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

我们在加德满都河谷南部的四个地点进行了泥质湖相沉积物的岩心钻探。总共800米长的岩心被恢复和研究，以重建在过去的一百万年在山谷中的古气候和古环境的变化。根据孢粉、硅藻、粘土矿物、有机质和碎屑颗粒的分析，重建了距今70万年以来的古气候曲线，时间间隔为200 ~ 2000年。对钻孔岩芯和地表暴露的古地磁和沉积学研究表明，勒湖诞生于大约1 Ma，由加德满都以南的Mahabharat山脉的隆起造成。盆地南部的地表地质调查表明，湖水在200和12 Ka时有两次排泄，有机湖泥的δ 13 C值至少有7次波动，波动范围为-30 ~-19 ‰。花粉组合的波动图与δ 13 C值的波动图相似，当δ 13 C值高时，指示气候寒冷干燥的花粉增多。这表明，包括C4植物在内的禾本科植物在冰期扩大了分布。总有机碳（TOC）和碳氮比（T/N）与d13 C呈负相关，花粉浓度与TOC和碳氮比有较好的相关性。这表明，土壤总有机碳和花粉浓度受河谷植被的控制。当气候温暖湿润时，C3陆生植物的分布范围似乎有所扩大。花粉浓度、硅藻总瓣数和C/N比。350 ka后的振幅和值均大于300 ka后的振幅和值。在中更新世，从约。650 ~ 350 ka，硅藻瓣的出现频率以小环藻sp1和sp2的单优势为特征。这些奇怪的事件很可能与MIS第11阶段问题有关。

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.