Unique transport paths of Cs from the Indian to Southern Oceans.

To assess ocean-scale transport systems, we examined the latitudinal cross-sectional distribution of Cs activity concentrations in the Indian and Southern Oceans between December 2019 and January 2020 using low-background γ-spectrometry. At 0°-20°S, Cs concentrations exhibited a gradual decrease below the mixing layer (1-0.1 mBq/L).

Fukushima-derived radiocesium in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2019 and 2020.

We measured dissolved radiocesium (Cs and Cs) in surface seawater collected in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2019 and 2020. The radiocesium released from the accident of the Fukushima Dai-ichi nuclear power plant (FNPP1) in 2011 was still observed in these areas (∼2 Bq m decay-corrected to the date of the accident). In 2019/2020, the FNPP1-derived radiocesium concentrations in the Bering Sea and the Chukchi Sea, which is a marginal sea of the Arctic Ocean connecting the Bering Sea to the Arctic Ocean, were within the range of those observed in 2017/2018.

Zonal and vertical transports of Fukushima-derived radiocesium in the subarctic gyre of the North Pacific until 2014.

The Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011 resulted in serious radiocesium contamination of the North Pacific Ocean. Most of the radiocesium was dissolved in seawater and transported by surface currents and subduction of mode waters. Within several years after the accident, a high-concentration water plume of the FNPP1-derived radiocesium at the sea surface had been transported from Japan to the North American continent across the subarctic gyre of the North Pacific Ocean.

Unique current connecting Southern and Indian Oceans identified from radium distributions.

We examined the spatial variations in Ra and Ra (activities) concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans from December 2019 to January 2020. Ra concentrations at the surface increased sharply from 30° S to 60° S along a ~ 55° E transect (1.4-2.

Temporal changes in tritium and radiocarbon concentrations in the western North Pacific Ocean (1993-2012).

The western North Pacific is one of the most studied oceanic basins due to its diverse structure and important role in connection with the adjacent reservoirs. Tritium (H) and radiocarbon (C) have been frequently exploited as oceanographic tracers due to their suitable properties; several extensive observation projects, such as GEOSECS, WOCE and WOMARS, used these two radionuclides to investigate different oceanographic processes, pathways, ocean currents and time scales of deep and bottom water formation. Here we evaluate temporal changes in H and C levels in seawater of the western North Pacific Ocean from 1993 to 2012.

Radiocesium in North Pacific coastal and offshore areas of Japan within several months after the Fukushima accident.

We measured activity concentrations of radiocesium (Cs and Cs) in seawater samples collected in North Pacific coastal and offshore areas of Japan within several months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011, including archived seawater samples whose radiocesium concentrations were previously reported to be below detection limits. By merging 329 new data with published results, we succeeded in reconstructing the temporal changes in activity concentrations and inventories of FNPP1-derived radiocesium in the coastal and offshore areas within several months after the accident for the first time. Cs directly-discharged from the FNPP1 was transported eastward within the coastal area about 250 km from the FNPP1 during two months after the accident due to complex movements of coastal surface currents.

Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident.

The marine environment is complex, and it is desirable to have measurements for seawater samples collected at the early stage after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident to determine the impact of Fukushima-derived radionuclides on this environment. Here Pu isotopes in seawater collected 33-163 km from the FDNPP site at the very early stage after the accident were determined (May 2011, within two months after the accident). The distribution and temporal variation of Pu and Pu were studied.

Time evolution of Fukushima-derived radiocesium in the western subtropical gyre of the North Pacific Ocean by 2017.

In 2015-2017, we measured activity concentration of radiocesium in the western subtropical gyre of the North Pacific Ocean and revealed the time evolution of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident between 2011 and 2017. The FNPP1-derived radiocesium deposited on the area south of the Kuroshio/Kuroshio Extension Currents in March 2011 was transported southward and westward through subsurface layers due to subduction of the subtropical mode water. In 2014, the radiocesium in the subsurface layers returned to the north and circulated within the Kuroshio recirculation area.

Radiocaesium derived from the TEPCO Fukushima accident in the North Pacific Ocean: Surface transport processes until 2017.

We report temporal variations of Cs activity concentrations in surface waters of six regions of the western and central North Pacific Ocean during 2011-2017 using a combination of 1264 previously published data and 42 new data. In the western and central North Pacific Ocean at latitudes of 30-42°N and longitudes of 140°E to 160°W, eastward transport of radiocaesium was clearly apparent. Cs activity concentrations in surface water decreased rapidly to ∼2-3 Bq m in 2015/2016, still a bit higher than Cs activity concentrations before the FNPP1 accident (1.

Tritium and radiocarbon in the western North Pacific waters: post-Fukushima situation.

Impact of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident on tritium (H) and radiocarbon (C) levels in the water column of the western North Pacific Ocean in winter 2012 is evaluated and compared with radiocesium (Cs) data collected for the same region. Tritium concentrations in surface seawater, varying between 0.4 and 2.

Temporal changes in radiocesium contamination derived from the Fukushima Dai-ichi Nuclear Power Plant accident in oceanic zooplankton in the western North Pacific.

We investigated temporal changes of the contamination of oceanic zooplankton with radiocesium (Cs and Cs) derived from the Fukushima Dai-ichi Nuclear Power Plant accident one month to three years after the accident at subarctic and subtropical stations (1900 and 900-1000 km from the plant, respectively) in the western North Pacific. The maximum activity concentrations of Cs in zooplankton were two orders of magnitude higher than the pre-accident level. In the first four months after the accident, the activity concentrations of radiocesium in subtropical zooplankton decreased rapidly, but no similar change was observed at the subarctic station.

Radiocesium in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014.

We measured radiocesium (Cs and Cs) in seawater from the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2013 and 2014. Fukushima-derived Cs in surface seawater was observed in the western subarctic area and Bering Sea but not in the Arctic Ocean. Vertical profile of Cs in the Canada Basin of the Arctic Ocean implies that Fukushima-derived Cs intruded into the basin from the Bering Sea through subsurface (150m depth) in 2014.

Fukushima-derived radiocesium in the western North Pacific in 2014.

In 2014, we measured activity concentration of radiocesium in the western North Pacific Ocean. In the north of Kuroshio Front high activity concentration of Fukushima-derived radiocesium in surface mixed layer in 2012 had been transported eastward by 2014. In the south of the front we found a radiocesium subsurface maximum in 200-600 m depth, which was similar to that observed in 2012.

Intrusion of Fukushima-derived radiocaesium into subsurface water due to formation of mode waters in the North Pacific.

The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium ((137)Cs and (134)Cs) into the North Pacific Ocean. Meridional transects of the vertical distribution of radiocaesium in seawater were measured along 147 °E and 155 °E in October-November 2012, 19 months after the accident. These measurements revealed subsurface peaks in radiocaesium concentrations at locations corresponding to two mode waters, Subtropical Mode Water and Central Mode Water.

Impact of Fukushima-derived radiocesium in the western North Pacific Ocean about ten months after the Fukushima Dai-ichi nuclear power plant accident.

We measured vertical distributions of radiocesium ((134)Cs and (137)Cs) at stations along the 149°E meridian in the western North Pacific during winter 2012, about ten months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident. The Fukushima-derived (134)Cs activity concentration and water-column inventory were largest in the transition region between 35 and 40°N approximately due to the directed discharge of the contaminated water from the FNPP1. The bomb-derived (137)Cs activity concentration just before the FNPP1 accident was derived from the excess (137)Cs activity concentration relative to the (134)Cs activity concentration.

Southward spreading of the Fukushima-derived radiocesium across the Kuroshio Extension in the North Pacific.

The accident of the Fukushima Dai-ichi nuclear power plant in March 2011 released a large amount of radiocesium into the North Pacific Ocean. Vertical distributions of Fukushima-derived radiocesium were measured at stations along the 149°E meridian in the western North Pacific during the winter of 2012. In the subtropical region, to the south of the Kuroshio Extension, we found a subsurface radiocesium maximum at a depth of about 300 m.

Fukushima-derived radiocesium in the northwestern Pacific Ocean in February 2012.

We measured radiocesium ((134)Cs and (137)Cs) in seawaters collected at stations in the northwestern Pacific Ocean in February 2012. Activity concentration of Fukushima-derived radiocesium was highest in the transition area between the subarctic and subtropical regions, which was due to the direct discharge. The direct discharged radiocesium was transported southwardly across the Kuroshio Extension along isopycnal mixing.