Geochemical Characteristics Of Loess-like Materials In The Nujiang Valley And Their Environmental Evolution Implications

The climate characteristics of Southwest China monsoon are relatively special.How Southwest China monsoon responds to global climate change since the Last Glacial Epoch,and what is the difference mechanism between Southwest China monsoon and global climate change,is worth further study.The results of previous studies show that the Nujiang River valley was formed in the Quaternary,and a certain amount of Quaternary aeolian sediments are distributed in Lujiangba,which is located in the middle and lower reaches of the Nujiang River valley,which provides a basis for studying the evolution of southwest monsoon in the Quaternary.In this paper,we selected loess-like sediments deposited in the Nujiang River valley,analyzed the mineral composition,geochemical elements and other environmental proxy-indices of the samples through field investigation and sample collection,discussed the formation age of loess-like materials in the Nujiang River,and reconstructed about 38 ka B.P.The evolution history of palaeoclimate environment since.The main research results are as follows:(1)According to the photoluminescence dating results of loess-like materials in the Nujiang River Valley,the age of the bottom section is about 38 ka B.p.,which is equivalent to the third stage of the Middle and Last Glacial Age of Late Pleistocene(MIS3b).(2)The loess-like materials in the Nujiang River Valley are in the initial stage of weathering(de-Ca and de-Na),which may be related to the climatic environment of the dry-hot valley.(3)according to the nujiang river valley loess substance and jinsha river valley over the standardization of sediments,luochuan loess,and the upper crust of trace element contrast values,similar to the distribution curve,showed that the nu river valley loess substance before deposit experienced fully mix homogenization,to represent the upper continental crust material,should be eolian origin.(4)The climate and environment in the Nujiang River Valley have experienced five obvious stage changes since 38 ka B.P.:38?36.4ka B.p.(late MIS3b): the temperature gradually rises and the precipitation is less;The climatic conditions are cold and dry;36.4?26.5ka B.P(MIS3a.):During this period,the precipitation increased and the temperature was relatively higher.The climate in this period was warm and humid as a whole.26.5?19.6ka B.P.(LGM):the precipitation decreased,the temperature was low and the climate was cold,which was in good agreement with the dry and cold environment of the global ice peak stage.19.6?11ka B.P.(the transitional period from the Last Glacial Age to the Holocene):the temperature showed a trend of gradual rise,with less precipitation,and the climate changed from cold-dry to warm-dry.11?4 ka B.p.(Early to Middle Holocene):The precipitation intensity in this period was relatively weak,and the climate was warm and dry.