Isotopic composition of interstitial fluids in sediment of the Nankai Trough, Deep Sea Drilling Project Leg 87

The isotopic compositions of dissolved CO2 and CH4 in sediments of the Nankai Trough indicate that CH4 is formed during early diagenesis by microbial reduction of CO2 . At the shallowest sampled depths, the CO2 dissolved in the pore water is unusually enriched in 12C (δ13C = - 35.2‰), indicating contribution of CO2 from oxidation of CH4 . 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 CH4 concentrations that are insufficient to saturate the pore water with respect to gas hy drate stability