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Report No.

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

Kinoshita, Norikazu*; Nagaoka, Mika  ; Nakanishi, Takashi*

The distribution of the anthropogenic radionuclide americium-241 ($$^{241}$$Am), a decay product of $$^{241}$$Pu discharged from atmospheric tests of nuclear weapons, was investigated to resolve its horizontal and vertical migration in the Tropical East Pacific. We analyzed $$^{241}$$Am concentrations in seawater samples collected in 2003. On comparing the $$^{241}$$Am concentrations with the previously determined concentrations of $$^{239+240}$$Pu in the same samples, the vertical profiles of $$^{241}$$Am were found to be similar to those of $$^{239+240}$$Pu. At some stations, the maximum concentration of $$^{241}$$Am occurred 100-200 m deeper than that of $$^{239+240}$$Pu. The $$^{241}$$Am/$$^{239+240}$$Pu ratios in the North Pacific and South Pacific were comparable to one another, and were typical of the ratio for the Pacific. The $$^{241}$$Am distribution was influenced by the water mass at depths below 400 m. The $$^{241}$$Am 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 $$^{241}$$Am vertical profile was explained by using a box model that considers the decay of $$^{241}$$Pu and adsorption and scavenging by suspended particles. The different depths for the maximum concentrations of $$^{241}$$Am and $$^{239+240}$$Pu observed at some stations were well explained by the model and by the distribution of CaCO$$_{3}$$ particles. The residence time of $$^{241}$$Am in the Pacific was also estimated by using the model.



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Category:Environmental Sciences



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