'Clumped isotope' thermometry in benthic foraminifera and ostracods: A novel tool for reconstructing deep-ocean temperatures

The benthic oxygen isotope (δ18O) record is an archive of past deep-water temperatures and ice volume. The carbonate 'clumped isotope' thermometer provides a new technique for reconstructing temperature. It is based on the principle that the proportion of 13C-18O bonds in carbonate minerals is temperature-dependent [1-3]. Unlike other temperature proxies, the clumping of heavy isotopes into bonds with each other is independent of the isotopic composition of the water in which the mineral precipitated. Abundances of 13C-18O bonds in carbonates are determined by measuring Δ47 values of CO2 extracted from carbonate by acid digestion; Δ47 values reflect enrichments, in per mil, of 13C18O16O in product CO2 relative to the amount expected by random chance. Paired measurements of Δ47 and δ18O can potentially yield accurate and quantitative determinations of past temperature and seawater δ18O. We have measured the Δ47 values of core-top benthic foraminifera and ostracods. Preliminary results show that multiple species with different ecologies exhibit the same temperature dependence as inorganic calcite [1]. In contrast to the benthic Mg/Ca thermometer [4], there does not appear to be a discernable influence of carbonate ion concentration on the Δ47 composition of benthic foraminifera (e.g. a carbonate ion effect). Calibration data will be shown for several benthic foraminifera, including the epifaunal taxa Planulina wuellerstorfi, Cibicidoides mundulus, Cibicidoides pachyderma, Hoeglundina elegans, Gyroidina sp. and the infaunal taxa Oridorsalis umbonatus, Melonis pompilioides, and Uvigerina sp. In addition, we will discuss results for the ostracod genus Krithe sp. and mixed ostracod species. [1] Wang, Z., Schauble, E., and Eiler, J., 2004, Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases, Geochimica et Cosmochimica Acta, 68, 4779-4797l [2] Schauble, E., Ghosh, P., and Eiler, J. 2006, Preferential formation of 13C-18O bonds in carbonate minerals, estimated using first-principles lattice dynamics, Geochimica et Cosmochimica Acta, 70, 2510-2529. [3] Ghosh, P., Adkins, J., Affek, H, Balta, B., Guo, W., Schauble, E, Schrag, D. and Eiler, J., 2006, 13C-18O bonds in carbonate minerals: A new kind of paleothermometer, Geochimica et Cosmochimica Acta, 70, 1439-1456. [4] Elderfield, H., J. Yu, P. Anand, T. Kiefer, and B. Nyland, 2006, Calibrations for benthic foraminiferal Mg/Ca palaeothermometry and the carbonate ion hypothesis. Earth and Planetary Science Letters, 250, 633-649.