Sea ice contribution to the air–sea CO2 exchange in the Arctic and Southern Oceans

Although salt rejection from sea ice is a key process in deep-water formation in ice-covered seas, the concurrent rejection of CO₂ and the subsequent effect on air–sea CO₂ exchange have received little attention. We review the mechanisms by which sea ice directly and indirectly controls the air–sea CO₂ exchange and use recent measurements of inorganic carbon compounds in bulk sea ice to estimate that oceanic CO₂ uptake during the seasonal cycle of sea-ice growth and decay in ice-covered oceanic regions equals almost half of the net atmospheric CO₂ uptake in ice-free polar seas. This sea-ice driven CO₂ uptake has not been considered so far in estimates of global oceanic CO₂ uptake. Net CO₂ uptake in sea-ice–covered oceans can be driven by; (1) rejection during sea–ice formation and sinking of CO₂-rich brine into intermediate and abyssal oceanic water masses, (2) blocking of air–sea CO₂ exchange during winter, and (3) release of CO₂-depleted melt water with excess total alkalinity during sea-ice decay and (4) biological CO₂ drawdown during primary production in sea ice and surface oceanic waters.