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

Abstract Where abundant at the sea floor, thermogenic gas hydrates impact bacterially-mediated processes in chemosynthetic communities dependent on methane and H2S. Our main gas hydrate sites are at ∼540 m water depth and relatively low temperature (∼7°C). Gas hydrates outcrop as vein-fillings in hemipelagic muds near gas vents within chemosynthetic communities. Molecular and isotopic properties of hydrate-forming C1–C5 hydrocarbons and CO2 provide insight to bacterially-mediated processes. Hydrate-bound methane is altered by bacterial oxidation, as indicated by enrichment of 13C and deuterium (D), and by CO2 depleted in 13C. 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 hydrocarbons yields a net production of CO2 depleted in 13C. Bacterial oxidation of hydrate-bound methane and free hydrocarbon gases in adjacent sediments could contribute to gas hydrate decomposition. Some thermogenic carbon in sediments could be recycled via methanogenesis to yield a net production of bacterial methane depleted in 13C. Our results strengthen the hypothesis that gas hydrates could favor life in other extreme environments at low temperatures.

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