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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. It is concluded that atmospheric-deposited radiocesium south of the Kuroshio Extension just after the accident had been transported not only eastward along with surface currents but also southward due to formation/subduction of subtropical mode waters within about 10 months after the accident. The total amount of decay-corrected 134Cs in the mode water was an estimated about 6 PBq corresponding to 10–60% of the total inventory of Fukushima-derived 134Cs in the North Pacific Ocean.

number of observational data in the open ocean cannot estimate the total oceanic deposition directly. Alternatively, that could be calculated indirectly from the total amount of radiocesium released to the atmosphere, which was derived primarily from measurements on land. Estimations of the total amount released to the atmosphere range widely, from 8.8 to 37 PBq 1,5,9,11,14,[21][22][23][24][25] . The 2.4 PBq deposited onto the islands of Japan suggests that most of the remaining radiocesium, 6.4-35 PBq, found its way into the North Pacific through atmospheric deposition. Atmospheric models have estimated independently the total oceanic deposition to be 5. 9,11,12,23,25 , similar to the range of 6.4-35 PBq. However, the deposition on land has been overestimated in many of the models.
Efforts to obtain observational data from the open ocean have continued. The marine monitoring from March 2011 by MEXT or the Nuclear Regulation Authority was extended eastward to the 144uE meridian in August 2011 7 . Radiocesium measurements in the area further east have been reported in several publications 8,17,[26][27][28][29][30][31] . Seawater sampling from April 2011 during commercial ship cruises has produced a valuable dataset across the North Pacific 28 , although as in many other previous studies, most of the samples were collected only at the surface. In June 2011 vertical profiles of the Fukushima-derived radiocesium were measured at stations along 147uE between 34.5uN and 38uN, and it was found that the radiocesium had penetrated to a depth of about 200 m roughly two months after the disaster 17 . Although these observational data are still insufficient for direct estimation of the total amount of radiocesium in the whole North Pacific, these data can be used to validate ocean model simulations that have predicted vertical and horizontal spreading of the radiocesium in the ocean 13,15,16,25,32,33 .
Here we report the vertical distributions of the Fukushima-derived radiocesium at stations along 149uE between 10uN and 42uN in the winter of 2012, about ten months after the accident. Our preliminary reports, which have already been published 31,34 , revealed that (1) the Fukushima-derived radiocesium activity was highest in the transition area between the subarctic and subtropical regions and (2) the radiocesium was transported southward across the Kuroshio Extension (KE) through subsurface layers. In this study, we discuss the causes of the southward spreading of the radiocesium based on temporal changes in the activity of surface waters. Secondly, we have estimated the vertical water-column inventory of radiocesium. These results will contribute to determination of the total inventory of radiocesium and will facilitate prediction of the spreading of the Fukushima-derived radiocesium in the North Pacific Ocean in the future. We measured both 134 Cs and 137 Cs activities (Methods). The ratio of decay-corrected 134 Cs/ 137 Cs in samples in which the 137 Cs activity was higher than 20 Bq m 23 was about 0.95. The small excess of 137 Cs was derived from another source of 137 Cs, global fallout due to the nuclear bomb testing in the 1950s and 1960s 35 36 . Therefore, only results for 134 Cs, which is a unique tracer of the FNPP1 accident, are presented in later sections.

Results
Temporal changes in 134 Cs activity in surface waters. Our sampling stations were located in the western North Pacific from cold subarctic to warm tropical regions, although information on sea surface temperatures estimated by satellite sensors was patchy in the northern area due to cloudy conditions during the sampling cruise ( Figure 1a). The image of sea surface height (SSH) implied that our observational line along 149uE crossed eastward-flowing currents around 35uN and 40uN where SSH gradient was relatively steep (Figure 1b). The northern and southern currents correspond to the subarctic and KE fronts, respectively. Here we define areas north of the subarctic and south of the KE fronts as the subarctic and subtropical regions, respectively. In addition, we designate the area between the two fronts as the transition area, in which the FNPP1 is situated (Figure 1). Although a boundary between the subtropical and tropical regions is not clear in Figure 1, we provisionally regarded the area south of 20uN as the tropical region because of the subtropical front around 20uN 37 . The distribution of SSH also suggests that the observational line crossed a southward meander of the KE front around 148uE (A in Figure 1b).
In surface seawaters, Fukushima-derived 134 Cs activity was detected at all the stations along the 149uE meridian from the subarctic to tropical regions in the winter of 2012 ( Figure 2). The radioactivity was highest (10-20 Bq m 23 ) in the transition area between 35uN and 40uN. In the subarctic region, north of 40uN, the activity decreased sharply at higher latitudes and fell to about 0.2 Bq m 23 at the northernmost station. To the south of the KE, between approximately 30uN and 35uN, the activity declined to a few Bq m 23 and then dropped to less than 1 Bq m 23 farther south of 30uN. We also collected seawater samples along a zonal transect at approximately 35uN, which crossed the southward meander of the KE (A in Figure 1b). Relatively high activity (about 8 Bq m 23 ) was observed at a station at 148uE, near the approximate center of the meander.
To discuss temporal changes in the surface 134 Cs activity, we also show in Figure 2 the activities measured in surface waters (0-20 m depth) between approximately 145uE and 152uE during previous studies 17, [26][27][28][29][30] . Just after the accident, in April-May 2011, the activities between 30uN and 40uN were high, though the range of activity was large (approximately 2-1000 Bq m 23 ). In the transition area (35uN-40uN), the activity increased significantly in the following period, June-August 2011. After that time, the activity decreased piecemeal and then fell to a few Bq m 23 in August 2012. The surface activity in the subarctic region to the north of 40uN also decreased monotonically from about 50 to a few Bq m 23 between June 2011 and August 2012. The transitory increase during June-August 2011, which was observed in the transition area, was indistinct in the subarctic region because of a lack of data in April-May 2011. To the south of the KE, between 30uN and 35uN, the high surface activity in April-May 2011 quickly decreased to a few Bq m 23 by June 2011. The magnitude of the temporal change of activity in the surface waters to the south of 30uN, including the southern subtropical and tropical regions, is uncertain, because 134 Cs activity was detected only in the winter of 2012. 134 Cs has a short half-life of only 2.07 years, and the activity decay-corrected to the sampling date decreased by 50-75% from April 2011 to September 2012. The fact that the observed activity decreased at a rate faster than the radioactive decay rate suggests that the surface 134 Cs activity was diluted by advection and diffusion.
Vertical profiles and inventories of 134 Cs activity. In the transition area between 35uN and 40uN, where surface 134 Cs activity was highest, 134 Cs activity from the surface to a depth of about 200 m was almost constant (Figure 3a). The homogeneity of the activity in the surface layer reflects surface cooling and vertical mixing in the winter and is consistent with the vertical uniformity of water temperature, salinity, density, and therefore the small potential vorticity at that time (Figs. 3b-3e). The activity then decreased sharply just below the winter mixed layer. The 134 Cs had penetrated to a depth of about 300 m by the winter of 2012. In the subarctic region, the 134 Cs activity in the surface mixed layer was also almost uniform vertically but lower than in the transition area. The depth of penetration was shallower than in the transition area, probably because the mixed layer was shallower, about 150 m deep. At the northernmost station, the activity in the mixed layer were lower as in the surface water. The vertical profiles of 134 Cs activity in the transition area and subarctic region can be largely explained by vertical diffusion between the surface mixed layer and deeper layers.
To the south of the KE, the surface activity was less than a few Bq m 23 in the winter of 2012 ( Figure 2). Figure 3a indicates that the 134 Cs www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 4276 | DOI: 10.1038/srep04276 activity was also low (but significantly above the detection limit) in the surface mixed layer from the surface to a depth of 150-200 m between approximately 25uN and 35uN. In contrast, to the south of 20uN the activity was not detected in the surface mixed layer to a depth of 100-150 m, except in surface waters collected with a bucket. Below the surface mixed layer, we found a conspicuous subsurface maximum centered at a depth of about 300 m throughout the subtropical region between 20uN and 35uN. This subsurface tongueshaped maximum appeared in a pycnostad between potential density anomalies of approximately 25.0 and 25.6 s h (Figure 3d Higher activities in the subsurface maximum were observed at 32uN and 34uN (10-20 Bq m 23 ), and the activity decreased at lower latitudes. We also note that the 134 Cs had penetrated into deeper layers, to depths of at least 600 m, between 32uN and 35uN.
We calculated vertically integrated (i.e., areal) 134 Cs inventories from the surface to a depth of 800 m in the winter of 2012 ( Figure 4). The areal inventories were corrected for radioactive decay to the date of the earthquake, 11 March 2011. High areal inventories were observed in the transition area, where surface activities were also high. Although the surface activities were low in the subtropical region between 30uN and 35uN, the areal inventories were comparable to those in the transition area because of the subsurface activity maximum. The areal inventories of 134 Cs activity in the subarctic region (40uN-42uN), transition area (35uN-40uN), and subtropical region (20uN-35uN) were calculated to be 0.

Discussion
In April-May 2011, just after the accident, the 134 Cs activity was as high as 1000 Bq m 23 in the surface waters of the transition area and just to the south of the KE (30uN-40uN) along approximately 145uE-152uE, more than 500 km from the FNPP1 (Figure 2). In April 2011, 134 Cs activity was also observed at stations in the subarctic and subtropical regions, more than 1000 km distant from the plant 26,28 . The wide dispersal of Fukushima-derived 134 Cs in the western North Pacific within about two months of the accident is consistent with patterns of atmospheric deposition of 134 Cs simulated by atmospheric models 13,25,38 . A low-pressure system traveling across Japan from 14-15 March 2011 was found to be effective in lifting particles containing 134 Cs from the surface layer to the altitude of the westerly jet stream, which carried the particles across the North Pacific within 3-4 days 39 .
In the transition area between 35uN and 40uN, the 134 Cs activities in surface waters during June-August 2011 were significantly higher than in April-May 2011 (Figure 2), which implied that contaminated waters discharged from the FNPP1 had been transported by the eastwardflowing North Pacific Current ( Figure 5). The radiocesium activities in surface seawater collected by commercial cruise ships revealed an eastward propagation of the main plume of the directly discharged 134 Cs. The zonal speed of the plume was estimated to be about 200 km month 21 , a speed that was consistent with trajectories of Argo floats launched near the FNPP1 28  The 134 Cs activity in the subarctic region was lower than in the transition area throughout the observational period; its pattern of temporal change, however, was similar to that in the transition area ( Figure 2). Whether there were intrusions of directly discharged 134 Cs from the transition area to the subarctic region is unclear, because the transitory increase in June-August 2011 was obscure in the subarctic region. Off the Kuril Islands, the activities in the surface waters of the Oyashio Current, which flows into the subarctic region ( Figure 5), were less than a few Bq m 23 in April 2011 27 . If the supply of directly discharged 134 Cs to the subarctic region had been blocked by the subarctic front, the surface activity in the subarctic region would have dropped more sharply because of the inflow of Oyashio Current water, the 134 Cs activity of which was low. In fact, the low activity at the northernmost station in the winter of 2012 implies an intrusion of Oyashio Current water (Figure 3a). Therefore, it is likely that the directly discharged 134 Cs was transported into the subarctic region through water exchanges between the transition area and the subarctic region. The gradual decrease of surface 134 Cs in the subarctic region indicates that the directly discharged 134 Cs was transported eastward and diffused vertically over time, as was also the case in the transition area.
Between 30uN and 35uN     KE current from the west probably flushed out the 134 Cs in the surface water between 30uN and 35uN. This process was also clearly demonstrated in ocean model simulations 12,13 and suggests that an exchange of surface seawater between the transition area and the subtropical region was restrained by the KE front. The 134 Cs activity in the surface mixed layer between 25uN and 35uN was low but detectable in the winter of 2012 (Figure 3a). The 134 Cs derived from atmospheric deposition just after the accident probably recirculated within the   western subtropical region ( Figure 5). Alternatively, the 134 Cs in the mixed layer could be explained by entrainment of 134 Cs from the subsurface maximum just below the mixed layer. To the south of 20uN, the 134 Cs was detected only in surface waters collected with a bucket. Although the cause of those surface activities is not sure, a little contamination on the bucket is possible.
In the subtropical region between 20uN and 35uN, we found a subsurface 134 Cs maximum just below the surface mixed layer in the winter of 2012 (Figure 3a). This tongue-shaped subsurface plume appeared on a pycnostad between 25.0 and 25.6 s h (Figure 3d) that resulted in a subsurface minimum of potential vorticity in the corresponding layers (Figure 3e). We conclude that the 134 Cs subsurface maximum was derived from formation and subduction of Subtropical Mode Water (STMW) 40 . To the south of the KE between approximately 30uN and 35uN, STMW is formed and penetrates to a depth of about 400 m (25.6 s h ) in late winter. This STMW then spreads to nearly the subtropical front 35 43 . The formation area of CMW is situated in the transition area in the central North Pacific. The CMW spreads eastward along the North Pacific Current, turns southward, and then turns westward ( Figure 5). Despite its similar water density anomaly (26.0-26.6 s h ), the path of the CMW as it spreads is likely to be to the south of approximately 30uN, along 149uE. In addition, a transit time as short as about 10 months (between March 2011 and January 2012) from the formation area to 149uE longitude is not plausible, because the renewal time of CMW is more than 20 years 44 .
Another possible explanation for the deeper penetration is conveyance of 134 Cs from the transition area across the KE. The satellite image of SSH indicates that stations at 32uN and 34uN were located near a cyclonic eddy centered at 33uN, 151uE (B in Figure 1b). This cyclonic eddy originated in a southward meander of the KE front around 158uE and pinched off southward from the meander in September 2011. Then the eddy moved westward and reached 151uE in January 2012. Similar to the relatively high activity at the station located near the center of the southward meander of the KE at 148uE (A in Figure 1b), the cyclonic eddy probably consisted of denser waters with a higher activity of 134 Cs, because the surface 134 Cs activity in the source area (the transition area) was more than 50 Bq m 23 in October 2011 29 . A model simulation has indicated that a cyclonic eddy detached from the KE front holds the transition area water in it, while small leakage occurs from layers denser than 26.0 s h 45 . Although the vertical profiles of temperature and salinity do not indicate the presence of a cyclonic eddy between 32uN and 34uN (Figs. 3b and 3c), a small amount of leakage of 134 Cs from such an eddy could explain the deeper penetration of the 134 Cs (Figure 3a).
Alternatively, the deeper penetration can be attributed to direct advection along subsurface isopycnals from the transition area. A salinity minimum observed just south of the KE has been explained by intrusion of Oyashio low-salinity water in the transition area; this intrusion was associated with the frontal wave structure of the KE 46,47 . The deeper 134 Cs penetration just south of the KE (Figure 3a) implies that a similar subsurface intrusion occurred in the winter of 2012.
In the winter of 2012 the areal inventory of 134 Cs (decay-corrected to the date of the accident) in the subtropical region (20uN-35uN) was estimated to be 1.6 6 0.1 kBq m 22 , which is about one-third of the areal inventory in the transition area (35uN-40uN), 4.6 6 0.3 kBq m 22 (Figure 4). The integral of the areal inventory along the meridian in the subtropical region, however, was 2.7 6 0.1 GBq m 21 , which was about twice the value of the integral in the transition area, 1.4 6 0.1 GBq m 21 . The large inventory in the subtropical region suggests that the 134 Cs released from the FNPP1 had been transported not only eastward but also southward. The average activity of the decay-corrected 134 Cs in the STMW was 5.6 6 0.4 Bq m 23 . We here assumed that this average activity could be regarded as the mean activity of the whole STMW in the North Pacific, because our observational line was located near the center of the area of STMW ( Figure 5). An estimation of the total volume of STMW (about 1 3 10 6 km 3 ) 44 implies that the STMW contained about 6 PBq of 134 Cs. Estimates of the total 134 Cs released to the North Pacific Ocean ranged from 10 PBq (direct discharge of 4 PBq 1 atmospheric deposition 6 PBq) to 46 PBq (16 1 30 PBq). Thus, the 6 PBq inventory accounts for 10-60% of the total release. However, the total inventory in the subtropical region derived from the activity in STMW may be underestimated, because CMW probably carried the radiocesium into the subtropical region, too ( Figure 5).
In this study we reconstructed the temporal change in Fukushimaderived radiocesium in surface water of the western North Pacific during about one year and a half after the accident. In April-May 2011 the 134 Cs activity between 30uN and 40uN arose from the atmospheric deposition (Figure 2). In the north of the KE front, the transition area and subarctic region the discharged 134 Cs was added while in the south of the KE front the atmospheric-deposited 134 Cs was flushed out by the KE current during the following period. We found the subsurface maximum of 134 Cs in the subtropical region about 10 months after the accident. The radiocesium that entered the ocean just south of the KE front via atmospheric deposition was subducted southward immediately because of formation of STMW. This process is reminiscent of the southward spreading of radiocesium derived from the nuclear bomb testing in the North Pacific via STMW formation 48 . In addition, there is an indication that the Fukushima-derived radiocesium in the transition area was conveyed southward across the KE by cyclonic eddies that detached from the KE and by subsurface intrusion under the KE. The rapid southward spreading of the 134 Cs through subsurface layers seems to not have been simulated well in ocean models 13,15,16,32,33 , probably because of problems associated with the simulation of processes responsible for formation/subduction of STMW in these models. The estimated inventory in the subtropical region (6 PBq or 10-60% of the total inventory) is probably a lower limit of estimation because contribution of CMW was not counted. The results in this study clearly suggest that radiocesium released from FNPP1 into the North Pacific Ocean had been transported not only eastward along with the surface currents but also southward due to formation/subduction of STMW within about 10 months after the accident.  10uN and 42uN (Figure 1). Surface samples were taken from the deck with a bucket or by pumping water from directly beneath the ship (a depth of about 4 m). The temperature and salinity of the surface water in the bucket were measured with a calibrated mercury thermometer and a salinometer (Autosal model 8400, Guildline Instruments), respectively. The temperature and salinity of the pumped water were measured with a sensor system for conductivity (or salinity), temperature, and pressure (SBE-11plus, Sea-Bird Electronics, Inc.). The salinity sensor on the system was calibrated with bottled seawater, the salinity of which had been measured with the salinometer. At 15 of the 31 stations, deeper seawater from depths of 25 to 800 m was collected with 12-liter, polyvinyl chloride bottles (Model 1010X NISKIN-X, General Oceanics, Inc.) equipped with another sensor system (SBE-11plus, Sea-Bird Electronics, Inc.). We collected about 20 dm 3 of seawater from each depth. The seawater was filtered through a 0.45 mm pore size membrane filter (HAWP14250, Millipore) and acidified on board by adding 40 cm 3 of concentrated nitric acid (Nitric Acid 70% AR, RCI Labscan, Ltd.) within 24 h after sampling.

Methods
Sample preparation. After the cruise, radiocesium in the seawater sample was concentrated on ammonium phosphomolybdate (AMP) in onshore laboratories for measurement of gamma-ray activity. The sample preparation was conducted in laboratories of four agencies: the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), the General Environmental Technos Co., Ltd. (KANSO), the Japan Marine Science Foundation (JMSF), and the National Institute of Radiological Sciences (NIRS). In the former two laboratories, the pH of the seawater sample was adjusted to 1.6, and 0.26 (or 0.39) g of cesium chloride (.98.0%, KANTO Chemical Co., Inc.) was added to the seawater as a carrier. Then 4 (or 6) g of AMP, made from hexaammonium heptamolybdate tetrahydrate (.98.0%, KANTO Chemical Co., Inc.) and phosphoric acid (85%, Wako Pure Chemical Industries, Ltd.), was added to the seawater and mixed well for two hours to form an AMP/Cs compound. The compound was stored overnight and then filtered onto a paper filter (Quantitative Filters Papers 5C, Tokyo Roshi Kaisha, Ltd.). After drying at room temperature, the compound on the filter was transferred to a teflon tube (5 cm 3 ) for gamma-ray measurement. The recovery of radiocesium from the seawater into the AMP/Cs compound in the tube was estimated to be about 95%. These procedures basically follow a protocol described in the literature 49 . The JMSF and NIRS laboratories used similar AMP methods 50,51 . The recoveries of radiocesium at the JMSF and NIRS laboratories were about 95 and 91%, respectively.
Analyses. The radiocesium activity in the AMP/Cs compound was measured in the laboratories of the Mutsu Oceanographic Institute/JAMSTEC, Low Level Radioactivity Laboratory/Kanazawa University (LLRL/KU), and the NIRS. In JAMSTEC, the radiocesium was measured with low-background Ge-detectors (Welltype GCW2022-7915-30-ULB, Canberra Industries, Inc.), which were calibrated with gamma-ray volume sources (Eckert & Ziegler Isotope Products) certificated by Deutscher Kalibrierdienst (DKD). The gamma counting time ranged from a day to a week, and 134 Cs and 137 Cs activities were evaluated from gamma-ray peaks at 605 and 661 keV, respectively. The averages of the detection limits (3 standard deviations) of the 134 Cs and 137 Cs measurements were calculated to be 0.53 and 0.20 Bq m 23 , respectively. In the case of the 605 keV photopeak from 134 Cs, the cascade summing effect was corrected. The factor for the summing effect was about 2, which was calculated as the difference between the 134 Cs/ 137 Cs ratios at a distance of 15 cm from the detector and in the well hole of the detector. The averages of the analytical uncertainties (standard deviations) for the 134 Cs and 137 Cs measurements were calculated to be 13% and 7%, respectively. These uncertainties arose from the gamma counting, the calibration, and the correction for the summing effect. The radioactivity of 137 Cs in a certified reference material for radionuclides, a water sample from Irish Sea (IAEA-443) 52 , was measured in the JAMSTEC laboratory. Results (0.36 6 0.02 Bq kg 21 , decay-corrected to 1 January 2007) agreed well with the radioactivity of 137 Cs in the certified seawater. The radiocesium activity was also measured in the LLRL/KU laboratory with low-background Ge-detectors 51,53 . The averages of the detection limits for the 134 Cs and 137 Cs measurements in the LLRL/KU laboratory were 0.16 and 0.05 Bq m 23 , respectively. The averages of the analytical uncertainties for 134 Cs and 137 Cs were calculated to be 11 and 6%, respectively. In the NIRS laboratory, the radiocesium activity was measured with Ge-detectors (GX-2019, Canberra Industries, Inc.). The uncertainties of radiocesium measurements in the NIRS laboratory (14% and 6% for 134 Cs and 137 Cs, respectively) were nearly equal to those in the JAMSTEC and LLRL/KU laboratories. The detection limits (2.2 Bq m 23 and 1.4 Bq m 23 for 134 Cs and 137 Cs, respectively), however, were higher than those in the JAMSTEC and LLRL/KU laboratories. Measurements of 134 Cs and 137 Cs activities in AMP/Cs compounds derived from certified reference materials (IAEA-443 and 445), which were prepared by KANSO, among the three laboratories resulted in good agreement within uncertainties. This agreement confirmed the comparability of the radiocesium measurements at the three laboratories.