Pore waters were taken from core sediments of Sites C0001, C0004, and C0008 on the landward slope of the Nankai Trough and Site C0002 in the forearc basin of the Nankai accretionary prism off Kumano during Integrated Ocean Drilling Program Expeditions 315 and 316. The carbon isotopic ratios of CH4 and total carbon dioxide (∑CO2) in dissolved gases were measured. The contribution of thermogenic CH4 was negligible at all sites, while carbon isotopic separation between CH4 and ∑CO2 indicated that CH4 formation was mainly by microbial hydrogenotrophic methanogenesis. Evaluation of the isotopic fraction of the initial substrate ∑CO2 pool showed larger fractionation at Site C0002 than at the other sites in the transect. In addition, the NH4+ concentration was higher at Site C0002 than at the other sites, indicating that organic matter degradation occurred more actively at Site C0002 than at the other sites. Therefore, CO2 and H2 as well as NH4+ were actively generated by the organic matter degradation at Site C0002, which could stimulate methanogenesis utilizing CO2 and H2 as substrates at Site C0002. The high sedimentation rate at Site C0002 in the forearc basin was due to the geomorphological setting of the site, within the outer ridge rimming the sediment-filled Kumano Basin, leading to organic matter burial without aerobic degradation on the surface of the seafloor, which preserve labile organic matter for utilization by methanogenesis. On the other hand, slope sediments were already exposed by organic matter degradation, which leaves scarce labile organic matter for supporting CH4 generation. Geomorphology was possibly an important factor controlling CH4 formation and accumulation, and the Kumano Basin sediments have greater potential as a CH4 hydrate reservoir than the landward slope sediments in the Nankai accretionary prism off Kumano.
2012 by The Geochemical Society of Japan