| Summary: | Sea ice is an active component of the Earth’s climate system, interacting with both the atmosphere and the ocean. However, a thorough understanding of its annual impact on exchanges of gases, with potential feedback on the climate, is still missing. Arctic sea ice is commonly covered by melt ponds during late spring and summer, with strong effects for sea ice physical and optical properties. Yet, little is known on how melt pond formation affects sea ice gas dynamics, with consequences for gas exchanges between sea ice and the atmosphere. Here we show how melt pond formation and meltwater percolation through the ice affect sea ice physical properties and sea ice gas composition with impacts on sea ice CO2 exchange with the atmosphere. Sea ice gas composition was mainly controlled by physical processes, with most of the gas being initially in the gaseous form in the upper ice layer. As sea ice warmed up, the upper ice gas concentration decreased, suggesting a release of gas bubbles to the atmosphere. However, as melt ponds formed, the ice surface became strongly depleted in gases. Due to the melt ponds development, meltwater percolated through the ice thickness resulting in the formation of an underwater ice layer also depleted in gases. Sea ice, including brine, slush, and melt ponds, was undersaturated in CO2 compared to the atmosphere, supporting an uptake up to –4.26 mmol m–2 d–1 of atmospheric CO2. However, this uptake weakened in the strongly altered remaining ice surface (the 'white ice') with atmospheric uptakes averaging –0.04 mmol m–2 d–1 as melt ponds formation progressed.
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