Abstract
AbstractWe report results of an intercomparison of stable carbon isotope ratio measurements in seawater dissolved inorganic carbon (δ13C‐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‰ (1σ). The multi‐lab average δ13C‐DIC value reported for the deep seawater sample was consistent within 0.1‰ 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‰. 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‰) is attainable, but only if an aqueous phase reference material for δ13C‐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 δ13C‐DIC can be assigned to various batches.
Highlights
Uncertainty target proposed by the Global Ocean Observing System (Æ0.05‰) is attainable, but only if an aqueous phase reference material for δ13C-DIC is made available and used by the measurement community
There are no agreed-upon standardization procedures, and liquid or soluble Certified Reference Materials (CRMs) that can be distributed among measurement groups are not available
Test waters and their suitability Two supplies of seawater were used for the intercomparison study: (1) “Certified Reference Material for oceanic CO2 measurements (Batch 157)” supplied by the University of California, San Diego, Scripps Institution of Oceanography; (2) samples of deep ocean seawater (DSW) collected in May 2017 from the northeastern basin of the Atlantic Ocean at depths of >4000 m during the 2017 Go-Ship A02 transAtlantic cruise (McGovern et al 2017; GO-SHIP; http://www. go-ship.org/)
Summary
Uncertainty target proposed by the Global Ocean Observing System (Æ0.05‰) is attainable, but only if an aqueous phase reference material for δ13C-DIC is made available and used by the measurement community. Over the past 200 years, the oceanic uptake of fossilfuel-derived CO2, with depleted values of δ13C, has caused time-dependent depletion of seawater δ13C-DIC This “13C Suess-effect” signal (Keeling 1979) is a useful tracer for estimation of the anthropogenic carbon (Cant) accumulation in ocean waters, either through examining the vertical or along-isopycnal distribution of δ13C-DIC or through assessments of the air sea δ13C-DIC disequilibrium (e.g., Tans et al 1993; Gruber and Keeling 2001; Körtzinger et al 2003; Olsen et al 2006; Quay et al 2007). Offsets between individual data sets ranged from −0.39‰ to +0.17‰, and likely provide a rough estimate of the reproducibility of historical data sets
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