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*Correspondin

E-mail addres1Present addre2Present addre

0967-0645/$ - see

doi:10.1016/S096

IAEA’97 expedition to the NW Pacific Ocean—results ofoceanographic and radionuclide investigations of the

water column

Pavel P. Povineca,*, Hugh D. Livingstona, Shigeki Shimab, Michio Aoyamac,Janine Gastauda, Ingo Goroncyd, Katsumi Hirosec, Lang Huynh-Ngoca,

Yoshihiro Ikeuchia,e, Toshimichi Itoa,f,1, Jerome La Rosaa,Laval Liong Wee Kwonga, Sang-Han Leeg,a, Hiroshi Moriyah, Sandor Mulsowa,

Beniamino Oregionia, Hakan Petterssona,2, Orihiko Togawaa,f

aMarine Environment Laboratory, International Atomic Energy Agency, 4 Quai Antoine 1er, Monte Carlo 98012, MonacobMutsu Marine Laboratory, Japan Marine Science Foundation, Mutsu 035-0064, Japan

cGeochemical Research Department, Meteorological Research Institute, Tsukuba 305-0052, JapandBundesamt fuer Seeschiffahrt und Hydrographie, Hamburg, Germany

eJapan Chemical Analysis Center, Office of Planning, Chiba-ken 263, JapanfJapan Atomic Energy Research Institute, Mutsu Establishment, Mutsu 035-0064, JapangKorea Ocean Research and Development Institute, Seoul 425-600, Republic of Korea

hTokai University, Institute of Oceanographic Research and Development, Shimizu 424-8610, Japan

Received 25 October 2002; received in revised form 28 March 2003; accepted 30 April 2003

Abstract

An international sampling cruise, the IAEA’97 NW Pacific Expedition, was carried out in 1997 with the primary

objective of contributing to better understanding of the present distributions of radionuclides in the open ocean and of

the processes affecting their distributions in the water column, to study the sources which have introduced radionuclides

to the Pacific Ocean, and to compare present results with historical data sets obtained from the previous expeditions

(e.g., the Geochemical Ocean Sections Programme, GEOSECS). The observed temperature, salinity, dissolved oxygen,

and silicate levels in water samples collected showed differences in the upper 1000m layer in all studied profiles.

Transuranics water profiles showed typical sub-surface maxima, with concentrations less by about a factor of 4 than the

GEOSECS results, at depths greater by about a factor of 2 and a decrease in the water column inventory by about 20%

over 24 years, which were less a function of particle-driven vertical processes and more in response to water mass

circulation in the region. Increases in concentrations were observed in deep water near the sea floor. Transuranics

inventories in the water column substantially exceeded those expected from global fallout. 90Sr and 137Cs data confirm

that the observed changes in concentration profiles have resulted from physical circulation of the regional water masses

and transport of radionuclides to deep waters. The IAEA’97 results present the most comprehensive recent study on the

g author. Tel.: +377-97-97-72-72; fax: +377-97-97-72-73.

s: p.povinec@iaea.org (P.P. Povinec).

ss: Japan Atomic Energy Research Institute, Tokaimura, Japan.

ss: University of Linkoeping, Department of Radiation Physics, S-58185 Linkoeping, Sweden.

front matter r 2003 Elsevier Ltd. All rights reserved.

7-0645(03)00138-3

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distribution of 239,240Pu, 90Sr and 137Cs in the NW Pacific Ocean with a description of the behaviour of these

radionuclides in the water column and the effects of the physical forcing of water mass circulation over the last 24 years.

r 2003 Elsevier Ltd. All rights reserved.

1. Introduction

The IAEA’s Marine Environment Laboratoryin Monaco carried out from 1995–2000 a researchproject on Worldwide Marine Radioactivity Stu-dies (WOMARS; IAEA, 2001a). The primaryobjectives of the part of the project devoted tothe Pacific Ocean were to contribute to betterunderstanding of the present open ocean distribu-tions of radionuclides in seawater and sediment, toquantify the contribution of different sources thathave introduced radioactivity to the Pacific Ocean,to provide new data on marine radioactivity andto compare them with other national and interna-tional surveys so that possible contributions fromnuclear weapons test sites and former radioactivewaste dumpsites in the Pacific Ocean could bereviewed objectively and comprehensively. Afurther objective of the WOMARS project wasto contribute to the scientific knowledge of theprocesses affecting tracer distribution by studyingradionuclides in the Pacific Ocean (Povinec andTogawa, 1998; Povinec et al., 1999a; IAEA,2001a, b).The major source of anthropogenic radionu-

clides in seawater and sediment in the PacificOcean can be traced to global and local falloutfrom atmospheric nuclear weapons testing. Othersources have introduced relatively smalleramounts of radionuclides into the Pacific Ocean(Livingston and Povinec, 2000, 2002). A numberof long-lived radionuclides persist and are stillmeasurable in the marine environment. Some ofthese radionuclides, such as 90Sr and 137Cs, areuseful tracers of water-mass movement. Thetransuranics, represented by 239,240Pu and 241Am,are highly particle-reactive and are useful tracersof deep-water movement and sedimentary pro-cesses in the ocean (Cochran, 1985). Concentra-tion data for these radionuclides are also valuableto establish baselines applicable for dose assess-ments from consumption of marine food products

and for understanding other biogeochemical pro-cesses in the ocean.There is a long list of papers (IAEA, 2001a) in

which 90Sr, 137Cs and 239,240Pu behaviour in thePacific Ocean has been discussed. The mostextensive studies were carried out during the1973–1974 GEOSECS programme, which pro-vided the first comprehensive data on the lateraland vertical distributions of 90Sr, 137Cs and239,240Pu in Pacific waters (Bowen et al., 1980).Traditionally, several Japanese institutes havebeen active in investigating these radionuclides,especially in NW Pacific waters and sediment (e.g.,Nagaya and Nakamura, 1976, 1984, 1987, 1992,1993; Aoyama and Hirose, 1995; Miyake et al.,1988; Nakanishi et al., 1990; Hirose et al., 1992,1999, 2001; Ikeuchi et al., 1999 and many others).The third group of investigations was connected tonuclear weapons testing sites in the MarshallIslands (e.g., Noshkin et al., 1983; Robison andNoshkin, 1999) and French Polynesia (e.g.,Bourlat et al., 1996; IAEA, 1998; Povinec et al.,1999b).The water-column distribution and behaviour of

90Sr, 137Cs and 239,240Pu, i.e., distributions, in-ventories, speciation and associations, have beenthe primary focus of previous work. Bowen et al.(1980) made the most comprehensive study ofthese radionuclides in water of the open PacificOcean obtained at 29 stations in 1973–74 as part ofthe GEOSECS programme. A number of otherstudies have been made subsequent to the GEO-SECS study (Nagaya and Nakamura, 1981, 1984,1987; Fowler et al., 1983; Miyake et al., 1988;Nakanishi et al., 1990; Hirose et al., 1992; Aoyamaand Hirose, 1995; Livingston and Noshkin,personal communication; Ikeuchi et al., 1999;Livingston et al., 2001. These studies providedata to augment the GEOSECS data and tocompare possible changes with time in thedistributions and inventories of radionuclides inthe water column.

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Fig. 1. The route of the IAEA’97 expedition and sampling locations.

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The GEOSECS expedition provided 90Sr, 137Csand Pu water profile data with high spatial anddepth resolutions (Bowen et al., 1980). Conse-quently, one of the objectives of the IAEAexpedition to the NW Pacific Ocean was to re-visit some of the GEOSECS stations so that theevolution of radionuclide concentrations in thewater column and their distribution could beevaluated with time. A similar comparison ispossible with the data obtained from theKNORR-1978 (Livingston and Noshkin, personalcommunication) and ‘‘Hakuho Maru’’-1980 and1982 (Nagaya and Nakamura, 1984; Livingston,1986) expeditions. The data compiled from thesefour expeditions, as well as data from otherexpeditions to the region, enabled the assessmentof time series for studying the behaviour ofradionuclides in the ocean with time over the last24 years (IAEA, 2001a).As part of the WOMARS project, the IAEA’s

Marine Environment Laboratory in Monacoorganized the NW Pacific Ocean Expedition(IAEA’97) in conjunction with several interna-tional institutes. The expedition took place from21 October to 20 November 1997 on board theR.V. ‘‘Bosei Maru’’ (Tokai University, Japan).

Seawater, sediment and biota were collected at 21stations on the route from the home port, Shimizu(Japan) to Milwaukee Sea-Mountain, then southto the Marshall Islands, Pohnpei (Micronesia),and back to Shimizu (Fig. 1).Recognizing that limitations in resources and

time make ocean-wide monitoring surveys impos-sible, the expedition (i) had a primary regionalfocus on the NW Pacific Ocean, (ii) carried outsampling and measurements that could providelinkages to extrapolate the results to contemporaryocean-wide surveys such as the tracer componentof the World Ocean Circulation Experiment(WOCE), (iii) collected water samples at locationswhere good quality historical radionuclide dataexist—such as at stations occupied in the GEO-SECS programme in the mid-1970s, (iv) sampledsediment (see Moon et al., 2003), plankton and fishto study the distribution of natural radionuclidesin the NW Pacific Ocean.This paper describes the results obtained from

the IAEA’97 NW Pacific Ocean Expedition, inparticular, oceanographic data, the distributionsof 239,240Pu, 137Cs and 90Sr in the water columnand their temporal trend from the GEOSECS era.The results of the investigation of sediment

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collected during the Expedition are presented inthe paper by Moon et al. (2003).

2. Background oceanography of the NW Pacific

Ocean

There are specific circulation patterns in the NWPacific that makes this area very interesting foroceanographic investigations. In the equatorialregion, the North Equatorial Current (NEC), isthe dominant circulation current, which flowswestward at the southern boundary of the surveyarea (around Pohnpei) and on reaching the islandof Honshu its western boundary current becomesthe Kuroshio Current (Wijffels et al., 1998). TheKuroshio Current leaves the Japanese coast atbetween 36�N and 39�N, and meets the OyashioCurrent (subarctic gyre). The Kuroshio Extensionbifurcates at around 155�E to the main currentand its northern branch (Kawabe and Taira,1998).Typical water masses in the North Pacific are

the North Pacific Tropical Water (NPTW), theSubtropical Mode Water (NPSTMW), and theNorth Pacific and Antarctic Intermediate Water(NPIW, AAIW). The NPTW is composed of salinesurface waters produced in the central region ofthe North Pacific. NPSTMW and NPIW aregenerated in the western North Pacific, whileAAIW enters from the South Pacific. NPSTMWis defined as a voluminous water mass in anisothermal layer around 16.5�C (Masuzawa, 1969)and a layer of potential vorticity minimumo2� 10�12 cm�1 s�1 (McCartney, 1982; Talley,1988; Suga et al., 1989), which is formed by deepvertical convection in winter just south of theKuroshio Current and the Kuroshio Extension(McCartney, 1982). NPIW and AAIW exist in theintermediate minimum salinity layers at isopycnal26.8sy (potential density) and at isopycnal 27.28sy;respectively (Reid, 1965). NPIW is formed in theregion where subtropical waters mix with subarcticwaters and extends to the subtropical gyre (Talley,1993), while AAIW extends to the tropics in theNorth Pacific. Deep water in the investigated areaare occupied by North Pacific Deep Water(NPDW).

3. Methods

3.1. Water sampling and CTD measurements

Sampling work in the NW Pacific Ocean wascarried out at 21 stations, including four GEO-SECS stations and seven stations close to Bikiniand Enewetak atolls (Fig. 1). Sea-surface tempera-ture (SST) and sea-surface salinity (SSS) weremeasured throughout the cruise using a Thermo-salinometer (SBE 24) fitted on board. A CTD(SBE 9 plus) with a Rosette Multi-bottle Sampler(CTD/RMS, SBE 32) was used to measureoceanographic parameters and to collect small-and medium-volume seawater samples. CTD/RMS sampling was done at least twice at eachstation and two or three bottles of seawatersamples were collected at each depth (to obtain asufficient volume of samples for chemical andradiochemical analyses). Two casts were made ateach station: a shallow cast down to 700m and adeep cast from 700m to bottom. The dataobtained from the deep casts were used as CTDprofile data. Seawater samples were collectedduring up-casts. Additionally, dissolved oxygen(DO) was continuously measured using the DOsensor (SBE 23) attached to the CTD. Thesampling depths at each station are shown inTable 1. These depths are different to those instandard oceanographic investigations because themain objective of the cruise was to collect newinformation on radionuclide distributions in theNW Pacific Ocean. After retrieving the CTD/RMSunderwater unit, seawater samples collected inNiskin bottles were divided into small portionsand used for chemical analyses on board (i.e.,dissolved oxygen, nutrients, pH). The radionuclidedeterminations and alkalinity measurements werecarried out on land in the laboratories of theparticipating institutes.Large-volume subsurface seawater samples were

collected at 5 stations by two to four casts.Fourteen 270-l and two 500-l ‘‘Gerard Ewing’’fiberglass reinforced plastic samplers were used forlarge volume water sampling. A pressure meterwas used to control water depth at all casts whereeither one 500-l sample or two 270-l samples weretaken and a pinger system was used for near-

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Table 1

Summary of CTD/RMS sampling

Station Bottom

depth (m)

Start

(date/time)

Location End

(date/time)

Location Sampling

depth (m)

Remarks

St. 1 5920 Oct-23/97 34–59.93N Oct-24/97 34–59.71N 9 49 100 150 249 346 496

23:28 145–59.85E 04:27 145–58.53E 694 992 1485 1979 2472 2960 3448

3937 4423 4908 4995

St. 2 6100 Oct-26/97 34–55.33N Oct-26/97 34–57.07N 10 40 79 130 199 300 445

01:12 151–54.69E 06:27 151–52.53E 597 801 1096 1979 2953 3933 4811

St. 2B 5920 Oct-28/97 32–36.99N Oct-28/97 32–37.44N 9 39 79 129 199 300 449

00:35 161–54.78E 06:02 161–53.89E 595 796 1093 1978 2960 3933 4909

St. 3 5470 Oct-30/97 30–33.98N Oct-30/97 30–33.75N 9 39 79 129 198 298 447

02:55 170–36.93E 07:06 170–36.62E 598 794 1092 1976 2956 3932 4911

St. 4 5980 Oct-31/97 24–59.94N Oct-31/97 25–00.01N 10 50 100 149 252 348 492

18:25 170–05.04E 23:15 170–05.33E 693 992 1484 2466 3449 4420 4908

St. 5 4860 Nov-01/97 15–00.21N Nov-01/97 15–00.21N 10 48 149 247 349 495 730 Sampling

interrupted

16:48 166–59.70E 20:02 166–59.56E

St. 6 4530 Nov-10/97 11–26.86N Nov-11/97 11–26.84N 9 39 79 128 198 297 448

20:03 164–52.60E 00:21 164–52.34E 595 792 989 1778 2761 4335 4500

St. 7 3880–3130 Nov-04/97 11–30.17N Nov-05/97 11–30.48N 9 48 98 149 247 347 495

21:53 161–44.63E 03:01 161–44.48E 693 991 1484 1975 2469 3525

St. 8 5520 Nov-14/97 15–30.60N Nov-14/97 15–30.13N 9 39 78 128 198 298 448

08:35 159–27.25E 13:00 159–30.03E 595 794 1091 1977 2763 4714 4908

St. 9 5600 Nov-16/97 22–22.69N Nov-16/97 22–22.26N 247 497 744 992 1485 1977 2468

01:54 152–40.25E 05:43 152–39.24E 2956 3447 3933 4910

St. 10 6000 Nov-18/97 29–29.61N Nov-18/97 29–28.91N 250 494 747 995 1485 1977 2467

01:08 146–50.75E 05:01 146–52.73E 2957 3447 3934 4906

St. P 4280-4200 Nov-06/97 07–19.96N Nov-06/97 07–18.66N 9 247 497 742 994 1486 Cast for

Pu only

02:38 158–24.92E 05:38 158–24.34E

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bottom samples. Large-volume surface watersamples of about 500 l were collected at all stationseither by using a submersible pump or a pump ondeck. All samples were pumped into precipitationtanks on deck for further processing.In order to separate particulate and dissolved

matter, all samples were pumped (90 lmin�1,B0.8 atm) through a membrane filter (0.45 mmpore � 293mm dia.) placed in a stainless steelholder.

3.2. Sample treatment and radionuclide analyses

The pre-concentration chemistry has been de-scribed by Lee et al. (2001) and La Rosa et al.(2001). After further radiochemical treatment ofsamples in laboratories 239,240Pu, 137Cs and 90Srmeasurements were carried in IAEA-MEL (Povi-

nec et al., 2001), Bundesamt fuer Seeschiffahrt undHydrographie (BSH), Meteorological ResearchInstitute (MRI, Aoyama et al., 2001a) and JapanChemical Analysis Centre (JCAC).

4. Results and discussion

4.1. Results of oceanographic observations

4.1.1. Physical and chemical measurements

The survey area extended from the tropics to theboundary between the subtropical and subarcticregions. Fig. 2 shows results of SST and SSSmeasurements along the track of the cruise. TheSST was 20–25�C at 26–35�N and 26–30�C below26�N. Lower SST was observed east of Honshu inthe Kuroshio Extension where subtropical waters

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20 22 24 26 28 30 32 32.5 33 33.5 34 34.5 35SSS (PSU)SST (˚C)

Sea surface temperature Sea surface salinity(a) (b)

Fig. 2. Sea surface temperature (SST) and sea surface salinity (SSS) measured along the track of the IAEA’97 cruise.

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and subarctic waters converge. The values for SSSranged from 33.5–35 PSU. Maximum SSS valueswere found around the Tropic of Cancer andminimum values near Pohnpei in the subtropicalgyre and the Pacific tropical current system. Thepatterns of SSS and SST reflected the circulationof surface water masses and were similar tocirculation patterns typical of the NW PacificOcean.Fig. 3 shows the CTD profiles for all sampling

stations. The temperature profiles showed that themixing layer in the subtropical Pacific was shallowand the widths of mixing layers in the mid-latituderegion were deeper than at lower latitudes. Watertemperature in the region of Bikini atoll decreasedregularly with depth. On the other hand, tempera-ture profiles at the station where subtropical andsubarctic waters converge, showed two thermo-clines and their trends with depths were differentto the trends found in the tropical zones.Especially, a discontinuous temperature profilewas found at 400–700m at St. 1. Salinity profiles inthe NW Pacific showed a minimum level in thesubsurface layer and the depth of the lowestsalinity level in the subtropical zone were shal-lower than at high latitudes. Irregular variations inlevels of salinity were observed at Sts. 1 and 2

which were located in the Kuroshio Extensionwhere subtropical and subarctic water masses mix.Dissolved oxygen measurements were per-

formed using two methods: the CTD/DO andthe titration procedure (t-DO) for water collectedby hydrocasts as the response rate of the DOsensor was very slow, of the order of severalseconds. The CTD/DO profiles are similar to thesalinity profiles, and their minimum levels are atshallower depths in tropical than in subtropicalareas.Fig. 4 shows vertical profiles of nutrients that

include SiO2–Si, PO4–P, NO2–N, NO3–N, pH andtotal alkalinity (TA), as well as vertical profiles ofpotential temperature, salinity, DO, and apparentoxygen utilizations (AOU) at sample collectiondepths. The depletion of nutrients was found insurface and subsurface layers because of biologicalactivity. The depth of the depleted nutrient-layerin the Kuroshio Extension is deeper than in thesubtropical area. Concentrations of nitrate andphosphate in the NW Pacific water columnsincreased with depth and were at a maximum atthe DO minimum layer and then decreasedgradually. The maximum silicate concentrationwas deeper than the maximum nitrate andphosphate concentrations, although concentra-

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JAPAN

Fig. 3. Profiles of potential temperature and salinity in all

stations in the upper ocean: potential temperature (top) and

salinity (bottom).

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tions of silicate increased with depth as did nitrateand phosphate. The pH levels ranged from 7.6 to8.25, the minimum value was found at the samedepth as the DO minimum layer and subsequentlyincreased gradually. The decrease of pH fromsurface to the pH minimum layer was due to thegeneration of acidic compounds (carbon dioxide,nitrate and others) by the decomposition oforganic matter which is produced in the surfacelayer. Values of TA were from 2000 to 2400 mMand generally increased up to the DO minimumlayer at all stations. Below the DO minimum layer,the TA values were nearly constant. AOU were

calculated by subtracting t-DO from saturatedoxygen concentrations at the correspondingdepths. The lowest AOU value was found in thesurface layer at each station and increased withdepth. Maximum AOU values appeared in the DOminimum layers and gradually decreased withdepth.Following the T2S diagrams (Fig. 5) stations

are classified into three groups. The first group, St.1, 2, 2B, 3, 4, 9 and 10 have salinity minimumlayers around sy ¼ 26:6� 27: The other stationsexcept for St. P, showed minimum salinity atshallower depths of around sy ¼ 26 (the secondgroup). The third group comprised St. P only,which did not show a clear salinity minimum.These differences are caused by the pattern ofocean circulation and the distribution of watermasses in this area. Moreover, there are somedifferences in T2S diagrams of the first group.The values of salinity minima at Sts. 1 and 2 areless than 34 PSU and fluctuations in salinity wereobserved around the minimum layer. These twostations are located in the Kuroshio Extensionwhere subtropical and subarctic waters meet.Assuming that all stations are at the same

longitude, the vertical sections of potential tem-perature, salinity and potential density (sy) aredepicted in Fig. 6. A low salinity water mass, theNPIW was found at a depth of approximately600m in high latitudes. Saline waters of above 35PSU were found at 100–200m at Sts. 4, 5, 8 and 9.AAIW, characterized by a minimum salinity ofsy ¼ 27:28; may appear at about 800m at Sts. 6and 7. Kawabe and Taira (1998) reported thatAAIW was observed at 800 dbar south of 12�N onthe WOCE P13 line in 1991. The water mass ataround 200m at 25–30�N is the STMW. However,there was no evidence of a water mass such as theCDW in deep waters at this latitude.Vertical migration of radionuclides from surface

waters to deep waters is partially supported by thepresence of small particles, for example, calcareousmatter and organic substances. The precipitationor formation of solid CaCO3 in surface waters andthe dissolution of solid CaCO3 in deep waters is avery important factor in the transfer of someradionuclides from surface to deep waters. Thesaturation index (O) of CaCO3 in seawater was

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0 50 100 200Silicate (µmol kg-1) Nitrate (µmol kg-1) Total alkalinity (µmol kg-1)

150 0 10 20 40 5030 2100 2200 2300 2400 2500

Stn-7Stn-1 Stn-2B Stn-6 Stn-8

Stn-9Stn-2Stn-10Stn-4

Stn-5Stn-3

Fig. 4. Vertical profiles of nutrients and related components.

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calculated from the product of the concentrationof Ca2+ and CO3

2�. The solubility product (K�sp)

was calculated from in situ measurements ofsalinity, temperature, and pressure. The values of[CO3

2�] can be calculated from the measuredcarbonate parameters, pH and TA. No cationswere analysed. However, as Ca2+ is a major

constituent of seawater, its concentration(mol kg�1) can be estimated from salinity asfollows: [Ca2+]=2.934� 10�4S (Millero, 1996).The saturation index in surface waters for calcitewas approximately 5.0 and decreased to below 1.0in deep waters (Fig. 7). Seawater became under-saturated (Oo1:0) at approximately 1000m. The

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33.5 34.5 35.534 35salinity

33.5 34.5 35.534 35salinity

Stn-1 Stn-2 Stn-2B Stn-3

Stn-4

Stn-P Stn-8 Stn-9 Stn-10

Stn-5 Stn-6 Stn-7

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

23.0

24.0

25.0

26.0

27.0

Fig. 5. T2S diagram for IAEA’97 stations. Thin lines and numbers in the diagram designate isopycnal surface and its potential

density.

200

400

600

800

1000

1200

1400

1600

1800

Dep

th (

dbar

)

10 20 30 Latitude (˚N)

10 20 30 Latitude (˚N)

10 20 30 Latitude (˚N)

(a) (b) (c)Potential temperature Potential densitySalinity

P 6,7 5,8 9 4 10 3 2B 2,1 P 6,7 5,8 9 4 10 3 2B 2,1 P 6,7 5,8 9 4 10 3 2B 2,1

Fig. 6. Vertical sections of (a) potential temperature (�C), (b) salinity (PSU) and (c) potential density (sy) in the upper ocean (assumingthat all stations are located at the same longitude; numbers and characters under the x-axis represent the sampling stations).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2615

ARTICLE IN PRESS

0

300

600

900

1200

1500

Dep

th (

m)

0

300

600

900

1200

1500

Dep

th (

m)

Calcite Aragonite

0 1 2 3 4 5 6 7Saturation index for calcite

0 1 2 3 4 5 6 Saturation index for argonite

unsat-urated

unsat-urated

saturated saturated

(a) (b)

Stn-1Stn-2Stn-2BStn-3Stn-4Stn-5Stn-6Stn-7

Stn-8Stn-9Stn-10

Fig. 7. Profiles of saturation indices for (a) calcite and (b) aragonite in the upper ocean (concentrations of calcium ions are estimated

from Ca-salinity relationship and those of carbonate ions are obtained from pH and total alkalinity).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372616

values were found at shallower depths in highlatitudes than in low latitudes (840m at St. 1 and1290m at St. 7). Although the solubility of CaCO3in seawater is not controlled by equilibrium but bykinetic constraints, the saturation index providesinformation on the fate of CaCO3 in deep waters.

4.1.2. Comparison of IAEA’97 oceanographic data

with GEOSECS and WOCE data

The locations of Sts. 2, 2B, 3, and 4 correspondto GEOSECS sites Gx-223, 225, 226 and 227,respectively, and are close to the WOCE P13 line(Kawabe and Taira, 1998). Sts. 5 and 6 are inproximity to the WOCE P13 line, where radio-nuclides such as tritium and carbon-14 wereanalysed. Moreover, St. 10 was close to thestations along the WOCE P10 line where noradionuclides were measured (Wijffels et al., 1998).Fig. 8(a–c) shows the comparison between water

temperature, salinity, DO, and silicate levelsdetermined from samples collected during thiscruise and measurements from samples collected

Fig. 8. (a) Comparison of potential water temperature between IAEA

(the upper part of the figure shows data for all water depths, the low

Comparison of potential salinity (PSU) between IAEA’97, GEOSECS

of the figure shows data for all water depths, the lower part designat

dissolved oxygen (top) and silicate (bottom) between IAEA’97, GEO

during the GEOSECS and WOCE cruises. Be-cause sampling depths for all data are differentand water masses move along isopycnal surfaces,potential density is adopted as the ordinate insteadof depth. Water characteristics measured duringthe IAEA’97 expedition are similar (taking intoaccount uncertainties of measurements), to thosemeasured during the GEOSECS and WOCEexpeditions.

4.2. Radionuclides in the water column

4.2.1. Transuranics

The measured concentrations of 239,240Pu, 238Puand 241Am in filtered surface waters are presentedin Table 2. The concentrations of 239,240Pu werefrom 1.270.2 to 7.470.7 mBq l�1, with a meanvalue of 2.570.2 mBq l�1. The lowest surfaceconcentration of 239,240Pu was observed at St. 6,whereas the highest surface concentration wasobserved at St. 7D due to mixing of top waterlayers after heavy storms caused by typhoons.

’97, GEOSECS and WOCE data for the same water mass areas

er part designates plots from the subsurface to the bottom). (b)

and WOCE data for the same water mass areas (the upper part

es plots from the subsurface to the bottom). (c) Comparison of

SECS and WOCE data for the same water mass areas.

ARTICLE IN PRESS

21

22

23

24

25

26

27

28

Pot

entia

l den

sity

(σ θ

)P

oten

tial d

ensi

ty (

σ θ)

0 10 20 30Potential temperature (˚C)

Potential temperature (˚C)

0 10 20 30Potential temperature (˚C)

0 10 20 30Potential temperature (˚C)

21

22

23

24

25

26

27

28

Pot

entia

l den

sity

(σ θ

)

21

22

23

24

25

26

27

28

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.01 3 5 7 9 11

Potential temperature (˚C)1 3 5 7 9 11

Potential temperature (˚C)1 3 5 7 9 11

IAEA'97

IAEA'97Stn-2,2B,3,4

Stn-5,6

IAEA'97Stn-10

GEOSECSseriesWOCE P3

WHP P13 WHP P10

(a)

21

22

23

24

25

26

27

28

Pot

entia

l den

sity

(σ θ

)P

oten

tial d

ensi

ty (

σ θ)

21

22

23

24

25

26

27

28

Pot

entia

l den

sity

(σ θ

)

21

22

23

24

25

26

27

28

Pot

entia

l den

sity

(σ θ

)

33.5 34.5 35.534.0 35.0salinity (PSU)

33.5 34.5 35.534.0 35.0salinity (PSU)

33.5 34.5 35.534.0 35.0salinity (PSU)

33.5 34.5 35.534.0 35.0salinity (PSU)

33.5 34.5 35.534.0 35.0salinity (PSU)

33.5 34.5 35.534.0 35.0salinity (PSU)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

IAEA'97

IAEA'97

Stn-2,2B,3,4

Stn-5,6

IAEA'97Stn-10GEOSECS

seriesWHP P3

WHP P13

WHP P10

(b)

(a)-1 (b)-1 (c)-1

(a)-2 (b)-2 (c)-2

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

Pot

entia

l den

sity

(σ θ

)

26.0

26.4

26.8

27.2

27.6

28.0

0 100 200 300Dissolved oxygen (µmol kg-1)

0 100 200 300Dissolved oxygen (µmol kg-1)

0 100 200 300Dissolved oxygen (µmol kg-1)

0 50 100 150 200Silicate (µmol kg-1)

0 50 100 150 200Silicate (µmol kg-1)

0 50 100 150 200Silicate (µmol kg-1)

IAEA'97Stn-2,2B,3,4GEOSECSseries

WOCE P13

IAEA'97IAEA'97Stn-2,2B,

3,4GEOSECSseries

WOCE P13

Stn-5,6

WOCE P13

IAEA'97Stn-10

WHP P10

IAEA'97Stn-5,6WOCE P13

IAEA'97Stn-10

WOCE P10

(c)

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2617

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P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372618

Similar conditions were found at St. 4B, whichalso shows 239,240Pu concentration higher by afactor of two in comparison with St. 4. Previousmeasurements by Miyake and Sugimura (1968)gave the concentrations of 239,240Pu in surfacewaters as from 4 to 51 mBq l�1, Imai and Sakanoue(1973) found 21–40 mBq l�1, and Bowen et al.(1980) found from 10 to 30 mBqkg�1 (GEOSECSSts. 223–226 in 1973). Higher concentrations of239,240Pu in surface waters in the mid-latitudes ofthe North Pacific were observed in the early 1980s(Nagaya and Nakamura, 1984). 239,240Pu concen-trations have decreased non-linearly with time(Fig. 9), the values observed in the 1960s and 1970swere 10 times higher than the present levels.However, the 239,240Pu concentrations have notchanged since the mid-1980s after the last atmo-spheric nuclear test conducted by China. Thissuggests that the 239,240Pu concentrations in sur-face waters were not only affected by globalfallout, but also by physical processes such asadvection and convection of water. In a recentstudy (Hirose et al., 2001), the observed 239,240Puconcentrations in surface waters of the NW Pacificwere relatively homogeneous (around 3 mBq l�1).The decrease rate of Pu in surface waters has beenfaster than that of 137Cs and 90Sr due to removal ofPu by scavenging processes.The spatial distribution of present 239,240Pu

concentrations in surface waters has depended onlatitude (Fig. 10). The stations located above 20�Nrevealed higher concentrations of 239,240Pu (asgenerally expected from global fallout). Similarresults were observed in previous studies (Miyakeand Sugimura, 1976; Nagaya and Nakamura,1984; Miyake et al., 1988; Hirose et al., 1992,2001), which suggests that the geographicaldistribution of 239,240Pu in surface waters of theNW Pacific is due to different rates of fallout input(Nagaya and Nakamura, 1984; Hirose et al.,2001). However, the difference between the max-imum and minimum 239,240Pu concentrations hasdecreased in comparison with previous results(Miyake and Sugimura, 1976). On the other hand,slightly higher 239,240Pu concentrations were foundin the low-latitude regions (5–12�N). The presentgeographical distribution of 239,240Pu in surfacewaters of the NW Pacific may be controlled by

different rates of fallout input following thebiochemical processes and advection from theNP current system. Higher 239,240Pu concentra-tions observed at Sts. 4B (19�N) and 7D (11�N)were due to vertical mixing of water masses afterheavy storms (typhoons).238Pu concentrations in surface waters of the

NW Pacific ranged from 0.0470.03 mBq l�1 to0.2470.06 mBq l�1, with a mean of 0.08770.017 mBq l�1. At Sts. 6 and 7B the low concentra-tions were low and only the detection limitcould be reported. Relatively higher 238Puconcentrations were found in the mid-latitudearea, similar to 239,240Pu concentrations. The238Pu/239,240Pu activity ratios in surface watersvaried from 0.01070.024 to 0.03570.015. Theaverage 238Pu/239,240Pu ratio (0.03470.005) is thesame as the global fallout ratio, which indicatesthat Pu isotopes present in surface waters in theNW Pacific are mainly derived from nuclearweapon tests.The lowest surface concentration of 241Am was

observed at St. 6 (0.1270.08 mBq l�1), whereas thehighest concentration was observed at St. 4B(1.4870.12 mBq l�1), the mean value being 0.5970.06 mBq l�1. However, no definite tendency to-ward a latitudinal gradient of 241Am concentra-tions was observed (Fig. 10). Data on 241Am in theNW Pacific are very sparse. Fowler et al. (1983)and Miyake et al. (1988) reported (2.470.3) mBq l�1 in 1980 and from 0.37 to 4.07 mBq l�1,with a mean of 2.4 mBq l�1 during late seventies.Nakanishi et al. (1990) reported a 241Am concen-tration of 1.870.8 mBq l�1 in surface waters in1983. These data show a decrease of 241Amconcentrations in surface waters since the1980s.The activity ratios of 241Am/239,240Pu varied

from 0.1070.03 to 0.5170.10, with a mean of0.1970.02. This ratio was similar to thoseobtained by Miyake et al. (1988) and Nakanishiet al. (1990). The observed average ratio of241Am/239,240Pu is lower than that calculated fromthe global fallout ratio (0.37). As 241Am inseawater originated mainly from the decay of241Pu, its scavenging rate is faster than that of239,240Pu due to its high particle-affinity (IAEA,1985). Therefore 241Am in surface waters may be

ARTIC

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S

Table 2

Concentrations of 239,240Pu, 238Pu, 241Am, 90Sr and 137Cs in surface seawater of the NW Pacific Ocean

St. Sampling

date

Location

(area)

239,240Pu

(mBq l�1)

238Pu

(mBq l�1)

241Am(mBq l�1) 238Pu/239,240Pu 241Am/239,240Pu 90Sr

(mBq l�1)

137Cs

(mBq l�1)

137Cs/90Sr

St. 1 23/10/97 34�5990N, 145�5990E 3.570.3 0.1670.04 0.6670.10 0.04570.012 0.1970.03 1.8570.04 2.870.3 1.570.2St. 2 26/10/97 34�5700N, 151�5250E 4.270.4 0.2170.07 0.6070.08 0.05170.018 0.1470.02 1.8370.04 3.670.1 2.070.1St. 2B 28/10/97 32�3690N, 161�5470E 3.870.3 0.1670.04 0.7370.08 0.04270.012 0.1970.02 1.8370.04 3.370.2 1.870.1St. 3 29/10/97 30�3380N, 170�3690E 4.070.3 0.2470.06 0.4070.11 0.06070.016 0.1070.03 2.2070.14 2.270.2 1.070.1St. 4 31/10/97 24�5990N, 170�5030E 2.470.2 0.2370.05 0.7570.10 0.09470.021 0.3170.05 1.5870.04 2.870.2 1.870.1St. 4B 02/11/97 19�4160N, 168�2490E 4.370.3 1.4870.12 0.3570.04 1.5870.04 2.770.2 1.770.1St. 5 03/11/97 14�5990N, 166�5970E 1.570.5 0.5770.13 0.3870.03 1.5770.04 2.670.2 1.770.1St. 6 10/11/97 11�2610N, 164�5240E 1.270.2 0.1270.08 0.1070.06 1.5070.04 1.870.1 1.270.1St. 6B 11/11/97 11�134’N, 163�3380E 1.270.2 0.3470.09 0.2870.09 1.5270.04 1.670.1 1.170.1St. 7 11/11/97 11�9490N, 161�5790E 1.770.2 0.8470.13 0.5170.10 1.7370.04 1.570.1 0.970.1St. 7B 05/11/97 9�1810N, 160�0450E 2.570.3 0.3370.08 0.1370.04 1.5370.04 2.270.2 1.470.1St. 7D 05/11/97 11�3010N, 161�4460E 7.470.7 0.9470.15 0.1370.02 1.6170.04St. 8 13/11/97 15�3010N, 159�3050E 1.470.2 0.6870.09 0.4870.09 1.4970.03 2.370.1 1.570.1St. 8B 15/11/97 19�4370N, 155�5910E 1.870.2 0.1470.04 0.4770.06 0.08070.022 0.2770.04 1.6770.04St. 9 16/11/97 22�2260N, 152�4020E 2.070.2 0.0770.03 0.7770.08 0.03570.015 0.3870.05 1.7870.04 3.070.3 1.770.2St. 9B 17/11/97 26�1360N, 150�1760E 2.770.2 0.1270.04 0.3470.06 0.04570.015 0.1370.02 1.8170.04St. 10 11/11/97 29�2960N, 146�5070E 3.170.2 0.1670.04 0.8070.09 0.05170.014 0.2670.04 1.7770.04 2.270.2 1.270.1St. P 06/11/97 7�1990N, 158�2480E 3.270.2 0.1570.04 0.5670.10 0.04770.014 0.1770.03 1.5370.04

P.P

.P

ovin

ecet

al.

/D

eep-S

eaR

esearch

II5

0(

20

03

)2

60

7–

26

37

2619

ARTICLE IN PRESS

1965 1970 1975 1980 1985 1990 1995 2000

239,

240 P

u ( µ

Bq

L-1

)

0

10

20

30

40

50

60

Imai and Sakanoue, 1973Miyake et al, 1988Nakanishi et al, 1990Hirose et al, 2001This study, 2002

Year

Year

1965 1970 1975 1980 1985 1990 1995 2000

239,

240 P

u ( µ

Bq

L-1

)

0

5

10

15

20

25

30

Miyake et al., 1988Nagaya andNakamura, 1984Hirose et al., 2001 This study, 2002

25-45 o N

0-25o N

Fig. 9. Temporal variations of 239,240Pu in surface seawater in the latitude belts 0–25�N and 25–45�N in the NW Pacific Ocean.

Latitude ( 0 - 40˚ N)0510152025303540

Act

ivity

(µB

qL-1

)

0

2

4

6

8

239, 240Pu241Am

Fig. 10. Latitudinal variations of 239,240Pu and 241Am concen-

trations in surface water of the NW Pacific Ocean.

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372620

preferentially scavenged with particles, whichcould induce the observed low activity ratio inthe surface waters. Higher activity ratios were

found around 10–25�N, which may be related tothe low concentrations of 239,240Pu and lowproductivity.As Pu was introduced into the Pacific Ocean

mainly by two pulse injections into the surfacelayer in the early 1950s (local tropospheric fallout)and in the early 1960s (global fallout), Pu in thewater column is in a transient state and itsconcentrations and inventories should change withtime according to oceanographic, geochemical andbiological processes that occur in the watercolumn. All the 239,240Pu profiles observed in thisstudy (Table 3, Fig. 11) showed features typical ofthose reported in previous studies (Bowen et al.,1980; Nagaya and Nakamura, 1984, 1987; Naka-nishi et al., 1990; Hirose et al., 2001). Theconcentrations of 239,240Pu gradually increasedfrom surface to mid-depth at around 500–800m(concentrations at subsurface maximum depthswere from 34.772.7 to 60.374.6 mBq l�1) andthen decreased with depth. The subsurface Pu

ARTICLE IN PRESS

239,240Pu (µBq L-1)0 5 10 15 20 25 30 35 40

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

238Pu/239,240Pu activity ratios

0.00 0.02 0.04 0.06 0.08

239,240Pu ( µBq L-1)0 10 20 30 40 50 60

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

238Pu/ 239.240Pu activity ratios

0.00 0.02 0.04 0.06 0.08

St. 1St. 3

239,240Pu ( µBqL-1)

0 10 20 30 40 50 60 70

Dep

th (

m)

0

1000

2000

3000

4000

5000

238Pu/239,240Pu activity ratios

0.00 0.02 0.04 0.06 0.08

St. 6

239,240Pu ( µBq L-1)

0 10 20 30 40 50 60 70D

epth

(m

)

0

1000

2000

3000

4000

238Pu /239,240

Pu activity ratios

0.00 0.03 0.06 0.09 0.12

St. 7

239,240Pu ( µBq L-1)0 10 20 30 40 50 60

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

238Pu /239,240

Pu activity ratios

0.00 0.03 0.06 0.09 0.12

St. 8

Fig. 11. Depth distribution of 239,240Pu concentrations (dots) and 238Pu/239,240Pu activity ratios (squares) in seawater of the NW Pacific

Ocean (IAEA’97).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2621

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P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372622

maximum has been explained by the verticaltransport of Pu associated with sinking particles,and the subsequent release of Pu from particulatematter to the water column (Bowen et al., 1971,1980; Livingston and Anderson, 1983). This occursthrough the breakdown of large organic-richparticles bearing Pu into small particles and theremineralization of Pu into the water. The AOUand nutrients profiles shown in Fig. 4 supportthese observations. High subsurface maximumconcentrations of 239,240Pu were found at Sts. 6,7 and 8, close to Enewetak and Bikini atolls. Manytests were conducted at or near ground level atthese atolls, and thus much of the resultantradioactivity was delivered to the tropospherewith substantial fallout to the regional ocean areas(Livingston et al., 2001; Livingston and Povinec,2002).The dissolved 239,240Pu in seawater in the NW

Pacific (Sts. 1, 6 and 7) shows elevated concentra-tions (by about a factor of 2) in samples takenclose to the sea bottom. This feature had notpreviously been observed in water profiles taken inthe NE Pacific Ocean (Bowen et al., 1980). Higherlevels of 239,240Pu near the bottom were attribu-table to either the dissociation of Pu from radio-nuclide-bearing particulate materials delivered tothe seafloor by benthic organisms and subsequentremobilization from the underlying sediment indeep water circulation, or release to overlyingwaters by oxidation prior to burial (Nagaya andNakamura, 1984). The observation of increasedconcentrations of thorium isotopes near thebottom in the open ocean supports the hypothesismentioned above (Nakanishi et al., 1990).The results of 238Pu measurements in the water

column (Table 3) show as usual much lowerconcentrations than 239,240Pu. The content of238Pu in nuclear weapons debris from globalfallout has been known to be about 3% of the239,240Pu content, whereas the 238Pu/239,240Puactivity ratio in radioactive debris from theMarshall Islands nuclear testing is less than 1%(Komura et al., 1984). The 238Pu/239,240Pu activityratios in shallower waters support the globalfallout origin of Pu in all the areas studied,including the close-in fallout area. The lower238Pu/239,240Pu activity ratios which occurred in

subtropical Pacific deep waters (Sts. 6 and 7)suggest that the major source of Pu in the NWPacific deep waters is close-in fallout from theMarshall Island testing grounds. This result isconsistent with the observed 240Pu/239Pu massratios in the water column (Buesseler, 1997). Thesefindings indicate that Pu isotopes observed in theNW Pacific originated from nuclear weapon tests.A noticeably higher 238Pu/239,240Pu activity ratiowas found in bottom waters at St. 7, and its causeis unknown.The most abundant Pu isotope found in the

water column of the NW Pacific is 241Pu, which isa b-emitter with a half-life of 14.4 years producedduring nuclear tests (it is a parent radionuclide of241Am). The 241Pu/239,240Pu activity ratio in globalfallout (Miyake et al., 1979) is 15. Its concentra-tion in the water profile at St. 7 (Fig. 12) showsfeatures similar to 239,240Pu and 241Am. The sub-surface maximum (peak concentration around150 mBq l�1) was observed at 1000m depth, deeperthan for 239,240Pu, but similar to 241Am, whosecause is due to the injection of close-in falloutplutonium with higher 241Pu/239,240Pu activityratios in deeper waters. The 241Pu/239,240Pu activityratios in shallower waters are less than 3,which correspond to around 15 in the early1960s. This finding confirms that plutonium inshallower waters of the NW Pacific is derived fromglobal fallout. On the other hand, the higher241Pu/239,240Pu activity ratios in deeper waters maybe from close-in fallout. Plutonium isotopicsignatures reveal that plutonium originating fromclose-in fallout from nuclear weapons testing atthe Pacific Proving Grounds was transported intodeeper layers than the plutonium from globalfallout.The 241Am profiles in the water column were

similar to the 239,240Pu profiles (Fig. 13). Theconcentrations of 241Am in surface waters werefrom 0.1270.08 to 0.8470.13 mBq l�1, graduallyincreasing at mid-depths of around 700–1000m(concentrations at subsurface maximum depthswere from 9.471.0 to 2273 mBq l�1), and thenfurther decreasing with depth. Higher subsurfacemaximum concentrations were found at Sts. 6, 7and 8 (similar to the 239,240Pu levels) as these sitesare close to the nuclear test area. The subsurface

ARTIC

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STable 3

Concentrations of 239,240Pu, 238Pu, 241Am, 90Sr and 137Cs in the water column of the NW Pacific Oceana

St. Sampling

date

Depth

(m)

239,240Pu

(mBq l�1)

238Pu

(mBq l�1)

241Am

(mBq l�1)

238Pu/239,240Pu 241Am/239,240Pu 90Sr

(mBq l�1)

137Cs

(mBq l�1)

137Cs/90Sr

St. 1 23/10/97 0 3.570.3 0.1670.04 0.6670.10 0.04570.012 0.1970.03 1.8570.04 2.870.3 1.570.224/10/97 100 4.870.4 0.2570.07 0.7370.11 0.05170.015 0.1570.03 1.9370.03 3.170.2 1.670.124/10/97 250 13.971.1 0.2570.11 1.6070.2 0.01870.008 0.1170.02 2.0470.03 3.170.1 1.570.124/10/97 500 3573 0.8370.12 6.4070.7 0.02470.004 0.1870.03 1.3970.03 2.270.5 1.670.424/10/97 700 3273 0.5870.08 8.9071.0 0.01870.003 0.2870.04 0.7370.03 0.8970.18 1.270.324/10/97 1000 3173 0.5370.09 9.4071.0 0.01770.003 0.3070.04 0.2570.01 0.9570.16 3.870.724/10/97 1500 2271 0.5370.09 7.8070.6 0.02470.004 0.3570.03 0.0670.01 o0.124/10/97 2500 15.570.8 0.4170.08 5.9070.4 0.02770.006 0.3870.03 0.1170.01 o0.124/10/97 3500 10.970.6 0.2570.06 4.9070.4 0.02370.006 0.4570.04 0.0670.01 o0.124/10/97 4500 11.670.6 0.4170.09 5.170.5 0.03670.008 0.4470.05 0.0770.01 o0.124/10/97 5850 17.570.8 0.3770.07 6.670.5 0.02170.004 0.3870.03 0.0870.01 o0.1 —

St. 3 29/10/97 0 4.070.3 0.2470.06 0.4070.11 0.06070.016 0.1070.03 2.2070.14 2.270.2 1.070.129/10/97 200 10.470.8 0.3170.06 1.270.2 0.03070.006 0.1270.03 2.2070.17 3.170.2 1.470.129/10/97 450 2672 0.5870.11 3.070.4 0.02270.005 0.1270.02 1.7570.13 3.270.2 1.870.229/10/97 600 4374 1.0670.13 7.270.8 0.02470.004 0.1170.02 1.3370.12 1.970.2 1.470.229/10/97 800 4774 1.1570.16 11.971.3 0.02470.004 0.1770.03 0.6370.16 0.870.2 1.370.529/10/97 1100 3772 0.8470.14 12.470.9 0.02270.004 0.2570.03 0.327 0.08 0.570.1 1.670.529/10/97 2000 19.470.8 0.4370.08 7.070.7 0.02270.004 0.3370.04 0.1270.04 o0.129/10/97 4000 17.070.7 0.1970.05 7.070.6 0.01170.003 0.3670.04 o0.129/10/97 5000 19.870.9 0.5770.09 5.270.4 0.02970.005 0.4170.02 o0.129/10/97 5400 19.170.9 0.4770.09 5.070.4 0.02570.005 0.2670.02 o0.1

St. 6 10/11/97 0 1.2170.20 o0.1 o0.2 1.5070.03 1.870.1 1.270.110/11/97 300 3673 1.3070.16 8.870.7 0.03670.005 0.3270.03 0.6170.02 0.8270.10 1.370.210/11/97 400 0.1970.0510/11/97 600 6075 1.0170.13 1972 0.01770.002 0.3270.03 0.177 0.02 o0.110/11/97 900 5775 0.7670.13 1972 0.01370.003 0.3470.03 0.0970.01 o0.110/11/97 1200 3473 0.3870.09 17.471.3 0.01170.003 0.5170.05 0.0670.01 o0.110/11/97 2000 2272 0.2370.06 11.371.3 0.01170.003 0.5270.07 0.0570.01 o0.110/11/97 3900 11.570.9 0.2870.07 3.870.7 0.02570.006 0.3370.07 0.0370.01 o0.110/11/97 4400 17.771.5 0.6270.12 4.770.4 0.03570.007 0.2770.03 0.0570.01 o0.1

St. 7 5/11/97 0 1.6570.22 0.870.1 — 0.5170.10 1.7370.04 1.570.1 0.970.111/11/97 250 3273 0.970.3 4.970.5 0.02870.009 0.1570.02 1.1470.03 1.270.1 1.170.111/11/97 500 5774 o0.2 1671 0.2870.03 0.2270.01 0.3070.04 1.470.211/11/97 750 4773 o0.2 2072 0.4370.04 0.1570.01 o0.111/11/97 1000 4673 0.970.2 2072 0.01970.005 0.4370.04 0.1270.02 o0.111/11/97 2000 23.571.5 12.170.9 0.5270.05 0.0770.01 o0.111/11/97 3100 18.871.2 7.970.7 0.4270.05 0.0470.01 o0.111/11/97 3600 3072 2.6970.35 7.870.6 0.09170.013 0.2770.03 0.0470.01 o0.1

P.P

.P

ovin

ecet

al.

/D

eep-S

eaR

esearch

II5

0(

20

03

)2

60

7–

26

37

2623

ARTICLE IN PRESSTable3(c

onti

nu

ed)

St.

Sampling

date

Depth

(m)

239,240Pu

(mBql�1)

238Pu

(mBql�1)

241Am

(mBql�1)

238Pu/239,240Pu

241Am/239,240Pu

90Sr

(mBql�1)

137Cs

(mBql�1)

137Cs/90Sr

St.8

13/11/97

01.470.2

o0.2

0.770.1

0.4870.09

1.4970.03

2.370.1

1.570.1

14/11/97

300

1871

0.4570.16

2.670.2

0.02570.009

0.1570.02

2.0370.04

3.170.1

1.570.1

14/11/97

600

5072

0.5570.15

2273

0.01170.003

0.4370.03

0.2770.02

0.4070.08

1.570.3

14/11/97

900

5373

0.6370.16

2271

0.01270.003

0.4170.03

0.1070.02

o0.1

14/11/97

1200

3372

0.5370.14

1971

0.01670.004

0.5670.04

0.0770.02

o0.1

14/11/97

2000

2872

0.2870.12

13.171.1

0.01070.004

0.4670.04

0.0770.01

o0.1

14/11/97

4900

1671

o0.2

6.070.4

0.3770.04

0.0570.01

o0.1

14/11/97

5400

1471

0.770.2

5.270.4

0.05470.018

0.3770.04

0.0770.01

o0.1

aAveragedvaluesfromIAE-MEL,BSH,MRIandJCAC.

241Pu (µBq L-1)

0 30 60 90 120 150 180

Dep

th (

m)

0

1000

2000

3000

4000

241Pu/239,240Pu activity ratio

0 2 4 6 8 10

241Pu

241Pu/

239,240Pu

St. 7

Fig. 12. Depth distribution of 241Pu concentrations and241Pu/239,240Pu activity ratios in seawater of the NW Pacific

Ocean (IAEA’97).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372624

maximum depth of 241Am is slightly deeper thanthat of 239,240Pu. This may be because the partitioncoefficient of 241Am between particles and sea-water is higher than that of 239,240Pu (IAEA, 1985)and 241Am is preferentially scavenged in the watercolumn.The observed 241Am/239,240Pu activity ratios at

all stations reflect the global fallout ratio (0.37).Lower ratios were observed near the surface due tothe preferential scavenging of 241Am. The activityratios increased with depth. An increase withdepth of 241Am/239,240Pu ratios was observed inparticles on filters in the North Pacific by Fowleret al. (1983), as well as in sediment from the NWPacific (1.25–3.76; Haque and Nakanishi, 1999)compared to the global fallout ratio (0.37). Theseobservations confirm that 241Am settles in thewater column more rapidly than 239,240Pu, as re-dissolved 239,240Pu and 241Am at subsurface max-imum depths are captured by large particles andmove to deeper waters. This could be the reason

ARTICLE IN PRESS

241Am (µBq L-1)

0 2 4 6 8 10 12

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

241Am /239,240

0.0 0.1 0.2 0.3 0.4 0.5 0.6

241Am (µBq L-1)

0 2 4 6 8 10 12 14

Dep

th (

m)

241Am /239, 240

0.0 0.1 0.2 0.3 0.4 0.5

St. 1 St. 3

241Am (µBq L-1)0 3 6 9 12 15 18 21 24

Dep

th (

m)

241Am /239, 240

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

241Am (µBq L-1)

0 3 6 9 12 15 18 21 24D

epth

(m

)

241Am /239, 240

0.1 0.2 0.3 0.4 0.5 0.6 0.7

St. 6 St. 7

241Am (µBq L-1)

0 3 6 9 12 15 18 21 24

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

241Am /239, 240Pu activity ratios

0.1 0.2 0.3 0.4 0.5 0.6 0.7

St. 8

0

1000

2000

3000

4000

5000

6000

0

1000

2000

3000

4000

0

1000

2000

3000

4000

5000

Pu activity ratios Pu activity ratios

Pu activity ratios Pu activity ratios

Fig. 13. Depth distribution of 241Am concentrations and 241Am/239,240Pu activity ratios in seawater of the NW Pacific Ocean

(IAEA’97).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2625

why the subsurface 239,240Pu and 241Am maximahave moved to deeper waters.In shallow and coastal locations large amounts

of Pu isotopes were deposited onto sediment by

enriched biogenic and inorganic particles. How-ever, in an open-ocean oligotrophic zone themajority of Pu remains in the water column(Livingston et al., 2001). The Pacific Proving

ARTICLE IN PRESS

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372626

Grounds are in the oligotrophic zone that issubjected to the North Equatorial Current(NEC). Therefore, high Pu concentrations origi-nating from nuclear tests were transported to thewest and upwards through the NEC by advectionmixing. Subsequently, Pu was redistributed byWestern North Pacific gyral circulation and theNEC, leading to the excess of Pu in the area. Anadditional source of plutonium in the NW Pacificcould be the remobilization of Pu from the atollsand could be a continuous source of plutoniumthere (Livingston et al., 2001).

4.2.2. 137Cs and 90Sr

The measured 137Cs concentrations in surfacewaters in 1997 ranged from 1.5 to 3.6mBq l�1

(Table 2). The highest surface 137Cs concentrationwas observed in the north-west, in the KuroshioExtension (St. 2). The lowest surface 137Csconcentration occurred in the Tropical Pacificnear 10�N, on the boundary between the NECand NECC. The geographical distribution ofsurface 137Cs in 1997 in the NW Pacific differedfrom that observed in the 1970s when 137Csconcentration maxima were observed in the mid-latitude region of the North Pacific (Saruhashiet al., 1975), which were then controlled by aninput process, i.e., global fallout. It is noteworthythat surface 137Cs is homogeneous in the Pacificcompared to 239,240Pu.The 137Cs concentration in surface waters of the

NW Pacific has decreased steadily since the early1960s to present day levels of between 2 and4mBq l�1. In a previous study (Saruhashi et al.,1975) a typical latitudinal distribution of 137Cs wasobserved, which was high in mid-latitudes (10–20mBq l�1) and low in the Equatorial region(around 5mBq l�1). The differences in 137Csconcentrations in surface waters between themid-latitudes and the Equatorial region havelessened over the past two decades. In theEquatorial region the 137Cs concentration in sur-face water showed no temporal change apart fromradioactive decay (Aoyama et al., 2001a). Thecurrent geographical distribution of surface 137Csin the NW Pacific is the result of global falloutinput distributed by surface circulation in the

subtropical region which includes subduction andequatorial upwelling processes.The 90Sr concentrations measured in surface

waters in 1997 were in the range of 1.49–2.20mBq l�1 (Table 2). The highest 90Sr surfaceconcentration was observed in the mid-latituderegion of the NW Pacific at St. 3. The geographicaldistribution of 90Sr in NW Pacific surface waters in1997 is homogeneous, as for 137Cs. However, stilllower surface 90Sr was observed in the Equatorialregion near 10�N. The difference between max-imum and minimum surface 90Sr concentrations inthe NW Pacific is less than for 137Cs. The waterprofile subsurface 137Cs concentration maxima(Fig. 14), which coincide with maximum salinityin water, were observed in the subtropical NWPacific, at Sts. 1, 3 and 8. Tropical Sts. 6 and 7showed an exponential decrease of 137Cs concen-trations from surface to bottom. Similar behaviourwas observed for 90Sr concentration profiles(Fig. 14), in which maxima were found at depthssimilar to 137Cs maxima at Sts. 1, 3 and 8.

4.3. Radionuclide water profile time series

4.3.1. Plutonium isotopes

The data from GEOSECS (early 1970s, Bowenet al., 1980), KNORR (late 1970s, Livingston andNoshkin, pers. comm.), Hakuho Maru (early1980s, Nagaya and Nakamura, 1984) and theIAEA’97 expedition (this paper) are used todevelop a radionuclide time series in the watercolumn of the NW Pacific (the positions of thesampling stations are shown in Fig. 15). Fig. 16shows 239,240Pu profiles in water columns of thecentral NW Pacific from the four expeditions. It isclear that 24 years later, the sub-surface maximumwas much smaller and less pronounced (a decreaseby about a factor of 4) and has moved to waterlayers deeper by a factor of 2 (from 450 to 850m).The development of the 239,240Pu profile in the

water column from the GEOSECS and KNORRstations situated west and east of Bikini atoll,respectively, and the IAEA’97 station situatedwest of Bikini atoll, are also compared in Fig. 16.The vertical spread in the 239,240Pu maxima ismuch narrower and the recent 239,240Pu concentra-tions maxima (Stn. 6) were higher than in the

ARTICLE IN PRESS

137Cs (mBq L-1)0 1 2 3 4 5 6 7 8

Dep

th (

m)

0

1000

2000

3000

4000

5000

90Sr (mBq L-1)

0.0 0.5 1.0 1.5 2.0 2.5

137Cs/90Sr activity ratios

0 1 2 3 4 5

137Cs90Sr137Cs/90Sr

St. 1

137Cs (mBq L-1)0 1 2 3 4 5 6 7 8

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

90Sr (mBq L-1)0.0 0.5 1.0 1.5 2.0 2.5

137Cs/90Sr activity ratios

0 2 4 6 8 10

St. 3

137Cs (mBq L-1)0 1 2 3 4 5 6 7 8

Dep

th (

m)

0

1000

2000

3000

4000

5000

90Sr (mBq L-1)

0.0 0.5 1.0 1.5 2.0 2.5

137Cs/90Sr activity ratios

0.0 0.5 1.0 1.5 2.0

St. 6

137Cs (mBq L-1)0 1 2 3 4 5 6 7 8

Dep

th (

m)

0

1000

2000

3000

4000

90Sr (mBq L-1)0.0 0.5 1.0 1.5 2.0 2.5

137Cs/90Sr activity ratios

0.0 0.5 1.0 1.5 2.0

St. 7

137Cs (mBq L-1)0 2 4 6 8

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

90Sr (mBq L-1)

0.0 0.5 1.0 1.5 2.0 2.5

137Cs/90Sr activity ratios

0.0 0.5 1.0 1.5 2.0

St. 8

Fig. 14. Depth distribution of 137Cs and 90Sr concentrations and 137Cs/90Sr activity ratios in seawater of the NW Pacific Ocean

(IAEA’97).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2627

ARTICLE IN PRESS

[1973]

[1978]

[1980]

[1982]

[1997]IAEA 97

KH-82-1

KH-80-2

Knorr73

GEOSECS

St.4St.13

St.14

Enewet akBikin i

Stn-1

Stn-3

Stn-6

Stn-8

CE-8

CY-11

St.2St.3

St.6-St.8

GX-2 23GX-2 24GX-2 25

GX-2 26

GX-2 29

130 135 140 145 150 155 160 165 170 175

East Longitude (deg.)

10

15

20

25

30

35

40

45N

orth

Lat

itude

(de

g.)

Fig. 15. GEOSECS (1973), KNORR (1978), Hakuho Maru (1980, 1982) and IAEA (1997) sampling stations in the NW Pacific Ocean

(data used for the evaluation of water profile time series).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372628

northern Sts. 1 and 3. The 239,240Pu concentrationsin the water column at St. 6 did not changesignificantly over the 24 years. As these stationsare all located in the westward-flowing NEC (withvery low biological productivity, oligotrophicconditions) and the eastward flowing NECC(North Equatorial Counter Current), the reasonfor the maintenance of the 239,240Pu concentrationprofiles and inventories in the upper ocean may bethat most of the 239,240Pu has been trapped inwater masses recirculating in near-permanent lowproductivity conditions. Although no difference isnoted in 239,240Pu profiles in the upper oceanstations, substantial differences can be found indeep water east of (upstream) and west of (down-stream) the atolls (Livingston et al., 2001). Adifference of about a factor of two can be observedin deep-water 239,240Pu concentrations in the east-ern (dotted lines) and western (solid lines) stationsoccupied between 1973 and 1982. Higher deep

Fig. 16. 239,240Pu profiles in the water column of the GEOSECS (19

stations: (a) central NW Pacific Ocean, (b) west of Bikini atoll, (c) ea

Bikini atoll.

water 239,240Pu levels were found in 1997 (IAEASts. 6 and 7, northwest of Bikini). This east/westgradient in deep waters may have been createdinitially from the large test fallout patterns andpreserved in different water masses separated bysills. A further explanation could involve thecontinuous remobilization of Pu to deep watersfrom heavily contaminated material down thesteep slopes from the atolls to the sea floor. Asdeep water circulation is slow, except to thenorthwest, elevated deep water levels to the westof the atolls could be maintained over time.239,240Pu in low-productivity oceanic areas is not

being as rapidly transferred to sediments as themore reactive actinides such as Am. Nevertheless,the particulate associations in transport to sedi-ments and retention there are an important aspectof Pacific Pu studies. Livingston and Anderson(1983) and Fowler et al. (1983) using sedimenttraps confirmed that the flux of sinking biogenic

73), KNORR (1978), Hakuho Maru (1980) and IAEA (1997)

st of Bikini atoll, (d) east (dotted lines) and west (solid lines) of

ARTICLE IN PRESS

0 0 0 0

0

0 0 0 0 0 0

50

0

1000

2000

3000

4000

5000

6000

0 0.02 .04 .06 .08 .1

239+240

Dep

th (

m) GEOSECS (1973)

GX-223

GX-224

GX-225

GX-226

KH-80-2 (1980)CY-11

IAEA 97 (1997)Stn-1

Stn-3

GEOSECS:GX-223-226

KH-80-2:CY-11

IAEA97:Stn 1+3

0

1000

1500

2000

2500

3000

3500

4000

4500

.02 .04 .06 .08 .1

239+240Pu (mBq/L)

Dep

th (

m)

Knorr73 (1978)

St. 6

St. 7

St. 8

IAEA 97 (1997)

Stn-6

Knorr73:St.6+7+8

IAEA 97:Stn-6

0

1000

2000

3000

4000

5000

6000

0 0.02 0.04 0.06 0.08 0.1

239+240Pu (m Bq/L)

Dep

th (

m)

GEOSECS (1973)

GX-229

Knorr73 (1978)

St. 2

St. 3

St. 4

GEOSECS:GX-229

Knorr73:St.2+3+4

0

1000

2000

3000

4000

5000

6000

0 0.02 0.04 0.06 0.08 0.1

239+240Pu (mBq/L)

Dep

th (

m)

GEOSECS (1973)GX-229

Knorr73 (1978)St. 2St. 3St. 4St. 6St. 7St. 8St. 13St. 14

KH-82-1 (1982)CE-8

IAEA 97 (1997)Stn-6

GEOSECS:GX-229

Knorr73:St.2+3+4

Knorr73:St.6+7+8+13+14

KH-82-1:CE-8

IAEA 97:Stn-6

Pu (mBq/L)(a) (b)

(c) (d)

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2629

ARTICLE IN PRESS

241Am (µBq L-1)

0 10 20 30 40

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

Nakanishi et al, 1990St. 83-17-N33o29'N,137o42'EOct.1983This study (St. 1)

241Am (µBq L-1)

0 10 20 30 40

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

Fowler et al, 1983St. VERTEX I35o48'N, 123o35'WSep. 1980This study (St.3)

Fig. 17. Comparison of 241Am water column profiles from

Nakanishi et al. (1990), Fowler et al. (1983) and IAEA’97.

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372630

particles fed by biological productivity in thesurface ocean, is the major vehicle transportingPu (or other reactive elements) from the surface tothe deep ocean. On its descent this material isexposed to oxidation and bacterial degradationprocesses, and both the organic material and itsassociated elements or radionuclides are to some

Fig. 18. 90Sr profiles in the water column of the GEOSECS (1973), K

(a) central NW Pacific Ocean, (b) west of Bikini atoll, (c) east of Bikin

(data were decay-corrected to 1 January 1997).

extent released into some kind of solution phase.About 1% of organic material originating at thesea surface actually arrives at the sea floor. Theother biogenic phases such as nutrients andCaCO3 (see Figs. 4 and 7) also influence thedistribution of Pu in the water column. Theseprocesses are responsible for the observed deep-water Pacific distributions of Pu below depthsventilated physically from the surface. Further-more, such material delivered to the sea floor isboth available for mixing by benthic organismsinto bottom sediments and to release to overlyingwaters by oxidation prior to burial (Cochran,1985). This is the kind of process that can beexpected to produce higher 239,240Pu concentra-tions at the bottom in Pacific deep water above thesea floor.Temporal variations in 241Am water profiles are

presented in Fig. 17 for 2 data sets from the samelatitude belt for 1980 (St VERTEX I, 35�480N,123�350W, Fowler et al., 1983) and 1997 (St. 3),and data for 1983 (St. at 33�N, 137�420E,Nakanishi et al., 1990) and 1997 (St. 1). In spiteof the limited data available, it could be seen thatthe behaviour of 241Am in the water column withtime is similar to that of 239,240Pu. The sub-surfacemaximum became much smaller and less pro-nounced (St. 1) and has moved to deeper waterlayers. While 239,240Pu maxima at Sts. 1 and 3 arecomparable, the difference in the 241Am maximumvalues at St. 3 is much smaller than expected fromSt. 1.

4.3.2. 137Cs and 90Sr90Sr and 137Cs profiles in the water columns of

the central NW Pacific Ocean at IAEA’97, as wellas GEOSECS and Hakuho Maru stations (decay-corrected to 1 January 1997) are very similar(Figs. 18 and 19, respectively). In the KuroshioExtension and subtropical regions, 137Cs concen-trations are higher by a factor of about 1.6, whichrepresents the typical global fallout ratio for theseradionuclides, whereas significant low 137Cs/90Sractivity ratios (around 1) occurred in the Equatorial

NORR (1978), Hakuho Maru (1980) and IAEA (1997) stations:

i atoll, (d) east (dotted lines) and west (solid lines) of Bikini atoll

ARTICLE IN PRESS

0

1000

2000

3000

4000

5000

6000

0 1 2 3 4

Dep

th (

m) GEOSECS (1973)

GX-223

GX-224

GX-225

GX-226

KH-80-2 (1980)CY-11

IAEA97 (1997)Stn-1

Stn-3

GEOSECS:GX-223-226

KH-80-2:CY-11

IAEA97:Stn-1+3

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0.0 0 .5 1.0 1.5 2.0 2.5

Dep

th (

m)

Dep

th (

m)

Knorr73 (1978)St. 7St. 8

IAEA97 (1997)Stn-6

Knorr73:St. 7+8

IAEA 97:Stn-6

0

1000

2000

3000

4000

5000

6000

0 1 2 3 4 5

90Sr (mBq/L)

Dep

th (

m)

GEOSECS (1973)GX -229

Knorr73 (1978)St. 3

GEOSECS:GX-229Knorr73:St.3

1000

2000

3000

4000

5000

6000

90Sr (mBq/L)

GEOSECS (1973)GX-229

Knorr73 (1978)St. 3

St. 7

St. 8

St.14

KH-82-1 (1982)CE-8

IAEA97 (1997)St n-6

GEOSECS:GX-229

Knorr73:St.3

Knorr73:St. 7+8+14

KH-82-1:CE-8

IAEA 97:Stn-6

00 1 2 3 4 5

a) (b)

c) (d)

90Sr (mBq/L) 90Sr (mBq/L)

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2631

(

(

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P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372632

Pacific near 10�N, as more 90Sr attached to CaCO3particles from atolls was deposited from close-infallout than for 137Cs. Low 137Cs/90Sr ratios werealso observed in soil and sediment samplescollected in Bikini, Enewetak and Rongelap atolls(Robison and Noshkin, 1999). 90Sr has beentransported to deeper water layers than 137Cs dueto its co-precipitation with Ca in carbonatematerials, which are later dissolved in the deepocean. There is also a remarkable decrease insurface and sub-surface concentrations fromGEOSECS to IAEA years—from about 4 to1.5mBq l�1 (decay-corrected peak values) for90Sr and from about 6.5 to 2mBq l�1 for 137Cs.As we have seen, the distribution pattern of

137Cs and 90Sr in the water column is quitedifferent from 239,240Pu, because these elementsare soluble and conservative with much lowerdistribution coefficients (IAEA, 1985) than Pu.Therefore, most of the 137Cs and 90Sr depositedfrom global fallout into the ocean still remain inthe water column with highest concentrations insurface waters.

4.4. Radionuclide inventories in the water column

Water-column inventories depend primarily onthe geographical position of the sampling stationdue to surface injection from global fallout. Ifsignificant amounts of radionuclides are trans-ported, the water column inventories depend onthe depth of the water column. 239,240Pu waterinventories in 1997 do not show any latitudinaldependence, varying between 100 (St. 1) and 130(St. 8) Bqm�2 (Table 4). The latitudinal distribu-tion of 239,240Pu water inventories in 1997 did notcoincide with global fallout. The correspondingGEOSECS 239,240Pu inventories in 1973 werebetween 130 and 150Bqm�2. At St. 3, at the samelocation as GEOSECS St. 226, the inventoriesdecreased from 150 to 120Bqm�2. On the otherhand, the 239,240Pu inventories in 1997 in theEquatorial Pacific were the same as the GEOSECSinventories. 238Pu water inventories in 1997 show atypical latitudinal distribution with highest valuesin the mid-latitudes and lowest values in theEquatorial Pacific. The differences in the latitu-dinal distributions between 239,240Pu and 238Pu

water inventories is attributable to the differencesin plutonium isotopic ratios between global falloutand close-in fallout (lower 238Pu content). 241Aminventories in 1997 show highest values at St. 8(57Bqm�2), whereas St. 1 shows the lowest 241Aminventories (35Bqm�2). The geographic distribu-tion of 241Am inventories also differs from globalfallout distribution. This can be explained by thedifference in plutonium global fallout isotopicratios and close-in fallout ratios (higher 241Pucontent). In the Equatorial Pacific, more 241Am isin-grown in deep waters from close-in 241Pufallout. These findings suggest that significantamounts of plutonium isotopes were injected intothe NW Pacific, especially deep waters, via close-infallout from the Pacific Proving Grounds.No correlation was found between water depth

and latitude in 239,240Pu inventories in this study.The high 239,240Pu inventories observed at mid-latitude stations (3 times higher than expectedfrom global fallout) are consistent with otherstudies (Nagaya and Nakamura, 1984; Hiroseet al., 2001). High inventories of 239,240Pu and241Am at Sts. 6, 7 and 8 are clearly due to close-infallout derived from large scale nuclear explosionsat Bikini and Enewetak atolls in the early 1950sand specific circulation processes in the area, asdiscussed earlier.As expected, the northern stations (Sts. 1 and 3)

showed the highest 90Sr inventories—1.69 and2.06 kBqm�2, respectively, and the highest 137Csinventories—2.77 and 2.77 kBqm�2, respectively.The 137Cs inventories in 1997 at the northernstations were similar to the previous values,2.8 kBqm�2 (decay-corrected to 1 January 1997)(5.0 kBqm�2 for GEOSECS St. 226 in 1973), and3.3 kBqm�2 (decay-corrected to 1 January 1997)(4.6 kBqm�2 for Hakuho Maru in 1983), and2.8 kBqm�2 in 1997 (St. 3), which are the sameorder of magnitude as the global fallout inven-tories in the mid-latitude region of the NorthernHemisphere (Aoyama et al., 2001a). The 90Srinventories were similar to the GEOSECS valuesof 1.9 kBqm�2 (decay-corrected to 1 January1997) (3.5 kBqm�2 for GEOSECS in 1973). The137Cs/90Sr activity ratios at Sts. 6 and 7 differ fromthe global fallout ratio (1 vs. 1.6), as discussed, dueto different depositional histories when more 90Sr

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(a)

0

1000

2000

3000

4000

5000

6000

Dep

th (

m)

0

1000

2000

3000

4000

5000

6000

0 1 2 3 4 5

137 Cs (mBq/L)

Dep

th (

m)

GEOSECS (1973)

GX-223

GX-224

GX-225

GX-226

KH-80-2 (1980)

C Y -11

IAEA97 (1997)

Stn- 1

Stn- 3

GEOSECS:GX-223-226

KH-80-2:CY-11

IAEA 97:Stn-1

0 1 2 3 4 5 6 7

GEOSECS (1973)GX-229

Knorr73 (1978)St . 2St . 3St . 4

GEOSECS:GX-229Knorr73:St. 2+3+4

0

1000

2000

3000

4000

5000

0 1 2 3 4 5 6 7

Dep

th (

m)

GEOSECS (1973)GX-229

Knorr73 (1978)S t. 2S t. 3S t. 4S t. 6S t. 7S t. 8S t. 13S t. 14

KH-82-1 (1982)CE-8

IAEA97 (1997)Stn-6

GEOSECS:GX-229

Knorr73:St. 2+3+4

Knorr73:St. 6+7+8+13+14

KH-82-1:CE-8

IAEA 97:Stn-6

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 1 2 3 4 5

Dep

th (

m)

GEOSECS (1973)St. 6St. 7St. 8

IAEA97 (1997)Stn-6

Knorr73:St. 6+7+8

IAEA 97:Stn-6

137 Cs (mBq/L)

137 Cs (mBq/L)137 Cs (mBq/L)

0

1000

2000

3000

4000

5000

6000

Dep

th (

m)

(a) (b)

(c) (d)

Fig. 19. 137Cs profiles in the water column of the GEOSECS (1973), KNORR (1978), Hakuho Maru (1980) and IAEA (1997) stations:

(a) central NW Pacific Ocean, (b) west of Bikini atoll, (c) east of Bikini atoll, (d) east (dotted lines) and west (solid lines) of Bikini atoll

(data were decay-corrected to 1 January 1997).

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–2637 2633

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Table 4

Inventories (Bqm�2) of Pu isotopes, 241Am, 90Sr and 137Cs in seawater in the NW Pacific Ocean

Station Location Water depth (m) 239,240Pu 238Pu 241Am 90Sr 137Cs

St. 1 34�5990N, 145�5990E 5927 10172 2.470.1 3571 1690720 27007200St. 3 30�3380N, 170�3690E 5472 12272 2.670.1 3871 20607120 28007200St. 6 11�2610N, 164�5240E 4537 11573 2.070.1 4572 630720 630780St. 7 11�3010N, 161�4460E 3690 11072 1.870.2 4471 780720 650740St. 8 15�3010N, 159�3050E 5557 12974 2.070.3 5772 12207120 19007300

P.P. Povinec et al. / Deep-Sea Research II 50 (2003) 2607–26372634

than 137Cs was attached to carbonate particlesfrom the atolls and deposited as close-in fallout.The typical features of the temporal variation

for the observed vertical profiles taken in 1973(Bowen et al., 1980), 1983 (Nagaya and Naka-mura, 1987) and 1997 (this study) are the decreaseof 90Sr and 137Cs concentrations in the uppermixed layer and the gradual increase in deeperlayers. The percentages of the 90Sr and 137Csinventories in water down to 1000m in the mid-latitude region of the North Pacific were 100% in1973, 90% in 1983, and 75% in 1997. Thesefindings suggest that a large portion of 90Sr and137Cs entering through the sea surface will remainin the upper mixed layer for more than a fewdecades (Aoyama et al., 2001b).

5. Conclusions

The Pacific Ocean has been a major repositoryof anthropogenic radionuclides released fromatmospheric tests of nuclear weapons as globalfallout, as well as close-in fallout from the tropo-sphere from the tests carried out at the MarshallIslands. Several observations have been made inthis study:

(i)

Oceanographic parameters (temperature, sali-nity, DO and nutrients) measured during theIAEA’97 cruise did not show remarkabledifferences in all studied profiles in compar-ison with GEOSECS data measured 20 yearspreviously.

(ii)

The transuranics sub-surface concentrationmaxima were observed over large areas of theNW Pacific. The decrease by about a factor of4 in 239,240Pu concentration maxima observed

in the central NW Pacific, at depths greater byabout a factor of 2, and the decline in levels inthe water column inventory by about 20%over the 24 year period, have been less afunction of particle-driven vertical processesand more in response to water mass circula-tion in the region. Increases in 239,240Puconcentrations were generally noted in deepwater near the sea floor. 239,240Pu and Aminventories in the water column substantiallyexceeded those expected from global fallout.Close-in fallout from the Marshall Islandsnuclear weapons test sites and specific circula-tion processes in the region were responsiblefor the observed phenomena. The plutoniumisotopic signatures suggest that plutoniumfrom close-in fallout in the NW Pacific waspreferentially transported to deep waters. Theclose-in fallout input of transuranics has beenin a more reactive physico-chemical form,which leads to their more rapid transport todeep waters.

(iii)

90Sr and 137Cs data confirmed that theobserved changes in concentration profileshave resulted from the physical circulation ofregional water masses, as these radionuclideshave not been affected as much by processesin the water column as transuranics. Low137Cs/90Sr activity ratios (around 1) observedin water profiles at stations close to theMarshall Islands indicate different deposi-tional histories when more 90Sr attached toCaCO3 particles than

137Cs was deposited asdebris from close-in fallout.

(iv)

239,240Pu, 90Sr and 137Cs concentrations insurface water 40 years after their mainintroduction to the NW Pacific still show alatitudinal dependence.

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(v)

The IAEA’97 results represent the mostcomprehensive recent data set on the distribu-tion of 239,240Pu, 241Am, 90Sr and 137Cs in theNW Pacific Ocean waters, with a descriptionof the behaviour of these radionuclides in thewater column and the physical forcing ofwater mass circulation over the last 24 years.

Acknowledgements

The authors thank the Captain and the crew ofthe research vessel ‘‘Bosei Maru’’ for their assis-tance during the IAEA’97 expedition. They arealso indebted to Messrs. S., Fukasawa, T. Kaza-ma, S. Ozawa, A. So and S. Yasuzumi (Universityof Tokai), S.-K. Park (KORDI), R. Rengarajan(Physical Research Laboratory, Ahmedabad) andH.-H. Uhde (BSH) for the assistance during thesampling and measurements carried out during theexpedition. The Expedition was organized byIAEA’s Marine Environment Laboratory in Mon-aco with the financial support of the Science andTechnology Agency of Japan (presently theMinistry of Education, Culture, Sports, Scienceand Technology) to which the authors expresstheir gratitude. IAEA-MEL operates under bilat-eral agreement between the IAEA and theGovernment of the Principality of Monaco.

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