Environmental Changes From Lacustrine Record Since Late Pleistocene In Ejina Basin, NW China

Ejina (Gaxun Nur) Basin is one of the largest inland basins in arid NW China, where has fragile ecosystem and unstable climate. Meanwhile the environment is controlled by atmospheric circulation and it is sensitivity to global climate changes in this region. There is a widespread lake sediment deposited since late Pleistocene , indicating the environment is wet. It was consistent with other stratigraphic evidence in NW China which was proved by the result of model. Therefore, a continue lake sediment collected from XHS, which was located in the north of Ejina Basin, was studied here. Through a variety of proxies, such as grain size, magnetic susceptibility, carbonate content, total organic carbon, organic carbon isotopes, inorganic carbon isotopes and mineral composition, the history of lake evolution since the late Pleistocene was reconstructed. It is important to study the history of climate change at this area since the late Pleistocene, which will help us to understand the process and mechanism of the climate change and provide a detailed record for the model.Grain size of sediment showed that it is sensitive to the history of lake evolution. The sediment was fine when the lake was extending, Grain size increased when the area of lake was decreasing. The distribution of Grain size was mainly controlled by the dynamic change of lake. The sensitive particles were calculated by the method of calculating standard deviation and can be used as an indictor of lake level.TOC and TN showed a similar variation through the profile. TOC/TN ratios were constant through the profile with an average value of 9.98, indicating that organic carbon was original from lake-internal input and detritus input. A high value of TOC occurred when the lake was during a high level period, indicating that TOC can be used as a proxy for productivity under warm climate conditions. The content of authigenic carbonate, which was mainly controlled by water temperature and biomass, can be used as a proxy for the variation of effective humidity in the study area. It is complex to interpret the variation ofd¹³C extracted form carbonate, which show high correlation coefficient with Grain size the content of carbonate and TOC. the variation ofd¹³C was controlled by the change of salinity of lake,The identified minerals were Quartz, calcite, aragonite, dolomite, feldspar and clay minerals with the method of Powder X-ray Diffraction (XRD). The concentration of major minerals was calculated as the semi-quantitative index. During the extending period of lake, quartz has a low concentration and feldspar which original from weathering has a high concentration. So the ratios of feldspar/quartz can be used as a proxy for weathering in the basin. The carbonate was made up of by calcite and dolomite at the bottom of the profile. The concentration of calcite decreased which was instead by the increasing of dolomite at the centre of profile. Dolomite became the primary composition at the top the profile. Aragonite showed a constant through the profile.According to the TOC, CaCO₃ contents, d¹³C and grain size variations of the sediments, and the lithofacies of the studied section, the history of climate change and paleolake evolution history of the studied section can be described by the following: 43.5～40.7ka BP, climate was dry and the sediment was made up of by sand; 40.7～39.3ka BP, the area of lake increased and the site of profile was at a stable under-water condition, indicating that the climate became to wet; 39.3～26.4ka BP, a large lake had been there and the productivity increased. All proxies showed that the climate was wet and it maybe the opportunity period since the late Pleistocene; 26.4～20.9ka BP, the climate fluctuated significantly and the water level was dropping, but it was wet and the paleolake was in the high level overall; 20.9～17.1ka BP, The climate was colder and dryer than before and the water level was dropping down, the effective humidity of the study area became smaller, and the paleolake retreated beyond the site of the study section.; 17.1～4.6ka BP, the lake level raised and the environment turned to be better at beginning, but the climate changed in the end, therefore, the environment became cold and dry again and the paleolake started to retreat until it disappeared.The records of lacustrine sediment at XHS were consistent with the record at DTL profile of Tengger Desert and BQD profile of Qaidam Basin. All the three records provide strong evidences for the existence of large area during 40-20ka BP, which showed a different environmental condition. A series of global climate change events which recorded by ice cores and stalagmites had been found at the records of XHS. During the period of H events, the area of lake decreased and weathering effect was weakly, indicating a dry condition. So the evolution of the lake was controlled by the global climate change.