Geochemical Characteristics Of The Quaternary Eolian Sediments In Mid-Subtropics

There are multiple types of Quaternary Aeolian sediments in the subtropical region of China, such as XiaShu Loess,aggradation red clay and old red sand, which recorded abundant information of the environment change. This paper selected three kinds of Quaternary Aeolian sediments as the study object, they includes seven typical aggradation red clay sections located in Anhui (XC section), Zhejiang (AJ,XA,TX and PJ section), Jiangxi (SH section) and Hunan (ZNL section),one typical XiaShu Loess section located in Nanjing,Jiangsu (XG section),and two typical Old red sand sections located in Zhejiang (DS section) and Fujian(QF section). Based on the characteristics of the physical and chemical indicators,such as major elements,trace elements and the tests of the granularity of the Old Red Sand(Particle size less than63μm), and by studying the aeolic features and the geochemical characteristics of the Aeolian sediments as well as extracting the fine components of the red sand(<63μm) and comparing it with other types of Aeolian sediments, the geochemistry contact between different types of Aeolian sediments were discussed preliminary. The main conclusions are as follows:(1) The sedimentary characteristics of the Quaternary Aeolian sediments in the Mid-subtropical zone are similar, but the grain-size characteristics are quite different. The Old Red Sand has a much coarse-grained level than that of the other two types of the Quaternary Aeolian sediments, which means sand (>63μm) is the dominant particle of the Old Red Sand, and its mean content ranges from43.77%to68.64%, the mean content of the silt (4～63μm) changes between19.16%and37.14%, and the average content of the clay (<4μm) is the least, which ranges from12.20%to19.09%, but the grain composition(<63μm) of the Old Red Sand is similar to that of the other types of Quaternary Aeolian sediments, which indicates the silt (4-63μm) is the main particle in the grain composition(<63μm) of the Old Red Sand, and its average content ranges73.29%to74.24%, the mean content of the clay (<4μm) is28.11%-29.30%,and the total content of the silt and the clay is93.82%-99.01%; as to the grain-size characteristics of the Aggradation Red Earth and the XiaShu Loess, silt (4-63μm) is their dominant particle and the content ranges from59.07%to76.91%, the average content of the clay(<4μm) ranges from19.76%to41.27%, the content of the sand(>63μm) is the least; It can be concluded from overall analysis that the material composition of the Aeolian sediments contains the nearly source and far source components in the Mid-subtropical area of China, and their grain composition of the fine part have some similarities.(2) The Quaternary Aeolian sediments in the Mid-subtropical zone share some similarities in the chemical composition of the major elements, the main major elements of the Quaternary Aeolian sediments are SiO2, Al2O3, T Fe2O3, their total contents range from85%to90%, K2O, Na2O, CaO, MgO, P2O5are leached out in large quantities; the content of SiO2, Al2O3, T Fe2O3of the Aggradation Red Earth changes from87.13%to90.49%, its total leaching loss ranges between1.65%and3.30%, mean content of SiO2, Al2O3, T Fe2O3of the XiaShu Loess is84.67%, and its total leaching loss is5.77%, the mean content of SiO2, Al2O3, T Fe2O3of the Old Red Sand is84.02%-84.87%, and its total leaching loss is2.78%-3.59%; by contrast with the upper continental crust-normalized(UCC) values for major elements, some differences among the major elements characteristics of different types of Aeolian sediments are found, contents of K, Na, Ca, Mg, P of the three kinds of the Aeolian sediments are lower, the contents of Al、Fe、Mn、Ti of the XiaShu Loess and the Aggradation Red Earth are higher, the contents of Mn, Ti of the old red sand are higher while the contents of Al, Fe are lower, and their mean content are11.57%-12.48%and2.92%-3.95%. From the spatial contrast of the sum content of T Fe2O3and Na2O, CaO, MgO, the content of T Fe2O3shows a slowly growing trend from the XiaShu Loess to the Aggradation Red Earth, while the sum content of the Na2O, CaO and MgO displays a slow decreasing trend; On the east-west direction, from the Old Red Sand to the Aggradation Red Earth, the T Fe2O3content shows a slow growing trend, while the sum content of Na2O,CaO and MgO performs a decreasing trend; the spatial variation of the CIA value shows an growing trend from the XiaShu Loess to the Aggradation Red Earth, the CIA value also increases gradually from the Old Red Sand to the Aggradation Red Earth, QF Old Red Sand located more southerly than that of the DS, but its CIA value is bigger than that of DS, thus, the weathering degree of the sediments, to a certain extent, changes with the latitude, and the features of the weathering intensity reflect a latitudinal effect.(3) The distribution and enrichment-loss characteristics of the trace elements of the Quaternary Aeolian sediments in the Mid-subtropical area have some similarities: Cr, Co, Cu, Zn, Rb, Sr, Y, Zr, Ba, Pb are the dominant elements, and the contents of Zr, Ba, Pb are higher than the other trace elements, which indicate that the changing rule of trace elements of the three types of the Quaternary Aeolian sediments have relative synchronicity; the contents of the Cr. Co. Y, Zr, Pb of the Aggradation Red Earth are higher, while that of the content of Cu, Zn, Rb, Sr, Ba are lower; the contents of Cr, Co, Y, Zr of the Old Red Sand are higher, while its Cu, Zn, Rb, Sr contents are lower; the contents of Cr, Co, Y, Zr of the XiaShu Loess are higher, but its Zn, Sr contents are lower.(4) Through the spatial contrast of grain size composition of the XiaShu Loess, the Aggradation Red Earth,the Old Red Sand and that of the Old Red Sand’s fine-grained component (<63μm), the grain-size frequency curves shows that the peak particle size of the Old Red Sand in DS is5.8φ to6.0φ and that of the Red Earth in XA is6.1φ to6.2φ, AJ and XC are6.2φ to6.3(p, the peak particle size is become smaller gradually from the direction of DS, XA to TX,PJ and SH. The scattering diagrams of SKcp-a and SKφ-Mcφ/φ shows that there is a gradual transitional characteristics between the fine-grained components of the Old Red Sand and that of the Aggradation Red Earth in Mid-subtropical area, and the particle size distribution curve of the XiaShu Loess have the same characteristics with that of the Aggradation Red Earth, the components of the grain from XG to XC and AJ also have a gradual transitional characteristics. By the analysis of the dust dynamics theory and nearly40years of wind data in Dongsha Zhujiajian, this paper presumed that there might be a connection between XG XiaShu Loess and XC, AJ Red Earth; moreover, the distribution features of the fine fraction (<63μm) of DS Old Red Sand were similar to that of the grain size frequency curves of XA red earth, and based on the graded properties of grain composition of DS, XA,TX and PJ, the author also presumed that the Old Red Sand of DS (<63μm) and the Red Earth of XA, TX, PJ may have a certain degree of connection.