Isotopic fractionation of noble gases by diffusion in liquid water: Molecular dynamics simulations and hydrologic applications

Bourg, Ian C.; Sposito, Garrison

doi:10.1016/j.gca.2008.02.012

Cited by 73 publications

(107 citation statements)

References 59 publications

(142 reference statements)

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“…Each simulation was carried out for 16 ns (with a 1-fs time step) in the NVE ensemble and was preceded by 202 ps of equilibration at the desired temperature. Due to the absence of a counter ion and the application of periodic boundary conditions, these simulations approximate conditions of infinite dilution (15)(16)(17)(30)(31)(32)(33)(34)(35). We chose to model all alkali and alkaline earth metal cations that have more than one naturally occurring stable isotope (Li + , K + , Rb + , Ca 2+ , Sr 2+ , Ba 2+ ), with the exception of Mg 2+ , for which preliminary calculations showed too few water-exchange events to accurately calculate k wex via direct simulation.…”

Section: Simulation Methodsmentioning

confidence: 99%

“…Growth of calcite from seawater-like aqueous solutions is not likely to be diffusion-limited for Ca, so the isotopic effects are inferred to be due to kinetic effects occurring at the solid/fluid interface (11,13). Diffusion in liquid water can result in kinetic isotopic fractionation (14)(15)(16)(17), but in the case of Ca isotopes, the available data suggest that the largest possible light-isotope enrichment from diffusion-limited growth is ∼0.4‰ in δ 44/40 Ca, which is too small to explain the observations (17).…”

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confidence: 99%

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Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Hofmann

Bourg

DePaolo

2012

Proc. Natl. Acad. Sci. U.S.A.

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Molecular dynamics simulations show that the desolvation rates of isotopes of Li + , K + , Rb + , Ca 2+ , Sr 2+ , and Ba 2+ may have a relatively strong dependence on the metal cation mass. This inference is based on the observation that the exchange rate constant, k wex , for water molecules in the first hydration shell follows an inverse power-law mass dependence (k wex ∝ m −γ ), where the coefficient γ is 0.05 ± 0.01 on average for all cations studied. Simulated waterexchange rates increase with temperature and decrease with increasing isotopic mass for each element. The magnitude of the water-exchange rate is different for simulations run using different water models [i.e., extended simple point charge (SPC/E) vs. four-site transferrable intermolecular potential (TIP4P)]; however, the value of the mass exponent γ is the same. Reaction rate theory calculations predict mass exponents consistent with those determined via molecular dynamics simulations. The simulation-derived mass dependences imply that solids precipitating from aqueous solution under kinetically controlled conditions should be enriched in the light isotopes of the metal cations relative to the solutions, consistent with measured isotopic signatures in natural materials and laboratory experiments. Desolvation effects are large enough that they may be a primary determinant of the observed isotopic fractionation during precipitation.aqueous geochemistry | kinetic isotope effect | ligand exchange N onequilibrium processes are generally recognized as important influences on isotopic fractionation during mineral precipitation, especially at temperatures below a few hundred degrees Celsius (1, 2). Recent work in "nontraditional" stable isotopes (Li, Mg, Ca, Fe, Cd, Cu, etc.) has confirmed this inference and shown that nonequilibrium fractionation must be accounted for to understand global biogeochemical cycles involving these elements (2-5). A key observation is that light isotopes are preferentially incorporated into precipitating solids (6-8)-the opposite direction of enrichment expected for equilibrium fractionation, which depends on bond energetics (9, 10). The origin of this nonequilibrium light-isotope enrichment is a key unknown in the isotopic geochemistry of mineral growth.Calcite grown from aqueous solutions provides an excellent illustration of light-isotope enrichment during precipitation. Synthetically precipitated and natural samples of calcite and aragonite are fractionated relative to aqueous Ca 2+ such that δ 44/40 Ca in calcite is lower by 0.5-2‰ (7, 11). Equilibrium Ca isotope fractionation between Ca 2+ (aq) and calcite has not been measured in the laboratory, but studies of deep-sea sedimentary pore fluids suggest that the equilibrium fractionation is negligible: ε eq ∼ 0.0 ± 0.1‰ (12). Growth of calcite from seawater-like aqueous solutions is not likely to be diffusion-limited for Ca, so the isotopic effects are inferred to be due to kinetic effects occurring at the solid/fluid interface (11, 13). Diffusion in liquid water can r...

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Section: Simulation Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Hofmann

Bourg

DePaolo

2012

Proc. Natl. Acad. Sci. U.S.A.

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“…Laboratory experiments have determined the isotope effect in solution for helium in water as a function of temperature (Benson and Krause, 1980). Given that the helium isotope ratio may be further affected by isotopic fractionation in molecular diffusion (Bourg and Sposito, 2008) associated with the balance between wave-induced bubble trapping and air-sea exchange (Fuchs et al, 1987;Jenkins, 1988b), we have used our observations of the full suite of noble gases on these samples to develop a much more complete model of this dynamic equilibrium isotope effect. Thus, the dynamic solubility equilibrium value for 3 He, C eq , was determined by adding 3 He isotopes to a one-dimensional Price-WellerPinkel (PWP) model (Price et al, 1986) subject to 6-hourly NCEP (National Center for Environmental Prediction) reanalysis forcing (Kalnay et al, 1996) and QuikSCAT winds from the BATS site (Stanley et al, 2006(Stanley et al, , 2009b.…”

Section: Calculation Of Fluxesmentioning

confidence: 99%

“…Thus, the dynamic solubility equilibrium value for 3 He, C eq , was determined by adding 3 He isotopes to a one-dimensional Price-WellerPinkel (PWP) model (Price et al, 1986) subject to 6-hourly NCEP (National Center for Environmental Prediction) reanalysis forcing (Kalnay et al, 1996) and QuikSCAT winds from the BATS site (Stanley et al, 2006(Stanley et al, , 2009b. The model used the temperature-dependent solubility of 3 He from Benson and Krause (1980) and the molecular diffusivity value from Bourg and Sposito (2008). The calculated dynamic solubility equilibrium is sensitive to the amount of air injection, and thus the other noble gases were used to constrain the air injection (Stanley et al, 2009b; subsequently referred to as S09).…”

Section: Calculation Of Fluxesmentioning

confidence: 99%

“…where α is the Bunsen solubility coefficient of the gas (Benson and Krause, 1980, for 3 He), D i is the diffusivity coefficient (for 3 He determined with the fractionation factor from Bourg and Sposito (2008) and the diffusivity coefficient of Jahne, 1987), D o is a normalization factor equal to 1 which is included in order to simplify the units (m 2 s −1 ), P i,b is the pressure of the gas in the bubble (Pa), and P i,w is the partial pressure of the gas in the water (Pa). P i,b is approximated by…”

Section: Calculation Of Fluxesmentioning

confidence: 99%

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The <sup>3</sup>He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site

et al. 2015

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Abstract. Significant rates of primary production occur in the oligotrophic ocean, without any measurable nutrients present in the mixed layer, fueling a scientific paradox that has lasted for decades. Here, we provide a new determination of the annual mean physical supply of nitrate to the euphotic zone in the western subtropical North Atlantic. We combine a 3-year time series of measurements of tritiugenic 3 He from 2003 to 2006 in the surface ocean at the Bermuda Atlantic Time-series Study (BATS) site with a sophisticated noble gas calibrated air-sea gas exchange model to constrain the 3 He flux across the sea-air interface, which must closely mirror the upward 3 He flux into the euphotic zone. The product of the 3 He flux and the observed subsurface nitrate-3 He relationship provides an estimate of the minimum rate of new production in the BATS region. We also apply the gas model to an earlier time series of 3 He measurements at BATS in order to recalculate new production fluxes for the 1985 to 1988 time period. The observations, despite an almost 3-fold difference in the nitrate-3 He relationship, yield a roughly consistent estimate of nitrate flux. In particular, the nitrate flux from 2003 to 2006 is estimated to be 0.65 ± 0.14 mol m −2 yr −1 , which is ∼ 40 % smaller than the calculated flux for the period from 1985 to 1988. The difference in nitrate flux between the time periods may be signifying a real difference in new production resulting from changes in subtropical mode water formation. Overall, the nitrate flux is larger than most estimates of export fluxes or net community production fluxes made locally for the BATS site, which is likely a reflection of the larger spatial scale covered by the 3 He technique and potentially also by the decoupling of 3 He and nitrate during the obduction of water masses from the main thermocline into the upper ocean. The upward nitrate flux is certainly large enough to support observed rates of primary production at BATS and more generally in the oligotrophic subtropical ocean.

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Influence of sample volume on nitrate N and O isotope ratio analyses with the denitrifier method

Zhou

Granger

Chang

2021

Rapid Comm Mass Spectrometry

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Rationale Analyses of the isotope ratios of nitrogen (15N/14N) and oxygen (18O/16O) in nitrate (NO3−) with the denitrifier method require relatively high sample volumes at low concentrations (≤1 μM) to afford sufficient analyte for mass spectrometry, resulting in isotopic offsets compared to more concentrated samples of the same isotopic composition. Methods To uncover the origins of isotopic offsets, we analyzed the N and O isotope ratios of NO3− reference materials spanning concentrations of 0.5–20 μM. We substantiated the incidence of volume‐dependent isotopic offsets, then investigated whether they resulted from (a) incomplete sample recovery during N2O sparging, (b) blanks – bacterial, atmospheric, or in reference material solutions – and (c) oxygen atom exchange with water during the bacterial conversion of NO3− to N2O. Results Larger sample volumes resulted in modest offsets in δ15N, but substantial offsets in δ18O. N2O recovery from sparging was less complete at higher volumes, resulting in decreases in δ15N and δ18O due to associated isotope fractionation. Blanks increased detectably with volume, whereas oxygen atom exchange with water remained constant within batch analyses, being sensitive to neither sample volume nor salinity. The sizeable offsets in δ18O with volume are only partially explained by the factors considered in our analysis. Conclusions Our observations argue for bracketing of NO3− samples with reference materials that emulate sample volumes (concentrations) to achieve improved measurement accuracy and foster inter‐comparability.

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Isotopic fractionation of noble gases by diffusion in liquid water: Molecular dynamics simulations and hydrologic applications

Cited by 73 publications

References 59 publications

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

The <sup>3</sup>He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site

Influence of sample volume on nitrate N and O isotope ratio analyses with the denitrifier method

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Isotopic fractionation of noble gases by diffusion in liquid water: Molecular dynamics simulations and hydrologic applications

Cited by 73 publications

References 59 publications

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

The &lt;sup&gt;3&lt;/sup&gt;He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site

Influence of sample volume on nitrate N and O isotope ratio analyses with the denitrifier method

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The <sup>3</sup>He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site