COSMOGENIC CL-36 ACCUMULATION IN UNSTABLE LANDFORMS .2. SIMULATIONS AND MEASUREMENTS ON ERODING MORAINES

Cosmogenic Cl-36 surface exposure ages obtained for multiple boulders from single landforms are usually characterized by a variance larger t han that of the analytical methods employed. This excessive boulder-to -boulder variability, progressively more profound with increasing age of landforms, is due to removal of soil and gradual exposure of boulde rs at the surface. In our gradual exposure model, boulders are initial ly buried in moraine matrix. With time, erosion lowers the moraine sur face and the boulders are gradually exposed to cosmic rays. Because th e cosmic ray intensity changes with depth, the boulders are subjected to variable production rates of the cosmogenic Cl-36. Initial depth of boulders and their chemical composition are variable, which results i n different amounts of the accumulated cosmogenic Cl-36 and thus diffe rent apparent ages of boulders. The shape of the resulting distributio n of the apparent ages and the coefficient of variation depend on the erosion depth, while the first moment is a function of the true surfac e age and the erosion depth. These properties of the apparent age dist ributions permit calculation of the surface age, the erosion depth, an d also the average erosion rate. We tested the model calculations usin g 26 boulders from a late Pleistocene moraine at Bishop Creek, Sierra Nevada, California. The set exhibited a bimodal distribution of the Cl -36 surface exposure ages. We interpreted the older mode as the result of gradual exposure and the younger one as the result of surficial pr ocesses other than soil removal. The 10 samples that constitute the ol der mode produced a distribution which closely matches the modeled dis tribution calculated using an age of 85 kyr and erosion depth of 570 g cm-2. This age is the same as an independent estimate obtained from c ation ratio studies, and the calculated erosion depth is very close to the erosion depth of 600 g cm-2 based on a simple analytical model of soil erosion. These results indicate that our statistical model adequ ately describes effects of soil erosion on accumulation of cosmogenic Cl-36. The approach can be used to simultaneously obtain the true land form age and the erosion rate from apparent Cl-36 ages and therefore m ay help in evaluation of surface exposure ages of eroding landforms.