Effects of warming daring a glacial-interglacial transition on the palaeogeography and macro aural assemblages on continental shelf in the Sea of Japan

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Global warming resulting from the increase of the greenhouse gases is one of the very serious problems. The effects of warming on the oceanography and macrofaunal assemblages have not yet examined on continental shelf in the Sea of Japan. Thus, we studied the stratigraphic distribution and abundance of both benthic molluscs and planktonic foraminifera from a particular horizon in the early Pleistocene Omma Formation. A particular horizon corresponds shelf sediment during the deposition at the initiation of flow of the warm Tsushima Current. We used planktonic foraminifera as indicator of the Tsushima Current, because they have short generation times and have responded rapidly relative to environmental changes rather than adult molluscs, which may lag behind changes.The result indicates that local extinction of cold-water molluscs lagged behind about 2,300-2,400 years after the initiation of the intrusion of the Tsushima Current. Then migration of a warm-water molluscan fauna to the shelf occurred shortly after the local extinction of cold-water molluscs. These lead to the following conclusions :1) the rate of increase in thickness of the Tsushima Current during the initiation of flow was faster than that of the sea-level rise ;2) the most important factor causing local extinction of them was probably the rise in temperature ; and3) an environment during local extinction of cold-water molluscs and appearance of warm-water molluscs may be unsuitable for both them. Perhaps such environment may be caused by the establishment of a climatic phase of high seasonality.There is a possibility that future warming will produce such condition on the continental shelf around Hokkaido which is southern limit of cold-water molluscs.