| Lake sediments are one of the prime archives for recording the past global climate and environment evolution.The studies on the lake sediments play an important role in many aspects,such as reconstructing the paleoclimatic evolution, finding the mechanism of driving climate change about some area,and exploring the relationship between human activities and nature processes,etc.Lake Lop Nur is located in the eastern Tarim Basin,which is the lowest part of the basin.In the periods of history it was the end point of several main rivers and gestated the famous "Loulan Civilization",which made it had been concerned by many scientists and explorers since 19th century.Being one of the aridest regions in China,Lop Nur lake basin receives precipitation of only 22mm annually with annual mean evaporation of 3000mm.The region is sensitive to global changes owing to its special location at the transition zone is affected by westerlies and monsoon circulation.Therefore the sediments in such an enclosed basin can provide good archives for paleoenvironmental or paleoclimatic changes.A late Pleistocene climate variability is inferred from the stratigraphy and geochemistry of sediment core taken from the Luobei depression(91°03′E,40°47′N), lake Lop Nur.Analyses,including magnetic susceptibility,granularity,chroma, carbonate contents,loss on ignition,trace element geochemistry,stable isotope and pollen analysis,were performed to reconstruct the environmental evolution of the area during 32-9 ka BP,which covers the special period from the last glacial maximum (LGM) to the early Holocene.The chronology is provided by the uranium/thorium disequilibrium dates.(1) The results suggested four paleoclimatic stages,which indicated the glacial variations between cold-humid and warm-arid environment.A period of extreme humidity,attributed to LGM,occurred at 31,900-19,200 yr BP.The period was followed by a warm-arid episode at 19,200-13,500 yr BP.Then a cold-humid interval at 13,500-12,700 yr BP may generally correspond to another cooling phases in high latitudes of the Northern Hemisphere.The last stage showed the obvious trend of turning warm and arid from 12,700 yr BP.(2) This work will give some help for understanding the relationship between human activities and nature processes for the arid tendency before dramatic human activities appeared. (3) The application of multivariate statistical methods to compositional variables is useful in interpreting geochemical data.Factor analysis and principal component analysis have been performed.The first principle component is refined as the warm-dry index,which is used by comparison with the isotope records of Greenland ice core and Hulu cave stalagmite.The climate variability of this district was restricted by global change,suggested by correlation with Dansgaard/Oeschger, Heinrich events and Younger Dryas event,which were observed in northern high latitudes.The synchroneity of the paleoclimatic events suggested that cold air activity in northern high latitudes was the most important factor to impact the climate evolution of this region.(4) A candidate mechanism involving immigration of westerly winds is tentatively proposed to account for this synchroneity.Lop Nur is a typical region affected by westerlies.In glacial period,the cold air activity at the high latitudes of the North Atlantic region has fundamentally influenced the westerly winds through pressure systems such as Mongolia high and Siberian high.The intensification of these two systems and southward shift of the polar front,which were consistent with the increase in global ice volume,induced the southward migration of westerlies and brought rainfall in Xinjiang.The continent-ocean-atmosphere interaction shows that the earth is an integrated climate-environmental system.The evolution of Lop Nur climate,which mainly reflects precipitation and temperature,is a result of the migration of westerly zone and changing intensity.It is closely related to the behavior of global climatic system.
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