| 引用本文: | 吕镔，刘秀铭.2021.福建漳浦砖红壤磁学特征及其环境意义[J].地球环境学报,12(1):19-31 |
| LÜ Bin, LIU Xiuming.2021.Magnetic properties of latosols from Zhangpu County, Fujian Province and their environmental significance[J].Journal of Earth Environment,12(1):19-31 |
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| 福建漳浦砖红壤磁学特征及其环境意义 |
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吕镔，刘秀铭
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1.湿润亚热带生态地理过程教育部重点实验室，福建师范大学 地理科学学院，福州 350007
2.福建师范大学 地理研究所，福州 350007
3. Department of Earth and Environmental Sciences, Macquarie University, Sydney NSW 2109, Australia
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| 摘要: |
| 福建南部漳浦零星分布有砖红壤，是中国砖红壤分布的北界。该地区属南亚热带，纬度地带性土壤为赤红壤，而砖红壤是热带地区的纬度地带性土壤。漳浦砖红壤形成时期气候条件如何？是否可以反映气候带的变迁？这是值得深入研究的问题。基于此，通过系统的环境磁学分析，结合漫反射光谱、色度和常量地球化学元素指标，类比漳浦砖红壤和热带地区的广东徐闻现代砖红壤磁学特征的异同，探讨其环境指示意义。结果表明：（1）漳浦砖红壤中的磁性矿物以磁赤铁矿和赤铁矿为主，赤铁矿含量高于磁赤铁矿，磁赤铁矿主导剖面的磁学性质。磁性矿物颗粒以细粒的超顺磁（SP）和单畴（SD）为主；（2）与徐闻现代砖红壤相比，漳浦砖红壤中的磁性矿物具有更高Fe3+/Fe2+比例、赤铁矿相对含量更高、磁性矿物颗粒更粗的特点，表明漳浦砖红壤形成时期气温更高；（3）漳浦砖红壤是古土壤，其形成时期的年均气温较现今高2.5℃以上；当时华南地区热带面积范围扩大，纬度北移4°以上。本研究为全球气候变化提供了土壤学的实证。 |
| 关键词: 砖红壤 环境磁学 磁性矿物 磁赤铁矿 古气候 |
| DOI：10.7515/JEE202008 |
| CSTR：32259.14.JEE202008 |
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| 基金项目:国家自然科学基金项目（41877435，41772180）；福建师范大学创新团队项目（IRTL1705） |
| 英文基金项目:National Natural Science Foundation of China (41877435, 41772180); Innovation Research Team Fund of Fujian?Normal University (IRTL1705) |
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| Magnetic properties of latosols from Zhangpu County, Fujian Province and their environmental significance |
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LÜ Bin, LIU Xiuming
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1. Key Laboratory for Humid Subtropical Eco-Geographical Processes of the Ministry of Education, College of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China
2. Institute of Geography, Fujian Normal University, Fuzhou 350007, China
3. Department of Earth and Environmental Sciences, Macquarie University, Sydney NSW 2109, Australia
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| Abstract: |
| Background, aim, and scope Environmental magnetism is useful and effective for obtaining paleoclimatic information from paleosols and sediments such as Quaternary loess-paleosol, Neogene red clay in the Chinese Loess Plateau, and Quaternary red soil in southern China. These paleosols, which are derived from aeolian deposition, are aggraded soils. The relationship between magnetic properties and climate has been well established and the magnetic parameters have been widely used as proxies for paleoclimate research in the aforementioned areas. Other types of soils, which are derived from crustal weathering, can be called weathering-residual type soils. These soils have received less attention with respect to research on environmental magnetism. In terms of latitudinal zones, the northern subtropics, middle subtropics, southern subtropics, and tropics are characterized by yellow-brown soils, red soils, lateritic red soils, and latosols, respectively. These soils are named in terms of their color. The content of goethite and hematite, as well as the ratio of these two minerals, directly control soil color. Climate is the most important factor influencing the relative contents of goethite and hematite in soils. Therefore, these soils are good materials for environmental magnetic studies. Zhangpu County of Fujian Province is the northern boundary for the distribution of Chinese latosols. However, Zhangpu County currently belongs to the southern subtropics. The latosols in Zhangpu are probably paleosols. In this study, we compared the magnetic properties of Zhangpu latosols and modern latosols in the tropics, and discussed their environmental significance in combination with other indexes. Materials and methods Samples of latosols derived from the weathering of basalt were collected from the Zhangpu section (ZP, 24°14′24″N, 117°59′24″E; 2.2 m a.s.l.) and Xuwen section (XW, 20°24′33.6″N, 110°8′18.6″E; 75.2 m a.s.l.). The mean annual temperature (MAT) values for Zhangpu and Xuwen are 21.3℃ and 23.8℃, respectively. The mean annual precipitation (MAP) values for Zhangpu and Xuwen are 1601.5 mm and 1428.4 mm, respectively. Rock magnetic parameters, including low-field magnetic susceptibility (χlf), frequency-dependent magnetic susceptibility (χfd), anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), and magnetic hysteresis loops and thermomagnetic curves (M—T & κ—T curves) were measured. The chromaticity index, diffuse reflectance spectroscopy (DRS), and major geochemical elements were also measured. All experimental procedures were completed at the Key Laboratory for Humid Subtropical Eco-Geographical Processes of the Ministry of Education, Fujian Normal University. Results The results show that: (1) There is no obvious trend in magnetic parameters with depth in the ZP section. (2) The average χlf, χfd%, and S-ratio values of the ZP section are lower than those of the XW section, and the average HIRM, χARM/χlf, SIRM/χlf, Bcr values of the ZP section are higher than those of the XW section. (3) Samples from the middle and bottom of the ZP section exhibited a Curie point of 645℃, whereas samples from the XW section exhibited a main Curie point of 580℃. (4) Hysteresis loops of both sections show that ferrimagnetic minerals contribute most of the χlf or SIRM. (5) DRS first derivative curves of the two sections show high peaks at about 575 nm, and very low peaks at about 435 nm, suggesting that the samples contain large quantities of hematite (α-Fe2O3) and small quantities of goethite (α-FeOOH). (6) The average chromaticity index brightness (L*) and redness (α*) of the two sections are very close, but the ZP section has a lower yellowness (b*). (7) The average SiO2 content, Al2O3 content, and chemical index of alteration (CIA) of the two sections are very similar, but the ZP section has a higher Fe2O3 content. Discussion The magnetic minerals in these two sections differ in terms of parameters such as category, magnetic domain, and relative content. As a whole, the ZP section has a higher Fe3+/Fe2+ ratio, higher hematite content, and coarser-grained magnetic minerals than the XW section. Climate is the most important factor that affects the magnetic properties of these two sections. Hematite and goethite indicate hot-dry and cold-wet environmental conditions. In general, the two latosol sections formed during a period in which the climate was warm. However, the magnetic properties of the ZP section suggest that it formed under a warmer climate than the XW section. Thus, the Zhangpu latosol cannot form in modern climatic condition. Conclusions Based on the above analysis, it can be concluded that the Zhangpu latosol is a paleosol that formed under a warm climate when the MAT was at least 2.5℃ higher than the present MAT and the tropic-subtropic boundary was 4° north of its current location. Recommendations and perspectives Although the studied soil is derived from crustal weathering and it is therefore difficult to constrain the age of this soil, this study provides evidence for ancient climate change in China’s low latitudes via pedological means. In addition, because late Quaternary sediments (e.g., red earth, loess-like sediments and old red sand) are widely distributed in Fujian and Guangdong provinces, further research should investigate whether there is a link in chronology and paleoclimate records among these soils. |
| Key words: latosols environmental magnetism magnetic minerals maghemite paleoclimate |
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