Towards the determination of the CO2 system in sea ice: The stoichiometric dissociation constants of carbonic acid in seawater and seawater-derived brines at sub-zero temperatures

The aquatic CO₂ system has been at the centre of scientific investigation of the geochemical and socio-economic effects of the rise in atmospheric CO₂ in the industrial era because of the interaction between the dissolved CO₂ pool in natural waters and atmospheric CO₂. The study of the CO₂ system in the polar oceans is incomplete, and one of the major obstacles is that the constants that describe the chemical equilibria of the weak acid-base components of the marine CO₂ system have only been well-defined for conditions of above-zero temperatures and practical salinity up to 50, whereas the polar oceans encompass conditions of sub-zero temperatures and salinities higher than 50 in the brines of their sea ice cover. The carbon chemistry in sea ice and the CO₂ fluxes across the atmosphere–ice–seawater interfaces are areas of active investigation in both polar regions, hampered by the lack of empirical characterization of the equilibria controlling the CO₂ system under polar-specific temperature and salinity conditions. To this end, we present the first determination of the stoichiometric (concentration-based) dissociation constants of carbonic acid in seawater and seawater-derived hypersaline natural brines at sub-zero temperatures. The results of our study will allow a leap forward on current computational and methodological limitations with regards the state of the CO₂ system in the globally influential sea-ice-covered polar oceanic regions. Wider use of these constants will aid investigation of ongoing and future changes in the CO₂ chemistry in high latitude oceans, setting important constraints on model predictions of climate excursions past, present, and future.