Linking the oxygen-17 compositions of water and carbonate reference materials using infrared absorption spectroscopy of carbon dioxide

Joint measurements of the ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios of carbonate minerals and waters are increasingly used to investigate various geochemical, physical and biological processes. Diverse analytical methods, each of them technically challenging in one way or another, have been developed or refined in recent years to measure oxygen-17 anomalies (Δ’¹⁷O) with instrumental precisions of 10 ppm or better. A critical underpinning of all these methods is how the international carbonate reference materials currently anchoring the VPDB ¹⁸O/¹⁶O scale are linked to the primary VSMOW-SLAP scale in (¹⁸O/¹⁶O, ¹⁷O/¹⁶O) space. For now, however, substantial systematic discrepancies persist between different groups and methods, even after all measurements are nominally standardized to VSMOW-SLAP.

Here we take advantage of VCOF-CRDS, a novel spectroscopic method combining the ease and simplicity of near-infra-red absorption measurements in pure CO₂ with metrological performance competitive with state-of-the-art IRMS techniques, to precisely characterize, based on previously reported equilibrium fractionation factors between water and CO₂, the relative triple oxygen isotope compositions of international water standards (VSMOW2, SLAP2, GRESP) and CO₂ produced by phosphoric acid reaction of carbonate standards (NBS18, NBS19, IAEA603, IAEA610, IAEA611, IAEA612). The robustness of our results derives from the demonstrated linearity of our measurements (RMSE ≈ 1 ppm), but also from the fact that, when equilibrated with or converted to CO₂, all of these reference materials yield analytes with closely comparable oxygen-18 compositions. In light of these observations, we revisit potential causes of the large inter-laboratory discrepancies reported so far. Collectively reconciling the different types of measurements constraining the relative ¹⁷O/¹⁶O ratios of the two standards most often used to normalize carbonate analyses (NBS18, IAEA603) is a matter of high priority.