[M-TT38] 地球化学の最前線

[MTT38-03] 人新世の正確なタイミングを記録する別府湾の極微量プルトニウム同位体と石垣島サンゴ骨格の放射性炭素

*横山 祐典^1,2,3,4,5、Tims Stephen⁵、Froehlich Michaela⁵、平林 頌子¹、阿瀬 貴博¹、Fifield L Keith⁵、Koll Dominik⁵、宮入 陽介¹、Pavetich Stefan⁵、加 三千宣⁶ (1.東京大学　大気海洋研究所 、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　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 生物地球化学センター、5.オーストラリア国立大学　物理学研究所、6.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

The term Anthropocene was originally suggested by Paul Crutzen and Eugene Stoemer in 2000 as name for the geological epoch in which human activities significantly impacted on Earth’s ecosystem. Mid-twentieth century is one of the candidate to mark starting of Anthropocene and the working group under the International Union of Geological Sciences (IUGS) has been taken place. Plutonium (Pu) has been used as a mid-twentieth century time-marker in various geological archives as a result of atmospheric nuclear tests mainly conducted in 1950s. Advancement of analytical techniques allows us to measure 239Pu and 240Pu more accurately and can thereby reconstruct the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here, we propose a novel method that couples annual banded reef building corals and nearshore anoxic marine sediments to provide a marker to precisely determine the start of the nuclear era which is known as a part of the Anthropocene. We demonstrate the efficacy of the methods using sediment obtained from Beppu Bay, Japan, and a coral from Ishigaki Island, Japan. The sedimentary records show a clear Pu increase from 1950, peaking during the 1960s, and then showing a sharp decline during the 1970s. However, a constantly higher 239Pu/240Pu ratio suggest an additional contribution to global fallout via ocean currents. Furthermore, single elevations in 240Pu/239Pu provide supportive evidence of close-in-fallout similar to previous studies. Coral skeletal radiocarbon displays a clear timing with the signatures supporting the reliability of the Beppu Bay sediments as archives and demonstrates the strength of this method to capture potential Anthropocene signatures.
[Reference: Yokoyama et al. (2022) Scientific Reports, 12:10068]