Geochemical Grain Size Effect Of Aggradational Red Earth And Its Environmental Significance

The aggradational red earth is a typical continental deposit in southern China,and it is a good geological carrier for studying the Quaternary paleoenvironmental records in the middle and low latitudes.There are two types of structures in its sedimentary profile:the first type is yellow brown soil,reticulated red earth,gravel layer and bedrock from top to bottom;the second type is homogeneous laterite,reticulated red earth,reticulated red earth from top to bottom.The main difference between the two types of sections is that after the reticulation process,some sections are overlaid with homogeneous laterite,while others are overlaid with yellow-brown soil.There have been many studies on the formation environment of the Pleistocene reticulated red earth,and it is generally believed that the reticulated red earth is the product of the hot and humid weathering during the abnormally strong summer monsoon.However,after the termination of reticulation process,the research on whether there are differences in the material composition and formation environment of the homogeneous laterite or yellow-brown soil that replaces it has not been in-depth.In this paper,taking the surrounding area of Poyang Lake as the research area,two types of representative profiles,Haihui Zhujiawan Profile（ZJW）and Nanchang Jiulong Dadao Profile（JLDD）,were selected to carry out geochemical research,and combined with particle size classification experiments to explore the Geochemical grain size effect of aggradational red earth,trying to extract the material composition and environmental change signals in detail,in order to deeply characterize the grain size effect and environmental significance of the aggradational red earth geochemistry,in order to further understand the homogeneous laterite,yellow Material differences and environmental information for yellow-brown soils laying the groundwork.The preliminary conclusions of this study are as follows:（1）Whole-rock geochemical studies show that the geochemical compositions of the reticulated red earth samples from the ZJW and JLDD profiles are very similar.The elemental composition is mainly Si O₂,Al₂O₃,Fe₂O₃,the sum of the content of the three is 92.46%and 92.87%,respectively,and the content of easily soluble elements such as K₂O,Na₂O,Ca O,Mg O is very low,respectively 1.79%,0.19%,0.08%,0.59%and 1.87%,0.20%,0.09%,0.69%,the leaching coefficient ba value and CIA index are very close,89.04,87.00 and 0.20,0.24,respectively.After the reticulation is terminated,the geochemical composition of homogeneous laterite and yellow-brown soil differs to a certain extent,which is mainly manifested in that the Si O₂ content（70.04%）of the yellow-brown soil is higher than that of the homogeneous laterite（66.16%）,while the content of Al₂O₃ and Fe₂O₃ is higher than that of the homogeneous laterite（66.16%）.It is lower than the homogeneous laterite,and the difference is large.The content of soluble elements is also higher in the yellow-brown soil.There is a difference in the degree of weathering between the two,and the homogeneous laterite is more weathered.The CIA and ba values of the homogeneous red soil are 86.97 and 80.20,respectively.And the two of yellow-brown soils are 0.28 and 0.48,respectively.,and there may also be differences in the material composition of the laterite and yellow-brown soils.（2）Further research on the geochemical characteristics of different grain sizes of homogeneous lateritic soil and yellow-brown soil shows that there is a grain size effect in the geochemical compositions of both.Compared with the whole-rock sample,the grain group<4μm is relatively depleted in Si,Na,and Ca,while Al,Fe,Mg,K,Mn,Ti,etc.are relatively enriched;Enriched,Al,Fe,Mg,Mn are relatively depleted.With the coarsening of the particle size,the contents of Si O₂,Na₂O and Ca O increased,while the contents of Al₂O₃,Fe₂O₃,K₂O and Mg O decreased.Al,Fe,Na,Mg elements have the most obvious differentiation among the particle sizes.Compared with the two types of samples,the enrichment degree of Al,Fe,Mg and the depletion degree of Na and Ca elements in the yellow-brown soil are smaller than those of the homogeneous laterite.In general,the weathering signals are mainly concentrated in the<4μm components,which are consistent with the change trend of the whole rock.The CIA value of homogeneous laterite（87.32）is significantly higher than that of yellow-brown soil（83.77）.In the silt composition,the stable elements Si and Ti are quite different,which may indicate that the provenance of the two is not completely consistent.（3）In view of the aeolian characteristics of the agglomerated laterite around Poyang Lake,using the northern loess and the sediments of the Yangtze River as the potential sources of long-range and short-range dust to discriminate endmembers,respectively defined as endmember 1 and endmember 2,homogeneous laterite The PI values of the 4～20μm and 20～63μm components were 0.43 and 0.61,respectively,while those of the yellow-brown soil were 0.38 and 0.44.It shows that far-source dust has a certain contribution in yellow-brown soil and homogeneous laterite,especially in fine-grained silt（4-20μm）components,and the contribution rate is higher in yellow-brown soil.The provenance index of the 20-63μm components of the homogeneous laterite is more inclined to the source of the Yangtze River,and the contribution of the yellow-brown soil is not excluded.Combined with the results of the detrital zircon U-Pb age spectrum,the provenance of the homogeneous laterite and yellow-brown soil in the>20μm composition is not consistent.The former is similar to the detrital zircon U-Pb age spectrum of the Yangtze block,and the latter is It is more similar to the U-Pb age spectrum of detrital zircon from the Cathaysian plate.（4）The research on the geochemistry and grain size effect of aggravated red soil shows that after the end of the laterite reticulation process,the regional difference of climate in the southern region increases.Since the late Mid-Pleistocene,the climate has turned to be dry and cool,and the long-distance transport capacity of the winter monsoon to dust has been enhanced,but the contribution of 4-20μm dust is mainly due to the increase of the exposed area of the Yangtze River bank,and the contribution to 20-63μm dust has increased.Homogeneous red soil and yellow-brown soil are not only the difference in weathering degree caused by the difference in latitude,but they may also have certain differences in material composition.