Sedimentological Characteristics Of Sandy Sediments And Regional Aeolian Activities In The Southeastern Margin Of Mu Us Desert Since Late Holocene

The Late Holocene is the closest geological historical period to the modern era,and environmental change in this period is one of the important research elements of past global change.The Mu Us Desert are not only located in the marginal zone of the East Asian monsoon,but also in the border zone of desert/loess and the interlacing zone of agriculture and animal husbandry in China.The unique geographical location makes the desert very sensitive to the response of environmental change,which is an ideal place to study the past global change.The widely developed aeolian sedimentary sequences in sandy land play an important role in the study of environmental evolution in arid and semi-arid regions.On the basis of comprehensive field investigation,this study selected typical sedimentary stratigraphic profiles,analyzed sedimentological characteristics such as grain size,magnetic susceptibility and geochemical elements of sediments,and identified the environmental indicative significance of grain size characteristics.Combined with OSL dating results,this study reconstructed the history of aeolsand activity in the study area,combined with other geological evidence in the surrounding area,The process of aeolian sand activity in the southeast margin of Mu Us Desert in Late Holocene is discussed,and the following conclusions are drawn:（1）The sediment grain size composition of the XHZ profile is dominated by very fine sand and fine sand,the content of both is more than 80%,the average content of silty sand is 16.77%,the content of clay is very little,the average grain size is 3.34Φ.The average grain size and grain size fraction show a consistent pattern of change with depth in the stratum,and the grain size composition and average grain size are fine-grained when the silty sand is developed.The grain size frequency distribution curves of all samples were positively or very positively skewed,with wide or narrow peaks.The top and bottom two sand sediment grain size frequency distribution curves were single-peaked,with peak particle sizes of 134μm and 147μm,respectively,with high and narrow peaks and good sorting.The grain size frequency distribution curve of the silt layer is bimodal,with the main peak grain size of 111μm and a weak peak at the fine end with a broad and slow peak size of 15μm.Three grain size End-member are separated using the Gen.Weibull function.EM1 indicates the intensity of regional dust storm activity,the amount of which is proportional to the intensity of dust storm activity;EM2 and EM3 are influenced by the same circulation and together indicate the intensity of East Asian winter winds.（2）The average value of low frequency magnetic susceptibility of the sediment is20.32×10^-8m³·kg^-1,with a variation range of 6.03～27.83×10^-8m³·kg^-1.The mean value of high frequency magnetic susceptibility is 20.04×10^-8m³·kg^-1,and the variation range is5.6～27.03××10^-8m³·kg^-1.The variation coefficients of both high and low frequency magnetic susceptibility are larger in the whole profile,and the variation trends of both are more consistent in the stratum,showing a high value in the silty sand and a low value in the sand layer.（3）From the geochemical characteristics,the sediment geochemical elements in the XHZ section are mainly composed of Si O₂ and Al₂O₃,with an average content of 69.41%and 10.74%,respectively.The changes of the major element oxide in the stratum show obvious phase characteristics,in which Al₂O₃ and Si O₂show opposite trends in the stratum and the same trend as Fe₂O₃,which is consistent with the general law of weathering into soil.The characteristics of the ratio of the major element oxide also show a certain regularity.The silica-alumina ratio,silica-alumina-iron ratio and leaching coefficient all show the characteristics of high values in the sand layer and low values in the silty sand,while the trend of chemical alteration index is opposite.The average content of each major element oxide in different stratum of the section compared with the average value of chemical elements corresponding to the upper continental crust（UCC）shows different degrees of deficit except for Si O₂.（4）Based on the grain size characteristics of the sediments in XHZ section and the OSL dating results,the variation of regional aeolian sand activities can be divided into three stages:～2.6 ka BP,strong winter wind,active aeolian sand activities,the formation accumulation of coarse sand particles.2.6 From ka BP to 100 years ago,the wind activity decreased in winter,the sand activity weakened,the sediment particles became finer,and the sand activity increased in the later period.In the past hundred years,the winter monsoon activity has become strong again,and the wind-sand activity has increased.（5）By combining the aeolian activity recorded by XHZ section and several sedimentary profiles in the surrounding areas,it is found that aeolian sand activity in the southeastern margin of Mu Us Desert since Late Holocene is more active than that in the Middle Holocene.The stratigraphic deposits are mainly aeolian sand and the dunes are in an activated state,which has a better correspondence with the regional climate background of the decline of East Asian summer monsoon and the decrease of precipitation in Late Holocene.In the process of aeolian sand accumulation,there are also stages of weak pedogenesis or paleosol development and dune reactivation,but there are regional differences in the response of these millennia-centennial aeolian activities to climate.In the historical period,especially in the last hundred years,the influence of human activities on the surface environment has intensified,which has created conditions for the enhancement of aeolian activities,and the local aeolian activities may also be affected by human activities.