MULTIPLE COSMOGENIC NUCLIDES AND LANDSCAPE EVOLUTION IN THE ATACAMA DESERT, NORTHERN CHILE

Prolonged hyperarid climate conditions in the Atacama Desert since the Miocene or earlier have been suggested for several decades, though it has been difficult to determine the onset of hyperaridity and to prove its constancy. Terrestrial cosmogenic nuclides (TCN) are used to trace surficial processes to estimate erosion rates and exposure ages. Such data provide valuable information on the interaction of climate and tectonics in the evolution of the Central Andes.

Multiple TCN data from hillslope interfluves and boulders for the northern part of the Atacama Desert acquired in a transect across the western slope of the Central Andes (Arica latitude) show a distinct trend with present day rainfall. Although timescales of present day rainfall records (decades) and apparent exposure ages determined by TCN (kys to mys) differ by up to 5 orders of magnitude, this suggests that hyperarid climate conditions persisted since the Pliocene/Miocene in the Coastal Cordillera and Western Escarpment (erosion rates 0.1-1 m/my), whereas the semiarid conditions of the Western Cordillera must have prevailed at least since the Pleistocene (erosion rates >10 m/my).

Catchment-averaged TCN (10Be, 21Ne) measured throughout the Lluta-catchment yield erosion rates of a similar order of magnitude as observed for hillslope interfluves in the Western Cordillera, suggesting coupling of channel and hillslope processes but also storage. In contrast, the hillslope erosion rates for the Western Escarpment and the Coastal Cordillera are offset by two orders of magnitude from the catchment-wide mean, implying decoupling of landscape processes, readily obvious also in the field.

Multiple long-living TCN (10Be, 21Ne, 26Al) indicate that erosion rates have been constant on long timescales, but may indicate outcrop-scale variations, whereas preliminary data from shorter living TCN (36Cl) from sanidine suggest that Quaternary climate variation can be identified, undetectable with longer living nuclides.