Journal Article Origin of methane-rich natural gas at the West Pacific convergent plate boundary

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  ,  Pinti, Daniele L.

72017-11-15 , Nature Publishing Group
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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