A high-resolution analysis of environmental changes during the last thousands years deduced from the cored sediments in Lake Kizaki, central Japan

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The surface cores of 2 to 3 m long were taken from the major basin floor at three sites of Lake Kizaki, using a new mechereth-type piston corer. Three ィイD114ィエD1C datings revealed that the cored sediments cover about 1500 to 1800 years before present. The sedimentation rate ranges from 1.3 to 1.9 mm/year (52 to 62 mg/cmィイD12ィエD1/year), and is lager than that estimated before this study. This high ratio is due to many turbidite layers which are estimated to be caused by large floods. The turbidite layers are characterized by the features such as relative high-density, low C and N contents and high C/N ratio. Major turbidite layers are enable to be correlated each other, suggesting large turbidity flows of basin scale. C/N ratio ranges from 10 to 13, suggesting that most organic materials are plankton of lake origin with minor amount of land plant materials. C and N contents fluctuate in short and long terms. The sort-term fluctuation is caused by turbidite dilution, and long-term fluctuation seems to be due to climatic changes. The warm period of the early Medieval Age and cool period of late Medieval Age (the Little ice Age) are identified in the proxy curve. Organic C and N contents of lake sediments are confirmed to be a good indicator of past climate.