Late Quaternary Loess And Environmental Records In The Yarlung Tsangpo Catchment

Tibetan Plateau(TP),kown as the water tower of Asia,the roof of the world,is the third pole of the earth.Loess,distributed widespreadly on the TP,is the product of long-term interaction between lithosphere,atmosphere,hydrosphere,cryosphere and biosphere.On the one hand,the climatic and environmental effects of dust aerosol have a profound impact on the plateau glacier change and the global climatic and environmental changes.On the other hand,loess deposition has provided a suitable habitat for human activities and is an important carrier to reveal the synergistic effect of prehistoric man-earth relationship on the TP.The study of loess and environmental changes on the TP in late Quaternary has become a hotspot in the field of geoscience.At present,there are only a few studies on loess from the TP,and mainly in the northeart of the TP.For the Yarlung Tsangpo River(YTR)catchment in the south of the TP,the study of loess is relatively scarce.The aeolian deposition and emission patterns in the basin and their provenance relationship with dust in the surroundings of the TP are not clear,and the dust emission potential of the valley has not been evaluated.The sedimentary chronology,spatio-temporal differentiation,climatic and environmental response and driving mechanism of aeolian deposits in the valley also need to be further studied.These studies are a necessary prerequisite for desertification control and ecological environment restoration in the YTR catchment,and are of great significance for the protecting of the ecological security barrier of the TP.In this research,the sources of aeolian materials in the YTR basin were systematically studied by using single mineral(detrital zircon),major/trace elements and Sr-Nd isotope.The model of aeolian deposition and dust emission in the YTR valley and its relationship with dust sources in the surroundings of the TP.At the same time,In order to clarify the driving mechanism of climatic environment behind the history of aeolian sediment activity recorded in the YTR basin,the OSL/p IRIR dating and environmental record reconstruction were used to study the aeolian sedimentary age,environmental response and Indian Monsoon variations.The main conlusions are as follows:1)The aeolian sedimentary process in YTR catchment is restricted by many factors,such as local climate environment,landform and underlying surface condition,forming a"dual"sedimentary structure model of loess on the top and aeolian sand on the bottom of the same valley.The geomorphic units of eolian deposits are very different in different valleys,which is a comprehensive natural process of regional climate change superimposed on local natural environment factors.The aeolian parent materials in this area are mainly derived from felsic bedrock debris of Lhasa block,which is in the early stage of chemical weathering.Due to the existence of complex structure,landform and intense surface processes,the geochemical characteristics of the different types of the surface sediments from different valleys show spatial differentiation.The results of single mineral,major and trace geochemistry and Sr-Nd isotope indicate that the aeolian deposits in the YTR valley are mainly derived from fluvial sand,supplied by near-source detrital components from local regions,which belongs to regional self-circulation sedimentation model.2)Significant isotopic fractionation of Sr-Nd between the<75?m and<5?m components of the surface sediments is not existing in the YTR Basin,which is not similar to the dust isotopic composition of the arid and semi-arid regions in the northern margin of the TP,indicating that the YTR basin is unlikely to receive a large amount of dust input from the deserts of Central Asian and other surroundings of the TP.The aeolian deposition and dust emission of the YTR catchment belong to regional self-circulation and in-situ release model,respectively.The Sr-Nd isotopic composition of surface sediments and dust in the YTR catchment is similar to that of the paleo-dust in the North Pacific Ocean,indicating that the regional dust can be transported to the Japan Sea and the North Pacific along with the westerly jet,and then contribute to the dust cycle in the northern hemisphere.Modern dust emission in the YTR valley mainly occurs in spring from March to May,and about 3?5%of dust is transported to the areas outside YTR catchment,with unit emission of 28.9?49.2 t/year·km~2.3)The aeolian deposits in the YTR basin are mainly distributed in the geological period after LGM,and mainly developed in different periods since the Late Glacial Period(?15 ka BP).However,the aeolian deposits in different valleys have spatio-temporal differentiation.The aeolian sediments in the basin since LGM are controlled by regional and global paleoclimate environment,and fluctuated greatly.The aeolian sediments in different periods have different signaling effects on the solar radiation in su?mer at 30°N,Indian Monsoon and westerly.However,it has a good record of global cold events,such as YD.The Aeolian sedimentary process of YTR catchment not only has synchronously respond to global paleoclimate,but also shows regional complexity.4)The Holocene aeolian sedimentary records in the middle and upper reaches of the YTR valleys have obvious spatial differentiation characteristics.The records of aeolian sediments in the mid-lower reaches of the Mainling valley show that the holocene suitable period is 8.3-4.0 ka BP,and its variation trend is consistent with the Indian Monsoon and solar radiation of 30°N in su?mer.The aeolian sedimentary profile in the Saga valley in the upper reaches of the YTR shows that the climate climate has gradually become wet since 8.3 ka BP,which is contrary to the trend of Indian Monsoon and solar radiation in su?mer at 30°N,indicating the characteristics of westerly wind in mid-latitude.Therefore,since the middle Holocene,the Indian Monsoon has a limited control over the upper reaches of the YTR through"Up-Over"mode.And the YTR catchment may be an ideal region to study the variations of Indian Monsoon and westerly wind strength and their synergistic processes.