Simultaneous onboard analysis of seawater dissolved inorganic carbon (DIC) concentration and stable isotope ratio (δ13C-DIC)

Dissolved inorganic carbon (DIC) and its stable carbon isotope (δ¹³C-DIC) are valuable parameters for studying the aquatic carbon cycle and quantifying ocean anthropogenic carbon accumulation rates. However, the potential of this coupled pair is underexploited as only 15% or less of cruise samples have been analyzed for δ¹³C-DIC because the traditional isotope analysis is labor-intensive and restricted to onshore laboratories. Here, we improved the analytical precision and reported the protocol of an automated, efficient, and high-precision method for ship-based DIC and δ¹³C-DIC analysis based on cavity ring-down spectroscopy (CRDS). We also introduced a set of stable in-house standards to ensure accurate and consistent DIC and δ¹³C-DIC measurements, especially on prolonged cruises. With this method, we analyzed over 1600 discrete seawater samples over a 40-d cruise along the North American eastern ocean margin in summer 2022, representing the first effort to collect a large dataset of δ¹³C-DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μmol kg⁻¹ for the DIC concentration and 0.03‰ for the δ¹³C-DIC value (1σ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μmol kg⁻¹ between CRDS and the coulometry-based results. The cross-validation of δ¹³C-DIC in the deep-ocean data exhibited a mean difference of only −0.03‰ ± 0.07‰, emphasizing the consistency with historical data. Potential applications in aquatic biogeochemistry are discussed.