Cosmogenic nuclides provide an important tool for determining the time a rock has been exposed to cosmic rays on the Earth's surface and also for determining erosion rates of geomorphic surfaces. Several important applications have already been demonstrated: dating past glacial episodes from exposure ages of rocks on moraines, dating volcanic eruptions, and dating meteor crater impacts. Although these studies have been successful in producing semi-quantitative results, reliable interpretation requires a better understanding of the fundamentals of production mechanisms of cosmogenic nuclides. The three aspects of surface exposure dating that have been studied least are the dependence of the cosmic ray flux on latitude, the dependence of the production rates on the rock matrix, and the dependence of production rates on the geometry of the rock. The limited data available on the cosmic ray flux does not include important effects of the air/ground boundary. This proposal requests funding to measure 10Be, 26Al, and 36Cl depth profiles in several rocks of same age and in the same location but of different lithology using accelerator mass spectrometry. The latitude correction will be investigated using granitic rocks of same age, but from different latitudes. All rocks will be dated independently and will show glacial polish to rule out erosion. A production rate model will be developed using Monte Carlo particle transport codes and measurements will be compared with theoretical values. The production rate model developed will then be available for others to predict surface production rates in other rocks of different lithology, other geometries, and other latitudes.
