Carbon dioxide is produced in brines formed during the growth of sea ice as a result of preferential precipitation of calcium carbonate. This process can explain the observed CO2 supersaturation in some arctic waters and could produce a CO2 flux into the ocean in ice-covered waters of 1.5 mol m-2 y-1 or a total of 6¿1013 mol y-1 for both the artic and antarctic regions.