Simultaneous onboard analysis of seawater dissolved inorganic carbon (DIC) concentration and stable isotope ratio (<i>δ</i>¹³C‐DIC)

Abstract Dissolved inorganic carbon (DIC) and its stable carbon isotope ( δ 13 C‐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 δ 13 C‐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 δ 13 C‐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 δ 13 C‐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 δ 13 C‐DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μ mol kg −1 for the DIC concentration and 0.03‰ for the δ 13 C‐DIC value (1 σ ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μ mol kg −1 between CRDS and the coulometry‐based results. The cross‐validation of δ 13 C‐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.