Winter to summer evolution of pCO2 in surface water of northern Greenland fjords

Ocean acidification induced by the absorption of anthropogenic CO2 and its consequences pose a potential threat to marine ecosystems around the globe. The Arctic Ocean, particularly vulnerable to acidification, provides an ideal region to investigate the progression and impacts of acidification before they manifest globally. Recent documentation of undersaturated surface waters in carbonate minerals in the Sherard Osborn fjord in northwest Greenland, a region visited for the first time in summer 2019, reveals inherent variability in biogeochemical processes. Associated with highly acidic surface waters, the partial pressure of CO2 (pCO2) was undersaturated relative to the atmosphere, indicating this study area as a CO2 sink. To comprehend variations in pCO2 in the northwest Greenland fjords and identify its drivers, we conducted a comparative study between two fjords in the region (Petermann and Sherard Osborn fjords) and used carbonate system data from the temperature minimum layer to examine the winter-to-summer evolution of pCO2 and influencing factors. Additionally, we evaluated pCO2 variations (δpCO2) concerning temperature, freshwater inputs, biological activity, and air-sea CO2 uptake to quantitatively assess the seasonal influencing factors on surface ocean pCO2. In the Sherard Osborn fjord, despite a substantial increase in surface temperature from winter to summer potentially increasing pCO2 and causing CO2 supersaturation relative to the atmosphere, freshwater inflow and biological activity reduced pCO2, resulting in CO2 undersaturation relative to the atmosphere. In the Petermann fjord, pCO2 remained lower than atmospheric levels due to a slight seasonal variation in surface temperature and significant biological activity, reducing pCO2 in surface water.