Appl. Radiat. lsot. Vol. 43, No. I/2, pp. 349-359, 1992 Int. J. Radiat. Appl. Instrum. Part A Printed in Great Britain. All rights reserved

0883-2889/92 $5.00 + 0.00 Copyright © 1991 Pergamon Press plc

Plutonium and 137Cs in the Western North Pacific: Estimation of Residence Time of

Plutonium in Surface Waters

K. HIROSE, Y. SUGIMURA AND M. AOYAMA*

Geochemical Laboratory, Meteorological Research Institute, Nagamine 1-1, Tsukuba, Ibaraki 305, Japan

*Nagasaki Marine Observatory, Minamiyamate 1 1-51, Nagasaki, Nagasaki 850 Japan

Long-term measurements of plutonium and |37Cs in the western North Pacific have been carried out. Plutonium concentrations in surface waters, for which the meridional distribution is high in the mid-latitude and low in the tropical region, decreased gradually during the period 1979 to 1987. The 137Cs concentration showed no decrease in the same period though surface 137Cs changed temporally and spatially. To determine the scavenging rate of plutonium from the mixed layer, we apply a simple box model to the long-term results of the pu/137Cs activity ratio in surface waters, in which an atmospheric fallout data set of plutonium and 137Cs on Tokyo is used. Numerical calculations give a rate constant of 0.25 y-I for the scavenging of plutonium from the mixed layer, which is lower than the typical values for thorium isotopes. The scavenging behaviour of these radionuclides in the mixed layer can be understood with a complexation-scavenging model, which is consistent with the reversible scavenging model.

Introduction

Artificial radionuclides from nuclear explosions and accidental releases from nuclear reactors or nuclear-fuel facilities have been injected into the ocean surface due to global fallout. These radionuclides, especially long-lived ones, are effective as indicators of ptysical and biogeochemical processes in the marine environment as are natural ra]ionuclides. For example, plutonium and americium, which are particle reactive in selwater, are considered to be indicators for scavenging processes of trace metals, whereas )37Cs and 90St in seawater may reflect mixing and diffusion processes rather than sc, tvenging.

One of the important problems regarding marine radioecology is to clarify the fate of fallout-derived radionuclides in the water column and these effects on .the marine ecosystem. However, knowledge of the scavenging rates of artificial radionuclides in the

349

350 Applications

open ocean is limited, although studies on the scavenging process using natural particle-reactive radionuclides have been widely carried out and lead to typical scavenging rate constants of thorium isotopes (Bacon and Anderson 1982; Nozaki etal., 1987; Murnase etal., 1990; Clegg and Whiffield, 1990). One of the reasons is that the steady-state approximation, which has been extensively used for the scavenging model of natural radionuclides, cannot be applied for the vertical transport of artificial radionuclides.

Our laboratory has measured concentrations of plutonium isotopes and 137Cs in the western North Pacific surface waters (Miyake and Sugimura 1976; Miyake et al., 1988). The data set of long-term measurements of surface plutonium and 137Cs will permit the estimation of the scavenging rate constants of these radionuclides according to a simple scavenging model.

In this report, we describe the results of long-term measurements of plutonium and 137Cs in surface waters of the western North Pacific during the period 1979 to 1987, and our estimate of the scavenging rate constants for plutonium and 137Cs. A model for the scavenging process of particle-reactive elements, containing the complexation equilibrium, will be discussed.

Sampling and Methods

Surface water samples were collected between 1979 and 1987 in the western North Pacific, on cruises of R.V. Ryofu, belonging to the Japan Meteorological Agency. All water samples were filtered through a fine membrane filter (Millipore HA, 0.45-gm-pore size).

One to two hundred liters of surface water were analysed for 137Cs. ~37Cs in seawater samples was adsorbed onto ammonium phosphomolybdate precipitates after acidification with concentrated HNO 3. Plutonium isotopes were coprecipitated with (Mg, Ca) hydroxides from 500 to 1000-liter water samples.

137Cs counting was carried out with a high-resolution ,/-ray spectrometer. Both dissolved and particulate plutonium were assayed using m-particle spectrometry following the radioanalytical separation described in detail by Hirose and Sugimura, (1985) and Miyake et al. (1988).

Results

Surface plutonium and 137Cs in the western North Pacific

Plutonium and 137Cs concentrations in surface waters of the western North Pacific are summarized in Table 1. The meridional distribution of total plutonium along the 137 ° longitude is shown in Fig. 1. During the sampling period of 1979 to 1986 surface plutonium in the western North Pacific showed a typical geographical distribution, high in

Applications 351

Table 1: Activity concentrations of plutonium and 137Cs in surface waters of the western North Pacific.

Sampling date:

Location: 239'240pU (IxBq L -l) 137Cs (mBq L q)

Dissolved: Particulate: Total:

1979 Jan. 29°59'N 20°05'N

6°31'N 0°30'N

1980 Jail. 27°00'N 20°00~ 10°002,4 5°00'N 2°30'N 0°03'N

1981 Jan. 15°00'N 10°02'N 5°02~ O°O0~N

1982 Jan. 30°00'N I 1 °00'N

1983 Jan. 29°59'N 24°00'N 14°00~ 9°55~ 4°55'N

1984 Jan, 30°00~ 20°0fiN 24°59W 14°00'N 5°00'N O°02q'q

1985 Jan. 30°OOW 25°00'N 20°007,I 14°59'N 10°05'N 5°027q 0°00~

1986 Jan. 24°00'N 17°0fiN 10 ° 0 0 ' N

4°30~ 0°25'N

1987 Mar. 27°46'N

137°17'E 27.0_+ 2,8 6,0_+ 1.3 137°00'E 20.7 _+ 1,7 6.1 + 1.0 137°00'E 8.5 -+ 0.7 2.1 -+ 0.5 137°00'E 11,1 _+ 0,9 5.2 _+ 1.2

137°00'E 31.0_+2.5 10.6_+ 1.6 137°00'E 9.6 -2-_ 0,8 137°00'E 2.3 _+ 0.2 5.4 _+ 1.2 137°00'E 5.2 + 0.4 137°00'E 5.2 -+ 0.4 137°00'E 3.0 -+ 0.3 3,6 _+ 0.7

137°00'E 4.3 _+0.3 0.41 _+ 0.10 4 .7+0.3 3 .6+0.7 137°02'E 6.9_+0.6 0.41_+0.10 7.3_+0.6 4.4_+0.8 137°02'E 9.8 _+ 0.8 0.59 + 0.15 10.4 + 0.8 3.7 _+ 0.7 137*00'E 5 .3±0.4 0.36+0.09 5 .7+0.4

137°00'E 9.6-+0.8 0.21-+0.05 9.8-+0.8 6.4-+1.2 137°00'E 4.4_+0.4 0.53_+0.10 4.9_+0.4 3 .4+0.7

137°02'E 6.5_+0.5 0.17+0.05 6.7_+0.5 9.0+ 1.3 137°00'E 9.2-+0.7 0.15_+0.04 9.4_+0.7 7.8_+ 1.4 137°00'E 6.5_+0.5 0.51 _+0.13 7.0_+0.5 7.0_+ 1.0 136°59'E 8 .8±0.7 0.35_+0.09 9,2 ± 0.7 6.4_+ 1.2 137°02'E 3.6 _+ 0.4 0.28 _+ 0.07 3.9 ± 0.4 2.7 _+ 0.6

137*00'E 8 .8±0.7 0.77_+ 0,15 9.6_+0.7 7.8± 1.5 137°00'E 6 .2±0.5 0 .15+0.04 6.4_+0.5 137°00'E 6.5 -+ 0.5 0.17 ± 0.07 6,7 _+ 0,5 137°00'E 5.9___0.5 0.42-+0.11 6.3_+0.5 3.5_+0.7 137°00'E 2.6-+0.3 0.15-+0.04 2.7±0.3 5.1±1.1 136°59'E 2.5-+0.3 0,13 ±0,03 2.6_+0.3

136°59'E 8.6 ± 0,7 0.14 _+ 0.04 8.7 _+ 0.7 5.5 _+ 1.0 137°00'E 4.8_+0.4 0 .22±0.06 5.0-+0.4 5.5_+ 1.0 136°59'E 3.7_+0.3 0 .14±0.04 3.8+0.3 5.5_+ 1.0 136°58'E 4.4-+0.4 0.42-+0.10 4.8-+0.4 6.1 -+ 1.0 136°59'E 2,3_+0.3 0 .16±0.04 2.5_+0.3 4 .3±0.9 137°00'E 1,8 _+ 0.2 0.37 _+ 0.09 2.2 _+ 0.2 4.1 + 0.6 137°00'E 2.3 _+ 0.3 0.36 _+ 0,09 2.7 _+ 0.3

136°59'E 5.1 _+0.4 0.46±0.11 5 .6±0.4 5.5_+0.8 137°00'E 3.7_+0.3 0.51 _+0.13 4,2_+0.3 5.6_+0.7 137°00'E 3.0_+0,3 0.37_+0.09 3,4+0.3 5.4-+0.8 137°00'E , 2 .3+0.3 0.41-+0.10 2.7+0.3 3.8_+0.5 137°00'E 2.1 ±0.2 0.30_+0.08 2.4_+0.2 3.5_+0.5

130°45'E 4 .2+0.3 0 .25±0.06 4.5-+0.3 5.5_+0.7

level of one estimated standard deviation for counting only, Uncertainties quoted are at a

352 Applications

the mid-latitude region and low in the tropical region. A similar distribution pattern was observed by Miyake and Sugimura (1976) and Miyake et al. , (1988), which suggests that the geographical distribution of surface plutonium in the ocean reflects a global deposition pattern. The 137Cs concentration in surface waters shows a distribution similar to that of surface plutonium. However the range of values for surface 137Cs is smaller than that for plutonium.

¢ r

G3 1 0 0

u") k_

-,-- 50

CD

CO

" - l O

O

~- 5 c'-" (1) r ) C O ¢,D

E e---

o

CL

P,

0

0

lib A

% • •

O

0

A A

& A A

I ! 1 I

O°N 3 0 ° 2 0 ° 10 ° 0 °

L a t i t u d e

Fig. 1: Mendional distribution of surface 239,24°pu in the western North Pacific along 137°E longitude. Sampling period: o : 1979, A: 1980, A: 1985, • : 1986.

The temporal variation of total plutonium concentrations in surface waters of the mid-latitude region (20 ° to 35 ° N) is shown in Fig. 2. The surface plutonium decreased by 1987 to about one fifth of the 1979 value. In this period, deposition of plutonium on Japan decreased from 270 mBq m-2y -1 in 1981 (a maximum) to 2°7 mBq m-2y -1 in 1986. The rate of decrease of plutonium in surface seawaters is lower than that of atmospheric fallout, which means that the residence time of plutonium in surface waters is longer than the atmospheric residence time of plutonium. On the other hand, 137Cs concentrations in surface waters show no clear change in this sampling period, which is consistent with the fact that most of the 137Cs remains in the surface layer.

Particulate plutonium in surface waters, ranged from 1.6% to 17% of the total, with an average of 6.9%, taken over 30 samples. In contrast, particulate 137Cs in surface waters is tess than 1% of the total.

Applications 353

CD

0

0

0

~ 20 U]

.~_~" C c r O a ~

r- 10 (.~ t - O

"m

a_

30 - f

1 + +++++

{+ , { 4

,I ,|

01975 1980 1985 1990 YEAR

Fig. 2: Temporal variation of surface 239,240pu ill tile mid-latitude region (35"N to 20°N) of the western North Pacific.

~stimation of scavenging rate constant of plutonium

Radionuclides in seawater, derived from atmospheric fallout, are generally ~ransported to deep waters via advection, diffusion and particle scavenging processes. A ximple scavenging model is introduced to determine the residence time of plutonium in the surface layer of open ocean waters. A scavenging process for plutonium and 137Cs is shown in Fig. 3. The plutonium and J37Cs concentrations in surface waters are controlled

Atmospt~ere

Fallout (Pu,~TCS)

Ocean Mixed loyer

Scavenging

Deep layer ~'M P'M = kM CM

Fig. 3: A scheme for a simple scavenging process of artificial radionuclides.

354 Applications

by atmospheric deposition and scavenging rates of plutonium and 137Cs in the surface layer and the depth of the surface mixed layer. • The concentrations of radionuclides in surface waters are expressed as follows;

CR(x,y,t) = Y~ F D t--l(l - A tkR) i (1) R(x,y,t) (x,y, / 1

where FR(x,y,t ) and D(x,y,t ) are the atmospheric deposition rate of the radionucfide and the depth of the mixed layer, respectively. The depth of the mixed layer shows a typical seasonal variation (deep in winter and shallow in summer: Bathen, 1972; Hanawa and Hoshino, 1988). k R is the scavenging-rate constant of the radion~clide in the surface mixed layer, which corresponds approximately to the euphotic layer.

(.~ I,._ q3)

0

t

03

c "

(..0 0

k_

,.l.--

, + - U

_.3

0-

10 -z

O

o o O O O

• •

•

. ~ . . [ _ . V

.

1:3 •

l o " ~9'70 19LZ5 19'80 ~9'85

YEAR

Fig. 4: 239,240pLb/137Cs activity raUos in surface waters of the western North Pacific. The solid line is calculated according to a simple scavenging model using the scavenging-rate constants of 0.25 and 0.11 y-I

for plutonium and z37Cs, respectively. The broken line is based on the 2-~9,24Opu/Z37Cs ratios in the total inventory of the water column. (o : Miyake and Sugimura, 1976, ~x : Bowen et al. 1980, CJ : Miyake et al.

1988, 7 : Nagaya and Nakamura, 1987, • : This work)

The scavenging-rate constant can be estimated from Eq. 1 by using time-series data sets for surface concentrations and values of the atmospheric deposition of radionuclides in a particular open ocean site. The atmospheric deposition rate of the radionuclide and the depth of the mixed layer are a function of time and location. However we have no data for atmospheric deposition in the Pacific Ocean site and there are only limited data for surface concentrations of radionuclides for given sites of the ocean.

To eliminate the geographic effect on the atmospheric deposition and surface concentrations of radionuclides, plutonium in surface waters are normalized with the

Applications 355

corresponding 137Cs, whose scavenging rate constant is considered to be lower than that of plutonium. The 239.24Opu/~37Cs activity ratio in the mixed layer is written as:

Rpu/C s = Cpu(x,y,t),lCcs(x,y,t) (2)

The scavenging-rate constants of plutonium and 137Cs in the mixed layer are estimated numerically from the best fit between the calculated and observed 239,24°pu/137Cs activity ratios° For the calculation, we used a time constant, t, of 0.25 y, which provides good approximate values to the exponential function, the corresponding quarterly fallout data of plutonium, and 137Cs observed in Japan (Hirose etal . , 1987) and the quarterly mean depths of the mixed layer (Hanawa and Hoshino, 1988). Since the long-term data set, shown in Fig° 4, of surface concentrations of plutonium and 137Cs in the North Pacific was collected by several workers (Miyake and Sugimura, 1976; Bowen e t a & 1980; Nagaya and Nakamura, 1987; Miyake et al., 1988), there will be systematic errors and a large uncertainty. In fact, in given sampling periods the experimental activity-concentration ratios vary widely; in some cases by more than an order of magnitude, as shown in Fig. 4. A further condition, therefore, is added that calculated 137Cs activity concentrations in surface waters coincide with the observed values in the mid-latitude of the western North Pacific within 50% uncertainty.

The calculated result is shown in Fig. 4. We obtain 0.25 and 0.11 yq for the scavenging-rate constants of plutonium and 137Cs, respectively, as typical values in )ligotrophic regions of the North Pacific.

Discussion

The removal processes of thorium isotopes, thorium being the most particle-reactive element, have been studied extensively. The vertical profiles of thorium isotopes are c uantitatively explained by the reversible scavenging model (Bacon and Anderson, 1982; Nozaki et al., 1987; Clegg and Whitfield, 1990). However, there is little information on tm chemical interactions between the sinking particles and metal ions. Some researchers l'ave proposed complexation with inorganic scavengers such as oxohydroxides of Fe and I~ln (Luoma and Davis, 1983). In open ocean waters, the concentrations of inorganic s:avengers are usually very low (Landing and Bruland, 1987; Martin and Gordon, 1988), c)mpared to hydroxide ion as a competitive inorganic ligand at concentrations of more titan 1 gmol L q. Recently, Hirose (1990) suggested that metal ions in particulate matter a'e directly associated with organic binding sites by complexation. This hypothesis is supported by the results of sequential leaching experiments for several metal ions ircluding copper, uranium, plutonium and thorium isotopes (Chester et al., 1988; Hirose and Sugimura, 1991a, b).

When metal ions in sinking particles are directly associated with organic binding sites b:' complexation, the scavenging process is represented as follows

M n+ + S m- = MS( n-m)+ : K M s (3)

MS (n-m)+ -> Scavenging : k s (4)

356 Applications

where M n* and S m- are the metal ion and the organic binding site in sinking parucles, respectively, MS (n-m)+ is the organic complex in sinking particles, KMS is the equilibrium constant, and k s is the scavenging-rate constant of the metal complex, which coincides with that of sinking particulate organic matter originating from biogenic materials. Since the metal complexation reaction as shown in Eq. 3 is a reversible process, this model is consistent with the reversible scavenging model proposed by Bacon and Anderson (1982) and Nozaki et aI. (1987).

The affinities of metal ions to particulate organic matter are conventionally represented by a partition coefficient, KD.M, defined by the following equation:

KD,M = Cp,M CD,M- i Cp- 1 (5)

where Cp, M and CD, M are the particulate and dissolved concentrations respectively of a metal in the seawater, and Cp is the concentration of the organic particulate matter in seawater. According to the equilibrium consideration (Hirose, 1990), the partition coefficient is equal to that for the sinking organic particles, KS.M:

KD, M = KS, M = [MS] CD,M-ICs -1 (6)

where [MS] is the concentration of the organic metal complex in the sinking particles and C s is the concentration of the sinking organic matter in the mixed layer.

The vertical flux of a metal on biogenic debris sinking out of the euphoric zone in a particular water column can be given as follows:

F M = k s [MS] (7)

where F M denotes the vertical flux of the metal. Substituting Eq. 6 into Eq. 7, we obtain the following equation:

F M = KD, M ks Cs CD,M (8)

where ksC s is equal to the vertical flux of the organic particulate matter out of the mixed layer, which means new productivity. The result implies that the vertical flux of metals in marine systems is directly linked to the flux of organic particulate matter. This equation clearly includes the idea that new productivity, rather than total primary production may determine the net scavenging rates of reactive elements from oceanic surface waters (Coale and Bruland, 1987). The relationship is also similar to the empirical equation proposed by Fisher et al., (1988), although the empirical equation contains the enrichment factor of phytoplankton instead of the partition coefficient of Eq. 8.

Equation 8 implies that the scavenging-rate constant determined for a dissolved metal is linearly related to the partition coefficient. Since available scavenging-rate constants are limited to thorium isotopes and plutonium, we examined Eq. 8 for two radionuclides. The scavenging-rate constant of thorium isotopes dissolved in the mixed layer of oligotrophic regions (1.4 to 5.2 y-l) (Broecker et al., 1973; Kaufman et al., 1981; Coale and Bruland, 1987) is about 5 to 20 times higher than that of plutonium (0.25 yd: this work). The partition coefficient of thorium is about 5 to 30 times higher than that of plutonium (Cochran et al., 1987). This result suggests that the scavenging-rate constant

Applications 357

for particle-reactive elements out of the surface layer is linearly related to the corresponding partition coefficient. This equation also permits us to estimate the vertical flux of organic matter out of the mixed layer. The new productivity is calculated to be 5 to 25 gC m-2y -1 from the scavenging-rate constant and partition coefficient of thorium, which is in good agreement with the value (18gC m-2y l ) obtained by Martin etal. (1987). These findings support the complexation scavenging model for the scavenging behaviour of particle-reactive elements from surface waters.

Conclusion

The long-term measurements of plutonium and 137Cs concentrations in surface waters of the western North Pacific reveal that both plutonium and 137Cs ale removed out of surface waters though plutonium is preferentially scavenged compared with 137Cs. The result is consistent with the fact that plutonium is more readily adsorbed on particle matter than 137Cs.

The residence times of plutonium and 137Cs in surface waters can be estimated from the long-term data sets of the atmospheric deposition and surface concentrations based on a simple scavenging model. The scavenging-rate constants of plutonium and 137Cs ou t of the mixed layer are 0.25 and 0.11 y-i (4 and 9.1 y of the residence time), respectively, which may be typical values in oligotrophic regions of the North Pacific.

We introduced a reversible scavenging model (complexation scavenging model) :ontaining the complexation concept to understand the scavenging behaviour of 9articlezreactive elements such as thorium and plutonium. The model leads to the simple • elationship that the scavenging-rate constant of a dissolved metal out of the mixed layer s a linear function of the partition coefficient of the corresponding metal and the vertical

]ux of organic particulate materials out of euphotic layers which corresponds to new i~roductivity. The result reveals that the scavenging behaviour of plutonium and thorium in surface waters of oligotrophic regions can be quantitatively explained by the t:omplexation scavenging model.

References

[ acon M.P. and Anderson R.F (1982) Distribution of thorium isotopes between dissolved and particulate forms in the deep sea. J. Geophys. Res., 87, 2045-2056.

Eathen K.H. (1972) On the seasonal changes in the depth of the mixed layer in the North Pacific Ocean. J. Geophys. Res., 77, 7138-7150,

B 3wen V.T., Noshkin V.E., Livingston H.D. and Volchok H.L. (1981) Fallout radionuclides in the Pacific ocean: Vertical and horizontal distributions, largely from GEOSECS stations. Earth Plant. Sci. Lett., 49, 411-434.

B,'oecker W.S., Kaufman A. and Trier R.M. (1973) The residence time of thorium in surface sea water and its implications regarding the rate of reactive pollutants. Earth Plant. Sci. Lett., 20, 35-44.

358 Applications

Chester R., Thomas A., Lin F.J., Basaham A.S. and Jacint G. (1988) The solid state speciation of copper in surface water particulates and oceanic sediments. Mar. Chem., 24, 261-292.

Clegg S.L. and Whitfield M. (1990) A generalized model for the scavenging of trace metals in the open ocean-II. Thorium scavenging. Deep-Sea Res., 38, 91-120.

Coale K.H. and BrulandK.W. (1987) Oceanic stratified eupbotic zone as elucidated by 234Th:238U disequilibria. Limnol. Oceanogr., 32, 189-200.

Cochran J.K., Livingston H.D., HirschbergD..I. and SurprenantL.D. (1987) Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean. Earth Plant. Sci. Lett., 84, 135-152.

Fisher N.S., Cochran J.K., Krishnawami So and Livingston H.D. (1988) Predicting the ocean flux of radionuclides on sinking biogenic debris. Nature, 335,622-625.

Hanawa K. and Hoshino I. (1988) Temperature structure and mixed layer in the Kuroshio region over the Izu Ridge. J. Mar. Res., 46, 683-700.

Hirose K. and Sugmaura Y. (1985) A new method of plutonium speciation in sea water. J. Radioanal. Nucl. Chem. Articles, 92, 363-369.

HiroseK., AoyamaM., Katsuragi Y. and SugimuraY. (1987) Annual deposition of 9°Sr, 137Cs and 239,240pll from the 1961 - 1980 nuclear explosions: A simple model. J. Meteor. Soc. Japan, 65, 259-277.

Hirose K. (1990) Chemical speciation of trace metals in seawater: Implication of particulate trace metals. Mar. Chem., 28, 267-274.

Hirose K. and Sugimura Y, (1991a) Chemical speciation of particulate uranium in seawater. J. Radioanal. Nucl. Chem. Articles, 149, 83-96.

Hirose K. and S ugimura Y. (1991b) Chemical speciation of particulate 23813, 239,240pu and thorium isotopes in seawater. Sci. Total Environ. in press.

Kaufman A., Li Y.-H. and Turekian K.K. (1981) The removal rates of 234Th .and 228Th from waters of the New York Bight. Earth Planet° Sci. Lett., 54, 385-392.

Landing W.M. and Bruland K.W. (1987) The contrasting biogeocbemistry of iron and manganese in the Pacific Ocean. Geochim. Cosmochim. Acta, 51, 29-43.

Luoma S.N. and Davis J.A. (1983) Requirements for modeling trace metal partitioning in oxydized estuarine sediments. Mar. Chem., 12, 159-181.

MamnJ.H., KnauerG.A. KarlD.M. and BroenkowW.W. (1987) VARTEX: carbon cycling in the northeast Pacific. Deep-Sea Res., 34, 267-285.

Martin J.H. and Gordon R.M. (1988) North Pacific iron distribution in relation to phytoplankton productivity. Deep-Sea Res., 35, 177-196.

Miyake Y. and Sugimura Y. (1976) The plutonium content in the western North Pacific waters. Proc. of Symp. on Transuranium Nuclides in the Environment, San Francisco, USERDA and IAEA, IAEA-SM-199/22 pp. 91-105.

Miyake Y., Saruhashi K., Sugimura Y., KanazawaT. and Hirose K. (1988) Contents of 137Cs, plutonium and americium isotopes in the Southern Ocean waters. Pap. Meteor. Geophys., 39, 95-113.

Applications 359

Murnase R.J., Sarminento J.L. and Bacon M.P. (1990) Thorium isotopes, particle cycling models, and inverse calculations of model rate constants. J. Geophys. Res., 95, 16195-16202.

Nagaya Y. and Nakamura K. (1987) Artificial radionuclides m the western North Pacific (II): 137Cs and 239,24OF~1 inventories in water and sediment columns observed from 1980 to 1986. J. Oceanogr. Soc. Japan, 43, 345-355.

Nozaki Y., Yang H.-S. and Yamada M, (1987) Scavenging of thorium in the ocean. J. Geophys. Res., 92, 772-778.