HR: 0800h
AN: PP11A-0552    [Abstracts]
TI: Theoretical estimates of equilibrium $^1^3C-^1^8O$ clumping in carbonates and organic acids
AU: * Schauble, E A
EM: schauble@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, UCLA, Box 951567, Los Angeles, CA 90095 United States
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, MC 100-23, Pasadena, CA 91125 United States
AB: The development of techniques for measuring small gas-phase molecules containing more than one rare stable isotope (e.g., $^1^3C^1^8O^1^6O$) at natural, ppm-level abundances$^{1,2}$ has made it possible to track sources and sinks of atmospheric gases from a new perspective. Similar measurements of $^1^3C-^1^8O$ clumping in ancient samples could improve our understanding of ancient climates, if the abundances of `clumped' rare stable isotopes in materials that retain isotopic signatures over geologic time can be measured with sufficient precision. This theoretical study estimates the abundances of such $^1^3C-^1^8O$ `clumps' in carbonates and organic acids and discusses their potential applications. Accompanying abstracts by Eiler et al. and Ghosh et al. will present the analytical methods and some initial data for carbonate minerals to examine the applicability of our theoretical models. Equilibrium isotopic speciations in carbonate minerals and organic acids are calculated from the reduced partition function ratios of isotopically substituted crystals and molecules. Vibrational frequencies used as input for these calculations come from {\it ab initio} force fields, determined using density functional theory. Our calculations indicate that carbonate minerals, including calcite, dolomite, and aragonite, when equilibrated at earth-surface temperatures, will have a slight overabundance of $CO_3^{2-}$ groups containing both $^1^3C$ and $^1^8O$ (i.e., $^1^3C^1^8O^1^6O_2^{2-}$) relative to what would be expected if carbon and oxygen isotopes were distributed randomly in the crystal lattice. Calcite and dolomite crystals are predicted to have 0.4$\permil$ excesses of $^1^3C^1^8O^1^6O_2^{2-}$ at 298 K; in aragonite the excess will be about 0.05$\permil$ larger. The excesses are smaller for crystals formed or equilibrated at higher temperatures, decreasing by 0.003\permil/$^o$C at room temperature and essentially disappearing at temperatures of 1000 K or higher. Similarly, there is an excess of both $^1^3C^1^8O^1^6OH$ and $^1^3C^1^6O^1^8OH$ groups in organic acids like formic acid ($HCOOH$) and pyruvic acid ($CH_3COCOOH$) that equilibrate at low temperatures. For gas-phase carboxylic acids, $^1^3C-^1^8O$ clumping in the $COOH$ group is strongest at the $C=O$ double bond, with an 1.0-1.1$\permil$ excess at room temperature. The $C-O-H$ subgroup has an ~0.4$\permil$ $^1^3C-^1^8O$ excess, and thus the average anomaly for the whole $COOH$ group is 0.7-0.8\permil. As with carbonate minerals, these excesses decrease at higher temperatures. The magnitude of $^1^3C-^1^8O$ clumping in carboxylic acid is similar to gas-phase $CO_2 ^{1,2,3}$, while in carbonate minerals the effect is about one-half as large. The temperature sensitivity of these isotopic clumping effects suggests that measurements of abundances of $^1^3C-^1^8O$ bonds in carbonates and organic acids could be useful for paleothermometry. The clumping equilibrium is an internal property of each phase, so temperature information can be obtained even when the isotopic composition of the fluid phase from which a sample precipitated is unknown. Clumping effects may also be able to distinguish pristine, unaltered sedimentary and biogenic carbonates and organic deposits from those that have undergone post-depositional diagenesis or metamorphism, even in samples that have not suffered extensive open-system exchange. Refs: $^1$Eiler et al. 2004, GCA {\it in press}; $^2$Schauble et al. {\it in prep.}; $^3$Wang et al. 2004, GCA {\it in press}.
DE: 3620 Crystal chemistry
DE: 4267 Paleoceanography
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1055 Organic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting