Banded iron formations (BIFs) record a time of extensive Fe deposition in the Precambrian oceans, but the sources and pathways for metals in BIFs remain controversial. Here, we present Fe- and ...Nd-isotope data that indicate two sources of Fe for the large BIF units deposited 2.5 billion y ago. High-ε Nd and -δ âµâ¶Fe signatures in some BIF samples record a hydrothermal component, but correlated decreases in ε Nd- and δ âµâ¶Fe values reflect contributions from a continental component. The continental Fe source is best explained by Fe mobilization on the continental margin by microbial dissimilatory iron reduction (DIR) and confirms for the first time, to our knowledge, a microbially driven Fe shuttle for the largest BIFs on Earth. Detailed sampling at various scales shows that the proportions of hydrothermal and continental Fe sources were invariant over periods of 10 â°â10 ³ y, indicating that there was no seasonal control, although Fe sources varied on longer timescales of 10 âµâ10 ⶠy, suggesting a control by marine basin circulation. These results show that Fe sources and pathways for BIFs reflect the interplay between abiologic (hydrothermal) and biologic processes, where the latter reflects DIR that operated on a basin-wide scale in the Archean.
Results from enriched 57Fe isotope tracer experiments have shown that atom exchange can occur between structural Fe in Fe(III) oxides and aqueous Fe(II) with no formation of secondary minerals or ...change in particle size or shape. Here we derive a mass balance model to quantify the extent of Fe atom exchange between goethite and aqueous Fe(II) that accounts for different Fe pool sizes. We use this model to reinterpret our previous work and to quantify the influence of particle size and pH on extent of goethite exchange with aqueous Fe(II). Consistent with our previous interpretation, substantial exchange of goethite occurred at pH 7.5 (≈ 90%) and we observed little effect of particle size between nanogoethite (average size of 81 × 11 nm; ≈ 110 m2/g) and microgoethite (average size of 590 × 42 nm; ≈ 40 m2/g). Despite ≈90% of the bulk goethite exchanging at pH 7.5, we found no change in mineral phase, average particle size, crystallinity, or reactivity after reaction with aqueous Fe(II). At a lower pH of 5.0, no net sorption of Fe(II) was observed and significantly less exchange occurred accounting for less than the estimated proportion of surface Fe atoms in the particles. Particle size appears to influence the amount of exchange at pH 5.0 and we suggest that aggregation and surface area may play a role. Results from sequential chemical extractions indicate that 57Fe accumulates in extracted Fe(III) goethite components. Isotopic compositions of the extracts indicate that a gradient of 57Fe develops within the goethite with more accumulation of 57Fe occurring in the more easily extracted Fe(III) that may be nearer to the surface.
The reaction of aqueous Fe(II) with Fe(III) oxides is a complex process, comprising sorption, electron transfer, and in some cases, reductive dissolution and transformation to secondary minerals. To ...better understand the dynamics of these reactions, we measured the extent and rate of Fe isotope exchange between aqueous Fe(II) and goethite using a 57Fe isotope tracer approach. We observed near-complete exchange of Fe atoms between the aqueous phase and goethite nanorods over a 30-day time period. Despite direct isotopic evidence for extensive mixing between the aqueous and goethite Fe, no phase transformation was observed, nor did the size or shape of the goethite rods change appreciably. High-resolution transmission electron microscopy images, however, appear to indicate that some recrystallization of the goethite particles may have occurred. Near-complete exchange of Fe between aqueous Fe(II) and goethite, coupled with negligible change in the goethite mineralogy and morphology, suggests a mechanism of coupled growth (via sorption and electron transfer) and dissolution at separate crystallographic goethite sites. We propose that sorption and dissolution sites are linked via conduction through the bulk crystal, as was recently demonstrated for hematite. Extensive mixing between aqueous Fe(II) and goethite, a relatively stable iron oxide, has significant implications for heavy metal sequestration and release (e.g., arsenic and uranium), as well as reduction of soil and groundwater contaminants.
Abstract Objective To evaluate the performance characteristics of florbetapir F18 positron emission tomography (PET) in patients with Alzheimer's disease (AD), mild cognitive impairment (MCI), and ...healthy control subjects (HCs). Methods Florbetapir PET was acquired in 184 subjects (45 AD patients, 60 MCI patients, and 79 HCs) within a multicenter phase 2 study. Amyloid burden was assessed visually and quantitatively, and was classified as positive or negative. Results Florbetapir PET was rated visually amyloid positive in 76% of AD patients, 38% of MCI patients, and 14% of HCs. Eighty-four percent of AD patients, 45% of MCI patients, and 23% of HCs were classified as amyloid positive using a quantitative threshold. Amyloid positivity and mean cortical amyloid burden were associated with age and apolipoprotein E ε4 carrier status. Conclusions: The data are consistent with expected rates of amyloid positivity among individuals with clinical diagnoses of AD and MCI, and indicate the potential value of florbetapir F18 PET as an adjunct to clinical diagnosis.
Magnetite is ubiquitous in the Earth's crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical Fe cycling. Magnetite is also the most abundant Fe oxide in ...banded iron formations (BIFs) that have not been subjected to ore-forming alteration. Magnetite is therefore an important target of stable Fe isotope studies, and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable Fe isotope fractionation factors for magnetite relative to fluids and other minerals. In this study, we utilized the three-isotope method (57Fe–56Fe–54Fe) to explore isotopic exchange via an enriched-57Fe tracer, and natural mass-dependent fractionation using 56Fe/54Fe variations, during reaction of aqueous Fe(II) (Fe(II)aq) with magnetite. Importantly, we employed a multi-direction approach to equilibrium by reacting four 57Fe-enriched Fe(II) solutions that had distinct 56Fe/54Fe ratios, which identifies changes in the instantaneous Fe isotope fractionation factor and hence identifies kinetic isotope effects. We find that isotopic exchange can be described by two 56Fe/54Fe fractionations, where an initial rapid exchange (∼66% isotopic mixing within 1 day) involved a relatively small Fe(II)aq–magnetite 56Fe/54Fe fractionation, followed by slower exchange (∼25% isotopic mixing over 50 days) that was associated with a larger Fe(II)aq–magnetite 56Fe/54Fe fractionation; this later fractionation is interpreted to approach isotopic equilibrium between Fe(II)aq and the total magnetite. All four Fe(II) solutions extrapolate to the same final equilibrium 56Fe/54Fe fractionation for Fe(II)aq–magnetite of −1.56±0.20‰(2σ) at 22 °C. Additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous Fe(II) yield final 56Fe/54Fe fractionations that are identical to those of the exchange experiments. Our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for Fe(H2O)62+ from Rustad et al. (2010) and stoichiometric magnetite from Mineev et al. (2007), and these relations may be combined with the experimental constraints to determine the temperature dependence of the Fe(II)aq–magnetite fractionation factor:103lnαFe(II)aq–magnetite=−0.145(±0.002)×106/T2+0.10(±0.02) where T is in K.
Part of the reason for large discrepancies in calculated Fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies, given the significant effect of octahedral versus tetrahedral Fe isotope fractionation that has been calculated. When our results are applied to BIF genesis, our experimentally determined Fe(II)aq–magnetite fractionation factor indicates that magnetite–siderite mineral pairs in ∼2.5 Ga BIFs did not form in Fe isotope equilibrium with each other, or with ancient seawater. Iron oxides in such BIFs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans, indicating that such BIF minerals may not be suitable proxies for ancient paleoenvironments.
•Isotopic exchange and fractionation between aqueous Fe(II) and magnetite is reported.•First multi-direction approach to equilibrium using iron isotopes is discussed.•Fe(II)aq–magnetite equilibrium fractionation factor is rigorously determined.•BIF minerals in isotopic disequilibrium may be poor proxies for paleoenvironments.
Before the Great Oxidation Event (GOE) 2.4–2.2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen-producing microorganisms. ...Recently, however, it has been proposed that transient oxygenation of shallow seawater occurred between 2.8 and 3.0 billion years ago. Here, we present a novel combination of stable Fe and radiogenic U–Th–Pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3.2 Ga, based on analysis of the Manzimnyama Banded Iron Formation (BIF), Fig Tree Group, South Africa. This unit is exceptional in that proximal, shallow-water and distal, deep-water facies are preserved. When compared to the distal, deep-water facies, the proximal samples show elevated U concentrations and moderately positive δ56Fe values, indicating vertical stratification in dissolved oxygen contents. Confirmation of oxidizing conditions using U abundances is robustly constrained using samples that have been closed to U and Pb mobility using U–Th–Pb geochronology. Although redox-sensitive elements have been commonly used in ancient rocks to infer redox conditions, post-depositional element mobility has been rarely tested, and U–Th–Pb geochronology can constrain open- or closed-system behavior. The U abundances and δ56Fe values of the Manzimnyama BIF suggest the proximal, shallow-water samples record precipitation under stronger oxidizing conditions compared to the distal deeper-water facies, which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time; this work, therefore, documents the oldest known preserved marine redox gradient in the rock record. The relative enrichment of O2 in the upper water column is likely due to the existence of oxygen-producing microorganisms such as cyanobacteria. These results provide a new approach for identifying free oxygen in Earth's ancient oceans, including confirming the age of redox proxies, and indicate that cyanobacteria evolved prior to 3.2 Ga.
•We propose a discrete redoxcline existed between deep and shallow seawater at 3.2 Ga.•Oxygen in the photic zone was produced by cyanobacteria.•Cyanobacteria evolved before 3.2 Ga.
The Mg isotope composition of biogenic and inorganic carbonate bears on paleoclimate and paleooceanography studies because of the potential for constraining temperatures, so-called “vital” effects, ...and marine Mg fluxes. Previous work has shown that marine organisms produce a wide range of Mg isotope compositions that are species dependent, where Δ26/24Mgcarb-sol fractionations vary from −1‰ to −5‰ (e.g., Hippler et al., 2009, GCA). Constraining Mg isotope fractionation during inorganic carbonate precipitation is important because this serves as a baseline with which to compare biogenic samples, as well as constrain Mg cycling in natural environments. We report Mg isotope fractionation factors between Mg-bearing calcite and aqueous Mg (Mg/Ca molar ratio between 3:1 and 13:1) from 20 free-drift and one chemo-stat experiment conducted at temperatures between 4°C and 45°C, for solutions buffered at PCO2 between 0.038% and 3%. Pure CaCO3 seed crystals were used to promote the heterogeneous growth of carbonate from solution, and to minimize kinetic isotope effects associated with nucleation and rapid precipitation from strongly super-saturated solutions. Under these conditions, calcite overgrowths that contained 0.8–14.9mol% MgCO3 precipitated on the seed crystals. The measured 26Mg/24Mg fractionation factors between Mg-calcite and solution (Δ26Mgcal-sol) are modestly correlated with temperature, changing from −2.70‰ at 4°C to −2.22‰ at 45°C. The fractionation factors are not correlated with experimental conditions (chemo-stat vs. free drift), Mg content of the overgrowth, PCO2, or the Mg/Ca ratio of the solution. The temperature-dependence of the Mg isotope fractionation is: Δ26Mgcal-sol=(−0.158±0.051)×106/T2−(0.74±0.56), where T is temperature in Kelvin. Fractionation of Mg isotopes in calcite is much less sensitive to temperature than oxygen isotope fractionation, which limits its application as a geothermometer. In contrast, the Mg isotope fractionations for biogenically precipitated Mg calcite vary greatly, suggesting its potential to discern “vital” effects in natural samples. Finally, the relatively small temperature effect on Mg isotope fractionation greatly simplifies use of Mg isotopes in modern or ancient marine systems to constrain Mg fluxes, including continental weathering.
► A systematic experimental investigation of Mg isotope fractionation during calcite precipitation. ► Experiments cover wide temperature and chemical ranges. ► Used both chemo-stat and free-drift experiments, used seed crystals to reduce kinetic isotope effects. ► Observed temperature-dependent Mg isotope fractionation: Δ26/24Mgcal-sol=(−0.158±0.051)×106/T2−(0.74±0.56).
The reaction between magnetite and aqueous Fe2+ has been extensively studied due to its role in contaminant reduction, trace-metal sequestration, and microbial respiration. Previous work has ...demonstrated that the reaction of Fe2+ with magnetite (Fe3O4) results in the structural incorporation of Fe2+ and an increase in the bulk Fe2+ content of magnetite. It is unclear, however, whether significant Fe atom exchange occurs between magnetite and aqueous Fe2+, as has been observed for other Fe oxides. Here, we measured the extent of Fe atom exchange between aqueous Fe2+ and magnetite by reacting isotopically “normal” magnetite with 57Fe-enriched aqueous Fe2+. The extent of Fe atom exchange between magnetite and aqueous Fe2+ was significant (54–71%), and went well beyond the amount of Fe atoms found at the near surface. Mössbauer spectroscopy of magnetite reacted with 56Fe2+ indicate that no preferential exchange of octahedral or tetrahedral sites occurred. Exchange experiments conducted with Co-ferrite (Co2+Fe2 3+O4) showed little impact of Co substitution on the rate or extent of atom exchange. Bulk electron conduction, as previously invoked to explain Fe atom exchange in goethite, is a possible mechanism, but if it is occurring, conduction does not appear to be the rate-limiting step. The lack of significant impact of Co substitution on the kinetics of Fe atom exchange, and the relatively high diffusion coefficients reported for magnetite suggest that for magnetite, unlike goethite, Fe atom diffusion is a plausible mechanism to explain the rapid rates of Fe atom exchange in magnetite.
The equilibrium Fe isotope fractionation factor between aqueous Fe(II) and goethite has been experimentally measured to be −1.05±0.08‰ in 56Fe/54Fe (2σ) at 22°C, using the three-isotope method. ...Experiments were done using two sizes of goethite (81×11nm and 590×42nm), and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface Fe(III) atoms have different isotopic properties than the bulk goethite. These experiments indicate that the interaction of Fe(II)aq and goethite is dynamic and results in complete or near-complete Fe isotope exchange over 30days, involving at least four components: Fe(II)aq, goethite, sorbed Fe(II), and Fe(III)surface. The equilibrium fractionation factor between Fe(II)aq and Fe(II)sorb is the same for both sizes of goethite, at Δ56FeFe(II)aq–Fe(II)sorb=−1.24±0.14‰; this fractionation factor is significantly different than the results of previous studies on Fe(II) sorption to goethite. The proportion of the Fe(III)surface component is greatest in the experiments that used the smallest goethite, and the Fe(III)surface–Fe(II)aq fractionation is estimated to be at least +2.1‰. The high Fe(III)surface–Fe(II)aq fractionation may exert a significant influence on the Fe isotope compositions of aqueous Fe(II) in natural systems that contain nanoparticulate goethite, including those involving bacterial iron reduction. These results demonstrate that the isotopic properties of nano-scale minerals may be distinct from micron-scale or larger minerals, as is the case for other thermodynamic properties of nanoparticles.
Hydrothermal experiments at 220, 160, and 130°C were performed to calibrate the Mg isotope fractionation factor between dolomite and aqueous Mg. Hydrothermal experiments included synthesis of ...dolomite using different starting materials, as well as exchange experiments that used poorly-ordered proto-dolomite. The morphology of synthesized dolomite was dependent on starting mineralogy, suggesting that dolomite was synthesized by different pathways. Hydrothermally synthesized dolomite was initially fine-grained disordered or poorly-ordered dolomite that, with time, recrystallized to coarser-grained ordered dolomite. Isotopic exchange was monitored using 87Sr/86Sr ratios and 25Mg tracers, and these indicated near-complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments. The Mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology, suggesting attainment of isotopic equilibrium. Combining results from synthesis and exchange experiments, the temperature dependent Mg isotope fractionation factor for ordered dolomite is:Δ26Mgdolo-aq=-0.1554(±0.0096)×106/T2where T is in Kelvin. In contrast, poorly-ordered dolomite has a Δ26Mgdolo-aq fractionation factor that is up to 0.25‰ lower than that of ordered dolomite, and this is attributed to longer Mg–O bonds in imperfectly ordered dolomite. The experimentally calibrated Δ26Mgdolo-aq fractionation factors lie between those calculated by Schauble (2011) and Rustad et al. (2010). The Δ26Mgdolo-aq fractionation factor extrapolated to lower temperatures using the Δ26Mg-T function of this study matches the Δ26Mgdolo-aq fractionation factor obtained by modeling of Mg isotope compositions of ODP drill core samples. This study shows that significant Mg isotope fractionation occurs during dolomite precipitation. These results collectively demonstrate that Mg isotopes in dolomite are a useful tool for studying Mg global cycling and dolomitization.
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