10Be distribution in soils from Merced River terraces, California

The distribution and residence time of cosmogenic 10Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of 10Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the 10Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of 10Be varies with soil horizon clay content, that the residence time of 10Be in these horizons exceeds 105 years, and that to a rough approximation the inventory of 10Be in a thoroughly sampled soil profile fits the equation: N = (q - Em)(1 - esup(-lambdat))/lambda where q is delivery rate, E is erosion rate, m is the concentration of 10Be in the eroding surface layer, lambda is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences. (author)