The Study Of Sedimentary Characteristics And Genetic Mechanism Of The Quaternary Red Clay In The Middle And Lower Reaches Of Yangtze River

Similar to the Loess,deep-sea deposits and ice cores,the widely distributed Quaternary red clay in southern China is a good carrier for reconstructing the Quaternary paleoclimate changes.Therefore,more and more scholars attach importance to it.Inour study,based on the previous research,6 Quaternary red clay profiles of Xuancheng(XC)in Anhui province,Jiujiang(JJ),Nanchang(NC)and Xinyu(XY)in Jiangxi province,Changsha(CS)in Hunan province,Tangxi(TX)in Zhejiang province were selected as the study objects.Based on the electron spin resonance(ESR)dating results,the chronology of the Quaternary red clay is established.Through the analysis of sediment grain size,quartz grain surface morphology,the variation of sedimentary characteristics along these Quaternary red clay profiles were discussed.Further combining with environmental geochemistry and magnetism analyses,the formation mechanism of the Quaternary red clay in the study area was discussed.This study will be helpful for better understanding of the Quaternary paleoclimate and paleoenvironmental changes in eastern China.The main understanding of this paper is as follows:(1)The dating of the Quaternary red clay in study areaElectron spin resonance(ESR)dating results show that the age of three Quaternary red clay profiles of CS,NC and XY did not occur inversion,the age increased with the increasing of depth.The dating results of these three Quaternary red clay profiles are consistent with the previous ESR and paleomagnetic dating results in Xuancheng city and Jiujiang city,indicating that the dating results in this paper are credible.According to the dating data and previous studies,it is concluded that the deposition of the vermicular red clay unit occurred in southeastern China since 1.0-1.2 Ma B.P.,and ended in approximately 0.4 Ma B.P..(2)Sedimentary dynamics of Quaternary red soilsBy using the training sets of typical eolian sediments,such as Chinese loess plateau loess,Xiashu loess,and floodplain deposits,a Fisher linear discriminant analysis function was established.The Quaternary red clay samples in the middle and lower reaches of the Yangtze River were analyzed by using this discriminant function.The results show that the homogeneous red clay samples and the yellow-brown earth samples were all identified as eolian sediments.The upper segment of vermicular red clay samples were mostly identified as eolian sediments.The lower segment of vermicular red clay samples were largely identified as floodplain deposit.The grain size characteristics of the Quaternary red clay indicate that the mean grain size of the red soil is finer than that of the Chinese loess plateau loess and Xiashu loess.The grain size characteristics of the quartz grains isolated from the red clay bulk samples indicate that the reason for the finer grain size of the Quaternary red clay is not only due to the effect of weathering,but also because of the finer original sedimentary material than the loess plateau loess and Xiashu loess.Combined with the results of linear discriminant analysis,it is concluded that the yellow-brown earth layer(or so called Xiashu loess)and homogeneous red caly layer were typical eolian sediments,and the main transport force is winter monsoon.Meanwhile there are many>100?m particles,which cannot be transported for a long distance by eolian in the vermicular red clay samples,distinguishing the vermicular red clay samples from the typical loess and Xiushu loess.Photomicrographs of quartz particles separated from vermicular red clay bulk samples show that the quartz grains always mixed with rounded and poor rounded grains.The morphology of the mixed quartz grains indicates that the transport dynamic of the vermicular red clay is complicated.Combined with linear discriminant results and grain size parameters,vermicular red clay is likely to be a kind of sediment mixed with near-sourced eolian deposits and far-sourced eolian deposits.The far-sourced eolian deposits shared similar provenance with Xiashu loess and were transported by the East Asian winter monsoon.The near-sourced eolian deposits were dust emitted from the adjacent floodplain.The rugged topography weakened the dustfall and strengthened the reconstructive effect of hydrodynamic forces during the Quaternary glacial periods.(3)Material sources of Quaternary red earthThe high chemical weathering indices(CIA)and low base/aluminium(ba)values of the Quaternary red clay in the lower reaches of the Yangtze River suggest that this kind of red clay underwent heavy chemical weathering.For the same profile,CIA value of the vermicular red clay layer is higher than that of the homogeneous red caly or yellow-brown earth layer,while the ba value is lower.These characteristics indicated that the weathering degree of vermicular red clay is higher than that of homogeneous red soil layer and yellowish brown earth layer.In JJ profile,SiO2/Al2O3 values of the heavy weathered vermicular red clay samples is smaller than that of relatively weak weathered yellowish brown earth and homogeneous red clay samples,although the geochemical behavior of SiO2/Al2O3 values tends to increase with the increasing of weathering degree.The ?Ce values of the vermicular red clay show slight negative anomalies,while the homogeneous red clay and yellowish brown earth are slightly positive.The ratio of light rare earth and heavy rare earth(?L/HREE)of the heavy weathered vermicular red clay samples is smaller than that of relatively weak weathered homogeneous red clay or yellow-brown earth samples.The values of?Ce and EL/HREE tend to increase with the increasing of weathered degree.It is easy to infer that if the material source of Quaternary red clay is stable,geochemical characteristics contradict the element geochemical behavior in Quaternary red clay profile.The TiOo2/Al2O3 vs.KO2/Al2O3 plots and clustering analysis of rare earth elements in JJ and NC profiles show that the geochemical compositions of the lower segment vermicular red clay layer,the upper segment vermicular red clay layer and the homogeneous red clay or yellow-brown earth layer are different.The magnetic susceptibility(x)curves of the JJ and NC Quaternary red clay profiles show similar vertical variations:the X of the yellow-brown earth and Homogeneous red clay in the profiles are higher than the heavy weathered vermicular red clay.The magnetic parameters suggest that there is no essential difference in magnetic grain sizes or major magnetic mineral categories between vermicular red clay in the upper segment and homogeneous red clay or overlying yellow-brown earth in the two profiles.Decreasing x values in the upper segment vermicular red clay layer are mainly attributable to different absolute concentrations of magnetic minerals.All of these results suggest that different material sources of yellow-brown earth(or homogeneous red clay)and vermicular red clay are key factors that are decreasing magnetic susceptibility levels in the vermicular red clay layer in the middle and lower reaches of the Yangtze River.Based on the above analyses,after peeling off the influence of weathering effects,this paper argues that material source is the key reason for the different geochemistry and environmental magnetism characteristics between the vermicular red clay layer and homogeneous red clay or yellow-brown earth layer in the Quaternary red clay profile.Combined with the results of grain size analysis,it is concluded that the Quaternary red clay in the middle and lower reaches of the Yangtze River is most likely to be a mixture of far-sourced and near-sourced eolian sediments.The far-sourced eolian deposits shared similar provenance with Xiashu loess and were transported by the East Asian winter monsoon.The near-sourced eolian deposits were dust emitted from the adjacent floodplain and were transported by the summer monsoon or local wind field.Because of the very short transport distance,near-sourced eolian deposits were poorly sorted and retained most of characteristics of floodplain deposits.Abundant fluviation introduced significant diluvium,which provided an abundant source of material for the vermicular red clay deposits.With continuous strengthening of the winter monsoon and river downcutting,the content of near-sourced eolian deposits gradually decreased and far-sourced eolian deposition increased until approximately 0.4 Ma.Different contents of Si,Al,Ce,Ti and rare earth elements in different material sources make the geochemistry charactoristics contradicted weathering charactoristics in the Quaternary red clay profile.On the one hand,different material sources with different background values of x resulted in the lower x value of vermicular red clay than the homogeneous red caly and yellow-brown earth.Moreover,the pedogenic ferromagnetic minerals transformed into hematite also resulted in the lower x value of vermicular red clay.