A 150,000-year Study On Climate Change And Sandstorm Activity Recorded In Liushumao Profile In Southern Yulin

The sequence of sand-loess-paleosoil,as the carrier of paleoclimate evolution information,has special advantages in reconstructing climate change and the history of aeolian activity,which received extensive attention.Located in the Desert loess transition area,Yulin is rich in sand-loess-palaeosol deposition and contains abundant quaternary paleo climate evolution and aeolian activity information.This paper selects Liushumao profile in the south of Yulin,establishes their stratigraphic sequence,and reconstructs the history of climate change and aeolian activity in the past 150,000 years,by analyzing grain size composition,elements,magnetic susceptibility,calcium carbonate content of 163 samples from the profile and comparing with other typical areas.The knowledge gained are as follows:(1)Two layers of aeolian sand,six layers of mixed sediment of loess and aeolian sand,three layers of loess and three layers of palaeosol have been deposited in the Liushulmao profile in the last 150,000 years.The sequence of strata formed in the Liushumao profile for 150,000 years are:loess layer L2L1,loess-aeolian sand mixed layer L2H1,palaeosoil layer S1,loess-aeolian sand mixed layer L1Hs,aeolian sand layer L1F2,loess-aeolian sand mixed layer L1H4,aeolian sand layer L1F1,loess-aeolian sand mixed LayerL1H3,paleosoil Layer L1S1,loess-aeolian sand mixed layer L1H2,loess layer L1L1,loess-aeolian sand mixed layer L1H1,paleosoil layer S0,loess layer L0.(2)Grain size composition shows that sand is the main component,silt sand is the second,and the clay content is the least of the whole profile,and their average content are 48.72%,45.14%,6.14%.High sand content indicates strong aeolian activity in this area.The total thickness of the aeolian sand layer and loess-aeolian sand mixed layer accounted for 54.6%of the whole profile,indicating that aeolian activity lasted for a long time in the deposition process.Grain size data shows that the order of grain size is aeolian sand layer>loess-aeolian sand mixed layer>loess layer>paleosoil layer.It indicates that when aeolian sand layer formed,the climate was relatively cold and dry,with strong winter monsoon and aeolian activity.When loess-aeolian sand mixed layer formed,the climate was cold and dry,and weaker aeolian activity than the former.When loess developed,the climate was relatively mild,with weak aeolian activity.The climate was relatively warm and wet during the development of palaeosol,with weakest winter monsoon and aeolian activity.Comparing the grain size characteristics of modern aeolian sand and aeolian sand in this profile,it is found that the aeolian activity in modern times is stronger than geological periods,but this is caused by unreasonable human activities.(3)The content differences of major element oxides and trace elements in different sedimentary phases reveal climate fluctuations.The major element oxides in Liushumao profile mainly are SiO2 and Al2O3,followed by CaO,Fe2O3,K2O,MgO,and Na2O.Al2O3,Fe2O3,CaO,and MgO are enriched in paleosoil,K2O,Na2O,and SiO2 are enriched in aeolian sand.Among trace elements,Cu,V,Rb,Zn,As,Mn,Ni and Ti are enriched in paleosoil,Ba and Sr are enriched in aeolian sand.These indicate that when aeolian sand layer formed,the climate was relatively cold and dry.When loess-aeolian sand mixed layer formed,the climate turned to dry and cold.When loess developed,the climate was relatively mild.The climate was relatively warm and wet during the development of palaeosol.CIA,Na2O/K2O,and ba indicate that the Liushumao profile belonged to a low-level chemical weathering stage.The values of CIA,Rb/Sr and SiO2/Al2O3 indicate that the winter monsoon was strongest and summer monsoon was weakest in the aeolian sand layer accumulation period.The winter monsoon was dominant during the formation of loess-aeolian sand mixed layer.During the formation of the loess layer,the summer monsoon gradually weakened and the winter monsoon gradually strengthened.The summer monsoon was strongest and winter monsoon was weakest during the development period of the paleosoil layer.(4)The peak-valley variation of the magnetic susceptibility curve also reveals climate fluctuations.The lowest value of magnetic susceptibility corresponds to the aeolian sand layer,the low value of magnetic susceptibility corresponds to the loess-aeolian sand mixed layer,the high value of magnetic susceptibility corresponds to the loess layer,the highest value of magnetic susceptibility corresponds to the palaeosoil layer.It indicates that when aeolian sand layer formed,the climate was relatively cold and dry,with weakest summer monsoon.When loess-aeolian sand mixed layer formed,better climate conditions than the former.When loess developed,the climate was relatively mild,with weak summer monsoon.The climate was relatively warm and wet during the development of palaeosol,with strongest summer monsoon.(5)The content of CaCO3 has different climate indicating significance in different regional environments.Among the aeolian sand-loess-palaeosol sedimentary sequence in the southern of Yulin,high content of CaCO3 indicates relatively strong pedogenesis and relatively warm and humid climate,while low content indicates relatively weak pedogenesis and relatively dry and cold climate.But in the central and southern part of the Loess Plateau,its indicating significance is opposite.(6)The results of physical and chemical properties and stratigraphic sequence reveal that the climate in the south of Yulin experienced an evolutionary process of"relatively mild and dry,relatively dry and cold,relatively warm and wet,relatively dry and cold,relatively warm and wet,relatively mild and dry" in the past 150,000 years.Specific manifestations of each period are as follows:in the late Middle Pleistocene,the climate was relatively mild in the early stage and relatively dry and cold in the late stage.In the early Late Pleistocene,the climate was relatively warm and humid,with several climate fluctuations.In the late Late Pleistocene,the climate was generally cold and dry,but there were climatic fluctuations.In the Holocene,the initial period was relatively warm and humid.Since the late Holocene,the climate developed in the direction of aridification,and had gradually approached modern climate conditions.(7)According to the theory that aeolian sand deposits represent the spread of sand,compared the stratigraphic sedimentary sequence of Liushuomao profile with adjacent areas,It is found that southern Yulin had experienced two obvious sand expansion periods,six weak expansion periods,three weak retreat periods and three obvious retreat periods in the past 150 000 years.During the deposition period of aeolian sand layer,the Mu Us sandy land expanded to the southeast obviously,with strong aeolian activity.During the period of loess aeolian sand mixed layer deposition,the Mu Us sandy land expanded to the southeast slightly,with relatively weak aeolian activity.During the loess development period,the Mu Us sandy land retreated slightly to the northwest,with weak aeolian activity.During the period of paleosol development,the Mu Us sandy land retreated to the northwest obviously,with the weakest aeolian activity.(8)The Malan loess contains a relatively thin and weakly developed palaeo-soil layer,indicating that there was a short period of relatively warm and humid climate and a period of shrinking sandy land in thelate of Late Pleistocene.