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

Abstract

The abundance of the doubly substituted CO_2 isotopologue, ^(13)C^(18)O^(16)O, in CO_2 produced by phosphoric acid digestion of synthetic, inorganic calcite and natural, biogenic aragonite is proportional to the concentration of ^(13)C–^(18)O 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 ^(13)C^(18)O^(16)O 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 ^(13)C and ^(18)O in the carbonate lattice (i.e., bound together vs. separated into different CO_3^(2−) units), and not the bulk δ^(18)O or δ^(13)C 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^(18)O of water and δ^(13)C 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^(18)O for carbonates will constrain the δ^(18)O of water from which they grew.

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