Fukushima-derived radiocesium in the western North Pacific in 2014
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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. The subsurface maximum spread southward from 18°N to 15°N between 2012 and 2014, which suggests spreading of Fukushima-derived radiocesium into the whole western subtropical area by 2014 due to formation and subduction of the subtropical mode water.
KeywordsFukushima Dai-ichi nuclear power plant accident Radiocesium North Pacific Ocean
The massive Tohoku earthquake and consequent giant tsunamis on 11 March 2011 resulted in serious damage to the Fukushima Dai-ichi nuclear power plant (FNPP1) in eastern Japan . Radiocesium (134Cs and 137Cs) released from the damaged FNPP1 caused radioactive contamination of the islands of Japan and the North Pacific Ocean mostly in March and April 2011 . Measurements of 134Cs and 137Cs activity concentrations in soil collected in Japan revealed that (1) the activities of 134Cs and 137Cs released from FNPP1 were equivalent at a 1:1 ratio approximately  and (2) the total deposition of 134Cs (or 137Cs) on the land was 2.4 PBq (1015 Bq) . On the other hand, the total deposition of 134Cs (or 137Cs) in the ocean had been estimated widely to be 9–37 PBq because of limited data obtained in the ocean [4, 5, 6, 7, 8, 9, 10, 11, 12]. In a recent study, observational data in surface water from commercial ships between April and June 2011 revealed that Fukushima-derived radiocesium deposited mainly in the north of the Kuroshio Front (30°N–35°N approximately), namely the subarctic area and a transition zone between the subarctic and subtropical areas . Then the total deposition of 134Cs (or 137Cs) on the North Pacific was calculated more narrowly to be 12–15 PBq [14, 15, 16], suggesting more than 80 % of the atmospheric-released radiocesium was deposited on the North Pacific Ocean. 134Cs (or 137Cs) was also discharged directly into the ocean due to leakage of contaminated water from FNPP1 mainly in April 2011, which was estimated to be 2–6 PBq [1, 6, 8, 17, 18, 19]. However, temporal or spatial extrapolation in the calculation resulted in a larger amount of 134Cs (or 137Cs) from 11 to 27 PBq [20, 21, 22]. The smaller amount of directly-discharged radiocesium indicates that the total amount of Fukushima-derived 134Cs (or 137Cs) in the North Pacific Ocean was 14–21 PBq.
Radiocesium was also released into the open ocean before the FNPP1 accident by atmospheric nuclear weapons testing mainly in the 1950s and 1960s, nuclear fuel reprocessing mainly in the 1980s , and the Chernobyl accident in 1986. In the North Pacific Ocean the major source of radiocesium was atmospheric deposition due to the nuclear weapons testing . The bomb-derived 137Cs deposited on the North Pacific was still there before the FNPP1 accident (1.0–2.5 Bq m−3 in surface seawater in the 2000s)  because its half-life is long, 30.17 y. In addition measurement of 137Cs in the North Pacific Ocean before the FNPP1 accident revealed that the bomb-derived 137Cs has been accumulated in the mid latitude, subtropical area . After the accident, the Fukushima-derived 137Cs was added to the bomb-derived 137Cs, which was estimated to be about 30 % of the bomb-derived 137Cs . In contrast, the 134Cs released before the accident had disappeared, because its half-life is only 2.06 y. Therefore 134Cs is a unique tracer for the FNPP1 accident and employed in this study in order to discuss temporal and spatial changes in the Fukushima-derived radiocesium.
Fukushima-derived 134Cs directly-discharged and atmospheric-deposited in the North Pacific Ocean was transported eastward along surface currents. In summer 2012, about one and half years after the accident, a water mass with high activity concentration of 134Cs (>5 Bq m−3) due to the direct discharge from FNPP1 (37.4°N/141°E) and deposition closed to it was observed in the central North Pacific around 165°E–170°W between 40°N and 50°N in surface seawater . This high-134Cs surface water then shifted northward gradually and reached to stations in the Gulf of Alaskan in summer 2014 [28, 29]. A zonal eastward speed of this radioactive water mass was estimated to be about 4–8 cm s−1 .
In the south of the Kuroshio Front, namely the subtropical area, the activity concentration of 134Cs in surface water was lower than that in the north of the front (the transition zone and subarctic area), where the Fukushima-derived radiocesium was discharged and deposited, because the Kuroshio Front restricted surface water exchange across it . In the western subtropical region, however, subsurface maxima of 134Cs (>10 Bq m−3) in approximately 200–600 m depth had been observed since about 10 months after the accident [29, 30, 31, 32, 33]. The subsurface layer of the 134Cs maxima correspond to density layers of subtropical mode water (STMW)  and central mode water (CMW)  in the North Pacific Ocean. Therefore it was concluded that the Fukushima-derived 134Cs was not only transported eastward in surface but also conveyed southward through subsurface due to formation and subduction of the mode waters.
Potential water density anomaly, σ θ (kg m−3) defined by [potential water density—1000] of STWM ranges about 25.0–25.6 kg m−3. STMW is formed in just south of the Kuroshio/Kuroshio Extension current in the mid-winter due to severe cooling by cold monsoon wind and then transported southward through subsurface . Therefore the main source of 134Cs observed in the STMW density was probably 134Cs deposited on the formation area just after the accident, in late March 2011. The total inventory of 134Cs decay-corrected to the accident date in STWM in 2012 was estimated to be about 4 PBq , which is 20–30 % of the total amount of 134Cs released in the North Pacific Ocean (14–21 PBq). CMW is denser than STMW (σ θ = 26.0–26.6 kg m−3) and formed in the north of the Kuroshio Front . However, mechanism of 134Cs entrainment into the CMW density is not clear because specific formation process of CMW has not been revealed yet. In addition, the total inventory of 134Cs in CMW is still unknown because the observational data are insufficient.
In 2014 we measured vertical profiles of radiocesium at 14 stations in the western North Pacific Ocean from the subarctic to subtropical areas (41°N–15°N) and discussed its temporal change between 2012 and 2014. As mentioned above temporal change in the Fukushima-derived radiocesium in surface layer in the north of Kuroshio Front has been roughly elucidated because there is enough data in surface water and the surface currents system in the North Pacific has been well understood. However, its temporal change in the mode waters in subsurface layer is still unknown due to limited number of vertical profile data. We compiled radiocesium data from this study and previous works and synthesized spreading of the Fukushima-derived radiocesium in the North Pacific Ocean by 2014.
After the cruises, radiocesium in the seawater sample was concentrated onto ammonium phosphomolybdate (AMP) for measurement of gamma-ray activity  in onshore laboratories of the Mutsu Institute for Oceanography at the Japan Agency for Marine-Earth Science and Technology (MIO/JAMSTEC), General Environmental Technos Co., Ltd., or Nihon University (NU). The radiocesium activity in the AMP/Cs compounds was measured using low-background Ge-detectors in Ogoya underground laboratory of the Low Level Radioactivity Laboratory at Kanazawa University (LLRL/KU) , MIO/JAMSTEC, or NU. The average of analytical uncertainty (one standard deviation or 68 % confidence level) for 134Cs and 137Cs measurements were 10–30 and 6–9 %, respectively. Detection limit for 134Cs/137Cs decay-corrected to the accident date (11 March 2011) for the measurement at LLRL/KU, MIO/JAMSTEC, and NU were calculated to be about 0.2/0.05, 0.4/0.1, and 0.7/0.2 Bq m−3, respectively. The detection limits of 134Cs measurement for samples from the KH14-06 were larger than the others because those measuring time were shorter. In MIO/JAMSTEC, the Ge-detectors were calibrated with gamma-ray volume sources (Eckert & Ziegler Isotope Products) certificated by Deutscher Kalibrierdienst (DKD). Measurements of 137Cs activity in AMP/Cs compounds derived from certified reference materials (IAEA-443 ) among the three laboratories resulted in good agreement within the uncertainty of the certificated value, which confirmed the comparability of the radiocesium measurements at the three laboratories.
Results and discussion
Vertical distribution of radiocesium in 2014
Temporal change in vertical distribution
About a half year later a sectional view along 165°E was achieved (Fig. 3b) . In the north of the Kuroshio Front, activity concentration of 134Cs decay-corrected to the accident date in the surface mixed layer along 165°E in June–July 2012 (3 Bq m−3 in average) was lower than that observed along 145°E–150°E in January–February 2012. By the summer of 2012, a main body of the 134Cs directly-discharged and deposited closed to FNPP1 had been transported to the central North Pacific . Thereby this 134Cs decrease in the surface layer could be explained by dilution due to water mixing during the eastward transportation. Below the surface mixing layer there was a subsurface maximum between 300 and 500 m depth approximately, which was not observed along 145°E–150°E in January–February 2012 (Fig. 3a). The water density anomaly of this subsurface maximum layer ranged 26.0–26.6 kg m−3, which corresponds to that of CMW denser than STMW. CMW is formed in an area in the north of the Kuroshio Front and the east of 150°E in the mid-winter, transported eastward, and then southward through the subsurface layer . The appearance of the subsurface maximum of 134Cs in the density layer of CMW along 165°E and not along 145°E–150°E suggests the formation area of CMW in the east of 150°E, where the Fukushima-derived 134Cs was subducted in the mid-winter (March) of 2011 and/or 2012. In the south of the front, another subsurface maximum around 200 m depth was observed. The water density anomaly of this shallower subsurface maximum ranged 25.0–25.6 kg m−3, which corresponds to that of STMW as same as that observed along 145°E–150°E in January–February 2012.
In March–July 2014 along approximately 142°E–155°E, the high concentration activity in the north of the Kuroshio Front observed in January–February 2012 disappeared (Fig. 3c). This is probably due to the eastward propagation of the contaminated surface water. A subsurface 134Cs maximum in the STWM density layer in the south of the front is similar to that observed in 2012, which contrasts to the disappearing of the high activity concentration of 134Cs in the surface mixing layer in the north of the front. STMW is transported eastward/southward and circulated within the western subtropical gyre although its horizontal speed is slower than that of the surface water current , which could result in persisting of the subsurface maximum of 134Cs in 2014. The activity concentration of the maximum peak around 300–400 m depth decreased by one-tenth at most and the peak shape was eroded vertically, which can be explained by entrainment into surface mixed layer and deep penetration due to vertical diffusion.
In December 2014 along 163°E–165°E between 15°N and 23°N, we also found a 134Cs maximum peak in subsurface layer of STWM (Fig. 3d). Although the activity concentration decay-corrected to the accident date of the maximum peak in 2014 at 23°N (about 3 Bq m−3) was almost equivalent with that observed in 2012, the subsurface maximum has been spread vertically from 100 to 400 m approximately in 2014. In addition the subsurface maximum spread southward from 18°N to 15°N at least between 2012 and 2014.
Temporal change in vertical inventory
At stations in the southern subtropical area between 15°N and 25°N along 163°E–165°E, the 134Cs vertical inventory increased thrice in average between 2012 and 2014 (Fig. 4) due to the vertical and horizontal (southward) spreading of its subsurface maximum. At stations in 23–24°N/165°E, 134Cs activity concentration decay-corrected to the accident date in the subsurface maximum peak around 200 m depth increased from 0.9 to 4.3 Bq m−3 between February 2012 and June 2012, and then decreased slightly in December 2014 to be 3.4 Bq m−3 (Fig. 5d). The vertical inventory, however, increased about twice between June 2012 and December 2014 (Fig. 5f) since the subsurface maximum spread vertically in 2014.
The subsurface maximum of Fukushima-derived 134Cs in the subtropical area appeared in the density layer of STMW, 25.0–25.6 kg m−3 σ θ (Fig. 5b, e) although its peak depths in the northern subtropical area were deeper than those in the southern area (Fig. 5a, d). In density layer of CMW (26.0–26.6 kg m−3 σ θ), a subsurface maximum of 134Cs was not observed in 2014, which implies that the subsurface maximum in the CMW layer observed in the transition zone in June 2012 (Fig. 3b) was transported southward across the Kuroshio Front. Therefore the erosion of the subsurface peak by diapycnal mixing (mixing between waters with different density) could result in downward (and upward) spreading of 134Cs in 2014. In short, the vertical inventory at the northern stations (34°N) decreased due to the decrease in the activity concentration of the subsurface maximum peak while that at the southern stations (23°N–24°N) increased because of the vertical spreading of the peak. The vertical inventory at 30°N/149°E did not change apparently between 2012 and 2014 (Fig. 4) because the decrease in activity concentration of the peak balanced with the increase in vertical spreading of the peak. These results indicate that the Fukushima-derived radiocesium in the western subtropical area has been transported southward due to subduction and advection of STMW with the vertical spreading due to diapycnal mixing.
We thank crew and staff of the research vessels for their help during onboard sample collection. This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan (KAKENHI), Interdisciplinary Study on Environmental Transfer of Radionuclides from the Fukushima Dai-ichi NPP Accident (#24110004).
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