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

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 17 O influences these discrepancies.
METHODS: We used CO2 -H2 O equilibration at known temperatures and phosphoric acid digested carbonates to generate CO2 samples with a wide range in 13 C and 18 O 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 δ13 C, δ18 O, δ47 , and Δ47 values. Data were compiled using a traditional correction ('Santrock') for the 17 O interference in m/z 45 as well as a more recently proposed correction ('Brand') for 17 O interference. Two reference frames using CO2 with distinct 13 C compositions were constructed to simulate an inter-laboratory comparison.
RESULTS: The traditional Santrock 17 O correction leads to a simulated Δ47 inter-laboratory comparison offset of 0.06 ‰, and a 0.1 ‰ Δ47 range in CO2 -H2 O 23°C equilibrations that is dependent on the 13 C composition. The more recent Brand 17 O correction removes these discrepancies. The traditional 17 O correction yields distinct temperature-Δ47 calibration curves for synthetic carbonates precipitated using different methods to degas CO2 , while the more recent 17 O correction collapses all calibration data onto a single curve.
CONCLUSIONS: The 17 O 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 17 O 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.