HR: 11:20h
AN: OS12A-05    [Abstracts]
TI: Origin and evolution of the interstitial waters of gas hydrate-bearing sediments, eastern margin of Japan Sea
AU: * Hiruta, A
EM: hiruta@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Department of Earth and Planetary Science, Hongo 7-3-1, Bunkyo-ku, Tokyo, Tokyo, 113-0033, Japan
AU: Matsumoto, R
EM: ryo@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Department of Earth and Planetary Science, Hongo 7-3-1, Bunkyo-ku, Tokyo, Tokyo, 113-0033, Japan
AU: Snyder, G
EM: gsnyder@rice.edu
AF: Rice University, Earth Science-MS126, Houston, TX, Houston, TX 77251-1892, United States
AU: Tomaru, H
EM: tomaru@mail.kitami-it.ac.jp
AF: Kitami Institute of Technology, New Energy Resources Research Center,165 Koen-cho, Kitami, Hokkaido, Kitami, 090-8507, Japan
AB: Gas plumes were observed on the Umitaka spur [1] and Joetsu knoll off Joetsu, eastern margin of Japan Sea. The plume sites are characterized by small mounds, active gas venting, seafloor gas hydrate and related chemical anomalies. More than 30 piston cores (4~12 m) were recovered. Some cores show characteristic depth profiles as to the concentration of inert chloride (Cl-) in the interstitial water. Cores recovered from plume sites often contain gas hydrate. In these cores, the Cl- concentrations of interstitial waters exhibit spiky anomaly due to gas hydrate dissociation during and after core recovery. Cores without gas hydrate showed linear decreasing (~-12mM/m) or increasing (48.5mM/m) profile of Cl- with depth. These linear profiles likely suggest diffusion between bottom sea water and brine or hyposaline water in deep sediment. Observed Cl- increasing profiles are likely to be explained as to the dissolution of evaporites, ion excursion by gas hydrate formation, whereas decreasing by inflow of fresh ground water and gas hydrate dissociation. The isotopic composition of water is useful proxy for the origin of such anomalous water. Cl- increasing cores exhibit linear decrease both in delta D and delta O-18. Such depth profiles likely correspond to the build up of the residual waters during gas hydrate formation. These cores are recovered only around plume sites and indicating that gas hydrate is currently formed in the sediment around plume sites. Cl- decreasing cores exhibit linear decrease both in delta D and delta O-18. Such depth profiles may have been caused by inflow of land derived ground water into the sediments. The isotopic compositions of the ground waters are estimated from covariation diagram between observed Cl- concentration, delta D and delta O-18. The estimated values (~-15 permil and ~-3 permil VSMOW for delta D and delta O-18) are different from those of ground waters near land (-61.9~-46.0 permil and -10.8~-8.2 permil VSMOW for delta D and delta O-18, respectively [2]). Thus, the low Cl- waters are not likely to be related with ground water. For alternate explanations, there are two other possibilities. First is deep diagenetic waters formed in such processes as dehydration of clay mineral, carbonate precipitation and basalt alternation [3]. The other possible mechanism is gas hydrate formation/dissociation. Gas hydrate water tends to become isotopically lower than ambient waters and residual waters are depleted in D and O-18. If gas hydrates continue to form in such residual water, the isotopic composition of gas hydrate could be depleted in D and O-18 relative to sea water. Dissociation of such gas hydrate would produce low Cl- and isotopically low waters. Wide distribution of low Cl- and isotopically low waters in the study area seem to indicate massive dissociation of gas hydrate after continual formation by active methane flux. Sea level change in last glacial maximum must have caused gas hydrate dissociation at base of gas hydrate stability zone. Reference [1] Aoyama C. et al., 2004, OCEANfS04 MTS/IEEE TECNO-OCEANf04 p. 1004-1009 [2] Mizota C. et. al., 1994, Geochem. J., 28, p. 387-410 [3] Tomaru H. et al., Pro. ODP, Sci. results, vol. 204, Ocean Drilling Program, College Station, TX, pp.1-20
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1724 Ocean sciences
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting