Boundary Conditions and Climate: Dual Controls on Hillslope Evolution in the Atacama Desert

The co-evolution of hillslopes and the soils that mantle them is a complex process controlled by many factors, the most important of which are boundary condition and climate. Models of hillslope geomorphic response to boundary condition predict that, if lithology, climate, and biota are similar, then bedrock erosion will be faster, soils thinner, and slopes greater on hillslopes bounded by active (bedrock-bedded) channels compared to hillslopes bounded by stable (non-eroding) surfaces. Two of the key assumptions of the models, the dependence of soil flux on slope and the dependence of bedrock erosion rate on soil thickness, have been supported with field data in semiarid and humid regions (eg. Heimsath et al., 1999; Roering et al., 2004), but the effects of boundary conditions on hillslope morphology and soils have not been systematically examined in the field, particularly on arid and hyperarid hillslopes.

We compared the bedrock erosion rates (calculated from ¹⁰Be and ²⁶Al) of active-boundary, gentle, convex hillslopes and stable-boundary hillslopes in three locations along a climatic gradient (semiarid to hyperarid) in northern Chile. Regardless of boundary condition, average bedrock erosion rates increase with increasing mean annual precipitation following a power law. This climatic sensitivity of bedrock erosion differs from a general insensitivity to rainfall observed in more humid regions. As predicted, active-boundary hillslopes have thinner soils, steeper slopes, and faster bedrock erosion rates compared to those with stable boundaries. Our work provides support for the key roles of both climate and boundary condition in the evolution of arid and hyperarid landscapes.