MIS027-08

会場:201A

時間:5 月 22 日 16:00-16:15

日本海上越沖ガスハイドレート中の炭化水素ガス組成および同位体比
Molecular and isotopic compositions of gas hydrate-bound hydrocarbons retrieved fromoff Joetsu, Japan Sea
八久保 晶弘 1∗ , 戸丸 仁 2 , 松本 良 2Akihiro Hachikubo1∗ , Hitoshi Tomaru2 , Ryo Matsumoto2
11北見工業大学, 2 東京大学
Kitami Institute of Technology, 2 University of TokyoGas hydrates are crystalline clathrate compounds composed of water and gas molecules that are stable at low temperature, highpartial pressure of each gas component, and high gas concentration. Recently, natural gas hydrates were obtained at the Umitaka
Spur and the Joetsu Knoll on the eastern margin of the Japan Sea (Matsumoto et al. , 2009; Hiruta et al. , 2009). We investigatedthe molecular and stable isotope compositions of hydrate-bound gas and dissolved gas in pore water in sediments of these areas.
The purpose of this study is to clarify the gas origin and to understand the migration system of the shallow gas hydrate accumulation in this area. Hydrate-bearing sediment cores were retrieved during the cruise onboard R/V Marion Dufresne in June 2010.
The Calypso piston coring system enabled us to take long sediment cores (up to 40m). Hydrate-bound hydrocarbons are primarily
thermogenic at the Umitaka spur (C1 delta 13 C: -36 permil-VPDB, C1 delta D: -165 permil-VSMOW), on the contrary, those at
the Joetsu knoll (about 30m depth from the sea floor) partly contain microbial methane because 13C and deuterium are both depleted (C1 delta 13 C: -55 permil-VPDB, C1 delta D: -194 permil-VSMOW) and close to the field of microbial methane via CO2reduction. Depth profiles of methane in sediment showed the minimum depth in delta 13 C, corresponded to the SMI depth wherehydrogen sulfide was concentrated. Methane concentration in sediment increased drastically beneath the depth of SMI. delta 13 Cof methane increased slightly along with depth, whereas delta D of methane decreased. Thermogenic ethane and propane aredetected both in hydrate-bound gas and dissolved gas in pore water. Heavy propane (delta 13 C: +5 permil-VPDB) indicated its
microbial oxidation. The molecular composition of thermogenic ethane in hydrocarbons was relatively high at the gas hydratesites, whereas microbial ethane is dominant in the peripheral area. High concentration of neopentane (2,2-dimethylpropane) wasdetected in the dissolved gas around the depths of gas hydrate at the both sites of Umitaka Spur and Joetsu Knoll. Since neopentane cannot be encaged to the crystallographic structure I of gas hydrate, it might be discharged from the hydrate crystal at theformation process and remained in the sediment.
This study was supported by the MH21 Research Consortium for Methane Hydrate Resources in Japan.
Hiruta, A., G.T. Snyder, H. Tomaru, R. Matsumoto (2009) Geochemical constraints for the formation and dissociation of gas
hydrate in an area of high methane flux, eastern margin of the Japan Sea. Earth Planet. Sci. Lett. , 279, 326-339, doi:10.1016/j.epsl.
2009.01.015.
Matsumoto, R., and 28 others (2009) Formation and collapse of gas hydrate deposits in high methane flux area of the Joetsu
Basin, eastern margin of Japan Sea. J. Geogr. , 118(1), 43-71.
キーワード: ガスハイドレート, 安定同位体, 日本海
Keywords: gas hydrate, stable isotope, Japan Sea