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Thermomagnetic curves of magnetic susceptibility (κ) are key to characterizing magnetic properties. We report hump‐shaped κ‐T curves of magnetite‐bearing basalt during heating‐cooling cycles to ∼340°C, with a large thermal hysteresis and similar starting and ending values, even in multiple repeated cycles, ruling out changes in magnetic mineralogy. Based on FORC diagrams and published results of engineered materials, we propose that thermal hysteresis arises from configurations of magnetic moments in clusters of single‐domain particles due to dipolar coupling, with different collective behavior during heating and cooling. This effect modifies the hump‐shaped thermal relaxation behavior of the individual nanoparticles. FORC and κ‐T results indicate an increase in effective particle sizes after 700°C‐heating. Our results are a warning against premature interpretation of a decreasing trend in κ‐T curves by maghemite inversion. Instead, fine particle behavior should be considered when a hump‐shaped κ‐T behavior is detected. Plain Language Summary Thermomagnetic curves of magnetic susceptibility (κ) are key to characterizing magnetic properties. A marked drop in κ‐T curves at ∼300–400°C is often considered to indicate the inversion of maghemite to hematite. Such a drop is often preceded by an increase in κ, creating a hump shape that is rarely noted in discussions. We report hump‐shaped κ‐T curves in magnetite‐bearing basalt. When heating up to ∼340°C and cooled subsequently, a large thermal hysteresis was observed. This hump shape and the thermal hysteresis behavior occur in a very similar way in repeated κ‐T cycles, ruling out changes in magnetic mineralogy. We hypothesize that the thermal hysteresis arises from configurations of coupled magnetic moments in clusters of fine particles, which is partly irreversible upon cooling. This effect modifies the hump‐shaped thermal relaxation behavior of the individual particle moments. When heated to 700°C, grain boundaries may weld and internal stress effects are reduced, increasing the effective particle sizes and shifting the hump‐peak to a higher temperature. Our results indicate that fine particle behavior should be considered for all types of natural materials when a hump‐shaped κ‐T curve is observed rather than interpreting the drop in κ as maghemite inversion. Key Points We observed reversible thermal hysteresis behavior in hump‐shaped partial magnetic susceptibility cycles of magnetite‐bearing basalts The thermal hysteresis may be caused by blocked states of coupled nanoparticle moments modulating thermal activation Descending susceptibility in hump‐shaped curves is often due to single‐domain thermal relaxation rather than maghemite inversion

Microorganisms are a primary control on the redox-induced cycling of iron in the environment. Despite the ability of bacteria to grow using both Fe(II) and Fe(III) bound in solid-phase iron minerals, it is currently unknown whether changing environmental conditions enable the sharing of electrons in mixed-valent iron oxides between bacteria with different metabolisms. We show through magnetic and spectroscopic measurements that the phototrophic Fe(II)-oxidizing bacterium Rhodopseudomonas palustris TIE-1 oxidizes magnetite (Fe3O4) nanoparticles using light energy. This process is reversible in co-cultures by the anaerobic Fe(III)-reducing bacterium Geobacter sulfurreducens. These results demonstrate that Fe ions bound in the highly crystalline mineral magnetite are bioavailable as electron sinks and electron sources under varying environmental conditions, effectively rendering magnetite a naturally occurring battery.

A range of ages have been proposed for the timing of India‐Asia collision; the range to some extent reflects different definitions of collision and methods used to date it. In this paper we discuss three approaches that have been used to constrain the time of collision: the time of cessation of marine facies, the time of the first arrival of Asian detritus on the Indian plate, and the determination of the relative positions of India and Asia through time. In the Qumiba sedimentary section located south of the Yarlung Tsangpo suture in Tibet, a previous work has dated marine facies at middle to late Eocene, by far the youngest marine sediments recorded in the region. By contrast, our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50.6–52.8 Ma, in broad agreement with the timing of cessation of marine facies elsewhere throughout the region. Double dating of detrital zircons from this formation, by U‐Pb and fission track methods, indicates an Asian contribution to the rocks thus documenting the time of arrival of Asian material onto the Indian plate at this time and hence constraining the time of India‐Asia collision. Our reconstruction of the positions of India and Asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early Eocene. We conclude the paper with a discussion on the viability of a recent assertion that collision between India and Asia could not have occurred prior to ∼35 Ma.

The onset of the India-Asia continent-continent collision in Tibet was accompanied by the elimination of oceanic crust and the development of a peripheral foreland basin on the subducting continental crust. However, owing to the progressively southward migration of this foreland basin, the appearance of flexural uplift (forebulge) predated flexural subsidence (foredeep) at the distal location, such as at Gamba and Tingri in the southern Tethyan Himalaya. Consequently, to trace and date this forebulge may provide a better constraint on the India-Asia collision. At Gamba and Tingri, our studies on the stratigraphy, paleontology, and paleoenvironment show that depositional cessation of the limestones occurred at Shallow Benthic Zone 7 (SBZ 7, ∼54-55 Ma) in the Gamba area and at the base of SBZ 10 (∼52.8 Ma) in the Tingri area. At Gamba, a conglomerate layer within the upper Zongpu Formation is suggested to represent the formation of the forebulge at the onset of the foreland basin, and the coincidence of the conglomerate layer with the Carbon Isotope Excursion (CIE) provides a precise age of ∼56 Ma (the Paleocene/Eocene boundary) for the possible initial India-Asia continental collision. Our results not only provide a reliable and precise age for the India-Asia collision but, for the first time, report time equivalence between the India-Asia collision and the CIE in Tibet.

Thermomagnetic susceptibility (κ‐T) curves are used widely for magneto‐mineralogical analysis. Humps in heating curves below the Curie temperature of magnetite are observed frequently. We present κ‐T curves with hump features in magnetite‐bearing flood basalt samples. Heating‐cooling experiments combined with theoretical principles suggest that these humps reflect primarily the transition from the stable single domain (SSD) to the superparamagnetic (SP) state, with possible contributions from maghemite decomposition and stress release. Scanning electron microscopy confirms magnetite intergrowths with <100 nm size, which supports the presence of SSD‐SP domain states and the diversity of hump shapes. Changes in magnetic interactions, internal stress distribution, and internal subdivision due to maghemitization likely contribute to different hump shapes in heating and cooling curves. The findings indicate that humps provide a diagnostic feature associated with thermal relaxation in fine magnetite in diverse lithologies and suggest caution in interpreting κ‐T decreases at intermediate temperatures solely by maghemite decomposition.

We investigated the magnetic and chemical properties of the roadside soil samples collected from five European and Asian countries. Spots in which cars slowed down and/or accelerated due to the traffic organization (speed limits, junctions, and traffic lights) were selected for sampling. Apart from the Zabrze site (Poland), the magnetic susceptibility and heavy metal contents decreased with increasing distance from the road edge. The highest mass-specific magnetic susceptibility values ( χ ) were observed in the samples collected from Mumbai (India) and Zabrze (Poland). Moreover, the high contents of Fe, Ni, Mn, and Co were observed in Mumbai, whereas in Zabrze, all the examined elements demonstrated high contents, except for Co. Analyses revealed that magnetite was the main magnetic mineral in the roadside soil samples. The high correlation coefficients ( r  = 0.87) between the magnetic susceptibility values and the total Fe content demonstrated that Fe occurred mainly as ferrimagnetic particles of technogenic origin resulting from traffic emissions. The traffic origin of the pollutants was also confirmed by the increased contents of the typically anthropogenic metals (Pb, Zn, Cd, and Cu) and a good correlation ( r  = 0.83) between the Ti and Mo contents, which do not occur in natural associations. The ratio between particular polycyclic aromatic hydrocarbons (PAHs) and high content of PAHs typical for car exhaust also implied traffic as their main source.

The increased aridification of Central Asia during the Miocene coincides in time with lake formations and the evolution of playa environments in the region. However, Miocene continental climate dynamics and the forcing of aridification are still not well constrained. Neogene lacustrine mudflat deposits in the Ili Basin in southeast Kazakhstan provide a well-exposed paleoclimate archive. Here, we present a detailed rock magnetic study of a middle Miocene playa cycle deposited in a closed basin. We use high-resolution rock magnetic parameters, lithological studies and geochemistry to reconstruct the playa environment and the depositional conditions. The rock magnetic mineralogy of the playa cycle is controlled by hematite and two fine-grained magnetite phases. Increased magnetic concentrations occur during dry mudflat conditions, with a lower groundwater table and increased aridity. The underlying processes controlling the observed variation in magnetic concentrations are a complex interplay of diagenetic processes during and after deposition. The data support an authigenic origin of both magnetite phases, one formed before and the other after sediment consolidation. Early diagenetic formation of fine-grained magnetite by microbial activity is followed by post-depositional formation of a secondary fine-grained magnetite phase. The rock magnetic results such as magnetic concentration-dependent parameters, ARM/SIRM and s-ratio indicate a sensitive record of (ground) water availability and aridity changes in the Ili Basin. We suggest that they can serve as an effective proxy for detailed paleo-environment reconstruction of playa evolution, not only in the middle Miocene Ili Basin but also in comparable floodplain/playa lake settings.

Previous studies indicated serious soil arsenic (As) pollution of large spatial extent related to tungsten mining. We performed systematic analyses of magnetic parameters and As contents of a slag covered soil profile close to the abandoned tungsten mine in southern China, in order to discuss the feasibility of using sensitive, non-destructive, and cost-effective magnetic methods for monitoring the soil arsenic content in such arsenic pollution areas. The results indicate that arsenic sulfide entered from slags into the underlying soil and changed to iron arsenate and moveable arsenic ion. The arsenic ions were transported from the upper to the lower part of the soil profile, leading to more serious arsenic pollution at lower levels of the section. Pedogenesis and oxidation of the entered iron and arsenic sulfide resulted in coexistence of magnetite/maghemite and hematite, with different contributions at depths of 125–195 cm, 60–125 cm, and 0–60 cm. The arsenic content is significant positively correlated with the hematite concentration given by the magnetic parameter HIRM and negatively correlated with the S −300 ratio that measures the relative contributions of magnetite(+maghemite) and hematite. The S −300 ratio is effective for semi-quantification of soil arsenic content, and may be also used for soil arsenic pollution assessment and monitoring in similar settings of tungsten mining.

Magnetic proxy approaches proved to be efficient for potentially toxic elements (PTEs) pollution assessment when targeting forests or areas with a homogenous background where anthropogenic magnetic signals could be easily distinguished. Here, we present a multidisciplinary approach for magnetic susceptibility ( x ) and HM assessment in a complex area in the Nile Delta, where geogenic input, land use, and various industries with different fly ash and surface water emissions interfere. Statistical analysis discriminates between the effects of lithologic elements and the concentrations of toxic anthropogenic elements. The studied elements are classified into lithogenic and anthropogenic-related (HMs, Au industry, and fertilizers industry) groups with maximum contamination levels of eight anthropogenic-related and highly toxic PTEs (Cu, Zn, Mo, Cd, Sb, Pb, Hg, and As) in the Akrasha industrial area (pollution load index = 15.84). Considering the whole data set, the numerical correlation of x with most PTE concentrations and the pollution load index (PLI) is weak, while it is moderate to strong with lithogenic elements. However, a comparison of lithogenic elements and PTE concentrations along with x- values in two separate clusters supports the correspondence of lithology with elevated x -values in silt and clay-rich soil samples as well as HM concentration in industrial sandy soils. Correspondence between magnetic maps and chemistry data with land use reflects the potential of magnetic proxy methods for qualitative PTE pollution pre-delineation of the polluted spots, provided that lithological conditions are carefully considered.

With the increasing number of vehicles and the development of transport systems, traffic contamination has become an important source of current environmental pollution. The study of roadside distribution patterns of traffic-related contaminants and its influencing factors can contribute to assess the scope and distribution process of the contaminants. A large number of works confirmed that magnetic methods which are fast and economic can be used to indicate the traffic-related pollution. Based on the previous study, we installed two new monitoring sites and conducted repeated in situ measurements of magnetic susceptibility ( κ ). From these data, we calculated the accumulation amount and accumulation rate of κ to reveal the detailed distribution process of traffic contaminants on the roadside surface over time. In addition, we collected daily meteorological data (rainfall and wind direction) and conducted a correlation analysis between the accumulation rate of κ and the meteorological data. The results show that the accumulation rate of traffic contaminants on the roadside surface is lower in periods of higher rainfall, and that the winds influence the distribution pattern of contaminants through changing their transmission path. The results confirm that magnetic methods can be used to reveal the detailed distribution process of traffic-related contaminants and its influencing factors.