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

RATIONALE: Measurements of oxygen-17 excess (O-17(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 O-17 and O-17(excess) values for NBS19 and NBS18, two international standards, for which O-18 values are already widely used as the references for carbonates. METHODS: The measurements are based on isotopic exchange at steady state between O-2 and CO2 over hot platinum sponge at 750 degrees C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the O-17 and O-18 values of this O-2 allow O-17 values of CO2 to be obtained with very high precision (0.01 to 0.03) and, correspondingly, accurate O-17(excess) values with a precision of 5 per meg. RESULTS: We measured, for the first time, the O-17 values and O-17(excess) of CO2 liberated from NBS19 at 25 degrees C (39.196 +/- 0.026; 20.276 +/- 0.015 parts per thousand and -227 +/- 4 per meg, respectively) and NBS18 (17.591 +/- 0.041 parts per thousand; 9.253 +/- 0.018 parts per thousand and 3 +/- 5 per meg, respectively). The values are given versus VSMOW. CONCLUSIONS: Accurate values of O-17 and O-17(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. Copyright (C) 2015 John Wiley & Sons, Ltd.