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Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3,191 m water depth on a spitlike extension of the continental rise (Blake Outer Ridge) on a passive margin in the Atlantic Ocean. Site 568, at 2,031 m water depth, is in upper-slope sediment of an active accretionary margin (Middle America Trench) in the Pacific Ocean. Both sites are characterized by high rates of sedimentation (greater than 30 m/m.y.), and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that crosscut reflections from sedimentary layers and parallel reflections from the sea floo suggested the presence of gas hydrates at both sites. During coring, small samples of gas
hydrate were recovered at subbottom depths of 238 m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates were methane, ethane, and CO2. Residual methane in sediments at both sites usually exceeded 10 ml per liter of wet sediment. Carbon isotopic compositions of methane, CO2, and ^SgrCO2 followed parallel trends with depth, suggesting that methane formed mainly as a result of biologic reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. The small samples of gas hydrates observed visually in cores confirm that gas hydrates are present at these sites, but much of the direct evidence for gas hydrates may be destroyed during the coring and recovery process.
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