Inorganic Carbon and pCO₂ Variability During Ice Formation in the Beaufort Gyre of the Canada Basin

Solute exclusion during sea ice formation is a potentially important contributor to the Arctic Ocean inorganic carbon cycle that could increase as ice cover diminishes. When ice forms, solutes are excluded from the ice matrix, creating a brine that includes dissolved inorganic carbon (DIC) and total alkalinity (A_T). The brine sinks, potentially exporting DIC and A_T to deeper water. This phenomenon has rarely been observed, however. In this manuscript, we examine a ~1 year pCO₂ mooring time series where a ~35-μatm increase in pCO₂ was observed in the mixed layer during the ice formation period, corresponding to a simultaneous increase in salinity from 27.2 to 28.5. Using salinity and ice based mass balances, we show that most of the observed increases can be attributed to solute exclusion during ice formation. The resulting pCO₂ is sensitive to the ratio of A_T and DIC retained in the ice and the mixed layer depth, which controls dilution of the ice-derived A_T and DIC. In the Canada Basin, of the ~92 μmol/kg increase in DIC, 17 μmol/kg was taken up by biological production and the remainder was trapped between the halocline and the summer stratified surface layer. Although not observed before the mooring was recovered, this inorganic carbon was likely later entrained with surface water, increasing the pCO₂ at the surface. It is probable that inorganic carbon exclusion during ice formation will have an increasingly important influence on DIC and pCO₂ in the surface of the Arctic Ocean as seasonal ice production and wind-driven mixing increase with diminishing ice cover.