Gulf of Mexico Hydrocarbon Seep Communities: Part IV Descriptions of Known Chemosynthetic Communities

ABSTRACT The Geochemical and Environmental Research Group (GERG) at Texas A&M University has discovered vent-type, chemosynthetic communities associated with our recent findings of widespread hydrocarbon seepage, gas hydrates and authigenic carbonate on the Texas/Louisiana continental slope. These vent type taxa (clams, mussels and tubeworms) are unique in that they are associated with active gas and oil seepage and harbor endosymbiotic, chemoautotrophic bacteria, including a proven methanotroph. INTRODUCTION Deep water chemosynthetic organisms were discovered by Geochemical and Environmental Research Group (GERG) scientists in 1984 while trawling at previously discovered sites of oil seepage and gas hydrates (1,2) on the Louisiana continental slope. The chemosynthetic communities consist of assemblages of tubeworms, clams, mussels, bacterial mats and other associated organisms. 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. These communities are characteristically associated with sources of hydrogen sulfide or methane in an oxygenated environment. The underlying geological processes supplying these reduced compounds vary from site to site. In the Gulf of Mexico chemosynthetic organisms assume an applied importance because we are faced with the potential impact upon a fauna that is uniquely associated with exploitable hydrocarbon reserves. The U. S. Dept. of Interior's Minerals 'Management Service Notice to Lessees (NTL-88-1l) requires "all operators of leases in water depths greater than 400 m ... to provide a consistent and comprehensive approach which will avoid damage to 'high density chemosynthetic communities". Thus, MMS .is requiring "prior to approvals of Applications of Permit to Drill (APDs) and Pipeline Applications, the operators shall delineate all seafloor areas which would be disturbed by the proposed operations. :Additionally, an analysis of geophysical information for these areas, as well as any other pertinent :information available, shall be furnished which discusses the possibility of disturbing geological phenomena (such as hydrocarbon charged sediments, seismic wipe-out zones, anomalous mounds or knolls, gas vents, or oil seeps) that could support chemosynthetic organism". Thus, the existence of chemosynthetic organisms on the continental slope is of practical importance to the oil industry. This paper summarizes the visual observations of chemosynthetic communities from submersible dives on the Johnson "Sea-Link" and U.S. Navy NR-l in 1986-1988. Chemosynthetic communities have been confirmed at sites (3) other than the ones summarized here based on the recovery of organisms from trawls. BACKGROUND Recent discoveries in the northern Gulf of Mexico are dramatically altering our understanding of the geological, chemical and biological processes which control the overall ecology of the continental slope. In the geological area, high resolution profiling has increasingly shown that salt tectonics and related processes dominate mesoscale topography and produce islands of hard substrate in a predominantly mud environment (4). Active and widespread geochemical systems involving hydrocarbons at or near the deep-sea sediment-water interface were first confirmed by the discovery of oil-stained cores and thermogenic hydrates by GERG (1).