Ancient East Asian Monsoon And Environmental Changes Recorded In The Xunyi Red Clay Sequence

During the Neogene,the dominant air flow over the Asian continent was gradually transferred from the planetary wind system to the monsoon wind system.The uplift of the Qinghai-Tibet Plateau resulted in a significant increase in the pressure gradient force,which further strengthened the Asian paleomonsoon of large-scale energy exchange between land and sea.The continuous and stable red clay sedimentary sequence widely distributed in the Loess Plateau of China records the evolution of the paleoclimate and environment of the Asian continent in the Late Cenozoic.Through various research methods,the paleoclimate and environment information contained in the red clay sedimentary sequence can be explored to reveal the evolution mechanism of the paleoclimate and environment of the Late Cenozoic in China.Xunyi located in the central part of the Loess Plateau,is characterized by continuous sedimentation,clear horizons,stable lithology,horizontal occurrence,good outcrops and abundant mammalian fossils.It is an important testing ground for excavating the paleoclimatic information of the Asian inland and even the world in the Late Cenozoic.Previous studies have taken the Xiaqiang aeolian sedimentary sequences located in the north of Xunyi as the research object,a reliable chronological framework has been established through biostratigraphy and magnetostratigraphy,and the paleoclimate and paleoenvironment of Xunyi red clay deposition period have been reconstructed from magnetic susceptibility,grain size,geochemical and micromorphological characteristics.However,the study on the evolution history of East Asian monsoon reflected by Xunyi red clay has not been carried out,and the study on the evolution history of paleoenvironment can be further strengthened.Therefore,based on previous research results,this research will introduce rock magnetism and magnetic texture research methods to excavate the East Asian monsoon evolution history and paleoenvironmental evolution history contained in Xunyi red clay from a microscopic perspective.The results of triaxial isothermal demagnetization and K-T experiments show that the magnetic minerals in Xunyi red clay are mainly magnetite,magnetite and hematite,and also contain goethite.The profile contains the same type of magnetic minerals as the Loess-paleosoil sequence and other red clay profiles.However,ferromagnetic minerals and magnetite with shape anisotropy are dominanting,and as the main contribution of magnetic susceptibility,they provide scientific support for the introduction and reconstruction of AMS ancient wind.The magnetic fabric parameters of the Shangqiang section of Xunyi show that the magnetic susceptibility ellipsoid of the whole red clay section is all pie shaped,and the magnetic facet F is more developed than the magnetic lineation L,which is in line with the characteristics of weak dynamic transport.This parameter also reflects Xunyi Shangqiang is the main eolian red clay,and generally the stable sedimentary environment,but unstable sedimentary environment during climate transition or unstable precipitation affected the directional alignment of magnetic minerals,destabilize the magnetic anisotropy,and shows a large fluctuation in the abnormal magnetic fabric parametersAMS parameters of the Shangqiang red clay section in Xunyi well record the evolution information of the East Asian monsoon since about 5.9～2.6Ma.The comparison of the chronological results and AMS parameters shows that the East Asian monsoon formed at5.9～2.6Ma and had a significant impact on the paleoenvironment in the study area.During5.9～2.6Ma,the dust carried by winter monsoon provided the provenance for the Shangqiang red clay in Xunyi,and the intensity was smaller during 5.9～4.3Ma.From 4.3 to2.6Ma,the cold and dry winter monsoon strengthened obviously and dominated,resulting in the gradual drying and cooling of the climate during 4.3 to 2.6Ma.During 5.9～4.3Ma,the intensity of winter monsoon was relatively weak at this time,and the intensity of summer monsoon was significantly greater than that of winter monsoon.The increasing summer monsoon brought abundant heat and abundant precipitation to Xunyi area,which made the region change from cold-dry under hot and humid conditions to completely hot and humid climate.The paleoenvironmental information reflected by the changes in wind direction and intensity of Asian paleomonsoon analyzed by AMS is consistent with the results of magnetic susceptibility and geochemistry analysis by Xue Xiangxu et al.(2001).