Changes In Eastern Asian Winter Monsoon Recorded By Aeolion Deposits At The Southeastern Margin Of Qaidam Basin Over The Past 5000 Years

Xiaxitai section(36°18'19"N,98°03'26"E),at the southeastern margin of the Qaidam Basin,is situated at the border of Dulan-Tiegui desert and 3 km far from Xiaxitai village.In the study area,mean annual precipitation is 179 mm,mane annual temperature is 2.9℃and the number of strong wind day per year is 31.6.Strong winds always appear from March to May.The average number of dust storm day is 9. The section is near to desert so that the sedimentation rate is high and the role of weathering to soil is weak.Therefore,the aeolian deposits can sensitively reflect changes in strength of near-surface wind fields in the study area.Qinghai-Tibetan Plateau,one of the dust source regions in Asia,plays an important role in long-range dust transportation.The Qaidam Basin is a major part of dust source area in Qinghai-Tibetan Plateau.In order to reconstruct winter monsoon evolution and understand the role of near-surface wind regime for atmospheric dust circulation, aeolian deposits in the dust source area are worth being investigated.The height of Xiaxitai section is 3.52 m.According to the dating results of(14)~C and OSL,temporal sequence of the Xiaxitai section was established.Based on grain size and elemental compositions of the aeolian deposits from the Xiaxitai section, variations in winter monsoon across the study area during the past 5000 years were investigated.In summary,the winter monsson evolution can be divided into four stages.Winter monsoon was very weak during 5300-4300 a BP and 2900-930 a BP; whereas during 4300-2900 a BP and 934 a BP to present it was rather stronger.Two abrupt events of winter monsson changes are recognized from the reconstructed sequence.One is that winter monsoon became weaker rapidly during 3960-3720 a BP; and the other demonstrates that winter monsoon was intensified dueing 1700-1400 a BP.The paleoclimatic records from Qinghai Lake and Gahai Lake indicate that a wetter climate prevailed during 5300-4 300 a BP,which is quite in agreement with the paleosol development in the bottom of the Xiaxitai section.This stage represents the end of the warm and humid climate pattern during early to mid Holocene demonstrated by many records.During late Holocene,climate of the surrounding areas became dry and cold,which perhaps is related to the stronger winter monsoon reflected by the aeolian deposits of the Xiaxitai section.Generally speaking,the results presented here are comparable to other records.The weak near-surface wind regime during 2900-930 a BP is most likely chatacterized of the local climate.Comparison of the variations in winter monsoon and the changes in the microparticulate concentration of Dunde ice core and nssK~+ of GISP2 ice core suggests that changes in the dust concentrations documented by ice cores was probably affected by winter monsoon.Near-surface wind regime in dust source areas may play an important role in global atmospheric dust circulation.Similarities between the strength of near-surface winds in the dust source regions and thenssK~+ flux of the Greenland ice core suggest that the Qaidam Basin may be one of source areas in eastern Asian for dust long-range transports.