Neglecting organic alkalinity introduces greater error than assuming boron to salinity ratios in Arctic sea ice brine carbonate system calculations

While total alkalinity (AT) is traditionally attributed to dissolved inorganic constituents, dissolved organic matter (DOM) can significantly contribute to AT as organic alkalinity (OrgAlk), introducing errors in calculated carbonate parameters, such as the CaCO3 saturation state (Ω) and partial pressure of CO2 (pCO2). This study presents measurements of OrgAlk in the Arctic Ocean sea ice system and assesses its influence on carbonate speciation, with OrgAlk contributing 0.1–1.0% to AT. Sea ice brine exhibited elevated DOM and OrgAlk, with an OrgAlk/DOC ratio of 0.13 ± 0.06 µmol kg− 1 µM− 1, consistent with global ocean values. Correcting AT for OrgAlk increased computed pCO2 up to 84 µatm and decreased Ω ≤ 0.2 for aragonite and ≤ 0.3 for calcite compared to un-adjusted values. Elevated brine pCO2 suggests that conventional estimates of Arctic sea ice CO2 uptake may be overestimated when AT is used as an input parameter, particularly in spring as OrgAlk is released. The omission of OrgAlk contributed greater errors to calculated carbonate parameters than the differences in boron from using direct measurements versus salinity based ratios, highlighting the necessity of accounting for even minor OrgAlk to refine predictions of surface pCO2, net air-sea CO2 flux, and the fate of CaCO3 minerals.