Quantification of oxygen-17 fractionation during phosphoric acid digestion: Implications for Δ’17O in carbonates

The application of the triple oxygen isotope systematics (Δ’¹⁷O) to carbonates has emerged as a powerful tool for reconstructing past hydroclimate dynamics and tracing the long-term oxygen and carbon cycles. Over the past decade, a wide range of analytical techniques has been developed to determine the Δ’¹⁷O value of carbonates. Most methodologies involve phosphoric acid digestion, a process introducing systematic oxygen isotope fractionation that varies with carbonate mineralogy. While oxygen-18 fractionation during acid digestion is well known, the corresponding effects on oxygen-17 remain poorly constrained for many carbonate phases. This unknown factor, is likely to contribute to interlaboratory variability in measured carbonate Δ’¹⁷O values, limiting the reliability of cross-study comparisons and raising concerns about the accuracy of reported Δ’¹⁷O values, particularly when mixed carbonate phases are involved. In this study, we characterize the oxygen-17 acid fractionation factor θ_{(CO₂–acid/CaCO₃)} for a range of carbonate minerals, including calcite, aragonite, dolomite, and witherite. Instead of relying on quantitative fluorination, we apply a new technique to determine the oxygen isotope composition of the carbonate phases. This method combines high-temperature conversion elemental analysis (TC/EA) with high-voltage glow discharge to achieve quantitative oxygen extraction from carbonate as CO and its subsequent conversion to CO₂. We compare the δ¹⁸O and Δ’¹⁷O values of the resulting CO₂ with those obtained via carbonate acid digestion at different temperatures. All measurements were performed using laser absorption spectroscopy (TILDAS). The results of this study will provide critical reference data to improve standardization, interlaboratory reproducibility and interpretation of carbonate Δ’¹⁷O values.