The production and evolution of mobile regolith: Modeled soil production and measured chemical weathering

Physical and chemical processes operate to release material from intact, immobile rock and render it susceptible to downslope transport. The transformative processes that create mobile material (soil or regolith) can exert control on landscape evolution in actively eroding, rock-dominated landscapes. This study examines rates of soil production in a variety of landscapes and the chemical evolution of mobile material on two granitic hillslopes.;Steady landscapes should display convex hilltops with uniform soil thickness and rates of release from the soil-bedrock interface. A hillslope at Bodmin Moor, UK displays parabolic hillslope form; however, depths to bedrock and soil production rates exhibit variability. We develop a theory of block-by-block lowering of the soil -- bedrock boundary, supported by a modeling exercise, to explain this inconsistency. Over long timescales a hillslope can be in steady state while at any point in time exhibiting a range of depths to bedrock.;We introduce a new soil production model that predicts rates of soil production by fitting published rates derived from in situ cosmogenic radionuclide data from different landscapes. The new model predicts soil production based on a variety of field parameters including precipitation and rock type to better account for the variability of soil production at different localities.;As material moves downslope, chemical weathering and mixing processes contribute to its evolution. We examine chemical and mineralogical data to quantify transformations that have occurred on two granitic hillslopes: Osborn Mountain, USA and Bodmin Moor, UK. The sites have experienced different temperature and moisture conditions throughout their history, with Osborn Mountain remaining cool and dry compared to Bodmin Moor. Our results indicate that temperature and water availability influence the mineralogical and chemical transformations that occur as material weathers.;We compare solid-phase and soil pore-water chemistry data from Bodmin Moor to elucidate chemical transformations. We use the chemical speciation model, PHREEQC, to show that conditions are favorable for secondary mineral precipitation below the soil O-horizon. The data are coupled with residence times to compute long-term and contemporary weathering rates. Contemporary rates of weathering are higher than the long-term average for a majority of chemical constituents.