The evolution of weathering profiles through time: New insights from uranium-series isotopes

The sustainability of soil is a major issue for society. In principle, the evolution of soil resources can be constrained by comparing the rates of soil erosion and production. Cosmogenic isotopes provide one measurement of soil erosion rates. They can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production. This implies that the evolution of soil resources (thinning, thickening or constant) since soil thickness is assumed to be constant with time. Here we utilise an independent method to estimate soil production rates, using uranium-series (U-series) isotopes. The study of a site in temperate Australia undisturbed by human activity shows that soil production rates inferred from U-series isotopes are similar to erosion rates derived from beryllium-10 (10Be) measurements, implying that at this site there is no net accumulation or loss of soil. Saprolite production rates (the migration rate of the weathering front into the bedrock) are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady-state. This study demonstrates that the combination of U-series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles. Preliminary observations suggest that the rate of bedrock weathering (i.e. saprolite production) in temperate Australia is of the same order of magnitude as that inferred for laterites in tropical climates. This may suggest that, for thick weathering profiles, although the extent of weathering strongly differs between temperate and tropical climates, the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present-day climatic conditions.