Evaluating the transport of surface seawater from 1956 to 2021 using ¹³⁷Cs deposited in the global ocean as a chemical tracer

Abstract. We analysed 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. 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, immediately before the Fukushima Nuclear Power Plant (F1NPS) accident. The ¹³⁷Cs inventory in the surface mixed layer in 1970 was estimated to be 187 ± 26 PBq. In 1975 and 1980, the ¹³⁷Cs inventory increased to 201 ± 28 and 210 ± 12 PBq, respectively, 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 48.1±12.1 PBq compared to that before the F1NPS accident, in which the contribution from the accident was estimated to be approximately 15.5 ± 3.9 PBq. The distribution and variation in ¹³⁷Cs in global surface seawater reflect basin-scale or global-scale transport. 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), South Atlantic Ocean (37.0 years from 1973 to 2004), and Central Atlantic Ocean (43.5 years from 1993 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. In the northern North Atlantic Ocean and its marginal sea, ¹³⁷Cs discharged from nuclear reprocessing plants is transported to the North Sea, Barents Sea and coast of Norway, and Arctic Ocean on a decadal scale. The dataset is available at http://dx.doi.org/10.34355/CRiED.U.Tsukuba.00085 (Aoyama, 2021), http://dx.doi.org/10.34355/Ki-net.KANAZAWA-U.00149 (Inomata and Aoyama, 2022a), http://dx.doi.org/10.34355/Ki-net.KANAZAWA-U.00150 (Inomata and Aoyama, 2022b), http://dx.doi.org/10.34355/Ki-net.KANAZAWA-U.00151 (Inomata and Aoyama, 2022c).