Linus Dep, Purdue University

Thesis for Ph.D., Purdue University

Cosmogenic nuclides produced in-situ in terrestrial rocks provide an important tool for dating and determining erosional histories of landforms. Reliable interpretation of nuclide contents in rocks requires a good understanding of the fundamentals of nuclide production mechanisms. This work completes an investigation of nuclide production dependence on depth and rock composition and also describes a new method for dating eroding landforms. Depth profiles of 36Cl, 10Be, and 26Al in rocks shielded and suddenly exposed to cosmic rays were measured by accelerator mass spectrometry. The depth dependence of 36Cl produced by neutron capture was determined by measuring 36cl in quartz containing fluid inclusions. The results show a maximum for nuclide production below the top surface of a boulder and are in good agreement with predictions by Monte Carlo simulation of low-energy neutrons near air/rock boundaries. The depth dependence of nuclide production by spallation was determined by measuring 10Be and 26Al in quartz. The production remains constant in the top 20 g/cm^2 and subsequently decreases exponentially with depth. The result supports the model prediction of Masarik and Reedy (1994).

The dependence of neutron-capture-produced 36Cl on elemental compostion and water content of rocks was determined by Monte Carlo simulation of low energy neutrons near the air/rock boundary. Chlorine-36 production varies with composition to a lesser extent than does the thermal neurton flux because a significant amount of production occurs at epi-thermal energies. Scaling factors for 36Cl production were determined for rock compostion and water content. The effect of snow cover and vegatation on 36Cl production was also determined.

A model was developed to determine exposure age of an eroding landform using a whole-rock 36Cl profile. The exposure age of a granodiorite glacial erratic boutlder were determined to be 93 +- 6 ka. The exposure age determined by this method agrees well with that from a 10Be depth profile (85 +- 5 ka). The average erosion rate of the boulder was 0.3 +- 0.2 mm/ka.