COSMOGENIC NUCLIDES IN ARID ENVIRONMENTS: EROSION RATES AND EXPOSURE HISTORIES OF LANDFORMS

In the last decade cosmic-ray-produced (cosmogenic) nuclides have been used to obtain exposure chronologies for a variety of landforms. Other studies have utilized cosmogenic nuclides to quantify the rates of erosion on scales ranging from individual bedrock surfaces to regional basins. While cosmogenic nuclides can be measured in samples taken from a wide variety of geologic and climatic settings, samples taken from arid environments, where removal of surface material is reduced, offer near-unique scientific opportunities. Cosmogenic nuclides have now been measured in a number of arid to semi-arid environments: Antarctica; Atacama Desert, Chile; Namibia; Australia; and others. The salient observation from these measurements is the relatively high concentration of cosmogenic nuclides in both bedrock and clasts, attesting to the antiquity of many of these materials. Minimum model exposure ages from these areas range from > 100 Kyr to > 1 Myr. Interpreted as limiting maximum erosion rates, the cosmogenic nuclide concentrations correspond to erosion rates ranging from ~2 m/Myr to < 0.2 m/Myr. In less arid environments, the inexorable effects of erosion, in a relatively short time (< 100 Kyr), dominate the inventory of cosmogenic nuclides, obscuring older events. While chronologies of events in arid and semi-arid landscapes can be extended back much further in time than those of wetter landscapes, arid landscapes also offer the opportunity to observe other geologic phenomena that affect cosmogenic nuclide concentrations, specifically complex exposure geometries. The measurement of cosmogenic nuclides in these environments not only provides quantitative information regarding the timing of major events, but may also provide clues regarding secondary events (burial or re-excavation) that are often difficult to discern. Finally, these environments are ideal testing grounds for the extension of cosmogenic nuclide techniques to additional problems in landform evolution.