Recent studies have introduced stable Ba isotopes (δ 138/134 Ba) as a novel tracer for ocean processes. Ba isotopes could potentially provide insight into the oceanic Ba cycle, the ocean's biological pump, watermass provenance in the deep ocean, changes in activity of hydrothermal vents, and land-sea interactions including tracing riverine inputs. Here, we show that aragonite skeletons of various colonial and solitary cold-water coral (CWC) taxa record the seawater (SW) Ba isotope composition. Thirty-six corals of eight different taxa from three oceanic regions were analysed and compared to δ 138/134 Ba measurements of co-located seawater samples. Sites were chosen to cover a wide range of temperature, salinity, Ba concentrations and Ba isotope compositions. Seawater samples at the three sites exhibit the wellestablished anti-correlation between Ba concentration and δ 138/134 Ba. Furthermore, our data set suggests that Ba/Ca values in CWCs are linearly correlated with dissolved [Ba] in ambient seawater, with an average partition coefficient of DCWC/SW = 1.8 ± 0.4 (2SD). The mean isotope fractionation of Ba between seawater and CWCs Δ 138/134 BaCWC-SW is-0.21 ± 0.08‰ (2SD), indicating that CWC aragonite preferentially incorporates the lighter isotopes. This fractionation likely does not depend on temperature or other environmental variables, suggesting that aragonite CWCs could be used to trace the Ba isotope composition in ambient seawater. Coupled [Ba] and δ 138/134 Ba analysis on fossil CWCs has the potential to provide new information about past changes in the local and global relationship between [Ba] and δ 138/134 Ba and hence about the operation of the past global oceanic Ba cycle in different climate regimes.

The boron isotopic ratio of 11B/10B (δ11BSRM951) 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 δ11BSRM951 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 δ11BSRM951 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 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 °C. These now well‐characterised solutions are useful reference materials to help the palaeoceanographic community build a comprehensive view of past ocean changes.

Extended calibration of cold-water coral Ba/Ca using multiple genera and co-located measurements of dissolved barium concentration. Chemge (2018), Abstract Biological productivity and ocean circulation are both important oceanographic variables that control the distribution of dissolved barium in the ocean interior ([Ba] sw ). The ability to accurately reconstruct [Ba] sw will provide key constraints on these processes in the past. The geochemistry of cold-water corals has the potential to unlock paleoceanographic records at spatial and temporal resolutions not available using other sedimentary archives. Previous studies have suggested that the Ba/Ca ratio of coral skeletons is linearly related to [Ba] sw .However, these efforts have used a limited number of species, sparse global seawater databases, or have not explicitly measured the Ba/Ca ratio. Here we investigate the Ba/Ca ratio in a well-constrained set of cold-water scleractinian (aragonitic) corals as a proxy for[Ba] sw , using 58 specimens from 7 coral genera along with co-located measurements of [Ba] sw . We find that traditional chemical cleaning procedures do not significantly affect the Ba/Ca ratio of cold-water coral skeletons, allowing rapid sample throughput. We also determine that intra-sample variation in Ba/Ca ratios can be reduced by using larger sample sizes (e.g. 20 mg). By combining our results with existing data, we find that cold-water coral Ba/Ca is linearly related to [Ba] sw according to the relationship: Ba/Ca µmol/mol = [0.15 ± 0.02] [Ba sw nmol/kg] + [2.5 ± 1.4], (R 2 = 0.7). We observe no species-specific 'vital effects' in cold-water coral Ba/Ca ratios, but site-specific effects could be a factor. Nevertheless, our results highlight the potential of Ba/Ca in cold-water corals to reconstruct biological and physical changes in the ocean interior.