CO₂ flux over young and snow-covered Arctic pack ice in winter and spring

Abstract. Rare CO₂ flux measurements from Arctic pack ice show that two types of ice contribute to the release of CO₂ from the ice to the atmosphere during winter and spring: young, thin ice with a thin layer of snow and older (several weeks), thicker ice with thick snow cover. Young, thin sea ice is characterized by high salinity and high porosity, and snow-covered thick ice remains relatively warm ( >  −7.5 °C) due to the insulating snow cover despite air temperatures as low as −40 °C. Therefore, brine volume fractions of these two ice types are high enough to provide favorable conditions for gas exchange between sea ice and the atmosphere even in mid-winter. Although the potential CO₂ flux from sea ice decreased due to the presence of the snow, the snow surface is still a CO₂ source to the atmosphere for low snow density and thin snow conditions. We found that young sea ice that is formed in leads without snow cover produces CO₂ fluxes an order of magnitude higher than those in snow-covered older ice (+1.0 ± 0.6 mmol C m⁻² day⁻¹ for young ice and +0.2 ± 0.2 mmol C m⁻² day⁻¹ for older ice).