Re-Assessing the Need for Apatite- and Dolomite-Specific Calibrations of the Carbonate Clumped Isotope Thermometer

Abstract

A few key methodological uncertainties remain for the carbonate clumped isotope community. One is how to compare data among published data sets that are not anchored to the InterCarb Carbon Dioxide Equilibrium Scale (I-CDES). A second is how temperature calibrations of calcite compare to those of other carbonate minerals in the I-CDES—particularly dolomite and apatite—which can elucidate several Earth system dynamics. Previous calibrations of the clumped isotope thermometer for dolomite are discrepant from one another and variably (dis)agree with calibrations developed for calcite; apatite calibrations have not yet been compared between laboratories using carbonate-based standardization. Here we report I-CDES standardized values for a suite of 11 carbonates that are commonly measured by the clumped isotope community to aid future comparisons of non-I-CDES data sets. In addition, 17 dolomite samples (25–1,200°C) and five apatite samples (1–38°C) of known precipitation temperature were measured using carbonate-based standardization. Excellent agreement between calcites and dolomites heated to similar temperatures (1,100–1,200°C) suggests no mineral-specific differences in absolute acid fractionation factor. We show that calcite and dolomite regressions largely agree but are sensitive to sample characteristics, regression method, and how equations are statistically compared. We suggest that there is no need for a dolomite-specific clumped isotope calibration, although our results suggest that further work is necessary to determine the influence of sample characteristics on this relationship. The apatite calibration equation defined in this study is statistically indistinguishable from calcite-based calibrations; we corroborate previous findings that an apatite-specific calibration is unnecessary.