Constraining organic alkalinity in Antarctic sea ice brines containing high concentrations of dissolved organic matter: Current limitations and future directions

Abstract. Organic alkalinity (OrgAlk), the component of total alkalinity (A_T) arising from non-inorganic species or uncharacterized bases, introduces error into calculated carbonate system parameters when A_T is used as an input. OrgAlk has been identified in Arctic sea ice brines with elevated dissolved organic carbon (DOC) concentrations, suggesting organic contributions to A_T may also be important in other sea ice environments. Antarctic sea ice brines are reported to contain higher DOC concentrations, yet estimating carbonate system components in Antarctic sea ice brines has relied on A_T-based calculations although no direct OrgAlk measurements exist for Antarctic sea ice brines and the potential magnitude of organic contributions to Antarctic brine A_T remains unknown. This study assesses potential OrgAlk contributions in Antarctic sea ice brines using published OrgAlk:DOC relationships as boundary conditions. Applying published OrgAlk:DOC relationships spanning 0.06–0.44 to Antarctic sea ice brine DOC concentrations produced a wide range of possible OrgAlk values, reinforcing the uncertainty of transferring DOC relationships across environments. These values are not interpreted as quantitative estimates of Antarctic sea ice brine OrgAlk, but rather as a sensitivity framework for evaluating plausible conditions. Sensitivity analyses indicate that unaccounted OrgAlk of this magnitude could affect calculated carbonate system parameters, including pCO₂, when A_T is used as an input parameter. These findings establish current uncertainty boundaries for OrgAlk in Antarctic sea ice brines, highlight a significant gap in Antarctic sea ice biogeochemistry and demonstrate the need for direct OrgAlk measurements to reduce uncertainty in Antarctic sea ice carbonate system calculations.