Climate Change In Otindag Sandy Land During The Holocene

Otindag sandy land,located at the edge of East Asian monsoon,is a region with fragile ecological environment and it is sensitive to climate change.The sedimentary sequence of aeolian sand and sandy paleosol interbedding exposed at the edge of sandy land,and aeolian sand/paleosol/limnetic sedimentary strata inside indicated the facts of paleoclimate changes and desert vicissitudes.On the basis of the previous studies,the TB section(aeolian sand/paleosol/limnetic sedimentary sequence)in the south of sandy land and LMM section(paleosol/aeolian sand/weak soil interbedded deposition)in the northeast of sandy land were taken as two stratigraphic sections to explore the regional paleoclimate in Otindag sandy land during the Holocene by analyzing the characteristics of sediments,which included the grain size,magnetic susceptibility and geochemical major elements as climate proxies,combined with the OSL dating.The following conclusions were obtained:(1)The climate changes reflected by grain size,magnetic susceptibility and geochemical elements were basically consistent in TB section and LMM section.Under the warm and humid climate,paleosol developed,as the content of fine particles increased due to the strong chemical weathering,and the grain size of the sediment was smaller.Meanwhile,the oxidation environment was conducive to the refinement and enrichment of ferromagnetic minerals,so the paleosol had a larger magnetic susceptibility,and there was more content of Al2O3 and Fe2O3 with relatively stable properties.In contrast,in cold and dry environment,the grain size of aeolian sand was coarse,the magnetic properties of the minerals weakened under reduction conditions,the magnetic susceptibility was in low value,and the content of Al2O3 and Fe2O3 was less.During the moisture period with much precipitation,the surface runoff put a big amount of coarse particles into the depression,so the grain size of lacustrine-swamp sediment was relatively coarse,the magnetic susceptibility was low,and compared with paleosol,the content of Al2O3 and Fe2O3 was relatively less than that in paleosol.(2)The interbedded deposition of aeolian sand,paleosol and limnetic facies is the manifestation of the strength changes of the East Asian monsoon.The TB section revealed that in 5.80±0.69~4.68±0.29 ka BP,lacustrine-swamp sediment was developed and the climate was relatively humid with much precipitation;in 4.68±0.29~1.49±0.11 ka BP,paleosol and aeolian sand were developed,as summer monsoon gradually weakened and winter monsoon was stronger than before,the climate became cold and dry;since 1.49±0.11 ka BP,paleosol has been developed and it was the response of warm and wet climate,but as the summer monsoon weakened,the climate has developed to aridity in the later period.The LMM section showed before 5.16±0.29 ka BP,there were 2 layers paleosol and 1 layer weak soil,and the strength of summer monsoon experienced the process of strong,weak and strong again;in 5.16±0.29~3.35±0.21 ka BP,2 layers of aeolian sand and 2 layers of paleosol were developed,and it showed two climate alternations of cold and warm;in 3.35±0.21~2.51±0.19 ka BP,2 layers of aeolian sand and 2 layers of paleosol were also developed,reflecting that there are two cold periods with strong winter monsoon and two warm periods dominated by summer monsoon respectively;since 2.51±0.19 ka BP,the sediments from the bottom to the top have been aeolian sand,paleosol and modern soil in turn,the climate has experienced cold and dry,warm and wet and gradually closed to modern climate.(3)Combined with previous studies and constructed the chronology sequence,which showed that aeolian sand was deposited in Otindag sandy land in the Early Holocene,paleosol was generally developed in the Middle Holocene,and in the Late Holocene,aeolian sand and paleosol were developed alternately.The transfer commensurated broadly with the climate change in the Holocene.The Holocene climate change in Otindag Sandy Land can be divided into 3 stages.The climate was unstable in the Early Holocene(11.5~8.8 ka BP),and there were still aeolian activities but the climate was becoming warm.In the Middle Holocene(8.8~4.2 ka BP),the summer monsoon became stronger and paleosol was generally developed in the sandy land,so the climate was warm and wet,and aeolion activities were curbed.During the Late Holocene(After 4.2 ka BP),the climate gradually became arid with frequent fluctuations.The climate change in Otindag sandy land is related to the strength of East Asian monsoon,which is the regional response to the global climate changes.