Mass spectrometry and natural variations of iron isotopes

Metal stable isotopes fractionation during adsorption

Isotope technology is an ideal tool for tracing the sources of certain pollutants or providing insights into environmental processes. In recent years, the advent of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has enabled the precise measurement of various metal stable isotopes. Due to the presence of "fingerprint" properties in various environmental samples, metal stable isotopes have been applied to distinguish the source of contaminants effectively and further understand the corresponding environmental processes. The environmental fate of metal elements is strongly controlled by adsorption, an essential process for the distribution of elements between the dissolved and particulate phases. The adsorption of metal elements on mineral and organic surfaces significantly affects their biogeochemical cycles in the environment. Therefore, it is crucial to elucidate the fractionation characteristics of stable metal isotopes during the adsorption process. In this review, three typical transitional metal elements were selected, considering Mo as the representative of anionic species and Fe and Zn as the representative of cationic species. For Mo, the heavier Mo isotope is preferentially adsorbed in the solution phase, pH has a more significant influence on isotope fractionation, and temperature and ionic strength are relatively insensitive. Differences in coordination environments between dissolved and adsorbed Mo during adsorption, i.e., attachment mode (inner- or outer-sphere) or molecular symmetry (e.g., coordination number and magnitude of distortion), are likely responsible for isotopic fractionation. For Fe, The study of equilibrium/kinetic Fe isotopic fractionation in aqueous Fe(II)-mineral is not simple. The interaction between aqueous Fe(II) and Fe (hydroxyl) oxides is complex and dynamic. The isotope effect is due to coupled electron and atom exchange between adsorbed Fe(II), aqueous Fe(II), and reactive Fe(III) on the surface of Fe (hydroxyl) oxide. For Zn, the heavier Fe isotope preferentially adsorbs on the solid phase, and pH and ionic strength are essential influencing factors. The difference in coordination environment may be the cause of isotope fractionation.

Role of Source, Mineralogy, and Organic Complexation on Lability and Fe Isotopic Composition of Terrestrial Fe sources to the Gulf of Alaska

Iron (Fe) is a key trace nutrient supporting marine primary production, and its deposition in the surface ocean can impact multiple biogeochemical cycles. Understanding Fe cycling in the subarctic is key for tracking the fate of particulate-bound sources of oceans in a changing climate. Recently, Fe isotope ratios have been proposed as a potential tool to trace sources of Fe to the marine environment. Here, we investigate the Fe isotopic composition of terrestrial sources of Fe including glacial sediment, loess, volcanic ash, and wildfire aerosols, all from Alaska. Results show that the δ56Fe values of glaciofluvial silt, glacial dissolved load, volcanic ash, and wildfire aerosols fall in a restricted range of δ56Fe values from -0.02 to +0.12‰, in contrast to the broader range of Fe isotopic compositions observed in loess, -0.50 to +0.13‰. The Fe isotopic composition of the dissolved load of glacial meltwater was consistently lighter compared to its particulate counterpart. The 'aging' (exposure to environmental conditions) of volcanic ash did not significantly fractionate the Fe isotopic composition. The Fe isotopic composition of wildfire aerosols collected during an active fire season in Alaska in the summer of 2019 was not significantly fractionated from those of the average upper continental crust composition. We find that the δ56Fe values of loess (<5 μm fraction) were more negative (-0.32 to +0.05‰) with respect to all samples measured here, had the highest proportion of easily reducible Fe (5.9-59.6%), and were correlated with the degree of chemical weathering and organic matter content. Transmission electron spectroscopy measurements indicate an accumulation of amorphous Fe phases in the loess. Our results indicate that Fe isotopes can be related to Fe lability when in the presence of organic matter and that higher organic matter content is associated with a distinctly more negative Fe isotope signature likely due to Fe-organic complexation.

Precise measurement of Fe isotopes in marine and biological samples by pseudo-high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS)

This paper introduces an enhanced technique for analyzing iron isotopes in complex marine and biological samples. A dedicated iron purification method for biological marine matrices, utilizing three ion exchange columns, is validated. The MC-ICPMS in pseudo-high-resolution mode determines precise iron isotopic ratios, with sensitivity improved through the DSN-100 desolvating nebulizer system and Apex-IR. Only 2 µg of iron on DSN versus 1 µg on Apex is needed for six replicates (30-60 times improvement) while 10 to 20 µg is required for a single measurement on a wet system considering the resolution power (Rp) is maintained at 11,000-13,000. The Ni-doping method with a Fe/Ni ratio of 1 yields more accurate isotopic ratios than standard-sample bracketing alone. Measurement reproducibility of triplicate samples from marine biological experiments on MC-ICPMS is ± 0.03‰ (2SD) for δ56Fe and ± 0.07‰ for δ57Fe (2SD). This study introduces a novel iron purification process specifically designed for marine and biological samples, enhancing sensitivity and enabling more reliable measurements with smaller sample sizes and reduced uncertainties. It proposes iron isotopic compositions for biological reference materials, offering a valuable reference dataset in diverse scientific disciplines.

Paleo-marine redox environment fluctuation during the early Cambrian: Insight from iron isotope in the Tarim Basin, China

The Ediacaran to Cambrian period is generally considered to be the vital transition in the history of marine redox environment and life evolution on earth. The ocean oxygenation levels during this transition period are still debated. Since iron is widely involved in biogeochemical cycles and undergoes redox cycling both in the seawater and sediments, it has become a significant proxy to reconstruct paleo-marine environment. In order to constrain the paleo-marine redox state in the early Cambrian, the iron isotope composition of bulk rock (δ56FeT) is interpreted combining with iron-speciation, redox sensitive elements and pyrite sulfur isotope (δ34Spy) of Yuertusi Formation in Tarim Block. The δ56FeT values varies from -0.39 ‰ to 0.48 ‰, with an average of 0.07 ‰, mainly controlled by pyrite mineral facies in this study. Based on the mechanism of pyrite generation in different redox condition, it is proposed that the marine environment of the lower Cambrian in the Tarim basin is dominated by anoxic with intermittent euxinic state. The dynamic evolution of redox environment can be divided into three intervals. The gradual decrease of δ56Fe in Interval I indicates the paleo-marine environment changed from anoxic ferruginous to euxinic, and the paleo-marine sulfate reservoir decreased to a limited level, which might be attributed to abundant burial of organic matter and pyrite. For Interval II, δ56Fe values first increase to evident positive because of partial oxidization then decreased to that of seawater (about 0 ‰) due to complete oxidization. In Interval III, the continuous decrease of δ56Fe values infers a sustaining oxidization. In summary, the paleo-marine environment of the lower Cambrian Yuertusi Formation evolved from anoxic ferruginous to euxinic and then oxidized continuous. Iron isotope statistics from geological historical periods indicate that seawater was relatively oxidized after the NOE event but did not reach the oxidation levels of present-day seawater.

Iron isotopic fractionation driven by low-temperature biogeochemical processes

Iron is geologically important and biochemically crucial for all microorganisms, plants and animals due to its redox exchange, the involvement in electron transport and metabolic processes. Despite the abundance of iron in the earth crust, its bioavailability is very limited in nature due to its occurrence as ferrihydrite, goethite, and hematite where they are thermodynamically stable with low dissolution kinetics in neutral or alkaline environments. Organisms such as bacteria, fungi, and plants have evolved iron acquisition mechanisms to increase its bioavailability in such environments, thereby, contributing largely to the iron cycle in the environment. Biogeochemical cycling of metals including Fe in natural systems usually results in stable isotope fractionation; the extent of fractionation depends on processes involved. Our review suggests that significant fractionation of iron isotopes occurs in low-temperature environments, where the extent of fractionation is greatly governed by several biogeochemical processes such as redox reaction, alteration, complexation, adsorption, oxidation and reduction, with or without the influence of microorganisms. This paper includes relevant data sets on the theoretical calculations, experimental prediction, as well as laboratory studies on stable iron isotopes fractionation induced by different biogeochemical processes.

Environmental implications of agricultural abandonment on Fe cycling: Insight from iron forms and stable isotope composition in karst soil, southwest China

Land-use change influences the fate of nutrient elements, including iron (Fe), and then threaten soil security. In this study, Fe forms and stable isotope composition (δ⁵⁶Fe) in soils were investigated to identify the variations in the processes of Fe cycling during agricultural abandonment in a karst region of Southwest China. Soil δ⁵⁶Fe compositions varied from -0.05‰-0.02‰ in croplands, 0.05‰-0.12‰ in abandoned croplands, to 0.30‰-0.80‰ in the native vegetation lands. In the croplands, Fe oxidation-precipitation process is considered as the main contributor to Fe migration and isotope fractionation, leading to a relatively enrichment of heavier Fe isotope in deeper soil layer. In the abandoned croplands and native vegetation lands, Fe isotope in the organic-rich layer (0-10 cm) was significantly lighter than that in subsurface layer (20-30 cm), mainly due to the recovery of soil organic carbon (SOC) and macro-aggregate after cropland abandonment. Moreover, the eluviation process mainly caused a decrease in soil Fe contents and enrichment of heavy Fe isotope in deeper soils (below 40 cm). The positive correlation between oxidized Fe and SOC contents suggested the accumulation of mobile Fe in soils after agricultural abandonment, which is beneficial for Fe uptake and assimilation by plants. This study suggests that agricultural abandonment significantly reduce soil Fe leaching loss and improve plant Fe supply by SOC accumulation in surface soil, which gives an environmental implication for the management of soil nutrients.

Zinc regulation of iron uptake and translocation in rice (Oryza sativa L.): Implication from stable iron isotopes and transporter genes

Iron (Fe) is an essential nutrient for living organisms and Fe deficiency is a worldwide problem for the health of both rice and humans. Zinc (Zn) contamination in agricultural soils is frequently observed. Here, we studied Fe isotope compositions and transcript levels of Fe transporter genes in rice growing in nutrient solutions having a range of Zn concentrations. Our results show Zn stress reduces Fe uptake by rice and drives its δ⁵⁶Fe value to that of the nutrient solution. These observations can be explained by the weakened Fe(II) uptake through Strategy I but enhanced Fe(III) uptake through Strategy II due to the competition between Zn and Fe(II) combining with OsIRT1 (Fe(II) transporter) in root, which is supported by the downregulated expression of OsIRT1 and upregulated expression of OsYSL15 (Fe(III) transporter). Using a mass balance box model, we also show excess Zn reduces Fe(II) translocation in phloem and its remobilization from senescent leaf, indicating a competition of binding sites on nicotianamine between Zn and Fe(II). This study provides direct evidence that how Zn regulates Fe uptake and translocation in rice and is of practical significance to design strategies to treat Fe deficiency in rice grown in Zn-contaminated soils.

Iron Isotopic Composition of Biological Standards Relevant to Medical and Biological Applications

Iron isotopes are fractionated by multiple biological processes, which offers a novel opportunity to study iron homeostasis. The determination of Fe isotope composition in biological samples necessitates certified biological reference materials with known Fe isotopic signature in order to properly assess external reproducibility and data quality between laboratories. We report the most comprehensive study on the Fe isotopic composition for widely available international biological reference materials. They consist of different terrestrial and marine animal organs (bovine, porcine, tuna, and mussel) as well as apple leaves and human hair (ERC-CE464, NIST1515, ERM-DB001, ERM-BB186, ERM-BB184, ERM-CE196, BCR668, ERM-BB185, ERM-BB124). Previously measured Fe isotopic compositions were available for only two of these reference materials (ERC-CE464 tuna fish and ERM-BB186 pig kidney) and these literature data are in excellent agreement with our data. The Fe isotopic ratios are reported as the permil deviation of the ⁵⁶Fe/⁵⁴Fe ratio from the IRMM-014 standard. All reference materials present δ⁵⁶Fe ranging from −2.27 to −0.35%0. Combined with existing data, our results suggest that animal models could provide useful analogues of the human body regarding the metabolic pathways affecting Fe isotopes, with many potential applications to medicine.

Determination of Picomolar Titanium in Seawater by Isotope Dilution Multicollector Inductively Coupled Plasma Mass Spectrometry after Mg(OH)2 Coprecipitation

A new isotope dilution inductively coupled plasma mass spectrometry (ICPMS) method is developed to determine picomolar concentrations of titanium (Ti) in seawater. The method applies Mg(OH)₂ coprecipitation to concentrate Ti from seawater, and uses a new ⁴⁹Ti-⁴⁷Ti isotope dilution to eliminate the need for separating Ti from seawater Ca, resulting in an isobaric interference-free analysis by high-resolution multicollector ICPMS. The method uses a 1.8 mL seawater sample with a detection limit of 1.6 pmol L^-1 that is determined mainly by Ti contamination during sample preparation rather than by ICPMS sensitivity, instrumental Ti background, or isobaric interferences. An oceanographically consistent vertical profile of dissolved Ti in the Sargasso Sea near Bermuda is measured with this method.

More than redox, biological organic ligands control iron isotope fractionation in the riparian wetland

Although redox reactions are recognized to fractionate iron (Fe) isotopes, the dominant mechanisms controlling the Fe isotope fractionation and notably the role of organic matter (OM) are still debated. Here, we demonstrate how binding to organic ligands governs Fe isotope fractionation beyond that arising from redox reactions. The reductive biodissolution of soil Fe(III) enriched the solution in light Fe isotopes, whereas, with the extended reduction, the preferential binding of heavy Fe isotopes to large biological organic ligands enriched the solution in heavy Fe isotopes. Under oxic conditions, the aggregation/sedimentation of Fe(III) nano-oxides with OM resulted in an initial enrichment of the solution in light Fe isotopes. However, heavy Fe isotopes progressively dominate the solution composition in response to their binding with large biologically-derived organic ligands. Confronted with field data, these results demonstrate that Fe isotope systematics in wetlands are controlled by the OM flux, masking Fe isotope fractionation arising from redox reactions. This work sheds light on an overseen aspect of Fe isotopic fractionation and calls for a reevaluation of the parameters controlling the Fe isotopes fractionation to clarify the interpretation of the Fe isotopic signature.

Coupled Fe-S isotope composition of sulfide chimneys dominated by temperature heterogeneity in seafloor hydrothermal systems

Seafloor hydrothermal chimneys are significant metal resources and have fundamental effects on marine chemistry balance. Previous studies on growth of the chimneys focused on mineral composition and elemental distribution in chimney walls, but the isotopic composition of different elements and their correlations remain poorly understood. Here, we report lateral Fe and S isotopes from sulfides across a chimney wall in detail from the Deyin-1 hydrothermal field (DHF) on the South Mid-Atlantic Ridge. Pyrite was characterized by increases in both δ³⁴S and δ⁵⁶Fe from the exterior to the interior within the chimney wall, which is likely related to enhanced S and Fe isotope fractionation between pyrite and fluids derived from the temperature gradient. Furthermore, δ⁵⁶Fe displayed a well-defined, linear, positive correlation with δ³⁴S in pyrite from the DHF as well as in pyrite from other areas. A model of hydrothermal pyrite formation suggests that, under conditions with uniform hydrothermal fluid, different temperatures and similar pyrite-fluid exchange degrees could result in the observed linear relationships between δ³⁴S and δ⁵⁶Fe. The regular and coupled Fe-S isotope variations within the chimney wall efficiently constrain the process of hydrothermal sulfide formation.

IUPAC Periodic Table of the Elements and Isotopes (IPTEI) for the Education Community (IUPAC Technical Report)

The IUPAC (International Union of Pure and Applied Chemistry) Periodic Table of the Elements and Isotopes (IPTEI) was created to familiarize students, teachers, and non-professionals with the existence and importance of isotopes of the chemical elements. The IPTEI is modeled on the familiar Periodic Table of the Chemical Elements. The IPTEI is intended to hang on the walls of chemistry laboratories and classrooms. Each cell of the IPTEI provides the chemical name, symbol, atomic number, and standard atomic weight of an element. Color-coded pie charts in each element cell display the stable isotopes and the relatively long-lived radioactive isotopes having characteristic terrestrial isotopic compositions that determine the standard atomic weight of each element. The background color scheme of cells categorizes the 118 elements into four groups: (1) white indicates the element has no standard atomic weight, (2) blue indicates the element has only one isotope that is used to determine its standard atomic weight, which is given as a single value with an uncertainty, (3) yellow indicates the element has two or more isotopes that are used to determine its standard atomic weight, which is given as a single value with an uncertainty, and (4) pink indicates the element has a well-documented variation in its atomic weight, and the standard atomic weight is expressed as an interval. An element-by-element review accompanies the IPTEI and includes a chart of all known stable and radioactive isotopes for each element. Practical applications of isotopic measurements and technologies are included for the following fields: forensic science, geochronology, Earth-system sciences, environmental science, and human health sciences, including medical diagnosis and treatment.

SciPhon: a data analysis software for nuclear resonant inelastic X-ray scattering with applications to Fe, Kr, Sn, Eu and Dy

The synchrotron radiation technique of nuclear resonant inelastic X-ray scattering (NRIXS), also known as nuclear resonance vibrational spectroscopy or nuclear inelastic scattering, provides a wealth of information on the vibrational properties of solids. It has found applications in studies of lattice dynamics and elasticity, superconductivity, heme biochemistry, seismology, isotope geochemistry and many other fields. It involves probing the vibrational modes of solids by using the nuclear resonance of Mössbauer isotopes such as ⁵⁷Fe, ⁸³Kr, ¹¹⁹Sn, ¹⁵¹Eu and ¹⁶¹Dy. After data reduction, it provides the partial phonon density of states of the Mössbauer isotope that is investigated, as well as many other derived quantities such as the mean force constant of the chemical bonds and the Debye velocity. The data reduction is, however, not straightforward and involves removal of the elastic peak, normalization and Fourier-Log transformation. Furthermore, some of the quantities derived are highly sensitive to details in the baseline correction. A software package and several novel procedures to streamline and hopefully improve the reduction of the NRIXS data generated at sector 3ID of the Advanced Photon Source have been developed. The graphical user interface software is named SciPhon and runs as a Mathematica package. It is easily portable to other platforms and can be easily adapted for reducing data generated at other beamlines. Several tests and comparisons are presented that demonstrate the usefulness of this software, whose results have already been used in several publications. Here, the SciPhon software is used to reduce Kr, Sn, Eu and Dy NRIXS data, and potential implications for interpreting natural isotopic variations in those systems are discussed.

Benthic iron cycling in a high-oxygen environment: Implications for interpreting the Archean sedimentary iron isotope record

The most notable trend in the sedimentary iron isotope record is a shift at the end of the Archean from highly variable δ⁵⁶ Fe values with large negative excursions to less variable δ⁵⁶ Fe values with more limited negative values. The mechanistic explanation behind this trend has been extensively debated, with two main competing hypotheses: (i) a shift in marine redox conditions and the transition to quantitative iron oxidation; and (ii) a decrease in the signature of microbial iron reduction in the sedimentary record because of increased bacterial sulfate reduction (BSR). Here, we provide new insights into this debate and attempt to assess these two hypotheses by analyzing the iron isotope composition of siderite concretions from the Carboniferous Mazon Creek fossil site. These concretions precipitated in an environment with water column oxygenation, extensive sediment pile dissimilatory iron reduction (DIR) but limited bacterial sulfate reduction (BSR). Most of the concretions have slightly positive iron isotope values, with a mean of 0.15‰ and limited iron isotope variability compared to the Archean sedimentary record. This limited variability in an environment with high DIR and low BSR suggests that these conditions alone are insufficient to explain Archean iron isotope compositions. Therefore, these results support the idea that the unusually variable and negative iron isotope values in the Archean are due to dissimilatory iron reduction (DIR) coupled with extensive water column iron cycling.

Tracing the oxygen isotope composition of the upper Earth's atmosphere using cosmic spherules

Molten I-type cosmic spherules formed by heating, oxidation and melting of extraterrestrial Fe,Ni metal alloys. The entire oxygen in these spherules sources from the atmosphere. Therefore, I-type cosmic spherules are suitable tracers for the isotopic composition of the upper atmosphere at altitudes between 80 and 115 km. Here we present data on I-type cosmic spherules collected in Antarctica. Their composition is compared with the composition of tropospheric O2. Our data suggest that the Earth's atmospheric O2 is isotopically homogenous up to the thermosphere. This makes fossil I-type micrometeorites ideal proxies for ancient atmospheric CO2 levels.

Iron Transformation Pathways and Redox Micro-Environments in Seafloor Sulfide-Mineral Deposits: Spatially Resolved Fe XAS and δ(57/54)Fe Observations

Hydrothermal sulfide chimneys located along the global system of oceanic spreading centers are habitats for microbial life during active venting. Hydrothermally extinct, or inactive, sulfide deposits also host microbial communities at globally distributed sites. The main goal of this study is to describe Fe transformation pathways, through precipitation and oxidation-reduction (redox) reactions, and examine transformation products for signatures of biological activity using Fe mineralogy and stable isotope approaches. The study includes active and inactive sulfides from the East Pacific Rise 9°50'N vent field. First, the mineralogy of Fe(III)-bearing precipitates is investigated using microprobe X-ray absorption spectroscopy (μXAS) and X-ray diffraction (μXRD). Second, laser-ablation (LA) and micro-drilling (MD) are used to obtain spatially-resolved Fe stable isotope analysis by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). Eight Fe-bearing minerals representing three mineralogical classes are present in the samples: oxyhydroxides, secondary phyllosilicates, and sulfides. For Fe oxyhydroxides within chimney walls and layers of Si-rich material, enrichments in both heavy and light Fe isotopes relative to pyrite are observed, yielding a range of δ(57)Fe values up to 6‰. Overall, several pathways for Fe transformation are observed. Pathway 1 is characterized by precipitation of primary sulfide minerals from Fe(II)aq-rich fluids in zones of mixing between vent fluids and seawater. Pathway 2 is also consistent with zones of mixing but involves precipitation of sulfide minerals from Fe(II)aq generated by Fe(III) reduction. Pathway 3 is direct oxidation of Fe(II) aq from hydrothermal fluids to form Fe(III) precipitates. Finally, Pathway 4 involves oxidative alteration of pre-existing sulfide minerals to form Fe(III). The Fe mineralogy and isotope data do not support or refute a unique biological role in sulfide alteration. The findings reveal a dynamic range of Fe transformation pathways consistent with a continuum of micro-environments having variable redox conditions. These micro-environments likely support redox cycling of Fe and S and are consistent with culture-dependent and -independent assessments of microbial physiology and genetic diversity of hydrothermal sulfide deposits.

Iron isotope characteristics of Hot Springs at Chocolate Pots, Yellowstone National Park

Chocolate Pots Hot Springs in Yellowstone National Park is a hydrothermal system that contains high aqueous ferrous iron [∼0.1 mM Fe(II)] at circumneutral pH conditions. This site provides an ideal field environment in which to test our understanding of Fe isotope fractionations derived from laboratory experiments. The Fe(III) oxides, mainly produced through Fe(II) oxidation by oxygen in the atmosphere, have high ⁵⁶Fe/⁵⁴Fe ratios compared with the aqueous Fe(II). However, the degree of fractionation is less than that expected in a closed system at isotopic equilibrium. We suggest two explanations for the observed Fe isotope compositions. One is that light Fe isotopes partition into a sorbed component and precipitate out on the Fe(III) oxide surfaces in the presence of silica. The other explanation is internal regeneration of isotopically heavy Fe(II) via dissimilatory Fe(III) reduction farther down the flow path as well as deeper within the mat materials. These findings provide evidence that silica plays an important role in governing Fe isotope fractionation factors between reduced and oxidized Fe. Under conditions of low ambient oxygen, such as may be found on early Earth or Mars, significantly larger Fe isotope variations are predicted, reflecting the more likely attainment of Fe isotope equilibrium associated with slower oxidation rates under low-O₂ conditions.

Precipitation of iron on the surface of Leptospira interrogans is associated with mutation of the stress response metalloprotease HtpX

High concentrations of free metal ions in the environment can be detrimental to bacterial survival. However, bacteria utilize strategies, including the activation of stress response pathways and immobilizing chemical elements on their surface, to limit this toxicity. In this study, we characterized LA4131, the HtpX-like M48 metalloprotease from Leptospira interrogans, with a putative role in bacterial stress response and membrane homeostasis. Growth of the la4131 transposon mutant strain (L522) in 360 μM FeSO4 (10-fold the normal in vitro concentration) resulted in the production of an amorphous iron precipitate. Atomic force microscopy and transmission electron microscopy analysis of the strain demonstrated that precipitate production was associated with the generation and release of outer membrane vesicles (OMVs) from the leptospiral surface. Transcriptional studies indicated that inactivation of la4131 resulted in altered expression of a subset of metal toxicity and stress response genes. Combining these findings, this report describes OMV production in response to environmental stressors and associates OMV production with the in vitro activity of an HtpX-like metalloprotease.

Iron isotope fractionation during Fe uptake and translocation in alpine plants

The potential of stable Fe isotopes as a tracer for the biogeochemical Fe cycle depends on the understanding and quantification of the fractionation processes involved. Iron uptake and cycling by plants may influence Fe speciation in soils. Here, we determined the Fe isotopic composition of different plant parts including the complete root system of three alpine plant species (Oxyria digyna, Rumex scutatus, Agrostis gigantea) in a granitic glacier forefield, which allowed us, for the first time, to distinguish between uptake and in-plant fractionation processes. The overall range of fractionation was 4.5 per thousand in delta(56)Fe. Mass balance calculations demonstrated that fractionation toward lighter Fe isotopic composition occurred in two steps during uptake: (1) before active uptake, probably during mineral dissolution and (2) during selective uptake of Fe at the plasma membrane with an enrichment factor of -1.0 to -1.7 per thousand for all three species. Iron isotopes were further fractionated during remobilization from old into new plant tissue, which changed the isotopic composition of leaves and flowers over the season. This study demonstrates the potential of Fe isotopes as a new tool in plant nutrition studies but also reveals challenges for the future application of Fe isotope signatures in soil-plant environments.

Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria

Iron isotope fractionations produced during chemical and biological Fe(II) oxidation are sensitive to the proportions and nature of dissolved and solid-phase Fe species present, as well as the extent of isotopic exchange between precipitates and aqueous Fe. Iron isotopes therefore potentially constrain the mechanisms and pathways of Fe redox transformations in modern and ancient environments. In the present study, we followed in batch experiments Fe isotope fractionations between Fe(II)(aq) and Fe(III) oxide/hydroxide precipitates produced by the Fe(III) mineral encrusting, nitrate-reducing, Fe(II)-oxidizing Acidovorax sp. strain BoFeN1. Isotopic fractionation in (56)Fe/(54)Fe approached that expected for equilibrium conditions, assuming an equilibrium Δ(56)Fe(Fe(OH)3 - Fe(II)aq) fractionation factor of +3.0 ‰. Previous studies have shown that Fe(II) oxidation by this Acidovorax strain occurs in the periplasm, and we propose that Fe isotope equilibrium is maintained through redox cycling via coupled electron and atom exchange between Fe(II)(aq) and Fe(III) precipitates in the contained environment of the periplasm. In addition to the apparent equilibrium isotopic fractionation, these experiments also record the kinetic effects of initial rapid oxidation, and possible phase transformations of the Fe(III) precipitates. Attainment of Fe isotope equilibrium between Fe(III) oxide/hydroxide precipitates and Fe(II)(aq) by neutrophilic, Fe(II)-oxidizing bacteria or through abiologic Fe(II)(aq) oxidation is generally not expected or observed, because the poor solubility of their metabolic product, i.e. Fe(III), usually leads to rapid precipitation of Fe(III) minerals, and hence expression of a kinetic fractionation upon precipitation; in the absence of redox cycling between Fe(II)(aq) and precipitate, kinetic isotope fractionations are likely to be retained. These results highlight the distinct Fe isotope fractionations that are produced by different pathways of biological and abiological Fe(II) oxidation.

Stable iron isotopes and microbial mediation in red pigmentation of the Rosso Ammonitico (mid-late Jurassic, Verona area, Italy)

The iron (Fe) isotopic composition of 17 Jurassic limestones from the Rosso Ammonitico of Verona (Italy) have been analyzed by Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). Such analysis allowed for the recognition of a clear iron isotopic fractionation (mean -0.8 per thousand, ranging between -1.52 to -0.06 per thousand) on a millimeter-centimeter scale between the red and grey facies of the studied formation. After gentle acid leaching, measurements of the Fe isotopic compositions gave delta(56)Fe values that were systematically lower in the red facies residues (median: -0.84 per thousand, range: -1.46 to +0.26 per thousand) compared to the grey facies residues (median: -0.08 per thousand, range: -0.34 to +0.23 per thousand). In addition, the red facies residues were characterized by a lighter delta(56)Fe signal relative to their corresponding leachates. These Fe isotopic fractionations could be a sensitive fingerprint of a biotic process; systematic isotopic differences between the red and grey facies residues, which consist of hematite and X-ray amorphous iron hydroxides, respectively, are hypothesized to have resulted from the oxidizing activity of iron bacteria and fungi in the red facies. The grey Fe isotopic data match the Fe isotopic signature of the terrestrial baseline established for igneous rocks and low-C(org) clastic sedimentary rocks. The Fe isotopic compositions of the grey laminations are consistent with the influx of detrital iron minerals and lack of microbial redox processes at the water-interface during deposition. Total Fe concentration measurements were performed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (confirmed by concentration estimations obtained by MC-ICP-MS analyses of microdrilled samples) on five samples, and resultant values range between 0.30% (mean) in the grey facies and 1.31% (mean) in the red facies. No correlation was observed between bulk Fe content and pigmentation or between bulk Fe content and Fe isotopic compositions. The rapid transformation of the original iron oxyhydroxides to hematite could have preserved the original isotopic composition if it had occurred at about the same temperature. This paper supports the use of Fe isotopes as sensitive tracers of biological activities recorded in old sedimentary sequences that contain microfossils of iron bacteria and fungi. However, a careful interpretation of the iron isotopic fractionation in terms of biotic versus abiotic processes requires supporting data or direct observations to characterize the biological, (geo)chemical, or physical context in relation to the geologic setting. This will become even more pertinent when Fe isotopic studies are expanded to the interplanetary realm.

Iron isotope fractionation during magmatic differentiation in Kilauea Iki lava lake

Magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks. The extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements. We report analyses of iron isotopes in basalts from Kilauea Iki lava lake, Hawaii. The iron isotopic compositions (56Fe/54Fe) of late-stagemeltveins are 0.2 permil (per thousand) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2 per thousand lighter than those of whole rocks. These results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales. This characteristic of iron relative to the characteristics of magnesium and lithium, for which no fractionation has been found, may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation.

Isotope Fractionation of Iron(III) in Chemical Exchange Reactions Using Solvent Extraction with Crown Ether

This work reports on the chemical isotope fractionation of Fe(III) by a solvent extraction method with a crown ether of dicyclohexano-18-crown-6. The (56)Fe/(54)Fe and (57)Fe/(54)Fe ratios were analyzed by multiple-collector inductively coupled plasma mass spectrometry. We determined the dependence of the isotope enrichment factors (epsilon) on the strength of HCl. The relative deviation of the (56)Fe/(54)Fe ratios relative to the unprocessed material (10(4) epsilon(56)) increases from -15.3 to -6.3 with [HCl] increasing from 1.6 to 3.5 mol/L. Likewise, 10(4) epsilon(57) increases from -22.8 to -9.6 under the same conditions. The correlation between epsilon(56) and epsilon(57) is mass dependent within the errors. The observed fractionation was broken down into the effects of competing extraction reactions and of a reaction between Fe(III) species (FeCl(2)(+) and FeCl(3)) in the aqueous phase. We found that the isotope fractionation between the Fe(III) species is mass dependent, which we confirmed by calculating the reduced partition function ratios.