Because marine carbonate rocks generally do not contain magnetic minerals, the red weathering crust formed by in-situ weathering of carbonate rocks and the magnetic minerals in soil are new minerals in the process of weathering and soil formation, especially the maghemite micro-nodules successfully extracted from the red weathering crust, which provide the most direct evidence for the study of the genetic mechanism of magnetic minerals in the red weathering crust. The research shows that the phenomenon of magnetic enhancement in the surface soil of red weathering crust of carbonate rocks is more prominent in the surface soil of red weathering crust that has experienced aluminization and developed for a long time, indicating that the appearance and evolution of maghemite, as the main contribution factor of red weathering crust magnetism, is closely related to aluminization of red weathering crust. Maghemite, as the only ferrimagnetic mineral in the red weathering crust soil of carbonate rocks in Guizhou, is the product of weathering of carbonate rocks in karst environment. The formation and evolution of red weathering crust of carbonate rocks in Guizhou can be roughly divided into three geochemical stages: decarbonization, magnesium, silicon-rich aluminum, iron-rich manganese and desilication-rich aluminum. The phenomenon of magnetic enhancement generally exists in the topsoil, especially in the topsoil of red weathering crust with high weathering degree and aluminum-rich desilication stage (such as the profile of red weathering crust of dolomite in Anshun), which shows that maghemite is the main contributor to the magnetism of red weathering crust of carbonate rocks, and its occurrence and evolution are related to carbonate rocks. The micro-nodules in maghemite are only found in the yellow-red soil on the surface of the red weathering crust of dolomite in Anshun, with a high degree of aluminization [the average content of the three highest element oxides in the surface soil samples of this profile is Al2O3 335%, SiO23 1%, Fe2O312%, and W (SiO2)/W (Al2O3) 0.88]. Therefore, maghemite in the surface soil of red weathering crust of carbonate rocks in Guizhou is mainly formed by the transformation of secondary iron oxide minerals in the surface soil of strongly weathered red weathering crust under humid and hot conditions. It was observed by transmission electron microscope that maghemite often has similar morphological characteristics to lepidolite, and its formation may have undergone the transformation process of dehydration and crystallization of lepidolite. The formation and transformation of maghemite in red weathering crust and its combination with other iron oxide minerals determine the magnetic change of red weathering crust, which quantitatively reflects the geochemical environment and process of carbonate weathered soil, and its environmental significance is mainly manifested in the following three aspects:
1) The variation of magnetic parameters along the vertical section of the red weathering crust is directly related to the biological assemblage of iron oxide minerals maghemite, goethite and hematite and the regular changes of their mineralogical characteristics. In fact, it records many periodic climate changes during the formation of red weathering crust. The mechanism that magnetic parameters of red weathering crust reflect climate is essentially the mechanism that magnetic minerals appear in paleoenvironment, that is, the temperature and humidity of paleoclimate determine the quantity and particle size of magnetic minerals (mainly maghemite). The higher the temperature and humidity are, the smaller and more magnetic minerals are produced. These two factors increase the magnetic parameters χ, χ rd, SIRM and "softness". However, the surface soil developed on the parent red weathered crust soil layer, which experienced stronger weathering and took longer to form soil, so the above magnetic parameters increased more in the surface soil. According to the variation characteristics of magnetic parameters of red weathering crust along the profile and the research results of profile mineralogy, it can be seen that frequency magnetic susceptibility (χ rd), saturated isothermal remanence (SIRM) and "soft" remanence are more sensitive to environmental changes, and they are more ideal environmental change indicators than the magnetic susceptibility commonly used in the past. It is worth mentioning that the characteristics of paleoclimate cycle reflected by magnetic parameters and magnetic minerals in the red weathering crust profile of Guizhou carbonate rocks are comparable to those recorded by loess-paleosol sequence in China since Quaternary, which may indicate that there is a certain correlation between paleoclimate changes in north and south China since Quaternary. This will undoubtedly contribute to the comparative study of red soil in southern China and loess in northern China and the study of global change.
2) Erosion and deposition are environmental processes with universal significance. Soil erosion is a very prominent environmental problem in karst areas at present, and it is also one of the manifestations of ecological fragility of red weathering crust of carbonate rocks. In this paper, the magnetic parameters of six red weathering crust profiles in Hongfeng Lake area, the largest artificial lake in Guizhou Plateau, were measured, and a relatively complete data system of magnetic parameters of red weathering crust profiles in Hongfeng Lake area was established, which not only provided a basis for the study of soil erosion degree and speed in Hongfeng Lake area, but also directly served for soil monitoring, ecological environment protection and soil and water conservation engineering evaluation in Hongfeng Lake area.
3) The variation of magnetic parameters along the longitudinal section of the red weathering crust is related to the variation of trace elements in the red weathering crust, especially the variation of heavy metal elements such as Cu, Pb, Zn, Co, Ni and Cr along the longitudinal section of the red weathering crust. The quantitative relationship between the contents of heavy metal elements such as copper, lead, zinc, cobalt, nickel and chromium in red weathering crust and magnetic parameters can be established by linear regression analysis.
Weathering soil formation of carbonate rocks and its environmental effects
Weathering soil formation of carbonate rocks and its environmental effects
The correlation between the content of heavy metal elements and magnetic parameters in red weathering crust essentially reflects the relationship between heavy metal elements and magnetic iron oxide minerals in occurrence and distribution. This relationship may be mainly based on the adsorption mechanism of iron oxide minerals to heavy metals, which will be discussed in later chapters.
According to the correlation between the magnetic parameters and mineralogical characteristics of red weathering crust and the content of heavy metal elements in red weathering crust, a quantitative relationship model of magnetic parameters-iron oxide minerals-heavy metal elements in red weathering crust is preliminarily established, which reveals the distribution characteristics and pollution mechanism of iron oxide minerals and heavy metal elements in red weathering crust to some extent, and can be directly applied to pollutant tracing and environmental treatment projects, and is suitable for environmental research of river and lake sediments.