(Table 1) Interstitial water d13C, tritation alkalinity, and sulphate concentrations of ODP Site 131-808A

The d13C values of total inorganic carbon (Sum CO2 ) were measured for interstitial waters extracted from the turbidite and hemipelagic sediments of the Nankai Trough accretionary prism during ODP Leg 131. At 3.0 mbsf in the bacterial sulfate reduction zone, a significant low d13C of -29.8 per mil (PDB) was found, suggesting oxidation of upward-migrated methane from the zone below. At 6.0 mbsf, below the sulfate reduction zone, the d13C value increases abruptly to a maximum value of +12.0 per mil, reflecting an enrichment of 13C in residual Sum CO2 from the bacterial reduction of some Sum CO2 to 12C-enriched CH4. Below 80 mbsf, the 13C values decrease almost linearly with depth to ~ -20 per mil at 1000 mbsf, indicating that the fraction of Sum CO2 derived from thermal degradation of organic material increases gradually with increasing in-situ temperature.