Enrichment And Distribution Of Pedogenic Iron Oxides And Its' Climatic And Tectonic Significance In Yunnan-Guizhou Plateau

Iron oxides and their hydrates are climate-sensitive minerals formed by weathering of iron bearing silicate minerals in supergene oxidation environment.It is usually attached to the surface of primary and secondary minerals in the form of Nano colloids,which dominate the optical and magnetic properties of soil and sediments.Under the condition of temperate precipitation,soil chromogenic and magnetogenic oxides are enriched synchronously with chemical weathering.Optical and magnetic indicators are widely used in Quaternary climate reconstruction based on northern loess.However,under the condition of subtropical high precipitation,chromogenic and magnetogenic oxides are controlled by the climate threshold,and decoupling of soil optical and magnetic indexes from the chemical weathering process,which limits the climate reconstruction of red layer sediments under the background of high temperature.Affected by the strong uplift of the Tibetan Plate,the Yunnan-Guizhou plateau has undergone differential uplift in the Late Cenozoic,and the climate pattern has changed significantly,becoming the region with the largest relative humidity gradient in China.A regional soil sequence of red soil yellow soil is developed on the surface of the plateau from west to East,and an obvious competitive distribution process of iron oxides has occurred.In this paper,soil characteristic layer samples,profile samples and complete weathering crust in the climate threshold conversion area from the transition area of Yunnan Guizhou Plateau are collected,comprehensively using geochemical methods,spectroscopy methods,and environmental magnetic methods to carry out research on pedogenic iron oxides of the Yunnan-Guizhou Plateau.Capture the climatic threshold of pedogenic iron oxide in enrichment and distribution process under the background of high temperature,clarify enrichment and distribution mechanism of pedogenic iron oxide on the regional and profile scales,and discuss the climatic and structural significance of pedogenic iron oxide in the Yunnan-Guizhou Plateau,and draw the following basic conclusions:1.,The total amount and the amorphous components of pedogenic iron oxides are equivalent from the red soil area of Yunnan to the yellow soil area of Guizhou.The distribution of chromogenic iron oxides Hm and Gt in the sequence is obvious.2.The total amount of pedogenic magnetic minerals in the Yunnan Plateau is significantly higher than that in the Guizhou Plateau,and the magnetic particles in the sequence are dominated by Superparamagnetic-Single domain particles,with synchronous enrichment of particles of various grain sizes.The enrichment process of magnetic particles in Yunnan Plateau was accompanied by the growth and transformation process of magnetic particles from fine to coarse.However,there was no obvious systematic transformation of various magnetic particles in Guizhou Plateau soils.3.The characteristic layer sequence of red-yellow soil on the Yunnan-Guizhou Plateau reflects the changes of pedogenic iron oxides in the horizontal direction.There are climatic thresholds of 1100 mmyr precipitation and 13 °C temperature for free iron and goethite.Hematite and magnetic susceptibility show monotonic variations with climate changes,but the rates of change are significantly different before and after 1100 mmyr precipitation and 13 °C temperature.4.The opposite changes in rainfall and temperature result in rapid change of relative humidity(RH),which could promote hematite and goethite and lead to competition between antiferromagnetic hematite and ferrimagnetic maghemite.Currently,dry and warm climate in the Yunnan Plateau favors the dehydration of amorphous iron oxides to form antiferromagnetic hematite and ferrimagnetic particles,while the wet and cool climate in the Guizhou Plateau mainly produces goethite and leads to competition between low concentration hematite and maghemite.5.The study of typical residual weathering crust profiles in the climate threshold transition area revealed a significant depth dependence in the pattern of soil iron oxides and associated magnetic particles.The soil reddening layer is associated with amount of ferromagnetic minerals due to the enrichment of hematite,and goethite is mainly enriched in the yellowing layer with the lowest magnetism,which compete with hematite and ferromagnetic minerals in the reddening layer.The enrichment of ferrous iron with the organic matter superimposed in magnetized layer may form magnetite to intensify the enrichment of topsoil magnetism,and the enrichment of ferrous iron in reduced layer due to the influence of groundwater leads to the enrichment of sulfur-containing ferrimagnetic mineral(Pyrrhotite et al.),but the extreme reduction process at the top of magnetized layer and bottom of reduced layer may lead to the general dissolution of iron oxides,including magnetic minerals.6.The pedogenic iron oxide on the Yunnan-Guizhou Plateau change linearly with altitude,but show different rates of change with 1800 m as the critical point.The change of hematite with altitude does not conform to the change law of modern soil,suggesting that it may have experienced a drying and dehydration process after the climate became dry in the later period.7.Based on the climate and altitude response of iron oxides in Yunnan Guizhou Plateau,it is inferred that the soil in Yunnan Guizhou Plateau is not formed under the modern climate environment,but it was formed by the original weathering crust remaining on the Plateau surface after being dehydration and transformation by dry climate or redeveloped on the basis of the original weathering crust.