Knowledge on the relative effects of biological activity and precipitation/dissolution of calcium carbonate (CaCO3) in influencing the air-ice CO2 exchange in sea-ice-covered season is currently lacking. Furthermore, the spatial and temporal occurrence of CaCO3 and other biogeochemical parameters in sea ice are still not well described. Here we investigated autotrophic and heterotrophic activity as well as the precipitation/dissolution of CaCO3 in subarctic sea ice in South West Greenland. Integrated over the entire ice season (71 days), the sea ice was net autotrophic with a net carbon fixation of 56 mg C m−2, derived from a sea-ice-related gross primary production of 153 mg C m−2 and a bacterial carbon demand of 97 mg C m−2. Primary production contributed only marginally to the TCO2 depletion of the sea ice (7–25 %), which was mainly controlled by physical export by brine drainage and CaCO3 precipitation. The net biological production could only explain 4 % of this sea-ice-driven CO2 uptake. Abiotic processes contributed to an air-sea CO2 uptake of 1.5 mmol m−2 sea ice day−1, and dissolution of CaCO3 increased the air-sea CO2 uptake by 36 % compared to a theoretical estimate of melting CaCO3-free sea ice. There was a considerable spatial and temporal variability of CaCO3 and the other biogeochemical parameters measured (dissolved organic and inorganic nutrients).