A PRELIMINARY STUDY ON RED EARTH AND CHANGES OF QUATERNARY ENVIRONMENT IN SOUTH CHINA
Zhao Qiguo; Yang Hao (Nanjing Institute of Soil Science, Chinese Academy of Sciences)
As a product of bioclimatic processes, red earth, developing extensively in tropics and subtropics in South China, recorded abundant information of environmental changes. Therefore, the regulation of environmental changes since Quaternary in South China could be expounded through studying the red earth.The Xuancheng profile of red earth in Anhui Province was studied in detail in this paper. According to the structure, color, compactibility and interlayer contact relationship of sidiments, the profile was divided into 17 layers and 9 groups. Each group was composed of upper paleosol and lower brown-yellow soil, with a erosional discontinuity between each two groups. We consider primitive sediments in this profile to be windgenetic according to the following reasons: (1) The stratified structure of the profile is very similar to that of the loess-paleosol sequence in Northern China, and both are an expression of accumulation of several times of deposition,with an erosion and a biochemical soil-forming process after each deposition. (2)In the profile there are easily weathering minerals such as hornblende, chlorite pyroxene, etc. (3) Electron microscope scanning shows that, most of quartz particles are prefectly round and spindle-like, some sharp edged. Again, frostedly glassifying, plate-shaped hollows and shell-like fractures can be seen on the surface of perfectly round particles. (4) According to 32 results of grain size analysis, the average particle diameter is in a range of 7-9, with a deviation (SK) of -0.7-0.9.However, a part of the sediments came from local dry riverbeds and lake banks.The red earth is paleosol that was formed by modifying of the sediments.To elucidate the chronological characteristics of the Xuancheng profile, the ESR age of 10 samples were determined. The results show that a silt layer (Layer 2was 0.817Ma B. P., the plinthitic red earth layers (Layers to 3-12) 0.701-0.426Ma B. P., and the isotropic red earth (Layers 13-15) 0.364-0.126Ma B. P. So the gravel layer and its overlying silt layer could be assigned to later Early Pleistocene.the plinthitic red earth to early Middle Pleistocene and the isotropic red earth to later Middle Pleistocene.To realized more clearly the characteristics of paleoclimatic cycles in this region,four indexes, namely the magnetic susceptibility, the total content of iron oxides,and the content and B13C of organic cotter,were studied. These indexes of the red paleosol are all larger and show peak-shape curves, while those of the brownyellow sediments are smaller and show valley-shape curves. Each layer in the Xuancheng Profile represented independently one special climatic event. The brownyellow sediment represented the wind-drift one formed during several alternations of dryer and cooler climate since Middle Pleistocene, and the red paleosol was formed in a warmer climate. Seven layers of paleosol in the profile represented 7 periods of more warm-humid climate than: at present since Middle Pleistocene. At that time, the plants with a well-developed root system was flourish and so the sediments underwent a biochemical soil-forming process, forming a plinthic structure.The older the sediments age was, the more developed,the plinthic structure.From the development degree of this structure and the δ13C value of organic matter,it could be inferred that the temperatures in warm-humid periods dropped gradually, while the wind's force in dry-cool periods rose gradually since Middle Pleistocene.Finally, compared with the loess-paleosol profile developed at the same epoch in Northern China, we found that although the both single-layer thickness of and the accumulation thickness of wind sediments, namely the deposition flux and the rate of deposition, in glacial periods in the South were smaller than those in the North loess region, the climatic environmental changes reflected in the South and the North had an obvious correlat ability, this is of significance for further studying the red earth and the Quaternary environmental changes in South China.
【CateGory Index】： P534.63
【CateGory Index】： P534.63