Laser spectroscopy-based oxygen and clumped isotope acid fractionation factors using two different carbonate digestion techniques

AbstractThe Aerodyne tunable infrared laser differential absorption spectrometer (TILDAS) measures stable oxygen (δ₆₂₈) and clumped (∆₆₃₈) isotopes in carbonates, analysing infrared absorbance of CO2 isotopologues produced during phosphoric acid digestion. These isotopes are influenced by acid fractionation, which varies with both digestion temperature and technique, making it essential to quantify the fractionation factor for data comparability and to assess isotopic exchange between the product CO₂ and H₂O. Here, we present analysis of Carrara Marble (MAR-J1) calcite, the same as primary standard NBS19 reacting with approximately 104% phosphoric acid at varying temperatures via two traditional digestion techniques: (i) break seal (BS)/McCrea-type digestion technique, which allows a complete equilibration between product CO₂ and H₂O; and (ii) individual acid bath (IAB) technique, which compromises the equilibration process. Our experiments reveal that the calcite acid fractionation for δ₆₂₈ and ∆₆₃₈ varies with digestion technique because CO₂–acid contact time controls exchange with H₂O. The IAB technique yields a shallower slope for δ₆₂₈ and a steeper slope for ∆₆₃₈, indicating minimal isotopic resetting, whereas the BS technique exhibits the opposite pattern, suggesting enhanced isotopic exchange during extended CO₂–acid interaction. These findings facilitate robust inter-laboratory comparisons of δ₆₂₈ and ∆₆₃₈ ratios in calcite.