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

Meteoric Be-10, 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 10(7) years. However, the evolution of the Be-10 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, Be-10 concentrations in the bulk and the 0-2 mu m (lutum) granulometric fraction of samples along Luvisols profiles developed from loess in Northern France have been measured. The bulk Be-10 concentrations are significantly higher in one of the three sites, likely reflecting differences in the inherited Be-10 concentrations of the loess parent material as well as in the accumulation rates of the later. However, the bulk Be-10 concentrations along all profiles are significantly correlated with the lutum (0-2 mu m fraction) content, the maximum Be-10 concentrations being evidenced in the Bt-horizon. Dominant adsorption of Be-10 to the lutum has been furthermore corroborated by the mass-balance calculations with as much as 79.8 +/- 9.0% of Be-10 being associated with the lutum. Contrary to the bulk Be-10 concentrations, the lutum Be-10 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 Be-10 by clay translocation was estimated. It ranges from 0.01 to 0.08 cm yr(-1). Inherited Be-10 in the loess parent material represented from 64 to 71% of the total Be-10 content in the simulated soils. Vertical Be-10 distributions and their maximum concentrations in the Bt-horizon thus mainly result from redistribution of the inherited Be-10 by clay translocation and bioturbation. (C) 2014 Elsevier B.V. All rights reserved.