13C– 18O bonds in carbonate minerals: A new kind of paleothermometer

Abstract

The abundance of the doubly substituted CO 2 isotopologue, 13C 18O 16O, in CO 2 produced by phosphoric acid digestion of synthetic, inorganic calcite and natural, biogenic aragonite is proportional to the concentration of 13C– 18O bonds in reactant carbonate, and the concentration of these bonds is a function of the temperature of carbonate growth. This proportionality can be described between 1 and 50 °C by the function: Δ 47 = 0.0592 · 10 6 · T −2 − 0.02, where Δ 47 is the enrichment, in per mil, of 13C 18O 16O in CO 2 relative to the amount expected for a stochastic (random) distribution of isotopes among all CO 2 isotopologues, and 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 (i.e., bound together vs. separated into different CO 3 2− units), and not the bulk δ 18O or δ 13C values. Current analytical methods limit precision of this thermometer to ca. ± 2 °C, 1σ. 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 δ 18O of water and δ 13C of DIC from which carbonate grew. Thus, this technique can be applied to parts of the geological record where the stable isotope compositions of waters are unknown. Moreover, simultaneous determinations of Δ 47 and δ 18O for carbonates will constrain the δ 18O of water from which they grew.