Glaciers produce abundant grains of reactive minerals that influence inorganic carbon cycling, yet the links between mineral weathering and CO2 dynamics in glacier-fed streams (GFSs) remain poorly understood. Here we combine streamwater geochemistry, solute end-member modelling, carbonate saturation calculations and suspended-sediment mineralogy across 170 GFSs worldwide. We show that 69% of GFSs were undersaturated with respect to atmospheric CO2 equilibrium, indicating widespread atmospheric CO2 uptake. Grain-scale analyses of selected carbonate-bearing GFSs reveal metastable Mg-bearing carbonate phases consistent with secondary mineral formation in transient microenvironments. We propose that CO2 released during sulfuric acid-driven carbonate weathering and secondary calcite precipitation can be partly consumed by carbonation of silicate- and clay-derived mineral surfaces, transferring dissolved inorganic carbon into particulate inorganic carbon. This sediment-mediated pathway can contribute to CO2 undersaturation under favourable lithological and geochemical conditions, revealing a previously underappreciated coupling between chemical weathering, sediment–water interactions and inorganic carbon storage in proglacial landscapes rapidly expanding under global glacier retreat.