Provenance Of The Loess In Lanzhou Area During The Last Glacial Period In Late Pleistocene And Its Tectonic And Paleoclimatic Implications

The special location of the Chinese Loess Plateau(CLP)and the continuous deposition,large thickness and wide distribution area of the loess make the Plateau an important location,and make the Loess an ideal focus for studying the global climate changes and the evolution process of the Asian monsoon.Provenance analysis of loess can be used to understand the aridity process,the ancient atmospheric circulation mechanism,and the coupling relationship between provenance evolution and surface processes in adjacent areas.So far,many achievements have been made in previous studies on loess provenance.However,there are still different points of view on the specific source area of the Quaternary loess in the Loess Plateau.Deserts in northwest of the CLP,arid areas in northwestern China,materials eroded from high mountains in active orogens,sediments of Yellow River are proposed to be the candidates of major source of the loess in CLP.The mechanism inducing the spatial-temporal variation of the provenance of the loess in CLP remains largely unknown.So,comparing the provenance of the loess deposited in different locations during specific periods will help us reveal the mechanism.Located in the western most of the CLP,the loess in Lanzhou area during the last glacial period in Late Pleistocene play an important role in studying the spatial variation of the loess provenance.This area is located in the flank of the east segment of the North Qilian Mountain,and may record detailed deposition signals related to surface process of the Northeastern Tibetan Plateau.Loess formation may have linked with the evolution of the Yellow River since most of loess mantled on the terrace of the Yellow River.Besides,the east Asian monsoon and westerlies interact in this area.Tracing the source of the Lanzhou last glacial loess will benefit our understanding of the palaeoatmospheric circulation during Late Pleistocene,the mechanism modulating the formation of the loess,and the relationship between loess deposition,evolution of Yellow River and tectonic process of the Northeastern Tibetan Plateau.Here,we studied three typical last glacial loess profiles in the Lanzhou-Baiyin area.On the basis of field observation,optical stimulated luminescence(OSL)dating,and ¹⁴C dating techniques,we identified the depositional age framework of the three last glacial loess profile.Based on zircon U-Pb geochronology,we carried out provenance study of the six samples collected form the top and bottom of the three profile.In order to trace the dominant source areas of the Lanzhou loess,we compare the zircon U-Pb age distribution of our samples with published data from:1)Proximal deserts;2)Upper reaches of the Yellow River;3)distal arid regions in the inland basins of northwest China;4)active orogens.We also employ a multidimensional scaling(MDS)statistical technique to explore these large detrital zircon datasets.In addition,we compared the zircon U-Pb age distribution of our samples with that of coeval loess throughout the CLP,aiming to reveal the spatial pattern of the loess provenance of the CLP and the possible inducing mechanism.The results show that the sediment thickness of the loess in the last glacial period of the study section is 12-65 m,and the age of the ¹⁴C sample from the top of the profile is 21.83±0.08 ka,and the ages of the three OSL samples from the base of profiles range of 30.2?50.7 ka,indicating that the sedimentary age span of the loess in the study area is about 8?29 ka.The detrital zircon U-Pb age spectra of loess samples from the bottom and top of the last glacial loess in the study area are similar,indicating that there is a stable source of dust material for the studied loess.Provenance comparation shows that the Qilian mountain and its piedmont are likely source area of the studied Lanzhou loess.The freezing-thawing weathering of alpine rocks and the erosion of mountain glaciers in the northeastern margin of the Tibet Plateau(including the Qilian Mountains)during the Late Pleistocene may be the important geological factors for the production of large amounts of the particulate dust.In consideration of the direction of dust source,the dust transport force should be near surface westerly winds.The source of the studied loess has a relatively close relationship with that of coeval L1 loess from Xining,Caoxian and Xifeng areas,while is different with that of coeval L1 loess from Lingtai and Heimugou areas,indicating that there is a gradient change of provenance of the last glacial loess from northwest to southeast in the CLP,which maybe related to the relative contribution of dust materials from the proximal deserts and the Northeastern Tibetan Plateau,respectively.This research will benefit the understanding of the spatial-temporal variation of the provenance of loess in the CLP,and highlight that the high mountain processes in active orogens during glacial period could act as an important dust material producing mechanism.