Choice of 17O correction affects clumped isotope (∆47) values of CO2 measured with mass spectrometry

Abstract

Rationale: The clumped isotope composition of CO2 (Δ47) 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 17O 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 13C and 18O 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 δ13C, δ18O, δ47, and Δ47 values. Data were compiled using a traditional correction (‘Santrock’) for the 17O interference in m/z 45 as well as a more recently proposed correction (‘Brand’) for 17O interference. Two reference frames using CO2 with distinct 13C compositions were constructed to simulate an inter‐laboratory comparison. Results: The traditional Santrock 17O correction leads to a simulated Δ47 inter‐laboratory comparison offset of 0.06 ‰, and a 0.1 ‰ Δ47 range in CO2‐H2O 23°C equilibrations that is dependent on the 13C composition. The more recent Brand 17O correction removes these discrepancies. The traditional 17O correction yields distinct temperature‐Δ47 calibration curves for synthetic carbonates precipitated using different methods to degas CO2, while the more recent 17O correction collapses all calibration data onto a single curve. Conclusions:The 17O correction strategy employed by CO2 and carbonate clumped‐isotope researchers can have a large effect on the accuracy of Δ47 values. Use of the traditional 17O correction may have caused errors in published studies as large as 0.1 ‰ and may account for Δ47 differences among laboratories and disagreement among previously published carbonate clumped isotope thermometry calibrations.