Kyushu University, Japan

Several major dissolved constituents and properties (pH, alkalinity, Ca, Cl, K, Mg, Na, Si (OH) (sub 4) , NH (sub 4) , SO (sub 4) , CH (sub 4) , and delta (super 13) C) were measured for interstitial waters retrieved from surface sediments of the Nankai Trough accretionary prism during the NGH99 cruise (Sept. 14-29, 1999). Detailed depth profiles of chemical components associated with early diagenesis indicate that sulfate reduction rapidly occurs to consume all sulfate within the upper 2 meters of the sediment column at one of five stations. It is suggested that the upward migration of methane plays an important role as a carbon source in addition to the sedimentary organic matter derived from the water column above. Based on the stable carbon isotope ratio of methane in the pore fluid (delta (super 13) C (sub PDB) < or =70 per mil), methane is thought to be biogenic through bacterial CO (sub 2) reduction. Various sulfate distributions suggest a localized CH (sub 4) supply from deeper zones.