Soil Magnetic Characteristics And Their Relationship With Soil Formation In Typical Climatic Zones In Yunnan

Soil magnetic measurement technology is fast and non-destructive,so it is widely used in Quaternary geology,pedology,sedimentology and other fields.Yunnan plateau soil has obvious advantages of latitude zonality and vertical gradient zone,so it is an ideal place to study soil magnetic characteristics and its relationship with soil formation process under different soil types and climatic conditions.This paper selects the soil profiles of different types(red soil,yellow brown soil,yellow soil,purple soil)located in different climatic zones of Yunnan Province as the research object.The magnetic characteristics of soil profiles in different climatic zones were studied by systematic environmental magnetic measurements,combined with geochemical major elements and other analytical methods.The relationship between magnetic characteristics and weathering soil formation process is discussed,and whether the magnetic parameters in the study area can be used as an index to indicate the soil formation process is discussed.Six sample plots were set up from south to north in Yunnan province,each with three forest soil profiles with a depth of 80-140cm.A total of 182 soil samples were collected.The low frequency magnetic susceptibility?_lf,frequency-dependent magnetic susceptibility?_fd,main soil physical and chemical parameters(BD,WC,pH,SOC)were measured.Select representative samples to determine the hysteresis loop,first-order reversal curve(FORC),IRM curves,?–T curves,etc.Nine major geochemical elements such as Si,Al,Fe and K were determined by representative profile,and the chemical weathering index of silica-alumina ratio and silica-sesquioxide ratio were calculated.Analysis and comparison of the characteristics of various geomagnetic parameters,physical and chemical parameters,geochemical elements and weathering index.As well as the relationship between various indexes and magnetic parameters,the understanding is as follows:1.The variation trend of low frequency magnetic susceptibility?_lf along the profile of 6 plots in the study area is consistent.The low frequency magnetic susceptibility of0-80cm depth is significantly higher than that of the bottom layer.The average values of low frequency magnetic susceptibility of different plots are significantly different,the lowest is Daguan plot(18.2×10^-8m³kg^-1),and the highest is Eshan plot(263.19×10^-8m³kg^-1).The variation characteristics of percentage frequency-dependent magnetic susceptibility profile are not consistent in various places;the average value of Daguan sample plots is the lowest,which is 7.24%;other sample plots are all more than 10%.2.The increase of low frequency magnetic susceptibility of 0-80cm soil layer in all plots is mainly due to the increase of the content of ferrimagnetic minerals(magnetite and magnetic hematite).With the increase of depth,the content of ferromagnetic minerals decreases and the content of antiferromagnetic minerals(hematite)increases obviously.The magnetic domain characteristics of magnetic minerals are mainly PSD,containing a certain amount of SP particles,and the bottom of part of the profile shows that MD particles have a certain contribution.3.There is a strong correlation between low frequency magnetic susceptibility and soil physical and chemical parameters.The results show that soil environmental factors have a great influence on magnetic properties.Soil organic carbon SOC is positively correlated with low frequency magnetic susceptibility;pH and volume weight BD are negatively correlated with low frequency magnetic susceptibility;The relationship between moisture content WC and low frequency mass magnetic susceptibility is inconsistent in various places,and there are three cases:positive correlation,negative correlation and ircorrelation.4.Chemical weathering index shows that the overall degree of soil weathering from strong to weak is Xuanwei,Jinghong,Eshan,Pu'er,Kunming and Daguan.There is a significant negative correlation between low frequency magnetic susceptibility and Indicating factor of weathering soil formation(Sa,Saf,Ba).The results show that the soil magnetism increases with the increase of weathering pedogenesis.Parent material,biology(organic carbon accumulation)and soil ages are the main soil-forming factors leading to various geomagnetic differences.In the study area,the low frequency magnetic susceptibility?_lf can be used as a good substitute index to indicate the pedogenesis to some extent.