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1.
This paper presents magnetic susceptibility results of 578 samples from three Devono-Carboniferous carbonate successions in the Yangtze Block. They indicate that the magnetic susceptibility variations in carbonates are controlled by ancient sea-level changes. It is particularly so if the supply of detrital materials was low, when the magnetic susceptibility was mainly controlled by the content of authigenic minerals. If the supply of detrital materials was high, the average susceptibility value would be higher, but there is still a correlation between susceptibility and sea-level curves. A sea level rise would lead to a decrease in the content of the detrital materials, and therefore to a decrease in the susceptibility values. On the top of HST, the uplift and exposure events may lead to elevated magnetic susceptibility values. Carbonate susceptibilities can therefore be considered as one of the environmental proxy data for the research of sequence stratigraphy.  相似文献   

2.
The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.  相似文献   

3.
在沙漠沉积环境中,成土作用对磁化率的贡献较小,往往被原生磁信号掩盖,因此分离两种磁组分对气候和粉尘代用指标的提取至关重要.本研究选取位于毛乌素沙地东缘的锦界风沙沉积剖面为研究对象,利用多变量一元线性回归中的"平均值概念"进行磁化率的原生碎屑组分0和次生成土组分Xpedo的分离,并探讨磁学参数所承载的气候和环境意义.结果显示,磁学比值参数(如Xpedo/X0、Xfd/HIRM、Xfd%和XARM/SIRM)彼此之间存在显著线性或指数/对数相关关系,它们对成土强度指示明确,可以在一定程度上减小或避免磁性矿物背景值差异所产生的误差,与磁化率相比更适宜用于该区的古降水量重建.HIRM主要由碎屑赤铁矿含量控制,HIRM与X0存在明显正相关关系,表明碎屑赤铁矿随原生磁性矿物总体含量的增加(减少)而增加(减少),在粉尘成因磁性矿物中所占比例大致稳定,从而HIRM可以指示源区粉尘通量的变化.锦界剖面的原生和次生磁性矿物浓度均明显低于黄土高原黄土,不同地层X0和Xpedo在磁化率中所占的比例存在较大差异,因此磁化率的环境意义比较复杂,在使用其恢复古气候古环境时需慎重.  相似文献   

4.
Low magnetic susceptibility, low Sr content and hence high Rb/Sr ratio in the lake sediment sequence indicate a weak chemical weathering process under arid and cold climate of the Little Ice Age in a single closed lake watershed. According to different geochemical behavior between rubidium and strontium in earth surface processes, variation of Rb/Sr ratios in the lake sediment sequence can be used as an effective geochemical proxy with definite climatic significance of chemical weathering in watershed. Unlike chemical weathering process in tropic zone and modern temperate-humid climate, concordant changes in both Sr content and magnetic susceptibility with δ18O values of Dunde ice core suggest that the weak chemical weathering was controlled by air temperature during the Little Ice Age maximum. After the Little Ice Age, chemical weathering intensity was controlled also gradually by precipitation with increasing in temperature.  相似文献   

5.
A map of bulk magnetic susceptibility was obtained on El Hongo trondhjemite, a small Cambrian pluton intruding the metamorphic basement in Eastern Sierras Pampeanas, Argentina, based on systematic magnetic susceptibility measurement at 450 sites using a SM30 susceptibility meter. Samples were collected on 58 sites and their bulk magnetic susceptibility was measured in laboratory with a Bartington MS2 system. Point-to-point comparison showed differences, that were attributed to the effect of roughness of the surveyed surfaces, and to the development of a weathered cap. However, the difference was systematic and in accordance with expected values predicted by manufacturer tables, whereby, once corrected with the appropriate factor, the obtained values with SM30 susceptibility meter were regarded as representative of fresh rocks. The resulting map was interpreted in terms of variation in abundance of magnetite, which is present in the rocks as a magmatic mineral, altered to hematite (martitized) in varying degrees. The map revealed that El Hongo trondhjemite is a weakly magnetic pluton, with a typical bulk susceptibility of about 500 × 10?6 SI, which would correspond to an abundance of magnetite below 0.2 vol%, but with conspicuous variations. Lows in the outer sector and in the vicinity of metamorphic xenoliths were interpreted as due to destruction of magnetic minerals linked to reactions between magma and host rock. A distinct concentric pattern in the western area could indicate the presence of a separate intrusion. Finally, alternate highs and lows in susceptibility follow the undulations in regional schistosity, which in turn would have controlled the emplacement of the pluton. Thus we provide a good example of the utility of magnetic susceptibility mapping in granitoid terrains, as an expeditious way for preliminary mapping that could guide further and more detailed research.  相似文献   

6.
In this work, we carried out a preliminary study of traffic-derived pollutants from primary sources (vehicles), and on roads (paved area), road borders and surroundings areas. The study is focussed on the identification, distribution and concentration of pollutants and magnetic carriers. Magnetic parameters and their analyses suggest that the magnetic signal of vehicle-derived emissions is controlled by a magnetite-like phase. Magnetic grain size estimations reveal the presence of fine particles (0.1–5 μm) that can be inhaled and therefore are dangerous to human health. Magnetic susceptibility results (about 175 × 10−5 SI) show a higher magnetic concentration — magnetic enhancement — in the central area of the tollbooth line that is related to higher traffic. In addition, magnetic susceptibility was computed on several roadside soils along a length of 120 km and used to generate a 2-D contour map, which shows higher magnetic values (100–200 10−5 SI) near the edge of the road. The observed distribution of magnetic values indicates that magnetic particles emitted by vehicles are accumulated and mainly concentrated within a distance of several meters (1–2 m) from the edge of the road. In consequence, the magnetic susceptibility parameter seems to be a suitable indicator of traffic-related pollution. Non-magnetic studies show an enrichment of some trace elements, such as Ba, Cr, Cu, Zn and Pb, that are associated with traffic pollution. Furthermore, statistical correlations between the content of toxic trace metals and magnetic variables support the use of magnetic parameters as potential proxies for traffic-related pollution in this study area.  相似文献   

7.
Summary Magnetic property variations in marine, lacustrine and loess-paleosol sequences have proved to be useful proxies in climate change studies. However in order to correctly interpret the record of the magnetic property variations it is absolutely necessary to have a good understanding of the cause of the observed variations. Most of the ambiguity in loess-paleosol studies is in distinguishing the role of pedogenesis from other climatic factors. Studying the mineral magnetic properties of the protected cave sediments which have not undergone pedogenesis allows us to determine the degree to which detrital input is climatically driven. These results will help us better understand the variations observed in the surficial loess-paleosol sequences. This study reports mineral magnetic data collected from entrance facies sediments deposited during the early Wurmian glacial stage in the Kůlna Cave. The entrance facies sediments consist of loess-like silts with varying amount of talus. The magnetic susceptibility record from these sediments shows higher values in layers originating in colder climates which is different to that commonly observed in surficial loess deposits. Higher values of magnetic susceptibility in Kůlna sediments are probably due to higher concentrations of ferromagnetic minerals (magnetite and maghemite) and due to an increased proportion of superparamagnetic grains. The magnetic mineralogy and the grainsize distribution (grains larger than superparamagnetic) appear not to change throughout the studied profiles. Higher magnetic susceptibility accompanied by an increase in the superparamagnetic fraction observed in the sediments deposited during colder periods can be explained by an increased input from a pedogenic source when the vegetation cover was reduced and the erosion rate increased.  相似文献   

8.
The concentrations of four selected heavy metals (Pb, Zn, Ni and Cr) were measured on 167 topsoil samples collected from the city of Xuzhou, China via inductively coupled plasma mass spectroscopy (ICP-MS). It was found that Pb and Zn were principally derived from anthropogenic inputs whereas Cr and Ni distributions were mainly controlled by parent materials. The spatial distribution patterns of Pb, and Zn were similar to that from low field magnetic susceptibility (χlf), suggesting interaction among them. Two threshold values for magnetic susceptibility and frequency-dependent susceptibility percentage (χfd) were applied to discriminate between polluted and unpolluted samples according to their magnetic susceptibility, resulting in 109 samples populating the “polluted” subset. The Pb and Zn concentrations of the “polluted” subset were statistically significantly higher than those measured in the “unpolluted” one. The heavy metal concentrations were also investigated varying the magnetic susceptibility thresholds to change the “polluted” subset.  相似文献   

9.
Magnetite is a natural component of many aquifers. Abiotic degradation of chlorinated solvents by magnetite can be an important mechanism for natural attenuation of these contaminants. The quantity of magnetite in aquifer materials can be estimated by measuring the magnetic susceptibility of the materials. This is most commonly done by determining the magnetic susceptibility of core samples in an analytical laboratory using a magnetic susceptibility meter. Unfortunately, the cost of acquiring core samples often makes an evaluation of abiotic degradation by magnetite economically unrealistic. Downhole sondes (probes) are available for the determination of magnetic susceptibility. In this study, a downhole sonde was evaluated as an affordable alternative to acquiring and analyzing core samples. The sonde was introduced into 10 monitoring wells. The data from the sonde were then compared to data from core samples that were collected from the same elevation as the sonde data. The core samples analyzed in the laboratory were used as the standard against which the sonde data were compared. The downhole sonde reported values that were similar to values reported on core samples. At most wells, the means of the two measurements could not be distinguished at the 95% confidence interval. When the means could be distinguished, they still agreed within a factor of two.  相似文献   

10.
During the Mesozoic (250–64 Ma) intervals of about 0.5 Myr were subject to severe environmental changes, including high sea-surface temperature and very low oxygen content of marine water. These Oceanic Anoxic Events, or OAEs, occurred simultaneously with profound disturbance to the carbon cycle. The carbon-isotope anomaly in the Early Jurassic that marks the Toarcian Oceanic Anoxic Event (T-OAE) at ~ 182 Ma is characterized in marine sections by a series of dramatic steps towards lighter values. Herein we present new carbon-isotope data from terrestrial organic matter (phytoclast separates), collected through a Late Pliensbachian–Middle Toarcian coastal and marginal marine succession in the Polish Basin, a setting where hinterland climate and sea-level change are well recorded. The results show that the shift to light carbon-isotope values in the woody organic matter, and therefore also in atmospheric carbon dioxide, similarly occurred in major steps. The steps are here correlated with those identified from marine organic matter, where they have previously been attributed to 100 kyr eccentricity forcing of climate. The results provide strong support for orbitally and climatically controlled release of isotopically light carbon from gas hydrates into the ocean–atmosphere system in a series of rapid bursts. Additionally, a link between the carbon-isotope steps and shoreline movements can be demonstrated. Individual peaks of the negative excursion are mostly associated with facies indicative of sea-level rise (flooding surfaces). However, at the same time inferred higher atmospheric carbon-dioxide content may be expected to have resulted in increased rainfall and temperature, leading to accelerated weathering and erosion, and consequently increased sediment supply, progradation and regression, causing some mismatches between isotope shifts and inferred sea-level changes. Enhanced abundance of megaspores derived from hydrophilic plant groups, and marked increase in kaolinite, are coincident with the overall development of the negative isotope excursion. The combined data suggest that each 100-kyr cycle in carbon-isotope values was characterized by increasingly severe palaeoclimatic change, culminating in extremely hot and humid conditions co-incident with the peak of the final most negative carbon-isotope excursion. The chemostratigraphic correlation allows very precise dating of the Late Pliensbachian–Middle Toarcian coastal and marginal marine sedimentary succession in the Polish Basin.  相似文献   

11.
Soil magnetometry was used for detailed mapping of immission load over the territory of the Giant Mountains National Park. This project is a continuation of our previous study, which suggested that low-field magnetic susceptibility of topsoils in this region is controlled by atmospherically deposited anthropogenic ferromagnetic particles. In the present study, we have compiled a map of topsoil magnetic susceptibility on the basis of measurements on more than 460 sites. Elevated values of magnetic susceptibility can be attributed to local sources of pollution, located within and at the margins of the Park. We have identified a group of heavy metals of anthropogenic origin, present in the topsoils, and found a positive correlation between the concentration of Pb and magnetic susceptibility. Our results prove that magnetic mapping is a sensitive, fast and robust method, which can be advantageously applied to regions with relatively low degree of pollution, such as the Giant Mountains National Park.  相似文献   

12.
Summary High resolution isothermal remanent magnetisation (IRM) acquisition was performed on forty-one Chinese loess samples with the aim of investigating the effects of weathering on the remanence carrying mineralogy. Magnetic susceptibility was taken as a measure of the degree of weathering. This is a valid assumption based on previous studies which have demonstrated a close relationship between climate and magnetic susceptibility in the Chinese loess. Detailed analysis, based on fitting accumulative log-gaussian curves to IRM acquisition data, reveals up to four distinct coercivity components. The remanences of the two harder coercivity components are found to be almost independent of magnetic susceptibility. On the other hand, the remanences of the two softer coercivity components are found to be positively correlated with magnetic susceptibility. Most probably the two harder components are associated with detrital iron-oxides in the primary dustfall, whereas the softer components were created during pedogenesis.  相似文献   

13.
镇江大港下蜀黄土剖面多个层位的磁化率出现异常降低,导致与北方黄土的磁化率记录难以对比.为探讨磁化率异常降低的原因,我们在大港钻探ZK孔获取了岩芯.在10个具代表性的层位采样并测量了其等温剩磁获得曲线.通过基于期望最大化算法的计算程序（Irmunmix V2.2）,定量分析了样品的磁性矫顽力组分（magnetic coercivity component）.结果显示这10个样品可大致分成3类,第一类不含中磁组分,第二类含有较少的中磁组分,第三类则含较多的中磁组分.矫顽力组分的含量与样品中铁锰结核的含量密切相关.铁锰结核含量高的样品,中磁组分、硬磁组分含量也高,而软磁组分含量低,与此对应的是这类样品的磁化率低.表明在铁锰结核的形成过程中,原始的软磁组分被溶解,而产生新的中磁组分、硬磁组分,这个次生变化过程导致样品磁化率异常降低.铁锰结核含量高的样品,经历过较强的还原作用,因此ZK剖面磁化率异常降低是还原作用的结果,可能是某时期降水量增大所致.  相似文献   

14.
Since the 1970s, environmental magnetism has found extensive applications in diverse research areas of geoscience[1]. In China, a lot of environmental magnetic studies have been carried out on loess, lake and marine sediments for the purpose of paleoclima…  相似文献   

15.
The role of single-domain (SD) magnetic particles in creation of inverse magnetic fabrics is investigated on simple mathematical models using a realistic estimate for SD intrinsic susceptibility. In contrast to the fraction created by multi-domain (MD) particles, in which the anisotropy of magnetic susceptibility (AMS) is controlled by both the grain AMS and intensity of the preferred orientation of the particles, the AMS of the SD fraction is controlled solely by the intensity of the preferred orientation. The degree of AMS of ensemble of SD grains with a preferred orientation is therefore much higher than that of the same ensemble of MD particles implying the existence of frequent inverse magnetic fabrics. However, the occurrence of inverse magnetic fabrics due to SD particles is more the exception than the rule. Consequently, the amounts of SD particles is probably in general low. Nevertheless, the presence of SD particles in amounts insufficient to create inverse magnetic fabrics may diminish the whole rock AMS substantially. This can be one of the reasons for relatively low AMS in volcanic rocks whose magnetic particles may be really small obeying the conditions for the existence of SD particles.  相似文献   

16.
In most Chinese loess–paleosol sequences, high magnetic susceptibility values are found in the soil horizons, with low values in the loess layers. The susceptibility signal has been widely used as a proxy climatic indicator. However, both the causes and mechanisms which control susceptibility still remain controversial. Our recent studies challenge some earlier interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments. First, the prevailing hypothesis of pedogenic origin cannot totally account for the susceptibility variations in many sections. Second, in some cases, the principal carrier of the magnetization seems not to be the ultrafine pedogenic minerals, rather coarse lithogenic magnetic minerals derived from local sources can also contribute significantly to the susceptibility signal. Finally, not all the Chinese soils have higher susceptibility values than that of loess or sand layers, opposite relationships do exist. It is concluded that the source and causes of susceptibility may be different from site to site or even at different times at a given site. Any single hypothesis cannot completely account for the susceptibility variations in the Chinese Loess Plateau and its surrounding regions. Besides the previous explanations of carbonate leaching, pedogenic processes and organic decomposition, the contribution of source materials to magnetic susceptibility must also be taken into account.  相似文献   

17.
Measurement of magnetic properties of steel sheets   总被引：1，自引：0，他引：1  
Magnetic methods are used in detection of environmental, engineering and military objects fabricated of thin ferromagnetic sheets having volume susceptibilities higher than 100 SI units. Magnetic modelling of such objects would be advantageous, but it requires knowledge of the susceptibility and remanence values of sheet materials, which is scarce. We introduce a magnetometer method for the determination of susceptibility and remanence on thin steel samples. The area of the sample must be so large that its within-sheet magnetization remains below the saturation state. The measurements are made in normal office surroundings in the Earth's magnetic field with an ordinary fluxgate magnetometer. The square-shaped sheet samples measured in this work have an edge length of 17.5 cm and a thickness in the range 0.5–1.0 mm. During the measuring procedure the sample is placed in four positions on a subvertical measurement board. For each position, the magnetic field in the dip direction of the board plane is measured on the opposite sides of the sample. The secondary field values are averaged for each sample position in order to reduce the effect of sample inhomogeneities. With these data, the susceptibility and remanence of the sample in its edge directions are then determined, based on a model curve which is calculated numerically using thin-sheet integral equations. The susceptibilities measured for different steel types (cold rolled and hot-dip zinc-coated steel sheets) varied in the range 200–500 SI units, and the remanence varied in the range 1000–20 000 A/m. No systematic differences were observed between the magnetic properties of various steel types. The repeatability of the susceptibility measurements was good (variations < 5%) but the remanence could be changed by 50% between repeated determinations. The measured susceptibility range signifies that pieces of steel with a typical thickness of 0.5 mm remain below magnetic saturation when their edge dimension is larger than 5 cm. Therefore magnetic modelling of larger steel pieces must be made using the thin-sheet theory with known magnetic properties, whereas smaller saturated pieces can be alternatively modelled as an equipotential system.  相似文献   

18.
《Geofísica Internacional》2014,53(4):473-490
We carried out a combined thermal and magnetic evaluation on experimentally knapped clasts of different lithologies (chert, quartzite, limestone, sandstone and obsidian) heated under controlled field and temperature conditions. The main aim of this study is to estimate the feasibility of use of these raw materials, which are commonly found in prehistoric archaeological sites for archaeomagnetic purposes. Rock magnetic analysis included measurements of low-field magnetic susceptibility, isothermal remanent magnetisation (IRM) acquisition curves, hysteresis loops and thermomagnetic curves of lithic clasts both before and after experimental heating. All lithologies, except the obsidian, recorded an increase of up two orders of magnitude in their magnetic concentration-dependent parameters revealing the formation of new ferrimagnetic minerals. Obsidian and sandstone are the most reliable magnetic carriers, followed by limestone, chert and quartzite. Magnetic susceptibility values show significant differences among lithologies. Isothermal remanent magnetisation proved also to be highly discriminatory as well as the room temperature hysteresis parameters. The main macroscopic alterations resulted in colour changes, rubefactions, potlids in cherts and the massive formation of internal fissures in obsidian specimens. The multispecimen palaeointensity technique was applied on selected samples yielding satisfactory results for heated obsidian and sandstone samples. The archaeological applicability of the results is discussed as well as their geomagnetic significance.  相似文献   

19.
断层带内的流体不仅可以通过水岩反应改变断裂岩的矿物组成和化学成分,从而导致化学性质和物理性质的变化,而且可以影响或控制断裂带的变形行为.断裂带中岩石磁学特征是由特定化学环境下磁性矿物的种类和含量所决定的,因此,从矿物学和地球化学角度探讨断裂岩的磁性变化,对揭示断层的变形行为和环境具有一定的指示作用.本文以汶川科钻WFSD-3P钻孔中龙门山灌县—安县断裂带断裂岩为研究对象,运用高分辨率磁化率测试、XRD矿物成分半定量分析、XRF元素扫描以及不同价态Fe元素含量分析等多种方法开展断层磁学变化和变形环境的研究.磁化率测试结果表明灌县—安县断裂带断层泥的磁化率值普遍低于对应的围岩磁化率平均值.结合前人研究成果表明造成该断层泥低磁化率异常的原因是在间震期的长期流体作用下,铁磁性矿物（例如磁铁矿）转变成顺磁性矿物（铁硫化物、菱铁矿或含铁的黏土矿物）.新生铁硫化物和含铁黏土矿物是在间震期缓慢形成的,而黏土矿物含量的增加弱化了断层强度,促进断层蠕滑,这说明断层泥低磁化率异常可能指示了该断裂在间震期长期缓慢活动,即为蠕滑变形.断层泥中黄铁矿的发育和高Fe2+和S元素、低Fe3+的特征显示灌县—安县断裂作用环境通常是在低温、还原环境中进行的.这些结果与低磁化率值的相关性暗示断层泥低磁化率异常可能对活动断层的低温还原环境具有指示意义.  相似文献   

20.
In gully erosion, the detached soil can be transported over long distances along the landscape. The eroded material can be redistributed and/or deposited on the soil surface along the landscape and then eventually be buried by newly eroded and deposited sediment. There can be significant variability of the soil conditions (e.g., texture and moisture content) over which the eroded material travels. The eroded material can be detected through the use of magnetic tracers attached to or mixed with the eroded soil. In this study we evaluated the degree to which the magnetic signal of the magnetite is conditioned by (i) burial depth of tracer, (ii) condition of soil covering the tracer and (iii) tracer concentration. In the laboratory containers were filled with a specific soil. In the filling process, a 0.5-cm layer of a soil–magnetite mixture was interspersed in the soil profile at a certain depth. Experiments encompassed three different soil–tracer concentrations (1000:1, 200:1, 100:1), four burial depths of tracer (0 cm, 3 cm, 5 cm and 10 cm from soil surface), and two different soils. In each case, the magnetic susceptibility was measured with a susceptometer. Experiments were repeated with different soil moisture contents. If the tracer is located under the soil surface, a minimum soil–tracer concentration of 200:1 is required for its correct detection. The intensity of the magnetic signal decreases dramatically with the vertical distance of the tracer from the soil surface. The maximum detection depth for the tracer's magnetic signal is strongly dependent on the natural magnetic susceptibility of the soil, which masks the tracer's signal. Variation in soil moisture content does not significantly affect the magnetic signal. For extensive field studies, the soil–tracer volume to be handled would be very high and therefore, it is necessary to explore new tracer application techniques.  相似文献   

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