The Discovery of Late Pleistocene Highstand Lacustrine Sediments of the Co Ngoin Lake and Adjacent Areas, Tibet

Co Ngoin(31°24′～31°32′N,91°28′～91°33′E), located at 47 km west of Nagqu County and the southeast part of the northern Tibet plateau, is a small lake with an area of 60 km~2. Highstand lacustrine sediments 18-271 m higher than the lake level ((4 515 m) a. s. l.) are distributed along the southeast coasts of Co Ngoin. U-series dating of carbonate-bearing lacustrine sediment samples indicates that the middle portion of highstand lacustrine sediments (108 m and 39 m higher than the lake level, respectively) was deposited in middle Late Pleistocene from 75.3±7.2 ka B.P. to 48.8±4.9 ka B.P. Based on the data of U-series dating and sporo-pollen analysis, this paper has discussed paleo-vegetation and paleo-climate in middle Late Pleistocene and their relationship with the Late Pleistocene giant paleo-lake (Lake Qiangtang) in northern Tibet Plateau. Sporo-pollen analysis shows that vegetation in the period from 75.3 ka B.P. to 48.8 ka B.P. consisted of two stages. At the early stage it was a forest-steppe consisting of Pinus, Artemisia and Betula, whereas at the late stage it was a forest-steppe or forest comprising Pinus, Artemisia and Polypodiaceae, suggesting a mild and slightly dry climate. At the late stage, however, the temperature and humidity were slightly higher than those of the early one. At the same time, the “Nagqu Basin”, which included the Co Ngoin Basin, was a portion of Lake East Qiangtan. At about 37 ka B.P., probably because the retrogressive erosion of the Nujiang River cut down the water divide between the “Lake East Qiangtang” and the Nujiang River, the lake water of the east portion of “Lake East Qiangtang” in Nagqu Basin was let out, making it a portion of external drainage. The Nagqu River, the source of Nujiang River, Co Ngoin, and probably Nairi Pingco may be the remnants of the paleo-lake. This is the reason why in the drill core of Co Ngoin which penetrates the whole Quaternary series the upper strata over the B/M boundary are relatively thin.