Paleoclimate And Provenance Changes On The Chinese Loess Plateau Based On Heavy Mineral And Zircon U-Pb Ages Of Loess And Red Clay

Aeolian sediments covers one tenth of the Earth terrestrial surface and are encoded with important information of past atmospheric circulation pattern.T The aeolian sediments on the Chinese Loess Plateau(CLP)record paleoclimatic information since at least the late Miocene and,thus,have been the focus of the paleoclimatic society during the past decades.Provenance analysis is an effective way to extract this sort of paleoclimatic information from aeolian sediments.Many methods had been used to trace the provenance of the Quaternary loess and the underlying Red Clay sequences on the Chinese Loess Plateau,and some potential sources had also been proposed,but because of the inherent complexities of provenance analysis,no concensus has been reached about provenance history of the Loess Plateau,nor its exact source regions.Although much progress has been made in understanding the source of Quaternary loess on the Chinese Loess Plateau(CLP),the provenance of the underlying upper Miocene-Pliocene Red Clay sequence is largely unknown.In addition,no consensus exists as to whether the bottom part of the Red Clay sequence is aeolian or water-lain.Here,we use the heavy mineral and detrital zircon U-Pb ages to tracing the provenance of loess and Red Clay on the Chinese Loess Plateau.The main results and conlusions are listed as follows:(1)The heavy mineral results of loess and Red Clay from the Chaona and Jiaxian reveal that the Red Clay samples from Chaona and Jiaxian have similar heavy mineral composition,and epidote and hornblende are the most dominant mineral for the Chaona and Jiaxian Red Clay samples and Jiaxian loess samples,respectively.(2)The heavy mineral results of loess(L26 and L33)from Chaona,Jiaxian,Xifeng and Luochuan reveal that the L26 loess at Jiaxian has the higher hornblende content than that of Xifeng,and the L33 loess of Jiaxian and Xifeng has the similar hornblende content and highter than that of Chaona and Luochuan.The magnetic susceptibility records and change of hornblende content from the Jiaxian,Xifeng and others section on the Chinese Loess Plateau reveal that the magnetic susceptibility record at Jiaxian(Jingchuan is currently much wetter than Jiaxian)significantly higher than in the Xifeng and Baishui sections before ~2.7 Ma,and the magnetic susceptibility record at Jiaxian changed to be significantly lower than that at Jingchuan,but closer to that in Xifeng and Baishui starting from ~2.7 Ma.We attribute this major change in climatic patterns at ~2.7 Ma to decreased northward moisture transportation associated with Northern Hemisphere glaciation and cooling in the Quaternary.(3)The heavy mineral results of Red Clay(~3 Ma and ~7 Ma)from Chaona,Jiaxian,Xifeng and Luochuan reveal that the Red Clay samples(~3 Ma and ~7 Ma)from Chaona,Jiaxian and Luochuan have the smilar hornblende content,and significantly lower than that of Xifeng.This suggest that the climate condition at Jaixian is wetter than that of Xifeng during the Red Clay deposited period.(4)The zircon U-Pb results from the loess and Red Clay reveal that both the Red Clay and loess-paleosol samples have most ages distributed between 600-200 Ma with elevated age peaks at 500-400 Ma and 300-200 Ma,and the Quaternary loess-paleosol samples have a more pronounced 300-200 Ma age population than the un-derlying Red Clay sequence.In addition,the age distribution of CN-8 Ma is markedly different from CN-3 Ma and the age distribution between CN-5.5Ma and CN-4Ma is similar.(5)Visual and statistical analyses of the U-Pb age populations and comparison with results from potential source regions reveals that the lowermost Red Clay of the late Miocene(depositional age of ~8 Ma)is likely sourced from the nearby Liupan Mountains and the Qaidam Basin;the middle Red Clay(5.5-4 Ma)of the early-mid Pliocene is sourced mainly from the Taklamakan desert;the upper Red Clay of the late Pliocene(~3 Ma)is sourced from mixed areas,although western source materials from middle-northern Tibetan plateau(including Qaidam Desert sediments and materials eroded from the Qilian Mountains)sediments appear to dominate;and the Quaternary loess is also sourced from mixed source regions,albeit with dominant northern CLP proximal desert sediments transported via winter monsoon winds,which in turn may be transported from mountain source regions of the northeastern Tibet and Gobi Altai via major river systems.This long term shift in sources suggests a progressive eastward aridification during the Pliocene in Asia with the specific timing of provenance shifts synchronous with large-scale climatic transitions and Tibetan uplift,demonstrating that Asian desertification is controlled by both factors.