Abstract. Eddy covariance observations of CO₂ fluxes were conducted during March–April 2012 in a temporally sequential order for 8, 4 and 30 days, respectively, at three locations on fast sea ice and on newly formed polynya ice in a coastal fjord environment in northeast Greenland. CO₂ fluxes at the sites characterized by fast sea ice (ICEI and DNB) were found to increasingly reflect periods of strong outgassing in accordance with the progression of springtime warming and the occurrence of strong wind events: F_CO2^ICE1 = 1.73 ± 5 mmol m⁻² day⁻¹ and F_CO2^DNB = 8.64 ± 39.64 mmol m⁻² day⁻¹, while CO₂ fluxes at the polynya site (POLYI) were found to generally reflect uptake F_CO2^POLY1 = −9.97 ± 19.8 mmol m⁻² day⁻¹. Values given are the mean and standard deviation, and negative/positive values indicate uptake/outgassing, respectively. A diurnal correlation analysis supports a significant connection between site energetics and CO₂ fluxes linked to a number of possible thermally driven processes, which are thought to change the pCO₂ gradient at the snow–ice interface. The relative influence of these processes on atmospheric exchanges likely depends on the thickness of the ice. Specifically, the study indicates a predominant influence of brine volume expansion/contraction, brine dissolution/concentration and calcium carbonate formation/dissolution at sites characterized by a thick sea-ice cover, such that surface warming leads to an uptake of CO₂ and vice versa, while convective overturning within the sea-ice brines dominate at sites characterized by comparatively thin sea-ice cover, such that nighttime surface cooling leads to an uptake of CO₂ to the extent permitted by simultaneous formation of superimposed ice in the lower snow column.