Plutonium isotopes in settling particles: transport and scavenging of Pu in the western Northwest Pacific

Effects of the Hydroxy-Regulated Li-Montmorillonite Atomic Interface Arrangement on Li Cycle of Marine Sedimentation and pH

The reaction of ubiquitous clay is related to the global cycle of the key metals, but the relationship between the Li occurrence interface and the sedimentation in the Li cycle remains unclear. We investigated the atomic interface arrangement of Li-montmorillonite (Li-Mt) during low-temperature water-rock reactions and Li migration. The results show that, in Cl-rich systems, deprotonation and exposure of Na adsorption sites cause Li enrichment and O pairing, which lead to the weakening of the shielding effect of Mt on anions and the formation of a Mt-Li-Cl atomically interfacial arrangement. Only up to 20.3% of the Li is contained in the atomic interface of Li-Mt. In F-rich system, the dehydroxylation of F paired with Al in octahedral sites causes Li accumulation via local crystallization of LiF, and co-complexation of F and Li forms a Mt(Al)-F-Li atomic interface, in which up to 46.8% of the Li is enriched by the Mt. The participation of F and Cl in the complexation intensifies lattice collapse of the Li-Mt edge. The sedimentation velocity decreases with the smaller particle size affected by the Li loading. Lithium leached from igneous rocks serves as the marine Li source, which contributes up to 99.8% and 99.5% of the Li in Cl- and F-rich systems, respectively. The response of Mt(OH) to Li migration with a time accumulating effect may make an important regulatory of oceanic pH by either acidification or alkalization.

Spatial distribution and modelling of 239+240Pu in the sediments and seawater columns of the South China Sea and Indian Ocean

In order to investigate the 239+240Pu potential influence in the ocean, and develop a new method for rapidly monitoring radioactive pollution, the 239+240Pu spatial distribution in the South China Sea (SCS) and the Indian Ocean (IND) sediments is analyzed by SF-ICP-MS (ELEMENT 2). The inventory-weighted mean activities of 239+240Pu were 0.413 ± 0.333 mBq/g, 0.128 ± 0.044 mBq/g, and 0.483 ± 0.606 mBq/g in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 239+240Pu activity spatial distribution in the SCS sediments was influenced by the current, the vertical distribution of Pu in seawater, and the transport of particulate matter. The 239+240Pu activity spatial distribution in the IND sediments could be impacted by Antarctic Intermediate Water. The average of 240Pu/239Pu atomic ratios were 0.258 ± 0.034, 0.219 ± 0.031, and 0.212 ± 0.028 in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 240Pu/239Pu atomic ratios in the SCS and IND indicate that Pu from the Pacific Proving Ground (PPG) is transported to the IND via the SCS internal current and transverse ocean currents within Indonesia. In addition, a seawater advection-dispersion equation (S-ADE) model is established based on the actual physical processes of radionuclides in the seawater column and well fitting results were obtained (R2 = 0.49 to 0.99). The 239+240Pu data and the geographic information from the sample site were used to correct the Pu distribution in the seawater. The calculated 239+240Pu mean concentrations in the surface seawater were 2.465 mBq/m3 and 2.205 mBq/m3 for the SCS and the eastern IND seawater, respectively, and the result is consistent with the previous measurements. Then, the 239+240Pu stored in the study area of SCS and eastern IND was estimated to be approximately 1.0-1.4% of the global ocean based on the model. This study provides a useful model for guiding and designing future monitoring of pollution by anthropogenic Pu and other isotopes.

Spatial distribution, source identification, and transportation paths of plutonium in the Beibu Gulf, South China Sea

To investigate the spatial distribution and source of plutonium isotopes in the Beibu Gulf, surface sediments were collected and analyzed using sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). The activities of 239+240Pu in surface sediments ranged from 0.012 to 0.451 mBq/g (mean: 0.171 ± 0.138 mBq/g, n = 36), indicating a decreasing trend in a counterclockwise direction from the southern bay mouth. The counterclockwise decreasing trend in the south of the bay mouth is similar to the current in the Beibu Gulf. The 240Pu/239Pu atom ratios in surface sediments ranged from 0.156 to 0.283 (mean: 0.236 ± 0.031, n = 36), slightly higher than that of the global fallout value of 0.18. This suggests that the Pu in the Beibu Gulf was a combination of global fallout and Pacific Proving Ground (PPG). The average contribution of the plutonium (Pu) derived from the PPG in the sediment was estimated to be 52 % ± 24 %.

Assessment of soil-soil solution distribution coefficients of global fallout 237Np and 239Pu in Japanese upland soils

Neptunium-237 and ²³⁹Pu are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular importance for their radiation dose evaluation; therefore, in this study, we have made the assessment of the soil-soil solution distribution coefficient (K_d, L/kg) using global fallout ²³⁷Np and ²³⁹Pu in Japanese upland soils. The K_d values were determined by extracting these radionuclides from 24 soil samples using a laboratory batch method. The desorption K_d values of ²³⁷Np ranged from 3.3 × 10² to 1.0 × 10⁴ L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 1.7 × 10³ L/kg and 2.6 × 10³ L/kg, respectively. The desorption K_d values of ²³⁹Pu were found to vary from 9.4 × 10³ to 7.1 × 10⁴ L/kg, and their GM and AM were 3.3 × 10⁴ L/kg and 4.0 × 10⁴ L/kg, respectively. In Japanese upland soils, the K_d value of ²³⁹Pu was one order of magnitude higher than that of ²³⁷Np.

Radioactivity research in mosses from typical Karst Regions in Leye Tiankeng, Southern China

Radionuclides in environmental ecosystems have ecotoxicity and health impact on human and environment, so radioactive contamination has always been one of the global concerns. This study mainly focused on the radioactivity of mosses collected from the Leye Tiankeng Group in Guangxi. The activities of ^239+240Pu measured by SF-ICP-MS and ¹³⁷Cs measured by HPGe in moss and soil samples are as follows: 0-2.29 Bq/kg in mosses and 1.5-11.9 Bq/kg in soils for ¹³⁷Cs, and 0.025-0.25Bq/kg in mosses and 0.07-0.51Bq/kg in soils for ^239+240Pu. The range of ²⁴⁰Pu/²³⁹Pu atom ratios (0.201 in mosses and 0.184 in soils) and ^239+240Pu/¹³⁷Cs activity ratios (0.128 in mosses and 0.044 in soils) indicated that the ¹³⁷Cs and ^239+240Pu in study area were mainly contributed by global fallout. ¹³⁷Cs and ^239+240Pu showed similar distribution in soils. However, their behaviors in mosses were quite different due to the differences in the growth environment of mosses. The transfer factors of ¹³⁷Cs and ^239+240Pu from soil to moss varied in different growth stages and specific environments. A weak positive correlation among ¹³⁷Cs, ^239+240Pu in mosses and soil-derived radionuclides suggested that resettlement was predominant here. The negative correlation between ⁷Be, ²¹⁰Pb and soil-derived radionuclides indicated that ⁷Be and ²¹⁰Pb came from atmospheric components, while the weak correlation between them suggested that their specific sources were different. The Cu and Ni were moderately enriched in mosses here due to the use of agricultural fertilizers, At the same time, Zn was at a high level in the Lilang area, where transportation was more developed.

Plutonium-based radiometric dating of rapidly accumulated sediments in the Sanyuan sinkhole, southern Chinese Loess Plateau

Radionuclides, such as ²¹⁰Pb, ¹³⁷Cs and ^239,240Pu, have been widely used for dating recent sediments in terrestrial and marine environments, while ¹²⁹I, as an important artificial radionuclide in the environment, is also a potential tracer for sediment dating and environmental process studies. However, they were not always successfully applied to sediment dating because of their different sources, half-lives, environmental behaviour and measurement techniques. The dating applicability of these nuclides in a sedimentary environment with rapid accumulation on land was investigated for sinkhole sediment from the southern Chinese Loess Plateau. Our results showed that ²¹⁰Pb and ¹³⁷Cs could not be adequately used for dating the sediments due to the difficulties in accurately measuring ¹³⁷Cs and excess ²¹⁰Pb (²¹⁰Pb_ex) signals caused by the dilution effect of rapid accumulation. ¹²⁹I is not an ideal dating tracer because of its multisource feature causing no remarkable peak value in the sediment cores. The depth distribution of ^239,240Pu in the sediment core showed a single peak corresponding to its maximum fallout in 1963 from the atmospheric nuclear weapons test, suggesting that Pu isotopes have significant advantages in dating recent sediments. The sensitive inductively coupled plasma-mass spectrometry (ICP-MS) measurement technique enables the determination of very low levels of ²³⁹Pu and ²⁴⁰Pu and makes ^239,240Pu a suitable tracer for dating the rapidly accumulated sediment. Based on the ^239,240Pu mass balance equation estimation and field observations, we proposed the water-eroded input from soil surrounding the sinkhole as another vital source of the sediments in addition to the aeolian contribution.

Enhanced boundary scavenging of 241Am on the continental margin of the East China sea

The 241Am concentrations were determined by sector field high-resolution inductively coupled plasma mass spectrometry for settling particles and surface sediments collected on the continental margin in the East China Sea to discuss the behavior of 241Am in the studied area. A large and regular increase was noted from 97 m depth to 592 m depth in 241Am concentrations in settling particles from the cylindrical sediment traps, whereas only a slight increase in 239+240Pu concentrations was evident. The gradual increase in 241Am/239+240Pu activity ratios from the cylindrical trap settling particles also was observed. These results were attributed to 241Am being scavenged by settling particles at a more rapid scavenging rate than 239+240Pu. There was an obvious tendency for 241Am fluxes to increase linearly with depth with the highest fluxes at near-bottom, and these fluxes were due to large advective import to the near-bottom. There was a tendency for 241Am concentrations in surface sediments to increase almost linearly with depth, showing the same tendency as 210Pb concentrations, and this tendency indicated that the enhanced boundary scavenging of 241Am as well as 210Pb occurred in the East China Sea continental slope region due to the greater abundance of reactive finer particles. The 241Am concentrations in settling particles from the time-series sediment trap ranged from 1.05 to 4.20 mBq g-1, showing large variation. The 241Am concentrations from the time-series trap varied inversely with the total mass fluxes, indicating that 241Am concentrations were diluted by particles when mass flux increased. The 241Am fluxes showed large time-series variations and these variations were primarily controlled by mass flux variation. It seems that the high total mass flux events are due to resuspension of the underlying surface sediments and episodic lateral transport of particles that flow down the continental slope in the East China Sea.

Plutonium isotopes in the North Western Pacific sediments coupled with radiocarbon in corals recording precise timing of the Anthropocene

Plutonium (Pu) has been used as a mid-twentieth century time-marker in various geological archives as a result of atmospheric nuclear tests mainly conducted in 1950s. Advancement of analytical techniques allows us to measure ²³⁹Pu and ²⁴⁰Pu more accurately and can thereby reconstruct the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here, we propose a novel method that couples annual banded reef building corals and nearshore anoxic marine sediments to provide a marker to precisely determine the start of the nuclear era which is known as a part of the Anthropocene. We demonstrate the efficacy of the methods using sediment obtained from Beppu Bay, Japan, and a coral from Ishigaki Island, Japan. The sedimentary records show a clear Pu increase from 1950, peaking during the 1960s, and then showing a sharp decline during the 1970s. However, a constantly higher isotope ratio between ²³⁹Pu and ²⁴⁰Pu suggest an additional contribution other than global fallout via ocean currents. Furthermore, single elevations in ²⁴⁰Pu/²³⁹Pu provide supportive evidence of close-in-fallout similar to previous studies. Coral skeletal radiocarbon displays a clear timing with the signatures supporting the reliability of the Beppu Bay sediments as archives and demonstrates the strength of this method to capture potential Anthropocene signatures.

Distributions and impacts of plutonium in the environment originating from the Fukushima Daiichi Nuclear Power Plant accident: An overview of a decade of studies

This paper reviews the current knowledge on plutonium (Pu) isotopic composition (the atom or activity ratios) and activity concentrations of ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu resulting from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. In this critical review, we document the characteristic values of Pu atom or activity ratios (fingerprints) and present their spatial distributions around the FDNPP site. Based on multiple Pu fingerprints (²³⁸Pu/^239+240Pu activity ratio, ²⁴⁰Pu/²³⁹Pu atom ratio, and ²⁴¹Pu/²³⁹Pu atom ratio), we clarify that Pu contamination from the FDNPP accident occurred in a restricted terrestrial area, while Pu in the Northwest Pacific Ocean is still predominately sourced from the Pacific Proving Grounds (PPG) and global fallout. Using a simple two end-member mixing model, we calculate average contributions of Pu from the FDNPP accident of 13 ± 20% (n = 180) in soil samples, 55 ± 32% (n = 38) in leaf litter samples, and 67 ± 26% (n = 129) in air dust/black substances. In the marine environment, the PPG source average contributions are 45 ± 15% (n = 76) in seawater and 42 ± 12% (n = 48) in sediments. The spatial distributions of Pu atom or activity ratios based on existing studies suggest that: 1) in the terrestrial region investigated 80 km northwest of the FDNPP site, the Pu contamination is mainly observed in an area within a 50 km distance, and 2) in the terrestrial region investigated 60 km southwest of the FDNPP site, the Pu contamination is mainly observed in an area within a 30 km distance. Studies of Cs-bearing radioactive particles indicate that Pu occurs as Pu oxide, and the fuel fragments containing Pu that were released from the reactors to the surrounding environment are associated with micron-scale Cs-bearing radioactive particles. We note that the fractionation between Pu and other radionuclides occurred after release. These new findings about the Pu fingerprints around the FDNPP site will help researchers to establish a reference background database for future environmental risk assessment and geochemical study there.

Soil-soil solution distribution coefficients of global fallout 239Pu and 237Np in Japanese paddy soils

²³⁹Pu and ²³⁷Np are long-lived radionuclides that emit alpha rays, and once released into the environment, they are present there for a long period of time. Therefore,²³⁹Pu and ²³⁷Np are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular interest; therefore, in this study, we have made the first-time determination of the soil-soil solution distribution coefficient (K_d, L/kg) using global fallout ²³⁹Pu and ²³⁷Np in soil. The K_d values were determined by extracting these radionuclides from 23 soil samples using a laboratory batch method. The desorption K_d values of ²³⁹Pu were found to vary from 3.2 × 10³ to 1.4 × 10⁵ L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 2.3 × 10⁴ L/kg and 3.2 × 10⁴ L/kg, respectively. The desorption K_d values of ²³⁷Np ranged from 8.9 × 10² to 2.1 × 10⁴ L/kg, and their GM and AM were 4.1 × 10³ L/kg and 6.4 × 10³ L/kg, respectively. A comparison between the obtained K_d values of ²³⁹Pu and ²³⁷Np indicated that the former were about 6-fold higher than the latter in Japanese paddy field soils. Pearson's correlation analysis suggested that the main factors contributing to the sorption and desorption of Pu isotopes in Japanese paddy soils were related to the amounts of Fe and Mn oxides, while the sorption and desorption of ²³⁷Np in them would be controlled by an ion-exchange reaction, and/or complex-formation with organic matter, especially in Fluvisols.

Distribution of plutonium isotopes in soils between two nuclear test sites: Semipalatinsk and Lop Nor

Plutonium (Pu) has attracted attention as an environmental tracer due to its radiotoxicity and the possibility of sources linked with nuclear accidents in recent years. Plutonium isotopes (^239,240Pu) were detected at trace levels in soils collected from the Xinjiang region located between the Semipalatinsk nuclear test site and China's Lop Nor nuclear test site. Little is known regarding the spatial variation of ^239,240Pu in soils from this region. This study reports the use of Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS) methods to distinguish between Pu isotopes derived from global fallout and nuclear weapon tests. We found that the ^239,240Pu activity concentrations ranged from 0.035 to 1.338 mBq/g; the ²⁴⁰Pu/²³⁹Pu atomic ratios were 0.157-0.223 with a weighted average of 0.180 ± 0.002, corresponding with the expected average global fallout ratio of 0.180 ± 0.014. This indicated that global fallout is the major source of Pu in the study region. The ^239,240Pu inventories in these soils ranged from 23.67 to 222.7 Bq/m², corresponding with those from other areas in China and other countries within the latitude range. Our Pu isotope data was supplemented with other published Pu data for soils collected in the vicinity of the Semipalatinsk nuclear test site and Lop Nor nuclear test site. Results indicate that ^239,240Pu inventories and ²⁴⁰Pu/²³⁹Pu atomic ratios in soils exhibit large variations with distance from the Semipalatinsk nuclear test site. High deposition and accumulation of Pu, and low ²⁴⁰Pu/²³⁹Pu ratios were observed in close-in fallout and downwind regions of the Semipalatinsk nuclear test site and China's Lop Nor nuclear test site.

Temporal trend of 240Pu/239Pu atom ratios in water columns in the Western North Pacific Ocean and its marginal seas

The ²⁴⁰Pu/²³⁹Pu atom ratios were determined by inductively coupled plasma mass spectrometry for seawater samples from the Okinawa Trough in the western North Pacific Ocean margin to identify their Pu sources and to elucidate the temporal variability in atom ratios in the water columns in the western North Pacific Ocean and its marginal seas. The ²³⁹Pu, ²⁴⁰Pu and ^239+240Pu water column inventories were 12.2 ± 0.5, 10.5 ± 0.5 and 22.7 ± 0.7 Bq m^-2, respectively. The observed ²⁴⁰Pu/²³⁹Pu ratios were clearly greater than the mean global fallout ratio of ∼0.18 with a mean value of 0.236 ± 0.014. The observed greater atom ratios confirmed the presence of close-in fallout Pu from nuclear testing at the Pacific Proving Grounds (PPG). The relative percentage of the PPG-derived Pu to global fallout Pu was estimated as 41 ± 10%. The ^239+240Pu water column inventory originating from the PPG close-in fallout was also calculated as 9.2 ± 0.4 Bq m^-2. The temporal variability in the mean ²⁴⁰Pu/²³⁹Pu atom ratios in the water columns in the western North Pacific Ocean and its marginal seas was obtained through comparison of the present study values and the previously reported values, and it had no significant temporal variation with a mean value of 0.237 ± 0.004 during the observation period of 1984-2015. The ²³⁹Pu and ²⁴⁰Pu derived from the two different sources of global fallout and the close-in fallout at the PPG were homogenized in the water masses in the studied areas.

Distributions of 239Pu and 240Pu Concentrations and 240Pu/239Pu Atom Ratios and 239+240Pu Inventories in a Water Column in the Eastern Indian Ocean: Transport of Pacific Proving Grounds-Derived Pu via the Indonesian Throughflow

The ^239+240Pu concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios in seawater from the eastern Indian Ocean were determined to identify their Pu sources and to propose the transport pathway of Pacific Proving Grounds (PPG)-derived Pu into the studied area. This is the first study by anyone on these Pu atom ratios in the Indian Ocean. In the West Australia Basin, the ^239+240Pu concentration was 2.89 mBq m^-3 in the surface water and increased with depth; a subsurface maximum was identified at 200 m depth and then decreased gradually with depth; its water column inventory was 32.8 Bq m^-2. The inventory-weighted mean ²⁴⁰Pu/²³⁹Pu atom ratios were 0.208 in the South Australia Basin, 0.226 in the Perth Basin, 0.242 in the West Australia Basin, 0.232 in the Bay of Bengal, and 0.225 in the Andaman Sea. The obtained ²⁴⁰Pu/²³⁹Pu ratios were clearly greater than the mean global fallout ratio of 0.18. These high atom ratios proved the presence of close-in fallout Pu from PPG nuclear tests. The relative contribution of global and PPG fallouts was evaluated using the two-end-member mixing model. The ^239+240Pu inventories originating from the PPG fallout were calculated as 2.9-14.9 Bq m^-2, which corresponded to 20-46% of the total ^239+240Pu inventory. A significant amount of the PPG-derived Pu has been transported to the eastern Indian Ocean. The proposed transport pathway accounting for the high ²⁴⁰Pu/²³⁹Pu ratio is the transportation of PPG-derived Pu by the North Equatorial Current followed by the Mindanao Current, Indonesian Throughflow, and then spreading over the Indian Ocean by its surface circulation system.

Plutonium isotopes and radionuclides in corals around Weizhou land in Beibu Gulf, China

Plutonium isotopes in the coral were determined with chemical separation method using AG 1-X8 and AG-MP-1M anion exchange resins and sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) in order to elucidate the activity concentration and source of Pu around Weizhou land in Beibu Gulf, China. Furthermore, the activity concentrations of other radionuclides (²³⁸U, ²²⁶Ra, ²³²Th, ¹³⁷Cs, ⁴⁰K and ²¹⁰Pb) were measured by a HPGe spectrometer. The activity concentration of ^240+239Pu in the coral is determined to be in the range of 8.95-27.84 mBq/kg. The ²⁴⁰Pu/²³⁹Pu atom ratios in the samples range from 0.173 to 0.225, indicating that the main source of plutonium in this area is global fallout while the contribution of PPG is about 30%. Further, the activity concentrations of ²³⁸U, ²³²Th, ⁴⁰K and ²²⁶Ra are determined to be in the range of 18.72-64.63, 1.37-20.8, 29.78-72.52 and 3.48-61.97 Bq/kg, respectively.

Vertical distribution of Pu in forest soil in Qinghai-Tibet Plateau

The ^239+240Pu and ¹³⁷Cs activities and ²⁴⁰Pu/²³⁹Pu atom ratios in surface soil and soil core collected from Qinghai-Tibet Plateau were investigated. The maximum of ^239+240Pu and ¹³⁷Cs activity concentrations in five soil cores were found in upper layers and have a same trend: the maximum followed by exponential decline. The ²⁴⁰Pu/²³⁹Pu atom ratio ranged from 0.160 ± 0.009 to 0.212 ± 0.012, clarified that the plutonium mainly came from the global fallout and the contribution of Lop Nor test sites was negligible. In addition, the vertical profiles of radionuclides (²¹⁰Pb, ²³⁸U, ²³²Th, ²²⁶Ra, ⁴⁰K) have been studied and a high correlation has been found between them. The correlations of Pu between organic matter (OM) and heavy metals were also studied. Person Correlation Coefficients revealed Pu had significant positive correlations with organic matter and Cd, Cu, Co, Zn, but negative correlation with Tl. The results have important implication for further understanding of the sources, records and environmental impacts of global and regional nuclear activities, which expanded the database of Pu activity level and atom ratio in Chinese soil and established a foundation for future environmental risk assessment.

Transuranic nuclides Pu, Am and Cm isotopes, and 90Sr in seafloor sediments off the Fukushima Daiichi Nuclear Power Plant during the period from 2012 to 2019

The ²³⁸Pu, ^239+240Pu, ²⁴¹Am, ²⁴²Cm, ^243+244Cm and ⁹⁰Sr concentrations in seafloor surface sediments collected at three sampling stations off the Fukushima Daiichi Nuclear Power Plant (FDNPP) site during the period from 2012 to 2019 were determined to elucidate the impacts of the FDNPP accident onto their concentrations in coastal sediments and to discuss the sources of the measured radionuclides. The ^239+240Pu, ²⁴¹Pu and ²⁴¹Am concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios in a sediment core were also determined to allow comparison of their inventories between this study and previously reported values and to identify the Pu sources. The ²³⁸Pu, ^239+240Pu, ²⁴¹Am and ⁹⁰Sr concentrations showed no remarkable temporal variations; no significant increases in concentrations after the FDNPP accident were observed; these concentrations were comfortably within the previously reported concentration range; and no detectable ²⁴²Cm and ^243+244Cm amounts were observed in surface sediments. The observed ²³⁸Pu/^239+240Pu activity ratios were approximately two orders of magnitudes lower than those for the damaged FDNPP reactor core inventories and the observed values in terrestrial samples after the accident. The ^239+240Pu, ²⁴¹Pu and ²⁴¹Am inventories in the sediment core were 389 ± 5, 503 ± 33 and 214 ± 3 Bq m^-2, respectively. The ^239+240Pu inventory was about an order of magnitude greater than the expected cumulative deposition density of global fallout from atmospheric nuclear weapons testing due to an enhanced scavenging effect. The ²⁴⁰Pu/²³⁹Pu atom ratios in the sediment core ranged from 0.239 to 0.246 with a mean value of 0.242 ± 0.002; these ratios were clearly greater than the mean global fallout ratio of 0.18. The results for ²³⁸Pu/^239+240Pu activity ratios and ²⁴⁰Pu/²³⁹Pu atom ratios reflected a mixture of global fallout and Pacific Proving Grounds (PPG) close-in fallout Pu rather than Fukushima accident-derived Pu. The sediment column inventory for ^239+240Pu originating from the PPG close-in fallout was calculated as 166 Bq m^-2, which corresponded to 43% of the total inventory. A significant amount of the PPG-derived Pu has been transported by ocean currents and then preferentially scavenged in the coastal waters of Japan.

Plutonium in Southern Yellow Sea sediments and its implications for the quantification of oceanic-derived mercury and zinc

The spatial distributions of mercury (Hg) and zinc (Zn) concentration and the isotopic composition of plutonium (Pu) were investigated in surface sediments and sediment cores collected from the Southern Yellow Sea (SYS) during May 2014. The variation of the ²⁴⁰Pu/²³⁹Pu atom ratio (0.18-0.31) in the surface sediments of the SYS clearly indicated a signal of close-in fallout input from the Pacific Proving Ground (PPG). The buried ^239+240Pu in the sediment of the SYS was estimated to be (4.7 ± 0.5) × 10¹⁰ Bq y^-1 during the period from 2011 to 2014, of which ∼33% (1.5 × 10¹⁰ Bq y^-1) was derived from the PPG by long-range transport via ocean currents (e.g., the North Equatorial Current and Kuroshio Current). The concentrations of Hg and Zn varied from 0.003 to 0.067 mg kg^-1 and from 43.9 to 137 mg kg^-1, respectively, and exhibited positive correlations with the ^239+240Pu activity both in the surface sediments (0-1 cm) and upper layers (7 cm) of the sediment cores. Therefore, by using Pu as a tracer, we estimated that the oceanic input contributed 2.0 tons y^-1 of Hg and 1.0 × 10³ tons y^-1 of Zn to the SYS sediments between 2011 and 2014, which accounted for 33% and 3% of total buried Hg and Zn, respectively. These findings indicate that environmental pollution control should also consider the oceanic contribution of some pollutants. The results of the present work help to elucidate the biogeochemical cycling of trace metals in marginal seas, and are helpful for managing environmental pollution in marine environments.

240Pu/239Pu atom ratios in water columns from the North Pacific Ocean and Bering Sea: Transport of Pacific Proving Grounds-derived Pu by ocean currents

The ²⁴⁰Pu/²³⁹Pu atom ratios in seawater from the northern North Pacific Ocean and Bering Sea were determined to elucidate the temporal variability in ^239+240Pu inventory, to identify their Pu sources and to propose the transport pathway of Pacific Proving Grounds (PPG)-derived Pu into the studied area. Water column ^239+240Pu inventory in the Bering Sea was lower than that reported for a comparable station in the Geochemical Ocean Sections Study, indicating that the Bering Sea inventory has been decreasing at the average rate of 2.6 Bq m^-2 yr^-1. The ²⁴⁰Pu/²³⁹Pu atom ratios ranged from 0.222 to 0.255 with a mean value of 0.235 in the northern North Pacific Ocean and from 0.215 to 0.237 with a mean value of 0.224 in the Bering Sea. The obtained ²⁴⁰Pu/²³⁹Pu ratios were clearly higher than the mean global fallout ratio of 0.18. These high atom ratios proved the presence of close-in fallout Pu from nuclear weapons testing at the PPG. The relative contribution of the global fallout and the PPG fallout were evaluated by using the two-end-member mixing model. The ^239+240Pu inventories originating from the PPG fallout were calculated as 17.9-25.4 Bq m^-2 in the northern North Pacific Ocean and 11.6-16.6 Bq m^-2 in the Bering Sea, which corresponded to 38-54% and 31-45% of the total ^239+240Pu inventory in the water column in the northern North Pacific Ocean and Bering Sea, respectively. A significant amount of the PPG-derived Pu has been transported to the northern North Pacific Ocean and Bering Sea. The proposed transport pathway accounting for the high ²⁴⁰Pu/²³⁹Pu ratio in the Bering Sea water column is the transportation of PPG-derived Pu by the North Equatorial Current followed by the Kuroshio Current, Kuroshio Extension, North Pacific Current and subsequently the Alaska Current, and then the Alaskan Stream.

An overview of current knowledge concerning the inventory and sources of plutonium in the China Seas

This study reviews the current understanding of the inventory and sources of plutonium (Pu) in the marine environment adjacent to China. The ^239+240Pu inventory in the China Seas was found to have large spatial variations. The quantity in sediments decreases away from the shore, generally tracing the sedimentation rate distribution. High ^239+240Pu inventories indicated that Pu in the water column was easily scavenged since Pu has a high particle affinity. Indeed, substantially higher ²⁴⁰Pu/²³⁹Pu atom ratios were observed in the sediment and seawater of the China Seas than are found in global fallout. We thus clarified that Pu sources in the China Seas were from both global fallout and the Pacific Proving Grounds (PPG) in the Pacific Marshall Islands. Plutonium from the latter source is transported into the China Seas through the North Equatorial Current (NEC) and Kuroshio. Using a two end-member mixing model, we revealed that the contribution of Pu from the PPG accounts for over 40% of the Pu in the East China Sea (ECS) and South China Sea (SCS), and less than 20% of the Pu in the Yellow Sea (YS). The distributions and isotopic composition of Pu in the China Seas indicate strong scavenging of Pu in the ECS and high Pu accumulation in the SCS. This information on the inventory and isotopic composition of Pu helps to establish a background for the future study of Pu in the China Seas.

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 ²³⁹Pu and ²⁴⁰Pu were studied. The results indicated that both ^239+240Pu activity concentrations (from 0.81 ± 0.16 to 11.18 ± 1.28 mBq/m³) and ²⁴⁰Pu/²³⁹Pu atom ratios (from 0.216 ± 0.032 to 0.308 ± 0.036) in these seawater samples were within the corresponding background ranges before the accident, and this suggested that Fukushima-derived Pu isotopes, if any, were in too limited amount to be distinguished from the background level in the seawater. The analysis of Pu isotopic composition indicated that the major sources of Pu in the seawater after the accident were still global fallout and the Pacific Proving Ground close-in fallout. The contribution analysis showed that the contributions of the Pacific Proving Ground close-in fallout in the water column of the study area ranged from 26% to 77% with the average being 48%.

Sources and scavenging of plutonium in the East China Sea

The ²⁴⁰Pu/²³⁹Pu atom ratio and ^239+240Pu activity of seawater in the East China Sea (ECS) was measured in order to examine the Pu sources and elaborate Pu scavenging process. High ²⁴⁰Pu/²³⁹Pu atom ratios (0.187-0.243, average = 0.221 ± 0.017) in the surface water and water column were observed during 2011, implying of non-global fallout Pu sources. The distribution of ²⁴⁰Pu/²³⁹Pu atom ratio in the ECS was in agreement with the introduction pathway of the Kuroshio, showing a decreasing trend away from the outer shelf. An even higher ²⁴⁰Pu/²³⁹Pu atom ratios (0.243-0.263, average = 0.253 ± 0.007) were observed in the Kuroshio, indicating the non-global fallout Pu signal from the Pacific Proving Grounds (PPG). Using a two end-member mixing model, the Pu source from the PPG contribution was calculated to be 36 ± 9% in the ECS seawater. The ^239+240Pu activities of surface seawater were in the range of 2.00-2.95 mBq m^-3 in the ECS. The spatial distribution of ^239+240Pu activity in the surface seawater showed an increasing trend from the outer shelf to the nearshore. Moreover, ^239+240Pu inventory of water column at the station DH23 in the ECS was calculated to be ~0.29 Bq m^-2, which was 1-3 orders of magnitude lower than the estimates of sediment cores in the ECS shelf (9-407 Bq m^-2). Such differences were determined by the high degree Pu scavenging efficiency in the ECS and high Pu input carried by terrestrial sediments from the Yangtze River. Finally, both ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activities were identical before and after the Fukushima nuclear accident (FNA), suggesting that the impact of the FNA on the ECS was negligible.

Sedimentary record of plutonium in the North Yellow Sea and the response to catchment environmental changes of inflow rivers

Plutonium (Pu) isotopes were first determined in surface and core sediment samples collected from the northern North Yellow Sea (NYS) to elucidate their source terms and deposition process as well as the response to catchment environmental changes of inflow rivers. ²⁴⁰Pu/²³⁹Pu atom ratios in all sediments showed the typical global fallout value of ∼0.18 without any influences from the nuclear weapons tests conducted recently in the North Korea or early in the Pacific Proving Ground. The large variation of ^239+240Pu activities (0.022-0.515 mBq/g) observed in surface sediments should be mainly attributed to the re-suspension and transportation of fine sediments influenced by the Liaonan Costal Current. Based on the two ^239+249Pu depth profiles with easily observed onset fallout levels (1952) and global fallout peaks (1963), ^239+240Pu served as a valid time mark in the coastal sedimentary system. Riverine input Pu contributed only 15-27% to the total global fallout inventory (92.5-108.8 Bq/m²) in the northern NYS, much lower than that in the Yangtze River estuary (77-80%), indicating a better soil conservation in the northeast China due to higher forest coverage compared to the Yangtze River's drainage basin. The increase of riverine input Pu after 1980s reflected the more intense soil erosion degree caused by the land use and cover change due to the increment of human activities in the northeast China at the same period. Our results demonstrated that plutonium is a good indicator for studying sedimentary process and its response to the environment in the coastal area.

Pacific Proving Grounds radioisotope imprint in the Philippine Sea sediments

Radionuclide concentrations were studied in sediment cores taken at the continental slope of the Philippine Sea off Mindanao Island in the equatorial Western Pacific. High resolution deposition records of anthropogenic radionuclides were collected at this site. Excess ²¹⁰Pb together with excess ²²⁸Th and anthropogenic radionuclides provided information about accumulation rates. Concentrations of Am and Pu isotopes were detected by gamma spectrometry, alpha spectrometry and ICP-MS. The Pu ratios indicate a high portion (minimum of 60%) of Pu from the Pacific Proving Grounds (PPG). This implies that the transport of PPG derived plutonium with the Mindanao Current southward is similarly effective as the previously known transport towards the north with the Kuroshio Current. The record is compared to other studies from northwest Pacific marginal seas and Lombok basin in the Indonesian Archipelago. The sediment core top was found to contain a 6 cm thick layer dominated by terrestrial organic matter, which was interpreted as a result of the 2012 Typhoon Pablo-related fast deposition.

Sources of plutonium isotopes and 137Cs in coastal seawaters of Liaodong Bay and Bohai Strait, China and its environmental implications

In order to investigate the sources of plutonium in seawaters of Liaodong Bay and Bohai Strait, China, surface seawater samples were collected and analyzed for ^239+240Pu and ¹³⁷Cs by radiochemical separation combined with ICP-MS and γ-spectrometry, respectively. A large variation of ^239+240Pu activities was observed, ranging from 1.993 to 29.677 mBq/m³ in the Liaodong Bay and from 0.932 to 10.183 mBq/m³ in the Bohai Strait. ¹³⁷Cs activities showed little variation between the investigated locations, which was attributed to the different behavior characteristics of ^239+240Pu and ¹³⁷Cs in seawaters. The ²⁴⁰Pu/²³⁹Pu atom ratios showed a significant variation between the Liaodong Bay (0.185) and Bohai Strait (0.225), indicating that Pu derived from the Pacific Proving Grounds might have been transported to the Bohai Strait but not yet to the Liaodong Bay, which could further provide valuable reference for evaluating the seawater exchange cycle between the Bohai Sea and Yellow Sea.

Establishing rapid analysis of Pu isotopes in seawater to study the impact of Fukushima nuclear accident in the Northwest Pacific

In order to assess the impact of the Fukushima derived Pu isotopes on seawater, a new analytical method to rapidly determine Pu isotopes in seawater by SF-ICP-MS including Fe(OH)₂ primary co-precipitation, CaF₂/LaF₃ secondary co-precipitation and TEVA+UTEVA+DGA extraction chromatographic separation was established. High concentration efficiency (~100%) and high U decontamination factor (~10⁷) were achieved. The plutonium chemical recoveries were 74–88% with the mean of 83 ± 5%. The precisions for both ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activity concentrations were less than 5% when 15 L of seawater samples with the typical ^239+240Pu activity of the Northwest Pacific were measured. It just needs 12 hours to determine plutonium using this new method. The limit of detection (LOD) for ²³⁹Pu and ²⁴⁰Pu were both 0.08 fg/mL, corresponding to 0.01 mBq/m³ for ²³⁹Pu and 0.05 mBq/m³ for ²⁴⁰Pu when a 15 L volume of seawater was measured. This method was applied to determine the seawater samples collected 446–1316 km off the FDNPP accident site in the Northwest Pacific in July of 2013. The obtained ^239+240Pu activity concentrations of 1.21–2.19 mBq/m³ and the ²⁴⁰Pu/²³⁹Pu atom ratios of 0.198–0.322 suggested that there was no significant Pu contamination from the accident to the Northwest Pacific.

Vertical distributions and source identification of the radionuclides 239Pu and 240Pu in the sediments of the Liao River estuary, China

Activity concentration of plutonium (Pu) and its isotopic compositions are extensively used for measuring transport processes of Pu and identifying its source. We investigated the spatial distribution characteristics of ^239+240Pu activity concentrations and ²⁴⁰Pu/²³⁹Pu atom ratio in several sediment cores collected from the Liao River coastal zone. Additionally, we calculated the ^239+240Pu inventories and based on the ²⁴⁰Pu/²³⁹Pu atom ratio to trace Pu source. The activity concentrations of ^239+240Pu in surface sediments of the Liao River estuary ranged between 0.103 ± 0.008 and 0.978 ± 0.035 mBq/g, with an average of 0.294 ± 0.024 mBq/g. The ²⁴⁰Pu/²³⁹Pu atom ratios, ranging from 0.173 ± 0.047 to 0.215 ± 0.061 (mean: 0.188 ± 0.049 (1σ)), were consistent with global fallout value, which indicates the global atmospheric fallout is the main source of Pu in sediment cores from the both sides of Liao River estuary. As for the tidal flat core LT-2, the mean ²⁴⁰Pu/²³⁹Pu atom ratio, slightly higher than that of the global fallout value, was 0.217 ± 0.050. Such pattern of Pu isotopic compositions indicated that Pu on the tidal flat in the Liao River estuary is sourced from a combination of global fallout and close-in fallout from the PPG by ocean currents transporting. And by using a two end-member mixing model, the results indicate the relative contribution of the PPG close-in fallout to core LT-2 is round 27% and 73% can be attributed to global fallout and river input. Therefore, these results clearly indicate that the direct global fallout is the main source of Pu in the Liao River estuary.

Temporal evolution of 137Cs, 237Np, and 239+240Pu and estimated vertical 239+240Pu export in the northwestern Mediterranean Sea

The evolution of ¹³⁷Cs, ²³⁷Np and ^239+240Pu at the DYFAMED station (NW Mediterranean) is discussed in relation to physical processes, downward fluxes of particles, and changes in the main input sources. The data set presented in this study represents the first complete ²³⁷Np vertical profiles (0.12-0.27μBqL^-1), and constitutes a baseline measurement to assess future changes. A similar behavior of Cs and Np has been evidenced, confirming that Np behaves conservatively. While the ¹³⁷Cs decrease has been driven by its radioactive decay, the vertical distribution of ²³⁷Np has not substantially changed over the last decade. In the absence of recent major inputs, a homogenization of their vertical distribution occurred, partly due to deep convection events that became more intense during the last decade. In contrast, ^239+240Pu surface levels in the NW Mediterranean waters have fallen in the past four decades by a factor of 5. This decrease in surface has been balanced by higher concentrations in the deep-water layers. A first estimate of the downward ^239+240Pu fluxes in the NW Mediterranean Sea is proposed over more than two decades. This estimation, based on the DYFAMED sediment trap time-series data and published ^239+240Pu flux measurements, suggests that sinking particles have accounted for 60-90% of the upper layer (0-200m) Pu inventory loss over the period 1989-2013. The upper layer residence time of Pu is estimated to be ~28years, twice as long as the residence time estimated for the whole western Mediterranean (~15years). This difference highlights the slow removal of Pu in the open waters of the NW Mediterranean and confirms that most of the Pu removal occurs along the coastal margin where sedimentation rates are high.

Investigation of low-level 242Pu contamination on nutrition disturbance and oxidative stress in Solanum tuberosum L

Plutonium associated with higher molecular weight molecules is presumed to be poorly mobile and hardly plant available. In our present study, we investigate the uptake and effects of Pu treatments on Solanum tuberosum plants in amended Hoagland medium at concentrations of [²⁴²Pu] = 100 and 500 nm, respectively. We found a direct proof of oxidative stress in the plants caused by these rather low concentrations. For the confirmation of oxidative stress, we explored the production of nitric oxide (NO) and hydrogen peroxide (H₂O₂) by epifluorescence microscopy. Oxidative stress markers like lipid peroxidation and superoxide radicals (O₂^•-) are monitored through histochemical analysis. The biochemical parameters i.e. chlorophyll and carotenoids are measured as an indicator of cellular damage in the tested plants including the enzymatic parameters such as catalase and glutathione reductase. From our work, we conclude that Pu in low concentration has no significant effects on the uptake of many trace and macroelements. In contrast, the content of O₂^•- , malondialdehyde (MDA), and H₂O₂ increases with increasing Pu concentration in the solution, while the opposite effects was found for NO, catalase, and glutathione reductase. These findings prove that even low concentration of Pu regulates ROS production and generate oxidative stress in S. tuberosum L.

Release of Pu isotopes from the Fukushima Daiichi Nuclear Power Plant accident to the marine environment was negligible

Atmospheric deposition of Pu isotopes from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has been observed in the terrestrial environment around the FDNPP site; however, their deposition in the marine environment has not been studied. The possible contamination of Pu in the marine environment has attracted great scientific and public concern. To fully understand this possible contamination of Pu isotopes from the FDNPP accident to the marine environment, we collected marine sediment core samples within the 30 km zone around the FDNPP site in the western North Pacific about two years after the accident. Pu isotopes ((239)Pu, (240)Pu, and (241)Pu) and radiocesium isotopes ((134)Cs and (137)Cs) in the samples were determined. The high activities of radiocesium and the (134)Cs/(137)Cs activity ratios with values around 1 (decay corrected to 15 March 2011) suggested that these samples were contaminated by the FDNPP accident-released radionuclides. However, the activities of (239+240)Pu and (241)Pu were low compared with the background level before the FDNPP accident. The Pu atom ratios ((240)Pu/(239)Pu and (241)Pu/(239)Pu) suggested that global fallout and the pacific proving ground (PPG) close-in fallout are the main sources for Pu contamination in the marine sediments. As Pu isotopes are particle-reactive and they can be easily incorporated with the marine sediments, we concluded that the release of Pu isotopes from the FDNPP accident to the marine environment was negligible.

Isotopic composition and distribution of plutonium in northern South China Sea sediments revealed continuous release and transport of Pu from the Marshall Islands

The (239+240)Pu activities and (240)Pu/(239)Pu atom ratios in sediments of the northern South China Sea and its adjacent Pearl River Estuary were determined to examine the spatial and temporal variations of Pu inputs. We clarified that Pu in the study area is sourced from a combination of global fallout and close-in fallout from the Pacific Proving Grounds in the Marshall Islands where above-ground nuclear weapons testing was carried out during the period of 1952-1958. The latter source dominated the Pu input in the 1950s, as evidenced by elevated (240)Pu/(239)Pu atom ratios (>0.30) in a dated sediment core. Even after the 1950s, the Pacific Proving Grounds was still a dominant Pu source due to continuous transport of remobilized Pu from the Marshall Islands, about 4500 km away, along the North Equatorial Current followed by the transport of the Kuroshio current and its extension into the South China Sea through the Luzon Strait. Using a simple two end-member mixing model, we have quantified the contributions of Pu from the Pacific Proving Grounds to the northern South China Sea shelf and the Pearl River Estuary are 68% ± 1% and 30% ± 5%, respectively. This study also confirmed that there were no clear signals of Pu from the Fukushima Daiichi Nuclear Power Plant accident impacting the South China Sea.

The release and persistence of radioactive anthropogenic nuclides

Atmospheric testing of nuclear weapons during the period 1945–1980 ushered in the ‘atomic age’ and released large quantities of anthropogenic radiogenic nuclides into the atmosphere. These radionuclides were subsequently deposited as fallout to the entire surface of the planet. While many have decayed to negligible levels, long-lived radionuclides persist and will do so for thousands of years. Isotopes of plutonium, 239Pu (half-life 24 100 years) and 240Pu (half-life 6563 years), provide the best chronological markers for the onset of this anthropogenic event both now and into the future due to their long half-lives, particle-reactivity, and the fact that they were present in negligible quantities prior to anthropogenic production and release. Chronostratigraphic markers established by distinct Pu concentration profiles and Pu isotope changes in sediment sequences and ice and coral cores can provide high-resolution dating over the last 60 years. However, even though fallout has ceased, it is found that the Pu inventory currently held in surface soil layers and the oceans will continue to supply Pu to sediment deposition zones for millennia and centuries, respectively. The delivery of this Pu will depend on soil erosion and bioturbation rates, and the rate of removal of dissolved Pu from the ocean.

A method of measurement of (239)Pu, (240)Pu, (241)Pu in high U content marine sediments by sector field ICP-MS and its application to Fukushima sediment samples

An accurate and precise analytical method is highly needed for the determination of Pu isotopes in marine sediments for the long-term marine environment monitoring that is being done since the Fukushima Dai-ichi Nuclear Power Plant accident. The elimination of uranium from the sediment samples needs to be carefully checked. We established an analytical method based on anion-exchange chromatography and SF-ICP-MS in this work. A uranium decontamination factor of 2 × 10(6) was achieved, and the U concentrations in the final sample solutions were typically below 4 pg mL(-1), thus no extra correction of (238)U interferences from the Pu spectra was needed. The method was suitable for the analysis of (241)Pu in marine sediments using large sample amounts (>10 g). We validated the method by measuring marine sediment reference materials and our results agreed well with the certified and the literature values. Surface sediments and one sediment core sample collected after the nuclear accident were analyzed. The characterization of (241)Pu/(239)Pu atom ratios in the surface sediments and the vertical distribution of Pu isotopes showed that there was no detectable Pu contamination from the nuclear accident in the marine sediments collected 30 km off the plant site.

(137)Cs, (239+240)Pu concentrations and the (240)Pu/(239)Pu atom ratio in a sediment core from the sub-aqueous delta of Yangtze River estuary

A sediment core collected from the sub-aqueous delta of the Yangtze River estuary was subjected to analyses of (137)Cs and plutonium (Pu) isotopes. The (137)Cs was measured using γ-spectrometry at the laboratories at the Nanjing University and Pu isotopes were determined with Accelerator Mass Spectrometry (AMS), measurements made at the Australian National University. The results show considerable structure in the depth concentration profiles of the (137)Cs and (239+240)Pu. The shape of the vertical (137)Cs distribution in the sediment core was similar to that of the Pu. The maximum (137)Cs and (239+240)Pu concentrations were 16.21 ± 0.95 mBq/g and 0.716 ± 0.030 mBq/g, respectively, and appear at same depth. The average (240)Pu/(239)Pu atom ratio was 0.238 ± 0.007 in the sediment core, slightly higher than the average global fallout value. The changes in the (240)Pu/(239)Pu atom ratios in the sediment core indicate the presence of at least two different Pu sources, i.e., global fallout and another source, most likely close-in fallout from the Pacific Proving Grounds (PPG) in the Marshall Islands, and suggest the possibility that Pu isotopes are useful as a geochronological tool for coastal sediment studies. The (137)Cs and (239+240)Pu inventories were estimated to be 7100 ± 1200 Bq/m(2) and 407 ± 27 Bq/m(2), respectively. Approximately 40% of the (239+240)Pu inventory originated from the PPG close-in fallout and about 50% has derived from land-origin global fallout transported to the estuary by the river. This study confirms that AMS is a useful tool to measure (240)Pu/(239)Pu atom ratio and can provide valuable information on sedimentary processes in the coastal environment.

Anthropogenic Pu distribution in Tropical East Pacific

The geographical distribution of the anthropogenic radionuclides (238)Pu and (239+240)Pu in the Tropical East Pacific in 2003 was studied from the viewpoint of material migration. We measured the contents of Pu isotopes in seawater and in sediment from the sea bottom. The distributions of Pu isotopes, together with those of coexisting nitrate and phosphate species and dissolved oxygen, are discussed in relation to the potential temperature and potential density (sigma-θ). The Pu contents in sediment samples were compared with those in the seawater. Horizontal migration across the Equator from north to south was investigated at depths down to ~800m in the eastern Pacific. The Pu distribution at 0-400m correlated well with the distribution of potential temperature. Maximum Pu levels were observed in the subsurface layer at 600-800m, corresponding to the depth where sigma-θ≈27.0. It is suggested that the Pu distribution depends on the structure of the water mass and the particular temperature and salinity. The water column/sediment column inventory ratio and the vertical distribution of Pu may reflect the efficiency of scavenging in the relevant water areas.

Plutonium isotopes as tracers for ocean processes: a review

Since the first nuclear weapons tests in the 1940s, pulsed inputs of plutonium isotopes have served as excellent tracers for understanding sources, pathways, dynamics and the fate of pollutants and particles in the marine environment. Due to the well-defined spatial and temporal inputs of Pu, the long half-lives of (240)Pu and (239)Pu and its unique chemical properties, Pu is a potential tracer for various physical and biogeochemical ocean processes, including circulation, sedimentation and biological productivity, and hence a means of assessing the impacts of global climate change. Due to the source dependency of the Pu isotopic signature, plutonium isotopes are beginning to be exploited as tools for the evaluation and improvement of regional and global ocean models that will enhance understanding of past and future changes in the oceans. This paper addresses the major sources of Pu and the physical and biogeochemical behaviour in the marine environment. Finally, the use of Pu isotopes as tracers for various oceanic processes (e.g. water mass transport, particle export, and sedimentation) is considered.

Determination of 240Pu/239Pu atom ratio in coastal surface seawaters from the western North Pacific Ocean and Japan Sea

Surface seawater samples were collected from a site in the vicinity of the nuclear fuel reprocessing facility at Rokkasho, Japan and sites along the Japan Sea coast. (239+240)Pu activities and (240)Pu/(239)Pu atom ratios were determined by alpha-spectrometry and isotope-dilution sector-field ICP-MS. The (240)Pu/(239)Pu atom ratio with the mean value of 0.227 +/- 0.006 was significantly higher than the mean global fallout ratio of 0.18. The contribution of the Pacific Proving Grounds close-in fallout was estimated to be 33% of the (239+240)Pu.

Environmental applications of single collector high resolution ICP-MS

The number of environmental applications of single collector high resolution ICP-MS (HR-ICP-MS) has increased rapidly in recent years. There are many factors that contribute to make HR-ICP-MS a very powerful tool in environmental analysis. They include the extremely low detection limits achievable, tremendously high sensitivity, the ability to separate ICP-MS signals of the analyte from spectral interferences, enabling the reliable determination of many trace elements, and the reasonable precision of isotope ratio measurements. These assets are improved even further using high efficiency sample introduction systems. Therefore, external factors such as the stability of laboratory blanks are frequently the limiting factor in HR-ICP-MS analysis rather than the detection power. This review aims to highlight the most recent applications of HR-ICP-MS in this sector, focusing on matrices and applications where the superior capabilities of the instrumental technique are most useful and often ultimately required.

240Pu/239Pu atom ratios in seawater from Sagami Bay, western Northwest Pacific Ocean: sources and scavenging

Seawater samples were collected in Sagami Bay, western Northwest Pacific Ocean, and their (239+240)Pu activities and (240)Pu/(239)Pu atom ratios were determined by alpha-spectrometry and sector field high-resolution ICP-MS. A few samples also were analyzed for (137)Cs activities. The (239+240)Pu inventory of 41.1 Bq m(-2) was equivalent to the expected cumulative deposition density of atmospheric global fallout at the same latitude and this inventory was considerably lower than inventories in the underlying sediment columns. This result indicated that a significant amount of (239+240)Pu has been removed into the underlying sediments through enhanced scavenging from the water column by the high fluxes of particles in this region. The atom ratio of (240)Pu/(239)Pu showed no notable variation from the surface to the bottom; the average value was 0.234+/-0.004. This atom ratio was significantly higher than the mean global fallout ratio of 0.18, proving the existence of close-in fallout plutonium originating from the Pacific Proving Grounds (PPG). The relative contributions of the global stratospheric fallout and the PPG close-in fallout were evaluated by using the two end-member mixing model. The contribution of the PPG close-in fallout was estimated to be 15.2 Bq m(-2), which corresponded to 37% of the (239+240)Pu inventory in the water column. Thus (239)Pu and (240)Pu from the two sources of global fallout and close-in fallout have been homogenized in the water masses in the western Northwest Pacific margin during the past three decades.

Precise Determination of Pu Isotopes in a Seawater Reference Material Using ID-SF-ICP-MS Combined with Two-stage Anion-Exchange Chromatography

In order to obtain the precise Pu isotope composition of Irish Sea water reference material issued by the International Atomic Energy Agency (IAEA-381), we analyzed the activities of (239)Pu, (240)Pu and the atom ratio of (240)Pu/(239)Pu by a highly sensitive isotope dilution SF-ICP-MS method combined with two-stage chromatographic separation and purification. With a mean chemical yield of 65% for (242)Pu tracer, the experimentally established values for (239)Pu, (240)Pu and (239+240)Pu activities are in good agreement with the certified ones. For the (240)Pu/(239)Pu atom ratio, we obtain a value of 0.2315 +/- 0.0008 with a high precision (RSD, 0.35%), which is much more precise than the information value of 0.22 +/- 0.03 (RSD, 13.6%) provided by the IAEA certification report. The precise determination of Pu isotopes in this seawater reference material will be useful for the validation of analytical methods for the study of radionuclides in the marine environment.