Study On The Variation Law Of Mountain Soil Magnetism In Mid-subtropical Monsoon Region

Environmental magnetism is a research method to determine the sedimentary environment by investigating the basic characteristics of magnetic minerals in sediments,and it is widely used in paleoclimate and paleoenvironmental research.In order to use the magnetic change characteristics of soil to invert the paleoclimate change process,the research on the response mechanism of soil magnetism to environmental factors（precipitation,temperature）is indispensable,and it is also an important content of soil magnetism research.Surveys of soils in temperate and tropical regions of the northern hemisphere show that the correlation between soil magnetic susceptibility and precipitation changes from positive to negative when the annual average precipitation exceeds 1000-1200 mm.However,areas with high precipitation in the study area usually correspond to areas with high temperature,so the controlled mechanism of soil magnetic changes needs to be further clarified.The vertical differentiation of climatic factors in mountainous areas is obvious.Usually,as the altitude increases,the precipitation increases and the temperature decreases.In a small spatial range,the foothills and the top of the mountain can form significantly different hydrothermal combination environments,which is more conducive to distinguishing the effects of temperature and precipitation on soil magnetism.This paper analyzes the weathering development characteristics and magnetic properties of soil profiles at different altitudes through the environmental magnetic analysis of the soil system in the mountainous area of the mid-subtropical monsoon region,combined with particle size,element geochemistry and diffuse reflectance spectroscopy.It is compared with the magnetic properties of soil profiles in the Loess Plateau,in order to explore the characteristics of soil magnetic changes and their corresponding climatic factors under different hydrothermal conditions on the basis of previous studies.Through the research,the main understandings are as follows:（1）The particle size components of the soil samples in the study area were mainly silt（4-63μm）.The average particle size（M_Z）and Standard Deviation（σ_g）of soil profile vary from 5.40～7.09φand 1.55～2.52 respectively,and the difference between the maximum and minimum values of each profile is small,indicating that the overall particle size of each soil profile is fine and uniform,and the soil structure and sedimentary dynamic environment are relatively stable.Compared with the average value of a-layer samples,the content of major elements at the bottom of the soil profile at different elevations in the study area shows that Si O₂ is depleted,while Fe₂O₃ and Al₂O₃ are relatively enriched,indicating that the study profiles have experienced A certain degree of desilication and iron-aluminum enrichment.The results of soil weathering parameters showed that the Chemical Index of Alteration（CIA）of each section ranged from 71.26 to 78.87,with an average of 75.69,indicating that the whole soil profile in the study area was in a medium weathering stage.The Coefficient of weathering leaching（ba value）ranged from 0.43 to 0.65,with an average value of 0.53,showing a low value,indicating that soil profiles in the study area had experienced strong leaching during development.The silica-aluminaratio（Sa）and silica-sesquioxide ratio（Saf）show consistent changes.The high altitude profile shows a lower value,while the low altitude profile shows a higher value,indicating that the former has a relatively high degree of soil weathering development,while the latter has a relatively low degree of soil weathering development..（2）Magnetite is the main secondary magnetic bearing mineral in the mountainous soils of the subtropical monsoon region,and there are a lot of hematite and goethite.The concentrations of magnetic minerals in soil profiles of different elevations in the study area were significantly different:the mean value of magnetic susceptibility(χ_lf)increased with the elevation.The meanχ_lf of soil profile in mountaintop region is 194.1510^-8m³kg^-1,while the meanχ_lf of soil profile in piedmont region is only 18.33×10^-8m³kg^-1,indicating that the magnetite content of the former is significantly higher than that of the latter,which has a higher relative hematite content.The results support that lower temperature is favorable to the formation of loamy magnetite and higher temperature is favorable to the formation of loamy hematite.（3）The low-altitude soil profile and high-altitude soil profile on the southern slope of Jinhua Mountain show that the loamy magnetite has coarser particle size than the primary magnetite in the parent material.The loamy magnetite at 400～800m altitude is finer than the primary magnetite in its parent material.Combined with the latter,the fitting slope ofχ_ARM andχ_lf increased significantly.This suggests that there may be a relatively favorable climate zone for superparamagnetic（SP）and single-domain（SD）magnetite particles,especially SD particles.（4）High annual precipitation（MAP>1000～1200 mm）can restrict the formation rate of strong magnetic minerals in soil.Soil temperature in central Asia of ferromagnetic minerals play a more important role in the process of loss:the condition of high temperature greatly limit the fine particle（SP and SD）magnetite formation rate,at the same time to speed up the transformation to the hematite,which leads to soil loss of hundreds of central Asia ferromagnetic particles,and show the formation of magnetic susceptibility decreased significantly.Therefore,the effect of precipitation on magnetic susceptibility has been overestimated in previous studies on soil environmental magnetism in humid regions.