A Study On Patterned Red Clay Formation And Environment Change In Gannan Region

The red clay in the south occurs from the southern bank of the Yangtze River in the North to South Sea Islands in the South, and from the islands of Taiwan archipelago in the east to Yunnan-Guizhou plateau in the west. It distributed mainly in Jiangxi Province, Hunan Province, Zhejiang Province, Fujian Province, Anhui Province, Hubei Province, Guangdong Province, Hainan Province and so on. The entire area covers about2000000km2. Along with the deeper research of global change, the red clay research have reached a great success, especially in weathering rate, soil forming process, formation date, environmental significance, geomagnetism significance, tectonic significance, regional correlation and so on. A large number of scientific and technical workers have made a lot of carefully and thoroughly study works and obtained many important results for southern patterned red soil for a long time. There are many problems that have not reached consistent recognition on red clay, such as the deposition fonnation, the question whether it can record geologic climate change or not, formation ages and so on. The research focused on the three scientific problems:red clay sedimentary characteristics, formation date and the quaternary red clay sedimentary environment change. In this paper, the Quaternary red clay sedimentary characteristics were analyzed, making comprehensive use of particle size analyse, mineral composition, chemical composition, magnetic susceptibility, sporo-pollen analyse and so on. The forming ages of the Quaternary Red Earth were infered in contrast with three section ESR dating. Combining mathematical model between time with silicon transport intensity, we can concluded that the forming ages of granite red weathering crusts. The environmental change about the Quaternary Red clay depositional dates could be infered in contrast with the red clay profile sporo-pollen Environmental Indication, susceptibility environment significance, by making comprehensive use of palynology, soil science, geology, geochemistry in studying the above scientific problems. It can help us to study the Quaternary climate change, and fill in geographical space on red clay research in Gannan region.On the basis of field investigations, it was found that the Quaternary red clay is distributed in non-continuous accumulations. The thickness of them are mainly about4m, some are above5m, which do not possess the same feature of eolian deposition in northern China. By comparing the chemical compositions of2red clay sections, it was indicating that there are big differences in important chemical composition contents between the north loess and the Southern Quaternary red clay in Gannan region. Through particle size analytical technique, the results showed that the average contents of coarse silt (10-50μm) were respectively29.14%,30.80%,38.15%in the three red clay sections. While the average contents of loess usually is greater than50%. Through the analysis above, we may conclude that the Quaternary red clay was not aeolian origin.By way of contrastive analysis Grain-size characteristics between the Quaternary red clay and granite red clay, the results showed there are some differences among Grain-size combination, Grain-size parameters and Grain-size curve. By using X diffraction analysis technology, it was found that there were difference of map and mineral composition between red clay and its beneath arenaceous shale. We may conclude that the Quaternary red clay did not derive from weathering of arenaceous shale.Through particle size analysis of huangjin (HJ), shahe (SH) and dangtang (DT) red clay sections, the results showed that the average contents of coarse sand grain (>63μm) were19.78%,21.84%,12.16%respectively. There were more coarse particles in red clay sections, and contained gravel (>2mm) in the whole red clay sections; suggesting be characteristic of alluvial deposition, by way of particle parameters criterion, and showed that the three red clay section were formed in alluvial sedimentary environment,By way of Pollen Analysis, it was founded that there are some aquatic herbs in huangjing(HJ) red clay section. Based on field investigations, we found that the Quaternary red clay distributed maily in the terrace II or III in Zhangjiang rive, Gongjiang rive and Ganjiang rive. So We may infer that the Quaternary red clay in Gannan region derive from pinnacled soil and weathering shell, which was washed down the mountain by the river and collected in low-lying depressions; and later the earth crust were elevated and stream were trenched. Nowadays the Quaternary red clay distributed in the red hill about100～200m along rives in Gannan region.The chemical compositions and magnetic susceptibility were tested and analyzed in Huangjin (HJ) section, Shahe (SH) section and Dangtang (DT) section. It was found that the major elements (SL,K,Ca,Mg,Na) show the characteristic of leaching and movement in Tiancun section, and increasing tendency from the bottom to the top. The accumulation of trace element (V,Cr,Mn) located between0.2～0.5, and belong to moderate accumulation, and the increasing tendency from the bottom to the top. All this points support the same conclusion: the desilica-allitization of red clay is in the pipeline on modern climate environment. The low frequency magnetic susceptibility of red clay horizon:the vermiculated red clay horizon is better than red soil horizon is better than soil horizon in magnetic susceptibility. The average magnetic susceptibility of the quaternary red clay is less than that of granite red, the magnetic susceptibility surface soil get lower than underneath red clay. This may have some connection with warm and wet climate environment in gannan region. The coefficient of SiO2/Al2O3and Al2O3/Fe2O3has a decreasing tendency from the bottom to the top, while the coefficient of R2O3/SiO2has an increasing tendency from the bottom to the top, reflecting that the degree of weathering were getting stronger and stronger from the bottom to the top. By the way of ESR dating for the bottom and top samples in Huangjin (HJ) section, Shahe (SH) section and Dangtang (DT) section, the results show a good approximation, age datas in three red clay section fluctuates between80～120Ka.B.P., belonging to the deposition of early late Pleistocene. Based on a mathematical model of relation between silicon migration intensity and time from Zhao qiguo academician, Ages of red clay of these samples in Huangjin (HJ) section range from80～360Ka.B.P.. Age datas for the superior part samples of section fluctuates between80～220Ka.B.P.,Belonging to early middle Pleistocene or later middle Pleistocene. Age datas for the underpart samples of section fluctuates between270～370Ka.B.P., belonging to early middle Pleistocene.Based on the sporo-pollen data of eight surface samples which include three red soil samples on granite type, three samples on the Quaternary red clay and two samples on sand shale in Gannan region, Luo-Pacias spore-pollen appraisal system identification of spore-pollen were used, sum up to131species, and Statistics valid pollen2694, it was found that the production of spores-pollen are rich in species and genus in red soil in Gannan region, surface sporo-pollen assemblages are composed of trees, shrubs herbs, pteridophytes; the vegetation structure of spores-pollen assemblage include arbors and shrubs. There were more acidity and barren palynoflora, such as Pinus massoniana, Podocarpus macrophyllla, Hicriopteris glauca, Nepholepis cordifolia and so on, especially Pinus massoniana and Hicriopteris glauca are strongly adaptable to the laterite environment. Spore-pollen assemblages showed that its proportions can reflects basic characteristics of botanical community in modern times.We took20cm intervals samples in Huangjin(HJ) red clay section. Through spores-pollen experiment, magnetic susceptibility value and Kd value have been studied by comparison of sporo-pollen assemblages change in Huangjin(HJ) section. It was concluded that the red sedimental date had experienced four stages climate change:warm and moisture stage, warm and wet stage, warm and dry stage, heat and moisture, there are consistent in big climate incident with Change of Global Climate, By comparison of surface and section sporo-pollen assemblages, we concluded that the geological climate of red sedimental were warmer and more moist than modern climate environment.