Palaeoenvironmental Evolution By Loess In Menyuan Basin

Menyuan basin is located in the northeastern margin of the Qinghai Tibet Plateau.It is not only the transition zone between the Qinghai Tibet Plateau and the Loess Plateau,but also the intersection area of the modern East Asian monsoon region and the mid latitude westerly region.It is an excellent experimental site for studying climate change and land surface response.However,there is little research work and lack of chronological records in this region,Therefore,the Aeolian Loess Section and other auxiliary sections in Menyuan basin are selected as the carriers to reconstruct the paleoenvironment and study the evolution process of the regional environment.In this study,the YHC loess section and nine other auxiliary sections in Menyuan basin were selected to carry out the quartz OSL dating and establish a reliable chronological framework.Combined with the analysis of paleoclimatic proxy indexes such as magnetic susceptibility,grain size,chromaticity,SOC and element geochemistry,the environmental change process of Menyuan basin since 39 ka has been reconstructed.Finally,the driving mechanism of Holocene climate change in the Menyuan basin is discussed by comparing the existing climate records of the Holocene westerly region and the East Asian monsoon region.Based on the above research,the following new understandings are obtained:(1)A large number of indicators in YHC loess profile have similar significance to the environmental change process in Menyuan basin,but there are some differences in details.Therefore,it is necessary to select a variety of indicators for comprehensive comparison in order to obtain more accurate information to study the regional environmental evolution process.(2)Based on the high-density OSL,the 39 ? 0 ka age framework of Menyuan basin is established.There are two obvious stratigraphic deficiencies in 35 ? 24 ka and 21 ?14 ka,which are caused by glaciation and wind erosion.(3)The paleoenvironmental evolution of Menyuan basin since 39 ka can be reconstructed by integrating various environmental indicators,which can be divided into seven stages: the climate changes from warm and wet to dry and cold in 39 ? 35 ka,and the climate fluctuation increases;35 ? 24 ka,the formation is missing;During24 ? 21 ka,the climate reached the driest and coldest period,and the glaciation was strong,which led to the formation erosion,moraine deposition,glacial water deposition and other special events occurred frequently;During 21 ? 14 ka,the climate improved,the glaciers subsided,the alluvial and proluvial events occurred frequently,the wind was strong,and the strata were eroded;From 14 ka to 8.5 ka,the climate tends to be warm and humid,precipitation increases significantly,and alluvial proluvial events occur frequently,reaching the warmest and wettest period in 8.5 ka;5 ? 4 ka,the warmest and wettest period;4 ? 0 ka,the climate changes from the warmest and wettest to aridity.(4)Comparing the environmental processes of the Northeast Qinghai Tibet Plateau with the East Asian monsoon region and the middle latitude westerly region,it is found that the Holocene climate change in this region was mainly controlled by the East Asian monsoon and also affected by the middle latitude westerly.