iodine in the deep water of the ocean

Deep-Sea Research, 1971. Vol. 18, pp. 913 to 919. Pergamon Press. Printed in Great Britain.

Iodine in the deep water of the ocean

SHIZUO TSUNOGAI*

(Received 10 March 1971; in revised form 27 April, 1971; accepted 4 May 1971)

Abstract The distribution of iodine in the ocean has been studied to ascertain the source of the iodide in deep water. Iodide in sea water was separated from iodate immediately upon sampling and both forms were determined. The concentration of iodide in the ocean is much smaller than that of iodate. The higher concentrations of iodide, however, were found in the surface water, near the bottom and in the water from some deep layers. The production rate of iodide from iodate in the bot tom (or on the bottom) has been estimated from the vertical distribution of iodide in the near-bottom water. This also gave the oxidation rate of iodide in the deep water. The production of iodide in the bottom water plays a fairly important role in the cycle of iodine in the ocean. Another source of iodide is presumed to be present in the deep water by taking into account the oxidation rate of iodide and the balance of iodide in the deep water. This source may be distributed locally and cause the higher concentrations of iodide found in some deep layers. The contribution of these two sources is much larger than the iodine from the decomposition of organic matter in the deep water.

I N T R O D U C T I O N

IODINE in the ocean has been studied by many researchers as reviewed by BAgKLEV and THOMPSON (1960). Recently, the behavior of iodine in the surface water has been clarified. The high concentration of iodine in the atmosphere is due to the evaporation of free iodine from the ocean where it is formed by the oxidation of iodide in surface water under solar irradiation (MIYAKE and TSUNOGAI, 1963). The iodide, a metastable form of iodine in oxygenated seawater, occurs in greater quantity in surface waters, though iodate, the thermodynamically stable form of iodine, is predominant even in the surface water (MIVAKE and TSUNOGAI, 1966). It was expected that the chief source of iodide is the reduction of iodate in the surface water by biological activity and a mechanism for its formation has been proposed (TsUNOGAI and SASE, 1969). The highest concentration of iodide apparently occurs in the surface water of the equatorial ocean (TsuNOGAI and HENMI, 1969). Its formation in the surface water and the evaporation of iodine from the ocean surface there is facilitated by the mechanisms just outlined (Fig. 1).

On the other hand, the distribution of iodide in the deep water has not been clear. Abnormally high concentrations of iodide have sometimes been found in some samples from the deep o c e a n (SUGAWARA and TERADA, 1957; MIYAKE and TSUNOGAI, 1966). However, the analysis of iodide is difficult and its concentration in deep water is so low that the analytical method must be very precise. Furthermore, the iodide con- centration changes during the storage of sea water samples. According to TSUNOGAI and SASE (1969), some iodate may be reduced to iodide by biological activity during storage. In this study, iodide was immediately separated from iodate after the sample was obtained and both forms were determined by an accurate analytical method.

*Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate, Japan.

913

914 SHIZUO TSUNOGA!

I o d i d e ( I - ) lodate ( 103 )

K >

R Rairl ppor ,ion Sun l ight

1 2 I-+ -~O 2 + 2 H20 ~ 12 ÷ 2 OH

~ ( < 5 I - + 6H ++ I0- 3 )

i

Rain$~/ 'R ive r

F q Ni t ra te reduc tase • ,c--- . . . . L ? rgan is m " '<

i ~Sinking

Fig. I. A schematic presentation of the cycle of iodine in the surface waters of the ocean. The left side of the figure is the domain of iodide and the right side is that of iodate.

SAMPLING AND ANALYTICAL METHOD

Sea water samples (Fig. 2) were collected in the North Pacific during cruises, KH 70-1 (Feb.-Mar. 1970) and KH 70-2 (Apr.-June, 1970) of the Hakuho Maru of the Ocean Research Institute of Tokyo University. The deep-water samples were obtained with Nansen bottles or Van Dorn type samplers which were attached to the wire at intervals of 500 m. The deepest water or the bottom water was collected with the aid of a sonar pinger (EG and G Model 220) fastened to the end of the wire. The lowest Nansen bottle was affixed 5-10 m above the sonar pinger. Hence, it was

',.. t \ ", \ o

,. "7 •

\ L

" " 1 6 o

"~ . . . . • i i - 3 0

- - - ~ " ..... 20

i*E 1 8 0 * W 160

--10

I i - - 4 0 * N 140 .

Fig. 2. The sampling stations. The double circles in the figure show stations with the surface water colder than 15°C and the solid circles show stations with warmer surface water.

Iodine in the deep water of the ocean 915

10--30 m above the bottom. The sample was treated without delay in the following way. While stirring with a magnetic stirrer, 10--20 ml of 0.1 N silver nitrate solution was added to 300-500 ml of a sea water within a few hours of sampling. After one day iodide in the silver halide precipitate was separated from the iodate in solution by filtration. The concentrations of iodide and iodate were determined in the laboratory on land by SUGAWAgA, KOYAMA and TEgADA'S method (1955) with some improve- ments (TStmOG~d, 1971a). The error of the method is about ___3% or _0.01 pg at./1. when repeated in duplicate.

R E S U L T S A N D D I S C U S S I O N

0 _ 0.1 0.2

- I

i od i de

- 2

- 3

km

- 4

- 5

L l

~ x 0 ~ 3 ~ a t / t

Totat I

I × 1

Fig. 3. The vertical distribution of iodine at 12°01'N, 158°02'E in the North Pacific where the depth is 5855 m. The concentrations of iodide (e), iodate (×) and total iodine (o) are indicated.

A vertical profile of iodine in the ocean (Fig. 3) based on a sample collected at 12°N, 158°E in the Pacific, is similar to those at all other stations (Table 1) except for a thin layer at the surface, where the concentration of iodide varies rather widely (see TSONOCAI and HErCMI, 1969). Assuming the distribution of iodine to be common in the whole ocean, the mean concentration and the total amount of iodine in the ocean has been estimated (Table 2).

At the surface, warm water generally has a greater concentration of iodide than water colder than 15°C (Table 1). A vertical profile often indicates a maximum in the surface layer, but nevertheless its concentration is much smaller than that of iodate.

In the deep water, the variation in the iodide content is less than previously

916 Smzuo TSUNOGAI

Table 1. Iodine in the ocean.

Depth No. of I - [03 - Total I (m) samples (ng at./1 .) (ng at./1 .) (ng at./1.)

(a) Samples taken from 8 stations shown in Fig. 2 whose surface temperature is warmer than 15°C

0-100 100-200 200-500 500-1000

1000-2000 >2000*

(b) Samples taken from 3

0-100 100-200 200-500 500-1000

1000-2000 >2000*

32 744-22 2644-28 338 4- 18 19 474-27 3074-51 3544-33 30 254-21 3624-43 3874-33 27 254-26 373 ± 5 2 398±36 24 224-17 4064-43 4284-34 43 144-17 413 4-31 4274-29

stations in the colder surface water region

15 34 4- 18 6 294-19

12 23 4- 15 12 19 4- 20 11 15 4- 13 14 164-13

(c) Bottom water samples taken from 9 stations whose depths

500-530 9 12 4- 11 10-30 9 42 4- 22

3 2 3 i 3 0 3574-20 3504-29 3794-22 3624-26 3854-22 4104-27 4294- 18 4094-19 4244-19 410 4- 13 426 4- 15

are more than 4500 m

403 4- 18 415 4- 20 3724-20 4144-20

*Excluding the bottom water.

Table 2. Mean concentration and total amount o f iodine in the ocean.

I - IOa- Total I

Mean concentration ng at./kg 19 388 407 ng at.ft. 19 399 418 /zg I/l. 2-3 51 53

I/C1 in weight ratio 2-73 ~" 10-"

Total content g at./m z 0.07 1.51 1.58 g I in the whole ocean 3"2 >( 1015 7"0 X 1016 7"3 × 1016

repor ted (e.g. SUGAWARA and TERADA, 1957; MIYAKE and TSUNOGAI, 1963). Even in the present determinat ions , the layers o f higher iodide (0.02-0.05 g at./1.) occurred i r regular ly and its concent ra t ion in o ther layers was lower than the analyt ical error. A n o t h e r r emarkab le feature is the higher concent ra t ion o f iodide observed in the b o t t o m water. Indeed, i t is more concent ra ted there than in the water 500 m above the b o t t o m (Table 1), as observed at the 9 stat ions with depths o f more than 4500 m.

The sources o f iodide in the deep water are discussed by using these results. Firs t , the concent ra t ion o f to ta l iodine is general ly larger in the deep water than in the surface water (Table 1). The d is t r ibut ion o f i d o i n e in the ocean is no t l ike tha t o f o ther halogens. These are conservative elements in the ocean, bu t iodine resembles tha t o f nutrients. This indicates that iodine in the ocean is incorpora ted into organic mat te r

Iodine in the deep water o f the ocean 917

at the surface and dissolved in the deep water after sinking as seston. This supplies iodide to the deep water.

In the second place the vertical profile of iodate seems to be inversely correlated with that of iodide, though the concentration of iodate increases with depth to about 1000 m. This then suggests that iodide in sea water is formed in situ from iodate and vice versa. The contribution of this source is estimated later.

As the third source, the iodide is presumably formed in the layer near the bottom or in the bottom because of the higher concentration of iodide in the bottom water. There must be some mechanism in the deep water other than that derived from the decomposition of organic matter. A previously proposed mechanism (TsUNOGAI and SASE, 1969) for iodide formation from iodate, however, cannot be applied to deep water. The details for this mechanism remain unknown but the amount of iodide produced and its oxidation rate in the bottom water can be calculated as follows. The chief source of iodide in bottom water is assumed to be evenly distributed on the bottom, while the sink of iodide is assumed to be uniformly operative in the water, i.e. the oxidation rate of iodide is of the first order and constant. The con- centration of iodide given in Table 1 is also assumed to be the same throughout the North Pacific (see Fig. 4). The balance of iodide in the near bottom water is written a s

aC D~2C ~-[ = ~ Z 2 - kC

where C is the concentration of iodide, Z is the distance above the bottom, t is time and k is the oxidation rate constant of iodide. The eddy diffusion constant, D in deep water as obtained by MUNR (1966) is 1"3 cm2/sec. The steady state is used in the

500 m

above n the

botton

Z

J i J i x

O0 20 40 lngat./L c ( [ - )

Fig. 4. The mean concentration of iodide in the bottom water and the conditions for the change of the concentration.

equation because daily or seasonal variations are not to be expected in the deep water of the North Pacific. Thus, the solution is given by

C = Co exp ( - Z)

918 Smzuo TSUNOGAI

where Co is the concentration of iodide in the bottom water. The flux of the iodide formed on the bottom, f is

f = - D Z = O = ~ / D . k C o.

By introducing the concentration of iodide given in Table 1, the constant k and f are obtained as

k = 2 . 5 x 10-2yr -1

f = 4 - 4 x 1 0 - 4 g a t . m -z yr -1.

There are large uncertainties in these figures so that the error may be as great as +_ :00%.

The annual production rate of iodide in the bottom water corresponds to the iodide contained in 10 m of the bottom water or to the iodate in 1.2 m of the same water. The amount of iodide reduced from iodate in this layer is not unexpectedly large when compared with radium derived from dissolution from the bottom. GOLDBERG and KOIDE (1963) determined the dissolution rate as ( 2 -5 )x 10 -21 g/cmZsec. The annual dissolution rate of radium thus corresponds to the radium in 6-16 m of the deep water.

These results make the cycle of iodine in the ocean clearer (Fig. 5). The amount of iodide reduced from iodate on the bottom is a few times greater than the amount of iodine supplied from the decomposition of organic matter in the deep water as estimated by TSUNOGAI (1971b).

~- ~o~

River. IRain 1011 0'05 l°°<ool \1o.o5 [£apo Rain R,ve, 00~01~' >

m

34 67(net)

100 t~-4- -~T_.~O r g3?i-~ - T 67

45 )_

18 12.0 1000 I 0 2 , 0 6 10rg112 " 7 (

12 t 44

Bottom 10.0O005

Fig. 5. The cycle of iodine in the ocean. The figures indicated are the annual rates in a unit of 10-4g at./m2yr or 3.6 × 101°g at./yr in the whole ocean. (Based on Fig. 1, TSUNOGAI and

SASE, 1969).

Iodine in the deep water of the ocean 919

On the other hand, the iodide produced on the bot tom is slowly oxidized to iodate in the deep water during diffusion. I f the oxidation rate of iodide, k can be applied to all the deep water, the amount of oxidized iodide in the deep water is about three times greater than that formed on the bottom. The large amount of oxidized iodide requires a source of iodide in deep water to maintain the steady state for the con- centration of iodide in the water, whether the mechanism of iodide formation is the same as the third source or not. This source seems to correspond to the second source of iodide in deep water described above. In that case, the contribution of the second source amounts to 7 x 10 -4 g at . /m 2 yr (Fig. 5), which is the largest source of iodide in deep water. The large production of iodide may be a cause of the high concentration of iodide found irregularly in some deep layers. The second source of the iodide would then be unevenly distributed. It is suggested that the mechanism for the iodide forma- tion is the reduction of iodide from iodate locally in deep water, where it may exist either in living micro-organisms or in seston. The proof of this mechanism, however, is a further problem to be considered.

C O N C L U S I O N

Iodide, the metastable form of iodine in oxygenated water is certainly a minor part of the iodine in the ocean. The main source of iodide is in the surface water. However, the iodide is produced in deep water f rom the three following sources. First is the iodide from the decomposition of organic matter. This is the smallest contribu- tion. The second is iodide reduced f rom iodate f rom unevenly distributed sources in the deep water. The second source is the largest, but the mechanism of the reduction is not well known. The third is iodide which is reduced f rom iodate in or near the bot tom.

Acknowledgements--I would like to thank Professor M. NlSnI~YRA for a critical reading of the manuscript. I am also grateful to Professor Y. HoRta~ and staff members of R. V. Hakuho Maru for collecting the samples.

REFERENCES

BARKLEY R. A. and T. G. THOMPSON (1960) The total iodine and iodate iodine content of sea water. Deep-Sea Res., 7, 24-34.

GOLI)aERO E. D. and M. KoII)E (1963) Rates of sediment accumulation in the Indian Ocean, pp. 90-102. In: Earth science and meteoritics, North Holland Publishing.

MIYAKE Y. and S. TSUNOOAI (1963) Evaporation of iodine from the ocean. J. geophys. Res., 68, 3989-3993.

MIYAKE Y. and S. Tstmo~AI (1966) Probl~me de l'iode dans les ocrans. Lamer, Bull. Soc. franco-japon, ocdanog., 4, 65-77.

MUNK W. H. (1966) Abyssal recipes. Deep-Sea Res., 13, 707-730. StrOAWARA K., T. KOYAMA and K. TERAI)A (1955) A new method of spectrophotometric

determination of iodine in natural water. Bull. chem. Soc. Japan, 28, 494--407. StJOAWARA K. and K. "I'sRAI)A (1957) Iodine distribution in the western Pacific Ocean.

J. Earth ScL, 5, 81-102. TstmooA1 S. (1971a) Decomposition rate of organic matter in the deep water of the Pacific.

Biological oceanography of the northern North Pacific Ocean, Prof. Motoda's Com- memoration Volume, Hakodate, Japan, (in press).

TstmooAI S. (1971b) Determination of iodine in sea water by an improved Sugawara's method. Analytica Chim. Acta, (in press).

Tstmot3AX S. and T. I-I~NMI (1969) The state of iodine in the tropical ocean. Chikyukagaku (Geochemistry) 3, 14-15.

TSUNooAI S. and T. SAS~ (1969) Formation of iodide-iodine in the ocean. Deep-Sea Res., 16, 489-496.