Variation of Benthic Foraminiferal Assemblage and Carbon and Oxygen Isotopic Composition for the Last 30000 Years on gas-hydrate-bearing Umitaka Spur, Joetsu, Eastern Margin of Japan Sea

Umitaka spur off Joetsu, eastern margin of Japan Sea, is characterized by large pockmarks, mounds and gas hydrate BSRs. Numerous gas plumes, about 600m height, were identified on the spur and sea water have high methane concentration. Moreover massive gas hydrates have been recovered from the seafloor of the spur. The environment around the spur is assumed to have been influenced methane. But active methane phenomena are only observed adjacent to pockmarks, not inside of them. If pockmarks have been made by large methane-release events, when and why did these events occur? To solve these questions, we conducted studies on foraminifer assemblages and Carbon-14 age determinations. Carbon and oxygen isotopic analyses of benthic foraminifera were carried out on three piston cores which were recovered during the KY06-08 cruise in 2005. Sediments and depositional environments Carbon-14 ages were measured by planktonic foraminifera (Globigerina. umbilicata) at 4 horizons of PC01 which recovered 7 m long sediment core. PC01 were dated as 22ka cal. BC at 3.9 m, and 3.2ka cal. BC at 6.4 m. The sediments are composed of bioturbated massive units (0-1.5m), which indicate oxidative conditions on the seafloor, and laminated unit (1.5-7.0m), which indicates reductive conditions on the seafloor. Benthic foraminifera At about 28kyr, the benthic foraminifera abundance and proportion of Stainforthia. rotundata increased. Afterwards, very little benthic foraminifera occurred. According to Akimoto (1996), Ruthefordoides corunuta (same as S. rotundata) occur with Calyptogena communities which are related to methane seeps. It is suggested that the methane flux in this area were more active than at present and the sea floor environment was oxygen-free and enriched in methane. Carbon and oxygen isotopic anomalies The delta13C of benthic foraminifera decreased at 24ka~27ka. At the same time, S. rotundte increased, so this suggests that delta13C-depleted dissolved inorganic carbon was made by methane oxidation at this time. The delta18O of planctonic and benthic foraminifera decreased toward 300cm. The decrease of delta 18O of planctonic foraminifera is explained by inflow of fresh water at Last Glacial Maximum. The decrease of delta 18O of benthic foraminifera possibly was influenced by a part of the cold and heavy surface water which flowed into the bottom waters. The period of 24 `28kyr was the Last Glacial Maximum, and the eustatic sea level fall may have led to the dissociation of methane hydrate. Consequent large methane release event promoted the methane-rich and reductive environment on the sea floor. Reference [1] Akimoto, K., T. Saji, R. Tsutsumi, and E. yoshihara (1996), Fossils, The Palaeontological Society of Japan, 60, p. 41-17. [2] Kennett, J. P., K. G. Cannariato, I. L. Hendy, L. Ingrid, and R. J. Behl (2000) Science, vol. 288, 128-133.