Remanent Magnetization Acquisition Research Articles

Biomagnetic Monitoring of Urban Pollution: The Case of Aburrá Valley, Colombia

This study aims to identify the most polluted areas and sites using the magnetic signal of ornamental plant leaves as an indicator of environmental pollution. Systematic sampling was conducted with 98 sampling sites described according to urban land use, such as road hierarchy and road surface, soil group, collected plant species, and municipality. The magnetic parameters analyzed were low- and high-frequency magnetic susceptibility and the isothermal remanent magnetization acquisition curves in order to calculate the magnetic enhancement factor. For the analysis of variance, a Kruskal–Wallis test was performed to compare urban land uses. Subsequently, the magnetic enhancement factor in dust and surface soil was used to prepare maps of environmental pollution for each urban area. Analyses of the different magnetic parameters of the dust deposited on leaves show that low-coercivity ferrimagnetic minerals dominated the magnetic signal, probably magnetite of anthropic origin, and were closely linked to vehicular traffic and, to a lesser extent, industrial activities.

Formation and preservation mechanisms of magnetofossils in the surface sediments of muddy areas in the yellow and Bohai Seas, China

Magnetofossils in the continental shelf sediments of the Yellow and Bohai Seas have long been overlooked. Based on the magnetic results of 88 surface sediments (0–10 cm depth), first-order reversal curve (FORC) diagrams, isothermal remanent magnetization (IRM) acquisition curves from 6 representative samples, and transmission electron microscopy (TEM) observations of 2 samples, the formation and preservation mechanisms of magnetofossils in this region are elucidated. The FORC diagrams consistently show a clear central ridge feature, which indicates the presence of intact magnetofossils in all representative samples. The morphologies observed by TEM are primarily equant and elongated, with minimal or no bullet-shaped (magnetite) magnetofossils. Analysis further reveals a widespread distribution of magnetofossils in the mud areas of the Bohai Sea, North Yellow Sea, and South Yellow Sea, with proportions (contribution to SIRM; SIRM is defined as the remanent magnetization that remains constant as the external magnetic field increases) of <32.5 %, 40.9 % ∼ 44.6 %, and 59.9 % ∼ 66.5 %, respectively. Despite the presence of non-biogenic single domain magnetite, the proportion of magnetofossils can be estimated by the χARM/SIRM value, as they are positively correlated. The surface sedimentary environment of these mud areas is primarily suboxic and characterized by abundant dissolved iron, which facilitate the formation of magnetofossils by magnetotactic bacteria (MTB). It is unlikely that the surface sedimentary environment becomes sulphidic, thereby enabling the preservation of magnetofossils after their formation. The redox state of the study area, crucial for magnetofossil formation, is mainly controlled by the total organic carbon (TOC) content. From north to south, the higher proportion of magnetofossils is coupled with higher TOC content, possibly due to the intensified reducing degree of the suboxic environment, promoting MTB proliferation and thus forming more magnetofossils. The mechanisms governing the formation and preservation of magnetofossils proposed in this study may also be applicable to geological records.

A Geochemical Mechanism for &gt;10 m Apparent Downward Offsets of Magnetic Reversals Inferred From Comparison of Two Scotia Sea Drill Sites

AbstractWe document an apparent downward displacement of the Matuyama‐Brunhes magnetic reversal by ∼20 m at Scotia Sea International Ocean Discovery Program Site U1538 (Pirie Basin) by comparison with the well‐defined paleomagnetic record at nearby Site U1537 (Dove Basin). Detailed stratigraphic correlation between the two sites is possible due to similar lithologic variations. However, the two sites have distinctly different porewater geochemistry. Notably, Site U1538 indicates a greater demand for electron acceptors to oxidize organic carbon and Fe2+ enrichment below the depth of SO42− depletion. Magnetic parameters indicate enrichment of an authigenic magnetic mineral with strong remanence properties around the depth of SO42− depletion (∼46 m at Site U1538) relative to magnetic parameters at correlative depths at Site U1537. Fe2+ enrichment below the depth of SO42− depletion is not predicted based on the energetically favorable order of electron acceptors for microbial respiration but is documented here and in other depositional settings. This indicates Fe2+ production exceeds the production of H2S by SO42− reduction, providing a geochemical environment that favors the production and preservation of ferrimagnetic remanence‐bearing iron sulfides over paramagnetic pyrite and, thus, a mechanism for deep chemical remanent magnetization acquisition at depths of tens of meters. The influence of authigenic ferrimagnetic iron sulfides on paleomagnetic signals can be difficult to demonstrate with magnetic properties alone; therefore, this finding has implications for evaluating the fidelity of magnetostratigraphic records with complementary geochemical data. Such situations should be considered in other depositional environments with similar porewater Fe2+ accumulation below the SO42− reduction depth.

Assessing lunar paleointensity variability during the 3.9 - 3.5 Ga high field epoch

Numerous paleomagnetic studies suggest that a lunar dynamo, with surface field intensities potentially as high as 40-100 µT, existed between ∼3.9 Ga and ∼3.5 Ga. This period is referred to as the High Field Epoch (HFE). However, the debate over the origin of magnetization recorded in lunar rocks still persists. In addition, whether the Moon could have sustained a continuously strong dynamo during the HFE remains unclear. To unravel the origin of magnetization preserved in lunar rocks and to better characterize the evolution of the ancient lunar dynamo, we conducted a comprehensive set of experiments including rock magnetic tests, electron microscopy, and paleomagnetic investigations on four HFE-aged Apollo 11 mare basalt samples: 10003, 10044, 10069, and 10071. Rock magnetic experiments and electron microscopy indicate that the remanence carriers are kamacite grains of varying sizes and domain states. Sample 10003 recorded a paleointensity of 54.10 ± 4.66 µT. Sample 10044, which was shocked (peak pressure >5 GPa), did not preserve a stable high coercivity remanent magnetization. Samples 10069 and 10071 recorded paleointensities of 61.46 ± 26.09 µT and 10.69 ± 2.87 µT, respectively. A series of hydrostatic pressure experiments, isothermal remanent magnetization (IRM) acquisition experiments, and viscous remanent magnetization (VRM) tests preclude the possibility of our samples containing shock remanent magnetization from transient impact-generated fields, IRM acquisition from exposure to spacecraft fields, and VRM acquisition from exposure to the Earth magnetic field. Overall, our study suggests that the source of these magnetizations was likely the lunar dynamo and may indicate nearly order-of-magnitude magnetic field fluctuations during the HFE.

Acquisition of natural remanence in the basaltic laterites of Deccan volcanic province (India): Implications to palaeomagnetic studies in laterites

Lateritization in the Deccan basalt province (India) represents a large spectrum of chemical weathering with up to 99% CIA values and complete ferri- to antiferromagnetic transformations. We combined rock magnetic, palaeomagnetic, XRD, and XRF analyses to investigate the mineralogical transformations and its relationship to the acquisition of natural remanent magnetization (NRM) by sampling a 25 m thick Deccan lateritic profile from the West Coast of India. Significant amount of amorphous hematite (ha), crystalline hematite (hc), goethite and their Al- substituted forms were detected along with gibbsite and kaolinite. The domain size association of ‘SP + SD and ‘SP + SD + MD’ in the ferri- as well as antiferromagnetic forms; and the combination of ha and hc yield strong NRMs amongst these laterites. Low field demagnetization (<5mT) allows removal of haNRM; and the secondary components due to ferrimagnetic oxides can be removed at < 25 mT. Thereafter, the hcNRM continued to decay up to > 150 mT. Palaeomagnetic tests (Koenigsberger ratio, MDF, intensity decay and Zijderveld diagrams) indicate stable Characteristic remanent magnetizations (ChRM) due to hc; and the Al-substitutions does not affect the ChRM stability, depicting these laterites as excellent palaeomagnetic recorders. The stable ChRMs due to Chemical/Crystallization RMs due to ‘hc’ also benchmarks the degree of maturity within the process of lateritization.

Palaeosecular variation in Northern Patagonia recorded by 0–5 Ma Caviahue–Copahue lava flows

SUMMARY Palaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (&amp;lt;3 per cent) superimposed on the GAD term, as reported by South American studies at mid southern latitudes.

Rockmagnetic Characterization of the Quaternary Fluviolacustrine Sediments of the Thimi Section, Kathmandu Valley, Nepal

The fine-grained silt/clay sediments sampled at 38 levels of a 21 m thick segment at the Thimi outcrop in the Kathmandu valley have been studied for their rockmagnetic properties. These sediments have yielded consistently normal magnetic polarity and 14C ages determined by various workers between 20 and 45 kyrs or even more. They were measured for a set of rock magnetic parameters: low-field magnetic susceptibility, parameters derived from isothermal remanent magnetization (IRM) acquisition and demagnetisation (IRM0.3T, IRM3T, MDFIRM), anhysteretic remanent magnetization (ARM) (ARMtot, ARM(30 mT)/ARMtot, χARM), hysteresis parameters, etc. Several relationships between these parameters have been suggested for the Kathmandu valley sediments. Four rockmagnetic zones, based on the minor variations in magnetic mineral types and grain size parameters, are proposed and their possible relationship to the published pollen data has been discussed.

Impact of turbulence on magnetic alignment in sediments

Rapidly deposited layers (RDL) such as turbidites or hyperpycnites are mostly studied for their sedimentological properties, but are carefully avoided in paleomagnetic studies due to the disturbances caused by such sudden and rapid sediment accumulation. Therefore, these layers can also be seen as potential indicators of sediment parameters susceptible of affecting the alignment of magnetic grains and ultimately the acquisition of the natural remanent magnetization (NRM). We have compiled 13 Holocene rapidly deposited layers from core MD99-2222 in the Saguenay Fjord, eastern Canada (St-Onge and al., 2004) with varying thicknesses (from 7.1 cm to 1,510 cm) and 4 Quaternary turbidites of different origins, to document the influence of sedimentary and magnetic parameters on natural remanent magnetization acquisition. We found a logarithmic relationship between rapidly deposited layers thickness on the one hand, and the amplitude of inclination changes and magnetic grain sizes on the other. Inclination and magnetic grain sizes are themselves correlated to each other by a logarithmic law. As there is no relationship between inclination deviation and stratigraphic depth, compaction alone cannot account for such large effects on inclination. Flocculation is grain size sensitive, but it is expected to affect mainly the natural remanent magnetization intensity, rather than its direction. Turbulence that prevails during the rapid deposition of sediments during such events is most likely the dominant factor.

Variably rotated lamproite dikes within a transpressive fault zone: Structural and paleomagnetic analysis of the Socovos Fault zone (Eastern Betics, SE Spain)

Rotations of large-scale crustal blocks deduced from paleomagnetism are usually invoked to support evolutionary tectonic models in orogens. However, the regional representativeness of local paleomagnetic rotations strongly depends on the structural context and can lead to conflicting results. This work analyzes an unusual cluster of almost coeval lamproite dikes (9.3–7.1 Ma) intruded along a 25 km-long segment of the Socovos transpressive fault zone in the Eastern Betics, with a view to determine rotations at the metre-scale. This fault zone has contrasting deformation styles, depending on the rheology of the rocks, which vary from ductile evaporites (gypsum) to brittle carbonates. The volcanic and subvolcanic rocks present a primary acquisition of high quality natural remanent magnetization, which defines a single directional component with very narrow α95. Matching the outcrop scale structures with the apparently dispersed paleomagnetic directions in the fault zone allows us to differentiate four deformational domains with distinctive characteristics: (1) a brittle zone with deformation concentrated in planes that bound blocks with no rotation or rotations below the expected secular variation ( ± 16° in inclination and ± 21° declination); (2) transpressive decametre-scale duplexes or single horses, also limited by brittle faults, where inclined axis rotations below 60° occur as rigid blocks; (3) buckling folds in ductile units associated with local reverse faulting; and (4), a triclinic transpressive ductile shear zone affecting a gypsum layer, where volcanic boudins have a rotation that is the sum of folding parallel to the shear zone walls plus an angular shear of 36°. These paleomagnetic results indicate that fault displacement continues after 7 Ma with the same kinematics as previously. Besides regional findings, this study illustrates the potential usefulness of small-scale paleomagnetic analysis for determining strain partitioning and deformation mode along large and complex transpressive fault zones, as well as the necessity to know the local structure to attribute regional meaning to the paleomagnetic rotation data.

Acquisition and Preservation of Remanent Magnetization in Carbonaceous Asteroids

Acquisition and Preservation of Remanent Magnetization in Carbonaceous Asteroids

Occurrence and Distribution Patterns of Magnetic Particles Within Stalagmite Growth Laminae

AbstractThe concentrations of magnetic particles in stalagmites depend greatly on the regional hydrologic condition, and thus can act as an important proxy for paleoclimate reconstruction. The central axis region of a stalagmite is usually selected for such magnetism study. However, little is known about the distribution and influence factors of exogenetic magnetic particles within the growth laminae. In this study, the occurrence and variations of magnetic particles along the growth laminae of three shape stalagmites were investigated by a set of magnetic measurements, including isothermal remanent magnetization (IRM) acquisition curves and low‐temperature magnetic analysis. The results show the main magnetic remanence carrier is detrital magnetite/maghemite, coexisting with small amounts of hematite and goethite. The distribution of soft magnetic particles along the growth laminae showed different patterns among the three shape stalagmites (dome, cone, and candle). For the dome shape stalagmite, the soft magnetic particle concentrations in all studied growth layers showed an increasing trend from center to edge. Similar patterns were also appeared in the bottom growth layers of the cone and candle shape stalagmites. This edge enrichment of magnetic particles is caused by drip water washing effect. However, such enrichment pattern became weaker or even inverse in the top growth layers of the cone and candle shape stalagmites, indicating that the shapes of stalagmites could also affect the surface distribution of soft magnetic minerals. These uneven distributions of magnetic particles imply that the central axis region of a stalagmite may not always be the optimal specimen for environmental magnetism study.

Rock Magnetism of Lapilli and Lava Flows from Cumbre Vieja Volcano, 2021 Eruption (La Palma, Canary Islands): Initial Reports

We present initial rock magnetic results for both lava flows and lapilli produced by the 2021 eruption of the Cumbre Vieja, La Palma (Canary Islands). Samples were taken during the eruption to minimize early alteration and weathering of the rocks and tephra. Standard procedures included progressive alternating field and thermal demagnetization, hysteresis curves, thermomagnetic experiments, progressive acquisition of isothermal remanent magnetization (IRM), and First-Order Reversal Curves (FORCs). Overall, our observations, including low to medium unblocking temperatures, isothermal remanent magnetization to 1 Tesla, and the abundance of wasp-waist hysteresis loops, strongly suggest the presence of Ti-rich titanomagnetites as the main remanence carriers in both lava flows and lapilli, in addition to some hematite as well. Whereas the former has been directly seen (SEM), hematite is elusive with nonmagnetic-based methods. Rock magnetic data, on a Day plot, also reveal that the magnetic grain size tends to be larger in the lava flows than in the lapilli.

Variation of sediment magnetic properties and their paleoenvironmental implications in Yangtze River Delta during 30–50 ka B.P.

Identifying the variability in and driving forces of relative sea-level (RSL) change in the Yangtze River Delta during 30–50 ka B.P. is vital for understanding global climatic and RSL changes. In this study, the environmental magnetism of sediments in the Yangtze River Delta was systematically studied for the period 30–50 ka B.P. based on the optically stimulated luminescence (OSL) dates. Detailed magnetic measurements, including high-temperature κ-T, magnetic hysteresis loop, isothermal remanent magnetization acquisition, and demagnetization curve measurements illustrate that the predominant magnetic contribution of core YZ07 is from magnetite, with a small contribution from hematite. Detrital input may be the primary source of magnetic minerals according to the χ/χfd% and χARM/χ correlations. In addition, the positive relationship between χ and the RSL curve indicates that χ is a valid indicator of RSL change. Additionally, comparing χ, RSL curves, and δ18O dates of the stalagmites demonstrated that climatic change influenced the RSL change, which mainly affected the concentration of magnetite. The results are conducive for understanding the response mechanism of magnetic indicators to relative sea-level change, and provide a scientific basis for encouraging for future work in understanding climate variability.

Demagnetization Energy and Internal Stress in Magnetite From Temperature‐Dependent Hysteresis Measurements

AbstractThe magnetization state of magnetite controls acquisition and stability of remanent magnetization in Earth and planetary rocks. Although commonly interpreted in terms of grain size, also stress, grain shape, and magnetostatic interactions can substantially modify magnetic stability. Here, we show that scaled reversible work (SRW) in the approach‐to‐saturation (ATS) of hysteresis curves is temperature independent if anisotropy is due to demagnetizing energy. Stress anisotropy vanishes with increasing temperature. With a new measurement and evaluation procedure stress‐induced anisotropy is separated from demagnetization effects. We calibrated the new method using theoretical ATS curves for different stress and demagnetization regimes. Experimental results for synthetic magnetite samples underpin the validity of the method and provide insight into the relationship between magnetostatic, magnetocrystalline, and stress energies. The SRW method provides a new tool to study the reliability of paleomagnetic recording mechanisms, and enables quantitative investigation of stresses due to tectonics, meteorite impacts, oxidation, exsolution, or quenching.

The Properties of Thermochemical Remanent Magnetization Acquired by Slow Laboratory Cooling of Titanomagnetite-Bearing Basalt Samples from Different Temperatures and the Results of the Thellier Method

—The experiments have been carried out on the acquisition of thermochemical remanent magnetization (TCRM) in basalt samples containing titanomagnetite (TM) with the Curie temperature Тс ~200°C by their rapid heating to maximum temperatures Т* from 450 to 530°C followed by slow cooling in the laboratory magnetic field Blab. At different stages of the preliminary thermal treatment of the initial samples, a set of magnetomineralogical studies including electron microscopy, X-ray diffraction and thermomagnetic analyzes, and measurements of magnetic hysteresis parameters were performed. It is shown that as early as the very beginning of the cooling process, all samples demonstrate explosive growth of TCRM corresponding to the stage of rapid single-phase oxidation of the initial titanomagnetite fraction of basalt, and that TCRM is acquired by the increase of Тс and volume of single-phase oxidized parts of TM grains as well as by the growth of the volume of Ti-depleted (relative to the initial TM) cells of microstructure of the subsequent oxidative exsolution. The Arai–Nagata diagrams for the samples carrying TCRM have a form of a broken line consisting of two linear segments. The low-temperature interval T < Т* corresponds to a mixture of thermochemical and thermoremanent (TRM) magnetizations and gives a slightly overestimated Blab because of the effect of a low cooling rate during the acquisition of TCRM and TRM. The high-temperature interval corresponds to pure TCRM and the Blab strength determined from this interval is underestimated by 20–27%. It is recommended to reject samples whose Araii–Nagata diagram has two or more linear segments against the background single-component NRM.

Archaeomagnetic evidence of a likely earlier occupation of “El Caracol” lava flow (Zacapu Malpaís, Western Mesoamerica)

A comprehensive archaeomagnetic investigation was carried out on seven ceramic fragments recovered at the "El Caracol" lava flow in the Zacapu Malpaís –Michoacan state, Mexico –. A full set of magnetic-mineralogy experiments (encompassing thermomagnetic curves, hysteresis loops, backfield curves, and isothermal remanent magnetization acquisition measurements) were carried out. The experimental methodology also considers magneto-chemical alteration detection, cooling rate, and remanence anisotropy effects to ensure reliable archaeointensity determinations. Five of the seven ceramic fragments – a pipe (potsherd I), vessel's supports (potsherd II and V), a vessel's wall-fragment (potsherd III), and a pipe's nozzle (potsherd VII) – yielded archaeointensity results within the established acceptance criteria to guarantee quality data. Thermal demagnetization of potsherds I, II, V, and VII shows between two and three magnetization components: one associated with potsherd's fabrication, the second with sample's reheating, and the third one, when present, with a viscous component. For each of the first two components, an intensity value was calculated. Potsherds I, II, III, and VII yielded archaeointensity values between 30 and 40 µT, while potsherd V had a much lower value <20 µT. Although secular variation models do not predict such low intensities, similar values have been reported for the center and south of Mexico. Archaeomagnetic dating was carried out with two regional Paleosecular Variation Curves. The "El Caracol" lava flow is an area with human occupation reported from AD 1200. The results obtained in this work, on the contrary, envision the possibility of an early occupation of the site.

Magnetic Properties of a Holocene Sediment Core from the Yeongsan Estuary, Southwest Korea: Implications for Diagenetic Effects and Availability as Paleoenvironmental Proxies

The sensitivity of magnetic properties, which characterize the mineralogy, concentration, and grain size distribution of magnetic minerals, to environmental processes may provide useful information on paleoenvironmental changes in estuarine environments. Magnetic property studies of estuaries are less common than other environments and, due to the west coast of South Korea having an abundance of estuaries, it provides a good place to study these processes. In this study, we analyzed a variety of magnetic properties based on magnetic susceptibility, hysteresis parameters, progressive acquisition of isothermal remanent magnetization and first-order reversal curve data from a Holocene muddy sediment core recovered from the Yeongsan Estuary on the west coast of South Korea. We examined diagenetic effects on magnetic properties and tested their availability as proxies of paleoenvironmental change. The presence of generally low magnetic susceptibility, ubiquitous greigite-like authigenic magnetic component, and very fine magnetic particle occurrence suggested that the analyzed sediments had undergone considerable early diagenetic alteration. Electron microscopic observations of magnetic minerals support this suggestion. Our results confirm that the use of initial bulk susceptibility as a stand-alone environmental change proxy is not recommended unless it is supported by additional magnetic analyses. We recognized the existence of ferromagnetic-based variabilities related to something besides the adverse diagenetic effects, and have examined possible relationships with sea-level and major climate changes during the Holocene. The most remarkable finding of this study is the two distinct intervals with high values in magnetic coercivity (Bc), coercivity of remanence (Bcr), and ratio of remanent saturation moment to saturation moment (Mrs/Ms) that were well coincident with the respective abrupt decelerations in the rate of sea-level rise occurred at around 8.2 and 7 thousand years ago. It is then inferred that such condition with abrupt drop in sea-level rise rate would be favorable for the abrupt modification of grain size distribution toward more single-domain-like content. We modestly propose consideration of the Bc, Bcr, and Mrs/Ms variability as a potential indicator for the initiation/occurrence of sea-level stillstand/slowstand or highstand during the Holocence, at least at estuarine environments in and around the studied area.

Magnetic signature of sewage polluted river sediments

The present study was designed to characterize the magnetic proxy for sewage-related pollution from river sediments. A suite of mineral magnetic analyses including low-field magnetic susceptibility, isothermal remanent magnetization (IRM) acquisition and demagnetization, back-field IRM treatment, and thermal demagnetization of composite IRMs are carried out for topsoil samples collected from river sediments in Daejeon, South Korea. A 350-m long in situ profile of magnetic susceptibility shows a background mean magnetic susceptibility of 22.4 ± 5.6 × 10−5 SI. The coercivity spectra display dual peaks of lower- and higher-coercivity fractions. The predominance of magnetic signal with maximum unblocking temperatures of 580 °C points to magnetite as the sole magnetic mineral. Hence the lower- and higher-coercivity fractions correspond to coarse-grained and fine-grained magnetite, respectively. Extraordinarily high magnetic susceptibility was observed from BTC02, near to the sewage discharge point. However, its mineral magnetic properties and particle morphologies are not different to those of other soil samples. Then, it is apparent that a high magnetic susceptibility simply reflects the high concentration of magnetites in BTC02. It is likely that a continuous discharge of heavy metal enriched water at the discharging point concentrated significant amount of magnetic particles. It is fortunate that anomalously high magnetic susceptibility (by implication heavy metal concentrations) diminished only several meters away from the discharge point. Perhaps, influences of anthropogenic magnetite were diluted as the high magnetic phases were washed away from the water cycles along the river. In practice, it can be proposed that a periodic replacement of soils near the sewage discharge point is efficient to reduce the heavy metal concentrations.

Integrated Magnetic Analyses for the Discrimination of Urban and Industrial Dusts

Industrial and urban dusts were characterized by investigating their magnetic properties. Topsoil composed of technogenic magnetic particles (TMP) originating from areas affected by three ironworks, street dust mainly composed of traffic-related pollution, and particulate matter (PM) from urban agglomeration in Warsaw, Poland were investigated. Several magnetic methods, namely magnetic susceptibility, thermomagnetic curves, hysteresis loops, decomposition of isothermal remanent magnetization acquisition curves, and first-order reversal curves, were performed to evaluate the magnetic fraction of dust. Magnetite was the main magnetic phase in all types of samples, with a small amount of high-coercive hematite within ironworks and street dust samples. Significant differences were observed in the domain structure (grain size) of industrial and traffic-related magnetic particles. The grain size of TMP obtained from steel production was in the range of 5–20 µm and was predominated by a mixture of single-domain (SD) and multidomain (MD) grains, with the prevalence of SD grains in the topsoil affected by Třinec ironwork. The traffic-related dust contained finer grains with a size of about 0.1 µm, which is characteristic of the pseudo-single-domain (PSD)/SD threshold. Street dusts were composed of a slightly higher proportion of MD grains, while PM also revealed the typical behavior of superparamagnetic particles.

Tracing of traffic-related pollution using magnetic properties of topsoils in Daejeon, Korea

The present study was designed to explore the possibility of roadside pollution screening using magnetic properties of topsoil samples in Daejeon, South Korea. Low-field magnetic susceptibility, frequency dependence of magnetic susceptibility, susceptibility of anhysteretic remanent magnetization, isothermal remanent magnetization (IRM) acquisition and demagnetization, back-field IRM treatment, and thermal demagnetization of composite IRM were determined for roadside topsoil samples. Magnetic susceptibility measured on 238 samples from the upper 5 cm of the topsoils ranged from 8.6 to 82.5 × 10–5 SI with a mean of 28.3 ± 10.8 × 10–5 SI. The proximal zone, 55 m wide area situated on either side of the main street, exhibited an enhancement of magnetic susceptibility. In areas distant from the main street, low magnetic susceptibility (< 50 × 10–5 SI) was observed. The topsoil samples exhibited significant susceptibility contrasts, suggesting that two dimensional magnetic mapping was effective in identifying traffic-related pollution. A few magnetic hotspots with intensities of magnetic susceptibility near or over 50 × 10–5 SI might reflect the difference in topographic elevation and surface morphology. Among various IRM-related parameters, remanence of coercivity was most significant statistically. In most samples, IRM component analysis provided dual coercivity components. Thermal demagnetization of composite IRM and morphological observation of magnetic separates suggest angular magnetite produced by vehicle non-exhaust emissions spherical magnetite derived from exhaust emission to be the dominant contributors to the magnetic signal. It is likely that lower- and higher-coercivity components represent the presence of coarse-grained angular magnetite and fine-grained spherical magnetite, respectively.