Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) – Signatures of stratospheric fallout in the Southern Hemisphere

The inventories of 137Cs (503 ± 34 Bq.m?2), 241Am (11.3 1.2 241Pu (33.7 3.4 238Pu (6.82 0.87 Bq.m?2) and 239+240Pu (113.0 5.9 sum 239Pu (100 11 240Pu (14.5 1.7 in the Gambier archipelago (23°S) French Polynesia, are well higher global fallout at this latitude, unequal proportions for different radionuclides. (240Pu/239Pu)AR (AR: atomic ratio) 0.0394 0.0062, (241Pu/239Pu)AR (2.03 0.39)10?4, confirm that overwhelmingly dominant source these radionuclides comes from local during 1970s atmospheric tests Moruroa Fangataufa located nearly 400 km Gambier. signatures were deduced excess its inventory mixing lines established fallout, some test sites isotopic ratios measured Signatures obtained 2.0 0.4 137Cs/239+240Pu, 0.045 0.008 238Pu/239+240Pu, 0.031 0.009 241Am/239+240Pu, 0.092 0.027 241Pu/239+240Pu, 0.0163 0.0049 (240Pu/239Pu)AR, (0.78 0.23)10?4 (241Pu/239Pu)AR. concordance [(240Pu/239Pu)AR, (241Pu/239Pu)AR] linear regression stratosphere (40°S) 1970s, indicates close-in deposition also those stratospheric injections tests. Southern Hemisphere evaluated by considering fission energy represents 11% Northern 89% remained same over period 1970–1974. activity deducted 21.9 0.1 0.11 0.05 1.03 0.12 241Pu/239+240Pu 0.35 0.04 241Am/239+240Pu. associated atom 0.157 0.011 (8.33 0.48)10?4 These appear to be consistent with results Hiva Oa, more than 1,000 north both sites, found Australian continent, regions not impacted UK-test debris. tropospheric about 90% They represent a proportion 40% 137Cs, 60% range between 80 Pu isotopes.