[DEEP WATERS] Sea-floor Geology, Biology and Hazards. Deepwater Fluid and Gas Expulsion

The Alvin dives of 2006 and the Jason dives of 2007 added valuable data for improving our understanding of the geological and biological impact of the fluid-gas expulsion process on the northern GOM’s middle to lower continental slope. A total of 15 sites ranging in water depth from slightly over 1,000m to over 2,800m were visited and sampled. Evidence of slow hydrocarbon seepage and/or more rapid venting and chemosynthetic communities were observed at each site. Most of the dive sites were positive sea-floor relief features that were represented on 3D seismic sea-floor reflectivity data as high positive amplitude zones. Seismic profiles across these features identified welldefined fluid-gas migration pathways from the deep subsurface to the modern sea floor. The high sea-floor reflectivity at every site corresponded to the occurrence of carbonate cementation of the sea floor, a by-product of microbial utilisation of hydrocarbons, as well as to communities of mussels and, in some cases, clams. At some sites, such as GC 852 and AT 340, huge slabs and boulders of carbonate created a hard and rough bottom setting. Tubeworm and mussel communities were found at the edges of carbonate slabs and in cracks and crevasses between the slabs and boulders. At the GC 852 site, large carbonate blocks provided the substrate for thriving soft and hard coral communities. The coral community at this site was the deepest and most prolific thus far observed in the northern GOM.

Several key sampling locations displayed evidence of significant brine seepage. Well-defined circular low-amplitude zones were found within areas of generally high positive amplitude on 3D seismic sea-floor reflectivity data. This relationship was especially true of AT 340, where brine flows were observed and sampled, and at AC 601, where a brine lake was found with scattered assemblages of mussels around the edges of the brine. However, tubeworms were not present. At the brine lake site it is also important to note that moribund organisms were found in the lake, but no living macro fauna. The lake waters were approximately three times the salinity of normal seawater. The white precipitate on the lake floor was found to be mostly barite. White flocs in the lake water were also found to be crystal rafts of barite. Data from the 2006 and 2007 dives proved that the processes of fluid-gas expulsion are active across the entire continental slope, to beyond the Sigsbee Escarpment. Symbiont-containing species that thrive on the products of hydrocarbon seepage were found at all dive sites. That these species, the tubeworms and mussels, form the foundation of the chemosynthetic communities in the GOM is well known. Species changes in these two groups of organisms were observed with depth. 3D seismic sea-floor reflectivity data can be used to identify deepwater oil and gas seep sites and the associated chemosynthetic communities protected by law.

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