Nature-based solutions for monitoring the impact of vehicular particulate matter and for the preventive conservation of the Palatine Hill archaeological site in Rome, Italy

Magnetic and chemical biomonitoring methodologies were applied to the southern slopes of the Palatine Hill archaeological area in Rome, Italy. Plant leaves and lichen transplants were respectively sampled and exposed between July 2022 and June 2023 to assess the impact of vehicular particulate matter from Via dei Cerchi, a trafficked road coasting Circus Maximus, towards the archaeological area upon the Palatine Hill. The magnetic properties of leaves and lichens, inferred from magnetic susceptibility, hysteresis loops and first order reversal curves, were combined with the concentration of trace elements. It was demonstrated that the bioaccumulation of magnetite-like particles, associated with tracers of vehicular emissions, such as Ba and Sb, decreased with longitudinal distance from the road, without any important influence of elevation from the ground. Lichens demonstrated to be more efficient biomonitors of airborne PM than leaves, irrespective of the plant species. Conversely, leaves intercepted and accumulated all PM fractions, including road dusts and resuspended soil particles. Thus, plant leaves are suitable for providing preventive conservation services that limit the impact of particulate pollution on cultural heritage sites within busy metropolitan contexts.

Magnetic enhancement in paleosols with hydroclimatic and vegetation cover variabilities (Holocene vs. late MIS 3) in the central Korean Peninsula

Magnetic susceptibility enhancement (kE) is useful for reconstructing terrestrial paleohydroclimate variabilities. However, kE and its driving process(es) in the Korean Peninsula remain uninvestigated. Therefore, this study investigated two kEs of similar magnitudes, dated MIS 1 (Holocene) and late MIS 3 (~ 29-36 ka), from a paleosol sequence in the upland of paleo-fluvial terrace in the central Korean Peninsula. We observed increased ferri- and antiferro-magnetic mineral components,including ultrafine particles, and stronger chlorite weathering for the two kEs, suggesting pedogenic component predominance. The Fe-bearing (phyllo)silicate weathering mechanism proposed for the Chinese Loess Plateau sequences can explain the pedogenesis-induced kEs for the studied site. Superparamagnetic-domain (SPD) to pseudo-single-domain sized particles of pedogenic magnetite are likely major contributors to kEs. Moreover, we recognized the younger kE interval as more SPD contribution but less in total ferrimagnetic contribution, and more antiferromagnetic contribution than the older ones. The magnetic differences between the periods can result from vegetation cover impact and surrounding hydroclimate conditions, consistent with the recent suggestion for part of the southeast Chinese sites with relatively more rainfall. Consequently, our study provides a baseline for improving the relationship between mineral magnetic signals and local/regional hydroclimatic and environmental variabilities.

Topsoil's magnetic and electrical properties in a volcanic and tropical region

Environmental monitoring by measuring topsoil's magnetic and electrical properties is one practical, quick, and low-cost approach. This method has been used worldwide as a proxy for the presence of potentially toxic elements. However, additional research must be conducted on diverse soil types, geology, and climates. We determined the magnetic and electrical properties of urban and agricultural topsoils in a volcanic region and analyzed them as possible proxies of potentially toxic elements for environmental monitoring. To identify topsoil characteristics, we employed the measurements of magnetic susceptibility and hysteresis, electrical conductivity, total dissolved solids, power of hydrogen (pH), particle morphology, and element contents that were statistically analyzed to identify relevant properties. The result was able to differentiate volcanic soils from urban, industrial, and agricultural areas as well. The value of low-frequency magnetic susceptibility (χLF) in soils from urban areas is higher than 866.0±249.9 ×10-8 m3 kg-1, while the value of χLF in agricultural areas is 208.0±67.8 ×10-8 m3 kg-1. This is reinforced by the relationship between low-frequency and frequency-dependent magnetic susceptibility (χLF-χFD%) in samples from urban areas that fall within the same cluster dominated by coarse-grained magnetic minerals originating from anthropogenic processes. In contrast, the agricultural area forms a separate cluster primarily influenced by pedogenic processes from acid igneous rock minerals. Caution is required for interpreting the magnetic signal due to the high contents of lithogenic magnetic particles inherited from the parent materials of Andisols.

Efficacy of green infrastructure in reducing exposure to local, traffic-related sources of airborne particulate matter (PM)

One aim of roadside green infrastructure (GI) is to mitigate exposure to local, traffic-generated pollutants. Here, we determine the efficacy of roadside GI in improving local air quality through the deposition and/or dispersion of airborne particulate matter (PM). PM was collected on both pumped air filters and on the leaves of a recently installed 'tredge' (trees managed as a head-high hedge) at an open road environment next to a primary school in Manchester, U.K. The magnetic properties of PM deposited on leaves and filters (size fractions PM10 and PM2.5) were deduced from hysteresis loops, first-order reversal curves (FORCs), and low-temperature remanence measurements. These were complemented with electron microscopy to identify changes in magnetic PM concentration downwind of the tredge/GI. We show that the tredge is permeable to airflow using a simple CO2 tracer experiment; hence, it allows interception and subsequent deposition of PM on its leaves. Magnetic loadings per m3 of air from filters (PM10 saturation magnetisation, Ms, at 5 K) were reduced by 40 % behind the tredge and a further 63 % in the playground; a total reduction of 78 % compared to roadside air. For the PM2.5 fraction, the reduction in magnetic loading behind the tredge was remarkable (82 %), reflecting efficient diffusional capture of sub-5 nm Fe-oxide particles by the tredge. Some direct mixing of roadside and playground air occurs at the back of the playground, caused by air flow over, and/or through gaps in, the slowly-permeable tredge. The magnetic loading on tredge leaves increased over successive days, capturing ~23 % of local, traffic-derived PM10. Using a heuristic two-dimensional turbulent mixing model, we assess the limited dispersion of PM < 22.5 μm induced by eddies in the tredge wake. This study demonstrates that PM deposition on leaves reduces exposure significantly in this school playground setting; hence, providing a cost-effective mitigation strategy.

Technogenic magnetic particles in topsoil: Characteristic features for different emission sources

Variations in mineralogical composition, grain size internal structure and stoichiometry of technogenic magnetic particles (TMPs) deposited in topsoil may provide crucial information necessary to trace main pollution sources and recognize various technological processes. The aim of the study was to characterize, by means of magnetic parameters and Mössbauer spectra, the TMPs from non-ferrous metallurgy, cement, coke, glass production as well as long range transport (LRT) and compare the obtained data with previous results focused on iron mining and metallurgy. This research shows that only certain pollution sources (e.g. mainly iron mining, iron metallurgy, LRT and partly glass production) can be successfully distinguished by the applied parameters. The main features characteristic for TMPs produced by Fe-mining are: high values of concentration-dependent magnetic parameters, low values of coercivity, significant contribution from coarse MD (multi-domain) grains and a relatively high stoichiometry of magnetite. The most discriminative feature for TMPs generated by the glass industry is the abundance of goethite in the topsoil samples, which is confirmed by magnetic and Mössbauer techniques. The TMPs released by the Ni-Cu smelter and the Pb-Zn waste exhibit significant differences in the Mössbauer parameters, indicating different stoichiometry of magnetite for each group. Such variations are due to replacement of Fe by other elements at tetrahedral sites in the case of TMPs released from the Ni-Cu smelter. TMPs characteristic for the LRT emissions contain higher amount of finer fraction of low-stoichiometry magnetite (mostly single-domain SD particles) than those originating from other sources. The TMPs accumulated in the topsoils around the coking plants cannot be clearly discriminated by the applied methodology due to strong influence of the local pollution sources. Magnetic studies of the TMPs generated by cement production are complicated, since their properties mainly depend on individual technology (e.g. additives) used by the local cement plants.

Barking up the Right Tree: Using Tree Bark to Track Airborne Particles in School Environment and Link Science to Society

Children's exposure to air pollution affects both their health and learning skills. Fine and ultrafine particulate matter (PM2.5, PM1), notably issued from traffic sources in urban centers, belong to the most potential harmful health hazards. However their monitoring and the society's awareness on their dangers need to be consolidated. In this study, raising teacher and pupil involvement for air quality improvement in their schools environment is reached through developing a passive monitoring technique (bio-sensors made of tree bark). The experiment was implemented in two urban elementary schools situated close to a main traffic road of the city of Toulouse (South of France). Magnetic properties, carbonaceous fraction measurements, and scanning electronic microscopy (SEM-EDX) investigations were realized both on passive bio-sensors and filters issued from active sampling. We find that traffic is the main PM1 source for both outdoors and indoors at schools. Higher levels of outdoor PM in the school's environments compared to urban background are reached especially in the cold period. The schools proximity to a main traffic source and lack of ventilation are the main causes for observed PM1 accumulation in classrooms. The co-working experiment with educational teams and pupils shows that the use of bio-sensors is a driver for children empowerment to air pollution and therefore represents a potential key tool for the teachers though limiting eco-anxiety. As PM accumulation is observed in many scholar environments across Europe, the proposed methodology is a step toward a better assessment of PM impact on pupil's health and learning skills.

Atmospheric Concentration of CO2 and PM2.5 at Salina, Stromboli, and Vulcano Islands (Italy): How Anthropogenic Sources, Ordinary Volcanic Activity and Unrests Affect Air Quality

Geogenic and anthropogenic sources of atmospheric particulate and CO₂ can lead to threats to human health in volcanic areas. Although the volcanic CO₂ hazard is a topic frequently debated in the related scientific literature, space and time distribution of PM_2.5 are poorly known. The results of combined CO₂/PM_2.5 surveys, carried out at Salina, Stromboli, and Vulcano islands (Aeolian archipelago, Italy) in the years 2020–2021, and integrated with investigations on bioaccumulation of metallic particulate matter by the mean of data on the magnetic properties of oleander leaves, are presented in this work. The retrieved results indicate that no significant anthropogenic sources for both CO₂ and PM_2.5 are active in these islands, at the net of a minor contribution due to vehicular traffic. Conversely, increments in volcanic activity, as the unrest experienced by Vulcano island since the second half of 2021, pose serious threats to human health, due to the near-ground accumulation of CO₂, and the presence of suspended micro-droplets of condensed hydrothermal vapor, fostering the diffusion of atmophile viruses, such as the COVID-19. Gas hazard conditions can be generated, not only by volcanic vents or fumarolic fields, but also by unconventional sources, such as the outgassing from shallow hydrothermal aquifers through drilled or hand-carved wells.

Intermediate field directions recorded in Pliocene basalts in Styria (Austria): evidence for cryptochron C2r.2r-1

Pliocene volcanic rocks from south-east Austria were paleomagnetically investigated. Samples were taken from 28 sites located on eight different volcanoes. Rock magnetic investigations revealed that magnetic carriers are Ti-rich or Ti-poor titanomagnetites with mainly pseudo-single-domain characteristics. Characteristic remanent magnetization directions were obtained from alternating field as well as from thermal demagnetization. Four localities give reversed directions agreeing with the expected direction from secular variation. Another four localities of the Klöch-Königsberg volcanic complex (3) and the Neuhaus volcano (1) have reversed directions with shallow inclinations and declinations of about 240° while the locality Steinberg yields a positive inclination of about 30° and 200° declination. These aberrant directions cannot be explained by local or regional tectonic movements. All virtual geomagnetic pole positions are located on the southern hemisphere. Four virtual geomagnetic poles lie close to the geographic pole, while all others are concentrated in a narrow longitude sector offshore South America (310°-355°) with low virtual geomagnetic pole latitudes ranging from - 15° to - 70°. The hypothesis that a transitional geomagnetic field configuration was recorded during the short volcanic activity of these five localities is supported by 9 paleointensity results and ³⁹Ar/⁴⁰Ar dating. Virtual geomagnetic dipole moments range from 1.1 to 2.9·10²² Am² for sites with low VGP latitudes below about 60° and from 3.0 to 9.3·10²² Am² for sites with higher virtual geomagnetic pole latitudes. The new ³⁹Ar/⁴⁰Ar ages of 2.51 ± 0.27 Ma for Klöch and 2.39 ± 0.03 Ma for Steinberg allow for the correlation of the Styrian transitional directions with cryptochron C2r.2r-1 of the geomagnetic polarity time scale.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s40623-021-01518-w.

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.

Identifying environmental pollution recorded in street dust using the magnetic method: a case study from central eastern China

Urban street dust constitutes important intermediate products for the transmission of solid organic and inorganic pollutants in the urban environment. In this study, 133 street dust samples were collected from Xinyang to explore their magnetic characteristics, spatial distribution, and environmental implications using magnetic measurements. The results are as follows. (1) There were ferrimagnetic, antiferrimagnetic, and paramagnetic (e.g., lepidocrocite) minerals in the dust. Among these, the dominant magnetic carriers were ferrimagnetic minerals. Furthermore, magnetite was a first-order ferrimagnetic carrier. (2) The magnetic domains of the dust were pseudo single-domain to multi-domain. (3) The magnetic concentration (χ and SIRM) of dust were 2.6 and 4.1 times higher than those of background samples that were not polluted by urban and anthropogenic activities, respectively. Therefore, we conclude that the dust consisted of high concentration of ferrimagnetic minerals and coarse magnetic particles. (4) The magnetic distribution was spatially different. The industrial area, which was the most polluted sampling area, had the highest magnetic concentration and the coarsest magnetic particles. This was attributable to industrial emissions, fossil fuel combustion, and exhaust emissions from heavy-laden trucks. Residential and commercial areas, which were the second most polluted areas, had higher concentration and coarser particles. This was primarily due to the high population density and traffic activities of mini-cars (i.e., high flux and exhaust emissions). Hence, the conclusion is that the magnetic characteristics, spatial distribution, and the sources of dust are dictated by anthropogenic activities. Our results indicate that the magnetic method is a highly effective tool to monitor urban environmental pollution.

Assessment of heavy metal pollution in Vistula river (Poland) sediments by using magnetic methods

The present study evaluated the level of heavy metal (HM) pollution in Vistula river sediments in a highly urbanized Warsaw agglomeration (Poland). Magnetometry was used to assess the pollution level by measuring the fine fractions (0.071 mm and < 0.071 mm) of sediments collected from the surface layer of the riverbank. The magnetic methods (e.g., mass magnetic susceptibility χ, temperature-dependence magnetic susceptibility, and hysteresis loop parameters) were supplemented by microscopy observations and chemical element analyses. The results showed the local impact of Warsaw's activity on the level of HM pollution, indicated by the maximum concentrations of magnetic particles and HM in the city center. The sediment fraction < 0.071 mm was dominated by magnetite and by a large amount of spherical-shaped anthropogenic magnetic particles. The pollution from the center of Warsaw was transported down-river over a relatively short distance of approximately 11 km. There was a gradual decrease in the concentrations of magnetic particles and HM in areas located to the north of the city center (down-river); furthermore, χ and concentrations of HM did not decrease to the values observed for the area to the south of Warsaw (up-river). The study showed two possible sources of sediment pollution: traffic-related and heat and power plant emissions. The influence of an additional source of pollution cannot be excluded as the amount of spherules in the sediments at the center was extremely high. The present study demonstrates that magnetometry has a practical application in detecting and mapping HM pollution in river systems.

Magnetic Emissions from Brake Wear are the Major Source of Airborne Particulate Matter Bioaccumulated by Lichens Exposed in Milan (Italy)

The concentration of selected trace elements and the magnetic properties of samples of the lichen Evernia prunastri exposed for 3 months in Milan (Italy) were investigated to test if magnetic properties can be used as a proxy for the bioaccumulation of chemical elements in airborne particulate matter. Magnetic analysis showed intense properties driven by magnetite-like minerals, leading to significant correlations between magnetic susceptibility and the concentration of Fe, Cr, Cu, and Sb. Selected magnetic particles were characterized by Scanning Electron Microscope and Energy Dispersion System microanalyses, and their composition, morphology and grain size supported their anthropogenic, non-exhaust origin. The overall combination of chemical, morphoscopic and magnetic analyses strongly suggested that brake abrasion from vehicles is the main source of the airborne particles accumulated by lichens. It is concluded that magnetic susceptibility is an excellent parameter for a simple, rapid and cost-effective characterization of atmospheric trace metal pollution using lichens.

Magnetic response of Arsenic pollution in a slag covered soil profile close to an abandoned tungsten mine, southern China

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.

Effective and universal tool for evaluating heavy metals-passive dust samplers

The study presents designating, accomplishing, optimizing, and validating a new tool - "passive sampler" (PS) that can be effectively used as a proxy to assess the level of traffic-related pollution. To construct the PS, a drainage pipe filled with a mixture of coarse sand and peat in a volume ratio of 1:1 was used; this was previously verified to exhibit high ability to accumulate pollutants. Magnetic methods supplemented with chemical method evaluating heavy metal content and electron microscopic observations were used to detect the effectiveness of the PS. The PS was validated in Warsaw, Poland, by observing the capacity and trends in the accumulation of traffic-related heavy metals as well as magnetic particles and by comparison of the properties of magnetic fraction of PS filling and street dust collected from the surface of road. A depth decreasing trend in distributions of magnetic susceptibility related to the concentration of magnetic particles and the content of heavy metals confirmed a very strong accumulation of pollution in the surface layer of samplers and their depth-migration. Magnetic fraction of PS filling and street dust revealed similarities in terms of magnetic mineralogy, grain size, domain state, morphology, and chemical composition. The good correlation of concentration of magnetic particles with traffic-related heavy metals indicates their similar transport pathway from road to sampler. Passive sampler is a compact, mobile, low-cost tool that does not require electricity for installation and can be effectively used for the identification of traffic-derived pollution. Moreover, the PS can overcome disadvantages of street dust arising from different geological backgrounds, cleaning of the road surface, runoff of deposited dust, etc., which cause the underestimation of pollution level.

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

Mixed-valent iron [Fe(II)-Fe(III)] minerals such as magnetite and green rust have received a significant amount of attention over recent decades, especially in the environmental sciences. These mineral phases are intrinsic and essential parts of biogeochemical cycling of metals and organic carbon and play an important role regarding the mobility, toxicity, and redox transformation of organic and inorganic pollutants. The formation pathways, mineral properties, and applications of magnetite and green rust are currently active areas of research in geochemistry, environmental mineralogy, geomicrobiology, material sciences, environmental engineering, and environmental remediation. These aspects ultimately dictate the reactivity of magnetite and green rust in the environment, which has important consequences for the application of these mineral phases, for example in remediation strategies. In this review we discuss the properties, occurrence, formation by biotic as well as abiotic pathways, characterization techniques, and environmental applications of magnetite and green rust in the environment. The aim is to present a detailed overview of the key aspects related to these mineral phases which can be used as an important resource for researchers working in a diverse range of fields dealing with mixed-valent iron minerals.

New Early Cretaceous palaeomagnetic and geochronological results from the far western Lhasa terrane: Contributions to the Lhasa-Qiangtang collision

To better constrain the Lhasa-Qiangtang collision, a combined palaeomagnetic and geochronological study of the far western Lhasa terrane was conducted on the Duoai Formation lava flows (~113-116 Ma), as well as on the Early Cretaceous Jiega Formation limestone. Following detailed rock magnetic, petrographical, and palaeomagnetic experiments, characteristic remanent magnetisation directions were successfully isolated from most samples using principal component analysis. The tilt-corrected direction groups yielded a palaeopole at 69.1°N, 319.8°E with A₉₅ = 4.8° (N = 19). A primary origin for the magnetisation is consistent with positive fold tests. Our results from the Early Cretaceous units, combined with published palaeomagnetic data obtained from Cretaceous strata from the Lhasa and western Qiangtang terranes, show that these two terranes had already collided by the Early Cretaceous, the Lhasa terrane had a relatively east-west alignment, and it remained at a relatively stable palaeolatitude during the entire Cretaceous. Comparing the Cretaceous palaeolatitude calculated for the western Lhasa terrane with those from Eurasia and Mongolia suggests a latitudinal convergence of ~1400 ± 290 km and ~1800 ± 300 km, respectively, since the Early Cretaceous.

Magnetic properties and element concentrations in lichens exposed to airborne pollutants released during cement production

The content of selected elements (Al, As, Ca, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, S, Ti, V and Zn) was measured in samples of the lichen Evernia prunastri exposed for 30, 90 and 180 days around a cement mill, limestone and basalt quarries and urban and agricultural areas in SW Slovakia. Lichens transplanted around the investigated quarries and the cement mill rapidly (30 days) reflected the deposition of dust-associated elements, namely Ca (at the cement mill and the limestone quarry) and Fe, Ti and V (around the cement mill and the basalt quarry), and their content remained significantly higher throughout the whole period (30-180 days) with respect to the surrounding environment. Airborne pollutants (such as S) progressively increased in the study area from 30 to 180 days. The magnetic properties of lichen transplants exposed for 180 days have been characterized and compared with those of native lichens (Xanthoria parietina) and neighbouring bark, soil and rock samples, in order to test the suitability of native and transplanted samples as air pollution magnetic biomonitors. The magnetic mineralogy was homogeneous in all samples, with the exception of the samples from the basalt quarry. The transplants showed excellent correlations between the saturation remanent magnetization (Mrs) and the content of Fe. Native samples had a similar magnetic signature, but the values of the concentration-dependent magnetic parameters were up to two orders of magnitude higher, reflecting higher concentrations of magnetic particles. The concentrations of As, Ca and Cr in lichens correlated with Mrs values after neglecting the samples from the basalt quarry, which showed distinct magnetic properties, suggesting the cement mill as a likely source. Conversely, Ti and Mn were mostly (but not exclusively) associated with dust from the basalt quarry. It is suggested that the natural geological characteristics of the substrate may strongly affect the magnetic properties of lichen thalli. Taking this into account, the results of this study point out the suitability of lichens as air pollution magnetic biomonitors.

Magnetic study of a mixture of magnetite and metallic iron in indoor dust samples

Magnetite in mixture with metallic iron in indoor dust samples was examined using several magnetic analyses, thermomagnetic curves of the magnetic susceptibility and the induced magnetization vs. temperature, hysteresis loops, and first-order reversal curves. The study of the magnetic properties was supplemented by the analysis of chemical elements and electron microscopic observation. The metallic iron in the mixture affects the values of hysteresis parameters, decreasing coercivity (B_c) and increasing saturation magnetization (M_s), and it is responsible for the magnetic enhancement of magnetic susceptibility. The thermomagnetic curves show several distinct features: the first Curie temperature of magnetite, the second one (∼764 °C) of iron, and the rapid decrease on the heating curves (between 600 and 750 °C) caused by the oxidation of iron to magnetite. Two magnetochemical processes appear during the thermal treatment of indoor dust: the oxidation of iron to magnetite and the neo-formation of magnetite as a result of chemical transformation of non-magnetic minerals. The shift of the hysteresis parameter ratios from the multi-domain (MD) region towards the single-domain (SD) region on the Day-Dunlop plot is controlled by the oxidation of iron in the thermally induced process and the grain size of the new formed magnetite. The magnetic properties of indoor dust are a potential indicator of indoor air pollution. Elemental iron plays an important role in the development of inflammation in humans via oxidative stress, so that the presence of metallic Fe in indoor dust can affect human health.

Validity of archaeomagnetic field recording: an experimental pottery kiln at Coppengrave, Germany

Palaeomagnetic data obtained from archaeological materials are used for reconstructions of the Earth's magnetic field of the past millennia. While many studies tested the reliability of this recorder for palaeointensity only a few studies did this for direction. The study presents an archaeomagnetic and rock magnetic investigation applied to an experimental pottery kiln, which was operated in 2003 to produce stone ware. This kind of high-quality pottery needs a temperature of at least 1160 °C. Shortly before heating of the kiln direct absolute measurements of the absolute geomagnetic field vector have been carried out close to it. After cooling of the kiln 24 oriented palaeomagnetic samples have been taken. Although Curie temperatures are about 580 °C, that is the typical temperature for magnetite, thermal as well as alternating field demagnetisations reveal also a considerable amount of hematite as magnetic carrier. This mixture of magnetite and hematite is dominated by pseudo-single domain grains. Demagnetisation removed in some cases weak secondary components, but in most cases the specimens carried a single component thermoremanent magnetisation. The mean characteristic remanent magnetisation direction agrees on 95 per cent confidence level with the directly measured field direction. Archaeointensity was obtained from five specimens with the Thellier-Coe method and with the multiple-specimen palaeointensity domain-state corrected method. Six of these specimens also provided a result of the Dekkers-Böhnel method, which overestimated the archaeointensity by about 9 per cent compared to the direct value, while after correction for fraction the value agrees very well. For the multiple-specimen palaeointensity domain-state corrected method only fractions between 25 and 75 per cent have been used and specimens showing alteration have been excluded. Above 450 °C many specimens showed alteration of the magnetic grains. Because median destructive temperatures were often above this value in most cases the fraction was less than 50 per cent. Nevertheless the obtained intensity (48.48 ± 0.24 μ) is on 95 per cent confidence level in agreement with the direct observation. Behaviour of the specimens during the Thellier-experiments was not ideal because of narrow unblocking temperature spectra and alteration. Nevertheless, the obtained mean archaeointensity is also in agreement with the direct field observation. Here the relative palaeointensity error is about 6 per cent and very high compared the multiple-specimen palaeointensity domain-state corrected method. The investigation demonstrates that a pottery kiln can provide a very precise estimate of the ancient geomagnetic field vector.

Rock magnetism of remagnetized carbonate rocks: another look

Authigenic formation of fine-grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks. Authigenic magnetite grains, dominantly in the superparamagnetic and stable single-domain size range, also give rise to distinctive rock-magnetic properties, now commonly used as a ‘fingerprint’ of remagnetization. We re-examine the basis of this association in terms of magnetic mineralogy and particle-size distribution in remagnetized carbonates having these characteristic rock-magnetic properties, including ‘wasp-waisted’ hysteresis loops, high ratios of anhysteretic remanence to saturation remanence and frequency-dependent susceptibility. New measurements on samples from the Helderberg Group allow us to quantify the proportions of superparamagnetic, stable single-domain and larger grains, and to evaluate the mineralogical composition of the remanence carriers. The dominant magnetic phase is magnetite-like, with sufficient impurity to completely suppress the Verwey transition. Particle sizes are extremely fine: approximately 75% of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single-domain. Although it has been proposed that the single-domain magnetite in these remagnetized carbonates lacks shape anisotropy (and is therefore controlled by cubic magnetocrystalline anisotropy), we have found strong experimental evidence that cubic anisotropy is not an important underlying factor in the rock-magnetic signature of chemical remagnetization.