cross equator transport of 137cs from north pacific ocean to south pacific ocean (beagle2003...

Progress in Oceanography 89 (2011) 7–16

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Progress in Oceanography

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Cross equator transport of 137Cs from North Pacific Ocean to South Pacific Ocean(BEAGLE2003 cruises)

M. Aoyama a,⇑, M. Fukasawa b, K. Hirose a,1, Y. Hamajima c, T. Kawano b, P.P. Povinec d, J.A. Sanchez-Cabeza e,f

a Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba 305-0052, Japanb Japan Agency for Marine-Earth Science and Technology, 2-15, Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japanc Low-Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, O24 Wake, Nomi, Ishikawa 923-1224, Japand Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska dolina F1, SK-842 48 Bratislava, Slovakiae Environmental Science and Technology Institute and Physics Department, Autonomous University of Barcelona, 08193 Bellaterra, Spainf Departamento de Medio Ambiente, CIEMAT, 28040 Madrid, Spain

a r t i c l e i n f o

Article history:Available online 13 December 2010

0079-6611/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.pocean.2010.12.003

⇑ Corresponding author. Tel.: +81 29 853 8683; faxE-mail address: [email protected] (M. Aoya

1 Present address: Faculty of Science and Technologcho, Chiyoda-ku, Tokyo 102-8554, Japan.

a b s t r a c t

The anthropogenic radionuclides such as 137Cs, 90Sr, 99Tc, 129I and some transuranics are important trac-ers of transport and biogeochemical processes in the ocean. 137Cs, with a half-life of 30 years, a major fis-sion product present in a dissolved form in seawater, is a good tracer of oceanic circulation at a time scaleof several decades.

At WOCE P6 line along 30�S during the BEAGLE cruise in 2003, surface seawater (around 80 L) was col-lected a few meters below the ocean surface by a pumping system. Water column samples (from 5 to20 L) were collected using a Rosette multisampling system and Niskin bottles. 137Cs was separated fromseawater samples using ammonium phosphomolybdate (AMP) and analysed for 137Cs in low-level HPGegamma-ray spectrometers. Results allowed to draw a detailed picture of the distribution of 137Cs in theSouth Pacific Ocean along P6 line.

A 137Cs depth section was depicted from about 160 samples. 137Cs concentrations in the subsurface lay-ers ranged from 0.07 ± 0.04 Bq m�3 to 1.85 ± 0.145 Bq m�3, high in the Tasman Sea and very low in theeastern region where upwelling occurs. Water column inventories of 137Cs from surface to 1000 dbardepth ranged from 270 ± 104 to 1048 ± 127 Bq m�2. It was concluded that the source of higher 137Cs con-centration and inventories in the Tasman Sea was 137Cs deposited in the mid latitude of the North PacificOcean and transported across the equator during four decades.

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1. Introduction

Global fallout radionuclides were injected into the oceansmainly in the late 1950s and early 1960s, after large-scale atmo-spheric nuclear weapons tests were carried out (United Nations,2000). There have been no major injections into the SouthernHemisphere since 1970, because the contribution from atmo-spheric nuclear weapons testing by China was negligible and theimpact of the Chernobyl accident has been limited to the BlackSea and some regions in the North Atlantic. The major depositionof global fallout radionuclides occurred in the Northern Hemi-sphere in the late 1950s and early 1960s, where about two-thirdsof the cumulative deposit is found (United Nations, 2000). Thehighest deposition occurred in the mid-latitudes of the western

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y, Sophia University, 7-1 Kioi-

North Pacific Ocean and western North Atlantic Ocean (Aoyamaet al., 2006). On the other hand, there is no significant source of137Cs in the Southern Hemisphere: the atmospheric nuclear testscarried out in French Polynesia did not contribute significantly tothe global fallout of 137Cs (Mittelstaedt et al., 1999); furthermore,the 137Cs outflow from Mururoa and Fangataufa atolls, whereunderground tests were carried out, has been negligible (Povinecet al., 1999).

Global fallout radionuclides in sea water have been analysedsince the 1950s to assess the radiological impacts of nuclear weap-ons testing on the marine environment and humans (Aarkrog et al.,1997). In recent years, depending on their half-lives and residencetimes in the ocean, these radionuclides have been used as transienttracers to follow the movement of water masses in oceans on time-scales from years to hundreds of years (Livingston and Povinec,2002; Tsumune et al., 2003). Although radionuclide data are tem-porally and spatially heterogeneous, they provide unique opportu-nities to trace water masses and to study biogeochemical processesin the water column, because the input function is relatively wellknown.

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8 M. Aoyama et al. / Progress in Oceanography 89 (2011) 7–16

A better understanding of the circulation of water masses in theworld ocean plays a crucial role in the protection of the marineenvironment against contamination from land based sources and,subsequently, transboundary transport as well as for climatechange studies, in which oceans are dominant players. Recentstudies on the global thermohaline circulation reveal complexitiesin the flow fields as shown in Fig. 1 (Schmitz, 1995; Macdonald andWunsch, 1996).

There has been direct evidence on outflow of surface water fromthe North Pacific into the Indian Ocean via the Indonesian seas,known as Indonesian throughflow (Gordon et al., 2003; Povinecet al., 2003a, 2004b; Fine, 1985), and further to the South Atlanticby the Benguela Current (Peeters et al., 2004). However, there hasnot been any evidence on an outflow of North Pacific surface waterto the South Pacific, as the zonal equatorial current system appearsto act as a boundary, preventing direct surface flows from theNorth to the South Pacific.

Fig. 1. 137Cs transects along 32�S in the Pacific Ocean. Dots in the section in the uppersample collected. The middle panel shows 137Cs activity in Bq m�3 by depth. The lowerequatorial current, NECC: north equatorial counter current, SEC: south equatorial curren

Recent observations of 137Cs in the North Pacific revealed thatsignificant amounts of 137Cs in the North Pacific water columnwere lost during the past four decades, when compared with pre-vious observations and the atmospheric input (Aoyama et al.,2001; Povinec et al., 2003b). The 137Cs time-series in the North Pa-cific show decreasing surface 137Cs concentrations during the pastfour decades, with variable decrease rates depending on the seaarea, being larger in the northern North Pacific than in the equato-rial and South Pacific (Inomata et al., 2009). After deposition on theocean surface 137Cs is mainly affected by advection, diffusion andradioactive decay (half-life 30 years), because there has not beena significant transport to bottom sediments, less than 10% of thewater column inventory (Lee et al., 2005).

In this paper, we show the results obtained from the BEA-GLE2003 cruise along 30�S, including a detailed distribution ofthe 137Cs concentration in the water column, and discuss theoceanographic implications of the 137Cs observations on the

panel represent a schematic surface circulation and the depth and location of eachpanel show 137Cs activity in Bq m�3 by density. KC: kuroshio current, NEC: northt, MC: Mindanao current, EU: equatorial under current.

Table 1Station Summary along P6 line.

Station Latitude Longitude Date Sampling type

P06-238 30.09�S 154.49�E 5 August, 2003 Water columnP06-234 30.08�S 156.52�E 5 August, 2003 Surface onlyP06-228 30.08�S 158.34�E 6 August, 2003 Water columnP06-221 30.09�S 161.50�E 7 August, 2003 Surface onlyP06-214 30.07�S 165.41�E 9 August, 2003 Water columnP06-211 30.09�S 167.00�E 10 August, 2003 Surface onlyP06-206 30.08�S 169.50�E 11 August, 2003 Water columnP06-200 30.09�S 172.01�E 12 August, 2003 Water columnP06-194 30.07�S 175.18�E 13 August, 2003 Water columnP06-191 30.57�S 177.01�E 14 August, 2003 Surface onlyP06-182 32.51�S 179.92�E 15 August, 2003 Surface onlyP06-175 32.50�S 177.66�W 18 August, 2003 Water columnP06-167 32.51�S 174.00�W 20 August, 2003 Water columnP06-162 32.50�S 171.91�W 21 August, 2003 Surface onlyP06-156 32.51�S 169.50�W 23 August, 2003 Water columnP06-150 32.49�S 166.50�W 24 August, 2003 Surface onlyP06-145 32.50�S 163.16�W 27 August, 2003 Water columnP06-142 32.50�S 161.15�W 27 August, 2003 Surface onlyP06-136 32.50�S 157.32�W 29 August, 2003 Water columnP06-132 32.49�S 154.00�W 30 August, 2003 Surface onlyP06-X16 32.51�S 150.50�W 31 August, 2003 Surface onlyP06-127 32.51�S 149.82�W 31 August, 2003 Water columnP06-124 32.50�S 147.35�W 1 September, 2003 Surface onlyP06-127A 32.51�S 149.82�W 12 September, 2003 Water columnP06-120 32.51�S 144.00�W 13 September, 2003 Water columnP06-117 32.50�S 141.49�W 14 September, 2003 Surface onlyP06-114 32.50�S 139.32�W 15 September, 2003 Water columnP06-108 32.49�S 135.33�W 16 September, 2003 Water columnP06-104 32.50�S 132.67�W 17 September, 2003 Surface onlyP06-100 32.49�S 130.00�W 18 September, 2003 Water columnP06-97 32.49�S 128.00�W 19 September, 2003 Surface onlyP06-92 32.51�S 124.67�W 21 September, 2003 Water columnP06-89 32.49�S 122.66�W 21 September, 2003 Surface onlyP06-84 32.49�S 119.34�W 23 September, 2003 Water columnP06-76 32.50�S 113.99�W 24 September, 2003 Water columnP06-71 32.50�S 112.00�W 25 September, 2003 Surface onlyP06-67 32.50�S 109.34�W 26 September, 2003 Surface onlyP06-62 32.50�S 106.02�W 27 September, 2003 Water columnP06-X18 32.50�S 103.00�W 28 September, 2003 Surface onlyP06-55 32.51�S 101.32�W 29 September, 2003 Surface onlyP06-52 32.50�S 99.33�W 30 September, 2003 Water columnP06-46 32.51�S 95.33�W 2 October, 2003 Water columnP06-43 32.51�S 93.33�W 2 October, 2003 Surface onlyP06-39 32.51�S 90.67�W 3 October, 2003 Surface onlyP06-34 32.50�S 87.34�W 4 October, 2003 Water columnP06-31 32.50�S 85.34�W 5 October, 2003 Surface onlyP06-26 32.50�S 82.00�W 6 October, 2003 Water columnP06-23 32.51�S 79.99�W 8 October, 2003 Surface onlyP06-18 32.50�S 76.65�W 9 October, 2003 Water columnP06-15 32.50�S 74.66�W 10 October, 2003 Surface onlyP06-10 32.50�S 72.49�W 11 October, 2003 Water column

M. Aoyama et al. / Progress in Oceanography 89 (2011) 7–16 9

transport process of 137Cs from the North to the South PacificOcean.

2. Sampling and method

2.1. Hydrographic observations

A CTD system with a Rosette Multi-bottle Sampler (CTD/RMS,12 L � 36 bottles) was used to measure oceanographic parametersand to collect seawater samples for measuring salinity, dissolvedoxygen, nutrients and carbonate system parameters. The on-boardCTD collected data from the surface to within 20 m of the bottom.Over the Tonga-Kermadec Trench in the western South PacificOcean the cast reached 6500 dbar. The details of water samplingand measurements are described elsewhere (Uchida and Fukasa-wa, 2005).

2.2. Water sampling of 137Cs

The round-the-globe BEAGLE2003 cruise (Uchida and Fukasawa,2005, 2007) provided water column samples in the South PacificOceans. Surface and water column samples were collected at 51stations as shown in Table 1, with sampling spacing of around2.5�. Surface water (around 80 L) was collected 5 m below theocean surface by a pumping system at 24 stations. Water columnsamples (from 5 to 20 L) were collected using a Rosette multisam-pling system and Niskin bottles at 26 stations. All water sampleswere filtered through a membrane filter (Millipore HA, 0.45 lmpore size) immediately after sampling. Filtered water sampleswere acidified by adding concentrated HNO3 (addition of 40 mLconc. HNO3 for 20 L seawater sample, making pH about 1.6). Thensamples were transported to the Meteorological Research Institute(MRI) for sample treatment and analysis.

2.3. Analytical methods

Recent developments in the analysis of very low 137Cs levels inunderground laboratories have made possible to determine 137Csconcentrations even in small seawater samples (7–10 L) with highsensitivity and precision (Hirose et al., 2005; Povinec et al., 2004a;Aoyama and Hirose, 2008). 137Cs was concentrated in seawatersamples by adsorption onto AMP (ammonium phosphomolyb-date) using a method described in detail elsewhere (Aoyamaet al., 2000; La Rosa et al., 2001; Aoyama and Hirose, 2008;Levy-Palomo et al., 2011). 137Cs activities were determined bylow-level c-spectrometry with high efficiency HPGe detectors inMRI and Kanazawa University (Hirose et al., 2005). A detaileddescription of analytical methods, quality assurance and resultsof intercomparison exercises are given in Levy-Palomo et al.(2011). This strategy made possible to analyse small volume sea-water samples and thus reach a high 137Cs data density, whichallowed us to draw a detailed picture on the distribution of137Cs in the South Pacific Ocean.

3. Results and discussion

3.1. 137Cs transect and inventory along P6

137Cs concentrations together with hydrographical data alongthe WOCE P6 line are shown in Table 2. Examination of the SouthPacific 137Cs transect (Fig. 1, middle panel) revealed the presence ofa water mass with 137Cs concentrations larger than 1.5 Bq m�3 inthe subsurface layers of the Tasman Sea, at the depths to 100–200 dbar, similar to those found in the subtropical gyre in theNorth Pacific Ocean (Aoyama et al., 2001, 2008; Povinec et al.,

2005). However, global fallout in the Southern Pacific was onlyabout 30% of that observed in the North Pacific (UNSCEAR, 2000).The water column inventories of 137Cs at each station were calcu-lated from surface to 1000 dbar depth (Table 3). The water columninventory of 137Cs in the Tasman Sea down to 1000 dbar in 2003ranged from 857 ± 133 Bq m�2 to 1048 ± 127 Bq m�2 at the sta-tions between P06-238 and P06-206 (Table 3). These inventoriesexceed the global 137Cs fallout in this region, which ranges from400 to 700 Bq m�2 (Aoyama et al., 2006). The 137Cs inventories inthe central part of the South Pacific Ocean along the P6 line also ex-ceed 600 Bq m�2 between stations P06-200 at 172�E and P06-62 at106�W (Table 3). Therefore, an additional source of the 137Cs in theSouth Pacific Ocean is necessary to explain the elevated 137Csinventories observed, especially in the Tasman Sea.

The 137Cs inventory at the stations located in the eastern part ofP6 line decreased from 577 ± 90 Bq m�2 at P06-52 to270 ± 104 Bq m�2 at P06-10 as shown in Table 3. The minimuminventory of 270 ± 104 Bq m�2 was only one fourth of the

Table 2137Cs activity and hydrographic data along P6 line.

Station Depth (dbar) 137Cs (Bq m�3) Nitrate (lmol kg�1) Silicate (lmol kg�1) Temperature (�C) Salinity (PSS) Sigma h

P06-238 S 1.71 ± 0.05 0.73 ± 0.03 0.69 ± 0.12 20.09 35.71 25.27102 1.48 ± 0.16 0.62 ± 0.03 0.68 ± 0.12 20.12 35.70 25.27202 1.14 ± 0.07 5.14 ± 0.05 1.50 ± 0.12 18.47 35.63 25.64404 1.23 ± 0.12 7.64 ± 0.05 2.41 ± 0.12 13.63 35.21 26.44602 0.31 ± 0.06 19.32 ± 0.09 8.52 ± 0.14 9.65 34.74 26.82803 0.60 ± 0.10 24.91 ± 0.10 17.82 ± 0.16 7.16 34.51 27.021002 0.20 ± 0.02 29.28 ± 0.11 36.04 ± 0.20 5.43 34.46 27.20

P06-234 S 1.74 ± 0.07 0.31 ± 0.03 0.71 ± 0.12 20.82 35.63 25.02

P06-228 S 1.45 ± 0.05 0.64 ± 0.03 0.77 ± 0.12 19.96 35.68 25.29100 1.54 ± 0.10 0.70 ± 0.03 0.78 ± 0.12 19.95 35.69 25.30199 1.52 ± 0.05 4.84 ± 0.05 1.50 ± 0.12 18.80 35.64 25.57398 1.46 ± 0.19 9.83 ± 0.06 3.48 ± 0.13 14.63 35.33 26.32600 0.85 ± 0.05 17.67 ± 0.08 7.37 ± 0.14 10.23 34.81 26.77800 0.49 ± 0.05 24.25 ± 0.10 15.98 ± 0.15 7.48 34.53 26.991000 0.34 ± 0.03 28.38 ± 0.11 30.59 ± 0.19 5.81 34.45 27.16

P06-221 S 1.64 ± 0.12 0.30 ± 0.03 0.64 ± 0.12 20.09 35.72 25.29

P06-214 S 1.50 ± 0.07 0.27 ± 0.03 0.66 ± 0.12 18.97 35.75 25.5998 1.78 ± 0.17 0.30 ± 0.03 0.67 ± 0.12 18.91 35.73 25.61200 1.54 ± 0.06 2.49 ± 0.04 1.00 ± 0.12 18.39 35.67 25.70402 1.13 ± 0.13 10.90 ± 0.06 3.90 ± 0.13 13.95 35.23 26.39604 0.75 ± 0.08 19.47 ± 0.09 9.21 ± 0.14 9.63 34.74 26.83801 0.46 ± 0.05 24.62 ± 0.10 17.85 ± 0.16 7.20 34.52 27.02999 0.25 ± 0.04 28.11 ± 0.11 31.82 ± 0.19 5.80 34.47 27.17

P06-211 S 1.59 ± 0.11 0.98 ± 0.04 0.70 ± 0.12 18.27 35.72 25.75

P06-206 S 1.47 ± 0.04 1.21 ± 0.04 0.83 ± 0.12 17.78 35.69 25.8599 1.85 ± 0.15 1.16 ± 0.04 0.79 ± 0.12 17.78 35.68 25.85198 1.38 ± 0.05 1.64 ± 0.04 1.00 ± 0.12 17.60 35.65 25.88400 0.99 ± 0.12 12.50 ± 0.07 4.59 ± 0.13 12.87 35.12 26.52602 0.85 ± 0.07 20.02 ± 0.09 9.93 ± 0.14 9.20 34.70 26.86800 0.54 ± 0.05 24.21 ± 0.10 16.59 ± 0.16 7.26 34.52 27.011000 0.29 ± 0.03 28.04 ± 0.11 31.20 ± 0.19 5.85 34.47 27.17

P06-200 S 1.59 ± 0.06 0.36 ± 0.03 0.70 ± 0.12 18.59 35.67 25.6399 1.72 ± 0.15 0.33 ± 0.03 0.70 ± 0.12 18.60 35.66 25.63198 1.57 ± 0.08 5.52 ± 0.05 1.76 ± 0.12 16.52 35.50 26.02399 1.17 ± 0.11 12.11 ± 0.07 3.98 ± 0.13 12.85 35.09 26.50598 0.55 ± 0.08 20.04 ± 0.09 7.38 ± 0.14 8.84 34.62 26.85797 0.49 ± 0.05 23.90 ± 0.10 11.96 ± 0.15 7.00 34.44 26.991003 0.15 ± 0.04 29.51 ± 0.12 29.96 ± 0.18 5.11 34.38 27.18

P06-194 S 1.37 ± 0.06 1.34 ± 0.04 1.08 ± 0.12 17.69 35.67 25.8599 1.30 ± 0.14 1.35 ± 0.04 1.08 ± 0.12 17.56 35.65 25.88200 1.41 ± 0.05 6.41 ± 0.05 2.26 ± 0.12 16.18 35.48 26.08399 1.30 ± 0.14 13.69 ± 0.07 5.56 ± 0.13 12.18 35.04 26.60599 0.67 ± 0.05 20.64 ± 0.09 9.78 ± 0.14 8.88 34.65 26.87798 0.69 ± 0.10 24.74 ± 0.10 15.73 ± 0.15 6.93 34.47 27.021001 0.24 ± 0.04 28.65 ± 0.11 30.17 ± 0.19 5.53 34.43 27.17

P06-191 S 1.54 ± 0.07 0.82 ± 0.04 1.36 ± 0.12 17.74 35.67 25.84

P06-182 S 1.20 ± 0.06 0.71 ± 0.03 0.82 ± 0.12 17.00 35.59 25.97

P06-175 S 1.26 ± 0.05 0.39 ± 0.03 0.75 ± 0.12 16.51 35.54 26.04101 1.52 ± 0.16 0.54 ± 0.03 0.70 ± 0.12 16.44 35.53 26.06201 1.04 ± 0.13 8.74 ± 0.06 2.73 ± 0.13 13.83 35.22 26.40401 0.53 ± 0.10 17.47 ± 0.08 5.21 ± 0.13 9.52 34.67 26.78598 0.53 ± 0.16 21.67 ± 0.09 7.19 ± 0.13 7.61 34.47 26.92799 0.29 ± 0.09 25.21 ± 0.10 12.94 ± 0.15 6.37 34.38 27.03999 0.15 ± 0.08 29.02 ± 0.11 25.01 ± 0.17 5.26 34.36 27.15

P06-167 S 1.49 ± 0.07 0.40 ± 0.03 0.79 ± 0.12 16.54 35.55 26.04100 1.22 ± 0.14 0.41 ± 0.03 0.86 ± 0.12 16.05 35.49 26.11199 1.11 ± 0.13 7.39 ± 0.05 2.49 ± 0.12 14.25 35.28 26.36401 1.18 ± 0.11 15.80 ± 0.08 4.74 ± 0.13 10.31 34.76 26.72600 0.76 ± 0.11 21.24 ± 0.09 6.64 ± 0.13 7.70 34.47 26.91799 0.43 ± 0.04 24.01 ± 0.10 10.17 ± 0.14 6.65 34.39 26.99999 0.31 ± 0.08 28.10 ± 0.11 20.84 ± 0.16 5.52 34.36 27.11

P06-162 S 1.40 ± 0.06 0.35 ± 0.03 0.87 ± 0.12 16.48 35.51 26.03

P06-156 S 1.20 ± 0.04 0.30 ± 0.03 0.80 ± 0.12 15.78 35.37 26.08101 1.15 ± 0.11 1.17 ± 0.04 0.94 ± 0.12 15.45 35.34 26.13200 1.35 ± 0.12 9.21 ± 0.06 2.63 ± 0.12 12.55 35.06 26.54400 0.85 ± 0.10 17.93 ± 0.08 5.58 ± 0.13 9.44 34.67 26.79602 0.75 ± 0.04 21.68 ± 0.09 7.12 ± 0.13 7.53 34.46 26.93801 0.63 ± 0.07 24.97 ± 0.10 12.21 ± 0.15 6.45 34.38 27.02998 0.35 ± 0.07 28.77 ± 0.11 24.26 ± 0.17 5.31 34.36 27.14


Table 2 (continued)


P06-150 S 1.50 ± 0.05 0.00 ± 0.03 0.44 ± 0.12 17.35 35.52 25.83

P06-145 S 1.42 ± 0.05 0.01 ± 0.03 0.68 ± 0.12 17.47 35.51 25.7899 1.59 ± 0.11 0.11 ± 0.03 0.70 ± 0.12 16.95 35.49 25.91196 1.42 ± 0.10 5.70 ± 0.05 1.68 ± 0.12 14.59 35.27 26.27399 0.97 ± 0.12 16.55 ± 0.08 4.24 ± 0.13 9.49 34.64 26.77600 0.83 ± 0.05 21.20 ± 0.09 6.22 ± 0.13 7.38 34.43 26.93800 0.63 ± 0.07 23.78 ± 0.10 9.56 ± 0.14 6.45 34.36 27.001000 0.29 ± 0.03 28.44 ± 0.11 21.58 ± 0.17 5.27 34.34 27.13

P06-142 S 0.91 ± 0.04 0.15 ± 0.03 0.57 ± 0.12 16.59 35.46 25.95

P06-136 S 1.26 ± 0.06 -0.04 ± 0.03 0.58 ± 0.12 17.09 35.48 25.85100 1.10 ± 0.13 1.36 ± 0.04 0.79 ± 0.12 16.46 35.40 25.95198 1.24 ± 0.10 6.65 ± 0.05 1.75 ± 0.12 14.24 35.18 26.28401 0.85 ± 0.11 15.75 ± 0.08 3.90 ± 0.13 9.48 34.64 26.76801 0.48 ± 0.05 24.50 ± 0.10 10.71 ± 0.14 6.22 34.35 27.021001 0.40 ± 0.07 29.00 ± 0.11 23.31 ± 0.17 5.02 34.32 27.15

P06-132 S 1.29 ± 0.07 -0.02 ± 0.03 0.63 ± 0.12 16.40 35.38 25.94

P06-X16 S 1.21 ± 0.04 0.01 ± 0.03 0.64 ± 0.12 17.14 35.43 25.80

P06-127 S 1.38 ± 0.04 0.08 ± 0.03 0.80 ± 0.12 16.97 35.39 25.8299 1.43 ± 0.09 0.03 ± 0.03 0.68 ± 0.12 16.96 35.39 25.82201 1.19 ± 0.08 2.30 ± 0.04 1.11 ± 0.12 15.11 35.16 26.08399 0.98 ± 0.14 16.14 ± 0.08 3.73 ± 0.13 9.46 34.58 26.72600 0.74 ± 0.05 21.35 ± 0.09 6.62 ± 0.13 7.12 34.40 26.94799 0.55 ± 0.11 24.73 ± 0.10 11.19 ± 0.14 6.04 34.33 27.02

P06-124 S 1.36 ± 0.05 -0.04 ± 0.03 0.32 ± 0.12 16.93 35.37 25.81

P06-127A S 1.48 ± 0.07 -0.02 ± 0.03 0.41 ± 0.12 16.93 35.41 25.84100 1.16 ± 0.11 -0.04 ± 0.03 0.41 ± 0.12 16.89 35.39 25.84202 1.01 ± 0.09 3.08 ± 0.04 0.88 ± 0.12 15.03 35.20 26.13399 0.39 ± 0.14 15.69 ± 0.08 3.36 ± 0.13 9.70 34.61 26.71601 0.62 ± 0.06 21.37 ± 0.09 6.21 ± 0.13 7.07 34.40 26.94801 0.65 ± 0.09 24.89 ± 0.10 10.90 ± 0.14 6.00 34.33 27.031000 0.19 ± 0.02 29.22 ± 0.11 22.71 ± 0.17 4.93 34.31 27.15

P06-120 S 1.50 ± 0.07 -0.03 ± 0.03 0.75 ± 0.12 17.70 35.42 25.66100 1.81 ± 0.16 0.75 ± 0.03 0.83 ± 0.12 16.83 35.34 25.82199 1.17 ± 0.10 4.96 ± 0.05 1.31 ± 0.12 14.30 35.08 26.19398 0.76 ± 0.13 16.85 ± 0.08 3.80 ± 0.13 8.99 34.53 26.76599 0.67 ± 0.04 22.13 ± 0.09 7.24 ± 0.13 6.82 34.38 26.96801 0.49 ± 0.06 25.55 ± 0.10 12.50 ± 0.15 5.84 34.32 27.041002 0.24 ± 0.03 29.60 ± 0.12 24.66 ± 0.17 4.79 34.31 27.16

P06-117 S 1.48 ± 0.07 0.01 ± 0.03 0.43 ± 0.12 16.86 35.30 25.78

P06-114 S 1.40 ± 0.05 0.00 ± 0.03 0.67 ± 0.12 17.21 35.28 25.69102 1.24 ± 0.11 -0.03 ± 0.03 0.63 ± 0.12 16.80 35.25 25.76200 0.89 ± 0.06 4.07 ± 0.04 1.05 ± 0.12 14.32 35.05 26.16399 0.56 ± 0.12 17.17 ± 0.08 4.01 ± 0.13 8.61 34.49 26.79600 0.67 ± 0.05 21.31 ± 0.09 7.04 ± 0.13 6.84 34.38 26.96802 0.59 ± 0.06 24.68 ± 0.10 12.20 ± 0.15 5.81 34.31 27.041000 0.20 ± 0.02 29.81 ± 0.12 26.25 ± 0.18 4.66 34.31 27.18

P06-108 S 1.31 ± 0.07 0.07 ± 0.03 0.66 ± 0.12 16.74 35.16 25.6999 1.00 ± 0.18 0.08 ± 0.03 0.71 ± 0.12 16.53 35.14 25.73201 1.09 ± 0.06 5.36 ± 0.05 1.08 ± 0.12 13.56 34.86 26.18400 0.79 ± 0.11 18.91 ± 0.09 4.55 ± 0.13 8.17 34.40 26.81599 0.55 ± 0.05 22.56 ± 0.10 7.70 ± 0.14 6.57 34.35 26.97801 0.40 ± 0.06 26.08 ± 0.11 13.45 ± 0.15 5.60 34.30 27.061001 0.19 ± 0.04 29.89 ± 0.12 26.93 ± 0.18 4.60 34.31 27.19

P06-104 S 1.19 ± 0.09 -0.03 ± 0.03 0.91 ± 0.12 16.60 35.09 25.67

P06-100 S 1.19 ± 0.06 -0.01 ± 0.03 0.59 ± 0.12 15.96 34.97 25.70101 1.01 ± 0.11 -0.05 ± 0.03 0.60 ± 0.12 15.94 34.92 25.70200 1.13 ± 0.05 4.20 ± 0.04 0.96 ± 0.12 13.75 34.84 26.12398 0.87 ± 0.11 19.05 ± 0.09 4.57 ± 0.13 8.10 34.43 26.81601 0.68 ± 0.05 22.25 ± 0.10 7.41 ± 0.14 6.66 34.36 26.97800 0.47 ± 0.04 25.63 ± 0.10 12.74 ± 0.15 5.72 34.31 27.051000 0.17 ± 0.03 30.26 ± 0.12 26.46 ± 0.18 4.62 34.31 27.18

P06-97 S 1.53 ± 0.11 0.04 ± 0.03 0.49 ± 0.12 16.91 35.17 25.64

P06-92 S 1.28 ± 0.06 -0.05 ± 0.03 0.62 ± 0.12 16.86 35.08 25.60102 1.24 ± 0.07 -0.07 ± 0.03 0.64 ± 0.12 16.85 35.08 25.61200 1.11 ± 0.04 4.52 ± 0.05 0.94 ± 0.12 13.96 34.84 26.07400 0.58 ± 0.10 20.00 ± 0.09 5.04 ± 0.13 8.10 34.41 26.80599 0.43 ± 0.06 23.43 ± 0.10 8.26 ± 0.14 6.35 34.33 26.99799 0.40 ± 0.04 26.78 ± 0.11 14.61 ± 0.15 5.40 34.29 27.081000 0.20 ± 0.03 31.37 ± 0.12 29.75 ± 0.18 4.38 34.32 27.21

(continued on next page)


Table 2 (continued)


P06-89 S 1.28 ± 0.10 0.06 ± 0.03 0.51 ± 0.12 16.29 35.00 25.68

P06-84 S 1.16 ± 0.09 0.04 ± 0.03 0.31 ± 0.12 15.55 34.58 25.53101 1.19 ± 0.12 0.01 ± 0.03 0.29 ± 0.12 15.57 34.59 25.53198 1.13 ± 0.08 9.11 ± 0.06 0.94 ± 0.12 12.15 34.47 26.15399 0.81 ± 0.09 20.70 ± 0.09 5.11 ± 0.13 7.41 34.36 26.86600 0.48 ± 0.02 23.08 ± 0.10 8.21 ± 0.14 6.25 34.32 26.99799 0.29 ± 0.06 26.75 ± 0.11 14.70 ± 0.15 5.32 34.28 27.081002 0.14 ± 0.07 31.60 ± 0.12 30.57 ± 0.19 4.32 34.31 27.21

P06-76 S 1.23 ± 0.07 0.08 ± 0.03 0.62 ± 0.12 17.05 34.96 25.50100 1.04 ± 0.13 0.06 ± 0.03 0.59 ± 0.12 16.94 35.00 25.53200 0.59 ± 0.07 5.57 ± 0.05 1.09 ± 0.12 14.02 34.81 26.04401 0.92 ± 0.08 20.10 ± 0.09 4.91 ± 0.13 7.80 34.36 26.81601 0.64 ± 0.05 22.66 ± 0.10 7.79 ± 0.14 6.38 34.33 26.98800 0.32 ± 0.03 26.00 ± 0.11 13.02 ± 0.15 5.40 34.28 27.061001 0.22 ± 0.03 31.21 ± 0.12 30.02 ± 0.19 4.31 34.31 27.21

P06-71 S 1.44 ± 0.07 0.03 ± 0.03 0.45 ± 0.12 17.17 34.96 25.42

P06-67 S 1.29 ± 0.07 0.34 ± 0.03 0.43 ± 0.12 16.06 34.66 25.50

P06-62 S 1.32 ± 0.08 -0.01 ± 0.03 0.87 ± 0.12 16.84 34.76 25.3399 1.70 ± 0.25 0.06 ± 0.03 0.57 ± 0.12 16.47 34.77 25.46199 1.02 ± 0.05 5.62 ± 0.05 0.83 ± 0.12 14.09 34.66 25.91400 0.55 ± 0.10 22.81 ± 0.10 6.39 ± 0.13 7.58 34.31 26.80602 0.10 ± 0.05 25.80 ± 0.11 10.40 ± 0.14 5.95 34.30 27.01798 0.45 ± 0.06 28.45 ± 0.11 16.65 ± 0.16 5.08 34.27 27.10998 0.12 ± 0.04 33.51 ± 0.13 35.91 ± 0.20 4.11 34.33 27.25

P06-X18 S 1.34 ± 0.07 0.55 ± 0.03 0.32 ± 0.12 16.07 34.43 25.30

P06-55 S 1.14 ± 0.07 0.43 ± 0.03 0.45 ± 0.12 16.22 34.50 25.32

P06-52 S 1.01 ± 0.05 0.16 ± 0.03 0.33 ± 0.12 16.70 34.74 25.3898 1.10 ± 0.08 0.13 ± 0.03 0.42 ± 0.12 16.68 34.72 25.38199 0.95 ± 0.04 5.64 ± 0.05 0.48 ± 0.12 14.35 34.61 25.81398 0.46 ± 0.13 23.29 ± 0.10 6.48 ± 0.13 7.32 34.31 26.83599 0.51 ± 0.04 23.60 ± 0.10 7.98 ± 0.14 6.02 34.30 27.00800 0.32 ± 0.03 27.21 ± 0.11 13.56 ± 0.15 5.14 34.26 27.081000 0.17 ± 0.02 32.69 ± 0.12 32.34 ± 0.19 4.11 34.31 27.24

P06-46 S 1.14 ± 0.07 0.97 ± 0.04 0.38 ± 0.12 16.17 34.52 25.3399 0.92 ± 0.05 0.09 ± 0.03 0.36 ± 0.12 17.23 34.92 25.40198 0.97 ± 0.05 6.13 ± 0.05 0.76 ± 0.12 14.05 34.63 25.88399 0.52 ± 0.12 23.97 ± 0.10 7.24 ± 0.13 7.07 34.29 26.86601 0.37 ± 0.05 24.34 ± 0.10 8.93 ± 0.14 5.92 34.29 27.01799 0.30 ± 0.05 28.53 ± 0.11 16.54 ± 0.16 4.99 34.26 27.101003 0.06 ± 0.02 34.70 ± 0.13 39.69 ± 0.21 4.00 34.34 27.27

P06-43 S 1.12 ± 0.05 1.31 ± 0.04 0.18 ± 0.12 15.97 34.36 25.26

P06-39 S 0.91 ± 0.06 1.02 ± 0.04 0.19 ± 0.12 16.41 34.47 25.24

P06-34 S 0.73 ± 0.07 2.32 ± 0.04 0.30 ± 0.12 15.78 34.31 25.2699 0.87 ± 0.11 2.86 ± 0.04 0.34 ± 0.12 15.04 34.26 25.40199 0.61 ± 0.05 11.39 ± 0.06 0.95 ± 0.12 12.11 34.19 25.95400 0.38 ± 0.11 26.79 ± 0.11 9.76 ± 0.14 7.10 34.30 26.86599 0.48 ± 0.05 25.53 ± 0.10 9.97 ± 0.14 5.69 34.27 27.02800 0.31 ± 0.05 30.89 ± 0.12 22.26 ± 0.17 4.79 34.28 27.141000 0.09 ± 0.03 35.28 ± 0.13 44.95 ± 0.22 3.82 34.36 27.30

P06-31 S 0.83 ± 0.07 1.52 ± 0.04 0.17 ± 0.12 16.29 34.44 25.23

P06-26 S 0.65 ± 0.06 -0.01 ± 0.03 0.35 ± 0.12 16.76 34.57 25.24100 0.78 ± 0.13 1.20 ± 0.04 0.33 ± 0.12 15.62 34.46 25.42801 0.21 ± 0.05 31.16 ± 0.12 23.66 ± 0.17 4.72 34.29 27.151000 0.07 ± 0.04 35.50 ± 0.13 46.71 ± 0.22 3.81 34.37 27.31

P06-23 S 0.58 ± 0.08 0.13 ± 0.03 0.36 ± 0.12 15.97 34.41 25.29

P06-18 S 0.46 ± 0.04 0.46 ± 0.03 0.27 ± 0.12 15.59 34.30 25.2999 0.63 ± 0.17 3.69 ± 0.04 0.36 ± 0.12 14.08 34.16 25.54201 0.59 ± 0.05 22.89 ± 0.10 8.67 ± 0.14 10.19 34.23 26.33401 0.28 ± 0.10 33.43 ± 0.13 17.57 ± 0.16 7.19 34.38 26.91601 0.31 ± 0.07 29.79 ± 0.12 16.88 ± 0.16 5.46 34.29 27.07799 0.12 ± 0.06 35.64 ± 0.13 39.21 ± 0.21 4.44 34.36 27.23

P06-15 S 0.67 ± 0.06 2.39 ± 0.04 0.16 ± 0.12 14.44 34.18 25.45


Table 2 (continued)


P06-10 S 0.07 ± 0.04 6.16 ± 0.05 0.59 ± 0.12 13.50 34.24 25.71100 0.19 ± 0.08 23.45 ± 0.10 21.03 ± 0.17 11.41 34.58 26.38200 0.56 ± 0.04 28.08 ± 0.11 24.73 ± 0.17 10.59 34.65 26.58399 0.29 ± 0.13 35.96 ± 0.13 23.21 ± 0.17 7.78 34.47 26.90600 0.26 ± 0.04 31.27 ± 0.12 18.68 ± 0.16 5.66 34.32 27.06801 0.21 ± 0.07 35.74 ± 0.13 37.34 ± 0.20 4.66 34.36 27.211001 0.11 ± 0.03 39.17 ± 0.14 63.67 ± 0.26 3.90 34.45 27.37

Note: Nitrate and silicate concentrations, temperature, salinity and density (sigma h) for the surface layer as show ‘‘S’’ in this table are obtained as nominal 10 m layer depth,while 137Cs samples were collected at 5 m depth.Uncertainties given in this table for 137Cs is one sigma of counting error and for nitrate and silicate are analytical precision expressed as one sigma, respectively.

Table 3137Cs inventory from surface to 1000 m.

Section – station Inventory Bq m�2

P06-238 857 ± 133P06-228 1048 ± 127P06-214 979 ± 127P06-206 970 ± 114P06-200 943 ± 119P06-194 965 ± 134P06-175 656 ± 169P06-167 868 ± 141P06-156 858 ± 116P06-145 956 ± 117P06-136 799 ± 126P06-127 693 ± 131P06-120 839 ± 124P06-114 712 ± 106P06-108 696 ± 118P06-100 751 ± 96P06-92 656 ± 87P06-84 676 ± 103P06-76 653 ± 90P06-62 620 ± 126P06-52 577 ± 90P06-46 538 ± 93P06-34 458 ± 99P06-26 446 ± 125P06-18 305 ± 100P06-10 270 ± 104


maximum inventory of 1048 ± 127 Bq m�2 in the Southern Pacificat station 228. At station P06-10, the 137Cs concentrations at sur-face and 100 dbar depths were 0.07 ± 0.04 Bq m�3 and0.19 ± 0.08 Bq m�3, respectively. These low 137Cs concentrationscaused the low 137Cs inventory at this station.

3.2. 137Cs time-series and cross equator transport of 137Cs from NorthPacific Ocean to South Pacific Ocean

The time evolution of surface 137Cs concentrations in the NorthPacific (box 1–6, Inomata et al., 2009), based on the HAM database(Aoyama and Hirose, 2004) suggests that global fallout 137Cs hasbeen continuously transported to the equatorial Pacific from thesubtropics in the North Pacific, where it arrives some 10 years later(from the mid 1960s to the late 1970s, Fig. 2). Using bomb tritiumdata, Fine and co-workers (1987) have shown that tritium- richsurface waters were subducted in the northern subtropics, thenflowed along isopycnals towards the equatorial thermocline ofthe central Pacific within a time scale of some 10 years (Fineet al., 1987). Liu and Philander (2001) have estimated that the trav-elling times from the subduction areas in the North Pacific to reachto 5�North are 12–15 years from central Pacific to the westernboundary pathway (Philippine–Mindanao coast) and 4–5 yearsfrom the North America coast to the central equatorial North Paci-fic. These travelling times are in good agreement with our findings(Fig. 2). There has been little discussion about how water crosses

the equator after the water masses enter the equatorial region. Thisis particularly important for the discussion of 137Cs sources of theTasman Sea (box 10 in Fig. 2) because substantial amounts of137Cs are observed there and along 32�S, despite the fact that most137Cs fallout is limited to the northern hemisphere. Tsumune et al.(2011) reveal that a coarse resolution model simulation is able toreproduce the water column 137Cs distribution with a high in thewestern part of the P6 line and a low in the eastern part, as was ob-served in this study. Their model simulation suggests that 137 Cs inthe high concentration core in the Tasman Sea was transportedfrom the North Pacific Ocean.

Recently, Nakano et al. (JGR, 2010) also show particle transportacross the equator in the Pacific Ocean using a Lagrangian modelapproach. Particles in the mid latitude region of the North Pacificenter the equatorial region through the interior pathway and theMindanao current, a southward-flowing current along the east-ward side of the southern Philippine Islands, extending from14�N to the south end of Mindanao near 6�N (MC in Fig. 1). Afterthat, the particles flow eastward along the equatorial undercurrent(EU) and finally enter the South Pacific surface water by Ekmantransport in the surface area. These pathways in the equatorial re-gion are consistent with previous studies (Rodgers et al., 2003;Goodman et al., 2005), in which particles released at the equatorare backtracked until they hit the mixed-layer of the high latitudi-nal region of the Pacific, and are also consistent with the map ofdepth-integrated model transport of Brown and Godfrey (2007).A new finding in Nakano’s experiment is a three-dimensional pic-ture of the explicit connection from the North to the South Pacific.A fraction of the particles entering the South Pacific via these path-ways return to the equatorial region, but some of them are trans-ported to the subtropical gyre in the Southern Pacific and remainthere. The particles enter the equatorial region from the North Pa-cific at the thermocline depth.

The interior pathway from the Northern Pacific to the equator issupported by the presence of the maximum 137Cs concentrationalong the 170�E section at 1�N. This 137Cs maximum correspondsto the tritium maximum identified by Fine et al., 1994, whosemechanics are discussed by Liu and Huang (1998). The particlesthat enter the South Pacific appear at the surface in the centraland eastern equatorial region. It is therefore concluded that higher137Cs concentrations in the western and central tropical SouthPacific were due to an input of 137Cs in the North Pacific.

3.3. 137Cs vs. property plots

The 137Cs vs. salinity plots are useful to investigate the source of137Cs in the South Pacific Ocean. They are characterized by having137Cs concentrations exceeding 1.5 Bq m�3 at high salinities (35.7)and 137Cs concentrations less than 0.5 Bq m�3 at low salinities(around 34.3) as shown in Fig. 3a. The 137Cs vs. salinity plot showedan S-shaped distribution in which 137Cs concentrations decreasewith salinity.

0.1

1

10

100

1960 1970 1980 1990 2000 2010

Box1

Box2

Box4

Box6

Box11

Box12

Box16

137C

s (B

q m

-3)

Year

Fig. 2. 137Cs time-series in surface waters of the Pacific and Indian Oceans (Inomata et al., 2009). The oceans were divided into 16 latitudinal boxes and 137Cs trends for boxes1, 2, 4, 6, 11, 12 and 16 are shown. 137Cs activity is decay corrected of the time of year at half year interval as appeared in x-axis.

Fig. 3. 137Cs vs. property plots. 137Cs vs. salinity. 137Cs vs. density. 137Cs vs. nitrate concentration. 137Cs vs. silicate concentration.



The 137Cs vs. density plot is more complex. The salinity mini-mum of 34.3 is likely Antarctic Intermediate Water (AAIW), andtherefore the lower 137Cs concentration water mass might beformed at the Antarctic region, where 137Cs deposition was verylow (Aoyama et al., 2006). At the eastern part of P6 line, the137Cs concentration in the surface layer was very low(0.07 ± 0.04 Bq m�3) where salinity was 34.24 and nitrate concen-tration was 6.16 ± 0.05 lmol kg�1. These indicate that the lower137Cs concentration was associated with strong upwelling of AAIW.

In the deeper layers, where density exceeds 26.5, the 137Cs con-centration tends to decrease with density as shown in Fig. 3b. Fordensities between 25.0 and 26.0, the 137Cs concentration rangedfrom 0.07 ± 0.04 Bq m�3 to 1.85 ± 0.15 Bq m�3 as shown in Fig. 1(lower panel) and Fig. 3b. In the easternmost station, P06-10, aminimum of 0.07 ± 0.04 Bq m�3 was observed while a maximumof 1.85 Bq m�3 was observed at P06-206 in the density range of25.0–26.0. Again, this indicates that higher 137Cs water is trans-ported from the equatorial South Pacific Ocean to the western partof the P6 line, while lower 137Cs activity water is transported fromthe Antarctic side to easternmost part of P6 line.

The 137Cs vs. nitrate and silicate plots indicate strong upwellingin the eastern part of P6 line, especially at the station P06-10. The137Cs concentration was less than 0.1 Bq m�3 (circled point in fig-ures (a–d) while the nitrate concentration was high (6 lmol kg�1)and salinity (34.24) was low (Table 2 and Fig. 3a and c). This indi-cates that the original water mass of the surface water at P06-10might have originally high nitrate concentration exceeding25 lmol kg�1, low salinity of 35.24 and low 137Cs concentration.After the original water mass is transported from mid depth to sur-face, nitrate is consumed by biological activity, but not salinity nor137Cs.

In the Tasman Sea, 137Cs concentrations exceeded 1.5 Bq m�3 inthe layers shallower than 200 dbar where salinity exceeded 35.5.The salinity in the subsurface layers shallower than 200 dbar inthe south of the Tasman Sea were less than 35.5, while the salinityin the subsurface layers shallower than 200 dbar in the north of theTasman Sea exceeded 35.5 (Talley, 2007). This is also evidence thathigher 137Cs concentration associated with higher salinity exceed-ing 35.5 must originate in the equatorial South Pacific.

4. Conclusions

Along the WOCE P6 line, 30–32.5�S in the South Pacific Ocean, avertical section of 137Cs was depicted using about 160 samples.137Cs concentrations in the subsurface layers ranged from0.07 ± 0.04 Bq m�3 to 1.85 ± 0.14 Bq m�3, high in the western re-gion, the Tasman Sea, and very low in the eastern region, whereupwelling occurs. Water column inventories of 137Cs from surfaceto 1000 dbar depth ranged from 270 ± 104 Bq m�2 to 1048 ±127 Bq m�2. The higher 137Cs concentration in the Tasman Seawas associated to higher salinity waters transported from theequatorial Pacific region, while very low 137Cs concentration inthe eastern region were associated to lower salinity and highnutrients waters, due to strong upwelling in this region.

It was concluded that the higher 137Cs concentration and inven-tory in the Tasman Sea was originated from 137Cs global fallout inmid-latitudes of the North Pacific Ocean and have crossed theequator during these four decades.

Acknowledgements

The authors thank the Captain and the crew of R/V Mirai andstaff of Marine Works Japan, Ltd. for their help with sample collec-tion on the BEAGLE2003 cruise. This work was supported by a grantfrom the Ministry of Education, Culture, Sports, Science and

Technology (MEXT) of Japan. This work was also supported by aMEXT Grant-in-Aid for Science Research (KAKENHI 18310017).The authors also thank Y. Kumamoto and K. Katsumata for theirvaluable discussions and comments on the early version of themanuscript, and we thank S. Ishikawa, A. Mori, S. Shimada and Y.Suda for their great help in drawing the figures and making tables.

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http://dx.doi.org/10.1029/2002GL016003

http://dx.doi.org/10.1029/2002JC001434

cross equator transport of 137cs from north pacific ocean to south pacific ocean (beagle2003...

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