Southern Ocean CO2 sink: the contribution of the sea ice

We report first direct measurements of the partial pressure of CO₂ (pCO₂) within Antarctic pack sea ice brines and related CO₂ fluxes across the air-ice interface. From late winter to summer, brines encased in the ice change from a CO₂ large oversaturation, relative to the atmosphere, to a marked undersaturation while the underlying oceanic waters remains slightly oversaturated. The decrease from winter to summer of pCO₂ in the brines is driven by dilution with melting ice, dissolution of carbonate crystals, and net primary production. As the ice warms, its permeability increases, allowing CO₂ transfer at the air-sea ice interface. The sea ice changes from a transient source to a sink for atmospheric CO₂. We upscale these observations to the whole Antarctic sea ice cover using the NEMO-LIM3 large-scale sea ice-ocean and provide first estimates of spring and summer CO₂ uptake from the atmosphere by Antarctic sea ice. Over the spring-summer period, the Antarctic sea ice cover is a net sink of atmospheric CO₂ of 0.029 Pg C, about 58% of the estimated annual uptake from the Southern Ocean. Sea ice then contributes significantly to the sink of CO₂ of the Southern Ocean.