Geochemical characteristics of the eolian deposits in southern China, and their implications for provenance and weathering intensity

摘要

The loess-paleosol sequences in the middle-lower reaches of the Yantgze River are valuable geological records of Quaternary environments that bear information about climate change for the subtropical zone of southern China. Type sections containing eolian deposits in this region mostly include the upper loess-soil sequence referred to as Xiashu Loess Formation, the middle Vermiculated Red Soil (VRS) and the underlying fluvial deposits or bedrock. In this study, Xiashu loess, paleosol formed in the Xiashu loess, and the VRS samples from Xuancheng (XC) and Jiujiang (JJ) sections were analyzed for their elemental (major and trace elements) and isotopic (Sm-Nd) geochemistry and compared with Pleistocene loess and paleosol samples from the Chinese Loess Plateau (CLP) in northern China. The results show an overall similarity in geochemical characteristics between the southern China loess and the average UCC, indicating that the dust materials were derived from well-mixed sedimentary protoliths which had undergone numerous upper crustal recycling processes. In comparison with the loess of northern China, the eolian deposits in southern China have higher TiO_2, Zr, Hf and lower Ba concentrations, higher TiO_2/Al_2O_3, U/Pb, Th/Pb and lower Ba/Rb, Zr/Hf, Eu/Eu-, Ce/Yb, Eu/Yb ratios, and lower -Nd(0) values with relatively greater range. These features coincide with the complicated source rock compositions in the middle-lower Yangtze River, which suggests local sources for the eolian deposits in this region. The eolian deposits in southern China have experienced the intermediate K removal weathering stage, Ca, Sr, Na, Mg and K are the main mobile elements in the weathering process. In comparison with the Xiashu Loess Formation, the VRS has greater depletion of Na, Mg, K and Mn, indicating a stronger weathering intensity or perhaps a longer period of weathering.