Using magnetic susceptibility to assess soil degradation in the Eastern Rif,Morocco

The soil in the Rif, Morocco, is at serious risk because increasing anthropogenic pressures are gradually transforming large natural areas into farmland. The distribution of magnetic minerals within the soil profile can be used to assess soil development and degradation. The soils in the study area are severely eroded because of a combination of highly erodible soils, intense rainstorms and scarce vegetation cover. To sample of representative soil profiles, lithology, slope gradient and land use were considered. The ranges of magnetic susceptibility in the soil profiles distinguished between two primary soil groups. Magnetic susceptibility varied in the soil profile and along the soil toposequence, and the variations were related to the differences in the original magnetic composition and the influence of main erosion factors. Lithology is the main factor contributing to the variation in magnetic susceptibility. The magnetic susceptibility values in soils on Tertiary marls (χ = 13·5 × 10^?8 m³ kg^?1) differed significantly from those on Quaternary terraces (χ = 122·1 × 10^?8 m³ kg^?1). Slope affected the distribution of magnetic susceptibility because of the continuous loss of topsoil in some parts of the slope and the deposition of eroded soil in others. Elimination of the natural vegetation cover and a shift to cultivated land for cereals has had a negative impact on soil development and, on similar slopes and substrates, magnetic susceptibility decreased significantly in cultivated soils. The soils on steep slopes that had natural vegetation cover retained the magnetic minerals better than did those on gentler slopes that were under cultivation. Grazing, clearing and, especially, tilling has weakened the soil and made it much more vulnerable to erosion. An analysis of the main factors causing erosion will help to promote rational use of the land and to establish conservation strategies in such fragile agroecosystems. Copyright © 2009 John Wiley & Sons, Ltd.     

MT探测居里面深度的可行性研究

居里面是地球内部铁磁性物质向顺磁性物质转换的界面,在温度略低于居里点时物质磁化率会快速升高,这被称为Hopkinsin峰.对于地球内部而言Hopkinsin峰是只有几百米至几公里厚的薄层,由于其与居里温度的关系,因此其底界面的深度可以作为居里面深度的估计.传统的居里面深度探测方法包括谱分析方法、等层模型方法和温度－深度剖面法.这些方法是人们研究地球内部热结构和居里面深度的重要手段,但是谱分析和等层模型的结果均有一些固有的缺点,如横向分辨率太低等;而地热方程的结果则受地表因素影响十分严重,并且地球内部的热源分布也不是十分清楚,这导致了其结果是不可靠的.本文提出用MT方法探测居里面深度,通过对几种简单一维模型进行的正反演数值试验,论证了该方法的可行性.结果表明,用MT方法研究居里面性质,不但可以得到居里面深度,还可以得到居里面顶部Hopkinsin峰所对应介质的电学和磁学性质,但必须同时反演岩层的电阻率和磁导率,才能获得较为可靠的居里面深度估计.     

磁化率及其环境意义

作为地质与环境演化过程的替代指标,磁化率被广泛地用来研究各种与古气候和环境演化相关的问题.然而,磁化率的复杂性和多解性常常被忽略.本文从磁化率的物理基础出发,系统地论述了磁化率的测量、影响磁化率的主要因素（包括磁性矿物的类型、粒径、温度、频率和外加场等）,简要介绍了磁化率在海陆古环境研究中的应用.     

Magnetic anisotropy of HED and Martian meteorites and implications for the crust of Vesta and Mars

We investigate the petrofabric of crustal rocks from Mars and Vesta through the measurement of the anisotropy of the magnetic susceptibility (AMS) of achondrites. Previous data are integrated with new measurements to obtain a dataset that provide macroscopic information about the magnetic fabric of 41 meteorites of the howardite–eucrite–diogenite clan (HED, falls only) and 16 Martian meteorites. The interpretation takes into account the large contribution of paramagnetism to the magnetic susceptibility of these meteorites. We use a model that allows the computation of the anisotropy degree of the population of ferromagnetic grains and provides a quantitative proxy for the degree of shape preferential orientation of these grains in HED and Martian meteorites. The results also provide quantitative information about the shape of the magnetic fabric (prolate, oblate).In HED achondrites, the ductile FeNi grains are sensitive strain recorders and our magnetic fabric data provide the first quantitative insights to the strain history of the crustal rocks of Vesta. Most HED achondrites are breccias but display a strong and spatially coherent magnetic anisotropy, indicating that intense deformation of FeNi grains took place after brecciation. The average fabric of eucrites, howardites is oblate (i.e. the texture is foliated) whereas the fabric of diogenites is more neutral. The howardite results suggest the existence of an isotropic fraction of ferromagnetic minerals that can be ascribed to the presence of carbonaceous chondrite clasts that have preserved their original magnetic fabric. In this hypothesis, howardites have an intensity of petrofabric very similar to eucrites and diogenites. Thermal metamorphism (itself possibly impact-related) plus lithostatic compaction occurring after brecciation appears as the best candidate to explain the observed petrofabric in eucrites and diogenites, whereas compaction by hypervelocity impacts may be reponsible for the fabric of howardites.Martian meteorites may still possess their primary magmatic fabric. Among Martian meteorites, basaltic shergottites and nakhlites display an oblate fabric (foliated texture) with only limited variations among each group. Olivine–phyric shergottites have a neutral fabric that points to a different petrogenesis. Nakhlites have weaker fabric intensity than shergottites. The fabric intensity is comparable to what is classically observed in terrestrial volcanic and plutonic rocks.     

Cyclostratigraphy of the Induan (Early Triassic) in West Pingdingshan Section, Chaohu, Anhui Province

The Induan sequence in the West Pingdingshan Section, Chaohu, Anhui Province, displays a series of superimposed mudstone/limestone cycles. The lithological character of the cycles, combined with power spectral and wavelet analysis of magnetic susceptibility readings, reveals 12 short eccentricity and 56 precession Milankovitch cycles - obliquity cycles are not apparent. The uniformity of cycle thicknesses indicates a stable depositional setting making this section ideal to perform various geo-logical studies. Accordingly, the Induan Stage is estimated to have lasted 1.1 Ma, and the depositional rate for this part of the section is about 3.7 cm/ka. This places the Induan-Olenekian boundary in the West Pingdingshan Section at about 251.5 Ma based on an age of 252.6 Ma for the Permian-Triassic boundary.     

海南岛双池玛珥湖全新世磁化率及其环境意义

海南岛双池玛珥湖沉积物磁化率高分辨率的时间序列分析表明,磁化率与粒度指标和粘土矿物含量存在相关关系,磁化率突变往往对应于沉积物的突变界面,因此与沉积相变化可以对比.磁化率与碳、氮含量呈负相关,表明湖泊的有机质生产力高时磁化率较低.多指标的对比分析进一步证明了中国热带全新世高温期为:7 200～2 700aBP,高温期从炎热而降雨量相对低的稳定时期过渡到频繁变动的气候不稳定阶段;晚全新世,尤其在2 700aBP左右,气候模式发生了重大转折,波动幅度变大,可能与厄尔尼诺-南方涛动(ENSO)活动性增强有关.海     

The thermal field in a basin after a sudden passive pure shear lithospheric extension and sublithospheric mechanical erosion:the case of the Tuscan Basin (Italy)

A simple new model for sudden lithospheric thinning that considers the crust to be stretched and the lower layer of the lithosphere to be partially stretched and partially mechanically eroded is proposed. This model allows calculation of the thermal field of the lithosphere during the initial warming phase and the surface uplift.
Application of this model to the Tuscan Basin explains the high regional heat flux density values (>100 mW m⁻² ), the tectonic subsidence (about 1 km) and the average uplift (>400 m) observed in this region well.     

Characterization of Magnetically Zoned Pluton, Tsushima, Japan

Abstract: Systematic measurements of magnetic susceptibility were carried out at the outcrops of the Uchiyama granitic pluton, Tsushima Islands. The pluton consists of hornblende-biotite granodiorite and biotite granite. It intrudes concordantly along an anticline axis of the Taishu Group in middle Miocene (16. 10. 5 Ma), and crops out in five areas. The results show that the low magnetic susceptibility facies (LM–facies, <100A-10^-6 emu/g) always occurs in the peripheral part of the pluton, while the high magnetic susceptibility facies (HM–facies, > 250 A 10^-6 emu/g) occupies the core through the medium magnetic susceptibility zone (MM–facies). It is pointed out that the pluton forms a magnetic zoning within a single plutonic body.
Chemical compositions of the granitoids show no conspicuous differences in major elements among the LM–, MM–, and HM–facies. Biotites from the LM–, MM–, and HM-facies also indicate uniform compositions with Fe/(Fe+Mg)=0. 6.
The pluton is principally surrounded by black mudstones of the Lower Formation of the Taishu Group, which is deltaic to shallow sea sediments of the Eocene to early Miocene age, and the carbon contents in them were analyzed. The analyses show that carbon was mostly decomposed and lost in the hornfels zone, while mudstones in the non-hornfels zone usually contain 0. 5 to 0. 7 % C.
From these facts, it is concluded that the magnetically zoned pluton at Tsushima was formed by an external volatile buffer system such as CH₄–CO₂ during the solidifying magma process.     

内蒙古嘎顺诺尔湖泊沉积物磁化率与粒度的古环境意义

湖泊沉积物磁化率和粒度已被广泛地应用于古环境和古气候研究,但在不同地区两者的关系不同,一般与磁性矿物来源、种类和赋存粒级有关。查明磁性矿物来源和赋存状态是利用粒度和磁化率恢复古环境的重要基础。通过测定干旱区嘎顺诺尔GXN剖面湖泊沉积物的磁化率与粒度及两者的相关关系,结合代表性样品的岩石磁学特征,揭示此类地区湖泊沉积物磁性矿物来源和赋存状态,探讨它们在干旱区湖泊沉积物中所反映的环境变化信息。结果显示,剖面下部（200~105 cm）和上部（36~0 cm）沉积物磁化率与粗砂含量呈正相关,中部（105~36 cm）与粉砂含量呈正相关,表明磁性矿物主要富集于粗颗粒和较粗颗粒组分中。剖面下部和中部沉积物以顺磁性矿物为主,含少量亚铁磁性矿物（如磁铁矿+磁赤铁矿）,含少量顺磁性矿物;剖面上部沉积物以亚铁磁性矿物为主（如磁赤铁矿+磁铁矿）,含少量顺磁性矿物。根据嘎顺诺尔晚全新世湖泊沉积物剖面的岩性、磁化率和粒度组合特征,可将其划分为河漫滩相沉积（200~105 cm）、浅湖相沉积（105~36 cm）和滨浅湖相沉积（36~0 cm）3个阶段。该湖粒度组分中<4 μm和4~64 μm颗粒主要反映了湖泊水动力的变化,受碎屑物来源和水动力条件的控制;而河漫滩相中>64 μm粗颗粒组分的峰值是风力作用的结果。河漫滩相及滨浅湖相的磁化率高值则指示了湖面较低,水动力较强,有较多的磁性矿物随粗颗粒入湖;浅湖相的磁化率低值则指示该时期湖面较高,水动力较弱,磁性矿物随入湖粗颗粒含量的减少而降低。     

洛川地区S4间冰期植被演化与气候变化

对洛川地区S₄间冰期磁化率与孢粉资料的分析表明：高分辨率的孢粉分析,比磁化率更细致地反映出气候的小尺度变化,能够更深入地研究区域气候变化,这主要得益于植被是对温度、降水量的直接反映,而磁化率则受多种因素的影响。S₄间冰期较为温暖湿润,当时亚热带的界线比现今纬度更高,可达到黄土高原中部地区。洛川地区S₄间冰期气候可以分为5个阶段,呈现为干冷-暖湿-湿热-暖湿-干冷的变迁过程,对应的植被类型应为旱生草原-森林草原-森林-森林草原-旱生草原。