西藏纳木错晚更新世以来湖面变化和湖相沉积中粘土矿物显示的环境信息

LAKE-LEVEL CHANGE OF NAM CO,TIBET,SINCE THE LATE PLEISTOCENE AND ENVIRONMENT INFORMATION OF CLAY MINERALS IN LACUSTRINE DEPOSITS

通过对西藏海拔最高、面积最大湖泊—纳木错周缘湖相沉积、湖岸堤的野外调查和湖岸阶地的水准测量,发现在纳木错沿岸拔湖48m以下,发育有6级湖岸阶地,拔湖48～139.2m发育有高位湖相沉积。湖相沉积物的同位素测年结果表明,纳木错湖泊发育与藏北高原东南部古大湖演化可划分为3个阶段:①116～37kaB.P.间的古大湖期;②37～30kaB.P.间的外流湖期;③30kaB.P.以来的纳木错期。根据纳木错晚更新世以来湖相沉积中粘土矿物的X光衍射分析结果,以及采用比值法、高岭石法和衍射峰法的研究,探讨了粘土矿物所显示的环境变化信息。粘土矿物成分变化表明,该区已具备了寒温带干旱、半干旱区的气候环境特征。为研究青藏高原的湖泊演化、气候变化、古地理变迁及其隆升过程等提供了新资料。

Field surveys of lacustrine sediments and lakeshore levees and leveling surveys of lake terraces at peripheries of Nam Co—the largest lake with the highest altitude above sea level in Tibet—indicate that there are 6 lake terraces below the lakeshore of Nam Co 48 m above the lake level and highstand lacustrine sediments from 48 to 1392 m above the lake level.Isotopic dating of the lacustrine sediments shows that the development of Nam Co and the evolution of the ancient large lake on the southeastern North Tibetan Plateau can be divided into three stages:① the ancient large lake stage at 116～37 ka BP,② the outflow lake stage at 37～30 ka BP,and ③ the Nam Co stage since 30 ka BP.Clay minerals in lacustrine sediments of Nam Co since the late Pleistocene were analyzed by X-ray diffraction,and according to the analytic results,the paper discusses the environment information of clay minerals by using the ratio method,kaolinite method and X-ray diffraction method.The changes of the clay mineral composition show the climatic and environmental features of a dry temperate arid and semi-arid area.The study provide new data for the study of the lake evolution,climate change,paleogeographic change and the plateau uplift process.