CO2 Hydration and Hydroxylation: The Origin of Carbonate Kinetic Isotope Effects

Open Access
Author:
Boettger, Jason Daniel
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 01, 2017
Committee Members:
  • Susan Louise Brantley, Dissertation Advisor
  • Susan Louise Brantley, Committee Chair
  • Matthew S Fantle, Committee Member
  • Katherine Haines Freeman, Committee Member
  • Lasse Jensen, Outside Member
Keywords:
  • isotopes
  • computational chemistry
  • isotope fractionation
  • carbonate
  • coral
Abstract:
Stable isotope ratios in carbonate minerals record the influence of several climatological and biogeochemical processes. This makes biogenic carbonates valuable inventories of climate proxy records, but also makes them challenging to interpret. Carbonate formation from carbon dioxide via the unequilibrated CO2 hydration and hydroxylation reactions is one process that may impact stable C, O and clumped C-O isotope ratios in carbonate minerals, particularly carbonate minerals formed by corals. This dissertation reports calculation of the isotopic influence of the CO2 hydration and hydroxylation reactions using computational chemistry models. Results are compared with observed isotopic trends in corals to determine whether these reactions are consistent with the observed vital effects in corals. Methodological considerations for the application of computational chemistry to the calculation of aqueous isotopic fractionation are reported. We analyzed several computational chemistry schemes for their ability to predict gas-phase isotope fractionation between CO2 and H2O. We also tested their ability to predict several other experimentally observable properties and whether success predicting each property correlated with success predicting isotope fractionation. Only successful prediction of harmonic vibrational frequencies correlated with successful prediction of isotopic fractionation; neither energies nor bond distances are good indications of a model chemistry useful in calculation of isotopic fractionation. B3LYP and X3LYP coupled with Pople triple-zeta basis sets were selected for application to fractionation in aqueous DIC species. Stable C and O isotope partitioning constants were calculated for both equilibrium fractionation between aqueous CO2 and H2CO3/HCO3- and for kinetic fractionation during the CO2 hydration and hydroxylation reactions. Predicted equilibrium fractionation agrees well with experimentally determined values. The CO2 hydration reaction is predicted to discriminate against both 13C and 18O by 10-11‰, while the CO2 hydroxylation reaction is predicted to discriminate against 13C by 13-16‰ and against 18O by 19-21‰. When calculating aqueous fractionation factors, it was necessary to analyze the H-bonding environment and interpolate to the expected H-bond environment experienced by each aqueous species, as individual H-bonds were found to have substantial effects on equilibrium and kinetic isotope fractionations, often of magnitude >1‰ and occasionally of magnitude >3‰ absolute deviation in predicted fractionation factor. H-bonds to hydroxyl groups from H2O always reduced the amount of 13C and 18O entering reaction products, while other H-bonds increased the amount of 13C and 18O entering reaction products when effects were statistically significant, except for H-bonds from attacking OH- during CO2 hydroxylation. Fractionation during CO2 hydration and hydroxylation is able to explain most but not all of the isotope disequilibrium observed in the skeletons of shallow-water corals. Isotope clumping between 13C and 18O was calculated during the CO2 hydration and hydroxylation reactions. When accounting for H-bond environment, hydration and hydroxylation increase clumping in product carbonates by 0.10‰ and 0.12‰ respectively. These results are consistent with the increased clumping observed in some shallow-water corals relative to other carbonates. All H-bonds are predicted to decrease the clumping in product carbonates except for H-bonds from water to an attacking OH- during CO2 hydroxylation. H-bond effects on clumping and single stable isotope fractionation are not consistent with a simple model of stiffer bonds favoring incorporation of both heavy isotopes and clumped isotopologues. Formation of transient carbonate precursor phases may also affect the isotopic composition of coral skeletons and other biogenic carbonates; however, isotopic compositions of these precursor phases are currently unknown. Synthesized amorphous calcium carbonate (ACC) observed under XRD, FTIR, and Raman bears similar structural features with biogenic stable and transient ACC, depending on concentration of metasilicate stabilizer used. The isotopic composition of ACC is affected by Ca2+ and CO32- concentrations in a manner different than calcite. The isotopic composition of ACC is unlikely to explain the anomalous isotopic composition observed in coral calcification centers.