Using Cosmogenic Radionuclides for the Determination of Effective Surface Exposure Age and Time-Averaged Erosion Rates

Abstract

We discuss the cosmogenic method of determining exposure ages and erosion rates of exposed surfaces. This method is recognized to date as the only reliable method of quantifying important processes in geochronology, provided the exposure history of the surfaces can be successfully characterized using two or more cosmogenic nuclides produced in situ in the surfaces of interest. The success of the cosmogenic technique is based on two important characteristics of the secondary cosmic ray beam interacting with the exposed surfaces: (1) cosmic ray labeling is sensitively dependent on the geometry of exposure of the solid, and (2) the mean distance characterizing the absorption of cosmic ray beam in typical rocks is only about 50 cm. These facts make it possible to determine the past exposure history of the exposed surfaces; this task can be put on much firmer ground if information on the exposure history can be constrained based on geological information. We summarize the early suggestions of Lal (Earth Planet Sci Lett 104:424–439, 1991) for using cosmogenic nuclides in geomorphology, as well as the complex exposure geometries considered subsequently in a paper by Lal and Chen (Earth Planet Sci Lett 236(3–4):797–813, 2005). These methods combined with the ongoing steady improvements in the AMS sensitivity should considerably widen the scope of applications of cosmic ray produced nuclides in geomorphology.