Direct Flux Measurements Reveal CO ₂ Sinks in Icelandic Glacial Streams Driven by Weathering and Hydrological Mixing

Abstract

Glacier‐fed rivers are increasingly recognized as important components of high‐latitude carbon cycling, yet direct observations of CO ₂ exchange in proglacial streams remain limited, especially in young basaltic catchments. Here, we quantify spatial and seasonal patterns of CO ₂ exchange along a 3‐km reach of the glacier‐fed stream Virkisá, originating from Falljökull, and complement these with measurements from four glacial streams draining the Öræfajökull ice cap in Iceland. Across four seasons, we conducted 154 direct CO ₂ flux measurements at six sites along Virkisá, combined with hydrochemical data, dissolved organic carbon (DOC) concentrations, and stable isotope‐based water source apportionment. The stream consistently acted as a net atmospheric CO ₂ sink, with the strongest uptake near the glacier outlet and progressively weaker uptake downstream. pH and distance from the glacier were the strongest predictors of CO ₂ flux. Elevated pH and CO ₂ uptake near the glacier outlet reflect rapid weathering of freshly eroded basaltic sediments generating CO ₂ ‐undersaturated meltwater in which dissolved CO ₂ is predominantly converted to bicarbonate. Uptake declined downstream as the water progressively equilibrated with the atmosphere, although net uptake persisted throughout the reach. The chemical signature of meltwater, shaped by the mixing of glacier ice melt, snowmelt, supraglacial runoff, and groundwater, further modified water chemistry and CO ₂ exchange. Despite seasonal variability in discharge composition, CO ₂ fluxes showed no significant seasonal or diurnal differences although peak uptake occurred during spring melt. Therefore, our findings identify proglacial streams as persistent CO ₂ sinks and highlight that water chemistry associated with basalt weathering—not biological activity—primarily controls CO ₂ dynamics.