Michio Aoyama

The Arctic Ocean is facing dramatic environmental and ecosystem changes. In this context, an international multiship survey project was undertaken in 2020 to obtain current baseline data. During the survey, unusually low dissolved oxygen and acidified water were found in a high-seas fishable area of the western (Pacific-side) Arctic Ocean. Herein, we show that the Beaufort Gyre shrinks to the east of an ocean ridge and forms a front between the water within the gyre and the water from the eastern (Atlantic-side) Arctic. That phenomenon triggers a frontal northward flow along the ocean ridge. This flow likely transports the low oxygen and acidified water toward the high-seas fishable area; similar biogeochemical properties had previously been observed only on the shelf-slope north of the East Siberian Sea.

The entire world was exposed to a global fallout of cesium-137 (¹³⁷Cs-GFO) produced from the atmospheric nuclear weapon tests examined mainly in the 1950s and 1960s. Clarifying the residual status of ¹³⁷Cs-GFO for an extended period (~50 years) after the fallout in Japan will provide strong evidence to predict the future of ¹³⁷Cs emitted by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident.Based on research conducted after the Chernobyl nuclear power plant accident, the FDNPP-generated ¹³⁷Cs fallout has been predicted to accumulate in the surface mineral soil and remain there for a long time. We questioned whether this insight could be applied to the FDNPP-generated ¹³⁷Cs falling on forest soils in Japan. This is because the geographical features of forests in Japan are characterized by steep terrain and heavy rainfall, different from forests in the Northern European continent.To confirm the prediction, that is, the long-term persistence of ¹³⁷Cs in forest soil, we explored the consequences of ¹³⁷Cs-GFO in forested areas across Japan after half a century from the fallout deposition. We determined the amount of residual ¹³⁷Cs-GFO in surface soils (0–30 cm depth) using the forest soil sample archives collected shortly before the FDNPP accident.The residual ¹³⁷Cs-GFO in forest soils was not significantly different from the cumulative ¹³⁷Cs-GFO obtained at observatories. We confirmed that most of the ¹³⁷Cs-GFO remained within 30 cm of the soil surface even half a century after the fallout. However, the spatially heterogeneous ¹³⁷Cs-GFO inventory within the forest was found to correspond to various vertical distribution patterns of ¹³⁷Cs-GFO. The correspondence between the ¹³⁷Cs-GFO inventory and the vertical distribution pattern indicates that the vertical distribution patterns resulted from active ¹³⁷Cs-GFO-contaminated sediment migration in the forest over the past half-century and not due to differences in the vertical infiltration rate of ¹³⁷Cs-GFO.Although most of the ¹³⁷Cs-GFO was assumed to remain within the forest surface soil, the ¹³⁷Cs-GFO inventory was considerably smaller than the cumulative deposition of ¹³⁷Cs-GFO (79%). Regarding the destination of the missing ¹³⁷Cs-GFO, in addition to sediment discharge into the water system, this study indicates the possibility of local storage of ¹³⁷Cs-GFO in soils deeper than 30 cm in the forest.Forest management that reduces sediment redistribution on the forest floor would help prevent the FDNPP-generated ¹³⁷Cs from flowing out of the forest.

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 other published data. The global ocean was divided into 37 boxes. When observing the 0.5-year median value of ¹³⁷Cs in each box in the Pacific Ocean, we noticed that the values gradually increased or had almost constant levels in the 1950s and 1960s, and then decreased exponentially in 1970–2010, immediately before the Fukushima Daiichi Nuclear Power Plant Station (F1NPS) accident. In the northern North Atlantic Ocean and its marginal sea, the 0.5-year median values of ¹³⁷Cs showed large variations in the directly discharged ¹³⁷Cs from the reprocessing plants. The ¹³⁷Cs inventory in the surface mixed layer in 1970, when ¹³⁷Cs was released into the surface seawater, was estimated to be 184±26 PBq. In 1975 and 1980, the ¹³⁷Cs inventory increased to 201±27 and 214±11 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 50.7±7.3 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. Mass balance analysis indicates that ¹³⁷Cs deposited by the global fallout in the western North Pacific Ocean moved to the eastern North Pacific Ocean. Subsequently, ¹³⁷Cs was transported southwards, followed by westward transport in the subtropical and equatorial Pacific Ocean, and then inflowed 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 (38.0 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 (20–30 years) and Atlantic oceans (30–40 years). 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 https://doi.org/10.34355/CRiED.U.Tsukuba.00085 (Aoyama, 2021), https://doi.org/10.34355/Ki-net.KANAZAWA-U.00149 (Inomata and Aoyama, 2022a), https://doi.org/10.34355/Ki-net.KANAZAWA-U.00150 (Inomata and Aoyama, 2022b), and https://doi.org/10.34355/Ki-net.KANAZAWA-U.00151 (Inomata and Aoyama, 2022c).

Radioactive cesium (137Cs) is distributed in the global ocean due to global fallout from atmospheric nuclear tests, release from reprocessing plants in Europe, and supply to the ocean due to the Fukushima Daiichi Nuclear Power Plant accident. In order to detect future contamination by radionuclides, it is necessary to understand the global distribution of radionuclides such as 137Cs. For this purpose, the IAEA is compiling a database of observation results (MARIS). However, since the spatio-temporal densities of observed data vary widely, it is difficult to obtain a complete picture from the database alone. Comparative validation using ocean general circulation model (OGCM) simulations is useful in interpreting these observations, and global ocean general circulation model (CESM2, POP2) simulations were conducted to clarify the behavior of 137Cs in the ocean. The horizontal resolution is 1.125° longitude and 0.28° to 0.54° latitude. The minimum spacing near the sea surface is 10 m, and the spacing increases with depth to a maximum of 250 m with 60 vertical levels. Climatic values were used for driving force. As a source term for 137Cs to the ocean, atmospheric fallout from atmospheric nuclear tests was newly established based on rainfall data and other data, and was confirmed to be more reproducible than before. Furthermore, the release from reprocessing plants in Europe and the leakage due to the accident at the Fukushima Daiichi Nuclear Power Plant were taken into account. 2020 input conditions were assumed to continue after 2020, and calculations were performed from 1945 to 2030. The simulated 137Cs activities were found to be in good agreement, especially in the Atlantic and Pacific Oceans, where the observed densities are large. On the other hand, they were underestimated in the Southern Hemisphere, suggesting the need for further improvement of the fallout data. 137Cs concentrations from the Fukushima Daiichi Nuclear Power Plant accident in March 2011 were generally in good agreement, although the reproducibility remained somewhat problematic due to insufficient model resolution. In other basins, the concentration characteristics were able to be determined, although the observed values were insufficient. Radioactivity concentrations of atmospheric nuclear test-derived 137Cs may continue to be detected in the global ocean after 2030. The results of this simulation are useful for planning future observations to fill the gaps in the database.

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).

Radiocaesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident was observed across a wide area of the North Pacific, not only in surface seawater, but also in the ocean interior. In this presentation, we summarized the time scale of Lagrangian transport of the FNPP1 derived radiocaesium in surface water during the period from the time of the accident to March 2021 in the North Pacific and the Arctic Oceans and its marginal seas as shown below.Initial observation results until December 2012 in the surface layer in the North Pacific Ocean by the global observations revealed that a typical feature within one year after the accident was a westward movement across the North Pacific Ocean, speed of which was reported at 7 km day-1 until August 2011. After that, the main body of FNPP1-derived radiocaesium moved east as 3 km day-1 and is separated from Japan in 2013. The arrival of the FNPP1 signal at the west coast of the American continent was reported in 2014. The elevation in the FNPP1 derived radiocaesium concentration in the Bering Sea in 2017 and in the Arctic Ocean in 2019 was reported. The northward bifurcation of the Kuroshio Extension made these obvious transport of the FNPP1 derived radiocaesium to the subarctic and arctic region while the transport by southward bifurcation was not observed. At Hawaii Islands in the subtropical gyre, there was no signal of the FNPP1 derived radiocaesium during the period from March 2011 and February 2017. At Yonaguni Island where the Kuroshio enters the East China Sea, the FNPP1 signal arrived at Yonaguni Islands eight years after the time of the accident, and these might be transported mainly from the subtropical gyre.At the marginal seas of the North Pacific Ocean, the elevation in the FNPP1 derived radiocaesium concentration in the northern East China Sea in 2014, in the Sea of Japan in 2014/2015 were observed.We also briefly summarize study results on nuclides other than radiocaesium (e.g., 90Sr, 239240Pu, and 129I).

Radioactive cesium (137Cs) is distributed in the global ocean due to global fallout by atmospheric nuclear weapons tests, releases from reprocessing plants in Europe, and supplied to the ocean by the Fukushima Daiichi Nuclear Power Plant (1F NPP) accident. In order to detect future contamination by radionuclides, it is necessary to understand the global distribution of radionuclides such as 137Cs. For this purpose, observed data have been summarized in a historical database (MARIS) by IAEA. The spatio-temporal density of the observations varies widely, therefore simulation by an ocean general circulation model (OGCM) can be helpful in the interpretation of these observations.In order to clarify the behavior of 137Cs in the global ocean, OGSM simulations were conducted. Parallel Ocean Program version 2 (POP2) of the Community Earth System Model version 2 (CESM2) is employed. The horizontal resolution is 1.125 degree of longitude, and from 0.28 degree to 0.54 degree of latitude. There are 60 vertical levels with a minimum spacing of 10 m near the ocean surface, and increased spacing with depth to a maximum of 250 m. The simulated period was from 1945 to 2030 with the circulation forced by repeating (“Normal Year”) atmospheric conditions. As input sources of 137Cs to the model, global fallout from atmospheric nuclear tests, releases from reprocessing plants in Europe, and input from the 1F NPP accident were considered. It was assumed that the input conditions in 2020 would continue after 2020.The simulated 137Cs activity agrees well with the observed data in the database, especially in the Atlantic and Pacific Oceans where the observation density is large. Since 137Cs undergoes radioactive decay with a half-life of 30 years, the inventory for each basin is the difference between the decay corrected cumulative input and flux. In the North Pacific, the inventory reached its maximum in 1966 due to the global fallout by atmospheric nuclear weapons tests. Fluxes from the North Pacific to the Indian Ocean, Arctic Ocean, and Central Pacific were positive, and the North Pacific was a source of supply for other ocean basins. The 1F NPP accident caused a 20% increase in the inventory in 2011. In the North Atlantic, the inventory reaches its maximum in the late 1970s, due to the releases from the reprocessing plant. The outflow flux from the North Atlantic to the Greenland Sea is larger than the other fluxes and is a source of supply to other ocean basins. After 2000, the inflow flux to the North Pacific from the Labrador Sea and the South Atlantic is larger than the outflow flux.The time series of 137Cs inventory in each ocean basin and the fluxes among ocean basins were quantitatively analyzed by OGCM simulations, and the predictions for the next 10 years were made.  The 137Cs activity concentrations by global fallout can be detected in the global ocean after 2030. The OGCM simulations will be useful in planning future observations to fill the gaps in the database.