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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.

Gas hydrate volume% filled in pore space of sediments by in situ bacterial methane production is cal culated as a function of total organic carbon (TOC) contents in sediments, assuming that all the excess amount of methane beyond the solubility in pore water can form the hydrate under ordinary conditions of outer continental margins. The results suggest that at least 0.5% TOC is required for the hydrate formation. Average volume of gas hydrates filled in pore space of hydrate-bearing sediments is estimated to be about 5-6% at a site on the Blake Ridge (Matsumoto et al., 1996). TOC required to fill the 5% pore volume as gas hydrate is calculated to be about 2% in the case of water depth of 3000 m and utilizable organic carbon for methanogenesis of 10%. This TOC value is comparable to the measured TOC values in the sediments at the site (0.8 to 2.3%, average 1.4%; Shipboard Scientific Party, 1996b). Therefore, hydrate formation by in situ bacterial methanogenesis can roughly explain the average amount of gas hydrates in the sediments. However, for the formation of locally concentrated massive gas hydrates, some accumulation processes are required. Accumulation of gas hydrates near the base of gas hydrate stability zone (BGHS) is possible by the recycling of methane and migration of methane from depths below the BGHS.