Abstract. 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 ± 10.2 Bq m^-2) compared to the reference inventory (40.5 ± 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 ± 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.

中文翻译：

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验证 Plutonium-239+240 作为新型土壤再分配示踪剂——与实测沉积物产量的比较

摘要。量化土壤再分布率是一项全球性挑战，可通过直接沉积物测量（例如陷阱）、模型和同位素、地球化学和放射性示踪剂来解决。钚的同位素 Pu-239+240 是这一挑战中相对较新的土壤再分布示踪剂。然而，Pu-239+240 作为土壤再分布的直接验证仍然缺失。我们使用意大利南部独特的沉积物产量时间序列，追溯到 Pu-239+240 的初始沉降物，以验证 Pu-239+240 作为土壤再分布示踪剂。在集水区收集分布的土壤样本 (n=55)，并在潜在的未受干扰参考点 (n=22) 提取 Pu-239+240，用 ICP-MS 测量并转化为土壤再分布率。最后，我们使用广义加性模型 (GAM) 对集水区的土壤再分配估计进行区域化。对于集水区，平均 Pu-239+240 存量显着减少 (16.8 ± 10.2 Bq m^-2 ) 与参考清单 (40.5 ± 3.5 Bq m ^-2, ) 相比，表明侵蚀占优势。将这些库存损失转化为土壤侵蚀率导致平均土壤流失量为 22.2 ± SD 7.2 t ha ^-1 yr ^-1。该方法的不确定性主要源于低活性样品的高测量不确定性，其中样品已随深度膨胀。因此，我们建议采取增量土壤样本并提取 ~ 20 克土壤。地理坐标和流量累积最好地描述了 GAM 模型中侵蚀率的空间格局。使用这些预测因子来提高整个集水区的 Pu 衍生土壤再分配率，导致现场平均损失 20.7 t ha ^-1yr ^{-1 ，这与在集水区出口测得的 18.7 t ha -1} yr ^-1的长期平均产沙量和 Cs-137 得出的土壤再分配率非常吻合。总的来说，这种 Pu 衍生的土壤再分配率与测量的产沙量数据的比较证实了 Pu-239+240 作为一种合适的回顾性土壤再分配示踪剂。