COSMOGENIC ANALYSES IN THE HYPERARID ENVIRONMENT

In hyperarid environments, concentrated flowing water, both in time and space, is necessary for geomorphic processes to operate at measureable rates. To estimate these rates, and the age of the landscape, choosing samples that are representative of the surface processes is crucial. Commonly, ages of ancient hyperarid landscapes have been determined by exposure dating of selected clast samples. Although anomalous in the landscape, such samples were erroneously assumed to represent the age of the landscape and the rates of surface processes that operate on it.

Calculated erosion rates inferred from cosmogenic nuclides in bedrock and sediment collected along the Ugab River, Namibia, show a clear discrepancy. Sediment samples imply consistent erosion at 5 mm/ka throughout the Ugab drainage basin. In contrast, bedrock erosion rate is half (1-3 mm/ka). Furthermore, these rates are an order of magnitude larger than erosion rates based on surface clasts (0.2 mm/ka) which were presented in previous studies. Similarly, the first measured cosmogenic nuclides in surface clasts and boulders from the Atacama Desert suggest a very ancient and stable landscape with rates of geomorphic processes approaching zero and exposure ages of >2-3 Ma. Recently, sediment and bedrock samples collected along transects from the Pacific Ocean to the Andean foothills, suggest decreasing rates of erosion from 100 mm/ka at the coastal plain to 1mm/ka in the heart of the desert and then increasing to 10 mm/ka at the Andean foothills. Although these erosion rates are low, they are (a) much greater than implied by the rates based on anomalous boulders, and (b) similar to rates in other hyperarid regions. The above examples demonstrate that analysis of anomalous clasts and boulders may skew our understanding of the landscape as a whole.

In contrast, surface clasts do represent typical exposure times, and thus, rates of geomorphic processes in the hyperarid Negev,. Here, clasts are not anomalous as they form the desert pavemented surface of extensive flat plains, By sampling the clasts that are representative of the landscape, the amalgamated clast samples yield exposure ages of 1.5–2 Ma, suggesting that erosion rates approach zero and indicating that it is one of the most stable landscapes in the world.