[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.

*Yusuke Yokoyama^1,2,3,4,5, Stephen Tims⁵, Michaela Froehlich⁵, Takahiro AZE¹, Keith Fifield⁵, Shoko Hirabayashi⁶, Dominik Knoll⁵, Michinobu Kuwae⁷, Yosuke Miyairi¹, Stefan Pavetich⁵ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 3.Graduate Program on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 4.Biogeochemistry Program, Japan Agency for Marine-Earth Science and Technology, 5.Research School of Physics, The Australian National University, 6.Faculty Geo-Environmental Science, Rissho University, 7.Center for Marine Environmental Studies, Ehime University)

Keywords:Anthropocene, Beppu Bay, Pu

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.