日本における湖沼掘削

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The author reviews scientific drilling projects investigating lacustrine sediments in Japan. The lake sediments are excellent archives for paleoenvironments and tectonic movements, and are especially useful when sediments have sequences of annual bands. The earliest deep drilling projects, namely 200 m and 1,400 m drillings, were performed at Lake Biwa in 1971 and 1982-1983, respectively. The results revealed detailed climate changes and tectonic movements of the Omi basin over the past 1.6 Ma. Sediment cores from Lake Suigetsu provide an excellent chronological key based on annual bands for the past 50 ka or more. Furthermore, studies on sediment cores from Lake Nojiri and the lacustrine Takano Formation provided high time resolution data on paleoclimate for the past 160 ka: these data are based on the total organic carbon content and pollen composition of the sediment cores. The impregnation method to make a thin section of lake sediment is essential for counting annual layers to avoid miscounting varves. Lacustrine sediments exposed on land may also be useful archives of the Pleistocene as shown by case studies on Takano and Yoshino Formations. Sample storage systems and sub-sampling method such as double-L channel are useful for systematic research on sediments.