Recent Sea-ice Retreat Accelerated CO2 Uptake and Carbon Cycling in the Arctic Ocean

As a result of severe sea-ice loss in recent summers, increases in biological production and uptake of atmospheric carbon dioxide (CO₂) have been suggested for the western Arctic Ocean based on remote sensing and numerical models. However, contrasting and contradictory works have been reported, emphasizing that our knowledge is insufficient for predicting ecosystem responses to future warming and sea-ice loss in the Arctic Ocean. We report an increase in biological removal of CO₂ and nutrients at sea surface in recent summers with the greatest difference between 2010 and 1994 occurring at about 80°N, which resulted in a low pCO₂ in the northern basin where sea-ice cover was still heavy and gas exchange was low but a high pCO₂ in the nearly ice-free southern basin due to compensation by atmospheric CO₂ uptake. We also report the expansion of the water mass with high total dissolved carbon (DIC), low pH and high nutrient concentrations to deeper depths and farther north in the basins. Using tracers and a mixing model, we attribute this high DIC and nutrient and low pH feature to both an increased Pacific Winter Water advection and enhanced local recycling of organic matter from a more productive sea surface in the northern basins. These contrasting observations indicate that recent sea-ice retreat has brought a major change in CO₂ distribution pattern and accelerated the rates of carbon cycle in the Arctic Ocean basins.