High-precision measurements of δ¹⁷O and ¹⁷O_excess of NBS19 and NBS18

Rationale Measurements of oxygen-17 excess (¹⁷O_excess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ¹⁷O and ¹⁷O_excess values for NBS19 and NBS18, two international standards, for which δ¹⁸O values are already widely used as the references for carbonates. Methods The measurements are based on isotopic exchange at steady state between O₂ and CO₂ over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ¹⁷O and δ¹⁸O values of this O₂ allow δ¹⁷O values of CO₂ to be obtained with very high precision (0.01 to 0.03 ‰) and, correspondingly, accurate ¹⁷O_excess values with a precision of 5 per meg. Results We measured, for the first time, the δ¹⁷O values and ¹⁷O_excess of CO₂ liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015 ‰ and -227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041 ‰; 9.253 ± 0.018 ‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW. Conclusions Accurate values of δ¹⁷O and ¹⁷O_excess of the international standards NBS19-CO₂ and NBS18-CO₂ are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories.