Weifu Guo | Woods Hole Oceanographic Institution (original) (raw)

Papers by Weifu Guo

Research paper thumbnail of Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation

Proceedings of The National Academy of Sciences, 2009

Research paper thumbnail of Multiple sulfur isotope analysis of volatile organic sulfur compounds and their sulfonium precursors in coastal marine environments

Research paper thumbnail of Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Geochimica Et Cosmochimica Acta, 2009

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate mine... more Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate 'clumped isotope' analysis. The CO 2 recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H 2CO 3 reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of 13C- 18O-containing isotopologues ('clumped' isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, 18O/ 16O ratios (1000 ln α∗) and three indices that measure proportions of multiply-substituted isotopologues (Δ47∗,Δ48∗,Δ49∗). We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where α=α18λ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of 13C- 18O bonds (the measured change in Δ 47 = 0.23‰) during phosphoric acid digestion of calcite at 25 °C. We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H 2CO 3 interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of 13C- 18O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of Δ47∗ for different carbonate minerals. Combined with previous theoretical evaluations of 13C- 18O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H 2CO 3.

![Research paper thumbnail of A `clumped-isotope' study of stratospheric CO2 reveals a new atmospheric process](https://a.academia-assets.com/images/blank-paper.jpg)

Research paper thumbnail of Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Geochimica Et Cosmochimica Acta, 2009

Research paper thumbnail of 13 C-18O bonds in carbonate minerals: A new kind of paleothermometer

![Research paper thumbnail of Carbonate `clumped isotope' thermometry: A status report](https://a.academia-assets.com/images/blank-paper.jpg)

Research paper thumbnail of 13C– 18O bonds in carbonate minerals: A new kind of paleothermometer

Geochimica Et Cosmochimica Acta, 2006

Research paper thumbnail of Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites

Geochimica Et Cosmochimica Acta, 2007

Research paper thumbnail of Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation

Proceedings of The National Academy of Sciences, 2009

Research paper thumbnail of Multiple sulfur isotope analysis of volatile organic sulfur compounds and their sulfonium precursors in coastal marine environments

Research paper thumbnail of Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Geochimica Et Cosmochimica Acta, 2009

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate mine... more Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate 'clumped isotope' analysis. The CO 2 recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H 2CO 3 reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of 13C- 18O-containing isotopologues ('clumped' isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, 18O/ 16O ratios (1000 ln α∗) and three indices that measure proportions of multiply-substituted isotopologues (Δ47∗,Δ48∗,Δ49∗). We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where α=α18λ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of 13C- 18O bonds (the measured change in Δ 47 = 0.23‰) during phosphoric acid digestion of calcite at 25 °C. We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H 2CO 3 interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of 13C- 18O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of Δ47∗ for different carbonate minerals. Combined with previous theoretical evaluations of 13C- 18O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H 2CO 3.

![Research paper thumbnail of A `clumped-isotope' study of stratospheric CO2 reveals a new atmospheric process](https://a.academia-assets.com/images/blank-paper.jpg)

Research paper thumbnail of Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Geochimica Et Cosmochimica Acta, 2009

Research paper thumbnail of 13 C-18O bonds in carbonate minerals: A new kind of paleothermometer

![Research paper thumbnail of Carbonate `clumped isotope' thermometry: A status report](https://a.academia-assets.com/images/blank-paper.jpg)

Research paper thumbnail of 13C– 18O bonds in carbonate minerals: A new kind of paleothermometer

Geochimica Et Cosmochimica Acta, 2006

Research paper thumbnail of Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites

Geochimica Et Cosmochimica Acta, 2007