青藏高原兹格塘错沉积物粒度组成及其环境记录的研究 *

兹格塘错是位于藏北高原腹地的一个封闭型湖泊,对其沉积物的粒度组成、粒度参数等粒度特征进行分析,揭示了湖面水位变化,水动力、风动力搬运强度等。并结合其他环境代用指标进行比较,探讨环境变化,揭示了兹格塘错区域全新世初期气候湿润、中期气候偏干、晚期干湿交替的气候变化特征。粒度参数所反映出的湖面波动与环境变化得到了其他环境代用指标较好的支持。

GRAIN-SIZE CHARACTERISTICS OF SEDIMENTS FROM THE ZIGETANG CO LAKE, TIBETAN PLATEAU AND THEIR ENVIRONMENTAL IMPLICATION

The Zigetang Co is an enclosed basin, located in the hinterland of the Tibetan Plateau(32°00'~32°09'N,90°44'~90°57'E). Its water surface area is about 187km² with an elevation of 4561m a.s.l. Its catchment area is 3430km², with a landscape of hills, plains, and desert grasslands. Neither glaciers nor permanent snow is in that area. The lake water is mainly supplied by 7 rivers, among which the longest is only about 60km. The water balance of the lake depends completely on precipitation and evaporation in the catchment, where the annual mean precipitation is ranged from 240~500mm, mainly concentrated in summer time from May to September, and the annual mean temperature is about -3.4~-0.4℃. Due to high elevation and harsh environmental condition, the area is little effected by human activity. Detailed observation of modern process has shown that this lake is a typical meromictic lake. The late is therefore sensitive to climatic changes and thus can be used as an ideal place to drill sediment cores for studying climatic evolution. The ZGT02 core, about 727cm long, was obtained at water depth of 30m in the Zigetang Co. The parameters of grain size, the carbonate content, the ages, as well as the geochemical characteristics of the sediment have been analysized in high resolution in the laboratory. This paper is dealt only with the grain size characteristics for whole profile of the core, with the purpose of discussing the sedimentary dynamics and the environmental changes during the Holocene.The results demonstrate that: during 10.6~8.7cal.kaB.P. , the sediments was characterized by fine median size, high content of clay and low content of sand which reflect an environment of deep-water lacustrine facies; during 8.7~5.0cal.kaB.P. , the parameters of grain-size imply a change of depositing dynamics. Eolian transportation and lacustrine endogenic depositing are the main depositional sources; during 4.15~4.27cal.kaB.P. , the grain-size features indicate an event of wind. By the analysis of grain-size parameters and the comparison with other proxies of the Zigetang Co sediments, three stages of environmental changes since Holocene have been distinguished. During 10.6~8.7cal.kaB.P. , the climatic was changed from dry to wet; During 8.7~5.0cal.kaB.P. , after several times of change, the climatic was converted from wet to dry. During this period a cold climate was detected at 8.2cal.kaB.P. In the Late Holocene since 5.0cal.kaB.P. , a wet-dry-wet fluctuation occurred.