Variation Of Illite Crystallization And Its Paleoenvironmental Significance In The Loess Plateau Since 2.6 Ma

The Chinese Loess Plateau has very important scientific significance in the study of paleoclimate.The complete loess-paleosol sequence of the Loess Plateau records abundant paleoclimate information since 2.6Ma,and provides a good research material for the study of Quaternary paleoenvironment and paleoclimate change.Clay minerals are an important component of loess and the most active part of the loess material composition.The study of clay mineralogy of loess has been one of the important fields in loess research.In this paper,five loess profiles from Huanxian,Xifeng,Changwu,Yongshou,and Weinan from the northwest to the southeast of the Loess Plateau were selected,and the qualitative and semi-quantitative analysis of clay minerals in the five profiles since the last interglacial period was performed by X-ray diffraction.The illite crystallinity of these five sections was analyzed in space-time.Based on the above studies,this paper conducts a preliminary study of paleoenvironmental information recorded by the illite crystallinity of the loess-paleosol sequence since 2.6 Ma in the Shaozhai section.The main findings and findings of this article are:（1）The qualitative and semi-quantitative analysis results of the Loess Plateau in China show that the main types of loess-paleosol clay minerals since the last interglacial period of the Loess Plateau are Illite,Chlorite,Kaolinite,and Smectite.There are also a small amount of Vermiculite,mixed layer minerals,and trace Hydroxy-interlayered Minerals（HIM）in the paleosol of the Yongshou and Weinan sections.The composition of clay minerals in each layer of the five loess-paleosol sections of the Highland Plateau is similar.Among them,Illite（69%-79.5%）was the main content,followed by Chlorite（7.3%-16.7%）and Kaolinite（6.3%-13.2%）,and Smectite（1.2%-7.2%）was the least.（2）In the changes of the profile and space of clay minerals,the relative content of illite in the paleosol is higher than that of the loess,and the illite of the ancient soil shows an increasing trend from northwest to southeast,while the change in loess is small.Chlorite is more in the loess,but less in the paleosol.The change in chlorite is more pronounced in the Yongshou and Weinan sections,which are located south.This change trend is related to the chlorite in the last interglacial and Holocene Affected by the hot and humid climate,weathering has formed a vermiculite/chlorite mixed layer mineral.The relative content of Kaolinite is relatively low in the five study sections,and the fluctuations are small.This may also be related to the material source,which can reflect the basic conditions of the original wind and dust,and is less affected by the climate.The relative content of Smectite is the least,but it is high in the loess and the paleosol is low,which may be related to the weathering of Smectite under the influence of abundant precipitation during the last interglacial and Holocene periods.（3）A study of the Illite Crystallinity KI values of the loess and paleosol in the Loess Plateau of China found that the illite KI values of the Huanxian,Xifeng,Changwu,Yongshou,and Weinan profiles since the last interglacial period basically show that the paleosol are even Higher than the underlying loess.The Illite Crystallinity KI is the highest in the last interglacial period S₁,the second in the Holocene S₀,and the lowest in the last glacial L₁.This change trend is more pronounced in the Changwu,Yongshou,and Weinan sections that are located south;In terms of space,the Illite Crystallinity KI values in the paleosol S₀ and S₁ showed a gradual increase trend from northwest to southeast,while the increase trend in the loess layer L₁ was smaller,which can better correspond to the change trend of magnetic susceptibility.We compared the increase amplitude in the KI value of the loess and the paleosol on the profile,and analyzed the spatial changes of it,and found that it gradually increased from northwest to southeast.Excluding the effect of particle size and sedimentation rate,it is found that the change of Illite Crystallinity in the loess is mainly controlled by the influence of the source material,and the Illite Crystallinity in the paleosol is closely related to weathering.The good correlation between Illite Crystallinity KI value and magnetic susceptibility can also better explain this problem.The above characteristics indicate that the Illite Crystallinity can be used as an effective index of the change of the East Asian summer monsoon.（4）From the change records of the illite crystallinity since 2.6 Ma in the Shaozhai section,it can be found that,overall,the KI value is increasing,and after comparing the KI value with the magnetic susceptibility,it is found that the glacial-interglacial cycle changes They have a good correspondence.This shows that the illite crystallinity can be a good indicator of chemical weathering in the short-time glacial-interglacial cycles.The KI values of illite crystallinity in the glacial loess layers and the interglacial paleosoil layers are not consistent.The KI value of the loess during the ice age gradually increased from 0.65 Ma,indicating the continuous expansion of the material source in the loess plateau,which may be mainly related to the periodic uplift of the qinghai-tibet plateau and global ice,and under its influence,it can lead to the addition of a large number of new surface biomass after various"alpine processes"in the material source area.The KI value of the Illite Crystallinity in the interglacial paleosol was 0.8 Ma as the limit of change.The value of KI was relatively low in the early Pleistocene and it gradually decreased,but the value of KI was relatively high after the Middle Pleistocene.It indicates the difference in the strength of the East Asian summer monsoon before and after the Middle Pleistocene.The East Asian summer monsoon is weaker in the Early Pleistocene and the summer monsoon is stronger in the Middle and Late Pleistocene.Our results also support the hypothesis that the Walker circulation caused by the sea surface temperature gradient affects the East Asian summer monsoon,but the stage of the Tibetan Plateau is not excluded Impact of sexual uplift on the strengthening of the interglacial summer monsoon.