Dating Fan Deposits with Cosmogenic Nuclides

Cosmogenic nuclides (³He, ¹⁰Be, ¹⁴C, ²¹Ne, ²⁶Al, ³⁶Cl) are produced in minerals due to nuclear reactions induced by cosmic rays. Measured cosmogenic nuclide concentrations are used to determine how long rocks and sediment have been exposed at or near the surface of the earth. The timing of abandonment of alluvial fan lobes can be determined directly with cosmogenic nuclide methods. For fans with abundant boulders, the top surfaces of the largest boulders are sampled for exposure dating which is based on the build-up of cosmogenic nuclides. ¹⁰Be, ²⁶Al, and ³⁶Cl have been most commonly applied. On fans with surfaces that were abandoned more than about 100-200,000 years ago, the boulders are often weathered, collapsed and crumbled. Such fans, as well as those that never had boulders at the surface, are dated with individual or amalgamated clast samples. Both post-depositional fan surface modification (cryo- or bioturbation), as well as the presence of inherited nuclide concentrations, may hinder obtainment of accurate ages. The former leads to exposure ages that are younger and the latter to ages that are older than the actual age of fan abandonment. Cosmogenic nuclide depth-profile dating is called upon. Samples are taken every 10-20 cm up to a depth of about 3 m. Alluvial fan sediment at greater depths can be dated with burial dating or burial isochron dating, which are based on the differential decay of two nuclides (for example ¹⁰Be and ²⁶Al). Optimally samples are taken from depths of more than 30 m. Although in the majority of published studies cosmogenic nuclide dating of fans has been used to determine tectonic slip rates, much has been learned about fan construction and, in turn, catchment processes.