A Comparative Study On Climate Instability Recorded By Loess In The Asian Westerlies And Monsoon Regions Since The Last Interglacial Period

The climate instability since the last interglacial is the focus of global change research.The study of climate instability is of great scientific significance for understanding the atmospheric circulation model and the climate response mechanism between regions.As a transitional zone between the North Atlantic region and the Asian monsoon region,Central Asia is strongly influenced by the Westerlies and very sensitive to the climate change.The loess deposits distributed widely in Central Asia preserve abundant paleoclimate information and are ideal archives for the study of climate instability.In this study,the grain sizes and magnetic susceptibility of the Daraikalon?DK?loess section located in Central Asia and the Caoxian?CX?loess section in the western part of the Loess Plateau were measured.On the basis of previous dating,the climate instability events recorded by the indicators were identified,and the degrees of response to the North Atlantic climatic instability were compared in central Asia and monsoon regions.Through the comparisons,the role of Westerlies was discussed in the process of propagating climate instability signals.The main conclusions are as follows:The grain sizes of loess are poorly sorted in Tajikistan and the Chinese Loess Plateau,showing the characteristics of fine deviation and normal peak state.The results of magnetic susceptibility show that DK profile have much higher value than that of CX section,which indicates that the pedogenesis of DK section is stronger.End-member?EM?analysis for the two sections shows that the particle sizes are more suitable for being fitted to 4 EMs.By using the parameter-end-member method,the four EM modes of the grain size of loess deposits in Tajikistan are 1.1?m?EM1?,5.1?m?EM2?,11.3?m?EM3?,20.5?m?EM4?,respectively.The EM1 are mainly controlled by the pedogenic processes.EM2 and EM3 are the particle size components by the Westerlies transporting.EM4 probably belongs to the dust storm component under the influence of polar cold air mass.The results of EMs of loess in CX section are 0.9?m?EM1?,6.6?m?EM2?,29.6?m?EM3?and 51.0?m?EM4?,in which EM1 and EM2 represent pedogenic components and Westerlies transporting components respectively,while both EM3 and EM4 are dust storm components under winter monsoon.The Westlies index about relative variation of Westerlies circulation with other circulation systems is obtained by using the ratio:W=EM??/?EM2+EM3+EM4?.Based on previous dating results,the Westerlies index?W?and frequency-dependent magnetic susceptibility(?_fd)of DK were compared with the NGRIP,and the results showed the indexes of DK section detailedly characterize the millennium-timescale signals during last glacial.And those climate fluctuations are superimposed on the variation tendency of global ice volume.At the same time,there is an inverse correlation between W and?_fd,which is probably because the variation of humidity in Central Asia was mainly controlled by the sea surface temperature in the North Atlantic region,while the cold events recorded by the W were caused by the Westerlies wind strength.The W-recorded cold events are caused by the Westerlies belt moving southward and the Westerlies strength increasing.The Westlies indexes show that the millennium-timescale events of the last glacial period in CX section are also very obvious,especially the H events.However,contrary to the DK profile,the Westlies weakened when the H events occurred.This is probably due to the southward shift of the westerly belt and the strengthening of the winter monsoon during the cold period.On the basis of the chronological model,it can be found that the DO cycles recorded by the W of DK section are clearer than that of CX section,which indicates that Westlies plays an important role in the transmission of the millennium-timescale signals.The DK section did not record obviously the climate fluctuations in last interglacial period,it is very likely that pedogenesis,Westerlies weakening and orbital factors covered up the millennium-scale signals.The?_fd and W of CX section show that the climate fluctuations were obvious in the last interglacial,and the fluctuations recorded in the two indexes are consistent.It is inferred that the climatic fluctuations in the last interglacial recorded by W of CX profile are closely related to the summer monsoon of East Asia.