Fine-crystalline, fabric-preserving dolostones in deep-time successions are difficult to reconcile with hightemperature burial models, suggesting the existence of a low-temperature formation pathway capable of overcoming both the kinetic hydration barrier of Mg2+ and the thermodynamic miscibility gap separating calcite from ordered dolomite. Here, we demonstrate a kinetically favourable route to self-assembling dolomite driven by the synergy of manganese redox cycling and carboxyl functionalization. Using a bio-inspired electrochemical reactor, we show that electrochemical valence-state modulation selectively regulates Mn2+ co-precipitation with dolomite reactants. Unlike inorganic controls where manganese is rapidly sequestered into non-templating phases, the functionalized system transiently stabilizes reactive Mn(III) intermediates. This sustains redox cycling and prevents irreversible oxide immobilization, which templates the nucleation of spheroidal, metastable magnesian-kutnahorite. Nanostructural characterization reveals a core-shell architecture where this metastable, isostructural precursor serves as a lattice-distorted scaffold, enabling the rapid heteroepitaxial growth of substitutionally disordered manganoan dolomite cortices. Mechanistically, localized acidity from redox cycling triggers a "proton-driven cation pump", actively releasing Mg2+ (and Ca2+) from the functionalized hydrogel reservoir to the mineralization front. This electrochemical route offers an extrapolable geological framework that links the massive fabric-retentive dolostones of the Precambrian to ancient redox-stratified shallow oceans, while explaining their punctuated scarcity in the Phanerozoic as a consequence of global oxygenation decoupling the manganese redox shuttle from shallow-marine environments.

This study aims to enhance the understanding of geomicrobiological processes and low-temperature dolomite formation, a field significantly developed by Judith A. McKenzie1. It focuses on shallow burial diagenetic dolomite from the onset of the Miocene Climatic Optimum (MCO, 16.9-14.7 Ma)2. Our investigation entailed examining interstitial ferroan and calcian dolomite in claystone (dolomitic mudstone), and integrates bulk-rock stable isotopes (C, N, Δ47 and Δ48) and elemental concentration analyses. The mineral is partially ordered (mean I(015)/I(110)=0.42), microcrystalline (2 to 12 μm) and predominantly subhedral (planar-s). Isotopic data revealed it formed under substantial benthic microbial activity, as evidenced by δ15N values suggestive of sustained N2 losses (+8.59 ± 2.51 ‰, median 9.50 ‰, N=19)—as typically observed in sedimentary settings featuring high rates of denitrification and anammox. Dolomite δ¹³C values (+1.41 to +11.44 ‰, median 7.58‰, N=19) record a mixture of dissolved inorganic carbon sources, dominated by methanogenic CO2. In the ca. 70 m-thick, partially eroded lacustrine succession, conspicuous correlation between dolomite abundances and bulk-rock potassium and barium levels provides evidence of episodic increases in chemical weathering during the warm and humid MCO climate2. The results reveal complex causal interactions linked to fluctuating pore-water dolomite precipitation potentials. Accordingly, Miocene oscillations in pCO2 levels accelerated silicate weathering in catchment areas dominated by alkaline igneous bedrocks, including trachybasalt3, K-rich peridotite and granitoids, thus enriching nearby prevalently anoxic rift paleolake with dolomite-ankerite reactants (i.e., reducible Fe3+, Mg2+ and Ca2+)3 and macronutrients (e.g., iron(III) oxide-bound PO43-). Overall, these sedimentary dynamics, coupled with the influx of soil-derived oxidized nutrients, enhanced benthic ferric iron-based respiration and the sediment redox buffering capacity, which was conducive to punctuated, interstitial dolomite cementation. The dolomite-bearing claystone levels, characterized by Post-Archean Shale-normalized positive europium anomalies, challenge traditional hydrothermal interpretations of dolomitization in rift lakes. Their coupled isotope values (∆47 and ∆48, Bernecker et al., 2024: EGU24-11056) point to formation at temperatures below 40°C, and in a shallow-burial diagenetic realm where the pore fluids and dolomite interacted within a closed system4. Climate variability during the onset of the MCO, along with changes in precipitation and weathering regimes, likely played a fundamental role in establishing the internal boundary conditions necessary for lacustrine dolomite formation.References1. Vasconcelos, C., McKenzie, J., Bernasconi, S. et al. Nature 377, 220–222 (1995).2. Kříbek, B., et al. J. Paleolimnol. 58, 169–190 (2017).3. Rapprich, V. et al. Depos. Rec. 9, 871–894 (2023).4. Staudigel, P., et al. Geochemistry, Geophysics, Geosystems 24, e2023GC011117 (2023).

Lake Medard (LM), a post-mining lake in the Czech Republic with stratified, sulfate-and iron-rich bottom waters, serves as a natural laboratory to study sediment-water-interface (SWI) dynamics where steep aqueous redox gradients are present. This study investigates the interplay of redox conditions, microbial activity, and sedimentary processes, revealing that short-term Eh fluctuations (80-100 mV) in the bottom water significantly mobilize rare earth elements (REE) and influence the partitioning of other redox-sensitive elements such as vanadium (V) and arsenic (As) from reactive iron (Fe)-and manganese (Mn)-oxyhydroxides during early diagenesis. While carbonate phases like siderite primarily retain their REE signatures, they can incorporate REE released during these redox shifts. Spectroscopic analyses confirms the presence of FeOOH polymorphs (goethite and lepidocrocite) in organo-mineral aggregates in the upper sediments. Sequential extractions shows that under stronger reducing conditions (Eh approximate to -190 mV), As predominantly associates with carbonates, shifting to Fe(III)-oxyhydroxides at higher Eh (approximate to-80 mV). Isotope analyses (813C) indicate that the bulk sediment carbonate is detrital, sourced from Miocene strata. Authigenic pyrite in LM sediments exhibits 834Spy values (-35.1 to-23.0%), reflecting microbial sulfate reduction. However, the accumulation of the byproduct sulfide, and thus pyrite stabilization, is limited by the low availability of labile organic substrates and the reoxidation of sulfide by Fe(III)-oxyhydroxides. These findings highlight the sensitivity of geochemical signals in sediments to subtle redox shifts and improve our interpretation of ancient deposits formed under dynamic water column redox conditions.

ABSTRACTThe strata encompassing the Permian–Triassic boundary interval capture a pivotal period in Earth's history, with significant changes in Phanerozoic Earth system dynamics, culminating in a severe mass extinction. In carbonate platforms, this boundary is marked by a shift from skeletal to microbial carbonate production. Whereas extensive research has focused on the End‐Permian Mass Extinction in open‐marine shelf environments, the transition within inner platform facies remains underexplored due to limited dating options and pervasive dolomitization. This study examines the Permian–Triassic boundary interval at the continuous dolostone, Brušane‐Sy section, in the External Dinarides (Croatia), that retains much of its original fabric. High‐resolution petrography, biostratigraphy and chemostratigraphy (δ13Ccarb and δ13Corg) were utilized to detail sedimentary responses across the boundary. The Upper Permian fine‐crystalline dolostone features well‐preserved cryptomicrobial/bioclastic, peritidal microfacies with calcareous algae and foraminifera. In contrast, the Lower Triassic dolostone, shows a transition to a medium‐crystalline, fabric‐destructive dolostone texture. The transition from fabric‐retentive Permian to fabric‐destructive Triassic dolostone is attributed to two dolomitization processes: (i) Late Permian transgression facilitating aragonite/high Mg‐calcite deposition, later transforming neomorphically into fabric‐retentive dolostone texture due to abundant precursor dolomite nuclei; and (ii) dispersed Early Triassic primary dolomite precipitation later stabilized during shallow burial with decaying microbial mats serving as loci for crystal growth but decreased nucleation. This shift is recorded by a minimal negative δ13Ccarb excursion (≤0.7‰) and a more pronounced shift in Δ13C (δ13Ccarb – δ13Corg; ca 4.6‰). Contrasting with typical open‐marine Permian–Triassic boundary excursions, such isotopic features reflect the localized shift in primary production to photoautotrophy (algae and cyanobacteria) and early dolomitization in the presence of seawater‐derived dissolved inorganic carbon. Understanding these sedimentary and diagenetic dynamics provides crucial insights into environmental changes and biogeochemical cycles affecting Permian–Triassic boundary dolomitization, offering a comprehensive view of the End‐Permian Mass Extinction across a wider range of shallow marine carbonate dominated depositional environments.

This study presents an integrated hydrological-hydrochemical approach to quantify reactive nitrogen (N-R) cycling in temperate mountain catchments. It employs stable isotope analyses (delta H-2, delta O-18 in water, delta N-15 in NH4+, and NO3- and delta O-18 in NO3-) to resolve interactions between water flow and N transformations. A two-component runoff model reveals groundwater as the dominant discharge contribution (75%-90%), with 10%-25% derived from rapidly infiltrating soil water-highlighting a swift hydrological response to precipitation. In addition, this work quantifies all N mineralization in vadose zone and denitrification in groundwater and their seasonal variation within a well-studied network of N-saturated temperate forests (i.e., the Czech GEOMON Network). Our results show that increased precipitation infiltration diminishes microbial N production and N-2 losses, but maximizes catchment N-R exports. Direct input of atmospheric N-R to runoff was recorded during a short period of spring snow melting only. Denitrification calculated from N-15 fractionation of NO3- in soil and groundwater accounts for 15%-24% of total mineralized N (N loss is 0.6-1.6 kg N ha(-1) year(-1)), with precipitation shifts markedly influencing N-R outflows. This framework enhances predictions of climate change impacts on nutrient transport and water quality in mountain catchments, which are critical water sources for ecosystems and human use. Overall, application of this approach can offer key insights for mitigating ecological risks from increased N-R mobilization, especially under rising atmospheric N deposition and global warming effects.

Records from the Miocene il-Blata section in Malta offer insights into the depositional environments of the Central Mediterranean following an Early Miocene restriction of the Mesopotamian Seaway (c. 20 Ma). Inorganic and organic stable carbon isotope values suggest relatively steady depositional environments, whereas authigenic iron dolomite abundances exhibiting substantial delta O-18 and delta C-13 variations at a sequence scale indicate dynamic sedimentation conditions leading to differential diagenesis. Petrographically, the dolomitic levels exhibit matrix-selective dolomitization, occasional silicification and phosphatization, and textural indicators pointing to subsurface microbial influences. These features collectively point to complex shallow burial diagenesis. In addition, the presence of framboidal pyrite and gypsum infilling foraminiferal chambers, along with the absence of large planktonic foraminifera, suggests the development of palaeoenvironmental stress imposed by a density-stratified water column affecting the pore waters. Towards the top of the studied succession, a shift from organic to siliceous deposits reflects water column perturbations possibly linked to changes in oceanic circulation associated with a temporary re-opening of the Mesopotamian Seaway. This study not only underscores the hydrochemical controls exerted by North African terrigenous fluxes over the Mediterranean, but also highlights the intricate interplay between shifting depositional environments and shallow burial diagenetic processes in shaping the geochemical and textural fabrics of authigenic mineral assemblages.

Electroactive microorganisms are pivotal players in mineral transformation within redox interfaces characterized by pronounced oxygen and dissolved metal gradients. Yet, their systematic cultivation from such environments remains elusive. Here, we conducted an anodic enrichment using anoxic ferruginous waters from a post-mining lake as inoculum. Weak electrogenicity (j = ∼5 µA cm-2) depended on electroactive planktonic cells rather than anodic biofilms, with a preference for formate as electron donor. Addition of yeast extract decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition but mainly comprised of sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum spp. and Stenotrophomonas spp.). A secondary enrichment strategy resulted in different bacterial communities composed of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium spp.). Secondary electron microscopy and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface, presumably impeding current production. Our findings indicate that (i) anoxic waters containing geogenically derived metals can be used to enrich weak electricigens, and (ii) it is necessary to specifically inhibit sulfate reducers. Otherwise, sulfate reducers tend to dominate over EAM during cultivation, which can lead to anode passivation due to biomineralization.

Investigating microorganisms in metal-enriched environments holds the potential to revolutionize the sustainable recovery of critical metals such as lanthanides (Ln3+). We observe Hyphomicrobium spp. as part of a Fe2+/Mn2+-oxidizing consortia native to the ferruginous bottom waters of a Ln3+-enriched lake in Czechia. Notably, one species shows similarities to recently discovered bacteria expressing proteins with picomolar Ln3+ affinity. This finding was substantiated by developing an in-silico ionic competition model and recombinant expression of a homolog protein (Hm-LanM) from Hyphomicrobium methylovorum. Biochemical assays validate Hm-LanM preference for lighter Ln3+ ions (from lanthanum to gadolinium). This is comparable to established prototypes. Bioinformatics analyses further uncover additional H. methylovorum metabolic biomolecules in genomic proximity to Hm-LanM analogously dependent on Ln3+, including an outer membrane receptor that binds Ln3+-chelating siderophores. These combined observations underscore the remarkable strategy of Hyphomicrobium spp. for thriving in relatively Ln3+ enriched zones of metal-polluted environments. Microorganisms in a metal-enriched post-mining lake reveal the potential of Hyphomicrobium spp. for lanthanide recovery. A H. methylovorum protein shows strong affinity for light lanthanides,providing insights into microbial metal uptake strategies.

Abstract Current biomining alternatives focus on exploiting metal-binding proteins with an exceptional affinity for lanthanides (Ln3+). Herein, we developed an in-silico competition model for ionic binding indicating the α-proteobacterium, Hyphomicrobium methylovorum, possesses a protein that potentially binds Ln3+ more efficiently than previous homologous protein studies demonstrate. Data regarding microbial receptors mediating acquisition of Ln3+ for intracellular transport, however, remain scarce. We therefore determined the in-silico binding capacity of an H. methylovorum outer membrane receptor for a chelating siderophore of Ln3+. These in-silico results directed us to examine the microbiome of a former coal mine, now metal-polluted lake in Czechia, where we identified twelve distinct Hyphomicrobium spp. indigenous to the bottom ferruginous waters. As consortia members of Fe(II)-/Mn(II)-oxidizers within waters often enriched in Ln3+, our findings suggest that select Hyphomicrobium possess a sufficient molecular armament for scavenging Ln3+. Hyphomicrobium scavenging capacity of Ln3+ can complement and diversify current alternatives implementing biotechnological mining methods.

Abstract Late Oligocene (ca 25 Ma) volcano‐sedimentary successions exposed on the western periphery of the Doupovské Hory Volcanic Complex reveal a complex sedimentation history influenced in various ways by decay of the alkali basalt volcanic edifice. Weathering of the volcanic rocks supplied abundant reactants that promoted carbonate precipitation in the peripheral palaeolakes—as evidenced by strongly non‐radiogenic 87Sr/86Sr values (0.7038–0.7041). On the other hand, the sediments of the initial shallow lake became deformed by the bulldozing effect of a debris avalanche. The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano. At this stage, surface water influx from the surrounding granites conferred a more radiogenic character (87Sr/86Sr values 0.7046–0.7049) to the calcrete deposits. Fossil assemblages as well as limestone textures suggest significant seasonal water‐level fluctuations, possibly reflecting the alternating rainy and dry‐seasons of a prevalently humid Central‐European Late Oligocene climate. The seasonal drying out of the ponds resulted in significant 18O enrichments. Although the ca 0‰ δ13C values might suggest mixing of atmospheric and volcanic CO2 during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity. The lower δ13C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present. This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non‐radiogenic Sr isotopic ratios (87Sr/86Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates. Contrary to the notion of carbonatites being present when highly non‐radiogenic Sr isotopes are found, these results show that Sr isotopes in carbonates formed in alkali basalt‐sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.

The sources of airborne particulate matter (PM10) emissions in Ostrava, Czech Republic, were investigated. Emphasis was placed on their organic carbon (OC) and elemental carbon (EC) contents, and their carbon stable isotope composition, 813C. Emission sources were identified using OC-613C and concentration values. To track the extent of long-term deposition, these sources were also identified using the black carbon (BC) 613C values of soil samples. At all sampling sites, wind flow is predominantly (65-80%) bidirectional in either SW-NE or NE-SW trajectories. Source apportionment along these dominant airflow trajectories was calculated from an isotopic 13C mass balance, and according to differences in the OC content and 613C values of PM10. Determined emission sources are: (i) combustion of Silesian hard coal (813C =-24.5%o); (ii) local Ostrava coal combustion (613C =-25.5 to-26%o), automotive emissions (613C =-26.5%o), and biogenic particles (613C =-28 to-28.5%o). Winter emissions (mean OC concentrations from 12 to 25 mu gm- 3) originated mostly from coal combustion (80%) in domestic and industrial point sources. Differences were ascribed to automotive emissions. Ostrava is located near the Czech-Polish border, transboundary emissions are transported under a southbound wind flow that transported from 40 to 80% of the collected PM10. Summer emissions were lower (mean OC concentration from 6 to 8 mu g m-3). Automotive emissions accounted for up to 40%, whilst biogenic production accounted for 60%. Absence of 13C isotope data of secondary OC (SOC 1.2-1.5 mu gm- 3) increases uncertainty in our source apportionment in summer; when SOC could comprise >20% of total OC. Contribution of SOC to the winter-measured OC is much lower (5-10%), and has no significant effect on mass balances. The upper soil layer analyses revealed long-term deposition of the same emissions sources. We conclude that the stable delta 13C isotope values of OC and EC are useful for discriminating against local sources of PM10 pollution in relatively small urban areas, containing discrete polluting sources. Such a simplified approach can be easily standardized and implemented to manage regulatory compliances in the increasingly commoditized carbon offset market.

Lake Medard is a recently established post-mining lake in the northwest of Czech Republic that displays significant concentrations of dissolved sulfate (dSO42-) and ferrous iron (Fe2+) in its density and redox stratified bottom water column. Siderite-buffered anoxic sediments, also rich in iron(III)-oxyhydroxides, underlie that water column characterized by limited labile organic substrates. This composition sustain a transitional redox state between nitrogenous/ferruginous and euxinic conditions. Our study focuses on the Lake Medard bottom water column elemental concentration profiles, sulfate-sulfur and -oxygen isotope compositions, bioactive ion concentrations, and planktonic microbiome data, combined with mineralogical and isotopic analyses of the upper anoxic sediments. This integrative approach reveals that the internal biogeochemical iron cycling is interlinked with that of nitrogen, sulfur and other redox sensitive metals. Minor seasonal oscillations in the monimolimnion redox potential impact mineral dissolution/(re)precipitation reactions, causing shifts in metal partitioning within anoxic sediments. Carbonate-buffered reactions appear to respond to a subsurface CO2 flux thereby influencing monimolimnial alkalinity and dissolved inorganic carbon concentrations. These hydrochemical modifications shift the sedimentary redox signals, occasionally favoring carbonate over oxyhydroxide metal-binding processes. Our findings address the fate of newly formed sedimentary oxyhydroxides in a transitional redox-stratified water column featuring ferruginous conditions without quantitative sulfate depletion to provide insights on interlinked biogeochemical processes within a concise framework

Soil solution chemistry depends largely on mineralogy and organic matter properties of soil horizons with which they interact. Differing lithologies within a given catchment area can influence variability in soil cation exchange capacities and affect solute transport. Zero-tension and tension lysimeters were used to evaluate the fast transport of solutes in the topsoil vs. slow diffusional matrix flow at the subsoil of three contrasting lithology catchments in a mid-elevation mountain forest. Our aim was to test the feasibility of lysimeters' hydrochemical data as a gauge for legacy subsoil pollution. Due to contrasting lithologies, atmospheric legacy pollution prevailing at the soil-regolith interface is differently yet consistently reflected by beryllium, lead, and chromium soil solution concentrations of the three catchments. Geochemical (dis)equilibrium between the soil and soil matrix water governed the hydrochemistry of the soil solutions at the time of collection, potentially contributing to decreased dissolved concentrations with increased depths at sites with higher soil pH. A complementary isotopic δ18O runoff generation model constrained potential seasonal responses and pointed to sufficiently long water-regolith interactions as to permit important seasonal contributions of groundwater enriched in chemical species to the topsoil levels. Our study also reflects subsoil equilibration with atmospheric solutes deposited at the topsoil and thus provides guidance for evaluating legacy pollution in soil profiles derived from contrasting lithology.

A bioelectrochemical method for arsenite scavenging from anoxic waters by Fe(iii) oxyhydroxides is feasible by boosting the microbial utilization of humic substance derivatives as electron shuttles for Fe mineral stabilization.

Atmospheric lead pollution has adverse health effects on humans. Identification of anthropogenic Pb sources is thus crucial for understanding of pollution-related risks and for formulation of efficient abatement strategies. We analyzed concentrations and isotope ratios of Pb in fine air-borne particulate matter (PM2.5) in three Central European cities. Aerosol sampling in 12-h intervals was performed during summer and winter in Hradec Kr & PRIME;alove & PRIME;, Olomouc and Brno, regional centers of highly industrialized Czech Republic, each with a population of more than 90 thousand. Lead analysis was complemented with concentration measurements of Zn, Cu, Cd, As, Sb, and S. Trace element and PM2.5 concentrations were higher in winter than in summer in all studied cities, on average by 47%. The overall mean concentration of PM2.5 in urban air was 21 & mu;g m  3, similar to that in Bern (Switzerland) and Vienna (Austria). Across the sites, 206Pb/207Pb ratios ranged from 1.142 to 1.178 in summer and from 1.143 to 1.173 in winter. The mean 206Pb/207Pb ratios were statistically indistinguishable in all three studied cities (1.165 in Hradec Kr & PRIME;alove & PRIME;, 1.163 in Olomouc, and 1.160 in Brno). In the 206Pb/207Pb vs. 208Pb/207Pb graph, Pb isotope composition of summer samples formed a straight line, whereas Pb isotope composition of winter samples was shifted toward higher 208Pb/207Pb ratios. A Pb isotope inventory of regional pollution sources indicated that winter-time Pb pollution was not caused solely by household heating and increased electricity production in coal-burning power plants. Recycling of industrial Pb originating from Variscan ores and waste incineration could have shifted the 208Pb/207Pb ratio of PM2.5 to higher values, however, such sources do not emit more Pb in winter than in summer. Remobilization of legacy alkyl-Pb from gasoline additives and Pb emissions from current unleaded gasoline and diesel could have shifted both Pb isotope ratios to lower values.

<p>The eastern Zagros Fold and Thrust Belt (ZFTB) in Iran includes a salt tectonic province with roughly 130 salt-gypsum diapirs emerging within the Neoproterozoic-Early Cambrian Hormuz Complex. The diapirs in the ZFTB differ in composition and in their distribution of exposed caprock mélanges (CRMs). Although there are numerous studies focused on the geochronology and geochemistry of igneous rocks as exotic blocks associated with CRMs, the geochemistry, and petrography of carbonates remain to be systematically investigated. The Paskhand Diapir is a unique little diapir with no visible salt rock at the surface. Its CRMs consist of massive and layered gypsum, carbonate, marlstone, and siltstone,  which are associated with diabase exotic blocks. The carbonate paragenesis is being examined. A grey fine crystalline  dolomite is considered to have originated early during diagenesis from a Neoproterozoic marine environment. Other carbonates can be distinguished on the basis of their microspar, and spar cements. In general, the major minerals are dolomite and calcite, with quartz and iron oxides being in minor abundance. Important trace minerals are pyrite, sphalerite, talc, mica, K-feldspar, malachite, bassanite, rutile, chlorite, and apatite. Their abundance in mineral assemblages is variable, also depending on the locality within the diapir. Later-stage calcitic veins frequently cross-cut through micritic and microspar cemented lithologies. Lithological mapping shows that the edge of the diapir commonly exhibits a greater variety of mineralization modes with extensive recrystallization as compared with its core. The <em>δ</em>13C values of dolomite range from –7.0 to +2.7 ‰ V-PDB. This range indicates that seawater was the principal source of reactants for dolomite precipitation, although with some inorganic carbon derived from organic matter oxidation. The <em>δ</em>18O values of dolomite range from –0.55 to –13.13‰ V-PDB, reflecting a temperature fractionation effect. The carbonate formation temperatures of the Hormuz complex (both veins and host rock) were determined for the first time by using the Δ47 (paleo)thermometer in dolomite. Δ47 values range between 0.422 ± 0.015 and 0.287 ± 0.015 ‰, indicating diagenetic closure temperatures of between 116.4 ± 11.7 and 271.2 ± 32.5  ºC. An intensive interaction of hydrothermal fluids with the host rock during localized carbonate recrystallization is thus evidenced.</p> <p>These results show that a correct interpretation of the mechanism(s) of carbonate alteration is critical for reconstructing the history of diapirism in the area. We hypothesize that carbonates in CRMs were reworked through a series of events largely influenced by thermochemical sulfate reduction (TSR) at T ≥ 110 ºC. </p> <p> </p> <p> </p>

In the oligotrophic bottom waters of a post-mining lake (Lake Medard, Czechia), ferruginous conditions occur without quantitative sulfate depletion. The dissolved organic matter supply to the deep waters is small and, accordingly, sulfate reduction promoting precipitation of stable ferrous sulfides is limited. In line with these observations, an isotopically constrained estimate of the rates of planktonic sulfate reduction (SRR) suggests that despite a high genetic potential—as determined by genome analyses, SRR are limited by substrate competition exerted by nitrogen and iron respiring prokaryotes. The microbial succession across the nitrogenous and ferruginous zones of the bottom water column also indicates a sustained genetic potential for chemolithotrophic sulfur oxidation, probably accompanied by disproportionation of S intermediates[1]. The bottom waters displayed dissolved Fe concentrations (~0.1 to 33 µM) and δ56Fe values (-1.77 ± 0.03 ‰ to +0.12 ± 0.05 ‰) that increase across the redoxcline and towards the anoxic sediment-water interface (SWI). These parameters pinpoint diffusive transport and partial oxidation of dissolved ferrous iron (Fe(II)) sourced from the lakebed, depletion of the residual Fe(II) in heavy isotopes at the redoxcline and enrichment near the SWI linked to monosulfide precipitation. In the carbonate-buffered lake sediments, however, sulfur re-oxidation appears to prevent substantial stabilization of iron monosulfides as pyrite, but it enables the interstitial precipitation of small proportions of equant microcrystalline gypsum. This gypsum isotopically fingerprints sulfur oxidation proceeding at near equilibrium with the ambient anoxic waters, whilst authigenic pyrite-sulfur displays a 38 to 27 ‰ isotopic offset from ambient sulfate, suggestive of incomplete sulfate reduction and indicative of the openness of the system[1]. Overall, our results demonstrate that under transitional redox states producing the meromictic stability described here, the simple biogeochemical zonation models based on energetic considerations of pure phases at standard conditions may not accurately describe the overlapping zonation of dissimilatory iron and sulfur reduction. Vigorous sulfur and iron co-recycling in the water column can be fuelled by ferric and manganic particulate matter and notably by the redeposited siderite stocks of the upper anoxic sediments. In the absence of ferruginous coastal zones today, the current water column redox stratification in the post-mining Lake Medard has scientific value for (i) testing emerging hypotheses on how a few interlinked biogeochemical cycles operated in low productivity nearshore paleoenvironments during transitional states between ferruginous and euxinic conditions; and (ii) to acquire insight on potential avenues for early diagenetic overprinting of redox proxy signals in ferruginous-type sediments. [1] Petrash, D. A., Steenbergen, I. M., Valero, A., Meador, T. B., Pačes, T., and Thomazo, C.: Aqueous system-level processes and prokaryote assemblages in the ferruginous and sulfate-rich bottom waters of a post-mining lake, Biogeosciences Discuss. [preprint], https://doi.org/10.5194/bg-2021-253, in review, 2021.

In the low-nutrient, redox-stratified Lake Medard (Czechia), reductive Fe(III) dissolution outpaces sulfide generation from microbial sulfate reduction (MSR) and ferruginous conditions occur without quantitative sulfate depletion. The lake currently has marked overlapping C, N, S, Mn and Fe cycles occurring in the anoxic portion of the water column. This feature is unusual in stable, natural, redox-stratified lacustrine systems where at least one of these biogeochemical cycles is functionally diminished or undergoes minimal transformations because of the dominance of another component or other components. Therefore, this post-mining lake has scientific value for (i) testing emerging hypotheses on how such interlinked biogeochemical cycles operate during transitional redox states and (ii) acquiring insight into redox proxy signals of ferruginous sediments underlying a sulfatic and ferruginous water column. An isotopically constrained estimate of the rates of sulfate reduction (SRRs) suggests that despite high genetic potential, this respiration pathway may be limited by the rather low amounts of metabolizable organic carbon. This points to substrate competition exerted by iron- and nitrogen-respiring prokaryotes. Yet, the planktonic microbial succession across the nitrogenous and ferruginous zones also indicates genetic potential for chemolithotrophic sulfur oxidation. Therefore, our SRR estimates could rather be portraying high rates of anoxic sulfide oxidation to sulfate, probably accompanied by microbially induced disproportionation of S intermediates. Near and at the anoxic sediment–water interface, vigorous sulfur cycling can be fuelled by ferric and manganic particulate matter and redeposited siderite stocks. Sulfur oxidation and disproportionation then appear to prevent substantial stabilization of iron monosulfides as pyrite but enable the interstitial precipitation of microcrystalline equant gypsum. This latter mineral isotopically recorded sulfur oxidation proceeding at near equilibrium with the ambient anoxic waters, whilst authigenic pyrite sulfur displays a 38 ‰ to 27 ‰ isotopic offset from ambient sulfate, suggestive of incomplete MSR and open sulfur cycling. Pyrite-sulfur fractionation decreases with increased reducible reactive iron in the sediment. In the absence of ferruginous coastal zones today affected by post-depositional sulfate fluxes, the current water column redox stratification in the post-mining Lake Medard is thought relevant for refining interpretations pertaining to the onset of widespread redox-stratified states across ancient nearshore depositional systems.

Abstract Soil solution chemistry depends largely on mineralogy and organic matter properties of soil horizons with which they interact. Differing lithologies within a given catchment area can influence variability in soil cation exchange capacities and affect solute transport. Zero-tension and tension lysimeters were used to evaluate fast transport of solutes in the topsoil vs. slow diffusional matrix flow at the subsoil of three contrasting lithology catchments in a mid-elevation mountain forest. Our aim was to test the feasibility of lysimeters hydrochemical data as a gauge for legacy subsoil pollution. Due to contrasting lithologies, atmospheric legacy pollution prevailing at the soil-regolith interface is differently yet consistently reflected by beryllium, lead, and chromium soil solution concentrations of the three catchments. Geochemical (dis)equilibrium between the soil and soil matrix water governed the hydrochemistry of the soil solutions at the time of collection, potentially contributing to decreased dissolved concentrations with increased depths at sites with higher soil pH. A complementary isotopic data constrained potential seasonal responses and pointed to sufficiently long water-regolith interactions as to permit important seasonal contributions of groundwater enriched in chemical species to the topsoil levels. Our study also reflects subsoil equilibration with atmospheric solutes deposited at the topsoil, and thus provides guidance for evaluating legacy pollution in soil profiles derived from contrasting lithology.

Complexes of organic matter (OM) with iron (Fe) oxyhydroxides (OM-Fe) prevent/reduce microbial degradation and enhance its stabilization in soils and sediments. Missing in most studies regarding the quality of OM-Fe complexes is in this study addressed by optical method characterization using absorbance and fluorescence, which is a unique approach in combination with sequential extraction methods. This study finds the quality of OM which promotes the stability of the OM-Fe aggregates with respect to the rate of redox dissolution of Fe mineral phases. Sediment samples from a mesotrophic freshwater reservoir were sequentially extracted to distinguish Fe oxyhydroxides solubility at different dissolution times. This allowed us to assess the OM-Fe association with easily reducible Fe-bearing minerals vs. more stable yet reducible fractions. The quality of the sequentially extracted DOM associated with different redox reactivity mineral phases was evaluated by UV–Vis and fluorescence spectroscopy using a parallel factor analysis (PARAFAC) model. Humic-like substances render soluble and labile OM-Fe associations, while non-humic substances enhance the stability of such associations under reducing conditions. In anoxic sediments, non-humic substances are stabilized in the OM-Fe associations. This study described the fate of OM-Fe in anoxic sediments by coupling sequential extractions with UV–Vis and fluorescence spectroscopy characterization. This is a new reliable and simple approach for evaluating the stability of OM-Fe aggregates in sediments, soils, and/or wastewater treatment sludge.