Simultaneous Onboard Analysis of Seawater Dissolved Inorganic Carbon (DIC) Concentration and Stable Isotope Ratio (&#948;13C-DIC)
Authors: Sun Zhentao , Li Xinyu , Ouyang Zhangxian , Li Qian , Featherstone Charles , Atekwana Eliot , Hussain Najid , Pan Yiwen , Cai Wei-Jun 
Published: March 2024

Dissolved inorganic carbon (DIC) concentration and stable carbon isotope value (&#948;13C-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 &#948;13C-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 &#948;13C-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 &#948;13C-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 &#948;13C-DIC calibration during long cruises. This work represents the first effort to collect a large dataset of &#948;13C-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 &#177;1.1 &#956;mol kg-1 for the DIC concentration and &#177;0.03&#8240; for the &#948;13C-DIC value (1&#963;). The duplicates taken from varying depths demonstrated high consistency with average standard deviations of 1.6 &#956;mol kg-1 for DIC concentrations ranging between 1900 and 2300 &#956;mol kg-1 and 0.04&#8240; for &#948;13C-DIC from -0.5&#8240; to 1.8&#8240;. The DIC concentration measurements of CRM displayed average discrepancies of 1.4&#177;1.7 &#956;mol kg-1 for Batch #188 and 1.0&#177;1.1 &#956;mol kg-1 for Batch #195 against certified values, indicating reliable accuracy. Our &#948;13C-DIC analysis of CRM from Batch #188 yielded an average of -0.20&#177;0.04&#8240;, closely matching the reference value of -0.19&#177;0.02&#8240; obtained by Isotope Ratio Mass Spectrometry (IRMS). Consistent standard deviations for &#948;13C-DIC of CRM from Batch #188 (0.04&#8240;, n = 36) and Batch #195 (0.03&#8240;, n = 7) further affirmed the potential utility of CRM as a viable liquid standard for &#948;13C-DIC measurements in seawater. An interlaboratory comparison of DIC analysis with NOAA/AOML revealed an average offset of 2.0&#177;3.8 &#956;mol kg-1 between onboard CRDS measurements and Coulometry results. Moreover, the cross-validation of &#948;13C-DIC against historical deep-ocean data exhibited a mean difference of only -0.04&#177;0.06&#8240;, emphasizing the high quality of our data.