The isotopic compositions of dissolved CO 2 and CH 4 in sediments of the Nankai Trough indicate that CH 4 is formed during early diagenesis by microbial reduction of CO 2. At the shallowest sampled depths, the CO 2 dissolved in the pore water is unusually enriched in 12 C (δ 13 C =-35.2‰), indicating contribution of CO 2 from oxidation of CH 4. The most intense microbiological activity appears to be confined to the uppermost 50 m of sediment, based on relative lack of change in the isotopic compositions below this depth. Gas hydrate probably is not present at these localities (Sites 582, 583) because of CH 4 concentrations that are insufficient to saturate the pore water with respect to gas hydrate stability.
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