OCS Study 

MMS 2002-035 

 

Stability and Change in Gulf of Mexico 

Chemosynthetic Communities 

Volume I: Executive Summary 

U.S. Department of the Interior

Minerals Management Service 

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

Minerals Management Service 

Gulf of Mexico OCS Region 

OCS Study 

MMS 2002-035 

Stability and Change in Gulf of Mexico 

Chemosynthetic Communities 

Volume I: Executive Summary 

Editor 

Ian R. MacDonald 

Prepared under MMS Contract 

14-35-0001-30813 

by 

Texas A&M University 

Geochemical and Environmental Research Group 

833 Graham Road 

College Station, Texas 77845 

Published by 

New Orleans

July 2002 
iii 

DISCLAIMER 

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

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

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

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

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

and compliance with the MMS editorial standards. 

REPORT AVAILABILITY 

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

at the following address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone (504) 736-2519 or 

 1-800-200-GULF 

CITATION 

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

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

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

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

2002-036. 456 pp. 

COVER ART 

Submersible sampling operations with submarine Johnson Sea Link. 
v 

ACKNOWLEDGMENTS 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

this program. 

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

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

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

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

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

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

to Ms. Tracie Robertson and Ms. Charlene Miller.
1

1.0 Executive Summary 

1.1 Introduction 

A multidisciplinary team of marine scientists has completed a program entitled Stability and 

Change in Gulf of Mexico Chemosynthetic Communities. The program was carried out under 

contract with the Department of Interior, Minerals Management Service, with technical 

supervision of the Gulf of Mexico Outer Continental Shelf Regional Office, New Orleans, 

Louisiana. The fundamental concern of the program was the effect that development of offshore 

energy reserves might have upon dense assemblages of deep-sea organisms, particularly 

chemosynthetic tubeworms, mussels, and clams, as well as fish and crustaceans, that live in 

association with them. Hydrocarbons from commercial oil and gas reserves escape into the sea 

bottom at natural seeps found commonly across the Gulf of Mexico’s northern continental slope. 

Chemosynthetic animals utilize chemical energy from hydrocarbons to maintain colonies that 

have unusually high biomass compared with the sea bottom elsewhere. Chemosynthetic 

communities at hydrocarbon seeps were discovered in 1984 and have been previously 

investigated in studies funded by MMS. However, more knowledge was needed about the life 

history and ecology of chemosynthetic communities in the Gulf of Mexico. The program has 

produced a substantial body of findings, which include numerous peer-reviewed publications and 

student theses in addition to the results detailed in the final report. Given the pace of publication, 

it is likely that this productivity will continue for some time to come. 

The program obtained results from an extensive program of field collections and laboratory 

analyses. Under the direction of the Geochemical and Environmental Research Group of Texas 

A&M University, sixteen investigators, associates, and students from seven separate institutions 

collaborated to formulate and carry out the work. During the course of two ambitious field 

seasons, program investigators collected samples from over fifty stations, which were distributed 

among four principal sampling sites with use of the submarine Johnson SEA-LINK. Extensive 

additional collections were made with use of the R/V GYRE and the US Navy Submarine NR-1. 

The primary geochemical samples included more than fifty sediment cores, which were 

extensively sub-sampled by sediment depth. Extensive collections of chemosynthetic and 

heterotrophic animals were obtained for investigations to determine, among other objectives, 

growth rates, health, genetic diversity, and food-web structure. A regional context for the 

targeted sampling was established through geophysical survey, sediment coring, and layered 

comparisons between community and regional-scale data with use of geographic information 

system (GIS) tools. Valuable time-series records of current, water temperature, and fluid 

discharge properties of the seep environment were obtained from targeted deployments of in-situ

instruments and moorings. Satellite remote sensing records were obtained through cooperation 

with industry to examine large-scale variation in seepage rates. 

Throughout the program, the investigators benefited from cooperative arrangements with other 

agencies and interests, including notably the NOAA National Undersea Research Center at the 

University of North Carolina, Wilmington, the Naval Research Laboratory Washington, DC 

laboratory, and the US Navy SUBLANT group, all of whom assisted with obtaining ship and 

submarine time over and above what would have been possible under the contract. 
2

1.2 Objectives and Basic Study Design 

The request for proposal that initiated this program specified the following objectives for study 

(taken verbatim from solicitation number 3813): 

Objective One. Review biotic and abiotic features of existing conceptual models of 

chemosynthetic communities which explain observed patterns of distribution 

and abundance, in order to develop an effective, refined plan for continued 

research. 

Objective Two. Further evaluate the physical-chemical factors (e.g., depth, temperature, water 

chemistry, sediment types, and dissolved gasses) which influence, limit, 

enhance, or control the distribution, abundance, and growth of chemosynthetic 

communities. 

Objective Three. Further investigate the sources (e.g., deep versus shallow or petrogenic versus 

biogenic) of any necessary dissolved gasses and the likelihood that petroleum 

production may ultimately deprive the animals of an energy source. 

Objective Four. Further determine if chemosynthetic communities are robust or fragile, and 

whether they are essentially permanent or ephemeral; characterize age, growth 

rate, turnover rates, reproduction and recruitment, and patterns of senescence 

and death in the dominant chemosynthetic animals; further examine recovery 

rates of communities damaged by physical disturbance. 

Objective Five. Further determine the reliability of methods for detecting chemosynthetic 

communities using remote acoustic and/or geophysical devices, imaging 

instrumentation, hydrocarbon measurements, and/or other available 

technologies. 

To address objective one, a thorough review of published literature and reports was completed. 

Two principal conclusions were reached. First, it was evident that the chemosynthetic fauna 

from Gulf of Mexico hydrocarbon seeps (tubeworms and mussels) were taxonomically and 

functionally similar what is found at hydrothermal vents at mid-ocean ridges, but included 

important differences. They are all dependent on symbiosis with bacteria that can utilize 

chemical energy. Tubeworms in both settings use hydrogen sulfide (H2S) to nourish their 

symbionts. Seep mussels have symbionts that can oxidize methane (CH4) from natural gas, 

while vent mussels rely only on H2S. However, communities at hydrothermal vents are attached 

a rocky substratum; they obtain needed H2S directly from vent fluids and are frequently 

disturbed by volcanic eruptions and other catastrophic events. Vent communities are often 

separated from each other by hundreds of miles of inhospitable habitat. Seep communities thrive 

on soft marine sediments where elemental cycling by microbial and chemical processes strongly 

influence the environment. Importantly, the H2S is produced by microbial consumption of seep 

organics—i.e. oil and gas. Many seeps appear to be stable features, in which colonies can persist 

for decades or longer without major natural disruptions. Seep also appears to be much more 

numerous and wide-spread than vents within comparable seafloor areas. This means that there is 

more opportunity for interaction between neighboring seeps (e.g. exchange of recruits) and a 

greater scope for non-seep fauna to utilize the productivity of seep animals as a food supply. It 

was concluded that new conceptual models of colonization, persistence, and interaction would 
3

have to be developed to describe seep chemosynthetic communities. 

The second finding of the review and study design process was that two styles of seepage have 

been described at Gulf of Mexico hydrocarbon seeps and contribute to habitant formation. In 

sediment diffusion habitats, mussels, clams, and tubeworms utilize reduced compounds – CH4

and H2S – by extending body parts into the sediment or by bathing their brachia (gills or plumes) 

in the steep gradients immediately above the sediment/water interface. Brine pooling habitats are 

found where concentrated brines are associated with petroleum migration and seepage. Because 

brines are much denser than sea water, brine discharge can form distinct pools on the seafloor. 

The brines contain high concentrations of CH4, which support dense mussel beds. Much of the 

community-level diversity in cold seep communities can be explained by examining the 

environmental consequences of these two styles of seepage. 

Spatial scale is an important factor in characterization and management of seep communities. 

Both habitat types occur in areas that are no more than a few hundred meters across. Within 

these habitats, the environmental gradients can be very high, leading to a heterogeneous 

patchwork of animal colonies. To meet the ambitious requirements of the request for proposal, a 

program of careful sampling in individual chemosynthetic communities was combined with 

observations at the regional level. All activities were conducted in on the continental slope south 

of Louisiana and Texas. Community level sampling was primarily carried out with use of the 

research submersible Johnson SEA LINK (JSL), deployed from a surface ship R/V EDWIN 

LINK. Regional sampling utilized the US Navy research submarine NR-1 and the R/V GYRE. 

1.3 Study Sites 

Most of the community-level sampling was carried out within four discrete sites. However, 

because the area of the Gulf of Mexico slope occupied by known chemosynthetic communities 

spans a depth range from about 500 to 2500 m, the geophysical survey was carried out within 

shallow- and deep-water areas. These two mega-sites were designated as survey sites for the 

development and testing of criteria to improve the prediction of the presence of chemosynthetic 

communities based on geophysical or other remote sensing data (Figure 1). Four principal 

community-level sites were designated for the intensive collection of samples and deployment of 

experiments and instrumentation (Table 1). Two of the sites were in brine-pooling settings; two 

others were sediment-diffusion settings. The sites differ principally in that two sites contain 

significant amounts of liquid hydrocarbons in the sediments and are dominated by tubeworms 

(GC185 and GC234) and two sites are brine seeps and are dominated by mussels (GC233 and 

GB425). These sites have been the subjects of historical studies, which in most cases provided 

ancillary geographic data. Because the sites contain the major faunal groups found in 

chemosynthetic communities (tubeworms and mussels) and cover a range of environmental 

conditions, hypotheses concerning abiotic controls on community ecology could be tested. 

Within each site, a series of sampling stations was established. The stations were chosen on the 

basis of the dominant chemosynthetic fauna found in the immediate vicinity, e.g. adult 

tubeworms, mussels, etc. Each station was marked so that it could be re-sampled. (Figure 1) 

shows the regional mega-sites and sampling sites. Table 1 details the attributes of the study sites 

and list the sampling stations. 
4

Figure 1.1 Northern Gulf of Mexico showing study site locations for submersible operations (triangles) and megasite areas for geophysical survey (shaded rectangles). Depth contours in meters. 

1.4 Sampling Methods 

A 24-day cruise on board the R/V EDWIN LINK, acting as tender ship for the submersible 

Johnson SEA-LINK II, was completed during 8 to 13 July 1997. This cruise (JSL97) comprised 

a major portion of the field effort for Year 1 of the program. The goals for this cruise were to 

complete an extensive series of sample collections, to release marked animals for growth studies, 

to deploy in-situ monitoring instruments, and to map the study sites for future effort. Data 

produced by this effort was crucial to the success of the program. Despite a very ambitious cruise 

plan, all goals were achieved thanks to the dedicated efforts of the cruise participants. A total of 

52 stations were occupied at four separate sites. A complete list of stations is provided in 

(Table 1). 

A 15-day cruise on board the R/V EDWIN LINK, tender ship for the submersible Johnson SEALINK I was completed during 4 to 18 July 1998. This cruise (JSL98) completed the sampling 

effort begun in the previous year. Most of the stations sampled in the JSL97 cruise were resampled. Sampling efficiency was improved on the basis of experience gained during the 

previous year so that more samples were collected in a shorter cruise. 

A highly integrated program of sample collection and site description was conducted during the 

two submersible cruises. All collections and stations were carefully documented with video and 

still photography and an exhaustive and carefully controlled list of collection, stations, and 

observations was made to ensure accurate sample identification and tracking. Principal sampling 

methods included sediment collection, water and gas collection, animal collections and 
5

manipulations, and photographic documentation. Methods are described in detail in the technical 

report volume and are only briefly summarized here. Sediment sampling was accomplished 

using inert plastic tubes, 7.5 cm in diameter and 30 cm in length. The tubes, called push cores, 

were pressed into the soft sediment using specially designed handles that could be manipulated 

with the submersible’s mechanical arm. Sediment sections were processed for geochemical 

analysis using methods to squeeze out pore fluids under carefully controlled conditions. Other 

sediment samples were frozen or preserved in analysis of the solid component, volatile gasses, or 

liquid hydrocarbon components. Push cores were taken in replicate at all stations unless the 

substrata were too rocky or too liquid for effective collection. 

Animal samples were collected using a variety of quantitative methods. Mussels were scooped 

from pre-measured frames into seal, separated containers. Tubeworms were collected with use of 

large, inverted net-bags that were placed over tubeworm clusters and cinched at the bottom with 

a hydraulic mechanism. Tubeworms and mussels were both marked for recapture in controlled 

studies of growth rate. Mussels were collected, measured, and marked with numbered tags prior 

to release. Tubeworms were marked in place using a device that stained the ends of their tubes 

with a vital dye. 

Auxiliary sampling methods included coring devices (box cores) lowered from surface ships, 

temperature and salinity measurements made from instruments attached to the submersible and 

lowered from surface ships. Several in situ measurement instruments were deployed and 

recovered using the submersible. 

Table 1.1 Stations Sampled According to the Experimental Design Criteria. 

Site Habitat Level within habitat Stations 

Adult AT1, AT2, AT3, AT4, AT5 

Tubeworm Senescent ST1, ST2 

Juvenile JT1, JT2, JT4, JT5, JT6 

Mussel M1, M2, M3, M4, M5 

White Mat B1 

Bacterial Mat 

Pigmented Mat B2 

GC185 (Bush Hill) 

Oily diffusion 

Station prefix = BH 

Reference Unoccupied boxcore from ship 

Adult AT1, AT2, AT3, AT4 

Senescent ST1, ST2 Tubeworm 

Juvenile JT1, JT2, JT4, JT5 

Mussel M1, M2 

GC234 

Oily diffusion 

Station prefix = GC 

Bacterial Mat B 

Tubeworm Adult AT1 

Inner edge M1, M4, M7 

GC233 (Brine Pool) 

Brine dissolution 

Station prefix = BP Mussel 

Outer edge M2, M3, M5 

GB425 

Brine dissolution 

Station prefix = GB 

Mussel M1, M2 
6

2.0 Major Findings 

2.1 Physical Oceanography 

Although the circulation of the overlying waters has been fairly well studied (Nowlin et al.

1998), surprisingly little is known about the oceanic environment of the sea floor on the 

continental slope of the Gulf of Mexico. Knowledge of the deep currents, temperature, and 

salinity is important in understanding how the environment affects the ecosystems found in the 

study region. Ocean currents give clues to the dispersal pathways taken by larvae of 

chemosynthetic species. Ocean temperatures can affect the reproductive cycling of organisms 

present. Ocean temperature also affects the stability of gas hydrate deposits. To learn more 

about the physical environment at the study sites, we made physical measurements as part of the 

two submersible cruises carried out during this program on the R/V EDWIN LINK. A current 

meter mooring was deployed in GC185 during the August 1997 cruise and recovered about nine 

months later on the May 1998 cruise. The mooring had two current meters, one 10 m above the 

bottom (530 m) and one approximately 200 m above the bottom (250 m). During these two 

cruises, measurements with a recording conductivity, temperature, and depth instrument (CTD) 

were made from the support ship, R/V EDWIN LINK, as well as from the SEA-LINK itself. 

This examination of the oceanic environment at chemosynthetic communities, although limited 

in scope, found that current speed and direction and water temperatures varied dynamically. The 

annual range in bottom water temperature equaled or exceeded 3°C in all measurement intervals 

(Figure 2). A strong diurnal periodicity in water temperature and current speed is clearly 

demonstrated in the long-term temperature records that were collected and analyzed. Currents 

speeds were significantly greater at the top meter and predominant current directions were 

different (Table 2). This periodic fluctuation is undoubtedly a predictable feature of sea floor 

habitats in the depth range of the study sites (500-600 m). Also evident from the temperature 

records are regular, but non-periodic episodes of temperature variations that tend to recur on a 

monthly or sub-monthly basis. These episodes result are evident as gradual fluctuations of the 

temperature baseline over a range of 2 to 3°C. Coupling between Loop Current eddies and 

transient deep-water currents are indicated, but the data are not extensive enough to define the 

mechanisms. 

Because chemosynthetic organisms are quite temperature-sensitive [Fisher, 1990 #100], the 

range and consistency of diurnal and longer-term temperature variations constitutes an 

environmental signal. This is noteworthy because the deep sea, which is devoid of sunlight, is 

often described as being temporally constant and unchanging. Moreover, because slight 

temperature increases can have an immediate and significant effect on the rates of venting from 

gas hydrate deposits [MacDonald, 1994 #187], a rise in temperature can influence the 

availability of nutrients in methanotrophic organisms such as seep mussels. Findings to date 

clearly indicate that events driven by Loop Current can have biological and possibly 

geochemical significance on the outer continental slope of the Gulf of Mexico. However, the 

observations we obtained have been quite limited geographically and in terms of water depth. 

Future work should prioritize obtaining temperature time-series from deeper sites to determine 

whether the diurnal periodicity and longer term fluctuation continues to be present as one moves 

further offshore. 
7

Figure 2.1 Time series of temperatures recorded by thermistors at GC185 and GB425. Time series of temperature 

collected at GC185 at the top and bottom current meter. 
8

Table 2.1 The frequency, in percent of time, that currents had a particulate range of speed and direction for the 

top and bottom meters. Rows with less than 0.5% have been deleted. The column at the left summarizes the speed 

ranges for all directions. The row at the bottom summarizes the percent of time for octant of direction for all 

speeds. 

CHEMO Top Current Meter 

Speed 

(cm/sec) N NE E SE S SW W NW Total 

< 2 7.48 0.17 0.35 0.06 0.15 0.25 0.41 0.41 9.28 

2 - 10 5.01 4.87 5.29 3.52 3.84 3.79 11.03 8.05 45.40 

10 - 20 1.48 2.47 5.20 2.92 1.16 2.46 10.74 3.57 30.00 

20 - 30 0.26 0.71 3.44 0.57 0.03 0.45 3.91 0.25 9.62 

30 - 40 0.00 0.33 2.89 0.01 0.01 0.17 1.05 0.01 4.47 

> 40 0.00 0.01 1.15 0.01 0.01 0.01 0.01 0.01 1.21 

Total 14.23 8.56 18.32 7.09 5.20 7.13 27.15 12.30 

CHEMO Bottom Current Meter 

Speed 

(cm/sec) N NE E SE S SW W NW Total 

< 2 24.32 1.70 3.28 2.00 1.12 0.68 3.19 1.68 37.97 

2 - 10 8.60 6.16 7.87 10.90 6.03 3.06 5.60 8.70 56.92 

10 - 20 1.02 0.57 0.77 0.75 0.01 0.09 0.12 1.65 4.98 

Total 33.94 8.44 11.93 13.66 7.17 3.84 8.92 12.04 

Geophysical Signatures of Chemosynthetic Communities 

Although continental slope and rise of the northern Gulf of Mexico technically fits into the 

category of “passive margin,” (i.e., it contains no plate boundary) in actuality it is very active 

because of the tectonic deformations of a massive layer of salt that underlies marine sediments. 

The marked irregularity of the slope is caused by rising salt diapirs or massifs and by basins 

resulting from salt withdrawal by (Bryant et al. 1990; 1991; Bouma and Bryant 1994). Gulf of 

Mexico chemosynthetic sites are found in water depths from several hundred meters to more 

than two thousand meters, seemingly wherever there are hydrocarbon seeps on the continental 

slope. However, much of the knowledge of chemosynthetic ecosystems in the Gulf of Mexico 

comes from a small number of sites, most found by chance. A fundamental unanswered question 

is whether lush chemosynthetic sites are rare and fragile or ubiquitous and robust. Although 

chemosynthetic organisms cover only a tiny fraction of the seafloor, they have the potential to 

cause the offshore energy industry to spend large sums of money to make sure that drilling rigs, 

pipelines, and other production equipment do not blunder into community sites and cause harm. 

Geophysical techniques can detect hydrocarbon seeps because the seeps change the physical 

properties of surface and near-surface sediments (Roberts et al. 1990). The task of remotely 

locating chemosynthetic organism sites is one of defining the small percentage of the seafloor 

affected by seepage and then checking within that area for chemosynthetic habitation. 

Active seeps are often visible owing to their release of oil and gas in the water. Slicks on the 

water surface can be imaged from space (and presumably lower altitudes), using photography or 

synthetic aperture radar (SAR), because oil reduces the roughness of the sea surface and cause it 

to appear bright: patches of floating oil are called “slicks”.(MacDonald et al. 1993; 1996). More 
9

often, seeps are recognized by their effects on the physiography of the seafloor and acoustic 

characteristics of near-surface sediments. Seeps give rise to several distinct signatures on highfrequency echo-sounder profiles and seismic reflection profiles. In echo-sounder records, seeps 

cause acoustic “wipeout,” which is the absence of subsurface reflectors. The effects of seepage 

are often noted on seismic reflection data, in particular the 2D and 3D multi-channel seismics 

often used for hydrocarbon exploration by the energy industry. Side-scan sonars also have the 

ability to show differences in surface sediment properties and thus may be useful in imaging 

seeps. 

The rationale of the study was to collect a high-resolution geophysical data set in these known 

seep areas and compare it with other geophysical and geologic data. This program was designed 

to focus on a few data types in limited areas. Two areas were chosen to be representative of the 

upper and lower slope morphologic regions. The areas were surveyed using long-range side-scan 

sonar. Figure 3 shows the side scan mosaic from the shallow mega-site. The side-scan images 

provided a data set that could be compared to pre-existing geophysical and submersible data. 

Piston cores, collected with a surface ship, as well as deep geophysical data and observations 

with submarine NR-1 were obtained from selected targets in the sonar images. 

Probably the most significant result from this study relative to the geophysical characterization 

of seeps is an increased awareness of their complexity in geophysical data. Generally, there is a 

tight link between faults, seeps, and chemosynthetic communities. In side-scan sonar data, 

seep related faults show high acoustic backscatter. Cores from the affected areas do not always 

show the same geologic characteristics, but are typically altered from normal hemipelagic 

property in a number of ways. Many of the cores exhibited carbonate nodules or layers, 

suggesting that much of the acoustic scattering in seep affected areas may result from authigenic 

carbonate material. Another potential disturbance is the formation of gas hydrate, which was 

recovered in several cores from GC185 and GC234 sites. Strong reflections might be caused by 

carbonate crusts and gas hydrate bodies. A feature of special importance to seep geology is the 

subcircular mud mound. The side-scan sonar mosaics imaged many subcircular, sonar-bright 

patches. In general, there were three different signatures: (1) uniformly high-backscatter (Figure 

6.26), (2) a high-backscatter center with a halo of lesser backscatter intensity (“bulls eye”, Figure 

6.26), and (3) high-backscatter with a central spot having little or no backscatter (“dead eye”, 

Figure 4). Some of the uniformly high backscatter patches were mud mounds, although none that 

we visited with submarine NR-1 was obviously active. A few subcircular sonar-bright patches 

were merely areas of the seafloor that appear to have had moderate seepage sometime in the past, 

but not enough to build a mound or support chemosynthetic communities. At several bulls eye 

sites, the near-bottom chirp sonar records from submarine NR-1 imply that the central highreflectivity zones are hard-bottoms, which we interpret as either carbonate or gas hydrate layers. 

The surrounding haloes seem to be wipeout areas (either attenuation or turbidity) caused by 

lesser seep-related disturbances to the sediments 

Knowledge about the geologic controls of chemosynthetic communities still has several 

significant gaps. Most research to date has focused on a small number of sites, so conclusions 

about chemosynthetic community occurrences, even on the upper slope, is limited by the 

generality of those sites. Increasing the number of seep sites investigated should be a high 

priority. This should include investigations into the number within a given area, so that we have 

better knowledge about the density of sites. It should also include an effort to obtain a broader 

picture of chemosynthetic sites in different structural settings. To date, most investigations have 
10

been on the Louisiana upper slope. The morphology of the lower slope is different from the 

upper slope, suggesting that controls on seeps may be different. In addition, the style of faulting 

changes from west to east across the continental slope and this may change the controls on seep 

occurrence. Because a large amount of geophysical data already exists for the Gulf of Mexico, 

collected by the energy industry, it would be prudent to consider obtaining geophysical data on 

seeps from a variety of settings and using computer GIS and database technology for 

assimilation and study. 

Figure 3. Side-scan sonar mosaic of the shallow mega site with locations of three of the community study sites. 

Overlay shows tracks of the 1998 submarine NR-1 dives. Thin dark lines are 50-m bathymetry 

contours. 
11

Figure 4. Side-scan sonar image of different mound backscatter characteristics. This record shows several 

subcircular high backscatter patche A., a large bulls eye mound, B. and another mound that has a dead 

spot (which shows better on another overlapping swath). C. The dead eye mound is where 

chemosynthetic organisms were found during NR-1 dives at a previously unknown seep. This record 

is from Green Canyon lease block 191. 

Geologic Controls on Chemosynthetic Communities 

Questions about the distribution and persistence of chemosynthetic communities in the northern 

Gulf of Mexico have to be posed in terms of scale. Over distances that span the Western and 

Central Lease areas, occurrences of chemosynthetic communities are determined by the presence 

of hydrocarbon reservoirs. At an intermediate scale, a major management concern remains the 

so-called lush chemosynthetic community and there are a number of questions that managers 

must consider: What constitutes a lush chemosynthetic community? How large an area does it 

cover? What densities of chemosynthetic species and what sorts of normal variation do such 

communities experience over a multi-year timeframe? The program study sites (Figure 1, Table 

1) were chosen because they typify lush communities. Quantifying the dimensions, densities and 

faunal composition of these sites provides a baseline against which communities newly 

discovered can be compared. Mapping the fine-scale limitations posed by seep geology at each 

site creates a template that can be used to refine the search for lush communities in future 

surveys. Case studies of evidence for variability or persistence are used to identify general 

tendencies or processes that are relevant to management timeframes. Available data included 

surveys conducted during the present program and data gathered during previous efforts and 

reanalyzed with use of updated techniques. Clearly, having a small number of sampling sites 

limits the generality of these results. However, the four sites represent two pairs of communities 

supported by distinct styles of seepage: sediment diffusion versus focused flow and the pooling 

of hypersaline fluids. Summary descriptions for each site are as follows. 

The GC185 site is also called Bush Hill. It is a sediment-diffusion type community and is 

probably the type-example for lush chemosynthetic communities [MacDonald, 1989 #132]. It 

occupies a broad, low mound, roughly hemispherical in cross-section, the rocky western margin 

of which aligns with high angle faulting. The biological community is concentrated in a 

relatively small portion of the mound, in a 40-m wide band that is offset, but largely parallel 
12

with, the axis of the fault. Gas hydrate outcrops on the eastern-most margin of this band, furthest 

from the fault, and appears to be a plug that waxes and wanes at the orifice of an active gas vent. 

The outcropping deposits of gas hydrate are dynamic structures in terms of their size, but occur 

persistently at predictable locations. Shallow deposits of gas hydrate extend from the community 

to locations well away from the principal axis of biological activity. Although these 

characteristics result from mapping a single site, which had been previously defined as a 

sediment diffusion site, comparison of similar attributes at the program’s second sediment 

diffusion site, GC234, provide a means for evaluating their generality. 

The GC234 site is the locality where the first documented collections of sediments containing 

thermogenic hydrocarbons were made in the Gulf of Mexico (Anderson et al. 1983). The 

geology of the region has been described by Behrens (1988) and Reilly et al. (1996). The 

extensive chemosynthetic community is situated on a slump block within a half-graben. Steep 

escarpments rise above the community on three sides, with extensive lithification on the western 

slope, and the bottom slopes away to the south. 

Visual observations from the SEA-LINK cruises at GC234 conclusively link several of the 

mounds with shallow deposits of gas hydrate. Notably, the HYD1 station, when first observed in 

the 1997 cruise, was a prominent outcrop of gas hydrate that exposed a broad expanse of gas 

hydrate on its downslope margin (Figure 5) The polychaete Hesiocaeca methanicola

(Desbruyeres and Toulmond 1998) was first observed on the exposed portion of this gas hydrate 

outcropping (Fisher et al. 2000). From these results, it is evident that gas hydrate forms in the 

upper few meters of the sediment column and produces broad mounds that breach the seafloor on 

the margin of steeper slopes (MacDonald et al. 1994). This exposes patches of gas hydrate, 

which subsequently dissolves so that the localized relief gradually deflates. A cycle of exposure, 

dissolution, re-growth and re-exposure repeats at fixed localities within individual mounds. 

Figure 5. Exposed gas hydrate at the HYD1 station in GC234 site, 1997. This is the location where the ice worm

(Hesiocaeca methanicola) was first collected. This deposit disappeared between the 1997 and 1998

SEA-LINK cruises. Note the band of sediment sandwiched between two layers of hydrate. 
13

The clustering of chemosynthetic animals within the site is apparent from characteristics 

observed at the sampling stations during SEA-LINK dives (Figure 6). As was the case at the 

GC185 site, sampling stations defined by juvenile, adult, and senescent tubeworms were readily 

located, as were stations defined by mussels and bacterial mats. Though tubeworm colonies were 

abundant in the immediate vicinity of the sampling stations, there were large areas that were 

unoccupied by tubeworms and other chemosynthetic animals. As was also seen at the GC185 

site, the association between tubeworm aggregations and gas hydrate at the site was consistent, 

but not deterministic. Dense aggregations of tubeworms extended beyond the limits of the 

hydrate mounds (e.g., Station AT1, Figure 6). 

Figure 6. Results of X-Star subbottom profile survey. (A) North-south tracklines overlaid on sediment style

showing location of hydrate mounds, faults, and sampling station locations. (B) X-Star subbottom

profiles showing abrupt transition between stratified and un-stratified sediments, faults, mounds and

piston core locations. 
14

It is evident from the results at GC234 that sharp discontinuities in seepage are prevalent within 

the study area. A probable cause of these discontinuities is ongoing fracturing of the sediment 

column during the movement of the slump block and the expansion of the graben found at the 

site. The effect upon community development is to generate a patchwork of animal clusters 

distributed across a wide area. Although the underlying cause of this heterogeneous distribution 

is differential seepage along fault margins, the patterns are obscured by the happenstance of 

settlement history and minor disturbances. This confused pattern contrasts sharply with the 

orderly gradients found at brine pooling habitats. 

The brine-filled pockmark and mussel bed at the GC233 site is a notable example of a 

chemosynthetic community supported by focused flow and methane dissolved in pooled 

hypersaline fluid (MacDonald et al. 1990b). This pool is situated on a low mound, which is 

elevated 6 to 8 m above the surrounding seafloor and has a basal diameter of approximately 100 

m (Figure 7). The fluid filling the pool has a salinity of 130 practical seawater units and is 

supersaturated in biogenic methane. The seep mussel community as this site has developed in the 

present day to a continuous band that completely surrounds the pool on the level margins of the 

crater. The persistence of these animals is strong evidence for conditions that have favored 

chemosynthetic animals over an extended time. The probable age of the larger seep mussels in 

this population exceeds 100 y (Nix et al. 1995). Stability of the level of brine filling the pool is 

essential because the brine is anoxic in addition to being hypersaline. Therefore, it would be fatal 

for mussels to be submerged in the brine. Examination of the inner edges of the pool, however, 

reveals that the mussel hold their siphons less than half their body lengths (about 4 cm) above the 

brine. The outer edges are less than 15 cm higher than the inner edges. Because mussels are 

sessile and bound in place by the byssal threads of their neighbors, even the slightest increase in 

the brine level would produce widespread stress or mortality. Subbottom profile data collected 

by NR-1 show a mound formed by repeated eruptions of fluidized mud through two distinct 

vents. The two piston cores that were taken from the pool edges provided strong evidence for 

continual chemosynthetic communities at the site. Core RN8-1 was collected at the outer edge of 

the mound. Mudstones and stiff clay in the upper sections are consistent with the strong reflector 

seen in the upper portion of the subbottom record. A lucinid shell recovered from 1 m subbottom 

indicates circumstances favorable to chemosynthetic animals in this stratum. Notably, at 3.7 m 

subbottom a seep mussel shell indicates presence of a mussel colony at a position about 80 m, 

laterally, from present mussel colonies. Core RN8-2 was recovered immediately south of the 

pool and mussel bed. Fragments of lucinid shell and mud stones at 0.5, 2, and 4 m subbottom are 

consistent with the depths of the upper layers in the subbottom profile. This site and its 

strikingly dense population represent an equilibrium state between the violent forces of 

formation and continual processes (e.g., dissolution of the brine) that are eroding the present 

conditions. Perception of stability in the face of potentially destabilizing forces is intuitively 

clear. However, not all brine pools are similarly stable, as findings from the fourth study site, 

GB425 illustrate. 
15

The GB425 site is a ~1-km wide plateau located at the edge of an intraslope basin that contains 

economically significant hydrocarbons in the Auger, Cardamom, and Macaroni fields and is 

bordered to the west by tabular salt bodies (McGee et al. 1993). Sediment cores from this zone 

have recovered high-molecular weight hydrocarbon and thermogenic gas hydrate. Fluids 

migrating up these faults disturb surface sediments due to the formation of carbonate nodules, oil 

and gas pockets, and biogenic debris. A lake of fluidized mud and brine is situated on the 

southeast edge of the plateau (Figure 8) 

Figure 7. Subbottom profile survey of GC233 brine pool. (A) Plan of site showing trackline and piston core

locations. (B) Example subbottom record from Track 11 taken along southwest to northeast track.

Water depth of submarine was held constant (within < 1 m) along trackline, so bottom profile is

accurate. Note horizontal banding marking sea floors buried by successive eruptions of the pool. 

RN8-1

RN8-2

A 

B 
16

Figure 8. Mud-filled crater at GB425. (A) Laser line-scan mosaic of GB425 brine pool, showing location of M1

sampling station (grid points are at 10 m centers). M1 station is located approximately 150 m to the

northeast. Mussel beds are restricted to southwestern (upslope) end of pool. (B) Photomosaic of M1

station looking south. Note marker float.

A 

B 

M1 

M2 
17

The mud lake at GB425 was sampled with use of the SEA-LINK on 29 July 1997 and 11 July 

1998. On the first visit a thermistor was suspended below a float anchored on a short tether. It 

recorded a reading every 20 min. The anchor was attached to a 75 by 75 cm plate to prevent it 

from sinking in the soft mud of the crater bottom. Samples of brine and mud were collected with 

use of a chamber that the SEA-LINK positioned below the fluid-seawater interface and sealed 

for recovery. The brine had a salinity of 133 practical salinity units. The fluid degassed violently 

when the collection chamber was opened on the surface, indicating that methane gasses in the 

fluid were super-saturated with respect to sea level temperature and pressure. 

Changes in the fluid properties of the mud lake over a one-year period indicated episodic activity 

related to fluid discharge. The geologic evidence of mud slides and the formation of large flattopped structure points to ongoing and massive fluid expulsion. Episodic events are difficult to 

capture with sampling cruises of a few days separated by months or years. To provide 

independent data on fluid discharge at GB425 and from the region as a whole, available satellite 

synthetic aperture radar (SAR) data were reviewed in collaboration with scientists from Unocal 

Corporation. Satellite SAR readily detects layers of floating oil that form over active seeps. It 

provides a means to survey the numbers of hydrocarbon seeps across oil-producing regions and 

to estimate the rates at which seeps are flowing. Comparison of SAR images collected in the 

Gulf of Mexico indicates large differences in the amount of oil seepage over short time lags. 

These changes are too great to be attributed to ambient sea surface conditions. 

Available time-series data from the 1997 through 1998 periods at GB425 comprised six SAR 

images and the records from the thermistor (Figure 9). Three of the SAR images were collected 

in May and June 1997, prior to deployment of the thermistor; three were concurrent with the 

deployment. The area of oil slicks overlying the fault zone repeatedly underwent order of 

magnitude changes between the May 1997 and the February 1998 observations (Figure 7.20B). 

During the 349-day thermistor deployment, fluid temperatures varied between 6.1° and 48.3°C 

with a mean of 26.1°C and standard deviation of 9.07. one can speculate that pulsed flows result 

when ongoing filling of hydrocarbon traps in western Auger Basin induces the failure of the 

permeable seals in the fault zone, injecting gas, oil, brine, and mud into surface strata and the 

ocean. Previous remote sensing surveys have depended on repeated detection of floating oil to 

confirm flowing seeps (Kornacki et al. 1994, MacDonald et al. 1993, MacDonald et al. 1996, 

Mitchell et al. 1999). This criterion would have overlooked seeps like the one described here. 

These results suggest that the time-dependence of seepage rates should be considered when 

evaluating the relative stability of seepage and seep dependent communities. 
18

Figure 9. Comparison of temperature time series to abundance of floating oil above site. (A) Three sub-scenes of

SAR data are reproduced to show, from left to right, large-, small-, and medium-size oil slicks. (B)

Time series of slick areas (in hectares) imaged by available SAR. Slick areas were compiled for an

18.7 by 13.1 km region overlying the Auger Basin fault zone (Figure 7.19). (C) Temperature time

series from fluid in diatreme recorded between 27 July 1997 and 11 July 1998. 

A 

B 

C 
19

Ecology of Seep Fauna 

The Gulf of Mexico seep communities exist as a direct consequence of the biological oxidation 

of the energy-rich pore fluids seeping from the sea floor and the concomitant production of 

biomass. The organisms responsible for this production are chemoautotrophic and 

methanotrophic bacteria. These bacteria exist as free-living cells throughout the seep 

environment and also occur as thick mats on the sediment or other surfaces. Chemoautotrophic 

sulfide-oxidizing bacteria also occur as symbionts in several different invertebrates, most notably 

vestimentiferan tubeworms (Figure 10A) and several species of mussels and clams. The seep 

mussel (Bathymodiolus childressi see Figure 10B), which is the dominant mussel species at these 

sites, harbors symbiotic methanotrophic bacteria. In each of these cases, the symbiotic bacteria 

provide the overwhelming bulk of the host’s nutritional needs, making the animals themselves 

chemoautotrophic in a sense. 

Three overlapping types of communities are present in the primary sites that were a focus for this 

study. Bacterial mat communities are apparent both separated from and immediately adjacent to 

aggregations of other types of seep fauna. These mats can be quite extensive in both size of 

individual mats and abundance of mats at a site. Because the mats produce organic biomass and 

oxidize potentially toxic substrates, they are potentially important primary producers and 

detoxifiers for the seep ecosystem. Tubeworms and mussels, because of their autotrophic 

lifestyle, gregarious settlement behavior, and relatively large size, function as “ecosystem 

engineers.” As such, they create substantial areas of habitat for numerous other species of 

endemic and non-endemic animals. Tubeworm and mussel communities can be relatively small 

or can completely cover areas up to thousands of square meters. Each can occur where the other 

is absent, or they can co-occur, each literally growing on one another. 

One of the long-term goals of the Minerals Management Service's programs to study seep 

communities has been to develop an understanding and appreciation of these relatively poorly 

known and recently discovered communities. The hope is to provide the knowledge base for 

well-informed management decisions. Over the past fifteen years, many other projects have also 

contributed to the database on these and related communities. However, only recently has there 

been available sufficient information to put together a cohesive picture of the biology and 

ecology of the seep animals. The present study specifically built on what was known about the 

physiology, genetics, and ecology of the three key faunal groups (bacteria, tubeworms and 

mussels) to build and test working models of the animal’s life histories and the communities they 

form. In some cases, studies were designed to address very fundamental physiological or genetic 

questions important to understanding the animals. In other cases, studies were designed to test 

system-level concepts arising from developing models of community succession, larval 

dispersal, or niche differentiation. 

To the greatest extent possible, experimental design included integrated sampling between the 

different components of the overall project. For example, before a mussel bed was sampled, 

water samples were taken from beneath and over the mussels for chemical analysis. The bed was 

then sampled in a quantitative approach that allowed the biomass and size of all inhabitants to be 

determined and the biomass of the entire bed to be calculated from photographs or mosaics. Each 

collection was further sub-sampled for all appropriate components (i.e., condition analyses, PAH 

tissue load determination, histopathology, population genetics, stable isotope food-web studies, 
20

Figure 10. Chemosynthetic fauna from the Gulf of Mexico. A. Tubeworms surround a mound 

of gas hydrate. Surface of mound is covered with bacterial mats. The mound is 

about 1 m wide. B. Seep mussels at gas seep. Note associated crustaceans and 

gastropods. The mussels are 8 to 10 cm long. 

and in some cases growth or transplant studies). This greatly enriched the interpretation and 
21

future usefulness of each individual study and is also an ecologically sound approach to 

obtaining the maximum amount of knowledge with minimal impact on the populations. 

Over the course of the program, numerous discoveries were made that fundamentally changed 

the views of the animals and the working models of these communities. Many of the studies 

remain ongoing because early results led to new directions and/or development of new 

techniques. Finally, as discovery usually does, the results of these studies spawned new and 

important questions that were not anticipated when the program was initiated. 

The key discovery that the tubeworms are extremely long-lived, in fact, the longest lived animals 

known to exist on the planet. (Figure 11) shows the age estimates for tube worm whose growth 

was monitored during the program. It indicates a maximum ages in excess of 200 years. This is 

in direct contrast to the hydrothermal vent tubeworms, which are a very fast-growing, 

opportunistic species. Because the tubeworms produce the framework for a complex ecosystem, 

the seep tubeworms are much more analogous to trees in the sense of being long-lived and 

ecosystem-structuring. As a result, activities that damage well-established communities may 

have an impact that would last for centuries. In fact, consideration of their long life in light of the 

non-persistent size distributions in the aggregations suggests that old communities might never 

recover from serious damage because the time window for vestimentiferan recruitment to the site 

would have passed. 

Figure 11. Individual ages predicted from tube length using the negative exponential model. The 1998 data 

set (solid circles) and the 1995-1998 data set (open circles) predict the average expected age of an individual. The 

upper bound of the 1998 data set (closed squares) predicts the minimum theoretical age of an individual constantly 

growing at the maximum growth rate of the species throughout its life.

A second discovery is that at least one of these species can acquire sulfide from buried sources 
22

and therefore may not require surface expression of seepage (into the water column). This may 

explain how the tubeworms live for centuries and also why the diversity of non-endemic fauna in 

these communities is much higher than at hydrothermal vents. It also suggests that the sulfide 

supporting these communities must be present at depths considerably greater than the depths of 

sulfide generation from seawater sulfate reduction. Thus, hydrocarbon seepage alone (with 

attendant production of sulfide in very shallow sediments) may not be sufficient for maintenance 

of large and long-lived tubeworm colonies. Therefore, the source of sulfide necessary to support 

these communities may be more patchily distributed than hydrocarbon seepage in general. 

Another completely unexpected finding was that the mussel beds in some areas of the Gulf of 

Mexico were so severely infected with parasites that mussels in these areas are effectively sterile 

and therefore cannot contribute to larval pool of the Bathymodiolus childressi in the Gulf of 

Mexico. (Figure 12) shows the histology of parasite infections in seep mussels from the study 

sites. 

Figure 12. Parasite infections of seep mussels from study sites. Plate 1. Gill rosettes -- 1a, b:

1000x; 1c: 25x. Plate 2. Rickettsial-like bodies in the digestive tubules and

diverticula -- 2a, b: 1000x. Plate 3. Gill ciliates between the demibranches -- 3a,

b: 1000x. Plate 4. Bucephalus in the gonad -- 10x. Plate 5. Rickettsial-like bodies

in the gill -- 5a: 1000x; 5b: 25x. 
23

By comparison, mussel populations at other sites were quite healthy. Thus, some populations 

may be key to the maintenance of the Gulf-wide meta-population, while other sites are 

inconsequential. Differential growth was evident in direct measurements of mussel growth made 

using mark and recapture methods. The results show significantly different rates of growth 

among populations from individual sites (Figure 13) 

Figure 13. Growth of mussels between 1997 and 1998 field seasons at the GC233 and at MMS study stations 

BHM1, GCM1, and GBM1. 

Results from stable isotope studies show that seep primary production not only supports a higher 

diversity of animals that are seep endemics or colonists than previously demonstrated, but also 

that significant amounts of seep primary production are being transferred into the surrounding 

deep-sea benthos. Of the eight species of larger mobile predators captured and analyzed for this 

study, individuals of six contained a significant amount of seep carbon and nitrogen in their 

tissues, indicating continued consumption of seep animals or bacteria over a significant period of 

time. Thus the seeps are not small islands of productivity inhabited by unique animals to be 

managed in isolation, but rather they are affecting and being affected by the surrounding deepsea fauna. 

Related to this finding is the discovery that significant numbers of the mat forming bacteria are 
24

present in situations when the mats are not visually apparent, and the strong implication that 

most of the fauna found among the tubeworms are not deriving their nutrition from tubeworm 

primary production. This finding, which is detailed in Section 8.2, suggests that primary 

production from free-living seep microbes may be much more important to the communities than 

usually recognized and underlies how little we know about the microbial ecology of these 

systems. 

The molecular genetics studies indicate that over the areas sampled for this study, the tubeworm 

and mussel populations are exchanging larvae between sites and therefore can be managed as 

single, large, meta-populations. Genetic diversity is not at risk through anthropogenic activities 

in this region. Another key finding is that the symbionts of four Gulf of Mexico tubeworm 

species are not specific to the host species, but rather to the geographic region where they are 

found. This strongly supports the hypotheses that the symbionts are not transmitted directly 

(rather they must be acquired de-novo each generation) and that niche-differentiation between 

species at a site is not a function of symbiont differences, and further suggests site-specific 

differences in the bacterial populations at different sites. All of this information contributes to 

our working models of these species. 

Based on our current level of understanding of the biology of the key ecosystem engineers and of 

the community ecology of the seep ecosystem, we have refined our working model of these 

communities. It is important to note that in the regions of the Gulf of Mexico where the seep 

communities occur the larval population is well mixed, and based on other studies, we know that 

the larvae of key species spend weeks or more in the water column. Thus, colonization and 

evolution of a new site will follow the same general pattern as colonization and evolution of 

patches of organisms within an established site, although the increased abundance of mobile 

consumers at established sites will likely influence the process through biological interactions. 

Biogeochemistry of Chemosynthetic Communities 

Upper Jurassic-sourced oil is found in anomalous amounts in sediments at or near four of our 

chemosynthetic community sites (GC185, GC234, GC233, GB425). The mean of 108 

measurements of oil concentration performed on the four study sites is about 11,778 ppm by 

sediment weight. The range between individual measurements is large, however, from 6 ppm to 

46,834 ppm. This wide range is expected in sediments of hydrocarbon seeps and even within 

single piston cores of seep sediments. Although oil is normally absent or present in low 

abundance in most Gulf slope sediments (<10 ppm), three of the study sites are clearly 

anomalous in terms of oil concentration. Differences in concentration of oil between three sites 

(GC185, GC234, GB425) are not thought to be meaningful. In contrast, the data do not 

demonstrate that crude oil is as widely distributed in sediments adjacent to the GC233 brine pool 

site. This, however, could be an artifact of sampling. 

Hydrocarbon gas is concluded to be more effective than oil at driving biogeochemical processes 

at all of our study sites (GC185, GC234, GC233, GB425). The biogeochemistry of hydrocarbon 

seeps is fundamentally related to the concentration, fluxes, sources, and sinks of dissolved gases. 

Gases, such as methane and other hydrocarbon components, which migrate from depth or are 

supplied by disassociating methane hydrate located close to the sediment-water interface, may 

represent a major source of reduced carbon in these environments. Carbon dioxide (CO2) and 

hydrogen sulfide (H2S) play critical roles as metabolic products of microbial sulfate reduction 
25

and exert a major influence on critical environmental variables such as pH and oxidationreduction potential (pe/Eh). The reoxidation of reduced sulfur also plays a critical role as an 

energy source for benthic organisms. 

Classification of the sampling stations 
The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

for most of our nationally owned public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; 

preserving the environmental and cultural values of our national parks and historical places; 

and providing for the 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 under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) 

primary responsibilities are to manage the mineral resources located on the Nation's Outer 

Continental Shelf (OCS), collect revenue from the Federal OCS and onshore Federal and Indian 

lands, and distribute those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

administers the OCS competitive leasing program and oversees the safe and environmentally 

sound exploration and production of our Nation's offshore natural gas, oil and other mineral 

resources. The MMS Minerals Revenue Management meets its responsibilities by ensuring the 

efficient, timely and accurate collection and disbursement of revenue from mineral leasing and 

production due to Indian tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially 

affected parties and (2) carrying out its programs with an emphasis on working to enhance the 

quality of life for all Americans by lending MMS assistance and expertise to economic 

development and environmental protection.