Journal article

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The clumped isotopic composition of carbonate‐derived CO2 (denoted Δ47) is a function of carbonate formation temperature, and in natural samples can act as a recorder of paleoclimate, burial or diagenetic conditions. The absolute abundance of heavy isotopes in the universal standards VPDB and VSMOW (defined by four parameters: R13VPDB, R17VSMOW, R18VSMOW and λ) impact calculated Δ47 values. Here, we investigate whether use of updated and more accurate values for these parameters can remove observed inter‐laboratory differences in the measured T‐Δ47 relationship. Using the updated parameters, we reprocess 14 published calibration datasets measured in 11 different laboratories, representing many mineralogies, bulk compositions, sample types, reaction temperatures, and sample preparation and analysis methods. Exploiting this large composite dataset (n=1253 sample replicates), we investigate the possibility for a “universal” clumped isotope calibration. We find that applying updated parameters improves the T‐Δ47 relationship (reduces residuals) within most labs and improves overall agreement but does not eliminate all inter‐laboratory differences. We reaffirm earlier findings that different mineralogies do not require different calibration equations, and that cleaning procedures, method of pressure baseline correction, and mass spectrometer type do not affect inter‐laboratory agreement. We also present new estimates of the temperature dependence of the acid digestion fractionation for Δ47 (Δ*25‐X), based on combining reprocessed data from four studies, and new theoretical equilibrium values to be used in calculation of the empirical transfer function. Overall, we have ruled out a number of possible causes of inter‐laboratory disagreement in the T‐Δ47 relationship, but many more remain to be investigated.

Publication date: 
Saturday, June 1, 2019