Groundwater Discharge Dominates CO2 and CH4 Fluxes through a Glacier-Fed Stream on the Qinghai-Tibet Plateau

Glacier-fed streams are pivotal yet poorly constrained components of cryospheric carbon cycling. While widespread CO₂ and CH₄ oversaturation suggests their potential as atmospheric sources, it remains unclear whether these fluxes are driven by in-stream biogeochemical processing or by the physical supply of terrestrially derived carbon via groundwater discharge. Here, we traced carbon transport using a suite of dissolved gases (²²²Rn, ⁴He, ⁴⁰Ar, ⁸⁴Kr, O₂, CO₂, CH₄) across a proglacial groundwater–stream–atmosphere continuum on the Qinghai-Tibet Plateau. Elevated ²²²Rn activities (up to 2.33 Bq L^-1), together with concomitant increases in streamflow, identified substantial groundwater discharge. Based on these observations, we established a ²²²Rn mass balance model to quantitatively constrain gas exchange velocities across both the groundwater–stream and stream–atmosphere interfaces. The stream remained persistently oversaturated with CO₂, whereas CH₄ remained near saturation. Paired ⁴⁰Ar and O₂ data indicated that O₂ dynamics were physically dominated, pointing to a limited in-stream metabolic contribution to CO₂. Flux results revealed that groundwater discharge supplied major CO₂ inputs (12–2144 mmol m^-2 d^-1), sustaining its oversaturation and driving rapid emission to the atmosphere (26–888 mmol m^-2 d^-1). Together, these results demonstrate that carbon emissions from the proglacial system was dominated by physical exchange across the groundwater–stream–atmosphere continuum, rather than by in-stream biological turnover. Our findings underscore that groundwater discharge as a critical yet underrepresented pathway is essential to be integrated into models of cryospheric carbon cycling to accurately project biogeochemical feedbacks under ongoing warming climate.