Vertical distributions of plutonium isotopes in marine sediment cores off the Fukushima coast after the Fukushima Dai-ichi Nuclear Power Plant accident

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident led to the release of large amounts of radionuclides into the atmosphere as well as direct discharges into the sea. In contrast to the intensive studies on the distribution of the released high volatility fission products, such as131I, 134Cs and137Cs, similar studies of the actinides, especially the Pu isotopes, are limited. To obtain the vertical distribution of Pu isotopes in marine sediments and to better assess the possible contamination of Pu from the FDNPP accident in the marine environment, we determined the activities of 239+240Pu and241Pu as well as the atom ratios of 240Pu/239Pu and241Pu/239Pu in sediment core samples collected in the western North Pacific off Fukushima from July 2011 to July 2012. We also measured surface sediment samples collected from seven Japanese estuaries before the FNDPP accident to establish the comprehensive background baseline data. The observed results of both the Pu activities and the Pu atom ratios for the sediments in the western North Pacific were comparable to the baseline data, suggesting that the FDNPP accident did not cause detectable Pu contamination to the studied regions prior to the sampling time. The Pu isotopes in the western North Pacific 30 km off the Fukushima coast originated from global fallout and Pacific Proving Ground close-in fallout.


Introduction
On 11 March 2011, a magnitude 9.0 earthquake, centered in the northwest Pacific about 130 km off the northeast coast of Japan, and the ensuing gigantic tsunami caused severe damage to the Fukushima Dai-ichi Nuclear Power Plant (FD-NPP).The cooling systems of some of the nuclear reactor units failed, resulting in hydrogen explosions in the reactor buildings and venting of gases, which caused large releases of radionuclides into the atmosphere.For example, the total released amount of 137 Cs to the atmosphere from the FDNPP reactors has been estimated to be in the range of 9.9-36.6PBq (Chino et al., 2011;Morino et al., 2011;Stohl et al., 2012).More than 70 % of the released radionuclides were deposited over the North Pacific (Yoshida and Kanda, 2012).In addition, highly contaminated water with large amounts of radionuclides, originating from desperate attempts to prevent reactor cores meltdowns by injecting water into the reactor units, was directly leaked or discharged into the North Pacific Ocean (Buesseler et al., 2011;Inoue et al., 2012;Tsumune et al., 2012).
Intensive studies on the high volatility fission products released into the ocean, such as 131 I, 134 Cs and 137 Cs, were carried out after the FDNPP accident, and data on the concentration and distribution of these products were immediately collected (Aoyama et al., 2013;Buesseler et al., 2011;Honda et al., 2012;Inoue et al., 2012).However, similar studies focusing on the possible released actinides, especially Pu isotopes, are limited.Pu isotopes are characterized by high chemical toxicity, radiotoxicity and long half-lives, and they have attracted much scientific and public concern.Investigating the impact of the FDNPP accident on the distribution of long half-live radionuclides in the environment is important for the long-term dose assessments (Yoshida and Kanda, 2012).In addition, as Pu isotopes are produced by the initial neutron capture reaction in 238 U in the nuclear reactors and directly used as the component of MOX fuel in the FDNPP unit 3 reactor (Burns et al., 2012), accurate determination of Pu isotopes in the environmental materials may provide important information to understand the reactors damages.
Abnormal atom ratios of 240 Pu/ 239 Pu (> 0.3) and activity ratios of 241 Pu/ 239+240 Pu (> 100) were reported in the surface soil and litter samples in the 20-30 km zone around the FDNPP, providing evidence for the atmospheric release of Pu into the terrestrial environment (Zheng et al., 2012a).Imanaka et al. (2012) and Yamamoto et al. (2012) also found 238 Pu/ 239+240 Pu activities ratios, higher than the global fallout value, for soil samples collected in Iitate Village and Okuma Town, at distances of 25-45 km from the plant.In the marine environment, we investigated the distribution of Pu isotopes in surface marine sediments in the Pacific 30 km off the FDNPP that were collected several months after the accident (Zheng et al., 2012b), and observed no significant variation in 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio compared with previously reported values in marine sediments in the western North Pacific.Meanwhile, Sakaguchi et al. (2012) found that Pu concentration in seawater sampled from the Pacific Ocean 50 km from the FDNPP site showed no extra component from the accident.However, the possible long-term Pu contamination in the marine environment remains unknown as no information is currently available on the Pu isotopes in the released radioactive liquids and in the FDNPP near-coastal (within 30 km) marine environment.Pu isotopes in the western North Pacific off Japan can be transported rapidly by oceanic currents, e.g. the Oyashio Current and the Kuroshio Current, and undergo advection and mixing (Buesseler et al., 2012;Lee et al., 2005;Zheng and Yamada, 2005).Thus, the possible Pu contamination from the accident needs further investigation before reaching a reliable conclusion.
Investigating the concentration of radionuclides in the marine sediments is important for the radiobiological assessment in the marine environment as the sediments are a possible source of continued contamination for the marine biota (Buesseler, 2012).The radiocesium ( 134 Cs and 137 Cs) originating from the accident was detected in the sinking particles collected from the deep sea (water depth 4810 m) in the western North Pacific, one month after the accident, suggesting the quick transportation of radionuclides to the deep sea via atmospheric deposition (Honda et al., 2013).Otasaka and Kobayashi (2012) observed that dissolved radiocesium from the FDNPP accident was advected southward where it deposited onto sediments along the Ibaraki coast early in the accident.Pu isotopes are more particle reactive than Cs and the sediment-water distribution coefficient (K d value) of Pu is two orders of magnitude higher than that of Cs (IAEA, 2004).In mid-April 1986, by a coincidence of timing, IAEA-MEL scientists moored automated time series sediment traps in the Ligurian Sea, and they found that in one month after the accident more than 50 % of the total 239+240 Pu inventory originating from Chernobyl and deposited in that region had transited through 200 m depth, while only 0.2 % of the corresponding 137 Cs deposition did so (Povinec et al., 1996).In the North Pacific, the Chernobyl-derived radiocesium was detected in the sinking particles collected at a depth of 780 m two months after the Chernobyl accident (Kusakabe et al., 1988).The resident time of Pu isotopes in the North Pacific was much shorter than that of radiocesium (Fowler et al., 1983;Honda et al., 2013).Thus Pu isotopes released from the FNDPP accident could be more quickly incorporated into sediments by the scavenging process than Cs, and the determination of Pu isotopes in the sediments should give reliable information about Pu contamination in the marine environment.
To compare the distributions of Pu isotopes in the marine sediments before and after the FNDPP accident, the background dataset needs to be established.Due to the potential applications of Pu isotopes as a chemical tracer for oceanic processes and as a source identifier for radioactive contamination, over the past decades, the distribution of Pu isotopes in the western North Pacific and its marginal seas have been studied intensively (Buesseler, 1997;Dong et al., 2010;Hong et al., 1999;Ito et al., 2007;Kim et al., 2003;Lee et al., 1998Lee et al., , 2003Lee et al., , 2004Lee et al., , 2005;;Lindahl et al., 2011;Liu et al., 2011a;Moon et al., 2003;Nagaya and Nakamura, 1992;Oikawa et al., 2011;Otosaka et al., 2006;Pettersson et al., 1999;Wang and Yamada, 2005;Zheng andYamada, 2005, 2006b, c).For the Japanese near-coastal marine environment, especially the estuaries, leaching of contaminated soils is another potential pathway for radioactive contamination in addition to the atmospheric deposition from the accident plume and the direct release of contaminated plant cooling waters (Bailly du Bois et al., 2012).Before the accident, 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in five estuaries in western and northern Japan had been investigated (Liu et al., 2011b;NIRS, 2010).However, for estuaries in eastern Japan, where it is geographically more possible to be contaminated from the FDNPP accident, the background data for Pu contamination assessment are limited.Our previous work reported the rapid assessment focused on only surface sediments (Zheng et al., 2012a).Although no detectable Pu was found, the possibility existed that quick downward migration of the deposited Pu in surface sediments could have occurred by diffusion and bioturbation effect since the sampling time was several months after the accident.Therefore, the vertical distributions of Pu in the sediment cores are important for a more complete assessment.In this work, we first measured Pu isotope concentrations in the surface sediments from seven eastern Japan estuaries facing the North Pacific, using samples collected from 2008 to 2010.Then we summarized the published results to establish the comprehensive baseline data of Pu distribution in the western North Pacific and its marginal seas.Next we determined the vertical distribution of Pu activities ( 239+240 Pu and 241 Pu) and Pu atom ratios ( 240 Pu/ 239 Pu and 241 Pu/ 239 Pu) in five sediment cores, collected in the western North Pacific after the FDNPP accident.Finally, we compared our obtained results with the baseline data to assess the impact of the possible Pu contamination in the marine environment due to the FDNPP accident.

Study area and sampling
Surface sediment samples (0-2 cm) were collected from seven estuaries (Sagami River, Oyodo River, Yoshino River, Oi River, Naka River, Kitakami River and Kako River) in eastern Japan facing the North Pacific during the years 2008 to 2010.For each estuary, 2-4 samples were collected.Five sediment core samples (ES4, 37 were collected by a multiple corer in the western North Pacific Ocean off the FDNPP site during the three cruises of MR-11-05, KH-11-07 and MR-12-02 from July 2011 to July 2012.The core samples were cut into 0.5 cm or 1 cm segments and stored in an on-board refrigerator until brought back to the land-based laboratory.The locations of the surface sediment sampling sites and the sediment core sampling sites in this study are shown in Figs. 1 and 2, respectively.

Analytical procedure
We modified the method for sample preparation from the literature work (Liao et al., 2008).Briefly, the sediment samples were dried at 105 • C for 24 h.Then they were placed in ceramic crucibles and ashed in a muffle furnace at 600 • C for 5 h.The ignition loss in each sample was determined by the decrease in sample weight before and after the ashing process.About 2.0 g dried sample was weighed out and spiked with 100 µL (ca. 1 pg 242 Pu) standard 242 Pu solution with a traceability to CRM 130 (plutonium spike assay and isotopic standard, New Brunswick Laboratory, USA) as yield monitor.The extraction of Pu was performed in a tightly covered Teflon tube with 20 mL concentrated HNO 3 on a hot plate at 180-200 • C for at least 4 h.A two-stage anion-exchange chromatographic method using AG 1X8 and AG MP-1M resins was employed to separate U and Pu and further purify Pu (Liao et al., 2008).The final sample was prepared in 0.8 mL 4 % HNO 3 media for Pu isotope analysis.The chemical yield of Pu for this employed sample preparation method was estimated to be in the range of 53 % to 85 % with a mean of 66 % ± 10 %.
The measurement of the concentration of Pu isotopes and Pu atomic ratios was done with a double-focusing SF-ICP-MS (Element 2, Thermo Finnigan, Bremen, Germany).An APEX-Q high-efficiency sample introduction system (Elemental Scientific Inc., Omaha, NE, USA) with membrane desolvation unit (ACM) and a conical concentric nebulizer was used.We used the SF-ICP-MS in the low resolution (LR) mode to utilize the maximum instrument sensitivity.
In addition, we replaced the normal skimmer cone with a high-efficiency cone (X-cone, Thermo Finnigan) to further increase the sensitivity of the SF-ICP-MS.All the measurements were made in the self-aspirating mode to reduce the risk of contamination by the peristaltic pump tubing.The detailed operation conditions and measurement parameters for this measuring method were described elsewhere (Zheng and Yamada, 2006a).A Pu isotope standard solution (NBS-947) with a known 240 Pu/ 239 Pu atom ratio was used for mass bias correction.Two ocean sediment reference materials (NIST-4357 and IAEA-368) were used for analytical method validation.The analytical results of the reference materials were in good agreement with the certified activity values and the reported Pu atom ratio values (Table 1S, see Supplement).

Baseline data for the Pu distribution in the western
North Pacific and its marginal seas before the FDNPP accident Pu isotopes in the western North Pacific and its marginal seas could be attributed to the global fallout and the Pacific Proving Ground (PPG) close-in fallout before the FNDPP accident.The PPG was the name used to describe a number of sites in the Marshall Islands and a few other sites in the Pacific Ocean, used by the United States to conduct nuclear testing between 1946 and 1962.The global fallout Pu has a 240 Pu/ 239 Pu atom ratio of 0.18 (Bowen et al., 1980;Kelley et al., 1999;Livingston and Povinec, 2002), while the PPG close-in fallout has been characterized by a higher 240 Pu/ 239 Pu atom ratio (> 0.30) (Buesseler, 1999;Diamond et al., 1960;Muramatsu et al., 2001).The 239+240 Pu activities and 240 Pu/ 239 Pu atom ratios in the surface sediments in Japanese estuaries as well as Tokyo Bay, Sagami Bay and Hiroshima Bay are summarized in Table 1 and the 239+240 Pu activities are presented in Fig. 1.As shown in Fig. 1, the concentration of 239+240 Pu activity in the Japanese estuaries is relatively low, ranging from 0.003 to 5.81 mBq g −1 .In particular, for the estuaries off the eastern coast of Japan facing the North Pacific, investigated in this study, the concentration of 239+240 Pu activities were typically below 0.400 mBq g −1 (Table 2S, Supplement).The highest concentration (5.81 mBq g −1 ) of 239+240 Pu activity was reported in the surface sediment of Sagami Nada (Zheng and Yamada, 2004).For all the estuary sediments investigated in this study, the 241 Pu activity was below the detection limit (2 mBq g −1 ) of the SF-ICP-MS.
The 240 Pu/ 239 Pu atom ratios in the Japanese near-coastal surface sediments are shown in Fig. 3, and they ranged from 0.170 to 0.270.All the values were generally higher than the value of global fallout, except for CM-04 and CM-06-08 in northern Japan.Zheng and Yamada (2006b) suggested CM-04 and CM-06-08 received only trace amounts of PPG close-in fallout Pu via atmospheric deposition.An average 240 Pu/ 239 Pu atom ratio of 0.231 ± 0.025 (n = 36) was obtained for the Japanese eastern estuaries facing the North Pacific, which indicated the mixing of global fallout Pu and PPG close-in fallout Pu.
A more comprehensive summarization about the distribution of Pu isotopes in the western North Pacific and its marginal seas in the literature (Buesseler, 1997;Dong et al., 2010;Hong et al., 1999;Ito et al., 2007;Kim et al., 2003;Lee et al., 1998Lee et al., , 2003Lee et al., , 2004Lee et al., , 2005;;Liu et al., 2011a;Moon et al., 2003;Nagaya and Nakamura, 1992;Oikawa et al., 2011;Otosaka et al., 2006;Pettersson et al., 1999;Wang and Yamada, 2005;Yamamoto et al., 1990;Zheng andYamada, 2005, 2006b, c) is given in Table 2.The concentration of 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in the surface sediments ranged from 0.002 to 5.38 mBq g −1 and from 0.150 to 0.281, respectively, except in the surface sediment collected near the Bikini Atoll, where the high concentration of 12.5 mBq g −1 and high atom ratio of 0.336 have been reported (Buesseler, 1997;Lee et al., 2005).In the Okinawa Trough and Sagami Bay, the 240 Pu/ 239 Pu atom ratios were found to increase with depth with a maximum of 0.30-0.33 in the deeper layer (> 5 cm) sediments (Lee et al., 2004;Wang and Yamada, 2005;Zheng and Yamada, 2006b), indicating clearly the early deposition of the PPG close-in fallout Pu.
Historically, 241 Pu was released into the environment by the nuclear weapon testing mainly conducted 50 yr ago and its half-live is relatively short (14.4 yr), so the current activity of 241 Pu in the environment is quite low.Yamamoto et al. (1990) determined the Pu isotopes in the sediments collected from the Nyu Bay in the Japan Sea, and they found that the 241 Pu activities and the 241 Pu/ 239+240 Pu activity ratios ranged from 4.5 mBq g −1 to 7.5 mBq g −1 and from 1.2 to 1.9, respectively.(For all the values related to 241 Pu discussed here, decay corrections have been made to 11 March 2011.)Similarly, Zheng and Yamada (2008) observed a 241 Pu activity of 8.4 mBq g −1 and a 241 Pu/ 239+240 Pu activity ratio of 1.2 at a depth of 18-20 cm in a sediment core collected from Sagami Bay.More comprehensive investigations were conducted by the Japanese government on the distribution of Pu isotopes in the surface marine sediments off Japan, and the results for the 241 Pu activity and 241 Pu/ 239+240 Pu activity ratio in the surface sediments were typically below 3.3 mBq g −1 and 1.1, respectively (MEXT, 2008).Due to the influence of the PPG close-in fallout, high 241 Pu activities (up to 19.3-33.4mBq g −1 ) and the 241 Pu/ 239+240 Pu activity ratios (2.2-2.7) were observed in the sediments near the Bikini Atoll (Lee et al., 2005).
For the inventory of 239+240 Pu, a wide range of values (1.38-693.33Bq m −2 ) have been observed.The vertical distribution of Pu isotopes in the sediments varied significantly due to the difference of bottom topography and sedimentation dynamics.For example, the enhanced particle scavenging in the continental margin resulted in higher Pu inventories in the sediments in the Okhotsk Sea (81-271 Bq m −2 ) compared to those from the global fallout areas (Zheng and   a : surface sediment 0-2 cm; ND: not determined.Yamada, 2006b).However, for the western North Pacific, the inventory of 239+240 Pu ranged from 2.8 to 71.8 Bq m −2 (Moon et al., 2003).Thus the inventory of 239+240 Pu in the sediments cannot provide a direct index for source identification.Isotopic composition information should be combined with the inventory values to reach an accurate conclusion.In summary, the 239+240 Pu activity and the 240 Pu/ 239 Pu atom ratio in the upper layer sediments (< 5 cm) in the western North Pacific and its marginal seas off Japan before the FDNPP accident could be considered as lower than 5.81 mBq g −1 and 0.28, respectively.For the deeper layer sediments (> 5 cm), the 240 Pu/ 239 Pu atom ratio could reach about 0.30 due to the earlier deposition of the PPG close-in fallout.The background values for 241 Pu activity and 241 Pu/ 239+240 Pu activity ratio in the sediments in the Japanese coastal areas could be lower than 10 mBq g −1 and 3, respectively.The inventories of Pu vary significantly depending on the different bottom topography and sedimentation dynamics.

Vertical distributions of Pu in sediment cores from
the western North Pacific after the FDNPP accident

Vertical distribution of Pu activities and Pu inventories
The analytical results of Pu isotopes in the sediment core samples are summarized in Table 3 and plotted in Fig. 4. The surface 239+240 Pu activities in the sediments of ES4, MC1, MC5 and FS1 ranged from 0.476 to 2.809 mBq g −1 .When  we compare them with the baseline data of the adjacent open oceans of the Pacific Ocean, there was no significant increase after the accident (Zheng et al., 2012b).For the F1 station, the surface 239+240 Pu activity was 1.774 mBq g −1 , which was comparable to that (1.580 mBq g −1 ) of the MC1 station.The F1 station is close to the MC1 station and the F1 sampling date (7 July 2012) was one year after the MC1 sampling date (18 July 2011).The similar surface 239+240 Pu activity for these two stations indicated that there was no remarkable change for 239+240 Pu activity in the surface sediments in the western North Pacific about 110 km southeast of the FDNPP during the last year (July 2011 to July 2012).The highest 239+240 Pu activity (3.079 mBq g −1 ) was observed in the upper layer (1-2 cm) of FS1, which was lower than the upper limit of the baseline data (5.38 mBq g −1 ) before the FNDPP accident as we concluded in Sect.3.1.As shown in Fig. 4, the vertical profiles of 239+240 Pu activities in the five sediment cores showed different patterns.For the cores of MC1 and FS1, a surface 239+240 Pu maximum was observed and the 239+240 Pu activities decreased with core depth.For the cores of F1 and MC5, approximately uniform 239+240 Pu activities were found from the surface to a depth of 5 cm and 8 cm, respectively.Similar observations were reported by Zheng et al. (2006b) for the 239+240 Pu distribution in two sediment cores (CM-03 and CM-04) from the western North Pacific (Fig. 4).Sediment particles mixing by bioturbation and/or physical factors after the deposition are thought to be the controllers of the surface mixing layer (SML) (Zheng et al., 2006b).For the marine environment near the Japanese coast, the M9.0 East Japan Earthquake was suggested to be another important factor that caused the displacement of the seafloor and resulted in turbulent diffusion of the seafloor sediments (Kawagucci et al., 2012).The SML depth (8 cm) in CM-04 was comparable with that of F1 and MC5.A subsurface maximum of 239+240 Pu activity was found in the sediment core of ES4.Usually, in areas with a high sedimentation rate, the subsurface maximum has corresponded to the year 1963, when the largest global fallout of Pu occurred (Hong et al., 1999).Moon et al. (2003) reported relatively low sedimentation rates (0.12-0.35 cm ky −1 ) in the western North Pacific after they investigated the 14 C and 230 Th ex distributions in the sediment profiles.As the water depth exceeds 5000 m at the ES4 station, the sedimentation rate is not a key factor controlling the vertical distribution of Pu isotopes there.However, the biological activity in the seafloor in the western North Pacific is high, evidencing a high bioturbation rate in the surficial marine sediment (Harada and Shibamoto, 2002).The penetration depth of radionuclides into the seabed can be measured and then related to mixing rate and sedimentation rate by incorporating the dispersion-advection equation (Demaster et al., 1985): where Z is the penetration depth (cm), D b is the mixing coefficient (cm 2 yr −1 ), T is the elapsed time (yr) since deposition and S is the sedimentation rate (cm yr −1 ).The particle mixing coefficient ranges from 0.02 to 1.00 cm 2 /y in the western North Pacific (Moon et al., 2003).If we assume that 48 yr have passed since 1963, the penetration depth of Pu in the western North Pacific can be roughly estimated to be 1.4-9.9cm by Eq. ( 1).This is consistent with the result (8 cm) we observed at the ES4 station.Similarly, Harada and Shibamoto (2002) observed that the penetration depth www.biogeosciences.net/10/2497/2013/Biogeosciences, 10, 2497-2511, 2013 of excess 210 Pb was 7.2 cm in the same sea area of ES4.The vertical profiles of 239+240 Pu activity and ignition loss in the sediment core samples are shown in Fig. 1S (see Supplement).No general relationships between Pu concentration and ignition loss were found from our results.High 241 Pu activities (4.5-34.8mBq g −1 ) and 241 Pu/ 239+240 Pu activity ratio (107.8) in the surface soil and litter samples from the 20-30 km zone around the FNDPP were observed, evidencing the contamination from the accident (Zheng et al., 2012b).However, for the five sediment cores investigated in this study, a detectable 241 Pu activity (7.42 mBq g −1 ) only occurred at the ES4 station at a depth of 3-4 cm where the 239+240 Pu activity peaked.This value was typically in the range (< 10 mBq g −1 ) of the background database before the FNDPP accident.Meanwhile, the 241 Pu/ 239+240 Pu activity ratio (2.5), comparable with reported results before the accident, was significantly lower than that derived from the FDNPP accident.
Plutonium inventory in sediment columns reflects the source function, transport of Pu in water and on particles, and the mixing process; thus it is an important parameter for geochemical study of Pu.The inventory of Pu in a sediment core can be estimated by summing the Pu concentration in each layer of the sediment (Livingston et al., 2001): where I s is the Pu inventory in a sediment column (Bq m −2 ), M is the number of sediment layers (the layer number is counted downward from the surface to the deeper layer), S i is the Pu concentration in layer i (Bq kg −1 , dry weight), D i is the dry bulk density of layer i (kg m −3 ), U i is the upper boundary of layer i (m) and L i is the lower boundary of layer i (m).The obtained Pu inventories for ES4, FS1, MC5, MC1 and F1 were 29.3, 131.7, 95.5, 32.6 and 77.2 Bq m −2 , respectively.As shown in Fig. 5, the inventories varied significantly.The lowest inventory was observed at ES4 station and the relatively low Pu accumulation may be due to the deep water depth and the low sedimentation rate (Bu et al., 2012).The highest inventory was found at the FS1 station, which was the nearest one to the Fukushima coast among the five sampling stations.High Pu inventories seem to be a common phenomenon in the coastal sediments in the western North Pacific and its marginal seas.For example, Lee et al. (2004) observed extremely high Pu inventories (201-693 Bq m −2 ) in the Southern Okinawa Trough.Zheng et al. (2005Zheng et al. ( , 2006a) ) reported that the Pu inventories in the Japan Sea and the Okhotsk Sea ranged from 5.7 to 241 Bq m −2 and from 81 to 271 Bq m −2 , respectively.The advective westward transport of Pu from the Marshall Islands by the North Equatorial Current followed by northward transport of Kuroshio Current was suggested to be an important factor causing the high Pu input (Lee et al., 2004;Zheng et al., 2004Zheng et al., , 2006c)).The Pu inventory (77.2 Bq m −2 ) at the F1 station is more than two times that (32.6 Bq m −2 ) at the MC1 station.These two stations are off the Ibaraki coast and are very close geographically.In the coastal area of Ibaraki, Otosaka et al. (2012) investigated the distribution of radiocesium in the sediments collected from near-coast to off-coast sites over some time after the FDNPP accident; and they found that the radiocesium, first appeared in the near-coast region, and then was transported to the off-coast region through suspended fine particles in a high-turbidity layer.However, the Pu inventory variation between the F1 and MC1 stations could not be simply explained as Pu accumulation from the near-coast region during the last year (July 2011 to July 2012).A more comprehensive comparison can be done with the results of the CM-03 station.The CM-03 station, which is located 200 km southeast of the FDNPP, is the only area close to the FNDPP for which the vertical distribution of 239+240 Pu activity before the accident is known.In the CM-03 sediment core, the Pu inventory was reported to be 74 Bq m −2 (Zheng and Yamada, 2006b), which was comparable with the result of F1.

Vertical distribution of Pu atom ratios and the possible sources of Pu
Pu atom ratios ( 240 Pu/ 239 Pu and 241 Pu/ 239 Pu) have been demonstrated to be good indicators for the Pu source identification.For instance, the atom ratio of 240 Pu/ 239 Pu in weapon grade Pu is typically around 0.05 and higher ratios can be expected with higher neutron fluxes associated with an increase in the yield of a nuclear detonation (Buesseler, 1997;Koide, et al., 1985).For a nuclear reactor, the Pu atom ratios can vary from 0.23 to 0.65 depending on the fuel characteristics, type and design of the reactor, operational conditions of the plant and cooling time since the fuel was last irradiated in the reactor (Ketterer and Szechenyi, 2008;Schwantes et al., 2012;Taylor et al., 2001).The 240 Pu/ 239 Pu and 241 Pu/ 239 Pu atom ratio fingerprints for the Pu isotopes released from the FDNPP accident were suggested to be 0.303-0.330and 0.103-0.135,respectively, and they were significantly higher than the global fallout values (Zheng et al., 2012b).
As shown in Table 3 and Fig. 4, the 240 Pu/ 239 Pu atom ratios in the five core samples have four major characteristics: (1) the ratios ranged from 0.188 to 0.293 and were typically higher than the reported global fallout value (0.18); (2) all the cores showed a nearly uniform distribution of 240 Pu/ 239 Pu atom ratio in the upper sediment layers; (3) the 240 Pu/ 239 Pu atom ratios increased from a definite depth in the cores of MC1, F1 and FS1, while the ratios stayed almost uniform in the cores of MC5 and ES4; and (4) the inventory-weighted 240 Pu/ 239 Pu atom ratio (0.202) at the ES4 station was lower than the ratios of the other four stations (FS1, 0.236; MC5, 0.240; MC1, 0.240; F1, 0.243).
We can see that almost all the sediment samples investigated in this study showed higher 240 Pu/ 239 Pu atom ratio than the global fallout.However, a higher 240 Pu/ 239 Pu atom ratio was observed in a wide range of sea areas in the western North Pacific before the FNDPP accident due to the presence of the PPG close-in fallout as discussed in Sect.3.1.As the majority of the tests at the PPG were conducted at or just above the earth's surface and the maximum deposition occurred in 1954, the injection of Pu from the PPG as tropospheric deposition to the western North Pacific happened before the global fallout injection.Therefore, the higher 240 Pu/ 239 Pu atom ratios in the deeper layers in the sediment cores of MC1, F1 and FS1 retained their validity as a record for the PPG close-in fallout.In Sagami Bay, on the western North Pacific margin, a similar distribution for the 240 Pu/ 239 Pu atom ratio in the sediment core was also observed (Zheng and Yamada, 2004); that is, a nearly uniform 240 Pu/ 239 Pu atom ratio of about 0.24 distributed from the surface down to the layer 12-14 cm deep, and a ratio higher than 0.27 starting from the layer at 16-18 cm depth.Our observed results provided further evidence for the hypothesis that the oceanic processes (North Equatorial Current and the Kuroshio Current) transported the PPG close-in fallout Pu westwards as far as the Japanese coast.The uniform distribution of the 240 Pu/ 239 Pu atom ratio in the sediment cores of MC5 and ES4 could be due to the mixing process caused by bioturbation and/or physical factors.Unlike the other four stations, the ES4 station is located in the pathway of the southward flowing Oyashio Current, which carries water bearing global fallout Pu.The CM-04 station is located in northern Japan in the western North Pacific and is affected by the Oyashio Current as well, and it showed a typical distribution of 240 Pu/ 239 Pu atom ratios which was the same as the global fallout.Thus the trace of PPG-derived Pu at the ES4 station could be from direct fallout in that region when the nuclear weapon tests were conducted.
We have concluded in Sect.3.1 that the 240 Pu/ 239 Pu atom ratio in the western North Pacific was typically below 0.28 in the upper layer (< 5 cm) sediments before the FDNPP accident.For all the sediment cores investigated in this study, the upper layer sediment 240 Pu/ 239 Pu atom ratio was below the background baseline limit.The deeper layer sediments, characterized by high 240 Pu/ 239 Pu atom ratio in FS1, MC1 and F1 were due to the early deposition of the PPG close-in fallout.Thus no significant amounts of Pu injection from the FDNPP accident, which has a high 240 Pu/ 239 Pu atom ratio (> 0.30), were observed in the investigated regions.The data on the distribution of 241 Pu give further support to this conclusion.No 241 Pu was detected for all the samples except one at the ES4 station with a concentration of 7.42 mBq g −1 and a 241 Pu/ 239 Pu atom ratio of 0.0029.This 241 Pu/ 239 Pu atom ratio was similar to the result (0.0033) we observed in the sediment at ES2 (Zheng et al., 2012a) and results (0.0031-0.0033) in the sediment near the Bikini Atoll reported by Lee et al. (2005).As the Fukushima-derived 241 Pu/ 239 Pu atom ratio was much higher than the observed result, the presence of 241 Pu in ES4 further evidenced the PPG source contribution to that region.
Presuming that the 240 Pu/ 239 Pu atom ratio for global fallout is 0.18 (Kelly et al., 1999), and that the representative 240 Pu/ 239 Pu atom ratio for the PPG close-in fallout is 0.36 (Buesseler, 1999;Diamond et al., 1960;Muramatsu et al., 2001) member-mixing model (Krey et al., 1976): where (Pu) is the 239+240 Pu activity; R is the 240 Pu/ 239 Pu atom ratio; and the subscript P , G and S refer to PPG closein fallout, global fallout and measured sediment samples, respectively.Via Eq. ( 3), we calculated the inventory-weighted percentages of the PPG close-in fallout in each sediment core.
As shown in Fig. 6, we found that the PPG close-in fallout contributed a nearly constant percentage (ca.40 %) of the Pu inventory in the sediment cores in the western North Pacific off the eastern coast of Japan except for ES4.As discussed above, ES4 is located in the pathway of the Oyashio Current, and received only a trace of directly deposited Pu from the PPG close-in fallout.For stations F1 and MC1, although the Pu inventories in these two close stations showed significant variation, the sources of Pu in them were identical (the PPG close-in fallout Pu and global fallout Pu), suggesting no detectable Pu contamination originating from the FDNPP accident injected to that region during the year since the accident.

Conclusions
The vertical profiles of Pu isotopes in five sediments cores obtained in the western North Pacific from July 2011 to July 2012 after the FNDPP accident as well as surface sediments in seven Japanese estuaries collected before the FNDPP accident were investigated.We first established the comprehensive background baseline data for the Pu distribution in the sediments in the western North Pacific and its marginal seas before the FNDPP accident.Then we compared the results of the five sediment cores with the baseline data.We could not identify any extra Pu injection from the FDNPP accident in the marine sediments collected outside the 30 km zone from the plant site up to the time of sampling (July 2012).The global fallout and the PPG close-in fallout Pu were the two main sources for the Pu contamination in the marine environment outside the 30 km zone.It has been modeled recently that if any Pu contamination from the FDNPP accident occurred in the sea, it would remain in a very close area around the power plant because of the low mobility of plutonium isotopes in the marine environment (Perianez et al., 2013).The Pu contamination situation in the marine environment within the 30 km zone around the FDNPP needs further investigation before a more comprehensive conclusion can be reached.

Fig. 1 .
Fig. 1.Map showing the Japanese estuaries and the Pu concentration (mBq g −1 ) in the surface sediments (0-2 cm) in the Japanese near-coast marine environment.The yellow circled sites represent the sampling sites in this study.The red circled sites are from the literature (NIRS, 2010; Zheng and Yamada, 2005, 2006b, c).The surface sediment Pu concentration range was between 0.003-5.81mBq g −1 .

Fig. 2 .
Fig. 2. Map showing the locations of the sediment sampling sites in the western North Pacific and the Kuroshio and Oyashio Currents.Stations CM-04 and CM-03 are from Zheng and Yamada (2006b).

Fig. 6 .
Fig. 6.The percentage of Pu that originated from global fallout and PPG close-in fallout in the sediment cores.The data for CM-03 were calculated from Zheng and Yamada (2006b).
, we could estimate the contribution of Pu source to the Pu inventory in the western North Pacific by using a two-end wwwet al.: Vertical distributions of plutonium isotopes in marine sediment cores

Table 1 .
Summary of the distributions of 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in the surface sediments (0-2 cm) from Japanese estuaries.
ND: not determined.

Table 2 .
Summary of the distributions of 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in sediments from the western North Pacific and its marginal seas.

Table 3 .
Results of Pu activities and Pu atom ratios in the sediment cores.