1°Be chronometry of bedrock-to-soil conversion rates

We report concentrations of cosmogenic l°Be ( t l / 2 = 1.5 x 10 6 y r s ) 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 1°Be 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 l°Be concentrations attain very low values ( ~ 4 × 106 atoms/g), and (3) the extremely low 1°Be concentrations in the underlying regolith (0.5 x 10 6 atoms/gram). The inventory of 1°Be in these soils is low, equivalent to about 6000 yrs of l°Be accumulation in a soil initially containing no 1°Be. On the basis of these measurements, and with the aid of simple models of soil (1°Be) motions on the hillslope, we conclude that l°Be 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 kin/106 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.