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Based on quantitative analyses of fossil diatom assemblages in four Piston Cores, the paleoceanographic changes during the last 340 kyr in the southern Bering Sea and western subarctic Pacific have been reconstructed. The total diatom accumulation rate (DAR) in each core is influenced by cold low-salinity surface waters. The cold low-salinity surface waters covered all over the southeast Bering Sea throughout the past 100 kyr, whereas such water masses expanded in the southwest Bering Sea and western subarctic Pacific during the glacial periods. Moreover, based on the changes in major diatoms, a model that predicts a flow through the Aleutian Island is proposed. The flow of the Alaskan Stream waters heads northward through the Aleutian Islands at present. As the climate became colder in the glacials, the shallow eastern and central passes were possibly climinated or restricted due to sea level drop. Therefore, the southeast Bering Sea was isolated from the North Pacific and the southeast Bering Sea was influenced by the Alaskan Stream during the glacial periods.

第4回極域科学シンポジウム横断セッション：[IC] 極域アイスコア・地形地質・モデリングから迫る古環境変動とそのメカニズム11月13日（水） 統計数理研究所 ３階セミナー室１（D305）

第3回極域科学シンポジウム/第34回極域生物シンポジウム 11月27日（火） 国立極地研究所 ３階ラウンジ

第3回極域科学シンポジウム 横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月） 国立国語研究所 ２階講堂

第3回極域科学シンポジウム 横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月） 国立国語研究所 ２階講堂

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究所 2階フロア

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究所 2階フロア

The warmest global temperatures of the past 85 million years occurred during a prolonged greenhouse episode known as the Early Eocene Climatic Optimum (52–50 Ma). The Early Eocene Climatic Optimum terminated with a long-term cooling trend that culminated in continental-scale glaciation of Antarctica from 34 Ma onward. Whereas early studies attributed the Eocene transition from greenhouse to icehouse climates to the tectonic opening of Southern Ocean gateways, more recent investigations invoked a dominant role of declining atmospheric greenhouse gas concentrations (e.g., CO 2 ). However, the scarcity of field data has prevented empirical evaluation of these hypotheses. We present marine microfossil and organic geochemical records spanning the early-to-middle Eocene transition from the Wilkes Land Margin, East Antarctica. Dinoflagellate biogeography and sea surface temperature paleothermometry reveal that the earliest throughflow of a westbound Antarctic Counter Current began ∼49–50 Ma through a southern opening of the Tasmanian Gateway. This early opening occurs in conjunction with the simultaneous onset of regional surface water and continental cooling (2–4 °C), evidenced by biomarker- and pollen-based paleothermometry. We interpret that the westbound flowing current flow across the Tasmanian Gateway resulted in cooling of Antarctic surface waters and coasts, which was conveyed to global intermediate waters through invigorated deep convection in southern high latitudes. Although atmospheric CO 2 forcing alone would provide a more uniform middle Eocene cooling, the opening of the Tasmanian Gateway better explains Southern Ocean surface water and global deep ocean cooling in the apparent absence of (sub-) equatorial cooling.