Southern Ocean CO 2 sink: The contribution of the sea ice

We report first direct measurements of the partial pressure of CO 2 (pCO 2 ) within Antarctic pack sea ice brines and related CO 2 fluxes across the air‐ice interface. From late winter to summer, brines encased in the ice change from a CO 2 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 pCO 2 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 CO 2 transfer at the air‐sea ice interface. The sea ice changes from a transient source to a sink for atmospheric CO 2 . 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 CO 2 uptake from the atmosphere by Antarctic sea ice. Over the spring‐summer period, the Antarctic sea ice cover is a net sink of atmospheric CO 2 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 CO 2 of the Southern Ocean.