刘鑫(1995—),男,山东滨州人,硕士研究生,主要从事土壤环境磁学研究。E-mail:
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。
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.
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.
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.
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
土壤环境磁学被广泛应用于古环境演化和气候变迁的研究[
与北半球同纬度的其他亚热带干旱地区不同,受季风影响的中国南方具有湿润多雨的气候特点和独特的水热组合方式,因此被称为湿润亚热带,具有独特性。该区山地受垂直地带性的影响,随着海拔升高,降水量升高而温度降低,有别于较大空间范围内温度与降水的同步变化,从而更有利于区分温度/降水对土壤磁性的影响[
周宁县地处福建省东北部,介于26°53′~27°19′ N,119°07′~119°21′ E之间。受季风环流和地形影响,该地形成了中亚热带季风山地气候,多年平均气温(MAT)15 ℃,多年平均降水量(MAP)2 049.3 mm(数据来源:中国气象局)。
周宁剖面(ZN剖面)的地理坐标为27°07′0.66″ N,119°20′21.38″ E,剖面底部海拔906.4 m(
周宁黄红壤剖面地理位置示意图(a)和剖面照片(b),图b中的红色虚线为B层与C层分界线
Location of Profile ZN of yellow-red soil(a)and photo of the profile(b), red dotted line is the boundary of horizon B and C
磁学测量、数据处理方法参照文献[
色度测量:色度仪的型号为Color Flex® EZ型,取适量200目以下的干燥样品平铺于测试皿,压平至不起皱,利用CIELAB表色体系,测定亮度(
地化测量:用压片机将适量干燥样品(无水硼酸覆在样品周围)压制成光滑圆饼,用帕纳科Epsilon3台式能量色散X射线荧光光谱仪对其进行元素测量。
漫反射光谱测量:取适量200目以下的干燥样品置于测试皿中,使用岛津UV-2 600+ISR-2 600PLUS紫外/可见光分光光度计测量,测量波长范围为380~700 nm,步长为5 nm。对结果进行一阶导数处理,用其曲线特征峰峰值来衡量针、赤铁矿相对含量。
ZN剖面各层次磁学参数的深度变化曲线如
周宁剖面常温磁学参数
Room temperature magnetic parameters of Profile ZN
周宁剖面代表性样品磁滞回线(实线:未进行顺磁校正的磁滞回线,虚线:顺磁校正后的磁滞回线)
Magnetic hysteresis loops of representative samples collected from Profile ZN(Solid line: hysteresis loop without paramagnetic correction, and dashed line: hysteresis loop after paramagnetic correction)
高温磁学测量主要包括对高温磁化率和高温磁化强度的测量,磁性矿物在加热过程中表现出不同的磁学特征,因此可以通过热磁曲线的转折温度(居里点、尼尔点或相变点)来鉴别磁性矿物[
周宁剖面代表性样品
其次,从加热-冷却旋回分析磁化强度变化。A层样品和0.5 m处B层样品的冷却曲线位于加热曲线上方,表明加热过程中生成了强磁性矿物;并且随着深度增加,磁铁矿的生成量逐渐降低,表现为磁化强度的增加幅度呈下降趋势(
CIELAB表色系统被广泛用于土壤研究[
周宁剖面色度参数
Chromatic parameters of Profile ZN
元素组成可以反映土壤淋溶程度和风化程度[
ZN剖面地化参数随深度变化曲线如
周宁剖面地球化学参数
Geochemical parameters of Profile ZN
漫反射光谱是衡量针铁矿(Gt)、赤铁矿(Hm)相对含量的有效方法。通常在漫反射光谱的一阶导数图谱中,代表针铁矿含量的特征峰峰值在435 nm左右,代表赤铁矿含量的特征峰峰值在565 nm左右[
代表性样品漫反射光谱和土壤层(A层和B层)色度参数与漫反射光谱参数的相关关系
Correlations of the diffuse reflectance spectrum parameters with the diffuse reflectance spectra of the representative samples and chromatic parameters of soil horizons(horizon A and B)
磁学参数的多解性、风化/成土过程中磁性矿物生成、转化途径的复杂性以及影响因素的多样性使得对土壤磁性定量研究非常困难。在湿润亚热带地区,土壤中磁性矿物的变化既有次生磁性矿物的生成,也有原生磁性矿物的破坏[
不同剖面常温磁学和色度特征值均值
Mean room temperature magnetic and chromaticity characteristic values of different profiles
剖面代码Profile code | χ |
χARM |
χfd% |
|||||
福建周宁ZN | 34.72 | 28.26 | 2.35 | 72.39 | 5.55 | 18.98 | 19.80 | 3.38 |
南平北站NPN | 85.23 | 122.06 | 5.21 | 67.55 | 16.90 | 31.36 | 35.65 | 1.86 |
浦城古楼PC-GL | 143.67 | 250.48 | 2.76 | 70.30 | 6.77 | 23.96 | 24.91 | 3.67 |
浦城上云PC-SY | 89.03 | 84.21 | 1.58 | 65.61 | 6.56 | 22.59 | 23.52 | 3.46 |
四个剖面均发育于中亚热带季风气候区的花岗岩风化壳上,受海拔影响,剖面的气候(主要为温度和降水量)、生物和局部环境等存在差异。四个剖面的气候条件与海拔高度不同:ZN剖面年均温度15 ℃,年均降水量2 049 mm,剖面底部海拔906 m;NPN剖面年均温度为19.3 ℃,年均降水量为1 660 mm,剖面底部海拔为109 m;PC-GL与PC-SY剖面所在的浦城县(海拔为276.9 m)年均温度17.7 ℃,年均降水量1 720 mm,PC-GL剖面底部海拔553 m,PC-SY剖面底部海拔为920 m。总体上,这四个剖面所在地区为湿热气候。但由于地形因素,福建较高海拔地区气温低降水多,较低海拔地区气温高降水少。为便于对比,用“相对湿冷”来描述较高海拔地区的气候状况。从对ZN剖面的磁学特征分析可知,ZN剖面以亚铁磁性矿物为主,与其余三个剖面的主导磁性矿物一致,因此成土时间不是磁性差异的主因。母质虽同为花岗岩风化壳,但其中磁性矿物、铁元素的含量及赋存形式均对原生磁性矿物存在影响,并进一步影响次生磁性矿物。在四个剖面中,ZN剖面C层的磁化率最低,约为PC-GL剖面磁化率的1/5,由此可知不同区域母质的本底磁性强弱有较大差别。此外,ZN剖面C层土壤Fe2O3的含量仅为1.24%,显著低于A层与B层,说明初始铁元素含量并不一定是磁化率升高的抑制因素。四个剖面的局地坡度均在5°以下,表明地形并不是影响磁性强弱的主导因素。生物(动植物与微生物)的生存和活动主要受气候条件的制约,因此暂将其纳入气候因素讨论。
气候(主要为温度和降水量)是影响土壤磁性变化的一个重要因素。降水量影响着细粒强磁性矿物的生成、转化和溶解。温度既能影响化学反应速率,又通过改变土体中的有效水分而影响磁性矿物。随着海拔升高,温度呈降低趋势,而降水量反之,土壤中的水分含量也明显升高。从四个剖面的磁学特征对比可知,海拔越高,磁颗粒越粗,因其母质本底磁性强弱不同,磁化率变化并无明显规律。对比四个剖面的色度结果,ZN剖面具有最低的
周宁剖面磁学、地球化学、色度参数对比
Comparison of magnetic, geochemical, and chromatic parameters of Profile ZN
ZN剖面中磁学、地化、色度等参数相互之间可以进行较好的比对,表明这些参数能够较为准确地指示亚热带地区花岗岩母质上土壤的风化。因此,本研究认为相对低温高湿地区的土壤发生层次之间的磁性差异主要受气候控制,体现在以下几个方面:(1)就磁性矿物种类而言,低温高湿的气候条件使土体处于较为湿润的环境,生成的次生磁性矿物更多的是针铁矿。亚铁磁性矿物如磁铁矿转化成弱磁性矿物甚至溶解;(2)就磁性矿物含量而言,由于较低的气温,化学风化强度较低,弱磁性矿物(针铁矿和赤铁矿)的总体含量低于低海拔的高气温地区;(3)就磁性矿物颗粒而言,原生磁性矿物受强烈的风化作用而使颗粒变细,新生成的次生磁性矿物也以细颗粒为主。
ZN剖面磁性矿物含量相对较少,从对磁性影响大小而言,以磁铁矿为主,同时含有少量的磁赤铁矿、赤铁矿和针铁矿、顺磁性矿物。从质量分数而言,针铁矿含量最高,赤铁矿次之,磁铁矿和磁赤铁矿含量极低。剖面下部磁颗粒较粗,以多畴颗粒为主,随着深度减小磁颗粒变细,单畴和超顺磁颗粒增多。母质和气候条件是不同剖面间总体磁学特征差异的两个重要影响因素。次生磁性矿物(特别是赤铁矿和针铁矿)的含量主要受气候条件控制。色度指标
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