The northern Gulf of Mexico (GOM) continental slope is the most thoroughly imaged and studied continental slope in the world’s oceans. Most of the slope is covered with 3D seismic, and much of the area has been imaged with high-resolution acoustic systems. These data describe a complex sea floor impacted by fluid-gas expulsion. The feedback relationship between sediment deposition and salt deformation creates many pathways for the upward migration of fluid and gases to the surface of the slope. Early studies discovered that the geology and biology of the sea floor at these sites of hydrocarbon and brine expulsion were different from surrounding areas.1-3

Although most data on these communities and hydrocarbon seep/vent sites have been derived from water depths above 1,000m, in 2006 and 2007 projects were fielded to add to upper slope data by concentrating observations and collections at middle to lower slope depths. Co-funded by the Minerals Management Service (MMS) and the National Oceanographic and Atmospheric Administration (NOAA), 25 days of deep submersible vehicle (DSV) Alvin’s time in 2006 and 35 days of remote operated vehicle (ROV) Jason’s time in 2007 were focused on the task of expanding our knowledge of hydrocarbon seeps/vents and their associated biological communities below a water depth of 1,000m. These dives have produced seabed observations and samples that have greatly improved our understanding of the distribution and variability in biological communities and their hydrocarbon seep-related habitats. A total of 15 dive sites were visited during the 2006 and 2007 cruises (see Figure 1). Site selection for the 2006 Alvin dives was accomplished though sea-floor reflectivity analysis of the MMS slope-wide 3D seismic database. From 80 potential sites, 20 were subjected to photo reconnaissance, of which 10 were selected for Alvin dives. Three sites, found in AC 601, GC 852 and AT 340, had impressive and diverse chemosynthetic communities in addition to well-defined fluid-gas expulsion geology. In addition to chemosynthetic communities, GC 852 had abundant hard and soft corals seated on substrates of authigenic carbonate boulders. At AC 601 (WD ~2,340m) a brine lake (4m deep and 180m wide, salinity ~90%) was investigated and sampled. At the three key sample locations, autonomous underwater vehicle (AUV) data were acquired prior to the 2007 ROV Jason cruise. Multibeam bathymetry from the AUV was used as a navigation underlay for detailed sampling by Jason. Depth and geographical ranges for key components of chemosynthetic communities were improved and our understanding of the geological aspects of hydrocarbon seep habitats was significantly advanced by data from the 2006 and 2007 dives. In this article, four of the high-priority sample sites will be discussed. The sites are found in the following lease blocks: Alaminos Canyon 601 (AC 601), Walker Ridge 269-270 (WR 269-270), Green Canyon 852 (GC 852) and Atwater Valley 340 (AT 340).

A better understanding of potential hazards to facility installation and pipelines on the slope have been gained through these studies, and more is now known about the location, density and diversity of the chemosynthetic communities associated with seeps. Additionally, these unique biological communities are protected by law and cannot be disturbed by industry activity. The dive programme proved that 3D seismic can be used to sucessfully identify these seep sites.

Alaminos Canyon Block 601
The AC 601 dive site (N 26º 23.3’; W 94º 30.8’; water depth ~2,340m) is located in a canyon-like re-entrant into the Sigsbee Escarpment. It is an anticlinal structure trending roughly east-to-west across the approximate mid-point of the canyon and has scattered fluid-gas expulsion features along its crest. Seismic profiles indicate that migration pathways from the deep subsurface are provided by faults and fractures that breach the anticline. The primary dive site in AC 601 is a circular depression with an elevated rim and high ground that appears to be an older expulsion centre (see Figure 2). Analysis of the 3D seismic sea-floor reflectivity data associated with this feature suggests it is a fluid-gas expulsion mound with evidence of small sediment flows running downslope away from the mound. This feature and a larger circular expulsion feature of very low relief to the south are easily recognisable on multibeam and 3D seismic sea-floor reflectivity maps, and both were observed and sampled during the 2006 and 2007 cruises.

The northernmost and smaller feature was investigated with Alvin in 2006. A previous MMS-sponsored ROV survey in 2005 found that the feature was a brine lake. The Alvin dive of 2006 allowed us to determine the dimensions of the lake, surrounding benthic communities and the origin of white ‘flocs’ floating in the brine as well as accumulated on the ‘shoreline’. The brine lake was found to be in water 2,334m deep, with a diameter of approximately 180m and a general depth of about 4m. Analysis of the brine indicated that it was nearly three times (~90% salinity) the salinity of normal seawater. The white material floating in the brine and precipitation on the lake bottom (see Figure 2) was found to be barite.4 No living organisms were found in the lake, but limited communities were observed and sampled around the lake margin. These communities consisted primarily of mussels and heart urchins. Authigenic carbonates were limited to very localised hardgrounds. High methane concentrations were observed in the sediments surrounding the lake and in the water column above the lake.4 In 2007, observation and sampling of the broad circular expulsion feature to the south verified the occurrence of fluidised mud and brine and extensive mussel beds. In fact, the largest mussel bed so far observed in the GOM was found at this site. However, more diverse chemosynthetic communities were not found.

Walker Ridge Blocks 269-270
A 3D seismic study of the Walker Ridge dive site5 (N 26º 41.2’; W 91º 39.9’; water depth ~1,950m) identified the dive objective as a ‘gas mound’ overlying gas hydrate in the shallow subsurface, as indicated by a distinct bottom-simulating reflector (BSR) crossing the stratigraphy of the area.5 The mounded area of interest occurs on the northern flank of a suprasalt sedimentary basin, where it appears that gas and perhaps fluids have migrated to the sea floor within porous and permeable upturned beds. The objective of the Alvin dive in 2006 was to explore the crestal area of a complex mounded sea floor (see Figure 3). On 3D seismic sea-floor reflectivity data, this area displays a high positive amplitude with a circular low amplitude centre, suggestive of an active vent and very soft sea floor. The Alvin dive confirmed hydrocarbon seep-related communities both on the flanks of the mound and at the apex (see Figure 3). At the crest, beds of very large mussels, numerous tubeworm ‘bushes’, exposed gas hydrate and abundant authigenic carbonates were found. We proved that the crestal mound was an active hydrocarbon seep site that supports a thriving chemosynthetic community. A dive in 2007 at this site using Jason improved on geological and biological sampling of this area.

Green Canyon Block 852
A prominent north-to-south trending elongated mound (N 27º 06.6’; W 91º 10.0’; water depth ~1,400m) that is nearly 4km long was the subject of Alvin dives in 2006 and Jason dives in 2007 (see Figure 4). The shallowest part of this feature (water depth ~1,400m) is characterised by large authigenic carbonate boulders. The ridge-like feature rises nearly 200m above the surrounding sea floor and in a regional context occurs at the south-east boundary of a well defined intraslope basin. The 3D seismic reflectivity data suggest that both the northern and the southern crestal areas as well as the western flank of the mound are characterised by localised hard seabed conditions. These areas are defined by high positive amplitudes and, on 3D seismic profiles, overlie regions of poor acoustic returns in the subsurface. These acoustic ‘wipeout’ zones are interpreted as migration pathways for fluids and gases to the sea floor. During the cruises, oil was observed on the sea surface over the southern crest of the GC 852 mound.

Hard bottom composed of authigenic carbonate hardgrounds and large carbonate boulders characterised these sites, and scattered but impressive benthic communities were observed. Gas was observed escaping from some mussel beds. In addition to chemosynthetic communities consisting of tube worms and mussels, the shallowest part of the mound supported well-developed hard and soft coral communities and other benthic species (see Figure 4). The corals are attached to the huge authigenic carbonate boulders (see Figure 4). During the dive at the coral site in 2006, a strong current was noted, which may supply the lush coral community with plankton for food and larvae for colonisation (see Figure 4). The GC 852 mound proved to be one of the key sampling sites during both the 2006 and the 2007 cruises. Not only did this site have abundant chemosynthetic communities, but the deepwater coral community was by far the most diverse and densely populated such community found so far in the northern GOM.

Atwater Valley Block 340
The AT 340 site (N 27º 38.8’; W 88º 21.9’; water depth ~2,200m) is the easternmost dive site and, regarding variability of surficial geology, is the most complex site visited during the 2006 and 2007 cruises. In a regional context, the site is a mound on the sea floor east of the mouth of the Mississippi Canyon where the canyon changes to a submarine fan. Seismic profiles across the site indicate that the mound is a salt-supported structure. Surface reflectivity patterns from 3D seismic data illustrate flow patterns radiating from a number of points of origin on the mound top. These data also discriminate high-amplitude mounds and low-amplitude depressions scattered over the mound surface. Multibeam bathymetry acquired by AUV clearly illustrates the complexity of this site (see Figure 5). Due to the variability of the surface geology, numerous chemosynthetic community sites and active seepage of hydrocarbons and brine occur at the AT 340 site. These interesting sea-floor conditions prompted us to make this a key sampling site after the first Alvin dive in 2006.

The individual mounded areas on top of the overall AT 340 site rise as much as 20m above the surrounding sea floor. These mounds are highly reflective on 3D seismic sea-floor amplitude data. They are composed largely of stacked carbonate blocks made up of mussel shells cemented by authigenic carbonate. Tube worms and living and dead mussels occur in the cracks and crevasses between carbonate blocks. Densely populated mussel beds can be found on the flanks of the mounds. Local areas between mounds support high densities of heart urchins where sediments are soft and reducing and show evidence of seepage. The trails caused by these mobile animals create an extremely complex pattern in the surface sediments (see Figure 5). In areas of brine seepage, flows and pools of brine-invaded fluidised mud occur without abundantly populated benthic communities. However, mussel aggregations are supported along the flanks (and sometimes within) the flows. The presence of shallow salt beneath the AT 340 mound suggests that dissolution of the salt body is probably the origin of the brine flows and seeps at the sea floor.

Source: Touch Oil & Gas| by Harry H Roberts, Jesse Hunt, Jr., William Shedd

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