青藏高原表土磁化率空间分布特征及影响因素

Spatial Distribution Characteristics and Influencing Factors of Magnetic Susceptibility of Topsoil on the Qinghai-Tibet Plateau

土壤磁化率是古环境重建的常用代用指标,对表土磁化率与现代环境的关系研究有助于理解磁化率产生差异的原因。当前,区域尺度上土壤磁化率变化成因认识尚不清楚,限制了磁化率作为古环境重建重要代用指标的精准应用。本研究系统调查了青藏高原的254个样点表层(发生层A层)土样,测定土壤磁化率和其他土壤属性,结合母质、气候、地形和植被等数据,阐明青藏高原地区土壤磁化率空间变化特征及其主要影响因素。结果表明:(1)不同母质类型之间,表层土壤磁化率(χ_(lf))无显著差异,百分频率磁化率(χ_(fd)%)差异显著,表现为黄土和砂页岩风化物>冰碛物和结晶盐风化物,其他母质类型之间无显著差异;不同土地利用之间,表层土壤χ_(lf)无显著差异,χ_(fd)%差异显著:森林和旱地>草地>荒地。(2)各因子对土壤磁化率影响表现为植被>理化性质>地形>母质。(3)空间分布上,χ_(lf)与χ_(fd)%均呈现由东南向西北降低的趋势。此外,χ_(lf)和χ_(fd)%的空间分布规律与青藏高原植被分区相吻合。因此,磁化率能更好地指示植被空间分布。

【Objective】Soil magnetic susceptibility is a commonly used proxy index for paleoenvironment reconstruction.The study on the relationship between topsoil magnetic susceptibility and the modern environment is helpful to understand the reasons for the difference in magnetic susceptibility.Researchers around the world had conducted numerous studies on the correlation between soil magnetic susceptibility of sediments and modern soils in different regions and environmental factors.However,previous studies mainly focused on the correlation between soil magnetic susceptibility and climate.This correlation between soil magnetic susceptibility and climate was inconsistently recognized by different researchers because climate is only one of the influencing factors for the variation of soil magnetization rate.The main controlling factors of soil development and soil magnetic susceptibility may vary from region to region,and the magnetic susceptibility reflects the combined effect of multiple environmental factors.At present,the lack of understanding of the causes of soil magnetic susceptibility changes at the regional scale limits the accurate application of magnetic susceptibility as an important proxy index for paleoenvironmental reconstruction.Thus,we hypothesized that(1)the spatial distribution patterns of soil magnetic susceptibility in different regions can respond to environmental variables besides climate and(2)the Normalized Difference Vegetation Index(NDVI),which is the result of the combined effect of climate,topography and soil,can better respond to magnetization rates.【Method】The Qinghai-Tibet Plateau is the youngest natural geographic unit in the world with closely integrated horizontal and vertical zonation,influencing regional and global energy and water cycles.It is one of the areas where global changes have had the most significant impact on the land surface because it had undergone six major geological tectonic events and has a variety of parent rock types.The natural environment of the Qinghai-Tibet Plateau is harsh and large-scale soil sampling is very difficult,so regional magnetic studies are limited.In this study,254 soil samples from the surface layer(layer a of the occurrence layer)of the Qinghai-Tibet Plateau were systematically investigated to determine the soil magnetic susceptibility and other soil properties.Combined with the data of parent material,climate,topography and vegetation,the spatial variation characteristics of soil magnetic susceptibility in the Qinghai-Tibet plateau were clarified and its main influencing factors were analyzed.【Result】The results showed that:(a)There was no significant difference in low-frequency magnetic susceptibility χ_(lf) of topsoil between different parent materials and the difference of percentage frequency magnetic susceptibility χ_(fd)% was significant.Specifically,the weathering of loess and sand shale>weathering of aeolian sand,moraine and crystalline salt,and there was no significant difference between other parent material types.Among different land uses,there was no significant difference in topsoil,but a significant difference in topsoil χ_(fd)%:forest and upland>grassland>wasteland.(b)The influence of each principal component on soil magnetic susceptibility was of the order vegetation component>soil physical property component>terrain component>parent material component.(c)Spatially,the variation law of soil magnetic susceptibility was most consistent with that of vegetation normalization index,χ_(lf) and χ_(fd)% in the Southeast both showed higher values.Also,the spatial distribution of χ_(lf) decreased from Southeast to Northwest,the high-value area was located on the southeast edge while the low-value area was located in the middle.Similarly,the spatial distribution of χ_(fd)% showed a high value in the southeast area and a low value in the western area.【Conclusion】Our study shows that magnetic susceptibility can better reflect the spatial distribution characteristics of the vegetation index.