Annual carbon dioxide flux over seasonal sea ice in the Canadian Arctic

Abstract. Continuous measurements of carbon dioxide (CO₂) flux were collected from a 10-m eddy covariance tower in a coastal-marine environment in the Canadian Arctic Archipelago over the course of a 17-month period. The extended length of data collection resulted in a unique dataset that includes measurements from two spring melt and summer seasons and one autumn freeze-up. These field observations were used to verify findings from previous theoretical and laboratory experiments investigating air-sea gas exchange in connection with sea ice. The results corroborated previous findings showing that full sea ice cover under winter conditions acts as a barrier to gas exchange. In the spring, CO₂ fluxes were downward (uptake) in both the presence of melt ponds and during ice break-up. However, diurnal cycles were present throughout the early spring melt period, corresponding to the opposing influences of freezing and melting at the ice surface. Fluxes measured during melt periods confirmed previous laboratory tank measurements that showed a gas transfer coefficient of melting ice of 0.4 mol m⁻² d⁻¹ atm⁻¹. Open water CO₂ fluxes showed outgassing in early summer and uptake in mid-to-late summer, tied closely to trends in surface water temperature and its effect on the partial pressure of CO₂ in the water. The winter field campaign represents the first eddy covariance CO₂ fluxes measured over naturally forming sea ice. Our measurements showed mean upward fluxes (outgassing) of 1.1 ± 1.5 mmol m⁻² d⁻¹ associated with the freezing of ice – the same order of magnitude found by previous laboratory tank experiments. However, peak flux periods during ice formation had measured fluxes that were a factor of 3 higher than the tank experiments, suggesting the importance of natural conditions (e.g., wind) on air-ice gas exchange. Conducting an Arctic-wide extrapolation we estimate CO₂ outgassing from the freezing period to be 5 to 15% of the magnitude of the estimated Arctic CO₂ sink. Overall, there was no evidence of dramatically enhanced gas exchange in marginal ice conditions as proposed by previous studies. Although the different seasons showed active CO₂ exchange, there was a balance between upward and downward fluxes at this specific location, resulting in a small net CO₂ uptake over the annual cycle of −0.3 g-C m⁻².