Isotopic compositions of methane-rich natural gases All argon contents are much lower than the air value of 0.934%, while helium abundances are ranging from 2.7 ppm to 285 ppm, mostly higher than air value of 5.24 ppm, confirming an elemental signature that differs from atmospheric noble gas. Nitrogen content varies significantly from 0.1% to 21%, while oxygen abundances are generally close to 0.1%, suggesting that air contamination is negligible. South Taiwan samples have intermediate characteristics compared to the samples from the Akita-Niigata and South Kanto gas fields. Akita-Niigata natural gas samples show appreciable amounts of ethane and propane (C 3H 8), while the samples from the South Kanto gas field are dominated by an almost pure methane phase. ![]() Except for three CO 2-rich samples in South Taiwan (CL-1, 2, 3), natural gas samples are dominated by hydrocarbon species such as methane and ethane (C 2H 6). In addition, we measured the nitrogen isotope composition of the sampled natural gases, which may provide further insights about their formation.Ĭhemical compositions of methane-rich natural gasesĬhemical compositions of 21 natural gas samples collected and analyzed in this work are listed in Supplementary Table S1 together with those from the literature 26, 27. Therefore, the CH 4/ 3He ratios of natural gases are expected to provide constraints on the origin of methane. Among various volatile elements in natural gases, helium-3 has been shown to be one of the most powerful tracers of fluid origin on Earth because of its mantle origin and since its isotopic variability depends on the geodynamic setting 21, 22, 23, 24, 25, which is characterized by geophysical parameters such as terrestrial heat flow, Bouguer gravity anomaly, seismic velocity (P-wave perturbation), crustal deformation and magnetic anomaly. This region is a window on a complete arc system with fore-arc, volcanic-arc and back-arc. 1) and measured their chemical and isotopic compositions. In order to elucidate the origin of geological methane and its relationship with the geodynamic setting of the explored area, we collected natural gases in the West Pacific region (Fig. In principle, the chemical and isotopic composition of this methane and the associated natural gases should correlate with the geodynamic setting of the region where these gases originate 17, 18, 19, 20. There are two types of geological methane (a) abiotic methane originated in volcanic and geothermal systems 14, 15, 16, and (b) methane derived from hydrocarbon generation processes in sedimentary basins 7. Much attention has recently focused on the sources of radiocarbon-free geological methane (i.e., older than 40kyrs) in the domain of petroleum geochemistry and microbiology 11, 12, 13. The deep geological methane cycle still has to be well constrained 8. The shallower biological methane cycle is relatively well quantified, such as biological release from wetlands, rice paddies, animals and termites 9 and its impact on the greenhouse gas budget of the atmosphere 10. If the geological carbon cycle is well documented for its oxide forms, such as CO 2 3, 4, 5, much work is needed to describe the geological cycle of its reduced forms, such as methane (CH 4) 6, 7, 8. 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. ![]() An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested. ![]() The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. 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 2H 6 ratios and the lowest CH 4/ 3He ratios. ![]() Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH 4/C 2H 6 ratios and low CH 4/ 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 CH 4/C 2H 6 and CH 4/ 3He ratios. 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 2H 6 and CH 4/ 3He ratios. 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. Methane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth.
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