Rare Earth Elements Characteristics Aeolian Deposits In Yili Basin, Xinjiang, And Their Implications For Provenance And Environmental

The Ili Basin lies in the Northeast of Asia, not only locates in the key part between the well-studied European loess sequences to the west and the extensive Chinese Loess Plateau(CLP) to the east, is a hinge to connect to the two regions. Under the effect of the westerly circulation, dust was carried to the piedmonts of the Ili Basin and deposited, several to nearly a hundred meters thickness loess sediments are exposed along the river terraces and piedmonts of Tianshan Mountains,authentically record the information of climate change, the vicissitude of desert, westerly circulation evolution in the vast interior of Asia and the climate interaction mechanism, relationship of climate change between the North Atlantic region and East Asia Monsoon Region. This work chose Zhaosu Poma loess section (ZSP) and a paleoaeolian sand section (TKP) in Takermohuer desert in Yili Basin generally along the dominant wind direction of the westerly,to systematically study characteristic of rare earth elements (REE) in whole rock and different grain fractions and phases, and to explore the provenance and paleoenvironmental significance of REE records. Using those REE data, this study investigated spatial differences of aeolian sediments in Yili Basin and their possible provenance.The preliminary results are shown as below:(1) REE characteristics in whole rock and different grain fractions and phases in ZSP loess section shown that:REE distributed patterns of whole rock samples are similar; EE concentrations show a general "increase firstly, then decrease" pattern in more coarse grain fraction of typical samples, the highest EE concentrations occur in 2-16μm fraction. Acid soluble tests for typical shown that with increasing grain size, the proportions of acid soluble fraction in whole rock samples display a initial decrease and a followed increase, that is the proportions are decreasing from<2μm to16-32μm fraction, then gradually increasing from 16-32 to >63μm fraction. The whole rock ∑REE concentrations show similar variations with mean grain sizes of the ZSP loess section, which fully demonstrated that the different transport processes display strong controls of the whole rock REE characteristics. The positive relationship between concentrations and mean grain size, inferring a proximal source material with ∑REE concentrations, and a distant source material,that is dust transported by the westerly, with low ∑REE concentrations. Based on the variations of grain size and REE data, the climate change since Last Glacial could be reconstructed.(2) REE characteristics in whole rock and different grain fractions and phases in TKP section shown that:REE distributed patterns,8Ce,8Eu and LREE/HREE ratios of whole rock samples are similar; ∑REE concentrations show observed decrease in more coarse grain fraction of typical samples; most of UCC-normalized REE patterns in the acid residue fraction and whole rock fraction display HREE-enriched, while those patterns for acid soluble fraction reveal clear MREE-enrich. TKP section REE characteristics perform high correlation between ∑REE and LREE/HREE ratios. EE and LREE/HREE ratios are also show good relationship with calcite content in the section, which is the sensitive proxy for climate change. Accordingly, we reconstructed climate change of Yili valley from 3.7 Ka.(3)The comparison between the two sections in Yili basin showed that UCC-normilized whole rock REE patters of each grain fractions in TKP section displayed middle and heavy REE enrichments, and the magnitude of enrichments for heavy REE is larger than middle REE, while those patters in ZSP section displayed observed heavy REE enrichments. The heavy REE enrichments in TKP section probably result from abundant heavy minerals with heavy REE enrichments, e.g., zircon and garnet, in those sandy sediments.(4) The comparison between loess in ZSP section of Yili region and that in Chaona section of Loess Plateau showed that both of EE concentrations are controlled by grain size of the sediments. The concentration difference in <2μm fraction of both regions probably arise from different extent of pedogenesis as well as different extent of the westerly influences.(5) Studies in whole rock and different grain fractions and phases in this study showed that REE bearing minerals in different grain size fraction and phases show dominating control of REE characteristics. The detailed investigation of REE characteristics in different grain size fractions and phases could improve our understanding of material composition, transport processes and climatic and environmental change in Yili region.