Sano, Yuji, Kinoshita, Naoya, Kagoshima, Takanori, Takahata, Naoto, Sakata, Susumu, Toki, Tomohiro, Kawagucci, Shinsuke, Waseda, Amane, Lan, Tefang, Wen, Hsinyi, Chen, Ai-Ti, Lee, Hsiaofen, Yang, Tsanyao F., Zheng, Guodong, Tomonaga, Yama, Roulleau, Emilie and Pinti, Daniele L. (2017) Origin of methane-rich natural gas at the West Pacific convergent plate boundary. Scientific Reports, 7 (Article number: 15646). DOI 10.1038/s41598-017-15959-5.

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Abstract

Methane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth. Here we report a clear relationship between the origin of methane-rich natural gases and the geodynamic setting of the West Pacific convergent plate boundary. Natural gases in the frontal arc basin (South Kanto gas fields, Northeast Japan) show a typical microbial signature with light carbon isotopes, high CH4/C2H6 and CH4/³He ratios. In the Akita-Niigata region – which corresponds to the slope stretching from the volcanic-arc to the back-arc –a thermogenic signature characterize the gases, with prevalence of heavy carbon isotopes, low CH4/C2H6 and CH4/³He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH4/C2H6 ratios and low CH4/³He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH4/C2H6 ratios and the lowest CH4/³He ratios. The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested.

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