Soil production, landscape evolution and vegetation dynamics in the Blue Mountains, Australia

Soil production is thought to relate to overlying soil depth by an inverse exponential function, as empirically derived using terrestrial in situ cosmogenic nuclides (TCN) at several study areas. This contrasts with a long held assumption that soil production is maximised under a thin soil cover, dm (Gilbert, 1877). Many sites in the Blue Mountains, Australia, display prima facie morphologic evidence for a `humped' soil production function. A sharp soil depth change occurs between proximal and distal spur extremities that accord with a change from forest to treeless heath, and shallow discontinuous soils are found on spur noses. Either of these features may indicate unstable conditions at depths less than dm implicit in a humped soil production function. We attempt to constrain the soil production function at our site using the TCN Be-10 from sandstone bedrock and saprolite, and morphometric analysis at Marrangaroo Creek. Although the soil depth change from forest to heath may be related to regional curvature not soil production, the peak in soil production under shallow mantles may explain alternating bands of soil and outcrop on spur noses. Soil production rates are mildly influenced by overlying soil thickness, suggesting that although thin layers of iron cemented sandstone only comprise a small percentage of the catchment bedrock, its resistance to weathering sets the pace of surface lowering. Furthermore, we present evidence that Marrangaroo Creek is a result of post-Miocene incision, similar to adjacent catchments in the region.