The Geochemical Characteristics And Environmental Implications Of Loess Deposits At Tengger Desert Margin During The Last Glacial Cycle

The last glacial cycle is a paleoclimate cycle that the nearest from modern, to research the evolution of ancient climate in this stage, not only understand the past of climate evolution, but also provide a similar reference type for prediction and researching our future climate change. Th erefore, More scholars pay much attention to the last glacial cycle, because it is different in the climate evolution. Loess sediments as a carrier that record important ancient climate information. It played a significant role in rebuilding the ancient climate and achieved fruitful results. But the Loess susceptibility enhanced mechanisms at the Desert Margin are different from the Loess Plat eau. It should not be used for reconstruction the summer monsoon. This paper selects the Baiya loess section and Caoxian loess section at the Desert Margin, We collected 1654 samples (Thick ness of Baiya loess section is 36.5m, continuous sampling with 5cm interval,730 samples were obtained; Thickness of Caoxian loess section is 46.2m, continuous sampling with 5cm interval, 924 samples were obtained),all of samples magnetic susceptibility, grain size, geochemical elements (major elements) and optically stimulated luminescence (OSL) samples were analyzed at the Key Laboratory of Western China's Environmental Systems (Ministry of Education) Lanzh ou University and Qinghai institute of Salt Lakes.We try to use Geochemistry, grain size and magnetic susceptibility to reconstruction the paleoclimate of Baiya Loess section and Caoxian Loess section at Desert Margin and to test the indicators application in the boundary belt of the Desert and the Loess. Get the following understanding by analyzing:(1) The results from geochemistry proxies analysis of Baiya section and Caoxian section: CIA can be more clearly reflect the change of summer monsoon of Baiya section and Caoxian section, while the Rb/Sr was not clear, the Rb/Sr were more complex than the Loess Plateau, Rb/Sr value can not be a good indicator of the climate change.Therefore,the Rb/Sr of Baiya section and Cao xian section were not suitable for reconstruction the paleoclimate evolution. SiO2/TiO2 and Zr/Rb reflect winter monsoon changes clearly and can be used as indicators of winter monsoon changes.(2) Magnetic susceptibility indicators in the Baiya section does not apply in the MIS3 stage, not a good indicator for summer monsoon changes. But it can make the response to climate chang es sensitive in the MIS1 and MIS5 stage; Caoxian section showed the case that both ends of magn etic susceptibility higher than the middle. In view of the formation mechanism of magnetic suscep tibility is complicated, be caution when use susceptibility index.(3) Through Baiya and Caoxian loess section different grain-size analysis:We found that the fine particles of Baiya section nearer the Desert was higher than Caoxian section, the Coarse-grain content below the Caoxian section, Which showed that the Content of coarse and fine part icles in two sections were controlled by different factors at different times. During the appropriate period, the source area of Baiya section is distance, during the cold period, Desert expansion made great contribution to the loess deposits. Caoxian profile content of each component smaller than the Baiya section, indicating the ancient climate information that grain-size recorded of Caoxian section farther away from the Desert becomes steadily through the handling-mixing procession.(4) Comparation constant element with the UCC of Baiya section and Caoxian section, the results showed that SiO2,Al2O3 is closer to UCC, CaO contents was enriched significantly relative to UCC; MgO,TiO2,Fe2O3 enriched relative to UCC; Na2O,K2O showed relatively weak loss to UCC. the chemical index of alteration (CIA) of the two sections are distributed between 50 to 60, indicating that two sections in the early stages of chemical weathering, In addition, Al-Ti-Zr triangular diagram showed that material sources change at different times, loess material mixed in the Holocene and last interglacial period indicating that the dust has experienced long distance transport and mixed process; the material source may be closer in glacial, eolian material mixed inhomogeneous.