The gas geochemistries of high-temperature and low-temperature hydrothermal fluids (HTHFs and LTHFs, respectively) were studied for the Suiyo Seamount hydrothermal system (140°39′E, 28°33′N) in the southern Izu-Bonin (Ogasawara) Arc of the western Pacific Ocean. HTHFs (290 ± 20°C) were collected from four active vents using a gas-tight fluid sampler, which prevented the loss of volatile components through degassing during sample processing. The end-member CH4 concentrations were homogeneous across the vent fields. LTHFs were collected from four diffuse flow zones using a funnel, which was deployed on the seafloor with replacement of the seawater inside the funnel by the LTHF. Strong linear relationships were found among the concentrations of CH4, ∑CO2, and Si of the obtained samples, which ensured that the end-member chemical composition of the LTHFs could be accurately estimated. While the ∑CO2 Concentrations of the HTHFs and LTHFs were similar, all four LTHFs showed enrichment of CH:4 as compared to the HTHFs. The carbon isotopic composition of CH4 also revealed a significant difference: the LTHFS (δ13C(CH4) = -4.8 ± 0.3‰PDB) showed 13C enrichment, as compared to the HTHFs (δ13C(CH4) = -5.8 0.4‰PDB). The 13C enrichment can be explained by a combination of microbial CH4 production and oxidation after formation of the LTHFs. On the basis of the observed differences in CH4 concentrations and carbon isotopic compositions of the HTHFs and LTHFs, microbial methane oxidation and production are estimated to have comparable impacts. These results support the notion of a subseafloor microbial consortium of methanogens and methanotrophs.
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