HR: 1340h
AN: OS23B-1309    [Abstracts]
TI: Stable Isotope Anomalies and Low Chloride Concentrations in Pore Water of CH$_{4}$-Rich Sediments at the Tanegashima Mud Volcano, Japan
AU: * Nakayama, N
EM: noriko@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Tsunogai, U
EM: urumu@ep.sci.hokudai.ac.jp
AF: Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810 Japan
AU: Ashi, J
EM: ashi@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Gamo, T
EM: gamo@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AB: Pore water from sediments collected at a Tanegashima mud volcano was analyzed for $\delta$$^{13}$C (PDB) of dissolved CH$_{4}$ together with other chemical components, Cl$^{-}$ and SO$_{4}$$^{2-}$, and the $\delta$$^{18}$O and $\delta$D (SMOW). The Tanegashima mud volcanoes are located at the water depths from 1400 m to 1800 m, off Tanegashima island between Ryukyu trench and Ryukyu arc of Japan. It is situated at the end of south-western convergent plate boundary on Nankai-trough, which forms a part of Philippine Sea plate subducting under Eurasian plate. This cruise was conducted as a part of the JNOC (Japan National Oil Corporation) geochemical survey by R/V Hakurei-maru II. The concentrations of CH$_{4}$ were generally higher than 100 micro-mol/kg. Its highest concentration (715 micro-mol/kg) was found in the crest core of a mud volcano. The $\delta$$^{13}$C values ranged from -32 to -50 $\permil$. C$_{2}$H$_{6}$ was detected only in the pore waters collected from the vicinity of the crest of the mud volcano. The highest $\delta$$^{13}$C (around -22 $\permil$) and low C1/C2 concentration ratios (less than 100) were measured at the crest site, supporting the thermogenical production of methane. Other geochemical anomalies were also observed in the crest pore water. The concentrations of Cl$^{-}$ in the pore water at this site were extremely depleted to a minimum of 350 mmol/kg. The Cl$^{-}$ anomaly has not been previously reported for pore water from mud volcanoes around Japan. An endmember of isotopic composition of the fluid is estimated to be +12 $\permil$ for $\delta$$^{18}$O and -40 $\permil$ for $\delta$D. From these results we conclude that the most likely process to reduce pore water salinity is primarily the mixing of clay mineral dehydration water with seawater. The thermogenic methane found in the crest pore waters of the Tanegashima mud volcano may be brought from the depths of sediments due to the migration of fluid evolved by mineral the dehydration process.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1015 Composition of the core
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting