Snow and sea ice melt preset pCO2 undersaturation in Arctic waters

The decline of summer sea ice in the Arctic Ocean is one of the most pronounced indicators of climate change. Reduced sea ice extent influences sea-air gas exchange. The Arctic Ocean is important in the global carbon cycle as it currently contributes 5% to 14% of global oceanic carbon uptake. Understanding how sea ice melt impacts the ocean's ability to act as a carbon sink is therefore crucial. In this study, we focus on Young Sound-Tyrolerfjord in Northeast Greenland to examine the interactions between the atmosphere, sea ice, and ocean during the transition from melt onset to melt pond drainage. High-frequency measurements of partial pressure of CO₂ (pCO₂) and seawater physical properties were taken 2.5 m below the sea ice. Our results reveal that pCO₂ in the seawater were undersaturated (252-355 μatm) compared to the atmosphere (401 μatm), suggesting that the seawater has the potential to take up atmospheric CO₂ as the sea ice breaks up. The undersaturation in pCO₂ was attributed to the mixing of under-ice seawater with low pCO₂ meltwater from snow and sea ice. Additionally, the drainage of meltwater from surface melt ponds, which had been in contact with the atmosphere, into the under-ice seawater caused small but clear fluctuations in pCO₂. This represents a connection between the atmosphere and under-ice seawater through meter-thick sea ice during the summer thaw. Our study demonstrates that snow and sea ice melt reduce pCO₂ in under-ice seawater, enhancing its potential for atmospheric CO₂ uptake during sea ice breakup.