Glacial streams in Iceland as CO2 sinks and CH4 sources

Glacial streams export organic matter (OM) derived from various sources, including atmospheric deposition, overridden soils, and in situ microbial production. In most inland freshwater systems, this OM is mineralised by microbial respiration, resulting in net CO₂ evasion to the atmosphere.
Here, we show that the glacier-fed stream Virkisá (Iceland) deviates from this paradigm and acts as a net carbon sink, sequestering atmospheric CO₂.

Using self-constructed, low-cost CO₂ chambers, we quantified CO₂ fluxes between the stream and the atmosphere at six sites along a 3 km transect downstream from the glacier terminus. Over four seasons (154 measurements conducted between March 2023 and August 2025), CO₂ uptake fluxes ranged from an average of -18.76 ± 13.87 mg m^-2 h^-1 at the glacier outlet to -4.48 ± 3.53 mg m^-2 h^-1 further downstream. CO₂ uptake was strongest in spring, weaker during summer and autumn, and decreased with distance from the glacier. Measurements from four additional glacial streams (Skaftafellsá, Svínafellsá, Kvíárjökull, and Fjallsá) consistently identified glacial streams as CO₂ sinks.

A strong correlation between CO₂ fluxes and pH indicates that negative CO₂ fluxes were primarily driven by enhanced chemical carbonate and silicate weathering, with electrical conductivity serving as a proxy for weathering intensity. Freshly eroded, highly reactive basaltic sediments originating from beneath the glacier may promote rapid weathering reactions, increasing pH and thereby consuming CO₂, overriding the biological and abiotic processes that typically dominate in non-glacierized catchments.

In contrast, chamber measurements using the same methodological principle reveal that these glacial streams act as sources of CH₄.

Overall, our findings highlight the role of glacial streams as significant carbon sinks and underscore the need for further investigation, particularly in the context of ongoing glacier retreat and the increasing exposure of reactive glacial sediments.