OCS Study 

MMS 95-0022 

Northern Gulf of Mexico 

Chemosynthetic Ecosystems Study 

Final Report 

Volume II : Technical Report 

TEXAS 

MISSISSIPPI ALABAMA GEORGIA 

FLORIDA 

LOUISIANA =- . 

~ .. ,' 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 95-0022 

Northern Gulf of Mexico 

Chemosynthetic Ecosystems Study 

Final Report 

Volume II : Technical Report 

Editors 

Ian R. MacDonald 

William W. Schroeder 

and 

James M . Brooks 

Prepared under MMS Contract 

14-35-0001-30555 

by 

Geochemical and Environmental Research Group 

Texas A&M University 

Texas A&M Research Foundation 

College Station, Texas 

Published by 

U .S. Department of the Interior 

Minerals Management Service New Orleans 

Gulf of Mexico OCS Region May 1996 
DISCLAIMER. 

This report was prepared under contract between the Minerals Management Service 

(MMS) and Geochemical and Environmental Research Group. This report has been 

technically reviewed by the MMS and approved for publication. Approval does not 

signify that the contents necessarily reflect the views and policies of the Service, nor 

does mention of trade names or commercial products constitute endorsement or 

recommendation for use . It is, however, exempt from review and compliance with 

MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of the report may be obtained from the Public Information Unit (Mail 

Stop 5034) at the following address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Unit (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone Number: (504) 736-2519 

1-800-200-GULF 

CITATION 

Suggested citation: 

MacDonald, I .R., W.W. Schroeder, and J.M . Brooks. 1995. Chemosynthetic 

Ecosystems Studies Final Report. Prepared by Geochemical and 

Environmental Research Group . U.S . Dept. of the Interior, Minerals Mgmt. 

Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 95-

0023 . 338 pp. 

COVER PHOTOGRAPH 

The foreground photograph shows the submersible Johnson Sea-Link I preparing for 

one of its many dives to study chemosynthetic ecosystems. The map depicts the 

locations of known chemosynthetic ecosystems in the northern Gulf of Mexico. 

iii 
PREFACE 

The Chemosynthetic Ecosystems Study concerns the prominent biological 

communities of tube worms, mussels, and clams that occur at natural hydrocarbon 

seeps on the continental slope and that derive their food supply from chemicals 

associated with the seeps. This is the Technical Report (Volume II) of the Final 

Report that will be issued by the Study, which is sponsored by the U.S. Department 

of Interior Minerals Management Service (MMS), Gulf of Mexico Region OCS Office 

(Contract 14-35-0001-30555) . 

The Study is being conducted by ten principal investigators (PIs) and four 

associates under the overall management of the Geochemical and Environmental 

Research Group (GERG) of Texas A&M University (see next page). The Program 

has completed all of the three scheduled research cruises and has completed 

processing material collected on these cruises. The first report of the Study 

(MacDonald 1992) presented a review of published literature pertinent to the subject 

and a limited synthesis of data collected prior to commencement of the Program. An 

appendix volume to this report reproduced the core, peer-reviewed literature pertinent 

to Gulf of Mexico seep communities An interim report described the methods, 

techniques, and equipment used during the field program, outlined the data sets and 

collections obtained during the field study, and discussed the analyses and 

interpretations employed to treat these materials. This report comprises some of the 

introductory material from the two previous reports in order to provide unfamiliar 

readers with a context for Study findings. In keeping with MMS guidelines, these 

chapters are addressed to an audience of knowledgeable lay-people. To enhance 

readability, extended listings of materials and methods will be given in an appendix 

volume or by citation to previous reports or publications . 

v 
Principal Investigators (*) and Associates for the Chemosynthetic Ecosystems 

Study. Affiliation abbreviations are: 

TAMU Texas A&M University 

GERG Geoctcemical and Environmental Research Group; 

TAMU 

OCNG Oceanography Department, Texas A&M University 

LSU Louisiana State University 

PSU Pennsylvania State University 

WHOI Woods Hole Oceanographic Institution 

UA/DISL University of Alabama, Dauphin Island Sea Lab 

Principal Investigator Affiliation Major Responsibility 

Javier Alcala-Herrera TAMU/GERG Chemical Analyses 

James M. Brooks* TAMU/GERG Project Director 

Robert S. Carney* LSU Ecology 

Charles R. Fisher* PSU Physiology 

Norman L. Guinasso, Jr.* TAMU/GERG Data Analysis 

Holgar W. Jannasch* WHOI Microbiology 

Mahlon C. Kennicutt II* TAMU/GERG Geochemistry 

Ian R. MacDonald* TAMU/GERG Ecology/Deputy Project Director 

Eric N. Powell* TAMU/OCNG Taphonomy 

William S. Sager* TAMU/OCNG Geology 

William W. Schroeder* UA/DISL Geology/Ecology 

Dan L. Wilkinson TAMU/GERG Field Logistics 

Carl O. Wirsen WHOI Microbiology 

Gary A Wolff TAMU/GERG Data Management 

vii 
TABLE OF CONTENTS 

Page 

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

List of Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .)cd 

1 .0 Chemosynthetic Ecosystems Study: Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 

1 .1 Chemosynthetic Communities in the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l-1 

1 .2 Summary of Study Accomplishments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l-3 

2.0 The Regional Distribution of Chemosynthetic Communities 

Across the Continental Slope in the Northern Gulf of Mexico . . . . . . . . . . . . . . . . 2-1 

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

2.2 Types of Seep Communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-2 

2.3 Zonal Distribution of Chemosynthetic Communities in the 

Northern Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-3 

2.3 .1 Local Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-3 

2.3 .2 Regional Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-6 

2.4 Remote-Sensing Detection of Natural Oil Seepage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8 

2 .4.1 Use of Geophysical Methods for Indirect Detection of 

Chemosynthetic Communities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-8 

2 .4.2 Formation and Detection of Surface Oil Slicks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-9 

2 .4.3 Remote Sensing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10 

2 .4.4 Estimates of Total Seepage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-16 

2 .4.5 Trial of the Laser Line Scan System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20 

2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-21 

3.0 Geological and Geophysical Characterization of Hydrocarbons . . . . . . . . . . .3-1 

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

3.1.1 Study Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-2 

3.1.2 Geologic Causes and Symptoms of Hydrocarbon Seeps in 

the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-2 

3.1 .3 Geophysical Signatures of Seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5 

3 .2 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-7 

3.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10 

3.4 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10 

3.5 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14 
3 .5 .1 Acoustic Echo Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-14 

3 .5 .2 Ground Truth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16 

3 .5 .3 Distribution of Echo Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19 

3 .5 .4 Backscatter Mosaics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25 

3 .5 .5 Connections to Deep Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31 

3 .5 .6 Relationship of Geophysical Signatures and Seep 

Organisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31 

3 .6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-34 

4.0 Geochemical Alteration of Hydrocarbons in Seep Systems . . . . . . . . . . . . . . . . . . . .4-1 

4.1 Introduction: A Descriptive Model of the Geochemical Setting of 

Seep Communities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-1 

4.2 Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-13 

4.2 .1 Samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-13 

4 .2 .2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-15 

4.3 R.esults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 

4.3 .1 Isolated Bacterial Mat at GC 234 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 

4.3 .2 Bacterial Mat and Tube Worms at GC 185 (Bush Hill) . . . . . . . . . . . . 4-20 

4.3 .3 Gas Hydrate at GC 185 (Bush Hill) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-20 

4.3 .4 Chemosynthetic Clams at GC 272 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-20 

4.3.5 Chemosynthetic Mussels at GC 233 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 

4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 

4.4 .1 Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-21 

4.4 .2 Seeps and Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 

4.4 .3 Sea-floor Modification at Seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-30 

4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-34 

5.0 Characterization of Habitats and Determination of Growth Rate 

and Approximate Ages of the Chemosynthetic SymbiontContaining Fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-1 

5.1 Mussel Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-2 

5.2 Water Sampling and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-3 

5 .3 Growth Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-5 

5 .4 Transplant Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-6 

5.4.1 Measures of Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-6 

5 .5 Statistical Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-8 

5 .6 Growth Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-9 

X 
5.7 Results for Vestimentiferans: Escarpicz sp. and Lamellibrachia 

sp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-9 

5 .7 .1 Vestimentiferan Growth Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10 

5 .7 .2 Vestimentiferan Habitat Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15 

5 .7 .3 Vestimentiferan Condition at Different Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18 

5 .8 Results for Vesicomyid Clams: Vesicomya chordates and 

Cczlyptogena ponderasa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21 

5 .9 Results for the Methane Mussel : Seep Mytilid Ia (SM Ia). . . . . . . . . . . . . . . . . . . . 5-24 

5.9.1 The Mussel Beds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25 

5.9 .1 .1 GC 184 Mussel Beds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25 

5.9 .1 .2 GC 234 Mussel Beds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29 

5 .9 .1 .3 GC 272 Mussel Bed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29 

5.9 .2 SM Ia Population Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-30 

5.9 .3 SM Ia Chemical Microhabitat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-30 

5.9 .4 Growth Rates and Ages of SM Ia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-36 

5.9 .5 Measures of Condition of SM Ia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-41 

5.9 .6 Transplant Studies with SM Ia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-42 

5.9 .7 Site Synopsis for SM Ia Beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-44 

5.10 Implications of Growth Experiments and Habitat 

Characterizations on MMS Management Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-46 

6.0 Sessile Macrofauna and Megafauna at Mussel Beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1 

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

6.2 Methods and Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-2 

6.2.1 Sampling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-2 

6.2.2 Sampling Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-3 

6.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-5 

6.3 .1 The Mytilid Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-6 

6.3 .2 Seep Mytilid - Bathynerites Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 

6.3 .3 Relationships with Provanna sculptcz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-9 

6.3 .4 Geographic Patterns of Subdominant Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 

6.4 Discussion: The Origins and Significance of Monotony. . . . . . . . . . . . . . . . . . . . . . . . . 6-10 

6.5 Description and Natural History of the Megafaunal Organisms . . . . . . . . 6-10 

6 .5 .1 Mollusca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 

6 .5 .2 Crustacea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15 

6 .5.3 Worms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17 

xi 
6 .6 Gulf-Wide Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-18 

6 .7 Potential for Adverse Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-24 

7.0 Stability and Fluctuation in Chemosynthetic Communities Over 

Time Scale of -1 Month to -1 Year. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 

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

7.1.1 Need to Understand Time-Scales of Seepage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 

7.1.2 Scope of the Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 

7 .2 Materials and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-3 

7.2 .1 Photomosaics and Other Quantitative Photography . . . . . . . . . . . . . . . . . . .7-3 

7.2 .2 Controlled Disturbance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-7 

7.2 .3 Monitoring Seafloor Gas Hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-9 

7.3 R,esults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 

7.3 .1 Photomosaics at GC 184: Site 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 

7.3 .2 Photomosaics at GC 184: Site 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11 

7.3 .3 Photomosaics at GC 234: Site 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 

7.3 .4 Photomosaics at GC 234: Site 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-14 

7 .3 .5 Photomosaics at GC 272: Site 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-16 

7 .3 .6 Photomosaics at GC 272 : Site 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17 

7 .3 .7 Photomosaics at VK 826 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17 

7.4 Disturbance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-19 

7 .4.1 Bush Hill Site, GC 184/185 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-19 

7 .4.2 GC 234. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-20 

7 .4.3 Site Three GC 272. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-20 

7 .4.4 Brine Pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-20 

7 .4.5 Inner Edge Markers - Brine Pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-21 

7 .4.6 Mat Surface Markers - Brine Pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-21 

8.0 

7 .5 In Situ Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-22 

7 .6 Geological Interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-25 

7 .7 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-28 

Evidence for Temporal Change at Seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-1 

8.1 Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-1 

8.2 Flow Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-2 

8.3 Size Frequency Distributions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-5 

8.4 Effects of Taphonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-6 

8.5 Taphonomy at Petroleum Seeps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-7 

8.6 Effects of Taphonomy on Seep Assemblages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-8 

8.7 Site Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-25 

xii 
8.7.1 Green Canyon 184 (Bush Hill). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-25 

8.7.2 Green Canyon 272 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-50 

8.7.3 Green Canyon 234 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-54 

8.7.4 Garden Banks 386 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-58 

8.7 .5 Garden Banks 425 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-59 

8.8 Community History, Persistence and Succession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61 

8.9 Summary and Management Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-64 

9.0 Age, Growth, and Reproduction in a Deep Sea Gorgonian from 

Hydrocarbon Seep Sites in the Northern Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . .9-1 

9.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1 

9.2 Year 1 Effort: Age and Growth Rate Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-3 

9.2.1 Methods and Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-3 

9.2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-3 

9.3 Year 2 Effort: Reproductive Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-5 

9.3.1 Methods and Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-5 

9.3.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-7 

9.3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-18 

9.3 .4 Management Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-18 

10.0 An Ecological Characterization of Hydrocarbon Seep 

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

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

10.1.1 The Gulf of Mexico Hydrocarbon System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2 

10.1.2 Salt Structures and Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2 

10.1 .3 Seep Zones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3 

10 .1 .4 The Seep Hydrocarbon System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4 

10 .1 .5 Seep Fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-5 

10.1 .6 Temporal Change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-7 

10 .2 Recommendations for Future Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-8 

11 .0 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1 

xiii 
LIST OF FIGURES 

Figure Page 

2.1 Locations where vestimentiferan tube worms, seep mytilids, or 

vesicomyid clams have been collected or photographed in the northern 

Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-5 

2.2 Overlap between oil slicks detected in remote sensing images from 

different dates reveals the locations of perennial oil seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-14 

2.3 Laser line scan image collected over Bush Hill during the June 1993 

trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-21 

3.1 Location of study areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-3 

3.2 3.5 kHz echo sounder profile over the flat-topped mud mound in GB 425. . . . . . . . 3-6 

3.3 Examples of acoustic echo types seen in 25 kHz subbottom reflection 

profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15 

3.4 Description of Core GB 386-A, from a Type 1 (hard substrate) echo 

character zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17 

3.5 Description of Core GB 425-A, from a Type 2 (buried hard substrate) 

echo character zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18 

3.6 Description of Core GB 425-D, from a Type IV (disseminated 

disturbance) echo character zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20 

3 .7 Patterns of acoustic echo types in the GB 425 survey area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-21 

3 .8 Patterns of acoustic echo types in the GB 386 survey area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22 

3 .9 Patterns of acoustic echo types in the GC 184 survey area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23 

3 .10 Patterns of acoustic echo types in the GC 234 survey area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26 

3.11 Side scan sonar mosaic of the GB 386 survey site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27 

3.12 Side scan sonar mosaic of the GC 234 survey site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28 

3.13 Comparison of GC 234 side scan sonar mosaic and fault traces (heavy 

black lines) mapped with multichannel seismic data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-29 

3.14 East-west 3.5 kHz echo sounder profile across the GB 386 mud mound 

(right of center), on the flank of a large salt diapir. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32 

3 .15 Model of the formation of the GC 184 mud mound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-36 

xv 
3 .16 East-west 3.5 kHz echo sounder profile across the GC 184 mud mound . . . . . . 3-38 

4 .1 Summary of seep site processes and indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-2 

4 .2 Oil seepage adds 13C-depleted organic carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-4 

4 .3 Oil seepage adds organic carbon at seep sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-5 

4 .4 Sources of carbonate at seeps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-6 

4 .5 Evidence of microbial hydrocarbon oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-7 

4 .6 GC 184 (Dive 3269) methane concentrations (ppm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 

4 .7 GC 234 (Dive 3265) methane concentrations (ppm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10 

4 .8 GC 272 (Dive 3280) methane concentrations (ppm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-11 

4 .9 GC 272 (Dive 3279) methane concentrations (ppm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-12 

4 .10 Detailed locations of core samples taken from a zoned orange and white 

Beggiatoa mat at the GC 2341ocality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14 

4 .11 Examples of C15+ chromatograms of hexane extracts from the GC 234, 

GC 185, GC 272, and GC 233 areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-19 

4 .12 Histograms illustrating that the concentrations of UCM in core samples 

of the GC 234 Beggiatoa mat are higher beneath the mat than in control 

cores away from the mat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-22 

4 .13 Histograms illustrating that the concentrations of total C1-C5 

hydrocarbons in core samples of the GC 234 Beggiatoa mat are higher 

beneath the mat than in control cores away from the mat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23 

4.14 Comparison of C15+ chromatograms of a subsurface reservoir sample of 

oil from Jolliet Field in GC 184 and a seep sample from GC 185 (Bush 

Hill) illustrates the effects of bacterial oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25 

4.15 Histograms illustrating differences in UCM concentrations between the 

four study areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-26 

4.16 Histograms illustrating differences in total C1-C5 hydrocarbon 

concentrations between the four study areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-28 

4.17 A C15+ chromatogram of sediments from a mud volcano on GC 143 . . . . . . . . . . . . 4-31 

5 .1 Relation between tube length and wet weight for hydrocarbon seep 

vestimentiferans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11 

5 .2 Size frequency distributions for SM la at three hydrocarbon seep sites . . . . . . . 5-27 

acvi 
5 .3 Relation between wet weight and shell length of SM la . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-28 

5 .4 Change in length as a function of initial shell length for SM la from three 

hydrocarbon seep sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-34 

5 .5 Age estimations for SM Ia from (a) GC 234 (open circles = 1991, solid 

circles = 1992) and (b) GC 184 (open circles = 1991, solid circles = 1992, 

solid triangles = transplant I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-37 

5.6 Change in length as a function of initial shell length for two GC 184 

transplant experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-40 

5.7 Relation between growth rate and condition indices in SM Ia from GC 

272. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-43 

7.1 Gas hydrate at a depth of 540 m on the continental slope of the Gulf of 

Mexico (27026.7'N and 91030 .4'W) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 

7.2 Data recorded by a bubblometer showing the release of gas and the 

water temperatures during a 44-day period in 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-26 

7.3 Photograph of a massive slab of mussel shells and carbonate cement 

that was up-ended in the floor of a large pockmark near the crest of 

Bush Hill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-29 

8.1 The species composition of the clam biofacies summed over all sites . . . . . . . . . . . . . .8-9 

8.2 The species composition of the lucinid biofacies summed over all sites. . . . . . . . . 8-11 

8 .3 The cumulative feeding guild structure of the seep clam biofacies 

summed over all sites defined by total and whole abundance, 

paleoproduction, and paleoingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 

8 .4 Predicted paleoguild structure of seep clam biofacies summed over all 

sites, defined by total and whole abundance, paleoproduction, and 

paleoingestion after taphonomic loss of all individuals in the lowermost 

three size classes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-14 

8 .5 The cumulative habitat tier structure for seep clam biofacies summed 

over all sites, defined by total and whole abundance, paleoproduction, 

and paleoingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-15 

8 .6 Predicted tier structure of seep clam biofacies summed over all sites, 

defined by total and whole abundance, paleoproduction, and 

paleoingestion after taphonomic loss of all individuals in the lowermost 

three size classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16 

8.7 The size-frequency distribution for the clam biofacies summed over all 

sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18 
8 .8 The apportionment of paleoproduction among the size classes for the 

clam biofacies summed over all sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-19 

8 .9 The apportionment of paleoingestion among the size classes for the 

clam biofacies summed over all sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-20 

8 .10 Predicted size-frequency distribution for the clam biofacies summed over 

all sites after taphonomic loss of all individuals in the lowermost three 

size classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-22 

8.11 Predicted apportionment of paleoproduction among size classes for the 

clam biofacies summed over all sites after taphonomic loss of all 

individuals in the lowermost three size classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-23 

8.12 Predicted apportionment of paleoingestion among size classes for the 

clam biofacies summed over all sites after taphonomic loss of all 

individuals in the lowermost three size classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-24 

8.13 The species composition of the lucinid bioFacies at GC 184 and GB 386. . . . . . . 8-26 

8.14 The size-frequency distribution for the clam biofacies at GC 184, GC 

234, GC 272, GB 386 and GB 425 (assemblage). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28 

8 .15 The apportionment of paleoproduction among the size classes for the 

clam biofacies (assemblage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-29 

8 .16 The apportionment of paleoingestion among the size classes for the 

clam biofacies (assemblage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30 

8 .17 The size-frequency distribution for the clam biofacies at GC 184, GC 

234, GC 272, GB 386 and GB 425 (species) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 

8 .18 The apportionment of paleoproduction among the size classes for the 

clam biofacies (species) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32 

8 .19 The apportionment of paleoingestion among the size classes for the 

clam biofacies (species) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-33 

8.20 The cumulative feeding guild structure for the seep clam biofacies at GC 

184, GC 234, GC 272, GB 386, and GB 425 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 

8.21 The cumulative habitat tier structure for the seep clam biofacies at GC 

184, GC 234, GC 272, GB 386, and GB 425 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-37 

8.22 The species composition of the lucinid biofacies at GC 184 from Core 1. . . . . . . 8-38 

8 .23 The size-frequency distribution of 5 cm core intervals at GC 184 Core 1 

(assemblage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-39 

8 .24 The apportionment of paleoproduction among the size classes for 5 cm 

core intervals at GC 184 Core 1 (assemblage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-40 

XVill 
8 .25 The apportionment of paleoingestion among the size classes for 5 cm 

core intervals at GC 184 Core 1 (assemblage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-41 

8 .26 The size frequency distribution for 5 cm core intervals at GC 184 Core 1 

(species) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-42 

8 .27 The apportionment of paleoproduction among the size classes for 5 cm 

core intervals at GC 184 Core 1(species). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-43 

8.28 The apportionment of paleoingestion among the size classes for 5 cm 

core intervals at GC 184 Core 1(species). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-44 

8.29 The cumulative feeding guild structure of several core intervals from the 

lucinid biofacies at GC 184 Core 1, defined by numerical abundance, 

paleoproduction, and paleoingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-46 

8 .30 The cumulative habitat tier structure of several core intervals from the 

lucinid biofacies at GC 184 Core 1, defined by numerical abundance, 

paleoproduction, and paleoingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-48 

8 .31 The numerical abundance, paleoproduction, and paleoingestion 

contributed by each 5 cm core interval from the lucinid biofacies at GC 

184 Core 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-49 

9 .1 Major bands and minor bands are visible around the central axis of 

Callogorgia sp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-4 

9 .2 Cross section of axial rod showing concentric ring structure in 

Leptogorgia hebes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-6 

9.3 Vacuole and lipid enriched cytoplasm of an egg from GC 234 in 1991 . . . . . . . . . . . . . . 9-9 

9.4 Rows of maturing sperm converge toward the center of a spermary from 

GC 234 in 1991 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-10 

9.5 A mature sperm with a characteristic head and tail from GC 234 in 

1991 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-11 

9.6 Relation of base diameter and height of Callogorgia sp. colonies. . . . . . . . . . . . . . . . . . . . 9-12 

9.7 Relation between height and reproductive state of Callogorgia sp . 

colonies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-13 

9.8 Relation of Callogorgia sp . colony height and collection site by year . . . . . . . . . . . . . 9-15 

9 .9 Relation between Callogoria sp . gamete number and collection site by 

year . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-16 

aix 
LIST OF TABLES 

Table Page 

2.1 Sites where chemosynthetic metazoans have been collected by trawl 

(Trl) or submarine (Sub), or definitively photographed by submarine, 

remotely operated vehicle (ROV), or photosled (Photosl) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 

2.2 Oil slicks detected in one or more remote sensing images from the 

northern Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12 

3.1 Description of piston cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 

3.2 Description of push cores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13 

4.1 Concentrations of UCM (ppm by weight) and total C 1-C5 hydrocarbons 

(ppm by volume) in sediment samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17 

4.2 Concentrations of individual C1-C5 hydrocarbons (ppm by volume) in 

sediment samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18 

5.1 Vestimentiferan tube growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12 

5 .2 Variability in replicate water samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16 

5 .3 Localization of H2S at seep vestimentiferan bushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-17 

5 .4 Means, standard deviations, and sample sizes are shown for condition 

indicators in Escarpia sp. (a) and Lamellibrczchia sp. (b) populations . . . . . . . . . . . . 5-19 

5 .5 Condition indicator models for seep vestimentiferans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21 

5.6 Sulfide levels in Louisiana slope clam habitats and freshly collected 

blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24 

5 .7 Methane concentrations in water samples from mussel beds . . . . . . . . . . . . . . . . . . . . . . . . . 5-26 

5 .8 Average growth rate of SMIa in size range 60-90 mm shell length from 3 

hydrocarbon seep sites and 2 transplant experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-32 

5 .9 Growth parameters of SMIa derived from Ford-Walford plots . . . . . . . . . . . . . . . . . . . . . . . . 5-32 

5 .10 Condition indices and size range for SMIa at three hydrocarbon seep 

sites and two transplant experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-33 

6.1 Sample collection for infaunal material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-4 

6.2 Site-specific Bathymodiolus-Bathynerites relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8 

6.3 Site-specific Provanna-Bathymodiolus-Bathynerites relationships . . . . . . . . . . . . . . . . . . . 6-9 
6.4 Composition of upper and lower slope seep heterotrophic fauna with 

reported species richness and dominant species of the background 

fauna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-20 

7 .1 Summary of locations at study sites where mosaics were compiled in 

order to observe short-term change in community type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 

7 .2 Classifications of community type used to evaluate photomosaics and 

video coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-6 

7 .3 Hydrocarbon composition (Cl-C5) of gas samples from the study site . . . . . . . . 7-25 

8.1 14C dates of mussel, lucinid, and thyasirid beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36 

9.1 Seep sites from which octocorals have been reported . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2 
1.0 Chemosynthetic Ecosystems Study: Overview 

The chemosynthetic communities associated with hydrocarbon seeps on the 

Louisiana/Texas continental slope are one of a series of functionally and 

taxonomically related assemblages in the deep-sea. For the purposes of this report, 

we define a chemosynthetic community as d persistent, largely sessile assemblage of 

marine organisms that depend upon the chemoautotrophic productivity of bacteria. 

These communities are characteristically associated with sources of hydrogen sulfide 

and/or methane in an oxygenated environment. The underlying geological processes 

supplying these reduced compounds vary from site to site. 

1 .1 Chemosynthetic Communities in the Gulf of Mexico 

The first discovery of chemosynthetic communities was made at hydrothermal 

vents in the eastern Pacific Ocean (Corliss et al. 1979). The principal components of 

these communities are tube worms, clams, and mytilids that derive their entire food 

supply from symbiotic bacteria, which were in turn supported by chemical 

components in the venting fluids and by the physiological mediations of their host 

organisms (Cavanaugh et al. 1981; Childress and Mickel 1985) . This series of 

discoveries contributed to what became a fundamental reordering of theories 

regarding life in the deep sea (Tunnicliffe 1992) . In 1984, a large quantity of bivalves 

and tube worms was recovered in a series of trawls that were pulled through oil and 

gas seeps on the northern Gulf of Mexico continental slope . It soon transpired that 

these then unfamiliar animals were close relatives of a fauna found in so-called 

"chemosynthetic communities" that had only recently been discovered at 

hydrothermal vents in the eastern Pacific Ocean (Kennicutt et al. 1985). 

Previously, the general concept of the continental slope of the Gulf of Mexico, 

held that the sea floor comprised a vast extent of soft sediment, sparsely populated 

1-1 
by diverse, but tiny burrowing animals and an assortment of depth-adapted fishes 

and crustaceans. Occurrence of species that were taxonomically and functionally 

similar to vent fauna on a passive continental margin meant that the distribution of 

chemosynthetic fauna was much more wide-spread. Dependence on hydrocarbon 

seepage also placed the communities in the one deep-sea benthic zone that is certain 

to be subject to possible impact from human activities . Addressing these concerns, 

Minerals Management Service (MMS) issued guidelines designed to protect the 

biological communities at oil seeps from possible harm due to disturbance associated 

with the energy industry (MMS 1988). Simultaneously, a review by MMS of 

information needed for prudent management of the seep communities concluded that 

additional study was required. Several critical questions were identified, including the 

following: 

1 . What are the specific geological, chemical, and ecological processes 

whereby seeping hydrocarbons support distinct communities? 

2 . How do these communities persist, and to what degree do physicalchemical and biological factors interact on different spatial and temporal 

scales? 

3. How quickly and to what degree will chemosynthetic communities 

recover from mechanical damage? 

4. How rare or common are dense chemosynthetic communities across the 

Gulf of Mexico? 

5. How much biomass do these communities comprise on the continental 

slope and what is the contribution to slope ecosystems? 

To evaluate possible impacts, it is important to determine how these 

communities persist in the natural environment and the extent to which they will be 

1-2 
resilient in the face of petroleum-related activities. In this regard, our overall goal is 

to determine to what extent the Gulf of Mexico deep water petroleum seeps fit into 

two possible categories : a robust or fragile community. 

Determination of the extent to which the hydrocarbon seep communities are 

robust or fragile entails coordinated geological, geochemical, and ecological research 

efforts that will develop an understanding of the spatial and temporal linkage pattern 

between hydrocarbon seepage and the formation of chemosynthetic community 

development on the seafloor. These investigations must determine how communities 

are established and persist within the particular geological and geochemical 

environments that support them. As stated above, the key to understanding 

potential impacts lies in understanding how the processes of geology, geochemistry, 

and biology interact. 

1 .2 Summary of Study Accomplishments 

The Study was initiated on 26 July 1991 . The Principal Investigators (PIs) 

made substantial resources available to the Study at no cost to MMS. These 

resources included a large and diverse amount of data gathered during previous 

investigations of chemosynthetic ecosystems in the Gulf of Mexico . Additionally, the 

North Carolina National Oceanographic and Atmospheric Administration (NOAA) 

National Undersea Research Center had granted the PIs a series of submarine dives 

with the Johnson Sea-Link during 1991 and 1992 . Consequently, the PIs were able to 

offer MMS a research program that was more extensive and which commenced field 

activities more expeditiously than anticipated in the Request for Proposal (RFP). 

The first submarine cruise (JSL-91) on this project utilized the Johnson SeaLink I, deployed from the support ship RN Seward Johnson during 25, 26, and 31 

August and 14-27 September 1991 . The cruise occupied locations of known 

chemosynthetic communities in water depths of 500 to 750 m (Figure 2 .1) . 

1-3 
Collectively, these sites were judged by the PIs to represent the faunal and 

environmental diversity of the chemosynthetic communities on the mid- to upper 

continental slope. A base-line set of water column, sediment, carbonate, and tissue 

samples from chemosynthetic and heterotrophic fauna were collected at each site. 

Additionally, extensive photographic and video documentation was obtained, 

particularly from stations delineated with durable sea-floor markers and floats. At 

selected sites, collections of tube worms, mussels, or clams were measured, marked, 

and returned alive to their habitats to determine in situ growth rates for the species . 

Samples of Beggiatoa mats were collected for shipboard experiments and laboratory 

assay. Several deployments of two time-lapse camera systems were carried out to 

determine short and long-term activity patterns within clusters of tube worms and 

mussels . Various markers, shell collections, and cages were also deployed for 

retrieval during subsequent cruises . 

The second submarine cruise (JSL-92) on this project utilized the Johnson SeaLink I, deployed from the support ship RN Seward Johnson during the period 3-31 

August 1992 . This cruise occupied the same sites as JSL-91 and successfully 

resampled many of the stations established previously. The long-term experiments 

were retrieved with uniform success, although there were some failures in 

performance. New techniques utilized during JSL-92 included an in situ pore-water 

sampling device and a small-diameter push-core array that made it possible to collect 

up to 15 sediment and bacteria samples from a 1 m2 grid. 

A limited cruise was recently completed using the RN Gyre during 4-9 

November 1992 (Gyre-92). This cruise collected sediment cores with a piston-corer 

specially designed for collecting intact shell samples from the surface strata (<_ 3 m 

subbottom). Two of the sites occupied during JSL-91 and JSL-92 were successfully 

cored. A third non-study site was also cored and subbottom profile data were 

collected at two sites. 

1-4 
The first report was submitted in final form on 30 November 1992 . For this 

report, each of the PIs wrote an independent overview of both the published literature 

in his topic area and of his unpublished results which are sufficiently advanced to 

release. In addition, a collection of the core literature pertinent to the Gulf of Mexico 

communities was assembled and reproduced in an appendix to this report. This 

collection included the discovery papers, review articles, and a series of papers 

entitled "Gulf of Mexico Hydrocarbon Seep Communities: I through VII," which has 

been dispersed among a variety of journals . 

The Interim Report was submitted in final form on June 29, 1993. It described 

the methods, techniques, and equipment used during the field program. It also 

outlined the data sets and analyses used by the principal investigators (PIs) . 

The major findings of the individual PIs were presented to the public during the 

MMS Information Transfer Meeting held in New Orleans, Louisiana on 15 December 

1993 . This report closely follows the format of those presentations. 

1-5 
2.0 The Regional Distribution of Chemosynthetic Communities 

Across the Continental Slope in the Northern Gulf of Mexico 

Ian R. MacDonald 

2.1 Introduction 

Hydrocarbon seepage is one of the major natural processes that shape the 

geological and geochemical characteristics of the Gulf of Mexico . Evidence for 

extensive natural hydrocarbon seepage in this region comes from historical records 

of floating and beached oil that predate modern offshore production and transport 

(Geyer and Giammona 1980), and from extensive collections of oil-stained marine 

sediments (Kennicutt et al . 1988b). Hydrocarbons migrate upward along faults 

from reservoirs situated at subbottom depths of 2000 m or greater (Kennicutt et al. 

1988a; Cook and D'Onfro 1991); oil and gas escape the sediment column in discrete 

regions known as seeps (MacDonald et al. 1989 ; MacDonald et al. 1990b); within 

seeps, there are often highly localized vents where very active release of gas and oil 

takes place ; finally, the released oil floats to the sea surface where it forms slicks 

that drift with wind and current until dispersed by evaporation, dissolution, and 

bacterial consumption (MacDonald et al. 1993). This overall sequence produces 

significant effects at length scales in the range of a few centimeters to a few 

kilometers. 

Appropriate management of human activities that might affect the 

chemosynthetic communities that are supported by hydrocarbon seepage must refer 

to reliable estimates of the abundance and distribution of the communities . 

Collectively, the historical data indicate substantial seepage across most of 

continental slope in the Gulf of Mexico; but they do not give a repeatable measure of 

the distribution of seepage nor of the biological communities associated with 

seepage. The small size and extreme inaccessibility of seep communities makes it 

difficult to determine where they occur without direct observation. During the 

2-1 
course of the Study, satellite and airborne remote sensing were successfully 

employed as means for detecting seepage on a regional basis (MacDonald et al . 

1993, in press). A newly developed optical sensor, the laser line scan system, was 

also evaluated in a demonstration deployment. Brief description of these results 

will be presented. 

Subsequent chapters in this report will describe geological, geochemical, and 

ecological effects of hydrocarbon seepage based on detailed observations and 

collections at representative seep sites made during this Study. This section will 

place these sites in their regional context by documenting the geographic range, 

frequency and possible magnitude of seepage . 

22 7~rpes of Seep Communities 

Four general community types were described by MacDonald et al . (1990b). 

Respectively, these were communities dominated by vestimentiferan tube worms 

(Lamellibrachia c .f. barhami and Escarpia n.sp.), mytilids (Seep Mytilid Ia, Ib, and 

III), epibenthic vesicomyid clams (Vesicomya cordacta and Calyptogena ponderosa), 

or infaunal lucinid or thyasirid clams (Lucinoma sp. and Thyasircz sp.) . These 

faunal groups display distinctive characteristics in terms of how they aggregate, the 

size of the aggregations, the geological and chemical properties of the habitats in 

which they occur, and to some degree, the heterotrophic fauna that occur with them. 

Tube worms form dense clusters that range in size from small tangles 20 cm 

in diameter to continuous bush-like stands tens of meters across . Tube worms are 

most often found in association with oil-laden sediment, which often contain up to 

10% extractable oil by weight (MacDonald et al. 1989). The oil contributes directly 

or indirectly to high local concentrations of sulfide . 

Mytilids generally occur in single- or double-layer beds that range in size 

from less than 0 .5 m2 to 500 m2 (MacDonald et al. 1990a; 1990b). The beds are 

2-2 
restricted to the localized areas where methane and oxygen are simultaneously 

available . Oxygen is present in the ambient seawater, methane from either direct 

evolution of gas bubbles or dissolved in brine (MacDonald et al. 1989). Mytilid beds 

often define the distinct boundaries of this availability. 

Living clams are either infaunal, in the case of the lucinaceans, or epibenthic, 

in the case of the vesicomyids. The latter often form distinctive furrow-like tracks 

as they plow through the surface sediment in search of sulfide (Fisher 1990). Areas 

which living vesicomyids occupy more or less continuously may be 100 to 150 m in 

width (Rosman et al . 1987) . The dead shells of both groups, however, tend to 

accumulate on the surface sediment, sometimes producing shell "pavements" that 

cover more extensive areas (Callender et al. 1990). The shells of the two groups are 

often difficult to distinguish from each other visually and dead vesicomyids often 

occur in life position. Visual observations of shell beds may therefore indicate a 

community of living vesicomyids or an infaunal community of lucinaceans . 

2.3 Zonal Distribution of Chemosynthetic Communities in the Northern 

Gulf of Mexico 

2 .3.1 Local Patterns 

The best evidence for long-term seepage is presence of chemosynthetic fauna 

at specific sites . Figure 2.1 shows the locations where the major chemosynthetic 

fauna have been collected or unambiguously observed . Table 2.1 lists the groups of 

chemosynthetic fauna found, nominal locations, depths, collection method and the 

citation . Occurrence of a chemoautotrophic community can indicate a range of 

environmental conditions; in all cases, however, this fauna is an indicator of 

hydrocarbon seepage (Brooks et al . 1987 ; Kennicutt et al. 1988a). Reliability of 

information on the areal extent, density, and continuity of the individual 

communities shown in Figure 2 .1 is uneven. Clearly, trawl samples will not provide 

2-3 
Table 2.1 Sites where chemosynthetic metazoans have been collected by trawl 

(Trl) or submarine (Sub), or definitively photographed by submarine, 

remotely operated vehicle (ROV), or photosled (Photosl). Fauna 

indicates the type of chemosynthetic fauna found: V=vestimentiferan 

tube worms, M=Seep Mytilids, C=vesicomyid or lucinid clams, 

PG=pogonophoran tube worms; codes in bold face followed by asterisk 

(e.g., 1* VM) are Sampling Sites for the present study . Lease block 

designators follow MMS standard abbreviations . Data sources give 

precedence to observations published in the open literature. 

Fauna Latitude Longitude MMS Depth Obs Data 

(North) (West) Lease Block (m) method source 

VM 26°21 .20' 94°29.80' AC0645 2200 Sub 1 

M 27°23.50' 94°29.45' EB0602 1111 1r1 2 

PG 27°27.55' 93°08.60' GB0500 734 Tr1 2 

VC 27°30.05' 93°02.01' GB0458 757 ZYl 2 

M 27°31.50' 92°10.50' GB0476 750 Sub 3 

MC 27°33.40' 92°32.40' GB0424 570 Sub 3 

V 27°35.00' 92°30.00' GB0425 600 Sub 3 

VC 27°34.50' 92°55.95' GB0416 580 Sub 3 

VC 27°36.00' 94°46.00' EB0376 776 Sub 3 

PG 27°36.15' 94°35.40' EB0380 793 Tr1 2 

MC 27°36.50' 92°28.94' GB0382 570 Sub 3 

VC 27°36.60' 94°47.35' EB0375 773 Trl 2 

VC 27°36.82' 92°1525' GB0386 585 Sub, Tr1 2,3 

VC 27°37.15' 92°14.40' GB0387 781 Sub, Zl-1 2,3 

V 27°37.75' 91°49.15' GC0310 780 1Yl 2 

VC 27°38.00' 92°17.50' GB0342 425 Tr1 2 

C 27°39.15' 94°24.30' EB0339 780 Tr1 2 

VC 27°39.60' 90°48.90' GC0287 994 Sub, Tr1 2 

C 27°40.45' 90°29.10' GC0293 1042 Tr1 2 

VC 27°40.50' 92°18.00' GB0297 589 1Y1 2 

VMC 27°40.88' 91°32.10' GC0272 720 Sub, TYl 2, 3, 4 

VC 27°42.65' 92°10.45' GB0300 719 Tr1 2 

V 27°43.10' 91°30.15' GC0229 825 Tr1 2 

VM 27°43.30' 91°16.30' GC0233 650 Sub 5 

VMC 27°43.70' 91°17.55' GC0233 813 Tr1 2 

VM 27°44.08' 91°15.27' GC0234 600 Sub 3,6 

VM 27°44.30' 91°19.10' GC0232 807 Sub 3 

VM 27°44.80' 91°13.30' GC0234 550 Sub 3,7 

VC 27°45.00' 90°16.31' GC0210 715 Sub 3 

C 27°45.50' 89°58.30' GC0216 963 Sub, Photosl 8,2 

VMC 27°46.33' 90°15.00' GC0210 796 Sub 3 

VM 27°46.65' 91°30.35' GC0184/5 580 Sub, Trl 2, 3, 9 

VM 27°46.75' 90°14.70' GC0166 767 Sub, Tr1 2,3 

VM 27°49.16' 91°31.95' GC0140 290 Sub 10 

V 27°50.00' 90°19.00' GC0121 767 Sub 3 

VM 27°53.56' 90°07.07' GC0081 682 Photosl 11 

VC 27°54.40' 90°11.90' GC0079 685 Tr1 2 

VM 27°55.50' 90°27.50' GC0030 504 Sub 3 

VPC' 27°56.65' 89°58.05' GC0040 685 Trl 2 

C 27°57.10' 89°54.30' MC0969 658 Tr1 2 

V 27°5725' 89°57.50' EW1010 597 Sub, 1r1 2,3 

V 27°58.70' 90°23.40' EW1001 430 Sub, Tr1 2,3 

VC 29°11.00' 88°00.00' VK0826 545 Sub, R.OV, Tr1 3, 4, 12 

Data sources: 1-Brooks et al . . (1989), 2-Kennicutt et al . (1988a,b), 3-GEftG unpubl. data, 4-Callender et al . 

(1990), 5-MacDonald et al . (1990b), 6-MacDonald et al . (1990b), 7-MacDonald et al . (1990a), 8-Rosman et al . 

(1987), 9-MacDonald et al . (1989), 10-Roberts et al . (1990), 1 1-Boland 1986, 12-Boss (1968), Gallaway et al . 

(1990), Volkes (1963) . 

2-4 
981W 96°W 94°W 92°W 90°W 88°W 860W 

32oN i 

30°N 

VK 826 

28°N 

26°N 

-------------

200- 1000 m 

IOOOT __ . ., 

--_,____ 

` 3000 m, 

Figure 2.1 Locations where vestimentiferan tube worms, seep mytilids, or vesicomyid clams have been collected or 

photographed in the northern Gulf of Mexico. Observations separated by less than 1 NM have been 

pooled. Study sites are indicated by their MMS lease block number. 
reliable data of this type; and photo surveys generally cannot provide an exhaustive 

mapping of individual communities. It is therefore difficult to determine what 

constitutes a chemosynthetic community in terms of management concerns. Does, 

for example, a single collection of clam shells signify a lush community? Likewise, 

in how much detail should offshore operators be required to search in order to 

certify that their operations will not impact a lush community? One approach to 

this issue is to examine the variable evidence for the typical length scales and 

geological dependency of well-documented chemosynthetic communities . 

An analysis of clustering frequencies for tube worms at GC 234 found 

significant clustering at spatial scales of about 5, 20, and 75 m (MacDonald 1990a), 

which may be indicative of clustering due to formation of tube worm "bushes" at the 

smaller scale, and to the size and spacing of fault zones at the larger scales . A 

review of the occurrence of chemosynthetic fauna along the submarine track-lines 

tends to confirm a characteristic size of 10 to 100 m for vestimentiferan and mytilid 

communities and 100 to 300 m for clam communities. We therefore have support 

for speculation that communities separated by less than 300 m probably share a 

common hydrocarbon reservoir. Multi-channel seismic data from the GC 184/185 

lease blocks (Cook and D'Onfro 1991) provided information on the spacing of 

reservoirs and migration pathways at this site . From these data, it appears that 

communities separated by distances of 1 km are not supported by seepage from a 

common reservoir. 

2.3.2 Regional Patterns 

Chemosynthetic fauna have been found in a 700 km-long corridor between 

88°W and 95°W and between the 290 and 2200 m isobaths . Distribution is uniform; 

the greatest number of communities were found between 91°W and 93°W between 

the 500 and 700 m isobaths . These observations have been influenced by the limits 

2-6 
of exploration; the envelope of occurrence suggests that the potential number of 

communities is much larger than those found to date. There was a gap in 

occurrences where the Mississippi Canyon intersects the corridor. Although this 

area has not been exhaustively explored for communities, geochemical exploration 

shows a general absence of oil-stained sediments from the Canyon, which suggests 

that the hiatus in community occurrence may be real (GERG unpublished data). 

The easternmost community, in the Viosca Knoll block 826, was found by 

investigations initiated by the Orb Energy Company (Gallaway et al. 1990). 

Interestingly, this area was trawled in 1955 by the U.S. Fish and Wildlife Service's 

MN Oregon. The resulting collections provided the holotype for the original 

description of Calyptogena cordata (Boss 1968) and a paratype for the original 

description of Acesta bullisi (Volkes 1963). The photograph of this paratype shows 

the unmistakable shell deformation that is caused by the commensal interaction 

with a vestimentiferan. The Viosca Knoll site represents early, unrecognized 

evidence for hydrocarbon seep communities in the northern Gulf, as well as a recent 

range extension. 

Each chemosynthetic community supported by hydrocarbon seepage depends 

on a reservoir of hydrocarbon and a migration pathway or pathways through which 

these fluids reach the seafloor. Implicit in the distribution shown in Figure 2 .1, or 

in a larger scale mapping of community distribution, is a determination of what 

spatial separation is required to consider two occurrences as distinct communities. 

The number of communities in a given area should be indicative of the number of 

hydrocarbon reservoirs, the number of migration pathways (i .e ., faults), the 

characteristic size of the faults, and interaction of the unconsolidated sediments, 

hydrates, or carbonate with seepage. This determination is also important for 

estimating the numbers of the communities and for management concerns 

regarding the potential zone of impact for a seafloor activity. 

2-7 
2.4 Remote-Sensing Detection of Natural Oil Seepage 

2.4.1 Use of Geophysical Methods for Indirect Detection of Chemosvnthetic 

Communities 

Our study suggests that geophysical techniques can be used to narrow the 

areas of detailed study needed to locate and classify chemosynthetic communities. 

Ultimately, no geophysical method will allow the reliable identification of such 

communities without in si tu observation. However, by using known geophysical 

signatures and affinities, likely locations can be identified for more detailed study. 

Seeps are located along faults and faults are usually identified in regional 

seismic exploration profiling surveys . Thus, the first step in locating 

chemosynthetic communities is to map the faults along which seeps could occur. 

The likeliest faults to harbor seeps are those that are active and thus extend to the 

surface and those that show a significant displacement . 

Having found the faults, the next step would be to look for seep related 

topography and sediment disturbance . Topographic features would be mud 

mounds, carbonate mounds, and pock-marks. The mud mounds are typically more 

than 10 m in height and more than 100 m in diameter and would be evident on 

high-resolution bathymetry maps and side-scan sonar images. Carbonate mounds 

can be somewhat smaller, one to a few meters high and 10 m wide, or less. In 

addition, pock-marks are variable in size, from small, shallow craters a few meters 

across to large craters many meters deep and hundreds of meters wide . 

Sediment disturbances can usually be found using acoustic profiling and 

imaging techniques . On side-scan sonar records, the disturbance can appear as an 

increase in backscatter, particularly in conjunction with small mounds . In 3 .5 kHz 

and other echo sounder profiles, the disturbance may show up as an attenuation of 

signal reflections ("wipe out"), as an enhancement ("hardbottom"), or as 

2-8 
reverberation ("turbidity") . These are caused mostly by gas, carbonate layers or 

disseminated nodules, or hydrate layers. In addition, if the seepage is particularly 

strong and contains much gas, the gas bubbles may be detected within the water 

column by these acoustic methods (Anderson and Bryant 1990). 

Having located areas of larger scale topographic features or sediment 

disturbances caused by seepage, a more detailed examination is necessary. To 

locate chemosynthetic communities, various ultra-high-resolution imaging methods 

are needed. Side-scan sonar images will show carbonate outcrops, and may even 

show tube worm bushes or bivalve clusters if the resolution is high enough and the 

organisms are dense enough. Typically, this will require a deep-towed sonar or 

sonar affixed to a submersible, so that the acoustic source and target are not greatly 

separated. However, many chemosynthetic organisms, such as bivalves, may be 

invisible . Underwater photography, such as with a camera sled or a laser line-scan 

device can provide visual sitings, as can submersible dives. Although these last are 

slow and painstaking techniques, presumably they can be directed, using a nested 

search technique employing increasingly higher resolution methods, so that the 

time consuming visual searches can be minimized. Likewise, new technologies, 

such as laser line-scanning imaging, may dramatically improve the search speed for 

visual survey. Another nesting procedure would combine geophysical methods with 

remote sensing of floating oil by use of airborne or satellite sensors. 

2.4 .2 formation and Detection of Surface Oil Slicks 

Oil, and other organic compounds, can be detected remotely in the ocean 

because they form a floating film that modifies the characteristics of the surface 

layer and locally increases specular reflection of radiant energy such as visible light 

or radar waves (Garrett 1986; Scott 1986 ; Stevenson et al . 1988). The phenomenon 

has been used to study small-scale oceanographic circulation (Cox and Munk 1954 ; 

2-9 
Soules 1970 ; La Violette and Arnone 1988), the effects of ship wakes (Peltzer et al. 

1992), and the surface slicks formed by natural oil seepage (Estes et al. 1985). 

Micro-layer surfactants originate from at least two sources: petroleum oil and the 

biological activity of plankton and fish (Garrett 1986). Although biological 

surfactants are physically and chemically different from oil, and can be 

distinguished remotely by their wave damping properties under some circumstances 

(Huhnerfuss et al. 1989), confirming the source of a remotely detected micro-layer is 

often difficult or impossible. By comparing remote sensing images that show 

evident oil slicks with locations of chemosynthetic communities (Table 2.1) and 

surface observation of floating oil (MacDonald et al. 1993) made it possible to 

confirm the locations of perennial oil slicks from natural seepage. 

2 .4 .3 Remote Sensing Methods 

The remote sensing data base comprised images collected from three sensor 

systems on board three satellites: 

1) European Radar Satellite (ERS-1, SAR, image) 

Orbit Frame Date 

9518 3051 8 May 1993 

9475 3051 8 May 1993 

11250 3051 9 Sept. 1993 

2) Landsat - (Landsat 5, Thematic Mapper image) 

Path, Row Collection Time and Date 

22,41 1105 CST 31 July 1991 

3) Space Shuttle (Atlantis with 120 mm format photograph, hand-held) 

Mission, Roll, and Frame Collection Time and Date 

STS-30, 151, 028 1535-1538 CST; 05 May 1989 

2-10 
The Space Shuttle photograph was scanned and digitally resampled to 

conform with the position of known coastal features and offshore installations . 

Suspected slicks in these images were outlined as closed polygons. The set of slick 

polygons for each image was then stored as a layer in a geographic information 

system (GIS) by use of MAPIX@ software. 

The objective of this exercise was to combine observations from the data sets 

described above, then to review these observations to identify probable sites for seafloor seeps. This was accomplished in a 2-stage fashion: First, all possible image 

pairs were compared to determine co-locations of slick polygons. Overlapping 

polygons were classified according to the style of conjunction by use of the following 

ranking scheme: 

Rank A: Overlap of the upwind ends of slicks in two or more images. 

Rank B: Overlap of slick ends irrespective of wind direction . 

Rank C: Ends of slicks within =2 km of each other. 

Rank D : Overlap of any portion of slick polygons in two or more images. 

Second, slick polygons were compared to locations of collections of floating, 

freshly surfaced oil and to locations of chemosynthetic communities. A location was 

considered to be a geochemical anomaly if a visibly oil-stained sediment sample was 

collected in the same lease block with a surface slick or community of 

chemosynthetic animals . 

By analyzing the remote sensing and ancillary data, we found 63 locations 

where multiple data sets indicated presence of perennial oil slicks (Table 2 .2, Figure 

2 .2). At six of these sites, we report collection of freshly surfaced oil from the 

surface at locations that coincide with a target in one of the remote sensing images . 

Oil-stained sediments and chemosynthetic fauna were also present in five and three 

2-11 
Table 2.2 Oil slicks detected in one or more remote sensing images from the 

northern Gulf of Mexico. Latitude and longitude are for point of 

overlap . Lease block designators follow standard MMS format. 

Depths given are mean depths for lease block. Slicks listed as having 

no overlap were confirmed by coincidence with sea truth collections or 

observation from submarine. 

ID Latitude Longitude Lease Depth Remote Sensing Seafloor Geoc emits Floating i 

Block (m) Overlap Manifestation Anomaly Collected 

1 27044.83'N 91013.33'W GC0234 831 no overlap Chemo fauna Oily core Floating oil 

2 27043.71'N 91036.57'W GC0226 825 no overlap - Oily core Floating oil 

3 27040.53'N 92018.00'W GB0297 589 no overlap Chemo fauna Oily core Floating oil 

4 27031.55'N 92010.58'W GB0476 891 no overlap Chemo fauna - Floating oil 

5 27021.99'N 92023.01'W GB0648 1058 no overlap - Oily core Floating oil 

6 27013.02'N 91003.12'W GC0766 1523 no overlap - Oily core Floating oil 

7 27057.88'N 90043.23'W EW0995 281 upwind ends - - - 

8 27055.63'N 91052.86'W SM0205 184 upwind ends - Oily core - 

9 27048.73'N 90051 .03'W GC0154 715 upwind ends - Oily core - 

10 27048.65'N 91007.20'W GC0148 551 upwind ends - - - 

11 27045.04'N 91029.48'W GC0229 825 upwind ends - - - 

12 27042.74'N 91019.14'W GC0232 807 upwind ends - Oily core - 

13 27040.52'N 90049.46'W GC0287 994 upwind ends Chemo fauna* - - 

14 27038.93'N 90048.39'W GC0331 1037 upwind ends - - - 

15 27037.26'N 91017.38'W GC0321 881 upwind ends - Oily core - 

16 27033.29'N 90058.55'W GC0415 1049 upwind ends - - 

17 27°32.55' N W60.00' W GC0415 1049 upwind ends - - - 

18 27032.10'N 90059.75'W GC0415 1049 upwind ends - - - 

19 27032.04'N 91024.65'W GC0451 1027 upwind ends - - - 

20 27031.96'N 91023.18'W GC0451 1027 upwind ends - Oily core - 

21 27031.41'N 91022.31'W GC0451 1027 upwind ends - - - 

22 27027.12'N 90055.64'W GC0504 1181 upwind ends - - - 

23 27°24.89' N 91022.67W GC0539 1288 upwind ends - - - 

24 27024.53'N 91021 .86'W GC0540 1307 upwind ends - - - 

25 27024.12'N 91039.70'W GC0577 1274 upwind ends - - - 

26 27023.88'N 91021 .05'W GC0584 1375 upwind ends - - - 

27 27°22.70' N 91045.02'W GC0576 1302 upwind ends - - - 

28 27022.56'N 91010.25'W GC0588 1502 upwind ends - - Floating oil 

29 27022.32'N 91010.45'W GC0587 1558 upwind ends - - - 

30 27021.39'N 91003.09'W GC0590 1404 upwind ends - Oily core - 

31 27021.31'N 90002.97W GC0611 1341 upwind ends - Oily core - 

32 27017.97'N 91011 .95'W GC0675 1622 upwind ends - - - 

33 27014.35'N 91036.36'W GC0711 1412 upwind ends - - - 

34 27°13.98' N 90054.12'W GCO725 1455 upwind ends - - - 

35 27013.62'N 91001 .21'W GC0723 1421 upwind ends - Oily core - 

36 27Q13.56' N 90047.80'W GC0727 1374 upwind ends - Oily core - 

37 27012.41'N 91002.04'W GC0766 1523 upwind ends - Oily core - 

38 27005.60'N 91021.69'W GC0892 1999 upwind ends - - Floating oil 

39 27°05.09' N 91023.75'W GC0891 2006 upwind ends - - - 

40 27044.24'N 91°18.83' W GC0232 807 ends undeter. Oil seep obs. Oily core Floating oil 

41 27042.74'N 91019.14'W GC0232 807 ends undeter. Chemo fauna Oily core - 

42 27°31.35' N 91024.25'W GC0451 1027 ends undeter. - - - 

43 27015.05'N 91035.83'W GC0711 1412 ends undeter. - - - 

44 27013.76'N 91002.02'W GC0722 1414 ends undeter. - Oily core Floating oil 

45 27013.57'N 90048.89'W GC0727 1374 ends undeter. - Oily core - 

46 27048.51'N 90051.07W GC0154 715 ends < 2km - - - 

47 27039.64'N 91022.03'W GC0319 909 ends < 2km - - - 

48 27024.73'N 91018.44'W GC0541 1374 ends < 2km - - - 

49 27010.42'N 91012.36'W GC0807 1676 ends < 2km - Oily core - 

50 27047.63'N 91007.23'W GC0148 551 random overlap - - - 

51 27046.67'N 91030.39'W GC0185 718 random overlap Oil seep obs. Oily core Floating oil 

52 27044.47'N 91012.75'W GC0235 841 random overlap - Oily core - 

2-12 
53 27043.62' N 91026.19'W GC0230 825 random overlap - - - 

54 27040.93'N 91°36.46' W GC0271 851 random overlap - Oily core - 

55 27032.79'N 90058.34'W GC0416 1072 random overlap - - - 

56 27032.60'N 91032.67'W GC0404 947 random overlap - Oily core - 

57 27028.39'N 90066.73'W GC0504 1181 random overlap - - - 

58 27027.09'N 90064.70'W GC0505 1172 random overlap - - - 

59 27026.18'N 91050.52'W GC0530 1176 random overlap - - - 

60 27022.43'N 91021.62'W GC0584 1375 random overlap - - - 

61 27020.55' N 91023.44'W GC0627 1395 random overlap - - - 

62 27°18.90' N 91038.93'W GC0666 1323 random overlap - - - 

63 27018.13'N 91010.80'W GC0675 1622 random overlap - - - 

cases, respectively. At 39 sites listed in Table 2.2, we make our primary 

determination based on overlap of the evident upwind ends of slicks in two or more 

of the remote sensing images . Floating oil was collected at two of these sites and 

oil-stained sediments were collected at an additional ten sites within this group. 

Among the remaining 23 locations listed in Table 2.2, four are distinguished as 

particularly strong candidates for perennial seeps because of positive indicators 

among the supporting data sets. The remaining sites fit into our scheme of 

overlapping targets in multiple remote sensing images, but they lack supporting 

evidence. 

Three locations were selected for dives with the research submarine Johnson 

Sea-Link because they coincided with remotely detected oil slicks. The dives had 

the objective of trying to locate a chemosynthetic community or other definitive 

evidence for long term seepage on the sea floor below each surface target. In 

particular, the concern was to explore potential new sites that were in greater water 

depths than previously known sites . All dives were made with the research 

submarine Johnson Sea-Link, which has a maximum depth rating of 1000 m. 

Johnson Sea-Link was supported by the mother ships R/V Seward Johnson and RN 

Edwin Link. Dives consisted of approximately 2 h of bottom time; during which, the 

submarine was first directed to the appropriate location by a surface ship, then 

carried out autonomous exploration based on what was visible to scientists in the 

submarine. 

2-13 
IJ 

92 91 

28 

LI 

r 

N. 1 

4v- 

. 

v 

t 

R 

,. 

-T 1~ 

7i 

. ERS1 11250, 1993 50 km .Space Shuttle LFC, 1989 

ERS1 11551 8851, 1993 Landsat TM, 1991 

figure 2.2 Overlap between oil slicks detected in remote sensing images from different dates reveals the locations of 

perennial oil seeps. (Rectangular outlines show coverage for individual remote sensing images.) ERS-1: 

European Radar Satellite 1, TM: Thematic Mapper, LFC: Linhoff Frame Camera. 
Sites selected for dives were located in the Green Canyon 232, 287 and 321 

lease blocks (Table 2.2). We were able to find chemosynthetic fauna at the first two 

of these sites, but not at the third . The surface slick in Green Canyon 232 was 

spotted from an airplane and was clearly visible to ship-board observers before it 

was noted in the satellite data. During two dives to depths of 700 m, we noted 

dense aggregations of tube worms and seep mussels similar to assemblages at the 

Bush Hill site (MacDonald et al., 1989) . One of us (Guinasso) inadvertently caused 

a copious discharge of oil and gas from the sea floor by attempting to collect some of 

the mussels. The mussels were clustered within a field of authigenic carbonate 

rubble. Probing the mussel cluster with a scoop sampler disturbed the carbonate 

substratum and initiated a burst of gas bubbles followed by a flow of oil droplets 

that continued unabated for over 30 min. Arrival of the oil droplets at the surface 

was noted by the shipboard observers at a location about 100 m from the 

submarine's sea floor position. 

The surface slick at Green Canyon 287 was noted in the LandSat TM and 

ERS-1 data sets. The site is the location is on a plateau that rises from a base depth 

of 1500 m to a crest depth of 760 m. A salt body has intruded to the near seafloor 

and salt movement has produced a series of shallow faults trending northwestsoutheast normal to the salt (see Reilly et al ., this volume) . The Johnson Sea-Link 

bottomed at a depth of 800 m and followed a southern course across the flank of the 

uplift block. This track took the submarine across a series of northwest-southeast 

trending valleys. Clam shells, carbonate outcrops encrusted with tube worms, and 

occasional gas seeps were observed in the floors of these valleys . Although clam 

shells were broadly distributed, the abundance of chemosynthetic fauna in the 

communities at this site was much less than at the Green Canyon 232 site. 

Although gas venting was evident at several locations, macro-seepage of oil was not 

2-15 
observed . Extractable hydrocarbons from sediment samples at the site were heavily 

biodegraded (MacDonald, unpubl. data) . 

Surface slicks at the Green Canyon 321 site were evident in all of the remote 

sensing data sets. We traversed the sea floor below these slicks repeatedly during 

more than 6 hours of submarine exploration that covered depths from 750 to 900 m. 

Occasional indications of oil seepage, i.e. mats of Beggiatoa bacteria and outcrops of 

authigenic carbonate, were observed, but neither significant aggregations of 

chemosynthetic fauna nor other definitive sea floor indications of macro-seepage 

were observed. 

2 .4.4 Estimates of Total Seepage 

Estimating the magnitude of seepage required to produce slicks of a given 

length requires knowledge of the detectable thickness of the visible oil slick, the rate 

at which it advects away from the point at which the oil arrives at the surface, and 

the rate of disappearance due to evaporation and dissolution. Direct measurement 

of slicks over oil seeps suggests slick thicknesses of 0 .1 to 1.0 ~tm (Allen et al. 1970). 

Duckworth (personal communication 1993) dispensed a variety of crude oils on 

seawater and observed the coverage . Although results depended upon oil type, 

water cleanliness, and evaporation, generally 1 ml of oil will spread to produce a 

stable silver-gray film that covers 10 sq m, which corresponds to a mean thickness 

of 0.1 gym. The stable film degraded to an invisible film that suppressed capillary 

waves and enhanced reflectivity, and that occupied an area consistent with a 

thickness of approximately 0 .01 gym . Slick thicknesses required to produce the 

rainbow sheen and mousse reported during the sea-truth trials probably exceeded 1 

gm; however, these observations were restricted to small areas within the overall 

slicks, and the slicks themselves contained patches of un-oiled water. A thickness 

of 0 .01 gm is therefore a reasonable lower limit for a detectable slick. 

2-16 
Oil floating on the sea degrades as a result of evaporation and dissolution. As 

a slick disperses from its point of origin, these effects will compound progressive 

thinning of the slick due to spreading until the slick is no longer detectable 

(Hollinger and Mennella 1973). A slick formed by an instantaneous release of oil 

will therefore have a set detectable life span . A persistent slick represents a 

continuous flux of oil to the surface (Fallah and Stark 1976). Oil coming to the 

surface continuously into an advective regime caused by current or wind forms a 

plume. An advective velocity of 20 cm sec-1 would move oil downstream from a 

point at about 17.2 km day-1. Plumes with lengths of the order of 5 km contain oil 

that has been on the surface for less than 1 day. Hollinger and Mennella (1973) 

noted that the oil slicks are thickest along a central axis and suggested that most of 

the oil in a slick is confined to a small fraction of its total area. 

More about the nature of these plumes can be learned by proposing a simple 

dispersion model of a plume . We posit a Gaussian distribution of oil thickness in 

the transverse cross-section of a plume because this is consistent with previous 

findings and because analytical solutions to Gaussian functions are well-described . 

A three-dimensional Gaussian plume can be described by 

c = q ex -rx _ y2 

2n u a ~ u 2a2 (1) 

where c is the thickness of the oil on the surface, cm; q is the seafloor seepage rate, 

cm3 sec-1 ; u is the advection velocity, cm sec-1; x is the downwind distance, cm; r is 

a degradation coefficient for the oil, sec-1; y is the cross plume distance, cm ; and s is 

the cross plume standard deviation . The standard deviation is given by assuming 

diffusion-like spreading so that 

= 21~dx /u (2) 

2-17 
where x is downwind distance, cm; and d is a dispersion coefficient, cm2 sec-1 . 

Similar models are described by Csandy (1972) to describe a plume resulting from a 

continuous oil spill . 

The x,y points that lie on the boundary of the detectable plume satisfy the 

equation 

9 ex -rx _ y

2 

-cd = 0 

2n u a 4 u 2a2 ~ (3) 

where cd is the minimum detectable thickness of oil, cm; r is the e-folding time or 

degradation constant (r = 0 .693/(86,400T), sec-1 ; T is a degradation half life, days; 

and s is given by (2). The model requires that the size and shape of the detectable 

plume is dependent on the parameters q, cd, u, r, and d, which we will refer to as 

the parameters. Some insight can be gained into these equations by scaling them in 

terms of some simple dimensionless variables . A natural scale length for the 

problem is given by 

L=d 

u (4) 

Substituting the dimensionless variables Y=y/L, X=x/I., R=rd/u2, and Q=q/(dcd) into 

(1) gives 

z 

C=1 Q exp(-RX - ) 

(5) 

2-18 
When Y, the dimensionless crossplume distance equals zero, then 

C = 2 Q exp(-RX) 

(6) 

The length of the plume is that value of X defined by (6) when C=c/cd = 1 . 

Substituting C=1 into (6) we can solve for Q as a function of the overall length of the 

plume XL 

Q = 2-~rn-XL exP(RXL) (7) 

The shape of the plume is given by solving (5) for Y with C=1. This gives 

Y = 2-1X -ln 

Qexp(-RX) 

We estimated the range of possible seepage rates required to produce the 

slicks in the two images by solving (7) for the lengths of slicks measured in the 

Shuttle and TM images. We used an advection rate, u = 20 cm s-1, which we 

estimate from the recorded wind speeds, and threshold thicknesses, cd = 0.01 and 

0.1 gym, and degradation half-life, T = 0.25 and 1 day, which we regard as a 

reasonable range for these parameters . This exercise suggests that natural seepage 

is on the order of 4.3 x 103 to 7.8 x 104 m3 y-1 in the 8,200 sq km area imaged in the 

TM scene and 1.1 x 104 to 4.8 x 105 m3 y-1 in the 15,000 sq km area imaged in the 

1989 Atlantis photograph. 

2-19 
2 .4.5 trial of the Laser Line Scan System 

At present there are two means for sea-floor imaging: seismic and optical. 

Seismic imaging, such as side-scan sonar or active sonar, is distinctly limited in 

resolution. Optical imaging, such as underwater photography, has long been 

limited in scope because of backscatter. The Laser Line Scan System (LLSS) is a 

state-of-the-art optical imaging system that overcomes the back-scatter problems 

plaguing underwater photography. Very recent design improvements by 

Westinghouse Electric Corp., Underwater Laser Systems have produced a new 

LLSS, Model SM2000S, that reduced power requirements to under 200 watt and 

reduced size and weight by roughly half. This improvement makes this LLSS a 

potential tool for much of the research submersible fleet including ROV's and towed 

fish, but as yet, the submarine user community has not undertaken any detailed 

scientific or engineering tasks with the system . In its current configuration, the 

LLSS provides high-resolution pictures of the seafloor along swaths up to 40 m 

wide. Despite the promise of this technology, it was unclear whether it could be 

used to image chemosynthetic fauna in adequate detail . 

A modification to the Study program was made to facilitate a trial with a 

laser system over Bush Hill. In this trial, the ART laser system, operated by SAIC, 

diverted from another mission and spent 12 hr surveying the GC 184 study site. 

This laser system was deployed in a towed fish by the RN Gyre. The laser produced 

excellent images in shallow water, but was severely limited by tow-fish stability in 

deeper waters. The results over Bush Hill were, therefore, disappointing in terms of 

providing a detailed map of the community. The effort did produce several quite 

satisfactory images of tube worm clusters and bacterial mats (Figure 2 .3) . This 

demonstrates that the technique can work, provided the stability problem is solved. 

2-20 
Figure 2.3 Laser line scan image collected over Bush Hill during the June 1993 

trials. Note the extreme image distortion caused by the roll, pitch and 

yaw in the towed vehicle. During a brief intervals of relative stability, 

the instrument captured a very clear image of several tube worm 

colonies . The largest colony is probably 1 to 2 m in diameter. Note the 

very abundant white patches of bacterial mats . 

2.5 Summary 

Chemosynthetic fauna occur across most of the continental slope in the 

northern Gulf of Mexico . Their distribution is more homogeneous over larger 

geographic and depth ranges than is the case for hydrothermal vent communities. 

Seep communities tend to be dominated by vestimentiferans, seep mytilids, or 

vesicomyid and lucinid clams, but intergradations among these community types 

are the rule, particularly between vestimentiferans and mytilids. The MMS 

Chemosynthetic Ecosystems Study encompasses a series of study sites which were 

2-21 
selected from the known list of communities on the basis of community type and 

geographic location. 

Methods for the remote sensing of chemosynthetic communities, many of 

which were developed or refined during this Study, can be applied to improve 

understanding of the distribution of hydrocarbon seepage. The distribution of 

macro oil seepage revealed by satellite remote sensing demonstrates that the 

conditions required for formation of hydrocarbon seep communities extend well 

beyond the artificial limits imposed by the exploration tools previously used. It is 

quite defensible to assume that hydrocarbon seep communities occur to depths of 

2000 m or greater across much of the northern Gulf of Mexico slope. 

2-22 
3.0 Geological and Geophysical Characterization of Hydrocarbons 

William W. Sager, Changshik Lee, and William W. Schroeder 

3 .1 Introduction 

Hydrocarbon seeps have been routinely studied by the oil industry using 

geophysical techniques . Often the deep subsurface structure is known through 

multichannel seismic reflection profiles acquired to find oil. Also, in areas where 

drilling rigs or pipelines are to be located, the oil industry has often acquired highresolution geophysical data for a "hazard" survey. Even the most precise of highresolution surveys, however, has insufficient resolution to image objects on scales of a 

few meters because of the separation of the observer at the sea surface from the 

target on the seafloor. Thus, small features and biologic communities at seeps are 

not routinely imaged by traditional sea surface geophysical techniques . 

In contrast, much of our knowledge of the biology or geochemistry of 

hydrocarbon seep communities comes from either dredge or trawl samples, or 

submersible viewing and sampling. Dredges and trawls provide spot samples, but are 

often difficult to relate to the seafloor in detail because they are typically dragged over 

the seafloor. Submersible studies provide a detailed view of the community 

inhabitants, relations, and small scale distribution, but are limited in scope by 

visibility and the slow speed at which observers can traverse the seafloor in a 

submersible. 

What is missing in this spectrum of studies is a link from the broad scale of 

geophysical sounding to the ultra small scale of the submersible viewport. In this 

report we examine the use of a hybrid technique, geophysical data collected from a 

submarine, as the basis of a study of the geological and geophysical characteristics of 

hydrocarbon seeps in the Gulf of Mexico. These geophysical data provide a synoptic 

3-1 
view on the scale of hundreds of meters, but with a resolution of less than a meter and 

this allows us to compare viewport-scale observations with broad scale seismic data. 

Our primary observations were made with two acoustic tools, a 25 kHz echosounder and a 77 kHz side-scan sonar. Both were used aboard the U.S . Navy nuclear 

submarine NR-1 to collect profile data in grids over four hydrocarbon seeps known to 

be inhabited by chemosynthetic communities. We chose these particular seeps 

because they have been the subject of intensive interdisciplinary studies and industry 

geophysical data are also available for all to some degree. This situation allowed us to 

examine the middle scale geologic characteristics and to link the ultra small and broad 

scale data sets. In this report, we attempt to synthesize the geological and 

geophysical characteristics of each of these four sites and then extrapolate this 

information to the broader population of chemosynthetic community sites. 

3.1 .1 Study Location 

The four seep sites examined in the geological/geophysical study are all located 

on the Louisiana continental slope in the Green Canyon (GC) and Garden Banks (GB) 

lease blocks areas (Figure 3.1). From east to west, the sites are GC234, GC 184/185, 

GB 386/387, and GB 424/425 (for brevity, we henceforth refer to the last three sites 

as GC1 184, GB 386, and GB 425). GC 234 is located along a portion of a tensional 

fault between salt diapir ridges (Behrens 1988) . The other sites are all mounds 

located either on the flank of a salt diapir (GC 386, GB 425) or on a fault radiating 

from a salt diapir (GC 184). The deepest seep site is GB 425 at 575 m, next is GB 

386 at 585 m, GC 184 at 540 m, and the shallowest is GC 234 at depth of 530 m. 

3 .1 .2 Geologic Causes and Symptoms of Hydrocarbon Seeps in the Gulf of Mexico 

Hydrocarbon seeps are widely scattered throughout the ocean basins because 

the two main ingredients that give rise to seeps, hydrocarbon source sediments and 

3-2 
sz4ow 9?20W 9TiQYY MOW 9190VY 91'20W 91'10'W 

-118'0011 

2T`,iON 

4T1011 

10 In 

figure 3.1 Location of study areas . 

3-3 

51'w gay 
tectonic fracturing, are relatively common (Hovland and Judd 1988) . In the Gulf of 

Mexico, the source layers are Cretaceous carbonate sediments and Cenozoic 

terrigenous sediments, both of which have layers containing abundant organic matter 

that can form hydrocarbons . 

Early in its history, during the Jurassic Period, the Gulf of Mexico was a 

shallow, periodically desiccated basin in which an extensive evaporite layer (the 

Louann Salt), up to several kilometers in thickness, was deposited (Buffler et al. 

1980; Worrall and Snelson 1989). Many additional kilometers of sediment, first 

carbonate and then terrigenous were piled atop the evaporites, as the basin margins 

aged and subsided (Worrall and Snelson 1989; Wu et al. 1990a). The organic material 

in these sediments was compressed and heated to form gas and oil . Salt is the main 

constituent of the evaporates. Because it is nearly incompressible, its density 

changes little with burial whereas the overlying sediments become denser through 

compaction . Moreover, under differential stress, salt deforms plastically on geologic 

time scales . The end result is that with continuing burial, the Louann Salt became 

mobile beneath the Gulf of Mexico shelf and slope during the Cenozoic Era. 

The mobile salt has greatly affected the physiography of the northern Gulf of 

Mexico (Jackson and Seni 1983; Jackson and Galloway 1984; Jackson and Talbot 

1986 ; Wu et al. 1990b; Simmons 1991). It has moved vertically to form diapirs and 

ridges, only some of which have reached the surface . It has moved laterally to form 

salt nappes and intraslope salt-withdrawal basins. Of greatest significance for 

chemosynthetic ecosystems, the movements have caused faulting, which is active 

today at many locations. The faults are typically tensional, listric, growth faults, 

often with parasitic antithetic faults (Worrall and Snelson 1989 ; Seni and Jackson 

1992) . These faults frequently tap reservoirs of hydrocarbons deep within the 

sediment column. Not only to they break the reservoir seal, but they also provide a 

conduit for upward hydrocarbon migration. Thus, hydrocarbon seeps are associated 

3-4 
with active salt movement and active faulting. Because the hydrocarbons cannot 

penetrate impermeable hemipelagic muds to any great degree, seeps are found along 

the faults that liberated the hydrocarbons (Roberts et al . 1990). Moreover, because 

it is the salt movement that causes the faults, usually seepage is on faults near salt 

bodies (Behrens 1988 ; Roberts et al. 1990; Kennicutt and Brooks 1990) . 

3.1.3 Geophysical Signatures of Seens 

For some time it has been realized that hydrocarbon seepage at the seafloor 

has effects that can be recognized by acoustic profiling and imaging methods 

(Hovland and Judd 1988). The effects can be generally broken into two categories: 

those that cause some sort of topographic feature and those which change the 

acoustic reflection characteristics of the sediments. 

Seep-related topographic features range from depression to uplift. The latter 

include mounds of authigenic (i .e ., formed in situ) carbonate (Roberts et al . 1990) or 

mud mounds (Hovland and Judd 1988; Neurauter and Bryant 1990). Authigenic 

carbonate mounds are formed by precipitation of carbonate during microbial 

degradation of hydrocarbons, whereas the mud mounds were probably formed by fluid 

mud entrained by the ascending hydrocarbons (e .g., Figure 3.2). The negative 

topographic features are pockmarks (small, subcircular, shallow craters) or blow-out 

craters, probably formed by escaping gas (Hovland and Judd 1988). 

The acoustic effects of hydrocarbon seeps are several . Gas bubbles within the 

sediment column can attenuate the acoustic signal, preventing further penetration 

and causing a "wipe out" zone in which deeper layers are hidden (Behrens 1988; 

Anderson and Bryant 1990). This signature can also be caused by near-surface 

biogenic gas, so it is not necessarily a sign of deep-rooted seepage. Authigenic 

carbonates change the reflection characteristics of the sediments, causing a 

reverberation or "turbidity" owing to the scattering of sound energy by disseminated 

3-5 
b 

0 

b 

b 

M 

N 

M 

N 

0 

m 

0 

~. 

a 

~. 

0 

0 

~. 

a 

a 

3-6 
nodules (Behrens 1988) . This may also be the source of strong acoustic 

backscattering sometimes seen around seeps using side-scan sonars . Gas hydrates 

within the sediment column also change the reflection characteristics . A hydrate 

layer may attenuate penetrating acoustic waves and it may also cause bottom 

simulating reflectors (BSRs) owing to the acoustic impedance contrasts at its edges 

Hovland and Judd 1988). 

3.2 Data 

As stated above, the primary data for this study were 25 kHz echo sounder 

profiles, 77 kHz side-scan sonar images, and bathymetry data acquired with the 

submersible NR-1 during surveys in 1991-1992 (GC 184/194, GC 234, GB 386) and 

1993 (GC 425). Both the echo-sounder and side-scan sonar were mounted on the 

lower hull of the submarine, which was sailed at an a more-or-less constant altitude of 

3 to 10 meters above the seafloor, typically at a velocity of 1 to 2 knots. The echo 

sounder records were recorded in analog fashion on videotape and have a vertical 

resolution of less than 10 cm. Side-scan sonar records were also recorded in analog, 

on paper records 20 inches wide. The sonar swath width was set at 100 m, and the 

images have a resolution of about 0 .5 m. Most surveys consisted of lines spaced 30 

to 100 m apart, giving more than 100% overlap of the side scan images from adjacent 

tracks . 

Although they are related, the side-scan sonar and echo sounder data show 

somewhat different aspects of the seafloor characteristics. The echo-sounder sends 

an acoustic wave to the seafloor at vertical incidence, so the received energy is 

proportional to the reflection coefficient of the seafloor and other interfaces (Urick 

1975; Dobrin and Savit 1988) . Some of the acoustic energy bounces back, but the 

rest penetrates into the seafloor, to be reflected by subsurface interfaces (e.g., 

sediment layers) or dissipated . Thus, a plot of received energy versus time gives a 

3-7 
picture of the interfaces at which significant acoustic impedance contrasts occur and 

these are interpreted as having geologic significance . Many investigators have 

employed the technique known as "seismic stratigraphy", using external and internal 

reflection characteristics as a scheme for the classification and interpretation 

sediment layers (Damuth 1975; Damuth and Hayes 1977). Our purpose for the echo 

sounder data was similar. Except at the nadir, acoustic waves from a side-scan 

sonar strike the seafloor obliquely, so the energy that returns to the instrument is not 

reflected, but is "backscattered" (Urick 1975) . Backscattering depends on many 

factors and chief among them are topography, seafloor roughness, and sediment 

texture (Johnson and Helferty 1990). 

Bathymetry data were also collected along submarine tracks, using a highfrequency echo sounder on the bottom of NR-1 . Bathymetric depths were determined 

from the two-way travel time of acoustic waves emitted and received by the echo 

sounder. Because the distance from the transceiver and seafloor was small, the 

separation was calculated using a constant sound velocity. This distance was added 

to the submarine depth, determined by pressure sensor, to arrive at total depth. 

Some NR-1 cruises produced data in the form of audio and video records of 

objects and organisms on the seafloor . These records were of from visual 

observations of features seen out the small forward viewing ports on the submarine. 

Positioning of the NR-1 was accomplished by inertial navigation between 

acoustic fixes from the mothership at the sea surface . Although the surface ship was 

positioned with the military Global Positioning System (GPS) satellite navigation, 

there are unknown errors in the acoustic ranging position of the submarine at fix 

locations. Furthermore, currents or inaccurate drift corrections can lead to 

significant horizontal positioning errors between fees, depending on the frequency and 

quality of fixes . However, comparison of side-scan sonar record overlaps on adjacent 

3-8 
tracks suggests that for most of the data, the navigation for any given survey is 

internally consistent on the order of 5 m or less. 

Geologic ground truth data were provided by 0 .5-m push cores, visual 

observations, and video photography obtained with the submersible Johnson SeaLink as well as 3-m piston cores acquired aboard the RN Gyre . Both the Gyre and the 

motherships for the Johnson Sea -Link (RN Seward Johnson and RN Edwin Link), 

were positioned by civilian GPS satellite navigation, with an accuracy of about 30 m. 

The submersible was positioned relative to the ship by an acoustic ranging system, 

which was repeatable to a few tens of meters. Although the Gyre's position was 

typically well determined, during coring both the ship's drift and currents pushing the 

coring wire off vertical can lead to errors in positioning the cores. 

Sea surface geophysical data were available for all four sites, but these data 

were diverse in type and parameters because most were borrowed from industry 

sources . For GC 184, GC 234, and GB 386, a few industry multichannel seismic 

reflection profiles were available . Likewise a variable number of hazard survey 

profiles, typically either run with a 3.5 kHz or similar echo-sounder or a high 

resolution multichannel seismic system, were also obtained. The navigation methods 

used for these surveys were typically unknown, since the data are proprietary, but 

industry standards are such that these data probably have navigation accuracies of 

less than 10 m. A more serious problem was offsets between different navigation 

data sets; these offsets were sometimes found to be on the order of a few hundred 

meters. In such instances, we had to resort to feature-matching to align submarine 

and sea surface data . We did not acquire industry data for GB 425, but instead 

carried out a 3.5 kHz and 12 kHz echo sounder survey with the RN Gyre. 

3-9 
3.3 Objectives 

The objective of this study was to use maps of geophysical (mainly acoustic) 

characteristics of the seafloor and shallow subbottom at hydrocarbon seep sites to 

provide a framework for assessing the effects of the seeps on the seafloor and the 

relationship to chemosynthetic communities. Using the principles of seismic 

stratigraphy, we wished to classify and map the extent of sediments affected by 

seepage. Our goal was to ground truth these classifications with geologic data (cores 

and submersible observations) and to correlate these characteristics with other 

geophysical and geologic data. A major objective of this task was to compare the 

geophysical framework with locations of chemosynthetic communities, surface 

features, and subsurface structures, such as faults . In addition, we wished to use the 

observational framework developed in this study for implications about the geologic 

formation and evolution of hydrocarbon seeps. 

3.4 Methods 

For each survey, the geophysical data were plotted at the same scale so that 

maps could be made for comparison of features. Bathymetric data were determined 

as described above and contoured at 5-m intervals . Where other sources of 

bathymetry data were available, the different data sets were compared to quantify 

and correct navigation offsets . The additional data were used to fill gaps or extend 

bathymetry contours outside the submarine survey area. 

Side-scan sonar records from NR-1 were automatically slant-range and speed 

corrected to produce deskewed images in which the along track and cross track 

dimensions are nearly the same. The slant-range correction method is internal to the 

sonar electronics and assumes a flat seafloor. Consequently, significant deviations 

from horizontal can cause errors in the plotting of features in the images. Sonar 

records were combined to make a mosaic of the acoustic backscatter variations over 

3-10 
each site . The paper side-scan records were electronically scanned as 8-bit gray scale 

images and were manipulated with a computer program (Photo Styler) to match 

common features on adjacent sonar swaths. This typically required some small 

adjustments of line navigation. The resultant image was a mosaic of the backscatter 

variations for each site. Some of the mosaics have gaps where it was impossible to 

manipulate the side-scan records to be consistent with adjacent tracks. This usually 

occurred at turns and where the submarine stopped or changed velocity in a manner 

that produced inaccurate speed corrections in the records . 

The 25 kHz echo sounder data were recorded with a video camera on 8-mm 

videotape. These images were played on a videotape player and a Macintosh 

computer with video frame grabbing board was used to capture the images digitally. 

Each image represented only about 2 minutes of data, so longer records were 

constructed by electronically mosaicking adjacent images. As described below, the 

echo sounder records were classified by reflection characteristics and these 

characteristics were plotted along navigation tracks and then contoured to fill in the 

gaps between lines . Where possible, the side-scan images were used to help the 

interpolation between lines. 

For ground truthing, piston cores and push cores (see Tables 3.1 and 3.2) were 

used along with visual observations and video data from submarine NR-1 and the 

Johnson Sea-Link submersible . The piston cores were obtained from a surface ship 

and examined for geologic features; samples were also taken for porosity and 

carbonate percentage analysis. Push cores were taken using the manipulator arm of 

the Johnson Sea-Link submersible. They were typically 30 to 50 cm in length, if full, 

and were extruded and examined visually. Video and voice records were used to 

identify the locations of chemosynthetic organisms for some surveys . The time of an 

observation was noted, so the observation could be plotted versus the submarine 

navigation and thus correlated to sediment acoustic character. 

3-11 
Table 3.1 Description of piston cores. 

Core ID Description Echo Core 

Type length 

(cm) 

GB 386-A Compact carbonate rock layer at the top of the I 225 

core 

GB 386-B1 Empty I 0 

GB 386-B2 Empty I 0 

GB 386-C Homogenous mud (no carbonate) VI 230 

GB 386-D Homogenous mud (no carbonate) VI 230 

GB 386-55 Carbonate rock layer in the top 50 cm I 182 

Cracks filled with oil 

GB 425-A 10 cm gas expansion void II 215 

Numerous vesicles and black-stained mud above 

the void 

GB 425-B Homogenous mud II 230 

Vesicles in the top 50 cm 

GB 425-C Homogenous mud with frequent empty spaces II 235 

Vesicles between empty spaces 

GB 425-D Carbonate nodules in the top 35 cm IV 212 

Shell fragments and oil stains throughout the core 

GB 425-E Homogenous mud II 240 

Vesicles in the top 70 cm 

3-12 
Table 3.2 Description of push cores. 

Core ID Echo Gas Oil Carbona Shell Bacterial 

Type to Mat 

GC 184-a II P tr - - - 

GC 184-b II P P P P - 

GC 184-c II P P P - - 

GC 184-d II P - - P - 

GC 184-e VI - - - - P 

GC 184-f II P - - P - 

GC 184-g II P - P P - 

GC 234-a VI - - - - P 

GC 234-b VI - - - - 

GC 234-c III P tr tr - - 

GC 234-d VI - - - - P 

GC 234-e III P P tr - - 

GC 234-f III P tr - - - 

GC 234-g VI - - - - P 

GC 234-h VI - - - - - 

GB 386-a II - - - - P 

GB 386-b VI - - - - P 

GB 386-c VI - - - - P 

(P = present, tr= trace) 

3-13 
To determine the interrelation of deep and shallow structure, faults and deep 

structure were interpreted on select multichannel seismic lines and extrapolated to 

the near surface . Structure maps were then compared with the side-scan sonar, 

bathymetry, and acoustic character maps. 

3.5 Results 

3 .5 .1 Acoustic Echo Types 

Echo sounder records were divided into six classes (Figure 3 .3) based on the 

amount and type of apparent disturbance of the sediment layers. Descriptions of 

these classifications are as follows: 

Type 1 : Characterized by a strong reflector at the seafloor and no evident 

acoustic penetration into the subbottom. The strong reflector is probably carbonate 

or gas hydrate at or near the seafloor and has a high reflection coefficient, causing 

much of the acoustic energy to be reflected or attenuated near the seafloor. 

Type 2 : Shallow penetration with an abrupt termination of acoustic 

penetration. This signature probably represents a thin layer of mud overlying a 

hardbottom, such as carbonate or hydrate. It may show areas in which recent mud 

flows have covered a pre-existing hardbottom. 

Type 3 . Scattered subbottom echoes without continuous horizontal internal 

reflectors. Such a pattern probably shows non-layered sediments disturbed by oil, 

gas, or disseminated carbonate precipitates . 

Type 4. Scattered subbottom echoes with indistinct, horizontally-continuous 

reflectors . This signature probably represents layered sediments disturbed by gas, 

oil, or carbonate precipitates . 

Type 5 . Zones in which acoustic energy is abruptly attenuated; typically 

laterally-continuous features disappear within these zones. Such zones are widely 

3-14 
::~ v^{ 

~s ate`' ~~,;~v 

., 

TYPE 1 

.F .r 

. N~r'

g 

i~tif . . lY ' N 

TYPE 2  b:. 

4:e% "y N 1 .< . 

: : . . .: f :. . . . . . . y . ., ~ °' °~ f 

TYPE 3 

!p ; 

~ ~- 

s r 

TYPE 4 

r .: 

9- 

E:.. . . . . 

J : . . . 

TYPE 5 

x v " z ~ 

` +~( 

ssf R,.:. Nf ~~+5 K" N. ~~ ~r.F 

TYPE 6 

z 

Figure 3.3 Examples of acoustic echo types seen in 25kHz subbottom 

reflection profiles. 

3-15 
recognized as the result of acoustic attenuation owing to scattering or absorption by 

gas bubbles within the sediments (Hovland and Judd, 1988). 

Type 6. Parallel, continuous subbottom reflectors . These characteristics 

denote normal, undisturbed hemipelagic sediments. 

3.5.2 Ground Truth 

In this section we describe the implications of our ground truth data for the 

interpretations of acoustic fades. Our interpretation of the Type 1 hardbottom zones 

is based on cores, side-scan images, and submersible observations. Cores within 

these zones were typically short and contained carbonate rocks, usually within the 

upper 20 cm, as seen for example in core GB 386-A from the seep mound in GB 386 

(Figure 3.4). Side-scan mosaics often showed rough topography in these regions and 

when viewed by submersible, carbonate outcrops were often seen. 

Type 2 is similar to Type 1, but is a hardbottom covered with a thin sediment 

layer. Such zones usually appeared no different from surrounding sediments when 

seen in side-scan images or from a submersible, because of the covering of sediment. 

Cores taken in these areas, however, showed similar features to Type 1 area cores, 

but the hard layers were buried below the surface. Push cores from a Type 2 area on 

the seep mound in GC 184 typically contained mud with disseminated carbonate, but 

one had nodular carbonate at its bottom. About half of these push cores showed 

traces of oil. Core GC 425-A, from a similar zone on the mound in GB 425, retrieved 

muds of two different colors, separated by an expansion void above which were small 

vesicles and a band of dark mud (Figure 3 .5). This core seems to show the boundary 

between two layers, presumably that seen in the echo sounder records. However, the 

two muds do not have sufficiently different properties to cause a strong reflection at 

their interface. Instead, the gas expansion features suggest that gas hydrate may 

have existed at the interface, but it evaporated when the core was raised. Thus, the 

3-16 
G8386-A 

carbonate rock 

(diameter 410 cm) 

light gray-colored 

silt a clay 

sharp color change 

gray or 

reddish brown-colored 

sift 

%Carbonab 

0 5 10 15 20 25 30 35 

Porosity 

X0.45 0.50 0.55 0.60 0.85 0.70 0.75 0. 80 

50 

100 

150 

e pprosi(y 

9iCarbonats 

Figure 3.4 Description of Core G:B386-A, from a Type 1 (hard substrate) 

echo character zone. At right are trends in carbonate percentage 

and porosity. 

200 

3-17 
%Carbonate 

GB425-A 0 s 10 1s 20 2s 30 3s 

Porosity 

(1 .45 0.50 0.55 0.60 0.85 0.70 0.75 0.e0 

light gray-colored 

sift or day 

vesicles 

(diameter <1mm-6mm) 

sharp color change 

dark-colored mud 

empty space 

(gas expansion) 

gray-colored 

Sift 

Figure 3.5 Description of Core C~B 425-A, from a Type 2 (buried hard 

substrate) echo character zone. At right are trends in carbonate 

percentage and porosity. 

0 

50 

~ 100 

150 

200 

3-18 
reflector at this location may have been hydrate. The Type 2 geometry suggests a 

hardbottom buried by a thin layer of sediment and may indicate recent seep-related 

sedimentation. We base this interpretation in part on observations of mudflows and 

active mud expulsion from seep vents on the mound in GB 425, where Type 2 

sediments cover much of the surface. 

Types 3 and 4 are similar, except for the existence of internal reflectors, which 

are likely related to the original deposition of the sediments. Core GC 425-D (Figure 

3.6) was recovered from a Type 4 zone (Figure 3.7) and it yielded mud permeated with 

carbonate nodules, oil stains, and shell fragments. All three components are potential 

scatterers of acoustic waves (particularly if the oil is accompanied by gas) and may 

therefore be the source of the apparent disturbance of the sediments noted in 

reflection profiles . 

We had no core samples of Type 5 acoustic wipeout zones because they 

occurred rarely in our study areas . Nevertheless, this type of acoustic behavior is 

widely recognized as the signature of gassy sediments (Hovland and Judd 1988; 

Behrens 1988; Anderson and Bryant 1990). 

Several cores were obtained from Type 6 zones in the GB 386 and GC 234 

study areas. These cores contained hemipelagic muds with little evidence of 

disturbance by hydrocarbon seeps and were similarly lacking in shell fragments and 

authigenic carbonates. Thus, we interpreted Type 6 as undisturbed hemipelagic 

sediment. 

3.5.3 Distribution of Echo Types 

Within the seep areas we studied, the acoustic fades appear in two different 

patterns. The seeps that are characterized by mounds, such as GB 386, GB 425, and 

GC 184, are characterized by either Type 1 (hardbottom) or Type 2 (buried 

hardbottom) over the mound itself (Figures 3.7, 3 .8, and 3.9) . These two reflection 

3-19 
G8425-0 

carbonate rock 

(diameter upto 8 cm) 

light gray-colored mud 

shell fragments 

oil trace 

sharp contact 

dark gray-colored mud 

shell fragments 

al trace 

%Carbonate 

0 5 10 15 20 25 30 35 

Porosity 

X0 .45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 

50 

100 

150 

200 

', e Porosity 

. . . - %Carbonate I 

Figure 3 .6 Description of Core GB 425-D, from a Type IV (disseminated 

disturbance) echo character zone . At right are trends in 

carbonate percentage and porosity. 

3-20 
Figure 3 .7 . Patterns of acoustic echo types in the GB 425 survey area. 

Bathymetry contours shown at 5-m intervals. Echo types are 

explained in Figure 3 .3 . Letters A- D show piston core locations 

(see Tables 3 .1 and 3.2); the letter V shows active vent locations. 

3-21 
W 

N 

N 

Figure 3.8 Patterns of acoustic echo types in the GB 386 survey area. Bathymetry contours shown at 5-m 

intervals . Echo types are explained in Figure 3.3. Capful letters show piston core locations; small 

letter show push core locations (see Tables 3.1 and 3.2). 
W 

N 

w 

Figure 3.9 Patterns of acoustic echo types in the GC 184 survey area. Bathymetry contours shown at 5-m 

intervals. Echo types are explained in Figure 3. Also shown are locations of tube, worm bushes and 

bivalve shells, denoted by filled and open circles, respectively. Lower case letters show the locations of 

push cores (see Tables 3.1 and 3.2). 
types show that hardbottoms are at or near the surface, owing to active or recently 

active carbonate precipitation or hydrate formation. The ape 2 signatures may 

indicate recent mudflows that have buried a previous hardbottom, as interpreted 

from submersible observations at GB 425 . The typical nearness of the hard 

substrate to the seafloor suggests that with time, the covering of sediments is 

probably incorporated into the hard substrate, so that the hard reflector moves 

upward through the sediment column. Alternatively, erosion may uncover some hard 

substrate. Given the uniformity of the burial or exposure of most hard substrate 

reflections, it seems that the upward movement is probably the most likely 

occurrence . 

Interestingly, patterns of Type 1 and Type 2 occurrence are slightly different 

at each site. GB 386 has Type 1 over the mound summit and Type 2 around its edges 

(Figure 3 .8), whereas the distribution on the GB 425 mound is almost opposite (Figure 

3.7). GC 184 appears different from the other two because the areas of Type 1 

returns are patchy and scattered (Figure 3.9) . These differences probably reflect a 

variability in the occurrence of carbonate or hydrate, which in turn may signify 

variability in the structure of the mounds and hence the geometry of hydrocarbon 

seepage. 

Around the mounds, the acoustic fades are variable, apparently dependent on 

the areal extent of the seepage zone. GB 386, seems to be an isolated seep mound, as 

it is surrounded by undisturbed, Type 6, sediments. Both GB 425 and GC 184 are 

evidently part of zones of seep disturbance that are larger than the survey areas. In 

GB 425 Type 1 and Type 4 sediments surround the mound (Figure 3 .8), whereas in 

GC 184, Type 3 sediments surround the mound (Figure 3 .9) . In both cases, sea 

surface geophysical survey data indicate that the study areas are only small parts of 

larger zones of seepage. 

3-24 
GC 234, which has no large mound structure, has acoustic facies patterns that 

are clearly different from the other sites (Figure 3 .10). Here, the distribution is 

complex, but mainly consists of small patches of Type 1 (hard substrate), Type 3 

(disturbed sediments), and Type 5 (acoustic wipe out) within an area of Type 6 

(undisturbed) sediments . The occurrence of Type 1 and Type 5 reflections, scattered 

in patches throughout the survey area, seems to indicate that carbonate or hydrate 

formation and gas seepage are widespread along the fault scarp and not as localized 

as at the mound sites . 

3 .5 .4 Backscatter Mosaics 

Acoustic backscatter patterns in the side-scan sonar records seem to mainly 

show the effects of small scale topography and this topography is extremely variable 

in its appearance at the sites we studied. In some instances, the topography is 

apparently related to mound structure or formation because the topographic features 

follow the bathymetric contours, such as curved ridges at the edges of the mounds in 

GB 386 (Figure 3.11) and GB 425. 

In some locations, the topography appears to be rock outcrop. In the images, 

such features appear as small scale roughness or clusters of mounds (Figure 3 .12). 

The roughness, blockiness, and clear acoustic shadows cast by these features 

suggests they are outcrops . Probably these outcrops consist mainly of authigenic 

carbonate ; however, in some areas it is possible that the small scale roughness is 

actually caused by clusters of tube worms (e.g., at GC 234) or even by hydrate 

mounds (MacDonald et al. 1990a) . Our study indicates that these outcrop zones are 

often correlated with Type 1 acoustic facies reflection patterns (compare Figs. 3 .10 

and 3 .12). Because the Type 1 character is a hard substrate, this is not surprising. 

Furthermore, at GC 234, at least, many of the "outcrops" are more-or-less linear and 

seem to be correlated with faults mapped in multichannel seismic data (Figure 3 .13) . 

3-25 
W 

N 

Figure 3.10 Patterns of acoustic echo types in the GC 234 survey area. Bathymetry contours shown at 5-m 

intervals . Echo types are explained in Figure 3 .3 . Also shown are locations of tube worm bushes, denoted 

by filled circles, and gas seeps, shown by an asterisk. Lower case letters show the locations of push cores 

(see Tables 3.1 and 3.2). 
W 

N 

figure 3.11 Side scan sonar mosaic of the GB 386 survey site. Bathymetry contours shown at 5-m intervals. Dark 

areas represent strong acoustic backscatter . 
W 

N 

00 

0 200m 505 

-2 

, - 5 5 

Carbonate . ~' y .. 

i 

. . 

V n C 34?2 ~a~3 :2` 

4 

T. 

V 

rv: r-' T 

27 4' 4 27°44' 

45"N ~ ~ 5 ' ~ ~t 45~~N 

r-- 1 13'30"W y - ~- r -~--r----- 13'W ,~t 

~" ` SV~~ r 

. 

o 1 

. 

?r 

,. 

v 

, . r - . 

,~

. 

Carbonate Outcrops ,.: . - .w . ., .y 

- 

,~ . .,: t ~~~y$ 

8 r ,h 

, 

> < .w; r ~.. ~ ~~ 55 

. . .~.5~ , . ..; . ..:: 

"f . 

.. .. .. 

, a 3 . 

bk 

,t : ., . 

. n F' x '.ih .. 5' F 

R . 

f'` 

} Carbonate OUICr 

I o S . . f< . it : :.9 'y A . 

. , . . a~9 ~~w 

' 

. .r. :.~. . _ . . . . 

figure 3.12 Side scan sonar mosaic of the GC 234 survey site. Bathymetry contours shown at 5-m intervals . 

Dark areas represent strong acoustic backscatter . 
0 201 

'45"N 

W 

N 

CD 

Figure 3.13 Comparison of GC 234 side scan sonar mosaic and fault traces (heavy black lines) mapped with 

multichannel seismic data (J. Reilly, in preparation) . 
Thus, the "outcrops" may represent the effects of seepage along the fault traces at 

the seafloor. 

The background acoustic backscatter from the seafloor appears remarkably 

constant on most of the side-scan mosaics . From experience in other areas with 

carbonate mounds (e.g., Laswell et al. 1990 ; 1994), we expected the sedimentary 

surface to show greater backscatter in disturbed areas where such factors as 

carbonate precipitation or shell deposits might increase the backscatter. In general, 

we did not find this to be true, perhaps because several of the survey areas were 

virtually entirely affected by seepage. Notably, in the survey area with the greatest 

acoustic fades variability (GC 234), it is clear that Type 1 outcrop features are 

generally surrounded by patches of seafloor with greater backscatter (darker in the 

mosaics; Figure 3.12). 

Some of the features in the mosaics have origins that are unclear. This is true 

of linear and curved ridges atop the two flat-topped mounds, GB 386 and GB 425. As 

seen from submersibles, such ridges are usually low (<1 m in height) and rounded. One 

possibility is that they are related to deformation of the mud in the mound; although, 

another is that they are formed by carbonate precipitation. The latter hypothesis 

arises from submersible observations in which some of these ridges were seen to be 

crusted with carbonate. 

At one site, GC 184, mysterious linear troughs, a few meters in width, were 

noted criss-crossing the mound. From submersible observations the troughs are seen 

to be shallow and smooth. Furthermore, available seismic data do not apparently 

show any structure beneath the troughs, such as fault offets . Consequently, we 

think these features are probably some type of drag or trawl marks. 

3-30 
3 .5 .5 Connections to Deep Faults 

In most of our study areas, it is still difficult to make detailed connections to 

deep faults . The faults are usually imaged with low-frequency multichannel seismic 

profiles, whose horizontal resolution is limited. In contrast, high-frequency, highresolution data, both 3.5 kHz echo sounder profiles taken at the sea surface and 25 

kHz profiles taken at depth, are unable to penetrate the Type 1 and Type 2 zones 

that characterize most of the seeps, so the near surface traces of the faults are 

unclear. Nevertheless, some relationships are clear . In each study, the seep is 

situated along the trace of a major fault, either caused by the uplift of a nearby salt 

diapir (GB 386, GB 425, and GC 184) or by faults between subsurface salt ridges (GC 

234). In GC 184, for example, the mound known as "Bush Hill" occurs on a fault that 

is active, as shown by the offset of near surface sediment layers (Figure 3.14) . The 

exact location of the fault beneath the mound is unclear because of the lack of 

acoustic penetration, but the geometry to the north and south of the feature suggest 

it is located near the steeper, western side of the mound where most of the Type 1 

reflection areas are located. GC 234 has no large mound, so there it is possible to 

map the traces of several faults that comprise the fault zone which crosses the study 

area. An excellent correlation is found among fault traces, carbonate outcrops, and 

chemosynthetic community locations (Figures 3.10, 3.12, and 3 .13) . These 

observations suggest that Type 1 and Type 2 reflection zones in some cases show the 

seafloor traces of faults, where seepage causes the precipitation of authigenic 

carbonate and supports chemosynthetic communities. 

3.5 .6 Relationship of Geophysical Signatures and Seep Organisms 

Within the limitations of our data set, we see a good correlation among hard 

substrate reflections, wipe out zones, "outcrops" on side-scan sonar images, and the 

locations of chemosynthetic organism clusters . One must remember that most of 

3-31 
W 

W 

N 

Figure 3.14 East-west 3.5kHz echo sounder profile across the GB 386 mud mound (right of center), on the 

flank of a large salt diapir. 
our data on the locations of chemosynthetic organisms comes from verbal 

observations recorded on tape during NR-1 cruises, so they record only the presence 

or absence of organisms that caused the observers, looking out the submarine 

portholes, to comment. Because tracks are located 30 to 100 m apart, the 

observations relate only the few tens of meters near the track line in which objects on 

the sea floor are visible. In addition, we cannot account for observer bias or 

variations in attention during the survey. Nevertheless, we were able to extract a 

large number of sitings, mainly of tube worm bushes, in the GB 184 and GC 234 

survey areas (Figures 3.9 and 3 .10). 

Comparison of acoustic facies and chemosynthetic organism sitings shows 

that at GC 184, 97% of tube worm bush locations and 92% of bivalve sitings are 

within areas classified as having Type 1 or Type 2 reflection characteristics (Figure 

3 .9). At this site, almost all of the seep mound shows these two reflection types, so 

we could also say that 96°l0 of all chemosynthetic organisms are on the mound. The 

lack of organisms off the mound, even within the small survey area, is striking. At 

the site in GC 234, the situation is slightly different. No Type 2 reflections were 

noted, but much of the seep zone is characterized by Type 1 or Type 3. Here there is 

a clear clustering of organisms near zones characterized by these two reflection types 

(Figure 3.10) . Although 86°l0 of the organisms are located on Type 1 or Type 3 

reflection zones, only 46% are located on Type 1 hard substrate reflectors, implying 

Type 1 areas are not required as community sites. 

These observations indicate a clear preference of chemosynthetic organisms 

for sediments showing Type 1, Type 2 (both hard substrates), and Type 3 (shallow 

disseminated seep products) characteristics . This is probably the result of two 

factors. First, these are areas where there has been a significant precipitation of 

authigenic carbonate, which may form a solid substrate to which organisms, such as 

tube worms, can attach themselves. Second, such areas also have high 

3-33 
concentrations of seeping hydrocarbons, hence the massive carbonate precipitation, 

so the organisms find at such locations the hydrocarbons needed to produce food. 

3.6 Discussion 

Tensional faults are one of the primary effects of deep salt movement on the 

sediment column of the Gulf of Mexico continental shelf and slope. These faults may 

dissect hydrocarbon reservoirs within the sediment column, allowing gas and oil to 

escape by providing both a break in the reservoir seal and an upward pathway. In 

order that the hydrocarbons reach the sea floor, it is probably necessary that the 

faults are large, or genetically related to one that is large, and also active. Only large 

faults will usually penetrate sufficiently deep to tap a mature reservoir and also 

provide sufficient offset to break through impermeable formations . Likewise, only 

active or recently active faults will break through to the seafloor, and this is probably 

necessary to let the hydrocarbons escape. 

Many faults that fit these qualifications exist within the Gulf of Mexico 

because salt body movements are ongoing. In our study areas, which are limited in 

geographic extent, all of the seeps occur either on the flanks of salt diapirs or along a 

fault system formed between two diapirs. This implies that the diapirs are the locus 

of most intense deformation and hence the most likely locations for seeps to occur. 

When hydrocarbons reach the seafloor, they have several geologic effects . The 

hydrocarbons change the characteristics of the sediments, causing disturbances that 

can be imaged by acoustical methods. They may also entrain sediments, causing the 

formation of seep mounds. 

The effects that perturb acoustic signals can be divided into two classes : the 

results of gas within the sediments and the results of materials precipitated within 

the sediments . The gas causes attenuation of the acoustic waves and the result is 

usually a zone of acoustic wipe out (signal loss) where subsurface reflections are not 

3-34 
seen or turbidity (reverberation) where reflectors may be masked (Hovland and Judd, 

1988; Anderson and Bryant, 1990). This former is the Type 5 reflection character 

noted in our study. Precipitates either cause acoustic turbidity, the scattered, 

amorphous reflections within the sediment column, or a hard substrate that reflects 

most of the acoustic energy. Hard substrates may be caused by the precipitation of 

massive carbonate layers and blocks (Roberts et al. 1990) or by the formation of gas 

hydrate layers. We see these as the Type 1 and Type 2 reflections within our 

surveys. Carbonates can also be disseminated as macro- or micronodules (Behrens 

1988; Roberts et al. 1990) and thus cause acoustic turbidity by the diffraction of 

acoustic energy throughout the affected sediment column. These effects cause Type 

3 and Type 4 reflection characters in our study areas . 

The formation of a seep mound implies the localization of the seep to nearly a 

point source. Interestingly, the three mounds we studied are all located on the flanks 

of salt diapirs (Figure 3.14), whereas the one site without a significant mound is along 

a fault system between diapirs . This may indicate that seeps next to diapirs are 

more likely to be localized, perhaps within a narrow fault, whereas those between are 

along wider fault zones are less likely to be constrained and localized. 

Our observations suggest that the mounds are formed by flows of low viscosity 

mud, like a "mud volcano," rather than by the diapiric rise of a mud body (Figure 

3.15). Evidence in favor of this hypothesis is as follows. Low viscosity, fluidized 

sediments are inferred from observations of fresh mud flows and the active eruption 

of gas and oil-charged mud from vents on the summit of the mound in GB 425. In 

addition, a layer of fresh, buried mussel shells in a core from near GC 234 indicate 

that the shells were quickly buried by a mud flow, before the shells could be degraded 

by post-mortem contact with sea water. That these mounds are not diapiric is 

inferred from industry multichannel seismic reflection profiles that appear to show 

3-35 
IV 2!!~ 

v 

Figure 3 .15 Model of the formation of the GC 184 mud mound. Faulting 

provided a conduit for fluid mud (top) which formed mound on fault 

scarp (middle) . Asymmetry of fault scarp may have caused 

mound asymmetry (bottom) . 

3-36 
continuous layers beneath the GB 386 and GC 184 mounds. This observation implies 

that the sediments were not deformed by diapiric action. 

Our observations also suggest that there are at least two types of mounds : 

those with rounded summits and those with flat-topped summits (Figures 3.2 and 

3.16). The one rounded-summit mound we studied, in GC 184, is asymmetric and has 

Type 1 reflections and chemosynthetic organisms occurring mainly on one side of the 

mound. These asymmetries suggest that the source of the hydrocarbons, probably 

the fault beneath the mound, lies under the west side of the mound (Figure 3 .9). The 

flat-topped mounds appear more symmetric in cross-section. In addition, observed 

hydrocarbon venting on the GB 425 mound was observed only at locations along the 

edge of the summit platform. We suggest that the middle of the summit, which shows 

mostly Type 2 hardbottom reflections, is impermeable to hydrocarbon migration, 

perhaps because it is capped by hydrate or carbonate, and this causes the 

hydrocarbons to vent around the edges of the summit. 

Interestingly, the flat-topped mound in GB 386, which has hardbottom at the 

surface (Figure 3 .8), is not a site of abundant and varied chemosynthetic 

communities, whereas GB 425, which has a thin sediment covering atop hardbottom 

(Figure 3 .7), is inhabited by many different species of chemosynthetic organisms. 

This difference may indicate that the GB 425 mound is more active, expelling more 

mud, gas, and oil, and this is the reason for the greater numbers and variety of 

chemosynthetic organisms . 

3-37 
W 

W 

OD 

27047'02"N 27047'02"N 

91031'45"W 0.5 0 1 km .~.... 91029'43"W . . . f 

= . . 

: .n .~a . 

. 

t 

Steep Slope- : Y?5m 

Y .1 0 .6 - .: 

^. t 

~~a . ~ :. . . . .. e'. . .. in seconds : . .: . ,~ . . . . : . ... . ~ ' >~ . 

t .i i 

/ . f S 

A 

- ~~ ~AcousUC ty K ,-~ r " Bush Hill y ~ ~ 

0.7 { ^ ,  ~- ~ 525m 

Y 

. 

.

.-

.~ ~ Transparent Layer 

. . . . .;. . 

. . . 

,. . . 

" ' . . 

. : . 0.8 

,. ~ ;- > s ~~q. ~ X,.. 

E r 3 ~ ~ , 

VE: X8.3 . . .:. . ; 

. .x_. . . .. . . . . . . . . . . . . . . . . .. . : . " s 

Figure 3.16 East-west 3.5kHz echo sounder profile across the GC 184 mud mound. Note offset of sedimentary 

layers on either side, indicating fault offset. 
4.0 Geochemical Alteration of Hydrocarbons in Seep Systems 

M.C . Kennicut II and Roger Sassen 

4.1 Introduction: A Descriptive Model of the Geochemical Setting of 

Seep Communities 

All chemosynthetic locations studied to date have a number of common 

geochemical features. These include oil seepage; gas charged sediments; hydrates; 

authigenic carbonate ; active hydrocarbon oxidation ; chemosynthetic biomass; 

elevated EOM, TOC, and salinity; and biodegraded oil (Figure 4.1) . While it is still 

not clear which features are essential, it is clear that these attributes are closely 

related. The general model describing geochemical settings that characterize seep 

communities was documented with collections made during the 1991 and 1992 

cruises. 

The management relevance of this portion of the program can be summarized 

as follows : 

1. Future releases of petroleum due to anthropogenic activities will be difficult 

to document because (a) ambient petroleum levels are high and 

heterogeneous and, (b) the most likely fluid to be spilled is the same as that 

which is naturally leaking. 

2. The issue of depletion or reduction of the hydrocarbon seepage due to 

extraction of petroleum reserves is still an unresolved question, however the 

potential does exist. 

3 . Due to chronic exposure to petroleum, as a result of fundamental life style, 

additional insults due to anthropogenic inputs may have little detrimental 

effect . However, if certain communities are perceived to already be in poor 

condition an additional insult could cause catastrophic failure of the 

ecosystem. 

4-1 
N 

Parameter Indicates 

Elevated TOC ~ ~ X ~_ X Oil Seepage 

Elevated CaC03 X Authigenic C03 

1-12S X 

;' ~ X 0 Anoxia 

Light P.W. C02 X, X X X Methane/Oil Oxidation 

Elevated IC02 X X X X 0, Hydrocarbon Oxidation 

Saline Waters 0 ~ 0 V y 0 Brine Seepage 

Fresh Water X X X 1 0 

Hydrate Decomposition 

Light Tissue b13C X- X X X X Chemo Autotrophy 

Figure 4.1 Summary of seep site processes and indicators. (Note, site descriptions refer to locations given in 

Figure 2 .1 and Table 2.1- except BH stands for Bush Hill, GC-184 and BP stands for Brine Pool Nr-1, 

GC-233. Light P.W. C02 indicates pore water C02 S 13C values.) 
A fundamental feature of the chemical setting of chemosynthetic 

communities dominated by tube worms is the presence of oil seepage (Kennicutt et 

al. 1988b ; MacDonald et al. 1989). Oil seepage is a source of labile carbon (Figure 

4.2) . This labile carbon can be oxidized by bacteria, most likely in conjunction with 

the reduction of seawater sulfate to H2S. This process leads to anoxic conditions 

through consumption of 02 . The production of sulfide also fulfills the energy needs 

of the dominant symbiont bacteria, sulfide oxidizers. Due to the 13C-depleted 

nature of the carbon in oil, a lowering of the sedimentary organic carbon stable 

isotopic compositions can be observed as the amount of organic matter increases 

(Figure 4.2). The introduction of oil to the system can increase the available organic 

carbon by several orders of magnitude (Figure 4 .3) . This latent source of both 

carbon and energy may provide the initial conditions that fuel the enhanced 

biomass. 

Production of large amounts of C02 during hydrocarbon oxidation leads to 

precipitation of authigenic carbonate (Figure 4.4) . The rapid microbial cycling of 

labile carbon allows for large increases in C02 in interstitial water. 13C-depleted 

carbonate, which ultimately forms if C02 is produced in excess of its solubility, is a 

pervasive feature at seep sites . The precipitation of carbonate is an important event 

for the ecology of an area since hard substrate appears to be essential to the 

establishment of some of the characteristic megafauna (i .e ., tube worms) . The 

presence of 13C-depleted carbonate represents an accumulation of materials over 

time (Figure 4.5). Once formed, the carbonate will remain in place unless a 

dissolution or erosional event occurs . The presence of hard substrate is an 

important control on the expression of community structure at chemosynthetic sites. 

A review of the stable isotopic compositions of both organic and inorganic 

pools of carbon at seep sites reveals the pervasive presence of 13C-depleted carbon 

due both to the infusion of 13C-depleted carbon into the system and preferential 

4-3 
9 

8 

o 

C 

6 

L 

v 5 

v 

4 

L' O 3 

O 

H 

1 

° GC-272 (CF) 

GC-234 

GC-184 

+ GB-386 

I x X 

x 

813C of Reservoired 

Oil in+Green Canyon 

0 . . . . 

~ D 

D 

OIL SEEPAGE 

x 

D 

x 

-27 .0 -26.0 -25.0 -24.0 

S13Corganlc matter (%o) 

Figure 4.2 Oil seepage adds 13C-depleted organic carbon. 

-23 .0 
411 

0 

o °, 

0 

`~ o 0 GC-272 (CF) 

x GC-234 

0 o GC-184 

o + GB-386 

X 0 

0 Xx a o 

0 0 

0 

0 20 40 60 80 100 120 

Extractable Organic Material (ppt) 

Figure 4.3 Oil seepage adds organic carbon at seep sites. 

9 

8 O 

C 7 

6 

5 

v 

4 

3 

-a 2 

f+ 

0 
O o 

O o° 

_ -10 

O "-

L 

0 vCL 

E -30 

-O 

41 

r _- 0.669 

O 

00 0~~ eo 00 o 

o 

o &" 0 o ° 

Ya o 

0 

O 

0 

D 

o 

0 

0 

o° 

0 

GC-272 (CF) 

x GC-234 

Bush Hill 

Shell hash (?) o 

MMMM)NIO0 

Authigenic 

Carbonate 

Formation 

0 

0 

0 

0 10 20 30 40 50 60 70 

CaC03 (%) 

Figure 4.4 Sources of carbonate at seeps . Stable carbon isotope values are expressed in S13C notation. 

-50 
8 

o GC-272 (CF) 

x GC-234 

o Bush Hill 

f GB-386 

6 

J 

4 

N 

v 

r"4 

2 

X 

X 

0 

13 

E03 

Hydrocarbon Oxidation / 

Carbonate Dissolution 

of 

o -t 

0 -10 -20 -30 

813C IC02 (%o) 

Figure 4 .5 Evidence of microbial hydrocarbon oxidation. 

-40 -50 
fixation of 12C carbon during chemosynthesis (Figure 4.4) . A closer examination of 

these data indicates the importance of various biogeochemical processes at these 

sites both now and in the past. 

One feature of chemosynthetic seep sites that make them difficult to describe 

in a quantitative manner are steep gradients in the concentrations of chemicals . 

Seep sites are characterized by a heterogeneous distribution of chemicals. This 

heterogeneity allows for incompatible elements essential to chemosynthesis to occur 

in close proximity, i .e ., reduced and oxidized compounds . Small scale studies (-1 

m2), which were first undertaken in 1992 at several sites, demonstrated this 

heterogeneity (Figures 4 .6 to 4 .9). Methane concentrations can vary by orders of 

magnitude over just a few centimeters . Hydrates are restricted in areal extent and 

can occur at or near the sediment/water interface . It is also apparent that 

heterogeneity occurs in three dimensions . These distributions suggest that 

relatively small scale microenvironments are pervasive and that individual 

organismal requirements can be satisfied by transport of chemicals over relatively 

short distances . Strong elemental gradients appear to determine the biotic and 

geological heterogeneity of seep communities. 

To test our model for geochemical control of community formation we 

undertook an intensive program of resampling of four of the main study sites during 

dives in 1993 . Our main objectives were (1) to document the processes that affect 

hydrocarbon composition and modify the seafloor, and (2) to establish the extent to 

which these processes impact the development of specific components of seep 

communities. 

4-8 
0-5 cm 

N~ 

^'C4 / i'C4 variable O.r1.0 

" Pushcore Locations ~ Possible Hydrate 

Bacterial Mat 

2,293 ppm 

oil seepage noted 

Figure 4.6 GC 184 (Dive 3269) methane concentrations (ppm). Contour intervals 

in ppm. 

49 

1 .0 m 

3-15 cm 
m ~ ~~"// 

1 .0 m 

33,250 ppm 

" Pushcore Locations Q Possible Hydrate 

Orange Mat with White Fringe 

Figure 4.7 GC 234 (Dive 3265) methane concentrations (ppm). Contour intervals 

in ppm. 

37,178 ppm 

0-10 cm 

>10cm 

410 
0-3cm 

11 - 25 cm 

" Pushcore Locations Q Possible Hydrate 

Figure 4.8 GC 272 (Dive 3280) methane concentrations (ppm). Contour intervals 

in ppm. 

4-11 

1 .0 m 0.1 

3-11 cm 
0-3cm 

9-15 cm 

Pushcore Locations 

Orange Bacterial Mat " White Bacterial Mat 

Figure 4.9 GC 272 (Dive 3279) methane concentrations (ppm). Contour intervals 

in ppm. 

412 

1 .0 m 

3-9 cm 
4.2 Experimental 

4.2.1 Samples 

Samples of sediments (45 cm push-cores) were acquired in 1993 at four of the 

Study sampling sites (GC 234, 185, 272, 233) in the Green Canyon area of the Gulf 

slope by the Johnson Sea-Link (JSL) research submarine . Samples were 

transferred to cans after collection, sodium azide bactericide was added, and the 

cans were then sealed under N2. Samples were frozen until analysis. 

Sediment samples were obtained from specific areas within chemosynthetic 

communities to document differences in hydrocarbon concentration and 

composition. Samples GC 234-1 through -12 were acquired during JSL dive 3525 

(270 44 .77'N, 910 13 .34'W) at a water depth of about 543 m. Small diameter core 

tubes (2 cm i .d.) were used to collect densely spaced samples within a 1 m2 grid 

straddling an isolated orange and white Beggiatocz mat (Figure 4.10) . Beggiactoa are 

H2S-oxidizing bacteria that are widely distributed at seep localities (Sassen et al. 

1993b) . The larger GC 234 locality is described by MacDonald et al. (1990a). 

Samples GC 185-1 through -12 were acquired during JSL dive 3529 at Bush 

Hill (270 46.92'N, 910 30 .46'W). Small diameter core tubes (2 cm i .d.) were used to 

collect sediment samples within a 1 m2 area beneath part of a Beggiatoa mat 

adjacent to a vestimentiferan tube worm cluster. Water depth at the sample site 

was approximately 541 m. The tube worms (Lamelliabraclziac n. sp.) contained 

bacterial symbionts that oxidize H2S (MacDonald et al . 1989). Sample GC 185-13 

was acquired from a gas hydrate that breached the sea floor nearby on Bush Hill 

(MacDonald et al . 1994) . No other samples of "outcropping" hydrates had been 

analyzed previously. 

Samples GC 272-1 through -7 were acquired using large diameter core tubes 

(6.5 cm i .d.) during JSL dive 3535 in an area (27041.28'N, 91032.45'W) characterized 

by anoxic sediments with epifaunal vesicomyid clams. Water depth at the site was 

4-13 
Bacterial Mat: 

Orange Mat 

White Mat 

10 cm 

Core Locations: 

Mat 

Control 

C 

(D 

Figure 4.10 Detailed locations of core samples taken from a zoned orange and white Beggiatoa mat at the GC 234 

locality. Numbers refer to Table 4.1 and 4.2 . 
approximately 575 m. The clams contained bacterial symbionts that oxidize H2S 

(MacDonald et al . 1990a). 

Sample GC 233-1 was acquired using a large diameter core tube (6.5 cm i .d.) 

during JSL dive 3539 (27043 .43'N, 91016 .78'W) adjacent to an anoxic brine pool 

rimmed with methanotrophic mussels (Seep Mytilid I) at a water depth of about 650 

m. The brine pool site is described by MacDonald et al. (1990b). 

4.22 Methods 

We have relied on basic geochemical measurements to provide preliminary 

insight to oil and C1-C5 hydrocarbons of sediments in this reconnaissance study. 

Methods used for geochemical analysis are similar to those described by Kennicutt 

et al. (1988a). 

Dried and powdered sediment samples were Soxhlet-extracted with hexane 

for 24 hours, and after reduction in volume, extracts were subjected to C15+ gas 

chromatography using a Hewlett-Packard 5880 chromatograph equipped with a 30 

m DB-5 capillary column. This approach yields the total hydrocarbon concentration 

in sediments. Chromatograms of recent sediments that do not contain oil typically 

consist of n-alkanes with strong odd-carbon predominance, and a flat baseline 

because the hydrocarbons that characterize oil tend to be absent or in low 

abundance. If oil is present, chromatograms typically show an elevated baseline. 

The area beneath the elevated baseline is called the unresolved complex 

mixture (UCM) because naphthenes and other compounds that are difficult to 

resolve by gas chromatography are present . The n-alkanes and isoprenoids, 

normally prominent in unaltered oils, were absent or in low abundance in our 

samples because these compounds had been altered by microbial oxidation. 

Moreover, oil was present in such great concentration as to generally overprint any 

higher-plant woes co-extracted from the sediments. For these reasons, no data on 

4-15 
n-alkanes or isoprenoids are presented. The concentrations of UCM are expressed 

as ppm by sediment weight. 

We analyzed the free C1-C5 hydrocarbons in the headspace of canned samples 

to define hydrocarbon types, and because they are sensitive indicators of bacterial 

oxidation (James and Burns 1984; Sassen et al . 1988). The C1-C5 hydrocarbon 

gases were separated using a Hewlett-Packard 5710 chromatograph equipped with 

a 2 m packed alumina column. The concentrations of individual C1-C5 

hydrocarbons are expressed as ppm by volume. 

4.3 Results 

Results of analysis of 33 sediment samples from the four chemosynthetic 

communities sampled in Green Canyon are shown in Table 4.1 (UCM, total C1-C5, 

gas ratios) and Table 4.2 (C1-C5 compositions). 

4.3 .1 Isolated Bacterial Mat at GC 234 

The C15+ chromatograms of hexane extracts from GC 234 showed a broad 

UCM characterized by the absence of n-alkanes and isoprenoids (Figure 4.11). 

UCM concentrations were in the 2,226-7,809 ppm range (mean = 4,039 ppm). 

Concentrations were highest beneath the mat (mean = 5,650 ppm), and decreased 

away from the mat (mean = 2,966 ppm). 

Total concentrations of C1-C5 hydrocarbons were in the 11.5 to 52,926 

ppm range (mean = 6,531 ppm) . The total C1-C5 concentrations were highest 

beneath the mat (mean = 12,979 ppm), whereas concentrations decreased away 

from the mat (mean = 83.6 ppm). Gas compositions were dominated by C1 

(predominantly methane), as indicated by gas wetness in the 0 .1 to 11 .7% range. 

The lowest gas wetness occurred beneath the orange mat. The i-C4/n-C4 ratios 

4-16 
Table 4.1 Concentrations of UCM (ppm by weight) and total C1-C5 hydrocarbons 

(ppm by volume) in sediment samples. Gas wetness - (C2-C4/C1-C4) X 

100. (Dashed numbers, e .g. GC 234-1, refer to core locations, see 

Figure 4.1 for example.) 

Sample UCM Total C1-C5 Gas Wetness i-C4/n-C4 

Green Canyon 234 

GC 234-1 2380 11.5 11.7 0.52 

GC 234-2 2684 20.1 10.8 0.51 

GC 234-3* ND 169.6 9.8 0.52 

GC 234-4* 3928 104.1 14.7 0.58 

GC 234-5 2226 45.9 2.7 0.56 

GC 234-6 3393 1652 1.7 1.77 

GC 234-7 4513 128.7 2.6 0.66 

GC 234-8* 7809 1257.9 2.9 0.34 

GC 234-9* ND 97.6 3.0 0.48 

GC 234-10* 4168 23316.1 0.1 0.59 

GC 234-11* 6693 52925.9 0.4 37.49 

GC 234-12 2597 130.1 2.6 2.04 

Green Canyon 185 

GC 185-1 35209 33035.2 1.8 39.02 

GC 185-2 38992 300.8 15.1 ND 

GC 185-3 23519 363.4 26.5 15.32 

GC 185-4 18702 2486.1 1.5 ND 

GC 185-5 35938 6780.2 69.4 585.62 

GC 185-6 21667 7180.7 0.5 ND 

GC 185-7 25802 10643.3 0.8 37.14 

GC 185-8 21587 12602.4 1.9 1.14 

GC 185-9 23145 31136.8 3.4 55.86 

GC 185-10 14849 8723.9 1 .0 2.92 

GC 185-11 20312 18057.6 7 .0 121 .75 

GC 185-12 17583 1138 .1 17.1 92.50 

GC 185-13 ND 153566 73.3 0.18 

Green Canyon 272 

GC 272-1 1060 47436.9 8.5 020 

GC 272-2 1464 26398.1 0.2 8.47 

GC 272-3 3294 39682.7 0.1 8.51 

GC 272-4 3312 38951.3 0.3 30.88 

GC 272-5 1089 23986.8 0.3 5.53 

GC 272-6 328 7426.7 5.6 3.76 

GC 272-7 1463 17476.5 0.3 5.25 

Green Canyon 233 

GC 233-1 82 8834.2 0.1 1.88 

* = mat samples 

4-17 
Table 4.2 Concentrations of individual C1-C5 hydrocarbons (ppm by volume) in sediment samples. 

Sample Methane Ethane Ethylene Propane Propylene i-Butane n-Butane i-Pentane n-Pentane 

Green Canyon 234 

GC 234-1 10.1 0.5 0.1 0.4 0.0 0 .2 0.3 0.0 0.0 

GC 234-2 17 .7 0.8 0.2 0.6 0.1 0 .2 0.5 0.0 0.0 

GC 234-3' 149.6 6.7 2.6 4.3 1.0 1 .8 3.5 0.0 0.0 

GC 234-4* 82.7 5.4 1.7 3.7 5.4 1 .9 3.3 0.0 0.0 

GC 234-b 44.b 0.9 0.1 0.2 0.1 0 .0 0.1 0.0 0.0 

GC 234-6 162.3 2.5 0.1 0.2 0.0 0 .0 0.0 0.0 0.0 

GC 234-7 125.1 2.7 0.2 0.3 0.1 0 .1 0.2 0.0 0.0 

GC 234-8' 1218.7 19.5 1.9 6.1 0.9 2 .8 8.1 0.0 0.0 

GC 234-9* 92.9 1.6 1.1 0.7 0.7 0 .2 0.4 0.0 0.0 

GC 234-10' 23289.5 23.7 0.6 1.0 0.3 0 .4 0.6 0.0 0.0 

GC 234-11' b2702.7 187.7 0.4 7.2 0.2 26 .9 0.7 0.0 0.0 

GC 234-12 124.4 1.8 1.4 0.6 0.9 0.7 0.3 0.0 0.0 

Green Canyon 185 

GC 185-1 31604.9 170.0 0.2 157.2 0.1 235 .1 6.0 961 .6 0.0 

GC 185-2 226.0 8.9 0.2 10.1 36.7 20.8 0.0 0.0 0.0 

GC 18b-3 266.9 43.8 0.1 26.7 0.0 24 .3 1.6 0.0 0.0 

GC 185-4 2449.0 13.7 0.3 4.0 0.1 18 .9 0.0 0.0 0.0 

GC 185-6 486.6 104.5 0.6 74.7 0.3 920.5 1.6 6192 .4 0.0 

GC 18b-8 7143.6 34.6 0.3 2.3 0.0 0 .0 0.0 0.0 0.0 

GC 185-7 10664.2 59.7 0.3 8.6 0.1 19 .9 0.5 0.0 0.0 

GC 18b-8 12362.9 97.9 0.0 36.7 0.0 b6 .0 48.9 0.0 0.0 

GC 186-9 28783.5 307.4 0.2 223.8 0.0 460.9 8.3 1362.7 0.0 

GC 186-10 8631.9 81.3 0.6 8.0 0.2 1 .4 0.6 0.0 0.0 

GC 186-11 14631.6 198.4 0.6 173.6 0.4 715 .6 5.9 2331 .7 0.0 

GC 186-12 942.6 92.7 0.6 45.0 0.2 56 .6 0.6 0.0 0.0 

GC 186-13 24152.8 13011.4 0.0 16634.3 0.0 6210.0 35074.3 67870.0 1632 .9 

Green Canyon 272 

GC 272-1 40594.9 22b2.5 0.0 607.4 0.2 15 1 .9 769.3 2218.7 852.1 

GC 272-2 26347.0 44.8 0.2 3.0 0.2 2 .6 0.3 0.0 0.0 

GC 272-3 39626.4 49.1 0.1 4.2 0.1 2 .5 0.3 0.0 0.0 

GC 272-4 38846.6 78.3 0.1 6.4 0.2 19 .0 0.6 0.0 0.0 

GC 272-6 23909.4 68.1 0.2 5.1 0.2 3 .3 0.6 0.0 0.0 

GC 272-6 7013.2 105.8 0.6 149.3 0.4 124 .4 33.1 0.0 0.0 

GC 272-7 17424.3 46.b 0.2 9.2 0.2 1 .7 0.3 0.0 0.0 

Green Canyon 233 

GC 238-1 8829.1 4.7 0.1 0.2 0.0 0.0 0.0 0.0 0.0 

* " mat Samples 
r 

cC 

Figure 4.11 Examples of C15. chromatograms of hexane extracts from the GC 234, GC 185, GC 272, and GC 233 

areas. All chromatograms from these areas show an UCM feature, and the absence of the n-alkanes 

and isoprenoids present in oil. 
the 0 .3-2.0 range, with the exception of a single orange mat sample (37.5). C5 

hydrocarbons were absent or below detection limits. 

4.3.2 Bacterial Mat and Tube Worms at GC 185 (Bush Hill) 

C15+ chromatograms from samples GC 185-1 through -12 exhibit an UCM 

characterized by the absence of n-alkanes and isoprenoids (Figure 4.11) . The UCM 

concentrations were in the 17,583 to 38,992 ppm range (mean = 24,775 ppm). 

Total concentrations of C1-C5 hydrocarbons were in the 301-33,035 ppm 

range (mean = 11,037 ppm) . Gas compositions were generally dominated by C1, but 

there were exceptions . The range of gas wetness was wide (0 .5-69.4%). In three 

samples, n-C4 was absent or below detection limits. i-Cq/n-C4 ratios in nine 

samples were in a wide range, 1 .1-586. Although n-C5 was below detection limits in 

these samples, i-C5 was a significant component in four samples (961-5,192 ppm) . 

4 .3 .3 Gas Hydrate at GC 185 (Bush Hill) 

Total C1-C5 concentration of the hydrate sample (GC 185-13) was 153,567 

ppm. The C2-C4 hydrocarbons were major components of the hydrate, as reflected 

by a gas wetness of 73 .3% . In contrast to other samples, i-C5 and n-C5 were 

significant components . The i-C4/n-C4 ratio was 0 .18 and the i-C5/n-C5 ratio was 

35.4. 

4.3 .4 Chemosvnthetic Clams at GC 272 

C15+ chromatograms of samples from an area characterized by vesicomyid 

clams only exhibited UCMs (Figure 4.11) . The UCM concentrations were in the 

328-3,312 ppm range (mean = 1,716 ppm) . Total concentrations of C1-C5 

hydrocarbons were in the 7,427-47,437 ppm range (mean = 28,766 ppm). Gas 

compositions were dominated by C 1, as indicated by gas wetness in the 0 .1-8.5% 

4-20 
range. The i-C4/n-C4 ratios varied in the 0.2-30 .9 range. The C5 hydrocarbons were 

either absent or below detection limits in all but one sample. 

4.3.5 Chemosvnthetic Mussels at GC 233 

The C 15+ chromatogram of sediment from the brine pool (Seep Mytilid I) only 

exhibited UCMs with slight overprinting from the co-extraction of higher-plant 

waxes from recent sediments (Figure 4.11). The UCM concentration was 82 ppm. 

The total concentration of C1-C5 hydrocarbons was 4,903 ppm. C1 dominated, as 

indicated by a gas wetness of about 0.1%. The C4 and C5 hydrocarbons were either 

absent or below detection limits . 

4.4 Discussion 

4 .4 .1 Hydrocarbons 

Hydrocarbon compositions at chemosynthetic communities appeared to be 

effected by multiple processes including (1) rates of migration of thermogenic 

hydrocarbons to sediments, (2) differing levels of bacterial oxidation, and (3) 

exclusions related to hydrate formation. The presence and type of hydrocarbon 

alteration appeared to strongly influence the chemoautotrophic fauna . We 

emphasize that large differences may occur within spatial scales of less than 1 m, 

helping to explain the great heterogeneity observed between and within 

chemosynthetic communities. 

The most basic observation concerning the Beggiatoa mat at the GC 234 

sampling locality is that concentrations of UCM (Figure 4 .12) and C1-C5 

hydrocarbons (Figure 4.13) were higher beneath the mat than in control cores near 

the mat. It should be stressed that the concentrations of Cl-C5 hydrocarbons 

beneath the mat were an order of magnitude higher than in control samples only 10 

cm from mat margins (Table 4 .1 and Figure 4.10). The result is consistent with the 

4-21 
4 

3 

V 

C 

d 

7 

Q 

N 

L 

T 

ntrol 

Figure 4.12 Histograms illustrating that the concentrations of UCM in core samples of the GC 234 Beggiatoa mat 

are higher beneath the mat than in control cores away from the mat. 

ut;m (PPM) uuuu gpoo 
N 

W 

3 

2 

A 

v 

c 

as 

Q 

as 

mtrol 

Figure 4.13 Histograms illustrating that the concentrations of total C1-C5 hydrocarbons in core samples of the GC 

234 Beggiatoaa mat are higher beneath the mat than in control cores away from the mat . 

Total Cj-CS (PPm) r N g ~ $ 8 

8 
tight coupling between mats and hydrocarbon gradients suggested by Sassen et al . 

(1993b). 

The effects of aerobic microbial oxidation can be recognized since the 

prominent n-alkanes and the branched-chain isoprenoids originally present in crude 

oil are preferentially degraded, leaving a residue of UCM (Sassen et al. 1993b). 

This sequence is illustrated in Figure 4.14 by comparison of an unaltered oil from a 

subsurface reservoir and an degraded seep extract. The C15+ chromatograms of 

extracts from the GC 234 sampling area were altered by bacterial oxidation, as 

shown by the example in Figure 4.11 . 

The i-C4/n-C4 ratios of low molecular weight hydrocarbons can provide a 

sensitive index of the degree of bacterial oxidation (Sassen et al. 1988). Since n-C4 

is preferentially oxidized by bacteria, i-C4 tends to become more significant as this 

process advances (Winters and Williams 1969) . The lowest gas wetness values and 

highest i-Cq/n-C4 ratio were from samples acquired from directly beneath the mat 

(Table 4.1). Bacterial oxidation effects (as indicated by the C1-C5 hydrocarbons) 

thus appeared more advanced beneath the mat than away from the mat. 

It should be emphasized that the highest hydrocarbon concentrations, as well 

as the most advanced bacterial oxidation, occurred beneath the mat we sampled 

rather than away from the mat. Hydrocarbons set into motion complex bacterial 

interactions within sediments (Sassen et al. 1993b). Hydrocarbon-oxidizing bacteria 

produce C02. When 02 is depleted, sulfate-reducing bacteria become active and 

produce H2S. Beggiatoa mats occupy the interface between anoxic sediments and 

the oxic water column. The mats oxidize H2S to first form granules of elemental 

sulfur within cells, and ultimately oxidize the elemental sulfur to form sulfate. 

The Beggiatoa mat adjacent to the tube worm cluster at GC 185 and the mat 

on GC 234 were similar in that thermogenic hydrocarbons altered by bacterial 

oxidation were present at both localities. However, the mean concentration of UCM 

4-24 
GC 185-2 Seep Sample 

N Cn 

NAL OXDA110N 

Figure 4 .14 Comparison of C15+ chromatographs of a subsurface reservoir sample of oil from Jolliet Field in GC 184 

and a seep sample from GC 185 (Bush Hill) illustrates the effects of bacterial oxidation . 
N Os 

4 

v 3 

c 

as 

lies 

Figure 4.15 Histograms of illustrating differences in UCM concentrations between the four study areas . Note the 

relatively high UCM concentrations measured in samples from GC 185 (Bush Hill). 

ut,m (PPM) `~' r~ a CO CD 

`+~ N c 
at GC 185 was higher (24,775 ppm) than at GC 234 (4,039 ppm) (Figure 4 .15). 

Mean concentrations of C1-C5 hydrocarbons at GC 185 are also higher (11,037 ppm) 

than at GC 234 (6,531 ppm) (Figure 4 .16). 

Bacterial oxidation decreases gas wetness by degrading the thermogenic C2- 

C4 hydrocarbons, a process accentuated in natural systems by addition of biogenic 

C 1 (James and Burns 1984; Sassen et al . 1988). GC 185 samples showed a much 

wider range of gas wetness (0 .5-69 .4%) than noted at GC 234 (0 .1-14 .7%). 

Unusually high gas wetness in some GC 185 samples argue that another process 

affected our results. The i-C4/n-C4 ratios at GC 185 were quite variable (1.1-586). 

Many of the i-C4/n-C4 ratios at GC 185 were high enough to imply more advanced 

bacterial oxidation effects than at the GC 234 area, where values were lower (0.3-

37.5). However, unusually low i-C4ln-C4 ratios in a few GC 185 samples also 

suggest that a process other than microbial oxidation affected our results . 

Relatively high concentrations of the i-C5 hydrocarbon in four GC 185 samples also 

needs to be explained. 

A core sample of gas hydrate that breached the seafloor (GC 185-13) was 

obtained near our other Bush Hill sample site . The abundant C2-C5 hydrocarbons 

(gas wetness = 74 .3%) contrasted with previously analyzed hydrate from the Gulf 

slope in which C1 is the dominant component (Brooks et al. 1984 ; 1986). The iC4/n-C4 ratio of the hydrate is quite low (0 .18), whereas the i-C5/n-C5 ratio was high 

(35 .4). It should be emphasized that the n-C4 and i-C5 hydrocarbons were strongly 

favored as guest molecules in Structure II hydrate (Ripmeester and Ratcliffe 1991). 

Thus, there is a straightforward explanation for our data on GC 185 sediments. 

Since the GC 185 area contained abundant hydrate (Sassen et al. 1993a), it is likely 

that a number of sediment samples contained small amounts of Structure II 

hydrate. 

4-27 
10 

9 

8 

7 

v 

c 

Q 

d 

'P 

00 

IC 185 

Figure 4.16. Histograms illustrating differences in total C,-C6 hydrocarbon concentrations between 

the four study areas. 

r 

Total CI-C5 (PPM) M " ~ ~ o 

LO LO 
The C15+ chromatograms of sediment extracts from a vesicomyid clam 

environment on GC 272 showed that oil has been affected by bacterial oxidation 

(Figure 4.10). The concentration of UCM was lower (mean = 1,716 ppm) than noted 

at GC 234 or GC 185 (Figure 4.15). C1-C5 concentrations (mean = 28,766 ppm), 

however, were higher than at other sites (Figure 4 .16). C1 dominated the 

compositions . The i-C4/n-C4 ratios tended to be lower than noted in samples 

adjacent to tube worm clusters at GC 185, suggesting that bacterial oxidation 

effects are less advanced. 

The sample of sediment from the anoxic brine pool on GC 233 colonized by 

the Seep Mytilid I was characterized by low UCM (82 ppm) . The dominant lower 

molecular weight hydrocarbon was C1 (8,829 ppm). The low gas wetness (0 .1%), 

and the absence of C4 and C5 hydrocarbons in our sediment sample, suggest a 

dominantly biogenic origin. This is consistent with the carbon isotopic composition 

of C1(S13C = -63.8) reported at the site (MacDonald et al. 1990b). The brine pool 

thus differs in that thermogenic hydrocarbons appear to be less significant than at 

other sites. 

4.4.2 Seeps and Vents 

Differences between seeps and vents need to be emphasized. Seeps are 

defined as areas where hydrocarbons accumulate in sediments and do not enter the 

water column in large volumes ; vents are specific sea-floor features where 

hydrocarbons and other fluids bypass the sediments and enter the water column, 

creating gas bubble trains and natural oil slicks . There is geochemical evidence 

suggesting that, at least in some cases, the geochemistry of hydrocarbons can 

differentiate seeps from vents. Our data showed that complex chemosynthetic 

communities were associated with sea-floor seeps where high concentrations of oil 

and gas altered by bacterial oxidation occurred in sediments . In contrast, 

4-29 
chemoautotrophic fauna other than Beggiatoa mats were not observed near a mud 

volcano on GC 143 where an oil and gas episodically vented to the water column 

(Roberts and Neurauter 1990). Since abundant hydrocarbons were present at both 

seeps and vents, it is surprising that we did not see complex communities in both 

situations . 

A C15+ chromatogram of oil-stained sediments from the active GC 143 mud 

volcano is shown in Figure 4.17. The chromatogram has unusual features including 

(1) a strongly elevated baseline (UCM) consistent with intense bacterial oxidation of 

oil, and (2) an envelope of n-alkanes and isoprenoids from oil that has not been 

significantly impacted by bacterial oxidation. This is consistent with episodic rather 

than continuous venting at the GC 143 mud volcano . Bacterial oxidation, at least at 

present, appears unable to keep pace with high rates of hydrocarbon venting. 

4.4.3 Sea-Floor Modification at Seeps 

Understanding why oil composition at seafloor seeps differs from that at 

vents could shed new light of the early development of complex chemosynthetic 

communities, particularly at GC 185 (Bush Hill). From the standpoint of the H2S 

requirements of chemosynthetic communities, venting is negative since 

hydrocarbons needed to promote bacterial processes in sediments are lost to the 

water column. Unless hydrocarbons are retained in sediments, hydrocarbonoxidizing bacteria will not deplete 02, and bacterial sulfate reduction will be 

inhibited. 

Beggiatoa mats appear to be so widely distributed because they are well 

adapted to rapid colonization of the many small or transient seep sites that provide 

energy and carbon sources on the Gulf slope (Sassen et al . 1993b). Our data showed 

a clear link to concentration gradients of thermogenic hydrocarbons, but mats also 

occurred at seeps of purely biogenic C1 where UCM was present in low abundance 

4-30 
'P 

w 

Figure 4.17. A C 15+ chromatogram of sediments from a mud volcano on GC 143. Unusual features include a 

strongly elevated baseline (UCM) consistent with the effects of bacterial oxication that is 

overprinted by the n-alkanes and isoprenoids characteristic of unaltered oil from recent venting. 
(Sassen et al. 1993b). Bacterial oxidation of any hydrocarbon type depletes 02 in 

sediments, permitting the bacterial sulfate reduction that provides the mats with 

H25. 

We propose that Beggiatoa mats and other bacteria play a role in starting a 

process of seafloor modification in areas of long term thermogenic seepage . 

Beggiatoa mats are thought to retard hydrocarbon loss to the water column, a 

function that would enhance production of the H2S needed by the mats . This 

"biologic barrier" at the interface between sediments and the water column could 

also enhance precipitation of authigenic carbonate rock to further seal the seafloor 

(Sassen et al. 1993b). 

Tube worms, in contrast, appear to occupy environments that have been 

modified by thermogenic seepage effects over a long span of time. The GC 185 

(Bush Hill) site, for example, is associated with active faults that serve as conduits 

for vertical migration of oil and associated gas from subsurface reservoirs of nearby 

Jolliet Field (Sassen et al . 1993a) . Since subsurface reservoirs of Jolliet Field were 

charged with oil and gas during the Pleistocene, hydrocarbons probably began to 

impact the seafloor at that time (Sassen et al. 1993a) . The abundant authigenic 

carbonate rock that accumulated at our tube worm site (MacDonald et al. 1989), as 

well as over shallow salt and faults north of Jolliet Field (Roberts et al. 1989), 

provides conclusive evidence of seepage over a long span of time. 

The highest UCM concentrations measured in the present study were 

adjacent to a vestimentiferan tube worm cluster on Bush Hill (GC 185). An 

empirical association of tube worms with oily sediments at GC 185 has been 

documented previously (MacDonald et al. 1989). Based on our new data, however, 

we can move further towards understanding why this relationship exists. Both 

migration and bacterial oxidation of hydrocarbons are rapid at this site. C15+ 

chromatograms only showed a UCM residue because the n-alkanes and isoprenoids 

4-32 
were destroyed by bacterial oxidation. The C1-C5 hydrocarbons from this site were 

particularly impacted by bacterial oxidation since the highest i-Cq/n-C4 ratios were 

measured there (Table 4.1). Furthermore, n-C4, i-C5, and n-C5 were absent or 

below detection limits in some samples (Table 4 .2) . 

We suggest that the vestimentiferan tube worms were so abundant at GC 185 

because modification of the site by seep-related bacterial processes was advanced . 

Relatively rapid seepage has occurred there since the Pleistocene, creating biologic 

barriers and relatively impermeable seals of authigenic carbonate rock and gas 

hydrate that retain hydrocarbons within sediments. Moreover, hydrate bodies could 

serve as buffers to maintain hydrocarbons in sediments for bacterial activity should 

hydrocarbon migration rates fluctuate over time . The intense bacterial oxidation of 

abundant hydrocarbons at GC 185 triggers the rapid production of the H2S required 

by the tube worms, favoring their proliferation. 

Another requirement of tube worms could help explain why they need a more 

specialized habitat than, for example, the adventitious Beggiatoa mats . Tube 

worms appear to need a hard substratum as a point of attachment (H.H. Roberts 

personal communication), and authigenic carbonate rock provides that in an 

otherwise unfavorable mud-dominated environment. A long span of time is 

required to precipitate enough carbonate rock to serve large populations of tube 

worms. 

Results from the GC 272 site move us closer to understanding the relation of 

hydrocarbons to the vesicomyid clam environment. The C15+ chromatograms 

showed evidence of bacterial oxidation, but UCM concentrations were low (1,716 

ppm) in comparison to GC 185 or GC 234. The C1-C5 hydrocarbon concentrations 

were high (mean = 28,766 ppm), but showed low gas wetness values consistent with 

both bacterial oxidation and strong dilution by biogenic Cl (Table 4.1) . It could be 

that the clams persist in a relict oil seep environment where the rate of oil seepage 

4-33 
has declined from higher levels in the past, or perhaps gas seepage triggers H2S 

production instead of oil. 

The composition of hydrocarbons sampled in association with the Seep 

Mytilid I (GC 233) was distinct since these organisms are methanotrophic. UCM 

was not present in significant concentration in sediments, and the main type of low 

molecular hydrocarbon was biogenic C1. This does not imply a negative correlation 

between the Seep Mytilid I and other environments where thermogenic methane 

predominates, however, since this organism is known to utilize either biogenic and 

thermogenic C 1 as available (MacDonald et al . 1990a). 

4.5 Conclusions 

1 . Sediment samples from beneath an isolated mat of H2S-oxidizing bacteria at 

GC 234 contained oil (mean = 5,650 ppm) and C1-C5 hydrocarbons (mean = 

12,979 ppm) that are altered by bacterial oxidation. Control cores away from 

the mat contained lower concentrations of oil (mean = 2,966 ppm) and C1-C5 

hydrocarbons (mean = 83.6 ppm) . Bacterial oxidation of hydrocarbons 

depletes 02, triggering bacterial sulfate reduction to produce the H2S needed 

for mat development. Moreover, C02 from the bacterial oxidation 

precipitates as authigenic carbonate rock, beginning a process that modifies 

the seafloor. 

2. Sediments at GC 185 (Bush Hill) contained high concentrations of UCM 

(mean = 24,775 ppm) and C1-C5 hydrocarbons (mean = 11,037 ppm). Tube 

worm communities requiring H2S occurred at GC 185 where the seafloor has 

already been greatly modified since the Pleistocene by accumulation of oil, 

gas hydrate, and authigenic carbonate rock in sediments. Venting is thus 

suppressed over time, favoring slow development of complex communities 

that depend on abundant hydrocarbons in sediments . 

4-34 
3. Sediment samples from an area with vesicomyid clams (GC 272) contained 

lower UCM (mean = 1,716 ppm) but C1-C5 concentrations were high (mean = 

28,766 ppm) . The clams appeared to persist at relict thermogenic seeps 

where the rate of seepage had declined from past highs, or H2S production 

was triggered by gas instead of oil . 

4. The sediment we sampled in association with the methanotrophic Seep 

Mytilid I was characterized by low UCM (82 ppm) and by dominance of 

biogenic C1 (8,829 ppm). 

4-35 
5.0 Characterization of Habitats and Determination of Growth Rate and 

Approximate Ages of the Chemosynthetic Symbiont-Containing Fauna 

Charles R Fisher 

One of the major factors that determines the ability of a species to recover 

from damage is the rate at which it grows. Growth and recovery in chemosynthetic 

fauna have been studied at hydrothermal vents (Hessler et al. 1988 #150), where 

they inhabit an environment subject to rapid and often catastrophic changes. 

Consequently, growth rates are high in these groups. Hydrocarbon seeps are believed 

to be geologically more stable; the chemosynthetic fauna dependent on these seeps 

would have less requirement for rapid growth. If true, recovery from damage would 

take longer. Direct measurements of growth rates in seep fauna were undertaken to 

increase understanding of this issue. Growth also depends on supply of nutrients, 

which can be quite heterogeneous in seeps; so it is necessary to combine the growth 

measurements with a characterization of the seep micro-habitats . The materials 

and methods section below will explain how all major work was carried out. The 

results section will present findings for each faunal group studied. 

The five dominant species of chemoautotrophic-symbiont containing animals 

fall into three taxonomic categories that have distinct metabolic requirements and 

physiological strategies for exploitation of their chemoautotrophic lifestyle (see Fisher 

1990 for review). Because of the differences in their lifestyles and their requirements 

(both from each other and especially from other non-symbiotic species), we devised 

unique methods to characterize their chemical habitat and measure growth and 

estimate age for each group . These animals live in a unique and dynamic 

environment that is virtually impossible to duplicate in the laboratory. In order to 

obtain meaningful growth rates for these animals, it was necessary to develop new 

techniques to measure their growth rates in situ, in their natural environment, using 

methods that were specifically adapted to each group's life styles. To characterize 

5-1 
their chemical habitats, we also developed new sampling apparatus and analysis 

methods which have provided the most comprehensive and biologically relevant data 

ever on the chemical microhabitats required by the symbiont-containing seep fauna. 

5.1 Mussel Methods 

Mussels used in this study were collected, from depths of 540 - 800 m, in 

September 1991, August 1992, and June 1993, using the deep-sea submersible 

Johnson Sea Link (Harbor Branch Oceanographic Institution) . They were placed in a 

temperature-insulated retrieval box mounted on the front of the submersible. On the 

deck of the ship, the mussels were immediately transferred to buckets of 8°C sea 

water and placed in a cold room (8°C) for all subsequent handling. Shell length and 

width of all mussels were measured with calipers to the nearest 0.1 mm. 

A subset of each collection (6-20 mussels, depending on the purpose) were 

processed for determination of morphometric relations and condition indices . 

Following shell measurements, the shells were drained of sea water by prying them 

open a few mm. The adductor muscles were then severed over an open, pre-weighed, 

specimen bag, so that all internal fluid was collected . Gill, mantle, and foot tissue 

subsamples were taken, placed in pre-weighed cryovials, and immediately frozen in 

liquid nitrogen. The remaining tissue (_ "bulk tissue") was removed from the shell, 

added to the specimen bag, and stored at -20°C. The shells were cleaned, dried and 

marked for future shell volume measurements. Tissue samples were transported to 

Pennsylvania State University in liquid nitrogen or on dry ice and upon arrival stored 

in a -70°C freezer. 

Mussel densities and size distributions were determined from collections made 

from within a 0 .5 m diameter ring placed firmly in a bed of mussels . The ring 

consisted of a 20 cm high band of 2 mm thick stainless steel, with three 15 cm spikes 

protruding below the ring to anchor it in place, and a handle for manipulation by the 

5-2 
submersible. Mussels had been collected from the beds used for the growth studies 

several times in the years previous to the ring collections . To avoid making 

quantitative collections from impacted beds, the ring collections at GC 184 and GC 

234 were made from nearby, undisturbed beds, which were visually similar. The 

similarity in water samples and size ranges of mussels collected from the beds within 

each site support the linked interpretation of the sites as presented here . In the case 

of the bed at GC 272, the ring collection was made from a relatively inaccessible and 

therefore undisturbed portion of the same bed as that used for the growth stu$y. 

Again the similarity in water samples, and size ranges support the joint 

interpretation of the two data sets. Placement of the collection ring was not random 

within the patchy, small mussel beds, but efforts were made to sample "typical" 

areas within each bed (the extreme periphery, obvious areas of dead shells, and 

mounds of mussels were avoided). After placement of the ring, essentially all mussels 

within the ring were collected (first by grab sampler, then with a suction sampler) and 

a video record was made of the ring to document the occasional mussel not collected . 

The mussels from each collection were placed in separate sections of the temperature 

insulated collection box and processed as described below . Between 0 .7 and 16% of 

the mussels were damaged during a given collection. These were counted and 

measured with as much accuracy as possible. 

5.2 Water Sampling and Analysis 

Water samples were collected using specially designed samplers which allowed 

collection of undiluted, small volume (1-5 ml) samples from specific areas above, 

within, or below the mussel beds. Samples were taken from three levels within the 

mussel beds; at the top of the shells near the siphons, 5 cm below that level, and 10 

cm below that level . The three water samplers used were constructed of 316 

stainless steel. Water was drawn through 2 ~tm fritted filters (the interstitial 

5-3 
samplers employ stainless steel filters and the "wand" sampler a plastic filter). The 

"wand" sampler consisted of a filter mounted on the end of a 70 cm tube fitted with a 

handle for the submersible. The interstitial water samplers were essentially large 

thumbtacks with the filters mounted below 14 cm diameter plates, above a 

sharpened point, and isolated from internal dead spaces by O-rings. The water was 

collected from a 1 cm length of filter centered either 5 or 10 cm below the plates with 

a flow rate of about 2 ml min-1 . 

The filters were connected to 7 m of 0.025 cm ID polyetheretherketone tubing 

using low dead volume fittings, and plumbed into the rear compartment of the 

submersible through stainless steel swagelock fittings (and a stainless ball valve), 

such that the tubing completely penetrates the hull of the submersible and is 

connected to a plastic 0.15 cm diameter valve and leer lock (syringe) fitting on the 

inside of the compartment. The entire sampling apparatus, from filter to syringe 

fitting, had a dead volume of < 1 .8 ml. Tubing from all three water samplers entered 

the rear compartment through the same ball valve which serves as an emergency 

cut off should an inner valve fail . All three samplers were carried in quivers mounted 

on the work platform of the submersible and could be picked up by the submersible 

manipulator and used at will . 

The samplers were positioned by the submersible pilots, the lines bled (2 mls) 

and the samples collected in 1 ml glass syringes fitted with low volume gas-tight 

valves . After collection of an 0 .5 ml sample, the sample was stored on ice until 

analyzed. To minimize the spontaneous oxidation of sulfide in the water samples 

before analysis, the sampling syringes were primed with 0.1 ml of a degassed pickling 

solution. In 1992 a solution of 1M Na4EDTA in 1N HCl was used, which inhibited the 

initial rate of the oxidation reaction by 65% . In 1993 this solution was replaced 

with a basic zinc acetate solution (5 :1, 2 .6% ZnAcetate (W/V):6% NaOH 

(W/V)), which precipitated the sulfide and inhibited the oxidation reaction 

5-4 
much more effectively (> 90% of the initial sulfide and oxygen were recovered after 20 

h of refrigerated storage in laboratory tests). Therefore, only the 1993 environmental 

sulfide data are presented here . Methane data from both years are given, as these 

data were not affected by the change in protocol. All water samples were analyzed on 

board ship for EC02, CH4, 02, EH2S, and N2 by gas chromatography (Childress et 

al. 1984) within 10 h of collection. 

5.3 Growth Studies 

Mussels were collected as previously described and the collection location was 

marked with a numbered float. A representative size range (-150 mussels) from 

each location was set aside for marking and an additional 20 were sampled as 

controls. In 1991, 3 different marking methods were tested; (1) notching the outer 

margin of the shell with a file, (2) gluing a commercial color-coded, numbered larval 

fish tag (6 .9 mm x 3 .2 mm), near the umbo of the shell, and (3) gluing a hand-made 

color-coded polypaper tag (9.8 mm x 3.3 mm), near the umbo. Before attaching the 

tags, shell length and width were measured. A small area of the shell was quickly 

dried and lightly abraded with #220 sand paper. The tags were attached with 

cyanoacrylate glue (Loctite #447 and/or #401), allowed to dry for 60 seconds, and 

rinsed in 2 changes of cold seawater. This entire procedure required the mussels to be 

removed from sea water for less than 2 min. All manipulations were conducted in the 

cold room (8°C) . Both types of tags persisted until 1992. Only the larval fish tags 

were used in subsequent years. Notching was found to be unnecessarily invasive and 

was also discontinued after 1991. The mussels used in this study were never kept on 

board ship for more than 15 h. 

For deployment, the mussels were placed in insulated acrylic containers filled 

with chilled sea water, returned to the site of collection, and released within 0.5 m of 

the marker float. 

5-5 
Marked mussels were retrieved in August 1992 and in June 1993. Twenty 

marked mussels and 20 unmarked mussels from each collection were processed as 

described above, with tissue subsamples removed from the first six. All other marked 

mussels were measured and re-released with additional newly marked mussels from 

the same collections (only the latter are presented in the growth analysis) . 

5.4 Transplant Experiment 

A transplant experiment (with two treatment levels, transplant 1 and 

transplant 2) was conducted at GC 184. Mussels were collected in September 1991 

and marked as described above. For deployment, 20 mussels were placed in each of 

two 42 cm x 19 cm coated wire cages and placed in the insulated collection box, in 

chilled 8°C sea water. The transplant cages were returned to the area of collection 

and deployed approximately 2 m (transplant 2) and 3-4 m (transplant 1) away from 

the collection (control) bed (the same bed used for the in situ growth study). Cages 

were retrieved in August 1992 and the mussels were treated as described for the 

growth studies. The twenty unmarked mussels from their original collection site (GC 

184 growth site, 1991 and 1992 collections) served as controls for the transplant 

experiment. 

5 .4 .1 Measures of Condition 

Bulk tissue was thawed and homogenized in a lOx dilution of distilled, deionized 

water using a Brinkman PT 3000 Polytron with a PT-DA 3020/2TM generator. 

Replicate samples (10 ml) were taken from this primary homogenate for 

determination of ash-free dry weight. A 1 ml aliquot was removed and further 

homogenized, in a hand held glass tissue grinder, then diluted an additional 50x for 

glycogen determination. This glycogen assay is a modification of several previous 

protocols . Briefly, glycogen was degraded to glucose by amyloglucosidase and glucose 

5-6 
was quantified spectro-photometrically following a dye-linked enzymatic oxidation 

(Sigma Diagnostics kit #510). Modifications included the use of phosphate buffer 

(0.067 M , pH 5 .8), which was found to be conducive to the action of all enzymatic 

steps, and a 2-h pre-incubation with amyloglucosidase (0.5 unit activity ml-1). This 

protocol yielded > 90% recovery of spikes and allowed quantification in tissue 

homogenates so that prior isolation of glycogen was not needed. Free glucose 

concentrations in the tissue were determined in separate aliquots (without the 

addition of amyloglucosidase) and were subtracted for determination of tissue 

glycogen levels . Replicate glycogen and glucose standards were run with each assay 

and the standard curves used to calculate the tissue glycogen levels. 

Condition index (CI), the ratio of the ash-free, dry mass of soft tissue to 

internal shell volume, is a commonly used measure of relative condition in bivalves 

and is a sensitive indicator of condition in related deep-sea mussels (Fisher et al . 

1988) . Soft-tissue dry weight was determined after drying replicate 10 ml samples of 

the homogenate described above to constant weight at 60°C . Ash weight was 

determined after ashing these samples in a muffle furnace, at 500°C, for 12 h. 

Internal shell volume was calculated based on the weight of sand contained in each 

valve . Each shell valve was filled with sand, leveled ofd' across the open edge, and the 

weight of sand recorded, 3 times per valve. Sand weight was converted to volume 

based on the empirically determined relation between the two. Tissue water content 

was also calculated from the ash-free dry weights. It provides an easily obtained 

measure of condition but, unlike CI, it is sensitive to residual mantle cavity water. 

5-7 
5.5 Statistical Methods 

Due to the nature of the condition indices data (percentages and ratios), an 

arcsine transformation (y' = 2 arcsineqy) was applied to all three condition 

parameters before testing for significant differences between sites and years. The 

parameters were normally distributed (Shapiro-Wilks test) and the variability of the 

condition parameters was analyzed with respect to site, year, and length by 

ANCOVA using a General Linear Model (GLM) procedure (SAS program, version 

607). If significant differences (p < 0.01) in condition index, water content, glycogen 

content, or length were supported by one-way analysis of variance, then multiple 

comparison tests (Fisher PLSD and Scheff tests, Statview II) were used to 

demonstrate significant differences between specific sites and years. 

Since the growth rate varied with initial length, statistical comparisons 

between sites and years were conducted on the size specific growth rates (AL/L ;, 

where AL = final length - initial length and L; = initial length (Bayne et al. 1982; 

Wootton 1991) and were compared by ANCOVA using a reduced model GLM 

procedure. Ages were estimated using the von Bertalanffy growth model and the 

variance of the age estimates were calculated using the following formula: 

[=ag/DK]2var(K) + 2cov(K, LI ax)[ag/aK][ag/aLmax] + [ag/aLmax]2var (Lmax) 

where g = age, Lmax = the extrapolated point of no growth , K = the rate at which LI ax 

is approached, L; = initial length and [ag/aK] _ (1/K2)[ln(1-L;/Lmax)] and [ag/aLmax] _ 

-Li/(K*Lmax*(Lmax-Li)~ 

5-8 
This formula was derived by M. Ghosh-Dastidar (Pennsylvania State 

University) using point estimates and standard errors for both K and L,na. and the 

results from an asymptotic Kramer-delta procedure. 

5.6 Growth Methods 

To determine growth rates for these species a device was constructed which 

deployed bands (tie-wraps) around the anterior end of the tubes. The hydraulically 

actuated device is positioned by a submersible and deploys up to eight individualized 

bands per dive, with adjustable, preset, tension for each band. Tension was adjusted 

to assure a tight fit, without crushing the worms' tubes . The position of the band with 

respect to the growing end of the worm is then documented by video . Two features of 

the bands are used for scale in the images. The most precise is the width of the outer 

surface of the band itself, which can be used to compensate for the angle of 

documentation, because the band lies flat on the surface of the tube and is 

approximately perpendicular to the long axis of the tube . Spherical stone beads were 

also attached to the bands for absolute scale from any angle. Re-video or collection of 

the banded worms in subsequent years was used to determine the yearly in situ 

growth rates of the tubes of individual animals. 

5.7 Results for Vestimentiferans : Escarpia sp. and Lamellibrachia sp. 

The two species of tube-worms present at these sites are in two different 

families (Escarpidae and Lamellibrachidae) but have similar gross anatomy, 

physiology, and lifestyles (Jones 1985) . The vestimentiferan larvae attach to hard 

substrate (including adult worm tubes in some species) when they settle and are 

immobile after metamorphosis to the adult form. The adults have no mouth, gut, or 

anus and apparently depend entirely on their symbionts to meet their nutritional 

needs. The symbionts constitute a significant proportion of their body mass (about 

5-9 
6% with another 40% accounted for by blood) and are housed in an internal organ 

inside the trunk of the animal with no direct connection to the outside environment 

(Childress et al. 1984; Jones 1985; Powell and Somero 1986). The symbionts require 

sulfide, oxygen, and inorganic carbon as chemoautotrophic substrates (Fisher et al. 

1988) and the immobile host must live in an environment with a constantly 

replenished supply of these nutrients. We measured sulfide and oxygen 

concentrations in the pools of water around their plumes and in the sediments around 

their tubes. Since the tissue of these tube-worms extends all the way to the base of 

the tubes and they grow from their anterior end, we measured growth in these 

animals by measuring tube growth over two one-year periods. Tube growth is related 

to tissue growth through the empirically determined relation between the two (Figure 

5.1). 

5.7 .1 Vestimentiferan Growth Rates 

The Vestimentiferan bander has been used successfully on over 100 

individuals. However, the limiting factor in our analyses turned out to be the quality of 

the video documentation obtained by the scientist in the sphere during the dive . Over 

the course of the past two years, we have obtained 28 measures of yearly growth 

increments (on 20 different individuals) . At the suggestion of the scientific review 

panel, most of the banded animals are still in situ, so their growth can be monitored 

into the indefinite future. At least one more year's data will be collected in September 

1994 during a cruise supported by NOAA. 

The relatively small number of measurements made at each site, and the 

relatively large percent error associated with measurements of very small growth 

increments precludes statistical comparisons between sites. However, the data set 

is clearly sufficient for general conclusions concerning these species (Table 5.1) No 

growth was detected in any individual between 1992 and 1993 (five 

5-10 
30 

-, 

bA 

20 

.. 

10 E 

Tube Length (cm) 

30 

CA 

20 

~. 

10 

. .., 

C 

Tube Length (cm) 

figure 5.1 Relation between tube length and wet weight for hydrocarbon seep 

vestimentiferans. a) Escarpia sp., y = -1 .931 + 0 .241x, R2 = 0 .78. b) 

Locmellibrachia sp., y = 0.523 +0.101x, R2 = 0 .67. 

5-11 

0 

0 100 200 300 

0 

0 20 40 60 80 100 120 
lamellibrachids and eight escarpids), although growth was documented in one of these 

individuals of each species in the previous year. It is possible that for some unknown 

reason, this was a "bad year" for the Louisiana Slope vestimentiferans, and the 

following calculations do not include data from that year (the averages for both years 

combined for each species are given in Table 5.1). Direct measures of tube growth 

from digitized images over the first year, were 0 mm for the escarpid and 0, 3 .1, 16.4, 

and 17.9 mm (0 to 3 rings, n = 4) for lamellibrachids. Of three additional escarpids 

monitored that year, one produced a single growth ring and the rest did not produce 

new tube material . Of the additional seven lamellibrachids monitored that year, one 

laid down three, one laid down two, and one laid down a single ring (the rest did not 

extend their tube that year). Thus, the average growth rate between 1991 and 1992 

(based on data from measured individuals and using the average between-ring 

increments for the other monitored individuals) was 2.5 mm (n = 4, SD = 5.7 mm) for 

the escarpids and 7 .1 mm (n = 11, SD = 8.3 mm) for the lamellibrachids . This 

represents an average increase in soft tissue biomass of 0.61°Io for the escarpids and 

0.51% for the lamellibrachids. 

Table 5.1 Vestimentiferan tube growth . Elongation rates of vestimentiferan tubes are shown. 

Since growth was determined to occur by secretion of new rings, only sample sizes (n) 

are shown for individuals which secreted no new rings (all determined rates for these 

individuals were 0 ± 1 mm/year). "*" indicates rates determined by measurement of 

digitized images; all other rates are based on calculations from increment counts . 

New Increments Lamellibrachia sp. Escarpia sp. 

(new rings) Elongation (mm/year) Elongation (mm/year) 

1991-1992 1992-1993 1991-1992 1992-1993 

0 0(n=5) 0(n=5) 0(n=3) 0(n=8) 

1 7, 3* 11 

2 13 

3 16*, 18*, 20 

Based on the full data set of 20 individuals at six different sites over 2 years 

(Table 5 .1), it is apparent that these cold-seep vestimentiferans grow at a much 

5-12 
slower rate than their hydrothermal vent relatives . However, ultimately they can 

reach lengths two to three times that of the largest known hydrothermal vent 

species . The total lengths of most of the banded worms is unknown because they are 

still in situ, partly buried with sediment . Lengths determined for the banded 

vestimentiferans collected to date and other vestimentiferans collected from the 

same aggregations are shown in Table 5.1 . Insufficient numbers of banded animals 

have been collected for statistical analysis of the relation between size and growth 

rate, but it should be noted that the largest banded individual collected (216 cm) 

deposited three growth rings between 1991 and 1992, and was thus among the 

fastest growing animals in this study. These collections are generally biased towards 

the smaller individuals in the aggregations because efforts were made to avoid 

collecting very large tube worms due to difficulties associated with handling the 

largest individuals in the submersible's insulated collection box. Individuals of 

Lamellibrachia sp . in excess of 3 m in length have been collected several times in the 

past, and one individual had the posterior 2 m of its tube stained from burial in the 

sediment documented photographically (personal observation CRF). The very slow 

growth rates and total lengths of these vestimentiferans indicate that they are a 

long-lived species . Based on these considerations, we suggest that the larger 

aggregations contain individuals in excess of 200 years old, perhaps greatly in excess 

of 200 years old. The current lack of growth data on smaller individuals precludes 

construction of allometric growth curves, which would allow more exact age 

determination of the adults at the sites studied. 

Both species of seep vestimentiferan deposit external rings on their tubes as 

they grow, the temporal meaning of which is unknown. The length of the increment 

between rings was measured on all visible rings on the anterior end of the tubes (the 

rings disappear towards the posterior, older, portion of the tubes) of 14 escarpids and 

20 lamellibrachids (a total of 411 rings were measured). The spacing between rings 

5-13 
varied from extremes of 1 .4 to 22.6 mm on escarpids and 0.9 to 17 .2 mm on 

lamellibrachids. However, the spacing of the rings was much less variable on a given 

individual, and the average ring spacing was 11.4 (SD = 4.3, n = 172) for the escarpids 

and 6.7 mm (SD = 3.0, n = 239) for the lamellibrachids . Based on examination of 

collected specimens and review of video footage, it appears that the rings mark the 

increment of tube deposited in a single growth "spurt", as suggested by Gardiner and 

Jones (1993). The terminal increment in all measured escarpids was within the 95% 

confidence interval of the mean of the adjacent 10 increments, suggesting that the 

increment had reached its full length . Based on the same criteria (95% confidence 

interval of the mean), the terminal increment of three out of 13 lamellibrachids 

measured was in the process of extension. The terminal tube increment of most of 

the individuals collected was noticeably thinner walled than adjacent sections of the 

tube, indicating that the second step in the deposition of a new tube increment is 

thickening the wall . We suggest that the rings are the relic of the flared tube ends, 

which strengthen the terminus of the tube and allows the worm to extend and retract 

without fouling the end of the tube, that the entire increment is deposited over a 

relatively short period of time, and then strengthened over an additional, longer period. 

This is also supported by the fact that no increases (±1 mm) in any pre-existing ring 

increments (including the original terminal increment) were detected on any of the 

animals measured in situ . We also conclude that there is no direct temporal 

significance to the rings, and they cannot be used to age the worms. 

The low growth rates and longevity of the cold seep vestimentiferansfmply life 

history strategies very different from their hydrothermal vent relatives . The deep 

sea in general is a relatively stable environment (Gage and Tyler 1991). In the case 

of the seep vestimentiferans and their source of nutrition (sulfide-based autotrophy) 

the environment may be even more stable as it is not tightly tied to vagaries in 

primary production on the ocean surface. At the other extreme are the hydrothermal 

5-14 
vent species whose habitats and source of reduced chemicals are ephemeral on time 

scales as short as years. Thus, the small phylum Vestimentifera, contains species 

that are among both the fastest growing and the longest lived deep-sea creatures. 

5.72 Vestimentiferan Habitat Characterization 

A second aspect of this study was to characterize chemically the 

vestimentiferan habitat, specifically with respect to their energy source, hydrogen 

sulfide. Dissolved gas concentrations (EC02, H2S, 02, N2, and CH4) were measured 

in samples of blood from individuals of both Lamellibrachia sp. and Escarpia sp., as 

well as in water samples from four different levels in the water column surrounding 

vestimentiferan bushes : plume level, midway between the plume level and the 

sediment surface, and 5 cm and 10 cm below the sediment surface. Blood levels of 

hydrogen sulfide have been shown to reflect exposure to sulfide in hydrothermal vent 

vestimentiferans (Childress et al. 1991) . One water sample with unrealistically high 

N2 and 02 concentrations was not included in the analysis because experience 

indicates that this is caused by introduction of a small air bubble during handling of 

the sample. 

Replicate samples of each type within two bushes were compared to determine 

the heterogeneity of the microhabitat within bushes (Table 5 .2) . Although seep 

environments are generally more stable than either hydrothermal vents or wood 

islands, the chemical environment at vestimentiferan bushes, as characterized by 

analysis of the precise water samples of this study, is quite patchy. A wide range of 

conditions are present at individual bushes. This unexpected variability most likely 

results from channeling of water flow in the vicinity of the vestimentiferan tubes, 

creating a patchy environment throughout and around each bush. 

5-15 
Table 5.2 Variability in replicate water samples. Ranges in dissolved gas concentrations (in 

p.M) are shown for plume level, midway between plume level and sediment surface, 

and 10 cm interstitial water samples for two representative seep vestimentiferan 

bushes. All values expressed here as "0" indicate the gases were not detectable, and 

where no gases were detected within a sample type, the range is expressed simply 

as "0." The detection limit of the procedure used to analyze these gases was 

approximately 3 gM, although occasionally the gas chromatograph could detect 

gases down to 1 NM in concentration . 

Type n C02 H2S 02 N2 CH4 

Site 1, Bush D 

Plume 2 2784, 2828 0 152,166 564,660 0,2 

level 

Midway 2 2722, 2762 0 80,172 258,660 0 

10 cm 2 3284, 4950 0,942 12~140 644,734 6,734 

Site 2. Bush D 

Plume 2 2600, 2952 0)15 5,148 602,700 0 

level 

Midway 2 2496, 2552 0 33,153 598,629 0 

10 cm 3 2776 -3436 0 - 100 0-58 655- 1167 0-20 

Because of the extremes within site variability of interstitial water chemistry, 

and the relatively low number of samples of each type taken at each site, no 

statistically valid comparisons of water chemistry between sites could be made. 

However, analysis of the full data set of water samples from the vestimentiferan 

habitats and blood samples of the vestimentiferans provide considerably new insight 

into the physiological ecology of the seep vestimentiferans and corroborate the 

conclusions of low growth rates in these animals . Sulfide was rarely detected at the 

plume level among the vestimentiferans (which is where hydrothermal vent 

vestimentiferans take up sulfide) (Table 5 .3). Thus, it seems likely that they access 

a different pool of H2S . The presence of H2S in over half of the 10 cm interstitial 

water samples suggests that these animals take up interstitial H2S across the 

buried posterior portion of their tubes, as had previously been suggested (Childress 

1987). Since the base of a seep vestimentiferan tube can be buried by up to a meter 

5-16 
of sediment, the H2S pool available for uptake across the thin tube posterior portion 

of the tube is likely much higher. 

Table 5.3 Localization of H2S at seep vestimentiferan bushes . Numbers of samples in which 

dissolved H2S was detectable and not detectable are shown for each sample type. 

*The detection limit of the procedure used to analyze these gases was approximately 

3 pM, although occasionally the gas chromatograph could detect gases down to 1 PM 

in concentration. 

Sample Type Not Detectable (< 3[iM*) Detectable (> 3~tM*) 

Plume level 8 1 

Midway 15 1 

5 cm Interstitial 9 1 

10 cm Interstitial 14 11 

Lamellibrachia sp. Blood 13 2 

Escarpia sp. Blood 4 2 

The H25 concentration in blood samples of freshly collected vestimentiferans 

also provides insight into their physiological ecology. These animals have sulfidebinding hemoglobins that bind H2S with very high affinity in equilibrium with 

environmental H2S concentrations (Childress and Mickel 1985). In the seep 

vestimentiferans of this study, only four of 21 blood samples contained detectable 

H2S (Table 5.3). The maximum concentration found in any of these samples was 32 

~tM (Escarpia sp. = 1 and 15 ~tM, Lamellibrachia sp. = 16 and 32 gM). An average of 

27gM sulfide was found in the blood of seven individuals collected by trawling in a 

previous study (Childress et al. 1986). All of this data implies equilibrium with a very 

low sulfide pool. For a full discussion of the implications of this data the reader is 

referred to Scott and Fisher (Submitted) and Simpkins (1994). In brief, we conclude 

that it is possible that the sulfide pool is around the plume but below our limits of 

detection, but more likely that it is an interstitial sulfide source in dynamic 

equilibrium with the vestimentiferan blood, limited by diffusional constraints. 

5-17 
5 .7.3 Vectimentiferan Condition at Different Sites 

A final aspect of this study was to compare the condition of individuals from a 

variety of environments. To do this, we used several different condition indicators and 

analyzed individuals of Lamellibrocchicz sp . in nine different bushes, and Escarpia sp. 

in seven different bushes, at four distinct hydrocarbon seep sites . Overall ranges in 

condition indicators were determined for each species and compared to means and 

standard deviations within bushes to ascertain the scale of variability within bushes 

(Table 5.4). This variability was analyzed following a General Linear Models 

procedure (SAS program, version 6.07) for each condition indicator to determine if it 

could be explained by variables measured in this study. Specifically, the GLM 

procedure was used to determine if the variability in each condition indicator (lipid 

content, water content, ash-free dry weight to tube-volume ratio, and RNA to DNA 

ratio) could be explained by the bush an individual was collected from (bush of origin), 

and/or the size of the individual (as reflected in wet weight, ash-free dry weight, or 

tube volume) . 

The variability in RNA/DNA ratios was not significantly explained by General 

Linear Models including all independent variables at a = 0 .05, although a model with 

bush of origin alone explained the RNA/DNA variability in escarpids significantly at p 

= 0 .065 (r2 = 0 .391). We believe that this is likely due to the temporal sensitivity of 

this analysis as compared to the other condition indices used, which reflect longer 

term, cumulative effects . In other words, short-term transient fluctuations in 

exposure to sulfide would likely be reflected in RNA/DNA ratios, where tissue lipid or 

water levels would change over longer time intervals and reflect conditions averaged 

over a longer time period. 

5-18 
Table 5.4 Means, standard deviations, and sample sizes are shown for condition indicators in 

Escarpia sp. (a) and Lamellibrachia sp. (b) populations (non-summarized data is 

presented for sample sizes of 2). The total ranges for all indicators in individuals 

sampled from all populations are shown for comparison. 

a: Condition indicators in Escarpia sn. noaulations. 

Water Content AFDWNoI. Lipid Content RNA/DNA 

(% Wet Weight) (9/ml) (% Wet Weight) (g/g) 

mean ± sd (n) mean t sd (n) mean t sd (n) mean t sd (n) 

Site 1, Bush A 83.2 ± 2.9 (4) 0.141 ± 0.027 (4) 2.1 ± 0.4 (4) nd 

Site 1, Bush B 76.6 ± 3.4 (3) 0.223 ± 0.060 (3) 2.5 ± 5.6 (3) nd 

Site 1, Bush C 78.5 ± 4.4 (6) 0.141 ± 0.035 (6) 3 .6 ± 1 .3 (4) 2.55 ± 0.47 (6) 

Site 1, Bush D 75.4 ± 2.4 (6) 0.219 ± 0.054 (5) 1 .8, 4.2 (2) 2.16 ± 0.32 (6) 

Site 2, Bush D 69.5, 75.0 (2) 0.217, 0.305 (2) 4 .7, 5.1 (2) 1 .68, 1 .86 (2) 

Site 3, Bush A 79.7 f 2.4 (5) 0.339 ± 0.106 (6) 4.8 f 1 .2 (5) nd 

Site 4, Bush A 72.8 ± 3 .2 (13) 0.271 ± 0.052 (13) 4.8 ± 1.9 (11) nd 

Total Range 66.3-85.4 (39) 0.084-0.553 (39) 1 .6-7 .8 (31) 1 .67-3 .07 (14) 

b: Condition indicators in Lamellibrachia SD. oonulations 

Water Content AFDWNoI. Lipid Content RNA/DNA 

(% Wet Weight) (9/ml) (% Wet Weight) (g/g) 

mean ± sd (n) mean ± sd (n) mean ± sd (n) mean ± sd (n) 

Site 1, Bush B 78.9 ± 0.8 (3) 0.232 ± 0.027 (3) 2.0 ± 0.1 (3) nd 

Site l, Bush C 85.0 ± 2.6 (6) 0.092 ± 0.030 (5) 1 .7 ± 0.3 (4) 3 .12 ± 0.49 (6) 

Site 1, Bush D 76.8 ± 1 .6 (6) 0.170 ± 0.072 (6) 2.7, 2.8 (2) 3.17 ± 0.65 (4) 

Site 2, Bush A 85 .6 ± 3.4 (17) 0.105 ± 0.047 (14) 1 .6 ± 0.8 (17) 3.30 ± 0.53 (8) 

Site 2, Bush B 83 .1 ± 4.5 (7) 0.181 ± 0.036 (6) 2.1 ± 0.7 (7) nd 

Site 2, Bush C 86.6 ± 1 .5 (10) 0.107 ± 0.023 (10) 1 .3 ± 0.3 (4) 3.06 ± 0.54 (6) 

Site 2, Bush D 72.4 ± 1 .8 (4) 0.201 ± 0.033 (4) 4 .7, 5.2 (2) 3.32 ± 0.62 (4) 

Site 3, Bush A 80.1 ± 4.1 (6) 0.154 ± 0.037 (6) 2 .4 ± 1 .2 (6) nd 

Site 3, Bush B 78 .3 ± 1 .0 (8) 0.216 ± 0.042 (8) 2 .8 ± 0.8 (7) 3.35 ± 0.80 (6) 

Total Range 70.4-91 .3 (68) 0.047-0 .324 (63) 0.4-5 .2 (53) 2.14-4.24 (35) 

5-19 
For each of the other three condition indicators analyzed (water content, ashfree dry weight/volume, and lipid content), bush of origin was found to explain 42 to 

71% of the variation in the indicator for both species (Table 5 .5), and size alone was 

found to explain between 0 and 30% . The combined models with the most 

explanatory power were determined by following stepwise GLM procedures (SAS 

version 6 .07; Table 5.5) . The bush an individual was collected from explains more 

about its condition than any single size variable, and only slightly less than a model 

that includes a size variable along with bush of origin (Table 5 .5). Thus, although 

some of the variation in the condition indicators in the vestimentiferans may be due 

to the size (and possibly age) of the animals, most of the variation is a reflection of 

the collection site and presumably the microhabitat conditions experienced by the 

animals. This implies that there are significant differences in the average 

environmental conditions experienced by vestimentiferans at different sites and that 

future work with a more intense water sampling program would provide valuable data 

that would directly relate environmental chemistry to vestimentiferan condition. 

Finally, all condition and size data were analyzed to determine if a size or 

condition indicator differential existed between the two seep vestimentiferan species. 

Using t-tests to compare all collected individuals of both species, Escacrpia sp. were 

found to be significantly shorter in tube length (55 .7 ± 25 .2 cm vs. 99.8 ± 50 .7 cm for 

Lamellibrczchia sp., p<_0 .0001), heavier in ash-free dry weight (2 .28 ± 1 .26 g vs . 1.51 ± 

1 .13 g for Lamellibrachia sp., p = 0 .00 14), and wider in anterior opening interior 

diameter (8 .24 ± 1.55 mm vs . 6.34 ± 1 .42 mm for Lamellibrachia sp ., p<_0 .0001) . 

Comparing condition variables between the two species with t-tests (Table 5 .4), 

Escarpia sp . was found to have significantly (p<_0 .0001) lower water content, higher 

ash-free dry weight/volume, and higher lipid content. Comparisons of escarpids and 

lamellibrachids collected from the same bushes corroborated these findings for 

individuals from the same site . 

5-20 
Table 5. 5 Condition indicator models for seep vestimentiferans . The explanatory power of the 

best statistical model*, determined using the stepwise General Linear Models 

procedures outlined in the text, as well as a single-variable General Linear Model of 

Bush of origin alone for each condition indicator for each species is indicated here by 

the expressed r2 values and associated p-values (Table 5.5a: Lamellibrachia sp., 

Table 5.5b : Escarpia sp. ) . The variables included in each model are abbreviated as 

follows : Bush of origin - Bush; Inner diameter of anterior increment = ID ; and Tube 

volume = Volume. 

a Lamellibrachia SD. 

Water Content AFDW/Volume Lipid Content 

Variables in Model Variables in Model Variables in Model 

Model r2 (p-value) Model r2 (p-value) Model r2 (p-value) 

hush Bush Bush 

0.717 ( :5 0.0001) 0.664 (<_ 0.0001) 0.536 (<_ 0.0001) 

Bush. ID* Bush, Wet Weieht, Volume* Bush, Volume* 

0.778 (5 0.0001) 0.782 (!5 0.0001) 0.666 (!5 0.0001) 

b. Escarnia SD. 

Water Content AFDW/Volume Lipid Content 

Variables in Model Variables in Model Variables in Model 

Model r2 (p-value) Model r2 (p-value) Model r2 (p-value) 

llush* DIUh hush 

0.552 (< 0.0001) 0.655 (5 0.0001) 0.425 (0.0265) 

Bush. Wet Weight. Volume* Bush. ID* 

0.739 (<_ 0.0001) 0.521 (0.0096) 

5.8 Results for Vesicomyid Clams: Vesicomya chordates and Calyptogena 

ponderosa 

The two species of vesicomyid clams associated with the hydrocarbon seeps 

contain abundant symbiotic chemoautotrophic bacteria in their gills (Brooks et al. 

1987). These bacteria use the same substrates as the symbionts of the tube worms 

- sulfide, oxygen, and dissolved inorganic carbon. The clams, however, are mobile 

5-21 
(Rosman et al . 1987) and while they acquire oxygen and C02 by normal means 

(across their gills), they apparently take up sulfide across the surface of their highly 

extensible and well vascularized foot (Arp et al. 1984; Fisher 1990). This is possible 

because their blood contains a sulfide binding protein, which has both a high affinity 

and capacity for sulfide (vesicomyids are able to concentrate sulfide by almost three 

orders of magnitude over environmental concentrations and the blood can bind sulfide 

to concentrations of 10 mM) (Arp et al. 1984). Based on our knowledge of these 

animals' physiology and ecology, we built and deployed clam corrals to contain 

marked clams in a 100 sq. foot area that we could characterize and return to in order 

to conduct growth experiments under a variety of environmentally relevant 

conditions. However, most clams did not survive the treatment (collection, recovery, 

marking, and redeployment). Only one experimental clam survived the treatment, 

and it did not grow a measurable amount in the following year. A second try in the 

subsequent year yielded similar results, with some indications of collection methods 

with better survival rate. This approach is still under study, however, for the 

purposes of this project we elected to apply our efforts to analyzing the microhabitat 

required by these clams by analysis of interstitial water from their habitat and 

analysis of the blood of freshly collected clams. 

In areas where vesicomyids are found on the Louisiana slope, they are 

distributed in patches and present in relatively low density, compared to tube worms 

and mussels on the Louisiana slope or vesicomyids at hydrothermal vents . 

Therefore, a study of this type consumes a substantial quantity of submersible time. 

Over the two years of the study, approximately 12 dives were made to collect clams 

and/or water samples from the vicinity of living clams. On some dives, only one or 

5-22 
two living clams were collected, which again emphasizes the patchy and low density 

distribution of these clams . Additional data on the chemistry of water around the 

clams can be found in the section on the mini push core arrays. One qualitative 

observation made by several observers was that V. chordate is most often found 

singly, leaving long trails in the sediment as it moves, and that C. ponderosa is often 

found is small aggregations of a few individuals near small carbonate outcrops with no 

significant trails around them. This would suggest that C. ponderosa is associated 

with more active seep environments than V. chordate. We were unable to confirm 

this with the small number of water samples analyzed in 1992 and 1993. However, it 

is clear that both species live in relatively benign environments where sulfide 

distribution is patchy and concentrations are low, even in water from 10 cm depth 

(Table 5.6). Sulfide was never detectable around the siphons of the clams (1-2 cm 

above the sediment), which also suggests that seepage is slow in these areas and that 

the clams are obtaining their sulfide through their vascularized feet, as has been 

proposed for their hydrothermal vent relatives . Analysis of the blood of the freshly 

collected clams supports the suggestion that C. ponderosa lives in an environment of 

higher sulfide concentrations than does V chordate . The blood of eight of nine 

specimens of V chordates analyzed contained no detectable sulfide and the other 

contained 59 ~tm sulfide. One of the two C. ponderosa had 322 p.m sulfide in its blood 

and sulfide was undetectable in the blood of the other. These data also support the 

general conclusions concerning the vesicomyid environments given above. For a full 

discussion of interpretation of vesicomyid blood sulfide levels and comparisons of seep 

and vent species, the reader is referred to Childress and Fisher (1992) and Scott and 

Fisher (submitted). 

5-23 
Table 5.6 Sulfide levels in Louisiana slope clam habitats and freshly collected blood. 

Sample type Mean (WM) Range (a) n(UD) n(total) 

Sediment surface UD 2 2 

5 cm interstitial 2 UD-15 6 7 

10 cm interstitial 36 14, 58 0 2 

Yesicomya chordate blood 107 UD-322 2 3 

Calyptogena ponderosa blood 7 UD-59 8 9 

UD=undetected ; detection limits for dissolved sulfide analyzed by gas chromatography were 1-3 pM 

depending somewhat on sample size. 

5.9 Results for the Methane Mussel: Seep Mytilid Ia (SM Ia) 

There are apparently at least 5 different species of symbiont-containing 

mytilids present associated with cold seeps in the Gulf of Mexico (Craddock et al., in 

press, Fisher 1993). None of these has yet been formally described, but their 

differences are apparent from physiological and molecular investigations (Fisher 

1993, Craddock et al., in press). Two of these species are present at the shallow seep 

sites, SM Ia and SM 3. SM 3 is very rare, has sulfur-oxidizing symbionts, and has 

never constituted more than about 0 .1% of the mussels collected from any station. 

The work presented here and most previous work has been on SM Ia. 

Seep Mytilid Ia contains methane oxidizing symbiotic bacteria in its gills ; the 

symbionts apparently provide the bulk of the animals' nutrition (Childress et al. 

1986; Fisher et al . 1987) . Because of the relative ease in handling methane (as 

compared to hydrogen sulfide), this species has proven relatively amenable to 

laboratory maintenance and study, and we have a lot of information on the 

physiology of both the intact mussel and the isolated symbionts. None-the-less the 

slow decline in the symbiont population in the laboratory, the sensitivity of the 

animals' growth and metabolism to methane and oxygen concentrations, and our 

5-24 
uncertainty of the microhabitat conditions the mussels are actually exposed to in situ 

required that growth rates be determined in the field. We used the quantitative 

collection rings (designed and built for this study), a variety of new water sampling 

apparatus (built for this study), unique marking methods (developed, tested, and 

employed for this study), and special temperature insulated deployment containers 

(designed for this study), which are described in a previous section. The mussels 

proved to be the most amenable to study of any of the symbiont-containing fauna, 

and we were able to characterize the population structure, growth rates, physiological 

condition, and microhabitats of three different areas with mussel communities (with 

data from a total of five mussel beds) . 

5.9.1 The Mussel Beds 

5.9.1.1 GC 184 Mussel Beds 

Sulfide was undetectable in the water samples from both experimental mussel 

beds in 1993 . Methane levels in the samples from the two beds spanned a similar, 

relatively low range of undetectable to <60 ~tM, in both 1992 and 1993 (Table 5 .7). 

Streams of bubbles (presumably methane) as well as large globules of oil were 

observed within both beds during the mussel collections . The retrieval container 

mounted on the sub, as well as the personnel handling the organisms, were always 

covered in an oil residue after recovery of mussels. 

Larger mussels predominated at this site, with the majority >55 mm in length 

and none <9 mm (Figure 5 .2a). Mussel density in the GC 184 bed was calculated to 

dbe 881 mussels m-2 based on the ring collection. Using the density and size 

frequency data from the ring collection, and the allometric equation derived from the 

logarithmic regression of tissue wet weight and shell length in 120 mussels between 

12 and 110 mm shell length [Figure 5.3: Tissue wet weight (g) = 2 .88 x 10-5 (shell 

length (mm))3.162], the soft tissue biomass for this bed was calculated to be 11 .3 kg 

5-25 
Table 5 .7 Methane concentrations in water samples from mussel beds. 

Mussel Bed Sampling Methane Concentration 

Depth (gm) 

(cm) 1992 1993 

GC 184: ring 0 56 ND 

5 ND 2 2 

10 - - 

GC 184 : growth 0 - 20 

5 ND ND 

10 10 - 

GC 272: ring 0 - 2 

5 - 4 

10 - - 

GC 272: growth 0 - 2 

5 35, 141, 290 4 

10 822, 27 -

GC 234 0 - 72, 832 

5 15, 16, 772, 802, 10744, 

869, 75 9466 

10 ND, 986 - 

ND = > non-detectable 

- - _ > not sampled 

5-26 
40 

30 

20 

10 

0 

160 

150 

140 

130 

120 

N 

110 

100 

90 

80 

i. 

70 

L 60 

Z so 

40 

30 

20 

10 

0 

40 

30 

20 

10 

0 

Figure 52 Size frequency distributions for SM Ia at three hydrocarbon seep sites. 

a) GC 184 (in 1992), b) GC 234 (in 1992), and c) GC 272 (in 1993): Total 

numbers from are -0.5 m ring, densities are given in the text. 

5-27 

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 

Shell Length (mm) 
100 

80 

60 

d 40 

20 

0 

0 20 40 60 80 100 120 

Shell Length (mm) 

Figure 5.3 Relation between wet weight and shell length of SM Ia. The equation 

that best describes the relation is: 

Tissue wet weight (g) = 228 x 10-5 [shell length (mm)]3.162 

5-28 
m-2 . Since the dissolved gas concentrations, and size ranges and distributions of 

mussels from both beds were similar, the beds are considered together in the 

discussion of "GC 184" mussels. 

5.9.1 .2 GC 234 Mussel Beds 

Both sulfide and methane levels in the water samples from the GC 234 beds 

were relatively high, especially in the samples taken 5 cm below the top of the 

mussels (between and below the shells), where sulfide and methane concentrations 

approached 8 and 11 mM, respectively (Table 5 .7) . Oil globules and streams of 

bubbles were also observed within these beds during sampling. The size distribution 

for the two beds sampled at GC 234 was dramatically different from the other two 

sites in that most of the mussels here were 15 to 40 mm long, with only a few larger 

mussels (Figure 5.2b). Mussel density for the bed sampled at GC 234 was calculated 

to be 3560 mussels m-2, with living biomass calculated to be 7.3 kg m-2. Since the 

dissolved gas concentrations, and size ranges and distributions of mussels from both 

beds were similar, the beds are considered together in the discussion of mussels from 

GC 234. 

5.9.1.3 GC 272 Mussel Bed 

Sulfide was not detected in any water samples from this mussel bed and 

methane levels decreased dramatically from 1992 (35 to 822 ~tM) to 1993 (2 - 4 ~LM). 

In contrast to the other two sites, no oil globules were released while collecting and 

neither the submersible or investigators were oil stained after handling the mussels. 

Gas bubbles were not observed in this mussel bed. This bed is dominated by large 

mussels, 78 - 105 mm long, which likely represent the last major settlement period at 

this site (Figure 5.2c). Mussel density in the bed was calculated to be 876 mussels 

m-2, with calculated mussel biomass of 37 .7 kg m-2~ 

5-29 
5 .9.2 SM Ia Population Structure 

The population structure of mussels from the three sites was strikingly 

different (Figure 5 .2). Clearly the beds sampled at GC 234 are in an active 

settlement phase, and the bed at GC 272 has not seen successful recruitment in 

many years (the GC 184 beds show an intermediate pattern) . It is not clear why 

there would not have been recruitment to GC 272 prior to 1992 as all measurements 

there (chemistry, growth rates, and condition indices) suggest that this was an 

excellent habitat at that time . The mussels there are currently relatively isolated 

from the rest of the population and larval supply there may have been limiting. 

Alternatively, if the variation in conditions documented during this study are 

characteristic of the recent history of this bed, this may explain the lack of juveniles 

there as they would be much more sensitive to significant perturbations in nutrient 

supply due to their higher metabolic demands. When one considers the spatial and 

temporal variation in seepage and the reliance of the mussels on methane in the 

seeping fluid, even large differences in population structure of mussel beds, which are 

only meters apart, is not surprising (MacDonald et al. 1990a). 

5.9.3 SM Ia Chemical Microhabitat 

The use of the new water sampling tools described here allowed collection of 

discrete water samples from specific points in and over the mussel beds, without the 

extensive dilution by ambient water, which had plagued previous efforts of this sort 

(personal observation; Fisher 1993). One fact that became abundantly clear in 

evaluation of the present data set (and confirmed by subsequent data sets) is that 

the presence of "seep fluid" is patchy. Even on scales of 0.1 m within a mussel bed, 

there are substantial vertical gradients on scales of centimeters as well. Because it 

is not known whether the mussels are able to acquire dissolved methane from their 

5-30 
siphons only or through their pedal gape as well, it is not clear exactly what methane 

concentrations they are actually exposed to. However, clear differences in methane 

and sulfide levels among the mussels at the different sites were evident, as was a 

change in conditions at GC 272 between 1992 and 1993. 

Methane concentrations measured in the two experimental beds at GC 184 

were consistently in the range 0 - 56 ~tM over both years (Table 5.7). Based on the 

laboratory studies of Kochevar et al . (1992), methane concentrations between 20 and 

60 ~tM are barely adequate for growth of this mussel, with methane as the major 

carbon and energy source . Although growth was easily detectable in the population 

as a whole (Table 5.8), the lack of detectable growth in over 1/3 of the mussels less 

than 60 mm in shell length (Figure 5.4a) supports the suggestion that nutrient levels 

(mainly methane) were barely sufficient for maintenance of this population. The low 

environmental methane levels are also reflected in the physiological condition of the 

mussels from GC 184. Although the only year in which they were significantly 

different from the other sites was in 1991, the average values for the GC 184 

collections indicated that those mussels were on the bottom of the condition scale 

every year. 

At the other end of the spectrum, methane concentrations in 5 cm interstitial 

water from the GC 234 beds were as high as 10744 ~tM, and a concentration of 832 

gM was documented around the siphons of mussels in 1993 (Table 5 .7) . The highest 

specific growth rates and average adult growth rates were found in the GC 234 

mussels for the 1992-1993 period (Tables 5 .9 and 5 .10) . The average values of the 

condition parameters were in general intermediate between those of the GC 184 and 

GC 272 mussels, with the GC 272 mussels having significantly higher condition index, 

lipid content, and significantly lower water content in both 1991 and 1992. This data 

implies that factors other than methane may be affecting the health of the mussels . 

5-31 
Table 5.8 Average growth rate of SMIa in size range 60 - 90 mm shell length from 3 

hydrocarbon seep sites and 2 transplant experiments . Rates were determined two 

ways: absolute growth, the measured change in length per time (between 340 and 

320 days) and normalized to a per year basis, and the specific growth rate, the 

measured change in length per time per unit length (calculated as a percent 

increase), also normalized to a per year basis. Standard deviation of the mean given 

in parenthesis. Values that are underlined are significantly different from the other 

two sites (p < 0 .05) . Sites marked with an asterisk are significantly different 

between years (p < 0.05). Growth at transplant I was significantly different from 

GC 184 over the same time period ( p < 0.01). 

Site 1991 - 1992 Growth n 1992 - 1993 Growth n 

Absolute Specific Absolute Specific 

(mm/Yr" ) (%/yT.) (IritTVyT .) (%lyT.) 

GC 184 0.1 (0.9) 0.2 (1.4) 31 0.4 (0.4) 0.6 (0.6) 31 

*GC 234 0.4(l .4) 0.4 (1.9) 21 1.21 2.9 11.1 21 

*GC 272 1 .7 (1 .2) 1 .9 (.1 ..7) 9 0.5 (0.6) 0.6 (1.2) 2 

Transplant I 1.1 (1 .0) 1.6 (1.4) 16 ---------- ----------- - 

Transplant II -0.2(0.4) -0.3(0.7) 14 ---------- ----------- - 

Table 5.9 Growth parameters of SMIa derived from Ford-Walford plots. 

Site K L Ma. 

1992 1993 1992 1993 

GC 234 0.205 0.210 74 .37 74.26 

GC 184 0.016 0.016 103.14 104.4 

GC 184 Transplant I 0.108 -------- 83.74 -------- 

5-32 
Table 5.10 Condition indices and size range for SMIa at three hydrocarbon seep sites and two 

transplant experiments . Values for each site and year are averages (±SD) . 

Underlined values were significantly different (p < 0.005) from collections at the 

same site in other years. Values marked with an asterisk are significantly different 

from values at the other sites in the same year (p < 0.01). The set of values 

underlined with a dashed line (% water in GC 234 in 1991 and 1992) were 

significantly different from each other (p < 0.01). Transplant values marked with a 

+ were significantly different (p < 0.05) from GC 184 values in the same year. $ 

Lengths between sites were significantly different every year (p < 0.01 between GC 

272 and the other two sites and p < 0.05 between GC 184 and GC 234). Lengths 

were not significantly different within the sites between years. TP = Transplant, 

WW = wet weight. 

Collection Glycogen Condition Water Length (mm)t n 

Site Year (% WW) Index (% WW) Range Average 

(g / ml) 

GC 184 - 1991 0.88 (0.63) 

GC 184 - 1992 1.13 (0.88) 0.053 (0.018) 

GC 184 - 1993 0.73 (0.44) 0.051 (0.010) 

GC 234 - 1991 1.55 (0.82) *0.061 (0.018) 

CG 234 - 1992 1.50 (0 .96) 0.062 (0.019) 

GC 234 - 1993 1.33 (0 .73) 0.062 (0.020) 

GC 272 - 1991 *2.94 (1.19) *0.079 (0.024) 

GC 272 - 1992 *3.58 (1 .24) *0.105 (0.025) 

GC 272 -1993 1 .07 (0.66) 0.056 (0.011) 

TP 1 -1992 1.47 (0.73) 0.061 (0.13) 

TP II -1992 0.41 (0.31) +0.036 (0.012) 

*90.4 (2.03) 58-83 73 (8.3) 17 

90.8 (2.28) 27-93 68 (15.7) 32 

90.5 (125) 59-92 74 (10.5) 12 

*.8 .8 .1-U-84) 47-75 67 (7.9) 20 

.9_9,3 . .0.92) 36-81 60 (13.9) 29 

89.3 (2.84) 45-83 63 (12.1) 12 

*86.0 (2.61) 72- 105 90 (9.6) 20 

*85.3 (2.43) 53- 116 91 (15.3) 30 

90.0 (1 .45) 64- 110 92 (11.5) 11 

89.9 (1.6) 52-92 70 (10.6) 6 

+93 .3 (1.28) 50-94 68 (11.1) 6 

5-33 
s 

., 

4 

0 o =O 0000 

o 

O ~NNUM~M~N~NIpM~NMN~no~""~~NO~NMN~NU~N. 

N p. ~" UY . O~ : 

-t 

-2 

so 0 

1$ o 

.-.16 

o 

s 12 4 

+r o 

C 10 ~ o 

o 0 

J g o 

.C o o0 ~ c 

o 0 0 

4 6060 1B 

o 

0 

60019 00 

Iyw11+1~11

o 

1~ .11

, 

0 .IIII.IIIII .....IIIII . .NIII..II "q4

. 

11H

.

.11

...1 O 

-2 

6 C 

4 

2 ~ . . b36 

O NN ...I..... N~~Q.I....1.I..I. N. . h....1..1111........ 

I.....I. ~MN 

u- V 

-2 

0 20 40 60 80 100 120 

initial Length (mm) 

Figure 5.4 Change in length as a function of initial shell length for SM Ia from three 

hydrocarbon seep sites. Solid circles = change in length from 1991 to 

1992 (365 days) as a function of 1991 length, Open circles = change in 

length from 1992 to 1993 (-321 days) as a function of 1992 length, a) 

GC 184, b) GC 234, and c) GC 272 . 

5-34 
Certainly the sulfide levels measured in these beds were the highest measured in this 

study, and all collections from this site were oil stained. 

At the single bed investigated at GC 272, the five water samples taken in 1992 

ranged from 27 to 822 p.M CH4 and the four taken in 1993 from 2 to 4 gM CH4. This 

indicates a substantial change in the microhabitat of the mussel bed, from sufficient 

to marginal methane concentrations . This was strongly reflected in the reduced 

growth rates and significant drop in the physiological condition of the population in 

1993 (Figure 5 .4c, Tables 5 .9 and 5 .10). In 1991 and 1992, all three condition indices 

indicated that the mussels from GC 272 were in the best condition of any of the 

collections, although methane levels were higher at GC 234 in 1992 . No crude 

hydrocarbon seepage associated with the GC 272 bed was apparent. The 

bioaccumulation of hydrocarbons in mussel tissue is known to adversely affect 

growth rates, reproductive processes, and nutritive storage capabilities (Bayne et al . 

1982; Lowe and Pipe 1986; Stromgren and Nielsen 1991; Stromgren et al. 1986) . As 

previously mentioned, crude hydrocarbons are abundant in both the GC 234 and GC 

184 mussel beds . Wade et al. (1989) found that hydrocarbon loading (mostly 

polycyclic aromatics) of animals at the seeps is high, especially for the sessile 

organisms . Among mussels examined from several beds in all three sites, McDonald 

(1990) found that the tissue hydrocarbon concentrations were highest at the GC 234 

site. Although McDonald (1990) demonstrated a cytochrome P450-dependent 

enzymatic detoxification mechanism at relatively high activities in SMIa (McDonald 

1990), this adaptation to their potentially toxic environment is likely to exert a 

substantial metabolic cost on seep mussels inhabiting areas with high hydrocarbon 

levels, and may explain the better condition of the GC 272 mussels. 

5-35 
5.9 .4 growth Rates and Ages of SM la 

The average growth rates, measured as increments in shell length, for adult 

SM Ia are roughly equal to growth rates reported for healthy mussels from littoral 

environments at similar temperatures (5-8°C) (Forster 1981; Sukhotin and 

Kulakowski 1992 ; Theisen 1973) . These rates (Table 5.8) are over an order of 

magnitude higher than rates reported for a small deposit-feeding deep-sea bivalve 

(0.084 mm yrl; Turekian et al . 1975), but are lower than those of the related vent 

mytilid, Bathymodiolus thermoplzilus, which grows at rates similar to littoral mytilids 

from similar temperatures (about 1 cm yr-1 at 10-20°C) (Lutz et al. 1985 ; Rhoads et 

al . 1981; Stromgren and Cary 1984). Unlike most deep-sea environments, vents and 

seeps are very productive (as a result of chemoautotrophy and methanotrophy) and 

thus, as long as the sites remain active, the fauna are less limited by nutrient supply 

than the surrounding slope benthos (see reviews by Fisher 1990 ; Lutz and Kennish 

1993; Tunnicliffe 1991). 

Like most animals, the juveniles grow much more rapidly, although mussels 

over a year old are still growing at rates over 1 cm yr-1 at some sites (Figures 5 .4b 

and 5 .5b). This is likely driven by allometric changes in size specific respiration rates 

and resource allocation to reproductive effort (Reiss 1989; Seed and Suchanek 1992; 

Thompson 1984). The fast growth rates of juveniles may also be of ecological 

importance to the species . There are several possible predators at the seeps, 

including several decapods, starfish, and fish (Carney et al. submitted) . These types 

of predators forage selectively on a size range of mussels in temperate habitats (Seed 

and Suchanek 1992), and likely behave similarly at seeps. The shells of these 

mytilids are relatively delicate (especially those below -15 mm shell length) and are, 

therefore, potentially easy prey. Rapid growth of the juveniles will shorten the time 

period of maximum vulnerability to selective predation (Seed and Suchanek 1992) . 

On the other hand, the slow growth rates of adults suggests that these bivalves may 

5-36 
to 

W 

v 

100 

90 

80 

E 70 

.c so 

.. 

rn 

d 50 

J 

co 40 p 

+r 

"c 30 

20 

10 0 

0 

Figure 5.5 Age estimations for SM Ia from a) GC 234 (open circles = 1991, solid circles = 1992) and b) GC 184 

(open circles =- 1991, solid circles = 1992, solid triangles = transplant I) . Estimates were calculated 

using the von Bertalanffy growth equation, Lt - Lmax [1-e-kt-toy, parameters for the equation, derived 

from a Ford-Walford plot, are given in Table 5.3. 

10 20 30 40 

Age (yrs) 

0 20 40 60 80 100 120 140 160 

Age (yrs) 
live a long time after reaching reproductive competence, even under sub-optimal 

conditions. 

Since the growth rate varied with initial length, statistical comparisons 

between sites were limited to the mature mussels between 60 and 90 mm in shell 

length (mussels in this size range were found at all sites and growth rate does not 

vary significantly as a function of shell length for mussels in this range). Both the 

average absolute growth rates and the average size specific growth rates (DL ~ T-1 ~ L1 ; Almada-Villela 1984; Wootton 1991) were compared using one-way analysis of 

variance. Statistical differences and trends between sites and years were the same 

for both methods and only absolute growth rates are discussed . 

No significant difference was found between the average growth rates of 

mature mussels at GC 184 and GC 234 in the 1991 to 1992 interval, but the average 

rate in GC 234 was higher in the 1992 to 1993 interval (p<0 .01). Comparisons of GC 

272 to GC 184 and GC 234 indicated a faster average rate at GC 272 (p<0.01) in 

1991/1992; however, in 1992/1993, the average growth rate at GC 272 fell sharply 

and was significantly lower than GC 234 (p<0.05) yet not significantly different from 

GC 184. 

Bivalve ages are commonly estimated by means of either the von Bertalanffy 

or the Gompertz growth equations (Bayne and Worrall 1980 ; Craeymeersch et al . 

1986; Rhoads et al . 1981). The von Bertalanffy equation, (Lt = Lmax [ 1-e-k(t-to)], 

where Lt is the shell length at the beginning of the experiment, LmaX is the 

extrapolated point of no growth, k is the rate at which Lmax is approached and t is 

time), provides a better description of growth for a population of more mature 

animals (Powell and Somero 1986 ; Seed and Richardson 1990) and the Gompertz 

equation, (logio Lt = logi0 LmaX (1-e-k(t-to)])' gives better descriptions of growth of 

juveniles (Theisen 1973). Due to the size disparity between the sites, ages were 

estimated for each population using both techniques . The age estimates from these 

5-38 
two equations were not notably different at GC 234 . However, at GC 184, the 

Gompertz equation yielded much greater ages (up to 350 years old) than did the von 

Bertalanffy equation. This is likely a reflection of poor representation of smaller 

mussels at this site. Therefore, the results shown in Figure 5 .5 are age estimations 

based on the von Bertalanffy growth equation. The parameters, Ln,a_, and k are 

estimated from a Ford-Walford plot (Craeymeersch et al . 1986; Seed and Richardson 

1990) and are given in Table 5 .9 . These parameters are obtained by plotting the 

length at a given time against the length a year before (Bayne and Worrall 1980). 

Age estimations for GC 234 and GC 184 are shown in Figures 5.5a and 5.5b, 

respectively, and were not calculated for GC 272 due to the complete lack of mussels 

below 50 mm in length (Figure 5 .4c) . 

The variable nature of methane seepage (Behrens 1988; Roberts et al . 1990) 

makes evaluation of the calculated ages problematic. Although this variability is on 

much longer scales than that documented around hydrothermal vent fauna (where 

significant differences in the chemical environment around animals can occur on 

scales of seconds to minutes, Johnson et al . 1988), it can certainly be significant on 

scales relevant to age calculations. For example, if one assumes that conditions have 

been constant at the GC 184 site for the past 160 years, then there are likely 

mussels present there at least that old . However, the strong possibility of changing 

conditions within a bed (as clearly documented for GC 272 within 3 years) suggests 

that the mussels there may have reached a significant portion of their present size 

under different (more favorable) environmental conditions, and that the calculated 

ages are, therefore, an overestimation . This possibility was empirically demonstrated 

by one of the transplant experiments where mussels from the GC 184 site were 

moved <3 meters and left for one year (transplant I, see section 5.4.6). Maximum 

ages calculated from the shell growth of the transplanted GC 184 mussels (Figure 

5 .6) dropped to 40 years (from 160 years for the source population) (Figure 5 .5a). 

5-39 
6 

5 

4 

s *r 

3 

J 

C 

1 

,= 0 

U 

-1 

-2 

40 50 60 70 80 90 100 

Initial Length (mm) 

Figure 5 .6 Change in length as a function of initial shell length for two GC 184 

transplant experiments, open circles = transplant I, located 3 m away 

from the original GC 184 mussel bed, solid circles = transplant II, 

located 2 m away from the original GC 184 mussel bed. Duration of this 

study was 325 days . 

5-40 
Thus, the age calculations assume growth over the entire life span of the organism 

under the current conditions, which is unlikely. However, these calculations do 

provide information on growth capabilities . Data from GC 234 in 1992 (Figure 5b) 

indicates that SM Ia can grow quite rapidly, and data from GC 184 in 1992 suggests 

they can also grow quite slowly without significant effects on condition. Based on 

these considerations and the fact that mussels up to 105 mm were collected at GC 

272, it is reasonable to conclude that SM Ia can live for at least 50 years, and 

perhaps much longer. 

5 .9 .5 Measures of Condition of SM Ia 

No significant differences were found between marked and unmarked mussels 

in terms of glycogen, water content, and condition index in any collection from any site 

during the duration of the study. We conclude that any differences between sites or 

years are site specific and not the result of the marking procedure. 

Due to the wide size range of mussels at GC 234 (and not at the other two 

sites), data from this site was used to check possible effects of size on the variation in 

the condition parameters (data from the three years was combined in order to 

achieve a more representative size range) . Regression analyses indicated no 

significant relationship between shell length and condition index, water content, or 

glycogen. 

Table 5 .10 shows the mean values (±SD) of each of the condition parameters 

in each year for the three growth sites. Significant differences were demonstrated 

between GC 234 and GC 184 for water content and condition index in 1991 (p<0.01). 

No significant differences were found, for these parameters, between the sites in 1992 

and 1993 or in glycogen content in any year. GC 272 mussels had significantly higher 

glycogen contents (p<0 .05), higher condition indices (p<0 .01), and lower water 

contents (p<0 .01) than the other two sites in both 1991 and 1992, but not in 1993 . 

5-41 
Mean shell lengths of mussels from all sites were significantly different in each year 

(p<0.05), but mean lengths within a site were not significantly different among years. 

Two lines of reasoning support the utility of the condition indices. First, all 

significant differences in pairwise comparisons (between sites in a given year, or 

among years at a single site) were corroborated by all three indices, with one 

exception: GC 184 and GC 234 in 1991, where a significant difference was not 

demonstrated in glycogen content but was for the other two parameters . Thus, the 

three indices are internally consistent. Secondly, all three parameters were 

significantly correlated with growth rates measured in adults at GC 272 (Figure 5 .7). 

This data set was chosen because of the range in growth rates among adults there 

and to avoid a data set with a large size range of individuals (because of the allometric 

effects on growth rate and not the other parameters) . The largest r2 and smallest 

relative 95°lo confidence interval on the slopes of these regressions was that relating 

growth and CI . This is likely due to the fact that determination of tissue water 

content is sensitive to residual mantle water and, therefore, draining technique (which 

is accomplished in a cold van on board ship), whereas CI is not, and the fact that 

glycogen content may be more sensitive to reproductive state and hydrocarbon 

exposure (Bayne 1976; De Zwaan and Mathieu 1992; Hummel et al. 1989; Lowe and 

Pipe 1986), The significant correlation between the growth rates recorded for 

individual mussels, and their condition as assayed by 

our indices, corroborates the efficacy of our measures of physiological condition: the 

mussels with high condition indices are growing faster. 

5 .9 .6 Transplant Studies with SM I 

The transplant experiments provided additional insight into to the ecology of 

the mussels . The initial purpose of these experiments was to deprive the mussels of 

methane by placing them away from their food source in unoccupied mud, and follow 

5-42 
6 

5 

4 

d 3 

O 

v 2 

0 

Figure 5.? 

0.16 

0.14 

N 

0.12 

C 

O 0.10 

+. 

C 0.08 

0 

U 

0.06 

0.04 

92 

90 

88 

0 

86 

L 

84 

82 

SO 

-1 0 1 2 3 4 

Change in Length (mm) 

Relation between growth rate and condition indices in SM la from GC 272; data for 

1992 (open circles) and 1993 (solid circles) are combined for the linear regression. a) 

Glycogen as a function of change in length, regression equation: y = 0.73x +2.313 

(p<0.05), 95% CI on slope is ±0.67, b) Condition index as a function of change in 

length, regression equation: y = 0.020x + 0.076 (p<0.001), 95% CI on the slope is 

±0.01, c) Water content vs. change in length (mm), regression equation : y = -2.03x + 

87.88 (p<0.005) 95% CI on slope is ±1.15 . The slopes for each parameter were also 

significant (p<0.05) when tested with the 1992 data alone. 

5-43 
the effects on condition and growth. The results from transplant II (2 m from the 

original GC 184 site) were as expected . The average growth was -0.22 mm yr-1 (but 

was not significantly different from zero, p > 0.3), all condition indices fell dramatically 

(from the already low levels at GC 184), and in fact six of the 20 mussels were dead 

upon recovery. However, the mussels from transplant I, (located a little further 

away from the original bed and about 2 m from transplant II), were in better 

condition and grew at a faster rate (Figure 5.6, Table 5.8) than mussels from their bed 

of origin, and none of the 20 mussels died. The differences in growth rates in mussels 

that were only 2 m apart indicate that very localized effects are responsible. These 

differences are most likely differences in exposure to seeping fluid and since the 

transplant I mussels fared better than in their bed of origin, site chemistry alone is 

insufficient for establishment of a bed. As with intertidal mussels, another essential 

factor for the establishment of a bed is apparently substrate upon which to settle . 

Thus, the production of authigenic carbonates, which is often associated with seepage 

in the Gulf of Mexico (Brooks et al. 1990), can contribute to the production of habitats 

suitable for colonization by mussels. 

5 .9.7 Site Synopsis for SM la beds 

When all data and observations are considered together, one can make some 

general conclusions concerning each site. The GC 184 beds have been active for at 

least 40 years, with appropriate conditions for larval settlement over most of that 

period. However, methane concentrations are currently low in the bed (although a 

stream of methane bubbles issue from one point in the bed), growth rates were low, 

and the population was dominated by large mussels, with a calculated mussel 

biomass of 11 .3 kg m-2. A plausible explanation is that there has been a recent 

decline in the quality of the habitat within this particular bed, which is supported by 

the mussel condition indices. At the beds in GC 234, there was a preponderance of 

5-44 
young mussels, indicating a recent active settlement period, and methane 

concentrations were high, as were growth rates. The mussel biomass in this bed was 

only 7.3 kg m-2. However, the condition indices were significantly higher than for GC 

184 mussels only in 1991 and significantly lower than GC 272 in both 1991 and 1992. 

Thus, although there were plenty of young, rapidly growing mussels, there may be 

some factor detrimental to condition. We suggest that may be due to exposure to 

high hydrocarbon levels and this may exert a considerable cost to the seep mussels. 

At GC 272 on the other hand, oil was never apparent during the collections . The 

mussels here were growing at appreciable rates in 1992 and were exposed to 

moderate to high levels of methane and the mussel biomass in this bed (37 .7 kg m-2) 

was considerably higher than in the other beds. Condition of animals in both 1991 

and 1992 was excellent by all criteria . However in 1993, growth rates, condition, and 

methane concentrations in the bed had all decreased. The almost complete absence 

of juveniles here cannot be explained by the environmental measurements. We 

conclude that none of the beds studied here are providing "perfect" conditions for the 

mussels, and that sites with higher growth rates and productivity are likely present 

elsewhere. 

Community methane oxidation rates were determined for each of the ring 

collection beds. According to the model of Kochevar et al. (1992), the seep mussels 

have a basal metabolic rate of 1 ~Imol 02 glhr-1 and methane oxidation rates 

between 4 and 10 ~imol g-lhr-1 for freshly collected mussels with non-limiting 

methane. It should be noted that these rate determinations were made on mussels 

with an average wet weight of 23 g. Since metabolic rate varies allometrically with 

wet weight in Mytilus edulis (Bayne 1976; Reiss 1989), the allometric equation BMR 

= aWb (Reiss 1989) [where BMR = basal metabolic rate, W= wet weight, and a and b 

are derived parameters taken from literature on intertidal mussels; a=0.102, b=0 .71 

(Bayne and Newell 1983)] was applied to the above rates and the minimum mussel 

5-45 
bed oxidation rates necessary to sustain basal metabolism, and mussel bed methane 

consumption rates under optimal conditions, were estimated for each site using the 

quantitative ring collection data (the allometric relationship between length and wet 

weight from Figure 5 .3 was used to determine wet weights for individual mussels in 

each ring collection) . Minimum calculated mussel bed oxidation rates range from 2 .9 

g d'im'2 at GC 234 to 49.4 g d-lm-2 at GC 272 (GC 184 = 7 .2 g d-lm-2). Under 

optimal conditions (of 02 and CH4 concentrations), maximal methane consumption 

rates range from 14 .6 g d-1 m-2 at GC 234 to 197.7 g d-1 m-2 at GC 272 (GC 184 = 

36 .2 g d-1 m-2). These values are in the same range as those calculated by Fisher 

(1993) and are several orders of magnitude higher than any other aquatic system 

(Fisher 1993 ; Frenzel et al : 1990; Kuivila et al. 1988). 

5 .10 Implications of Growth Experiments and Habitat Characterizations on 

MMS Management Concerns 

One of the main purposes of the studies described above was to address MMS 

management concerns on the robustness of these various communities . The very 

slow growth rates and long lives of the vestimentiferans indicates that a community 

destroyed by a physical disturbance would not recover in 100 years . In fact, as these 

animals require a hard exposed substrate to settle on, and very active seepage at 

that point in space, sedimented communities that are physically damaged would 

likely never recover. 

The vesicomyid clams, on the other hand, are sparsely distributed and mobile. 

It is unlikely that a physical disturbance would impact either the population in the 

area or the habitat they require. 

The bathymodiolid mussels, which use methane as a carbon source (seep 

mytilid Ia), have strict chemical requirements that tie them tightly to areas of the 

most active seepage in the Gulf of Mexico. Where conditions are right, they consume 

5-46 
methane at very high rates and produce biomass at rates that could be significant to 

the surrounding deep-sea fauna. Both individual mussels and communities appear to 

be relatively long lived, although the mussels are also capable of growing to 

reproductive size very quickly. Physical disturbance of an active mussel bed is 

unlikely to have a long lasting effect on the community due to the high growth rates of 

the individuals. Furthermore, the unexpected results of the transplant experiments 

suggests that the mussel distribution is limited by hard substrate and these animals 

may even increase in number if that substrate is supplied on the ocean floor. 

5-47 
6.0 Sessile Macrofauna and Megafauna at Mussel Beds 

Robert S. Carney 

6.1 Introduction 

Prior to initiation of systematic sampling, it was generally expected that the 

fauna associated with the upper continental slope seeps off Louisiana would show 

some difference in composition between sites, among mussel beds within a single 

site, and even between the outer and inner portions of individual beds . These 

expectations can be developed both from succession and from island zoogeographic 

theory. If there is a succession in the fauna of mussel beds, then beds and positions 

of beds at different ages should also have a distinctive fauna. Alternately, if there is 

only random immigration from the normal background or distant seeps, then 

different mats and sites should show a faunal difference . The results of 61 Johnson 

Sea-Link grabs, detailed below, support neither succession nor island zoogeographic 

models . Edge to inner, bed to bed, and site to site, there is a monotonous 

consistency in the species compliment of the seeps . 

Originally intended as an analysis of variance exercise looking at substrate, 

site, individual bed, and edge versus interior of the bed differences, in practice this 

study of the larger bed fauna ended as a general descriptive effort. Several aspects 

of the seep environment contributed to this. First, it is not feasible to target the 

substrate type during sampling. Except for the Brine Pool area, all beds proved to 

be underlain by rock, even if hidden by a thin veneer of mud. Second, it proved very 

difficult to locate patches suitable for sampling. Only a single mussel area was 

located at GC 272, and it was too small to sustain repeated sampling for a variety of 

purposes. Even at Bush Hill and GC 234, where beds seem to be abundant, finding 

a bed more than a few centimeters across accessible to the submersible was difficult . 

6-1 
Finally, even locating a definitive edge and inner area was difficult . However, these 

sampling problems did not mask the basic finding of a monotonous fauna . 

6.2 Methods and Design 

As originally envisioned and designed, megafaunal sampling would take the 

form of an analysis of variance allowing conclusions to be drawn about composition 

and abundance differences within between seep sites, within between different 

types of patches (mussel beds and tube worm clumps), and within between the 

interior portions of a bed and the outer edge. The underlying questions all 

addressed the issue of how variable seep community composition was in space and, 

to some degree, in time . Unfortunately, we encountered three major problems in 

sampling for differences between types of patches. First, we could not devise a 

means of sampling either tubeworm clumps or clam beds that would assure capture 

of associated organisms. The worm clumps proved too large and well anchored for 

the methods available to us, while finding live clam beds suitable for sampling 

proved an elusive task. Second, my preliminary classification of mussel beds as 

being on rock or on sediment proved to be useless in the field ; beds that appeared to 

be on mud usually proved to be on rock. Third, there was insufficient bottom time 

to search for appropriate bed types for sampling. As a result, this final report is 

restricted to the fauna of the mussel beds, which were well sampled. With the 

exception of the Brine Pool bed, all samples came from rock based mats. 

6.2 .1 Sampling Meth 

All megafaunal sampling used the grab of the Johnson Sect-Link (JSL) 

submersible . The JSL grab is an integral part of the hydraulic manipulator on the 

Johnson Sea-Link. It consists of two semi-cylindrical jaws 19.2 cm long and 17 .8 cm 

(T. Askew, personal communication) wide when open, which rotate closed about a 

6-2 
common axis . Under ideal conditions, the bite of the JSL grab samples 341 cm2 

square meters of bottom. The JSL grab is powerful, extremely easy to use and can 

take repeated samples in just a few minutes . The real problem with the grab is that 

samples must be transferred into the "critter Bitter" of the submersible . This is 

done by positioning the closed grab over a collection chamber and opening the grab 

so that the sample falls into the chamber . It is obvious that large amounts of 

sediment are lost during this transfer, and it is to be suspected that much of the 

smaller fauna is also lost. However, shelled and less mobile megafauna are 

successfully transferred . 

6.2.2 Sampling Design 

Due to the limitations of submersible sampling, the final sampling reflected 

opportunities that arose at the moment rather than prior planning. A total of 61 

JSL grabs suitable for analysis were taken at four sites, Bush Hill, Green Canyon 

272, Green Canyon 234, and Brine Pool NR-1 (Table 6.1) . Sampling two different 

beds at each site proved impractical due to long search times, therefore all grabs 

were taken either from the same large bed at intervals separated by at least two 

meters or at adjacent small beds. In most instances, the planned sampling at the 

interior and at the edge was carried out, while at other sites bed geometry 

prevented this . All sampling was paired with two grabs being taken less than 50 

cm. apart. 

For data management, samples were designated by the dive number, a 

decimal point, and the chamber receiving the sample . Samples were classified as to 

site, bed 1 or 2, and replicate . A total of 63 samples were taken and logged, but two 

were rejected as having missed the mussel mat or lost material during transfer. 

Sample Analysis 

6-3 
Table 6.1 Sample collection for infaunal material. 

Sample Date Latitude Longitude Depth Site Bed 

Dive/ m/d/y feet Block Location & 

Chamber Replicate 

2598.02 09/14/89 27°43.38 91° 16.64 2149 GC-233 S outh End Pool 1 

2598.03 09/14/89 27°43.38 91° 16.64 2149 GC-233 South End Pool 2 

2598.04 09/14/89 27°43.38 91° 16.64 2149 GC-233 South End Pool 3 

2598.05 09/14/89 27°43.38 91° 16.64 2149 GC-233 South End Pool 4 

2598.07 09/14/89 27°43.38 91° 16.64 2149 GC-233 South End Pool 5 

2598.08 09/14/89 27°43.38 91° 16.64 2149 GC-233 South End Pool 6 

2598.09 09/14/89 27°43.38 91° 16.64 2149 GC-233 North End Pool 1 

2598.10 09/14/89 27°43.38 91° 16.64 2149 GC-233 North End Pool 2 

2598.11 09/14/89 27°43.38 91° 16.64 2149 GC-233 North End Pool 3 

2598.12 09/14/89 27°43.38 91° 16.64 2149 GC-233 North End Pool 4 

3137.11 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 1 Interior Rep. 1 

3137.12 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 11 nterior Rep. 2 

3137.01 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 1 Edge Rep. 1 

3137.02 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 1 Edge Rep.2 

3137.03 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 2 Edge Rep. 1 

3137.04 09/19/91 27°41 .27 91* 32.35 2372 GC-272 Bed 2 Edge Rep. 2 

3137.05 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 2 Interior Rep. 1 

3137.06 09/19/91 27°41 .27 91° 32.35 2372 GC-272 Bed 2 Interior Rep. 2 

3139.12 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 1 Edge Rep 1 

3139.01 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 1 Edge Rep. 2 

3139.02 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 1 Interior Rep. 1 

3139.03 09/21/91 27°46.92 91' 30.46 1790 CG-184 Bed 1 Interior Rep. 2 

3139.04 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 2 Interior Rep. 1 

3139.05 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 2 Interior Rep. 2 

3139.06 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 2 Edge Rep. 1 

3139.07 09/21/91 27°46.92 91° 30.46 1790 CG-184 Bed 2 Edge Rep. 1 

3142.08* 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 1 Interior Rep. 1 

3142.09 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 1 Interior Rep. 2 

3142.10 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 1 Interior Rep. 3 

3142.11 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 1 Edge Rep. 1 

3142.12 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 1 Edge Rep. 2 

3142.01 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 2 Interior Rep. 1 

3142.02 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 2 Interior Rep. 2 

3142.03 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 2 Edge Rep. 1 

3142.04 09/25/91 27°44.72 91° 13.21 1787 CG-234 Bed 2 Edge Rep. 2 

3145.12* 09/24/91 27°43.38 91° 16.63 2149 GC-233 Bed South Edge Rep. 1 

3145.01 09/24/91 27°43.38 91° 16.63 2149 GC-233 Bed South Edge Rep. 2 

3145.02 09/24/91 27°43.38 91° 16.63 2149 GC-233 Bed South Interior Rep. 1 

3145.03 09/24/91 27°43.38 91° 16.63 2149 GC-233 Bed South Interior Rep. 2 

3145.04 09/24/91 27°43.38 91° 16 .63 2149 GC-233 Bed North Edge Rep. 1 

3145.05 09/24/91 27°43.38 91° 16.63 2149 GC-233 Bed North Edge Rep 2 

3145.06 09/24/91 27°43.38 91° 16.63 2149 GC-233 Bed North Interior Rep. 1 

3145.07 09/24/91 27°43.38 91° 16 .63 2149 GC-233 Bed North Interior Rep. 2 

6-4 
Table 6 .1 Continued 

3265.04 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 1 Edge Rep. 1 

3265.06 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 1 Interior Rep. 1 

3265.07 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 1 Edge Rep. 2 

3265.08 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 1 Interior Rep 2 

3265.09 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 2 Edge Rep. 1 

3265.10 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 2 Interior Rep. 1 

3265.11 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 2 Edge Rep. 2 

3265.12 08/09/92 27°44.76 91° 13.33 1770 GC-234 Bed 2 Interior Rep.2 

3268.05 08/10/92 27°44.78 91° 13.28 1771 GC-234 Bed 1 Interior Rep. 1 

3268.06 08/10/92 27°44.78 91° 13.28 1771 GC-234 Bed 1 Interior Rep. 2 

3268.07 08/10/92 27°44.78 91° 13.28 1771 GC-234 Bed 1 Edge Rep. 1 

3268.08 08/10/92 27°44.78 91° 13.28 1771 GC-234 Bed 1 Edge Rep. 2 

3269.01 08/11/92 27°47 .00 91° 30.47 1825 GC-184 Bed 1 Edge Rep. 1 

3269.02 08/11/92 27°47 .00 91° 30.47 1825 GC-184 Bed 1 Edge Rep. 2 

3269.11 08/11/92 27°47 .00 91° 30.47 1825 GC-184 Bed 1 Interior Rep. 1 

3269.12 08/11/92 27°47 .00 91° 30.47 1825 GC-184 Bed 1 Interior Rep. 2 

3274.12 08/14/92 27°46.99 91° 30.46 1785 CG-184 Bed 1 Edge Rep. 1 

3274.01 08/14/92 27°46.99 91° 30.46 1785 CG-184 Bed 1 Edge Rep. 2 

3274.02 08/14/92 27°46.99 91° 30.46 1785 CG-184 Bed 1 Interior Rep. 1 

3274.03 08/14/92 27°46.99 91° 30.46 1785 CG-184 Bed 1 Interior Rep. 2 

* Sample considered bad due to lack of seep fauna and omitted from analysis. 

Final analysis was very much influenced by the results obtained. The faunal 

composition proved to be so monotonous site to site, bed to bed, and edge to inner, 

that simple graphics and descriptive statistics were used. The very high dominance 

of a few gastropods and the general lack of correlation among fauna eliminated the 

utility of multivariates and made description the most useful analytical tool. 

6.3 Results 

The mussel bed samples were dominated by only 13 species. Of these, only 

the mussels and the gastropods, Bathynerites and Prouanna, were sufficiently 

abundant to support statistical analysis. The counts for the remaining species were 

6-5 
dominated by zeroes, introducing major sample-size artifacts into any analyses 

undertaken. There were no significant correlations among the faunal groups 

overall, thus eliminating multivariates as a useful analytical tool . 

6.3.1 The Mvtilid Substrate 

A total of 1542 specimens of the seep mytilid la-type mussel were taken in 61 

JSL grabs. Collectively, specimens between 3 and 8 cm predominated. Juveniles 

less than 1 cm were abundant, but restricted to only four samples (3265-11 and 

3142-3 outer edge grabs at GC 234, and 3137-5 and 3137-6 inner area grabs at GC 

272). Specimens from 8 to 13 cm were few in number, but predominated in 17 

samples from all sites . There were distinct inter-sample differences in size 

structure of the population, but these did not correspond to the factors of the 

sampling scheme (inner versus edge, different patches at a site, differences in 

patches) in any consistent manner. Ten Bush Hill (GC 184) and GC 234 samples 

had a wide size range of individuals . Only one location showed an exclusive 

predominance of smaller forms, a clump at CG 234. All other samples, regardless of 

site, clump, or position were dominated by animals in the 5 cm and greater size 

classes. 

At the Brine Pool (GC 233), with its assumed simple gradient geometry, size 

segregation with smaller specimens predominating at the inner edge was apparent 

in observation and reinforced by sampling. The results of sampling at other sites 

(as well as observation) indicated that a similar segregation was taking place at all 

sites, but over a much smaller and complex spatial scale . Approximately 33% of the 

areas are predominated by specimens larger than 5 cm with no, or very few, smaller 

specimens . Of the 67% of the area with a wider size range, including smallest 

forms, only 7%, or four, samples showed recent recruitment to the smallest size 

class. The GC 272 results are unique in that the outer edge samples proved to be 

6-6 
articulated dead shells with no living mussels, and the inner edge samples were 

bimodal, with a few large specimens and many in the 1 cm class. 

When the size frequency data taken from the Johnson Sea-Link grabs is 

combined with diver observation, it is apparent that successful recruitment of 

juvenile mussels into the smallest size class is relatively uncommon and restricted 

to discrete small areas within existing mussel beds. Clumps of small mussels have 

been observed al all four dive sites, even though such a clump was actually sampled 

only once. Unfortunately, at our current level of understanding we can not ascribe 

this recruitment pattern to rare and aggregated larval settlement in chemically 

suitable habitats, spatially heterogeneous post-settlement survival, or size 

segregating post-settlement migration. 

Due to the great size range in mussels, count is a poor expression of the 

extent to which the mussels provide a habitat for associated species. Biomass, being 

an expression of volume, is more informative . The average biomass was 24.21 g 

AFDW/ sample (SD = 20 .39) with a maximum 85 grams. Only 11 (approx. 20%) of 

the samples had a mussel biomass greater than 40 grams, and these came from all 

four sites . Although extrapolation to a larger size area is of questionable validity, it 

facilitates comparisons . The average mussel biomass over all sites and samples was 

710 g AFDW/M2 with a maximum observed of 2491 grams. 

6.3.2 Seep Mvtilid - Bathvnerites Relationships 

Bathynerites naticoidecz (Gastropoda) was the dominant heterotroph in 

mussel beds. Much smaller than the mussels, it was actually the numerically 

dominant animal collected in the mats; 2,571 specimens were collected in 50 out of 

61 Johnson Sea-Link samples. While present at all four sites, there were distinctive 

inter-site differences . Bathynerites naticoidea was most abundant at Bush Hill and 

GC 234 sites, but was less abundant at the Brine Pool and GC 272 . Both of these 

6-7 
low abundance sites contained relatively large specimens. Where Bathynerites 

naticoidea was numerous, there was a wide size range . The highest abundance (317 

specimens) was predominated by small (<4mm) specimens. That sample (collected 

on JSL dive 3142) coincided with a bed of numerous smaller mussels at the GC 234 

site. 

One sample was dominated by many small mussels and many small 

Bathynerites : otherwise, correlation between mussel and snail abundance was low 

and indicated little direct dependence on the scale of the sampling. A changing 

relationship between Bathynerites populations and the Bathymodiolus substrate is 

suggested by the statistical summaries in Table 6.2 . The variances of counts and 

biomass values are so great that all four sites may be viewed as values along a 

continuum from high densities of mixed size mussels down to low densities of a few 

older individuals. Bathynerites was distinctly less abundant at GC 272 and the 

Brine pool, both in samples and in observations, and no mussel-snail correlation 

exits . Where the snail is abundant, Bush Hill and GC 234, higher correlation 

suggest some relationship between mussel and snail abundance. 

Table 6.2 Site-specific Bathymodiolus-Bathynerites relationships . 

Mussel Count 

Snail Count 

Mussel Biomass 

Correlation 

Count-Count 

Biomass - Count 

Bush Hill, n=16 

68.06 ± 87.95 

26.50 ± 19.54 

30.57 ± 21.96 

0.777 

0.474 

GC 234 n=20 

46.95 ± 51.6? 

62.25 ± 85.26 

15.95 ± 15.04 

0.540 0.337 

0.558 0.119 

GC 272 n=8 

13.37± 24.68 

3.00 ± 4.11 

20.45 ± 30.12 

Brine Pool n=17 

8.35 ± 4.84 

12.53 ± 11.95 

28.34 ± 16.57 

0.172 

.0400 

6-8 
6.3.3 relationships With Provanna scuplta 

A total of 511 specimens of Provanna scupltcz were taken in 37 out of 61 

submersible grabs. On a site basis, P. scuplta appeared to be rare at the Brine Pool, 

but ubiquitous elsewhere. It showed no consistent relationship with either the 

extent of substrate, or with the abundance of Bathynerites, Table 6.3 . 

Table 6 .3 Site-specific Provacnna-Bathymodiolus-Bathynerites relationships . 

Bush Hill, n=16 GC-234 n=20 GC-272 n=8 Brine Pool n-17 

Mussel Count 68.06 ± 87.95 46.95 ± 51.67 13.37± 24.68 8.35 ± 4.84 

Provana Count 8.38 ± 15.80 4.71 ± 8 .82 9.00 ± 11 .8 0. 06 ± 0.24 

Correlation 

Count - Count 0.107 -0.260 0.396 0.141 

Biomass - Count 0.224 0.009 0.260 .090 

Nerite-Provanna 0.122 -0.004 0.974 -0.19 

6.3 .4 graphic Patterns of Subdominant Species 

Species counts are presented in Appendices (Volume III). Except for the two 

most abundant gastropods (Bathynerites and Provanna), all other species occurred 

in half or fewer samples . Any discerned patterns are likely to be influenced by 

sampling artifacts . The highly mobile shrimp, Alvinocaris stactophila, and squat 

lobster, Munidopsis, n. sp . probably avoid capture and have been observed to flee 

from the sample during transfer. As such, they may be seriously under-sampled . 

As many as 15 of the former have been taken in a single sample, but Alvinocaris 

stactophila was only taken in 31 of the 61 JSL grabs. It was not found in the few 

samples at GC 272. Munidopsis n. sp . was taken 25 times, with a maximum count 

of five; it was collected at all sites. The large orbinnid polychaete now undergoing 

taxonomic study by R.E . Ruff was taken 18 times, most frequently at the Brine Pool. 

It was not found at GC 272 . Due to its observed aggregating behavior, it mighty 

6-9 
easily go unsampled. The gastropod, Cataegis, was found at all sites, a maximum 

abundance of 24 per grab, and present in 21 samples. The chitin, Ischnochiton, was 

taken in only four samples, exclusively at GC 272 . However, other species have 

been taken at all sites. Similarly, an unidentified nemertine, a sipunculid, a limpit, 

and a small crab, Trichopeltarion nobile, were collected four or fewer times with no 

obvious geographic pattern . 

6.4 Discussion: The Origins and Significance of Monotony 

Since all the mussel mat-associated fauna are mobile, it should be expected 

that parts of a bed and beds separated by a few meters should share a common 

fauna . It is the similarity between sites that is most puzzling . It has two 

interpretations. First, conditions for successful colonization of a bed by a 

heterotroph must be so restrictive that only a particular suite of species can inhabit 

them. Second, all sites must be roughly at the same stage of succession or nonsuccessive immigration . This seems only feasible if all are relatively old - old 

enough for the faunal composition to have reached an equilibrium. The failure to 

find simple relationships among species is disappointing since it underscores this 

project's failure to identify population regulating mechanisms. From a management 

perspective, the monotony of the mussel bed fauna across sites suggests that local 

damage to populations would probably be mitigated by recolonization from adjacent 

sites. There is a pool of common inhabitants largely restricted to the mussel bed 

system, but site-specific endemics do not appear to exist among the heterotrophs. 

6.5 Description and Natural History of the Megafaunal Organisms 

6.5.1 1Mo lusca 

The upper slope seep communities are mollusc-dominated with respect to 

both chemosynthetic and heterotrophic forms. Mussels of the Bathymodiolus 

6-10 
species complex form the primary biogenic substrate. On the basis of external 

morphology, all specimens collected in this component of the study were the same 

species, and may be referred to as the seep Mytilid Ia. All further reference to 

"mussel" in this section refers to this form. Resolution of specific subdivisions of the 

five or more forms in the Gulf of Mexico Bathymodiolus complex is under study by 

Lutz, Turner, and associates . Sediment dwelling chemosynthetic forms 

Calyptogena ponderosa and Vesicomya cordata, Lucinoma atlantis, Lucinomcz sp ., 

and Thyasira oleophila are also associated with the seeps (Callender et al . 1990). 

There is a marked absence of heterotrophic bivalves in the seep community. A Timid 

bivalve, Acesta bullisi, appears to be heterotrophic, but in addition to being found 

attached by byssal fibers to the rocks, some specimens enclose the anterior opening 

of tube worms suggesting a complex mutualism (Kohl and Vokes 1994). 

The mussel-shell habitat supports an abundant fauna of heterotrophic 

gastropods (summarized in Waren and Bouchet 1993). In approximate sequence of 

dominance, the most common species present are Bathyneritac naticoidea, Provana 

sculptor, Cataegis meroglyptd, and Buccina canatae . Less common gastropods 

include Cancellaria rosewateri, Cantrainia macleani, Crymnobela extensa, a chiton 

Ichnochiton sp., and a rarely-collected limpet. Hyalorisia galea and Gaza fisheri 

were reported from the Bush Hill site based on material collected prior to the 

current study. The former was not collected during subsequent sampling, and the 

latter was observed on the sediments in seep areas, but not in mussel mats. 

The question of recognizing endemism is greatly complicated by uncertainty 

as to specific and generic identities of deep-sea gastropods. The two most abundant 

species Bathynerites naticoidea and Provanna scuplta appear to be true endemics . 

The former genus seems to be restricted to the upper slope Louisiana seeps. 

Provanna is a cosmopolitan seep/vent associate, with P. scuplta being restricted to 

the Louisiana seeps . 

6-11 
Bathynerites naticoidea (Clarke 1989) was the most abundant heterotrophic 

animal collected in association with mussel mats and sometimes exceeded even the 

mussels in abundance. The genus and species were new to science prior to the seep 

studies. Clarke (1989) produced the original description based on Bush Hill 

material; Waren and Bouchet (1993) extended the description based on additional 

material . Naticoidea seems to be endemic to the Louisiana slope seeps. It has not 

been found at either the Alaminos Canyon or the Florida Escarpment sites . A 

similar fossil nerite has been found at Miocene sites in Italy (Taviani, personnel, 

communications), but Warren and Bouchet point out that a lack of traits in the 

fossil form makes equating the two species problematic. 

Considerable information on the natural history was developed by Zande 

(1994). Batlzynerites naticoidea is a radular browser consuming a mixture of 

mineral material, organic detritus, bacteria, and periostricum from mussel shells, 

tube worms, and the substrate in the vicinity of chemosynthetic assemblages. Egg 

cases of 100 or more larvae are laid on convenient substrates, with development to 

juvenile benthic forms taking place in the capsule. The size frequency distribution 

of collected specimens suggests episodic settlement. The largest specimens were 

associated with the brine pool site . Although closely associated with the 

chemosynthetic fauna, episodes of roaming beyond the bed edge were observed 

during the 1993 dives series . 

Provanna sculpta (Waxen and Ponder 1991) was new to science, with the type 

species coming from Bush Hill (Waxen and Ponder 1991) . Although the genus 

Prouanna has been collected away from chemosynthetic communities, its 

association with these systems has lead to the view that it is primarily a seep and 

vent endemic (Waxen and Ponder 1991) . The genus has been placed in the family 

Abyssochrysidae, a relict of a largely extinct group (Houbrick 1979) . Provanna 

sculpts lacks a planktotropic form, but it is not known if it has a dispersive 

6-12 
developmental stage or not. A comprehensive monograph of the genus Provacnna 

has not yet be published, but Warren and associates have produced a series of 

papers describing new species from seep and vent systems and refining the 

description of the genus (Waren and Ponder 1991). Ten or more species exist 

associated with the chemosynthetic systems of the Pacific. P. sculpts seems to be 

the only member of the genus yet found in the Atlantic. 

Provanna is a radular browser. Being small and matching mussel operculum 

in color, it is largely unobserved, but very numerous . Present at all sites sampled, it 

is notably rare at the Brine Pool site. The shell usually shows deep dissolution pits 

which are often the settlement site for juvenile mussels. Dissolution of the shell 

makes measurement of length difficult. In the present study, the maximum length 

of the shell opening was used as the most reliable indication of size . The size range 

of specimens is quite narrow with no indication of recruitment. 

Cataegis meroglypta (McLean and Quinn 1989) - The genus Cdtaegis was 

erected for two species of continental slope trochid, C. finkli and C. meroglypta that 

are apparently opportunistic grazers (McLean and Qinn 1989). The former species 

was found at slope depths associated with seagrass falls, and the latter in the upper 

slope seep systems. Waren and Bouchet (1993) have reviewed current information 

about the species. Since C. meroglypta has not been reported from outside of a seep 

environment it may be endemic. However, the genus Cataegis seems to be a rare, 

but more widely distributed deep-sea form. 

Catacegis meroglypta is a radular browser. There is a conflict between 

video/photo observation and the results of JSL grab sampling. Grab sampling 

indicates that C. meroglypta is consistently less abundant than B. naticoidea . 

However, some photographs and observations suggest that it is locally very 

abundant and dominant. This discrepancy may be due to aggregation of C. 

meroglypta or behavior during sampling. Aggregates on mussels or adjacent to 

6-13 
mussel beds may capture the attention of observers and give the false impression 

that C. meroglypta predominates over a larger area. Alternately, the species may 

readily drop from mussels during the disturbance of collecting, while Bocthynerites 

holds on, thus biasing the estimates . 

Buccina canatae - The genus Buccina is a cosmopolitan species complex 

common on continental slopes . B. canatae, a predatory neogastropod, has been 

collected from seep and non-seep environments in the Gulf of Mexico and Caribbean 

(Warn and Bouchet 1993). Initially observed on the sediment surface of clam 

areas, it appears to forage widely and in aggregates . Aggregation may be related to 

mating. One mound of specimens collected from the sediment contained 48 

individuals. During the 1992 dive series B. canatae were conspicuous within mussel 

beds at Bush Hill. The species was found at all sites . 

Ischnochiton mexicanus (Kaas 1993) - The chiton, Ischnoclziton mexiccznus, 

was encountered in four of the JSL grab samples and eight bulk collections of 

mussels. The species is externally unremarkable, but posses a distinctive radula 

used in feeding. The species may be endemic to the upper slope seep, but the genus 

is more widely distributed . Due to the relatively few specimens and the status as a 

species new to science, ash-free dry weights were not determined . The new species 

was examined and diagnosed (Kaas 1993) by Piet Kaas of the National Museum of 

Natural History, Leiden, The Netherlands . 

Minor Gastropods - Six species of gastropods were also collected incidental to 

large samples of mussels for physiological study. Of these, only an unidentified 

limpit was seen in the grab sample series . The remaining five species are listed 

below: 

1. Cancellaria rosewateri (Petit 1983) - It has been speculated (Waxen and 

Bouchet 1993) that C. rosewateri is a suctorial feeder parasitizing other 

6-14 
organisms in the seep environment. It appears to inhabit seep and non-seep 

environments. The genus is cosmopolitan and most common at shelf depths . 

2 . Cantrainea macleani (Waren and Bouchet 1993) - The species was new to 

science, but occurred in samples from both in and remote from seep 

communities. Its range includes the continental slope in both the Gulf of 

Mexico and Caribbean. 

3. Gymnobela extensa (Dall 1889) - The genus Gymnobella is a common deep-sea 

group of selective invertebrate predators. Since G. extensor was reported by 

Dall from the Yucatans straights, it is most likely to not be specific to the 

seep environment. 

4. Phymorhynchus sp. (Bouchet and Waren 1980) 

5. Hyalorisia galaea (Dall 1889) 

6.52 Crustacea 

Crustaceans are an abundant and highly mobile component of the upper 

slope seep heterotrophic biota. About four species have been encountered in the 

JSL grab samples and an additional six to eight observed or taken as incedental 

specimens. In addition to those collected, large forms (these include the decapods, 

Paralomis cubensis, Roclzina crasser, Chaceon quinquedens, Chaceon fenneri, and 

the giant isopod, Bathynomus giganteus), smaller zooplankton, and crustacean 

nekton often swarmed around the submersible lights at the Bush Hill site 

suggesting some bentho-pelagic coupling. It is assumed that the ecological role of 

the seep crustaceans is that of predator/scavenger, the same as typical benthic crabs 

and shrimp . In mussel bed grab samples, the two most abundant species, 

Aluinocaris stactophilia and Munidopsis sp., nov., are endemic . Alvinocaris belongs 

to a cosmopolitan seep/vent family. The genus Munidopsis is a cosmopolitan form 

common in deep water that also inhabits seeps and vents . 

6-15 
Alvinocczris stacctophilid (Williams 1988) - Hydrothermal and hydrocarbon 

chemosynthetic communities often have high populations of three or fewer caridean 

shrimp genera, Aluinocaris, Rimicaris, and Chorocaris. Since these genera are new 

to science, their taxonomic position is not well established. However, they appear to 

represent a distinct group with common phylogenetic affinities, originally placed in 

the family Bresiliidae (Williams and Chace 1982) . Subsequently, the new family 

Alvinocarididae was proposed (Christoffersen 1989) to contain these forms . 

Two species of Aluinocaris have been found in Gulf of Mexico chemosynthetic 

systems . The hydrocarbon seeps on the upper continental slope off Louisiana are 

populated by A. stactophilia and both known deeper Gulf systems are populated by 

A. muricola . A key to the three species of Alvinocaris has been developed by 

Williams (1988) and was the basis of identification of the specimens collected in this 

study. These species are easily distinguished by the form of the rostrum. That of A. 

muricola is longer and has numerous spines on both the dorsal and ventral 

surfaces. That of A. stactophilia is shorter and bears only one or no ventral spines. 

It is especially noteworthy that none of the 42 benthic shrimps reported on 

the typical deep-sea floor are found in the seep community. This failure of the 

background fauna to exploit the seeps is most dramatic for the most abundant 

species. Pequegnat's (1983) inventory of carideans lists 33 species in the Gulf of 

Mexico, of which at least 25 are found below the shelf break. Nematocarcinus 

rotundus is dominant on the upper slope and N. ensifer dominant on the lower 

slope. Three species of Glyphocrangon are sub-dominant but common: G. nobilis, G. 

aculeata, and G. alispina. These three sub-dominants have overlapping ranges 

mostly above 2000 m. None of these five have been collected or observed in the seep 

sites. Pequegnat (1983) lists 22 penaid shrimp, with 17 found below the shelf break. 

Benthysicymus carinatus is only slightly less common than N. rotundus and shares 

an overlapping range on the continental slope . 

6-16 
Although Aluinocaris stczc tophilia appears to have the typical caridean 

feeding habit of predator/scavenger, perhaps grazing upon free living 

chemosynthetic bacteria . One member of the Alvinocarididae, Rimicaris exoculata, 

is specially adapted to feed upon epibiotic chemosynthetic bacteria living on its own 

cuticle. This symbiosis involves epibiotic bacteria on the mouth parts (Van Dover et 

al . 1988) and in the prebranchial chamber (Gebruck et al. 1992; Segonzac et al . 

1993). A general review and detailed examination of the feeding of three species of 

Alvinocarididae at a mid-Atlantic ridge site has been given by (Segonzac et al. 

1993). 

Munidopsis sp, nou. - Galatheid crabs are common in the deep benthos and 

have invaded most seep and vent communities . There is distinctive sexual 

dimorphism in the chelae . Larger males may have chelae longer than the rest of 

the body. These animals appear to be opportunistic scavengers/predates. 

Low Abundance Crustacea - Due to loss of smaller and lighter weight 

material during the transfer from the JSL grab to the storage chambers of the 

submersible, retention of copepods, isopods, tanaiids, etc. was unlikely. Sieving 

washings from the mussel samples through a 250 micrometer screen did not 

produce a macrofauna size class of specimens. One juvenile decapod crab was 

collected in three grabs and could be seen in video images of the mussel mats . 

Tentatively identified as Trichopeltarion nobile, it is a member of the background 

fauna with massive claws suitable for shell crushing. Adults were not observed. 

6 .5 .3 Worms 

Although polychaetes are usually a very diverse component of the benthos, 

the mussel beds contained few forms . A large (up to 20 cm long) polychaete related 

to the Orbinnidae occurred in dense clumps of several 10's of specimens. At the 

brine pool, these worms were conspicuous waving the anterior end about above the 

6-17 
mussel substrate. They were assumed to be detrital feeders . Nereiids of similar 

size were encountered in the orbinnid clusters. As with the smaller crustaceans, a 

smaller worm fauna was absent or lost during sampling. When rock material is 

taken, it frequently contains an undescribed sipunculid. Nemertines were also 

encountered. 

6.6 Gulf-Wide Patterns 

Chemosynthetic communities afford both a complex biogenic habitat and a 

rich food source exploited by heterotrophic species. Unlike the chemosynthetic 

producers, these exploiters may not be geochemically restricted in distribution. 

Therefore, a primary question is to what extent those species are colonists from the 

typical deep-sea biota, or are specialists endemic to chemosynthetic systems . It is to 

be expected for cold seeps, that like hydrothermal systems (Tunnicliffe 1991 ; 1992), 

the associated heterotrophic species reflect mixed sources. Some may be truly 

endemic to chemosynthetic communities . Some may be colonists from the 

surrounding benthos, which have established resident seep populations, while 

others are vagrants making temporary use of the seep site but not developing 

resident populations. The tremendous difficulty in actually assigning species to 

these classifications lies in a lack of knowledge of the species composition of the 

surrounding benthos . That environment can be very species-rich and is inhabited 

by such an abundance of rare species that a complete inventory is virtually 

impossible . Apparent seep and vent endemics may just be colonists or vagrants 

whose more widely scattered kin simply have not made it into a museum collection. 

Fortunately, the Gulf of Mexico has been extensively sampled and the 

bathymetric distribution of the larger normal (megafauna) benthos well 

documented . Therefore, it is possible for many species to determine which are 

colonists and vagrants . Absolute determination of endemics remains uncertain; 

6-18 
however, the bathymetric range of samples affords an indirect method of 

assessment. Should apparent endemics show a bathymetric species turnover akin 

to the normal fauna, then their endemism may be suspect. 

A comparison of heterotrophic forms is presented in Table 6.4. The 

comparison is limited to larger organisms (greater than 0.5 cm) and the more 

common groups. The species listed were primarily collected by manipulator grabs 

from the Johnson Sea-Link submersible and retained on a 500 micrometer sieve. 

Large crabs and fishes were recorded in video or still camera images . A more 

complete discussion of the data can be found in MacDonald (1992) . The faunal 

tables in Pequegnat (1983) were used to establish the preliminary classifications. 

Species that have never been recorded from outside a seep are classified as 

endemics. Colonists have been recorded outside seeps, but obtain high population 

levels only within seeps . Vagrants are those species which are found outside seeps 

and do not appear to achieve unusually high population levels within . An 

"unresolved" classification is applied to all species in which the taxonomic position 

is uncertain at this time. The list of dominant chemosynthetic species is restricted 

to molluscs, crustacea, echinoderms, and fishes . Other taxa occurred too rarely to 

be of use in the comparisons. 

Several conclusions are readily apparent. First, the upper slope and 

Alaminos Canyon heterotrophic fauna are very dissimilar. If elements are shared, 

their relative abundance at the two sites must be so dramatically different as to 

have escaped detection at this level of sampling. Second, endemism is low at the 

species level - five endemics out of 39 species . At the level of genus, only the two 

coiled gastropods, Batlzynerita naticoidea and Prouanna sculpts, possibly the 

Alaminos Canyon limpet, and the shrimp, Aluinocaris are endemic to 

chemosynthetic systems. Third, the colonists and vagrants reflect only 1/10th of the 

surrounding species richness, 23 species in seeps at the upper slope sites versus 251 

6-19 
Table 6.4 Composition of upper and lower slope seep heterotrophic fauna with reported species richness and 

dominant species of the background fauna. 

0 

UPPER. SLOPE SEEP SITES UPPER SLOPE BACKGROUND ALAMINOS CANYON SITES LOWER SLOPE BACKGROUND 

GASTROPODS 

Bathynertia naticoidea Endemic 

Provanna scuplta Endemic 

Cataegis meroglypta Colonist 

Buccina canatae Colonist 

Ichnochiton sp Colonist 

Cantrainia macleani Colonist 

Gymnobela extensa Colonist 

Cancellaria rosewateri Vagrant 

Hyalorisia gales Vagrant 

400 - 1000m 

102 species 

Gemmula periscelida 

Leucosyrinx tenocer¢s 

Gymnobela ipara 

2000-3000m 

32 species 

GASTROPODS Dominants 

limpets Endemic Leucosyrix tenoceras 

Buccina sp. Unresolved Gynobela bairdi 

CRUSTACEA 

Galatheiid Crabs 

Munidposis sp. Endemic 

Eumunida pict¢ Vagrant 

Other Crabs 

Paralomis cubensis Vagrant 

Rochina crassa Vagrant 

Chaceon quinquedens Vagrant 

Chaceon fenneri Vagrant 

Trichopeltarion nobile Vagrant 

Large Isopods 

Bathynomus giganteus Vagrant 

Shrimp-Like Forms 

Alvinocaris statophilus Endemic 

18 spp 

Munidops¢ sigsbei 

Munich v¢lida 

Munich longipes 

33 spp 

Parapagurus sp. 

Bathyplax typhla 

Chaceon quinquedens 

I spp. 

Bathynomus giganteus 

35 spp. 

Systellapsis pellucida 

Pasiphaea merriami 

Glyphocrangon alispina 

Glyphocrangon nobilis 

Penaeopsis serrata 

Parapenaeus longirostris 

Benthesicymus bartletti 

CRUSTACEA 

Galatheiid Crabs 6 spp 

Munidopsis sp Unresolved Munidopsis bermudezi 

Other Crabs 1 spp 

none Chaceon quinquedens 

Large Isopods 

none 

Shrimp-Like Forms 

Aluinocaria muricola Colonist 

1 spp. 

Bathynomus giganteus 

13 spp 

Nematocarcinus acanthitelsonis 

Benthesicymus cereus 

Hemipenaues carpenters 
ECHINODERMS 

Asteroidea 

Sclerasterias sp . Colonist 

Ophiuroidea 

none 

Fishes 

Eptatretus sp 

Synaphobranchus sp 

Urophycis cirratus 

Hoplostethus sp. 

Chaunax suttkusi 

Table 6.4 

25 species 

Plutonaster intermedius 

Nymphaster arenatus 

Astropecten americanus 

26 spp 

Ophiernus adspersum 

Bathypectinura hems 

111 spp. 

Vagrant Gadomus longifilus 

Vagrant Dibranchus atlanticus 

Vagrant Nezumia aequalis 

Vagrant Dicrolene intronigra 

Vagrant Synaphobranchus SP. 

Continued. 

ECHINODERMS 

Asteroidea 

none 

Ophiuroidea 

Ophiomusium sp Colonist 

Fishes 

Liparid Unresolved 

21 spp 

Dytaster insignis 

Ampheraster alaminos 

Benthopecten simplex 

13 spp 

Ophiomusium planum 

Bathyplectinura hero 

21 spp. 

Dicrolene intronigra 

Coryphaenoides m¢crocephalus 

N 

r 
species in the background fauna. Fourth, the distinction between vagrant and 

colonist seems to be related to size and mobility. Colonists tend to be smaller, 

slower, and subdominant in the background. Vagrants tend to be larger, highly 

mobile, and may include dominant elements from the background. 

The low endemism and changing heterotrophic fauna of sites at 400-1000 m, 

and 2000 m are generally consistent with colonization from a vertically changing 

deep-sea fauna. However, the factors that limit successful colonization to such a 

small percent of the background species remain obscure. Purely random 

colonization can be ruled out, since all of the more than lO locations sampled in this 

and other programs at the upper slope site had the same suite of colonizing species. 

Perhaps the species of this suite are the only ones of all the background fauna 

capable of persisting in the unique chemical environment of the seep sites. Or, they 

predominate locally due to contagious colonization in which the very high 

populations densities of initial successful colonists make subsequent colonization of 

nearby sites highly probable. 

The observed distribution of apparent endemics is puzzling. The possible 

absence of the gastropods Bathynerites and Provanna from the deeper site would 

suggest that these abundant forms are actually colonists from the vertically zoned 

background fauna. However, other evidence points to true endemism. A nerite 

similar to Bathynerites appears to have had a long association with seeps, being 

found in Miocene deposits in Italy (Taviani, personal communications), but has 

been reported from no other sites . Other species of the genus Provanna are 

associated with vent and seep sites in the Pacific (Waxen and Boucet 1993). 

Deep-sea species richness with its many potential colonists makes it highly 

likely that colonization is a probable mechanism influencing the species composition 

of seep assemblage to some degree. If the heterotrophic component is 

predominantly colonists, then such fauna should parallel deep-sea community 

6-22 
changes bathymetrically, geographically, and in the fossil record. If, however, 

endemism is predominant then the heterotrophic component should not parallel the 

surrounding benthos, and may share with the taxa of the chemosynthetic 

assemblage common origins and convergent adaptations for colonization and 

persistence . It is even tempting to suggest that heterotrophic and chemotrophic 

endemics may have linked life histories . A possible example of this has been found 

in the upper slope sites. Juvenile Bathymodiolus are common on shells of 

Bathynerities and Provcznnac, and the cuticles of Munidopsis sp . Tube worms have 

been found growing from the shells of Bathynerities. Such epizootic associations 

provide the sessile members a post-settlement means of dispersion that may be 

important in maintaining viable populations as the chemical gradients of seeps and 

vents undergo local shifts. 

The low endemism of the Gulf of Mexico sites, especially the upper slope, and 

the high degree of colonization by fauna from the surrounding seafloor is markedly 

different from that found for hydrothermal systems where 95% of the species are 

endemic. As noted by Tunnicliffe (1991; 1992) as much as 95% of hydrothermal 

vent fauna is endemic and with little taxonomic similarity to continental margin 

systems except the chemosynthetic of the assemblage . On the basis of the results 

presented herein, it can be proposed that the hydrothermal versus cold vent faunal 

distinction may be due to the geographic and bathymetric gradients of the 

surrounding benthos. Seep systems associated with continental margin geology 

exist within the biodiversity framework of the continental slope . High overall 

species richness at the base of the slope and increasing large predator diversity on 

the upper slope afford many opportunities for colonization and exploitation of seep 

systems. Displacement by colonists and extermination by predators may eliminate 

many hydrothermal vent endemics from continental slope communities . 

Hydrothermal systems, due to their association with spreading centers, tend to be 

6-23 
remote from continental margins. While overall surrounding species richness may 

be high, low overall abundance and substrate dissimilarity may limit opportunities 

for colonization. Large predators, especially decapod crustaceans are absent, thus 

reducing predation . 

6.7 Potential for Adverse Impact 

Now that we have some knowledge of the species composition of 

chemosynthetic mussel beds, it is possible to consider management issues with 

respect to the whole local community rather than just the chemosynthetic species . 

In this regard the main findings are: 

1 . that the associated fauna is monotonous in composition at the sites sampled; 

2 . that the dominant associated species are seep specific and may be endemic; 

3 . that the associated species consists mostly of larger and possibly older 

individuals ; 

4 . that the associated species at the sites studied differ from seeps elsewhere in 

the Gulf of Mexico; 

5 . that large predators and scavengers from the normal benthos do exploit seep 

productivity. 

In certain respects the heterotrophic component of mussel beds may be 

relatively robust . Sessile forms unique to mussel beds are virtually absent, and the 

mobile species are heavily shelled. Thus, acute physical disruption of a bed should 

have minimal effect so long as the chemosynthetic organisms survive . However, the 

ecology of the component species may make the assemblage fragile if exposed to 

chronic disturbances. This fragility stems from the facts that recruitment seems to 

be sporadic, and the whole community (mussels included) are subject to predation 

by at least one sea star and a variety of fish and invertebrates . Chronic disruption 

6-24 
might trip the balance in populations such that predators exterminate local 

populations faster than they can be resettled . 

The Gulf-wide distribution of seep associates is difficult to interpret since the 

deeper sites are so poorly studied. However, it appears that mussel beds are locally 

monotonous on the scale a ten's of kilometers and hundreds of meters depth, but 

regionally different on a scale of one hundred meters distance and a thousand 

meters depth. Since there seems to be some degree of zoogeographic restriction for 

seep species, the question of how much disturbance can take place without 

extinction arises . At this time, we know that the upper slope seep associates occur 

at many sites giving them some protection from extinction . However, the extent of 

deeper sites (2000m +) in the Gulf of Mexico is unknown. If limited, the species at 

sites may be at greater extinction risk and warrant greater environmental 

protection than sites above 1000 meters. 

6-25 
7.0 Stability and Fluctuation in Chemosynthetic Communities 

Over Time Scale of -1 Month to -1 Year 

Ian R» MacDonald and Robert S. Carnet' 

7.1 Introduction 

7 .1 .1 Need to Understand Time-Scales of Seepage 

For management, the major question about hydrocarbon seep communities is 

how much regulation is required to protect them from the possibly detrimental 

activities of oil and gas development and production. Are these communities uniquely 

impervious to industrial impact due to their adaptation to oiled and otherwise 

chemically hostile environments, or do these same adaptations suggest the existence 

of an easily perturbed system occupying a very narrow and fragile geochemical niche? 

Although long-term monitoring and experimentation is needed for a definitive answer 

to the robust or sensitive question, the 5-year series of submersible observations of 

hydrocarbon seep communities between 1989 and 1993 provided an opportunity for 

initial observations as to how communities change naturally and in response to 

intentional perturbation. 

7.1.2 Scone of the Work 

Chemosynthetic communities generally occupy areas affected by hydrocarbon 

seepage that are a few hundred meters in width. Within these sites, tube worms 

form clusters a few meters to a few tens of meters across and mussels form clusters 

that are less that one meter to a few meters in width (MacDonald et al. 1990a). To 

test whether major faunal changes were occurring as a natural process within these 

communities and to make observations that would allow us repeatedly to examine 

the type and density of fauna at sub-meter scales, we needed to acquire 

measurements of variations in the seepage process that might affect the 

communities. Our approach was to photograph repetitively functional clusters of 

7-1 
typical chemosynthetic fauna at each of the Study sites, to deploy in situ monitoring 

devices to observe the seepage process, and to examine the geological record for 

evidence of events that would have changed the habitat structure of the seeps. 

The repetitive photography operated at two scales. We assembled mosaics of 

faunal clusters from about 5 to 25 sq m in area and looked for changes at a 1 m scale . 

We also initiated disturbance (to mussel clusters) and monitored recovery . Two 

programs of controlled disturbance were initiated . The first simply placed marker 

rods at the edge of mussel mats to facilitate long-term observation, and the second 

consisted of placing recovery experiments at three locations . Mussel clusters were 

mechanically disturbed, a recovery cage placed over the disturbed portion of bottom, 

and re-population by mussels monitored over time. 

In situ monitoring focused on gas hydrate mounds that we observed breaching 

the seafloor at GC 184 and elsewhere . Gas hydrate is an ice-like substance that 

forms, under pressure, at temperatures above the freezing point of water by inclusion 

of methane (and other gases) into a lattice of water molecules. Most often detected 

by seismic means as deeply buried layers in marine sediment (Singh et al. 1993), gas 

hydrate has also been collected from <5 m below the floor of the continental slope in 

the Gulf of Mexico (Brooks et al. 1984; 1986). Globally, subaquatic gas hydrate 

comprises a 1016 kg reservoir of organic carbon (Kvenvolden 1988) that may interact 

with the atmosphere to influence climate cycles . Regionally, they are potential 

energy resources as well as potential hazards to seafloor structures (Kvenvolden et 

al . 1993). The interplay of hydrate formation and community structure is significant 

and contributes a source of short-term fluctuation with measurable effects. 

7-2 
7.2 Materials and Methods 

7.2 .1 Photomosaics and Other Quantitative Photography 

Both video and emulsion photographs were used to obtain images in which seep 

fauna could be identified and the aerial coverage of the image was known. Studioquality videos were provided by the cameras on the Johnson Sea-Link . The emulsion 

camera used was either a Benthos@ 35 mm camera with 28 mm lens or a Photosea@ 

stereo camera, which took photographs stereo-paired as an aid to determining threedimensional details in the communities. The cameras were oriented vertically and 

generally took pictures at altitudes of 1 to 3 m above the seafloor. Lasers, mounted 

on the cameras in parallel arrays of two or four, provided a reliable faducial mark for 

the photographs by projecting red dots at known distance apart in the final images. 

Additional analysis of video sequences was occasionally employed to obtain a 

qualitative characterization of the fauna at sampling stations. In all cases, the time 

and date were recorded on the imagery. This time tag, together with the navigation 

records of the support ship and the dive notes, provided a reliable means for 

determining where each image was taken. We also deployed durable markers at the 

sites to facilitate collection of abiotic samples and photographic survey from 

repeatable locations . During collection of the mosaic imagery, the operator traversed 

the area in the immediate vicinity of the marker in a series of short, overlapping 

tracks about 10 m in length. 

Mosaics were assembled from a series of digital files made by either scanning 

the 35 mm photographs by use of a Nikon 3510AF slide scanner, or by "capturing" 

individual frames of video by use of a Targa 64 video capture board. Digital 

mosaicking was conducted using AGIS software . This involved a series of homologous 

transforms whereby daughter images were joined to an initial image after rotation, 

translation, and scale adjustment to render the daughter image equivalent to the 

7-3 
parent image. Rotation, etc. was performed on the basis of matching two points in 

the daughter image to two identical or nearly identical points in the parent. This 

process specifically did not adjust for error due to differences in look angle between 

successive images, nor did it adjust for parallax differences due to the topography of 

the seafloor features. This meant that as more and more images were added to a 

mosaic, errors tended to accrue in the scale and relative position of bottom features. 

However, for areas on the order of 10 to 20 sq m, the method provided a robust 

technique for determining the structure and stability of fine-scale faunal clusters. 

Table 7.1 summarizes the locations of the study sites where mosaics were 

compiled in order to observe short-term change in community type. Table 7.2 lists 

the classification of community type used to evaluate photomosaics and video 

coverage. Percent coverage for these classifications and change in per cent coverage 

over time was determined using gridded (hard copy) mosaics and gridded video 

renderings of the same areas for all three years. Some grids were not covered 

completely by photomosaic elements, making occasional estimates necessary. 

Accuracy was achieved by simultaneously inspecting the hard copy(s) and on-screen 

views of the same grids for both the same and different years. 

7-4 
Table 7 .1 . Summary of locations at study sites where mosaics were compiled in 

order to observe short-term change in community type. 

Study Site Sampling Aerial Years Data Sources 

Location Coverage 

(sq m) 

GC 184, Bush Hill 

1. Marker Bucket #1 

2. Marker Bucket #2 

GC 272 

1. Milk-crate 

2. Aluminum frame 

VK 826, Yiosca Knoll 

1. Marker Bucket 

GC 234 

1. Site #1 (Milk-crate) 

2. Site #2 (Mussel bed) 

C-buoy 

38 1991 color mosaic 

22 1992 color mosaic 

25 1993 B&W high-alt . mosaic 

6 1992 Hi8 video: JSL92 dive 327? 

6 1993 color mosaic 

25 1991 color mosaic 

25 1992 Hi8 video: JSL92 dives 3276 

and 3277 

8 1991 color mosaic 

8 1992 Hi8 video: JSL92 dive 3261 

20 1991 Hi8 video: JSL91 dive 3142 

16 1992 color mosaic 

4 1991 color mosaic 

4 1992 Hi8 video: JSL92 dive 3268 

7-5 
Table 7.2. Classifications of community type used to evaluate photomosaics and 

video coverage. 

1 . Tube Worms (T 

Dense Dome shaped bush; cannot Td 

see sediment 

Medium Recumbent; cannot see Tm 

sediment 

Sparse Isolated; sediment visible Ts 

2. Mussels - assumed to be Seep Mytilid Ia - (M_ -) 

Second term describes the density of the mussels: 

Dense Solid mass; cannot see Md_ 

sediment. 

Medium Patchwork; some sediment Mm_ 

is visible . 

Sparse Scattered; sediment visible Ms_ 

Third blank describes the color for lining mussels : 

Brown Live mussels M -b 

periostracum intact 

White Live mussels M -W 

periostracum eroded 

Hyphenate abbreviation describes dead mussels. 

Dead Dead; occur as scattered M-d 

debris 

(Example: Msb, Live, sparse, brown mussels) 

3. Bacterial Mats (B_) 

Second blank describes color of bacteria, when interpretable . Specific color 

discrimination cannot be reproduced, so a simplified approach is employed. 

White White bacteria Bw 

Colored Orange, Pink, Yellow Bc 

4. Sediment (S_) 

Second blank refers to surficial morphology 

Featureless No visible features; Sf 

smooth 

Bioturbated Visible burrows ; intense Sb 

reworking of sediment 

Hyphenated terms refer to "rocky" debris or fragments 

Rubble Scattered rocky debris; cm S-r 

scale assumed 

Boulders Large (+20 cm) rocks S-B 

7-6 
7 .2.2 Controlled Disturbance 

Plastic rods (3/4 in PVC pipe) were deployed at dive sites starting in 1987 to 

serve as simple reference scales. Of special interest is the deployment of both 0.5 m 

and 3 m rods at the NR-1 Brine Pool site (GC 233) . This site has a simple overall 

geometry, making it an ideal study site . Seven 3 m rods were deployed along the long 

axis of the pool. Pole 2596 was laid horizontally across the mussel mat at the 

northern end of the pool on JSL dive 2597, Sept. 13, 1989 . On dive 2598, 3 m rod 

2598 was placed at the southern end on the mussel mat. On dive 2599, rod 2599-1 

was inserted vertically at the northern edge of the pool and 2599-2 at the southern 

edge. These poles were within 20 cm of the inner edge of the mussel mat, but not 

touching it . A 2 m long rod was planted upright in the brine pool approximately 2 m 

from the mussel mat on the northern and southern ends of the pool. Approximately 2 

m from the northern end, rod B was inserted in the bottom of the brine pool and rod C 

similarly placed at the southern end. A third rod sank from sight when placed in the 

center of the pool. 

Outer Edge Markers - The edge markers were 50 cm 0 .52-inch PVC rods 

painted with 10 cm wide white, red, blue, and yellow bands . These bands allowed for 

both scale and color balance . Edge markers were inserted more or less vertically into 

the sediments at the outer edge of the mussel mat. At the northern end of the pool, 

five rods were placed at 1 to 2 m intervals along the well-defined edge of the mat. At 

the southern end, where the solid mat was replaced by scattered clumps of mussels, 

four rods were placed at the edges of clumps. 

Inner Edge Markers - A 2 m long 1 .5 inch PVC pipe was inserted into the pool 

bottom within 10 cm of the mussel mat inner edge, but not touching the mat. 

Approximately 40 cm of rod remained above the surface of the brine. 

Rectangular recovery frames were constructed of 21 mm outside diameter 

PVC water pipe . Fifty cm lengths were used to make 0 .25 m2 top and bottom 

7-? 
sections, and the remaining sides constructed of 20 cm lengths . Corners were 

fastened with 3-way right angle joints and commercial PVC pipe adhesive. The 

corners were drilled to allow for flooding of cavities. Half the interior of the cage was 

enclosed in 3.5 cm stretch mesh plastic fruit tree bird netting. The line of the netting 

was less than 1 mm in diameter, was easy to see through, and its black color limited 

obscuring of a video image. Nylon line was stretched across the diagonal of the cage, 

and then netting was secured with tie-wraps to these diagonal lines and adjacent 

sides. The 50 cm rods used had been sanded and painted white, blue, red, and yellow 

with an epoxy-based paint. The 10 cm long color bands provided visual scale, 

orientation, and color balance in subsequent image analysis . 

Cages were deployed in conjunction with massive mussel sampling during the 

1991 dive series . Two cages were deployed at Bush Hill on dives JSL-3129 (Sept. 5, 

1991) and JSL-3139 (Sept. 20, 1991). Single cages were deployed at Green Canyon 

272 on dive JSL-3137 (Sept . 19, 1991) and Green Canyon 234 on dive JSL-3144 

(Sept. 24, 1991). Disturbance consisted of repeatedly grabbing mussels with the JSL 

manipulator until more than 100 individuals had been collected. Since the contiguous 

area denuded was typically less than 0.25 m2, the cages were placed over the denuded 

area and adjacent bare substrate . The mussels collected were used in other 

shipboard studies . When marked mussels were returned to the site, they were 

deployed away from the recovery frames. 

The cages were revisited during the August 1992 dive series, approximately 

one year after deployment. The cages were visually inspected and then surveyed 

with the JSL video camera. Still frames from this video were printed using a 

Mitsubishi CP210U video copy processor, and the content and position of biota within 

the frame recorded . The rods at NR-1 Brine Pool were video surveyed in 1991 and 

1992 . 

7-8 
7 .2 .3 Monitoring Seafloor Gas Hydrates 

In 1991, we noticed a mound of gas hydrate that was breaching the seafloor 

near the bucket 1 sampling station at GC 184. In 1992, the feature was 

photographed to document its size and shape . In 1993, an intensive sampling effort 

was conducted to characterize this feature. All observations and sampling were 

conducted using the research submarines NR-1 and Johnson Sea-Link I. Samples of 

bubbles were collected in an acrylic vessel fitted with a funnel, inlet and outlet valves, 

which allowed the gas to enter the vessel and the displaced water to escape, and a 

check valve to release excess pressure during ascent . Aliquots of the sample were 

released under pressure and stored chilled in glass containers . Inert and volatile 

components of the bubble samples were analyzed at sea using a gas chromatograph 

with a thermal conductivity detector (Childress et al . 1984) . 

Sediment samples were collected in 45 cm long by 6.5 cm diameter coring 

tubes that were pressed into the seafloor with the manipulator arm of the Johnson 

Sea-Link. The cores were cut into 15 cm sections, poisoned with sodium azide and 

stored frozen under nitrogen in sealed cans. Gaseous hydrocarbons were recovered 

from the head space of the cans for analysis . Hydrocarbon analysis for all gas 

samples was performed using a gas chromatograph with flame ionized detection 

(Kennicutt et al. 1988a). 

Sediment temperatures under the hydrate mound were measured with a semiautonomous temperature probe. The probe consisted of eight thermistors (YSI model 

44006) mounted 20 cm apart along a 1.5 m long fiberglass tube. Thermistor readings 

were recorded at 30 s intervals by use of a Tattletales data logger mounted inside a 

pressure housing at the handle end of the probe. The thermistors were calibrated by 

comparison to a Hewlett Packard quartz thermometer and recorded temperature to 

an accuracy of at least ±0 .05°C. The probe was pushed into the sediment by the 

7-9 
mechanical arm of NR-1 and was left in place for -30 min to allow temperatures at 

the thermistors to stabilize. 

To determine the influence of fluctuating water temperature upon gas hydrate 

stability, we deployed a bubblometer -25 m away from the mound in a bed of mussels 

where we suspected that hydrate was present in the underlying sediment. This 

device, fitted with thermistors and a Tattletales data logger, used a 50 cm by 50 cm 

hood to funnel gas bubbles into a four-chambered wheel. Rotation of the wheel was 

retarded by a sliding counterweight; at 540 m water depth, a buoyant volume of gas 

equivalent to -3600 mL at standard temperature and pressure was required to lift 

the counterweight and rotate the wheel. Each chamber rotation tripped a magnetic 

switch; rotations and water temperatures were recorded at 5 min intervals . 

Deployment was accomplished with NR-1, and recovery was made with Johnson SeaLink. 

7.3 Results 

7.3.1 Photomosaics at GC 184: Site 1 

This site encompasses approximately 200 sq m that contained two beds of live 

mussels and a scattering of dead mussel shells. The entire region comprises a low 

knoll near the top of Bush Hill; minimum water depth is 540 m. The area covered by 

the photomosaic was surrounded three sides by tube worm bushes. Gas vents were 

evident in several sites within the mussel beds and the gas hydrate outcrop described 

in section 7 .5 was situated approximately 10 m to the northwest. The site is marked 

with a white, weighted bucket (5 gal) inscribed with a large number one . Although 

photomosaics were collected over about 25 sq m of the site during 1991-1993, the 

effective overlap was limited to a 6 sq m area, which is designated grids A through F . 

The features observed in A through F for 1991 to 1993 were as follows: 

7-10 
Grid A. 1. Mdb Grid B. 1. Mdb 4. M-d 

2. Mdb+Ts 2. Msb 

3. Ts 

Grid C. 1. Msb Grid D. 1. Tm 

2. Ts 2. Bw 

3. M-d 3. Sf 

4. Sf 4. Ts 

Grid E. 1. Ts Grid F. 1. Ts 

2. M-d 2. Bw 

3. Bw 3. Sf 

4. Sf 4. M-d 

The assessment of aerial variation in feature coverage over 1991 to 1993 

revealed no observable change . Evaluation of 1991 and 1992 data showed direct 

correlations for all grids. No observable change in population densities or 

distributions were seen for tube worms, mussels, or bacterial mats. The 1993 data 

was evaluated in a similar manner, but since it was a b/w mosaic, less accuracy in 

identifying color-dependent bacterial mats was obtained. Therefore, the 1993 mosaic 

was evaluated for the presence or change in those features, which are not color 

dependent for identification . 

From 1991 through 1993, the 6 sq m area around Bucket #1 did not reveal any 

discernible change in the aerial distributions or densities for tube worms, living 

mussels, dead mussels, featureless sediment, or bacterial mats. It was evident that 

some individual mussels had died during the study period, a smaller (50 cm) grid may 

detect some slight variations, but none were evident at this scale . 

7.3 .2 Photomosaics at GC 184: Site 2 

This site consist of a medium dense cluster of tube worms bordered by an 

expanse of uncolonized sediment . The sedimented area included several small 

patches of bacteria and a scattering of mussel shells . The area sampled by the 

7-11 
photomosaics encompassed a total of 8 sq m marked by a white weighted bucket (5 

gal) inscribed with a large number two . Water depth at the site was 550 m. 

The area evaluated was determined by the extent of video coverage available 

from 1992, as no usable photo coverage exists for that year. The area evaluated was 

8 sq m extending southwards from the bucket. Grids are designated A though H. 

Features observed in A through H for 1991 and 1992 were as follows: 

Grid A, 1 . Sf Grid B. 1 . Sf 

2. M-d 

Grid C. 1 . Sf Grid D. 1 . Sf 

2 . M-d 

3 . Ts 

Grid E. 1 . Tm Grid F. 1 . M b (density . . ?) 

2 . Ts 2 . Ts 

3. Sf 3 . Sf 

4. M-d 

Grid G. 1 . Ts Grid H 1 . Sf 

2 . Bc *1 2. M-d 

3.Sf*1 3. Ts *2 

4. Msb *2 

*1 - Cannot discriminate Bc from Sf on video owing to the oblique view presented, and variable 

illumination conditions. 

*2 - The "sparse" and "medium" descriptors are unclear on video for the same reasons as stated 

above. 

Between 1991 and 1992, no signs of colonization of the open sediment area 

was observed. No visible changes in tube worm occurrence or density was observed. 

Mussels were more difficult to evaluate owing to their small size and scattered 

occurrence . Nonetheless, no variations in the mussel population was observed . 

Video from Johnson Sea-Link dives 3270 and 3274 (1992) were examined for 

comparison to the color 1991 mosaic. It was not always possible to determine an 

accurate percent aerial coverage (distribution) for the desired features . Therefore, 

the video was examined to establish the presence or absence of features seen in 1991 . 

7-12 
Wherever possible, percent change was estimated. Review of the data reveal no 

evidence for change in the distribution or density of any observed fauna. 

The same features observed in 1991 were seen in 1992 with no noticeable 

change in distribution or density characteristics . Live video, which provided 

occasional (although accidental) close-ups, did not reveal any change in the character 

of the open sediment area or in fauna. 

7.3.3 Photomosaics at GC 234: Site 1 

This site was characterized by a large (approximately 8 sq m) mussel bed that 

graded into medium dense tube worms at the edges. These chemosynthetic fauna 

had colonized a broad terrace in a north-south slope. The site is marked by a green 

plastic milk crate and a 50 cm framework of pvc pipe. The water depth at the site is 

535 m. The area defined by 1992 coverage was 16 sq m. Grids are labeled A through 

P. Features observed in A through P for 1991 and 1992 were as follows: 

Grid A. 1 . Tm Grid B. 1. Tm Grid C l. Tm 

2. Msb 

3. Sf 

Grid D. 1 . Tm Grid E. 1 . Tm Grid F 1. Tm 

2. Ts 

3. Msb 

4. M-d 

5. Sf 

Grid G. 1 . Tm Grid H. 1 . Tm Grid I . 1. Msb 

2 . Ts 2. Ts 2.M-d 

3 . Msb 3. Sf 3. Sf 

4. M-d 

5. Sf 

Grid J. 1 . Tm Grid K. 1. Tm Grid L 1. Tm 

2 . Ts 2. Ts 2. Ts 

3 . Msb 3. Sf 3. Sf 

4. M-d 

5. Sf 

7-13 
Grid M. 1 . Tm 

2 . Msb 

3 . M-d 

Grid P. 1 . Ts 

2 . Sf 

3 . Tm 

Grid N. 1 . Tm Grid O 1 . Tm 

2 . Ts 

3 . Sf 

The mosaic and dive video provided sufficient quality and resolution to allow for 

accurate interpretation . In all 16 sq m evaluated for 1991 and 1992, identical 

features were observed. Tube worms revealed no visible change in density or 

distribution between 1991 and 1992. Here, were arranged in a ring, which forms a 

barrier around the open, central area of the bush. The center was dominated by open 

sediment and was colonized by mussels and sparse tube worms. 

In all 16 sq m evaluated for 1991 and 1992, identical features were observed. 

Both live (sparse) brown mussels and dead mussels were evident. Some brown 

mussels were borderline in classification between "sparse" and "medium" in density. 

No detectable change between 1991 and 1992 was observed for density or 

distribution . Some re-arrangement of mussels was observed, though no large-scale 

movements are seen. Finally, no new colonization in the open sediment area was 

observed for any fauna type . Except for small scale movements of mussels within 

the central open area of the bush, no detectable changes in distribution or density for 

any fauna was observed. 

7.3.4 Photomosaics at GC 234: Site 2 

This site is a cluster of mussels that had colonized a gas chimney about 75 cm 

in diameter and 1 .2 m high. A steady stream of gas bubbles was observed escaping 

through the mussel bed during observations in both 1991 and 1992 . The chimney 

was surrounded on three sides by very dense tube worm bushes . The entire 

aggregation was perched on the edge of a steep dropoff to the south. The site is 

7-14 
marked by an orange float labeled with the letter C . Water depth at the site was 545 

m. Because no scale controls (i .e ., laser dots, beer cans, etc.) were included, the size of 

the grids was estimated. For convenience, the mosaic was divided into 4 grid areas 

that are easily assessed on the 1992 video. Grids are labeled A through D . Features 

observed in A through D for 1991 and 1992 were as follows: 

Grid ,k 1 . Mdb Grid B. 1 . Mdb 

2. Mmb 2 . Mdw 

3 . Ts 3 . Ts 

4. Tm 4. Tm 

5 . Sf 5 . Sf 

Grid C. 1 . Tm Grid D. 1 . Tm 

2 . Sf 2 . Mdb 

3 . Ts 3 . M-d 

* It is difficult to separate dead mussels from live white mussels on the mosaic. This is also not 

possible on the live video . 

All features observed in 1991 year were readily detected in 1992 . The only 

noticeable change in faunal distribution occurred with sparse tube worms in Grid A. 

The 1991 mosaic revealed a small, isolated clump of brown mussels intermixed with 

sparse tube worms (and several stellaroids) . In the 1992 video the sparse tube 

worms, though still present, appeared to be greatly reduced in number. Estimated 

reduction was approximately one half. Live brown mussels were easily seen in both 

years . However, the white mussels were only seen in the 1991 coverage. The same 

look angle was not available in the 1992 video, therefore, positive discrimination 

between Mdw and M-d was speculative. This reduced the confidence in interpretation 

for Grid B. The variation in look angle between coverage types limited our confidence 

in establishing a high degree of accuracy for the entire evaluation area. However, 

those areas that were well represented in both years of data coverage revealed no 

clear evidence for change in faunal distribution or density. 

7-15 
7 .3 .5 Photomosaics at GC 272 : Site 1 

This site consists of a mussel bed approximately 10 sq m in area and situated 

amidst a number of prominent carbonate outcrops . Tube worms were present in 

limited numbers around the western edge of the bed. Depth increases sharply to the 

south and west. Markers at the site include a Gas seeps were observed in 1991, but 

were absent or less evident in 1992. Water depth at the site is 719 m. The area 

photographed defined 9 sq m, but 1993 coverage was limited to 6 sq m. Grids 

evaluated were designated A through F Features observed in A through F for 1992 

and 1993 were as follows : 

Grid A. 1. Tm Grid B. 1. Mdb 

2. Ts 2. MdW 

3. MmW 3. Sf 

4. Msb 

5. Sf 

Grid C. 1. Tm Grid D. 1. Sf 

2. Sf 2. M-d 

3. Mdb 3 . MsW 

4. MmW 

Grid E. 1. Sf Grid F. 1 . Sf 

2. Mdb 2. M-d 

3. M-d 3 . MsW 

4 . Msb 

Numerical evaluation was performed on the 1993 mosaic and qualitative 

estimates (low, medium, high) were attributed to the features seen in the 1992 video. 

Correlation was interpreted by comparing the qualitative terms with the percentages 

between features . High quality of both the video and the mosaic permitted accurate 

assessments. 

Tube worms showed no evidence of change in distribution or density. A minor 

variation for mussels was observed in Grid B. The Mbd (i .e., dense brown mussels) 

appeared to have 'rearranged' but with no discernible change in aerial distribution. It 

is probable that this change was due to disturbance during sample collection. The 

7-16 
percentage of "white" mussels (those in which the periostracum was eroded) did 

increase in this grid between 1992 and 1993 . The percent change in white mussels 

was estimated at 100°l0 (i .e ., the number of w. mussels doubled) between 1992 and 

1993 . No tube worm or other mussel colonization was observed in the open sediment 

areas. Except for the white mussel variations, the lack of any perceptible change in 

distribution or density for any other features suggested that the GC 272 area 

observed was very stable. Mussels appeared to move about, but did not show any 

large-scale shifts in density or distribution, and their percent of aerial coverage 

remained essentially constant. 

7.3.6 PhQtomosaics at GC 272: Site 2 

This site is found within an extensive region of vesicomyid clams. The site 

consists of flat and relatively featureless sediment. It is marked by a "clam corral," a 

square aluminum frame measuring 3 m on a side and marked with a float labeled with 

the number three. Water depth at the site was 728 m. A high level of detail was 

observable for the sediment surface in both years of evaluation. Any changes were 

readily seen . Comparison of 1991 and 1992 revealed, however, no observable 

changes in fauna type, distribution, or density. In both years, only a few clams were 

observed outside the corral. No visible colonization of sediment was observed by any 

organisms. No significant signs of bioturbation were noted, although a few, rare trails 

were seen and a spider crab was observed at Corral #3 . The area appeared 100% 

stable with no discernible changes between observational periods . 

7 .3 .7 Photomosaics at VK 82 

This site consisted of a carbonate hard-ground colonized by sparse tube worms 

and bacterial mats along two intersecting fractures. The site is marked with 

weighted, orange bucket (5 gal). Water depth at the site is 455 m. Grids were 

7-1? 
designated labeled A through F. Features observed in A through F for 1991 and 1992 

were as follows: 

Grid A, 1 . Bw Grid B. 1 . Mdb 

2. Sf 

Grid C. 1 . Sf Grid D. 1 . Bw 

2. M-d 2 . M-d 

3 . Ts 3 . Sf 

4. Ts 

Grid E. 1 . Sf Grid F. 1 . Sf 

2 . Bw 2 . Bw 

3 . Ts 3 . Ts 

We evaluated both mosaics simultaneously after inspecting each mosaic 

separately for greater accuracy. Numerical variation, when viewed in this manner, 

were seen as functions of estimates made where substantial areas of no coverage 

exist. One-to-one comparisons (made on screen) were more revealing and eliminated 

this problem. 

Replication of percent cover were observed for all features. Tube worms 

revealed no change in distribution or density. One noteworthy observation was the 

appearance of the bacterial mats located in (and near to) the fracture set. In both 

years, bacteria was highly visible. In fact, individual, small (- 6 - 84 cm diameter) 

patches of bacteria were specifically identifiable from 1991 and 1992. We initially 

suspected a change in bacteria from the mosaics alone ; however, upon review of dive 

tape 3261, we observed exactly the same patches observed in 1991. Bacteria 

showed no signs whatsoever for change in distribution. We estimated the same 

bacteria at 50 cm grid scale, and again found no visible change. Viosca Knoll 

appeared to be very stable. No observable changes in any feature distributions were 

observed. As with bacteria mats, it was possible to identify specific tube worm 

individuals from 1991 in the 1992 coverage, further suggesting a very stable setting. 

7-18 
7.4 Disturbance 

7.4 .1 hush Hill Site, GC 184/185 

Two cages were deployed approximately 40 cm apart at a site of dense mussel 

and mussel-tube worm cover. The cages were placed on sediments that had been 

exposed by the collection of mussels for experimentation. In cage one, approximately 

15 large (greater than 7 cm) mussels occurred as a cluster in the open area of the 

frame. Approximately 10 animals of similar size were attached to the frame of the 

cage on the open side . The netted side appeared to be devoid of mussels. The 

crustacean Munidopsis and Alvinocaris were present in both the open and netted 

area. The bottom in the open side not occupied by mussels was colored white, 

suggesting chemical and/or bacterial activity. The bottom on the netted side 

appeared to have no special coloration. 

Cage two was quite noticeably different . The netted section was completely 

covered with white Beggiatoa film obscuring observation of any contents. The open 

side was devoid of mussels, and the bottom was covered with both white and red 

Beggiatoa mats. Approximately two large mussels were attached to the outer edge of 

the frame, but none were in the open area. Munidopsis and Alvinocaris could be seen 

adjacent to the frame, but not within it . 

The area between the two cages contained a cluster of approximately 20 large 

mussel specimens both clumped and singly. This clump was an extension of a large 

mussel aggregation located adjacent to the cages . The bottom between the cages also 

had reddish Beggiatoa cover. Two specimens of the sea star Sclerasterias were near 

the to cages . 

7-19 
7 .4.2 GC 234 

A single cage was deployed on an area of mud and exposed low relief 

carbonates . The site contained both dense mussel aggregations and mussel-tube 

worm assemblages . The larger area around the cage was whitish due to bacteria or 

chemical activity. Two mussels were attached to the frame in this section. The 

netted section contained no mussels, but approximately three large specimens 

appeared to be attached to the exterior of the frame. The netted side contained 

numerous specimens of Alvinocczris, Munidopsis, and the gastropod, Buccinium . 

7.4 .3 Site Three GC 272 

A single cage was deployed in a field of mussel clumps . Tube worm bushes 

were in the area, but greater than 2 m from the cage . The cage abutted two mussel 

clumps . The open side contained a single mussel attached to the inner side of the 

frame. The bottom of the open side was soft bottom with no distinctive features. The 

netted side contained two mussels attached to the frame, one under the netting on the 

bottom and the other attached to the outside of the frame. At the edge of the netting, 

nearly centered within the frame was a large burrow approximately 30 cm across . 

The legs of a large crustacean, possibly an anamuran crab, could be seen within the 

burrow. During the period of observation, both a hake and a hag fish swam through 

the open side of the cage. 

7.4.4 Brine Pool 

The rods in the brine had a coating of "fuzz" that appeared to be hydroids. No 

other organisms were visible by simple inspection. There were no juvenile mussels 

obvious at any position on the rods . Rods around the periphery of the mussel mat 

had little or no such fuzz covering. Rods lying horizontally on the mat appeared to be 

devoid of a covering. 

7-20 
7.4.5 Inner Edge Markers - Brine Pool 

Only the southern edge marker was repeatedly surveyed in 1991, 1992, and 

1993 . With respect to the relationship with the mat edge, there was no major change 

between 1989 and 1993. The rods remained centered in a 10 cm radius indentation in 

the mussel mat with no noticeable advance in the mat edge. Faunsitically, two years 

after placement, the rod was covered with an unidentified tubiculous organism. Only 

in the 1993 survey were juvenile mussels, approximately 2 cm in maximum 

dimension observed. These were settled at the brine pool interface, where a reddish 

precipitate appeared to have formed. In addition to the tubicullous covering, limpets 

and sepulid worm tubes could be observed on the rod. 

The rod at the northern end was never well surveyed since its emplacement. 

By 1991, it was covered by tubiculous organism and still stood approximately 10 cm 

from the mat edge. Video footage taken in 1993 showed the tubiculous organisms to 

still be present. The rod appeared to still be at the very edge of the mat, but 

resolution was insufficient to resolve its exact position. 

7 .4.6 Mat Surface Markers - Brine Pool 

Only three edge markers remained upright and visible at the southern end. 

The surface rod had obviously moved since its deployment and rested against one of 

the vertical rods . Its ends lie upon living mussel clumps, while its central portion 

spans a mud channel . That is largely devoid of mussels and marked by dark chemical 

stains. No mussels, either juveniles or adults were observed on the rod in 1991, 1992, 

or 1993. Major movement of the rod from its deployment to its present location 

occurred between 1989 and 1991 . Since then, there has been apparently progressive 

movement of the rod in the direction of its length. This movement was quite evident 

by monitoring the point at which the vertical edge marker touches the surface 

7-21 
marker. Between 1991 and 1992, this point of contact moved 20 cm. Between 1992 

and 1993, there was an additional 10 cm movement in the same direction . 

Already by 1991, adult mussels were moving on to the northern edge although 

only -50% of the surface was obscured by mussels . In 1993, only the inner 1/2 of the 

rod was well surveyed. That end was -80% covered with adult mussels, greatly 

obscuring the rod . Even the small bare spaces bore marks of abyssal attachment. 

7.5 In Situ Observations 

During submarine dives at Bush Hill in July 1991, we observed streams of 

bubbles issuing from a low mound at a depth of 540 m. Covered with bacterial mats 

and surrounded by chemoautotrophic tube worms and mussels, the mound was quite 

conspicuous . Although seismic and geochemical evidence (Kennicutt et al. 1988a) 

indicated that shallow gas hydrate was present throughout Bush Hill, we did not 

recognize the mound as gas hydrate until H. H. Roberts (1991 personal 

communication) described finding gas hydrate in bubbling mounds at a hydrocarbon 

seep -10 km from Bush Hill . We now realize that hydrate mounds are common at 

hydrocarbon seeps in the Gulf of Mexico. 

The size and shape of the mound altered significantly over the course of 

<1 year. On August 28, 1992, the hydrate mound comprised three distinct lobes 

protruding in a row that extended -2.5 m along the crest of a low hillock (Figure 7 .1 

top) . The central lobe, the most prominent, was lens shaped and -50 cm in diameter. 

A deep undercut on its downslope side exposed yellow hydrate material (Figure 7.1 top 

right) . A steady stream of tiny bubbles and occasional bursts of larger bubbles 

escaped along the edges of the central lobe. The right-hand and left-hand lobes had 

some fractured sediment at their bases but lacked the undercut of the central lobe . 

Small pieces of hydrate broke free and floated away when the right-hand lobe was 

probed. We returned to the mound in 1993 for a series of dives that began on May 

7-22 
Figure 7 .1 Gas hydrate at a depth of 540 m on the continental slope of the Gulf of 

Mexico (27°26.7'N and 91°30 .4'W). A frame of video taken in 1992 

(upper) shows a prominent central lobe of hydrate breaching the 

seafloor between two smaller lobes (scale bar is 50 cm). The yellow 

material in the color inset photograph shows gas hydrate exposed near 

the point of the arrow; similar material was visible around the entire 

edge of the lobe (scale bar is 10 cm). Note the small bubbles on the left 

side of the frame. A mosaic of four photographs taken in 1993 (lower) 

shows that the central lobe of gas hydrate observed in 1992 had 

disappeared in 1993, leaving an area of disturbed sediment and exposed 

gas hydrate . The right-hand lobe increased in volume and elevation 

above the surrounding sea floor during this period. 

7-23 
15th. Comparison of photographs and videotapes showed that the entire central lobe 

had disappeared between the 1992 and 1993 observations, leaving an area of 

disturbed sediment and patches of exposed gas hydrate (Figure 7 .1 bottom). The lefthand lobe remained relatively unchanged, but the right-hand lobe had grown and had 

developed a deep undercut on its downslope margin. 

The temperature probe was inserted under the down-slope side of the left-hand 

lobe (Figure 7 .1 lower) on June 2, 1993 . We were able to insert the bottom 60 cm of 

the probe before it grounded against an impenetrable layer. The angle of insertion 

was about 50° off vertical . So, the lower-most thermistor was buried beneath the 

hydrate lobe at a subsurface depth of -26 cm; the middle thermistor was buried at 

13 cm; the third was at the seafloor; and the remaining thermistors were in the 

water column. After 26 min, the lower and middle thermistors had equilibrated to 

temperatures of 7.55°C and 7.41°C, respectively. The seafloor and water column 

temperatures were 7 .35°C and 7 .28°C, respectively. Insertion of the probe initiated a 

copious discharge of oil and gas that persisted for 30 min. Samples of the gas stream 

that vented continuously around the hydrate mound consisted of 0 .2% 02, 8% C02, 

11 .4% N2, 69 .6% methane, 6.3% ethane, 1 .7°lo propane, 0.2% i-butane, 0.9% nbutane, 0.3% i-pentane, and < 0 .1% n-pentane. The relative concentrations of C2+ 

gases in all samples collected at Bush Hill ranged from <0 .5% to >12% (Table 7.3). 

Highest C2+ concentrations were found in samples of the bubble stream; lowest 

concentrations were recovered from sediment cores . 

7-24 
Table 7.3. Hydrocarbon composition (C1-C5) of gas samples from the study site. 

Sample Methane Ethane Propane Iso-Butane N-Butane ISO- N-Pentane 

(%) (%) (%) (%) (%) Pentane (%) 

Bubble 88 .0 8.0 2.1 0.3 1 .2 0.4 <0.1 

Bubble 88 .0 7.5 2.2 0 .5 1 .1 0 .6 <0 .1 

Sediment 89 .4 8.0 0.9 0 .1 1 .0 N.D. 0.3 

Sediment 93 .5 5.3 0.8 0 .4 0 .4 0 .2 N.D. 

Sediment 94.3 4.2 0.8 0 .4 0 .3 N.D. N.D. 

Sediment 99.5 0.5 N.D . N.D. N.D. N.D. N.D. 

Note: N.D.-not detected 

The bubblometer was deployed from May 15 to June 27, 1993. Over this 44 

day period, spontaneous gas releases were only recorded during a 10 day interval 

when water temperatures exceeded 8 .0°C (Figure 7.2). Small gas releases began 

when the water temperature first exceeded 8.0°C on May 19 . The temperature 

remained >8.0°C and additional releases were recorded on May 21 and 23. Beginning 

on May 25, a series of major releases caused up to 23 rotations of the chamber per 

hour. Cessation of major releases coincided with a decline of water temperature to 

below 8 .0°C . Temperatures remained <8 .0°C for the subsequent 30 days, and no 

additional gas releases were recorded. The total volume of the gas release recorded by 

the bubblometer amounted to 900 L at standard temperature and pressure. 

7.6 Geological Interpretations 

The near-surface geology of hydrocarbon seeps is described in detail elsewhere 

in this report. Our purpose here is to examine the geological record for evidence of 

sudden, localized processes that could disrupt or destroy portions of the 

chemosynthetic community. The tectonic history of a seep will clearly result in 

gradual changes as hydrate mounds - such as Bush Hill - increase in size, or as 

active faults separate to form grabbens or half-grabbens such as can be seen at the 

GC 234 site. The progression of these processes is probably sufficiently slow that the 

7-25 
71 

N 

8 .5 

U 

°... 8.0 

a 

E 

t' 7.5 

7.0 

V 

V 

V 1 

., 

r 

f 

- ------------------- 

17 

May 

1993 

24 31 7 

June 

14 21 

40 

35 

30 

c 

25 

0 

20 

15= 

U 

10 

5 

0 28 

Figure 7.2 Data recorded by a bubblometer showing the release of gas and the water temperatures during a 44-day period in 1993. 

Temperatures detected by a thermistor resting in contact with the sea floor were recorded at 5-min intervals (solid line). 

The trends contained in the oscillations was determined with use of a 40-h low-pass filter (heavy line). The hourly 

rotation count (broken line) shows a series of intense gas releases interspersed with long periods of inactivity. Rotations 

at the beginning of the deployment (left arrow) resulted from gas released by our disturbance of the mussel bed. Five 

manual rotations were recorded upon recovery (right arrow) to check the system. 
communities are able to persist in the face of the ongoing change. A disruptive event, 

in contrast, causes relative sudden changes in conditions that render a habitat 

uninhabitable or prevent colonization . Several distinct processes meet this criterion, 

which are related to active fluid migration through the sediment-water interface . 

Active discharge of brine or brine and mud mixtures have been observed at 

numerous sites. The largest example of the latter was described by on the basis of 

observations made from Submarine NR-1 in May of 1993 . The site is located the 

GB 425 lease block (near 27°33.6'N and 92°32 .3'W) at a water depth of 590 m 

(Figure 2.1). It consists of a flat-topped, semicircular mound with a diameter of about 

1100 m and a bathymetric relief of about 20 m above the surrounding seafloor. The 

top of the mound was covered with thin crusts of calcium carbonate . Extensive mud 

flows were evident on the southern and northeastern flanks of the mound. Active 

mud vents were found by following the traces of these flows onto the upper periphery 

of the mound. These vents were roughly circular pools of undetermined diameter from 

which liquefied mud was overflowing at the down-slope edges. Continual streams of 

gas bubbles and oil droplets were erupting from these pools, creating a localized cloud 

of suspended sediments. A temperature probe, lowered into the mud recorded a 

temperature of 19°C, about 12°C warmer than ambient sea water. Several clusters 

of mussels (SM Ia) were found at locations adjacent to the mud vents where the 

substratum had stabilized sufficiently to permit colonization . Notably, no tube 

worms colonies were observed on this feature. 

The evidence of the mud volcano in GB 425 suggests that fluid discharge must 

stabilize before chemosynthetic fauna can make effective use of a seep site. 

Pockmarks offer a further example of this relationship . Formation of pockmarks has 

been attributed to vigorous discharge of gas through the seafloor (Hovland and Judd 

1988). The brine-filled pockmark found in GC 233 is an example of the combination of 

pockmark formation and brine flow (MacDonald et al. 1990a,b). The very high 

7-27 
concentrations of dissolved methane in the brine provide favorable circumstances for 

seep mytilids, which have formed a contiguous mat around the edges of the pool . In 

the current status of the mussel bed surrounding this brine pool, we see a highly 

productive and evidently stable community, but earlier in the geologic history of this 

feature, the edges of the pool would have been untenable. Even in the present 

configuration, the anoxic, hypersaline brine is a potential hazard to mussels and to 

fish. A slight change in the level of the brine in the pool would inundate the mussels 

around the inner edge and cause high mortality. 

The tendency for hydrate or authigenic carbonate to temporality cap active 

vents creates a potential instability in chemosynthetic habitats. Effects of pressure 

release due to failure of a hydrate plug is discussed earlier in this section; the 

geological record also provides evidence for eruptions that dislodged carbonate caps . 

Examples of disruptive or catastrophic events have been found at GC 184. Figure 

7.3 shows a massive slab of mussel shells and carbonate cement that was upended in 

the floor of a large pockmark near the crest of Bush Hill. The combination of the 

cemented shells in what is essentially a debris field left by an eruptive event clearly 

demonstrates the potential for localized mortality in chemosynthetic communities 

due to catastrophic pressure releases. 

7.7 Conclusions 

Over periods of one to two years and at scales of 1 m we could not detect any 

change in the distribution or composition of the chemosynthetic fauna at seven 

sampling stations in four separate study sites. These stations included tube worm, 

mussel, and vesicomyid clam habitats, as well as areas of bacterial mat and 

uncolonized sediment. As a corollary, neither were we able to detect recovery of 

mussel clusters from artificial disturbances when patches of about 0.25 sq m were 

removed. 

7-28 
co 

figure 7.3 Photograph of a massive slab of mussel shells and carbonate cement that was up-ended in the floor of a 

large pockmark near the crest of Bush Hill. 
On a larger scale, submarine observations at three of these site date to 1986 

or 1987 . Over this period as well, the character of the communities - the 

composition, the density, and the general distribution of the sessile chemosynthetic 

fauna has remained unaltered . That is to say that markers, both natural and 

artificial, that were associated in 1987 with clusters of tube worms or mussels, or 

with an area in which living vesicomyid clams could be found were in all cases 

associated with the equivalent fauna in 1993 . No mass die-offs or large scale shifts in 

faunal composition have been documented during the 8-year history of investigation 

of hydrocarbon seep communities in the northern Gulf of Mexico. Unless a longer 

time series can be obtained to challenge these results, the conclusion must stand that 

hydrocarbon seep communities show fidelity of composition and distribution over time 

scales of at least 5 years and probably 10 years and over length scales of meters to 

tens of meters. 

At the sub-meter scale, the surface markers at the brine pool (GC 233) 

indicate that there can be appreciable movement of mussels in some portions of the 

mat, while in other areas the mussels are much more sedentary. This can be 

interpreted in at least two ways . First, it might indicate that movement to maintain 

position on a gradient is more intensive at the northern end . Here, there is a low beret 

directly behind the mussel mat restricting the mat and the edge of the brine pool . 

Even slight vertical fluctuations in the brine layer would be easily manifest on the 

fauna, attempting to move up to avoid the brine or move down to obtain greater 

methane. By contrast, the southern end resembles an overflow plain in which any 

change in the surface level of the brine pool would simply flow through the outflow 

channels, and not create a major change in the vertical gradient experienced by the 

south end mussel clumps. 

An alternate interpretation is that movement of mussels does not reflect 

repositioning to stay in a desired range of a moving gradient, but that it is due to 

7-30 
growth of individuals wedging themselves deeper into the mat. In this view, the 

apparent lack of movement onto the rod at the southern end may reflect lower 

growth rates and/or recruitment rates in that region. 

The main conclusion that can be drawn from controlled disturbance 

observations is that recovery from the experimental disturbance does not proceed 

rapidly. Immigration of adult mussels into a disturbed area was minimal over a year, 

and there is no conspicuous recruitment of juveniles over the period of a year. These 

results are the same for all three sites with cages . The massive bacterial overgrowth of cage two at Bush Hill indicates that development of bacterial mats can 

take place in less than a year. 

The absence of apparent larval recruitment was the most interesting finding. 

However, the relatively short duration of the experiment coupled with the lack of any 

microscopic examination of the caged bottom limits the certainty of this observation. 

Mussels may have settled, but the field growth rates could be so slow that recruited 

individuals are still in the undetectable 1-2 mm size range. Alternatively, larvae may 

not have settled at all or, have recruited but failed to survive to a detectable size 

range. 

The observation that mussels have not colonized the brine pool rods for three 

years supports the conclusion that larval recruitment is a slow, and possibly rare 

event. This is especially important in the case of rods crossing the brine/seawater 

interface just a few centimeters from dense mussel clumps . It may be safely 

assumed that the rods are situated in a chemically suitable environment, yet no 

mussel clumps have appeared on the rods. 

In the introductory section, we mentioned that interspecies competition may 

be important. The hydroid cover on the brine pool rods is a hint that such processes 

are in operation. The vertical rods may provide an artificial refuge from predatory 

browsing by gastropods and crustacea. Isolated from the mussel mat's heterotrophic 

7-31 
fauna, the rods in the brine have escaped browsing by gastropods and crustacea and 

developed a dense hydroid cover. Rods lying horizontally on the mussel mat are 

exposed to browsing that crops back the hydroids. Similar dense hydroid cover is 

found on some tube worms in the Bush Hill area. It may be assumed that some 

peculiarities of local topography tend to restrict browsers on such bushes . 

Unfortunately, this grazer control explanation for the hydroids fails to explain the 

absence of new recruits from all rods at the brine pool . 

Recovery of artificially displaced mussel clumps by migration of mature 

mussels had begun one year after disturbance . Except for cage 1 at Bush Hill, very 

few specimens participated in this migration. More important to recovery, after one 

year there was no obvious recruitment of juvenile mussels. At this preliminary stage, 

it appears that recovery from disturbance that denudes mussel cover will be slow 

with successful recruitment of juveniles taking more than 3 years . 

Significant changes have occurred in these communities, however, within 

recent time. The animals are indisputably growing - albeit quite slowly, as we 

describe earlier in this volume. Presence of juvenile tube worms (Gardiner personal 

communication) and mussels demonstrates that spawning and settlement are ongoing. Observational evidence documents the presence of defunct mussel clusters 

and of dead tube worms in the process of being buried by sediment. These are 

localized loss, however, probably tied to subsurface rechanneling of seepage or to 

catastrophic, but again local events such as mud flows, blow-outs, or development of 

hydrate mounds. Geological evidence that small scale catastrophes that punctuate 

overall stability is overwhelming. The upended pavements, pockmarks, mud-flows, 

brine flows, and hydrate formation and dissociation are all consequences of a 

environment in which the final few meters of the sediment column alternately 

channel, check, and release fluids migrating from much greater depths . Resulting 

disturbances are neither so pervasive, nor so devastating as the volcanism of the 

7-32 
mid-ocean ridges, but will clearly affect sessile animals that form part of the barrier 

to seepage. 

The evidence from our observations of the outcropping hydrate at GC 184 

suggests that shallow gas hydrate acts as a kind of pressure relief system, 

alternately checking and releasing the flow of hydrocarbons from localized vents. 

Gases accumulate as components of hydrate; gas and oil are also trapped beneath 

layers of hydrate . The accumulated volume escapes when the plug of gas hydrate 

either dislodges because of excess buoyant force, or dissociates because water 

temperature rises above the limit of stability. The range of stable temperature is 

defined by hydrostatic pressure and by the chemical composition of individual hydrate 

bodies. Under these circumstances, intermittent formation and loss of shallow gas 

hydrate occurs over periods of less than one year. 

Shallow gas hydrate initially forms when gas, migrating from deep hydrocarbon 

reservoirs, encounters barriers such as authigenic carbonates and layers of biota 

near the seafloor. Lateral displacement of the flow of gas and temperature gradients 

such as we measured would contribute to the growth of gas hydrate layers . 

Observations indicate that these near-surface layers can be -10 m in diameter; 

morphologically, they resemble pingos. The exposed gas hydrate lobe (Figure 7 .1) is 

merely the edge of a larger formation. When a layer of gas hydrate breaches the 

seafloor, the flow of gas around its emergent edge will entrain fine sediment and 

undercut the base. The exposed lobe of gas hydrate is not a permanent feature. 

Eventually, its buoyant force and that of free gas trapped below will exceed the 

strength of its attachment; the hydrate lobe will then break free with a burst of gas 

and oil. A detached lump of gas hydrate will float upward in the water column for a 

considerable distance before dissociating completely in warm surface waters . 

Whether an entire layer of shallow gas hydrate could break free in a similar manner is 

7-33 
a matter for speculation, but clearly a catastrophic event of this type has the 

potential to affect local communities of chemosynthetic fauna. 

Intermittent releases of gas during warming events are a second mechanism 

that can release pulses of hydrocarbon to the water column . The gas venting 

recorded by the bubblometer (Figure 7 .2) was consistent with dissociation of methane 

hydrate and subsequent release of trapped gas caused by a temporary rise in water 

temperature. The brief lag observed between maximum temperature and maximum 

release would result from time required for the temperature wave to propagate 

downward into the sediment. No metabolic activity of the mussels or sediment 

bacteria could possibly produce a gas release of such magnitude or duration. To 

support this argument, we need to estimate the maximum stable temperature for 

hydrate that might form from the gases present in the shallow sediment of Bush Hill. 

Maximum stable temperatures for gas hydrate depend upon its chemical 

composition. The C2+ gases, particularly propane, are preferentially incorporated by 

the hydrate lattice and increase the dissociation temperature of gas hydrate. With 

the use of the program CSMHYD presented in Sloan (1990), we calculated that at a 

hydrostatic pressure of 5.4 MPa, the maximum temperature for a hydrate of pure 

methane is 7 .5°C ; whereas a hydrate formed at equilibrium with a gas mixture of 98% 

methane and 2% propane will contain 70% methane and 30% propane and remain 

stable to 14 .5°C . Inert gases will also increase the stability temperature of gas 

hydrate. 

Understanding gas hydrate formation and dissociation under these conditions 

is incomplete. It seems unlikely that natural gas hydrate at Bush Hill forms in 

equilibrium with its source gases . Preferential inclusion of C2+ gases during hydrate 

formation, and methanogenesis caused by microbial degradation (James and Burns 

1984), will increase the proportion of C 1 in gas trapped near the seafloor. The gases 

7-34 
sampled at Bush Hill have a range of compositions (Table 7 .3). Dissociation 

temperatures for hydrates formed from these gases should, therefore, also vary. 

Formation and loss of shallow gas hydrate is much more dynamic than is the 

case for deep gas hydrate. Generally, deep gas hydrate has probably been stable 

during recent geologic time . Dissociation of deeply buried gas hydrate over 10 ka 

cycles may result from sea level change and atmospheric warming and may influence 

climate cycles (Kvenvolden et al . 1993 ; McDonald 1990). More rapid dissociation 

might occur if large-scale slumping of marine sediments reduced overburden pressure. 

Our results indicate that formation and loss of shallow gas hydrate occurs at or near 

the seafloor within a year or less and that transient temperature increases can 

markedly influence this process . Warm cored eddies form in the northern Gulf of 

Mexico every 10-12 months, on the average (Sturges 1992); large eddies can reach 

14°C at 500 m depth and can force warm water onto the continental slope (Walker et 

al. 1993). High temperatures could cause dissociation of gas hydrate in near-seafloor 

sediment . Free oil and gas trapped beneath the gas hydrate would then escape . 

Given the widespread occurrence of shallow gas hydrate in this region, normal water 

circulation processes may intermittently trigger pulses of hydrocarbon released into 

the water column over a large geographic area. It is, therefore, reasonable to consider 

the possible effect of prolonged warming upon the total input of hydrocarbon into the 

Gulf of Mexico and the World Ocean. 

7-35 
8.0 Evidence for Temporal Change at Seeps 

Eric N. Powell 

8.1 Perspective 

As a population of benthic animals die, some fraction is buried and preserved in 

the sediments. This collection of dead remains is called a death assemblage. Because 

sediment is continually collecting, the death assemblage produces a more-or-less 

continuous record of the community. Unless long-term monitoring has been 

conducted, the death assemblage will be the only record of the history of the 

community. We have used the death assemblage to study the long-term history of 

petroleum seeps . Our primary objectives were to : (1) determine how stable or 

persistent seep communities are over long time spans ; (2) document the types of 

temporal variability present in seep communities and to suggest reasons for that 

variability; and (3) examine the response of seep communities to disturbance and 

determine their resilience. 

We utilize three measures of community structure to examine persistence, 

resilience, and temporal variability in seep communities : numerical abundance, 

paleoproduction, and paleoingestion . We evaluate persistence, resilience and 

temporal variability by using these measures to follow changes in guild and their 

structure and habitat optimally. Paleoproduction and paleoingestion are the 

paleontological analogues of biomass and energy flow through the consumer food 

chain in living communities (Powell and Stanton submitted a; b) . Biomass and 

ingestion as preserved are lifetime attributes, as all preserved individuals have 

completed their life spans. The prefer "paleo" is used to distinguish time-averaged 

attributes of this kind from their ecological counterparts. Accordingly, we define 

paleoproduction, the paleontological analogue of biomass, as the net production of 

somatic tissue over the animal's lifetime. In the same vein, we define paleoingestion, 

8-1 
the paleontological analogue of energy demand or daily consumption, as the amount 

of energy required to maintain the animal's necessary physiological processes 

(respiration, reproduction, growth) over its life span. We use a hindcasting model to 

calculate paleoproduction and paleoingestion . 

8.2 Flow Model 

The hindcasting model is formulated as follows. Estimation of paleo-energetics 

followed the approach described by Powell and Stanton (1985): 

Alt = Pglt +Prlt + Rlt (1) 

where Alt is the energy assimilated (in calories) over the individual's life span, Pglt, 

paleoproduction, is the portion of net production devoted to somatic growth over the 

individual's life span, Prlt is the portion of net production devoted to reproduction over 

the individual's life span, and Rlt is the amount of energy respired over the individual's 

life span . Paleoingestion, the amount of energy consumed (in calories) over the 

animal's life span is, then, the assimilated energy divided by the assimilation 

efficiency. 

Ilt = Alt (A/)"1 (2) 

The parameters of equation (4) are estimated as from the study of recent 

bivalve species follows: 

Pglt = KB (3) 

8-2 
where B is biomass-at-death in g ash-free dry weight (AFDW) and x is a caloric 

conversion (5675 cal g AFDW-1 for gastropods, 4452 cal g AFDW-1 for bivalves) . 

Biomass is calculated from biovolume (in mm3) by 

logloB = mblogloV + bb; (4) 

where, for bivalves, mb = 0.9576, bb = -4 .8939, and V = L3 where L is the maximum 

anterior-posterior length (in mm); and, for gastropods, mb = 0.7708, bb = -3 .2421, and 

V is an operational equivalent of shell volume derived, for most gastropods, from the 

equation for a cone, V = 1/3n(W/2)2L, where W (in mm) is the maximum width and L 

(in mm) is the apex-adapical tip length. For some cylindrical species, alternative 

equations are used (Staff et al. 1985 ; Powell and Stanton 1985) . 

t~ [m 

P~it = 18 or 6.8% of J RdR - j RdR (5) 

1 to to I 

where to is age at birth; tn,, age at maturity; and t , age at death. The values of 18% 

or 6.8% are for bivalves and gastropods, respectively. 

Pit = J / 1 ST 

r Mb

1(1Om`bb+b` )I [IOg10(T + 1)]3m~m~ ~ ~(1T 

J 

w 

The constants in equation (6) come from three equations: (a) biomass versus 

respiration at 10°C 

1og10R = mr1og10B + br; (7) 

8-3 
(b) size (biovolume) versus biomass [equation (4)] ; and (c) size versus age. 

V = S3 = fsTloglo(T+1)]2, (8) 

where S (in mm) is a linear measurement of size. The values of bb and mb are those 

used in equation (4). The values of mr and br were taken from the equation relating 

biomass to respiration at 10°C, mr = 0.8573 ; br = 1 .4984. The constant sT was 

obtained by solving equation (8) using a known maximum size and maximum age for 

each species. 

Maximum size was obtained from Ode (1975-1988) and our unpublished data. 

Maximum ages were obtained from Powell and Cummins (1985), Powell and Stanton 

(1985), Comfort (1957), Heller (1990), and Callender and Powell (in press). In cases 

where a maximum age could not be obtained from the literature either through 

reference to the species in question or a closely-related taxon, we assigned a 

maximum age of two years. This resulted in an underestimation of paleo-energetics 

for these species . Probably, the most egregious underestimate was setting lucinid 

maximum age at two years. No maximum age data are available for Lucinidae . This 

probably represents an underestimate of 400% or more . Consequently, estimates of 

energy flow derived from the model are also very conservative. 

Trophic guilds and habitat tiers were assigned using information in Andrews 

(1977), Staff et al. (1985), Callender and Powell (in press) and Stanton and Nelson 

(1980) supplemented by our own observations and other references cited in Staff et 

al. (1985) and Callender and Powell (in press). The set of trophic guilds comprised the 

following six guilds: herbivores, chemosynthetic species, low-level suspension feeders, 

high-level suspension feeders, predators, and deposit feeders. Parasites and predators 

were combined into one feeding category. No deep subsurface deposit feeders, which 

might require a fifth guild, were present. We divided the fauna into four habitat tiers 

8-4 
based on consideration of the location of their body mass and also their feeding 

structure: infauna, semi-infauna, low-level epifauna, and high-level epifauna. Rather 

than distinguishing habitat by mobility [e.g., vagrant as used by Scott (1978) and 

Miller (1982)], we used the high and low-level epifaunal categories of Ausich and 

Bottjer (1982) to distinguish habitat type . Low-level epifauna include species whose 

bodies rest on or just off the sediment surface and whose feeding structures are 

confined to near sediment-surface water. Mussels are good examples. The high-level 

epifaunal tiers comprise species that can raise their feeding structures significantly 

above the surrounding sediment either by being erect forms, forms attached to other 

taxa, or mobile forms, which typically climb on top of other species. Most vagrant 

benthos were included among the high-level epifauna. Semi-infauna are those 

animals whose body mass straddles the sediment-water interface, such as the 

vesicomyids . 

8.3 Size Frequency Distributions 

We examined the size-frequency distribution in two ways depending upon how 

an individual's size was standardized with respect to the remainder of the 

assemblage . In one case, we standardized individual size relative to the maximum 

size of the species. In this case, all adults, regardless of size, are relatively large, but 

some juveniles, considered small by this definition, may be of the same size as adults 

of another species, themselves considered large. Stunting is frequently a result of 

local shortages in food supply, such individuals fail to reach the species' maximum 

size. The absence of adults can indicate a suboptimal environment. Accordingly, 

large, by this criterion, is an important population attribute. In the second case, we 

standardized size relative to the size of the largest shell found in the assemblage. We 

chose the individual attaining the maximum linear dimension measured in all of the 

biofacies to designate this maximum size. By this definition, some adults are small. 

8-5 
An assemblage comprised exclusively of small individuals may indicate a generalized 

impoverishment in food supply. 

In both cases, the size-frequency distribution of each assemblage was divided 

into 10 size classes computed as the tenth percentiles of the chosen maximum size. 

We also calculated the fraction of the species' paleoproduction and paleoingestion 

contributed by the members of each size class . These plots are not frequency 

distributions where the size classes were redefined according to the biomass or energy 

flow parameter plotted. Rather, we retained the size of each linear size class in the 

species' or assemblage's total paleoproduction or paleoingestion. Accordingly, for 

paleoproduction and paleoingestion, the frequency spectra indicate the contribution 

by that linear size class to the whole assemblage . This follows the convention of 

Powell and Stanton (1985). 

8.4 Effects of Taphonomy 

Reconstruction of the long-term record of any community from its death 

assemblage requires an understanding of the processes that formed the preserved 

assemblage from the original community of living organisms. The processes that 

affect the organisms' remains after death are termed taphonomic processes . 

Taphonomic processes bias the death assemblage in several predictable ways. Not 

all species and individuals are preserved. Preservation is poor for species having few 

or no hardpans. Smaller individuals are generally less likely to be preserved even 

among the shelled biota, where preservation is better. Those that are preserved may 

be broken into fragments . Taphonomic processes are, therefore, an important bias 

that alters the death assemblage from its original composition. 

A second important bias is called time averaging. Because the process of 

sedimentation gradually buries the older remains, death assemblages provide a 

record, not only of the organisms that were alive, but also the sequence in which they 

8-6 
were alive. The degree to which this temporal record of events is recorded depends 

upon the sedimentation rate, the rate of sediment resuspension and redeposition, and 

a suite of biological processes that homogenize the sediment or otherwise disrupt the 

sequential record. The gradual degradation of the temporal record by these processes 

is called time averaging. Interpretation of the preserved record requires an 

understanding of the degree to which time averaging biased the assemblage. 

8.5 Taphonomy at Petroleum Seeps 

Taphonomic information was obtained in two ways: the death assemblage was 

examined for evidence of taphonomy, called the taphonomic signature, using a 

semiquantitative method developed by Davies et al. (1990) ; and shells were deployed 

for extended periods to measure the rate of taphonomic processes and time averaging 

at Bush Hill only. The results of this research will be summarized here. 

Dissolution and breakage are the principal taphonomic processes operating at 

petroleum seeps . Both processes occur in most death assemblages . However, 

petroleum seeps differ from most other environments of deposition by the severity of 

the taphonomic process. Dissolution, probably driven by sulfide oxidation that 

produces sulfuric acid, is of greater consequence at petroleum seeps than elsewhere 

on the Texas continental shelf, continental slope, or in most Texas estuaries and 

lagoons. Disarticulation and breakage is primarily caused by crabs or other animals 

and affects a significant fraction of the preserved biota. The resulting biases are of 

three kinds : (a) juvenile preservation is extremely poor; (b) small species are poorly 

preserved (unlike many environments, the bias against small individuals extends to 

the adults of small species) ; and (c) mussels are poorly preserved in comparison to 

the various clam species at all sizes. Thus the rarity of mussels in the death 

assemblage should not be taken as evidence of the rarity of mussel communities in 

the site's history. Little record of this biofacies is preserved under normal 

8-7 
sedimentary conditions. Catastrophic burial is required. A detailed discussion of the 

taphonomic data can be found in the Appendix. 

8.6 Effects of Taphonomy on Seep Assemblages 

We now consider the effect of taphonomy on species composition, guild and tier 

structure, and size-frequency distribution . We did this by recalculating each of the 

assemblage attributes under the assumption that taphonomy biased the assemblage 

against the lowest three size classes as defined by the assemblage's maximum size. 

We include, for comparison, a fossil analogue from Tepee Buttes, which represents 

the expected state after extensive taphonomic alteration. The Tepee Buttes of the 

Pierre Shale (middle late Campanian) near Pueblo, Colorado, are classic examples of 

cold seep deposits (Arthur et al. 1982 and Howe et al. 1986). The Buttes are aligned 

in quasi-linear group that represent fault traces along which hydrogen sulfide-rich 

fluids migrated from the underlying Pierre and Niobrara strata (Howe et al. 1986). 

Each linear group of buttes consists of asymmetrical conical carbonate mounds, 

ranging from 1 to 20 m tall and 4 to 60 m wide, surrounded by shale (Petta and 

Gerhard 1977). Portions of the limestone core contain a low diversity fauna 

dominated by lucinid bivalves. The low diversity and high density lucinid-dominated 

assemblages, stratigraphic relations between the carbonate mounds and adjacent 

shale, and the quasi-linear arrangement of the buttes suggest that the Tepee Buttes 

were formed by ancient hydrogen sulfide seeps, very similar to the Louisiana 

petroleum seep sites . 

Species richness was relatively high in the vesicomyid and lucinid biofacies, 

but low in the thyasirid biofacies and the Tepee Buttes assemblage (Figure 8.1). 

Numerically, Calyptogena ponderosa dominated the vesicomyid biofacies ; however, 

four additional taxa contributed at least 5% and 12 other taxa as much as 1% of the 

abundance . By paleoproduction or paleoingestion, Cdlyptogena ponderosa dominated 

8-8 
Lucinid biofades-coat slope 

Lucfnotna sp. 

cf. Bathymodtolus :p. 

Luctnoma attantis 

Calyptogtna ponderosa 

0 50 100 150 200 250 300 

Cumulative Percent 

Thyasirid biofacies-coat. slope 

Thyaiira oleophila 

Luctnoma sp. 

0 20 40 60 80 100 120 140 160 180 

Vesicomyid biofacies-coat. slope 

Calyptogena ponderosa 

Lucinoma n. sp. 

Bathyncodtolus n. sp. 

Tumid sp . 

Vericomya cordata 

Serpulid Tube 

Crab 

Lucinid biofacies-Tepee Buttes 

Nymphalucina occidentals: :: 

0 SO 100 150 200 250 300 

Whole abundance . Paleoproduction E3 Paleoingestion 

figure 8.1 The species composition of the clam biofaciee summed over all sites. 

Rank orders by numerical abundance, paleoproduction, and 

paleoingestion of taxa contributing 1% or more to the death assemblage. 

8-9 

0 50 100 150 200 250 300 
all the other taxa, contributing over 90°Io of the total. The Lucinoma biofacies was 

dominated by a species of Lucinidae, Lucinoma sp., and by the vesicomyid, Vesicomya 

cordata . Typically three to four other taxa accounted for 1% or more of the numerical 

abundance. V cordata frequently was more abundant than the lucinids. Both 

paleoproduction and paleoingestion were overwhelmingly dominated by Lucinoma sp. 

The thyasirid biofacies was dominated numerically by Thyasira oleophila; however, 

by paleoproduction or paleoingestion, Lucinoma sp. contributed over 60% to the 

assemblage's composition. Unlike the previous two biofacies, few other species were 

present. Nymphalucina occidentalis was the dominant species at Tepee Buttes in 

terms of numerical abundance, paleoproduction, and paleoingestion . The mactrid 

bivalve, Cymbophora ashbunerii, was the second most common species, but 

contributed only 2 .3% of the individuals and less than 1% of the paleoproduction. 

After taphonomic loss, species richness dropped precipitously in the lucinid and 

vesicomyid biofacies, but changed relatively little in the thyasirid biofacies and in the 

Tepee Buttes assemblage (Figure 8.2). The precipitous drop in species richness in the 

former two indicates the importance of size-selective taphonomic loss in assemblages 

numerically dominated by small species and juveniles of large species. In contrast, 

the presence and rankings of species by paleoproduction or paleoingestion was 

changed relatively little by taphonomic processes . 

The guild structures of the lucinid and vesicomyid beds at petroleum seeps 

were dominated by chemoautotrophs, with minor contributions from the predator, 

suspension-feeder, and herbivore guilds, when evaluated by numerical abundance 

(Figure 8.3). Chemoautotrophs accounted for over 90% of the paleoproduction and 

paleoingestion . The thyasirid biofacies was somewhat unique in that the guild 

structure was exclusively chemoautotrophic, and thus the guild structure consisted 

exclusively of low-level suspension feeders . The guild structure of the fossil analogue 

from Tepee Buttes was almost identical to that of the thyasirid biofacies . 

8-10 
A. 

Lucinoma n. 

Vesicomya cord 

Lucinoma aUa 

Thyasira oleop~ 

Gymnobela judiU 

Chama 

Pleurotomella 

Pleuromeris 

Bathymodiolus n. 

Alvania cerifann 

CuspTdaria obi 

Lu&noma r . 

8athymodiolus n. 

Calyptogena ponders 

LuGnoma aft 

Solemya 

VesFcomya cord 

Ilidnoma n. 

Batttymodiolus n. 

Calyptogena pondsn 

Vesicomya cord 

Cantrainea 

Laanoma afiai 

B . 

Calyptogena ponderosa 

Thyasira oleophila 

Vesicomya cordata 

CanUainea sp. 

Lucinoma n. sp . 

8athymodiolus n. sp. 

Cataegis cf . meroglypta 

Pleurotomella sp. 

Tumdae sp . 

Chama sp. 

Luanoma atlantis 

Unidentified gastropod 

Unidentified gastropod 

Trochidae sp. 

Ervillia sp. 

Unidentified limpet 

Gymnobela sp. 

Calypfogena ponderosa 

Ludnoma n. sp. 

Bathymodlolus n. sp. 

Vesiaomya aordata 

Calyptogena ponderosa 

Bafhymodiolus n. sp. 

l .ucainoma n. sp. 

M Individuals 

B(omass 

Paleolngesflon 

Individuals 

Biomass 

Paleofngest(on 

figure 82 A. The species composition of the lucinid biofacies summed over all 

sites. Rank orders by numerical abundance, paleoproduction, and 

paleoingeetion of tags contributing 1% or more to the death assemblage. 

B. The species composition of the vesicomyid biofacies . Rank orders by 

numerical abundance, paleoproduction, and paleoingestion of taxa 

contributing 1% or more to the death assemblage . 

8-11 

0 10 20 30 40 50 60 70 80 90 

Percent (%) 

o 10 2o 3o 4o 5o 6o 7o eo 9o 1oo 

Percent (%) 
Lucinld biofacies-coot, slope 

1 

'a0 ' 

F 

0 

r 

C 

a 

Vesicomyid biofacies-contslope 

1 

30 

1 

F 

w 0 

.. C 

v 

a 

120 

100 

90 

60 

40 

20 

0 

Thyas[rid biofacies-cont slope 

Total whole TAW- !Paeoa6urr&nct o6undcrue production ingestion 

Lucinid coquina-Tepee Buttes (unmodiC~cd) 

Total Whole !MCO- 'l'0lLOa6unJana a6urrdarue production ingution 

High Filtcrcr Dcposit Fccdcr 

Low Filtcrcr ~ Grazcr 

Prcdatar ~ Chcmaautotroph 

120 

100 

80 

60 

40 

20 

0 

figure 8.3 The cumulative feeding guild structure of the seep clam biofacies 

summed over all sites, defined by total and whole abundance, 

paleoproduction, and paleoingestion . 

8-12 

Total' Whole Volta- Palcoa6wrdana abundance production rogation 

AbJl7LdG71LL TOICO' r0/CO-iriBlltlOA 

production 
Following taphonomy, the guild structures of the lucinid and vesicomyid beds 

was overwhelmingly dominated by chemoautotrophs (Figure 8 .4) . Few of the 

remaining guilds were represented at all and none were important contributors . 

Chemoautotrophs accounted for over 99% of the paleoproduction and paleoingestion. 

Unlike the lucinid and vesicomyid biofacies, the guild structure of the thyasirid 

biofacies was unchanged and it remained exclusively chemoautotrophic. 

The tier structures of lucinid and vesicomyid beds, when evaluated 

numerically, were a complex mixture of infauna, semi-infauna, and low-level epifauna 

(Figure 8.5). Infauna, however, dominated the tier structure of lucinid beds and semiinfauna the tier structure of vesicomyid beds when evaluated by paleoproduction or 

paleoingestion . Infauna dominated the tier structure of the thyasirid biofacies by any 

measure - numerical abundance, paleoproduction, or paleoingestion . The fossil 

analogue from Tepee Buttes, like the thyasirid assemblage from the continental 

slope, was dominated by infauna when evaluated numerically, by paleoproduction or 

by paleoingestion . 

Following further taphonomic loss, infauna came to dominate the tier structure 

of lucinid beds numerically as well as by paleoproduction or paleoingestion (Figure 

8 .6) . Semi-infauna came to dominate the tier structure of vesicomyid beds 

numerically as well as by paleoproduction and paleoingestion. Infauna dominated the 

tier structure of the thyasirid biofacies by any measure - numerical abundance, 

paleoproduction, or paleoingestion. Thus, like guild structure, the thyasirid biofacies 

was unchanged by further taphonomic loss. 

The size-frequency distribution was divided into 10 size classes computed as 

the tenth percentiles of the chosen maximum size . We defined the assemblage's 

maximum size according to the largest shell collected in the assemblage. We also 

calculated the fraction of the species' paleoproduction and paleoingestion contributed 

by the members of each size class . These plots are not frequency distributions where 

8-13 
120 

100 

H 80 

0 60 

40 

so 

0 

Grciaid btotacia-conL elope 

Whore roko- 4areoa6.6. producNoa bigcstfon 

Vesicomyid biofacies-coat slope 

120 

100 

H 

60 

40 

a Z° 

0 

l16ux&nee Valco- :aleop-ducH°n bWrtlon 

120 

100 

60 

ao 

Zo 

0 

Lucinid coquina-Tepee Buttes (unmodifkd) 

120- 

1007 

60 

40 

20 

0 

Thyasicid blofaciercont slope 

whore rareoa6undaaa productioR 3grstion 

High Flterer Deposit Feeder 

Low Filterer Grazer 

Predator 0 Chemoautotroph 

Figure 8.4 Predicted paleoguild structure of seep clam biofaciea summed over all 

sites, defined by total and whole abundance, paleoproduction, and 

paleoingestion after taphonomic loss of all individuals in the lowermost 

three size classes. 

8-14 

sakwJrws6ara produeNox Ingrstton 
Ludnid biotaclea-cont.slope 

1-4 

.+ 

120 

100 

60 

40 

20 

0 

lbyssirid biofacies-cont.slope 

TOW Whole Taco- Yaleosbw.4= amdoux pmdaW 

120 

3 ioo 

F 80 

0 60 

40 

a 20 

0 

a lEgh Epifeim 

Low Epifaima 

Semi-Infaima 

WMma 

Lucinid biofactes-Tepee Buttes (unmodiAed) 

Figure 8.5 The cumulative habitat tier structure for seep clam biofacies summed 

over all sites, defined by total and whole abundance, paleoproduction, 

and paleoingestion . 

Vestcomyid biofacles-cont.slope 

AbUXAMM rawproduetl°a 

8-15 

?ota! 'Whole Palm- 4aleo- 

?ota! whole rolco- 4o(eoa6wr6ana o6ur,6ana pnnQucdon bgrstlon 
Lucinid biotacles-cont .abpe 

0 

E+ 

w 

Thyasirid biofacles-conislope 

Lucinid coquina-Tepee Buttes (unmodified) 

1 . . 

D M8h Epifauna 

Low FVifMM 

Semi-Infauna 

WE= 

1 

Figure 8 .6 Predicted tier structure of seep clam biofacies summed over all sites, 

defined by total and whole abundance, paleoproduction, and 

paleoingestion after taphonomic loss of all individuals in the lowermost 

three size classes. 

w 

00 

Sao 

60 

40 

20 

0 

wnok iauo- fauo-

~6~aiLanes pmd--tlon 6gtttlon 

Vesicomyid biofacies-cont.slope 

8-16 

7Vho(t Paleo- 4aleodw6ana production bgutiax 

llbux"nee ?oleo- 2aleoprodutti°a hBtttbn 

Wkole 4ako- Paleo-

.6w&da= prodvctiorc eVcsctora 
the size classes were redefined according to the biomass or energy flow parameter 

plotted. Rather, we retained the size classes defined by linear dimension to show the 

relative importance of each linear size class in the assemblage's total paleoproduction 

or paleoingestion. Accordingly, for biomass and paleoingestion, the frequency spectra 

indicate the contribution by that linear size class. 

The size-frequency distributions for the vesicomyid, lucinid, and thyasirid 

assemblages were bimodal (Figures 8 .7 to 8 .9). The lucinid biofacies contained a 

mode in the smaller two size classes and a mode in the middle four size classes. Most 

of the paleoproduction was present in the central four size classes . Much of the 

paleoingestion was also, however, a significant component present in the uppermost 

size classes . The largest specimen in this assemblage was 92 mm. The largest 

specimen in the thyasirid assemblage was much smaller, 59 mm. This assemblage's 

size-frequency distribution was distinctly bimodal, with modes in size classes two to 

four and in the uppermost three . The two modes were produced by two 

chemosynthetic species of differing size, Lucinomd sp. and Thyasira oleophila . The 

larger individuals, all lucinids, were about the same size as those occupying the 

central mode in the lucinid assemblage. Both species were present primarily as 

adults. Consequently, the assemblage's paleoproduction and paleoingestion were 

distributed relatively evenly between the two modal size groupings. (Had the lower 

mode been dominantly by juveniles, it would have contributed little to the 

assemblage's paleoproduction and even less to the assemblage's paleoingestion.) The 

vesicomyid biofacies also contained two modes, one in the lower two size classes, and 

the other in the upper four . The upper three size classes contained most of the 

paleoproduction and paleoingestion. The largest specimen collected was 92 mm. The 

largest specimen from Tepee Buttes was 39 mm. The size-frequency distribution 

was monomodal; the mode occupied the central size classes. The single mode shifted 

to somewhat larger sizes for paleoproduction and took on a slightly bimodal 

8-17 
Numerical abundance by biofacies 

90 

e0 

1 

100 

>p 

60 

50 

r, 

Perce4t 

30 

Z0 

10 

0 

'1bY ~ . ~ ~ '`Vt ~ 

. 

she 

14A 

Oak 

Figure 8.7 The size-frequency distribution for the clam biofacies summed over all 

sites. Size classes are defined as the tenth percentiles of the size of the 

largest individual in each assemblage. Listed values are the upper 

boundaries of the size classes. Numerical abundance represents the 

fraction of the total number of individuals is each size class . Each 

distribution is color coded for clarity. 

8-18 
Figure 8 .8 The apportionment of paleoproduction among the size classes for the 

clam biofacies summed over all sites. Size classes are defined as the 

tenth percentiles of the size of the largest individual in each assemblage. 

Listed values are the upper boundaries of the size classes . 

Paleoproduction represents the fraction of each assemblage total 

contributed by the individuals in each size class. Each distribution is 

color coded for clarity. 

8-19 

Paleoproduction by biofacies 
Paleoingestion by biofacies 

P 

erIZ11.t 

Perce4t 40 of ~~ 

MZe 

-r% ja 

Oak 

Figure 8.9 The apportionment of paleoingestion among the size classes for the 

clam biofacies summed over all sites. Size classes are defined as the 

tenth percentiles of the size of the largest individual in each assemblage. 

Listed values are the upper boundaries of the size classes. 

Paleoingestion represents the fraction of each assemblage total 

contributed by the individuals in each size class. Each distribution is 

color coded for clarity. 

8-20 
appearance for paleoingestion because some individuals of large species attained 

near-maximum size . Overall, the size-frequency distribution resembled the lucinid 

biofacies but lacked the smaller mode that was produced by the juvenile size classes. 

After further taphonomic loss, only the large mode in the lucinid and 

vesicomyid biofacies' size-frequency distribution remained (Figures 8.10 to 8 .12). In 

contrast, the thyasirid assemblage remained as distinctly bimodal after further 

taphonomic loss as it had been originally . Paleoproduction and paleoingestion 

remained distributed after further taphonomic loss as they had been in all original 

biofacies . 

Thus, taphonomic loss modifies the guild, tier, and size-frequency structure of 

the lucinid and vesicomyid biofacies towards that observed in the fossil analogue from 

Tepee Buttes, and also toward the thyasirid biofacies at petroleum seeps . The latter 

is a recent analogue that evidently has not received significant living input in the near 

term. The thyasirid biofacies resembled the Tepee Buttes assemblage in most of the 

important attributes that are consistent with its taphonomic age. The taphonomic 

process uniformly resulted in simplification of tier and guild structure, a substantial 

decline in species richness, and a lesser, but important, change in size-frequency 

distribution (when evaluated by numerical abundance). Taphonomic loss affected the 

evaluation of community structure relatively little when paleoproduction and 

paleoingestion were used as the defining variables . These two variables, then, are 

robust indicators of community structure . 

Time averaging, in assemblages deposited below storm wave base, occurs by 

one of four mechanisms: (1) sedimentation rate is so slow that individuals of widely 

different ages are deposited in effectively the same sampling interval; (2) bioturbation 

mixes the sedimentary column and reorients the contained shells; (3) physical 

processes of sediment degassing and seepage mix the sedimentary column and 

reorient the contained shells ; and (4) infauna die at depth in sedimentary layers 

8-21 
Numerical abundance by biofacies 

loo 

9p 

8p 

>0 

60 

_ 5U 

+o 

30 

20 

1-0 

vu 4~ T 

~t ~f 

P 

erce4t 

y T 

biw 

414~ 

40k 

Figure 8.10 Predicted size-fi equency distribution for the clam biofacies summed 

over all sites after taphonomic loss of all individuals in the lowermost 

three size classes. Size classes are defined as the tenth percentiles 

of the size of the largest individual in each assemblage . Listed values 

are the upper boundaries of the size classes. Numerical abundance 

represents the fraction of the total number of individuals is each size 

class . Each distribution is color coded for clarity. 

8-22 
size 

lou 

9p 

e0 

?0 

6p 

Sp 

p~~~'At 

3p 

Z0 

lp 

0 

figure 8.11 Predicted apportionment of paleoproduction among size classes for 

the clam biofacies summed over all sites after taphonomic loss of all 

individuals in the lowermost three size classes. Size classes are 

defined as the tenth percentiles of the size of the largest individual in 

each assemblage . Listed values are the upper boundaries of the size 

classes. Paleoproduction represents the fraction of each assemblage 

total contributed by the individuals in each size class . Each 

distribution is color coded for clarity. 

8-23 

Paleoproduction by biofacies 
Paleoingestion by biofacies 

loo 

90 

80 

70 

610 

5p 

to per1Z,,,t 

30 

Z0 

lp 

°U 

1` fit 50 

i 

qze 

V . 

"~'Tep~s 

00 `~~ aq ' 

Figure 8.12 Predicted apportionment of paleoingestion among size classes for the 

clam biofacies summed over all sites after taphonomic loss of all 

individuals in the lowermost three size classes . Size classes are 

defined as the tenth percentiles of the size of the largest individual in 

each assemblage. Listed values are the upper boundaries of the size 

classes. Paleoingestion represents the fraction of each assemblage 

total contributed by the individuals in each size class . Each 

distribution is color coded for clarity. 

8-24 
containing the remains of much older epifaunal species. Bioturbation is relatively 

unimportant at petroleum seeps . Physical processes capable of mixing shells have 

been observed. These include mud volcanoes and gas blowout craters. Their overall 

importance cannot yet be ascertained but clearly can have local significance . 

Measurements of time averaging, now available from single cores taken at four sites 

(GC 184, GC 272, GC 234, and GB 425) show no evidence of these processes . 

Infauna dying at depth is, however, a significant process, which, combined with a slow 

sedimentation rate, assures a certain degree of time averaging. Nevertheless, amino 

acid analyses show that individuals of single infaunal species gradually become older 

downcore, consistent with the expectation that time averaging occurs on a limited 

scale defined by the living depth of the adults of these species. Thus, a significant 

temporal record is present that would permit examination of temporal trends on the 

scale of about 300 years for lucinid beds and less that 300 years for the vesicomyid 

and mussel assemblages. 

8.7 Site Descriptions 

Site descriptions will be limited to clam beds, with one exception, because the 

keystone species of the other important chemosynthetic associations found in seep 

habitats, tubeworms and mussels, are rarely preserved in significant quantities . 

8.7.1 Green Canyon 184 (Bush Hill) 

Samples from Green Canyon 184 (Bush Hill) were dominated numerically by 

vesicomyids and lucinids (Figure 8 .13) . Although some Lucinoma atlantis and 

Calyptogena ponderosa were collected, Lucinoma sp . and Vesicomya cordata 

predominated. Lucinoma sp . accounted for the majority of the paleoproduction and 

paleoingestion. Samples were obtained by box coring, so penetration rarely exceeded 

50 cm. The lucinid biofacies at GC 184 was typical of lucinid biofacies at all the sites 

8-25 
GC-184 Ludnid biofacies 

Vesicomya cordata 

Lucinoma n. sp. 

Luctnoma atlanttr 

Bathymodiolur n. sp. 

Calyptogina ponderosa 

Alvania ceritanmur 

Pleurotomtlla tp. 

Thyaiira oleophila 

Vttrinella 

Unknown Gastropod 

Gymnobela judithae 

Plturomeris cf. armilla 

Gyncnobela sp. 

Chama :p. 

G&386 Lucinid biofacies 

Lucinoma n. rp. 

Luctnoma atlantir 

Plturomerir cf. armilla 

Thyarira oleophila 

Cantrainea rp. 

Vericomya cordata 

Solemya sp. 

Thyasira cf. exavata 

Gymnobela judtthae 

BathyncodioIus n. rp. 

Batrrchia canatae 

Unknown bivalve 

Owenta ip. A 

Abundance " Paleoproduction 0 Paleoingestion 

Figure 8.13 The species composition of the lucinid biofacies at GC 184 and GB 

386. Rank orders by numerical abundance, paleoproduction, and 

paleoingestion of taxa contributing 1% or more to the death 

assemblage. 

8-26 

o 2o 4o 6o 8o 1oo 12o 14o 16o 18o 2oo 

Cumulative Percent 

0 SO 100 150 200 250 300 

Cumulative Percent 
in its size-frequency composition. Modes occurred in the lowermost two size classes 

and the middle four (Figure 8.14). The central mode and the largest two size classes 

accounted for the bulk of the paleoingestion and paleoproduction (Figures 8 .15 and 

8.16). These size frequencies were nearly identical to that observed at GC 234, very 

similar to GC 272, and somewhat different from GB 386 where the largest two size 

classes were not equivalently represented. 

Categorizing the individuals by species' maximum size showed that the 

majority of the individuals were juveniles (Figure 8 .17). Adults rarely exceeded 70% of 

the species' maximum size. Individuals greater than 70% of the species' maximum 

size were more abundant at GC 234, GC 272, GB 386 and GB 425 indicating that GC 

184 was a less optimal site for lucinid growth than the other sampled sites . Those 

few large individuals did account for proportionately more paleoproduction and 

paleoingestion than at the other sites, however (Figures 8 .18 and 8 .19). One 

explanation for this oddity would be occasional periods near optimal for growth 

interspersed by periods of much longer duration that were marginal enough to limit 

survivorship to adulthood . The fraction of individuals of 10% maximum size was 

highest in the lucinid biofacies from GC 184 indicating that survivorship was 

probably lower here than at GC 272, GB 386, and GB 425, assuming that the 

taphonomic process was similar at all sites . 

Guild structure at GC 184 was overwhelmingly chemosynthetic when 

evaluated numerically and almost exclusively chemosynthetic when evaluated by 

paleoproduction or paleoingestion . Comparison to lucinid biofacies at other sites 

shows that chemosynthetic dominance was this high only at GC 184 and GB 386 

(Figure 8 .20). Tier structure was complex, when evaluated numerically, because of 

the presence of vesicomyids and some true epifauna . Infauna dominated tier 

structure when evaluated by paleoproduction or paleoingestion. The infaunal 

8-27 
Numerical abundance by biofacies 

100 

SIo 

80 

>o 

60 

So 

40 percent 

30 

C'8_42S 

G8_3~ Lu . ~~d g~ . 

GC_~Z Vet ~ b~f ~~$ 

GC c`2'2 ~CiNd . ~ld $~f ~s 

) S1Ze 

`234 Luc, , _ _ ~f ~a 

N~- a 

figure 8.14 The size-frequency distribution for the clam biofacies at GC 184, GC 

234, GC 272, GB 386 and GB 425 . Size classes are defined as the 

tenth percentiles of the size of the largest individual in each 

assemblage . Listed values are the upper boundaries of the size 

classes. Numerical abundance represents the fraction of the total 

number of individuals is each size class . Each distribution is color 

coded for clarity. 

8-28 
Paleoproduction by biofacies 

100 

90 

80 

>o 

60 

50 

40 percent 

30 

Zo 

lp 

0 

:z~~s+Q C'B.a2S 

Gg,3~ ``ry~d o bi 

GC.2~2 V~~~ ~ d 

G GC.2~2 ~~ ' my~ bi 

s 

Size 

K V-234 

Aciea 

Figure 8.15 The apportionment of paleoproduction among the size classes for the 

clam biofacies at GC 184, GC 234, GC 272, GB 386 and GB 425. 

Size classes are defined as the tenth percentiles of the size of the 

largest individual in each assemblage. Listed values are the upper 

boundaries of the size classes. Paleoproduction represents the 

fraction of each assemblage total contributed by the individuals in 

each size class. Each distribution is color coded for clarity. 

8-29 
Paleoingestion by biofacies 

100 

90 

70 

60 

So 

40 

P 

erizent 

30 

io 

pe ~a 

rceht of 

Sloe 

-'t3y ~,°facka 

GC-184~__"Q b~.~a 

'"ndbjo~,, 

"~8 

figure 8.16 The apportionment of paleoingestion among the size classes for the 

clam biofacies at GC 184, GC 234, GC 272, GB 386 and GB 425. 

Size classes are defined as the tenth percentiles of the size of the 

largest individual in each assemblage. Listed values are the upper 

boundaries of the size classes. Paleoingestion represents the fraction 

of each assemblage total contributed by the individuals in each size 

class. Each distribution is color coded for clarity . 

8-30 
Numerical abundance by biofacies 

Figure 8.17 The size-frequency distribution for the clam biofacies at GC 184, GC 

234, GC 272, GB 386 and GB 425. Size classes are defined as the 

tenth percentiles of the size of the largest individual for each species. 

Listed values are the upper boundaries of the size classes. 

Paleoingestion represents the fraction of each assemblage total 

contributed by the individuals in each size class. Each distribution is 

color coded for clarity. 

8-31 
Paleoproduction by biofacies 

90 

80 

>o 

60 

50 

4,0 

percent 

30 

Illy Qla .386 kk4d 

V 

QC!-2.72 

, V 

-'4,d biofacies 

bi 

ie, "I - 100 S) 

size 

~Ir- ~234 01~ 

Figure 8.18 The apportionment of paleoproduction among the size classes for the 

clam biofacies at GC 184, GC 234, GC 272, GB 386, and GB 425. 

Size classes are defined as the tenth percentiles of the size of the 

largest individual for each species . Listed values are the upper 

boundaries of the size classes. Paleoproduction represents the 

fraction of each assemblage total contributed by the individuals in 

each size class. Each distribution is color coded for clarity. 

8-32 
Paleoingestion by biofacies 

ip0 

90 

80 

60 

50 

40 Percent 

30 

0 

1-1 0 v G8

.4Z51~G8_3&6 

~ . _`1y~d bi°f~ ca 

GC-272 

Vc~ ~ ~f ~~s 

GC_272 ~°~yid 

bi 

°F~ cs 

90 (spe~leS) 

S12e 

lpp 

`234 b f"`lc s 

. 

~`'`~ , a 

194 La,~ c 

`mod 

`qCj~ 

Figure 8.19 The apportionment of paleoingestion among the size classes for the 

clam biofacies at GC 184, GC 234, GC 272, GB 386, and GB 425. 

Size classes are defined as the tenth percentiles of the size of the 

largest individual for each species. Listed values are the upper 

boundaries of the size classes. Paleoproduction represents the 

fraction of each assemblage total contributed by the individuals in 

each size class . Each distribution is color coded for clarity. 

8-33 
i 

N 

9 

i 

v 

0. 

1 

0. 

GC-184 LucWd biofactes 

00 

s0 

60 

40 

20 4 1 

0 

A6uxdana ?ako- 4aleopffl6utt7°a iwtstton 

GC-272 Vesicomyid biofactes 

zu 

00 

80 

60 

40 

20 

0 

ssowiaanec :aao- rampProd-don *W-UM 

0 High Filterer 

Low Filterer 

Predator 

Deposit Feeder 

Crrazer 

Chemoautotroph 

Anunaana Pra :uo- racouett°n Lgrstfon 

figure 820 The cumulative feeding guild structure for the seep clam biofacies at 

GC 184, GC 234, GC 272, GB 386, and GB 425, defined by numerical 

abundance, paleoproduction, and paleoingestion. 

8-34 

GC-Z34 LucWd biofacies 

l16uxdanee ta(to- takotrodueti°x bwestfort 

GC-272 LucWd biofacies 

J16andana Polco- 4aleoprodrrctio- bgrstlon 

GB-425 Thysslrid btofacies 

116undana 4ako- talcop-ducti°a LrBution 

GB-386 LucWd btofacies 
contribution to tier structure, when evaluated by energy flow, was typical of most 

lucinid biofacies (Figure 8.21) . 

A number of cores were obtained with lucinids spanning the coring interval of 

50 to 70 cm . We know from piston coring elsewhere that the accumulated remains 

extend much deeper than this at most, if not all, sites. Living lucinids rarely are found 

deeper than 5 cm. An estimation from sedimentation rate would give the lowermost 

lucinids an age of 667 to 4000 yr. given Behren's (1988) range in sedimentation rate. 

14C dates identified the oldest shell as 3,500 yr. old (Table 8 .1). 13C/12C ratios for 

shell carbonate indicated that nearly all shell carbonate came from seawater 

bicarbonate and 14C dates of living shells were effectively present day so that shell 

ages were not compromised by the inclusion of 14C-dead carbon, obtained as 

interstitial or metabolic bicarbonate, into the shell . Amino acid dating showed that 

lucinid beds in this area have been gradually accumulating for many years . No 

evidence of catastrophic processes was present. 

Figures 8.22 through 8 .28 show an example of one core through a lucinid bed at 

GC 184 . Juveniles accounted for the majority of the individuals in this core 

throughout its length (Figure 8 .23) . Large individuals were most frequent above 20 

cm and below 35 cm. Large individuals were rare in-between . The largest shells were 

found primarily above 20 cm. Plots of paleoproduction and paleoingestion (Figures 

8.24 and 8 .25) show that community energy flow was dominated by small individuals 

from 20 to 35 cm (and at 5 cm). Categorizing size relative to species' maximum size 

(Figures 8 .26 through 8.28) shows that adults were uncommon between 25 and 35 

cm and that large adults were only common in the upper 10 cm of the core . Juveniles 

dominated the assemblage throughout the core. Thus, most small individuals were 

juveniles, not adults of small species. Plots of paleoproduction and paleoingestion 

show that the largest individuals accounted for the most energy flow (compare Figure 

8-35 
Table 8.1 14C dates of mussel, lucinid, and thyasirid beds. 

Seep Site Sample Estimated Age (years) 

GC-234 Mussel Bed 1 (95 cm) 2000- 3000 

GC-234 Mussel Bed 2 (195 cm) 3000 -4000 

GC-184 Lucinid Bed (68 cm) > 3500 

GB-425 Thyasirid Bed (40 cm) 500- 1000 

8-36 
GC-184 Lacinid biofictes 

a 

GC-272 Lncinid blofacies 

w 

00 

60 

40 

20 

0 

samo1"" 

~1 

Ez go 

60 

40 

20 4 

GB-386 LucWd biofacies 

GC-234 Luclnld btofacies 

12(Y 

1 

60 

40 

?A 

0 

Abundance Paeo- MOojnvduett°x tngts"on 

GC-272 Vesicomyid biofacies 

120 

100 

80 

60 

40 

20 

n 

GB-425 Thyssirid biotacies 

A6undaue Pa4o- fa4oP-ductl°a bgut7on 

High Epifauna E] Low Epifauna 0 Semi-Infauna M Infauna l 

Figure 8.21 The cumulative habitat tier structure for the seep clam biofacies at 

GC 184, GC 234, GC 272, GB 386, and GB 425, defined by numerical 

abundance, paleoproduction, and paleoingestion . 

8-37 

l16wa(oxa ta(io- taleoPnLucti= 5genbrc 

ra&obe-tiA6uxdarac !alto- PolcoprnLuceion bgution 
GC-184 Lucinid biofacies-core 1 

0 cm 

Vesicomya cordata 

Lucinoma n. sp. : . :::: : : . . : : . :::..... . 

Pleurotomella 

Vitrintlla 17 

0 50 100 150 200 250 

Cumulative Percent 

10 cm 

Vesicomya cordata 

Plaurotoraella 

Vitrintlla 

Lucfnoma n. sp. 

. . . . . .... . . . . . . . . ........ . . . . . . . . ....... . . . . . . 

Gymnobtla judithae 

0 20 40 60 80 100 120 140 160 180 200 

2 $ cm 

Vesicomya cordata 

Gymnobtla judithae . : : :.:: : . : : : . : : : 

0 SO 100 150 200 250 

35 cm 

Vesicomya cordata 

Calyptogena ponderosa u' °°°°°°°°°°°°'"""'~ 

Mr 

Chama rp. 

1# 1 T= 

0 20 40 60 80 100 120 140 160 

50 cm 

Lucinoma n. rp. 

Lucisoma atlantis 

Vesicomya cordata 

Alvania ceritanmus 

Pleuromcrif cj armilla 

10 Abundance / Paleoproduction 0 Paleoingestion 

Figure 822 The species composition of the lucanid biofacies at GC 184 from Core 1 . 

Rank orders by numerical abundance, paleoproduction, and 

paleoingestion of taxa contributing 1% of more to the death 

assemblage. 

8-38 

0 50 100 150 200 250 
Numerical abundance by core interval 

peat 

Green Canyon 184 

Figure 823 The size-frequency distribution of 5 cm core intervals at GC 184 

Core 1 . Size classes are defined as the tenth percentiles of the size of 

the largest individual in each assemblage. Listed values are the 

upper boundaries of the size classes . Numerical abundance 

represents the fraction of the total number of individuals is each size 

class. Each distribution is color coded for clarity. 

8-39 
Paleoproduction by core interval 

loo 

9p 

e0 

70 

6p 

5p 

lcqt 

30 

20 

lp 

0 

Green Canyon 184 

Figure 8.24 The apportionment of paleoproduction among the size classes for 5 

cm core intervals at GC 184 Core 1. Size classes are defined as the 

tenth percentiles of the size of the largest individual in each 

assemblage. Listed values are the upper boundaries of the size 

classes . Paleoproduction represents the fraction of each assemblage 

total contributed by the individuals in each size class . Each 

distribution is color coded for clarity. 

8-40 

-""1a e, ~_ -up `` 
Paleoingestion by core interval 

loo 

9p 

80 

70 

60 

50 

sp 

30 

20 

lp 

0 

Green Canyon 184 

Figure 825 The apportionment of paleoingestion among the size classes for 5 cm 

core intervals at GC 184 Core 1. Size classes are defined as the 

tenth percentiles of the size of the largest individual in each 

assemblage. Listed values are the upper boundaries of the size 

classes. Paleoproduction represents the fraction of each assemblage 

total contributed by the individuals in each size class . Each 

distribution is color coded for clarity. 

8-41 
Numerical abundance by core interval 

loo 

9p 

80 

70 

60 

Sp 

40 lpftl-~ 

t 30 

lp 

D 

Green Canyon 184 

Figure 826 The size frequency distribution for 5 cm core intervals at GC 184 

Core 1. Size classes are defined as the tenth percentiles of the size of 

the largest individual for each species . Listed values are the upper 

boundaries of the size classes. Numerical abundance represents the 

fraction of the total number of individuals in each size class. Each 

distribution is color coded for clarity. 

8-42 

-

.ea) 

alze 

" w 
Paleoproduction by core interval 

loo 

90 

80 

70 

60 

SO 

so P ~t 

at 

tp 

10 

0 

Green Canyon 184 

Figure 827 The apportionment of paleoproduction among the size classes for 5 

cm core intervals at GC 184 Core 1. Size classes are defined as the 

tenth percentiles of the size of the largest individual for each species. 

Listed values are the upper boundaries of the size classes. 

Paleoproduction represents the fraction of each assemblage total 

contributed by the individuals in each size class. Each distribution is 

color coded for clarity. 

8-43 

.,q 
Paleoingestion by core interval 

loo 

9p 

e0 

>0 

60 

5p 

40 plerc~ 

3p 

20 

0 

Green Canyon 184 

figure 828 The apportionment of paleoingestion among the size classes for 5 cm 

core intervals at GC 184 Core 1. Size classes are defined as the tenth 

percentiles of the size of the largest individual for each species. Listed 

values are the upper boundaries of the size classes. Paleoproduction 

represents the fraction of each assemblage total contributed by the 

individuals in each size class . Each distribution is color coded for 

clarity. 

8-44 

-yea) ~ `'o 
8.28 with Figure 8 .25 for example). Long-lived small species were not important 

anywhere in the core. 

Changes in species composition occurred coincident with changing optimality 

for the lucinaceans . Below 35 cm, habitat optimality was about average for 

Lucinidae . Lucinoma sp. was abundant and frequently reached 70% of the species' 

maximum size . The habitat became suboptimal for Lucinidae at about 30 cm and 

remained so to 20 cm. In this interval, vesicomyids dominated the community by 

number, paleoproduction and paleoingestion. However, few adults were found, so that 

the habitat was distinctly suboptimal for the vesicomyids. Interestingly, few juvenile 

lucinids were found, indicating that the reduction in habitat optimality for the 

lucinaceans included an essentially complete failure of settlement. Lucinoma sp. 

returned at 15 cm and again dominated the paleoproduction and paleoingestion of the 

assemblage, despite continuing settlement of vesicomyids, because this species 

routinely grew to 70% or larger of maximum size . Thus, the history of this site was 

defined by the disappearance and reappearance of lucinids and the persistent 

settlement of vesicomyids few of which, however, grew to adulthood. The lucinid 

assemblage was not persistent, but was resilient in this context. Although specific 

dates are unavailable, the 15 cm hiatus probably accounts for 500 to 1000 yr. 

The changing structure of the biota at this site can also be visualized through 

guild and tier structure . Chemoautotrophs dominated in the upper and lower portions 

of the core and were overwhelmingly dominant in terms of paleoproduction and 

paleoingestion. In the central portion of the core, however, predators were dominant 

(Figure 8 .29). This is one of the characteristics of the non-seep or normal slope fauna 

and indicates a transition from a chemosynthetic community where most of the 

primary consumers are shelled organisms to a community where most of the primary 

consumers are soft-bodied organisms . Changes in tier structure were complex. 

Infauna dominated deep in the core and, by paleoproduction and paleoingestion, at the 

8-45 
GC-184 Core l Lucinid biofacies 

0 cm 

120 

100 

H 80 

0 60 . .,., 

e 40 

a, Zo 

0 

Abundance Paleo- Paleoproduction ingestion 

25 cm 

121 

-3is 

o & 

H 

. . 

o 

C 

a> 44 

a °~ 21 

1 

Abundance Paleo- Patioproduction ingestion 

50 cm 

120 

100 

o 

H 80 

0 60 ++ 

C 

40 

a 20 

0 

Abundance Paleo- Paleoproduction ingestion 

120 

100 

80 

60 

40 

Zo 

0 

10 cm 

Abundance Paleo- Paleoproduction ingestion 

120 

ioo 

80 

60 

40 

20 

n 

35 cm 

Abundance Paleo- Paleoprortuction ingestion 

High filterer 

Low Filterer 

Predator 

Deposit Feeder 

Grazer 

Chemoautotroph 

Figure 829 The cumulative feeding guild structure of several core intervals from 

the lucinid biofacies at GC 184 Core 1, defined by numerical 

abundance, paleoproduction, and paleoingestion. 

8-46 
top of the core . In-between, epifauna and semi-infauna were important. This, too, is 

a characteristic of the non-seep or normal slope fauna. The absence of a significant 

infaunal component in the shelled biota in the central region of this core is 

particularly significant because most chemosynthetic species that are preservable 

are also infaunal (Figure 8.30). 

Figure 8 .31 shows the downcore distribution of total faunal abundance, 

paleoproduction, and paleoingestion . The presence of a chemosynthetic biota 

coincides with high faunal abundances, paleoproduction, and paleoingestion . The 

presence of the non-seep biota coincides with low faunal abundance and extremely 

low paleoproduction and paleoingestion, in keeping with the dependency by the nonseep biota on planktonic production as the sole food supply. Faunal abundance, 

paleoproduction and paleoingestion were higher in the deeper chemosynthetic section 

of the core, even though the largest animals were found in the upper section, 

assuming roughly equivalent sedimentation rates. A second core at GC 184 sampled 

the background (non-seep) fauna for the region. Although the species composition 

varied downcore, no pattern could be discerned in the size-frequency distributions 

evaluated by any measure: numerical abundance, paleoproduction or paleoingestion. 

Most individuals were small, as is typical for the non-seep benthos. With a few spotty 

exceptions, these small individuals accounted for a large fraction of the 

paleoproduction and paleoingestion in each core interval because many of these were 

adults of small species. Categorizing the size classes by species' maximum size 

shows that adults were routinely collected in most core layers and some individuals 

normally attained 80% or more of species' maximum size . Juveniles of 

chemosynthetic species dominated the numerical abundance throughout the core 

indicating their persistent settlement but failure to survive. Thus, the absence of 

chemosynthetic adults was not due to settlement failure, but was due to poor 

survivorship. The standard slope fauna dominated energy flow at the site . 

8-47 
GC-184 Core l Lucinid biofacies 

0 cm 

120 

100 

F° 80 

60 

0. 

20 

25 cm 

v 

0. 

Id -V 7 

so 

60 

ao 

20 

0 

.... ... . 

xnunaancc 7auo- Paleoproduction rogation 

1 

0. 

50 cm 

20 

00 

80 

60 

40 

20 

0 

Abundance TWO- TalcopnvducHon ingestion 

10 cm 

, . , 

35 cm 

121 

is 

81 

44 

2( 

Abundance Nalco- Paleoprrvductton ingestion 

High 

D Epirauna 

0 

LOW 

Epifauna 

SemiInfauma 

Infauna 

Figure 8.30 The cumulative habitat tier structure of several core intervals from 

the lucinid biofaciee at GC 184 Core 1, defined by numerical 

abundance, paleoproduction, and paleoingestion. 

8-48 

Abundance Palco- PalcoprnducNon ingestion 

Abundance Palco- Paleoproductlon ingestion 
0 

5 

10

15 

20 

25 

30 

35 

40 

45 

50 

0 cm 

5 cm 

10 cm 

15 cm 

20 cm 

25 cm 

30 cm 

35 cm 

40 cm 

45 cm 

50 cm 

0 

5 

70 

15 

20 

25 

30 

35 

40 

45 

50 

0 100000 200000 300000 400000 500000 60ooov iouuUu 

Paleoingestion 

Figure 8.31 The numerical abundance, paleoproduction, and paleoingestion 

contributed by each 5 cm core interval from the lucinid biofacies at GC 

184 Core 1. 

8-49 

0 10 20 30 40 50 60 70 80 

Abundance 

0 5000 10000 15000 20000 25000 30000 35000 40000 

Paleoproduction 

GC-184 Core 1 Lucinid biofacies 
Tier and guild structure provide a good example of the structure of the normal 

slope community. Most core intervals were dominated by predators . Predators are 

routinely over-represented in shelf and slope biofacies . This over-representation is 

particularly apparent in paleoproduction and paleoingestion. Chemoautotrophs are 

only important numerically, indicating their small size and insignificant contribution 

to community energy flow. Similarly, tier structure is weighted towards epifauna, as 

expected from the normal slope biota. Infauna were unimportant in community 

energy flow (as preserved). 

The highest values of abundance, paleoproduction, and paleoingestion are low 

in comparison to the other core for GC 184 that included chemosynthetic faunas. A 

slight trend in increasing abundance was observed downcore. Paleoingestion and 

paleoproduction were highest in the upper section of the core and sporadically high 

lower in the core . 

8.7 .2 Green Canyon 272 

Both lucinid (Lucinoma sp .) beds and vesicomyid (dominantly Calyptogena 

ponderosa) beds were sampled at GC 272. Samples were obtained exclusively by box 

core . Although penetration routinely exceeded 40 cm, the assemblage was always 

contained in the top 10 cm of the core. Evidence from piston cores obtained at other 

sites, discussed subsequently, prevent the unequivocal designation of GC 272 as a 

relatively new seep. Deeper samples would be required . Living vesicomyids were 

occasionally collected . Living lucinids were very rare . Lucinids in the top 10 cm, 

dated by amino acid dating using a 14C calibration curve obtained from GC 184 

samples, showed that most shells were no older than about 500 yr. Many specimens 

could not be distinguished from living individuals, indicating a very recent origin. 

The lucinid biofacies at GC 272 was dominated numerically, by 

paleoproduction, and by paleoingestion by Lucinoma sp. The lucinid biofacies at GC 

8-50 
272 was nearly identical to that observed at GC 184 in most aspects . The sizefrequency distribution was dominated by small individuals with a second smaller 

mode in the central 5 size classes (Figure 8 .14) . Larger individuals were 

proportionately somewhat more common that at GC 184 . Unlike the lucinid beds at 

GC 184, GC 234, and GB 425, the smaller size classes accounted for a significant 

fraction of the assemblage's paleoingestion indicating that a suite of small species, 

represented by adult individuals, was also present at this site (Figure 8 .16). These 

species accounted for little of the paleoproduction, however, because they were small 

(Figure 8 .15) . When the individuals were tallied according to the fraction of species' 

maximum size, the assemblage was characterized by a proportionately greater 

fraction of adult individuals than observed at GC 184 or GC 234. In this 

characteristic, the lucinid beds at GC 272 and GB 386 were similar (Figure 8.17). As 

in the other biofacies, the adult individuals accounted for the bulk of the 

paleoproduction and paleoingestion (Figures 8 .18 and 8 .19). 

Guild structure at GC 272 was overwhelmingly chemosynthetic when 

evaluated numerically, by paleoproduction or by paleoingestion . Comparison to 

lucinid biofacies at other sites shows that chemosynthetic dominance was lower than 

at GC 184 and GB 386 (Figure 8 .20). Tier structure was complex, when evaluated 

numerically, because of the presence of vesicomyids and some true epifauna . 

Infauna dominated tier structure when evaluated by paleoproduction or 

paleoingestion. The infaunal contribution to tier structure was typical of most lucinid 

biofacies (Figure 8.21). 

The vesicomyid biofacies was dominated numerically, by paleoproduction, and 

by paleoingestion by Calyptogena ponderosa . Like the lucinid beds, the vesicomyid 

beds at GC 272 were characterized by a bimodal size-frequency distribution, one 

mode in the smallest size classes and another in the upper 4 size classes (Figure 

8 .14). A larger proportion of the shells collected were large (80% or better of the 

8-51 
assemblage's maximum size) than in the other clam biofacies . Like most of the other 

clam biofacies sampled, the upper five size classes accounted for the bulk of the 

paleoproduction and paleoingestion, indicating that most of the smaller individuals 

were juveniles (Figure 8 .15, 16) . When the individuals were tallied according to the 

fraction of species' maximum size, the assemblage was characterized by a 

proportionately greater fraction of adult individuals than observed in the lucinid beds 

at GC 184 or GC 234, but a similar proportion as observed in the lucinid beds at the 

same site and at GB 386 (Figure 8.17). As in the other biofacies, the adult individuals 

accounted for the bulk of the paleoproduction and paleoingestion (Figures 8 .18 and 

8 .19) . 

Guild structure of the vesicomyid biofacies at GC 272 was overwhelmingly 

chemosynthetic when evaluated numerically and nearly exclusively chemosynthetic 

when evaluated by paleoproduction or by paleoingestion . Numerically, guild structure 

was somewhat more complete than observed in the lucinid biofacies (Figure 8.20). 

Tier structure was complex, when evaluated numerically, because of the presence of 

vesicomyids and some true epifauna. Semi-infauna dominated tier structure when 

evaluated by paleoproduction or paleoingestion (Figure 8 .21). 

The downcore trends in species composition and size-frequency distribution for 

the lucinid biofacies at GC 272 showed that the larger individuals were restricted to 

the upper 5 cm of the core . From 5 to 15 cm, only small individuals were found. No 

specimens were taken below 15 cm. Most of the assemblage's paleoproduction and 

paleoingestion in the top 5 cm of the core were contributed by the upper 5 size 

classes . Defining the size classes by the species' maximum size showed that most of 

the individuals in most core intervals were juveniles . However, some adults were 

present at 5 and 10 cm indicating that the intervals below the surface were 

characterized by small species, some of which reached a significant fraction of 

maximum size. Thus, in intervals down to 15 cm, adults accounted for most of the 

8-52 
paleoproduction and paleoingestion, even though, below the surface, these adults were 

of small species. The species composition of this core shows that lucinids were 

common down to 10 cm, however conditions were adequate to produce large 

individuals only in the upper 5 cm. Thus, below the top 5 cm, the habitat was 

suboptimal for chemosynthetic species. Below the top 5 cm, the assemblage was 

composed of a series of small species, many of which occurred in the central zone of 

the core described from GC 184 (Figure 8 .22) and in the non-seep biofacies cored at 

the same site . Guild structure also documents the introduction of the 

chemosynthetic fauna at the top of this core . Predators dominate the lower core 

intervals as is typical of the normal slope biofacies. Chemoautotrophs dominate the 

upper portion of the core. Tier structure shows the transition from a dominantly 

epifaunal shelled biota to a dominantly infaunal shelled biota. Thus the assemblage 

below 5 cm at GC 272 contained those species characteristic of the non-seep 

biofacies. Whether this site is a new site, or whether the sampled horizons below 5 

cm represent a period of time of suboptimal conditions for chemosynthesis such as 

recorded at GC 184, cannot presently be determined. 

Six additional cores were taken from GC 272, none of which were subsectioned. 

These cores record the extent of local variability in conditions at this site. Small 

individuals are common in only three of the cores, both lucinid samples and one of the 

non-seep samples. The remaining two cores were composed of relatively large 

individuals (as compared to their respective biofacies elsewhere). Recalculation in 

terms of paleoproduction and paleoingestion show that the small individuals included 

a fair percentage of adults of small species in both lucinid samples. When examined 

according to species' maximum size, two of the lucinid cores contain a substantial 

proportion of juveniles. The other had fewer. Juveniles are important in only one of 

three of the sampled non-seep biofacies . Paleoproduction and paleoingestion show 

substantial variation in the size structure of the larger individuals . Nevertheless, 

8-53 
individuals routinely grew to 80% or more of species' maximum size in these biofacies, 

as they did in the one described previously from the subsectioned core indicating that 

this site has been characterized, at least recently, by near optimal conditions for 

chemosynthetic species. The absence of juveniles, however, suggests recruitment 

failure in most recent times (although severe taphonomic loss cannot be excluded as 

an alternative explanation) . 

In considering the guild and tier structures for these cores, only two cores are 

dominantly chemosynthetic . Two other cores containing chemosynthetic individuals 

retain significant non-chemosynthetic traits . Predators and deposit-feeders are 

common. Thus, local variability in guild structure is high, which might be indicative of 

a site recently colonized by chemosynthetic species. Analysis of tier structure 

supports these conclusions. In particular, infauna are uncommon. 

Overall, then, GC 272 is characterized by substantial small scale variability in 

conditions . However, most locales conducive to chemosynthetic species provided 

near-optimal conditions in the recent past. Few living individuals and the paucity of 

preserved juveniles, however, suggests that optimal conditions may no longer be 

present for either the vesicomyids or the lucinids at this site. Moreover, examination 

of guild and tier structure suggests that this site has only recently harbored a 

chemosynthetic biota . Perhaps GC 272 is in transition between the normal slope and 

a well-developed seep habitat and so might be expected to contain a highly spatially 

variable biota and widely varying conditions of optimality for the various biotas. 

8.7.3 Green Canyon 234 

Samples from Green Canyon 234 "Mussel Beach" were of two types: lucinid 

dominated or mussel dominated. Samples were obtained by box core and piston core. 

Lucinid-dominated assemblages were generally upslope of the mussel beds and 

contained mostly Lucinoma sp. No dates are available for the lucinids. Several large 

8-54 
diameter piston cores were taken in the mussel bed locale at GC 234. Two of these 

cores recovered buried mussel beds . In one core, a buried mussel bed was found at 85 

to 100 cm depth and another at 195 to 200 cm depth. The shell material was almost 

exclusively Batlzymodiolus sp., with a few Lucinoma shells mimed in. In a second core, 

a buried mussel bed was also recovered at 85 to 100 cm. Like the former, most shell 

material was mussel . A few scattered lucinids and gastropods were recovered 

between and above these buried mussel beds in both cores, but no other significant 

shell accumulations were observed. In a third core, mussels were collected in the 

upper 10 cm, but few additional specimens were recovered in the remaining about 

200 cm of core length. Thus, although mussel shells were not distributed 

continuously with depth, no other biofacies was present to take the place of the 

mussels in sedimentary units lacking them. 

Both buried mussel beds had been killed catastrophically, probably by mud 

flows. 14C dates indicate ages of about 2500 and 3500 years for these beds (± 500 yr. 

due to the variable amount of 14C-dead carbon incorporated in their shells) . Thus, 

this site has been inhabited, sporadically, by extensive mussel populations for at 

least 3500 yr. 

The lucinid biofacies at GC 234 was similar to that observed at GC 184 in 

most respects (Figure 8 .14 through 8 .19) . The size-frequency distribution was 

bimodal . Many small individuals were present and most of them were juveniles. 

Adults, also common, frequently lived to 80% or greater of maximum size . Adults 

accounted for most of the assemblage's paleoproduction and paleoingestion . The 

biofacies was unusual, however, in that the vesicomyids and thyasirids were 

dominant numerically, however Lucinoma sp . accounted for most of the assemblage's 

paleoproduction and paleoingestion . Thus, the vesicomyids and thyasirids were 

mostly juveniles that failed to reach adulthood. Five box cores from GC 234, three 

from lucinid beds and two from nearby non-seep habitats were analyzed. Of the three 

8-55 
lucinid beds, two are numerically dominated by Lucinoma, which also account for 

most of the paleoproduction and paleoingestion. One is numerically dominated by 

Thyasira oleophila, however Lucinoma and Vesicomya cordata dominate 

paleoproduction and paleoingestion . Each of these cores is characterized by a 

preponderance of small individuals. Large specimens are only abundant in core 342. 

Core 341, dominated by a mixture of chemosynthetic species, is composed exclusively 

of small individuals. Few of the individuals in any of the other cores were adults ; core 

341 contained only a few adult individuals and none approaching the species' 

maximum size. Accordingly, in core 341, unlike most others, juveniles accounted for a 

substantial fraction of paleoproduction and paleoingestion, an indicator of particularly 

poor survivorship . Accordingly, habitat optimality varied considerably among these 

three cores. The core containing the mixed chemosynthetic assemblage was 

distinctly suboptimal for all three species. Larval settlement, however, seemed to be 

an ongoing process, unlike at GC 272 (once again, this conclusion assumes an 

equivalent taphonomic milieu). 

Guild structure in the lucinid biofacies at GC 234 was overwhelmingly 

chemosynthetic when evaluated numerically and nearly exclusively chemosynthetic 

when evaluated by paleoproduction or by paleoingestion . Comparison to lucinid 

biofacies at other sites shows that chemosynthetic dominance was lower than at GB 

386, but about equivalent to GC 184 (Figure 8.20) . Tier structure was complex, when 

evaluated numerically, because of the presence of semi-infauna and some true 

epifauna. Infauna dominated tier structure when evaluated by paleoproduction or 

paleoingestion. The infaunal contribution to tier structure was typical of most lucinid 

biofacies (Figure 8 .21). Guild structure varied among the three cores examined. Core 

341, unique for other reasons, had a much lower dominance by chemosynthetic 

species. Predators were more common, a normal slope characteristic . However, 

infauna were important contributors to all three cores. Thus these cores represent a 

8-56 
range of conditions from distinctly suboptimal to near-optimal for the lucinid 

biofacies . 

Two non-seep biofacies were also sampled in the GC 234 area . Individuals 

were nearly or exclusively small. The two cores are, however, quite distinctive. Core 

301, which was not subsectioned, shows a standard non-seep fauna with many adult 

individuals of small species, a significant number of which reached near-maximum 

size . This core, then represents an optimal normal slope habitat near the seep sites. 

The second core was taken in a suboptimal habitat for seep and non-seep species 

combined . This area has been suboptimal for an extended period of time. All 

individuals are small. Most are juveniles. Thus juveniles account for a substantial 

fraction of paleoproduction and paleoingestion throughout these cores. A look at 

species composition suggests that this site is gradually becoming more favorable for 

vesicomyids. The numerical contribution of vesicomyids increases upcore as the 

contribution of non-seep species declines. Moreover, the fraction of paleoproduction 

and paleoingestion contributed by chemosynthetic species increases substantially in 

the upper 5 cm. The core may document, then, the incipient colonization of the area 

by Vesicomyidae . 

Guild and tier structure support these observations. Predators and deposit 

feeders dominate the guild structure at the two non-seep cores as is usual for the 

normal slope fauna. However, the upper section of one of these cores shows a 

significantly increased fraction of the paleoproduction coming from chemosynthesis . 

Chemosynthetic taxa dominate in the cores taken in the lucinid biofacies. The 

productivity of the biota increases upcore in the non-seep core. Abundance in the 

upper 10 cm is typical of chemosynthetic communities. Lower abundance downcore 

is typical of non-seep communities. Relatively low paleoingestion and 

paleoproduction throughout, however, indicate the preponderance of juveniles in the 

fauna, even at the nearsurface where these variables reach maximal values. Finally, 

8-57 
epifauna and semi-infauna dominate the tier structure of both non-seep cores, 

although infauna are increasingly important upcore, whereas infauna dominate the 

tier structure of cores taken in the lucinid biofacies. Thus, the incipient development 

of a chemosynthetic fauna is evident in the non-seep cores, suggesting an ongoing 

development of the chemosynthetic community. 

8.7.4 Garden Banks 386 

One piston core and one box core were taken from Garden Banks 386 . The 

piston core contained a mixture of mostly Lucinoma with some thyasirids to a depth 

of 35 cm. No shell material was collected in deeper core intervals. The box core 

sampled to a depth of 35 cm and sampled lucinids throughout that depth range. The 

biofacies was dominated numerically, by paleoproduction, and by paleoingestion by 

Lucinomcz sp. (Figure 8.13). Lucinoma sp. dominance persisted throughout the upper 

35 cm of the sediment. The biofacies was bimodal containing a mode in the small size 

classes and one in the central 5 (Figure 8.14) . This size distribution persisted 

throughout the core . Paleoproduction and paleoingestion were overwhelmingly 

contributed by the upper 5 size classes (Figures 8.15 and 8.16). This distribution 

persisted throughout the core. A substantial fraction of the assemblage consisted of 

large juveniles and adults (Figure 8 .17). The proportion of adult individuals increased 

upcore, particularly in the upper 10 cm . Paleoproduction and paleoingestion were 

primarily contributed by large juveniles and small adults, however (Figures 8.18 and 

8.19), a phenomenon that persisted throughout the core. Thus, the large adults found 

upcore were adults of small species . The site, though persistently conducive to 

dominance by chemosynthetic clams, was persistently suboptimal for them. 

Chemosynthetic clams rarely exceeded 60% of species' maximum size . Thus, this 

site sits in stark contrast to GC 184 where the habitat oscillated between nearoptimal habitat and distinctly suboptimal habitat for the lucinids. At GB 386, the 

8-58 
habitat was invariantly suboptimal, but not sufficiently so to exclude the 

chemosynthetic clams from a dominant position in community energy flow. 

The guild and tier structure of the lucinid biofacies at GB 386 was nearly 

exclusively chemoautotrophic and infaunal. This represents the extreme condition 

among the lucinid biofacies sampled. Guild and tier structure of the subsectioned core 

show that this site was nearly exclusively chemoautotrophic and infaunal throughout 

the sampled record of the site . Very little variation occurred downcore in guild and tier 

structure, in comparison to GC 184 for example. Very little variation occurred in 

numerical abundance, paleoingestion or paleoproduction as well, although the latter 

two reached highest values in the lowermost core section. The high values are typical 

of chemosynthetic faunas . 

8 .7 .5 Garden Banks 425 

Cores from GB 425 predominately contained thyasirids (Tlzyasira oleophila) 

and lucinids (Lucinoma sp .) . Samples were obtained by box core and piston core. 

Lucinomd sp. was commonly encountered alive . Thyasira was not. One piston core 

recovered form GB 425 contained Thyasira, nearly exclusively, to a depth of 210 cm. 

Large accumulations were observed between 50 and 60 cm and again at 70 to 85 cm. 

A second piston core recovered a thyasirid layer at 70 to 75 cm depth with scattered 

thyasirid shells throughout the remainder of the core . A box core was sectioned into 

5-cm intervals to a depth of 35 cm. This core resembles the piston cores in all 

significant respects. Thyasirids were numerically dominant in all intervals . Lucinids 

accounted for somewhat less than half to more than half of the paleoproduction and 

paleoingestion. However, juvenile thyasirids and lucinids were rare (Figures 8 .14 and 

8.17) . This distribution persisted throughout the core. The size-frequency 

distribution of the assemblage was distinctly bimodal, the modes being produced by 

the smaller thyasirid and larger lucinid individuals (Figure 8.14). This trend persisted 

8-59 
throughout the core . The two species accounted for an equivalent share of the 

paleoproduction and paleoingestion (Figures 8.15 and 8.16). This trend occurred in 

most core intervals . Intervals at 10 cm and 20 cm, however, contained no large 

lucinids. Nearly all individuals were adults, a much higher fraction than any other 

biofacies or site (Figure 8 .17) . This was true in all core intervals. Individuals near 

100% of maximum size were frequently encountered. In some cases, a near majority 

exceeded 80% of maximum size. Not surprisingly, large adults accounted for most of 

the paleoproduction and paleoingestion in all core intervals (Figure 8 .20). 

Significantly, in core intervals where lucinids were not found, thyasirids still 

persistently reached near-maximum size. 

This site, then, is unique in maintaining near-optimum conditions for two 

chemosynthetic species for a substantial length of time . Thyasirids at 30 cm depth 

dated as old as 1000 yr. (Table 1) . The site was near-optimal for thyasirids 

throughout most of that time period . Optimality for lucinids varied. Lucinids 

disappeared from and reappeared in the assemblage twice over the 30 cm of core . At 

each reappearance, they attained adult size, however they rarely exceeded 70% of 

maximum size, indicating, along with their lower persistence, a distinctly less 

favorable habitat than was present for the thyasirids . Piston cores, as yet undated, 

indicate that this site has been conducive to thyasirids and lucinids for many 

thousands of years. Curiously, the absence of living thyasirids indicates a recent 

change in conditions unlike that present previously. Similarly, despite the number of 

adult lucinid clams, juveniles were not present in the death assemblage, although 

they were at most other sites . Although higher than normal taphonomic loss cannot 

be discounted, the suggestion is that this area is becoming less favorable for 

chemosynthetic clams than it has been over the last thousand or so years. 

Guild and tier structure closely resembled that observed at GB 386 despite the 

difference in faunal composition . The biofacies was chemosynthetic and infaunal . 

8-60 
Guild and tier structure varied little downcore. This site and GB 386 are interestingly 

different from the Green Canyon sites in their simplified guild and tier structure 

evaluated by all three measures, numerical abundance, paleoproduction and 

paleoingestion . Very few heterotrophic individuals were preserved at this site and 

very few non-infaunal chemosynthetic individuals were present. 

Abundance, paleoingestion and paleoproduction increased downcore, 

particularly below 10 cm. Values were typical of well-developed chemosynthetic 

faunas. The interval at 20 cm containing no lucinids was characterized by lower 

paleoingestion and paleoproduction than other core sections below 10 cm. The upper 

10 cm were low in all three variables. Densities and energy flow variables, however, 

did not drop to unusually low values for chemosynthetic faunas. In fact, the values 

observed in the upper 10 cm were typical of most chemosynthetic faunas; the deeper 

densities and energy flow values were substantially higher than observed in most 

other cores, indicating periods of unusually high chemosynthetic productivity. 

8.8 Community History, Persistence and Succession 

Unlike mussel beds, clam beds may persist as a sediment surface phenomenon 

for extended periods without continual living input because taphonomic loss rate and 

sedimentation rate are low. Thus the proportion of small species and juveniles of 

large species in the assemblage can be used to evaluate whether recruitment is a 

continuing process in the assemblage or whether the clam bed represents an area no 

longer supporting chemosynthesis. Most clam beds were inactive. Living individuals 

were rarely encountered and only commonly encountered at GB 425 . Juveniles were 

usually an important component of the death assemblage, however, so that larval 

settlement was a persistent feature at most sites . Like many shallower water 

habitats, however, larval survivorship varied greatly. Sites frequently failed to 

support adult individuals and only a few sites consistently supported individuals 

8-61 
larger than 70°!0 of species' maximum size . Thus habitat optimality varied 

significantly amongst these clam beds and the widespread occurrence of clam beds, 

due to their high degree of preservability, belied the fact that thriving beds were 

probably no more common than were the less preservable mussel beds at most sites . 

Some sites retained optimal habitat for some species continuously over 

geologically-long periods of time. More commonly, however, habitat optimality varied 

substantially over time scales of 300 to 500 yr. Local extinctions and recolonizations 

occurred and these seemed to occur fairly rapidly in the context of the time span 

represented by the entire core or a portion of the core recording times conducive to a 

selected fauna. These events, however, although occurring rapidly on a geological 

time scale, nevertheless occurred rarely over a human time scale. Over a 50 yr. time 

span, local extinctions and recolonizations should be gradual and exceedingly rare. 

Thresholds of optimality seem to be present. Thus, it is more typical for large adults 

to appear and vanish rather than for size to gradually increase or decrease. 

Triggering mechanisms producing the rapid (on a geological time scale) faunal 

changes are unknown, but could include small changes in sediment chemistry. 

Thresholds and triggers are important in most bivalve population dynamics . 

In the case of the mussel beds, catastrophic burial was implicated in 

preservation, but the importance of catastrophic burial in fostering local extinctions 

seems to be minor. Most extinctions occurred quickly, but without hiatuses of 

faunally depauperate sediments that might be typical of catastrophic sedimentary 

events. Thus, one important characteristic of these sites was the degree of 

persistence of the chemosynthetic biota . A fauna typically was persistent over a 

time span of a few hundred years, but was typically not persistent over a time span 

of 300 to 500 yr. 

8-62 
Faunal succession was not observed. When local extinctions occurred in the 

chemosynthetic biota, the biota was replaced by a normal slope biota or a mixture of 

a normal slope biota and the juveniles of chemosynthetic species that failed to 

survive to adulthood. Thus the only faunal transitions were between chemosynthetic 

and non-chemosynthetic faunas . Not one distinctive faunal transition between two 

chemosynthetic species was observed. Sites were always recolonized by the 

chemosynthetic species that had previously become locally extinct. Accordingly, the 

chemosynthetic faunas were resilient over long time scales; time scales of geological 

importance . The inescapable conclusion is that sites are more or less permanently 

conducive to only one, or at most two, chemosynthetic species. Site specificity is 

retained over geologically-long time frames. This suggests that site chemistry 

remains within a narrow range for geologically-long times and that the local 

variations in chemistry are sufficient simply to control the presence or absence of the 

locally preferred chemosynthetic fauna at the expensive of the background 

heterotrophic fauna. 

The relatively low persistence but high resilience of the seep faunas indicates 

that sites retain a degree of uniqueness in their capacity to support seep faunas over 

geologically-significant time periods . In this study, only the Lucinoma sp. biofacies 

was found at more than one site. Significantly, this species was rarely persistent 

over long time spans and rarely did a significant number of individuals attain nearmaximum size, so that this species was simply more capable of surviving in a variety 

of suboptimal habitats than were the other species. At least as important was the 

failure to sample habitats optimal for several important chemosynthetic species, 

namely the solemyids (a few were taken at GB 386), Lucinoma atlantis (occasionally 

collected at GB 425) and Vesicomya cordata . We suggest that at least three 

additional unique habitats remain unsampled and unsurveyed. Given the uniqueness 

8-63 
of sites, we would expect these biofacies to dominate sites that are as yet unexplored 

rather than to be present but unsampled within already explored sites. 

8.9 Summary and Management Implications 

The purpose of this portion of the study was to evaluate the persistence of 

seep communities, their resilience and the frequency of disturbance, and successional 

relationships in recovery from disturbance . 

Overall, seep communities were persistent on time scales of human life spans, 

but not over periods of 500 to 1,000 years. Although a substantial amount of 

variability existed, most sites maintained conditions conducive to the optimal growth 

of one or more seep species over periods of several hundred to over a thousand years . 

During these intervals, species composition, size-frequency distribution and 

productivity were relatively stable components in the community. Not all sites were 

characterized by optimal habitat during these intervals, however, if optimal is defined 

as promoting individual growth to >_ 80% of species maximum size. Some sites clearly 

provided better conditions than others and did so for an extended period of time . 

Despite this persistence, disturbances did occur in nearly every site . The 

mechanisms included catastrophic burial, but other more subtle mechanisms were 

also present. The results of disturbances were uniformly similar. For a time, a 

normal slope biota occupied no site. Chemosynthetic species normally recruited to 

these areas during this time, but failed to survive, so recruitment failure was 

generally not important in restricting recovery . Recovery normally took several 

hundreds of years, but, realistically, sampling intervals would have restricted 

observation of more rapid events, so short term episodes of disturbance and recovery 

might have been common but gone unnoticed in our analysis. Nevertheless, the time 

of recovery suggests that the natural cycle of disturbance and recovery will be very 

difficult to distinguish from a permanent loss due to anthropogenic impacts . 

8-64 
Recovery times are simply too long. Surprisingly, when recovery occurred, the same 

chemosynthetic species reoccupied the site. Species composition returned as before, 

but community productivity was often altered to some degree . No successional 

changes were observed, nor was replacement of one chemosynthetic species by 

another observed after the disturbance . 

Accordingly, the singularly most significant observation in this study is the 

nearly-perpetual uniqueness of these sites . Sites seem to support only a restricted 

species complement for hundreds to thousands of years. Thus each site is relatively 

unique, and a management strategy whereby a certain number of sites are preserved 

unharmed while others are permitted to be impacted seems sensible until a wider 

range of sites can be studied to better assess the degree of uniqueness of each site 

over geologically-long time periods . Simply put, as of today, we have no site 

duplicates; that is, we have no sites that are sufficiently similar to be considered 

equivalent to each other for management purposes . Nor can we be sure that a site, 

once lost, will ever be replaced. Consequently, management policy should be 

formulated extremely conservatively based on the assumption that each site is 

unique and unreplaceable. 

8-65 
9.0 Age, Growth, and Reproduction in Deep Sea Gorgonian 

from Hydrocarbon Seep Sites in the Northern Gulf of Mexico 

S.E. Beasley, W.W. Schrceder and M.R. Dardeau 

9.1 Introduction 

The purpose of this research element was to characterize the age and growth 

rate patterns, and reproductive strategy of a non-chemosynthetic faunal component 

of seep-sites. This effort was intended to provide input to two of the sections in Task 

3 (Field Operations and Sampling) set forth in the RFP (Solicitation No . 3555). The 

first, task 3.c .5 (Chemosynthetic Epifauna and Associated Non-chemosynthetic 

Fauna), addresses estimating the biomass and size frequency of representative fauna 

at seep-sites while the second, Task 3.d (Temporal Variability), focuses on detecting 

differences over time in biomass, growth, numbers, and general health of 

representative fauna at seep-sites. 

Octocorals have been acknowledged as a principal member of biological 

assemblages at numerous hydrocarbon seep sites in the Gulf of Mexico (e.g., Brooks 

et al. 1989; MacDonald et al. 1989; 1990a), but have not been the focus of any 

organized research to date . Although not represented in every seep community, they 

have been reported from at least thirteen seep sites (Table 9 .1) and are considered 

non-chemosynthetic members of the assemblages. Their taxonomy is still being 

resolved. In this component of the study, two aspects of the basic life history of 

octocorals are examined; age and growth rate estimates, and reproductive 

characteristics . 

The axial skeleton of sea whips is composed of gorgonian, which is deposited in 

concentric layers around a central canal (Wainwright et al . 1982). The degree to 

which these rings are visible varies from species to species. Little is known about 

growth rings, particularly their periodicity. Annual growth rings have been 

documented in Muricea californica and M. fruticosa (Grigg 1974), and have been 

9-1 
Table 9.1 Seep sites from which octocorals have been reported. 

Lease Block Source 

East Break 376 2 

Garden Banks 341 5 

Garden Banks 382 5 

Garden Banks 386 1 

Garden Banks 388 2 

Garden Banks 416 5 

Green Canyon 53 4 

Green Canyon 121 5 

Green Canyon 166 5 

Green Canyon 184/185 1,2,3 

Green Canyon 232 5 

Green Canyon 234 1,2 

Viosca Knoll 826 1 

1 - Present Study 

2 - Brooks et al. 1989 

3 - MacDonald et al 1989 

4 - Roberts et al. 1989 

5 - GERG Unpublished Data 

proposed for Leptogorgia hebes, and L. uirgulata (Mitchell et al . 1993). Nothing, 

however, is known about the periodicity of growth ring formation in deep-sea 

gorgonians . Information from analysis of growth rings can be used as a form of 

environmental hindcasting. The frequency of extinction events and recolonizing 

episodes can be estimated by determining population age structure . Further, 

comparative stability of different seep-sites can be evaluated by comparing growth 

rates of sessile organisms . When combined with a detailed examination of the 

physical processes impacting the various sites, these data may provide critical 

insight into the short- and long-term health of both the individual faunal components 

and the community at-large of these unique habitats . Reproductive cycles of these 

gorgonians are likewise undescribed. For example, it is not known if the deep-sea 

9-2 
communities are maintained by asexual and/or sexual means, when colonies become 

reproductively active, or if they exhibit a regular propagation cycle. 

9.2 Year 1 Effort: Age and Growth Rate Estimates 

9.2 .1 Methods and Materials 

In September 1991, 55 colonies were collected from the Green Canyon 234 

(GC 234) study site using the Johnson Sea-Link submersible . They were identified as 

a new subspecies of an undescribed species of Callogorgia (Bayer personal 

communication) . Upon return to the ship each colony was counted and corresponding 

height and basal diameter recorded . Seventeen of the colonies were randomly 

collected along a transect. These specimens were numbered, tagged, placed in Helly's 

fixative for eight hours, transferred to several washes of sea water over a 24 hr period 

and stored in 70°lo undenatured ethanol prior to laboratory analysis . The remaining 

38 colonies were attached to rock samples and used only for measurements of height 

and basal diameter. 

Ring morphology in the axial skeleton was determined by embedding a two 

centimeter-long basal section of each colony (n=17) in epoxy resin. A basal section 

was defined as the part of the colony between the basal plate and the first branch. A 

transverse thin section of the embedded base was made by using a Buehler low speed 

saw. The section was polished using 0 .3 gm alumina paste to facilitate counting of 

growth rings under a binocular microscope. 

9.2.2 Results and Discussion 

Two types of layering were visible around the central axis of Callogorgia sp. ; 

major bands and minor bands (Figure 9.1). Major bands are comprised of dense 

gorgonin material, burnt orange in color, which appears as either symmetrical or 

asymmetrical dark bands completely encircling the central axis . Major bands are 

9-3 
Figure 9.1 Major bands and minor bands are visible around the central axis of 

Callogorgia sp. The major bands shown here appear incomplete as the 

result of uneven polishing. 

9-4 
easily distinguished from each other. The number of major bands present in 

Callogorgiac sp. (n=17) ranges from one to three. Minor bands, generally light brown 

in color, are made up of less dense gorgonin material and are located between the 

darker bands. Individual minor bands are often difficult to distinguish because of their 

abundance and proximity to one another. In addition, many of the minor bands do not 

appear to be completely concentric and therefore might more accurately be described 

as incomplete bands around the central axis. A comparison with the more uniform 

concentric ring structure observed in Leptogorgia hebes can be seen in Figure 9.2. 

If major bands of C¢llogorgia sp . are annual, then the oldest colony could not 

exceed three to four years of age. If, however, the minor bands are laid down 

annually, then some colonies could be up to 70 years old. Although major bands 

probably represent periods of reduced growth, the large number of minor growth 

bands or layers make interpretation difficult and offer a poor scale against which 

occurrence of long term cycles could be verified. This is in contrast to two shallow 

water gorgonians of the northeastern Gulf of Mexico, Leptogorgia hebes and L. 

virgulata, whose axial skeletons are composed of concentric rings (e .g ., see Figure 9.2) 

which appear to exhibit annual periodicity and whose population age structures 

suggest event-related (e.g., hurricane impacts) rather than biologically mediated 

mortality (Mitchell et al. 1993). It was concluded that the complex internal skeletal 

structure of Callogorgia sp. would not permit any meaningful estimates of age or 

growth rates. Therefore, the Year-2 field and laboratory research effort was modified 

to focus on the reproductive ecology of Callogorgia sp. 

9.3 Year 2 Effort: Reproductive Cycle 

9.3 .1 Methods and Materials 

In August 1992, additional Callogorgia sp. collections were made from the 

Viosca Knoll 826 WK 826), Green Canyon 184 (GC 184) and GC 234 study sites, 

9-5 
Figure 9.2 Cross section of axial rod showing concentric ring structure in 

Leptogorgia hebes. 

9-6 
again utilizing the Johnson Sea-Link submersible . Colony height and corresponding 

basal diameter were recorded for the colonies collected from VK 826 (n=6), GC 184 

(n=10), and GC 234 (n=6). Of these colonies, four from VK 826 and five from GC 234 

were tagged and preserved following the same procedures employed in 1991 . No 

colonies from GC 184 were preserved for histological analysis . 

Many corals are hermaphroditic; to determine if Callogorgia sp. is represented 

by separate sexes, the gametes present were examined to establish the gender of 

each colony. This was accomplished by randomly selecting 10 one centimeter long 

segments from the upper branches of each colony for sectioning. Once the one 

centimeter sections were selected, they were cut from the branch using a razor blade 

and subsequently stained using Cason's trichome stain technique (Cason 1950). 

Stained sections were infiltrated several times with Spurr's resin and absolute 

ethanol (1 :1) to allow the resin to penetrate all soft tissue of the coral before 

embedding in 100% Spurr's resin. Embedded samples were polymerized in a dry heat 

oven at over 700C. Using a Buehler Isomet slow speed saw, longitudinal thin sections 

were made of each branch sample and then mounted on petrographic slides, polished, 

and coverslipped. A Wild M-20 microscope was used to record presence and gender of 

gametes from each colony. Because sections do not expose polyps equally, each one 

centimeter segment was examined for polyps with the greatest number of gametes 

present. The mean maximum number of gametes per polyp is the mean of all polyps 

counted from 10 segments from each colony. 

9.3 .2 Results and Discussion 

Total number of colonies examined for reproductive activity from collections 

made at VK 826 and GC 234 during September 1991 and August 1992 was 26 ; of 

these 13 were female and 13 were male. Of 17 colonies collected from GC 234 in 

September 1991, seven were female and ten were male . Four additional female 

9-7 
colonies and one male colony were obtained from the same site in August 1992 (n=5) 

and two female colonies and two male colonies were taken from VK 826 in August 

1992 (n=4). In female colonies, oocytes are attached to sulcal septa of the gastric 

cavity by a short mesogleal stalk. The peripheral layer of a mature oocyte is made 

up of entoderm which encompasses a layer of mesoglea. Lying to the interior of the 

mesogleal layer is the vacuole and lipid enriched cytoplasm which surrounds the 

nucleolus and nucleus of the egg (Figure 9.3). The mean maximum number of oocytes 

per polyp for all female colonies was 3.5 . Large oocytes, having a mean maximum 

diameter of 300 gym, occurred in colonies having a minimum height of 30 cm. Small 

gametes were present in the one colony < 30 cm in height but were too immature for 

gender to be determined. In male colonies, spermaries are attached to sulcal septa of 

the gastric cavity. Maturing spermaries contain thousands of sperm and are easily 

distinguished from yolk-laden eggs by the presence of spermatocytes which become 

mature sperm while converging into rows towards the center of the spermary where 

there is typically a hollow area (Figure 9.4) . The mean maximum number of 

spermaries found in male colonies was 3.3 per polyp. A transmission electron 

micrograph of a colony collected from GC 234 in 1991 showed mature sperm present 

with a characteristic head and tail (Figure 9.5). 

Regression analyses were performed to examine the relationship between 1) 

basal diameter and colony height, and 2) number of gametes in each polyp and colony 

height. Colony height increased significantly (P<0 .001, r2 =0.98) as the basal 

diameter of each colony increased (Figure 9.6) . Number of oocytes per polyp also 

increased significantly (P=0.004) as colony height increased (Figure 9.7) for female 

colonies . However, no relationship (P=0 .101) was found between number of 

spermaries present per polyp and colony height (Figure 9 .7), although males did 

produce a maximum number of spermaries per polyp when a minimum height of 70 -

90 cm was reached. 

9-8 
Figure 9.3 Vacuole and lipid enriched cytoplasm of an egg from GC 234 in 1991. 

9-9 
LS =r 

Figure 9.4 Rows of maturing sperm converge toward the center of a spermary from 

GC 234 in 1991. 

9-10 
Figure 9.5 A mature sperm with a characteristic head and tail from GC 234 in 

1991 . 

9-11 
0 = GC 234 NE 1991 

t = VK 828 1992 

v = GC 234 NE 1992 

= GC 184 1992 

200 

U 

150 

d 

C7 

a 

0 

0 100 

a a 

d 

U 

w 

0 

E,., 5 0 

x 

w 

x 

0 

0 1 2 3 4 5 6 7 8 9101112131415 

BASE DIAMETER (mm) 

Figure 9.6 Relation of base diameter and height of Callogorga sp. colonies. The 

height of each colony tends to increase significantly (p<0 .001) as the 

basal diameter of each colony increases. 

9-12 
Y, = 22.237 + 13 .96X 

w 200 

o y2= 18.218 + 12 .82X 

0 180 

U 

160 

140 

94 120 

c7 

0 100 

a 

a 

U eo O 0 

O 60 " D 

E 

40 

D 

x 20 

O = FEMALE VK828, 1992 

V - FEMALE GC234NE, 1992 

O = FEMALE GC294NE. 1991 

0 = MALE VK828, 1902 

= MALE GC234NE, 1992 

= MALE GC234NE, 1991 

Y1 (YALE) 

Y. (FEMALE) 

v 

o ~ 

0 1 2 3 4 5 6 7 8 

MEAN MAXIMUM NUMBER OF 

GAMETES PER POLYP 

Figure 9.7 Relation between height and reproductive state of Callogorgia sp . 

colonies. Female colonies are reproductive by the time a height of 30 cm 

is reached. The number of oocytes per polyp tends to increase 

significantly (p=0.004) as the height of the colony increases. Male 

colonies are reproductive by the time a height of 20 cm is reached. The 

number of spermaries present per polyp does not tend to increase 

significantly (p=0.101) with increasing height. 

9-13 
One-way ANOVA's were performed to determine if: 1) height of reproductive 

colonies differed between study sites from September 1991 and August 1992 ; and 2) 

number of gametes per polyp differed significantly between study sites from 

September 1991 and August 1992. Mean height of reproductive colonies from GC 

234 1992 (n=5) was greater than colonies from both GC 234 1991 (n=17) and VK 826 

1992 (n=4). The differences, however, were not significant (P=0 .271) (Figure 9.8). 

Colonies from GC 234 1992 (n=5) did have significantly more (P<0.05) gametes than 

GC 234 1991 (n=17) and VK 826 1992 (n=4) (Figure 9.9). Though sample size was to 

small for statistical analysis, the mean maximum number of oocytes per polyp for 

female colonies collected from GC 234 1991 (2.7; n=17) did not differ from VK 826 

1992 (2 .7; n=2). However, female colonies from study site GC 234 1992 (n=4) 

contained more oocytes per polyp (5 .1) than earlier collections from GC 234 1991 and 

VK 826 1992 (Figure 9.4). Although size differences were not significant at Green 

Canyon between 1991 and 1992, colonies at GC 234 1992 were larger, which may 

have resulted in the increase in fecundity at GC 234 1992 . Again, because of small 

sample size, statistical analysis could not be performed on mean maximum number 

of spermaries present per polyp in male colonies . However, there appeared to be no 

difference in the mean maximum number of spermaries present per polyp for male 

colonies collected from GC 234 1991 (3 .2; n=10), GC 234 1992 (4 .3 ; n=1), and VK 826 

1992 (3.3; n=2). Caution must be taken in interpretation of differences in fecundity; it 

is difficult to draw conclusions based on the small number of specimens collected from 

Green Canyon and Viosca Knoll but differences in fecundity are consistent with 

colony size . Colonies from both GC 234 and VK 826 1992 were larger in overall 

height and possessed a greater number of gametes per polyp than colonies from GC 

234 1991. 

The possibility also exists that differences in fecundity resulted from the timing 

of our collections relative to spawning cues. Two distinct modes of sexual 

9-14 
125 

w 

z 0 

0 100 

U 

W 

7 

F 

75 

A 0 

x 

a 

w 

50 

w 0 

E+ 

x 

25 w 

x 

z 

d 

W 

Figure 9.8 Relation of Callogorgia sp. colony height and collection site by year. 

Mean height of reproductive colonies from GC 234 1991, GC 234 1992, 

and VK 826 1992 did not differ significantly (p=0 .271) from each other. 

Error bars indicate standard error for each sample . 

9-15 

GC234 GC234 VK828 

1991 1992 1992 
= FEMALE COLONIES 

0 = MALE COLONIES 

w 

E 

W 

d 

Ch 

w 

o 

o 14 

z o a 

a 

d 

z 

d 

W 

6 

5 

4 

3 

2 

1 

0 

2 

3 

4 

5 

6 

N=7 ~ N=2 

N=10 

~~ N=2 

N=1 

N=4 

GC234 

1991 

N=17 

GC294 

1992 

N=6 

VK828 

1902 

N=4 

figure 9.9 Relation between Callogorgia sp . gamete number and collection site by 

year. Colonies from GC 234 1992 have significantly more (p<0 .05) 

gametes per polyp than GC 234 1991 and VK 826 1992. Error bars 

indicate standard error for each sample. 

9-16 
reproduction occur among gonochoric corals ; internal fertilization followed by brooding 

and broadcast spawning followed by external fertilization . Dunn (1975) defined 

brooding in marine invertebrates as "the retention of offspring by a parent through 

the embryonic stages usually passed in the plankton." Corals exhibiting a brooding 

reproductive strategy produce small eggs (Rinkevich and Loya 1979), have an 

extended breeding season (Harrison and Wallace 1990) and facilitate localized 

recruitment (Stimson 1978). Broadcast-spawning corals contain large eggs, and 

have a distinct annual spawning period, either mass-spawning-spawning or minor, 

during which gametes are released from male and female colonies (Harrison and 

Wallace 1990) . Mass broadcast-spawning occurs when gametes are released over 

one to several consecutive days (Babcock et al. 1986; Gittings et al . 1992), however, 

minor broadcast-spawns occur over an extended period of time lasting from one to 

several weeks (Hartnoll 1975 ; Gittings et al. 1992) . Synchrony of spawning 

generates high gamete concentrations in the water column thus maximizing spermegg encounters and increasing the probability of successful fertilization (Oliver and 

Babcock 1992). Seasonal timing of broadcast-spawning events has been shown to 

coincide with lunar patterns . Mass spawning of hermatypic corals from the 

northwestern Gulf of Mexico (Gittings et al. 1992), Cura~ao (Van Veghel 1993), and 

the Caribbean (Szmant 1991), occurring about a week following full moons in August 

and September, have been reported. In 1991, colonies were collected from Green 

Canyon on 24 September, one day after the September full moon and 29 days 

following the August full moon . In 1992, colonies from GC 234 were collected on 10 

August, three days prior to the full moon which took place on 13 August. A minor 

spawning event occurring sometime after the full moon in August 1991 could have 

decreased the mean number of gametes present per polyp in colonies from GC 234 

1991 relative to GC 234 1992 . The possibility of participation in a region-wide 

9-17 
spawning event by these geographically isolated communities suggests a mechanism 

for gene flow. 

9.3 .3 Conclusions 

Like many other octocorals, Callogorgia sp. is dioecious, with a sex ratio of 1 :1 . 

All colonies >20 cm were reproductive although mature gametes were observed only 

in colonies having a minimum height of 30 cm. Results of histological analysis from 

the same time of year for two consecutive years suggest that Callogorgia sp. exhibits 

an annual cycle of gonad development that are confirmed by the presence of mature 

sperm having fully developed head and tail regions. Presence of tails in sperm has 

been used to distinguish mature sperm and as an estimate of spawning period. 

Brazeau (personal communication) noted that spawning in the gorgonian Plexaura 

spp. occurred a couple of days after tails developed. Mature sperm present in 

Callogorgia sp. suggests that a broadcast-spawning event probably occurred within a 

few days after collecting in September 1991 and August 1992. Timing of sperm 

maturation in Callogorgia sp. coincides with the shallow water gorgonian Leptogorgia 

hebes (Beasley personal observation), as well as several hermatypic corals (Szmant 

1991; Gittings et al . 1992; Van Veghel 1993), which spawn in late August or early 

September. 

9.3.4 Management Implications 

Reproductive fitness in benthic invertebrates results from successful 

recruitment. High fecundity and fertilization success lead to the availability of 

larvae for settlement. Fecundity is high in the populations of Callogoria sp . studied 

but broadcast-spawning puts an entire year class at risk to both natural (e.g. benthic 

storms) and anthropogenic (e.g. temporary burial of hard substrate surfaces as a 

result of oil/gas related drilling or construction activities) perturbations during a 

9-18 
relatively short period each year. On the other hand, the annual cycle ensures that, 

as long as the original population remains reproductively viable, the loss of one (and 

possibly more) year class to recruitment failure can be made up in successive years . 

Taller (older) colonies are clearly more fecund and should be given priority in any 

management considerations . 

9-19 
10.0 An Ecological Characterization of Hydrocarbon Seep Communities 

Ian R. MacDonald 

10.1 Introduction 

Ecological theory proposes that every species occupies a niche uniquely 

defined by the organisms adaptation to a restrictive combination of geological space, 

physical and chemical tolerances, nutritional requirements, predators, feeding 

adaptations, prey, mobility, reproductive needs, etc . (Hutchinson 1953). By 

extension, an aggregate of niches forms an interdependent community of organisms; 

classes of communities, generally united by some more or less arbitrary theme form 

an ecosystem (the lake in Hutchinsori s example) . This study has the formal title: 

"The Chemosynthetic Ecosystem Study"; it is appropriate to consider the fauna of 

deep-sea hydrocarbon seeps in these terms. 

In practice, species are described by their morphology rather than their 

ecological niche or function; and communities and ecosystems are often defined as 

convenient catchalls for organisms that seem to cluster together. We simply know 

too little about too many species to adequately explain why they are where we find 

them. For example, the fauna of the deep sea is characterized by a great abundance 

(numerical, not biomass) of highly diverse animals that burrow or crawl on the 

seafloor. Deep sea biologists are generally unable to explain why groups are found 

together, to predict what their geographic range will be, or how they will persist in 

the face of disturbance. In this context, the chemosynthetic communities found at 

cold seeps and hot vents offer the distinct opportunity for rigorous ecological 

definition. As Fisher (Chapter 5, this volume) has shown, these animals depend, 

directly or indirectly, upon chemoautotrophic production by bacteria and their 

metazoan hosts; the bacteria have well-defined chemical requirements that are met 

by a very narrow range of geological circumstances. We argue that chemosynthetic 

io-i 
communities at hydrocarbon seeps form a distinct subset of these communities ; a 

subset which, to date, is known only from the northern Gulf of Mexico. 

This section will attempt to identify deductively the factors that make the 

Gulf seep communities unique . The exercise progresses in steps from the regional 

to the local and from the general to the specific. At each level, we attempt to list 

key characteristics that distinguish a specific component of Gulf of Mexico seep 

ecosystem from its broader context. 

10.1.1 The Gulf of Mexico Hydrocarbon System 

The northern Gulf of Mexico is one of the most productive petroleum basins 

in the world . In contrast with other well-developed marine hydrocarbon systems, 

for example the North Slope of Alaska or the North Sea, the Gulf of Mexico has 

multiple hydrocarbon source rocks and appropriated reservoirs. Because of the 

timing of subsidence and Cenozoic salt tectonics, large amounts of liquid 

hydrocarbons have been preserved in the system instead of being cracked to gas. 

The circum-basin distribution of source rocks create wide-spread potential for 

natural seepage of hydrocarbon to shallow slope sediments . 

10.1.2 halt Structures and Faults 

The continental slope in the Gulf of Mexico has been extensively modified in 

response to sediment loading and halokinesis resulting in the formation of 

extensional faults and tectonic uplift providing both a means of structural "focusing" 

of migrating hydrocarbons and the development of vertical conduits capable of 

delivering the reservoired hydrocarbons to the seafloor (Worrall and Snelson 1989; 

Kennicutt et al. 1988a). Absent the conduits for vertical migration, macroseepage of 

hydrocarbons could not persist. Therefore, deltaic hydrocarbon basins or basins 

10-2 
otherwise overlain by a less fractured sediment column are less likely to support 

extensive biological communities that are affected by seepage. 

10.1.3 deep Zones 

Seeps zones are sub-sets of the hydrocarbon system and are generally less 1 

km in extent. The entire extent of a seep zone is transformed by the hydrocarbons ; 

however the most vigorous migration is localized in gas, brine, or mud vents, which 

are orifices 1 to 10 m wide. Thus, a seep is the entire zone affected by hydrocarbon 

migration, a vent is the most active portion of the seep . Chemical and thermal 

gradients are distinct. At least two styles of seep zone can be identified among the 

seeps studied; both are defined by the circumstances of migration. Salt-related 

seeps are regimes in which a salt piercement structure penetrates the shallow 

subbottom. The salt-related seeps studied have shown evidence for vigorous, 

localized, and intermittent migration, perhaps because authigenic carbonates and/or 

hydrate layers generate capping structures that deflect or retard migration such 

that the focus of venting evidently shifts in position across the area of the seep. 

Dissolution of the salt stock produces hypersaline fluids that generally contain high 

concentrations of biogenic methane . Migration of these briny fluids frequently 

entrains large volumes of fluid mud that can radically transform surficial sediment, 

producing such features as mud mounds, brine pools, and pockmarks . Fault-related 

seeps occur at kinks or intersections in faults, frequently the reliever or antithetic 

faults related to growth or transform faults . The zones studied have shown 

evidence for continual activity along the fault over recent geological time . When the 

fault remains active, the conduit for migration stays open, with the result that 

seepage is channelized along a distinct linear zone . Evidence for this includes 

massive (and old) colonies of seep fauna arrayed along faults and the lithification of 

the fault zones due to protracted precipitation of authigenic carbonates . 

10-3 
10.1.4 The Seep Hydrocarbon System 

The fundamental property of the chemosynthetic habitat, i.e ., a spatially 

contiguous source of reduced compounds and oxygenated water, is the same at 

hydrothermal vents and at hydrocarbon seeps; however, the secondary 

characteristics of the two environments are quite different. The following factors 

are particularly distinct: 1) the attachment substratum at hydrothermal vents is 

always litified, whether basalt or poly-metallic sulfide, whereas the substratum at 

seep habitats almost always has an unconsolidated sediment component; 2) the 

thermal gradients at hydrothermal vents are very strong, whereas at seeps they are 

generally slight or non-existent; and 3) the variability of fluid migration is on the 

order of months to years at vents, versus years to tens of years or more at seeps. 

Geochemical processes interact with the physical environment to create these 

differences. 

Maintenance of the chemosynthetic habitat in the Gulf of Mexico relies on the 

delivery of biogenic and/or thermogenic hydrocarbons to the seafloor and the 

shallow subbottom in sufficient volumes and rates. The seep hydrocarbon system is 

a discrete region defined laterally by the extent of the migration conduit, vertically 

by sediment porosity and a complex of bacterially mediated geochemical 

transformations, and temporally by the persistence of migration. Qualitatively, the 

geochemical character of any portion of the seep zone is defined by the type and 

phase of migrating fluid, by the timing of migration, and by the history of migration 

at the particular locality. 

The specific components of the chemosynthetic fauna are highly selective for 

geochemical properties. Sediments beneath isolated mats of H2S-oxidizing bacteria 

(Beggiatoa sp.) generally contain thermogenic hydrocarbons that are altered by 

bacterial oxidation. Sediments near, but not immediately under the mats contain 

10-4 
lower concentrations of hydrocarbons. Bacterial oxidation of hydrocarbons depletes 

02, triggering bacterial sulfate reduction to produce the H2S needed for mat 

development and creating an abrupt oxic-anoxic interface at, or very near, the 

seafloor. Moreover, C02 from the bacterial oxidation precipitates as authigenic 

carbonate rock, beginning a process that modifies the seafloor. Tube worm 

communities requiring H2S occur at GC 185 where the seafloor has already been 

greatly modified since the Pleistocene by accumulations of oil, gas hydrate, and 

authigenic carbonate rock in sediments . Venting is thus suppressed over time, 

favoring slow development of complex communities that depend on abundant 

hydrocarbons in sediments. Sediment samples from an area with vesicomyid clams 

contain lower UCM (mean = 1,716 ppm) but C1-C5 concentrations are high (mean = 

28,766 ppm) . The clams appear to persist at relict thermogenic seeps where the 

rate of seepage has declined from past highs, or H2S production is triggered by gas 

instead of oil. The sediment associated with the methanotrophic Seep Mytilid Ia is 

characterized by low infaunal diversity and by dominance of biogenic methane. 

10.1.5 Seep Fauna 

The life histories of the dominant seep species reflect the ecological 

characteristics of their habitat. Compared to their vent counterpart Riftia 

pachyptils, seep vestimentiferans (Lamellibrachia and Escarpia) are slow growing 

(probable life-span is 100+ years) and slender. The S13C values for seep tube worm 

tissues are -30 to -23%o whereas vent tube worms are typically -12 to -8%0. The gas 

exchange organ, a very prominent plume in Riftia, is very reduced in both seep 

species, consistent with lower overall metabolic rates and growth. Although they 

require a hard attachment substratum as juveniles, adults are often rooted in soft 

sediment and circumstantial evidence suggests that they are able to acquire 

reduced compounds through their tube walls . The presence of large tube worm 

10-5 
aggregations probably tends to increase localized sedimentation (i .e. like dune 

vegetation). 

Seep mytilids are hosts to methanotrophic symbionts and are able to satisfy 

their entire energy and carbon requirement from methane . No such symbionts have 

been described from vent mytilids . Seep mytilids are also slow growing; their 

probable life-span is 50+ years . Tissue S13C values are low and quite variable (-60 

to -30°x), which reflects the variable mixture of biogenic (isotopically lighter) to 

thermogenic (isotopically heavier) methane available among the collection sites. 

Taxonomically, seep mytilids are quite speciose in the Gulf of Mexico with at least 

six probable species having been proposed (but not, as yet, described) by workers 

studying the group. 

Less is known about the life history of the epifaunal seep clams 

(Vesicomyidae). Their reliance upon soft sediment clearly distinguishes them from 

the hydrothermal vent clams of the same family, which inhabit fissures in a basalt 

substrate. However, the family is quite cosmopolitan and has been found in 

sedimentary habitats in many other localities. The infaunal seep clams (Lucinacea) 

are little known in terms of life history. All of these bivalve species are presumed to 

be slow-growing and all are spatially less restricted in occurrence around seep 

zones . Hydrothermal vent communities contain high numbers of endermic species; 

in contrast, there is comparatively little endemism among seep heterotrophs . The 

infauna is depauperate compared to any sedimentary deep-sea habitat . However, 

the existence of any infaunal component is sufficient to distinguish the seep habitat 

from the hydrothermal vent habitat, where the absence of a sedimentary cover 

precludes survival of infauna. 

10-6 
10 .1 .6 Temporal Change 

The paleontological record, which is primarily the shells of seep bivalves 

preserved in the sediment column and dating to about 4000 yr. before present, 

documents the stability of seep sites in terms of bivalve community composition and 

spatial location of the communities . In this regard, hydrocarbon seeps appear to be 

quite anomalous among soft-bottom bivalve communities. Generally, the paleorecord of bivalve communities on the continental slope and shelf shows marked 

shifts in the abundance and diversity of soft-bottom communities over time-scales of 

this order. The paleo-record also documents catastrophic burial events that 

repeatedly, but temporarily extirpated localized portions of a seep mytilid 

community. 

The composition and distribution of living (and sessile) seep fauna shows 

equivalent stability during the course of the study. This is not unusual when 

compared to non-chemosynthetic analogs such as coral reef communities, but quite 

different from hydrothermal vent communities. Studies of vent communities show 

radical shifts in the composition, as well as marked declines in the abundance and 

apparent health of vent chemoautotrophs over one to five year time periods . 

Likewise, vent communities have been observed to recover completely from total 

extirpation (due to a volcanic eruption) in slightly over one year. The dynamic 

nature of the vent environment requires rapid colonization and growth. Seep fauna 

undoubtedly are more slow-growing and long-lived then their vent counterparts, but 

do not recover as rapidly from disturbance . The types of disturbance that are most 

common in seeps (blow-outs and mud or brine flows) are events that render portions 

of the habitat uninhabitable but lack the destructive force of a lava flow. In 

essence, seep communities can afford to recover slowly from disturbances because 

the seep environment is stable over comparatively long time and length scales. In 

10-7 
terms of human disturbance, this means that prospects for recovery from localized 

damage are good, but would require periods of years to tens of years to complete . 

10.2 Recommendations for Future Study 

The Chemosynthetic Ecosystem Study depended heavily upon the use of 

submarines for collection of samples and deployment of monitoring devices. The 

amount of submarine time available for the Study was very substantially 

augmented by grants to the principal investigators from agencies other than MMS. 

This augmentation was necessary to meet MMS's objectives . Even with this nonMMS funded submersible time, the sampling effort was necessarily restricted in 

time (both the duration of the program and the restriction to summer field work), in 

number of sites, and in water depths sampled. It is clear that the study of seep 

communities is an expensive and time-consuming undertaking. To be cost effective, 

future efforts should focus on aspects most pertinent to management objectives and 

should utilize approaches that minimize the need for submarine time and other 

resource-intensive field efforts . The following recommendations suggest several 

topics that have potential management relevance . 

Archival and dissemination of imagery data collected during the Study - A 

substantial portion of the Study data takes the form of imagery. Image types 

include large format maps, video tapes, 35 mm color positives, and digital image 

data files. These data are an essential component of the geological, short-term 

change, and growth-rate investigations carried out under the Study. They should 

be available to MMS to investigate outside the program, and to the interested 

public; however, neither MMS nor the PIs fully anticipated the importance of these 

data sets at the outset of the Study. Consequently, there is no provision in the 

scope of work for dissemination of images in any format that approximates their 

10-8 
original resolution. A modest level of support would be adequate to provide for the 

archival of the data for future use. 

An approach would be to reproduce and distribute a large selection of the 

images in a CD-ROM format. Using CD-ROM technology, it is possible to store up 

to 600 35 mm slides as high resolution digital files . Alternatively, a CD-ROM can 

store up to 30 min. of live video. The digital format are highly standardized and are 

readable by anyone with a desktop computer. Reproduction costs are modest. 

Temporal change and growth rates of chemosynthetic fauna - Study results 

demonstrate that seep chemoautotrophs are slow-growing animals living in habitats 

that are generally stable; however, the period of observation has been short 

compared to the usual exploration and production cycle for offshore platforms . 

There is a need to better constrain processes that might tend to be confused with 

human impact on communities . Given the habitat stability and slow growth rates, 

near-term replication of the growth and change experiments is not indicated. 

Revisiting the mosaic sites after a period of three to five years to test whether local 

distribution of fauna has shifted is more likely to provide the needed information. 

Likewise, continued growth studies can be conducted with the marked mussels and 

tube worms currently deployed at the study sites. 

Use of remote sensing methods for detection of seep communities - Although 

submarines offer investigators the capability of fine-scale sampling and 

manipulation, these studies target only a fraction of the total seeps on the Gulf of 

Mexico continental slope. The submarines used most often in the Gulf of Mexico, 

Johnson Sea-Link and NR-1, are limited in their operations to depths of 1000 m and 

700 m, respectively; this artificially excludes a major portion of the potential habitat 

from consideration. Furthermore, submarine investigations proceed one site at a 

time, which will bias the results toward the characteristics of the small number of 

sites that can be investigated. 

10-9 
Satellite and airborne remote sensing have been used to determine 

distribution of seepage across parts of the continental slope and could provide a 

basin-wide estimate for the total number of active seep sites -- as well as an 

estimate for the total quantity of hydrocarbons entering the water column. The cost 

of image acquisition is significant for remote sensing, but cooperative research 

programs are underway that will compile complete coverage of the Gulf slope over 

at least two and possibly three time intervals. These programs are being carried 

out through the collaborative auspices of GOSAP (Gulf Offshore Satellite 

Applications Project), which is a consortium of industry and academic research 

institutions and under the sponsorship of the Environmental Task Force, which is a 

federal initiative convened by Vice President Gore. The Naval Research Laboratory 

has taken a lead role in providing interpretation of remote sensing data through the 

dual use of Navy remote sensing assets . Additional airborne remote sensing data 

are also being collected by the Marine Spill Response Corporation, with cooperation 

and participation of various state and federal agencies including the Texas General 

Land Office and the U.S. Coast Guard. Participation of MMS should, at a 

minimum, include consideration of these efforts to ascertain how data collected can 

be of use to MMS. Further participation could include supplementary programs to 

analyze or compile portions of the data set of most interest to MMS. 

Laser line scan imaging of seep communities - The laser line scan system, 

which received it first trial at seep communities during the Study, offers a potential 

means for greatly decreasing the time and resources needed to map the dimensions 

and estimate the abundance of seep fauna. A program to test the system aboard the 

submarine NR-1 is scheduled for November 1994. Three of the study sites (GC 234, 

GC 233 and GC 184) may be mapped with the laser line scanner. MMS may wish to 

evaluate the results of this exercise and could commission a presentation of the data 

10-10 
in a format that would augment the results obtained in the present Study and 

provide a baseline for future efforts . 

Study of gas hydrate and in situ monitoring devices for gas flux - One 

unexpected result of the Study was discovery of the potential for water circulation to 

influence seepage rates by regulating the formation and dissociation of gas hydrate. 

Catastrophic release of oil and gas due to loss of a hydrate seal under conditions of 

prolonged warming is a potential that should not be ignored. Studies of properties 

of gas hydrate under the conditions that prevail at seeps would help to constrain the 

possibility of such an occurrence . Methods available to conduct such studies include 

pressurized collection vessels that allow recovery of hydrate samples under near in 

situ conditions and the use of autonomous flux meters that can monitor gas release 

over protracted deployments. Both methods are designed around submarine 

deployment, but could be adapted to unmanned ROV applications. 

Collaboration with on-going investigations - An noted above, the Study has 

been subsidized in part, by the willingness of the principal investigators to apply 

resources from other programs toward the goals that mutually benefit MMS and the 

principal investigators . Although the major collaboration ends with this program, 

prospects for future collaboration at a more modest scale continue. Two possible 

programs that would continue to develop Study results toward management 

concerns are outlined below. 

Condition indices should be developed that will allow investigators to 

determine the relative health of chemoautotrophs from analysis of tissue samples . 

The method has application for ascertaining possible human impact to communities 

and for determining a baseline of condition among sites with different geochemical 

properties. On-going investigations of seep chemoautotrophs by Dr. Charles Fisher 

offers a source of material upon which development of these indices could be based. 

10-11 
Seep mussels are a major component of the chemosynthetic ecosystem, but 

the net productivity of these animals, and their subsequent contribution to the slope 

ecosystem, are difficult to determine because their productivity is a host-symbiont 

system. The productivity of the host is merged with productivity of the bacterial 

populations. Dr. Eric Powell has developed a host-parasite model for studying 

oyster populations. This simulation model could be readily adapted to the seepmussel symbiont case and applied to predict productivity of mussel aggregations. 

Coupled with mapping data from seeps, such a model could be used to predict the 

contribution of seep mussel productivity -- and hence of methane flux -- to large 

portions of the Gulf continental slope ecosystem. 

In conclusion, the Study has benefited from a broad agency and academic 

collaboration for submarine science . Numerous management-relevant programs are 

indicated by the Study results . The potential for on-going collaboration to improve 

our understanding of hydrocarbon seep ecology is stronger than ever. 

10-12 
11.0 Literature Cited 

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Point, Santa Barbara, California. Science 170:974-977. 

Almada-Villela, P.C. 1984. The effects of reduced salinity on the shell growth of small 

Mytilus edulis. J. Mar. Biol. Assoc. 64:171-182 . 

Anderson, A.L. and W.R. Bryant. 1990 . Gassy sediment occurrence and properties ; 

Northern Gulf of Mexico. Geo-Marine Letters 10:209-220. 

Andrews, J . 1977 . Shells and Shores of Texas. University of Texas Press, Austin, 

Texas, 365 pp. 

Arp, A.J ., J.J. Childress, and C .R. Fisher Jr. 1984 . Metabolic and blood gas transport 

characteristics of the hydrothermal vent bivalve, Calyptogena macgnifica . 

Physiol. Zool. 57 :648-662 . 

Arthur, M.A., E.G. Kauffman, P.A. Scholle, and R. Richardson. 1982. Geochemical 

and paleobiological evidence for the submarine spring origin of carbonate 

mounds in the Pierre Shale (Cretaceous) of Colorado. Geol. Soc. of Am. 

Abstracts with Programs 14 :435. 

Ausich, W.I . and D.J . Bottjer. 1982. Tiering in suspension-feeding communities on 

soft substrata throughout the Phanerozoic. Science 216 :173-174 . 

Babcock, R.C., G.D . Bull, P.L. Harrison, A.J. Heyward, J.K. Oliver, C .C. Wallace, and 

B.L . Willis. 1986. Synchronous spawnings of 105 scleractinian coral species 

on the Great Barrier Reef. Mar. Biol. 90:379-394 

Bayne, B.L. (ed). 1976. Marine Mussels, Their Ecology and Physiology. Cambridge 

University Press, New York. 

Bayne, B.L . and C.M. Worrall . 1980 . Growth and production of mussels Mytilus 

edulis from two populations . Mar. Ecol. Prog. Ser. 3 :317-328 . 

Bayne, B .L., J . Widdows, M.N. Moore, P . Salkild, C.M . Worrall, and P. Donkin. 1982 . 

Some ecological consequences of the physiological and biochemical effects of 

petroleum compounds on marine molluscs . Phil. Trans . R. Soc. Lond. B 

297 :219-239. 

Bayne, B.L . and R.C . Newell. 1983 . Physiological energetics of marine molluscs. pp. 

407-515. In A.S.M . Saleuddin and K.M . Wilbur . The Mollusca. Academic 

Press, New York. 

Behrens, E.W. 1988. Geology of a continental slope oil seep, northern Gulf of Mexico. 

AAPG Bulletin 72 :105-114. 

Boland, G.S. 1986. Discovery of co-occurring bivalve Acesta sp . and chemosynthetic 

tube worms Lamellibrachia . Nature 323:759 . 

11-1 
Boss, K.J . 1968 . New species of Vesicomyidae from the Gulf of Darien, Caribbean 

Sea (Bivalvia; Mollusca). Bull. Mar. Sci. 18:731-748 . 

Bouchet, P. and A. Waren . 1980. Revision of the North-east Atlantic bathyal and 

abyssal Turridae (Mollusca, Gastropoda). J. Mol. Studies Supplement 8:1-119. 

Brooks, J .M., M.C . Kennicutt II, R.R. Fay, T.J . McDonald, and R. Sassen. 1984. 

Thermogenic gas hydrates in the Gulf of Mexico. Science 226:965-967 . 

Brooks, J.M ., H.B. Cox, W.R. Bryant, M.C. Kennicutt, R.G. Mann, and T.J. McDonald. 

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11-14 
S~,ENT OF Ty 

° 

The Department of the Interior Mission ~Q ~ 

F~F ' 

a `' y ;, As the Nation's principal conservation agency, the Department of the Interior has responsibility for 

most of our nationally owned public lands and natural resources. This includes fostering sound use 

of our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

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4 $ 

environmental and cultural values of our national parks and historical places; and providing for the 

ACH 3 ~ enjoyment of life through outdoor recreation . The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

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As a bureau of the Department of the Interior , the Minerals Management Service's (MMS) primary 

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