Low methane supersaturation observed in southwestern Greenland fjords

Climate-driven retreat of Greenland’s marine-terminating glaciers in rapidly transforming fjord systems has potential implications for marine methane cycling. Because the Greenland ice sheet and its basal meltwaters have been identified as sources of methane, it has been hypothesized that fjords fed by marine-terminating glaciers should be associated with elevated dissolved methane concentrations and enhanced sea-air exchange. Here, we test this hypothesis by comparing two adjacent fjords in southwest Greenland (Narsaq region), one currently fed by a marine-terminating glacier, and one supplied by a pro-glacial river draining a glacier that has retreated inland. Measurements collected during the summers of 2023 and 2024 reveal remarkably similar dissolved methane concentrations, vertical distributions and saturation levels in both fjords. Methane concentrations were highest in surface meltwaters and fjord waters and lower in intruding oceanic waters into the fjords, with no evidence for significant methane inputs from the seafloor. Overall, the observed methane concentrations (4.94 – 9.30 nM) correspond to a surface layer (0-3m) saturation of 129-226% resulting in small atmospheric fluxes from 0.49 to 3.31 µmol m -2 d -1 . High-resolution measurements using a membrane inlet laser spectrometer (MILS) further revealed no detectable fine-scale methane enrichment near the glacier front. We attribute the similarity in dissolved methane distributions between the two fjords to the dominance of surface meltwater discharge during summer, even in the presence of a marine-terminating glacier, suggesting that southern Greenland fjords represent a relatively weak source of atmospheric methane under summer conditions. Our results suggest that ongoing retreat from marine-terminating to land-terminating glaciers is unlikely to substantially enhance methane emission from Greenland fjords under present and near-future summer conditions. Wintertime measurements remain needed to fully constrain annual methane budgets in these fjords.