| Red earth,which is widely distributed in south China,records abundant regional climate information and is an important material for the study of environmental magnetism.The red earth formation in south China is varied,and weathering crust type and aggradation type are two common red earth formation modes.What are the characteristics of red earth produced by the two types of soil-forming methods,and what are the effects of different genesis on the magnetic characteristics of the soil?The discussion and solution of these problems are the basis to find out the"climate signal"in the magnetic characteristics of red earth,and also have unique significance for the comparison of the results of soil environmental magnetic research in the south and north.In the mid-subtropics of Zhejiang Province,both typical weathering crust red earth and aggregation red earth are distributed in the region,which is an ideal place to carry out relevant research.This paper selected the subtropical region of Zhejiang Province as the research area,and collected 10 red earth profiles with similar altitude and relatively uniform spatial distribution as the research object.4 profiles belong to weathering crust type and 3 belong to aggregation type.We conducted normal temperature magnetic measurement on all soil samples,selected representative samples for high temperature magnetic measurement,and reflected the situation of antiferromagnetic minerals by chromaticity and diffuse reflectance spectroscopy experiment results.The characteristics of geochemical elements were used to reflect the composition and change of soil elements and the intensity of pedogenesis.By discussing and comparing the magnetic characteristics of the two types in detail,this paper summarizes the similarities and differences of the magnetic characteristics of the subtropical weathering crust red earth and the aggregation red earth in Zhejiang Province,and tries to preliminarily analyze the principle and mechanism behind the similar or different phenomena,so as to find the appropriate climate proxy index.Through this research,the following understanding and conclusions are obtained:(1)In the process of soil formation,there are differences in magnetic characteristics between weathering crust type and aggradation type soil in the mid-subtropics of Zhejiang Province,mainly through the influence of parent material and climate factors.(2)The two types of magnetic minerals in red earth include ferromagnetic minerals and antiferromagnetic minerals.The main types and sources of ferromagnetic minerals in different of the same type are different:the types of ferromagnetic minerals include magnetite,hematite,pyrrhotite,etc,and the sources are divided into two types:parent material residue and soil formation.These mainly depend on the parent material,and have little relation with climate and other factors.The antiferromagnetic minerals of the two types of red earth,hematite and goethite,are secondary.(3)The particle size of magnetic minerals is influenced by both climate and parent material.The particle size of magnetic particles in weathering crust profile decreases with decreasing depth,which indicates that soil formation causes soil magnetic particles to become smaller.The magnetic particles in the parent material were coarser,and the magnetic particles in the developed soil were coarser,mainly pseudo-single domain(PSD)and multi-domain(MD)particles.Superparamagnetic(SP)and single domain(SD)fine particles occupy an absolute proportion advantage in soil if there are more fine particles of primary magnetic minerals in parent material.Compared with the weathering crust red earth,the aeolian aggradation profiles are generally characterized by finer primary magnetic particles and lower Fe elements.So the aeolian aggradation profiles have finer magnetic particles and higher SP and SD components.However,in the paleosoil of aggradation profiles,especially in the reticulated layer,because the paleoclimate at that time was more humid and hot,some fine iron oxides may have escaped from the soil in the later stage of soil formation,resulting in the decrease of the proportion of fine magnetic particles.(4)Ferromagnetic minerals of weathering crust red earth and hydrogenic aggradation red earth are greatly influenced by parent material,and the transformation is more complex and the mechanism is more diverse,which is characterized by high content level,many vertical variation patterns and large amplitude.The distribution and variation of ferromagnetic minerals in aeolian aggradation profiles mainly depend on the later pedogenesis,which is more regular.The content of hematite has nothing to do with the profile type,and the content of hematite in the two profiles is generally higher,and the matrix effect is obvious,which often interferes with the determination of goethite content by optical methods.(5)The magnetic minerals in the soils of the subtropics of Zhejiang province are affected by many factors.In a small area,temperature and precipitation increase and decrease simultaneously with little difference.Therefore,it is difficult to quantify the climate signals in magnetic minerals,and many parameters reflecting the characteristics of magnetic minerals cannot be used as climate proxies,such as magnetic susceptibility,yellow-red ratio(b~*/a~*),etc.However,the laterite in the subtropical region has high hematite content,high secondary proportion and is sensitive to climate,especially has obvious correlation with annual average temperature.The redness(a~*)and characteristic peak height of hematite,which can accurately indicate the content of hematite in soil,have climatic significance.However,when it is used as a temperature indicator,due to the genesis of the subtropical laterite in Zhejiang province and the complexity of parent material species,a~*and the characteristic peak height of hematite in a few sections may lose the significance of climate indication.Therefore,it is necessary to comprehensively consider and analyze the application with other information to ensure its correctness. |
