Insights into the Pu isotopic composition (Pu, Pu, and Pu) and U in marshland samples from Madagascar.

This work provides new insights into the presence of Pu, Pu, Pu, and U in the Southern Hemisphere through the study of peat bog cores from marshlands in Madagascar (19°S). Pb, Pu and Pu activities were characterized by alpha spectrometry in previous studies. Here, Pu from alpha-spectrometry discs corresponding to 10 peat-bog cores (85 samples) was reassessed for the aim of completing its isotopic composition (Pu, Pu, and Pu) by Accelerator Mass Spectrometry. In addition, U was studied in a single core exhibiting unusually low Pu/Pu ratios. Integrated Pu/Pu atom ratios in the single cores ranged above and below the (0-30°S) fallout average ratio, 0.173 ± 0.027, from 0.126 ± 0.003 to 0.206 ± 0.002, without a regional pattern, thereby demonstrating the heterogeneous distribution of the Pu and Pu signal. However, such a variability was not observed for Pu/Pu, ranging from (6 ± 1) · 10 to (11 ± 1) · 10 and consistently below the (0-30°S) fallout ratio of (9.7 ± 0.3) · 10 (2012). The integrated U/Pu atom ratio in the studied core, 0.147 ± 0.005, was also significantly lower than the values reported for the global fallout in the Northern Hemisphere, in the 0.20-0.23 range. Our results point out to stratospheric fallout as the main source of both U and Pu at the studied site, whereas Pu and Pu signals show the influence of tropospheric fallout from the low-yield tests conducted in Australia (1952-1958) by United Kingdom and in French Polynesia (1966-1975) by France despite the long relative distances (i.e. about 15,000 and 8500 km). It was also demonstrated that a representative number of samples is necessary in order to assess Pu contamination and its various origins in a specific region in the Southern Hemisphere due to the heterogeneous distribution, and results based on single sample analysis should be interpreted with caution.