Geomorphic process rates in the central Atacama Desert, Chile: insights from cosmogenic nuclides and implications for the onset of hyperaridity

Water plays a critical role in erosion and sediment transport and this relationship is most evident in the hyperarid Atacama Desert of Northern Chile, a region characterized by erosion rates that fall to near zero and cobbles and boulders with cosmogenic nuclide concentrations indicative of exposure for many millions of years. Cosmogenic nuclide concentrations from the Atacama are used to both determine the age of hyperaridity onset and to place important constraints on rates of geomorphic processes in this uniquely arid environment. Prior determinations of cosmogenic nuclide concentrations from the Atacama Desert focus primarily on exposure ages from boulders/cobbles or erosion rates from bedrock. However, recent determinations of cosmogenic nuclide concentrations from boulders, bedrock, and sediment at the same location suggests that material from diverse sample types have different cosmogenic nuclide concentrations. Therefore, it is critical to determine which concentrations of cosmogenic nuclides are most representative of overall erosion rates from the Atacama. Here, concentrations of cosmogenic nuclides in more than 100 samples across two east-west transects within the central Atacama Desert (22-26°S) of Northern Chile are considered. Concentrations of 10Be and 26Al were measured in samples of bedrock, alluvial sediment, active stream sediment, and boulders within several sub-regions of the Atacama Desert, including: the western and eastern Coastal Cordillera, the inner absolute desert (including the Cerro de los Tetas), the Cordillera Domeyko, and the western flank of the Andes. This data allows detailed comparisons of cosmogenic nuclides concentrations both within diverse sample types at a given site and between major geomorphic sub-regions of the Atacama. The general pattern of cosmogenic nuclide concentrations in hyperarid environments is characterized by concentrations that are higher in boulders, moderately high in bedrock, lower in hillslope sediment, and lowest in channels that flow across the desert. At many locations in the central Atacama, boulders and bedrock have erosion rates as much as an order of magnitude slower than that of finer grained sediment at the same location, a relationship that is attributed to the fact that boulders sit above thick gypcrete soils. The hillslope to basin concentrations of cosmogenic nuclides within each sub-region, along with 26Al/10Be ratios, suggests that concentrations of 10Be in most bedrock and sediments reflects erosion rates. However, in the western Coastal Cordillera 10Be concentrations in sediment also reflects transport time. The overarching trend in this data set is that inferred erosion rates are lower to both the east and the west, corresponding with increases in both precipitation and erosion rates towards both the Andean Flank and in the Coastal Cordillera. This trend is previously noted for the Atacama; however, this large data set allows the first observation of the influence of other variables upon erosion rates. Most notably, another clear influence on erosion rates in the Atacama is slope. In some cases, differences in slope are enough to overcome the influence of aridity. For example, erosion rates on the flanks of the Cordillera Domeyko are faster (>1 m/Ma) than that at the crest (