Origin of methane-rich natural gas at the West Pacific convergent plate boundary
Résumé
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 CH 4 /C 2 H 6 and CH 4 / 3 He 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 CH 4 /C 2 H 6 and CH 4 / 3 He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH 4 /C 2 H 6 ratios and low CH 4 / 3 He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH 4 /C 2 H 6 ratios and the lowest CH 4 / 3 He 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. Volatile elements are transported from the Earth's interior to the hydrosphere and the atmosphere through volcanic and hydrothermal systems in addition to micro-and macro-seepages from active tectonic areas 1,2. If the geological carbon cycle is well documented for its oxide forms, such as CO 2
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