[M-TT38] Frontiers in Geochemistry

[MTT38-03] Precise timing of the Anthropocene recorded as trace amount of Plutonium isotopes in the Beppu Bay sediments and radiocarbon in corals from Ishigaki, Japan

*Yusuke Yokoyama^1,2,3,4,5, Stephen Tims⁵, Michaela Froehlich⁵, Shoko Hirabayashi¹, Takahiro AZE¹, L Keith Fifield⁵, Dominik Koll⁵, Yosuke Miyairi¹, Stefan Pavetich⁵, Michinobu Kuwae⁶ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Department of Earth and Planetary Science, University of Tokyo, 3.Graduate Program on Environmental Sciences, University of Tokyo, 4.Biogeochemistry Research Center, Japan Agency for Marine-Earth Science and Technology, 5.Research School of Physics, The Australian National University, 6.Center for Marine Environmental Studies, Ehime University)

Keywords:Anthropocene, Beppu Bay, Plutonium

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]