240Pu/239Pu signatures allow refining the chronology of radionuclide fallout in South America

Atmospheric nuclear tests (1945–1980) have led to radioactive fallout across the globe. French tests in Polynesia (1966–1974) may influence the signature of fallout in South America in addition to those conducted by USA and former USSR until 1963 in the Northern hemisphere. Here, we compiled the 240Pu/239Pu atom ratios reported for soils of South America and conducted additional measurements to examine their latitudinal distributions across this continent. Significantly lower ratio values were found in the 20–45° latitudinal band (0.04 to 0.13) compared to the rest of the continent (up to 0.20) and attributed to the contribution of the French atmospheric tests to the ultra-trace plutonium levels found in these soils. Based on sediment cores collected in lakes of Chile and Uruguay, we show the added value of measuring 240Pu/239Pu atom ratios to refine the age models of environmental archives in this region of the world. [Display omitted] •240Pu/239Pu atom ratios in soils and sediments of South America were analysed.•The contribution of French atmospheric nuclear tests was calculated.•Misinterpretation in sediment core dating may have occurred over the last decades.•240Pu/239Pu atom ratios allow refining age models of environmental archives.