How do silicate weathering rates in shales respond to climate and erosion?

Mineral weathering, a major control on long term atmospheric CO₂, can be limited by processes such as reaction kinetics, supply of fresh mineral, or water throughput in the weathering zone. In these cases, weathering fluxes increase with temperature, erosion, and runoff, respectively, and we refer to the regimes as kinetically limited (KL), erosive transport limited (ETL), and runoff limited (RL). Larger watersheds can exhibit mixtures of these behaviors in a transition regime (TR). To understand these weathering regimes, we focused on the lithology (shale) that is one of the most widespread lithologies exposed on continents globally. We identified and analyzed 142 shale watersheds across the United States, spanning a diverse range of climatic (mean annual temperature (MAT): 3–17 °C; mean annual precipitation (MAP): 220–1975 mm yr⁻¹; potential evapotranspiration (PET): 908–2059 mm yr⁻¹), topographic (mean catchment elevation: 78–2346 m; mean catchment slope: 0.4–28°), and hydrologic (annual runoff (q): 0.002–1.3 m yr⁻¹) conditions. For these sites, we calculated silica fluxes (i.e., F_SiO2) using SiO₂ concentrations and river discharge measurements. We observed that when the humidity index (HI; MAP/PET) is <0.55, F_SiO2 increases with MAP and q; however, when HI > 0.55 F_SiO2, does not increase with MAP nor q. We posit that sites with HI < 0.55 are RL. The river chemistry for the RL sites is consistent with equilibrium between smectite and kaolinite. Fitting the data for sites with HI > 0.55 to an exponential temperature dependence, we find that the sensitivity of weathering to temperature is consistent with an apparent activation energy, Ea, of 56 ± 8 kJ mol⁻¹. If sites with HI > 0.55 but average slopes <10°are removed, the value increases to 92 ± 10 kJ mol⁻¹. This value is consistent with the Ea measured experimentally for the dissolution of the common shale clay, chlorite. To extrapolate from our measurements, we use geospatial-based definitions to classify HUC12 watersheds across the United States. This reveals that 48%, 37%, 9%, and 7% of US shale watersheds are RL, ETL, TR, and KL, respectively. Globally, we estimate that silicates weathering in shales consume 2.6 × 10¹² mol CO₂ yr⁻¹, which is 22% of the total CO₂ sequestered by silicate weathering.