Variation Of Asian Summer Monsoon Intensity In Late MIS6 Derived From Grayscale And Mg/Ca,Sr/Ca,Ba/Ca Records Of A Stalagmite In Wanxiang Cave,Eastern Qinghai-Tibet Plateau

As an important reference body of the current and future Earth's climate system,the study of paleoclimate evolution and its change mechanism is conducive to better coping with the current climate challenges.The Asian monsoon is an important part of the global climate system,and changes in its temporal distribution can lead to changes in the distribution of heat and water resources within the Asian monsoon region,which affect the lifestyles of nearly half of the world's population.Therefore,the study of the evolution of the Asian monsoon paleoclimate has always been the focus of global paleoclimate change research.The late MIS6 period belongs to the penultimate glacial period,and its climatic background state is very different from today,and the study of the evolution of the Asian monsoon in this period can allow us to deeply understand the evolution characteristics of the monsoon under different boundary conditions,which will help us to more fully understand the evolution mechanism of the Asian monsoon.However,there are still insufficient studies on the evolution of the Asian monsoon in the late MIS6 period,mainly due to the problems of less research records in the relevant period,lower resolution of existing data records,and relatively single data indicators.As the fourth pillar of paleoclimate record after ice cores,loess and deep-sea sediments,stalagmites have their unique research advantages and are widely used in Quaternary climate reconstruction.Wanxiang Cave is located on the edge of the modern Asian monsoon region,and the stalagmites in the cave can sensitively record the changes in monsoon strength,and is an important cave base for studying the evolution of asian summer winds.Based on 4 U-Th isotope ages,1118 grayscale scans,and1071×3×2 sets of Mg/Ca,Sr/Ca,Ba/Ca trace element ratio data of Wanxiang Cave stalagmite(number WXZ05-10),the sequence of high-resolution grayscale and trace element ratio in late MIS6 was established.By comparing the grayscale sequence with the trace element ratio sequence with other geological records,the climatic significance of the ratio of grayscale to trace elements of stalagmite WXZ05-10 was revealed;the evolution process of the Asian monsoon in late MIS6 was discussed according to the variation characteristics of the high-resolution grayscale to trace element ratio sequence;the monsoon enhancement event reflected by stalagmite WXZ05-10 was marked;and finally,the driving factors of the influence of late MIS6 on climate change were discussed by spectral analysis.The main conclusions are as follows:(1)The grayscale record of the Wanxiang Cave stalagmite WXZ05-10 can indicate changes in Asian summer monsoon intensity.The intensity of the monsoon increases,the greater the gray value;the reverse is also true.The changes in Mg/Ca,Sr/Ca,and Ba/Ca on the centennial scale mainly reflect the changes in monsoon rainfall,the monsoon rainfall increases,Mg/Ca,Sr/Ca,Ba/Ca values decrease,and vice versa,representing the control mechanism of prior Calcite Precipitation(PCP);at the millennium scale,the ratio of trace elements is affected by monsoon precipitation,and is also affected by the regional environment.The influence of multiple factors,such as karst systems,may represent the intensity of surface chemical weathering in the region.In addition,at different stages of stalagmite growth,the control mechanism of trace element ratio may also vary due to changes in the climate background state.(2)The evolution of Asian summer winds in late MIS6(143155-139067 yr B.P.)can be divided into two stages: in the I stage(143155-140415 yr B.P.),the Asian summer winds showed an enhancing trend,but there were four complete enhancementattenuation rotations in the strengthening process.During Phase II(140415-139067 yr B.P.),the H12 event led to a weakening of the Atlantic meridional overturning circulation,a southward shift of the westerly and tropical convergence belts,and a significant weakening of the Asian monsoon.(3)There were five centennial monsoon enhancement events in the late MIS6period(143155-139067 yr B.P.),the event number and time of occurrence were: B4.2(142986-142732 yr B.P.),B4.1(142190-141935 yr B.P.),B3.3(141604-141411 yr B.P.),B3.2(141154-140939 yr B.P.),B3.1(140210-139550 yr B.P.).The first four events occurred in Phase I of monsoon evolution.As the duration of the four monsoon enhancement events gradually decreased,it was also clear that the time interval between the events was getting shorter and shorter,similar to the Chinese interstadial(CIS)event in China.(4)The gray level of WXZ05-10 and Mg/Ca,Sr/Ca,Ba/Ca in Wanxiang Cave stalagmite is influenced by multiple cycles on decadal-centennial scale.The main significant periods are ?700 a and ?500 a.The 700 a cycle has been recorded in some Marine and lake geological vectors,but the driving mechanism of this cycle remains unclear and needs further study.The ?500 a cycle is mainly driven by thermohaline circulation.In addition to the main cycle,there are also some non-major cycles at the decadal scale and ?200 a time scale,which are mainly controlled by solar activity and Pacific decadel oscillation,and may also be regulated by natural variability within the monsoon system.