LAKE BIWA SEDIMENTS IN JAPAN: RECORDERS OF GLOBAL PALEOENVIRONMENT AND ISLAND ARC TECTONCIS SINCE PLIOCENE

Lake Biwa is the largest freshwater lake in Japan, measuring 22.6 km side by 68 km long and having a maximum depth of 104 m. The lake has a long history from the early Pliocene based on the geological survey and paleogeographical study of lake sediments around Lake Biwa (Kobiwako Group). The sediments revealed the paleoenvironmental change and tectonic events at the convergent margin of the Eurasian plate since early Pliocene times. Deep drillings for paleolimnological studies were carried out 1970's and 1980's. These studies showed that the present Lake Biwa Basin bears a sedimentary sequence of about 900 m thickness, which were deposited in lacustrine or fluvial environments in the Pliocene and the Quaternary. The upper most unit is a continuous and massive sequence of lacustrine clay of 250m thick intercalating more than 50 layers of volcanic ashes. The tephrochronological and paleoclimatological data suggest that the 250 m clay unit has been deposited from the last 430 kyrs and is correlated to major glacial-interglacial cycles. We will show the summary of the result from drilling studies in Lake Biwa. Pioneer studies on the long sediment core from Lake Biwa revealed that the fluctuations of climatic signals were successively correlated to the Milancovitch cycles. However, the upper part of long core was highly disturbed because of relatively poor coring techniques. In 1990's, 15 to 20 m long piston cores were obtained from three sites in Lake basin, and analytical studies at higher-resolution have been carried out. These studies suggested that the Lake Biwa sediments record climatic fluctuations at millennial to century order, which can be correlated to the Heinlich events or Dansgaard EOeschger cycles. The paleolimnological setting of Lake Biwa offers outstanding advantages for high-resolution study of the East Asian monsoon climate of the Late Quaternary.