| The framework of climate change is mainly based on Marine sediments in global change research. In recent years, the quaternary climate research has attracted a great attention of geologists with the development of the global change research. However, the reasearch on global climate change using ancient soils have become a hot subject of earth science. The loess-palaeosol sequences in the Loess Plateau, northwest China, contain an excellent record of multiple-cycle change of Quaternary climate, which is quite consistent with the results revealed by deep-sea isotope and has attracted great attention of the palaeoclimatic workers at home and abroad. China is located in a typical monsoon-influenced region. The summer monsoon brings abundant moisture and heat to the continent, while the winter monsoon transports large volumes of aeolian dust to areas of east and south China. Recent studies of Quaternary sediments in south China suggest that deposits from the Pleistocene onwards cover the mid-lower reaches of the Yangtze River. In response to a warmer and wetter climate, soil development migrated progressively southwards. Aeolian deposits in south China may be subject to relatively intense syndepositional pedogenic alteration, resulting in the removal of some imprints of Aeolian provenance and consequently making it difficult todecipher the climatic and environmental information documented in the sediments. The Quaternary sediments in south China were accretionary in nature, reflecting some degree of syndepositional pedogenesis under tropical to subtropical climate conditions. Unlike the loess-palaeosol sequence with clearly defined soil layers of different colours and structures in northwest China, Quaternary sediments in south China were mainly red earth. Although in recent years the deposits have attracted the interest of many pedologists and geologists for the purpose of reconstructing palaeoenvironments and palaeoclimates in south China, interpretations of the climatic significance of the red earth are still debatable. Investigations of the Fe2O3content and organic13C composition of the Xuancheng red earth profile in the mid-lower reaches of the Yangtze River, south China suggested that the red earth in fact comprised aeolian deposits with intense pedogenic modification, which could be stratigraphically correlated with the loess deposits in northwest China. However, the correlation between the red earth and the loess deposits was not clear, as the differences in the multiple cyclic changes in magnetic susceptibility, Fe2O3content and organic13C composition were very small. Moreover, there was a major pedogenic discrepancy in the soil-forming periods between the two materials. Pedogenic alteration indeed removed some imprints indicating the environmental and climatic conditions. However, this process would also have produced clay minerals corresponding to the conditions of weathering or hydrolysis of the materials. Hence, evidence for environmental and climatic conditions is probably preserved in the clay fraction of the sediments.Previous studies have mainly focused on magnetic strata, organic carbon isotope and grain size analysis, etc., and the clay mineralogy of red earth is rarely lack. Although the asseblage, relative content and crystallinity of clay minerals in sediments have been widely used in the reconstruction of the paleoclimate. However, the characteristic mineral phases must be used to obtain more accurate climate information. As a special intergrade clay mineral,1.4-nm Spacing intergrade mineral, widely existing in red earth of southern China, is usually considered as a intergrade mineral transforming from vermiculite to montmorillonite or chlorite to montmorillonite. Due to the limited methods and ideas in the past,1.4-nm Spacing intergrade minteral was mistaken as chlorite, vermiculite or smectite, which would influence the understanding the internal factors of physical and chemical properties of soil,also the clay mineral evolution in the ancient soil, as well as soil classification and its climate significance. Therefore, specific species, Genesis, evolution, and climate environment of1.4nm intergrade minteral from different soil layers of Jiujiang and Xuancheng section are investigated using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), High resolution transmission electron microscope (HRTEM), Scanning electron microscope (sem) and Inductively coupled plasma emission spectrum (ICP-AES), which is of great importance to clay mineralogy, soil mineralogy, and even the global change research and so on.The XRD patterns of the clay fractions show that1.4-nm Spacing intergrade minteral is widely distributed in Jiujiang and Xuancheng red earth section, with the highest levels in yellow sandy soil of Jiujiang section. The1.4nm peak of1.4-nm Spacing intergrade minteral did not expand after Mg saturation and glycerol solvation, and the1.4nm peak shifted to1.38nm and afterwards1.20nm with temperature, indicating that1.4-nm Spacing intergrade minteral is hydroxy-interlayered vermiculite (HIV). The XRD patterns of Air-dried, Mg-saturated and glycerol solvated, Mg-saturated and heated and K-saturated and heated samples show that the clay mineral assemblages of yellow sandy soil were mainly illite, HIV and kaolinite, with minor illite-vermiculie or illite-HIV and kaolinite-vermiculite or kaolinite-HIV mixed-layer clays. The clay mineral assemblages of homogeneous red earth were dominated by illite, vermiculite and kaolinite, with minor HIV, illite-vermiculie or illite-HIV and kaolinite-vermiculite or kaolinite-HIV mixed-layer clays. The clay mineral assemblages of net-like red earth were composed of kaolinite, illite, illite-vermiculie and vermiculite, with kaolinite-vermiculite or kaolinite-HIV mixed-layer clays. The clay mineral assemblages of homogeneous red earth in Xuncheng section were dominated by illite and kaolinite, with minor HIV, illite-vermiculie or illite-HIV and trace kaolinite-vermiculite or kaolinite-HIV mixed-layer clays. Net-like red earth had the same clay assemblages with homogeneous red earth, but contained more kaolinite and illite-vermiculie or illite-HIV mixed-layer clays. The increase of illite and vermiculte and the decrease of kaolinite and illite-vermiculie or illite-HIV mixed-layer clays occurred from the bottom up along the two sections, indicating that since middle pleistocene the climate advanced toward cold and dry in the mid-lower reaches of the Yangtze river. The relatively high content of HIV in Jiujiang yellow sandy soil suggests that yellow sandy soil apparently experienced a different pedogenic process with homogeneous red earth and net-like red earth.Under HRTEM, illite-vermiculite or illite-HIV mixed-layer minerals and kaolinite-vermiculite or kaolinite-HIV mixed-layer minerals can be clearly observed. Meanwhile, lattice fringe image presented the transformation of illite layers to vermiculite or HIV layers, and also showed the transformation of vermiculite or HIV layers to kaolinite layers. These observations show that the weathering sequence of clay mineral of Jiujiang and Xuancheng red earth section may be as this:illiteâ†'illite-vermiculite or illite-HIV mixed-layer mineralsâ†'vermiculite (HIV)â†'kaolinite-vermiculite or kaolinite-HIV mixed-layer mineralsâ†'kaolinite. An increased weathering trend towards the bottom of section, the transformation of illite and vermiculite or HIV to kaolinite was accelerated, causing the downward increase of illite-vermiculite or illite-HIV mixed-layer minerals and the downward decrease of illite and vermiculite or HIV.Under the observation of SEM, the clay minerals in the Jiujiang and Xuancheng section occurred as flakiness shape with irregular and fragmentary edges, indicating the clays have undergone intensive weathering during its formation. The crystallization of clay minerals exhibited a decreased tendency from the yellow sandy soils to homogenous red earth and to net-like red earth, suggesting the weathering of the section enhanced downwards, meanwhile showing the climate in mid-Pleistocene was warmer and wetter than that in late Pleistocene. The EDS showed that the chemical composition of the flakiness shape clays in the red earth of the Jiujiang and Xuancheng section were mainly composed of Si, Al, Fe, K, Mg and Na, with enrichment of K and Mg in the yellow sandy soils. These results with more enrichment of K and Mg might mean the higher content of illite and vermiculite. The element Mg presented a lower value in the homogenous red earth and net-like red eath, indicating a lower content of vermiculiteic minerals in these soils. However, the homogenous red earth and net-like red earth were richer in Fe and showed distinctly red hues, suggesting Fe might occur as goethite and/or hematite microcrystalline minerals adsorbed onto the surface of the clay minerals.The ICP-AES results showed that total extracted cations of clay minerals in Jiujiang yellow sandy soils ranged from45.1to67.3cmol (+)/kg, and the (Mg+Ca+Na)/Al raios ranged from11.9to22.7; total extracted cations of homogeneous red earth ranged from85.1to116.2cmol (+)/kg, and the (Mg+Ca+Na)/Al raios ranged from358.4to577.6; total extracted cations of net-like red earth ranged from89.1to116.9cmol(+)/kg, and the (Mg+Ca+Na)/Al raios ranged from465.9to644.2. Total extracted cations of clay minerals in Xuancheng homogeneous red earth ranged from52.4to88.2cmol (+)/kg, and the (Mg+Ca+Na)/Al raios ranged from193.1-406.6; Total extracted cations of clay minerals in Xuancheng net-like red earth ranged from41.9to96.7cmol (+)/kg. It is clear that the (Mg+Ca+Na)/Al raios and total extracted cations of Jiujiang yellow sandy soils were smaller than those of homogeneous and net-like red earth from Jiujiang and Xuancheng, suggesting that The Al cations are adsorbed as hydroxyl ions in the interlayer of clay minerals. The activity of hydroxyl-Al not only controlled the transformation of clay minerals, also influenced their total extracted cations.The yellow sandy soils of Jiujiang in the lower reaches of Yangtze River formed since100ka, corresponding to late-pleistocene. However, homogeneous and net-like red earth formed during the period of400ka to100ka and the period of800ka to400ka, which were the product of mid-pleistocene. Al was adsorbed in the interlayer region of vermiculite as hydroxyl-Al, resulting in the formation of HIV. Illite and vermiculite or HIV might transform into smectite wih weathering, and kaolinite formed as weathering proceeded. The occurrence of illite-vermiculite or illite-HIV mixed-layer minerals and the transformation of illite to vermiculite or HIV indicated that vermiculite or HIV was the weathering product of illite. Therefore, HIV may have an illite precursor. Since Mg, Ca, Na occurred to be simple ions in the interlayer of clay minerals from acid red earth, whereas Al entered in the interlayer of vermiculite in the form of hydroxyl-Al, altering mineral composition and icon exchange capacity. A gradual crease of HIV and illite and a gradual decrease of illite-vermculite or illite-HIV mixed-layer minerals suggest that a gradual climate change from warm and humid to cool and dry in the mid-lower reaches of Yangtze River region since mid-pleistocene, in good agreement with global climate change. Thus, not only HIV can reveal the climate change during the forming period of red earth, but also can be used as a characteristic mineral to define the boundary of Middle Pleistocene and Late Pleistocene. |
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