­­BERYLLIUM-7 AND LEAD-210 CHRONOMETRY OF MODERN SOIL PROCESSES

Environmental archives developed from ice core, sediment and peat deposits provide critical constraints on modern histories of global environmental change. Soil systems could provide new opportunities to reconstruct histories of environmental perturbations, but the lead-210 (²¹⁰Pb; half-life 22 years) chronometers so powerful in sedimentary systems lack a conceptual basis for their application to soil systems. Here we develop and test a new radionuclide age model for dating modern soil processes. The coupled radionuclide accumulation model that we propose, R^Be:Pb, incorporates measurements of beryllium-7 (⁷Be; half-life 54 days) to account for ²¹⁰Pb behavior during depositional processes which are endemic to soils and omitted from conventional ²¹⁰Pb age models. We test the R^Be:Pb model using the 1963-1964 peak in nuclear weapons testing fallout, which we trace using measurements of the plutonium-241 daughter americium-241 (²⁴¹Am; half-life of 432 years). In three different soils we locate the ²⁴¹Am weapons horizon at depths ranging from 2.5 to 6 centimeters. Despite their disparate depths all weapons horizons yield concordant ages averaging 1967±4 via the R^Be:Pb model. In comparison, the conventional Constant Rate of Supply (CRS) age model shows a minor bias, with ages averaging 1972±4. Using a numerical model to compare the age models at broader timescales we show that the CRS bias increases dramatically with decreasing soil ages. The R^Be:Pb model is shown to be unbiased at these depths, but at depths below the occurrence of ⁷Be it suffers from high uncertainties. At intermediate depths corresponding to ages of approximately 50 years, the CRS and R^Be:Pb models converge. The ²¹⁰Pb models described here demonstrate that advection rates of soil colloids on the order of 1 mm yr^-1 are typical, but also that they are variable with depth, and are likely to be controlled by horizon-specific processes. We thus interpret our calculated ²¹⁰Pb ages to represent (i) exposure of the soil system to atmospheric aerosol deposition in the context of (ii) organic carbon assimilation, colloid production, and advection through the soil profile. We propose that ²¹⁰Pb chronometry is valuable for tracing colloidally-mediated transport of Pb and similarly-refractory metals, as well as the mobile pool of carbon in soils.