[U-13] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia:

[U13-P03] Anthropocene signals recorded as Plutonium isotopes and Cesium-137 in Beppu Bay marine sediments, Eastern Kyushu, Japan.

*横山 祐典^1,2,3,4,5、ティムス スティーブン⁵、ミカエラ フローリッヒ⁵、阿瀬 貴博¹、ファイフィールド キース⁵、平林 頌子⁶、ノール ドミニク⁵、加 三千宣⁷、宮入 陽介¹、シュテファン パヴェティヒ⁵ (1.東京大学　大気海洋研究所 高解像度環境解析研究センター、2.東京大学　大学院　理学系研究科　地球惑星科学専攻、3.東京大学　大学院　総合文化研究科　国際環境学教育機構、4.国立研究開発法人　海洋研究開発機構、5.オーストラリア国立大学 物理学専攻、6.立正大学　地球環境科学部、7.愛媛大学 沿岸環境科学研究センター)

キーワード：人新世、別府湾、プルトニウム

Above ground nuclear weapon tests conducted between 1945 and 1980, have lead to the distribution of various anthropogenic radionuclides in the environment including Plutonium (Pu). It has been used as a time-marker of mid twentieth century in various geological archives similar to the ¹³⁷Cs fall out signature. Further, advancement of analytical techniques allows us to measure ²³⁹Pu and ²⁴⁰ Pu accurately that can fingerprint the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here we report the record of Pu isotopes between 1950 to 1990 for the sediments obtained from Beppu Bay, in Eastern Kyushu, Japan. Approximately 8m length of core was taken from 70m water depth and radiocarbon dates suggested the high sedimentation rate (2-3mm/yr) throughout (Kuwae et al., 2013). The record shows a clear Pu increase in 1950, which peaked during the1960s, followed by sharp decline in the section corresponding to the 1970s. A minor increase in the 1980s is also observed in the sediment, namely the sediment follows global fallout trends. However, a constantly higher isotope ratio between ²³⁹Pu and ²⁴⁰ Pu suggest an additional contribution other than global fallout via ocean current. Further, distinguished higher points provided supportive evidence of close-in-fallout that are previously reported in open ocean environments.