The Relationship Between Strontium Isotope And Environment Variation In Paleo-lake Sediments During Late Period Of Late Pleistocene In Western China

The Qaidam and Ejina basins possess the important significance in the late Pleistocene climate studies since their sensitive localities. Both of them are located at the arid area of northwest China, where has fragile ecosystem and unstable climate. Meanwhile the northwest arid region is one of the most distinct regions which are influenced by global warming. So the climate studies related to Qaidam basin and Ejina Basin will help us to understand the process and mechanism of the climate change. Besides, the systematic research about the arid, semi-arid area of northwest China will provide crucial theoretical basis for the development of the western China.In this paper, we selected lacustrine sediments and shells collected from the shell bar section in Qaidam and Xiaohushan section in the Ejina basins as the study objects based on the precise dating technique and reconstructed the late Pleistocene climate variation in the study regions mainly based on the strontium isotopes variations combined with a series of other indexes. The results are showed as follows:1. The evolution processes during the high lake level development period could be divided into following stages:â‘ the climate was warm and dry from 43.9Ka BP to 39.9Ka BP;â‘¡the climate was warm and humid between 39.9Ka BP and 31.7Ka BP, 31.7Ka BP and 26.9Ka BP with some abrupt cold events.â‘¢the climate was warm and dry from 26.9 to 22.5Ka BP. Generally speaking, the climate during the high lake level period was warmer and wetter than the present. The climate evolution revealed by Xiaohushan section are as follow: the climate was warm/wet from 949 to 304cm (38.15-17.37Ka BP), reflecting the warmer/wetter climate environment condition than today, comparable to that recorded by the shell bar section in Qaidam basin.2. The strontium isotope analysis results show that the ⁸⁷Sr/⁸⁶Sr as well as Rb/Sr ratios of the inland lake sediments can be treated as the effectively chemical weathering indicators. The ⁸⁷Sr/⁸⁶Sr ratios of the acid soluble fraction indicate the strontium isotopic variation during the sedimentation processes in the lake system and the ⁸⁷Sr/⁸⁶Sr ratios of acid residual fraction reveals the strontium isotopic variation in drainage area, which displayed opposite trend to lake systems. It shows that the behaviors of the strontium isotope in the lake system sedimentary are different to that in the source region. The strontium isotope and organic carbon values exhibited different ranges in the two sections, indicating the regional difference of the chemical weathering and climate characteristics.3. The strontium isotope and element variation can preserve the chemical properties of paleo-lake water body. Meanwhile the ⁸⁷Sr/⁸⁶Sr and Rb/Sr variation reflected the chemical weathering process of the drainage area. The growth process of Shell had biological fractionation which reflected the different responding mechanisms to the biological environment. The ⁸⁷ Sr/⁸⁶Sr, Sr/Ca and Sr/Ba values of shells could be an indicator of water salinity to a certain extent; however, the Mg/Ca values were not related to the paleo-environment because of the biological effects and seasonal factors.4. Study results show that the Sr/Ca, Sr/Ba and Mg/Ca ratios in the shell bar section responded to the lake salinity and temperature variations, typically reflecting lake salinity shifts and exhibited negative correlation. Moreover, the Mg/Ca ratio is impacted by the temperature greater than salinity. In brief, all the proxies are influenced by mult-factors. The Sr/Ca ratio in the calcite and dolomite-bearing sediments could better reflect the lake salinity than in the aragonite-bearing sediments. It is good index of the aquatic environment.5. The ⁸⁷Sr/⁸⁶Sr and Mg/Ca ratios of Xiaohushan section in Ejina basin responded to environment temperature and humidity variations, demonstrating that ⁸⁷Sr/⁸⁶Sr ratio also is a good indicator for salinity. However, the Sr/Ca and Sr/Ba ratio must be combined with other indexes because of the inaccuracy when applied them to the salinity studies. Generally speaking, strontium isotope and the element ratio must combine with other indexes when applied them to the paleo-environment studies.6. The lacustrine sediments in the Qaidam basin and Ejina basin reveal the paleoclimate variation characteristics in the MIS3 stage. The contemporary climate was warm/wet and existed high lake level stages, forming a large lake period which was comparable with the Tengger desert and Tibetan Plateau mega-lake period. All the results demonstrated that the climate change in the northwest China was unstable in MIS3 stage, existing distinct fluctuations and some typical cold events, but had the same pace with the globe climate change.7. The cold event recorded in the North Atlantic area also observed in the Qaidam and Xiaohushan section. Meanwhile the magnitude of the climate events in the last glacier preserved in sediments in northwest China decreased gradually from the west to the east. We suppose it caused by the westerlies and East Asia monsoon.