HR: 10:20h
AN: OS12A-01    [Abstracts]
TI: Types and Evolution of Gas Hydrate System along the Tectonically Active Zones of the Western Pacific: Nankai Trough vs. Eastern Margin of Japan Sea
AU: * Matsumoto, R
EM: ryo@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Tomaru, H
EM: tomaru@mail.kitami-it.ac.jp
AF: University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Takeuchi, L
EM: lika@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Hiruta, A
EM: hiruta@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Ishizaki, O
EM: ishizakios@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Aoyama, C
EM: aoyamac@hotmail.com
AF: Japan's Independent Institute, 3-1-10 Shimbashi Minato-ku, Tokyo, 105-0004, Japan
AU: Machiyama, H
EM: bucci@jamstec.go.jp
AF: JAMSYEC, Nasushima, Yokosuka, 237-0061, Japan
AU: Goto, T
AF: JAMSYEC, Nasushima, Yokosuka, 237-0061, Japan
AB: A series of sea-going surveys of marine gas hydrates around Japan Islands for more than a decade has revealed characteristic and contrasting features and evolution of gas hydrate system between the Nankai subduction zone and the Joetsu Basin of Japan Sea. Gas hydrate of the Nankai trough largely occurs as pore-filling type, laterally extending in turbidites. Methane is depleted in C-13, mostly derived from microbial brake-down of organic matters. Strong and continuous BSRs occur at around 270 mbsf, corresponding to the base of gas hydrate stability (BGHS). Furthermore, double BSRs with weak reflector (BSR-2) 20-30 m below BSR appear in uplifting knolls. BSR-2 is explained as a relic BSR, which coincides with BGHS. Combination effects of uplifting of gas hydrate bearing sediments and sea-level fall are likely to have caused shoaling of BGHS and BSRs, dissociation of gas hydrate between old and new BRSs, and further accumulation of gas hydrates above BSR. Thus the recycling of methane at BGHS triggered by glacial sea level fall contributed for the development of subsurface gas hydrate deposits. Joetsu basin is located on a newly formed convergent boundary between Eurasia and Philippine Sea Plates. Well-defined circular pockmarks with ca.500 m in diameter develop on the folded and faulted Umitaka spur and Joetsu knoll in the basin. A number of circular swells and mounds, 200-500 m in diameter, have been also recognized nearby the pockmarks. Thus the Umitaka spur and perhaps Joetsu knoll are characterized by rough topography of pockmarks and mounds. Methane of plumes and gas hydrate originates in deep-seated thermogenic gases with relatively heavy carbon. 3D seismic profiles clearly depict gas chimney structures below pockmark-mound zones, and gigantic methane plumes stand on the mounds not in the pockmarks. Pockmarks are often considered as vent holes, however, those of the Joestu Basin are quite. BSRs occur at about 150 mbsr, corresponding to very high heat flow, and are widely distributed throughout the area, while no double BSRs are observed. BSRs within gas chimneys are very strong and often exhibit pull-up structure. A number of piston corers have recovered chunks of massive gas hydrate from the mounds. ROV dives observed gas hydrates exposed atop the mounds. Furthermore, electric ocean floor survey has revealed that sediments below the pockmark-mound zones were not conductive. These lines of evidence suggest that the mounds are more-or-less composed of or at least contain significant amounts of methane. Sea-level fall during the last glacial, 120 m in Japan Sea, should have caused instability of gas hydrate, in particular, those within pockmarks. Pull-up structures within the chimney seem to support the model that the mounds are gas hydrate dome and the pockmark, probably a relic hydrate mound. Glacial sea level fall should have caused massive dissociation of subsurface methane hydrate as in case of the Nankai trough. However the methane from the dissociation of massive hydrate in the chimney should escape to seawater to form a crater-like depression pockmarks. Considering active venting, gigantic plumes, inferred violent venting and perhaps floating of massive gas hydrates, gas hydrate deposits are to be formed during warmer, high-sea level periods, and episodic dissociation and massive emission of methane to ocean/atmosphere system.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
DE: 1120 Isotopic disequilibrium dating
DE: 1641 Sea level change (1222, 1225, 4556)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting