HR: 1340h
AN: OS23A-1048    [Abstracts]
TI: Methane-derived carbonates from the Joetsu basin, eastern margin Japan Sea: possible evidence for strong methane seepage
AU: * Sanno, R
EM: risa@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Department of earth and planetary science, 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, Department of earth and planetary science, 7-3-1,Hongo,Bunkyo-ku, 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, 7-3-1,Hongo,Bunkyo-ku, Tokyo, 113-0033, Japan
AB: A number of gas plumes, sea floor gas hydrate, carbonate nodules, pockmarks and mounds are observed at Umitaka spur (UT) and Joetsu knoll (JK) off Joestu, eastern margin of Japan Sea. Methane derived carbonate nodules were recovered by piston cores, dredges and ROV hyper dolphin from plume sites on the mounds and around pockmarks. Three types of occurrence of carbonate nodules have been identified; (i) those associated with gas hydrates nearby plumes, (ii) exposed on the sea floor, (iii) existing in sediments without gas hydrates far away from gas plumes. Type (iii) carbonates indicate that strong methane seepage occurred in the area in the past. Mineral composition, carbon and oxygen isotopic composition of carbonates have been analyzed and U-Th ages have been determined to examine the past active methane seep events related to the dissociation of gas hydrates in the study area. Carbonate nodules are grouped into (i) aragonite dominant type (70-100 percent in peak high ratio), (ii) calcite dominant type (almost 100 percent in peak high ratio) and (iii) mixture of calcite and aragonite. Dolomite occurs as minor constituents. Carbon isotopic composition of these three types of nodules from JK are between -27 and -32 permil (VPDB) and between -5 and -25 permil (VPDB) from UT. The difference is likely to reflect the different contribution of methane derived bicarbonates. Gas hydrate dissociation events in JK is considered to have been larger than UT. The oxygen isotopic compositions of aragonite dominant nodules from both JK and UT are mainly between +3.5 and +5.5 permil (VPDB). Assuming that the oxygen isotope value of sea water was +0.3 permil (VSMOW), paleo temperatures are calcurated to be very low, -2.2 to -6.6 degree. This strongly suggest that the oxygen isotope value of aragonite precipitating water was +3.3 to +3.4 permil (VPDB), much heavier than sea water value. Heavy oxygen water have been possibly derived from the dissociation of gas hydrates, so aragonite dominant nodules may indicate past gas hydrate dissociation events. To the contrary, the oxygen isotopic compositions of calcite dominant nodules are observed to be +1.5 to +4.5 permil (VPDB), suggesting that these nodules precipitate either under higher temperature (more than 3 degree) or heavy-oxygen- depleted waters. Considering that the bottom water temperature is constantly low at around 0.3 degree throughout the last glacial-interglacial periods, calcite dominant nodules are explained to have been formed in light-oxygen waters, perhaps in the residual waters of gas hydrate formation. Preliminary results of the U-Th dating for carbonate nodules from UT (Watanabe et al, 2006) indicate that nodules were precipitated between 12 and 35 ka, centered around 20 ka. More U-Th dating needs to clarify the relationship between climate change and gas hydrates formation / disossiation. Reference Watanabe et al. (2006) ISC 2006 FUKUOKA, JAPAN, (abstract)
DE: 4803 Analytical chemistry
DE: 4835 Marine inorganic chemistry (1050)
DE: 4851 Oxidation/reduction reactions (0471)
DE: 4860 Radioactivity and radioisotopes
DE: 4875 Trace elements (0489)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting