BE-10 CHRONOMETRY OF BEDROCK-TO-SOIL CONVERSION RATES

We report concentrations of cosmogenic Be-10 (t1/2 = 1.5 x 10(6) yrs) in soil excavated from a soil-mantled hillslope in Black Diamond Mines Regional Park, Contra Costa County, California. The most striking features of the data are: (1) the similarity in the downward decreasing trends of Be-10 concentrations in two soil profiles collected 75 m apart, (2) the coincidence in each soil profile of the soil/bedrock interface (as defined by visual inspection of soil pits) and the level at which Be-10 concentrations attain very low values (approximately 4 x 10(6) atoms/g), and (3) the extremely low Be-10 concentrations in the underlying regolith (0.5 x 10(6) atoms/gram). The inventory of Be-10 in these soils is low, equivalent to about 6000 yrs of Be-10 accumulation in a soil initially containing no Be-10. On the basis of these measurements, and with the aid of simple models of soil (Be-10) motions on the hillslope, we conclude that Be-10 loss from the surface is dominated by its removal in soil by creep. We calculate local rates of bedrock-to-soil conversion of between 0.15 and 0.27 km/10(6) yrs. Comparing these with uplift rates determined for coastal regions of California indicates that soil creep alone is capable of removing soil from the local geomorphic system at a rate equivalent to the rate of uplift of much of the coast.