The Climate And Environmental Evolution And Aeolian Activity Since The Holocene In The Gonghe Basin Recorded By Masi Section

The sediments of desertified grassland in alpine region represent the significant changes of sedimentary external force with the climate change of warm-wet and cold-dry,but also record the forward and adverse process of desertification and the change of desertified strength during the geological history,so the sediments can serve as the information carrier to recovery the evolution of ancient sedimentary environment.A variety of indicators of elements,particle size,magnetic susceptibility and organic matter contain abundant environmental information in sediments,which can preferably reveal ancient sedimentary environmental evolution.The Gonghe Basin in the northeastern Qinghai-Tibetan Plateau lies at the convergence of the Asian summer monsoon,the winter monsoon,and the westerlies which is an important component of the transition zone between desert and loess and the fragile landscape belt in northern China.This paper selected a typical paleosoil-aeolian sand sediment profile of Masi in the middle of Gonghe Basin,and utilized the ¹⁴C,according to the comprehensive analysis of the environmental proxies of grain size,magnetic susceptibility,organic matter,and geochemical elements,and compared with typical profile in the eastern Qinghai-Tibetan Plateau,and rebuit the sedimentary environmental evolution process of Gonghe Basin since the Holocene.Research results showed that the sand composition is the main component of the MS sediments,and the fine sand is major sediment type of the sand component.This is consistent with the results of characteristics grain-sizes in the modern sand.But the climate of the MS profile is slightly warmer than the modern one.The particle size distribution shows that the depositional environment of the MS profile is relatively stable and the source of aeolian sand is single.The sand content in aeolian land deposition was higher than paleosoil,on the contrary,clay,Md,Mz,SC/D,magnetic susceptibility and organic matter content in paleosoil were higher than aeolian land deposition,which refered to the warm-wet environment in paleosoil,cool-humid environment in aeolian land deposition.The results show that the variation tendency of each constant element are relatively small in the whole profile,which indicates that the elements of the MS section have a high degree of uniformity.The constant elemental content of the section is different from that of the upper part.Compared with UCC,the MS profile shows that Mg,K,Na,Fe and Al are loss.Expect Na element,the content of other elements in the relative enrichment showed a loss,and the chemical composition of the ancient soil layer was closer to the UCC mean value.In addition,the ratio of ferrosilicon was found to be more accurately and accurately recorded than the Na/K.The results show that the order of change of the trace elements is Sr>Ni>Ce>Ga>Rb>As>Mn>Y>Ba>Pb.Ba,Rb,Mn and Y elements Which can reflect the depositional environment information of the MS section more clearly,while Sr,Ni and Ce are relatively vague in reflecting the difference of depositional environmental information in the study area.The trace element ratios Rb/Sr,Ba/Sr and CaO/Sr are more clearly which reflects the wet and dry changes of the regional depositional environment.The climate change information characterized by Cu/Mn is relatively obscure.Through the analysis of grain size,magnetic susceptibility,organic matter and geochemical elements of Langqu profile in MS profile,and combined with ¹⁴C,the environmental evolution in Gonghe Basin since the Holocene could be divided into 5stages.First stage,the winter and summer monsoon strength presented a frequent fluctuation in Holocene and the climate trended to be warm in 11.98⁹.75 ka BP due to the weaker winter monsoon strength,accompanying with developed paleosol,although there were frequently climatic fluctuations;Second stage,from 9.75ka BP to8.09 ka BP,this stage can be regarded as a transitional period from the early Holocene to the middle Holocene,the climate turned from drought to moisture;Third stage,from 8.09 ka BP to 4.78 ka BP,the climate was the warmest and wettest during the entire Holocene due to the weakest winter monsoon emerged,the pedogenesis was stronger and accompanying with prominently developed paleosol;Fourth stage,the climate has become cold and dry in 4.78².90 ka BP due to the winter monsoon became stronger,the development of paleosol was interrupted and aeolian sand was formed;Last stage,the warm and humid degree increased during 2.90⁰.67 ka BP,accompanying with developed paleosol.Overall,the characteristics of regional winter and summer monsoonal variation was that when one was rising,the other was falling.