The Provenance And Paleoclimatic Implications Of Loess Deposits In The Eastern Piedmont Of Liupan Mountains Since The MIS 2

Aeolian sediments on the Chinese Loess Plateau（CLP）are important carriers to research of paleoclimate changes.To probe into the provenances of loess can also be used to further understand the evolution of past atmospheric circulation.Although lots of fruitful work have been done about the loess provenances and paleoclimate changes on the CLP,some debates still remain,for example,the specific source area is uncertain,are the sediments derive from the mountain process or the Yellow River alluvial sediments.The loess area in front of the eastern piedmont of Liupan Mountains is the ideal place to do research of sediment provenances and paleoclimate changes,because of its special geographic location.Here,we present a loess-paleosol section（CF2019）located in the middle-eastern CLP,obtain the data by AMS ¹⁴C dating,grain size and X-ray fluorescence spectrometric analysis,discuss the possible provenances of loess particles in the section as well as the paleoenvironmental meaning of elements,determine the paleoclimate sensitive proxies of the winter monsoon and the variation of the desert margin,and reconstruct the paleoclimate changes of this piedmont area since the Marine Isotope Stage 2.The main conclusions are as follows:1.The provenances of sediments in the study area.Being compared with sections in other areas,the CF2019 section has a finer particle distribution than sections located in the western area as well as the northern area,and coarser than the southern area,indicating that the Liupan Mountains and the variation of the desert margin played important roles in changing the characteristics of sediments.Furthermore,the uplift of the Liupan Mountains also contributed to produce the vast amounts of coarse particles.According to the analyses of grain size end-member and the general grain size parameters,the fine particles might be carried by the Westerlies from the deserts,gobi and the sand areas located in the western China,while the coarse particles of CF2019section might be derived from the Tengger Desert and Mu Us Desert as well as the mountain processes of Liupan Mountains（tectonic processes,frost weathering,etc.）.2.The sensitive proxies of winter monsoon and the variation of the desert margin are determined.After comparing the distributions curves of different ranges of grain size levels with the variation curves of deep-sea oxygen isotope and 30°N insolation intensity in July,we choose the>30μm grain size component and EM3 section as the winter monsoon sensitive proxy,and considering the distance of CF2019 section to the northern deserts,we choose the>70μm grain size component to indicate the variation of the desert margin.3.The AMS ¹⁴C ages show that the CF2019 section are the loess-paleosol sequence since the Marine Isotope Stage 2（26.1-0 ka BP）.Depending on the dating ages,we create the age-depth model of the CF2019 section,and then combine the grain size and geochemical proxies to reconstruct the history of paleoclimate changes of this area since26.1 ka BP,which can be roughly summarized as follows:（1）the Last Glacial Maximum（26.1-20 ka BP）,the winter monsoon strengthened during thisphase,the sedimentation rate was also the highest of the whole section,loess deposited rapidly,and the climate conditions were generally cold and dry;（2）the Last Deglaciation（20-11.6 ka BP）,the climate changed rapidly and largely,but the climate conditions were better comparing to the last phase,and become much wetter and warmer after 15 ka BP;（3）the Holocene（11.6-0 ka BP）,the winter monsoon kept weakening in this period,the sedimentation rate lowered significantly,the pedogenesis processes strengthened and paleosol deposited widely especially during the Holocene Optimum（8.6-4.2 ka BP）.The tone of Holocene climate was warm and humid,but there were still several unstable periods existed.Moreover,the CF2019 section also recorded several climate change events,for instance,the Melt-Water Pulse 19 ka event,the Younger Dryas event as well as the cold events around 8.2 ka BP,5.5 ka BP and 2.8 ka BP,indicating that the paleoclimate changes recorded in the CF2019 section are generally consistent with the global climate changes.