Abstract. Calcium carbonate precipitation in sea ice can increase pCO₂ during precipitation in winter and decrease pCO₂ during dissolution in spring. CaCO₃ precipitation in sea ice is thought to potentially drive significant CO₂ uptake by the ocean. However, little is known about the quantitative spatial and temporal distribution of CaCO₃ within sea ice. This is the first quantitative study of hydrous calcium carbonate, as ikaite, in sea ice and discusses its potential significance for the carbon cycle in polar oceans. Ice cores and brine samples were collected from pack and land fast sea ice between September and December 2007 during an expedition in the East Antarctic and another off Terre Adélie, Antarctica. Samples were analysed for CaCO₃, Salinity, DOC, DON, Phosphate, and total alkalinity. A relationship between the measured parameters and CaCO₃ precipitation could not be observed. We found calcium carbonate, as ikaite, mostly in the top layer of sea ice with values up to 126 mg ikaite per liter melted sea ice. This potentially represents a contribution between 0.12 and 9 Tg C to the annual carbon flux in polar oceans. The horizontal distribution of ikaite in sea ice was heterogenous. We also found the precipitate in the snow on top of the sea ice.