| Aeolian sediments with good stratigraphic continuity and contrast are commonly used as proxy records in paleoclimatic studies,as they have relatively simple formation mechanisms and clear environmental implications.Especially in glacial-interglacial cycles,aeolian sediments show good correspondence with paleoclimatic changes.The Tibetan Plateau,as the “driver” and “amplifier” of the global climate system,has a significant effect on the global climate system through its plateau effect.However,the aeolian sediments widely distributed in the Qinghai Lake Basin in the northeastern Tibetan Plateau are often affected by sedimentary discontinuities in previous studies based on these sediments to reconstruct the paleoclimatic evolution in this region,which makes it difficult to establish a high-resolution chronology and to reconstruct the millennial-scale climate change history.Therefore,more continuous aeolian profiles are needed for research.In this paper,we selected the HK aeolian profile in the southeast of Qinghai Lake for high-density optically stimulated luminescence(OSL)sampling.Based on the different sedimentary properties,we sampled more densely in the paleosol layers.In the 5.8 m thick aeolian profile,we collected 31 OSL samples and one charcoal sample for radiocarbon dating as a verification material.We established a millennial-scale chronology by using the undatable age-depth model and analyzed the sedimentary history of the HK profile.We found that this profile is a continuous aeolian profile with a millennial-scale sedimentation.We also collected 276 environmental samples at intervals of 2,5,and 10 cm for grain size parameters,magnetic susceptibility,elemental geochemistry,and total organic carbon(TOC)analyses.We obtained sensitive aeolian end-members(EMs)by using the non-parametric end-member modelling analysis(EMMA)and compared them with published paleoclimatic proxy records.We reconstructed the millennial-scale climate change and aeolian activity history in the Qinghai Lake Basin and the expansion and contraction history of the Langmaxiexiang sand field.The main conclusions are as follows:(1)We performed high-density OSL dating on the aeolian sediments in the Qinghai Lake Basin and compared them with the radiocarbon ages in the strata.The results showed that the OSL ages in this study are reliable.We combined the undatable age-depth model with the OSL ages and analyzed the sedimentary history of the HK profile.We found that this profile is a continuous aeolian profile with a millennial-scale sedimentation.(2)The grain size parameters,chemical index of alteration(CIA)and index of compositional variability(ICV),low-frequency magnetic susceptibility and TOC of the HK profile are good climatic proxies,which can indicate the historical aeolian activity intensity,weathering degree,pedogenesis intensity and vegetation cover of the aeolian sediments.(3)We compared the ages of the HK profile with those of other aeolian sediments in the Qinghai Lake Basin and found that the aeolian sediments in the Qinghai Lake Basin mainly formed after the late last deglaciation.The aeolian sediment accumulation rate was the highest during 10-8 ka,reflecting strong aeolian activity.The late last deglaciation-early Holocene was mainly the accumulation period of aeolian sand and loess,and the mid-Holocene was the development period of paleosol.(4)We reconstructed the climate change in the Qinghai Lake Basin over the past 13.5 ka and divided it into four stages:(1)13.5-11.7 ka,cold and dry climate,strong aeolian activity,possibly influenced by the BA and YD events;(2)11.7-7 ka,climate fluctuation,enhanced soil development and chemical weathering,weakened aeolian activity,with three cold events(8.9 ka,8.2 ka and 7.7 ka).During this period,the changes of CIA values,low-frequency magnetic susceptibility values and other environmental indicators of the profile samples were basically consistent with the changes of East Asian winter monsoon intensity,so we inferred that the Qinghai Lake Basin was mainly influenced by the East Asian winter monsoon.(3)7-4 ka,this period was characterized by a warm and humid climate,increased biomass,stagnant aeolian activity,enhanced pedogenesis,and mainly influenced by the East Asian summer monsoon;(4)after 4 ka,the sand content increased continuously,indicating an intensification of aeolian activity in this stage,especially after 2 ka,the CIA value,low-frequency magnetic susceptibility value and other environmental indicators decreased rapidly,suggesting a cold and dry climate in this stage.Meanwhile,two important climatic events were identified in the change trend of the CIA value of the aeolian deposits: the MWP warm event and the LIA cold event.(5)Using the non-parametric grain size end-member analysis method,the aeolian deposits of the HK profile were divided into seven end-members(EMs),and the transport modes of each end-member were simulated by probability cumulative curves.It was found that EM3-7 reflected the expansion process of the Langmaxiegang Sandy Land,indicating that the period of maximum expansion of the Langmaxiegang Sandy Land since the late Last Glacial Period was 12.5-11.5 ka and 8.8-8.4 ka.(6)Compared with previous studies,it was found that the climate change process of the Qinghai Lake basin indicated by the low-frequency magnetic susceptibility value and sand content of the HK profile in the early Holocene was consistent with the TRACE21 model reconstruction of the East Asian winter monsoon intensity based on orbital radiation,atmospheric greenhouse gases,continental ice sheets and other climatic forcing factors.The regional climate change process indicated by the CIA value of the profile in the middle and late Holocene was strongly consistent with the reconstruction of the East Asian summer monsoon intensity using the pollen record of the Gonghai Lake lacustrine sediments.Therefore,it can be inferred that the precipitation distribution pattern and climate change of the Qinghai Lake basin since the late Last Glacial Period may be related not only to the evolution of the East Asian winter and summer monsoons,but also to the regional change characteristics of the Tibetan Plateau,mainly manifested as the amplification effect of the plateau effect on climate change,which is conducive to the identification and analysis of key climatic abrupt events. |
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