Isotope Fractionation Factors Research Articles

Monitoring Cr toxicity and remediation processes - combining a whole-cell bioreporter and Cr isotope techniques

Bioremediation is a sustainable and cost-effective means of contaminant detoxification. Although Cr(VI) is toxic at high concentrations, various microbes can utilise it as an electron accepter in the bioremediation process, and reduce it to the less toxic form Cr(III). During remediation, it is important to monitor the level of toxicity and effectiveness of Cr(VI) reduction in order to optimize the conditions. This study employed a whole-cell bioreporter Acinetobacter baylyi ADPWH-recA to access the degree of toxicity of different species of Cr over a range of initial concentrations. It also investigated whether Cr isotope fractionation factors were impacted by different levels of Cr toxicity (related to its concentration) and Cr(VI) reduction rates by Cr resistant bacteria Pseudomonas fluorescens LB 300. The results show that, of both Cr2O72− and CrO42−, the whole-cell bioreporter was efficient in indicating the level of genotoxicity of Cr(VI) at low concentrations and cytotoxicity at high concentrations via variations of bioluminescence. High concentrations (> 100 mg/L) of Cr(III) could also strongly induce the luminescence in the bioreporter, indicating DNA damage at such abundance. Pseudomonas fluorescens LB 300 was found to be effective in reducing Cr(VI) even when the concentration was high (40 mg/L); however, complete Cr(VI) reduction was only observed at low concentrations (< 5 mg/L), since the toxicity of high concentrations of Cr(VI) impacted the effectiveness of reduction by the bacteria. During reduction, the C53r/C52r ratio of remaining Cr(VI) increased from its initial value, and the calculated fractionation factor by bacterial Cr(VI) reduction (ε) was −3.1±0.3‰. The fractionation factor was independent of the initial Cr(VI) concentration. Therefore, a single Cr isotope fractionation factor can be effectively applied in indicating the extent of bioremediation processing of Cr(VI) over a wide range of concentrations. This significantly simplified monitoring of Cr(VI) depletion in bioremediation, since variations of ε normally indicate a change in the reduction mechanism and therefore would complicate the elucidation of processes driving the remediation.

Cr isotope systematics in the Connecticut River estuary

Stable chromium (Cr) isotopes (expressed in δ53Cr as the deviation of 53Cr/52Cr in samples from the international standard NIST SRM 979) are an emerging paleoredox proxy that has been used to track Earth's redox evolution over a range of timescales. There have been several major advances in our understanding of the Cr isotope system, but we are still developing a basic knowledge of the global Cr isotope mass balance and there are key Cr fluxes with poorly constrained isotopic compositions. The Cr isotope behavior in estuaries is one such poorly constrained aspect of the modern Cr cycle. Therefore, we collected water and suspended particulate samples from the estuary of the Connecticut River, and present the first Cr isotope dataset from a salinity gradient. As pH and salinity increases from brackish to salt water, dissolved Cr increases from 118.1 ± 25.7 ng/kg (1σ, n = 6) to 225.1 ± 26.0 ng/kg (1σ, n = 8), while particulate Cr decreases from 341.3 ± 319.8 ng/kg (1σ, n = 6) to 208.4 ± 364.5 ng/kg (1σ, n = 8); dissolved δ53Cr decreases from 1.30 ± 0.27‰ (1σ, n = 6) to 0.64 ± 0.16‰ (1σ, n = 8), whereas particulate δ53Cr remained stable at 0.11 ± 0.07‰ (1σ, n = 9). These data suggest that Cr is lost from particles to solution when transitioning from brackish to salt water in the Connecticut River estuary. If these observations can be confirmed in other estuaries, estuary systems can potentially enhance riverine dissolved Cr fluxes and thus lower the Cr oceanic residence time, which has important implications for the Cr isotope system as a paleoceanographic redox proxy. There is a clear inverse correlation between dissolved Cr and δ53Cr in the Connecticut River estuary, which yields a Cr isotope fractionation factor of ~0.89‰ that is indistinguishable from previous estimates based on open seawater samples. Large δ53Cr variation in oxygenated open and estuary seawater suggests that marine δ53Cr variations need not necessarily be exclusively tied to Cr(VI) reduction in low oxygen zones. Therefore, interpreting δ53Cr in the sedimentary record requires taking multiple processes into consideration.

Seasonal trends of the stable nitrogen isotope ratio in particulate nitrogen compounds and their gaseous precursors in Akita, Japan

Particulate matter (PM) can have adverse effects on human health. Moreover, because the mechanisms of PM formation and behavior in the atmosphere are notably complicated, to reduce PM concentrations effectively and meet environmental standards, source–receptor relationships must be clearly understood. Stable isotope ratios can be used to detect chemical processes and distinguish sources. In environmental science, especially in research on aerosols, stable isotope ratios have proven to constitute a powerful tool for source identification. However, there are few long-term studies of isotope fractionation during secondary aerosol formation. In this study, stable nitrogen isotope ratios (δ15N) of ammonia gas (NH3), nitrogen dioxide gas (NO2), nitric acid vapor (HNO3), particulate nitrate (NO3−), and ammonium (NH4+) in suspended PM (SPM) were analyzed to investigate seasonal trends and isotope fractionation during aerosol formation for long term sampling in Akita, Japan. The results indicated that δ15N-NH4+ in SPM and δ15N-NH3 gas ranged from 1.3‰ to 38.5‰ (mean 16.1‰) and from −33.6‰ to −0.0‰ (−16.9‰), respectively. Furthermore, δ15N-NO3− (SPM) and δ15N-NO2 and δ15N-HNO3 (gaseous) ranged from −4.6‰ to 4.8‰ (mean −0.5‰), from −8.2‰ to −3.1‰ (−5.4‰), and from −7.5‰ to 2.7‰ (−5.0‰), respectively. The mean annual isotope fractionation factors for transformations from gaseous NH3 to NH4+ in SPM, from gaseous NO2 to gaseous HNO3, and from HNO3 gas to NO3– in SPM in the atmospheric environment were +33.3‰, +0.5‰, and +4.9‰, respectively. Isotope fractionation of NH4+ in SPM was much higher than that of NO3– in SPM. As the chemical reaction from gaseous precursors progressed, δ15N-NO3– in SPM became steadily heavier.

Quantification of Methanogenic Pathways Using Stable Carbon Isotopic Signatures.

In many anaerobic environments methane (CH4) is produced by methanogens, with either H2/CO2 or acetate (i.e., the methyl group) as precursors, through what are referred to as hydrogenotrophic and acetoclastic methanogenic pathways respectively. Their relative contribution to total CH4 production can be quantified by determining the stable carbon isotopic fractionation factors for both pathways as well as the isotopic signatures of CO2, CH4, and the methyl group in acetate of the sample. The procedures for measuring carbon isotopic fractionation factors of both methanogenic pathways and isotopic composition of these compounds by isotope ratio mass spectrometry are described in this chapter. The results are very helpful in evaluating the activity of the methanogens involved in each methanogenic pathway as well as those of other biological pathways with different fractionation factors.

Simulating speleothem growth in the laboratory: Determination of the stable isotope fractionation (δ13C and δ18O) between H2O, DIC and CaCO3

Here we present novel cave-analogue experiments directly investigating stable carbon and oxygen isotope fractionation between the major involved species of the carbonate system (HCO3−, CO2, CaCO3 and H2O). In these experiments, which were performed under controlled conditions inside a climate box, a thin film of solution flew down an inclined marble or glass plate. After different distances of flow and, thus, residence times on the plate, pH, electrical conductivity, supersaturation with respect to calcite, precipitation rate as well as the δ18O and δ13C values of the dissolved inorganic carbon (DIC) and the precipitated CaCO3 were obtained.Progressive precipitation of CaCO3 along the plate is accompanied by degassing of CO2 and stable isotope fractionation, and the system is driven out of isotope equilibrium. We observe a strong enrichment of the δ13C values with increasing residence time and a smaller enrichment in δ18O. The temporal evolution of the δ18O and δ13C values of both the DIC and the precipitated CaCO3 can be explained by a Rayleigh fractionation model, but the observed enrichment in δ13C values is much larger than expected based on isotope equilibrium fractionation factors.Our setup enables to determine the fractionation between CaCO3 and HCO3−, i.e., εCaCO3/HCO3−. Carbon isotope fractionation, 13εCaCO3/HCO3−, is strongly negative for all experiments and much lower than equilibrium isotope fractionation (0–1‰). In addition, 13εCaCO3/HCO3− decreases with increasing residence time on the plate, and thus decreasing supersaturation with respect to calcite. Thus, isotope fractionation depends on precipitation rate and consequently occurs under kinetic conditions. This is in contrast to previous studies, which found no rate-dependence and no or even a positive carbon isotope fractionation between CaCO3 and HCO3−. Oxygen isotope fractionation, 18εCaCO3/HCO3−, is also negative and dependent on precipitation rate. Since no literature values for 18εCaCO3/HCO3− are available, we calculated 18εCaCO3/HCO3− using equilibrium oxygen isotope fractionation factors between water and calcite and water and HCO3−, respectively. At the beginning of the plate, the fractionation is in agreement with the fractionation calculated using fractionation factors determined in cave systems.The observed fractionation between CaCO3 and water, 1000ln18α, is also in good agreement with the values determined in cave systems and shows a very similar temperature dependence 1000ln18α=16.516±1.267∗103T−26.141±4.356. However, with progressive precipitation of CaCO3 along the plate, the system is forced out of isotope equilibrium with the water, and 1000ln18α increases.The large, negative, rate-dependent isotope fractionations observed in this study suggest that precipitation of speleothem calcite is strongly kinetically controlled and may, thus, have a large effect on speleothem δ18O and δ13C values. Since these values may erroneously be interpreted as reflecting changes in past temperature, precipitation and/or vegetation density, these results have important implications for paleoclimate reconstructions from speleothems.

First-principles investigation of the effect of crystal structure on sulfur isotope fractionation in sulfide polymorphs

In order to study the effect of crystal structure on S isotope fractionation properties of sulfide polymorphs, the reduced partition function ratios of S-34/S-32 (10(3)ln beta(34-32)) for FeS2, CoAsS, ZnS, CdS and MnS polymorphs are calculated in the present study. Our calculations show that 10(3)ln beta(34-32) decreases in the order of pyrite > marcasite > cobaltite > alloclasite > sphalerite approximate to wurtzite > gamma-MnS > hawleyite approximate to greenockite > alabandite. The S coordination number exerts a strong influence on the equilibrium partitioning of S isotopes in these sulfides, and there exist significant S isotope fractionations between the sulfides with different S coordination numbers (CNS), such as alabandite (CNS = 6) and other sulfides (e.g., FeS2, CoAsS, ZnS and CdS polymorphs) (CNS = 4). There is a broad negative correlation between the average bond lengths of all the bonds formed by S and 10(3)ln beta(34-32) values. The compactness of atom packing plays an important role in determining S isotope fractionation properties of sulfides, and S-34 generally tends to concentrate in the phases with higher atomic packing factor (APF). The linear correlation between APF and 10(3)ln beta(34-32) can predict the change of 10(3)ln beta(34-32) with increasing pressure, but pressure has a negligible effect on S isotope fractionation factors between sulfides.

Using stable isotope fractionation factors to identify Cr(VI) reduction pathways: Metal-mineral-microbe interactions

Microbes interact with metals and minerals in the environment altering their physical and chemical states, whilst in turn metals and minerals impact on microbial growth, activity and survival. The interactions between bacteria and dissolved chromium in the presence of iron minerals, and their impact on Cr isotope variations, were investigated. Cr(VI) reduction experiments were conducted with two bacteria, Pseudomonas fluorescens LB 300 and Shewanella oneidensis MR-1, in the presence of two iron oxide minerals, goethite and hematite. Both minerals were found to inhibit the rates of Cr(VI) reduction by Pseudomonas, but accelerated those of Shewanella. The Cr isotopic fractionation factors generated by Shewanella were independent of the presence of the minerals (ε = −2.3‰). For Pseudomonas, the ε value was the same in both the presence and absence of goethite (−3.3‰); although, it was much higher (ε = −4.3‰) in the presence of hematite. The presence of aqueous Fe(III) in solution had no detectable impact on either bacterial Cr reduction rates nor isotopic fractionation factors. The presence of aqueous Fe(II) induced rapid abiotic reduction of Cr(VI). The different effects that the presence of Fe minerals had on the Cr fractionation factors and reduction rates of the different bacterial species may be attributed to the way each bacteria attached to the minerals and their different reduction pathways. SEM images confirmed that Pseudomonas cells were much more tightly packed on the mineral surfaces than were Shewanella. The images also confirmed that Shewanella oneidensis MR-1 produced nanowires. The results suggest that the dominant Cr(VI) reduction pathway for Pseudomonas fluorescens LB 300 may have been through membrane-bound enzymes, whilst for Shewanella oneidensis MR-1 it was probably via extracellular electron transfer. Since different minerals impact differentially on bacterial Cr(VI) reduction and isotope fractionation, variations of mineralogies and the associated changes of bacterial communities should be taken into consideration when using Cr isotopes to quantify Cr redox behaviour in the environment.

Deconstructing the Dissimilatory Sulfate Reduction Pathway: Isotope Fractionation of a Mutant Unable of Growth on Sulfate.

The sulfur isotope record provides key insight into the history of Earth's redox conditions. A detailed understanding of the metabolisms driving this cycle, and specifically microbial sulfate reduction (MSR), is crucial for accurate paleoenvironmental reconstructions. This includes a precise knowledge of the step-specific sulfur isotope effects during MSR. In this study, we aim at resolving the cellular-level fractionation factor during dissimilatory sulfite reduction to sulfide within MSR, and use this measured isotope effect as a calibration to enhance our understanding of the biochemistry of sulfite reduction. For this, we merge measured isotope effects associated with dissimilatory sulfite reduction with a quantitative model that explicitly links net fractionation, reaction reversibility, and intracellular metabolite levels. The highly targeted experimental aspect of this study was possible by virtue of the availability of a deletion mutant strain of the model sulfate reducer Desulfovibrio vulgaris (strain Hildenborough), in which the sulfite reduction step is isolated from the rest of the metabolic pathway owing to the absence of its QmoABC complex (ΔQmo). This deletion disrupts electron flux and prevents the reduction of adenosine phosphosulfate (APS) to sulfite. When grown in open-system steady-state conditions at 10% maximum growth rate in the presence of sulfite and lactate as electron donor, sulfur isotope fractionation factors averaged −15.9‰ (1 σ = 0.4), which appeared to be statistically indistinguishable from a pure enzyme study with dissimilatory sulfite reductase. We coupled these measurements with an understanding of step-specific equilibrium and kinetic isotope effects, and furthered our mechanistic understanding of the biochemistry of sulfite uptake and ensuing reduction. Our metabolically informed isotope model identifies flavodoxin as the most likely electron carrier performing the transfer of electrons to dissimilatory sulfite reductase. This is in line with previous work on metabolic strategies adopted by sulfate reducers under different energy regimes, and has implications for our understanding of the plasticity of this metabolic pathway at the center of our interpretation of modern and palaeo-environmental records.

Predicting High-Dimensional Isotope Relationships from Diagnostic Fractionation Factors in Systems with Diffusional Mass Transfer

A dual or multiple stable isotope relationship, for example, a trajectory in a δ−δ (or δ′−δ′) space, can be used to deduce the relationship of underlying diagnostic isotope fractionation factors (α) and therefore reveal the mechanism of a reaction process. While temporal data sampled from a closed-system can be treated by a Rayleigh distillation model, spatial data should be treated by a reaction-transport model. Owing to an apparent similarity between the temporal and spatial trajectories, the research community has often ignored this distinction and applied a Rayleigh distillation model to cases where a reaction-transport model should be applied. To examine the potential error of this practice, here we compare the results of a Rayleigh distillation model to a diffusional reaction-transport model by simulating the trajectories in nitrate’s δ′18O−δ′15N space during a simple denitrification process. We found that an incorrect application of a Rayleigh distillation model can underestimate the degree of a di...

Modelling the mercury stable isotope distribution of Earth surface reservoirs: Implications for global Hg cycling

Abstract Mercury (Hg) stable isotopes are useful to understand Hg biogeochemical cycling because physical, chemical and biological processes cause characteristic Hg isotope mass-dependent (MDF) and mass-independent (MIF) fractionation. Here, source Hg isotope signatures and process-based isotope fractionation factors are integrated into a fully coupled, global atmospheric-terrestrial-oceanic box model of MDF (δ202Hg), odd-MIF (Δ199Hg) and even-MIF (Δ200Hg). Using this bottom-up approach, we find that the simulated Hg isotope compositions are inconsistent with the observations. We then fit the Hg isotope enrichment factors for MDF, odd-MIF and even-MIF to observational Hg isotope constraints. The MDF model suggests that atmospheric Hg0 photo-oxidation should enrich heavy Hg isotopes in the reactant Hg0, in contrast to the experimental observations of Hg0 photo-oxidation by Br. The fitted enrichment factor of terrestrial Hg0 emission in the odd-MIF model (5‰) is likely biased high, suggesting that the terrestrial Hg0 emission flux (160 Mg yr−1) used in our standard model is underestimated. In the even-MIF model, we find that a small positive atmospheric Hg0 photo-oxidation enrichment factor (0.22‰) along with enhanced atmospheric HgII photo-reduction and atmospheric Hg0 dry deposition (foliar uptake) fluxes to the terrestrial reservoir are needed to match Δ200Hg observations. Marine Hg isotope measurements are needed to further expand the use of Hg isotopes in understanding global Hg cycling.

Anaerobic methane oxidation by nitrate: kinetic isotope effect

The ratio of stable carbon isotopes (13C/12C) in different environments serves as a significant limitation in estimating the global balance of methane [Hornibrook et al., 2000]. In this case, the value of 13C/12C largely depends on the kinetic isotope effect associated with the metabolism of microorganisms that produce and consume CH4. The article suggests a dynamic model of the processes of methane formation and its anaerobic oxidation with nitrate by methanotrophic denitrifying microorganisms (DAOM), which allowed estimating the fractionation factor of stable carbon isotopes. In the experiment with peat from the minerotrophic bog [Smemo, Yavitt, 2007], the dynamics of the amount of methane and was measured. The dynamic model showed that the introduction of nitrate leads to a slow decrease in the partial pressure of methane. Since methane in the DAOM process is a substrate, methane is enriched with heavier carbon 13C in the system under study. This leads to an increase in the value . The carbon isotope fractionation factor during methane oxidation with nitrate was equal to 1.018 and comparable with the fraction of carbon isotope fractionation in the process of acetoclastic methanogenesis (1.01). Model calculations have shown that during incubation the apparent fractionation factor of carbon isotopes with the simultaneous formation of methane and DAOM slowly decreases. The ratio of 13C/12C isotopes in dissolved and gaseous methane practically does not differ. The model showed that an increase in the initial concentration of nitrate increases the rate of DAOM, which leads to a decrease in the concentration of dissolved methane. In this case, the value of 13C/12C increases. In field studies, Shi et al. (2017) showed that the presence of DAOM in peat bogs in which fertilizers penetrate can be controlled by the amount of nitrate used and the depth of penetration into the anoxic layer. Two MATLAB files describing DAOM are attached to the article.

Low temperature stable mineral recrystallization of foraminiferal tests and implications for the fidelity of geochemical proxies

Fossilized foraminiferal tests are widely used as proxies to constrain paleoenvironments, including past sea surface and bottom water temperatures and the chemical composition of ancient seawater. However, relatively little is known about the extent to which calcareous foraminiferal tests exchange with seawater prior to burial. In this study, we used a 45Ca radiotracer approach to quantify the extent and rate of foraminiferal calcite–fluid exchange over a three-month period. Modern foraminiferal tests exhibited faster exchange with a 45Ca-spiked solution (2.6⋅10−4 to 2.0⋅10−5 mol/m2/d) than fossil foraminiferal tests (7.6⋅10−5 to 5.3⋅10−6 mol/m2/d) from ODP Site 807. The impact of dissolved Si on the exchange rate of modern foraminifera was minimal (2.6⋅10−4 to 9.2⋅10−6 mol/m2/d). Unlike foraminiferal calcite, inorganic calcite exchanged at remarkably slow rates (6.6⋅10−6 to 5.9⋅10−7 mol/m2/d) suggesting that biogenic calcite is more reactive than inorganic calcite. Increased Ca2+, Mg2+, and Sr2+ concentrations in fluids from parallel tracer-free reactors indicated preferential dissolution of tests, though there was no overt change in test morphology during experiments (as imaged by scanning electron microscopy and micro-CT scanning). Time-dependent box model simulations of 45Ca, as well as aqueous elemental chemistry, support the hypothesis that calcite–fluid exchange occurred via dissolution–reprecipitation of foraminiferal tests that was induced by intra-test chemical heterogeneity. The short-term exchange reactions investigated in this study are comparable to the reactions that can occur in the seawater column and at the sediment–seawater interface. This work suggests that dissolution–reprecipitation in such settings has the potential to overprint the chemistry of foraminiferal tests without overt physical alteration, which can lead to a potential bias in paleoenvironmental reconstructions. Model-based estimates suggest a maximum of 2–3°C alteration of Mg/Ca-based temperatures over experimental time scales. Model efforts also suggest that the diagenetic trajectories in Mg/Ca–δ18O space, for instance, are impacted by the partition coefficient of Mg in calcite (λMg) as well as by the mechanism of alteration. This is demonstrated for (i) the ratio of exchange to dissolution and (ii) the relative rate of high-Mg-calcite to low-Mg-calcite reaction. We have illustrated that the mechanism of alteration, in addition to water-rock ratios, determine diagenetic trajectories. Finally, surface reaction has implications for our ability to constrain fractionation factors in experimental and natural systems. The experimental observations suggest that exchange between tests and solution is nominal over ∼100 days. Experiments aimed at deducing fractionation factors rely on the equilibration of bulk solid and fluid, since these are the readily analyzed reservoirs. We quantify the fraction of carbonate (>5%) that must react so that surface reaction does not impact the interpretation of Ca isotopic fractionation factors, and discuss the implications for more “fluid-dominated” elements such as Mg and O.

Triple oxygen isotope fractionation in the DIC-H2O-CO2 system: A numerical framework and its implications

Triple oxygen isotope composition of carbonate minerals reflects the isotope composition of the water from which carbonates precipitate, and is emerging as a promising proxy for constraining past changes in Earth hydroclimate and surface environment. However, quantitative interpretation of this proxy is not straightforward when carbonate minerals do not form under isotope equilibrium with water. For these carbonates, isotope composition of dissolved inorganic carbon (DIC) in the precipitating solution usually exerts the dominant control on their isotope composition. Here we examine the systematics of triple oxygen isotope fractionation in the DIC-H2O-CO2 system by deriving the fundamental equilibrium and kinetic triple oxygen isotope fractionation factors in this system and simulating the evolution of triple oxygen isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H2O isotope exchange, CO2 degassing and CO2 absorption.We show that under thermodynamic equilibrium dissolved HCO3– and CO32– both exhibit similar Δ′17O as calcite and aragonite (within 10 per meg at 25 °C), with temperature dependences around 0.61 per meg/°C. However, kinetic isotope fractionations associated with CO2 hydration and hydroxylation and their reverse reactions can produce a range of disequilibrium triple oxygen isotope effects in DIC during all three processes we simulated. The magnitudes of these disequilibrium effects vary with both time and the physicochemical conditions of the solution, e.g., temperature, pH and initial composition. Particularly, we predict correlated enrichments in Δ′17O and δ18O of DIC during CO2 degassing but depletions during CO2 absorption. The slopes of these correlations vary mainly as a function of solution pH but not temperature, DIC concentration or air pCO2, yielding values of 8.8 and 12.0 per meg/‰ at pH = 8 and 9, respectively, at 25 °C. Such disequilibrium isotope effects in DIC are expected to be inherited by carbonate minerals that form from these solutions (e.g., speleothem, coral skeleton, and high pH travertine) and, if not accounted for, could lead to inaccurate estimates of the triple oxygen isotope compositions of the parent water and carbonate formation temperatures. Our numerical model provides a quantitative framework for interpreting triple oxygen isotope composition of DIC and for correcting disequilibrium triple oxygen isotope effects in natural carbonates.

Iron isotope exchange and fractionation between hematite (α-Fe2O3) and aqueous Fe(II): A combined three-isotope and reversal-approach to equilibrium study

Hematite is the most thermodynamically stable Fe oxide at the Earth’s surface and its isotopic composition may record past biogeochemical Fe cycling and paleo-environmental conditions. Proper interpretation of its isotopic values requires an understanding of the equilibrium Fe isotope fractionation factor between hematite and other Fe-bearing minerals and aqueous species. Here, we use the three-isotope method (54Fe-56Fe-57Fe, with 56Fe/54Fe and 57Fe/56Fe ratios expressed in delta notation relative to the IRMM-014 isotope standard) to simultaneously determine Fe isotope exchange, via tracking an enriched tracer-isotope (i.e. 57Fe), and measure mass-dependent isotope fractionation between aqueous Fe(II) (Fe(II)aq) with structural-Fe(III) in hematite, as determined from variations in 56Fe/54Fe ratios. Specifically, we used a reversal-approach to equilibrium by reacting three hematite samples of varying particle size and reactivity with two Fe(II)aq solutions that had initial 56Fe/54Fe ratios above and below the presumed equilibrium value. We confirm that Fe(II)aq readily exchanges with hematite at low temperature, and that the extent of exchange depends on hematite particle size. In three-isotope plots, δ56Fe values of Fe(II)aq exhibit unique trajectories depending on the initial hematite particle size. Rapid exchange occurring between Fe(II)aq and small hematite particles are affected by an apparent mixing reaction, which results in the δ56Fe value of Fe(II)aq to approach the isotopic composition of hematite. These trajectories exhibit inflections to more negative δ56Fe values during continued exchange. The location of these inflections depend on particle size and are interpreted to represent a change in the recrystallization mechanism from rapid “heterogeneous” recrystallization, which is limited to surface exchange, to “homogeneous” recrystallization involving the bulk hematite. Slow isotopic exchange occurring at longer reaction times appears to approximate equilibrium for all particle sizes with measured Fe(II)aq-hematite Fe isotope fractionation factors at 22 °C ranging from −2.61 ‰ (±0.26‰, 2σ) to −3.14 ‰ (±0.34‰, 2σ) in δ56Fe, and an average value of −2.8‰. This value is consistent with estimated fractionation factors measured during microbial Fe reduction experiments and with calculated and spectroscopically derived reduced partition function ratios. The three-isotope method provides a robust measure of equilibrium fractionation if proper constrains are utilized to eliminate extrapolation from partial exchange. Furthermore, it can provide insight into the mechanism of mineral-fluid exchange which is necessary for accurate quantification of the extent of mineral recrystallization when using isotopic tracers.

Experimental determination of equilibrium CH4–CO2–CO carbon isotope fractionation factors (300–1200 °C)

Carbon isotope fractionation in the CO2–CO–CH4–C system was investigated at 300–1200°C at near-atmospheric pressures by thermally decomposing a variety of organic materials in sealed quartz tubes. Measured gas speciations correspond well to the expected range from thermodynamic calculations. We show that chemical and isotopic equilibrium among gas species is obtained when applying a nickel catalyst for CO2/CH4, CH4/CO, and CO2/CO at ≤600°C or without a catalyzing agent for CO2/CO at ≥800°C. The experiments define carbon isotope fractionation factors for the CO2/CH4, CO2/CO and CH4/CO pairs as(i)103ln⁡αCO2/CH4=8.9(±0.6)⋅105⋅(1T2)0.825(±0.005) (200–1200°C)(ii)103ln⁡αCO2/CO=1.07(±0.05)⋅106⋅(1T2)0.830(±0.003) (300–1200°C)(iii)103ln⁡αCH4/CO=1.1(±0.2)⋅103⋅(1T2)0.462(±0.001) (300–1200°C), which reproduce the experimental values within 0.2‰ for CO2/CH4 and CO2/CO and within 0.12‰ for CH4/CO (T in K, 1σ fit uncertainties in brackets, CO2/CH4 includes the ≤600°C experimental data of Horita, 2001). Carbon isotope fractionation factors at 1000°C are still large for CO2/CH4 and CO2/CO (6.6 and 7.5‰, respectively) but only 1.5‰ for CH4/CO.Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ13C values that depend on the X(O) = O/(O + H) of the organic starting material, i.e. the initial oxidation state of carbon in the organics. We further observe a catalytic effect of the quartz walls on chemical and isotopic exchange in the CO2/CO system, probably due to the activation of the silicate surface by H+ and OH− ions at >650°C. Our experimental results yield improved calibrations of the CO2/CH4 equilibria and the first experimental calibration of CO2/CO and CH4/CO carbon isotope fractionation. Applications are in the tracing of magmatic hydrothermal gas emissions, in carbon-precipitating COH-fluids, and in monitoring of coal-seam fires, but our results may also be applied for quality control during steel-making processes.

Special Issue: Diagnostic Tools to Assess In Situ Remediation System Performance

Special Issue: Diagnostic Tools to Assess In Situ Remediation System Performance

Dual QM and MM Approach for Computing Equilibrium Isotope Fractionation Factor of Organic Species in Solution.

A dual QM and MM approach for computing equilibrium isotope effects has been described. In the first partition, the potential energy surface is represented by a combined quantum mechanical and molecular mechanical (QM/MM) method, in which a solute molecule is treated quantum mechanically, and the remaining solvent molecules are approximated classically by molecular mechanics. In the second QM/MM partition, differential nuclear quantum effects responsible for the isotope effect are determined by a statistical mechanical double-averaging formalism, in which the nuclear centroid distribution is sampled classically by Newtonian molecular dynamics and the quantum mechanical spread of quantized particles about the centroid positions is treated using the path integral (PI) method. These partitions allow the potential energy surface to be properly represented such that the solute part is free of nuclear quantum effects for nuclear quantum mechanical simulations, and the double-averaging approach has the advantage of sampling efficiency for solvent configuration and for path integral convergence. Importantly, computational precision is achieved through free energy perturbation (FEP) theory to alchemically mutate one isotope into another. The PI-FEP approach is applied to model systems for the 18O enrichment found in cellulose of trees to determine the isotope enrichment factor of carbonyl compounds in water. The present method may be useful as a general tool for studying isotope fractionation in biological and geochemical systems.

Morphological and isotopic changes of heterocystous cyanobacteria in response to N2 partial pressure.

Earth's atmospheric composition has changed significantly over geologic time. Many redox active atmospheric constituents have left evidence of their presence, while inert constituents such as dinitrogen gas (N2 ) are more elusive. In this study, we examine two potential biological indicators of atmospheric N2 : the morphological and isotopic signatures of heterocystous cyanobacteria. Biological nitrogen fixation constitutes the primary source of fixed nitrogen to the global biosphere and is catalyzed by the oxygen-sensitive enzyme nitrogenase. To protect this enzyme, some filamentous cyanobacteria restrict nitrogen fixation to microoxic cells (heterocysts) while carrying out oxygenic photosynthesis in vegetative cells. Heterocysts terminally differentiate in a pattern that is maintained as the filaments grow, and nitrogen fixation imparts a measurable isotope effect, creating two biosignatures that have previously been interrogated under modern N2 partial pressure (pN2 ) conditions. Here, we examine the effect of variable pN2 on these biosignatures for two species of the filamentous cyanobacterium Anabaena. We provide the first in vivo estimate of the intrinsic isotope fractionation factor of Mo-nitrogenase (εfix =-2.71±0.09‰) and show that, with decreasing pN2 , the net nitrogen isotope fractionation decreases for both species, while the heterocyst spacing decreases for Anabaena cylindrica and remains unchanged for Anabaena variabilis. These results are consistent with the nitrogen fixation mechanisms available in the two species. Application of these quantifiable effects to the geologic record may lead to new paleobarometric measurements for pN2 , ultimately contributing to a better understanding of Earth's atmospheric evolution.

Nitrogen isotope fractionation factors (α) measured and estimated from the volatilisation of ammonia from water at pH 9.2 and pH 8.5

ABSTRACTThis paper examines the nitrogen isotope fractionation factors (α) associated with the volatilisation of ammonia from water under controlled conditions at two pH values (8.5 and 9.2). This experiment assumed the continuous removal of ammonia at a single purge rate of 10 mL air min−1. The fractionation resulting from the removal of total ammonia from the water into an acid trap was named the observed isotope fractionation factor (αobs), and it was measured as 1.019 (±0.0025) at pH 8.5 and 1.030 (±0.0025) at pH 9.2. The observed isotope fractionation factor includes the equilibrium isotope fractionation factor (αeq) and the kinetic isotope fractionation factor (αkin), each one mathematically derived from the experimental data. The equilibrium and kinetic isotope fractionation factors were estimated as αeq = 1.036 (±0.0014) and αkin = 1.050 (±0.003), respectively. Our results are compared to other previously measured and estimated fractionation factors.

Calculation of Stable Isotope (O, C, Cа) Fractionation Factors for Apatite

Calculation of Stable Isotope (O, C, Cа) Fractionation Factors for Apatite