Long-term erosion rates from in-situ accumulation of cosmogenic nuclides in soil matrix of glacial moraines, eastern Sierra Nevada, California

 J. Scott Shomer and Marek G. Zreda, Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721

 Knowledge of erosion rates on different temporal scales is essential for studies of landscape evolution. Short-term erosion rates can be measured directly. Long-term rates however, are more difficult to determine directly and must be extrapolated from shorter-term rates. Such extrapolations are uncertain and usually overestimate the true long-term rates. In situ accumulation of cosmogenic nuclides can be used to approach the problem of determining erosion rates on small spatial and long temporal scales.

 Previous studies measuring accumulation of Cl-36 in boulders have yielded boulder exposure ages for thirteen moraines ranging in age from 17 ky to 150 ky on the eastern slopes of the Sierra Nevada range near Bishop, California. Complex exposure histories of the individual boulders dictated large sample numbers to determine the maximum possible age of the moraine. The oldest morainal boulders, which are most likely to have been exposed at the surface since their deposition, have a greater accumulation of cosmogenic nuclides than the surrounding soil matrix which has been continuously eroding. This study utilizes the previously determined ages and the in-situ accumulation of Cl-36 in the top 0.5 meter of soil at the crests of the moraines to calculate a time-integrated erosion rate for the surfaces . Since the soil has been continuously eroding, the concentration of cosmogenic nuclides in the soil is much lower than in the exposed boulders. The difference in apparent ages can be used to determine a time-integrated erosion depth (in g/cm2) and erosion rate (in g/(cm2yr).

 Preliminary results indicate that the youngest moraines (16 to 20 ky) have experienced a modest erosion rate of 1.75 x 10-3 to 2.1 x 10-3 g/cm2yr. Using a rock bulk density of 2.5 g/cm3, the erosion is equivalent to 7.0 to 8.4 mm/ky. The oldest moraines (125 to 150 ky) have a higher erosion rate on the order of 25 to 35 mm/ky. We explain the difference in calculated rates as variations in external and internal factors that control the rate of erosion. External factors include variable climatic conditions with wetter and colder climates favoring high erosion rates. Internal factors include a materials degree of weathering and its increasing susceptibility to erosion with increased weathering