End-Member Model Analysis Of Loess Grain-Size In The Eastern Tibetan Plateau And Its Environmental Implications

Aeolian loess is the result of multiple sedimentary processes,including different material sources,multiple transport forces and complex sedimentary environments,which will sort the dust and mix the different grain-size populations.Loess is widespread in the eastern Tibet Plateau(ETP).Due to the unique geographical environment and atmospheric circulation conditions,it recorded paleoenvironmental informations of the ETP,which can be deciphered through the End-Member decomposition.Moreover,End-Member analysis of the regional loess can identify the transport processes and the material sources of the loess.In this paper,the grain-size characteristics of loess in the ETP and two loess sections are studied by the traditional grain-size methods and the grain-size End-Member methods.The spatial and temporal changes of loess grain-size and its environmental implications in this area are explored,which is significant to understand the paleoenvironment and air circulation changes in the ETP since the Last Glaciation.Some main conclusions are drawn as follows:(1)Five aeolian End-Members(EM)are obtained by the parameterized End-Member analysis of the loess grain-size in the ETP,which can be divided into: the fine-grained EMs which are transported by the high altitude air flow chronically,its main wind force may be the westerly;the coarse-grained EMs that are suspended by low altitude near surface wind intermittently,its main wind force may be the plateau winter monsoon;and the sand EM produced by the influence of non-aeolian coarse population after deposition.The loess grain-size in the ETP is different from that of the Chinese Loess Plateau;loess in the ETP is greatly influenced by source area,it has lower degree of Sorting property,more coarse-grained material,closer and more complex material source.The difference of loess grain-size in different areas of ETP also reflects the influence of sedimentary factors on loess sedimentation.(2)The loess grain-size EM characters of the Ganzi(XS)section and the Jinchuan(MC)section are consistent with the EM results of loess in the ETP area,thus those can make analogies.The dominant loess grain-size population of the Ganzi loess is the EM4(the mode size is 63 um),which may indicate the Ganzi area is mainly influenced by the plateau winter monsoon.While the dominant loess grain-size population and sedimentary contribution population of Jinchuan loess is the EM2(7 um),which may indicate the plateau winter monsoon influence the Jinchuan area weakly.The difference of grain-size dominant populations also shows that the two sections have different recording effects on regional circulation change since the Last Glaciation.(3)The proportion of EM in the ETP reveals the westerly has a large fluctuation in the stadial epoch and is stable during the interstadial epoch of the Last Glaciation;it indicates the continuous enhanced trend of the plateau winter monsoon in the ETP and the high correlations between the climate system of high latitude region and the Tibet Plateau.(4)The loess grain-size is closely related to the air circulation characteristics and topographic features of in the ETP,which makes its dominant grain-size populations and the recorded air circulation changes information difference in different regions.Further detailed work on loess grain-size in the western Sichuan Plateau are needed for better understanding the history and mechanism of the environment changes in the ETP.