Coupling uranium series and 10Be cosmogenic radionuclides to evaluate steady-state soil thickness in the Betic Cordillera

On Earth, the Critical Zone is the near-surface environment where interactions between the atmosphere, lithosphere, hydrosphere and biosphere take place. The development of the regolith mantle is controlled by the downwards propagation of the weathering front into the bedrock and denudation at the surface of the regolith by mass movements, water and wind erosion. When the removal of surface material is approximately balanced by the production of weatherable material at the bottom of the regolith profile, the soil system is assumed to be in steady-state. Although recent literature on chemical weathering often assumes that soils have a steady-state thickness, the concept has rarely been validated with empirical field data. In this study, we present and compare analytical data from two independent isotopic techniques: in-situ produced cosmogenic nuclides and U-series disequilibria to constrain soil development under semi-arid climatic conditions. Three soil profiles were sampled across the Betic Ranges, at the ridge crest of zero-order catchments with distinct topographic relief, hillslope gradient and 10Be-derived denudation rate. 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. Our results show the potential of a combined U-series disequilibria and in-situ 10Be isotope approach to evaluate steady-state soil thickness. Our study also illustrates the frontiers in applying U-series disequilibria to track soil production in environments characterized by weak soil development.