Plutonium isotopes and 241Am in surface sediments off the coast of the Japanese islands before and soon after the Fukushima Dai-ichi nuclear power plant accident

The concentrations of Pu239+240, Pu-241 and Am-241 in the sediments at the coast of Japanese islands from 2008 to 2011 varied widely from one sampling site to another and were generally lower in sandy sediments at shallower sites, and higher in clayey sediments at deeper sites. In contrast, there seemed to be no temporal variation in the concentrations during the survey period. The Am-241/Pu239+240 activity ratio was relatively higher than the global fallout presumably owing to the ingrowth of Am-241 from the ancestor, Pu-241 originating mainly from the hydrogen bomb explosion tests in the Marshall Islands in the 1950s.