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In 2015 Barkan et al performed high‐precision measurements of the δO and Oexcess values of the international carbonate standards NBS19 and NBS18. In 2016 a new standard (IAEA‐603) was introduced, aimed to replace NBS19 that had been exhausted, as the new international standard for δC and δO analyses. In order to use it also as a standard for Oexcess, we present here high‐precision measurements of three oxygen isotopes in this standard. The measurements were carried out by direct oxygen isotope exchange between CO2 extracted from CaCO3 and O2 of known three oxygen isotopic composition, followed by analysis of the resulting O2 for δ O and δO values. These values were used to calculate Oexcess as: Oexcess = 10 6 [ln(10δO + 1) − 0.528 ln(10δO + 1)]. All methodological details, including those for CO2 extraction from carbonates, were given previously. In short, CO2 samples were extracted from CaCO3 by digestion with 1.92 g/cm 3 phosphoric acid at 25°C. The CO2 underwent isotopic exchange with O2 of a known isotopic composition over hot platinum. After isotope exchange, the O2 samples were measured by dual‐inlet mass spectrometry versus an O2 working reference gas that was calibrated with respect to VSMOW. The obtained values were then normalized to the VSMOW–SLAP scale, assuming that the Oexcess value of SLAP equals zero. As the O isotopic fractionation for phosphoric acid digestion is unknown, the reported values are for the CO2 rather than for CaCO3. 4

Calibration of δ 17 O and 17 O excess values of three international standards: IAEA‐603, NBS19 and NBS18