The abundance of the doubly substituted CO2 isotopologue, 13C18O16O, in CO2 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: Δ47 = 0.0592 · 106 · T-2 -0.02, where Δ47 is the enrichment, in per mil, of 13C18 O16O in CO2 relative to the amount expected for a stochastic (random) distribution of isotopes among all CO2 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 CO32- units), and not the bulk δ18O or δ13C 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 δ18 O of water and δ13C 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 Δ47 and δ18O for carbonates will constrain the δ18O of water from which they grew.
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This study was conducted in response to Michael Bender’s good-natured prodding of J.M.E. and J.F.A.; we thank him for his insightful instincts and persistence. We gratefully acknowledge the help of Lisa Welp, who measured the δ 18 O values of water from which we grew inorganic calcites, Ma Chi, who helped with XRD analyses of synthetic carbonates, and Dr. Willi Brand, who provided aliquots of MAR-J1 carbonate standard. We also thank the Smithsonian Institute for lending us deep-sea coral samples. This work made use of an instrument purchased with the help of NSF Grant EAR-0220066 and the Packard Foundation, and benefited from salary support provided by NSF Grant EAR-0345905.