Insights into the Pu isotopic composition (239Pu, 240Pu, and 241Pu) and 236U in marshland samples from Madagascar

This work provides new insights into the presence of (PU)-P-239, Pu-240, Pu-241, and 236 U in the Southern Hemisphere through the study of peat bog cores from marshlands in Madagascar (19 degrees S). Pb-210, (PU)-P-238 and Pu239+240 activities were characterized by alpha spectrometry in previous studies. Here, Pu from alpha-spectrometry discs corresponding to 10 peal-bog cores (85 samples) was reassessed for the aim of completing its isotopic composition (Pu-239, Pu-240, and Pu-241) by Accelerator Mass Spectrometry. In addition, U-236 was studied in a single core exhibiting unusually low Pu-240/Pu-239 ratios. Integrated Pu-240/Pu-239 atom ratios in the single cores ranged above and below the (0-30 degrees 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-239 and Pu-240 signal. However, such a variability was not observed for Pu-241/Pu-239, ranging from (6 +/- 1) . 10(-4) to (11 +/- 1) . 10(-4) and consistently below the (0-30 degrees S) fallout ratio 01(9.7 +/- 0.3) . 10(-4) (2012). The integrated U-236/Pu-239 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 020-023 range. Our results point out to stratospheric fallout as the main source of both U-236 and Pu-241 at the studied site, whereas Pu-239 and Pu-240 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. (C) 2020 Elsevier B.V. All rights reserved.