末次间冰期以来新疆巴里坤湖蒸发盐的沉积环境记录

EVAPORATION SALT RECORDS OF ENVIRONMENTAL RESPONSE TO CLIMATE CHANGE IN BARKOL LAKE BASIN, NORTHWESTERN CHINA

沉积物岩芯的碳酸盐、石膏等蒸发盐含量和矿物组成的证据显示了新疆巴里坤湖末次间冰期以来一直是一个水位变化频繁的浅水盐湖，有时甚至为间歇式的湖泊，约在34000aB.P.前后巴里坤湖沉积相发生了显著的变化，34000aB.P.之前为泥坪-湖滨相沉积，34000aB．P．之后为成湖相沉积。尽管我们对巴里坤湖34000aB．P．前后的沉积相变难以理解，但是长期处于浅水状态的巴里坤湖沉积物中蒸发盐的含量和矿物组成仍然与气候变化密切相关。在泥坪-湖滨相形成的碳酸盐中白云石相对的增加是地表强烈蒸发的结果。以石膏为主的蒸发盐含量增加是温度和降水增加所致，而其含量减少则是冰川前进致使补给水矿化度降低造成的。这使我们获得了该区古气候变化并不服从于冰期与雨期同步模式的信息。

Evaporation salts, for example, chlorite, nitrate, sulfate, carbonate etc., are common minerals of sediments in a saline lake, and crystallized by strongly evaporation of lake water. They could be a good indicator in evolutionary history of the lake since chemical precipitation in a lake is closely controlled by chemical composition of the lake water, in turn, by input water. In this paper, we take an example to show how to get paleoenvironmental information from the evaporation salts in sediments related to a saline lake. Barkol Lake is a shallow saline lake at present day, and located at northern Xinjiang, China, where is an arid-semiarid and cold region. A core was taken from sediments in this lake in 1994. The sediment core was sub-sampled for analyses of CaO, MgO, carbonate minerals, and gypsum. The evaporation salts in the sediment core are mainly composed of gypsum, calcite, aragonite, and dolomite. Based on observation of the sediment core section, geochemical indices and compositions of the evaporation salt minerals, we can conclude as followings:(1) Ratio of gypsum / total evaporation salts is a proxy of sedimentary facies.Gypsum was dominantly deposited during lake factes periods. Barkol has been a shallow saline lake, even intermission lake since 150 000 aB. P., and its level was frequently changed due to climatic changes. A dramatic shift of sedimentary facies was happened at about 34 000a B.P. The sedimentary factes before and after 34 000 a B.P. are beach-shore deposits and shallow lake deposits respectively.(2) Ratio of dolomite/ total carbonates in the beach-shore deposits is an indicator of evaporation of the pore water of the sediments, increasing with temperature.(3) The total salts for lake factes deposits must be an index of chemical deposition rate which depends mainly on evaporation rate of the lake water and chemical composition of the input water. Concentration of the total salts in the sediment core is correlated with climate, increasing with temperature and precipitation. (4) Paleoclimatic change in Barkol Lake Basin is characterized as dry-cold to wet-warm.