安徽郎溪县十字镇第四纪红土理化性质及其古环境意义

The Quaternary red soil is widely distributed in the middle and lower reaches of the Yangtze River, and also an important medium reflecting the Quaternary paleo-environment in southern China. The soil profile, located in Shizi town of Langxi County, Anhui Province, has a special double-layer structure with Yellow-brown Earth overlaying the reticulate red clay. Based on the analysis of particle-size, soil magnetism, color index and diffuse reflectance spectra of the 155 samples, combining with the research methods of Chronostratigraphy and Magnetic stratigraphy, comparing the study profile with that of Xiashu loess in the middle and lower reaches of the Yangtze River, this paper aims to explore the material source of the profile. We also compare double-layer structure with Yellow-brown Earth overlaying the reticulate red clay, in order to reveal the implications of paleo-climate and paleo-environmental change about the transition of the double-layer structure. The results show that: 1) The studied profile of the section contains no gravel (>2 mm), low content of sand (> 63μm), and aeolian particle (10~50 μm) is the major type of particle in all samples, clay (<2 μm) is the second major type. All the above conditions reflect the aeolian characteristics of the studied profile. Generally speaking, the grain-size frequency curves of the reticular red clay horizons show the double-peak mode-the 10~50 μm particle group is “the dominated mode” and the one less than 2 μm is “the secondary mode” or “the dominated mode” of some samples. As compared with the Yellow-brown Earth range in the upper part, the underlying reticular red clay range is finer in particle-size and more intensively weathered. 2) The results of soil magnetism, color index and diffuse reflectance spectra for the Yellow-brown Earth range and the reticular red clay range samples in Langxi shows that the reticular red clay range has more hematite and goethite than the Yellow-brown Earth range. This is because the reticular red clay development in southern China was under an extremely humid condition, pedogenesis actually occurred under waterlogged conditions. So the fine-grained magnetite and magnetite were gradually dissolved, and finally converted into goethite and hematite. 3) There is significant negative correlation between magnetic susceptibility (MS) values and clay fraction contents, indicating the MS values are controlled by the weathering degree of the reticulate red clay. The studied profile has significant changes about each physical and chemical indicators from upper part’s Yellow-brown Earth to lower part’s reticulate red clay, that may reflect a significant climate change event in southern China, and can be considered to be a response to the middle Pleistocene major global climate change in the middle and lower reaches of the Yangtze River.