CARBONATES IN LACUSTRINE SEDIMENTS OF LAKE BALIKUN, XINJIANG, CHINA

The Lake Balikun of Xinjiang is a saline lake in an arid and semi-arid region of northwest China. The source of carbonates of the lake includes authigenic and allochthonous carbonates. Based on the data of grain-size and X-ray diffraction, and taking the dry climate and regional landscape into account, it is believed that the aeolian carbonate is the major component of the allochthonous carbonates, and the carbonate from rivers is limited. However, the aeolian carbonate is less than 10% of the total carbonate in the central part of the Lake. Dusts were collected from April to June in 2009 from three sites. Mineralogical examination suggests that the carbonate in the dust is predominated by calcite, and the main components of lacustrine carbonates are calcite (Mg-calcite) and aragonite. It suggests that only a part of calcite of the lake sediments is aeolian and the rest is authigenic. The quantitative change in lacustrine carbonates is related to biological activity, especially the activity of algae and brine shrimps. Algae bloom in spring before surface brine water formed. The fecal pellets of brine shrimp are aragonite, calcite and gypsum. The Lake Balikun includes three depositional sub-environments:lake margin and shallow water sub-environment, transitional sub-environment and lake center lacustrine sub-environment. And the content of carbonate is different in sub-environments. In this regard, the boundary conditions of the sub-environment, including the physical and chemical properties of the water body, biological activities, wind and lake level, are the factors in control over the distribution pattern of carbonates in the lake. Authigenic minerals, such as chlorides (halite), sulphates (gypsum, anhydrite, mirabilite thenardite) and carbonates (calcite, aragonite), are derived from the seasonal change of surface brine water rich in Na-SO₄-Cl-Mg. As the result, the composition of authigenic minerals is different in different sub-environments. Generally, the amount of calcite is opposed to aragonite. From the lake margin to the shallow water sub-environment, the main authigenic carbonate is calcite, since there is limited biological activity and low Mg/Ca ratio. With the increase in biological activity and Mg/Ca ratio, aragonite deposits increase in the transitional sub-environment and central part of the lake. In conclusion, all the factors mentioned above must be taken into account when paleoclimatic interpretation is made with lacustrine carbonates.