Calcium Carbonate and Water Pyrolysis Measurements Suggest Minor Adjustment to the VPDB and VSMOW-SLAP δ(18)O Scale Relation.

RATIONALE: Measurements of the oxygen isotopic composition of materials are widely used in many fields. These oxygen isotopic compositions are expressed using different scales. These scales are only linked indirectly, because water and calcium carbonate reference materials, used to establish these oxygen isotope delta scales, have to be converted to CO(2) first, and the isotopic fractionation of these conversions has only been measured a few times in the past. The anchoring of the two oxygen isotope delta scales is therefore currently suboptimal. METHODS: Primary reference materials, both waters and calcium carbonates, were pyrolyzed within a single measurement sequence in a high-temperature elemental analyzer-pyrolysis system connected to an isotope ratio mass spectrometer in continuous-flow mode. As calcium carbonate is difficult to pyrolyze completely, additives were added to reach a 100% yield. The δ(18)O of the calcium carbonates were normalized on the VSMOW-SLAP scale using VSMOW2 and in-house water references. RESULTS: The average results of 6 international calcites references measured in five independent sequences of pyrolysis measurements showed a difference of 0.07 to 0.09‰ with the presently described relationship in literature between the δ(18)O VPDB and δ(18)O VSMOW-SLAP scale. CONCLUSIONS: The outcome of the study made the direct comparison of the two different (18)O scales possible. Our results demonstrate a small discrepancy in the presently recommended relation between the two oxygen isotope delta scales.