Saturation Isothermal Remanent Magnetization Research Articles

Late Miocene tectonic forcing of climate transition in the northeastern Tibetan Plateau

Interactions among late Cenozoic global cooling, tectonic activity and regional climate evolution of the northeastern Tibetan Plateau remain elusive. To further understand the relative contributions of global climate change and tectonism, we conducted a comprehensive study of heavy minerals, rock magnetism, and branched glycerol dialkyl glycerol tetraethers (brGDGTs) in late Cenozoic sediments of the Laojunmiao section (ca. 12.7 Ma to 5 Ma) in the Jiuxi Basin. Our results reveal a notable increase in magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM) at 8.5 Ma, aligning with an accelerated decrease in temperature captured by brGDGTs. Combined with heavy mineral analysis, we propose that the profound changes in magnetic parameters and temperature are closely linked to the intensive uplift of the northern Qilian Shan since 8.5 Ma. Furthermore, this ongoing uplift has occurred simultaneously with persistent aridification in the Jiuxi Basin and its adjacent regions. The Qilian Shan might have reached a threshold elevation and began to act as a moisture barrier to northwestward moisture since 8.5 Ma. By synthesizing hydroclimate evolution records from the northeastern Tibetan Plateau, we deduce that changing topography resulting from tectonic uplift may have exerted a significant influence on regulating the moisture distribution in this region during the mid- to late Miocene.

A Fuzzy-Based Analysis of Air Particle Pollution Data: An Index IMC for Magnetic Biomonitoring

Airborne magnetic particles may be harmful because of their composition, morphology, and association with potentially toxic elements that may be observed through relationships between magnetic parameters and pollution indices, such as the Tomlinson pollution load index (PLI). We present a fuzzy-based analysis of magnetic biomonitoring data from four Latin American cities, which allows us to construct a magnetic index of contamination (IMC). This IMC uses four magnetic parameters, i.e., magnetic susceptibility χ, saturation isothermal remanent magnetization SIRM, coercivity of remanence Hcr, and SIRM/χ, and proposes summarizing the information to assess an area based exclusively on magnetic parameters more easily. The fuzzy inference system membership functions are built from the standardization of the data to become independent of the values. The proposed IMC is calculated using the baseline values for each case study, similar to the PLI. The highest IMC values were obtained in sites close to industrial areas, and in contrast, the lowest ones were observed in residential areas far from avenues or highways. The linear regression model between modeled IMC and PLI data yielded robust correlations of R2 > 0.85. The IMC is proposed as a complementary tool for air particle pollution and is a cost-effective magnetic approach for monitoring areas.

Changes in rock magnetic properties of sediments along the Carlsberg Ridge: Inferences on active/extinct vent fields

Hydrothermal vents at Mid Ocean Ridges (MOR) are known to leave metalliferous, bacterial, temperature-induced signatures in sediments. Knowledge of active/extinct vents along the Carlsberg Ridge (CR) is sparse; further conventional methods to distinguish relevant areas are tedious/expensive. We show that basic rock magnetic measurements, including magnetic susceptibility and laboratory-induced remanent magnetizations, can help to characterize sediment samples near and away from hydrothermally activity/extinct areas at the CR. Eight spade cores from the Carlsberg ridge (of which three are in the vicinity of the Tianxiu hydrothermal field and one from the ridge flank) collected on board R.V. Sindhu Sadhana during 77th and 83rd cruises are investigated for the present study. Two spade cores consist of top 2 cm and six core lengths ranged between 22 and 38 cm. Coarse fraction and magnetic minerals were separated and studied under a binocular microscope and scanning electron microscope (SEM). Magnetic susceptibility (χlf) of sediments ranges between 1.3 and 37.1 × 10−8 m3 kg−1 with higher values for ridge valley and lower values for ridge flank. Ridge flank sediments are associated with low χlf, Saturation Isothermal Remanent Magnetization (SIRM), Hard Isothermal Remanent Magnetization (HIRM) and high χARM/SIRM, which are taken as base values. Sediments in the vicinity of the Tianxiu hydrothermal vent field are associated with high χlf, SIRM, HIRM, and low Anhysteretic remanent magnetic susceptibility (χARM)/SIRM, coarse Stable Single Domain (SSD) and fine SSD & Mixture grains with magnetite as the dominant mineral. Similar signatures are observed in two more spade cores ∼250 km south of the Tianxiu vent field, suggesting new active/relict site/sites in the vicinity. Signatures like that of flank sediments are also observed in two cores from the ridge valley indicating, the absence of hydrothermal activity in the vicinity. In the marine environment, identifying/locating metalliferous hydrothermal sediment/sites along extensive MOR is expensive and time-consuming; rock magnetic could be a non-destructive method to shortlist the areas for detailed studies.

Inverse magnetic fabric of remagnetized limestones in the Zaduo area, Eastern Qiangtang Terrane: Implications for oroclinal bending in the Eastern Himalayan Syntaxis

Magnetic fabric analysis is a common technique to assess the strain regime during mountain building processes. Here, we use this approach to evaluate the tectonic evolution of the Tibetan Plateau and the Eastern Himalayan Syntaxis by analyzing the limestones of the Jurassic Buqu Formation in the Zaduo area, Eastern Qiangtang Terrane (China). These limestones were chemically remagnetized during the Cenozoic. For a proper assessment, it is relevant to understand how the mineralogy of the remagnetized limestones affects their magnetic fabric and how the magnetic fabric can improve our understanding of the tectonic strain and regional deformation. The role of the authigenic magnetite in the development of the magnetic fabric should thus be explored. Comparison of the bulk susceptibility (Km) with various natural and laboratory rock magnetic properties (Km versus natural remanent magnetization, Km versus saturation isothermal remanent magnetization, and Km versus saturation magnetization) indicates that susceptibility and remanences are both carried by authigenic magnetite. Most of the magnetite grains show axial ratios <1.3:1 according to the Néel diagram, and fall within the single-domain range based on the mass magnetic susceptibility (χ) and DC field-normalized anhysteretic remanent magnetization (χARM) ratio, giving rise to the inverse magnetic fabric observed. Twelve sites (120 specimens) are divided into four groups based on the magnetic fabric and rock magnetic behaviors. Overall, there is a clear trend of decreasing Km, natural remanent magnetization, saturation isothermal remanent magnetization, ferromagnetic percentage and shape parameter from Group I to IV. The K1 axis of all four groups documents a NNE-SSW oriented compression during remagnetization, contrasting with the Eocene NE-SW compression in the Gongjue area farther east. This different compressional regime likely resulted in different rotations and structural trends surrounding the Eastern Himalayan Syntaxis.

Magnetic characteristics of atmospheric particulate matter and its indication of atmospheric pollution during winter in Lanzhou, NW China

In order to elucidate the significance of environmental magnetic methods in monitoring atmospheric particulate matter pollution, this study conducted an analysis of the magnetic and chemical properties of PM10 and PM2.5 samples collected during winter in Lanzhou City. The magnetic measurements revealed significant variations in the saturation isothermal remanent magnetization (SIRM) values between daytime and nighttime, with higher SIRM values during nighttime. The magnetic properties were dominated by low-coercivity ferrimagnetic minerals exhibiting multidomain and pseudo-single domain characteristics. SIRM exhibited a significant correlation with the mass concentration of particulate matter. The correlation indicated that during the day, the SIRM of PM2.5 showed a significant correlation with elements such as Zn, Pb, Cl−, K+, K, and organic carbon (OC). Additionally, the SIRM of PM10 was also related to Mg2+, Ca2+, and some crustal elements. During nighttime, the SIRM of PM10 and PM2.5 was primarily associated with Cl−, K+, K, and OC. By comparing the magnetic and chemical properties of particulate matter under clean and non-clean air conditions, similar trends were observed between SIRM and indicators such as NO3−/SO42−, As/Pb, K+/K, and Ca2+, which are commonly associated with traffic emissions, biomass burning, and construction dust sources. Factor analysis was applied to further investigate the sources of magnetic particles, unequivocally demonstrating that traffic emissions constituted the primary source of magnetic particles in both PM10 and PM2.5 during daytime, whereas biomass burning emerged as the predominant source during nighttime. Consequently, our study highlights the innovative use of magnetic parameters as reliable indicators for assessing atmospheric particulate pollution and discerning the sources of particulate matter.

The influence of plant species, leaf morphology, height and season on PM capture efficiency in living wall systems

Green infrastructure (GI) is already known to be a suitable way to enhance air quality in urban environments. Living wall systems (LWS) can be implemented in locations where other forms of GI, such as trees or hedges, are not suitable. However, much debate remains about the variables that influence their particulate matter (PM) accumulation efficiency. This study attempts to clarify which plant species are relatively the most efficient in capturing PM and which traits are decisive when it comes to the implementation of a LWS. We investigated 11 plant species commonly used on living walls, located close to train tracks and roads. PM accumulation on leaves was quantified by magnetic analysis (Saturation Isothermal Remanent Magnetization (SIRM)). Several leaf morphological variables that could potentially influence PM capture were assessed, as well as the Wall Leaf Area Index. A wide range in SIRM values (2.74–417 μA) was found between all species. Differences in SIRM could be attributed to one of the morphological parameters, namely SLA (specific leaf area). This suggest that by just assessing SLA, one can estimate the PM capture efficiency of a plant species, which is extremely interesting for urban greeners. Regarding temporal variation, some species accumulated PM over the growing season, while others actually decreased in PM levels. This decrease can be attributed to rapid leaf expansion and variations in meteorology. Correct assessment of leaf age is important here; we suggest individual labeling of leaves for further studies. Highest SIRM values were found close to ground level. This suggests that, when traffic is the main pollution source, it is most effective when LWS are applied at ground level. We conclude that LWS can act as local sinks for PM, provided that species are selected correctly and systems are applied according to the state of the art.

Magnetite in Muong Nong‐Type Australasian Tektites From South China

AbstractSouth China belongs to the northern geographic branch of the Australasian strewn field (AASF) of tektites and microtektites, and this area is assumed to be part of the uprange region of the putative impactor trajectory that formed the yet undiscovered source crater. Ferromagnetic minerals in impact glass may record the magnetization process and thermal history of impact melt, but the possible identity of ferromagnetic minerals in AASF tektites from South China is elusive. Here, we perform a rock magnetism and crystallography study of iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant single‐domain and pseudo single‐domain nano‐grains of magnetite within the spherules. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Compared to the large value ranges of magnetic properties of tektites in the entire strewn field, individual specimens of both Muong Nong‐type and single splash‐form tektites have relatively homogeneous magnetic properties in terms of magnetic susceptibility, natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM) and NRM/SIRM ratio. The results indicate that the impact melt that formed each tektite specimen experienced similar thermal history, and most AASF tektites from South China were cooled in an ambient magnetic field that is the same order of magnitude as the Earth's magnetic field.

Tracing titanomagnetite alteration with magnetic measurements at cryogenic temperatures

SUMMARY Titanomagnetite containing up to 0.6–0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3−xTixO4 (x ≈ 0.46) at temperatures of 355, 500 and 550 °C for up to 375 hr, by examining their magnetic properties over a wide range of temperatures. The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30 per cent for the samples annealed for 375 hr at 355 °C, and for 110 hr at 500 and 550 °C. The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205 °C) by up to 350 °C. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, that is, not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties.

Magnetoclimatological Record of Late Pleistocene Loess in the Southern Hunshandake Sandy Land, Inner Mongolia: A Threshold Response to the East Asian Summer Monsoon Variations

AbstractWe report the first complete loess–paleosol record spanning the last 130 kyr from the southern extremity of the Hunshandake Sandy Land (HSL) in central‐eastern Mongolia. Our combined mineral magnetic and geochemical results demonstrate that during the last interglacial, the front of the East Asian summer monsoon (EASM) extended to central‐eastern Inner Mongolia, consistent with modern observations of climate change. However, during the last glacial, typical magnetic parameters (e.g., magnetic susceptibility and anhysteretic remanent magnetization and their ratios), which have been successfully adopted to denote the EASM variations on the Chinese Loess Plateau (CLP), show only muted temporal variations and cannot be readily correlated with equivalent records from the main body of the CLP. Remarkably, the Zr/Rb ratio, a robust geochemical indictor of the winter monsoon, is positively correlated with saturation isothermal remanent magnetization and saturation magnetization during the last glacial, demonstrating the strong control of wind vigor on high‐field magnetization signals dominated by lithogenic ferrimagnetic minerals. In contrast to the variable response of magnetic parameters to regional paleoclimatic fluctuations, the Rb/Sr and Ba/Sr ratios (two summer monsoon indicators) faithfully track weak chemical weathering processes and fine‐scale monsoon oscillations, especially during the last glacial. Additionally, we found that higher proportions of pedogenic high‐coercivity magnetic minerals were formed during the last interglacial, which may reflect relatively low rainfall but higher evapotranspiration in the southern extremity of the HSL compared with the CLP. We therefore attribute this complex magnetic record to a threshold response to the EASM variations in eastern Inner Mongolia.

Non-destructive techniques for the determination of magnetic particle and element contents in grapevine leaves and soil as an eco-sustainable tool for environmental pollution assessment in the agricultural areas.

The concentration of magnetic particulate matter (PM) on the leaf surface (an indicator of current pollution) and topsoil (an indicator of magnetic PMs which have geogenic natural signal or historical pollution origin) was assessed in agricultural areas (conventional and organic vineyards). The main aim of this study was to explore whether magnetic parameters such as saturation isothermal remanent magnetization (SIRM) and mass-specific magnetic susceptibility (χ) can be a proxy for magnetic particulate matter (PM) pollution and associated potentially toxic elements (PTEs) in agricultural areas. Besides, wavelength dispersive X-ray fluorescence spectroscopy (WD-XRF) was investigated as a screening method for total PTE content in soil and leaf samples. Both magnetic parameters (SIRM and χ) pinpoint soil pollution, while SIRM was more suitable for evaluating magnetic PM accumulated on leaves. The values of both magnetic parameters were significantly (p < 0.01) correlated within the same type of sample (soil-soil or leaf-leaf), but not between different matrixes (soil-leaf). Differences between magnetic particles' grain sizes among vegetation seasons in vineyards were obtained by observing the SIRM/χ ratio. WD-XRF was revealed to be an appropriate screening method for soil and leaf total element contents in agricultural ambient. For a more precise application of WD-XRF leaf measurements, specific calibration using a similar matrix to plant material is required. In parallel, measurements of SIRM, χ, and element content (by WD-XRF) can be recommended as user-friendly, fast, and eco-sustainable techniques for determining magnetic PM and PTE pollution hotspots in agricultural ambient.

The magnetic signal from trunk bark of urban trees catches the variation in particulate matter exposure within and across six European cities.

Biomagnetic monitoring increasingly is applied to assess particulate matter (PM) concentrations, mainly using plant leaves sampled in small geographical area and from a limited number of species. Here, the potential of magnetic analysis of urban tree trunk bark to discriminate between PM exposure levels was evaluated and bark magnetic variation was investigated at different spatial scales. Trunk bark was sampled from 684 urban trees of 39 genera in 173 urban green areas across six European cities. Samples were analysed magnetically for the Saturation isothermal remanent magnetisation (SIRM). The bark SIRM reflected well the PM exposure level at city and local scale, as the bark SIRM (i) differed between the cities in accordance with the mean atmospheric PM concentrations and (ii) increased with the cover of roads and industrial area around the trees. Furthermore, with increasing tree circumferences, the SIRM values increased, as a reflection of a tree age effect related to PM accumulation over time. Moreover, bark SIRM was higher at the side of the trunk facing the prevailing wind direction. Significant relationships between SIRM of different genera validate the possibility to combine bark SIRM from different genera to improve sampling resolution and coverage in biomagnetic studies. Thus, the SIRM signal of trunk bark from urban trees is a reliable proxy for atmospheric coarse to fine PM exposure in areas dominated by one PM source, as long as variation caused by genus, circumference and trunk side is taken into account.

Magnetic properties of gas hydrate-bearing sediments and their association with iron geochemistry in the Sea of Marmara, Turkey

The anaerobic oxidation of methane, a key geochemical process that is involved in the cycling of sulfate and iron (oxyhydr)oxides in marine sediments, results in the formation of iron sulfides. Although ferrimagnetic iron sulfides have been identified in seepage systems, the link between iron migration and sediment magnetic properties remains poorly understood. Here, we investigate two cores from the Sea of Marmara to evaluate biogeochemical iron cycling and iron sulfide mineralogy in gas hydrate-bearing sediments. Magnetic analyses indicate the presence of greigite and pyrrhotite in a core from a hydrate-rich site with a high hydrocarbon flux, which contrasts with a lack of these minerals in a core characterized by only mild seepage. This is supported by the results of rock magnetic and scanning electron microscope analyses of the sediments. The presence of authigenic greigite is critical for assessing local redox records and together with the occurrence of monoclinic pyrrhotite may suggest specific diagenetic processes in gas hydrate environments. Our analysis demonstrates the usefulness of these ferrimagnetic minerals, with a high saturation isothermal remanent magnetization to magnetic susceptibility ratio (SIRM/χ > 15 kAm−1) and a high index of hysteresis parameters (DJH > 0.2) indicative of magnetic mineralogy changes, for evaluating variability in the intensity of seepage fluxes and for estimating gas hydrate distributions.

Selection of Plant Species for Particulate Matter Removal in Urban Environments by Considering Multiple Ecosystem (Dis)Services and Environmental Suitability

To select plant species for particulate matter (PM) removal from urban environments, it is important to consider the plant species’ ecosystem (dis)services and environmental suitability in addition to their effectiveness in PM removal. In this study, 61 plant species were ranked for PM removal using three separate models: (i) leaf traits, (ii) leaf saturation isothermal remanent magnetization (SIRM), and (iii) ecosystem services and disservices. The plant species’ effectiveness in PM accumulation and the effective leaf traits were identified using leaf SIRM. In each model, plant species were assigned scores and weights for each criterion. The weighted average or the product (Π)-value was calculated for each plant species. The weighted average of each plant species was multiplied by the scores of leaf longevity and leaf area index (LAI) to scale up to a yearly basis and per unit of ground surface area. The preference ranking organization method for enrichment of evaluations (PROMETHEE) method was employed for the services and disservices model because of the lack of precise weights for the included criteria in the model. A scenario analysis was performed to determine a change in the ranking of plant species when the weights of the criteria were modified in the services and disservices model. The plant species with increased ecosystem services and reduced ecosystem disservices were Tilia cordata (Mill.), Tilia platyphyllos (Scop.), Alnus incana (L.), Acer campestre (L.), and Picea abies (L.). The findings of this study can be relevant to urban planners for recommending suitable choices of plant species for the development of urban green spaces.

Environmental magnetic signatures in mangrove ecosystems in northern Persian Gulf: Implication for pollution assessment in marine environment

Magnetic properties of root, bark, and leaf of mangrove (Avicenna marina) and sediment were determined for pollution assessment at three locations in the northern coast of the Persian Gulf. The study revealed that the sources of the particles deposited on leaf surfaces can be discriminated via saturation isothermal remanent magnetization (SIRM) values and heavy metal. However, different factors including wind direction, size of the magnetic particles and crown density, play a role using SIRM for biomonitoring of atmospheric particulate matter. For leaves, the significant correlations between SIRM and leaf elemental contents indicated that the deposited particles on their surface mainly have geogenic sources. The magnetic analyses revealed that leaves are more suitable than bark for monitoring atmospheric pollution using mangrove trees due to the effect of different factors including dense crown of trees, washing of tree trunk by sea waves, and elements translocation from roots and sediments. Instead, the positive and significant correlation between the SIRM values for sediments and mangrove roots, and no or negative correlation between sediments and roots with barks and leaves indicates that the magnetic properties of the sediments and mangrove roots are suitable indicators of pollution in aquatic environment.

Rock Magnetic Characterization of Returned Samples From Asteroid (162173) Ryugu: Implications for Paleomagnetic Interpretation and Paleointensity Estimation

AbstractIn this study, systematic rock magnetic measurements and saturation isothermal remanent magnetization (SIRM) paleointensity calibration experiments were conducted for the returned samples from C‐type asteroid (162173) Ryugu and two carbonaceous chondrites (Orgueil and Tagish Lake) to evaluate the remanence carriers of the Ryugu sample and its ability as a paleomagnetic recorder. Our magnetic measurements show that Ryugu samples exhibit signatures for framboidal magnetite, coarse‐grained magnetite, and pyrrhotite, and that framboidal magnetite is the dominant remanence carrier of Ryugu samples in the middle‐coercivity range. The SIRM paleointensity constant was obtained for two Ryugu samples, and the median value was 3,318 ± 1,038 μT, which is close to the literature's value based on the average among magnetite, titanomagnetite, pyrrhotite, and FeNi alloys and is widely used for SIRM paleointensity experiments. The paleointensity values estimated using the obtained SIRM paleointensity constant indicate a strong magnetic field of the protoplanetary disk, suggesting that Sun's protoplanetary disk existed at the disk location of Ryugu's parent planetesimal when framboidal magnetite precipitated from the aqueous fluid.

Magnetic and elemental characterization of the particulate matter deposited on leaves of urban trees in Santiago, Chile

Airborne particulate matter is a serious threat to human health, especially in fast-growing cities. In this study, we carried out a magnetic and elemental study on tree leaves used as passive captors and urban dust from various sites in the city of Santiago, Chile, to assess the reliability of magnetic and elemental measurements to characterize particulate matter pollution from vehicular origin. We found that the magnetic susceptibility and saturation isothermal remanent magnetization measured on urban tree leaves is a good proxy for tracing anthropogenic metallic particles and allow controlling the exposure time for particulate matter collection, in agreement with other studies carried out in large cities. Similar measurements on urban soil can be influenced by particles of detritic (natural) origin, and therefore, magnetic measurements on tree leaves can help to identify hotspots where fine particles are more abundant. Elemental particle-induced X-ray emission analysis of tree leaves showed the presence of a number of elements associated with vehicular emissions, in particular Cu, Zn, Fe, K and S which are present at every site, and As, Se, V, Ni, Sr, Zr, Mo and Pb identified at some sites. We observed a correlation between magnetic parameters and the concentrations of S and Br as well as Cu to a smaller extent. Moreover, this study shows the importance of selecting carefully the tree species as well as the location of trees in order to optimize phytoremediation.

Abundance and characteristics of sediment-bound magnetic minerals and trace elements in karst ditch wetland: A case study from Guizhou Province, China

This study investigated sediment-bound magnetic properties and selected trace elements level in the karst ditch wetland, Caohai National Nature Reserve, Guizhou Province, China. Sediment-bound magnetic signals were quantified using low-frequency mass magnetic susceptibility (χLF), anhysteretic remanent magnetization susceptibility (χARM), saturation isothermal remanent magnetization (SIRM), and percentage frequency-dependent susceptibility (χfd%). Concentrations of Cd, Cr, Sb and Zn were determined using inductively coupled plasma mass spectrometry. Sediment χLF, χARM, SIRM, and χfd% were higher than those of bedrocks and mainly altered by the pedogenic processes. The estimated χfd% ranged from 6.15 % to 14.62 % and reflected the magnetic grain sizes were largely concentrated in the range of superparamagnetic particles. The elevated concentrations of sediment-bound Cd, Cr, Sb, and Zn supported the significant contribution of the anthropogenic sources in the karst ditch wetlands. The weak relationship between magnetic signals and selected trace elements (p < 0.05) suggested the occurrence of few sediment-bound iron-containing minerals associated with selected trace elements. These results indicated that a minor contribution of anthropogenic sources of selected trace elements to the elevated sediment magnetic signals in the karst ditch wetlands.

Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments

AbstractMarine sediments can preserve continuous paleomagnetic intensity records. Different magnetic minerals may acquire remanent magnetizations differently, so that compositional variations of magnetic mineral assemblages in sediments may hinder extraction of reliable relative paleointensity (RPI) records. To better understand this issue, we conducted a paleo‐ and rock magnetic study of a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. RPI estimated by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) decreases downcore with an inverse correlation with the ratio of ARM susceptibility to saturation isothermal remanent magnetization. This relationship indicates that the RPI signal weakens as the proportion of biogenic magnetite increases. The NRM–ARM demagnetization diagrams obtained have concave‐down curvature. These observations indicate that the RPI recording efficiency of the biogenic component is lower than that of the terrigenous component if we assume that the magnetizations of the high‐ and low‐coercivity windows are carried dominantly by biogenic and terrigenous components, respectively. This assumption is supported by first‐order reversal curve measurements, transmission electron microscope observations, low‐temperature measurements, and extraction of silicate‐hosted magnetic inclusions from the sediments. Previous studies have suggested that the RPI recording efficiency of biogenic magnetite is higher than that of the terrigenous component, which disagrees with our results. Different concentrations of silicate‐hosted magnetic inclusions in different sedimentary environments might explain this contradiction. We conclude that biogenic magnetite contributes to RPI records with lower efficiency than unprotected terrigenous magnetic minerals in the studied sediments. Changing biogenic magnetite proportions distort ARM‐normalized RPI estimations.

Magnetic response of urban topsoil to land use type in Shanghai and its relationship with city gross domestic product

Due to its effectiveness in terms of time consumption and the high sensitivity to magnetic particles, the magnetic technique has been widely used in the last decades to detect urban soil pollution. As one super metropolis in China, Shanghai is greatly affected by industrial activities and traffic emission. In this study, a total of 192 topsoil samples were collected from four different functional areas, namely, traffic, residential and commercial, unpolluted, and industrial area, covering all administrative districts of Shanghai. Based on the magnetic technique, magnetic properties of urban topsoil were systematically investigated. The results showed that the average values of magnetic susceptibility (χlf) and saturation isothermal remanent magnetization (SIRM) were higher than those of the background value in Shanghai, suggesting that enrichment of ferromagnetic mineral in topsoils. Magnetic susceptibility exhibited high values in industrial area and traffic area, but low values in unpolluted, residential and commercial area. Combined the Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX), it is indicated that the magnetic mineralogy of topsoil is dominated by iron sheets due to traffic activities and spherules due to fossil fuel combustion. Regarding administrative districts, high concentrations of magnetic materials were from districts having greater industrial activities, while low values were in less-polluted agriculture regions. Additionally, a comparison of topsoil χlf value and Gross Domestic Product (GDP) between Shanghai and other cities in China indicated that χlf is generally positively correlated with the city GDP. These results suggested that the urban topsoil in Shanghai was impacted by human activities and GDP. The results of this study are furthermore testifying the effectiveness of the magnetic technique in evaluating the pollution of urban soil.

Species-specific dynamics in magnetic PM accumulation and immobilization for six deciduous and evergreen broadleaves

Numerous studies have demonstrated the effectiveness of plants in accumulating airborne particulate matter (PM) on their leaf surfaces. However, it is not fully clear whether leaf traits or atmospheric conditions influence the accumulation and immobilization dynamics of magnetic PM on leaf surfaces. In this study, leaves of two deciduous broadleaf trees (Quercus petraea, Quercus robur), two deciduous broadleaf shrubs (Prunus padus, Sambucus nigra) and two evergreen broadleaf shrubs (Prunus laurocerasus, Rhododendron sp.) were sampled from a common garden every 48 to 72-h to determine the dynamics of magnetic PM accumulation and immobilization. The mass of water-insoluble removable PM was estimated using gravimetric analysis. The ferro-magnetic and magnetizable component of leaf surface accumulated PM, leaf immobilized PM and water-insoluble removable PM in three size fractions (PM > 10, 3–10, and 0.2–3 μm) was determined using Saturated Isothermal Remanent Magnetization (SIRM). Leaf SIRM for both surface accumulated PM and leaf immobilized PM differed in the following order Q. robur < Q. petraea < S. nigra < P. laurocerasus < P. padus < Rhododendron sp. indicating that PM immobilization on leaves of plant species is a function of net accumulated PM. In proportion to the SIRM signal of leaf surface accumulated PM, on average 4 % was recovered in the SIRM of the water-insoluble removable PM and 63 % was found in the SIRM signal of the immobilized PM while 33 % of the SIRM signal of the leaf surface accumulated PM could not be recovered in the immobilzed or water-insoluble removable PM. The leaf surface accumulated SIRM related with leaf wettability. The mass and SIRM of water-insoluble removable PM were significantly affected by leaf traits and meteorological conditions i.e., (i) negatively by leaf wettability, precipitation and wind speed and (ii) positively by relative humidity and ambient PM2.5 concentrations. These results indicate that magnetic PM accumulation is influenced by both the atmospheric conditions as well as by the micro-morphological leaf traits of plant species. Leaves of Rhododendron sp. followed by Q. robur showed the highest median net deposition velocities for both coarse and fine-particles. Based on the results of this study we recommend S. nigra a deciduous broadleaf shrub species, as a preferred choice to mitigate PM pollution in urban environments. As it accumulated the highest proportion of ferro-magnetic particles in the mass of water-insoluble removable PM considering the harmful effects of these particles on human health. In addition, S. nigra is known to have low biogenic volatile organic compound (BVOC) emissions and provides a good provision for birds and insects.