Studies on the changes in Indian monsoon by drilling in the Paleo-Kathmandu Lake

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We carried out core drilling of muddy lacustrine sediments at four sites in the southern part of the Kathmandu Valley. The total 800 m long cores were recovered and studied in order to reconstruct changes in paleoclimate and paleoenvironments in the valley during the last one million years. We reconstructed paleoclimatic curves of the last 700 kyr, on the bases of analyses of pollen, diatom, clay mineral, organic materiall and detrital grains, with 200 to 〜2000 years interval. Paleomagnetic and sedimentological studies on the drill cores and surface exposures revealed that le lake was born at about 1 Ma, caused by the uplift of the Mahabharat range to the south of Kathmandu. Surface geological survey in the southern part of the valley indicates that the lake-water drained two times at about 200 Ka and 12 Ka.δ 13C value of organic lacustrine mud shows at least seven times fluctuation ranging from -30 to -19 ‰. A fluctuation diagram of pollen assemblage shows similar pattern to that of δ 13C value, and indicative pollens of cold and dry climate increase when d13C has high value. It suggests that grasses including C4 plants expanded their distribution during the glacial period. Total organic carbon(TOC) and carbon/ nitrogen ration (T/N) shows inversed correlation with d 13C, and pollen concentration has good correlation with TOC and C/N. It implies that TOC and pollen concentration were controlled by land plants vegetated in the valley slope. When it was warm and wet climate, C3 land plants seem to have expanded their distribution. Pollen concentration, total valves of diatom and C/N ratio before ca. 350 ka show larger amplitude and larger value than those after 300 ka. During the middle Pleitocene from ca. 650 to 350 ka, frequency of diatom valves is characterized by monodominance of Cyclotella sp 1 and sp 2. These curious incidents are likely to be related to MIS stage 11 problem.