Beschreibung: We here report on specific chemical and microbial compositions observed at the Archaean hydrothermal site in the Southern Mariana backarc spreading center, which produces two remarkably different hydrothermal fluids. One was black smoker hydrothermal fluid at 340 °C containing a low concentration of methane with a relatively high carbon isotope ratio (d13C of methane relative to VPDB), -7.8 per mil, indicating that methane originated from a magmatic source; thus, this is a fairly ordinary hydrothermal fluid for basalt-hosted hydrothermal activity. In contrast, the other fluid was clear smoker hydrothermal fluid at 117 °C containing a high concentration of methane with a very low carbon isotope ratio (-49.7 per mil). The host rock of the Archaean hydrothermal site is basalt, and therefore Fischer-Tropsch-type reactions resulting from serpentinization of mantle rocks are not feasible as a source for the "light" carbon in methane. Additionally, the carbon isotope ratio of carbon dioxide demonstrated that the sources of CO2 and CH4 were not organic materials. The remaining possibility, biogenic methane, was confirmed by cultivation of hyperthermophilic hydrogen-oxidizing methanogens from both rocks covering the vent of clear smoker hydrothermal fluids and in situ cultivation systems. Although the dominance and abundance of methanogens were very low in this ecosystem, the number was consistent with the relationship between hydrogen concentration and methanogen abundance (Takai et al., 2015). These results suggest that phase separation led to concentration of hydrogen and subsequent persistence of a hyperthermophilic subsurface lithoautotrophic microbial ecosystem in both serpentine-hosted and basalt-hosted hydrothermal systems. This type of ecosystem may occur in similar settings elsewhere on modern earth; in addition, similar communities may have existed in other types of deep-sea hydrothermal systems in the geological past.

Beschreibung: 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.