HR: 14:25h
AN: PP33D-04    [Abstracts]
TI: ¹³ C-¹⁸ O bonds in carbonate minerals: A new kind of paleothermometer
AU: * Ghosh, P
EM: pghosh@gps.caltech.edu
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125
AU: Adkins, J
EM:
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125
AU: Affek, H
EM:
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125
AU: Balta, B
EM:
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125
AU: Guo, W
EM:
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125
AU: Schauble, E
EM:
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567
AU: Schrag, D
EM:
AF: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, MA 02138
AU: Eiler, J
EM:
AF: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125
AB: Abundances of bonds between ¹³C and ¹⁸O in carbonate minerals are predicted to increase with decreasing temperature due to the isotope exchange equilibrium:¹³C¹⁶O₃⁼ + ¹²C¹⁸O¹⁶O₂^{= = 13}C¹⁸O¹⁶O2⁼+ ¹²C¹⁶O₃⁼. We show that the abundance of the doubly substituted CO₂ isotopologue, ¹³C¹⁸O¹⁶O, in CO₂ produced by phosphoric acid digestion of calcite is correlated with the growth temperature of that calcite, presumably because abundances of ¹³C¹⁸O¹⁶O in product CO₂ are proportional to abundances of ¹³C¹⁸O¹⁶O₂= in reactant calcite. We calibrate this relationship by analysis of calcite precipitated at four temperatures between 1 and 50 ° C,and document the applicability of this calibration to biogenic aragonites. We report abundances of ¹³C¹⁸O¹⁶O in CO₂ using the Δ {47} value, which describes the enrichment or depletion, in per mil, of mass-47 isotopologues (mostly ¹³C¹⁸O¹⁶O)relative to the amount expected for stochastic (random) distribution of all stable isotopes among all CO2 isotopologues. Using this nomenclature, the temperature-dependence of the formation of ¹³C-¹⁸O bonds in calcite is described by the function: Δ⁴⁷={(0.0592*10⁶)/T²}-0.02 where T is the temperature in Kelvin. This relationship can be used for a new kind of carbonate paleothermometry, where the temperature-dependent property of interest is the state of ordering of ¹³C and ¹⁸O in the carbonate lattice (bound together vs. separated into different CO3= units), and not the bulk δ¹⁸O or δ¹³C values. We refer to this method as the `clumped isotope thermometer' because it measures the temperature-dependent `clumping' of ¹³C and ¹⁸O into bonds with each other in the carbonate mineral lattice. A key feature of this thermometer is that it is thermodynamically based, like the traditional carbonate-water paleothermometer, and so is suitable for interpolation and even modest extrapolation, yet is rigorously independent of the δ¹⁸O of water and δ¹³C of DIC from which carbonate grew.
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005