Magnetic characteristics of purple soils in Wuyishan and their significance for pedogenesis
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摘要:
紫色土是非地带性土壤, 具有磁性本底值低的特征, 是研究成土初始阶段土壤磁性矿物与成土因素关系的良好材料。目前, 国内紫色土系统的环境磁学研究处于起步阶段。本研究以福建省武夷山市的紫色土剖面(简称WYS剖面, 地理坐标为27°44'24''N, 118°00'36''E; 海拔216.3m)为分析对象。该剖面发育于白垩系紫红色粉砂质泥岩之上, 厚度为0.9m。通过系统的环境磁学研究, 结合漫反射光谱一阶导数曲线、色度指标和常量地球化学元素指标, 分析其磁学特征的层次分异, 探讨磁学参数对成壤的指示意义。结果表明: 1)WYS剖面以反铁磁性矿物赤铁矿为主。A层的亚铁磁性矿物磁铁矿含量略有增加, 超顺磁(SP)颗粒含量更高, 磁颗粒相对更细。成壤作用是造成A层和C层的磁学特征差异的主导因素。2)WYS剖面母岩具有易于物理风化、不易化学风化的特征。由于成土时间短, 即便在湿热气候条件下, 母质中磁性矿物转化微弱。母质和成土时间是影响紫色土磁学性质的主导因素。3)成壤过程中, 少部分赤铁矿转化为磁铁矿。磁学参数的微小变化响应于弱成壤作用, 表明紫色土磁学特征可以非常敏感地指示成壤强度。
Abstract:Purple soils are a type of azonal soils with weak background magnetic properties, which make it useful for studying the relationship between soil magnetic minerals and soil-forming factors in the initial stage of pedogenesis. However, so far there are few environmental magnetic studies of purple soils in China. We studied a purple soil profile (27°44'24''N, 118°00'36''E; altitude 216.3m) in Wuyishan City, Fujian Province, China, termed the WYS section. The profile is developed on Cretaceous purplish red silty mudstone and is 0.9-m thick; it has a typical humus-rich A horizon, overlying the parent material C horizon. We collected seven samples from the A horizon(0~30cm)at 5-cm intervals, and six samples from the underlying C horizon, at 10-cm intervals. Magnetic parameters, chromaticity index, diffuse reflectance spectroscopy(DRS)and major element contents were measured on all the samples, with the aim of determining the pedogenic significance of the magnetic properties. The principal results are as follows: (1)The most abundant magnetic mineral throughout the profile was the imperfect antiferromagnetic mineral hematite. There were minor differences in the magnetic properties of the A and C horizons. The content of the ferrimagnetic mineral magnetite was slightly higher in the A horizon, and a small quantity of the iron oxyhydroxide goethite was also present. Additionally, the content of superparamagnetic(SP)particles was higher in the A horizon. (2)The parent material of the WYS section, purplish red silty mudstone, is susceptible to physical weathering, but the degree of chemical weathering is relatively weak, which is consistent with the short duration of pedogenesis. Therefore, the degree of magnetic mineral transformation of the parent rock was limited, despite the humid and warm climate. The parent material and the duration of pedogenesis are the dominant factors affecting the magnetic properties of the purple soil. (3)During pedogenesis, a small quantity of hematite in the weathered crust of the purple silty mudstone is reduced under the combined influences of organic matter, moisture, and other factors, forming fine-grained magnetite, including SP particles. Additionally, a magnetite rim may develop around a core of hematite, which contributes to the observed slight enhancement of the magnetic properties of the A horizon. The magnetic properties of this purplish soil are only slightly altered by the weak pedogenic environment, which indicates that its magnetic properties are a sensitive indictor of pedogenic intensity.
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Key words:
- environmental magnetism /
- magnetic susceptibility /
- hematite /
- pedogenesis /
- purple soil
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图 1
武夷山紫色土剖面地理位置示意图(a)和剖面照片(b)
Figure 1.
Location of WYS purple soil section (a) and the section photo (b) from Wuyishan. The length of ice axe is 75cm
图 3
武夷山剖面代表性样品磁滞回线和热磁曲线
Figure 3.
Magnetic hysteresis loops and M-T curves of typical samples from Wuyishan section. Solid lines are hysteresis loops without paramagnetic correction, and dashed lines are hysteresis loops after paramagnetic correction
图 4
武夷山剖面代表性样品DRS一阶导数曲线
Figure 4.
DRS First derivative curves of typical samples from Wuyishan section
图 6
武夷山剖面紫色土元素组成和化学蚀变指数CIA随剖面深度变化
Figure 6.
Sequential variations in chemical composition and chemical index alteration of purple soil from Wuyishan section
表 1
武夷山剖面常温磁学、色度、地球化学元素数值范围和平均值
Table 1.
The value range and the average values of room temperature magnetic parameters, chroma indexes, and geochemical element indexes of Wuyishan section
指标 A层 C层 范围 均值 范围 均值 χlf(×10-8m3/kg) 9.83~12.00 10.80 9.58~9.91 9.72 SIRM(×10-5Am2/kg) 262.57~278.26 268.92 259.83~269.19 265.07 HIRM(×10-5Am2/kg) 141.65~158.53 152.85 161.72~170.32 166.52 χfd%(%) 1.69~6.35 4.43 0~1.69 0.71 χARM/χlf 0.15~0.62 0.31 0.18~0.42 0.26 SIRM/χlf(×103A/m) 22.99~27.00 25.01 26.20~27.75 27.27 Bcr(mT) 311.56~349.24 331.49 354.42~370.62 364.35 S-ratio(%) -19.93~-5.62 -13.74 -28.07~-23.14 -25.63 L* 51.98~52.61 52.21 51.51~52.39 51.86 a* 12.97~13.69 13.39 13.31~13.68 13.42 b* 19.17~19.82 19.58 18.13~19.37 18.63 Ca*b* 23.15~24.16 23.72 22.55~23.71 22.98 b*/a* 1.45~1.48 1.46 1.35~1.42 1.38 CIA 80.30~81.04 80.61 78.61~80.41 79.25 
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