Geochemical Evidence For The Provenance Of Xiashu Loess And Its Paleoenvironmental Implications

The widespread loess deposits named Xiashu Loess,which are located in the middle and lower reaches of the Yangtze River,are middle or late Pleistocene loess deposits.As the southernmost eolian dust deposits in China yield valuable paleoclimatic records and well preserved terrestrial climate records with a long time span in subtropical regions,Xiashu Loess are payed great attention.Provenance of aeolian dusts has long been regarded as one of the major interests in the loess research.Distinguishing the provenance can not only reveal the paleoenvironmental information of the source regions,but also possibly provide the key evidences for the ancient dust transportation and deposition.However,the provenance of Xiashu Loess remains controversial.Up to now,sections which have been explored the provenance using geochemical methods are usually located in the south of Yangtze River,and only limited sections studied.The provenance of Xiashu Loess located in the north of Yangtze River and the spatial changes of the chemical compositions for Xiashu Loess are indefinitely.In order to comprehensively explore the provenance of Xiashu Loess in space,22 sections from three districts were investigated,respectively the eastern of the Tongbai-Dabie Mountains,the Jianghuai Hill and the south bank of Yangtze River.The major element,trace element as well as Pb isotope of the <20?m fraction of 107 samples mainly from the first loess layer of the 22 sections were analyzed and compared with the Xifeng loess which is the typical loess deposits from the Chinese Loess Plateau.Meanwhile 9 equal-space sampling sites selected along the bank of Yangtze River from Tongling,AnHui province,to the estuary of Yangtze River.The major element as well as trace element of the <20?m fraction of 22 samples mainly from alluvial flat were analyzed and compared with the Xiashu Loess.In this study,Pb isotopic composition characteristics of the <20?m fraction of Xiashu loess are measured for the first time as well as been used to explore the provenance of Xiashu Loess.Based on the spatial changes of the chemical compositions for Xiashu Loess from a large quantity of sections.Results and conclusions are drawn as follows:(1)The normalized element distribution patterns of Xiashu Loess and Xifeng loess are similarity,indicating the mixed characteristics of the Xiashu Loess possess.The chemical compositions show significant variation with different profiles with no specific trend for Xiashu Loess(2)The immobile element ratios and Pb isotopic composition of the <20?m fraction in Xiashu Loess are different from those of loess from the Chinese Loess Plateau indicate that the loess deposits in the two regions have different provenances.And the immobile element ratios of the <20?m fraction in Xiashu Loess are similarity to alluvial flat indicate that the alluvial flat is one of the source of the the Xiashu Loess.The immobile element ratios and Pb isotopic composition show significant variation with different profiles with no specific trend suggesting diversity of the source for Xiashu Loess.(3)The fact that Xiashu Loess sourcing from the adjacent fine deposition point to the aridification of local floodplains in the present northern subtropical area during glacial periods,And the role of the East Asian winter monsoon in spreading dust from northern deserts to southern China has often been overestimated.