Aeolian Deposits On The Southeastern Edge Of The Mu Us Sandy Land:Implications For Climatic And Environmental Changes During The Late Quaternary

The Mu Us Sandy Land is located in marginal areas dominated by the Asian summer monsoon,and thus ecological environments of this region is fragile and very sensitive to climate change to the sand land is adjacent to the Chinese Loess Plateau(CLP),constituting one of the most typical desert-loess transitional zones in northern China.Strata consisting of well-sorted aeolian sand,loess and/or paleosol are widely distributed in this area,which recorded regional climate changes and the history of the sandy land as well.Therefore,this region is an ideal region to study the spatiotemporal changes of the desert-loess boundary zone.In this study,two aeolian profiles(sections BJ and JJ)on the southeast margin of the Mu Us Sandy Land were investigated.Based on the Quartz Optically Stimulated Luminescence(OSL)and the K-feldspar post-IR Infra-Red Stimulated Luminescence(pIRIR)dating and stratigraphic variability,in combination with climate proxies,i.e.,grain size,magnetic susceptibility,loss of ignition(LOI)and elements compositions,we attempt to investigate the process of loess deposition,the temporal and spatial evolution of the Mu Us Sandy Land,and involved climate changed during the mid-to Late-Quaternary.The main findings are as follows:1.The loess-paleosol sedimentary characteristics of the study profiles on the southeast margin of the Mu Us Sandy Land are largely consistent with those of typical sections on the Loess Plateau,but the coarse grain contents in the study sections are generally higher.The loess-paleosol-aeolian sand formation in the late mid-Pleistocene to the last interglacial period was deposited at the Baijie site,which is well corresponding to the typical loess paleosol sequences in the CLP.The JJ section was dominated by aeolian sand sedimentary stratigraphic,and the age of bottom aeolian sand unit is 120.7±11.0 ka.2.When a warm and wet climate changed to a cold and dry one along the southeast margin of Mu Us Sandy Land,vegetation was degraded,surface wind erosion was intensified,and erosional discontinuities were prone to occur in eolian deposits in the study area,which is manifested by the rapid conversion of loess to well-sorted sand deposition as observed in the study sections.The Mu Us Sandy Land expanded to the east and south,and the loess deposition areas contracted southwards.When the climate changes from cold and dry to warm and wet,the vegetation is widely developed,and the sediment type was gradually transitioned from aeolian sand to loess or sandy paleosol,and the sedimentation was relatively continuous,and the range of loess deposition expanded northwards.3.The Mu Us Sandy Land existed at least during the late Middle Pleistocene,the sandy loess at the bottom of the BJ section shows that the Mu Us Sandy Land was close to the BJ area before 230.1 ka,and the paleosol developed during 230.1-225.4 ka shows that the Mu Us Sandy Land was fixed during this period,and that the sand land area contracted northwards.During the MIS 5d period,the aeolian sand occurred in the BJ and JJ region,indicating that the Mu Us Sandy Land expanded southwards during this period,accompanied by strong erosion,and erosional hiatuses may appear in the BJ section.During the MIS 5c-5a period,the loess/paleosol in the BJ area shows that the Mu Us Sandy Land contracted northward again.The aeolian sand strata deposited on JJ section showed that the Mu Us Sandy Land expanded around 120.7 ka,44.7 ka and 1.4 ka.4.Combined with the fact that the loess deposition has hiatuses in the northern part of the CLP,our results show that during the glacial period,the typical loess distributed in the upwind areas relative to the modern Loess Plateau suffered from strong wind erosion and acted as an important homogeneous source of the leeward loess deposition,which may have led to the corresponding increase of mass accumulation rates of loess in the same period,but the grain size did not change much.This process undoubtedly increases the complexity of the interpretation of the applied proxies of loess sedimentary sequence,which deserves further in-depth understanding.