福建周宁黄红壤的磁学特征及其磁性矿物转化
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P318

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国家自然科学基金项目(41877435)和福建师范大学创新团队项目(IRTL1705)资助


Magnetic Characteristics of Yellow-Red Soil and Transformation of Its Magnetic Minerals, in Zhouning, Fujian Province
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    摘要:

    对我国亚热带地区发育于花岗岩之上的一个黄红壤剖面进行了系统的环境磁学测量,对土壤样品的磁化率、等温剩磁、磁滞回线等常温磁学参数进行测量,对代表性样品进行热磁分析,并结合色度、常量地球化学元素和漫反射光谱参数,探讨亚热带黄红壤的磁性特征,以及在相对湿冷的气候条件下,黄红壤中的磁性矿物具有怎样的转化规律。结果表明:亚热带黄红壤中强磁性矿物为亚铁磁性的磁铁矿、磁赤铁矿,弱磁性矿物为反铁磁性的赤铁矿、针铁矿。随着成土作用/风化作用增强,磁性矿物颗粒变细。母质和气候条件是影响区域磁性差异的重要因素,次生磁性矿物(特别是赤铁矿与针铁矿)的含量主要受气候条件控制。在相对湿冷的气候条件下,磁性矿物的转化以强磁性的磁铁矿与磁赤铁矿转化为弱磁性的赤铁矿与针铁矿为主。气温(而非降水)是湿润亚热带地区花岗岩风化壳上发育土壤中针铁矿和赤铁矿含量以及相对比例的主导影响因素。

    Abstract:

    [Objective] In order to explore magnetic characteristics of the yellow-red soil in the subtropical region and transformation of the magnetic minerals in the soil under relatively humid and cold climate conditions, the author selected a profile of yellow-red soil (Profile ZN) derived from granite weathering crust in Zhouning County, northeast of Fujian Province. Under a mid-subtropical monsoon mountain climate, this area had an average annual temperature of 15℃, an average annual rainfall of 2 049.3 mm, and an elevation of 906.4 m counted from the bottom of the profile. The profile was about 1.9 m thick. With the floating soil on the surface removed, a total of 20 samples were collected at 10 cm intervals from the soil and weathering crust layers of the profile.[Method] In this study, room temperature magnetic parameters of the samples were measured, and thermomagnetic analysis conducted of representative samples, in combination of chroma, major geochemical elements and diffuse reflectance spectrum analyses.[Result] Results show:The profile was low in magnetic susceptibility, and relatively low in content of magnetic minerals, the upper part of the profile contained relatively more superparamagnetic particles, while the lower part did relatively more multi-domain particles. a*, b* and Ca*b* varied in the same trend, being the highest in Horizon B and the lowest in Horizon C, while b*/a* was high in the profile surface. The content of Fe2O3 increased with the degree of pedogenesis, indicating that during the soil forming process, other elements were leached while iron accumulated relatively. Iron content was not a major factor limiting magnitude of magnetic susceptibility. The profile was lower than those in the humid and hot low altitude tropical and subtropical regions in CIA (chemical index of alteration).[Conclusion] By comparing this profile with the three (NPN, PC-GL, PC-SY) in the adjacent area, the following conclusions were drawn:(1) Profile ZN contains relatively less magnetic minerals, which are composed of mainly ferrimagnetic mineral and small portions of maghemite, antiferromagnetic minerals hematite and goethite as well as paramagnetic minerals. Moreover, its content of goethite is higher than that of hematite. The magnetic particles in the lower part of the profile are coarser, and mainly multi-domain particles. And the particles get finer, and the portions of single-domain and superparamagnetic particles increase with decreasing soil depth. (2) Parent material and climatic conditions are two important factors contributing to the difference in magnetic characteristics between different profiles, and the climate conditions are the main ones controlling the content of secondary magnetic minerals (especially hematite and goethite). Chroma index b*/a* can be used to measure the content of goethite/hematite. (3) Under a relatively humid and cold climate the transformation of magnetic minerals in the soil is dominated by the transformation of strong magnetic minerals (magnetite and maghemite) into weak magnetic minerals (hematite and goethite). (4) Temperature, instead of precipitation, is the dominant factor influencing the content and relative proportion of goethite and hematite in the soil derived from granite weathering crust in humid subtropical regions.

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刘鑫,吕镔,郑兴芬,陈梓炫,杜佳昊.福建周宁黄红壤的磁学特征及其磁性矿物转化[J].土壤学报,2022,59(4):987-998. DOI:10.11766/trxb202008240424 LIU Xin, Lü Bin, ZHENG Xingfen, CHEN Zixuan, DU Jiahao. Magnetic Characteristics of Yellow-Red Soil and Transformation of Its Magnetic Minerals, in Zhouning, Fujian Province[J]. Acta Pedologica Sinica,2022,59(4):987-998.

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