ORGANIC GEOCHEMISTRY OF SEDIMENTS FROM CHEMOSYNTHETIC COMMUNITIES, GULF-OF-MEXICO SLOPE

We used a research submersible to obtain 33 sediment samples from chem osynthetic communities at 541-650 m water depths in the Green Canyon ( GC) area of the Gulf of Mexico slope. Sediment samples from beneath an isolated mat of H2S-oxidizing bacteria at GC 234 contain oil(mean = 5 650 ppm) and C-1-C-5 hydrocarbons (mean = 12,979 ppm) that are altered by bacterial oxidation. Control cores away from the mat contain lower concentrations of oil (mean = 2966 ppm) and C-1-C-5 hydrocarbons (mea n = 83.6 ppm). Bacterial oxidation of hydrocarbons depletes O-2 in sed iments and triggers bacterial sulfate reduction to produce the H2S req uired by the mats. Sediment samples from GC 185 (Bush Hill) contain hi gh concentrations of oil (mean = 24,775 ppm) and C-1-C-5 hydrocarbons (mean = 11,037 ppm) that are altered by bacterial oxidation. Tube worm communities requiring H2S occur at GC 185 where the sea floor has bee n greatly modified since the Pleistocene by accumulation of oil, therm ogenic gas hydrates, and authigenic carbonate rock. Venting to the wat er column is suppressed by this sea-floor modification, enhancing bact erial activity in sediments. Sediments from an area with vesicomyid cl ams (GC 272) contain lower concentrations of oil altered by bacterial oxidation (mean = 1716 ppm) but C-1-C-5 concentrations are high (mean = 28,766 ppm). In contrast to other sampling areas, a sediment associa ted with the methanotrophic Seep Mytilid I (GC 233) is characterized b y low concentration of oil (82 ppm) but biogenic methane (C-1) is pres ent (8829 ppm).