ENVIRONMENTAL CHANGES OF LAKES NAKAUMI AND SHINJI

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Lakes Nakaumi and Shinji are maritime coastal lakes a long the Sea of Japan, and have been surveyed the entire areas by various methods of echo-sounding and on the basis of analysing samples obtained by gravity cores and bottom sampling. These data have made it possible to illustrate with precise paleogeographic maps eight stages of lake development from 20,000 y.B.P to 100 y.B.P. Especially the geohistory of the lakes since the Hudoki (Nara) Period (8th Century) was analysed in detail in the present study, which have clarified the Medieval Warm Period in the Middle Age and the global cooling during the Little Ice Age. These are reflected quite sharply in the paleogeographic maps. Archeological data around Lakes Nakaumi and Shinji have been collected and analysed from the viewpoint of sea-level changes and the historic site at Eshima Island was excavated to reconstruct the history and environment of Nara Age and to correlate these to the written history of Izumonokuni Fudoki in 8th Century. These were referred to illustrate the above paleogeographic maps. The Honjo Area, which has long been appointed as a largest reclamed land in Lake Nakaumi, was surveyed in detail by echo-soundings of Sidescan Sonar and Atlas Deso 20 and Uniboom to provide useful data for the better utility of this appointed area in near future. Surface sediments of Lake Nakaumi were collected carefully without disturbing the top layer and vertical elementary distributions of C,S and Fe within the core sediments were investigated by a color mapping method of Electron Prove Microanalyser. It as been clarified that the concentration of total nitrogen and sulfate decrease with depth.