Simultaneous Onboard Analysis of Seawater Dissolved Inorganic Carbon (DIC) Concentration and Stable Isotope Ratio (δ13C-DIC)

Dissolved inorganic carbon (DIC) concentration and stable carbon isotope value (δ¹³C-DIC) are valuable for studying aquatic carbon cycles. These parameters reveal significant geochemical insights, such as the discernible effect of ocean anthropogenic CO₂ uptake, the primary control of surface δ¹³C-DIC distribution by photosynthesis and respiration against a meridionally variable air-sea equilibrium background, and the notable impact of terrestrial carbon inputs in estuarine environments. However, one cannot take full advantage of this coupled pair as only 15% or less of water samples during past ocean cruises and very few coastal ocean samples have been analyzed for δ¹³C-DIC as the traditional isotope analytical technology is labor-intensive and limited in shore-based laboratories. This study reports a rapid and cost-effective method based on Cavity Ring-Down Spectroscopy (CRDS) for automatically and simultaneously analyzing DIC concentration and δ¹³C-DIC on shipboard. Compared to traditional techniques, our analyzer is more portable and operational-friendly. We also prepared and preserved a set of stable in-house NaHCO₃ standards for seawater δ¹³C-DIC calibration during long cruises. This work represents the first effort to collect a large dataset of δ¹³C-DIC onboard on any oceanic transect; here along the North American eastern ocean margins in summer 2022. We efficiently processed 30 samples daily per analyzer over a 40-day expedition with excellent on-site uncertainty of ±1.1 μmol kg^-1 for the DIC concentration and ±0.03‰ for the δ¹³C-DIC value (1σ). The duplicates taken from varying depths demonstrated high consistency with average standard deviations of 1.6 μmol kg^-1 for DIC concentrations ranging between 1900 and 2300 μmol kg^-1 and 0.04‰ for δ¹³C-DIC from -0.5‰ to 1.8‰. The DIC concentration measurements of CRM displayed average discrepancies of 1.4±1.7 μmol kg^-1 for Batch #188 and 1.0±1.1 μmol kg^-1 for Batch #195 against certified values, indicating reliable accuracy. Our δ¹³C-DIC analysis of CRM from Batch #188 yielded an average of -0.20±0.04‰, closely matching the reference value of -0.19±0.02‰ obtained by Isotope Ratio Mass Spectrometry (IRMS). Consistent standard deviations for δ¹³C-DIC of CRM from Batch #188 (0.04‰, n = 36) and Batch #195 (0.03‰, n = 7) further affirmed the potential utility of CRM as a viable liquid standard for δ¹³C-DIC measurements in seawater. An interlaboratory comparison of DIC analysis with NOAA/AOML revealed an average offset of 2.0±3.8 μmol kg^-1 between onboard CRDS measurements and Coulometry results. Moreover, the cross-validation of δ¹³C-DIC against historical deep-ocean data exhibited a mean difference of only -0.04±0.06‰, emphasizing the high quality of our data.