Late Cenozoic Climate Change Of The Northwest China: Evidences From LS2 Core Of Lop Nor

Since the Late Cenozoic,the most important features of the global environment are the rapid step-like cooling,the growth and gradual expansion of bipolar ice sheet towards mid and low Latitude Mountains,the increase of global continent areas,the remarkable expanding of arid areas.Uplift of the mountain is one of the most important driving forces for the foregoing global environmental changes.The uplift of the Tibetan Plateau since the Late Cenozoic especially,has not only led to the intensive tectonic deformation in the inner Asian continent,but also exerted a significant influence on the landform pattern,climate and environmental changes around the Tibetan Plateau,which is a key issue in geosciences research.Taklimakan Desert,situated in Tarim Basin is the largest moving desert as well as the aridest area in China and even Asian.It is the typical example of arid evolution. Lop Nur,which lies in eastern Taklimakan Desert,is a typical arid core and was known as the death center.In addition,it is also the confluence basin of the Tarim Basin,Aerjin Mountains,Tianshan Mountains,the Qilian Mountains and northwest part of the Hexi Corridor.The gradual uplift of the Qinghai-Tibet Plateau directly controls the development of sedimentary environment,deposition cycles as well as evolution history of Tarim Basin.As a result,The Lop Nur,whose stratigraphic sequences faithfully indicate region tectonics,climate and environment features and the arid history since the late Cenozoic is considered to be one of the idealist places to study the history of plateau uplift and aridity in westem China,.Therefore, High-quality(1056.3m)deposition core LS2(all core-hole was obtained more than 95 percent),led by An Zhisheng,in 2003 was obtained in The Lop Nur.This is so far the first high-quality in-land lake sediment obtained in China.It provides complete information for the study of ancient climate and environment changes in the Northwest China as well as the history of aridity and uplift of the Tibetan plateau.Based on analyzing and contrasting about the deposits multiple indexes such as grain-size,chloride and sulfate ions,the environment evolution and the tectonic uplift history of the northern Tibetan Plateau since the late Miocene are reconstructed primitively.Our results show that the aridity in Chinese inland area was probably caused by interaction of Tibetan plateau uplift and Arctic ice sheet since 7Ma.The conclusion is summarized as follows:1.According to the record of the chloride and sulfate ions of Lop Nor LS2, combined with previous studies,we concluded that:Differention ratios in a same lake sediment can reflect the evolutionary phase and history of the lake,because different ions stemmed from different lake evolution stages have different sensitivity on environmental changes.2.Based on detailed indexes' analysis of LS2 core,we clearly divided the 1056.3m long LS2 core into six parts,which are 1056.3-905.7m,905.5-746.25m, 746.05-528.85m,528.65-210.2m,210-16.8m and 16.55-0m respectively.The age sequence of LS2 core(7Ma-0)was established based on the age model of lake sediments as well as grain size,chloride and sulfate ions records of LS2 core.The six climate stages are:7-6.18Ma(1056.3-905.7m),arid period;6.18-5 Ma(905.5-746.25m),intensified arid period;5-3.6Ma(746.05-528.85m), Sub-humid and semi-arid period;3.6-1.2Ma(528.65-210.2m),periodically increased aridity(Lop Nur has experienced four periodic,step-like aridity and 3.3-2.45 Ma is the obvious transition period of the Lop Nur Environmental evolution);1.2-0.14Ma (210.2—16.8m),persistent drought stage;0.14-0Ma(16.55-0m)extreme arid period(Lop Nur shrink progressively during this stage).3.On the basis of detailed study of the deposition rate changes of LS2 core,in combination with the principle of basin-mountain coupling as well as Cenozoic variation features of the basins from Tibetan Plateau Cenozoic era,we suggest that the evolution of Lop Nur correlated well with uplift and deformation of the northern Tibetan Plateau.Nine tectonic uplift of Tibetan plateau,which happened respectively at 6.36-6.13Ma,5.72-5.62Ma,5.39-5.29Ma,3.62-3.39Ma,2.55-2.4Ma,2.17-1.87Ma,0.82-0.62Ma,0.32-0.2Ma,0.15-0.07Ma indicate that the northern Tibetan Plateau underwent an multistage,non-uniform velocity and inhomogeneity complex process during the late Cenozoic era.4.The arid history of Asian interior and tectonic uplift of Tibetan plateau,which are reflected by environment evolution of Lop Nur Since 7Ma are directly linked to the formation and expansion of the p Arctic ice sheets.All this analysis suggest that the development of the aridity in Asian inland since 7 Ma might be caused by combined action between the Tibetan Plateau uplift and the extension of the Arctic ice sheets.5.The early pliocene humid event(5-3.6Ma)was probably caused by higher global sea surface temperature and higher sea level during that time.