Environmental Significance Of The Chemical Composition Of Surface Aeolian Sediments In The Badain Jaran Desert,NW China

Surface aeolian sediments in deserts include aeolian sands,as well as lake surface sediments in the hinterland of desert.This is because lakes in desert hinterlands that are not recharged by river runoff,sediment is input solely by wind transport.Those surface aeolian sediments are regarded as indispensable archives for the study of global and regional environmental evolution.However,the weathering,erosion,transport and deposition processes in hyper arid desert regions are different from those in other arid and semi-arid areas,therefore,the paleoenvironmental signifances and modern processes of aeolian transportation and deposition of aeolian sediment are particular.Among them:?1?the mechanisms controlling the spatial variability of chemical elements in the aeolian sands and lake surface sediment;?2?the environmental significance revealed by elemental proxies in aeolian sand and lake surface sediments;?3?how to extract and separate reliable elemental proxie from lake sediments in lakes in the hinterland of desert for paleoenvironmental reconstruction?At present,further research is needed.Solutions to the above-mentioned issues can provide reliable elemental proxie with clear environmental indicators for the internal links between regional and global environmental changes,and provide new evidence for the hot issue of the process and mechanism of the interaction of the earth's surface layer.In this study,a total of 115 samples of aeolian sand,lacustrine sediment,and exposed bedrock were collected across the entire Badain Jaran Desert in the hyperarid region of northwestern China and analyzed for major elements and trace elements by X-ray fluorescence spectrometry.Meanwhile,100 surface sediment samples were collected from five lakes in the hinterland of the Badain Jaran Desert,NW China,and examined for their major,trace and rare earth element?REE?contents and mineral assemblages.XRF results of aeolian sediments showed that significant differences in the chemical compositions of aeolian sediments were observed among different grain size fractions.The concentrations of most measured elements were higher in the fine?<250?m?than coarse fractions.The enrichment of CaO,Fe₂O₃,MgO,Cr,Ni,Ti,and Zn in fine aeolian sands showed that elemental composition was highly dependent on grain size.Compared with the average chemical composition of the upper continental crust?UCC?,the aeolian sands in the Badain Jaran Desert were enriched in SiO₂,while other major elements?i.e.,Al₂O₃,Fe₂O₃,MgO,CaO,Na₂O,K₂O,and Ti?showed significant depletion and possible leaching characteristics,This result indicates that the aeolian sands have not been subjected to intense weathering and recycling.The CIA values of aeolian sediments in the Badain Jaran Desert were all below 50,varying between 41.2 and 49.9,which were significantly lower than those of shale but close to those of unaltered granitic rocks.Badain Jaran Desert can be divided into four different zones from north to south?A–D?,based on differences in the principle component scores.The concentrations of major and trace elements varied largely among the geochemical zones in the Badain Jaran Desert.XRF and X-ray diffraction results of aeolian sediments revealed significant differences in the chemical compositions of surface sediments among the five lakes,which had different TDS values.In SB Lake,Z Lake,T Lake and B Lake,the SiO₂/Ti ratios of the lake surface sediments decrease from the nearshore to the offshore regions.In contrast,their Na₂O/K₂O ratios increase from the nearshore to the offshore regions.The Rb/Sr ratios increase from the nearshore to the offshore regions in SB Lake,Z Lake and T Lake and decrease from the nearshore to the offshore regions in B Lake.The LREE/HREE ratio and?Eu results indicate that the lake surface sediments are LREE-enriched and positive Eu anomalies.The main minerals in the lake sediments are of both detrital and saline species,large amounts of dissolved substances in groundwater were deposited in saline minerals in saline lake.The PCA results show that the sediments of the salt lakes and the brackish lake have different element group characteristics.We concluded that:?1?The elemental concentrations of aeolian sediments varied considerably among different regions,particularly between the marginal and central zones,which could be related to differences in the chemical composition of the source rocks.The variations in the chemical composition of aeolian sediments in the central zone were affected mainly by sedimentary sorting rather than spatial heterogeneity in precipitation.?2?Given that the changes in Chemical Index of Alteration?CIA?were not a function of the chemical weathering in this desert,the exact environmental significance indicated by CIA should be stated explicitly before using this index as a proxy for paleoenvironmental reconstructions in hyperarid regions.?3?Geochemical characteristics of the lake sediments and their spatial variations are influenced by aeolian sands,saline minerals,and sediment sorting.Aeolian sands dominate the immobile and non-soluble elements,whereas authigenic minerals are important in influencing variations in the mobile and soluble elements.Sediment sorting is reflected in the spatial variation of the sediment chemical composition.?4?The positive relationship between the SiO₂/Ti of sediment and lake area indicates that changes in the SiO₂/Ti ratio can reveal approximate lake area.The Na₂O/K₂O ratio in sediments can reveal the TDS of salt lakes more effectively than can the Rb/Sr ratio.