青藏高原东北部希门错湖岩心粒度特征及其环境意义

位于青藏高原东北部的希门错为一冰川深水湖,流域面积小,受人类活动影响较小,忠实地记录了湖泊的自然演化过程。结合希门错湖区特点,对沉积物的粒度特征进行了详细分析。希门错岩心的黏土含量在20%~60%之间,粗砂含量在0~10%之间,粉砂含量在20%~80%之间。粒度参数用矩值法求得,平均粒径(Φ值)为6.0~8.5,标准偏差为0.8~3.0,偏度分布范围在-1.0~1.0之间,峰度为1.0~25.0。频率分布曲线存在众数粒径为0.6、1.0、8、30和200 μm的次总体。众数粒径为1 μm的次总体在7.3~6.0 m成为沉积物的主体,显示了极弱的水动力条件,为湖面结冰的情况下形成。众数粒径为8和30 μm的次总体为沉积物的主体,分别代表了弱的水动力环境和强的水动力环境,指示了深水和浅水两种环境。结合有机质含量和磁化率两个指标,对整段岩心的沉积特征进行了分析,发现整段岩心经历了5次较大的环境变化:12.80~7.30 m有机质含量最低,磁化率最高,粗砂含量几乎为零,黏土含量较高,湖水较深,水动力条件较弱;7.30~6.25 m有机质含量低,磁化率下降,沉积物主要由黏土组成,湖面结冰时间更长,沉积环境更加稳定;6.25~5.00 m有机质含量和磁化率剧烈波动,粗砂明显增多,湖水较浅,水动力条件较强;5.00~3.55 m有机质含量快速升高,磁化率下降,粗砂含量仍然较高,水动力条件较强,但水位比前一阶段有所升高;3.55~0 m有机质含量最高,磁化率最低,粗砂含量降低,湖水较深,降水量较大,水动力条件减弱。

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.