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Plutonium isotopy as a dating tool to reconstruct the recent history of land use in South America

​A French-South American consortium led by the LSCE (CEA-CNRS-UVSQ) is working to explain the chronology and spatial distribution of radionuclides deposited in South America following atmospheric nuclear testing. Their research, based on measurements of plutonium isotopes in soil and sediment samples, will enable the precise reconstruction of the evolution in land use since the 1960s.

Published on 11 July 2022

The agricultural revolution of the 1960s in South America – coinciding with the principal period of artificial radionuclide fallout – led to a significant increase in the area of cultivated land, to the detriment of natural ecosystems. This rapid agricultural expansion accelerated soil erosion and increased sediment production and transport, with considerable environmental and economic impacts. But what exactly is the contribution of anthropogenic activities in recent decades to these transfers?

To find out, researchers at the LSCE and their partners relied on the potential of artificial radionuclide fallout (cesium and plutonium) resulting from atmospheric nuclear weapons testing between 1945 and 1980.

During this period, the USA, UK, USSR, France and China conducted atmospheric nuclear weapons tests, traces of which are detectable in the soil. Once deposited on the soil surface, under the effect of precipitation, trace and ultra-trace radionuclides can strongly bind to fine soil particles. These particles travel over continental surfaces before accumulating, over time and in successive layers, at the bottom of lakes and hydroelectric dam reservoirs.

The scientists first had to establish an age model of the different layers of radionuclide-labeled sediments in sites that have remained intact for many decades. They relied on cesium-137, a temporal marker capable of distinguishing several sequences throughout the era of atmospheric nuclear tests (first tests with detectable global fallout as early as 1954, maximum fallout between 1963 and 1965, etc.).

The consortium also studied, for the first time, the temporal evolution of two plutonium isotopes (240Pu and 239Pu), present as ultra-trace elements in sedimentary layers collected from lakes in Chile and Uruguay.

Using an empirical two-source mixing model, the scientists were able to differentiate between radiological fallout from US, UK, and USSR nuclear tests prior to 1963, and French tests conducted later in Polynesia from 1966 to 1974, in the southern hemisphere.            

On the strength of this result, they propose a revised chronology of artificial radionuclide fallout in South America, both spatially and temporally; furthermore, they encourage the scientific community to measure as much as possible the 240Pu/239Pu isotope ratios in sediment cores taken between 20° and 50° south latitude.

They conclude that the analysis of these isotope ratios would help to avoid future misinterpretations in environmental reconstructions due to incorrect dating of the sedimentary record by artificial radionuclides in this region of the world.

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