Study on migration, disappearance and evolution of Ostatcoda with relation to the opening and clostire of the Tsushima and Korean Straits during the late Cenozoic

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The aim of this study is to clarify the relationships between the opening and closure of the Thushima and Korean Straits and migration, disappearance and evolution of Ostracoda during the late Cenozoic. Surface sediment samples for the analysis of recent Ostracoda were collected from enclosed bays along the Tsushima Island. (Shush, Aso and Miura Bays) and Omura Bay. Concerning fossil. Ostracoda, I collected samples from the upper Pliocene Kuwae Formation, Niigata Prefecture, the Miyazaki Group, Miocene deposits in the Chugoku Mountains and so on. Based on these results and taking account of earlier studies, I led to the four conclusions as follows;The Korean Strait has been a geographic bather far embayment ostracodes alter the middle Pleistocene. Namely dominant species in Japanese enclosed bays are different from those in Korean and Chinese enclosed baysWarm water ostracode species living in marine areas shallower than 50 m in water depth were recognized in the interglacial deposits of the middle Pliocene Kuwae Formation. Especially, warm water inflows occurred certainly at about 2.9 Ma. However, there are only a few ostracode species which lived dominantly in shallow seas under the influence of the Kuroshio Current at that time. Thus warm water currents having flowed into the Japan Sea were weaker and thinner than today.Temperate intermediate waters warmer than today (more than 6 ℃) were developed in each interglacial period of the Middle Pliocene. However, inflow routes of the waters were not clarified in this study.Tropical to subtropical ostracode species were flourished through the northern to southern Japan during the Miocene climatic optimum (ca. 16 Ma) because the southern strait opened widely and strong warm water currents flowed into the Japan Sea. These ostracode assemblages were somewhat different from those in the late Early Miocene (ca. 17-18 Ma).