Research On The Paleoenvironmental Changes Of Tengger Desert Since The Mid-Pleistocene Transition

The paleoenvironment change of desert is an important part of the global paleoclimate study.The history of paleoenvironment changes in different sandy regions in China not only reflect the influence of global climate on arid areas,but also directly indicates the relative influence of Westerlies and East Asian Summer Monsoon during different time scales.The Tengger Desert is one of the important sandy deserts in China,borders the northeastern Tibetan Plateau and the northwestern Chinese Loess Plateau,located in the interaction zone between the East Asian Summer Monsoon（EASM）and the Westerlies.Previous studies have carried out research on its formation and evolutionary history.Several core records have revealed a history of wet and dry variability at tectonic scales and 400 ka eccentricity cycles.However,the low sedimentation rate of the desert since its formation makes it difficult to record orbital cycles on shorter time scales,limiting the insight into the mechanisms driving the humidity changes in the arid zone.Therefore,obtaining higher resolution sedimentary records in desert has become a key to paleoenvironmental studies in arid zone.Based on a 122.63 m core（WEDP06）in the hinterland of Tengger Desert,paleomagnetism,optically stimulated luminescence（OSL）and ²⁶Al/¹⁰Be burial dating were carried out to determine the dating framework of the core.Lithological observation,grain size analysis,carbonate content and total organic carbon（TOC）content were applicated to determine the sedimentary facies and reconstruct the paleoenvironmental changes.The main results of this study are as follows:（1）The core is divided into three main lithological units from bottom to top:122.63-98.95 m is dominated by yellowish and grayish fine sand layers interbedded with calcareous sand layers;98.95-45.47 m is dominated by grayish fine sand layers interbedded with grayish-brown and grayish-yellow silty clay layers,with clear stratigraphic rotation;45.47-0 m is dominated by yellowish and grayish fine sand layers,with several thin layers of calcareous silt.（2）Magnetic stratigraphic results from the WEDP06 indicate that the Matuya-ma/Brunhes boundary（781 ka）is located at 87.5 m.The bottom age of the core is determined to be～940 ka by orbital tuning in combination with OSL and ²⁶Al/¹⁰Be burial dating.（3）Ten typical humid lithologic layers can be identified in the WEDP06,which are dominated by gray and gray-white lacustrine and swampy isostatic deposits,exhibiting fine grain size,high carbonate content and TOC;corresponding to the marine isotope stages MIS 1,MIS 5,MIS 7,MIS 9,MIS 11,MIS 13-15,MIS 17,MIS 19,MIS21,and MIS 23,respectively.It indicates that the Tengger Desert is influenced by the East Asian summer monsoon during interglacial periods,with increased precipitation and lake expansion.Whereas it becomes arid and cold during glacial periods and develops thick aeolian sand layers.The humidity variability of the desert has distinct100 ka and 41 ka cycles,with the 41 ka cycles dominating from 500-940 ka and 100 ka cycles dominating from 0-500 ka,which is mainly controlled by the north Hemisphere ice volume and the change of tilt.（4）The desert became drier during the interglacial period after 500 ka,as evidenced by increase>125μm fractions and decrease carbonate and TOC content.We suggest that the drying of the desert during interglacials was caused by sand source changes combine with the warmer temperature during interglacials after the Mid-Brunhes Event（MBE）.After 500 ka,the increased supplying of abundant grains from the northeastern Tibetan Plateau,as the Qilian Mountains,leaded to the developing of thick aeolian sand layers in the desert,which could obviously lowered the groundwater table.At the same time,the increased temperature during interglacials after MBE enhanced the evapotranspiration.As a result,the effective water available to plants is decrease,the dunes cannot be incomplete fixed shown as enhanced drought and ecological degradation during the warmer interglacials after 500 ka.