Abstract The Woodfjorden area in northwestern Svalbard hosts the world's northernmost onshore thermal springs, extinct Quaternary volcanoes (the Bockfjorden Volcanic Complex), late Miocene lava flows (the Seidfjellet Formation) that overlie a Devonian sedimentary basin, and a long‐lived regional fault zone. To contribute to a better understanding of subsurface fluid flow in this volcanic region, we conducted a 3D magnetotelluric (MT) survey at 12 sites, combined with water and gas sampling at the thermal spring clusters (Trollkjeldene and Jotunkjeldene). While the MT survey has a limited aperture due to the remoteness of the region and complex logistics, the identified structural features put several constraints on the evolution of this poorly studied hydrothermal system. The electrically conductive zones (1–10 m), under Trollkjeldene and the Halvdanpiggen volcanic center, are interpreted as signatures of deep hydrothermal fluids or mineralized zones; the resistive anomalies (1–10 km) at 1–4 km depth under the Sverrefjllet volcano indicate buried magmatic or basement rocks. Stable isotope data indicate both meteoric and crustal components for thermal waters. Carbon and noble gas isotope ratios support a mixed gas source involving crustal, atmospheric, and mantle components. Helium isotope ratios suggest a mantle‐derived fraction of 6%–8%. These findings suggest extinct magmatic activity and ongoing hydrothermal circulation, likely prolonged due to fluid compartmentalization at depth by low‐permeability magmatic or metamorphic rocks.

Abstract Titanomagnetite in alkaline maar deposits exhibits variable syn‐ and post‐crystallization alteration linked to wet emplacement and groundwater fluctuations, which remain poorly documented. Magnetic and mineralogical analyses of lapilli, scoria, and basalt from ICDP Eger drill cores S4 and S4b (Bažina maar, western Eger Graben, Czechia) reveal systematic depth‐dependent changes in magnetic properties, fabric, and mineralogy. Effusive basaltic units host Ti‐rich titanomagnetite with Curie temperatures of 170°–240°C, field‐dependent susceptibility, low Δ T C , and positive A40 values, indicating juvenile compositions and minimal alteration. Oblique magnetic foliation and steep lineation from in‐phase anisotropy of magnetic susceptibility define an upward flow fabric consistent with a feeder dike. In contrast, lapilli and scoria show multiple Curie temperatures between 400° and 570°C, strongly negative Δ T C and A40, and frequency‐dependent susceptibility, reflecting rapid quenching and progressive maghemitization. Hydrothermally altered basalts exhibit Curie temperatures between 440° and 520°C and lower negative Δ T C , suggesting late‐stage reducing fluids. Chemical remanent magnetization due to maghemitization in the lapilli parallels original thermal remanent magnetization in basalt. Reverse polarity and moderate to steep inclinations across all lithologies indicate monogenetic emplacement older than 0.78 million years. The transition from fresh titanomagnetite to maghemitized, hydrothermally overprinted titanomaghemite records a complete sequence from magma ascent to post‐eruptive alteration. Out‐of‐phase magnetic susceptibility measured at different temperatures proves a sensitive proxy for distinguishing primary magmatic signals from secondary overprints in maar‐diatreme systems.

Sverrefjellet is the remnant of an extinct alkali basaltic stratovolcano located in NW Svalbard, representing a distinctive phase of Quaternary magmatism in the High Arctic. It is part of the Bockfjorden Volcanic Complex, which consists of several eruption centers within the Woodfjorden-Bockfjorden area. The volcanism occurred during the northern hemisphere’s glaciations and reveals evidence for magma interactions with glaciers. The Quaternary eruption centers are localized along the Breibogen Fault and were probably linked to the evolution of the Knipovich mid-oceanic ridge, yet the exact age of the magmatic activity remains uncertain. Sverrefjellet is renowned for its high abundance of mantle-derived xenoliths, which have become a focal point in most publications on this volcano to date. However, the magmatic architecture and physical volcanology of Sverrefjellet have received only limited attention after the initial mapping by Skjelkvåle et al. (1989). In July 2023, an international multi-disciplinary geoscientific expedition to Woodfjorden-Bockfjorden was undertaken. One of the primary objectives was to perform detailed mapping and systematic sampling of volcanic-related units within Sverrefjellet volcano, with the aim of exploring and refining magma emplacement processes. To facilitate this, drones were utilized to acquire high-resolution 3D digital textured models over the best-exposed outcrops of the volcano. The in-field sampling of the main volcanic units was accompanied by extensive (∼100) magnetic susceptibility measurements with a hand-held Kappameter (SM-30). In total, 20 rock samples have been prepared for petrographic, SEM and EPMA analyses. We observed the presence of mantle-derived xenoliths in all volcanic units, which include dyke intrusions, pillow basalts with associated lava tubes, basaltic lava flows, and various volcanogenic sediments. The slopes of the extinct volcano display predominant frost weathering, with the southern slope adorned with olivine sand and gravel sourced from 'bomb-shaped' nodules or clasts that typically contain peridotite xenoliths as their cores. The presence of pillow lavas and associated 1 to 2 meter large lava tubes suggests subglacial magma emplacement. This is supported by their relatively high elevation at 200-300 meters above sea-level, which makes interaction with seawater highly unlikely. In between lava flows and dykes, texturally distinctive zones characterized by platy tops and bottoms as well as numerous flattened boulder-sized xenolithic nodules were observed. Petrographic and SEM analyses of xenoliths and host basalts revealed no preferred alignment of crystals within the platy zones, suggesting that these schistose textures developed due to rapid magma cooling and subsequent freeze-thaw action rather than tectonic shearing. The basalts display typical ferrimagnetic susceptibilities (average: 3.24 × 10-3 SI), whereas the volcanogenic sediments exhibit low paramagnetic susceptibility (0.38 × 10-3 SI), indicating rapid magma quenching during fragmentation, which is characteristic of subglacial emplacement. Our preliminary results support a subglacial origin for the Sverrefjellet eruptions. Ongoing detailed mapping and thorough magnetic mineralogy analyses, coupled with geochronological and geomorphological studies, will enhance our understanding of subglacial volcanic processes at the extinct Sverrefjellet volcano and more broadly. Additionally, these findings will contribute to a better understanding of the nature and origin of High Arctic Quaternary magmatism and its paleogeographic setting.

Underground hydrogen storage in porous rocks is a promising method to stabilize renewable energy fluctuations. However, data on the geochemical reactivity of hydrogen with reservoir rocks and its potential effects on reservoir performance are limited. This study investigates the geochemical reactivity of hydrogen with Buntsandstein reservoir sandstones from northern Germany, collected at a depth of about 2.5 km. Experiments were performed at 100 degrees C and 150 bar hydrogen partial pressure for four weeks, examining scenarios with dry hydrogen, synthetic saline fluid with hydrogen, synthetic saline fluid with helium (as a control), and an oxidation environment (air). We measured permeability, porosity, magnetic susceptibility, and fluid element concentration before and after the experiments. Results showed no significant mineral changes attributed to hydrogen. Magnetic susceptibility indicated no formation of magnetic minerals, such as magnetite and pyrrhotite. Minor variations in permeability and porosity were attributed to anhydrite dissolution from fluid chemistry nonequilibrium. Overall, our findings suggest hydrogen interactions with Buntsandstein sandstone (no pyrite content) at temperatures up to 100 degrees C do not risk hydrogen loss or reservoir performance degradation.

Large-scale impact events are some of the most catastrophic and instantaneous geological processes in nature, and leave in their wake conspicuous geological structures with characteristic magnetic anomalies. Despite magnetic anomalies in craters being well-documented, their relationship with the magnetic mineral composition of the target and impactites is not always straightforward. Furthermore, the influence of impact shock and post-impact events in the magnetism of natural craters remains elusive. In the Ries crater, Germany, the negative magnetic anomalies are attributed to a reverse polarity remanent magnetization in the impact-melt bearing lithologies. We report new chemical, rock-, and mineral-magnetic data from the shocked basement and impactites, from surface samples, NR73 and SUBO-18 boreholes, and explore how temperature and hydrothermalism may influence the magnetic mineralogy in the crater. We identified shocked, pure magnetite in the basement, and low-cation substituted magnetite in the impactites as the main magnetic carriers. The shocked basement is demagnetized but remains largely unaltered by post-impact hydrothermalism, while the impactites show weak magnetization and are extensively altered by neutral-to-reducing post-impact hydrothermalism. We suggest that the magnetic mineralogy of the demagnetized uplifted basement may contribute significantly to the magnetic anomaly variation, in line with recent findings from the Chicxulub peak-ring.

Large impact craters on Earth are associated with prominent magnetic anomalies, residing in magnetite of the shocked target rocks and impactites. Shock experiments on magnetite suggest that up to 90% of magnetic susceptibility is lost at pressures >5 GPa, but can be partially restored by post-shock thermal annealing. The magnetic property changes are caused by shock induced grain size reduction and fragmentation, as well as domain wall-pinning at crystal lattice defects. A recent study of granitoids from the peak-ring of the Chicxulub crater found that annealing may occur naturally, but can also be overprinted by high-temperature hematite-to-magnetite transformation in non-oxidizing environments. In this study, we isolate the effect of defect annealing and hematite-to-magnetite transformation using the evolution of hysteresis, isothermal remanent magnetization components and first order reversal curve (FORC) diagrams at different high-temperature steps. We used a laboratory-shocked magnetite-quartz ore, a non-shocked naturally oxidized granite, and a naturally shocked and oxidized granite. Our findings suggest that annealing of shock-induced lattice defects partially restores some pre-shock magnetic behavior and causes an apparent average bulk-sample domain state increase. Hematite-to-magnetite transformation creates new fine-grained magnetite that strongly overprints the original signal, and decreases the average bulk-sample domain state. Where annealing and hematite-to-magnetite transformation both occur, the new magnetite masks the annealing-induced property restoration and apparent domain state modification in the shocked magnetite. As magnetite oxidation is a ubiquitous process in surface rocks, these findings are fundamental to understand hematite-to-magnetite transformation as a potential overprint mechanism, and could have broad implications for paleomagnetic interpretations.

Millions of people worldwide are exposed to arsenic (As) contaminated groundwater. Despite decades of research and evidence of As mobilisation in anoxic aquifers being caused by reductive dissolution of iron minerals, the mechanisms behind the local scale variability of dissolved As remains unclear. Therefore, the trans-disciplinary AdvectAs project investigates the environmental behaviour and spatial heterogeneity of dissolved As in groundwater in the Red River delta, Vietnam. Here we present the results from hydrochemical and water isotope investigations. In particular, we will show how the large As variability (0.1–510 µg/L) is related to consecutive As (im)mobilisation steps, depending on site hydrology, geology and the interplay of Fe, Mn, S and organic matter cycles. Such complexity of (im)mobilisation processes can be simplified in 5 major hydro(geo)chemical zones, providing a conceptual tool with potential for application at other sites in Asia affected by geogenic As contamination of groundwater.

Magnetic susceptibility behaviour around the Verwey transition of magnetite (approximate to 125 K) is known to be sensitive to stress, composition and oxidation. From the isotropic point (approximate to 130 K) to room temperature, decreasing magnetic susceptibility indicates an increase in magnetocrystalline anisotropy. In this study, we present a model which numerically analyses low-temperature magnetic susceptibility curves (80-280 K) of an experimentally shocked (up to 30 GPa) and later heated (973 K) magnetite ore. To quantify variations of the transition shape caused by both shock and heating, the model statistically describes local variations in the Verwey transition temperature within bulk magnetite. For the description, Voigt profiles are used, which indicate variations between a Gaussian and a Lorentzian character. These changes are generally interpreted as variations in the degree of correlation between observed events, that is between local transition temperatures in the model. Shock pressures exceeding the Hugoniot elastic limit of magnetite (>= 5 GPa) cause an increase in transition width and Verwey transition temperature, which is partially recovered by heat treatment. Above the Verwey transition temperature, susceptibility variations related to the magnetocrystalline anisotropy are described with an exponential approach. The room temperature magnetic susceptibility relative to the maximum near the isotropic point is reduced after shock, which is related to grain size reduction. Since significant oxidation and cation substitution can be excluded for the studied samples, variations are only attributed to changes in elastic strain associated with shock-induced deformation and annealing due to heat treatment. The shocked magnetite shows a high correlation between local transition temperatures which is reduced by heat treatment. The model allows a quantitative description of low-temperature magnetic susceptibility curves of experimentally shocked and subsequently heat-treated polycrystalline magnetite around the Verwey transition temperature. The curves are accurately reproduced within the experimental uncertainties. Further applications for analysing magnetite-bearing rocks seem possible if model parameters, such as for oxidation are included into the model.

The Chicxulub impact event at ca. 66 Ma left in its wake the only complex crater on Earth with a preserved peak ring, characterized by a well-developed magnetic anomaly low. To date, little is known about its magnetic properties. The joint Integrated Ocean Drilling Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drill core M0077A revealed that the peak ring consists of uplifted and strongly deformed granitoid basement rocks overlain by a 130-m-thick impact melt and suevite layer. Pre- and postimpact hydrothermal systems affected this basement with maximum temperatures up to 450 °C. We used microscopy, mineral chemistry, temperature-dependent magnetic susceptibility, and hysteresis properties to characterize the magnetic mineralogy of pre-, syn-, and postimpact rocks. Compared to its amount of pure, stoichiometric shocked magnetite, the granitoid basement shows low magnetic susceptibility, which is in line with earlier experimental studies indicating that shock reduces magnetic susceptibility. Cation-substituted magnetite with varying compositions in the melt rocks carries a higher induced and remanent magnetization compared to the basement. In the granitoid basement, magnetite was partially oxidized to hematite by a pre-impact hydrothermal event, but at lithological contacts with high-temperature impact melt rock, this hematite was locally retransformed back to magnetite. Elsewhere in the granitoid basement, the temperature reached in the hydrothermal system was too low for hematite retransformation. It was also too low to anneal all the lattice defects in the shocked magnetite, which likely occurs above 540 °C. The presence of shocked magnetite in the granitoid basement well explains the magnetic anomaly low due to its unusually low induced magnetization.

The Lockne impact structure in Sweden formed due to a meteorite impact into the Proterozoic basement rocks in the Ordovician. The dykes of Asby dolerite are part of the target basement and are located outside the crater rim. We investigated the variation in thermomagnetic properties and X-ray diffraction (XRD) spectra of Asby dolerite with distance from the crater centre to find evidence of a potential low shock overprint. Ti-poor, (Ti)-magnetite with dominantly pseudo-single domain behaviour is the main magnetic carrier in the studied samples. Towards the centre of the crater, the Verwey transition temperature (Tv) and Curie temperature (Tc) decrease, whereas alteration indices increase systematically. Higher alteration indices and lower Tv near the crater centre may be owed to more intense fracturing of the silicate minerals which facilitated low-temperature oxidation of (Ti)-magnetite and alteration of mafic dykes by improving fluid circulation in a network of fractures in the silicate minerals. Higher deformation near the crater centre is supported by our XRD results, as the peak position (2θ value) of plagioclase (040), pyroxene (3¯11) and magnetite (311) peaks decrease, while the width of the plagioclase (040) and pyroxene (3¯11) peaks increase towards the centre. This study shows that changes in thermomagnetic properties and lattice parameters of magnetite are a powerful tool to characterise low shock pressure deformation in impacted target rocks.

The Woodfjorden area of northern Spitsbergen (NW Svalbard) offers access to the world’s northernmost onshore thermal springs, extinct Pleistocene alkali basaltic volcanoes and Miocene flood basalts including extensive hyaloclastites. In July 2023, we undertook a 14-day international multi-disciplinary geoscientific expedition to Woodfjorden-Bockfjorden to investigate the Cenozoic geological evolution of the area. The expedition objectives spanned a wide range of scientific topics from sampling of fluids and gas in the thermal springs to constraining the lithosphere by acquiring magnetotelluric data and sampling volcanic rocks. More specifically, we have 1) conducted gas, fluid and travertine sampling at the thermal springs of Gygrekjelda, Jotunkjeldene and Trollkjeldene, 2) mapped and sampled the Quaternary volcanic centers at Sverrefjellet and Halvdanpiggen, 3) sampled the Miocene basalts of the Seidfjellet Formation along seven profiles plus the underlying Devonian sedimentary rocks, 4) acquired magnetotelluric data at 12 stations along both coasts of Woodfjorden and Bockfjorden and 5) collected extensive digital geological data (digital outcrop models and photospheres) using unmanned aerial vehicles (UAVs; also known as drones). The collected samples are currently being analyzed for, amongst others, petrology, geochemistry and geochronology. In this contribution, we report on the expedition’s background, scientific objectives and present selected preliminary results such as field parameters from the thermal springs (temperature, pH, electrical conductivity), magnetic susceptibility of volcanic rocks and digital outcrop models plus photospheres.

Pore space in siliciclastic rocks is one of the most important petrophysical properties in geothermal and hydrocarbon reservoir rock characterization. We used the anisotropy of magnetic susceptibility (AMS) of ferrofluid-impregnated Permo-Triassic sandstones of different Buntsandstein and Rotliegend facies as a proxy for pore space anisotropy and preferred flow direction as a case study for reservoir characterization. We compared the calculated ferrofluid porosity (2–21%) with He porosity (2–26%) and permeability (0.002–214 mD) and described the sediment microstructure using petrographic point-counting analysis. For water- and oil-based ferrofluid impregnation, we observed a positive correlation with He porosity and mass and susceptibility impregnation efficiency were used to control the quality of the impregnation process. Triaxial to oblate magnetic rock fabrics were mostly mimicked by the magnetic pore fabrics, except for some of the water-based ferrofluid impregnated samples, where magnetic ellipsoid shapes changed from oblate to prolate. AMS of the unimpregnated sandstones reflects well defined primary sedimentary to diagenetic fabrics with grain imbrication and cross bedding along with more laminated sedimentary structures. Deviation in ferrofluid-impregnated AMS axes orientation can be related either to the low anisotropy < 1.07 in sandstones from the Lower and Upper Buntsandstein, or the low impregnation efficiency. The mimicry is mostly better when the magnetic susceptibility of the sandstone is higher due to a higher concentration of phyllosilicates while micro-porosity is controlled by the clay fabric. A comparison of sediment petrography with magnetic pore fabrics suggests that the pore space is controlled by the bedding of the sandstones with mostly no preferred flow direction within the bedding plane. Graphical Abstract

In this pilot study, we analyzed the response of magnetic fabrics and X-ray diffraction to low strains (≤ 0.0103) applied during quasi-static and dynamic deformation. Four cylindrical samples of Maggia gneiss were deformed under uniaxial compression oriented parallel to the foliation of the gneiss. Out of these four, two cylinders were deformed at dynamic strain rates (> 50/s) with a split Hopkinson pressure bar and two at quasi-static strain rates (< 10–4/s) using a hydraulic press. From each deformed cylinder, five to six specimens were retrieved. Under the polarizing microscope, the original samples and those deformed in the laboratory do not show any difference in microstructures. However, X-ray diffraction reveals gradual straining of the quartz and biotite lattice with increasing experimental strain. Moreover, after the experimental deformation, the maximum and intermediate principal magnetic susceptibility axes (K1 and K2) form a girdle, which is clearly different from the triaxial distribution in the undeformed samples. The corrected degree of anisotropy and the oblateness of the magnetic fabrics increase. The magnitude of the intermediate susceptibility axis (K2) increases, and of the minimum susceptibility axes (K3) decreases. In this pilot study, we are able to show that XRD and magnetic fabrics are very sensitive to strain and capable of recording even low strains which do not present any other apparent evidence of deformation. With further studies, XRD can be established as a practical technique for measuring low mechanical strain in rocks.

In this study a modus operandi to investigate site-specific nanostructures in thin films (lamellae) excavated “in-plane” across (sub)grain boundaries is presented. This is done by discussing the case of a magnetite grain hosted in a thin section of banded iron formation (Norway) that is prepared parallel to the kinematic reference frame (XZ section of the strain ellipsoid). SEM-EBSD analysis reveal that the magnetite grains do not develop a strong crystallographic preferred orientation, although individual grains are strained and show evidence of intracrystalline deformation in form of low angle grain boundaries (LAGB's). Two “in-plane” lamellae using focused ion beam (FIB) technique are excavated from a magnetite grain in the kinematic reference frame, and nanostructures are studied along three LAGB's using high resolution transmission electron microscopy imaging followed by Fourier transformation (FT), inverse FT and estimation of dislocation densities. Our data establish an empirical relationship for the studied LAGBs, namely, the smaller the angle between LAGB and X-direction, the larger are the shear strain and dislocation density. This relationship is validated from numerical simulations of viscoplastic deformation and dynamic recrystallisation of polycrystalline aggregates of halite, which is also a cubic mineral analogous to magnetite. In addition to the site-specific “in-plane” FIB lamella information, this study also shows that in a deformed mineral the different orientations of the LAGB compared to the principal strain axes show a different dislocation density. This approach of full tracking of the extension direction (X) from the macroscopic to the nano-scale could play an important role in forward modelling of microstructure evolution in future studies.

Abstract Most pyroclastic deposits of Popocatépetl volcano were emplaced at high temperatures and have similar mafic to more evolved compositions, suggesting a long‐lived, interconnected magma environment. We performed a magnetic and microscopic study on different eruptive sequences <14 ky in age and found that temperature and field dependence of magnetic susceptibility are suited to separate eruption phases. We observed homogeneous titanomagnetite with Curie temperatures (TC) of 50–200°C and 200–400°C, together with different amounts of oxy‐exsolved titanomagnetite with TC ∼ 570°C. Some block‐and‐ash flow deposits show remarkably irreversible TC in heating and cooling branches with a positive ΔTC (TC heating–TC cooling) of up to 130°C in the center. The central part of this sequence is characterized by decreasing magnetic susceptibility and low field dependence of magnetic susceptibility (<10%), which is atypical for ulvöspinel‐rich titanomagnetite. The nonreversibility of heating and cooling runs measured with rates of around 10 K/min is probably related to vacancy‐enhanced nanoscale chemical clustering, which seems to occur preferentially during rapid quenching, possibly combined with subtle maghemitization. In contrast, pumice layers have the highest field dependence (∼20%) and contain Ti‐rich and intermediate titanomagnetite with TC < 100 and ∼300°C, which are in line with mafic and more evolved magma composition. In intermediate phases, irreversibility of TC is more common but with a relatively low ΔTC of ±20°C. We suggest that magneto‐mineralogy in pyroclastic density currents is complex but offers a complementary tool to the paleomagnetic directional analysis for emplacement temperature and contribute information on the volcanic material history and their emplacement conditions.

SUMMARY Cyclic loading at elevated temperatures occurs either naturally during tectonic or volcanic-induced earthquakes or can be human-induced due to various geological engineering activities. The aim of this study is to test if mechanical fatigue in rocks can be monitored by magnetic methods. For this purpose, the effect of cyclic-mechanical loading (150 ± 30 MPa) on the magnetic susceptibility and its anisotropy of a magnetite-bearing ore with varying temperatures (400 and 500 °C) and environment (air and vacuum) was investigated. Our study shows that magnetic susceptibility decreases significantly (up to 23 per cent) under air conditions and in vacuum (up to 4 per cent) within the first ca. 1000 cycles. Further loading does not significantly affect the magnetic susceptibility which then remains more or less constant. The decrease of susceptibility parameters is stronger at 500 °C compared to 400 °C under both experimental conditions. Magnetic susceptibility was always measured after decompression of the loaded sample at room temperature so that magnetostriction can be excluded as a reason for these changes. The higher the temperature at which samples were loaded the more pronounced is the oxidation of magnetite to haematite. The transformation of magnetite into haematite under ambient conditions is the most important mechanism influencing bulk magnetic properties. The weak changes in magnetic susceptibility after vacuum loadings are probably caused by intragranular microcracks formed on the surface of magnetite grains. These surface deformation structures are accompanied by the refinement of magnetic domains, which is observed by magnetic force microscopy. Bulk magnetic grain size modifications are also confirmed by hysteresis parameters as well as by the increasing Hopkinson peak ratios determined from magnetic susceptibility measurements over Curie point. The degree of magnetic anisotropy and shape factor only change for the air-treated samples and are therefore related to the haematite formation and not to irreversible ductile deformation in magnetite. Our experimental study shows that cyclic loading can change significantly the magnetic properties of a rock due to mineral transformation below < 1000 cycles and that the first stages of mechanical fatigue, which are a precursor of the failure of rock, are closely associated with these transformations.

Fe(III) minerals play a crucial role for arsenic (As) mobility in aquifers as they usually represent the main As-bearing phases. Microbial reductive dissolution of As-bearing Fe(III) minerals is responsible for the release of As and the resulting groundwater contamination in many sites worldwide. So far, in most studies mainly abiogenic iron minerals have been considered. Yet, biogenic minerals that possess different properties to their abiogenic counterparts are also present in the environment. In some environments they dominate the iron mineral inventory but so far, it is unclear what this means for the As mobility. We, therefore, performed an in-situ aquifer Fe(III) minerals exposure experiment i) to evaluate how different biogenic and abiogenic Fe(III) minerals are transformed in a strongly reducing, As-contaminated aquifer (25 m) compared to As-free moderately reducing aquifer (32 m) and ii) to assess which microbial taxa are involved in these Fe(III) minerals transformations. We found that higher numbers of bacteria and archaea were associated with the minerals incubated in the As-contaminated compared to the non-contaminated aquifer and that all Fe(III) minerals were mainly colonized by Fe(III)-reducing bacteria, with Geobacter being the most abundant taxon. Additionally, fermenting microorganisms were abundant on minerals incubated in the As-contaminated aquifer, while methanotrophs were identified on the minerals incubated in the As-free moderately reducing aquifer, implying involvement of these microorganisms in Fe(III) reduction. We observed that biogenic Fe(III) minerals generally tend to become more reduced and when incubated in the As-contaminated aquifer sorbed more As than the abiogenic ones. Most of abiogenic and biogenic Fe(III) minerals were transformed into magnetite while biogenic more crystalline mixed phases were not subjected to visible transformation. This in-situ Fe(III) minerals incubation approach shows that biogenic minerals are more prone to be colonized by (Fe(III)-reducing) microorganisms and bind more As, although ultimately produce similar minerals during Fe(III) reduction.

High arsenic (As) concentrations in groundwater are a worldwide problem threatening the health of millions of people. Microbial processes are central in the (trans)formation of the As-bearing ferric and ferrous minerals, and thus regulate dissolved As levels in many aquifers. Mineralogy, microbiology and dissolved As levels can vary sharply within aquifers, making high-resolution measurements particularly valuable in understanding the linkages between them. We conducted a high spatial resolution geomicrobiological study in combination with analysis of sediment chemistry and mineralogy in an alluvial aquifer system affected by geogenic As in the Red River delta in Vietnam. Microbial community analysis revealed a dominance of fermenters, methanogens and methanotrophs whereas sediment mineralogy along a 46 m deep core showed a diversity of Fe minerals including poorly crystalline Fe (II/III) and Fe(III) (oxyhydr)oxides such as goethite, hematite, and magnetite, but also the presence of Fe(II)-bearing carbonates and sulfides which likely formed as a result of microbially driven organic carbon (OC) degradation. A potential important role of methane (CH4) as electron donor for reductive Fe mineral (trans)formation was supported by the high abundance of Candidatus Methanoperedens, a known Fe(III)-reducing methanotroph. Overall, these results imply that OC turnover including fermentation, methanogenesis and CH4 oxidation are important mechanisms leading to Fe mineral (trans)formation, dissolution and precipitation, and thus indirectly affecting As mobility by changing the Fe-mineral inventory.

Iron minerals are the most important arsenic host in As-contaminated deltaic sediments. Arsenic release from Fe minerals to groundwater exposes millions of people worldwide to a severe health threat. To understand the coupling of Fe mineralogy with As (im)mobilization dynamics, we analyzed the geochemistry and mineralogy of a 46 m long sediment core drilled into the redox transition zone where a high As Holocene aquifer is juxtaposed to a low As Pleistocene aquifer in the Red River delta, Vietnam. We specifically concentrated on mm- to cm-scale redox interfaces within the sandy aquifer. Various Fe phases, such as Fe- and Mn- bearing carbonates, pyrite, magnetite, hematite and Fe-hydroxides (goethite, lepidocrocite) with distinct As concentrations were identified by a combination of high-resolution microscopic, magnetic and spectroscopic methods. The concentration of As and its redox species in the different Fe-minerals were quantified by microprobe analysis and synchrotron X-ray absorption. We developed a conceptual model integrating Fe-mineral transformations and related As (im)mobilization across the redox interfaces. Accordingly, As is first mobilized via the methanogenic dissolution of Fe(III) (oxyhydr)oxide mineral coatings on sand grains when reducing groundwater from the Holocene aquifer intruded into the Pleistocene sands. This stage is followed by the formation of secondary Fe(II)-containing precipitates (mainly Fe- and Mn-bearing carbonates with relatively low As < 70 mu g/g), and minor pyrite (with high As up to 5800 mu g/g). Due to small-scale changing redox conditions these Fe(II) minerals dissolve again and the oxidative behavior of residual Fe(III)-phases in contact with the reducing water leads to the formation of abundant Fe(III)/Fe(II) (oxyhydr)oxides especially at the studied redox interfaces. Microcrystalline coatings and cementations of goethite, magnetite and hematite have intermediate to high As sorption capacity (As up to 270 mu g/g) creating a key sorbent responsible for As (im)mobilization at interfingering redox fronts. Our observations suggest a dynamic system at the redox interfaces with coupled redox reactions of abiotic and biotic origin on a mm to cm-scale. In a final stage, further reduction creates magnetite with low As sorption capacity as important secondary Fe-mineral remaining in reduced gray Pleistocene aquifer sands while considerable Fe and As is released into the groundwater. The presented redoxdependent sequence of Fe phases at redox interfaces provides new insights of their role in As (im)mobilization in reducing aquifers of south and southeast Asia. (C) 2021 Elsevier Ltd. All rights reserved.