During sea ice formation in polar areas, brine rejection increases the density in the underlying water column and thereby contributes to the formation of deep and intermediate water masses in the world ocean. Here we present evidence that dissolved inorganic carbon (TCO2) is rejected together with brine from growing sea ice and that low temperatures may result in a significant change in the ratio of TCO2 and alkalinity in Arctic sea ice compared with surface waters. Model calculations show that this sea ice--driven carbon pump affects surface water partial pressure of CO2 significantly in polar seas and potentially sequesters large amounts of CO2 to the deep ocean.