Lawrence Berkeley National Laboratory

Abstract 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 CO 2 dynamics, groundwater chemistry, and subsurface flow. Our results reveal that seasonal snowmelt enhances carbonate weathering by promoting the infiltration of CO 2 ‐rich water to depth, while pyrite oxidation is primarily sensitive to low water saturation that facilitates O 2 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 CO 2 consumption (1% of soil‐derived CO 2 ), 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 CO 2 . 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. Plain Language Summary The breakdown of rocks, or weathering, in mountain regions affects both water quality and the Earth's carbon cycle. Sedimentary rocks, such as shales, are important because they contain large amounts of carbon. However, it is difficult to assess how much weathering contributes to the movement of carbon in and out of the ground, since it involves many interconnected reactions influenced by changing environmental conditions. In this study, we use a computer model to understand how water, gases, and minerals interact beneath a snow‐covered shale hillslope in Colorado and to calculate the contributions of the different reactions. The melting of snow in spring greatly affects how rocks break down and how CO 2 moves deeper in the ground, where it reacts with carbonate and silicate minerals. Steeper slopes enhance water movement, which accelerates weathering. Overall, shale weathering temporarily removes CO 2 from the atmosphere, making it a short‐term carbon sink. However, over long times, it releases more CO 2 than it stores because much of the carbon comes from the rocks. Key Points Hydroclimatic events, especially snowmelt, increase carbonate weathering and soil‐derived CO 2 drawdown in shale‐underlain hillslopes Steeper slopes enhance the vertical transport of reactants, increasing dolomite and pyrite dissolution rates and enhancing CO 2 drawdown Shale weathering acts as a short‐term CO 2 sink but is a long‐term CO 2 source due to sulfuric acid‐driven carbonate weathering