Chlorite Weathering And Evolution Of East Asian Summer Monsoon In The Chinese Loess Plateau Since 1.6 Ma

The loess-paleosol deposits in the Chinese Loess Plateau provide one of the most complete land records of global climate and Asian monsoon during the Quaternary period.The Chinese Loess Plateau is controlled by different climate systems in winter and summer:Winter is mainly controlled by the risk of northwest winter,and dust accumulates to form the loess layer.Summer is mainly controlled by the southeast summer risk and the paleosol layer is developed due to pedogenesis.The alternation of loess and paleosol layers records the evolution of east Asian monsoon.Based on X-Ray Diffraction（XRD）,this study interprets the changes of the Quaternary East Asian summer monsoon by studying loess-paleosol clay minerals in the Chinese Loess Plateau.Firstly,qualitative and semi-quantitative analysis of clay minerals in the study area is carried out and their indicative significance is discussed.The spatial-temporal variation of chlorite weathering and its relationship with the East Asian summer monsoon were discussed by analyzing representative samples from six profiles from northwest to southeast since L₆（700ka）.Finally,the chlorite weathering index Ch（003）/（002）was used to analyze the chemical weathering of chlorite since 1.6Ma in the Xifeng profile,and compared with the climatic records of high and low latitudes,to study the evolution process of east Asian summer monsoon and its driving mechanism.The preliminary conclusions are as follows:1)Qualitative and semi-quantitative:the composition and content of all regions are consistent,mainly illite,followed by chlorite,montmorillonite,and kaolinite.Illite and chlorite exceed 80%,montmorillonite and kaolinite are less.2)Indicative significance of clay minerals:Taking Xifeng profile as an example,illite and chlorite increase gradually with the decrease of profile depth.While montmorillonite is the opposite,kaolinite content basically keeps equilibrium.This may indicate the regional weathering process on a long-time scale.The relative content of clay minerals varies greatly among different types.The illite content in the paleosol layer is higher than that in the loess layer,while chlorite and montmorillonite show an opposite trend.This means that on the orbital scale,the variation of clay mineral composition and content was caused by weathering during glacial-interglacial period,which may be related to the evolution of East Asian summer monsoon.3)Temporal and spatial variation of chlorite:Temporal and spatial observations of chlorite weathering since L₆（700ka）in six profiles from northwest to southeast of the Loess Plateau were studied.Chlorite weathering index Ch（003）/（002）can be used as an effective proxy for the evolution of east Asian summer monsoon.And it can be an important indicator of weathering intensity and environmental evolution in this region.At the spatial scale,chlorite weathering index Ch（003）/（002）showed an increasing trend from northwest to southeast,and the increasing rate was higher in interglacial period than in glacial period,showing a spatial gradient change of climate.At the same time,chlorite weathering index Ch（003）/（002）is consistent with the trend of magnetic susceptibility and has a strong correlation.Based on the weathering characteristics of chlorite,it is speculated that chlorite may provide free iron for the formation of fine secondary ferromagnetic minerals in the Chinese Loess Plateau during the weathering and leaching process at an older age,which leads to the enhancement of mineral magnetism in this region.4)Chlorite weathering and evolution of East Asian summer Monsoon:There is a close relationship between chlorite weathering and evolution of East Asian summer monsoon since 1.6Ma in Xifeng loess-paleosol.From glacial to interglacial Angle,chlorite weathering index Ch（003）/（002）has a strong correlation with stratigraphic changes,and the paleosol layer is much higher than the loess layer.The chlorite weathering index Ch（003）/（002）showed a decreasing trend in the long series,and showed different amplitude and frequency at different stages,which may indicate the long-term weathering trend and the characteristic events of"mid-Pleistocene climate transition"in this region.5)Driving mechanism of East Asian summer Monsoon:Based on ensemble empirical mode analysis（EEMD）,we analyzed the long series time period signals of chlorite weathering index Ch（003）/（002）,deep-sea oxygen isotope LR04 and sea surface temperature SST,and discussed the evolution of East Asian summer monsoon and driving mechanism of high and low latitude.The results show that the east Asian summer monsoon indicated by chlorite weathering index Ch（003）/（002）has obvious quasi-periods of～20ka,～41ka and～100ka.The first two may be controlled by the precession period of earth orbit and the tilt of earth axis influenced by solar radiation at low latitudes.The quasi-period of～100ka May be related to the global ice volume.Therefore,the east Asian summer monsoon since 1.6Ma in the Loess Plateau may be driven by both earth orbit and global ice volume.The periods of>100ka indicated by the deep-sea oxygen isotope LR04 are not significant in the high latitude records,while the low latitude records and the east Asian summer monsoon have significant long periods of 400,000～500,000 and 800,000 years.These two long periods can be regarded as evidence that the low latitude processes independently drive the changes of the east Asian summer monsoon.On the long scale,both the EAST Asian summer monsoon and the high and low latitude records show a decreasing trend since 1.6Ma,but the East Asian summer monsoon is the most significant,which may be affected by the uplift of the Tibetan Plateau.