrations at Cruise 1B near-field stations, Cruise 1B far-field 

stations, and Cruise 3B far-field stations. However, a significant difference was detected between 

sediment SBF concentrations in Cruises 1B and 3B near-field stations (the post-drilling 

concentrations were higher). There was no significant difference among sediment PAH 

concentrations at near-field stations and far-field stations during both cruises. 
Figure 9.3. Spatial distribution of synthetic-based fluid (SBF) concentrations at Viosca Knoll (VK)

Block 916 on Cruise 1B (pre-drilling) and Cruise 3B (post-drilling). The area of

geophysically mapped cuttings is shown for the post-drilling cruise.

500 m radius

o

1

2

3

4

5

6

7

8

9

11

12

10

Possible 

Cuttings

DS-3

DS-2

DS-1

VK 916

Cruise 1B 

(pre-drilling)

FF1

FF2

FF3

FF4

FF5

FF6

FF1

FF2

FF3

FF4

FF5

FF6

Concentrations 

at Far-Field Sites

500 m radius

Legend

Wellsite

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

Numbers next to dots are 10,000-100,000 μg/g

box core station numbers

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

1

5

3

6

9

12

8

4 11

7

10

2

Concentrations 

at Far-Field Sites

Note: Far-field values are background concentrations of 

hydrocarbons eluting in the SBF range

VK 916

Cruise 3B 

(post-drilling)

*

*

*

Numbers next to dots are 

box core station numbers

Note: Far-field values are background concentrations of 

hydrocarbons eluting in the SBF range

*

Legend

333 
334 

Sediment SBF concentrations in the three discretionary box core samples all displayed high 

concentrations (>10,000 μg/g) in the 0 to 2 cm section. SBF concentration decreased with depth 

in all three box cores. Sediment PAH concentrations in discretionary box cores generally 

decreased with depth, but all of the 0 to 2 cm sections could not be analyzed due to the high SBF 

concentrations. 

9.3.2 Garden Banks Block 516 – Exploration/Development Site 

Sediments from the near-field and far-field stations at the GB 516 location were sampled during 

Cruise 1B (October/November 2000) and Cruise 2B (July 2001). In addition to the box cores 

collected at both the near-field and far-field locations, three discretionary box cores were located 

in suspected cuttings depositional areas during Cruise 2B. These box cores were sampled in 2-cm 

intervals from 0 cm to 10 cm. The sediment SBF and total PAH concentrations are listed in 

Tables 9.3 and 9.4. 

Table 9.3. Sediment synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) concentrations at Garden Banks Block 516. 

SBF (μg/g)a Total PAH (ng/g) 

Sample Cruise 1B Cruise 2B Cruise 1B Cruise 2B 

Nov. 2000 July 2001 Nov. 2000 July 2001 

NF-B01 775 1,283 561 171 

NF-B02 25,131 1,093 3,470 215 

NF-B03 557 4,520 344 405 

NF-B04 93.3 162 473 191 

NF-B05 195 420 359 335 

NF-B06 1,091 29,167 402 431 

NF-B07 1,926 4,666 502 351 

NF-B08 12.0 418 241 351 

NF-B09 526 6,170 246 306 

NF-B10 24.0 7.0 218 165 

NF-B11 11.4 152 194 151 

NF-B12 5.4 56.0 230 142 

FF1-B01 9.4 3.0 224 193 

FF1-B02 5.7 3.0 182 205 

FF2-B01 3.6 4.0 227 162 

FF2-B02 3.3 7.0 138 203 

FF3-B01 6.1 4.0 242 199 

FF3-B02 4.2 3.0 177 152 

FF4-B01 3.6 3.0 444 203 

FF4-B02 3.3 3.0 120 192 

FF5-B01 2.0 5.0 115 175 

FF5-B02 3.6 4.0 230 249 

FF6-B01 3.4 4.0 174 217 

FF6-B02 3.0 4.0 149 250 

FF = far-field; NF = near-field. 

a Far-field values represent background concentrations of hydrocarbons that coelute with SBFs. 
335 

Table 9.4. Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon (PAH) 

sediment concentrations in the Garden Banks Block 516 discretionary box core 

samples from Cruise 2B (July 2001). 

Sample SBF (μg/g) Total PAH (ng/g) 

DS-B1 (0-2) 5,148 87.4 

DS-B1 (2-4) 21,908 146 

DS-B1 (4-6) 3,984 123 

DS-B1 (6-8) 1,073 701 

DS-B1 (8-10) 1,173 331 

DS-B2 (0-2) 117,280 23,840 

DS-B2 (2-4) 136,297 19,214 

DS-B2 (4-6) 119,992 18,041 

DS-B2 (6-8) 98,940 16,878 

DS-B2 (8-10) 13,271 1,667 

DS-B3 (0-2) 3,353 43.0 

DS-B3 (2-4) 162 215 

DS-B3 (4-6) 37 194 

DS-B3 (6-8) 21 98.3 

DS-B3 (8-10) 25 89.0 

DS = discretionary sample. 

SBF concentrations in Cruise 1B near-field sediments ranged from 5.4 to 25,131 μg/g and in 

far-field sediments ranged from 2.0 to 9.4 μg/g. SBF concentrations in Cruise 2B near-field 

sediments ranged from 7.0 to 29,167 μg/g and in far-field sediments ranged from 3.0 to 7.0 μg/g. 

Surface sediment SBF concentrations in the near-field and discretionary box core samples are 

displayed in Figure 9.4. Total PAH concentrations in the Cruise 1B near-field sediments ranged 

from 194 to 3,470 ng/g and in far-field sediments ranged from 115 to 444 ng/g. Total PAH 

concentrations in Cruise 2B near-field sediments ranged from 142 to 431 ng/g and in far-field 

sediments ranged from 152 to 250 ng/g. 

Using the Tukey’s HSD test, a significant difference was detected between the sediment SBF 

concentrations at near-field and far-field stations during both cruises. Near-field SBF 

concentrations were higher than far-field concentrations on both cruises. There was no 

significant difference between the sediment PAH concentrations at the near-field and far-field 

stations during both cruises. 
Figure 9.4. Spatial distribution of synthetic-based fluid (SBF) concentrations at Garden Banks (GB)

Block 516 on Cruise 1B and Cruise 2B. Areas of geophysically mapped cuttings are 

shown for Cruise 2B (not available for Cruise 1B).

Legend

Wellsite

2

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

500 m radius

6 10

1

9

5

4

8

12

7

11

3

2 500 m radius

6

10 1

9

5

4

8

12

7

11

3

DS-1

DS-2

DS-3

Possible

Cuttings

>100,000 μg/g

FF1

FF2

FF3

FF4

FF5

FF6

Concentrations 

at Far-Field Sites

FF1

FF2

FF3

FF4

FF5

FF6

Concentrations 

at Far-Field Sites

Numbers next to dots are 

box core station numbers

Note: Far-field values are background concentrations of 

hydrocarbons eluting in the SBF range

GB 516 

Cruise 1B

GB 516 

Cruise 2B

Legend

Wellsite

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

>100,000 μg/g Numbers next to dots are 

box core station numbers

Note: Far-field values are background concentrations of 

hydrocarbons eluting in the SBF range

*

*

*

*

336 
337 

Sediment SBF concentrations in the three discretionary box core samples all displayed high 

concentrations (>3,000 μg/g) in the 0 to 2 cm section. SBF concentration generally decreased 

with depth in all three box cores, although two of the discretionary box cores had significant 

(>1,000 μg/g) SBF concentrations, even 10 cm. Sediment PAH concentrations in discretionary 

box cores generally decreased with increasing depth. Sediment PAH concentrations in DS-B2 

had elevated levels ranging from 23,840 ng/g at 0 to 2 cm to 1,667 ng/g at 8 to 10 cm. The 

naphthalenes, biphenyl, and acenaphthene dominated the total PAH in these samples. The source 

of these PAHs is suggested to be from some other contaminant from the drilling activity, as SBF 

does not contain PAHs. 

9.3.3 Garden Banks Block 602 – Post-Development Site 

Sediments from the near-field and far-field sampling stations at the GB 602 location were 

sampled during Cruise 2B (July 2001). In addition to the box cores collected at both the 

near-field and far-field locations, three discretionary box cores were located in suspected cuttings 

depositional areas. These box cores were sampled in 2-cm intervals from 0 cm to 10 cm. The 

sediment SBF and total PAH concentrations are listed in Tables 9.5 and 9.6. 

Table 9.5. Sediment synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) concentrations at Garden Banks Block 602 on Cruise 2B (July 2001). 

Sample SBF (μg/g)a Total PAH (ng/g) 

NF-B01 462 132 

NF-B02 585 495 

NF-B03 8,446 552 

NF-B04 1,978 256 

NF-B05 4,339 208 

NF-B06 3,263 499 

NF-B07 1,954 245 

NF-B08 13,792 539 

NF-B09 1,430 170 

NF-B10 1,351 405 

NF-B11 2,027 219 

NF-B12 985 189 

FF1-B01 6.0 118 

FF1-B02 5.0 127 

FF2-B01 5.0 115 

FF2-B02 4.0 152 

FF3-B01 3.0 119 

FF3-B02 4.0 93.8 

FF4-B01 5.0 98.2 

FF4-B02 8.0 119 

FF5-B01 10.0 110 

FF5-B02 6.0 143 

FF6-B01 8.0 136 

FF6-B02 5.0 128 

FF = far-field; NF = near-field. 

a Far-field values represent background concentrations of hydrocarbons that coelute with SBFs. 
338 

Table 9.6. Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon (PAH) 

sediment concentrations in the Garden Banks Block 602 discretionary box core 

samples from Cruise 2B (July 2001). 

Sample SBF (μg/g) Total PAH (ng/g) 

DS-B1 (0-2) 10,572 582 

DS-B1 (2-4) 8,798 653 

DS-B1 (4-6) 3,742 436 

DS-B1 (6-8) 1,293 150 

DS-B1 (8-10) 291 136 

DS-B2 (0-2) 3,749 436 

DS-B2 (2-4) 2,706 357 

DS-B2 (4-6) 764 220 

DS-B2 (6-8) 62.0 123 

DS-B2 (8-10) 24.0 134 

DS-B3 (0-2) 930 358 

DS-B3 (2-4) 532 221 

DS-B3 (4-6) 37.0 123 

DS-B3 (6-8) 11.0 136 

DS-B3 (8-10) 12.0 135 

DS = discretionary sample. 

SBF concentrations in Cruise 2B near-field sediments ranged from 462 to 13,792 μg/g and in 

far-field sediments ranged from 3.0 to 10.0 μg/g. Surface sediment SBF concentrations in 

near-field and discretionary box core samples are displayed in Figure 9.5. Total PAH 

concentrations in Cruise 2B near-field sediments ranged from 132 to 552 ng/g and in far-field 

sediments ranged from 93.8 to 152 ng/g. 

Using the Tukey’s HSD test, a significant difference was detected between the sediment SBF 

concentrations in the Cruise 2B near-field and far-field stations (near-field concentrations were 

higher). There was no significant difference between the sediment PAH concentrations in 

Cruise 2B near-field and far-field stations. 

Sediment SBF concentrations in the three discretionary box core samples all displayed high 

concentrations (>900 μg/dry g) in the 0 to 2 cm section. SBF and PAH concentrations in 

discretionary box cores generally decreased with increasing depth. 
Figure 9.5. Spatial distribution of synthetic-based fluid (SBF) concentrations at Garden Banks (GB)

Block 602 on Cruise 2B. Areas of geophysically mapped cuttings are shown.

500 m radius

Legend

Wellsite

1

5

2 9

6

10

11

12

3

7

4

8 DS-2

DS-1

DS-3

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

GB 602 

Cruise 2B

FF1

FF2

FF3

FF4

FF5

FF6

Concentrations 

at Far-Field Sites

*

Numbers next to dots are 

box core station numbers

Note: Far-field values are background concentrations of 

hydrocarbons eluting in the SBF range

*

Possible 

Cuttings

339 
340 

9.3.4 Mississippi Canyon Block 292 – Post-Development Site 

Sediments from the near-field and far-field sampling stations at the MC 292 location were 

sampled during Cruise 2B (July 2001). In addition to the box cores collected at both the 

near-field and far-field locations, three discretionary box cores were located in suspected cuttings 

depositional areas during Cruise 2B. These box cores were sampled in 2-cm intervals from 0 cm 

to 10 cm. The sediment SBF and total PAH concentrations are listed in Tables 9.7 and 9.8. 

Table 9.7. Sediment synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon 

(PAH) concentrations at Mississippi Canyon Block 292 on Cruise 2B (July 2001). 

Sample SBF (μg/g)a Total PAH (ng/g) 

NF-B01 2,408 563 

NF-B02 782 200 

NF-B03 86.1 297 

NF-B04 <1.4 256 

NF-B05 584 743 

NF-B06 7.8 174 

NF-B07 12.8 247 

NF-B08 920 452 

NF-B09 12.4 222 

NF-B10 101 310 

NF-B11 8.9 272 

NF-B12 10.0 269 

FF1-B01 6.0 395 

FF1-B02 <1.4 748 

FF2-B01 19.0 545 

FF2-B02 <1.4 453 

FF3-B01 8.0 305 

FF3-B02 <1.4 279 

FF4-B01 8.0 339 

FF4-B02 <1.4 329 

FF5-B01 6.0 249 

FF5-B02 28.0 333 

FF6-B01 <1.4 210 

FF6-B02 <1.4 218 

FF = far-field; NF = near-field. 

a Far-field values represent background concentrations of hydrocarbons that coelute with SBFs. 
341 

Table 9.8. Synthetic based fluid (SBF) and total polycyclic aromatic hydrocarbon (PAH) 

sediment concentrations in the Mississippi Canyon Block 292 discretionary box core 

samples from Cruise 2B (July 2001). 

Sample SBF (μg/g) Total PAH (ng/g) 

DS-B1 (0-2) 59.1 302 

DS-B1 (2-4) 76.0 521 

DS-B1 (4-6) 18.0 280 

DS-B1 (6-8) 10.0 233 

DS-B1 (8-10) 9.0 210 

DS-B2 (0-2) 714 552 

DS-B2 (2-4) 60.0 329 

DS-B2 (4-6) 16.0 203 

DS-B2 (6-8) 14.0 155 

DS-B2 (8-10) 9.0 94.0 

DS-B3 (0-2) 618 524 

DS-B3 (2-4) 103 398 

DS-B3 (4-6) 33.0 285 

DS-B3 (6-8) 15.0 140 

DS-B3 (8-10) 12.0 130 

DS = discretionary sample. 

SBF concentrations in Cruise 2B near-field sediments ranged from <1.4 to 2,408 μg/g and in 

far-field sediments ranged from <1.4 to 19.0 μg/g. Surface sediment SBF concentrations in the 

near-field and discretionary box core samples are displayed in Figure 9.6. Total PAH 

concentrations in the Cruise 2B near-field sediments ranged from 174 to 743 ng/g and in far-field 

sediments ranged from 210 to 748 ng/g. 

Using the Tukey’s HSD test, a significant difference was detected between the sediment SBF 

concentrations in Cruise 2B near-field and far-field stations. No significant difference was 

detected between the sediment PAH concentrations in Cruise 2B near-field and far-field 

locations. 

Sediment SBF concentrations in the two of three discretionary box core samples displayed 

elevated concentrations (>600 μg/g) in the 0 to 2 cm section. The first discretionary box core 

samples displayed concentrations slightly above background (>50 μg/g) in the 0 to 2 cm section. 

SBF and PAH concentrations in discretionary box cores generally decreased with increasing 

depth. 
10

6 2

5

1

8

9

12

DS-1

DS-2

DS-3

4

3

7

11

500 m radius

Legend

Wellsite

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

Figure 9.6. Spatial distribution of synthetic-based fluid (SBF) concentrations at Mississippi Canyon

(MC) Block 292 on Cruise 2B. Areas of geophysically mapped cuttings are shown.

FF1

FF2

FF3

FF4

FF5

FF6

Concentrations 

at Far-Field Sites

*

Numbers next to dots are 

box core station numbers

Note: Far-field values are background concentrations of 

hydrocarbons eluting in the SBF range

*

MC 292

Cruise 2B

Possible 

Cuttings

342 
343 

9.4 RESULTS – TISSUE SAMPLES 

Two species of deepwater organisms, the isopod Bathynomus giganteus and the crab Chaceon 

quinquedens, were collected during the July 2001 (Cruise 2B) using traps located at both the 

near-field and far-field stations at two of the well locations (GB 602 and MC 292). Three 

organisms from each of the sampling locations were selected at random. The internal organs 

were removed, combined, and homogenized into one sample. Both species were analyzed for the 

presence of PAHs (Table 9.9). 

Table 9.9. Summary of polycyclic aromatic hydrocarbons (PAH) in tissue samples from Garden 

Banks Block 602 (GB 602) and Mississippi Canyon Block 292 (MC 292). 

Sample Location Species Total PAH (ng/g dry wt) 

MC 292 NF Trap 1 (HC) Bathynomus giganteus 86.8 

MC 292 NF Trap 2 (HC) B. giganteus 91.9 

MC 292 FF3 (HC 1 of 2) B. giganteus 416 

MC 292 FF4 (HC 1 of 2) B. giganteus 109 

GB 602 NF Trap 1 (HC) B. giganteus 132 

GB 602 NF Trap 2 (HC) B. giganteus 157 

GB 602 FF5 (HC 1 of 2) B. giganteus 131 

GB 602 FF5 (HC 2 of 2) B. giganteus 97.7 

MC 292 NF Trap 1 (HC) Chaceon quinquedens 136 

MC 292 NF Trap 2 (HC 1 of 2) C. quinquedens 38.6 

MC 292 FF3 (HC 1 of 2) C. quinquedens 108 

MC 292 FF4 (HC 1 of 2) C. quinquedens 107 

GB 602 NF Trap 1 (HC) C. quinquedens 176 

GB 602 NF Trap 2 (HC) C. quinquedens 94.9 

FF = far-field; NF = near-field. 

PAH body burden concentrations ranged from 38.6 to 416 ng/dry g. Using the student t-test, no 

significant difference was found between the total PAH body burden concentrations in the two 

deepwater organisms (α= 0.05). Additionally, no significant difference was found between the 

total PAH body burden concentrations at near-field and far-field stations (α= 0.05). 
344 

9.5 DISCUSSION 

The lateral distribution and concentrations of SBFs in the deep-sea sediments during this study 

were the direct result of drilling activities. Drilling activities at each of the four locations have 

been summarized in Chapter 3. The lateral distribution and concentration of SBFs in the 

sediments were extremely heterogeneous. This patchy distribution of SBFs in marine sediments 

has been reported in other studies (Gallaway et al. 1997; McIlroy 1998; Terrens et al. 1998; 

Fechhelm et al. 1999). It has been suggested that the cuttings discharge depth, rate, and total 

mass of discharge, currents, and storm currents are the main determinants of the dimension of the 

cuttings accumulations near offshore platforms (Neff et al. 2000). 

One way to assess the significance of the PAH contaminant load in sediments is to compare the 

concentrations measured in the near-field and far-field locations with previous MMS studies and 

the National Oceanic and Atmospheric Administration (NOAA) Status and Trends Mussel Watch 

Program (Brooks et al. 1989). Sediment PAH values detected at the four deepwater sites are 

similar to the concentrations that were detected in sediments collected during the Gulf of Mexico 

Offshore Operations Monitoring Experiment (GOOMEX) Study (Kennicutt et al. 1996a) and an 

American Petroleum Institute study (Brooks et. al. 1989). The concentration ranges of the 

detected PAHs in the near-field and far-field locations were similar, suggesting that 

concentrations of PAHs in the sediments were not significantly increased due to drilling. 

However, there was a significant increase in the concentration of PAHs in the second 

discretionary box core from GB 516 during Cruise 2B. Also, some samples from VK 916 

near-field locations on Cruise 3B could not be analyzed for PAHs due to the high concentrations 

of SBF in the sediments. 

The total PAH body burden concentrations in the two test species of deepwater organisms, the 

isopod Bathynomus giganteus (152 ng/dry g ± 109 ng/dry g) and the crab Chaceon quinquedens 

(110 ng/dry g ± 45.5 ng/dry g) collected during Cruise 2B from the near-field and far-field sites in 

GB 602 and MC 292, were similar in concentration to other benthic samples collected during the 

GOOMEX Study (Kennicutt et al. 1996a). Even though the concentrations of the majority of the 

individual PAH analytes were at or below the MDL (see Appendix H), the presence of 

contaminants in the internal organs does represent exposure to mature hydrocarbons. The PAH 

contaminants detected in the internal organs of these two species are probably due to their 

intimate relationship with the sediments (e.g., sediment particles retained in the gut). No 

statistically significant differences between near-field and far-field tissue PAH concentrations 

were detected. Thus, there was no clear evidence of PAH bioaccumulation from drilling 

discharges. This issue is discussed further in Chapter 16. 
345 

Chapter 10 

Sediment Microbes 

Paul A. LaRock 

School of the Coast and Environment 

Dept. of Oceanography and Coastal Sciences 

Louisiana State University 

10.1 INTRODUCTION 

The thrust of the current effort in this chapter was to determine the effects of oil drilling activities 

on sedimentary microbes using adenosine triphosphate (ATP) determinations as the primary 

analytical tool. ATP serves as a useful measure of bacterial colonization and survival in extreme 

or altered environments as it is directly tied to the physiological functioning of the community. 

The nature and magnitude of the sedimentary bacterial community has been the subject of a 

number of studies. The initial approach to quantify sedimentary bacteria was to separate them 

from the sediment matrix and count the resultant cells using epifluorescent microscopy. A 

consensus developed that the size of the bacterial community was inversely proportional to 

sediment grain size (Dale 1974; DeFlaun and Mayer 1983; Yamamoto and Lopez 1985), and that 

it also varied directly with organic carbon. The reasoning for the increase in bacterial numbers 

with decreasing grain size was the greater availability of sediment surface area for colonization. 

Early experiments at sediment oxygen uptake indicated that more oxygen was consumed by small 

grain-size sediments than by coarser materials. However, the experiment was carried out by 

suspending less than a gram of material into 28-mL biochemical oxygen demand bottles and 

following oxygen loss (Hargrave 1972). This approach ignores in situ conditions and the fact that 

very fine materials may have more interstitial (pore) space, but the pores are so small that flow 

into the sediments is greatly inhibited, thus limiting the introduction of nutrients and oxygen into 

the sediment matrix (Maier et al. 2000). The work of Yamamoto and Lopez (1985) revealed that 

increased surface was only one aspect of microbial colonization and recognized that the size and 

shape of sediment particles affect sediment packing. Larger more sorted particles, as found in 

sand, would actually make nutrients more available to the sedimentary bacterial community. 

Both DeFlaun and Mayer (1983) and Yamamoto and Lopez (1985) noted that clays were a poor 

substrate for bacterial attachment. 

Biochemical approaches were subsequently employed to probe the microbial activities in 

sediments. One such example was the fine work of Koster and Meyer-Reil (2001), who 

experimented with shallow coastal sediments whose compositions varied from sand to sand-mud 

mixtures. Measurements of phospholipid, chlorophyll a, and ATP were made to assess structural 

components, photosynthetic producers, and physiologically active biomass, respectively. Parallel 

determinations of total carbon (C) and nitrogen (N) were carried out to assess the relative 

availability of nutrient resources based on the notion that sediments with low C:N ratios indicated 

the presence of easily available dissolved organic carbon (DOC), or new material, whereas high 

C:N ratios indicated the presence of resilient compounds. Their results indicated that in sandy 

sediments, both the phospholipid and ATP were high, but as the sediments became progressively 

more muddy (finer grain size), the phospholipid became the dominant biomarker (accumulated), 
346 

but ATP decreased significantly. Similarly, the carbon analysis revealed that available DOC was 

a large fraction (about 25%) of the total organic pool in the sandy sediments, but as the sediment 

mud content increased, the available DOC fraction became an absolute minor component of the 

carbon pool. In sum, Koster and Meyer-Reil (2001) found that as sediment grain size decreased, 

available carbon resources were diminished, and that compounds that are microbial structural 

components accumulated, but the community itself was less physiologically active. 

The ATP assay offers considerable advantage over conventional microbiological methods when 

assessing the effect of the environment on microbial communities because it is non-cultural, 

avoids the issue of nutrient limitations, and reflects the in situ properties of the community. ATP 

is present in physiologically active cells and thus offers a means of assessing the relationship 

between the microbial community and the suitability of the prevailing habitat. Environmental 

ATP determinations can, therefore, provide quantification of the effects of toxicants, nutrient 

enrichment, or other alteration of the environment. A few of the more recent applications of ATP 

technology have been to examine issues in marine ecology (Karl 1986; Karl 1993; Bjorkman and 

Karl 2001), study the effects of nutrient loading in coastal waters (Malin et al. 2001), quantify 

hydrothermal communities (Atkinson et al. 2000), assess river water and sediment quality (Dutka 

et al. 1991), follow the toxic effects of zinc released from trout farms (Martinez-Tabche et al. 

2000), determine the toxicity of pollutants on wastewater treatment (Dalzell and Christofi 2002; 

Dalzell et al. 2002), and assess the effects of wastewater-borne heavy metals in mangroves (Yim 

and Tam 1999). 

As the ATP assay became more widespread, it became evident that there were substances that 

resulted in a loss of ATP during extraction and that we needed to compensate for the loss. It was 

shown that suspended material in water samples, for example, was capable of reducing ATP 

quantification by either affecting the extraction efficiency or by inhibiting the luciferin-luciferase 

reaction (Lee et al. 1971; Sutcliffe and Orr 1976). Sutcliffe and Orr (1976) found that high levels 

of suspended particles reduced the yield of ATP by as much as 50% to 66% of that known to be 

in the sample. 

The nature of the ATP extractant also plays a role in the overall efficacy of the determination. 

Bancroft et al. (1976) suggested that extractant interactions with the individual sample must be 

considered, especially when using an acid as this promotes the mobilization of otherwise 

insoluble ions. This brings up the second type of difficulty encountered with the ATP assay, that 

of ionic interference. To address this issue, ethylenediaminetetraacetic acid (EDTA) was initially 

used to complex cations (Karl and LaRock 1976), but Bancroft et al. (1976) indicated that EDTA 

addition was a time-consuming step that lost an additional 25% of the sample ATP, and 

consequently it is no longer used. Along these lines, Cunningham and Wetzel (1978) found that 

samples that were acid-extracted had a problem with fulvic acids, which accounted for a loss of 

70% to 80% of the sample ATP. These acid-soluble organic compounds sequester and bound 

charged species including ATP, and in the case of fulvic acids, the inhibitory effect was not 

reversible. The preceding discussion serves to illustrate the importance of establishing a protocol 

that incorporates ATP standards that allow correction for all the potential losses encountered 

along the way. An obvious mechanism is to prepare replicate samples that contain a known ATP 

standard and note the efficiency of recovery. This necessitates multiple samples and perhaps a 

range of ATP standards in order to cover all possibilities (Koster and Meyer-Reil 2001). The use 
347 

of a radioactive ATP standard circumvents the need for duplicates, as will be demonstrated later 

in this chapter. 

The work we now report used ATP determinations as the primary analytical tool. Given the work 

schedule, the number of samples to be processed, the chemical nature of the material, and the 

very fine muds encountered, it became necessary to refine the ATP assay protocol and assess the 

effects of sediment grain size on ATP recovery. Efforts were made to assess the turnover of 

14C-labeled substrates, but the anoxic nature of the sediments prevented us from doing so given 

the current methodology. Efforts also were made to extract and polymerase chain reaction (PCR) 

amplify sediment deoxyribonucleic acid (DNA) to determine whether there were differences 

between the pre-drilling and post-drilling materials, but unidentified inhibitory compounds in all 

of the post-drilling samples apparently blocked amplification, although the pre-drilling sediments 

were successfully extracted and amplified. 

10.2 MATERIALS AND METHODS 

As discussed in Chapter 2, sites were established at a depth of approximately 1,000 to 1,150 m in 

VK 916, GB 516, GB 602, and MC 292, off the Louisiana coast. VK 916 was an exploration site 

that was sampled before and after drilling of a single exploration well. GB 516 was an 

exploration/development site that was sampled once after exploration drilling and again after 

several development wells were drilled. The other two locations (GB 602 and MC 292) were 

post-development sites that were sampled only once, after several exploration and development 

wells had been drilled. The sampling strategy was to take 12 box cores in the immediate vicinity 

of where drilling had been or would be taking place, termed the near-field sites, and 2 box cores 

each at six far-field sites at least 10 km distant from the near-field site. Near-field samples are 

identified by the location and station number – for example, VK 916 NF-B01 through NF-B 12. 

Far-field stations are identified by the far-field site number (1 through 6), and box core number 

(01 or 02) – for example, VK 916 FF1-B01 and FF1-B02. Maps of the near-field and far-field 

sample locations are presented in Chapter 2. Numbers of microbiotal samples analyzed are 

summarized in Table 10.1. 

Table 10.1. Numbers of microbiotal subsamples collected from box cores. 

Near-Field Far-Field Cruise and Site 

NF FF1 FF2 FF3 FF4 FF5 FF6 

Cruise 1B 

GB 516 11 2 2 2 2 2 2 

VK 916 12 --a 2 2 2 1 2 

Cruise 2B 

GB 516 12 2 2 2 2 2 2 

GB 602 10 2 2 2 2 2 2 

MC 292 12 1 2 2 2 2 2 

Cruise 3B 

VK 916 12 2 2 2 2 2 2 

FF = far-field; NF = near-field; GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll. 

a

 No box cores collected. 
348 

10.2.1 ATP Assay 

Conceptually, the ATP assay can be divided into two components, the initial extraction and the 

subsequent laboratory assay of the ATP using the luciferin-luciferase reaction. ATP was 

extracted using 0.6 or 1.25N sulfuric acid as previously described (Karl and LaRock 1976) since 

phosphoric acid resulted in a copious precipitate. The major alteration in the technique was 

separate corrections for adsorptive loss encountered during sediment extraction and subsequent 

interference during the assay procedure. 

As described in Chapter 2, a 0.25-m2

 box core was used to obtain sediment from the seafloor. 

A sample for microbial analyses was obtained from a subcore in each box core. The top 2 cm of 

sediment were collected, for a total volume of 60 cc, which was then quickly stirred and 

subsampled using syringe corers. Two cc of sediment were added to each of four centrifuge 

tubes (15 x 100 mm) and 5 mL of 0.6N sulfuric acid and 0.1 mL of 14C-ATP (total activity 

10,000 dpm, NEC-417 or equivalent, New England Nuclear/Perkin-Elmer, Boston, MA). The 

mixtures were vortexed for 5 min, centrifuged in a clinical centrifuge until settled (generally 

15 min), and 4 mL withdrawn to be adjusted to pH 7.8 with a graded series of sodium hydroxide 

solutions. The volume was then brought up to 7.0 mL with TRIZMA 7.8 (Sigma-Aldrich 

Chemical Co., St. Louis, MO), and the extract was frozen for subsequent laboratory analysis. 

The addition of a radioactive ATP internal standard allowed every sample to be corrected for 

adsorptive losses, which was a critical concern when working with very fine-grained sediments. 

The ATP was measured by adding 1 mL of the extract to a 22-mL scintillation vial along with 

0.5 mL TRIZMA 7.8 buffer to which 200 μl of luciferin-luciferase mixture (FL-AAM ATP kit, 

Sigma-Aldrich Chemical Co., St. Louis, MO) was injected. After a 30-second delay, the emitted 

light was detected using a Turner 20e Luminometer (Turner Industries Inc., Sunnyvale, CA) and 

the ATP determined relative to standards run with each enzyme preparation. Substances 

interfering with the assay were corrected for by recounting each sample and substituting a 10 to 

50 ng ATP standard in 0.5 mL of TRIZMA for the plain TRIZMA used in the initial count. 

Adsorptive loss was corrected by determining the 14C-ATP recovered in the extract relative to 

that which was added. In all calculations, careful attention was paid to all the dilution factors 

used in the various steps of the process. 

The use of ATP is a powerful means of determining factors affecting microbial biomass. There 

are, however, corrections that must be applied to overcome inaccuracies that are associated with 

both the extraction of the ATP from the sediments and its subsequent assay by the 

luciferin-luciferase method. One approach to correct for these interferences is to add an internal 

standard to a second set of replicates and determine the recovery after subtracting the actual 

sample ATP from the sample plus standard mixture. This approach essentially doubles the 

workload, and the ultimate accuracy of the correction depends upon the number of replicates 

used. For this reason, we decided it would be better to differentiate between the adsorptive loss 

in the extraction process and the interference encountered during the actual ATP assay itself. 

The approach we used to determine adsorptive loss was to add radioactive ATP, either 14C or 3

H 

labeled, to every sample and calculate an extraction efficiency as the difference between the 

added and recovered radiolabeled standard. The use of a radiolabeled material allows every 
sample to be treated, is the more sensitive approach, and does not add measurable ATP to the 

sample. To demonstrate the efficacy of this approach, we conducted a series of experiments in 

which a stable internal ATP standard was added to one set of samples (done in triplicate) and 

14C-ATP to a second set of samples (also done in triplicate), and the recovery of each standard 

was determined. The results of this experiment are shown in Figure 10.1 and reveal close 

agreement between the two approaches (r2

 = 0.82). However, the stable ATP standard has a 

recovery that is only about 80% of that determined by the use of the radioisotope. We attribute 

the difference to the increased sensitivity of the radioassay and the fact that isotope detection is 

not affected by potential ionic interference as might be encountered when assaying the stable 

internal ATP standard. 

As may be noticed in Figure 10.1, the recovery of the extracted ATP varied from 10% to 100%. 

The principal factor affecting ATP loss in the extraction process is sediment grain size, with the 

finer materials being more adsorptive relative to the coarser sands. To illustrate the point, we 

compared the ATP extraction efficiency (determined with a radioactive standard) to sediment 

grain-size ratio. The results are seen in Figure 10.2 and demonstrate that as the proportion of 

fines increases (i.e., the grain size decreases), the ATP extraction efficiency decreases. 

Figure 10.1. A comparison between the recovery of a stable internal adenosine triphosphate 

(ATP) standard to the recovery of the 14C ATP adsorption standard. 

349 
Figure 10.2. The effect of sediment grain size on the recovery of the 14C ATP extraction 

standard. Note that as the sediment grain size decreases, adsorption of the extracted 

ATP becomes significant, reducing the efficiency of extraction to as little as 10%. 

As discussed in Chapter 5, sediments from the study sites were predominantly clay with a mean 

sedimentary particle diameter ranging from about 1 to 3 μm. Over 80% of the sediment consisted 

of components smaller than 10 μm; thus, we were dealing with materials that are very 

fine-grained and impermeable (Maier et al. 2000). 

10.2.2 Microbial Respiration 

The method used for microbial respiration was essentially that of Deming (1993), except that 

pressurization was not required and that aspect of the protocol was omitted and the experiments 

were carried out as time series. For each box core retrieved, approximately 60 cc of sediment 

were collected, homogenized, and 2 cc added via a syringe corer to 10, 50-mL serum bottles, 

followed by 10 mL of filtered seawater to form a slurry. Formaldehyde at a final concentration of 

2% was added to two of the bottles to serve as a poisoned control. The remaining eight bottles 

were incubated at in situ temperature (6oC), with duplicates being sacrificed after 30, 60, 90, and 

120 minutes of incubation, and the respired 14CO2 collected and counted by liquid scintillation 

counting. Three different 14C-labeled compounds were used, glutamic acid (NEC 290), acetic 

acid (NEC 553), and glucose (NEC 042). Acetic acid was used for all samples, while the 

glutamic acid was used on Cruises 1B and 2B and glucose on Cruise 3B to provide assurance that 

the results obtained were not substrate-specific. 

350 
351 

 

10.2.3 Sediment DNA Extraction, Quantification, and Amplification 

A variety of methods were used to extract DNA from the sediments that was suitable for PCR 

amplification (Atlas 1993; Steffan et al. 1988; Tsai and Rochelle 2001). Extracted DNA was 

quantified fluorometrically using ethidium bromide as described by van Lancker and Gheyssens 

(1986). The presence of interfering organic substances was assessed by changes in the 

spectrophotometric absorption of DNA over a range of 250 to 700 nm (Chen et al. 1977; Edzwald 

et al. 1985). 

10.2.4 Toxicity Bioassay 

A bioluminescent assay (Lumitox®) using the dinoflagellate Pyrocystis lunula was used to test the 

relative inhibitory effects of sediment extracts. A detailed methodology for the luminescent assay 

may be found in American Society for Testing and Materials (ASTM) Protocol E 1924, and the 

procedure as reported here was performed by the Lumitox Co., Slidell, LA, which developed the 

technique. In this particular instance, a 10-g aliquot of the desired sediment sample was weighed 

and diluted to a final volume of 50 mL with artificial seawater, continuously shaken for 

75 minutes, and then allowed to settle. Five mL of the cleared suspension was put in a 20-mL 

scintillation vial, and 50 μl of this suspension was used to challenge 300 P. lunula suspended in 

3 mL of artificial seawater contained in a 20-mL LSC vial. Pentuplicates were prepared for each 

sediment and control sample assayed. Each set of vials was incubated in darkness for 4 hours and 

then monitored for light output in a luminometer especially designed for the purpose. Negative 

controls consisted of a P. lunula suspension without any added substances (i.e., an uninhibited 

cell suspension). Positive controls consisted of P. lunula suspensions to which two different 

concentrations of sodium dodecyl sulfate (SDS) were added. The relative inhibitory effect of the 

SDS and the sediment extracts was assessed by a reduction in the light output of P. lunula. A 

total of 24 separate sediment samples was tested and was chosen such that there were two 

near-field and two far-field samples for both the pre-drilling and post-drilling samplings at 

GB 516 and VK 916. For GB 602 and MC 292, only post-drilling materials were available, and 

two near-field and two far-field samples were used. 

10.3 RESULTS1

10.3.1 ATP Analysis 

The ATP results for all stations and cruises are summarized in Table 10.2, and although there are 

some hot spots, they show that for all the post-drilling conditions, the ATP ranges between 2 to 

5 ng g-1. 

A comparison of the pre- and post-drilling ATP values at GB 516 and VK 916, on the average, 

show a marked decrease of about 5- to 15-fold after drilling. At GB 516, the average ATP 

concentration on Cruise 1B was 44.2 ng g-1 for the near-field site and 43 ng g-1 for the far-field 

sites. The Cruise 2B averages were 4.3 ng g-1 for the near-field site and 2.5 ng g-1 for the far-field 

sites. At VK 916, the average pre-drilling ATP concentration was 32 ng g-1 for the near-field site 

1

 Editor’s note: In this chapter, the author classifies Cruise 1B data from GB 516 as “pre-drilling” because of 

subsequent development drilling there. Elsewhere in this report these samples are classified as post-drilling due to two 

previous exploratory wells (see Chapter 3). See Chapter 15 for an alternate interpretation of these data.
352 

and 36 ng g-1 for the far-field sites. The post-drilling averages for VK-916 near-field (7.7 ng g-1) 

and far-field (7.1 ng g-1) indicate a five-fold reduction in the microbial ATP biomass, although it 

should be noted that again there is spatial heterogeneity in the data. The difference between the 

pre-drilling and post-drilling ATP concentrations is statistically significant (p<0.0001 for GB 516 

and p<0.0002 for VK 916). Statistically, there was no significant difference between the 

near-field and far-field ATP levels for either GB 516 or VK 916 (p<0.137 for GB 516 and 

p<0.157 for VK 916) or for GB 602 and MC 292 (p<0.083). Simply stated, the far-field stations 

did not serve as true, unaffected controls of the drilling operation and thus the similar effect on 

ATP levels in both near-field and far-field locations cannot be attributed solely to drilling. 

Table 10.2. Sediment adenosine triphosphate (ATP) concentrations (ng g-1 dry wt) for all 

near-field and far-field sites. 

GB 516 VK 916 GB 602 MC 292 Site Samplea

Cruise 1B Cruise 2B Cruise 1B Cruise 3B Cruise 2B Cruise 2B 

NF B01 288.3 2.1 47.5 <1.7b -- 4.5 

B02 418.2 0.6 42.1 <1.3b 3.4 1.2 

B03 13.1 7.9 30.2 <1.8b 3.3 1.4 

B04 27.9 0.5 34.5 <1.8b 2.3 0.9 

B05 26.4 1.1 29.3 15.3 14.1 4.4 

B06 -- 11.9 48.3 25.6 4.7 4.4 

B07 33.5 4.4 30.1 41.5 2.3 1.1 

B08 104.1 1.5 21.9 <0.3b 17.6 5.4 

B09 39.2 9.6 25.1 1.1 2.7 1 

B10 29.2 0.7 23.3 1.3 15.2 1.6 

B11 56.3 5.1 23.4 <1.5b 16.2 5.5 

B12 68.7 6.8 29.1 1.1 -- 1.7 

FF1 B01 28.7 5.5 -- 61.1 2.1 2.9 

B02 43.4 5.8 -- 1.1 1.1 -- 

FF2 B01 102.4 2.3 57.9 1.3 1.7 2.5 

B02 25.3 2.8 61.4 1.1 1.9 66.6 

FF3 B01 92.3 0.5 34.2 2.7 3.6 1.7 

B02 35.3 3.1 29.6 <1.5b 2.4 2.8 

FF4 B01 29.1 5.4 19.7 2.7 1.1 2.1 

B02 38.1 1.1 28.4 2.3 2.2 0.7 

FF5 B01 64.4 0.5 24.5 2.3 3.2 1.1 

B02 14.2 0.6 -- 2.7 216.3 2.2 

FF6 B01 29.3 1.4 36.4 0.9 2.6 1.5 

B02 16.7 0.8 32.1 5.6 2.7 1 

a Sample locations were randomly selected on each cruise; therefore, the samples collected on two cruises 

at GB 516 and VK 916 are not paired samples from the same location. 

b A < symbol indicates that interference reduced the sensitivity of the assay to the value indicated; thus, 

ATP may be present at a lower concentration but be undetectable. 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll. 
353 

These findings are shown more clearly in Figure 10.3, noting that the graphs are plotted 

logarithmically. Based on previous data from the eastern Gulf of Mexico, we might expect 

reasonable sedimentary ATP values at GB 516 and VK 916 (pre-drilling) of 30 to 40 ng g-1 at a 

mean grain size of 2 to 3 μm, provided that sediment grain size was the controlling factor. Our 

measured ATP results are in good agreement with the projected values based on grain-size 

criteria for GB 516 and VK 916 pre-drilling conditions, but the post-drilling results are 

significantly lower. 

There are three possibilities that may explain the reduction in ATP levels as a result of drilling: 

1) the addition of drill cuttings simply overlays the existing sediments, creating a new and 

uncolonized environment; 2) the new material may contain some inhibitory compound; and 3) a 

combination of the above two events. One approach to sort out the possibilities is to compare the 

DNA content of the pre- and post-drilling samples using ethidium bromide (van Lancker and 

Gheyssens 1986). The DNA in the post-drilling materials is about one-fifth that found in the 

pre-drilling materials, but that only partially accounts for the 10- to 15-fold reduction in ATP. 

10.3.2 Toxicity Bioassay 

A toxicity bioassay was performed that measured the relative changes in light output of the 

dinoflagellate P. lunula as it was exposed to extracts made from the various pre- and post-drilling 

sediments. The addition of SDS, either 10 or 20 μl (labeled X2 in the figures), was used as a 

positive control to elicit a measurable response that can be compared to the negative control 

(P. lunula with no additives). The results obtained for VK 916 are shown in Figure 10.4 and 

indicate that all of the pre-drilling materials yielded the same response as the unamended 

P. lunula control, but the post-drilling samples exhibited some inhibition, with the near-field 

sediments being more inhibitory than the far-field sediments. Bear in mind that the design of 

these tests was only to indicate the relative effects of the various sediments, and that only a small 

aliquot of the extract was used, which may not indicate the full inhibitory potential of the 

materials. 

The results for GB 516 (Figure 10.5) essentially paralleled those obtained for VK 916; namely, 

the post-drilling samples were inhibitory with the near-field materials exerting a stronger effect 

than the far-field samples did. Tests on GB 602 (Figure 10.6a) post-drilling sediments showed 

some inhibition in the bioassay, but the results for MC 292 (Figure 10.6b) indicated no inhibition 

whatsoever, which may reflect different drilling parameters or perhaps different environmental 

conditions affected by the proximity of the Mississippi River. 
Figure 10.3. Microbial adenosine triphosphate (ATP) concentrations at the four study sites. Note the log scale.

Near-field Far-field

LEGEND

GB 602

NF

Cruise 2B

(post-drilling)

FF

0.1

10

100

1000

Microbial ATP (ng g-1 dry wt) 1 MC 292

Microbial ATP (ng g-1 dry wt) 0.1110 100 1000 NF

Cruise 2B

(post-drilling)

FF

VK 916

NF

Cruise 1B

(pre-drilling)

FF NF FF

Cruise 3B

(post-drilling)

Microbial ATP (ng g-1 dry wt) 1 1000 10010 0.1 GB 516 NF

Cruise 1B

(post-drilling)

FF NF F

Cruise 2B

(post-drillin

0.1

1

10

100

1000

Microbial ATP (ng g-1 dry wt)

354 
Figure 10.4. The relative toxicity of sediment extracts for selected sampling sites at VK 916 

before and after drilling. The cross-hatched bars indicate the light output of 

P. lunula alone, the stipled bars are P. lunula with two different concentrations of 

SDS added, the black bars are P. lunula plus the pre-drilling sediment extract, and 

the white bars are P. lunula plus the post-drilling extract. 

Figure 10.5. The relative toxicity of sediment extracts for selected sampling sites at GB 516 

before and after drilling. Symbols are the same as in Figure 10.4. 

355 
(a) (b)

Figure 10.6. The relative toxicity of sediment extracts for selected sampling sites at a) GB 602 

and b) MC 292. Symbols are the same as in Figure 10.4, except that there were no 

pre-drilling samples. 

10.3.3 DNA Assay 

Efforts were made to PCR amplify the extracted sedimentary DNA to look at changes in bacterial 

community structure that might have resulted from drilling. A variety of methods were used, 

beginning with the established protocols to secure DNA that could be used for PCR, which 

worked on the pre-drilling samples but failed on all the post-drilling samples. Attempts were 

made by three different laboratories to obtain clean DNA on the post-drilling materials, but all 

were without success. The extracts did contain DNA as determined by ethidium bromide, but 

some inhibiting compounds were collected as well as DNA. Drilling fluids are known to contain 

organic compounds, and high levels of some metals (see Chapter 8), both of which are known to 

interfere with PCR. Efforts to eliminate these potential sources of interference used Sepharose 

4B for organics (Jackson et al. 1997) and Chelex 100 (Walsh et al. 1991) for the removal of 

metals, as well as specialized clean-up kits, and none had a positive effect. 

10.3.4 Respiration 

Efforts also were made to assess the heterotrophic activity of the sedimentary microflora by 

measuring the respiration of 14C-labeled compounds by capturing the resultant 14CO2 produced. 

The method entails making a sediment-water slurry, incubating at in situ temperatures, and 

sacrificing samples over a given time period. A total of 72 separate samples was challenged in 

the heterotrophic activity assay, and all yielded negative respiratory curves as seen in 

Figure 10.7, regardless of whether the labeled substrate was acetate, glutamate, or glucose. The 

process is carried out in a normal laboratory setting (i.e., in air), which would shut down 

anaerobic bacteria or those facultative organisms functioning as anaerobes, and consequently, the 

utilization of substrate would diminish as the redox conditions of the experiment became more 

356 
positive with time. An interesting implication of these observations is that the sediments in the 

areas tested are essentially anoxic with respect to the microflora. 

 Figure 10.7. Respired carbon dioxide from acetate in sediments from GB 516, Cruise 1B. These 

curves were representative of all the heterotrophic respiration experiments and 

suggest that the sediments had trace amounts of oxygen or were in fact anoxic. 

10.4 DISCUSSION 

The major issues covered in this effort were 1) the relationship between sediment grain size and 

living microbial ATP biomass levels; 2) the effects of drilling on microbial ATP biomass; and 

3) the inhibitory effects of oil production on sediment microflora. Given the nature of the 

sedimentary matrix we had to deal with, we had to devise a new method of measuring ATP and 

compensating for interferences on a very rigorous sampling program. There are a variety of 

means of extracting sedimentary ATP, but a major problem for all of them is a means of 

correcting loss of ATP during sediment extraction by adsorption to the sediments, followed by a 

secondary interference encountered during the luciferin-luciferase assay. The simplest means to 

tackle the problem would be to add an internal standard to a duplicate set of samples and to 

determine the overall loss in the final counting. This approach essentially doubles the work effort 

and limits the number of replicates that can reasonably be processed, and for very fine-grained 

materials, the initial loss from adsorption to the sediments may be so great that too much of the 

standard is lost to be of value. 

Our solution was to use radioactive ATP to follow the adsorptive loss followed by an internal 

ATP standard added at the time of assay to determine counting efficiency. By separating the 

interference into two components, every sample can be used to determine adsorptive loss (the 

radioactive ATP is essentially at a concentration that is below detection limits), thus expediting 

the field aspect. When counting the extracted ATP, the internal standard can be adjusted in the 

laboratory if, as was the case with our work, there is severe interference in the counting process. 

As seen in Figure 10.1, there is agreement between both approaches, although the radioisotope 

357 
358 

approach gave a better estimate of recovery. Our findings also show that as the sediments 

became finer in composition, the extraction efficiency fell off appreciably (Figure 10.2). 

Samples taken from our sites typically had an extraction efficiency of 8%. 

There are a considerable number of reports describing the bacterial numbers found in a variety of 

sediments. One general conclusion based on direct microscopic evaluation is that bacterial 

numbers increase as sediment grain size decreases. The question that arises is whether these 

bacteria are active, or whether an alternative metabolic assay, such as ATP, would be a better 

indicator of sediment suitability for microbial colonization? In a separate analysis of microbial 

data from the Mississippi-Alabama-Florida (MAFLA) program in the eastern Gulf of Mexico 

(LaRock, unpublished data), ATP levels declined with reduced sediment particle size, which is 

contrary to the microscopic observation but in good agreement with the findings of Koster and 

Meyer-Reil (2001). Furthermore, by extrapolating the ATP-grain size regression lines to a zero 

ATP concentration, we arrived at a theoretical minimum grain size of about 2 μm, which would 

support bacterial development. 

Information on deep-sea sedimentary microbial biomass is limited, and information that is 

integrated to sediment grain size is even more scarce. One of the first measurements of deep-sea 

ATP biomass was reported by Karl et al. (1976), who found that the upper 2 cm of sediment 

collected from the abyssal plain in the Atlantic Ocean had a value of 2.4 ng g-1. More recent 

work by Egeberg (2000), done in conjunction with the Ocean Drilling Program, suggested that 

particle size may be a determining factor in regulating sediment microbes. Egeberg made 

down-core measurements from 1.38 to as deep as 57.8 m below the seafloor and reported ATP 

values as high as 6 to as low as 0.23 ng g-1, with an average value of 0.75 ng g-1. No 

determinations were made on surface sediments, however. 

Koster and Meyer-Reil made a suite of measurements in shallow coastal waters of the Baltic Sea, 

which included ATP, chlorophyll a, phospholipid, and carbon species. The sediments they 

worked with were characterized as sandy, slightly muddy sand, and muddy sand. The TOC 

increased as the sediments became finer, although the available DOC (the more labile 

component) decreased, leading to the conclusion that the higher organic content is composed of 

more refractory, or aged material, which is less capable of supporting microbial metabolism. 

With increasing mud content, the microbial biomass, based on phospholipid determinations, 

increased, whereas the active biomass, based on ATP determinations, decreased. This result 

neatly illustrates the disparity that we observe between measurements that seek to quantify the 

total quantity or number of microbes without regard to whether they are functional or not. In this 

case, as well as with direct acridine orange direct counts (AODCs), the measured parameter 

(phospholipid or cell number) reflects only whether bacteria are present, and not whether they are 

growing or in a metabolically active state. In other words, the finer sediments may harbor more 

microbes, but based on ATP measurements, their geochemical potential is diminished. 

The ATP values obtained by Koster and Meyer-Reil are not directly comparable to our 

measurements because they reported their results as a function of sediment volume (i.e., ng cm-3

as opposed to ng g-1). Since our sediments were about 60% to 70% water, 1 g of sediment is a 

close approximation to 1 cm3

 of sediment slurry. With this difference in mind, the ATP values 

reported ranged from 673 to 2,843 ng cm-3, which agree well with the values we reported for the 
359 

Florida shelf off the Gulf coast. The question now is what ATP levels might one reasonably 

expect at 1,000 m in the sediments of the Gulf of Mexico if there were no anthropogenic impacts? 

A reasonable expectation might range from 1,000 ng g-1 in the shallow coastal zone to 2 ng g-1 in 

the most distant and deepest regions of the open ocean. Under the present circumstances, an 

approximate value might be between 50 and 100 ng g-1. 

The Cruise 1B ATP values for GB 516 (Table 10.2) show considerable spatial variability that 

may, in part, be the result of gas and oil seeping from the seafloor (see the dedicated issue of Geo 

Marine Letters 14:2/3, 1994 for an extensive overview of the Louisiana seep zone). In 

calculating an average ATP concentration for the area, we did not include the exceptionally high 

values of NF-B01 and NF-B02, as these samples may be affected by gas or oil seepage. 

One effect of drilling was the addition of drilling muds and cuttings to the seafloor. This is seen 

in the chemical analyses in Chapter 8, where it is apparent that the primary change is with the 

addition of barium to the sedimentary matrix. Barite is a major constituent of the drilling muds 

that are used to aid in the drilling process. Based on the change in barium concentrations alone, 

more of the coarser drilling discharges were deposited in the near-field sites than were found in 

the far-field locations, as were also seen in the GOOMEX project (Kennicutt 1995; Kennicutt et 

al. 1996a). In other words, heavier materials are quickly deposited, whereas the finer materials 

are more dispersed. Interestingly, the mean grain size for these sediments borders on the 

hypothetical 2-μm threshold that we postulate to be the lower limit that will support bacterial 

growth. Clay sediments, because of the extremely small particle size, are highly impervious. 

This has the effect of inhibiting bacterial movement either into or out of the clay sediments, as 

well as limiting the passage of oxygen or nutrients (Maier et al. 2000). 

How might drilling discharges affect the sedimentary microbial community? There are three 

possibilities: 1) the solid phase of the SBM simply buries the existing community; 2) there is an 

inhibitory component in the SBM that affects bacterial metabolism or the environment; or 3) the 

outcome is a combination of 1 and 2. From the oxygen data (see Chapter 8), it is obvious that the 

sediments quickly became anoxic as a result of drilling. The base chemical of SBMs 

(representing up to 75% of the total volume) is selected to be biodegradable as mandated by 

USEPA regulations; however, this organic enrichment has the effect of creating anoxia. The 

synthetic chemicals, usually internal and linear alpha olefins, have low solubility and 

bioavailability, but emulsifiers that include detergents and other additives may contribute some 

level of toxicity (Jerry Neff, pers. comm.; Neff et al. 2000). 

Toxicity bioassays were performed in the GOOMEX project using the sea urchin Arbacia 

punctulata and following either its fertilization or embryological development (Carr and 

Chapman 1992; Carr 1993). Of the two approaches, the embryological development test proved 

to be the more sensitive in assessing pore water effects in production areas relative to an 

uncontaminated control site. Inhibitory effects were found only within a 150-m radius of a 

platform, and no inhibitory effects were found at any of the far-field sites. Based on the ATP 

data, it appears that bacteria are severely impacted, even at the far-field locations. Whether this 

was due to toxicity remained an open question. In order to add some degree of quantification to 

the problem, a bioassay was done by the Lumitox Co., Inc., based on the relative reduction in 

light emission from the dinoflagellate P. lunula when exposed to water extracts made from the 
360 

pre- and post-drilling sediments. The results (Figures 10.4 and 10.5) showed that prior to 

drilling, the sediments at GB 516 and VK 916 had no negative effect on the test organism, and 

light output from P. lunula was essentially equal to that of the negative controls. The 

post-drilling samples, however, caused a reduction in light emission, with the near-field materials 

exerting a more deleterious effect than the far-field materials. In a similar fashion, the 

post-drilling samples from GB 602 had a negative effect, but surprisingly, the samples from 

MC 292 had no effect at all. At the time the MC 292 samples were taken, the sea surface was 

noticeably green, even though we were in 1,000 m of water, suggesting terrestrial influences that 

might in some fashion mitigate the effects of production activities. How do these results translate 

into an effect that the bacterial community might experience? We cannot say precisely, but these 

findings together with the ATP results indicate that a stress condition had been established. 

What is the nature of the inhibitory substance(s)? In the GOOMEX program it was considered to 

be metals, specifically barium, but in our study, the barium concentration at the far-field stations 

did not increase significantly after drilling, compared to a one to two orders of magnitude 

increase at the near-field locations. This suggested that some other factor was responsible for the 

ATP and toxicity test results, which we believe is organic in nature. Attempts were made by 

three different laboratories to extract and amplify DNA for characterization using a variety of 

techniques and clean-up kits, which worked with the pre-drilling sediments but all failed when 

applied to the post-drilling sediments. DNA was extracted from the sediments as determined by 

the ethidium bromide technique, but the problem arose in the polymerase amplification. In a 

similar vein, the sensitivity of the ATP assay was greatly reduced when we counted the 

post-drilling samples. The analytical system we use is routinely able to detect 0.1 ng g-1, but the 

limit of detection for the post-drilling samples was raised to 1.5 to 1.8 ng g-1, which accounts for 

the less than symbols in Table 10.2. Note also that the interfering effect was most pronounced 

for the near-field stations. In other words, something is interfering with enzyme function. 

How long do these deleterious effects last? That is difficult to say, but they do not appear to be 

permanent. GB 516 had been developed and then abandoned a number of years ago and then 

redeveloped in 2000-2001. Based on ATP and toxicity measurements prior to the secondary 

development, it is apparent that the sediments had fully recovered from the earlier drilling. 
361 

Chapter 11 

Meiofauna and Macroinfauna 

Barry A. Vittor & Associates, Inc. 

Continental Shelf Associates, Inc. 

11.1 INTRODUCTION 

Responses of benthic organisms to offshore Gulf of Mexico petroleum exploration and 

production have been studied extensively on the continental shelf. Investigations of acute effects 

of drilling muds and cuttings on macroinfaunal communities have been sponsored by the MMS 

and American Petroleum Institute in particular, and include studies such as the Central Gulf of 

Mexico platform project (Baker et al. 1981), drilling discharges fate and effects in shallow 

nearshore waters in south Texas (Continental Shelf Associates, Inc. 1989), and the Buccaneer gas 

and oil field benthic community study (Harper et al. 1981). Some studies have addressed 

meiofaunal and macroinfaunal community structure and detected effects on these organisms by 

surveying around drilling/production sites of varied duration. The Gulf of Mexico Offshore 

Operations Monitoring Experiment (GOOMEX) investigated potential chronic, sublethal, and 

long-term oil and gas production effects on meiofauna and macroinfauna, as well as other 

ecosystem components on the continental shelf (Montagna and Harper 1996). That program 

detected changes in benthic assemblages due to both organic enrichment and toxicity of drilling 

muds and cuttings discharged to the bottom. The present investigation used some of the same 

methods but focused on meiofaunal and macroinfaunal community responses to drilling 

discharges in the deepwater Gulf of Mexico. Previous studies that provide some information 

about benthic effects of SBMs on the Gulf of Mexico continental slope include an opportunistic 

survey of a wellsite in the Mississippi Canyon area (Gallaway et al. 1997; Fechhelm et al. 1999) 

and a study of several continental slope and shelf sites by Continental Shelf Associates, Inc. 

(2004). 

Background information on deepwater benthic communities is available from the MMS northern 

Gulf of Mexico continental slope study (Gallaway 1988; Pequegnat et al. 1990). More recently, 

the MMS deep Gulf of Mexico benthos study has expanded the depth range and geographic 

coverage of the previous continental slope study (Rowe and Kennicutt 2002). The study included 

stations at depths from 300 m to greater than 3,000 m. 

11.2 MATERIALS AND METHODS 

Study design has been described in Chapter 2. Briefly, benthos were sampled at four sites on the 

northern Gulf of Mexico continental slope. VK 916 was an exploration site sampled before and 

after drilling of a single exploration well. GB 516 was an exploration/development site that was 

sampled once after exploration drilling and again after several development wells were drilled. 

GB 602 and MC 292 were post-development sites sampled once after several exploration and 

development wells had been drilled. 

Benthic samples were collected on three cruises. Cruise 1B (October-November 2000) was a 

pre-drilling survey of VK 916 and a snapshot of GB 516 after exploration drilling and prior to 
362 

development drilling. Cruise 2B (July 2001) was a post-drilling survey following development 

drilling at GB 516, GB 602, and MC 292. Cruise 3B (August 2002) was the post-drilling survey 

at VK 916. On each cruise, box core samples were collected at 12 randomly selected locations 

within the 500–m near-field radius at each study site visited. Samples also were collected at 

12 far-field locations (two locations selected randomly within each of six far-field sites). 

Macroinfauna (animals retained on a 300-μm screen) and meiofauna (those passing through a 

300-μm screen but retained on a 63-μm screen) were counted and identified to major group 

(order or higher taxonomic level in most cases). In most cases, annelids were identifiable to class 

(Polychaeta or Oligochaeta), and the overwhelming majority of these were polychaetes. 

However, in some data sets (e.g., all Cruise 3B samples), they were identified only as annelids. 

For consistency in making comparisons among sites and cruises, all of these observations are 

reported here as Annelida. Meiofaunal nematodes from GB 602 and MC 292 during Cruise 2B 

were identified to family to determine changes in feeding types among these taxa; feeding types 

of nematode families were adopted from Jensen (1986). Harpacticoid copepod reproductive 

condition also was noted at these two sites to detect potential sublethal effects of synthetic 

drilling muds. Benthic data reduction included individual and taxon abundance, taxon diversity, 

evenness, and richness. Statistical treatment of the data involved ANOVA for differences in 

meiofaunal or macroinfaunal individual abundances between survey sites and cruises. 

Twenty-four stations were selected for more in-depth taxonomic identification of macroinfauna to 

the Lowest Practical Identification Level (LPIL). These analyses included near-field and far-field 

samples from each site and each cruise. For the 24-station analysis of macroinfaunal data, spatial 

and temporal patterns were examined using multivariate techniques, including cluster analysis, 

non-metric multidimensional scaling (MDS), and similarity percentage breakdown (SIMPER). 

These analyses were performed on a similarity matrix constructed from a raw data matrix 

consisting of taxa and samples (station-cruise). To weight the contributions of common and rare 

taxa, raw counts of each taxon in a sample (n) were transformed to logarithms [log10 (n+1)] prior 

to similarity analysis. Cluster analysis was followed by MDS ordination of the similarity matrix 

to corroborate cluster results. Species accounting for observed assemblage differences among 

groups and within groups of stations were identified using the SIMPER procedure, which 

determines the average contribution of each species characterizing a station group or 

discriminating between pairs of station groups (Clarke 1993). These analyses (MDS and 

SIMPER) were performed with the PRIMER v5 package (Clarke and Gorely 2001). 

11.3 MEIOFAUNAL RESULTS AND DISCUSSION 

A complete list of meiofaunal taxa collected is presented in Appendix I1. 

11.3.1 Viosca Knoll Block 916 

VK 916 was sampled on Cruise 1B (November 2000) and Cruise 3B (August 2002). The two 

cruises served as pre- and post-drilling surveys for this site. 
363 

Table 11.1 summarizes meiofaunal community statistics for VK 916 including density, diversity, 

evenness, and richness. There was considerable spatial and temporal variation in all of the 

community statistics. As shown in the table and illustrated in Figure 11.1, total densities 

decreased by about an order of magnitude between the two cruises at both near-field and far-field 

sites. Mean densities on Cruise 1B were 608 and 544 individuals/10 cm2

 for near-field and 

far-field, respectively, as compared with 96 and 19 individuals/10 cm2

 for Cruise 3B. On both 

surveys, there was considerable overlap in the range of densities between near-field and far-field 

sites (i.e., densities at some near-field stations were similar to those at far-field stations). On the 

pre-drilling cruise (1B), the near-field site showed a greater range of densities, including both 

higher and lower values than the far-field sites. On the post-drilling cruise (3B), near-field 

densities tended to be higher than far-field densities, with some overlap. 

Figure 11.2 illustrates the taxonomic composition of the meiofaunal samples at VK 916. On 

Cruise 1B, both the near-field and far-field sites were numerically dominated by nematodes and 

harpacticoid copepods, which together accounted for over 90% of the total numbers. On 

Cruise 3B, the composition of the near-field samples was markedly different, with annelids and 

aplacophoran molluscs contributing over 50% of the individuals. At the far-field site, nematodes 

accounted for about the same proportion as on the pre-drilling survey, but annelids were 

relatively more abundant and harpacticoids relatively less abundant than on the pre-drilling 

cruise. 

It also is instructive to look at the two near-field stations having the highest density on each 

survey. On Cruise 1B, these were Stations NF-B02 and NF-B08. Nematodes accounted for 90% 

of the individuals at NF-B02 and 83% of the individuals at NF-B08. On Cruise 3B, the two 

highest densities were observed at NF-B05 and NF-B08. Annelids were the most abundant group 

at NF-B05, accounting for 61% of the total. At NF-B08, nematodes were the most abundant 

group (48%), but aplacophorans (31%) and annelids (19%) were significant components. 

ANOVA was conducted for the most abundant groups in the VK 916 samples to test for spatial 

and temporal differences (Table 11.2). Significant (p<0.05) differences were as follows: 

• Harpacticoid copepods and nematodes were significantly less abundant during Cruise 3B than 

during Cruise 1B. The same was true for aplacophorans, but only at the far-field sites. 

• On Cruise 3B, annelids and aplacophorans were more abundant at the near-field than at the 

far-field sites. There were no significant near-field vs. far-field differences in mean density 

for nematodes or harpacticoids. 

11.3.2 Garden Banks Block 516 

GB 516 was sampled on Cruise 1B (October-November 2000) and Cruise 2B (July 2001). Both 

cruises were post-drilling, with exploration wells drilled prior to the first cruise and several 

development wells drilled between cruises (see Chapter 3). 
364 

Table 11.1. Meiofaunal community statistics for Viosca Knoll Block 916.

Station No. 

Taxa 

Density 

No./10 cm2 H’ 

Diversity 

J’ 

Evenness 

D 

Richness 

Cruise 1B (October-November 2000) 

Near-Field 

NF-B01 10 122 1.77 0.77 1.87 

NF-B02 7 1,452 0.41 0.21 0.82 

NF-B03 9 387 0.89 0.40 1.34 

NF-B04 9 468 1.15 0.52 1.30 

NF-B05 9 700 0.60 0.27 1.22 

NF-B06 6 398 0.96 0.53 0.84 

NF-B07 11 898 0.80 0.33 1.47 

NF-B08 14 1,872 0.69 0.26 1.73 

NF-B09 7 230 0.54 0.28 1.10 

NF-B10 2 18 0.21 0.31 0.35 

NF-B11 6 141 0.77 0.43 1.01 

Far-Field 

FF2-B1 10 896 0.57 0.25 1.32 

FF2-B2 8 574 1.16 0.56 1.10 

FF3-B1 8 596 0.80 0.39 1.10 

FF3-B2 9 514 1.03 0.47 1.28 

FF4-B1 8 744 0.75 0.36 1.06 

FF4-B2 7 414 0.89 0.46 1.00 

FF5-B1 6 217 0.40 0.22 0.93 

FF5-B2 4 74 0.56 0.40 0.70 

FF6-B1 8 783 0.61 0.29 1.05 

FF6-B2 7 626 0.45 0.23 0.93 

Cruise 3B (August 2002) 

Near-Field 

NF-B01 4 40 1.10 0.80 0.81 

NF-B02 10 136 1.50 0.65 1.83 

NF-B03 7 92 1.45 0.75 1.33 

NF-B04 8 92 1.69 0.81 1.55 

NF-B05 6 187 1.03 0.58 0.96 

NF-B06 4 107 0.95 0.69 0.64 

NF-B07 4 21 1.00 0.72 0.99 

NF-B08 7 245 1.14 0.59 1.09 

NF-B09 3 29 0.76 0.69 0.59 

NF-B10 3 19 0.86 0.79 0.68 

NF-B11 6 119 1.35 0.75 1.05 

NF-B12 6 67 1.17 0.65 1.19 

Far-Field 

FF1-B01 5 32 1.16 0.72 1.15 

FF1-B02 1 11 0.00 N/A 0.00 

FF2-B01 4 30 0.73 0.53 0.88 

FF2-B02 4 17 0.89 0.64 1.06 

FF3-B01 1 4 0.00 N/A 0.00 

FF3-B02 2 3 0.64 0.92 0.91 

FF4-B01 4 40 0.80 0.57 0.81 

FF4-B02 4 38 0.53 0.38 0.82 

FF5-B01 2 24 0.56 0.81 0.31 

FF5-B02 3 23 0.80 0.73 0.64 

FF6-B01 1 1 0.00 N/A N/A 

FF6-B02 2 6 0.45 0.65 0.56 

N/A = not applicable.
VK 916

NF

Cruise 1B

(pre-drilling)

FF NF FF

Cruise 3B

(post-drilling)

GB 516

NF

Cruise 1B

(post-drilling)

FF N

Cr

(pos

GB 602

NF

Cruise 2B

(post-drilling)

FF

Figure 11.1. Total meiofaunal densities at the four study sites. Note the log scale.

Meiofaunal Density (no./10 cm2)

NF

Cruise 2B

(post-drilling

MC 292

104

103

102

1 1

10

Meiofaunal Density (no./10 cm2)

104

103

102

10

Meiofaunal Density (no./10 cm2)

104

103

102

1

10

Meiofaunal Density (no./10 cm2)

104

103

102

1

10

Near-field Far-field

LEGEND

365
VK 916 Near-field

Cruise 1B (pre-drilling)

Figure 11.2. Taxonomic composition of meiofauna at Viosca Knoll Block 916.

Total Individuals = 6,686

VK 916 Far-fieldCruise 1B (pre-drilling)

VK 916 Near-fieldCruise 3B (post-drilling)

VK 916 Far-fieldCruise 3B (post-drilling)

Total Ind

Total Individuals = 1,154 Total Indiv

Nematoda 38%

Annelida 36%Aplacophora 17%Harpacticoida 4%

Other 5%

Nematoda 79%

Harpacticoida 14%

Annelida 3%

Other 4%

366
367 

Table 11.2. ANOVA results for meiofaunal groups. Significance level α =0.05. In the 

interaction column, cruise/site combinations are listed in order of decreasing 

abundance, and vertical lines group cruises/sites that are not significantly different. 

ANOVA Result Site Group Cruise Site Interaction 

Viosca Knoll 916 Annelida 

Interaction 

significant; see 

last column 

Interaction 

significant; see 

last column 

 │Cr 3B NF 

││Cr 1B FF 

││Cr 1B NF 

│Cr 3B FF 

Aplacophora 

Interaction 

significant; see 

last column 

Interaction 

significant; see 

last column 

 │Cr 3B NF 

 │Cr 1B FF 

││Cr 1B NF 

││Cr 3B FF 

Harpacticoida Cr 1B > Cr 3B n.s. n.s. 

Nematoda Cr 1B > Cr 3B n.s. n.s. 

Garden Banks 516 Annelida n.s. NF > FF n.s. 

Harpacticoida n.s. n.s. n.s. 

Nematoda Cr 1B > Cr 2B n.s. n.s. 

Garden Banks 602 Annelida N/A n.s. n.s. 

Harpacticoida N/A n.s. n.s. 

Nematoda N/A n.s. n.s. 

Mississippi Canyon 292 Annelida N/A NF > FF n.s. 

Harpacticoida N/A n.s. n.s. 

Nematoda N/A n.s. n.s. 

FF = far-field; NF = near-field; N/A = not applicable (only one cruise at GB 602 and MC 292); 

n.s. = not significant. 

Table 11.3 summarizes meiofaunal community statistics for GB 516 including density, diversity, 

evenness, and richness. There was considerable spatial and temporal variation in all of the 

community statistics. As shown in the table and illustrated in Figure 11.1, densities apparently 

decreased between the two cruises at both near-field and far-field sites. Mean densities on 

Cruise 1B were 122 and 68 individuals/10 cm2

 for near-field and far-field, respectively, as 

compared with 38 and 22 individuals/10 cm2

 for Cruise 2B. On both surveys, there was 

considerable overlap in the range of densities between near-field and far-field sites. On 

Cruise 1B, the near-field site included both the highest and lowest values. On Cruise 2B, the 

near-field site had the highest densities, and the far-field site had the lowest density. 

Figure 11.3 illustrates the taxonomic composition of the meiofaunal samples at GB 516. At the 

far-field sites on both cruises, nematodes and harpacticoid copepods together contributed 88% to 

89% of the total population. At the near-field site, these groups were less numerically dominant 

(66% to 79% combined percentage), and the proportion of annelids was higher. 
368 

Table 11.3. Meiofaunal community statistics for Garden Banks Block 516.

Station No. 

Taxa 

Density 

No./10 cm2 H’ 

Diversity 

J’ 

Evenness 

D 

Richness 

Cruise 1B (October-November 2000) 

Near-Field 

NF-B01 2 8 0.66 0.95 0.48 

NF-B02 3 21 0.98 0.89 0.66 

NF-B03 3 170 1.07 0.97 0.39 

NF-B04 7 226 1.13 0.58 1.11 

NF-B05 5 86 0.91 0.56 0.90 

NF-B06 4 152 0.89 0.64 0.60 

NF-B07 4 15 1.17 0.84 1.11 

NF-B08 4 171 0.93 0.67 0.58 

NF-B09 4 71 0.87 0.63 0.70 

NF-B10 6 288 0.90 0.50 0.88 

NF-B11 4 131 0.79 0.57 0.62 

Far-Field 

FF1-B1 6 137 1.02 0.57 1.02 

FF1-B2 6 66 1.02 0.57 1.19 

FF2-B1 5 80 1.34 0.83 0.91 

FF2-B2 4 106 0.81 0.59 0.64 

FF3-B1 4 32 0.95 0.69 0.87 

FF3-B2 5 56 1.37 0.85 0.99 

FF4-B1 4 18 1.04 0.75 1.04 

FF4-B2 6 62 0.73 0.41 1.21 

FF5-B1 9 146 1.04 0.47 1.61 

FF5-B2 8 72 1.52 0.73 1.64 

FF6-B1 2 13 0.43 0.62 0.39 

FF6-B2 4 22 0.98 0.71 0.97 

Cruise 2B (July 2001) 

Near-Field 

NF-B01 1 3 0.00 N/A 0.00 

NF-B02 4 6 1.24 0.90 1.67 

NF-B03A 5 107 1.11 0.69 0.86 

NF-B04 5 156 0.69 0.43 0.79 

NF-B05 3 7 0.96 0.87 1.03 

NF-B06 3 13 0.54 0.49 0.78 

NF-B07 3 10 1.05 0.96 0.87 

NF-B08 3 9 0.85 0.77 0.91 

NF-B09 4 84 0.79 0.57 0.68 

NF-B10 3 28 0.74 0.67 0.60 

NF-B11 4 26 1.08 0.78 0.92 

NF-B12 3 12 1.10 1.00 0.80 

Far-Field 

FF1-B1 3 13 0.54 0.49 0.78 

FF1-B2 1 1 0.00 N/A N/A 

FF2-B1 2 6 0.64 0.92 0.56 

FF2-B2 3 12 0.92 0.84 0.80 

FF3-B1 2 30 0.68 0.99 0.29 

FF3-B2 4 12 1.08 0.78 1.21 

FF4-B1 6 35 1.43 0.80 1.41 

FF4-B2 3 30 0.80 0.73 0.59 

FF5-B1 4 21 0.90 0.65 0.99 

FF5-B2 4 33 0.88 0.64 0.86 

FF6-B1 5 41 0.89 0.55 1.08 

FF6-B2 3 34 0.71 0.65 0.57 

N/A = not applicable.
Nematoda 46%

Harpacticoida 33%

Annelida 19%

Other 2%

N

HP

O

O

Figure 11.3. Taxonomic composition of meiofauna at Garden Banks Block 516.

Harpacticoida 46%

Nematoda 20%

Annelida 32%

Other 2%

GB 516 Near-fieldCruise 2B

GB 516 Near-fieldCruise 1B

GB 516 Far-fieldCruise 1B

N

H

A

O

GB 516 Far-fieldCruise 2B

Total Individuals = 461 Total Individual

Total Individuals = 1,339 Total Individuals 

369
370 

On Cruise 1B, the two highest densities at the near-field site were observed at NF-B10 and 

NF-B04. Harpacticoids and nematodes together accounted for 85% of total individuals at 

NF-B04 and 95% at NF-B10. On Cruise 2B, harpacticoids and nematodes also dominated at the 

station with the highest abundance (combined 95% of individuals at NF-B04). However, at the 

station with the second highest abundance (NF-B03A), annelids accounted for 50% of the total 

individuals. 

ANOVA was conducted for the most abundant groups at GB 516 to test for spatial and temporal 

differences (Table 11.2). Results were as follows: 

• Nematodes were significantly more abundant on Cruise 1B than on Cruise 2B, whereas the 

abundance of annelids and harpacticoids did not differ between cruises. 

• Annelids were significantly more abundant in the near-field than the far-field. 

• There were no significant differences in nematode or harpacticoid copepod abundance 

between the near-field and far-field. 

11.3.3 Garden Banks Block 602 

GB 602 was sampled only on Cruise 2B (July 2001). Table 11.4 summarizes meiofaunal 

community statistics for GB 602 including density, diversity, evenness, and richness. There was 

considerable spatial variation in all of the community statistics. As shown in Table 11.4 and 

Figure 11.1, there was considerable overlap in the range of densities between near-field and 

far-field sites. However, several near-field stations had higher densities than any of the far-field 

stations. Mean densities were 93 individuals/10 cm2

 for the near-field and 26 individuals/10 cm2

for the far-field. 

Figure 11.4 illustrates the taxonomic composition of the meiofaunal samples at GB 602. At the 

far-field sites, nematodes and harpacticoid copepods together contributed 87% of the total 

population. However, at the near-field site, annelids were the second most abundant group (31%, 

compared with 49% for nematodes), and nematodes and harpacticoids together contributed only 

66% of the total population. 

The two near-field stations having the highest density were NF-B03 and NF-B12. At NF-B03, 

annelids were dominant (54%), followed by nematodes (40%). At NF-B12, nematodes were the 

most abundant group (46%), followed by annelids (38%). 

ANOVA did not detect any significant differences between near-field vs. far-field sites with 

respect to abundance of the most abundant meiofaunal groups at GB 602 (Table 11.2). No 

temporal comparisons are possible because the site was sampled only on Cruise 2B. 
371 

Table 11.4. Meiofaunal community statistics for Garden Banks Block 602, Cruise 2B 

(July 2001).

Station No. 

Taxa 

Density 

No./10 cm2

H’ 

Diversity 

J’ 

Evenness 

D 

Richness 

Near-Field 

NF-B01 4 16 0.69 0.50 1.08 

NF-B02 10 140 0.57 0.26 1.62 

NF-B03 7 163 0.91 0.57 0.79 

NF-B04 8 86 1.31 0.67 1.35 

NF-B05 6 48 1.31 0.81 1.03 

NF-B06 6 126 0.82 0.51 0.83 

NF-B07 5 81 1.16 0.72 0.91 

NF-B08 4 5 1.33 0.96 1.86 

NF-B09 4 28 1.09 0.79 0.90 

NF-B10 5 9 1.43 0.89 1.82 

NF-B11 9 107 1.42 0.68 1.50 

NF-B12 11 305 1.18 0.54 1.40 

Far-Field 

FF1-B1 12 35 1.78 0.81 2.25 

FF1-B2 7 26 1.30 0.73 1.53 

FF2-B1 14 39 1.57 0.63 3.00 

FF2-B2 10 25 1.77 0.85 2.17 

FF3-B1 3 4 1.04 0.95 1.44 

FF3-B2 8 30 1.39 0.67 2.06 

FF4-B1 2 4 0.56 0.81 0.72 

FF4-B2 9 84 1.59 0.76 1.58 

FF5-B1 5 17 1.30 0.81 1.41 

FF5-B2 3 7 0.96 0.87 1.03 

FF6-B1 6 17 1.22 0.76 1.41 

FF6-B2 11 27 1.84 0.80 2.73 
Figure 11.4. Taxonomic composition of meiofauna at Garden Banks Block 602 and Mississippi Canyon Bloc

GB 602 Near-fieldCruise 2B

Total Individuals = 1,114

Nematoda 49%

Annelida 31%Harpacticoida 17%

Other 3%

GB 602 Far-fieldCruise 2B

Total Indivi

MC 292 Near-fieldCruise 2B

Nematoda 69%Harpacticoida 15%Annelida 11%Other 5%

Total Individuals = 1,248 Total Individu

MC 292 Far-fieldCruise 2B

372
373 

11.3.4 Mississippi Canyon Block 292 

MC 292 was sampled only on Cruise 2B (July 2001). Table 11.5 summarizes meiofaunal 

community statistics for MC 292 including density, diversity, evenness, and richness. There was 

considerable spatial variation in all of the community statistics. As shown in Table 11.5 and 

Figure 11.1, there was considerable overlap in the range of densities between near-field and 

far-field sites. However, the highest densities were seen in the near-field. Mean densities were 

104 individuals/10 cm2

 for the near-field and 51 individuals/10 cm2 for the far-field (including 

one zero value). 

Figure 11.4 illustrates the taxonomic composition of the meiofaunal samples at MC 292. At the 

far-field sites, nematodes were by far the dominant group, contributing 80% of the total 

population. At the near-field site, nematodes were slightly less dominant (69%), and harpacticoid 

copepods and annelids were relatively more abundant than at the far-field sites. 

The two near-field stations having the highest density were NF-B01 and NF-B05. At NF-B01, 

nematodes were dominant (78%). At NF-B05, nematodes were the most abundant group (50%), 

followed by annelids (20%). 

ANOVA indicated that annelids were more abundant in the near-field than in the far-field, but 

there were no significant differences for nematodes or harpacticoids (Table 11.2). No temporal 

comparisons are possible because the site was sampled only on Cruise 2B. 

11.3.5 Individual Groups and Environmental Correlations 

The three most abundant groups from the meiofaunal data (annelids, harpacticoid copepods, and 

nematodes) were selected for further plotting and analysis. Figure 11.5 shows densities of these 

individual groups in all post-drilling samples, with each point representing an individual sample. 

The figure shows that for all three groups, there is a tendency for at least some near-field stations 

to have higher densities than their far-field counterparts. This pattern is most pronounced for 

annelids, which clearly tended to be more abundant in the near-field (as confirmed by significant 

ANOVAs, see Table 11.2). Although the ANOVA did not detect a significant difference 

between near-field and far-field means for nematodes or harpacticoids, Figure 11.5 shows there 

were higher densities at some near-field stations. 
374 

Table 11.5. Meiofaunal community statistics for Mississippi Canyon Block 292, 

Cruise 2B (July 2001).

Station 

No. 

Taxa 

Density 

No./10 cm2

H’ 

Diversity 

J’ 

Evenness 

D 

Richness 

Near-Field 

NF-B01 9 279 0.85 0.41 1.24 

NF-B02 6 104 1.10 0.61 1.08 

NF-B03 16 112 1.93 0.73 2.76 

NF-B04 12 130 1.25 0.54 1.85 

NF-B05 12 202 1.41 0.61 1.70 

NF-B06 5 13 1.44 0.89 1.56 

NF-B07 9 67 1.71 0.78 1.90 

NF-B08 9 100 1.56 0.75 1.52 

NF-B09 9 124 1.17 0.56 1.45 

NF-B10 8 94 1.43 0.69 1.54 

NF-B11 4 10 0.94 0.86 0.87 

NF-B12 4 13 1.31 0.94 1.17 

Far-Field 

FF1-B1 11 65 1.42 0.62 2.16 

FF1-B2 7 97 1.03 0.53 1.31 

FF2-B1 0 0 N/A N/A N/A 

FF2-B2 8 100 1.38 0.66 1.52 

FF3-B1 3 43 0.99 0.90 0.53 

FF3-B2 7 60 0.97 0.54 1.22 

FF4-B1 7 51 1.46 0.75 1.53 

FF4-B2 10 72 1.46 0.66 1.87 

FF5-B1 5 23 1.35 0.84 1.28 

FF5-B2 5 31 1.29 0.80 1.16 

FF6-B1 7 45 1.25 0.70 1.31 

FF6-B2 4 29 0.88 0.64 0.89 

N/A = not applicable. 
Harpacticoids

Harpacticoids per 10 cm2

VK 916 GB 516 GB 602 MC 292

NF NF FF FF FF FF NF NF

(Cruise 1B)

GB 516

NF FF

(Cruise 2B)

Nematodes per 10 cm2

Nematodes

(Cruise 3B) (Cruise 2B) (Cruise 2B)

VK 916 GB 516 GB 602 MC 292

NF NF FF FF FF FF NF NF

(Cruise 1B)

GB 516

NF FF

(Cruise 3B) (Cruise 2B) (Cruise 2B) (Cruise 2B)

Figure 11.5. Comparison of near-field and far-field abundances of meiofaunal nematodes,

harpacticoids, and annelids in post-drilling samples.

1

103

VK 916 GB 516 GB 602 MC 292

NF FF NF FF NF FF FF NF

(Cruise 1B)

GB 516

NF FF

(Cruise 3B) (Cruise 2B) (Cruise 2B) (Cruise 2B)

Annelids

Annelids per 10 cm2

102

10

Near-field

Far-field

LEGEND

1

103

102

10

1

103

102

10

375
376 

Correlations were calculated between densities of these meiofaunal groups and environmental 

variables (barium, SBF, and TOC concentrations; sediment sand, silt, and clay percentages). 

Only stations exposed to drilling discharges were included in this analysis (i.e., near-field, 

post-drilling). Results are summarized in Table 11.6. Most of the correlation coefficients were 

not significant. There was no consistent relationship between any of the selected environmental 

variables and the abundance of annelids, harpacticoid copepods, or nematodes. At MC 292, 

annelid densities were positively correlated with barium and SBF. Annelid densities also were 

positively correlated with barium at GB 516 (Cruise 2B), but there was little or no relationship 

with barium or SBF at VK 916 or GB 602. Harpacticoid densities were correlated negatively 

with SBF at GB 516 (Cruise 1B), and at GB 602, were correlated positively with sand and 

negatively with silt. Nematode densities were negatively correlated with barium and SBF at 

GB 516 (Cruise 1B) and negatively correlated with silt at GB 602. 

11.3.6 Nematode Feeding Groups and Harpacticoid Reproductive Status 

During Cruise 2B, samples from GB 602 and MC 292 were selected for further analysis. Where 

possible, nematodes were identified to family, and the families were assigned to feeding types 

(deposit feeder or predator) based on Jensen (1986). Harpacticoid reproductive condition was 

also noted (ovigerous vs. non-ovigerous females). 

Table 11.7 summarizes the results of these analyses. The proportion of the nematode population 

that could be identified to family level ranged from 6% to 36% among the sites. The following 

results were noted: 

• Densities of deposit feeders were about six times higher at MC 292 than at GB 602. 

• Densities of predators also were apparently higher at MC 292 than at GB 602. 

• Near-field vs. far-field sites had similar densities of deposit feeders at both GB 602 and 

MC 292. 

• Near-field sites had higher densities of predators at both locations. 

• The ratio of deposit feeders to predators was 3.5 and 4.2 at the two near-field sites and 13.9 

and 14 at the two far-field sites. That is, predators were more abundant (relative to deposit 

feeders) at the near-field sites. 

The ratio of non-ovigerous females to ovigerous females was higher at GB 602 (7.0 and 6.5 for 

near-field and far-field sites, respectively) than at MC 292 (2.3 and 4.0 for near-field and far-field 

sites, respectively). There was no indication that near-field sites were consistently higher or 

lower than far-field sites with respect to this ratio, or with respect to densities of ovigerous or 

non-ovigerous females. Due to the small number of ovigerous females noted at any site, this 

analysis was not considered very useful. 

11.3.7 Discussion 

Deep-sea meiofaunal organisms typically occupy the top 2 cm of sediment. Data compiled from 

a variety of studies by Bett et al. (1994) indicated that about two-thirds of meiofauna occur in this 

layer of surficial sediment, on average. These organisms are therefore readily exposed to surface 

sediment disturbance, such as deposition of drilling muds and cuttings from gas and oil 

exploratory and development activities. 
377 

Table 11.6. Rank correlation coefficients (Spearman’s rho) between meiofaunal densities and 

environmental variables within areas exposed to drilling discharges (near-field sites 

on post-drilling cruises). Statistically significant correlation coefficients (p<0.05), 

i.e., those large enough that they are probably not due to chance, are bold and 

double-underlined. 

Correlation Coefficient vs. Environmental Variable Group Site Cruise 

Barium SBF TOC Sand Silt Clay

Annelida VK 916 3B -0.052 -0.147 -0.242 0.105 0.115 -0.150

GB 516 1B 0.365 0.050 0.041 0.153 -0.265 0.265

GB 516 2B 0.764 0.416 0.203 0.316 0.082 -0.195

GB 602 2B 0.049 0.219 0.117 0.638 -0.198 -0.109

MC 292 2B 0.653 0.832 0.533 0.179 0.502 -0.582

Harpacticoida VK 916 3B -0.340 -0.201 -0.294 0.044 -0.007 0.026

GB 516 1B -0.501 -0.733 0.000 -0.224 0.068 -0.068

GB 516 2B -0.067 -0.446 -0.312 0.049 0.004 -0.186

GB 602 2B 0.189 0.133 -0.308 0.594 -0.622 0.427

MC 292 2B 0.473 0.374 0.171 0.444 0.155 -0.388

Nematoda VK 916 3B 0.017 0.018 -0.165 0.164 -0.115 -0.049

GB 516 1B -0.600 -0.845 -0.191 -0.068 0.145 -0.145

GB 516 2B -0.032 -0.172 -0.054 -0.116 -0.025 -0.056

GB 602 2B -0.252 -0.228 -0.249 0.501 -0.595 0.368

MC 292 2B 0.352 0.493 0.272 -0.021 0.049 -0.084

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; SBF = synthetic-based fluid; 

TOC = total organic carbon. 

Table 11.7. Additional meiofaunal analyses from Cruise 2B: nematode trophic groups and 

harpacticoid copepod reproductive condition. Densities are number of individuals 

per 10 cm2

. 

Garden Banks Block 602 Mississippi Canyon Block 292 Statistic 

Near-Field Far-Field Near-Field Far-Field 

Nematode density (total) 45.8 14.2 71.5 41.0 

Deposit feeders a 2.1 2.8 11.7 13.9 

Predators a 0.6 0.2 2.8 1.0 

Not determined 43.1 11.2 57.0 26.1 

Ratio deposit/predators 3.5 14.0 4.2 13.9 

Harpacticoid density (total) 15.3 8.5 15.9 4.0 

Non-ovigerous females 1.4 1.3 1.4 0.4 

Ovigerous females 0.2 0.2 0.6 0.1 

Ratio non-ovig./ovigerous 7.0 6.5 2.3 4.0 

a Nematode trophic groups are based on identification to family, with feeding types of families assigned 

based on Jensen (1986).
378 

In the present study, we observed a wide range in meiofaunal abundance among the four study 

sites and between surveys. Average meiofaunal densities ranged from 19 to 608 per 10 cm2

. For 

comparison, a regression equation developed during the MMS northern Gulf of Mexico 

continental slope study (Gallaway et al. 1988) predicts a meiofaunal density of about 490 per 

10 cm2

 for a water depth of 1,000 m. Preliminary data from the new MMS deep Gulf of Mexico 

benthos study indicate typical meiofaunal densities at a water depth of 1,000 m are in the range of 

100 to 200 per 10 cm2

 (Rowe and Kennicutt 2002). An inverse relationship between water depth 

and meiofaunal abundance has been widely reported (Jumars and Gallagher 1982; Montagna and 

Harper 1996; Rowe and Kennicutt 2002). Bett et al. (1994) reported higher densities at similar 

water depths in the northeast Atlantic (over 1,000 per 10 cm2

) due to higher nematode densities 

than in the present study. 

Two general patterns are evident in the meiofaunal data and are discussed further below: 

• Densities declined between Cruise 1B and the subsequent cruises. 

• Densities tended to be higher in the near-field than the far-field. 

The meiofaunal data indicate a decrease in density over time at both near-field and far-field sites. 

At GB 516, average densities decreased by a factor of three at both the near-field and far-field 

sites between Cruise 1B (October-November 2000) and Cruise 2B (July 2001). At VK 916, 

average densities decreased by a factor of six at the near-field site and by a factor of 28 at the 

far-field sites between Cruise 1B and Cruise 3B (August 2002). No cruise-to-cruise comparisons 

are possible for GB 602 or MC 292; however, the Cruise 2B densities for GB 602 far-field sites 

are similar to densities at nearby GB 516 during this time. 

The reason for this decline over time is unknown. It is similar to the decline in microbial biomass 

noted in Chapter 10 between cruises. There was no area-wide change in sediment grain size (see 

Chapter 5) or TOC (see Chapter 8) between cruises. At VK 916, TOC increased by about 

one-third at the near-field site, but not at the far-field site. At GB 516, there was no significant 

difference between TOC between cruises for either far-field or near-field sites. The decline is not 

attributable to drilling discharges because little or no exposure of far-field sites to drilling 

effluents was detected (e.g., elevated barium or SBF concentrations; see Chapters 8 and 9); this 

issue is discussed further in Chapter 15. 

The broad decline in meiofaunal abundance between cruises complicates interpretation. Because 

of the magnitude of this change, it would be difficult to determine whether differences between 

cruises are due to drilling impacts. Therefore spatial comparisons (near-field vs. far-field) are 

more useful in detecting impacts. In particular, although there was considerable overlap between 

near-field and far-field densities, the highest densities were always seen in the near-field. 

The high total densities in the near-field reflected the increased abundance of several groups. As 

shown in Figure 11.5, in post-drilling samples, the highest densities of annelids, harpacticoid 

copepods, and nematodes were always seen in the near-field. 

VK 916 offers the only straightforward pre- vs. post-drilling comparison. On average, the 

magnitude of population decline between cruises was much greater at the far-field sites than at 

the near-field site, suggesting a possible enrichment effect in the near-field. Groups that showed 
379 

higher densities in the near-field on the post-drilling cruise were annelids and aplacophoran 

molluscs. These two groups contributed over 50% of the individuals in the near-field on 

Cruise 3B. In contrast, the far-field sites (and the pre-drilling near-field site) were numerically 

dominated by nematodes and harpacticoid copepods, which together accounted for over 90% of 

total numbers. 

GB 516 provides evidence of an apparent numerical response of annelids to drilling discharges 

(higher densities near drilling). At the far-field sites, nematodes and harpacticoid copepods 

together contributed 88% to 89% of the total population. At the near-field site, nematodes and 

harpacticoids were less numerically dominant (67% to 79% combined percentage), and there was 

a noticeable population of annelids on both cruises. 

At MC 292, annelid densities were significantly higher in the near-field than the far-field. But at 

GB 602, there were no significant differences between near-field and far-field sites with respect 

to densities of individual meiofaunal groups. However, there is some indication of differences in 

community composition at GB 602 that could reflect drilling impacts. At the GB 602 near-field 

site, annelids were the second most abundant group (31%, compared with 49% for nematodes), 

much different from the far-field sites, where nematodes and harpacticoid copepods together 

contributed 87% of the total population. 

Our results contrast with those found previously in the GOOMEX study. Montagna and Harper 

(1996) found that nematodes were more abundant near platforms and harpacticoids were less 

abundant. They attributed the nematode increase to organic enrichment and the harpacticoid 

decrease to toxicity of metals in drilling discharges. In our study, harpacticoid densities were not 

reduced in the near-field. A similar result (higher densities in the near-field) is reported in 

Chapter 13 for one particular harpacticoid, Bathycletopsyllus sp. The only group that had lower 

densities in the near-field was ostracods at GB 516, but this group was not common enough in the 

meiofauna to establish a general pattern (see further information on this group in the 

Macroinfauna section). 

Although meiofaunal annelid, nematode, and harpacticoid copepod densities tended to be highest 

in the near-field, there were no consistent correlations between abundances of these groups and 

drilling discharge indicators (e.g., barium and SBF concentrations) within near-field sites. 

The detailed analyses of nematode feeding types and harpacticoid copepod reproductive status 

were not very useful because of the small number of organisms identified. The nematode 

analysis indicated that nematode families classified as predators were more abundant (relative to 

deposit feeders) in the near-field than the far-field. This seems to be the opposite of what one 

might expect in the presence of near-field organic enrichment (Montagna and Harper 1996). The 

ratio of harpacticoid non-ovigerous females to ovigerous females was higher at GB 602 than at 

MC 292, but there was no indication that near-field sites were consistently higher or lower than 

far-field sites with respect to this ratio. 
380 

11.4 MACROINFAUNAL RESULTS AND DISCUSSION 

A complete list of macroinfaunal taxa is presented in Appendix I2. 

11.4.1 Viosca Knoll Block 916 

VK 916 was sampled on Cruise 1B (October-November 2000) and Cruise 3B (August 2002). 

The two cruises served as pre- and post-drilling surveys for this site. 

Table 11.8 summarizes macroinfaunal community statistics for VK 916 including density, 

diversity, evenness, and richness. There was considerable spatial and temporal variation in all of 

the community statistics. As shown in Table 11.8 and illustrated in Figure 11.6, generally, 

densities decreased by a factor of six between the two cruises at both near-field and far-field sites. 

Similar declines were observed in microbial biomass (see Chapter 10) and meiofaunal densities 

(see Section 11.3.7). 

Mean densities on Cruise 1B were 8,517 and 8,751 individuals/m2

 for near-field and far-field, 

respectively, as compared with 1,633 and 1,256 individuals/m2

, respectively, for Cruise 3B. 

There was considerable overlap between near-field and far-field densities on both cruises. On the 

pre-drilling cruise (1B), the highest densities were seen at the far-field sites. On the post-drilling 

cruise (3B), the highest densities were seen in the near-field. 

Figure 11.7 illustrates the taxonomic composition of the macroinfaunal samples at VK 916. On 

Cruise 1B, the community was similar at near-field and far-field sites, with annelids the most 

abundant group, followed by ostracods and aplacophoran molluscs. On the post-drilling cruise 

(3B), the near-field and far-field sites differed noticeably, with the near-field site having higher 

proportions of annelids and gastropods and lower proportions of ostracods and amphipods than 

the far-field. 

On the post-drilling cruise, the two near-field stations with the highest densities were dominated 

by annelids (98% at NF-B06) or a combination of annelids and gastropods (68% and 30% 

respectively, at NF-B07). 

ANOVA was conducted for individual macroinfaunal groups to test for spatial and temporal 

differences (Table 11.9). The significant (p<0.05) differences were as follows: 

• For annelids, amphipods, aplacophorans, bivalves, gastropods, harpacticoids, ostracods, 

rhyncocoels, and tanaids, Cruise 1B densities were higher than Cruise 3B densities. 

• For ostracods, far-field densities were higher than near-field densities. 
381 

Table 11.8. Macroinfaunal community statistics for Viosca Knoll Block 916. 

Station 

No. 

Taxa 

No. 

Individuals 

Density 

(no./m2

) 

H’ 

Diversity 

J’ 

Evenness 

D 

Richness 

Cruise 1B (October-November 2000)

Near-Field 

NF-B01 14 933 9,330 1.56 0.59 1.90

NF-B02 15 435 4,350 1.68 0.62 2.30

NF-B03 16 1,034 10,340 1.69 0.61 2.16

NF-B04 17 1,254 12,540 1.67 0.59 2.24

NF-B05 18 1,240 12,400 1.70 0.59 2.39

NF-B06 15 1,186 11,860 1.49 0.55 1.98

NF-B07 16 759 7,590 1.34 0.48 2.26

NF-B08 14 1,252 12,520 1.58 0.60 1.82

NF-B09 15 726 7,260 1.59 0.59 2.13

NF-B10 14 399 3,990 1.43 0.54 2.17

NF-B11 14 825 8,250 1.62 0.61 1.94

NF-B12 1 177 1,770 0.00 N/A 0.00

Far-Field 

FF2-B1 15 1,356 13,560 1.64 0.61 1.94

FF2-B2 17 1,144 11,440 1.59 0.56 2.27

FF3-B1 17 1,738 17,380 1.62 0.57 2.14

FF3-B2 14 344 3,440 1.30 0.49 2.23

FF4-B1 16 1,110 11,100 1.75 0.63 2.14

FF4-B2 16 838 8,380 1.67 0.60 2.23

FF5-B1 14 220 2,200 1.42 0.54 2.41

FF5-B2 13 586 5,860 1.61 0.63 1.88

FF6-B1 14 1,251 12,510 1.76 0.67 1.82

FF6-B2 11 164 1,640 1.57 0.65 1.96

Cruise 3B (August 2002)

Near-Field 

NF-B01 6 61 610 1.13 0.63 1.22

NF-B02 8 140 1,400 1.66 0.80 1.42

NF-B03 12 193 1,930 1.80 0.72 2.09

NF-B04 12 158 1,580 1.72 0.69 2.17

NF-B05 4 217 2,170 0.29 0.21 0.56

NF-B06 4 391 3,910 0.09 0.07 0.50

NF-B07 4 447 4,470 0.68 0.49 0.49

NF-B08 12 130 1,300 2.01 0.81 2.26

NF-B09 2 49 490 0.10 0.14 0.26

NF-B10 8 67 670 0.85 0.41 1.66

NF-B11 7 71 710 1.50 0.77 1.41

NF-B12 7 36 360 1.32 0.68 1.67

Far-Field 

FF1-B1 8 87 870 1.45 0.70 1.57

FF1-B2 13 186 1,860 1.68 0.65 2.30

FF2-B1 13 162 1,620 1.80 0.70 2.36

FF2-B2 12 142 1,420 1.86 0.75 2.22

FF3-B1 8 84 840 0.99 0.48 1.58

FF3-B2 12 147 1,470 1.81 0.73 2.20

FF4-B1 11 123 1,230 1.74 0.73 2.08

FF4-B2 11 153 1,530 1.69 0.70 1.99

FF5-B1 9 145 1,450 1.75 0.80 1.61

FF5-B2 9 119 1,190 1.51 0.69 1.67

FF6-B1 5 29 290 0.92 0.57 1.19

FF6-B2 10 130 1,300 1.74 0.76 1.85

N/A = not applicable.
Figure 11.6. Total macroinfaunal densities at the four study sites. Note the log scale.

VK 916

105

Macroinfaunal Density (no./m2)

NF

Cruise 1B

(pre-drilling)

FF NF FF

Cruise 3B

(post-drilling)

GB 516

NF

Cruise 1B

(post-drilling)

FF N

C

(po

Macroinfaunal Density (no./m2)

GB 602

NF FF

Cruise 2B

(post-drilling)

Macroinfaunal Density (no./m2)MC 292

NF 

Cruise 2

(post-drilli

Macroinfaunal Density (no./m2)

104

103102

105

104

103102

105

104

103102

105

104

103102

Near-field Far-field

LEGEND

382
VK 916 Near-field

Cruise 1B (pre-drilling)

Figure 11.7. Taxonomic composition of macroinfauna at Viosca Knoll Block 916.

Total Individuals = 1,960

VK 916 Far-fieldCruise 1B (pre-drilling)

VK 916 Near-fieldCruise 3B (post-drilling)

VK 916 Far-fieldCruise 3B (post-drilling)

Total Indivi

Annelida 68%Gastropoda 7%Bivalvia 7%Ostracoda 5%Aplacophora 5%Other 8%

Total Individuals = 10,220 Total Individ

Annelida 48%

Ostracoda 11%

Aplacophora 11%

Bivalvia 8%

Decapoda 7%

Other 15%

383
384 

Table 11.9. ANOVA results for macroinfaunal groups. Significance level α =0.05. In the 

interaction column, cruise/site combinations are listed in order of decreasing 

abundance, and vertical lines group cruises/sites that are not significantly different. 

ANOVA Result Site Group Cruise Site Interaction 

VK 916 Annelida 1B > 3B n.s. n.s. 

Amphipoda 1B > 3B n.s. n.s. 

Aplacophora 1B > 3B n.s. n.s. 

Bivalvia 1B > 3B n.s. n.s. 

Decapoda n.s. n.s. n.s. 

Gastropoda 1B > 3B n.s. n.s. 

Harpacticoida 1B > 3B n.s. n.s. 

Ostracoda 1B > 3B FF > NF n.s. 

Rhyncocoela Interaction significant; 

see last column 

Interaction significant; 

see last column 

│ 1B NF 

│ 1B FF 

│ 3B FF 

│ 3B NF 

Tanaidacea 1B > 3B n.s. n.s. 

GB 516 Annelida n.s. NF > FF n.s. 

Amphipoda Interaction significant; 

see last column 

Interaction significant; 

see last column 

│ 2B FF 

│ 1B FF 

│ 1B NF 

 2B NF 

Aplacophora n.s. n.s. n.s. 

Bivalvia n.s. NF > FF n.s. 

Gastropoda n.s. NF > FF n.s. 

Harpacticoida 2B > 1B n.s. n.s. 

Nematoda 2B > 1B n.s. n.s. 

Ostracoda n.s. FF > NF n.s. 

Rhyncocoela n.s. n.s. n.s. 

Tanaidacea n.s. FF > NF n.s. 

GB 602 Annelida N/A NF > FF -- 

Aplacophora N/A NF > FF -- 

Bivalvia N/A NF > FF -- 

Gastropoda N/A NF > FF -- 

Harpacticoida N/A NF > FF -- 

Nematoda N/A FF > NF -- 

Rhyncocoela N/A NF > FF -- 

Tanaidacea N/A FF > NF -- 

MC 292 Annelida N/A NF > FF -- 

Amphipoda N/A n.s. -- 

Aplacophora N/A n.s. -- 

Bivalvia N/A NF > FF -- 

Harpacticoida N/A NF > FF -- 

Nematoda N/A n.s. -- 

Ostracoda N/A n.s. -- 

Tanaidacea N/A/ n.s -- 

FF = far-field; NF = near-field; N/A = not applicable (only one cruise at GB 602 and MC 292); 

n.s. = not significant. 
385 

11.4.2 Garden Banks Block 516 

GB 516 was sampled on Cruise 1B (October-November 2000) and Cruise 2B (July 2001). 

Exploration wells were drilled at this site prior to the first cruise, and therefore both cruises were 

post-drilling. Several development wells were drilled between the two cruises (see Chapter 3). 

Table 11.10 summarizes macroinfaunal community statistics for GB 516 including density, 

diversity, evenness, and richness. There was considerable spatial and temporal variation in all of 

the community statistics. As shown in Table 11.10 and illustrated in Figure 11.6, both near-field 

and far-field densities generally increased between the two cruises. Mean density at the 

near-field site was 4,901 individuals/m2

 on Cruise 1B and 11,087 individuals/m2 on Cruise 2B. 

At the far-field sites, mean density was 1,692 individuals/m2

 on Cruise 1B and 

2,838 individuals/m2

 on Cruise 2B. There was considerable overlap between near-field and 

far-field densities on both cruises. However, the range of densities was much greater in the 

near-field than in the far-field. Most notably, on both cruises, the near-field site included several 

stations that had much higher densities than any far-field sample. 

Figure 11.8 illustrates the taxonomic composition of the macroinfaunal samples at GB 516. At 

both near-field and far-field sites, annelids were the most abundant group on both cruises (37% to 

50%). However, gastropods were the second most abundant group in the near-field (25% to 

30%), compared with only 1% at the far-field sites. 

Gastropod and/or annelid dominance also was evident when looking at some individual samples 

in the near-field. On Cruise 1B, the two near-field stations with the highest densities were 

NF-B02 and NF-B06. At NF-B02, gastropods accounted for 89% of the total population. At 

NF-B06, annelids were 78% of the total. On Cruise 2B, the two near-field stations with the 

highest densities were NF-B03 (48% annelids and 26% gastropods) and NF-B06 (66% 

gastropods). 

ANOVA was conducted for individual macroinfaunal groups to test for spatial and temporal 

differences (Table 11.9). The significant (p<0.05) differences were as follows: 

• For harpacticoids and nematodes, densities were significantly higher on Cruise 2B than on 

Cruise 1B. 

• For annelids, bivalves, and gastropods, near-field densities were significantly higher than 

far-field densities. 

• For ostracods and tanaids, far-field densities were significantly greater than near-field 

densities. 

For amphipods, densities at far-field sites (both cruises) and the near-field on Cruise 1B were 

higher than for the near-field on Cruise 2B. 
386 

Table 11.10. Macroinfaunal community statistics for Garden Banks Block 516. 

Station 

No. 

Taxa 

No. 

Individuals 

Density 

(no./m2

) 

H’ 

Diversity 

J’ 

Evenness 

D 

Richness 

Cruise 1B (October-November 2000)

Near-Field 

NF-B01 10 636 6,360 0.90 0.39 1.39

NF-B02 8 1922 19,220 0.43 0.21 0.93

NF-B03 13 237 2,370 0.87 0.34 2.19

NF-B04 13 225 2,250 1.11 0.43 2.22

NF-B05 12 244 2,440 1.15 0.46 2.00

NF-B06 12 907 9,070 0.84 0.34 1.62

NF-B07 14 196 1,960 1.43 0.54 2.46

NF-B08 16 336 3,360 1.68 0.61 2.58

NF-B09 13 317 3,170 1.29 0.50 2.08

NF-B10 13 263 2,630 1.53 0.59 2.15

NF-B11 16 335 3,350 1.71 0.62 2.58

NF-B12 15 263 2,630 1.70 0.63 2.51

Far-Field 

FF1-B1 14 153 1,530 1.62 0.61 2.58

FF1-B2 13 226 2,260 1.57 0.61 2.21

FF2-B1 17 251 2,510 2.10 0.74 2.90

FF2-B2 12 75 750 1.89 0.76 2.55

FF3-B1 14 202 2,020 1.78 0.67 2.45

FF3-B2 12 78 780 1.80 0.72 2.52

FF4-B1 15 170 1,700 1.89 0.70 2.73

FF4-B2 16 152 1,520 1.63 0.59 2.99

FF5-B1 14 129 1,290 1.76 0.67 2.68

FF5-B2 15 181 1,810 1.85 0.68 2.69

FF6-B1 14 185 1,850 1.64 0.62 2.49

FF6-B2 13 228 2,280 1.72 0.67 2.21

Cruise 2B (July 2001)

Near-Field 

NF-B01 7 177 1,770 1.42 0.73 1.16

NF-B02 12 716 7,160 1.59 0.64 1.67

NF-B03 11 3,738 37,380 1.42 0.59 1.22

NF-B04 14 380 3,800 1.16 0.44 2.19

NF-B05 12 542 5,420 1.93 0.78 1.75

NF-B06 5 2,446 24,460 0.71 0.44 0.51

NF-B07 16 1,302 13,020 1.34 0.48 2.09

NF-B08 15 862 8,620 1.37 0.51 2.07

NF-B09 8 2,434 24,340 0.85 0.41 0.90

NF-B10 12 173 1,730 1.68 0.67 2.13

NF-B11 12 126 1,260 1.75 0.70 2.27

NF-B12 16 408 4,080 1.82 0.66 2.50

Far-Field 

FF1-B1 17 174 1,740 2.29 0.81 3.10

FF1-B2 21 359 3,590 2.15 0.71 3.40

FF2-B1 12 142 1,420 1.92 0.77 2.22

FF2-B2 15 268 2,680 1.83 0.67 2.50

FF3-B1 15 284 2,840 1.86 0.69 2.48

FF3-B2 12 296 2,960 1.75 0.70 1.93

FF4-B1 17 174 1,740 2.20 0.78 3.10

FF4-B2 15 308 3,080 2.04 0.75 2.44

FF5-B1 16 248 2,480 2.12 0.76 2.72

FF5-B2 15 538 5,380 2.00 0.74 2.23

FF6-B1 17 296 2,960 1.94 0.68 2.81

FF6-B02 16 318 3,180 1.87 0.67 2.60
Figure 11.8. Taxonomic composition of macroinfauna at Garden Banks Block 516.

GB 516 Near-fieldCruise 2B

GB 516 Near-fieldCruise 1B

GB 516 Far-fieldCruise 1B

GB 516 Far-fieldCruise 2B

Total Individu

Total Individuals = 13,304

Total Individuals = 5,881 Total Individua

Annelida 49%Gastropoda 30%Bivalvia 8%Rhyncocoela 3%Harpacticoida 2%Other 8%

Annelida 48%Gastropoda 25%Harpacticoida 8%Bivalvia 7%Nematoda 6%Other 6%

387
388 

11.4.3 Garden Banks Block 602 

GB 602 was sampled only on Cruise 2B. Table 11.11 summarizes macroinfaunal community 

statistics for GB 602 including density, diversity, evenness, and richness. There was considerable 

spatial and temporal variation in all of the community statistics. As shown in Table 11.11 and in 

Figure 11.5, there was considerable overlap between near-field and far-field densities. However, 

the range of densities was much greater in the near-field than in the far-field. The near-field site 

included several stations that had much higher densities than any far-field sample. The lowest 

densities were seen at the far-field sites. Mean densities were 9,251 individuals/m2

 in the 

near-field and 1,743 individuals/m2

 in the far-field. 

Figure 11.9 illustrates the taxonomic composition of the macroinfaunal samples at GB 602. 

Annelids were the most abundant group in the near-field (47%) followed by harpacticoid 

copepods (20%) and gastropods (11%). The far-field was different in that nematodes were the 

most abundant group (34% vs. 26% for annelids), and gastropods accounted for less than 1% of 

the total. 

Annelid and/or harpacticoid dominance was evident at near-field stations with high densities. 

The two near-field stations with the highest densities were NF-B03 and NF-B05. At NF-B03, 

annelids accounted for 88% of the total population. At NF-B05, harpacticoid copepods were 60% 

of the total. 

ANOVA was conducted for individual macroinfaunal groups to test for spatial and temporal 

differences (Table 11.9). The significant (p<0.05) differences were as follows: 

• Annelids, aplacophorans, bivalves, gastropods, harpacticoids, and rhyncocoels were more 

abundant at the near-field site than at the far-field sites. 

• Nematodes and tanaids were more abundant at the far-field sites than at the near-field site. 

11.4.4 Mississippi Canyon Block 292 

MC 292 was sampled only on Cruise 2B. Table 11.12 summarizes macroinfaunal community 

statistics for GB 602 including density, diversity, evenness, and richness. There was considerable 

spatial and temporal variation in all of the community statistics. As shown in Table 11.12 and in 

Figure 11.5, there was considerable overlap between near-field and far-field densities. The 

near-field site included several stations that had much higher densities than any far-field sample. 

The lowest density was observed at one far-field site. Mean densities were 10,327 individuals/m2

in the near-field and 5,448 individuals/m2

 in the far-field. 

Figure 11.9 illustrates the taxonomic composition of the macroinfaunal samples at MC 292. 

Annelids were the most abundant group in the near-field (39%) followed by harpacticoid 

copepods (20%) and nematodes (18%). The far-field was different in that nematodes were the 

most abundant group (30% vs 24% for annelids), and harpacticoid copepods were less than 1% of 

the population. 

The two near-field stations with the highest densities were NF-B01 and NF-B03. At NF-B01, 

harpacticoids were 48%, and annelids accounted for 41% of the total population. At NF-B03, 

nematodes and annelids each accounted for 34% of the total. 
389 

Table 11.11. Macroinfaunal community statistics for Garden Banks Block 602, 

Cruise 2B (July 2001). 

Station 

No. 

Taxa 

No. 

Individuals

Density

(no./m2

) 

H’ 

Diversity

J’ 

Evenness 

D 

Richness 

Near-Field 

NF-B01 14 278 2,780 1.80 0.68 2.31 

NF-B02 7 1,144 11,440 1.31 0.67 0.85 

NF-B03 6 2,310 23,100 0.47 0.26 0.65 

NF-B04 8 196 1,960 1.77 0.85 1.33 

NF-B05 13 2,192 21,920 1.10 0.43 1.56 

NF-B06 7 761 7,610 1.08 0.55 0.90 

NF-B07 5 170 1,700 1.45 0.90 0.78 

NF-B08 9 1,045 10,450 0.62 0.28 1.15 

NF-B09 10 1,218 12,180 1.25 0.54 1.27 

NF-B10 9 554 5,540 1.52 0.69 1.27 

NF-B11 7 538 5,380 1.65 0.85 0.95 

NF-B12 13 695 6,950 1.73 0.67 1.83 

Far-Field 

FF1-B1 11 143 1,430 1.92 0.80 2.01 

FF1-B2 16 315 3,150 1.73 0.62 2.61 

FF2-B1 11 151 1,510 1.81 0.76 1.99 

FF2-B2 20 176 1,760 2.01 0.67 3.67 

FF3-B1 13 196 1,960 1.91 0.74 2.27 

FF3-B2 14 125 1,250 1.72 0.65 2.69 

FF4-B1 16 252 2,520 2.02 0.73 2.71 

FF4-B2 12 186 1,860 1.69 0.68 2.10 

FF5-B1 14 80 800 2.13 0.81 2.97 

FF5-B2 14 150 1,500 2.18 0.83 2.59 

FF6-B1 16 227 2,270 2.08 0.75 2.77 

FF6-B2 10 91 910 1.87 0.81 2.00 
Figure 11.9. Taxonomic composition of macroinfauna at Garden Banks Block 602 and Mississippi Canyon B

GB 602 Near-fieldCruise 2B

Total Individuals = 11,101

GB 602 Far-fieldCruise 2B

Total Indiv

MC 292 Near-fieldCruise 2B

Total Individuals = 12,392 Total Ind

MC 292 Far-fieldCruise 2B

Annelida 47%Harpacticoida 20%Gastropoda 11%Bivalvia 10%Aplacophora 6%Other 6%

Annelida 39%Harpacticoida 20%Nematoda 18%Aplacophora 9%Bivalvia 6%Other 8%

390
391 

Table 11.12. Macroinfaunal community statistics for Mississippi Canyon Block 292, 

Cruise 2B (July 2001). 

Station 

No. 

Taxa 

No. 

Individuals

Density

(no./m2

) 

H’ 

Diversity

J’ 

Evenness 

D 

Richness 

Near-Field 

NF-B01 11 4,580 45,800 1.13 0.47 1.19 

NF-B02 9 492 4,920 1.15 0.52 1.29 

NF-B03 15 1,423 14,230 1.69 0.62 1.93 

NF-B04 17 785 7,850 1.95 0.69 2.40 

NF-B05 14 820 8,200 1.70 0.64 1.94 

NF-B06 16 1,186 11,860 1.51 0.54 2.12 

NF-B07 17 814 8,140 1.71 0.60 2.39 

NF-B08 9 538 5,380 1.11 0.50 1.27 

NF-B09 14 811 8,110 1.77 0.67 1.94 

NF-B10 14 691 6,910 1.63 0.62 1.99 

NF-B11 8 89 890 1.13 0.55 1.56 

NF-B12 13 163 1,630 1.59 0.62 2.36 

Far-Field 

FF1-B1 15 560 5,600 1.90 0.70 2.21 

FF1-B2 13 846 8,460 1.46 0.57 1.78 

FF2-B1 2 46 460 0.57 0.83 0.26 

FF2-B2 17 683 6,830 1.62 0.58 2.30 

FF3-B1 14 350 3,500 1.69 0.64 2.22 

FF3-B2 14 215 2,150 1.97 0.75 2.42 

FF4-B1 15 725 7,250 1.88 0.69 2.13 

FF4-B2 16 831 8,310 1.86 0.67 2.23 

FF5-B1 13 509 5,090 1.88 0.73 1.93 

FF5-B2 12 95 950 1.85 0.74 2.42 

FF6-B1 16 1,133 11,330 1.78 0.66 1.99 

FF6-B2 14 545 5,450 1.83 0.69 2.06 
392 

ANOVA was conducted for individual macroinfaunal groups to test for spatial and temporal 

differences (Table 11.9). The significant (p<0.05) differences were as follows: 

• Annelids, bivalves, and harpacticoid copepods were more abundant at the near-field site than 

at far-field sites. 

Taxa that did not exhibit differences in abundance between near-field and far-field sites were 

amphipods, aplacophorans, nematodes, ostracods, and tanaids. 

11.4.5 Individual Groups and Environmental Correlations 

The six most abundant groups from the macroinfaunal data (annelids, gastropods, bivalves, 

amphipods, ostracods, and aplacophorans) were selected for further plotting and analysis. 

Figures 11.10 and 11.11 show densities of these individual groups in all post-drilling samples, 

with each point representing an individual sample. For annelids, gastropods, and bivalves, there 

was a strong, consistent tendency for the highest densities to occur in the near-field. 

Aplacophorans showed a similar trend at GB 516 and GB 602, though not at VK 916. The 

opposite was true for amphipods and ostracods, with the highest densities occurring at one or 

more far-field stations (except for GB 516 on Cruise 1B). 

Correlations were calculated between densities of these macroinfaunal groups and environmental 

variables (barium, SBF, and TOC concentrations; sediment sand, silt, and clay percentages). 

Only stations exposed to drilling discharges were included in this analysis (i.e., near-field, 

post-drilling). Results are summarized in Table 11.13. 

Selected relationships between densities of individual groups and barium, SBF, and TOC 

concentrations are illustrated in Figures 11.12 and 11.13 (annelids), Figure 11.14 (gastropods), 

Figure 11.15 (amphipods), and Figure 11.16 (ostracods). 

11.4.5.1 Annelids 

Annelid densities tended to be positively correlated with sediment barium, SBF, and TOC, though 

the correlation was significant in only a few cases (Table 11.13). Overall, barium was the 

strongest correlate. Nearly all far-field stations and some near-field stations had annelid densities 

less than 2,000 individuals/m2

. Some near-field stations on post-drilling cruises had up to 

10 times higher densities. Generally, stations with elevated annelid densities had barium 

concentrations of 10,000 μg/g or higher (Figure 11.12) and SBF concentrations of 1,000 μg/g or 

higher (Figure 11.13). 

11.4.5.2 Gastropods 

Gastropod densities tended to be positively correlated with sediment barium, SBF, and to a lesser 

extent, TOC (Table 11.13). Densities also tended to be positively correlated with sand 

percentages. Only a few of the correlations were significant. 
VK 916 GB 516 GB 602 MC 292

NF FF NF FF NF FF NF FF

(Cruise 1B)

GB 516

NF FF

(Cruise 2B)

Annelids per m2

Annelids

(Cruise 3B) (Cruise 2B) (Cruise 2B)

Figure 11.10. Comparison of near-field and far-field abundances of macroinfaunal annelids, gastropods, 

and bivalves in post-drilling samples. Note the log scale.

Near-field

Far-field

LEGEND

105

104

103

102

<10

102

103

104

Bivalves per m2

Bivalves

VK 916 GB 516 GB 602 MC 292

NF NF FF FF NF FF NF FF

(Cruise 1B)

GB 516

NF FF

(Cruise 3B) (Cruise 2B) (Cruise 2B) (Cruise 2B)

Gastropods

<10

105

104

103

102

Gastropods per m2

VK 916 GB 516 GB 602 MC 292

NF FF NF FF NF FF NF FF

(Cruise 1B)

GB 516

NF FF

(Cruise 3B) (Cruise 2B) (Cruise 2B) (Cruise 2B)

393
VK 916 GB 516 GB 602 MC 292

NF NF FF FF NF FF NF FF

(Cruise 1B)

GB 516

NF FF

(Cruise 2B)

Amphipods per m2

Amphipods

(Cruise 3B) (Cruise 2B) (Cruise 2B)

Figure 11.11. Comparison of near-field and far-field abundances of macroinfaunal amphipods, 

aplacophorans, and ostracods in post-drilling samples. Note the log scale.

Near-field

Far-field

LEGEND

103

102

10

0

0

1

0

102

103

104 Aplacophorans

Aplacophorans per m2

VK 916 GB 516 GB 602 MC 292

NF NF FF FF NF FF NF FF

(Cruise 1B)

GB 516

NF FF

(Cruise 3B) (Cruise 2B) (Cruise 2B) (Cruise 2B)

Ostracods

1

1

0

104

103

102

Ostracods per m2

VK 916 GB 516 GB 602 MC 292

NF NF FF FF NF FF NF FF

(Cruise 1B)

GB 516

NF FF

(Cruise 3B) (Cruise 2B) (Cruise 2B) (Cruise 2B)

394
395 

Table 11.13. Rank correlation coefficients (Spearman’s rho) between macroinfaunal densities 

and environmental variables within areas exposed to drilling discharges (near-field 

sites on post-drilling cruises). Statistically significant correlation coefficients 

(p<0.05), i.e., those large enough that they are probably not due to chance, are bold 

and double-underlined. 

Correlation Coefficient vs. Environmental Variable Group Site Cruise 

Barium SBF TOC Sand Silt Clay 

Annelida VK 916 3B 0.319 0.459 0.347 0.291 0.084 -0.277

GB 516 1B 0.452 0.263 0.280 -0.049 -0.035 0.035

GB 516 2B 0.706 0.748 0.126 0.210 0.566 -0.483

GB 602 2B 0.420 0.210 0.585 0.090 -0.035 -0.035

MC 292 2B 0.671 0.504 0.046 -0.007 -0.357 0.147

Gastropoda VK 916 3B 0.324 0.505 0.257 0.373 0.516 -0.516

GB 516 1B 0.399 0.135 -0.170 0.731 0.089 -0.089

GB 516 2B 0.883 0.669 0.221 0.595 0.462 -0.493

GB 602 2B 0.429 0.615 0.866 0.169 0.018 -0.109

MC 292 2B 0.557 0.444 0.405 0.296 -0.138 -0.208

Bivalvia VK 916 3B -0.148 -0.163 -0.281 0.113 0.049 -0.039

GB 516 1B 0.438 0.322 0.193 -0.014 0.165 -0.165

GB 516 2B 0.442 0.411 -0.311 0.151 0.295 -0.133

GB 602 2B -0.287 -0.504 -0.515 0.077 -0.357 0.385

MC 292 2B 0.566 0.504 0.298 0.329 0.014 -0.231

Amphipoda VK 916 3B -0.475 -0.507 -0.637 -0.354 -0.007 0.239

GB 516 1B -0.850 -0.691 -0.081 -0.189 -0.131 0.131

GB 516 2B -0.168 -0.409 -0.307 -0.219 -0.226 0.307

GB 602 2B -0.158 -0.138 0.039 -0.296 0.237 -0.217

MC 292 2B -0.488 -0.526 -0.633 -0.091 0.021 -0.067

Ostracoda VK 916 3B -0.262 -0.142 -0.250 -0.046 0.145 0.060

GB 516 1B -0.620 -0.635 0.004 0.208 -0.212 0.212

GB 516 2B -0.014 -0.134 0.032 0.003 -0.049 0.063

GB 602 2B -0.385 -0.196 0.315 -0.599 0.136 0.004

MC 292 2B -0.281 -0.446 -0.640 0.098 -0.007 -0.091

Aplacophora VK 916 3B -0.235 -0.326 -0.456 -0.003 0.249 -0.084

GB 516 1B 0.269 0.276 0.322 -0.182 -0.594 0.594

GB 516 2B 0.339 0.152 -0.527 0.244 -0.046 0.018

GB 602 2B -0.361 -0.491 -0.576 0.102 -0.425 0.470

MC 292 2B 0.000 -0.035 -0.098 0.091 -0.091 -0.140
Figure 11.12. Annelid (predominantly polychaete) densities vs. barium concentrations on post-drilling cruises. Note 

barium concentrations and the different density scale for VK 916.

GB 516

Annelids per m

2

GB 602 MC 292

102

Barium Concentration (μg/g dry wt)

Annelids per m

2

Near-field, Cruise 3BFar-field, Cruise 3B

103 104 105 106

Barium Concentration (μg/g dry wt

102 103 104 105 10

VK 916

Barium Concentration (μg/g dry wt)

Annelids per m

2

Near-fieldFar-field

102 103 105 106 104

Barium Concentration (μg/g dry wt)

Annelids per m

2

102 103 104 105 10

0

1,0002,0003,0004,000

0

5,000

10,00015,00020,00025,000

0

5,000

10,00015,00020,00025,000

0

5,000

10,00015,00020,00025,000

396
Figure 11.13. Annelid (predominantly polychaete) densities vs. synthetic-based fluid (SBF) concentrations on post-dril

Note the log scale for SBF concentrations and the different density scale for VK 916.

GB 516

Annelids per m

2

GB 602 MC 292

SBF Concentration (μg/g dry wt)

Annelids per m

2

Far-field, Cruise 3B

SBF Concentration (μg/g dry wt)

VK 916

Annelids per m

2

Near-fieldFar-field

SBF Concentration (μg/g dry wt)

Annelids per m

2

1 10 102

SBF Concentration (μg/g dry wt)

Near-field, Cruise 3B

103 104 105

1 10 102 103 104 105 1 10 102 103 104 10

1 10 102 103 104 10

1,0002,0003,0004,000

0 0

5,000

10,00015,00020,00025,000

0

5,000

10,00015,00020,00025,000

0

5,000

10,00015,00020,00025,000

397
0

5000

10000

15000

20000

1 10 102

SBF Concentration (μg/g dry wt)

Gastropods per m2

0

5000

10000

15000

20000

Barium Concentration (μg/g dry wt)

Gastropods per m2

VK916-Cr3 NF

GB516-Cr1 NF

GB516-Cr2 NF

GB602-Cr2 NF

MC292-Cr2 NF

0

5000

10000

15000

20000

01234

Total Organic Carbon (%)

Gastropods per m2

VK916-Cr3 NF

GB516-Cr1 NF

GB516-Cr2 NF

GB602-Cr2 NF

MC292-Cr2 NF

Figure 11.14. Gastropod densities vs. sediment barium, synthetic-based fluid (SBF), and total 

organic carbon concentrations at near-field (NF), post-drilling stations. Note the log

scale for barium and SBF concentrations.

103 104 105

103 104 105 106

VK916-Cr3 NF

GB516-Cr1 NF

GB516-Cr2 NF

GB602-Cr2 NF

MC292-Cr2 NF

398
Figure 11.15. Amphipod densities vs. sediment barium concentrations on post-drilling cruises. Note the log scale for b

VK 916 GB 516

MC 292

Amphipods per m

2

0

100200300400

102

Barium Concentration (μg/g dry wt)

Amphipods per m

2

Near-field, Cruise 3BFar-field, Cruise 3B

Barium Concentration (μg/g dry wt)

0

100200300400

Barium Concentration (μg/g dry wt)

Amphipods per m

2

Near-fieldFar-field

0

100200300400

Barium Concentration (μg/g dry wt)

Amphipods per m

2

GB 602

103 104 105 106

102 103 104 105 106 102 103 104 105 106

102 103 104 105 106

0

100200300400

399
Figure 11.16. Ostracod densities vs. sediment barium concentrations on post-drilling cruises. Note the log scale for ba

VK 916 GB 516

MC 292

Ostracods per m

2

Ostracods per m

2

Near-field, Cruise 3BFar-field, Cruise 3B

Barium Concentration (μg/g dry wt)

Ostracods per m

2

Near-fieldFar-field

Barium Concentration (μg/g dry wt)

Ostracods per m

2

GB 602

102 103 104 105 106

0

200400600

Barium Concentration (μg/g dry wt)

0

200400600

Barium Concentration (μg/g dry wt)

0

200400600

102 103 104 105 106

102 103 104 105 106 102 103 104 105 10

0

1,0002,0003,000

400
401 

As shown in Figure 11.14, the relationship between gastropods and drilling fluid indicators was 

striking. All far-field stations and most near-field stations had low gastropod densities (generally 

less than 100 individuals/m2

). However, a few near-field stations at GB 516 and GB 602 had 

much higher densities, up to 17,000 individuals/m2

. The extremely high densities were associated 

with SBF concentrations of 4,500 μg/g or higher and barium concentrations of 55,000 μg/g and 

higher. Much weaker relationships were observed at VK 916 and MC 292. VK 916 had several 

stations with high SBF concentrations and few gastropods (or none). Figure 11.14 also shows 

there is no overall consistent relationship between gastropod densities and TOC. 

11.4.5.3 Bivalves 

There were no strong, consistent correlations between bivalve abundance and sediment barium, 

SBF, TOC, or grain size fractions. There was a significant positive correlation with barium at 

MC 292 on Cruise 2B (Table 11.13). 

11.4.5.4 Amphipods 

Amphipod densities tended to be negatively correlated with barium, SBF, and TOC 

concentrations, though only a few of the correlations were significant (Table 11.13). The 

weakest correlations were at GB 602. As shown in Figure 11.15, amphipod densities at far-field 

stations ranged from 0 to 320 individuals/m2

. Generally, near-field stations with barium 

concentrations higher than about 10,000 μg/g and/or SBF concentrations greater than about 

1,000 μg/g had low amphipod densities. 

11.4.5.5 Ostracods

Ostracod densities tended to be negatively correlated with barium and SBF concentrations, 

though the correlations were significant only at GB 516 on Cruise 1B (Table 11.13). At GB 602, 

ostracod abundance was negatively correlated with sand. At MC 292, ostracod abundance was 

negatively correlated with TOC. With the exception of GB 516, near-field stations with barium 

concentrations higher than about 10,000 μg/g (Figure 11.16) and/or SBF concentrations greater 

than about 1,000 μg/g had low ostracod densities. 

11.4.5.6 Aplacophorans 

There were no consistent correlations between aplacophoran abundance and sediment barium, 

SBF, TOC, or grain size fractions (Table 11.13). At GB 516 on Cruise 1B, aplacophoran 

abundance was correlated negatively with silt and positively with clay. At GB 602, aplacophoran 

abundance was negatively correlated with TOC. 

11.4.6 Detailed Macroinfaunal Taxonomic Analysis 

11.4.6.1 Community Characteristics 

Twenty-four stations were selected for detailed taxonomic analysis. These included near-field 

and far-field locations at each site on each cruise (Table 11.14). 
402 

Table 11.14. Samples included in the 24-station analysis of macroinfauna. 

Site Cruise Samples Included 

Viosca Knoll Block 916 1B 

NF-B04 

NF-B11 

FF2-B01 

FF6-B01 

3B 

NF-B03 

NF-B07 

FF1-B02 

FF5-B01 

Garden Banks Block 516 1B 

NF-B02 

NF-B06 

FF1-B02 

FF6-B02 

2B 

NF-B03 

NF-B08 

FF3-B02 

FF5-B02 

Garden Banks Block 602 2B 

NF-B01 

NF-B03 

FF1-B01 

FF4-B01 

Mississippi Canyon Block 292 2B 

NF-B03 

NF-B06 

FF4-B02 

FF6-B01 

In the detailed taxonomic analysis, 266 macroinfaunal taxa were identified, including 

119 polychaetes, 86 crustaceans, 26 bivalves, 21 gastropods, two echinoderms, two rhynchocoels, 

two scaphopods, one oligochaete, and seven other taxa that were identified only to phylum 

(nematodes, sipunculids, sponges, turbellarians), order (actiniarians), or class (aplacophorans, 

enteropneusts) (Appendix I2). The most abundant macroinfauna were the polychaete 

Aphelochaeta sp. B (17.2% of collected individuals), the gastropods Solariella sp. A (8.5%) and 

Ganesa sp. A (6.7%), Nematoda (LPIL) (5.7%), and Aplacophora (LPIL) (5.5%) (Table 11.15). 

The most widespread taxa were non-identified bivalves and spionid polychaetes, both of which 

were collected at 22 of the 24 stations, followed by non-identified aplacophorans, maldanid 

polychaetes, and rhychocoels found at 20 stations. 

The basic community statistics for each of the 24 stations are summarized in Table 11.16. 

(The values differ from those presented earlier for the same stations because individuals were 

identified to LPIL for this analysis.) The total number of taxa per sample ranged from 13 to 123. 

Total number of individuals ranged from 138 to 3,818. H’ ranged from 0.58 to 3.85, J’ ranged 

from 0.22 to 0.90, and D ranged from 1.72 to 17.21. 
403 

Table 11.15. Ten most abundant macroinfaunal taxa for the 24-station analysis. 

Taxon Type of Organism Count 

Aphelochaeta sp. B Polychaete 3,426 

Solariella sp. A Gastropod mollusc 1,701 

Ganesa sp. A Gastropod mollusc 1,326 

Nematoda (LPIL) Nematode 1,143 

Aplacophora (LPIL) Aplacophoran mollusc 1,093 

Harpacticoida (LPIL) Harpacticoid copepod 845 

Maldanidae (LPIL) Polychaete 796 

Bivalvia (LPIL) Bivalve mollusc 682 

Actinocythereis sp. E Ostracod 620 

Ophryotrocha sp. D Polychaete 464 

Table 11.16. Macroinfauna community statistics for the 24-station analysis. 

Station Cruise No. 

Taxa 

No. 

Indiv. 

H’ 

Diversity

J’ 

Evenness 

D 

Richness

Viosca Knoll Block 916 

VK916-NF-B04 1B 123 1,197 3.79 0.79 17.21 

VK916-NF-B11 1B 86 655 3.48 0.78 13.11 

VK916-FF2-B01 1B 94 1,108 3.48 0.77 13.27 

VK916-FF6-B01 1B 82 1,117 3.13 0.71 11.54 

VK916-NF-B03 3B 60 191 3.63 0.89 11.23 

VK916-NF-B07 3B 13 451 0.82 0.32 1.96 

VK916-FF1-B02 3B 35 184 2.88 0.81 6.52 

VK916-FF5-B01 3B 45 139 3.43 0.90 8.92 

Garden Banks Block 516 

GB516-NF-B02 1B 14 1,903 0.58 0.22 1.72 

GB516-NF-B06 1B 56 878 2.55 0.63 8.12 

GB516-FF1-B02 1B 65 215 3.44 0.82 11.92 

GB516-FF6-B02 1B 72 214 3.85 0.90 13.23 

GB516-NF-B03 2B 34 3,818 2.31 0.65 4.00 

GB516-NF-B08 2B 55 796 2.67 0.67 8.08 

GB516-FF3-B02 2B 59 270 3.43 0.84 10.36 

GB516-FF5-B02 2B 72 488 3.65 0.85 11.47 

Garden Banks Block 602 

GB602-NF-B03 2B 19 2,316 0.75 0.25 2.32 

GB602-NF-B01 2B 42 270 2.54 0.68 7.32 

GB602-FF1-B01 2B 53 138 3.54 0.89 10.55 

GB602-FF4-B01 2B 65 149 3.60 0.86 12.79 

Mississippi Canyon Block 292 

MC292-NF-B03 2B 99 1,433 2.90 0.63 13.48 

MC292-NF-B06 2B 84 638 3.43 0.77 12.85 

MC292-FF4-B02 2B 63 612 2.86 0.69 9.66 

MC292-FF6-B01 2B 72 736 3.26 0.76 10.76 
404 

Figure 11.17 shows the same data, but with stations grouped into two subsets: 

• Near-drilling = all near-field stations at GB 516, GB 602, and MC 292, plus near-field 

stations at VK 916 on Cruise 3B (post-drilling). 

• Away from drilling = all far-field stations plus VK 916 near-field stations from Cruise 1B 

(pre-drilling). 

As summarized in the figure, some stations “near drilling” had higher densities, lower diversity, 

lower evenness, and lower richness indices compared with stations “away from drilling.” Other 

“near-drilling” stations had community characteristics similar to those of the “away from drilling” 

stations. 

11.4.6.2 Community Structure Comparisons 

Figure 11.18 summarizes the results of MDS ordination for the 24-station dataset. MDS 

ordination identified four main station (sample) groups (A through D) that were similar with 

respect to species composition and relative abundance (Table 11.17). Two individual stations 

were dissimilar to the others and were designated as Groups E and F. 

Some geographic influence is evident in the groupings. Groups A and B consist of stations from 

GB 516 and GB 602, which are very close geographically (within tens of kilometers). Groups C 

and D include stations from VK 916 (Group C) or both VK 916 and MC 292 (Group D), which 

are relatively close to each other, but about 400 km from the two Garden Banks sites. 

Drilling impacts also are evident in the groupings. The two “outlier” stations (Groups E and F) 

are both near-field, post-drilling stations, and they had the highest SBF concentrations among the 

24 stations (47,920 μg/g and 25,131 μg/g, respectively) (Figure 11.19). Both were dominated by 

taxa that were not found at other stations. Group E (Station VK916-NF-B07) was numerically 

dominated by the polychaete Capitella capitata, a classic indicator of organic enrichment, and 

Group F (Station GB516-NF-B02) was numerically dominated by the gastropod Solariella sp. A. 

Group A includes only near-field, post-drilling stations at GB 516 and GB 602. These stations 

had barium concentrations ranging from 13,500 to 351,000 μg/g and SBF concentrations ranging 

from 418 to 8,446 μg/g. Groups B, C, and D presumably reflect little or no drilling impact. Most 

of the stations are far-field or near-field pre-drilling. Each group included one or two near-field 

post-drilling stations, but those tended to be at the low end of the range compared to the high 

barium and SBF concentrations seen for Groups A, E, and F. 

Taxa typifying station groups, determined with SIMPER, are presented in Table 11.18. As 

indicated by lower average levels of similarity, Groups A and B had more assemblage variability 

between stations than Groups C and D. Group A was distinguished from other groups mostly due 

to high abundances of the polychaete Aphelochaeta sp B. This genus has been identified as an 

indicator of organic enrichment in some studies (Llansó et al. 1998). Group B stations yielded 

relatively high abundances of non-identified spionid polychaetes and nematodes. Group C 

stations yielded certain taxa that were rare at other stations, such as the polychaete 

Sthenolepis sp. D and the amphipod Harpiniopsis sp. A. Group D stations yielded high 

abundances of non-identified maldanid polychaetes and aplacophorans, as well as polychaetes 

such as Cossura soyeri and Scoletoma verrilli, which were rare or absent at stations in Groups A 

through C. 
Figure 11.17. Macroinfaunal community characteristics for the detailed, 24-station analysis. Diversity (H')

(a) Density

0

10,000

20,000

30,000

40,000

Near

Drilling

Density (no./m2

)

Away from

Drilling

(b) Diversity

Away from 

Drilling

Near

Drilling

Evenness (J')

0

0.2

0.4

0.6

0.8

1

Away from 

Drilling

Near

Drilling

(c) Evenness

0

1

2

3

4

(d) Richness

Richness (D)

0

5

10

15

20

Away from 

Drilling

Near

Drilling

"Away from Drilling" = all far-field 

stations plus near-field pre-drilling 

stations at VK 916

"Near Drilling" = all near-field stations 

at GB 516, GB 602, and MC 292, plus 

near-field post-drilling stations at 

VK 916

405
Figure 11.18. Station/cruise groups resolved by multidimensional scaling of macroinfaunal data from the 24-station d

VK916FF

VK916NF

VK916FF

GB516-NF

Near Drilling(near-field, post-drilling)

LEGEND

GB516FF

GB516FF

GB516FF

GB516FF

GB602NF

GB516NF

GB516NF GB516NF

GB602NF

GB602FF

GB602FF

MC292FF

MC292FF

MC292NF

MC292NF

VK916NF

VK916FF

VK916FF

VK916NF

Away from Drilling(all far-field plusnear-field pre-drilling)

406
407 

Table 11.17. Station groups (A through D) resolved by multi-dimensional scaling (MDS). 

MDS plot labels refer to Figure 11.18. 

Station Group MDS Plot Label Full Label Cruise 

GB516-NF GB516-NF-B03 2B 

A GB602-NF GB602-NF-B03 2B 

GB516-NF GB516-NF-B06 1B 

GB516-NF GB516-NF-B08 2B 

GB602-FF GB602-FF1-B01 2B 

GB602-FF GB602-FF4-B01 2B 

GB602-NF GB602-NF-B01 2B 

B GB516-FF GB516-FF3-B02 2B 

GB516-FF GB516-FF5-B02 2B 

GB516-FF GB516-FF1-B02 1B 

GB516-FF GB516-FF6-B02 1B 

VK916-FF VK916-FF5-B01 3B 

C VK916-FF VK916-FF1-B02 3B 

VK916-NF VK916-NF-B03 3B 

MC292-FF MC292-FF6-B01 2B 

MC292-NF MC292-NF-B03 2B 

VK916-NF VK916-NF-B04 1B 

D MC292-NF MC292-NF-B06 2B 

VK916-FF VK916-FF2-B01 1B 

VK916-NF VK916-NF-B11 1B 

MC292-FF MC292-FF4-B02 2B 

VK916-FF VK916-FF6-B01 1B 

E VK916-NF VK916-NF-B07 3B 

F GB516-NF GB516-NF-B02 1B 
Figure 11.19. Comparison of sediment barium, synthetic-based fluid (SBF), and total organic carbon 

concentrations at station groups identified from multidimensional scaling ordination.

Barium

102

AB CDEF

Station Group

Barium Concentration (μg/g)

Synthetic-Based Fluids

1

10

SBF Concentration (μg/g)

Total Organic Carbon

0

1

2

3

4

Total Organic Carbon (%)

A B CDE F

Station Group

ABCDEF

Station Group

103

104

105

106

102

103

104

105

408
409 

Table 11.18. Average abundance of infaunal species accounting for at least 50% of the 

within-group similarity in station (sample) groups A through D. Groups E and F 

(not shown) were dominated by the polychaete Capitella capitata and the gastropod 

Solariella sp. A, respectively. Numbers in bold represent the average similarity for 

each group as a whole.

Group Taxa Average Abundance Average Similarity 

Aphelochaeta sp. B 848.00 11.22 

Harpacticoida (LPIL) 133.75 2.95 

A Phyllodocidae Genus E 36.00 2.47 

Prionospio sp. O 30.25 2.45 

Bivalvia (LPIL) 58.00 2.31 

38.50 

Spionidae (LPIL) 17.86 2.94 

Harpacticoida (LPIL) 22.29 2.17 

Nematoda (LPIL) 36.86 1.74 

Bivalvia (LPIL) 4.57 1.56 

Paraonidae (LPIL) 6.00 1.45 

B Aplacophora (LPIL) 4.57 1.41 

Maldanidae (LPIL) 8.14 1.19 

Pseudotanais (LPIL) 6.00 1.17 

Syllidae (LPIL) 5.14 1.13 

Monticellina dorsobranchialis 1.86 1.08 

Ostracoda (LPIL) 2.14 0.92 

Paraonidae Genus B 9.57 0.89 

34.14 

Sthenolepis sp. D 16.33 4.69 

Harpiniopsis sp. A 9.67 3.57 

Aplacophora (LPIL) 11.33 3.55 

C Cirratulidae (LPIL) 10.00 3.24 

Aricidea (LPIL) 9.33 3.12 

Actinocythereis sp. E 16.33 2.88 

Ophelina sp. F 3.67 2.23 

46.07 

Maldanidae (LPIL) 89.50 3.44 

Aplacophora (LPIL) 119.75 2.95 

Bivalvia (LPIL) 51.63 2.59 

Ophelina sp. F 26.63 2.03 

Cirratulidae (LPIL) 26.63 1.93 

Actinocythereis sp. E 71.38 1.71 

D Cossura soyeri 22.38 1.69 

Aricidea (LPIL) 24.00 1.68 

Sthenolepis sp. D 29.00 1.62 

Podocopida (LPIL) 19.13 1.22 

Harpacticoida (LPIL) 19.25 1.15 

Scoletoma verrilli 8.88 1.15 

Levensenia sp. T 7.75 1.06 

47.77 
410 

In conclusion, the ordination identified groups of stations that reflect both geographic location 

and drilling impacts. The three station/cruise groups most likely affected by drilling (Groups A, 

E, and F) were dominated by high abundances of one or a few deposit-feeding species. 

11.4.7 Discussion 

In the present study, we observed a wide range in macroinfaunal abundance among the four study 

sites and between surveys. Mean densities ranged from 1,256 to 11,087 individuals/m2

. This 

compares well with preliminary data from the new MMS deep Gulf of Mexico benthos study, 

which show macroinfaunal densities at a water depth of 1,000 m range from a few thousand to 

over 10,000 individuals/m2

, with an average of about 6,000 individuals/m2 (Rowe and 

Kennicutt 2002). Earlier data from the MMS northern Gulf of Mexico continental slope study 

indicated somewhat lower densities of about 2,500 individuals/m2

 for a water depth of 1,000 m 

(Gallaway et al. 1988). 

The study results correspond well with prior deepwater observations of macroinfaunal 

community structure. Jumars and Gallagher (1982) noted that polychaetes usually constitute 50% 

to 75% of the individuals in deepwater sediments, and usually include the most abundant species. 

In the 24-station analysis, the most speciose polychaete families were Paraonidae (11 species), 

Cirratulidae (9 species), Lumbrineridae (8 species), Capitellidae (7 species), Lumbrineridae 

(7 species), Pilargidae (7 species), Syllidae (7 species), and Spionidae (6 species). The taxa 

census for the present study is similar to that found in other studies (Hartman 1965; 

Fauchald 1972). Fauchald (1972) reported that in deepwater benthic collections from western 

Mexico and adjacent areas of the eastern Pacific, the families Ampharetidae, Capitellidae, 

Lumbrineridae, Maldanidae, Onuphidae, Paranoidae, and Spionidae were well represented. 

Hartman (1965) reported similar deepwater macroinfauna from the western Atlantic, including 

members of Syllidae, in a list of commonly occurring taxa. 

 

Unlike the meiofaunal data, macroinfaunal densities did not consistently decrease between 

cruises. At VK 916, mean densities decreased by a factor of six between Cruises 1B and 3B at 

both near-field and far-field sites. However, at GB 516, mean densities increased (approximately 

doubled) at both near-field and far-field sites. 

Nematodes are normally considered meiofaunal taxa, but due to the small screen mesh used in 

deep sea studies (300 μm), the larger individuals can be retained with the macroinfauna. For 

example, at both GB 602 and MC 292, nematodes were the most abundant group in the far-field 

macrofaunal samples. Nematodes were significantly more abundant in the far-field than in the 

near-field at GB 602. This is the opposite of the result seen in the meiofaunal data, where 

nematodes were more abundant in the near-field. However, at most stations, the macrofaunal 

nematodes accounted for <2% of the total population. If both macrofaunal and meiofaunal 

nematodes are combined, the pattern is the same as discussed previously under Meiofauna – i.e., 

the highest densities tend to occur in the near-field (see Figure 11.5). 

Drilling impacts were evident in spatial comparisons. In post-drilling samples, there was 

considerable overlap in macroinfaunal densities between near-field and far-field sites, but the 

highest total densities were always seen in the near-field (Figure 11.6). The elevated total 

densities reflect higher numbers of annelids, gastropods, and bivalves in the near-field 
411 

(Figure 11.10). A similar but weaker trend was evident for aplacophorans. Two groups that 

tended to have higher abundances away from drilling were amphipods and ostracods. In terms of 

community statistics, some stations “near drilling” had lower diversity, lower evenness, and 

lower richness indices compared with stations “away from drilling” (Figure 11.17). 

In previous studies, drilling discharges have been shown to affect macrofaunal communities in a 

variety of ways, including burial and smothering, alterations in sediment grain size and 

mineralogy, and toxicity of drilling fluid components (National Research Council 1983; Neff 

1987; Continental Shelf Associates, Inc. 1989; Montagna and Harper 1996). Where SBM 

cuttings are discharged, organic enrichment and anoxia are additional factors (Neff et al. 2000). 

Some of the impacts noted here are similar to those observed in the GOOMEX study, which 

focused on long-term production sites on the continental shelf (Montagna and Harper 1996). 

Although GOOMEX did not involve SBMs, an organic enrichment effect was noted, with higher 

macrofaunal densities near platforms – particularly, increased abundance of deposit-feeding 

polychaetes. In GOOMEX, the organic enrichment apparently was due to sloughing of 

biofouling organisms from submerged structures over a period of years, whereas in the present 

study, SBM cuttings are the presumed source of organic enrichment. Montagna and Harper 

(1996) attributed reduced abundances of amphipods and harpacticoid copepods near platforms to 

toxicity of metals from water-based drilling fluids, though no metal toxicity studies were 

conducted. In the present study, harpacticoid abundance was actually higher near drilling (see 

Meiofauna section), and reduced abundances of amphipods and ostracods in the near-field were 

not significantly correlated with metal concentrations other than barium (drilling fluid indicator). 

Potential toxicity is addressed further in Chapter 14. 

There have been few previous studies of SBM cuttings impacts on the Gulf of Mexico continental 

slope. An opportunistic survey was conducted after development drilling at the Pompano 

development site in MC 28 (water depth of 565 m) (Gallaway et al. 1997; Fechhelm et al. 1999). 

All observations were within 90 m of the drillsite, and this area had elevated macroinfaunal 

densities (up to 13,724 individuals/m2

) and low diversity relative to baseline conditions. 

Macroinfaunal populations were dominated by epibenthic scavengers including cyclopoid 

copepods and gastropods, and the authors suggested that true infaunal dwellers (e.g., 

tube-dwelling polychaetes) had been buried by cuttings discharges. There was no consistent 

pattern of increasing or decreasing density in relation to distance or SBF concentration. The 

increased macrofaunal densities are consistent with the results seen in the present study. 

However, the benthic effects observed here are more complex and patchy than observed in the 

Pompano study. Sediment profile imaging data (see Chapters 6 and 7) indicate a range of 

post-drilling conditions including areas of azoic sediment column, areas of dominance by 

surface-dwelling pioneering assemblages, and areas where recent disturbance has selectively 

compromised near-surface species relative to deeper-living infauna. Differences between the 

studies include the range of distances from the drillsite (within 90 m in Pompano, within 500 m in 

this study) and the amount of time elapsed since the most recent drilling discharges (4 months in 

Pompano, 5 months to 2 years in this study). 

More recently, a comprehensive study of SBM impacts was conducted at several continental 

slope and shelf sites in the Gulf of Mexico (Continental Shelf Associates, Inc. 2004). However, 
412 

that study did not include box core sampling at continental slope sites. Benthic community 

impacts were documented at three shallower sites on the continental shelf. At one site with 

relatively high SBF concentrations, the abundance and diversity of the benthic community were 

reduced within 250 m of the site center. There was evidence of recovery during the year between 

sampling cruises at this site. Near- and mid-field sediments at two other sites (with lower SBF 

concentrations) had only moderately disturbed benthic community structure. Variability of 

diversity and evenness was greatest in the near-field zone and generally much lower in the 

far-field zone. In the near-field zone, this variability was probably due to variations in sediment 

textures and patchy distributions of cuttings. 

11.5 CONCLUSIONS 

11.5.1 Meiofauna 

• Meiofaunal densities declined by about an order of magnitude between October-November 

2000 (Cruise 1B) and the subsequent surveys (Cruise 2B, July 2001 and Cruise 3B, 

August 2002) at both near-field and far-field sites. The cause is unknown, but the decrease is 

not attributable to drilling activities. 

• Meiofaunal densities tended to be higher near drilling. Although there was considerable 

overlap in near-field vs. far-field densities, the highest densities of nematodes, harpacticoid 

copepods, and especially annelids occurred in the near-field. 

• Meiofaunal densities in the near-field were not consistently correlated with drilling indicators 

(barium, SBF) or other sediment variables (TOC, grain size fractions). 

• The elevated meiofaunal densities seen here are similar to the nematode enrichment effect 

previously reported in the GOOMEX study by Montagna and Harper (1996) for platforms in 

shallower Gulf waters. However, we did not see lower harpacticoid densities near drilling as 

reported in GOOMEX. 

11.5.2 Macroinfauna 

• Macroinfaunal densities tended to be higher near drilling. Although there was considerable 

overlap in near-field vs. far-field densities, the highest densities occurred in the near-field. 

• For annelids (predominantly polychaetes), gastropods, and bivalves, there was a strong, 

consistent tendency for the highest densities to occur in the near-field. Aplacophorans 

showed a similar trend at GB 516 and GB 602, though not at VK 916. The opposite was true 

for amphipods and ostracods, with the highest densities generally occurring at one or more 

far-field stations. 

• In terms of community statistics, some stations “near drilling” had lower diversity, lower 

evenness, and lower richness indices compared with stations “away from drilling.” 

• Annelid (predominantly polychaete) densities in the near-field were positively correlated with 

drilling indicators (barium, SBF). Some near-field stations with barium concentrations higher 

than about 10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g had elevated 

polychaete densities. 

• Gastropod densities in the near-field were positively correlated with drilling indicators 

(barium, SBF). A few near-field stations at GB 516 and GB 602 had very high gastropod 

densities, which were associated with barium concentrations of 55,000 μg/g or higher and 

SBF concentrations of 4,500 μg/g or higher. Much weaker relationships were observed at 

VK 916 and MC 292. 
413 

• Amphipod densities in the near-field were negatively correlated with drilling indicators 

(barium, SBF). Generally, near-field stations with barium concentrations higher than about 

10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g had low amphipod 

densities. 

• A detailed taxonomic analysis of 24 stations showed that species composition reflects both 

geographic location and drilling impacts. The three station/cruise groups most likely affected 

by drilling (as indicated by high barium and SBF concentrations) were dominated by high 

abundances of one or a few deposit-feeding species. 
415 

Chapter 12 

Megafauna and Image Analysis 

Robert S. Carney 

Coastal Ecology Institute 

Louisiana State University 

12.1 INTRODUCTION: IMAGE-BASED SAMPLING 

Image-based sampling is extremely appealing but must be considered critically in designing 

studies. What is the purpose of the images to be obtained? Hecker's camera sled work is 

noteworthy both in terms of relating animals to structure (Hecker 1982) and sampling in deep-sea 

canyons where coring is inapplicable and trawling impossible (Hecker 1990). Excellent 

quantification and aggregation analysis for dominant megafauna have been obtained from 

submersibles (Grassle et al. 1975) and cameras mounted to beam trawls (Wakefield and Genin 

1987). However, the literature presenting successful studies gives no hint of the vast majority of 

photo and video surveys, which are never analyzed. Bulk-load survey cameras can take 

approximately 800 frames per deployment, while video takes tens of thousands. Extracting 

useful final data from such massive raw data usually proves to be overwhelming. Imaging 

technology is appealing, but the actual benefits are not often evident. Unless specimens are 

required for other analysis, image-based sampling generally gives a better measure of items on 

the seafloor, but not those buried (Kidd and Huggett 1981). 

The most extensive photographic survey of the deep Gulf of Mexico was carried out as a 

component of the Northern Gulf of Mexico Continental Slope Study (Gallaway 1988). Sixty 

surveys were conducted, producing a total of 48,000 images. An Edgerton-type pressure housed 

35-mm camera and strobe were configured to drift 2 m above bottom. The length of survey was 

determined by drift rate, ranging from 1,500 to 5,000 m long. Analysis of the images was 

conducted by G. Boland, with data processing and management by J. Baker. The daunting task of 

analyzing so many images was made manageable by randomly subsampling 100 or 200 images. 

In all, 9,147 images (24,590 m2

 of bottom) were analyzed for fauna and lebensspuren (life 

marks). The primary use of the results was to provide density and patchiness estimates for 

dominant megafauna. 

Due to the overall paucity of fauna captured in bottom photos and the comparable abundance of 

tracks, trails, burrows, and mounds, there has been a persistent interest in obtaining information 

about the bottom habitat from analyses of lebensspuren. The present study did not include a 

study of specific marks but did include an exploratory effort to quantify the overall extent of 

biological structure seen on the bottom. As such, the study sought to bridge the three ways in 

which lebensspuren has been examined: 

1. A description of an obvious and pervasive small-scale topography (Ewin and Davis 1967; 

Heezen and Hollister 1971). 

2. A surrogate for observations of actual animals that provide estimates of population density. 

3. As a record of habitat-influenced foraging behavior (Seilacher 1967). 
416 

Tried alone, none of these three approaches has proven to be informative in either an ecological 

or management sense. Local patterns of specific marks have been found, but no obvious 

relationships to habitat difference could be identified (Gallaway 1988). There is the confusing 

factor of erasure of marks both by erosional events and by bioturbation (Wheatcroft et al. 1988). 

Predictions that habitat changes in foraging patterns might be found have been disproved in the 

case of depth (Kitchell 1979). 

The approach taken in this study was less ambitious. It was assumed that lebensspuren could be a 

surrogate of overall sediment processing by organisms. If simply quantified, it would be an 

easily assessed parameter that might indicate impacts in the vicinity of development. 

12.2 METHODS 

12.2.1 Field Sampling 

A Benthos Edgerton-type camera and single strobe were sled mounted and dragged along the 

bottom by cable (see Chapter 2). A down weight on the cable reduced the amount of wire played 

out. Time of exposure was recorded by an image data chamber, while ship position and time 

were logged by the towing vessel. The camera was oriented straight down as in the Northern 

Gulf of Mexico Continental Slope Study, unlike other camera sled surveys, in which the camera 

looks forward of the sled. There are pros and cons to all methods. Down-facing cameras tethered 

above bottom give excellent large area coverage, but fixed distance to bottom is hard to maintain. 

Sleds assure a fixed distance at the cost of area imaged. Angled cameras photograph a larger 

panoramic scene. However, fauna quantification becomes somewhat problematic, and 

inconsistent illumination limits digital analysis. In effect, the lighting pattern becomes the 

dominant information in the image. 

The intended design was to carry out photographic surveys at four locations: VK 916, GB 516, 

GB 602, and MC 292. At each, three tows crossed a near-field 500-m radius circle centered 

around existing or planned drilling activity (Figures 12.1, 12.2, 12.3, and 12.4). Exact tracks 

across that circle were determined by sea conditions and obstacle avoidance. Three additional 

tows were carried out in the far-field at stations greater than 10 km from the near-field. This 

near-field and far-field pairing was intended to test for effects of development. The design was 

also to include before/after comparisons at GB 516 and VK 916, with the three near-field and 

three far-field samplings being carried out prior to and after drilling was initiated. Unfortunately, 

this part of the study was compromised by prior drilling at the GB 516 site (see Chapter 3). 

12.2.2 Image Processing 

Images were scanned on a Nikon Cool-Scan 4000 at a density of 1,208 by 1,937 pixels using 

Nikon Scan software as an Adobe Photoshop plug-in in a Microsoft® Windows 2000 

environment. The resulting raw scan was cropped as necessary and stored as a tagged image 

format (tif) file without compression, 7.1 megabytes. The actual pixel sizes were somewhat 

arbitrary. They produced high-resolution images, and images could be stored on an ordinary 

CD-ROM. Auto exposure was defeated during scanning so that actual image-to-image 

differences were maintained for later analysis. The name of each image file was derived from the 

information of the data chamber (roll number, day, hour, minute, and second). 
Figure 12.1. Locations of camera sled transects at Viosca Knoll (VK) Block 916 near-field site. 

Arrows indicate direction of tow.

VK 916 (Exploration Site)

Cruise 1B (pre-drilling)

VK 916 (Exploration Site)

Cruise 3B (post-drilling)

Sled NF2-R3

Sled NF4-R7

Sled NF1-R2

Sled NF1-R6 Sled NF3-R15

Sled NF2-R10

SITE CENTER

29° 06' 24.31"N

87° 53' 19.48"W

Camera sled transect

LEGEND

Arrows indicate 

transect direction

New wellsite

(drilled during this study)

500 m radius

500-m radius 500 m radius

500-m radius

417
GB 516 (Exploration/

Development Site)

Cruise 2B

GB 516 (Exploration/

Development Site)

Cruise 1B

SITE CENTER

27° 29' 23.81"N

92° 23' 08.26"W

-

500-m radius

500-m radius

Figure 12.2. Locations of camera sled transects at Garden Banks Block (GB) Block 516 near-field (NF) 

site. Arrows indicate direction of tow.

LEGEND

Camera sled transect

Previous wellsite

New wellsite

(drilled during this study)

Sled NF3-R5

Sled NF2-R4

Sled NF1-R1

Sled NF3-R3

Sled NF1-R3

Sled NF2-R2

Arrows indicate 

transect direction

418
GB 602 (Post-Development Site)

Cruise 2B (post-drilling)

Sled NF1-R7

Sled NF2-R10 Sled NF3-R11

SITE CENTER

27° 22' 38.02"N

92° 27' 35.79"W

500-m radius

Figure 12.3. Locations of camera sled transects at Garden Banks Block (GB) Block 602 near-field (NF) 

site. Arrows indicate direction of tow.

LEGEND

Camera sled transect

Previous wellsite

Arrows indicate 

transect direction

419
Figure 12.4. Locations of camera sled transects at Mississippi Canyon (MC) Block 292 near-field (NF) 

site. Arrows indicate direction of tow.

Camera sled transect

LEGEND

Previous wellsite

MC 292 (Post-Development Site)

Cruise 2B (post-drilling)

Sled NF1-R14

Sled NF2-R15

Sled NF3-R18

Arrows indicate transect direction

SITE CENTER

28° 42' 13.08"N

88° 35' 44.19"W

500-m radius

420
421 

Visual inspection was based upon a dual examination. The film was examined under 

magnification, and the digital image was examined subsequently. Data logged were data 

chamber content, image quality, confirmation that the camera was moving (non-redundant 

image), and an inventory and count of all fauna and objects. Handwritten logs were then 

converted into Microsoft® Excel tabular files. 

Analysis of image color and texture was carried out using custom written programs employing 

the Precision Visuals Workstation Analysis and Visualization (PV-WAVE) image processing and 

statistics tool kits. These were run on a 64-bit Unix workstation. Color analysis (calculation of 

hue) and textural analyses were run on a 1,000 x 1,000 pixel area in the center of each image. 

This area was unaffected by color bleeding from the red data chamber insert and was uniformly 

lighted by the strobe. 

12.2.3 Color Analysis 

The color of the seafloor can be a useful indicator of ecological condition. In typical deep-sea 

habitats, the top 1 to 2 cm of sediment are well oxidized and appear brownish in color; underlying 

these are more reduced sediments that are more gray. When bioturbation is active, mounds of the 

excavated gray sediment are conspicuous on top of the brown surface. The more extensive the 

burrowing, the greater the shift from brown to gray. The deep-sea floor normally does not 

become sulfidic in the top few centimeters, but sulfate reduction can occur when there is an 

usually high input of organics, either from detritus, seeping hydrocarbons, and/or man-made 

sources. The resulting sulfidic sediments are typically black and may be colonized by white to 

orange bacterial mats. 

12.2.4 Color Models 

There are several methods of mathematically describing the perceived colors. Computer imaging 

typically uses the RGB (red, green, and blue) model in which each pixel (picture element) is 

represented by three values. In the most common 8-bit systems, values range from 0 to 255 for 

each. These data are easily converted to a displayed image via the RGB primary colors of display 

devices. In analysis of seafloor images, RGB data are not especially useful since we are more 

interested in describing colors than displaying them. 

Hue is a basic concept in color perception, selecting colors, and describing colors. Descriptions 

such as “brownish,” “bluish,” etc. all describe the hue of an image. Full description of a color 

requires two other terms, S (saturation) and L (lightness). Of these, L is more readily understood 

in terms of exposure, being a measure of where between maximum lightness (white) and 

minimum lightness (black) an image falls. S refers to the intensity of the hue. For example, the 

brightest red possible for a given film image is said to be fully saturated. A poorly saturated red 

would appear to be a grayish pink ranging from very light pink at high L to a dark gray at low L. 

H, S, and L are readily calculated from RGB by conversion from the RGB to hue/saturation/light 

color models. Hue is expressed as an angle between 1 and 360 degrees, much like a color wheel. 

The algorithm employs scalings between the maximum and minimum values of the RGB 

representation. The conversion was carried out using the COLOR_CONVERT function of 

PV-WAVE. The average hue was calculated for a 1,000 x 1,000 pixel area. 
422 

Texture analysis was considered highly exploratory. One hundred images were selected by 

examination, which ranged from a uniform, almost featureless bottom up to images with multiple 

mounds and burrows with associated large shadows. Texture statistics were then calculated using 

three texture models in the PV-WAVE Image Processing Toolkit. Each model produced a single 

value or suite of texture statistics. The ranking of the calculated statistic was then compared to 

the initial visual assessment. 

12.3 RESULTS 

12.3.1 Texture Analysis 

The trial effort at digital texture analysis was not successful. Objectively, some sites had distinct 

and consistent textures. Notably, most smooth bottom images were found at GB 516, and most 

coarsely burrowed bottom images were found at MC 292. None of the three texture models 

reflected these subjective patterns. The primary difficulty is identification of the most appropriate 

scale at which to seek patterns. The pixel-level analyses attempted were probably at too fine of 

scale to reflect burrows and mounds a few hundred pixels across. 

12.3.2 Color Analysis 

Calculation of hue and plotting of values against time of image were exceptionally successful, but 

for unanticipated reasons. The anticipated local shifts in hue due to bioturbation were not found. 

Rather, all or most camera tows in near-field areas where development had taken place revealed 

areas where the normal brownish-gray bottom shifted progressively to areas of dark blue-black. 

These dark bottoms are most likely due to anoxia caused by drilling muds adhering to cuttings. 

No such shifts from normal to dark bottom were found in any far-field tow or in the VK 916 

near-field prior to development. 

12.3.2.1 Viosca Knoll 916 (Figures 12.5, 12.6, 12.7, and 12.8) 

There were no hue shifts on any Cruise 1B tows, a result consistent with the lack of drilling 

activity. On Cruise 3B (post-drilling), all three near-field tows crossed one or two dark bottom 

areas. Locations of dark bottom areas were consistent with geophysically mapped cuttings 

deposits (Figure 12.8). Bacterial mats were not observed (though they were seen in some 

sediment profile images from this site; see Chapter 6). Far-field images showed no hue shifts. 

12.3.2.2 Garden Banks 516 (Figures 12.9, 12.10, and 12.11) 

The dark bottom in the near-field was crossed by all three tows on Cruise 1B. It appears to be a 

feature at least 250 m across and lying to the north of the center. The dark bottom is covered in 

some images by white to red mats consistent with the sulfide-using bacteria Beggiatoa. These 

bacterial areas may reflect site of local bottom anoxia supported by bacterial degradation of 

discharged drilling fluids. Cruise 2B results were similar except that no bacterial mats were seen. 
Figure 12.5. Image analysis results for Viosca Knoll (VK) Block 916, Cruise 1B, near-field transects.

0 2,000 4,000

0 2,000 4,000

0 2,000 4,000

VK 916 Near-Field Transect 2, Cruise 1B

VK 916 Near-Field Transect 1, Cruise 1B

VK 916 Near-Field Transect 3, Cruise 1B

423
Figure 12.6. Image analysis results for Viosca Knoll (VK) Block 916, Cruise 3B, Near-field Transect 1.

0 1,000 2,000 3,000 4,000 5,000

VK 916 Near-Field Transect 1, Cruise 3B

424
Figure 12.7. Image analysis results for Viosca Knoll (VK) Block 916, Cruise 3B, Near-field Transect 2.

0 1,000 2,000 3,000 4,000 5,000

Dark Bottom

VK 916 Near-Field Transect 2, Cruise 3B

425
1 2

3

4

= Geophysically mapped cuttings

LEGEND

Transect 1

1 2

3

4

Transect 2

1 = Example photograph location

Figure 12.8. Locations of example photographs in relation to geophysically mapped cuttings deposits at Viosca Knol

Cruise 3B.

= Wellsite

0 1,000 2,000 3,000 4,000 5,000

0 1,000 2,000 3,000 4,000 5,000

VK 916 Near-Field Transect 2, Cruise 3B

VK 916 Near-Field Transect 1, Cruise 3B

426
Figure 12.9. Image analysis results for Garden Banks (GB) Block 516, Cruise 1B, Near-field Transect 1.

0 500 1,000 1,500 2,000 2,500

Seconds on Bottom

4

5

1 2

3

GB 616 Near-Field Transect 1, Cruise 1B

427
Figure 12.10. Image analysis results for Garden Banks (GB) Block 516, Cruise 2B, Near-field Transect 1.

0 2,000 4,000

GB 516 Near-Field Transect 1, Cruise 2B

428
Figure 12.11. Image analysis results for Garden Banks (GB) Block 516, Cruise 1B, far-field transects.

0 500 1,000 1,500

0 400 1,000 1,200

0 500 1,000 1,500 0 2,000 4,000

0 2,000 4,000 200 600 800

429
430 

12.3.2.3 Garden Banks 602 (Figures 12.12 and 12.13) 

The normal-dark shift in near-field images was somewhat more difficult to identify since most 

images produced hue values indicative of dark bottoms. Only the third tow had the dramatic shift 

found at other sites. It appears as though GB 602 has cuttings more thinly spread over a larger 

area. Far-field results were typical, with no indication of dark bottom. 

12.3.2.4 Mississippi Canyon 292 (Figures 12.14 and 12.15) 

The normal-dark shift was again obvious in the near field, and far-field images showed only 

normal bottom. As an interesting note, the bottom at MC 292 showed a distinctive texture in all 

tows. There was a mosaic of overlapping circles about 10 cm across. These appear to be filled-in 

feeding pits. The maker of the pits was never photographed. 

12.4 FAUNAL ANALYSIS 

Results of faunal analysis are summarized in Table 12.1. Thirty-eight rolls of film had usable 

images on them. Camera difficulties required extra tows at GB 516 on Cruise 1B, and only five 

tows were successful at VK 916 on Cruise 2B. Otherwise, the original design was reasonably 

well achieved. Fauna were counted in 16,770 images. For the entire data set, approximately 

7.5% of images contained identifiable fauna. At all sites except MC 292, this value ranged from 

a high at GB 516 of 9% to 6.6% at GB 602. Distinctively lower was the 2% at MC 292. MC 292 

had a distinctive bottom texture that both indicates a unique habitat and may obscure fauna. 

The small area of the images proved to be more of a problem than anticipated. Originally, it was 

assumed that higher resolution images of small areas would miss large animals but allow 

identification and enumeration of many animals at the lower limit of the megafauna size range, 

1 cm. This did not prove to be the case. Large fishes, crabs, and the rare holothuroid usually 

appear in only part of the image frame. Some distinctive small fauna was seen, notably retracted 

anemones and hermit crabs as small as 1 cm, but little else. Animals in the 1 to 2 cm size range 

are either very rare, buried, or obscured by microtopography. 

For statistical analysis, fauna were placed into six categories: 

1. Benthic coelenterates were most abundant, but the category has a high degree of uncertainty. 

It contains a few obvious burrowing anemones and sea pens, along with a great many 

organisms that might be retracted anemones. All were benthic and partially buried. 

2. Shrimps consisted of unidentifiable penaeids usually seen a few centimeters off bottom along 

with Glyphocrangon sp. seen on the bottom. 

3. Ophiuroids could not be identified to species. Partially buried specimens were common, 

suggesting that a large number of fully buried specimens were not imaged. 

4. Fishes were seldom imaged whole; that and the camera angle made identification uncertain. 

Coryphanoidids were most common, followed by synaphobranchid eels and a few rays 

identifiable as Bathyraja sp. 

5. Crabs were predominantly small (<2 cm) hermit crabs. Chaceon quinquedens was imaged in 

very low numbers. 

6. Gastropods were small (<2 cm). Most were found on a trail, but there is no certainty that 

they were live or occupied by a withdrawn hermit crab. 
Figure 12.12. Image analysis results for Garden Banks (GB) Block 602, Cruise 2B, Near-field Transect 3.

1,00

0

2,00

0 0 1,50 0 500 2,500

GB 602 Near-Field Transect 3, Cruise 2B

431
Figure 12.13. Image analysis results for Garden Banks (GB) Block 602, Cruise 2B, far-field transects.

0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500

Light Fogging

Corrected

Light Fogging

Corrected

Light Fogging

Corrected

432
Figure 12.14. Image analysis results for Mississippi Canyon (MC) Block 292, Cruise 2B, Near-field Transect 3.

0 500 1,000 1,500 2,000 2,500 3,000

44

5

1 2

3

GB 292 Near-Field Transect 3, Cruise 2B MC 292 Near-Field Tow 3, Cruise 2B

433
Figure 12.15. Image analysis results for Mississippi Canyon (MC) Block 292, Cruise 2B, far-field transects.

0 2,000 4,000

0 2,000 4,000

0 2,000 4,000

434
Table 12.1. Results of photographic faunal survey. Data are counts of each type of animal seen in photographs from e

No. of Animals Site Far/Near Cruise/Film No. of Images Total Coelenterates Shrimps Ophiuroids Fishes 

Viosca Knoll Block 916 Cruise 1B “pre-drilling” 

VK 916 FF3 C-1-R-14 490 49 12 2 17 3 VK 916 FF4 C-1-R-13 64 2 1 0 0 VK 916 FF4 C-1-R-16 442 19 2 2 8 6 VK 916 FF5 C-1-R-11 433 30 2 3 13 8 VK 916 NF-1 C-1-R-6 246 15 3 3 3 2 VK 916 NF-2 C-1-R-10 406 23 6 3 7 3 VK 916 NF-3 C-1-R-15 453 26 2 2 6 4 

Average 362.00 23.43 4.00 2.14 9.00 3.71 

Image/Counta 0.0647 0.0110 0.0059 0.0249 0.0103 

Cruise 3B “post-drilling” 

VK 916 FF4 C-3-R-6 698 68 16 30 7 7 VK 916 FF5 C-3-R-5 710 56 19 20 10 3 VK 916 NF-1 C-3-R-2 695 47 17 7 7 12 VK 916 NF-2 C-3-R-3 712 47 9 6 3 23 VK 916 NF-4 C-3-R-7 753 50 26 6 6 6 

Average 713.60 53.60 17.40 13.80 6.60 10.20 

Image/Counta 0.0751 0.0244 0.0193 0.0092 0.0143 

435
Table 12.1. (Continued). 

436

No. of Animals Site Far/Near Cruise/Film No. of Images Total Coelenterates Shrimps Ophiuroids Fishes 

Garden Banks Block 516 Cruise 1B 

GB 516 FF1 C-1-R-19 240 28 9 5 12 0 GB 516 FF3 C-1-R-18 560 36 9 3 15 5 GB 516 FF3 C-1-R-4 580 17 2 5 8 1 GB 516 FF5 C-1-R-17 300 25 4 5 12 3 GB 516 FF5 C-1-R-5 280 15 1 5 8 1 GB 516 NF-1 C-1-R-1 300 28 9 5 12 0 GB 516 NF-2 C-1-R-2 650 96 70 12 2 12 GB 516 NF-3 C-1-R-3 580 71 59 6 1 3 

Average 436.25 39.5 20.375 5.75 8.75 3.125 

Image/Counta 0.0905 0.0467 0.0132 0.0201 0.0072 

Cruise 2B 

GB 516 FF1 C-2-R-6 340 4 0 3 0 GB 516 FF2 C-2-R-1 510 58 16 19 17 5 GB 516 FF6 C-2-R-2 440 40 6 14 15 2 GB 516 NF-1 C-2-R-3 460 28 11 12 0 5 GB 516 NF-2 C-2-R-4 440 60 31 16 1 11 GB 516 NF-3 C-2-R-5 440 28 1 10 5 11 

Average 438.33 36.33 10.83 12.33 7.60 5.67 

Image/Counta 0.0829 0.0247 0.0281 0.0173 0.0129 
Table 12.1. (Continued). 

437

No. of Animals Site Far/Near Cruise/Film No. of Images Total Coelenterates Shrimps Ophiuroids Fishes 

Garden Banks Block 602 Cruise 2B “post-drilling” 

GB 602 FF2 C-2-R-9 370 10 3 4 0 1 GB 602 FF3 C-2-R-8 380 11 1 3 3 2 GB 602 FF6 C-2-R-12 435 12 1 3 3 2 GB 602 NF-1 C-2-R-7 180 18 0 15 0 3 GB 602 NF-2 C-2-R-10 175 24 1 22 0 1 GB 602 NF-3 C-2-R-11 342 50 2 39 2 7 

Average 313.67 20.83 1.33 14.33 1.33 2.67 

Image/Counta 0.0664 0.0043 0.0457 0.0043 0.0085 

Mississippi Canyon Block 292 

Cruise 2B “post-drilling” 

MC 292 FF2 C-2-R-13 348 8 1 1 3 2 MC 292 FF3 C-2-R-16 413 11 1 2 2 5 MC 292 FF6 C-2-R-17 481 9 1 4 2 1 MC 292 NF-1 C-2-R-14 540 5 0 1 2 2 MC 292 NF-2 C-2-R-15 412 11 0 1 2 3 MC 292 NF-3 C-2-R-18 472 12 2 2 1 5 

Average 444.33 9.33 0.83 1.83 2.00 3.00 

Image/Counta 0.0210 0.0019 0.0041 0.0045 0.0068 

Total 16,770 1,147 356 301 215 170 

Image/Counta 0.0684 0.0212 0.0179 0.0128 0.0101 

FF = far-field; NF = near-field. a Proportion of images that contained identifiable fauna. 
438 

Fauna imaged but omitted from analysis include the holothuroid Mesothuria lactea and the 

starfish Dytaster sp. Fewer than five were seen of each in the entire survey. 

The primary hypothesis behind the image analysis task was that fauna in the near-field and in the 

far-field are equal within natural variation. A traditional approach to testing this hypothesis 

might have estimated faunal densities near and far, and then tested for uniformity of the densities. 

Due to the small area of bottom captured in each photograph, most images contained no fauna, 

and with very few exceptions, only one animal was seen in any picture. These facts made a 

goodness-of-fit analysis, based on percentages of images with fauna, a simple and informative 

approach. 

Roll 13 of Cruise 1B at VK 916 was omitted due to the very limited number of images, leaving 

37 camera tows to be divided into two categories: close to development (n=15) and far from 

development (n=22). The division followed the original plan of near- and far-fields, with two 

important exceptions. The VK 916 near-field pre-drilling sites were considered far from 

development. The GB 516 Cruise 1B near-field tows were categorized as being in the vicinity of 

development due to prior drilling activity. Results are presented in Table 12.2. 

Table 12.2. Results of comparisons between near-field and far-field tows. 

Images Total Coel. Shrimps Ophiuroids Fishes Crabs Gastropods 

Near-field w. fauna 575 238 160 44 104 17 12 

Far-field w. fauna 570 117 141 171 66 37 38 

Kolmogorov Z 1.0316 0.8506 0.7692 1.701 1.384 0.8959 0.9049 

Prob. greater by 

chance 0.1639 0.3408 0.4644 0.0022* 0.0239* 0.2937 0.2776 

* Significant result (null hypothesis rejected). 

A Kolmogorov two-sample test was performed using the statistical package of PV-WAVE. One 

sample consisted of results from each of the near-field tows, and the other consisted of results 

from each of the far-field tows. The differences (higher or lower) in proportion of images with 

and without fauna may occur by chance alone, with a probability of 0.1639. Thus the hypothesis 

of faunal independence is accepted using the traditional criteria of α = 0.05. No clear effect of 

proximity for all fauna could be convincingly detected. 

For fishes and ophiuroids, significant results were obtained. For fishes, the probability of the 

observed differences in proportions being to chance alone were 0.0239. Thus, for fishes, the 

hypothesis of similar fauna must be rejected by the criteria of α = 0.05. Fishes are more abundant 

near development than far from it. For ophiuroids, a similar probability of the results by chance 

was 0.0022, also significant. The direction of the near-field versus far-field changes is opposite. 

Fishes were significantly more abundant near development activity, while ophiuroids were more 

abundant remote from development. 

The intended before/after comparison at GB 516 and VK 916 was seriously compromised by the 

fact that the GB 516 site had been drilled prior to the “before” sampling (as well indicated in the 
439 

color analysis). The VK 916 site produced problematic results. Both near- and far-field sites 

produced generally higher numbers during the “after” cruise. By the design criteria, these 

changes must be attributed to an overall increase independent of drilling effects. Conceptually, it 

is still possible to carry out a low power before/after control comparison. The low number of 

images with fauna, however, causes small changes to have disproportionate effects. Thus, natural 

increase cannot be partitioned from any drilling effects. 

12.5 DISCUSSION 

The results of this image analysis task are important from three perspectives: 

1. The study did produce definitive indications of near-field versus far-field effects. 

2. The color analysis greatly increased the data utility of images. 

3. The pros and cons of small area images could be considered. 

12.5.1 Faunal Results 

The faunal results were consistent with previous findings in shallow water. Color analysis, 

combined with the results of geophysical surveys (Chapter 4) and chemical analyses (Chapters 8 

and 9), suggest that the near-field images were taken in or very close to deposits of drilling 

discharges. Thus, there is no uncertainty as to the presence of a potentially impacting process. 

Overall, small megafauna showed little impact; there was no gross reduction or increase in 

imaged fauna. Localized significant shifts were found for two component groups. Fishes were 

more abundant in the near-field consistent with attraction to disturbance and structure. 

Ophiuroids were less abundant in the near-field. An exact cause is less easily suggested. The 

very flat organisms might have been buried by cuttings, or there may be a behavior shift, which 

resulted in fewer animals being seen in the near-field. 

12.5.2 Color Analysis 

Using hue rather that digital RGB as a descriptive parameter for color proved exceptionally 

successful. Subjective classification of images was quantified and made more useful for 

(GIS)-type display. The patterns of dark bottom coincided with acoustic mapping of cuttings 

reasonably well. Such a linkage between biological sampling and geological survey has the 

potential to greatly increase the reliability of impact studies. 

12.5.3 Consideration of Size 

The use of small-area, straight-down imaging in this study was unique. It probably contributed to 

the detection of a difference in the ophiuroids, and it did not prevent detection of a difference in 

the larger fishes. It did not, however, make available a new suite of species for observation. 

Making some simplistic assumptions about density-area relationships, it can be predicted that 

quadrupling the area of the image (doubling the linear dimensions) would produce a four-fold 

increase in the number of images capturing fauna, about 28% based on the current studies. Such 

an increase in size would not lose too many animals below the limits of resolution. 
441 

Chapter 13 

Genetic Diversity of 

Harpacticoid Copepods 

Christopher S. Gregg, David W. Foltz, and John W. Fleeger 

Louisiana State University 

13.1 INTRODUCTION 

The deep sea in the northern Gulf of Mexico is an increasingly active area for exploration of oil 

resources. The ecological (Peterson et al. 1996) and genetic (Street and Montagna 1996) impacts 

of offshore oil exploration and production have been studied in shallow continental shelf habitats; 

however, there are currently no data on the effects of oil exploration and production on genetic 

diversity of deep-sea infauna (Creasy and Rogers 1999). 

In shallow water, reductions in genetic diversity have been observed in populations of 

harpacticoid copepods living near offshore oil platforms compared to reference sites (Street and 

Montagna 1996). Studies of natural populations of fishes (Murdoch and Hebert 1994) and 

aquatic insects (Beaty et al. 1998) have also noted reductions in genetic diversity in populations 

exposed to contaminants compared to those from uncontaminated sites. Additionally, laboratory 

experiments on harpacticoid copepods have shown reductions in genetic diversity when copepods 

were exposed to xenobiotics such as PAHs (Street et al. 1998) or pesticides (Schizas et al. 2001). 

Two mechanisms that may cause the observed reductions in genetic diversity in natural 

populations exposed to contaminants are natural selection and population bottlenecks (reviewed 

in Hebert and Luiker 1996; Bickham et al. 2000; Belfiore and Anderson 2001). Natural selection 

reduces genetic diversity by the differential survival of tolerant genotypes compared to less 

tolerant genotypes. Schizas et al. (2001) demonstrated that Microarthridion littorale that 

possessed one mitochondrial lineage had greater survival when exposed to pesticides compared to 

those possessing other lineages, thus bolstering the argument for natural selection. Genetic 

diversity can be decreased in a population that has undergone a rapid decrease in population size, 

a bottleneck, through the loss of rare alleles by the random process of genetic drift. Additionally, 

a small population that has undergone a rapid increase in size may show low diversity because 

mutation and gene flow would not have had time to introduce new alleles (reviewed in 

Harpending et al. 1998). 

Undetected cryptic species (i.e., morphologically similar but genetically divergent species) may 

also be a factor in observed reduction in genetic diversity. Molecular analysis within some 

previously well-studied species has revealed genetic divergence suggestive of interspecific 

differences (Knowlton 1993, 2000). Cryptic species have been found to occur in widely-studied 

pollution indicator species (e.g., Grassle and Grassle 1976; Lobel et al. 1990) and in harpacticoids 

(Ganz and Burton 1995; Rocha-Olivares et al. 2001). The presence of cryptic species may 

contribute to an observed loss in genetic diversity, if they differ in their tolerance to contaminants 

and if they are sympatric at uncontaminated sites. This would represent a loss of species diversity 

as opposed to genotypic selection. Fleeger et al. (2001) demonstrated that cryptic species within 

the genus Cletocamptus, which co-occur in Louisiana salt marshes, exhibited differential 
442 

tolerances to heavy metals. The possibility of cryptic species must be accounted for when 

examining the effect of pollution on genetic diversity. 

In the present study, we intended to determine if genetic diversity in harpacticoid copepods was 

reduced in populations near development drilling sites compared to populations from reference 

sites (sensu Street and Montagna 1996). Sites adjacent to (near-field) and distant from (far-field) 

development drilling sites were sampled at two locations off the coast of Louisiana at water 

depths slightly greater than 1,000 m. Analysis of samples taken near the drillsites and at 

reference sites indicated that two very different harpacticoid communities with few species in 

common were present. Near-field sites were characterized by a relatively low species diversity, 

high dominance community. Large numbers of a single species, Bathycletopsyllus sp., typified 

near-field sites, although other species were present in unusually high densities for the deep sea. 

Far-field sites were characterized by high species diversity and low dominance, more typical of 

undisturbed deep-sea communities. Almost no species were in common at near-field sites and 

far-field sites (Bathycletopsyllus sp. was found in only one far-field site, represented by one 

individual). We concluded that Bathycletopsyllus sp. was likely an opportunistic species, present 

in low numbers in undisturbed deep-sea habitats, but able to undergo a population expansion in 

response to disturbance associated with oil drilling and production activities. 

Bathycletopsyllus sp. is a benthic harpacticoid copepod in the family Cletopsyllidae (Huys and 

Lee 1998/99). Thus far, only one species, Bathycletopsyllus hexarthra, of this genus has been 

described from the Indian Ocean off Le Réunion at a depth of 400 m in muddy sediments (Huys 

and Lee 1998/99). The species examined in the present study has not been described, but it is 

approximately 0.9 mm in body length. The females brood their larvae in an attached egg sac 

(personal obs.), typical of harpacticoid copepods (Hicks and Coull 1983). 

The objectives of the present study were to examine the genetic diversity and population structure 

of Bathycletopsyllus sp. at the two development drilling sites, which are approximately 407 km 

distant from each other in the northern Gulf of Mexico. Additionally, DNA sequence data were 

used to determine whether the harpacticoids examined were a single species or a complex of 

cryptic species. We examined DNA sequence variation of the mitochondrial cytochrome c 

oxidase subunit I (COX I) gene. Mitochondrial genes have been used extensively to study the 

population genetics of harpacticoid copepods (Burton and Lee 1994; Burton 1998; Schizas et al. 

1999; Edmands 2001; Schizas et al. 2001) as well as to examine the effects of pollution on 

harpacticoids (Street et al. 1998; Fleeger et al. 2001; Schizas et al. 2001). The presence of cryptic 

species among harpacticoid copepods (Ganz and Burton 1995; Rocha-Olivares et al. 2001) has 

also been detected using the COX I gene. The patterns of population genetic diversity and 

genetic structure or the presence of cryptic species were used to determine how Bathycletopsyllus

sp. may have responded to environmental disturbance associated with oil production activities. 

The present study was also the first to examine the population genetics of a deep-sea harpacticoid 

species using data from a mitochondrial gene. 
443 

13.2 METHODS 

13.2.1 Collection and Processing of Samples 

Two post-development drilling sites were sampled in July 2001. GB 602 and MC 292 are located 

off the coast of Louisiana at depths of 1,125 m and 1,034 m, respectively (see Chapter 2). 

Sediment samples were collected by box core from which subsamples were taken by coring. 

Three 7.6-cm diameter cores were taken from each box core to obtain harpacticoid copepods for 

genetic analysis. Twelve box cores were collected at both GB 602 and MC 292 from randomly 

selected locations within 500 m of each post-development site. Subsamples from box cores were 

combined into one collection jar and were preserved onboard ship in 95% ethanol. Details of the 

sampling sites and general sampling methods are given in Chapter 2 of the present report. 

Sediment samples were stained with rose bengal and stored at 4°C upon arrival at Louisiana State 

University. Processing consisted of sieving on a 63-μm sieve, then sorting and removing the 

harpacticoid copepods under a stereo dissection microscope. All sorting was done in SED buffer 

(Seutin et al. 1991) to minimize DNA degradation. Identification of the harpacticoids to the 

lowest taxonomic level was done by one of us (John W. Fleeger), and one species in the genus 

Bathycletopsyllus was found to be abundant at near-field sites at both GB 602 and MC 292. 

13.2.2 Genetic Methods 

DNA was extracted from individual copepods using the Gene Releaser (BioVentures, Inc.) 

protocol described by Schizas et al. (1997). A 710-base pair portion of the mitochondrial COX I 

gene was amplified by the PCR using primers developed by Folmer et al. (1994)(LCOI: 

5’-GGTCAACAAATCATAAAGATATTGG-3’; HCOI: 5’-TAAACTTCAGGGTGACCAAAAAATCA-3’). 

Reactions were done at 50-μl final volume, and the final concentrations of the reagents were as 

follows: 1X Promega Reaction Buffer A; 2.5 mM MgCl2; 0.2 mM total dNTPs; 1 μM of both 

forward and reverse primers; three units Promega Taq polymerase; 2-μl DNA extract; sterile H20 

to bring volume to 50 μl. The PCR was done in a Perkin-Elmer DNA Thermal Cycler under the 

following conditions: 1 cycle 95°C for 3 min; 40 cycles of 95°C for 30 sec, 47°C for 45 sec, and 

74°C for 1 min; 1 cycle 74°C for 7 min. Three μl of each PCR product was electrophoresed into 

a 2% agarose gel, stained with ethidium bromide, and visualized by ultraviolet transillumination. 

Because of the small size of the organisms (<1 mm) and poor preservation of some samples, 

initial amplifications with the LCOI and HCOI primers of Folmer et al. (1994) had a low success 

rate, and DNA sequencing revealed contaminating coamplicons. Primers internal to LCOI – 

HCOI were designed, BathyLCOI (5’-ATATTGGTACCTTGAATTTGTTAGC-3’) and 

BathyHCOI (5’-CCAAAAAATCAAAATAAATGCTGGT-3’), by examining preliminary 

sequence data from the LCOI – HCOI fragment. This led to a higher success rate and elimination 

of contaminating coamplicons. Additionally, two primers were designed from the middle of the 

PCR fragment, BathyCOI K (5’-GGGCHCCTGATATGGCTTTTCC-3’, modified from Burton 

and Lee 1994) and Bathy iR (5’-ATTATTTAATCGAAGAAAAGCCATA-3’) to provide greater 

overlapping coverage of the COX I gene for unambiguous assembly of contigs. 

DNA sequences of the COX I gene were obtained from individuals chosen at random; a total of 

33 individuals from GB 602 and 28 from MC 292 was analyzed. The PCR products were 
444 

purified using Exo-SAP IT (USBiochemicals) following the manufacturer’s protocols, then 

sequenced using the ABI-Prism Big-Dye cycle sequencing chemistry (Applied Biosystems). The 

sequencing reactions were scaled down to 10-μl reaction volumes, as described by 

Rocha-Olivares et al. (2001), and run on an ABI-377 Gene Analyzer (Applied Biosystems). 

13.2.3 Data Analysis 

A genealogy of the COX I gene in Bathycletopsyllus sp. was constructed to show the 

relationships among the different haplotypes observed in the two populations. A 

minimum-spanning tree was constructed using Arlequin v. 2.0 (Schneider et al. 2000), and a 

statistical-parsimony network was constructed using TCS v. 1.13 (Clement et al. 2000), with 

genealogies from both methods producing identical topologies. Network approaches were chosen 

over traditional phylogenetic approaches, represented by a bifurcating tree, because in 

intraspecific studies, ancestral haplotypes are usually extant and give rise to multiple descendent 

haplotypes (Smouse 1998; Posada and Crandall 2001). 

Genetic subdivision of the populations was examined by analysis of molecular variance 

(AMOVA; Excoffier et al. 1992) using the program Arlequin v. 2.0 (Schneider et al. 2000). A 

hierarchical design was implemented to test the null hypotheses of no heterogeneity of haplotype 

frequencies between sites, among box cores within sites, and within box cores. The distance 

matrix used in the analysis consisted of pairwise differences among haplotypes; additionally, 

conventional F-statistics were determined based only on allele frequencies where distances 

between all pairs of non-identical haplotypes were set to one. The null distribution for 

significance tests was obtained by 644 random permutations of the data matrix. 

Two methods were used to make inferences regarding population demographics of 

Bathycletopsyllus sp. at the two drilling sites: comparison of haplotype and nucleotide diversities 

and mismatch distribution analysis. Haplotype diversity (probability that two randomly chosen 

haplotypes are different; h) and nucleotide diversity (probability that two homologous nucleotides 

are different; πn) were calculated for both populations. Low values for h and/or πn are expected 

in recently founded populations or populations that have undergone a demographic bottleneck, 

and high values are expected in large populations in equilibrium (Bowen and Grant 1997; Avise 

2000). Mismatch distributions, the frequency distribution of the number of nucleotide differences 

between pairs of individuals, were determined for both populations and compared to a sudden 

population expansion model (Rogers and Harpending 1992; Rogers 1995). Slatkin and Hudson 

(1991) and Rogers and Harpending (1992) have demonstrated that populations that have 

undergone a relatively recent population expansion exhibit a unimodal mismatch distribution, 

while populations in equilibrium exhibit a multimodal distribution. A Kolmogorov-Smirnov 

two-sample test was used to test the null hypothesis that the mismatch distributions for the 

populations at both sites did not differ (Sokal and Rohlf 1995). Also, the fit of the observed 

mismatch distribution for both populations to a sudden expansion model (Li 1977) was tested 

using the procedure of Schneider and Excoffier (1999). The sudden expansion model determines 

a frequency distribution of the expected number of nucleotide differences between two randomly 

chosen haplotypes in a population that has undergone an expansion. The molecular indices 

(h and πn) were calculated, and the mismatch distribution analysis was performed using 

Arlequin v. 2.0 (Schneider et al. 2000). 
445 

13.3 RESULTS 

13.3.1 Numerical Response to Disturbance by Bathycletopsyllus sp. 

One species of harpacticoid copepod in the genus Bathycletopsyllus was present in sufficient 

numbers at near-field samples from both GB 602 and MC 292 for genetic analysis (Table 13.1). 

At GB 602, a total of 459 Bathycletopsyllus sp. was found and removed from all 12 near-field 

samples, where 9 of 12 samples had 10 or more individuals, and 4 of 12 had greater than 

50 individuals. Bathycletopsyllus sp. was less common at MC 292, with 183 individuals sorted 

from all 12 near-field samples, but was still present in sufficient numbers for genetic analysis. 

Two out of 12 samples had more than 10 individuals. 

The density of Bathycletopsyllus sp. appeared to be related to the concentration of SBF 

hydrocarbons (see Chapter 9) in a complex fashion. At near-field GB 602, SBF concentrations 

were an order of magnitude greater than at the MC 292 site, and there was no relationship 

between SBF concentration and Bathycletopsyllus sp. density (r2

 = 0.04, Figure 13.1a). At the 

MC 292 near-field site, there was a positive relationship between SBF concentration and 

Bathycletopsyllus sp. density (r2

 = 0.85, Figure 13.1b). Regression of SBF concentrations with 

copepod numbers at GB 602 with observations from box cores with SBF concentrations less than 

2,500 μg/dry g revealed a positive linear relationship (r2

 = 0.52, Figure 13.1c). The SBF 

concentration above 2,500 μg/dry g was chosen because it was near the maximum concentration 

observed at MC 292. These data suggest that Bathycletopsyllus sp. responds positively to 

increasing levels of disturbance/hydrocarbons up to a certain threshold level, but then density 

decreases with increasing concentration above the threshold. 

13.3.2 Genetic Divergence 

The analyzed dataset consisted of 605 base pairs of the mitochondrial COX I gene with sequence 

data collected from 33 Bathycletopsyllus sp. from GB 602 and 28 from MC 292. Among the 

COX I gene sequence data, 9 haplotypes and 13 polymorphic sites were observed from GB 602 

individuals, while 11 haplotypes and 14 polymorphic sites were observed from MC 292 

individuals. The average transition to transversion ratios for the GB 602 and MC 292 data were 

8.9 and 3.6, respectively. The nucleotide composition showed that thymine (T) was the most 

common nucleotide, and cytosine (C) was the least common at both sites, with C = 14.9%, 

T = 35.9%, adenine (A) = 26.3%, and guanine (G) = 22.9% for GB 602, and C = 14.7%, 

T = 36.0%, A = 26.5%, and G = 22.8% for MC 292. The maximum P-distance was 1.6%, and the 

mean P-distance was 0.5% as calculated in MEGA 2.1 (Kumar et al. 2001). The maximum 

observed P-distance was within the range of values expected for within-species variation; thus the 

two populations likely represent a single species and not a complex of cryptic species. 
446 

y = -0.0021x + 45.25

0

20

40

60

80

100

120

140

0 5000 10000

1

15000

y = 0.0559x - 7.7438

 r2

= 0.85

0

50

100

150

200

0 500 1000 1500 2000 2500

y = 0.0322x - 4.3147

0

20

40

60

80

0 500 1000 1500 2000

# Bathycletopsyllus sp. Boxcore -1

2500

y = -0.0021x + 45.25

0

20

40

60

80

100

120

140

0 5000 10000

1

y = 0.0559x - 7.7438

0

50

100

150

200

0 500 1000 1500 2000

y = 0.0322x - 4.3147

0

20

40

60

80

0 500 1000 1500 2000

Figure 13.1. Relationship between synthetic based fluid (SBF) concentration and 

Bathycletopsyllus sp. per box core at the near-field sites. (a) Relation

Block 602 including all box cores. (b) Relationship at Mississippi Ca

including all box cores. (c) Relationship at Garden Banks Block 602

with SBF concentrations greater than 2,500 μg dry g-1.

SBF Concentration (μg dry g-1)

r

2 = 0.04

r

2 = 0.50

2500

the number of 

ship at Garden Banks 

nyon Block 292 

 excluding box cores 
447 

Table 13.1. Numbers of Bathycletopsyllus sp. collected from box cores at the near-field sites of 

Garden Banks (GB) Block 602 and Mississippi Canyon (MC) Block 292. The 

numbers are the total number of Bathycletopsyllus sp. found in three, 7.6-cm 

diameter cores taken from box core samples. 

Site Box Core GB 602 MC 292 

B01 19 151 

B02 17 6 

B03 5 3 

B04 41 1 

B05 131 1 

B06 10 0 

B07 70 0 

B08 4 20 

B09 61 0 

B10 0 1 

B11 74 0 

B12 27 0 

Total 459 183 

The haplotype cladogram revealed the presence of three distinct clades, which will henceforth be 

referred to as Clades BI, BII, and BIII for Bathycletopsyllus clade (Figure 13.2). Clade BI was 

separated from both Clades BII and BIII by three mutational steps, and Clades BII and BIII were 

separated from each other by a minimum of six mutational steps. The distribution of haplotypes 

was spatially heterogeneous as there were few haplotypes shared among sampling sites. 

Eighty-three percent of the individuals in Clade BI was from GB 602, while 88% and 89% of the 

individuals from Clades BII and BIII were from MC 292, respectively (Table 13.2). Haplotype 

A was the most frequently occurring haplotype in Clade BI, from which all other Clade BI 

haplotypes appear to have arisen. Haplotype I was the most frequently occurring haplotype in 

Clade BII; however, the relationships among the other haplotypes were more complex in that they 

do not all radiate from this haplotype in a “star phylogeny” (Slatkin and Hudson 1991) as in 

Clade BI. Only three haplotypes were observed in Clade BIII, with no single haplotype 

numerically dominant. 

13.3.3 Population Structure 

The AMOVA indicated genetic heterogeneity between Bathycletopsyllus sp. populations at the 

GB 602 and MC 292 sites. Significant genetic structuring was noted between the two sites 

sampled using both pairwise distances (Φst = 0.35; P <0.0001) and haplotype frequencies 

(Fst = 0.21; P <0.0001) (Table 13.3). There was no evidence of genetic structure among the 

subpopulations represented by box cores within sites in AMOVAs using both pairwise 

differences (Φsc = 0.043; P = 0.075) and haplotype frequencies (Fsc = 0.0013; P = 0.37) 

(Table 13.3). 
448 

C

D

B A

G

H

O K I

E

L J

N

M

F

Clade BI

Clade BIII

Clade BII

C

D

B A

G

H

O K I

E

L J

N

M

F

Clade BI

Clade BIII

Clade BII
449 

Table 13.2. Distribution of Bathycletopsyllus sp. haplotypes at each sampled location in Garden 

Banks Block 602 and Mississippi Canyon Block 292. Clades correspond to those 

shown in Figure 13.2. 

Haplotype Box 

core A B C D E F G H I J K L M N O 

Total 

Garden Banks 602 

B01 3 1 1 5 

B02 3 1 1 5 

B04 4 1 5 

B05 1 1 2 

B07 1 1 1 1 4 

B08 1 1 

B11 4 1 1 6 

B12 4 1 5 

Mississippi Canyon 292 

B01 2 2 1 2 3 1 11 

B02 3 1 4 

B03 1 1 

B04 1 1 

B08 1 3 1 1 2 1 1 1 11 

Total 21 9 1 1 2 2 1 1 7 4 2 1 4 4 1 61 

Total Clade BI = 36 Total Clade BII = 16 Total Clade BIII = 9 

Table 13.3. Analysis of molecular variance across the populations from the Garden Banks 602 

and Mississippi Canyon 292 sites. For each analysis, the percentage of the variance 

explained (%) and the probability of more extreme values from 644 permutation 

tests (P) are presented. Values outside the parentheses are from the analysis of 

molecular variance (AMOVA) weighted by pairwise differences between 

haplotypes. Those within parentheses are from the unweighted AMOVA. 

Source of Variation % P 

Among sites 32 (21.4) 0.0015 (<0.0001) 

Among box cores within sites 3 (0.10) 0.075 (0.37) 

Within sites 65 (78.5) <0.0001 (<0.0001) 

Φst = 0.35; Φsc = 0.043; Φct = 0.32. 

Fst = 0.21; Fsc = 0.0013; Fct = 0.21. 
450 

13.3.4 Genetic Diversity 

The indices measuring molecular, haplotype, and nucleotide diversity indicated different 

demographic histories for Bathycletopsyllus sp. at GB 602 and MC 292. Haplotype diversity was 

0.61 (standard deviation [SD] ± 0.089) and 0.89 (SD ± 0.033), and nucleotide diversity was 

0.0020 (SD ± 0.0014) and 0.0066 (SD ± 0.0038) at GB 602 and MC 292, respectively. 

Haplotype diversity was significantly lower at GB 602 compared to MC 292 (t = 15.95, df = 59, 

P <0.001). Nucleotide diversity was also significantly lower at GB 602 compared to MC 292 

(t = 6.43, df = 59, P <0.001). These diversity measures reflect the distribution of and relationship 

among the haplotypes at the two sites. GB 602 was numerically dominated by Haplotypes A (20) 

and B (6) in Clade BI, whereas haplotypes were more evenly distributed among Clades BI, BII, 

and BIII at MC 292, with no overwhelming numerical dominance by any haplotype at this site 

(Table 13.2). The low haplotype and nucleotide diversities observed for Bathycletopsyllus sp. at 

GB 602 were comparable to other harpacticoid copepods in the genus Cletocamptus and some 

populations of the species Microarthridion littorale (Figure 13.3). Bathycletopsyllus sp. 

collected at MC 292 had high haplotype diversity and low to moderate nucleotide diversity 

compared to other harpacticoids. The haplotype diversity for this population was similar to that 

observed in some populations of Microarthridion littorale, while the nucleotide diversity 

remained low to moderate (Figure 13.3). 

The mismatch distributions from GB 602 and MC 292 populations differed significantly from 

each other (D = 0.54 > D.01 = 0.11), but neither distribution differed significantly from a sudden 

expansion model. Pairwise comparisons of the DNA sequence data from the GB 602 population 

resulted in a unimodal frequency distribution that did not differ significantly from the sudden 

expansion model (P = 0.071; Figure 13.4a). The observed mean number of mismatches among 

randomly chosen sequences was 1.21. The mismatch distribution for MC 292 was multimodal, 

but it also did not differ significantly from the sudden population expansion model (P = 0.068; 

Figure 13.4b). The distribution produced by the MC 292 sequence data had a mean number of 

mismatches of 3.97. These data indicate that, while both populations may have undergone a 

population expansion, processes affecting the mismatch distribution differed at MC 292 

compared to GB 602. 

13.4 DISCUSSION 

13.4.1 Numerical Response to Disturbance by Bathycletopsyllus sp. 

Bathycletopsyllus sp. appeared to be the most abundant harpacticoid at both the GB 602 and 

MC 292 near-field sites; more than 100 individuals were found in some box core samples 

(Table 13.1). In the far-field sites, only one Bathycletopsyllus sp. individual was observed in 

over 24 box cores from both GB 602 and MC 292. These data suggest that this species responds 

to disturbance by increasing in population size. SBF concentrations may serve as a non-biotic 

indicator of disturbance. SBF hydrocarbons were elevated at near-field sites compared to 

far-field sites (see Chapter 9). At GB 602, the average SBF concentration in μg/dry g was 

3,384.3 (SD ± 3,940.7) at the near-field site and 5.75 (SD ± 2.0) at the far-field sites. At MC 292, 

the average SBF concentration in μg/dry g was 411.2 (SD ± 712.6) at the near-field site and 6.95 

(SD ± 8.4) at the far-field sites. 
451 

0

0.005

0.01

0.015

0.02

0.2 0.4 0.6 0.8 1

Cs Cf A L Cf LA Cd Ml Bsp

h

πn

0

0.005

0.01

0.015

0.02

0.2 0.4 0.6 0.8 1

Cs Cf A L Cf LA Cd Ml Bsp

h

πn

Figure 13.3. Relationship between haplotype diversity and nucleotide diversity of COX

five species of harpacticoid copepods including Bathycletopsyllus sp. Th

haplotype diversity (h), and the y-axis is nucleotide diversity (TTn). Cs = 

stimsoni, Cf AL = C. fourchensis from Alabama, Cf LA = C. fourchensis

Cd = C. deborahdexterae (all Cletocamptus data were from Rocha-Olivar

Ml = Microarthridion littorale (Schizas et al. 2002). Bsp = Bathycletopsyl

were taken from COX I sequences except Ml, which were from cytochrom

 I sequences for

e x-axis is

Cletocamptus

 from Louisiana,

es et al. 2001).

lus sp. All data

e b sequences.
452 

# of Base Pair Differences

0

0.1

0.2

0.3

0.4

0.5

01234567

f(obs)

f(model)

0

0.1

0.2

0.3

0.4

0.5

0123456789 10

0

0.1

0.2

0.3

0.4

0.5

01234567

f(obs)

f(model)

0

0.1

0.2

0.3

0.4

0.5

0123456789 10

Figure 13.4. Mismatch distributions for pairwise haplotype distances between Bathyc

individuals collected at Garden Banks (GB) Block 602 and Mississipp

Block 292. The x-axis represents the number of base pair differences bet

and the y-axis represents the frequency. (a) Mismatch distribution for in

(b) mismatch distribution for individuals at MC 292. f(obs) = frequency

number of base pair differences. f(model) = frequency of the number of b

expected with a sudden population expansion model.

letopsyllus sp. 

i Canyon (MC) 

ween individuals, 

dividuals at GB 602, 

 of the observed 

ase pair differences 
453 

Although the data in the present study were limited, there was a suggestion of a positive 

correlation between abundance of Bathycletopsyllus sp. and SBF hydrocarbons (see Figure 13.1). 

The relationship held only when SBF concentrations less than 2,500 μg/dry g were considered 

(Figure 13.1a and 13.1c) at GB 602. These data suggest that Bathycletopsyllus sp. responds to 

disturbance/hydrocarbons by increasing in population size, but only up to a threshold 

concentration of SBF hydrocarbons, then this species decreases in number. These data must be 

interpreted with caution, however, because no information is available on the biology of this 

copepod species, let alone its tolerance to contaminants. It is possible that SBF concentrations at 

values below the threshold may serve as an indicator of physical disturbance to which 

Bathycletopsyllus responds. For example, a change in sediment granulometry may accompany 

the increased SBF concentrations associated with drilling activities. Laboratory experimentation 

and additional field sampling are necessary to determine cause-and-effect relationships. 

13.4.2 Genetic Divergence 

Three discreet COX I lineages (BI, BII, and BIII) were observed in Bathycletopsyllus sp. 

populations from both GB 602 and MC 292. Clade BI was separated from Clades BII and BIII 

by three nucleotide differences, and Clades II and III were separated from each other by a 

minimum of six nucleotide differences. The maximum uncorrected difference among haplotypes 

was 1.6%, and the average uncorrected difference was 0.5%, which is within the range expected 

for intraspecific variability of the COX I gene. Therefore, the individuals sampled likely 

represent two populations of a single species and not a complex of cryptic species. The low level 

of divergence was surprising given the distance separating the two populations (407 km) and the 

usually low dispersal capabilities of harpacticoid copepods. High levels of divergence have been 

observed in other harpacticoid species over similar or even smaller spatial scales. For example, 

Rocha-Olivares et al. (2001) noted uncorrected genetic distances of up to 25% at the COX I gene 

in population comparisons of four populations of the morphologically described species 

Cletocamptus deitersi. Two of these diverged lineages are sympatric in populations from 

Louisiana and Alabama. Closer examination of the four genetically diverged lineages has 

revealed morphological differences, and they have been described as four distinct species 

(Gómez et al. in press). High levels of genetic differentiation at the COX I locus also have been 

noted in Tigriopus californicus populations from southern California over relatively small spatial 

scales (Burton and Lee 1994; Burton 1998; Edmands 2001). Burton (1998) has shown that 

T. californicus populations inhabiting the north and south sides of Santa Cruz Island exhibit as 

much as 22% sequence divergence at the COX I locus. In a genetic survey of Microarthridion 

littorale populations from South Carolina, Georgia, Florida, and Louisiana, Schizas et al. (1999) 

found high genetic divergence at another mitochondrial gene, cytochrome b (cyt b). Populations 

from Louisiana showed uncorrected nucleotide divergence of as much as 36%, while 10% 

divergences were observed among populations from South Carolina, Georgia, and Florida. In a 

smaller scale survey of M. littorale populations from South Carolina, three distinct cyt b clades 

that diverged by as much as 4.3% were observed (Schizas et al. 2002). The three cyt b lineages 

were found to co-occur at most of the sampled sites in South Carolina. 

Genetic surveys of benthic invertebrates from the deep sea using mitochondrial genes have also 

revealed high genetic divergence over sometimes small spatial scales in morphologically similar 

taxa. For example, France and Kocher (1996) showed by comparing 16S rRNA sequences that 
454 

populations of the deep-sea amphipod, Eurythenes gryllus, diverged by 10% between depth 

zones. Also using the 16S rRNA gene, Chase et al. (1998) showed that populations of the 

bivalve, Deminucula atacellana, above 2,500 m, diverged from populations below 2,500 m by 

4.1%. In contrast, Quattro et al. (2001), using cyt b sequences, found that populations of the 

deep-sea gastropod, Frigidoalvania brychia, at the same depth level, exhibited nucleotide 

divergences of up to 23%. These highly diverged populations were separated by a maximum 

distance of ≈ 80 km at the depth of 500 m. The levels of nucleotide divergence seen in these 

deep-sea organisms are consistent with divergence levels between species in shallow-water taxa, 

suggesting they may be cryptic species (Etter et al. 1999). 

13.4.3 Population Structure 

The Bathycletopsyllus sp. populations at GB 602 and MC 292 were genetically heterogeneous as 

indicated by the significant fixation indices (Φst = 0.35; Fst = 0.21) in the AMOVA (Table 13.3). 

Examination of the distribution of haplotypes at the two sites indicates an absence of gene flow 

between the two populations (Table 13.2). Most of the individuals sampled from GB 602 were in 

Clade BI, while most of the individuals from MC 292 were in Clades BII and BIII. Harpacticoid 

copepods in general have low dispersal potential because most species brood eggs in an egg sac, 

which then hatch into a benthic naupliar stage (Hicks and Coull 1983). At the present time, the 

type of development possessed by Bathycletopsyllus sp. is unknown; however, females in our 

samples had attached egg sacs (pers. obs.), indicating that this species likely exhibits larval 

development typical of harpacticoid copepods. Given the likely low dispersal potential and the 

distance between the two sites (407 km), the observed genetic heterogeneity was not surprising. 

Other species of harpacticoid copepods have shown genetic heterogeneity among populations, 

sometimes over small geographic distances. Surveys of allozyme variation have revealed 

significant genetic structure between T. californicus populations, separated by as little as 1.5 km, 

along the southern California coast (Burton et al. 1979; Burton and Lee 1994). T. californicus

inhabits tidepools in the supralittoral zone of the rocky intertidal, and dispersal of individuals 

across barriers, such as sandy beaches, is thought to be rare (Burton et al. 1979). The estuarine 

harpacticoid copepod, M. littorale, has also been found to exhibit significant genetic 

differentiation among populations from South Carolina in surveys of the mitochondrial cyt b gene 

(Schizas et al. 2002). M. littorale can potentially be dispersed within estuarine creeks through 

resuspension and advection by tidal currents (Palmer and Gust 1985). However, little is known 

about the transport of estuarine meiofauna between adjacent estuaries, and the genetic data 

indicate it does not frequently occur. 

Determining the underlying genetic structure of a population may be confounded if the locus 

examined has alleles that are selectively advantageous. An allele with a strong selective 

advantage is likely to become fixed in a population despite high levels of gene flow of other, less 

advantageous alleles (Takahata 1991). Schizas et al. (2001) demonstrated that one of three cyt b 

lineages of the harpacticoid copepod, M. littorale, survived in higher numbers when exposed to a 

pesticide treatment. Frequencies of this lineage are highest in M. littorale populations inhabiting 

contaminated estuaries compared to relatively pristine estuaries (Schizas et al. 2001; Schizas et al. 

2002). In the present study, levels of SBF hydrocarbons were, on average, an order of magnitude 

higher at the GB 602 site compared to the MC 292 site (see Chapter 9). Bathycletopsyllus sp. at 
455 

GB 602 was dominated by the BI lineage of the COX I gene, whereas a more even distribution of 

all three lineages was observed at the MC 292 site. Therefore, it is plausible that individuals 

possessing a haplotype in the BI lineage had a selective advantage over the other lineages, 

although further study is needed to test this hypothesis. 

13.4.4 Genetic Diversity 

Levels of haplotype diversity and nucleotide diversity observed in Bathycletopsyllus sp. 

populations differed between the two sites: GB 602 (h = 0.61, SD = 0.089; πn = 0.0020, 

SD = 0.0014); MC 292 (h = 0.89, SD = 0.033; πn = 0.0066, SD = 0.0038). These values of 

haplotype and nucleotide diversity fall within the range of values observed for other harpacticoid 

copepod species that live in muddy habitats similar to Bathycletopsyllus sp. (see Figure 13.3). 

For the population at GB 602, genetic diversities, both haplotype and nucleotide diversities, were 

moderate to low. In contrast, the population at MC 292 had high haplotype diversity and low to 

moderate nucleotide diversity. 

Several factors may contribute to the patterns of genetic diversity observed in the 

Bathycletopsyllus sp. populations, including short- and long-term demographic processes, gene 

flow, and natural selection. The general demographic history of a population can be inferred by 

examination of haplotype and nucleotide diversities (Bowen and Grant 1997; Avise 2000). High 

haplotype and nucleotide diversities are usually observed in populations that have remained 

constant in size over a long time period, thus allowing for the accumulation of alleles through 

mutation. Low values of haplotype and/or nucleotide diversities signal that a population has 

undergone population fluctuations and alleles have been lost due to genetic drift while population 

sizes were small. Migration among populations may increase or decrease genetic diversity 

depending on the number of migrants between populations and the frequencies of alleles in the 

populations. Natural selection may decrease genetic diversity in a population by the differential 

survival of individuals possessing a favorable genotype. Each of these mechanisms may explain 

the observed genetic diversity in Bathycletopsyllus sp. at the two sites. 

The haplotype diversity and nucleotide diversity for the population at the GB 602 site were low, 

and the values were comparable to those seen in three species of harpacticoids in the genus 

Cletocamptus (data from Rocha-Olivares et al. 2001; Figure 13.3). C. deborahdexterae, which is 

found in the Salton Sea, CA, had the lowest calculated haplotype diversity (h = 0.32) and a low 

nucleotide diversity (πn = 0.0038). The low genetic diversities seen in this population are likely a 

result of the recent creation of the Salton Sea (in 1905) and its colonization by a small founder 

population. C. stimpsoni, collected from an inland brine seep in Alabama, also had low genetic 

diversity (h = 0.43; πn = 0.00067). This brine seep is subject to periodic flood-drought cycles 

causing extreme fluctuations in salinity and water level/flooding. Species in the genus 

Cletocamptus have been shown to increase population sizes in response to increases in salinity 

(Dexter 1995; Simpson et al. 1998). Therefore, it is plausible that the brine seep populations 

undergo demographic fluctuations, which cause reductions in genetic diversity. Laboratory 

microcosm experiments have demonstrated that C. fourchensis increased in abundance compared 

to other harpacticoid species in sediments contaminated with diesel fuel (Carman et al. 1997). 

This species has subsequently been found to be tolerant of heavy metal contamination (Fleeger et 

al. 2001) and may increase in numbers due to a release from competition with other meiofauna 
456 

(Carman et al. 1997). Species in the genus Cletocamptus seem to undergo fluctuations in 

population size caused by environmental factors, which may explain the low genetic diversity 

observed in these populations. Processes similar to those affecting genetic diversity in 

Cletocamptus may have occurred in the Bathycletopsyllus sp. population at GB 602. 

Selection for individuals from a lineage that is tolerant to contamination may also result in the 

low genetic diversity observed in the population at GB 602. Different intraspecific (Schizas et al. 

2001) and interspecific (Fleeger et al. 2001) mitochondrial lineages of harpacticoid copepods 

have been shown to tolerate exposure to contaminants. This apparent selection for certain 

haplotypes would cause the elimination of other less tolerant types, thus reducing the genetic 

diversity. Since mitochondrial genes have no known detoxification functions, it is difficult to put 

forward a mechanistic explanation for how selection acts on the mtDNA molecule itself; 

however, selection acting on nuclear loci may constrain mtDNA variation (Kilpatrick and Rand 

1995). 

The Bathycletopsyllus sp. population at the MC 292 site had high haplotype diversity and low to 

moderate levels of nucleotide diversity compared to the other harpacticoid species. This pattern 

of genetic diversity suggests that the population at MC 292 also may have undergone fluctuations 

in numbers, but not to the same extent as at GB 602 (Bowen and Grant 1997; Avise 2000). There 

are three likely explanations to account for the observed genetic diversity at MC 292. First, the 

source populations contributing to the expansion at MC 292 may have been larger, but still 

unobserved in our sampling, compared to the GB 602 site. With less dramatic population 

fluctuations, there would be a decreased chance of losing rare haplotypes, resulting in higher 

haplotype diversity. Second, the population at MC 292 may be an admixture from two or more 

populations that diverged in the past and have recently come into contact. Individuals falling into 

Clades BII and BIII have the highest frequency at the MC 292 site, and Figure 13.2 shows that 

these are the most divergent clades. The maximum P-distance between haplotypes in these two 

clades is 1.6%. Assuming a mutation rate at the COX I gene of 1.4%/Myr (Knowlton and Weigt 

1998) to 2.4%/Myr (Knowlton et al. 1993), the haplotypes observed at MC 292 have diverged 

over 0.67 to 1.1 Myr. Given the apparent patchy distribution of this species, the most 

parsimonious explanation is that the population at MC 292 has resulted from secondary contact of 

two allopatric populations. Lastly, as mentioned above, selective pressure may be reduced at 

MC 292 because of the decreased level of disturbance, thus allowing more haplotypes to coexist 

in this habitat. 

A way of visualizing genetic diversity in a population is to plot the mismatch distribution, or the 

frequencies of the observed number of differences between pairs of haplotypes. The shape of this 

distribution is affected by the demographic history of a population. A multimodal distribution 

indicates a population that has been at demographic equilibrium for a long period of time, and a 

unimodal distribution indicates a population that has undergone a sudden bottleneck or expansion 

(Slatkin and Hudson 1991; Rogers and Harpending 1992). Figure 13.4 shows that the mismatch 

distributions at the two sites differ in shape: GB 602 is unimodal and MC 292 is multimodal, but 

still not significantly different from a sudden population expansion model. Again, alternative 

hypotheses can be used to explain the shapes of the observed mismatch distributions, such as 

natural selection (GB 602) and admixture of diverged populations (MC 292). 
457 

13.5 CONCLUSIONS 

The present study suggests that the harpacticoid copepod, Bathycletopsyllus sp., responds to 

disturbance caused by oil drilling in the deep sea. Three lines of evidence support this 

conclusion. First, Bathycletopsyllus sp. was absent from far-field sites but present in large 

numbers at near-field sites. Second, the abundances were related to concentration of SBF in the 

sediments up to a concentration of about 2,500 μg/dry g. Third, the low genetic diversity 

observed in both Bathycletopsyllus sp. populations was consistent with what would be expected 

from a population that underwent an expansion from a small population size. Only two drilling 

sites were sampled in the present study, so our conclusions are tentative at this time. 

Further study of Bathycletopsyllus sp. is warranted to help explain the high abundance of this 

species near the drilling sites. For example, this species may be common at natural hydrocarbon 

seeps in the northern Gulf of Mexico and therefore may have evolved a tolerance to 

hydrocarbons. Additionally, other disturbances in the deep sea, which lead to localized carbon 

enrichment in the sediment (e.g., whale carcasses and phytoplankton settlement events), may 

provide suitable environmental conditions for opportunistic colonization by this species. If 

further sampling at other deep-sea oil platforms and organically enriched habitats shows similar 

results, then Bathycletopsyllus sp. may prove to be a useful marine pollution indicator species in 

the deep sea. 
459 

Chapter 14 

Sediment Toxicity 

Roy Kropp, Battelle Marine Sciences Laboratory 

and Jerry Neff, Battelle Duxbury 

14.1 INTRODUCTION 

Acute solid-phase (sediment) toxicity tests were performed with near-field and far-field 

sediments from two post-development sites – Garden Banks 602 (GB 602) and Mississippi 

Canyon Block 292 (MC 292). The tests were conducted according to standard USEPA and 

ASTM protocols. 

14.2 MATERIALS AND METHODS 

14.2.1 Sediment Sampling Locations 

Surficial sediments were collected for laboratory sediment toxicity tests from GB 602 and 

MC 292 during Cruise 2B (8 to 25 July 2001). Samples were collected from 12 randomly 

selected near-field stations and 12 far-field stations at each site (see Chapter 2). Site 

characteristics are summarized in Table 14.1. Near-field stations were within 500 m of the 

discharge site; far-field stations were 10 to 25 km from the discharges. 

14.2.2 Test Organism Collection 

One species of test animal, the estuarine amphipod Leptocheirus plumulosus, was used to 

evaluate the toxicity of test sediments. Although not a deepwater species, this amphipod is useful 

as a surrogate because it is easily cultured, exhibits high survival under a variety of environmental 

test conditions, and displays sensitivity to reference toxicants similar to those of other amphipods. 

The amphipods were obtained from in-house cultures maintained at the Battelle Marine Sciences 

Laboratory (MSL) and from Chesapeake Cultures, Hayes, Virginia. Organisms exhibiting 

abnormal behavior or appearance were not used in toxicity tests. 

Table 14.1. Characteristics of the two development drilling sites from which sediments for solid 

phase toxicity testing were collected. 

Development Drilling Site Parameter 

Garden Banks 602 Mississippi Canyon 292 

Water depth (m) 1,125 1,034 

Distance to Mississippi River (km) 370 69 

Wells within 5-km radius 7 5 

Time since last well drilled (months) 6 24 

Total mud and cuttings discharge (bbl) 159,302 120,003 

Synthetic based fluid types IO, LAO, ester IO 

IO = internal olefin; LAO = linear-α-olefin. 
460 

14.2.3 Sediment Sample Preparation

Sediments were received at MSL on 31 July 2001. Cooler temperatures, which were measured 

upon arrival, ranged from 1.2oC to 4.7oC. All of the jars were in good condition. Laboratory 

control sediment, which is used to validate the test, was collected from Sequim Bay, WA. Prior 

to its use, the laboratory control sediment was pressed through a 0.5-mm-mesh sieve to remove 

any live organisms that might interfere with the test results. All sediment samples were stored in 

a walk-in cold room at 4oC ± 2oC until used in toxicity tests. 

14.2.4 Toxicity Test 

Acute toxicity testing was performed in accordance with guidance in USEPA (1994, 2001a) and 

ASTM (1999). The tests used subadult L. plumulosus in a 10-day static exposure with mortality 

as the endpoint. The animals were exposed to 48 Gulf of Mexico test sediments and a laboratory 

control sediment. Because of the number of sediments to be tested, the toxicity test was 

conducted in two batches of 24 samples. Each batch included a laboratory control sample and a 

concurrent reference toxicant test. Batch 1 included sediments from MC 292 and test animals 

from MSL in-house cultures. Batch 2 included sediments from GB 602 and test animals obtained 

from Chesapeake Cultures on 16 August 2001. 

After preliminary water-quality measurements of the conditions (temperature, pH, dissolved 

oxygen, and salinity) in each container were made, the test was initiated by introducing 

20 organisms into each test chamber. During the test, these water quality parameters were 

measured in one replicate on days 2, 4, 6, and 8, and were measured in all replicates at test 

termination. Organisms were randomly allocated to treatments, and treatment replicates were 

randomly positioned on water tables (Figure 14.1). Random positions for test chambers were 

assigned using the discrete random number generator in Microsoft Excel 97® spreadsheet 

software. Amphipods were not fed during the test. Each batch of the bioassay included a 

concurrent, 96-h, water-only, reference toxicant (cadmium chloride at seven concentrations) test 

to assess the sensitivity of each test population. Specific conditions for the benthic toxicity test 

are summarized in Table 14.2. 
Figure 14.1. Sediment toxicity test system. 

Table 14.2. Toxicity test conditions. 

Parameter Test Conditions 

Treatments: 48 Gulf of Mexico sediments (2 batches of 24) 

1 laboratory control (Sequim Bay) sediment with 

each batch 

Replicates: 5 

Test population per replicate: 20 individuals, total per treatment, n = 100 

Temperature: 25° ± 2°C 

Dissolved Oxygen: > 50% saturation (> 3.6 mg/mL at 25°C, 20‰) 

pH: 7.8 ± 0.5 

Salinity: 20‰ ± 2‰ 

Feeding: None 

Reference Toxicant: 

(1 concurrent with each batch) 

Cadmium chloride at 0, 0.156, 0.312, 0.625, 1.25, 

2.5, 5.0, and 7.0 mg/L 

14.3 RESULTS 

Results of toxicity tests are summarized here. Test endpoint data for individual replicates and 

reference toxicant test data, water quality data summaries, and a sediment receipt summary are in 

Appendices J1 and J2. 

14.3.1 Mississippi Canyon Block 292 

Toxicity tests with near-field and far-field sediments from MC 292 were initiated on 7 August 

2001, using amphipods purchased from MSL in-house cultures, and completed 17 August 2001. 

During the 10-day test, most water quality parameters remained within the required ranges. 

Dissolved oxygen values for most replicates exceeded the hypothetical maximum concentration 

value on Day 0 and Day 10 of the test (Appendix J1). At test completion, three test chambers 

contained more than the intended stocking density (n = 20) of amphipods (Table 14.3). These 

samples and the assumed stocking densities are shown in the table. 

461 
462 

Table 14.3. Stocking density for test chambers found at test completion to have more than 

20 amphipods. 

Sediment Replicate 

Number of 

Amphipods at 

Termination 

Assumed 

Stocking Density 

Survival Using 

Assumed 

Stocking Density 

MC292-NF-B06 1 21 21 1.00 

MC292-FF2-B02 2 24 25 0.96 

MC292-FF4-B02 2 38 40 0.95 

Fractional survival of amphipods in the laboratory control sediments tested concurrently with 

MC 292 sediments was 0.99, exceeding the acceptability criterion of 0.90 survival. Mean 

fractional survival among the test sediments ranged from 0.41 to 0.99 (Table 14.4; Appendix J1). 

Mean fractional survival of amphipods in replicate chambers containing sediments from the 

near-field stations (0.76 ± 0.21) was significantly lower than mean survival among replicates 

from far-field stations (0.93 ± 0.08). Mean fractional survival in all near-field and far-field 

samples was 0.84 ± 0.15. 

Table 14.4. Amphipod survival in the toxicity tests with sediments from 

Mississippi Canyon Block 292, performed 7 to 17 August 2001. 

Proportion Surviving Sediment Mean Standard Deviation CV 

Laboratory Control 0.99 0.02 2 

MC292-NF-B01 0.47 0.22 46 

MC292-NF-B02 0.41 0.18 43 

MC292-NF-B03 0.84 0.11 14 

MC292-NF-B04 0.84 0.10 11 

MC292-NF-B05 0.71 0.12 18 

MC292-NF-B06 0.94 0.08 9 

MC292-NF-B07 0.92 0.04 5 

MC292-NF-B08 0.71 0.13 19 

MC292-NF-B09 0.74 0.22 30 

MC292-NF-B10 0.71 0.25 36 

MC292-NF-B11 0.96 0.02 2 

MC292-NF-B12 0.81 0.13 16 

MC292-FF1-B01 0.92 0.10 11 

MC292-FF1-B02 0.93 0.10 11 

MC292-FF2-B01 0.92 0.08 8 

MC292-FF2-B02 0.89 0.11 12 

MC292-FF3-B01 0.94 0.04 4 

MC292-FF3-B02 0.88 0.16 18 

MC292-FF4-B01 0.93 0.10 10 

MC292-FF4-B02 0.98 0.03 3 

MC292-FF5-B01 0.99 0.02 2 

MC292-FF5-B02 0.91 0.09 10 

MC292-FF6-B01 0.94 0.04 4 

MC292-FF6-B02 0.97 0.04 5 

CV = coefficient of variation; NF = Near-Field; FF = Far-Field. 
463 

The reference toxicant (cadmium) test was conducted 7 to 11 August 2001. The LC50 for 

cadmium was 0.64 mg/L (Appendix J1), which was within the control chart range (mean 

± 2 standard deviations) calculated for the species during the previous 20 tests at MSL (0.05 to 

1.38 mg/L) indicating that the test population was about as sensitive as previous populations 

tested. 

14.3.2 Garden Banks Block 602 

Benthic toxicity tests of sediments from GB 602 were initiated on 20 August 2001 using 

amphipods purchased from Chesapeake Cultures, Inc., and was completed 30 August 2001. 

During the 10-day test, most water quality parameters remained within the required ranges. 

Dissolved oxygen concentrations for most replicates exceeded the hypothetical maximum 

concentration value on Day 0 of the test (Appendix J2). Some values were above this value on 

Days 2 and 4. The dissolved oxygen concentration in one test chamber (sediment 

GB602-NF-B08, replicate 2) was below the minimum target value on Day 0; aeration in this 

chamber was increased slightly. All dissolved oxygen concentrations were above the minimum 

limit for the remainder of the test. 

Fractional survival of amphipods in the laboratory control sediments during tests with GB 602 

sediments was 1.0, which exceeds the acceptability criterion of 0.90 control survival. Mean 

fractional survival among the test sediments ranged from 0 to 0.81 (Table 14.5; Appendix J2). 

The reference toxicant (cadmium chloride) test was performed 20 to 24 August 2001. The 

calculated LC50 was 0.57 mg/L (Appendix J2), which was within the normal range (mean 

± 2 standard deviations) calculated for the species during the previous 20 tests at MSL (0.11 to 

1.36 mg/L). However, fractional control survival was 0.45; therefore, the results of this reference 

toxicant test may not provide an accurate description of the test population’s sensitivity. 

Mean fractional survival among the test sediments ranged from 0 to 0.81 (Table 14.5; 

Appendix J2). Mean fractional survival of amphipods in replicate chambers containing sediments 

from the near-field stations (0.17 ± 0.18) was significantly lower than mean survival among 

replicates from far-field stations (0.68 ± 0.13). Mean fractional survival in all near-field and 

far-field sediments was 0.42 ± 0.28. Amphipod survival in both near-field and far-field 

sediments from GB 602 was much lower than survival in sediments from MC 292. This may 

have been due in part to the poorer condition of the amphipods used in the tests with GB 602 

sediments than those used in tests with MC 292 sediments. 
464 

Table 14.5. Amphipod survival in the toxicity tests with sediments from 

Garden Banks Block 602, performed 20 to 30 August 2001. 

Proportion Surviving Sediment Mean Standard Deviation CV 

Laboratory Control 1.00 0.00 – 

GB602-NF-B01 0.36 0.24 66 

GB602-NF-B02 0.41 0.12 30 

GB602-NF-B03 0.27 0.20 75 

GB602-NF-B04 0.14 0.10 69 

GB602-NF-B05 0.20 0.29 146 

GB602-NF-B06 0.04 0.06 161 

GB602-NF-B07 0 0 – 

GB602-NF-B08 0 0 – 

GB602-NF-B09 0.08 0.13 163 

GB602-NF-B10 0.20 0.12 59 

GB602-NF-B11 0.08 0.06 71 

GB602-NF-B12 0.23 0.10 45 

GB602-FF1-B01 0.71 0.11 16 

GB602-FF1-B02 0.59 0.19 32 

GB602-FF2-B01 0.64 0.13 20 

GB602-FF2-B02 0.66 0.16 24 

GB602-FF3-B01 0.73 0.06 8 

GB602-FF3-B02 0.74 0.08 11 

GB602-FF4-B01 0.67 0.07 10 

GB602-FF4-B02 0.74 0.05 7 

GB602-FF5-B01 0.53 0.14 27 

GB602-FF5-B02 0.81 0.07 8 

GB602-FF6-B01 0.74 0.12 16 

GB602-FF6-B02 0.62 0.20 33 

CV = coefficient of variation; NF = Near-Field; FF = Far-Field. 

14.4 DISCUSSION 

14.4.1 Mississippi Canyon Block 292 

A total of about 120,000 bbl (19.1 million liters) of drilling muds and cuttings had been 

discharged from the drilling operations at MC 292 before the sampling cruise (Table 14.1). The 

total included about 10,331 bbl of water based mud and 1,409 bbl of cuttings discharged directly 

to the sea floor before the risers were in place and about 93,960 bbl of water based drilling mud 

and 12,813 bbl of associated cuttings from the platform after the riser was in place. About 

1,490 bbl of SBM cuttings were discharged, with about 10% internal olefin (Novaplus) adhering 

to the cuttings. The last discharge was about 24 months before the sampling cruise. 

Sediments from two near-field stations, NF-B01 and NF-B02, were moderately toxic to the 

amphipods (fraction surviving ranged from 0.15 to 0.85 among 10 replicate test containers). Four 

far-field sediment replicates had fractional survival between 0.60 and 0.75. Thus, sediments from 

just two near-field stations can be considered moderately toxic to benthic amphipods. Station 

NF-B01 was in an area mapped geophysically as cuttings, but Station NF-B02 was not. 
There was not a clear relationship between sediment toxicity at MC 292 and concentrations of 

metals in sediments (Chapter 8). The strongest relationship (r2

 = 0.50) was between sediment 

barium concentration and mean fraction survival of amphipods in near-field and far-field 

sediments (Figure 14.2). The sediment in which fractional survival was 0.47 contained 

39,700 mg/kg barium; however, the sediment with a fractional survival of 0.41 contained 

8,040 mg/kg barium. Regressions for all other metals had lower r2

 values than that for barium. 

MC 292 - Survival vs. Barium

y = -40676x + 39217

r 2 = 0.50

0

5000

10000

15000

20000

25000

30000

35000

40000

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Fraction Surviving

Ba Concentration (mg/kg)

Figure 14.2. Regression of barium concentration against fraction of amphipods 

surviving in a 10-day sediment toxicity test with near-field and far-field 

sediments from Mississippi Canyon Block 292. r2

 = 0.50. 

There was a slightly stronger relationship (r2

 = 0.60) between the concentration of synthetic based 

fluids (SBF) in sediment and toxicity to benthic amphipods (Figure 14.3). Mean amphipod 

fractional survival was less than 0.74 at nearly all locations where sediment SBF concentration 

was higher than about 100 mg/kg. Sediments in which mean amphipod fractional survival was 

0.41 and 0.47 contained 782 mg/kg and 2,408 mg/kg SBF, respectively. Two far-field sediments 

contained more than 15 mg/kg SBF. However, SBF was measured as total resolved and 

unresolved hydrocarbons in the boiling point range of the synthetic base fluid (Chapter 9). Thus, 

some of the material in these samples may be hydrocarbons or extractable organic chemicals from 

other sources than SBM cuttings. Water-based muds sometimes contain small amounts of 

petroleum hydrocarbons from lubricity additives (e.g., Mentor 26, gilsonite) added to the mud. 

465 
Fraction Surviving

MC 292 - Survival vs. SBF

y = -2728.6x + 2512.3

r

2

 = 0.60

0

500

1000

1500

2000

2500

0.3 0.4 0.5 0.6 0.7 0.8 0.9

SBF Concentration (mg/kg)

1

Figure 14.3. Regression of synthetic based fluid (SBF) concentration against 

fraction of amphipods surviving in a 10-day sediment toxicity test 

with near-field and far-field sediments from Mississippi Canyon 

Block 292. r2

 = 0.60. 

Drilling muds and cuttings usually do not contain more than traces of polycyclic aromatic 

hydrocarbons (PAH), unless a diesel fuel pill was added to the mud system to free stuck pipe, or 

formation oil got into the mud system. Oil-contaminated drilling muds and cuttings are not 

permitted for ocean discharge in the Gulf of Mexico. Near-field sediments at MC 292 contained 

174 to 743 μg/kg total PAH. The highest concentration was in sediments from Station NF-B01 

where mean fractional amphipod survival was 0.47. Far-field sediments actually contained 

higher PAH concentrations than near-field stations did (concentration range: 210 to 748 μg/kg). 

The regression of mean fractional amphipod survival against PAH concentration had an r2

 of 

0.03, indicating no relationship between PAH concentration in sediment and amphipod survival. 

Mean fractional amphipod survival in the sediment containing 748 μg/kg PAH was 0.95, 

indicating that PAH probably were not causing the sediment toxicity. 

14.4.2 Garden Banks 602 

A total of nearly 160,000 bbl (25.4 million liters) of drilling muds and cuttings had been 

discharged from the drilling operations at GB 602 before the sampling cruise (Table 14.1). The 

total included about 23,946 bbl of water based mud and 3,266 bbl of cuttings discharged directly 

to the sea floor before the risers were in place and about 91,301 bbl of water based drilling mud 

and 12,450 bbl of associated cuttings discharged from the platform after the riser was in place. 

About 28,339 bbl of SBM cuttings were discharged, with about 10% internal olefin, 

linear-α-olefin, or ester adhering to the cuttings. The last discharge was about 6 months before 

the sampling cruise. Because of the more recent and larger mass of mud and cuttings discharged 

466 
at GB 602 than at MC 292, it was expected that impacts on the benthic environment would be 

more severe. 

Amphipod survival was low in nearly all replicate near-field sediment samples from GB 602. 

Amphipod fractional survival in sediment replicates ranged from 0 to 0.75. There were only four 

sediment replicates with amphipod fractional survival greater than 0.50. 

The far-field sediments from GB 602 were moderately toxic to the amphipods. Amphipod 

fractional survival in replicate far-field sediment samples ranged from 0.30 to 0.90. This poor 

survival, plus the poor survival among amphipods exposed to control sediments indicates that the 

amphipods may have been in poor condition at the start of the bioassay. However, near-field 

sediments clearly were more toxic than far-field sediments at GB 602. 

There was a moderate relationship (r2

 = 0.57) between barium concentration and mean fraction 

surviving in sediments at near-field and far-field stations at GB 602 (Figure 14.4). The 

relationship between barium concentration and sediment toxicity was stronger for GB 602 than 

for MC 292. 

GB 602 - Survival vs. Barium

y = -89739x + 63914

r 2 = 0.57

0

20000

40000

60000

80000

100000

120000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fraction Surviving

Ba Concentration (mg/kg)

Figure 14.4. Regression of barium concentration against fraction of amphipods 

surviving in a 10-day sediment toxicity test in near-field and far-field 

sediments at Garden Banks Block 602. r2

 = 0.57. 

467 
The relationship between amphipod survival and SBF concentrations in near-field and far-field 

sediments at GB 602 is weaker than the relationship between barium concentration and survival 

(Figure 14.5). The r2

 for the regression of SBF concentration versus fraction amphipod survival 

is 0.35. 

There also was a weak relationship between PAH concentration in sediments and amphipod 

survival (Figure 14.6). The r2

 for this regression is 0.40. Near-field sediments contained higher 

PAH concentrations (326 ± 152 ng/g) than far-field stations did (122 ± 17 ng/g). Overall, these 

results suggest that there is a relationship between toxicity to benthic amphipods and 

concentrations of drilling mud and cuttings in sediments, as indicated by elevated concentrations 

of barium (from drilling mud barite), SBF, and PAH. 

GB 602 - Survival vs. SBF

y = -6716.8x + 4542.4

r 2 = 0.35

0

2000

4000

6000

8000

10000

12000

14000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fraction Surviving

SBF Concentration (mg/kg)

Figure 14.5. Regression of synthetic based fluid (SBF) concentration against 

fraction of amphipods surviving in a 10-day sediment toxicity 

test with near-field and far-field sediments from Garden Banks 

Block 602. r2

 = 0.35. 

468 
GB 602 - Survival vs. PAH

y = -339.53x + 367.72

r 2 = 0.40

0

100

200

300

400

500

600

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fraction Surviving

PAH Concentration (ng/g)

Figure 14.6. Regression of polycyclic aromatic hydrocarbon (PAH) concentration 

against fraction of amphipods surviving in a 10-day sediment toxicity test 

with near-field and far-field sediments from Garden Banks Block 602. 

r

2

 = 0.40. 

14.5 CONCLUSIONS 

Acute solid-phase (sediment) toxicity tests were performed with near-field and far-field 

sediments from two post-development sites, MC 292 and GB 602. The tests were conducted 

according to standard USEPA and ASTM protocols. The principal conclusions are as follows: 

• At both MC 292 and GB 602, mean amphipod survival was significantly lower in sediments 

from near-field stations than in sediments from far-field stations. 

• Mean amphipod survival was lower at GB 602 than at MC 292 in both near-field and far-field 

sediments, possibly indicating that the amphipods used for the GB 602 tests were in relatively 

poor condition at the start. 

• Mean amphipod survival was negatively correlated with drilling mud and cuttings indicators 

(barium and SBF) for both sites. The highest correlations were with SBF at MC 292 

(r2

 = 0.60) and barium at GB 602 (r2 = 0.57). 

• The toxicity of near-field sediments apparently was unrelated to sediment total PAH 

concentrations at MC 292, but there may have been a weak negative relationship at GB 602. 

Sediment toxicity is discussed further in the Ecological Risk Assessment (Chapter 16). 

469 
471 

Chapter 15 

Synthesis 

Neal W. Phillips and Alan D. Hart 

Continental Shelf Associates, Inc. 

15.1 INTRODUCTION 

This study focused on four sites on the northern Gulf of Mexico continental slope in water depths 

of 1,033 to 1,125 m. Two sites were in the Western Planning Area (GB 516 and GB 602), and 

the other two were in the eastern part of the Central Planning Area (VK 916 and MC 292). 

The six cruises provide insight into impacts of both exploration and development. Two sites were 

sampled post-exploration (VK 916 on Cruises 3A/3B; GB 516 on Cruise 1B). Three sites were 

sampled post-development (GB 516, GB 602, and MC 292, all on Cruises 2A/2B). Both WBMs 

and SBMs were used in drilling at these sites. 

The general objectives of this study were to assess the physical, chemical, and biological impacts 

of oil and gas development at selected exploration and development wellsites on the Gulf of 

Mexico continental slope. Specific objectives were to document (1) drilling mud and cuttings 

accumulations; (2) physical modification/disturbance of the seabed due to anchors and their 

mooring systems; (3) debris accumulations; (4) physical/chemical modification of sediments; and 

(5) effects on benthic organisms. 

This chapter synthesizes findings from the preceding chapters and focuses on several topics: 

• Types of impacts and their areal extent; 

• Variations in impacts among sites and relationships to level of drilling activity; 

• Duration/persistence of impacts; and 

• Methodology evaluation. 

There have been few previous environmental studies of drilling impacts on the Gulf of Mexico 

continental slope. A post-drilling survey of a development site (Pompano) in MC Block 28 

(water depth 565 m) was reported by Gallaway et al. (1997) and Fechhelm et al. (1999). More 

recently, a comprehensive study of SBM impacts was conducted at several continental slope and 

shelf sites in the Gulf of Mexico (Continental Shelf Associates, Inc. 2004). The latter study is 

referred to as the SBM monitoring program. Findings from these studies are cited where 

appropriate in this synthesis. 

15.1.1 Baseline Conditions 

“Baseline” characteristics of the four sites can be summarized based on far-field data. All of the 

sites are located on the northern Gulf of Mexico continental slope and are similar with respect to 

the overall environmental setting. However, water depths at GB 516 and MC 292 were about 

120 m shallower than at VK 916 and GB 602 (Table 15.1). 
472 

Table 15.1. Comparison of baseline environmental characteristics based on data from far-field 

sites. 

Sediment Characteristics 

Site 

Water 

Depth 

(m) 

Approx. 

Dist. to 

Miss. R. 

(km) 

Sedimentation 

Rate (cm/yr) 

Mean 

Grain Size 

(μm) 

Mean Total 

Organic 

Carbon (%) 

Mean 

Integrated 

Oxygen 

(nmol/cm2

) 

Mean 

Barium 

(μg/g) 

VK 916 1,125 120 0.06 2.44 

1.82 

1.10 

1.53 

90 

84 

805 

866 

GB 516 1,033 357 0.07 1.96 

2.40 

0.52 

1.08 

153 

165 

1,281 

1,478 

GB 602 1,125 370 0.04 2.03 1.10 227 821 

MC 292 1,034 69 0.14 1.64 1.44 65 754 

The two Garden Banks sites are located in the Western Planning Area and are more than 350 km 

from the Mississippi River mouth (Table 15.1). The other two sites are much closer. The likely 

influence of the Mississippi River discharge on sedimentation rate and sediment oxygen levels 

has been noted in Chapter 8. MC 292, being the closest to the river mouth, has a sedimentation 

rate 2 to 3 times higher than the other sites. Baseline sediment oxygen levels were lower at both 

VK 916 and MC 292 than at the two Garden Banks sites. 

Far-field sediment barium levels were about 50% higher at GB 516 (1,281 to 1,478 μg/g) than at 

the other three sites (754 to 866 μg/g). Background barium concentrations in the Gulf of Mexico 

typically are in the range of 200 to 800 μg/g (Anderson et al. 1981; Trefry et al. 1981). The slight 

elevation at GB 516 may be due to the relatively large number of previous wells in the area 

compared with the nearby GB 602 site (Chapter 3). 

Figure 15.1 shows selected biological data for the far-field sites. Comparisons among sites are 

complicated by temporal differences. At the VK 916 site, there was an order of magnitude 

decrease in microbial biomass as well as meiofaunal and macroinfaunal densities between 

cruises. A similar decrease was evident at GB 516 for microbes and meiofauna but not for 

macroinfauna. 

Excluding the high values for VK 916 and GB 516 on Cruise 1B, nearly all microbial biomass 

(ATP) levels at far-field sites were in the range of 1 to 5 μg/g. Excluding the Cruise 1B samples 

at VK 916, most meiofaunal densities were in the range of 1 to 100 individuals/10 cm2

. Typical 

far-field macroinfaunal densities were a few thousand individuals per square meter, except for 

several stations exceeding 5,000 individuals/m2

 at MC 292 (Cruise 2B) and VK 916 (Cruise 1B). 

Analysis of macroinfaunal data suggests there are some geographic differences in benthic 

community composition (Chapter 11). In the ordination analysis, two groups consisted of 

stations from GB 516 and GB 602, which are very close geographically (stations within tens of 

kilometers), whereas two other groups included stations from VK 916 and/or MC 292, which are 

relatively close to each other but about 400 km from the two Garden Banks sites. 
Figure 15.1. Microbial biomass and meiofaunal and macroinfaunal densities at far-field sites. 

Points are individual stations.

VK 916 GB 516

GB

602

MC

292

Cr.1B Cr.3B Cr.1B Cr.2B

Microbial Biomass (ATP)

0

50

100

150

200

250

Microbial ATP (ng/g)

Total Meiofauna

0

200

400

600

800

1,000

Meiofaunal Density 

(no. per 10 cm2

)

Total Macroinfauna

0

5,000

10,000

15,000

20,000

Macroinfaunal Density 

(no. per m2

)

VK 916 GB 516

GB

602

MC

292

Cr.1B Cr.3B Cr.1B Cr.2B

VK 916 GB 516

GB

602

MC

292

Cr.1B Cr.3B Cr.1B Cr.2B

Far-Field Characteristics

473
474 

In conclusion, the four sites are broadly similar environmentally. Some physical/chemical and 

biological differences are evident between the western (GB 516 and GB 602) and central 

(VK 916 and MC 292) sites. Proximity to the Mississippi River mouth is an important influence 

on ambient conditions, as exemplified by lower sediment oxygen levels at the two central sites 

and the high sedimentation rate at MC 292. Significant temporal changes unrelated to drilling 

(e.g., in microbial biomass and meiofaunal and macroinfaunal densities) were noted at VK 916 

and GB 516, and therefore, the impact analysis focuses on spatial comparisons (near-field vs. 

far-field). 

15.1.2 Drilling Activities 

The four sites represent a range of drilling activities (Chapter 3). VK 916 is the simplest case, 

with no previous drilling in the near-field site and a single exploration well drilled during this 

study. A semisubmersible drilling rig was used, and one set of anchors was deployed. Both 

WBMs and SBMs were used during drilling. This site was sampled before (Cruises 1A/1B) and 

after (Cruises 3A/3B) exploratory drilling. 

GB 602 and MC 292 are development sites with several previous exploration and development 

wells and no new drilling during this study. Drilling occurred at various times over several years, 

so there were multiple drilling rigs, anchor deployments, and sets of drilling discharges. An 

important difference is that most of the drilling at MC 292 was done with WBM, with SBM used 

only for two sidetracks. Therefore, the total quantity of SBM cuttings discharged at MC 292 was 

the smallest among the four sites. These sites were sampled once during Cruises 2A/2B to 

provide a snapshot of post-development status. 

GB 516 is the most complex case, with two previous exploration wells and five new development 

wells drilled during this study. Overall, activities were similar to those at the other development 

sites (GB 602 and MC 292) in that multiple wells were drilled over time, with multiple anchor 

deployments and drilling discharges; and subsea wellheads, manifolds, and flowlines were 

installed connecting to a remote platform. The difference is that two “snapshots” were obtained – 

once during Cruise 1B after exploration and again on Cruises 2A/2B after development. 

The general sequence of events was similar for most wells (Chapter 3). The initial well interval 

was drilled (jetted) using a water-based spud mud. During this time before the marine riser was 

set, there were no returns to the drilling rig, and all mud and cuttings were released at the 

seafloor. Spud muds consisting primarily of water and bentonite clay were used for this jetting 

process. The wellhead was then connected to a marine riser system that returned muds and 

cuttings to the drilling rig during the remainder of the drilling process. Subsequently, one or 

more well intervals were drilled using WBM, with muds and WBM cuttings discharged from the 

rig. Finally, in most cases, the mud system was switched to SBM for drilling of remaining well 

intervals. In some cases, there were bulk discharges of remaining WBM prior to switching to 

SBM. During SBM intervals, SBM cuttings were discharged, but muds were retained (except for 

those adhering to cuttings). In addition, some quantities of SBMs were lost to the formation, lost 

during casing/cementing operations, or left in the wellbore for future sidetracks. Sidetracks at 

these sites were drilled primarily using SBMs. As noted previously, at MC 292, SBMs were used 

only during drilling of sidetracks. 
475 

Table 15.2 summarizes the number of wells and estimated drilling fluid and cuttings discharges 

for each site. The GB 516 wells and discharges are divided into Exploration and Development. 

Table 15.2. Drilling summary for wells within each near-field site. 

Estimated Seafloor 

Discharges (bbl) 

Estimated Drilling Rig 

Discharges (bbl) Site 

Total No. 

Wells Within 

500-m Radius WBM WBM 

Cuttings WBM WBM Cuttings 

SBM 

Cuttings

SBM Type 

Exploration 

VK 916 1 3,074 419 12,807 1,746 2,510 Syn-Teq (IO) 

GB 516 2 6,501 887 14,731 2,009 1,904 Novaplus (IO) or 

Petrofree LE (LAO)

Development 

GB 516 7a 13,002 1,774 29,462 4,018 7,313 

Novaplus (IO) and 

possibly Petrofree LE 

(LAO)

GB 602 7 23,946 3,266 91,301 12,450 28,339 

Novadril (IO) or 

Petrofree ester (1 well); 

Novaplus (IO) or 

Petrofree LE (LAO) (all 

others) 

MC 292 5 10,331 1,409 93,960 12,813 1,490 Novaplus (IO) 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; IO = internal olefin; LAO = linear-α-olefin; 

SBM = synthetic-based mud; WBM = water-based mud. a Totals for GB 516 post-development include the two exploration wells listed above. 

The most important quantities in the table include the volume of SBM cuttings, the volume of 

WBM cuttings (including both seafloor and rig discharges), and the volume of WBM discharged 

at the seafloor. These are the discharges most likely to accumulate in detectable deposits within 

the near-field sites. WBM discharges from the rig are assumed to be less important because these 

fine particles would be widely dispersed in the water column while settling to the seafloor 

(Brandsma 1990). 

In comparing discharges at the four sites: 

• Quantities of most discharges were less at the two exploration sites than at the 

post-development sites, due to the smaller number of exploration wells. 

• The GB 602 post-development site had by far the largest quantities of SBM cuttings 

discharged (about four times as much as GB 516). 

• The MC 292 and GB 602 post-development sites had comparable quantities of WBM cuttings 

and mud discharges. The main difference is the much smaller quantity of SBM cuttings 

discharged at MC 292. Most of the drilling at MC 292 was done with WBM, with SBM used 

only for two sidetracks. 

The timing of drilling activities vs. sampling cruises differed among sites. The interval between 

cessation of drilling and the date when the surveys began ranged from 5 months to about 2 years 

depending on the site (see Section 15.3.3). 
476 

In addition to the drilling activity, the three development sites (GB 516, GB 602, and MC 292) 

had structures placed on the seafloor, including subsea wellheads, flowlines and umbilicals, and 

associated equipment. At GB 516, these structures were emplaced during this study, between 

Cruises 1A/1B and 2A/2B. At the other two sites, the structures were installed prior to this study. 

15.2 NATURE AND EXTENT OF IMPACTS 

Table 15.3 summarizes the physical, chemical, and biological impacts noted during this study. 

These are discussed subsequently in individual subsections. 

Two types of information are available concerning the extent of impacts. Geophysical surveys 

(Cruises 1A, 2A, and 3A) were used to map the extent of anchor scars, debris, and drilling 

deposits within a radius of about 3 km around each near-field site. Biological/chemical sampling 

(Cruises 1B, 2B, and 3B) focused on detecting physical, chemical, and biological changes within 

the zone of most likely impact. Within each near-field site, eight stations were within 300 m, and 

the other four were between 300 m and 500 m from the site center. This stratification can be used 

to evaluate the extent of impacts within the near-field. 

15.2.1 Physical 

15.2.1.1 Anchor Scars 

Based on the geophysical surveys, anchor scars extended to the limit of the surveyed area (about 

3-km radius) around the wellsites (Chapter 4). The anchor radius for a drilling rig is a function of 

water depth (typically about three times water depth in this depth range). Individual anchor scars, 

all in soft bottom areas, ranged from less than 100 m to over 3 km in length. 

VK 916 had only one set of anchor scars associated with the exploratory drilling at this site. 

However, other scars remained from previous drilling in adjacent blocks (VK 872 and VK 873). 

The other three sites, surveyed post-development, had several sets of anchor scars, reflecting the 

use of different drilling rigs to drill various wells over time. 

The extent of anchor scars was greater than initially expected. Exploration and development 

plans submitted to the MMS typically show the planned anchor locations and the catenary 

(“touch-down”) point where anchor chains may contact the seafloor once the anchors are 

tensioned from the rig. Based on these diagrams, anchor scars would be expected mainly along 

the outer portion of each anchor line, i.e., between the catenary point and the anchor location (a 

distance of a few hundred meters at this water depth). However, when anchors are being carried 

out to their pre-plotted locations by the anchor-handling vessel, longer portions of the anchor 

cable or chain may temporarily rest on the seafloor. This evidently is the cause of some of the 

longer scars detected by the side-scan sonar. Most of these wells were drilled at least 5 years ago 

and probably did not use taut-line anchoring systems. Such systems, which are now used for 

most deepwater anchoring, consist of polypropylene line and wire, with little or no chain that 

could drag on the seafloor during deployment (J. Hood, Transocean Inc., personal communication 

2006). 
477 

Table 15.3. Impact summary. 

Parameter and 

Chapter Reference Impact Extent Notes 

PHYSICAL 

Anchor scars 

(Chapter 4) 

Presence of anchor scars Within about 3-km 

radius from wellsites, 

8 radial directions 

Individual scars <100 m to 

>3 km in length 

Seafloor equipment 

and debris (Chapter 4) 

Presence of structures 

and debris on bottom 

Subsea wellheads etc. 

within few hundred 

meters; flowlines and 

umbilicals extend 

many kilometers to 

nearby blocks; 

occasional debris 

within 3-km radius 

VK 916 had the smallest 

number of side-scan 

“debris” contacts 

Mappable mud and 

cuttings deposits 

(Chapter 4) 

Presence of 

geophysically detectable 

mud and cuttings zones 

around wellsites 

Well jetting deposits 

mostly within 100 m 

of wellsites. Rig 

discharge deposits 

extending from 

several hundred 

meters to about 1 km 

Areal extent greatest at 

GB 516 and GB 602, 

smallest at VK 916 

Sediment grain size 

(Chapter 5) 

Increased percentage of 

(mainly coarse) sand at 

some stations 

Primarily within 

300-m radius 

Cuttings similar in grain 

size to surficial sediments 

Sediment color/ 

appearance 

(Chapter 12) 

Areas of black bottom 

within near-field 

Extent not 

determined. Most 

consistent at GB 602; 

patchy at other sites 

Black bottom is presumed 

due to the presence of 

cuttings and/or mud 

Thickness of drilling 

deposits (Chapter 4, 

Chapter 8) 

Isopach maps from 

subbottom profiling at 

VK 916 indicate 

thickness ranging from 

0 to 45 cm 

Extent not 

determined 

Sectioning of a few 

discretionary cores at 

VK 916, GB 516, and 

GB 602 indicates layer of 

mud/cuttings several 

centimeters thick. Full 

range of thicknesses was not 

sampled by these cores 

CHEMICAL 

Sediment barium 

(Chapter 8) 

Elevated by orders of 

magnitude at nearly all 

near-field stations 

Nearly all stations 

within 500-m radius; 

highest values within 

300-m radius 

One discretionary station 

1,000 m from GB 602 site 

center had elevated barium; 

far-field data suggest slight 

effects at 2 to 3 km 

Sediment SBF 

(Chapter 9) 

Elevated by orders of 

magnitude at most 

near-field stations 

Most stations within 

500-m radius; nearly 

all stations within 

300-m radius 

One discretionary station 

1,000 m from GB 602 site 

center had elevated SBF; 

far-field data suggest slight 

effects at 2 to 3 km 

Sediment PAH 

(Chapter 9) 

Elevated at two GB 516 

stations 

Within 300-m radius Not attributable to SBM 

since these discharges do 

not contain PAH 
Table 15.3. (continued). 

478 

Parameter and 

Chapter Reference Impact Extent Notes 

Metals other than 

Barium (Chapter 8) 

Elevated concentrations 

of As, Cd, Cr, Cu, Hg, 

Pb, and Zn at some 

stations; lower 

concentrations of Al, Fe, 

Mn, Ni, and V 

Few stations Differences are related to 

concentrations of these 

metals in barite, or reductive 

dissolution in the case of 

Mn 

Sediment TOC 

(Chapter 8) 

Elevated at some 

near-field stations 

Primarily within 

300-m radius 

High TOC is associated 

with high SBF 

concentrations 

Sediment oxygen/Eh 

(Chapter 8) 

Low oxygen and 

negative Eh at some 

near-field sites 

Primarily within 

300-m radius 

Poor redox conditions are 

associated with high SBF 

levels; least severe MC 292 

Redox potential 

discontinuity 

(Chapters 6 and 7) 

Decreased depth of the 

oxidized layer in the 

near-field 

Patchy within 

near-field 

Poor redox conditions are 

associated with high SBF 

levels; least severe MC 292 

Metals in isopods 

Bathynomus giganteus

(Chapter 8) 

Elevated Ba, Cr, and Pb 

in isopods at GB 602 but 

not at MC 292; lower 

Cd and Hg in isopods at 

GB 602, but not at 

MC 292 

Extent not 

determined 

Metals in red crabs 

Chaceon quinquedens

(Chapter 8) 

Elevated Ba and reduced 

As, Cd, Cu, Hg, Ni, V, 

and Zn in crabs at 

GB 602; elevated Ba, 

Cd, Cr, and V in crabs at 

MC 292 

Extent not 

determined 

Most consistent finding is 

elevated barium (in isopods 

from GB 602 and in crabs 

from both GB 602 and 

MC 292). Possibly due to 

sediment particles in gut 

Hydrocarbons in 

isopods and red crabs 

(Chapter 9) 

-- -- 

No impact on tissue PAH 

concentrations detected 

BIOLOGICAL 

Benthic communities 

(general conditions) 

(Chapters 6 and 7) 

Azoic areas; areas of 

pioneering communities; 

poor organism-sediment 

index 

Patchy within 

near-field 

MC 292 appears least 

affected 

Microbiota 

(Chapter 10) 

Elevated microbial 

biomass (ATP) at some 

stations 

Patchy within 

near-field 

Generally, stations with 

high ATP had high SBF 

levels; order of magnitude 

decrease in ATP between 

cruises at VK 916 and 

GB 516 (both near- and 

far-field) 

Microbial mats 

(Chapters 6 and 7) 

Microbial mats seen in 

SPI images from several 

near-field stations 

Patchy within 

near-field 

Mats not seen at MC 292 or 

at GB 516 on Cruise 2B 

Meiofauna 

(Chapter 11) 

Increased meiofaunal 

densities in near-field 

including annelids, 

harpacticoids, and 

nematodes 

Primarily within 

300-m radius 

Order of magnitude 

decrease in meiofauna 

between cruises at VK 916 

and GB 516 
Table 15.3. (continued). 

479 

Parameter and 

Chapter Reference Impact Extent Notes 

Meiofauna - 

harpacticoid genetics 

(Chapter 13) 

Low genetic diversity in 

near-field populations of 

Bathycletopsyllus sp. 

Extent not 

determined 

Bathycletopsyllus sp. absent 

from far-field sites, but 

abundant at near-field sites; 

low genetic diversity is 

consistent with expansion 

from a small population 

Macroinfauna 

(Chapter 11) 

Increased polychaete, 

gastropod, and bivalve 

densities in near-field; 

reduced amphipod and 

ostracod densities 

Primarily within 

300-m radius 

-- 

Megafauna 

(Chapter 12) 

Higher fish densities and 

lower ophiuroid 

densities within 

near-field 

Extent not 

determined 

Effects evident at VK 916 

and GB 516 but not at 

GB 602 or MC 292 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; Al = aluminum; ATP = adenosine triphosphate; 

Ba = barium; Cd = cadmium; Cr = chromium; Cu = copper; Eh = redox potential; Fe = iron; Hg = mercury; Mn = 

manganese; Ni = nickel; PAH = polycyclic aromatic hydrocarbon; Pb = lead; SBF = synthetic-based fluid; ; SBM = 

synthetic-based mud; TOC = total organic carbon; V = vanadium. 

15.2.1.2 Seafloor Equipment and Debris 

At the three sites where development drilling has occurred and production is ongoing (GB 516, 

GB 602, and MC 292), the geophysical survey identified known structures on the seafloor, 

including pipelines and flowlines, subsea wellheads, umbilical termination modules, etc. The 

wellheads and associated equipment were mostly within 200 m of the site centers. Pipelines and 

flowlines extended many kilometers to platforms or other structures in nearby lease blocks. 

At all four sites, there were side-scan sonar contacts identified as debris. These could include 

pieces of pipe, etc. that presumably fell from the drilling rig or other vessels. VK 916 had the 

smallest number of contacts (4), whereas there were numerous contacts at the other sites. Debris 

locations were not investigated further in this study. Visual observations of debris around former 

drillsites in the Gulf of Mexico have been reported by Shinn et al. (1993). 

15.2.1.3 Mappable Cuttings and Mud Deposits 

As noted in Chapter 4, side-scan sonar and chirp sonar subbottom records were used to map 

deposits of drilling mud and cuttings around the wellsites. However, there are no unique 

geophysical signatures for these deposits. The mapped distribution of mud and cuttings was 

inferred from a combination of geophysical data and information about discharges. 

Two zones were recognized. Although these were identified as “drilling muds” and “drill 

cuttings” in the geophysical cruise reports, both zones probably consist of both muds and 

cuttings, and primarily the latter. First, in the area immediately around wellsites, there typically 

was a zone recognized by a combination of a smooth seafloor (little backscatter on side-scan 

sonar records) and a high amplitude response at the seafloor on high resolution subbottom 

profiles. This zone, identified as “drilling muds” in the geophysical cruise reports, probably 
480 

represents an area of cuttings and spud mud deposition from initial jetting of wells. Cuttings 

probably account for most of the materials deposited during this stage, because spud muds are 

primarily seawater with low solids concentrations. Typically, these initial deposits, along with 

excess cement from setting of the well casing, create a shallow, compact mound. Other activities 

including installation of subsea wellheads, flowlines, and umbilical modules at the three 

post-development sites (GB 516, GB 602, and MC 292) (see Chapter 3) may also have 

contributed to the observed reflectance pattern around the wellsites. 

More extensive areas where side-scan sonar showed high reflectivity extending in a radial pattern 

around the wellsites were interpreted as primarily cuttings (with adhering muds) from rig 

discharges. Cuttings are derived from subsurface sediments that are much more compacted and 

therefore denser than surface hemipelagic sediments that drape most of the northern Gulf 

continental slope. When clumps of these dense, coarse sediments are scattered across the seafloor 

by surface discharges, they create an acoustic impedance difference that translates into a higher 

amplitude reflection on side-scan sonar records. Although water-based drilling muds were also 

discharged from the rig at all wellsites, they probably were widely dispersed before reaching the 

seafloor (Brandsma 1990) and would not likely be detected geophysically. Cuttings discharged 

during both WBM and SBM well intervals presumably included adhering muds that sank with the 

cuttings. 

Figure 15.2 shows the mapped distribution of these zones on post-drilling cruises at each of the 

four sites. For simplicity, these zones are referred to as well jetting deposits and rig discharge 

deposits. The only post-exploration map is for VK 916 on Cruise 3A; there was no 

post-exploration geophysical survey at GB 516. Post-development maps are available for 

GB 516, GB 602, and MC 292 from Cruise 2A. 

Generally, areas mapped as well jetting deposits were identified within about 100 m of wellsites. 

Areas mapped as rig discharge deposits typically extended several hundred meters from wellsites, 

with the greatest extent (about 1 km) observed at GB 602 and GB 516. The estimated area of 

mud and cuttings accumulation was greatest at GB 516 (Cruise 2B) and smallest at VK 916 

(Table 15.4). 

Table 15.4. Geophysically estimated areal extent of drilling mud and cuttings deposits at each 

near-field site. Both well jetting deposits and rig discharge deposits are assumed to 

consist of both drilling muds and cuttings (primarily the latter). See text for caveats 

concerning the identification of these zones. 

Site Cruise Stage Well Jetting 

Deposits (ha) 

Rig Discharge 

Deposits (ha) 

VK 916 3A Post-exploration None mapped 13.37 

GB 516 2A Post-development 2.48 108.53 

GB 602 2A Post-development 1.08 43.18 

MC 292 2A Post-development 0.78 25.61 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll.
Figure 15.2. Extent of geophysically mapped drilling mud and cuttings deposits around (a) Viosca Knoll 916 

on Cruise 3A; (b) Garden Banks 516 on Cruise 2A; (c) Garden Banks 602 on Cruise 2A; and 

(d) Mississippi Canyon 292 on Cruise 2A. See text for assumptions and caveats concerning 

mapping of these zones.

500 m radius

VK 916 Cruise 3A 

(post-exploration)

500 m radius

GB 516 Cruise 2A 

(post-development)

Well no. SE002 and

sidetracks ST01 and ST02

Nov 2000-Feb 2001

Well no. SE001 and 

sidetrack ST01

Jul-Aug 1999

Well no. SE001

sidetracks ST02 and ST03

Nov-Dec 2000

Well no. 1

Nov-Dec 2001

Well no. 1

Sep 1995-Jul 1996 Legend

Wellsite

Rig discharge deposits

Well jetting deposits

N

S

W E

N

S

W E

(a)

(b)

481
Figure 15.2. (continued).

500 m radius

MC 292 Cruise 2A 

(post-development) 500 m radius

GB 602 Cruise 2A

(post-development)

Well no. 1

Sep 1995

Well no. 2

+ sidetrack

Sep 95-Mar 1997

Well no. 5

+ sidetrack

Dec 1998-May 1999

Well no. 4

+ sidetrack

Dec 1998-Jan 2001

Well no. 1

(May-Sep 1995)

Well no. 3 and sidetrack

(Feb-Jun 1999)

Well no. 4 and sidetrack

(Feb-Jul 1999)

N

S

W E

N

S

W E

Legend

Wellsite

Rig discharge deposits

Well jetting deposits

(c)

(d)

482
483 

Identification of these mapped zones is not definitive. No “ground truth” sampling or analysis 

was conducted to verify that cuttings or drilling mud particles were present in these zones, or to 

determine the minimum thickness that could be detected. In Chapter 4, it was noted that other 

processes could produce the reflectance patterns mapped as cuttings, including seafloor 

disturbances (grooves, furrows, etc.) caused by the process of setting anchors and the dropping of 

sediment clumps to the seafloor from the anchor cables as they are pulled into place. However, 

several lines of evidence (discussed below) suggest that the geophysically mapped areas are due 

to drilling discharges rather than clumps from anchor cables or other seafloor disturbances. 

Typically, eight anchors are deployed radially around the drilling rig. If the high reflectivity 

areas were related to anchoring (scars, grooves, and clumps), they should be distributed radially. 

However, the high-reflectivity areas are predominantly along a SW to NE or WSW to NNE axis 

and are not lined up with plotted anchor locations (see Figures 15.3 and 15.4 for VK 916 and 

GB 516; the other two side-scan sonar mosaics are presented in Appendix K). The distribution 

tends to be along isobaths, consistent with the expected current patterns along the continental 

slope (Nowlin et al. 2001; Sturges et al. 2004). Also, drilling rigs generally were present at each 

site for only a few months during drilling of each well. Based on what is known about 

colonization of submerged structures, this probably is not long enough to develop a substantial 

fouling biota on the anchor cables, making it unlikely that the observed patterns were created by 

substantial clumps of organic matter falling to the seafloor. In addition, if the deposition were 

caused by clumps from anchor cables, one would expect to see the patterns extending all the way 

out to the anchor locations, whereas the high-reflectively areas are primarily within the 500-m 

near-field radius. 

The association of the mapped deposits with the drilling rig (rather than anchor cables) is also 

evident in the isopach map (Figure 15.5) produced for VK 916. The plot shows thicknesses 

ranging from 0 to 45 cm within the mapped cuttings zone. The thickness of the deposits and the 

proximity to the wellsite strongly suggest that the geophysically mapped zone consists of drilling 

discharges. 

Further confirmation is available from chemical analysis of sediments. Concentrations of barium 

and SBF tended to be higher in areas mapped as drilling deposits (see plots in Section 15.2.2). 

VK 916 shows the clearest pattern, with stations within the zone of rig discharge deposits having 

higher barium and SBF concentrations than those outside the zone. At GB 516, only one station 

was not in the zone of rig discharge deposits, and this station had relatively low barium and SBF 

concentrations. Stations in the zone of well jetting deposits at GB 516 had very high barium 

concentrations, and one station had very high SBF concentrations. Patterns at GB 602 and 

MC 292 were less clear-cut. Overall, the results suggest that the geophysically mapped zones are 

due to drilling discharges, though the exact boundaries do not necessarily correspond to barium 

and SBF contours. 
N

S

W E

500 m radius

VK 872 wellsite

Legend

Wellsite

Anchor location

(approximate)

VK 916 wellsite

Figure 15.3. Side-scan sonar mosaic showing highly reflective seafloor (dark areas, presumed rig 

discharge deposits) at Viosca Knoll 916 on Cruise 3A (post-exploration). Dark areas are 

oriented predominantly WSW to ENE whereas anchors are distributed radially. A similar 

but fainter pattern is evident around a wellsite in VK 872 (drilled about 3.5 years earlier).

484
Figure 15.4. Distribution of highly reflective seafloor (dark areas, presumed rig discharge deposits) at 

Garden Banks 516 on Cruise 2A (post-development). Note that dark areas are 

predominantly oriented WSW to ENE around the wellsite and do not appear to be

associated with anchor locations.

Legend

Wellsite

Anchor location

(approximate)

Area of Complex 

Seafloor Features

(unrelated to drilling)

500 m radius

N

S

W E

485
486 

Figure 15.5. Isopach (thickness) plot of geophysically mapped drilling deposits at Viosca 

Knoll 916. Adapted from the isopach map in Appendix C2. 

15.2.1.4 Sediment Grain Size 

Sediments at all four sites were predominantly clay and silt. There were no major, gross changes 

in sediment grain size distribution around the wellsites (Chapter 5). There were no gravel-sized 

particles (>2 mm) at most stations. On Cruise 3B at VK 916, one station (NF-B01) had 

0.74% gravel associated with high concentrations of barium (171,000 μg/g) and SBF 

(10,283 μg/g). 

A few near-field samples had slightly elevated percentages of sand-sized particles. Most stations 

had <10% sand. Higher percentages were evident at several stations at GB 516 on Cruise 2B 

(up to 18%) and at one MC 292 station (14%). GB 602 had higher sand percentages both 

near-field and far-field, but one near-field station had the highest value (21%). Near-field, 

post-drilling elevations in sand percentages were detected statistically at VK 916, GB 516 

(Cruise 2B), and MC 292, but not at GB 602 nor at GB 516 on Cruise 1B (Chapter 5). 

Figure 15.6 shows sand, silt, and clay percentages for stations grouped into three categories: 

• Far-field stations (>10 km) (also includes near-field pre-drilling stations at VK 916); 

• Near-field stations 300 to 500 m from site center; and 

• Near-field stations within 300 m of site center. 

Thickness contours are shown in the original

units (ft). 1 ft = 30 cm

0.25

0.5

1.0

0.75 1.25

0

0.25

0.5

0.75

1.0

1.25

1.5

0 50 100 150 200 METERS

Legend

Wellsite

Geophysically mapped drilling

deposits (mud and cuttings)
Figure 15.6. Sediment percentages of sand, silt, and clay in relation to proximity to drilling. 

Points are individual stations including non-random "discretionary" stations. 

"Far-field" includes near-field, pre-drilling stations at Viosca Knoll 916.

Silt

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

0

20

40

60

80

100

Silt (%)

Clay

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

0

20

40

60

80

100

Clay (%)

0

5

10

15

20

25

Sand (%)

Sand

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

487
488 

The range of sand percentages at the far-field and 300- to 500-m groups was similar. The 

<300-m group had two values that were slightly higher than those seen in the far-field, and 

overall, this group tended to have higher sand percentages than the far-field. In the near-field, 

both silt and clay percentages were within the range of values observed in the far-field. 

Grain size variations were patchy within near-field sites and the spatial pattern did not necessarily 

correspond to the drilling discharge zones mapped in Chapter 4. Also, except for VK 916, there 

were no strong correlations between percentages of sand, silt, or clay and concentrations of 

drilling indicators such as barium and SBF. These findings suggest that grain size distribution 

per se is not a very good indicator of drilling discharge impacts. Other physical measures have 

been determined to be more definitive in identifying cuttings, such as visual analysis for angular 

or tabular particles (Continental Shelf Associates, Inc. 2004). 

We assume that the materials deposited within the near-field sites consist mainly of (1) cuttings 

and WBMs released at the seafloor during the early stage of drilling; and (2) surface-discharged 

cuttings with small percentages of adhering SBMs. The latter is probably the main contributor at 

the near-field stations. While very coarse particles presumably attributable to seafloor discharges 

were noted near some drillsites in the SBM monitoring program (Continental Shelf Associates, 

Inc. 2004), these deposits are unlikely to have been sampled at our stations. Also, it is expected 

that any water-based drilling fluids discharged at the surface would be widely dispersed while 

settling to the bottom in a water depth of 1,000 m or more and would not appreciably affect grain 

size of bottom sediments. 

Cuttings with adhering SBMs tend to clump together and sink rapidly when discharged 

(Brandsma 1996). It is this aggregation that results in detectable accumulations of SBM cuttings 

on the seafloor near the drillsite. Once deposited, these clumps are eventually broken up due to 

biological activity. Grain size determinations would not show the presence of loosely adhering 

clumps. However, rough seafloor texture is evident in some near-field photographs (Chapter 12). 

15.2.1.5 Sediment Color/Appearance 

Color analysis of seafloor photographs (Chapter 12) showed that areas of “black bottom” were 

present on post-drilling surveys at all four near-field sites, but not at far-field sites (or VK 916 

near-field on the pre-drilling survey). Black surface layers also were seen in some sediment 

profile images (Chapters 6 and 7). The black bottom was covered in some images by 

white-to-red mats consistent with the sulfide-using bacteria Beggiatoa. These bacterial areas may 

indicate sites of local bottom anoxia supported by bacterial degradation of SBM. Although 

impacts were patchy at all sites, most images at GB 602 showed dark bottom, possibly indicating 

that this site has cuttings more thinly spread over a larger area. The patterns of black bottom 

coincided with geophysical mapping of cuttings reasonably well where direct comparisons could 

be made. 

15.2.1.6 Thickness of Mud and Cuttings Deposits 

The thickness of mud and cuttings deposits was estimated during the post-drilling geophysical 

survey at VK 916. The isopach map (Figure 15.5) shows thicknesses ranging from 0 to 45 cm 

around the wellsite. For comparison, the Pompano study estimated maximum cuttings 

thicknesses of 20 to 25 cm near a development well (Fechhelm et al. 1999) and the SBM 
489 

monitoring program detected layers ranging from a few centimeters to about 20-cm thick around 

various wellsites (Continental Shelf Associates, Inc. 2004). 

In addition, discretionary samples taken in likely mud/cuttings areas during Cruises 2B and 3B 

provide information about the thickness of mud and cuttings at a few stations (Chapter 8). Cores 

were subsectioned in five, 2-cm thick intervals over the top 10 cm. At VK 916, the thickness of 

layers containing drilling discharges during Cruise 3B (post-exploration), based on concentrations 

of barium and TOC, was about 4 cm in each of three discretionary cores. At GB 516 on 

Cruise 2B (post-development), excess 210Pb data for one station identify the presence of drilling 

discharges in the top 2 cm. At GB 602, a 6-cm thick layer of sediment containing drilling 

discharges was observed at the NF-DS2 station on Cruise 2B (post-development). The thickness 

of drilling deposits at MC 292 was not estimated. As indicated by the isopach map produced for 

VK 916 (see above), these values do not represent the thickest deposits since the area 

immediately surrounding the wellsites was not sampled. 

15.2.2 Chemical 

15.2.2.1 Drilling Fluid Tracers (Barium and SBF) 

Barium and SBF are sensitive tracers of drilling fluids in this study. Barite (BaSO4) is a major 

ingredient in both WBMs and SBMs. SBF concentrations are presumed to be related to the 

amount of SBF adhering to discharged cuttings. Current USEPA regulations limit the SBF 

retention on cuttings to 6.9% internal olefins and 9.4% ester-based fluids (USEPA 2001b). 

However, several of the wells within the near-field sites were drilled prior to those regulations. 

Historical data for wells drilled in the 1990's in the Gulf of Mexico indicate an average retention 

on cuttings of 12.8% for poly-alpha-olefin fluids and 9.2% for internal olefin fluids (Annis 1997, 

as cited by Neff et al. 2000). 

Neither barium nor SBF are ideal tracers of drilling fluids or cuttings. SBFs biodegrade over 

time. Although barium persists, its fate does not necessarily represent that of other drilling fluid 

components with different masses, shapes, solubilities, specific gravities, etc. Once drilling fluids 

and cuttings are released to the water column or the seafloor, the components may react, degrade, 

and move with different characteristics. 

Figure 15.7 shows the range of barium and SBF concentrations at far-field sites. Far-field 

barium values typically were between 500 and 1,500 μg/g, except at GB 516, where values as 

high as 2,780 μg/g were observed. Far-field SBF concentrations generally were less than 

10 μg/g, except for the Cruise 3B values at VK 916, which were in the 10 to 15 μg/g range. 

There was one higher SBF concentration at VK 916 FF2 (111 μg/g), which may be due to drilling 

discharges from wells in an adjacent lease block (see Chapter 3). 

All of the far-field sites had non-zero SBF concentrations (i.e., above the detection limits). 

However, these values do not necessarily represent the presence of SBM cuttings. These 

concentrations may be due to small quantities of natural hydrocarbons that eluted in the same 

range as SBFs in the analytical procedure. Table 15.5 shows the confidence intervals for 

far-field barium and SBF concentrations. 
Figure 15.7. Far-field concentrations of barium and synthetic-based fluids. Points are individual

stations including non-random "discretionary" stations. "Far-field" includes near-field, 

pre-drilling stations at Viosca Knoll 916.

Note: Cruise 3B plot 

excludes FF2-B02 

(SBF=111 μg/g). This 

sample may have 

been exposed to 

drilling discharges 

from another well.

Note: MC 292 plot 

excludes FF6-B01 

(SBF=28 μg/g)

and FF1-B02 (19 μg/g).

These samples may 

have been exposed to 

drilling discharges from 

another well.

VK 916

0

5

10

15

0 500 1000 1500 2000 2500 3000

Barium Concentration (μg/g)

SBF Concentration (μg/g)

Cr 1 NF

Cr 1 FF

Cr 3 FF

GB 516

0

5

10

15

0 500 1000 1500 2000 2500 3000

Barium Concentration (μg/g)

SBF Concentration (μg/g)

Cr 1 FF

Cr 2 FF

GB 602 and MC 292

0

5

10

15

0 500 1000 1500 2000 2500 3000

Barium Concentration (μg/g)

SBF Concentration (μg/g)

GB 602 FF

MC 292 FF

490
491 

Table 15.5. Confidence intervals for far-field barium and synthetic-based fluid (SBF) 

concentrations. 

95% Confidence Interval for an Observation Site Cruise Barium (μg/g) SBF (μg/g) 

VK 916 1B 546 – 1,064 2.96 – 7.74 

VK 916 3B 565 – 1,166 1.49 – 17.91 

GB 516 1B 532 – 2,031 -0.01 – 8.55 

GB 516 2B 254 – 2,702 1.38 – 6.45 

GB 602 2B 169 – 1,473 1.38 – 10.12 

MC 292 2B 516 – 993 -13.46 – 31.46 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll. 

As discussed in Chapters 8 and 9, barium and SBF concentrations were significantly higher at 

sites near drilling (near-field, post-drilling) than at sites far from drilling (i.e., far-field sites or the 

near-field, pre-drilling site at VK 916). Post-drilling, near-field barium concentrations were as 

high as 351,000 μg/g. The maximum barium concentration was measured at GB 516 Station 

NF-B03 on Cruise 2B and represents a sediment content of ~66% barite (using a barium level of 

53% for industrial barite, Trefry et al. 2003). All near-field stations except two at VK 916 had 

barium concentrations exceeding the 95% confidence interval for far-field concentrations. 

Post-drilling, near-field SBF concentrations ranged from <1.4 to 117,280 μg/g. The highest 

concentration was measured in a discretionary sample at GB 516 on Cruise 2B. All near-field 

stations at GB 602 and 11 of 12 stations at GB 516 had SBF concentrations that exceeded the 

95% confidence interval for far-field concentrations. SBF increases were not as widespread at 

the other two sites. At VK 916, 8 of 12 stations exceeded the far-field confidence limits, and at 

MC 292, only 6 of 12 stations did. (VK 916 had only a single well drilled, and MC 292 had the 

lowest volume of SBM discharges.) 

Figures 15.8, 15.9, 15.10, and 15.11 show barium and SBF concentrations at each of the four 

sites in relation to geophysically mapped zones. For VK 916 and GB 516, stations with elevated 

barium and SBF were mainly within these zones. For GB 602, elevated concentrations seemed to 

be distributed throughout the near-field site, and most stations were in or near the mapped zones 

in any case. MC 292 does not show the very high ranges of barium and (especially) SBF values 

as compared with the other sites. 

Figure 15.12 shows concentrations of barium and SBF for all sites combined, with stations 

grouped into the same three categories cited previously (far-field, near-field 300 to 500 m, and 

near-field <300 m). The figure shows that although the highest barium concentrations were seen 

within 300 m, most near-field stations including those between 300 m and 500 m from the center 

had elevated barium concentrations. Similarly, for SBF, although a 300-m station had the highest 

concentration, the range was fairly similar for both 300-m and 500-m radii. For both barium and 

SBF, some near-field stations, even within 300 m, had concentrations within the range observed 

for far-field stations. 
Figure 15.8. Barium and synthetic-based fluid (SBF) concentrations on Cruise 3B at Viosca Knoll 916 

site in relation to geophysically mapped rig discharge deposits.

500 m radius

1

2

3

4

5

6

7

8

9

11

12

10

Rig Discharge 

Deposits

DS-3

DS-2

DS-1

o

1

2

3

4

5

6

7

8

9

11

12

10

Rig Discharge 

Deposits

DS-3

DS-2

DS-1

Barium

SBF

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

500 m radius

Legend

Wellsite

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

Legend

Wellsite

10,001 - 100,000 μg/g

2,001 - 10,000 μg/g

1 - 2,000 μg/g

Barium Concentration

>100,000 μg/g

492
Figure 15.9. Barium and synthetic-based fluid (SBF) concentrations on Cruise 2B at Garden Banks 516 

in relation to geophysically mapped mud and cuttings deposits.

Barium

Legend

Wellsite

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

10,001 - 100,000 μg/g

2,001 - 10,000 μg/g

1 - 2,000 μg/g

Barium Concentration

>100,000 μg/g

SBF

Legend

Wellsite

2

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

500 m radius

6

10 1

9

5

4

8

12

7

11

3

DS-1

DS-2

DS-3

Rig Discharge 

Deposits

2 500 m radius

6

10 1

9

5

4

8

12

7

11

3

DS-1

DS-2

DS-3

Rig Discharge 

Deposits

Well Jetting 

Deposits

>100,000 μg/g

Well Jetting 

Deposits

493
Figure 15.10. Barium and synthetic-based fluid (SBF) concentrations on Cruise 2B at Garden Banks 602 

in relation to geophysically mapped mud and cuttings deposits.

500 m radius

Barium

Legend

Wellsite

1

5

2 9

6

10

11

12

3

7

4

DS-2 8

DS-1

DS-3

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

10,001 - 100,000 μg/g

2,001 - 10,000 μg/g

1 - 2,000 μg/g

Barium Concentration

>100,000 μg/g

500 m radius

SBF

Legend

Wellsite

1

5

2 9

6

10

11

12

3

7

4

8 DS-2

DS-1

DS-3

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

Rig Discharge 

Deposits

Well Jetting 

Deposits

Rig Discharge 

Deposits

Well Jetting 

Deposits

μ

494
10

6 2

5

1

8

9

12

DS-1

DS-2

DS-3

4

3

7

11

500 m radius

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

Barium

10

6 2

5

1

8

9

12

DS-1

DS-2

DS-3

4

3

7

11

500 m radius

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

SBF

Figure 15.11. Barium and synthetic-based fluid (SBF) concentrations on Cruise 2B at Mississippi

Canyon 292 site in relation to geophysically mapped mud and cuttings deposits.

Legend

Wellsite

10,001 - 100,000 μg/g

2,001 - 10,000 μg/g

1 - 2,000 μg/g

Barium Concentration

>100,000 μg/g

Legend

Wellsite

1,001 - 10,000 μg/g

101 - 1,000 μg/g

1 - 100 μg/g

SBF Concentration

10,000-100,000 μg/g

Rig Discharge 

Deposits

Well Jetting 

Deposits

Rig Discharge 

Deposits

Well Jetting 

Deposits

495
Figure 15.12. Sediment concentrations of barium, synthetic-based fluids (SBF), and polycyclic 

aromatic hydrocarbons (PAH) in relation to proximity to drilling. Points are individual 

stations, including non-random "discretionary" stations. "Far-field" includes near-field, 

pre-drilling stations at Viosca Knoll 916.

> 10 km from

Drilling

Barium Concentration (μg/g)

300-500 m

from Drilling

<300 m

from Drilling

10

SBF Concentration (μg/g)

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

102

103

104

105

SBF

10

102

103

104

PAH Concentration (ng/g)

PAH

Note: PAH could not be analyzed at 

four <300 m stations at VK 916 due to 

high SBF concentration

Far-Field Near-Field

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

102

103

104

105

106

Barium

1

1

GB 516 Cruise 2B

NF-DS2

GB 516 Cruise 1B

NF-B02

496
497 

The near-field data are essentially limited to the 500-m radius. However, one discretionary 

station at GB 602 on Cruise 2B (Station NF-DS3) was about 1,000 m southwest of the site center 

and had a barium concentration of 14,800 μg/g; in comparison, nearly all far-field values were 

less than 2,000 μg/g. The SBF concentration was 930 μg/g, compared with nearly all of the 

far-field values being less than 15 μg/g. The geophysical map does not show this station within 

the cuttings zone, though it does show cuttings (from rig discharges) extending about this 

distance from the site center. 

There are some indications of more distant effects. One far-field site (VK 916 FF2) had slightly 

elevated barium and SBF concentrations on both cruises. Four wells were drilled between April 

and July 1997 in VK 914, approximately 3.1 km from the FF2 site center. Subsequently, four 

wells were drilled in VK 915 within the 3 months preceding Cruise 3B, at a distance of 2.6 km. 

Drilling plans indicate that SBMs would be used in VK 915 but not in VK 914. On Cruise 3B, 

barium concentrations at FF2 averaged 1,315 μg/g, compared with a mean of 865 μg/g for the 

other VK 916 far-field sites. The SBF concentration at one of the VK 916 FF2 stations was 

111 μg/g, about 10 times higher than the mean of other VK 916 far-field sites on this cruise. 

Based on the extent of detectable barium and SBF increases, it seems possible that prior to the 

first cruise, the VK 916 near-field site also may have been exposed to small quantities of drilling 

fluids from the two previous wells in VK 872, approximately 2.3 km to the NNE. Near-field 

barium concentrations averaged 1,076 μg/g, whereas far-field concentrations (excluding FF2, as 

noted above) averaged 865 μg/g. However, the barium difference is not statistically significant. 

There was no indication of elevated SBF concentrations (drilling plans did include SBMs). 

The results are similar to those noted in the SBM monitoring program (Continental Shelf 

Associates, Inc. 2004). Elevated barium and SBF were detected mainly within 250 m of 

wellsites, with small amounts detected at greater distances. 

15.2.2.2 Polycyclic Aromatic Hydrocarbons (PAHs) 

With two exceptions, sediment PAH concentrations ranged from 43 to 748 ng/g dry wt 

(Chapter 9). Two samples at GB 516 had considerably higher concentrations. On Cruise 1B, 

Station NF-B02 had 3,470 ng/g, and on Cruise 2B, one of the discretionary stations (NF-DS2) 

had PAH levels of 23,840 ng/g in the top 2 cm. Both of these stations were within the 300-m 

radius (Figure 15.12). 

The source of the PAHs is suggested to be from some other contaminant from the drilling or 

production activity, as SBMs do not contain PAHs. Although the geohazards report for this lease 

block notes that shallow gas may be seeping through the seafloor in GB 516 (Fugro-McClelland 

Marine Geosciences, Inc. 1997), hydrocarbon seepage probably is not the source of the elevated 

PAH in these two samples, for several reasons. The concentrations are much higher than the 

highest levels (1,040 ng/g) measured during the Deepwater Gulf of Mexico Benthos (DGoMB) 

program, which included stations near seep areas (Rowe and Kennicutt 2002). The naphthalenes, 

biphenyl, and acenaphthene dominated the total PAH in our samples, rather than perylene, which 

was the major PAH in many DGoMB samples (Rowe and Kennicutt 2002). Also, PAH 

concentrations in the discretionary core decreased vertically with depth (highest values were at 

the surface). This issue is discussed further in the Ecological Risk Assessment (Chapter 16). 
498 

15.2.2.3 Sediment Metals Other Than Barium 

Near-field concentrations of arsenic, cadmium, chromium, copper, lead, mercury, and zinc were 

elevated in some near-field sediment samples as compared with far-field samples (Chapter 8). 

Generally, elevated concentrations of these metals were associated with high barium 

concentrations (i.e., drilling fluid). However, these elevated concentrations are within the 

expected range of background concentrations for uncontaminated marine sediments (Neff 1987). 

Relatively low concentrations of aluminum, iron, nickel, and vanadium were measured in some 

near-field samples and were attributed to dilution of ambient sediments with barite, which 

contains no significant amounts of these metals. Also, concentrations of manganese were lower 

and more variable at near-field sites, a result attributed to reductive dissolution of this metal at 

stations where the presence of drilling discharges created reducing conditions. 

Considerable interest has been generated regarding concentrations of mercury in sediments 

adjacent to drilling sites because concentrations of total mercury near drilling sites are often 2 to 

10 times higher than in nearby background sediments (Neff 2002a). This mercury is known to be 

a natural impurity in barite (Kramer et al. 1980; Trefry and Smith 2003). Calculations in 

Chapter 8 indicate that the mercury concentrations in barite deposited at these sites are in line 

with USEPA regulations, which allow a maximum level of 1 mg/kg in barite (USEPA 1993). 

15.2.2.4 Metals and Hydrocarbons in Biota 

Concentrations of 11 metals (arsenic, barium, cadmium, chromium, copper, iron, lead, mercury, 

nickel, vanadium, and zinc) were determined in samples of the giant isopod Bathynomus 

giganteus and the red crab Chaceon quinquedens from near-field and far-field stations at sites 

GB 602 and MC 292 (Chapter 8). 

In isopods from GB 602, levels of barium, chromium, and lead were greater at near-field stations, 

whereas concentrations of cadmium and mercury were greater at far-field stations. No significant 

differences were detected for isopods at MC 292. 

For red crabs from GB 602, barium concentrations were higher in the near-field, whereas 

concentrations of arsenic, cadmium, copper, mercury, nickel, vanadium, and zinc were higher in 

the far-field specimens. For MC 292, concentrations of barium, cadmium, chromium, and 

vanadium were greater at near-field stations. 

The most consistent finding in the metals data is elevated barium (in isopods from GB 602 and in 

crabs from both GB 602 and MC 292). The only other metal with more than one significant 

result is chromium (elevated in isopods from GB 602 and crabs from MC 292). All other metals 

either had contradictory findings between sites (cadmium and vanadium in crabs); were 

significant in only one organism at one site (lead in isopods at GB 602); were higher at far-field 

sites (arsenic, copper, nickel, and zinc in crabs at GB 602; cadmium and mercury in both isopods 

and crabs from GB 602); or had no significant differences (iron). 
499 

Studies have consistently shown that industrial barite (and its trace components) used in drilling 

muds has low bioavailability to marine organisms (Neff et al. 1989b,c). The elevated barium 

concentrations detected in isopods and crabs may reflect small amounts of sediment particles 

retained in the gut. 

Both species also were analyzed for tissue PAH concentrations (Chapter 9). PAH body burden 

concentrations ranged from 38.6 to 416 ng/g dry. No significant difference was found between 

the total PAH body burden concentrations in the two deepwater organisms. No significant 

difference also was found between the total PAH body burden concentrations at near-field and 

far-field stations. 

15.2.2.5 Total Organic Carbon 

Sediment TOC concentrations at far-field and pre-drilling near-field stations ranged from 0.28% 

to 1.73%. Near-field concentrations ranged from 0.26% to 7.16%. 

Elevated TOC was patchy within near-field sites. Numbers of near-field stations exceeding the 

95% confidence interval for far-field sites were as follows: 

• VK 916 – 4 of 12 stations; 

• GB 516 – 0 of 12 stations (Cruise 1B) and 5 of 12 stations (Cruise 2B); 

• GB 602 – 4 of 12 stations; and 

• MC 292 – 1 of 12 stations. 

Figure 15.13 shows TOC concentrations with stations grouped into the same three categories 

noted previously. With one exception, elevated TOC (greater than 2%) was observed only within 

300 m of the site center. The exception was one VK 916 discretionary station at a distance of 

about 315 m with a TOC concentration of 3.07%. 

All stations with elevated TOC had high SBF and barium concentrations. The high ratio of SBF 

to barium suggests that drilling deposits at those stations were primarily SBM cuttings rather than 

WBM cuttings or muds. TOC was strongly correlated with SBF concentration within near-field 

sites (Figure 15.13), supporting the conclusion that the organic-rich areas were due to deposition 

of SBM cuttings. Most of the TOC concentrations exceeding 2% occurred at either VK 916 or 

GB 516 (Cruise 2B). There was little or no elevated TOC at MC 292 or at GB 516 

post-exploration (Cruise 1B). 

15.2.2.6 Sediment Redox Conditions 

Chapter 8 presents detailed discussions of sediment oxygen levels and redox conditions at each of 

the four sites. The sediment profile imaging studies (Chapters 6 and 7) also provide visual 

information about redox conditions. 

As a useful summary measure, Figure 15.14 compares integrated oxygen levels in the sediment 

column at each site. Included in these plots are both near-field and far-field stations, plus 

discretionary stations placed deliberately in likely cuttings deposits. With a couple of exceptions, 

oxygen levels were markedly reduced at near-field, post-drilling sites. This effect was most 

pronounced at GB 602 and at GB 516 on Cruise 2B, where all near-field and discretionary 

stations had much lower oxygen than the far-field. 
Figure 15.13. Relationship between total organic carbon (TOC) and synthetic-based fluid (SBF) 

concentrations in near-field sediments at the four sites.

TOC vs. SBF

0

2

4

6

8

0 25000 50000 75000 100000 125000

SBF Concentration (μg/g)

Total Organic Carbon (%)

VK 916 Cr3

GB 516 Cr2

GB 602 Cr2

MC 292 Cr2

Far-Field TOC Range

TOC

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

0

2

4

6

8

Total Organic Carbon (%)

500
DISC N

F FF DISC NF FF

0

50

100

150

200

Integrated O2 (nmol/cm2

)

GB 516

Post-Exploration

122

153

25 17

165

Post-Development

NF FF DS NF FF

Figure 15.14. Integrated amounts of oxygen in the sediment column at near-field (NF), far-field (FF),

and discretionary (DS) stations at all four sites. Markers and numbers show means 

and lines show standard deviations. The standard deviation for some DS samples is 

smaller than the marker.

Post-Development

Legend

Discretionary stations

Near-field stations

Far-field stations

DIS

C NF FF

DIS

C NF FF

0

50

100

150

200

250

GB 602

22

227

55

38

65

NF FF DS NF FF

MC 292

DS

25

Integrated O2 (nmol/cm2

)

Post-Development

NF MF FF NF MF FF 0

30

60

90

120

150

Integrated O2 (nmol/cm2

)

VK 916

Pre-Exploration

86

44

11

84

NF FF DS NF FF

90

Post-Exploration

501
502 

On Cruise 1B at VK 916, where there was no previous drilling, most near-field stations had 

oxygen levels similar to those seen in the far-field. Similarly, at GB 516, although there was 

previous drilling prior to Cruise 1B (two exploration wells), oxygen levels at many near-field 

stations initially were similar to those in the far-field. However, after subsequent drilling of five 

development wells, the near-field oxygen levels were greatly reduced on Cruise 2B 

(Figure 15.14). 

At MC 292, differences between near-field and far-field stations were less striking. The degree 

of similarity is partly due to lower levels of oxygen at the MC 292 far-field site as compared with 

the other sites. The difference in background conditions may be related to the proximity of 

MC 292 to the Mississippi River delta and the higher sedimentation rate at this site (~0.14 cm/yr 

vs. 0.04 to 0.07 cm/yr for the other three sites). MC 292 and VK 916 had lower far-field oxygen 

levels than the two western sites (GB 516 and GB 602). In addition, MC 292 had the smallest 

quantities of SBM discharges and the longest time since drilling (see Section 15.3, Site

Comparisons). 

Sediment Eh profiles were obtained at most stations (Chapter 8). Eh represents the sum of all 

oxidation/reduction reactions in the sediment. Here, we use the Eh value near the sediment/water 

interface (1 to 3 cm depth in the sediment column, depending on the station) as an index of redox 

conditions. Far-field stations had Eh values ranging from 100 to 692 mV. Many near-field 

stations also were within this range. However, numerous near-field stations had low or negative 

Eh, including values as low as -296 mV. GB 602 had the most stations with negative values. 

Figures 15.15 and 15.16 show that areas of low Eh were patchy/localized at the post-exploration 

sites (VK 916 Cruise 3B and GB 516 Cruise 1B) but more extensive at two of the 

post-development sites (GB 602 and GB 516 Cruise 2B). At the other post-development site 

(MC 292), only one near-field station had negative Eh; the others were similar to the far-field. 

All stations with negative Eh at 1 to 3 cm depth had elevated barium and SBF concentrations, 

indicating that anoxic and reducing conditions were associated with areas of cuttings deposition. 

Sediment profile imaging (Chapters 6 and 7) included measurements of apparent RPD depth (i.e., 

the depth of the oxidized layer) in each photograph. The most important findings were as 

follows: 

• RPD depths were shallower at near-field sites on post-drilling surveys; that is, anoxic 

conditions were closer to the sediment surface. 

• Instances of zero RPD depth were noted only on near-field, post-drilling transects. 

• Among the four sites, near-field vs. far-field differences were smallest at MC 292. 

In Chapter 7, some sediment profile images were interpreted as indicating hypoxic or anoxic 

conditions in near-bottom waters. However, direct measurements in Chapter 8 show that 

near-bottom waters were not anoxic and oxygen levels rarely approached zero at the 

sediment/water interface. While the two sets of observations are not directly comparable because 

the SPI transects did not sample the same locations as the box cores, the difference highlights the 

difficulty of inferring the oxidation state from sediment color. 
Figure 15.15. Redox potential (Eh) values at 1 to 3 cm depth in the sediment column at Viosca Knoll 916 

and Garden Banks 516 on post-exploration cruises in relation to geophysically mapped mud 

and cuttings deposits.

500 m radius

1

2

4

5

6

7

8

9

12

10

Rig Discharge 

Deposits

DS-3

DS-2

DS-1

VK 916 Cruise 3B

(post-exploration)

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

Legend

Wellsite

0 - 100 mV

101 - 200 mV

> 200 mV

Eh (mV) at 1-3 cm depth

< 0 mV

GB 516 Cruise 1B

(post-exploration)

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

2

500 m radius

6

10 1

9

5

4

8

12

7

11

Rig Discharge 3

Deposits

Well Jetting 

Deposits

Legend

Wellsite

0 - 100 mV

101 - 200 mV

> 200 mV

< 0 mV

Eh (mV) at 1-3 cm depth

503
Figure 15.16. Redox potential (Eh) values at 1 to 3 cm depth in the sediment column at Garden Banks 516, 

Garden Banks 602, and Mississippi Canyon 292 on post-development cruises in 

relation to geophysically mapped mud and cuttings deposits.

GB 602 Cruise 2B

(post-development)

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

MC 292 Cruise 2B

(post-development)

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

10

6 2

5

1

8

9

12

DS-1

DS-2

DS-3

4

3

7

11

500 m 

radius

500 m 

radius 1

5

2

6

10

11

12

3

7

4

8

DS-2

DS-1

DS-3

GB 516 Cruise 2B

(post-development)

Legend

Wellsite

0 - 100 mV

101 - 200 mV

> 200 mV

< 0 mV

FF1

FF2

FF3

FF4

FF5

FF6

Far-Field

2 500 m 

radius 6

10 1

9

5

8

12

7

11

3

DS-1

DS-2

DS-3

Cuttings

Muds

Eh (mV) at 1-3 cm depth

Rig Discharge 

Deposits

Well Jetting 

Deposits

Rig Discharge 

Deposits

Well Jetting 

Deposits

Rig Discharge 

Deposits

Well Jetting 

Deposits

504
505 

15.2.3 Biological 

This study included several biological components. Sediment profile imaging (Chapters 6 and 7) 

provides an overview of benthic community health and successional status. Box core samples 

were analyzed for microbial ATP (Chapter 10), and meiofauna and macroinfauna (Chapter 11). 

Additional studies of harpacticoid copepod genetics were conducted at GB 602 and MC 292 

(Chapter 13). Photographs taken with the camera sled were examined for the presence of 

megafauna (Chapter 12). 

15.2.3.1 Benthic Community Status 

Sediment profile imaging was used to characterize benthic communities at each site (Chapters 6 

and 7). Images were analyzed to determine successional stage using a paradigm based on 

recolonization of disturbed shallow-water habitats (Rhoads and Germano 1982). Application of 

this method to our study sites is an extrapolation, as detailed knowledge about infaunal 

successional stages is not available for these sites. Stage I organisms live very close to the 

sediment-water interface, and are pioneers as they require little oxidized sediment. By their 

feeding and burrowing activities, these Stage I animals, often small annelids, deepen the RPD, 

preparing the sediment for colonization by somewhat larger animals such as amphipods 

(Stage II). (No Stage II assemblages were identified here because detailed knowledge of 

successional dynamics is not available for the study sites.) Stage III organisms are large, 

deep-burrowing, head-down deposit feeders, usually polychaetes and echinoderms, which aerate 

the sediment to several centimeters in depth. Their presence indicates an equilibrium community 

and a healthy environment. 

In addition, an overall organism-sediment index (OSI) was calculated for each image in 

Chapter 6. The index takes into account the depth of the apparent RPD, successional stage, 

presence or absence of sediment methane bubbles, and presence or absence of reduced sediment 

near the sediment-water interface. The value of the index can range from +11 (best habitat value) 

to –10 (worst habitat value). Values of the OSI below +6 tend to be associated with disturbed 

benthic habitats. Negative OSI values reflect a highly disturbed benthos. 

Post-drilling surveys indicated that the near-field zone at three of the four sites (VK 916, GB 516, 

and GB 602) consisted of a mosaic of poor benthic conditions (patchy areas of low or negative 

OSI and azoic or retrograde benthic communities) alternating with areas of moderately high 

benthic habitat quality. The degraded near-field habitats stand in marked contrast to those seen at 

far-field sites that exhibit aerobic boundary layer conditions with well-developed Stage I and I-III 

communities. 

Table 15.6 summarizes OSI data and successional status for VK 916 and GB 516 as an example. 

Negative OSI values indicative of highly disturbed benthic habitats were observed only in the 

near-field, post-drilling images. 
506 

Table 15.6. Organism-sediment index (OSI) and successional stage for Viosca Knoll 916 

(VK 916) and Garden Banks 516 (GB 516). 

Stations with 

Negative OSI (%) Site and Successional Stage 

Cruise Stage 

FF NF FF NF 

VK 916 

Cruise 1B Pre-drilling 0 0 Stage I-III (mostly) Stage I-III (mostly) 

VK 916 

Cruise 3B Post-exploration 0 25 Stage I-III (mostly) Stage III dominates; 

Stage I rare 

GB 516 

Cruise 1B Post-exploration 0 36 Stage I-III (mostly) About 80% of stations are 

azoic or Stage I 

GB 516 

Cruise 2B Post-development 0 18 Stage I-III (mostly) About 50% of stations are 

azoic or Stage I 

FF = far-field; NF = near-field. 

Post-drilling communities at GB 516 were dominated by pioneering infauna (Stage I), which are 

quick to colonize disturbed habitats. Microbial mats were observed at some stations on 

Cruise 1B. There was some indication of slight improvement in benthic habitat conditions 

between Cruises 1B and 2B. This is puzzling because box core data show that redox conditions 

deteriorated between cruises; however, the sediment profile transects may simply have crossed 

less severely affected areas than those sampled by box cores on Cruise 2B. 

At VK 916, the successional picture was different in that Stage I seres were rarely encountered. 

This observation suggests that recent surface disturbance (deposition, resuspension, and/or 

erosion) may have selectively compromised near surface-dwelling species relative to 

deeper-living infauna. A similar phenomenon has been noted in studies of the ecological effects 

of bottom trawling (Rosenberg et al. 2003). These observations are based on near-field sampling 

only; no far-field data were available for the post-drilling survey at this site. 

Results for the GB 602 post-development site appear qualitatively similar to those for GB 516. 

However, at the other development site (MC 292), only a small number of stations appeared to 

have been visibly affected by drilling muds and cuttings. Only a few stations along one transect 

had signs of depleted oxygen conditions. There was evidence of infaunal activity at most 

stations, and no microbial mats were observed at this site. The relative lack of impacts is 

consistent with box core redox measurements and may be related to the low volume of SBM 

cuttings discharged at this site, as discussed further in Section 15.3. 

15.2.3.2 Microbiota 

In Chapter 10, LaRock concluded that microbial biomass was reduced at both near-field and 

far-field sites due to drilling discharges. However, alternative explanations can be offered that 

are more consistent with both the microbial data and findings in other chapters. The results 

suggest that microbial biomass was higher, not lower, in some areas where drilling discharges 

were present. 
507 

Figure 15.17 shows the microbial ATP data for the four study sites. A temporal difference in 

microbial biomass is evident at VK 916 and GB 516. Specifically, ATP concentrations measured 

on Cruise 1B were about an order of magnitude higher than those measured on Cruises 2B or 3B. 

The difference is statistically significant for both VK 916 and GB 516, as noted by LaRock. In 

Chapter 10, this difference is attributed to drilling discharges affecting both near-field and 

far-field sites. However, this interpretation has several problems: 

• The decrease occurred regardless of the degree of exposure to drilling discharges. Between 

cruises, the near-field sites received large accumulations of drilling deposits as indicated by 

orders of magnitude increases in barium and SBF concentrations, whereas most far-field sites 

were exposed to undetectable amounts of drilling fluids, if any. 

• The high ATP concentrations seen at GB 516 on Cruise 1B are not consistent with inhibition 

of microbial populations by drilling discharges. If drilling discharges reduce microbe 

populations, then this site should already have been inhibited on Cruise 1B because two wells 

were drilled there 14 months earlier. (The presence of drilling discharges is evidenced by 

barium concentrations as high as 135,000 μg/g and SBF concentrations as high as 

25,131 μg/g.) Instead, both near-field and far-field ATP concentrations were in the same 

range as those at VK 916, where there was no previous drilling. 

• No plausible mechanism is offered that could cause microbial inhibition at a distance of more 

than 10 km away. Moreover, if drilling discharges reduce microbial biomass at stations more 

than 10 km away, then nearly all of the Cruise 1B far-field stations, as well as the VK 916 

near-field site, could already have been affected. All of the VK 916 and GB 516 far-field 

sites, as well as the VK 916 near-field site, had previous wells within 10 km (see Chapter 3). 

The cause of the decrease between cruises is unknown, but we do not believe it to be attributable 

to drilling discharges. Temporal differences apparently unrelated to drilling are evident in some 

other data sets, including meiofauna and macroinfauna and megafauna. For example, meiofaunal 

densities declined by about an order of magnitude between Cruise 1B and the subsequent surveys 

at both near-field and far-field sites (Chapter 11). Macroinfaunal densities decreased between 

surveys at VK 916 (both near- and far-field) but increased at GB 516 (both near- and far-field) 

(Chapter 11). 

There is ample evidence that microbial biomass, though patchy, was higher in areas where 

drilling discharges accumulated. For example, sediment profile camera images commonly 

showed microbial mats at near-field sites but not far-field sites (Plate 15.1; Table 15.7; 

Chapters 6 and 7). 
Figure 15.17. Microbial adenosine triphosphate (ATP) concentrations at the four study sites. Points are individual station

VK 916

01

10

100

1000

Microbial ATP (ng/g dry wt)

GB 516

01

10

100

1000

Microbial ATP (ng/g dry wt)

GB 602

01

10

100

1000

Microbial ATP (ng/g dry wt)

MC 292

01

10

100

1000

Microbial ATP (ng/g dry wt)

Cruise 1B Cruise 3B

Cruise 2B Cruise 2B

Near-Field Far-Field

Near-Field Far-Field Near-Field Far-F

Near-Field Far-Field

Cruise 1B Cru

Near-Field Far-Field Near-Fie

Near-field Far-field

LEGEND

508
509 

Plate 15.1. Sediment profile image from the near-field 

site at Garden Banks 516 during Cruise 1B 

showing a microbial mat and with the 

sediment column below completely gray to 

black. Image width is 15 cm. 

Microbial Mat 

Table 15.7. Presence of microbial mats as recorded in sediment profile images. 

Presence of Microbial Mats Site Cruise Stage FF NF 

VK 916 1B Pre-drilling -- -- 

3B Post-exploration -- Present at several stations 

GB 516 1B Post-exploration -- Present at several stations (29%) 

2B Post-development -- -- 

GB 602 2B Post-development -- Present at several stations 

MC 292 2B Post-development -- -- 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; FF = far-field; NF = near-field. 

Within the VK 916 and GB 516 sites, the stations with the highest microbial biomass 

corresponded to those with the highest SBF concentrations (Figure 15.18). This effect was 

evident at SBF concentrations greater than 1,000 μg/g. A similar effect was not evident at 

GB 602 or MC 292 (Figure 15.19); both of these sites, as well as VK 916, had one apparent 

“outlier” value among the far-field stations, which complicates interpretation. For example, at 

GB 602, Station FF5-B02 had an ATP concentration of 216.3 ng/g (compared with 1.1 to 

3.6 ng/g for other GB 602 far-field samples). If this value is included, the near-field and far-field 

means are not significantly different (p=0.52), but if it is omitted, the near-field mean is 

significantly higher (p=0.02). 

The increased microbial abundance at some near-field stations is consistent with an organic 

enrichment effect due to the addition of SBM cuttings. The possibility that some inhibition of 

microbial activity also occurs where SBM cuttings accumulate, as hypothesized in Chapter 10 

cannot be ruled out. However, if it exists, this inhibition seems to be overwhelmed by the organic 

enrichment effect. 
Figure 15.18. Microbial adenosine triphosphate (ATP) concentrations at Viosca Knoll 916 and 

Garden Banks 516 on post-drilling cruises in relation to sediment synthetic-based fluid 

(SBF) concentrations. Points are individual stations. Note the different ATP scales on 

each graph.

VK 916 - Cruise 3B 

(post-exploration)

0

20

40

60

80

1 10 102 103 104 105

SBF Concentration (μg/g dry wt)

ATP (ng/g dry wt)

Near-Field

Far-Field

GB 516 - Cruise 1B

(post-exploration)

SBF Concentration (μg/g dry wt)

Near-Field

Far-Field

GB 516 - Cruise 2B

(post-development)

0

5

10

15

SBF Concentration (μg/g dry wt)

ATP (ng/g dry wt)

Near-Field

Far-Field

1 10 102 103 104 105

1 10 102 103 104 105

0

100

200

300

400

500

ATP (ng/g dry wt)

510
Figure 15.19. Microbial adenosine triphosphate (ATP) concentrations at Garden Banks 602 and 

Mississippi Canyon 292 on Cruise 2B (post-drilling) in relation to sediment 

synthetic-based fluid (SBF) concentrations. Points are individual stations. Note the 

different ATP scales on each graph.

GB 602 - Cruise 2B

(post-development)

0

50

100

150

200

250

SBF Concentration (μg/g dry wt)

ATP (ng/g dry wt)

Near-Field

Far-Field

MC 292 - Cruise 2B

(post-development)

0

20

40

60

80

SBF Concentration (μg/g dry wt)

ATP (ng/g dry wt)

Near-Field

Far-Field

1 10 102 103 104 105

1 10 102 103 104 105

511
512 

15.2.3.3 Meiofauna 

Meiofaunal densities declined by about an order of magnitude between October-November 2000 

(Cruise 1B) and the subsequent surveys (Cruise 2B, July 2001 and Cruise 3B, August 2002) at 

both near-field and far-field sites (Chapter 11). The decrease is not attributable to drilling 

activities, for the same reasons discussed under Microbiota. However, because of the magnitude 

of change, impacts must be inferred from near-field vs. far-field comparisons within cruises. 

Meiofaunal densities tended to be higher near drilling. Although there was considerable overlap 

in near-field vs. far-field densities, the highest densities of nematodes, harpacticoid copepods, and 

especially annelids occurred in the near-field. As shown in Figure 15.20, this effect was seen 

mainly within 300 m of the wellsite(s), although a few stations in the 300- to 500-m range also 

had elevated numbers. The elevated meiofaunal densities are similar to the nematode enrichment 

effect previously reported in the GOOMEX study by Montagna and Harper (1996) for platforms 

in shallower Gulf waters. In addition, unlike GOOMEX, higher (not lower) harpacticoid 

densities were found near drilling. 

Meiofaunal densities in the near-field were not consistently correlated with drilling indicators 

(barium, SBF) or other sediment variables (TOC, grain size fractions). 

A study of harpacticoid genetic diversity was conducted at GB 602 and MC 292 (Chapter 13). 

The study focused on Bathycletopsyllus sp., which was absent from far-field sites but abundant at 

near-field sites. Results showed low genetic diversity in near-field populations of 

Bathycletopsyllus sp., which is consistent with expansion from a small, pre-drilling population. 

15.2.3.4 Macroinfauna 

Macroinfaunal densities tended to be higher near drilling (Chapter 11). Although there was 

considerable overlap in near-field vs. far-field densities, the highest densities occurred in the 

near-field. As shown in Figure 15.20, this effect was seen almost entirely within 300 m of the 

wellsite(s). 

For annelids (predominantly polychaetes), gastropods, and bivalves, there was a strong, 

consistent tendency for the highest densities to occur in the near-field. Aplacophorans showed a 

similar trend at GB 516 and GB 602, though not at VK 916. The opposite was true for 

amphipods and ostracods, with the highest densities generally occurring at one or more far-field 

stations. 

Annelid (predominantly polychaete) densities in the near-field were positively correlated with 

drilling indicators (barium, SBF). Some near-field stations with barium concentrations higher 

than about 10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g had elevated 

polychaete densities (Figure 15.21). 

Gastropod densities in the near-field were positively correlated with drilling indicators (barium, 

SBF). A few near-field stations at GB 516 and GB 602 had very high gastropod densities, which 

were associated with barium concentrations of 55,000 μg/g or higher and SBF concentrations of 

4,500 μg/g or higher (Figure 15.22). Much weaker relationships were observed at VK 916 and 

MC 292. 
Figure 15.20. Meiofaunal and macroinfaunal densities in relation to proximity to drilling. Points

are individual stations including non-random "discretionary" stations. The pre-drilling

survey at Viosca Knoll 916 has been excluded from these plots (higher densities both

near- and far-field would obscure other patterns).

Total Macroinfauna

0

10000

20000

30000

40000

50000

Macroinfaunal Density 

(no. per m2

)

Total Meiofauna

0

100

200

300

400

Meiofaunal Density 

(no. per 10 cm2

)

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

> 10 km from

Drilling

300-500 m

from Drilling

<300 m

from Drilling

Far-Field Near-Field

513
Figure 15.21. Annelid (predominantly polychaete) densities vs. synthetic-based fluid (SBF) concentrations on post-dr

Note the log scale for SBF concentrations and the different density scale for Viosca Knoll 916.

GB 516

Annelids per m

2

GB 602 MC 292

1000200030004000

SBF Concentration (μg/g dry wt)

Annelids per m

2

Far-Field, Cruise 3B

SBF Concentration (μg/g dry wt)

VK 916

Annelids per m

2

Near-FieldFar-Field

SBF Concentration (μg/g dry wt)

Annelids per m

2

0

1000200030004000

1 10 102

0

5000

10000150002000025000

0

5000

10000150002000025000

SBF Concentration (μg/g dry wt)

0

5000

10000150002000025000

Near-Field, Cruise 3B

103 104 105

1 10 102 103 104 105 1 10 102 103 104 

1 10 102 103 104 

514
Synthetic Based Fluids

0

5000

10000

15000

20000

1 1

0 102

SBF Concentration (ug/g dry wt)

Gastropods per m2

VK916-Cr3 NF

GB516-Cr1 NF

GB516-Cr2 NF

GB602-Cr2 NF

MC292-Cr2 NF

Barium

0

5000

10000

15000

20000

Barium Concentration (ug/g dry wt)

Gastropods per m2

VK916-Cr3 NF

GB516-Cr1 NF

GB516-Cr2 NF

GB602-Cr2 NF

MC292-Cr2 NF

Total Organic Carbon

0

5000

10000

15000

20000

01234

Total Organic Carbon (%)

Gastropods per m2

VK916-Cr3 NF

GB516-Cr1 NF

GB516-Cr2 NF

GB602-Cr2 NF

MC292-Cr2 NF

Figure 15.22. Gastropod densities vs. sediment barium, synthetic-based fluid (SBF), and total organic 

carbon concentrations at near-field (NF), post-drilling stations. Note the log scale for barium 

and SBF concentrations.

103 104 105

103 104 105 106

515
516 

Amphipod densities in the near-field were negatively correlated with drilling indicators (barium, 

SBF). Generally, near-field stations with barium concentrations higher than about 10,000 μg/g 

and/or SBF concentrations greater than about 1,000 μg/g had low amphipod densities 

(Figure 15.23). 

A detailed taxonomic analysis of 24 stations showed that species composition reflects both 

geographic location and drilling impacts. In terms of community statistics, some stations “near 

drilling” had lower diversity, lower evenness, and lower richness indices compared with stations 

“away from drilling” (Figure 15.24). Station/cruise groups most likely affected by drilling (as 

indicated by high barium and SBF concentrations) were dominated by high abundances of one or 

a few deposit-feeding species. For example, at VK 916, Station NF-B07 was numerically 

dominated by the polychaete Capitella capitata, a classic indicator of organic enrichment, and at 

GB 516, Station NF-B02 was numerically dominated by the gastropod Solariella sp. A. 

15.2.3.5 Megafauna 

Overall, small megafauna identified in photographs showed little impact; there was no gross 

reduction or increase in imaged fauna (Chapter 12). However, localized significant shifts were 

found for two component groups. Fish were more abundant in the near-field, consistent with 

attraction to disturbance and structure (and possibly, increased food supplies in the form of 

elevated macroinfaunal densities). Ophiuroids were less abundant in the near-field, perhaps 

because these flat organisms were buried by cuttings, or perhaps there was a behavior shift, which 

resulted in fewer animals being seen in the near-field. Figures 15.25 and 15.26 show that the 

near-field vs. far-field differences for fish and ophiuroids were seen mainly at VK 916 and 

GB 516. There was little or no difference between near- and far-field at GB 602 or MC 292. 

15.2.3.6 Discussion 

The geophysical and chemical data reviewed previously indicate that a layer of cuttings and muds 

up to 45 cm thick was deposited within the near-field sites. Based on previous studies, one would 

expect impacts to include burial and smothering and changes in benthic assemblages due to 

altered sediment grain size and mineralogy (National Research Council 1983; Neff 1987; 

Continental Shelf Associates, Inc. 1989; Hinwood et al. 1994). Because SBM cuttings were 

discharged, toxicity, organic enrichment, and anoxia (including potentially toxic concentrations 

of sulfide and ammonia) are additional factors (Neff et al. 2000). 

Observations consistent with burial and/or stressed conditions include 

• Azoic areas observed in near-field sediment profile images from VK 916, GB 516, and 

GB 602; 

• Absence or rarity of surface-dwelling macroinfauna in near-field sediment profile images 

from VK 916; 

• Reduced densities of macroinfaunal amphipods and ostracods in the near-field box cores; and 

• Reduced densities of ophiuroids in near-field photographs. 
Figure 15.23. Amphipod densities vs. sediment barium concentrations on post-drilling cruises. Note the log scale for b

VK 916 GB 516

MC 292

Amphipods per m

2

0

100200300400

102

Barium Concentration (μg/g dry wt)

Amphipods per m

2

Near-Field, Cruise 3BFar-Field, Cruise 3B

Barium Concentration (μg/g dry wt)

0

100200300400

Barium Concentration (μg/g dry wt)

Amphipods per m

2

Near-FieldFar-Field

0

100200300400

Barium Concentration (μg/g dry wt)

Amphipods per m

2

GB 602

103 104 105 106

102 103 104 105 106 102 103 104 105 10

102 103 104 105 10

0

100200300400

517
Figure 15.24. Macroinfaunal community characteristics for the detailed, 24-station analysis. Diversity (H')

(a) Density

0

10,000

20,000

30,000

40,000

Near

Drilling

Density (no./m2

)

Away from

Drilling

(b) Diversity

Away from 

Drilling

Near

Drilling

Evenness (J')

0

0.2

0.4

0.6

0.8

1

Away from 

Drilling

Near

Drilling

(c) Evenness

0

1

2

3

4

(d) Richness

Richness (D)

0

5

10

15

20

Away from 

Drilling

Near

Drilling

"Away from Drilling" = all far-field 

stations plus near-field pre-drilling 

stations at VK 916

"Near Drilling" = all near-field stations 

at GB 516, GB 602, and MC 292, plus 

near-field post-drilling stations at 

VK 916

518
Figure 15.25. Fish abundance along camera sled transects at each site. Points are individual transects.

GB 602

05

10152025

No. of Fish/Transect

MC 292

05

10152025

No. of Fish/Transect

GB 516

05

10152025

No. of Fish/Transect 

VK 916

05

10152025

No. of Fish/Transect Cruise 1B Cruise 3B

FarField Near- Field

FarField Ne

F

Far- Field Near- Field

NearField Far-Field (and Near-Field

Pre-Drilling)

519
Figure 15.26. Ophiuroid abundance along camera sled transects at each site. Points are individual transects.

VK 916

05

101520

Far-Field 

(and Near-Field

Pre-Drilling)

No. of Ophiuroids/Transect Cruise 1B Cruise 3B

GB 516

05

101520

No. of Ophiuroids/Transect 

GB 602

05

101520

No. of Ophiuroids/Transect

MC 292

05

101520

No. of Ophiuroids/Transect

NearField Far- Field Near- Field

FarField Near- Field Far- Field Near- Field

520
521 

Sediment toxicity may be a contributing factor in the observed negative effects. Acute toxicity 

tests with near-field and far-field sediments from MC 292 and GB 602 showed that mean 

amphipod survival was significantly lower in sediments from near-field stations than in sediments 

from far-field stations (Chapter 14). Amphipod survival was negatively correlated with drilling 

mud and cuttings indicators (barium and SBF) for both sites. However, the bioassays were 

conducted with estuarine amphipods (rather than indigenous deep-sea fauna) and do not simulate 

actual field conditions. The potential for toxicity is addressed further in the Ecological Risk 

Assessment (Chapter 16). 

On the other hand, there is evidence of elevated biological activity associated with a disturbed, 

organically enriched habitat. This includes 

• Microbial mats observed in sediment profile images; 

• Elevated microbial biomass (ATP) associated with high SBF concentrations; 

• Elevated near-field densities of meiofauna, including annelids, harpacticoid copepods, and 

nematodes; 

• Elevated near-field densities of macroinfauna, including annelids, bivalves, and gastropods; 

• Predominance of pioneering stage macroinfaunal assemblages at some near-field stations; 

and 

• Elevated near-field densities of fishes at VK 916 and GB 516. 

Similar benthic impacts of SBM cuttings on the Gulf of Mexico continental slope were noted 

during an opportunistic survey after development drilling at the Pompano development site in 

MC 28 (water depth 565 m) (Gallaway et al. 1997; Fechhelm et al. 1999). All observations were 

within 90 m of the drillsite, and this area had elevated macroinfaunal densities (up to 

13,724 individuals/m2

) and low diversity relative to baseline conditions. Macroinfaunal 

populations were dominated by epibenthic scavengers, including cyclopoid copepods and 

gastropods, and the authors suggested that true infaunal dwellers (e.g., tube-dwelling polychaetes) 

had been buried by cuttings discharges. There was no consistent pattern of increasing or 

decreasing density in relation to distance or SBF concentration. The increased macrofaunal 

densities are consistent with the results seen in the present study. However, the benthic effects 

observed here are more complex and patchy than observed in the Pompano study. Sediment 

profile imaging data indicate a range of post-drilling conditions, including areas of azoic sediment 

column, areas of dominance by surface-dwelling pioneering assemblages, and areas where recent 

disturbance has selectively compromised near-surface species relative to deeper-living infauna. 

Differences between the studies include the range of distances from the drillsite (within 90 m in 

Pompano, within 500 m in this study) and the amount of time elapsed since the most recent 

drilling discharges (4 months in Pompano, 5 months to 2 years in this study). 

Direct comparisons with the SBM monitoring program are not possible because that study did not 

include box core sampling at continental slope sites (Continental Shelf Associates, Inc. 2004). 

However, data were collected at three shallower sites on the continental shelf. At one site with 

relatively high SBF concentrations, the abundance and diversity of the benthic community were 

reduced within 250 m of the site center. There was evidence of recovery during 1 year between 

sampling cruises at this site. Near- and mid-field sediments at two other sites (with lower SBF 

concentrations) had only moderately disturbed benthic community structure. Variability of 

diversity and evenness was greatest in the near-field zone and generally much lower in the 
522 

far-field zone. In the near-field zone, this result was probably due to variations in sediment 

textures and patchy distributions of cuttings. 

15.3 SITE COMPARISONS 

15.3.1 Areal Extent of Impacts 

Table 15.8 compares the areal extent of impacts among the four sites. Measures of areal extent 

include the areas of geophysically mapped mud and cuttings zones and the number of near-field 

stations with elevated barium and/or SBF concentrations. The latter is indicated by values 

exceeding the far-field 95% confidence limits. In addition, the number of stations with SBF 

concentrations above 1,000 μg/g and barium concentrations above 10,000 μg/g are tabulated 

since macroinfaunal data indicate some impacts are associated with these high levels. 

Table 15.8. Comparison of sites with respect to areal extent of mud and cuttings deposits. 

Geophysically Mapped

Areal Extent (ha) Barium SBF 

Site and Cruise Well 

Jetting 

Deposits 

Rig 

Discharge 

Deposits 

No. NF 

stations 

>FFa

No. NF stations 

>10,000 μg/g 

No. NF 

stations 

>FF 

No. NF stations 

>1,000 μg/g 

Post-Exploration 

VK 916 Cruise 3A/B 

None 

mapped 13.37 10 5 8 4 

GB 516 Cruise 1A/B (no data) (no data) 12 8 11 3 

Post-Development 

GB 516 Cruise 2A/B 2.48 108.53 12 10 11 6 

GB 602 Cruise 2A/B 1.08 43.18 12 12 12 9 

MC 292 Cruise 2A/B 0.78 25.61 12 3 6 1 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; FF = far-field; NF = near-field. 

a Number of NF stations exceeding 95% confidence limits for FF stations (maximum = 12). 

Overall, the areal extent of impacts was greatest at two post-development sites (GB 516 and 

GB 602). These two sites had relatively large mapped cuttings zones, and most stations had high 

barium and SBF concentrations. Impact extent was smallest at the VK 916 exploration site, 

where only a single well was drilled. Among the post-development sites, MC 292 had the 

smallest extent of impacts based on both the mapped extent of cuttings deposits and the relatively 

few stations having very high barium and SBF concentrations. The latter result may reflect the 

limited use of SBM at this site and the longer elapsed time since drilling ended there (see 

Section 15.3.3). 
523 

The geophysically mapped areal extent of cuttings was positively correlated (r = 0.70) with the 

total number of wells; the relationship with estimated SBM cuttings volume was weaker 

(Figure 15.27). For GB 602 and MC 292, which had five and seven wells respectively, the total 

cuttings area was not much greater than that mapped for the single VK 916 well. Only for 

GB 516 (post-development) was the increase in cuttings area roughly proportional to the increase 

in number of wells (i.e., 7-fold). Although discharges from multiple wells presumably would fall 

in approximately the same pattern on the seafloor, some increase in area due to multiple wells 

would be expected due to variations in current patterns over time as well as redistribution of the 

initial deposits of muds and cuttings. 

The number of stations with very high SBF concentrations (>1,000 μg/g) was positively 

correlated with the volume of SBM cuttings discharges (r = 0.62) (Figure 15.28). Similarly, the 

number of stations with very high barium concentrations (>10,000 μg/g) was positively correlated 

with the estimated SBM cuttings discharges (r = 0.78). Correlations with WBM cuttings 

discharges were much weaker, even for barium (0.14). This is consistent with the prediction that 

SBM cuttings are more likely to settle rapidly near the discharge due to clumping of hydrophobic 

SBM-coated cuttings particles (Brandsma 1996; Neff et al. 2000). 

15.3.2 Severity of Impacts 

Table 15.9 compares the sites with respect to several measures of impact severity. These include 

maximum and mean barium and SBF concentrations in the near-field. Geometric mean is used 

for barium and SBF (i.e., log transformation) because of the order of magnitude differences 

among stations. Also included are maximum TOC concentration (as a measure of organic 

enrichment) and the difference in mean integrated sediment oxygen levels between near-field and 

far-field sites (values previously presented in Figures 15.13 and 15.14). 

Table 15.9. Comparison of near-field sites with respect to impact severity. 

Barium Concentrations SBF Concentrations Site and Cruise 

Max Meana Max Meana TOC Max Sediment O2

Differenceb

Post-Exploration

VK 916 

Cruise 3A/B 171,000 9,895 47,920 291 3.45 -40 

GB 516 

Cruise 1A/B 135,000 11,486 25,131 199 0.62 -31 

Post-Development

GB 516 

Cruise 2A/B 351,000 29,960 29,167 672 2.87 -140 

GB 602 

Cruise 2A/B 116,000 43,385 13,792 2,073 1.99 -202 

MC 292 

Cruise 2A/B 39,700 5,532 2,408 56 1.63 -10 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; SBF = synthetic-based fluid; 

TOC = total organic carbon. 

a Geometric mean concentration. 

b Difference between mean sediment integrated oxygen concentrations at near-field and far-field sites 

(negative values mean less oxygen in the near-field). 
Figure 15.27. Geophysically mapped areal extent of drilling discharge deposits in relation to 

(a) number of wells drilled and (b) estimated synthetic-based mud (SBM) cuttings 

discharges. Viosca Knoll 916 (red dots) is the only exploration site represented 

(Garden Banks 516 geophysical data are for post-development only).

Areal Extent vs. No. of Wells

0

20

40

60

80

100

120

1234567

No. of Wells Drilled

VK 916

MC 292

GB 516

GB 602

Areal Extent vs. SBF Cuttings Discharge

0

20

40

60

80

100

120

0 5000 10000 15000 20000 25000 30000

Estimated SBM Cuttings Discharge (bbl)

Areal Extent of Drilling 

Discharge Deposits (ha)

VK 916

MC 292

GB 516

GB 602

(a)

(b)

Areal Extent of Drilling 

Discharge Deposits (ha)

524
Figure 15.28. Areal extent of cuttings deposits (number of near-field stations with very high barium or synthetic-ba

concentrations) in relation to estimated synthetic-based mud (SBM) and water-based mud (WBM) cu

The maximum number of stations is 12.

02468

1012

0 5000 10000 15000 20000 25000 30000

Estimated SBM Cuttings Discharge (bbl)

No. of Stations with Sediment

Barium >10,000 μg/g

02468

1012

0 5000 10000 

Estimated WBM Cuttings Discha

02468

1012

0 5000 10000 15000 20000 25000 30000

Estimated SBM Cuttings Discharges (bbl)

Areal Extent (Stations with Elevated Barium) Areal Extent (Stations with Eleva

No. of Stations with Sediment

Barium >10,000 μg/g

Areal Extent (Stations with Elevated SBF)

No. of Stations with Sediment

SBF >1,000 μg/g

02468

1012

0 5000 10000 

Estimated WBM Discharg

No. of Stations with SBF

>1,000 μg/g

Areal Extent (Stations with E

MC 292

VK 916

GB 602

GB 516 Cruise 1B

GB 516 Cruise 2B

MC 292

VK 916

GB 516 Cruise 1B

GB 516 Cruise 2B

GB 602

VK 916

GB 516 Cruise 1B

GB 516 Cruise 2B

VK 916

GB 516 Cruise 2B

GB 516 Cruise 1B

525
526 

By these measures, the two Garden Banks post-development sites were the most severely 

affected, having high mean barium and SBF concentrations and the greatest reduction in sediment 

integrated oxygen levels. The VK 916 post-exploration site had the highest individual SBF and 

TOC values and the second highest barium value, but mean barium and SBF concentrations were 

not as high as at GB 516 and GB 602 because fewer stations were affected. By all measures, 

MC 292 was least severely affected, having low maximum and mean barium and SBF 

concentrations and the smallest decrease in sediment integrated oxygen levels. 

Figure 15.29 shows that two measures of impact severity, the geometric mean barium and SBF 

concentrations, were positively correlated with estimated SBM cuttings volume. GB 602 had the 

highest mean barium and SBF concentrations, corresponding to the largest SBM cuttings 

discharge. 

The decrease in mean sediment oxygen levels was strongly negatively correlated with both mean 

sediment SBF concentrations and the estimated SBM cuttings discharges (Figure 15.30). The 

decrease in oxygen was greatest at the site with the highest SBF concentrations and the highest 

estimated SBM discharge (GB 602). 

15.3.3 Duration of Impacts 

The timing of drilling activities vs. sampling cruises differed among sites. The interval between 

cessation of drilling and the date when the surveys began ranged from 5 months to nearly 2 years 

depending on the site (Table 15.10). 

Table 15.10. Timing of drilling vs. cruise dates. 

Site Previous Drilling 

Ended 

Subsequent 

Cruise Cruise Dates Time Interval (approx) 

VK 916 18 Dec 2001 3A (Geophysical) 

3B (Biol/chem) 

7-12 Aug 2002 

4-14 Aug 2002 

8 months 

GB 516 21 Aug 1999 1B (Biol/chem) 23 Oct-17 Nov 2000 14 months 

5 Feb 2001 2A (Geophysical) 

2B (Biol/chem) 

24 June-7 July 2001 

8-25 July 2001 

5 months 

GB 602 3 Jan 2001 2A (Geophysical) 

2B (Biol/chem) 

24 June-7 July 2001 

8-25 July 2001 

6 months 

MC 292 16 July 1999 2A (Geophysical) 

2B (Biol/chem) 

24 June-7 July 2001 

8-25 July 2001 

24 months 

GB = Garden Banks; MC = Mississippi Canyon; VK= Viosca Knoll. 

Over time, SBF residues are decomposed by microbes, resulting in significant decreases in SBF 

concentration (Candler et al. 1995). Figure 15.31 shows that both time and volume of SBM 

cuttings discharges are factors affecting SBF concentrations in near-field sediments. On 

Figure 15.31a, the size of each circle is proportional to the geometric mean SBF concentration in 

the near-field. Mean SBF concentrations tend to increase with increasing SBM discharge 

volume, and tend to decrease with longer elapsed time since drilling. 
Figure 15.29. Relationship between post-drilling sediment barium and synthetic-based fluid (SBF) 

concentrations within near-field sites and estimated volumes of SBF cuttings discharges.

0

500

1000

1500

2000

2500

0 500

0

1000

0

1500

0

2000

0

2500

0

3000

0

Estimated SBF Cuttings Discharge (bbl)

Sediment SBF Concentration (μg/g)

(geometric mean)

GB 602

LEGEND

VK 916

GB 516 Cruise 1B

GB 516 Cruise 2B

MC 292

Sediment Barium vs. SBF Cuttings Discharges

0

10000

20000

30000

40000

50000

0 5000 10000 15000 20000 25000 30000

SBF Cuttings Discharge (bbl)

Sediment Barium Concentration (μg/g)

(geometric mean)

Sediment SBF vs. SBF Cuttings Discharges

527
Figure 15.30. Difference between mean near-field (NF) and far-field (FF) sediment oxygen levels in 

relation to sediment synthetic-based fluid (SBF) concentrations and estimated SBF 

cuttings discharges.

-250

-200

-150

-100

-50

0

0 500 1000 1500 2000 2500

Sediment SBF Concentration (μg/g)

(geometric mean)

-250

-200

-150

-100

-50

0

0 5000 10000 15000 20000 25000 30000

Estimated SBF Cuttings Discharge (bbl)

Difference between Mean 

Near-Field and Far-Field 

Integrated Oxygen (nmoles/cm2

)

Difference between Mean 

Near-Field and Far-Field 

Integrated Oxygen (nmoles/cm2

)

GB 602

GB 516 Cr. 2B

GB 516 Cr. 1B

MC 292

VK 916

GB 602

GB 516 Cr. 2B

GB 516 Cr. 1B

MC 292

VK 916

528
Figure 15.31. Plots of (a) geometric mean synthetic-based fluid (SBF) concentration and (b) geophysically 

mapped areal extent of drilling discharge deposits in relation to estimated synthetic-based 

mud (SBM) cuttings discharges. Red circles indicate post-exploration data; all others are 

post-development.

Areal Extent vs. Time and SBM Discharge

0

5000

10000

15000

20000

25000

30000

35000

0 5 10 15 20 25 30

Months Since Drilling Ended

Estimated SBM Cuttings 

Discharge (bbl)

VK 916 MC 292

GB 516, Cr. 2A

GB 602

Size of the circles is 

proportional to 

geophysically mapped 

areal extent of drilling 

discharge deposits

0

5000

10000

15000

20000

25000

30000

35000

0 5 10 15 20 25 30

SBF Concentrations vs. Time and SBM Discharge

Estimated SBM Cuttings 

Discharge (bbl)

Months Since Drilling Ended

Size of the circles is 

proportional to sediment 

SBF concentration 

(geometric mean) in the 

near-field

GB 602

GB 516, Cr. 2B

VK 916 GB 516, Cr. 1B MC 292

(b)

(a)

529
530 

Similar findings were noted in the SBM monitoring program, which showed an overall trend of 

decreasing sediment SBF concentrations with elapsed time after drilling (Continental Shelf 

Associates, Inc. 2004). In addition, SBF concentrations decreased significantly when 

development sites were resampled 1 year after the first sampling cruise. Barium concentrations 

did not consistently decrease during this time. Drilling mud barite is stable and persistent in 

sediments near drill cuttings discharge sites. Although it can be dispersed and diluted by bed 

transport or burial, this apparently occurs over a longer time period than a year or two at this 

water depth.

In the broader view, data from the sites and adjacent blocks provide insight into the duration of 

drilling impacts. In addition, as discussed in Chapter 4, persistent cuttings and/or anchor scars 

from previous activities at nearby wellsites also were evident. These observations are 

summarized in Table 15.11. They suggest that geophysically detectable mud/cuttings deposits 

persist for 5 years or more and anchor scars may persist for 14 years or more. Because no 

chemical or biological sampling was done in adjacent blocks, it is not known if the mapped 

mud/cuttings deposits from older wells are associated with persistent elevations in barium and 

SBF, anoxic conditions, or altered benthic communities. 

Table 15.11. Impacts observed at various time intervals after drilling, including wells in blocks 

adjacent to the study sites. 

Impacts Noted Block Date Drilling 

Ended 

Time Elapsed 

Since Drilling Physical Chemical 

Study Sites

GB 516 

(Cruises 2A, 2B) 

5 Feb 2001 5 months Mud/cuttings deposits 

Anchor scars 

Debris 

Elevated barium, SBF, 

TOC; low oxygen and Eh 

GB 602 

(Cruises 2A, 2B) 

3 Jan 2001 6 months Mud/cuttings deposits 

Anchor scars 

Debris 

Elevated barium, SBF, 

TOC; low oxygen and Eh 

VK 916 

(Cruises 3A, 3B) 

18 Dec 2001 8 months Mud/cuttings deposits 

Anchor scars 

Debris 

Elevated barium, SBF, 

TOC; low oxygen and Eh 

GB 516 

(Cruise 1B) 

21 Aug 1999 14 months No data Elevated barium and SBF; 

low oxygen and Eh 

MC 292 

(Cruises 2A, 2B) 

16 July 1999 24 months Mud/cuttings deposits 

Anchor scars 

Debris 

Elevated barium and SBF, 

but more patchy and less 

elevated than at other sites 

Adjacent Blocks

MC 248 (adjacent to 

MC 292) 

14 Mar 2000 15 months Mud/cuttings deposits 

Anchor scars 

No data 

VK 872 (adjacent to 

VK 916) 

18 Dec 1998 ~3.5 years Mud/cuttings deposits 

Anchor scars 

No data 

MC 291 (adjacent to 

MC 292) 

20 Oct 1997 ~4 years Mud/cuttings deposits 

Anchor scars 

No data 

GB 602 (Well No. 3 

and two sidetracks) 

7 Aug 1996 ~5 years Mud/cuttings deposits 

Anchor scars 

No data 

GB 516 (Well No. 1) 23 July 1996 ~5 years Mud/cuttings deposits 

Anchor scars 

No data 

VK 873 (adjacent to 

VK 916) 

25 Mar 1988 ~14 years Anchor scars No data 

GB = Garden Banks; MC = Mississippi Canyon; VK = Viosca Knoll; Eh = redox potential; 

SBF = synthetic-based fluid; TOC = total organic carbon. 
531 

15.4 METHODOLOGY EVALUATION 

This study used a combination of conventional and innovative techniques, some of which have 

not been used previously in environmental monitoring of deepwater drilling in the Gulf of 

Mexico. 

15.4.1 Selection of Far-Field Sites 

The six far-field sites for each near-field site were chosen to be at similar water depth and 10 to 

25 km away. The intention also was to locate these sites away from other previous wellsites, to 

the extent possible. However, due to the large number of wells in the Gulf of Mexico, it was 

difficult to find far-field sites that were not within 10 km of previous drilling. For example, all of 

the VK 916 far-field sites had 25 or more previous wells within 10 km (Chapter 3). 

As discussed in Chapter 3, four far-field sites had previous wells within 3 km. These were 

VK 916 FF1 and FF2, and MC 292 FF1 and FF2. Analysis of barium and SBF concentrations 

suggests that two of these (VK 916 FF2 and MC 292 FF1) may have been exposed to small 

amounts of drilling discharges (see Appendix K). At VK 916 FF2, the most likely source was 

four wells drilled between March and May 2002 in VK 915. This drilling occurred during the 

program and could not have been foreseen when the site was selected. However, the other three 

far-field sites had previous wells that probably could have been avoided. The initial mapping 

effort did not include a large enough area (15-km radius from near-field site center), and eventual 

mapping of previous wells for this report turned out to be a complex, iterative exercise, even 

working with the MMS databases. In any future study, it is recommended that more effort be 

devoted to initial mapping to ensure that far-field sites are not near previous drilling. This 

problem also highlights the importance of having multiple reference sites. 

Statistically, elevated barium and SBF concentrations at some far-field sites could make it more 

difficult to detect near-vs.-far-field differences by increasing both the far-field mean and the 

variance among far-field sites. However, the far-field concentrations seen here are orders of 

magnitude lower than most barium and SBF concentrations in the near-field. (Keep in mind that 

SBF concentrations measured at most far-field sites represent background concentrations of 

hydrocarbons that co-elute with SBF in the analytical procedure.) For statistical comparisons of 

the near-field site as a whole vs. the far-field, the effects of slight elevations in the far-field are 

negligible. For example, even though VK 916 FF2 and MC 292 FF1 were included in statistical 

analyses in Chapters 8 and 9, significant near-field vs. far-field differences for both barium and 

SBF were detected. 

The elevated barium and/or SBF levels at some far-field sites represent very small amounts of 

drilling fluids or fine cuttings particles. For example, a barium increase of 500 μg/g would 

represent about 943 μg of industrial barite per gram of sediment, or 0.094% (assuming industrial 

barite averages 53% barium; Trefry et al. 2003). Similarly, an SBF increase of 100 μg/g would 

represent approximately 1,000 μg of cuttings particles per gram of sediment, or 0.1% (assuming 

retention on cuttings averages about 10% on a dry weight basis). 
532 

15.4.2 Geophysical Surveys 

The use of an autonomous underwater vehicle (AUV) for the Cruise 2A and 3A geophysical 

surveys was a marked improvement over the deep-tow methods used on Cruise 1A (Chapter 4). 

The AUV saved time because it could accurately follow a pre-programmed track and did not 

require a large turning radius when moving from one transect to the next. In addition, this system 

was less vulnerable to weather delays. Geophysical data collected with the AUV proved useful in 

mapping the extent of drilling mud and cuttings deposits. The maps identified areas of possible 

cuttings, which could then be targeted for additional sampling. Other techniques, including 

sediment hydrocarbon and metal analyses, sediment profile imaging, and color analysis of bottom 

photographs generally confirmed the mapped distribution of mud and cuttings. 

Although the geophysically derived maps generally corresponded with zones of suspected drilling 

mud and cuttings deposits, there was no “ground truthing” to determine whether deposits were 

accurately mapped or to estimate the minimum thickness that could be detected. Since positive 

identification of cuttings particles is difficult even with samples in hand, there probably is no 

practical way to ensure the accuracy of the mapping. Except where thickness of deposits can be 

estimated from subbottom profiles, the geophysical mapping is most useful as a guide to aid in 

station placement rather than a detailed, quantitative picture of mud and cuttings deposition. 

15.4.3 Seafloor Photography 

In the Northern Gulf of Mexico continental slope study, bottom photographs were taken using a 

camera drifted approximately 2 m above the bottom (Gallaway 1988). At this altitude, the area of 

each photograph was about 2.3 m2

. In the present study, a dragged sled was used to obtain 

close-up images of the seafloor looking straight down from a height of 0.6 m. The area of each 

photograph was much smaller, about 0.22 m2

. While the closeup views of the seafloor obtained 

with this method made it easier to identify biota, the area was found to be too small for useful 

megafaunal counts, as most images showed no fauna or at most, one organism. Originally, it was 

assumed that higher resolution images of small areas would miss large animals but allow 

identification and enumeration of many animals at the lower limit of the megafaunal size range, 

1 cm. This did not prove to be the case. Large fish, crabs, and the rare holothuroid usually 

appeared in only part of the image frame. Some distinctive small fauna was seen, notably 

retracted anemones and hermit crabs as small as 1 cm, but little else. 

Making some simplistic assumptions about density-area relationships, it can be predicted that 

quadrupling the area of the image (doubling the linear dimensions) would produce a four-fold 

increase in the number of images capturing fauna, about 28% based on the current studies. Such 

an increase in size would not lose too many animals below the limits of resolution. 

A digital photographic analysis of seafloor color was developed (Chapter 12) that identified 

patchy areas of dark, blue-black bottom attributed to a covering of cuttings and/or drilling fluids. 

However, an exploratory attempt to develop a digital texture analysis was not successful. 
533 

15.4.4 Detecting Benthic Community Impacts 

Sediment profile imaging (Chapters 6 and 7) proved a useful tool in identifying benthic 

communities affected by SBM cuttings. Each image included not only biological data, but also 

information about the physical/chemical context (e.g., redox conditions). The usefulness of this 

technique is based on a paradigm developed from sampling disturbed shallow-water habitats. 

However, it appears to be applicable to detecting SBM cuttings impacts on the continental slope. 

Sediment profile imaging and box coring provide somewhat different and complementary 

pictures of benthic impacts. While sediment profile images showed areas dominated by 

pioneering assemblages, they did not provide any direct information about the very high densities 

of some macroinfauna and meiofauna species near drilling. Also, sediment profile imaging 

classified some stations as “azoic” (i.e., lacking visible benthic macroinfauna). In contrast, none 

of the box cores had zero macroinfauna, although some had relatively few organisms. The 

patchiness of macroinfaunal distribution and the different scale of observations may help to 

explain this discrepancy. Box core macroinfaunal samples consist of a cube of sediment 32 cm 

x 32 cm and 20 to 30 cm deep. The sediment profile images are 15 cm wide by 20 cm deep 

(maximum) and are looking at a single plane through the sediment column. In images classified 

as “azoic,” there may be macrofauna in the sediment column that are not in the plane of the 

photograph. 

In the present study, most box core specimens were identified only to major group (e.g., 

Annelida), which is not very helpful in understanding impacts. More detailed analysis such as 

identification of polychaete families is necessary to understand whether the community is 

changing – for example, from deep, tube-dwelling species to a pioneering assemblage of surface 

scavengers. A subset of 24 stations was processed in greater detail by the principal investigators 

but was beyond the required scope of this study. For cost-effective sampling, it would appear that 

sediment profile imaging supplemented by box core samples processed at least to the level of 

family would be most useful. 

15.5 CONCLUSIONS 

The following are the main conclusions of the study: 

• Geophysical and chemical measurements indicated that a layer of SBM cuttings and muds 

was deposited within the near-field radius. Geophysically mapped cuttings zones ranged 

from 13 to 109 ha in area, with larger zones observed at post-development sites. Areas 

mapped as cuttings typically extended several hundred meters from wellsites, with the 

greatest extent (about 1 km) observed at GB 602 and GB 516. The cuttings deposits were 

estimated to be up to 45 cm thick at one site (VK 916). 

• Concentrations of drilling fluid tracers (barium and SBF) were elevated by several orders of 

magnitude within near-field sites. Mean sediment concentrations of barium and SBF were 

positively correlated with estimated discharge volumes of SBM cuttings. 

• Areas of SBM cuttings deposition were associated with elevated TOC and anoxic conditions, 

including low dissolved oxygen, negative Eh, and shallow depth of the oxidized layer. Sites 

with larger volumes of SBM cuttings discharges and higher mean sediment SBF 

concentrations had the greatest reduction in mean sediment oxygen levels. 
534 

• Sediment profile imaging indicated that the near-field sites had patchy zones of disturbed 

benthic communities, including microbial mats, areas lacking visible benthic macroinfauna, 

zones dominated by pioneering stage assemblages, and areas where surface-dwelling species 

were selectively lost. 

• Macroinfaunal and meiofaunal densities generally were higher near drilling, although some 

faunal groups were less abundant in the near-field (amphipods, ostracods). Among 

megafauna, increased fish densities and reduced ophiuroid densities were noted in the 

near-field of two sites (VK 916 and GB 516). 

• Microbial biomass (ATP) was elevated in some samples near drilling and positively 

correlated with SBF concentrations above about 1,000 μg/g at VK 916 and GB 516, but not at 

GB 602 or MC 292. The ATP data were problematic, however, with major temporal changes 

and apparent far-field “outliers” complicating the interpretation. 

• Meiofaunal densities in the near-field were not consistently correlated with drilling indicators 

(barium, SBF) or other sediment variables (TOC, grain size fractions). 

• Annelid (predominantly polychaete) and gastropod densities in the near-field were positively 

correlated with drilling indicators (barium, SBF). Some near-field stations with barium 

concentrations higher than about 10,000 μg/g and/or SBF concentrations greater than about 

1,000 μg/g had elevated polychaete densities. A few near-field stations at GB 516 and 

GB 602 had very high gastropod densities, which were associated with barium concentrations 

of 55,000 μg/g or higher and SBF concentrations of 4,500 μg/g or higher. 

• Amphipod densities in the near-field were negatively correlated with drilling indicators 

(barium and SBF). Generally, near-field stations with barium concentrations higher than 

about 10,000 μg/g and/or SBF concentrations greater than about 1,000 μg/g had low 

amphipod densities. Separately, acute toxicity tests with near-field and far-field sediments 

from MC 292 and GB 602 showed that mean amphipod survival was significantly lower in 

sediments from near-field stations than in sediments from far-field stations. Amphipod 

survival in the toxicity tests was negatively correlated with drilling indicators. 

• Detailed taxonomic analysis of a subset of the macroinfaunal samples showed some stations 

near drilling had lower diversity, lower evenness, and lower richness indices compared with 

stations away from drilling. Species composition varied in relation to both geographic 

location and drilling impacts. Station/cruise groups most likely affected by drilling were 

dominated by high abundances of one or a few deposit-feeding species, including known 

pollution indicators. 

• At all four near-field sites, impacts were patchy, with some stations showing conditions 

similar to those at the far-field sites. Impacts generally were less extensive and less severe at 

post-exploration sites than at post-development sites. 

• Impacts attributable to SBM cuttings such as elevated TOC, poor redox conditions, and 

associated biological changes were least severe at MC 292, where the smallest quantities of 

SBM cuttings were discharged. However, the time elapsed since drilling also was longer at 

this site (about 2 years) than at the other three sites (5 to 14 months), and the less severe 

impacts may reflect recovery of this site over time. 

• Observations from the study sites and adjacent lease blocks suggest that geophysically 

detectable mud/cuttings deposits may persist for 5 years or more and anchor scars may persist 

for 14 years or more. Because no chemical or biological sampling was done in adjacent 

blocks, it is not known if the mapped mud/cuttings from older wells are associated with 

persistent elevations in barium, anoxic conditions, or altered benthic communities. 
535 

Chapter 16 

Ecological Risk Assessment 

Jerry Neff, Battelle Duxbury 

16.1 INTRODUCTION 

An ecological risk assessment is the estimation of the likelihood of undesirable effects of human 

activities or natural events and the accompanying risks to non-human organisms (Suter 1997). 

Ecological risk assessment methods are being used with increasing frequency to predict or 

forecast the consequences to the environment of human activities, particularly releases of 

chemicals (USEPA 1992, 1998; Suter 1993, 1997). The results of ecological risk assessment 

should be used by risk managers and environmental regulators to make decisions about how to 

manage operations in such a way as to minimize or mitigate adverse environmental impacts 

(Maciorowski 1997). 

The objective of this chapter is to estimate and evaluate the ecological risk of drilling discharges 

to the ocean from drilling rigs or platforms on the continental slope (≈ 1,000 m) of the northern 

Gulf of Mexico. This risk assessment is based on the results of the study as presented in 

Chapters 3 through 14 as well as the preceding Synthesis (Chapter 15). 

An ecological risk assessment often includes four steps (USEPA 1992, 1998; Suter 1993, 1997): 

• Problem formulation 

• Exposure assessment 

• Effects assessment 

• Risk characterization 

In the problem formulation, the study sites are briefly described and their physical and biological 

characteristics are summarized, as well as the types and amounts of drilling discharges at each 

site. Current understanding of the marine environmental fates and effects of drilling waste 

discharges (water based and synthetic based drilling muds and cuttings) is summarized. The 

latter summary is used as the basis for a simple conceptual model for potential impacts of drilling 

discharges and the four deepwater discharge sites. 

The exposure assessment includes a description of the concentrations of chemicals of concern 

(synthetic base chemicals, hydrocarbons, metals) in near-field and far-field sediments at the four 

sites. The exposure assessment also contains a description of selected physical and chemical 

changes (sediment grain size, geochemistry, and redox state) in near-field sediments that could be 

caused directly or indirectly by accumulation of drilling discharge solids on the seafloor. 

The effects assessment includes a brief summary of the toxicity and marine ecological impacts of 

the chemicals of concern (water based drilling muds and cuttings, synthetic based mud cuttings, 

hydrocarbons, metals), based on laboratory and field studies from the scientific literature. These 

published effects data are used to evaluate changes (measured mainly as difference between 

near-field and far-field) in benthic micro-, meio-, macro-, and mega-biota and sediment toxicity 

as possible direct or indirect effects of drilling discharge accumulations on the seafloor. 
536 

The risk characterization integrates and evaluates the relationships between exposure to 

chemicals of concern or physical/chemical alteration of the seafloor and biological changes in the 

benthic organisms of concern. The risk characterization produces a quantitative estimate of the 

risk of injury to valued ecosystem components from exposure to the chemicals of concern. 

“Risk” is used in this context as the probability that an organism will incur injury (harm) as a 

direct or indirect result of exposure to the chemicals of concern or other potential stressors 

evaluated. 

16.2 PROBLEM FORMULATION 

16.2.1 Deepwater Drilling in the Gulf of Mexico 

Today, the outer continental shelf of the U.S. Gulf of Mexico is the most extensively developed 

and mature offshore petroleum province in the world (Kaiser and Mesyanzhinov 2005). In 2003, 

there were 7,800 active leases on the outer continental shelf and slope of the U.S. Gulf of Mexico, 

54% of them in deep water (Richardson et al. 2004). Since 1947, the international oil industry 

has drilled more than 40,000 wells in the northern Gulf and placed more than 6,300 structures in 

water depths to more than 2,300 m (Kaiser and Mesyanzhinov 2005). 

Technological developments have permitted oil and gas exploration in the U.S. Gulf of Mexico to 

move progressively farther offshore to depths exceeding 2,300 m (MMS 2000a; Richardson et al. 

2004). More than 1,675 wells have been drilled in the Gulf at greater than 200 m water depth, 

culminating in drilling of two discovery wells in 2004 in more than 2,200 m of water in the 

Walker Ridge lease area, and the first well drilled in more than 3,000 m of water in 2003. There 

were 121 producing wells in water depths greater than 1,000 m the northern Gulf of Mexico in 

the first half of 2005 (MMS 2005). 

Exploration for and development and production of offshore oil and gas resources is a massive, 

long-term undertaking that may cause physical, chemical, and biological disturbance to the local 

marine environment (Neff 1987, 2005). Offshore oil and gas operations can be divided into three 

phases: geophysical evaluation, exploration, and development/production. All three phases may 

occur simultaneously during development of large offshore fields. The objective of the current 

study was to evaluate the environmental effects of exploration and development activities at 

continental slope depths in the northern Gulf of Mexico. 

Drilling of wells is the main activity in exploration and development. Exploration, appraisal, and 

development drilling are essentially the same, though the facilities from which drilling occurs 

sometimes are different (Figure 16.1). Drilling at the four sites evaluated in this risk assessment 

was performed from anchored semi-submersible rigs (Chapter 3). 
Figure 16.1. Facilities from which offshore wells are drilled change with water depth. As water 

depth increases beyond 400 m, drilling usually is performed from floating drilling 

units or floating production facilities. From International Association of Oil & Gas 

Producers (OGP 2003). 

16.2.2 Drilling Practices and Discharge Histories 

Similar practices were used for drilling exploration and development wells at the four sites 

evaluated in this risk assessment. Both water based drilling muds (WBM) and synthetic based 

drilling muds (SBM) were used to drill most wells. The surface section (to about 300 m) of 

exploration and most development wells (except offset or deviated wells from an existing 

well-bore) was drilled (spudded) or jetted with a water based spud mud, composed primarily of a 

slurry of bentonite clay or gel (e.g., cellulose polymers) and sometimes barite in seawater. The 

spud mud was pumped down the drill pipe and exited through holes in the drill bit, where it 

picked up cuttings (chips of crushed rock produced by the drill bit) and carried them to the 

seafloor surface. The spud muds and associated cuttings were discharged directly to the seafloor 

until the riser was put in place. 

After the riser was installed, the drilling fluids and cuttings were pumped through the well 

annulus back to the drilling rig, where drilling fluids and cuttings were passed through the solids 

control equipment on the rig to separate the drilling fluid from the cuttings. The drilling fluids 

were returned to the mud pit to be recirculated down-hole (Neff et al. 1987). The cuttings were 

discharged to the ocean as allowed by the National Pollutant Discharge Elimination System 

(NPDES) permit. 

WBM were used for the shallower segments of the wells drilled at the four sites. The treated 

cuttings, containing about 10% to 15% WBM solids, were discharged to the ocean during 

537 
538 

drilling, which occurred approximately half the time during development. There also were 

periodic discharges of WBM to the ocean, particularly when changing the WBM system or 

switching over to SBM. The deeper segments of most wells, particularly if deviated, were drilled 

with SBM. Cuttings were removed from the SBM and cuttings returns and discharged to the 

ocean if they met effluent limitation guidelines in the NPDES permit. The SBM themselves were 

recycled down hole and eventually returned to shore for regeneration or disposal. 

16.2.2.1 Viosca Knoll Block 916 

VK 916 was an exploration site; only one well was drilled within the near-field site. Although 

25 wells were drilled within 10 km before or during the study, most were more than 4 km away 

and the nearest was 2.3 km (Chapter 3). The near-field exploration well was drilled from an 

anchored semi-submersible rig. Eight anchors were deployed in a radial array with a radius of 

about 2,440 m around the site; each anchor had about 450 m of anchor chain on the seafloor. 

Scour by anchor chains at all locations contributes to physical disturbance of the seafloor at the 

rig site. 

Both WBM and SBM were used to drill the exploratory well (Table 16.1). A total of about 

15,880 bbl of WBM and 2,165 bbl of WBM cuttings was discharged to the ocean during 

spudding and drilling of the upper segments of the well. An additional 2,510 bbl of SBM cuttings 

(probably containing about 10% SBF chemical) also were discharged during drilling of the 

deepest sections of the well. The SBM was an internal olefin (IO). A total of about 20,556 bbl of 

drilling mud and cuttings was discharged to the ocean during drilling of this exploratory well. 

Table 16.1. Summary of drilling activities within 500 m of the four drillsites before and during 

this study. bbl = barrel: 42 gallons or 159 L. 

Wells and Discharges VK 916 GB 516 GB 602 MC 292 

Wells within 500 m 

Prior to study 0 2 7 5 

During study 1 5 0 0 

Est. Seafloor Discharges (bbl) 

WBM mud 3,074 13,002 23,946 10,331 

WBM cuttings 419 1,774 3,266 1,409 

Est. Drill Rig Discharges (bbl) 

WBM mud 12,807 29,462 91,301 93,960 

WBM cuttings 1,746 4,018 12,450 12,813 

SBM cuttings 2,510 7,313 28,339 1,490 

Total Discharges 20,556 55,569 159,302 120,003 

SBM Type Syn-Teq (IO) Novaplus (IO)

Petrofree LE 

(LAO) 

Novadril (IO) or 

Petrofree ester; 

Novaplus (IO) or 

Petrofree LE (LAO) 

Novaplus (IO) 

IO = internal olefin; LAO = linear alpha olefin; SBM = synthetic based mud; WBM = water based mud. 
539 

16.2.2.2 Garden Banks Block 516 

Two exploration wells (one straight well and one sidetrack) were drilled within 500 m of the site 

center at GB 516 before the study, and five development wells were drilled during the study 

(Table 16.1). An additional 49 wells had been drilled within 10 km of the site between 1987 and 

2000. Most of these had been drilled in Blocks GB 426/471, 7.5 to 9.1 km from the site. 

Both WBM and SBM were used to drill the wells at GB 516. The SBM segments drilled during 

exploratory drilling before the study were drilled with either an IO or a linear alpha olephin 

(LAO) SBM; IOs were the only SBM used to drill the five development wells drilled in 

2000-2001 during the study. A total of 55,569 bbl of drilling muds and cuttings was discharged 

from the wells within 500 m of site center, including 42,464 bbl of WBM, 5,792 bbl of WBM 

cuttings, and 7,313 bbl of SBM cuttings (Table 16.1). A variety of gas production structures and 

flowlines are positioned on the seafloor at the site (Chapter 3). 

16.2.2.3 Garden Banks Block 602 

GB 602 is a post-development site in 1,125 m of water. A total of seven wells were drilled within 

500 m of the site and an additional 11 wells were drilled 1.3 km to 8.3 km away. Drilling at the 

site was from an anchored semi-submersible rig. The anchor radius was about 2,377 m, and 

about 914 m of anchor chain from each anchor was on the seafloor. A variety of gas production 

structures and flowlines are positioned on the seafloor at the site (Chapter 3). 

Nearly 160,000 bbl of drilling muds and cuttings were discharged during drilling of the seven 

wells at the site center (Table 16.1). Discharges included 115,247 bbl of WBM, 15,716 bbl of 

WBM cuttings, and 28,339 bbl of SBM cuttings. This was the largest volume of SBM cuttings 

discharged at the four sites. 

16.2.2.4 Mississippi Canyon Block 292 

MC 292 is a post-development site in about 1,034 m of water. A total of 16 wells had been 

drilled within 10 km of the site, including five wells within 500 m of site center at MC 292. The 

wells near site center are producing wells with a subsea manifold and flowlines carrying natural 

gas to VK 900, about 44 km to the north. A variety of gas production structures and flowlines are 

positioned on the seafloor at the site. 

Three of the wells near the center of the site were drilled with WBM. Two sidetracks were 

drilled with an IO SBM system. A total of 120,003 bbl of drilling mud and cuttings was 

discharged to the ocean at this site, including 104,291 bbl of WBM, 14,222 bbl of WBM cuttings, 

and 1,490 bbl SBM cuttings (Table 16.1). 

16.2.3 The Benthic Environment of the Study Area 

The continental slope of the U.S. Gulf of Mexico (≈ 200 to 2,700 m) supports unique biological 

communities distinct from those on the continental shelf or Sigsbee and Florida Escarpments 

(Rowe and Kennicutt 2002; Gallaway et al. 2003). Several investigators have identified a distinct 

zonation of benthic community structure with depth on the slope (Rowe and Menzies 1969). This 

apparent zonation is actually the result of interactions among a variety of physical and chemical 
540 

parameters that vary with distance from shore and water depth, the most important of which is the 

availability of organic nutrients (Rowe and Kennicutt 2002). 

Slope communities are food-limited. Nutrients reach the slope benthos as a rain of dissolved and 

particulate organic matter and inorganic nutrients, derived primarily from production in surface 

waters and runoff from land. Deposition rate declines with distance from land and increasing 

water depth, except where water currents and bottom physiography concentrate nutrients, as in 

the Mississippi Trough and DeSoto Canyon (Rowe and Kennicutt 2002). Local nutrient sources, 

such as oil and methane seeps and brine pools, which are abundant on the upper slope of the 

north-central Gulf of Mexico, also increase biological production over large areas (MacDonald et 

al. 1996; MacDonald 1998, 2002; Gallaway et al. 2003; Sager et al. 2003). Outside these local 

nutrient-enriched areas, benthic biomass decreases with decreasing nutrient flux to the benthos 

with depth on the slope. This decrease in biomass with depth is steeper on the slope of the Gulf 

of Mexico than in other oceans studied (Rowe 1983), perhaps because of the lower production in 

surface waters of the Gulf of Mexico. 

The slope communities in the Gulf of Mexico tend to be depauperate in numbers and biomass. 

The mean size of benthic macrofauna tends to be smaller than that of the macrofauna occurring at 

similar depths in the Atlantic (Rowe and Menzel 1971; Rowe et al. 1974; Gallaway et al. 2003). 

On the slope, the biomass of meiofauna tends to be greater than that of macrofauna, which in turn 

exceeds that of the megafauna (Pequegnat et al. 1990). The meiofauna are dominated by 

nematodes, the macrofauna by polychaetes, and the megafauna by echinoderms (particularly 

holothurians) or decapod crustaceans. 

Rowe and Kennicutt (2002) are developing a conceptual model of the slope benthic community 

of the northern Gulf of Mexico. The components of the benthic/demersal food web of the slope, 

classed by size and trophic position, are microbiota, meiofauna, macrofauna, megafauna, and 

demersal fishes. Site-specific information about benthic communities of the study area have been 

presented in Chapters 6 and 7 (sediment profile imaging), Chapter 10 (sediment microbes), 

Chapter 11 (meiofauna and macroinfauna), and Chapter 12 (megafauna and image analysis). 

16.2.3.1 Microbiota 

Benthic microbiota such as bacteria and protists, including foraminifera, are the primary 

consumers of the particulate organic carbon raining down from above, as well as any organic 

matter derived from oil/gas seeps and brine pools. Bacteria are the most abundant benthic 

microbiota, with densities of about 109

 per mL (Rowe and Kennicutt 2002). Protists are less 

abundant, but most are larger than bacteria. Foraminifera are shelled protozoans that often are 

extremely abundant in surface layers of slope sediments, particularly where sediments are 

enriched with organic matter and are hypoxic. Foraminifera often are as large as or larger than 

most meiofauna. Foraminiferal biomass may exceed that of meiofauna, and abundance often 

exceeds that of meiofauna and macrofauna combined. 

16.2.3.2 Meiofauna 

Meiofauna are metazoans between 63 and 300 μm in size, and include nematodes, harpacticoid 

copepods, ostracods, kinorhynchs, small polychaetes, and several other taxa. Nematodes and 
541 

harpacticoids usually are most abundant, but polychaetes and ostracods contribute most to the 

meiofaunal biomass in slope sediments (Gallaway et al. 2003). Most meiofauna feed on benthic 

microbiota, other meiofauna, and small detritus particles. Meiofaunal production and turnover 

often is high, sometimes comparable to that of many microbiota. Production seems to be slower 

in cold, low nutrient slope sediments than in warmer, organically enriched continental shelf 

sediments. Nevertheless, meiofaunal biomass often is greater than that of the macrofauna in 

slope sediments of the Gulf of Mexico (Pequegnat et al. 1990; Rowe and Kennicutt 2002). 

Because of their relatively high biomass, meiofauna may be directly or indirectly responsible for 

much of the organic matter metabolism in slope sediments. Meiofauna graze on particulate 

organic carbon directly and on the microbiota that are the primary consumers of the organic 

matter from surface waters and seeps. They probably are responsible for maintaining bacterial 

populations in log-phase growth and, therefore, are indirectly responsible for maintaining high 

rates of nutrient cycling (Rowe and Kennicutt 2002). 

16.2.3.3 Macroinfauna 

Macroinfauna are invertebrates larger than about 0.3 mm (this group of benthic animals is 

operationally defined by the sieve size used for sampling benthic fauna). About 50% of the 

benthic macroinfauna of slope sediments are polychaetes (Rowe and Kennicutt 2002). Bivalve 

mollusks and crustaceans also are abundant. The most abundant taxa of crustaceans often are 

isopods, amphipods, and tanaids, though penaeid shrimp also may be abundant. Diversity of the 

slope macroinfaunal community in the Gulf of Mexico is relatively high (Gallaway et al. 2003). 

The production-to-biomass ratio of the slope macroinfauna may be well under 1.0, indicating a 

low turnover rate of macrofaunal biomass. The abundance and biomass of macroinfauna decline 

sharply with increasing water depth on the slope, reflecting the depth-dependent decline in food 

availability. Abundance declines less steeply than biomass with depth, indicating that 

macroinfaunal size decreases with depth (Gallaway et al. 2003). Macroinfaunal abundance is 

much lower than that of meiofaunal abundance at the four sites evaluated in this risk assessment. 

For example, the maximum meiofaunal abundance at far-field sites at VK 916 at the time of 

Cruise 1B was almost two orders of magnitude greater than that of the macroinfauna 

(90 individuals/cm2

 vs. 1.7 individuals/cm2) (see Chapter 11). 

The mean size of the Gulf of Mexico slope macrofauna is smaller than that of the western North 

Atlantic slope macroinfauna. The slope macroinfauna feed on one another, on meiofauna, and on 

organic detritus. Most of the slope polychaetes are selective or non-selective deposit feeders, 

followed by carnivores, and then scavengers (Gallaway et al. 2003). The macroinfauna in turn 

are consumed by megafauna and demersal fishes. 

16.2.3.4 Megafauna 

Megafauna are benthic epifauna that can be sampled with a trawl or other net with a 2.5-cm 

stretch mesh and that can be seen easily in bottom photographs (about 1 cm or greater in 

diameter). The dominant megafauna at many slope locations are echinoderms, particularly 

holothurians. Decapod crustaceans and cnidarians, such as sea anemones, soft corals, and sea 

pens, also may be common. The giant isopod, Bathynomus giganteus, a benthic scavenger, 
542 

appears to be most abundant on the upper slope at water depths less than those of the four sites in 

this study (Barradas-Ortiz et al. 2003). 

16.2.3.5 Demersal Fishes 

Demersal fishes live and feed on or near the seafloor. Gallaway et al. (2003) identified 153 taxa 

of demersal fishes in their trawl collections from the continental slope of the Gulf of Mexico. 

Fishes are more abundant in bottom photographs than estimated by trawls. Fish abundance is 

variable with depth on the slope of the Gulf of Mexico between 300 and 1,200 m, and then 

declines steeply to low abundances between 1,300 and 2,900 m (Gallaway et al. 2003). 

Photographic observations indicated a fish density of about 200 to 300 individuals/ha in the 

1,000- to 1,200-m depth interval of the sites evaluated in this risk assessment (Gallaway et al. 

2003). 

16.2.4 Conceptual Model 

The conceptual model for risks of drilling discharges to continental slope waters of the Gulf of 

Mexico describes the chemicals of potential concern (COPCs), their sources, and transport 

pathways. It describes valued ecosystem components (receptors of concern) at potential risk and 

the most likely exposure-biological response scenarios and consequences at the population or 

community level. 

16.2.4.1 Chemicals of Potential Concern 

The largest volume discharges to the ocean associated with drilling of exploration and 

development wells for oil and gas on the continental slope of the Gulf of Mexico are drilling 

fluids (also called drilling muds) and drill cuttings. Three types of drilling muds are used 

offshore (Neff et al. 2000). They include WBM and two types of non-aqueous drilling fluids, 

oil based drilling fluids (OBF) and SBM. Only WBM and SBM were used to drill the wells at 

the sites evaluated in this risk assessment. Bulk WBM and cuttings generated with WBM or 

SBM are permitted for discharge to Federal waters of the Gulf of Mexico, if they meet effluent 

limitation guidelines. 

WBM is a suspension of particulate materials, dissolved salts, and organic compounds in fresh 

water, seawater, or a concentrated brine. The base fluids of synthetic based muds, as the name 

suggests, are well-characterized chemical compounds synthesized specifically for formulation of 

the mud product (Neff et al. 2000). In SBM, the continuous phase is a synthetic organic ester, 

ether, acetyl, or olefin (ether and acetyl SBMs are not used in the U.S.). Most SBM used today in 

the Gulf of Mexico are LAO, internal olefins (IO), or esters (Table 16.2). Some SBM 

formulations contain a mixture of two or three synthetic organic chemicals, usually olefins and 

esters. LAO, IO, and ester SBM were used to drill SBM sections of the wells drilled at the sites 

evaluated in this risk assessment (Table 16.1). The synthetic chemical usually represents 30% to 

90% of the volume and about 20% to 40% of the mass of the SBM (Kenny 1993). The other 

ingredients in SBM usually are similar to those in WBM. However, larger amounts of 

emulsifiers are used in SBM than in WBM. 
543 

Table 16.2. Names and chemical structures of synthetic chemicals used most frequently in the 

Gulf of Mexico for SBM (From Neff et al. 2000).

Synthetic Chemical Type Generic Chemical Structure 

Linear-α-Olefin (LAO) CH3 - (CH2)n - CH = CH2

Internal Olefin (IO) CH3 - (CH2)m – CH =CH - (CH2)n - CH3

Ester CH3 - (CH2)n - C = O 

 \ 

 O - (CH2)m – CH3

Drill cuttings are particles of crushed rock produced by the grinding action of the drill bit as it 

penetrates into the earth (Neff et al. 1987). Drill cuttings range in size from clay-sized particles 

(~2 μm) to coarse gravel (>30 mm) and have an angular configuration. Their chemistry and 

mineralogy reflect that of the sedimentary strata being penetrated by the drill and the amount of 

drilling mud ingredients adhering to the cuttings at the time of disposal. For example, the metals 

concentrations in drill cuttings discharged to offshore waters of California are similar to those of 

the drilling muds used to drill the wells (Phillips et al. 1998). Most of the metals associated with 

cuttings are in immobile forms in minerals from the geologic formations. 

Most of the mass of the ingredients of drilling muds and cuttings is composed of inert solids, 

particularly barite and clay. Liquid or water-soluble ingredients, other than water, usually are 

present at low concentrations or have a very low toxicity when diluted in the ocean (most 

inorganic salts). The few more toxic chemicals, such as bactericides and emulsifiers, usually are 

used in small enough quantities that they do not pose a risk to the marine ecosystem after 

discharge to and initial dilution in the ocean. 

The ingredients of drilling muds and cuttings of major environmental concern, because of the 

volumes used and discharged or their potential for toxicity, are metals and hydrocarbons, 

including SBF chemicals. Most of the metals in drilling muds and cuttings are associated with 

the drilling mud solids, barite and clay, or with cuttings particles and are in solid, immobile 

forms. Several metals are present in most WBM and SBM (Table 16.3). The metals of greatest 

concern, because of their potential toxicity and/or abundance in drilling fluids, include arsenic, 

barium, cadmium, chromium, copper, iron, lead, mercury, nickel, and zinc (Neff et al. 1987, 

2000). Some of these metals are added intentionally to drilling muds as metal salts or 

organo-metallic compounds. Others are present as trace impurities in major mud ingredients, 

particularly barite and clay. 
544 

Table 16.3. Concentration ranges of several metals in water based muds (WBM) from different 

sources and in typical soils and marine sediments. Concentrations are mg/kg dry wt 

(ppm). Modified from Neff et al. (1987). Metals that sometimes are present in 

drilling muds at concentrations more than 10 times those in natural sediments are 

shaded and bold. 

Metal Drilling Muds1 Clay-Loam Soils2 Sediments3

Barium 720 – 449,000 150 − 1,500 1 − 2,000 

Chromium 0.1 – 5960 20 − 100 36 − 110 

Cadmium 0.16 – 54.4 0.01 − 7 0.1 – 0.6 

Copper 0.05 – 307 7 − 70 7 −33 

Iron 0.002 – 27,000 --- 20,000 – 60,000 

Mercury 0.017 – 10.4 <0.01 – 0.90 0.03 − 0.14 

Lead 0.4 – 4226 <10 – 70 10 − 33 

Zinc 0.06 – 12,270 20 − 220 27 − 88 

Nickel 3.8 – 19.9 5 − 50 13 − 45 

Arsenic 1.8 – 2.3 1.7 − 27 6.9 − 26 

Vanadium 14 – 28 --- 63 − 238 

Aluminum 10,800 --- 10,000 – 90,000 

Manganese 290 – 400 50 − 2,000 100 – 10,000 

1 From Neff (1982); 2 from Breckenridge and Crockett (1995); 3 from Robertson and Carpenter (1976), 

Siegel et al. (2000), and Neff (2002b).

The metals most frequently present in drilling muds at concentrations more than 10-fold greater 

than natural concentrations in soils and sediments are barium, chromium, lead, and zinc 

(Table 16.3). Mercury sometimes is present at elevated concentrations in U.S., Canadian, and 

North Sea drilling muds; it is derived from mercury present in drilling mud barite (Neff 2002a). 

The USEPA (1993) placed restrictions on the maximum concentration of mercury in drilling mud 

barite destined for ocean disposal. With wider use of low-trace-metal barite for drilling muds, 

average mercury concentrations in U.S. WBM have declined, though concentrations usually still 

are higher than background concentrations in clean marine sediments. Barite in drilling muds 

used offshore in U.S. waters since 1993 contains an average of about 0.5 ppm mercury (Candler, 

personal communication 2002). 

The most abundant metal in most WBM is barium. Nearly all the barium in drilling mud is from 

barite (BaSO4) added to the mud to increase its density. Clays and clay-rich shales may contain 

high concentrations of barium. Natural marine sediments and the cuttings themselves also 

contain barium; barium concentration in sediments tends to be inversely related to sediment grain 

size. Clean, fine-grained marine sediments may contain more than 1,000 mg/kg barium. Most of 

the barium in sediments is in barite. Many hydrocarbon seeps in the Gulf of Mexico contain high 

concentrations of barium, which precipitates rapidly upon mixing with seawater (rich in sulfate) 

forming barite chimneys (Aharon et al. 2001). 

Barite in drilling muds and sediments has a low solubility in seawater (81 μg barite/L or 48 μg 

Ba/L), because of the high natural concentration of sulfate in the ocean. Because it is insoluble in 

seawater, it has a low bioavailability and low toxicity to marine organisms. 
545 

When present at elevated concentrations, compared to concentrations in clean sediment, drilling 

mud chromium is derived primarily from chrome- or ferrochrome-lignosulfonates or chromate 

salts added intentionally to the mud for viscosity control. Barite and bentonite clay also may 

contain traces of chromium. The chromium in a used drilling mud, even that added as chromate, 

is in the trivalent, chromic valence state. Trivalent chromium salts have low solubilities and 

limited mobility in the environment. They usually have a low toxicity to plants and animals. 

Because of concerns about the environmental impacts of chromium in drilling muds, operators in 

most offshore areas have replaced chrome lignosulfonates with chrome-free mud thinners. Most 

modern WBM and SBM contain low concentrations of chromium. 

Most of the other metals sometimes detected in drilling muds are present primarily as trace 

impurities in barite, clay, or the drill cuttings in the formation penetrated by the drill bit. The 

most abundant metals in barite are barium (not a true metal), lead, zinc, and iron. Lead, zinc, and 

copper also may enter drilling mud in pipe thread compound (pipe dope) or drill collar dope used 

to lubricate the threads and promote electrical conduction between pipe sections (Ayers et al. 

1980). The dopes contain several percent metallic metal; some of the dope gets into the drilling 

mud, contaminating it. Various zinc salts may be added intentionally to drilling mud for control 

of hydrogen sulfide. 

The COPCs used for this risk assessment were selected based on the information discussed above 

and include: 

• SBF chemicals (IO, LAO, ester) 

• Petroleum hydrocarbons (particularly polycyclic aromatic hydrocarbons [PAH]) 

• Total organic carbon (TOC) 

• Cuttings particles 

• Metals (barium, chromium, lead, mercury, and zinc) 

16.2.4.2 Sources and Transport Pathways 

Offshore discharge of drilling wastes to Federal waters of the Gulf of Mexico is limited to those 

drilling muds and cuttings that meet the regulatory requirements of the general NPDES permit. 

Only WBM and cuttings generated with WBM or SBM are permitted for discharge, if they meet 

effluent limitations guidelines. Spud muds, WBM containing bentonite clay and (sometimes) 

barite, and cuttings from the upper interval of a well meet effluent limitations guidelines and are 

discharged directly to the seafloor. After the riser is installed, cuttings recovered from the solids 

control equipment on the platform are flushed with seawater into a central discharge pipe (shunt 

line) that releases the cuttings just above or below the sea surface (Canadian Association of 

Petroleum Producers 2001). Occasionally, if the platform is in an environmentally sensitive area, 

the MMS may require that the shunt line extend to near the seafloor. None of the mud and 

cuttings discharges at the four sites evaluated in this risk assessment were shunted. 

WBM or SBM cuttings are discharged continuously during actual drilling, which usually occurs 

about half the time during drilling of a well. Drill cuttings containing 5% to 10% adsorbed WBM 

solids usually are discharged to the ocean at a rate of 0.2 to 2 m3

/hour (53 to 530 gallons/hour) 

(Neff et al. 1987). SBM cuttings discharged to the Gulf of Mexico during the years of this 
546 

investigation usually contained 7% to 15% SBF chemical (Annis 1997). The general NPDES 

permit for the Western Gulf of Mexico, introduced in February 2002, requires that retention of 

synthetic base chemical on cuttings not exceed 6.9% (internal olefins) or 9.4% (esters). All 

drilling in this study occurred before the new permit came into effect, so there probably was an 

average of about 10% SBF chemical on SBM cuttings discharged at the four sites. Drilling rates 

often are higher with SBM than with WBM, with a concomitant increase in the rate of SBM 

cuttings discharge, compared to WBM cuttings discharge, during drilling. However, there are no 

bulk discharges of drilling muds during drilling with SBM; therefore, total volumes of drilling 

wastes discharged are much smaller when SBM rather than WBM are used. 

There often are periodic bulk discharges of used WBM, particularly when the composition of the 

WBM system must be changed to address down-hole conditions or at the time of switch-over 

from a WBM to an SBM during drilling of deep or deviated sections of the well. In addition, 

when drilling a clay-rich formation, WBM mud viscosity may be controlled by dilution (addition 

of make-up water) of the return from the shale shaker to maintain an optimum bentonite clay 

concentration and viscosity. This WBM management strategy requires frequent bulk discharges 

of drilling mud. 

Estimated volumes of drilling muds and cuttings discharged at the four sites are summarized in 

Table 16.1. Total discharges within 500 m of site center ranged from 20,556 bbl during drilling 

of a single exploratory well at VK 916 to 159,302 bbl during drilling of seven wells at GB 602. 

The largest drilling discharge at all four sites was bulk WBM, followed by SBM cuttings. As 

discussed above, WBM was used to drill the upper sections of the wells. In most cases, there was 

a switch-over to the more expensive SBM systems for drilling the deepest sections of the wells or 

for sidetracks where the superior technical properties of the SBM are needed. 

When WBM and WBM cuttings are discharged to the ocean, the larger particles, representing 

about 90% of the mass of the mud solids, form a plume that settles quickly to the bottom (or until 

the plume entrains enough seawater to reach neutral buoyancy) (Figure 16.2). About 10% of the 

mass of the mud solids consisting of fine-grained unflocculated clay-sized particles and a portion 

of the soluble components of the mud form another plume in the upper water column that drifts 

with prevailing currents away from the platform and is diluted rapidly in the receiving waters 

(Ayers et al. 1980; Brandsma et al. 1980; National Research Council 1983). The fine-grained 

solids in the upper plume settle slowly over a large area of the seafloor. Recent WBM dispersion 

modeling predicts rapid dilution of WBM and WBM cuttings in the receiving water environment 

(Nedwed et al. 2004; Smith et al. 2004). 
Figure 16.2. Dispersion and fates of drilling muds and cuttings following discharge to the ocean. 

When discharged to the ocean, SBM cuttings containing more than about 5% adhering SBM 

solids tend to clump together in discrete masses that settle rapidly to the seafloor (Brandsma 

1996; Delvigne 1996). Water cannot penetrate the hydrophobic mass of cuttings, so the cuttings 

do not disperse efficiently. Ester-based SBM cuttings disperse more readily than cuttings 

containing a synthetic olefin (Growcock et al. 1994). 

Because most SBM cuttings do not disperse effectively in the water column following discharge, 

they settle rapidly through the water column and accumulate on the seafloor near the discharge 

site (Neff et al. 2000). Because of the rapid settling of SBM cuttings, water depth has relatively 

little effect on the depth and areal extent of SBM cuttings accumulations on the bottom (as 

compared with those of WBM cuttings). SBM cuttings accumulate in a very patchy distribution 

within a short radial distance of the discharge point. 

SBM cuttings accumulations were surveyed near a drilling template at the Pompano II 

exploratory drilling site in Mississippi Canyon Block 28 (MC 28) in 565 m of water south of the 

Mississippi River in the northern Gulf of Mexico (Gallaway et al. 1997; Fechhelm et al. 1999). 

Discharges from the rig included 7,700 bbl of WBM cuttings, 5,150 bbl of SBM cuttings, and an 

estimated 7,695 bbl of Petrofree LE (an SBF base chemical containing 90% LAO and 10% ester), 

which was associated with the cuttings. Remotely operated vehicle (ROV) surveys revealed a 

thin layer of cuttings spread in a patchy distribution on the seafloor near the drilling template. 

547 
548 

Maximum cuttings accumulations appeared to be 20 to 25 cm thick in some locations. These 

larger cuttings accumulations probably were derived from direct drilling returns to the seafloor 

during initial drilling with spud WBM before the riser was installed. There was no clear gradient 

of SBM cuttings concentrations with distance in the small radius (<90 m) surveyed. 

More recently, a monitoring program was performed near eight WBM and SBM discharge sites 

in 37 to 556 m of water in the Gulf of Mexico (Continental Shelf Associates, Inc. 2004). There 

was clear evidence of accumulation of drilling discharge solids in seafloor sediments near all the 

discharge sites; however, it was not possible to determine the relative proportions of WBM, 

WBM cuttings, and SBM cuttings in sediments. Most mud and cuttings appeared to be deposited 

within 100 and 250 m of the discharge site at both continental shelf and continental slope water 

depths. Thus, most drilling discharge solids accumulate in sediments near the discharge sites; 

only small amounts are dispersed over a wide area of the seafloor. 

16.2.4.3 Receptors of Concern 

A study panel of the National Research Council (1983) examined all the information available at 

that time on the environmental fates and effects of WBM and cuttings discharges to marine 

waters and concluded: 

“Based on laboratory and field studies to date, most water-based drilling fluids used on the U.S. 

OCS have low acute and chronic toxicities to marine organisms, (because of) the expected or 

observed rates of dilution and dispersal of drilling muds in the ocean after discharge. Their 

effects are restricted primarily to the ocean floor in the immediate vicinity and for a short 

distance downcurrent from the discharge. The bioaccumulation of metals from drilling fluids 

appears to be restricted to barium and chromium and is observed to be small in the field.” 

The same generalizations apply to SBM cuttings (Neff et al. 2000; OGP 2003; Melton et al. 

2004). As a general rule, effects of drilling mud and cuttings discharges on the benthic 

environment are related to the total mass of drilling solids discharged and the relative energy of 

the water column and benthic boundary layer at the discharge site. Near-bottom water currents 

usually are low at continental slope depths in the Gulf of Mexico. Thus, drilling discharge solids 

tend to accumulate on the bottom near the discharge and persist there. 

A sediment quality triad analysis was performed at three continental shelf sites where WBM and 

WBM and SBM cuttings had been discharged (Continental Shelf Associates, Inc. 2004). The 

analysis indicated gradients of decreasing sediment physical and biological disturbance with 

distance from the discharge site and with time after the last drill cuttings discharge, showing that 

ecosystem recovery was occurring. The most important direct effect of accumulation of SBM 

cuttings in near-field sediments was a depletion of oxygen in surficial sediments as a result of 

microbial activity stimulated by organic enrichment of the sediments. This led to an alteration of 

macrofaunal community structure. Thus, biological affects of the cuttings accumulations 

appeared to include microbial, meiofaunal, and macrofaunal communities, and possibly 

megafaunal communities as well. 
549 

Based on the preceding discussion, the receptors of concern for this risk assessment are the 

near-field benthic and demersal communities at the discharge sites. Water column organisms will 

not be included because they are at low risk of harm from drilling discharges. 

16.3 EXPOSURE ASSESSMENT 

Several approaches were used to assess exposure of benthic biological communities to physical 

disturbance and the COPCs from drilling discharges at the four deepwater discharge monitoring 

sites. Side-scan sonar and chirp sonar subbottom profiling were used to map physical disturbance 

and cuttings accumulation and distribution on the seafloor. Surface sediment samples and 

sediment cores were collected in the near-field (≤ 500 m) and far-field (≥ 10,000 m) of each 

discharge site and analyzed for SBF chemicals, PAH, and TOC to document the distribution and 

exposure concentrations of organic chemicals associated with drilling discharges. Thirteen 

metals also were analyzed in the sediment samples. Barium was used as the main tracer of the 

distribution and relative concentration of drilling mud solids on the seafloor. Several metals may 

occur in drilling muds and cuttings at concentrations significantly higher than their concentrations 

in uncontaminated marine sediments. These metals, including arsenic, cadmium, chromium, 

copper, lead, mercury, nickel, vanadium, and zinc, were analyzed in sediments and tissues of 

benthic marine animals as an indication of exposure of benthic fauna to excess amounts of metals 

from drilling discharges. Three other metals were analyzed as an aid to determining the phase 

associations and possible sources of the metals in sediments and the redox state of the sediments. 

These metals are aluminum, iron, and manganese. Finally, two species of benthic/demersal 

megafauna were collected from two sites in July 2001 and analyzed for PAH and 11 metals as an 

indication of direct exposure to bioavailable forms of organic and metal ingredients of drilling 

muds and cuttings. 

16.3.1 Physical Disturbance of Sediments 

16.3.1.1 Anchor Scars 

Side-scan sonar surveys and chirp sonar subbottom profiling revealed evidence of anchor scars in 

the near-field (approximate 3-km radius) at all four sites (see Chapter 4). Anchors (usually at 

least eight) are deployed in a radial pattern around most floating drilling rigs (Figure 16.1). The 

anchor radius depends primarily on water depth. At the water depths of the study sites (1,033 to 

1,125 m), anchor radius ranged from 2,377 m to >3,000 m (see Chapter 3). Some of the anchor 

chain between the anchor and the platform rests on or just above the bottom. The anchor chains 

scour the bottom, producing a radial array of scars on the seafloor. Individual anchor scars 

ranged in length from about 100 m to more than 3,000 m. Anchor scars from other wells drilled 

within the block or in adjacent blocks were identified at all four drilling sites. The largest number 

of anchor scars was at GB 602 (over 80 individual scars, including those associated with wells in 

adjacent blocks). These linear anchor scars caused a localized disturbance of the soft-bottom 

seafloor between about 500 and 3,000 m from the site center. 

16.3.1.2 Seafloor Equipment 

Development of deepwater oil and gas resources requires emplacement of substantial amounts of 

equipment on the seafloor (Regg et al., 2000; Ward, 2000). This equipment includes the template 

and wellhead for each well, pipelines and flowlines, umbilical termination modules, etc. The 
550 

seafloor equipment causes localized disturbance of the seafloor. If bottom currents and bed 

transport are high, sediment scour may occur around the seafloor structures. Physical disturbance 

from seafloor structures is highly localized and probably has minimal impacts on area-wide 

benthic communities (MMS, 2000a). However, at sites with low sediment accumulation rates 

and low bottom current speeds, the disturbance of bottom topography by the seafloor structures 

usually is persistent. Production-related equipment was emplaced on the seafloor at the three 

development sites (GB 516, GB 602, and MC 292), but not at the exploration site (VK 916). 

16.3.1.3 Accumulation of Drilling Muds and Cuttings 

The most important physical disturbance of the seafloor from drilling activities, based on areal 

extent of disturbance and potential for alteration of benthic communities, is accumulation of 

solids on the seafloor from drilling mud and cuttings discharges. Accumulation of drilling mud 

and cuttings solids on the seafloor may cause changes in sediment texture and mineralogy, 

sediment redox depth, and bottom topography. Large accumulations of solids in a small area 

(primarily when cuttings are shunted to the bottom) may bury benthic fauna. These physical 

changes to the seafloor may cause changes in benthic community structure and function. 

The areal distribution of drilling muds and cuttings on the seafloor in the near-field of the four 

sites was estimated in Chapter 4. Tentative identification of mud and cuttings solids on the 

seafloor was based on intensity of acoustic signals returned from side-scan sonar and chirp sonar 

subbottom profiles of mud and cuttings particles, which are denser than surficial sediments. 

Concentrations of barium and SBF chemical usually were higher in sediments from areas where 

mud and cuttings accumulations were detected. This observation lends support to the hypothesis 

that cuttings accumulations were observed in the sonar images. 

Zones identified as drilling muds/cuttings were mapped in a small area (0.8 to 2.5 ha) around the 

site center at three of the four drilling sites (see Chapter 15, Table 15.4). The discharges that 

accumulated in these zones probably were a mixture of spud WBM and cuttings released to the 

seafloor during initial spudding of the wells before the marine riser was set. These areas also 

were the sites of bottom equipment installations, such as templates and wellheads, which may 

have contributed to the local bottom sediment disturbance. 

Cuttings, presumably from rig discharges, were tentatively identified over a larger area at each 

drilling site. The area of cuttings accumulations ranged from 13.4 ha at VK 916, where a single 

exploration well was drilled, to 108.5 ha at GB 516, a post-development site where two 

exploration wells had been drilled before the study and five development wells were drilled 

during the study. The areal extent of mud and cuttings accumulations on the bottom increased as 

the number of wells drilled before or during the study increased (see Chapter 15). 

Surficial sediments at the far-field sites were predominantly hemipelagic silts and clays with 

median grain sizes of 1.6 to 2.4 μm (Chapter 5). Mean TOC concentration in far-field sediments 

ranged from 0.5% to 1.5%. Drill cuttings span a wide range of grain size from fine clays to 

coarse sands and gravels. Thus, accumulation of cuttings in sediments should result in an 

increase in mean grain size or the fraction of sandy sediments in the near-field. 
551 

Sediment profile imaging (Chapter 6) identified a thin layer of sand-sized organic-mineral 

aggregates overlying a uniform layer of clay sediments in the near-field at GB 516 and VK 916 

after drilling. This mineral aggregate layer also was present in near-field sediments at VK 916 at 

the time of the pre-drilling survey in October-November 2000. Therefore, it is uncertain if these 

aggregates represent drill cuttings. Surface accumulations of coarse particles in near-field 

sediments were not reported from sediment profile imaging surveys at GB 602 and MC 292 

(Chapter 7). All surficial sediments at GB 602 were silt-clay, whereas those at MC 292 were 

predominantly clay. A few near-field sediments at all four sites contained slightly elevated 

concentrations of sand. The highest sand concentration (21%) was in a near-field sediment 

sample from GB 602 (Chapter 5). The sediment grain size data provide some evidence of the 

presence of small amounts of cuttings in near-field sediments at the four sites. 

16.3.2 Chemical Alteration of Sediments 

16.3.2.1 Synthetic Based Fluids 

Because SBF base chemicals are well-characterized synthetic mixtures of a small group of 

organic isomers, they are good tracers of the fate and distribution of the organic phase of SBM 

cuttings in the marine environment. SBM cuttings discharged offshore in the Gulf of Mexico 

during this monitoring study probably contained an average of 9% to 12% SBF chemical (Annis 

1997). SBF chemicals biodegrade slowly following discharge of SBM cuttings to the ocean 

(Neff et al. 2000); therefore, concentrations in sediments near a discharge site tend to decrease 

with time after completion of drilling and cessation of discharges. SBF chemicals in sediments 

may adversely affect benthic communities by direct toxicity or by organic enrichment and 

associated development of sediment hypoxia (including production of toxic sulfide and ammonia) 

caused by microbial degradation of the organic chemicals (Neff et al. 2000; Hartley et al. 2003; 

Continental Shelf Associates, Inc. 2004). 

Far-field sediments from the four sites contained <1.4 to 111 μg/g dry wt (parts per million) SBF 

chemical (Table 16.4). The highest value (for VK 916 FF2) may represent contamination from 

discharges from wells drilled in an adjacent block (see Chapter 3). Most far-field sediments 

contained 2 to 10 μg/g SBF chemical, which probably represents background concentrations of 

biogenic and possibly petroleum hydrocarbons that coelute with synthetic olefins in the analytical 

procedure. Near-field sediments collected in November 2000 from VK 916 contain background 

concentrations, reflecting the lack of drilling at this site before the November 2000 field survey. 

Far-field sediments containing more than about 10 μg/g SBF chemical may have received small 

amounts of SBM cuttings from the drillsites monitored or from other drilling activities in the 

vicinity of the monitored sites. 

SBF chemical concentrations in near-field and discretionary sediment samples ranged from 

<1.4 mg/kg to more than 136,000 μg/g (13.6%) (Table 16.4). These sediment samples were 

collected at 12 randomly-selected stations and three discretionary stations, selected to maximize 

the likelihood of detecting drilling discharge solids, within 500 m of site center at each drillsite. 

The near-field samples were collected from the upper 2 cm of sediment and the discretionary 

samples were collected from five depth intervals between 0 and 10 cm in sediments to document 

the vertical distribution of SBF residues. 
552 

Table 16.4. Concentrations of synthetic based fluid (SBF) chemical in near-field, far-field, and 

discretionary box core sediments from the four sites monitored in this investigation. 

Near-field and far-field sediments were from the upper ≈ 2 cm; discretionary 

samples were from different depth intervals between 0 to 2 cm and 8 to 10 cm. 

Data from Chapter 9. 

SBF Chemical Concentration Range (μg/g dry) Site Date Near-Field Discretionary Far-Field 

VK 916 Nov 2000 2.4 − 6.6 --- 4.3 − 7.2 

Aug 2002 7.5 − 47,920 159 − 28,138 2.2 − 111a

GB 516 Nov 2000 5.4 − 25,131 --- 2.0 − 9.4 

July 2001 56.0 − 29,167 21 − 136,297 3.0 − 7.0 

GB 602 July 2001 462 − 13,792 11.0 − 10,572 3.0 − 10.0 

MC 292 July 2001 <1.4 − 2,408 9.0 − 714 <1.4 − 28.0 

a

 One far-field site may have become contaminated by drilling discharges from an adjacent lease block. 

The wide range of SBF chemical concentrations observed in sediments collected within 500 m of 

the four sites demonstrates the extremely patchy and heterogeneous distribution of SBM cuttings. 

Highest concentrations generally were located in regions mapped geophysically as cuttings 

(Chapter 15). Highest SBF chemical concentrations were in near-field surface sediments 

collected at VK 916 in August 2002 shortly after completion of a single exploratory well. Four 

(of 12) near-field samples contained more than 5,000 mg/kg SBF chemical. The 0 to 2-cm layer 

of sediment from the three discretionary stations contained more than 10,000 μg/g SBF chemical 

(range, 10,431 to 28,138 μg/g). Deeper layers in these discretionary samples contained less than 

4,000 mg/kg SBF chemical, and concentration decreased with depth in the sediments. Thus, 

SBM cuttings were present in near-field sediments as a surface layer. There was no evidence of 

accumulation of SBM cuttings in far-field sediments at VK 916 (a single high concentration was 

attributed to discharges from drilling activities in an adjacent lease block). 

Near-field sediments at GB 516 and GB 602 contained similar concentration ranges of SBF 

chemical (Table 16.4). The SBF concentration range was smaller at GB 602 than at GB 516, 

reflecting a more uniform distribution of SBM cuttings in surface sediments at the former site. 

SBF concentrations in all but one near-field sample from the two sites were higher than in all 

far-field samples, suggesting that most SBM cuttings deposition was within about 500 m of the 

discharge location. 

The highest concentration of SBF chemical in two of the three discretionary samples from 

GB 516 was in the 2 to 4-cm depth interval. All three samples in the 0 to 6-cm depth interval in 

one discretionary sample contained more than 10% SBF chemical, indicating nearly exclusively 

cuttings solids in the sample. The near-surface, 0 to 2-cm interval contained the highest SBF 

chemical concentration in the three discretionary samples from GB 602. Drilling had occurred in 

the near-field at GB 516 between July 1999 and February 2001 (within five months of the July 

2001 survey) and at GB 602 between September 1995 and January 2001 (within six months of the 

July 2001 survey). Thus, the vertical distribution of SBM cuttings in sediments at the two sites 

was not related to time between completion of drilling and sample collection. 
553 

Sediments at MC 292 contained the lowest concentrations of SBF chemical. Only 5 of 

12 near-field samples and 2 of 15 discretionary samples contained more than 100 μg/g SBF 

chemical. This site was the closest of the four sites to the mouth of the Mississippi River (69 km) 

and had the highest sediment accumulation rate (0.14 cm/year). Five wells had been drilled 

within 500 m of site center between February and July 1999, more than two years before the 

survey. Thus, the low SBF chemical concentrations in near-field sediments at MC 292 can be 

attributed to dilution of cuttings with natural sediments and biodegradation of the SBF chemical. 

MC 292 also had the lowest volume of SBM discharges (Chapter 3). 

16.3.2.2 Polycyclic Aromatic Hydrocarbons 

PAH are composed of two or more fused benzene rings. PAH in the environment are derived 

primarily from petroleum and from combustion of organic matter (Neff 2002b). Their 

concentrations were determined in this monitoring program as an indication of possible petroleum 

contamination of deepwater sediments from drilling discharges. 

Modern WBM and SBM rarely contain more than traces of PAH. OBF formulated with diesel 

fuel or mineral oil may contain PAH. Enhanced mineral oils used to formulate modern OBF 

contain less than 0.001 weight percent (1 μg/g) total PAH (Jacques et al. 1992). OBF and OBF 

cuttings, even those prepared with enhanced mineral oil, have never been permitted for discharge 

to U.S. territorial waters. 

Cuttings produced during drilling with WBM or SBM may contain small amounts of petroleum 

hydrocarbons. The hydrocarbons may come from spotting fluids and lubricants added to the 

mud, or from the geologic strata being penetrated by the drill. Drilling mud and cuttings from all 

depths in a well off California contained petroleum hydrocarbons, including PAH (Table 16.5). 

The drilling muds contained higher concentrations of total petroleum hydrocarbons (TPH) and 

lower concentrations of total and individual PAH than the cuttings at all depths, suggesting that 

the PAH were derived primarily from the formations being drilled. Cuttings from well intervals 

through hydrocarbon-bearing formations contain high concentrations of petroleum hydrocarbons 

and are not permitted for ocean discharge. However, some may be discharged with cuttings from 

shallower or deeper strata. 

Table 16.5. Concentrations of hydrocarbons in composite samples of water based mud (WBM) 

and drill cuttings (in parentheses) from three drilling depths in a well in the Point 

Arguello Field, California. Concentrations are μg/g dry wt (ppm). From 

Steinhauer et al. (1994). 

Chemical Surface Mid-Well Bottom Average 

Total Petroleum Hydrocarbons1 159 (600) 137 (95) 988 (526) 390 (407) 

Total PAH2 0.87 (2.3) 8.0 (12) 51 (121) 25 (45) 

Naphthalenes3 0.27 (1.2) 5.4 (8.9) 39 (96) 18 (35) 

Fluorenes3 ND (ND) 0.38 (0.35) 4.1 (8.2) 2.8 

Phenanthrenes3 0.34 (0.79) 0.94 (0.64) 4.5 (9.3) 2.8 (3.6) 

Dibenzothiophenes3 0.03 (ND) 0.71 (0.40) 3.9 (8.1) 1.9 (2.8) 

1 Total resolved + unresolved hydrocarbons. ND = Not detected. 

2 Total 2- through 5-ringed PAHs plus alkyl homologues. 3

 Includes parent PAH and alkyl homologues.
554 

The source of the high TPH in the cuttings from near the surface is unknown. Total and 

individual PAH concentrations increased with depth in the well. TPH and PAHs in the cuttings 

from the bottom of the well closely resembled the crude oil in the reservoir and probably were 

from the hydrocarbon-bearing formation. 

Uncontaminated marine sediments usually contain small amounts of PAH (Neff 2002b). Most of 

the PAH in offshore marine sediments are from human sources, including petroleum product use 

and combustion of fossil fuels. Concentrations in sediments tend to decrease with distance from 

human activities and with increasing water depth. Background concentrations of total PAH in 

sediments from the continental slope and rise off southern New England are about 0.1 μg/g in 

surface layers and 0.01 to 0.02 μg/g in the 24 to 35-cm depth interval of sediment cores 

(Venkatesan et al. 1987). Deep-sea sediments from 2,505 to 5,906 m water depth in the central 

Pacific Ocean contain 0.0008 to 0.061 μg/g total PAH (sum of 17 analytes) (Ohkouchi et al. 

1999). In areas where there is offshore oil production activity or natural oil seeps, sediment PAH 

concentrations may be much higher. Continental shelf and estuarine sediments off Texas have a 

background total PAH concentration of about 0.03 μg/g and 0.1 ng/g, respectively (Brooks et al. 

1990). Sediments near three platforms in 20 to 150 m of water in the Gulf of Mexico, monitored 

in the GOOMEX program, contained 0.008 to 1.24 μg/g total PAH (Kennicutt et al. 1996b). 

Surface sediments from the continental slope of the central and western Gulf of Mexico, 

including seep areas, sampled during Cruise 1 of the Deepwater Gulf of Mexico Program 

contained <0.01 to 1.03 μg/g total PAH (Rowe and Kennicutt 2002). Highest concentrations 

were in lower shelf and upper slope sediments off the Mississippi River and in areas of the upper 

slope near seeps and brine pools. 

Concentrations of total PAH in near-field and far-field sediments from the four sites ranged from 

0.012 to 23.84 μg/g dry wt (Table 16.6). The highest concentrations were in the surface layer of 

a discretionary sediment core collected at GB 516. The sediment core containing the highest 

PAH concentration contained more than 16 μg/g total PAH in the four depth intervals between 

0 to 2 cm and 6 to 8 cm, with concentration decreasing slightly with depth in the core. This 

sediment core also contained high concentrations (≥ 10,000 μg/g) of SBF chemical. The 

sediments in this core also contained elevated (compared to other sediment samples) 

concentrations of barium, mercury, lead, and TOC (see Chapter 8). The PAH in these samples 

probably were derived from drilling discharges. 

Table 16.6. Concentrations of total polycyclic aromatic hydrocarbons (PAH) in near-field, 

far-field, and discretionary box core sediments from the four sites. Near-field and 

far-field sediments were from the upper ≈ 2 cm; discretionary samples were from 

different depth intervals between 0 to 2 cm and 8 to 10 cm. Data from Chapter 9. 

Total PAH Concentration Range (μg/g dry) Site Date Near-Field Discretionary Far-Field 

VK 916 Nov 2000 0.16 − 0.29 --- 0.18 − 0.39 

Aug 2002 0.25 − 0.44 0.012 − 2.16 0.18 − 0.40 

GB 516 Nov 2000 0.19 − 3.47 --- 0.12 − 0.44 

July 2001 0.14 − 0.43 0.043 − 23.84 0.15 − 0.25 

GB 602 July 2001 0.13 − 0.55 0.12 − 0.65 0.094 − 0.15 

MC 292 July 2001 0.17 − 0.74 0.094 − 0.55 0.21 − 0.75 
555 

Most sediment samples from the four sites contained total PAH concentrations higher than 

background for deepwater sediments. There was no difference in total PAH concentrations in 

near-field and far-field sediments (Table 16.6). Thus, with the exception of the discretionary 

sediment core from GB 516 discussed above, it is uncertain if the excess PAH came from drilling 

discharges or other development and production activities. 

One of the sediment samples from the discretionary cores at VK 916 contained more than 2 μg/g 

total PAH. Because no development and production activities occurred at this site, it is possible 

that the excess PAH in near-field sediments were from drilling discharges. However, two wells 

were drilled 2.3 km from the site in VK 872 in late 1998 , and eight wells were drilled 4.6 to 

7.4 km from the site in VK 915 in between January 2001 and June 2002, before the August 2002 

field survey (see Chapter 3). The sediment PAH and SBF concentrations may have come from 

these development activities. 

16.3.2.3 Total Organic Carbon 

Mean TOC concentrations in far-field sediments at the four sites ranged from 0.82% to 1.53% 

(Table 16.7) (see Chapter 8). Highest concentrations were in sediments from sites nearest the 

mouth of the Mississippi River (MC 292 and VK 916). These concentrations are typical for 

fine-grained continental shelf and slope sediments. 

Table 16.7. Mean (± standard deviation) and range of concentrations of total organic carbon in 

sediments from the four study sites. Data from Chapter 8. 

Total Organic Carbon (%) Site Date Near-Field Far-Field 

VK 916 Nov 2000 1.54 ± 0.21 (1.3 − 1.9) 1.44 ± 0.11 (1.3 − 1.6) 

Aug 2002 2.02 ± 0.77 (1.4 − 3.4)a 1.53 ± 0.13 (1.3 − 1.7) 

GB 516 Nov 2000 1.25 ± 0.69 (0.8 − 3.3)b 0.82 ± 0.08 (0.7 − 1.0) 

July 2001 1.32 ± 0.51 (0.8 − 2.9)b 1.08 ± 0.07 (1.0 − 1.3) 

GB 602 July 2001 1.44 ± 0.28 (1.1 − 2.0)b 1.12 ± 0.13 (0.9 − 1.3) 

MC 292 July 2001 1.35 ± 0.12 (1.2 − 1.6) 1.41 ± 0.08 (1.3 − 1.5) 

a NF post-expl. >NF pre-expl.; b NF > FF.

There was a statistically significant increase in sediment TOC concentrations in near-field 

sediments between the pre-drilling and post-drilling surveys at VK 916. TOC concentrations 

were significantly higher in near-field than in far-field sediments at GB 516 and GB 602 

(Table 16.7). The discretionary sediment core from GB 516 containing high concentrations of 

SBF chemical and total PAH also contained higher TOC concentrations (6.5% to 8.9% in July 

2001) than any of the other sediment samples from the four sites. These results suggest that part 

of the sediment TOC was associated with drilling discharges. 
556 

16.3.3 Metals in Sediments 

Trefry et al. (Chapter 8) measured concentrations of 13 metals in near-field, far-field, and 

discretionary core sediments collected in November 2000 and August 2002 at VK 916, in 

November 2000 and July 2001 at GB 516, and in July 2001 at GB 602 and MC 292. The metals 

include aluminum, arsenic, barium, cadmium, chromium, copper, iron, lead, manganese, mercury, 

nickel, vanadium, and zinc. Aluminum, iron, and manganese are useful for evaluating the solid 

phase associations of the other metals. The metal COPCs in this risk assessment, because of their 

frequent presence at elevated concentrations in drilling muds (Table 16.3), are barium, 

chromium, mercury, lead, and zinc. 

16.3.3.1 Barium 

Background concentrations of barium in uncontaminated marine sediments increase with 

decreasing sediment grain size and increasing water depth and range from about 1 to 2,000 mg/kg 

(Table 16.3). 

Mean barium concentrations in far-field sediments from the four sites range from 850 mg/kg to 

1,480 mg/kg (Table 16.8). Barium concentrations in near-field sediments collected in November 

2000 at VK 916 are comparable to those in far-field sediments and reflect the lack of drilling 

operations in the immediate vicinity of the site center previous to the November sampling. These 

sediment barium concentrations are somewhat higher than expected background concentrations 

for slope sediments and probably reflect drilling waste inputs from previous drilling operations at 

nearby sites. 

All near-field sediments, except those collected in November 2000 at VK 916, contained 

significantly higher barium concentrations than far-field sediments collected at the same time 

(Table 16.8). Highest mean concentrations were in near-field sediments collected in July 2001 at 

GB 516 and GB 602. Barium concentrations were highest in surface layers of sediments from 

discretionary cores collected at all four drilling sites, and barium concentration decreased with 

increasing depth below the surface in the cores (Chapter 8). This suggests recent deposition of 

barium, as solid barite, in near-field sediments at all four sites. There is little doubt that the 

enrichment in barium concentrations in near-field sediments at the four sites was caused by 

deposition of drilling mud solids from WBM, WBM cuttings, and SBM cuttings discharges. 

Mean barium enrichments (NF [barium]/FF [barium]) ranged from 12- to 60-fold. 

Table 16.8. Mean (± standard deviation) and range of concentrations of barium in sediments 

from the four study sites. Data from Chapter 8. 

Barium Concentration (mg/kg dry wt) Site Date Near-Field Far-Field 

VK 916 Nov 2000 1,090 ± 160 (800 − 1,290) 870 ± 170 (700 − 1,180)

Aug 2002 35,900 ± 52,600 (1,050 − 171,000)a 940 ± 220 (670 − 1,390)

GB 516 Nov 2000 24,500 ± 38,300 (3,100 − 135,000) 1280 ± 340 (760 − 1,970)

July 2001 59,000 ± 95,700 (8,600 − 351,000)b 1480 ± 560 (790 − 2,770)

GB 602 July 2001 50,800 ± 31,900 (16,600 − 116,000)c 850 ± 300 (570 − 1,550)

MC 292 July 2001 9,000 ± 11,000 (1260 − 39,700)c 760 ± 110 (630 − 920)

a NF Aug 2002 > NF Nov 2000; b NF July 2001 > Nov 2000 and NF > FF; c NF > FF.
557 

16.3.3.2 Chromium 

Chromium concentrations were similar in near-field and far-field sediments from all four sites 

(see Chapter 8). Concentrations ranged from about 31 mg/kg (near-field at VK 916 and GB 516) 

to 103 mg/kg (far-field at MC 292). The average concentration of chromium in marine sediments 

is about 72 mg/kg (Trefry et al. 2002), and concentrations in uncontaminated marine and 

estuarine sediments range from 50 to 100 mg/kg (Neff 2002b). Thus, there is no evidence that 

sediments near the discharge sites were contaminated with chromium from drilling discharges. 

16.3.3.3 Mercury 

WBM used offshore in the past sometimes contained elevated (compared to concentrations in 

clean marine sediments) concentrations of mercury. Much of the mercury in drilling discharges 

is from drilling mud barite (Neff 2002a). Since the USEPA (1993) placed a limit of 1 mg/kg on 

mercury in drilling mud barite destined for ocean disposal, offshore operators have been able to 

reduce mercury concentrations in drilling muds to a mean of about 0.5 mg/kg through use of 

cleaner grades of barite. 

The mean background concentration of mercury in marine sediments is about 0.2 mg/kg (Neff 

2002b; also see Chapter 8). Sediment samples collected along several transects perpendicular to 

the shoreline from offshore Corpus Christi, TX, to offshore Panama City, FL, in 210 to 3,140 m 

of water as part of the Deepwater Program: Northern Gulf of Mexico Continental Slope Habitats 

and Benthic Ecology Program, contained mercury at concentrations ranging from 0.009 to 

0.043 mg/kg; concentrations decreased slightly with distance from shore (Rowe and Kennicutt 

2001). Mean mercury concentrations in far-field sediments from the four sites ranged from 0.07 

to 0.12 mg/kg (Table 16.9), slightly higher than those recorded by Rowe and Kennicutt (2001). 

Mercury concentrations were slightly but significantly higher in near-field sediments than in 

far-field sediments at MC 292 and in near-field sediments from the second than those from the 

first survey at VK 916 and GB 516 (Table 16.9). The differences were small, but do indicate that 

small amounts of mercury associated with drilling mud barite were accumulating in near-field 

sediments at some continental slope drilling sites. All mercury concentrations were in the lower 

part of the range expected for uncontaminated marine sediments. 

Table 16.9. Mean (± standard deviation) and range of concentrations of mercury in sediments 

from the four study sites. Data from Chapter 8. 

Mercury Concentration (mg/kg dry wt) Site Date Near-Field Far-Field 

VK 916 Nov 2000 0.071 ± 0.003 (0.065 − 0.075) 0.071 ± 0.004 (0.062 − 0.078) 

Aug 2002 0.090 ± 0.028 (0.071 − 0.15)a 0.077 ± 0.008 (0.064 − 0.087) 

GB 516 Nov 2000 0.10 ± 0.015 (0.079 − 0.13) 0.11 ± 0.009 (0.092 − 0.12) 

July 2001 0.12 ± 0.039 (0.088 − 0.24)b 0.12 ± 0.020 (0.095 − 0.15) 

GB 602 July 2001 0.10 ± 0.027 (0.070 − 0.17) 0.098 ± 0.007 (0.084 − 0.11) 

MC 292 July 2001 0.082 ± 0.012 (0.061 − 0.11)c 0.070 ± 0.006 (0.061 − 0.082) 

a NF Aug 2002 > NF Nov 2000; b NF July 2001 > Nov 2000; c NF > FF.
558 

16.3.3.4 Lead 

Drilling muds and cuttings often contain elevated concentrations of lead. It is present as an 

impurity in drilling mud barite and clay, probably as insoluble lead sulfide (galena), and in pipe 

thread compound used in the joints of drilling pipe (Neff 2002b). Background concentrations of 

lead in uncontaminated marine sediments usually are in the range of 5 to 30 mg/kg, with a mean 

background concentration of 19 mg/kg (Chapter 8). Most lead concentrations in far-field 

sediments from the four sites were in the upper part of the range of background concentrations 

(Table 16.10). 

Table 16.10. Mean (± standard deviation) and range of concentrations of lead in sediments from 

the four study sites. Data from Chapter 8. 

Lead Concentration (mg/kg dry wt) Site Date Near-Field Far-Field 

VK 916 Nov 2000 30.3 ± 3.3 (25.0 − 34.8) 28.3 ± 1.8 (25.3 − 30.9) 

Aug 2002 29.9 ± 3.5 (26.3 − 36.9) 25.4 ± 1.4 (22.4 − 27.4) 

GB 516 Nov 2000 24.1 ± 2.5 (18.3 − 28.7) 24.5 ± 1.6 (20.9 − 27.3) 

July 2001 33.6 ± 10.3 (21.0 − 60.3) 24.0 ± 2.0 (20.9 − 26.8) 

GB 602 July 2001 41.8 ± 10.0 (22.1 − 57.0)a 21.6 ± 1.5 (19.5 − 24.1) 

MC 292 July 2001 30.1 ± 5.0 (24.6 − 44.4) 31.4 ± 1.1 (30.2 − 34.0) 

a NF > FF. 

A few near-field sediment samples from GB 516 and GB 602 contained slightly elevated lead 

concentrations. The mean lead concentration in near-field sediments from GB 602 was 

significantly higher than the mean concentration in far-field sediments from the same site. The 

difference was small but may represent input of lead-contaminated solids from drilling discharges 

or platform operations (paint chips or sacrificial anodes). 

16.3.3.5 Zinc 

Zinc concentrations are extremely variable in uncontaminated marine sediments. Most zinc is 

present in highly insoluble residual forms as part of the mineral lattice of sediment particles or in 

heavy minerals, such as chromite, ilmenite, and spalerite (zinc sulfide) (Neff 2002b). Zinc 

concentrations in the sandy sediments of Georges Bank are in the range of 1.2 to 70 mg/kg; they 

are much higher in fine-grained sediments of Terra Nova Bay, Antarctica, ranging from 75 to 

155 mg/kg. Sediments from the coastal zone of Louisiana contain 52 to 133 mg/kg zinc. 

Reflecting this variability in sediment zinc concentrations, the mean zinc concentrations in 

far-field sediments from the four sites range from 77.4 to 113 mg/kg (Table 16.11). 
559 

Table 16.11. Mean (± standard deviation) and range of concentrations of zinc in sediments from 

the four study sites. Data from Chapter 8. 

Zinc Concentration (mg/kg dry wt) Site Date Near-Field Far-Field 

VK 916 Nov 2000 108 ± 2.0 (106 − 110) 104 ± 3.0 (99 − 106) 

Aug 2002 106 ± 13 (88 − 142) 107 ± 4.0 (101 − 116) 

GB 516 Nov 2000 94.4 ± 14.9 (84.1 − 133) 90.2 ± 3.3 (83.4 − 95.4) 

July 2001 96.2 ± 38.3 (63.1 − 213) 86.8 ± 3.5 (81.5 − 92.6) 

GB 602 July 2001 90.3 ±8.9 (76.2 − 101)a 77.4 ± 5.2 (71.8 − 89.3) 

MC 292 July 2001 120 ± 58 (50 − 294) 113 ± 12 (90 − 141) 

 a NF > FF. 

The means and range of zinc concentrations in near-field sediments are similar to those in 

far-field sediments (Table 16.11). Highest concentrations are in a few near-field sediment 

samples from MC 292 and GB 516. The near-field mean sediment zinc concentration was 

significantly higher than the far-field mean only at GB 602. Thus, there is only limited, weak 

evidence of zinc contamination of near-field sediments from drilling discharges. 

16.3.4 Polycyclic Aromatic Hydrocarbons and Metals in Tissues of Benthic Megafauna 

Two species of deep-sea benthic/demersal animals were collected at near-field and far-field sites 

at MC 292 and GB 602 in July 2001 and analyzed for total PAH and 11 metals. The giant isopod 

Bathynomus giganteus and the red crab Chaceon quinquedens were collected in baited traps 

(Chapter 2). 

16.3.4.1 Polycyclic Aromatic Hydrocarbons 

Concentrations of total PAH in soft tissues of giant isopods and red crabs ranged from 0.039 to 

0.18 μg/g dry wt (parts per million) (Table 16.12). Concentrations tended to be slightly higher in 

Bathynomus than in Chaceon. Concentrations were slightly higher in far-field than in near-field 

Bathynomus at MC 292; the opposite trend was evident at GB 602. Soft tissues of Chaceon from 

near-field and far-field locations at MC 292 and at near-field sites at GB 602 were similar. No 

samples were available from the far-field at GB 602. Thus, there was no clear evidence of PAH 

bioaccumulation from drilling discharges, which would have been indicated by substantially 

higher PAH body burdens in crustaceans from near-field than from far-field sites. 

Tissue PAH concentrations in nearly all samples from MC 292 and GB 602 were lower than 

concentrations in surface near-field and far-field sediment samples collected at the same time as 

the tissue samples (Table 16.12). Thus, biota/sediment accumulation factors were less than 1, 

indicating limited bioavailability of PAH from site sediments. The PAH in the isopods and crabs 

probably were derived from sediment ingestion or consumption of benthic infauna that were in 

equilibrium with local sediments. 
560 

Table 16.12. Concentrations of total polycyclic aromatic hydrocarbons (PAH) (μg/g dry wt) in 

whole soft tissues of giant isopods Bathynomus giganteus and red crabs Chaceon 

quinquedens collected in July 2001 from MC 292 and GB 602. PAH 

concentrations in two composite samples from each site are shown. PAH 

concentrations in sediments are included for comparison. Data from Chapter 9. 

MC 292 GB 602 Species Near-Field Far-Field Near-Field Far-Field 

Bathynomus giganteus 0.087 − 0.092 0.11 − 0.42 0.13 − 0.16 0.098 − 0.13 

Chaceon quinquedens 0.039 − 0.14 0.107 − 0.108 0.095 − 0.18 No Sample 

Surface sediment 0.17 − 0.56 0.21 − 0.75 0.13 − 0.55 0.094 − 0.15 

16.3.4.2 Metals 

The 11 metals analyzed in whole soft tissues of Bathynomus and Chaceon include arsenic, 

barium, cadmium, chromium, copper, iron, lead, mercury, nickel, vanadium, and zinc. Three to 

six replicate composite tissue samples of each species were analyzed for each metal at near-field 

and far-field sites at MC 292 and GB 602. 

Concentrations of most metals were highly variable in the soft tissues of the two species of 

benthic crustaceans (Table 16.13). The coefficient of variation often was 50% or greater. 

Because of this variability and the small numbers of replicates for each location, significant 

differences in tissue metals concentrations were difficult to detect. 

There were no significant differences between near-field and far-field sites in metals 

concentrations in giant isopods from MC 292 (Table 16.13). However, at GB 602, 

concentrations of barium, chromium, and lead were significantly higher in near-field than in 

far-field isopods. Concentrations of cadmium and mercury were significantly higher in soft 

tissues of isopods from far-field than in those from near-field stations. 

Mean barium concentration was about 10-fold higher in tissues of Bathynomus from near-field 

than from far-field sampling sites at GB 602. It is highly likely that the excess barium in the 

near-field isopods was derived primarily from drilling discharges. 

Cadmium and mercury were present at high concentrations in soft tissues of isopods from both 

near-field and far-field sites at both MC 292 and GB 602. Most of the cadmium and mercury in 

drilling discharges is associated with drilling mud barite (Neff 2005). Because there was no 

relationship between concentrations of barium and either cadmium or mercury in the isopods, it is 

unlikely that the higher than expected concentrations of cadmium and mercury in the crustacean 

tissues were derived from drilling discharges. 

Many non-decapod crustaceans contain high concentrations of cadmium, probably sequestered in 

insoluble granules in excretory tissues (Neff 2002b). Most crustaceans contain less than 1 μg/g 

total mercury in soft tissues (Neff 2002b). However, no data are available for Bathynomus from 

other locations. The unusual life habits of this giant deepwater isopod (Barradas-Ortiz et al. 

2003) may predispose it to bioaccumulate mercury. 
561 

Table 16.13. Mean (± standard deviation) concentrations of 11 metals in whole soft tissues of 

giant isopods Bathynomus giganteus and red crabs Chaceon quinquedens collected 

in July 2001 from MC 292 and GB 602. Three to six replicate samples were 

analyzed for each site. Significant differences between near-field and far-field are 

shaded and bold. Concentrations are μg/g dry wt. Data from Chapter 8. 

MC 292 GB 602 

Metal Near-Field Far-Field Near-Field Far-Field 

Bathynomus giganteus 

Arsenic 39 ± 21 28 ± 23 39 ± 28 61 ± 33 

Barium 17 ± 8 16 ± 15 164 ± 136a 15 ± 9 

Cadmium 14 ± 10 16 ± 17 7 ± 4b 14 ± 6 

Chromium 1.1 ± 0.8 1.5 ± 1.3 1.2 ± 1.2a 0.43 ± 0.04 

Copper 75 ± 86 73 ± 59 51 ± 50 95 ± 86 

Iron 1,310 ± 1,220 2,250 ± 2,530 979 ± 549 1450 ± 540 

Lead 0.38 ± 0.27 0.46 ± 0.38 0.61 ± 0.39a 0.26 ± 0.06 

Mercury 3.3 ± 2.8 4.3 ± 4.4 3.1 ± 1.3b 8.1 ± 6.8 

Nickel 0.9 ± 0.5 0.9 ± 0.6 1.3 ± 0.8 0.55 ± 0.45 

Vanadium 2.8 ± 1.1 3.0 ± 1.7 2.8 ± 1.5 1.7 ± 1.5 

Zinc 264 ± 183 360 ± 339 344 ± 272 327 ± 94 

Chaceon quinquedens 

Arsenic 133 ± 37 131 ± 46 65 ± 25b 170 ± 29 

Barium 43 ± 38a 3.9 ± 1.8 54 ± 46a 11 ± 7 

Cadmium 101 ± 13a 2.9 ± 2.3 2.1 ± 1.1b 12 ± 5 

Chromium 2.5 ± 2.0a 0.6 ± 0.4 1.0 ± 0.6 0.7 ± 0.2 

Copper 248 ± 73 199 ± 70 59 ± 45b 151 ± 65 

Iron 1,340 ± 1,100 612 ± 215 216 ± 111 254 ± 81 

Lead 0.89 ± 0.70 0.25 ± 0.12 0.26 ± 0.10 0.32 ± 0.14 

Mercury 1.0 ± 0.4 0.8 ± 0.4 0.54 ± 0.14b 1.5 ± 0.4 

Nickel 14 ± 4.6 10 ± 3.2 3.4 ± 1.6b 9.2 ± 2.4 

Vanadium 6.0 ± 4.0a 2.8 ± 1.3 1.3 ± 0.7b 3.3 ± 1.0 

Zinc 230 ± 107 226 ± 27 148 ± 61b 261 ± 31 

a

 NF > FF; 

b FF > NF.

The pattern of metal bioaccumulation in red crabs was different from that in giant isopods 

(Table 16.13). Near-field crabs from MC 209 contained significantly higher concentrations of 

barium, cadmium, chromium, and vanadium than far-field crabs did. Only barium was present at 

a significantly higher mean concentration in near-field than far-field red crabs at GB 602. 

However, concentrations of arsenic, cadmium, copper, mercury, nickel, vanadium, and zinc were 

significantly higher in far-field than in near-field Chaceon. As for Bathynomus, the higher 

concentration of barium in crabs from near-field stations is an indication of exposure to drilling 

discharges containing high concentrations of drilling mud barite. It is probable that much of the 

excess barium in the soft tissues of the isopods and crabs from near-field sites is present as 

unassimilated solid barite particles in the gut, digestive gland, and gills (Jenkins et al. 1989). 

The high concentrations of several metals in tissues of red crabs from far-field sites at GB 602 

may be related to the proximity up-slope of mud mounds and hydrocarbon seeps (Sager et al. 
562 

2003) that may be rich sources of metals to bottom waters. The unusually high concentration of 

cadmium in near-field crabs at MC 292 is unexplained. 

16.3.5 Summary of Exposure 

Several physical and chemical parameters for sediments and benthic megafauna were used to 

characterize the distribution and relative amounts of drilling discharges (the disturbance or 

exposure) near four deepwater drilling sites. Evidence of exposure of the benthic environment to 

drilling discharges was defined as a higher parameter value in near-field than in far-field 

sediments at a site. The relative magnitude of benthic disturbance or exposure at the four sites 

was estimated based on the relative areas of seafloor covered by cuttings, as indicated by acoustic 

surveys, and the relative concentrations of chemicals, particularly SBF chemical and barium, in 

near-field sediments. 

Near-field sediments at all four sites showed evidence of disturbance by or exposure from drilling 

operations at site center (Table 16.14). The ranking of the four sites with respect to increasing 

area of seafloor covered by cuttings is: VK 916, MC 292, GB 602, and GB 516. This ranking 

from lowest to highest level of exposure and disturbance is consistent with the data for other 

indicators of physical disturbance or drilling discharge accumulation on the seafloor, as 

summarized in Table 16.14. 

Table 16.14. Summary of evidence of physical disturbance to or chemical exposure of the 

near-field benthic environment at the four study sites. + = evidence of exposure or 

disturbance; +- = weak or uncertain evidence of exposure; − = no evidence of 

exposure or disturbance. The area of seafloor with geophysical evidence of cuttings 

accumulation is summarized. 

Parameter VK 916 GB 516 GB 602 MC 292 

Anchor scars + + + + 

Seafloor equipment – + + + 

Cuttings area 13.4 ha 108.5 ha 43.2 ha 25.6 ha 

Sediment SBF NF > FF + + + + 

Sediment PAH NF > FF − +– − −

Sediment TOC NF > FF + + + −

Sediment Ba NF > FF + + + + 

Sediment Cr NF > FF − − − −

Sediment Hg NF > FF + + − + 

Sediment Pb NF > FF − − + −

Sediment Zn NF > FF − − + −

Tissue PAH NF > FF NS NS − −

Tissue Ba NF > FF NS NS + + 

Other metals NF > FF NS NS +- +- 

NS = no sample; PAH = polycyclic aromatic hydrocarbon; SBF = synthetic based fluid; TOC = total 

organic carbon. 
563 

Barium concentration can be used as a rough indication of the concentration of total drilling 

waste solids (WBM, WBM cuttings, and SBM cuttings) in sediments. SBF chemical 

concentration can be used as a rough indication of the concentration of SBM cuttings solids in 

sediments. However, neither barium nor SBF chemical concentration is a conservative tracer of 

drilling discharges. There was a relatively poor correlation in this study and in the earlier study in 

shallower water (Continental Shelf Associates, Inc. 2004) between barium and SBF chemical 

concentrations in near-field sediments (see Chapter 15). The poor covariance can be attributed to 

highly variable ratios of relative amounts of WBM/WBM cuttings and SBM cuttings discharged 

and accumulating in different regions near the different discharge sites. Because WBM and SBM 

are used sequentially, not concurrently, for drilling, WBM cuttings and SBM cuttings discharges 

occur at different times under different tidal and residual current regimes, resulting in different 

distributions of WBM cuttings and SBM cuttings solids on the seafloor. In addition, the total 

amount of WBM and WBM cuttings discharged per unit depth interval of well drilled is much 

greater than the amount of SBM cuttings discharged. Thus, the WBM mud and cuttings signal in 

sediments often swamps the weaker SBF chemical signal. 

16.4 EFFECTS ASSESSMENT 

Several biological parameters were measured in this investigation as indices of biological effects 

of physical disturbance or chemical contamination of sediments (exposure) from drilling 

activities. Sediment toxicity tests were performed with near-field and far-field sediments from 

the two post-development sites, MC 292 and GB 602 (Chapter 14). Sediment oxygen 

concentration was measured in sediments as an index of organic enrichment from microbial 

biodegradation of organic matter from drilling discharge. Benthic microbial, meiofaunal, 

macrofaunal, and megafaunal communities were examined by sampling and sediment profile 

imaging. In most cases, effects were measured as a difference between near-field and far-field 

biological parameters at each site. 

16.4.1 Sediment Toxicity 

Sediment samples were collected in July 2001 from the two post-development sites, MC 292 and 

GB 602, for laboratory sediment toxicity tests (Chapter 14). The sediment bioassays (10-day 

exposure to sediments from near-field and far-field stations) were performed with the benthic 

amphipod Leptocheirus plumulosus according to standard USEPA and ASTM protocols. 

Differences in survival of amphipods in near-field and far-field sediments from each site were 

used as an indication of biological effects of sediment disturbance or contamination from 

previous drilling discharges at the site. 

For sediments from both MC 292 and GB 602, there was significantly lower mean survival of 

amphipods in near-field than in far-field sediments (Table 16.15). For MC 292 sediments, 

survival of amphipods in far-field sediments was comparable to that in control sediments, 

indicating that the animals were healthy and the far-field sediments were non-toxic. In the 

GB 602 tests, survival of amphipods in control sediments was good; however, survival of controls 

in the reference toxicant test was poor, suggesting that the amphipods used in the test may not 

have been healthy. 
564 

Table 16.15. Survival of amphipods Leptocheirus plumulosus in 10-day sediment toxicity tests 

with 12 replicate sediment samples each from near-field and far-field stations at 

MC 292 and GB 602. Mean survival was significantly lower in near-field than in 

far-field sediments at both discharge sites. Data from Chapter 14. 

Percent Survival (Mean ± SD, n = 12) Discharge Site Near-Field Far-Field 

MC 292 76 ± 21 93 ± 8 

GB 602 17 ± 18 68 ± 13 

SD = standard deviation. 

In summary, near-field sediments from both MC 292 and GB 602 were toxic to benthic 

amphipods. Far-field sediments from GB 602 also were toxic, but some of this effect could have 

been due to the poor health of the test animals. The toxicity of near-field sediments may have 

been caused in part by chemicals from drilling discharges in the sediments. 

16.4.2 Sediment Microbial Activity 

Biodegradable organic matter often has a greater effect than sediment texture and deposition rate, 

or in some cases potentially toxic chemicals, on the structure and function of benthic biological 

communities in accumulations of drilling muds and cuttings on the seafloor (Hartley et al. 2003). 

Bacteria and fungi indigenous to marine sediments degrade the organic matter and, in the process, 

may deplete the oxygen in the pore water of near-surface layers of sediment, reduce sediment 

redox potential, and generate potentially toxic concentrations of sulfide and ammonia (Wang and 

Chapman 1999; Gray et al. 2002; Wu 2002). 

The redox potential (Eh) is a measure of the relative intensity of oxidation or reduction reactions 

or of the concentration of electrons in solution. The value of Eh (measured in millivolts: mV) 

reflects the sum of all oxidation/reduction reactions that are occurring in the sediment. Eh values 

greater than zero indicate oxidizing conditions; those less than zero indicate reducing conditions. 

The redox potential discontinuity (RPD) is the location in sediment where Eh is zero. Microbes 

use different inorganic oxidants (electron acceptors) to oxidize organic matter. As the primary 

oxidant, oxygen (O2), is consumed, Eh and oxygen concentration decline; different bacterial 

consortia then use first denitrification, manganese oxide reduction, iron hydrous oxide reduction, 

sulphate reduction, and methane formation as energy sources for organic matter oxidation (Van 

Cappellen and Wang 1995). Additional oxygen is consumed in the sediment column to 

re-oxidize these alternate electron acceptors by abiotic or microbially-mediated 

oxidation/reduction reactions. Microbial reduction of nitrate to ammonia begins as Eh falls below 

about +200 mV; sulfate reduction to sulfide begins at an Eh of about -100 mV. The slope of the 

vertical dissolved oxygen and Eh gradient in sediment and, therefore, the depth of the RPD and 

oxygen depletion depend on the oxygen concentration in the overlying water, the permeability of 

the sediment, and the availability of biodegradable organic matter and reducible inorganic 

substrates in the sediment (Van Cappellen and Wang 1995). 

SBM cuttings and some WBM and WBM cuttings, particularly gel muds containing high 

concentrations of carbohydrates, contain a large amount of biodegradable organic matter. Most 

of this organic matter is oxidized more efficiently under aerobic than anaerobic conditions. When 
565 

SBM cuttings accumulate on the seafloor, oxygen in the pore water of the cuttings layer is 

consumed rapidly, resulting in a steep gradient of decreasing oxygen concentration with depth in 

the sediment (Figure 16.2). Sediments contaminated with high concentrations of organic matter 

from SBM cuttings often become anoxic within about 1 to 2 mm of the surface. This steep 

gradient of decreasing oxygen concentration with depth in the cuttings pile is caused by a 

combination of rapid microbial degradation of labile organic matter and the low permeability of 

the cuttings material, which slows diffusion of oxygen into sediment from the overlying water 

(Shimmield and Breuer 2000; Shimmield et al. 2000). This process is called organic enrichment 

and results in substantial changes in sediment physical/chemical properties and in benthic 

community structure (Pearson and Rosenberg 1978). 

As the sediment organic matter concentration increases as a result of SBM cuttings accumulation 

in surface layers, microbial activity increases and the gradient of declining oxygen concentration 

with depth in sediment steepens, resulting in a change in benthic community structure. The 

number of species, faunal abundance, and biomass (the SAB ratio) of the benthic community 

change in characteristic ways described in the Pearson-Rosenberg model (Nilsson and Rosenberg 

2000) along a gradient of decreasing sediment organic matter concentration with distance from a 

source of sediment organic enrichment. The changes in the benthic community are caused 

primarily by sediment hypoxia, rather than directly by organic enrichment (Gray et al. 2002). 

The microbially-mediated increase in hydrogen sulfide and ammonia concentrations in sediments 

that accompanies sediment hypoxia also may contribute to benthic community alterations 

(Vismann 1991). 

Severely organically enriched sediments may support white mats of the sulfate-oxidizing 

bacterium Beggiatoa spp. They have been observed on the surface of OBF and SBM cuttings 

piles in the North Sea and Gulf of Mexico (Breuer et al. 1999; Schimmield and Breuer 2000; 

Continental Shelf Associates, Inc. 2004). These bacterial mats overlie anoxic sulfide-rich marine 

sediments (Gundersen et al. 1992); Beggiatoa bacteria are chemoautotrophs that oxidize sulfide 

through elemental sulfur to sulfate. 

Most of the benthic meiofauna and macroinfauna of continental shelf and slope sediments live in 

the upper few centimeters of the sediment column, particularly if the sediments are fine-grained 

(Gray 2002). Most of these benthic fauna utilize the oxygen in the thin surface oxidized layer of 

sediment. Therefore, the best way to express the critical oxygen concentration for benthic fauna 

is as the depth-integrated amount of oxygen per unit area of sediment (the oxygen inventory of 

the sediment) (Trefry et al. 2004). The oxygen inventory is the total amount of oxygen per square 

centimeter of sediment surface integrated to the depth of the RPD. The Eh gradient in sediments 

also is a useful indication of the quality of the benthic habitat. 

16.4.2.1 The Oxygen Inventory of Sediments 

Because Eh and dissolved oxygen concentration have a marked effect on speciation, 

bioavailability, and toxicity of most metals in sediments, these parameters were measured at 

different depths in sediment samples collected for metals analysis in this study (see Chapter 8). 

Lower oxygen inventory and Eh and shallower RPD depth in near-field than in far-field 

sediments are circumstantial evidence of organic enrichment of near-field sediments, possibly by 

accumulation on the bottom of drilling discharges containing biodegradable organic matter. 
566 

Mean oxygen inventory in near-field and far-field sediment samples at VK 916 before initiation 

of exploratory drilling and in far-field sediments after exploratory drilling was 86 nM/cm2

(Chapter 8). Discretionary and near-field sediments collected after exploratory drilling contained 

much lower mean oxygen inventories, 11 and 44 nM/cm2

, respectively. This is a strong 

indication of increased oxidative activity in near-field sediments after exploratory drilling, 

probably caused by microbial oxidation of organic matter in deposited drilling waste solids. 

There was a similar pattern of change in oxygen distribution in sediments at GB 516 (Chapter 8). 

Before development drilling began, oxygen inventories in near-field and far-field sediments were 

similar and high. After development, discretionary and near-field sediments contained much 

lower oxygen inventories than far-field sediments did. There was a similar distribution, 

post-development, at GB 602. 

Oxygen inventories were low in all sediments at MC 292 after completion of development 

drilling. This probably was caused by the close proximity to the mouth of the Mississippi River 

and resulting relatively high sedimentation rate, TOC concentration, and small grain size of 

sediments at this site. 

16.4.2.2 Sediment Eh and RPD Depth 

As discussed above, organic enrichment of sediments causes a steeper gradient of declining Eh 

with depth in sediment, resulting in a shallowing of the RPD depth (the depth in sediment at 

which Eh is 0 and oxygen is depleted). The range of RPD depth and Eh1 cm (the Eh at a depth of 

1 cm) were lower in near-field than in far-field sediments at all four sites after discharges 

(Table 16.16). These parameters were less variable among replicate samples of far-field 

sediments than of near-field sediments at each discharge site. The greater variability in near-field 

sediments may reflect a patchy distribution of drilling discharge solids on the near-field seafloor. 

RPD depth and Eh1 cm tended to be lower in discretionary than in near-field sediments, because 

discretionary samples were collected at near-field sites with the highest likelihood of containing 

drilling discharge material. 

There was a decline in RPD depth and Eh1 cm in near-field but not far-field sediments between the 

pre-exploration and post-exploration surveys at VK 916 (Table 16.16). There was a much 

smaller change at near-field stations at GB 516 between the pre-development and 

post-development surveys. RPD depth and Eh1 cm were similar in far-field sediments before and 

after exploratory or development drilling at both sites. 

Only post-drilling samples were collected at GB 602 and MC 292. RPD depth and Eh1 cm of some 

near-field and discretionary sediment samples were less than values for far-field samples. Some 

near-field and discretionary sediments from GB 602 and near-field sediments from MC 292 had 

negative Eh1 cm values, indicating shallow sediment hypoxia. All these results provide consistent 

support for the hypothesis that patchy distributions of drilling discharge solids containing 

biodegradable organic matter in near-field surface sediments at all four sites caused localized 

sediment hypoxia. 
567 

Table 16.16. Ranges of redox potential discontinuity (RPD) depths and Eh at 1 cm in sediments 

from near-field (NF), discretionary (DS), and far-field (FF) zones at the four study 

sites. Data from Chapter 8. 

RPD Depth (cm) Eh1 cm (mV) Site Zone Min Max Max Min 

VK 916 (Nov 2000) NF 1.6 3.5 +675 +328 

Pre-Exploration FF 2.4 3.1 +614 +135 

NF 0.2 3.0 +529 -79 

DS 0.1 0.5 +31 -65 VK 916 (Aug 2002) 

Post-Exploration FF 0.0 3.2 +514 +218 

GB 516 (Nov 2000) NF 0.3 5.7 +664 -163 

Pre-Development FF 3.4 5.8 +692 +406 

NF 0.2 2.5 +506 -107 

DS 0.1 0.3 +196 -171 GB 516 (July 2001) 

Post-Development FF 2.7 5.7 +492 +104 

NF 0.1 2.8 +402 -167 

DS 0.2 1.7 +424 -296 GB 602 (July 2001) 

Post-Development FF 2.8 >6.1 +462 +225 

NF 0.2 3.0 +350 -163 

DS 0.8 1.7 +321 +29 MC 292 (July 2001) 

Post-Development FF 0.2 >3.3 +449 +158 

16.4.2.3 Sediment Microbiology 

Due to microbial degradation of organic matter from SBM drilling waste solids deposited on the 

seafloor, one would expect that sediments exhibiting organic enrichment would also have higher 

microbial metabolism. This possibility was examined by determining concentrations of 

adenosine triphosphate (ATP) in near-field and far-field sediments from the four sites (see 

Chapter 10). ATP is the main energy-containing cofactor in biochemical pathways in all living 

aerobic and anaerobic prokaryote and eukaryote organisms. Therefore, ATP concentration is a 

good index of biological activity in sediments. Much of the biological activity in fine-grained 

sediments is from microscopic bacteria and protozoa. Therefore, ATP concentration has been 

adopted as an indication of microbial activity in sediments. 

ATP concentrations in near-field and far-field sediments collected during three field surveys at 

four sites varied widely, from <0.3 to 418.2 ng/g (Table 16.17). Mean concentrations were 

consistently higher in sediments collected in November 2000 (pre-exploration and 

pre-development) than in near-field and far-field sediments collected in July 2001 or August 2002 

(post-drilling). LaRock (Chapter 10) interpreted this decline in ATP concentration as evidence of 

a drilling related biological effect on the benthic environment. However, as observed in 

Chapter 15, the magnitude of apparent effects was similar at near-field and far-field stations and 

did not vary with concentrations of drilling discharge markers, barium and SBF chemical, in 

sediments. It is possible that the inter-annual difference reflected a long-term, area-wide change 

in benthic microbial productivity, or differences in application of the ATP analytical method 

among batches of samples. 
568 

Table 16.17. Sediment adenosine triphosphate (ATP) concentrations (ng/g dry wt) in surface 

sediments collected at 12 near-field and 6 far-field stations at the four study sites. 

Data from Chapter 10. 

Site Zone Mean ATP (± SD) ATP Range 

VK 916 (Nov 2000) NF 32.1 ± 9.2 21.9 − 48.3 

Pre-Exploration FF 36.0 ± 14.3 19.7 − 61.4 

VK 916 (Aug 2002) NF 7.9 ± 13.1 <0.3 − 41.5 

Post-Exploration FF 7.1 ± 17.1 0.9 − 61.1 

GB 516 (Nov 2000) NF 100.4 ± 130.7 13.1 − 418.2 

Pre-Development FF 43.3 ± 28.5 14.2 − 102.4 

GB 516 (July 2001) NF 4.4 ±3.9 0.5 − 11.9 

Post-Development FF 2.5 ± 2.1 0.5 − 5.8 

GB 602 (July 2001) NF 8.2 ± 6.6 2.3 − 17.6 

Post-Development FF 20.1 ± 61.8 1.1 − 216.3 

MC 292 (July 2001) NF 2.8 ± 1.9 0.9 − 5.5 

Post-Development FF 7.7 ± 19.5 0.7 − 66.6 

Given uncertainties about temporal differences in sediment ATP concentration, the best approach 

for examining possible effects of drilling discharges is to compare ATP concentrations between 

near-field and far-field at each site and sampling time. There were no significant differences in 

ATP concentrations between near-field and far-field sediments at any of the four sites (see 

Chapter 10). Near-field sediments collected at GB 516 in November 2000 (pre-development) 

contained a higher mean ATP concentration than far-field sediment samples collected at the same 

time. This difference can be attributed to two near-field samples containing higher ATP 

concentrations than any other sediments sampled in this project: 288.3 and 418.2 ng/g dry wt. 

Similarly, the far-field mean ATP concentration for GB 602 was higher than the near-field mean, 

because of one sample containing 216.3 ng/g ATP. Thus, there was no real difference in ATP 

concentrations in near-field and far-field sediments at any site. Apparently, organic enrichment 

of sediments results in variable responses in the benthic microbial community. An influx of 

biodegradable organic matter to sediments from drilling wastes would be expected to increase 

microbial community metabolism and biomass. However, onset of sediment hypoxia or anoxia 

may reduce total microbial biomass because aerobic metabolism usually far exceeds anaerobic 

metabolism in oxidized and reduced sediments, respectively. 

16.4.2.4 Microbial Mats 

There was evidence of enhanced benthic microbial activity in the form of microbial mats at 

several near-field stations. Microbial mats were observed in sediment profile images at some 

near-field stations at VK 916, GB 516, and GB 602 (see Chapters 6 and 7). Most microbial mats 

are constructed by oxidation of sulfide to sulfate (white) by sulfide-oxidizing bacteria such as 

Beggiatoa spp. They occur on the surface of reduced marine sediment layers rich in sulfide; they 

frequently occur on sediments near hydrocarbon vents on the upper slope of the Gulf of Mexico 

(MacDonald 2002). 
569 

16.4.3 Meiofaunal and Macroinfaunal Communities 

16.4.3.1 Meiofauna 

Most meiofauna, except foraminifera, are short-lived, rapidly reproducing invertebrates or the 

early life stages of larger benthic invertebrates. Meiofaunal communities often respond rapidly to 

changes in the physical disturbance and chemical contamination of sediments (Coull and 

Chandler 1992; Carman et al. 1997). They also recover rapidly from acute disturbances. 

There was a tendency for meiofaunal density to decline between Cruise 1B and subsequent 

cruises at VK 916 and GB 516, and to be lower in far-field than in near-field sediments at all four 

sites (Chapter 11) (Figure 16.3). At VK 916, mean meiofaunal densities decreased by about an 

order of magnitude between the pre-drilling (November 2000) and post-drilling (August 2002) 

cruises. Mean meiofaunal densities were higher at near-field (608 individuals/10 cm2

) than at 

far-field (544 individuals/10 cm2

) stations at the time of the pre-drilling cruise and at near-field 

(96 individuals/10 cm2

) than at far-field (19 individuals/10 cm2) stations at the time of the 

post-drilling cruise. The decline between cruises and between near-field and far-field stations 

was accompanied by a significant increase in the relative abundance of annelids in near-field 

compared to far-field samples at the time of the post-drilling cruise. 

A similar pattern of temporal change in meiofaunal density was observed at GB 516 

(Figure 16.3). Mean meiofaunal densities decreased between the pre-development cruise 

(November 2000) and the post-development cruise (July 2001). Meiofaunal density also was 

lower at far-field (68 individuals/10 cm2

) than at near-field (122 individuals/cm2) stations at the 

time of the pre-development cruise and at far-field (22 individuals/10 cm2

) than at near-field 

(38 individuals/cm2

) stations at the time of the post-development cruise. There was considerable 

variability and overlap in meiofaunal densities among all near-field and far-field samples. As at 

VK 916, annelids were significantly more abundant in near-field than in far-field sediments. 

Meiofaunal densities were low and highly variable in near-field and far-field sediments from 

GB 602 and MC 292 at the time of the single post-development sampling in July 2001 

(Figure 16.3). Mean densities at GB 602 were 93 individuals/10 cm2 at near-field stations and 

26 individuals/10 cm2

 at far-field stations. Mean densities at MC 292 were 

104 individuals/10 cm2

 near-field and 51 individuals/10 cm2 far-field. Meiofaunal annelids were 

significantly more abundant in near-field than far-field sediments at MC 292. 

Higher total meiofaunal densities in the near-field at all four sites reflected the increased 

abundance of several groups. The highest densities of annelids, harpacticoid copepods, and 

nematodes were always seen in the near-field. There were no consistent correlations between 

abundances of these groups and drilling discharge indicators (e.g., barium and SBF 

concentrations) within near-field sites (Chapter 11). 
VK 916

NF

Cruise 1B

(pre-drilling)

FF NF FF

Cruise 3B

(post-drilling)

GB 516

NF

Cruise 1B

(post-drilling)

FF NF FF

Cruise 2B

(post-drilling)

GB 602

NF

Cruise 2B

(post-drilling)

FF

Meiofaunal Density (no./10 cm2

NF

Cruise 2B

(post-drilling)

FF

)

MC 292

104

103

102

1 1

10

Meiofaunal Density (no./10 cm2)

104

103

102

10

Meiofaunal Density (no./10 cm2)

104

103

102

1

10

Meiofaunal Density (no./10 cm2)

104

103

102

1

10

Near-field Far-field

LEGEND

Figure 16.3. Total meiofaunal densities in near-field (NF) and far-field (FF) sediments 

from the four study sites. From Chapter 11. 

VK 916 105

Macroinfaunal Density (no./m2)

NF

Cruise 1B

(pre-drilling)

FF NF FF

Cruise 3B

(post-drilling)

GB 516

NF

Cruise 1B

(post-drilling)

FF NF FF

Cruise 2B

(post-drilling)

Macroinfaunal Density (no./m2)

GB 602

NF FF

Cruise 2B

(post-drilling)

Macroinfaunal Density (no./m2)

MC 292

NF FF

Cruise 2B

(post-drilling)

Macroinfaunal Density (no./m2)

104

103

102

105

104

103

102

105

104

103

102

105

104

103

102

Near-field Far-field

LEGEND

Figure 16.4. Total macroinfaunal densities in near-field (NF) and far-field (FF) sediments 

from the four study sites. From Chapter 11. 

570 
571 

Benthic community parameters were highly variable at the four sites (Table 16.18). The survey 

in November 2000 at VK 916 occurred before any drilling in the immediate vicinity of the site 

center. Community parameters were similar for near-field and far-field stations, indicating that 

the area around the drilling site was fairly uniform before drilling. 

All other sampling at VK 916 and the other three sites occurred after some drilling. At all sites, 

mean macrofaunal density was greater at near-field than at far-field stations (Table 16.18). 

However, mean diversity (H') evenness (J'), and richness (D) were higher at far-field than at 

near-field stations. The number of benthic taxa was the same or higher in far-field as in near-field 

sediments. In many cases, the differences were small and probably not statistically significant. 

These patterns of benthic community change at near-field sites are consistent with mild organic 

enrichment of near-field sediments (Gray et al. 2002; Rosenberg et al. 2004). 

Most identifications were performed to the family level, decreasing the power of most community 

parameters to differentiate among sampling sites. However, there was a trend at all sites for 

highest densities of annelids, gastropods, and bivalve mollusks to be at near-field stations. 

Amphipods, ostracods, and tanaids tended to be more abundant at far-field than at near-field 

stations. Some of the taxa that were most abundant in near-field sediments were well-known 

opportunistic species, such as the annelid Capitella spp., characteristic of disturbed, eutrophic 

sediments. This benthic faunal composition also is consistent with organic enrichment of 

near-field sediments. 

Table 16.18. Benthic macrofaunal community parameters for near-field (NF) and far-field (FF) 

sediments at the four study sites. Data from Chapter 11. 

Site Zone No. Taxa Density 

(no/m2

) 

Diversity

(H') 

Evenness 

(J') 

Richness 

(D) 

VK 916 (Nov 2000) NF 14.1±4.3 8517±3672 1.45±0.47 0.53±0.17 1.94±0.64 

Pre-Exploration FF 14.7±1.9 8751±5319 1.59±0.14 0.60±0.06 2.10± 0.19 

VK 916 (Aug 2002) NF 7.2±3.4 1633±1332 1.10±0.68 0.54±0.27 1.31±0.71 

Post-Exploration FF 10.1±2.4 1256±420 1.58±0.31 0.69±0.09 1.89±0.36 

GB 516 (Nov 2000) NF 12.9±2.3 4901±4961 1.22±0.41 0.47±0.13 2.06±0.51 

Pre-Development FF 14.1±1.5 1692±558 1.77±0.15 0.67±0.05 2.58±0.24 

GB 516 (July 2001) NF 11.7±3.5 11087±11585 1.42±0.37 0.59±0.13 1.71±0.62 

Post-Development FF 15.7±2.4 2838±1036 2.00±0.17 0.73±0.05 2.63±0.42 

GB 602 (July 2001) NF 9.0±3.0 9251±7109 1.31±0.44 0.61±0.21 1.24±0.48 

Post-Development FF 13.9±2.8 1743±671 1.92±0.16 0.74±0.07 2.53±0.50 

MC 292 (July 2001) NF 13.1±3.1 10327±11780 1.51±0.30 0.59±0.07 1.87±0.44 

Post-Development FF 13.4±3.9 5448±3261 1.69±0.38 0.69±0.07 2.00±0.58 

16.4.3.2 Benthic Community Status 

Sediment profile imaging (SPI) was used to characterize benthic community structure along 

near-field and far-field transects at the four sites (Chapters 6 and 7). Two parameters derived 

from SPI image analysis, infaunal successional stage and organism sediment index (OSI), were 

estimated from images generated at VK 916 and GB 516 (Chapter 6). These parameters were not 

calculated for SPI images from GB 602 and MC 292 because of the uniformity of the images and 
572 

general lack of biogenic activity (Chapter 7). However, Chapter 7 does provide a written 

summary of benthic biological conditions at these two discharge sites. 

Infaunal successional stage is based on observed patterns of recolonization of disturbed 

shallow-water marine sediments (Rhodes and Germano 1986). Stage I infauna are small and live 

close to the sediment-water interface. They are pioneers and opportunists in that they do not 

require a deep layer of oxidized sediment. The burrowing and feeding activity of Stage I fauna, 

mainly small annelids, deepens the RPD depth, allowing colonization by larger, more aerobic 

benthic animals, such as amphipods (Stage II). Stage II benthos are transitional species that often 

are unique to different benthic habitats. The Stage II community at these deepwater, continental 

slope sites has not been characterized. Stage III animals are large, deep-burrowing, head-down 

deposit feeders, usually polychaetes and echinoderms, that aerate the sediments to a depth of 

several centimeters. Stage I through III benthos can co-occur at a site and their presence together 

often indicates a healthy, diverse benthic community. 

The OSI takes into account the depth of the apparent RPD, successional stage, presence or 

absence of sediment methane bubbles, and the presence or absence of reduced sediments near the 

sediment-water interface. OSI values can range from +11 (best habitat quality) to -10 (worst 

habitat quality). OSI values below about +6 often are associated with disturbed benthic habitats. 

Negative OSI values reflect a highly disturbed benthos. 

Most near-field and far-field sediments collected at VK 916 before exploratory drilling were 

characterized by Stage I-III successional stages and positive OSI values, both indicative of a 

healthy benthic community (see Chapter 15, Table 15.6). There were small patches of sediment 

with Stage I seres and OSI values between +2 and 6 along both near-field and far-field transects, 

indicating a patchy distribution of mild sediment disturbance. 

The frequency of Stage I or azoic near-field sediments was higher at the time of the 

post-exploratory drilling in August 2002 at VK 916. Many near-field sediments near the drilling 

site were azoic or were depauperate of benthic fauna. The surface of some of these sediments 

contained deposits of black, sulfidic sediment and apparent RPD depth was at the sediment 

surface, both indicative of severe organic enrichment. OSI values ranged from -7 to +7. The 

azoic sediments with 0-cm RPD depths had OSI of -4 of lower; sediments with Stage I-III 

communities had OSI values mostly higher than +5. The post-drilling SPI survey shows that the 

near-field benthic ecosystem at VK 916 is near or just below the OSI threshold of +6 (disturbed 

benthic habitat) and 25% of sediment stations had OSI below 0, indicating very poor habitat 

quality. Most far-field sediments collected in August 2002 from VK 916 contained Stage I−III 

successional stages. 

SPI imagery revealed evidence of a slight improvement in benthic habitat quality at GB 516 

between the post-exploration survey (November 2000) and the post-development survey (July 

2001). In November 2000, most near-field sediments were either Stage I or azoic. Microbial 

mats were observed at 29% of near-field stations. More than 35% of near-field sediments had 

negative values for OSI. 
573 

At the time of the post-development survey, about half the sediments were Stage I or azoic. No 

microbial mats were observed; however, black, sulfidic sediments extended to the sediment 

surface at 15% of near-field stations. OSI values were negative at 18% of near-field stations. 

Far-field sediments were mostly healthy before and after development drilling at GB 516. 

Although successional stage and OSI were not estimated for GB 602 and MC 292, some 

comparisons with VK 916 and GB 516 can be made. Post-development conditions in near-field 

sediments at GB 602 were similar to those in near-field sediments at GB 516 at the time of the 

post-development survey (Chapter 7). There were no Stage III animals at near-field stations; less 

than half the stations supported Stage I benthic fauna. There also was evidence of microbial mats 

and sulfidic sediments at the sediment surface. Far-field sediments appeared to be well-oxidized 

and aerobic. However, benthic fauna were scarce. 

Only a few near-field sediments at the other post-development site, MC 292, appeared to be 

disturbed. More than 80% of near-field sediments contained evidence of benthic fauna, and more 

than 20% of sediments contained deep-dwelling benthic infauna (Stage III). Sediments appeared 

well-oxygenated with deep RPD depths. Far-field sediments at MC 292 had biological activity 

similar to that at near-field sediments. Overall, sediments at MC 292 showed minimal evidence 

of benthic disturbance. 

16.4.4 Demersal/Benthic Megafauna 

Benthic megafaunal abundance and distribution were estimated from bottom photographs taken 

along near-field and far-field transects at each of the four sites (see Chapter 12). Most of the 

megafauna detected in the photographs were small. Overall, relative abundance of small 

megafauna was similar along near-field and far-field transects and at the four sites. The average 

number of megafauna along individual transects at a site ranged from 9.33 animals at MC 292 in 

July 2001 to 53.6 animals at VK 916 in August 2002 after exploratory drilling. Coelenterates 

(sea anemones) often were the most abundant megafauna. 

The only statistically significant differences in megafaunal abundances at near-field and far-field 

sites were a higher abundance of fish and a lower abundance of ophiuroids in the near-field. The 

differences in abundances of these two taxa in the near-field and far-field were greatest at VK 916 

(post-exploration) and GB 516 (both post-exploration and post-development). There was little or 

no difference in megafaunal abundances in the near-field and far-field at GB 602 and MC 292. 

16.4.5 Summary of Injury 

Evidence of biological injury to the deepwater benthic ecosystem from drilling discharges was 

evaluated based on sediment toxicity and various aspects of the structure and function of different 

components of the benthic community in the near-field and far-field of the four discharge sites. 

Although pre-drilling and post-drilling surveys were performed at two locations, VK 916 and 

GB 516, it was difficult to distinguish drilling-related effects from changes attributable to 

seasonal or interannual changes in environmental conditions on the continental slope. Therefore, 

biological characteristics of the near-field and far-field benthic environment at each site and each 

survey were evaluated for evidence of drilling-related biological effects (injury). There were 
574 

differences in most biological parameters between near-field and far-field locations at the four 

sites, or between pre-drilling and post-drilling surveys at VK 916 (Table 16.19). 

Table 16.19. Summary of evidence of biological injury to the near-field benthic environment at 

the four study sites. + = evidence of biological effects; +- = weak or uncertain 

evidence of biological effects; – = no evidence of biological effects. 

Parameter VK 916 GB 516 GB 602 MC 292 

Sediment toxicity NS NS + + 

Oxygen inventory + + + +- 

Eh and RPD + + + + 

Microbial ATP – – – – 

Microbial mats + + + – 

Meiofauna + + + + 

Macroinfauna + + + + 

Benthic community status + + + – 

Megafauna + + – – 

ATP = adenosine triphosphate; NS = no samples; RPD = redox potential discontinuity. 

Sediment toxicity was evaluated at the two post-development sites, MC 292 and GB 602. 

Near-field sediments at both sites were more toxic than far-field sediments (Table 16.19). This 

toxicity could have been caused by accumulation of toxic drill cuttings ingredients in sediments, 

or it could be a secondary effect of organic enrichment (sediment hypoxia and increase in 

sediment sulfide and ammonia). 

Changes in sediment Eh, RPD depth, and oxygen inventory in sediments are secondary effects of 

sediment microbial activity. There was evidence of lower Eh, RPD depth, and smaller oxygen 

inventory at some near-field than at far-field stations at all four sites (Table 16.19). However, it 

was difficult to detect changes in microbial activity. Nevertheless, the patterns of change in these 

parameters are consistent with a drilling-discharge-mediated organic enrichment in some 

near-field sediments. Organic enrichment was patchy, probably reflecting the patchy distribution 

of drill cuttings solids on the near-field seafloor. 

Near-field and far-field meiofaunal and macroinfaunal communities were different at all four sites 

(Table 16.19). Near-field sediments tended to have higher densities of benthic fauna but lower 

diversity, evenness, and richness than far-field sediments. This is a characteristic feature of mild 

organic enrichment (Pearson and Rosenberg 1978). Near-field sediments are dominated by a 

small number of abundant opportunistic species. 

There was little evidence of biological effects of drilling discharges on near-field megafaunal 

communities (Table 16.19). Fishes tended to be more abundant and ophiuroids tended to be less 

abundant in near-field than in far-field sediments. This probably was caused by accumulation of 

solids on the near-field seafloor. 
575 

16.5 RISK CHARACTERIZATION 

Data on exposure of the species of concern to the chemicals of concern is compared to data on 

biological effects (injury) in the risk characterization. The relationships between the magnitude 

of exposure and severity of injury are examined. If the risk assessment is based solely or 

primarily on field monitoring data, as this one has been, it may be possible to estimate the area of 

the affected environment exhibiting different levels of risk (or severity) of injury attributable to 

the chemicals or disturbances of concern. 

16.5.1 Causes of Sediment Toxicity 

Some near-field sediment samples from MC 292 and GB 602 were toxic to benthic amphipods 

(Chapter 14). SBF chemical, PAH, and barium concentrations in sediments were used as 

quantitative exposure parameters to evaluate possible causes of sediment toxicity. Regression of 

SBF chemical concentration, PAH concentration, and barium concentration in individual samples 

of near-field and far-field sediments from GB 602 and MC 292, separately or combined against 

fraction surviving in each sediment sample, yielded negative slopes: an inverse relationship 

between concentrations of chemicals in sediment and survival of amphipods in the toxicity test. 

The correlation coefficients for the regression between sediment barium concentration and 

fractional survival were r2

 = 0.50 for MC 292 and r2 = 0.57 for GB 602. For the regression of 

SBF versus fractional survival, the correlation coefficients were r2

 = 0.60 for MC 292 and 

r

2

 = 0.35 for GB 602. There was no relationship between sediment PAH concentration and 

survival of benthic amphipods at MC 292, but there was a weak negative relationship for GB 602 

(r2

 = 0.40). 

Barium is a better indicator than SBF chemical of the total drilling solids discharge accumulation 

on the seafloor, because barium (as barite) is more persistent in sediments than SBF chemical, 

and barium is derived from WBM and WBM cuttings as well as SBM cuttings. The total mass of 

WBM-related discharges was nearly 10 times higher than the mass of SBM cuttings discharged at 

all four sites (Table 16.1). These results suggest that chemicals associated with both WBM and 

SBM waste solids in near-field sediments are contributing to sediment toxicity. The highest 

concentrations of SBF chemical and barium in sediment associated with significant sediment 

toxicity (≤ 50% survival) are about 1,000 mg/kg and 20,000 mg/kg, respectively. Barite and SBF 

chemicals have low toxicities to marine animals (Neff et al. 2000), so it is uncertain what drilling 

waste ingredients actually are causing toxicity. 

16.5.2 Causes of Organic Enrichment 

There was evidence of sediment organic enrichment at all four sites. Organic enrichment was 

indicated by low Eh and oxygen inventories and shallow RPD depths, as well as the presence of 

microbial mats on the sediment surface at three sites. Evidence from microbial production was 

unclear. 

Negative Eh and low oxygen inventories (both indicative of a severely disturbed benthic habitat) 

occurred at stations where barium, SBF chemical, and TOC concentrations were high, indicating 

that the organic enrichment was being caused all or in major part by accumulation of drilling 

wastes on the seafloor. WBM cuttings typically have a lower biological oxygen demand than 
576 

SBM cuttings (Dow et al. 1990; Neff 2005). SBF chemicals are required by the effluent 

limitations guidelines in NPDES permits to be biodegradable. Esters are highly biodegradable; 

IOs and LAOs are somewhat less so (Neff et al. 2000; Melton et al. 2004). There are 

hydrocarbon seeps in the vicinity of the Garden Banks discharge sites; these could have 

contributed to organic enrichment of regional sediments. However, this contribution probably 

would be similar in near-field and far-field sediments. Thus, it is probable that SBF chemicals 

and other biodegradable organic chemicals in SBM (e.g., emulsifiers) are the major contributors 

to the enhanced organic enrichment in near-field sediments at the four sites. 

16.5.3 Changes in Benthic Communities 

Meiofaunal and macrofaunal densities and community parameters were different in near-field and 

far-field sediments after exploratory or development drilling at all four sites. Abundances of 

several species of benthic fauna tended to be higher in near-field than in far-field sediments, 

possibly an effect of organic enrichment of near-field sediments. 

Statistical correlations were examined between densities of different meiofaunal and macrofaunal 

groups and the values of several exposure parameters (SBF chemical, barium, TOC, and percent 

sand) at near-field stations. Most correlations were not significant for meiofaunal groups. There 

was no consistent relationship between any of the exposure parameter values and the abundance 

of meiofaunal annelids, harpacticoid copepods, or nematodes. Annelid abundance was positively 

correlated with sediment barium concentration at GB 516 and MC 292, and with SBF chemical 

concentration at MC 292, but not at GB 602 and VK 916 (Chapter 11). Harpacticoid and 

nematode densities were negatively correlated with SBF chemical concentration and percent silt 

at GB 516 and GB 602. These results suggest that the near-field meiofaunal community at some 

sites was affected by accumulation in sediments of fine-grained material, possibly from drilling 

discharges. 

The abundance of macrofaunal annelids and gastropods in near-field sediments at GB 516, 

GB 602, and MC 292 was positively correlated with sediment, barium, and/or SBF chemical and 

TOC concentration. There also was a positive correlation between bivalve mollusk densities and 

sediment barium concentration at MC 292. Gastropod abundance also was positively correlated 

with percent sand at GB 516. 

In general, near-field sediments supporting elevated densities of macrofaunal annelids had barium 

concentrations of 10,000 μg/g or higher and SBF chemical concentrations of 1,000 μg/g or 

higher. Extremely high densities of gastropods in near-field sediments at GB 516 and GB 602 

were associated with SBF chemical concentrations of 4,500 μg/g or higher and barium 

concentrations of 55,000 μg/g or higher. 

The abundance of amphipods, ostracods, and aplacophorans in near-field sediments was inversely 

correlated with one or more of sediment barium, SBF chemical, or TOC concentration, or percent 

sand, silt, or clay in sediments. Strongest negative correlations were for amphipods and ostracods 

and barium and SBF chemical concentrations in sediments at GB 516 in November 2000 (before 

development drilling). These observations suggest that the abundance of annelids, gastropods, 

and bivalves tends to increase and the abundance of amphipods, ostracods, and aplacophorans 
577 

tends to decrease in response to deposition on the sediments of WBM and SBM cuttings solids. 

This could be a response to organic enrichment or to changes in sediment texture. 

16.5.4 Summary 

This risk assessment has shown that drilling mud and cuttings ingredients accumulated in a 

patchy distribution in near-field (<500 m from discharge site) sediments at four exploration or 

development sites in approximately 1,000 m of water on the continental slope of the Gulf of 

Mexico. Excess barium and SBF chemical were the best indicators of the presence of drilling 

wastes in sediments. A few far-field sediment samples also contained excess barium or SBF 

chemical and may have been contaminated by discharges from the drilling activities monitored 

during this study or from other nearby exploration and development operations. 

The accumulation of drilling wastes in near-field sediments caused a variety of changes in the 

benthic environment at the sites. It appears that the primary effect of the discharges was organic 

enrichment of near-field sediments. The influx of biodegradable organic matter to the sediments 

associated with the discharged solids stimulated microbial metabolism and reduced sediment 

permeability, causing oxygen depletion and redox changes in surficial sediments. Benthic 

meiofaunal and macrofaunal communities responded to the reduced oxygen inventories and 

shallower RPD depths. 

The abundance of some species, particularly annelids, increased and the abundance of some other 

species, particularly amphipods and ostracods, increased in sediments with the greatest organic 

enrichment. The benthic communities were denser but less diverse in organically-enriched 

sediments than in far-field, unenriched sediments. These patterns of benthic community change 

at near-field sites are consistent with mild organic enrichment of near-field sediments (Gray et al. 

2002; Rosenberg et al. 2004). 

It is possible that organic enrichment contributed to the toxicity of near-field sediments in 

laboratory toxicity tests, by stimulating production of toxic sulfide and ammonia. The major 

ingredients of WBM and SBM cuttings have a low toxicity (Neff et al. 2000; Neff 2005). 

Elevated toxicity in laboratory bioassays was observed in sediments containing more than 

10,000 to 20,000 μg/g barium and 1,000 μg/g SBF chemical. These also were the concentrations 

in near-field sediments associated with increased abundance of annelids. Gastropod abundance 

was increased markedly in sediments containing more than 55,000 μg/g barium. Thus, sediment 

toxicity was not caused by barium or SBF chemicals. 

Concentrations of 10,000 μg/g barium or 1,000 μg/g SBF chemical in sediments can be used as 

an indicator of a drill cuttings concentration above which benthic biological community changes 

can be expected. Sediments with barium or SBF chemical concentrations exceeding this indicator 

concentration were distributed in a highly patchy fashion in the near-field of the four sites. Even 

in this relatively small area around a deepwater discharge (≈ 78 ha based on the near-field radius 

of 500 m), effects on benthic communities are extremely uneven, ranging from unaffected to 

severely disturbed. The patchy distribution of drilling waste solids and disturbed benthic 

communities on the near-field seafloor should facilitate ecological recovery as organic matter in 

the deposited drilling solids is depleted and oxygen penetrates deeper into sediments. The 
578 

unaffected substrates can serve as a source of recruits to nearby areas affected by drilling 

discharges. 

The near-field data are primarily from within a 500-m radius around each site. Based on the 

geophysical surveys, areas mapped as cuttings extended beyond this radius at three of the sites, 

with the greatest extent (about 1 km) observed at GB 602 and GB 516 (Chapter 4). As discussed 

in Chapter 15, areas mapped as cuttings generally had higher barium and SBF concentrations, but 

the results were not consistent enough to allow for extrapolation of barium and SBF 

concentrations outside the 500-m radius. One discretionary station at GB 602, located about 

1,000 m southwest of the site center (and not within the mapped cuttings zone), had a barium 

concentration of 14,800 μg/g and an SBF concentration of 930 μg/g. These values either exceed 

(barium) or are just under (SBF) the likely thresholds for biological effects cited above. Barium 

and SBF concentrations at far-field sites, including those located 2 to 3 km from previous 

unrelated drilling (Chapter 3), were far below these thresholds. The results suggest that 

biological effects may have occurred in some areas between about 500 and 1,000 m from the 

development wellsites. 
579 

Chapter 17 

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

for most of our nationally owned public lands and natural resources. This includes fostering 

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As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) 

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Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

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