Intertwined weathering and metamorphic controls on river dissolved inorganic carbon in the interior of a mountain belt

Carbon dioxide fluxes associated with weathering and carbon export in river basins are highly spatially variable, with some locations acting as sinks whilst others represent CO 2 sources. Chemical weathering and the associated carbon fluxes are strongly modulated by bedrock geology, with orogenic systems constituting locations where metamorphic lithologies can be found juxtaposed with sedimentary rock mélanges. Hydrological processes also converge in mountain belt interiors, where water in the shallow critical zone can intersect with gas-rich fluids derived from deeper tectonic processes. The Engadin Valley in the Swiss Alps provides a natural geological setting to examine how these competing controls impact weathering and dissolved inorganic carbon (DIC) fluxes in alpine rivers. Using the radiocarbon and stable isotope composition of river DIC, alongside the dissolved products of weathering, we observed an increase in the fractional contribution of riverine DIC derived from metamorphic processes coincident with a distinct lithological transition from the upper to the lower Engadin Valley. DIC export from the upper Engadin Valley is dominated by silicate weathering of igneous and metamorphic terrains, while Δ 14 C DIC values < -100 ‰ suggest some of this DIC could originate from pre-aged organic sources from soils, permafrost thaw or glaciated catchments. Riverine DIC concentrations increase from headwaters to the most downstream station in the lower Engadin Valley, mirrored by a sharp drop in Δ 14 C DIC values, while the magnitude of this decrease varies seasonally. This decline in Δ 14 C DIC values, which is counter to the trend expected due to increasing exchange with atmospheric CO 2 downstream, is consistent with contributions from geogenic carbon sources, specifically DIC from weathering of carbonate minerals and petrogenic organic carbon (OC), spatially intertwined with inputs CO 2 from metamorphic processes. Overall, seasonal variations in hydrological processes and associated flow pathways appear to modulate DIC contributions from bedrock weathering, while metamorphic CO 2 inputs occur in a quasi-constant manner. This study demonstrates that geogenic CO 2 sources in the shallow critical zone contribute significantly to the riverine DIC pool, leading in the Engadin valley to a potential shift in CO 2 fluxes from uptake via silicate weathering upstream to potential CO 2 release downstream, highlighting both the importance and complexity of carbon cycle processes in mountainous river basins.