TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 46 (No. 2), pp. 89-106, 2012

Methane production and accumulation in the Nankai accretionary prism: Results from IODP Expeditions 315 and 316

Tomohiro Toki,1 Yuto Uehara,1 Kazunari Kinjo,1 Akira Ijiri,2 Urumu Tsunogai,3 Hitoshi Tomaru4 and Juichiro Ashi5

1Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
2Kochi Institute for Core Sample Research, JAMSTEC, B200, Monobe, Nankoku 783-8502, Japan
3Earth and Planetary System Science, Faculty of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
4Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
5Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan

(Received July 1, 2011; Accepted November 30, 2011)

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 (ΣCO2) in dissolved gases were measured. The contribution of thermogenic CH4 was negligible at all sites, while carbon isotopic separation between CH4 and ΣCO2 indicated that CH4 formation was mainly by microbial hydrogenotrophic methanogenesis. Evaluation of the isotopic fraction of the initial substrate ΣCO2 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.
Key words: Nankai accretionary prism, IODP, origin of methane, carbon isotope, methanogenesis

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