Triple oxygen isotope equilibrium between calcite and water: new constraints over 8–200 °C and their metrological implications

High-precision measurements of carbonate oxygen-17 compositions are increasingly used for studying paleo-environments, hydrology, diagenesis, or biomineralization processes. Calibrating triple oxygen isotope equilibrium fractionations between calcite and water as a function of temperature provides a critical baseline for many applications. Here we report new observations constraining these fractionations between a suite of natural and synthetic carbonates and their parent waters, obtained using VCOF-CRDS, a new spectroscopic method providing ppm-level metrological linearity. Our observations on slow-growing natural calcites collected from Laghetto Basso and Devils Hole and those of our synthetic carbonates that closely approach δ18O equilibrium are in good agreement with the latest theoretical models of calcite/water equilibrium fractionations between 8 °C and 200 °C. Data/model agreement is maximized when assuming that reference materials IAEA-603 and NBS18 have Δ17OVSMOW values of −0.068 ‰, and −0.042 ‰ respectively. These estimates are consistent with some but not all of the observations reported so far by other groups. A sensitivity analysis shows that varying these parameters can strongly bias estimates of equilibrium 17θ[calcite/water], whereas reformulating the equilibrium calibration in terms of chemically homogeneous comparisons (Δ17O[carbonate/IAEA-603] − Δ17O[water/VSMOW]) is entirely unaffected by Δ17O[IAEA-603/VSMOW] but sensitive to the Δ17O difference between IAEA-603 and NBS18. We conclude that pinning down the relative Δ17O values of NBS18 and IAEA-603 is the most pressing short-term need for paleo-reconstructions. We also recommend systematically normalizing carbonate Δ17O measurements using two carbonate reference materials with sufficiently different δ18O, and fully describing these normalization choices when reporting new Δ17O measurements.