A refined chronology and spatial distribution of artificial radionuclide fallout (239,240Pu, 137Cs) in South America

Sedimentary sequences have received a growing interest as a support for conducting climatic and environmental reconstructions covering the 20th century period, which has been highly impacted by socio-environmental changes. South-America is one of the regions of the world the most impacted by these changes (e.g. agricultural expansion, extreme climatic events) which induce many deleterious consequences (e.g. increase of soil erosion, transfer of contaminants). However, quantitative information regarding soil erosion and sediment accumulation processes at the catchment scale is currently lacking to determine the magnitude of these phenomena and promote effective policies to mitigate their environmental and economic impacts.

Fallout of anthropogenic radionuclides (¹³⁷Cs, ²³⁹Pu and ²⁴⁰Pu) emitted by atmospheric nuclear weapon tests conducted between 1945 and 1980 provides an opportunity to overcome this lack of information. Indeed, artificial radionuclides bound to fine-grained sediment have been increasingly recognized as powerful tools to conduct environmental, climatic and soil redistribution rate reconstructions during the Anthropocene. Although spatial and temporal reconstructions of this fallout have been conducted worldwide, this information remains scarce in South America. In addition, scientific controversies emerged regarding the contribution of French atmospheric nuclear tests to the deposition of artificial radionuclides in this region of the world, requiring further investigation.

Based on a compilation of ¹³⁷Cs inventories in undisturbed soil profiles (n=96) and a digital soil mapping approach, an open-access baseline map of ¹³⁷Cs fallout at the subcontinental scale of South America was created. The results showed that the ¹³⁷Cs inventory technique should be appropriate to reconstruct soil erosion in intensive agricultural landscapes of Chile, Argentina, Uruguay and southern Brazil and theoretically applicable in Paraguay, Bolivia and Peru. Compared to previous estimations, higher levels of ¹³⁷Cs fallout were observed between 20 and 60° South latitude. Additional measurements were therefore conducted in undisturbed soils and lake sediment cores collected at these latitudes by analyzing the ²⁴⁰Pu/²³⁹Pu atomic ratios, which is a powerful tool to determine the sources and their respective contributions to the deposition of anthropogenic radionuclides. Significantly lower plutonium atom ratios were found and attributed to the higher contribution (up to 60% in Uruguay) of the fallout following French atmospheric nuclear tests between 1966 and 1974.

This refined chronology and spatial distribution of bomb-derived fallout will undoubtedly be useful to avoid misinterpretations of sediment core dating and reconstruct soil redistribution rates during the Anthropocene in South America.