Oxide geochemical characteristics and paleoclimate records of “Wushan loess”

Abstract：In this paper, based on analytical results of 73 oxide samples of "Wushan loess", the authors analyzed and discussed oxide geochemical characteristics of "Wushan loess" and their recorded paleoclimate characteristics. In the average content of the main chemical compositions in "Wushan loess", SiO2, Al2O3, Fe2O3, are higher than those in Luochuan loess and Ganzi loess. The values are close to the values of Luochuan paleosols but lower than those in Xuancheng aeolian red clay in Anhui and Xiashu loess profile in Zhenjiang. The values of SiO2, Al2O3, Fe2O3 and TiO2 in "Wushan loess" gradually decrease with depth, whereas those of FeO, CaO, Na2O and P2O5 gradually increase. In Wushan section, the Fe3+/Fe2+ ratio curve shows a significant positive correlation with the total iron content curve but a negative correlation with the curve of SiO2/(Al2O3+Fe2O3). From old to young, values of residual coefficients and weathering indexes become smaller and smaller, but de-alkali coefficients become larger. In the period of accumulation of "Wushan loess", the ancient climate in this area was partially warm and partially humid in general. The intensity of leaching and oxidation that sediments experienced was equivalent to Luochuan paleosols, it was significantly stronger than the north Luochuan loess and Ganzi loess of western Sichuan but weaker than the eolian red clay in Xuancheng of Anhui and Xiasu loess in Zhenjiang. In this profile, from bottom upward, the acidity in the deposition was moderately weakened, the alkalinity was enhanced, the weathering intensity that the supergene band suffered became weaker and weaker. Eolian depositional environment changed from the beginning of partially hot and partially humid gradually to dry and cold. In the period of accumulation of "Wushan loess", paleoclimate evolution showed stage characteristics. The section was divided into upper and lower parts with 6-6.5 m as the boundary; the paleoclimate in the lower part was partially hot and humid, and partially cold (dry) and warm (wet), with frequent fluctuation, whereas the upper part was relatively dry and cold, its climate was relatively stable, and the fluctuation was relatively small.