The study of air-ice CO2 exchange emphasize the importance of gas bubble transport during sea ice growth

We report air-ice CO2 fluxes measured continuously using automated chambers over artificial sea ice from freezing to decay. We observed an uptake of CO2 as seawater was cooling down prior to sea ice formation. As soon as the first ice crystals started to form, we observed a shift from a sink to a source. Sea ice released CO2 until we initiated the ice decay by warming the atmosphere. Sea ice then returned to be a CO2 sink. Direct measurements of the fluxes were consistent with the depletion of dissolved inorganic carbon in sea ice. Measurements of bulk partial pressure of CO2 in sea ice and of atmospheric CO2 allowed us to assess a gas exchange coefficient for CO2 at the air-sea ice interface during the grow stage. We compared these observations with a 1D biogeochemical model. Discrepancies between the model and the observations lead us to emphasize the role of gas bubbles in CO2 transport through sea ice.