HR: 1340h
AN: PP43B-1276    [Abstracts]
TI: Isotope Fractionations Associated With Degassing of CO2 Aqueous Solutions and its Implications for Carbonate Clumped Isotope Thermometry
AU: * Guo, W
EM: wfguo@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, 1200 E. California Blvd., Pasadena, CA 91125,
AU: Niles, P
EM: paul.b.niles@nasa.gov
AF: NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058,
AU: Daeron, M
EM: daeron@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, 1200 E. California Blvd., Pasadena, CA 91125,
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, 1200 E. California Blvd., Pasadena, CA 91125,
AB: Bicarbonate dehydration (HCO3^-+H⁺→H₂CO₃→CO₂+H₂O) and dehydroxylation (HCO₃^-→CO₂+OH^-) are important reactions in solutions containing dissolved inorganic carbon (DIC) and are involved in multiple geologic processes, including cryogenic carbonate formation, speleothem deposition and air-sea CO₂ exchange. Current understandings of the isotope fractionations that accompany these reactions are very limited. Here we present a model of the isotopic fractionations accompanying dehydration of carbonic acid in aqueous solution, using techniques from ab initio, transition state and statistical thermodynamic theory, and tests of this model based on measurements of experimental and natural carbonates produced by degassing of CO₂ from aqueous solutions. Our model predicts that the isotopologues of carbonic acid containing heavy isotopes dehydrate more slowly than the normal isotopologues, such that fractionations between product CO₂ and reactant H₂CO₃ are ~-25‰ for δ¹³C and ~-9‰ for δ¹⁸O at 300K. Expression of these isotope fractionations during degassing of CO₂ from aqueous solutions should lead to increases in the δ¹³C and δ¹⁸O of residual DIC species, and in carbonate minerals that precipitate from that DIC; this phenomenon could explain the non-equilibrium isotopic compositions of some cryogenic carbonates and speleothems. The carbonate clumped isotope thermometer constrains carbonate formation temperatures based on the proportions of ¹³C-¹⁸O bonds in the carbonate mineral lattice [1]; the lower the formation temperature, the greater the proportion of ¹³C-¹⁸O bonds. By extending our above model to include the multiply- substituted isotopologues, we predict that isotope fractionations accompanying dehydration of carbonic acid decrease the proportion of ¹³C-¹⁸O bonds in the remaining DIC pool relative to their expected equilibrium abundances, and therefore lead to an apparent overestimation of carbonate formation temperatures as determined by the carbonate clumped isotope thermometer; the size of this effect, if unaccounted for, is ~10°C overestimation for every 1‰ kinetic enrichment in carbonate δ¹⁸O. We test the predictions of our model by measuring the isotopic compositions of carbonates formed by rapid freezing of CO₂-saturated CaCl₂ solutions. Results from these cryogenic carbonate precipitation experiments generally closely follow our predicted trend of isotope fractionations, i.e. higher in δ¹³C and δ¹⁸O, and lower in proportion of ¹³C-¹⁸O bonds. In many natural systems where CO₂ degasses from aqueous solution, much of the evolved CO₂ is produced by dedydroxylation of HCO₃^- (particularly in relatively high pH solutions). We are currently extending our modeling and experimental efforts to encompass this type of systems. Finally, we will present isotopic data for natural speleothems and compare them with the predictions of our physical chemistry model of kinetically controlled carbonate growth. Reference [1] Ghosh et. al. (2006) GCA, 70: 1439-1456.
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 4540 Ice mechanics and air/sea/ice exchange processes (0700, 0750, 0752, 0754)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting