First-principles calibration of the carbonate clumping paleothermometer in apatite (Invited)
Abstract
Chemical properties in solutions and complex crystals may be determined at the nanoscale, in the sense that they exhibit behavior distinct from both isolated molecules and pure bulk materials. The potential use of trace and minor substituents in biominerals to make inferences about the environments in which they formed is dependent on such small-scale phenomena. The focus of this study is first-principles calibration of the 13C-18O “clumping” paleo-thermometer in apatite, which records body temperature in vertebrates(1) and possibly ambient environmental temperatures in samples from cold-blooded (poikilothermic) organisms and inorganic sources. The clumping paleothermometer depends on the temperature-dependent tendency of 13C and 18O to form bonds with each other, rather than the more common 12C and 16O isotopes in carbonate molecules. An important unresolved question is whether this equilibrium is established in solution or in the mineral host, and how it might be modified by biochemical kinetics. In the case of apatite, carbonate molecules may substitute into the apatite structure in multiple ways, replacing monovalent anions (OH, F, Cl) or phosphate groups. The 13C-18O distribution might also be affected by the chemical composition of the host lattice. Energies of 13C18O16O22- groups relative to species with only one rare isotope were estimated using electronic structure (density functional theory) models of hydroxylapatite and fluorapatite, for a variety of carbonate-substitution arrangements. These calculations indicate almost the same level of clumpiness for all sampled carbonate/phosphate substitution geometries and bulk calcite, but a slightly greater tendency to form 13C18O16O22- when carbonate substitutes for monovalent anions. The dominance of carbonate/phosphate substitution mechanisms in typical bioapatites suggests that this difference is unlikely to introduce significant error in estimated body temperatures, but it may make it possible to test whether isotopologue distributions are set in the lattice or inherited from aqueous solution. 1Eagle et al., 2010, PNAS, doi:10.1073/pnas.0911115107.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.B13I..04S
- Keywords:
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- 0419 BIOGEOSCIENCES / Biomineralization;
- 0424 BIOGEOSCIENCES / Biosignatures and proxies;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry