Geochemical characteristics of soils derived from the lower-Cambrian black shales distributed in central hunan, China

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    The geochemistry of major and trace elements of soils derived from the Lower-Cambrian black shales (the black-shale soils) were studied by samples of typical soils and corresponding parent rocks from Dongping, Yanxi, Taojiang and Ningxiang in central Hunan province (China), using ICP-MS and XRF techniques. Results show that the black-shale soils were strongly weathered, with chemical weathering index (CIA) values of higher than 73. Due to the higher intensity of chemical weathering, the chemical compositions of the soils are characterized by distinct depletion of CaO and Na2O, and by significant enrichment of Al2O3and Fe2O3. The soils were also enriched with heavy metals including Mo, Cd, Sn, Sb, U, V, Cr, Co, Ni, Cu, Zn, Tl, Pb and Th, when compared to the China average soils, and the complex enrichment indexes (EI) are higher than 3 in average with the highest up to 17.0. This heavy metal enrichment of the soils probably reflected the chemical components of the parent rocks. Assessment using the geoaccumulation index ( Igeo) shows that the soils are contaminated by heavy metals Mo, Cd, Sb, Sn, U, V, Cu, Tl and Ba, for which the contamination of metals Mo, Cd and Sb is the most heavy, reaching moderate to extreme pollution degrees. Significant correlations of heavy metals to major elements indicate that heavy metals in the soils are hosted mostly in clay minerals and Fe-oxides (goethite), among which metals Ba, Sn, Th and Sc are hosted in clay minerals, metals Zn, Ni, Mn, Co, Cd, Tl and Pb in Fe-oxides, and metals Cr, V, Mo, Sb and U neither in clay minerals nor in Fe-oxides (goethite). Elemental ratios of Zr/Hf, Ta/Nb and Nd/Sm of the soils are similar to each other with values of 36.20, 0.085 and 5.30, respectively, and these ratios are also consistent to those of their corresponding parent rocks. In addition, the REE distribution patterns of the soils from different area are similar, and similar to that of their corresponding parent rocks. The rare earth elements were not differentiated during pedogenesis. Therefore, it is suggested that the heavy metal contamination of the soils was caused originally by metals from the parent rocks (black shales) in the area, which is a natural source of soil heavy-metal contamination.

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Yu Changxun, Peng Bo, Tang Xiaoyan, Xie Shurong, Yang Guang, Yin Chunyan, Tu Xianglin, Liu Qian, Yang Kesu. Geochemical characteristics of soils derived from the lower-Cambrian black shales distributed in central hunan, China[J]. Acta Pedologica Sinica,2009,46(4):557-570.

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