EROSION RATES IN THE ATACAMA DESERT, NORTHERN CHILE (~24°S) FROM MULTIPLE COSMOGEIC NUCLIDES

<10>Prolonged hyperarid conditions since 10 Ma are suggested for the Atacama Desert, Northern Chile. Erosion, sediment transport, and deposition rates are slow, and remain poorly characterized. Terrestrial cosmogenic nuclides (TCN) are used to trace surface processes and to estimate exposure ages and erosion rates along a transect across the broadest portion of the Atacama, providing valuable insights into the interaction of climate and tectonics in the landscape evolution.

The core of the Atacama (~24°S) constitutes a hyperarid zone of ~250 km between the Pacific Ocean and the vegetated flank of the Andes. A transect at this latitude includes: the eastern and western sides of the costal range, the hyperarid core of the Atacama desert, mountain ranges (e.g. the Tettas and the Cordillera Domeyko) within it, and the western flank of the Andes. Multiple TCN data from bedrock, relict alluvial boulders, surface grus and/or colluvium, and active alluvial channels show a distinct trend with aridity.

We present< >Be data from a representative subset of samples (n=24). Generally, cosmogenic nuclide concentrations in all sample types are high and indicate very long surface residence times. These concentrations are interpreted as extremely long exposure ages and/or slow erosion rates of all components in the geomorphic system. Erosion rates in the hyperarid desert and on the eastern side of the costal range are less than 1 m/My. Although rates are uniformly low in this region, higher rates are observed in proximity to high relief areas, which also capture slightly more rainfall and are more tectonically active. The oldest surfaces are relict boulder fields whose ages quantify the time gap since the Atacama experienced significant geomorphic power. TCN in grus around such boulders indicates very slow erosion and transport rates.

Erosion rates on the western costal range and on the western Andean flank are one to two orders of magnitude higher (several meters to tens of meters per My). In the costal range, TCN concentrations in sediment collected from active fans yield faster erosion rates than those from exposed bedrock suggesting increasing relief, a typical characteristic of an extremely active tectonic mountain range. North-South gradients in erosion rates may exist for both the costal range and the western Andean flank.