Trace The Source Of Xiashu Loess In Ningzhen Region Using Isotope Geochemical Tools

Deciphering the eolian sources is critical to understand the paleo-significance of the Quaternary eolian deposits(the Xiashu loess)in the lower reaches of the Yangtze River.Both a local source from the fluvial sediments of the Yangtze River and a distal source from the northern deserts similar to that of the loess on the Chinese Loess Plateau(CLP)have been proposed.Geochemical tool is widely used to trace the eolian source of atmospheric dust.This research systematically compare the detrital zircon ages,the Sr,Nd,and U isotope of the Xiashu loess(in Ningzhen region),the loess on the CLP,the fluvial sediments of the Yangtze River and potential source materials,in order to decipher the source region of Xiashu loess.Recent researches show great advantages of detrital zircon ages as a source tracer for Asian dust based on the laser ablation U-Pb isotopic dating technique.This work presents the U-Pb ages of zircon grains extracted from the Xiashu loess.The results indicate that the Xiashu loess has a very different age distribution of zircon grains from that of the loess on the CLP as well as the materials in the arid lands of the Asian Interior.Instead,the zircon ages of the Xiashu loess are indistinguishable from the fluvial sediments of the Yangtze River,indicating the dominance of proximal dust source for the coarse particles of Xiashu loess.For the fine grained fraction of Xiashu loess,the 20-25?m grains are analyzed by the Sr,Nd and U isotope.It is difficult to distinguish the source of Xiashu loess by the evidence of Sr and Nd isotope.As a geomorphological tracer,the technique of U isotope could reflect the comminution ages and transport time of eolian particles,thus provide a new way to trace the transport path of source particles.Our results of U isotope show that the recoil loss factor(f)and the transport time of Xiashu loess is significantly higher than that of the loess on CLP.The initial 234U/238U ratio of Xiashu loess when it deposits should be lower if the Xiashu loess has a similar source with the loess on CLP.The 234U/238U value of modern sediments from the Yangtze River is much lower,which might be introduced by a longer transport time.The sources of Xuancheng red earth and Jiujiang deposits might be different from that of Xiashu loess,according to the evidence of Sr,Nd and U isotope.A direct comparison between Xiashu loess and the sediments of Yangtze River during glacial periods is lacked,which could provide a more robust evidence for the eolian source of Xiashu loess?A local source from the fluvial sediments of the Yangtze River for Xiashu loess is suggested,which implies that extensive eolian processes might have existed in the currently wet South China,possibly in response to the full glacial conditions after the middle Pleistocene transition of global climate.