The Spatial Distribution Of Geochemical Composition Of Eolian Sediments In The Alashan Desert And Its Environmental Sensitivity

The variation of sediment elemental composition and related proxies can indicate regional climate change and trace sediment sources.However,when factors such as climate conditions and physical and chemical properties of source rocks differ in different regions,the significance of elemental proxies for climate change and sediment sources may also change.Therefore,exploring the environmental sensitivity of elemental proxies that indicate climate change and sediment sources in specific regions is not only helpful to improve the reliability of regional past climate reconstruction,but also a prerequisite for accurately tracing sediment sources using elemental proxies.Deserts are one of the most widely distributed surface landscapes in arid and semi-arid regions,accounting for about 1/5 of the global land area.However,due to the scarcity of precipitation in desert regions,the weathering,erosion,transportation,and deposition processes of wind-blown sand are significantly different from those in other regions.Therefore,it is currently unclear whether elemental proxy indicators that indicate climate change and sediment sources in desert regions have environmental sensitivity.In this study,the Alashan Desert（including the Badain Jaran,Tengger,and Ulan Buh deserts）was chosen as the study area,and 249 modern surface wind-blown sand samples,36 desert and Gobi samples outside the desert periphery,32 rock debris samples,and 14 sediment samples from rivers and lakes were collected.The constant,trace,and rare earth element contents of the above samples and different grain sizes were tested by X-ray fluorescence and ICP-MS.Based on the element composition,content,and spatial variation of the above samples,the spatial distribution patterns of wind-blown sand elements in the Alashan Desert and their influencing factors were revealed,and the controlled factors and climate sensitivity of multiple chemical weathering indices of desert/sand dune wind-blown sand were explored.New precipitation proxy indicators in desert regions were discovered,and rare earth element indicators that accurately reveal sediment sources in wind-blown sand in arid desert regions were determined.The following conclusions were mainly drawn:（1）The composition of source rocks,sedimentary sorting,and re-cycling play a significant role in controlling the spatial variation of chemical element contents in aeolian sand in the Alashan Desert.The difference in the composition of source rocks not only causes significant variations in the content of elements such as Cr,Co,Ni between the Badain Jaran,Tengger,and Ulan Buh sand deserts but also leads to significant differences in the element contents of inland aeolian sand in a single desert compared to the peripheral regions.At the same time,sedimentary sorting affects the spatial variation characteristics of Si O₂,Al₂O₃,Fe₂O₃,Mg O,Ca O,Na₂O,and Sr element contents in the inland aeolian sand of the Badain Jaran and Tengger deserts.Sedimentary recycling,on the other hand,affects the variation of Si O₂,K₂O,and other element contents among the three deserts.In addition,since regional climatic conditions have a small impact on the spatial distribution of chemical element contents in aeolian sand in the Alashan Desert,element contents cannot directly reflect regional climate changes.（2）In the aeolian sand of the Alashan Desert,CIA and Rb/Sr do not change with climate factors（temperature and precipitation）.This suggests that climate conditions have little impact on CIA and Rb/Sr.Instead,non-climatic factors have an extremely significant impact on the ratios of CIA and Rb/Sr,among which the differences in source rock composition,carbonate content,and sedimentary sorting-induced particle size changes are the main factors affecting the spatial variation of CIA and Rb/Sr in the Alashan Desert.In arid desert areas,these non-climatic factors interfere with the indicative significance of CIA and Rb/Sr for climate change,and thus,CIA and Rb/Sr cannot reflect climate changes in this region.（3）In the northern desert/sandy areas of China,more than ten chemical weathering indices show significant regional differences in the whole wind-blown sand samples,which are mainly controlled by the differences in source rock composition and particle size rather than precipitation changes.Since precipitation in desert areas is usually low and varies less,the intensity of chemical weathering is low,and these chemical weathering indices do not respond significantly to precipitation changes in desert areas.Therefore,the majority of chemical weathering indices cannot accurately reflect the changes in chemical weathering intensity and precipitation.In addition,although most chemical weathering indices in fine-grained aeolian sand samples（<125μm）are not sensitive to precipitation,when precipitation changes are large,a strong correlation between WIP and annual average precipitation in fine-grained aeolian sand samples reveals that it can indicate the changes in chemical weathering intensity and precipitation in desert/sandy regions.（4）A new index,the DW index,which is the（Al₂O₃+K₂O）/Na₂O,has been proposed to indicate the intensity of chemical weathering of plagioclase in desert areas and changes in precipitation.A high value of the DW index indicates more precipitation,and vice versa.As the DW index can reveal the early weathering process of plagioclase in desert areas and has advantages such as independence from authigenic carbonate and low dependence on grain size,it is more suitable than other chemical weathering indices for indicating changes in precipitation in desert areas.When the source rock composition does not change significantly,the variation of the DW index in the surface aeolian sand of the desert corresponds well with precipitation,indicating that the DW index is sensitive to spatial changes in precipitation.In addition,the precipitation reconstruction of the Xiangshan section in the southern part of the Tengger Desert,based on the DW index,shows good consistency with the precipitation reconstruction results of the Tengger Desert and its surrounding areas over the past 19 ka,demonstrating that the DW index can indicate changes in precipitation in aeolian sediment sequences.（5）In arid deserts,Ce/Ce*,（La/Sm）_N,（La/Gd）_N,Y/Tb,Y/Er,and Ce/Nd are sensitive to provenance and can be used as reliable indicators for tracing the origin of eolian sands.In the Badain Jaran Desert,sedimentary sorting and recycling significantly affect the REE content and（La/Yb）_N and Y/REE,and therefore,（La/Yb）_Nand Y/REE cannot be used to trace the origin of eolian sands.However,Ce/Ce*,（La/Sm）_N,（La/Gd）_N,Y/Tb,Y/Er,and Ce/Nd are primarily controlled by provenance and are less affected by chemical weathering,sedimentary sorting,and recycling.Based on these REE indicators,we determined that the coarse fraction of eolian sands in the Badain Jaran Desert mainly originates from the sediments of Heihe river and Gobi materials from the north and west of the desert.The<125μm fraction of eolian sands in the Badain Jaran Desert has a different source from other size fractions and may originate from lacustrine sediments in the hinterland of the Badain Jaran Desert.This work assessed the sensitivity of various element proxies in detecting environmental changes,and provided some robust paleoclimatic proxies and reliable provenance tracers for paleoclimate reconstruction and provenance research in deserts.The discussion of the above issues is not only helpful to reconstruct desert’s climatic and environmental changes during the geological history,further reveal regional differences in response to global change in arid desert areas,but also of great significance to determine the provenance of desert sediment,and to further understand the"source-sink"process of desert sediment.