Kinetically limited weathering at low denudation rates in semi-arid climates

On Earth, the Critical Zone supports terrestrial life, being the near-surface environment where interactions between the atmosphere, lithosphere, hydrosphere, and biosphere take place Quantitative understanding of the interaction between mechanical rock breakdown, chemical weathering, and physical erosion is essential for unraveling Earth's biogeochemical cycles. In this study, we explore the role of soil water balance on regulating soil chemical weathering under water deficit regimes. Weathering rates and intensities were evaluated for nine soil profiles located on convex ridge crests of three mountain ranges in the Spanish Betic Cordillera. We present and compare quantitative information on soil weathering, chemical depletion and total denudation that were derived based on geochemical mass balance, 10Be cosmogenic nuclides and U-series disequilibria. Soil production rates determined based on U-series isotopes (238U, 234U, 230Th and 226Ra) are of the same order of magnitude as 10Be-derived denudation rates, suggesting steady state soil thickness, in two out of three sampling sites. The chemical weathering intensities are relatively low (∼5 to 30% of the total denudation of the soil) and negatively correlated with the magnitude of the water deficit in soils. Soil weathering extents increase (nonlinearly) with soil thickness and decrease with increasing surface denudation rates, consistent with kinetically limited or controlled weathering. Our study suggests that soil residence time and water availability limit weathering processes in semi-arid climates, which has not been validated previously with field data. An important implication of this finding is that climatic regimes may strongly regulate soil weathering by modulating soil solute fluxes.