Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Quantifying soil redistribution rates is a global challenge addressed with direct sediment measurements (e.g., traps), models and isotopic, geochemical and radiogenic tracers. The isotope of Plutonium, namely Pu-239+240, is a relatively new soil redistribution tracer in this challenge. Direct validation of Pu-239+240 as soil redistribution is, however, still missing. We used a unique sediment yield time series in Southern Italy, reaching back to the initial fallout of Pu-239+240 to verify Pu-239+240 as a soil redistribution tracer. Distributed soil samples (n=55) were collected in the catchment, and at potential undisturbed reference sites (n=22), Pu-239+240 was extracted, measured with ICP-MS and converted to soil redistribution rates. Finally, we used a Generalized Additive model (GAM) to regionalize soil redistribution estimates for the catchment. For the catchment sites, mean Pu-239+240 inventories were significantly reduced (16.8 &plusmn; 10.2 Bq m^-2) compared to the reference inventory (40.5 &plusmn; 3.5 Bq m^-2,) indicating the dominance of erosion. Converting these inventory losses into soil erosion rates resulted in an average soil loss of 22.2 &plusmn; SD 7.2 t ha^-1 yr^-1. The uncertainties of the approach stemmed mainly from the high measurement uncertainties of low-activity samples where samples have been bulked over depth. Therefore, we recommend taking incremental soil samples and extracting ~20 g of soil. The geographic coordinates and the flow accumulation best described the spatial pattern of erosion rates in the GAM model. Using those predictors to upscale Pu-derived soil redistribution rates for the entire catchment resulted in an average on-site loss of 20.7 t ha^-1 yr^-1, which corresponds very well to the long-term average sediment yield of 18.7 t ha^-1 yr^-1 measured at the catchment outlet and to Cs-137 derived soil redistribution rates. Overall, this comparison of Pu-derived soil redistribution rates with measured sediment yield data validates Pu-239+240 as a suitable retrospective soil redistribution tracer.