Mapping the spatial distribution of global 137Cs fallout in soils of South America as a baseline for Earth Science studies

Abstract

Owing to the rapid expansion of agriculture in South America in recent decades, soil erosion and fine 26 sediment supply to river networks, which lead to deleterious on-site and off-site environmental 27 impacts, are exacerbated in intensively cultivated catchments. Measuring soil inventories of bomb-28 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 30 phase of agricultural intensification (1960s-2020). However, information about the spatial distribution 31 of 137 Cs fallout across the soils of South America remains scarce, and the published data has not been 32 synthesized at the scale of this subcontinent so far. The objective of the current research is therefore 33 to quantify and map the initial 137 Cs fallout at the scale of South America, based on the compilation of 34 published 137 Cs inventories, additional measurements conducted on undisturbed soil profiles and 35 digital soil mapping as this baseline information may be useful for a wide range of Earth Science 36 applications. A database of 137 Cs inventories at 96 reference sites (i.e. areas without soil erosion nor 37 accumulation) has been compiled for a variety of soil profiles (Argentina = 10, Brazil = 34, Chile = 46, 38 Uruguay = 5, French Guiana = 1) located between 5.3° North latitude and 53° South latitude. The spatial 39 distribution of 137 Cs fallout was shown to be highly latitude-dependent, with a maximum in the 30-50° 40 South latitude band. There were higher fallout levels than expected between 20 to 60° South latitude 41 compared to the previous estimations made by UNSCEAR. A partial least square regression approach 42 based on rainfall data and geographical information as covariates was used to create a baseline map 43 of 137 Cs fallout in soils of continental South America. This baseline map provides a powerful reference 44 dataset to anticipate the order of magnitude of 137 Cs inventories in undisturbed soil profiles collected 45 in Brazil and Southern Chile and for numerous other applications in Earth Sciences. The potential 46 application of the 137 Cs inventory technique in countries of South America in general, and in regions 47 investigate soil redistribution rates in most of South American countries where detectable levels of 52 137 Cs can be expected to be found in sites exposed to erosion.

Natural Denudation Versus Anthropogenically Accelerated Erosion in Central Brazil: A Confrontation of Time and Space Scales

“…In the Cerrado soil, the inventory of 137 Cs is roughly 422 Bq m −2 , which is within the range of values predicted by Chaboche et al. (2021) for this region of Brazil. Since both profiles are only distant by 500 m, we did not interpret the difference in 137 Cs activities as a result in a difference of localized precipitations.…”

“…Indeed, the 137 Cs activities measured in the Cerrado soil are lower than 1.9 Bq kg −1 , as evidenced for areas located around the Equator, between 20°N and 20°S (e.g., Evrard et al, 2020). In the Cerrado soil, the inventory of 137 Cs is roughly 422 Bq m −2 , which is within the range of values predicted by Chaboche et al (2021) for this region of Brazil. Since both profiles are only distant by 500 m, we did not interpret the difference in 137 Cs activities as a result in a difference of localized precipitations.…”

“…Since both profiles are only distant by 500 m, we did not interpret the difference in 137 Cs activities as a result in a difference of localized precipitations. We rather interpret the absence of 137 Cs in the agropastoral soil due to the occurrence of erosion processes at the time of the land use change, which prevented this radionuclide to fix and migrate downward in the soil (e.g., Chaboche et al., 2021). This suggests a soil erosion that is not constant in time, probably faster at the beginning and during the agricultural period, and rather weak since the plot is under pasture because of a higher vegetation cover by grasses.…”

When climate and human interactions threaten soil and water resources through hyper sedimentation: example of El Niño events (1978–2019 period), Poechos Reservoir, Northern Peru

“…Fallout radionuclide measurements could not be used for the establishment of the core chronology due to the low activities of 210 Pbex and the absence of 137 Cs in the core sediment. Past studies had demonstrated the low 137 Cs activities found in soils and sediment in this low-latitude region of South-America (Chaboche et al, 2021) whereas the 210 Pbex was supposed to be present in the upper part of the soil (≈5cm) before decreasing with depth. The very low lead-210 activities detected in the sediment core can provide information on the intensity of erosion processes in this region by mobilizing subsoil sources (channel bank, gullies) depleted in 210 Pbex, and consequently highly impacted by erosion processes (Evrard et al, 2020).…”

“…Most of the existing limnological studies focusing on EN events were consequently focused on marine sediment cores (Hendy et al, 2015). The relative lack of studies can also be explained by the complexity to date these recent sediment archives (>1950) using radionuclides in this this low-latitude area with limited thermonuclear bomb fallout (Chaboche et al, 2021).…”