Soil Magnetic Characteristics And Its Response To Climate In Humid Subtropical Regions

Environmental magnetism has been successfully used in study of paleoenvironment and paleoclimate change.In this paper,54 Humid subtropical soil profiles collected in Fujian Province were selected.Through magnetic measurement,chroma measurement and diffuse reflectance spectroscopy,the soil layer differentiation and spatial differentiation of soil profiles in subtropical regions were systematically understood.The semi-quantitative study of goethite and hematite content,as well as the response of secondary magnetic mineral goethite and hematite in the climate,we can conclud the following conclusions:(1)There are obvious differences in the magnetic characteristics and variation laws of the soil profiles under the control of the south subtropical and Central subtropical climate in the study area.The soil profiles under the control of the south subtropical climate are developed on different parent rocks.Most of the profile magnetic susceptibility(c)show an increasing trend from bottom to top.The magnetic enhancement profile is primarily due to changes in the type and content of magnetic minerals and changes in the content and size of magnetic particles.The soil profiles in the mid-subtropical zone can be roughly divided into two categories according to the magnetic characteristics.One is similar to the soil profile in the southern subtropical region,and the magnetic susceptibility decreases with the increase of the profile depth.The other one is the dissolution of coarse magnetic particles of ferrimagnetic minerals or the leaching of fine-grained magnetic particles during the process of soil formation,and the magnetic susceptibility increases with the increase of the depth of the profile.Two types of magnetic characteristic profiles are staggered in the mid-subtropical region,and there is no obvious regional boundary line and distribution law.(2)The study of the magnetic characteristics of the NPN profile and the NF profile on different parent rocks under the same climate control shows that the profile developed on the strong magnetic parent rock,the magnetic properties of the profile slightly decreased during the process of soil formation.The profile developed on the weak magnetic parent rock,the magnetic property increases from the parent horizon to the elavial horizon.The parent material is one of the dominant factors in the magnetic characteristics of the Humid subtropical soil profile.The soil in the humid subtropical zone is deeply affected by the parent material,so traditional magnetic parameters(c,SIRM and HIRM and other magnetic parameters and their ratios)applicable to temperate regions cannot reflect the climate change law of the region.Secondary magnetic minerals respond more directly to climatic factors such as temperature and precipitation.(3)Using the two methods of chroma and diffuse reflectance spectrum to measure the content of goethite and hematite,and the measurements of the two methods were mutually verified to accurately achieve semi-quantitative determination of goethite and hematite.Both a~*and spectrum h(Hm)can better reflect the content of hematite,b~*/a~*and h(Gt/Hm)can better reflect the relative content of goethite and hematite in the soil than a~*and h(Hm).b~*indicates that the content of goethite is not very good,and h(Gt)can better indicate the content of goethite compared to b~*.Correlation between b~*,a~*and b~*/a~*and h(Gt),h(Hm)and h(Gt/Hm)between the elavial horizon and the illuvial horizon is basically the same as that of the whole profile in the humid subtropical regions.It indicates that the secondary magnetic mineral goethite and hematite are basically not affected by the parent material,and can be used as a climate proxy indicator for humid subtropical regions.(4)Correlation studies were conducted between the chroma parameters(a~*,b~*and b~*/a~*)which characterize the relative content of goethite and hematite,and the climatic(precipitation,temperature)data of modern instruments.In the region with small spatial scale,the soil color parameter changes are complicated,the relationship between goethite and hematite and climate is not significant on this spatial scale.In the regions with large spatial scale,the soil chroma parameters a~*,b~*and b~*/a~*show a certain zonality,and the content of hematite has a certain positive correlation with the latitude,the relationship between goethite and latitude is unclear,the ratio of hematite content and the relative content of goethite to hematite(Gt/Hm)is continuous in latitude.Hematite and Gt/Hm have a slightly higher correlation with annual precipitation,but Gt/Hm is more suitable as a substitute for precipitation due to the strong competitiveness of goethite and the possible dissolution of hematite in a high humidity environment,it is more suitable as an alternative indicator of precipitation.Gt/Hm has the highest correlation with temperature,and it can be used as a proxy for temperature.In the spatial distribution,the degree of response of hematite and Gt/Hm to temperature is higher than that of precipitation.In short,Gt/Hm is more sensitive to temperature,which is beneficial to the study of temperature and soil magnetic properties.