THE IMPACT OF CLIMATE AND BOUNDARY CONDITIONS ON HILLSLOPE EROSION RATES IN THE ATACAMA DESERT, CHILE

Erosion rates on hillslopes are generally expressed as functions of curvature and soil depth. However, they are also dependent on climate and tectonics (both of which affect geomorphology and the soil mass balance). Can we explicitly include climate in an expression of soil production? In the Atacama Desert in northern Chile, we measured erosion rates (using cosmogenic radionuclides ¹⁰Be and ²⁶Al) on granitic hillslopes along a climate gradient (hyperarid to semiarid) with differing boundary conditions (actively incising channels and stable landforms) to determine how rainfall and hillslope base level control erosion rates.

There is a strong positive correlation between rainfall and average erosion rates, primarily due to the shift from slow, abiotic, erosive processes in the hyperarid region to faster biotic processes in the semiarid region. In the hyperarid region, this effect is compounded by a thick (10-10² cm) accumulation of dust and salt which decreases the effectiveness of near-surface erosional processes on the underlying bedrock.

Additionally, hillslopes bounded by active channels (active slopes) have higher erosion rates than those with stable boundaries (stable slopes). Active slopes appear to be maintaining their grade with channel incision, and thus reflect climatically-driven, regional erosion. In contrast, stable slopes are relaxing towards stable base levels and their erosion rates reflect the combined effects of inherited geomorphology and climate.

The Atacama Desert reached its present state of aridity 2-9 Mya. Given the extremely slow rates of bedrock erosion in the hyperarid and arid regions, it is difficult to correlate erosion rates with geomorphology since most of the landscape is a relic from wetter periods 10³-10⁹ years ago. However, it appears that erosion rates on the active slopes have adjusted to climate and may provide the necessary data to develop an expression for bedrock erosion as a function of climate, rather than geomorphology. A production function like this could bridge the gap in erosion studies from local to regional scales.