Tritium concentrations of the deep sea-water in the Japan Sea and the Pacific Ocean

J.RADIOANAL.NUCL.CHEM.,LETTERS 127 /6/ 447-456 /1988/

TRITIUM CONCENTRATIONS OF THE DEEP SEA-WATER IN THE JAPAN SEA AND THE PACrFIC OCEAN

T. Kaji, N. Momoshima x, Y. Takashima M

Radioisotope Center, Kyushu University,

MDepartment of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku,

Fukuoka 812, Japan

Received 14 May 1988 Accepted 27 May 1988

Tritium concentrations were determined for sea-water samples collected from the Sapan Sea and the Pacific Ocean. In the Japan Sea, it was recognized that tritium was distribut- ed around 2000 m in depth. This means that the vertical mixing of water mass is taking place. On the other hand, in the Pacific Ocean, the tritium concentration appears to reach zero at about i000 m although more than 1 TU concentration are detected for samples collected from deeper water. Hypo- thetical origins of tritium in the deep water in the Pacific Ocean are discussed.

INTRODUCTION

Owing to the wide-scale utilization of nuclear fusion

energy, tritium will be an important radioactive pollutant

in the environment. Tritium released from nuclear faci-

lities can contaminate a variety of environmental materials

where hydrogen is present as a constituent element. There-

fore, it is necessary to know the behaviour of tritium

447 Elsevier Sequo~ ~ A., Lausanne A k ~ m ~ i K~d~, Budapest

KAJI et al,: TRITIUM CONCENTRATIONS OF SEA-WATER

containing substances in nature in order to avoid hazard

against human beings.

Especially, the dynamics of tritium in the ocean is

important because the ocean is one of the largest sinks

of tritium. Since tritium originally added onto the sur-

face of the ocean descends down to the bottom water through

the intermediate water, the vertical distribution of tri-

tium provides information about the time scale of vertical

mixing.

We have already reported on the vertical profiles of

tritium in lakes and in oceans measured with a newly

developed device I'2 The sea-water samples were provided

by the Ocean Research Institute, the University of Tokyo.

Since unexpected vertical profiles were obtained, we tried

to find out the possible tritium sources for the explana-

tion of the unusual phenomenon.

EXPERIMENTAL

Sea-water samples used in this study were provided by

the Ocean Research Institute, the University of Tokyo.

These samples were collected during the Antares Expedition

/KH84-3/ of the R/V Hakuho Maru of the Ocean Research In-

stitute. Sampling locations are shown in Fig. i. The col-

lection dates are given in the exposition of each figure

showing vertical profile of tritium concentration. The

analytical techniques including the pretreatment of water

samples are reported elsewhere I'2.

RESULTS AND DISCUSSION

All the tritium data mentioned below are normalized to

the date of sampling by correcting for radioactive decay.

These statistical errors are shown by one-sigma error

448

KAJI et al.: TRITIUM CONCENTRATIONS OF SEA-WATER

100 200 300 km

Depth . . . .

o d - - 200 ,. - 2000 ~ ~ooo y ~ !

6oo0 AN-7 �9 ~[,ooo

: . , 1 , t AN -I(

., . ~ l ~ m ( ~ "a "35o

' ,a

!30o ;,I

2...~ ,.~

g~25o 130 ~ 135 ~ 140 ~ E

Fig. i. Sampling locations

bars in figures. Vertical profiles of tritium concentration,

potential temperature and specific conductivity at the

sampling locations in the Japan Sea are shown in Figs 2

and 3. Studies of vertical profiles of tritium concentra-

tion for the Japan Sea were first carried out by the

authors, though there are some reports on other radio-

nuclide concentrations.

449


5

. . . . .

~u 3 U C O

E 2

~" 1 -

0 1 0 1

I l l ~IiD aid i iO

Tritium conc. Temperature

. . . . Sotinity

2 3 4 Depth,xlO3m

"-134.2

~ - 3 4 . o ~

E 5 ~ - 3 3 . 8 N

- 3 3 . 6 0

- 33.4

Fig. 2. Tritium concentration, temperature, and salinity vs. depth at station 7 /11-12 Sep. '84[

g

8 E : _~ :{ ~ 2

O I O I

Tritium conc. Temperatu re SaLinity

2 3

Depth, xlO3m

- 4oo~

zo

-35 -~ o ~

E

- 34

- 33

Fig. 3. Tritium concentration, temperature, and salinity vs. depth at station 16 /16 Sep. '84/

The Japan Sea is a marginal sea of the western Pacific

Ocean, and it is almost landlocked but connected to the

Pacific and other marginal seas through the straits of

Tsushima, Tsugaru, and Soya. As these straits have very

shallow sill depths of less than 150 m, it is considered

450

KAJI et al.: TRITIUM CONCENTRATIONS OF SEA.WATER

that there is no flux of deep water from the surrounding

seas and the deep water of the Japan Sea originates from

somewhere within the Japan Sea itself. For this reason,

the abyssal circulation in the Japan Sea is of special

interest. Moreover, the average depth of the Japan Sea

is about 1350 m and the deepest site is 3700 m. Because

these values are much smaller than those of the Pacific

Ocean, vertical mixing of tritium in the Japan Sea is

expected to reach the deeper region by the winter con-

vection.

Tritium concentrations in surface layers at both of

the sampling stations show slight intricate variations.

In general, the tritium concentration in the Japan Sea

is higher than that in the lower latitude region of the

western North Pacific Ocean. At the station AN-7 the tri-

tium concentration decreases gradually from near the

i00 m in depth and reaches about the background level at

about 2000 m depth. This shows a rather active mixing

caused by vertical diffusion in this region. At the

station AN-16 the tritium data between iO00 m and 2000 m

in depth were missing because of experimental difficulty.

Therefore, the decreasing tendency of tritium concentra-

tion in this region was not verified, but its concentra-

tion was estimated to be close to the background level

near 2250 m. Besides, at the deeper than 2500 ~ a slight

increase in tritium concentration was observed. This

phenomenon cannot be explained so far, but it must be

clarified in the near future by getting more information

about the region. From the measurements of potential

temperature, and amounts of dissolved oxygen and sili-

cate, Gamo and Horibe 3 found that the water in the Japan

Sea was composed of two water masses, the upper layer

/the Deep Water/ and the lower layer /the Bottom Water/

451


I - -

Tritium conc. (Ist run) --~30 . Tritium conc. (2nd run) I

{ ,. Tempera,ure . | ~ E ~ l ! 5'0 - - - . . . . . . . . . .

X ~

I �9 I I I 1 2 3 4 5 6 8 9 10

Depth, xlO ~ m

Fig. 4. Tritium concentration, temperature, and salinity vs. depth at station 1 /23-29 August, '84[

and that the interface of these water masses existed

near 2000 m in depth. Our results on tritium concentra-

tion coincide with their inference.

Vertical profiles of tritium concentration, potential

temperature and specific conductivity at the sampling

locations in the Pacific Ocean are shown in Figs 4 and

5. In the Pacific Ocean, the penetrations of tritium

below iO00 m have not been verified in the GEOSECS ex-

pedition during 1973-1974 and also in our previous

measurements I. At the AN-I station /which belongs to the

Izu-Ogasawara Trench with a depth of up to i00OO m/ the

itritium concentrations exceed 1 TU below iOO0 m, at

7000 m it is 2.4 TU, though the concentration falls

nearly to zero at about iOOO m.

This is a quite unbelievable phenomenon from the con-

ventional concept. Similarly in the AN-4 station /which

belongs to the Japan Trench with the depth of up to

7500 m/ 1-2 TU of tritium concentrations are detected

at 2000, 4000, and 6000 m, though: it drops tO zero at

1500 m. In order to examine the possibility of sample

452

KAJI et a].: TRITIUM CONCENTRATIONS OF SEA-WATER

~ 5

"63 8 C

3z E ~ 1

Oi ~ - 0 I I

5 6 7 8 Depth, xlO s m

T , - ~ ~; Tritium conc. (Ist run) - ~" ~ Tr i t ium conc. (2nd run) -

,, Tempera tu re a . . . . Sat in i ty r { -

1 2 3 4

2s o~ 7 35.0

-134.o T8 1o ~ I

33.5

5 J33.0

Fig. 5. Tritium concentration, temperature, and salinity vs. depth at station 4 /2-3 Sep. ,84/

contamination, we measured other samples collected at

the same locations but at different depths. These re-

sults are also shown in Figs 4 and 5 in the open circles.

Unfortunately, however, a more puzzling feature of the

tritium profiles were obtained.

To discuss the reliability of the vertical profiles

of tritium in the AN-I and AN-4 stations, mass balance

of tritium was calculated. Without data about the trl-

tium concentrations of precipitations in these locations,

we used 9Osr and tritium inventories in the Japmn Sea

and calculated their ratio to estimate tritium mass

balance because our tritium data in the Japan Sea

fitted to other oceanographic data reasonably. The

calibration factor (3H/9Osr) was about 253. This value

was multiplied by the 9Osr inventories by Nagaya et al. 4

The results are shown in Table i. Tritium inventories

(mCi km -2) integrated along the vertical profile are

shown in the first column in Table i. The values of A]B

in Table 1 correspond to the input[inventory ratios

453


TABLE 1

Comparison of 3H inventories in the water column and those estimated ,from 3 H / 9 O s r r a t i o /mCi k m - 2 /

AN-7 AN-I AN-4

3 H A 1.97xi04 4.67xi04 3.05xlO 4

B x _ 1.90xlO 4 1.90xlO 4

A/B ~ i.OO 2.46 1.61

~Calibrated by using the data of 90Sr inventories by Nagaya et al. /1981/.

/so-called tritium mass balance]. Therefore, the closer

the ratio to unity, the more the reliability of Vertical

profile increases. The ratio in AN-I is as great as 2.46.

From the results mentioned above, the three main pos-

sibilities are listed up as the cause for nonzero tri-

tium concentrations in the deep or bottom water. These

are shown below together with problems left for future

investigations.

i. Possibility caused by measuring system.

With respect to contamination of samples, we have no

evidence for it because we detected zero levels for other

samples treated in the same electrolytic enrichment series.

The influence of the chemiluminescence in liquid scintil-

lation counting is also excluded since almost the same

values are obtained in the re-measurements after a month.

However, some influence is expected if variation of back-

ground counting rate during the measurement and change

of volume reduction ratio in the electrolytic enrichment

happeni

454


2. Possibility of the inflows of sea-water containing

high tritium concentrations from other sea regions.

The residence time of the deep sea-water in the Paci-

fic Ocean is between 200 and i000 y. Therefore we have

to examine further whether rapid circulation occurs or

not, by collecting plenty of other oceanographic data.

3. Possibility of the redissol~tion of tritium due to

organic particulates which include tissue-bound tri-

tium after settlingand depositing on the sea floor.

These kinds of phenomena are found in the cases of

90Sr, 137Cs, and 239'240pu /Refs 4-6/. In order to cla-

rify this fact, the tritium concentrations in the sur-

face layer soils of the sea floor have to be measured

directly after sampling by a box-core sampler and com-

bustion. It is necessary to perform the electrolytic en-

richment to measure such a low-level tritium concentra-

tion. Moreover, it needs a small-volume electrolytic en-

richment cell to perform the electrolysis of such a small

amount of water collected from samples combusted. We are

developing such a device and technique.

The authors wish to thank Dr. T. Gamo of the Ocean Re-

search Institute, the University of Tokyo for providing

the sea-water samples for this study. This work is sup-

ported in part by The Grant-Aid for Fusion Research, The

Ministry of Education, Science and Culture.

455


REFERENCES

i. T. Kaji, N. Momoshima, Y. Nakamura, Y. Takashima, Mem. Fac. Sci. Kyushu University, Ser. C, 14 /1984/ 269.

2. T. Kaji, S. Nakashima, N. Momoshima, Y. Takashima, Mem. Fac. Sci. Kyushu University, Ser. C, 15 /1986/ 201.

3. T. Gamo, Y. Horibe, J. Oceanogr. Soc. Japan, 39 /1983/ 220.

4. Y. Nagaya, K. Nakamura, J. Oceanogr. Soc. Japan, 37 /1981/ 135.

5. S.W. Fowler, S. Ballestra, J. LaRosa, R. Fukai, Deep-Sea Res., 30 /1983/ 1221.

6. E.R. Sholkovitz, Earth Sci. Rev., 19 /1983/ 95.

456

Documents

Tritium concentrations of the deep sea-water in the Japan Sea and the Pacific Ocean