Montebello Islands marine sediment retains nuclear weapons-derived radionuclide contamination 70 years after detonations

Three nuclear weapons tests were conducted in the 1950s at the Montebello Islands, an Indian Ocean archipelago in NW Australia. Anthropogenic radionuclides were introduced into the surrounding waters, where little quantification of the marine environment has occurred since. Here, we assess the distribution of anthropogenic radionuclides and plutonium (Pu) atom ratios in marine sediment throughout the Montebello Islands and the Western Australian (WA) coastline. Surface sediment was analysed for ⁹⁰Sr, ¹³⁷Cs, ^{238,239,240,241}Pu and ²⁴¹Am, alongside ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu atom ratios. The highest activity concentrations in sediment were from ^239,240Pu, at 4- to 4500-fold higher in the Montebello Islands than the WA coastline, with 4-fold higher levels in the northern area of the archipelago compared to the south. Activity concentrations of ^239,240Pu (>600 Bq kg^-1) were similar to sediment at the Marshall Islands test sites despite 700-fold higher cumulative detonation yields at the latter. Maximum activity concentrations of ⁹⁰Sr, ¹³⁷Cs and ²⁴¹Am across the study were 140, 58 and 20 Bq kg^-1 respectively. Our data indicate that the source of anthropogenic radionuclides in marine sediment in the archipelago was dominated by the local detonations, whereas WA coastline samples were additionally influenced by global fallout and Marshall Islands testing. The Pu ratios were very low, and activity concentrations relatively high, compared to world and hemispheric data, representing a distinct and ongoing Pu source that should be considered in regional and hemispheric oceanic studies.