Understanding long-term soil processes using meteoric 10Be: A first attempt on loessic deposits

Meteoric 10Be, due to its high affinity with soil and sediment particles, is widely used in geomorphologic and environmental studies attempting to evaluate the soil production/denudation rates or soil ages up to 107 years. However, the evolution of the 10Be distribution as a function of depth is poorly known in soils as revealed by recent reviews (Graly et al., 2010; Willenbring and von Blanckenburg, 2010). In this study, 10Be concentrations in the bulk and the 0–2 μm (lutum) granulometric fraction of samples along Luvisols profiles developed from loess in Northern France have been measured. The bulk 10Be concentrations are significantly higher in one of the three sites, likely reflecting differences in the inherited 10Be concentrations of the loess parent material as well as in the accumulation rates of the later. However, the bulk 10Be concentrations along all profiles are significantly correlated with the lutum (0–2 μm fraction) content, the maximum 10Be concentrations being evidenced in the Bt-horizon. Dominant adsorption of 10Be to the lutum has been furthermore corroborated by the mass-balance calculations with as much as 79.8 ± 9.0% of 10Be being associated with the lutum. Contrary to the bulk 10Be concentrations, the lutum 10Be concentrations showed several maxima coinciding with shifts in the coarse to fine silt ratio. This was interpreted as a change in the loess deposit dynamic. Finally, using numerical modeling approach based on the advection-diffusion equation, an average downward migration of 10Be by clay translocation was estimated. It ranges from 0.01 to 0.08 cm yr−1. Inherited 10Be in the loess parent material represented from 64 to 71% of the total 10Be content in the simulated soils. Vertical 10Be distributions and their maximum concentrations in the Bt-horizon thus mainly result from redistribution of the inherited 10Be by clay translocation and bioturbation.