Estimates of ikaite export from sea ice to the underlying seawater in a sea ice–seawater mesocosm

Abstract. The precipitation of ikaite and its fate within sea ice is still poorly understood. We quantify temporal inorganic carbon dynamics in sea ice from initial formation to its melt in a sea ice–seawater mesocosm pool from 11 to 29 January 2013. Based on measurements of total alkalinity (TA) and total dissolved inorganic carbon (TCO₂), the main processes affecting inorganic carbon dynamics within sea ice were ikaite precipitation and CO₂ exchange with the atmosphere. In the underlying seawater, the dissolution of ikaite was the main process affecting inorganic carbon dynamics. Sea ice acted as an active layer, releasing CO₂ to the atmosphere during the growth phase, taking up CO₂ as it melted and exporting both ikaite and TCO₂ into the underlying seawater during the whole experiment. Ikaite precipitation of up to 167 µmolkg⁻¹ within sea ice was estimated, while its export and dissolution into the underlying seawater was responsible for a TA increase of 64–66 µmolkg⁻¹ in the water column. The export of TCO₂ from sea ice to the water column increased the underlying seawater TCO₂ by 43.5 µmolkg⁻¹, suggesting that almost all of the TCO₂ that left the sea ice was exported to the underlying seawater. The export of ikaite from the ice to the underlying seawater was associated with brine rejection during sea ice growth, increased vertical connectivity in sea ice due to the upward percolation of seawater and meltwater flushing during sea ice melt. Based on the change in TA in the water column around the onset of sea ice melt, more than half of the total ikaite precipitated in the ice during sea ice growth was still contained in the ice when the sea ice began to melt. Ikaite crystal dissolution in the water column kept the seawater pCO₂ undersaturated with respect to the atmosphere in spite of increased salinity, TA and TCO₂ associated with sea ice growth. Results indicate that ikaite export from sea ice and its dissolution in the underlying seawater can potentially hamper the effect of oceanic acidification on the aragonite saturation state (Ω_aragonite) in fall and in winter in ice-covered areas, at the time when Ω_aragonite is smallest.