Calcium Carbonate and Water Pyrolysis Measurements Suggest Minor Adjustment to the VPDB and VSMOW-SLAP δ18O 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 CO₂ 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 δ¹⁸O 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 δ¹⁸O VPDB and δ¹⁸O VSMOW-SLAP scale.

Conclusions

The outcome of the study made the direct comparison of the two different ¹⁸O scales possible. Our results demonstrate a small discrepancy in the presently recommended relation between the two oxygen isotope delta scales.