THE GRAIN SIZE CHARACTERISTICS OF XIMENCUO LAKE CORE IN THE NORTHEAST TIBET PLATEAU AND ITS ENVIRONMENTAL SIGNIFICANCE

Ximencuo(33°23',101°08') in the northeast of Tibet Plateau is a modern glacial lake.The mean water depth is more than 40 m,and there is little disturbance of human activity, so the core can record the environmental changes. A continuous 12.8 m-long core was taken from the center of the lake in 2004. We analyzed the grain size characteristics and reconstructed the sedimentary environment of the core.
The high percentage of clay (20%~80%) indicates that the water depth of the sampling place is deep at most time of the core. The high sand content means low lake level and high lake energy. The high standard deviation of the core was caused by the glacial environment. The grain size frequency distribution curves and cumulative distribution curves that reflect different grain size contents are also very useful to reconstruct paleo-environment of the lake. 7.30~6.25 m section with high clay content (40%~80%) indicates that the lake level was high and the lake surface frozen. 6.25~5.00 m section shows that the sand content increased sharply (8%~10%), which means the lake level dropped suddenly because of dry and cold climate. 5.00~3.55 m section indicates that organic content increased sharply with the magnetic susceptibility decreasing accordingly,which means the climate became warmer and wetter. 3.55~0 m shows the highest organic content and the lowest magnetic susceptibility. The sand content decreases and the clay content increases,meaning a warm climate and high lake level except two abrupt dry events at the beginning. Further researches,together with other indexes, are needed to get more detailed information.