[M-AG34] Radioisotope migration: New development for radioisotope migration study related to 1F NPP accident

[MAG34-10] Water masses circulation and residence time in the surface seawater in the global ocean based on radiocaesium

*Yayoi Inomata¹, Michio Aoyama² (1.Kanazawa University, Institute of Nature and Environmental Technology, 2.University of Tsukuba, Center for Research in Isotopes and Environmental Dynamics)

Keywords:Radiocaesium, residence time, surface seawater in the global ocean, inventory, long term variation

With a half-life of 30.17 y, ¹³⁷Cs is regarded as a tracer to investigate the sea water circulation in the global ocean. The dominant sources of ¹³⁷Cs have originated from global and local fallout due to atmospheric nuclear weapon tests by the United States and Russian Federation, direct discharges from nuclear fuel reprocessing plants at Sellafield and La Hague, the Chernobyl accident in 1986, and direct release and atmospheric deposition from the Fukushima Nuclear Power Plant (F1NPS) accident in March 2011. We analyzed the spatiotemporal variations in the ¹³⁷Cs activity concentrations in global ocean surface seawater from 1956 to 2021 using the HAMGlobal2021: Historical Artificial radioactivity database in Marine environment, Global integrated version 2021 and IRSN database (N=56447). The global ocean was divided into 37 boxes. The 0.5-yr average value of ¹³⁷Cs in each box, except in the northern North Atlantic Ocean and its marginal sea, decreased exponentially in 1970–2010, before the F1NPS accident. The apparent half residence time (Tap) of ¹³⁷Cs in the surface seawater was estimated to be 8.8-38 years. Mass balance analysis indicates that ¹³⁷Cs deposited by the global fallout in the western North Pacific Ocean moves to the eastern North Pacific Ocean. Subsequently, ¹³⁷Cs is transported southwards, followed by westwards transport in the subtropical and equatorial Pacific Ocean and inflow into the Indian Ocean via the Indonesian Archipelago. The longer apparent half residence times in the Indonesian Archipelago (36.7 years from 1973 to 1997) and Central Atlantic Ocean (43.5 years from 1992 to 2016) also support the interpretation of the global-scale transport of ¹³⁷Cs from the western North Pacific Ocean to the Indian and Atlantic Oceans. The ¹³⁷Cs inventory in the surface mixed layer in 1970 was estimated to be 184 ± 26 PBq. The ¹³⁷Cs inventory increased to 214 ± 11 PBq in 1980 due to direct discharge from the Sellafield and La Hague nuclear fuel reprocessing plants. In 2011, the ¹³⁷Cs inventory in the global ocean mixed layer increased to 50.7±7.3 PBq due to the F1NPS accident, in which the contribution from the accident was estimated to be approximately 15.5 ± 3.9 PBq.