Cosmogenic Production Near the Air-Rock Interface

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Many important Earth science questions can be addressed by determining the time at which specific rock samples became exposed at the Earth's surface. For example, a terminal moraine consists of boulders deposited at the front of a glacier as it melts; hence the surface exposure age of the boulders constrains the timing of glacial retreat. Over the last fifteen years great progress has been made in the development of techniques to measure such ages via the accumulation of rare isotopes produced by interactions in the uppermost ~ 1 meter of Earth's surface between high energy cosmic rays and the atoms in rocks. Accurate "cosmogenic dating" requires that the rate and systematics of isotope production be accurately known. This project will investigate a particularly poorly understood part of these systematics - the processes occurring at the air-rock interface, where cosmic rays transition from one medium (atmosphere) in to a very different one (rock). Nuclear processes at this interface will be investigated through high spatial resolution analyses in the uppermost few cm of rock of several different isotopes produced by cosmic rays, including helium-3 and neon-21. In addition, synthetic mineral targets will be exposed to cosmic ray irradiation at high elevation in the Andes, where the cosmic ray flux is very high. These targets will be retrieved and analyzed at high spatial resolution for the same set of isotopes. The synthetic targets complement the work on natural rocks because they permit a far more controlled environment in terms of chemical composition and exposure history. The ultimate goal of this research is to increase the accuracy and reliability of using cosmogenic isotopes as tools for determining the age of the Earth's surface. This research has widespread application to investigations of many relevant processes such as rates of erosion, timing of fault movements, and in documenting Quaternary climate change.