Probing dissolved CO 2 and CH 4 in glacial streams of the European Alps

Glacier meltwater streams are a source of methane (CH4) but can act as a sink for atmospheric carbon dioxide (CO2) through the weathering of minerals. Though both processes have been observed at glaciers in other regions of the world, the European Alps have so far been overlooked. Thus, we currently do not know whether, and to what extent, ongoing glacial melt in the Alps affects global warming through the release or storage of greenhouse gases (GHGs). We assess the source and sink behavior of alpine glacier streams with regard to CH4 and CO2 by analyzing meltwater immediately at the outlet of twenty-six glaciers located in the Eastern Alps (Italy, Austria, and Switzerland) once during the melt season of 2023. We found that most glacier streams were supersaturated with CH4 compared to the atmosphere. In contrast, CO2 concentrations varied considerably across the sampled streams. Overall, these glacier-fed rivers did not show elevated CH4 or CO2 concentrations nor fluxes compared to other headwater streams. However, they illustrate the complexities behind dynamic outgassing of GHGs associated with glacial melt and may thus serve as model systems to understand and predict the behavior of the globally shrinking cryosphere.