Mapping the spatial distribution of global $^{137}$Cs fallout in soils of South America as a baseline for Earth Science studies
Résumé
Owing to the rapid expansion of agriculture in South America in recent decades, soil erosion and fine sediment supply to river networks, which lead to deleterious on-site and off-site environmental impacts, are exacerbated in intensively cultivated catchments. Measuring soil inventories of bomb-derived fallout radiocesium ($^{137}$Cs) bound to fine particles is one of the few techniques available to reconstruct soil redistribution rates and evaluate the sustainability of farming practices over the recent phase of agricultural intensification (1960s–2020). However, information about the spatial distribution of $^{137}$Cs fallout across the soils of South America remains scarce, and the published data has not been synthesized at the scale of this subcontinent so far. The objective of the current research is therefore to quantify and map the initial $^{137}$Cs fallout at the scale of South America, based on the compilation of published $^{137}$Cs inventories, additional measurements conducted on undisturbed soil profiles and digital soil mapping as this baseline information may be useful for a wide range of Earth Science applications. A database of $^{137}$Cs inventories at 96 reference sites (i.e. areas without soil erosion nor accumulation) has been compiled for a variety of soil profiles (Argentina = 10, Brazil = 34, Chile = 46, Uruguay = 5, French Guiana = 1) located between 5.3° North latitude and 53° South latitude. The spatial distribution of $^{137}$Cs fallout was shown to be highly latitude-dependent, with a maximum in the 30–50° South latitude band. There were higher fallout levels than expected between 20 and 60° South latitude compared to the previous estimations made by UNSCEAR. A partial least square regression approach based on rainfall data and geographical information as covariates was used to create a baseline map of $^{137}$Cs fallout in soils of continental South America. This baseline map provides a powerful reference dataset to anticipate the order of magnitude of $^{137}$Cs inventories in undisturbed soil profiles collected in Brazil and Southern Chile and for numerous other applications in Earth Sciences. The potential application of the $^{137}$Cs inventory technique in countries of South America in general, and in regions with vulnerable ecosystems threatened by the expansion of agricultural activities in particular, is discussed in light of this comprehensive literature review. Furthermore, the regions (i.e. the North-Western part of the continent) where additional samples should be collected in priority to improve this baseline map are outlined. Our results demonstrate that $^{137}$Cs inventories are sufficiently high to investigate soil redistribution rates in most of South American countries where detectable levels of $^{137}$Cs can be expected to be found in sites exposed to erosion.
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