Glacial meltwater and primary production are drivers of strong CO₂ uptake in fjord and coastal waters adjacent to the Greenland Ice Sheet

Abstract. The Greenland Ice Sheet releases large amounts of freshwater, which strongly influences the physical and chemical properties of the adjacent fjord systems and continental shelves. Glacial meltwater input is predicted to strongly increase in the future, but the impact of meltwater on the carbonate dynamics of these productive coastal systems remains largely unquantified. Here we present seasonal observations of the carbonate system over the year 2013 in the surface waters of a west Greenland fjord (Godthåbsfjord) influenced by tidewater outlet glaciers. Our data reveal that the surface layer of the entire fjord and adjacent continental shelf are undersaturated in CO₂ throughout the year. The average annual CO₂ uptake within the fjord is estimated to be 65 g C m⁻² yr⁻¹, indicating that the fjord system is a strong sink for CO₂. The largest CO₂ uptake occurs in the inner fjord near to the Greenland Ice Sheet and high glacial meltwater input during the summer months correlates strongly with low pCO₂ values. This strong CO₂ uptake can be explained by the thermodynamic effect on the surface water pCO₂ resulting from the mixing of fresh glacial meltwater and ambient saline fjord water, which results in a CO₂ uptake of 1.8 mg C kg⁻¹ of glacial ice melted. We estimated that 28% of the CO₂ uptake can be attributed to the input of glacial meltwater, while the remaining part is due to high primary production. Our findings imply that glacial melt\-water is an important driver for undersaturation in CO₂ in fjord and coastal waters adjacent to large ice sheets.