Distribution and settling behavior of americium-241 in the tropical East Pacific

Abstract

The distribution of the anthropogenic radionuclide 241Am, a decay product of 241Pu discharged from atmospheric tests of nuclear weapons, was investigated to resolve its horizontal and vertical migration in the Tropical East Pacific. We analyzed 241Am concentrations in seawater samples collected in 2003. On comparing the 241Am concentrations with the previously determined concentrations of 239+240Pu in the same samples, the vertical profiles of 241Am were found to be similar to those of 239+240Pu. At some stations, the maximum concentration of 241Am occurred 100–200 m deeper than that of 239+240Pu. The 241Am/239+240Pu ratios in the North Pacific and South Pacific were comparable to one another, and were the typical ratio for the Pacific. The 241Am distribution was influenced by the water mass at depths below 400 m. The 241Am data support the view there is a current flowing at depths of 400–3000 m from the North Pacific through the Equator to the South Pacific. In addition, the 241Am vertical profile was explained by using a 1-D scavenging model that considers the decay of 241Pu and adsorption and scavenging by suspended particles. The different depths for the maximum concentrations of 241Am and 239+240Pu observed at some stations were well explained by the model and by the distribution of CaCO3 particles. The residence time of 241Am in the Pacific was also estimated by using the model.