ABSTRACT

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.