Thermogenic gas hydrates and hydrocarbon gases in complex chemosynthetic communities, Gulf of Mexico continental slope

Where abundant at the sea floor, thermogenic gas hydrates impact bacteriall y-mediated processes in chemosynthetic communities dependent on methane and H2S. Our main gas hydrate sites are at similar to 540 m water depth and re latively low temperature (similar to 7 degrees C). Gas hydrates outcrop as vein-fillings in hemipelagic muds near gas vents within chemosynthetic comm unities. Molecular and isotopic properties of hydrate-forming C-1-C-5 hydro carbons and CO2 provide insight to bacterially-mediated processes. Hydrate- bound methane is altered by bacterial oxidation, as indicated by enrichment of C-13 and deuterium (D), and by CO2 depleted in C-13. The degree of gas hydrate alteration appears related to duration of exposure at the sea floor . In hydrate-associated sediments, bacterial oxidation of a mixed pool of h ydrocarbons yields a net production of CO2 depleted in C-13. Bacterial oxid ation of hydrate-bound methane and free hydrocarbon gases in adjacent sedim ents could contribute to gas hydrate decomposition. Some thermogenic carbon in sediments could be recycled via methanogenesis to yield a net productio n of bacterial methane depleted in C-13. Our results strengthen the hypothe sis that gas hydrates could favor life in other extreme environments at low temperatures. (C) 1999 Elsevier Science Ltd. All rights reserved.