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The weathering of sedimentary rocks in high-elevation catchments influences freshwater quality and the global carbon cycle. While individual biogeochemical mechanisms involved in this process are relatively well understood, quantifying their contributions to solute export and carbon fluxes under natural, transient conditions remains challenging. Here, we implement a numerical multidimensional and multiphase model to simulate coupled hydrological and biogeochemical processes in a shale-underlain, snow-dominated hillslope in the Rocky Mountains, Colorado. The model captures the dynamic interplay between soil respiration, mineral weathering, and climate-driven hydrological forcing, reproducing observed soil CO₂ dynamics, groundwater chemistry, and subsurface flow. Our results reveal that seasonal snowmeltmore » enhances carbonate weathering by promoting the infiltration of CO₂-rich water to depth, while pyrite oxidation is primarily sensitive to low water saturation that facilitates O₂ diffusion through the regolith. Topography modulates the spatial distribution of shale weathering, as steeper slopes enhance lateral drainage, favoring the delivery of reactants to greater depths. While shale weathering at our site acts as a transient carbon sink, with silicates and carbonates buffering acidity and promoting atmospheric CO₂ consumption (1% of soil-derived CO₂), the exported dissolved inorganic carbon is predominantly geogenic (∼73%). Consequently, when accounting for long-term marine carbonate precipitation. The current weathering regime represents a net source of carbon to the atmosphere. The oxidation of pyrite and petrogenic organic carbon together release approximately 0.9 mol·m⁻²·yr⁻¹ of CO₂. Our findings highlight the role of topography, hydroclimate, and the coupling between acid-base reactions in shaping the carbon balance and the solute exports in mountainous critical zones.« less

Climate influences near-surface biogeochemical processes and thereby determines the partitioning of carbon dioxide (CO₂) in shale, and yet the controls on carbon (C) weathering fluxes remain poorly constrained. Using a dataset that characterizes biogeochemical responses to climate forcing in shale regolith, we implement a numerical model that describes the effects of water infiltration events, gas exchange, and temperature fluctuations on soil respiration and mineral weathering at a seasonal timescale. Our modeling approach allows us to quantitatively disentangle the controls of transient climate forcing and biogeochemical mechanisms on C partitioning. We find that ~3% of soil CO₂ (1.02 mol C/m²/y) ismore » exported to the subsurface during large infiltration events. Here, net atmospheric CO₂ drawdown primarily occurs during spring snowmelt, governs the aqueous C exports (61%), and exceeds the CO₂ flux generated by pyrite and petrogenic organic matter oxidation (~0.2 mol C/m²/y). We show that shale CO₂ consumption results from the temporal coupling between soil microbial respiration and carbonate weathering. This coupling is driven by the impacts of hydrologic fluctuations on fresh organic matter availability and CO₂ transport to the weathering front. Diffusion-limited transport of gases under transient hydrological conditions exerts an important control on CO₂(g) egress patterns and thus must be considered when inferring soil CO₂ drawdown from the gas phase composition. Our findings emphasize the importance of seasonal climate forcing in shaping the net contribution of shale weathering to terrestrial C fluxes and suggest that warmer conditions could reduce the potential for shale weathering to act as a CO₂ sink.« less