| Red earth in subtropical-tropical areas of southern China is known for the production of monsoon climate of Quaternary period. It is an important continental sedimentary carrier of paleoenvironment and climate changes, and contains abundant information of environmental succession. Much research on red earth is focused mainly on material source, stratigraphic, geochemical and magnetic characteristics. An in-depth and systematic research of its origins, chronology and paleoclimatology were also carried out. But, there were still some divergences in its origins, pedogenesis and the relationship of magnetic susceptibility and its paleoclimatic changes. Based on above divergences, we conducted a thorough research on red earth of Yunnan plateau and Vermiculate red earth (VRE) of Yangtze valley, to explain its pedogenesis and environmental changes from three aspects:(1) to study the paleopedogenic of magnetic mineral and its environmental response;(2) to explore the origins of nano-scale iron oxide minerals and its environmental response;(3) and to establish the relationship between pedogenesis of iron oxide minerals and geochemistry process. This study was conducted to establish a relationship between pedogensis and paleoenvironment factors on red earth in subtropical-tropical areas of southern China. The conclusions from the study are as follows:1. Pedogenic magnetite and maghemite is main carrier of magnetic minerals of red earth in tropical and subtropical regions of southern China. The Uniform red clay (URC) of VRE in Yangtze valley contains more maghemite and hematite of SP and SSD grains. And the reticulate red clay (RRC) of VRE contains more antiferromagnetism minerals of SSD grains. Red earth in Yunnan plateau are mainly compose of ferromagnetic minerals (magnetite and maghemite) of SP grains, and contains few of goethite and hematite.2. Geochemical analysis shows that RRC of VRE has experienced high weathering pedogenesis. Magnetism and mineralogy research explains that the loss of magnetic susceptibility in RRC is due to the highly magnetic mineral of maghemite converted into weak magnetic mineral of hematite promoted by highly weathering pedogenesis.3. Rock magnetism and iron oxide mineral analysis indicated that the climate conditions of Yangtze valley is due to alternation of wetting and drying seasons with short term heavy rains. That is in the initial period of the VRC formation, the pedogenic environment was moist and warm, and suited for significant formation of maghemite. After this, a period with a high temperature and seasonal drying climate prevailed, which favored the transformation from maghemite to hematite Pedogenes is largely depends on the pedogenic environment, So magnetic parameters (HDRM and SIRM/x) caused by pedogenic environment can be used to indicate the changes of paleoclimatic.4. Thermomagnetic curves of red earth in Yunnan plateau showed that red earth with some magnetite and goethite has higher isomorphous substitution. The geochemical analysis showed a significant relationship between element of Ti and Fe and the correlation coefficient was greater than0.83. The TEM micrograph of nanoparticles also showed that red earth in Yunnan plateau contain Ti content, combined with Fe, exist as titanomagnetite and titanomaghemite in red earth. Geochemical analysis of highly magnetic red earth showed that titanomagnetite and its isomorphous replacement products mainly exist as primary mineral of titanomagnetite and titanomaghemite in magnetic particles of red earth.5. Magnetism data showed that the topsoil of red earth in Yunnan plateau has high hematite, maghemite contents while magnetite is in less quantity. Formed higher content of super paramagnetic magnetic minerals by the weathering Pedogenese. It shows magnetic enhancement in topsoil, This magnetic enhancement in topsoil is due to highly weathering under hot-wet alternate climate. We suspect that this kind of highly magnetic red earth could not have formed above the elevation of2000m, It should had been uplifted by the neotectonics after its formation. According to the difference in elevations between the red paleosols and modern red soil base, We can estimate that the red earth in Yunnan plateau had been lifted about1600m since its formation, the maximal uplift occurred in the middle Pleistocene other than in the early Pleistocene. So the highly magnetic susceptibility confirmed that Yunnan plateau has experienced strong uplifted by the neotectonics in the mid and late quaternary.6. The loss of magnetic susceptibility before and after DCB treatment indicated that magnetic minerals in red earth of Yunnan plateau mainly consist of pedogenic fine ferrimagnetic minerals (FM)(superparamagnetic particles (SP) and/or stable single domain (SSD). Paleomagnetic and HRTEM analysis found that pedogenic nanoscale magnetite and maghemite are the main reasons for magnetic enhancement produced under aerobic conditions. The pedogenic fine ferrimagnetic minerals (FM) is an intensity index of pedogenesis. The presence and amount of iron oxide provide effective information for the pedogenesis under different climate condition and help in the reconstruction of paleoclimate.7. Magnetic parameters of highly magnetic red earth in different climate area showed that magnetic susceptibility and the content of free iron oxide (Fed) in Yangtze valley are significantly lower than red earth of Yunnan plateau. Yunnan is located in the hot zone near the equator. High temperature and rainfall, small difference in temperature and high intensity of weathering lead to high leaching of mobile elements (e.g. Ca, Mg, K and Na) and relative accumulation of immobile elements (e.g. Fe and Al), and kaolinite, quartz, gibbsite, hematite, maghemite and magnetite as the dominant mineral.8. The field emission scanning electron microscope (FESEM) showed the magnetic minerals in the subtropical-tropical soils, southern China are existed in forms of strip, sphericity, clava, stratiform, schistose, octahedron and quadrilateral. With a Magnetic-particle contains high content of iron and with the result of EDS we can concluded that these magnetic-particles include:strip, sphericity, clava shapes, are the pure iron oxide minerals while some of the strip magnetic mineral with aluminosilicate minerals were adhere to the suface. Magnetic minerals with Stratiform and schistose shapes of contains higher content of Ti while octahedron and quadrilateral shapes have less magnetic mineral. Based on the atom ration analysis of Fe and Ti, we can concluded that stratiform and schistose shapes of magnetic mineral are the titanomagnetite while as octahedron and quadrilateral shapes are titanomaghemite.9. Magnetic minerals of highly magnetic soils are highly enriched with heavy metals and trace elements such as:Ba, Cd, Co, Cr, Ga, La, Mn, Mo, Pb, Sn, Zn, Ti. Primary magnetic minerals were enriched with five elements like Co, Ga, Mn, Sn, Ti while the rest of the elements were present in the secondary iron oxide minerals. Red earth is highly enriched in common heavy metals, such as Zn, Cd, Cr, Pb and the enrichment intensity in turn is Cd> Zn> Cr> Pb. |
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