Calibration of the method of exposure dating of geomorphic surfaces on the earth using in-situ-produced cosmogenic radionuclides, and applications in the central Andes
Abstract
In-situ-produced cosmogenic nuclides provide an important tool for studying terrestrial landforms. This work refines our current understanding of cosmogenic nuclide production processes, outlines procedures for obtaining geomorphologic information from Accelerator Mass Spectrometry data, and uses these techniques to study the exposure histories of eight landforms in the Central Volcanic Zone of the Andes. The effects of geometric shielding, from local obstructions and from burial on sloped surfaces, are modeled theoretically for spallation-based production. Numerical solutions are obtained for several common geometries where analytic solutions are not possible, and equations are presented to model the results. Neutron activation-based production processes are modeled using Monte Carlo simulations, and results are presented to model neutron activation-based production at the base of high, vertical cliffs. In addition, depth dependence is modeled for andesites, rhyolites, and dacites following the procedure of Dep (1995). For the interpretation of cosmogenic radionuclide concentrations, a FORTRAN code has been developed. This code uses fits of measured cosmic ray attenuation lengths as a function of geomagnetic latitude, Monte Carlo simulations of production profiles, and the presented numerical solutions for geometric shielding to interpret nuclide concentrations. Results are discussed from a theoretical perspective, with particular emphasis on utilizing multiple mineral separates from a specific sample to quantify both exposure age and erosion rate for landforms with simple exposure histories. Production rate calibration appropriate to the 100 ka age range is determined from comparisons of cosmogenic nuclide data with age estimates resulting from Ar-Ar analysis. Exposure histories for the sampled landforms have been calculated from cosmogenic isotope data, with ages as low as a few thousand years for samples from Tata Sabaya to samples that are in erosional equilibrium at a few millimeters per thousand years of denudation for samples from Ucumasi and San Martin.
Degree
Ph.D.
Advisors
Elmore, Purdue University.
Subject Area
Radiation|Geophysics
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