Comparative Study On Quaternary Red Earth And Xiashu Loess In Physical And Chemical Characteristics And Its Environmental Significance

As Quaternary sediments, aggradation red earth in mid-subtropical zone and Xiashu Loess in northern subtropical zone are frequently seen in the middle and lower reaches of the Yangtze River region in China. It has a lot of study results about red earth and Xiashu Loess environmental record, but it lacks of the comparative study between the two types of sediments. Paper chooses the middle and lower reaches of Yangtze River region in China as study area, which is the transition zone between south China and north China. This area is a climate change sensitive area. Study object includes four typical aggradation red earth sections located in southern Anhui (XC section), northern Jiangxi (JL section), Zhejiang (TX and AJ section), and one typical Xiashu Loess section located in Nanjing, Jiangsu (XG section). Based on some methods of sedimentology and geochemistry, we analyzed the Grain-size, quartz surface characteristics, major elments, trace elments and total organic carbon (TOC), and compared the physical and chemical characteristics of red earth and Xiashu Loess. Paper study bottom-up sedimentary sequence from red earth to Xiashu Loess at the same section, and Xiashu formation sediments deposited in the same epoch but different areas, then their physical and chemical characteristics and paleo-environment records were analyzed, to study their paleo-environment differences. The main preliminary conclusions are:(1) The sedimentary characteristics of all aggradation red earth are similar, which indicate aeolian characteristics. Their grain-size characteristics are generally uniform:Silt (5～50μm) is the advantages particle, mean content of red earth ranges from 55.74% to 65.23%; the content of dust particle (10～50μm) is high, the average value between 34.27%～46.22%; the content of clay (<5μm) is also high, the average value of red earth is between 33.86%～42.62%. The content of gravel (>50μm) is least, the average value between 0.9%～2.84%. Grain-size frequency curves mostly are positive offset with single-peak shape and small-tail, some samples contain coarse tail. Grain-size characteristics and surface micro-structure features of quartz particles indicate aeolian features and strong weathering. The sedimentary characteristics of aggradation red earth and Xiashu Loess are similar, the advantages particle of Xiashu Loess is silt, mean content is 68.61%, the average content of clay is 24.42%, gravel is 6.97%, and the content of dust particle is high(53.39%). However, comparing with Xiashu Loess, aggradation red earth has smaller particle size, poorer sorting, and stronger weathering of quartz surface, all above reflect the red earth's weathering is stronger, or intensity of sedimentation is weaker.(2) The red earth and Xiashu Loess elements geochemical characteristics of all samples have good consistency, but red earth has stronger weathering. The red earth total contents of SiO2, Al2O3 and Fe2O3 range from 85.19%～91.71%, Xiashu Loess 84.68%, K2O, Na2O, CaO, MgO are leached out in large quantities. Contrast with the upper continental crust-normalized (UCC) values for major elements, contents of Ti, Mn, Fe, Al are higher, but contents of K, Na, Ca, Mg, P are lower. The trace elements contents of them are similar. Contrast with UCC, Sr contents are lower, but contents of Cr and Co are higher. TC or TOC contents are lower. The chemical indexs indicate: CIA average value of red earth (between 77.86%～90.63%, average value is 86.31%) is higher than Xiashu Loess (between 70.32%～82.36%, average value is 73.7%), but ba's average value of red earth (0.17) is lower than Xiashu Loess (0.36), Rb/Sr value of red earth (1.42～3.34) is higher than Xiashu Loess (1.09～1.65). They all reflect red earth has stronger chemical weathering than Xiashu Loess.(3) From the view of the same section of the aggradation red earth, vermicular red earth layer, homogeneous red earth layer and yellow-brown earth layer arrange in the bottom-up sequence, bottom layer is old and upper layer is young. By contrast with physical and chemical characteristics, all layers of the aggradation red earth are similar. But from the vermicular red earth layer to homogeneous red earth layer or yellow-brown earth layer, sediment particles become thicker, sorting turn better, CIA turn gradually weaker:Mφof vermicular red earth is between 7.11～7.52φ, homogeneous red earth is 7.479, yellow-brown earth is between 6.85～7.04φ; CIA Corresponding is 86.1%～88.07%,83.3% and 81.54%～83.83%. It indicates that when the vermicular red earth forms, sedimentation weakens, weathering strengthens; while yellow-brown earth forms, sedimentation significantly enhances, weathering process weakens strongly. Sedimentary sequence in the same red earth section from vermicular red earth to yellow-brown Xiashu earth may record the overall trend of paleoclimate:from the hot and humid to dry and cold during the Quaternary, which also records the growing of East Asian winter monsoon profile from the Middle Pleistocene to the Late Pleistocene.(4) The ESR age of yellow-brown Xiashu earth of JL section is 365 ka B.P.. Through the contrasting of layers, XG Xiashu Loess and yellow-brown earth belonging to Xiashu Loess upper the JL red earth profile have the same deposition age, belonging to the late period of Mid-Pleistocene. Compared with XG Xiashu Loess, JL yellow-brown earth locates more southerly, and its accumulation thickness is thinner, but their physical and chemical characteristics are similar overall, showing aeolian features. So it is possible that they both belong to the eolian deposits during the same period with the similar characteristics in different latitude zones. Compared with XG Xiashu Loess, the yellow-brown earth upper the JL section has some different features, the average grain-size is smaller (the former Mφis 6.85φ, the latter Mφis 6.46φ), sorting is poorer, degree of chemical weathering is higher (the former CIA value is 81.54%, ba value is 0.23; the latter CIA value is 73.7%, ba value is 0.36), the features above may indicate the level of soil weathering gradually increased from northern subtropics to mid-subtropics and show latitude effect and horizontal zonality of paleo-climate conditions and monsoon circulation.All above show:red earth and Xiashu Loess are eolian sediments with similar physical and chemical charcteristics, but they are different sedimentary bodies. When red earth and Xiashu Loess in the same section, they compose young-old sequence, and the grain-size of red earth is smaller, chemical weathering is higher than Xiashu Loess, it indicates the trend of paleoclimate from the hot and humid to dry and cold during the Quaternary; When Xiashu Loess with the same deposition age, they may appear in different latitude in Yangtze River zone, and Xiashu Loess uppear red earth has smaller particle size and stronger weathering than typical Xiashu Loess, it reflects the latitude effects of paleoenvironmental changes and horizontal zonality.