The longevity of hillslope soil in SE and NW Australia

Abstract

The cosmogenic nuclide 10 Be that is produced in the atmosphere and falls out in rainfall (‘garden-variety’ 10 Be) is used at a site in SE Australia and one in NW Australia to estimate soil formation rates and natural erosion rates. Soil formation rates of 1–7 m/Ma are found at the SE Australian site, while higher rates of 10–27 m/Ma are found in NW Australia. These are compared with estimates of modern erosion rates based on surveys of sedimentation in farm dams or 137 Cs in hillslope soils. In both cases, modern erosion rates are markedly higher than either soil formation rates or natural erosion rates measured here. At the NW Australian site, the 10 Be data are complemented by measurements of in situ 36 Cl in nearby limestone cuesta surfaces. These show that the hard-rock surfaces are eroding more slowly than the soils, and that relief is therefore increasing in this landscape. This contribution to a small but growing body of studies of the likely longevity of hillslope soils in Australia shows that ‘garden-variety’ 10 Be can be a very useful tracer, but only if its transport both within the solum and saprolite is taken in account.