An international intercomparison of stable carbon isotope composition measurements of dissolved inorganic carbon in seawater

We report results of an intercomparison of stable carbon isotope ratio measurements in seawater dissolved inorganic carbon (delta C-13-DIC) which involved 16 participating laboratories from various parts of the world. The intercomparison involved distribution of samples of a Certified Reference Material for seawater DIC concentration and alkalinity and a preserved sample of deep seawater collected at 4000 m in the northeastern Atlantic Ocean. The between-lab standard deviation of reported uncorrected values measured with diverse analytical, detection, and calibration methods was 0.11 parts per thousand (1 sigma). The multi-lab average delta C-13-DIC value reported for the deep seawater sample was consistent within 0.1 parts per thousand with historical measured values for the same water mass. Application of a correction procedure based on a consensus value for the distributed reference material, improved the between-lab standard deviation to 0.06 parts per thousand. The magnitude of the corrections were similar to those used to correct independent data sets using crossover comparisons, where deep water analyses from different cruises are compared at nearby locations. Our results demonstrate that the accuracy/uncertainty target proposed by the Global Ocean Observing System (+/- 0.05 parts per thousand) is attainable, but only if an aqueous phase reference material for delta C-13-DIC is made available and used by the measurement community. Our results imply that existing Certified Reference Materials used for seawater DIC and alkalinity quality control are suitable for this purpose, if a Certified or internally consistent consensus value for delta C-13-DIC can be assigned to various batches.