Over-determination of the carbonate system in natural sea-ice brine and assessment of carbonic acid dissociation constants under low temperature, high salinity conditions

•We report carbonate system measurements (DIC, TA, pHF, pCO2) in natural sea-ice brine.•Seawater-derived carbonic acid dissociation constants not valid for high salinity brine.•Determination of K1⁎ and K2⁎ at high salinity, low temperature urgently needed.

By over-determining the carbonate system in natural sea-ice brine samples, we have confirmed that stoichiometric equilibrium constants for the dissociation of carbonic acid in seawater (K1⁎, K2⁎) derived for general oceanic conditions may not be readily extrapolated to sea-ice brine systems. Dissolved inorganic carbon, total alkalinity, pH, and pCO2 measurements in sea-ice brine have allowed us to rigorously examine the validity of K1⁎ and K2⁎ at salinities as high as 82 and in-situ temperatures as low as − 4.8 °C. The use of seawater-derived constants yields average offsets between calculated and measured values ranging from 10% to 43% for brine salinities of 38–82. Future high-accuracy determination of sea-ice brine carbonate system components will require the specific determination of carbonate system equilibrium constants under appropriate temperature and salinity conditions. Furthermore, the community is in need of a means to evaluate the accuracy of carbonate system measurements in natural, high-salinity brine samples using the presently available analytical methods.