5. CONCLUSIONS AND RECOMMENDATIONS
With this work, the VSMOW-SLAP scale has in fact become metrologically traceable to the System International (SI) units for both isotopes: the combination of the absolute isotope 18O abundance for VSMOW (Baertschi7 and our present result for SLAP with respect to VSMOW leads to the absolute 18O abundance for SLAP of 1.88798 (43) x 10-3. For2H, this traceability has long been accomplished. In this work, however, we also produce a new and probably more accurate value for δ2H of SLAP with respect to VSMOW of -430.3 ± 0.3‰. This value is significantly lower than the values by Hagemann4, de Wit5 and Tse6. However, like in the 18O case, the value for 2R for VSMOW influences the value we obtain for δ2H of SLAP. In case we would use the2RVSMOW value reported by de Wit (155.95 x 10-6) in combination with the2RSLAP value of Hagemann (89.02 x 10-6), the difference would translate into -429.2‰, whereas our value for SLAP would change into -429.8‰. This ”friction of values” calls for a new gravimetric mixing experiment, now making use of the better and easier optical measurements of δ2H of water, combined with NMR determination of the purity of the2H and 1H waters. We plan to perform such an experiment in the near future. When that is successful, both the δ2H and δ18O isotope scales would become SI-traceable. That would be a first.
Best estimates for the absolute 13C abundance so far, for the VPDB-scale have been determined by Malinovsky25 (further work is in progress). The18O-side of this carbonate scale is much more complicated, due to the fractionating process that is at the basis. Furthermore, there still is no consensus on scale normalization, let alone on the absolute 13C and 18O abundances of such materials. For 18O, coupling the18O VPDB scale to VSMOW-SLAP using the CO2 - H2O equilibration process is probably a more fruitful route towards pinpointing this18O VPDB-scale to SI units, certainly for non-carbonate materials such as atmospheric CO2.
Acknowledgements
We acknowledge the valuable personal communication with Tyler Coplen (USGS) and Manfred Groening (IAEA) about the history of the assigned δ18O value for SLAP. We are grateful to the IAEA Terrestrial Environment Laboratory in Seibersdorf for providing ampoules of 1 mL of the original VSMOW and SLAP. We also want to thank Len Wassenaar, from the IAEA Isotope Hydrology laboratory in Vienna for providing the Antarctic water that formed the basis of this study. We would like to express special thanks to Johan Kemmink (RuG) who performed the NMR analysis of the 2H2O water.
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Table 1