12. CARBON ISOTOPE RATIO OF TOTAL INORGANIC CARBON IN PORE WATERS ASSOCIATED WITH DIAGENESIS OF ORGANIC MATERIAL AT SITE 808, NANKAI TROUGH1

The δ13C values of total inorganic carbon (ΣCO 2) 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 δ13C of -29.8‰ (PDB) was found, suggesting oxidation of upward-migrat ed methane from the zone below. At 6.0 mbsf, below the sulfate reduction zone, the δ13C value increases abruptly to a maximum value of+12.0‰, reflecting an enrichment of 13 C in residual ZCO2 from the bacterial reduction of some ΣCO 2 to 12C-enriched CH4. Below 80 mbsf, the 13 C values decrease almost linearly with depth to —20‰ at 1000 mbsf, indicating that the fraction of ZCO2 derived from thermal degradation of organic material increases gradually with increasing in-situ temperature.