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1.
ARM (anhysteretic remanence)/SIRM (saturation isothermal remanence) and TRM (thermoremanence)/SIRM were measured as a function of the concentration (volume fraction) of single-domain magnetite (3 × 10?6 ? C ? 2 × 10?2), ARM/SIRM increases with decreasing concentration, showing that there is magnetic interaction between fine particles. The role of magnetic interaction in TRM acquisition is also important at higher concentrations of magnetite (C ? 0.1%), where the value of TRM/SIRM increases with decreasing concentration. It is only for concentrations ofC ? 0.1% that the value of TRM/SIRM is fairly constant and interactions among magnetite grains seem to be ignored. The ratio TRM/ARM decreases from seven to almost unity as the concentration of magnetite decreases.  相似文献   

2.
The combination of magnetic and geochemical methods was used to determine the mineralogy, grain size and domain structure of magnetic particles in indoor dust collected in 195 sites in Warsaw, Poland. Data show an asymmetric distribution of magnetic susceptibility (χ) in the wide range of 20–1514 × 10?8 m3 kg?1. Comparison of magnetic parameters shows that the internal dust contains outside pollution characteristic for air and soil. More than 90% of indoor dust samples were characterized by roughly uniform magnetic mineralogy, typical for fine grained magnetite (diameter of 0.2–5 μm), and grain size between pseudo-single-domain and small multi-domain with small contribution of superpara-magnetic particles (～10%). Samples with χ larger than 220 × 10?8 m3 kg?1 contain mainly magnetite and an anthropogenic metallic Fe with T C > 700°C. The indoor dust contains, characteristic for the urban areas, spherical magnetic particles originated from fossil fuel combustion processes and mixture of irregular angular iron-oxides grains containing other elements, including Na, Ca, Al, Si, K, S, Mn, Cl, and Mg.  相似文献   

3.
The physical and chemical properties of soils are closely controlled by the climate factors and thus are significant for paleoclimate reconstruction.In this study,two adjacent soil profiles(LP and LPM) with different slopes from Yan'an,Shaanxi Province,were investigated using magnetic methods to determine the impact of topography on magnetic properties of soil.Our results show that although LP and LPM have similar magnetic minerals and grain size distribution,both the average and maximum magnetic susceptibility(x) of LP are almost doubled compared to those of LPM.In addition,the ratios of susceptibility enhancement to the background(Ex) for LP and LPM are 2.27 and 2.04,respectively;the ratios of saturation isothermal remnant magnetization(SIRM) enhancement(ESIRM) for these two profiles are 1.80 and 1.86,respectively.The slopes of the linear regression trends between frequency-dependent susceptibility(χFD) and "hard" isothermal remnant magnetization(HIRM)(χFD/HIRM),Anhysteretic remnant magnetization(ARM) and HIRM(ARM/HIRM),are almost free from the effect of parent material.χFD/HIRMs for LP and LPM are 28.7×10x5 and 28.9×10x5 A mx1,respectively;ARM/HIRMs are 0.35 and 0.33 for LP and LPM,respectively.These results demonstrate that parameters,χFD/HIRM,ARM/HIRM,Ex and ESIRM,are affected less by parent material and topography,which are better than the x for bulk samples to indicate the paleoclimatic conditions(e.g.,the paleorainfall) in a large-scale region.  相似文献   

4.
Intense rainfall following wildfire can cause substantial soil and sediment redistribution. With concern for the increasing magnitude and frequency of wildfire events, research needs to focus on hydrogeomorphological impacts of fire, particularly downstream fluxes of sediment and nutrients. Here, we investigate variation in magnetic enhancement of soil by fire in burnt eucalypt forest slopes to explore its potential as a post‐fire sediment tracer. Low‐frequency magnetic susceptibility values (χlf) of <10 µm material sourced from burnt slopes (c. 8·0–10·4 × 10?6 m3 kg?1) are an order of magnitude greater than those of <10 µm material derived from long‐unburnt areas (0·8 × 10?6 m3 kg?1). Susceptibility of anhysteretic remanent magnetization (χARM) and saturation isothermal remanent magnetization (SIRM) values are similarly enhanced. Signatures are strongly influenced by soil and sediment particle size and storage of previously burnt material in footslope areas. Whilst observations indicate that signatures based on magnetic enhancement show promise for post‐fire sediment tracing, problems arise with the lack of dimensionality in such data. Magnetic grain size indicators χfd%, χARM/SIRM and χfd/χARM offer further discrimination of source material but cannot be included in numerical unmixing models owing to non‐linear additivity. This leads to complications in quantitatively ascribing downstream sediment to source areas of contrasting burn severity since sources represent numerical multiples of each other, indicating the need to involve additional indicators, such as geochemical evidence, to allow a more robust discrimination. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

5.
In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/ titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S-200 values (S-200 = IRM-200/ SIRM, where IRM-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.  相似文献   

6.
Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock δ18O data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic (18O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities (χ > 1 × 10?4 G/Oe), and S-type granites by low susceptibilities (χ < 1 × 10?4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.  相似文献   

7.
We test the possibility of using the pseudo-Thellier method as a means of determining absolute paleointensity. Thellier analysis of anhysteretic remanent magnetization (ARM) and pseudo-Thellier analysis of thermoremanent magnetization (TRM) have been carried out on a large collection of sized synthetic magnetites and natural rocks. In all samples, the intensity of TRM is larger than that of ARM and the ratio R (=TRM/ARM) is strongly grain size dependent. The best-fit slope (bTA) from pseudo-Thellier analysis of TRM shows a linear correlation with R. The ratio bTA/R yielded approximately correct paleointensities, although uncertainties are larger than in typical Thellier-type determinations. For single-domain and multidomain magnetites, alternating field and thermal stabilities of ARM and TRM are fairly similar. However, for ∼0.24 μm magnetite, ARM is both much less intense and less resistant to thermal demagnetization than TRM, reflecting different domain states for the two remanences and resulting in severely non-linear Arai plots for Thellier analysis of ARM.  相似文献   

8.
Magnetic signature of different vegetation species in polluted environment   总被引：2，自引：0，他引：2  
Detailed magnetic study on vegetation samples from several strongly polluted and clean sites in Bulgaria is carried out in order to evaluate suitability of different species as passive dust collectors in magnetometry. From each location, available species among lichens, mosses, poplar leaves, dandelion, needles have been sampled. Magnetic susceptibility calculated on mass-specific basis shows wide variability between diamagnetic signal up to 846 × 10−8 m3/kg. Lichens and mosses are found to be the species, showing magnetic signals with the strongest contrast between clean and polluted environment. The main magnetic phase is magnetite-like according to the results from thermomagnetic analysis of susceptibility on magnetic extracts. Scanning electron microscopy (SEM) microphotographs reveal the presence of abundant particulate matter on vegetation surface both with anthropogenic (spherules) and lithogenic origin. Magnetic grain size deduced by the ratio of saturation remanent magnetization (SIRM) and mass-specific magnetic susceptibility (χ) and coercivities (Bc and Bcr) suggest that different species accumulate preferentially small SD-like grains from pollution emissions. Contrasting relationship of the ratio of anhysteretic remanent magnetization (ARM) and χ for polluted vs clean sites deduced by needles and lichens may be related to transformation of the accumulated dust particles within lichens’ tissue. This finding indicates that the exact species used as biological dust collector is of importance when studying spatial grain size distribution of magnetic dust particles. Pilot study on polycyclic aromatic hydrocarbons (PAH) content and its relation to magnetic parameters shows good correspondence between high levels of PAHs and high SIRM values for locations affected by non-ferrous industrial production.  相似文献   

9.
Absolute18O content of standard mean ocean water   总被引：1，自引：0，他引：1  
The absolute values of the18O/16O ratio (Rs) and the relative18O content (Xs) in SMOW have been determined by comparing SMOW mass spectrometrically with well-defined synthetic mixtures of pure D218O and H216O. The results are:RS = (2005.20 ± 0.45) × 10?6, XS = (2000.45 ± 0.45) × 10?6  相似文献   

10.
This paper focuses on the study of the correlation between magnetic parameters with the level of contamination by heavy metals in urban soils. We report a magnetic investigation of urban soil samples from Merida, state of Yucatan, Southern Mexico. It appears that most of our samples contain ferrimagnetic minerals as the magnetic carriers, probably coming from the titanomagnetites/titanomaghemites series. This is inferred by the acquisition of isothermal remanent magnetization, which shows that most of samples are almost completely saturated at about 200 mT. The S−200 value (factor characterizing stability of remanent magnetization) is between 0.8 and 1.0, characteristic of ferrimagnetic minerals. The susceptibility vs. temperature measurements also point to some titanomaghemites and titanomagnetites as probable responsible for magnetization. On the other hand, measurements of magnetic susceptibility at high and low frequencies helped us to determine the high content of superparamagnetic grains in the majority of the samples, although not all of these showed high values of magnetic susceptibility. We found that the most contaminated samples by Pb and Zn showed the higher saturation isothermal remanent magnetization values, whereas the higher values in magnetic susceptibility correspond to samples contaminated by Cr. Finally, we noted that a high level in Sr decreases the magnetic susceptibility.  相似文献   

11.
Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ～1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ～375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.  相似文献   

12.
Saturation remanent hysteresis studies were carried out on numerous dolerite dyke samples from the peninsular India. These studies result in four types of remanent hysteresis curves which indicate that the magnetic material is magnetite or titanomagnetite with variable grain-size having remanent coercive forces (H cr ) of 8 to 30 mT, requiring saturating fields (H s ) up to 250 mT. Two extreme types of samples with (1) low coercive forces requiring high saturating fields and (2) high coercive forces requiring low saturating fields are noticed along with the generally observed ones. The Granulometric and Lowrie-Fuller Tests on these samples indicated that the magnetic material i.e., magnetite or titanomagnetite in these rocks is in the form of Multi-Domain (MD), Cation Deficient (CD) and a mixture of these two forms (MD+CD) within.  相似文献   

13.
Gega lake, in southeastern Tibet, was formed by the blocking of Yarlung Tsangpo gorge by a glacier and is a well-known example of geomorphological damming. However, the evolution of the damming process at the site is still not understood in detail. Here, we use measurements of multiple magnetic parameters of the sediments from the Yusong (YS) 3 section, which is well-dated by optically stimulated luminescence, to provide a detailed history of the late stage of Gega dammed lake since 17.0 ka. Low-frequency field magnetic susceptibility (χlf) increases upwards gradually from 25 to 79?×?10?8 m3/kg above 5.5 m, but other magnetic properties, such as frequency-dependent magnetic susceptibility (χfd % and χfd), susceptibility of anhysteretic remanent magnetization (χARM), and saturation isothermal remanent magnetization (SIRM) did not show a similar degree of enhancement. The magnetic grain size indicators of χARM/χlf, χARM/SIRM, and χlf/SIRM all indicate a trend of increasing magnetic grain size from the 5.5 m to the top of section; however, the bulk sediment grain size decreases gradually within the same interval. The total organic carbon is very low (0.2–0.7%), and thus, it is unlikely that the sedimentary environment is sufficiently strongly reducing to lead to the dissolution of magnetic minerals. Therefore, we infer that the coarsening-upwards of the magnetic grain size, and the increasing magnetic susceptibility from 5.5 m, reflect the gradual preferential preservation of magnetic minerals, caused by the deepening of the lake. Thus, we interpret the magnetic record of the section as reflecting the gradual deepening of the late stage of Gega ice-dammed lake. Last, the abrupt disappearance of the lake may have resulted in an outburst flood.  相似文献   

14.
The Tuoku region in northern Tarim Basin of China is a key area for studying oil/gas reservoir rocks. The magnetic and mineralogical parameters of well cuttings from two wells, well S7, situated on oil/gas field, and well S6, at an oil/water interface, were measured. The two wells are located in the same structure with similar strata and types of lithology, but well S6 is a showing well of oil and gas 5 km northwest of well S7. The purpose of this paper is to evaluate the possibility and distribution of secondary magnetic alteration that may have occurred due to hydrocarbon migration above an oil/gas accumulation. It is concluded that the magnetism of well cuttings from major strata in well S7, including source rocks, oil reservoir rocks and cap rocks, and in Quaternary (Q) soil is higher than that from well S6. The Cambrian oil-bearing strata and cap rocks have even higher magnetism in well S7. The shape and parameters of magnetic hysteresis loops indicate that soft (H c<20 mT,H s<0.3 T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S7, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S6 (for example, low coecivityH c and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iron oxide (magnetite, maghemite and hematite) in well S7 are often higher than those in well S6. The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S7, and in strata Ekm and N1j are 1.215% and1.498%, respectively. Typical spherical magnetite grains are found within the main source rocks and the soils in well S7. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is composed of aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background. Project supported by the National Natural Science Foundation of China and the Geological Industry Foundation of China (Grant No. 49374216) and Foundation of the State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation of China (Grant No. 9608).  相似文献   

15.
A phenomenological model based on a linear relationship between the magnetic coercivity field and the reciprocal of the grain diameter is applied to explain the anhysteretic remanent magnetization (ARM) imparted to artificial samples with different concentrations of a very well characterized magnetite powder. By analyses of scanning electron microscopy images, the spherically shaped single domain synthetic magnetite is found to follow a lognormal grain size distribution with ~86 nm of mean diameter. The proposed model, fitted to ARM measurements up to a peak alternating field of 100 mT, yields a very good agreement. The coercivity behaviour predicted by micromagnetism theory disagrees with the experimental results of this work. A likely explanation for the discrepancy is that the magnetite particles, which consist of a mixture of grains in coherent rotation and curling modes, produce similar observations as domain processes.  相似文献   

16.
Thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling. Rocks that cool slowly commonly contain magnetic grains of millimeter scale. This study investigated 1-mm-sized magnetic minerals of iron, iron–nickel, magnetite, and hematite and concluded that the thermoremanent magnetization (TRM) acquired by these grains did not accurately record the ambient magnetic fields less than 1 μT. Instead, the TRM of these grains fluctuated around a constant value. Consequently, the magnetic grain ability to record the ambient field accurately is reduced. Above the critical field, TRM acquisition is governed by an empirical law and is proportional to saturation magnetization (Ms). The efficiency of TRM is inversely proportional to the mineral's saturation magnetization Ms and is related to the number of domains in the magnetic grains. The absolute field for which we have an onset of TRM sensitivity is inversely proportional to the size of the magnetic grain. These results have implications for previous reports of random directions in meteorites during alternating field demagnetization, or thermal demagnetization of TRM. Extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude. Extraterrestrial rocks commonly contain large iron-based magnetic minerals as a common part of their composition, and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates.  相似文献   

17.
Low-temperature oxidation under atmospheric conditions affects the magnetic properties of magnetite in natural rocks: the coercivities of magnetite grains increase and other parameters change accordingly. It was recently shown that heating to 150°C largely removes the effects of low-temperature oxidation (van Velzen and Zijderveld, 1995). Heating may therefore serve as a detection tool for the presence of the effect of low-temperature oxidation. In the present study, a collection of loess and paleosol samples from various loess regions of the world is examined for the influence of low-temperature oxidation. In all samples of the collection a decrease of coercivities was found after heating to 150°C. Generally loess samples were affected to a larger extent than paleosol samples. The original range of remanent coercivities(B cr)of 21-58 mT changed to 20-42 mT after heating. The IRM capacity of the samples decreased from 0 up to 25%. ARM showed changes between a decrease of 10% and an increase of 15%. The grain-size indicative parameter IRM/ARM is considerably influenced by the heating and therefore by low-temperature oxidation. The changes in susceptibility are limited and will not influence the interpretation of large-scale features of the susceptibility record as a paleoclimate proxy. Small variations, however, may be obscured by the varying influence of oxidation in the outcrop, which can significantly modify the rock-magnetic record. Rock-magnetic parameters used to determine magnetic mineral content and grain sizes should be corrected for the effect of low-temperature oxidation. To this end heating to 150°C is recommended. The occurrence of the changes is in itself already an indication for the presence of magnetite. Low-temperature oxidation will not only be due to recent weathering in the outcrop, but also to earlier oxidation processes in the source area, during transport and deposition of the loess and during pedogenesis. Truly fresh sediment samples are only influenced by this earlier oxidation. In that case heating will reveal the degree of ancient low-temperature oxidation, which may be related to climate at the time of deposition and pedogenesis.  相似文献   

18.
Results of a detailed petromagnetic study of sediments of the Koshak section, including the Mesozoic/Cenozoic (K/T) boundary, are presented. The rocks are shown to have a very low magnetization. A relatively high magnetization is characteristic of two thin clayey beds, one at the K/T boundary and the other 0.6 m above it: x up to 2.5 × 10?9 m3/kg, M s up to 0.6 × 10?3 A m2/kg, and M rs up to 0.4 × 10?3 A m2/kg. This is related to relatively high concentrations of hemoilmenite (up to 0.2%), magnetite (up to 0.01%), and goethite (up to 0.24%) in these beds. It is evident that the distribution of these magnetic minerals is lithologically controlled (the predominant occurrence in clayey beds), which is expressed, in particular, in the relation between the paramagnetic (clayey) and diamagnetic (carbonate) contributions to the magnetization of the sediments. Thus, clayey interbeds are sharply distinguished by the value of the paramagnetic magnetization (M p = (83–86) × 10?5 A m2/kg) as compared with purely diamagnetic chalk (M d = ?(26–35) × 10?5 A m2/kg). Minor concentrations of metallic iron (up to ～0.002%) discovered in the sediments have a lithologically uncontrolled distribution (metallic iron is more often observed near the K/T boundary rather than in clayey beds). Most probably, magnetite, hemoilmenite, and goethite were accumulated mostly with clay and other terrigenous material, while fine particles of iron are likely to have been dispersed by air. The whole set of the data of this work suggests that the K/T boundary is not distinguished by characteristic magnetomineralogical and magnetolithologic features.  相似文献   

19.
The low-temperature (K1) transition has been measured in prepared samples with a low concentration of magnetite to test the validity of the technique for identifying magnetite in weakly magnetized rocks. Using an astatic magnetometer, magnetite concentrations as low as 1 part in 100,000 can be satisfactorily detected.Measurements on natural samples show the presence of magnetite in a variety of limestones which are known to have a stable natural remanent magnetization.  相似文献   

20.
Summary Rock-magnetic characteristics of late Pleistocene loess-paleosol sequences in the Czech Republic show patterns of variation that reflect climate-related depositional and diagenetic processes which acted on the sedimentary profiles. Mass-normalized magnetic susceptibility is high in interglacial and interstadial paleosols, while uniformly low values are measured in unweathered loess horizons. Normalized ferrimagnetic susceptibility and anhysteretic remanent magnetization show an enhancement of ultrafine (superparamagnetic, SP) and fine (single-domain, SD, and pseudo-single-domain, PSD) grains in chernozem paleosols correlated with δ18O substages 5c and 5a as well as in the Holocene soil. The parabraunerde paleosol associated with peak interglacial conditions, correlated with δ18O substage 5e, shows evidence of diagenetic loss of fine grained magnetic minerals, although coarse (multidomain, MD) grains appear to be preserved. Low temperature remanence behavior plus high temperature susceptibility measurements of representative samples from each lithologic unit indicate that magnetite and maghemite are the dominant magnetic minerals within the sediments. Variations in concentration-independent rock-magnetic parameters are therefore primarily a function of grain size variations through the profile. It is anticipated that with additional magnetic and non-magnetic sedimentological and geochemical tests, a quantitative rock-magnetic — paleoclimate model can be developed for the central European loess region.  相似文献   

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