plutonium and 137cs in surface water of the south pacific ocean

ent 381 (2007) 243–255www.elsevier.com/locate/scitotenv

Science of the Total Environm

Plutonium and 137Cs in surface water of the South Pacific Ocean

K. Hirose a,b,⁎, M. Aoyama a, M. Fukasawa c, C.S. Kim d, K. Komura a,b,P.P. Povinec e, J.A. Sanchez-Cabeza f

a Meteorological Research Institute, Nagamine 1-1, Tsukuba, Ibaraki 305-0052, Japanb Low Level Radioactivity Laboratory, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan

c Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15, Natsushima, Yokosuka 237-0061, Japand Korea Institute of Nuclear Safety, Daeduk-Danji Daejeon 305-336, Republic of Korea

e Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska dolina F1, SK-842 48, Bratislava, Slovakiaf Marine Environment Laboratories, International Atomic Energy Agency, 4 Quai Antoine 1er, MC98000, Monaco

Received 8 December 2006; received in revised form 15 March 2007; accepted 19 March 2007Available online 25 April 2007

Abstract

The present plutonium and 137Cs concentrations in South Pacific Ocean surface waters were determined. The water sampleswere collected in the South Pacific mid-latitude region (32.5 °S) during the BEAGLE expedition conducted in 2003–04 byJAMSTEC. 239,240Pu concentrations in surface seawater of the South Pacific were in the range of 0.5 to 4.1 mBq m−3, whereas137Cs concentrations ranged from 0.07 to 1.7 Bq m−3. The observed 239,240Pu and 137Cs concentrations in the South Pacific werealmost of the same level as those in the North Pacific subtropical gyre. The surface 239,240Pu in the South Pacific subtropical gyreshowed larger spatial variations than 137Cs, as it may be affected by physical and biogeochemical processes. The 239,240Pu/137Csactivity ratios, which reflect biogeochemical processes in seawater, were generally smaller than that observed in global fallout,except for the most eastern station. The 239,240Pu/137Cs ratios in the South Pacific tend to be higher than that in the North Pacific.The relationships between anthropogenic radionuclides and oceanographic parameters such as salinity and nutrients wereexamined. The 137Cs concentrations in the western South Pacific (the Tasman Sea) and the eastern South Pacific were negativelycorrelated with the phosphate concentration, whereas there is no correlation between the 137Cs and nutrients concentrations in theSouth Pacific subtropical gyre. The mutual relationships between anthropogenic radionuclides and oceanographic parameters areimportant for better understanding of transport processes of anthropogenic radionuclides and their fate in the South Pacific.© 2007 Elsevier B.V. All rights reserved.

Keywords: Plutonium; 137Cs; 239,240Pu/137Cs activity ratio; South Pacific; Nutrients; Temporal change

1. Introduction

Long-lived anthropogenic radionuclides (plutoniumisotopes and 137Cs) in seawater of the South Pacificwere introduced on the ocean surface by global fallout

⁎ Corresponding author. Tel.: +81 29 853 8718; fax: +81 29 8538728.

E-mail address: [email protected] (K. Hirose).

0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2007.03.022

due to atmospheric nuclear weapons testing, from whichthe major fallout occurred in the mid 1960s. Globalfallout amounts of anthropogenic radionuclides in theSouthern Hemisphere were about one third or less thanin the Northern Hemisphere (UNSCEAR, 2000;Aoyama et al., 2006a). Anthropogenic radionuclides(mostly plutonium) were also injected into seawater byclose-in fallout from French nuclear explosions con-ducted at the French Polynesia (Chiappini et al., 1999).

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http://dx.doi.org/10.1016/j.scitotenv.2007.03.022

244 K. Hirose et al. / Science of the Total Environment 381 (2007) 243–255

As a result, the South Pacific waters have been widelycontaminated by anthropogenic radionuclides (Hamil-ton et al., 1996; Hirose and Aoyama, 2003; Povinecet al, 2005).

In order to have better understanding of presentlevels of anthropogenic radionuclides in the marineenvironment, it is necessary to determine their concen-trations in seawater over the wide areas of the SouthPacific Ocean. However, the available data for the SouthPacific are very sparse when compared with that in theNorthern Hemisphere seas (Aoyama and Hirose, 2004;Hirose and Aoyama, 2003; Povinec et al., 2004).

In 2003–2004, JAMSTECconducted a 7-month roundthe world Blue Earth Global Expedition (BEAGLE),visiting the South Pacific (winter), Atlantic (late spring)and Indian (summer) Oceans (Uchida and Fukasawa,2005). This cruise provided rare opportunity to collect alot of water samples for radioactivity measurements inSouthern Hemisphere oceanic waters, as did the mostprecious hydrographic data set to investigate an oceanresponse to climate change.

At present we can determine concentrations ofanthropogenic radionuclides (239,240Pu and 137Cs) insmall volume seawater samples thanks to recent devel-opments in radioanalytical techniques which havesensitivities of more than one order of magnitude betterthan before (Aoyama et al., 2000; Hirose et al., 2005;Kim et al., 2002). Therefore, we can present here acomprehensive data set on 239,240Pu and 137Cs concen-trations in seawater, which are also useful as oceano-graphic tracers.

Fig. 1. Sampling locations

In this paper, we describe geographic distributionsand temporal trends of 239,240Pu and 137Cs concentra-tions, together with oceanographic parameters such assalinity and nutrients, in surface waters in the mid-latitude region of the South Pacific, and compare theirdistribution in surface waters of the North Pacific.

2. Sampling and methods

Surface water samples were collected using a sub-mersible pump at about 5 m depth during the BEAGLEcruise on board of the R/V Mirai organized by JapanAgency for Marine-Earth Science and Technology(JAMSTEC). Large volume (80 L) surface seawatersamples were collected in the South Pacific at 50 stations(Fig. 1). The sampling spacing was about 2.5°. All watersamples were filtered through a fine membrane filter(Millipore HA, 0.45 μm pore size) immediately aftersampling. Filtered water samples were transported to theMeteorological Research Institute (MRI) and subjected to137Cs and plutonium analyses. Small volume samples(less than 10 L)were transported to the Korean Institute ofNuclear Safety (KINS) for ICPMS (Inductively CoupledPlasma Mass Spectrometry) analysis of plutonium.

137Cs was concentrated in 20 L seawater samples byadsorption onto AMP (ammonium molybdophosphate)using a method described in detail elsewhere (Aoyamaet al., 2000; Hirose et al., 2005). 137Cs activities weredetermined by γ-spectrometry with high efficiencyHPGe detectors (Hirose et al., 2005). Plutonium for α-spectrometry measurements was concentrated by co-

in the South Pacific.

Table 1137Cs and 239,240Pu concentrations in South Pacific surface waters⁎

Latitude Longitude 137Cs (Bq m−3) 239,240Pu (mBq m−3)

−30.09 154.49 1.71±0.05 2.5±0.5−30.08 156.52 1.74±0.07 1.8±0.5−30.08 158.34 1.45±0.05 2.8±0.5−30.09 161.50 1.64±0.12 1.9±0.5−30.07 165.41 1.50±0.07 2.9±0.6−30.09 167.00 1.59±0.11 2.2±0.4−30.08 169.50 1.47±0.04 1.7±0.4−30.10 172.01 1.59±0.06 1.0±0.4−30.07 175.18 1.37±0.06 2.0±0.6−30.58 177.01 1.54±0.07 2.0±0.3−32.51 179.92 1.20±0.06 1.7±0.3−32.50 −177.66 1.26±0.05 1.6±0.4−32.51 −174.00 1.49±0.07 1.4±0.5−32.50 −171.91 1.40±0.06 1.2±0.3−32.51 −169.50 1.20±0.04 2.5±0.9−32.49 −166.50 1.50±0.05 4.1±1.0−32.50 −163.17 1.42±0.05 2.9±0.4−32.50 −161.15 0.91±0.04 1.8±0.3−32.50 −157.32 1.26±0.06 1.1±0.2−32.49 −154.00 1.29±0.07 1.5±0.6−32.50 −147.35 1.36±0.05 1.5±0.4−32.51 −144.00 1.50±0.07 2.7±0.6−32.50 −141.49 1.48±0.07 1.8±0.5−32.50 −139.32 1.40±0.05 2.3±0.4−32.49 −135.33 1.31±0.07 4.0±1.2−32.50 −132.67 1.19±0.09 3.0±0.3−32.49 −130.00 1.19±0.06 2.0±0.4−32.49 −128.00 1.53±0.11 2.2±0.6−32.51 −124.67 1.28±0.06 1.4±0.3−32.49 −122.66 1.28±0.10 0.8±0.2−32.49 −119.34 1.16±0.09 1.7±0.5−32.49 −117.32 1.08±0.09 1.7±0.5−32.50 −113.99 1.23±0.07 1.3±0.4−32.50 −112.00 1.44±0.07 1.0±0.5−32.50 −109.34 1.29±0.07 2.0±0.4−32.50 −106.02 1.32±0.08 1.9±0.5−32.50 −103.00 1.34±0.07 1.9±0.6−32.51 −101.32 1.14±0.07 2.3±0.5−32.50 −99.33 1.01±0.05 2.2±0.7−32.51 −95.33 1.14±0.07 1.4±0.4−32.51 −93.33 1.12±0.05 1.3±0.4−32.51 −90.67 0.91±0.06 3.2±0.5−32.50 −87.34 0.73±0.07 0.5±0.3−32.50 −85.34 0.83±0.07 1.7±0.2−32.50 −82.00 0.65±0.06 1.1±0.3−32.51 −79.99 0.58±0.08 1.9±0.6−32.50 −76.65 0.46±0.04 0.8±0.3−32.50 −74.66 0.67±0.06 1.1±0.6−32.50 −72.49 0.07±0.04 1.5±0.4

⁎The total uncertainty is given at 1 sigma level.

245K. Hirose et al. / Science of the Total Environment 381 (2007) 243–255

precipitation with Fe hydroxides from about 60 L ofseawater samples. 239,240Pu was analysed by α-spectrometry (Hirose et al., 2001) and/or SF-ICPMSfollowing radioanalytical separation using anion ex-change resin and extraction chromatographic resin(TEVA), described in detail elsewhere (Kim et al.,2000, 2002). The chemical yield was determined by theaddition of a known amount (2 mBq) of 242Pu tracer. Adetail description of analytical methods, quality assur-ance and results of intercomparison exercises usingIAEA-381 reference material (Irish Sea water) are givenin Data Book No. 3 (Kumamoto and Watanabe, 2007).

3. Results and discussion

3.1. Surface 137Cs in the South Pacific

137Cs concentrations (Table 1) in surface waters inthe mid-latitude region of the South Pacific (along32.5°S) were in the range from 0.07 to 1.74 Bq m−3, notsignificantly different from the North Pacific mid-latitude region (Aoyama et al., 2004, 2006b; Povinecet al., 2003). Large 137Cs concentration gradients, withhigh values in the North Pacific mid-latitude region, andlow ones in the South Pacific were observed in the1970s and 1980s (Bowen et al., 1980; Miyake et al.,1988; Hirose and Aoyama, 2003).

The high density 137Cs data revealed a typical longi-tudinal distribution (Fig. 2), which depends on sea areas.Taking into account oceanographic conditions in theSouth Pacific, the mid-latitude region can be dividedinto three sea areas (Schmitz, 1996):

i.) the western South Pacific (WSP; 154.5°E–180°)including the Tasman Sea, corresponding to theEast Australian Current (EAC) System;

ii.) the South Pacific subtropical gyre (SPSG; 180°–100°W), and

iii.) the eastern South Pacific (ESP; 100°W–72.5°W).

The 137Cs concentrations in the WSP ranged from1.37 to 1.74 Bq m−3, and they were higher than in theSPSG. They gradually decreased fromwest to east, whichcorresponds from upstream and downstream of the EAC(Sokolov and Rintoul, 2003), respectively. The surface137Cs concentration was high in the upstream of the EAC,and low in the downstream of the EAC. The surface 137Csconcentrations in the SPSG varied spatially from 0.91 to1.53 Bq m−3, but we did not observe a longitudinalgradient. In the ESP, the 137Cs concentrations ranged from0.07 to 1.14Bqm−3, having lower values than in theWSPand SPSG. The pronounced feature of the 137Cs

concentrations in the ESP is a steep decrease from westto east; especially, a low 137Cs concentration occurred inthe most eastern station of the ESP. The low 137Cs con-centration in this region is due to the effect of advection ofwater masses in the Southern Ocean. The present 137Csconcentrations in the mid-latitude surface water of the

Fig. 2. Longitudinal distribution of 137Cs concentrations in surface water of the South Pacific.


South Pacific are controlled by physical processes in theocean rather than by input processes.

3.2. 239,240Pu in the South Pacific

239,240Pu concentrations in surface water in the mid-latitudes of the South Pacific were in the range of 0.5 to4.1mBqm−3 (Table 1), the same order ofmagnitude as thatin the subtropical gyre in the North Pacific (1.5 to 9.2 mBqm−3) (Hirose et al., 2001, 2006; Povinec et al., 2003;Yamada et al., 2006). The surface 239,240Pu concentrationsin the Pacific in the 1970s and 1980s showed a latitudinaldistribution with high values in the mid-latitude region ofthe North Pacific, and low values in the South Pacific,which reflects deposition patterns of global fallout (Hiroseand Aoyama, 2003; Miyake and Sugimura, 1976; Miyake

Fig. 3. Longitudinal distribution of 239,240Pu concen

et al., 1988). The current observation of the surface239,240Pu concentrations suggests that there has not been amarked difference between the North and South Pacific,although rather large spatial variations of the surface239,240Pu concentrations in each sea area were observed.

The close sampling spacing revealed that the surface239,240Pu concentrations show a different longitudinaldistribution (Fig. 3) from that of 137Cs. The 239,240Puconcentrations in the WSP ranged from 1.0 to 2.9 mBqm−3, and showed a similar longitudinal distribution as did137Cs. In the SPSG, there was no longitudinal gradient inthe surface 239,240Pu concentrations, which ranged from0.8 to 4.1 mBq m−3, although peaks of higher 239,240Puconcentrations were observed near 165°Wand 135°W. Inthe ESP, the surface 239,240Pu concentrations ranged from0.5 to 3.2 mBq m−3. The surface 239,240Pu concentrations

trations in surface water of the South Pacific.


in the mid-latitude region of the South Pacific broadlydecreased from west to east.

3.3. 239,240Pu/137Cs activity ratios in surface water

The 239,240Pu/137Cs activity ratios in surface watersreflect biogeochemical processes in the surface layer(Hirose et al., 1992; Yamada et al., 2006). Thelongitudinal distribution of the 239,240Pu/137Cs activityratios in surface waters is shown in Fig. 4. The239,240Pu/137Cs activity ratios in the WSP, the SPSG,and the ESP were in the range of 0.63 to 1.93×10−3,0.63 to 2.52×10−3, and 0.68 to 21×10−3, respectively.They were lower, except for the most eastern station,than that in global fallout (0.015; decay corrected to2003), because plutonium was effectively removed fromsurface layers by adsorption onto sinking particles(Livingston and Anderson, 1983; Hirose, 1997).

The 239,240Pu/137Cs activity ratios in the WSP surfacewaters show a gradient from high values observed in westto low ones in east, which implies that a preferentialscavenging of plutonium occurs within transport ofradionuclides by the EAC in the WSP. In fact, surfacedivinyl chlorophyll content, observed during the BEA-GLE expedition, increased from the west to the east WSP(Bouman et al., 2006). The typical feature of the239,240Pu/137Cs activity ratios in the SPSG was theoccurrence of two peaks near 165°W and 135°W, whichcorrespond to the peaks of 239,240Pu concentrations insurface waters. The 239,240Pu/137Cs activity ratios in theESP showed relatively large spatial variations. As a result,the 239,240Pu/137Cs activity ratios in the mid-latituderegion of the South Pacific showed a broad gradient withlow values in west and high ones in east. This trend

Fig. 4. Longitudinal distribution of 239,240Pu/137Cs acti

suggests that particle production, i.e., biological activity,in the ESP is weaker than that in theWSP (as documentedby Bouman et al., 2006), and/or an input of water masseswith higher 239,240Pu/137Cs activity ratios due to recyclingprocesses, such as the deep convection and upwelling.

In order to elucidate plutonium behaviour in theSouth Pacific, we introduced a scavenging index (SI),defined as follows:

SI ¼ 239;240Pu=137Cs� �

sw=239;240Pu=137Cs� �

GF;

where (239,240Pu/137Cs)sw and (239,240Pu/137Cs)GF arethe decay-corrected 239,240Pu/137Cs activity ratios in thesurface water and global fallout, respectively. The re-lationship between the SI and the surface 239,240Puconcentration in each area of the South Pacific wasexamined. The results presented in Fig. 5 (a lower SIvalue corresponds to a higher particle scavenging) arewell correlated with the surface 239,240Pu concentrationsin each area of the South Pacific. The fair correlations(the correlation factors are 0.97 for the WSP, 0.95 for theSPSG and 0.81 for the ESP) suggest that a significantpart of the plutonium concentrations in surface watersare affected by the particle scavenging, which dependson a local biological activity.

In the WSP, an intercept of this relationship was aminus value (−0.005), which means that an increase (ordecrease) of 137Cs concentration leads to the decrease(or increase) of 239,240Pu levels. To explain this phe-nomenon, we consider these mechanisms:

i) an input of water with lower 239,240Pu and higher137Cs concentrations occurs by the advection inthe WSP, and

vity ratios in surface water of the South Pacific.

Fig. 5. The relationship between the scavenging index (SI) and the 239,240Pu concentration in South Pacific surface water. A: the Tasman Sea (WSP),B: the South Pacific subtropical gyre, C: the eastern South Pacific.


ii) water mass is affected by an input of water withhigher 239,240Pu and lower 137Cs concentrations,which reflect deeper waters.

On the other hand, intercepts of the relationship in theSPSG and the ESP were positive values (0.009 and0.034), which means that the decrease of the 137Cs


concentration leads to the decrease of 239,240Pu; in otherword, the decrease of the 137Cs and 239,240Pu concen-trations in these sea areas may occur due to a con-tribution of water mass with low 137Cs and low 239,240Puconcentrations. A slope of the SI–239,240Pu relationshipin the WSP (0.045) is similar to that in the SPSG(0.047), whereas a rather steep slope appears in the ESP(0.059). Causes of the different slopes between theSPSG and ESP are unknown, probably they reflectdifferent biogeochemical processes.

3.4. Relationship between radionuclides and salinity/nutrients

In order to have better understanding of thebehaviour of anthropogenic radionuclides in the South

Fig. 6. The property plots between salinity and radionuclide concentration inTasman Sea (WSP); Closed circle: the South Pacific subtropical gyre; Open

Pacific, it is fruitful to examine the relationshipsbetween oceanographic parameters and radionuclideconcentrations. Nutrients in oceanic waters are tradi-tional oceanographic tracers to characterize water mass,as is salinity. Higher nutrient concentrations in surfacewaters have been used as an indicator of physicalprocesses such as upwelling and deep convection, andbiological processes, because nutrient concentrations ina major part of the Pacific (including the subtropicalgyre) are extremely low due to low biological activity,and as a result low-nutrient seas show low biologicalproductivity.

We examine the relationships between radionuclidesand salinity/nutrients (phosphate and silicate) to depictthe oceanographic characteristics of each sea areausing anthropogenic radionuclides. The property plots

the South Pacific surface water. A: 137Cs, B: 239,240Pu. Open circle: thesquare: the eastern South Pacific.


between the radionuclide concentrations and salinity insurface waters (Fig. 6) indicate that the central SouthPacific is divided into three sea areas, corresponding to theWSP, SPSG and ESP. This finding suggests that differentoceanographic processes occur in each sea area. On theother hand, the property plots between the radionuclideconcentrations and phosphate in surface waters (Fig. 7)indicate that the central South Pacific may be divided intotwo groups, which correspond to the WSP+SPSG andESP, whereas characteristics of each sea areas, i.e., theWSP, SPSG and ESP, appeared from the property plotbetween the 137Cs and silicate concentrations (Fig. 8).The water mass in the WSP showed higher 137Cs andsilicate concentrations, while the ESP consists of waterwith lower 137Cs and silicate concentrations. In contrast to

Fig. 7. The property plots between phosphate and radionuclide concentrationthe Tasman Sea (WSP); Closed circle: the South Pacific subtropical gyre;corresponding to a low correlation between the nitrate and phosphate conce

the 137Cs-silicate plot, the water masses cannot bedistinguished from the property plot of the 239,240Pu andsilicate concentrations. The correlations between theradionuclide concentrations and salinity were examinedin each sea area to elucidate the oceanographic character-istics of the area. The results of the correlation analysisbetween the radionuclide and nutrient concentrations ineach sea area are summarized in Table 2.

3.4.1. WSPIn the WSP, which is characterized as water mass

with higher salinity (N35.5), there is a weak correlationbetween the 137Cs concentration and salinity in thesurface waters (the correlation factor: 0.40). Similarly, aweak correlation (the correlation factor: 0.49) was found

s in the South Pacific surface water. A: 137Cs, B: 239,240Pu. Open circle:Open square: the eastern South Pacific. Open rhomb shows values

ntrations.

Fig. 8. The property plots between silicate and radionuclide concentrations in the South Pacific surface water. A: 137Cs, B: 239,240Pu. Open circle: theTasman Sea (WSP), closed circle: the South Pacific subtropical gyre, open square: the eastern South Pacific.


between the 239,240Pu concentration and salinity in theWSP surface water. These findings may imply that asingle mixing process along the flow of the EACoccurred in the WSP, in which water masses with high137Cs and 239,240Pu concentrations, and high salinities(corresponding to the upstream of the EAC), was altered

Table 2Correlation factors of the relationships between radionuclide concentrations asea areas

Sea area S–137Cs S–239,240Pu P–137Cs

WSP 0.40 0.49 −0.57SPSG 0.31 0.07 −0.32ESP 0.53 0.46 −0.93

The italic thick number is a statistical significant correlation with a confidenconfident level range of 0.75 to 0.95.

by mixing with water masses having low 137Cs and239,240Pu concentrations and low salinities, observedsouth of the EAC. The surface salinity map in the Pacific(Levitus et al., 1994) indicates that high salinity waters(corresponding to the upstream of the EAC), exist in thecentral tropical South Pacific.

nd oceanographic parameters (salinity, phosphate and silicate) in each

P–239,240Pu Si–137Cs Si–239,240Pu

−0.11 −0.32 −0.10−0.17 0.09 0.11−0.33 −0.57 −0.11

t level of N0.95, and the italic numbers are weak correlations with a


In the WSP, the surface 137Cs concentrations weaklyanti-correlated with the phosphate concentrations (thecorrelation factor: −0.57), whereas there was no correla-tion between the 137Cs and silicate concentrations. Thisfinding suggests that thewatermasswith higher 137Cs andlow phosphate concentrations, corresponding to the up-stream of the EAC, was partly altered by mixing with thewater mass having lower 137Cs and higher phosphate, aprocess which is consistent with the radionuclides–salinity plots. The water mass with higher phosphateappeared south of the EAC together with meso-scaleeddies (Bradford et al., 1982). On the other hand, wefound no correlation between the 239,240Pu and nutrientconcentrations in the WSP surface water.

3.4.2. SPSGIn the SPSG, the surface 137Cs concentrations seem

to be almost independent on salinity, although there is aweak correlation between the 137Cs concentration andsalinity. This suggests that the surface 137Cs is trappedin the subtropical gyre and homogenized in the SPSG.On the other hand, the surface 239,240Pu in the SPSGshowed irregular variations on salinity, probably af-fected by biogeochemical processes. In this connection,biological activities in the SPSG showed large spatialvariations (Bouman et al., 2006).

In the SPSG surface waters, there is no definite rela-tionship between the radionuclide and nutrient con-centrations, although the surface 137Cs concentrationsweakly anti-correlated with phosphate. The property plotsbetween the 137Cs and silicate concentrations in the SPSGare located between the WSP and ESP. The SPSG is atypical oligotrophic ocean, in which nutrient concentra-tions in surface layer are generally low; especially, nitratewas lacking in the surface waters. This situation mayexplain that no correlation was found between the radio-nuclide and nutrient concentrations in the surface waters.

3.4.3. ESPIn the ESP, consisting of the east edge of SPSG and

the Peru Current, there was a weak correlation betweenthe 137Cs concentration and salinity in the surface waters(the correlation factor: 0.53), and a very weak corre-lation was observed between the 239,240Pu concentrationand salinity (the correlation factor: 0.46). This findingsuggests that the SPSG surface water with higher 137Csand 239,240Pu concentrations and higher salinities(N34.5) in the ESP is affected by advection of thewater mass with low 137Cs and 239,240Pu concentrationsand low salinities (b34.5), which is originating from theSouthern Ocean as low salinity water (salinity around34) (Levitus et al., 1994).

In the ESP, we found a fair negative correlationbetween the 137Cs and phosphate concentrations in thesurface waters (the correlation factor: −0.93) for pointswith a good N:P correlation, whereas there is a weaknegative correlation between the 137Cs and silicate con-centrations. This finding suggests that the ESP surfacewater consists of the water mass with higher 137Cs andlow phosphate concentrations (originating from theSPSG), and one with lower 137Cs and higher phosphateconcentrations (corresponding to the Southern Oceanwater characterised by low salinity). On the other hand,we found no correlation between the 239,240Pu and nu-trients concentrations in the ESP surface water.

It should be noted that the nutrient concentrationsin surface waters are affected by biological activity,whereas 137Cs shows no-bioactive behaviour in seawa-ter. Therefore, the relationship between 137Cs and nu-trients should be only an apparent correlation observedduring winter in the South Pacific. However, nutrients inseawater are one of the potential tools to be used forbetter understanding of the oceanographic behaviour ofanthropogenic radionuclides in the ocean.

3.5. Temporal changes of surface 137Cs and 239,240Puconcentrations

In order to predict future concentrations of anthro-pogenic radionuclides and their fate, it is important toelucidate their history in the ocean. The temporal changepatterns of radionuclides concentrations in surface waterdepend on the sea area because the temporal change ofsurface radionuclide concentrations has been affected byinput patterns, current systems, and other physical andbiogeochemical processes. We examined temporalchanges of surface 137Cs and 239,240Pu concentrationsin three sea areas (WSP, SPSG, and ESP) of the SouthPacific using present data and the data stored in theHAM database (Aoyama and Hirose, 2004). The results(Fig. 9) suggest that the surface 137Cs and 239,240Puconcentrations decrease gradually in each area of theSouth Pacific, although available data are very sparsewhen compared with the North Pacific. Assuming thatthe surface 137Cs and 239,240Pu concentrations decreasedexponentially during the past four decades, we calcu-lated apparent residence times of radionuclides in sur-face water of each sea area. As a result, the surface 137Csconcentrations in the WSP and the SPSG exhibit longerapparent residence times of 30 (+9−5) and 29 (+5−4)years, respectively, whereas a lower apparent residencetime of 20 (+6−4) years was found for the ESP. Theapparent residence times of the surface 137Cs in theWSP and SPSG were longer than that in the mid-latitude

Fig. 9. Temporal variations of 137Cs and 239,240Pu concentrations in South Pacific surface waters. A1: 137Cs, the Tasman Sea (WSP), B1: 137Cs, theSouth Pacific subtropical gyre, C1: 137Cs, the eastern South Pacific. A2: 239,240Pu, the Tasman Sea (WSP), B2: 239,240Pu, the South Pacific subtropicalgyre, C2: 239,240Pu, the eastern South Pacific. Open and closed circles shows historical data and data from this work, respectively.


region of the North Pacific (Hirose and Aoyama, 2003).The surface 239,240Pu concentrations in the SPSG andthe ESP decreased with apparent residence times of 19(+2−1) and 22 (+2−1) years, respectively, whereas theapparent residence time of the surface 239,240Pu in theWSP, which is calculated to be 34 (+17−8) years, did notchange during the past decade. The apparent residencetime of the surface 239,240Pu in the SPSG was shorterthan that of the corresponding 137Cs residence time,which is in agreement with previous observations(Hirose and Aoyama, 2003). This finding has been

explained by preferential removal of surface 239,240Pudue to particle scavenging. On the other hand, theapparent residence time of the surface 239,240Pu in theWSP and the ESP was longer than that of thecorresponding 137Cs. To compare the residence timeof 239,240Pu with that of 137Cs, the radioactive decay of137Cs (half life: 30.17 years) must be taken into account.The oceanic half-residence times of the surface 137Cs inthe WSP, the SPSG and the ESP are calculated to be 63(+6−3), 52 (+4−3) and 24 (+4−3) years, respectively,whereas the half-residence times of the surface 239,240Pu


in the WSP, the SPSG and the ESP are 24 (+12−6), 15(+2−1) and 12 (+2−1) years, respectively. The half-residence time of the surface 239,240Pu is shorter thanthat of 137Cs in the corresponding sea area, which isexplained by preferential removal of 239,240Pu due tobiogeochemical processes (Hirose, 1997). A longer half-residence time of the surface 239,240Pu occurred in theWSP. Similarly, a longer half-residence time of thesurface 239,240Pu was also observed in the Sea of Japan,in which a rapid recycling of scavenged plutoniumoccurs by deep convection in winter (Hirose et al.,2002). It is unlikely that a similar recycling process ofplutonium could occur in the western Southern Ocean.These findings may suggest, however, that an input ofsurface 239,240Pu by advection could exist in the WSP.

3.6. Transport processes of radionuclides

Time-series data in each area of the Pacific revealedthat the decrease rate of the surface 137Cs concentrationdepends on the sea area; especially, the decrease rate ofsurface 137Cs in the North Pacific is larger than that in theSouth Pacific (Hirose and Aoyama, 2003; Aoyama et al.,2006a,b). As a result, the difference in the current 137Csconcentrations between the North and the South Pacifichas decreased. The relationships between radionuclidesand oceanographic parameters suggest that the observed137Cs and 239,240Pu concentrations in the Australian sideof the WSP (the Tasman Sea) are supported by an inflowof the EAC with high 137Cs and 239,240Pu concentrations,higher salinity and low nutrients, which originates in thecentral tropical South Pacific. The current 137Cs and239,240Pu surface concentrations in the WSP seem tomaintain at constant levels. The WSP waters are alteredby mixing with the water masses having low 137Cs and239,240Pu concentrations, lower salinity and higher nu-trients concentrations during the eastward flow of theEAC, whose water mass is produced by upwelling ac-companied with meso-scale eddies south of the EAC(Bradford et al., 1982).

In the SPSG, the current surface 137Cs concentrationwas longitudinally homogeneous, and exhibited a weakcorrelation with salinity, although no correlation wasobserved between the radionuclide and nutrient concen-trations, in contrast to the WSP and ESP. Time-seriesdata indicated that the surface 137Cs in the SPSG showeda long half-residence time. These findings suggest thatanthropogenic radionuclides have been stored in theSPSG during a longer time scale. Therefore, the SPSG isthe largest reservoir of fallout 137Cs in the Pacific.

In the ESP, the surface 137Cs showed the shortest half-residence time within the South Pacific, which means

that the surface 137Cs has been actively diluted by theSouthern Ocean waters. The distribution of the surface137Cs concentration in the ESP shows that the SPSGwaters in the east edge of the gyre have been altered byadvection of water masses (the Peru Current) with lower137Cs concentrations, high nutrients concentrations andlow salinity, originating in the Southern Ocean.

4. Conclusions

The current levels of 137Cs and 239,240Pu in the SouthPacific surface waters were evaluated using high densitydata obtained after the BEAGLE expedition. The surface137Cs and 239,240Pu concentrations in the Pacific, whichare around 1 Bq m−3 and several mBq m−3, respectively,show no significant differences between the North andSouth Pacific subtropical gyres, although lower 137Cs and239,240Pu concentrations occurred in the eastern SouthPacific.We can depict the oceanographic characteristics ofthe South Pacific from the relationship between the radio-nuclide concentrations and oceanographic parameters (sa-linity and nutrient concentrations), and temporal trends ofthe radionuclide concentrations in the surface water. The137Cs concentrations in surface waters of the mid-latituderegion of the South Pacific have been supported by inputofwater masseswith higher 137Cs concentrations from thecentral tropical South Pacific. The South Pacific subtrop-ical gyre is the largest reservoir of anthropogenic radio-nuclides in the Pacific. The surface 137Cs concentrationsin the east edge of the east South Pacific have beendecreasing by the inflowof the SouthernOceanwaterwithlow 137Cs concentrations. However, there are still largeareas in the eastern tropical South Pacific and the SouthernOcean where no data exist at all.

The world-wide contamination of the world oceansby long-lived radionuclides has been investigatedduring the past four decades. The global fallout 137Csand 239,240Pu found in the ocean have been useful toolsfor tracing water motion and geochemical processes attime scales of several decades.

Acknowledgements

The authors thank the Captain and the crew of the R/V“Mirai” for their assistance during the BEAGLE2003expedition. They are also indebted to Takeshi Kawano,Shuuichi Watanabe and Koji Yoshikawa, all at JAM-STEC, for their help during the BEAGLE2003 cruise.Several cruise participants from Marine Works JapanLtd. assisted during water sampling. The authors arealso indebted to Akira Takeuchi for sample collection,and Ikuo Koshino for radiochemical separations. This


research was partly supported by a Grant-in-Aid forScientific Research from the Ministry of Education,Culture, Sport, Science and Technology of Japan (No.18310017). The authors also thank three anonymousreviewers for their constructive comments. InternationalAtomic Energy Agency is grateful to the Government ofthe Principality of Monaco for support provided to itsMarine Environmental Laboratories.

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