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

熱帯東太平洋におけるAm-241分布及び沈降挙動

木下 哲一*; 永岡 美佳 ; 中西 孝*

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

大気圏内核実験により放出された$$^{241}$$Puの壊変生成物である$$^{241}$$Amの熱帯東太平洋における水平及び垂直移動を解明するために、分布を調査した。2003年に採取した海水試料の$$^{241}$$Am濃度を測定し、既に得られている$$^{239+240}$$Pu結果と比較した。その鉛直分布は$$^{239+240}$$Puと同様であったが、一部地点では、極大濃度が$$^{239+240}$$Puよりも100mから200m深いところで観測された。$$^{241}$$Am/$$^{239+240}$$Pu濃度比は、太平洋における他地点と同様の値であった。得られた$$^{241}$$Am濃度の分布は、400m以深の水塊の影響を受けており、400mから3000mの深さで北太平洋から赤道を通って南太平洋に流れる海流を裏付けていた。また、$$^{241}$$Amの鉛直分布について、$$^{241}$$Puからの壊変、懸濁粒子への吸着、沈降挙動をボックスモデルを用いて説明し、滞留時間を算出した。

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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パーセンタイル:84.99

分野:Environmental Sciences

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