Evolution Of East Asian Monsoon Recorded By Mineral Composition Characteristics Of Aggradational Red Earth

The aggradational red earth is a typical wind dust deposit in the middle subtropical zone,which has the characteristics of deposition and weathering.It is a good carrier of Quaternary environmental interpretation in the middle subtropical zone.Based on the mineralogical evidence,this paper preliminarily discusses the information of mineral species,relative content and characteristic index of JL profile on the evolution of paleoenvironment in this area,and further discusses the evolution of East Asian monsoon recorded by JL profile in combination with previous research results of geochronology,element geochemistry and grain size.The results are as follows:The aggradational red earth is a typical wind dust deposit in the middle subtropical zone.It was formed in a long-term humid and hot weathering environment.It has the characteristics of aggradational red earth with deposition and weathering.To a certain extent,the characteristics of aggradational red earth affect the accurate interpretation of environmental information,so it is necessary to carry out the research on particle size.Therefore,in this paper,92 clay minerals with less than 2?m component and heavy minerals with more than 10 ?m component were selected from JL section at the northern foot of Lushan Mountain are selected to analyze their mineral characteristics and explore the weathering mechanism of aggradational red earth in the subtropical region.Combined with the previous research results of geochronology,element geochemistry and grain size,this paper attempts to reveal the East Asian monsoon evolution information recorded in the aggradational red earth.The results are as follows:(1)JL section is 1846 cm thick,(1)?(5)(1846?1400cm)is reticulated red earth,formed in ? 1.2 ? 0.44Ma;(6)?(8)(446?0cm)is yellowish brown sediment,formed in0.44 Ma.The composition of clay minerals in different stratigraphic units of JL section is basically the same,mainly illite and kaolinite,followed by vermiculite,with little montmorillonite;In addition,there are some hydroxy interlayer vermiculite(HIV)and a small amount of 0.71-1.0nm illite-kaolinite mixed layer minerals in the yellowish brown soil deposit,which indicates that the climatic conditions at this time are not as good as those in the formation period of reticulated red earth.From the relative content of clay minerals,illite,kaolinite,vermiculite and montmorillonite were 46.85%,36.90%,15.22% and 1.04% respectively;Illite showed a gradually increasing trend,while kaolinite showed an obvious decreasing trend on the contrary,vermiculite(+HIV)showed a first decreasing and then increasing trend,which may be related to the content of hydroxy interlayer vermiculite in yellow brown soil.In addition,the whole rock CIA of JL profile has a weak correlation with illite crystallinity,and a strong correlation with K/I,which can be used as a climatic indicator to indicate the strength of weathering soil.The indicator gradually decreases from bottom to top,indicating that the climate in Jiujiang area has become dry and cold since 1.2Ma.(2)The major heavy minerals are zircon(19.51%),ilmenite(32.35%)and hematite-limonite(19.32%),with the relative contents of anatase,rutile,anatase,tourmaline and epidote ranging from 1% to 10%,while the other minerals are relatively less,mainly in extremely stable and stable mineral assemblages;In addition,the contents of epidote,hornblende pyroxene and garnet in yellowish brown sediments increase significantly.The mineral maturity ZTR and weathering coefficient W of yellow brown sediments and reticulated red earth are 65.79 and 89.82,0.28 and 0.02,respectively.The principal component analysis of heavy minerals and element geochemistry of silt grain size in JL section shows that the difference of heavy mineral species content,combination type and characteristic index between reticulated red earth layer and yellow brown deposit mainly reflects the change of weathering intensity caused by climate change since 1.2Ma,which proves that climate change is an important factor controlling heavy mineral composition.(3)Based on ESR chronological framework,the principal component analysis of the grain size,magnetization,kaolinite and illite content,K/I index,ZTR index and chromaticity of clay minerals is carried out in JL profile.It is revealed that the climate has been in a continuous dry and cold trend since 1.2Ma,and its climate change has experienced three evolution stages: Extreme warm and wet stage(0.8?1.2Ma),enhancement stage of winter monsoon(0.4?0.5Ma)and winter monsoon strengthens again(Since 0.4Ma).At the same time,there are two winter monsoon enhancement events.During the period of 0.8?1.2Ma,the enhancement of winter monsoon caused widespread aeolian accumulation in the south of China,which may be the dual driving effect of global cooling and the increase of Arctic ice,which is the corresponding response of southern China to the Middle Pleistocene Transformation(MPT).During the period of 0.4?0.5Ma,the development of reticulation weakened or stopped,and there were ferromanganese glue film or nodules,which indicated that the climate turned colder again,and the contrast of winter and summer monsoon was further enhanced.may be related to the uplift of the Tibetan Plateau and the expansion of the polar glaciers.In addition,during MIS13-21,the East Asian summer monsoon(EASM)was relatively strong,especially the abnormal strong EASM in MIS13-21,which may play an important role in the formation of reticulated red earth in the south China.