The contribution of sea-ice-covered regions to the global air-sea CO2 exchange was, until recently, assumed to be insignifi cant primarily because sea ice was considered impermeable. The discovery of a sea ice CO2 pump and the recognition that extensive and productive microbial communities exist within sea ice, however, has changed this general perception. Therefore, an improved understanding of the current role of sea ice in the overall carbon budget is needed. The main focus of this PhD study was: 1) to investigate the factors that control the spatial and temporal distribution of calcium carbonate and other important biogeochemical parameters in Greenland sea ice, 2) to discuss the potential interactions between these parameters, and 3) to assess how these parameters aff ect the sea ice CO2 system. Here, I report measurements of calcium carbonate dynamics, biological activity, total alkalinity (TA), total inorganic carbon (TCO2) and other biogeochemical parameters from sea ice in Greenland. First, I look at the dynamics of these parameters at temporal and spatial scales in Subarctic sea ice and, secondly look at the dynamics of these parameters in High Arctic winter sea ice on a more patchy level. Altogether, my results indicate that the TCO2 depletion of the Subarctic and High Arctic sea ice is mainly controlled by physical export through brine drainage and CaCO3 precipitation/dissolution – a conclusion that, therefore, strengthens the concept of the sea-ice-driven carbon pump in high latitude waters. Furthermore, the diff erent studies combined revealed that the relative contribution of primary production to TCO2 depletion is minor compared to the contribution of calcium carbonate precipitation. However, the biological contribution to the TCO2 depletion might be much higher in areas with high primary production. Consequently, the evaluation of the sea ice sink described in this thesis may not be representative of the Arctic as a whole since the uptake of CO2 by biological activity seems to be much lower in Greenland sea ice compared to other regions. Extensive investigations are, however, still needed to elucidate local and regional variation in biological activity in sea ice in Greenland and in other Arctic regions. The highest concentrations of calcium carbonate ever reported in natural sea ice was measured in approximately 5-month-old High Arctic land-fast sea ice, followed by high concentrations in newly formed High Arctic polynya sea ice; whereas the lowest concentrations observed during our studies were in Subarctic land-fast sea ice. Variations in sea ice properties such as temperature, salinity, pH, ice texture and freshwater input are likely responsible for some of the diff erences found in calcium carbonate concentrations between sites. Consequently, the diff erent studies revealed large variations in calcium carbonate concentration and other biogeochemical parameters at diff erent temporal and spatial scales, emphasising the importance of full-season studies covering the meter-hundred meter spatial scale in order to make reliable carbon budgets. This PhD thesis also presents a survey of the infl uence of biological processes and glacier runoff on the pCO2 dynamics in Subarctic coastal waters. The study revealed that the Subarctic Godthåbsfj ord system in SW Greenland can be considered as a strong sink of CO2 and that the CO2 uptake is highly regulated by biological processes and by mixing glacial meltwater and coastal waters. Moreover, the CO2uptake is strongest nearest to the outlet from the Greenland Ice Sheet. If our estimates are representative of similar Subarctic fj ord system in Greenland, then the coastal areas of Greenland constitutes a larger sink than anticipated and this knowledge should be included in future global carbon budgets.