Subglacial input of CH4 in coastal polar waters: challenges and impacts in the context of glacial retreat

It has been reported that microbial processes below the Greenland Ice Sheet produce significant amounts of methane that is transported by subglacial streams to proglacial rivers, where it is rapidly released into the atmosphere. While such process has been suspected for the Antarctic Ice Sheet, there has been little evidence reported so far. During the late Austral summer of 2023, we carried out three surveys in the West Antarctic Peninsula (WAP) from Horseshoe Island (67° 514 south) to the Northern tip of the Peninsula to document the distribution of CH4 in surface waters. We observed striking features in two bays of Dodman Island in the Grand Didier Channel, with a marked supersaturation of methane (up to 400%) in the island's bay, whereas saturation (maximum of 260%) was observed elsewhere. Our main hypothesis is that this supersaturation is linked to meltwater from the glacier on the island, which acts as a source of methane in the water column. This hypothesis is supported by vertical profiles of CH4 concentration, and benthic incubations. These suggest that CH4 is produced below the Antarctic ice sheet and transported at sea.
However, our observations indicate that the subglacial release of CH4 in Antarctic coastal waters is not a ubiquitous process, especially within bays with land-terminating glaciers. This interrogates the overall amount of CH4 released by subglacial streams, the role of the CH4 oxidation in the water column in hampering CH4 release to the atmosphere, and the positive feedback related to the transition from marine-terminating glaciers to land-terminating glaciers.