^240+239Pu depositional signatures as a viable geochronological tool in the Amazon Basin

Abstract

Anthropogenic radionuclide signatures associated with nuclear testing are increasingly utilized in environmental science to explore recent sedimentation. In this study, we assess the suitability of Pu radioisotope analysis in floodplain lake environments in the Amazon Basin to form geochronologies during the 20 th century. The 240 Pu + 239 Pu ( 240+239 Pu) signatures in six sediment cores indicate sediment accumulation rates in the floodplain lakes of the major rivers; Amazon (2.3 mm year -1 ), Tapajos (10.2 and 2.4 mm year -1 ) and Madeira (3.4, 4.2 and 6.2 mm year -1 ). The results from this study show that 240+239 Pu fallout activities, and the well documented ( 240 Pu/ 239 Pu) atomic ratios of the above ground nuclear tests which began in the 1950's, are sufficient and well preserved in Amazon floodplain lake sediments to infer chronologies. Lead-210 dating analyses in the same sediment cores produced comparable sediment accumulation rates at three of the six sites. The differences between dating methods may be attributed to the different time scale these dating methods represent and/or in the solubility between Pb and Pu along the sediment column. The geochronologies derived from the 240+239 Pu and 210 Pb dating methods outlined in this work are of interest to identify the effects of changing sediment accumulation rates during the previous century as a result of development, including deforestation, along the Amazon Basin which increased towards the middle of the 20 th century. This study shows that Pu dating provides a viable alternative geochronology tool for recent sediment accumulation (previous ~60 years) along the Amazon Basin.

Rapid determination of plutonium isotopes in small samples using single anion exchange separation and ICP-MS/MS measurement in NH3–He mode for sediment dating

“…Since the 1940s, plutonium (Pu) isotopes have been released into the environment by a series of human nuclear activities. 239 Pu (T 1/2 = 24,100 y), 240 Pu (T 1/2 = 6561 y) and 241 Pu (T 1/2 = 14.4 y) have been not only the focus of the attention in the field of radioecology because of their high chemical toxicity and radiotoxicity [1,2], but also increasingly applied as powerful tracers for various environmental studies in recent years, such as ocean processes [3,4], lake deposition [5,6] and soil erosion [7][8][9]. More recently, plutonium isotopes were even suggested as a good primary marker for the Anthropocene by researchers [10,11].…”

Application of ^239,240 Pu, ¹³⁷Cs and heavy metals for dating of river sediments

“…The peak of radiocaesium in the SBI profile is higher than on the floodplains of other European rivers and much higher than in lakes and estuaries of the southern hemisphere (Hancock et al, 2011;Sanders et al, 2017), whereas the peak of plutonium activity can be comparable to some extreme values from the Rhone River valley, which is affected by pollution from the Marcoule spentfuel reprocessing plant (Provansal et al, 2010). The very high value of radioisotopes can easily be explained by the topography of the sampling site, which is located outside the levee zone, in the centre of the undrained depression.…”

“…In most of the presented profiles distinct radioisotope peaks correspond reasonably with the peaks of metals; however, the long tails of metals below the peaks in the CIII and CII profiles and the blurred peaks in CI exhibit the important role of post-depositional metal migration processes in overbank sediments. Such processes are negligible in reservoir sediments with stable anoxic conditions hampering organic matter degradation, which is able to affect the fate and transport of radionuclides and heavy metals (Sanders et al, 2017;Kalbitz and Wenrich, 1998).…”