Carbon isotopic evidence of methane oxidation through sulfate reduction in sediment beneath cold seep vents on the seafloor at Nankai Trough

During the Kaiko-Tokai project, we took a total of 15 near-surface sediment push cores from inside and outside seafloor clam colonies and bacterial mats in the Nankai Trough. In this study, we present the chemical and isotopic composition of pore fluids extracted from the sampled cores, with special emphasis on those taken at the Ryuyo Canyon, and discuss (a) the chemical relation between the seeping fluid and biological communities, and (b) the origin of cold seep fluids. Compared with bottom seawater, up to 5% depletion of chloride concentration, extremely high CH4 concentration of more than 300 mumol/kg, and slight deltaD(H2O) depletion are observed in pore fluids at the Ryuyo Canyon, which suggests that the Ryuyo Canyon is an active seep zone of low chloride, CH4-rich, low deltaD(H2O) fluid from the sea bottom. Remarkable depletion of sulfate and significant enrichment of dissolved inorganic carbon and of hydrogen sulfide are also observed in the pore fluids, particularly those taken within bacterial mats, while the carbon isotopic composition of pore fluid dissolved methane (delta(13)C(CH4)) shows large heterogeneity (up to 30parts per thousand) in and around the vents. These chemical and isotopic characteristics might be due to active anaerobic methane oxidation and hydrogen sulfide production (methane oxidation through sulfate reduction) in the sedimentary layer just below (within 30 cm) the vents on the seafloor. The carbon isotopic composition of methane can be considered direct evidence of the mediation process between methane-enriched cold seep fluid and the H2S-dependent biological community on the cold seep vent. After correcting for the effect of methane oxidation at the sediment surface, we estimate the end-member carbon isotopic composition of methane as less than -90parts per thousand PDB, and the end-member CH4/C2H6 ratio as more than 10(3), which suggests a microbial, shallow layer is the origin of methane in the fluid. (C) 2002 Elsevier Science B.V. All rights reserved.