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 − e^−λι)/λ where q is delivery rate, E is erosion rate, m is the concentration of ¹⁰Be in the eroding surface layer, λ 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.