Magnetic Characteristics And Environmental Responses Of Soil In The Vertical Zone Of Daiyun Mountain,Fujian Province

Magnetic variation of natural soil is a mirror of the environmental change processes which magnetic minerals will ensue migration,transformation,dissolution and etc in different soil environment.It can reconstruct soil and environmental changes in process through study the changes of soil magnetism which can guide human beings to adapt to and make use of the environment better and promote the harmonious development of human-earth relations.It is difficult to establish the spatial variation rule of soil magnetism on regional scale due to the complexity of the tropical and subtropical regional geographic environment,while mountains condense climate gradients across the land.It's helpful to clarify the relationship model between soil magnetism and environmental change through studying the variation mechanism of magnetic minerals in the mountain soil.We choose the soil of Daiyun Mountain,representative area among the southern subtropical mountains,as the research object which have many soil types and the vertical zonality climate characteristics significantly.Based on the methods of elemental geochemistry,environmental magnetism and spectroscopy,this paper studies the weathering and development characteristics of soil in the vertical zone of Daiyun Mountain and the relationship between soil magnetism and environmental elements in order to clarify the magnetic variation of magnetic minerals in subtropical mountain soil and its relationship with environmental factors.The research conclusions are as follows:(1)The variation range of the content of major elements of bedrock is very small at different elevations of Daiyun Mountain which indicate the lithology is basically the same and provides a unified prerequisite for the analysis of the soil development characteristics and magnetic properties in the study area.Weathered parameters of the major elements show that SiO₂ content increases with elevation and the Al₂O₃ content is decreasing in the vertical zone soil of Daiyun Mountain.It suggests that desilication and aluminization abate as the altitude rises,the climate tends to be cold and wet,and the characteristics of Fe₂O₃ content increases with elevation is related to the enhancement of leaching in the study area.The results of weathering parameters showed that Sa,Saf,Fe₂O₃/Al₂O₃ and Na₂O/K₂O all increased with elevation which indicate that the degree of soil development is weakening,while increased in the planation area of mountain.(2)The magnetic characteristics of vertical zone soil profiles are obviously different in Daiyun Mountain:Its main magnetic minerals are magnetite and maghemite for lithologic consistent soil profiles.Secondary lepidocrocite occurs in the soil when elevation exceeds 1400m which suggest that the soil moisture is higher above this elevation and the soil is in the environment of redox alternation for a long time.Magnetic parameters ?fd%,?ARM/?lf,SIRM/?lf and the shapes of hysteresis loops indicate that the magnetic domain particles gradually changed from SP particles to SD or PSD as the elevation increased when the effects of topography were excluded.In other words,the magnetic domain particles gradually became coarser which indicate the degree of soil development gradually decreased as elevation increased.In addition,it is considered that?ARM/?lf is not suitable to be used as a proxy of soil development in this region because it was affected by the magnetic minerals in the parent layer.(3)The relationships between magnetic characteristics and the soil forming elements are complex of the soil in the study area.It is concluded that the difference of parent materials(lithology)can significantly affect the initial magnetism of soil and it has a great influence on the analysis of the spatial variation rule of soil magnetism with weak development and regional scale.Topographic differences,including slope and aspect,will also affect the degree of soil development and thus the change of soil magnetism.The effects of organisms on soil magnetism mainly occur in the surface layer.The magnetic enhancement or decrease of surface soil is related to the content of soil organic matter and the characteristics of soil microorganisms.Climate factors have the greatest influence on the process of soil formation.In this study,it was considered that the contents of magnetic domain particles and secondary magnetic minerals referred to in ?fd%?HIRM and S-ratio of the normal temperature magnetic parameters have a certain response ability to climate factors,while it is still necessary to search for more specific proxies for climate factor response.(4)It is conclud that climate factors significantly affect the content changes of hematite and goethite(and lepidocrocite)by analyzing the spectral characteristics of soil in the vertical zone of Daiyun Mountain.According to the results of chromaticity of soil profile B layer with increasing altitude,climate tends to be cold and wet,b*shows an obvious increasing trend,while a*shows a decreasing trend,which was consistent with the change of goethite(and lepidocrocite)and hematite content indicated by diffuse reflection spectrum,indicating that the content of goethite(and lepidocrocite)increased while the content of hematite decreased with the elevation.Contrast before and after heating goethite and hematite characteristic peak of diffuse reflection spectrum and half quantitative analysis shows that hematite content decreased in soil with elevation and it disappears when the rainfall of more than 1900 mm and annual average temperature is lower than 13.5? while the content of goethite(and lepidocrocite)continue to increase.It is proved that goethite(and lepidocrocite)and hematite have clear indication of climate gradient in subtropical vertical zonal soil.