Geochemical Characteristics And Provenance Of Surface Sediments From Alxa Area

The westerly jet is considered an important climatic system that affects the East Asian area.The Alxa area is located in arid areas of the west China in the hinterland of the East Asia.The widespread distribution of surface sediments in the area is strongly influenced by the westerly jet.Different types of surface sediments contain different provenance information,and the research on the source material is of great significance to the analysis of the dust source area and the transportation path in the westerly area.This paper collected different types of surface sediments from the Alxa,Southern Xinjiang,and Dunhuang areas.Based on the end-member analysis of bulk samples,the major,trace and rare-earth geochemistry characteristics of different size fractions,and the provenance of surface sediments in the Alxa area are discussed:1.The results of the end-member analysis of the grain-size–frequency curve show that the average mode size of the fine-grained component C1 in the Alxa,Dunhuang,and Southern Xinjiang areas are 6.17 ?m,8.44 ?m,and 10.56 ?m,respectively,similar to the westerly dust particle size decreasing slowly from the upwind direction to the downwind direction.This indicates that the westerly jet plays an important role in the dust transport in the Alxa area.The C2,C3,and C4 components of the coarse-grained end members in the Alxa area reflect the characteristics of near-source materials.The material sources may be primarily the large amount of detrital materials produced by the high mountains around the study area under the action of glacier abrasion,weathering,and denudation.2.The analysis of the UCC-normalized patterns of major elements and trace elements shows that there are certain differences in the performance of major elements and trace elements in the Alxa,Dunhuang,and Southern Xinjiang areas,but the overall distributions in the fine fractions such as 2–16 ?m and 16–32 ?m are relatively close,where the 2–16 ?m components show the highest similarity and the 32–75 ?m and >75?m components are quite different.The geochemical characteristics of major elements and trace elements may indicate that the Alxa,Dunhuang,and Southern Xinjiang areas demonstrate a strong correlation with substances in the <2 ?m,2–16 ?m,and 16–32 ?m fractions.In the 32–75 ?m and >75 ?m fractions,there were significant source differences.The analysis of the UCC-normalized patterns of rare-earth elements,?Eu value,trace element ratio,and rare-earth element characteristic parameter discriminant diagram shows that the similarity of the UCC-normalized patterns of the Alxa area and other areas in the <2 ?m fraction is poor.The 2–16 ?m fraction displays a high similarity,whereas the 32–75 ?m and >75 ?m fractions are quite different.The ?Eu value is closer in the 2–16 ?m and 16–32 ?m fractions.The trace element ratio discriminant diagram and the characteristic parameter diagrams of rare-earth elements show that the similarity of the scattered points in the <2 ?m fraction is poor,the distribution range of the scattered points in the 2–16 ?m and 16–32 ?m fractions is small,and the similarity is high.In the32–75 ?m and >75 ?m fractions,the scattered points show a wide distribution range.The result may indicate that the Central Asia shows a certain influence on the Alxa,Dunhuang,and Southern Xinjiang areas in the <2 ?m and 2–16 ?m fractions,but in addition,there may be other sources.In the 2–16 ?m fractions,the remote dust from the Central Asia shows a greater impact on the Alxa,Dunhuang,and Southern Xinjiang areas.In the 32–75 ?m and >75 ?m fractions,the material sources in each area are mainly affected by the region.The weathering and denudation products of the bedrock exposed in the mountains near the source provide material contributions to the coarse-grained fractions of the area.3.Based on the end-member analysis of the grain-size–frequency curve,the UCC-normalized patterns and characteristic parameter diagrams of major,trace,and rare-earth elements in surface sediments from different areas,and the ?Eu values in different grain-size fractions,combined with the topography and prevailing wind direction of the study area,it is concluded that the <2 ?m fractions in the Alxa area are carried by the upper westerly jet from the remote Central Asian desert area.In addition,the fine particulate matter from the Gobi Desert area in Mongolia was carried by the winter monsoon to the Alxa area for deposit.The 2–16 ?m fractions were deposited in the Alxa area primarily by the westerly jet,so more information on the westerly jet could be recorded.The 16–32 ?m fractions in the surface sediments of the Alxa area,in addition to the westerly jet that transported the dust from the Central Asian desert area to the area,were also influenced by near-source materials.The 32–75 ?m fractions were mainly contributed by the mixture of materials from the Qilian Mountain through the Heihe River and Central Gobi,and primarily contributed by the Qilian Mountains.In addition,the widespread lake sand in the study area can also influence the northeast and southeast parts of the study area.In the >75 ?m fractions,except for the Qilian Mountains and the Central Gobi,the weathering and denudation products of the bedrock exposed in the Bei Mountains and Yabulai Mountains also provide the main materials for the area.