| Summary: | Rare CO 2 flux measurements from Arctic pack ice show that two types of ice contribute to the release of CO 2 from the ice to the atmosphere during winter and spring: young, thin ice with athin 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 CO 2 flux from sea ice decreased due to the presence of the snow, the snow surface is still aCO 2 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 CO 2 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).
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