西藏错鄂及邻区晚更新世高位湖相沉积的发现及其意义

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

西藏那曲县西的错鄂(31°24′～31°32′N,91°28′～91°33′E,湖面海拔4515m)东南岸(嘎杂西北坡),发育了拔湖18～271m的湖相沉积。错鄂及邻区湖相沉积的U系法测年和孢粉分析结果表明,至少在75.3～48.8kaBP间,当地植被经历了早期以松、蒿、桦为主的森林草原和晚期以松、蒿、水龙骨科为主的森林草原或森林等两个阶段,反映了气候由温和较干到温和偏干的状况,但晚期温度与湿度均较早期为高,而干燥度则略有降低。同时,包括错鄂盆地在内的“那曲盆地”则是古“羌塘东湖”的东半部。约在37kaBP左右,可能由于怒江的溯源侵蚀,切开了“羌塘东湖”与怒江的分水岭,使“羌塘东湖”的东半部古湖外泄,而成为外流区———怒江(上游那曲)流域的一部分。正因为有古湖外泄的一段历史,故可以理解钻穿了整个第四纪地层的错鄂孔B/M界线以上部分的地层较薄的原因了。

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, respectivdy） 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 discuxsed 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.