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 (δ¹¹B_SRM951) and trace element composition of marine carbonates are key proxies for understanding carbon cycling (pH) and palaeoceanographic change. However, comparability of results between laboratories requires carbonate reference materials. Here we report results of an inter‐laboratory comparison study to both assign δ¹¹B_SRM951 and trace element compositions to new marine carbonate reference materials (RM), NIST RM 8301 (Coral) and NIST RM 8301 (Foram) and to assess analytical variability among laboratories. Non‐certified reference values and expanded 95% uncertainties for δ¹¹B_SRM951 in NIST RM 8301 (Coral) (+24.17‰ ± 0.18‰) and NIST RM 8301 (Foram) (+14.51‰ ± 0.17‰) 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 values among laboratories (e.g., Mg/Ca ± 5%) was often greater than analytical precision (% RSD) within a single laboratory (e.g., Mg/Ca < 2%). Such differences potentially alter proxy‐reconstructed seawater temperature by more than 2 °C. These now well‐characterised solutions are useful reference materials to help the palaeoceanographic community build a comprehensive view of past ocean changes.