NIST RM 8301 Boron Isotopes in Marine Carbonate (Simulated Coral and Foraminifera Solutions): Inter-laboratory δ11B and Trace Element Ratio Value Assignment

The boron isotopic ratio of B-11/B-10 (delta B-11(SRM951)) and trace element composition of marine carbonates are key proxies for understanding carbon cycling (pH) and palaeoceanographic change. However, method validation and comparability of results between laboratories requires carbonate reference materials. Here, we report results of an inter-laboratory comparison study to both assign delta B-11(SRM951) and trace element compositions to new synthetic marine carbonate reference materials (RMs), NIST RM 8301 (Coral) and NIST RM 8301 (Foram) and to assess the variance of data among laboratories. Non-certified reference values and expanded 95% uncertainties for delta B-11(SRM951) in NIST RM 8301 (Coral) (+24.17 parts per thousand +/- 0.18 parts per thousand) and NIST RM 8301 (Foram) (+14.51 parts per thousand +/- 0.17 parts per thousand) solutions were assigned by consensus approach using inter-laboratory data. Differences reported among laboratories were considerably smaller than some previous inter-laboratory comparisons, yet discrepancies could still lead to large differences in calculated seawater pH. Similarly, variability in reported trace element information among laboratories (e.g., Mg/Ca +/- 5% RSD) was often greater than within a single laboratory (e.g., Mg/Ca < 2%). Such differences potentially alter proxy-reconstructed seawater temperature by more than 2 degrees C. These now well-characterised solutions are useful reference materials to help the palaeoceanographic community build a comprehensive view of past ocean changes.

Automated summary

This study assigned boron isotopic ratios and trace element compositions to new synthetic marine carbonate reference materials and assessed data variance among laboratories. The results provide useful reference materials for the palaeoceanographic community and highlight the importance of method validation and comparability of results.