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

• Published in: Marine geology 2002-07, Vol.187 (1), p.145-160
• Main Authors: Tsunogai, U., Yoshida, N., Gamo, T.
• Format: Article
• Language: English
• Subjects: bacterial mat; hydrogen isotopic composition; low chloride; Marine; methane; stable carbon isotopic composition
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/S0025-3227(02)00263-3

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 CH 4 concentration of more than 300 μmol/kg, and slight δD 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, CH 4-rich, low δD 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 (δ 13C CH4) shows large heterogeneity (up to 30‰) 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 H 2S-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 −90‰ PDB, and the end-member CH 4/C 2H 6 ratio as more than 10 3, which suggests a microbial, shallow layer is the origin of methane in the fluid.