ent 382 (2007) 342–350www.elsevier.com/locate/scitotenv

Science of the Total Environm

137Cs in the western South Pacific Ocean

Masatoshi Yamada a,⁎, Zhong-Liang Wang b

a Nakaminato Laboratory for Marine Radioecology, Environmental Radiation Effects Research Group, National Institute of Radiological Sciences,Isozaki 3609, Hitachinaka, Ibaraki 311-1202, Japan

b State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guanshui Road 46, Guiyang,Guizhou Province 550002, China

Received 18 January 2007; received in revised form 23 April 2007; accepted 27 April 2007Available online 29 May 2007

Abstract

The 137Cs activities were determined for seawater samples from the East Caroline, Coral Sea, New Hebrides, South Fiji andTasman Sea (two stations) Basins of the western South Pacific Ocean by γ spectrometry using a low background Ge detector. The137Cs activities ranged from 1.4 to 2.3 Bq m−3 over the depth interval 0–250 m and decreased exponentially from the subsurface to1000 m depth. The distribution profiles of 137Cs activity at these six western South Pacific Ocean stations did not differ from eachother significantly. There was a remarkable difference for the vertical profiles of 137Cs activity between the East Caroline Basinstation in this study and the GEOSECS (Geochemical Ocean Sections Study) station at the same latitude in the Equatorial PacificOcean; the 137Cs inventory over the depth interval 100–1000 m increased from 400±30 Bq m−2 to 560±30 Bq m−2 during theperiod from 1973 to 1992. The total 137Cs inventories in the western South Pacific Ocean ranged from 850±70 Bq m−2 in theCoral Sea Basin to 1270±90 Bq m−2 in the South Fiji Basin. Higher 137Cs inventories were observed at middle latitude stations inthe subtropical gyre than at low latitude stations. The 137Cs inventories were 1.9–4.5 times (2.9±0.7 on average) and 1.7–4.3 times(3.1±0.7 on average) higher than that of the expected deposition density of atmospheric global fallout at the same latitude and thatof the estimated 137Cs deposition density in 10° latitude by 10° longitude grid data obtained by Aoyama et al. [Aoyama M, HiroseK, Igarashi Y. Re-construction and updating our understanding on the global weapons tests 137Cs fallout. J Environ Monit2006;8:431–438], respectively. The possible processes for higher 137Cs inventories in the western South Pacific Ocean than that ofthe expected deposition density of atmospheric global fallout may be attributable to the inter-hemisphere dispersion of theatmospheric nuclear weapons testing 137Cs from the northern stratosphere to the southern one and its subsequent deposition, andwater-bearing transport of 137Cs from the North Pacific Ocean to the western South Pacific.© 2007 Elsevier B.V. All rights reserved.

Keywords: 137Cs activity; γ spectrometry; Vertical profile; Latitudinal distribution; Water column inventory; Western South Pacific Ocean

⁎ Corresponding author. Tel.: +81 29 265 7130; fax: +81 29 2659883.

E-mail address: m_yamada@nirs.go.jp (M. Yamada).

0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2007.04.039

1. Introduction

The main introduction routes of the artificialradionuclide 137Cs (half life of 30.07 yr) into the PacificOcean are worldwide global fallout from atmosphericnuclear weapons testing and close-in fallout from U. S.tests conducted on the Bikini and Enewetak Atolls in the

343M. Yamada, Z.-L. Wang / Science of the Total Environment 382 (2007) 342–350

northern Marshall Islands (UNSCEAR, 2000; Aarkrog,2003; Hamilton, 2004). Hamilton (2004) has estimatedthat the globally dispersed 137Cs deposited in the PacificOcean was 182 PBq (decay-corrected to January 1,2000), which included an estimated 58 PBq of 137Cs inlocal and regional fallout. In the South Pacific Oceanthere is another source of artificial radionuclides fromthe French nuclear testing at the Mururoa andFangataufa Atolls from 1966 to 1974 (Bourlat et al.,1995; Hamilton et al., 1996; Chiappini et al., 1999). Theactivities of artificial radionuclides are generally lowerin southern hemisphere waters because they receivedless fallout from atmospheric nuclear weapons testing; itwas estimated that approximately 3/4 of test falloutaffected the northern hemisphere and 1/4, the southernhemisphere (UNSCEAR, 2000; Aarkrog, 2003).

In the western South Pacific Ocean, oceanic ridges andisland arcs separate the ocean into many semi-isolatedbasins. This area includes the East Caroline Basin, CoralSea Basin, New Hebrides Basin, South Fiji Basin andTasman Sea Basin; all of these are deeper than 4000 m.The 137Cs activities and inventories in the water columnshave been well investigated in the North Pacific Ocean(e.g., Bowen et al., 1980; Nagaya and Nakamura, 1981,1984, 1987; Aoyama and Hirose, 1995; Aoyama et al.,2000; Povinec et al., 2003a), however, investigations onvertical profiles of 137Cs activities and inventories havebeen limited in the South Pacific Ocean, especially in the

Fig. 1. Map showing the sampling locations. Black circles represent samplinget al. (1980).

present study area of the western South Pacific (Fig. 1).Bowen et al. (1980) reported the 137Cs and 239+240Puactivities at stations in the open ocean east of Fiji andNewZealand (GEOSECS-246, 251 and 263; Fig. 1) during theGEOSECS (Geochemical Ocean Sections Study) Expe-dition. A few measurements of 137Cs activities werecarried out at the Mururoa and Fangataufa Atolls in theFrench Polynesia (Bourlat and Martin, 1992; Bourlatet al., 1996; Hamilton et al., 1996).

The Sagittarius Expedition of the R/V Hakuho-Maruaimed at collecting a geochemical dataset in the westernSouth Pacific Ocean (Kudo et al., 1996; Zhang andNozaki, 1996). The objectives of the present study are tomeasure the 137Cs activities in water columns collectedduring the Sagittarius Expedition in the East Caroline,Coral Sea, New Hebrides, South Fiji and Tasman SeaBasins of the western South Pacific Ocean, to tracetemporal changes by comparing with measurementsreported by the GEOSECS Expedition, to comparethe 137Cs inventories with the integrated depositiondensity of atmospheric global fallout and to discuss theprocesses controlling the 137Cs inventory in the westernSouth Pacific Ocean.

2. Materials and methods

Water samples for 137Cs determinations wereobtained during the Sagittarius Expedition (KH-92-4

locations for this study and black squares represent sites from Bowen

Table 1137Cs activities in the western South Pacific Ocean a

Station Sampling date Depth 137Cs b

(mm/dd/yy) (m) (Bq m−3)

SA-5 (East Caroline Basin) 9/25/92 0 2.31±0.109/25/92 67 2.03±0.099/25/92 100 2.32±0.109/23/92 250 1.40±0.079/23/92 497 0.34±0.049/24/92 749 b0.079/24/92 1000 b0.079/24/92 1244 b0.079/24/92 1500 b0.079/24/92 2008 b0.079/24/92 3008 b0.079/24/92 4004 b0.079/24/92 4490 b0.079/24/92 4781 b0.079/24/92 4979 b0.079/24/92 5086 b0.079/24/92 5136 b0.079/23/92 5145A b0.079/23/92 5145B b0.07

SA-7 (Coral Sea Basin) 9/28/92 0 2.14±0.109/28/92 54 1.81±0.099/28/92 106 1.89±0.099/28/92 212 1.51±0.089/28/92 250 1.44±0.079/28/92 502 0.43±0.079/29/92 755 b0.079/29/92 1001 b0.079/29/92 1242 b0.079/29/92 1508 b0.079/28/92 2016 b0.079/28/92 3004 b0.079/29/92 3983 b0.079/29/92 4226 b0.079/28/92 4393 b0.079/28/92 4592 b0.07

SA-10 (New Hebrides Basin) 10/2/92 0 1.84±0.0810/2/92 108 2.33±0.1010/2/92 162 2.02±0.0810/3/92 256 1.73±0.0810/3/92 498 1.04±0.0410/3/92 751 0.27±0.0410/3/92 999 0.13±0.0410/3/92 1257 b0.0710/3/92 1501 b0.0710/2/92 2008 b0.0710/2/92 3001 b0.0710/2/92 3992 b0.0710/2/92 4977 b0.0710/3/92 5112 b0.0710/3/92 5362 b0.07

SA-12 (South Fiji Basin) 10/5/92 0 2.23±0.1010/5/92 56 2.18±0.0710/5/92 97 2.26±0.1010/5/92 150 2.14±0.1010/5/92 256 1.90±0.0710/5/92 499 1.03±0.08

Table 1 (continued)

Station Sampling date Depth 137Cs b

(mm/dd/yy) (m) (Bq m−3)

10/6/92 747 0.53±0.0710/6/92 982 0.29±0.0810/5/92 1480 b0.0710/6/92 1984 b0.0710/6/92 2966 b0.0710/6/92 4014 b0.0710/6/92 4403 b0.0710/5/92 4557 b0.0710/5/92 4615 b0.07

SA-16 (Tasman Sea Basin) 10/17/92 0 1.70±0.0910/18/92 50 1.54±0.0910/18/92 100 1.70±0.0810/18/92 150 1.58±0.0710/18/92 255 1.54±0.1310/18/92 499 0.99±0.1110/18/92 754 0.37±0.0910/18/92 1009 b0.0710/18/92 1500 b0.0710/18/92 2009 b0.0710/18/92 3006 b0.0710/18/92 4000 b0.0710/18/92 4439 b0.0710/17/92 4453 b0.0710/17/92 4582 b0.0710/17/92 4635 b0.07

SA-19 (Tasman Sea Basin) 10/21/92 0 2.01±0.0910/21/92 50 2.10±0.0910/21/92 100 2.25±0.0910/22/92 150 1.95±0.0910/22/92 253 1.95±0.0710/22/92 501 1.23±0.1010/22/92 756 0.29±0.0910/22/92 1004 b0.0710/22/92 2004 b0.0710/21/92 3001 b0.0710/21/92 3988 b0.0710/21/92 4591 b0.07

a The 137Cs activities are corrected for decay to the date of samplecollection.b The errors quoted for 137Cs activities are 1 σ values derived from

counting statistics.

SA-12 (South Fiji Basin)

344 M. Yamada, Z.-L. Wang / Science of the Total Environment 382 (2007) 342–350

Cruise, from September 16 to October 27, 1992) of theR/V Hakuho-Maru (Fig. 1). Seawater samples (about250 L each) were collected at six stations in the EastCaroline Basin (SA-5: 0°01′S, 149°57′E), the Coral SeaBasin (SA-7: 14°19′S, 154°20′E), the New HebridesBasin (SA-10: 19°09′S, 167°41′E), the South Fiji Basin(SA-12: 27°18′S, 175°24′E) and the Tasman Sea Basin(SA-16: 40°05′S, 155°05′E; SA-19: 27°14′S, 155°12′E)with a double barrel PVC large-volume sampler from thesubsurface (below 250 m) to the bottom. Seawatersamples (about 250 L each) from the surface to thesubsurface (above 200 m) were collected from the ship's

Fig. 2. The latitudinal distributions of 137Cs activities. Black circles arefrom this study and open circles are from Momoshima et al. (2005).

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deck by using a pumping system. As described inprevious papers (Nagaya and Nakamura, 1984, 1987;Nakanishi et al., 1990, 1995), Pu isotopes, 241Am, 230Thand 137Cs were sequentially separated from every watersample. About 200–250 L of an unfiltered water samplewas acidified with hydrochloric acid to a pH of about 2and then 3 g of Fe3+ and 20 mg of Cs+ were added. Afterallowing the solution to stand for more than 2 h, it wasneutralized with ammonia solution to form ironhydroxide precipitate. The iron hydroxide precipitatewas separated from most of the supernatant water andtransferred to a land-based laboratory for furthertreatment. The supernatant was acidified with nitricacid to a pH of 1–2 for measurement of 137Cs activities,and then Cs was adsorbed onto 200 g of ammoniummolybdophosphate (AMP) and brought back to the land-based laboratory.

The analytical procedure for 137Cs determination inseawater was the same as that described previously(Yamada et al., 2006a). The AMP was dried and put intoa plastic container. The activities of 137Cs on AMP weredetermined by γ-spectrometry using a low backgroundtype high-purity Ge detector (EGPC-60-210-R, EurisysMesures). The chemical yields of 137Cs were determinedby the recovery of the added stable Cs+ carrier. Thestable Cs concentrations were measured by a quadru-pole-type ICP-MS (HP-4500, Yokogawa AnalyticalSystems) after the dissolution of AMP inNaOH solution.

3. Results and discussion

3.1. 137Cs activities in the surface waters

Analytical results for 137Cs activities are given inTable 1. The 137Cs activities were corrected for decay tothe date of sample collection. The 137Cs activities insurface waters in 1992 ranged from 1.7 Bq m−3 in theTasman Sea Basin (SA-16) to 2.3 Bq m−3 in the EastCaroline Basin (SA-5). Very few data have been reportedto date on the 137Cs activities in surface waters in thepresent study area, as shown by the Global MarineRadioactivity Database (GLOMARD)maintained by theInternational Atomic Energy Agency's Marine Environ-ment Laboratory in Monaco (Povinec et al., 2004).Bourlat et al. (1996) have reported 137Cs activities in1993 for surface water of 2.77±0.55 Bq m−3 from theEast Caroline Basin and 1.93±0.39 Bq m−3 from theCoral Sea Basin. The present data are on the same levelas their data. 137Cs activities also were obtained byPovinec et al. (2003b) as about 1.5 Bqm−3 in the TasmanSea (around 40°S) in 1998 and byMiyake et al. (1988) as4.4–6.7 Bq m−3 between 0° and 40°S in 1978.

The latitudinal distributions of 137Cs activities insurface waters between 5°N and 40°S are shown inFig. 2, together with data by Momoshima et al. (2005)which were obtained during the same cruise of the R/VHakuho-Maru in 1992. The 137Cs activities monoton-ically decreased from 5°N to 40°S. The expected 137Csdeposition density of atmospheric global fallout in thesouthern hemisphere showed a relative maximumbetween 40°S and 50°S and a slight decrease towardsthe equator (UNSCEAR, 2000; Aarkrog, 2003). Thelatitudinal 137Cs distributions in the western SouthPacific Ocean showed the opposite trend to the expecteddeposition density from global fallout.

3.2. Vertical profiles of 137Cs activities

Vertical profiles of 137Cs activities in the western SouthPacific Ocean are shown in Fig. 3. The 137Cs activitiesfrom surface waters to subsurface waters of 250 m depthranged from 1.4 to 2.3 Bq m−3 and showed no significantdecrease with depth at each station. A difference wasobserved in 137Cs activities in the 0–250 m depth betweenthe southernmost station (SA-16) of the Tasman Sea Basinand the other five stations, i.e., the 137Cs activities (1.65 Bqm−3 on average) at SA-16 were relatively lower than those(2.07 Bqm−3 on average) in the other five stations. TheT–S diagram for the six stations is given in Fig. 4. The near-surface layers in thewestern South Pacific Ocean consist ofTropical Surface Water (TSW) and Subtropical LowerWater (SLW). The Antarctic Intermediate Water (AAIW;T=5–6 °C) occupies the intermediate layer around 800±200 m just below the Subantarctic Mode Water (SAMW).AAIW is characterized by low salinity (34.4–34.5) andhigh dissolved oxygen content. AAIW and SAMW areexported from the Southern Ocean into the subtropicalgyres. TSWand SLW were not found in the surface layersonly at SA-16 (Fig. 4) where the surface 137Cs activities

Fig. 3. Vertical distributions of 137Cs activities at: (a) SA-5 (East Caroline Basin), (b) SA-7 (Coral Sea Basin), (c) SA-10 (New Hebrides Basin), (d)SA-12 (South Fiji Basin), (e) SA-16 (Tasman Sea Basin) and (f) SA-19 (Tasman Sea Basin). The horizontal tick marks plotted below the verticaldistributions indicate the depth of the bottom.

346 M. Yamada, Z.-L. Wang / Science of the Total Environment 382 (2007) 342–350

were relatively lower than those in the other five stations.On the basis of water properties and geostrophicvelocities measured on a meridional section along155°E through the East Australian and Coral Sea Basins,Sokolov and Rintoul (2000) described the circulation nearthe western boundary of the South Pacific Ocean asfollows: the primary inflow to the Tasman and Coral Seasis supplied by the South Equatorial Current (SEC), whichcrosses the section along 155°E as a wide band ofwestward flow between 14°S and 18°S; the SECbifurcates at the Australian coast near 18°S to formnorthward and southward flowing boundary currents; thesouthern branch feeds the East Australian Current (EAC);the core of the EAC lies over the continental slopebetween 18°S and 30°S; after the EAC separates from the

coast at 30°S, more than half of the EAC recirculatesnorth and then west, and the remainder continues east as ameandering jet across the Tasman Sea; an anticycloniccirculation facilitates the exchange of water between thesouthern part of the Tasman Sea and the Southern Ocean;subantarctic water spreads north to 36–38°S, and thenrecirculates back to the west. Lower surface 137Csactivities at SA-16 than in the other five stations mightbe attributable to inflow of the 137Cs-poor waters (Miyakeet al., 1988) to the southern Tasman Sea from theSouthern Ocean.

The 137Cs activities decreased exponentially from thesubsurface to 1000 m depth, where the SAMW andAAIW are found, and the distribution profiles of 137Csactivities at the six western South Pacific Ocean stations

Fig. 4. T–S diagram in the western South Pacific Ocean (Nozaki,1992). Abbreviations are: Tropical Surface Water (TSW), SubtropicalLower Water (SLW), Subantarctic Mode Water (SAMW), AntarcticIntermediate Water (AAIW), Southern Pacific Deep Water (SPDW)and Antarctic Bottom Water (AABW).

Fig. 5. Vertical distributions of 137Cs activities at SA-5 (open circles)from this study and GEOSECS-246 (black squares) from Bowen et al.(1980).

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did not differ from each other so much. 137Cs in theocean is only weakly associated with particulate matterand it has been shown to behave predominantly as asoluble water tracer. The present striking feature wasthat the 137Cs activity at 1000 m depth in the South FijiBasin (SA-12) was significantly higher than those at the1000 m depth in the other five stations. This indicatedthat the 137Cs-bearing waters might be being conveyedinto a deeper layer in the South Fiji Basin than in theother basins.

3.3. Comparison with the GEOSECS data

In order to elucidate the temporal variation in thevertical profile of 137Cs activities, the distributionpatterns and inventories of 137Cs were compared be-tween SA-5 (0°01′S, 149°57′E) and GEOSECS-246(0°0′S, 179°0′E; sampling date: December 21, 1973;Bowen et al., 1980) at the same latitude in the EquatorialPacific Ocean. The 137Cs activities at GEOSECS-246were corrected for decay to the date of SA-5 samplecollection, September 24, 1992.

There was a significant difference for the verticalprofiles of 137Cs activities between SA-5 and GEO-SECS-246; the surface water 137Cs activities in the 0–100 m depth were higher at GEOSECS-246 than at SA-5; the intermediate water 137Cs activities below 250 m atGEOSECS-246 were lower than those at SA-5 (Fig. 5).The 137Cs inventory over the depth interval 0–100 m

decreased from 280±15 Bq m−2 at GEOSECS-246 to220±10 Bq m−2 at SA-5 during the period from 1973 to1992. On the other hand, the 137Cs inventory of 400±30 Bq m−2 over the depth interval 100–1000 m atGEOSECS-246 corresponded to approximately 70±7%of that (560±30 Bq m−2) at SA-5. The increasing rate of137Cs over the depth interval 100–1000 m could beestimated to be 8.7 Bq m−2 yr−1 based on the differencein the 137Cs inventories between SA-5 and GEOSECS-246. The Pacific equatorial surface currents areprimarily wind-driven: The surface flows are dominatedby the westward SEC between about 3°N and 20°S andthe North Equatorial Current (NEC) between about10°N and 20°N; a weaker eastward-flowing SouthEquatorial Countercurrent (SECC) extends eastwardfrom the region of the western boundary; the subsurfaceflows are dominated by the eastward EquatorialUndercurrent (EUC) which is the strongest equatorialPacific current; the much weaker Southern SubsurfaceCountercurrent (SSCC) flows eastward below thepoleward flanks of the EUC; the westward EquatorialIntermediate Current (EIC) is found directly belowthe EUC across the Pacific; along the western Pacificboundary, the New Guinea Coastal Undercurrent(NGCUC) flows westward along the north coast ofNew Guinea with a maximum near 200 m, but ex-tending to at least 800 m (Lukas, 2001). These resultsindicated that higher activities and inventory of 137Csobserved in the intermediate water at SA-5 than atGEOSECS-246 were probably due to water transportinto the intermediate layer on a time scale of about twodecades.

Table 2137Cs inventories in the water column, deposition densities of atmospheric fallout and their ratios a

Station Location 137Cs inventory(Bq m−2)

Global fallout b

(Bq m−2)10°×10° data c

(Bq m−2)Ratio to GFd Ratio to grid data e

Minimum f Maximum f Minimum f Maximum f Minimumf Maximum f

SA-5 0°01′ S 149°57′ E 650±30 910±80 420 g 305 1.5±0.1 2.2±0.2 2.1±0.1 3.0±0.2SA-7 14°19′ S 154°20′ E 620±30 850±70 270 235 2.3±0.1 3.2±0.3 2.7±0.1 3.6±0.3SA-10 19°09′ S 167°41′ E 930±30 1220±90 270 285 3.4±0.1 4.5±0.3 3.3±0.1 4.3±0.3SA-12 27°18′ S 175°24′ E 1090±50 1270±90 450 375 2.4±0.1 2.8±0.2 2.9±0.1 3.4±0.2SA-16 40°05′ S 155°05′ E 800±60 1020±90 540 h 415 1.5±0.1 1.9±0.2 1.9±0.1 2.4±0.2SA-19 27°14′ S 155°12′ E 980±40 1190±90 450 715 2.1±0.1 2.6±0.2 1.4±0.1 1.7±0.1a The decay of 137Cs is normalized to January 2000.b The expected deposition density of atmospheric global fallout (UNSCEAR, 2000).c The estimated deposition density in 10°×10° grid data (Aoyama et al., 2006).d Ratio of the measured 137Cs inventory to the expected deposition density of atmospheric global fallout at the same latitude.e Ratio of the measured 137Cs inventory to the estimated deposition density in 10°×10° grid data.f The errors quoted are 2 σ values.g The average deposition density in the latitude of 10°N–10°S.h The average deposition density in the latitude of 30°S–50°S.

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3.4. Water column inventories

Each station's 137Cs inventory was calculated byintegrating the 137Cs activity vertically from the surface tothe bottom. This was estimated by linear interpolation ofthe 137Cs activity measured at each depth in this study.137Cs activity below the detection limit of 0.07 Bq m−3

was approximated as 0 Bq m−3 for the minimum valueand 0.07 Bq m−3 for the maximum value in the inventorycalculation. The 137Cs inventories in the water column aregiven in Table 2, together with the latitudinal 137Cscumulative deposition density of atmospheric globalfallout calculated from UNSCEAR (2000) and thegeographical 137Cs deposition density as grid data in10° latitude by 10° longitude reported by Aoyama et al.(2006). Fallout data of 137Cs over the oceans are lacking,so that it was supposed that the latitudinal depositiondensity patterns over land and sea were comparable. The137Cs activities were corrected for decay to January 2000.

The 137Cs inventories in the western South PacificOcean between the equator and 40°S ranged from 620±30 Bqm−2–850±70 Bqm−2 in the Coral Sea Basin (SA-7) to 1090±50 Bq m−2–1270±90 Bq m−2 in the SouthFiji Basin (SA-12). Higher 137Cs inventories wereobserved at middle latitude stations (SA-10, SA-12 andSA-19) in the subtropical gyre than at low latitude stations(SA-5 and SA-7). The ratios of the measured 137Csinventory to the expected deposition density of atmo-spheric global fallout at the same latitude were between1.5±0.1–3.4±0.1 for the minimum value and 1.9±0.2–4.5±0.3 for the maximum value. At all stations in thewestern South Pacific Ocean, the 137Cs inventories weresignificantly higher than that of the expected deposition

density of global fallout (UNSCEAR, 2000). The 137Csinventories were also 1.4±0.1–3.3±0.1 times higher forthe minimum value and 1.7±0.1–4.3±0.3 times higherfor the maximum value than that of the estimated 137Csdeposition density in 10° latitude by 10° longitude griddata as obtained by Aoyama et al. (2006). Furthermore, inspite of the lower expected deposition density of globalfallout at the same latitude region in the South PacificOcean than that in the North Pacific Ocean, the 137Csinventories of 930±30–1220±90 Bq m−2 (SA-10) at thelatitude of 10–20°S in the western South Pacific Oceanwere remarkably higher than those (590–610 Bq m−2;decay-corrected to January 2000) reported at the latitudeof 10–20°N (IAEA97-6: 11°27′N, 164°53′E; IAEA97-7:11°30′N, 161°45′E) in the western North Pacific Oceanby Povinec et al. (2003a).

The latitudinal distributions of 137Cs inventories overthe depth interval 0–500 m and total 137Cs inventory inthe whole water column between 10°N and 50°S areshown in Fig. 6a and b, respectively. Grey shadings inFig. 6b indicate the latitudinal distributions of theexpected 137Cs deposition density of atmospheric globalfallout (decay-corrected to January 2000; UNSCEAR,2000). The expected 137Cs deposition density ofatmospheric global fallout in the southern hemispheredecreased in value on going between 50°S and 40°Stowards the equator. The total 137Cs inventory maximumwas observed between 20°S and 30°S in the subtropicalgyre in the South Pacific Ocean (Fig. 6b). The latitudinaldistributions of total 137Cs inventory showed a differenttrend to the expected 137Cs deposition density of globalfallout. The 137Cs inventories over the depth interval 0–500 m ranged from 570±30 Bq m−2 to 770±30 Bq m−2

Fig. 6. The latitudinal distributions of 137Cs inventories in: (a) 0–500 mdepth interval and (b) the whole water column. The errors quoted for137Cs inventories are 2 σ values. Grey shadings in b indicate thelatitudinal distributions of the expected 137Cs deposition density ofatmospheric global fallout (decay-corrected to January 2000;UNSCEAR, 2000). Total 137Cs inventories for minimum and maximumvalues in each station are plotted in b.

349M. Yamada, Z.-L. Wang / Science of the Total Environment 382 (2007) 342–350

and also had a maximum in the subtropical gyre (Fig. 6a).The percentages of the 137Cs inventory in the 0–500 mdepth interval ranged from 59% to 67% (63%±3% onaverage). This value was comparable to that of 63%observed in the South China Sea (Yamada et al., 2006b),suggesting that a large portion of 137Cs still remained inthe upper layer in the western South Pacific Ocean.

The possible sources of excess 137Cs inventories inthe western South Pacific Ocean might be attributable toboth the inter-hemisphere dispersion of the atmosphericnuclear weapons testing 137Cs from the northernstratosphere to the southern one and its subsequentdeposition, and water-bearing transport of 137Cs fromthe North Pacific Ocean to the South Pacific. Unfortu-nately, it is difficult to discriminate the relativecontributions to the higher inventories by stratosphericand water-bearing transport. Further data are needed toresolve the relative contributions of these processes.

4. Conclusions

Six vertical profiles of 137Cs activities in the EastCaroline, Coral Sea, New Hebrides, South Fiji andTasman Sea Basins were provided in this study. The137Cs activities in surface waters ranged from 1.7 Bqm−3 in the Tasman Sea Basin to 2.3 Bq m−3 in the EastCaroline Basin. The latitudinal 137Cs distributions insurface waters showed the opposite trend to theexpected deposition density from global fallout. The137Cs activities from surface waters to subsurface watersof 250 m depth showed no significant decrease withdepth at each station. The 137Cs activities decreasedexponentially from the subsurface to 1000 m depth.There was a significant difference for the verticalprofiles of 137Cs activities between SA-5 and GEO-SECS-246, indicating that their temporal variationoccurred on a time scale of about two decades. Thetotal 137Cs inventories were significantly higher thanthat of the expected deposition density of global falloutand that of the estimated 137Cs deposition density byAoyama et al. (2006). The higher 137Cs inventories inthe western South Pacific Ocean may be due to the inter-hemisphere transport of 137Cs from the northern strato-sphere to the southern one and water-bearing transportof 137Cs from the North Pacific Ocean to the westernSouth Pacific. Further studies are needed to resolve therelative contributions of the stratospheric and water-bearing transports.

Acknowledgements

The authors would like to thank the late Dr. Y.Nozaki, and the captain, officers and crew of the R/VHakuho-Maru for their collaboration in the samplingduring the KH-92-4 cruise. The authors wish to expresstheir gratitude to Drs. T. Nakanishi, H. Narita and J.Zhang for their help with sample collection and to Dr. J.Zheng for his valuable suggestions. The authors are alsograteful to the three anonymous reviewers for usefulcomments on the manuscript. Z.-L. Wang acknowledgesa postdoctoral fellowship from the Japan Society for thePromotion of Science.

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