| The Badain Jaran Desert（BJD）,with an area of 5.2×104 km2,is the second largest and located at the western of Alxa Plateau in China.The BJD is famous around world for the more than 105 lakes and thousands of mega-dunes in its southern and hinterland.Climatologically,the BJD is located at the edge of the East Asian summer monsoon and is a typical arid zone of the monsoon-westerly transition zone.The precipitation is little in the desert and surrounding areas,and the lakes in the desert are recharged by groundwater.In recent years,the lakes in the desert and surrounding areas are shrinking and drying up with the global warming and increasing aridity.How will the desert environment evolve in the current climate context.Understanding the evolution pattern of desert environment and the effect factors is of great significance to regional ecological environment management and government planning.In order to solve the above problems,the southern margin of BJD was selected as study area,which are more sensitive to climate change.And the research on paleohydrologic-environmental evolution was carried out.A series of alluvial-flood deposits occur on the southern edge of the desert,which record the hydrological processes and climate change.The alluvial-flood deposits profiles and borehole core were selected as the object of study.Sedimentary facies of the deposits were analyzed by field sedimentary characteristics analysis,grain size analysis and scanning electron microscopy of the Quartz in the deposits.The alluvial-flood are mainly composed of channel deposits and slack-water deposits.And,the features of channel deposits and slack-water deposits are systematically summarized.The age of the alluvial-flood deposits on the southern edge of the desert were determined by using optically stimulated luminescence（OSL）dating.Then,the formation mechanism and climate background of alluvial sediments were analyzed and its hydrogeological significance for groundwater replenishment in desert regions was discussed.For the lacustrine deposit in the core BD06,the age sequence was established by using 14C dating.The evolution history of the lake in the south margin of desert was reconstructed since15.5 ka by analyzing the environmental proxies,such as total organic carbon,grain size analysis,geochemistry analysis and mineral composition.By comparing the paleohydrological-environmental evolution history recorded by overbank deposits and lacustrine sediments around the southern margin of the BJD with other desert palaeoenvironmental evolutions in northern China,we explored the similarities and differences of paleoenvironmental evolution in the BJD during the Late Quaternary.The main results and conclusions of this study are as follows:（1）The alluvial-flood deposits are mainly exposed in the depression or low-lying areas among mega-dunes in the southern edge of the BJD.The alluvial-flood deposits are mainly composed of channel deposits and slack-water deposits.The former is mainly composed of medium sand and coarse sand with parallel bedding,fine gravel and argillaceous fragments.The latter is mainly composed of silty silt and silty silt,forming multiple cycles,with the characteristics of mud cracks,erosion surface and horizontal bedding.（2）Based on the OSL dating results in this study and the dating ages of the previous studies,the alluvial-flood deposits are mainly formed in the humid climate period of glacial-interglacial cycle.A small amount of channel sediments also developed in relative dry climate.The alluvial-flood deposits are mainly formed in the Middle Holocene period with relatively humid regional climate in millennial scale since the deglaciation period.（3）Based on the location,sedimentary features and ages of alluvial-flood deposits,we proposed the groundwater recharge model of the intermittent precipitation recharge in the mountainous area around the desert:When the climate is humid or the rainfall concentration season,the rainfall in the desert edge or the adjacent mountains（such as Beida Mountain）forms intermittent rivers or local floods,which converge at the edge of the sand mountain along the direction of terrain decline,and generate runoff that permeates through the surface detrite deposit or aeolian sand deposit to the desert hinterland for groundwater recharge.（4）The Badain Lake in the BJD edge was formed in the last deglaciation period（15.5 ka）.According to the changes of hydrological environmental indicators,the lake evolution can be divided into the following stages.During the last deglaciation period,the lake has a lower water level with saline water.In the Early and Middle Holocene,the lake level rose and the water was desalted.Around～7 ka,the lake experienced a decrease in water level and water salinization about one millennium.After～5 ka,the lake began to shrink gradually.Based on comprehensive analysis,we suggest that the formation and evolution of desert lakes were affected by meltwater during the deglaciation and Early Holocene;The lake high water level in the Middle Holocene was due to the forward movement of East Asian Summer Monsoon;After～5 ka,the East Asian summer monsoon subsided and the lake gradually shrank.（5）Compared with other deserts in North China,the hydrological-environment evolution of the BJD during late Quaternary is consistent with the most deserts in the orbital scale.And,the millennial-scale evolution since the last deglaciation is more influenced by the replenishment of ice and snow meltwater in the recharge area and the advance and retreat of the East Asian summer monsoon,which is different from the deserts in the westerly wind region（Xinjiang region）and other deserts（sandy land）in the edge of the East Asian summer monsoon. |
