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

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.