| The climate in southern China is humid and hot,and its combination of water and heat is different from other regions in the same latitude,so this region is known as the oasis on the Tropic of Cancer.This region has a unique significance for the study of soil environmental magnetism,which is helpful to understand the response of soil magnetic minerals under different climatic conditions.In this study,south subtropical region of China was selected as the research area,starting from Chaozhou in the east and going southwest to Xuwen County,15 soil profiles with similar altitude and relatively uniform spatial distribution were collected systematically along the coast of Guangdong as the research objects.Among them,12 profiles belong to south subtropical region,and 3profiles belong to marginal tropical region.We conducted detailed room temperature magnetic measurements on all soil samples collected,selected representative samples for thermomagnetic measurements,and combined chroma,diffuse reflectance spectroscopy and iron content analysis based on X-ray fluorescence spectroscopy to preliminarily explore the magnetic characteristics of soil profiles in south subtropical region,and then analyzed the leading factors of magnetic differences between different profiles,and determined the content changes of secondary magnetic minerals—goethite and hematite,and looked for reliable climate proxy indicators in south subtropical region.Through the research in this article,the following understandings of the magnetic characteristics of the soils in south subtropical region and their climatic significance have been gained:(1)The types,contents and particle sizes of magnetic minerals in different soil profiles in the study area,as well as the transformation of magnetic minerals during soil-forming process are quite different.Generally speaking,the main ferrimagnetic mineral in south subtropical soil are magnetite,and the antiferromagnetic minerals are hematite and goethite.Some profiles also contain a large amount of paramagnetic minerals and a small amount of lepidocrocite.Whether the soil profile is derived from igneous rock weathering crust or sedimentary rock weathering crust,it can be divided into two types according to the bottom-up change of magnetic susceptibility with depth(Zengcheng profile is not zonal soil profile,and its development intensity is not balanced with climate,so it is excluded):The profile with increased magnetic susceptibility and the profile with reduced magnetic susceptibility.There are at least two completely different processes involved in the transformation of strong magnetic minerals in the soils of south subtropical region:1)The transformation of iron-containing silicate minerals,hematite or goethite into fine-grained secondary strong magnetic minerals;2)The coarse-grained primary strong magnetic minerals are dissolved or transformed into weak magnetic minerals.These two processes can well explain the change in magnetic susceptibility of most profiles with depth:The profile with increased susceptibility is dominated by process 1),and the profile with reduced susceptibility is dominated by process 2).The three soil profiles in marginal tropical region all belong to the profile with increased magnetic susceptibility.In addition,under high-humidity conditions,the conversion of fine-grained ferrimagnetic minerals into goethite and even lepidocrocite is the mechanism of the decrease in magnetic susceptibility in the upper part of the Enping profile.(2)The iron content of the 15 soil profiles in the study area is not the limiting factor for the increase in magnetic susceptibility.However,there are significant differences in the magnetic properties of the parent horizon between different profiles.On the whole,the magnetic properties of the soil profiles developed on the weathering crust of the strong magnetic parent rock tend to decrease during soil-forming process,while the magnetic properties of most soil profiles developed on the weathering crust of weak magnetic parent rock tend to increase from the bottom to the top.The parent material usually directly causes the magnetic difference by controlling the background magnetic strength of the soil profile.In addition,due to the influence of the parent material,?,SIRM reflecting the content of magnetic minerals,and?fd%,?ARM/?and other magnetic parameters reflecting the relative content of magnetic particles cannot be used as the climate proxy indicators in this region.Therefore,the parent material is one of the leading factors in the magnetic difference of the soil profiles in south subtropical region.(3)In the chromatic study of 14 soil profiles(excluding the Zengcheng profile),it is found that the hematite content of the parent material horizon of the reduced magnetic susceptibility profile is always lower than that of the sedimentary horizon,while the hematite content of the increased magnetic susceptibility profile is always lower than that of the sedimentary horizon.The parent material horizon has the highest hematite content,followed by the sedimentary horizon,and the leached horizon has the lowest hematite content.The measurement results of diffuse reflectance spectroscopy also confirmed this rule.Through the study of the correlation between the leached horizon and sedimentary horizon chromatic parameters and the diffuse reflectance spectrum parameters of 14 soil profiles,it is found that the a*and the peak height at 575 nm,b*/a*and the ratio of the peak heights at 435 nm and 575 nm of south subtropical and marginal tropical soil profiles have a good positive correlation,while the correlation between b*and the peak height at 435 nm is poor,indicating that a*and b*/a*can accurately reflect the relative content of goethite and hematite in soils of south subtropical and marginal tropical regions,while b*can not reflect the goethite content in this region.(4)In south subtropical and marginal tropical regions,the content and proportion of secondary hematite and goethite are mainly controlled by climatic conditions,and have nothing to do with the original iron content and occurrence form,and the background content and transformation of magnetic minerals.The content of goethite is related to precipitation and temperature.Its content increases with the increase of precipitation and decreases with the increase of temperature.Whether it is the b*/a*in the chromatic parameters or the ratio of the peak heights at 435 nm and 575 nm in the diffuse reflectance spectrum parameters,they are all significantly correlated with precipitation.Therefore,the ratio of goethite to hematite can be used as a sensitive precipitation index in south subtropical and marginal tropical regions. |
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