A new approach for dating recent silting soils of debris flow deposits from the changes of 210Pbex concentration in surface layers

The 210Pbex accumulation-decay model is widely applied for sediment dating while not applicable for dating sediments derived from debris flows because the sources of excess lead-210 (210Pbex) are diverse and changeable. In this study, we developed a new model (n=-1λln1-C(n)Cr) for dating recent silting soils of debris flow deposits using changes in 210Pbex concentration in surface soil layers, where C(n) is the 210Pbex concentration in the top layer of the silting soil after n years since final sedimentation (Bq/kg), Cr is the 210Pbex concentration in the top layer at a reference site (no erosion or sedimentation in the long term), and coefficient λ = 0.03114 year−1 is the decay constant of 210Pbex. The model was validated with data obtained from the Hunshui Gully watershed in southwestern China. Six sampling sites with different sedimentation stages were selected in the Hunshui Gully watershed. The 210Pbex concentration increased from 51.20 ± 4.61 for the site with debris flow deposits in 2003 to 112.56 ± 7.88 Bq/kg for those in 1965. The derived time of the new model fits the actual value well, and the root-mean-square error of this new model was 3.23 years. Our results confirm that the fallout radionuclide 210Pbex can be applied for dating recent silting soils in debris flow deposits. However, further studies are needed to verify the model and determine the coefficients for various environments.