OCS Study 

MMS 2002-036 

 

Stability and Change in Gulf of Mexico 

Chemosynthetic Communities 

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 2002-036 

Stability and Change in Gulf of Mexico 

Chemosynthetic Communities 

Volume II: Technical Report 

Editor 

Ian R. MacDonald 

Prepared under MMS Contract 

14-35-0001-30813 

by 

Texas A&M University 

Geochemical and Environmental Research Group 

833 Graham Road 

College Station, Texas 77845 

Published by 

New Orleans

July 2002 
iii 

DISCLAIMER 

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

the Texas A&M Research Foundation. This report has been technically reviewed by the MMS, 

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

reflect the views and 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 the MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office (Mail Stop 5034) 

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, Louisiana 70123-2394 

Telephone (504) 736-2519 or 

 1-800-200-GULF 

CITATION 

MacDonald, I.R., ed. 2002. Stability and Change in Gulf of Mexico Chemosynthetic 

Communities. Volume II: Technical Report. Prepared by the Geochemical and 

Environmental Research Group, Texas A&M University. U.S. Dept. of the Interior, 

Minerals Mgmt. Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 

2002-036. 456 pp. 

COVER ART 

Submersible sampling operations with submarine Johnson Sea Link. 
v 

ACKNOWLEDGMENTS 

This report would not have been possible without the contributions of a large number of people 

from Texas A&M University, Louisiana State University, Pennsylvania State University, The 

University of Virginia, The University of Texas at Austin, The University of California at Davis, 

and Rutgers University. Thanks go to the principal investigators who contributed text to this 

volume: Dr. Robert Carney (Trophic Relationships), Dr. Charles F. Fisher (Physiological 

Ecology), Dr. Norman Guinasso (Physical Oceanography), Dr. Samantha Joye 

(Electrochemistry), Dr. Mahlon C. Kennicutt II (Environmental Chemistry), Dr. Paul A. 

Montagna (Statistical Design), Dr. John W. Morse (Inorganic Chemistry), Dr. Douglas C. Nelson 

(Microbial Ecology), Dr. Eric Powell (Histopathology and Community Health), Dr. William 

Sager (Geophysics), Dr. Roger Sassen (Hydrocarbon Chemistry), Dr. Stephen Schaeffer 

(Genetics), and Dr. Gary Wolff (Data Management). For logistical support during at-sea 

operations, the program participants are grateful to the captains and crews of the R/V Gyre, R/V 

Edwin Link, and DSRV Johnson Sea-Link. Cooperation with the University of North Carolina at 

Wilmington, National Undersea Research Center (NOAA), including research grants to Drs. 

Fisher, Guinasso, MacDonald, and Sassen, immeasurably augmented the at-sea operations. The 

Harbor Branch Oceanographic Institution went beyond its contractual obligations in support of 

this program. 

The editors were assisted greatly by the efforts and contributions of the staff at TAMU. Special 

thanks to Ms. Rebecca Von Gonten, Mr. Carl Perkins and Ms. Debbie Paul for their help 

compiling and editing this report. Graduate students Ms. Susie Escorcia, Mr. Michael Peccini, 

and Ms. Alice Pichahachy provided much valuable assistance at sea and in the office. 

Thanks are also due to the Texas A&M Research Foundation (TAMRF) staff that contributed to 

the financial administration of this program. At the TAMRF, a special debt of gratitude is owed 

to Ms. Tracie Robertson and Ms. Charlene Miller.
vii

Table of Contents 

Page 

List of Figures...................................................................................................................xv 

List of Tables .................................................................................................................xxv 

1.0 Stability and Change in Gulf of Mexico Chemosynthetic Communities 

Program .............................................................................................................. 1-1 

1.1 Introduction.............................................................................................. 1-1 

1.2 Key Personnel .......................................................................................... 1-2 

1.3 Program Milestones ................................................................................. 1-3 

1.4 Publications, Abstracts, and Graduate Theses ......................................... 1-3 

2.0 Review of Literature.......................................................................................... 2-1 

2.1 Introduction.............................................................................................. 2-1 

2.1.1 Microbiology of chemosynthesis................................................. 2-1 

2.1.2 Symbiosis with invertebrates ....................................................... 2-1 

2.2 Discoveries of Chemosynthetic Communities......................................... 2-2 

2.2.1 Vestimentiferan tubeworms......................................................... 2-3 

2.2.2 Seep mussels ................................................................................ 2-3 

2.2.3 Vesicomyid clams........................................................................ 2-4 

2.2.4 Lucinid clams............................................................................... 2-4 

2.2.5 Polychaete “ice worms”.............................................................. 2-4 

2.3 Types of Chemosynthetic Communities in the Northern Gulf of 

Mexico ..................................................................................................... 2-4 

2.4 Distribution of Chemosynthetic Communities in the Northern Gulf of 

Mexico .................................................................................................... 2-5 

2.4.1 Evidence from energy prospecting .............................................. 2-6 

2.4.2 Evidence from remote sensing and direct observations............... 2-8 

2.5 Habitat Process at Gulf of Mexico Hydrocarbon Seeps .......................... 2-9 

2.5.1 Sediment diffusion..................................................................... 2-11 

2.5.2 Brine pooling ............................................................................. 2-13 
viii

2.6 Model Components and Interactions....................................................... 2-14 

2.6.1 Microbial communities .............................................................. 2-14 

2.6.2 Mussel colonies.......................................................................... 2-15 

2.6.3 Tubeworm colonies.................................................................... 2-16 

2.6.4 Energy flow and food web......................................................... 2-16 

2.6.5 Change at the community level.................................................. 2-16 

2.6.6 Regional characteristics ............................................................. 2-17 

2.6.7 Geologic stability....................................................................... 2-17 

3.0 Study Sites, Field Efforts, and Sampling Design............................................. 3-1 

3.1 Introduction.............................................................................................. 3-1 

3.2 Regional Studies ...................................................................................... 3-1 

3.3 Community Level Sampling Sites .......................................................... 3-3 

3.4 Sediment Diffusion Communities............................................................ 3-4 

3.4.1 GC185 ........................................................................................... 3-4 

3.4.2 GC234 ........................................................................................... 3-5 

3.5 Brine-Pooling Comminiutes .................................................................... 3-5 

3.5.1 GC233 ........................................................................................... 3-5 

3.5.2 GB425 ........................................................................................... 3-5 

3.6 Station Selection and Sample Tracking ................................................... 3-6 

3.7 Fieldwork ................................................................................................. 3-7 

3.7.1 Geophysical survey with R/V GYRE,1997................................... 3-7 

3.7.2 Detailed sampling with submarine SEA-LINK,1997.................... 3-8 

3.7.3 Geophysical survey with submarine NR-1,1998........................... 3-9 

3.7.4 Detailed sampling with submarine SEA-LINK, 1998................... 3-9 

3.8 Sampling Design...................................................................................... 3-9 

3.8.1 The relationship between sites and habitats .................................. 3-9 

3.8.2 Univariate statistical model for the sampling plan...................... 3-10 

3.8.3 Multivariate analyses for the sampling plan................................ 3-12 

4.0 Field Methods..................................................................................................... 4-1 

4.1 Introduction.............................................................................................. 4-1 

4.2 Submarine Johnson SEA-LINK............................................................... 4-1 

4.3 Navigation................................................................................................ 4-2 

4.4 Markers and Sketch Maps........................................................................ 4-2 
ix

4.5 Sediment Collection................................................................................. 4-4 

4.6 Mussel and Associated Fauna Collection and Shipboard Processing...... 4-5 

4.7 Mussel Growth and Transplant Studies ................................................... 4-8 

4.8 Tubeworm and Associated Faunal Collections and Shipboard 

Processing ................................................................................................ 4-8 

4.9 Tubeworm Growth Studies...................................................................... 4-9 

5.0 The Physical Environment at the Seep Sites .................................................. 5-1 

5.1 Background.............................................................................................. 5-1 

5.2 Methods.................................................................................................... 5-1 

5.3 Measurements of Currents and Temperature........................................... 5-2 

5.4 Temperature Records............................................................................... 5-4 

5.5 Effects of Bottom Water Fluctuation on Hydrate Stability ................... 5-19 

5.6 Conclusions and Recommendations for Future Measurements............. 5-19 

6.0 Geophysical Detection and Characterization of Chemosynthetic 

Organism Sites .................................................................................................. 6-1 

6.1 Introduction.............................................................................................. 6-1 

6.1.1 Overview of continental slope geology......................................... 6-1 

6.1.2 Overview of seep geophysical signatures ..................................... 6-2 

6.1.3 Geologic controls on chemosynthetic communities...................... 6-5 

6.1.4 Finding chemosyntheic communities using geophysics ............... 6-6 

6.1.5 Study overview and rationale ........................................................ 6-6 

6.2 Surveys................................................................................................... 6-10 

6.2.1 Study sites.................................................................................... 6-10 

6.2.2 Cruise 97G4: side-scan sonar survey and coring ........................ 6-12 

6.2.3 Submarine NR-1 cruise ............................................................... 6-15 

6.3 Data and Methods .................................................................................. 6-16 

6.4 Results.................................................................................................... 6-17 

6.4.1 GB site mosaic............................................................................. 6-18 

6.4.2 GC Shallow mosaic..................................................................... 6-20 

6.4.3 GC Deep mosaic.......................................................................... 6-27 

6.4.4 Submarine NR-1 dives ................................................................ 6-30 

6.4.5 Core data...................................................................................... 6-31 

6.4.6 Comparison with other geophysical data .................................... 6-32 
x

6.5 Discussion.............................................................................................. 6-36 

6.5.1 Geophysical signature of seeps ................................................... 6-36 

6.5.2 Geologic controls on chemosynthetic communities.................... 6-42 

 6.5.3 Searching for chemosynthetic community sites.......................... 6-43 

6.5.4 Directions for future research...................................................... 6-45 

7.0 Spatial and Temporal Patterns in Seep Communities .................................. 7-1 

7.1 Introduction.............................................................................................. 7-1 

7.2 Materials and Methods............................................................................. 7-2 

7.2.1 Survey results from submarine NR-1............................................ 7-2 

7.2.2 Laser line-scan mosaics................................................................. 7-3 

7.2.3 Subbottom profiling ...................................................................... 7-4 

7.2.4 Geographic co-registering ............................................................. 7-4 

7.2.5 Satellite image processing and interpretation................................ 7-4 

7.3 Results and Discussion ............................................................................ 7-5 

7.3.1 GC185 ........................................................................................... 7-5 

7.3.2 GC234 ......................................................................................... 7-12 

7.3.3 GC233 ......................................................................................... 7-22 

7.3.4 GB425 ......................................................................................... 7-31 

7.4 Conclusions............................................................................................ 7-38 

8.0 Ecology of Seep Fauna ...................................................................................... 8-1 

8.1 Introduction.............................................................................................. 8-1 

8.2 Bacterial Mats .......................................................................................... 8-4 

8.2.1 Objectives...................................................................................... 8-4 

8.2.2 Introduction ................................................................................... 8-4 

8.2.3 Results ........................................................................................... 8-5 

8.2.4 Discussion ..................................................................................... 8-9 

8.3 Mussel Community Ecology.................................................................. 8-12 

8.3.2 Objectives.................................................................................... 8-12 

8.3.3 Laboratory methods..................................................................... 8-12 

8.3.4 Results ......................................................................................... 8-14 

8.3.5 Discussion ................................................................................... 8-27 
xi

8.4 Vestimentiferan Community Ecology ................................................... 8-32 

8.4.1 Introduction ................................................................................. 8-32 

8.4.2 Objectives.................................................................................... 8-32 

8.4.3 Materials and Laboratory Methods ............................................. 8-33 

8.4.4 Results ......................................................................................... 8-35 

8.4.5 Discussion ................................................................................... 8-49 

8.5 Histopathy and Health of Seep Mussels ................................................ 8-52 

 

8.5.1 Introduction ................................................................................. 8-52 

8.5.2 Methods....................................................................................... 8-54 

8.5.3 Results: the parasitic fauna of seep mussels................................ 8-55 

8.5.4 Dynamics of parasitism............................................................... 8-62 

8.6 Food Chain Dynamics............................................................................ 8-72 

8.6.1 Introduction ................................................................................. 8-72 

8.6.2 Background: Expected Seep-Background Interactions............... 8-72 

8.6.3 Objectives.................................................................................... 8-74 

8.6.4 Methods....................................................................................... 8-74 

8.6.5 Results ......................................................................................... 8-75 

8.6.6 Discussion ................................................................................... 8-93 

8.6.7 Conclusion-scientific questions and regulatory 

 consequences ............................................................................... 8-94 

8.7 Mussel and Vestimeniferan Population Ecology................................... 8-95 

8.7.1 Objectives.................................................................................... 8-95 

8.7.2 Materials and Laboratory Methods ............................................. 8-96 

8.7.3 Results ....................................................................................... 8-100 

8.7.4 Discussion ................................................................................. 8-112 

9.0 Inorganic Biogeochemistry of Cold Seep Sediments ...................................... 9-1 

9.1 Overview.................................................................................................. 9-1 

9.2 Pore Water and Solid Phase Chemistry ................................................... 9-1 

9.2.1 Introduction ................................................................................ 9-1 

9.2.2 Results ........................................................................................ 9-4 

9.2.3 Discussion .................................................................................. 9-5 
xii

9.3 Stable Isotope Geochemistry ................................................................. 9-41 

9.3.1 Introduction .............................................................................. 9-41 

9.3.2 Results ...................................................................................... 9-41 

9.3.3 Discussion ................................................................................ 9-65 

9.4 Microelectrode Profiling........................................................................ 9-65 

9.4.1 Introduction............................................................................. 9-65 

9.4.2 Results..................................................................................... 9-68 

9.4.3 Discussion............................................................................... 9-78 

9.5 Conclusion ............................................................................................. 9-81 

10.0 Hydrocardon Geochemistry of Gulf of Mexico Chemosynthetic 

Communities..................................................................................................... 10-1 

10.1 Introduction............................................................................................ 10-1 

10.2 Petroleum Source Rocks........................................................................ 10-2 

10.3 Oil in Chemosynthetic Communities..................................................... 10-2 

10.4 Gas Hydrates.......................................................................................... 10-7 

10.5 Hydrocarbon Oxidation Rates................................................................ 10-8 

10.6 Brine Pools........................................................................................... 10-10 

10.7 Biogeochemical Synthesis ................................................................... 10-10 

10.8 Conclusions.......................................................................................... 10-11 

11.0 Synthesis of Results and Recommendations for Future Study.................... 11-1 

11.1 Synopsis ................................................................................................. 11-1 

11.2 Objectives and Basic Study Design ....................................................... 11-1 

11.3 Physical Factors and the Distribution and Abundance of 

Chemosynthetic Communities............................................................... 11-3 

11.4 Regional and Local Geophysical Signatures of Chemosynthetic 

Communities.......................................................................................... 11-5 

11.5 Program-wide Hypotheses..................................................................... 11-6 

11.5.1 Integrated geochemistry............................................................. 11-7 

11.5.2 Surface geochemistry................................................................. 11-8 

11.5.3 Comparison of geochemistry with tubeworms .......................... 11-8 

11.6 Geochemical Dependence of Tube Worm and Beggiatoa Colonies ... 11-17 

11.6.1 Model contribution of Beggiatoa to maintaining extreme 

rates of sulfate reduction.......................................................... 11-18 
xiii

11.7 A conceptual model of seep community establishment, growth, and 

evolution .............................................................................................. 11-23 

11.8 Model of Seep-background Tropic Interaction.................................... 11-25 

11.9 Recommended Approach to Sampling Design for Future Efforts....... 11-27 

11.9.1 Survey, characterization, and enumeration of seeps and seep 

communities............................................................................. 11-28 

11.9.2 Process-related Studies ............................................................ 11-29 

12.0 Data Management............................................................................................ 12-1 

12.1 Data Management .................................................................................. 12-1 

12.2 The Project Data Base............................................................................ 12-1 

12.3 Data Entry .............................................................................................. 12-3 

12.4 Geospatial applications ........................................................................ 12-12 

13.0 Literature Cited ............................................................................................... 13-1 
xv

LIST OF FIGURES 

Figure Page 

2.1 Offshore lease holdings illustrate the trend to deeper water over the past 

fifteen years.......................................................................................................... 2-7 

2.2 Distribution of known chemosynthetic communities and perennial oil slicks 

detected in available remote sensing data.......................................................... 2-10 

2.3 Illustration of the chemosynthetic community formation in a sediment 

diffusion style hydrocarbon seep. ...................................................................... 2-12 

2.4 Illustration of the chemosynthetic community formation in a brine pool 

style hydrocarbon seep....................................................................................... 2-14 

3.1 Northern Gulf of Mexico showing study site locations for submersible 

operations and mega-site areas for geophysical survey....................................... 3-2 

4.1 Submersible sampling operations with submarine Johnson SEA-LINK............. 4-3 

4.2 Combination float and bottom marker used to mark sampling stations .............. 4-5 

4.3 Hand-drawn sketch maps of locations for individual stations and the 

relative bearings between stations. ...................................................................... 4-7 

5.1 Power spectra of the current speed from the top and bottom meters on the 

GC185 meter mooring. ........................................................................................ 5-5 

5.2 Rotary spectra from the GC185 current meters. .................................................. 5-6 

5.3 Time series of temperatures recorded by thermistors at GC185 and GB425. ..... 5-7 

5.4 Power spectra from near bottom temperature sensors showing pronounced 

peaks near the 12-and 24-hour period.................................................................. 5-8 

5.5A Stick plots of currents at GC185 from September and October 1997 

together with the associated temperature measurements at each meter............. 5-10 

5.5B Stick plots of currents at GC185 from November and December 1997 

together with the associated temperature measurement at each meter. ............. 5-11 

5.5C Stick plots of current at GC185 from January and February 1998 together 

with the associated temperature measurement an each meter. .......................... 5-12 
xvi

5.5D Stick plots of currents at GC185 from March and April 1998 together with 

the associated temperature measurement at each meter. ................................... 5-13 

5.5.E Stick plots of currents at GC185 during May 1998 together with the 

associated temperature measurement at each meter. ......................................... 5-14 

5.6.A Charts of sea surface height anomaly every 20 days during the 1997-1998 

current meter deployment (Sept.-Oct. 1997). .................................................... 5-15 

5.6.B Charts of sea surface height anomaly every 20 days during the 1997-1998 

current meter deployment (Nov. 1997-Jan. 1998)............................................. 5-16 

5.6.C Charts of sea surface height anomaly every 20 days during the 1997-1998 

 current meter deployment (Feb-April 1998).................................................... 5-17 

5.6.D Charts of sea surface height anomaly every 20 days during the 1997-1998 

current meter deployment (April-June 1998) ................................................... 5-18 

5.7 Plot of the stability fields in Structure I, II, and H hydrate................................ 5-20 

6.1 Chemosynthetic site search protocol proposed by Reilly (1995) ........................ 6-8 

6.2 Location map of study sites on the Louisiana continental slope. ........................ 6-9 

6.3 Oil slicks on the sea surface over the Louisiana slope, imaged from Space 

Shuttle photography and synthetic aperture radar images. ................................ 6-10 

6.4 Bathymetry, lease blocks, and ship tracks for the GC Shallow site survey....... 6-11 

6.5 Bathymetry, lease blocks, and ship tracks for the GC Deep site survey. .......... 6-12 

6.6 Bathymetry, lease blocks, and ship tracks for the GB site survey..................... 6-13 

6.7 Core locations in GC Shallow survey area. ....................................................... 6-15 

6.8 Tracks of the 1998 submarine NR-1dives. ........................................................ 6-16 

6.9 TAMU2

 side-scan sonar mosaic of GB site........................................................ 6-19 

6.10 Surface geology interpretation map of GB site.................................................. 6-20 

6.11 TAMU2 side-scan sonar mosaic of GC Shallow site.......................................... 6-21 

6.12 Surface geology interpretation map of GC Shallow site. .................................. 6-22 

6.13 Side-scan sonar image of carbonate mounds in Green Canyon lease block 

139...................................................................................................................... 6-22 
xvii

6.14 Side-scan sonar image of the Bush Hill and Bush Lite chemosynthetic 

community sites ................................................................................................. 6-23 

6.15 Side-scan sonar image of faults and probable sediment flow caused by 

seepage from fault.............................................................................................. 6-24 

6.16 Side-scan sonar mosaic of vents and sediment flows in Green Canyon 

leaseblocks 271, 272, 315, and 316. .................................................................. 6-25 

6.17 Side-scan sonar mosaic of area including Green Canyon leaseblocks 232 to 

236...................................................................................................................... 6-26 

6.18 TAMU2

 side-scan sonar mosaic of GC Deep site............................................... 6-27 

6.19 Surface geology interpretation map GC Deep site. ........................................... 6-28 

6.20 Side-scan sonar mosaic of large sediment flow on the floor of Tiger Basin 

in the GC Deep survey area. .............................................................................. 6-29 

6.21 Zones of acoustic wipeout and turbidity in GC Shallow survey area................ 6-33 

6.22 Zones of acoustic wipeout and turbidity in GB survey area.............................. 6-36 

6.23 Seismic amplitude rendered on the seafloor from 3D multichannel seismic 

data over Bush Hill. ........................................................................................... 6-37 

6.24 Seismic amplitude rendered on the seafloor from 3D multichannel seismic 

data over the Garden Banks 425 mound............................................................ 6-38 

6.25 Seismic amplitude rendered on the seafloor from 3D multichannel seismic 

data over Green Canyon lease blocks 233 and 234. .......................................... 6-38 

6.26 Side-scan sonar image of different mound backscatter characteristics. ............ 6-40 

6.27 Dead-eye mounds............................................................................................... 6-41 

7.1 Generalized view of fault system and mound formation at the GC185 site. ....... 7-6 

7.2 Detailed bathymetry (1-m contours) of the GC185 site....................................... 7-8 

7.3 Detailed photomosaic of station JT1 in GC185 shows scattering of dead 

shells and live mussels, sparse tubeworms and carbonate rubble........................ 7-9 

7.4 First and last frame from a time-lapse series recorded at the HYD1 station. .... 7-13 

7.5 Overview of the GC234 study site, situated in the head of a canyon formed 

between active fault-lines. ................................................................................. 7-14 
xviii

7.6 Merger of 1990 survey data with 1998 survey data and sampling stations....... 7-15 

7.7 Results of X-Star subbottom profile survey....................................................... 7-17 

7.8 Results of X-Star subbottom profile survey with station locations. .................. 7-18 

7.9 Exposed gas hydrate at the HYD1 station in GC234 site, 1997. ....................... 7-19 

7.10 Tubeworm abundance at GC234. ...................................................................... 7-20 

7.11 Brine Pool NR-1 in the GC233 study site.......................................................... 7-24 

7.12 Subbottom profile survey of GC233 brine pool. ............................................... 7-25 

7.13 Three-dimensional interpretations of combined data for GC233 brine pool..... 7-27 

7.14 Descriptions of piston cores collected at GC233 study site............................... 7-28 

7.15 Temperature profiles of two brine pools............................................................ 7-31 

7.16 Regional setting for study site in GB425........................................................... 7-32 

7.17 Mud-filled crater at GB425................................................................................ 7-34 

7.18 Thermistor array deployed in mud filled crater at GB425................................. 7-35 

7.19 Synthetic aperture radar (SAR) images (collected by RADARSAT) provide 

a means to compare the size of natural oil slicks on 29 July and 11 August 

1997 on the sea surface immediately above the GB425 site. ............................ 7-36 

7.20 Comparison of temperature time series to abundance of floating oil above 

site...................................................................................................................... 7-37 

8.2.1 Evolutionary relationships among selected members of the gamma 

roteobacteria as determined by 16S rRNA sequencing. ..................................... 8-7 

8.2.2 Depth integrated Beggiatoa biomass (0 to 5 cm) for 10 representative cores 

from Gulf of Mexico hydrocarbon seeps............................................................. 8-8 

8.2.3 Comparison of sediment depth distributions of Monterey Canyon 

Beggiatoa sp. (75-m cell diameter) at a cold seep (900-m depth) and a 

representative Gulf of Mexico Beggiatoa sp. (42-m cell diameter) at a 

hydrocarbon seep (600 m depth)........................................................................ 8-10 

8.3.1A Size frequency distribution of mussels in each of the MMS primary study 

sites and from six collections at the GC233 in 1995. ........................................ 8-17 

8.3.1B Size frequency distribution of mussels in each of the MMS primary study 

sites (G-L). ........................................................................................................ 8-18 
xix

8.3.1C Size frequency distribution of mussels in each of the MMS primary study 

sites (M-R) . ....................................................................................................... 8-19 

8.3.1D Size frequency distribution of mussels in each of the MMS primary study 

sites (S-T)........................................................................................................... 8-20 

8.3.2 Density of mussels, as individuals per sq. m, in each of the MMS primary 

study sites and from six collections at the GC233 in 1995................................ 8-21 

8.3.3 Biomass of mussel soft tissue in each of the MMS primary study sites and 

from six collections at the GC233 in 1995. ....................................................... 8-22 

8.3.4 Growth of mussels between 1997 and 1998 field seasons at the GC233 and 

at MMS study stations BHM1, GCM1, GBM1and BPM1/M4. ........................ 8-27 

8.3.5 Growth and condition of transplant and control mussels from GC185 – 

GC233 reciprocal transplant study..................................................................... 8-28 

8.3.6 Growth and condition of transplant and control mussels from the GC234 – 

GC233 reciprocal transplant study..................................................................... 8-31 

8.4.1 Size specific growth rate in Lamellibrachia sp. ................................................ 8-36 

8.4.2 Individual ages predicted from tube length using the negative exponential 

model.................................................................................................................. 8-36 

8.4.3 Size-specific growth rate in the Escarpia-like species. ..................................... 8-37 

8.4.4 Mean growth rates (±s.d.) of Lamellibrachia sp. and the Escarpia-like 

species at all principal sampling stations........................................................... 8-37 

8.4.5 Mean condition indices (±s.d.) of Lamellibrachia sp. and the Escarpia-like 

species at all principal sampling stations........................................................... 8-40 

8.4.6 Mean tube lengths (±s.d.) of subsamples of Lamellibrachia sp. and the 

Escarpia-like species taken from the principal sampling stations for 

determination of growth rate and condition....................................................... 8-40 

8.4.7 Abundances of Lamellibrachia sp. and the Escarpia-like species at seven 

principal sampling stations from which all vestimentiferans were collected 

and measured. .................................................................................................... 8-41 

8.4.8 Size frequency histograms of Lamellibrachia sp. and the Escarpia-like 

species at the seven principal sampling stations from which all 

vestimentiferans were collected and measured.................................................. 8-42 

8.4.9 Species richness and abundance in Bushmaster Jr. Collection of 

BHJFIGURE 8.4.2............................................................................................. 8-45 
xx

8.4.10 Species richness and abundance in Bushmaster Sr. Collection of 

GCAFIGURE 8.4.1............................................................................................ 8-46 

8.4.11 Species richness and abundance in Bushmaster Sr. Collection of 

BHAFIGURE 8.4.1............................................................................................ 8-47 

8.4.12 Species richness and abundance in Bushmaster Sr. Collection of 

GCSFIGURE 8.4.1 ............................................................................................ 8-48 

8.4.13 Sulfide permeability of Lamellibrachia sp. tubes.............................................. 8-49 

8.5.1 Plate 1. Gill rosettes -- 1a,b: 1000x; 1c: 25x. Plate 2. Rickettsial-like 

bodies in the digestive tubules and diverticula -- 2a,b: 1000x. Plate 3. Gill 

ciliates between the demibranchs -- 3a,b: 1000x. Plate 4. Bucephalus in the 

gonad -- 10x. Plate 5. Rickettsial-like bodies in the gill -- 5a: 1000x; 5b: 

25x...................................................................................................................... 8-56 

8.5.2 Plot of total PAH body burden versus population for B. childressi

populations from 5 major sites........................................................................... 8-68 

8.5.3 Proportional differences between the sample mean and the global mean for 

each compound in the PAH compound series. .................................................. 8-69 

8.5.4 Plot of total PAH body burden for mussels in the transplant experiment. ........ 8-70 

8.5.5 Proportional differences between the sample mean and the global mean for 

each compound in the PAH compound series for the three transplant 

experiments........................................................................................................ 8-71 

8.6.1 Univariate histograms of 13C determinations for faunal groups. ..................... 8-81 

8.6.2 Univariate histograms of 15N determinations for faunal groups...................... 8-82 

8.6.3 Univariate histograms of 34S determinations for faunal groups....................... 8-83 

8.6.4 Apparent utility of 13C and 34S for elimination of food source ambiguity 

in seep and background systems........................................................................ 8-87 

8.6.5 GC233 13C versus 15N and 13C versus 34S scatter plots for main faunal 

groups showing strong influence of mixing and minor influence of trophic 

shifts................................................................................................................... 8-89 

8.6.6 GC185 13C versus 15N and 13C versus 34S scatter plots for main faunal 

groups showing greater indication of mixing from 34S.................................... 8-90 
xxi

8.6.7 GC234 13C versus 15N and 13C versus 34S scatter plots for main faunal 

groups showing indication of mixing but with insufficient 34S 

determinations for more definitive statements................................................... 8-92 

8.7.1 Frequency distribution of microsatellite alleles for the 4-10D microsatellite 

locus in Escarpia sp. collected from four hydrocarbon seeps in the Gulf of 

Mexico. ............................................................................................................ 8-105 

8.7.2 Frequency distribution of microsatellite alleles for the 3-2D microsatellite 

locus in Escarpia sp. collected from four hydrocarbon seeps in the Gulf of 

Mexico. ............................................................................................................ 8-106 

8.7.3 Frequency distribution of microsatellite alleles for the gA3C microsatellite 

locus in B. childressi collected from six stations within the GC185 

hydrocarbon seep in the Gulf of Mexico. ........................................................ 8-107 

8.7.4 Frequency distribution of microsatellite alleles for the gA3C microsatellite 

locus in B. childressi collected from five hydrocarbon seeps in the Gulf of 

Mexico ............................................................................................................. 8-108 

8.7.5 Phylogenetic relationships among vestimentiferan tubeworms from the 

hydrocarbon seeps and hydrothermal vents..................................................... 8-109 

8.7.6 Phylogenic tree of tubeworms based on the COI gene of the mitochondrial 

DNA................................................................................................................. 8-110 

8.7.7 Phylogenic relationships among sulfur-oxidizing endosymbionts based on 

16S rDNA sequences. ...................................................................................... 8-111 

9.1 Organic carbon and grain size distributions....................................................... 9-15 

9.2 Characterization of chemosynthetic sample stations by sulfate reduction. ....... 9-17 

9.3 Sulfate reduction stoichiometry by station. ....................................................... 9-30 

9.4 Sulfate reduction stoichiometry, all stations by year......................................... 9-31 

9.5 Carbonate mineral precipitation......................................................................... 9-32 

9.6 Sedimentary reactive iron. ................................................................................. 9-34 

9.7 Sedimentary sulfide (TRS and AVS)................................................................. 9-35 

9.8 Reactive iron availability. .................................................................................. 9-36 

9.9 Iron reactivity towards sulfide. .......................................................................... 9-37 

9.10 Sulfate depletion depth. ..................................................................................... 9-39 
xxii

9.11 Beggiatoa mat transect (GCB3)......................................................................... 9-40 

9.12 Brine geochemisty. ............................................................................................ 9-42 

9.13 Stable isotopes in carbonates. ............................................................................ 9-63 

9.14 Sulfur stable isotopes in H2S and TRS .............................................................. 9-64 

9.15 Calcium carbonate concentration ...................................................................... 9-66 

9.16 DIC stable carbon isotopes. ............................................................................... 9-67 

9.17 Control H2S profiles........................................................................................... 9-69 

9.18 Beggiatoa H2S profiles. ..................................................................................... 9-70 

9.19 Tubeworm H2S profiles. ................................................................................... 9-71 

9.20 Transect H2S profiles. ....................................................................................... 9-72 

10.1 Sketch of typical gas hydrate mound (~1-3 m across) modeled after the 

Bush Hill site including sampling stations BHHYD-1 and BHST-2................. 10-5 

10.2 Diagrams summarizing isotopic and molecular properties of sediments 

overlying gas hydrate mounds and from near tube worm colonies at GC133 

(BHST-2) and GC234 (GCAT-2) sampling stations. ........................................ 10-6 

10.3 Diagrams summarizing isotopic and molecular properties of vent gas and 

gas hydrate samples at the BHHYD-1 and -2 sample stations, GC133............. 10-9 

10.4 Geochemical overview of the gas hydrate environment.................................. 10-11 

11.1 Plot of the rate of change of bottom water temperature (dT/dt) versus the 

northerly component of the current velocity shows a weak, but non-random 

correlation (R2

 = 0.17) ...................................................................................... 11-4 

11.2 Variable loadings on principal component axes (PC1 and PC2) for 

inorganic and organic chemicals integrated over a depth range of 11cm........ 11-10 

11.3 Variable for elements in the experimental design for inorganic and organic 

chemicals integrated over a depth range of 11cm (sites and years)................. 11-11 

11.4 Variable scores for elements in the experimental design for inorganic and 

organic chemicals integrated over a dept range of 11cm (sites and habitats).. 11-12 

11.5 Variable loadings for inorganic and organic chemicals in upper-most layers 

of surface sediments (2 - 4 cm)........................................................................ 11-13 
xxiii

11.6 Variable scores for elements in the experimental design for inorganic and 

organic chemicals in surface sediments (upper 2 - 4 cm)................................ 11-14 

11.7 Variable loadings for inorganic and organic chemicals in surface sediments 

only (2 – 4 cm) averaged over both sampling years. Calculated for 

comparison to tubeworm condition and growth .............................................. 11-15 

11.8 Schematic of the tubeworm bush and bacterial mat habitats sampled during 

the program ...................................................................................................... 11-16 

11.9 Apparent diffusion coefficients........................................................................ 11-19 

11.10 Temporal cycle of sulfate reduction and enhanced sulfate supply .................. 11-21 

11.11 Comparison of apparent diffusion coefficients and sulfate reduction rates..... 11-22 

11.12 A descriptive trophic model for an open seep system ..................................... 11-26 

12.1 Example of data collection form used for CTD casts........................................ 12-2 

12.2 Table and relationships in the program DBMS ................................................. 12-4 

12.3 Table listing of the program DBMS .................................................................. 12-5 

12.4 Sediment sample table ....................................................................................... 12-6 

12.5 Form designed to record surface operations with pull down menus for 

selecting site....................................................................................................... 12-7 

12.6 Merged modules forming a database file for headspace.................................... 12-8 

12.7 Geospatial figure with results layered over bathymetry at Bush Hill site ....... 12-12 

12.8 Metadata file and contents for program data ....................................................12.13 
xxv

LIST OF TABLES 

Table Page 

1.1 Program Team and Roles..................................................................................... 1-2 

1.2 Program Milestones ............................................................................................. 1-3 

1.3 Technical Publications Resulting from Preliminary Findings of the Stability 

and Change in Gulf of Mexico Chemosynethic Communities Program. ............ 1-4 

3.1 Mega-Site Regional Survey Areas....................................................................... 3-3 

3.2 Principal Community-Level Sampling Sites and Pertinent Characteristics ........ 3-4 

3.3 Stations Sampled According to the Experimental Design Criteria...................... 3-8 

3.4 Experimental design for tubeworm sampling program. .................................... 3-11 

3.5 Experimental design for the mussel program. ................................................... 3-11 

3.6 Experimental design for the microbial mat sampling program. ........................ 3-12 

5.1 GC185 Current meters ......................................................................................... 5-2 

5.2 The frequency, in percent of time, that currents had a particulate range of 

speed and direction for the top and bottom meters.............................................. 5-3 

5.3 Tidal analysis of two current meters showing magnitudes of the semi-major 

and semi-minor tidal constituents and the direction of the resulting tidal 

ellipse. .................................................................................................................. 5-4 

5.4 Statistics of recorded bottom water temperatures................................................ 5-8 

6.1 Cruise 97G4 Core Locations.............................................................................. 6-14 

6.2 Core disturbance properties ............................................................................... 6-32 

6.3 Chemosynthetic community site indicators....................................................... 6-46 

7.1 Summary of datasets analyzed and discussed in this chapter.............................. 7-3 

7.2 Areas occupied by components of the chemosynthetic community at 

GC185 that were identified in laser line-scan mosaic. ...................................... 7-10 
xxvi

7.3 Field notes on piston cores taken at sites GC185 and GC234. .......................... 7-11 

7.4 Comparison of surface amplitudes from 3D seismic data to tubeworm 

abundance .......................................................................................................... 7-22 

7.5 Relative areas of GC233 brine pool as calculated from laser line scan 

mosaic. ............................................................................................................... 7-23 

7.6 Shells collected from living mussels (SEA LINK) at GC233 and GC185 

sites and in piston cores from GC233 site. ........................................................ 7-29 

8.2.1 Depth distribution of Beggiatoa filaments in Gulf of Mexico cores, 1998. ........ 8-6 

8.2.2 Evidence that 37-44 m wide Beggiatoa from Gulf of Mexico have 

vacuoles that lack typical bacterial cytoplasm..................................................... 8-6 

8.2.3 Seep Beggiatoa, Gulf of Mexico. ........................................................................ 8-9 

8.2.4 Comparison of Gulf of Mexico Beggiatoa spp. with non-vacuolate, narrow 

Beggiatoa cultures and other native populations of vacuolate bacteria............. 8-11 

8.3.1 Methane levels in water samples from among mussels at MMS stations.......... 8-15 

8.3.2 Characteristics of mussel collections from each primary MMS study site 

and GC233 in 1995. ........................................................................................... 8-23 

8.3.3 Density (number of individuals per sq. m) of associated fauna in mussel 

beds. ................................................................................................................... 8-24 

8.3.4 Concentrations of Methane (CH4) and Oxygen (O2) at GC233 inner zone 

and the short-term transplant locations for 1997. .............................................. 8-25 

8.3.5 Mean (standard deviation) condition index, glycogen content, water 

content, and length for the inner zone of the GC233, the control boxes and 

the short-term transplant boxes in 1997............................................................. 8-26 

8.3.6 Von Berltalanffy growth parameter estimates for GC233, GCM1, BHM1, 

and GBM1 mussel populations.......................................................................... 8-26 

8.4.1 Characteristics of Lamellibrachia sp. and the Escarpia-like species in 

juvenile, adult and senescent aggregations. ....................................................... 8-38 

8.4.2 Characteristics of 1998 bushmaster collections................................................. 8-43 
xxvii

8.4.3 Seawater sulfide concentrations among hydrocarbon seep tubeworm 

bushes................................................................................................................. 8-43 

8.4.4 Interstitial sulfide levels beneath vestimentiferan aggregations in the Gulf 

of Mexico........................................................................................................... 8-44 

8.4.5 Plume CO2 flux prior to during, and after root H2S exposure ........................... 8-44 

8.5.1 Semi-qualitative scales used for evaluation of gonadal stage, gill and 

digestive gland atrophy, and Bucephalus infection. .......................................... 8-54 

8.5.2 Prevalences and infection intensities in year one for the five types of 

parasites identified for each of the 4 primary sites. ........................................... 8-58 

8.5.3 Comparison of sites for which greater than one sample was available in 

year one.............................................................................................................. 8-59 

8.5.4 P values ( = 0.05) for Spearman's Rank correlations for parasite 

prevalence and measures of general health, based on year one samples........... 8-60 

8.5.5 P values ( = 0.05) for Spearman's Rank correlations for parasite infection 

intensity and measures of general health based on year one samples................ 8-60 

8.5.6 Health indices and PAH body burden (ppb) in year one for mussels taken 

from each of the 4 primary sites. ....................................................................... 8-61 

8.5.7 Mean prevalences, infection intensities, and physiological indices for 

mussel populations sampled in year two. .......................................................... 8-63 

8.5.8 Relationship of the transplanted individuals to the donor and receiver 

population in cases where a nonparametric ANOVA identified a significant 

population effect. ............................................................................................... 8-65 

8.6.1 Deployments and recovery of large surface-deployed traps.............................. 8-76 

8.6.2 Small within-site traps deployed and recovered by submersible....................... 8-77 

8.6.3 Summary table for stable isotope analyses from program study sites ............... 8-78 

8.6.4 Product-moment correlation of isotope pairs as an indication of utility in 

any future multivariate model............................................................................ 8-84 

8.6.5 Summary of immunoassys................................................................................. 8-93 

8.7.1 Samples of Escarpia spicata collected from four localities in the Gulf of 

Mexico. .............................................................................................................. 8-96 
xxviii

8.7.2 Samples of Lamellibrachia sp. collected from four localities in the Gulf of 

Mexico. .............................................................................................................. 8-96 

8.7.3 Samples of Bathymodiolus childressi collected from four localities in the 

Gulf of Mexico................................................................................................... 8-97 

8.7.4 PCR primers for nine microsatellite loci in Lamellibrachia sp. from the 

Gulf of Mexico. ............................................................................................... 8-101 

8.7.5 Estimates of the number of migrants exchanged between four populations 

of Lamellibrachia sp........................................................................................ 8-102 

8.7.6 PCR primers for 25 microsatellite loci in Escarpia sp. from the Gulf of 

Mexico ............................................................................................................. 8-103 

8.7.7 Estimates of the number of migrants exchanged between four populations 

of Escarpia sp .................................................................................................. 8-104 

8.7.8 PCR primers for four microsatellite loci in Bathymodiolus childressi from 

the Gulf of Mexico........................................................................................... 8-106 

8.7.9 Estimates of the number of migrants exchanged between four populations of 

Bathymodiolus childressi ..................................................................................... 8-108 

9.1 Porewater pH, carbon, sulfur, and anions............................................................ 9-6 

9.2 Summary of sulfate reduction............................................................................ 9-16 

9.3 Major cations. .................................................................................................... 9-19 

9.4 Dissolved organic carbon and nutrients............................................................. 9-43 

9.5 Solid phase properties........................................................................................ 9-56 

9.6 Sulfate Reduction Rate (35S).............................................................................. 9-73 

9.7 Comparative sulfate reduction rates................................................................... 9-77 

10.1 Molecular and istopic properties of vent gases and gas hydrates ...................... 10-4 

10.2 Molecular and isotopic properties of hydrocarbons and carbon dioxide in 

selected sediment samples. ................................................................................ 10-4 

11.1 Tubeworm condition and growth data correlated to chemical (by station) 

principal components (PC1 and PC2)................................................................ 11-9 

12.1 Sampling summary at each site.......................................................................... 12-2 
xxix

12.2 Summary of the samples collected .................................................................... 12-9 

12.3 Tracking table .................................................................................................. 12-10 
1-1 

1.0 Stability and Change in Gulf of Mexico Chemosynthetic Communities Program 

1.1 Introduction 

This volume is the Final Report for the research program entitled Stability and Change in Gulf of 

Mexico Chemosynthetic Communities. It describes findings obtained from natural hydrocarbon 

seeps on the continental slope of the northern Gulf of Mexico. This section of the report will 

outline the purpose and scope of the program as well as the program team jointly responsible for 

the results. Major milestones of the program are listed along with a compilation of technical 

articles and scientific presentations that have been produced while the program was underway. 

The program was conducted under the auspices of the Texas A&M Research Foundation under 

contract (Contract No. 1435-01-96-CT-30813) to the Department of the Interior, Minerals 

Management Service (MMS). The contract was awarded on 31 October 1996 and has been 

completed over 42 months through the efforts of a multi-disciplinary team from a number of 

research institutions. The present research program is the second major research effort funded by 

the Minerals Management Service to address this topic. The Chemosynthetic Ecosystems Study

(CHEMO I) was conducted between 1992 and 1995 as the primary assessment of the ecology of 

natural oil seeps on the Gulf of Mexico continental slope (MacDonald et al. 1995). 

The continued research has been designed to aid the Minerals Management Service in the 

scientifically sound management of living resources found at hydrocarbon seeps on the northern 

Gulf of Mexico continental slope. An integrated, multi-disciplinary approach was proposed to 

address the complex issues associated with the protection of these biological assemblages. As 

such, the program encompasses ecological studies at both regional and local scales as well as an 

evaluation of temporal changes in these communities. Stability and change within these 

communities should be understood in the context of their interactions within the geological, 

chemical, and oceanographic setting. Knowledge of the processes that control the distribution, 

health, and succession of communities in these environments is necessary to forecast and 

forestall potential impacts derived from exploration and exploitation of fossil energy reserves on 

the northern Gulf of Mexico continental slope. Integrated studies were designed to collect 

ecological, geological, chemical, and oceanographic information related to the longevity, 

robustness, senescence, and recovery of chemosynthetic communities. 

The team of Principal Investigators (PI) assembled to conduct this research represents some of 

the foremost researchers in their respective fields. The present program includes community 

ecological, regional geological, microbial, and site-specific chemical and oceanographic studies. 

Each PI and his or her associates were responsible for individual components of the program and 

contributed jointly to the fieldwork and overall synthesis. At the community level, efforts focus 

on the abiotic factors that control the distribution, abundance, and health of the major 

chemosynthetic and associated fauna. Investigations of the life history of these organisms are 

also included. At the regional level, efforts focus on the geological, chemical, and oceanographic 

processes that support communities including larval dispersion and circulation processes that 

maintain the genetic stability of these communities. 
1-2 

1.2 Key Personnel 

The principal investigators, their area of expertise, and the sections of this report to which they 

contributed are listed in Table 1.1. A three-person Scientific Review Board has provided 

oversight and review of the program and is also listed in Table 1.1. 

Table 1.1. Program Team and Roles. Bold-Face Text 

Indicates Chapters of the Present Report to which the Team Members Contributed.

Texas A&M University Investigators

Department of Oceanography

Dr. John W. Morse 

Inorganic Chemistry 

Section 9

Dr. Rolf Arvidson 

Inorganic Chemistry 

Section 9

Dr. William Sager 

Geophysics 

Section 6

Geochemical and Environmental Research Group

Dr. Norman L. Guinasso, Jr. 

Physical Oceanography 

Section 5

Dr. Ian MacDonald 

Program Manager 

Imaging & GIS 

Section 7 and editorial review

Dr. Gary A. Wolff 

Data Management 

Section 11

Dr. Mahlon C. Kennicutt II 

Deputy Program Manager 

Environmental Chemistry 

Section 8.5

Dr. Roger Sassen 

Hydrocarbon Chemistry 

Section 10

Principal Investigators Not at Texas A&M University

Dr. Steven Macko 

University of Virginia 

Trophic Relationships 

Sections 8.6 and 9.3

Dr. Douglas C. Nelson 

University of California, Davis 

Microbial Ecology 

Section 8.1

Dr. Eric Powell 

Rutgers University 

Histopathology 

Section 8.5

Dr. Robert Carney 

Louisiana State University 

Trophic Relationships 

Section 8.6 

Dr. Paul Montagna 

University of Texas at Austin 

Statistical Design 

Section 11

Dr. Samantha Joye 

University of Georgia 

Electrochemistry 

Section 9.4

Dr. Charles R. Fisher 

Pennsylvania State University 

Physiological Ecology 

Sections 8.3 and 8.4

Dr. Steve Schaeffer 

Pennsylvania State University 

Molecular Ecology and Genetics 

Section 8.7

Scientific Review Board

Dr. James Barry 

Monterey Bay Aquarium Research 

Institute 

Dr. Cindy Lee Van Dover 

William and Mary University 

Dr. William W. Schroeder 

The University of Alabama 
1-3 

1.3 Program Milestones 

Stability and Change in Gulf of Mexico Chemosynthetic Communities (hereafter abbreviated as 

CHEMO II) was awarded in 1996. It has progressed through planning, sampling, and analytical 

phases. The program now concludes with the reporting phase. The program milestones listed in 

Table 1.2 summarize major field efforts, workshops, and presentations of program results to 

public audiences. 

Table 1.2 Program Milestones. 

October 1-8, 1996 Program award and post-award meeting of investigators 

June 3-15, 1997 Geophysical survey cruise with R/V GYRE 

July 8-31, 1997 Community sampling cruise with submarine Johnson SEA-LINK and R/V 

EDWIN LINK 

December 18, 1997 Presentations at the MMS Information Transfer Meeting, New Orleans, LA 

May 18-22, 1998 Ground-truth survey with Submarine NR-1 

July 1-15, 1998 Community sampling cruise with submarine Johnson SEA-LINK and R/V 

EDWIN LINK 

September 1998 Interim Report Completed 

December 4, 1998 Presentations at the MMS Information Transfer Meeting, New Orleans, LA 

May 21-23, 1999 Synthesis Workshop, College Station, TX 

November 30, 1999 Presentations at the MMS Information Transfer Meeting, New Orleans, LA 

January 27, 2000 Presentations in dedicated session at American Geophysical Union Ocean 

Sciences Meeting, San Antonio, TX 

1.4 Publications, Abstracts, and Graduate Theses 

The program team has been very active in presenting and publishing technical articles 

concerning research results. Although the present report is intended as a definitive presentation 

to the Minerals Management Service, these publications, as well as follow-on articles that will 

continue to emerge after the report has been submitted, are critical for recording findings in the 

peer-reviewed literature and for offering the opportunity for critical review by the scientific 

community. In addition, graduate student research is vital for training the next generation of 

marine scientists. Table 1.4 lists open literature references produced by the program. 
1-4 

Table 1.3. Technical Publications Resulting from Preliminary Findings 

of the Stability and Change in Gulf of Mexico Chemosynthetic Communities Program. 

Peer-reviewed articles published or in press

Bergquist, D. C., F. M. Williams, and C. R. Fisher. 2000. Longevity record for deep-sea 

invertebrate. Nature 403: 499-500. 

Callender W. R. and Powell, E. N. 1999. Why did ancient chemosynthetic seep and vent 

assemblages occur in shallower water than they do today? Intl. Jnl. of Earth Sci. 88 

(3): 377-391. 

Callender R, Powell E.N. 2000. Long-term history of chemoautotrophic clam-dominated faunas 

of petroleum seeps in the northwestern Gulf of Mexico. FACIES. 43: 177-204. 

Fisher, C. R., I. R. MacDonald, R. Sassen, C. M. Young, S. Macko, S. Hourdez, R. Carney, S. 

Joy, and E. McMullin. 2000. Methane ice worms: Hesiocaeca methanicola colonizing 

fossil fuel reserves. Naturwissenschaften 87 (4): 184-187. 

Freytag, J.K., Girguis, P.R., Bergquist, D.C., Andras, J.P., Childress, J.J., Fisher, C.R. 2001. A 

paradox resolved: Sulfide acquisition by roots of seep tubeworms sustains net 

chemoautotrophy. PNAS. 98(23): 13408-13413. 

Gardiner, S.L., McMullin, E., Fisher, C.R. 2001. Seepiophila jonesi, a new genus and species of 

vestimentiferan tube worm (Annelida : Pogonophora) from hydrocarbon seep 

communities in the Gulf of Mexico. Proc. of the Biol. Soc. of Wash. 114(3): 694-707. 

Hourdez, S., L. A. Frederick, A. Schernecke, and C. R. Fisher. 2001. Functional respiratory 

anatomy of a deep sea orbiniid polychaete from the Brine Pool NR-1 in the Gulf of 

Mexico. Invertebrate Biology. 120(1): 29-40. 

Julian, D., F. Gaill, E. Wood, A. J. Arp, and C. R. Fisher. 1999. Roots as a site of hydrogen 

sulfide uptake in the hydrocarbon seep vestimentiferan Lamellibrachia sp. J. Exp. Biol. 

202; 2245-2257. 

Lanoil BD, Sassen R, La Duc MT, Sweet ST, Nealson KH. 2001. Bacteria and Archaea physically

 associated with Gulf of Mexico gas hydrates. Applied and Env. Microbiol. 67 (11): 5143-5153. 

Macavoy, S.E., Carney, R.S., Fisher, C.R., and Macko, S.A. 2002. Use of chemosynthetic 

biomass by large, mobile, benthic predators in the Gulf of Mexico. Mar. Ecol. Prog. Ser. 

225: 65-78. 

MacDonald, I.R. 1998. Habitat formation at Gulf of Mexico hydrocarbon seeps. Cahiers de 

Biologie Marine. 39 (3-4): 337-340. 
1-5 

Table 1.3. Continued 

MacDonald, I. R., D. Buthman, W. W. Sager, M. B. Peccini, and N. L. Guinasso Jr. 2000. 

Pulsed flow of hydrocarbons from a mud volcano. Geology. 28(10): 907-910. 

McMullin, E., D. C. Bergquist, and C. R. Fisher. 2000. X-metazoans: Real animals in extreme 

environments. Space Biol. Bull. 13: 1-12. 

Nelson, K. and C. R. Fisher. 2000. Absence of cospeciation in deep-sea vestimentiferan tube

 worms and their bacterial endosymbionts. Symbiosis. 28(1): 1-15. 

Powell, E.N., R.D. Barber, M.C. Kennicutt, and S.E. Ford. 1999. Influence of parasitism in 

controlling the health, reproduction and PAH body burden of petroleum seep mussels. 

Deep Sea Research. 46. 2053-2078. 

Pruski, A. M., A. Fiala-Médioni, C. R. Fisher, and J. C. Colomines. 2000. Free amino compound 

composition of symbiotic invertebrates from the Gulf of Mexico hydrocarbon seeps. Mar. 

Biol . 136: 411-420. 

Sager, W. W., C. S. Lee, I. R. Macdonald, and W. W. Schroeder. 1999. High-frequency nearbottom acoustic reflection signatures of hydrocarbon seeps on the northern Gulf of 

Mexico continental slope. Geo Marine Letters. 18 (4). 267-276. 

Sager, W. W., I. R. MacDonald, W. R. Bryant, R. L. Carlson, and D. B. Prior, TAMU2

 digital 

side-scan sonar survey of Louisiana Slope areas containing active oil seeps and 

halokinetic sediment modification, paper 8595, Proceedings of the 1998 Offshore Technology Conference, Houston, 4-7 May 1998, 57-69, 1998. 

Sager, W. W., M. C. Kennicutt II and Gas Hydrate Science Team, Proposal to the Ocean Drilling 

Program for Drilling Gas Hydrate in the Gulf of Mexico, paper 12111, Proceedings of the 

2000 Offshore Technology Conference, Houston, 1-4 May 2000. 

Sassen R, Sweet ST, Milkov AV, DeFreitas DA, Kennicutt MC. 2001. Thermogenic vent gas 

and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant? 

Geology. 29(2): 107-110. 

Scott, K. M., M. Bright, and C. R. Fisher. 1998. The burden of independence: Inorganic carbon 

utilization strategies of the sulfur chemoautotrophic hydrothermal vent isolate 

Thiomicrospira crunogena and the symbionts of hydrothermal vent and cold seep 

vestimentiferans. Cah. Biol. Mar. 39: 379-382. 

Smith, E. B., K. M. Scott, E. R. Nix, C. Korte, and C. R. Fisher. 2000. Growth and condition of 

seep mussels at a Gulf of Mexico Brine Pool. Ecology 81:2392-2403. 
1-6 

Table 1.3. Continued 

Urcuyo, I.A., Massoth, G.J., MacDonald, I. R., Fisher, C.R. 1998. In situ growth of the

 vestimentiferan Ridgeia piscesae living in highly diffuse flow environments in the main

 Endeavour Segment of the Juan de Fuca Ridge. Cahiers De Biologie Marine 39(3-4):

 267-270. 

Published abstracts

Arvidson, R. S., and J. W. Morse. 1999. Sulfate reduction rates in chemosynthetic communities, 

Gulf of Mexico. EOS. 80 (49). OS242-243. 

Bergquist, D. C., C. R. Fisher, and F. M. Williams. 1999. Life history characteristics and 

population structures of two co-occurring Vestimentiferan species at hydrocarbon seeps 

on the upper Louisiana Slope. EOS. 80 (49). OS243. 

Buthman, D., I. R. MacDonald, M. Peccini*, W. W. Sager, and N. L. Guinasso Jr., Episodic 

High Temperatures and Hydrocarbon Seepage from a Mud Volcano, EOS, Trans. AGU, 

Ocean Sciences 2000 Meeting Supplement, OS244, 1999. 

Escorcia, S. P., I. R. MacDonald, and S. B. Joye. Spatial and inter-annual variation of sulfide 

fluxes in chemosynthetic communities in the Gulf of Mexico. 1999. in Navigating into 

the next century. pp. 63, American Society of Limnologists and Oceanographers, Santa 

Fe, NM. 

Fisher, C. R., R. T. Ward, B. Begley, and R. Kosoff. 1999. Compositional and successional 

patterns of cold seep communities on the Louisiana Slope of the Gulf of Mexico. eos. 80 

(49). os243. 

Freytag, J. K., P. R. Girguis, J. P. Andras, D. C. Bergquist, and C. R. Fisher. 1999. Sulfide 

uptake by buried portions of cold seep vestimentiferans can sustain autotrophic carbon 

fixation. EOS. 80 (49). OS5. 

Guinasso, N. L., I. R. MacDonald, M. Peccini, and L. L. Lee. 1999. Near-bottom phyiscal 

oceanographic measurements near oil-seep sites on the Gulf of Mexico slope and their 

relationship to hydrate stability. EOS Supplement. 80 (49). OS242. 

Kastner, M., I. R. MacDonald, A. Paytan, and S. Sweet. 1999. Isotopic and molecular 

composition of shallow gas hydrates from Gulf of Mexico hydrocarbon seeps. EOS 

Supplement. 80 (49). OS242. 

MacDonald, I. R., W. W. Sager, N. L. Guinasso Jr., and E. Powell. 1999. Evidence of long-term 

fluctuation in fluid expulsion at hydrocarbon seeps. EOS Supplement. 80 (49). OS242. 
1-7 

Table 1.3. Continued 

MacDonald, I. R. W. W. Sager, N. L. Guinasso, Jr. Gas in continental slope sediments, northern 

Gulf of Mexico: Examples from two distinct seafloor environments (extended abstract), 

Proceedings, International Conference on Gas in Marine Sediments, 7-11 September, 

1998, Bologna, Italy. 

Macko, S. A., S. E. MacAvoy, R. S. Carney, and C. R. Fisher. 1999. Stable isotope analysis of 

chemosynthetic and heterotrophic animals associated with hydrocarbon seeps in the Gulf 

of Mexico. EOS. 80 (49). OS242. 

McMullin, E., J. Wood, S. Schaeffer, and C. Fisher. 1999. Genetic structure of hydrocarbon seep

 vestimentiferan communities. EOS. 80 (49). OS5. 

Mitchell, R., I. R. MacDonald, and K. K. Kvenvolden. 1999. Estimates of total hydrocarbon 

seepage into the Gulf of Mexico based on satellite remote sensing images. EOS 

Supplement. 80 (49). OS242. 

Nelson, D. C., S. C. McHatton, A. A. Ahmad, and J. P. Barry 1999. Mats of vacuolate, 

filamentous sulfur bacteria: adaptation to anoxia at vents and seeps? Abstracts, Annual 

Meeting, American Society of Limnology and Oceanography, Santa Fe, New Mexico. 

Powell, E. N., K. A. Ashton-Alcox, R. D. Barber, and S. E. Ford. 1999. Parasite transmission and

 infection intensification in petroleum seep mussels. EOS. 80 (49). OS5-6. 

Sager, W. W., and I. R. MacDonald. Long-range side-scan sonar imaging of hydrocarbon seeps 

and sediment mass wasting on the Louisiana continental slope, Gulf of Mexico. Society 

of Exploration Geophysicists, Annual Meeting, Abstracts CD, October 31 – November 5, 

1999. 

Sager, W. W., and I. R. MacDonald. Shallow and deep acoustic mapping of hydrocarbon seep 

sites, Northern Gulf of Mexico, EOS, Trans. AGU, Fall Meeting Supplement, 79, F818, 

1998. 

Sager, W. W., and I. R. MacDonald. Side-scan sonar survey of halokinetic and hydrocarbon seep 

sediment modification on the Louisiana Slope, Amer. Assoc. Petrol. Geol., Annual 

Meeting, April 11-14, 1999, Program with Abstracts, p. A121, 1999. 

Sager, W. W., and I. R. MacDonald. Shallow and deep acoustic mapping of hydrocarbon seep 

sites, northern Gulf of Mexico. 1998. in American Geophysical Union Fall Meeting, San 

Francisco. 

Sager, W. W., I. R. MacDonald, W. R. Bryant, R. L. Carlson, and D. B. Prior. TAMU2

 digital 

side-scan sonar survey of Louisiana Slope areas containing active oil seeps and 

halokinetic sediment modification. 1998. in Offshore Technology Conference. pp. OTC 

Paper 8595, Offshore Technology Conference, Houston, TX. 
1-8 

Table 1.3. Continued 

Sassen, R., A. V. Milkov, D. A. DeFreitas, and S. T. Sweet. 1999. Geologic setting of Gulf of 

Mexico gas hydrates: significance to climate change. EOS. 80 (49). OS233. 

Sassen, R., I. R. MacDonald, N. L. Guinasso Jr., S. Joye, A. G. Requejo, S. T. Sweet, J. AlcalaHerrera, D. A. DeFreitas, and D. R. Schink. 1998. Bacterial methane oxidation in seafloor gas hydrate: significance to life in extreme environments. Geology. 26 (9). 851-854. 

Sassen, R., S. Joye, S. T. Sweet, D. DeFreitas, A. V. Milkov, and I. R. MacDonald. 1999a. 

Thermogenic gas hydrates and hydrocarbon gases in complex chemosynthetic 

communities, Gulf of Mexico continental slope. Organic Geochemistry. 30. 485-497. 

Graduate theses completed

Bergquist, D.C. 2001. Life history characteristics and habitat-structuring by vestimentiferan

 tubeworms from Gulf of Mexico cold seeps. Ph.D. dissertation. Pennsylvania State

 University. State College, PA. 138 pp. 

Davidson, N. E. 1999. Immunochemical confirmation of predation upon seep mussels and 

tubeworms in the Gulf of Mexico. Master of Science thesis. University of South Carolina. 

Columbia, SC. 67 pp. 

Escorcia, S. P. 2000. Sulfide distribution in chemosynthetic communities at hydrocarbon seeps in 

the Gulf of Mexico. Master of Science thesis. Texas A&M University. College Station, 

TX. xx pp. 

Macavoy, S.E. 2000. The exploitation of variable nutrient pools by aquatic predators in tidal 

freshwater and deep chemosynthetic communities: a multiple stable isotope and 

compound specific approach. Ph.D dissertation. University of Virginia. Charlottesville, 

VA. 233 pp. 

Ward, R. T. 2000. Community composition and successional trends in Gulf of Mexico Upper 

Louisiana Slope seep vestimentiferan communities. Master of Science thesis. 

Pennsylvania State University. State College, PA. xx pp. 
2-1 

2.0 Review of Literature 

2.1 Introduction 

2.1.1 Microbiology of chemosynthesis 

Chemosynthesis is a mode of life practiced by numerous groups of bacteria that are able to 

oxidize simple compounds such as hydrogen sulfide (H2S) and methane (CH4) (Jannasch 1989). 

The sulfide-oxidizing forms use energy released by the oxidation process to drive the cellular 

machinery of carbon fixation. These bacteria produce carbohydrates, proteins, and other complex 

organic compounds starting with the basic building blocks of nutrients and water. Like 

photosynthetic plants, chemosynthetic bacteria are thus able to form new organic compounds at 

the base of the food chain. Ecologically, chemosynthetic bacteria differ from green plants 

because they do not need light and do require free oxygen. 

In their free-living forms, chemosynthetic bacteria are found where a substrate is enriched with 

H2S or CH4. Such conditions often occur in the anaerobic sediments of marshes or sewage 

treatment ponds where the decomposition of organic matter produces these chemically reduced 

compounds in abundance. Because they require the means for oxidizing their chemical nutrient 

source, chemosynthetic bacteria typically live at the interface between reduced sediments and 

oxygenated water. A common form is Beggiatoa, which lives in long filaments that form palecolored mats on sediment surfaces (Larkin et al. 1994). In shallow aquatic habitats, 

chemosynthetic bacteria are one component of complex systems comprising numerous pathways 

for producing and recycling organic matter. 

Below the photic zone — in depths of 300 m or more — photosynthesis is no longer possible and 

nutrient limits sharply constrain the possibilities for community structure. Where seepage of 

hydrocarbons, venting of hydrothermal fluids, or other geological processes supply abundant 

reduced compounds, chemosynthesis becomes the dominant component of the ecosystem. In the 

northern Gulf of Mexico, these conditions are met where oil and gas seep into seafloor sediments 

at depths of approximately 400 m and greater. Although chemosynthesis remains an exclusively 

microbial process at the cellular level, chemosynthetic communities in the deep sea achieve 

prominence because of the symbiotic partnership between chemosynthetic bacteria and 

invertebrate hosts (Fisher 1990). 

2.1.2 Symbiosis with invertebrates 

Free-living chemosynthetic bacteria are limited to interfaces because they simultaneously require 

oxygen and reduced compounds that would spontaneously oxidize in the presence of oxygen. 

Symbiotic partnership with invertebrate hosts greatly extends the possible habitat for the 

chemosynthetic mode of life (Tunnicliffe et al. 1998). In basic arrangements, bacteria live within 

specialized cells in the host organism; however, specific adaptations can vary. The host 

physiology supplies oxygen and chemosynthetic substrates to the bacteria, often by means of 

specialized blood chemistry (Arp et al. 1984, Arp et al. 1987) and exploits the resulting bacterial 

productivity (Nelson and Fisher 1995). 
2-2 

2.2 Discoveries of Chemosynthetic Communities 

Awareness of chemosynthetic communities in the Gulf of Mexico came in a series of steps. The 

initial recognition that certain locations in the seafloor might harbor dense assemblages of 

invertebrates occurred in 1977 when anomalous concentrations of clam shells were 

photographed in the vicinity of hydrothermal vents in the eastern Pacific Ocean (Lonsdale 1977). 

The connection between chemical enrichment and this fauna was not made until the submarine 

ALVIN visited the area (Corliss et al. 1979) and discovered assemblages of vestimentiferan 

tubeworms, Riftia pachyptila (then completely new to science), the vent mussel, Bathymodiolus 

thermophilis, and a clam from the family Vesicomyidae, which had actually first been described 

from material collected in the Gulf of Mexico (Boss 1968). Perhaps because the first ALVIN 

cruise to the vents did not include a biologist, the importance of chemoautotrophic symbionts for 

these species was not elucidated for at least two years (Cavanaugh et al. 1981, Felbeck et al.

1981). 

Because the Gulf of Mexico is not a tectonically active region and lacks hydrothermal venting, 

chemosynthetic fauna were not anticipated here. Another ALVIN cruise, this time to the base of 

the Florida Escarpment in the eastern Gulf, unexpectedly found vestimentiferans and 

Bathymodiolus-like mussels at so-called cold seeps near the location 26 02' N and 84 55' W at 

a water depth of approximately 3200 m (Hecker 1985, Paull et al. 1984). This site is a 

continental margin brine seep, also called a cold seep, where brines formed by dissolution of the 

limestone Florida Platform seep out into hemipelagic sediments at the base of the Florida 

Escarpment (Paull and Neumann 1987). The brines are enriched in sulfides and the stable isotope 

signature of the source fluid can be seen in the tissue of the chemosynthetic fauna (Paull et al.

1985). The vestimentiferan and their symbionts depend upon this sulfide source (Cary et al.

1989, Cavanaugh 1985). The mussels possessed methylotrophic symbionts (Cavanaugh et al.

1987), which could be supported by methane dissolved in the brine. Because of the depth of 

occurrence, the Florida Escarpment site can only be sampled with ALVIN, which is generally 

occupied elsewhere. Therefore, the site has not been well sampled and was last visited during 

two dives in June 1992. The reader can review summaries of ALVIN dives in the Gulf of Mexico 

via the world wide web at the URL www.marine.whoi.edu/webpub/divelog/. 

Concurrent with discoveries on the Florida Escarpment, chemosynthetic fauna were found at 

hydrocarbon seeps on the northern Gulf of Mexico Continental Slope. The initial discovery came 

as a result of sampling by otter trawl, which targeted several known oil seeps (Kennicutt et al.

1985). The trawl recovered vesicomyid clams (Calyptogena ponderosa, and Vesicomya cordata), 

a new species of mussel, Bathymodiolus childressi (B. childressi would remain undescribed for 

over ten years after the discovery), and two still undescribed vestimentiferan species, 

Lamellibrachia n. sp. c.f. barhami Webb and Escarpia n. sp. The first photographic evidence of 

these communities (Boland 1986) was obtained in the course of a research program sponsored by 

the Minerals Management Service (Gallaway 1988), which also sponsored the first submarine 

dives to Gulf of Mexico hydrocarbon seeps. These species and their associated benthic 

communities will be discussed in more detail below. Additional findings about hydrocarbon seep 

communities published during the initial discovery phase included the following: 
2-3 

1. Demonstration that these fauna derived their nutritional requirements from hydrocarbons 

(Brooks et al. 1987) 

2. Demonstration of methanotrophy by the seep mussel (Childress et al. 1986) 

3. Recognition of the association of the fauna with specific seep attributes (Kennicutt et al.

1988a, Kennicutt et al 1988b) 

4. Description of the first submarine dives to visit a seep community (MacDonald et al. 1989). 

Subsequently, researchers from several institutions have continued the incremental process of 

discovery, documentation, and scientific research at Gulf of Mexico hydrocarbon seeps. 

Louisiana State University (LSU) scientists have completed numerous cruises using submersible 

and geophysical survey methods and have developed a body of theory and documentation to 

explain the geological styles associated with seeps and seep communities (Roberts and Aharon 

1994, Roberts et al. 1990a , Roberts and Carney 1997, Roberts et al. 1992). LSU researchers 

contributed heavily to two special issues of the Journal Geo-Marine Letters (volume 10:4, 1990 

and volume 14:2/3, 1994), which offer the reader important collections of contemporary 

research. Pennsylvania State University scientists have completed extensive research on the life 

history and physiology of seep megafauna (tubeworms and mussels). 

2.2.1 Vestimentiferan tubeworms 

These highly adapted polychaetes lack a mouth, gut, and anus. They live in a tough 

polysaccharide tube, typically 1 cm in diameter and up to 2 m long. Gas exchange and oxygen 

uptake occurs via a vascularized plume (red in color) that extends 1 to 2 cm from the anterior 

tube end. The tube is often held 1 m or more above the seafloor. Their symbionts utilize H2S, 

which the tubeworm absorbs from root-like structures that extend below the buried portions of 

the tubes. Buried tube length may be as much as one third of the body length. Two species are 

common to the Gulf of Mexico, Lamellibrachia n. sp. cf. barhami and Escarpia n. sp. The 

identification of the latter is currently being investigated (S. Gardener, personal communication). 

Lamellibrachia is the larger animal and typically forms bush-like clusters of several hundred 

individuals. These animals grow at rates typically less than 1 cm per year, so that a large adult 

may be 200 or more years old. A single large cluster marks a location where hydrocarbon 

seepage has continued unabated for several hundred years or more (Fisher et al. 1997).

2.2.2 Seep mussels 

These deep-sea mussels possess methanotrophic-oxidizing symbionts that live in the linings of 

greatly enlarged gills (Childress et al. 1986). Methane and oxygen are supplied to the symbionts 

through the ventilation of the gills. The mussels retain functional feeding grooves and guts. 

Excess bacteria are sloughed out of the gills and digested normally. Because their symbionts 

require dissolved CH4, seep mussels are restricted to locations where CH4 concentrations are 

high, for example near active gas vents. At such sites, mussels may completely cover the seafloor 

in mats that are bound together by byssal threads and extend for several meters or more. The 

maximum length of an adult mussel is 12 to 13 cm. The growth rates are slow, with juveniles 

requiring at least 20 years to reach maturity and large adults frequently surviving 40 years (Nix

et al. 1995). The most common species is Bathymodiolus childressi. 
2-4 

2.2.3 Vesicomyid clams 

Vesicomyid clams are surface-dwelling bivalves that plow long, curving furrows across the 

seafloor (Rosman et al. 1987). The foot is thrust forward and down into anoxic soils while the 

siphon is extended into the bottom water with the exposed portion of the shell. This allows the 

animal to absorb H2S across the foot epithelium where it is transported to symbiont-lined gills 

via specialized blood chemistry. Adults are 75 to 90 cm long with a deep, heavy shell. The two 

species known from the Gulf of Mexico are Calyptogena ponderosa and Vesicomya cordata. 

Nothing is known of the growth rates, but deep-sea bivalves are typically long-lived. 

Accumulations of dead shells with clusters of live individuals suggest persistent occupation of 

active seep sites. These aggregations have been found on the flow fields where expulsion of oilrich mud generates shallow anoxic layers. 

2.2.4 Lucinid clams 

Lucinid clams are possibly the most ubiquitous chemosynthetic invertebrates in the Gulf of 

Mexico. Living adults are almost never seen in photo-surveys, only the accumulations of dead 

shells. These animals live in deep, U-shaped burrows and manipulate the oxygen tension in their 

burrows by moving up and down in the passage to the surface. The chalky shells are subcircular, 

shallow, and have a small but distinct beak at the hinge. Symbionts live in enlarged gills and 

utilize H2S. Growth rates and life spans are unknown. The most common species in the Gulf of 

Mexico are Lucinoma atlantis and Thiasira oleophila (Callender and Powell 1997). 

2.2.5 Polychaete "ice worms" 

The polychaete “ice worm,” Hesiocaeca methanicola, received attention in the press following 

its discovery in 1997, but relatively little is published about its life history or ecology 

(Desbruyeres and Toulmond 1998). The worm inhabits shallow burrows on the surface of 

shallow gas hydrate deposits. It does not possess chemosynthetic symbionts; however, the stable 

carbon isotope ratios of its tissue are consistent with a diet derived from chemosynthetic 

production. (Fisher et al 2000), have recently published discovery and preliminary ecological 

descriptions of the ice worm. 

2.3 Types of Chemosynthetic Communities in the Northern Gulf of Mexico 

There have been several attempts by Roberts and Carney (1997) to categorize the distinguishing 

features of slowly seeping oil and gas seeps, rapid, mud-prone expulsion features (mud 

volcanoes), and quiescent, mineral-prone seeps. Reilly et al. (1996) categorizes complex 

communities that comprise a mixture of tubeworms and seep mussels, and simple communities, 

which consist of a single species — usually seep mussels or vesicomyid clams. MacDonald et al.

(1998a, submitted) identified brine-pooling and sediment-diffusion habitats, noting that so-called 

simple and complex communities can occur in close proximity. At slow oil and gas seeps, fluids 

migrate to the seafloor from deep (3000-5000 m subbottom) reservoirs that are broadly 

distributed across the continental slope. In the upper section of the sediment column, a layer of 

unconsolidated hemipelagic sediment is several hundred meters thick. The upper sediment 

column diffuses and retains oil and gas over areas considerably larger than the fault axis (Reilly
2-5 

et al. 1996). In the upper meter or so of the sediments, microbial degradation of the labile carbon 

in the oil and gas area depletes available oxygen and reduces seawater sulfate to H2S. This 

provides chemosynthetic substrates for invertebrates with sulfide oxidizing symbionts, notably 

vestimentiferan tubeworms. Increased alkalinity due to microbial productivity causes extensive 

precipitation of carbonate. Accumulating fluid and carbonate produces low mounds with basal 

diameters of 10 m to over 500 m and slopes of 10% or greater. At localized vents, methane 

bubbles through near-bottom waters and generates sufficient local concentrations to support seep 

mussels. Gas hydrates form where free gas is trapped beneath layers of rock or other shallow 

obstructions. The result is often a patchwork of tubeworm clusters and carbonate boulders 

extending over the surface of the seep with the greatest concentrations along fault axes. 

At mud volcanoes, formation of chemosynthetic communities is controlled by the intensity and 

frequency of mud discharge. Rapid fluid flux often includes abundant hydrocarbons, but burial 

of slow-growing fauna will limit community formation at active sites. Because the fluid flux is 

associated with shallow salt in most cases, halite dissolution produces concentrated brines. The 

increased density of these briny fluids tends to create pools or channels with distinct, stable 

interfaces. Seep mussels can colonize the stable edges of mud-filled craters or channels. 

Repeated burial over thousands of years of activity is indicated by recovery of mussels shells in 

cores taken at mud-prone sites. 

Mineral-prone seeps occur with decreased rates of venting and formation of surface domes 

capped with lithified layers. Lithified surface layers and crusts represent the terminal phase of 

seeps regardless of whether the active seepage mode was slow, oil and gas driven, or rapid and 

mud prone. Lithification greatly reduces sediment porosity and limits seepage to faults and 

fissures in the crust. Layers of bivalve shell may remain over large areas for many years after 

most seepage and all chemosynthetic production has ceased. Type cases of "typical" 

chemosynthetic communities are given below. 

2.4 Distribution of Chemosynthetic Communities in the Northern Gulf of Mexico 

Seismic survey methods can be used to detect seeps and chemosynthetic communities by looking 

for migration conduits — also called seismic wipeouts — that coincide with surface mounds and 

low-angle faults (Reilly et al. 1996, Roberts and Carney 1997). Side-scan sonar has also shown 

promise and may be more cost-effective in some applications (Sager and MacDonald 1998). As 

the discussion above indicates, the timing of migration and seepage is not necessarily predicted 

by structures that are detected with seismic data. The geochemistry of hydrocarbon seeps has 

been thoroughly described (Anderson et al. 1983, Brooks et al. 1986, Brooks et al. 1984, 

Kennicutt et al. 1988a, Kennicutt et al 1988b, Kennicutt et al. 1987) and will reliably predict 

regional occurrences of chemosynthetic communities for at least the so-called "lush" 

communities of most concern for management (MMS 1988). However, brine-pool communities 

like Brine Pool NR1 (GC233) are not necessarily associated with thermogenic hydrocarbons in 

the surface sediments (MacDonald et al. 1990b). Submersible and photo-surveys have been 

executed haphazardly and with a definite bias toward sites greater than 1000 m in depth, this due 

to the cost and depth limitations of available submersibles. Surveys of chemosynthetic 

communities need to be evaluated critically, therefore, with an eye to the underlying limits of the 
2-6 

data and the motivating goals of the survey. The following sections briefly summarize different 

evidence for the regional distribution of chemosynthetic communities in the Gulf of Mexico. 

2.4.1 Evidence from energy prospecting 

Sassen et al. (1993a) demonstrated that, where data permit comparison, all major oil fields 

located in the deeper waters of the Gulf of Mexico are associated with chemosynthetic 

communities. Recent exploration and production have not been thoroughly documented by 

submersible observations, and there is some question as to community development in water 

depths between 1000 and 2000 m where data are lacking (MacDonald 1998b). However, these 

and other authors (Abrams, 1996 #158; Kaluza and Doyle 1998) note that salt tectonism 

generates migration conduits across the entire Gulf of Mexico slope. All hydrocarbon fields in 

the region are, therefore, highly susceptible to macro seepage. Dependence upon seeping 

hydrocarbons places Gulf of Mexico chemosynthetic fauna in a deep-sea locality that may be 

affected by human activities. The offshore energy industry has experienced several expansive 

episodes in the past twenty years. All of these have increased activities at ever-greater depths. 

The amount of seafloor influenced by seepage is quite small compared to the extent of the 

subbottom hydrocarbon system; therefore, industry engineers generally strive to avoid the 

unstable substrate at seeps (Reilly et al. 1996). Current interest lies in improving the capacity to 

predict where seep communities will occur and in understanding the processes that contribute to 

either stability or change in this environment so that anthropogenic changes can be distinguished 

from natural processes. A map of hydrocarbon discoveries and production should predict the 

general localities where chemosynthetic communities will be found. A good surrogate for 

discoveries is in active lease areas, i.e. areas where oil companies have leased development. The 

map in Figure 2.1 summarizes active leasing over the past 15 years. Increased leasing in the 

deeper waters of the Gulf of Mexico is the trend. Thus, one can surmise that abundant 

chemosynthetic communities will be found in these regions. However, first-hand observations 

are required to confirm community occurrence at scales of 1 km or less. 
2-7 

Figure 2.1. Offshore lease holdings illustrate the trend to deeper water over the past fifteen years. Offshore leases remain active for 

in the period 1985 through 1990 but not renewed are not current while leases obtained in the 1995 through 1999 timedevelopment. The southern-most lease blocks are in water depth approaching or exceeding 3000 m. 
2-8 

2.4.2 Evidence from remote sensing and direct observations 

The inventory of known seep communities in the Gulf of Mexico has been developed 

incrementally as direct observations of chemosynthetic fauna are obtained and reported. 

Kennicutt et al. (1988b) conducted what remains as the most comprehensive exploratory survey 

of localities where high-molecular weight hydrocarbons were found in surface sediments when 

systematically sampled by trawl net. Many of these reports occur in a haphazard manner when 

seep fauna are discovered during the course of sampling designed for other objectives. Examples 

of unanticipated discoveries include the finding by Boland (1986) of tubeworms during the 

course of photography using a remote camera sled. Voucher collections are often not obtained 

from animals that live at sites found by camera sleds, and navigation is often approximate. 

Gallaway et al. (1990) were able to document several aggregations of tubeworms and 

vesicomyid clams with the use of a remote operating vehicle (ROV). Other ROV operations have 

reported sighting chemosynthetic fauna, but accurate records are often unavailable. The US Navy 

research submarine NR-1 has completed several broad-area surveys discovering previously 

unknown chemosynthetic communities (MacDonald et al. 1990b, Reilly et al. 1996). A more 

recent survey with NR-1 was completed in 1998 and will be described herein. Exploration in 

deep water (>1000 m) areas has been carried out by the submarine ALVIN at sites on the Florida 

Escarpment noted above, along the Sigsbee Escarpment in the central Gulf of Mexico (Brooks et 

al. 1990), and along Alaminos Canyon and Green Knoll. Most of the dives with submersibles 

and the vast majority of detailed sample collections have been made by the submarine Johnson 

SEA-LINK. SEA-LINK has a maximum depth rating of 1000 m, so there is a distinct bias in the 

inventory of seep communities toward sites above this depth. Aharon (1994), MacDonald et al.

(1996), and Roberts and Carney (1997) have all published inventories or maps of known 

chemosynthetic communities in the Gulf of Mexico. The map shown in Figure 2.2 is derived 

from MacDonald et al. (1996). 

Indirect evidence for chemosynthetic communities can be obtained by satellite remote sensing 

methods. Oil that escapes from most seeps floats to the surface and forms a thin (<1m) layer 

that drifts downstream when driven by wind and currents. This layer dampens the capillary 

waves on the water's surface, thereby improving its specular reflectivity (Espedal and Wahl 

1999). Electro-magnetic energy (including visible light and RADAR energy) reflected from 

films of this type creates areas of water that are bright or dark depending on whether the energy 

is reflected toward or away from the sensor (Huhnerfuss et al. 1989, MacDonald et al. 1993). 

This technique has been used with increasing frequency as an exploration tool in frontier basins, 

because it verifies the presence of source material for hydrocarbon generation as well as, 

pressure-charged reservoirs (Kornacki et al. 1994). A satellite image is a snapshot in time and 

other processes besides oil seepage can produce light-reflecting films on the ocean. Therefore, 

the rule of thumb is to look for so-called perennial slicks, that is, slicks that appear in the same 

location over multiple samplings. The slicks’ locations, 63 in all, that are mapped in Figure 2.2 

were developed by analysis of a limited number of satellite images made available for research 

purposes (MacDonald et al. 1996). Subsequent work, much of which remains proprietarily held, 

has greatly expanded this inventory of perennial slicks. Mitchell et al. (1999) document over 200 

perennial slicks in a complete survey of the northern Gulf of Mexico. The northern Gulf of 

Mexico is potentially a very large and relatively contiguous province within which 
2-9 

chemosynthetic communities supported by hydrocarbon seepage will be found. It is reasonable 

to predict that the chemosynthetic habitat in this region has a few, basic characteristics. 

2.5 Habitat Processes at Gulf of Mexico Hydrocarbon Seeps 

At hydrocarbon seeps, reduced compounds are generated as by-products of microbial 

consumption of hydrocarbons in the upper few meters of the sedimentary column and are 

directly supplied by upward migration from deeply buried sources. Hydrogen sulfide is produced 

and consumed by biotic and abiotic processes at hydrocarbon seep communities at much higher 

rates than in normal sediments (Lin and Morse 1991). Biogenic reaction products are responsible 

for the formation of iron sulfide minerals and, in some cases, massive deposits of carbonate 

minerals at seep sites (Morse 1994). Seep sediments exhibit geochemical processes that differ 

from most models of normal marine sediment diagenesis in many important ways. Organic 

enrichment by hydrocarbon seepage is an important process in producing reduced compounds 

(Brooks et al. 1987). Hydrocarbon generation and migration occur over timeframes of millions 

to tens of millions of years (Sassen 1987). Chemosynthetic communities occupy areas of the 

seafloor that are very small compared to the extent of the hydrocarbon reservoirs that feed them. 

Temporally, chemosynthetic communities are ephemeral features in comparison to the overall 

hydrocarbon system of the Gulf of Mexico basin. The volume of hydrocarbons withdrawn from 

reservoirs by humans is unlikely to impact the survival of chemosynthetic communities, since 

this volume is insignificant when compared to the total volume of the extant hydrocarbon 

system. Two styles of seepage have been described at Gulf of Mexico hydrocarbon seeps: 

sediment diffusion and brine pooling. Much of the community-level diversity in cold seep 

communities can be explained by examining the environmental consequences of these two styles 

of seepage. 
2-10

Figure 2.2. Distribution of known chemosynthetic communities and perennial oil slicks detected in available remote sensing data (ad

in MacDonald et al. 1996). Depth contours are at 200 m intervals. 
2-11 

2.5.1 Sediment diffusion 

Sediment-diffusion hydrocarbon seeps have been discovered at many locations on the upper 

continental slope during energy prospecting activities. It has been widely recognized that upward 

migration of reservoir oils can result in high concentrations of liquid and gaseous hydrocarbons 

in seafloor sediments (Anderson et al. 1983). In these habitats mussels, clams, and tubeworms 

utilize reduced compounds – methane and H2S – by extending body parts into the sediment or by 

bathing their brachia (gills or plumes) in the steep gradients immediately above the 

sediment/water interface (Figure 2.3). Gas seeps and gas hydrates, ice-like solids that form when 

methane and water combine at high pressure, are also recognized as important components of 

slope seep systems (Brooks et al. 1984, Shipley et al. 1979). The important characteristics of the 

sediment diffusion habitat are abundant, highly altered hydrocarbons that include thermogenic 

gases, liquid petroleum, and tars, which are distributed throughout the sedimentary section 

overlying a fault or fault nexus (Behrens 1980, Kennicutt et al. 1987, Reilly et al. 1996, Roberts 

and Aharon 1993). At the seafloor, a complex veneer tends to entrap and further contribute to the 

alteration of seeping fluids. Roughly in order of greatest relative age, major components of this 

veneer are as follows (note that the citations provided refer to descriptions of the components, 

not their influence on habitat formation): 

1. Authigenic carbonates in the form of rubble and consolidated pavements (Roberts and 

Aharon 1994, Sassen et al. 1994a). 

2. Extensive aggregations of vestimentiferans, the "roots" of which extend into the upper 

sediment as dense tangles (Fisher et al. 1997). 

3. Beds of bivalves, including living mussels and/or clams and layers of shell (Callender et al.

1990, MacDonald et al. 1990a). 

4. Layers of gas hydrate, which can entrap the buoyant phases of hydrocarbons but are subject 

to gradual decomposition and possible catastrophic failure due to temperature fluctuations in 

bottom waters (MacDonald et al. 1994). 

5. Mats of the sulfide-oxidizing bacteria, Beggiatoa (Larkin et al. 1994), which occur in 

pigmented and non-pigmented forms (Sassen et al. 1993b). 

Although there is good evidence for structural instability in rapidly forming seep habitats, 

particularly where mud volcanism produces massive fluid discharge (Neurauter and Roberts 

1994, Roberts and Neurauter 1990), the formation of large chemosynthetic communities takes 

place in the context of increasing lithification and general seafloor stability (Roberts and Aharon 

1994). The chemosynthetic fauna themselves will, with time, retard the escape of hydrocarbons 

from the sediments (Sassen et al. 1994b). 
2-12 

Figure 2.3. Illustration of the chemosynthetic community formation in a sediment diffusion style hydrocarbon 

seep. Gas and oil migrate along fault planes that penetrate reservoirs (lower), and then diffuse through 

unconsolidated sediments approaching the seafloor (middle left). Formation of gas hydrate, layers of 

biota, and authigenic carbonate entrap hydrocarbons in uppermost sediment (middle right), facilitating 

microbial degradation of hydrocarbons and subsequent reduction of seawater sulfate to sulfides 

needed for tubeworm symbiosis (upper right). (Art by C. Bruce Morser.) 
2-13 

2.5.2 Brine pooling 

Concentrated brines are often associated with petroleum migration and seepage (Behrens 1980). 

Where they discharge, their density allows for the formation of distinct pools on the seafloor. 

The discovery of brine-filled depressions in the Gulf of Mexico predates the discovery of 

chemosynthetic communities (Bright et al. 1980, Paull and Neumann 1987, Shokes et al. 1977). 

The discovery of chemosynthetic fauna on the Florida Escarpment demonstrated that brine could 

be a carrier of reduced compounds (Cary et al. 1989, Paull et al. 1984). The significance of the 

pooling process became evident when a dense colony of Bathymodiolus childressi was 

discovered around the edges of a small, brine-filled crater (MacDonald et al. 1990b). Where 

brines accumulate on the seafloor, mussels can obtain methane through the uptake of water by 

placing their inhalant siphons just above the brine. High concentrations of gaseous hydrocarbons 

are often dissolved in Gulf of Mexico brines. The defining characteristics of the brine pooling 

habitat are an extremely sharp (~1 cm) density-driven interface between the brine and seawater 

and a surface, which allows for the development of a thin layer of brine that intercepts the 

sediment surface. Dense colonies of mussels become established at these 

seawater/sediment/brine interfaces. Where the brine interface is less sharp, for example in the 

large brine pool filling Orca Basin that has a meters-thick separation between seawater and 

saturated brine (Shokes et al. 1977), the mussels are unable to bridge the distance between the 

anoxic brine and the oxygen-rich seawater; consequently, mussel communities appear to be 

absent (Brooks et al. 1990). The brine pooling habitat results from brine seeps on nearly level sea 

bottoms or from brine-filled pockmarks or craters that exhibit a brine/seawater interface at the 

edges of the depression (Figure 2.4). 
2-14 

Figure 2.4. Illustration of the chemosynthetic community formation in a brine pool style hydrocarbon seep. Salt 

diapir pressurizes a shallow reservoir of methane – also known as a shallow gas hazard (left). 

Subsequent release of gas excavates a surface crater and diatreme (middle). High concentrations of 

methane are available to seep mytilids around the edges of the brine pool formed by dissolution of 

underlying salt (right). (Art by C. Bruce Morser.) 

2.6 Model Components and Interactions 

The following sections provide a brief description of the various biological components of an 

overall model of chemosynthetic communities including microbial communities, mussel 

colonies, and tubeworm colonies. Interactions between the environmental setting and biology are 

also considered including change at the community level, geological stability, regional 

characteristics, energy flow, and food webs. 

2.6.1 Microbial communities 

Bacteria are a fundamental component of the seep community interacting with the chemical 

environments both as free-living bacteria and endosymbionts. The importance of bacterial 

occurrences and their role in mediating chemical transformations at seep sites is poorly 
2-15 

understood. It is believed that autotrophic microbial (sulfide oxidizing) productivity is dependent 

on heterotrophic microbial (sulfate reducing) productivity. Beggiatoa filaments can extend 8 to 

12 cm down into the anoxic sediment. Beggiatoa mats in the Gulf of Mexico sites have the 

potential for competing with higher organisms for sulfide over at least the top 10 to 12 cm of 

these anoxic sediments. Beggiatoa exhibits two growth forms: pigmented and non-pigmented. 

Enzyme activities in the pigmented and non-pigmented forms are consistent with heterotrophic 

and autotrophic metabolism, respectively. Beggiatoa may also be facultative consumers of 

hydrocarbons. Beggiatoa and the related genus Thioploca are the only bacteria known to contain 

a central vacuole. Similar filamentous, thio-autotrophic bacteria from Guaymas Basin 

hydrothermal vents (Gulf of California, 2000 m depth) and the Clam Field Seeps of Monterey 

Canyon (Monterey Bay, depth 900 m) contain intracellular nitrate at concentrations as high as 

160 mM (4,000- to 8,000-fold above ambient levels). Expanded investigations are needed to 

more fully understand the role microbes play in seep ecology. 

2.6.2 Mussel colonies 

Bathymodiolus childressi, which is the major mussel species in these habitats, has evolved in 

symbiosis with methanotrophic endo-bacteria (Childress et al. 1986, Fisher 1990). B. childressi 

was formally described by Gustafson et al. (1998) and had previously been given the provisional 

taxonomic designation Seep Mytilid I. It is often described by this name in mussel colonies and 

is most abundant where gaseous hydrocarbon flux is high. Compared to tubeworms, mussel 

colonies recruit more rapidly and are more mobile. Mussels, which inhabit the sediment 

diffusion habitat, experience greater variability in nutrient supply due to hydrate formation and 

decomposition and carbonate precipitation and are more ephemeral than mussel colonies at brine 

pools. Differences in the longevity, “health,” and stability of mussel colonies depend on the 

following features of their life history and physiology: 

1. Mussels colonize a new site with motile larvae. 

2. Seep Mytilid Ia spawning is not seasonal, and recruitment is episodic. 

3. The presence of settlement substrate enhances the chances of establishment of a mussel 

community. 

4. Mussels require high concentrations of methane around their siphons for growth and 

reproduction. 

5. The final size of individuals as well as the density of the population is primarily a function of 

ambient methane concentrations. 

6. Mussels can grow extremely fast as well as survive over long time periods. 

Mussel communities in sediment diffusion habitats may begin to die out as carbonate 

precipitation blocks the seepage of methane into the community. In the absence of genetic 

isolation, a mussel community at a sediment diffusion habitat removed by physical disturbance 

can resettle and mature in less than twenty years. Catastrophic mud burial is another cause for 

the demise of mussel communities. Mussel communities associated with brine seepage can be 

very long lived. 
2-16 

2.6.3 Tubeworm colonies 

Tubeworms are the visually dominant biomass in sediment diffusion habitats. A significant seep 

community will typically contain tens of bushes distributed across several hectare of seafloor. A 

mature bush indicates that petroleum flux and associated microbial activity has persisted at a 

location for more than 100 years. However, a bush forms and declines though a series of lifehistory stages. Vestimentiferan bushes display distinctive growth forms, species compositions, 

and levels of productivity at different stages of their development (newly recruited, mature, 

senescent). Vestimentiferan spawning is not seasonal, and recruitment is episodic. 

Vestimentiferans colonize a new site with motile larvae. A larval settlement substrate is required 

for the establishment of a vestimentiferan colony. This substrate of choice is authigenic 

carbonate, which is formed in areas of active seepage. Both species of vestimentiferan recruit 

equally to new sites. Juveniles of both species grow relatively quickly to lengths of about 30 cm 

at sites where sulfide is found in the water around the plumes of the juveniles. Flow of seep 

fluids from the sediment column to the water column slows down as the bush matures. As a 

result of carbonate precipitation and sedimentation, the bases of vestimentiferan bushes become 

covered with an ever-increasing amount of sediment. After 10 to 20 years, the transfer of sulfide 

to the water column is insufficient to carry sulfide to the plumes of the vestimentiferans, and they 

begin to take up sulfide across the buried, terminal portion of their tube. Under these conditions, 

the Lamellibrachia out competes the Escarpid-like species and begins to dominate the 

assemblage. The Lamellibrachia continues to grow very slowly. Individuals can reach lengths of 

three meters and live for hundreds of years (Fisher et al. 1997). Eventually, as a result of a 

combination of continued authigenic carbonate precipitation and sulfide depletion by animals on 

the edge of the aggregation, the animals in the center of the bush begin to die. Re-channeling of 

seepage due to carbonate precipitation eventually cuts off the supply of sulfide to the 

aggregations, and the demise of the entire aggregation ensues. 

2.6.4 Energy flow and food web 

Chemical energy is converted into biomass through chemoautotrophic fixation by free-living and 

symbiotic bacteria. Because of the advantages inherent in multi-cellular anatomy and 

physiology, both mussels and tubeworms are very efficient at extracting chemicals from the 

environment and converting them into biomass. The endemic heterotrophic fauna rely almost 

entirely on seep chemosynthesis for their food. Free-living bacteria are the major source of food 

for most of the endemic heterotrophic fauna. Primary production of seeps finds its way into the 

food chain of the deep Gulf of Mexico through crustaceans, fish, and other associated fauna that 

feed on the seep communities. 

2.6.5 Change at the community level 

Three processes contribute to change at the community level: recruitment, succession of 

aggregations, and episodic instability due to localized processes (e.g., gas hydrate 

formation/dissociation, mud expulsion). The current model suggests that changes in 

chemosynthetic community structures in seep communities occur over time from a few years to 

tens of years. Substantial change is expected to be observed in the abundance and distribution of 

bacterial mats. Significant changes may occur in the abundance and distribution of mussels that 

are greatest for mussel communities, which occur in the sediment diffusion habitats. Lesser 
2-17 

changes will be evident in the tubeworm colonies, particularly among mature bushes. 

Catastrophic events may cause localized effects in the community, but the frequency of these 

occurrences is unknown. The occurrence of these “rare” events can only be determined through 

long-term monitoring of the community including the in-situ measurement of various 

physical/chemical variables. 

2.6.6 Regional characteristics 

The major regional feature of Gulf of Mexico communities is their prevalence across the 

northern continental slope. This suggests that these communities are routinely subjected to the 

effects of eddy-driven circulation events and variations in temperature, pressure, and 

sedimentation rates. Although physical oceanography is not a major focus of this program, the 

need for a minimum set of oceanographic observations is indicated by previous results. 

Genetically isolated populations are inherently unstable and thus susceptible to extinction as a 

result of disturbance. In contrast, populations characterized by extensive gene flow are insulated 

from catastrophic decline by the capacity for resettlement. Proximity of communities on the 

slope and circulation characteristics of the slope suggest that the gene pool of chemosynthetic 

species should be well-mixed, although existing taxonomic descriptions are not sufficient to 

determine if this is true. Modern molecular techniques can provide the information needed by 

delineating corridors of gene flow. 

2.6.7 Geologic stability 

Geological evidence demonstrates that hydrocarbon and brine seepage has persisted on the 

northern Gulf of Mexico slope for thousands of years. Seeps are associated with migration 

conduits that connect the seafloor to deep sub-surface reservoirs containing gaseous and liquid 

hydrocarbons that were generated millions of years ago. Defining the processes that determine 

the geological cycle of seepage, i.e., reservoir dynamics, large-scale halokinesis, sedimentation 

rates, etc., is largely beyond the scope of this program. It is possible, however, to make 

qualitative evaluations of the relative age of the primary study sites and to identify additional 

sites of various ages. This will improve prediction of community presence/absence and increase 

the understanding of the long-term fate of communities. 
3-1 

3.0 Study Sites, Field Efforts, and Sampling Design 

3.1 Introduction 

This chapter describes the study sites where program activities were conducted and the selection 

criteria by which the sites and the individual sampling stations were chosen. The program was 

conducted in the northern Gulf of Mexico south of Louisiana where, as the previous chapter has 

indicated, the largest concentration of known hydrocarbon seeps is located. The program 

objectives called for evaluating factors that controlled the distribution of chemosynthetic 

communities over a large portion of the Gulf of Mexico and for detailed study of the ecology of 

representative communities. It was therefore necessary to conduct research at a range of different 

scales. At a regional scale, the seismic signatures of chemosynthetic communities were evaluated 

to improve detection capabilities, and a limited exploratory survey was undertaken to search for 

previously unknown communities. For the detailed studies, specific sites for biological and 

chemical samplings from submersibles were selected at the scale of individual communities. 

Separate field programs were conducted to accomplish detailed collections for biology and 

geochemistry. The community-level sites were selected based on previous information. They 

represent, respectively, brine pooling and sediment diffusion seeps, which are the two distinct 

seepage settings described in the models outlined in the previous chapter. At an intermediate 

level, detailed maps and geographic information system (GIS) datasets were compiled to 

investigate the correlation between biological and geological/geochemical components and the 

communities. Locations of the mega-sites and community-level sites are shown in Figure 3.1. 

Details of the characteristics of the sites and the rationale for their selection are given below. 

3.2 Regional Studies 

Field activities during the program included regional geophysical surveys designed to determine 

seismic and remote-sensing characteristics of active seeps and seep communities. Because the 

area of the Gulf of Mexico slope occupied by known chemosynthetic communities spans a depth 

range from about 500 to 2500 m, the geophysical survey was carried out within shallow- and 

deep-water areas. These two mega-sites were designated as survey sites for the development and 

testing of criteria to improve the prediction of the presence of chemosynthetic communities 

based on geophysical or other remote sensing data (Figure 3.1). Comparison of the geophysical 

signatures obtained from known or probable seeps in the two mega-sites provides a test of 

whether fundamental changes occur in the types of signatures when depth increases. In addition, 

regional-level efforts considered the geological and oceanographic processes that support 

communities, including larval dispersion and circulation processes that maintain the genetic 

stability of these communities. 
3-2 

Figure 3.1. Northern Gulf of Mexico showing study site locations for submersible operations (triangles) and mega-site areas for g

rectangles). Depth contours in meters. 
3-3 

The size and location of these mega-sites were designed to optimize survey operations and 

provide significant regional coverage. Both areas encompassed several types of geological 

formations. Both contained more than ten perennial sea surface slicks detected by remote sensing 

techniques. Both sites were surveyed with a swath-mapping sonar system (TAMU2 System). 

Additional survey data were collected at the Garden Banks sampling site (GB425) in support of 

the community-level studies. The submarine NR-1 also visited selected targets in the shallow 

mega-site during 1998 in an attempt to verify the interpretations made of the TAMU2

 data. The 

characteristics of the two mega-sites are summarized in Table 3.1. 

Table 3.1. Mega-Site Regional Survey Areas. 

Mega-site 1 (“Shallow,” extended 4 NM South to include GC321) - area 1214 km2

- Boundaries: 91°35'W-91°10'W; 27°36'N-27°50'N 

- Contains GC185, GC233, and GC234 sampling sites 

- Contains approximately 13 slicks 

- Water depths range from 400 to 900 m 

Mega-site 2 (“Deep”) – area 1214 km2

- Boundaries: 91°35'W-91°10'W; 27°10'N-27°26'N 

- Contains northern Pygmy Basin; eastern Longhorn Basin; and most of Tiger Basin (intrasalt basins) 

- Contains 12 to 15 slicks 

- Water depths range from 950 to 1250 m 

3.3 Community-Level Sampling Sites 

At the community level, efforts focus on the abiotic factors that control the distribution, 

abundance, and health of the major chemosynthetic and associated fauna. Investigations of the 

life history of these organisms are also included. Four principal community-level sites were 

designated for the intensive collection of samples and deployment of experiments and 

instrumentation. Two of the sites were in brine-pooling settings; two others were sedimentdiffusion settings. The sites differ principally in that two sites contain significant amounts of 

liquid hydrocarbons in the sediments and are dominated by tubeworms (GC185 and GC234) and 

two sites are brine seeps and are dominated by mussels (GC233 and GB425). These sites have 

been the subjects of historical studies, which in most cases provided ancillary geographic data. 

The sites contain the major faunal groups found in chemosynthetic communities (tubeworms and 

mussels) and cover a range of environmental conditions so that hypotheses concerning abiotic 

controls on community ecology could be tested. To simplify sample identification, each site was 

given a unique, two-letter abbreviation. A brief description of each site's setting and 

community’s characteristics is presented in Table 3.2. 
3-4 

Table 3.2. Principal Community-Level Sampling Sites and Pertinent Characteristics. 

Sampling Site 

(Alternate names) 

Latitude, 

Longitude Depth (m) 

Fauna present 

(by dominance) Seepage properties 

GC185 

(BH, Bush Hill) 

27°46.9’ N 

91°30.4’ W 550-580 

tubeworms 

heterotrophs 

mussels 

high molecular weight hydrocarbons 

free methane to pentane gases 

GC234 

(GC) 

27°44.1’ N 

91°131’ W 525-560 

tubeworms 

heterotrophs 

mussels 

high molecular weight hydrocarbons 

free methane to pentane gases 

GC233 

(BP, Brine Pool) 

27°43.4’ N 

91°16.8’ W 640 

mussels 

heterotrophs 

tubeworms 

brine 

free and dissolved methane gas 

GB425 

(GB) 

27°33.2’ N 

92°32.4’ W 600 

mussels 

heterotrophs (?) 

tubeworms 

brine 

free and dissolved methane gas

high molecular weight hydrocarbons

3.4 Sediment Diffusion Communities 

3.4.1 GC185 

The "Bush Hill" site (27°47'N, 91°30.4'W) described by MacDonald et al (1989) was the first 

hydrocarbon seep community to be sampled from a submersible. Reilly et al. (1996) describe it 

as the type-example of a complex chemosynthetic community. The Conoco tension legwork 

platform (TLWP) was installed more than 2 km west southwest from the mound and began 

producing oil in the late 1980's. The major seep area is a 300 m-EW by 500 m-NS mound with a 

crest depth of 570 m rising about 40 m above the surrounding seafloor and composed of mud, 

carbonate, and shallow gas hydrate (Roberts and Carney 1997). The north-south axis of the 

mound is situated along the surface trace of a west-dipping fault that is the conduit by which oil 

and gas reaches the surface from deposits located at approximately 1200 m subbottom depth. 

Surface sediments contain, by weight, up to 10% liquid hydrocarbons, which Kennicutt et al.

(1988) described as having fingerprints identical to oil produced by the TLWP wells. However, 

reservoirs tapped by TLWP wells were located at subbottom depths of 3000 m or greater. Cook 

and D’Onfro (1991) suggest that the field comprises a complex of many reservoirs charged from 

a common source but seeping only from the shallowest strata. Sulfide concentrations in shallow 

sediments (<10 cm) associated with tubeworm clusters have been measured in the 100- to 250-

M range with use of a microelectrode technique (Escorcia et al. 1999). Methane concentrations 

in near-bottom waters are 30- to 60-M in the vicinity of active gas vents, and below 1-M 

elsewhere (MacDonald et al. 1989, Nix et al. 1995). Shallow gas hydrates have been recovered 

by coring at GC185 (Brooks et al. 1986). Layers of gas hydrate breach the sediment layer near 

the highest point of the mound (MacDonald et al. 1994). Tubeworm clusters extend over much 

of the mound crest while mussels are confined to the active gas vents. 
3-5 

3.4.2 GC234 

The “Green Canyon” site is a very extensive complex of tubeworm clusters, seep mussel beds 

and outcropping gas hydrates located in the northeastern corner of the GC234 lease block 

(27°44'50"N, 91°13'30"W). The geology of this region was first described by Behrens (1980), 

who perceived a network of diapir crests that converge to the west of the site as an east-west 

trending diapir ridge. Seismic wipeouts are widespread and substantial quantities of oil are 

dispersed in the sediments (Brooks et al. 1986). The surface topography is uneven, sometimes 

with massive deposits of authigenic carbonate either exposed or thinly draped with sediment. 

Tubeworm communities appear to be restricted to terraces where the seismic reflections (25 

kHz) are wiped out within approximately 3 m of subbottom, and extensional faults approach the 

mud-line (MacDonald et al. 1990a). Nix et al. (1995) report sulfide and methane concentrations 

approaching 8 and 11 mM, respectively. The type specimen of the "ice worm" Hesiocaeca 

methanicola was collected from an outcropping layer of gas hydrate at this site (Desbruyeres and 

Toulmond 1998, Fisher et al. 2000). 

3.5 Brine-Pooling Communities 

3.5.1 GC233 

The focus of the GC233 chemosynthetic community is a small (190 m

2

) pool of brine (salinity 

121.35 psu) found near 27°43.4’N and 91°16.5’W at a water depth of 650 m (MacDonald et al.

1990b). Brine fills a crater at the center of an approximately 100 m wide mound. The mound 

rises about 6 m above the surrounding seafloor, but the crater and its diatreme extend at least 30 

m below the surface. The brine contains microbial methane (13C = -63.8) in concentrations that 

are supersaturated at standard temperature and pressure. Streams of CH4 bubbles emanate 

continually from the center of the pool. The pool is ringed by a large (540 m

2

) bed of seep 

mussels (MacDonald et al. 1990b). Mussels settled on the “shoreline” of the pool include 

numerous juveniles; however, the periphery of the bed comprised a single settlement class 

without juveniles. Sulfide levels are below levels of detection in the pool but rise sharply in 

fluids collected beneath the surrounding mussel bed (Fisher, personal communication). The bed 

of seep mussels comprises a striking example of a mono-specific aggregation of chemosynthetic 

fauna, but numerous other species of heterotrophic animals are also commonly observed at the 

site (MacDonald 1992). Recent findings challenge the Reilly et al. (1996) designation that this 

brine pool, NR-1, is a “simple” community because small but significant clusters of 

vestimentiferans are known to occur to the south of the pool. 

3.5.2 GB425 

GB425 is on the western edge of Auger Basin, an intraslope basin that contains economically 

significant hydrocarbons in the Auger, Cardamom, and Macaroni fields. Potential hydrocarbon 

traps in Auger Field are sealed against the up-thrown side of faults along a paleo-ridge that rises 

to the east and northeast of the basin, thus enabling hydrocarbon accumulation and commercial 

production (Shew et al. 1993). In the western portion of Auger Basin, the potential hydrocarbon 

strata are pierced by down-thrown faults along the flanks of tabular salt bodies (McGee et al.
3-6 

1993). Geophysical records show that these faults have provided scant barriers to fluid 

migration. Sediment samples from this zone have recovered high-molecular weight 

hydrocarbons and thermogenic gas hydrate. Previous work had identified a steep-sided and flattopped mound that extends approximately 0.7 x 1.0 km at a summit depth of 570 m located 

midway along the seep-affected region of Auger Basin. Active fluid expulsion has created a 50-

m wide, sub-circular mud lake on the southwestern edge of the summit, which is one of two such 

expulsion features on the mound. Fine, fluidized mud emanates from a central diatreme, 

overflows the western margin of the lake, and moves down the southern flank of the mound. The 

mud is fluidized in hypersaline brine (133 practical salinity units) that is supersaturated with 

methane at standard temperature and pressure (STP). Beds of the seep mussel Bathymodiolus 

childressi rim the southern edge of the diatreme. 

3.6 Station Selection and Sample Tracking 

The objectives of the program required collection of diverse samples and deployment of in-situ 

growth experiments. Another important consideration was the need to compare, to the extent 

possible, results produced by the different disciplines. Because the program was primarily 

concerned with biological communities and their ecological interactions, the collection stations 

for detailed samples were chosen according to robust biological characteristics. That is to say, 

each individual station was chosen because particular species or groups of species were living in 

that precise location. To this degree, each station was intended as a repeatable example of the 

habitat for chemosynthetic species. Within the four individual sampling sites (GC185, GC234, 

GC233, and GB425), three major chemosynthetic habitats were studied: tubeworm dominated 

habitats, mussel dominated habitats, and microbial mat habitats. Control stations – locations 

where chemosynthetic fauna were not evident – were designated within and away from the sites. 

Deposits of gas hydrate constituted another habitat type, although it was somewhat problematic 

because it was not initially considered in the sampling design and because the extent of gas 

hydrate deposits in the shallow subbottom was often uncertain. 

Another level of spatial variability was examined to determine the abiotic factors that control 

growth and development of chemosynthetic communities within a habitat. This level of spatial 

resolution is called the “subhabitat.” In the tubeworm habitat, subhabitats are related to colony 

size and age. There are (small) juvenile assemblages, (large) adult assemblages, and senescent 

assemblages, which were thought to be relatively old. In the mussel habitat at the brine pool 

sites, there were mussel beds on the inner and outer edges of brine pools. There are two types of 

heterotrophic bacterial mats: pigmented and non-pigmented. Within the limits of program 

resources, stations and sites were replicated to provide statistically valid comparisons. 

Considerable effort was expended to coordinate sampling activities and track samples that were 

distributed among the various investigators. All samples were catalogued according to a scheme 

that allowed investigators to determine the site and station type for each sample. As noted, each 

site had a two-letter abbreviation (BH, GC, BP and BH). In addition, each subhabitat type was 

further abbreviated. Tubeworm stations were abbreviated JT, AT, and ST, respectively for 

juvenile, adult and senescent aggregations. Mussel and Beggiatoa stations were respectively 

abbreviated M and B. As detailed below, each sampling dive with the submersible was given a 
3-7 

unique number. Thus, by recording the dive number and station number with every sample, it 

was readily possible to determine not only the site and subhabitat for each sample but also the 

time and additional details of collection. For example, a sample identified as Dive 2883 station 

BHAT1 is immediately identifiable as coming from GC185, from an adult tubeworm station, and 

from a specific collection. 

3.7 Fieldwork

Fieldwork during the program comprised two geophysical survey cruises onboard the R/V 

GYRE and submarine NR-1 in 1997 and 1998, respectively, and two cruises using the submarine 

Johnson SEA-LINK onboard the R/V EDWIN LINK in 1997 and 1998. Although the majority of 

ship-time expenses were funded through MMS, significant cost sharing with other agencies and 

programs was an important component of the fieldwork that extended the amount of sampling 

that could be carried out. Contribution of ship days with R/V GYRE was received from the State 

of Texas/Texas A&M University ship operations program. Use of the submarine NR-1 was 

provided courtesy of the United States Navy. Dive days with SEA-LINK were awarded by the 

National Undersea Research Program (NURP) center (in grants to PIs Fisher and MacDonald) 

and by the Office of Naval Research. Logistic details of these cruises are briefly recapitulated 

below. 

3.7.1 Geophysical survey with R/V GYRE, 1997

The geophysical survey cruise was conducted with the R/V GYRE (Cruise 97-G-4). The goal 

was to use the TAMU2

 digital side-scan sonar and a 3.5kHz echo sounder to map the character 

and distribution of seafloor features in three areas of the Louisiana continental slope where 

hydrocarbon seeps are known to occur. The three areas are as follows: 

1. A 36 sq. NM (123 km2

) area centered on the mud mound at the border of lease blocks 

GB424/425 (henceforth “GB425” site). 

2. A 315 sq. NM (1050 km2

) area on the upper slope including the intensively studied 

chemosynthetic community sites Bush Hill, Brine Pool NR-1, Mussel Beach, and GC234 

(henceforth the “Shallow” site). 

3. A 360 sq. NM (1228 km2

) area on the lower middle slope due south of the Shallow site 

(henceforth the “Deep” site). After completing the geophysical surveys, the strategy was to 

collect cores for 36 hours at the end of the cruise to "ground truth" the geophysical images. 

The R/V GYRE departed Galveston on 3 June 1997. GB425 was surveyed from 4 to 5 June, 

collecting 45.5 NM (84.2 km) of trackline data. Seven north-south tracks were surveyed and 

were spaced 1500 m apart with the sonar swath width set at 3000 m. In addition, an east-west 

track was collected over the GB425 vent mound and a sub-circular feature to the west. The 

“Shallow” site was surveyed from 5 to 9 June with data collected on 28 north-south tracks (387.0 

NM/716.0 km), spaced 1500 m apart, and 7 north-south tracks (38.5 NM/71.2 km) situated 

between the main lines in the northwest corner (to fill in a shallower water area). As before, the 

sonar swath-width was set to 3000 m. Seventeen north-south lines (272.0 NM/503.2 km) were 
3-8 

run at the “Deep” site on 9 to 11 June with a spacing of 2500 m and a sonar swath-width of 5000 

m. All of the surveys produced high quality data showing faults, carbonate mounds, mud 

volcanoes, mudflows, sediment flows, and other features whose identity is uncertain. On 13 

June, R/V GYRE met a crew boat to exchange technical personnel and obtain core liners. The 

ship returned to the “Shallow” site for coring. Thirty-one cores (16 piston cores and 15 gravity 

cores) were collected during a 30-hour period on 13 and 14 June. With the coring finished, the 

R/V GYRE returned to Galveston, docking on 15 June 1977. 

3.7.2 Detailed sampling with submarine SEA-LINK, 1997 

A 24-day cruise on board the R/V EDWIN LINK, acting as tender ship for the submersible 

Johnson SEA-LINK II, was completed during 8 to 13 July 1997. This cruise (JSL97) comprised 

a major portion of the field effort for Year 1 of the program. The goals for this cruise were to 

complete an extensive series of sample collections, to release marked animals for growth studies, 

to deploy in-situ monitoring instruments, and to map the study sites for future effort. Data 

produced by this effort was crucial to the success of the program. Despite a very ambitious cruise 

plan, all goals were achieved thanks to the dedicated efforts of the cruise participants. A total of 

52 stations were occupied at four separate sites. A complete list of stations is provided in Table 

3.3. All collections and observations were logged in a computerized database to facilitate data 

management. A detailed dive log has been issued as an Appendix to this volume. 

Table 3.3. Stations Sampled According to the Experimental Design Criteria. 

Site Habitat Level within habitat Stations 

Adult AT1, AT2, AT3, AT4, AT5 

Tubeworm Senescent ST1, ST2 

Juvenile JT1, JT2, JT4, JT5, JT6 

Mussel M1, M2, M3, M4, M5 

White Mat B1 

Bacterial Mat 

Red Mat B2 

GC185 

Oily diffusion 

Station prefix = BH 

Reference Unoccupied boxcore from ship 

Adult AT1, AT2, AT3, AT4 

Senescent ST1, ST2 Tubeworm 

Juvenile JT1, JT2, JT4, JT5 

Mussel M1, M2 

GC234 

Oily diffusion 

Station prefix = GC 

Bacterial Mat B 

Tubeworm Adult AT1 

Inner edge M1, M4, M7 

GC233 

Brine dissolution 

Station prefix = BP Mussel 

Outer edge M2, M3, M5 

GB425 

Brine dissolution 

Station prefix = GB 

Mussel M1, M2 
3-9 

3.7.3 Geophysical survey with submarine NR-1, 1998 

Submarine NR-1 and her tender M/V CAROLYN CHOUEST arrived in the sampling area and 

commenced operations on 18 May 1998. NR-1 was equipped with two survey instruments. a 2-

12kHz subbottom profiler (EdgeTech X-Star Chirp system) and a laser line scanner (Raytheon 

LS4096). The subbottom profiler was used to obtain high-resolution imaging of the near-bottom 

sediment layers. The laser line scanner was initially used to produce high-resolution, large-area 

images of the seafloor. The instrument experienced difficulties and was eventually disabled due 

to a flooded connector. The mission continued for several days, collecting detailed subbottom 

data from study sites GC234 and GC233 as well as fifteen other targets that had been identified 

in the TAMU2

 data. After four days, a flooded connector disabled the NR-1, and the mission was 

cut short. Despite the abbreviated cruise, invaluable and unique data were collected in support of 

program objectives. 

3.7.4 Detailed sampling with submarine SEA-LINK, 1998 

A 15-day cruise on board the R/V EDWIN LINK, tender ship for the submersible Johnson SEALINK I was completed during 4 to 18 July 1998. This cruise (JSL98) completed the sampling 

effort begun in the previous year. Most of the stations sampled in the JSL97 cruise were resampled. Sampling efficiency was improved on the basis of experience gained during the 

previous year so that more samples were collected in a shorter cruise. Two ancillary dive days 

were added to the end of the cruise by cost sharing with the Office of Navel Research, which 

provided the opportunity for intensive sampling of gas hydrate deposits. A complete list of 

stations is provided in Table 3.3. 

3.8 Sampling Design 

The key elements of the design for the tubeworm, mussel, and microbial mat studies are similar. 

There are three main effects: time, subhabitat, and replicate site. The study designs differ only in 

that there are a different number of subhabitats for tubeworms (3) and mussels and mats (2). 

With the addition of the two reference stations at each site, the total number of subhabitats 

increases to five for tubeworms and three for mussels and mats. However, because many of the 

habitats occur in the same sites, the reference stations were sampled only once, and the data was 

used for all three statistical analyses. In addition, the mussel and mat sites have site as a design 

difference, because two sites contain liquid hydrocarbons and two contain brine pools. 

3.8.1 The relationship between sites and habitats 

All three habitats occur only at the sites containing liquid hydrocarbons. Therefore, most of the 

study can be accomplished at GC234 and GC185. The tubeworm study is to be carried out only 

at liquid hydrocarbon containing sites. In this case, the site is a form of replication. The mussel 

study is being carried out at all four sites, so site is a major design element for the mussel study. 

The bacterial mat study could be carried out at all four sites or at just the liquid hydrocarbon 

containing sites, as in the tubeworm study. Reference habitats (i.e., sediment unoccupied by 

mussels, mats, or worms) are used to determine the effect of biological communities on 

background sediment geochemistry. Unoccupied sites are available at all study sites. 
3-10 

3.8.2 Univariate statistical model for the sampling plan 

A statistical model is a mathematical representation of the relationships amongst the variables in 

the experimental design of the study. The statistical model is used to test null hypotheses. Model 

development is based on the elements of the design. The following 3-way analysis of variance 

(ANOVA) model represents the experimental design for the common sampling plan for time, 

habitats, and sites that can be used to test the first three null hypotheses (Ho1 - Ho3); 

 Yijkl =  + I + k + jk + l + jl + kl + jkl + i(jkl) (3-1) 

where Yijkl is the ith observation,  is the overall sample mean, I is the main treatment effect for 

time where there are 2 levels of j, k is the main treatment effect for subhabitat where there are 

four or five levels of k, jk is the time-habitat interaction, l is the main treatment effect for site 

where there are two or four levels of one, jl is the time-site interaction, kl is the habitat-site 

interaction, jkl is the second order interaction for all main effects, and i(jkl) is the random 

error associated with each of the three replicates. Time and subhabitat are fixed effects, but site is 

a random effect, because the two sites sampled were only chosen to represent the generality of a 

larger number of actual sites available. Because this is a mixed model with both random and 

fixed effects, the expected mean squares for each term must be calculated, and the appropriate 

error term used for each null hypothesis test. The expected mean squares and F tests are the same 

for all habitats but differ in the number of degrees of freedom, because there is a different 

number of sites and subhabitats sampled (compare Tables 3.4, 3.5, and 3.6). 

Post hoc multiple comparison tests, such as the Tukey Test, are used to detect differences among 

sample means for main effects where there are more than two levels. Subhabitat is the only main 

effect that falls into this category. A priori linear contrasts will also be used to test if the 

reference station is different from the chemosynthetic subhabitat stations. A priori linear 

contrasts will be used to determine if there are site differences when both the liquid hydrocarbon 

and brine seep sites are sampled. 
3-11 

Table 3.4. Experimental design for the tubeworm sampling program.Sources are Time (T), Subhabitat (H), 

Replicate Site (S), Degrees of Freedom (DF), Variance Components that make the Expected Mean Squares 

(EMS) for the mixed model and Appropriate Mean Square (MS) term used as the denominator in the F Test for 

each source. There are four subhabitats: young, mature, senescent, and reference. There are just two sites. 

Source DF EMS Error Term 

T 2 - 1 = 1 (Error) +  (T*H*S) +  (T*S) + (T) MS(T*S) 

H 4 - 1 = 3 (Error) + (T*H*S) +  (H) MS(T*H*S) 

S 2 - 1 = 1 (Error) +  (T*H*S) +  (T*S)+(S) MS(T*S) 

T*H (2 - 1)(4 - 1) = 3 (Error) +  (T*H*S) + (T*H) MS(T*H*S) 

T*S (2 - 1)(2 - 1) = 1 (Error) + (T*H*S) +  (T*S) MS(T*H*S) 

H*S (4 - 1)(2 - 1) = 3 (Error) + (T*H*S) + (H*S) MS(T*H*S) 

T*H*S (2 - 1)(4 - 1)(2 – 1) = 3 (Error) + (T*H*S) MS(Error) 

Error (2)(4)(2)(2 - 1) = 16 (Error) 

Total (2)(4)(2)(2) - 1 = 31 

Table 3.5. Experimental design for the mussel program. Sources are Time (T), Subhabitat (H), Replicate Site 

 (S), Degrees of Freedom (DF), and Variance Components that make the Expected Mean Squares 

(EMS) for the mixed model and Appropriate Mean Square (MS) term used as the denominator in 

the F Test for each source. There are two subhabitats: in the center and at the edge of the brine 

pool. There are four sites: two oily (liquid hydrocarbons) and two brine seeps. 

Source DF EMS Error Term 

T 2 - 1 = 1 (Error) + (T*H*S) + (T*S) + (T) MS(T*S) 

H 2 - 1 = 1 (Error) + (T*H*S) + (H) MS(T*H*S) 

S 4 - 1 = 3 (Error) + (T*H*S) + (T*S) + (S) MS(T*S) 

T*H (2 - 1)(2 - 1) = 1 (Error) + (T*H*S) + (T*H) MS(T*H*S) 

T*S (2 - 1)(4 - 1) = 3 (Error) + (T*H*S) + (T*S) MS(T*H*S) 

H*S (2 - 1)(4 - 1) = 3 (Error) + (T*H*S) + (H*S) MS(T*H*S) 

T*H*S (2-1)(2-1)(4-1) = 3 (Error) + (T*H*S) MS(Error) 

Error (2)(2)(4)(2 - 1) = 16 (Error) 

Total (2)(2)(4)(2) – 1 = 31 
3-12 

Table 3.6. Experimental design for the microbial mat sampling program. Sources are Time (T), Subhabitat 

(H), Replicate Site (S), Degrees of Freedom (DF), and Variance Components that make the Expected Mean 

Squares (EMS) for the mixed model and Appropriate Mean Square (MS) term used as the denominator in the F 

Test for each source. There are two subhabitats, pigmented and non-pigmented, in three sites. 

Source DF EMS Error Term 

T 2 - 1 = 1 (Error) + (T*H*S) + (T*S) + (T) MS(T*S) 

H 2 - 1 = 1 (Error) + (T*H*S) + (H) MS(T*H*S) 

S 3 - 1 = 2 (Error) + (T*H*S) + (T*S) + (S) MS(T*S) 

T*H (2 - 1)(2 - 1) = 1 (Error) + (T*H*S) + (T*H) MS(T*H*S) 

T*S (2 - 1)(3 - 1) = 2 (Error) + (T*H*S) + (T*S) MS(T*H*S) 

H*S (2 - 1)(3 - 1) = 2 (Error) + (T*H*S) + (H*S) MS(T*H*S) 

T*H*S (2 - 1)(2 - 1)(3 - 1) = 2 (Error) + (T*H*S) MS(Error) 

Error (2)(2)(3)(2 - 1) = 12 (Error) 

Total (2)(2)(3)(2) - 1 = 23 

3.8.3 Multivariate analyses for the sampling plan 

The models in the above section are univariate and can be used to test for differences among the 

main effects for the variables measured. The models are also used to test the first three null 

hypotheses concerning differences due to temporal change and spatial variability among and 

within specific chemosynthetic habitats. However, to test the fourth null hypothesis, about 

differences among the types of chemosynthetic communities, a multivariate analysis is 

necessary. In this case, all data collected during the study can be synthesized in one general 

analysis where the experiment-wide error rate is controlled. The purpose of the analysis is to 

identify the abiotic factors that control biotic factors among the three different chemosynthetic 

habitats. 

In the end, the multivariate data set is a matrix consisting of rows of observations for each 

sample (identified with time, site, habitat, and subhabitat) and columns of values for each 

measurement taken at each station. Many of the columns of data in this matrix are auto 

correlated, or co-vary; therefore, a simple bivariate test, such as calculation of correlation 

coefficients for all possible combinations of variables, is inappropriate and introduces multiple 

testing errors. Multivariate analysis is used so that important relationships are not ignored among 

the variables and to control the experiment-wide error rate. 

Principal Components Analysis (PCA) is the multivariate technique of choice. PCA is a 

transformation of the data set to create another data set with two desirable attributes – the 

principal components are mutually orthogonal (which means that the columns are now 

uncorrelated), and the components are extracted in order of decreasing variance. This gives a 
3-13 

data set that has a reduced number of variables but still contains most of the information in the 

original data set. The reduced number of variables is used to test hypotheses or make predictions 

about how abiotic factors are related to biotic factors. Hypotheses are tested by visualizing plots 

of PCA scores for each row (i.e., observation) where the various elements of the experimental 

design are plotted as the symbols for the observation. This technique has been used successfully 

in many environmental studies. 
4-1 

4.0 Field Methods 

4.1 Introduction 

Extensive sampling of the community stations was completed with the use of the submersible 

Johnson SEA-LINK during research cruises in 1997 and 1998. The logistics and sampling design 

for these cruises was presented in the previous chapter. Many of the sampling methods, 

equipment, and procedures were developed expressly for this program and related research. This 

chapter provides a description of the field methods and procedures used and details the unique 

aspects of the effort. 

4.2 Submarine Johnson SEA-LINK 

SEA-LINK is a four-person, battery-powered vehicle that operates to a maximum depth of 1000 

m (Figure 4.1A). The SEA-LINK typically carries two crew members and two scientists or 

observers. The pilot and one observer sit forward in an acrylic sphere. One other observer and a 

safety officer ride in a second chamber in the aft of the vehicle. SEA-LINK is launched from the 

deck of the tender, which during this program was R/V EDWIN LINK, by use of a heavy-lift Aframe. Launch and recovery operations can be carried out in moderately heavy seas. Loss of dive 

due to weather was minimal during this program. In normal operations, SEA-LINK dives twice 

per day and remains on the bottom for approximately 2.5 hours each dive. The monitoring 

equipment, which is standard to the SEA-LINK, includes video and still cameras. The video 

camera is mounted on a pan-and-tilt assembly under the control of the forward observer. The still 

camera is either fired manually or by an interval-trigger. A macro-focus camera system is also 

available for detailed documentation. Samples are collected by use of a seven-function 

mechanical arm that can be equipped with a clamshell-type scoop. Additional sampling 

equipment includes a suction-sampler that feeds into as many as 12 discrete containers. Samples 

of the ambient water can be collected through several intake lines that are plumbed into the aft 

chamber. 

A sampling plan for a typical sampling day was agreed to during a series of pre-dive conferences 

and the observers were provided with detailed check lists of stations to survey, samples to 

collect, and other tasks to be accomplished. The sequence of collections would be planned to 

accommodate the sample-processing schedule during the morning and afternoon activities. The 

SEA-LINK was generally launched so that it would reach bottom at or near the stations that 

comprised the sampling objectives for the dive. Once on bottom, the forward observer would 

start recording video sequences and audio notes on a high-band 8-mm recorder connected to the 

SEA-LINK video camera, recording that continued uninterrupted throughout the dive. These 

videos, along with written notes, comprised the scientific documentation for the samples 

collected during the dive. When the dive finished and the SEA-LINK was recovered, the 

observers would compare and combine notes to ensure an accurate record of the collections was 

taken. Problems that required immediate attention were discussed with the chief scientist so that 

missed tasks could be completed in the second dive of the day. Science meetings were held each 
4-2 

evening after the dives and immediate sample processing had been completed. During these 

meetings, the observers who had dived that day would recount their experiences and shared any 

difficulties that had been encountered. The lists of samples would then be reviewed to determine 

the priorities and tasks for the sampling activities to be accomplished the next day. The following 

sections detail significant aspects of collection and other field methods. 

4.3 Navigation 

During the 1997 and 1998 sampling cruises with SEA-LINK, the tender ship R/V EDWIN LINK 

was navigated with differentially corrected global positioning system (GPS) (accurate to 

approximately 5 m). The position of the submarine on the bottom was determined by use of a 

short base-line acoustic system. An estimate of the submarine's absolute position was obtained 

from the offset between the two vessels. Although this method is adequate for reoccupying 

stations, it was sensitive to factors such as how the submarine was oriented and changes in the 

water column that affect acoustic transmission. Consequently, the navigation system might 

indicate that the submersible was occupying a particular station, while the estimated position of 

the station was only accurate to about 10 m. In many cases, errors of as much as 30 m apparently 

occurred. Efficient submarine sampling at multiple stations requires the pilot and scientists to 

know the relative bearings between stations. Putting this in perspective, the study sites 

encompass areas on the order of 100 x 100 m and include from 5 to 15 sampling stations. If 

station positions are only known with an error of 10 m, then bearings between stations will not 

be accurate enough for reliable navigation. Consequently, valuable bottom time is expended and 

replication of samples is difficult to achieve. Acoustic pingers were also placed at prominent 

points at the stations. By homing on these pingers, the submarine pilots were usually able to find 

sampling stations with minimal searching. 

4.4 Markers and Sketch Maps 

To maximize spatial control, pragmatic operational procedures during the dives and image 

processing and geographic information system (GIS) methods were combined and applied to the 

best available information on the study sites. The pragmatic methods included standard markers 

for the stations and a set of simple sketch maps designed to be used by the submersible scientist 

and pilot during the dives. 

Each station was marked with a combination float and visual scale (Figure 4.2). The float, which 

was painted with reflective paint, provides high-visibility marking from the vantage point of the 

submersible. The marker, a 30 x 30 cm square painted with a black and white checker pattern, 

provides a marking for an over-head vantage point and a convenient scale for future visual 

survey efforts. Each marker was labeled with an abbreviated form of the station name, e.g., AT1 

for BHAT1. Hand-drawn sketch maps included locations of individual stations and relative 

bearings between stations (Figure 4.3). These sketches were continually updated and modified as 

stations were marked and occupied, or as better information became available about station 

locations, adjacent landmarks, etc. 
4-3 

A B 

C D 

E F 

Figure 4.1. Submersible sampling operations with submarine Johnson SEA LINK. SEA LINK is launched

from the deck of the tender (A). Plexiglass box is used for recovering mussels and other animals at

bottom temperatures (A and B). Push cores are used for sediment collection (B). On the bottom,

SEA LINK collects samples with use of a mechanical arm shown deploying the tubeworm-staining

device (C) and a push core (D). Tubeworms were collected with use of large (E) and small (F)

bushmaster devices. 
4-4 

4.5 Sediment Collection 

An important component of the sampling program was to collect sediment from the stations 

defined by the study design. The majority of sediment samples were taken with so-called push 

cores. These were butyrate tubes with 7 cm internal diameter and 40 cm length. The cores were 

mounted on nylon plugs fitted with tee-handles and one-way valves. The tee-handle allowed the 

mechanical arm of the SEA-LINK to grasp the core and force it into the sediment. The valves 

allowed water displaced by sediment to escape from the tube. Typically, SEA-LINK carried 

eight push cores on a rack mounted of the front of the work platform (Figure 4.1B). Each core 

was marked with a sequence number that was clearly visible to the forward observer. When 

collecting a core, the observer would note the station number, the placement of the core relative 

to the station marker and to faunal clusters, the core sequence number, and any other 

peculiarities of the collection. An important distinction was the relation of cores to tubeworm 

clusters. Cores could not be collected within a tubeworm cluster because of the interference from 

the tube mass. Cores were collected as close as possible to the base of the cluster (Figure 4.1D) 

often under the overhang of the longer worms. Other cores were collected through or adjacent to 

bacterial mats. Whenever possible, core collection was recorded with the video camera. As the 

pilot slowly pushed the core tube into the sediment, ideally in a vertical orientation, the observer 

closely monitored the circle of sediment inside the tube. The pilot would continue to push down 

until the circle began to move below the level of the surrounding sediment, at which point 

insertion was stopped to avoid compacting the sample. Push cores generally recovered 20 to 25 

cm of sediment. When the submarine was recovered, the core tubes were quickly removed from 

their holders, capped, taped and labeled, and then stored in a shipboard cold room until processed 

by the individual investigators. In 1998, each core was photographed prior to processing to retain 

a record of sediment visual characteristics. 

A series of control sediment samples was collected using a Gulf of Mexico box core deployed 

from the surface by R/V EDWIN LINK. These cores were taken within the approximate 

boundaries of the sampling stations or at approximately 2 km distances from the stations. 

Because they did not sample tubeworm clusters or mussels, they were deemed control samples of 

the seep area un-colonized by chemosynthetic fauna. When the box cores were brought onboard 

the ship, sediments were sub-sampled from within the core by manually inserting a core tube, 

which was then treated like a standard push core. Limited sediment samples were also collected 

by use of the clam-shell scoop on the SEA-LINK mechanical arm. 
4-5 

Figure 4.2. Combination float and bottom marker used to mark sampling stations. 

4.6 Mussel and Associated Fauna Collection and Shipboard Processing 

Quantitative collections (“ring collections”) of mussels and associated fauna were made from 

each mussel bed to assess mussel density and size-frequency distributions, to characterize the 

faunal associates, and to provide samples for other investigators. Two different shapes of “rings” 

were used over the course of this study. Both “rings” were constructed of 20 cm wide and 2 mm 

thick stainless steel. One, a 0.5 m diameter circular ring, was used previous to 1997 and again in 

1998 (described by Nix et al. 1995). At the suggestion of the submersible pilots a new 0.5 m 

square ring was built and used in 1997. This “ring” had the advantage that the SEA-LINK 

collection scoop, which has a square footprint, goes right up to the edge of the “ring,” making it 

more efficient for pilots to clear out the ring. However, it was discovered while at sea that it had 

the significant disadvantage of sinking mussel aggregations over very soft substrate (at the Brine 

Pool) rather than cutting out an area to subsample. Therefore, none of the quantitative (density) 
4-6 

data on Brine Pool collections from 1997 is considered reliable, and we have substituted data 

Brine Pool collections made in 1995 to allow comparison to the other study sites. The other 

collections made with the square ring in 1997 appeared satisfactory based on submersible 

observations and video records, and the data is included here. 

The placement of the rings for these collections was not random; the sub pilots were instructed to 

place the rings in an undisturbed portion of the mussel bed that appeared representative of the 

general area and amenable to collection. The ring was placed firmly in the mussel bed and all 

fauna within the inside of the ring were collected first by scooping into the insulated box on the 

front of the submersible and then by suctioning into other containers. Video documentation of 

the ring was taken during and after collection to record animals that “escaped” or were not 

collected (left behind); later, counts were corrected for these missing animals. After recovery of 

the submarine, the entire collection was transferred to chilled water for separation and 

enumeration of the fauna. The lengths of all living mussels collected were measured. An 

average of about 6% of the living mussels were damaged during the collections. These damaged 

mussels were counted, and the best possible estimate of their length was made. After 

measurement, mussels were removed from the collection for other studies, including genetic 

analyses, histopathology, determination of hydrocarbon loads, stable isotope determinations, and 

determination of condition indices. The associated fauna was subsampled at sea for stable 

isotope analyses and the remainder placed in 10% formalin in seawater for transport back to the 

laboratory. 

Six to twelve mussels from each collection were sampled for determination of physiological 

condition. The shell length of each mussel was measured. The shell was then opened a few 

millimeters to drain seawater from the mantle cavity. The abductor muscles were cut over a preweighed sample bag, collecting all internal fluids, and the tissue scraped into the sample bag and 

stored at -20C. The empty shell was cleaned, dried, and marked for later determination of shell 

volume. The samples were transported back to Pennsylvania State University on dry ice and 

stored at -70C until further analysis. 
4-7 

Figure 4.3. Hand-drawn sketch maps of locations for individual stations and the relative bearings between stations. 
4-8 

4.7 Mussel Growth and Transplant Studies 

One hundred and fifty mussels collected at GBM1 in 1997 were measured and marked as 

described below for a growth study of mussels at this newly discovered site. The marked animals 

were re-collected in 1998 and re-measured. 

For the reciprocal transplant studies, ring collections from a site were augmented with several 

scoops of mussels (into a separate recovery container) to assure sufficient material for the study. 

In a cold room (8C) on board the ship, the length of each mussel was measured. Then a 

commercial, color-coded and numbered larval fish tag (6.9 x 3.2 mm) was glued to the umbo for 

identification (Nix et al. 1995). Three hundred individuals of a representative size range from 

each location were marked in 1997 and half redeployed at their original collection site (marked 

controls) while the other half were deployed at the transplant site. Immediately before the launch 

of the submersible, the marked mussels were placed in temperature insulated deployment 

containers filled with chilled seawater (8C). Upon arrival at the experimental site, the lids of the 

deployment containers were removed and the mussels were re-deployed. All mussels were 

deployed within 24 h of their collection. Approximately one year later, the marked mussels were 

re-collected, and their lengths re-measured. Ten additional mussels were processed at sea for 

later determination of condition index from each of the collection and deployment sites in both 

1997 and 1998 (unmarked controls). 

A short-term transplant experiment was conducted in 1997 to provide information on the time 

course of change in condition index, water content, and glycogen content in mussels removed 

from a favorable environment. Mussels were collected from the inner zone of the Brine Pool and 

placed in boxes constructed of one-half inch mesh plastic coated fence wire that were designed 

such that the lid could be shut on the bottom by the submersible. Between 8 and 14 mussels 

were placed in each of four boxes, and an additional 12 mussels were brought to the surface after 

the dive. Two of the boxes were returned to the original collection site (controls), and two were 

transplanted onto bare mud substrate about 100 meters from the Brine Pool. One box from each 

location was picked up at the end of the cruise (after a 13-day deployment), and the other two 

were collected after a 42-day deployment during another cruise. After collection, the mussels 

were sampled for determination of physiological condition as described above. 

4.8 Tubeworm and Associated Faunal Collections and Shipboard Processing

Intact vestimentiferan aggregations were collected along with all associated fauna using the socalled bushmaster devices, which were designed specifically for this collection and required by 

this program. Each collection device consists of an outer mesh net suspended in a frame of 

fiberglass rods and four hydraulic pistons (Figure 4.1E, F). The first deployment of the 

bushmasters in the 1997 SEA-LINK cruise made use of nets lined with 1 cm mesh, which 

allowed many smaller animals to escape. During the 1998 SEA-LINK cruise, the mesh was lined 

with 4 mm nylon netting. The frame provides support during deployment and keeps the net open 

while it is positioned over a vestimentiferan aggregation. The four hydraulic pistons are attached 

to a steel “draw-string” cable that is threaded through the open end of the net so that it can be 
4-9 

closed hydraulically, and an intact aggregation can be securely contained during collection and 

recovery. The large bushmaster has a maximum open diameter of 150 cm and a height of 120 cm 

(Figure 4.1E). The small bushmaster has a maximum open diameter of 70 cm and a height of 67 

cm (Figure 4.1F). When closed, the bottom portion of the devices can be cinched to a diameter of 

less than 10 cm. These devices have also been used to collect tubeworm aggregations from 

hydrothermal vents on the Juan de Fuca Ridge allowing direct comparison between these 

habitats. 

Immediately after collection, samples of the major associated macro fauna were removed and 

frozen for isotopic analysis, samples of tubeworms were removed for genetic and physiological 

studies, and some stained tubeworms were removed for the processing described below. The 

intact aggregation with the remaining associated fauna was preserved in 10% formaldehydeseawater, bagged and stored in a 55-gallon drum for transportation to the laboratory for 

subsequent processing. 

4.9 Tubeworm Growth Studies 

Staining the anterior ends of their tubes and monitoring the deposition of new tube material 

above this stain mark followed growth of over 600 tubeworms from 22 aggregations. The tubes 

were stained using an in-situ staining apparatus manipulated by the SEA-LINK submersible. The 

apparatus used a 20-liter collapsible plastic reservoir to hold the stain, which was plumbed 

through a reversible hydraulic pump to a 25 cm diameter clear plastic dome. The dome was 

placed over a group of vestimentiferans (normally 20 to 40 individuals) in an aggregation, and 

the hydraulic pump delivered a positively buoyant saturated solution of the dye Acid Blue 158 

(in 40% seawater) from the reservoir into the dome (Figure 4.1C). The tubeworm tubes inside 

the device were continuously exposed to this staining mixture for approximately five minutes. 

Then the staining mixture was redirected from the dome back to the reservoir by reversing the 

flow of the hydraulic pump. Up to six groups of vestimentiferans were stained during a single 

dive. During the staining process, the tubeworms retracted inside their tubes and were therefore 

not directly exposed to the dye solution. The tubeworms appeared healthy during follow-up dives 

to the sites. There was no evidence of mortality due to staining or of differential tube growth 

compared to adjacent unstained tubeworms. Subsequent anterior tube growth is readily apparent 

the following year as white tube increments above the blue-stained tube section (Bergquist et al.

2000). To confirm that tube growth does not occur in the middle region of the tubes of these 

species, several aggregations of smaller tubeworms were stained entirely, either in situ or in 

buckets on board ship, and then re-deployed. When appropriate and possible, aggregations 

containing stained individuals were collected intact using one of the bushmaster devices 

described above. In other cases, the stained animals were collected using the manipulator arm of 

the SEA-LINK and recovered in a temperature-insulated box mounted on the front of the 

submarine (Figure 4.1A, B). 

When time allowed, ten stained worms from each collection were processed at sea for 

calculation of growth rate, determination of biomass, and subsequent calculation of condition 

index. In some collections, ten additional tubeworms were taken as back-up samples. These 

animals were placed in a recirculating water bath (~8o

C), and the tube length prior to staining, 
4-10 

new tube growth, and anterior tube diameter of each individual were measured. Each animal was 

removed from its tube, dissected and placed into two separate sample bags: one containing the 

vestimentum, plume, and upper body wall and the other containing the trophosome and lower 

body wall. All samples were individually labeled and frozen on board the ship and returned to 

Pennsylvania State University on dry ice for further processing. All remaining stained 

tubeworms were fixed in formalin and returned to the Pennsylvania State University. 
5-1 

`5.0 The Physical Environment at the Seep Sites*

5.1 Background 

Chemosynthetic communities are found in the Gulf of Mexico at depths greater than 500 m. 

Several sites in Green Canyon and Garden Banks were investigated during two cruises in 1997 

and 1998. The study region is sandwiched between these two well-studied areas. 

To the north, a large anti-cyclonic gyre dominates the Texas-Louisiana Shelf. Currents along the 

coast are predominantly southerly resulting from the prevailing wind stress and buoyancy forcing 

of the Mississippi River outflow water. A return flow brings water east along the shelf break 

(Cochrane and Kelly 1986). The Louisiana-Texas Shelf Circulation Study confirmed this general 

scheme, although it still remains uncertain as to whether or not the easterly flow returns to the 

Louisiana coast to fully close the coastal gyre (Nowlin et al. 1998). 

To the south, the currents of the waters of the deep northern Gulf of Mexico are dominated by 

the passage of anti-cyclonic, warm-core eddies or rings that break off the Loop Current. These 

large, circular, baroclinic structures, 250 to 300 km in diameter, transit slowly to the west along 

the continental slope losing their energy as they interact with the slope off the eastern coast of 

Mexico (Forristall et al. 1992, Kirwan et al. 1984, Lewis and Kirwan 1987). These warm-core 

eddies are often associated with cyclonic, cold-core eddies. Interactions between the cyclonic 

and anti-cyclonic rings can produce very large currents capable of moving large amounts of 

water on or off of the shelf (Biggs and Muller-Karger 1994, Brooks 1984). 

Although the circulation of the overlying waters is fairly well understood, surprisingly little is 

known about the physical environment of the sea floor at these sites. Knowledge of the deep 

currents, temperature, and salinity is important in understanding how the environment affects the 

ecosystems found in the study region. Ocean currents give clues to the dispersal pathways taken 

by larvae of chemosynthetic species. Ocean temperatures can affect the reproductive cycling of 

organisms present. Ocean temperature also affects the stability of gas hydrate deposits. 

During this program we made a series of measurements to help characterize the temperature and 

current regimes that affect the chemosynthetic communities. We sought to characterize the 

temperature range and variability and acquire measurements of near-bottom current speed and 

direction. 

5.2 Methods 

To learn more about the physical environment at the study sites, we made physical 

measurements as part of the two submersible cruises carried out during this program on the R/V 

EDWIN LINK. A current meter mooring was deployed in GC185 during the August 1997 cruise 

and recovered about nine months later on the May 1998 cruise. During these two cruises, 

 

*

 This section was authored by Norman L. Guinasso, Jr. 
5-2 

measurements with a recording conductivity, temperature, and depth instrument (CTD) were 

made from the support ship, R/V EDWIN LINK, as well as from the SEA-LINK itself. 

The mooring, described in more detail in MacDonald (1998b, Figure 3.8), consisted of two 

Oregon Environmental Inc. 9407 vector-averaging current meters. These current meters were 

selected mainly on the basis of cost as were the plastic fishing floats used for buoyancy. Again to 

lower cost, the mooring used a single Benthos 866A acoustic release and incorporated a weak 

link that could have been cut by the Johnson SEA-LINK if the release had failed. Table 5.1 gives 

details of the mooring deployment. The mooring was planned to observe temperature currents 

near the top of the water column and near the bottom as well. The top meter, located at a depth of 

250 m, was intended to pick up any currents associated with the passage of rings. The bottom 

meter was placed about 10 m above the bottom raising it above most of the bottom Ekman layer 

that might be present. The units were set to record a 10-minute average each hour allowing them 

to record up to a year of data. 

Table 5.1. GC185 Current meters. 

Location 27° 46.95’ N; 91° 30.28’ W 

Water depth 547 m 

Depth of bottom meter 537 m (10 m above bottom) 

Depth of top meter 247 m (300 m above bottom) 

Sampling interval 10-minute average every hour 

Deployed 9 August 1997 

Recovered 22 May 1998 

The CTD instruments used were a Seabird SBE19 (serial #1992) owned by Texas A&M and a 

Seabird SBE25 (serial #254334-0041) owned by the Harbor Branch Oceanographic Institute. 

Both recorded temperature depth and conductivity internally. The times and type of CTD 

measurements made during the two cruises are given in the appendix volume. During the 1997 

cruise, nine CTD profiles were made from the R/V EDWIN LINK. Other profiles were collected 

from CTD instruments mounted on the submarine or, in the case of lowering into the brine pool 

or mud volcano, from a small winch mounted on the submarine. 

5.3 Measurements of Currents and Temperature 

The current meter mooring was successfully recovered during the 1998 SEA-LINK cruise. After 

recovery of the mooring, data was downloaded from both meters and provided records for a 

duration of 286 days for both temperature and current. Table 5.2 shows the speed and direction 

statistics for the top and bottom meters. The values in the body of the table are the fraction of the 

time the current had a particular speed and direction. 

For the top meter, the right-hand column shows that the current was less than 10 cm/sec 55% of 

the time. The bottom row shows the number of measurements in each octant centered at the 
5-3 

direction shown. Currents were about twice as likely to be toward the eastern or western octant 

than in the other six direction groupings. For the bottom meter, currents were below 10 

cm/second 93% of the time and predominantly to the north. 

Table 5.2. The frequency, in percent of time, that currents had a particulate range of speed and direction for the 

top and bottom meters. The column at the left summarizes the speed ranges for all directions. The row at the 

bottom summarizes the percent of time for octant of direction for all speeds. 

CHEMO Top CM 

Speed 

(cm/sec) N NE E SE S SW W NW Total 

< 2 7.48 0.17 0.35 0.06 0.15 0.25 0.41 0.41 9.28 

2 - 10 5.01 4.87 5.29 3.52 3.84 3.79 11.03 8.05 45.40 

10 - 20 1.48 2.47 5.20 2.92 1.16 2.46 10.74 3.57 30.00 

20 - 30 0.26 0.71 3.44 0.57 0.03 0.45 3.91 0.25 9.62 

30 - 40 0.00 0.33 2.89 0.01 0.01 0.17 1.05 0.01 4.47 

> 40 0.00 0.01 1.15 0.01 0.01 0.01 0.01 0.01 1.21 

Total 14.23 8.56 18.32 7.09 5.20 7.13 27.15 12.30 

CHEMO Bottom CM 

Speed 

(cm/sec) N NE E SE S SW W NW Total 

< 2 24.32 1.70 3.28 2.00 1.12 0.68 3.19 1.68 37.97 

2 - 10 8.60 6.16 7.87 10.90 6.03 3.06 5.60 8.70 56.92 

10 - 20 1.02 0.57 0.77 0.75 0.01 0.09 0.12 1.65 4.98 

20 - 30 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.07 

30 - 40 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

> 40 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 33.94 8.44 11.93 13.66 7.17 3.84 8.92 12.04 

Table 5.3 from DiMarco (2000) has provided the amplitudes of the tidal components derived 

from the analysis of these two current meter records using the method of cyclic descent in the 

manner of DiMarco and Reid (1998). Shown is the period in hours for each of the tidal 

constituents, the amplitudes of the semi-major and semi-minor axes, and the orientation of the 

tidal ellipse. Negative values indicate clockwise "rotation." The amplitudes of the tidal 

components are generally less than about 1 cm/sec at the top meter and less than 0.6 cm/sec at 

the bottom meter. 

Figure 5.1 shows the power spectra from the top and bottom current meter data. Evident are 

significant peaks near 12 and 24 hours. The inertial period at 27.7N is 25.7 hours so the peak 

near one cycle per day can be expected to contain inertial as well as tidal signals. 

Inertial currents can be expected to rotate clockwise. Figure 5.2 shows rotary power spectra 

from the top and bottom current meter records calculated by the method of Gonella (1972). The 
5-4 

top meter shows a predominant clockwise rotation as would be expected from inertial currents. 

Rotary spectra from the bottom meter do not seem to have a rotational preference. 

Table 5.3. Tidal analysis of the two current meters showing magnitudes of the semi-major and semi-minor tidal 

constituents and the direction of the resulting tidal ellipse. 

Top Meter 

 Period (hours) Major (cm/s) Minor (cm/s) 

Direction (N) 

degrees 

M2 12.42 1.02 -0.12 1117.9 

S2 12.00 0.19 0.01 131.5 

N2 12.66 0.34 0.01 120.5 

K2 11.97 0.07 -0.02 85.1 

K1 23.93 0.99 -0.30 174.1 

O1 25.82 0.98 0 145.8 

P1 24.07 0.28 0.06 65.7 

Q1 26.87 0.47 -0.02 179.0 

Bottom Meter 

Period (hours) Major (cm/s) Minor (cm/s) 

Direction (N) 

degrees 

M2 12.42 0.27 0.07 20.9 

S2 12.00 0.12 0.02 51.8 

N2 12.66 0.15 -0.08 139.6 

K2 11.97 0.09 0.02 83.9 

K1 23.93 0.56 0.20 185.4 

O1 25.82 0.54 0.09 133.8 

P1 24.07 0.40 -0.33 19.1 

Q1 26.87 0.34 0.10 239.0 

5.4 Temperature Records 

Temperature was measured along with current speed and direction by the sensors in the current 

meters. A long-term temperature record in this region was also collected by Ian MacDonald from 

December 1993 through August 1995 using a recording thermistor located about one meter off 

the bottom. This recording thermistor was first deployed in GC185, recovered in September 

1994, and then moved to a nearby site in GC234. 

The top and middle panels in Figure 5.3 shows the temperature time series collected at GC185 in 

1993 and 1994 and at GC234 in 1994 and 1995. The bottom panel in Figure 5.3 shows the 

temperature time series from the bottom and top meters during the 1997-1998 deployments. The 

range of temperatures in the bottom meter and the near-bottom thermistor deployment are shown 

in Table 5.4. Considerable variability is evident in the records from both the top and bottom 

meter. 
5-5 

Figure 5.1. Power spectra of the current speed from the top and bottom meters on the GC185 meter mooring. The 

bar shows 95% confidence limits of the spectral estimates. Vertical lines mark the period of the M2 

and K1 tides. The vertical line labeled “Inertial” is the inertial period at the mooring site. 
5-6 

Figure 5.2. Rotary spectra from the GC185 current meters. The left panel for each meter gives the clockwise 

spectra; the right panel gives the counterclockwise spectra. 
5-7 

Figure 5.3. Time series of temperatures recorded by thermistors at GC185 and GB425. Time series of temperature 

collected at GC185 at the top and bottom current meter. 
5-8 

Table 5.4. Statistics of recorded bottom water temperatures. 

 Temperature 

Location Deployed Recovered Mean Minimum Maximum 

Standard 

Deviation 

GC185 January 1993 September 1994 7.9° 6.9° 10.8° 0.49° 

GC234 October 1994 August 1995 8.0° 6.9° 9.9° 0.58° 

GC185 August 1997 May 1998 7.5° 6.4° 9.6° 0.50° 

Figure 5.4. Power spectra from near bottom temperature sensors showing pronounced peaks near the 12- and 24- 

hour period. 

Figures 5.5A-E show stick plots of currents along with temperature time series for the entire 

GC185 mooring deployment. Current speeds are greater at the top meter and show considerable 

mesoscale variability. The diurnal and semi-diurnal periods are in evidence in the bottom record. 

In September and October 1997 much of the flow seems to be directed along a north-south or 

northwest-southeast line. This is most likely due to the larger scale topographic features as 
5-9 

shown in Figure 6.11, referenced in section six of this report. A large mound rises up to about 

250 m depth about 5km to the NW of the current meter site. A northeast-southwest canyon-like 

feature is formed between the surrounding topography and this mound. Flows up and down the 

axis of this feature would direct currents at the mooring site north-south or northwest-southeast 

as observed. It is particularly interesting that the large near-bottom currents observed during 

November and December 1997 tend to align with the canyon direction. This is particularly 

evident the first week in November, the period from November 22 to December 6, 1997, and 

throughout most of January, February and early March of 2000. 

Much of the variability of currents in the region results from the propagation of mesoscale 

eddies. These eddies or rings are largely driven by density gradients in the water column, which 

in turn produce changes in elevation of the sea surface of the Gulf of Mexico. These elevation 

changes are observed by microwave radar altimeters deployed in satellites orbiting the earth 

(Leben and Born 1993, Leben et al. 1990). Figures 5.6a-d show a series of maps of a sea surface 

height anomaly for the period of deployment produced by Leben (2000) and downloaded from 

his web site http://www-ccar.colorado.edu/. In the Gulf of Mexico, currents flow clockwise 

around regions of positive sea surface anomaly and counterclockwise around negative regions. 

A prominent feature in the data is Eddy El Dorado, which moves westward toward the current 

meter sight during September through December 1997. By the end of December 1997, Eddy 

El Dorado was directly south of the current meter mooring at GC185. Strong westward currents 

and elevated temperatures in December 1997 at the top meter may be the result of the approach 

of Eddy El Dorado. Of interest is the sharp temperature drop accompanied by the reversal to 

westerly flow at the top meter. This is the result of a cold-core cyclone associated with Eddy El 

Dorado. Near the beginning of 1998, Eddy Fourchon breaks off from the Loop Current and 

begins heading west toward the GC185 mooring. In March, the top meter and associated 

temperature rise indicates the arrival of the effects of Eddy Fourchon at the mooring. Current 

velocities at the top meter in April and May are more complicated and may be associated with 

the cold-core feature to the northwest of Eddy Fourchon. 

Currents at the bottom may intensify during some of the stronger current episodes higher in the 

water column. Strong easterly and northeasterly currents in early November 1997, for example, 

are associated with increased north-south fluctuations of water velocity at the bottom. High 

currents in December 1997 at the top meter are associated with large fluctuations directed 

northwest-southeast in the bottom meter. Currents at the bottom may intensify during some of 

the stronger current episodes higher in the water column. Strong easterly and northeasterly 

currents in early November 1997, for example, are associated with increased north-south 

fluctuations of water velocity at the bottom. High currents in December 1997 at the top meter are 

associated with large fluctuations directed northwest-southeast in the bottom meter. 
5-10 

Figure 5.5A. Stick plots of currents at GC185 from September and October 1997 together with the associated 

temperature measurement at each meter. The top panel shows the record from the current meter at a 

depth of 247 m. The bottom panel shows the record from the current meter 10 m off the bottom. 
5-11 

Figure 5.5B. Stick plots of currents at GC185 from November and December 1997 together with the associated 

temperature measurement at each meter. The top panel shows the record from the current meter at a 

depth of 247 m. The bottom panel shows the record from the current meter 10 m off the bottom. 
5-12 

Figure 5.5C. Stick plots of currents at GC185 from January and February 1998 together with the associated 

temperature measurement at each meter. The top panel shows the record from the current meter at a 

depth of 247 m. The bottom panel shows the record from the current meter 10 m off the bottom. 
5-13 

Figure 5.5D. Stick plots of currents at GC185 from March and April 1998 together with the associated temperature 

measurement at each meter. The top panel shows the record from the current meter at a depth of 247 

m. The bottom panel shows the record from the current meter 10 m off the bottom. 
5-14 

Figure 5.5E. Stick plots of currents at GC185 during May 1998 together with the associated temperature 

measurement at each meter. The top panel shows the record from the current meter at a depth of 247 

m. The bottom panel shows the record from the current meter 10 m off the bottom. 
5-15 

Figure 5.6A. Charts of sea surface height anomaly every 20 days during the 1997-1998 current meter deployment 

(Sept.-Oct. 1997). 
5-16 

Figure 5.6B. Charts of sea surface height anomaly every 20 days during the 1997-1998 current meter deployment 

( Nov. 1997-Jan. 1998). 
5-17 

Figure 5.6C. Charts of sea surface height anomaly every 20 days during the 1997-1998 current meter deployment 

(Feb.-April 1998). 
5-18 

Figure 5.6D. Charts of sea surface height anomaly every 20 days during the 1997-1998 current meter deployment 

(April-June 1998). 
5-19 

5.5 Effects of Bottom Water Fluctuation on Hydrate Stability 

A major goal of the program of physical measurements was to learn how temperature 

fluctuations at the sites of chemosynthetic communities affect the stability of gas hydrates 

outcropping and buried in the sediments (Brooks et al. 1994). Figure 5.7 shows a chart of 

Pressure-Temperature stability fields, calculated using the 1998 version of CSMHYD, the 

Colorado School of Mines Hydrate Stability Program (Sloan 1989) for several types of gas 

hydrates that might occur at the near bottom near GC185. 

The dotted curve is the stability line for a structure II gas hydrate. Structure II hydrates contain 

methane and significant amounts of C2-C4 hydrocarbons. These hydrates are stable at 

temperatures and pressures beneath and to the left of the dotted line. 

The dashed curve is for pure methane structure I hydrate. Again this hydrate is stable to the left 

and beneath the dashed line. Finally, the solid line shows a stability curve for structure H 

hydrate. This hydrate, postulated to exist in nature in the Gulf of Mexico (Sassen and 

MacDonald 1994), can contain significant amounts of isopentane in addition to the smaller 

hydrocarbon gas molecules. 

Shown for comparison using a box is the range of pressures and temperatures measured during 

this program (Table 5.4). Structure II hydrate is calculated to be stable over the entire 

temperature range measured at the GC234 canyon sites. Structure I hydrate is stable only part of 

the time at these sites and the measured temperature fluctuations would lead to the disassociation 

of structure I hydrate. The calculations show that structure H hydrate is not stable at the 

pressures and temperatures found at the GC185. 

5.6 Conclusions and Recommendations for Future Measurements 

This examination of the oceanic environment at chemosynthetic communities, although limited 

in scope, found that current speed and direction and water temperatures varied dynamically. A 

strong diurnal periodicity in water temperature and current speed is clearly demonstrated in the 

long-term temperature records that were collected and analyzed. This periodic fluctuation is 

undoubtedly a predictable feature of sea floor habitats in the depth range of the study sites (500-

600 m). Also evident from the temperature records are regular, but non-periodic episodes of 

temperature variations that tend to recur on a monthly or sub-monthly basis. These episodes 

result are evident as gradual fluctuations of the temperature baseline over a range of 2 to 3°C. 

Coupling between Loop Current eddies and transient deep-water currents are indicated, but the 

data are not extensive enough to define the mechanisms. Because chemosynthetic organisms are 

quite temperature sensitive ([Fisher, 1990 #100), the range and consistency of diurnal and 

longer-term temperature variations constitutes an environmental signal. This is noteworthy 

because the deep sea, which is devoid of sunlight, is often described as being temporally constant 

and unchanging. Moreover, because slight temperature increases can have an immediate and 

significant effect on the rates of venting from gas hydrate deposits (MacDonald, 1994 #187), a 

rise in temperature can influence the availability of nutrients in methanotrophic organisms such 
5-20 

as seep mussels. These observations have been quite limited geographically and in terms of 

water depth. Future work should prioritize obtaining temperature time-series from deeper sites 

to determine whether the diurnal periodicity and longer term fluctuation continues to be present 

as one moves further offshore. Determining the relationship between Loop Current processes and 

deep-water circulation will require a much more ambitious program than the present, limited, 

observations can support. However, findings to date clearly indicate that events driven by Loop 

Current can have biological and possibly geochemical significance on the outer continental slope 

of the Gulf of Mexico. 

Figure 5.7. Plot of the stability fields in Structure I, II, and H hydrate. The box shows the pressure temperature 

region measured at GC185. Structure II hydrate is most likely the only stable hydrate species at the 

site. 
6-1 

6.0 Geophysical Detection and Characterization of Chemosynthetic Organism Sites1

6.1 Introduction 

Chemosynthetic organisms flourish at oil seeps, deep beneath the ocean surface in an 

environment that is difficult for humans to explore. As a result, much of our knowledge of 

chemosynthetic ecosystems in the Gulf of Mexico comes from a small number of sites, most 

found by chance. Gulf of Mexico chemosynthetic sites are found in water depths from several 

hundred meters to more than two thousand meters (MacDonald et al. 1996). They seem to exist 

wherever there are hydrocarbon seeps on the continental slope. Despite the apparently large 

depth range, the distribution of such organisms is poorly known owing to sparse data. A 

fundamental unanswered question is whether lush chemosynthetic sites are rare and fragile or 

ubiquitous and robust. Although chemosynthetic organisms cover only a tiny fraction of the 

seafloor, they have the potential to cause the offshore energy industry to spend large sums of 

money to make sure that drilling rigs, pipelines, and other production equipment do not blunder 

into community sites and cause irreparable harm. 

Because chemosynthetic organisms are small, they are difficult to detect using geophysical 

methods. Even near-bottom geophysical records do not readily show chemosynthetic organisms. 

This means that confirmation of chemosynthetic organisms at a given site depends on nearbottom optical imaging, for example, submersible observations or bottom photography, or 

recovery of physical samples, such as coring or trawling. Nevertheless, geophysical techniques 

can detect hydrocarbon seeps because the seeps change the physical properties of surface and 

near-surface sediments (Roberts et al. 1990). Thus, the task of remotely locating chemosynthetic 

organism sites becomes one of defining the small percentage of the seafloor affected by seepage 

and then checking within that area for chemosynthetic habitation. 

In this study, our goal was to compare and contrast geophysical methods of detecting and 

characterizing hydrocarbon seep sites inhabited by chemosynthetic organisms. Because of this 

focus, we ran the risk that the small number of sites that were studied are not entirely 

representative and are insufficient in number to make significant tests of hypotheses. 

Nevertheless, the results of this study are consistent with what is known about seeps and 

chemosynthetic organisms. Our overarching conclusion is that there is no one method of 

detecting chemosynthetic sites that is always accurate. Ultimately, confirmation of the existence 

of chemosynthetic organisms at a given location depends on actually seeing the organisms, by 

submersible, deep-camera platform, or recovery of recently live specimens in trawl or core. 

Defining areas of the seafloor that have been affected by seepage is more straightforward. This 

can limit the area of seafloor that must be considered to a fraction of the deep slope. Making the 

next step, to document whether a chemosynthetic site exists by remote techniques, is a process of 

collecting sufficient evidence to make a reasonable conclusion as to their presence or absence. 

 

1

 This section was authored by William W. Sager. 
6-2 

6.1.1 Overview of continental slope geology 

The continental slope and rise of the northern Gulf of Mexico technically fits into the category of 

“passive margin,” (i.e., it contains no plate boundary) and is very active owing to salt tectonics. 

This activity is reflected in the physiography of the slope. The seaward incline is broken by 

uplifts caused by rising salt diapirs or massifs and basins caused by salt withdrawal (Bryant et al.

1990; 1991; Bouma and Bryant 1994). Indeed, the base of the slope is an escarpment (Sigsbee 

Escarpment) resulting from the seaward migration and coalesence of large salt sheets (Amery 

1969; Humphris 1979). Salt movement has caused the slope to be broken by numerous regional 

and local growth faults and related structures (Worrall and Snelson 1989). Topographic gradients 

are much higher than is the norm for passive margins, with slope angles often greater than 40° 

(Bryant et al. 1995). 

Modern-day salt tectonism in the Gulf of Mexico has its roots in the early geologic history of the 

Gulf. The basin first opened with the rifting of North and South America during the Jurassic 

(e.g., Pindell 1985). A thick salt sheet was deposited on syn-rift sediments in the young basin, 

and the salt was buried in turn by Late Jurassic continental sediments, carbonate-dominated 

Cretaceous sediments, and finally siliciclastic Tertiary sediments (Worrall and Snelson 1989). 

The Neogene saw huge volumes of sediment poured into the northern Gulf of Mexico, ultimately 

creating a thick (up to 16 km) continental-margin sedimentary wedge (McGookey 1975). Salt is 

incompressible, so the weight of Neogene sediments caused the salt layer to deform and 

mobilize, moving upward and toward the south (Humphris 1979). Today the salt is found high in 

the sediment column where it has taken on a variety of morphologies, two of the principal being 

diapirs and sheets (Seni and Jackson 1983; Jackson and Talbot 1986). 

The rough topography of the northern Gulf of Mexico slope reflects the deformation caused by 

salt tectonics. Salt diapirs are common on the shelf and upper slope, but the middle and lower 

slope are characterized by laterally extensive salt sheets stretching south to the Sigsbee 

Escarpment (Humphris 1979). Atop this sheet, the slope character is dominated by small 

subcircular to elongate basins and salt massifs, the former occurring where the salt is thin or 

absent (Bryant et al. 1991). The mobile salt has extensively fractured the overlying sediments 

with regional growth (normal) faults and associated fault types (Rowan et al. 1999). These faults 

act as conduits for the migration of hydrocarbons, which ascend from older, deeper layers to 

shallow reservoirs at 2-3 km depths and from there to the seafloor (Kennicutt et al. 1988). 

Late Cenozoic to Recent sedimentation in the Gulf of Mexico has occurred at a moderate to high 

rate, especially during sea level lowstands when rivers emptied onto the shelf edge and upper 

slope (Lowrie 1987; Coleman et al. 1991). Given this situation, a blanket of Pleistocene to 

Holocene sediments is expected at the seafloor. Although the northern Gulf of Mexico slope is 

indeed generally draped by recent sediments (Coleman et al. 1991), salt tectonics and deep 

currents have caused mass wasting, erosion, and sediment reworking (Bryant and Roemer 1983; 

van den Bold et al. 1987; Bryant et al. 1999). As a consequence, it is difficult to predict the age 

and texture of seafloor sediments at an arbitrary location. 
6-3 

6.1.2 Overview of seep geophysical signatures 

Active seeps are often visible owing to their release of oil and gas in the water. Slicks on the 

water surface can be imaged from space (and presumably lower altitudes), using photography or 

synthetic aperture radar (SAR), because oil alters the roughness of the sea surface (MacDonald et 

al. 1993; 1996). It is not clear, however, how continuous such slicks are because analysis of SAR 

data at one Gulf of Mexico site suggests intermittent activity (MacDonald et al., in press). Gas 

released into the water column by seeps can scatter sound waves and give rise to a vertical plume 

on records from high-frequency echo-sounders (Anderson and Bryant 1990). Such plumes are 

not always observed over active seeps, perhaps because the gas plume must reach a large volume 

before enough sound is scattered to register on the echo-sounder. 

More often, seeps are recognized by their effects on the physiography of the seafloor and 

acoustic characteristics of near-surface sediments. Seeps are a result of tensional faulting of 

continental margin sediments allowing upward migration of hydrocarbons and other formation 

fluids from buried reservoirs (e.g., Kennicutt et al. 1988). In recent times active faults have often 

broken the seafloor, leading to rough topography caused by fault scarps, horsts and grabens, and 

mass wasting of sediments (Roberts et al. 1990b). Fluids and gas expelled from vents along the 

faults cause a number of “fluid expulsion features” including mud volcanoes, mud diapirs, and 

pock marks (Behrens 1988; Neurauter and Bryant 1990; Roberts et al. 1990a; 1990b; Neurauter 

and Roberts 1994; Kaluza and Doyle 1996). In addition, authigenic carbonate buildups, up to 

tens of meters in size, occur at the seafloor as a result of microbial oxidation of hydrocarbons by 

bacteria and the subsequent release of carbon dioxide and bicarbonate that catalyze the 

production of carbonate in near-surface interstitial waters (Roberts et al. 1990). 

Seeps give rise to several distinct signatures on high-frequency echo-sounder profiles and 

seismic reflection profiles. In echo-sounder records, seeps cause acoustic “wipeout,” which is the 

absence of subsurface reflectors. This can be a result of the near total reflection of sound owing 

to a hard ground at the seafloor or attenuation owing to dispersion of the sound waves by gas 

bubbles within the sediment (Behrens 1988; Anderson and Bryant 1990). Generally, two types of 

wipeout are noted on echo-sounder records: simple wipeout (subsurface layers vanish) and 

reverberant wipeout (a prolonged bottom echo). The former probably results simply from 

attenuation owing to gas and the latter may occur because of reverberation caused by gas 

bubbles, carbonate, biogenic debris, or other seep-related material (Behrens 1988). Evidently, 

gas may cause either simple wipeout or reverberation, depending on gas bubble size and density 

(Anderson and Bryant 1990). 

Similar effects were noted in ultra-high frequency echo-sounder records acquired over seeps 

from near the seafloor (Sager et al. 1999). Seafloor acoustic characteristics ranged from total 

reflection from hard bottoms, to limited reflection penetration in shallow buried hard bottom 

regions, to acoustic reverberation, to wipeout. Because of rapid attenuation of sound at high 

frequency, such records can only be obtained from deep-tow vehicles or submersibles. 

Consequently, such data are rare and typically lack wide lateral coverage. 

The effects of seepage are often noted on seismic reflection data, in particular the 2D and 3D 

multi-channel seismics often used for hydrocarbon exploration by the energy industry. The 

seafloor topographic expression of a seep is often not well represented because marine seismic 
6-4 

systems are designed for imaging deep reflectors rather than the seafloor. The low sound 

frequency has poor vertical resolution because the long wavelength and the geometry of long 

streamers do not image the near-surface well. However, the high lateral data density afforded by 

3D seismic data sets often makes up for these problems. As a consequence, an interpreter can 

usually recognize seafloor mounds and craters if they are large (tens of meters vertically and 

hundreds of meters laterally). The most common seismic signature of a seep is a signal loss akin 

to the wipeout in higher frequency reflection records. Typically, this appears as a columnar 

region for up to 1-2 seconds (two-way travel time) beneath the seep in which reflectors are weak 

or absent owing to signal attenuation. These features are commonly called “gas chimneys,” 

although it is uncertain whether the effect is caused solely by gas or whether other factors related 

to seepage also contribute (Abrams 1996; Roberts et al. 1999). Although gas chimneys are often 

nearly vertical, sometimes they are inclined, following the path of an inclined fault that acts as a 

conduit for gas and formation fluids. In some instances, reflectors at the edges of these chimneys 

appear to turn downward (“pull-down”) as a result of lower seismic velocities in the sub-seep 

sediments caused by significant volumes of gas (Corthay 1997). 

Using modern seismic workstations, digital multi-channel seismic data can be processed in many 

ways to enhance certain signal characteristics. As a result there are many different characteristics 

that can be used to define regions with anomalous signal characteristics, including seeps. One 

technique is to extract the amplitude of the seafloor reflection. The seafloor signal strength 

depends on the reflection coefficient, which in turn depends on characteristics of the near-surface 

sediments, so seafloor amplitude anomalies have been used to define seep areas (Roberts 1996; 

Roberts et al. 1996). Using this technique, the high lateral density of 3D data is especially useful 

in showing the geometry of anomalous sediments, which often provides clues about the nature of 

the geologic cause (Roberts et al. 1996). In addition, the sign of the amplitude anomaly can be 

used to infer whether significant volumes of gas are present. Positive amplitude anomalies 

indicate an increase in density or seismic velocity, which suggests a hard bottom, such as that 

caused by carbonate crusts, gas hydrate, or other dense material accumulation (Roberts et al.

1996; Reilly et al. 1996). Positive amplitude anomalies are the most common. In contrast, 

negative amplitude anomalies indicate lower velocity and density, which can occur when 

significant concentrations of gas are present (Roberts et al. 1996; Reilly et al. 1996). Low signal 

anomalies in 3D data time slices can also be used to identify gassy regions in the subsurface, 

using similar logic (Corthay 1997). Other signal characteristics can be used to define anomalous 

regions, either at the surface or in the subsurface. Virtually any attribute that is used to highlight 

anomalous signal characteristics, such as amplitude-velocity-offset (AVO), coherency, signal 

envelope, or rms deviation, can be used to define areas affected by seeps. 

A similar digital seismic technique that has the potential to delineate seep-affected areas is highresolution 3D seismic data. Some energy companies reprocess standard exploration 3D data to 

maximize the resolution and frequency content. Only the first hydrophone group in the streamer, 

or first several groups of hydrophones, are used to mitigate the adverse geometry caused by the 

lengths of modern exploration streamers. The data are also high-pass filtered to improve 

resolution. The result is a 3D data set with about twice the near seafloor resolution of the original 

data. Another similar technique is to acquire short-offset, high-resolution 3D multi-channel data. 

Such data sets are acquired with short streamers (typically 100-m in length) and higherfrequency sound sources. Furthermore, they are processed to more carefully position the 

streamers. The result is a data set with three to five times better resolution than exploration 3D 
6-5 

data. Because of their high resolution and dense data coverage, these data sets are very useful for 

interpreting seep geology (Corthay 1997). Unfortunately, few of these data sets exist, because 

they represent additional field expenses. 

Side-scan sonars also have the ability to show differences in surface sediment properties and thus 

may be useful in imaging seeps. Such sonars construct an image by displaying the amplitude of 

sound returned from the seafloor perpendicular to the sonar transducer arrays. Unlike seismic 

reflection profiling, which relies on vertical-incidence reflection, the side-scan sonar makes an 

image from sound that is “backscattered” from the seafloor. Backscatter depends on scattering of 

sound from surface roughness and particles within the near-surface sediment (Johnson and 

Helferty 1990), so side-scan sonars are often used to examine variations in sediment type at the 

seafloor. Around seeps, side-scan sonars typically show enhanced backscatter owing to the 

physical disturbance of seafloor sediments by seepage (Kaluza and Doyle 1996; Vogt et al.

1997; Roberts 1998; Sager et al. 1998). The processes that cause enhanced backscatter have not 

been widely investigated. In addition, large side-scan sonar image mosaics are not common 

because extensive side-scan sonar mapping is also not frequently done for hydrocarbon 

exploration and production. 

6.1.3 Geologic controls on chemosynthetic communities 

It is clear that chemosynthetic communities occur along faults where hydrocarbons seep to the 

surface. To have hydrocarbon seeps, two ingredients must be present: (1) hydrocarbons in 

subsurface sediment layers and (2) conduits for the hydrocarbons to reach the sea floor. In the 

Gulf of Mexico, rapid sedimentation buried copious amounts of organic matter and turned it into 

hydrocarbons by the right combination of elevated temperature and pressure. Conduits are faults 

which are a result of the mobilization of a Jurassic salt layer by the accumulation of a thick postJurassic sediment pile. 

In virtually every geophysical data set in which seeps, faults, and chemosynthetic organisms can 

be mapped, the chemosynthetic communities occur along the surface expressions of active faults 

(Behrens 1988; Roberts 1995; Reilly 1996; Sager et al. 1999). In the northern Gulf of Mexico, 

many of these faults have developed in response to rapid sedimentation in Pliocene and 

Pleistocene time and resultant adjustment of the unstable sediment pile (Worral and Snelson 

1989; Cook and D’Onfro 1991). Although much research has gone into the faulting in the Gulf 

of Mexico and its relation to structure and morphology (e.g., Worral and Snelson 1989; Rowan et 

al. 1999), our understanding of the mechanics of seepage is poor. Recent research suggests that 

fluid flux at seeps varies with time on time scales that range from months (MacDonald et al., in 

press) to centuries or longer (Reilly et al. 1996; Roberts and Carney 1997), but the forcing 

mechanisms are uncertain. 

Reilly et al. (1996) hypothesized that faulting style and salt activity are two major controls. They 

maintain that the faults that yield the most consistent long-term flux of hydrocarbons are 

antithetic faults rooted on regional growth faults (they term these “fault-dominated” seeps). Such 

faults are generated by fracturing and adjustment of the continental slope sediment pile in 

response to downslope gravity sliding and regional salt movements. Alternatively, they argue 

that faults that root into salt diapirs may cause seeps, but that these seeps (“salt-dominated” 

seeps) are not continuous enough for colonization by complex chemosynthetic communities. 
6-6 

Their reasoning is that rotational shear on the hanging wall of regional growth faults continually 

produces tension consistent with the sustained rapid flux of hydrocarbons needed by complex 

chemosynthetic communities. In contrast, they contend salt diapir faults are only intermittently 

active in response to the upward movement of the salt diapir. 

6.1.4 Finding chemosynthetic communities using geophysics 

Although geologists working for energy companies frequently use geophysical data to define 

seep and potential chemosynthetic organism sites, few publications exist on the subject. 

Dr. Harry Roberts has published a series of papers describing submersible dives to 

chemosynthetic community sites, which were based on geophysical data and other knowledge 

(e.g., Roberts and Aharon 1994). Many were based on high-frequency echo-sounder profiles 

showing fluid expulsion mounds, bathymetry, or multi-channel seismic data. Roberts did not 

examine the process of finding the chemosynthetic sites using these geophysical data. However, 

he did suggest that seismic amplitude data rendered at the seafloor may be useful for locating 

seep sites hospitable to chemosynthetic organisms if the data show low amplitude, phase reversal 

spots caused by gas (Roberts et al. 1996; Roberts and Doyle 1998). 

Reilly (1995) tackled the issue directly and developed a protocol for identification of 

chemosynthetic organism sites (Figure 6.1). He suggested using satellite imagery to detect a sea 

surface slick, and if no slick was detected a chemosynthetic site was unlikely because they are 

only likely to form at continuous seeps. The next decision was whether the site showed 

prolonged, high strength returns from the seafloor on multi-channel seismic data. If so, and the 

reflection coefficient is negative, further investigations were warranted. A positive coefficient 

was deemed to be indicative of hard bottom development and unlikely to be a chemosynthetic 

organism site. At the negative reflection coefficient location, the next observation was to acquire 

piston cores and examine the geochemical signature of the sediments. If significant C1-C5

hydrocarbons were found, especially elevated C5, the organic matter for chemosynthetic 

organisms are present and additional consideration is needed. The next step is to consider the 

geologic model. First fault conduits need to be traced and then correlated to a model of fault 

development. Reilly contends that faults directly related to salt dome uplifts are too 

intermittently leaky to sustain lush chemosynthetic organism sites, whereas inter-domal faults are 

more likely to be continually active. If the site in question is on the latter, he recommended either 

avoiding the site, imaging the seafloor with cameras, or making an observation with a 

submersible or ROV. 

6.1.5 Study overview and rationale 

A complete examination of the geophysical characteristics of hydrocarbon seeps and their 

relation to chemosynthetic organism sites is a project of such large magnitude as to be beyond 

the grasp of the present study. Instead, we chose to focus on a few data types in limited areas. 

Prior chemosynthetic organism site studies have focused on areas typically restricted to a few 

kilometers on a side or less; consequently, we thought it important to investigate a larger area to 
6-7 

see how much is affected by seepage and how many chemosynthetic organism sites are located 

there. Three study areas were chosen, all on the Louisiana continental slope (Figure 6.2). The 

two larger areas were chosen to be representative of the upper and lower slope morphologic 

regions. In addition, both contain seeps as shown by about 10 slicks in each (Figure 6.3) on the 

images studied by MacDonald et al. (1993; 1996). The third area was much smaller and centered 

on a known chemosynthetic organism site. 
6-8 

Figure 6.1. Chemosynthetic site search protocol proposed by Reilly (1995). 
6-9 

Figure 6.2. Location map of study sites on the Louisiana continental slope. Dark gray shading shows land area. 

Light lines show bathymetry contours and leaseblock areas. Boxes with shaded bathymetry denote 

study areas. 

The rationale of the study was to collect a high-resolution geophysical data set in these known 

seep areas and compare it with other geophysical and geologic data. For the high-resolution data 

coverage, we surveyed using long-range side-scan sonar because seepage affects the physical 

properties of surficial sediments providing a contrast that can be imaged with side-scan sonar. In 

addition, side-scan sonars yield detailed acoustic images akin to aerial photographs in which 

much can be learned about the geometry and spatial characteristics of seafloor features owing to 

the high data density. The side-scan images provided a data set that could be compared to preexisting geophysical and submersible data. In addition, as a part of the project, we planned to 

collect piston cores with a surface ship as well as deep geophysical data and observations with 

submarine NR-1 from selected targets in the sonar images. We hoped that with NR-1 we could 

learn how many of the sites that appeared to be seeps in the geophysical data were actually 

inhabited by chemosynthetic organisms. 
6-10 

Figure 6.3. Oil slicks on the sea surface over the Louisiana slope, imaged from Space Shuttle photography and 

synthetic aperature radar images. Boxes show study areas. Redrawn from MacDonald et al. (1993; 

1996). 

6.2 Surveys 

6.2.1 Study sites 

The two larger study areas are in the Green Canyon leaseblocks. The “GC Shallow” area is 25 x 

41 km in size and includes water depths from 300 to 1050 m on the upper slope (Figure 6.4). It 

covers all or parts of 54 lease blocks. This area contains several chemosynthetic organism sites 

that have been the subject of ongoing studies (GC185, GC233, and GC234) in addition to other 

locations known as seeps (GC139-GC140, Roberts and Aharon 1994; GC272, Roberts 1996). 

This area was chosen because it is one in which prior data from seep sites could be used to better 

understand regional geophysical data. 
6-11 

Figure 6.4. Bathymetry, lease blocks, and ship tracks for the GC Shallow site survey. Bathymetry contours are 

shown at 50-m intervals from NOAA multibeam soundings. Dots show shotpoints along ship tracks 

for the TAMU2

 side-scan sonar survey of cruise 97G4 on R/V GYRE. Squares represent lease block 

boundaries. Stars show chemosynthetic community sites GC185 (BH), GC233 (BP), and GC234 

(GC). 

The second large study area, “GC Deep,” is nearly the same size as the GC Shallow site (30 x 41 

km), but is located directly south of the shallow site on the deeper continental slope in water 

depths of 1000 to 2250 m (Figure 6.5). It contains approximately the same number of sea surface 

slicks as GC Shallow (Figure 6.3) and covers all or parts of 63 lease blocks. Although little is 

known about the specific geology of this area, other than bathymetry, we studied the GC Deep 

area for comparison with GC Shallow. GC Deep is representative of the deeper slope where salt 

massifs and intra-salt basins characterize the topography, whereas GC Shallow has a smoother 

topography less perturbed by salt tectonics. 

The third survey site is located on the upper slope in the Garden Banks lease blocks. It is smaller, 

only 12 x 13 km, and centered on an active mud volcano, which is a chemosynthetic organism 

site, at the border of blocks GB424 and GB425. Water depths in this study area range from 550 

to 850 m. 
6-12 

Figure 6.5. Bathymetry, lease blocks, and ship tracks for the GC Deep site survey. Conventions as in Figure 6.4. 

Small filled circles show locations of cores 21-1 and 22-1. 

6.2.2 Cruise 97G4: side-scan sonar survey and coring 

Cruise 94G4 occurred from 3 to 15 June 1997 on board the R/V GYRE. The three study sites 

were surveyed using the TAMU2

 side-scan sonar, which uses frequencies of 11 and 12 kHz 

(Hilde et al. 1991). In all, a total area of approximately 2400 km2 was surveyed. The sonar body 

was towed behind the GYRE at a depth of approximately 50 m and with a layback of about 240 

m. The ship itself was positioned using a differential global positioning system (DGPS), with an 

accuracy of <5 m, and the tow body was located relative to the ship using an ultra-short base line 

(USBL) acoustic ranging system. A 3.5 kHz echo-sounder was run simultaneously with all lines 

to collect subbottom profiler records along the tracks. 

Survey lines were run north-south (N-S) at a speed of 5 to 6 knots. In the GC Shallow and GB 

sites, ship tracks were spaced 1500 m apart, and the sonar swath width was set at 3000 m. This 

provided 200% coverage, allowing each area of the seafloor to be insonified from two different 

angles and allowing the zone beneath the sonar (which is poorly imaged) of one track to be filled 

by data from adjacent tracks. With this swath width, pixel sizes of about 1.5 m were achieved on 

the original sonar swaths. Seven N-S lines were run in the GB area (Figure 6.6) and one crossline was made to look at the mud volcano from a different angle and to run the subbottom 

profiler across the crater in the western part of the study area. In the GC Shallow study area, 21 
6-13 

N-S lines were run across the entire extent of the survey box (Figure 6.4). Additionally, 21 short 

lines were run in between the original lines on the north side of the survey. These were fill lines 

designed to fill bathymetry coverage in the shallow parts of the survey. Fill lines were needed 

because it is only possible to calculate bathymetry to a distance of 3.5 times the water depth with 

the TAMU2

 sonar and this left gaps with a track spacing of 1500 m. 

Figure 6.6. Bathymetry, lease blocks, and ship tracks for the GB site survey. Star labeled “GB” shows 

chemosynthetic community site on a mud volcano. Conventions as in Figure 6.4. 
6-14 

In the GC Deep area, the swath width and track spacing were increased to make the survey 

shorter. Seventeen N-S lines were run, spaced 2.5 km apart (Figure 6.5). A 5500-m swath width 

was used, giving a pixel dimension of about 2.7-m on the original sonar swaths. 

After the side-scan sonar surveying, 31 gravity and piston cores were collected from features 

thought to represent seeps in the sonar images (Table 6.1). Core penetration ranged from 4 cm to 

5.2 m. Most cores were taken in the GC Shallow survey area (Figure 6.7) because it was believed 

better to concentrate on one study area rather than to spread the ground truth data too thin. 

Table 6.1. Cruise 97G4 Core Locations 

 On Bottom Time Depth Latitude Longitude Length Lease 

Core Date (UT) (m) (d) (m) (d) (m) (m) Block Type*

GC1-1 13-Jun-97 13:32 420 27 48.097 -91 13.025 4.61 GC146 P 

GC1-2 13-Jun-97 14:12 420 27 47.832 -91 13.370 1.42 GC146 G 

GC2-1 13-Jun-97 14:52 405 27 46.351 -91 12.471 5.20 GC191 P 

GC2-2 13-Jun-97 15:33 407 27 46.363 -91 12.475 0.04 GC191 G 

GC3-1 13-Jun-97 16:18 519 27 44.277 -91 11.337 5.51 GC235 P 

GC3-2 13-Jun-97 16:51 519 27 44.228 -91 11.513 1.47 GC235 G 

GC4-1 13-Jun-97 17:55 641 27 43.475 -91 15.560 5.43 GC234 P 

GC4-2 13-Jun-97 18:20 641 27 43.469 -91 15.644 1.03 GC234 G 

GC5-1 13-Jun-97 19:04 809 27 42.401 -91 15.661 4.27 GC278 P 

GC6-1 13-Jun-97 19:53 832 27 42.425 -91 15.193 4.16 GC278 P 

GC7-1 13-Jun-97 20:50 559 27 43.762 -91 18.052 2.18 GC233 P 

GC7-2 13-Jun-97 21:10 575 27 43.729 -91 17.946 1.34 GC233 G 

GC8-1 13-Jun-97 21:42 639 27 43.436 -91 17.050 4.57 GC233 P 

GC8-2 13-Jun-97 22:16 592 27 43.371 -91 17.417 1.30 GC233 G 

GC9-1 13-Jun-97 22:55 688 27 43.457 -91 20.950 4.20 GC232 P 

GC9-2 13-Jun-97 23:38 688 27 43.542 -91 21.004 1.48 GC232 G 

GC10-1 14-Jun-97 0:36 766 27 38.859 -91 20.906 4.25 GC320 P 

GC10-2 14-Jun-97 0:57 766 27 38.906 -91 20.684 1.36 GC320 G 

GC11-1 14-Jun-97 1:46 766 27 38.910 -91 20.473 4.00 GC320 P 

GC12-1 14-Jun-97 2:39 795 27 38.320 -91 20.340 4.08 GC320 P 

GC13-1 14-Jun-97 3:34 890 27 35.779 -91 23.133 4.08 GC363 P 

GC14-1 14-Jun-97 5:29 985 27 39.972 -91 33.805 3.78 GC315 P 

GC15-1 14-Jun-97 6:44 726 27 40.363 -91 32.215 3.93 GC272 P 

GC16-1 14-Jun-97 15:36 676 27 41.574 -91 32.398 0.74 GC272 G 

GC17-1 14-Jun-97 16:17 630 27 43.054 -91 32.108 1.41 GC228 G 

GC18-1 14-Jun-97 17:26 279 27 49.369 -91 33.055 1.28 GC140 G 

GC19-1 14-Jun-97 17:52 327 27 49.234 -91 33.256 1.92 GC140 G 

GC19-2 14-Jun-97 18:46 263 27 48.396 -91 33.129 0.25 GC140 G 

GC20-1 14-Jun-97 19:21 401 27 47.947 -91 35.434 1.33 GC139 G 

GC21-1 14-Jun-97 10:37 1152 27 23.764 -91 33.345 1.50 GC580 G 

GC22-1 14-Jun-97 11:43 1542 27 23.365 -91 34.530 1.67 GC579 G 

*Type: P = piston; G= gravity 
6-15 

Figure 6.7. Core locations in GC Shallow survey area (small filled circles). Conventions as in Figure 6.4. 

6.2.3 Submarine NR-1 cruise 

In the summer of 1998, we were scheduled for three weeks of time on the submarine NR-1 in the 

Gulf of Mexico. Two weeks of that time was to be spent investigating sites within the GC 

Shallow and GB study sites. Two remote sensing tools were installed on the submarine for the 

cruise, X-Star 2-12 kHz chirp sonar for making near-bottom echo-sounder profiles and a 

Raytheon laser line-scanner for making optical images. Submarine positions were obtained using 

inertial navigation tied to daily fixes provided by acoustic ranging to the tender ship, which was 

itself navigated with P-code global positioning system (GPS). 

Owing to technical difficulties, the NR-1 was late leaving port, and the available survey time was 

decreased. The submarine reached the GC Shallow study area on 18 May 1998 and operations 

were commenced. For the next several days, repeated problems with the laser line-scanner and 

its data storage unit made it necessary to return to the surface several times. Eventually the laser 

line-scanner developed a short in a cable through the hull and was unusable for the rest of the 

survey. Although there were problems with a preamplifier on the chirp sonar, it performed well 

near the sea bottom and delivered good data. Seventeen targets from the side-scan sonar mosaics 

were investigated in four days (Figure 6.8). These included dense grids of X-star lines over the 

GC234 chemosynthetic organism site and the GC233 site. Finally on 22 May, the NR-1

developed problems with a thruster and the cruise was terminated. 
6-16 

Figure 6.8. Tracks of the 1998 submarine NR-1 dives. Background is the TAMU2 side-scan sonar mosaic. Thin 

dark lines are 50-m bathymetry contours. Light lines are submarine tracks. 

6.3 Data and Methods 

TAMU2

 side-scan sonar swaths were georeferenced and combined to make mosaics by C&C 

Technologies, Inc., using proprietary software. The result was a side-scan sonar image of the 

whole of each survey area with pixel resolutions of 4 m for the GC Shallow and GB sites and 8 

m for the GC Deep site. For comparison with other data, such as core locations and NR-1 

surveys, the mosaics were entered into two GIS computer programs, ERMapper and ArcView. 

Interpretations for features in the side-scan mosaics were made from inference using the twodimensional geometry and reference to the 3.5 kHz subbottom profiles collected along ship 

tracks, cores, and submersible observations. In many ways, the mosaic interpretation is similar to 

that of aerial photographs in which the interpreter attempts to recognize features from their shape 

and morphology using results from limited ground-truth data to extrapolate into other similar 

areas. Comparison with 3.5 kHz echo-sounder data was useful because these subbottom profiles 

give the show geometry in the vertical plane as well as detail within the sediments. For example, 
6-17 

the 3.5 kHz data were critical in recognizing faults from the offset of buried layers and 

confirming mounds from the change in seafloor elevation. 

Chirp sonar data from the NR-1 cruise were plotted from tapes recorded during the cruise. 

Navigation data were corrected for offsets occurring between fixes from the mother ship. Tracks 

from the dense surveys over the GC233 and GC234 chemosynthetic organism sites were entered 

into ArcView for more detailed spatial analysis and comparison with pre-existing data. Seafloor 

acoustic signature maps were constructed using criteria similar to those of Lee (1995) and Sager 

et al. (1999), developed from higher frequency subbottom profiler data from submarine NR-1. 

Submarine tracks were plotted for surveys over individual site surveys and areas of similar 

acoustic reflection characteristics were plotted and contoured. The result was a series of microsurveys, typically a few hundred meters across, similar to those in Sager et al. (1999). 

Cores were sectioned and capped at sea. On shore, the cores were split across a diameter to 

expose the interior for description. All cores were photographed and described using techniques 

and sedimentological terminology similar to that used by the Ocean Drilling Program at Texas 

A&M University. The description produced a graphical representation that shows layering, color, 

and large objects such as shells and nodules. For a subset of the cores, a 1-cm slab was cut from 

the core split face and X-rayed to look for variations in density and structure. The X-radiographs 

often show layering, structure, and textural features not evident to the naked eye. 

Several geophysical data maps were gathered from outside sources for comparison with the sidescan sonar and NR-1 profiler data. A map of wipeout zones from 3.5 kHz profiles, published in 

Behrens (1988), was obtained for the eastern half of the GC Shallow area. Two seafloor 

amplitude anomaly maps, derived from exploration 3D multi-channel data, were obtained for 

lease blocks GC184, GC185 and GC272. In addition, a shallow geohazard map, derived from 3.5 

kHz subbottom profiler data, was obtained for lease blocks GC233, GC234, and GC235. 

3D multi-channel seismic data sets were also procured from Western Geophysical, Inc., for 

several blocks in the GC Shallow and GB sites. Data from lease blocks GB424 and GB425 were 

acquired from the GB site, whereas data from blocks GC184, GC185 and GC233, GC234 were 

obtained from the GC Shallow site. These data were examined using PC workstations running 

Kingdom Suite interpretation software. Because these data were received late in the project, only 

limited interpretation was possible. The seafloor horizon was picked on all data sets, and 

bathymetry maps and seafloor amplitude extractions were made. A few lines within each block 

were examined in comparison to existing knowledge of the structure of each of the seep areas. 

6.4 Results 

The TAMU2

 side-scan sonar mosaics yielded wonderfully detailed images of the seafloor that 

show areas disturbed by faulting, seepage, and mass wasting. Areas of dark and light on the 

mosaics are determined by the strength of the acoustic pulse returned to the side-scan sonar from 

the seafloor. Unlike reflection profiling methods, which show variations in reflection coefficient 

across interfaces, side-scan sonar images represent the amount of signal scattered back to the 

sonar from the seafloor. This “backscatter,” as it is called, depends on topography, surface 
6-18 

texture and roughness, and acoustic scattering within the near-surface sediments (Johnson and 

Helferty, 1990). On the mosaics shown in this report, dark areas represent high backscatter 

(“sonar bright”) and light gray areas, low backscatter (“sonar dim”). 

6.4.1 GB site mosaic 

The predominant feature on the GB site mosaic is a sonar bright zone running from the northeast 

corner to the center and from there south (Figure 6.9). A second zone of sonar bright returns 

occurs several kilometers to the west of the first. Both zones represent seep alterations of 

sediments along normal faults (Figure 6.10). The faults themselves are particularly evident in the 

sharp, linear sonar bright zones in the south center and central west parts of the mosaic. The area 

to the east of the central sonar bright zone is a slope that inclines southeastward (Figure 6.6). 

From the central bright zone westward is a plateau of rough bathymetry caused by faulting. The 

north part of the mosaic covers the south side of a salt dome, notable in the bathymetry contours 

(Figure 6.6). 

Several subcircular sonar bright zones, approximately 500 to 700 m in diameter, are evident in 

the mosaic. One is located in the northeast corner, two are in the center, and a fourth is in the 

west central part of the survey. These features are typical of mud mounds formed by fluid 

expulsion at hydrocarbon seep vents (e.g., Neurauter and Bryant 1990; Kaluza and Doyle 1996). 

The southern of the two mounds in the center has been visited during several dives of the 

submarine NR-1 and submersible Johnson SEA-LINK. It has active gas and oil vents on its 

northeast and southwest sides but little activity is evident over its flat summit. The vent on the 

southeast side is also the site of a brine pool, filled with warm, sediment-rich fluid. Around this 

vent, chemosynthetic organisms have been noted, in particular a mussel bed near the brine pool. 

Submersible observations of the seafloor reveal extensive flow-like features suggesting ongoing 

expulsion of fluidized mud. 

An interesting feature of this active mound is a small spot atop the mound, no more than about 

100 m in diameter, that gives little or no sonar return. Such a sonar “dead” spot has been noted 

on several other mounds that have active vents with brine pools. This dead spot may result from 

absorption of the sonar energy by extremely gassy sediments or by specular reflection of the 

sonar waves away from the sonar. 

The subcircular features on the west and northeast sides of the survey are not well known. The 

northeastern feature appears to be a fluid expulsion vent described by Roberts and Carney (1997, 

their Figure 6.11). The feature in the western part of the mosaic appears to be a crater that has 

been partly filled by sediments. On three sides it is a depression, but on its east side, the flat floor 

is raised. Features that look like sediment faulting, failure and collapse appear on its western 

side, implying the feature was originally a crater. Because the sonar bright zone in the center of 

the crater corresponds to acoustic wipeout in 3.5 kHz profiles, the sediments filling the crater 

appear gassy. 

In several locations, linear sonar bright streaks run down slope. Based on their geometry and 

where they start, these are probably sediment flows. A large flow appears to emanate from the 

area between the two central mounds and may be from the vent on the northeast flank of the 

large, southern mound. Streaks emanate from other locations along the main fault in the center of 
6-19 

the survey area, implying fluid expulsion from many points in and along the fault. Seafloor with 

mottled backscatter appears to the north central area of the mosaic. This zone corresponds to the 

steeper flanks of the salt diapir. Thus, the variegated backscatter may represent tectonic 

disturbance and slope failure on the side of the diapir. 

Figure 6.9. TAMU2

 side-scan sonar mosaic of GB site. Dark shades indicate areas of high acoustic backscatter, 

whereas lighter shades represent low backscatter. Thin lines are bathymetry contours at 25-m 

intervals with multiples of 50 shown as solid. Square labeled GB shows the location of the 

chemosynthetic community on the mud volcano. 
6-20 

Figure 6.10. Surface geology interpretation map of GB site. Geologic interpretation was developed using TAMU2

side-scan sonar mosaic and 3.5 kHz echo-sounder profiles along ship tracks. 

6.4.2 GC Shallow mosaic 

Seepage along faults and large salt domes characterizes the GC Shallow mosaic. On the west 

side of the survey, two large salt diapirs cause domes, both associated with sonar bright patches 

(Figure 6.11). The northern diapir, in GC139 and GC140, is conical in shape (Figure 6.4) and has 

faults radiating to its southwest and southeast sides (Figure 6.12). Its summit has a mottled, sonar 

bright appearance caused by a cap of reflective material. In detail, the sonar bright zones contain 

many small, subcircular features, some with shadows on the side away from the sonar (Figure 

6.13). These are likely mounds constructed from precipitated carbonate material that have been 

described on this mound by Roberts and Aharon (1994). In some places, these mounds form 

lineaments that likely indicate seepage and mound formation along a fault. 
6-21 

Figure 6.11. TAMU2

 side-scan sonar mosaic of GC Shallow site. Dark shades indicate areas of high acoustic 

backscatter, whereas lighter shades represent low backscatter. Thin light lines are 50-m bathymetry 

contours, with multiples of 100 m shown as solid lines. Known lush chemosynthetic community sites 

are labeled (BH= Bush Hill, T17=TAMU-17, BP=Brine Pool NR-1, GC=GC234, T6=TAMU-6). 

GC185 is located on the southeast flank of this dome. It appears as a comma-shaped sonar bright 

patch (Figure 6.14), of which the large subcircular portion corresponds to the mound mapped in 

prior work (Sager et al. 1999). Nearby mottled sonar bright zones imply that seepage has 

occurred in other spots near GC185. Particularly interesting is a dark, subcircular sonar bright 

patch about 2 km south of GC185. Like the active mud volcano in the GB425 survey, this mound 

has a light, sonar dead spot in its center. This mound is Bush Lite, an active vent and 

chemosynthetic organism site that has been visited by previous submersible dives (H. H. 

Roberts, personal communication 1995). 

The southern diapir has a circular, pancake shape with steep sides and a relatively flat, rounded 

top (Figures 6.4, 6.11). Steep sides on the dome circumference appear to be fault scarps. Only a 

few sonar bright patches are seen on the top of this dome but on its northeast side are numerous 

highly reflective areas. Those sonar bright patches occur where a fault complex appears to 

connect this dome with one farther north with faults that trend out the southern side of the survey 

area. Within this zone of sonar bright patches are several different types of sonar features. Linear 
6-22 

Figure 6.12. Surface geology interpretation map of GC Shallow site. Geologic interpretation was developed using 

TAMU2

 side-scan sonar mosaic and 3.5 kHz echo-sounder profiles along ship tracks. 

Figure 6.13. Side-scan sonar image of carbonate mounds in Green Canyon lease block 139. The mounds appear as 

small subcircular features with strong backscatter toward the ship track and weak backscatter (or 

shadow) away. Many mounds occur in lineaments that possibly indicate formation along a fault. 
6-23 

Figure 6.14. Side-scan sonar image of the Bush Hill and Bush Lite chemosynthetic community sites. Both appear 

as subcircular, high backscatter features that correspond to the mud mounds at each site. Bush Lite 

has a “dead spot” near its center from which there is no sonar return. Image is from Green Canyon 

lease blocks 184 and 185. 

or curvilinear sonar bright areas correlate to faults and evidently show faults on which seepage 

has occurred. In one location to the northwest of the dome, a lobate sonar bright patch trends 

downslope from the faults (Figure 6.15). This feature may be a sediment flow that has emanated 

from the fault. In the southwest part of this sonar bright area is a dark area that has streaks 

oriented downslope (Figure 6.16). Some of these streaks appear to radiate from certain spots. 

This sonar bright zone appears to be a broad sediment flow (or multiple flows) that has been 

emitted from vents along the faults. Several active mud volcanoes and oil/gas seeps have been 

discovered in the area to the north and east of the vents during previous submersible dives 

(Roberts 1996; Roberts and Carney 1997). 
6-24 

Figure 6.15. Side-scan sonar image of faults and probable sediment flow caused by seepage from fault. Image is 

from Green Canyon lease block 228. 

The east side of the GC Shallow mosaic is dominated by sonar bright patches associated with a 

fault system that rings the head of a shallow canyon that trends southeastward out of the study 

area (Figures 6.4, 6.11, 6.12). Particularly notable is the fault zone located in the eastern central 

part of the survey. Along this fault are located several chemosynthetic organism sites: GC233, 

GC234, Mussel Beach, and two that were discovered during the NR-1 dives, TAMU-17 and 

GC191 (Figure 6.17). In this fault system, most of the sonar bright patches occur very near the 

faults. Indeed, many are located in grabens, i.e., downdropped zones between normal faults. 
6-25 

 

Figure 6.16. Side-scan sonar mosaic of vents and sediment flows in Green Canyon leaseblocks 271, 272, 315, and 

316. Dark, high-backscatter patches indicate pervasive seepage in area along faults at flank of large 

salt dome. A dark area with features trending downslope indicates a large sediment flow or flows that 

appear to emanate from several vents. 

As in the western part of the survey, the eastern fault complex contains many subcircular sonar 

bright patches ranging in size from a few hundred meters to about a kilometer across. These 

appear to be mud mounds constructed by fluid expulsion. With further examination, several 

classes of mound were noted. Some are uniformly dark (sonar bright) and some are uniform, but 
6-26 

lighter. A number of mounds have brighter centers surrounded by less bright haloes. These were 

dubbed “bulls eye” mounds owing to their appearance. Also noted were two mounds that are 

sonar bright with sonar dead spots in their centers. These differences are likely related to the 

degree of mud volcano activity and the progressive buildup of hard material, such as carbonate 

or gas hydrate, atop these mounds. Mound appearances will be discussed in more detail in a later 

section. 

Figure 6.17. Side-scan sonar mosaic of area including Green Canyon leaseblocks 232 to 236. A fault system 

trends east-west across the center of the image and is the locus of much seepage. Feature labels as 

follows: F=fault, M=mound, SF=sediment flow. Known chemosynthetic community sites are labeled 

(T17=TAMU-17, BP=Brine Pool NR-1, T10=TAMU-10, GC=GC234, T7=TAMU-7, T6=TAMU-6). 

At the head of the canyon is a broad, fan-shaped sonar bright area with streaks trending 

downslope. This bright area appears to emanate from a mound at the intersection of three sets of 

faults in the western part of GC234. The geometry of the fan-shaped bright zone implies that it is 

a large sediment flow or series of sediment flows that have originated from a vent at the location 

of the mound. Elsewhere along the faults that ring the head of the canyon are curvilinear sonar 

bright streaks trending downslope. These may be smaller sediment flows caused either by fluid 

expulsion or mass wasting. 

In several places along the eastern fault zone, linear and curvilinear sonar bright features are seen 

along the faults (Figures 6.12, 6.17). These features probably result where faults experience a 

small amount of seepage. At other spots along these faults, small lobate sonar bright zones are 
6-27 

seen trending downslope, probably indicating sites where greater seepage has occurred, leading 

to small sediment flows. 

The fault zone along the eastern central part of the survey area contains four known significant 

chemosynthetic organism sites. Two have similar appearances – dark, subcircular sonar bright 

mounds with dead spots (TAMU-17 and GC191). The two others are remarkable in that their 

locations do not stand out in the mosaic (Figure 6.17). Both GC233 and GC234 are in broad, 

sonar bright zones that do not stand out among the many sonar bright patches in this area. The 

geophysical signatures of these sites are discussed in more detail below. 

6.4.3 GC Deep mosaic 

The appearance of the GC Deep site mosaic is startlingly different than the GB and GC Shallow 

mosaics (Figure 6.18). Sonar bright patches in the shallow mosaics cover a small percentage of 

the total area and are arrayed along faults, mainly resulting from seepage. The GC Deep mosaic 

has a more chaotic appearance with many sonar bright areas scattered throughout the survey 

area. Although little ground truth data is available from the GC Deep area, many features can be 

interpreted from a combination of bathymetry, geometry, and character on subbottom profiles. 

 

Figure 6.18. TAMU2

 side-scan sonar mosaic of GC Deep site. Dark shades indicate areas of high acoustic 

backscatter, whereas lighter shades represent low backscatter. Thin light lines are 50-m bathymetry 

contours. 
6-28 

Many of the sonar bright features in the GC Deep area appear to be a result of sediment mass 

wasting caused by slumping related to uplift and subsidence between salt withdrawal basins and 

salt massif highs. Three salt withdrawal basins are seen in the study area: Longhorn Basin in the 

northwest corner, Pygmy Basin on the south edge, and Tiger Basin in the northeast (Figure 6.5). 

Many of the features in the sonar images appear to be related either to faulting or tectonic 

disturbance of the sediments. Longhorn Basin, in the northwest corner of the survey, has steep 

flank slopes that display sharp headwall scarps where sediments have failed and cascaded down 

into the basin. The flanks appear rough as is expected for faulted, failed slopes. Similar features 

are seen on the northern flank of Pygmy Basin and the eastern flank of Tiger Basin. In fact, faults 

appear to rim the sides and upper flanks of many of the salt massifs between basins (Figure 

6.19). 

Figure 6.19. Surface geology interpretation map of GC Shallow site. Geologic interpretation was developed using 

TAMU2 side-scan sonar mosaic and 3.5 kHz echo-sounder profiles along ship tracks. 

Broad sonar bright patches occur on the floor of all three basins where sediments have 

accumulated owing to slope failure. In Longhorn Basin, such sonar bright sediments ring the 

edge of the basin. Similar sediments are evident in on the northeast side of Pygmy Basin. On the 

floor of Tiger Basin is an especially interesting sonar bright zone. Its shape on the floor of the 

basin is lobate and appears to have curved from east-west to north-south trending (Figure 6.20). 

Furthermore, it terminates on the eastern basin flank and appears to emanate from two small 
6-29 

sources. This bright zone appears to be a large sediment flow that cascaded down the basin flank, 

turned to follow the slope toward the south, and ponded in the basin. The fact that this flow 

traces back to two possible vents suggests that it may have resulted from fluid expulsion, rather 

than slope failure. 

Figure 6.20. Side-scan sonar mosaic of large sediment flow on the floor of Tiger Basin in the GC Deep survey 

area. Dark, lobate feature is flow, which has emanated from sources on the salt uplift flanks that 

border the basin. Lobate flow end suggests ponding on basin floor. Image includes Green Canyon 

leaseblocks 586, 587, 630 and 631. 
6-30 

The side-scan sonar images also show many zones with parallel linear or curvilinear features that 

have the appearance of grooves from the geometry of shadows and bright returns. Typically, 

these grooves trend downslope on the flanks of the salt massifs. These features may be erosion 

gullies caused by rapid downslope transport of sediments on the massif flanks. 

Because of the greater complexity, fewer 3.5 kHz profiles, fewer ground truth data, and lack of 

known seeps in the GC Deep area, it is difficult to pinpoint seeps in this area. Dark, irregular 

sonar bright patches occur in places, such as the north central part of the survey, and these may 

indicate the effects of seepage. Likewise, the flow in Tiger Basin may result from seeps. 

However, it is clearly much more difficult to find seeps with side-scan sonar in the more highly 

tectonized area of the lower slope than on the upper slope. 

6.4.4 Submarine NR-1 dives 

During the NR-1 cruise, the submarine visited 17 targets defined from the side-scan sonar 

mosaic and made detailed mini surveys over 12 sites (Figure 6.8). Two of the mini surveys 

provided extensive mapping of subbottom characteristics from known chemosynthetic organism 

sites, GC234 and GC233. The other sites were previously unexplored. 

One of the goals of the submarine observations was to see which sites are inhabited by 

chemosynthetic organisms. Depauperate communities of chemosynthetic organisms were found 

in numerous locations. These might have included one or a few small, stunted tubeworm bushes, 

bacterial mats, or a collection of chemosynthetic bivalve shells. This observation implies that 

there are many places in the sonar bright zones where hydrocarbons are available near the 

surface in small amounts, insufficient to support lush communities. Clusters of chemosynthetic 

organisms were observed at six new locations during the NR-1 survey. Two sites had particularly 

large concentrations of tubeworms. One is at site TAMU-6 in the southwest corner of GC191 

(27o

 45.8’N, 91o 12.7’W). The other is site TAMU-17, located in the west central part of GC233 

(27o 43.7’N, 91o 18.1’W). In the side-scan sonar mosaics, both appear as subcircular sonar bright 

areas 500 to 700 m in diameter. Furthermore, both have sonar dead spots near their centers. From 

the submarine, the seafloor at both sites was characterized by extensive sediment flows, 

indicating recent and ongoing fluid expulsion. In addition, brine pools were found at both 

locations. Both sites contained abundant tubeworm bushes. The other sites are TAMU-7 (27o

44.1’N, 91o

 11.5’W), TAMU-9 (27o 44.6’N, 91o 14.9’W), TAMU-10 (27o 43.4’N, 91o 15.6’W), 

and TAMU-14 (27o

 42.9’N, 91o 16.6’W). TAMU-9 and TAMU-14 both appear to be medium 

diameter (200-400 m) mud mounds, although the former is a bulls eye mound whereas the latter 

is more homogeneous in appearance. TAMU-10 is the large mound that appears to be the source 

of the large sediment flow seen in the mosaic. In contrast, TAMU-7 displays no obvious mound 

but is a circular high backscatter feature within a large aureole of moderate backscatter. 

Subbottom profiler records in the mini surveys show characteristics that include normal, 

unaffected sediments, acoustic turbidity, acoustic wipeout, hard bottoms, and buried hard 

bottoms (e.g., Sager et al. 1999). In general, the highest backscatter zones correspond to hard 

bottoms. At least one bulls eye feature appears to have a chirp sonar reflection pattern (a central 

buried hard bottom surrounded by acoustically turbid sediments) that matches its side-scan sonar 

image. Other mini surveys are more difficult to interpret because lateral heterogeneity in chirp 

sonar profile character is high. Most sites have combinations of hard bottoms, buried hard 
6-31 

bottoms, turbidity, and wipeout. Interestingly, the relief of the subcircular sonar bright zones 

ranges from nothing to tens of meters, implying that not all circular features are mounds. The 

consistent aspect of these surveys is that these features occur over sites that have the appearance 

of seeps in the side-scan mosaic (usually a subcircular sonar bright feature), whereas other zones 

have lesser evidence of seep disturbance or undisturbed sediments. 

Although observations from the submarine NR-1 found chemosynthetic organism communities 

at several of the hypothesized seep sites, it is notable that many sites appeared barren. Many of 

the mini surveys were limited in extent, so it is possible that by chance chemosynthetic 

communities were missed at some sites where they exist. However, it appears that many of the 

apparent seeps are dormant. In particular, some sites have extensive hard bottoms but no obvious 

vents and chemosynthetic communities. These may be older sites whose lack of evident activity 

or chemosynthetic organisms may reflect the “paving over” of these sites by authigenic 

carbonate precipitation with time (e.g., Sager et al. 1999). This implies that side-scan sonar, or 

indeed any geophysical exploration tool, may have difficulty recognizing which seep sites are 

active, because the geophysical techniques are affected by anomalous physical properties in the 

upper few meters of sediment, which have been deposited over a few thousand years. 

6.4.5 Core data 

Twenty-nine of the 31 cores recovered on cruise 97G4 were examined for evidence of seepage or 

chemosynthetic organisms. The other two were short cores containing copious amounts of oil 

that were given over to chemical analysis. Two additional cores from GC233 were also studied. 

Physical characteristics of the cores were extremely variable. A few showed little evidence of 

seep disturbance (Table 6.2). Typically these were cores that missed high backscatter zones or 

were at their edges. Most cores showed one or two of several types of disturbance and few were 

highly disturbed. The following were disturbances noted in these cores: dense layers, high degree 

of consolidation, gas expansion cracks, strong hydrogen sulfide (H2S) smell, carbonate (and 

clay/carbonate) nodules, crusts, or gravels, shells, and oil. Often the highly disturbed cores 

showed multiple types of disturbance and these cores are typically from near known or suspected 

seep vents. The stiffening, formation of dense layers, and carbonate nodules all probably come 

from precipitation of authigenic carbonate within the pores of the sediment owing to the 

byproducts of microbial degradation of hydrocarbons. Gas cracks are caused by the expansion of 

gas within the pore space as the pressure is relieved from the core; some gas expansion voids 

may result partly from gas hydrate decomposition as well. The strong H2S smell comes from the 

production of that gas in near surface sediments driven to anoxic conditions by microbial 

activity. In sum, it appears that surface sediments from sonar bright zones typically show 

disturbances related to seepage. However, the properties of these sediments show high lateral 

variation, so it is difficult to predict the degree and type of core disturbance from the side-scan 

sonar mosaic. 
6-32 

Table 6.2. Core disturbance properties. 

Core Back-scatter Disturbances Feature 

No disturbance 

GC5-1 M sediment flow in GC278 

GC9-1 H south edge high BS, vent-like zone 

GC19-1 L near high BS on top of salt dome 

 

Low-moderate disturbance 

GC1-1 L gas cracks, oil at bottom between high BS features 

GC1-2 M clay/carbonate nodules, highly consolidated small bulls eye 

GC2-2 carbonate pebbles large bulls eye 

GC3-2 H H2S smell high BS vent-like feature 

GC4-2 M clay/carbonate nodules, highly consolidated south flank, large flow-source mound 

GC6-1 M dense layers sediment flow in GC278 

GC8-2 H highly consolidated south edge of high BS zone 

GC10-1 M dense layers west edge of high BS zone 

GC10-2 M highly consolidated patchy high BS zone 

GC11-1 H clay/carbonate nodules, gas cracks patchy high BS zone 

GC12-1 H clay/carbonate nodules, dense layers high BS zone between faults 

GC13-1 L dense layers near south edge of high BS zone 

GC14-1 H dense layers sediment flow in GC215-316 

GC15-1 M clay/carbonate nodules patchy high BS zone 

GC17-1 M highly consolidated edge of high BS near fault 

GC18-1 M carbonate nodules, gravel, highly consolidated patchy high BS zone atop salt dome 

GC19-2 M highly consolidated patchy high BS zone atop salt dome 

GC20-1 M highly consolidated, H2S smell high BS zone near mound 

GC21-1 L highly consolidated failed basin flank in GC Deep area 

GC22-1 H highly consolidated basin floor in GC Deep area 

 

High disturbance 

GC2-1 H clay/carbonate nodules, H2S smell, oil stain large bulls eye 

GC3-1 H clay/carbonate nodules, H2S smell, oil edge of high BS vent-like feature 

GC4-1 M clay/carbonate nodules, oil stain, gas expansion south flank, large flow-source mound 

GC7-1 H carbonate gravel, shells center of active mud volcano w/chemo 

GC7-2 H oil, highly consolidated SE edge of active mud volcano w/chemo

GC9-2 H abundant oil, tar south edge high BS, vent-like zone 

GC8-1 H Oil high BS area near mound 

GC16-1 H abundant oil, gas expansion, carbonate nodules high BS area near mound 

RN8-001 H oil, gas cracks, clay/carbonate nodules, shells GC233 

RN8-002 H oil, gas cracks, clay/carbonate nodules, shells GC233 

6.4.6 Comparison with other geophysical data 

Wipeout zones. Wipeout on high-frequency echo-sounder profiles is a reflection character 

frequently interpreted as indicating the presence of shallow gas (Hovland and Judd 1988). It is 

also a common characteristic of mud mounds and mud volcanoes (e.g., Neurauter and Bryant 

1990). This characteristic is frequently used to define shallow seeps. The term wipeout actually 

encompasses two slightly different acoustic reflection characters. With true wipeout, subsurface 
6-33 

reflectors disappear laterally, leaving an apparent “void” (Behrens 1988). Acoustic turbidity, 

which is a prolonged bottom echo owing to scattering and reverberation, is often accompanied 

by the loss of subbottom reflectors, so it is sometimes called wipeout (Behrens 1988). Both may 

be caused by free gas because the reflection character of gas depends on the bubble 

concentration and size (Anderson and Bryant 1990). However, acoustic turbidity may also be 

caused by scattering resulting from introduced matter, such as carbonate nodules and shell 

fragments (Sager et al. 1999). 

Comparison of wipeout zones in the GB and GC Shallow sites with the side-scan sonar mosaics 

shows that there is a correlation between wipeout and high backscatter. This is to be expected as 

both are a result of seepage. The cause of the correlation is likely a link between the cause of the 

high backscatter and gas. The cores suggest that carbonate precipitates are common in the high 

backscatter zones and form due to bacterial utilization of the hydrocarbons. 

Examined in detail, the wipeout zones are slightly more extensive than the sonar bright areas 

(Figures 6.12, 6.21). This is likely caused by the fact that high frequency echo-sounders 

penetrate through tens of meters of sediment, and wipeout may occur within that zone, whereas 

sonar backscatter is dependent on the properties of surficial sediments. Calculations indicate that 

most of the returned sonar energy at 12 kHz comes from the upper 1 to 2 m of sediment (Liu 

1997). 

Figure 6.21. Zones of acoustic wipeout and turbidity in GC Shallow survey area. Interpretation made from 3.5 

kHz echo-sounder profiles collected on cruise 97G4 of the R/V Gyre. 
6-34 

Another difference is the amount of detail in the two types of records. Side-scan sonar mosaics 

show far more detail in surface features than do the wipeout maps (Figures 6.12, 6.21). In part, 

this is a result of the difference in resolution, which is determined by the way the data are 

collected. Wipeout is interpreted from single profiles, so the lateral resolution is defined by the 

ship track spacing. In contrast, the side-scan sonar insonifies all of the seafloor and the resolution 

is dependent on the pixel size and the footprint of the sonar (Blondel and Murton 1997). For the 

GC Shallow and GB site mosaics, the pixel size is 4 m, and variations in seafloor backscatter are 

recognizable at a resolution of about 2 pixels. Closely spaced reflection profiles might be able to 

attain similar resolution, as is suggested by modern high-resolution 3D geohazard surveys, but at 

substantially greater cost. The side-scan sonar also achieves greater detail by its dynamic range. 

Wipeout is typically interpreted as present or not, whereas the side-scan sonar can distinguish 

differences in backscatter amplitude, which add detail to the image for interpretation. 

High frequency echo-sounder data do have one distinct advantage over side-scan sonar data in 

defining seeps. Topography and faults are more easily recognizable in the profiler records. The 

topography affects side-scan sonar images but not to the extent of seafloor texture differences. 

On profiler records, topography is simply the undulation in the level of the seafloor return. Faults 

show up on echo-sounder records as offsets in the layering and sometimes the seafloor, if the 

fault is active. On the side-scan sonar images, faults are not evident unless they cause significant 

topography that gives rise to a strong return off a scarp face or a shadow or if there is disturbance 

of the physical properties of the sediments at the fault trace. In the GC Shallow and GB site 

mosaics, many faults were visible. Faults that were not visible do not break the seafloor or 

display significant seepage. 

The best interpretation can be made with a combination of side-scan sonar and echo-sounder 

records, one to show seafloor property variations and the other to show topography and faults. 

This combination was used to make the detailed interpretations shown in Figures 6.10, 6.12, and 

6.19. 

Surface amplitudes from 3D multi-channel seismic (MCS) data. The energy industry and 

support companies have spent considerable time and energy devising ways to use multi-channel 

3D seismic data to highlight small differences in sediment geometries and physical properties. 

With modern seismic workstations, it is often possible to look at numerous signal attributes to 

enhance variations in properties. One readily available attribute is the amplitude of the reflected 

signal at the seafloor. Obviously, this will correlate with the reflection coefficient of the seafloor 

interface, which depends on the change in seismic velocity and elastic parameters (i.e., stiffness). 

Because seeps affect those properties, they should be evident in maps of reflection amplitude 

rendered at the seafloor. This has been suggested as a method of finding seeps and the 

chemosynthetic communities that inhabit them (Roberts 1996; Roberts et al. 1996; Reilly 1996). 

Specifically, these authors suggest that chemosynthetic community sites are characterized by a 

negative reflection coefficient (and consequent phase reversal of the seafloor reflector) owing to 

the presence of near surface gas. 

In this study, the author visited William Shedd at the Minerals Management Service to view the 

results of his study of the correlation of chemosynthetic sites with multi-channel seismic 

amplitude anomalies at dozens of Gulf of Mexico sites. In addition, amplitude anomalies were 

compared with the locations of several known chemosynthetic organism sites in the GB and GC 
6-35 

Shallow survey areas (GB mud volcano, GC185, GC233, GC234, TAMU-17, and TAMU-6). 

The conclusion is that the situation is more complicated than just simply finding a circular 

anomaly with the right coefficient. 

At the very first, it is necessary to note differences in data. Not all multi-channel data are created 

equal. Standard exploration multi-channel data were collected with long streamers and low 

frequencies and are optimized for deep penetration. This limits resolution near the seafloor. 

Indeed, with many exploration data, the seafloor return can be difficult to recognize, especially 

where topographic features intervene. Reprocessed multi-channel data are better (Roberts 1996). 

These are derived from signals from only a small part of the seismic streamer and have been 

filtered to raise the frequency content. Better still are high resolution, short-offset 3D multichannel data optimized for near seafloor imaging (Corthay 1997; Sonnier and Gerlach 1999; 

Campbell 1999). 

Examination of the many study areas in Shedd’s study indicate that in some areas, 

chemosynthetic organism-inhabited seeps are visible as subcircular amplitude anomalies that 

probably reflect a mud volcano. However, in many places there are widespread surface 

amplitude anomalies making it difficult to determine where the inhabited seeps occur. In the data 

from the GB and GC Shallow sites, there was much variation in amplitude signature. None of the 

known chemosynthetic organism sites clearly showed a phase reversal at the seafloor; however, 

this may have been partly a result of the limited resolution of the exploration 3D multi-channel 

seismic data at our disposal. GC185 shows a subcircular, negative amplitude anomaly that 

corresponds to the well-known mud mound (Figure 6.22). With better data, this mound may 

show a phase reversal. The GB mud volcano shows little amplitude anomaly at the seafloor 

(Figure 6.23), despite the fact that it is an active mud volcano. Small features in the amplitude 

image may or may not reflect actual differences owing to the limited resolution of the 3D seismic 

data. Interestingly, the GB mud volcano has a large reflection contrast in the upper few tens of 

milliseconds below the seafloor, as evidenced by strong seabottom multiples (Figure 6.24). 

However, this is not evident at the seafloor itself as seen in Figure 6.23. 

In GC233 and GC234 there are three chemosynthetic sites – TAMU-17, GC233, and GC234. 

Surface amplitude anomalies are highly variable and show features such as flows and bright 

spots around the TAMU-10 mound on which only a few chemosynthetic organisms were found. 

The TAMU-17 site shows up as a strong, bright spot, but GC233 and GC234 do not stand out 

(Figure 6.25). In addition, we also compared a low-resolution surface amplitude anomaly map of 

GC272 with the side-scan sonar map and published figures (Roberts 1996). This is a complicated 

area with many sonar bright features, many amplitude anomalies, and several known mud 

volcanoes (Roberts 1996). Owing to the complexity and number of anomalies, it appears difficult 

to sort out which anomalies result from active seeps that might be inhabited. 
6-36 

Figure 6.22. Zones of acoustic wipeout and turbidity in GB survey area. Interpretation made from 3.5 kHz echosounder profiles collected on cruses 97G4 and 92G14 of the R/V Gyre. 

6.5 Discussion 

6.5.1 Geophysical signature of seeps 

Probably the most significant result from this study relative to the geophysical characterization 

of seeps is an increased awareness of their complexity in geophysical data. The near-bottom NR1 profile data reiterate prior interpretations that the lateral variability of seep effects is high at
6-37 

Figure 6.23. Seismic amplitude rendered on the seafloor from 3D multi-channel seismic data over Bush Hill. Dark 

shades are high amplitude, whereas low amplitudes are light. Bush Hill is at center and displays a 

low amplitude anomaly coincident with the mound. Depth contours in two-way travel time are shown 

at 25 millisecond intervals (18.8 m). 

seeps and chemosynthetic organism sites (e.g., Sager et al. 1999). Often only meters to 

centimeters separate areas with little or no alteration from areas replete with carbonate rock 

outcrops, gas hydrate mounds, gas and oil vents, and chemosynthetic organisms. When one 

attempts to characterize such spatial heterogeneity with an acoustical technique from afar, 

particularly from the sea surface, the resultant signature depends in large part on the type of 

technique and its resolution. Even long range side-scan sonar data and 3D multi-channel seismic 

data, with resolutions of several (side-scan) to several tens (3D) of meters, tend to give a 

blended, average signature that belies the true complexity shown by near-bottom data. 

Despite the uncertainties, we can nevertheless make some general statements. One thing that is 

abundantly clear from our study is the tight link between faults, seeps, and chemosynthetic
6-38 

Figure 6.24. Seismic amplitude rendered on the seafloor from 3D multi-channel seismic data over the Garden 

Banks 425 mound. Dark shades are high amplitude reflections from the seafloor, whereas low 

amplitudes are light. High amplitude returns are seen over the flanks of the flat-topped mud volcano. 

A sediment flow, trending downslope to the east, also displays high amplitude returns. Depth 

contours in two-way travel time are shown at 20 millisecond intervals (15 m). 

Figure 6.25. Seismic amplitude rendered on the seafloor from 3D multi-channel seismic data over Green Canyon 

lease blocks 233 and 234. Dark shades are high amplitude reflections from the seafloor, whereas low 

amplitudes are light. High amplitude anomalies are seen in a ring around the TAMU-10 mound and 

in elliptical patches over two low mounds to the west, including the TAMU-17 site. Neither the Brine 

Pool NR-1 (BP) or GC234 (234) sites stand out in this amplitude anomaly map. Depth contours in 

two-way travel time are shown at 20 millisecond intervals (15 m). 
6-39 

organism sites. We have no example of a significant chemosynthetic community site that is not 

on a significant, active fault. Indeed, all of the large chemosynthetic sites that we have studied 

are on large regional faults or regional fault complexes (a series of related, nested faults). Along 

these faults the intrusion of gas and oil into the shallow sediment column changes the acoustic 

reflection properties in ways that can be exploited by acoustic remote sensing. 

Seeps appear to be associated with the classic high-frequency echo-sounder wipeout signature, 

which is thought to result from attenuation and reverberation owing to free gas bubbles (Behrens 

1988; Hovland and Judd 1988). This is true of the GB425, GC185, Bush Lite, TAMU-17, 

GC234, and TAMU-6 chemosynthetic sites. From the sea surface, most of these sites are 

characterized by large columnar wipeout zones that lie beneath the seep mounds. In near-bottom 

profiler records, the wipeout zones are patchier, reflecting the greater resolution of having the 

acoustic sensor near the seafloor. The same may also be true of the GC233 site, but this site is 

more difficult to interpret from sea surface records because it is small and lost in a geologically 

complex zone. From near the seafloor, the brine pool mound only shows a slight wipeout directly 

beneath the crater lake. This may be a result of the main gas reservoir being deeper than the 

penetration of the chirp sonar. 

In side-scan sonar data, seep related faults show high acoustic backscatter. Cores from the 

affected areas do not always show the same geologic characteristics, but are typically altered 

from normal hemipelagic property in a number of ways (Table 6.2). Changes include 

overconsolidation (stiff clays), formation of hard layers, formation of authigenic carbonate 

nodules, gas and oil charging, and the introduction of chemosynthetic biogenic debris. Another 

potential disturbance is the formation of gas hydrate, but because any hydrate in our cores would 

have dissociated before reaching the surface or shortly thereafter, we have not been able to 

document it. With the exception of free gas bubbles, all of these disturbances should increase the 

amount of acoustic reflection, primarily as a result of scattering. We note that many of the cores 

exhibited carbonate nodules or layers, suggesting that much of the acoustic scattering in seep 

affected areas may result from authigenic carbonate material. Strong reflections might be caused, 

in particular, by carbonate crusts and gas hydrate bodies. 

A feature of special importance to seep geology is the subcircular mud mound. In older surveys, 

with widely spaced high-frequency echo-sounder profiles, these were found primarily when a 

mound was crossed by a ship track (e.g., Neurauter and Bryant 1990). Increasingly, with swath 

bathymetry, side-scan sonars, and 3D multi-channel data, this is no longer necessary. Many such 

mounds appear to be mud volcanoes formed at high flux seep vents. Often they rise several tens 

of meters above the surrounding seafloor and have widths from hundreds of meters to 

kilometers. In side-scan sonar data they appear as subcircular sonar-bright zones, sometimes with 

high-backscatter features indicative of sediment flows emanating from their centers. In multichannel 3D seismic data, mud mounds often show anomalous reflection properties at the 

seafloor. Typically, they have strong reflections, often with a positive reflection coefficient 

(Roberts 1996), but sometimes with a negative coefficient indicative of gas (Reilly 1995). 

In the side-scan sonar mosaics, we imaged many subcircular, sonar-bright patches in the GB and 

GC Shallow areas. In general, there were three different signatures: (1) uniformly highbackscatter (Figure 6.26), (2) a high-backscatter center with a halo of lesser backscatter intensity 

(“bulls eye,” Figure 6.26), and (3) high-backscatter with a central spot having little or no 
6-40 

backscatter (“dead eye,” Figure 6.27). Some of the uniformly high backscatter patches were mud 

mounds, although none that we visited with submarine NR-1 was obviously active. A few 

subcircular sonar-bright patches were merely areas of the seafloor that appear to have had 

moderate seepage sometime in the past, but not enough to build a mound or support 

chemosynthetic communities. At several bulls eye sites, the near-bottom chirp sonar records 

from submarine NR-1 imply that the central high-reflectivity zones are hard-bottoms, which we 

interpret as either carbonate or gas hydrate layers. The surrounding haloes seem to be wipeout 

areas (either attenuation or turbidity) caused by lesser seep-related disturbances to the sediments 

Figure 6.26. Side-scan sonar image of different mound backscatter characteristics. This record shows several 

subcircular high backscatter patches, a large bulls eye mound, and another mound that has a dead spot 

(which shows better on another overlapping swath). The dead eye mound is the TAMU-6 site where 

chemosynthetic organisms were found. This record is from Green Canyon lease block 191. 

and introduction of gas. Interestingly, none of the bulls eye mounds visited with submarine NR-1 

appeared to have significant chemosynthetic communities or evidence for high-flux seepage. We 

speculate that these mounds may be older and have become inactive or sites for only limited 

seepage, perhaps because of plugging of the vents by carbonate or hydrate formation. Dead eye 

mounds in all cases were the site of active brine discharge or brine pools (GB425 mud volcano, 

Bush Lite, TAMU-17, and TAMU-6). We think the dead eye feature may result either from the 

specular reflection of sonar signals away from the sonar off a brine pool interface (recall that the 

sonar “sees” objects by backscatter) or from the near total absorption of sonar signals by 

extremely gassy, fluid-sediment slurries in these brine vents. 

An interesting aspect of our examination of seep mounds is the recognition that many of the 

mounds appear presently inactive when visited by submarine NR-1. This is important because it 

suggests that there are limits to our ability to infer active seep sites from sea surface exploration 

geophysics alone. That active and inactive mounds would appear much alike in side-scan sonar, 

high-frequency echo-sounder, and multi-channel 3D data is reasonable because all of these 

techniques respond to the physical properties of the upper several meters or more. Thus a 
6-41 

recently active, but now dormant mound may appear geophysically very similar to an active 

mound. This implies that other evidence is necessary to decide whether a particular mud mound 

is inhabited by chemosynthetic organisms. 

Figure 6.27 Dead eye mounds. Partial side-scan sonar swaths are shown with three dead eye mounds in different 

parts of the study areas. (A) Bush Lite mound, Green Canyon lease block 184; (B) GB425 mound, 

Garden Banks lease block 425; (C) TAMU-17 mound, Green Canyon lease block 233. 

Another important finding from this study is the realization that the geophysical signatures of the 

upper and lower slopes are so different. On the upper slope, disturbances to the seafloor 

sediments are largely a result of seepage, so seep affected regions stand out. In contrast, on the 

lower slope, seep affected regions are almost lost in a complex tapestry of disturbances that 

mainly result from mass wasting owing to salt tectonics around intra-salt basins and salt massifs. 

These severely limit the effectiveness of side-scan sonars to image seeps on the lower slope. In 
6-42 

addition, high-frequency echo-sounders are almost useless in the same areas owing to the poor 

resolution caused by the large separation of source and reflector (and consequent beam 

spreading). In addition, the geologic complexity also adds to the problem by the reflection of 

sound off rough topography. Although we had no lower slope 3D multi-channel data to examine 

in our lower slope study region, we suspect that such data also are inadequate to the task of 

finding seeps in similar terrains. One reason is the same spreading of sound waves with long 

distances from source to receiver. Another is that the same disturbances to surficial sediments 

that show up on side-scan sonar records will also cause reflection anomalies. In sum, the usual 

geophysical methods that have worked well on the upper slope may be limited in their ability to 

detect seeps on the lower slope. 

6.5.2 Geologic controls on chemosynthetic communities 

The present study has not greatly changed our picture of the geologic factors that constrain 

chemosynthetic community locations. This is not unexpected given that many of the sites studied 

here are those examined by prior workers (Reilly et al. 1996). If anything, this study has 

cemented the link between complex chemosynthetic sites and faults, which is especially evident 

in the geophysical data from the shallow sites (e.g., Figures 6.10, 6.12). In the GB and GC 

Shallow study sites, the known, large chemosynthetic communities are found along regional 

faults. What is less clear is whether all of these sites occur on antithetic faults because we did not 

have the data to map the faults in detail or at depth. Furthermore, we recognize that it is very 

difficult to delineate faults in complex structural terrains, especially when the acoustic 

perturbations caused by seepage interfere. Defining faults near the sea floor with exploration 3D 

multi-channel data is difficult owing to the low resolution. High-resolution high-frequency data 

is often limited in its subsurface penetration and by acoustic wipeout. As a result, we still cannot 

make detailed maps of faults in the shallow subsurface and this limits our ability to understand 

how faults control seepage. 

One thing made clear by the chirp sonar data collected near the seafloor by submarine NR-1 is 

the high lateral variability of seep effects along the fault zones that were investigated. The high 

variability implies that small-scale structure is important in determining the location, intensity, 

and duration of seepage. While large-scale faults likely determine where seeps occur in the broad 

sense, the small scale structure of seep sites is likely controlled by near surface faulting and 

modification of the conduits by seep products, such as gas hydrate and authigenic carbonate. In 

several locations in our study area, it appears that large faults are modified near the seafloor by 

breakage of the near-surface sediments into small blocks by slope instabilities (Reilly et al.

1996). The result is that the distribution of seeps is controlled in part by the system of nearsurface faults that develop. 

The side-scan sonar data from the GC Deep survey area suggest a somewhat different picture 

than do the shallow data. In the deep area we have been hampered in understanding seep 

occurrence by having few ground truth data and by the fact that high-resolution is difficult to 

achieve using geophysical data collected at the sea surface. Nevertheless, the structural style of 

the GC Deep survey area is very different because much of the surface is disturbed by sediment 

mass wasting owing to slope failure caused by salt uplift and because many of the faults are 

directly related to the boundaries between salt massifs and salt-withdrawal basins. Others have 

suggested that most seepage occurs along faults ringing the basins (Sassen et al. 1999) and this 
6-43 

seems to fit our data. Current data sets available to us are inadequate for comparison of these 

faults to the shallower structures and test models such as that proposed by Reilly et al. (1996). 

Furthermore, because energy companies have only recently begun exploration of the deep slope, 

few studies have been done about seeps in this environment. This is clearly an area needing 

future research. 

6.5.3 Searching for chemosynthetic community sites 

One of the primary motivations for this study was to consider how geophysical remote sensing 

methods may be used to find chemosynthetic community sites. The good news is that 

geophysical methods can identify a small fraction of the seafloor that might be inhabited by 

chemosynthetic organisms by finding evidence of seep-related disturbances to surficial 

sediments. The bad news is that seeps are geologically complex and have high lateral variability, 

so there is no geophysical method that unequivocally and reliably identifies the inhabited seep 

sites. 

It is useful to compare our data and knowledge to the chemosynthetic community site 

identification protocol proposed by Reilly (1995, Figure 6.1). The present study cannot provide a 

definitive test of Reilly’s approach because the number of sites that were studied is small. 

Nevertheless, our data suggest certain shortcomings. 

The topmost level of Reilly’s search is to look at the sea surface for an oil slick using highaltitude or space-platform aerial photography, radar imagery, or a similar method (e.g., 

MacDonald et al. 1993; 1996). The slick map published by MacDonald (1996) shows slicks that 

emanate from the area of GC185, TAMU-17, and GC234. In addition, slicks have been detected 

over the GB site (MacDonald et al., in press). While this is encouraging, it is difficult to trace a 

given slick to a particular chemosynthetic community site because the slicks are large, the 

chemosynthetic community sites small, and there is likely some drift of the slick as the 

hydrocarbons ascend through the water column. Furthermore, as noted by MacDonald et al.

(1996), the correspondence of slicks with chemosynthetic community sites is good, but not 

universal. This may occur because the flux of hydrocarbons at the seafloor is variable. 

MacDonald et al. (1996) suggest that the most robust slicks are those that emanate from high 

flux vents, such as mud volcanoes. Additionally, the flux may change with time owing to 

changes to the hydrocarbon supply to the vent. At the GB site, which has shown active venting 

on several visits by submersible, it was noted that a prominent slick is sometimes present, but at 

other times is absent (MacDonald et al., in press). In short, it appears that the absence of a slick 

does not mean the absence of a chemosynthetic community, but the presence of a slick greatly 

increases the chance that such a community exists on the seafloor somewhere in the vicinity of 

the upwind end of the slick. 

Wipeout zones constitute the second level indicator in Reilly’s protocol. He specifically 

mentions this signature on high-frequency echo-sounder profiles, such as 3.5 kHz sonars, but 

wipeout may also occur on records at other frequencies. In addition, Reilly does not distinguish 

between the two different types of wipeout signature, attenuated signal and turbidity. Since we 

have no example of a Gulf of Mexico chemosynthetic community site that does not display 

wipeout in its environs, this appears to be a reliable indicator. However, it was observed that 

wipeout zones are typically much more extensive in area than the chemosynthetic sites (for 
6-44 

example, Figs. 6.21, 6.22). Thus, it is usually not possible to use the wipeout signature to define 

the exact location of a chemosynthetic community site. 

Reilly (1995) contends that chemosynthetic community sites are indicated by negative amplitude 

anomalies at the seafloor in multi-channel seismic data. The negative anomalies are recognized 

by a phase reversal at the seafloor and are a result of low acoustic velocities in the surficial 

sediments owing to gas. Furthermore, he contends that amplitude anomalies characterized by a 

positive reflection coefficient (i.e., no phase reversal) are indicative of hard bottoms and are 

unlikely to be the sites of significant chemosynthetic communities. Although the present study 

has only a limited amount of multi-channel data over a small number of sites for comparison, it 

appears that this indicator is unreliable. A phase reversal is not evident at the seafloor at known 

chemosynthetic organism sites GB, GC, GC185, and TAMU-17. Part of the problem may be that 

the data we examined is exploration 3D multi-channel data in which the long acoustic streamers 

used make for a poor geometry to define the seafloor. As a result, it is often difficult to see a 

phase reversal over a small patch of seafloor. High-resolution data collected for seafloor 

geohazards studies might work better. However, given the results of our limited analysis, it 

appears that the reflection coefficient is not diagnostic. 

The occurrence of C2-C5 hydrocarbons in piston cores is suggested by Reilly (1995) to be 

another indicator of lush chemosynthetic community sites using the logic that if a highermolecular weight hydrocarbon signature is present, it is likely that the full range of 

chemosynthetic organisms are also present. We could not do an adequate test of this hypothesis 

in the present study because only a small number of the cores were analyzed for geochemistry. 

Oil or oil stains were found in cores GC1-1, GC2-1, GC3-1, GC4-1, GC7-2, GC8-1, GC9-2, 

GC16-1, RN8-001 and RN8-002. Of these, cores GC1-1, GC2-1, and GC8-1 were obtained from 

sites where no significant occurrence of chemosynthetic organisms was noted during the 1998 

NR-1 dives. Cores GC3-1 and GC4-1 were recovered from sites where only small, limited 

chemosynthetic sites were found (TAMU-7 and TAMU-10, respectively). Although it is 

possible that significant concentrations of chemosynthetic organisms exist at these locales, but 

were missed by the NR-1, the indication is that higher-molecular weight hydrocarbon occurrence 

in the surficial sediments is more widespread than the occurrence of lush chemosynthetic 

communities. Chemosynthetic communities seem to require sustenance by sustained, robust 

seeps, but it is probable that heavier hydrocarbons appear in locations along lower flux seeps 

where the hydrocarbon availability is insufficient to support a lush community. Thus, the 

appearance of higher molecular-weight hydrocarbons in a piston core is a necessary, but not 

sufficient, indicator of chemosynthetic community occurrence. 

Reilly (1995; 1996) recognized the importance of faults in controlling the location of seeps and 

chemosynthetic community sites. Implied in his protocol is the assumption that all 

chemosynthetic sites occur on faults. He went one step farther, distinguishing two types of fault: 

(1) a regional fault, typically connecting uplifted salt masses, and (2) a local, salt dome fault, 

usually directly above a rising salt diapir. According to Reilly (1995; 1996), only the regional 

faults are sufficiently active to produce the continuity of seepage needed to support lush 

chemosynthetic communities. In contrast, he proposed that salt dome flank faults leak 

hydrocarbons intermittently and are too intermittent for the development of large, multi-species 

chemosynthetic communities. Our data are not inconsistent with this view. GC185 and Bush Lite 

are on an antithetic fault rooted in a regional growth fault connecting diapirs (e.g., Cook and 
6-45 

D’Onfro 1991). Several chemosynthetic communities occur in GC232 to GC236 (TAMU-17, 

GC233, TAMU-10, GC234, TAMU-7, and TAMU-6) along a set of regional faults that occur 

around the rim of an intra-salt basin and which connect the crests of buried salt bodies (Figure 

6.17; e.g., Behrens 1988). In addition, the seep site on the mud volcano at the boundary of 

GB424 and GB425 also occurs on a major regional growth fault. While this dichotomy seems to 

apply to the limited data set in this study, caution is probably in order for a broader application. 

In many cases, local salt dome flank faults may connect with or grade into larger regional faults, 

making a distinction between the two problematic. Furthermore, there are few examples of seeps 

on salt dome flanks, so a generalization may be premature. 

The final step in Reilly’s (1995) chemosynthetic community search protocol is confirmation of 

inhabitation by sea bottom photography or submersible observations. Given the complexities of 

seeps and the patchiness of chemosynthetic communities, direct observation is probably the only 

method of absolute confirmation. 

Reilly’s approach to locating chemosynthetic community sites was to make a cascading flow 

chart of increasingly detailed observations (Figure 6.1). Virtually all of these observations are 

cogent, but the representation as a decision tree, in which certain observations lead to a rejection 

of a given location as a potential site, has some suspect elements and should be used with 

caution. Because of the uncertainties in our knowledge of chemosynthetic community sites, 

deciding whether a site is likely to be inhabited should be a process of gathering evidence either 

for or against (Table 6.3). Unless data exists that directly confirms habitation, such as bottom 

photographs of chemosynthetic organisms, submersible observations of the same, or cores or 

trawls containing chemosynthetic organisms, one must consider the preponderance of geologic 

and geophysical evidence and decide whether there are sufficient indicators that are consistent 

with the presence of chemosynthetic organisms. Mostly these are indicators of a robust seep. 

For many purposes, it may be sufficient to define seep-affected areas along surficial faults as 

potential chemosynthetic community sites to be avoided in offshore production operations. In the 

GC Shallow area, these areas cover about 11% of the total seafloor, although some lease blocks 

are covered 50% or more (e.g., GC139, GC140, GC232-GC235, GC272, GC320-GC321, and 

GC363-GC364). Definition of seep-affected area might be approached with 3.5 kHz data by 

looking for wipeout signatures, with side-scan sonar images by looking for high backscatter 

areas, or with multi-channel 3D seismic data by looking for anomalous reflection at the seafloor 

or shallow subsurface. If an area apparently affected by seepage is deemed important enough for 

operations, more detailed investigations, including high-resolution geophysics, coring, ROV 

photography, or submersible observations can be made. 

6.5.4 Directions for future research 

Knowledge about the geologic controls of chemosynthetic communities still has several 

significant gaps. Most research to date has focused on a small number of sites, so conclusions 

about chemosynthetic community occurrences, even on the upper slope, is limited by the 

generality of those sites. We had hoped that the submarine NR-1 dives on many targets in the 

shallow seep study areas would illuminate the distribution of chemosynthetic sites, but bad luck 

dictated that only a small number of sites was visited. Increasing the number of seep sites 

investigated should be a high priority. This should include investigations into the number within 
6-46 

Table 6.3. Chemosynthetic community site indicators. 

Indicator Data type Likelihood

Deep rooted fault Multi-channel 2D or 3D seismic data M 

Acoustic wipeout High frequency echo-sounder profiles M 

Strong amplitude anomaly Multi-channel 2D or 3D seismic data M 

Mound along fault Swath bathymetry; 3D multi-channel seismics, 

geohazard survey M 

Confluence of regional faults Multi-channel 2D or 3D seismic data H 

Sea surface oil slick Aerial photography; space photography; 

synthetic-aperature radar H 

Gas stream in water column High frequency echo-sounder profiles H 

High backscatter along fault Side-scan sonar H 

Chimney-shaped acoustic wipeout Multi-channel 2D or 3D seismic data H 

Strong amplitude anomaly along fault coincident 

with mound 

Multi-channel 2D or 3D seismic data H 

High backscatter along fault coincident with 

mound 

Side-scan sonar VH 

Strong amplitude anomaly along fault, negative 

reflection coefficient, coincident with mound 

Multi-channel 2D or 3D seismic data VH 

Presence of small mounds Near bottom high frequency echo-sounder profiles VH 

Presence of small mounds Near bottom side-scan sonar swaths VH 

Oil, gas Cores VH 

Chemosynthetic shell debris Cores C 

Images of chemosynthetic organisms ROV video, camera sled, submersible C 

*Likelihood of chemosynthetic community code: M = moderate, H = high, VH = very high, C = confirmed 

a given area, so that we have better knowledge about the density of sites. It should also include 

an effort to obtain a broader picture of chemosynthetic sites in different structural settings. To 

date, most investigations have been on the Louisiana upper slope. The morphology of the lower 

slope is different from the upper slope, suggesting that controls on seeps may be different. In 

addition, the style of faulting changes from west to east across the continental slope and this may 

change the controls on seep occurrence. Because a large amount of geophysical data already 

exists for the Gulf of Mexico, collected by the energy industry, it would be prudent to consider 

obtaining geophysical data on seeps from a variety of settings and using computer GIS and 

database technology for assimilation and study. 
 7-1

7.0 Spatial and Temporal Patterns in Seep Communities* 

7.1 Introduction 

Questions about the distribution and persistence of chemosynthetic communities in the northern 

Gulf of Mexico have to be posed in terms of scale. Over distances that span the Western and 

Central Lease areas, occurrences of chemosynthetic communities are determined by the presence 

of hydrocarbon reservoirs. Leasing and production trends demonstrate conclusively that a 

massive hydrocarbon system extends across virtually the entire continental slope (Figure 2.1). 

The tendency of this system to produce viable chemosynthetic habitats is confirmed, to the limits 

of exploration, by direct observation and collection of the fauna (Figure 2.2). Likewise, the 

prevalence of mineral deposits formed by biologically mediated processes (Roberts and Aharon 

1994, Roberts and Aharons 1993, Sassen et al. 1989) and the abundant fossil remains of 

chemosynthetic species (Powell et al. 1998) convincingly show the persistence of 

chemoautotrophic processes in the Gulf of Mexico throughout recent geologic time and probably 

the entire Pleistocene. Therefore, chemosynthetic fauna supported by hydrocarbon seepage are a 

broadly distributed and well-established component of the continental slope ecosystem. 

However, effective management of this ecological component requires the capacity to predict 

where and in what configuration individual chemosynthetic communities occur and to detect 

unusual changes in these communities; this capacity must be scaled to the activities under 

management, that is energy exploration and production. The various components of this program 

address different ranges of this scale. Findings concerning the physical oceanography of the 

shallow mega-site (Chapter 5) document circulation patterns and temperature ranges that typify 

much of the central slope. Geophysical surveys (Chapter 6) identify seep-affected zones and 

show the influence of geological features such as salt diapirs and growth faults. The surveys also 

indicate major differences in seepage style between the shallow and the deep zones — 

differences that have a largely undetermined effect on community formation. At the other end of 

the scale, the ecological and geochemical investigations (Chapters 8, 9, and 10) are largely 

directed toward processes within individual clusters of chemosynthetic species, although site-tosite comparisons certainly apply, particularly in the results from the trophic and the genetic 

studies. 

At an intermediate scale, however, a major management concern remains the so-called lush 

chemosynthetic community. What constitutes a lush chemosynthetic community? How large an 

area does it cover? What densities of chemosynthetic species and what sorts of normal variation 

do such communities experience over a multi-year timeframe? The program study sites were 

chosen because they typify lush communities. Quantifying the dimensions, densities and faunal 

composition of these sites provides a baseline against which communities newly discovered can 

be compared. Mapping the fine-scale limitations posed by seep geology at each site creates a 

template that can be used to refine the search for lush communities in future surveys. Temporal 

change is problematic because observations have been sporadic, due to the availability of 

sampling platforms, and because the time-scales over which observations have been carried out 

 

* This section was authored by Ian R. MacDonald. 
 7-2

are relatively short. Case studies of evidence for variability or persistence can nonetheless 

identify general tendencies or processes that are relevant to management timeframes. 

This chapter will marshal evidence for spatial and temporal patterns at the four principal sites. 

Available data includes surveys conducted during the present program and data gathered during 

previous efforts and reanalyzed with use of updated techniques. Clearly, having a small number 

of sampling sites limits the generality of these results. However, the four sites represent two pairs 

of communities supported by distinct styles of seepage: sediment diffusion versus focused flow 

and the pooling of hypersaline fluids. The specific objectives of this work are as follows: 

1. To provide a general description of the relative locations of sampling stations within the four 

sites, along with the overall dimensions and bathymetry of the sites; 

2. To demonstrate the clustered distribution of tubeworms and mussels within the study sites 

and quantify the spatial coverage of these fauna; 

3. To detail the evidence for geological processes, such as faults, gas hydrate deposits, or fluid 

flows, that facilitate or limit the ability of chemosynthetic fauna to colonize the sites; 

4. To examine evidence of temporal change within the sites by use of geological interpretation, 

time-series observations, and satellite remote sensing of oil venting. 

7.2 Materials and Methods 

Datasets and other materials discussed in this chapter are summarized in Table 7.1. All data were 

compiled as GIS data layers to facilitate comparison over multiples scales or time periods. The 

individual datasets and analytical methods are described in further detail below. 

7.2.1 Survey results from submarine NR-1

Submarine NR-1 provides unique capabilities for surveying chemosynthetic communities. NR-1 

is nuclear powered and can remain submerged in continuous operation for tens of days. 

Compared with other scientific submarines, it is a very stable platform from which to deploy 

optical and acoustic imaging systems. It employs a precise inertial navigation system that tracks 

its relative position during near-bottom operations. Notably, it continually records operational 

data such as its position, heading, and altitude above bottom, which makes it possible to 

reproduce highly accurate tracklines and to mosaic data collected along track into large area 

images or interpreted maps. NR-1 has been used for several surveys of chemosynthetic 

communities in the Gulf of Mexico beginning in 1987 and continuing through the cruise made in 

1998 under the auspices of the CHEMO II program. Survey methods and capabilities employed 

with NR-1 have been described in several publications (MacDonald et al. 1990b, MacDonald et 

al. 1997, Reilly et al. 1996, Sager and MacDonald 1998). In addition, detailed bathymetric maps 

and side-scan sonar mosaics from study sites GC185, GC234, and GB425 were presented in the 

previous program, The Chemosynthetic Ecosystem Study (MacDonald et al. 1995). Wherever 

possible, these datasets were re-analyzed and co-referenced, geographically, with newer results 

to provide continuity and synergy with previous work (Table 7.1). 
 7-3

Table 7.1. Summary of datasets analyzed and discussed in this chapter. 

Datasets are organized by study site, with collection details in parentheses. 

GC185 

Laser line-scan mosaic and bathymetric data (collected with NR-1, 1994). 

Time-lapse video sequence of surface gas hydrates 

 (recorded Sept. 1995 through Jan. 1996). 

Ground-truth observations and photo-mosaics 

 (collected with SEA-LINK, 1997 and 1998). 

Piston cores (collected from surface ship June 1998). 

GC234 

Side-scan sonar mosaic and bathymetric data 

 (collected with NR-1, 1990, revised for present work). 

X-Star subbottom profiles (collected May 1998). 

Ground-truth observations and photo-mosaics 

 (collected with SEA-LINK, 1997 and 1998). 

Piston cores from surface ship (collected June 1998) 

3D seismic survey data (obtained from Western Geophysical, Inc.). 

GC233 

CTD profiles of brine pool (collected with SEA-LINK, 1991, 1992, and 1998). 

Laser line-scan mosaic and bathymetric data (collected with NR-1, 1994). 

Ground-truth observations and photo-mosaics 

 (collected with SEA-LINK, 1997 and 1998). 

Piston cores from surface ship (collected June 1998). 

X-Star subbottom profiles (collected May 1998). 

GB425 

Laser line scan mosaic and bathymetric data (collected with NR-1, 1994). 

Ground-truth observations and photo-mosaics 

 (collected with SEA-LINK, 1997 and 1998). 

CTD profiles of brine pool (collected with SEA-LINK, 1997 and 1998) 

Temperature time-series from brine pool (recorded 1997-1998) 

Satellite synthetic aperture radar images (collected by RADARSAT, 1997-1998) 

7.2.2 Laser line-scan mosaics 

Laser line-scan system (LLSS) technology uses a solid-state laser and a rotating mirror to 

illuminate individual scan-lines. By passing the device across the bottom in a track perpendicular 

to the scan-lines, a LLSS can image a swath of the seafloor up to 30 m wide with centimeter 

resolution. During a cruise in 1994, Submarine NR-1 was used for collection of contiguous 

LLSS images that could be assembled into a geo-rectified mosaic covering thousands of square 

meters of the seafloor. Construction of mosaics requires ER-MAPPER and PV-WAVE 

computational programs on UNIX workstations and additional image-processing routines on PC 

workstations. The methods are described in detail by MacDonald et al. (1997). 
 7-4

During a research cruise in 1994, data were collected for large-area mosaics of three of the four 

sampling sites: GC185, GC233, and GB425. The mosaics for these sites were completed prior to 

the 1997 SEA-LINK cruise. At three of the four study sites, the sketch maps were based on laser 

line-scan mosaics covering all or most of the site. These mosaics provided detailed information 

about the location of prominent landmarks and faunal clusters. Mosaics also provided an 

accurate distance and bearing scale for locating individual sampling stations. 

To combine the information in the laser line-scan mosaics with the selection of sampling stations 

accomplished during the submersible cruise, the following procedure was adopted. First, the 

mosaics were geo-rectified in the ER-MAPPER software environment. Each image was 

registered, without warping, to the best available coordinates of some prominent feature within 

the site, preserving the scale and rotation information calculated when the mosaic was 

constructed. Latitude and longitude for any point in the mosaic was then available through the 

software. The video information and notes from the scientists in the submersible’s sphere were 

used to locate each sampling station within the mosaic. Details of the mosaic are adequate for 

this in most cases. Finally, the geo-rectified images were queried in ER-MAPPER to determine 

the interpolated coordinates of each sampling station. Comparison of the interpolated coordinates 

with coordinates obtained by the SEA-LINK acoustic navigation system confirmed that there 

was general agreement between the two methods. 

7.2.3 Subbottom profiling 

Subbottom profiles and large-scale visual reconnaissance were conducted in May 1998 from 

submarine NR-1. The profiler was a 2-12 kHz chirp system (Edgetech X-Star), which employed 

a deep-ocean transducer array hard-mounted on the submarine’s box keel. Operation of the 

instrument and data logging was done from inside the submarine. During subbottom profiling, 

the submarine was steered at a constant depth along a straight trackline – generally east-west or 

north-south. Navigation was accomplished with the submarine’s inertial positioning system, 

regularly updated with fixes from a short baseline acoustic tracker operated by the support vessel 

(DSR CAROLYN CHOUEST). The subbottom profiles were recorded digitally and could be 

replayed, filtered, and contrast-enhanced during analysis. 

7.2.4 Geographic co-registering 

Extensive information was collected and compiled from the GC234 site during a 1990 cruise 

with submarine NR-1. Although positions recorded by the inertial navigation system were 

precise, this cruise pre-dated availability of GPS and acoustic positioning data for NR-1 and her 

tender. A small but unknown bias, therefore, offset the grid of side-scan, bathymetric, and visual 

observations made during this cruise. To co-register these data with the grid of observations 

recorded during the 1998 cruise, the bathymetric data generated by both surveys were fitted to 

surfaces or digital elevations models (DEM) with a grid size of 1 m. The two surfaces were then 

iteratively compared until an alignment was found that produced the minimum difference 

between the two surfaces. The 1990 data were then shifted by the offset required to bring the two 

surfaces into alignment. 
 7-5

7.2.5 Satellite image processing and interpretation 

SAR images that were obtained from RADARSAT (a Canadian satellite operator) and were 

analyzed to monitor oil venting in the vicinity of the GB425 site have the following reference 

numbers and acquisition dates: COO115630 wide 1 (19 May 1997), 8374-1219180 wide 1 (5 

June 1997), MO120437 wide (29 June 1997), M0193062 SN1 (6 Aug. 1997), M0192826 wide 1 

(27 Nov. 1997), C0014225 wide 1 (24 Feb. 1998). This is the complete inventory of images that 

were archived after collection and were collected under weather conditions compliant with 

visualizing floating oil. 

Wind speeds in surface areas examined in the SAR images were in the range of 2 to 7 m s-1, 

based on estimates provided by Enfotec Technical Services Inc. Surface current speeds were in 

the range of 0.1 to 0.3 m s-1 as estimated from satellite altimetry. Radar signals received from 

layers of floating oil have a low backscatter coefficient and appear “dark” compared to adjacent 

radar “bright” areas. Natural layers of floating oil—so called oil slicks—are distinguished from 

biogenic surfactants by having a few broad and parallel bands, which have distinct termini and 

often describe acute-angle curves as floating oil drifts with wind and current (Espedal and Wahl 

1999). 

7.3 Results and Discussion 

7.3.1 GC185 

The GC185 site is one of two study sites that were defined in the sampling design as sediment 

diffusion sites. The second sediment diffusion site is GC234. A chemosynthetic community at 

GC185, which is also known as Bush Hill, has been very widely studied since its discovery in 

1986 (MacDonald et al. 1989). It has become a de facto type specimen for the so-called lush 

chemosynthetic community defined by management concerns (MMS 1988). Notably, the 

community has, since 1989, coexisted with an energy production in Jolliet Field, where the 

world’s first tension leg well platform was installed in 536 m of water. The field consists of 

multiple, stacked reservoirs that formed in association with a major growth fault and slump 

block. The gravity slide occurred when sediment was tilted by a salt diapir rising to the westnorthwest during Lower Pleistocene (Cook and D’Onfro 1991). Regional bathymetry in the 

vicinity of the GC185 site clearly shows it to be situated on the flank of a canyon created by this 

slide (Figure 6.4). Part of the secondary or antithetic fractures emanating from the growth fault 

intersects the seafloor along a north-south trace that defines the GC185 community, as well as a 

smaller seep (Bush Lite) to the south (Figure 7.1). Reilly et al. (1996) consider the site to be a 

clear example of the “fault-dominated” seepage style that, these authors assert, uniquely supports 

complex communities of tubeworms, mussels and associated species. Roberts and Carney (1997) 

describe the setting as an example of a transitional seep, and cite it as “the most extensive 

[known] development of a chemosynthetic community.” Additionally, the influence of shallow 

gas hydrate deposits upon the surface geology and chemosynthetic community development has 

been widely noted and discussed (Carney 1994, MacDonald et al. 1994, Reilly et al. 1996, 

Roberts and Carney 1997, Sassen et al. 1994b). In this context of previous results, a careful reexamination of the biological and geological features of the site serves to refine and extend 

understanding of Gulf of Mexico chemosynthetic communities. 
 7-6

Figure 7.1. Generalized view of fault system and mound formation at the GC185 site (adapted from Reilly et al.

1996). (A) Plan view of fault trace and mound location. Chemosynthetic community labeled Bush Hill

corresponds to the GC185 study site. (B) Schematic diagram of subsurface faulting and salt structures 

in the region near GC185 site. Note the high-angle or antithetic fault that forms the principal migration 

conduit for hydrocarbons reaching the community. 

A 

B 
 7-7

Use of laser line-scan survey data, processed with satellite image processing techniques, yields a 

definitive map of the major community components extending over an area of 27,650 sq. m, 

which is more than 80% of the mound (Figure 7.2). The laser line-scan survey was completed 

with coverage of virtually the entire mound, omitting only the steep escarpments on the western 

margin where imaging was problematic. Review of the raw data revealed that much of the area 

was unoccupied by the larger chemosynthetic animals (tubeworms and mussels). These portions 

of the survey were not processed for inclusion in the final map. Interpretation of the image was 

greatly enhanced by ground-truth observations of the fauna during the 1997 and 1998 SEALINK cruises. Tubeworm clusters and mussel beds were quite evident, for the most part, in this 

image. Components could be quantified in terms of the area each feature occupied (Table 7.2). 

Tubeworm clusters, a major component of the community were categorized according to their 

densities. Questions of identification were resolved, for the most part, by review of the video and 

photographs taken by SEA-LINK. 

One very abundant component of the community was bacterial mats, which were evident as 

mottled patches virtually throughout most of the mosaic, becoming less abundant to the eastern 

side of the area. Mats tend to be ephemeral and are therefore impossible to verify. Bacterial mats 

were not quantified. Another problematic feature component was areas of dead mussel shells and 

rubble that were often found on the margins of living assemblages. Figure 7.3 shows this mixture 

of living and dead components near the juvenile tubeworm station BHJT1. Difficulties 

distinguishing the defunct remains of former aggregations from living animals posed a minor 

source of uncertainty regarding the total areas affected by seepage and occupied by 

chemosynthetic animals. 

Despite these difficulties, the laser line-scan mosaic of GC185 probably represents the largest 

area over which a comprehensive census of a deep-sea community has been completed. Analysis 

of this product shows that chemosynthetic animals occupied less than 2% of the total area of the 

mosaic. Authigenic carbonate, an additional indicator of hydrocarbon seepage (Roberts and 

Aharons 1993), occupies only an additional 2.2% of the area. Most of the area of the community 

consists of soft sediments that are widely colonized by Beggiatoa. The distribution of 

chemosynthetic animals and carbonate describes a distinctly linear pattern that closely parallels 

the fault trace described by Reilly et al. (1996). Tubeworms were the most dispersed component 

of the community (apart from bacterial mats), while most of the mussels and all of the 

outcropping gas hydrate, both of which are dependent on rapid gas flux, were focused in two 

small areas near the crest of the mound. This result is qualitatively similar to the original 

mapping of the GC185 community by MacDonald et al. (1989). However, owing to limited 

navigation and sampling, that work failed to discern the linear, north-south alignment of the 

community. Additionally, the previous mapping used a kernel smoothing routine to predict a 

broad, smoothly extended region over which tubeworms would be found. Discontinuities in the 

occurrence of tubeworms distinguished in the laser line-scan data contradict this finding. 

Carbonate, although it occupied the largest area of the community, was also largely restricted in 

distribution to the western margin of the site. Much additional carbonate structure could be found 

on the western slope of the mound. This region comprises the active margin of the fault, and the 

offset from the surrounding seafloor indicates protracted activity of this fault margin. The active 

biological community, however, is displaced 20 to 30 m easterly from this margin. 
Figure 7.2. Detailed bathymetry (1-m contours) of the GC185 site (see Fig. 6.4 for regional bathymetry). (A) Mosaic of 18 lase

mound overlaid with sampling stations, current meter mooring, and piston core sites (BHPST1-3). (B) Areas of individumussels, and carbonate outcrops are plotted based on interpretation of the full-scale mosaic 

7-8 
7-9

Figure 7.3. Detailed photomosaic of station JT1 in GC185 shows scattering of dead shells and live mussels, sparse

tubeworms and carbonate rubble. Black and white marker is 30 cm wide. Juvenile tubeworms have a 

coiled growth form and have settled on hard substrate next to the marker. 
7-10

The outcropping deposit of gas hydrate near the center of the community (point HY1 in Figure 

7.2A) has been the source of much discussion in the literature. MacDonald et al. (1994) noted 

the tendency of the mound to change radically in shape from one year to the next and speculated 

that pieces of the deposit may break free and float away. Sassen and MacDonald (1994) found 

pentane in the sediments overlying this mound and speculated that the hydrate might include 

structure-H crystal forms. Although this was not confirmed by subsequent work (Sassen et al.

1998), the proximity of tubeworm aggregations to the hydrate outcrop was noted. In a general 

sense, Carney (1994) has suggested that gas hydrate might act as a buffer that would retain 

hydrocarbons in surface sediments and extend the possibility for biological interactions. The 

dynamics of gas hydrate at the site and possible interaction between it and the biological 

community is therefore of interest. 

Table 7.2. Areas occupied by components of the chemosynthetic 

community at GC185 that were identified in laser line-scan mosaic. 

Community Component Area (sq. m) 

Tubeworms, Dense 22.7 

Tubeworms, Recumbant 69.0 

Tubeworms, Senescent 24.6 

Tubeworms, Sparse 215.3 

Gas Hydrate Outcrop 11.9 

Seep Mussels 71.9 

Carbonates 610.0 

Total Area of Mosaic 27,650 

Compiled results show that gas hydrate deposits are a perennial feature of this specific location; 

markers and dropped objects confirm this placement. The outcrop forms on the sloping southern 

margin of a large mussel bed, which is surrounded on three sides by recumbent tubeworms 

(Figure 7.2). The location is offset from the top of the mound on a gradual slope. The 

surrounding tubeworms are numerous, but by no means the densest aggregation in the 

community. Exposed hydrate appears on the downslope side of the deposit, which evidently 

extends laterally underneath the mussels. 

A time-lapse camera positioned to monitor this hydrate outcrop recorded 1-sec video sequences 

at hourly intervals over a 132-day deployment (Figure 7.4). Beginning as a prominent mound, 

with approximately 30 cm of relief above the surrounding seafloor and patches of exposed gas 

hydrate clearly visible, the mound gradually deflated over the monitoring interval. By the end, 

the exposed hydrate had completely disappeared and the mound was leveled. Biological activity 

was high on and around the mound, with fish and invertebrates appearing in numerous video 

sequences. Visible in the video images was a bacterial mat, which covered the mound and the 

sediment directly in front of the mound. During the monitoring interval, this mat was repeatedly 

disturbed by the activity of crabs and bivalves. The animals would appear in several of the 

hourly images, where they could be seen plowing through the surface sediments until the 
7-11

bacterial mat had been completely obscured and overturned. Subsequently, over the course of 

several days the mat would re-establish itself. A similar pattern was observed during most 

disturbance events. 

Piston cores provide additional information on gas hydrate and basic sediment characteristics of 

the site (Table 7.3). Significant recoveries of gas hydrate were made from 1 m subbottom depths 

at a location 70 m south of the outcropping deposit (Figure 7.2A, BHPST-3). The core closest to 

the outcropping deposit (BHPST-1) recovered carbonate and oily mud, but no intact hydrate. 

Both of these cores were short, indicating a hard substratum at shallow subbottom depths. The 

third core, which was collected closest to the rocky margin of the mound, failed to penetrate a 

rocky bottom location. Interestingly, the recovery of gas hydrate came from a locality well 

removed from the major concentrations of chemosynthetic animals. This demonstrates that, 

although shallow gas hydrate deposits may be associated with tubeworm aggregations, presence 

of shallow gas hydrate does not guarantee that tubeworms will occur at a location. The rocky 

substratum encountered by the BHPST-2 core is consistent with the observation of carbonate 

outcrops in the laser line-scan mosaic. 

Table 7.3. Field notes on piston cores taken at sites GC185 (BHPST1-3: 

See Figure 7.2 for locations.) and GC234 (GCPST1-3: See Figure 7.7 for locations). 

Core ID Number Length (m) Description 

BHPST-1 1.2 Soft, liquid-rich clay. Oil vugs and veins mixed with small carbonate nodules. 

Strong odor of H2S. 

BHPST-2 0.1 Oil-rich carbonate rubble and small pieces of tubeworm tube in tip of core. 

BHPST-3 1.2 

Upper 90 cm comprise soft, olive gray clay with oil veins. Small carbonate 

nodules at 60 cm subbottom. Lower 20 cm of core comprise a solid cylinder of 

gas hydrate, laced with veins of oil and sediment nodules. 

GCPST-3 0.1 Recovery from tip only, which contained carbonates and vestimentiferan 

pieces. 

GCPST-2 2.4 

Firm clay with multiple oil stains, veins, and vugs. Strong odor of H2S. Color 

olive gray. Hydrates from 0.5 to 8 cm in sections 4, 5, and 6. Extremely gassy 

with gas cuts and fractured in all sections. 

GCPST-1 0.8 Liquid filled core. Strong odor of H2S with oil staining throughout. Hydrates 

from 0.5 to 3.0 cm in diameter in section 3. Gassy and bubbling. 

In summary, the present compilation of results portrays the GC185 site as a broad, low mound, 

roughly hemispherical in cross-section, the rocky western margin of which aligns with high 

angle faulting. The biological community is concentrated in a relatively small portion of the 

mound, in a 40-m wide band that is offset, but largely parallel with, the axis of the fault. Gas 

hydrate outcrops on the eastern-most margin of this band, furthest from the fault, and appears to 

be a plug that waxes and wanes at the orifice of an active gas vent. The outcropping deposits of 
7-12

gas hydrate are dynamic structures in terms of their size, but occur persistently at predictable 

locations. Shallow deposits of gas hydrate extend from the community to locations well away 

from the principal axis of biological activity. Although these characteristics result from mapping 

a single site, which we previously defined as a sediment diffusion site, comparison of similar 

attributes at the program’s second sediment diffusion site, GC234, provide a means for 

evaluating their generality. 

7.3.2 GC234 

The GC234 site is the locality where the first documented collections of sediments containing 

thermogenic hydrocarbons were made in the Gulf of Mexico (Anderson et al. 1983). The 

geology of the region has been described by Behrens (1988) and Reilly et al. (1996). The 

extensive chemosynthetic community is situated on a slump block within a half-graben. Steep 

escarpments rise above the community on three sides, with extensive lithification on the western 

slope, and the bottom slopes away to the south. In 1990, the site was thoroughly surveyed with 

submarine NR-1, which completed a series of east-west oriented tracks through the site at a line 

spacing of about 30 m over a total area of approximately 1200 by 700 m (Figure 7.5). Video 

records of the chemosynthetic animals, bathymetric soundings, and side-scan sonar records 

comprise a baseline description of the site. The side-scan sonar mosaic, which was assembled 

digitally from scanned paper-chart records, delineates several broad areas of high-reflectance 

seafloor (Figure 7.5). Visual survey confirms abundant carbonate hardground and outcroppings 

across the western escarpment. Several large, solitary promontories are visible as isolated hard 

targets in the side-scan image; these include a massive structure approximately 15 m wide and 5 

m tall that is situated in the center of the side-scan mosaic. Visual survey confirmed that the 

remaining regions of high reflectance comprise large colonies of tubeworms situated on an 

undulating terrain. Notably there are two broad bands that trend northwest-southeast across the 

center of the image. Within these regions, tubeworms colonies occur at densities ranging from 

sparse aggregations, to bushes measuring 1 to 2 m across, to continuous stands that extend for 10 

m or more (MacDonald et al. 1990a, MacDonald et al. 1995, Reilly et al. 1996). 

On the basis of this information, the array of sampling stations for the present program were 

established within a sub-area of the seep that measured approximately 300 x 200 m (Figure 7.6). 

Operations during the 1997 cruise with SEA-LINK deployed permanent markers at these stations 

and provided detailed descriptions of the features present at each station (see Chapter 3, this 

volume). When the site was surveyed in 1998, with submarine NR-1, these markers were used as 

benchmarks to align NR-1 survey results with station locations. NR-1 completed a dense array of 

north-south and east-west tracklines. Depth soundings along these tracks were used to compile 

an independent bathymetric surface. The side-scan sonar mosaic and the visual observations 

from the 1990 NR-1 cruise were aligned with the recent data on the basis of this surface. Piston 

cores, collected after the NR-1 cruise, used a coring device that dropped marker buoys at the 

location of each core (Table 7.3). These markers were located with SEA-LINK during its 1998 

sampling cruise. Buoy locations were confirmed by use of the SEA-LINK’s acoustic navigation 

system. Thus, it is possible to compare numerous layers of geographic information from the site. 
7-13

Figure 7.4. First (A) and last (B) frame from a time-lapse series recorded at the HYD1 station. Exposed hydrate is

clearly visible in the first image as an outcropping patch on a low mound. During the intervening time

the mound gradually deflated and the hydrate entirely disappeared. Crossbar is 40 cm wide. 

Day 1 

Day 132 

A 

B 
7-14

Figure 7.5. Overview of the GC234 study site, situated in the head of a canyon formed between active fault lines. 

(A) Bathymetry of canyon overlaid with the trackline of submarine NR-1 during 1992 cruise. (B) Sidescan sonar mosaic of site compiled from data collected by NR-1 in 1992. Note regions of high surface 

reflectivity. 

A 

B 
7-15

Figure 7.6. Merger of 1990 survey data with 1998 survey data and sampling stations. (A) Inset box overlaid on

GC234 bathymetry shows detailed trackline followed by NR-1 in 1998 cruise. (B) Bathymetry of XStar survey area generated from 1998 depth data and GC234 sampling station locations. The 1990 a

and 1998 datasets were co-located by shifting the 1990 data to match the 1998 bathymetric contours. 

A 

B 
7-16

Subbottom profiles from the 1998 survey revealed complex and abrupt transitions from regions 

in which surface strata appear as distinct, closely spaced layers to areas in which the strata were 

blanked by continuous, moderate-intensity returns. These so-called wipeout zones have been 

described in previous investigations of Gulf of Mexico seeps (Sager and MacDonald 1998) and 

are accepted as indicators of gassy surface sediments that dampen or obscure high-frequency 

seismic reflections. The present data, however, made it possible to trace the transitions between 

normally stratified sediments and the wipeout zone (Figure 7.7). The wipeout zone begins 

abruptly, over a distance of less than 10 m, and describes a zigzag margin that bisects the study 

area. East-west tracks show a series of faults that appear to be separating along this line 

direction. These faults evidently begin as slight offsets in the alignments of substrata (e.g., Fault 

A in Figure 7.7) and open into larger conduits. The north-south track indicates an equally abrupt 

transition to the wipeout zone that begins where the slope increases (Figure 7.8). The wipeout 

zone is not homogeneous. Distinct, arc-shaped reflectors were detected at numerous points 

within the wipeout zone, usually at levels 2-3 m subbottom (e.g., Track M2-12, Figure 7.7). 

Within the wipeout zone and adjacent to the normally stratified strata, the seafloor had a 

distinctively undulating profile along many of the tracks (e.g., Track M2-8, Figure 7.6). This 

seafloor irregularity was lacking in the regions where the subbottom was visibly stratified. By 

comparing adjacent tracklines, these irregularities were resolved as a series of mounds with base 

areas 15 to 20 m across and relief of 1 to 3 m. The reflection intensity at the seafloor was not 

consistent among all the mounds. Most of the sampling stations occupied during the program 

were on or adjacent to these mounds (Figure 7.8). Some mounds had distinct, intense reflections 

at the seafloor. However, the prominent mound at station M2 had a very indistinct seafloor 

reflector. 

Visual observations from the SEA-LINK cruises at GC234 conclusively link several of the 

mounds with shallow deposits of gas hydrate. Notably, the HYD1 station, when first observed in 

the 1997 cruise, was a prominent outcrop of gas hydrate that exposed a broad expanse of gas 

hydrate on its downslope margin (Figure 7.9). The polychaete Hesiocaeca methanicola

(Desbruyeres and Toulmond 1998) was first observed on the exposed portion of this gas hydrate 

outcropping (Fisher et al. 2000). A similar exposure of gas hydrate was seen at the HYD2 

station. The gas hydrate deposit at HYD1 completely disappeared between the 1997 and 1998 

SEA-LINK cruises, while the HYD2 deposit was reduced to a small remnant. Gas hydrate 

deposits were also observed immediately beneath the surface at the M2 station, where small 

pieces of the substance were dislodged by push core collections. Piston core recovery of gas 

hydrate from the M2 station (Table 7.3) confirmed that the visible deposit observed from SEALINK was part of a larger, buried deposit that extended down the flanks of the mound. 

From these results, it is evident that gas hydrate forms in the upper few meters of the sediment 

column and produces broad mounds. As at GC185, it was seen that individual deposits might 

expand to breach the seafloor on the margin of steeper slopes (MacDonald et al. 1994). This 

exposes patches of gas hydrate, which subsequently dissolves so that the localized relief 

gradually deflates. It appears that a cycle of exposure, dissolution, re-growth and re-exposure 

repeats at fixed localities within individual mounds. It also appears that the mounds are 

concentrated along the margin between the wipeout zone and the region in which shallow 

sedimentary strata are visible. 
7-17

Figure 7.7. Results of X-Star subbottom profile survey. (A) East-west tracklines overlaid on sediment style 

showing location of hydrate mounds, faults and ground-truth piston cores. (B) X-Star subbottom

profiles showing abrupt transition between stratified and un-stratified sediments, faults, mounds, and 

piston core locations. 

A 

B 
7-18

Figure 7.8. Results of X-Star subbottom profile survey with station locations. (A) North-south tracklines overlaid 

on sediment style showing location of hydrate mounds, faults, and sampling station locations. (B) XStar subbottom profiles showing abrupt transition between stratified and un-stratified sediments, faults, 

mounds and piston core locations. 

A 

B 
7-19

The clustering of chemosynthetic animals within the site is apparent from characteristics 

observed at the sampling stations during SEA-LINK dives (Figure 7.7). As was the case at the 

GC185 site, sampling stations defined by juvenile, adult, and senescent tubeworms were readily 

located, as were stations defined by mussels and bacterial mats. Though tubeworm colonies were 

abundant in the immediate vicinity of the sampling stations, there were large areas that were 

unoccupied by tubeworms and other chemosynthetic animals. As was also seen at the GC185 

site, the association between tubeworm aggregations and gas hydrate at the site was consistent, 

but not deterministic. Dense aggregations of tubeworms extended beyond the limits of the 

hydrate mounds (e.g., Station AT1, Figure 7.7). Conversely, the prominent hydrate outcrop at 

HYD1 occurred in an area locally devoid of tubeworm colonies (Figure 7.9). To examine the 

discontinuities in distribution of tubeworms in light of the subbottom profiles and other 

geological data, the visual survey data obtained in the 1990 survey with NR-1 were reanalyzed as 

a layer in the GIS dataset (Figure 7.10). 

Figure 7.9. Exposed gas hydrate at the HYD1 station in GC234 site, 1997. Location where the ice worm

(Hesiocaeca methanicola) was first collected. This deposit disappeared between the 1997 and 1998

SEA-LINK cruises. Note the band of sediment sandwiched between two layers of hydrate. 
7-20

Figure 7.10. Tubeworm abundance at GC234. (A) Tubeworm clusters scaled according to area occupied and

relative density of each cluster; analysis was based on visual survey done by NR-1 in 1990. Cluster

locations are overlaid on a krige surface calculated from log-transformed tubeworm abundance with

use of software routines in the S-Plus spatial statistics package. Dark areas in krige surface indicate

areas of greater predicted abundance for tubeworms; black areas show limits of interpolated data. (B)

Surface amplitude map from 3D seismic data for area. (Low amplitude areas are plotted in dark

shades.) Rectangular outline delineates 1998 subbottom survey area and stations from Figures 7.7 and

7.8. 

A 

B 
7-21

The first step was to fit a continuous surface to the tubeworm abundance estimates calculated 

from the NR-1 survey data from 1990 (Figure 7.10A). Kriging is a statistical technique for 

spatial prediction frequently used by geologists to delineate the distribution of ore deposits or 

other features that have a relatively continuous distribution, but are sampled at discrete locations 

(Cressie 1991). In calculating the krige surface, scales of spatial auto-correlation, threshold 

levels, and nugget effects (tendency to occur in large isolated concentrations) were estimated 

from the survey data using standard techniques (Kaluza and Doyle 1998); the parameters for the 

krige surface fit were adjusted accordingly. Least-square error values were calculated and several 

surfaces were produced until a minimum error was obtained. The resulting surface delineates 

several zones of high abundance, which are separated by larger areas in which tubeworms were 

evidently vacant. As expected, the predicted regions of high abundance correspond closely to the 

observed tubeworm clusters. However, the best fit predicts high abundance in areas where no 

tubeworms were observed. Notably, in the region of the program sampling sites, the krige 

surface predicts a zone of high abundance centered on the M2 station and extending over an 

ovoid approximately 200 m by 150 m. To the northeast of station M2, the predicted surface 

extends over the area in which the subbottom survey found normally stratified sediments (little 

evidence of seepage) and where the 1990 visual survey with NR-1 failed to detect tubeworm 

colonies. 

The second treatment of the tubeworm survey data was to compare the observed tubeworm 

clusters to available surface amplitude values from a 3D seismic survey (Figure 7.10B). Surface 

amplitudes process the first return from a 3D-processed dataset. They are standard products that 

would be available to operators assessing a region prior to siting exploration or production 

facilities (See Chapter 6 for details). The data are coarse, spatially, because of array limits and 

the present plot distinguishes only 8 levels of return. Nonetheless, the correspondence between 

tubeworm abundance and low surface amplitude is striking. Table 7.4 summarizes the surface 

amplitudes observed along the 1990 survey tracklines and at the locations where tubeworm 

clusters were encountered. Discounting area where amplitude data were missing, this analysis 

shows that approximately 81% of the tubeworm population was found in that 25% of the 

surveyed area having low to moderate seismic amplitude. A goodness-of-fit test on these results 

indicates that the probability this distribution would arise by random chance is less than 5%. 

Examining the zone around the sampling stations, an area of low amplitude clearly contains 

much of the observed tubeworm population. Unlike the krige surface, however, the low 

amplitude zone does not extend to the northeast. Instead it appears roughly conformal to the 

wipeout zone delineated by the independent subbottom survey. Low surface amplitudes appear 

to be a good predictor of tubeworm abundance. 

It is evident from the results at GC234 that sharp discontinuities in seepage are prevalent within 

the study area. A probable cause of these discontinuities is ongoing fracturing of the sediment 

column during the movement of the slump block and the expansion of the graben found at the 

site. The subbottom data delineate distinct zones generated by these small-scale faults or 

fractures. In areas where subbottom strata are obscured, one finds abundant evidence of active 

fluid and gas migration, including widespread colonies of tubeworms and shallow gas hydrate 

deposits. Results of surface fitting to a detailed survey of chemosynthetic animals tended to 

overestimate the occurrence of the animals. The reason for this is probably that the surface fitting 

routines used do not deal well with sharp discontinuities. This result suggests that an evaluation 
7-22

 of the extent of chemosynthetic animals in a seep-affected region should make use of seismic 

amplitude data combined with visual surveys. 

Table 7.4. Comparison of surface amplitudes from 3D seismic 

data to tubeworm abundance (Figure 7.10). For details 

on the 1990 NR-1 survey tracklines see Figure 7.5. 

Surface amplitude 

levels: increasing 

intensity 

Proportion 1990 

survey area along 

trackline 

Proportion total 

tubeworm 

abundance 

no data 0.07680 0.01460 

1 0.00046 0.00190 

2 0.00184 0.03420 

3 0.06839 0.28030 

4 0.17458 0.47800 

5 0.55663 0.12340 

6 0.16920 0.05330 

7 0.00799 0.00670 

8 0.01322 0.00750 

7.3.3 GC233 

The brine-filled pockmark and mussel bed at GC233 is a notable example of a chemosynthetic 

community supported by focused flow and methane dissolved in pooled hypersaline fluid 

(MacDonald et al. 1990b). This pool is situated on a low mound, which is elevated 6 to 8 m 

above the surrounding seafloor and has a basal diameter of approximately 100 m (Figure 7.11). 

The fluid filling the pool has a salinity of 130 practical seawater units and is supersaturated in 

biogenic methane. Samples of fluid, decanted from ambient pressure into one atmosphere, will 

boil vigorously as gas comes out of solution. The solids dissolved in the fluid are primarily NaCl, 

derived from halite deposits at depth. Added density due to the excess salt is what maintains the 

brine as a distinct fluid in the pool. However, the salinity of the brine is grossly out of 

equilibrium with seawater. Brine is certainly mixing with seawater by molecular and turbulent 

diffusion. The elevation of the mound, sediment slides, and bacterial mats on the downslope 

(southern) end of the pool, and a raised dike around the upslope (northern) edge of the pool are 

among the abundant visual evidence that the pool was excavated by a vigorous discharge of 

fluid. The persistence of a mussel community, developed in the present day to a continuous band 

that completely surrounds the pool on the level margins of the crater, is strong evidence for 

conditions that have favored chemosynthetic animals over an extended time (Table 7.5). The 

probable age of the larger seep mussels in this population exceeds 100 y (Nix et al. 1995). 

Stability of the level of brine filling the pool is essential because the brine is anoxic in addition to 

being hypersaline. Therefore, it would be fatal for mussels to be submerged in the brine. 

Examination of the inner edges of the pool, however, reveals that the mussel hold their siphons 

less than half their body lengths (about 4 cm) above the brine. The outer edges are less than 15 

cm higher than the inner edges. Because mussels are sessile and bound in place by the byssal 
7-23

threads of their neighbors, even the slightest increase in the brine level would produce 

widespread stress or mortality. This site and its strikingly dense population represent an 

equilibrium state between the violent forces of formation and continual processes (e.g., 

dissolution of the brine) that are eroding the present conditions. Perception of stability in the face 

of potentially destabilizing forces is intuitively clear. The purpose of this section is to present the 

basic evidence for change during the past and stability during the present at the GC233 site. In 

the final section of this chapter, this evidence will be contrasted with a site in a much more active 

state. 

Table 7.5. Relative areas of GC233 brine pool (Brine Pool NR-1) 

as calculated from laser line-scan mosaic (Figure 7.11). 

Site Characteristic Area (sq. m) 

Contiguous mussel bed 434 

Peripheral mussel beds 214 

Open brine 253 

Total area of mosaic 1658 

To investigate the sedimentary evidence for the formative history of Brine Pool NR-1, a 

thorough subbottom survey was completed with submarine NR-1 during the 1998 cruise (Figure 

7.12A). Two piston cores were also taken at locations intended to penetrate the downslope edge 

of the pool and the periphery of the mound (Figure 7.12A). As with piston cores at other 

program sites, the coring device was rigged to drop markers, which were relocated to confirm 

positions. Subbottom records revealed considerable complexity in the mound and pool. On the 

flanks of the mound, indurate layers underlie layers that gave weak seismic reflections (Figure 

7.12B), suggesting surface flows of loosely consolidated material over hardened ground. A 

narrow dike, raised about 25 cm, forms a rim that surrounds three sides of the pool outside of the 

mussel bed. The mussels appear as a layer distinct from the brine, which forms a very level 

reflecting surface. Below the brine, sediments are homogeneous and contain a large, irregular 

reflector near the bottom of the record. The most regular subbottom features were a sequence of 

regular layers buried under the present day mound. Distances between these layers were greatest 

in the center of the mound and least around the edges. The deepest layer formed a funnel shaped 

base to the outline of the mound. 
7-24 

Figure 7.11. Brine Pool NR-1 in the GC233 study site (north is topmost). (A) Local bathymetry of site (contours in meters) showin

low, circular mound. Inset box shows outline of laser line-scan mosaic. (B) Laser line-scan mosaic of brine pool and 

GC233 site. Marked points show sampling station locations. Gas vents mark persistent bubble streams that emerge flocation of temperature profiles. 

Gas vents

A B
7-25 

From the bathymetric and subbottom records, it is evident that the pool and mound have a 

complex three-dimensional structure. Two separate studies were completed to quantify this 

structure. The depth of the brine pool was measured by a series of lead-line casts into the brine. 

These were accomplished by lowering a weight from the SEA-LINK by use of a small winch and 

by counting depth markings on the line. The position of each cast was estimated by sighting from 

the submarine to markers visible around the pool edge. These sightings were then compared to a 

copy of the laser line-scan mosaic to mark the position of each cast. Depths from the completed 

Figure 7.12. Subbottom profile survey of GC233 brine pool. (A) Plan of site showing trackline and piston core

locations (). (B) Example subbottom record from Track 11 taken along southwest to northeast track. 

Water depth of submarine was held constant (within < 1 m) along trackline, so bottom profile is

accurate. 

RN8-1

RN8-2

northeast 

A 

B 
7-26 

series of 30 casts was interpolated by hand and then fit to a continuous bathymetric surface 

(Figure 7.13A). The survey reveals the pool is a shallow shelf, pierced by two deep and narrow 

diatremes. In plan view, the pool is broadest at its northern end. The two diatremes are 

positioned in the two ends of this broad portion of the pool (Figure 7.11B) and can be recognized 

by the continual streams of bubbles that escape from the pool at these points. The other effort to 

delineate the three-dimensional structure of the site was to georectify the individual subbottom 

profiles, to trace out the subbottom layers visible within the mound, and to fit these layers into 

continuous surfaces for enumeration. This was successfully competed for the surface of the 

mound, including the brine-water interface, and three additional subbottom layers (Figure 

7.13A). Additional reflectors were visible between the deepest layer and the second layer, but 

were not distinct enough to trace them through multiple records. The total volume between the 

deepest layer and the mound surface was 63,530 cu. m. The surface, middle, and bottom layers 

contributed 20,410 cu. m, 7,712 cu. m, and 35,410 cu. m, respectively to this total. 

What these data show is a mound formed by repeated eruptions of fluidized mud through two 

distinct vents. The present-day diatremes have evidently closed in to a fraction of their greatest 

diameter, but remain open well below the deepest visible layers in the mound. Examining Figure 

7.12C, it is clear that the maximum depth recorded in the deepest diatreme (28 m) would extend 

from the pool surface to below the deepest extent of the subbottom profile. The surface 

properties of each layer evidently hardened enough to appear as distinct reflectors. Additional 

data from the GC233 site were examined to determine whether these hardened layers supported 

chemosynthetic communities and to gauge the recent stability of the feature. 

The two piston cores that were taken from the pool edges provided strong evidence for continual 

chemosynthetic communities at the site (Figure 7.14). Core RN8-1 was collected at the outer 

edge of the mound. Mudstones and stiff clay in the upper sections are consistent with the strong 

reflector seen in the upper portion of the subbottom record. A lucinid shell recovered from 1 m 

subbottom indicates circumstances favorable to chemosynthetic animals in this stratum. Notably, 

at 3.7 m subbottom a seep mussel shell indicates presence of a mussel colony at a position about 

80 m, laterally, from present mussel colonies. Core RN8-2 was recovered immediately south of 

the pool and mussel bed. Fragments of lucinid shell and mud stones at 0.5, 2, and 4 m subbottom 

are consistent with the depths of the upper layers in the subbottom profile. However, gas 

expansion pockets in the core make measurements problematic and this result should be regarded 

with caution. 
7-27 

Figure 7.13. Three-dimensional interpretations of combined data for GC233 brine pool. (A) Laser line-scan mosaic draped ov

comprising the mound and the measured depth of the brine pool. (B) Individual surfaces interpolated from subbottom pcontiguous reflectors, which are interpreted as relic mound surfaces. 

A B
7-28 

The cores do show layering and chemosynthetic occupation at the site. Carbon dating of the 

recovered shell material provides an indication of the ages of substrata around pool (Table 7.6). 

However, interpretation is somewhat problematic due to contamination with ancient carbon that 

is depleted in 14C. To have a baseline for comparison, shell samples were taken from living 

specimens at GC233 and GC185. Each shell from the living specimens was sub-sampled from 

the umbo and the ventral margin to determine ages for the oldest and youngest parts of the shells, 

respectively. These results show 14C “age” at settlement of 2020 y BP for a 120 mm mussel from 

GC233 and 1760 y BP for a 91 mm mussel from GC185. Because a zero age mussel could not 

have had an age of approximately 2000 y, these values indicate incorporation of bicarbonate 

from a source depleted in 14C. The result is somewhat problematic because the slightly negative 

13C values do not appear to balance the dilution of modern carbon if one assumes that living 

mussel soft tissue will have strongly negative 13C values. Also there is a fractionation with age 

of about one per mill between the young and old values. However, it is highly suggestive that 

both specimens are near the asymptotic lengths found in their respective populations and should 

therefore represent the maximum age of the population. Strikingly, carbonate from the ventral 

(youngest) margin of both specimens exhibited 14C ages about 200 y older than the umbo. This is 

Figure7.14. Descriptions of piston cores collected at GC233 study site (see Figure 7.12 for locations). The larger

shells or shell fragments are depicted with their positions within the cores indicated. See Table 7.6 for

14C ages of shells. Sediments comprised stiff clay or looser mud. 
7-29 

consistent with the maximum ages estimated from previous growth studies of seep mussels (Nix

et al. 1995). Additional analysis and modeling would be required to model the carbon pools 

utilized by mussel shell growth. The apparent ages do provide a minimum age, which could be 

subtracted from fossil material to approximate a more accurate age. 

Table 7.6. Shells collected from living mussels (SEA-LINK) at GC233 and GC185 sites and in piston cores 

from GC233 site. Shell samples from living specimens were taken from the umbo and ventral margin to 

compare, respectively, the oldest and youngest portions of the shell. Living mussel from GC233 

(BP outer) was 120 mm; living mussel from GC185 was 91 mm. Figure 7.14 shows relative 

positions within cores — at least three bivalve species were found. Ages estimated 

by 14C (accelerator mass spec. determination). Stable carbon isotopes 

(13C) are a check on carbonate source. 

Site (Station) 

Collection 

Method 

Stratum depth m 

(specimen type) 

14C Age 

(y BP) 

Error 

(y) 13C 

GC233 (BP outer) SEA-LINK 0.0 (umbo) 2020 35 -6.77 

GC233 (BP outer) SEA-LINK 0.0 (margin) 2220 35 -7.77 

GC185 (BHM1) SEA-LINK 0.0 (umbo) 1760 40 -1.66 

GC185 (BHM1) SEA-LINK 0.0 (margin) 1950 35 -2.46 

 

GC233 (RN8-2) piston core 2.80 (lucinid) 13750 35 -2.78 

*GC233 (RN8-2) piston core 2.80 (lucinid) 13750 30 -2.65 

GC233 (RN8-2) piston core 4.00 (lucinid) 14850 35 -1.55 

 

GC233 (RN8-1) piston core 0.05 (mussel) 7270 40 -1.97 

GC233 (RN8-1) piston core 0.70 (lucinid?) 17450 55 -22.89 

GC233 (RN8-1) piston core 0.85 (mussel) 12200 35 -1.11 

GC233 (RN8-1) piston core 3.40 (mussel) 15800 40 -2.3 

GC233 (RN8-1) piston core 3.40 (mussel) 16200 50 -5.16 

GC233 (RN8-1) piston core 3.69 (vesicomyid ?) 16800 45 6.07 

*replicate 

With these caveats, the apparent ages of the fossil shell material from the cores show reasonably 

consistent trends. The strata in core RN8-2, closest to the brine pool and mussel community, 

have ages of 13750 y BP at 2.8 m subbottom and 14850 y BP at 4 m subbottom. These ages are 

in excess of what could be explained by burial through sedimentation at rates typical of the upper 

slope (10 cm per 1000 y). Similarly, strata in core RN8-1 show increasing age with depth. One 

outlier in the trend has excessively negative 13C values. Radiocarbon ages in shell material from 

the cores suggest prolonged occupation of this site by chemosynthetic animals over the past 10 to 

13 x 103

 y. Relative differences in the apparent ages of the strata, and the depth of the strata, are 

consistent with episodic burial events related to discharges from the brine pool feature. Brine 

Pool NR-1 appears from these data to be an ancient site, continually occupied by chemosynthetic 

bivalves, but subject to major perturbations over time. The length of time that the pool 
7-30 

community has been in its present state cannot be estimated from the core stratigraphy. The best 

evidence for recent stability comes from the properties of the brine and the age structure of the 

mussel population. 

Seepage of gas is continuous through the northern end of the pool at the GC233 site, but has 

been reduced to a few sparse streams of bubbles. Temperature profile through the upper part of 

the pool (Figure 7.15A) shows a fluid column that is sharply stratified by salinity and 

temperature. Fluid samples indicate that the maximum conductivity values correspond to a 

salinity of 130 PSU. The abrupt increase at the brine water interface indicates a fluid volume that 

is at near equilibrium with the overlying seawater. Mass and thermal fluxes across the interface 

appear to be dominated by diffusive rather than advective processes. A second, sharper 

thermocline beginning about 2 m below the brine surface corresponds to an abrupt decrease in 

conductivity and to a marked increase in amount of suspended sediment in the fluid column. The 

conductivity effect is an artifact upon the CTD instrument produced by the suspended sediment. 

Similar temperature profiles in the pool have been measured in 1991, 1992, and 1998, indicating 

recent stability of the pool’s density structure. The persistence of a colony of methanotrophic 

mussels and their uniform distribution around the pool is further evidence for long-term stability 

of the feature. The presence of a mussel whose age may be as great as 200 y on the outer portion 

of the mussel bed surrounding the pool suggests that the pool has not erupted or overflowed in 

recent history. However, if the supposition of episodic eruptions is valid, examples should exist 

of brine-filled pools with much more dynamic behavior. As an example of a more active brine 

pool and mud volcano, we turn to the final study site examined in this program, namely GB425. 
7-31 

7.3.4 GB425 

The GB425 site is a mud volcano located at the edge of Auger Basin (Figure 17.16A), an 

intraslope basin that contains economically significant hydrocarbons in the Auger, Cardamom, 

and Macaroni fields and is bordered to the west by tabular salt bodies (McGee et al. 1993). A 

tension-leg platform installed at Auger Field is producing from an estimated 100 million barrels 

of extractable oil (Shew et al. 1993), but large quantities of pressurized fluid have evidently 

escaped through anticline faults along the flanks of the salt at the basin margin. Sediment cores 

from this zone have recovered high-molecular weight hydrocarbon and thermogenic gas hydrate. 

Fluids migrating up these faults disturb surface sediments due to the formation of carbonate 

nodules, oil and gas pockets, and biogenic debris. These anomalies are readily detectable by 

seismic and acoustic imaging (Figure 17.16B). 

Figure 7.15. Temperature profiles of two brine pools. (A) Temperature profiles in Brine Pool NR-1 (GC233 site) 

taken at three times over an eight year time span. (B) Temperature profiles in mud-filled crater at

GB425 site taken in 1997 () and 1998 (#). Temperature profiles shown are from points where oil and 

gas venting was most evident. Pool depths are relative to sea level. Depths of interface (arrows) were

derived from conductivity response of CTD and comparison of pictures that showed change in the

height of mud on thermistor mooring tether. 

A B
7-32 

Active fluid expulsion is localized in a 50-m wide, sub-circular crater on the southwestern edge 

of the summit, which is one of two such vents on the mound (Figure 7.17A). Fine, fluidized mud 

overflows the northeastern margin of the crater and moves down the southern flank of the 

mound. The mud is fluidized in hypersaline brine (133 practical salinity units) that is 

supersaturated with methane at STP. Beds of the seep mussel rim the southern edge of the crater 

(Figure 7.17B). 

The mud lake at GB425 was sampled with use of the SEA-LINK on 29 July 1997 and 11 July 

1998. On the first visit a thermistor was suspended below a float anchored on a short tether. It 

recorded a reading every 20 min. The anchor was attached to a 75 by 75 cm plate to prevent it 

from sinking in the soft mud of the crater bottom. Samples of brine and mud were collected with 

use of a chamber that the SEA-LINK positioned below the fluid-seawater interface and sealed 

for recovery. The brine had a salinity of 133 practical salinity units. The fluid degassed violently 

Figure 7.16. Regional setting for study site in GB425. (A) Larger stippled box shows the area that was covered in

satellite remote sensing (SAR) images (Figure 7.19) while the inner stippling shows coverage of a side

scan sonar mosaic and of SAR sub-scenes (Figure 7.20). (B) Mosaic of 75 kHz side-scan sonar tracks 

shows acoustic texture of western Auger Basin. Production facilities for Auger oil field are centered 

approximately 6 km to the east. A salt wall and fault system border the basin to the west and

northwest. Migration of fluid along the flanks of this salt unit provides natural release of pressures

accumulated within the basin. Seafloor mounds, mud flows (arrows), and slump scars indicate dynamic

material fluxes at numerous points along the fault. The study site is situated at a small, active diatreme

on the southern margin of a flat-topped mound located mid-way along the fault (<). 

A 

B
7-33 

when the collection chamber was opened on the surface, indicating that gases were supersaturated with respect to sea level temperature and pressure. The gas that evolved was less than 

99% CH4. Solvent extractions of sediments suspended in the brine contained high-molecular 

weight hydrocarbons, as did sediments collected in short cores. Exposed deposits of gas hydrate 

were observed around a gas vent (M2) that was located about 150 m northeast of the lake. 

Visual observations indicated that gas and oil discharges were minor in 1997 and the feature 

appeared less active than had been previously noted. In 1998, however, a continual stream of 

small gas bubbles and drops of oil emanated from the central diatreme, while bursts of larger 

bubbles, oil drops, and suspended sediment periodically escaped from the surrounding mud. 

Although drops of oil were observed reaching the sea surface, this flow did not generate a 

conspicuous slick. In 1997, a thermistor was suspended 30 cm below the interface at a position 

about 15 m west of the apparent vent. When the thermistor was recovered in 1998, the apparent 

level of mud in the pool, as measured by its height on the mooring tether, had risen by 50 cm 

(Figure 7.18). 

On both occasions, a temperature profile was recorded by lowering a CTD into the fluid. The 

1997 profile penetrated only 0.7 m into the muddy fluid (Figure 7.15B). In 1998, however, active 

venting pointed the way to a deeper part of the lake and the CTD was lowered 3.2 m below the 

interface (Figure 7.15B). Changes in characteristics of pooled fluid over the ~1 year interval 

demonstrate increased flow. In both profiles, abrupt increases in conductivity demarcated a ~1 

cm interface between the fluid mud and overlying seawater. In 1997, convective cooling across 

this interface produced the monotonic 2.7°C decrease in temperature between the pool bottom 

and the interface. In 1998, a subsurface maximum temperature of 26.7°C at 34 cm below the 

interface and the apparent rise in the level of mud in the pool, provide evidence for discharge of 

warm mud that spread in a surficial layer, eventually flowing downslope from the mud lake 

outlet. Similar sheet-flows of mud would have been required to form the flow-fields evident in 

the side-scan sonar (Figure 7.16B). 

Changes in the fluid properties of the mud lake over a one-year period indicated episodic activity 

related to fluid discharge. The geologic evidence of mud slides and the formation of large flattopped structure points to ongoing and massive fluid expulsion. Episodic events are difficult to 

capture with sampling cruises of a few days separated by months or years. To provide 

independent data on fluid discharge at GB425 and from the region as a whole, available satellite 

synthetic aperture radar (SAR) data were reviewed in collaboration with scientists from Unocal 

Corporation. Satellite SAR readily detects layers of floating oil that form over active seeps. It 

provides a means to survey the numbers of hydrocarbon seeps across oil-producing regions and 

to estimate the rates at which seeps are flowing. Comparison of SAR images collected in the 

Gulf of Mexico (Figure 7.16A) indicates large differences in the amount of oil seepage over 

short time lags (Figure 7.19). These changes are too great to be attributed to different sea surface 

conditions. Seafloor measurements from the mud volcano explain how large short-term 

differences in seepage might occur. 
7-34 

Figure 7.17. Mud-filled crater at GB425. (A) Laser line-scan mosaic of GB425 brine pool, showing location of M1 

sampling station (grid points are at 10 m centers). M1 station is located approximately 150 m to the

northeast. Mussel beds are restricted to southwestern (upslope) end of pool. (B) Photomosaic of M1

station looking south. Note marker float.

A 

B 

M1 

M2 
7-35 

Available time-series data from the 1997 through 1998 periods at GB425 comprised six SAR 

images and the records from the thermistor (Figure 7.20). Three of the SAR images were 

collected in May and June 1997, prior to deployment of the thermistor; three were concurrent 

with the deployment. The area of oil slicks overlying the fault zone repeatedly underwent order 

of magnitude changes between the May 1997 and the February 1998 observations (Figure 

7.20B). During the 349-day thermistor deployment, fluid temperatures varied between 6.1and 

48.3C with a mean of 26.1C and standard deviation of 9.07. 

Some fresh oil can be seen surfacing in SAR images near the location of the diatreme (Figure 

7.19 inset). Although lateral offsets of ~1000 m are typically seen between a sea-floor vent and 

the location where rising oil reaches the sea surface (MacDonald et al. 1996), only a portion of 

the oil slicks that were detected could have originated from this specific source, because the 

analyzed portions of the SAR images covered a much wider area than the vent. Much additional 

oil presumably came from other discharge points along the seep-affected zone (Figure 7.16B); 

some may have drifted into the scene from sources external to the study area. Temperature data 

confirm pulsed flow at a point source within the zone. SAR data indicate that these flows were 

part of a regional pattern of discharge. Active seepage has not been identified in this region 

during remote sensing surveys (MacDonald et al. 1996, Mitchell et al. 1999). Therefore, the 

floating oil detected by satellite SAR during 1997-98 represents a higher level of seepage activity 

than was previously known from the locality. 

Figure 7.18. Thermistor array deployed in mud filled crater at GB425. (A) Distance between float and mud

interface was 110 cm eight hours after deployment. (B) Distance was reduced to 61 cm when device

was recovered one year later, apparently due to an increase in the level of mud in pool. 

A 

B
7-36 

Figure 7.19. Synthetic aperture radar (SAR) images (collected by RADARSAT) provide a means to compare the

size of natural oil slicks on 29 July (A) and 11 August 1997 (B) on the sea surface immediately above

the GB425 site. Outlines in both images indicate area overlying Auger Basin fault (Figure 7.19B) that

was analyzed in detailed studies of slick area (Figure 7.20) and show the location of mud volcano.

Inset image details fresh oil surfacing close to the seafloor location of the site. Cross-marks indicate

location of Auger platform. 

A 

B 
7-37 

Figure 7.20. Comparison of temperature time series to abundance of floating oil above site. (A) Three sub-scenes of

SAR data are reproduced to show, from left to right, large-, small-, and medium-size oil slicks. (B) 

Time series of slick areas (in hectares) imaged by available SAR. Slick areas were compiled for an 

18.7 by 13.1 km region overlying the Auger Basin fault zone (Figure 7.19). (C) Temperature time

series from fluid in diatreme recorded between 27 July 1997 and 11 July 1998. 

A 

B 

C 
7-38 

Previous studies of mud volcano systems have identified pulsed flow in sealed boreholes based 

on temperature increases of at most 4°C over periods of months (Carson and Screaton 1998). 

Seafloor temperature anomalies recorded in diatremes have been ascribed to shallow convection 

within mud-filled chambers, perhaps driven by dissociation of gas hydrates (Henry et al. 1996). 

The extreme, rapidly fluctuating temperature of pooled fluids reported here, and the coincidence 

of large oil slicks, demonstrate energetic flow from considerable subbottom depth. Vertical 

transfer of geothermal heat is intensified in the presence of subbottom halite (McBride et al.

1998), so the rate of increase in subbottom temperature is likely to be greater in the vicinity of 

the salt unit bounding Auger Basin. The temperature in a borehole, located approximately 8 km 

northwest of the mound over shallow salt, was 48C at 2309 m subbottom. This suggests that 

fluids with temperatures recorded at 48C in the diatreme would have leaked from at least this 

depth. 

Present results mitigate against two processes that could explain occurrence of pulsed flow in 

this fluid expulsion feature. Gas hydrate layers trap large quantities of free gas in marine 

sediments (Dickens et al. 1997). In the Gulf of Mexico, where massive gas hydrates occur near 

the seafloor, precipitous failure of these deposits will release quantities of oil and gas 

(MacDonald et al. 1994). Although gas hydrate has been collected from the flank of the mound 

peripheral to the mud vent, it cannot act as a fallible seal in the diatreme because, at pressures of 

~5 to 20 megapascal, the high temperature and salinity of the fluidized mud would prevent gas 

hydrate from forming (Sloan 1989). Some evidence suggests that earthquakes stimulate the 

activity of marine hydrocarbon seeps (Macgregor 1993). Although the continental slope of the 

northern Gulf of Mexico is not a seismically active region, many faults on the slope intersect and 

offset the seafloor (including the fault on which this fluid expulsion mound lies), which indicate 

ongoing activity (Rowan and Weimer 1998). There is no record, however, of significant seismic 

activity in the northern Gulf of Mexico during 1997-1998. 

The expulsion episodes described by the present data are part of an ongoing process that has 

prevented formation of commercial hydrocarbon reservoirs in the western portion of Auger 

Basin. Rejecting the possibility of gas hydrate dissociation or seismic activity, one can speculate 

that pulsed flows result when ongoing filling of hydrocarbon traps in western Auger Basin 

induces the failure of the permeable seals in the fault zone, injecting gas, oil, brine, and mud into 

surface strata and the ocean. Previous remote sensing surveys have depended on repeated 

detection of floating oil to confirm flowing seeps (Kornacki et al. 1994, MacDonald et al. 1993, 

MacDonald et al. 1996, Mitchell et al. 1999). This criterion would have overlooked seeps like 

the one described here. These results suggest that the time-dependence of seepage rates should 

be considered when remote sensing results are used for evaluating the hydrocarbon potential of a 

new basin or extrapolating to estimate a regional seepage rate. Such studies are needed to 

constrain the balance of hydrocarbon generation, entrapment, and natural seepage into the 

biosphere.

7.4 Conclusions 

This chapter examined the distribution of chemosynthetic animals and geologic features 

associated with hydrocarbon seepage and fluid discharge at the four study sites. Results indicated 
7-39 

that temporal and spatial variability in chemosynthetic communities have distinct styles and 

characteristic scales. Temporally, the main process that causes short-term variation at the 

sediment-diffusion style sites (GC185 and GC234) appears to be formation and dissociation of 

gas hydrate outcrops. These features can wax and wane over periods of a few months and cause 

major upheavals to local sediment. Mounds, pockmarks, and small fissures are local and 

transient features that can be ascribed to gas hydrate formation and dissociation. There is little 

data resulting from this study to constrain the variability of sediment-diffusion communities over 

longer timeframes — although the extreme longevity of tubeworms described elsewhere in this 

report (Chapter 8) is clear evidence of longer-term stability. A much broader spectrum of 

temporal variability was found in the brine-pooling sites. Satellite evidence of oil venting and 

temperature fluctuations in the GB425 mud volcano indicates rapid cycling of fluid expulsion 

activity over periods of days or weeks. Brine pools are also able to come to a dynamic 

equilibrium between gas venting, fluid mixing, and molecular diffusion, as the temperature 

profiles from GC233 demonstrate. The result is establishment of extremely steep gradients that 

evidently persist relatively unperturbed for years or tens of years. Over geologic time, fluid 

expulsion can remain focused in zones a few meters across. 

Spatially, communities at the so-called brine pooling sites were tightly constrained by the 

discharge and pooling properties of expelled fluids. The mussel beds at these sites exhibited 

markedly higher densities — at a community scale — than were found in the sediment diffusion 

sites. Mussels were concentrated along gradients defined by the edges of standing bodies of 

methane-saturated fluids. Overflow patterns sharply constrain the habitable area around brine 

pools. The result is communities focused into a few large and dense patches with well-defined 

boundaries. Sediment-diffusion sites are characterized by generally smaller and more widely 

dispersed patches scattered over much greater areas. Although boundaries defined by fluid 

migration processes are evident (for example, the transition between stratified sediments and 

wipeout zone in GC234), they are blurred by the distribution of chemosynthetic animals. Linear 

trends were evident at GC185 and GC234, reflecting alignment with fault traces. However, the 

bulk of the chemosynthetic biomass was displaced away from the high-amplitude regions above 

the most active regions of the faults and showed a distinct prevalence for sediment with low 

seismic amplitude. The across-trend dimension of the two sites studied was on the order of 50 to 

100m, while the along-trend dimension was several-fold greater. 

For management concerns, these patterns indicate that different approaches should be used to 

evaluate the regions requiring protection depending on whether the seepage style is dominated 

by focused flow in mud volcano-like features or by diffuse migration along fault planes. In the 

former case, chemosynthetic communities could be effectively avoided by defining the axes of 

discharge and, from available evidence, the total area of concern would be relatively small. In the 

latter case, geophysical evidence could be used to help define zones where chemosynthetic 

animals could occur, but these zones will be larger and may lack distinct boundaries. 
8-1 

8.0 Ecology of Seep Fauna 

8.1 Introduction*

The Gulf of Mexico seep communities exist as a direct consequence of the biological oxidation 

of the energy-rich pore fluids seeping from the sea floor and the concomitant production of 

biomass. The organisms responsible for this production are chemoautotrophic and 

methanotrophic bacteria. These bacteria exist as free-living cells throughout the seep 

environment and also occur as thick mats on the sediment or other surfaces. Chemoautotrophic 

sulfide-oxidizing bacteria also occur as symbionts in several different invertebrates, most notably 

vestimentiferan tubeworms and several species of mussels and clams. The dominant mussel at 

these sites, Bathymodiolus childressi, harbors symbiotic methanotrophic bacteria. In each of 

these cases, the symbiotic bacteria provide the overwhelming bulk of the host’s nutritional needs, 

making the animals themselves chemoautotrophic in a sense. 

Three overlapping types of communities are present in the primary areas that were a focus for 

this study. Bacterial mat communities are apparent both separated from and immediately 

adjacent to aggregations of other types of seep fauna. These mats can be quite extensive in both 

size of individual mats and abundance of mats at a site. Because the mats produce organic 

biomass and oxidize potentially toxic substrates, they are potentially important primary 

producers and detoxifiers for the seep ecosystem. Tubeworms and mussels, because of their 

autotrophic lifestyle, gregarious settlement behavior, and relatively large size, are ecosystem 

engineers. As such, they create substantial areas of habitat for numerous other species of 

endemic and non-endemic animals. Tubeworm and mussel communities can be relatively small 

or can completely cover areas up to thousands of square meters. Each can occur where the other 

is absent, or they can co-occur, each literally growing on one another. 

One of the long-term goals of the Minerals Management Service's programs to study seep 

communities has been to develop an understanding and appreciation of these relatively poorly 

known and recently discovered communities. The hope is to provide the knowledge base for 

well-informed management decisions. Over the past fifteen years, many other projects have also 

contributed to the database on these and related communities. However, only recently has there 

been available sufficient information to put together a cohesive picture of the biology and 

ecology of the seep animals. The present study specifically built on what was known about the 

physiology, genetics, and ecology of the three key faunal groups (bacteria, tubeworms and 

mussels) to build and test working models of the animal’s life histories and the communities they 

form. In some cases, studies were designed to address very fundamental physiological or genetic 

questions important to understanding the animals. In other cases, studies were designed to test 

system-level concepts arising from developing models of community succession, larval 

dispersal, or niche differentiation. 

To the greatest extent possible, experimental design included integrated sampling between the 

different components of the overall project. For example, before a mussel bed was sampled, 

water samples were taken from beneath and over the mussels for chemical analysis. The bed 

 

*

 This section was authored by Charles R. Fisher 
8-2 

was then sampled in a quantitative approach that allowed the biomass and size of all inhabitants 

to be determined and the biomass of the entire bed to be calculated from photographs or mosaics. 

Each collection was further sub-sampled for all appropriate components (i.e., condition analyses, 

PAH tissue load determination, histopathology, population genetics, stable isotope food-web 

studies, and in some cases growth or transplant studies). This greatly enriched the interpretation 

and future usefulness of each individual study and is also an ecologically sound approach to 

obtaining the maximum amount of knowledge with minimal impact on the populations. 

Over the course of the program, numerous discoveries were made that fundamentally changed 

the views of the animals and the working models of these communities. Many of the studies 

remain ongoing because early results led to new directions and/or development of new 

techniques. Finally, as discovery usually does, the results of these studies spawned new and 

important questions that were not anticipated when the program was initiated. 

Each of the separate studies presented in this chapter produced findings that have important 

implications for management decisions even when considered in isolation. These findings and 

studies are briefly highlighted here. More discussion of these discoveries and the related studies 

that have contributed to the working models are contained in the synthesis chapter of this report 

(Chapter 11). 

Section 8.4 details the key discovery that the two species of tubeworms are extremely long-lived, 

in fact, the longest lived animals known to exist on the planet. This is in direct contrast to the 

hydrothermal vent tubeworms, which are a very fast-growing, opportunistic species. Because 

the tubeworms produce the framework for a complex ecosystem, the seep tubeworms are much 

more analogous to trees in the sense of being long-lived and ecosystem-structuring. As a result, 

activities that damage well-established communities may have an impact that would last for 

centuries. In fact, consideration of their long life in light of the non-persistent size distributions 

in the aggregations suggests that old communities might never recover from serious damage 

because the time window for vestimentiferan recruitment to the site would have passed. 

A second discovery discussed in Section 8.4 is that at least one of these species can acquire 

sulfide from buried sources and therefore may not require surface expression of seepage (into the 

water column). This may explain how the tubeworms live for centuries and also why the 

diversity of non-endemic fauna in these communities is much higher than at hydrothermal vents. 

It also suggests that the sulfide supporting these communities must be present at depths 

considerably greater than the depths of sulfide generation from seawater sulfate reduction (see 

Chapter 9). Thus, hydrocarbon seepage alone (with attendant production of sulfide in very 

shallow sediments) may not be sufficient for maintenance of large and long-lived tubeworm 

colonies. Therefore, the source of sulfide necessary to support these communities may be more 

patchily distributed than hydrocarbon seepage in general. 

Section 8.5 demonstrates that the mussel beds in some areas of the Gulf of Mexico were so 

severely infected with parasites that mussels in these areas are effectively sterile and therefore 

cannot contribute to larval pool of the Bathymodiolus childressi in the Gulf of Mexico. By 

comparison, mussel populations at other sites were quite healthy. Thus, some populations may be 

key to the maintenance of the Gulf-wide meta-population, while other sites are inconsequential. 

The life history of seep mussels is further detailed in Section 8.3. 
8-3 

A finding with implications for heterotrophic fauna is presented in Section 8.6. Results from 

stable isotope studies show that seep primary production not only supports a higher diversity of 

animals that are seep endemics or colonists than previously demonstrated, but also that 

significant amounts of seep primary production are being transferred into the surrounding deepsea benthos. Of the eight species of larger mobile predators captured and analyzed for this study, 

individuals of six contained a significant amount of seep carbon and nitrogen in their tissues, 

indicating continued consumption of seep animals or bacteria over a significant period of time. 

Thus the seeps are not small islands of productivity inhabited by unique animals to be managed 

in isolation, but rather they are affecting and being affected by the surrounding deep-sea fauna. 

Related to this finding is the discovery that significant numbers of the mat forming bacteria are 

present in situations when the mats are not visually apparent, and the strong implication that 

most of the fauna found among the tubeworms are not deriving their nutrition from tubeworm 

primary production. This finding, which is detailed in Section 8.2, suggests that primary 

production from free-living seep microbes may be much more important to the communities than 

usually recognized and underlies how little we know about the microbial ecology of these 

systems. 

The molecular genetics studies, which are presented in Section 8.7, are still underway due to a 

combination of unforeseeable complications in the samples themselves and in the methodology. 

However, this work to date indicates that over the areas sampled for this study, the tubeworm 

and mussel populations are exchanging larvae between sites and therefore can be managed as 

single, large, meta-populations. Genetic diversity is not at risk through anthropogenic activities 

in this region. Another key finding is that the symbionts of four Gulf of Mexico tubeworm 

species are not specific to the host species, but rather to the geographic region where they are 

found. This strongly supports the hypotheses that the symbionts are not transmitted directly 

(rather they must be acquired de-novo each generation) and that niche-differentiation between 

species at a site is not a function of symbiont differences, and further suggests site-specific 

differences in the bacterial populations at different sites. All of this information contributes to 

our working models of these species. 

Based on our current level of understanding of the biology of the key ecosystem engineers and of 

the community ecology of the seep ecosystem, we have refined our working model of these 

communities. It is important to note that in the regions of the Gulf of Mexico where the seep 

communities occur the larval population is well mixed, and based on other studies, we know that 

the larvae of key species spend weeks or more in the water column. Thus, colonization and 

evolution of a new site will follow the same general pattern as colonization and evolution of 

patches of organisms within an established site, although the increased abundance of mobile 

consumers at established sites will likely influence the process through biological interactions. 
8-4 

8.2 Bacterial Mats*

8.2.1 Objectives 

This section addresses the following objectives: 

1. To characterize and compare of the morphology of Gulf of Mexico Beggiatoa to ascertain 

their affinity to ecologically similar groups elsewhere; 

2. To examine the dominant energy motifs of Beggiatoa with respect to their vertical 

distribution, their change in internal nitrogen ions, and their impact on sediment sulfide; 

3. To verify that Beggiatoa are capable of consuming energy sources (e.g., sulfide or methane) 

at sediment depths up to 10 cm; 

4. To assay bacterial mats for diagnostic enzymes. 

8.2.2 Introduction 

Colonies of the bacteria Beggiatoa spp. are prominent indicators of hydrocarbon seepage in the 

outer continental shelf and slope of the Gulf of Mexico. They are among the earliest known 

chemoautotrophs, and their ability to synthesize new carbon by use of chemical energy obtained 

by oxidizing H2S was suspected as early as the late 1880s (Brock and Schlegel 1989). Beggiatoa

are members of a diverse group of sliding and gliding bacteria that occupy large filaments and 

have the ability to position themselves along chemical gradients in the upper-most sediments 

(Larkin et al. 1994, Larkin and Strohl 1983, Nelson et al. 1989). Beggiatoa are very common in 

Gulf of Mexico seeps. Indeed the present work found that individual filaments were nearly 

ubiquitous among sediment samples from the study sites and often infiltrated the upper 5-10 cm 

of sediment cores. Beggiatoa are most visible when they form thick surface aggregations, often 

termed "bacterial mats," on the periphery of seep habitats occupied by tubeworms and mussels 

(MacDonald et al. 1989, Nikolaus 1995). These mats usually extend well beyond the immediate 

vicinity of tubeworm or mussel aggregations. They are highly visible due to their white or 

yellowish color and will frequently be encountered in visual surveys of seep localities. 

Recently, several studies have been published on natural populations of morphologically diverse 

marine chemoautotrophic, sulfur-oxidizing bacteria belonging to the genera Thioploca, 

Beggiatoa, and Thiomargarita (Fossing et al. 1995, McHatton et al. 1996, Nelson et al. 1989, 

Otte et al. 1999, Schulz et al. 1999). Individual cells of these uncultivated bacteria are unusually 

wide ranging from 12 to 750 m in diameter versus a typical bacterial cell dimension of 

approximately 1 m. Each of the wide sulfur-oxidizers examined has been found to contain a 

central vacuole as the majority of the vital volume of each cell and whenever examined these 

bacteria have been observed to accumulate nitrate at intracellular concentrations ranging from 40 

to 800 mM – roughly 10,000-fold higher than the corresponding ambient concentration. The 

possession of vacuoles is an extremely unusual bacterial trait, and until the work of Fossing et al.

(1995) accumulation of nitrate by bacteria was unknown. The physiological role of nitrate, which 

is presumably stored in the vacuole, has been fairly well characterized for two natural 

 

*

 This section was authored by Douglas C. Nelson and Sarah C. McHatton. 
8-5 

populations (McHatton 1998, McHatton et al. 1996, Otte et al. 1999). It appears these bacteria 

employ nitrate as an electron acceptor that allows them to fuel their metabolic processes via 

oxidation of hydrogen sulfide even in the absence of oxygen. This respiratory process, which 

converts sulfide to elemental sulfur plus sulfate and nitrate to ammonia, allows the studied 

populations of Beggiatoa and Thioploca to dominate the top 10 to 20 cm of marine sediments 

and to reach biomass densities exceeding 1.0 kg wet wt./m2. When dense mats of these bacteria 

are observed at the sediment/water interface, they are at the one point in their migration cycle 

where it is possible for them to take up and store nitrate. All of the vacuole-containing, nitrateaccumulating bacteria that have been characterized to date by molecular analysis (Ahmad 1999, 

Ahmad et al. 1999, Teske et al. 1999) fall into a very restricted evolutionary affinity group 

(Cluster 3, Figure 8.2.1), suggesting that they also share numerous other metabolic properties. 

8.2.3 Results 

Based on samples from a number of sediment cores, the dominant Gulf of Mexico Beggiatoa

populations are in two main width classes, averaging approximately 40 and 80 m in diameter 

(Table 8.2.1). Any core taken in the vicinity of the hydrocarbon seeps, regardless of whether a 

surface mat was present, contained significant biomass of these bacteria distributed over the 

upper 3 to 5 cm of the sediment (Table 8.2.1). There is some tendency for the depth-integrated 

Beggiatoa biomass to be lower for the sites where no surface mat was present (Figure 8.2.2), but 

the no-surface-mat core designated GCB3 C9 (Table 8.2.1) has a total biomass and vertical 

distribution typical for those of the orange and white mats (Table 8.2.1, Figure 8.2.2). For 

representative samples of the narrower (ca 40 m) Gulf of Mexico Beggiatoa tested, the protein 

to vital volume ratio (Table 8.2.2) indicates that they contain a non-cytoplasm-filled vacuole as 

approximately 80% of their vital volume. That is, like the vacuolate strain from Monterey 

Canyon, they contain only about 20% of the protein predicted if their vital volume was occupied 

by typical bacterial cytoplasm. 
8-6 

Table 8.2.1. Depth distribution of Beggiatoa filaments in Gulf of Mexico cores, 1998. 

Sample Cell Width Beggiatoa (grams wet wt./ m sq.) by depth interval 

 (um) 0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm Total Comments 

BPB2 C5 40 1.4 0.7 0.1 0 0 202 No surface mat 

BPB2 C7 37 2.2 1.8 0.3 0 0 4.3 dense Orange mat 

BPB3 C7 40, 81 6.1 3.5 2.3 0.7 0 12.6 Orange mat 

GCB1 C3 38 4.7 2.9 2.3 1.1 0.7 11.7 White/Orange mat 

GCB1 C4 44, 80 8.4 1.5 1.5 0.2 0.2 11.8 great Orange mat, 

harvested

GCB2 C7 41 4.1 2 1.5 0.5 0 8.1 Orange mat, 

harvested

GCB3 C1 42 9.2 5.3 2.6 1.4 0.7 19.2 great Orange mat, 

harvested

GCB3 C2 42, 81 7.6 3.8 2.1 0.8 0.5 14.8 White/Orange mat 

GCB3 C9 40 4.8 3.2 1.8 1.3 0.3 11.4 No surface mat 

GCBXC1 C3 37 2.6 0.4 0.4 0 0 3.4 No mat, box core 

Table 8.2.2. Evidence that 37 – 44 m wide Beggiatoa

from Gulf of Mexico have vacuoles that lack typical bacterial cytoplasm. 

Beggiatoa sample Filament Diameter Central Vacuole? 

Protein Vital volume 

(mg/cm3

) 

Control strain MS-81-6 

(pure culture) 

4 μm No 121 ± 17 (n = 12) 

GCB2 C7 

Gulf of Mexico Seeps 

43 μm Yes 25 – 27 

Monterey Canyon Seeps 75 μm Yes 24 
8-7 

Figure 8.2.1. Evolutionary relationships among selected members of the gamma roteobacteria as determined by 

16S rRNA sequencing (Ahmad 1999, Ahmad et al. 1999). The majority of the strains shown are 

autotrophic sulfur bacteria. Cluster 3 includes all vacuolate sulfur bacteria known to accumulate 

intracellular nitrate. Clusters 1 and 2 include all known sequences for, respectively, non-vacuolate 

freshwater and non-vacuolate marine pure cultures of Beggiatoa spp.
8-8 

Figure 8.2.2. Depth integrated Beggiatoa biomass (0 to 5 cm) for 10 representative cores from Gulf of Mexico 

hydrocarbon seeps. Groupings depend on color of surface Beggiatoa mat or its absence. Sites and 

biomass versus depth details are presented in Table 8.2.1. 

Table 8.2.3 summarizes the physiological properties for surface Beggiatoa populations taken 

from various Gulf of Mexico cores. All populations tested accumulated nitrate to internal 

concentrations of 8 to 110 mM when averaged over their entire vital volume (cytoplasm plus 

vacuole). Activity of the enzyme alpha-ketoglutarate dehydrogenase (AKGdH), indicative of the 

ability to oxidize organic matter by the Kreb’s cycle (also called the citric acid cycle or TCA 

cycle) was present in three of the eight samples tested. It was present in some, but not all, orange 

mats tested and in the one mixed orange/white mat tested. However, it was not detected in the 

only pure white mat tested (Table 8.2.3). The enzyme ribulose-1,5-bisphosphate 

carboxylase/oxygenase (RuBPC/O), diagnostic for carbon dioxide fixation via the Calvin cycle, 

was present in 9 of the 11 samples tested. Although the highest RuBPC/O activity was observed 

for a white mat (BHB3 C1, Table 8.2.3), other white mats had less activity than some orange 

mats, and one white 
8-9 

mat had no detectable activity. No pure orange mat that was assayed had zero detectable 

RuBPC/O activity (Table 8.2.3). 

Several 80-m wide Gulf of Mexico filaments hybridized with a 16S rRNA oligonucleotide 

probe specific to Monterey Canyon Beggiatoa sp. (Ahmad et al. 1999). 

Table 8.2.3. Seep Beggiatoa, Gulf of Mexico. Enzyme activities and internal nitrate concentrations. 

Core/Subcore RuBPC/O 

(nmol min-1 mg-1 prot) 

AKGdH 

(nmol min-1 mg-1 prot) 

Int. NO3

-

(mM) 

Beggiatoa Color 

BHB1 C3 0 1220 22 Orange/White 

BHB3 C1 4.8 - 110 White 

BPB2 C1 0.22 125 - Orange 

BPB2 C5 - 1270 - Orange 

BPB2 C7 slight. Activity - - Orange 

BPB3 C7 0.16 below detection limit 12 Orange 

GCB1 C3 0 - 19 White 

GCB1 C4 - below detection limit 26 Orange 

GCB1 C7 0.35 below detection limit - White 

GCB1 C8 0.44 below detection limit - Orange 

GCB2 C2 0.066 - 36 Orange 

GCB2 C7 0.11 - 8 Orange 

GCB3 C4 0.05 below detection limit - Orange 

8.2.4 Discussion 

Compared to Monterey Canyon Beggiatoa populations, the Gulf of Mexico Beggiatoa mats have 

shallower deep distributions in the sediment and lower overall biomass densities (Figure 8.2.3). 

Monterey Canyon Beggiatoa sp. and Thioploca spp. appear to consume sulfide completely over 

the zone they occupy while respiring internal nitrate stores to ammonia (McHatton 1998, Otte et 

al. 1999, see figure 8.2.3). It is assumed, based on possession of a nitrate-filled vacuole and the 

close evolutionary relationships of all vacuolate filamentous bacteria (Figure 8.2.1), that the 

dominant Gulf of Mexico Beggiatoa spp. will have similar metabolic properties with respect to 

sulfide oxidation and nitrate reduction. By hypothesis, their oxidation of sulfide or stored 

elemental sulfur to sulfate at depths of up to 5 cm in the sediment will serve to re-supply this 

electron acceptor to the very active sulfate reducing bacterial populations present there (see 

Section 9). Their relatively low internal nitrate stores (Table 8.2.4) may reflect either low 

availability of this compound or rapid consumption of nitrate to drive the oxidation of reducedsulfur substrates. 
8-10 

Figure 8.2.3. Comparison of sediment depth distributions of (A) Monterey Canyon Beggiatoa sp. (75-m cell 

diameter) at a cold seep (900-m depth) and (B) a representative Gulf of Mexico Beggiatoa sp. (42-

m cell diameter) at a hydrocarbon seep (600 m depth). For panel A, note the absence of free H2S 

throughout the range occupied by Beggiatoa and the ammonia maximum centered in the same 

region. These chemical signatures are assumed to be due to energy conserving bacterial oxidation 

of sulfide to sulfate at the expense of the reduction of internally stored nitrate to ammonia. 

Although the scale is compressed for panel B, similar metabolism is assumed. For panel A 

biomass is measured in mg dry wt. per cm3

; for panel B it is in gm wet wt. per m2 over a 1.0-cm 

depth interval. 
8-11 

The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC/O), diagnostic for 

carbon dioxide fixation via the Calvin cycle, was present in 9 of the 11 samples tested. Although 

one white Beggiatoa mat (BHB3 C1) had high activity of this enzyme (Table 8.2.3), the average 

value for these populations (Table 8.2.4) is 3- to 30-fold lower than the corresponding activity 

found in chemoautotrophic populations from Monterey Canyon seeps and the Guaymas Basin 

hydrothermal vents. Conversely, certain Gulf of Mexico Beggiatoa populations contain alphaketoglutarate dehydrogenase activity, an enzyme diagnostic for the ability to oxidize organic 

matter in the process of energy generation. This is the first such demonstration of this activity for 

any native Beggiatoa population (Table 8.2.4). Obligate chemoautotrophic Beggiatoa strains and 

facultative strains grown as chemoautotrophs lack this activity (Table 8.2.4). Among 

chemoautotrophs, only facultative strains grown as chemoheterotrophs (e.g., strain 81-6 in the 

presence of acetate, Table 8.2.4, see also McHatton et al. 1996) possess this activity, which is 

essential for oxidizing organic matter to carbon dioxide in energy conserving reactions. Thus, 

although we did not completely confirm the finding of Nikolaus (1995) that orange Beggiatoa

mats lack Calvin cycle activity while white mats possess it, it seems that the Gulf of Mexico 

mats are, on average, less autotrophic and more heterotrophic than any natural Beggiatoa mats 

previously studied. Given the high organic content of the sediments they occupy, this should not 

be surprising. These high enzyme activities associated with organic respiratory processes do not 

preclude the possibility that sulfide oxidation rates are also high. 

Table 8.2.4. Comparison of Gulf of Mexico Beggiatoa spp. with 

non-vacuolate, narrow Beggiatoa cultures and other native populations of vacuolate bacteria. 

Non-Vacuolate 

Beggiatoa 

Controls 

Source RuBisC/O Activity 

 (nmol min-1 mg-1

prot) 

KDGH Activity 

(nmol min-1 mg-1

prot) 

Internal 

[NO3

-

] 

(mM) 

Fil. Width 

(μm) 

strain 81-1c 

(obligate 

chemoautotroph) 

sulfide medium 

sulfide+acetate 

20 ± 2.5 

14 ± 1.6 

0 

0 

0.0003 2 

strain 81-1c 

(facultative 

chemoautotroph) 

sulfide medium 

sulfide+acetate 

23 ± 9.2 

3.3 ± 0.2 

0 

153 ± 11 

not tested 4 

Vacuolate 

Bacterial Genus 

 

Beggiatoa spp. Gulf of Mexico 

seeps (this study) 

0.56 

(n = 11) 

330 

(n = 8) 

32 

(n = 8) 

40 & 80 

Beggiatoa sp. Monterey Canyon 

seeps 

7.5 - 15 0 160 

(n = 5) 

75 

Beggiatoa spp. Guaymas Basin 

hydrothermal vents 

1.47 

(n = 8) 

not tested 130 

(n = 3) 

24 - 32 

40 - 42 

116 - 122 

Beggiatoa sp. Bay of Concepion not tested not tested 42 ± 27 

(n = 16) 

35 – 40 

Thioploca spp. Chilean oxygen 

minimum zone 

not tested not tested 150 - 500 12 – 20 

30 – 43 

Thiomargarita 

namibiensis

Namibian oxygen 

minimum zone 

not tested not tested 100 – 800 100 – 750 
8-12 

8.3 Mussel Community Ecology*

8.3.2 Objectives 

The overall objectives of the mussel community ecology study fall into five general categories as 

follows: 

To characterize the primary MMS study sites with respect to the biomass and numerical 

density of mussels, the size frequency of mussel populations, methane concentrations in 

water among the mussels, physiological condition of the mussels, and biomass and 

species composition of the associated fauna; 

To characterize in a more general sense the species richness and biomass of associated fauna 

supported by this type of chemosynthetic community at the shallow Gulf of Mexico seep 

sites; 

To determine the growth rate of mussels from GB425 for comparison to previous studies; 

To determine the time course of changes in condition of mussels after removal from a 

favorable habitat in order to provide increased interpretive power in the previous and 

current transplant experiments; 

To conduct reciprocal transplant experiments to test the hypotheses that differ in growth 

rates, condition, and maximum size of the mussels in different study sites found in the 

previous MMS study that are due to habitat characteristics; 

8.3.3 Laboratory methods 

Measures of physiological condition in mussels. Three analyses were initially employed to 

document variations in the physiological condition of mussels in the short-term transplant 

experiment. After evaluation of that data and data from previous studies (Nix et al. 1995; Smith 

et al. 2000), it was determined that glycogen content provided little or no additional information 

on the physiological condition of the mussels from different sites, so that analysis was not 

performed on animals from other collections. Water content (as a percent of wet weight) and 

standard bivalve condition index (CI, the ratio of ash free dry tissue mass to shell volume) were 

determined on 6 to 12 mussels from all treatments of the reciprocal transplant experiment and for 

6 to 12 mussels from each primary MMS study site. Two techniques were used to determine the 

dry and ash free dry weights of the mussel tissue, depending on whether the mussels were also 

analyzed for glycogen content. For mussels also analyzed for glycogen content (primarily the 

short-term transplant mussels), the mussel tissue was first homogenized in nine times its mass of 

deionized water. Ten mm of the homogenate was dried at 60C to constant weight (usually 48 

hours) and then ashed at 500C to obtain an ash weight. Animals not analyzed for glycogen were 

diced in a glass petri dish, and three non-uniform tissue samples were dried and then ashed. 

Tissue water content was calculated from the wet weight and dry weight. Shell volumes were 

calculated from the weight of sand needed to fill the shells and an empirically determined 

relationship between sand weight and volume. Glycogen content of the tissue of the short-term 

 

*

 This section was authored by Charles R. Fisher and Tracie Ward. 
8-13 

transplant treatment mussels was measured using a modified spectrophotometric enzyme assay 

(Nix et al. 1995) and references therein on a subsample of the primary homogenate further 

diluted to a final concentration of 1:499 with deionized water. 

Separation and biomass determination of associated fauna. In the laboratory, the faunal 

specimens were transferred from formalin into a 70% ethanol solution. They were then sorted 

into provisional taxonomic groups, hereafter called morphospecies. Members of the same 

morphospecies from each location were counted and sorted by size. A random subsample was 

then selected for biomass analysis from the size classes that represented the majority of the 

collection. 

The subsampled individuals were weighed (wet weight) and placed in a 60C oven until a 

consistent weight was obtained (dry weight), usually about 30 hours. The samples were then 

ashed at 500C for six hours to obtain an ash weight. From these measurements, ash free dry 

weight (AFDW) was calculated by subtracting the ash weight from the dry weight, and the ash 

free wet weight (AFWW) was calculated by subtracting the ash weight from the wet weight. To 

obtain the biomass for an entire morphospecies of a site, the overall wet weight of the 

morphospecies collection was recorded and multiplied by the average percent-AFWW of the 

collection in question. Percent-AFWW was calculated as ash free wet weight divided by wet 

weight. An estimate of a morphospecies with many individuals across the same site was found 

using the average percent-AFWW for the subgroup of the site in question. For less wellrepresented individuals, percent-AFWW from all collections in which the morphospecies was 

present was averaged. The average percent-AFWW was then used for all biomass estimation of 

the under-represented species at all sites. 

Mussel lengths were used to estimate biomass using the empirically determined equation, 

W = 2.88  10-5  L3.1862 (8.3-1) 

where W is tissue wet weight in grams, and L is shell length in millimeters (Fisher, 1993). 

Preliminary statistical analyses. Significant differences between the size-frequency 

distributions in the different zones and years were detected using a continuity-adjusted chisquared test to correct for small sample sizes (SAS Institute, 1989). If all years of the same zone 

were not significantly different (p > 0.05), then the size-frequency distributions for all years were 

combined and used to test for differences between zones. 

Due to the nature of the condition indices data (percentages and ratios), the data for all three 

condition measurements were transformed using an arcsine function to approximate a normal 

distribution prior to statistical analysis. Significant differences between zones and years were 

tested using ANCOVA with length as the covariate (SAS Version 6.07, Proc GLM). If the model 

was significant (p < 0.05), then differences between specific sites and years were tested using 

Tukey’s pairwise comparisons (SAS Version 6.07, Proc GLM). 
8-14 

The von Bertalanffy growth parameters of L (maximum size) and k (the rate at which L is 

approached) were estimated from mark-recapture data using Fabens’ method (Fabens, 1965) and 

asymptotic standard errors were obtained for these estimates (SAS Version 6.07, Proc NLIN). 

However, the estimation of von Bertalanffy growth parameters from mark-recapture data when 

the recapture interval of each individual is the same is not straightforward. Simulation studies 

have shown that when individual variability in growth parameters exists, the parameter estimates 

obtained may be extremely biased even when Fabens’ method, the best method available at this 

time, was used (Smith et al. 1997). Therefore, although the estimates provided here are internally 

consistent and consistent with size-frequency data, they may be biased from the true population 

parameters. 

Due to the potential bias in parameter estimates obtained for the von Bertalanffy growth model, 

it is not possible to test with adequate power for differences between populations using standard 

t-tests on these estimates (Smith et al. 2000). Therefore, it is necessary to use alternative 

techniques such as nonlinear regression with indicator variables (modified from Juliano and 

Williams 1985). First, Equation 8.3.1 (for comparing two sites or years) 

Lt+1 = (L + e

-(k +I) dt) + e-(k + I) dt  Lt (8.3-2) 

where Lt+1 is the recapture length (mm), Lt is the initial length (mm), dt is the recapture interval 

(years), and I is the indicator variable with a value of 0 for the first site or year, or 1 for the 

second site or year, was fit for the two sites or years of interest. Then standard F-tests were used 

to test for significant differences between the full model (Equation 8.3.1) and the reduced model 

with no indicator variables and for the significance of the model parameters (and ), which 

would indicate significant differences between the growth parameters. This technique provides 

adequate power in testing for differences in growth between populations in spite of the potential 

bias in parameter estimates. However, tests for the significance of individual parameters, either 

L or k, using this method does not provide adequate power in most cases (Smith et al. in 

preparation). Therefore, this nonlinear regression technique can detect differences in growth 

between two populations, but it cannot adequately assign this difference to either L or k. 

8.3.4 Results 

Site characterizations and differences. Methane was consistently detectable at the Brine hosted 

sites but was only detected at three of the eight mussel beds at GC185 and GC234 (Table 8.3.1). 

The detection of methane was correlated with the presence of recent recruitment events at a site 

as reflected in the size frequency distribution of mussels (Figure 8.3.1). Recent recruitment had 

occurred at the BHM3, GCM2, and GC233 (Figure. 8.3.1) but not at BHM1 where methane was 

consistently detected in 1997. 
8-15 

Table 8.3.1. Methane levels in water samples from among mussels at MMS stations. 

Range 

Station Year n 

Min ∑CH4 

(mM) 

Max ∑CH4 

(mM) 

Mean ∑CH4 

(mM) 

1997 4 0.009 0.101 0.039 

BHM1 

1998 4 0 0.055 0.0014 

BHM2 1997 3 0 0 0 

1997 3 0 0.010 0.003 

BHM3 

1998 3 0 0 0 

BHM4 1997 0 0 0 0 

BHM5 1997 3 0 0 0 

BHM6 1998 3 0 0 0 

1997 3 0 0 0 

GCM1 

1998 2 0 0 0 

1997 3 0.016 0.068 0.035 

GCM2 

1998 2 0 0 0 

1997 3 0.253 0.797 0.452 

GBM1 

1998 6 0 4.496 1.359 

1997 8 0.021 0.162 0.077 BPM1 (middle) 1998 3 0.049 0.061 0.055 

BPM2 (outer) 1997 3 0.690 0.926 0.794 

BPM3 (middle) 1997 3 0.690 2.162 1.483 

1997 3 0.142 0.937 0.433 

BPM4 (inner) 1998 4 1.052 0.356 0.141 

BPQ (inner) 1997 3 0.015 0.028 0.019 

BPR (outer) 1997 3 0 0 0 

BPT (inner) 1998 6 0.132 0.475 0.245 

BPV (inner) 1998 3 0 0.058 0.019 

BPW (outer) 1998 3 0.077 0.240 0.155 

The wide variation in the density and area-specific biomass of mussels in the different 

collections provides a picture of the range in these parameters found in this type of 

chemosynthetic community around shallow seep sites in the Gulf of Mexico (Figure 8.3.2 and 

8.3.3). No consistent difference between types of sites, or sites, is apparent in this data set that 

emphasizes the variability within a site and even within a large mussel bed. 

The variation in the condition indices (percent-water and CI) also reflects significant bed-to-bed 

variation (Table 8.3.2). The two beds at GB425 are significantly different with respect to both CI 

(p < 0.01) and percent-water (p < 0.005). The two beds at GC234 are significantly different with 

respect to CI (p < 0.001). Similarly, GC185 bed M5 had a significantly lower CI than any other 

BH bed, and M4 was also significantly lower than M1. At GC233, the M1 and M2 collections 

had significantly different CI values and were significantly lower than the M3 and M4 

collections. However, the large sample size of this data set also allows robust comparisons 
8-16 

between types of sites and sites. Both indices indicate that, overall, mussels at the brine 

dominated sites are in better condition than mussels from petroleum dominated sites (CI, p < 

0.001; percent-water, p < 0.005) and that the mussels at GC233 are in better condition than 

mussels from GC234 or GC185 (CI p < 0.001; percent-water, p < 0.05). 

Associated fauna. The gastropod, Bathynerita naticoides, and the Alvinocarid shrimp were 

present in all mussel beds sampled and were often abundant (Table 8.3.3). The undescribed 

orbinid polychaete was very abundant in most collections from GC233 and both collections from 

GC234 but was absent in the collection from GB425. Only a single individual was found in a 

single collection from GC185. The galatheid crab, Munidopsis sp. 1, and the gastropods, 

Provanna sculpta, Cantrainea meroglypta, and Buccium canetae, were abundant in GC185 

collections but absent or very rare in GC233 collections. 
8-17 

Figure 8.3.1A. Size frequency distribution of mussels from six collections at the GC233 in 1995. 
8-18 

Figure 8.3.1B. Size frequency distribution of mussels in each of the MMS primary study sites (G-L). 
8-19 

Figure 8.3.1C. Size frequency distribution of mussels in each of the MMS primary study sites (M-R). 
8-20 

Figure 8.3.1D. Size frequency distribution of mussels in each of the MMS primary study sites (S-T). 

Short-term transplant experiments. Water sampling data showed non-detectable levels of 

methane (< 2 M) at the short-term transplant location while the inner edge of GC233 (BP I) had 

moderately high levels of methane (Table 8.3.4). Both sites were similar in oxygen 

concentration; however, the oxygen at the short-term transplant site could potentially be much 

higher as the highest observed concentration here was over two times as high as the maximum 

observed at GC233. The three physiological condition indices showed no significant differences 

between the inner zone mussels and either the 13- or 42-day transplants (Table 8.3.4). Only the 

13-day control mussels were found to have significantly lower CI and higher water content from 

all other samples, indicating that the mussels in this control box sustained damage through 

manipulation or were placed in a very stressful microhabitat at GC233. Glycogen content in all 

of the mussels sampled including the original population was highly variable (Table 8.3.5), 

possibly obscuring differences between transplant groups. However, this variability did not 

correspond with any experimental factor, such as order run or assay date, in the assay protocol, 

indicating that the variability was most likely present in the inner zone mussel population and 

was not a result of the transplant experiment. 
8-21 

Figure 8.3.2. Density of mussels, as individuals per sq. m, in each of the MMS primary study sites and from six collections at the G
8-22 

Figure 8.3.3. Biomass of mussel soft tissue in each of the MMS primary study sites and from six collections at the GC233 in 1995. 
8-23 

Table 8.3.2. Characteristics of mussel collections from each primary MMS study site and GC233 in 1995. 

Year Dive Station 

L max 

(mm) 

L min 

(mm) 

L mean 

(mm) 

L median 

(mm) 

Density 

(indiv/sq. 

m) 

Biomass 

(g/sq. m) 

C

1995 2626 BP-inner 128.8 2.4 58.2 (±32.9) 53.9 1360 32210 ND  2630 BP-inner 130.0 14.9 59.5 (±28.1) 53.7 1650 34990 ND  2628 BP-middle 125.0 31.0 101.3 (±18.0) 106.7 499 34550 ND  2630 BP-middle 127.3 2.5 102.2 (±27.2) 110.5 443 34100 ND  2626 BP-outer 125.2 13.4 102.8 (±15.3) 106.6 423 29730 ND  2630 BP-outer 134.5 8.3 86.2 (±42.0) 109.0 265 16950 ND 1997 2852 BHM1 93.4 13.3 65.5 (±15.1) 67.5 1380 25213 0. 2857 BHM2 100.7 53.8 84.4 (±11.2) 86.7 255 9148 0. 2865 BHM3 79.7 10.5 59.8 (±27.1) 75.5 632 13259 0. 2868 BHM4 93.4 16.1 59.2 (±15.6) 62.2 918 12832 0.0 2873 BHM5 97.4 14.6 61.6 (±11.9) 61.3 2140 31488 0.0 2851 BPM1 117.7 27.5 73.9 (±23.0) 72.5 ND ND ND  2870 BPM2 128.9 9.5 75.0 (±36.7) 81.3 ND ND 0. 2870 BPM3 125.8 5.0 81.4 (±24.2) 78.7 ND ND 0. 2876 BPM4 124.1 7.4 82.2 (±24.1) 81.9 ND ND 0. 2877 GCM1 78.9 9.6 55.8 (±9.1) 56.5 2237 23298 0.0 2883 GCM1 134.5 75.2 103.7 (±11.1) 103.4 456 34362 0.0 2889 GBM2 77.9 3.8 51.8 (±21.8) 60.0 748 8258 0.01998 4038 BP-inner 95.5 2.2 30.5 (±22.4) 24.1 2424 11560 0. 4038 BP-outer 119.9 86.9 105.7 (±10.4) 107.8 132 9890 0. 4342 BHM3 96.6 1.5 50.6 (±28.1) 53.7 723 10230 0. 4342 BHM6 85.2 11.1 49.6 (±13.9) 49.1 1075 9020 0. 4040 GBM1 118.4 58.5 107.3 (±14.5) 112.4 158 12630 0.0 4041 GBM1b 122.2 13.2 85.2 (±30.0) 96.5 408 20110 0.ND – Not determined 
8-24 

Table 8.3.3. Density (number of individuals per sq. m) of associated fauna in mussel beds. 

Year Dive Station Bathynerita naticoidea 

Alvinocaris stactophila 

Alvinocaris stactophila affinis 

Bersilliidae sp.

Orbiniidae 

Munidopsis n. sp. 1

Provanna sculpta 

Cantrainea sp. 

Buccinidae sp. 

Polynoidae sp.

Xanthidae sp.

Phascolosoma turnerae 

Lamellibrachia sp.

Escarpid-like sp.

1995 2626 BP-inner 87 346 31 0 5265 0 0 5 0 15 0 0 0 0  2630 BP-inner 26 87 15 0 3657 0 0 0 0 5 0 0 0 0  2628 BP-middle 51 138 5 0 52 0 0 0 0 0 0 0 0 0  2630 BP-middle 31 51 5 0 1323 10 0 0 0 0 0 0 0 0  2626 BP-outer 76 82 0 0 3631 0 0 0 0 0 0 0 0 0  2630 BP-outer 66 10 5 0 1219 0 0 0 20 0 0 0 0 0 1997 2852 BHM1 827 24 49 0 6 30 67 0 0 0 0 0 0 0  2865 BHM3 2674 176 0 0 0 0 55 30 6 0 0 0 0 0  2868 BHM4 2273 164 30 0 0 18 760 711 24 0 6 0 0 0  2873 BHM5 1106 97 12 6 0 97 590 6 140 0 0 0 0 0  2851 BPM1 6 650 6 6 1167 24 0 0 0 0 0 0 0 0  2870 BPM2 280 511 0 0 0 6 0 0 24 0 0 0 0 0  2870 BPM3 201 37 0 0 0 12 0 0 0 0 0 0 0 0  2876 BPM4 67 201 67 0 0 0 0 0 0 0 0 0 0 0  2877 GCM1 979 286 18 0 693 6 55 0 0 0 0 0 0 0  2889 GCM2 1787 565 37 0 2383 0 164 213 0 0 0 0 0 0  2883 GBM1 468 225 12 0 0 24 12 6 0 0 0 0 0 0 1998 4042 BHM3 1253 158 0 0 0 20 316 5 10 5 0 46 0 15  4042 BHM6 2241 285 5 0 0 20 234 31 0 20 0 5 10 10 
8-25 

Growth studies and long-term reciprocal transplant experiments. Growth was monitored in 

four mussel beds, three of which were used for the reciprocal transplant experiments. Very large 

mussels dominated the newly discovered mussel bed at GB425 (GBM1), so an experiment was 

initiated to monitor growth in that population. Although the limited size range and small number 

of marked mussels recovered limits the conclusions that can be drawn from this study, the fact 

that larger animals are clearly still growing is apparent (Figure 8.3.4) Two reciprocal transplant 

experiments were conducted. In this section, changes in growth and condition indices of the 

transplanted mussels with respect to the host populations are reported. Differences in the 

histopathology and hydrocarbon loads of the mussels are discussed in section 8.5. Tagged 

mussels were compared to untagged mussels (controls) at the end of the experiment to test for an 

effect of the tagging process. There was no significant difference in either percent-water or CI 

between tagged and untagged mussels at any of the four sites, indicating no detectable effect of 

the tagging process. 

Table 8.3.4. Concentrations of Methane (CH4) and Oxygen (O2) 

at GC233 inner zone and the short-term transplant locations for 1997. 

Sampling depth was 2.5 cm. 

Sample CH, (μM) O, (μM) 

GC233-inner mean 48 67 

 median 32 46 

 low 12 30 

 high 101 120 

Short-term transplant mean ND 66 

 median ND 45 

 low ND 34 

 high ND 285 

ND – non-detectable 

The first experiment was a transplant between the GC185 mussel bed, BHM1, and the GC233 

collection site BPM1. The growth of the two populations over the period of the transplant 

experiment (marked controls) was significantly different (p < 0.001, Table 8.3.6, Figure 8.3.4). 

Growth of the transplanted mussels was significantly different from growth of their populations 

of origin (p < 0.001) but not significantly different from the mussels they were transplanted 

among. The condition indices (percent-water and CI) of the two host populations were not 

significantly different in the year of collection, precluding analysis of transplant induced changes 

in condition in this experiment (Figure 8.3.4). 
8-26 

Table 8.3.5. Mean (standard deviation) condition index, glycogen content, water content, and 

length for the inner zone of the GC233, the control boxes and the short-term transplant boxes in 1997. 

 Inner Zone 13-day Control 

42-day 

Control 

13-day 

Transplant 

42-day 

Transplant 

CI (g/ml3) 0.13A

(0.01) 

0.06B

(0.03) 

0.13A

(0.03) 

0.11A

(0.03) 

0.10A

(0.02) 

PercentGlycogen 1.6A 

(1.3) 

1.0B

(0.9) 

1.6A

(0.6) 

1.3A,B

(1.0) 

1.4A

(0.8) 

Percent Water 83.7A,C

(2.6) 

92.6B

(4.1) 

80.8A

(2.3) 

85.5C

(2.5) 

82.7A,C

(2.4) 

Length (mm) 74 

(32) 

75 

(25) 

94 

(20) 

81 

(27) 

90 

(21) 

N 6 7 6 12 12 

A,B,C Categories with different letters indicate a significant difference (p < 0.05) between them. 

The second transplant was between the GC234 M1 mussel bed and another collection site in 

GC233, BPM4. Growth of the marked controls at each site was significantly different (p < 

0.0001, Table 8.3.6, Figure 8.3.4). Growth of both transplanted populations was significantly 

different from their populations of origin and the mussels they were transplanted among, with 

patterns intermediate between the two extremes of the host populations (Figure 8.3.5). The 

condition indices of the two host populations were significantly different in 1997 and 1998. The 

condition indices of the transplanted populations were significantly different from their 

populations of origin but not significantly different from the mussels they were transplanted 

among (Figure 8.3.6). 

Table 8.3.6. von Berltalanffy growth parameter estimates 

For GC233, GCM1, BHM1, and GBM1 mussel populations. 

 Parameter Estimates 

Station Lmax k 

BPM1 

124.48 

(116.52, 138.43) 

0.16 

(0.11, 0.21) 

BPM4 

103.78 

(92.02, 115.49) 

0.24 

(0.14, 0.35) 

BHM1 

94.47 

(85.10, 103.84) 

0.17 

(0.10, 0.23) 

GCM1 

66.60 

(57.18, 76.02) 

0.08 

(0.01, 0.14) 

GBM1 

124.02 

(106.40, 141.64) 

0.05 

(0.01, 0.08) 
8-27 

Figure 8.3.4. Growth of mussels between 1997 and 1998 field seasons at the GC233 and at MMS study stations 

BHM1, GCM1, GBM1, and BPM1/M4. 

8.3.5 Discussion 

There was considerable heterogeneity between collections of mussels with respect to all of the 

“community ecology” parameters. Care should be exercised in interpreting the differences 

because there is also considerable heterogeneity within a single mussel bed. The scientists and 

pilots in the submarine for each collection were asked to make the collections from “typical 

appearing” regions of the bed they were sampling. However, review of photo mosaics and video 

records of each collection site confirms the heterogeneity present within most mussel beds. For 

example, in many of the beds there were regions where no juveniles were present, but 

disarticulated shells were abundant. Other regions had no juveniles or disarticulated shells 

apparent while yet in other regions, all the large mussels were literally covered with juveniles. 

Nonetheless, all analyses reported in this section were conducted on collections from within a 

0.5-meter ring or square, so correlation between parameters such as mussel population 

characteristics, condition, faunal associates, and chemistry are appropriate. Also, because of the 

replication within some sites (notably GC185 and GC233), comparisons between faunal 

associates are appropriate. 
8-28 

Figure 8.3.5. Growth and condition of transplant and control mussels from GC185 – GC233 reciprocal 

transplant study. Bars with different letters over them are significantly different from each other. 

Mussel density at the sites ranged from 255 to 2424 individuals m2 and from 8.3 to 35 kg m2 of 

mussel tissue in mussel beds at these sites. This is, of course, a very high biomass compared to 
8-29 

the rest of the deep Gulf of Mexico and would be expected to be a source of nutrition for 

numerous other animals from the nutrient limited background fauna. In fact, the greatest species 

richness found in any single collection was only eight species, and the total species richness for 

this type of community at any one site was 13 species (from a total of six collections at GC185). 

The overwhelming bulk of the biomass of the associated fauna is from seven species, five of 

which are endemic to the seeps. This is in contrast to the communities associated with tubeworm 

aggregations, which have a much higher species richness and host significant numbers of nonendemic species. Although the identity of the associated fauna is quite variable between sites, the 

total biomass of associated fauna between sites is remarkably constant. 

Two species were always found associated with mussel beds – the gastropod, Bathynerita

naticoides, and the Alvinocarid shrimp, suggesting that these endemic species have excellent 

dispersal abilities and can tolerate a wide range of conditions. A different pattern was seen with 

the undescribed orbinid polychaete that was very abundant in most collections from GC233 and 

both collections from GC234 but was absent in the collection from GB425. Only a single 

individual was found in the single collection from GC185. Its abundance in one of the oil and 

salt dominated sites and absence in the others suggests that dispersal abilities rather than 

physiological tolerance may limit its distribution. The fact that Bathynerita naticoides was found 

in much lower abundance at GC233, and the other gastropods were absent or rare, suggests that 

this habitat may not be suitable for these gastropods, perhaps because of the steep gradients in 

dissolved gases and brine among the mussels (Smith et al. 2000). 

The short-term transplant experiment found a very high level of individual variation in glycogen 

content between each of the treatments, which would mask moderate differences between sites. 

Because this is also a very time-consuming assay, we decided not to use this assay for the other 

studies but instead increased the number of animals assayed for condition by other methods. 

Neither CI nor percent-water was significantly different between the control animals and the 

animals transplanted off site, with the exception of one control cage whose animals crashed in 

condition and are not considered for this analysis. This indicates that it takes a minimum of 

months for changes to occur in these conservative indices of animal condition and that they are, 

therefore, reliable indicators of average environmental conditions experienced over at least the 

previous several months and not sensitive to short-term fluctuations in environmental conditions. 

The two reciprocal transplant experiments indicate a significant effect of environment on animal 

growth and condition but also suggest that mussels moved into very different environments may 

take more than one year to fully feel the effects of the new environment. In the GC185-to-GC233 

experiment, the initial populations had similar (but significantly different) growth patterns and 

physiological condition that were not significantly different between populations (Figure 8.3.5). 

When transplanted, the mussels showed the same growth pattern as the mussels they were 

transplanted among and were significantly different from their bed of origin. There was no 

evidence of a historic effect on the mussels’ growth pattern after transplantation, but the initial 

populations were very similar. In the second experiment, the initial populations had very 

different growth parameters and physiological condition indices. After transplantation, the 

mussels were in the same condition as those around themselves (and different from their bed of 

origin), but the growth characteristics were significantly different from either population (Figure. 

8.3.6). Sample size and limitations in statistical power for analysis of growth parameters prevent 

conclusions concerning where the differences in growth parameters occur, but visual 

examination of the data suggests that the growth characteristics of the transplanted mussels were 
8-30 

intermediate between the two host populations. Hypothetically, this could reflect either genetic 

differences between the populations, a residual effect of their original physiological condition 

affecting growth for at least the first few months of the experimental interval or some other 

carry-over condition such as a pathology or lack thereof. The other studies reported here indicate 

that genetic differences are unlikely to contribute to this effect. Pathology and condition are 

likely contributors. 
8-31 

Figure 8.3.6. Growth and condition of transplant and control mussels from the GC234 – GC233 reciprocal 

transplant study. Bars with different letters over them are significantly different from each other. 
8-32 

8.4. Vestimentiferan Community Ecology*

8.4.1 Introduction 

Vestimentiferan tubeworms are the biomass dominant fauna at many Gulf of Mexico 

hydrocarbon seeps and are undoubtedly the best known chemosynthetic species occurring in the 

region. Within the program study sites, tubeworms are prevalent at the sediment diffusion sites, 

GC185 and GC234, and occur in reduced abundance on the margins of the two brine-pooling 

sites, GC233 and GB425. The Gulf of Mexico species are taxonomically and functionally similar 

to Riftia pachyptila, the tubeworm species first discovered at hydrothermal vents in the eastern 

Pacific Ocean. Both forms lack any alimentary apparatus and live by harboring sulfide-oxidizing 

bacteria in a symbiotic and chemosynthetic partnership. Both forms occur in prominent clusters 

of hundreds of individuals. These clusters offer habitat substrata to diverse, non-chemosynthetic 

invertebrates and fishes. Environmental evidence has suggested that the Gulf of Mexico species 

are distinguished from their hydrothermal vent analogs by their ability to subsist on sulfides 

dissolved in the sedimentary pore fluids instead of an actively venting fluid source. The molar 

strength of MMS demonstrated that tubeworms grew at rates of less than one centimeter per year 

and lived in excess of one hundred years (Fisher et al. 1997). 

8.4.2 Objectives 

The tubeworm community ecology study comprised six overall objectives as follows: 

To characterize the principal sampling stations for integration with data collected by other 

investigators. Each primary tubeworm aggregation was characterized with respect to 

growth rate and physiological condition of the tubeworms. Seven of these aggregations 

were collected intact and were also characterized with respect to tubeworm species 

composition, size frequency and biomass. Four of those aggregations were collected with 

a bushmaster device that recovered associated fauna. Biomass and species composition 

of the associated fauna were quantified for these four aggregations. Data on sulfide levels 

above and below the sediment water interface are available for most aggregations. 

To characterize generally the species richness and biomass of associated fauna supported by 

this type of chemosynthetic community at the shallow Gulf of Mexico seep sites. 

To determine precisely the growth rates of tubeworms over their life cycle and calculate ages 

for animals of representative sizes. 

To study the differences in occurrence and life history of the two co-occurring species of 

tubeworms to refine working models of their interactions. 

To investigate the hypothesis that adult tubeworms take up sulfide from their buried posterior 

ends. 

 

*

 This section was authored by Derk Bergquist and Charles R. Fisher 
8-33 

To develop new life history models for the tubeworms and the communities of fauna 

associated with tubeworm aggregations. 

8.4.3 Materials and Laboratory Methods 

Tubeworm growth measurement and bulk collection of tubeworm aggregations. The length 

and anterior tube diameter were measured for all individuals of the two co-occurring 

vestimentiferans, Lamellibrachia cf luymesi. and the Escarpia-like species, in seven 

aggregations collected intact. This allowed the construction of size frequency histograms for 

these populations. 

Condition and biomass of vestimentiferans. Subsamples taken at sea for determination of 

physiological condition and biomass were thawed in the laboratory and dried to a uniform weight 

(60°C). Individual samples were then ashed at 500°C for at least six hours and weighed again for 

calculation of ash-free dry weight (AFDW). The tube of each of these animals was cut into 

segments of 10 cm or less, and the diameters of the ends of each segment were measured to +/- 

0.05 mm for diameters less than 4 mm and +/- 0.1 mm for diameters larger than 4 mm using a 

precision-machined device similar to a ring-sizer. The volume of each tube was calculated as the 

sum of the volumes of each segment estimated as the frustrum of a cone: 

V = (1/3)(l)(r1

2

 + r1r2 + r2

2

) (8.4-1) 

where l is the section length, r1 is the radius of one end of the section and r2 is the radius of the 

opposite end of the section. Condition was then calculated as the ratio of tissue-AFDW to tubevolume. The relationship between individual biomass and total tube length for each species was 

determined using linear regression. This was used to estimate the biomass of whole aggregations 

based upon their size distributions. 

Associated fauna. During the July 1997 cruise, three trial collections were made using the large 

and small bushmaster devices. At this time, the bushmaster devices did not have linings, and the 

openings in the mesh bag were four millimeters. These collections were analyzed for studies on 

the tubeworm populations, but associated fauna was not quantified. In 1998, four collections 

were made using the bushmaster devices lined with 64-μm mesh. These four collections were 

analyzed with respect to composition, density, and biomass of associated fauna. Tubeworm 

aggregations at stations BHJT2, GCAT1, and BHAT2 were collected in their entirety. 

Approximately 50% of the aggregations at station GCST1 was enclosed in the bushmaster device 

and recovered intact with its associated fauna. Because the data were analyzed on the basis of the 

density of the associated fauna, this collection is treated the same as the others. 

Contents of the mesh-lined bag were filtered sequentially through a 240-μm and 64-μm sieve. 

Fauna retained on the sieves was stored in 70% ethanol. The formalin from each preserved 

tubeworm aggregation (bush) was also sieved and the retained fauna transferred to 70% ethanol. 

Each aggregation was rinsed twice with 25 to 50% ethanol, preserved in 50% ethanol (because 
8-34 

of fire danger) in glass aquaria where they were carefully searched, and the remaining associated 

fauna removed. All animals removed from the aggregation were pooled, sorted to the lowest 

practical taxonomic level, and enumerated. 

A haphazard sample of up to 30 individuals of each species in a collection was measured for 

length with calipers. In some collections, all individuals of a species were measured. Crustaceans 

were measured from the tip of the rostrum to the tip of the telson. Mussel lengths were measured 

from the longest points of the shell edge. For clams, shell length was measured from the umbo to 

the maximum shell distance. Pulmonate gastropod shells were measured from the spiral tip to the 

base of the apex. Minimum, maximum, and average lengths were determined for each species in 

each collection. A complete presentation of these data are made in Appendix Table 8.4.1. The 

trends and modes are discussed in this section. 

Wet weights and ash-free dry weights were determined for a subsample of six to 15 individuals 

of each species measured for length. Weighed samples were placed in pre-ashed aluminum pans 

and dried at 60°C. The samples were then ashed at 500C for a minimum of six hours and 

weighed. A conversion factor for wet weight to AFDW was calculated from this data and total 

species wet weights per collection were then converted to total AFDW for each species in a 

collection. 

Species residency status (endemic versus nonendemic) was determined from the literature where 

possible. Specimens not identified by the tubeworm ecology group were sent to specialists for 

classification (see appendix volume for species identifications). 

The number of vestimentiferans in each aggregation and total surface area of tubeworm tubes in 

each aggregation was estimated from haphazard sub-sampling of aggregations. The length of 

each tubeworm tube above the root ball and the outside diameters of the posterior and anterior 

tube ends were used to calculate tube surface area using the equation for the surface area of a 

frustrum: 

  l (r1 + r2) (8.4-2) 

where l is the length of the tube, and r1 and r2 are the radii of the two ends. 

Statistical analyses. Non-linear regression was used to fit a variety of growth models, including 

the von Bertalanffy, Gompertz, Richards, Logistic, Negative Exponential, and various linear 

transformations, to the relationship between growth rate and tube length. The best model was 

chosen based upon visual inspection and analysis of residuals. The 

age of each individual, i, was determined by integrating the growth model and solving over the 

definite integral (0, length of i). 

In three aggregations, growth data was successfully collected on both Lamellibrachia cf luymesi

and the Escarpia-like species found within the aggregation. To test for within-aggregation 

growth rate differences between the two species, individual t-tests were employed and the 

sequential Bonferroni method was used to correct for multiple comparisons (Rice 1989). 
8-35 

Additionally, comparisons between and within aggregations of different developmental stages 

(juvenile–JT, adult–AT, and senescent–ST) were performed using analysis of variance and linear 

regression to determine whether growth and physiological condition vary according to the 

previously proposed model of bush development (Simpkins 1994). Because growth rate is 

known to vary with animal length, length was incorporated as a covariate (ANCOVA, Minitab 

12) in all models involving growth rate; when length was not found to be significant it was 

removed and an analysis of variance was performed (ANOVA). All multiple comparisons were 

corrected using the Tukey-Kramer method, and all p-values shown are adjusted. 

8.4.4 Results 

Growth rates and ages. Data from over 600 individual Lamellibrachia cf luymesi, including 

both data collected as part of the study and data from animals stained previously to this study, 

were combined for a statistically robust analysis of their growth and calculation of age for this 

species. A simple negative exponential model of the form, 

 dL/dt = ae-bL (8.4.3) 

where a and b are parameters and L is the standardized animal length, consistently provided the 

best and most robust fit to the relationship between yearly growth rate and tube length in 

Lamellibrachia cf luymesi (Figure 8.4.1). For this species, our results indicate the ages of 

animals in the upper range of sizes collected for this study (greater than two meters in length) are 

in excess of 170 to 250 years (Bergquist et al. 2000, Figure 8.4.2). These results provide a 

minimum age for individuals of various lengths. For example, assuming a two-meter long 

tubeworm grew at the maximum rate throughout its life (a very unlikely scenario), its minimum 

age would be 80 years. 

Because the Escarpia-like species grows very close to the sea floor, only juvenile aggregations 

were successfully stained. Within the limited size range sampled, no significant relationship was 

found between growth rate and standardized tube length in this species (Figure 8.4.3). The mean 

overall growth rate for all the Escarpia-like species collected in this study was 2.19 cm/yr (SE = 

0.20, n = 80) and was significantly less than the mean growth rate for Lamellibrachia cf luymesi

of the same size range (mean = 2.92, SE = 0.09, n = 373; df = 116, t = 1.98, P = 0.0012). The 

mean growth rate of Lamellibrachia cf luymesi was greater than that of the Escarpia-like species 

at two of the three stations for which we had data on both species (Figure 8.4.4). Mean growth 

rates were only significantly different between the two species at one of the stations (BHJT2; ttest, P = 0.0154; sequential Bonferroni method). 
8-36 

Figure 8.4.1. Size specific growth rate in Lamellibrachia sp. Negative exponential model fit to data collected in 

1998 (solid line), all data collected between 1995 and 1998 (broken line) and the upper bound of 

the 1998 data (dotted line). (Modified from Bergquist et al. 2000.) 

Figure 8.4.2. Individual ages predicted from tube length using the negative exponential model. The 1998 data 

set (solid circles) and the 1995-1998 data set (open circles) predict the average expected age of an 

individual. The upper bound of the 1998 data set (closed squares) predicts the minimum 

theoretical age of an individual constantly growing at the maximum growth rate of the species 

throughout its life. 
8-37 

Figure 8.4.3. Size-specific growth rate in the Escarpia-like species. All data collected between 1995 and 1998 

are shown. 

Figure 8.4.4. Mean growth rates (+s.d.) of Lamellibrachia sp. (solid bars) and the Escarpia-like species (open 

bars) at all principal sampling stations. As sample size was low and growth rates were not 

significantly different, they were pooled to decrease sample variance. 

 
8-38 

Site-specific comparisons. Table 8.4.1 summarizes the length, growth rate, and condition of 

Lamellibrachia cf luymesi and the Escarpia-like species of the three vestimentiferan community 

developmental stages collected in this study. The mean growth rates of Lamellibrachia cf 

luymesi at each principal sampling station are shown in Figure 8.4.4. As expected, growth rates 

of Lamellibrachia cf luymesi in the juvenile aggregations were significantly greater than the 

growth rates in both adult and senescent aggregations (ANCOVA; P < 0.0001 and P = 0.0485, 

respectively), but unexpectedly, there was no significant difference in growth rates between adult 

and senescent aggregations (P = 0.9083). The growth rates of Lamellibrachia cf luymesi varied 

significantly between different juvenile and adult aggregations (ANCOVA; P < 0.001 and P = 

0.002, respectively), but growth rates did not vary significantly between different senescent 

aggregations (P = 0.245). Among the juvenile stations, BHJT1 and BHJT2 showed significantly 

less growth than GCJT2N and GCJT2S (0.0003 < P < 0.0346). Among the adult stations, 

GCAT1 showed significantly more growth than BHAT1 and BHAT2 (P = 0.0193 and P = 

0.0229, respectively). The growth rates of the Escarpia-like species varied significantly between 

different juvenile aggregations (ANOVA, p = 0.028) with GCJT2S growing faster than BHJT2 

(P = 0.0270). 

Table 8.4.1. Characteristics of Lamellibrachia sp. and the Escarpia-like species in juvenile, adult and senescent 

aggregations. Included are the number of individuals of each species and the number of stations from which 

individuals were sampled from each developmental stage. Length, growth rate, and condition are all given as 

means and standard deviation (s.d.). 

Species Stage Stations Count Length in cm 

(s.d) 

Growth 

rate in cm/yr 

(s.d.) 

Condition in g/cm3

(s.d.) 

Lamellibrachia sp. Juvenile 6 244 36.3 (16.8) 2.75 (1.94) 0.131 (0.031) 

Adult 4 136 119.5 (37.7) 0.61 (0.91) 0.094 (0.026) 

Senescent 3 59 155.5 (121.2) 0.59 (0.56) 0.119 (0.045) 

Escarpia-like Juvenile 5 57 24.5 (10.4) 2.84 (2.22) 0.148 (0.055) 

Adult 2 20 41.7 (8.3) ------- 0.144 (0.025) 

 Senescent 1 5 53.6 (21.5) ------- 0.165 (0.037) 

The condition of Lamellibrachia cf luymesi and the Escarpia-like species at each station is 

shown in Figure 8.4.5. Overall, the relationship between condition and growth rate in 

Lamellibrachia cf luymesi is positive and significant (P < 0.001, Pearson’s r = 0.359), indicating 

that growth rate reflects relative animal health over a large size range. No such relationship was 

found for the Escarpia-like species (P = 0.473). In a two-way ANOVA, the interaction between 

site and developmental stage was significant (P < 0.001), so the two sites were analyzed 

separately with two one-way ANOVAs. At BH, juvenile and senescent aggregations has higher 

condition indices than adult aggregations (P = 0.002 and P < 0.001, respectively), but juvenile 

and senescent aggregations did not have significantly different condition indices (P = 0.261). At 

GC, juvenile aggregations had significantly higher condition indices than adult or senescent 

aggregations (P < 0.001 in both cases), but senescent and adult aggregations were not 
8-39 

significantly different (P = 0.78). No similar trends were apparent in the Escarpia-like species, 

but less data were available for this species because of collection methods. The condition of 

Lamellibrachia cf luymesi differed significantly between aggregations within all developmental 

stages (ANOVA; P < 0.033 for all), but in the Escarpia-like species only varied significantly 

between aggregations within the juvenile stage (P < 0.0001). Among juvenile aggregations, the 

condition of Lamellibrachia cf luymesi was significantly higher at GCJT2N than at BHJT2, 

GCJT1a and GCJT1b (P < 0.001 for all); at GCJT2S condition was significantly higher than at 

BHJT2 or GCJT1b (P = 0.006 and P = 0.002, respectively). The condition of Lamellibrachia cf 

luymesi among adult stations was higher at BHAT1 than at BHAT2 (P = 0.038) and among 

senescent stations was lowest at GCST2 (P < 0.0001), the lowest condition found in any 

aggregation collected. The condition of the Escarpia-like species was significantly lower at 

BHJT2 than at any other juvenile station except GCJT1a (ANOVA; P < 0.0005 for all) and was 

significantly lower at GCJT1a than at BHJT1 and GCJT2S (P = 0.0231 and P = 0.0438, 

respectively). The condition of Escarpia-like species did not vary significantly among adult 

stations. The mean lengths of all the vestimentiferans subsampled from each of the primary 

stations for growth rate and condition are shown in Figure 8.4.6. Mean lengths varied 

significantly between the three developmental stages in both species (P < 0.001 for both) and 

varied significantly between stations within all developmental stages in Lamellibrachia cf 

luymesi (P < 0.001 for all) and between stations within the juvenile stage in the Escarpia-like 

species (P < 0.001). 

Vestimentiferan population composition and size structure. Both the Escarpia-like species and 

Lamellibrachia cf luymesi were present in every aggregation that was collected intact. The 

Escarpia-like species was the less abundant of the two vestimentiferans in six of the seven 

aggregations (Figure 8.4.7) and, overall, was significantly less abundant within the aggregations 

than Lamellibrachia cf luymesi (Paired t-test, df=6, P = 0.032). Lamellibrachia cf luymesi was 

significantly longer than the Escarpia-like species in all of these aggregations (paired t-tests, p < 

0.001 in all cases, sequential Bonferroni method). 

The size distributions of both species in all but one aggregation (BHJT2) were non-persistent 

(Figure 8.4.8a-g). The smallest size classes were present in aggregations of only small animals 

indicating that new recruitment was occurring or had recently occurred (Figure 8.4.8a, b). In 

these aggregations the two species show similar modes in their size distributions. The smallest 

size classes were present in only one aggregation of larger individuals (GC-AT1); when 

collected, a cluster of smaller individuals was partially entangled with one edge of this 

aggregation. With the exception of GC-AT1, aggregations of larger individuals in the smallest 

size classes were not represented indicating that recruitment to these sites had ceased (Figure 

8.4.8c-g). In addition, the modes of the Lamellibrachia cf luymesi size distributions were much 

higher than the Escarpia-like species. 
8-40 

Figure 8.4.5. Mean condition indices (+s.d.) of Lamellibrachia sp. (solid bars) and the Escarpia-like species 

(open bars) at all principal sampling stations. 

Figure 8.4.6. Mean tube lengths (+s.d.) of subsamples of Lamellibrachia sp. (solid bars) and the Escarpia-like 

species (open bars) taken from the principal sampling stations for determination of growth rate and 

condition. 

0

0.05

0.1

0.15

0.2

0.25

BH-JT1BH-JT2

BH-AT1BH-AT2BH-ST1

GC-JT1aGC-JT1b

GC-JT2NGC-JT2S

GC-AT1GC-AT2GC-ST1GC-ST2

S ta tio n

Condition (g/cubic cm)
8-41 

Figure 8.4.7. Abundances of Lamellibrachia sp. (solid bars) and the Escarpia-like species (open bars) at seven 

principal sampling stations from which all vestimentiferans were collected and measured. 

Associated faunal collections. The complete list of species collected with the four bushmaster 

collections is presented in appendix volume. The resident status (endemic, non-endemic, or 

unknown) is given in this table as well. A total of 60 species of animals from 11 phyla (4,624 

individuals) was collected with the four aggregations. Thirty-five species had not been 

documented from previous collections of seep fauna and many of those may turn out to be new 

species. Eight species were present in all collections. Four of these species, the shrimp Alvinocaris

stactophila, the crab Munidopsis sp. 1, the annelid Nicomache sp., and the sponge Ectyomyxilla

methanophila are thought to be endemic to the seeps. The four other species, the nemertean Lineus 

sp., the hydroid Acryptolaria sp., the amphipod Stephonyx sp., and the polychaete Harmothoe sp., 

are likely endemic but confirmation of this awaits complete identification by experts. 

A collection of juvenile tubeworms yielded 19 species of associated fauna, and the highest 

biomass of associated fauna of any collection (Table 8.4.2). Most of this biomass was a result of 

the very high numbers of a few species of endemic animals (Figure 8.4.9). Communities 

associated with adult aggregations are much more diverse, with about twice the species richness 

of the juvenile community (Table 8.4.2). The communities are still dominated by endemic 

species, but numerous species of non-endemic animals are present in small to moderate numbers 

(Figure 8.4.10 and 8.4.11). The biomass of associated fauna in the adult aggregations is almost 

one order of magnitude less (per unit area) than in the juvenile aggregation. A single collection 

of a senescent aggregation contained 20 species, half of which were represented by a single 

individual (Figure 8.4.12). This aggregation was supporting about 20% of the biomass of the 

adult aggregations (Table 8.4.2). Even endemic fauna was only present in small numbers if 

present at all in the senescent aggregation. 

0

200

400

600

800

1000

1200

BH-JT1BH-JT2

GC-JT2N

GC-JT2S

GC-ST1GC-AT1BH-AT1

S ta tio n

Number
8-42 

Figure 8.4.8. Size frequency histograms of Lamellibrachia sp. (solid bars) and the Escarpia-like species (open 

bars) at the seven principal sampling stations from which all vestimentiferans were collected and 

measured. Histograms ordered by increasing Lamellibrachia sp. modal size: a) BHJT2*, b) 

BHJT1, c) GCJT2N, d) GCJT2S, e) BHAT1*, f) GCAT1*, g) GCST1**. (*Size frequencies 

presently based on subsamples; **one half of the aggregation was collected and all individuals in 

that half were counted and measured.) 

A B

C D

E F

G
8-43 

Table 8.4.2. Characteristics of 1998 bushmaster collections. 

Aggregation 

Name 

Volume 

(m3

) Number 

Tube SA 

(m2) 

No. 

Species 

No. 

Individuals 

Ind/m2

Density 

gAFDW/m2 Biomass 

BHJT2 0.02 1236 1.3 19 3263 2453 100 

GCAT1 0.1 1112 6.2 34 323 52 15 

BHAT2 0.3 800 5.3 37 976 185 20 

GCST1 0.4 306 4.7 20 64 14 3 

Pore water and wand samples associated with tubeworm aggregations. Water samples taken 

above the sediment and associated with tubeworm bushes were analyzed using a method that 

allowed detection of sulfide at concentrations as low as 0.1 μM (Table 8.4.3). Sulfide was never 

detected in control samples taken by the submarine between 100 and 300 meters above the 

bottom. Sulfide was only detected in 2 of 25 samples taken at plume level among tubeworm 

aggregations and both of these samples were taken in juvenile aggregations. Sulfide was detected 

in 4 of 23 samples taken at a point approximately half way between the sediment and the plumes 

of the tubeworms in the middle of aggregations. Even at the sediment water interface sulfide was 

below the limits of detection in 15 of 22 samples. Of the 13 samples with detectable sulfide, 

eight were associated with juvenile aggregations and four with a senescent aggregation station 

found to be in close proximity to a juvenile aggregation. 

Table 8.4.3. Seawater sulfide concentrations among hydrocarbon seep tubeworm bushes. 

Sample Location 

Range of [H2S] 

(μM) Samples Taken 

Samples with 

No Detectable Sulfide % ND 

Water Column ND 24 24 100 

Plume Level ND – 0.7 25 23 92 

Mid-Bush ND – 1.9 23 19 83 

Sediment Level ND – 3.7 22 15 68 

Using the deep probe, interstitial water samples were taken from beneath the tubeworm 

aggregations. Depth of the sample was determined by the depth where the probe met solid 

substrate; in some cases this was likely the top or edge of the carbonate rock the aggregations 

were attached to. Juvenile aggregations tended to be attached to shallowly buried rocks, and the 

larger aggregations to much deeper buried rocks. Distribution of sulfide in the interstitial spaces 

was very patchy. In one case “replicate” samples taken from the same depth beneath an 

aggregation yielded 0 and 2.75 mM sulfide. Because of the patchiness and the relatively small 

number of samples taken in association with each aggregation it is not appropriate to use this 

data to draw conclusions about the exposure of individual aggregations to sulfide. Nonetheless it 

is apparent from the data set that high levels of sulfide are often present in interstitial pools and 

that high levels of sulfide are found at least to 70 cm depth, the deepest depth sampled (Table 

8.4.4). 
8-44 

Table 8.4.4. Interstitial sulfide levels beneath vestimentiferan aggregations in the Gulf of Mexico. Included are 

depth ranges from which the samples were taken, the number of those samples in which sulfide was not detected 

(ND), the range of quantified sulfide concentrations, and the mean sulfide concentration (+/-1 s.d.). 

Bush 

Type 

Depth 

Range (cm) 

Number 

Samples 

Number 

ND 

Sulfide-Range 

(mM) 

Sulfide 

Concentration (mM) 

Juvenile 10-40cm 9 5 0.000-2.725 0.5610 (+/-1.097) 

Adult 15-70cm 12 2 0.000-0.6865 0.1682 (+/-0.2451) 

Senescent 27.5-65cm 12 6 0.000-7.885 0.6644 (+/-2.2739) 

0.0080 (+/-0.0106)* 

*mean sulfide concentration when the single very high value (7.885 mM) was removed

Sulfide uptake by posterior extensions (roots). Using the new bushmaster collection devices, it 

was confirmed that root-like extensions were present on all intact individuals examined from 

larger aggregations and even most individuals from juvenile aggregations. Permeability studies 

on these and other translucent buried tubes, “roots,” confirmed that they are quite permeable to 

sulfide and that this increased permeability is not due solely to the thickness of the root tubes 

(Figure 8.4.13, Julian et al. 1999a). Incubation of live animals in split respirometers 

demonstrated unequivocally that Lamellibrachia cf luymesi is capable of taking up sulfide across 

its posterior end (root) at rates sufficient to fuel net CO2 uptake (autotrophy) (Table 8.4.5, 

Freytag et al. submitted). 

Table 8.4.5. Plume CO2 flux 

prior to, during, and after root H2S exposure* 

Experimental Conditions 

CO2 Flux 

(μmoles/gm./hr.) 

Pre-H2S Exposure - 2.36 ± 0.76 (n = 73) 

500 μM H2S in posterior chamber 2.13 ± 0.98 (n = 78) 

Post-H2S Exposure (17 hrs) - 4.87 ± 1.81 (n = 38) 

*From Freytag et al. submitted 
8-45 

Figure 8.4.9. Species richness and abundance in Bushmaster Jr. Collection of BHJFIGURE 8.4.2. 

Vestimentiferans are indicated by the bars with ascending hash marks, mussels by bars with 

descending hash marks, arthropods with white bars, annelids by black bars, and all other fauna by 

gray bars. 
8-46 

Figure 8.4.10. Species richness and abundance in Bushmaster Sr. Collection of GCAFIGURE 8.4.1. 

Vestimentiferans are indicated by the bars with ascending hash marks, mussels by bars with 

descending hash marks, arthropods with white bars, annelids by black bars, and all other fauna by 

gray bars. 
8-47 

Figure 8.4.11. Species richness and abundance in Bushmaster Sr. Collection of BHAFIGURE 8.4.1. 

Vestimentiferans are indicated by the bars with ascending hash marks, mussels by bars with 

descending hash marks, arthropods with white bars, annelids by black bars, and all other fauna by 

gray bars. 
8-48 

Figure 8.4.12. Species richness and abundance in Bushmaster Sr. Collection of GCSFIGURE 8.4.1. 

Vestimentiferans are indicated by the bars with ascending hash marks, mussels by bars with 

descending hash marks, arthropods with white bars, annelids by black bars, and all other fauna by 

gray bars. 
8-49 

Figure 8.4.13. Sulfide permeability of Lamellibrachia sp. tubes. Closed symbols represent root tube sections, 

open symbols represent trunk tube sections. Circles represent flow-through measurements, and 

triangles represent recirculating measurements. The diffusion coefficient (D, inset) was calculated 

by multiplying P by the wall thickness. (Julian et al. 1999a) 

8.4.5 Discussion 

Two key discoveries from this study have profound implications to the understanding of the 

tubeworm communities in the Gulf of Mexico. One is the presence and function of the long 

posterior extensions of the tubeworms, their roots. It is now known that Lamellibrachia cf 

luymesi at least, and most likely the Escarpia-like species as well, can take up sulfide from 

interstitial fluids, which are tens of centimeters deep in the sediment. This adaptation provides 

the tubeworms access to a stable and longer lasting source of sulfide and provides the 

explanation for the growth and abundance of tubeworms in areas where sulfide is not detectable 

in the water above the sediments. It also has significant implications for the structure of the 

associated faunal communities. Tubeworm communities can flourish in areas where very little 
8-50 

potentially toxic seep fluid is present in the water above the sediments. Access to a stable and 

long-lived source of sulfide may also be a key factor in explaining the longevity of tubeworms, 

which was the second key discovery of this study. When the current study began, it was known 

that the tubeworms could live in excess of one hundred years, but we had no proof that they 

could live much beyond a century. The current data set clearly indicates that Lamellibrachia cf 

luymesi lives in excess of 170 to 250 years (Bergquist et al. 2000) making them one of the 

longest lived non-colonial animals we know of. The data was interpreted conservatively 

(Bergquist et al. 2000) because those estimates were based on a maximum size of two meters, 

and animals up to three meters have been collected at these sites and are presumably much older. 

It is conceivable that they can live considerably longer still. 

There are a variety of lines of evidence that suggest Escarpia-like species lives at least as long as 

Lamellibrachia cf luymesi and perhaps even longer. Because both species appear to settle 

simultaneously over a period of time before recruitment to a substratum ceases, the presence of 

the Escarpia-like species in every aggregation recovered, particularly in aggregations with the 

largest Lamellibrachia cf luymesi, strongly suggests that this species is living at least as long as 

Lamellibrachia cf luymesi. The fact that aggregations comprising only large Escarpia were 

observed and that aggregations comprising only Lamellibrachia cf luymesi were not observed 

suggests that Escarpia might outlive Lamellibrachia cf luymesi. If this is in fact the case, the 

Escarpia-like species may be living significantly longer than Lamellibrachia cf luymesi. 

Characteristics of the vestimentiferan populations varied significantly between different stations. 

As was expected, vestimentiferans display the highest growth rates in juvenile aggregations, but 

the variation found between different juvenile aggregations and the lack of any difference in 

growth rates between the adult and senescent aggregations clearly suggest that developmental 

phase is not the only factor driving growth rate. The positive relationship between growth rate 

and condition in Lamellibrachia cf luymesi and the fact that those juvenile aggregations showing 

the lowest growth rates typically had the lowest condition index values indicate that the ability of 

this vestimentiferan to grow is tied to its physiological health. This is particularly interesting 

when considering the high condition index of juvenile and BH senescent aggregations as 

compared to the relatively low condition of the adult aggregations. One possibility is that in 

juvenile aggregations, the local seepage rate can support the relatively low vestimentiferan 

biomass, but as the populations and the individuals within those populations grow to the adult 

phase, standing-stock biomass increases and competition begins to play an important role. As the 

population size decreases as a result of competition, the aggregation enters the senescent phase 

where fewer individuals are vying for the limited local seepage. The large number of individuals 

occupying a very limited space at BHJT2 may be reflecting this competition dynamic in their 

low growth rate and condition relative to the other juvenile aggregations. 

The bushmaster collections yielded an unexpected wealth of associated fauna. A total of 59 

different species were collected in association with these four aggregations of tubeworms. 

Thirty-five species had not been documented from previous collections of seep fauna and many 

of those may turn out to be new species. Many of these species were found in more than one 

collection indicating that their occurrence was not unusual. Since 17 of the species were 

represented in only a single collection it is likely that we have only begun to scratch the surface 

of the species richness of fauna associated with vestimentiferans at these sites. One discovery of 

special relevance to the study that also emphasizes the superiority of this new collection method 

over previous methods was the association of the chemosynthetic mussel Tamu fisheri with the 
8-51 

bases of some vestimentiferan aggregations. A significant effort was made during CHEMO I to 

collect this species, but its natural habitat was unknown (like the new species of Escarpia-like 

vestimentiferan discovered during this study, only a few individuals of Tamu fisheri were 

collected during the CHEMO I project). A second surprise was the consistent collection of fish 

and eels in the aggregations. 

The limited number of collections precludes a statistical comparison of bushes of different ages, 

but several trends stand out in the data. The juvenile aggregation had the highest biomass of any 

aggregation even though it also had the lowest surface area of exposed tube for colonization. 

Species richness was about half of the adult aggregations and dominated by known endemic 

species. Mollusks dominated the associated fauna as they do in mussel beds and the fauna in 

general was similar to that found associated with mussel beds. All of these observations are 

consistent with juvenile aggregations occurring in the areas of most active seepage colonized by 

vestimentiferans and with a high level of productivity in the aggregations. The abundance of 

grazers suggests an abundant food source for this feeding group, which is likely to be free-living 

bacteria. 

Both adult aggregations had about twice the species richness of the juvenile and senescent 

aggregations. However, the biomass supported by the adult aggregations was 15 – 20% of that 

supported by the juvenile aggregation. Although mollusks are still abundant in these 

aggregations, arthropods are more dominant. Even though the two collections were from 

different sites, the six most abundant species (morphotypes) of arthropod were the same. 

Although many of the species in these aggregations are endemic to the seeps, the presence of 

many known vagrant species suggests this habitat is tolerable by a wider range of fauna. The 

moderately high biomass suggests that there is still significant primary productivity associated 

with the adult aggregations. 

The senescent aggregation was notable for the very low biomass of associated fauna, 69% of 

which was accounted for by a crab and two fish. Mollusks were rare in this aggregation (a single 

individual) and non-endemic species abundant. Over half of the species present were represented 

by a single individual, and the density of associated fauna was less than 1% of that found in the 

juvenile aggregation. All of this is consistent with a habitat of low productivity and low toxicity. 

The model for seep tubeworm life history and community succession that results from the 

present work is as follows: Tubeworm aggregations begin with larval tubeworms settling in areas 

of active seepage where the precipitation of carbonates forms the hard substrate they need for 

recruitment. In some cases, this is in areas where mussels are also settling or have previously 

colonized. For a few years, the high level of seepage is maintained at this point, the carbonate the 

first recruits have settled on may continue to grow, and recruitment to this aggregation continues 

as long as sulfide is released from the seafloor and the carbonate remains exposed. Because the 

young aggregation is in a microhabitat of active seepage, only fauna that can tolerate these 

conditions are associated with the young aggregations. These fauna include the animals found in 

mussel beds and is dominated by endemic seep animals that can occur in large numbers with 

significant biomass. During this time the very young tubeworms obtain sulfide across their 

plume, grow a posterior extension of their tube (a “root”), and begin to supplement their sulfide 

uptake from interstitial pools. Over the next century or two, the tubeworms continue to grow 

while seepage of sulfide from the sediment into the water column progressively decreases. 

During this period, there is sufficient primary production associated with the aggregation to 
8-52 

maintain a moderately high biomass community and yet the toxicity of the habitat has decreased 

to the point that a wide variety of non-endemic fauna can colonize or visit the aggregations. At 

this point, the analogy between the tubeworms and long-lived ecosystem-structuring plants is 

quite strong. Although the tubeworms may not be the prime food source for most of the 

associated fauna, they provide a habitat for numerous species and stable isotope data suggests 

most of the fauna is incorporating seep primary production. As the aggregation continues to age, 

flow of sulfide into the water column continues to decrease and some thinning of the aggregation 

occurs. Non-endemic and non-mobile fauna often colonizes the tubes heavily, primary 

production by free-living bacteria associated with the aggregation decreases significantly, and 

the biomass of the associated faunal community drops significantly. This stage in their life 

history may also last a very long time, because the tubeworms in these less dense old 

aggregations continue to grow and are in very good condition, presumably in part because of 

reduced competition from the remaining vestimentiferans. 

8.5 Histopathy and Health of Seep Mussels*

8.5.1 Introduction 

Petroleum seep mussels (Bathymodiolus childressi) are often exposed to high hydrocarbon

concentrations in their natural habitat. And, like many mollusks, they are susceptible to 

parasitism and disease. Thus, they offer the opportunity to examine the relationship between 

parasitism, disease, and contaminant exposure under natural conditions. Because petroleum seep

mussels are closely associated with hydrocarbon seepage, the prevalence and infection intensity 

of parasites, diseases, and tissue pathologies may vary with site chemistry and, as a consequence, 

may provide a useful early warning signal of long-term changes in the health of seep

communities that may eventually result in local extinction. Discriminating populations that are in 

decline from those that are healthy is a key component to understanding the processes controlling

the structure and persistence of seep communities.

This is the first report on the histopathology of cold-seep mussels. The goals of this study were 

as follows: 

1. To document the parasite body burdens in petroleum seep mussels; 

2. To determine whether parasite body burdens varied spatially between nearby populations 

and between populations separated on larger scales; 

3. To evaluate the degree to which parasites might impact population dynamics and the 

persistence of seep communities; 

4. To document the rapidity with which parasite infections and physiological condition 

respond to changes in host population structure and environment; 

 

 

*

 This section was authored by Evic N. Powell. 
8-53 

5. To examine the relationship between parasites levels and polycyclic aromatic 

hydrocarbon (PAH) body burdens. 

The importance of parasitism and disease in seep mussel populations has not been studied. 

Mussels commonly harbor parasites and diseases of a variety of types (Gee and Davey 1986; 

Kent et al. 1989; Kim et al. 1998) and, in some cases, these parasites and diseases can 

significantly affect their health and fecundity (Bierbaum and Ferson 1986; Coustau et al. 1991; 

Pérez et al. 1997). The concept that reduced health, which is brought on by pollutants,

limitations in food availability, and other stressors, results in increased susceptibility to a range 

of parasites and diseases in mollusks was initially propounded by Laird (1961) and has received 

support from a wide array of subsequent studies (Kim and Powell 1998). 

Understanding the role that parasites might play in mussel population dynamics and health 

requires an understanding of the rapidity with which parasite infections respond to changes in 

host population structure and environment. Parasite transmission rates vary substantially between 

parasite types. Some oyster parasites, such as Perkinsus marius and Haplosporidium nlesoni, can 

reach 100% prevalence in less than six months (Hofmann et al. 1995; Ford et al. in press). Other 

parasites, including many trematodes, have very slow rates of transmission (Sousa 1990; Curtis 

1996, 1997). Proliferation of parasite infection also varies according to the response time to

environmental change and physiological limitations on parasite growth. 

In this report, we examine these processes in populations of the seep mussel, Bathymodiolus 

childressi. Following the methods section, we will first discuss the results of the population 

survey in year one and then address the specific process-oriented questions studied in year two. 

8.5.2 Methods

The Johnson SEA-LINK collected seep mussels, B. childressi, from the primary study sites 

GC185, GC233, and GC234. Additional limited collections were taken from GB425 and a fifth 

site designated as TAMU-17. Table 8.5.1 lists the sampling stations and the 1997 and 1998 

collections for the study. For most collections, sampling focused on the largest mussels in the 

population. This permitted sufficient tissue for PAH analysis and provided a direct comparison 

between PAH analysis and parasite body burden. In theory, this sampling protocol focused on 

the most heavily parasitized fraction of the population. At selected sites, the importance of size 

on parasitism was evaluated by taking a wider range of size classes for analysis.

Collected mussels were fixed in Davidson's fixative immediately after surfacing following the 

NOAA Status and Trends (NS&T) protocols (Ellis et al. 1998a). Dissection, embedding, 

sectioning, and staining procedures followed the same protocol. All assessments were based on 

quantitative measures or semi-quantitative scales so that reproductive stage and

parasite/pathology infection intensity could be rigorously statistically evaluated. Direct appraisal 

of this approach during the MMS GOOMEX program (Wilson-Ormond et al., in press) showed 

the power of using quantitative scales of infection intensity rather than just prevalence. Nearly all 

statistically significant relationships were observed using quantified measures. Earlier work in 

the NS&T program had indicated that this approach would be advantageous (Wilson et al. 1990, 

1992). Friedman et al. (1997) provides a recent comparison example for abalone.

Reproductive stage and certain pathologies like digestive gland atrophy were assigned semiquantitative scales describing stage of development or severity of the effect, respectively. Most
8-54 

parasites were quantified by tallying the number of specimens in standardized tissue crosssections (Ellis et al. 1998b; Sericano et al. 1993). The exception was widespread, ramifying, or

invasive parasites such as certain trematode infections. These were also quantified using a semiquantitative infection scale. Table 8.5.1 describes the scales used for these evaluations. 

Table 8.5.1. Semi-qualitative scales used for evaluation of gonadal stage (Ellis et. al 1998a), gill and 

digestive gland atrophy, and Bucephalus infection (Ellis et al 1998b). 

Variable Scale description Scale values 

Gonadal Stage Resting 0 

 Developing to fully ripe 1 to 5 

 Spawning to spent 4 to 1 

Gill and 

Digestive- gland 

atrophy 

No atrophy to severe, widespread tissue degeneration 0 to 3 

Parasitic 

infestation 

Uninfected to complete or near complete filling of organism 0 to 4 

Mussels used for PAH analysis were frozen immediately after collection. PAHs were analyzed 

using the NS&T Mussel Watch protocol (Wade et al. 1993). In order to permit easy comparison 

to Mussel Watch data, a series of 18 PAHs were summed and reported as total PAHs (Wade et 

al. 1988; Jackson et al. 1994). These PAHs were naphthalene, 2-methylnaphthalene, 1-

methylnaphthalene, biphenyl, 2,6-dimethylnaphthalene, acenaphthalene, fluorene, phenanthrene, 

anthracene, 1-methylphenanthrene, fluoranthene, pyrene, benz[a]anthracene, chrysene, 

benzo[e]pyrene, benzo[a]pyrene, perylene, and dibenz[a,h]anthracene.

In addition, the experimental design included a reciprocal transplant between pairs of 

populations, which were located at separate study sites. These populations generally correspond 

to mussel beds or mussel stations (e.g., BHM1) which are described elsewhere and are generally 

referred to by their study site descriptor. Mussels (B. childressi) were collected from one site 

(e.g., GC184), hereinafter termed the donor population, by submersible and brought to the 

surface in a temperature-controlled container. Mussels were kept onboard ship for up to 18 hr at 

in situ temperatures. Each mussel was marked with a numbered tag affixed with epoxy. Half of 

each collection, hereinafter termed the control site, was returned by submersible to the donor 

population from which the collection was made. The remainder, hereinafter termed the transplant 

population, was deployed by submersible at a second site (e.g., GC233) into a population 

hereinafter termed the receiver population. At the same time, mussels from the natural 

population at this second site were collected, returned to the surface, and marked as before. Once 

again, one group was returned to the same population, and a second group was transplanted into 

the population from which the first transplant was obtained. Thus, each population in the 

reciprocal pair had mussels of three types. They are as follows: 

1. The natural population untouched by the experimental protocol; 

2. The control mussels taken to the surface and returned into the same population; 

3. The mussels transplanted from the reciprocal site. 
8-55 

In addition, each population served as both donor and receiver depending on the direction of 

transplant. The reciprocal transplant pairs were GC185-GC233 and GC234-GC233. The two 

GC233 collections came from different, albeit nearby, stations (BPM1 and BPM4). One year 

later, each of these stations was revisited and samples of each of the three groups were collected 

– the natural mussels, the controls, and the transplanted mussels.

Statistical analyses used non-parametric analysis of variance (ANOVA) (Kruskal-Wallis) and

Tukey's Studentized Range Test on ranked data to identify significant differences within the 

ANOVA. In some cases, a principal components analysis (PCA) was run first, and the first three 

or four PCA factors were used as variables in the ANOVA.

For parasite prevalence, data were initially encoded as 1 (infected) or 0 (uninfected). For semiquantitative variables, data were encoded as their scores. For quantitative variables, data were 

normalized in two ways. First, parasite counts per section were normalized to an estimate of

cross-sectional area using L2

/4.75 as the divisor. The latter was deemed sufficient because a 

regression of anterior-posterior length (L) against dorsal-ventral height (H) or left-right width 

(W) did not yield a significant slope. That is, mussel shape did not change significantly with size 

and, hence, cross-sectional area bore a simple relationship to length (L/H = 2.5, L/W = 1.9). The 

term "whole body'' cross-sectional area is used because the divisor (L2

/4.75) is not an actual 

measure of the tissue section cross-sectional area but simply a normalizing tool to put animals of 

varying sizes into a common frame of reference. In the second normalized treatment method, 

wet-weight (gr) was divided by length (mm) to obtain the condition index. Although some 

measure of mantle cavity volume is typically used for condition indices (Rainer and Mann 1992; 

Smith 1995), the regular growth form of B. childressi permitted the simpler measure of 

normalized length to be used.

8.5.3 Results: the parasitic fauna of seep mussels 

Description of parasites found. Collections and assays completed during the first year of the 

program demonstrated significant parasite infestations of seep mussels. These results provided a 

characterization of the parasitic fauna and comparisons of the intensity of parasitic infestations 

among the program study sites. Five types of parasites were identified in sectioned preparations: 

gill rosettes, gill inclusions, gill ciliates, body inclusions, and Bucephalus-like trematodes. Figure 

8.5.1 illustrates these parasites. They are described below. 
8-56 

Gill rosettes. Gill rosettes of unknown affinity are associated with the bacteriocytes in the gill. 

Bacteriocytes are the specialized cells in B. childressi that harbor methanotrophic symbionts 

(Fisher 1990). The cells show no internal structure at the light microscope (Figure 8.5.1: Plates 

1a, 1b, and 1c). Each rosette is characterized by a group of 12 to 28 variably sized (2 to 5 m) 

rounded cells within or breaking through the gill. Except where the rosette breaks through the 

gill surface, no tissue pathology was observed. Gill rosettes resemble the free inclusion bodies 

observed by Johnson and Pennec (1995) (their Figures 1 through 4) in Loripes lucinalis. 

Gill inclusions and body inclusions. Gill inclusions and body inclusions are similar to 

chlamydia/rickettsia inclusions (Buchanan 1978; Meyers 1979; Fries et al. 1991) but have a 

more visible structure and often appear as a group of tightly compacted strands that are usually in 

a swirled configuration. For simplicity, they are hereinafter referred to as rickettsia. These 

rickettsia-like bodies were usually located at or near the distal end of the thickened tissue of the 

gill filaments that surround the water tubes (Figure 8.5.1: Plates 5a and 5b). Their average size is 

9.9 m by 15 m; however, they can vary greatly in size. 

Figure 8.5.1. Plate 1. Gill rosettes -- 1a, b: 1000x; 1c: 25x. Plate 2. Rickettsial-like bodies in the

digestive tubules and diverticula -- 2a, b: 1000x. Plate 3. Gill ciliates between the

demibranches -- 3a, b: 1000x. Plate 4. Bucephalus in the gonad -- 10x. Plate 5. Rickettsiallike bodies in the gill -- 5a: 1000x; 5b: 25x. 
8-57 

Body inclusions are also chlamydial/rickettsial-like inclusions but have a more visible structure, 

often appearing as a group of tightly compacted short rods that are in multidirectional groups. 

Occasionally, similar bodies are observed that appear smooth, without internal structure. Johnson 

and Pennec (1995) observed a very similar organism in Loripes lucinalis (their Figures 1 through 

3), which they believed to be a Chlamydia. For simplicity, they are herinafter referred to as 

rickettsia. These rickettsia are found in the digestive diverticula or the digestive tubules (Figure 

8.5.1: Plates 2a and 2b). Size is highly variable but averages 26.6 m by 20.9 m, about twice 

the size of the rickettsia in the gills. Besides the substantial size difference, significant 

differences in the distribution pattern among sites strongly suggest that body and gill rickettsia 

are distinctive organisms. No obvious tissue pathology was observed.

Gill ciliates. Extracellular gill ciliates, averaging 21 m by 11.4 m, are found between the gill 

filaments or near the base of the gill demibranches (Figure 8.5.1: Plates 3a and 3b). The ciliates 

appear to be freely mobile on the gill surface. Whether these ciliates are parasitic or adventitious 

commensals has not been determined.

Bucephalus-like trematodes. Bucephalus-like trematodes (e.g., Hopkins 1957, 1958; Wardle 

1990) were observed invading the gonadal material and, in heavier infections, other body 

compartments. The sporocysts are very similar to the Bucephalus trematodes found in shallowwater oysters and mussels, except that the mature cercariae are very large (Figure 8.5.1: Plate 4).

Hereinafter, these will be referred to as Bucephalus for simplicity although, we caution the 

reader that the trematodes observed in these mussels have not been unequivocally assigned to 

that genus.

Results of the population survey in the first year showed that infection intensity of gill parasites 

was significantly higher at GC233 than at the other sites (Tables 8.5.2 and 8.5.3). Much of this 

difference was due to a significantly higher prevalence and infection intensity of gill rickettsia.

Gill parasites were not observed at GC234 or at three of four GC185 sites but were present in 

over half of all GC233 specimens. Body rickettsia was prevalent at all sites and in all 
8-58 

Table 8.5.2. Prevalences and infection intensities in year one for the five types of parasites identified for each of the 4 primary sites

were sampled more than once, each sample came from a discrete mussel bed. Infection intensities are counts per tissue cross-s

Bucephalus which is rated on a 0-to-4-point scale. 

 Gill Rosettes Gill Inclusions Gill Ciliates Body Inclusions 

Location N Prevalence 

Infection 

Intensity Prevalence 

Infection 

Intensity Prevalence 

Infection 

Intensity Prevalence 

Infection 

Intensity P

GC-233 

M1 10 30% 4.3 30% 0.9 30% 0.5 80% 28.9 M2 3 0% 0.0 67% 25.3 0% 0.0 67% 19.7 M3 8 88% 22.6 100% 51.5 25% 0.6 88% 30.9 

GC-184 

M1 11 0% 0.0 0% 0.0 0% 0.0 100% 58.0 M2 8 0% 0.0 13% 0.1 13% 0.1 88% 4.9 M4 10 0% 0.0 0% 0.0 0% 0.0 90% 46.0 M5 10 0% 0.0 0% 0.0 0% 0.0 90% 7.7 

GC-234 

M1 10 0% 0.0 0% 0.0 0% 0.0 90% 46.0 M2 10 0% 0.0 0% 0.0 0% 0.0 100% 24.1 

GB-425 

M1 7 29% 0.6 86% 3.7 0% 0.0 100% 30.3 
8-59 

Table 8.5.3. Comparison of sites for which greater than one sample

was available in year one. Like letters indicate no significant 

difference at  = 0.05 using Tukeys Studentized Range Test. Totals do 

not include Bucephalus. 

 GC233 GC185 GC234 

Gill Rosettes 

Prevalence A A A 

Infection Intensity A A A 

Gill Chlamydia/Rickettsia 

Prevalence A B B 

Infection Intensity A B B 

Gill Ciliates 

Prevalence A A A 

Infection Intensity A A A 

Total Gill Parasites 

Infection Intensity A B B 

Body Chlamydia/Rickettsia 

Prevalence A AB B 

Infection Intensity A A A 

Total Parasites 

Infection Intensity A A A 

Bucephalus sp. 

Prevalence A B B 

Infection Intensity A B AB 

Gonadal Stage A A A 

Gill Tissue Atrophy A A A 

Digestive Gland Atrophy A A A 

PAH Body Burden A A A 

populations, never less than 67%; however, prevalence at GC234 was significantly higher than at 

GC233 (Table 8.5.3). Prevalence of Bucephalus was much higher at GC185 than at the other 

sites. Bucephalus was not recorded at GC233 or GB425 but reached prevalence as high as 100% 

at GC185 and 80% at GC234. Mussel bed-to-bed variation within a site was high. All gill

parasites recorded from GC185 were recorded from only one of the four GC185 samples. 

Bucephalus prevalence ranged from 18% to 100% GC185. Body rickettsia infection intensity 

ranged from 4.9 to 58 at GC185 (Table 8.5.2).

Prevalence and infection intensities of the three gill parasites were normally highly correlated. 

That is, mussel beds having high prevalence or infection intensities for one gill parasite normally 

had high prevalence or infection intensities for all gill parasites (Tables 8.5.4 and 8.5.5). The 

only exception was gill ciliates, an extra cellular commensal that was not correlated with gill 

rickettsia, an internal parasite. The two body parasites, body rickettsia and Bucephalus, were 

essentially uncorrelated with each other or with any gill parasite, with the exception of 

Bucephalus prevalence, which was weakly correlated with gill rickettsia prevalence. 

Comparison with shallow-water mussels. Gill ciliates were commonly observed in Mytilus 

edulis and Mytilus californianus at nearly every locale on both coasts. Overall, in comparison to 

shallow-water mussel populations, parasite infection intensities were higher by a factor of 10 in 

seep mussels than in shallow-water populations sampled by the Mussel Watch program.
8-60 

Table 8.5.4. P values ( = 0.05) for Spearman's Rank correlations for parasite prevalence and measures of general 

health, based on year one samples. Parentheses indicate negative correlations. 

 

 Gill 

Inclusions 

Gill 

Ciliates 

Body 

Inclusions

Bucephalus Gonadal

Stage 

Gill 

Atrophy 

Digestive 

Gland 

Atrophy 

PAH 

Body 

Burden 

Gill Rosette .009 .02 — — — — — — 

Gill Inclusions — — (.05) .05 — — — 

Gill Ciliates — — — — — (.04) 

Body Inclusions — — — — .03 

Bucephalus — — — 

Gonadal Stage (.01) (.01) — 

Gill Atrophy .0003 — 

Digestive Gland 

Atrophy 

 — 

Table 8.5.5. P values ( = 0.05) for Spearman's Rank correlations for parasite infection intensity 

and measures of general health based on year one samples. Parentheses indicate negative 

correlation. 

 Gill 

Inclusions 

Gill 

Ciliates 

Total 

Gill 

Parasites

Body 

Inclusions

Total 

Parasites

Gonadal

Stage 

Gill 

Atrophy

Digestive 

Gland 

Atrophy 

PAH 

Body 

Burden

Gill Rosette .001 .002 .0001 — .02 — — — — 

Gill Inclusions — .0001 — .008 — — — — 

Gill Ciliates .02 — — — — — — 

Total Gill 

Parasites 

 — .005 — — — — 

Body 

Inclusions 

 — — (.02) (.03) — 

Total Parasites — (.04) (.01) — 

Bucephalus (.002) — .03 — 

Gonadal Stage (.01) (.01) — 

Gill Atrophy .0003 — 

Digestive 

Gland Atrophy 

 — 

Implications for reproduction. Bucephalus infections are initiated in the gonadal tissue of a 

mollusk (infection intensity 1). As the sporocysts grow and the infection spreads, the gonadal 

tissue is completely consumed, and the parasite begins to invade the body tissue (infection 

intensity 2). Although mortalities have rarely been recorded (see Allen 1979 for an example), the 

loss of reproductive potential in infected animals is well known (Hopkins 1957). The evidence 

presented here indicates that Bucephalus exerts a significant influence on seep mussel population 

dynamics. Entire populations at the study sites are no longer reproductively active. Seep mussel 

populations were often found to have parasite prevalence exceeding 70% and infection 

intensities as high as 3.8 on the four-point scale (Table 8.5.2). Of even more interest is the fact 

that 50% to 75% of populations at the two primary sites, GC185 and GC234 (Table 8.5.2), are 

heavily impacted by this parasite. Thus, the populations of seep mussels at certain primary sites 
8-61 

may have insufficient fecundity to maintain themselves. Very likely, only a fraction of petroleum 

seep mussel populations are maintaining the entire beta-level population structure of this species.

Measures of tissue atrophy. Measures of tissue atrophy (Table 8.5.6) were inversely correlated 

with gonadal stage, which was determined, in large measure, by Bucephalus infection. Healthy 

mussels, those with low atrophy scores, were reproductively active, averaging a 3 or 4 on the 

five-point scale for gonadal stage. Unhealthy mussels with high atrophy scores rarely advanced 

in reproductive condition. Furthermore, gill tissue atrophy was negatively correlated with 

gonadal stage, which suggests that tissue atrophy in seep mussels, as we measured it, is a 

pathological condition. Body rickettsia was most common in animals with low atrophy scores, 

which possibly indicates that healthy tissue is required for infection intensification in this 

parasite. Generally, rickettsia/chlamydia are not thought to be harmful in mollusks (Otto et al.

1977; Elston and Peacock 1984; Cajaraville and Angulo 1991); however, mortality is frequently 

associated with infections in crustaceans (Johnson 1984; Sparks et al. 1985; Bower et al. 1996)

and occasional reports of pathologies associated with high prevalence are seen in bivalves (Le 

Gall et al. 1988). 

Table 8.5.6. Health indices and PAH body burden (ppb) in year one for mussels taken 

from each of the 4 primary sites. In cases where sites were sampled more than once, 

each sample came from a discrete mussel bed.

 Gonadal Gill Tissue Digestive PAH Body 

Location Stage Atrophy Gland Atrophy Burden 

GC233 

M1 3.5 0.7 0.5 565 

M2 3.0 0.7 0.7 — 

M3 4.4 0.5 0.1 214 

 

GC185 

M1 3.1 0.0 0.3 1346 

M2 0.0 2.1 2.4 283 

M4 0.5 0.7 1.1 697 

M5 0.0 2.7 2.4 1135 

 

GC234 

M1 0.8 1.1 0.5 614 

M2 3.8 0.5 0.5 1065 

 

GB425 

M1 4.0 0.6 1.0 1160 

PAH body burden. PAH body burden was high in petroleum seep mussels, exceeding 1 ppm in 

four of nine seep populations sampled in 1997 (Table 8.5.6). Variation in two parasites, gill 

ciliates and Bucephalus, explained most of the variation in PAH body burden between mussel 

populations. Although the data are admittedly few, so that relationships of this sort must be

considered with caution, the amount of variation explained (81%) is extremely high by NS&T 

standards – where the infection intensity of any three parasites rarely explains more than 40% of 

the PAH body burden (Kim et al. 1998). The higher average body burdens in seep mussels may 
8-62 

be responsible for this greater predictability, as many Mussel Watch locales were characterized 

by low PAH body burdens (<400 ppb).

The partial regressions for gill ciliates and Bucephalus on PAH body burden indicate that PAH 

burden decreases with increased infection of both parasites. The data do not permit a 

determination of causality, but the size and infection intensity of gill ciliates do indicate that 

ciliates are a surrogate for some other physiological variable that would decrease sequestration of 

PAH. In the case of Bucephalus, the probable cause of decreased PAH burden is more apparent. 

At infection intensities of one or higher, Bucephalus replaces a substantial fraction of the host 

tissue, including all of the gametic tissue. PAHs are known to be sequestered in lipid-rich 

gametic tissue at much higher concentrations than somatic tissue (Friocourt et al. 1985; Ellis et 

al. 1993) in some bivalves. This is due to the increased lipid concentration of the gametes. The 

relationship holds true for both eggs and sperm (Ellis et al. 1993). Bucephalus would be 

expected to reduce overall body burden, at high infection intensities, by replacing gametic tissue. 

The three highest PAH body burdens came from populations without Bucephalus and with few 

gill parasites. The simplest explanation for the observed relationship between PAH and parasites 

is that PAH body burden was low in some mussels due to the absence of gametic tissue and that 

PAH body burden was low in other mussels in which overall health was low, accounting for the 

frequency of gill ciliates that would not have been found in healthier animals. This result leads to 

the unanticipated conclusion that higher body burdens are found more frequently in healthy 

petroleum seep mussels. The generally high PAH body burdens found in all seep mussel 

populations and the finding of highest PAH burdens in the healthiest population is evidence that 

seep mussels are not as sensitive to PAH exposure as are the shallow-water mytilids.

8.5.4 Dynamics of parasitism 

Sampling in the second year (1998) was designed to confirm certain conclusions made in the 

first year (1997) and to investigate the dynamics of parasitism in mussel populations. The results 

from 1997 collections suggested that the parasite species and infection intensities differ 

significantly in mussel populations at separate sites. A larger survey was conducted in 1998 to 

determine the validity of this conclusion (Table 8.5.7). No additional parasite types were 

recognized in 1998 collections, which suggest that the five parasite types already described 

comprise the overwhelming majority of all parasites plaguing B. childressi. A more extensive

sampling of mussel beds in the study sites confirmed the absence of Bucephalus in GC233, its 

relative rarity at GC234 and its abundance at GC185. Bucephalus was also found at TAMU-17

and at one site at GB425. More extensive sampling confirmed the absence of gill rosettes at 

GC185 and GC234, and the ubiquity of this parasite at GC233. (One light infection at GCM1

may have occurred by introducing this parasite to this site via the transplant experiment.) Gill 

rickettsia were once again not observed at GC234 (One light infection at GCM1 may have

occurred by introducing this parasite to this site via the transplant experiment.). 
8-63 

Table 8.5.7. Mean prevalences, infection intensities, and physiological indices for mussel populations sampled in year two. For g

rickettsia, and body rickettsia infection intensities, parasite counts per section were normalized to an estimate of cross-sectional a

L2/4.75. N is the number of individuals used to generate the mean in this table and used in most statistical analyses. (N) is the numbe

used in the size-frequency analyses. 

Gill Rosettes Gill Inclusions Gill Ciliates

Body 

Inclusions Bucephalus Indices

Atr

Location N 

Prevalence Infection Intensity Preva- lence Infection Intensity Preva- lence Infection Intensity Preva- lence Infection Intensity Preva- lence Infection Intensity Gonadal Stage Condition Index G

Tis

GC-233 

M1 15 13.3% 0.024 73.3% 0.253 26.7% 0.017 80.0% 1.508 0.0% 0.000 3.733 0.338 0.M4 9 33.3% 0.033 33.3% 0.216 11.1% 0.005 100% 2.610 0.0% 0.000 3.444 0.266 0.AT1 5 0.0% 0.000 0.0% 0.000 0.0% 0.000 100% 3.404 0.0% 0.000 0.000 0.103 1.IBP 10(10) 50.0% 0.099 60.0% 0.128 40.0% 0.042 100% 4.990 0.0% 0.000 3.800 0.244 0.OBP 3 33.3% 0.018 100% 6.323 0.0% 0.000 100% 3.715 0.0% 0.000 3.000 0.341 0.

GC-184 

M1 9 0% 0.000 0.0% 0.000 0.0% 0.000 100% 4.936 77.8% 1.556 2.667 0.233 0.

M2 10 0% 0.000 0.0% 0.000 10.0% 0.008 80.0% 2.001 100% 3.000 0.000 0.270 1.M3 10(9) 0% 0.000 0.0% 0.000 20.0% 0.012 100% 3.567 40.0% 1.200 1.400 0.208 0.M4 8 0% 0.000 0.0% 0.000 0.0% 0.000 100% 11.442 87.5% 2.625 0.250 0.138 0.

M5 6(10) 0% 0.000 50.0% 0.048 16.7% 0.006 100% 1.521 33.3% 1.000 2.333 0.400 0.M6 10(13) 0% 0.000 0.0% 0.000 0.0% 0.000 100% 1.823 70.0% 2.100 0.900 0.151 0.JT2 8(7) 0% 0.000 0.0% 0.000 0.0% 0.000 100% 4.529 62.5% 1.875 0.750 0.121 0.

GC-234 

M1 10 10.0% 0.010 10.0% 0.787 0.0% 0.000 100% 5.361 20.0% 0.600 1.300 0.100 0.

M1A 10 0.0% 0.000 0.0% 0.000 0.0% 0.000 100% 6.419 40.0% 1.200 0.800 0.107 0.M2 13 0.0% 0.000 0.0% 0.000 7.7% 0.013 92.3% 3.273 15.4% 0.308 1.846 0.156 0.

GB-425 

M1 13 23.1% 0.020 76.9% 0.515 23.1% 0.011 92.3% 5.584 7.7% 0.231 3.462 0.338 0.

M1A 4 0.0% 0.000 25.0% 0.088 0.0% 0.000 100% 1.769 0.0% 0.000 3.000 0.390 0.M2 12 0.0% 0.000 58.3% 0.446 8.3% 0.004 100% 7.428 0.0% 0.000 2.417 0.277 0.

TAMU-17 8 0.0% 0.000 37.5% 0.053 0.0% 0.000 100% 2.691 50.0% 1.250 1.875 0.302 1.
8-64 

Thus, an extended survey conducted in 1998 confirms the endemic nature of many of the 

parasites in B. childressi populations. Program study sites differ significantly in their parasite 

complement. The similarity in parasite burden between GB425 and GC185 and the dissimilarity 

between the much closer GC185 and GC233 suggest that environmental effects are the likely 

cause of these differences. 

A high force of infection combined with either short parasite lives or rapid adult mortality could 

sustain the parasite prevalence and infection intensities seen in seep mussel populations. The first 

alternative is rare among invertebrates, while the second alternative is not consistent with the 

long life spans observed in B. childressi (Nix et al. 1995). Therefore, it is most likely that the 

parasite prevalence and infection intensity observed resulted from the slow, persistent 

accumulation of infection. 

We investigated the dynamics of transmission, proliferation, and loss in two ways: by 

transplanting mussels from one place to another and by examining the influence of host size on 

prevalence and infection intensity.

Results of Transplant. Donor and receiver populations varied significantly in prevalence and

infection intensity for most parasites and in physiological condition for most physiological 

indices. Therefore, the infection intensity and/or prevalence (typically both) of transplanted 

populations had to rise or fall significantly over one year in order to come into equilibrium with 

receiver populations. Transplanted populations came into equilibrium with the receiver

populations much faster in cases where a decrease in infection intensity or prevalence was 

required (Table 8.5.8). Quite the opposite was true for cases where the transplanted population 

had to gain infections or increase in infection intensity to come into equilibrium with the receiver

population. Very few cases reached equilibrium after increased infection. In many cases, the 

transplanted population retained the donor population condition for relatively uninfected 

populations. In most of the remainder, the transplanted population clearly fell between the donor 

and receiver population condition. All transplant results are summarized in Table 8.5.8. Overall, 

the single-celled rickettsia responded more rapidly to the environment of the receiver population 

than did the multi-celled Bucephalus.

In cases where the transplant was made to a receiver population of lower prevalence and 

infection intensity, infection intensity and prevalence rapidly declined so that the transplanted 

mussels were indistinguishable from the receiver population after one year. The data strongly 

suggest that rickettsial infections are rapidly lost without a constant source of infective elements 

renewing infections and the force of infection as well as factors controlling within-host 

proliferation probably significantly influences that infection intensity.

The rapid loss rates explain why gill rickettsial prevalence is maintained well under 100% in the 

GC233 populations despite the long life span of B. childressi. This suggests that the same rates 

for body rickettsia are distinctly lower, perhaps because the life span of a rickettsial body is

significantly longer, and as a result prevalence hovers around 100%. The information clearly 

suggests that a more effective method of transmission exists at GC233 (both sites) than at GC234 

or GC185 for gill rickettsia and that conditions for proliferation are equally superior there. What 

precisely these conditions are we do not yet know.
8-65 

Table 8.5.8. Relationship of the transplanted individuals to the donor and receiver population in cases where a nonparametric ANOV

population effect. Brackets indicate situations where a Tukey's Studentized Range Test did not resolve the source of the variation. In 

population is listed under the population of greatest affinity.

Receiver Population Significantly Greater than Donor Population 

 Change required an increase in the variable value 

No Change Moderate Change Complet

(Donor Population 

Not Significantly Different)

(Donor and Receiver Population 

Not Significantly Different)

(Receiver 

Not Significa

Bucephalus prevalence (GC184) Gill rickettsia infection intensity (GC233M1) Gonadal stag

Bucephalus infection intensity (GC184) Gonadal stage (GC233M1) Gill rickettsia prev

Gill rickettsia prevalence (GC233M1) Digestive gland atrophy (GC233M1) Gill rickettsia infectio

Gill tissue atrophy (GC233M1) Condition index (GC233M4) 

Condition index (GC233M1) [ Bucephalus prevalence (GC234)] 

 [ Bucephalus infection intensity (GC234)] 

 [Body rickettsia prevalence (GC184)]  [Body rickettsia infection intensity (GC184)] 

Receiver Population Significantly Lower than Donor Population 

 Change required a decrease in the variable value 

No Change Moderate Change Complet

(Donor Population 

Not Significantly Different)

(Donor and Receiver Population 

Not Significantly Different)

(Receiver 

Not Significa

Bucephalus prevalence (GC233M4) Gonadal stage (GC184) Gill rickettsia pr

Bucephalus infection intensity (GC233M4) Condition index (GC184) Gill rickettsia infect

Bucephalus preva

Bucephalus infection 

 Gill tissue atr Digestive gland  Condition in Body rickettsia infectio Gonadal sta [Gill rickettsia pr
8-66 

The multi-celled Bucephalus shows precisely the opposite condition. Again, transmission rates 

could be estimated in two cases because Bucephalus was not found in either GC233 population. 

Rates were 0.12 to 0.25 y-1. These transmission rates are surprisingly high given the prevalence

observed in mussels transplanted to GC234 but are in accordance with the high prevalence 

observed in mussels transplanted to GC185. This suggests that loss rates are much higher at 

GC234. Observations support the potential for rapid loss of infections, but are ambiguous 

overall. Mussels transplanted from GC234 to GC233-BPM1 retained donor population

prevalence. Infections were not lost measurably over one year. Mussels transplanted from 

GC185 to GC233-BPM1 lost about 75% of their infections in one year.

B. childressi is considered to have a life span measured in decades. Measured transmission rates

are relatively high, yet measures of population prevalence are often well below 100%. The strong 

suggestion is that these trematode sporocysts have life spans considerably less than their hosts, a 

fact that is of some consequence since, while infected, the mussels lose any capacity to

reproduce. From these arguments, however, we cannot explain the persistence of Bucephalus in 

mussels transplanted from GC234 in comparison to those originating at GC185.

The simplest explanation for the high parasite prevalence is the slow, steady accumulation of 

infections over a long life span. However, transplanted mussels rapidly approached the condition 

of the receiver population in most instances, both in prevalence and infection intensity. Clearly, 

the parasite body burden in these mussels is a dynamic process, rapidly responding to changes in 

infection pressure in the environment. The rates of proliferation and loss must also be relatively 

rapid. This infection milieu must be dramatically different from that experienced by shallowwater mussels on the east and west coast of the United States, where low prevalence and 

infection intensities are the norm (Kim et al. 1998). 

Local versus regional control on parasitism. Nearby populations were as different, in many 

respects, as populations from the different major sites (e.g., GC233, GC185). This observation,

made after 1997, was confirmed by the more extensive sampling in 1998 (Table 8.5.7). Thus the 

dynamics controlling parasite acquisition, proliferation, and loss are predominately determined 

by local environmental and/or biological conditions varying from population to population, not 

major site to major site. The single exception is the absence of certain parasite types from certain 

major sites. Some of the within-major-site patchiness may be ascribed to the size and remoteness

of populations within the major sites. For example, both samples of mussels from tubewormdesignated collections (BPAT1 and BHJT2) had prevalence measures of zero for all gill 

parasites. On the other hand, two neighboring collections at GB425 (GBM1 and GBM1A) 

differed considerably in gill parasite infection intensity, as did some of the samples from the 

extensive mussel bed surrounding the brine pool at GC233 so that within-population patchiness 

is also likely to be responsible.

Gill parasites, all of which probably have a nearest neighbor type of transmission, exhibit the 

greatest degree of patchiness, in comparison to Bucephalus, which infects from what is likely a 

mobile definitive host. Transmission of the nearest-neighbor kind would sustain patchiness and 

limit transmission to remote, small populations. Of course, the same type of transmission might 

be ascribed to the ubiquitous body rickettsia, yet its ubiquitously high prevalence would demand 

a more efficient mode of transmission.
8-67 

PAH Body Burdens. PAH body burdens averaged lower in 1998 than in 1997. Nevertheless, 8 

of 21 populations had body burdens exceeding 500 μg (g dry wt) and two exceeded 1 μg (g dry 

wt) in animals collected in 1998. Body burden was not related to major site. Some populations 

were high and some were low at each major site (Figure 8.5.2). Body burden depended upon 

small-scale processes. The same could be said when the proportional contribution of individual 

PAHs to the total was examined. A major-site dependency did not exist. However, local

populations differed substantially (Figure 8.5.3). As examples, mussels from GCM1 and BHM6 

were relatively depleted in naphthalenes. Mussels from BPOBP and BPM1 were relatively 

enriched in methylnaphthalenes. Mussels from TAMU-17 were relatively enriched in chrysenes.

Transplanted populations came into equilibrium with receiver populations in PAH body burden 

after one year, in cases where body burden was higher in the receiver population. Body burden 

remained high in the one case of mussels transplanted to populations with lower body burden 

(Figure 8.5.4). However, an examination of the proportional contribution of 44 PAHs to the PAH 

body pool indicated that proportions came into equilibrium in all cases (Figure 8.5.5). Therefore, 

the retention of high body burdens in the case where mussels were transplanted to a population 

with lower body burden was not due to low depuration rates. Likely, it was due to an inherently 

higher storage capacity in mussels obtained from populations with high body burden. What 

biological process is responsible is not yet clear. Changes in total lipid, for example, did not 

explain the pattern.

In 1998, the in-depth analysis of a larger data set confirms the conclusions made in 1997 that 

parasites are much more closely related to PAH body burden than physiological indices such as 

condition index or tissue atrophy scores. They further confirm that gill ciliate prevalence and 

infection intensity and Bucephalus infection intensity explain much of this relationship. 

Bucephalus certainly changes the tissue composition of the host. Gonadal tissue is reduced or 

absent, and trematode tissue attains a relatively high proportion of total tissue. It seems likely 

that a differential capacity for PAH sequestration may most easily explain the relationships 

observed. Gill ciliates are likely too small and infection intensities too low to themselves damage 

the host in any way. Gill ciliates probably are symptomatic of some exposure or physiological 

process impacting the animal, recalling the original proposal of Laird (1961). Regardless, the 

comparisons identify an important relationship between PAH body burden and parasitism in 

petroleum seep mussels and raise the question of using caution in accepting PAH body burden as 

a simple measure of exposure in these populations.
8-68 

Figure 8.5.2. Plot of total PAH body burden versus population for B. childressi populations from 5 major sites. The PAHs summ

are listed in Figure 8.5.3. 
8-69 

Figure 8.5.3. Proportional differences between the sample mean and the global mean for each compound in

the PAH compound series. For this calculation, each mean variable value for each sample was

standardized to parts per thousand, as was the global mean value for the variable for all samples.

The difference between the global mean standardized value and the sample mean standardized

value was calculated and these mean standardized differences were plotted. The PAH compound

series is as follows: 1, naphthalene; 2, c1-naphthalenes; 3, c2-naphthalenes; 4, c3-naphthalenes;

5, c4-naphthalenes; 6, biphenyl; 7, acenaphthylene; 8, acenaphthene; 9, fluorene; 10, c1-

fluorenes; 11, c2-fluorenes; 12, c3-fluorenes; 13, phenanthrene; 14, anthracene; 15, c1-

phenanthrenes/anthracenes; 16,c2-phenanthrenes/anthracenes; 17, c3-

phenanthrenes/anthracenes; 18, c4-phenanthrenes/anthracenes; 19, dibenzothiophene; 20, c1-

dibenzothiophenes; 21, c2-dibenzothiophenes; 22, c3-dibenzothiophenes; 23, fluoranthene; 24,

pyrene; 25, c1-fluoranthenes/pyrenes; 26, benzo[a]anthracene; 27, chrysene; 28, c1-chrysenes;

29, c2-chrysenes; 30, c3-chrysenes; 31, c4-chrysenes; 32, benzo[b]fluoranthene; 33,

benzo[k]fluoranthene; 34, benzo[e]pyrene; 35, benzo[a]pyrene; 36, perylene; 37, indeno[1,2,3-

c,d]pyrene; 38, dibenzo[a,h]anthracene; 39, benzo[g,h,i]perylene; 40, 2-methylnaphthalene; 41,

1-methylnaphthalene; 42, 2,6-dimethylnaphthalene; 43, 1,6,7-trimethylnaphthalene; 44, 1-

methylphenanthrene. 
8-70 

Figure 8.5.4. Plot of total PAH body burden for mussels in the transplant experiment. The donor population for the GC233M1 tr

The reciprocal was true for the GC185M1 transplant. The mussels transplanted to GC233M4 came from GC234M1. 

this measurement are listed in Figure 8.5.3. 
8-71 

Figure 8.5.5. Proportional differences between the sample mean and the global mean for each compound in the

PAH compound series for the three transplant experiments. For this calculation, each mean

variable value for each sample was standardized to parts per thousand, as was the global mean

value for the variable for all samples. The difference between the global mean standardized

value and the sample mean standardized value was calculated and these mean standardized

differences were plotted. Donor populations are identified in Figure 4 and the PAH compound

series in Figure 8.5.3.
8-72 

8.6 Food Chain Dynamics*

8.6.1 Introduction 

There is an obligate relationship between the habitat-forming chemosynthetic hosts—tubeworms 

(vestimentiferans), mussels (modiolids), and clams (vesycomids)—and seafloor emissions of 

hydrogen sulfide and/or methane. This relationship is so obvious and strong that declaration of 

these ecosystems to be chemically and physically controlled would meet with little debate. 

Uncritical acceptance of the position that a special geochemical environment is both necessary 

and sufficient for the establishment and persistence of chemosynthetic communities is unwise 

from the management perspective. As in most ecosystems, seeps are likely to experience both 

abiotic and biotic control, which renders the geochemical environment necessary but insufficient 

for community establishment and persistence. If this is the case, effective management to protect 

seep environments must consider the role of biotic control. 

Rigorous demonstration of biotic control, starting with models and progressing into field 

situations, has dominated much of theoretical and practical ecology for the past fifty years. 

Unfortunately, strict requirements of experimental design, replication, and use of controls may 

make it impractical formally to identify the nature and extent of biotic control in any deep-sea 

system, especially seeps and vents. This is the case when non-lethal competition among species 

is a factor in biotic control. It is possible, however, to gauge the nature and extent of predation 

through analyses that determine the dietary history of a sampled organism. This section presents 

results from this investigation. 

There are two basic questions about seep trophic relations that are of both scientific and 

management importance: within-seep feeding patterns and interactions between the seep and 

background systems. The background system is defined as the surrounding deep-sea 

environment. It is established that the numerous consumer organisms associated with seeps and 

vents derive all or some nutrition from chemosynthetic sources. If the details of feeding indicate 

that the habitat-forming species are being preyed upon, then predatory-mediated population 

control could impact system stability and persistence. Similarly important are seep-background 

interactions. If the background fauna exploits the seep system or the seep somehow exploits the 

background, then the zone of interaction must extend beyond the confines of the special 

geochemical environment. Regulatory policies, therefore, need to protect such a larger area. 

8.6.2 Background: expected seep-background interactions 

The oasis of seeps versus the desert of the background benthos. In spite of different geological 

settings, deep-sea chemosynthetic communities at continental margin cold seeps and seafloor 

spreading sites share ecological features. Both are known to have high standing stock biomass 

levels of producer and consumer organisms in spatially compressed areas. Interactions of these 

productive systems with the comparatively meager background of typical deep-sea benthic fauna 

are poorly known. Observations within seep and vent systems support the view that only a very 

small fraction of background species actually exploit the chemosynthetic production (Carney 

1994). 

 

*

 This section was authored by Robert S. Carney, Steven Macks and Stephan Macavoy. 
8-73 

Initial observational evidence of linkage. Analysis of stable isotope content in tissues from hot 

vent and hydrocarbon seep organisms has been instrumental in confirming the chemosynthetic 

nature of productivity and apparent trophic linkages within the producers and consumers 

community (Fisher et al. 1994; Kennicutt et al. 1992; Rau 1981; Rau and Hedges 1979; 

Vandover and Fry 1989, 1994). Successful trophic tracing requires unambiguous end points. 

With some cautions, this condition is met at the hydrocarbon seeps. For most non-seep fauna, 

phytodetritus will be the ultimate food source and sole organic carbon pool. Phytodetritus is 

organic material created photosynthetically in the euphotic zone. Depending on the species 

composition of the phytoplankton and various oceanographic conditions, the isotopic 

composition of the phytodetritus reflects fractionation of the available seawater inorganic carbon 

pool (Degens et al. 1968a; Degens et al. 1968b; Goericke and Fry 1994; Goericke et al. 1994). In 

the Northern Gulf of Mexico, the non-seep deep benthic fauna has a very well defined 13C value 

17.7‰ 1.2 (Carney et al. unpublished manuscript). For consumers in and near seeps, the 

abundant biomass of the symbiont-hosting organisms is a possible food source. In the 

methanotrophic mussels at Gulf of Mexico seeps, the methane source affords a pool of carbon 

distinguishable by the stable carbon isotope signature of methane (Kennicutt et al. 1992). Studies 

of the isotope composition of Gulf of Mexico seep biota have been undertaken since the earliest 

discoveries of the systems and have proven extremely informative. They served to identify 

methane as an energy and carbon source for the methanotrophic mussels, proved some degree of 

trophic dependence by the heterotrophic seep residents upon chemosynthetic productivity, and 

suggested that certain mobile fauna caught away from seeps, notably large crabs like Chaceon

and fish like Synaphobranchus had foraged in the seep areas at some time (Carney et al.

unpublished manuscript). 

However, as results accumulated, it became evident that the carbon pools available to the seep 

organisms are complex and do not always have distinguishing isotopic signatures. Therefore, 

interpretation has been more speculative than desired. This was most noticeable for the seepresident gastropods and crustaceans that displayed isotope values from (-45‰ up to -17‰). This 

wide range can be interpreted in many ways. Falling between mussel tissue and phytodetritus, it 

simply could be a mixture of sources. But, how could the animals get so much phytodetritus at 

those depths? Alternately, there could be multiple carbon pools, which are essentially 

chemosynthetic but mimic chemo-phyto mixed sources. Such mimics could be tubeworms that 

have been found to have very wide ranges (Kennicutt et al., 1992) or free-living bacteria that 

have yet to be effectively sampled. A similar suggestion of a confounding bacterial carbon pool 

has been put forward by VanDover and Fry (1989, 1994). 

Whether seeps and vents are trophically open or closed is fundamental in respect to how both 

those systems and the deep-sea background function. There is a prevalent idea in deep-sea 

ecology that the deep benthos is food limited (reviewed in Gage and Tyler 1991). Food limitation 

means that the inventory of sustained species populations is more impacted by food supply than 

other factors. This condition is assumed to have persisted over very long time periods giving rise 

to considerable foraging specialization (Jumars et al. 1990). If the deep background were food 

limited, then one would predict a high level of exploitation of seep/vent systems. This 

investigation examines the interaction of these two systems by looking for evidence of trophic 

links in samples from the background rather than the seep or vent site. Inquiry directed at the 

background is a necessary complement to site-directed work and may prove to be a better means 

of assessing larger-scale effects of seep and vent systems. 
8-74 

8.6.3 Objectives 

The objective of this work is to provide a trophic component to the study of interdependencies 

and interrelationships between chemosynthetic and non-chemosynthetic organisms. The task is 

intended to improve upon preliminary findings and is intended to provide the following: 

A definitive means of determining which seep-dwelling species actually exploit 

chemosynthetic production rather than some other habitat value of seep-related 

topographic features; 

A definitive means of determining if mobile deep-sea species found adjacent to seep 

communities exploit chemosynthetic production; 

A definitive means of identifying prey-predator pairs, which might indicate predatory control 

on seep populations. 

8.6.4 Methods 

Field methods. The animals examined in this study were collected during the summers of 1997 

and much more extensively in 1998. Mobile predatory fauna within seep communities were 

collected either with small (~75x75x50 cm) wire mesh traps deployed within and around 

tubeworm and mussel beds at GC185, GC234, and GC233 by the Johnson SEA-LINK 

submersible or directly by the submersible. Mobile predators were caught off-site with surface 

deployed Z-frame traps, which measured approximately 150x200x75cm, and were deployed 

roughly 2 km off the location of the seep communities in areas known from prior surveys to lack 

active seep communities. Both small and Z-frame traps were intended to capture fish and 

crustaceans. Both were constructed of 1-in trap mesh, equipped with two 20-cm entry mouths, 

and baited with menhaden in a wire bait cage to minimize consumption. Trap soak time (duration 

of deployment) was determined by cruise logistics and ranged from one to six days. 

Isotope analysis. Tissue samples were taken from captured organisms and frozen until shipment 

to the stable isotope laboratory. Samples were dried at 60°C for three days and homogenized. 

The dried tissues were then ground to powder and, if they contained carbonate, were acidified 

with 10% HCl, and re-dried. The powder was placed in a tin cup and weighed (typically 0.5 mg 

was used). The isotope compositions were determined using a high temperature combustion 

system that incorporated a Carlo-Erba CHN analyzer interfaced to an OPTIMA stable isotope 

mass spectrometer using a continuous flow interface. The samples were combusted in an 

oxidizing furnace at 1020°C. The gases from the combustion were passed through a reduction 

furnace at 650°C and a chemical water trap, followed by a gas chromatograph, which separated 

the evolved carbon dioxide from the nitrogen gas. These pulses of combustion gases were 

isotopically assessed following calibration of the mass spectrometer with pulses of laboratory 

gases of known isotopic composition. 

As an auxiliary activity, sulfur isotopes were also being examined. For these determinations, the 

furnace temperature was higher (1030°C), and the gas chromatographic separation was 

accomplished at a higher temperature. A total of 5 to 6 mg of sample was required for sample 

analysis. The evolved sulfur dioxide was analyzed versus a laboratory tank of calibrated gas and 

corrected for cross mass overlap of oxygen. Typical precision of the measurements was 0.2‰. 
8-75 

The isotope compositions will be reported relative to standard material and follow the same 

procedure for all stable isotopic measurements, as follows: 

x

E = [(xE/yE)sample/(xE/yE)standard] - 1) * 1000 (8.6-1) 

where E is the element analyzed (carbon, nitrogen or sulfur), x is the atomic weight of the 

heavier isotope, and y the lighter isotope (x = 13, 15, 34 and y = 12, 14, 32 for carbon, nitrogen, 

and sulfur, respectively). The standard materials to which the samples are compared are PDB 

(Pee Dee Belemnite) for carbon, air (N2

) for nitrogen, and CDT (Canyon Diablo Triolite) for 

sulfur. Reproducibility of all measurements is typically better than 0.3‰ for 

34S, and 0.2‰ for 



13C and 

15N. 

Immunoassay. Immunoassay, or serological assay, can be a reliable means of identifying 

ingested tissues when tissue cropping, mastication, and digestion have rendered stomach 

contents otherwise unrecognizable. Application of this technique to the identification of preypredator links in the marine benthos has been extensively developed by Dr. Robert Feller of the 

University of South Carolina (Feller et al. 1979). The technique is based on the fact that immune 

systems create antibodies to foreign proteins (antigens). This process is exploited as an analytical 

tool by taking possible prey tissue and injecting it into lab rabbits to trigger antibiotic creation. 

These antibodies are then extracted and purified. If successful, the resulting antibody will be 

highly specific to the prey species and the antigen-antibody reaction used as an assay when 

unknown tissues are encountered. In the current task, immunoassay was attempted only on a 

trial, or proof of method, basis. The main limitation in this approach is that the antibodies 

produced may lack the required species specificity. When this is the case, the antibody may give 

false positive reactions to potential prey tissue (Feller and Gallagher 1982). While it is hoped that 

mussels, tubeworms, and bacteria are sufficiently unrelated and that highly specific antibodies 

will be produced, this must first be confirmed. 

8.6.5 Results 

General sampling results. The collection methods described above were tested and refined during 

the 1997 cruise. Most importantly, it was shown that traps could be deployed and recovered in 

over 500 m of water from the surface ship with minimal gear requirement. Three traps were 

deployed by the submersible in separate seep sites – GC185, GC233, and GC234. Off-site 

deployments made use of a hydraulic pot hauler. The use of a commercial pot-hauling line 

proved to be a simple operation requiring approximately 1 hour from start to finish to deploy or 

recover. Operations of such short duration did not conflict with inter-site steaming or divesupport use of the ship. 

In the following year, many short-duration deployments around GC185, GC233, and GC234 

were planned in order to sample as much of the spatial variation as possible. Unfortunately, the 

highly effective line hauler used in 1997 was not available, and hauling was done on a berthingline capstan, requiring 2.5 hours to haul. Including positioning, the time needed from start to 

finish in a retrieval was about four hours. This proved far too long to allow for the planned 

deployments. As a result only seven deployments were made with longer soak times ranging 

from two to seven days (Table 8.6.1). 
8-76 

Table 8.6.1. Deployments and recovery of large surface-deployed traps. 

Date Latitude Longitude Station Site Action 

7/10/97 27°47.19 91°31.05 BHR1 GC185 Deployed 

7/11/97 27°47.19 91°31.05 BHR1 GC185 Recovered 

7/11/97 27°44.35 91°16.74 BPR2 GC233 Deployed 

7/13/97 27°44.35 91°17.74 BPR2 GC233 Recovered 

7/13/97 27°44.8 91°14.6 GCR1 GC234 Deployed 

7/16/97 27°44.8 91°14.6 GCR1 GC234 Recovered 

7/3/98 27°47.20 91°29.000 BHTP1 GC185 Deployed 

7/5/98 27°47.20 91°29.000 BHTP1 GC185 Recovered 

7/5/98 27°47.17 91°30.032 BHTP2 GC185 Deployed 

7/12/98 27°47.173 91°30.032 BHTP2 GC185 Recovered 

7/12/98 27°47.173 91°30.032 BHTP3 GC185 Deployed 

7/16/98 27°47.173 91°30.032 BHTP3 GC185 Recovered 

7/6/98 27°45.900 91°11.60 CGTP1 GC234 Deployed 

7/10/98 27°45.900 91°11.60 GCTP1 GC234 Recovered 

7/10/98 27°46.330 91°13.480 GCTP2 GC234 Deployed 

7/15/98 27°46.330 91°13.480 GCTP2 GC234 Recovered 

7/9/98 27°43.430 91°16.624 BPTP1 GC233 Deployed 

7/13/98 27°43.340 91°16.624 BPTP1 GC233 Recovered 

7/13/98 27°43.860 91°16.400 BPTP2 GC233 Deployed 

7/16/98 27°43.869 91°16.400 BPTP2 GC233 Recovered 

An unexpected benefit of the fewer settings with longer soak times may have been larger 

catches. The planned one- and two-day deployments may have produced very few specimens. 

The possibility exists, however, that predation and scavenging by Bathynomus giganteus and 

Eptatretus sp. in the traps may have removed soft tissue animals when traps were deployed for 

several days. The very low catch of fish other than hagfish may be due to this cause. 

On-site small trap deployments proved feasible but complicated by shifting dive sites and 

equipment priorities. As a matter of policy, traps were not deployed unless it was highly likely 

that the site would be revisited and the trap recovered. Traps were deployed at GC233, GC185, 

and GC234 (Table 6.6.2). However, only the GC233 trap with nine specimens was recovered 

while the task participants were on board. Workers retrieved the other traps as a matter of course 

near the end of the dive, but faunal samples were not logged. 
8-77 

Table 8.6.2. Small within-site traps deployed and 

recovered by submersible. 

Dive Site Date Action 

2851 GC233 7/9/97 Deploy 

2854 GC233 7/11/97 Recover 

2857 GC185 7/12/97 Deploy 

2860 GC233 7/13/97 Deploy 

2878 GC233 7/24/97 Recover 

2862 GC234 7/14/97 Deploy 

2881 GC234 7/25/97 Recover 

4025 GC185 7/4/98 Deploy 

4027 GC185 7/5/98 Recover 

4029 GC234 7/6/98 Deploy 

4032 GC234 7/7/98 Recover 

4043 GC234 7/12/98 Deploy 

4052 GC234 7/16/98 Recover 

4039 GC233 7/10/98 Deploy 

4054 GC233 7/17/98 Recover 

General isotope results. An overall summary of isotope results is presented species-by-species 

in Table 8.6.3. The following sections present this same information on an isotope-by-isotope 

and a site-by-site basis. 

General isotope results–carbon. 13C determinations were made for 86 producer organisms in 

six taxa, 155 resident consumers in 23 taxa, and 76 vagrant species in 15 taxa. Figure 8.6.1 

illustrates the single-isotope results. The results replicate those obtained in previous studies. The 

13C values in the chemosynthetic producers range from close to that of background deep-sea 

organisms to very distinctive values. This range mimics two-food source mixing and severely 

limits the utility of 13C alone in trophic studies of these systems. Bathymodiolus childressi from 

multiple sites show a broad range of values (minimum = -68.96‰, average = -46.23‰, 

maximum = -26.19‰) consistent with the site-varying sources of methane. The much rarer 

thiotrophic mussel Tamu fisheri was collected at a single site and had a narrower range and less 

depleted values (minimum = -38.68‰, average = -37.04‰, maximum = -35.75‰). The 

thiotrophic tubeworms Lamellibrachia sp. and Escarpia sp. showed even less depletion 

(Lamellibrachia, minimum = -24.76‰, average = -21.32‰, maximum = -18.18‰; Escarpia, 

minimum = -35.59‰, average = -25.73‰, maximum = -21.71‰). 
8-78

Table 8.6.3. Summary table for stable isotope analyses from program study sites. 

 13C  15N  32S 

Faunal Element Site n min. average max. n min. average max. n min. aveChemosynthetic-host fauna Bathymodiolus childressi GB425 3 -59.0 -53.6 -43.1 3 -9.2 -2.0 4.3 1 4.3 Bathymodiolus childressi GC185 26 -44.5 -38.6 -26.2 26 0.3 3.9 8.3 17 -24.2 Bathymodiolus childressi GC233 11 -69.0 -63.6 -60.8 11 -20.9 -16.6 -14.0 7 5.8 Bathymodiolus childressi GC234 6 -45.5 -43.7 -41.2 6 1.8 3.1 4.1 1 6.7 Bathymodiolus childressi Escarpia sp. GC185 5 -22.5 -22.1 -21.7 5 2.5 3.2 3.8 3 -35.1 -Escarpia sp. GC234 3 -35.6 -31.8 -29.7 3 2.5 3.6 5.4 Lamellabrachia sp. GC185 15 -23.2 -20.9 -18.2 15 -0.5 1.5 2.7 9 -35.1 -Lamellabrachia sp. GC234 2 -24.8 -24.5 -24.2 2 3.2 3.5 3.7 Lamellabrachia sp. Tamu fisheri GC185 14 -40.3 -37.3 -35.8 14 -1.6 2.0 6.6 10 -29.0 -

Tamu fisheri 

Lucinid clam GC185 1 -31.6 -31.6 -31.6 1 1.9 1.9 1.9 1 -20.8 -Seep-dwelling heterotrophic fauna 

 Mobile taxa Bathynerita naticoidea GB425 1 -46.9 -46.9 -46.9 1 -3.1 -3.1 -3.1 1 6.4 Bathynerita naticoidea GC185 8 -31.5 -28.0 -20.3 8 5.8 7.8 10.7 6 -7.8 Bathynerita naticoidea GC233 2 -52.4 -51.9 -51.4 2 -6.5 -6.0 -5.6 5 6.6 Provanna sculpta GC185 3 -32.3 -31.4 -30.6 3 3.4 4.0 4.5 1 -9.9 Cataegis meroglypta GC185 1 -28.5 -28.5 -28.5 1 4.1 4.1 4.1 Munidopsis sp. type1 GC185 34 -41.0 -31.5 -23.3 34 0.1 6.1 8.9 5 -13.0 Munidopsis sp. type1 GC233 2 -52.3 -50.2 -48.2 2 -10.7 -9.6 -8.4 1 17.3 Munidopsis sp. type1 GC234 3 -36.1 -33.1 -30.2 3 5.8 6.3 7.0 Munidopsis sp. type2 GC185 2 -28.9 -28.7 -28.6 2 9.2 9.3 9.3 Munidopsis sp. type2 GC234 2 -27.4 -27.1 -26.7 2 6.7 8.2 9.6 1 6.1 Alvinocaris sp. GC185 1 -20.6 -20.6 -20.6 1 5.4 5.4 5.4 1 -0.4 Alvinocaris stactophilia GC185 14 -38.5 -28.6 -21.3 14 2.2 5.9 9.1 5 -6.4 Alvinocaris stactophilia GC233 8 -51.3 -43.7 -36.5 8 -9.2 -7.7 -4.4 4 16.1 Alvinocaris stactophilia GC234 3 -30.0 -29.1 -28.5 3 6.8 8.3 10.1 Buccina canatae GC233 3 -53.7 -52.6 -50.5 3 -11.5 -10.6 -9.8 1 7.4 Buccina canatae GC234 1 -3.5 
8-79

Table 8.6.3. Continued. 

 13C  15N  32S 

Faunal Element Site n min. average max. n min. average max. n min. aveBuccina canatae GC185 2 -35.6 -29.2 -22.8 2 5.0 6.2 7.3 4 -9.6 ScotchBonnet-like gastropod GC185 6 -23.4 -14.8 22.4 6 3.2 4.0 4.8 4 -4.3 Sclerasterias tanneri GB425 0 1 10.7 Sclerasterias tanneri GC185 5 -36.3 -34.4 -32.5 5 0.7 4.6 5.6 1 4.2 orbinid worm GC233 4 -62.6 -61.4 -60.0 4 -10.4 -9.4 -9.0 9 12.6 orbinid worm GC234 1 -41.0 -41.0 -41.0 1 4.2 4.2 4.2 sipunculid GC185 14 -41.1 -30.0 -27.3 14 -5.9 4.5 8.9 9 -12.3 sipunculid GC234 3 -31.1 -29.9 -28.2 3 7.0 7.4 7.8 1 1.5 Atelecylidae crab GC185 1 -24.8 -24.8 -24.8 1 9.7 9.7 9.7 Nephropsis sp. GC185 1 -35.2 -35.2 -35.2 1 3.1 3.1 3.1 White shrimp GC185 1 -31.0 -31.0 -31.0 1 3.0 3.0 3.0 Axiidae GC185 1 -33.2 -33.2 -33.2 1 2.6 2.6 2.6 Nereiid polychaete GC185 2 -31.2 -30.6 -29.9 1 5.4 5.4 5.4 2 -9.8 

Nicomache sp. GC185 1 -28.5 -28.5 -28.5 1 7.5 7.5 7.5 1 -3.4 polychaete unidentified GC185 5 -36.4 -29.1 -23.7 5 3.4 4.1 4.9 polychaete unidentified GC234 2 -30.9 -30.4 -29.8 2 8.9 9.7 10.6 

 Harmothoe sp. GC185 2 -23.7 -23.6 -23.5 2 4.3 5.7 7.0 2 -12.6 - Harmothoe sp. GC233 2 15.6 Eurythoe sp. GC185 1 -40.6 -40.6 -40.6 1 8.1 8.1 8.1 3 -12.0 Nemertean GC185 5 -29.0 -25.5 -22.2 5 1.1 4.4 7.5 2 -9.6 Nemertean GC234 2 -40.1 -40.0 -39.9 2 5.1 5.4 5.8 Platyhelmenthes GC233 1 15.9 Pycnogonia GC185 1 -26.1 -26.1 -26.1 1 6.7 6.7 6.7 Ophiuroid GC185 1 21.9 Bellottia sp. GC234 2 -27.4 -27.3 -27.1 2 9.6 9.9 10.1 Bellotia sp. gut content GC185 1 -21.7 -21.7 -21.7 1 8.7 8.7 8.7 Unidentified eel GC185 1 -30.4 -30.4 -30.4 1 7.4 7.4 7.4 Diodora sp. GC185 2 -31.8 -30.2 -28.6 2 9.2 9.6 10.0 1 4.9  Sessile taxa sponge GC185 4 -34.2 -31.4 -25.3 4 3.4 6.7 8.7 1 3.6 Hydroid GC185 5 -32.6 -28.4 -26.2 5 1.9 4.7 7.3 2 -18.5 Hydroid GC234 1 -24.5 -24.5 -24.5 1 8.5 8.5 8.5 0 0.0 Scalpellid barnacle GC185 1 -19.5 -19.5 -19.5 1 9.9 9.9 9.9 1 14.3 
8-80

Table 8.6.3. Continued. 

 13C  15N  32S 

Faunal Element Site n min. average max. n min. average max. n min. aveScalpellid barnacle GC234 1 -17.7 -17.7 -17.7 1 12.7 12.7 12.7 1 20.6 Verrucamorph barnacle GC234 1 18.8 Acesta bullisi GC233 1 -5.6  Vagrant taxa Synaphobranchus sp. GC185 1 -33.4 -33.4 -33.4 1 7.1 7.1 7.1 1 -13.3 -Synaphobranchus sp. GC233 1 -42.5 -42.5 -42.5 1 -3.0 -3.0 -3.0 Synaphobranchus sp. GC234 1 -24.9 -24.9 -24.9 1 8.5 8.5 8.5 Nezumia GC233 1 -32.7 -32.7 -32.7 1 2.2 2.2 2.2 Urophycis cf. cirratus GC234 1 12.3 12.3 12.3 1 -17.5 -17.5 -17.5 1 20.0 Bathynomus giganteus GC185 7 -18.4 -16.0 -14.2 7 12.3 14.0 15.8 9 15.9 Bathynomus giganteus GC233 5 -16.7 -15.9 -14.4 5 11.4 14.6 16.3 7 9.3 Bathynomus giganteus GC234 3 -15.9 -15.1 -14.0 3 13.4 14.1 14.6 7 18.3  gut contents GC233 3 -30.6 -21.8 -15.6 3 3.7 8.2 13.4 1 20.5 

 gut contents GC234 4 16.3 Eptatretus sp. GC185 5 -28.3 -23.3 -19.6 5 7.8 9.9 11.5 29 -8.8 Eptatretus sp. GC233 7 -28.6 -21.1 -17.7 7 8.2 10.4 13.3 1 18.4 

Eptatretus sp. GC234 7 -18.3 -17.0 -15.8 7 7.3 10.4 12.2 12 12.1  gut contents GC233 2 -62.8 -42.1 -21.4 2 -15.5 -2.6 10.4 1 19.1  gut contents GC234 6 12.7 Rochinia crassa GC185 14 -22.9 -18.6 -16.4 14 9.2 10.3 13.2 12 -12.0 Rochinia crassa GC233 1 -26.2 -26.2 -26.2 1 5.1 5.1 5.1 1 12.8 Rochinia crassa GC234 3 -18.8 -18.0 -17.3 3 10.1 10.5 11.0 3 20.1  gut contents GC185 5 -20.4 -19.1 -17.8 5 6.5 9.5 11.1 3 12.7  Background taxa meso-pelagic fish GC233 3 -18.9 -18.5 -18.3 3 6.5 7.8 9.5 Shark GC233 6 -19.9 -18.3 -17.4 6 6.7 9.7 12.9 4 17.5 Chaceon sp. GC185 1 19.7  Miscellaneous Sediment GC185 2 -26.8 -26.6 -26.5 2 11.7 13.0 14.4 1 1.3 Sediment GC234 2 -35.9 -31.1 -26.3 2 3.1 5.5 7.9 
8-81 

Figure 8.6.1. Univariate histograms of 13C determinations for faunal groups. 

All 39 taxa of seep-resident consumers showed depleted carbon values relative to typical deep Gulf 

fauna. The most depleted values were found in an orbinid worm from GC233 (-62.55‰) and the least 

depleted from a nerite snail at GC185 (-20.30‰). Such a range could be explained by consumption of 

chemosynthetic production or a mixed diet incorporating typical deep-sea phytodetritus material. In 

effect, 13C alone is ambiguous. There is less ambiguity in the case of vagrant consumers. The eight 

taxa considered vagrants showed dramatically different results. All except the eel, Synaphobranchus

sp., showed values on average indistinguishable from background deep-sea fauna, in spite of close 

association with seeps. Scattered individuals of the hagfish Eptatretus and the crab Rochinia crassa

had depleted values consistent with seep feeding. 

General isotope results–nitrogen. 15N determinations were paired with 13C, and the same number 

and distribution of values were obtained. Figure 8.6.2 illustrates the single-isotope results. The results 

largely mirror those for 13C with some informative differences. 
8-82 

15

Figure 8.6.2. Univariate histograms of 15N determinations for faunal groups. 

Bathymodiolus childressi shows a large range (minimum = -20.92, average = -1.51, maximum = 8.30) 

directly caused by the distinctly low values at GC233. Rather than having a progression of values as in 

the case of 13C, all three thiotrophs showed a consistent range of values centered at about 2.0‰. 

Resident consumers appeared to be bimodal with a mode of -11‰ and less depleted mode at about 

+6‰. As with carbon, the lower values coincided with GC233 (19 out of the 24 samples in the lower 

mode were from GC233). Vagrant consumers showed additional shifts to less depleted values and had 

a modal value (+10‰) consistent with typical deep Gulf benthos. Again, scattered individuals of 

Eptatretus and Rochinia crassa had values indicative of seep feeding. A shift to less depleted values is 

expected in consumers due to trophic effects or could be due to a diet incorporating tissue from the 

background. This possibility of confounding trophic shifts with mixed food sources reduces the utility 

of 15N. 

General isotope results–sulfur. 34S determinations were made as a developmental effort. For 143 

specimens, 34S was determined in conjunction with 13C and 15N. An additional 91 determinations of 

34S alone were made. Figure 8.6.3 illustrates the single-isotope results. 34S values proved to be 
8-83 

extremely informative and less affected by the ambiguities associated with 15N and 13C. 

Chemosynthetic producers showed a species-specific shift with thiotrophs being more depleted than 

methanotrophic mussels. This is directly opposite the case for 13C and 15N. Resident consumers had 

values more easily interpreted, coming from a diet of mussel tissue rather than tubeworm. Again, 

vagrant consumers formed a much narrower group with most values reflecting the deep Gulf 

background food web. 

Figure 8.6.3. Univariate histograms of 34S determinations for faunal groups. 

General isotope results–carbon, nitrogen and sulfur. When bivariate isotope data, 13C and 15N, are 

collected from a wide variety of organisms in an ecosystem, two non-exclusive relationships can be 

expected. The first is produced by a system with isotopically homogenous food sources in which 

isotope partitioning produces a shift at each trophic level. This shift is usually in the form of a line 

passing through the food source and extending along a slope of (+3‰ N)/(+1‰C). Position along this 

trophic-shift line can be used to assess the number of levels in a given system. The second expected 

pattern in 13C and 15N data is produced by a system in which the food sources are isotopically 

heterogeneous. Consumers with a mixed diet will fall on lines between the sources describing mixing. 
8-84 

Mixed results were obtained with respect to a definitive determination of whether Gulf hydrocarbon 

seep systems act as single food source systems with trophic shifts or complex food-source systems in 

which mixing obscures determination of trophic levels (Figure 8.6.4). In the case of the dominant 

mussel, the methanotroph Bathymodiolus childressi, carbon again proved to be a definite marker. 

Nitrogen, however, proved unexpectedly variable in this chemosynthetic producer. The great range of 

values (-20.92‰ to 8.3‰) can completely mask the trophic shifts that might be produced by an 

improbable ten trophic levels. Within a given site the variance in nitrogen is less, but the range seen at 

GC233 underscores the difficulty of interpreting trophic structure. When carbon alone was used in 

previous studies, tubeworm values overlapped typical deep-sea phytodetritus values making it 

impossible to discern the food source of associated fauna. It was hoped that the addition of nitrogen 

would provide better resolution. Unfortunately, this objective was only partially met. Nitrogen values 

associated with tubeworms were distinct from those associated with the background fauna. The 

combined carbon and nitrogen values for tubeworms did, however, fall close to background values and 

directly upon an ambiguous line that might equally as well describe trophic shifting by tubeworm 

consumers or mixing of food sources. 

Multiple isotope considerations. Ideally, determination of three or more isotopes will lead to a 

multivariate model of food webs. Such a model cannot be developed from the current study because 

the dynamics of nitrogen and sulfur fractionation remain poorly understood, and the sampling was 

unbalanced with respect to the number of samples per species. Nevertheless, the considerable potential 

for such a model is evident from examination of inter-isotope correlation (Table 8.6.4). Correlation or 

covariance matrices are the starting point for a wide range of multivariate analyses such as principal 

components and factor analysis. 

Table 8.6.4. Product-moment correlation of isotope 

pairs as an indication of utility in any future multivariate model. 

Correlation 

Pair 

All Data 

n = 144 

Producers 

n =42 

Resident 

Consumers n = 50 

Vagrant Consumers 

n = 52 

13C vs 34S + 0.23 - 0.84 - 0.54 + 0.78 

13C vs 15N +0.77 + 0.50 + 0.78 + 0.59 

15N vs 34S +0.39 - 0.33 - 0.46 + 0.34 

For 34S and 13C, the results were unexpected. The strong negative correlation in producers must 

reflect unknown linked geochemical processes in the sources or linked metabolic chemistry in the 

producers. The weaker negative correlation in resident consumers reflects retention of this seep foodsource signal with some effect of mixing. The strong correlation of reversed sign in the vagrant 

consumers indicates that food-source mixing has completely obscured the relationship in the original 

seep source. For 13C versus 15N, the correlations are as expected. There is limited positive linkage in 

the producer population that is increased in the resident consumers due to an unresolvable combination 

of trophic shifts and food-source mixing. This positive correlation remains in vagrant consumers due to 

trophic effects but declines as mixing reaches its endpoint, and the signal of the seep source becomes 
8-85 

obscured. For 15N versus 34S, the correlation is low and less informative than the other two pairings. 

The sign of the correlation indicates that variation in sulfur is the dominant component. 

The most informative combination of isotopes for food web tracing proved to be 34S and 13C. The 

utility of this pair has three causes. First, unlike 15N these are minimally influenced by trophic shift. 

Second, there is a linear or nearly linear relationship between the two isotopes in chemosynthetic 

organisms. Third, the slope of the linear relationship is such as to minimize ambiguity. The 34S shift 

going from methanotroph-thiotroph-background is opposite in direction from the 13C shift. As a 

result, in specimens where 13C cannot distinguish food sources, 34S can, and visa versa. 

Trapped fauna results. Trapping on- and off-site was a qualified success in spite of the logistical 

inability to carry out the number of deployments planned. The principal organisms collected were a 

spider-like crab Rochinia crassa, a hagfish Eptatretus sp., and the giant isopod Bathynomus giganteus. 

The results on a species basis are presented in Table 8.6.1. On a per-site basis, the data were plotted as 

vagrant species in Figures 8.6.5 through 8.6.6. 

At GC233, a number of different predator species were trapped on-site. In addition to the three most 

common vagrants, Bathynomus giganteus (seven specimens), Eptatretus (four specimens), and 

Rochinia (three specimens), the rattail fish Nezumia sp. (one specimen) was captured. Direct collection 

by the submersible produced two additional vagrants, the eel Synaphobranchus sp. and a small 

demersal shark, as well as an additional specimen of Eptratretus. Eight specimens have markedly 

depleted 13C values indicating heavy usage of the chemoautotrophic production, while the remaining 

15 reflected greatest reliance on photosynthetic food sources. 

The off-site trap, deployed approximately two km from GC233, caught eleven Bathynomus and one 

Rochinia. The Rochinia was distinct, being depleted in 13C relative to typical ocean predators and to the 

same species collected off-site at GC234. Rochinia captured on-site at GC233 were more depleted than 

those captured off-site at GC233. These values, as well as depleted34S, were indicative of consumerutilizing chemosynthetically fixed sulfur. Based on the 34S values, the Rochinia had derived 

approximately 48% of its sulfur from methanotrophic material. There was no statistical difference in 

13C or 15N between the Bathynomus caught on- and off-site at GC233; however, those caught off-site 

were significantly more enriched in 34S (p = .03) relative to those on-site, indicating less reliance on 

seep food sources. 

At GC234, two species of vagrant predators were captured in the small on-site trap– Bathynomus 

giganteus (two specimens) and Eptatretus sp. (11 specimens)–along with a single specimen of the 

predatory gastropod Buccinum canetae, which is likely a seep colonist rather than a vagrant (Figure 

8.6.2). The on-site vagrants yielded values more reflective of heavy reliance on photosynthetically 

derived foods than the surrounding chemoautotrophic production. The relatively large sample size for 

Eptatretus did, however, reveal a pattern for 13C and 34S consistent with limited consumption of 

chemoautotrophic tissue. The range of values for captured Eptatretus 13C, 15N, and 34S was -18.3 to 

-16.3‰, 7.3 to 12.2‰, 12.1 to 18.5‰, respectively, with a trend toward depletion in 13C and 34S. 

Several of the Eptatretus have 34S values that were quite depleted relative to ocean sulfate (20‰) or 

animals utilizing ocean sulfate, 19 to 20‰ (Peterson and Howarth, 1987). A simple regression of 13C 

with 34S was significant (p = 0.02). The two Bathynomus giganteus were significantly more enriched 

in 13C and 15N (p = .05 and .03, respectively) than the Eptatretus and were isotopically similar to 

“typical” ocean fauna. 
8-86 

Three species of vagrant predators were caught off-site in Z-frame traps approximately 2 km from a 

known seep community at GC234. In addition to Rochinia (three specimens) and Bathynomus (two 

specimens), one hake (Urophycis) was captured. All individuals had isotope values similar to those 

seen in other pelagic Gulf of Mexico predators (Roelke and Cifuentes 1997). Due to the small sample 

size and mismatch of species captured, statistical comparison within the off-site specimens had limited 

power. Bathynomus on-site was the more enriched in 13C and 15N than off-site, although not 

significantly so (p= .08). Off-site, there was no statistical difference between the 34S of Bathynomus

and Rochinia. As a group, the predators collected off-site in the Z-frame trap were significantly more 

enriched in 34S compared to those caught in the small on-site trap (p = .0002). It is unlikely that these 

off-site vagrants consumed any appreciable amount of chemosynthetically derived food. 

Samples at GC185 were limited to numerous off-site specimens of Eptatretus, Rochinia, and 

Bathynomus collected at 27°47.173' N, 91°30.032' W. The relatively large sample sizes for the three 

species allowed a more thorough analysis of the data. For Eptatretus, the mean off-site 13C at GC185 

was equal to that on-site at GC233 consistent with incorporation of chemosynthetic organic matter. The 

same extent of incorporation at both sites, however, cannot be assumed due to the site-specific 

differences in the isotopic signatures of the food sources. The chemosynthetic fauna at GC185 are not 

as depleted in 13C (or 15N) as the GC233 fauna for two reasons. First, the GC233 population of 

Bathymodiolus childressi utilizes biogenic methane (13C < 55‰), while the Bathymodiolus childressi

population at GC185 utilizes thermogenic methane and has more 13C enriched values (around -40‰). 

Second, there is a substantial community of Lamellibrachia and Escarpia tubeworms at GC185 that 

harbor thiotrophic bacteria and have 13C values of approximately 21‰. Additionally, the thiotrophic

bivalves Tamu fisheri and lucinid clams are present at GC185 and have much more depleted 13C 

values than the tubeworms. 

Rochinia were also depleted in 13C and 34S (to a greater extent) relative to background ocean isotopic 

values. Their 13C and 15N values overlap the range observed for the Eptatretus (Figure 8.6.4), but 

their 34S values are significantly depleted relative to Eptatretus (p = .002). The relatively low 34S 

(substantially lower than that of marine plankton) combined with 13C values that seldom fall below -

22‰ suggests that tubeworm material or organisms that feed on tubeworms contribute to the diet of the 

Rochinia. The fact that the Rochinia are approximately 8‰ higher in 15N than tubeworms strongly 

suggests that they are not directly grazing upon tubeworms but are consuming tubeworm predators. 
8-87 

Bathymodiolus childressi

Lamellibrachia sp.

Figure 8.6.4. Apparent utility of 13C and 34S for elimination of food source ambiguity in seep and background 

systems. 

Site-specific community results. 

GC234 and GC233. Principal sampling at GC233 consisted of mussel ring collections, trapping onand off-site, and supplemental discretionary sampling with the submersible manipulator. Tissue from 

284 specimens was frozen in 1997 and 105 additional were frozen in 1998. Isotopic determinations 

were made on 81 samples. Of these, 55 had both carbon and nitrogen determined, and 19 had carbon 

and sulfur determined (Figure 8.6.5). Within the immediate area of GC233, Bathymodiolus childressi is 

the sole producer with a very distinctive isotope signature (13C = -63.59 ±16.58, 15N = -16.58 ±1.87, 

n = 11). The associated fauna is comparatively simple with only five taxa analyzed. These consumers 

showed the distinctive depletion of the site (13C = -50.35 ±7.76, 15N = 8.55 ±1.79, n = 19) but were 

clearly shifted towards background values. Seven species of background organisms were analyzed 

(13C = -20.66 ±6.25, 15N = 9.64 ±4.42, n = 24), posing a definite contrast to the producers. 

Figure 8.6.5 illustrates that the data are an excellent example of the processes discernable from isotopic 

testing. The scatter of points suggests that both in the seep and in the background there is trophic 

enrichment of nitrogen producing the expected "T" line. Far more of the data variance, however, is 
8-88 

associated with the mixing "Mix" line caused by feeding on two endpoints. Examination of the 13C 

and 34S data produces an unexpected result in spite of the limited number of analyses. The consumers 

tend to be more depleted in sulfur than the Bathymodiolus childressi. This suggests multiple food 

sources at GC233. 

GC185. Contrasted to the simplicity of the trophically open GC233, GC185 is a more complex system 

(Figure 8.6.6). Principal sampling at GC185 consisted of bushmaster, mussel ring collections, trapping 

on- and off-site, and supplemental discretionary sampling with the submersible manipulator. Tissues 

from 230 specimens were frozen in 1997, and 98 additional specimens were frozen in 1998. Isotopic 

determinations were made on 261 samples. Thirty-eight taxa were collected with substantial producer 

populations represented by mixed populations of thiotrophs, Lamellibrachia, Escarpia, and Tamu 

fisheri, as well as by the methanotroph, Bathymodiolus childressi. Resident consumers comprised 

twenty taxa, and vagrant consumers four taxa. Tubeworm tissue, as exemplified by Lamellibrachia, 

had 13C values only slightly more depleted than background fauna but distinctively lower 15N values 

(13C = -20.90 ±1.3, 15N = 1.50 ±0.87, n = 15). The associated fauna from these worm clumps tended 

to be more depleted with respect to carbon and sometimes with respect to nitrogen, suggesting that the 

tubeworm tissue per se was not a food source. Rather, resident consumers at GC185 tended to fall 

within a lose cluster framed by tubeworms, Bathymodiolus childressi (13C = -38.7 ±4.38, 15N = 3.78 

±2.48, n = 27) and background consumers ((13C = -19.36 ± 3.95, 15N = 10.8 ± 1.15, n = 32). This 

pattern is not especially informative and might be interpreted as reflective of many possible 

combinations of trophic effects and mixing of the three isotopic end-members. 

 Despite the complexity contributed by the apparent trophic enrichment of nitrogen, 13C and 34S 

determinations added some clarity. Most resident consumers had values consistent with incorporation 

of mussel production, either Bathymodiolus childressi or Tamu fisheri, with background food sources. 

There was little indication of direct consumption of tubeworms alone or mixed with mussel or 

background. 
8-89 

Figure 8.6.5. GC233 13C versus 15N and 13C versus 34S scatter plots for main faunal groups showing strong 

influence of mixing and minor influence of trophic shifts. 
8-90 

Bathymodiolus childressi

Bathymodiolus childressi

15

Figure 8.6.6. GC185 13C versus 15N and 13C versus 34S scatter plots for main faunal groups showing greater 

indication of mixing from 34S. 

GC234. Principal sampling at GC234 consisted of bushmaster, mussel ring collections, trapping on- 
8-91 

and off-site, and supplemental discretionary sampling with the submersible manipulator. Tissue from 

55 specimens was frozen in 1997 and 233 additional were frozen in 1998. Isotopic determinations were 

made on 58 samples (Figure 8.6.7). This site is similar to GC185 in that production is multi source with 

three species having been sampled: Lamellibrachia, Escarpia and Bathymodiolus childressi. Resident 

consumers comprised only five taxa due to limited sampling, and vagrant consumers five taxa. 

Tubeworm tissue, Lamellibrachia and Escarpia, combined had 13C values distinctly more depleted 

than background fauna and distinctively lower 15N values (13C = -28.89 ±4.64, 15N = 3.11 ± 0.88, n 

= 5). The second potential food source, Bathymodiolus childressi (13C = -43.66 ± 1.56, 15N = 3.11 ± 

0.88, n = 6) formed a distinct cluster and background consumers (13C = -17.29 ± 2.37, 15N = 11.32 ± 

2.00, n = 16) a third. The associated fauna (13C = -31.11 ± 4.81, 15N = 7.62 ± 1.85, n = 19) produced 

an ambiguous pattern consistent with both mixing of mussel and background sources, or exclusive use 

of tubeworms followed by trophic shifts. Analysis of sulfur was too limited to improve upon the 

interpretation. 

Immunoassay results. A modest trial program was initiated to determine the utility of immunoassay to 

confirm sub-lethal predation (cropping) by seep heterotrophic consumers. This effort did not conform 

to the overall sampling design but assumed that given species had the same trophic role at all sites. 

Intended prey species included Beggiatoa, Escarpia, and Bathymodiolus. Intended predators included 

ten each of the dominant seep associates. The complete results of the immunoassay component are 

presented in the appendix volume. The material presented here is intended as a summary (Table 8.6.5). 

The effort was generally successful with only minor departures from the original proposal. Antisera 

were successfully developed and tests for specificity were run against a variety of marine invertebrates. 

Antisera for tubeworms were usable but showed lower specificity. Analyses were run on stomach 

contents for 54 specimens. With respect to prey, antisera for Beggiatoa were not made, while 13 taxa 

were analyzed rather than four with a total of 54 specimens being suitable for assay. 

All taxa assayed had indications of having eaten Escarpia and/or Bathymodiolus. There appears to be 

no feeding specialization in the taxa examined. On a specimen level, 26 animals had neither in the gut 

(using a low sensitivity test) indicating an alternate diet. Most of these 26 were hagfish or crustaceans. 

Buccina contributed the majority of 13 specimens that showed evidence of both Escarpia and 

Bathymodiolus in the stomach contents. 
8-92 

Bathymodiolus childressi

Lamellibrachia sp.

Bathymodiolus childressi

Lamellibrachia sp.

15

Figure 8.6.7. GC234 13C versus 15N and 13C versus 34S scatter plots for main faunal groups showing indication of 

mixing but with insufficient 34S determinations for more definitive statements. 
8-93 

Table 8.6.5. Summary of immunoassays. 

Tubeworm Feeding Mussel Feeding 

Responses to Test Sensitivity Responses to Test Sensitivity 

Species Assayed n Low High Low High 

Eptatretus sp. 11 2 3 2 9 

Bathynomus 7 0 6 1 7 

Munidopsis I 7 3 4 3 3 

Buccina 5 4 3 5 5 

Alvinocaris 5 2 2 4 3 

Atelecylida 4 1 2 1 2 

Bathyneritia 4 0 1 4 3 

Synaphobranchus 2 1 0 2 1 

Munidopsis II 3 0 3 0 2 

Nereid worms 2 1 0 2 0 

Rochinia 1 1 1 1 1 

Chaceon 2 0 0 0 1 

8.6.6 Discussion 

Seep residents–evidence of a two-way open system. Since seeps are known to have very high 

biomass and are assumed to have very high productivity, it is easy to assume that they would be 

exploited by the food-limited background species. Indeed, the more relevant question is not whether 

the background exploits seeps, but why so few background species are apparently active in this 

exploitation. As found in this study, this exploitation with transfer into the background is indicated by a 

faint but distinct seep isotope signature in trap-collected scavengers and predators. A far more 

unexpected result was the finding of seep-associated consumers such as the galatheid crabs and nerite 

gastropods with signatures of a diet-mixing seep and background production. At the site, GC233, 

where the determinations were most distinctive, some seep endemics may have consumed as much as 

50% of their assimilated diet from the background. 

The presence of an apparent isotope signature for a mixed seep-background diet in animals thought to 

be intimately associated with seeps has other explanations that cannot be refuted from the data at hand. 

1. Wide foraging–The intimacy of the seep association may be more apparent than real, with 

significant foraging into the background an event so far unobserved during submersible dives. In 

effect, seep consumers may range and forage widely. 

2. Bacterial mimics–Seep detritus may contain bacteria that produce a biomass that exactly mimics 

the mixed diet signal. 

3. Mixed detrital pool–Seep detritus may be the location of the diet mixing containing both seep 

production and organic detritus from the background. 
8-94 

Conflicting evidence of direct predation. The full immunoassay component is presented in the 

appendix volume. The discussion here is limited to apparent conflicts concerning the extent to which 

tubeworms serve as prey to resident and vagrant consumers. Isotopic tracing indicates that tubewormassociated fauna can have an apparent thiotrophic 13C and 34S signature but is not consistent with 

consumption of tubeworm tissue per se. Immunoassay indicates the contrary, with gut contents 

commonly testing positive for tubeworm tissue. The difference in results cannot be resolved from the 

data on hand but could be due to three factors. 

1. It simply reflects the actual feeding history of the individual organisms sampled. Some had recently 

eaten tubeworms while others had not. The sample size was too small to establish the general 

pattern. 

2. It reflects a difference between ingestion and assimilation with the worm tissue in the gut not being 

assimilated. 

3. The test for tubeworm tissue in the gut gave false positives due to cross-reactions with polychaete 

tissue noted in the cross-reaction study. 

8.6.7 Conclusion – scientific questions and regulatory consequences 

Seep systems interact with the background, posing two questions. 

Scientific. What role does predation by background species play in determining the establishment, 

species composition, persistence, and ultimate stability of seep systems? 

1. Disruption background predators exert a predatory pressure on (a) the browsing, (b) resident 

predator population, and (c) substrate producing organisms. Similar predation is generally accepted 

as an important factor in biodiversity preservation in other systems and may play a similar role in 

the seep systems. Alteration of this predation due to environmental impacts in the background may 

have a detrimental effect upon seep systems as a whole. 

2. Disruption of seep predation on background will eliminate a food source that may be critical to 

sustaining the full biodiversity of seep communities during times of reduced chemosynthetic 

production. 

3. Contamination of background detritus that is then transported and mingles with the seep detritus 

pool may result in food-web amplification and toxic effects upon seep fauna. 

Regulatory. What should the appropriate scales and nature of restrictions on development be to 

protect both local seeps and the larger area of the bottom with which they interact? Present regulations 

of offshore oil and gas consider environmental impact within a region one kilometer in diameter a 

likely consequence of development but acceptable when weighed against the vastness of the softbottom benthic environment. Present regulations governing protection of seep communities view seep 

communities as isolated systems threatened only by local physical disruption. 
8-95 

8.7 Mussel and Vestimentiferan Population Ecology*

8.7.1. Objectives 

Molecular genetic methods provide a powerful tool with which to infer the evolutionary history of 

populations. Each of the major evolutionary forces—mutation, recombination, natural selection, and 

migration—leaves a characteristic signature on the genomic DNA carried by an organism. The 

evolutionary history of individuals within and between populations as well as relationships among 

species can be inferred with molecular genetic methods. The chemosynthetic communities that live at 

the cold seeps of the Gulf of Mexico provide a unique opportunity to examine the role that dispersal plays 

in the evolution of the dominant hydrocarbon seep organisms. 

The vestimentiferan tubeworms and mussels, in association with their endosymbiotic bacteria, are the 

main ecosystem structuring organisms at the cold seeps (Fisher et al. 1997). The tubeworms are sessile 

organisms that lack a mouth, gut, and anus, but thrive at the cold seeps through their association with 

endosymbiotic bacteria that live a specialized tissue known as the trophosome. Two aspects of the 

vestimentiferan life history are not clearly understood and can be clarified with molecular genetic 

approaches. First, how are endosymbionts acquired each generation? Male and female tubeworms 

issue their gametes into the water column where fertilization takes place. Whether the endosymbionts 

are transmitted with the egg or acquired later in development is not known. Efforts to detect 

endosymbionts in the egg have failed to find evidence for vertical transmission of the bacteria (Cary et 

al. 1993), yet newly settled juveniles always have endosymbionts. Thus, it appears that the 

endosymbionts are acquired each generation. Second, the distance that larvae can travel to colonize 

new seep habitats has profound implications for the long-term stability of these communities. Limited 

dispersal ability of seep fauna would suggest that the chemosynthetic communities are fragile where 

biodiversity is at risk. Genetic variation is the raw material of the evolutionary process that allows 

populations to change in response to environmental flux. Destruction of seep habitats, in this model, 

causes a profound loss of genetic diversity and may lead to an uncertain future for these fragile 

communities. Extensive dispersal ability of vestimentiferans and mussels would indicate that 

chemosynthetic communities are genetically robust to disturbance. This model of genetic exchange 

suggests that loss of a single community may be overcome through recolonization of seep sites from 

other localities within the Gulf of Mexico. Under this scenario, the main limitation to the survival of 

global chemosynthetic communities is the availability of suitable habitat for new communities to be 

established. 

In this study, appropriate types of DNA variation were used to investigate the following: 

1) the migratory history of three seep species found in chemosynthetic communities, two 

vestimentiferan tubeworms (Escarpia sp. and Lamellibrachia sp.) and one mussel (Bathymodiolus 

childressi); 

2) the phylogenetic history of tubeworms from cold seeps and hydrothermal vents; 

3) the phylogenetic history of tubeworm endosymbionts. These data are used to address the 

robustness of the chemosynthetic communities to disturbance. 

 

*

 This section was authored by Steve Schaeffer. 
8-96 

8.7.2. Materials and laboratory methods 

Mussel and tubeworm samples. Tubeworms and mussels were collected from four hydrocarbon 

seeps during the summer of 1997 (see Tables 8.7.1-8.7.3). For each species, individuals were sampled 

from each of four populations to examine short, intermediate, and long range movement of larvae 

among seep communities. Gene flow among local aggregations of tubeworms and mussels within 

GC185 were used to estimate short distance dispersal where sampled localities were separated by less 

than one kilometer. Gene flow among the three Green Canyon populations were used to estimate 

intermediate distance dispersal where sampled sites were separated by 4 to 25 kilometers. Finally, 

gene flow between GB425 and the Green Canyon populations was used to estimate long distance 

dispersal where sampled localities were separated by over 100 kilometers. 

Table 8.7.1. Samples of Escarpia spicata collected from four localities in the Gulf of Mexico. 

Sampling Site Latitude, Longitude Depth (m) Station ID Individuals Sampled 

GC185 {Bush Hill (BH)} 27°46.9' N, 91°30.4' 

W 

550-580 JT1 10 

 JT2 10 

 JT5 10 

 AT1 10 

 AT5 10 

 TT6 10 

GC233 {Brine Pool (BP)} 525-560 AT1 10 27°43.4' N, 91°16.8' 

W 

GC234 {Green Canyon 

(GC)} 

27°44.1' N, 91°15.3' 640 NM1 10 

W 

 

GB425 {Garden Banks 

(GB)} 

27°33.2' N, 92°32.4' 

W 

600 10 

Table 8.7.2. Samples of Lamellibrachia sp. collected from four localities in the Gulf of Mexico. 

Sampling Site Latitude, Longitude Depth (m) Station ID Individuals Sampled 

GC185 {Bush Hill (BH)} 550-580 JT1 10 27°46.9' N, 91°30.4' 

W JT2 10 

 JT5 10 

 AT1 10 

 AT5 10 

 TT6 10 

GC233 {Brine Pool (BP)} 525-560 AT1 10 27°43.4' N, 91°16.8' 

W 10 

GC234 {Green Canyon 

(GC)} 

27°44.1' N, 91°15.3' 640 NM1 10 

W 

 10 

GB425 {Garden Banks 

(GB)} 

27°33.2' N, 92°32.4' 

W 

600 10 
8-97 

Table 8.7.3. Samples of Bathymodiolus childressi collected from four localities in the Gulf of Mexico. 

Sampling Site Latitude, Longitude Depth (m) Station ID Individuals Sampled 

GC185 {Bush Hill (BH)} 550-580 M1 10 27°46.9' N, 91°30.4' 

W M2 10 

 M3 10 

 M4 10 

 M5 10 

 M6 10 

GC233 {Brine Pool (BP)} 525-560 M1 10 27°43.4' N, 91°16.8' 

W M2 10 

GC234 {Green Canyon 

(GC)} 

27°44.1' N, 91°15.3' 640 M1 10 

W 

 M2 10 

GB425 {Garden Banks 

(GB)} 

27°33.2' N, 92°32.4' 600 M1 10 

W 

 M2 17 

Microsatellite isolation and statistical analysis. The study of dispersal and gene flow in tubeworms 

and mussels utilizes highly variable genetic loci that are likely to be different among individuals. The 

rationale for choosing these loci was that genetic markers that can change each generation are likely to 

give an accurate reflection of how much gene flow occurs on intermediate time scales. For this work, 

microsatellite loci, which are di-, tri-, and tetranucleotide sequences that change rapidly among 

individuals of the same species, were used to estimate levels of gene flow among four seep populations 

(from samples at sites GB425, GC185, GC233, and GC234). 

Microsatellite loci were cloned with standard genetic engineering methods (Armour et al. 1994; 

Sambrook et al. 1989). Genomic DNA was purified with standard methods (Ausubel et al. 1989). The 

genomic DNA was partially digested with the restriction enzyme Sau3A and size separated on a 2% 

agarose gel. Digested DNA were size selected for fragments between 200 and 800 base pairs. These 

fragments were cloned either into the Blue Script (Stratagene) cloning vector directly or were subject 

to a procedure that enriched the sequences for microsatellite repeats and then cloned them into the Blue 

Script vector (Armour et al. 1994). The plasmid vector was moved into the bacterial host Escherichia 

coli via the process of transformation to create a genomic library for the species. The genomic library 

was screened nonisotopically with probes that contained di-, tri-, and tetranucleotide repeats. 

Nucleotide sequences of clones that hybridized strongly to the microsatellite repeat probes were 

determined with polymerase chain reaction (PCR) mediated sequencing (Saiki et al. 1988). Clones that 

contained a valid microsatellite whose position was near the middle of the recombinant DNA insert 

were used to design PCR primers that could be used to test organisms from the sample collections for 

repeat polymorphisms. Microsatellites that were shown to be identical among all individuals tested in 

the tubeworms or mussels were not investigated further. Polymorphic microsatellite loci were used to 

screen all individuals collected from the four populations. 

The frequencies of each microsatellite allele within each locus were estimated with standard population 

genetic methods (Nei 1987). The microsatellite allele frequencies were used to estimate Fst , the 

probability that two alleles from two subpopulations will be identical by descent (Nei 1987). The 

neutral migration parameter, 4Nem, is estimated directly from Fst (Slatkin 1985), where Ne is the 
8-98 

effective population size and m is the migration rate. This value is interpreted as the number of 

migrants exchanged between populations per generation (Wright 1931, 1943). 

Nucleotide sequencing of a new escarpid species. The morphological identification of tubeworms 

from bushmaster collections of entire vestimentiferan communities revealed a new species of Escarpia. 

To clarify the relationship of this new vestimentiferan species, the cytochrome oxidase I (COI) gene 

from the mitochondrial DNA molecule was used to establish what affinity this escarpid-like species 

showed toward previously identified species of tubeworms from the Gulf of Mexico and the Pacific 

Ocean. Genomic DNA was prepared with standard procedures (Ausubel et al. 1989). The COI was 

amplified from the new escarpid species with the polymerase chain reaction (Saiki et al. 1988) using 

the vestimentiferan specific COI gene primers (Black et al. 1997). The COI gene was sequenced on an 

ABI 373 and a Beckman CEQ2000 automated sequencers. The gene was sequenced on both DNA 

strands and the complete sequence was assembled with the SEQMAN II program with the 

LASERGENE package of DNA analysis software (DNAStar, Inc.). The sequences were aligned with 

the MEGALIGN program within the LASERGENE package using Clustal V (Higgins et al. 1992). 

The nucleotide sequence alignments were adjusted manually to allow for the placement of insertions 

and deletions in the alignment. 

Molecular evolutionary relationships among sequences were examined by the neighbor-joining method 

of tree construction (Saitou and Nei 1987), based on pairwise genetic distances estimated from the 

proportion of differences and corrected for multiple substitutions by the Jukes and Cantor formula 

(Jukes and Cantor 1969). The significance of the branching order was evaluated by bootstrap analysis 

of 1000 computer-generated trees. All analyses were performed with MEGA (Kumar et al. 1994). 

An AluI restriction endonuclease assay of the COI gene was designed to discriminate between the 

"spiked" morph of Escarpia and the "normal" Gulf of Mexico Escarpia. The COI gene of each species 

had three DNA fragments when digested with AluI, but DNA fragments in the "spiked" morph were 

shorter than the "normal" morph of Escarpia. This analysis was performed on all tubeworms collected 

during the 1997 field season to insure that the genomic libraries used to isolate microsatellites came 

from the dominant Escarpia species and to insure that collected tubeworms used in the gene flow study 

were identified correctly. 

Nucleotide sequencing to determine host-endosymbiont relationships. Vestimentiferan tubeworms 

were collected by manned submersibles DSV ALVIN and the Johnson SEA-LINK II from the 

following deep-sea hydrothermal vent and cold seep sites: 

1) Middle Valley, Northeast Pacific (NEP), 2420 m, Lamellibrachia barhami, n = 1; Ridgeia piscesae, 

n = 1; 

2) Gorda Ridge, NEP, 2720 m, Ridgeia piscesae, n = 1; 

3) 90N, East Pacific Rise (EPR), 2500 m, Oasisia alvinae, n = 1; Riftia pachyptila, n = 2; Tevnia 

jerichonana, n = 2.; 

4) Alaminos Canyon, Gulf of Mexico, 2020 m, Escarpia laminata, n = 2; Lamellibrachia sp. nov. 1., 

n = 1; 

5) GC185, Gulf of Mexico, 545 m, "escarpid-like", n = 1; Lamellibrachia sp. nov. 2., n = 1; and Green 
8-99 

Canyon 234, Gulf of Mexico, 540 m, "escarpid-like", n = 1; Lamellibrachia sp. nov. 2., n = 1 

(GC185 and Green Canyon 234 are separated by approximately 10 km.); 

6) Florida Escarpment, Gulf of Mexico, 3700 m, Escarpia laminata, n = 2. 

Total nucleic acids were extracted from the symbiont-containing trophosome tissue by previously 

described methods (Nelson and Selander 1994). Most of the bacterial small subunit ribosomal gene 

(SSU rDNA) was amplified by PCR with primers (fD1 and rP1) that are highly conserved among 

eubacteria (Weisburg et al. 1991). The sequence of the 3’ end of the SSU rDNA was obtained by 

amplification with primers (1406f and 242r) located in the 3’ end of the SSU rDNA and the 5’ end of 

the large subunit rDNA, respectively (Lane 1991). 

Nearly complete SSU rDNA sequences (exclusive of the 5’ primer site) were obtained by Taq cyclesequencing, and the resulting products were analyzed on an ABI 373 automated sequencer. Internal 

primers for Taq cycle sequencing were selected from a list of conserved eubacterial primers (Lane 

1991). For each symbiont, 1515-1516 base pairs (bp) of the SSU rDNA gene were completely 

sequenced with multiple overlapping sequences on both strands, with the exception of the 

approximately 30 bp near the 5’ most primer, which was single-stranded. Individual sequencing runs 

were assembled and edited by the use of SEQMAN and SEQMANED (DNASTAR, Inc.). Final 

symbiont sequences were aligned and compared with the Eyeball Sequence Editor (Cabot and 

Beckenbach 1989). 

To establish host relationships for five of the tube worm taxa, a portion of the mitochondrial 

cytochrome oxidase I gene (COI) was sequenced and compared with published data for other 

vestimentiferans (Black et al. 1997; Kojima et al. 1997). For these studies, total nucleic acids were 

extracted from vestimentum tissue. The COI gene was amplified by PCR with primers designed from 

conserved sequences of other invertebrates: 

 COIf: TC(CA)ACTAATCA(CT)AA(GA)GA(CT)ATTGG(ATGC)AC, and (8.7-1) 

 COIr: CC(ATG)CTTAG(TA)CCTA(GA)(GA)AA(GA)TGTTG(ATCG)GG. 

These primers amplified an approximately 1250-bp segment, and from this segment 350 bp were 

sequenced as described above. Sequences were obtained from the following individuals and localities: 

1) Lamellibrachia barhami, Middle Valley, site 1; 

2) Escarpia laminata, Alaminos Canyon, site 4; 

3) "escarpid-like", GC185, site 5; 

4) Lamellibrachia sp. nov. 2., GC185, site 5; 

5) Escarpia laminata, Florida Escarpment, site 6. 

Molecular evolutionary relationships among sequences were examined by the neighbor-joining method 

of tree construction (Saitou and Nei 1987), based on pairwise genetic distances estimated from the 

proportion of differences and corrected for multiple substitutions by the Jukes and Cantor formula 
8-100 

(Jukes and Cantor 1969). The significance of the branching order was evaluated by bootstrap analysis 

of 1000 computer-generated trees. All analyses were performed with MEGA (Kumar et al. 1994). 

To determine the branching pattern of the deep phylogenetic relationships, we utilized four-cluster 

analysis (Rzhetsky et al. 1995). This method may be used to infer the relationships among groups of 

species, of which each group may contain many species. 

8.7.3 Results 

Overview of the microsatellite work. The proposed use of DNA microsatellites for this project was 

based on the use of these hypervariable loci in model organisms, where they were used for DNA 

forensics, for population genetic studies, and for human disease gene localization (Rubinsztein et al.

1995). Three unexpected observations in the collections of tubeworms and mussels initially impeded 

progress in isolating microsatellite loci. First, early in the second year, the histopathology study of 

mussels from GC185 (Section 8.5) revealed a high infection rate with metazoa parasites. DNA 

isolation procedures from these parasite-infested mussels do not discriminate between the DNAs of the 

two organisms. The organismal origin of any microsatellites isolated from the GC185 mussels could 

be from either mussel or parasite. Therefore, the mussel microsatellite project had to start anew using 

B. childressi from the GC233 Brine Pool site where parasites had not been detected. Second, the 

discovery of a second Escarpia morph in year two set back the microsatellite project because it was 

unclear how many individuals of each species had been collected. A molecular biological 

identification procedure was developed to screen all sampled tubeworms in our collections before the 

microsatellite isolation project could continue. Finally, the screens of microsatellites in the studies of 

tubeworms and mussels suggest that while these species do have microsatellites, the number of 

polymorphic loci may be quite small as has been observed in a variety of species (Rubinsztein et al.

1999; Samadi et al. 1999).

Vestimentiferan tubeworms microsatellites. The screen for microsatellites in Lamellibrachia sp. 

yielded 104 positive clones that hybridized strongly to the repetitive DNA probes out of ~1,000 total 

clones that were screened. Of the 104 positive clones, 45 clones had an insert size greater than 200 

base pairs, the minimum size capable of containing a viable insert for microsatellite primer design. Of 

the 45 clones that were sequenced, nine microsatellite clones could be used to design PCR primers for 

screening Lamellibrachia populations. Two of these nine microsatellite loci, 1-2B and JW7, were 

polymorphic and could be used for studies of gene flow (Table 8.7.4). 

A second screen of this species has been conducted on a DNA library that was not pre-enriched for 

microsatellite sequences. This second screen has generated 70 positive clones out of roughly 500, 20 

of which have been sequenced. Five of these 20 clones contained microsatellites and one PCR primer 

have been designed to test for levels of variability. The JW7 microsatellite locus shows that 2.0 

Lamellibrachia larvae are estimated to be exchanged among the three Green Canyon populations each 

generation (Table 8.7.5). A total of 3.6 Lamellibrachia larvae are estimated to be exchanged between 

GB425 and the three Green Canyon populations each generation (Table 8.7.5). The 1-2E microsatellite 

locus is currently being run on all of the Lamellibrachia sp. samples. 
8-101 

Table 8.7.4. PCR primers for nine microsatellite loci in Lamellibrachia sp. from the Gulf of Mexico. 

Name Size 

(bp) 

 PCR Primers Variability Temp (C) Repeat 

A10 75 f 5' CAA TTA CGA GTC AGT GTG AG 

3' 

N 65 (GA)12

 r 5' CGC AAT GAT GGT GAA TGT C 3' 

JW1 130 f 5' CGC TAG CCA CGA CTA CCG AT 

3' 

N 51 (GGA)6

 r 5' ACG CGG GAG CGA ATA CGA 

GT 3' 

 

JW8 320 f 5' GGC AAT TGT TGA GGA CGT GT 

3' 

N 51 (GAC)20+(CA)30

 r 5' GGA AGT GAA CCA ATG CTC TG 

3' 

 

JW3 160 f 5' CAT CAC CAA GAC CAA AGC AG 

3' 

N 55 (GGA)5

 r 5' ATA GTG AGC CAG TCT CGA TG 

3' 

 

JW4 250 f 5' CGC TTT ACT CAT CCA CAG TT 

3' 

N 55 (CCAGA)6

 r 5' GCT GGC ACT GAC GTA ATC AA 

3' 

 

JW5 50 f 5' CTC GAC GAA GCC CTC AAA GA 

3' 

N 57 (GGA)9

 r 5' CTT CAC TCA TCA TCG CTT TC 

3' 

 

L3.7B 270 f 5' CAG CAG AAT GAG GTG TGG AT 

3' 

N 55 (CCT)48

 r 5' CCT ACG TGG AAG ATG GTT GA 

3' 

 

JW2 160 f 5' GAT GGT AAT GAA ATG TCT GC 

3' 

P 51 (CT)20

 r 5' ATG ACA GTA CCC AAT TAC GA 

3' 

 

JW7 300 f 5' GCA GAG CAT TGG TTC ACT TC 

3' 

P 53 (CACG)13

 r 5' GAA TCG TGC AAC TGG TTG AC 

3' 

 

1-2E 250 f 5' TGT TGA GGA CGT GTT ACA AG 

3' 

P (CACG)15

 r 5' ACG TGC ATG TGT TTA CTA GC 

3' 

 

f, forward PCR primer; r, reverse PCR primer; N, PCR primers could not be developed for the 

microsatellite locus; P, microsatellite locus was polymorphic; Temp, annealing temperature for the PCR 

reactions; and Repeat, length of the original microsatellite clone. 
8-102 

Table 8.7.5. Estimates of the number of migrants 

exchanged between four populations of 

Lamellibrachia sp. 

 GC185 GC233 GC234 

GB425 2.06 2.43 2.33 

GC185 1.55 6.79 

GC233 1.63 

Two screens have been conducted for microsatellite DNA in Escarpia sp. The first screen was of a 

library constructed using repeat-enriched DNA and it generated 75 positive clones out of roughly 150. 

Of the 75 positive clones, 56 had inserts larger than 150 base pairs, and were sequenced. Of the 56 

clones that were sequenced, only 17 were suitable for PCR primer design. Four of these primers 

successfully amplified the target microsatellite sequence. Two of the four were invariant and were not 

useful for estimating levels of gene flow among Escarpia populations. Three primers, 4-10d, 3-2d, and 

2-2A, were polymorphic and were used in a population screen on this species. A second screen of this 

species has been conducted on a DNA library that was not pre-enriched for microsatellite sequences. 

This second screen has generated 110 positive clones out of roughly 1000, 45 of which have been 

sequenced. Eighteen of these 45 clones contained microsatellites and four PCR primers have been 

designed to test for levels of variability. Table 8.7.6 shows the primers designed for the microsatellite 

loci found in our genomic library screens of Escarpia sp. 
8-103 

Table 8.7.6. PCR primers for 25 microsatellite loci in Escarpia sp. from the Gulf of Mexico. 

Name Size (bp) PCR primers Var Temp (oC) Repeat 

4-8H 200 f 5' CAA TAT TTG ACA GCT CGG AC 3' N (GT)30

 r 5' CTG TAC GCA TCA TGT TTG AC 3' 

1-4d 77 f 5' GAT CCT GCC ATG CAA ACG TG 3' N (GT)10

 r 5' CCA TAA CTG ACT AAC GTC TC 3' 

1-5d 180 f 5' GTA AAC ATT GTC TAT ACA G 3' N (GT)20

 r 5' CCT ATT GGA TCA ATA TTT AC 3' 

3-2e 220 f 5' GAT CTA CCG TAC CTA TAA TG 3' N (GT)50?

 r 5' GAT CAA GGT AAA CGT GGA TG 3' 

3-1a 190 f 5' GAT CGG CAC AAT AAT GTA AGA G 3' N (GT)40

 r 5' TAG TGG ATC ACG CGC GCA AGT A 3' 

1-2g 180 f 5' GCG TGC GTG CGT AAG TGA GT 3' N (CGTG)8

 r 5' GGA TCA CAA CAC GCC TGG GT 3' 

1-2g#2 180 f 5' TCG CTG CGT AAG TGA GTG TA 3' N (CGTG)8

 r 5' CAC AAC ACG CCC TGG TAT CAC AAC 

1-3b 150 f 5' CAT TAT GTG TGA GTG CGT CC 3' N (GT)40

 r 5' CTG TAA GAA GAC ACC CAA CC 3' 

1-3b#2 150 f 5' CAT TAT GTG TGA GTG CGT CC 3' N (GT)40

 r 5' CTA TCT GTA AGA AGA CAC CCA ACC 

1-3g 300 f 5' TCC GGT GTA CTA TGA GGT CG 3' N (GT)80?

 r 5' TAG TGG ATC ACA CAC GCA CG 3' 

3-5f 225 f 5' GGA TCA GCA GCA CAG TCT TA 3' N (GT)50?

 r 5' TCA CCA TAC GCA CGT ATG AA 3' 

3-1h 150 f 5' CTC AGT GTG ACA GGT ATG TG 3' N (GT)26

 r 5' GGA TCA TGA CTG TAG GAG TG 3' 

3-5h 120 f 5' TGG TAT TAG CAT TTG TCC CA 3' N (GT)15

 r 5' TCC ATA ACT GAC CAA CGT CT 3' 

3-6b 170 f 5' GGA TCT TGG CAC AAA TGT CT 3' N (GT)45

 r 5' GGA TCA CAT CAC ATA CAC AC 3' 

1-2H#2 209 f 5' ACA TAC ATG ACC GAC TAA CG 3' N (GT)35

 r 5' ATC CAG GGT TGA ACG AGA TA 3' 

4-10E 200 f 5' TCT TAA ATA TGC GAC TGC CT 3' N (GT)22

 r 5' TCA ACC ATG AGT GAT GTG TA 3' 

3-3h 120 f 5' TGG TAT TAG CAT TTG TCC CA 3' M (GT)18

 r 5' TCC ATA ACT GAC CAA CGT CT 3'

1-2H 185 f 5' GAC TAA CGT CTC TGT ACC GA 3' M (GT)35

 r 5’ GGT TGA ACG AGA TAG GTT GC 3’ 
8-104 

Table 8.7.6. Continued.

3-2D 150 f 5' GAT CTC TTC TGG CAG GCG TT 3' P (CGTG)10

 r 5' CTC CAT TGC ATC TAC GGG TC 3' 

4-10D 200 f 5' GGA CAA GGA TGA GAG CAA CA 3' P (GT)26

 r 5' TCG CTC CAC TAC TCA CAC AA 3'

E2-2A 150 f 5' CAG TGT CAT ACA GCG GGC TT 3' P (GCGG)9

 r 5' ATC CGC CTC TGC CTA CGT TT 3' 

2-7E 550 f 5' GAT AAA CTG CAA GCC TCA GAG P (CA)40

 r 5' GGT AAG AGT GGT TTC ATT CG 3' 

2-9E 400 f 5' CAC AAC TGC TGT TAT TGG AA 3' M (CACG)5

 r 5' ACA GAG TGC GTG CAT ATG TG 3' 

2-9H 450 f 5' GTA TTA GCT CGT GAA CGC CA 3' N (TATT)5,(TAA

 R 5' GGT CTA TAG TTA TCG GAC TG 3' 

f, forward PCR primer; r, reverse PCR primer; N, PCR primers could not be developed for the microsatellite locus; M, microsatellite locus was 

monomorphic; P, microsatellite locus was polymorphic; Temp, annealing temperature for the PCR reactions; and Repeat, length of the original 

microsatellite clone.

Figures 8.7.1 and 8.7.2 show the allele frequencies of the two variable microsatellite loci (4-10D and 3-2D) in Escarpia sp. 

collected from four hydrocarbon seeps. The allele frequencies of these three microsatellites are currently being collected 

for the stations within GC185. A total of 11.3 Escarpia larvae are estimated to be exchanged among the three Green 

Canyon populations each generation (Table 8.7.7). A total of 1.4 Escarpia larvae are estimated to be exchanged between 

GB425 and the three Green Canyon populations each generation (Table 8.7.7). 

Table 8.7.7. Estimates of the number of migrants 

exchanged between four populations of Escarpia sp. 

 GC185 GC233 GC234 

GB425 2.0 1.8 0.5 

GC185 12.7 1.0 

GC233 21.2 

Mussel microsatellites. Three screens have been or are being conducted for microsatellite DNA in 

Bathymodiolus childressi. The first screen for microsatellites yielded five positive clones out of 1000 

total clones that were tested. One of the five positive clones that were nucleotide sequenced, gA3C, 

had a large (GA) repeat and had an insert size sufficient to design PCR primers for further analysis. 

This microsatellite locus was highly polymorphic and was used to screen the individuals in the four 

seep populations for genetic diversity. The microsatellite locus has been run on all of the B. childressi 

samples from the four Gulf of Mexico populations. 
8-105 

The second microsatellite screen yielded 30 positive clones out of 850 total recombinant clones that 

hybridized strongly to the repetitive DNA clones. Of the 30 positive clones, 20 clones had an insert 

size greater than 200 base pairs, the minimum size capable of containing a viable insert for 

microsatellite primer design. Of the 20 clones that were sequenced, three microsatellite clones could 

be used to design PCR primers for screening B. childressi populations. The three microsatellite loci 

from the second screen were monomorphic and could not be used for studies of gene flow (Table 

8.7.8). A third screen for microsatellites is currently underway. 

Figure 8.7.1. Frequency distribution of microsatellite alleles for the 4-10D microsatellite locus in 

Escarpia sp. collected from four hydrocarbon seeps in the Gulf of Mexico.
8-106 

Table 8.7.8. PCR primers for four microsatellite loci in Bathymodiolus childressi from the Gulf of Mexico. 

Name Size 

(bp) 

 PCR Primer Variab

ility 

Temp 

(C) 

Repeat 

gA3C f 5' TAT TAA CAC GAC GCG TGG CA 3' P 55 (GA)28

 r 5' TTC GCG ACA ACT ACC AGA GA 3' 

B8-01 157 f 5' GGC TTC CTC CAC CAC TAA TA 3' M 60 (TCAA)4

 r 5' TGA GCG TGG CAG TAA TGA GA 3' 

C9-2 119 f 5' GGT ACG CAA TCT TCA ACG TC 3' M 55 (GAA)5

 r 5' ACG AGT TGG TCA AGG TAC GA 3' 

2C9-2 117 f 5' GGT ACG CAA TCT TCA ACG TC 3' M 50 (GAA)5

 r 5' ACG AGT TGG TCA AGG TAC GA 3' 

f, forward PCR primer; r, reverse PCR primer; M, microsatellite locus was monomorphic; P, microsatellite locus was polymorphic; Temp, 

annealing temperature for the PCR reactions; and Repeat, length of the original microsatellite clone. 

Figure 8.7.2. Frequency distribution of microsatellite alleles for the 3-2D microsatellite locus in 

Escarpia sp collected from four hydrocarbon seeps in the Gulf of Mexico
8-107 

A total of 23 microsatellite alleles were observed at the gA3C locus in B. childressi. The allele 

frequencies of gA3C in the six collection stations at GC185 are found in Figure 8.7.3. Most 

microsatellite alleles are shared among the six collection stations. The allele frequencies of gA3C at 

the four collection localities are found in Figure 8.7.4. Again, although there are frequencies 

differences in gA3C microsatellite alleles among populations, the alleles that are present in the five 

populations are similar. A total of 2.47 B. childressi larvae are estimated to be exchanged among the 

GC185 mussel stations. A total of 8.7 B. childressi larvae are estimated to be exchanged among the 

three GC234 populations each generation (Table 8.7.9). A total of 14.5 B. childressi larvae are 

estimated to be exchanged between GB425 and the three GC234 populations each generation (Table 

8.7.9). 

Figure 8.7.3. Frequency distribution of microsatellite alleles for the gA3C microsatellite locus in B. childressi collected from 

six stations within the GC185 hydrocarbon seep in the Gulf of Mexico. 
8-108 

Table 8.7.9. Estimates of the number of migrants exchanged 

between four populations of Bathymodiolus childressi. 

 GC185 GC233 GC234 

GB425 20.3 9.4 13.8 

GC185 16.9 13.2 

GC233 6.7 

Phylogenetics of vestimentiferans. The phylogenetic relationships of the vestimentiferan tubeworms 

based on the COI sequences are shown Figure 8.7.5. The newly discovered "spiked morph" of 

Escarpia is closely related to other escarpid species that are found in the Gulf of Mexico where this 

new morph clusters with an escarpid collected at GC234. The genetic distance between 

the "spiked" escarpid and the "normal" escarpid from GC234 is similar to that which separates a 

Lamellibrachia from the Gulf of Mexico and that of Pacific Ocean species of Lamellibrachia.

Figure 8.7.4. Frequency distribution of microsatellite alleles for the gA3C microsatellite locus in B. childressi

collected from five hydrocarbon seeps in the Gulf of Mexico. 
8-109 

Phylogenetic relationships of vestimentiferan endosymbionts. Nelson and Fisher (2000) in a paper 

supported by the MMS contract showed that the seep vestimentiferans fall into two easily 

distinguishable groups. One group consists entirely of species in the genus Lamellibrachia. The other 

includes all of the described species in the genus Escarpia and an undescribed “escarpid-like” species, 

which is distantly related to the named species of Escarpia (Figure 8.7.6; S. Gardiner personal 

communication). Lamellibrachia and Escarpia (or the “escarpid-like” species) co-occur at many seep 

sites in the Gulf of Mexico as well as at several locations around Japan (Kojima et al. 1993). 

Figure 8.7.5. Phylogenetic relationships among vestimentiferan tubeworms from the hydrocarbon seeps and hydrothermal 

vents. Note the two species of Escarpid-like tubeworms from the MMS study sites are different from each 

other and from the other described Escarpids
8-110 

In contrast to the evolutionary relationships among the hosts, the present findings identified three 

groups of symbionts from seep vestimentiferans that do not correlate with host taxonomic classification 

(Figure 8.7.7). The first group includes the symbionts from Lamellibrachia sp. nov. 2. and the 

“escarpid-like” tube worms from shallow seep sites in the Gulf of Mexico. In fact, sequence 

differences between symbionts from two individuals of the same host species are the same or greater 

than the differences between the sequences of symbionts of the two different host species sampled 

from the same site (Figure 8.7.6). Average divergence (5.8 sites, 0.38%) among the symbionts in four 

host individuals from the shallow seep sites in the Gulf of Mexico is approximately three times the 

divergence (2.1 sites, 0.14%) found among all vent symbionts analyzed. 

A second group of cold seep symbionts is found in vestimentiferans collected from Alaminos Canyon, 

a seep site that occurs in the deeper waters of the Gulf of Mexico (Brooks et al. 1990) Similar to the 

Figure 8.7.6. Phylogenic tree of tubeworms based on the COI gene of the mitochondrial DNA. This tree is based on 

data collected by Nelson and Fisher (2000), Feldman et al. (1997), Black et al. (1997).
8-111 

shallow sites, two species of tube worms, Lamellibrachia sp. nov. 1. and Escarpia laminata, occur at 

the site and harbor symbionts from the same population. Only a single nucleotide difference was found 

among the three symbiont sequences analyzed from Alaminos Canyon tube worms. These sequences 

differed from those of the shallow Gulf of Mexico species by an average of 19 nucleotide differences 

(1.3%). 

Escarpia laminata was collected from 3,700 meters on the Florida Escarpment in the Gulf of Mexico. 

The SSU rDNA sequences of the symbionts from the two individuals analyzed from this site were quite 

similar (99.9%) but differed by 1.8% from symbionts of the other Gulf of Mexico sites (Figure 8.7.7). 

Figure 8.7.7. Phylogenetic relationships among sulfur-oxidizing endosymbionts based on 16S rDNA 

sequences. Included results presented in Feldman et al. (1997).
8-112 

The large divergence (27.1 nucleotide differences) between the symbionts of Escarpia laminata from 

the Florida Escarpment and Escarpia laminata from Alaminos Canyon is not reflected in the 

evolutionary relationships of the hosts, which show few differences in the COI gene (Figure 8.7.6, 2.5 

sites). Clearly, the seep symbiont phylogeny is not congruent with that of the hosts, but rather with 

geography. 

8.7.4. Discussion 

Gene flow in vestimentiferans. Wright's (1931, 1943) mathematical model of migration shows that 

one migrant per generation is sufficient to homogenize gene frequencies among populations. The data 

from Lamellibrachia and Escarpia sp. collected from four seeps in the Gulf of Mexico show that 

microsatellite alleles are largely not restricted to a single habitat, but are shared among the tube worm 

populations examined in this study (Tables 8.7.5 and 8.7.7). These data suggest that disturbance of a 

local community will not have a profound effect on genetic diversity within the communities in the 

Gulf of Mexico. There is potential for large amounts of gene flow provided there are sufficient habitats 

for the tubeworms to settle. Thus, the amount of suitable habitat in the Gulf of Mexico will limit how 

many chemosynthetic communities that can be established not the biological ability of the tubeworms 

to disperse. The estimates of gene flow in Lamellibrachia and Escarpia are consistent with those 

found for the tubeworms that inhabit the hydrothermal vents of the Pacific Ocean (Black et al. 1994). 

Gene flow in mussels. The single variable microsatellite for Bathymodiolus childressi indicates that 

the migration parameters are greater than one for all comparisons between study sites (Table 8.7.9). 

This is evidence that this species is capable of extensive gene flow over short, intermediate, and long 

ranges. Again, this level of exchange is capable of homogenizing gene frequencies among mussel 

populations and that mussels have the potential to recolonize new seep sites in the Gulf of Mexico. 

These results are quite striking given the variation in the growth abilities and condition of the mussels 

in the different populations. The migration data suggest that the condition of endemic mussels is not 

controlled genetically, but by the selective constraints of the local environment. 

Phylogenetics of vestimentiferans. The relationship of the newly discovered "spiked morph" of 

Escarpia to vestimentiferans from the Gulf of Mexico and the Pacific Ocean suggest that this morph is 

a new species of escarpid-like tubeworm. These results should be viewed with caution because our 

conclusion is based on a single genetic locus that resided within the mitochondrial DNA; however, the 

molecular differentiation of the COI genes in the two morphs of Escarpia is substantiated by the 

extensive morphological characterization of these tubeworms (Stephen Gardner; Bryn Mar, 

unpublished data). This new escarpid-like species was rare in our collections of tubeworms taken with 

the Bushmaster suggesting that this new type should be considered fragile because of its low 

abundance. 

Phylogenetics of vestimentiferan endosymbionts. The lack of concordance between the evolutionary 

relationships of the seep lineages of hosts and symbionts argues strongly against vertical transmission 

of symbionts from one generation to the next. Although this lack of congruence stands in contrast to 

other deep-sea metazoan endosymbioses that have been studied to date (Cary et al. 1993; Distel et al.

1994; Kreuger et al. 1996; Peek et al. 1998), it is not without precedence. An environmental 

transmission of the sulfur-oxidizing gill symbiont of the shallow water lucinid bivalve has been 

proposed (Durand et al. 1996; Gros et al. 1996). However, a strategy that requires the de-novo 

acquisition of symbionts each generation would seem risky for a host that is absolutely dependent on 

the symbiont for its nutritional needs as an adult, and for a host that occupies a habitat that is as patchy 
8-113 

and ephemeral as the hydrothermal vents. Thus, there must be some mechanism of host-symbiont 

recognition that assures the uptake and establishment of a symbiont population within larval or early 

juvenile tubeworms. 

An unexpected finding was the close relationship between the Escarpia laminata symbionts from the 

Florida Escarpment and Lamellibrachia barhami from Middle Valley in the northeast Pacific. Not 

only are the two locations separated by land masses, but Middle Valley is a hydrothermal vent site 

located on a mid-ocean ridge. However, Middle Valley is a sedimented site with some areas of very 

low flow (Southward et al. 1996). Thus, in some ways, areas of Middle Valley may be very similar to 

a cold seep environment. The same species, L. barhami, is found at the cold seep sites (400 km 

distant) on the continental margins off the coast of Oregon and California (Southward et al. 1996; 

Suess et al. 1985; Webb 1969). At the Middle Valley site where these collections were made, the 

hydrothermal vent species, Ridgeia piscaesae, appeared emaciated, apparently reflecting the relatively 

inactive status of this site (J.J. Childress personal communication), and harbored the expected vent 

symbiont and not the seep symbiont hosted by L. barhami. This site is only one that we know of where 

two taxa of tube worms co-occur but do not host the same species of symbiont. (At Guyamas Basin, 

Riftia pachyptila and Escarpia spicata co-occur, and we predict that the symbionts from these two 

species would also be very different.) The fact that they house very divergent symbionts argues for 

host/symbiont co-evolution at least at the level of the divergence into the two very different habitats. 

Overall Conclusions. This effort has improved our understanding of the genetic structure of seep 

communities and has implications for protection of these environments. The currents in the Gulf of 

Mexico are sufficient to allow transport of mussel and tube worm larvae among the four seep 

communities sampled for this study. This would suggest that the seep communities are robust to 

disturbance, but gene flow data provides no insights into the number of suitable habitats that the 

tubeworms and mussels can colonize. The microsatellite loci developed in this study will provide 

useful tools for further examination of tubeworm and mussel populations to further refine how these 

chemosynthetic organisms are transported around the Gulf of Mexico. Each depth community supports 

a unique form of free-living bacterial endosymbiont for the tubeworms that is likely to be transmitted 

environmentally to the host. The movement of hosts from different seep habitats may or may not be 

allowed because of the unique bacterial flora present in each habitat. 
 9-1

9.0 Inorganic Biogeochemistry of Cold Seep Sediments*

9.1 Overview 

This component of the program was designed to provide a more detailed examination of the 

inorganic biogeochemistry occurring in cold seep sediments. The primary focus of the research 

was on understanding the processes influencing the carbon dioxide and sulfide systems. A 

review of the basic marine chemistry of these systems is given below. The sections of this 

chapter have been divided according to major activity and are synthesized at the end of this 

chapter. The topic areas for each section are as follows: 

9.2 Pore water and solid phase chemistry

This section uses a “classical” approach in which cores were sectioned to establish depth profiles 

of dissolved and solid phase chemical components of interest. 

9.3 Stable isotope chemistry 

Fractionation of the stable isotopes of carbon, oxygen and sulfur occurs by both biotic and 

abiotic processes. Stable isotope ratios of these elements were determined at selected sites for 

both dissolved and authigenic mineral phases to aid in the interpretation of the processes 

occurring in cold seep sediments. 

9.4 Microelectrode profiling 

An “emerging” new technology was utilized to obtain very detailed depth profiles of dissolved 

sulfide near the sediment-water interface. This was done in order to obtain better estimates of 

fluxes across the sediment-water interface and behavior of sulfide in the highly dynamic region. 

9.5 Synthesis

In this section the findings made in the above sections are integrated to establish more general 

relationships among these parameters. This is the first step toward the overall project integration 

and modeling of research results. 

9.2 Pore Water and Solid Phase Chemistry*

9.2.1 Introduction 

The biogeochemistry of hydrocarbon seeps is fundamentally related to the concentration, fluxes, 

sources, and sinks of dissolved gases. Gases, such as methane and other hydrocarbon 

 

*

 This section was authored by John Morse and Rolf Aruitsm. 
 9-2

components, which migrate from depth or are supplied by disassociating methane hydrate 

located close to the sediment-water interface, may represent a major source of reduced carbon in 

these environments. Carbon dioxide (CO2) and hydrogen sulfide (H2S) play critical roles as 

metabolic products of microbial sulfate reduction and exert a major influence on critical 

environmental variables such as pH and oxidation-reduction potential (pe/Eh). The reoxidation 

of reduced sulfur also plays a critical role as an energy source for benthic organisms. 

The speciation systematics of carbon dioxide and hydrogen sulfide in natural waters share some 

important similarities. Both behave as acids that dissolve via stepwise hydration and diprotic 

dissociation reactions, producing components whose relative concentrations vary as a function of 

pH. CO2 dissolves in water according to the following reactions: 

2 ( ) 2 (aq)

K

CO g CO

H

 (9-1a) 

 CO  H O HCO  H

K

2(aq) 2 3

1

 (9-1b) 

  2

3 3

2

HCO H CO

K

 (9-1c) 

Because seawater pH is ~ 8, roughly the average of pK1 (6.4) and pK2 (10.3) at 25°C, 

bicarbonate ion is the primary dissolved inorganic carbon component. In similar fashion, 

hydrogen sulfide dissolves as 

2 ( ) 2 (aq)

K

H S g H S H(9-2a) 

 H S HS  H

K

aq

1

2 ( ) (9-2b) 

   2 2

HS H S

K

 (9-2c) 

Bisulfide ion is also the most common component at seawater pH. CO2 and H2S are both 

produced when sulfate is used as an electron acceptor in the remineralization of simple organic 

matter, 

 CH O  SO HS  HCO3  H 2 2 (9-3) 2 4 2

or methane (here written as the complete oxidation of CH4 to CO2), 

CH SO HS HCO3 H2O 2

4  4      (9-4) 

The bisulfide and bicarbonate produced in these reactions may react further to form sulfide and 

carbonate minerals within the sediment. 
 9-3

Important sulfide minerals consist of pyrite (FeS2) and the metastable, so-called acid-volatile 

sulfides (AVS: i.e., amorphous FeS, mackinawite, and possibly greigite). The abundance of 

pyrite + AVS (total reduced sulfur, or TRS) is limited by the availability of sulfide and/or iron 

(Canfield 1989). These minerals are thus important in terms of buffering dissolved sulfide and 

may also serve to sequester toxic metals (Morse 1994) and thus control their bioavailability. 

Carbonate minerals such as calcite and aragonite (CaCO3), and more rarely, dolomite 

(CaMg(CO3)2) form via reactions represented as 

3 2 2

2 2HCO3  Ca CaCO  H O  CO  (9-5) 

and may act as inorganic cements in binding of primary sedimentary particles or, in the case of 

carbonate mounds, form important sedimentary structures themselves. 

In the presence of reactive iron oxides, reactions (9-3) and (9-5) may combine to yield both 

carbonates and AVS as products, 

CH O SO Ca FeOOH FeS CaCO3 CO2 H2O 2 2

2 4 4

11

4

5

4

9

       (9-6) 

In oxic regimes, remineralization of organic matter may also lead to the dissolution of carbonate 

minerals, 

    2 O2 CH2O CaCO3 2HCO3 Ca (9-7) 

Dissolved sulfide produced in reaction (9-3) may also be reoxidized to sulfate by oxygen, nitrate, 

Fe(III) or Mn(IV) compounds, depending on their availability. 

Sulfide may be used as an electron donor by microbial sulfide oxidizers yielding sulfate, 

intermediate thiosulfate, or elemental sulfur as metabolic products. Thus sulfide is important 

both as a waste product of sulfate reduction, and as a substrate itself for new energy production. 

In addition, despite the important differences in geometry that distinguish chemosynthetic seep 

systems from other benthic habitats, it is not clear what limitation is imposed on these systems in 

terms of the supply of metabolizable organic matter. Evidence of bacterial oxidation of both 

crude oil hydrocarbons (Sassen et al. 1994a) and methane hydrates (Sassen et al. 1999a) suggests 

a complex relationship between the organic matter pools and their relative lability, sulfate 

reducing bacteria, and sulfide oxidizers. 

The production and remineralization of organic matter in chemosynthetic seep communities are 

thus intimately linked to the cycling of inorganic CO2 and H2S via reactions such as those 

described above. A practical understanding of the location and distribution, productivity, and 

dynamics of seep populations cannot be made without careful measurement of pore water and 

solid phase carbon and sulfur components. These measurements were undertaken with the 

following objectives: 

1. Identification of carbon dioxide and hydrogen sulfide sources: Variations in CO2 and H2S 

concentrations may derive entirely from local reactions occurring within the sediment. It is 
 9-4

also possible, however, that deep-seated fluids migrating to the surface may deliver these 

gases. For example, if seep fluids are a significant source of sulfide, these chemosynthetic 

communities might be reasonably compared with those associated with seafloor 

hydrothermal vent systems. It will thus be determined whether pore waters reflect extra local 

sources of these inorganic gases. 

2. Quantification of sulfate reduction rates: Understanding the dynamics of sulfur and carbon 

cycling in these chemosynthetic ecosystems, particularly with respect to sulfide oxidizers, is 

not possible without measurement of the rate of microbial sulfate reduction. This parameter 

serves as a basic discriminator between sites that are biologically “active” and those that are 

in relatively quiescent or in decline, and as a means of general comparison of these 

communities with “normal” marine systems. 

3. Understanding of the relationship between supply of organic components and electron 

acceptors (sulfate), and identification of limiting components: Despite apparent widespread 

abundance of organic material in the form of both hydrocarbon gases and crude oil 

components, chemosynthetic communities are localized and heterogeneously distributed. A 

critical step to understanding these systems thus lies in identifying those components 

(organic, inorganic, physical-chemical, sedimentologic, etc.) responsible for overall 

limitation and localization of these systems. 

4. Determination of the role of brines in mineral formation: Because brines can possess solute 

concentrations much greater than seawater, their arrival from depth and mixing with pore 

water could substantially alter solute concentrations and form an important source of 

components for mineral formation. It is thus important to determine the end member 

composition of brine fluids and what component they represent in pore water. 

5. Quantification of sulfide and carbonate mineral formation: Diagenetic processes such as 

precipitation and dissolution of sedimentary mineral phases also buffer concentrations of 

pore water sulfide and carbon. These phases (AVS/TRS, calcite, reactive iron oxides) act as 

sources and sinks for dissolved components and thus affect their bioavailability as well as 

physical sediment properties. 

9.2.2 Results 

Complete results of all solid phase and pore water work are tabulated in the Appendix and 

organized in the following manner: pH, DIC, sulfide, sulfate, and chloride (Table A.9.1); major 

cations (Table A.9.2); nutrients and dissolved organic carbon (Table A.9.3); solid phase 

properties (Table A.9.4); and sulfate reduction rates (Tables A.9.5 and Table 9.1). Stable isotope 

data are presented later in this chapter. The results of organic geochemistry appear in a separate 

chapter. In these tables, any differences in coverage with respect to the various analytical 

components at each station reflect the difficulties imposed by undertaking a large number of 

analyses from a relatively small number of cores. Cores that showed obvious evidence of upset 

during retrieval were discarded. Missing data arise from inadequate core material or pore water 

sample volume to accommodate analysis of all components (many deeper intervals yielded less 

than 5 mL), differences in depth of core penetration, and infrequently due to poor analyses; blank 
 9-5

cells indicate these. In addition, certain analyses (e.g., sulfate reduction assays, dissolved organic 

carbon, carbon and sulfur isotopes) focused only on specific cores and intervals therein. 

9.2.3 Discussion 

The following discussion will focus on key aspects of the inorganic geochemistry that relate to 

biotic relationships and activities. The most important reaction in these communities is 

dissimulatory sulfate reduction. As will be seen below, the overall vigor of sulfate reduction 

serves as a principle discriminator in the geochemical characterization of these highly dynamic 

chemosynthetic communities. 
 9-6

Table 9.1. Porewater pH, carbon, sulfur, and anions. 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

2871 GC234 GCST1 0–2 7.948 2.845 .0 – 560.82 

 2–4 7.979 3.998 .0 28.189 563.11 

 4–6 8.076 3.995 .0 22.414 565.41 

 6–8 8.119 4.434 .0 27.271 567.70 

 8–10 – – – – – 

 10–12 8.080 4.103 .0 27.465 565.47 

 12–14 8.088 4.323 .0 27.865 565.24 

 14–16 8.014 3.914 .0 26.948 544.05 

 16–18 7.956 3.682 – 25.804 522.86 

 18–20 7.937 – .0 26.837 571.06 

2873 GC235 BHAT1 0–2 7.976 3.349 .0 27.161 537.13 

 2–4 7.949 4.056 .0 26.473 534.66 

 4–6 8.010 4.396 .0 25.353 552.15 

 6–8 7.978 5.186 2.1 26.017 551.08 

 8–10 7.879 4.134 28.4 25.983 542.77 

 10–12 8.011 4.517 6.6 25.687 544.65 

 12–14 7.890 4.187 30.0 24.649 533.51 

 14–16 7.872 4.148 28.1 26.459 555.78 

2873 GC235 BHM4 0–2 7.809 – 2784.4 17.585 569.72 

 2–4 7.826 7.478 5026.9 11.086 575.74 

 4–6 7.724 8.893 5300.4 1.983 591.39 

 6–8 7.919 12.015 6519.0 .0 577.48 

 8–10 7.940 10.770 5322.6 .0 572.96 

 10–12 7.939 12.854 5352.2 .0 606.34 

 12–14 7.860 13.336 5248.7 .0 577.64 

 14–16 7.878 12.621 5300.4 .0 580.95 

 16–18 7.936 11.875 5359.6 0.037 559.62 

 18–20 7.895 10.212 5307.8 .0 580.03 

2874 GC235 BHST1 0–2 8.024 – .0 28.435 600.92 

 2–4 7.953 4.527 48.2 25.668 549.51 

 4–6 7.834 – 276.4 24.411 546.77 

 6–8 7.893 6.263 1219.7 23.240 557.81 

 8–10 7.800 6.002 1931.9 22.138 546.00 

 10–12 7.825 7.000 1948.4 21.572 545.01 

 12–14 7.886 6.693 1828.0 21.606 571.14 

 14–16 – – – – – 

 16–18 7.875 8.674 1866.3 20.630 564.13 

2875 GC235 BHST2 0–2 7.963 3.531 0.8 26.202 546.57 

 2–4 7.912 4.371 0.9 25.753 536.27 

 4–6 7.907 4.484 0.6 26.609 545.08 

 6–8 7.992 3.814 .0 22.627 453.56 

 8–10 7.911 3.440 .0 26.625 539.69 

 10–12 7.879 4.088 .0 27.212 552.15 

2875 GC235 BHUN1 0–2 7.941 2.927 .0 27.246 558.07 

 2–4 7.869 2.421 .0 27.217 555.83 

 4–6 7.877 3.093 .0 27.372 553.59 

 6–8 8.052 3.429 .0 26.837 551.34 
 9-7

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

 8–10 8.067 3.060 .0 26.972 549.10 

 10–12 8.095 3.322 .0 27.618 548.77 

 12–14 8.121 3.268 .0 26.988 548.44 

 14–16 8.104 3.259 .0 26.765 548.11 

 16–18 8.083 3.537 .0 26.614 547.78 

 18–20 8.081 2.823 .0 26.904 547.45 

 22–24 8.044 3.584 .0 26.739 546.79 

 26–28 8.019 3.186 1.0 27.765 554.97 

2876 GC233 BPR2 0–2 7.932 3.244 .0 26.198 597.77 

 2–4 7.841 3.165 .0 23.584 492.36 

 4–6 7.993 3.626 2.0 23.638 498.75 

 6–8 7.771 4.163 .0 22.907 491.77 

 8–10 7.734 5.186 2.0 24.933 518.49 

 10–12 7.958 4.088 17.9 26.569 550.64 

2880 GC234 GCB1@GCAT1 0–2 7.920 4.295 0.9 27.426 559.46 

 2–4 7.852 6.314 452.4 24.936 560.40 

 4–6 7.905 9.668 1186.2 22.838 561.34 

 6–8 7.839 7.119 1247.7 22.559 562.28 

 8–10 7.856 9.059 1242.1 21.863 563.22 

 10–12 7.815 8.725 1257.5 21.998 564.16 

 12–14 7.755 8.490 2126.1 21.035 559.20 

 14–16 7.773 8.306 2153.5 19.526 554.23 

 16–18 7.758 8.553 2142.5 18.888 549.27 

 18–20 7.667 6.912 2139.8 17.663 – 

 22–24 7.695 10.360 3418.2 16.482 – 

2882 GB425 GBB1@GBM2 0–2 7.544 6.069 1407.0 18.482 640.49 

 2–4 7.437 6.399 2012.5 8.992 799.65 

 4–6 7.343 6.936 870.8 2.954 1079.00 

 6–8 7.371 7.342 724.6 0.729 1284.56 

 8–10 7.300 8.114 824.4 0.526 1439.06 

 10–12 7.367 9.403 338.2 0.848 1533.00 

 12–14 7.385 15.895 21.7 0.391 1621.28 

 14–16 7.358 9.826 25.8 0.390 1629.44 

 16–18 7.383 10.278 2.1 0.341 1607.36 

 18–20 7.348 9.763 1.6 0.344 1698.21 

 22–24 7.418 7.987 1.7 0.246 1697.65 

 26–28 7.497 9.134 1.7 0.473 1669.09 

2883 GB425 GBM1 0–2 7.957 3.623 .0 27.678 – 

 2–4 7.746 4.524 .0 – – 

 4–6 7.806 3.699 .0 27.002 – 

 6–8 7.901 – .0 27.285 – 

 8–10 7.867 4.245 1.0 26.720 576.33 

 10–12 – 4.140 0.0 26.518 – 

 12–14 7.886 4.512 24.8 26.716 – 

 14–16 7.849 4.516 20.7 25.818 – 

 16–18 7.724 4.318 2.1 26.979 – 

2885 GB425 GBM2 0–2 7.725 10.613 374.3 16.226 779.66 

 2–4 7.488 15.378 719.6 2.425 1139.57 
 9-8

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

 4–6 7.513 12.747 364.8 0.419 1497.99 

 6–8 7.346 14.565 359.2 0.103 1367.35 

 8–10 7.415 10.095 – 0.385 992.21 

 10–12 7.444 11.811 42.0 – – 

 12–14 7.446 11.033 39.1 0.324 1562.10 

 14–16 7.569 8.546 – – – 

 16–18 7.521 – 3.6 – – 

2886 GC234 GCAT2 0–2 7.741 5.825 4585.7 18.961 571.15 

 2–4 7.777 7.508 6832.9 13.816 564.66 

 4–6 7.818 4.379 7810.8 12.106 568.64 

 6–8 7.779 10.943 10157.7 5.472 568.15 

 8–10 7.807 11.034 10018.0 0.384 568.65 

 10–12 7.819 18.914 .0 557.18 

 12–14 7.787 13.098 10353.3 .0 451.04 

 14–16 7.820 14.172 10618.8 .0 573.10 

 16–18 7.797 11.915 11191.5 .0 562.83 

 18–20 7.822 12.770 10297.4 .0 588.10 

 20–22 7.793 18.469 – – – 

2888 GC234 GCAT1 0–2 7.918 3.956 0.7 27.146 572.59 

 2–4 7.803 4.576 4.7 26.217 551.83 

 4–6 7.730 4.661 51.5 26.344 548.12 

 6–8 7.792 4.204 44.9 25.531 564.32 

 8–10 7.530 4.867 47.2 25.997 551.32 

 10–12 7.770 5.119 18.8 – – 

 12–14 7.736 5.302 19.8 – – 

2891 GC235 BHAT2 0–2 7.776 4.027 1.5 26.080 567.65 

 2–4 7.609 3.403 0.8 24.232 567.87 

 4–6 7.628 2.779 0.6 26.324 535.49 

 6–8 7.857 3.817 1.9 26.441 543.78 

 8–10 8.019 4.320 1.1 26.264 546.95 

 10–12 8.654 4.215 1.5 26.595 567.82 

 12–14 – – – – – 

 14–16 7.972 4.706 32.7 26.505 559.51 

box GC235 BHBC2 0–2 7.986 2.803 .0 27.638 567.27 

 2–4 7.854 2.756 .0 27.719 553.30 

 4–6 7.868 3.601 .0 27.433 555.71 

 6–8 7.988 2.833 .0 27.166 563.94 

 8–10 8.046 3.244 .0 26.735 549.69 

 10–12 8.145 3.574 .0 26.319 538.95 

 12–14 8.140 3.454 – 26.319 546.00 

 14–16 7.870 3.504 .0 25.669 530.73 

 16–18 8.002 3.860 .0 25.533 528.43 

 18–20 7.976 4.328 1.8 25.808 532.11 

 22–24 8.007 3.124 0.9 25.663 537.35 

4025 GC185 BHAT1 0–2 7.272 4.521 .0 27.085 551.42 

 2–4 7.551 4.654 .0 27.359 552.05 

 4–6 7.570 5.044 .0 26.979 558.44 

 6–8 7.477 4.686 .0 27.646 561.54 
 9-9

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

 8–10 7.465 6.468 .0 27.526 559.80 

 12–14 7.516 – .0 27.840 588.32 

4027 GC185 BHST1 0–2 7.229 3.582 .0 27.674 557.98 

 2–4 7.718 2.960 .0 27.333 555.98 

 4–6 7.933 4.101 .0 27.362 554.56 

 6–8 8.072 3.637 1.3 27.485 548.95 

 8–10 7.959 4.448 0.0 27.500 552.22 

 10–12 7.903 3.975 25.3 27.798 555.46 

 12–14 7.918 3.346 20.1 28.406 571.24 

 16–18 7.742 3.385 34.9 27.614 552.74 

 20–22 7.592 5.037 123.3 27.438 559.31 

4028 GC185 BHB3 0–2 7.409 9.414 457.5 23.883 564.10 

 2–4 7.433 13.225 5296.3 13.223 573.05 

 4–6 7.532 28.631 – 2.260 560.16 

 6–8 7.443 – 10598.0 .0 556.26 

 8–10 7.459 27.117 9756.0 .0 – 

 10–12 7.399 34.952 8390.0 .0 560.84 

 12–14 7.310 – 9175.9 .0 – 

 14–16 7.517 11.617 4759.8 – 560.27 

4029 GC234 GCAT2 0–2 7.812 4.050 0.0 22.345 552.63 

 2–4 7.783 5.655 386.1 22.025 553.81 

 4–6 7.516 6.789 892.1 18.362 550.31 

 6–8 7.468 12.821 791.0 14.294 556.91 

 8–10 7.437 20.076 6090.7 7.368 561.71 

 10–12 7.304 24.289 5073.3 3.824 584.54 

 12–14 – – – 2.338 – 

 14–16 7.390 20.136 2843.5 1.382 – 

 16–18 – – – .0 – 

 18–20 7.366 24.338 2634.5 .0 – 

 22–24 7.509 23.305 3958.4 .0 576.48 

4031 GC234 GCST1 0–2 7.464 4.078 0.0 27.982 558.34 

 2–4 7.625 4.486 0.0 27.702 556.57 

 4–6 7.826 5.231 3.4 27.602 564.75 

 6–8 7.913 4.889 100.1 27.529 – 

 8–10 7.754 3.569 247.5 27.403 553.26 

 10–12 8.015 5.859 87.3 27.355 542.09 

 12–14 7.964 – 113.2 27.486 559.87 

 14–16 8.019 5.048 48.8 27.684 555.40 

 18–20 8.017 7.304 34.9 27.829 555.82 

 22–24 7.870 4.669 39.4 27.959 569.33 

4031 GC234 GCST1 0–2 – – .0 – – 

 2–4 – – .0 – – 

 4–6 – – .0 – – 

 6–8 – – .0 – – 

 8–10 – – .0 – – 

 10–12 – – .0 – – 

 12–14 – – .0 – – 

 14–16 – – 11.2 – – 
 9-10

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

 18–20 – – 42.7 – – 

 20–22 – – 67.6 – – 

4032 GC234 GCAT1 0–2 7.372 3.733 .0 27.899 553.83 

 2–4 7.541 3.158 .0 27.699 557.84 

 4–6 7.829 4.413 .0 27.556 555.81 

 6–8 8.026 4.751 .0 27.405 549.04 

 8–10 8.144 4.865 .0 27.291 542.55 

 10–12 8.132 2.395 .0 27.079 551.90 

 12–14 7.898 4.946 24.3 27.833 597.08 

 14–16 7.675 3.636 98.5 27.374 – 

4036 GC234 GCJT1 0–2 7.324 3.810 .0 27.223 556.27 

 2–4 7.171 3.692 .0 27.352 588.80 

 4–6 7.309 4.551 .0 27.363 557.10 

 6–8 7.334 6.826 .0 28.238 574.86 

 8–10 7.648 4.168 .0 28.484 583.98 

 10–12 7.804 4.034 .0 27.975 573.88 

 12–14 7.585 3.710 52.7 27.577 523.17 

 16–18 7.683 3.963 .0 28.798 564.26 

4037 GC233 BPAT1 0–2 7.306 4.067 .0 28.847 577.69 

 2–4 7.234 3.913 .0 28.873 599.75 

 4–6 7.601 1.793 .0 28.059 546.25 

 6–8 7.771 3.980 .0 28.425 600.49 

 8–10 7.986 3.724 .0 28.977 580.26 

 10–12 7.936 3.276 .0 28.618 567.77 

 12–14 7.878 4.361 .0 25.459 – 

4038 GC233 BPB2 0–2 7.579 5.525 638.4 25.564 641.13 

 2–4 7.555 4.810 811.7 21.759 743.96 

 4–6 7.304 7.833 2175.4 19.127 741.15 

 6–8 7.318 7.900 1901.3 18.092 783.13 

 8–10 7.263 8.307 2565.2 15.220 840.98 

 10–12 7.077 11.975 4656.0 1.774 – 

 12–14 – – 4496.9 – – 

4040 GB425 GBUN1 0–2 7.397 2.285 .0 28.929 585.36 

 2–4 7.414 3.575 .0 28.136 567.02 

 4–6 7.547 .0 27.574 587.10 

 6–8 7.710 4.411 .0 27.302 577.89 

 8–10 7.731 4.008 6.6 27.188 593.64 

 10–12 7.747 4.5 – – 

 12–14 7.436 3.681 250.9 27.806 606.73 

 14–16 7.634 4.034 .0 27.058 575.50 

 16–18 7.641 3.788 17.2 26.450 638.28 

 18–20 – – – – – 

4042 GC185 BHM3 0–2 7.317 4.968 123.7 28.121 587.19 

 2–4 7.503 5.443 300.0 26.184 703.06 

 4–6 7.480 4.415 378.3 26.440 626.02 

 6–8 7.416 6.123 572.9 26.941 – 

 8–10 7.416 4.076 572.9 26.446 593.45 

 10–12 7.411 5.034 481.3 26.042 676.85 
 9-11

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

 14–16 7.532 5.088 355.4 25.979 576.14 

 18–20 7.690 4.340 124.8 28.385 561.10 

4043 GC234 

GCB3 

orange

Beggiatoa

0–2 7.431 6.415 992.6 26.619 565.32 

 2–4 7.395 8.700 2893.3 21.209 558.91 

 4–6 7.400 7.297 3863.6 22.107 – 

 6–8 7.539 6.065 2244.9 24.543 606.71 

 8–10 7.443 8.921 2533.4 22.277 576.81 

 10–12 7.535 7.806 2500.9 21.322 573.50 

 12–14 7.508 7.902 2528.9 20.438 559.46 

 14–16 7.631 4.800 2368.5 21.139 – 

 16–18 7.568 5.638 2006.0 24.489 473.32 

 20–22 7.624 7.286 2079.2 23.413 600.58 

4043 GC234 GCB3 

off mat 0–2 7.501 3.322 .0 25.193 589.99 

 2–4 7.766 4.729 .0 26.121 628.57 

 4–6 7.892 3.705 .0 25.428 601.37 

 6–8 8.037 – .0 25.614 574.05 

 8–10 8.132 4.218 .0 25.186 578.05 

 10–12 8.083 7.117 6.5 26.935 596.32 

 14–16 7.756 4.884 251.6 25.869 567.14 

 18–20 7.547 3.550 409.5 26.958 613.84 

 22–24 7.810 6.342 229.6 26.477 – 

 26–28 7.527 5.211 306.0 27.464 677.01 

4043 GC234 GCB3 

white Beggiatoa 0–2 7.410 6.849 515.7 29.067 558.71 

 2–4 7.614 4.519 1097.0 25.401 567.54 

 4–6 7.356 8.049 1712.9 25.523 586.51 

 6–8 7.468 – – – – 

 8–10 7.538 4.802 1992.2 24.729 605.03 

 10–12 7.250 7.795 2158.6 24.234 554.39 

 12–14 7.487 9.913 2579.9 24.638 573.37 

 14–16 7.532 8.514 2382.6 23.734 561.92 

 16–18 7.525 7.255 2063.2 23.405 553.71 

 18–20 7.545 7.168 1383.7 23.706 556.21 

4044 GC185 BHAT2 0–2 7.204 4.368 .0 28.380 680.18 

 2–4 7.099 4.907 .0 27.605 604.20 

 4–6 7.298 4.452 .0 27.803 637.58 

 6–8 7.571 7.458 .0 27.521 557.04 

 8–10 7.731 5.284 .0 28.358 677.69 

 10–12 7.712 4.256 .0 28.757 597.19 

 12–14 7.895 5.070 .0 28.209 597.50 

 14–16 7.796 5.533 .0 28.482 618.21 

 18–20 7.739 5.186 .0 28.268 – 

 22–24 7.539 3.522 29.0 28.042 627.28 
 9-12

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

4046 GC185 BHM4 0–2 7.287 5.138 1675.5 19.359 634.32 

 2–4 7.528 24.715 7255.3 9.300 613.08 

 4–6 7.462 17.071 3980.9 7.302 565.59 

 6–8 7.437 29.688 10918.2 2.670 567.45 

 8–9 7.552 27.497 4947.9 0.664 665.26 

4047 GC234 GCJT2 0–2 7.917 6.100 427.9 27.659 663.92 

 2–4 7.810 4.535 672.1 26.624 588.61 

 4–6 7.737 5.771 584.3 27.508 595.70 

 6–8 7.579 6.683 1656.5 25.078 575.78 

 8–10 7.884 7.735 1580.2 24.515 588.20 

 10–12 7.797 11.748 1931.8 23.408 595.25 

 12–14 7.858 5.821 1901.3 24.200 623.18 

4047 GC234 GCJT2 16–18 7.804 9.252 1992.9 23.846 620.93 

 20–22 7.493 14.098 4831.7 15.013 612.62 

 22–24 7.629 15.575 4600.6 11.481 738.28 

4048 GC234 GCAT2 0–2 7.585 6.293 0.4 27.984 597.10 

 2–4 7.902 4.818 6.3 27.896 605.84 

 4–6 7.957 6.640 204.7 26.409 902.69 

 6–8 7.604 8.949 2328.0 22.999 577.19 

 8–10 7.380 13.103 4460.1 15.363 597.57 

 10–12 7.453 22.202 9272.3 6.604 661.53 

 12–14 – – – 3.569 – 

 14–16 7.423 24.515 12672.5 1.006 570.93 

 16–18 – – – 0.401 – 

 18–20 7.428 26.595 9641.8 .0 595.71 

 20–22 – – – .0 – 

 22–24 7.413 27.196 7942.1 .0 579.04 

 24–26 7.404 26.900 10205.5 – 595.38 

4053 GB425 GBM2 0–2 7.435 11.637 2921.7 22.525 601.31 

 2–4 7.438 18.448 8012.8 .0 – 

 4–6 7.604 18.561 8808.3 – 608.80 

 6–8 7.564 11.602 7530.4 .0 – 

 8–10 7.486 16.109 – .0 695.04 

 10–12 7.364 4.555 8819.6 .0 – 

 14–16 7.154 14.083 4455.5 .0 951.52 

 18–20 6.978 13.287 3055.3 .0 870.30 

 22–24 6.998 11.662 2541.9 .0 1189.27 

 28–30 7.116 13.137 2940.7 .0 1044.96 

box GC185 BHBXC1 0–2 8.096 3.656 1.3 28.339 570.15 

 2–4 7.933 4.223 .0 27.881 558.15 

 4–6 7.921 3.681 .0 27.689 558.09 

 6–8 7.846 2.609 .0 27.177 549.63 

 8–10 7.881 4.110 .0 27.712 556.50 

 10–12 7.845 4.138 .0 27.238 552.69 

 14–16 7.843 4.126 .0 26.801 547.97 

 18–20 7.775 3.332 .0 26.785 551.69 

 22–24 7.588 4.872 .0 26.588 – 
 9-13

Table 9.1. Porewater pH, carbon, sulfur, and anions. (Continued) 

Dive 

Number 

Site Station Depth 

(cm) 

pH Dissolved

Inorganic

Carbon 

(mM) 

Total 

Dissolved 

Sulfide 

(M) 

Sulfate 

(mM) 

Chloride 

(mM) 

 

box GC185 BHUN2 0–2 7.466 11.382 961.3 23.552 629.35 

 2–4 7.473 15.324 7018.2 12.955 759.90 

 4–6 7.507 18.954 6988.2 11.112 638.97 

 6–8 7.532 24.270 2783.3 7.486 556.55 

 8–10 7.517 28.919 11070.8 .0 565.28 

 10–12 7.473 28.444 9697.2 – 619.89 

 12–14 7.536 26.824 8819.6 .0 658.24 

 14–16 7.546 35.647 11304.9 .0 562.09 

 18–20 7.485 26.122 9907.0 – 611.22 

 20–22 7.486 26.900 12710.4 .0 616.33 

box GC234 GCBX2 0–2 7.367 3.941 .0 29.425 – 

 2–4 7.290 3.598 .0 26.881 – 

 4–6 7.348 4.332 .0 27.206 – 

 6–8 7.441 4.168 .0 27.994 – 

 8–10 7.484 3.834 .0 24.480 603.77 

 10–12 7.492 4.569 .0 25.715 559.29 

 14–16 – – – – – 

 18–20 7.806 4.648 .0 – – 

 22–24 7.952 4.569 .0 – – 

 26–28 7.707 4.411 .0 – – 

box GC234 GCBXC1 0–2 7.907 4.676 .0 27.252 554.19 

 2–4 8.030 577.8 26.047 550.44 

 4–6 7.902 3.740 630.8 26.725 555.21 

 6–8 7.856 4.701 275.8 26.885 556.82 

 8–10 8.026 6.248 320.3 28.017 573.35 

 10–12 7.958 4.137 131.6 27.639 556.79 

 14–16 8.009 4.404 69.1 28.060 566.86 

 18–20 7.897 4.521 218.7 27.551 – 

 22–24 7.870 3.759 206.9 27.420 564.79 

 28–30 7.427 3.184 192.4 27.707 – 

box GC234 GCBXC1 0–2 – – .0 – – 

 2–4 – – – – – 

 4–6 – – 299.9 – – 

 6–8 – – 154.5 – – 

 8–10 – – 304.3 – – 

 10–12 – – 129.0 – – 

 14–16 – – 149.1 – – 

 18–20 – – 293.4 – – 

 22–24 – – 346.4 – – 

 28–30 – – 363.2 – – 
 9-14

As described in the Introduction, hydrogen sulfide and dissolved bicarbonate produced by 

bacterially mediated reduction reactions, Equations (9-3) and (9-4), can be in turn consumed in 

sulfide and carbonate mineral formation. The relationship of these processes determines the 

availability of dissolved sulfide, for example, for use as a reductant in the fixation of CO2 by 

chemolithoautotrophic bacteria such as Beggiatoa and other sulfide oxidizers. Carbonate mineral 

formation within the surface sediment also creates a hard substrate for attached biota such as 

vestimentiferan tubeworms. Because of the central importance of sulfate reduction in these 

processes, most of this discussion describes how reduction rates vary throughout the study area 

and how this variation relates to other measured pore water and sediment properties. 

Limitation of sulfate reductor. As can be seen from reaction (9-3), the extent of sulfate 

reduction is limited by two factors: the supply of metabolizable organic matter and/or seawater 

sulfate. One of the critical differences between seep and normal benthic marine environments is 

the overall abundance of organic matter in the form of liquid hydrocarbons and dissolved 

methane gas. At most stations in which extensive sulfate reduction was observed (stations 

classified as active, see below), gas was also observed freely bubbling from cores during 

recovery. This indicates super saturation under surface conditions. Organic carbon 

concentrations are substantially higher and more variable than elsewhere in the northern Gulf of 

Mexico. Continental shelf and slope transects by Linn and Morse (1991) showed organic carbon 

to vary between 0.3 and 1.5 wt %, with an overall mean of 0.71 ± 0.27 wt %. In contrast, organic 

carbon concentrations here (see Table 9.1) ranged from 0.2 to 13.1 wt %. Organic carbon data 

are presented in Figure 9.1. Figure 9.1A shows the asymmetric frequency distribution of organic 

carbon, in which the greatest frequency of samples contained between 1 and 1.5 wt % organic 

carbon. Mean values for 1997 and 1998 cruises were 4.4 ± 2.6 and 3.9 ± 2.7 wt %, respectively. 

Figure 9.1B also indicates that these sediments are predominantly fine-grained, with the greatest 

frequency having a mass fraction less than 65 m in diameter of approximately 90% (mean 

84±12%). Furthermore, Figure 9.1C also suggests that samples with decreasing grain sizes 

tended to be richer in organic carbon. DOC concentrations vary widely throughout the study area 

(range 5.8-42289 M, mean 10105 ± 7701 M), but do not correlate well with other measured 

parameters. Nutrients concentrations also do not tend to show any obvious relationship to other 

pore water or solid phase data. However, concentrations do tend to be higher at “M” (mussel) 

stations, and ammonia levels at GBM2 are more than an order of magnitude higher than 

elsewhere (3940 versus a mean of 286 M). 

Stations classified by sulfate reduction rates. The variation in the extent and intensity of 

sulfate reduction is large, as quantified both by measured sulfate reduction rates and the relative 

distribution of sulfate versus sulfide. These parameters are used here to distinguish between 

stations that are either active or inactive; they are summarized in Table 9.2. The variation in 

integrated sulfate reduction rates is also shown graphically in Figure 9.2. The choice of these 

terms is somewhat arbitrary, the important distinction being that active areas show clear evidence 

of intense sulfate reduction, either in terms of a high integrated sulfate reduction rate (SRR > 50 

mmoles-sulfate m-2 day-1), or in terms of depleted sulfate and calcium and elevated 

concentrations of sulfide and DIC. By comparison, inactive areas lack evidence of significant 
 9-15

Figure 9.1. Organic carbon and grain size distributions. (A) Organic carbon frequency distribution. Note here that 

the distribution for both years is similar and distinctly asymmetric, with the greatest number of samples 

falling between 1 and 1.5 wt % organic carbon. Mean values for 1997 and 1998 cruises were 4.4±2.6 

and 3.9±2.7 wt %, respectively. These sediments are thus richer in organic carbon than is typical: Lin 

and Morse (1991) reported a mean of 0.71±0.27 wt % for Mississippi River delta and adjacent 

continental shelf and slope sediments. (B) Grain size distribution. These sediments are predominantly 

fine-grained. The largest number of sediment samples have a mass fraction less than 65 m in diameter 

of ~90%, with a mean of 84±12%. (C) Relationship of organic carbon and grain size distributions. As 

sediments become more fine-grained (increasing mass fraction of grains < 65 m in diameter), organic 

carbon content increases as well. This may reflect dilution of seep sediment by influx of coarsergrained material relatively poor in organic carbon. 
 9-16

Table 9-2. Summary of sulfate reduction 

Dive Site Station 

integrated 

SRR3

mmoles/m2/day 

Excess 

Chloride 

>10% 

calcium 

depletion 

>10% 

sulfate 

depletion 

DIC 

> 4 mM 

H2S 

>1000 M 

4043 GC234 GCB3 off mat — 

2876 GC233 BPR2 — 

box GC234 GCBX2 — 

box GC185 BHBC2 — 

box GC234 GCBXC1 — 

4040 GB425 GBUN1 — 

4037 GC233 BPAT1 — 

2873 GC185 BHAT1 13.8 

4025 GC185 BHAT1 — 

4027 GC185 BHST1 14.4 

2888 GC234 GCAT1 16.5 

4032 GC234 GCAT1 — 

2875 GC185 BHST2 19.7 

box GC185 BHUN2 (1997) 20.0 

2875 GC185 BHUN1 20.7 

2871 GC234 GCST1 21.3 

4042 GC185 BHM3 — 

2873 GC185 BHM4 22.1 

2874 GC185 BHST1 23.0 

box GC185 BHBXC1 — 

4031 GC234 GCST1 28.6 

box GC185 BHBC2 29.8 

2891 GC185 BHAT2 32.2 

4044 GC185 BHAT2 — 

inactive 

2883 GB425 GBM1 55.0 

2882 GB425 GBM2 

(GBB1@GBM2) 1541.22 x 2 x x 2 

2885 GB425 GBM2 — x 2 x x 2 

4053 GB425 GBM2 348.0 x 2 x x 2 brines 

4038 GC233 BPB2 57.7 x 2 x x 2 

4028 GC185 BHB3 8.2 x x x x 

4043 GC234 GCB3 

white Beggiatoa 13.7 x x x x 

2880 GC234 GCAT1 

(GCB1@GCAT1) 13.8 x x x x 

4047 GC234 GCJT2 23.4 x x x x 

active 

4046 GC185 BHM4 6.0 x x x x 

4043 GC234 GCB3 

orange Beggiatoa 244.3 x x x x 

box GC185 BHUN2 (1998) 374.5 x x x x high 

4048 GC234 GCAT2 458.5 x x x x 

4029 GC234 GCAT2 1585.21 x x x x 

extreme 2886 GC234 GCAT2 1635.9 x x x x 

2872 GC234 GCAT2 2452.5 x x x x 

Notes 

1

 Indicates a maximum value; incubations of shorter duration gave lower integrated rates. 

2

 Sulfate reduction (and resulting sulfide accumulation and calcium depletion) associated with brines is limited to 

the mixing zone between seawater and the brine end member. Brine end member is Na-Ca-Cl in composition, and 

contains essentially no sulfide or sulfate. 3

 Integrated rates were calculated by summing the area under the sulfate reduction rate curve with respect to depth. 

Missing data (–) indicate that no sulfur-35 core was available, or no extraction was done due to negligible sulfide 

concentrations observed in porewater sample core. 
 9-17

Figure 9.2. Characterization of chemosynthetic sample stations by sulfate reduction. Stations are categorized 

as active or inactive according to overall evidence of sulfate reduction, either in terms of a high 

integrated sulfate reduction rate (SRR > 50 mM-sulfate m-2 day-1) or depleted sulfate and calcium 

and elevated concentrations of sulfide and DIC. Inactive areas lack evidence of significant sulfate 

reduction in terms of low integrated reduction rates (<10 to 50 mmoles-sulfate m-2 day-1) and 

porewater compositions that are only slightly modified from normal seawater (sulfide 

concentrations less than ~250 M, negligible depletion of sulfate, DIC concentrations less than 

two times seawater). Although not all active stations showed high measured sulfate reduction rates, 

they clearly exhibit significant production of sulfide at the expense of sulfate. Active-high stations 

record integrated rates from 50 to 1000 mM-sulfate m-2 day-1), and rates at active-extreme stations 

exceed >1000 mM-sulfate m-2 day-1. Brines stations may show high measured sulfate reduction 

rates confined to the shallow interval where brine mixing is minimal. 
 9-18

sulfate reduction. Inactive areas have low integrated reduction rates (<10 to 50 mmoles-sulfate 

m-2 day-1) and pore water compositions that are only slightly modified from normal seawater. 

Furthermore, sulfide concentrations at inactive stations are typically less than approximately 250 

M with negligible depletion of sulfate, DIC concentrations that are less than two times 

seawater, and the chloride and major ion compositions that are more or less invariant with 

respect to standard seawater. This group comprises the largest number of stations, and with the 

possible exception of bacterial mats, does not distinguish itself biologically in any obvious way. 

According to the station descriptors (AT, JT, ST, M, etc.), it is variously dominated by 

tubeworms (adult, juvenile, senescent) and mussels. Although the discrimination between active 

and inactive is an operational one, sulfate reduction rates measured at inactive stations agree with 

the averages of (non-seep) rates measured in the northern Gulf of Mexico (Table 9.3). 

Active stations include BHUN2 (1998 core), BHM4 (dive 4046), BHB3, GCAT1 (dive 2880), 

GCAT2, GCB3, and GCJT2. Note that although not all active stations showed high measured 

sulfate reduction rates (SRR), they clearly exhibit significant production of sulfide at the expense 

of sulfate. In addition, integrated rates at certain active sites are so high as to merit further 

division of this category to high (50 to 1000 mmoles-sulfate m-2 day-1) and extreme (>1000 

mmoles-sulfate m-2 day-1). Active-high stations thus include BHUN2 (1998 core), GCAT2-dive 

4048 and GCB3 (orange Beggiatoa mat); active-extreme stations are limited to GCAT2 (dives 

2872, 2886, 4029). The most important station in terms of defining the relationship between SRR 

and other measured parameters is GCAT2. In addition, SRR experiments using cores recovered 

during dive 4029 used variable incubation times of 4.1, 8.5, and 13.3 hours and gave integrated 

rates of 1585, 86, and 84 mmols/m2

/d, respectively. This behavior of increasing apparent rate 

with decreasing reaction time suggests that these rates probably represent minimum values, 

although it is also possible (but unlikely) that this variation reflects lateral heterogeneity as well. 

These rates probably constitute the highest ever measured in marine sediments. Comparison with 

data compiled from various sources in Table 9.3 indicates that they exceed the highest rates 

measured in the Gulf of Mexico (which agree well with those of inactive stations) by at least two 

orders of magnitude (Lin and Morse 1991). In addition, certain stations show clear evidence of 

mixing between seawater and a sodium-calcium-chloride brine end member, and these stations 

(GBM2 and BPB2) form a separate category in Table 9.2. Sulfate reduction in these stations may 

also be very high but is limited to the shallow interval just below the sediment-water interface. 

Brine mixing is only locally important and is discussed below. 

The observed co-variations in sulfate, DIC, sulfide, and calcium at active sites can be understood 

by recognizing the essential stoichiometric relationships among these components. As sulfate 

reduction proceeds according to Equations (9-3) or (9-4), DIC and sulfide are produced at the 

expense of sulfate and organic material. These relationships may be used to calculate an apparent 

extent of sulfate reduction, by comparing the amount of so-called reduced sulfate with excess 

DIC. Reduced sulfate is calculated as the difference between an initial seawater concentration of 

sulfate and the sulfate concentration observed in pore fluid. This difference is normalized to 

chloride to remove any differences in overall salinity. If there is no sulfate reduction, the reduced 

sulfate value will be zero. If sulfate reduction is completed, the reduced sulfate value will be 

28.898 mM/L (the standard concentration at a salinity of 35 psu). Excess DIC concentrations are 
9-19 

Table 9.3. Major cations. 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

2871 GC234 GCST1 0-2 .0 12.266 59.447 12.899 498.75 0.1100 19.3 

 2-4 2.96 10.927 50.937 12.419 454.16 0.0946 21.1  4-6 .0 9.033 40.883 9.095 354.45 0.0637 21.3 3 6-8 .0 10.038 49.873 11.841 454.33 0.0839 19.7 2 10-12 .0 16.224 50.751 12.139 452.29 0.0884 19.5  12-14 .0 9.983 50.349 11.671 444.70 0.0855 21.2  14-16 .0 10.061 49.724 11.722 441.31 0.0874 –  16-18 3.02 8.968 40.087 10.700 357.56 0.0798 –  18-20 .0 10.140 50.239 11.914 449.33 0.0842 37.9 3

2873 GC235 BHAT1 0-2 4.20 10.758 49.410 11.493 437.40 0.0998 22.9 

 2-4 .0 9.804 48.717 10.695 431.25 0.0819 24.3  4-6 .0 9.903 49.542 11.092 438.38 0.0817 17.6  6-8 .0 10.798 49.183 10.944 428.04 0.0837 25.0  8-10 .0 10.229 48.707 11.543 436.93 0.0883 21.0  10-12 .0 9.629 49.411 10.862 448.46 0.0788 21.7  12-14 .0 9.539 47.497 10.663 424.45 0.0783 19.3  14-16 .0 9.877 49.202 11.272 438.15 0.0825 17.8 3

2873 GC235 BHM4 0-2 4.58 9.175 54.916 12.329 505.02 0.0844 20.9 

 2-4 5.05 7.782 50.042 10.992 450.41 0.0660 22.0  4-6 .0 1.560 52.628 11.983 474.26 0.0178 16.5  6-8 6.67 5.312 53.839 13.374 495.71 0.0465 4.5  8-10 4.25 4.803 48.034 11.509 445.84 0.0419 4.1 4 10-12 .0 0.620 68.180 16.112 642.92 0.0000 2.4 5 12-14 4.78 5.157 49.042 12.765 457.27 0.0486 6.4  14-16 5.75 5.473 49.610 12.476 462.03 0.0505 18.6  16-18 5.43 4.357 48.101 12.722 445.97 0.0504 2.8  18-20 4.17 2.664 51.575 13.242 479.82 0.0327 2.1 

2874 GC235 BHST1 0-2 3.06 11.291 54.740 12.747 471.77 0.0850 19.3 

 2-4 2.68 10.346 48.773 11.933 440.29 0.0894 21.1  4-6 .0 9.214 48.138 11.154 443.04 0.0773 –  6-8 .0 9.656 50.020 11.379 450.46 0.0733 15.8  8-10 .0 9.246 48.528 10.876 437.34 0.0762 17.6  10-12 .0 9.054 47.697 11.492 437.31 0.0765 17.6  12-14 4.19 9.710 51.365 12.257 477.49 0.0834 20.8  14-16 .0 9.978 50.451 10.454 440.29 0.0823 20.6  16-18 .0 8.904 49.718 11.935 454.79 0.0761 6.1 
9-20 

Table 9-3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

2875 GC235 BHST2 0-2 .0 9.898 49.156 11.102 433.33 0.0797 18.2 

 2-4 .0 9.550 47.743 10.723 428.57 0.0783 21.4  4-6 .0 10.191 48.843 11.511 441.20 0.0875 19.4  6-8 .0 8.041 41.856 8.909 379.19 0.0612 19.2  8-10 2.57 10.402 48.816 12.009 438.95 0.0914 21.1  10-12 .0 10.011 49.797 11.269 445.67 0.0857 19.4 

2875 GC235 BHUN1 0-2 .0 9.909 50.012 10.485 446.22 0.0838 15.8 2

 2-4 .0 10.083 49.807 11.072 446.20 0.0866 12.3  4-6 .0 9.813 50.060 11.451 452.05 0.0812 12.2  6-8 .0 9.689 49.048 11.415 439.74 0.0808 12.3  8-10 .0 9.757 48.756 11.670 442.13 0.0825 19.3  10-12 2.33 10.583 49.674 12.809 454.84 0.0932 13.8  12-14 .0 9.769 48.887 11.907 444.79 0.0816 15.9  14-16 .0 9.759 48.236 12.086 447.71 0.0828 14.1  16-18 .0 9.701 48.827 11.261 431.86 0.0772 14.0  18-20 .0 9.724 48.606 11.825 441.30 0.0820 12.2  22-24 .0 9.848 48.041 11.705 434.99 0.0821 36.3  26-28 .0 9.965 49.684 12.541 454.58 0.0851 21.2 

2876 GC233 BPR2 0-2 .0 9.514 47.688 10.648 423.96 0.0785 21.1 

 2-4 .0 8.598 43.345 9.860 385.70 0.0755 14.7  4-6 .0 8.654 43.163 10.541 393.69 0.0756 15.8  6-8 .0 8.488 41.899 10.483 386.95 0.0738 16.8  8-10 .0 9.393 45.633 11.381 421.72 0.0813 14.0 2 10-12 2.23 10.289 48.801 12.492 446.92 0.0951 15.9 

2880 GC234 GCB1@GCAT1 0-2 .0 10.336 51.522 11.309 466.32 0.0883 15.9 3

 2-4 .0 9.562 49.290 11.328 450.56 0.0816 14.0 2 4-6 .0 9.615 48.501 11.534 432.61 0.0893 0.8  6-8 .0 8.981 49.417 12.061 451.85 0.0838 4.2  8-10 .0 8.293 47.883 10.957 431.80 0.0794 1.8  10-12 .0 8.811 49.134 11.243 441.55 0.0852 3.7  12-14 2.66 9.274 49.326 12.593 446.48 0.0946 1.6 2 14-16 .0 8.944 48.521 11.491 435.75 0.0889 3.4  16-18 .0 8.093 47.970 11.126 433.53 0.0776 8.8  18-20 .0 7.891 47.382 11.242 427.08 0.0778 – 2 22-24 .0 8.373 48.096 11.529 435.59 0.0849 13.9 
9-21 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

2882 GB425 GBB1@GBM2 0-2 .0 12.851 44.241 10.006 504.87 0.1420 3.2 

 2-4 5.91 18.180 40.523 12.236 642.02 0.2377 17.9  4-6 87.35 27.644 38.123 14.160 894.70 0.6063 18.2  6-8 1555.48 35.978 36.776 15.683 1070.21 0.9585 21.1  8-10 .0 43.669 36.873 17.428 1267.27 1.2258 18.0  10-12 .0 43.939 35.419 17.059 1292.72 1.2119 17.9  12-14 .0 44.246 36.038 17.532 1359.84 1.2011 –  14-16 .0 43.527 36.029 17.618 1403.87 1.1662 –  16-18 .0 46.415 35.227 18.934 1352.86 1.2439 –  18-20 .0 49.776 37.044 20.277 1437.00 1.3139 –  22-24 .0 48.540 36.604 20.163 1434.48 1.2886 –  26-28 .0 45.231 36.664 22.988 1417.89 1.2349 – 

2883 GB425 GBM1 0-2 .0 10.432 50.306 11.111 456.16 0.0876 12.1 

 2-4 – – – – – – 12.3  4-6 2.92 11.040 49.558 11.742 442.40 0.1014 14.0  6-8 .0 10.398 50.188 11.683 461.64 0.0923 –  8-10 2.71 11.103 49.388 12.053 452.64 0.1096 1.7  10-12 .0 10.887 49.288 11.548 460.28 0.1108 –  12-14 .0 11.114 49.719 11.979 479.38 0.1098 17.4  14-16 .0 11.455 48.027 11.796 464.76 0.1207 12.3  16-18 3.51 11.969 49.972 13.270 473.06 0.1174 – 

2885 GB425 GBM2 0-2 .0 16.302 45.588 9.460 615.22 0.1147 16.3 

 2-4 40.99 27.457 38.365 8.914 982.14 0.4249 19.6 2 4-6 .0 33.038 32.922 9.270 1253.33 1.0885 17.9  6-8 .0 37.910 34.763 10.985 1363.76 1.2961 17.8  8-10 .0 34.624 31.983 9.641 1348.74 1.1705 –  10-12 – – – – – – –  12-14 .0 37.636 31.817 10.201 1335.30 1.1885 –  14-16 – – – – – – 50.8  16-18 – – – – – – 16.4 

2886 GC234 GCAT2 0-2 3.30 9.070 49.006 11.578 445.33 0.0884 16.6 

 2-4 .0 7.819 47.891 10.331 444.55 0.0727 18.6  4-6 .0 8.371 47.831 10.734 436.58 0.0796 18.4  6-8 .0 4.916 47.836 11.087 445.10 0.0684 16.4  8-10 .0 5.034 47.636 11.181 446.64 0.0684 18.7  10-12 .0 3.976 45.752 10.626 431.99 0.0638 – 
9-22 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

 12-14 .0 2.554 48.955 11.916 473.15 0.0601 20.7  14-16 4.90 5.098 47.011 10.832 439.90 0.0658 17.1  16-18 5.24 5.107 46.166 10.730 431.02 0.0666 –  18-20 5.37 2.312 48.279 11.505 451.28 0.0636 3.5  20-22 4.77 5.213 47.774 11.546 451.02 0.0680 5.6 2

2888 GC234 GCAT1 0-2 .0 9.285 50.285 9.981 452.12 0.0786 12.3 

 2-4 .0 9.703 48.879 10.963 439.56 0.0848 14.5  4-6 .0 9.768 49.175 10.816 435.48 0.0867 16.5  6-8 .0 9.236 47.526 10.195 425.45 0.0810 17.2  8-10 .0 9.807 47.985 11.175 441.86 0.0833 –  12-14 – – – – – – – 

2891 GC235 BHAT2 0-2 .0 9.587 47.764 10.216 425.22 0.0831 16.4 4

 2-4 .0 9.484 46.492 10.269 419.95 0.0805 29.6 5 4-6 .0 9.788 47.961 11.014 434.47 0.0834 18.5  6-8 .0 9.800 48.183 11.175 438.94 0.0849 –  8-10 .0 9.567 48.184 11.546 445.21 0.0829 18.2  10-12 .0 9.691 48.827 11.720 451.38 0.0845 –  14-16 .0 9.840 49.745 12.253 445.53 0.0866 – 

box GC235 BHBC2 0-2 .0 10.043 49.930 12.121 454.87 0.0890 12.3 

 2-4 .0 9.717 49.780 11.018 451.77 0.0861 13.3  4-6 .0 9.800 49.264 11.853 457.31 0.0840 14.1  6-8 .0 9.910 49.466 12.072 450.85 0.0857 13.9  8-10 .0 10.356 48.580 12.271 438.90 0.0926 14.2  10-12 .0 9.671 47.740 11.881 434.04 0.0831 12.3  12-14 .0 9.619 47.429 11.848 439.58 0.0826 12.1  14-16 .0 9.242 46.354 11.068 422.76 0.0779 12.8  16-18 .0 9.837 46.455 11.619 421.74 0.0865 12.2  18-20 .0 9.028 46.899 10.694 436.74 0.0724 12.2  22-24 .0 9.710 46.811 11.541 422.85 0.0852 12.3 

4025 GC185 BHAT1 0-2 0.43 10.048 51.486 11.010 455.74 0.0908 .0 

 2-4 0.36 10.206 51.250 11.129 455.07 0.0900 36.7  4-6 0.32 10.283 51.701 11.266 456.24 0.0916 23.5  6-8 0.37 10.596 52.553 11.459 469.44 0.0922 24.1  8-10 0.34 10.704 52.664 12.105 469.75 0.0964 33.9 4 12-14 0.31 10.689 52.202 11.829 471.96 0.0934 21.2 
9-23 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

4027 GC185 BHST1 0-2 0.49 10.279 52.200 11.635 460.63 0.0914 .0 

 2-4 0.39 9.945 50.716 11.032 462.76 0.0853 25.8 0 4-6 0.34 9.835 49.776 11.271 454.36 0.0875 15.5  6-8 0.40 10.194 51.384 11.715 459.26 0.0883 29.3 0 8-10 0.39 9.936 50.143 11.493 458.54 0.0897 12.0  10-12 0.37 9.984 50.289 11.655 460.29 0.0900 18.9  12-14 0.41 10.120 51.265 11.766 469.76 0.0904 14.9  16-18 0.45 9.868 49.829 11.741 461.51 0.0884 9.8  20-22 0.51 9.838 49.890 11.961 463.91 0.0905 .0 

4028 GC185 BHB3 0-2 0.86 9.572 51.405 11.009 462.13 0.0871 6.3 

 2-4 1.18 6.908 50.066 11.280 458.85 0.0759 13.8 5 4-6 1.15 3.811 48.544 11.083 444.85 0.0599 11.5  6-8 0.24 0.880 47.560 11.110 445.40 0.0113 12.0  8-10 1.41 1.930 48.112 11.304 448.33 0.0392 24.1  10-12 2.71 2.648 48.010 11.414 462.89 0.0531 9.8  12-14 3.43 3.795 46.488 12.117 467.02 0.0642 3.4 0 14-16 5.71 3.999 44.310 10.834 401.62 0.0568 13.2 3

4029 GC234 GCAT2 0-2 0.39 9.866 47.820 11.362 428.83 0.0880 11.5 

 2-4 0.38 9.791 48.047 11.311 434.19 0.0872 10.9  4-6 0.04 9.497 48.010 11.470 430.84 0.0858 10.9  6-8 0.42 8.754 47.675 11.493 443.47 0.0862 2.3  8-10 0.08 0.889 47.784 11.484 440.89 0.0099 8.0  10-12 0.13 1.779 47.605 11.069 438.19 0.0321 13.2  14-16 0.54 5.404 47.850 11.237 439.17 0.0751 13.2  18-20 0.69 5.340 48.274 11.310 443.94 0.0731 19.6 3 22-24 1.04 4.297 46.003 12.026 436.70 0.0708 24.2 2

4031 GC234 GCST1 0-2 0.42 10.266 50.697 10.483 454.25 0.0857 18.4 

 2-4 0.52 10.186 51.323 10.733 458.79 0.0881 11.5  4-6 0.36 9.996 50.455 11.511 456.64 0.0869 13.8 0 6-8 0.29 10.113 51.329 11.135 472.87 0.0886 12.7  8-10 0.24 10.117 52.117 10.708 469.83 0.0846 – 2 10-12 0.26 10.198 51.730 11.593 470.81 0.0893 9.2  12-14 0.27 9.926 50.331 11.746 462.82 0.0928 12.1 0 14-16 0.24 10.201 51.710 11.560 470.99 0.0900 14.4 0 18-20 0.26 10.308 52.533 11.429 476.86 0.0894 19.0 0 22-24 0.26 10.217 50.931 11.463 467.39 0.0886 12.7 2
9-24 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

4032 GC234 GCAT1 0-2 0.34 10.257 52.030 10.910 464.37 0.0904 26.5 

 2-4 0.35 10.094 51.762 11.334 469.12 0.0903 21.3  4-6 0.36 10.125 52.106 11.335 468.05 0.0886 12.1 0 6-8 0.27 9.859 51.116 11.821 467.56 0.0908 16.1 0 8-10 0.25 9.984 51.463 11.680 462.34 0.0884 25.3  10-12 0.26 9.691 50.047 11.575 461.36 0.0895 12.1  12-14 0.32 10.109 52.322 12.301 485.29 0.0939 –  14-16 0.30 10.042 51.308 11.721 466.50 0.0906 15.5 0

4036 GC234 GCJT1 0-2 0.37 10.693 53.782 11.436 483.89 0.0942 .0 

 2-4 2.60 10.458 52.929 11.349 473.19 0.0909 .0 0 4-6 0.23 10.269 51.693 11.567 473.23 0.0910 .0 0 6-8 0.21 10.458 52.551 11.955 482.74 0.0939 .0 0 8-10 0.23 10.542 52.779 12.307 482.68 0.0958 .0 2 10-12 0.39 10.543 53.237 11.872 482.38 0.0932 .0  12-14 0.03 10.329 52.174 11.754 472.72 0.0921 .0  16-18 0.42 10.074 51.173 11.801 464.06 0.0916 .0 

4037 GC233 BPAT1 0-2 0.44 10.458 53.381 12.067 484.65 0.0966 42.6 0

 2-4 0.53 10.419 53.200 12.056 479.51 0.0966 35.7  4-6 0.45 10.245 51.931 11.884 470.11 0.0937 16.1  6-8 0.70 10.308 51.624 12.376 476.49 0.0977 43.2  8-10 0.59 12.430 52.600 12.203 475.57 0.0974 5.8  10-12 4.89 12.702 52.697 12.210 486.33 0.0976 12.1 3 12-14 9.04 11.879 47.586 10.870 423.73 0.0861 5.8 2

4038 GC233 BPB2 0-2 13.40 14.272 47.895 11.272 436.99 0.0911 0.2 

 2-4 3.16 12.675 49.499 11.374 585.82 0.2030 0.5 0 4-6 5.48 14.079 48.068 11.138 643.65 0.2588 0.6 0 6-8 3.41 13.619 47.534 10.319 658.97 0.2663 11.8  8-10 2.34 13.370 45.540 10.810 712.97 0.3092 .0 0 10-12 11.86 17.016 44.561 10.052 870.64 0.4048 .0  12-14 18.75 15.877 41.796 10.488 842.05 0.4377 .0 

4040 GB425 GBUN1 0-2 5.98 11.153 52.741 11.172 465.14 0.0979 1.2 0

 2-4 3.34 10.441 50.022 11.266 453.11 0.0949 3.0  4-6 2.57 10.816 51.352 11.138 467.74 0.0837 2.4 0 6-8 5.22 10.888 50.541 10.725 469.78 0.0952 0.6  8-10 0.27 10.684 51.714 11.663 477.23 0.1008 1.2  12-14 4.85 11.138 51.467 11.087 480.47 0.1035 .0 
9-25 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

 14-16 3.93 10.738 51.163 11.984 471.42 0.1138 7.7 0 16-18 3.46 11.174 52.819 12.644 510.42 0.1199 4.7 0

4042 GC185 BHM3 0-2 2.00 11.108 53.556 11.511 470.73 0.0923 1.8 0

 2-4 1.19 10.831 53.795 11.987 479.26 0.0945 .0  4-6 5.07 10.916 52.412 11.225 467.29 0.0880 3.6  6-8 2.97 11.249 54.901 12.202 490.04 0.0959 4.1  8-10 1.53 10.569 52.482 11.467 462.72 0.0880 2.4  10-12 2.43 10.905 53.483 11.525 471.98 0.0904 1.8 0 14-16 2.22 11.187 54.530 12.347 483.82 0.0937 .0  18-20 1.66 10.490 51.808 11.889 465.71 0.0915 3.6 

4043 GC234 GCB3 

orange Beggiota 0-2 2.47 10.597 51.965 11.301 473.29 0.0943 0.6 

 2-4 1.31 7.185 37.454 8.448 342.87 0.0669 .0 

 4-6 2.55 9.931 51.278 11.615 471.73 0.0923 0.6  6-8 3.03 10.288 52.800 11.548 481.76 0.0874 0.6  8-10 1.62 9.484 51.620 11.512 467.92 0.0885 0.6  10-12 1.88 9.021 50.048 11.536 466.00 0.0885 .0  12-14 1.65 8.841 50.386 10.576 458.04 0.0810 5.9  14-16 0.87 8.630 50.276 11.221 454.32 0.0863 2.4  16-18 1.07 9.343 51.276 11.629 467.35 0.0888 3.0  20-22 1.29 9.119 51.097 11.701 467.76 0.0900 .0 

4043 GC234 GCB3 off mat 0-2 0.95 10.191 51.860 11.401 465.30 0.0927 0.6 

 2-4 1.82 10.649 53.293 12.114 484.81 0.0937 .0  4-6 1.41 10.436 51.866 11.981 476.23 0.0927 0.6 0 6-8 1.34 10.287 50.980 11.805 463.63 0.0910 4.7  8-10 1.90 10.076 49.475 11.866 452.05 0.0895 1.2 0 10-12 0.19 11.067 53.147 12.039 476.96 0.0923 .0  14-16 2.86 10.726 49.928 10.856 456.24 0.0845 12.0 2 18-20 2.52 11.113 51.802 11.170 474.08 0.0885 .0  22-24 1.60 10.453 49.184 11.426 453.11 0.0853 .0  26-28 1.98 10.827 50.186 11.893 465.38 0.0935 2.4 

4043 GC234 GCB3 white Beggiota 0-2 1.56 10.206 50.937 10.412 457.18 0.0874 0.6 

 2-4 1.51 9.931 50.568 11.114 457.18 0.0893 1.2  4-6 1.83 10.685 54.034 11.717 489.43 0.0947 .0 
9-26 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

 8-10 1.01 9.841 50.819 11.363 463.36 0.0901 .0  10-12 1.32 9.881 51.459 11.448 466.54 0.0916 .0  12-14 0.96 9.619 50.195 11.793 468.48 0.0932 .0  14-16 1.14 9.635 51.045 11.963 466.70 0.0940 1.8  16-18 0.84 9.794 51.323 11.570 468.83 0.0913 1.2  18-20 0.83 9.852 50.806 11.589 463.06 0.0929 1.2 

4044 GC185 BHAT2 0-2 1.81 10.725 53.079 11.209 474.85 0.0970 3.5 

 2-4 1.46 10.678 52.721 11.747 469.12 0.0945 78.0  4-6 0.95 10.344 51.343 11.981 471.16 0.0955 17.0  6-8 0.83 9.933 49.506 12.118 467.56 0.0949 5.3  8-10 0.92 10.350 51.700 13.289 485.29 0.1027 5.3  10-12 1.53 10.451 51.919 12.511 485.23 0.0985 7.0  12-14 1.17 10.214 51.392 12.472 480.93 0.0979 2.3 2 14-16 1.31 10.271 51.344 12.594 483.37 0.0983 2.9  18-20 0.95 10.299 52.044 12.626 491.52 0.1000 16.4 2 22-24 1.27 10.410 51.871 12.718 488.40 0.0989 0.6 

4046 GC185 BHM4 0-2 1.83 8.512 43.124 9.302 376.48 0.0754 .0 0

 2-4 16.66 9.773 54.831 11.195 479.80 0.0838 0.6  4-6 45.61 5.922 49.136 11.324 445.34 0.0757 1.2 4 6-8 49.66 7.903 49.961 11.790 445.70 0.0795 2.9 0 8-9 41.63 7.862 50.542 11.717 460.45 0.0733 – 

4047 GC234 GCJT2 0-2 1.61 10.278 54.068 11.247 478.59 0.0923 .0 0

 2-4 1.46 9.914 53.107 11.422 471.50 0.0904 2.3  4-6 1.74 10.176 53.774 10.938 478.40 0.0885 1.8  6-8 1.18 9.911 51.674 11.581 471.26 0.0896 3.5  8-10 1.12 9.572 51.406 11.809 474.00 0.0893 3.5  10-12 1.07 10.203 54.952 12.477 505.37 0.0947 .0  12-14 1.36 6.092 32.508 7.423 294.98 0.0546 2.3  16-18 1.28 9.485 51.918 12.137 482.48 0.0900 .0  20-22 1.41 7.600 50.955 12.146 476.12 0.0857 .0 0 22-24 1.35 3.975 51.813 12.550 486.24 0.0718 13.5 0

4048 GC234 GCAT2 0-2 1.59 9.941 49.374 10.289 438.03 0.0864 85.2 

 2-4 1.64 10.692 55.105 12.135 495.24 0.0953 85.2 2 4-6 1.65 11.410 59.292 13.333 534.08 0.1033 0.7 0 6-8 1.58 9.528 51.431 11.771 470.37 0.0898 8.2  8-10 2.81 9.272 52.179 11.235 468.60 0.0851 71.0 
9-27 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

 10-12 3.72 7.474 51.978 11.850 475.45 0.0839 4.1 0 14-16 4.77 6.832 50.123 11.856 463.74 0.0819 4.1 0 18-20 4.56 6.319 50.558 11.615 467.31 0.0778 2.9  22-24 5.24 5.925 49.739 11.498 462.35 0.0751 2.3 0 24-26 4.77 5.765 49.491 11.659 465.11 0.0738 1.8 

4053 GB425 GBM2 0-2 5.54 8.434 52.338 12.180 469.85 0.0958 .0 2

 2-4 18.53 5.493 51.487 12.012 492.58 0.0936 7.0 0 4-6 7.45 4.436 48.544 11.172 490.60 0.0850 1.8  6-8 9.70 5.604 48.517 11.645 523.99 0.1190 2.9 0 8-10 10.86 6.632 54.697 13.483 590.95 0.1384 1.8 0 10-12 2.30 1.686 51.639 12.894 622.34 0.0380 4.7  14-16 91.55 11.550 44.662 12.886 809.55 0.4630 21.7  18-20 .0 17.772 38.667 11.693 920.12 0.6621 1.8 0 22-24 .0 21.495 41.126 11.571 972.91 0.6626 1.8  28-30 .0 15.510 43.155 12.972 898.18 0.5602 2.3 2

box GC185 BHBXC1 0-2 0.46 10.072 51.256 11.378 471.04 0.0897 2.3 0

 2-4 0.42 9.916 50.333 11.559 462.87 0.0888 21.8  4-6 0.41 10.123 52.100 11.437 461.04 0.0903 7.5  6-8 0.45 9.834 50.201 11.407 449.05 0.0873 18.4 0 8-10 0.45 10.045 51.220 11.691 457.40 0.0897 6.9  10-12 0.50 10.130 51.295 11.650 459.94 0.0901 38.4  14-16 0.52 9.940 50.136 11.404 453.34 0.0879 28.1  18-20 0.46 9.704 49.802 11.814 456.90 0.0893 18.4 0 22-24 0.43 9.820 50.471 11.340 457.26 0.0888 37.3 0

box GC185 BHUN2 0-2 2.19 8.923 51.911 11.877 476.18 0.0877 .0 0

 2-4 1.64 6.008 56.631 12.311 513.07 0.0765 1.8 0 4-6 1.52 6.005 50.655 11.782 465.25 0.0757 .0  6-8 1.56 5.666 48.951 12.838 503.52 0.0795 .0  8-10 1.77 5.999 49.047 11.297 448.02 0.0713 8.2  10-12 2.35 6.784 50.115 12.435 471.76 0.0799 .0 0 12-14 1.80 2.320 49.712 12.117 469.89 0.0586 .0  14-16 2.74 1.732 47.447 12.006 460.72 0.0562 0.6 0 18-20 5.23 4.514 50.556 14.999 568.08 0.0785 .0  20-22 2.26 2.065 47.517 11.506 436.55 0.0482 2.9 0
9-28 

Table 9.3. Major cations. (continued) 

Dive 

Number 

Site Station Depth

(cm) 

Ba 

(M) 

Ca 

(mM) 

Mg 

(mM) 

K 

(mM) 

Na 

(mM) 

Sr 

(mM) 

Fe

(M)

M

(

box GC234 GCBX2 0-2 1.99 11.313 56.913 11.969 508.61 0.1040 2.3 2

 2-4 4.00 10.474 52.664 11.533 484.48 0.0995 10.6 0 4-6 2.16 10.109 52.115 11.593 475.58 0.0941 3.5 5 6-8 1.81 10.251 54.452 12.571 509.25 0.1018 18.8  8-10 4.16 10.044 53.576 11.779 483.37 0.0950 20.5 3 10-12 1.77 9.554 52.193 11.215 485.97 0.0879 21.7  14-16 – – – – – – –  18-20 0.49 8.699 52.525 11.702 471.74 0.0884 4.6  22-24 0.57 8.799 53.194 12.508 494.34 0.0936 7.5  26-28 – – – – – – 1.2 

box GC234 GCBXC1 0-2 0.52 9.972 51.736 11.558 467.42 0.0911 9.2 0

 2-4 0.27 9.545 50.612 11.920 462.09 0.0903 9.8  4-6 0.24 9.576 50.454 11.885 463.62 0.0892 17.8 0 6-8 0.21 9.629 50.250 11.652 460.11 0.0889 9.2 0 8-10 0.22 10.023 51.706 12.215 480.79 0.0918 .0 0 10-12 0.21 9.823 50.446 12.129 466.83 0.0909 10.4 0 14-16 0.19 10.064 51.824 11.708 476.01 0.0896 17.8 0 18-20 0.21 10.087 51.471 12.252 475.91 0.0907 1.7 0 22-24 0.03 10.059 51.661 11.835 472.76 0.0905 .0 0 28-30 0.26 10.063 51.801 12.076 477.69 0.0921 2.9 
9-29 

calculated as the difference between the observed concentration and a nominal seawater 

concentration (2.400 mols/L). Excess DIC is also affected by calcium carbonate precipitation 

(Equation 9-5) and dissolution. This effect can be corrected by calculating the difference 

between a standard seawater calcium concentration 10.512 mM/L at S = 35‰) and the observed 

value. Positive and negative differences indicate net precipitation and dissolution, respectively. 

These relationships are important because they constrain the source of the carbon undergoing 

oxidation. Figure 9.3 shows excess DIC versus reduced sulfate for the following stations: 

GCAT2 (dives 4029 and 4048), BHB3, BHM4 (dive 4046), BHUN2, GCB3, GCBXC1, and 

GCJT2. These data are compared in each station with the two reaction paths that would result 

from sulfate reduction involving either a simple organic matter (CH2O) or methane (CH4). The 

complete data set from both the 1997 and 1998 cruises, less those stations showing brine 

compositions, is shown in Figure 9.4. Where the extent of reduction is less than ~30%, excess 

DIC values tend to scatter between these two bounding slopes. In contrast, where reduction 

varies from less than 30% to approximately 100%, excess DIC concentrations are consistent with 

methane oxidation. The fact that sulfide concentrations shown in Figure 9.3 do not show the 

same stoichiometric relationship with DIC as they do with sulfate suggests that sulfide is either 

consumed by sulfide, or possibly reacts to form solid phase AVS, as discussed below. 

Solid-phase buffers. Although the concentrations of dissolved carbon and sulfide clearly reflect 

the intensity of biogenic sulfate reduction, they also reflect mineral precipitation reactions. The 

DIC that is produced promotes the non-organic precipitation of calcium carbonate. Increases in 

DIC should thus be correlated to increases in solid phase inorganic carbon. This pattern is well 

illustrated in Figure 9.5A, which shows a steep rise in inorganic CaCO3 with pore water DIC up 

to approximately 10 mM DIC (calcium concentrations significantly greater than seawater reflect 

the addition of Ca-rich brines). Over this region, DIC concentrations are well buffered by CaCO3

precipitation. However, extensive CaCO3 precipitation also brings about significant depletion in 

calcium concentration. This depletion is shown in Figure 9.5B, which shows that dissolved 

calcium falls to approximately 10% of its nominal seawater value (10 mM) at the very elevated 

DIC concentrations. Thus the scatter of points at very high DIC concentrations (associated with 

methane oxidation) in Figure 9.5A reflects the limitations of DIC buffering by CaCO3 as calcium 

is removed from the system. 

The formation of solid-phase sulfides is an important buffer to dissolved H2S concentrations. It is 

important to recognize that a dearth of dissolved sulfide does not necessarily imply a low rate of 

sulfate reduction, but may instead reflect excess reactive iron to form iron sulfide minerals. 

Conversely, high pore water sulfide concentrations may reflect a limitation in the availability of 

reactive iron. High sulfate reduction rates, low concentrations of solid phase sulfide minerals 

(acid-volatile sulfide, or AVS, and pyrite), and low concentrations of reactive iron suggests that 

the very high pore water sulfide concentrations in these seep sediments indicates constraints to 

sulfide mineral formation. Compared with CaCO3 mineral formation, precipitation of solid phase 

sulfides is substantially more complex. It involves the reduction of sedimentary iron oxides by 

dissolved H2S, and reaction paths that lead to formation of either AVS and/or pyrite. Moreover, 

substantial differences exist in the reactivity of various iron oxides phases, and analyses of 

“total” sedimentary iron, which may involve contributions from iron silicate phases, 
9-30 

Figure 9.3. Sulfate reduction stoichiometry by station. Reduced sulfate (solid symbols) is plotted versus excess 

DIC for stations GCAT2 (dives 4029 and 4048), BHB3, BHM4 (dive 4046), BHUN2, GCB3, 

GCBXC1, and GCJT2, and is used to constrain the source of the carbon undergoing oxidation. These 

data are compared in each station with the reaction paths that would result from sulfate reduction 

involving either a simple organic matter (CH2O) or methane (CH4). Dissolved sulfide (open symbols) 

is also shown for comparison. 
9-31 

Figure 9.4. Sulfate reduction stoichiometry, all stations by year. Reduced sulfate versus excess DIC is plotted

for 1997 (squares) and 1998 (circles) stations, excluding those classified as brines in Figure 9.2. 

Data for both years tend to scatter between the bounding reaction paths for simple organic matter

such as crude oil or detritus and methane (see Figure 9.3) for those stations showing less than

~30% extent of reduction. Where reduction varies from >30 to ~100%, excess DIC concentrations

are consistent with a methane oxidation reaction path. 
9-32 

Figure 9.5. Carbonate mineral precipitation. (A) Inorganic carbon (as CaCO3) versus porewater DIC. Up to 

about 10 mM DIC, increases in porewater concentration correlate strongly with increases in solid 

phase CaCO3. Above 10 mM DIC, depletion of dissolved calcium limits CaCO3 formation. (B) 

Porewater calcium. Concentrations above ~10 mM record influence of calcium-rich brines. 

Precipitation of CaCO3 at high DIC concentrations brings about calcium depletion to ~10% of its 

nominal seawater value (10 mM). 
9-33 

may overestimate the iron that is potentially available for incorporation as sulfide. This 

variability has led to the notion of “reactive” Fe, and is the motivation for using buffered citrate 

dithionite (CD) versus HCl for metal extractions (see Methods). The differences in the effect of 

these solvents are easily seen in Figure 9.6, which plots HCl- and CD-extracted iron versus grain 

size. The important point here is that CD-Fe concentrations are substantially less than those of 

HCl-Fe, and may be a more accurate reflection of the iron that is available for sulfide mineral 

formation. 

These relationships between dissolved sulfide, Pyrite-S (TRS) concentrations (mean 111.8±35.4 

mol/g), greatly exceed those of AVS (mean 4.3 ± 4.9 mol/g) in these sediments. Thus, AVS 

represents a very minor fraction of the reduced sulfur present in these sediments, particularly in 

comparison to the H2S present in the pore water. AVS and TRS are compared in Figure 9.7, 

which plots both these values versus dissolved sulfide (Figure 9.7A) and organic carbon (Figure 

9.7B). TRS also shows little variation with respect to H2S, which implies that pyrite-S generation 

may be largely decoupled in time or space from the process of sulfate reduction. 

In much the same way that reduced sulfate was compared to excess DIC as a way of constraining 

the stoichiometry of the reacting organic matter, iron concentrations determined from CD 

extracts can be compared with AVS concentrations to constrain the composition of the sulfide 

forming in these sediments. This is shown in Figure 9.8A, which also includes the reaction paths 

for two AVS minerals, mackinawite and greigite. Points which lie to the left of the greigite line 

suggest that greigite is incompletely dissolved in citrate dithonite buffer (Morse 1994) but 

completely liberated by the AVS distillation. However, the fact that the data also tend to scatter 

to the right of the mackinawite line implies incomplete sulfidization of “reactive” iron and is 

problematic in the face of high H2S concentrations. CD-Fe and H2S are plotted in Figure 9.8B 

(bottom), which does show a crude inverse relationship between these two parameters, but only 

when relatively high concentrations of CD-Fe are included in the dataset, at slightly less than 20 

mol/g CD-Fe, H2S concentrations rapidly exceed 1 mM. 

Resolution of this apparent paradox of reactive iron limitation (low AVS and high pore water 

sulfide) with “excess” reactive iron may come about through consideration of the kinetics of iron 

oxide mineral dissolution and sulfidization. Using data from (Crill and Martens, 1987), Figure 

9.9 tabulates the reactivity of iron phases by their first order “decay” constant, k. If we assume 

that CD must extract all iron having a k greater than 10-4, this iron would have a half-life of 

approximately 7000 years. This implies that the maximum age of the as-yet unsulfidized reactive 

iron in this system is on the order of 104

 years. This time span is long compared with the 

apparent dynamics of this environment; for example, substantial changes in near surface sulfide 

concentrations were noted between 1997 and 1998 at GCAT2. 

In summary, the observation of high dissolved sulfide reflects both the high rates of sulfate 

reduction and low availability of reactive sedimentary iron. Although sedimentary iron is 

present, it is apparently not reactive in sufficiently brief time scales as to effectively buffer 

dissolved sulfide. Sulfide that is not incorporated as AVS and pyrite is then available for 

biological consumption by sulfide oxidizers. Because sulfide oxidizers may play critical roles in 

chemosynthetic communities, a low concentration of sedimentary reactive iron may be an 
9-34 

Figure 9.6 Sedimentary reactive iron. Iron extracted from sediments using citrate dithionite (CD, open circles)

and HCl (closed circles) increases qualitatively as a function of grain size. However, CD-Fe 

concentrations are substantially less than those of HCl-Fe and are probably a more accurate 

reflection of the iron that is available for sulfide mineral formation. 
9-35 

Figure 9.7. Sedimentary sulfide (TRS and AVS). (A) Dissolved versus solid phase sulfide. Pyrite sulfur (TRS, 

open circles) is substantially greater (mean 111.8±35.4 mol/g) than metastable, iron monosulfide 

(AVS, closed circles, mean 4.3±4.9 mol/g) in these sediments. Although the TRS concentrations

are largely invariant with respect to dissolved sulfide, there is some suggestion that AVS

concentrations are lowest in those samples having very high dissolved sulfide concentrations,

consistent with iron limitation. (B) Organic carbon and solid phase sulfide. AVS (closed circles) 

and TRS (closed circles) show little control by organic carbon concentration. 
9-36 

Figure 9.8. Reactive iron availability. (A) Reactive iron versus AVS. Data are cast relative to reaction paths for

AVS minerals mackinawite and greigite. Data which lie to the left of the greigite line suggest this

phase is incompletely dissolved in citrate dithonite buffer (Morse 1994) but completely liberated by

the AVS distillation. Conversely, data that scatter to the right of the mackinawite line implies

incomplete iron sulfidization. This latter result points to variations in iron reactivity with respect to

sulfide that may reflect kinetic controls discussed in text. (B) CD-Fe and dissolved H2S. Iron

limitation is consistent with an inverse relationship, albeit crude, between these two parameters:

dissolved sulfide is negligible at relatively high concentrations of CD-Fe, whereas at less than ~20 

mol/g CD-Fe, H2S concentrations rapidly exceed 1 mM. 
9-37 

Figure 9.9. Iron reactivity towards sulfide. The variation in the reactivity of iron phases is reflected by the first 

order “decay” constant, k, which spans many orders of magnitude. If k ~10-4 is set as a lower limit 

for CD-extractable Fe, this iron would have a half-life of ~7000 years, implying that the maximum

age of the unsulfidized reactive iron is on the order of 104

 years. This time span is long compared 

with the apparent dynamics of this environment; for example, substantial changes in near surface 

sulfide concentrations were noted between 1997 and 1998 at GCAT2. After Canfield et al. (1992). 
9-38 

important feature of these environments. In addition, the excess DIC produced during sulfate 

reduction promotes extensive precipitation of calcium carbonate. This must in turn produce 

critical environmental changes in terms of substrate stability, permeability, and related physical 

sediment characteristics. Moreover, because the observed extent of sulfide mineralization is low, 

the development of calcium carbonate cements and hardgrounds may be the only truly physically 

durable signature of ancient seeps where no extant community is detectable. 

Sulfate depletion. In addition to variations in the overall intensity of sulfate reduction discussed 

above, the depth at which sulfate was quantitatively depleted also varied at active sites. This 

variation is shown in Figure 9.10 in sulfate versus depth profiles for stations GCAT2, BHB3, and 

BHUN2 (1998). These profiles show production of DIC and H2S for comparison. At BHB3, 

sulfate depletion is essentially complete at 7 cms; at BHUN2 this occurs at approximately 9 cms. 

At GCAT2 (dives 4029 and 4048) depletion is not complete until 17 to 19 cms. There are also 

apparent temporal changes in this depth. Comparison of the 1997 data for GCAT (dive 2886) 

with that of the 1998 dives (4029 and 4048) shows a relaxation of the sulfate gradient and an 

expansion of the zone of sulfate reduction over this one-year period, with sulfate persisting to 

deeper depths in the later dives. This change is also reflected in the presence of high 

concentrations of sulfide (> 5 mM) within 2 cm of the surface in 1997. These levels are not 

encountered until 9-10 cms in the 1998 cores. 

Despite variation in the depth at which sulfate depletion is complete, all of the above stations 

show greater than 50% depletion by 10 cm. This pattern is in contrast with that observed of the 

bacterial mat transect at GCB3. Three cores were taken here: one from the interior of a mixed 

orange and white Beggiatoa mat, one from the outer perimeter of the mat consisting of white 

Beggiatoa only, and a control core taken outside the mat entirely. This transect shows a clear 

influence of pore water chemistry by the mat itself. For example, comparison of on- and off-mat 

profiles in Figure 9.11A shows a development of a shallow pH maximum in the off-mat profile 

that is absent in the on-mat cores. The off-mat core contains negligible sulfide concentrations, 

whereas both the orange and white mats have sulfide concentrations over 2.5 mM (Figure 

9.11B). However, although the orange Beggiatoa mat core shows a high-integrated sulfate 

reduction rate (Table 9.2), all the cores collected at GCB3 exhibit a pattern of sulfate depletion 

with respect to depth that is distinctly different from that seen at other stations of comparable 

productivity (Figure 9.11C). GCB3 contains only modest overall depletions in sulfate, with 

orange Beggiatoa showing greater reduction (<30%) than white. This is in contrast to other 

active sites that show more extensive shallow sulfate depletion. In addition, the depletion profile 

for the orange mat shows considerable scatter. Although GCB3 is a significant site because it is 

dominated by Beggiatoa, the weak and noisy sulfate depletion curves coupled with relatively 

high integrated rates (at least in the case of the orange mat) are difficult to reconcile and suggest 

that these bacteria may significantly influence transport of pore water sulfate. 

Brines. Brines were clearly distinguished on the basis of elevated chloride concentrations. In 

these stations (GBM2 and BPB2), pore waters directly below the sediment-water interface 

typically exhibit chloride values close to seawater values. With increasing depth, there is a 

shallow zone of variable thickness with chloride concentrations intermediate between those of 

seawater and the brine end member. Because of its variable thickness, this zone can best be seen 

by calculating a mixing line between seawater and brine end members, with the latter having 
9-39 

Figure 9.10. Sulfate depletion depth. Depth profiles of dissolved sulfate (open circles), sulfide (closed circles), and DIC (diamond

depth at which sulfate was quantitatively depleted varies at active stations GCAT2 (A-C), BHUN2 (1998 core, D), an

sulfate depletion is essentially complete at 7 cms; at BHUN2 this occurs at ~ 9 cms, and at GCAT2 (dives 4029 and 4

complete until 17 to 19 cms. Comparison of the 1997 data for GCAT (dive 2886) with that of the 1998 dives (4029 anrelaxation of the sulfate gradient and an expansion of the zone of sulfate reduction, with sulfate persisting to deeper dSulfide concentrations are also higher (> 5 mM) within 2 cm of the surface in 1997 at GCAT2 compared to the 1998 G
9-40 

Figure 9.11. Beggiatoa mat transect (GCB3). Depth profiles at GCB3 of pH (A), dissolved sulfate (B), and sulfide (C) differ both w

profiles at other stations. Open circles are off-mat, filled circles are orange mat, and filled squares denote orange mat a mid-depth maximum with is absent in the on-mat cores. The off-mat core contains negligible sulfide concentrationsorange and white mats have sulfide concentrations over 2.5 mM. Sulfate shows a noisy and mild depletion in the on-mfrom the pattern of sulfate depletion seen at other stations of comparable productivity. 
9-41 

maximum chloride values three times that of standard seawater, negligible sulfide and sulfate, 

elevated calcium, strontium, and sodium, and depleted magnesium concentrations relative to 

standard seawater. The pattern of chloride, sulfide, sulfate, and DIC concentrations with extent 

of mixing for GBM2 (dives 2882 and 4053) is shown in Figure 9.12. The pattern of rapid sulfate 

depletion with mixing clearly reflects the absence of sulfate in the end member brine, and 

confines the entire zone of sulfate reduction to a thin zone less than approximately 5 cm (< ~50 

% brine) in thickness at GBM2. Despite being limited to a relatively thin depth interval, sulfate 

reduction rates at GBM2 were among the highest measured (Table 9.2). There is considerably 

less brine dilution at BPB2, also shown for comparison in Figure 9.12. 

9.3 Stable Isotope Geochemistry*

9.3.1 Introduction 

Carbon, sulfur, and oxygen each have more than one stable isotope. By use of a mass 

spectrometer it is possible to measure the ratio of the stable isotopes for a given element. These 

ratios are then reported relative to a standard using the “” notation, which is in ‰ because the 

differences are usually small. The isotope ratios of these elements vary depending on their source 

and as a result of chemical and biological processes that cause fractionation of the isotopes. 

Therefore, they are often good indicators of sources and tracers of biogeochemical processes. 

A modest effort was made in this study to use carbon and oxygen isotopes to investigate the CO2

system and sulfur isotopes to investigate the H2S system. Stable isotope ratios of dissolved 

components (DIC and H2S) and solids (CaCO3 and TRS) were determined on selected samples 

from sites of particular interest. 

9.3.2 Results 

Analytical results are reported in Tables 9.4 and 9.5.

Carbon and oxygen stable isotope ratios for DIC and CaCO3 were plotted against each other and 

against other components including sulfur isotope ratios and concentrations of dissolved and 

solid components. In many cases, no relationships were observed, and those results are not 

presented here. However, interesting relationships were apparent. A good (R2

 = 0.85) linear 

relationship was observed (Figure 9.13) between carbon and oxygen isotope ratios in CaCO3. 

However, virtually no (R2

 = 0.08) relationship occurred between the carbon isotope ratios in DIC 

and CaCO3 or between the DIC concentration and its stable isotope ratio (R2

 =0.006). 

There were generally no good correlations between sulfur stable isotope ratios in TRS and H2S 

and with other components. For example, R2

s are 0.06 for TRS 34S vs. SO4, 0.11 for TRS 34S 

vs. TRS, 0.19 for H2S-34S vs. DIC, and 0.30 for H2S-34S vs. H2S. Figure 9.14 shows that with 

the exception of two TRS samples, TRS has a large range of quite negative 34S values at both 

low and high (low SO4 concentrations) extents of sulfate reduction. In contrast, many H2S-34S

values are close to zero with an increasingly large scatter to high positive values with increasing 

extent of sulfate reduction. 

 

*

 This section was authored by John W. Morse and Steven Macks. 
9-42 

Figure 9.12. Brine geochemisty. (A) Variation in chloride concentration as a function of depth at GBM2, normalized with respect to 

Open circles are dive 2882, closed squares are 2885, and open squares are 4053 data. (B) Sulfate concentrations at GBfraction brine. (C) Sulfide concentrations at GBM2 brines, expressed as fraction brine. (D-F) Variations with respect to (D), GBM2 (dive 4053), and BPB2 (dive 4038). Open circles are sulfate, closed circles are sulfide, and open squares avariation in the sulfate depletion with respect to fraction of brine mixing. 
9-43 

Table 9.4. Dissolved organic carbon and nutrients. 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

2871 GC234 GCST1 0-2 6262 135.39 10.45 2.91 12.79 

2-4 8417 158.12 9.66 4.15 11.30 4-6 8272 186.18 9.70 8.30 8.45 6-8 8425 190.50 9.13 10.38 6.78 

8-10 9070 217.38 8.72 5.81 7.40 

10-12 8892 228.05 9.43 17.44 2.54 12-14 9157 264.43 8.34 8.30 2.11 14-16 10275 229.91 7.48 9.55 2.24 16-18 – – – – – 18-20 – – – – – 

2873 GC235 BHAT1 0-2 7181 244.83 53.75 9.55 0.00 

2-4 9044 322.13 71.57 34.04 0.12 4-6 7850 330.95 78.38 9.55 0.24 6-8 9556 337.45 82.89 9.55 0.37 

8-10 9995 299.10 52.29 10.38 0.40 

10-12 11768 268.47 34.42 12.45 0.72 12-14 13289 239.01 39.96 16.19 0.54 14-16 – – – – – 

2873 GC235 BHM4 0-2 18310 472.90 146.42 22.47 1.78 

2-4 18285 454.56 120.24 27.51 2.48 4-6 18318 431.84 72.15 21.38 1.76 6-8 – – – – – 

8-10 – – – – – 

10-12 – 398.05 30.64 16.96 0.62 12-14 – 390.70 30.26 21.52 4.31 14-16 – 391.39 31.62 16.80 3.53 16-18 – 408.18 38.31 14.39 1.71 18-20 – 397.89 22.45 12.92 2.69 

2874 GC235 BHST1 0-2 6011 152.52 5.03 24.49 1.94 

2-4 5391 145.63 12.19 9.55 0.95 4-6 4016 109.33 10.81 8.72 0.00 
9-44 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

6-8 7448 95.86 10.54 4.57 1.14 

8-10 5945 80.99 8.93 4.57 0.08 

10-12 3056 107.72 7.39 7.47 0.39 12-14 5670 101.81 9.15 12.04 1.07 14-16 4229 106.87 8.89 12.45 1.14 16-18 – 106.11 7.88 16.19 1.61 

2875 GC235 BHST2 0-2 12064 291.60 64.94 25.75 4.02 

2-4 12287 209.80 39.51 23.61 0.05 4-6 18979 187.83 33.17 22.28 0.68 6-8 – – – – – 

8-10 – – – – – 

10-12 – – – – – 

2875 GC235 BHUN1 0-2 2398 78.86 7.46 6.23 10.58 

2-4 2150 79.82 7.50 7.47 1.82 4-6 2339 88.69 8.14 5.40 0.99 6-8 2609 93.77 8.62 7.06 1.11 

8-10 3999 99.53 8.66 7.89 1.34 

10-12 3814 90.19 7.86 5.29 0.62 12-14 3939 98.98 7.68 7.23 0.65 14-16 – – – – – 16-18 – – – – – 18-20 – – – – – 22-24 – – – – – 26-28 – – – – – 

2876 GC233 BPR2 0-2 6725 176.48 6.05 8.76 0.63 

2-4 3462 196.53 6.30 10.29 0.49 4-6 4301 232.62 6.19 15.94 0.88 6-8 4846 230.06 5.36 7.97 0.51 

8-10 – – – – – 

10-12 – – – – – 

2880 GC234 GCB1@GCAT1 0-2 4433 278.70 19.11 7.43 0.19 
9-45 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

2-4 9483 96.60 28.77 11.85 1.76 4-6 4400 70.57 12.51 5.12 2.64 6-8 6134 83.85 14.44 1.68 4.11 

8-10 10005 70.93 13.09 1.56 4.86 

10-12 8223 61.78 12.91 3.50 1.09 12-14 5402 55.75 12.29 3.79 1.54 14-16 – – – – – 16-18 4134 57.30 11.45 2.84 1.22 18-20 3969 63.69 12.21 2.30 1.90 22-24 7198 54.21 11.44 0.93 0.39 

2882 GB425 GBB1@GBM2 0-2 34392 1970.00 169.43 81.09 15.66 

2-4 38426 2610.00 89.86 91.90 10.38 4-6 29993 2820.00 43.99 68.66 6.13 6-8 14992 3040.00 4.47 86.80 5.94 

8-10 7751 3250.00 10.50 67.42 0.74 

10-12 6038 3540.00 8.63 77.07 2.52 12-14 5385 3690.00 5.32 64.73 3.22 14-16 5358 3910.00 6.44 67.46 2.00 16-18 5874 3940.00 6.09 55.14 3.09 18-20 5105 3900.00 4.25 44.47 4.92 22-24 – – – – – 26-28 – – – – – 

2883 GB425 GBM1 0-2 6740 220.73 33.26 29.02 0.62 

2-4 8517 198.39 47.93 29.71 0.04 4-6 12577 242.14 80.13 29.99 0.67 6-8 10744 242.04 95.73 26.63 0.91 

8-10 11770 309.52 112.94 26.92 0.78 

10-12 11208 364.76 103.53 35.68 1.35 12-14 12127 406.47 52.93 25.06 4.53 14-16 – – – – – 16-18 – – – – – 
9-46 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

2885 GB425 GBM2 0-2 6056 224.23 14.46 26.55 5.23 

2-4 8257 303.86 46.94 28.85 4.71 4-6 10208 334.01 98.38 21.47 6.43 6-8 11562 367.10 115.48 18.92 6.49 

8-10 11871 361.34 79.51 23.64 6.47 

10-12 11321 340.06 68.13 23.12 8.13 12-14 – – – – – 14-16 – – – – – 16-18 – – – – – 

2886 GC234 GCAT2 0-2 23204 313.91 97.45 21.38 21.88 

2-4 24558 321.55 73.48 5.56 15.98 4-6 21681 292.64 40.37 0.21 0.76 6-8 11149 277.82 25.43 10.14 10.59 

8-10 23291 250.45 19.36 4.39 12.06 

10-12 15209 222.47 15.08 7.89 10.32 12-14 – – – – – 14-16 – – – – – 16-18 – – – – – 18-20 – – – – – 20-22 – – – – – 

2888 GC234 GCAT1 0-2 8415 198.04 29.07 18.23 9.81 

2-4 6488 201.07 26.20 12.87 10.50 4-6 9846 204.75 18.73 8.96 5.43 6-8 8357 209.47 13.10 7.95 5.87 

8-10 – – – – – 

10-12 – – – – – 12-14 – – – – – 

2891 GC235 BHAT2 0-2 5473 136.61 9.50 26.46 9.44 

2-4 6657 145.72 15.47 8.72 4.97 4-6 7517 153.73 13.85 6.32 3.63 6-8 6637 145.64 13.10 8.09 3.52 
9-47 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

8-10 4233 132.43 11.48 10.80 3.16 

10-12 – – – – – 12-14 – – – – – 14-16 – – – – – 

box GC235 BHBC2 0-2 5618 130.18 14.40 18.98 0.43 

2-4 5446 140.33 12.46 17.20 1.31 4-6 4855 111.62 8.28 15.02 0.81 6-8 4659 104.91 7.30 11.18 1.26 

8-10 3568 122.98 7.26 10.23 1.55 

10-12 3967 132.44 7.51 11.34 1.11 12-14 3754 148.08 7.55 13.28 0.96 14-16 3919 147.91 8.31 11.92 1.43 16-18 4879 163.53 7.77 8.90 1.94 18-20 4578 149.98 6.86 5.89 0.29 

4025 GC185 BHAT1 0-2 – 144.46 30.38 8.4 122.12 2

2-4 – 183.98 53.77 6.61 3.81 4-6 – 228.61 51.86 7.13 7.28 6-8 – 194.67 45.47 6.49 1.92 

8-10 – 145.92 23.89 6.43 1.18 

10-12 – 142.18 22.83 5.23 3.15 12-14 – 116.64 12.82 5.17 2.16 14-16 – 461.24 9.77 5.11 1.61 16-18 – 95.41 9.29 6.19 2.16 

4027 GC185 BHST1 0-2 6788 109.45 12.08 5.78 87.29 2

2-4 6387 140.09 8.39 7.95 82.35 24-6 5367 224.78 15.14 6.23 216.2 26-8 5048 183.14 9.92 5.07 70.65 1

8-10 5229 172.43 8.82 5.56 13.2 

10-12 4729 187.94 10.17 5.51 6.93 12-14 4731 204.3 11.25 5.46 5.58 14-16 4663 136.25 6.41 5.4 5.17 
9-48 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

16-18 4342 110.05 5.65 4.25 2.46 18-20 4691 97.59 5.09 5.29 0.29 20-22 5037 89.19 5.55 4.15 0.33 22-24 4372 81.52 5.67 6.28 2.01 24-26 4753 77.9 5.12 4.05 2.05 26-28 5246 71.7 5.51 4.54 2.79 28-29 5773 77.99 4.48 5.57 2.16 

4028 GC185 BHB3 0-2 8930 195.9 23.72 7.3 109.2 

2-4 6429 57.64 7.31 4.43 25.29 4-6 7031 37.39 5.11 1.04 5.07 6-8 6666 38.76 4.69 0.98 4.83 

8-10 6419 29.91 5.03 1.49 1.93 

10-12 5666 35.5 5.15 0.88 1.45 12-14 6216 47.49 8.03 0.28 1.57 14-16 7494 75.39 10.83 4.1 1.91 

4029 GC234 GCAT2 0-2 7718 159.61 12.34 6.63 264.34 2

2-4 6311 167.08 7.5 3.73 154.18 4-6 9748 114.26 5.33 3.68 110.67 6-8 7140 104.92 4.78 6.46 103.83 

8-10 10127 105.36 7.33 1.32 60.45 

10-12 7351 68.29 4.48 3.52 15.56 12-14 7419 83.08 7.37 1.22 52.76 14-16 5797 66.2 5.6 3.42 22.08 16-18 7554 45.45 3.37 1.13 15.12 18-20 10256 83.95 9.36 0.52 21.54 20-22 10640 80.39 10.03 1.03 13.68 22-24 9835 73.2 7.98 2.1 15.96 24-26 11755 66.02 6.54 0.38 13.6 26-28 11323 41.94 5.79 0.34 6.3 28-30 13368 40.51 5.23 0.29 2.16 

4031 GC234 GCST1 0-2 5516 144.91 11.48 9.05 75.97 2
9-49 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

2-4 6238 117.15 13.68 5.62 18.84 4-6 4575 104.32 11.58 3.56 119.94 6-8 11543 88.96 9.4 3.87 21.24 

8-10 6018 74.2 8.15 2.14 33.11 

10-12 5769 64.86 8 3.81 8.37 12-14 9660 59.31 8.74 -4.74 29.11 14-16 10886 48.27 7.97 -5.11 4.1 16-18 11378 45.01 8.23 -8.21 1.83 18-20 21224 42.85 8.97 -91.76 10.09 20-22 10866 35.6 7.24 -4.16 3.33 

4032 GC234 GCAT1 0-2 7516 147.36 6.62 8.83 301.91 1

2-4 6940 170.14 6.21 9.98 78.3 14-6 5795 166.4 3.91 8.68 66.04 6-8 5503 147.55 3.76 7.38 18.73 

8-10 5183 124.87 4.56 7.93 34.63 

10-12 4548 115.34 4.01 8.47 14.42 12-14 4489 81.68 3.26 7.17 9.12 14-16 4568 80.65 3.04 6.48 8.65 16-18 4845 71.78 3.1 7.02 9.31 18-20 4797 71.47 3.16 6.95 9.15 20-22 5328 85.09 3.55 6.88 6.56 

4036 GC234 GCJT1 0-2 – 117.25 16.47 10.8 14.88 1

2-4 – 136.89 14.45 11.36 6.56 14-6 – 141.23 9.5 11.35 3.04 6-8 – 123.28 6.17 11.34 0.78 

8-10 – 108.71 6.17 11.33 1.08 

10-12 – 94.24 5.76 10.75 1.07 12-14 – 88.45 5.48 11.31 1.67 14-16 – 86.61 5.41 9.59 1.82 16-18 – 83.34 5.34 10.71 1.83 18-20 – 85.45 5.54 10.14 2.07 
9-50 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

20-22 – 82.54 5.61 11.26 2.27 22-23 – 82.51 5.74 11.26 1.41 

4037 GC233 BPAT1 0-2 – 112.38 10.77 24.53 42.61 

2-4 – 108.33 11.09 8.57 13.35 14-6 – 105.1 10.54 5.17 5.67 6-8 – 102.69 13.02 6.1 44.41 

8-10 – 98.3 11.79 8.36 16.12 

10-12 – 95.91 8.64 6.62 4.08 

4038 GC233 BPB2 no frozen core available4040 GB425 GBUN1 0-2 3950 47.76 6.47 5.55 123.08 

2-4 7015 121.09 20.13 8.47 66.27 4-6 3589 69.9 4.83 7.41 19.19 6-8 3412 88.39 7.22 4.01 10.04 

8-10 3083 88.76 5.01 1.6 3.33 

10-12 2850 91.09 4.55 2.2 6.43 12-14 3078 80.59 4.04 2.79 1.78 

4042 GC185 BHM3 0-2 9331 11.39 2.38 0.62 1.28 

2-4 13046 282.15 46.17 6.97 17.52 4-6 10601 240.5 34.28 5.8 6.21 6-8 11014 225.58 29.68 6.96 4.72 

8-10 10826 213.38 25.52 5.79 1.42 

10-12 9873 213.69 29.7 5.78 3.19 12-14 9757 201.93 23.68 5.78 3.16 14-16 10056 194.72 17.42 6.93 1.37 16-18 10939 186.79 16.89 6.35 1.36 18-20 21325 183.36 14.95 5.76 0.47 20-22 25818 178.82 14.62 10.97 1.27 22-24 25104 155 13.14 8.64 1.9 

4043 GC234 GCB3 

orange Beggiotoa 0-2 8797 291.35 37.51 9.79 9.65 

2-4 9458 193.21 21.99 5.74 10.15 
9-51 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

4-6 10577 137.53 12.27 5.16 6.93 6-8 24492 126.86 11.74 2.86 6.4 

8-10 31250 121.71 11.2 0.57 4.94 

10-12 39172 119.47 10.99 4.57 4.19 12-14 42289 98.4 11.06 2.28 2.64 14-16 15743 87.91 16.52 2.85 0.95 16-18 17545 83.91 20.35 2.84 1.22 18-20 17966 75.99 13.76 1.14 0.92 20-22 17692 56.73 11.06 0.57 1.74 22-24 17110 48.55 5.78 0 0.07 24-26 16816 42.17 6.11 2.83 0.06 

4043 GC234 GCB3 off mat 0-2 3485 65.17 8 7.42 204.35 

2-4 3609 85.54 8.46 7.35 135.8 4-6 3724 98.96 6.64 6.66 33.5 16-8 5142 107.03 8.71 7.82 11.14 

8-10 5107 125.13 5.81 5.9 9.98 

10-12 5174 111.9 5.73 6.44 5.13 12-14 5428 111.22 5.58 5.75 1.71 14-16 5451 103.95 5.57 7.54 2.83 16-18 5186 89.47 4.14 7.46 2.57 18-20 6329 89.88 4.54 4.3 2.94 20-22 5869 89.57 4.53 7.32 1.94 22-24 5641 95.77 5.26 7.87 1.98 24-26 6230 95.82 4.71 7.79 1.98 26-28 6813 96.96 5.52 5.86 1.77 28-30 7035 98.83 5.85 6.41 1.96 30-32 6854 91.95 4.68 6.95 3.35 

4043 GC234 GCB3 

white Beggiotoa

no frozen core available
9-52 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

4045 GC185 BHAT2 0-2 – 198.43 25.06 13.22 144.35 

2-4 – 220.11 26.85 8.28 65.94 14-6 – 230.48 23.64 7.42 20.93 16-8 – 206.37 21.7 5.96 12.24 

8-10 – 179.9 15.3 5.09 8.43 

10-12 – 164.17 10.67 5.38 3.95 12-14 – 168.07 8.97 6.31 2.26 14-16 – 133.72 6.83 5.73 1.73 16-18 – 131.42 6.55 5.73 8.73 18-20 – 140.07 7.11 6.3 1.5 20-22 – 142.47 8.29 7.44 1.06 22-24 – 122.62 6.34 5.72 1.29 24-26 – 121.11 6.55 5.71 1.11 26-28 – 123.47 6.62 7.43 3.53 

4046 GC185 BHM4 no frozen core available4047 GC234 GCJT2 0-2 10372 13.54 1.3 0.54 5.41 

2-4 8519 7.95 0.62 0.51 1.8 4-6 7962 4.95 0.42 0.54 2.64 6-8 12063 6.86 0.75 0.46 1.81 

8-10 24608 6.45 0.96 0.28 1.37 

10-12 22034 2.33 0.32 0.23 0.72 12-14 23805 1.9 0.31 0.2 0.58 14-16 29024 1.69 0.32 0.02 0.34 16-18 29398 2.08 0.83 0.03 0.26 18-20 34789 1.92 0.36 0.2 0.3 20-22 32307 2.04 0.41 0.03 0.07 22-24 26844 1.78 0.27 0.03 0.1 24-26 26030 1.95 0.41 0.01 0.05 26-27 25609 1.65 0.31 0.04 0.02 

4048 GC234 GCAT2 0-2 9251 201.46 59.93 8.04 148.31 

2-4 12334 125.35 20.73 2.65 32.04 
9-53 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

4-6 11173 72.13 15.11 1.92 14.72 6-8 26860 76.32 14.96 1.84 12.71 

8-10 9950 43.41 10.07 -4.9 6.37 

10-12 27003 45.24 10.38 -6.65 7.51 12-14 24415 35.8 10.24 -10.74 3.17 14-16 22517 29.35 9.96 -5.11 6.27 16-18 11497 29.37 11.99 -10.88 6.31 18-20 18385 27.24 11.51 -9.61 12.14 20-22 23185 29.39 14.83 -8.34 6.36 22-24 23705 27.98 12.8 -17.47 7.95 24-26 24085 32.63 12.98 -12.17 6.72 26-28 25253 33.36 12.83 -29.77 2.18 28-30 7829 36.24 11.5 -22.4 3.32 30-32 8798 43.84 10.83 -14.05 2.69 32-34 9178 55.2 12.12 -83.66 5.4 

4053 GB425 GBM2 0-2 9860 288.8 59.06 5.5 114.26 

2-4 10070 276.83 43.49 -8.23 49.34 4-6 8853 335.27 21.11 -80.02 15.5 6-8 7456 646 7.59 1.9 11.71 

8-10 6195 1040 3.24 0.97 9.74 

10-12 5814 1244 2.78 2.64 4.15 12-14 5253 1604 2.82 30.37 8.1 14-16 3687 1940 4.51 40.34 9.63 16-18 3689 2165 2.78 29.36 2.23 18-20 3122 2508 2.64 37.01 2.15 20-22 3913 2686 2.78 41.32 3.2 22-24 3203 3116 3.08 40.32 3.52 24-26 3714 3461 2.74 40.31 5.63 26-28 3546 3683 3.28 20.33 105.9 2
9-54 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

box GC185 BHBXC1 0-2 – 61.36 9.55 3.45 364.83 

2-4 – 51.07 10.87 2.6 76 4-6 – 49.02 10.34 2.88 41.18 6-8 – 46.71 11.64 4.09 24.87 

8-10 – 53.77 10.53 5.3 31.67 

10-12 – 60.56 13.96 8.48 72.86 12-14 – 59.98 22.91 5.74 23.09 14-16 – 66.13 23.02 2.35 14.65 16-18 – 65.9 21.52 3.56 19.54 18-20 – 76.06 25.5 4.76 33.73 20-22 – 76.92 22.83 4 33.71 22-24 – 81.04 14.79 7.83 16.96 24-26 – 80.08 13.61 2.81 7.12 26-28 – 89.85 14.56 5.39 7.8 28-30 – 97.94 13.77 3.37 12.89 30-32 – 102.02 14.25 13.49 6.7 

box GC185 BHUN2 0-2 23872 388 90.81 28.24 34.2 

2-4 10351 – – – – 4-6 9095 166.53 24.3 13.91 55.8 6-8 8539 138.7 21.71 10.49 74.98 

8-10 8640 97.76 15.39 11.18 51.13 

10-12 9028 121.26 8.68 11.19 10.37 12-14 9589 79.73 23.86 4.37 12.55 14-16 10332 62.77 25.05 0.97 6.69 16-18 12641 44.65 19.25 0.31 8.05 18-20 26398 33.89 14.76 0.34 8.24 20-22 22148 150.69 8.21 11.21 27.81 22-24 29285 36.29 13.37 1.08 1.17 

box GC234 GCBX2 0-2 – 29.11 6.88 6.01 18.44 

2-4 – 23.76 8.72 4.63 6.07 4-6 – 30.61 8.23 5.62 9.3 
9-55 

Table 9.4. Dissolved organic carbon and nutrients. (continued) 

Dive Number Site Station Depth 

(cm) 

Dissolved

Organic 

Carbon 

M) 

NH4 

(M) 

PO4 

(M) 

Urea 

(M) 

NO3 

(M) 

N

(

6-8 – 41.5 6.97 5.93 3.33 

8-10 – 56.21 6.82 2.5 1.32 

10-12 – 56.24 8.39 6.89 1.12 12-14 – 63.33 14.81 4.49 4.76 14-16 – 70.84 7.4 7.86 3.93 16-18 – 74.26 6.14 3.41 2.73 18-20 – 90.24 6.75 0.66 1.12 20-22 – 82.65 6.05 3.02 0.51 22-24 3689 83.45 5.2 7.07 0.43 24-26 – 91.11 6.02 4.32 0.35 

box GC234 GCBXC1 0-2 10203 195.49 26.23 8.74 119.34 6

2-4 9475 226.1 24.75 9.39 11.68 4-6 5071 173.96 11.22 2.57 2.53 6-8 3469 83.65 9.71 5.26 8.55 

8-10 2679 61.48 6.35 1.49 9.35 

10-12 2988 50.41 6.47 2.49 3.1 12-14 2674 45.3 6.53 0.08 3.49 14-16 2704 37.67 5.2 1.41 3.57 16-18 2580 42.44 5.67 0.02 2.58 18-20 2798 35.92 6.28 15.93 2.45 20-22 2798 34.14 5.72 2.69 2.98 22-24 3136 33.81 5.29 2.66 3.52 
9-56 

Table 9.5. Solid phase properties. 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

2871 GC234 GCST1 0-2 0.799 0.909 8.25 5.91 19.50 19.24 1.76 0.69 0.15 

2-4 0.776 0.851 9.04 6.47 21.42 20.94 0.85 0.49 0.09 

4-6 0.760 0.908 1.0 9.67 7.07 21.67 19.75 0.54 0.47 0.06 

6-8 0.751 0.911 8.73 6.20 21.08 20.18 0.39 0.48 0.00 

8-10 0.739 0.831 9.07 6.40 22.25 18.93 0.37 0.45 0.08 

10-12 0.742 0.947 7.89 5.22 22.25 21.69 0.36 0.46 0.05 

12-14 0.736 0.898 118.5 7.60 5.94 13.83 15.73 0.41 0.37 0.07 

14-16 0.736 0.922 7.76 6.31 12.08 19.80 0.40 0.46 0.07 

16-18 0.000 0.942 8.14 6.19 16.25 20.06 0.38 0.44 0.06 

18-20 0.000 0.903 7.64 5.55 17.42 24.47 0.52 0.50 0.00 

2873 GC185 BHAT1 0-2 0.809 0.873 39.0 10.90 8.21 22.42 24.46 2.96 0.48 0.09 

2-4 0.795 0.857 40.0 13.00 10.10 24.17 21.97 1.88 0.41 0.09 

4-6 0.775 0.878 3.5 14.10 10.80 27.50 27.00 1.85 0.45 0.09 

6-8 0.745 0.889 14.90 11.50 28.33 23.08 1.54 0.43 0.07 

8-10 0.764 0.885 13.70 10.90 23.33 20.73 1.11 0.39 0.08 

10-12 0.776 0.894 13.90 10.90 25.00 17.91 0.65 0.37 0.05 

12-14 0.763 0.866 118.7 13.80 11.70 17.50 20.97 0.60 0.37 0.07 

14-16 0.000 0.864 10.90 8.30 21.67 23.03 0.66 0.46 0.05 

2873 GC185 BHM4 0-2 0.813 0.710 6.7 11.80 7.63 34.75 20.22 0.97 0.31 0.00 

2-4 0.809 0.715 12.00 6.94 42.17 20.29 0.89 0.33 0.00 

4-6 0.814 0.746 5.4 11.60 6.61 41.58 20.51 1.01 0.35 0.08 

6-8 0.768 0.751 7.2 11.30 7.89 28.42 17.91 0.68 0.33 0.00 

8-10 0.781 0.766 5.0 12.00 7.83 34.75 23.20 0.42 0.43 0.00 

10-12 0.761 0.789 5.6 12.20 8.47 31.08 20.60 0.70 0.41 0.00 

12-14 0.772 0.835 4.1 119.6 11.30 8.32 24.83 20.08 1.07 0.36 0.00 

14-16 0.754 0.857 4.1 10.50 7.94 21.33 23.44 0.62 0.40 0.00 

16-18 0.757 0.825 3.7 11.50 8.29 26.75 24.54 1.01 0.42 0.09 

18-20 0.757 0.792 11.20 8.52 22.33 22.61 0.82 0.40 0.00 

2874 GC185 BHST1 0-2 0.791 0.903 6.61 4.33 19.00 30.76 4.47 0.45 0.08 

2-4 0.765 0.944 6.27 4.64 13.58 45.39 3.96 0.57 0.08 

4-6 0.742 0.883 8.8 6.81 4.83 16.50 44.66 2.32 0.55 0.08 

6-8 0.731 0.961 8.16 4.61 29.58 28.65 1.84 0.74 0.07 

8-10 0.734 0.884 11.10 6.73 36.42 30.42 1.54 0.61 0.07 

10-12 0.724 0.873 10.80 8.22 21.50 26.08 0.99 0.52 0.11 

12-14 0.727 0.823 9.21 6.72 20.75 29.12 0.77 0.51 0.08 

14-16 0.000 0.953 129.8 9.01 6.62 19.92 31.72 1.06 0.52 0.05 

16-18 0.000 0.915 8.99 6.38 21.75 31.66 0.64 0.51 0.07 

2875 GC185 BHST2 0-2 0.764 0.765 7.96 4.69 27.25 13.82 3.32 0.30 0.14 

2-4 0.777 0.829 7.41 4.64 23.08 17.98 1.92 0.30 0.13 

4-6 0.777 0.756 2.3 6.88 4.93 16.25 20.67 1.07 0.36 0.13 

6-8 0.000 0.891 7.96 5.03 24.42 16.71 2.07 0.31 0.11 

8-10 0.000 0.850 7.32 5.69 13.58 16.00 1.92 0.30 0.14 

10-12 0.000 0.868 163.4 7.76 4.94 23.50 16.55 3.86 0.29 0.14 

2875 GC185 BHUN1 0-2 0.811 0.922 4.69 3.54 9.58 30.62 17.19 2.06 3.75 

2-4 0.799 0.932 4.93 4.11 6.83 20.56 9.92 0.71 1.25 

4-6 0.779 0.916 1.6 6.58 4.84 14.50 22.27 3.65 0.40 0.42 

6-8 0.769 0.918 6.11 4.84 10.58 20.07 2.74 0.34 0.24 

8-10 0.743 0.953 6.40 5.20 10.00 28.19 2.60 0.37 0.29 

10-12 0.741 0.930 6.54 4.99 12.92 13.83 1.36 0.21 0.17 

12-14 0.735 0.931 117.13 6.15 5.22 7.75 29.60 1.33 0.38 0.15 

14-16 0.000 0.930 6.36 4.67 14.08 20.54 1.30 0.33 0.14 

16-18 0.000 0.941 6.28 4.84 12.00 26.11 3.13 0.35 0.19 

18-20 0.000 0.930 6.12 4.91 10.08 28.45 1.46 0.36 0.18 

22-24 0.000 0.962 5.95 5.58 3.08 21.63 1.21 0.36 0.16 

26-28 0.000 0.891 6.30 4.90 11.67 22.35 1.69 0.28 0.19 

2876 GC233 BPR2 0-2 0.797 0.878 4.29 2.45 15.33 42.40 18.16 0.54 0.43 
9-57 

Table 9.5. Solid phase properties. (continued) 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

2-4 0.794 0.938 4.92 2.84 17.33 34.67 30.87 0.54 0.66 

4-6 0.773 0.898 8.3 4.35 2.84 12.58 33.97 14.68 0.44 0.40 

6-8 0.739 0.916 4.66 2.96 14.17 39.01 7.17 0.55 0.23 

8-10 0.000 0.839 5.57 2.54 25.25 34.15 8.21 0.45 0.48 

10-12 0.000 0.873 145.4 6.07 2.52 29.58 35.95 5.52 0.76 0.57 

2880 GC234 GCB1@GCAT1 0-2 0.802 0.901 17.0 3.05 1.59 12.17 83.24 20.79 1.99 0.15 

2-4 0.794 0.943 10.0 3.12 1.29 15.25 41.69 6.10 0.76 0.08 

4-6 0.767 0.955 4.0 2.76 1.25 12.58 41.69 2.15 1.05 0.16 

6-8 0.742 0.974 2.5 3.17 1.34 15.25 28.51 1.86 0.80 0.08 

8-10 0.729 0.961 2.6 3.00 1.20 15.00 31.72 0.23 0.77 0.08 

10-12 0.726 0.945 3.9 3.17 1.24 16.08 26.06 0.81 0.74 0.08 

12-14 0.732 0.955 3.0 118.5 3.12 1.77 11.25 30.53 0.69 0.81 0.05 

14-16 0.724 0.979 4.3 3.10 1.26 15.33 26.65 0.62 0.76 0.07 

16-18 0.729 0.948 5.0 3.09 1.26 15.25 32.95 0.63 0.84 0.11 

18-20 0.724 0.933 2.0 3.14 1.29 15.42 31.32 0.61 0.91 0.06 

22-24 0.721 0.966 4.7 2.99 1.50 12.42 28.19 0.53 0.83 0.07 

2882 GB425 GBB1@GBM2 0-2 0.744 0.784 4.54 1.73 23.42 46.94 5.11 0.93 0.04 

2-4 0.636 0.761 4.43 1.09 27.83 46.14 1.44 1.27 0.08 

4-6 0.637 0.874 1.2 3.79 1.03 23.00 39.83 1.41 1.07 0.16 

6-8 0.686 0.791 3.49 1.03 20.50 33.45 0.71 1.39 0.08 

8-10 0.661 0.859 3.18 1.16 16.83 31.59 1.22 1.53 0.08 

10-12 0.643 0.871 2.65 1.17 12.33 32.30 0.76 1.20 0.05 

12-14 0.620 0.859 153.5 2.86 1.07 14.92 36.76 0.80 1.27 0.16 

14-16 0.610 0.879 2.74 1.00 14.50 37.71 1.83 1.33 0.24 

16-18 0.575 0.893 4.00 0.99 25.11 32.25 1.64 1.26 0.24 

18-20 0.580 0.853 2.94 1.52 11.83 43.98 1.50 1.55 0.18 

22-24 0.000 0.912 2.94 1.43 12.58 44.24 1.67 1.83 0.27 

26-28 0.000 0.843 2.92 1.70 10.17 42.84 1.45 1.60 0.25 

2883 GB425 GBM1 0-2 0.761 0.810 1.7 3.16 2.10 8.83 42.23 4.83 1.04 0.33 

2-4 0.744 0.839 6.7 4.56 3.11 12.08 50.43 7.79 1.24 0.42 

4-6 0.692 0.823 4.9 4.19 3.54 5.42 63.56 6.89 1.50 0.36 

6-8 0.667 0.842 6.5 5.79 2.00 31.58 42.20 6.46 1.02 0.37 

8-10 0.619 0.710 4.5 3.86 2.76 9.17 54.07 3.40 1.47 0.18 

10-12 0.538 0.764 1.6 3.14 2.41 6.08 34.35 5.26 0.95 0.30 

12-14 0.476 0.761 2.3 180.9 2.96 1.96 8.33 43.68 3.58 1.24 0.34 

14-16 0.000 0.646 2.5 2.37 1.59 6.50 40.18 3.81 1.07 0.40 

16-18 0.000 0.718 2.5 2.23 1.40 6.92 58.88 3.94 1.70 0.44 

2885 GB425 GBM2 0-2 0.674 0.209 1.6 4.27 1.32 24.58 35.21 4.94 1.96 0.23 

2-4 0.684 0.581 1.8 3.62 1.22 20.00 33.99 2.71 1.34 0.16 

4-6 0.661 0.800 0.9 4.04 0.79 27.10 64.76 1.75 3.40 0.45 

6-8 0.661 0.887 0.8 3.60 1.30 19.17 54.49 1.11 3.04 0.49 

8-10 0.611 0.911 2.3 2.95 0.81 17.87 49.62 1.11 2.06 0.30 

10-12 0.604 0.885 0.7 2.78 0.86 16.00 52.07 0.96 2.34 0.28 

12-14 0.000 0.919 0.6 132.6 2.63 1.02 13.42 54.44 1.19 2.33 0.33 

14-16 0.000 0.914 0.7 2.51 0.81 14.18 68.30 0.92 2.81 0.30 

16-18 0.000 0.926 1.3 2.52 0.79 14.42 59.69 0.78 2.23 0.22 

2886 GC234 GCAT2 0-2 0.774 0.887 9.7 6.04 3.37 22.25 37.67 2.53 0.81 0.07 

2-4 0.800 0.913 6.6 6.18 2.80 28.17 30.19 2.56 0.72 0.07 

4-6 0.965 0.868 3.7 6.02 2.77 27.08 29.56 1.39 0.71 0.07 

6-8 0.769 0.916 3.3 6.14 2.51 30.25 24.47 0.96 0.63 0.09 

8-10 0.749 0.900 3.3 5.72 2.17 29.58 26.60 0.39 0.77 0.06 

10-12 0.742 0.920 3.4 6.04 2.21 31.92 23.54 1.04 0.78 0.07 

12-14 0.916 2.4 124.9 5.45 2.20 27.08 27.14 0.53 0.90 0.08 

14-16 0.930 2.5 5.52 2.09 28.58 24.07 0.93 0.75 0.09 

16-18 0.926 2.1 5.72 2.29 28.58 25.14 0.73 0.86 0.07 

18-20 0.918 1.5 5.54 2.36 26.50 25.84 1.17 0.86 0.16 

20-22 
9-58 

Table 9.5. Solid phase properties. (continued) 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

2888 GC234 GCAT1 0-2 0.829 0.917 2.9 7.66 5.77 15.75 25.42 1.43 0.47 0.06 

2-4 0.802 0.881 2.3 7.47 5.95 12.67 23.51 1.60 0.41 0.07 

4-6 0.796 0.949 1.1 8.84 6.65 18.25 18.13 0.57 0.34 0.06 

6-8 0.794 0.850 1.1 8.92 7.39 12.75 23.78 0.80 0.42 0.07 

8-10 0.926 1.3 8.06 6.11 16.25 17.70 0.55 0.33 0.06 

10-12 0.906 0.9 8.46 6.54 16.00 19.83 0.55 0.35 0.00 

12-14 0.892 0.9 83.3 8.02 6.45 13.08 20.95 0.50 0.29 0.00 

2891 GC185 BHAT2 0-2 0.815 0.920 7.18 5.13 17.08 21.96 1.43 0.53 0.07 

2-4 0.795 0.908 6.75 5.17 13.17 23.34 2.07 0.40 0.00 

4-6 0.784 0.941 6.0 6.33 4.93 11.67 33.00 1.05 0.36 0.00 

6-8 0.767 0.962 6.56 4.83 14.42 0.35 0.00 

8-10 0.767 0.942 6.89 5.47 11.83 34.65 2.32 0.38 0.00 

10-12 0.940 7.86 4.20 30.50 28.79 0.97 0.39 0.00 

12-14 0.929 132.4 7.43 5.66 14.75 17.84 0.42 0.23 0.00 

14-16 0.906 7.97 5.89 17.33 20.36 0.26 0.30 0.00 

box GC185 BHBC2 0-2 0.846 0.903 4.70 3.82 7.33 24.40 3.55 1.22 0.40 

2-4 0.867 0.906 4.71 3.28 11.92 24.16 2.91 0.77 0.18 

4-6 0.759 0.894 3.1 5.76 4.01 14.58 16.83 2.62 0.32 0.12 

6-8 0.748 0.879 6.65 4.66 16.58 18.27 2.33 0.34 0.07 

8-10 0.740 0.901 7.09 5.36 14.42 14.61 2.84 0.32 0.08 

10-12 0.738 0.906 6.77 4.91 15.50 27.10 1.12 0.54 0.06 

12-14 0.727 0.900 84.6 7.45 5.40 17.08 28.74 1.11 0.56 0.07 

14-16 0.711 0.899 7.88 5.75 17.75 23.00 1.59 0.42 0.06 

16-18 0.719 0.907 7.99 5.87 17.67 25.75 1.06 0.46 0.00 

18-20 0.736 0.910 8.16 5.86 19.17 25.25 1.29 0.45 0.07 

22-24 0.888 7.50 5.78 14.33 25.70 0.76 0.48 0.08 

4025 GC185 BHAT1 0-2 0.845 4.2 11.10 7.59 29.25 52.38 14.47 1.38 0.47 

2-4 0.813 3.7 10.50 6.93 29.75 124.1 16.36 1.10 0.62 

4-6 0.799 5.8 11.60 8.52 25.67 9.24 7.58 0.14 0.54 

6-8 0.789 3.5 13.00 9.79 26.75 35.94 6.76 0.42 0.43 

8-10 0.789 1.7 17.40 12.90 37.50 41.00 5.74 0.72 

10-12 0.793 2.9 105.8 14.30 9.96 36.17 18.12 7.40 0.71 0.44 

12-14 0.804 15.20 9.61 46.58 40.60 1.12 

14-16 0.808 15.80 10.80 41.67

16-18 0.805 15.30 8.25 58.75

4027 GC185 BHST1 0-2 0.867 0.870 7.46 5.20 18.83 71.31 30.58 1.96 0.86 

2-4 0.819 0.810 10.10 6.36 31.17 59.04 19.31 0.93 1.36 

4-6 0.812 0.781 4.6 10.00 7.46 21.17 30.51 9.45 0.66 0.37 

6-8 0.813 0.833 8.91 6.19 22.67 67.16 3.16 1.23 0.12 

8-10 0.792 0.830 9.21 6.37 23.67 0.00 8.64 1.19 

10-12 0.761 0.868 113.8 9.01 6.66 19.58 52.59 10.36 0.93 0.38 

12-14 0.802 0.788 110.5 9.19 5.99 26.67 55.40 8.49 0.91 0.86 

14-16 0.796 0.748 9.73 7.61 17.67

16-18 0.777 1.000 11.40 8.10 27.50 59.27 6.34 1.18 

18-20 0.769 0.616 12.00 11.50 4.17

20-22 0.768 0.715 11.20 9.00 18.33 68.29 6.23 1.05 0.84 

22-24 0.780 0.844 10.80 8.83 16.42

24-26 0.819 0.722 10.40 7.23 26.42

26-28 0.775 0.713 11.90 10.50 11.67

28-30 0.780 0.773 12.20 8.81 28.25

4028 GC185 BHB3 0-2 0.821 0.844 10.50 7.83 22.25 116.6 10.87 1.26 0.46 

2-4 0.790 0.843 10.90 9.11 14.92 46.74 7.91 1.25 2.84 

4-6 0.793 0.880 3.0 10.70 8.03 22.25 51.24 6.32 0.55 

6-8 0.787 0.893 11.80 8.45 27.92 44.85 0.39 1.01 

8-10 0.790 0.722 14.10 13.10 8.33 55.57 7.22 1.13 
9-59 

Table 9.5. Solid phase properties. (continued) 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

10-12 0.784 0.722 11.40 8.31 25.75 63.15 8.44 0.57 0.48 

12-14 0.783 0.728 119.9 10.00 7.03 24.75 23.31 10.58 0.88 0.43 

14-16 0.789 0.820 11.30 7.61 30.75 79.46 12.42 1.22 

4029 GC234 GCAT2 0-2 0.830 0.562 6.15 2.60 29.58 60.49 24.25 0.93 1.95 

2-4 0.775 0.683 6.68 2.78 32.50 65.72 22.08 1.48 0.41 

4-6 0.785 0.716 4.0 6.47 2.88 29.92 73.13 12.06 1.44 0.77 

6-8 0.783 0.778 6.39 3.13 27.17 56.02 8.88 1.27 0.58 

8-10 0.769 0.717 6.03 2.50 29.42 0.00 6.06 1.23 

10-12 0.732 0.741 110.9 5.56 2.13 28.58 39.68 7.00 1.19 1.05 

12-14 0.764 0.801 6.02 2.90 26.00

14-16 0.754 0.827 101.7 5.18 2.33 23.75 0.00 6.70 0.00 0.92 

16-18 0.746 0.807 6.06 3.87 18.25

18-20 0.742 0.846 6.41 2.90 29.25 56.66 5.08 0.74 1.03 

20-22 0.767 0.737 5.70 1.92 31.50

22-24 0.736 0.760 5.90 2.79 25.92 56.28 5.04 1.73 1.12 

24-26 0.751 0.680 6.11 3.11 25.00

26-28 0.748 0.714 6.55 2.13 36.83

28-30 0.750 0.824 5.90 2.34 29.67

4031 GC234 GCST1 0-2 0.828 0.657 4.90 3.55 11.25 48.47 25.58 1.17 0.47 

2-4 0.862 0.614 6.77 4.90 15.58 0.00 13.34 0.47 

4-6 0.840 0.725 1.5 8.25 5.63 21.83 24.34 11.14 0.60 0.51 

6-8 0.832 0.795 9.03 6.62 20.08 52.37 11.64 0.90 

8-10 0.825 0.705 8.64 6.44 18.33 61.45 12.68 0.82 

10-12 0.804 0.774 8.70 5.50 26.67 55.50 12.86 0.99 

12-14 0.798 0.763 8.95 8.70 2.08 49.10 11.21 1.13 0.27 

14-16 0.796 0.717 9.37 5.88 29.08 54.99 6.46 1.43 

16-18 0.809 0.758 9.30 7.13 18.08

18-20 0.805 0.744 8.18 5.75 20.25 57.91 0.37 1.28 0.02 

20-22 0.799 0.682 8.29 6.01 19.00 34.61 5.54 0.40 

22-24 0.788 8.61 5.45 26.33 57.67 12.94 1.12 

4032 GC234 GCAT1 0-2 0.881 5.01 3.69 11.00 62.79 15.63 2.24 1.59 

2-4 0.874 4.88 3.32 13.00 48.09 7.00 1.51 1.02 

4-6 0.845 3.5 5.83 3.73 17.50 36.80 11.10 0.83 0.38 

6-8 0.841 8.62 5.42 26.67 53.06 4.02 1.12 0.74 

8-10 0.827 7.02 3.59 28.58 48.95 2.61 1.46 1.33 

10-12 0.813 6.71 3.11 30.00 44.46 4.11 0.90 0.60 

12-14 0.820 145.9 5.57 3.06 20.92 28.94 5.95 1.26 0.86 

14-16 0.821 5.27 2.80 20.58 44.23 2.46 1.28 0.63 

4036 GC234 GCJT1 0-2 0.810 4.2 8.38 6.28 17.50 58.70 22.11 0.44 

2-4 0.810 3.6 7.57 5.77 15.00 0.00 16.84 0.50 

4-6 0.802 3.3 6.75 4.66 17.42 60.50 14.40 0.74 0.64 

6-8 0.796 2.7 9.27 5.08 34.92 50.74 9.82 1.72 0.42 

8-10 0.808 2.5 8.29 7.91 3.17 0.00 

10-12 0.797 2.9 10.20 8.86 11.17 61.15 1.88 0.63 

12-14 0.795 2.6 61.5 8.72 8.17 4.58 26.27 1.16 

16-18 0.799 3.1 7.73 7.12 5.08 34.88 20.03 1.14 0.87 

4037 GC233 BPAT1 0-2 0.862 5.05 3.13 16.00 88.80 2.82 

2-4 0.879 4.57 2.68 15.75 58.45 25.31 0.99 0.63 

4-6 0.786 5.9 4.61 2.61 16.67 42.85 17.99 1.15 0.48 

6-8 0.754 4.69 2.46 18.58 72.88 21.59 0.89 1.05 

8-10 0.774 4.61 2.54 17.25 57.90 20.88 0.96 0.61 

10-12 0.746 5.31 2.41 24.17 75.89 17.53 1.51 0.69 

12-14 105.4 35.64 20.66 1.07 0.48 

4040 GB425 GBUN1 0-2 0.753 2.35 1.12 10.25 57.29 1.42 

2-4 0.729 2.37 1.11 10.50 58.82 1.42 

4-6 0.684 3.9 1.97 0.94 8.58 45.64 12.57 0.55 0.32 

6-8 0.615 2.04 0.62 11.80 9.13 
9-60 

Table 9.5. Solid phase properties. (continued) 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

8-10 0.616 1.86 0.59 10.58 45.28 0.26 1.07 0.02 

10-12 0.607 2.36 0.80 12.97 49.45 4.40 1.40 0.41 

12-14 0.601 102.5 2.14 0.62 12.66 5.23 0.20 

14-16 0.23 0.02 

16-18 18.76 0.19 0.76 0.03 

18-20 1.82 0.32 0.10 0.10 

4042 GC185 BHM3 0-2 0.857 9.29 6.18 25.92 20.23 8.33 0.62 0.47 

2-4 0.816 0.812 9.58 6.37 26.75 23.16 8.77 0.43 0.47 

4-6 0.806 0.846 3.9 9.58 6.25 27.75 19.71 6.92 0.28 

6-8 0.804 0.790 8.62 5.42 26.67 20.44 7.41 0.42 

8-10 0.760 0.703 8.97 3.75 43.50 22.09 3.99 0.46 0.24 

10-12 0.760 0.533 126.5 8.56 4.03 37.75 24.67 11.42 0.68 

12-14 0.792 0.636 8.90 5.31 29.92

14-16 0.771 0.419 122.1 9.09 6.40 22.42 26.75 9.57 0.78 0.51 

16-18 0.781 0.664 9.33 8.82 4.25

18-20 0.789 0.624 11.30 8.42 24.00 29.99 7.79 0.58 0.44 

20-22 0.791 0.608 11.20 6.46 39.50

22-24 0.769 0.736 8.38 5.38 25.00

4043 GC234 GCB3 

orange Beggiatoa

0-2 0.840 0.438 3.5 5.92 2.89 25.25 46.09 8.44 1.67 0.48 

2-4 0.794 1.000 3.8 6.31 3.34 24.75 55.27 11.40 1.39 1.50 

4-6 0.789 0.555 3.7 6.87 3.19 30.67 30.29 9.24 0.79 0.39 

6-8 0.785 0.464 5.3 6.62 4.59 16.92 47.34 8.11 1.42 0.39 

8-10 0.761 0.389 6.9 8.06 3.48 38.17 55.38 10.94 1.63 1.79 

10-12 0.764 0.441 5.5 8.33 3.69 38.67 41.46 10.85 1.11 1.25 

12-14 0.758 0.518 4.2 117.3 7.08 3.53 29.58 25.38 11.47 1.03 0.90 

14-16 0.756 0.484 4.1 7.95 5.77 18.17 36.02 5.31 1.53 1.12 

16-18 0.754 0.621 0.8 7.69 3.54 34.58 55.26 6.09 1.46 0.95 

18-20 0.755 0.581 6.32 2.74 29.83

20-22 0.756 0.603 4.7 7.45 4.14 27.58 36.62 4.53 0.99 0.66 

22-24 0.753 1.000 6.80 3.41 28.25

24-26 0.677 0.569 7.53 3.63 32.50

4043 GC234 GCB3 

off mat 

0-2 0.862 0.960 1.8 4.50 2.66 15.33 25.51 18.32 1.23 1.03 

2-4 0.847 0.955 5.2 5.12 3.00 17.67 35.51 14.06 1.54 0.95 

4-6 0.842 0.963 4.0 4.61 2.64 16.42 26.54 10.55 1.00 0.49 

6-8 0.806 0.827 2.9 5.75 3.20 21.25 24.37 10.86 1.24 0.46 

8-10 0.805 0.827 2.2 5.75 3.54 18.42 10.95 7.67 0.00 0.37 

10-12 0.814 0.897 2.2 94.6 5.72 3.16 21.33 26.53 11.37 1.12 0.44 

12-14 0.772 0.871 5.91 3.14 23.08

14-16 0.778 0.833 2.4 94.6 5.74 2.83 24.25 21.62 7.06 1.19 0.45 

16-18 0.759 0.805 6.38 3.24 26.17

18-20 0.747 0.768 2.3 7.26 4.18 25.67 25.59 6.38 1.08 

20-22 0.756 0.808 6.61 3.13 29.00

22-24 0.757 0.823 1.5 6.34 4.99 11.25 10.82 0.00 

24-26 0.767 0.858 6.33 3.22 25.92

26-28 0.749 0.813 6.79 4.84 16.25 22.16 0.94 

28-30 0.747 0.836 6.27 3.22 25.42

30-32 0.749 0.697 7.08 3.51 29.75

4045 GC185 BHAT2 0-2 0.857 8.81 5.80 25.08 56.09 31.70 1.00 1.24 

2-4 0.819 8.07 7.34 6.08 0.00 28.46 0.00 0.49 

4-6 0.821 5.4 7.26 5.83 11.92 35.38 13.13 0.73 0.44 

6-8 0.812 7.24 5.37 15.58 53.46 19.95 0.72 

8-10 0.791 7.18 5.56 13.50 62.72 17.71 0.50 0.44 

10-12 0.806 7.41 5.42 16.58 59.19 17.17 0.58 0.50 

12-14 0.811 129.2 7.14 5.40 14.50 30.68 7.08 0.60 0.48 

14-16 0.802 7.24 5.23 16.75 58.65 12.38 0.39 0.48 
9-61 

Table 9.5. Solid phase properties. (continued) 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

16-18 0.792 7.38 5.83 12.92

18-20 0.786 7.11 5.84 10.58 53.19 9.56 0.45 0.49 

20-22 0.789 7.15 5.54 13.42

22-24 0.783 7.22 5.67 12.92 56.92 10.91 0.76 

24-26 0.775 6.89 5.63 10.50

26-28 0.786 7.11 6.01 9.17

4047 GC234 GCJT2 0-2 0.833 0.532 5.47 2.48 24.92 67.58 1.62 1.71 0.08 

2-4 0.814 0.731 5.31 2.38 24.42 54.14 12.69 1.34 0.50 

4-6 0.825 0.804 2.1 4.68 3.04 13.67 23.85 7.84 0.61 0.70 

6-8 0.812 0.749 4.58 2.60 16.50 54.43 0.32 1.06 0.03 

8-10 0.767 0.791 4.21 1.90 19.25 57.24 1.94 0.00 

10-12 0.746 0.809 4.11 1.81 19.17 61.27 0.18 1.42 0.02 

12-14 0.730 1.000 134.0 4.23 1.68 21.25 26.89 7.04 1.37 0.86 

14-16 0.711 0.870 4.45 2.32 17.75

16-18 0.739 0.854 4.76 2.17 21.58 72.01 0.67 1.38 0.08 

18-20 0.749 0.764 4.00 1.44 21.33

20-22 0.754 0.860 4.27 1.50 23.08 54.18 0.27 1.15 0.03 

22-24 0.765 0.830 3.90 1.95 16.25 58.06 0.95 1.62 0.07 

24-26 0.754 0.889 3.80 1.64 18.00

26-28 0.765 1.000 3.68 1.51 18.08

4048 GC234 GCAT2 0-2 0.864 0.875 4.7 5.72 2.94 23.17 65.71 12.92 1.28 

2-4 0.809 0.824 3.6 6.14 3.13 25.08 77.54 15.93 1.36 0.45 

4-6 0.790 0.862 5.1 6.68 3.54 26.17 58.21 10.83 1.11 0.54 

6-8 0.778 0.900 5.3 5.63 2.69 24.50 63.63 7.47 1.53 0.52 

8-10 0.776 0.880 1.8 5.74 2.34 28.33 53.17 4.13 1.54 

10-12 0.778 0.767 2.5 71.0 5.79 3.05 22.83 11.82 7.61 0.71 1.06 

12-14 0.770 0.870 0.00

14-16 0.763 0.889 2.3 116.4 5.52 3.39 17.75 22.94 43.54 0.97 1.01 

16-18 0.761 0.897 5.51 2.49 25.17

18-20 0.766 4.4 5.40 2.54 23.83 66.03 4.60 0.82 0.43 

20-22 0.763 0.904 5.64 2.56 25.67

22-24 0.765 0.857 4.6 5.68 2.95 22.75 62.51 5.51 1.44 0.47 

24-26 0.768 0.847 2.0 5.33 2.51 23.50 64.72 4.97 1.37 0.42 

26-28 0.767 0.913 5.03 2.63 20.00

28-30 0.771 0.888 5.06 2.45 21.75

30-32 0.745 0.866 5.48 2.45 25.25

32-34 0.739 1.000 5.25 2.36 24.08

4053 GB425 GBM2 0-2 0.863 0.946 2.68 1.55 9.42 89.05 18.45 2.29 1.07 

2-4 0.740 0.909 3.10 1.10 16.67 106.1 19.30 3.72 1.64 

4-6 0.662 0.861 1.4 3.03 0.70 19.45 108.8 14.89 4.73 2.91 

6-8 0.635 0.850 2.68 0.74 16.13 113.5 8.71 4.91 1.11 

8-10 0.616 0.880 2.50 0.83 13.95 74.39 6.03 3.36 0.56 

10-12 0.598 0.895 117.3 2.35 0.78 13.12 96.17 14.61 4.25 2.21 

12-14 0.594 0.876 2.38 0.77 13.46

14-16 0.629 0.890 114.4 2.50 0.69 15.09 86.00 16.57 4.11 2.30 

16-18 0.619 0.913 2.30 0.69 13.40

18-20 0.607 0.874 2.33 0.71 13.49 11.29 12.46 0.38 0.49 

20-22 0.624 0.883 2.39 0.94 12.06

22-24 0.645 0.938 2.25 0.75 12.53 42.89 4.31 1.41 0.97 

24-26 0.626 0.871 2.18 0.91 10.62

26-28 0.611 0.847 2.39 0.23 18.00

28-30 98.47 18.54 4.77 1.80 

box GC185 BHBXC1 0-2 0.851 2.82 0.85 16.38 36.54 23.53 2.80 2.20 

2-4 0.838 2.69 0.93 14.68 64.67 19.83 3.25 1.65 

4-6 0.827 2.69 0.94 14.55 36.25 31.83 3.46 2.87 

6-8 0.808 2.56 0.99 13.06 78.74 17.48 3.58 1.75 

8-10 0.802 2.79 1.04 14.58 79.18 19.24 3.56 1.73 
9-62 

Table 9.5. Solid phase properties. (continued) 

Sulfur Extractable Metals mols/g) 

mols/g) 

Carbon 

 (weight percent) Fe Mn 

Dive 

Number Site Station Depth (cm) 

Porosity (

) 

Grain Size 

>65 m 

Acid Volatile 

AVS 

Total Reduced 

TRS 

Total 

Organic 

Inorganic 

( as CaCO3) 

HCl 

Citrate Dithonite 

HCl 

Citrate Dithonite 

10-12 0.804 97.5 2.69 0.96 14.40 41.51 4.50 2.11 0.41 

12-14 0.805 2.83 0.98 15.43

14-16 0.804 2.59 1.01 13.17 50.63 21.17 9.45 7.17 

16-18 0.798 2.46 0.92 12.88

18-20 0.804 2.53 0.98 12.91 50.25 30.18 5.04 3.78 

20-22 0.802 2.64 0.98 13.88

22-24 0.804 2.81 0.98 15.28 42.43 11.44 4.72 0.78 

24-26 0.797 2.59 0.93 13.88

26-28 0.800 2.46 0.92 12.87

28-30 0.776 2.46 1.02 12.00

30-32 0.771 2.68 0.90 14.86

box GC185 BHUN2 0-2 0.812 0.818 4.71 3.44 10.58 79.56 23.80 0.94 0.69 

2-4 0.785 0.704 4.60 2.36 18.67 81.75 21.55 1.03 0.61 

4-6 0.812 0.918 6.00 3.98 16.83 65.70 16.12 0.00 0.53 

6-8 0.795 0.917 6.23 4.40 15.25 86.13 1.05 0.53 

8-10 0.792 0.910 6.61 5.49 9.33 74.54 11.52 1.11 0.51 

10-12 0.787 0.861 6.85 5.46 11.58 70.46 14.68 1.04 0.55 

12-14 0.778 0.937 121.6 7.74 5.93 15.08 44.12 7.54 0.00 0.43 

14-16 0.777 0.933 7.35 5.64 14.25 61.96 10.98 0.94 0.49 

16-18 0.790 0.948 7.06 5.68 11.50

18-20 0.787 0.913 7.69 6.27 11.83 57.01 9.76 0.57 1.65 

20-22 0.758 0.932 7.85 6.04 15.08 71.27 12.00 0.86 0.94 

22-24 0.792 0.925 8.15 6.01 17.83

box GC234 GCBX2 0-2 0.908 2.59 1.49 9.17 77.36 40.03 40.89 25.24 

2-4 0.872 2.68 1.53 9.58 64.95 68.42 28.30 50.96 

4-6 0.835 0.2 2.83 1.45 11.50 47.70 51.89 3.41 4.83 

6-8 95.11 36.08 3.18 2.19 

8-10 66.44 54.94 1.35 2.13 

10-12 3.5 44.28 44.24 2.76 1.90 

14-16 2.4 39.07 37.82 1.42 1.79 

18-20 0.792 2.86 1.53 11.08 125.0 53.45 3.10 1.86 

22-24 0.778 2.89 1.21 14.00 112.3 58.38 2.21 3.33 

26-28 0.796 3.24 1.33 15.92 89.19 53.03 2.24 1.88 

box GC234 GCBXC1 0-2 0.859 3.15 1.82 11.08 56.64 43.46 1.35 0.81 

2-4 0.816 0.924 12.6 2.98 1.36 13.50 43.16 32.81 1.51 0.49 

4-6 0.814 0.924 8.4 2.94 1.23 14.25 87.01 27.78 1.52 0.92 

6-8 0.808 0.955 2.76 1.34 11.83 54.72 21.88 1.46 0.89 

8-10 0.786 0.959 130.0 2.86 1.11 14.58 1.71 16.60 0.03 0.96 

10-12 0.762 0.967 124.1 2.43 1.20 10.25 47.16 10.03 1.41 0.44 

12-14 0.793 0.964 2.60 1.14 12.17

14-16 0.797 0.961 106.0 2.69 1.09 13.33 34.94 10.45 1.32 0.44 

16-18 0.781 0.953 3.28 1.24 17.00

18-20 0.776 0.962 2.72 1.20 12.67 32.22 11.07 0.74 0.46 

20-22 0.768 0.958 3.05 1.11 16.17

22-24 0.773 0.967 113.8 2.68 1.10 13.17 29.26 10.58 1.35 0.57 

24-26 0.776 2.90 1.14 14.67

26-28 0.765 3.03 1.24 14.92

28-30 0.755 111.6 3.46 2.72 6.17 39.10 10.22 1.42 

30-32 0.755 2.86 1.18 14.00
9-63 

Figure 9.13. Stable isotopes in carbonates. The relationship between 18O and  13C (0/00 vs PDB standard) in

CaCO3. 
9-64 

Figure 9.14. Sulfur stable isotopes in H2S and TRS. Sulfur stable isotopes in H2S and TRS in 0/00 plotted against 

dissolved sulfate concentration. 
9-65 

9.3.3 Discussion 

The probable explanation for the observed relationship between CaCO3 13C and 18O is that it 

represents a mixture of biogenic CaCO3, largely from pelagic organisms, and CaCO3, which is 

being formed authigenically in the sediments. Support of this hypothesis is made by the 

increasingly negative 13C values and increasingly positive 18O values with increasing CaCO3 

concentration (Figure 9.15A), and an increasing CaCO3 content with increasing organic matter 

concentration (Figure 15B). Further evidence of extensive precipitation in these sediments is the 

major loss of dissolved calcium from pore waters. Figure 9.16 shows the relationship between 

DIC concentrations and 13C-DIC. It includes an approximate line for the trend that would be 

expected if oxidation of marine organic matter were the primary source of added DIC. Although 

many points fall close to this line, many others fall both to the right and left of the line. A 

possible explanation, for the C13-DIC data falling to the negative side of the line, is that a more 

isotopically negative source of organic matter is being oxidized (e.g., thermogenic methane). In 

cases where the C13-DIC is on the positive side of the line, methane formation is likely, leaving 

behind isotopically heavier DIC. 

The sulfur isotope ratios in TRS are, while relatively wide ranging, well within the range 

observed for marine muds for over a quarter of a century. Thus, even though the sedimentary 

geochemistry and extremely high sulfate reduction rates at the active sites are not typical for 

marine sediments, the sulfur stable isotopes do not indicate anything unusual is occurring in the 

sulfur system. The explanation for the causes of the strong positive sulfur stable isotope ratios in 

H2S is not certain. A major factor influencing sulfur stable isotope ratios is how “open or closed” 

the system is with respect to the sulfate that is the source for sulfide. The situation is further 

confounded by sulfur cycling within the sediment where sulfides can be oxidized to sulfate, 

which may then again be reduced to sulfide. Certainly, one way to produce positive sulfide 

values is to have a system that approaches being closed as is evidenced by reduction of almost all 

dissolved sulfate. As this occurs, the sulfate must become more positive, and the resulting H2S 

that is produced from the residual sulfate will also become more positive. The scatter to higher 

stable isotope ratios with increasing extent of sulfate reduction (Figure 9.14) is consistent with 

this explanation but does not necessarily prove it is the only possibility. Since there is vastly 

more solid-phase sulfide than dissolved sulfide, this “end member” affect will not be readily 

observed in TRS sulfur isotope ratios. 

9.4 Microelectrode profiling*

9.4.1 Introduction 

As discussed in the first section of this chapter, the biogeochemical cycling of sulfur, mainly 

sulfate reduction, H2S mineral formation and H2S oxidation, fuels important heterotrophic 

(sulfate reduction coupled to organic carbon oxidation) and autotrophic (H2S oxidation coupled 

to CO2 reduction) processes in Gulf of Mexico hydrocarbon seep chemosynthetic communities. 

Solid-state gold-mercury (Au-Hg) amalgam microelectrodes and square and cyclic wave 

voltammetry were used to acquire millimeter scale concentration profiles of dissolved H2S in the 

 

*

 This section was authored by Samatha Joye, Susan Escortia and John W. Morse. 
9-66 

Figure 9.15. Calcium carbonate concentration. The relationship between calcium carbonate concentration and (A) 13C and (B) o
9-67 

Figure 9.16. DIC stable carbon isotopes. The concentration of DIC versus its 13C (o/oo) value. 
9-68 

upper six centimeters of sediment (Brendel 1995; Brendel and Luther 1995) with the goal of 

constraining sediment-water fluxes of H2S. The dissolved species profiles obtained using 

microelectrodes can easily document the steep, mm-scale concentration gradients that are either 

obscured or erased by more traditional scale-sampling techniques, such as squeezing or 

centrifuging cm-scale core sections, used to collect pore water for chemical analyses. While 

solid-state Au-Hg microelectrodes have clear advantages over traditional membrane-diffusion 

microelectrodes (mainly they are easier and less expensive to fabricate), they are prone to 

technical difficulties under harsh environmental conditions (Brendel 1995). Performance must be 

carefully documented when conditions are sub-optimal, particularly if the goal is to micro profile 

sediments characterized by high concentrations of DOC and/or H2S. 

The objectives of the microelectrode work were as follows: 

1. Acquire mm-scale depth profiles of dissolved H2S in sediment pore water: The upper six 

centimeters of the sediment is a region of active material cycling and represents the region 

where aerobic and anaerobic processes may be coupled as oxygen diffuses into the sediment 

from the overlying water column. Documenting the H2S profiles within this region allowed 

us to calculate sediment-water interface exchange (i.e., fluxes) of H2S. 

2. Calculate sediment-water fluxes of H2S: The sediment-water flux of H2S may serve as a 

means to group chemosynthetic community types (bacterial mat versus tubeworm, etc.) and 

provide insight to the mechanisms by which different chemosynthetic communities cycle 

H2S in the upper reaches of the sediment. 

9.4.2 Results 

Despite precautions and monitoring of the electrode performance, saturation of the electrode 

surface was frequently inescapable. Thus, the concentrations obtained using microelectrodes 

sometimes underestimated concentrations determined using traditional (colorimetric) techniques 

(Escorcia 2000). On some occasions, no usable data were obtained, e.g., when the detectable 

range was exceeded (Escorcia 2000). These results present the internally consistent findings 

from a selected subset of the data. 

Micro profiles of H2S concentration in cores collected from Beggiatoa mats (Figures 9.17 and 

9.18) and tubeworm bushes (Figure 9.19). The strategy in 1997 was to collect cores from 

representative environments (i.e., tubeworm bushes, bacterial mats, mussel beds, and control 

sites) to document the pore-water H2S distribution in different community types. After 

examining the 1997 results, the sampling strategy was altered in 1998 to collect cores from 

transects crossing from white to orange bacterial mats (Figure 9.18) and from the drip line and 

periphery of tubeworm bushes (Figure 20B and C). Cores were classified by macro community 

type following confirmation via video or the dive log. Table 9.6 summarizes the sediment-water 

interface fluxes of H2S calculated using Fick’s First Law of Diffusion, H2S concentrations from 

the core water overlying and a depth of 0.4 cm, and diffusion constants for H2S described earlier 

in this chapter. Table 9.7 summarizes the sediment oxygen demand estimated from measured 

sulfate reduction rates (see Tables A.9.5 and 9.1) and underscores the inherent problem 

associated with microelectrode profiling aboard ship (as opposed to in situ micro profiling with a 

benthic lander). 
9-69 

Figure 9.17. Control H2S profiles. Microelectrode profiles of the average pore water H2S concentration in control cores from 1997 (A
9-70 

Figure 9.18. Beggiatoa H2S profiles. Microelectrode profiles of the average pore water H2S concentration in 

cores from white Beggiatoa mats in 1997 (A) and 1998 (B) and orange Beggiatoa mats in 1997 

(C) and 1998 (D). 
9-71 

Figure 9.19. Tubeworm H2S profiles. Microelectrode profiles of the average pore water H2S concentration in cores from tubeworm 

and 1998 (B: > 1m from bush and C: at the bush dripline). 
9-72 

Figure 9.20. Transect H2S profiles. Microelectrode profiles of the average pore water H2S concentration in 

cores collected along a transect from no mat (small circles), to orange/white mat (medium 

circles), to orange mat (large circles). 
9-73 

Table 9.6. Sulfate reduction rate (35S). 

Dive 

Number Site Station Core Incubation Time 

(h) 

Injection Depth 

(cm) 

Sulfate Reduction Rate 

(mmols/cm3

/sec x 109) 

1 2.21

3 2.78

5 1.95

7 1.70

9 1.92

11 1.22

2871 GC234 GCST1 5 11.3 

13 2.22

1 647.05

3 157.21

5 382.91

7 387.22

9 10.58

2872 GC234 

GCAT2 

(SO4 data from 

dive 2886) 

3 11.0 

17 2.66

1 3.85

3 9.13

5 1.47 2873 GC185 BHM4 6 10.6 

17 0.03

1 1.94

3 2.63

5 1.48

7 1.72

2873 GC185 BHAT1 4 9.8 

9 1.38

1 1.18

3 2.15

5 1.10

7 2.51

9 1.79

11 0.81

13 1.74

15 1.65

2874 GC185 BHST1 5 11.5 

17 1.33

1 1.65

3 1.78

5 2.86

7 2.14

9 2.18

2875 GC185 BHST2 4 12.0 

11 2.46

1 0.44

3 0.75

5 1.49

7 0.90

9 1.30

11 0.76

13 1.02

15 1.87

17 1.24

19 1.31

21 0.70

2875 GC185 BHUN1 5 18.0 

23 0.67
9-74 

Table 9.6. Sulfate reduction rate (35S). (continued) 

Dive 

Number Site Station Core Incubation Time 

(h) 

Injection Depth 

(cm) 

Sulfate Reduction Rate 

(mmols/cm3

/sec x 109) 

1 246.59

3 431.15

5 15.63

7 2.33

9 1.58

11 1.12

13 1.05

17 1.43

19 1.37

box GC185 BHBC2 1 18.0 

21 2.26

1 1.94

3 1.33

5 1.35

7 0.72

9 1.12

11 1.29

13 0.19

2880 GC234 GCAT1 

(GCB1@GCAT1) 5 10.0 

15 1.03

1 83.15

3 791.97

5 17.10

7 10.32

2882 GB425 GBM2 

(GBB1@GBM2) 8 17.0 

9 20.53

1 12.61

5 11.85

9 5.33 2883 GB425 GBM1 8 9.7 

15 5.18

2886 GC234 GCAT2 6 12.0 

1 

3 

5 

7 

9 

 26.94

 171.81

 273.98

 443.53

 74.36

1 1.57

3 5.04

7 1.46 2888 GC234 GCAT1 8 12.0 

11 1.86

1 12.73

3 2.33

7 3.79 2891 GC185 BHAT2 7.8 

11 2.95

1 1.72

3 1.65

5 1.36

7 1.18

9 0.75

11 0.75

13 0.60

4027 GC185 BHST1 1 14.3 

17 0.77

1 4.20

4028 GC185 BHB3 6 12.0 3 2.50

5 0.33
9-75 

Table 9.6. Sulfate reduction rate (35S). (continued) 

Dive 

Number Site Station Core Incubation Time 

(h) 

Injection Depth 

(cm) 

Sulfate Reduction Rate 

(mmols/cm3

/sec x 109) 

1 10.10

3 12.21

5 45.55

7 40.85

9 176.75

11 291.48

13 0.00

4029 GC234 GCAT2 5A 4.1 

15 47.69

1 4.62

3 7.22

5 7.27

7 9.58

9 8.43

11 6.87

13 5.23

4029 GC234 GCAT2 5B 8.5 

15 3.64

1 7.57

3 10.30

5 11.76

7 7.67

9 7.22

11 3.82

13 1.69

4029 GC234 GCAT2 8A 13.3 

15 0.89

1 0.60

3 3.13

5 2.44

7 1.62

9 1.47

11 1.84

13 1.57

15 1.65

17 1.82

4031 GC234 GCST1 8 11.8 

19 1.16

1 10.01

3 9.06

5 6.22

7 5.20

9 4.56

4038 GC233 BPB2 7 12.0 

11 1.70

1 12.25

3 10.88

5 13.34

7 19.49

9 28.53

11 20.34

13 29.89

4043 GC234 GCB3 

orange Beggiatoa 1 13.1 

15 19.45

1 3.21

3 2.44

5 2.39 4043 GC234 GCB3 white Beggiatoa 9 12.4 

7 1.78
9-76 

Table 9.6. Sulfate reduction rate (35S). (continued) 

Dive 

Number Site Station Core Incubation Time 

(h) 

Injection Depth 

(cm) 

Sulfate Reduction Rate 

(mmols/cm3

/sec x 109) 

1 70.04

3 79.71

5 65.75 box GC185 BHUN2 1 12.1 

7 37.49

1.5 2.74

2.5 1.79

3.5 1.30 4046 GC185 BHM4 5 4.2 

4.5 1.55

1 3.24

3 1.39

5 1.70

7 1.70

9 1.25

11 1.41

13 0.52

15 0.85

17 0.78

19 0.99

21 0.67

4047 GC234 GCJT2 3 13.7 

23 0.45

1 37.11

3 57.86

5 75.52

7 48.35

9 27.03

11 17.77

13 7.27

15 3.18

4048 GC234 GCAT2 5 11.9 

17 1.01

4053 GB425 GBM2 5 12.3 1 186.89
9-77 

Table 9.7. Comparative sulfate reduction rates 

Locality 

water 

depth 

(m) 

Integrated SO4

Reduction rate 

(mmoles-SO4/m2

/day) 

Great Sippewissett Marsh, Massachusetts1 ~ 107- 420 

Cape Lookout Bight, North Carolina2 8 149.9 

Black Hole, Long Island Sound (i) 10 35.6 

Saanich Inlet, British Columbia (ii) 225 16.5 

Gulf of Mexico shelf, Mississippi delta (iii) 59 13.8 

Skagerrak, Danish coast (iv) 200 12.4 

Gulf of Mexico shelf, Texas (reference) 151 2.11 

MANOP, Station M, Pacific (v) 3100 0.004 

 

i. J. T. Westrich, Ph.D. dissertation, Yale University (1983). 

ii. A. H. Devol, J. J. Anderson, K. Kuivia, J. W. Murray, Geochim. Cosmochim. Acta 48, 993 (1984). 

iii. S. Lin and J. W. Morse, Amer. J. Sci. 291, 55 (1991). 

iv. N. Iversen and B. B. Jørgensen, Limnol. Ocean. 30, 945 (1985). 

v. This rate is not obtained from direct measurement but from a model calculation in B. Bender and D. T. Heggie, 

Geochim. Cosmochim. Acta 48, 977 (1984). 

Though the microelectrode system we used is capable of simultaneous quantification of 

dissolved oxygen and dissolved H2S concentration, we never documented any dissolved oxygen 

in the water overlying sediment cores or in the sediment pore water. The lack of dissolved 

oxygen in these cores probably reflects an artifact stemming from the unavoidable time delay 

between core collection and microelectrode profiling. On a typical day, approximately 4-8 hours 

elapsed between core collection using the JSL submersible and initiation of micro profiling. An 

additional 2-3 hours elapsed between the completion of the first micro profile and the initiation 

of the second micro profile in a given core. These delays were unavoidable. 

Submarine dives lasted 2-4 hours, retrieval and cataloging of cores required 1-2 hours, and 

preparing the cores and microelectrodes (repeated standardization) required an additional 1-2 

hours. Approximately 4-6 hours were required to complete two 6-cm deep profiles in one core, 

meaning that there was an addition 2-3 hour lag between replicate profiles in a given core. We 

hypothesized that during this time, dissolved oxygen in the overlying water was rapidly 

consumed given the (predicted) high sediment oxygen demand. We attempted to remedy this 

artifact in 1998 by aerating the overlying waters with an aquarium pump once the cores were on 

deck; however, aeration for one hour did not restore oxygen to detectable levels. To evaluate the 

potential oxygen demand associated with H2S oxidation, we used the integrated sulfate reduction 

 

1

 Higher rates are reported by R. W. Howarth and J. M. Teal, Limnol. Ocean. 24, 999 (1979); Lower rates are from 

G. M. King, B. L. Howes, J. W. H. Dacey, Geochim. Cosmochim. Acta 49, 1561 (1985). 2

 A maximum sulfate reduction rate is taken from July 1983 data reported in P. M. Crill and C. S. Martens, 

Geochim. Cosmochim. Acta 51, 1175 (1987). 
9-78 

rates to estimate oxygen consumption due to complete oxidation of H2S oxidation to sulfate. This 

calculation does not consider the oxygen demand associated with other metabolic processes, 

such as aerobic respiration or methane oxidation, so should be considered a conservative 

estimate of the sediment oxygen demand. This calculation requires the following assumptions: 

1. The sediment surface area in a core was flat and averaged 38.5 cm2

; 

2. Two moles of oxygen were consumed per mole of H2S oxidized completely to SO4

2-; 

3. The overlying water was saturated with oxygen at bottom water temperature (7°C) and 

salinity (35‰), i.e., the concentration of dissolved oxygen was ≈ 300 μM (Pilson 1998); 

4. A 100 mL water phase was present above the core. 

For example, consider a core having a daily depth-integrated sulfate reduction rate (SRR) of 232 

mmol S m-2 d-1 (this value represents the average rate at sites with high SRR; Tables A.9.5 and 

9.1), the hourly SRR was 9.7 mmol S m-2 hr-1. Reoxidation of the generated H2S resulted in an 

oxygen demand of 19.3-mmol m-2 hr-1. Given a surface area of 38.5 cm2

, the total sediment 

oxygen demand in the core was 0.074-mmol hr-1 or 74-μmol hr-1. A 100 mL overlying water 

phase saturated with oxygen when the core was collected would have contained only 

approximately 30 μmoles of oxygen. At the calculated oxygen consumption rate (74 μmol hr-1), 

this oxygen (~ 30 μmoles) would have been consumed in approximately 25 minutes. Values of 

the oxygen consumption due to H2S oxidation in sediments exhibiting low, high, and extremely 

high SRR rates are shown in Table 9.7. In all cases, complete consumption of oxygen from the 

overlying water would have occurred by the time micro profiling began. This calculation 

considers only one sink for oxygen (i.e., H2S oxidation) so actual oxygen consumption rates are 

likely to be even faster. Even sites characterized by ‘low’ rates of SRR (i.e., low for CHEMO 

communities) were prone to rapid oxygen depletion. Once oxygen was depleted, sediment 

oxygen consumption could have maintained oxygen concentrations below detectable levels, even 

if the water column was aerated with an aquarium pump. The oxygen demand in CHEMO 

sediments is extreme. For comparison, the oxygen demand in sediments of the South Atlantic 

and Danish coastal regions are 0.07 to 0.2 and 0.6 to 0.8-mmol m-2 hr-1, respectively (Glud et al.

1994; Gundersen and Jorgensen 1990). The oxygen demand in sediments from Galveston Bay, 

Texas, was 0.3-mmol m-2 h-1 (An 1999; An and Joye 2000). The rates from deep-sea sediments 

are two orders of magnitude lower than those typical of low SRR GOM hydrocarbon seep sites, 

while the rates from estuarine environments are approximately half of that estimated at “low” 

SRR GOM hydrocarbon sites. 

9.4.3 Discussion 

Sediment micro profiles of H2S concentration revealed significant differences between 

chemosynthetic community types. The following section presents the average H2S concentration 

profiles for cores collected from control sites, within Beggiatoa mats and near tubeworm bushes. 

Cores from mussel beds were not profiled with microelectrodes, because it was difficult to 

collect viable cores. However, several cores from Beggiatoa mats adjacent to mussel beds were 

collected and sampled (these are classified as Beggiatoa cores). From these profiles, sedimentwater interface H2S fluxes were calculated for each year and community type. The most striking 
9-79 

and consistent conclusion based on the microelectrode data is that Beggiatoa mats exploit 

regions characterized by high H2S flux. 

Average H2S profiles in cores collected from white (A, B) and orange (C, D) Beggiatoa mats in 

1997 (A, C) and 1998 (B, D) are presented in Figure 9.17. Profiles of H2S concentration obtained 

from Beggiatoa mats exhibit sharp concentration gradients near sediment water interface (SWI). 

In one core from 1997 (GCB-1), a large drop in concentration was observed at approximately 1.5 

cm (Figure 9.17A). We attribute this to a loss of sensitivity in the microelectrode (most likely 

due to amalgam degradation). This feature did not reflect the actual pore water concentration. 

Oxygen was never documented in the water overlying these cores; however, H2S was commonly 

present in the overlying water (at concentrations up to 170 μM), suggesting that our assumption 

of high sediment oxygen demand and rapid oxygen consumption is valid. Concentration changes 

were most obvious within the upper two cm; below that depth, concentrations increased 

gradually. 

Three cores collected along a transect grading from an uninhabited sediment to sediment 

dominated by orange and/or white Beggiatoa were micro profiled in 1998 (Figure 9.18). The 

core collected from the area without a visible Beggiatoa mat exhibited negligible H2S 

concentrations (< 2 μM). The core from the center of the mat, which was dominated by orange 

Beggiatoa, exhibited H2S concentrations ranging from negligible (in overlying water) to 280 μM 

(at 5 cm). The transition zone between orange Beggiatoa and no Beggiatoa mat was a mixture of 

white and orange Beggiatoa. This region exhibited the highest H2S concentrations (745 μM at 3 

cm). Both cores collected from Beggiatoa inhabited sediments exhibited sharp increases in H2S 

concentration near the SWI. 

Pore water H2S profiles from sediment cores collected near tubeworm bushes are presented in 

Figure 20. In 1997 (Figure 9.20A), profiles from cores (n = 4) collected at the drip line of 

tubeworm bushes exhibited H2S depletion in the upper 3 cm. Below this depth, H2S 

concentration gradually increased. One core was collected approximately 0.5 m away from the 

bush drip line (BHST2-7) and higher H2S concentration were observed in this core. To 

investigate the hypothesis that sediments adjacent to the tubeworm bush (i.e., at the drip line) 

exhibited lower H2S concentration, we collected sets of cores at the drip line (Figure 9-19C) and 

away from the drip line (> 1 m, Figure 9.20B) in 1998. In 10 of 12 cores, H2S was depleted 

throughout the upper 3.5 (or more) cm. In the remaining two cores (GCAT1-9A on Figure 9-19B 

and GCJT2-4 on Figure 9.20C), higher H2S concentrations were observed but there was no 

consistent pattern over depth. Data from 1997 and 1998 support the hypothesis of Julian et al.

(1999a) that tubeworm roots actively take up H2S from the sediment contributing to H2S 

depletion, particularly within the upper 4 cm. 

Sediment-water interface fluxes of H2S were compared between community types to identify 

distinguishing community features (Table 9.6). Negative values predict a flux from the sediment 

to the overlying water while positive values predict a flux from the overlying water to the 

sediment. However, we do not believe that H2S fluxes from the overlying water into the 

sediment. Our large geochemical database shows clearly that large reservoirs of H2S are found at 

depths greater than 6 cm in these sediments. We believe that (calculated) positive fluxes are an 

artifact related to the depletion of oxygen from the overlying water during core processing. 

Oxygen depletion could restrict H2S oxidation, thereby shifting the H2S profile up into the water 
9-80 

column. Microbially-mediated H2S oxidation coupled to nitrate reduction within upper sediment 

layers could consume H2S in the absence of oxygen, yielding profiles that suggest an H2S flux 

from the overlying water into the sediment. Explanation of these profiles requires an active H2S 

oxidant (other than oxygen) at the sediment-water interface, which is entirely feasible based on 

the known ability of some microorganisms to couple H2S oxidation with NO3 reduction. 

Fluxes from Beggiatoa mats were not related to mat coloration. Negative fluxes (from the 

sediment to the water column) were consistently documented for both mat types and fluxes 

ranged from 0 to -200 mmol m-2d-1. In cores collected along a transect grading from no mat 

(outside) to orange/white mat (transition) to orange mat (interior), fluxes varied from 0 (outside) 

to -137 (transition) and -44 (interior) mmol m-2 day-1. Because of the variability between 

replicate profiles, the calculated fluxes in the transition zone were not significantly different from 

those observed in the interior. In 1997, both positive and negative fluxes were calculated from 

H2S profiles in cores collected near tubeworm bushes. Positive fluxes (i.e., a sediment sink) of 

H2S within the top cm were observed in some sites (0-6 mmol m-2d-1), while large gradient 

supported negative fluxes were observed at one site (BHST2-7). In 1998, only 2 of 13 profiles 

supported an H2S flux (an efflux). Fluxes from the remaining cores were insignificant (not 

distinguishable from zero). 

Hydrogen sulfide concentrations and distribution in sediment collected from within Beggiatoa

mats were distinct from those obtained in sediment associated with tubeworm bushes. Sediment 

beneath Beggiatoa mats was characterized by sharp gradients in H2S concentrations and 

increasing concentrations with depth. In contrast, sediment from areas inhabited by tubeworm 

bushes exhibited H2S deletion within the upper 2-3 centimeters and slight increases in H2S 

concentrations with depth. While calculated fluxes from Beggiatoa cores were similar in 1997 

and 1998, fluxes from tubeworm were not detectable (in 11 of 13 cores) in 1998. This difference 

probably arose because of our effort to reestablish oxygen concentrations in the overlying water 

in 1998. By aerating the overlying water for 1 hour, we prevented upward migration of the H2S 

profiles. Thus, a ‘zero’ sediment-water interface H2S flux is probably more representative of 

cores collected from tubeworm bushes than the “positive” fluxes we calculated in 1997. 

However, pore water H2S concentrations in sediment associated with tubeworm bushes were 

lower at depths greater than 3 cm in 1998 compared to 1997, suggesting that hydrocarbon 

seepage rates may have declined between 1997 and 1998. Short-term (annual) variations in

seepage rates are difficult to document, but would not be surprising. Since tubeworms are sessile 

organisms, they must cope with environmental variability. Changes in tubeworm “condition” and 

in the organic geochemistry of the sediments associated with tubeworm environments may 

provide more definitive proof of inter-annual variability in hydrocarbon seepage rate. H2S 

profiles from tubeworm bushes support the theory that tubeworm roots consume H2S from 

sediments (Julian et al. 1999a), as the upper sediments consistently exhibit H2S depletion. 

Though our microelectrode H2S profiles only penetrated to 6 cm, additional H2S measurements 

to depths of 20 cm confirm an increase in H2S concentration below the average depth of the root 

ball (Table A.9.1). 

No reduction in pore water H2S concentration was observed in cores collected from Beggiatoa

mats in 1998. Beggiatoa mats have the ability to exploit their environment by gliding into 

favorable chemical positions under their own locomotion. These microorganisms may “colonize” 
9-81 

other areas following disturbance via gas or oil ebullition or bottom currents. Beggiatoa mats are 

not found in regions where there is no significant sedimentary H2S source (e.g., Figure 9.20), 

suggesting Beggiatoa do not proliferate in regions where the H2S flux is insignificant. This 

feature explains the apparent insensitivity of these Beggiatoa mats to changes in hydrocarbon 

seepage rates. Rather, Beggiatoa are opportunistic and occur only where environmental 

conditions are optimal. 

9.5 Conclusion 

The observations discussed above can be summarized as follows: 

SRR is a strong discriminator. The overall geochemistry of chemosynthetic communities is 

strongly controlled by the rate and extent of sulfate reduction, and SRR in concert with coupled 

reactions serves to characterize stations as active versus inactive. Maximum measured sulfate 

reduction rates are extremely high at certain stations, and are associated with quantitative 

reduction of sulfate to sulfide. Comparison of the corrected DIC concentrations with sulfate 

depletion calculations implies methane oxidation. However, this activity is highly focused and 

localized, and represented only by a minority of the stations visited. The majority of sites appear 

to have sulfate reduction rates that are typical of the northern Gulf of Mexico. 

Elemental distributions reflect sulfate reduction and iron limitation. Consumption of sulfate is 

balanced by H2S production, and the extent of reaction reflects local dynamics. Production of 

bicarbonate during sulfate reduction serves to precipitate CaCO3 – this reaction is detectable both 

as TIC and by depletion of dissolved calcium. Production of AVS appears to be limited by low 

quantities of reactive iron. Of the total solid phase sulfide pool, well over 95% is present as 

pyrite (TRS). Iron limitation gives rise to the very high dissolved sulfide concentrations observed 

at these stations. Reactive (citrate-dithonite) iron probably varies considerably in reactivity, and 

substantial “excess” iron apparently remains unsulfidized despite high H2S concentrations. 

Constraints imposed by kinetic rate constants suggest that this iron has been available no longer 

than approximately 104

 years, fixing an approximate maximum age for this system. 

The depth of sulfate depletion probably reflects a localized balance between activities of nearsurface sulfide oxidizers and sulfide reducing bacteria (SRB) at depth. The pattern of sulfate 

depletion with respect to depth is variable and possibly related to specific habitat. Beggiatoa

mats, although located over high rates of sulfate reduction, appear to exert strong control on 

profiles of pore water sulfate, which show only modest depletion. This pattern stands in contrast 

with profiles observed at GCAT, which may thus reflect both tubeworm and bacterial mat 

influences. Observations of freely bubbling gas and high methane concentrations, particularly at 

GCAT2, suggest that organic matter is not limiting, and it is likely that methane fluxes are high 

enough that even intense activity by SRB brings about little depletion. However, these conditions 

are strongly localized: methane concentrations are also observed to be highly variable, and the 

flow of gas from depth may be focused along preferred avenues of escape. Rapid decomposition 

of shallowly buried methane hydrate could also modify sedimentary structure and introduce 

methane fluxes that are very high but also transient and short lived. 
9-82 

Brines are localized influences only, appear to be Na-Ca-Cl in composition with a salinity 

roughly three times seawater, and are depleted in Mg and Sr. Brine fluids lack sulfate and 

sulfide. Significant sulfate reduction at these sites is thus confined to the shallow, near-surface 

mixing zone. 

Carbon and oxygen stable isotope ratios strongly support the formation of authigenic carbonates, 

which can frequently quantitatively dominate over carbonates from biogenic sources in the upper 

water column. 

Sulfur stable isotope ratios are consistent with the highly complex biogenic sulfur system present 

in many of the sediments. This results in considerable variability in their ratios that does not 

reflect simple relationships to other chemical parameters. 

The microelectrode studies demonstrated the difficulties associated with working in sediments 

where high concentrations of DOC are present and dissolved sulfide concentrations vary by 

orders of magnitude. The considerable time period between core collection and when the cores 

became available for study also made precise determinations of interfacial gradients difficult. In 

spite of these difficulties clearly different patterns of near-interfacial dissolved sulfide were 

observed in association with different types of benthic biota. Data from 1997 and 1998 support 

the hypothesis of Julian et al. (1999a) that tubeworm roots actively take up H2S from the 

sediment contributing to H2S depletion, particularly within the upper 4 cm. Fluxes from 

Beggiatoa mats were not related to mat coloration. Some data are consistent with the hypothesis 

that significant changes in gas/hydrocarbon input at a site can change within a year. 
10-1 

10.0 Hydrocarbon Geochemistry of Gulf of Mexico Chemosynthetic Communities*

10.1 Introduction 

In an ecological sense, stability connotes the ability of a community to return to a prior condition 

following perturbation (May 1973). Unique complex chemosynthetic communities were first 

discovered in the Gulf of Mexico during trawling in areas characterized by sediments containing 

free hydrocarbon gases, gas hydrates, and bacterially oxidized crude oil rich in toxic aromatic 

hydrocarbons at water depths in the 600 to 700 meter range (Kennicutt et al. 1985). These lowtemperature complex chemosynthetic communities derive energy from reduced carbon, mainly 

hydrocarbon gases and bacterial H2S. Later research confirmed that a number of complex 

chemosynthetic communities are spatially associated with hydrocarbon production (Sassen et al.

1993a). Hence, an important issue is whether or not hydrocarbon production in the Gulf of 

Mexico slope will impact the stability of complex chemosynthetic communities. 

It is important to recognize that the impact of seepage on biologic communities in the Gulf of 

Mexico is not restricted to the present time (i.e., the Holocene). The geologic record of 

hydrocarbon seepage in the Gulf of Mexico Salt Basin spans tens of millions of years as 

successive waves of hydrocarbon migration have impacted the sea floor. Salt and faults have 

served as conduits for vertical migration in the Gulf of Mexico during much of the Tertiary Era, 

impacting the sediments and fauna immediately above shallow salt domes. One well-documented 

example is Damon Mound, an exposed salt dome cap rock onshore in Brazoria County, Texas, 

where seepage of hydrocarbons was active from the Late Oligocene to Early Miocene Epochs, 

roughly 20 to 30 million years before present. 

The geochemical literature on salt dome cap rocks shows that complex biogeochemical processes 

in the Gulf of Mexico tens of millions of years ago had already assumed the general pattern that 

we observe today (Sassen 1980; Sassen et al. 1994a). Bacterial activity in sea floor seeps 

encompasses oxidation of oil and gas to generate CO2. Geologically enormous volumes of 

authigenic carbonate rock occur as disk-shaped deposits over most shallow oil-productive salt 

domes that have been drilled in the northern Gulf of Mexico Salt Basin. The authigenic 

carbonate rock is strongly depleted in 13C, a clear indication of an origin from bacterial 

hydrocarbon oxidation (Posey et al. 1987; Prikryl 1990; Sassen 1980). The same can be said for 

the enormous economic reserves of elemental sulfur in the Gulf of Mexico. There is general 

agreement that bacterial sulfate reduction to produce H2S, and bacterial oxidation of H2S to 

elemental sulfur have been the main factors in formation of the Gulf Coast’s reserves of 

elemental sulfur (Sassen 1980; Sassen et al. 1994a; Sassen et al. 1994b). When we look at the 

biogeochemical processes that drive the chemosynthetic communities of the present day Gulf of 

Mexico slope, we see the continuation of a basic biogeochemical pattern over a long span of 

geologic time. 

 

*

 This section was authored by Roger Sassen. 
10-2 

10.2 Petroleum source rocks 

Understanding the Gulf slope subsurface hydrocarbon system over time and space is necessary to 

address community stability. A large structural complex formed during the Tertiary Era across 

much of the Gulf slope by syndepositional loading of an unstable salt substrate, creating 

temperature conditions within salt withdrawal basins that generate enormous volumes of oil and 

gas from deeply buried petroleum source rocks (Sassen et al. 1999b). Biomarker analysis of oil 

from the GC185 area, plus numerous unpublished biomarker analyses performed at GERG, is 

consistent with a single petroleum source rock of varying lithology and maturity. Upper Jurassic 

source rocks are thought to be the main source of hydrocarbons across much of the upper Gulf 

slope from Garden Banks (GB) east to southern Mississippi Canyon (Gross et al. 1995). Actively 

moving salt bodies and concomitant faulting are the migration conduits from depth where oil and 

gas reservoirs occur and to the sea floor upon which chemosynthetic communities are found 

(Sassen et al. 1993a). The main conclusion is that the occurrence of chemosynthetic 

communities of the Gulf slope is largely related to a single Upper Jurassic hydrocarbon system in 

which hydrocarbon generation is ongoing or geologically recent. This source rock has probably 

been generating oil and feeding hydrocarbon seeps since the Miocene Epoch, leading to the 

assumption that we are looking at a geologically long-term process. 

10.3 Oil in chemosynthetic communities 

Upper Jurassic-sourced oil is found in anomalous amounts in sediments at or near four of our 

chemosynthetic community sites (GC185, GC234, GC233, GB425). The mean of 108 

measurements of oil concentration performed on the four study sites is about 11,778 ppm by 

sediment weight. The range between individual measurements is large, however, from 6 ppm to 

46,834 ppm. This wide range is expected in sediments of hydrocarbon seeps and even within 

single piston cores of seep sediments. Although oil is normally absent or present in low 

abundance in most Gulf slope sediments (<10 ppm), three of our sites are clearly anomalous in 

terms of oil concentration. Differences in concentration of oil between three sites (GC185, 

GC234, GB425) are not thought to be meaningful. In contrast, our new data and previous studies 

(Sassen et al. 1994b) do not demonstrate that crude oil is as widely distributed in sediments 

adjacent to the GC233 brine pool site. This, however, could be an artifact of sampling. 

C15+ chromatography of crude oil from chemosynthetic communities as part of the present and 

previous studies shows a common pattern of alteration. If in significant concentration, all oils are 

heavily impacted by bacterial oxidation that did not necessarily occur within surficial sediments 

of chemosynthetic communities. Bacteria selectively oxidize the n-alkanes and isoprenoid 

hydrocarbons originally present in the oil, eventually leaving only an elevated baseline on 

chromatograms called the unresolved complex mixture (UCM) (Sassen et al. 1994b). The work 

of Sassen (1980) implies that the lack of a continuous alteration series from altered to unaltered 

oil shows that the process of bacterial oxidation of oil is either inactive or only active at a low 

rate. Thus, slow bacterial oxidation of increasingly toxic crude oil could have a minor effect on 
10-3 

driving geomicrobiologic processes but cannot be assumed as a major driving force driving 

bacterial activity at present in shallow sediments (<20 cm) of our study areas. This uniformly 

advanced bacterial oxidation of oil was also observed in studies of three of the same localities in 

previous years (Sassen et al. 1993a; Sassen et al. 1994b; Sassen et al. 1993b). Other areas not 

part of the present study (i.e., the GC142 mud volcano) show the continuous alteration series in 

oil samples but such areas are generally not occupied by chemosynthetic communities (Sassen et 

al. 1994a). This bias is inherent to our study based on our choice of sampling sites. 

Hydrocarbon gas is concluded to be more effective at driving bacterial processes at all of our 

study sites (GC185, GC234, GC233, GB425). Molecular and isotopic properties provide 

abundant evidence of the classic bacterial alteration series of hydrocarbon gases commonly 

observed in geologic environments, particularly at GC185 and GC234 (Sassen et al. 1999a). This 

bacterial alteration series is observed in subsurface oil reservoirs, (James and Burns 1984; 

Winters and Williams 1969), in cap rocks of Gulf Coast salt domes (Sassen 1980), and in 

laboratory simulations (Stahl 1980). 

Relatively unaltered thermogenic hydrocarbon gases from the subsurface hydrocarbon system 

were sampled at the vent sites of GC185 and GC234 as shown in Table 10.1 and 10.2 (Sassen et 

al. 1999a; Sassen et al. 1998). These gases resemble the gases from subsurface reservoirs of 

nearby Jolliet Field on GC184 (Kennicutt et al. 1988a). The sites are complex chemosynthetic 

communities including various species of tubeworms, mussels, clams, and other organisms. The 

bulk of thermogenic gases and CO2 from the subsurface hydrocarbon system bypass shallow 

sediments and exit by venting to the water column (Sassen et al. 1999a). 

An extensive interpretation of the molecular and isotopic properties of vent gases and gas 

hydrates at the GC185 and GC234 localities of the present study is given by Sassen et al. (1999a, 

1998). The present report provides additional summary data. We studied C1-C5 hydrocarbon 

gases using a new analytical procedure for gas chromatography developed at GERG starting in 

1994. See Sassen et al. (1999a) for analytical details. 

We subjected less than 164 samples of gas from sites of chemosynthetic communities to C1-C5

gas chromatography, including gas-hydrate associated samples. The analysis included methane 

plus thermogenic hydrocarbon gases including ethane, propane, isobutane, normal butane, 

isopentane, normal pentane, and neopentane. No previous studies have included neopentane, a 

compound that was not previously known to occur in chemosynthetic communities. In addition, 

we looked at distinct bacterial hydrocarbon gases including ethylene and propylene. 

Background in the Gulf slope of C1-C5 hydrocarbons is low. Methane is generally present in 

sediments (at concentrations of <10 ppm) but the higher hydrocarbon gases are generally absent 

or in low abundance (<1 ppm). The concentrations we see in our sample suite from 

chemosynthetic communities are anomalous, linking hydrocarbon gases to chemosynthetic 

communities of all sites. The mean total concentration of C1-C5 hydrocarbons from 164 samples 

is 200,518 ppm. The range is from 4 ppm to 1,047,626 ppm. The wide range is typical of seeps 

in natural geologic settings. 
10-4 

Table 10.1. Molecular and isotopic properties of vent gases and gas hydrates. 

Station Location Sample 

JSL 

Dive 

13C 

C1 

D 

C1 % C1

δ13C 

C2 % C2

13C 

C3 % C3 

13C 

i-C4 

% 

i-C4 

13C 

n-C4 

% 

n-C4

13C

i-C5 

BHHYD-1 GC185 Vent Gas* 2874 -46.0 -198 95.9 -29.7 2.4 -25.0 1.2 <0.1 -22.6 0.3 -26.1 BHHYD-1 GC185 Vent Gas* 2875 -44.1 -200 90.4 -30.2 4.5 -26.3 3.7 -27.9 0.6 -23.3 0.6 -26.1 BHHYD-1 GC185 Gas Hydrate* 2892 -42.9 -163 83.1 -28.6 7.6 -24.9 8.1 -27.2 0.9 -22.1 0.2 BHHYD-1 GC185 Gas Hydrate* 2637 -42.2 -190 71.7 -29.0 10.6 -25.5 12.6 -27.6 2.6 -22.8 1.7 -26.2 BHHYD-1 GC185 Gas Hydrate* 2637 -43.5 -177 80.2 -29.7 9.4 -25.5 7.3 -27.9 1.6 -23.0 1.2 -24.8 BHHYD-1 GC185 Gas Hydrate* 2637 -42.5 -193 72.1 -29.2 12.4 -25.7 11.4 -27.8 2.3 -22.7 1.6 -24.7 BHHYD-2 GC185 Gas Hydrate 2891 -42.9 -115 85.7 -28.6 6.3 -25.6 6.1 -26.8 1.1 0.8 GCHYD-1 GC234 Vent Gas 2881 -48.7 -203 93.7 -29.2 2.4 -26.4 2.4 -27.9 0.5 -25.2 0.6 -26.5 GCHYD-1 GC234 Gas Hydrate 2881 -48.4 -203 73.8 -29.4 12.6 -26.4 9.9 -28.4 1.6 -24.9 1.8 

BPRN-1 GC233 Vent Gas 2876 -64.3 -165 97.8 0.1 2.1 0.0 0.0 

BPRN-1 GC233 Vent Gas 2860 -65.5 -200 97.4 0.1 2.5 0.0 0.0 

* Data from Sassen et al. (1998) 

Table 10.2. Molecular and isotopic properties of hydrocarbons and carbon dioxide in selected sediment sample

Station Location Sample 

JSL 

Dive

C1-C5

ppm 

13C 

C1 

D 

C1 % C1

13C 

C2 % C2

13C 

C3 % C3 

13C 

i-C4 

% 

i-C4

13C 

n-C4 

% 

n-C4

13C

i-C5 

BHHYD-1 GC185 Sediment* 2637 241,808 -45.4 -140 61.0 -29.5 16.4 -24.7 13.7 -28.5 3.3 -21.5 3.7 -24.9 BHHYD-1 GC185 Sediment* 2641 88,687 -42.4 -134 57.1 -28.8 30.7 -20.3 2.4 -31.0 0.7 -19.6 3.4 -25.3 BHHYD-1 GC185 Sediment* 2641 107,902 -42.3 -158 61.5 -29.6 25.4 -20.5 5.6 -28.5 5.8 -20.4 0.6 -21.3 BHHYD-1 GC185 Sediment* 2651 225,079 -51.0 -136 71.5 -29.3 17.4 -22.9 4.6 -28.4 1.1 -20.6 1.6 -18.1 BHST-2 GC185 Sediment** 2875 137,631 -41.6 -125 90.8 -24.7 9.2 0.0 0.0 0.0 GCHYD-1 GC234 Sediment* 2881 138,864 -49.3 -121 60.5 -26.7 24.5 -22.1 5.7 -27.8 2.7 -19.8 3.1 -23.7 GCAT-2 GC234 Sediment** 2886 75,249 -53.1 -125 85.2 -27.7 1.4 -19.3 0.9 -28.3 1.8 -21.9 2.9 -22.4 GCAT-2 GC234 Sediment** 2886 46,530 -53.9 -119 80.8 -27.2 1.3 -20.5 0.7 -28.2 1.8 -22.4 1.0 -25.2 GCAT-2 GC234 Sediment** 2886 12,960 -54.2 -104 73.3 1.4 0.6 -26.5 2.0 -22.8 3.5 -25.1 *Hydrate mound ** Tubeworm 
10-5 

There are some similarities between sites that contain abundant tubeworms and mussels, sites 

GC185 and GC234 (e.g. Figure 10.1). These sites also contain abundant gas hydrates impacting 

the geochemistry of hydrocarbon gases. Figure 10.2 shows results of C1-C5 hydrocarbon gases 

from 23 samples associated with tubeworms at GC185 (mean = 38,408 ppm). Methane is the 

single most abundant light hydrocarbon, at concentrations up to 666,642 ppm (mean = 36,538 

ppm). Sassen et al. (1999a) used carbon and hydrogen isotopes to show that methane in GC185 

sediments is a complex mixture of both thermogenic and bacterial methane that has been 

subjected to intense bacterial oxidation. Hydrogen isotopes are diagnostic in this regard (Sassen

et al. 1994b). Ethane (mean = 1603 ppm), propane (mean = 0.5 ppm), isobutane (mean = 18.4 

ppm), normal butane (mean = 90.2 ppm), isopentane (mean = 76.9 ppm), normal pentane (mean 

= 3.8 ppm), and neopentane (0.4 ppm) are present. Carbon isotopes suggest that bacteria show 

some preference for oxidation of straight-chain compounds such as propane, normal butane, and 

normal pentane. Ethylene and propane are very minor components (means = <1 ppm). 

Figure 10.1 Sketch of typical gas hydrate mound (~1-3 m across) modeled after the Bush Hill site including 

sampling stations BHHYD-1 (gas hydrate and mound sediment cover) and BHST-2 (nearby tube worm 

sediments). From photograph in Sassen et al. (1998). 
10-6 

-225

-175

-125

-75

-25

25

Carbon and Hydrogen Isotopes 

13 C (D*) 0

/00 

*C1 C1 C2 C3 i-C4 n-C4 i-C5 nSediments 

Tube Worm 

Hydrate Mound

0%

25%

50%

75%

100%

Normalized C1 to C5 

 C1 C2 C3 i-C4 n-C4 i-C5 n-C5 

Molecular Compositions 

Figure 10.2. Diagrams summarizing isotopic (A) and molecular (B) properties of sediments overlying gas

hydrate mounds and from near tube worm colonies at GC133 (BHST-2) and GC234 (GCAT-2) 

sampling stations. 

A 

B 
10-7 

C1-C5 hydrocarbon gases from 21 samples associated with tubeworms at GC234 (mean = 21,906 

ppm) were analyzed. Methane is the single most abundant light hydrocarbon, at concentrations 

up to 94,030 ppm (mean = 18,557 ppm). Carbon and hydrogen isotopes show the methane in 

GC185 sediments is a complex mixture of both thermogenic and bacterial methane that has been 

subjected to intense bacterial oxidation (Sassen et al. 1999a). Ethane (mean = 326 ppm), propane 

(mean = 155.6 ppm), isobutane (mean = 332 ppm), normal butane (mean = 551 ppm), isopentane 

(mean = 1665 ppm), normal pentane (mean = 211 ppm), and neopentane (12.1 ppm) are present. 

Carbon isotopes suggest that bacteria show some preference for oxidation of straight-chain 

compounds such as propane, normal butane, and normal pentane. Possibly because of its 

molecular structure, causing its exclusion during gas hydrate crystallization, the branched-chain 

isopentane molecule appears to be preferentially preserved in some sediment samples. Ethylene 

and propane are very minor components (means = <1 ppm). 

C1-C5 hydrocarbon gases from six samples associated with mussels at GC185 (mean = 341,593 

ppm) were analyzed. Methane is the single most abundant light hydrocarbon, at concentrations 

up to 885,790 ppm (mean = 296,791 ppm). Ethane is in significant abundance (mean = 23338 

ppm), along with propane (mean = 6945 ppm), isobutane (mean = 2498 ppm), normal butane 

(mean = 1389 ppm), isopentane (mean = 10,542 ppm), normal pentane (mean = 29 ppm), and 

neopentane (12.1 ppm). Carbon isotopes suggest that bacteria show some preference for 

oxidation of straight-chain compounds such as propane, normal butane, and normal pentane 

(Sassen et al. 1999a), but bacteria do not heavily oxidize these gases. Possibly because of its 

molecular structure, causing its exclusion during gas hydrate crystallization, the branched-chain 

isopentane molecule appears to be preferentially preserved in some sediment samples. Ethylene 

and propane are very minor components (means = <1 ppm). Only one sample of sediment from a 

mussel site at GC234 was obtained during sampling and analyzed. 

The tubeworms at GC185 and GC234 are associated with abundant gas hydrate masses and share 

a geochemical environment in sediments with high concentrations of C1-C5 hydrocarbon gases 

that are heavily altered by bacterial oxidation. Bacterial hydrocarbon oxidation is geochemically 

associated with bacterial sulfate reduction. The hydrocarbon gases at tubeworm sites show 

evidence of being involved in progressive gas hydrate crystallization. The relative abundance of 

isopentane as a free gas, which is excluded during crystallization of gas hydrate, serves as a 

marker for the gas hydrate crystallization process. In contrast, the concentrations of C1-C5

hydrocarbon gases at GC234 mussel sites are about an order of magnitude higher than at 

tubeworm sites and consequently higher concentrations of less altered hydrocarbons are present. 

Mussel sites in complex chemosynthetic communities are interpreted to be directly associated 

with high-flux vent sites. Video records of mussel sites at complex communities could show a 

relation to visible gas bubble trains in the water column. 

10.4 Gas Hydrates 

Vent gas undergoes a rapid phase change under ambient temperature and pressure and 

crystallizes as abundant solid gas hydrate at the GC185 and GC234 localities. Conclusions 

cannot be extended to all chemosynthetic communities but, where present in abundance, gas 
10-8 

hydrates can have a profound localized effect on diverse bacterially mediated geochemical 

processes in complex chemosynthetic communities dependent on hydrocarbon gases and H2S 

(Sassen et al. 1999a). Gas hydrates occur physically as vein-fillings in hemipelagic muds near 

gas vents and are surrounded by chemosynthetic organisms such as tubeworms at GC185. 

Molecular and isotopic properties of the gas hydrate forming C1-C5 hydrocarbons and CO2

provide key insight to bacterially mediated processes (Figure 10.3). Hydrate-bound methane is 

altered by bacterial oxidation at GC185, as indicated by enrichment of 13C and deuterium (D) 

and by hydrate-bound CO2 depleted in 13C (Sassen et al. 1998). Results suggest that the degree 

of gas hydrate alteration is related to duration of exposure of gas hydrate at the sea floor (Sassen

et al. 1999a). On this basis, we speculated that gas hydrates could play a role in initiation and 

development of complex chemosynthetic communities and in the presence of abundant 

tubeworms. 

10.5 Hydrocarbon Oxidation Rates 

The rate of hydrocarbon gas oxidation is high in the sediments associated with gas hydrates at 

GC185 and GC234. In sediments associated with gas hydrates, bacterial oxidation of a mixed 

pool of C1-C5 hydrocarbon gases yields a net production of CO2 highly depleted in 13C (Sassen

et al. 1999a, Table 2). The bacterial oxidation rate is assumed to be rapid based on molecular and 

isotopic evidence, suggesting concomitant bacterial sulfate reduction and precipitation of 

authigenic carbonate in sediments are also rapid. The rate of bacterial oxidation is such that it 

could destabilize gas hydrates by removing the free hydrocarbon gases necessary to maintain 

their stability (Sassen et al. 1999a). 

Sediments at both GC185 and GC234 often contain methane depleted in 13C relative to both 

thermogenic vent gases and gas hydrates (Sassen et al. 1999a). This is strong geochemical 

evidence that thermogenic carbon in sediments (i.e., mainly gaseous n-alkanes) can be recycled 

via methanogenesis to yield a net production of bacterial methane depleted in 13C at 

chemosynthetic communities (Sassen et al. 1999a). This observation has significance to life in 

extreme environments (Sassen et al. 1999a) and also to the geochemistry of the dominant 

hydrocarbon gas at our other study sites (GC233 and GB425). 
10-9 

Bush Hill 

Vent Gas 

Hydrate Gas

-225

-175

-125

-75

-25

25

13 C (D*) 0

/00 

Carbon and Hydrogen Isotopes 

*C1 C1 C2 C3 i-C4 n-C4 i-C5 n-C5

0%

25%

50%

75%

100%

Normalized C1 to C5 

C1 C2 C3 i-C4 n-C4 i-C5 n-C5

Molecular Compositions 

A 

B 

Figure 10.3. Diagrams summarizing isotopic (A) and molecular (B) properties of vent gas and gas hydrate

samples at the BHHYD-1 and -2 sample stations, GC133. 
10-10 

10.6 Brine Pools 

The origin of flowing methane at brine pools at GB425 and GC233 provides a key example of 

bacterial recycling of thermogenic carbon. Measurements of both 13C and D unambiguously 

identify thermogenic methane and bacterial methane of varying origin (Coleman et al. 1996). 

Isotopic properties identify the volumetrically significant methane at brine pool sites as mainly 

from reduction of CO2 via bacterial methanogenesis (Sassen et al. 1999a and references therein). 

 

The molecular geochemistry of C1-C5 hydrocarbons at the GC233 and GB425 brine pool sites is 

surprisingly similar. No intact gas hydrate has been recovered and analyzed from either site. We 

conclude that gas hydrates are probably less abundant at these sites because of gas composition 

(dominantly methane) and suppression of gas hydrate crystallization by high salinity. 

 

The gas at both sites is typically less than 99% methane. We analyzed 30 samples from the 

GC233 site for C1-C5 hydrocarbons. Concentration of bacterial methane at GC233 is as much as 

999,515 ppm (mean = 422,374 ppm). We analyzed 14 samples from the GB425 site. At GB425, 

concentration of bacterial methane is as much as 940,402 ppm (mean = 125,662 ppm). Ethane 

and other higher hydrocarbon gases in the carbon number range we studied are relatively minor 

components. These mussels are associated with high flux bacterial methane, and the same could 

be said of mussel sites studied at GC234. There, as noted above, the methane is more 

thermogenic in origin. The mussels appear to be adventitious to high flux methane vents and 

brine pools. 

The main source of CO2 at the brine pool sites is assumed to be bacterial oxidation of 

thermogenic hydrocarbons (gas and oil), reinforcing the relationship of the Upper Jurassic source 

system even to some simple chemosynthetic communities of mussels on the Gulf slope. The 13C 

and D values of some methane from adjacent sediments are consistent with the atypically high 

rates of bacterial oxidation, suggesting that concomitant bacterial sulfate reduction rates are also 

high, tending to increase alkalinity and to drive precipitation of abundant authigenic carbonate 

rock. 

10.7 Biogeochemical Synthesis 

The hydrocarbon geochemist looks at starting and ending products of chemical reactions in a 

natural environment; the task is then to infer the reactions most likely to explain complex 

observations (Sassen 1980). The biogeochemical cycles at chemosynthetic communities are not 

unique. Instead, they appear to be similar to those long observed in salt dome cap rocks of the 

Gulf Coast. However, this study leads to an improvement of the old models, as summarized in 

Figure 10.4. Hydrocarbons are oxidized to bicarbonate or CO2, and some oxidized carbon 

precipitates as authigenic carbonate rock. Sassen (1980) suggested that this net accumulation of 

authigenic carbonate rock is a perturbation in the carbon cycle. However, the present work 

diminishes that emphasis because much thermogenic carbon (gas and oil) is apparently cycled to 

bacterial methane via CO2 reduction. This also explains the common observation of bacterially 

altered oil and gas from subsurface reservoirs in association with bacterial methane (James and 

Burns 1984). The sulfur cycle appears to involve bacterial sulfate reduction, H2S oxidation, 

accumulation of elemental sulfur, and then bacterial oxidation back to sulfate (Figure 10.4). 
10-11 

10.8 Conclusions 

All study sites share the presence of hydrocarbons that drive multiple bacterial processes 

including bacterial hydrocarbon oxidation to produce CO2, sulfate reduction to produce H2S, and 

methanogenesis to produce methane, fueling the chemosynthetic communities. However, gas 

hydrocarbon geochemistry differs. 

GC185 and GC234 represent active seep environments with ongoing charge of thermogenic gas 

at obvious vent sites related to deep subsurface reservoirs. Thermogenic gas hydrates are present 

in abundance and impact sediment geochemistry. Thermogenic hydrocarbon gas is bacterially 

oxidized. Bacterial gas oxidation promotes bacterial sulfate reduction and concomitant 

precipitation of authigenic carbonate rock. GC185 and GC234 host complex communities where 

tubeworms are important elements. Mussel sites within complex communities are associated 

with very localized high-flux methane-rich thermogenic gas. 

In contrast, at the GC233 and GB425 sites, the main hydrocarbon gas is bacterial methane, and 

thermogenic hydrocarbon gases are minor constituents. The bacterial methane is speculated to be 

CaCO3

CO2 H2

CH4 SO4

- S0

Seawater 

Gas 

Hydrate 

Oxidation Reduction Oxidation 

Figure 10. 4. Geochemical overview of the gas hydrate environment. 
10-12 

the result of recycling of thermogenic hydrocarbons (gas and oil) largely destroyed by bacterial 

activity in the past. Mussels dominate at the GC233 and GB425 sites. It could be argued that 

mussels are adventitious and merely prefer high-flux vents and brine pools where methane is 

abundant. Origin of methane (i.e., bacterial versus thermogenic) is not a preference. 

Geologically, the brine pool sites at GC233 and GB425 might appear to be more active than the 

GC185 and GC234 sites. However, geochemically, thermogenic hydrocarbon charge to GC233 

and GB425 occurred in the geologic past, and the GC185 and GC234 sites are more active 

receiving thermogenic gas at present. 

Given the prolific volumetrics of the Upper Jurassic source system, hydrocarbon charge to 

migration conduits can be expected to continue on a scale of millions of years. Oil and gas 

production is unlikely to significantly affect overall flow of hydrocarbons to the sea floor or the 

stability of communities. 
11-1 

11.0 Synthesis of Results and Recommendations for Future Study 

11.1 Introduction 

The program obtained results from an extensive program of field collections and laboratory 

analyses. Under the direction of the Geochemical and Environmental Research Group of Texas 

A&M University, sixteen investigators, associates, and students from seven separate institutions 

collaborated to formulate and carry out the work. During the course of two ambitious field 

seasons, program investigators collected samples from over fifty stations, which were distributed 

among four principal sampling sites with use of the submarine Johnson SEA-LINK. Extensive 

additional collections were made with use of the R/V GYRE and the US Navy Submarine NR-1. 

The primary geochemical samples included more than fifty sediment cores, which were 

extensively sub-sampled by sediment depth. Extensive collections of chemosynthetic and 

heterotrophic animals were obtained for investigations to determine, among other objectives, 

growth rates, health, genetic diversity, and food-web structure. A regional context for the 

targeted sampling was established through geophysical survey, sediment coring, and layered 

comparisons between community and regional-scale data with use of geographic information 

system (GIS) tools. Valuable time-series records of current, water temperature, and fluid 

discharge properties of the seep environment were obtained from targeted deployments of in-situ 

instruments and moorings. Satellite remote sensing records were obtained through cooperation 

with industry to examine large-scale variation in seepage rates. This section reiterates the study 

objectives as specified by the Mineral Management Service’s Request for Proposal and outline 

how they were addressed by the separate work efforts that were completed during the program. It 

concludes by synthesizing major finding as descriptive models for geochemical and ecological 

processes at Gulf of Mexico hydrocarbon seeps and outlining knowledge gaps for future study. 

11.2 Objectives and Basic Study Design 

The request for proposal that initiated this program specified the following objectives for study 

(taken verbatim from solicitation number 3813): 

Review biotic and a biotic features of existing conceptual models of chemosynthetic 

communities which explain observed patterns of distribution and abundance, in order to develop 

an effective, refined plan for continued research. 

Further evaluate the physical-chemical factors (e.g., depth, temperature, water chemistry, 

sediment types, and dissolved gasses) which influence, limit, enhance, or control the distribution, 

abundance, and growth of chemosynthetic communities. 

Further investigate the sources (e.g., deep versus shallow or petrogenic versus biogenic) of any 

necessary dissolved gasses and the likelihood that petroleum production may ultimately deprive 

the animals of an energy source. 

Further determine if chemosynthetic communities are robust or fragile, and whether they are 

essentially permanent or ephemeral; characterize age, growth rate, turnover rates, reproduction 

and recruitment, and patterns of senescence and death in the dominant chemosynthetic animals; 
11-2 

further examine recovery rates of communities damaged by physical disturbance. 

Further determine the reliability of methods for detecting chemosynthetic communities using 

remote acoustic and/or geophysical devices, imaging instrumentation, hydrocarbon 

measurements, and/or other available technologies. 

To address Objective One, a thorough review of published literature and reports was completed. 

Two principal conclusions were reached. First, it was evident that the chemosynthetic fauna 

from Gulf of Mexico hydrocarbon seeps (tubeworms and mussels) were taxonomically and 

functionally similar what is found at hydrothermal vents at mid-ocean ridges, but included 

important differences. They are all dependent on symbiosis with bacteria that can utilize 

chemical energy. Tubeworms in both settings use hydrogen sulfide (H2S) to nourish their 

symbionts. Seep mussels have symbionts that can oxidize methane (CH4) from natural gas, 

while vent mussels rely only on H2S. However, communities at hydrothermal vents are attached 

a rocky substratum; they obtain needed H2S directly from vent fluids and are frequently 

disturbed by volcanic eruptions and other catastrophic events. Vent communities are often 

separated from each other by hundreds of miles of inhospitable habitat. Seep communities thrive 

on soft marine sediments where elemental cycling by microbial and chemical processes strongly 

influence the environment. Importantly, the H2S is produced by microbial consumption of seep 

organics—i.e. oil and gas. Many seeps appear to be stable features, in which colonies can persist 

for decades or longer without major natural disruptions. Seep also appear to be much more 

numerous and widespread than vents within comparable seafloor areas. This means that there is 

more opportunity for interaction between neighboring seeps (e.g. exchange of recruits) and a 

greater scope for non-seep fauna to utilize the productivity of seep animals as a food supply. It 

was concluded that new conceptual models of colonization, persistence, and interaction would 

have to be developed to describe seep chemosynthetic communities. 

The second finding of the review and study design process was that two styles of seepage have 

been described at Gulf of Mexico hydrocarbon seeps and contribute to habitat formation. In 

sediment diffusion habitats, mussels, clams, and tubeworms utilize reduced compounds – CH4

and H2S – by extending body parts into the sediment or by bathing their brachia (gills or plumes) 

in the steep gradients immediately above the sediment/water interface. Brine pooling habitats are 

found where concentrated brines are associated with petroleum migration and seepage. Because 

brines are much denser than sea water brine discharge can form distinct pools on the seafloor. 

The brines contain high concentrations of CH4, which supports dense beds of mussel and 

associated species. The study hypothesizes that much of the community-level diversity in cold 

seep communities can be explained by examining the environmental consequences of these two 

styles of seepage. Although there was a clear interest in exploring the Gulf of Mexico slope for 

new communities, this was balanced against the need for intense sampling to answer process 

questions. The program proceeded with four representative sampling sites for process studies and 

the shallow and deep mega-sites for studies of regional distribution and remote sensing 

investigations. 

11.3 Physical Factors and the Distribution and Abundance of Chemosynthetic 

Communities 

Two factors in the physical oceanography of the Gulf of Mexico slope have the potential to 
11-3 

control the distribution and abundance of chemosynthetic communities: the temperature range at 

depths where the communities occur and the water circulation patterns that disperse larvae 

among communities or potential communities. 

Temperature tolerances of the dominant tubeworms and mussels are relatively narrow. As noted 

in Chapter 4, collections made for the physiology and growth experiments during the program 

made use of an insulated container to preserve living specimens at near in-situ temperatures. 

Observations of seep mussels under laboratory conditions show that exposure to temperatures 

over 10° C was sufficient to cause mussels to slough their symbionts (Fisher, 1990). The mean 

temperature recorded at 550m by the thermistor deployed at GC185 was 7.2° C, with occasional 

excursions approaching a maximum temperature of 9° C. The historic record, reproduced from 

thermistor deployments made prior to this program, indicates that temperature spikes as high as 

11° C are a regular occurrence at these localities on the Gulf slope. At no time, however, were 

temperatures above 9° C sustained for more than a few hours. From this information, one can 

infer that temperature stresses are a routine component of the seep mussel’s habitat in this depth 

range. Although systematic comparisons are lacking, there is general agreement that the upper 

depth limit for chemosynthetic communities in the Gulf of Mexico seems to occur between 400 

m and 500 m. This upper limit corresponds to the approximate isotherm above which sustained 

exposures (days rather than hours?) to temperature >10° C will occur. Seep mussel aggregations 

at lesser depths would have to endure regular episodes of temperatures that would produce 

extreme stress in laboratory populations. 

The case for a depth-temperature limit in the range of vestimentiferan tubeworms is less straight–

forward than for the mussels, but available information on behavior of larvae from seep 

vestimentiferans does indicate a mechanism that would affect this life stage. Observations of 

seep vestimentiferans spawned under laboratory conditions Young et al. (1996) showed that the 

larvae will swim or float upward in the water column for approximately eight days prior to 

settlement. Larvae spawned at shallower depths might therefore be carried into warm surface 

waters where temperature would be stressful. What is the upward rate of motion and how far up 

in the water column could a larva get in 8 days from 500 m? This mechanism would not preclude 

colonization from tubeworm populations that were established at greater depths, but could limit 

the genetic viability of populations on the upper slope. 

Regardless of how the mechanism of temperature effect plays out to limit or enhance 

chemosynthetic community viability, temperature fluctuations are correlated with circulation 

events, which in turn have the potential to affect larval transport between communities. Current 

meter records from GC185 indicate a weak but non-random positive correlation (statistics?) 

between the magnitude of temperature change over time (dT/dt) and the strength of currents 

(Figure 11.1). During the deployment, most of this movement (which movement?) was in a 

northerly (v) direction. Ecologically, the implication is that temperature stress indicates increased 

current speed. 

Many invertebrates, including seep fauna, respond to temperature spikes by spawning (C. 

Young, personnel communication). One can conjecture that rapid change in water temperature is 

sensed by seep mussels and tubeworms. Gametes released in response to this signal be advected 

away from the immediate vicinity of the seep community and could potentially colonize new 

sites or provide migrants to other communities. 
11-4 

Figure 11.1 Plot of the rate of change of bottom water temperature (dT/dt) versus the northerly component of the

current velocity shows a weak, but non-random correlation (R2

 = 0.17). 
11-5 

Results from the DNA fingerprinting of seep mussels and tubeworms clearly show that larval 

exchange among sites takes place with sufficient frequency to maintain gene pools for seep 

mussels (Bathymodiolus childressi) and two species of tubeworms (Escarpid and Lamellibrachia 

sp.) that are integrated among the four study sites (see Section 8.7). There is no evidence that any 

populations of these species are isolated from the others to the extent that divergence would 

occur at the species level. The metric of genetic exchange is the number of migrants per 

generation exchanged between pairs of populations (Tables 8.7.7, 8.7.8, and 8.7.9). The higher 

the rate of exchange, in theory, the greater the larval transport between sites. Comparing genetic 

exchange with distance between sites (Figure 3.1), it is apparent that physical separation does not 

act to limit exchange over the distance scales that separate chemosynthetic communities. This 

pattern is consistent with a mechanism in which larvae that migrate to the upper layers of the 

water column are transported over greater distance due to the more energetic water movement in 

these strata. The ability of larvae to survive transport and remain viable will also be limited by 

temperature-induced stresses. 

11.4 Regional and Local Geophysical Signatures of Chemosynthetic Communities 

Classification systems proposed by Reilly et al. (1996) and Roberts and Carney (1997) as a 

means of locating chemosynthetic communities from remotely sensed data sets were reviewed at 

the mega-site and local scales during the course of the program. Comparison of known 

chemosynthetic organism sites with side-scan sonar and other data types highlights the difficulty 

in attempting to find a diagnostic geophysical signature that will identify inhabited seep sites. 

Several localities in the data are characterized by large, focused-flow fluid discharges—also 

called mud volcanoes. The data from mud volcanoes (i.e. the GB425 site, the Bush Lite site 

located in the southern half of GC185, and the TAMU-6 and TAMU-17 sites discovered in lease 

blocks GC191 and GC233) indicate extensive and recent discharges of mud, which are evident 

as flow patterns on the edges of mounds. The GB185 study site, continuously vents oil and gas, 

but shows no evidence of recent sediment expulsion. In contrast, the GC234 and GC233 sites are 

seeps that support significant chemosynthetic communities, but are not evident (resolvable) in 

either the side-scan sonar or multi-channel seismic data. Both occur in areas of high acoustic 

backscatter, but the locations of the chemosynthetic communities do not stand out against the 

complex anomalies located along fault zones in the vicinity. 

Several different types of mound character have been noted in the geophysical data. The sidescan sonar data show three types: 1) nearly homogeneous high backscatter, 2) high-backscatter 

center with a halo of lesser backscatter (“bulls-eye”), and 3) high backscatter with a “dead spot” 

from which the sonar receives no return (“dead-eye”). The GC185 study site is an example of 

type 1, and this appears to be a result of a mound charged with gas and gas hydrate but without 

an obvious fluid expulsion vent. Bulls-eye mounds (type 2) are characterized in chirp sonar 

surveys by a central hard-bottom, surrounded by a region of acoustic wipeout likely caused by 

gas charged sediments. These mounds were not inhabited by chemosynthetic organisms and 

appeared to be senescent or dormant. Dead-eye mounds (type 3) were all highly active mud 

volcanoes with brine pools or brine vents. We speculate that the central dead spots are caused by 

specular reflection of the sonar signal away from the sonar (i.e., no backscatter) or nearly 

complete absorption of the sonar signal by an extremely gas- charged, sediment/fluid slurry in 
11-6 

the vent region. 

MCS data shows that seafloor amplitude anomalies associated with known chemosynthetic sites 

and mounds are variable. The GC185 site has a negative reflection coefficient, implying a 

reduction of seismic velocity within the mound and a reversal of the signal polarity, probably as 

a result of gas-charged sediments. In contrast, the Garden Banks 425 mound shows a ring of 

slightly greater reflection around the mound perimeter, with the mound center showing no 

anomaly. This type of perimeter signature was noted on several other mounds. The TAMU-17 

mound, a highly active mud volcano with abundant tubeworm clusters, displays a positive 

amplitude anomaly. The GC233 and GC234 sites were difficult to detect with MCS amplitude 

anomaly data. 

Two geological features identified in the combined remote-sensing survey and ground-truth 

studies played critical roles in establishment of chemosynthetic communities. These features 

were standing pools of hyper-saline brine formed in the craters of mud volcanoes and shallow 

deposits of gas hydrate overlying actively venting faults. Both features have the capacity to trap 

large volumes of hydrocarbons at the seafloor. Both should be considered diagnostic of 

chemosynthetic communities. That is, a brine pool or a shallow deposit of gas hydrate would 

have a very high likelihood of being colonized by a significant concentration of chemosynthetic 

fauna. The exception would be possible cases where sediment discharge was so vigorous or so 

recent that colonization was impeded. The prospects for detecting colonization in MCS data are 

slight, although associated attributes such as mounds, faulting, and wipe-out zones will be 

evident. Satellite remote sensing can be used to confirm active oil discharge, but significant 

seeps could go undetected in satellite data due to highly episodic discharge. Shallow gas hydrate 

is readily collected by coring and analysis of surface geochemistry. There is no single 

characteristic geophysical signature that will allow an interpreter to find all of the known 

chemosynthetic organism sites in our study areas. Geophysical data can be used to define areas 

affected by seepage, and these generally make up only a small fraction (about 11%) of the total 

survey area (although the percentage can be much higher within a localized area, such as a single 

lease block). Whether chemosynthetic organisms inhabit a given locality can ultimately be 

determined only by direct observations, such as submersible observations, camera sled, or ROV 

photos. However, the likelihood of chemosynthetic organism occurrence can be inferred by using 

a number of geologic and geophysical observations to build a case for their occurrence and to 

screen out false positives (Table 6.1). 

11.5 Program-wide Hypotheses 

Objectives two and three required and examination of the chemical factors that may control the 

distribution, abundance, and growth of chemosynthetic communities. Physiological requirements 

for chemosynthesis require availability of chemical substrates, CH4 and H2S, at localities where 

tubeworms and mussels form stable aggregations. However, as we have seen in the course of 

this program, the spatial and temporal gradients of chemical compounds at seeps tend to be 

abrupt and heterogeneous over small space and time scales (see Chapter 7). Moreover, 

biogeochemical processes that produce, maintain, or consume chemical substrates—particularly 

H2S—exhibit a wide range of activity (see Chapter 9). Thus, although we know that tubeworms 

require H2S, and can measure high concentrations of H2S, in association with tubeworms, and 

can demonstrate that the H2S is produced by reduction of seawater SO4 driven by oxidation of 
11-7 

methane the specific temporal and spatial availability of H2S needed to support and tubeworm 

bush has not yet been determined. The sampling program was designed to examine whether 

there is a predictable relationship between the prominent chemosynthetic fauna (primarily 

tubeworms) and sediment geochemistry. A further refinement was to compare tubeworm 

aggregations that were in different stages of development (juvenile, adult, senescent). Basically, 

there appear to be two possibilities: 1) There is a robust threshold level of H2S (or some other 

chemical variable) in the sediments next to all tubeworm bushes; so that the distribution of 

tubeworms in a seep community mirrors the availability of hydrocarbon gas and H2S. Or, 2) 

chemical concentrations were not consistent in the sediments next to tube worm bushes; 

implying that a high degree of spatial and temporal haphazardness. 

The approach to the integrated statistical analysis was to merge data sets from various 

geochemical and biological components to characterize environments and test hypotheses on 

distribution patterns gleaned from the sampling design. First we examine the variability of 

geochemical variables alone, then we test for correlation between geochemistry and the 

biological community. Principal components analysis (PCA) was used to characterize 

environmental patterns by grouping stations along two axes of a PCA plot. PCA is a variable 

reduction technique with three important characteristics: Because it is a transformation of data, 

there are as many components as there are variables, the components are mutually orthogonal 

(i.e., the components are uncorrelated), and the components are extracted in order of decreasing 

variance. Therefore, the new variables resulting from the PCA contain most of the variance and 

none of the redundancy of the original data set. The sampling plan can be thought of as simply a 

spatial surrogate for environmental heterogeneity due to geochemical differences in the 

chemosynthetic habitats. A PCA of all the geochemical variables was used to reduce this large 

number of variables to two new variables, which are uncorrelated and contain most of the 

variance in the data set. These new variables can then be used as dependent variables to assess 

biological relationships between tubeworms and the geochemical environment (Green and 

Montagna 1996; Carr et al. 2000). 

Three different PCAs were performed. Most geochemical variables were measured at different 

depths in sediment to create a vertical profile of the chemical within the sediment. These profiles 

were converted to integrated values over the sediment horizons to a depth of 11 cm to reduce the 

profile data to one number (Arvidsson, personal communication). The second analysis was 

performed on surface sediments only. A third analysis was performed by using the average 

surface values over both sampling years at a station to create a data set to compare with the 

tubeworm data set. All data were log transformed prior to PCA treatment. Patterns of the PCA 

scores from the analyses allow us to test the program-wide hypotheses that there are no 

differences or changes in chemosynthetic communities over time, sites, habitats, and subhabitats. 

The hypotheses are tested by looking for groupings on the PCA scores from the sampling design. 

11.5.1 Integrated geochemistry 

The first two principal components for the integrated geochemical data set explained 55% of the 

variance. Sr, Na, and Cl had the highest positive loadings on PC1, whereas TOC, SO4 and Mg 

had the most negative loadings on PC1 (Figure 11.2). The total C1 - C5 gases, DIC, and H2S also 

loaded highly on PC1. The PC1 axis represents the major geochemical process of brine seepage 

(high salt) and high sulfate reduction due to seeping gases and sulfide production. Interestingly, 
11-8 

brine seepage does not appear to be separating from hydrocarbon seepage. Because of the lack of 

separation of indicators of brine and hydrocarbon seepage (Figure 11.2), it is not surprising that 

sites do not separate when scores are examined (Figure 11.3). There is overlap in patterns for all 

habitats (Figure 11.2), and stations and years (Figure 11.4). This lack of a pattern among scores 

for elements of the sampling design indicates there are no differences in integrated values of 

geochemical variables among sites, habitats, stations, and years. The only outliers are values 

taken from GB425 in 1997, where the two samples group together along PC2. The second major 

axis, PC2, had high positive values for Ca, pH, and K. 

11.5.2 Surface geochemistry 

The first two principal components for the surface sediment geochemical data set explained 49% 

of the variance. More hydrocarbon and inorganic values could be retained for this data set, and 

nutrient values were added. In some respects the results are different, but the major outcome is 

the same. Again Na, Cl, methane, total C2 - C5 gases, DIC, and H2S loaded highly on PC1. 

However, Sr loaded negatively on PC1. The most negative loadings on PC1 were TOC, SO4, Ca 

and NO2 (Figure 11.5). PC2 was very different from the integrated analysis, containing mostly 

high positive values for nutrients NH4, Urea, PO4, and SIO3. The PC 1 axis again represents the 

major geochemical process of brine seepage (high salt) and high sulfate reduction due to seeping 

gases and sulfide production. Once again brine seepage does not appear to be separating from 

hydrocarbon seepage. The lack of separation of indicators of brine and hydrocarbon seepage 

(Figure 11.5) again made it difficult to distinguish sites and habitats or stations and years (Figure 

11.6). Again, no pattern among scores for elements of the sampling design indicates there are no 

differences in integrated values of geochemical variables among sites, habitats, stations, and 

years. 

Slight patterns do exist in the PCA scores. Three of the GB station had the highest PC1 loadings 

(Figures 11.5), indicating they were the most active seeping sites. No clear pattern emerges 

among any of the other sites. The three highest PC1 scores were all for mussel stations (Figures 

11.5). One other mussel station also had a positive score, but the remaining two stations were 

close to zero. All of the senescent tubeworm stations (ST) were close to the origin (0, 0), 

indicating these were likely among the least active stations. 

The lack of clear patterns for sites, stations, and habitats can be caused by two mechanisms. 

First, chemosynthetic communities are broadly homogeneous. Second, the variance at individual 

sites is so great that the sites and habitats appear homogeneous when they are in fact 

heterogeneous and the appearance of homogeneity is simply a function of small sample sizes. 

The most likely explanation is probably a combination of both mechanisms. There is a great deal 

of variance from core to core, or sample to sample. There is also likely a great similarity of 

geochemical processes across broad areas of the chemosynthetic communities. 

11.5.3 Comparison of geochemistry with tubeworms. 

The tubeworm data was by station, so a third analysis was run by averaging the temporal 

samples by site. The analysis of means by stations was similar to the previous analysis, which 

included years. The total C1 - C5 gases, DIC, and H2S had the highest positive loadings on PC1, 

whereas TOC, SO4 and Ca had the most negative loadings on PC1 (Figure 11.7). PC2 again 

contained high positive values for nutrients NH4, Urea, PO4, and SIO3. One difference was that 
11-9 

Na and Cl had intermediately high values for both PC1 and PC2, thus falling between both axes. 

The PC scores for stations were correlated to the growth and condition scores for tubeworms 

(Table 11.1). There was no significant correlation between geochemical properties and 

Lamellibrachia condition, growth, or length. Escarpid condition did correlate to both PC1 and 

PC2, indicating that better tubeworm condition was found where higher seepage activity 

occurred. 

Table 11.1. Tubeworm condition and growth data correlated to chemical (by 

station) principal components (PC1 and PC2). Pearson correlation 

coefficients (r), probability r=0, and number of observations (N). 

Species 

 Metric 

Chemical PC1 

r 

N 

Chemical PC2 

r 

N 

Lamellibrachia 

 Condition 

0.32 

0.4368 

8 

-0.44 

0.2743 

8 

Escarpid 

 Condition 

0.99 

0.0013 

5 

-0.87 

0.0554 

5 

Lamellibrachia 

 Growth 

-0.02 

0.9542 

8 

-0.34295 

0.4056 

8 

Lamellibrachia 

 Length 

0.47 

0.2356 

8 

-0.08 

0.8538 

8 

In summary, we have not been able to demonstrate a predictable relationship between tubeworm 

bushes and the concentrations of chemical forms in the nearby surface sediments. This provides 

very important guidance for study design in ecological investigations of chemosynthetic 

communities. The results demonstrate an overwhelming heterogeneity at the level of individual 

aggregations of tubeworms, bacterial mats, and seep mussels. In the face of this variability and 

the proven longevity of tubeworms and mussels, the importance of colonization history is 

paramount. A seep community—at least in a sediment diffusion habitat similar to GC185 and 

GC234—comprises a mosaic of individual colonies, active gas vents, and shallow hydrate 

deposits (Figure 11.8). Within this array, the intensity of venting and the geochemical response 

to hydrocarbons is highly variable. Large numbers of unbiased samples would be required to 

reduce the variance of sample means. In practical terms, the number of samples exceeds what is 
11-10 

possible for a program operating from a submarine with limited time on bottom and sampling 

payload. A further complication is the problem of collection bias. Locations for sediment 

collection were chosen in a non-random manner. Although this sample choice was clearly 

necessary to establish the ecological context for geochemical analyses, the statistical power of 

the study design was compromised. Consequently, the results are most useful for setting the 

upper and lower bounds for relevant processes such as sulfate reduction. 

Figure 11.2. Variable loadings on principal component axes (PC1 and PC2) for inorganic and organic chemicals

integrated over a depth range of 11 cm. Variables are abbreviated as follows: calcium (Ca), chlorine

(Cl), dissolved inorganic carbon (DIC), extractable organic material (EOM), hydrocarbon gases 

methane through pentane (C1-C5), sulfate (SO4), hydrogen sulfide (H2S), iron (Fe), manganese (Mn),

magnesium (Mg), potassium (K), strontium (Sr), sodium (Na), total inorganic carbon (TIC), total

organic carbon (TOC), and pH. For a discussion of variables see Chapter 9. 
11-11 

Figure 11.3. Variable scores for elements in the experimental design for inorganic and organic chemicals integrated

over a depth range of 11 cm. Sites and program years are plotted as symbols. See Chapter 3 for site

descriptions. 
11-12 

Figure 11.4. Variable scores for elements in the experimental design for inorganic and organic chemicals integrated

over a depth range of 11 cm. Sites and habitats are plotted as symbols. Habitats are abbreviated as

follows: adult tubeworm (AT), bacterial mat (B), mussel bed (M), senescent tubeworm (ST),

unoccupied seep station (U). 
11-13 

Figure 11.5. Variable loadings for inorganic and organic chemicals in upper-most layers of surface sediments (2 - 4 

cm). Variables abbreviated as in Figure 11.1 and as follows: ammonia (NH4), hydrocarbon gases 

ethane through pentane (TOTC2C5), nitrite (NO2), phosphate (PO4), and silicate (SIO3). 
11-14 

Figure 11.6. Variable scores for elements in the experimental design for inorganic and organic chemicals in surface

sediments (upper 2 - 4 cm). Upper panel shows stations (see Table 3.3 for station definitions), lower

panel show program years 1997 and 1998. 
11-15 

Figure 11.7. Variable loadings for inorganic and organic chemicals in surface sediments only (2 - 4 cm) averaged 

over both sampling years. Calculated for comparison to tubeworm condition and growth. 
11-16

Figure 11.8. Schematic of the tubeworm bush and bacterial mat habitats sampled during the program. The bush shown is a photograp

BHAT1 station. The root ball was recovered following a collection with the bushmaster and is superimposed to shoposition. Proximity of push core sediment samples to tubeworm bushes was limited by the physical barrier of the tub

concentrations of H2S and rates of sulfate reduction were measured beneath Beggiatoa mats adjacent to tubeworm bush

Sulfide production is presumed to be enhanced by biologically mediated transport of oxidants into the sediments and cyc
11-17 

11.6 Geochemical Dependence of Tube Worm and Beggiatoa Colonies 

Sulfide-based chemoautotrophy, as exhibited by the tubeworm symbiosis and by Beggiatoa, 

plays a central role in the seep ecosystem. The program examined this overall process from two 

semi-distinct perspectives. Tubeworm growth, condition, and habitat geochemistry were studied 

through methods described in Chapters 4 and 8.4. Notably, a unique sample set of pore fluids 

from within the so-called "root ball" of several tubeworms bushes was collected with a wand that 

routinely penetrated 50 cm or more into the subbottom. Results reported by Julian et al. (1999) 

demonstrated that the roots, which are slender extensions of the worm tubes buried in the 

sediments beneath the bush, are the site of sulfide uptake in Lamellibrachia and Escarpids. The 

geochemistry of seep sediments was extensively studied based on samples collected in push 

cores. A sub-set of the geochemical sediment samples were cross-compared with collections of 

Beggiatoa. As the schematic in Figure 11.8 illustrates, these tubeworm and sediment 

geochemistry samples are only partially comparable. The geochemical samples were taken from 

a shallower subbottom depth adjacent to the tubeworm bushes while the wand-collected fluids 

from the root balls were small-volume samples on which only a limited suite of analyses could 

be performed. A synthesis of these studies provides a better delineation of seep geochemistry, 

but leaves details of the process to be resolved by future study. 

With their high biomass and evident impact on seafloor topography, the large colonies (bushes) 

of tubeworms exert a strong influence on seep ecology. Consumption of sulfide by the 

aggregation of individual tubeworms and symbionts in a large bush must be substantial over 

time. Given their extremely long life spans (hundreds of years) the persistence of a tubeworm 

bush must mark a stable supply of sulfide in its immediate locality; however, as the mapping 

efforts undertaken by the program have shown, the distribution of tubeworms within a seep 

complex is notably patchy and discontinuous. The Beggiatoa, in contrast, are ubiquitous. All 

cores collected from seep stations contained visible filaments of Beggiatoa. The Beggiatoa 

filaments concentrate into conspicuous mats on surface sediments where conditions are optimal. 

The sulfide is produced through reduction of seawater sulfate in an undetermined consortium of 

bacteria and is a byproduct of hydrocarbon consumption. Based on the observed variability in the 

distribution of mats, these optimal conditions are presumed to be ephemeral and spatially 

transient. Beneath Beggiatoa mats, infiltration of bacterial filaments ten or more centimeters into 

the sediment and transport of NO3 in filament vacuoles enhance consumption of sulfide (large 

arrows in Figure 11.8). 

Where tubeworms or Beggiatoa are lacking, the supply of seawater sulfate at depth in the 

subbottom is limited by molecular diffusion (small arrows in Figure 11.8) and the rates of sulfate 

reduction are comparatively low. Many of the stations classified as inactive in Figure 9.2 fall into 

this category. However, these mechanisms specifically do not address sulfide production and 

consumption at the root level beneath tubeworm bushes. The wand-collected samples 

consistently found moderate to high sulfide concentrations in this zone. The probable effect of 

the root mass upon the diffusion of seawater into the subbottom is significant. A tangle assembly 

of tubes displaces sediment and greatly increases its permeability to seawater. The highest sulfate 

reduction rates were seen beneath mats adjacent to tubeworm bushes. The following sections 

develop a model for how Beggiatoa and possibly tubeworms can enhance sulfate reduction rates 

as well as mediate overall sulfide concentrations. 
11-18 

11.6.1 Model contribution of Beggiatoa to maintenance of extreme rates of sulfate reduction 

In geochemical studies, transport fluxes are typically calculated from observed gradients. In the 

seep sites investigated during this program, we find a conundrum: sulfate gradient profiles and 

sulfate reduction rates indicate that the sulfate flux into the sediment porewaters needed to 

sustain the observed sulfate-reduction rates greatly exceeds the sulfate flux calculated from the 

gradients. What is the mechanism for enhanced supply of sulfate? Because sulfur plays a central 

role in the biological productivity in seep communities, the mechanisms by which sulfate 

reduction rates are enhanced are critical. 

Insight into the possible importance of transport can also be had through comparison of the 

sulfate depletion and reduction rate curves in the context of a simple diagenetic model. By Fick’s 

law, the gradient expressed in the sulfate depletion curve at a given depth is proportional to the 

diffusive flux of sulfate occurring over this interval. By making the assumption that sulfate 

reduction is the only sink for sulfate, the flux of sulfate required to sustain the measured 

reduction rate can be compared with that implied by the depletion curve. This has been done for 

station GCAT (dive 4029) in Figure 9.12, by fitting the sulfate concentration curve and 

comparing the apparent rate diffusion with the reduction rate according to the diagenetic 

equation [Berner, 1980 #382], 

R

z

C

z

C D D

t z

C

S I  

 



 









	




 ( ) ( ) ( )   (11.1) 

Here C is the concentration of sulfate,  is porosity, DS and DI are the sediment diffusion and 

irrigation coefficients, respectively,  is the depositional rate, and R is the reduction rate. This 

equation essentially states that the change in concentration at a fixed depth per unit time is the 

sum of diffusive, advective, and reaction processes. Assuming that the depositional rate is 

negligible compared to diffusion, and that the profile reflects a steady state condition (i.e., that 

the sum of the right hand terms is zero), the ratio of the reduction rate and the second derivative 

of concentration (R / {2C(z)/z

2

}) should approximate the sum DS +DI.. The average value of 

this ratio over the fitted part of the curve shown in Figure 11.9 is 236 x 106

 cm2 sec-1. The 

accepted value for DS is ~ 5 x 10-6 cm

2

 sec-1 [Berner, 1980 #382]. This implies that the diffusive 

flux of sulfate to the zone of maximum sulfate reduction (10-12 cm) is relatively minor, and thus 

that transport via other processes (described generally as bioirrigation) is a critical feature in 

sustaining these very high reduction rates. The fact that highest rates also occur in areas in which 

there is a high abundance of Beggiatoa mats and tubeworms are found together suggests that 

tubeworms and Beggiatoa may interact to increase the rate of sulfate reduction. 

It is possible that surface-bound sulfide oxidizers such as tubeworms and Beggiatoa may play a 

critical role in sustaining the observed high sulfate reduction rates. What follows is speculative, 

but it is at least consistent with the biotic distributions and the overall pattern of 1) sulfate 

reduction rates, 2) depletion of sulfate at depth, and (3) the association of bacterial mats. Figure 

11.10 depicts a possible temporal sequence in which the interaction between methane flux, 

sulfate reduction rate, and the depth at which sulfate is depleted (i.e., the thickness of the zone of 

sulfate reduction, and thus the magnitude of the integrated rate) is mediated by the activities of 

surface-bound sulfide oxidizers. Assuming an unlimited methane supply, sulfate reduction rates 

are limited by the diffusive supply of sulfate to the zone of sulfate reduction (stage 1 in Figure 

11.10). Sulfate depletion gradients here are very high, with sulfate typically depleted within 
11-19 

0

5

10

15

20

25

30

0 1 10-7 2 10-7 3 10-7

0 5 10 15 20 25 30

mmols/cm 3/sec

mmols/L

sulfate

reduction

rate

Figure 11.9. Apparent diffusion coefficients. Diffusion coefficients computed from sulfate reduction rate and

sulfate depletion profile data are composed of a molecular component and an “excess” term. In cases

where this excess term is large, enhanced transport or recycling of sulfate is indicated. 
11-20 

 5 cms. The gradient may approach zero concentration axis asymptotically as shown in Figure 

11.10, or linearly. Convex-upward curvature indicates consumption of sulfate through the entire 

zone; no curvature indicates consumption at the base. As sulfate cycling is progressively 

enhanced, reduction rates increase, sulfate gradients relax in the near-surface section and 

decrease the second derivative of the concentration curve, thus increasing the apparent diffusion 

coefficient. The zone of sulfate reduction increases in depth, and the integrated rate increases 

proportionally (stage 2 in Figure 11.10). 

With increased reduction rate, sulfide concentrations (poorly buffered by reactive iron) increase 

as well. However, Beggiatoa may be unable to maintain sulfide gradients under very high 

sulfide concentrations, and thus may be forced to confine their activities to progressively 

shallower depths in the sediment. As this trend continues, enhanced sulfate cycling decays, and 

the zone of sulfate reduction progressively shrinks (stage 3 in Figure 11.10). As Beggiatoa are 

ultimately forced out, the system ultimately returns to its initial diffusion-limited state. 

Alternatively, ebullition of methane gas could displace sulfate-depleted porewater, which would 

then be replaced by seawater from the sediment-water interface. This transport mechanism was 

suggested by Fossing et al. [, 2000 #375] as a means of reconciling measured high sulfate 

reduction rates with sulfate depletion profiles, and has also been discussed by O’Hara et al. [, 

1995 #379]. It is noteworthy that the cores from GCAT2 were characterized by extensive gas 

fractures to a degree not seen in most sediment samples. This indicates that gas venting was 

extremely active at this station and may have contributed to the apparent high diffusion rates. 

These speculations may be inaccurate and require extensive microbiological data for 

corroboration. In particular, if Beggiatoa utilize nitrate as an intermediate electron acceptor, they 

must be shown to be capable of sustaining rates of nitrate reduction close to the measured 

“excess” sulfate reduction (see Section 8.2). In addition, stage 2 profiles are typical of those seen 

at adult tubeworm station GCAT2 (dives 2886, 4029, and 4048). This station is also significant 

in having a dripline Beggiatoa mat. In Figure 11.11, the apparent diffusion coefficient (DS + DI

computed from equation 11.1) is plotted against the sulfate reduction rate measured at that depth. 

“Pure” bacterial mats (those having no immediate association with tubeworms) and, similarly, 

tubeworms unassociated with bacterial mats both tend to plot at the lower end of enhanced 

coefficients and rates. However, it is where tubeworms and mats are closely associated that 

extreme enhancements of rates is seen. This may indicate some interaction between mats and 

tubeworms: e.g., if tubeworms sequester sulfur and do not return it as sulfate to the porewater, 

they may be able to buffer sulfide to levels that Beggiatoa mats can tolerate. Lastly, it must be 

noted that sulfate-reducing bacteria have not been observed to oxidize methane (or other 

hydrocarbons) directly. There is field evidence that anaerobic methane oxidation occurs close to 

or at the base of the sulfate reduction zone [Reeburgh, 1976 #380; Reeburgh, 1980 #381; 

Martens, 1977 #378; Devol, 1981 #374; Iversen, 1985 #377; Blair, 1995 #373]. Hoehler et al. [, 

1994 #376] have hypothesized that this process may be accomplished by methanogenic bacteria 

who oxidize methane through a “reversed” methanogenic reaction: 

CH4 + 3H2O = 4H+

 + HCO3

-

(11.2) 
11-21 

The H2 produced in this reaction can then be utilized directly by sulfate-reducing bacteria, who 

apparently are able to out-compete methanogens for this substrate and maintain it at low 

concentrations: 

 

Figure 11.10. Temporal cycle of sulfate reduction and enhanced sulfate supply. Cartoon shows a possible sequence

in which sulfate reduction rate is mediated by the activities of surface-bound sulfide oxidizers. Lefthand panel, unlimited methane supply; right-hand panel, organic matter limitation. Stage 1: sulfate 

reduction rates are limited by the diffusive supply of sulfate to the zone of sulfate reduction. Stage 2: 

sulfate cycling is progressively enhanced by activities of sulfide oxidizers, giving rise to increasing

reduction rates increase. Stage 3: sulfide oxidizers cannot keep pace with sulfide flux at extreme SR

rates and are gradually forced out, resulting in an eventual return to initial diffusion-limited state. 
11-22 

 4H4 + SO4

2-

 + H+ = 4H2O HS -(11.2) 

The net reaction above is equivalent to reaction (9-4). A sulfate reduction peak located close (or 

at least within) the interval over which sulfate is being depleted (e.g., at GCAT2) is consistent 

with observations of anaerobic methane oxidation in other environments. 

0.01

0.1

1

10

100

1 10 100 1000

GCAT2-4048

GCB3-white

GCAT2-4029

BHB3-4028

GCAT2-2886

GCB1@GCAT1

BHST1

SRR (

mols-SO

4 cm-3 day-1)

Apparent D*S (cm2 s-1 X 106) molecular diffusion

low gradients

Figure 11.11. Comparison of apparent diffusion coefficients and sulfate reduction rates. Although there is evidence

of enhanced cycling of sulfate in both tubeworm and mat habitats, extreme rates are dominated by

GCAT2, where tubeworm and mats are found in close proximity. 
11-23 

11.7 A Conceptual Model of Seep Community Establishment, Growth, and Evolution 

Objective fours required and examination of the life history and ecology of chemosynthetic 

species and their community components. How long do the tubeworms live and individual 

aggregations persist? A variety of observations suggest that they may live much longer even 

than we have previously suggested (Fisher et al. 1997; Bergquist et al. 2000). The empirical data 

on growth rates of the vestimentiferans indicates that most are 170 – 250 years old by the time 

they reach a length of 2 m. It also indicates that their growth rate slows down dramatically at this 

time. However, we often collect animals in excess of 2 m in length, sometimes as much as 3 m in 

length. The oldest aggregations sampled contained animals in good health, with no signs of 

increased mortality. Occasional sightings of aggregations of very large individuals of the 

Escarpid-like species and the total absence of monospecific aggregations of L. cf luymesi in these 

areas suggest that the Escarpid-like species may outlive L. cf luymesi by a considerable margin. 

When all of the above observations and data are taken into consideration, it is not unrealistic to 

suggest that present estimates of longevity up to 250 years may be low by a factor of at least two. 

The following model has been developed from size-based classification of tubeworms or mussels 

as juveniles and from geochemical analyses of fluids associated with physiological uptake by 

these species. New seep communities normally develop in specific areas where active surface 

expression of seepage results in deposition of carbonate substrate for the larvae to settle upon in 

an environment enriched in methane or sulfide or both. Bacterial mats may proliferate during the 

early stages before significant carbonate deposition has occurred. Mussel or tubeworm larvae 

settle on the solidifying carbonate substrate, and the tubes and shells of the young animals 

provide additional substrate for further settlement. Mussel communities will continue to flourish 

as long as methane is released in substantial quantities from the sediments. In some cases this 

can last for periods measured in hundreds of years at least, such as at the GC233 site and perhaps 

over methane hydrate deposits. Parasitism can radically alter the reproductive capability of 

mussel communities such that, even though individuals are living and contributing to community 

structure, they are unable to reproduce. In most cases, deposition of carbonate slows down and 

channels seepage over a period of decades, such that the release of seep fluid from that point on 

the ocean floor is greatly diminished. During the early decades while the high level of seepage is 

maintained, the carbonate upon which the first recruits have settled may continue to grow. 

Recruitment of tubeworms or mussels to this point continues as long as sulfide and/or methane is 

released from the seafloor and the carbonate remains exposed. The majority of recruits are 

derived from established colonies in the immediate vicinity, but significant recruitment occurs 

from communities that are tens of kilometers distant. Because the young communities are in 

microhabitats of very active seepage, only fauna that can tolerate this chemistry are associated 

with the young communities. Mussels cannot bind and accumulate methane (as tubeworms do 

sulfide), and therefore only occur in the areas of most active seepage. As a result, associated 

fauna found in mussel beds is completely dominated by endemic seep animals that can tolerate 

these conditions (the highest species richness documented in any mussel bed was 8, and a total of 

only 16 species was found in all mussel beds sampled). However, these well-adapted endemic 

species occur in large numbers with significant biomass. During this time, the very young 

tubeworms obtain sulfide across their plumes and may begin to grow a posterior extension of 

their tubes (a “root”) to supplement their sulfide uptake from interstitial pools. Young tubeworm 

aggregations support not only large numbers and biomasses of endemic animals like the mussel 

beds, but significant numbers of non-endemic animals as well. 
11-24 

Over the next century or two, the tubeworms continue to grow while seepage of sulfide from the 

sediment into the water column progressively decreases because 1) the tubeworms absorb it at 

depth in the sediment and 2) the accumulation of carbonates and the “root ball” impede seepage. 

The tubeworms continue to expand their “roots” into the sediment and at least Lamellibrachia cf

luymesi increases its dependence on this source of sulfide. The Escarpid-like species continues to 

take up significant amounts of sulfide across its plume which is maintained near the sedimentwater interface. During this period, there is sufficient primary production associated with the 

aggregation to maintain a moderately high biomass community while the toxicity of the habitat 

has decreased to the point that a much wider variety of non-endemic fauna colonizes or visits the 

aggregations. At this point, the analogy between the tubeworms and long-lived ecosystemstructuring plants (e.g., trees) is quite strong. Although the tubeworms are not a food source for 

most of the associated fauna, they provide a habitat for numerous species that are supported by 

seep primary production. During this time we infer that significant primary production by freeliving bacteria associated with tubeworm tubes is supporting the diverse community of 

associated fauna. 

As the aggregation ages, flow of sulfide into the water column continues to decrease and some 

thinning of the aggregation occurs. “Roots” are the primary route of sulfide acquisition by all 

remaining tubeworms. Non-endemic and non-mobile fauna often colonize the tubes heavily; 

primary production by free-living bacteria associated with the aggregation decreases 

significantly, and the biomass of the associated faunal community drops significantly. This stage 

in their life history may also last a very long time, because the tubeworms in these less dense old 

aggregations continue to grow and are in very good condition, presumably in part because of 

reduced competition from the remaining vestimentiferans. 

Clearly program results have contributed substantially to understanding of the seep fauna. 

However, there are still very significant and important questions to be addressed concerning this 

fauna that would provide essential information for management decisions. This includes 

furthering understanding of the relatively shallow slope communities, and determining the 

general applicability of findings to the functionally similar but taxonomically distinct, deeperwater communities. The latter may be especially critical to the MMS as energy company 

activities continue to expand into the deep-water environment. Some examples of key questions 

that remain to be addressed are summarized as follows: 

1. What is the source of the sulfide that sustains the vestimentiferan tubeworm communities 

over centuries? 

1a. If it is sulfide produced by reduction of seawater sulfate, then how does it reach the roots of 

 the tubeworms, 0.5 m and more below the seawater-sediment surface? 

1b. What rate of sulfide flux is required to sustain a typical old tubeworm aggregation? 

1c. What rate of sulfide flux is required to sustain the very large aggregations? 

2. Is transfer of seep organic production to the surrounding deep sea even more extensive than 

documented to date? 

2a. Is transfer through zooplankton significant? 

2b. Is transfer through as yet untrapped mobile predators significant? For example we observe 

 large sharks and fishes—even swordfish—at seeps. Do these animals obtain a food 

 supply at seeps? 
11-25 

3. Is the working model of faunal succession correct? 

3a. Is diversity and biomass inversely correlated with each other and both correlated with 

 toxicity and production? 

3b. Does this model hold in other biogeographical settings with different species filling 

 similar roles? 

4. What is the total biodiversity in the tubeworm ecosystems in these relatively well known 

sites? 

4a. What is the biodiversity of the ecosystems at much more poorly sampled sites where quick 

 looks suggest differences, such as sites 100 – 200 miles west and barite hosted mussel 

 beds? 

5. Do any of our findings apply to the taxonomically distinct communities at deeper sites? 

5a. What and who is there? (biodiversity) 

5b. Are the sites and species long-lived? 

5c. Are roots a universal feature of seep tubeworms? 

5d. Do the mussels with two types of symbionts (at places like Alaminos Canyon and the 

Florida Escarpment) compete with and eventually overgrow the tubeworms as mussels do 

at hydrothermal vents on the East Pacific Rise? 

5e. Are these sites more productive as some photographs seem to suggest? 

5e. Do our successional models for the associated fauna hold in the context of a much 

reduced surrounding biota? 

5f. Is the importance of seep production to the surrounding fauna enhanced in deeper (and 

more nutrient limited) settings? 

11.8 Model of seep-background trophic interaction. 

The principal finding of the trophic interaction study was that hydrocarbon seep systems are to 

some degree trophically linked to the surrounding background system (Figure 11.12). Since the 

beginning of deep chemosynthetic community studies, there has been an implicit assumption 

based in dramatically different biomass and species composition that such systems were largely 

isolated from the surrounding background benthic system. Reinforcing this assumption has been 

the considerable depth and remoteness from coastal productivity of hydrothermal sites, which 

place them within comparatively food-limited background regions. 

The typical background deep-sea phytoplankton-derived detritus fuels the system that 

accumulates as newly arrived material at the sediment-water interface. As this labile material is 

consumed by detritivores, it is also mixed downward into the sediment by their action and 

diluted with mineral components of the sediment. Over time, a residue of old refractory detritus 

builds up in the sediment mixed layer, which may reenter the food web via a microbial 

intermediary. Determination of the exact importance of old versus new detritus is an active area 

of deep-sea research. Collectively, old and new carbon supports an extremely diverse but low 

biomass detritivore community on all size scales (meiofauna to megafauna). These, in turn, 

support predator and scavenger populations on the same size scale. Although meager compared 

to the influx of detritus from the photic zone, both detritivores and the predators that prey upon 

them contribute via a fecal route to the detritus pool in the sediments. Isotopically, this 

phytodetritus-based system displays quite narrow ranges in isotope values. 

Seep systems differ from the background in many obvious respects. First and foremost, they are 
11-26 

a bioherm in the sense of having a biogenic structure comprised of shells, tubes of the metazoan 

hosts for thiotrophic and methanotrophic bacteria, and cementing authigenic carbonates. Most of 

the associated consumer fauna can be effectively categorized as browsers, opportunistically 

consuming mucous films, free-living bacteria, and bits of tissue from the substrate-forming 

mussels, tubeworms, and clams. Within this browser group, there is probably a continuum from 

strict detritivores such as orbinid polychaetes and sipunculids through film grazers such as nerite 

and limpet gastropods to non-lethal predators that can feed on either snips of tissue or detritus, 

such as the Munidopsis species. At the highest trophic level are resident predator/scavengers 

such as the seastar Sclerasterias tanneri and the gastropod Buccina canatae that feed 

opportunistically on both the substrate formers and the browsers. 

Figure 11.12. A descriptive trophic model for an open seep system. Established links drawn with solid lines and

conjectural links with dashed line. 
11-27 

The present work has confirmed one seep-background link and provided evidence of possibly 

three more. Explanation of why seep browsers and resident predator/scavengers seem to have 

various amounts of phytodetritus-derived tissue may depend on the existence of three other links 

flowing from the background into the seep. 

Link A: Predation on seep fauna by background vagrants. The proven link is trophic flow 

from the seeps to the background in the form of predation by the vagrant fauna. Our results 

indicate that this takes place but at a relatively low level. Analyses of stable isotope ratios, 

immunoassays, gut content examinations, and direct observations prove that species from the 

background prey upon all three major species in the seep system. 

Link B: Predation on the background by seep residents. Simple predation by the seep 

residents upon background fauna would introduce a background isotope signature into the seep. 

Such predation or scavenging could involve wide-area foraging episodes but would most likely 

be due to background fauna entering the seep community. Such an inflow of tissue would be 

aided by mortality caused by the toxicity of the environment. 

Link C: Mingling of the detritus pool. The browser component of the seep fauna is composed 

of detritivores exploiting thin films of detritus rather than the sediment-mixed detritus exploited 

by the background. If the spatially vast detritus pool of the background mingles with the on-site 

detritus pool of the seep systems, the isotope signal of the background would be introduced to all 

consumers. As with all benthic systems, the detritus of seeps is the most poorly defined and 

sampled ecosystem component. Given the lack of knowledge of both background and seep 

detritus, it is easier to delineate expected differences than to describe each separately. Unlike 

other bioherms in which the substrate organisms are heterotrophic filter feeders, seep mussels, 

tubeworms, and clams should produce no fecal contribution to a detritus pool. There may, 

however, be an unmeasured contribution of mucous pseudofeces associated with gill and plume 

cleaning. Seep consumers will contribute fecal material to the detritus pool. Unlike the 

background detritus pool, seep detritus is unaffected by bioturbation and is available to browsers 

on the surface of the biogenic substrate. 

The detrital pool in the seep may be open to import from the background, thus explaining the 

apparent food source mixing seen in the browser component. Such import may be of two forms. 

Biological import occurs because when background vagrants visit the seep sites, they will make 

a fecal contribution to the local detritus pool, which will carry the isotopic signature of the 

background. If the topography of the seeps serves as fish attractors, this fecal influx may be 

appreciable. Physical import may occur because seeps experience net influx by being a passive 

trap for re-suspended material, even though the area of a seep site is too small to receive much 

direct vertical influx of phytodetritus. The mesh of tubeworm bushes and the pumping of 

mussels both produce elevated local sedimentation, drawing laterally transported detritus from 

the benthic boundary layer. While not a food source for the substrate forming organisms, the 

seep browsers along with its background isotope signature consume this imported detritus. 

11.9 Recommended Approach to Sampling Design for Future Efforts 

The following recommendations for future study have been developed in consultation with 
11-28 

program investigators, members of the scientific review committee, and MMS representatives. 

They are based on a review of program results, assessment of future needs, and appraisal of 

available technology. The recommendations fall into two basic categories: 1) Expanded survey 

and enumeration of individual seeps and seep communities to measure the effect of regional 

variables upon community composition and abundance, and 2) studies focused on processedrelated questions germane to seep communities. Specific suggestions are grouped under these 

respective headings. 

11.9.1 Survey, characterization, and enumeration of seeps and seep communities 

Extend the investigation of Gulf of Mexico chemosynthetic communities to the entire slope. 

The results of the present program are qualified in light of the relatively narrow depth and 

geographic range of study sites. Essentially, existing data comprised findings from the midslope (500 to 700 m) and a single site at the 2250 m base of the escarpment (Alaminos Canyon). 

As the deep megasite sonar data clearly show, the slope between 1000 and 2000 m comprises 

sedimentary regimes that are very different from the deeper or shallower slope. Because bottom 

water temperatures are colder in the deep slope, gas hydrate stability will presumably be 

enhanced at the expense of active gas venting. The abundance of ecosystem forming species 

such as tubeworms may be radically altered. The consequence for community formation and 

composition on the deep slope needs to be investigated because the regulatory model is based on 

mid-slope and shallow slope models for the most part. Likewise, the geographic range of 

known communities is too selective. Most of the work to date has focused on a narrow range of 

the central slope. Rates of sedimentation, heat flow, and salt tectonism differ to the east or west 

of the Mississippi Fan. These factors will impact the development of chemosynthetic 

communities. 

Employ cost-effect sampling methodologies and analytical suites. The use of submarines for 

further investigation of the deep slope beyond the 1000 m limit of the SEA-LINK submarines 

will be sharply constrained by vessel availability. The national facility for deep submergence, 

submarine ALVIN, is typically occupied in other marine regions. Consequently, expanded 

sampling from surface ships and ROVs should be utilized to expand regional coverage. Use of 

standard methods of box coring and piston coring proved very effective during the program and 

can be a cost-effective component of future studies. The numbers of inorganic chemical species 

measured can be safely reduced to a few key parameters, e.g. H2S, sulfate reduction, chlorinity. 

However, depth profiles are important and should be extended at least to the ~1 m subbottom 

depths where tubeworm roots interact with the sulfur cycle. What is envisioned is a two or three 

stage process for investigating the deep slope. First, potential sites are identified by use of 

regional geological and geophysical characters; seep-affected zones within the sites can be 

further defined by review of MCS data to identify areas of low seismic amplitude and by using 

piston coring to screen for thermogenic gas and high-molecular weight hydrocarbons. The result 

should define broad zones as active seeps, marginal, or non-seep areas. Second, a more intensive 

program of sediment collection from surface ships can stratify sampling within the identified 

zones. The resulting collections from each could match the sample numbers achieved during the 

present program at a fraction of the cost. Ultimately, investigations of chemosynthetic 

communities require direct observation and sampling from the seafloor. By accomplishing the 

bulk of the sediment collection for site characterization from the surface ship, seafloor operations 

can be focused on photodocumentation of biomass, fine-scale mapping, and tissue collection for 

molecular genetics. 
11-29 

Extend the database of DNA fingerprints for seep tubeworms and mussels. Work 

accomplished during the program has established a powerful set of tools for further defining 

rates of genetic exchange, speciation, and population stability among Gulf of Mexico 

chemosynthetic fauna. The present results demonstrate a mixed gene pool and robust populations 

for the four study sites, but do not go far enough to address cross-slope and along-slope 

exchange. Consequently, it remains undetermined whether there are geographic- or depth-related 

barriers to population migrants or genetically isolated communities that might be of special 

management concern. The DNA fingerprinting technique is cost-effective because the sample 

collection can be accomplished with an ROV without requirements for specialized collection 

equipment. Collections can be accomplished in conjunction with regional survey expeditions. 

Continue histopathology and host-parasite modeling. Like DNA fingerprinting, mussel 

histopathology provides very useful data on community health without the need for elaborate 

collection equipment. In fact, the same specimens can be analyzed for both purposes. The most 

important consideration is to determine to what extent parasite epizootics are responsible for 

local extinctions of mussels. The extent to which PAH exposure facilitates the development of 

epizootics must be considered as well. The possibility that other seep mollusks are severely 

impacted by parasites should also be considered. In shallow water, gastropods usually have 

higher trematode body burdens than bivalves. This, by analogy, would mean that gastropods at 

seeps are heavily impacted. In shallow water, clams tend to be somewhat less impacted than 

mussels, on the average. A check of the lucinids and vesicomyids would seem in order. A hostparasite model should be developed to provide some predictive capability with respect to 

epizootic generation. The management of shallow water molluscan keystone species now often 

involves numerical modeling. Hard clams, mussels and oysters are good examples. The influence 

of disease is now a topic of active research. Coupled host-disease models are being developed 

and routinely used now in the case of oysters. One of the realizations is that small changes in 

environment can destabilize host-parasite population dynamics leading to widespread epizootics. 

Only a numerical model can investigate this phenomenon. 

Continue to compile and maintain a GIS database for Gulf of Mexico chemosynthetic 

communities. Because of the success of the investigations to date, continued applied and basic 

research at seeps will generate ever more data that is potentially important for effective 

management. Gas hydrate research, for example, promises to produce a diverse array of results 

over the next decade. The collective aggregate results will be multi-disciplinary and 

coordination will be sporadic. A central database utilizing GIS tools should be established and 

actively maintained. A common set of descriptors for Gulf of Mexico chemosynthetic 

communities should be developed and periodically reviewed and expanded. Efforts should be 

made to incorporate non-standard indicators on seep numbers and activity such as have been 

obtained from satellite remote sensing. 

11.9.2 Process-related studies 

Examine rate-limiting processes at the sediment-water interface and at the level of 

tubeworm roots. Results during the program indicated that extreme fluxes of sulfide species 

occur at seeps. These rates are calculated from measurements of sulfate reduction and 

assumptions regarding sediment porosity and molecular diffusion. Solid state instruments such 

as micro-electrodes and benthic flux chambers are becoming proven tools for making direct, insitu measurements. This work would be directed from a human-occupied submarine. 
11-30 

Deployment of this type of instrument would be a valuable component of a submersible or ROV 

expedition that focused on process-related questions at the existing study sites or similar habitats 

on the mid-slope. 

Characterize the guild of sulfate-reducing micro-organisms. Reduction of seawater sulfate to 

H2S as a byproduct of microbial hydrocarbon consumption is the key process supporting the lush 

communities of chemosynthetic communities at Gulf of Mexico seeps. However, the consortium 

of taxa which carry out this process, their distribution within surface sediments, and the 

intermediate chemical markers produced remain undetermined. A significant first step toward 

answering these questions would be to conduct phylogenetic studies with use of 16-S RNA 

probes or similar molecular techniques. Automated sample processing facilities have become 

more widely available and could analyze depth profiles from sediment cores. 

Extend collections of the tubeworm epifauna. The bushmaster collection device, which was 

perfected during the present program, should be used to collect tubeworm bushes and associated 

fauna at other seep sites. This work would provide important information concerning variation 

in the species diversity of the tubeworm fauna. 

Focus on gas hydrate interaction with seep ecology. The role of gas hydrate deposits in 

formation or maintenance of chemosynthetic communities has not been clearly defined. The 

evidence to date is often anecdotal concerning biological interactions with hydrate. The stability 

characteristics of gas hydrate change significantly over the depth range on the Gulf of Mexico 

slope. Its ecological role may also change. The ice worm (Hesiocaeca methanicola) increases 

the surface area of exposed hydrate by forming an extensive network of burrows. This may 

significantly affect persistence of shallow hydrate deposits. However, the functional relationship 

of the polychaete with hydrate has not been determined. 
12-1 

 12.0 Data Management 

12.1 Data Management 

Data management plays a key role in complex environmental programs. It is an important 

component of the program because it serves as the central node for the flow of data. As this 

central node, data management monitors, controls and facilitates data flow, ensuring the integrity 

of the data through each phase of the program. 

Data management initially prepared a detailed summary of the planned data and sample 

collections for the initial phase of the program (Table 12.1). Using this summary, a unique 

sample designation was assigned to each planned sample collection element. These sample 

designators were then incorporated into the sample tracking system. This sample designation 

scheme formed the central component for the sample tracking system and the basis for 

documenting the collection and labeling of samples in the field, monitoring their progress 

through the laboratories, and submission of analytical results. 

Appropriate forms for sample collection and data transmittal were developed prior to collection 

(Figure 12.1). The uniformity gained by preparation of standardized forms facilitated 

documentation and ensured the completeness of data entered into the program database. As each 

field effort was completed, collection information was entered into the database. 

As the project investigators completed their sample analysis, they submited their data on task 

specific electronic data submission forms. Before an investigator’s data file was incorporated 

into the program database, it was subjected to a data screening analysis; a quality control 

procedure designed to minimize errors. 

12.2 The Project Database 

A computerized relational database management system (DBMS) is invaluable for effective 

search and retrieval of data to support project management and to provide data products and 

analyses to report authors. Relational systems insulate the end-user from the physical links in the 

database and allow the relationships between data to be easily restructured, thus responding 

quickly to changing needs. The program database is an integrated repository for all information 

relevant to the study. 
12-2 

Table 12.1. Sampling summary at each site. 

Site Habitat Sub- habitat Stations 

Adult AT1, AT2, AT3, AT4, AT5 

Tubeworm Senescent ST1, ST2 

Juvenile JT1, JT2, JT4, JT5, JT6 

Mussel M1, M2, M3, M4, M5 

White Mat B1 

Bacterial Mat 

Red Mat B2 

GC185 (BH) 

Oily diffusion 

Reference Boxcore 

Adult AT1, AT2, AT3, AT4 

Tubeworm Senescent ST1, ST2 

Juvenile JT1, JT2, JT4, JT5 

Mussel M1, M2 

GC234 (GC) 

Oily diffusion 

Bacterial Mat B 

Tubeworm Adult AT1 

Inner edge M1, M4, M7 

GC233 (BP) 

Brine dissolution Mussel 

Outer edge M2, M3, M5 

GB425 (GC) 

Brine dissolution 

Mussel M1, M2 

Figure 12.1. Example of data collection form used for CTD casts. 
12-3 

The central component of a relational DBMS is the table and was the first design step in 

constructing a database. A table is a set of information concerning a given topic. By definition, a 

properly designed relational database contains multiple tables. 

The project database contains reference tables (e.g., Dive Log, Dive Activities, samples 

sediment, etc.) and tables for each sampling activity (e.g., Brine Samples, Hydrate Samples, 

Boxcores, etc.) and the relationship among each of the tables (Figure 12.2). The DBMS contains 

tables for each of the research component results (e.g., PAH, Histo, etc.)0 is shown in (Figure 

12.3). Tables are in turn broken down into records, which give information about a certain item 

in a table, and fields, which contain discrete pieces of information about each record. As an 

example, the sediment sample table is organized into records (rows), with each record 

representing a dive, and fields (columns) named dive number, station, sample type, etc. and 

disposition of the sample (Figure 12.4). 

12.3 Data Entry 

Having designed the database tables, the database was then populated with data from the various 

program components. Forms for data entry standardized the data files. Forms that are more 

complex were created with lookup boxes that allowed a selection from acceptable values for 

entry into a given field. List boxes were also used. These permit more than one item to be 

associated with a given record, as is the case when one station contains multiple samples (Figure 

12.5). 

The program database is divided into two modules. The first module is sample/data status 

inventory. It consists of the files necessary to track the status and custody of all data and samples 

collected during the program. 

The second module of the database contains the analytical results of the samples. The merging of 

these two modules results in the project database, as represented in (Figure 12.6). 

A summary of the samples collected over the course of the program is shown in (Table 12.2). 

Many of the samples for a particular matrix were sub-sampled and distributed among the 

researchers. As results arrived from the researchers, a tracking table was populated with 

information on the source, analysis type and disposition of the data (Table 12.3). 
12-4 

Figure 12.2. Table and relationships in the program DBMS. 
12-5 

Figure 12.3. Table listing of the program DBMS. 
12-6 

Figure 12.4. Sediment sample table. 
12-7 

Figure 12.5. Form designed to record surface operations with pull down menus for selecting site. 
12-8 

 

Figure 12.6. Merged modules forming a database file for headspace. 
12-9 

 

Table 12.2. Summary of the samples collected. 

Year Site Matrix Sample 

 

All GB425 Gas 19 

All GB425 Hydrate 19 

All GB425 Sed 64 

1 GB425 Mussels 83 

 

All GC185 Gas 31 

All GC185 Hydrate 31 

All GC185 Sed 157 

1 GC185 Mussels 1002 

1 GC185 Tubeworm 1162 

 

All GC233 Gas 29 

All GC233 Hydrate 29 

All GC233 Sed 38 

1 GC233 Mussels 187 

 

All GC234 Gas 41 

All GC234 Hydrate 41 

All GC234 Sed 164 

1 GC234 Mussels 741 

1 GC234 Tubeworm 1769 
12-10 

Table 12.3. Tracking table. 

 Matrix Collection Analysis Cru1-Jun97 Datafile Cru2-Jun98 

NOTE - DB file

- italics, XLS 

file – bold 

Fisher –

TW 

TW CI, 

GROWTH, 

Biomass 

Sub Cond Index combined yr1&2 tubeworm growth and condition d

for the MMS stations collected in 

Fisher –

Mus 

Mussel Size , 

CI - Sites Sub 

1997 Gulf of Mexico ring collection 

mussel shell length and wet weight 

data. 

97rclwt.xls 

condition index and percent water 

of mussels collected at study site

1998 

Mussel Site 

Condition 

Index - Ring 

Sub Cond Index 1997 Gulf of Mexico ring collection mussel condition index and percent water data. 97rc_ci.xls 

shell lengths, condition index, an

water in the tissue of the mussels 

the two reciprocal transplant 

experiments initiated in 1997 a

collected in 1998 

Mussel 

Summary Ring Sub Mussel Summary 

summary information pertaining to 

the mussel populations at the primary

study sites from the 1997 ring 

collections 

97rc_sum.xls biomass of the mussel soft tissu

Fisher –

other Assoc. fauna Sub 97 ring collection assoc. fauna 97rc_afau.xls 

summary table for data on associ

fauna from bushmaster collection

vestimentiferan aggregations 19

Water Sub H2S, CH4 

Gas Chromatography measurements 

of Sulfide and Methane 

concentrations (mM) from water 

sampled at dive stations in 1997 H2S,

CH4 only 

F_watgc1.xls CO2,H2S, O2, N2, CH4, CO 14 sam

Sassen Sediment Samples Cores EOM 52 samples Eomdata, D_eom_c1 36 samples 

Sediment 

Samples HS,Fl, GC 15 samples - PISTON 

Pisco_hs, 76333_fl 

76333_gc*, 

D_sager_core76333

Hydrate C15-C32, UCM 6 samples hdrate, D_hyd_c1 C1-C5, delC13 

 MudSuck 

Brine/Gas 

Samples brinetrap C1-C5 JSL97_hs, D_HS_JSL97 C1-C5, delC13, Hydrate ; C1-C

brinetrap 

Sediment 

Samples C1-C5 C1-C5, delC13 

Carbonate 

Samples 
12-11 

Table 12.3. (Cont.) 

 Matrix Collection Analysis Cru1-Jun97 Datafile Cru2-Jun98 

M.Joye Core Samples 

Mudsucker 

Samples Mn+2, S-2, Fe 17 dives Mudsjoye.xls 

D-mudsk_joye1 

Kennicutt Tissue PAH 62 samples pah, D_pahc1 21 samples, pooled 

Tissue %Lip, Tot PAH 11 samples totpahc1 21 samples 

J.Morse Sediment Core Pore Water Metals, Core Metals, Sulf Red Morse ,D_por_mc1, D_so4_mc1 D_sol_mc1 

Sediment Mudsuck 

E.Powell Mussels Histo Histopath powell.xls ? 

D_histo1 

Macko C,N,S trap fauna macko97.xls del34S corr, box and dive 

Carney del 13C, del 15N, del S (both cruises in 1 file) isotopic determinations on fauna collected during dives. 

Nelson bacterial ND 
12-12 

12.4 Geospatial applications 

Export files were prepared from the database for inclusion into geospatial applications. 

Arcview/Arcmap (ESRI) was used to generate layered displays of the program results (Figure 

12.7) and spatial analyses. 

Metadata files, data dictionaries, data relations and all other database documentation files were 

produced for all data. These data products provide all the requested data documentation 

information for program data and follow the guidelines established by the Federal Geographical 

Data Committee. Spatial information, data quality and other categories of data information are 

provided in this document that accompanies the program data (Figure 12.8). 

Figure 12.7. Geospatial figure with results layered over bathymetry at Bush Hill site. 
12-13 

Figure 12.8. Metadata file and contents for program data. 
13-1 

13.0 Literature Cited 

Abrams, M.A. 1996. Distribution of subsurface hydrocarbon seepage in near-surface marine 

sediments in Hydrocarbon Migration and Its Near-surface Expression. Edited by 

Schumacher, D. and M.A. Abrams. AAPG Memoir 66. Amer. Assoc. Petrol. Geol. 

Tulsa, OK. Pp. 1-14. 

Aharon, P. 1994. Geology and biology of modern and ancient submarine hydrocarbon seeps and 

vents: An Introduction. Geo-Marine Letters.14: 69-73. 

Ahmad, A. 1999. Phylogenetic and physiological studies of filamentous sulfur-oxidizing bacteria 

belonging to the genus Beggiatoa. Ph.D. dissertation thesis. University of California, 

Davis. 

Ahmad, A., J.P. Barry, and D.C. Nelson. 1999. Phylogenetic affinity of a wide, vacuolate, 

nitrate-accumulating Beggiatoa sp. from Monterey Canyon, California, with Thioploca 

spp. Applied and Environmental Microbiology. 65(1): 270-277. 

Allen, R.L. 1979. A yield model for the Foveaux Strait oyster (Ostrea lutaria) fishery. Rapports 

et Proces-Verbaux des Reunions Conseil International pour l'Exploration de la Mer. 175: 

70-79. 

Amery, G.B. 1969. Structure of Sigsbee Scarp, Gulf of Mexico. American Association of 

Petroleum Geologists Bulletin. 53: 2480-2482. 

An, S. 1999. Nitrogen cycling and dentrification in Galveston Bay. Ph.D. dissertation thesis. 

Texas A&M University. College Station, TX. 

An, S., and S.B. Joye. In press. Enhancement of coupled nitrification-dentrification by benthic 

photosynthesis in shallow estuarine sediments. Limnol. Oceanogr. 

Anderson, A.L., and W.R. Bryant. 1990. Gassy sediment occurrence and properties: Northern 

Gulf of Mexico. Geo-Marine Letters 10: 209-220. 

Anderson, R.K., R.S. Scalan, P.L. Parker, and E.W. Behrens. 1983. Seep oil and gas in Gulf of 

Mexico sediment. Science 222: 619-621. 

Arp, A.J., J.J. Childress, and J. Fisher, C. R. 1984. Metabolic and blood gas transport 

characteristics of the hydrothermal vent bivalve Calyptogena magnifica. Physiological 

Zoology 57: 648-662. 

Arp, A.J., J.J. Childress, and R.D. Vetter. 1987. The sulfide-binding protein in the blood of the 

vestimentiferan tube-worm, Riftia pachyptila, is the extracellular haemoglobin. Journal of 

Experimental Biology 128: 139-158. 
13-2 

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

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