The Response Of Reversing Dune Morphology And Sedimentology To Seasonal Wind Regime

Wind is the dynamic factor in shaping the dunes.The formations of reversing dunes or the appearances of reverse aeolian sand accumulation are influenced by seasonal wind regimes with two winds that blow from opposite directions.Seasonal alternation of wind regimes not only reshape dune morphologies and internal sedimentary structures,but also reorganize grain size compositions.The studies on feedback mechanisms among dune forms,airflow,dune sands and sedimentary structures are helpful to understand dune formation and evolution,and reveal the response of dune features to seasonal wind regimes.Based on wind data analysis,field observation,grain-size analysis,dune drone images reconstruction and numerical simulation,this paper investigates the morphologies,grain-size characteristics and internal sedimentary structures of reversing dunes in Mu Us Desert and reveals their formation and evolution mechanism under the influence of seasonal wind regimes,and primary understandings are as follows.Under the influence of the East Asian monsoon system,the near-surface wind regimes are characterized by the seasonal variation of strong northwest winds in spring and weak southeast winds(reversing winds)in summer.As northwest winds dominate,barchan dunes or transverse dunes usually develop.As reversing winds dominate,former lee sides are subjected to wind erosion,their slopes become gentler and sand compositions were reorganized from fining to coarsening.In contrast,former stoss sides experience wind deposition,which cause the formations of reversing slip slopes and grain fining characteristics.In addition,internal sedimentary structures of reversing dunes are composed of high-angle(30°)cross-strata and low-angle(10°)bounding surfaces.The dip angles of the bounding surfaces gradually decrease from the bottom to the top,indicating the wind erosion intensity increases with height caused by speed-up effect.Therefore,the change of erosion and deposition patterns on dune surface induced by seasonal wind regimes plays a critical role in dune morphological transformation,grain-size composition and internal sedimentary structure.Based on field observation and numerical simulation,the morphological and sedimentary features of reversing dunes were investigated under the influence of seasonal wind regimes,and their formation and evolution processes were revealed.Furthermore,our results strengthen the understanding of aeolian geomorphologies classification and their environments,and provide a basis for sand-fixing and investigating the atmospheric circulation under the effect of East Asian monsoon system.