Choice of 17O correction affects clumped isotope (Δ47) values of CO2 measured with mass spectrometry

RATIONALE: The clumped isotope composition of CO2 (Delta(47)) derived from carbonate is widely used as a paleotemperature proxy with broad applications in geoscience. Its accuracy is, however, limited by inter-laboratory discrepancies of reference materials and disagreement among carbonate geothermometer calibrations. Here we show how the correction for the abundance of O-17 influences these discrepancies. METHODS: We used CO2-H2O equilibration at known temperatures and phosphoric acid digested carbonates to generate CO2 samples with a wide range in C-13 and O-18 compositions. All samples were purified using an offline vacuum line. We used a Thermo MAT 253 isotope ratio mass spectrometer with a Faraday collector array for m/z 44-49, to measure delta C-13, delta O-18, delta(47), and Delta(47) values. Data were compiled using a traditional correction ('Santrock') for the O-17 interference in m/z 45 as well as a more recently proposed correction ('Brand') for O-17 interference. Two reference frames using CO2 with distinct C-13 compositions were constructed to simulate an inter-laboratory comparison. RESULTS: The traditional Santrock O-17 correction leads to a simulated Delta(47) inter-laboratory comparison offset of 0.06%, and a 0.1% Delta(47) range in CO2-H2O 23 degrees C equilibrations that is dependent on the C-13 composition. The more recent Brand O-17 correction removes these discrepancies. The traditional O-17 correction yields distinct temperature-Delta(47) calibration curves for synthetic carbonates precipitated using different methods to degas CO2, while the more recent O-17 correction collapses all calibration data onto a single curve. CONCLUSIONS: The O-17 correction strategy employed by CO2 and carbonate clumped-isotope researchers can have a large effect on the accuracy of Delta(47) values. Use of the traditional O-17 correction may have caused errors in published studies as large as 0.1% and may account for Delta(47) differences among laboratories and disagreement among previously published carbonate clumped isotope thermometry calibrations. Copyright (C) 2016 John Wiley & Sons, Ltd.