Observed Fractionation Factors Research Articles

High temperature evaporation and isotopic fractionation of K and Cu

The chemical and isotopic signatures of moderately volatile elements are useful for understanding processes of volatile depletion in planetary formation and differentiation. However, the fractionation factors between gas and melt phases during evaporation that are required to model these planetary volatile depletion processes are still sparse. In this study, twenty heating experiments were conducted in 1 atm gas-mixing furnaces to constrain the behavior of K, Cu, and Zn evaporation and isotopic fractionation from basaltic melts at high temperatures. The temperatures range from 1300 °C to 1400 °C, and durations are from 2 to 8 days. Oxygen fugacities (fO2) range from one log unit below to ten log units above that of the iron-wüstite buffer (IW–1 to IW + 10, corresponding to logfO2 of –10.7 to –0.68 at 1400 °C). The conditions were selected to achieve an evaporation-dominated regime (where timescales of diffusion ≪ evaporation for trace elements) in order to avoid diffusion-limited evaporation. Our results show during evaporation Zn behaved as the most volatile, followed by Cu and then K, regardless of temperature and oxygen fugacity. Partitioning of Zn into spinel layers within experimental capsules, however, has been observed, which has substantial effects on the Zn isotope fractionation factor. Therefore, Zn results are presented but further discussion is excluded. Element loss depends on both temperature and oxygen fugacity, where higher temperatures and lower oxygen fugacities promote evaporation. However, with varying temperature and oxygen fugacity, the kinetic isotopic fractionation factors, α (where, RR0=fα-1), for K and Cu remain constant, thus these factors can be applied to a wider range of conditions than those in this study. The experimentally determined fractionation factors for K, and Cu during evaporation from basaltic melts are 0.9944, and 0.9961, respectively. The fractionation factors for these elements with varying volatilities are significantly larger than the “apparent observed fractionation factors,” which approach one and are inferred from lunar basalts relative to the Bulk Silicate Earth. This observation suggests near-equilibrium conditions during volatile-element loss from the Moon as the “apparent observed fractionation factors” of lunar basalts are similar for all three elements.

A modified Monod rate law for predicting variable S isotope fractionation as a function of sulfate reduction rate

Abstract Microbial sulfate reduction is associated with characteristic S isotope partitioning, which can aid in determining the rate of this ubiquitous reactive pathway in a wide variety of reducing environments. Here we introduce a new simple yet predictive model that expands the range of environmental conditions over which this functional relationship can be applied through the use of a modified Monod rate expression constrained by a novel set of experiments. A series of continuously-fed batch reactors containing an equivilant biomass of Desulfovibrio vulgaris, a sulfate reducing bacteria, were subjected to differing continuous mass addition rates of formate to precisely control the rate of sulfate reduction via electron donor limitation. Based on growth yield calculations, additional biomass accumulation was negligible. The isotopic composition (δ34S) of the residual sulfate pool was measured through time for each experiment. This approach resulted in five steady state reduction rates of 2.06, 1.22, 0.83, 0.52 and 0.28 μmol * h−1 that enriched the unreacted sulfate in 34S by unique characteristic enrichment factors (αobs) of 0.9976, 0.9962, 0.9938, 0.9924 and 0.9903, respectively. This relationship was used to calibrate a new coupled set of isotope-specific Monod rate laws that have been modified to incorporate (1) a minimum (α1) and maximum (α2) fractionation factor and (2) a rate-controlling electron donor factor (DF). Application of these parameters in the updated model reproduce most of our data and simulate realistic shifts in the observed fractionation factor (αobs) as a function of reduction rate in an electron donor limited system. The model was applied to our current dataset as well as three previous studies, which collectively provide a broad range in both rates and αobs, and support validation across substantially different conditions. We show that this approach offers a reasonable approximation to more detailed microbial reactive network models, while still maintaining sufficient simplicity and versatility to allow incorporation into multi-component reactive transport simulations. Thus, the current study provides a foundation for accurate simulation of the relationship between αobs and sulfate reduction rates in open, transient, and through-flowing systems.

A specialist detritivore links Spartina alterniflora to salt marsh food webs

Because most plant production is subject to senescence and is eventually consumed by detritivores, the factors that drive detritivore diet choice are pivotal to the flow of energy and materi- als through food webs. Here, we investigated the common salt marsh amphipod Gammarus palustris, which is a habitat specialist that feeds specifically on the dead leaves of its living host plant, salt marsh cordgrass Spartina alterniflora. Restricted use and consumption of dead S. alterniflora was reinforced by superior amphipod performance (survival, size, and sexual development) on dead S. alterniflora relative to other diets, and was driven at least in part by amphipods being physically able to feed on soft, decaying plant tissues but not live, turgid tissues. Stable isotopes from field surveys and laboratory assimilation assays suggest that amphipods also feed on S. alterniflora in the field, and that the important marsh fish Fundulus hetroclitus feeds on amphipods. Thus, consumption of G. palustris by F. heteroclitus may be an important trophic pathway linking cordgrass production to nearshore food webs. Importantly, direct isotopic analyses of amphipods and their known food sources demonstrated substantial deviation of observed fractionation factors from idealized stan- dards. This suggests caution when using idealized trophic shifts to describe food web linkages, and a renewed focus on assimilation assays to determine the realized fractionation of dietary isotopes.

Fractionation factors for stable isotopes of N and O during N2O reduction in soil depend on reaction rate constant

Nitrous oxide (N(2)O) is a major greenhouse gas that is mainly produced but also reduced by microorganisms in soils. We determined factors for N and O isotope fractionation during the reduction of N(2)O to N(2) in soil in a flow-through incubation experiment. The absolute value of the fractionation factors decreased with increasing reaction rate constant. Reaction rates constants ranged from 1.7 10(-4) s(-1) to 4.5 10(-3) s(-1). The minimum, maximum and median of the observed fractionation factors were for N -36.0 per thousand, -1.0 per thousand and -9.3 per thousand and for O -74.0 per thousand, -6.9 per thousand and -26.3 per thousand, respectively. The ratio of O isotope fractionation to N isotope fractionation was 2.4 +/- 0.3 and it was independent from the reaction rate constants. This leads us to conclude that fractionation factors are variables while their ratio in this particular reaction might be a constant.

Isotopic view on nitrate loss in Antarctic surface snow

Massive post‐depositional processes alter the nitrate concentration in polar firn where the annual snow accumulation is low. This hinders a direct atmospheric interpretation of the ice core nitrate record. Fractionation of nitrate isotopes during post‐depositional nitrate loss may allow estimating the amount of nitrate loss in the past. We measured δ15N of nitrate in two Antarctic surface cores from the Dome C area. In concert with the known concentration decrease with depth we observe an increase in the isotopic signature. Assuming a Rayleigh type process we find an isotope effect of ɛ = −54‰. We measured the fractionation factor for photolysis in the laboratory and obtained ɛ = −11.7 ± 1.4‰. As the observed fractionation factor in the firn is much lower this rules out that photolysis in the surface snow is the main process leading to the dramatic nitrate loss in the top centimeters of the firn.

Carbon and hydrogen isotope fractionation by moderately thermophilic methanogens

A series of laboratory studies were conducted to increase understanding of stable carbon (13C/12C) and hydrogen (D/H) isotope fractionation arising from methanogenesis by moderately thermophilic acetate- and hydrogen-consuming methanogens. Studies of the aceticlastic reaction were conducted with two closely related strains of Methanosaeta thermophila. Results demonstrate a carbon isotope fractionation of only 7‰ (α = 1.007) between the methyl position of acetate and the resulting methane. Methane formed by this process is enriched in 13C when compared with other natural sources of methane; the magnitude of this isotope effect raises the possibility that methane produced at elevated temperature by the aceticlastic reaction could be mistaken for thermogenic methane based on carbon isotopic content. Studies of H2/CO2 methanogenesis were conducted with Methanothermobacter marburgensis. The fractionation of carbon isotopes between CO2 and CH4 was found to range from 22 to 58‰ (1.023 ≤ α ≤ 1.064). Greater fractionation was associated with low levels of molecular hydrogen and steady-state metabolism. The fractionation of hydrogen isotopes between source H2O and CH4 was found to range from 127 to 275‰ (1.16 ≤ α ≤ 1.43). Fractionation was dependent on growth phase with greater fractionation associated with later growth stages. The maximum observed fractionation factor was 1.43, independent of the δD-H2 supplied to the culture. Fractionation was positively correlated with temperature and/or metabolic rate. Results demonstrate significant variability in both hydrogen and carbon isotope fractionation during methanogenesis from H2/CO2. The relatively small fractionation associated with deuterium during H2/CO2 methanogenesis provides an explanation for the relatively enriched deuterium content of biogenic natural gas originating from a variety of thermal environments. Results from these experiments are used to develop a hypothesis that differential reversibility in the enzymatic steps of the H2/CO2 pathway gives rise to variability in the observed carbon isotope fractionation. Results are further used to constrain the overall efficiency of electron consumption by way of the hydrogenase system in M. marburgensis, which is calculated to be less than 55%.

Hydrogen isotope fractionation between methanol and diphenylphosphine or dimethylphosphine in the gas phase and in aprotic solvents

D/H fractionation factors between MeOH and Ph2PH in dilute solutions of tetrachloroethylene, benzene, tetrahydrofuran, pyridine, and acetonitrile and T/H fractionation factors between MeOH and Me2PH vapors were measured. The experimental results agree very well with values calculated from the statistical theory of isotope effects formulated by Bigeleisen and Mayer. There are correlations between observed fractionation factors and solvent polarity, and the interaction energy of methanol with the given solvent. Another correlation has been found between enthalpy of the exchange reactions and the interaction energy between methanol and the given solvent. Key words: isotope effects, fractionation factor, diphenylphosphine, methanol.

Oxygen isotope exchange between sulphate and water during bacterial reduction of sulphate

The bacterial reduction of sulphate is accompanied by oxygen isotope exchange reactions. The mechanisms were not investigated in detail but it is suggested that the formation of sulphate-enzyme complexes as intermediary reaction products are facilitating the exchange of 18O between water and sulphate. Under the experimental conditions the observed fractionation factor is close to 25‰ at 30°C and approaches 27‰ at 17°C. Extrapolated values from a field study indicate a difference of ∼ 29‰ at ∼ 5°C.

Experimental study of sulfur isotope fractionation factors between sulfate and sulfide in high temperature melts.

Sulfur isotope fractionation factors between sulfate and sulfide at 600 to 1, 000°C were experimentally determined by decomposing anhydrous Na2SO3 in melts of NaCl and LiCl-KCl mixture (6 : 4 molar ratio). Sulfur isotope exchange equilibrium was attained between sulfate and sulfide formed by decomposition of sodium sulfite within 120 minutes at 600°C in the LiCl-KCl melt and within 30 minutes at 810°C in the NaCl melt. The observed fractionation factors (α) obey the relationship in the experimental temperature range: 1000 lnαsulfate-sulfide = 7.4 × 106/T2 − 0.19. In alkali chloride melts, sulfide would exist as HS-, S2-, NaHS and NaS-, and sulfate as SO42- and NaSO4-, respectively, in an analogy with aqueous sodium chloride solution. The observed fractionation factors are considered to represent those between bulk sulfate and sulfide ions. The fractionation factor between gaseous H2S and dissolved sulfate in melt at 600 and 700°C is about 1 and 0.7‰ smaller than those between dissolved sulfide and sulfate in melt, respectively, and may be approximated by the following equation in the experimental temperature range: 1000 lnαsulfate-H2S = (6.5 ± 0.3) × 106/T2.

A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO 2 laser, area ∼ 8 cm2, initiates an explosive reaction between SF 6 and SiH 4 . This occurs at a minimum energy of 4 J [full width at half maximum (FWHM) \sim 1.5 /mu s] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 p (SiH 4 )/ p (SF 6 ) 2 ( B^{3}\Sigma-_{u} \rightarrow X^{3}\Sigma-_{g} ). Transitions \upsilon' (0-4) \rightarrow \upsilon (2-15) were recorded. In the3 \Sigma-_{u} state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH 4 )/(SF 6 ) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH 4 )/(SF 6 ) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF 6 (0.2-0.5 percent of initial amount) is enriched in34SF 6 ; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO 2 laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.

Calculations of the Oxygen Isotope Fractionation between Hydration Water of Cations and Free Water

The oxygen isotope fractionation factors between the hydration complex of the alkali ions in the gas phase and a free water molecule have been computed on the basis of the energy surfaces calculated by Kistenmacher, Popkie and Clementi for a water molecule in the field of an alkali ion. For comparison with recently measured oxygen isotope fractionation factors in aqueous alkali halide solutions, the gas phase values are multiplied with the corresponding separation factors between water vapor and liquid water thus relating the hydration complex in the gas phase with pure water. Qualitative agreement between computed and observed fractionation factors has been found for H2O and D2O even neglecting the isotope effect connected with the transfer of the hydration complex from the gas phase to the solution. This transfer effect is estimated for H2O by a quantitative comparison of computed and observed oxygen isotope fractionation factors.

Helium Isotope Effect in Solution in Water and Seawater

The isotope effect in the solution of helium in water from 0 degrees to 40 degrees C has been determined by microgasometric measurements of the solubilities of pure helium-3 and helium4. At 0 degrees C helium-3 is less soluble than helium-4 in both distilled water and sea-water by 1.2 percent. The observed fractionation factor is 0.988+/-0.002 at 0 degrees C and appears to decrease with increasing temperature at the rate of 0.0001 per degree Centigrade, although the existence of this trend is of limited statistical certainty. The measured isotope effect is in agreement with the ratio of helium-3 to helium-4 in surface ocean water reported by Clarke, Beg, and Craig.

Analysis of Isotope Exchange Reactions among Nitrogen Oxides Involving N2O3

The fundamental frequencies of isotopic N2O3 molecules and equilibrium constants of isotope exchange reactions involving N2O3 have been calculated from available spectroscopic data. The calculations are compared with observed fractionation factors for nitrogen isotope exchange between N2O3 and NO, reported previously, and with experimental fractionation factors for oxygen isotope exchange, reported in this paper. The effect of phase changes on the isotopic equilibrium constant has been estimated.

Fractionation of sulphur isotopes in volcanic gases

The 34S 32S -ratios in several samples of SO 2 and H 2S from the fumaroles of Volcano Showashinzan, Japan, were measured. The outlet temperature of the fumaroles from which the samples were collected ranged from 100 to 661 °C. It was observed that the lower the outlet temperature of the fumarole, the higher the content of the heavy isotope in the SO 2. The isotopic ratio of H 2S was found, on the other hand, to be almost constant throughout the range of temperature given above. This isotopic fractionation between SO 2 and H 2S was ascribed to the reaction: H 2 S + 2 H 2 O ⇌ SO 2 + 3 H 2 The temperatures estimated from the observed fractionation factors and the theoretical ones were in fairly good agreement with the observed outlet temperatures. The isotopic ratio of both compounds can be reasonably explained by a simplified system containing only H 2S, H 2O, SO 2 and H 2. But, some difficulty arises when the chemical compositions of the actual fumaroles are taken into consideration.

Isotopic Composition of Oxygen in the Catalytic Decomposition of Hydrogen Peroxide

The isotopic composition of oxygen liberated in the catalytic decomposition of hydrogen peroxide using MnO2, Fe2O3, colloidal gold, metallic platinum, and soluble catalase as catalysts has been measured. All the catalysts except catalase produced a measurable fractionation of the oxygen isotopes in the case of complete decomposition. A few rate experiments were also carried out, and the kinetics studied from the standpoint of isotope separation as a function of fraction of hydrogen peroxide decomposed. It is demonstrated that none of the liberated oxygen comes from the substrate water or from the potassium permanganate when hydrogen peroxide is oxidized. Observed fractionation factors are somewhat smaller than the maximum values calculated on the basis of Bigeleisen's equations. A minimum in the O18 percentage of the evolved oxygen in the case of the catalase experiments suggests that two mechanisms are operative for this type of catalysis. The initial enhancement of the O18 percentage of the evolved oxygen over that in the peroxide remains unexplained.