HR: 1340h
AN: OS43A-0614    [Abstracts]
TI: High Concentration of Methane and Magnificent gas Plumes Over gas Hydrate Field in the Eastern Margin of Japan Sea
AU: * Ishida, Y
EM: yasushi@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science University of Tokyo, Hongo 7-3-1 Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Matsumoto, R
EM: ryo@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science University of Tokyo, Hongo 7-3-1 Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Hiruta, A
EM: hiruta@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science University of Tokyo, Hongo 7-3-1 Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Aoyama, C
EM: aoyamac@dokken.co.jp
AF: JAPAN'S INDEPENDENT INSTITUTE Co.,Ltd., ISHII Bldg. 7Fl., 3-1-10 Shinbashi Minato-ku, Tokyo, 105-0004 Japan
AU: Tomaru, H
EM: hitoshi@earth.rochester.edu
AF: Department of Earth & Environmental Sciences, University of Rochester, 227 Hutchison Hall,Rochester, New York, 14627 United States
AU: Hiromatsu, M
EM: mineo-hiro@graduate.chiba-u.jp
AF: Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AB: Gas hydrates and prominent pockmarks have been observed on the Umitaka Spur in the eastern margin of Japan Sea, at the depth of about 900 m.Magnificent methane plumes, 550 to 600 m high, were detected by echo sounder for fish school, and massive gas hydrates were recovered by piston coring during the UT04 cruise of R/V Umitaka-maru (2004). The seawater over this area was collected by CTD and the samples of interstitial waters were extracted from sediment cores by hydraulic squeezer. The ratio of methane to ethane concentration (C1/C2) and the isotopic (δ ¹³C) composition of methane in the plume sites are less than 10³ and from -40 to -50 (‰ PDB) respectively, suggesting that the origin of such gases are mostly thermogenic, whereas the gases in the sediments away from plumes are mostly microbial. The seawater samples demonstrated anomalously high concentration of methane over the plume sites. Maximum concentration is 160nmol/L above the methane plume site. The methane concentration values of most samples ranged from 4 to 6nmol/L. When it compared with the Nankai Trough (1 to 4nmol/L), even the base level methane is quite high. Seawater samples collected at the depth of 200 m exhibit sharp anomalies of 16 to 34nmol/L. With the intension to check the possibility of the inflow from the shelf and river waters, we collected surface waters far away from the Umitaka spur. Methane concentration was only 7nmol/L. Therefore, we conclude that anomalously high concentration at 200 m level over the spur is not likely to be explained by inflow of shelf waters, but also by methane seeps. The temperature of waters are extremely low from 0.25^°C to 1.0^°C below 300 m, then abruptly increases in shallow waters to about 25^°C at surface water. Thus, bottom and intermediate waters are within the stability condition of methane hydrate. Under these conditions, gases from the sea floor would form gas hydrate within bottom water mass. Gas hydrate crystals would float up shallow to the water mass where they dissociate to supply methane at around 300 m due to abrupt temperature increase.
DE: 3004 Gas and hydrate systems
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005