The distribution of particulate iodine in the Atlantic Ocean

Earth and Planetary Science Letters, 32 (1976) 441-450 ©Elsevier Scientific Publishing Company; Amsterdam - Printed in The Netherlands

441

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THE DISTRIBUTION OF PARTICULATE IODINE IN THE ATLANTIC OCEAN * GEORGE T.F. WONG 1, PETER G. BREWER and DEREK W. SPENCER Department o f Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543 (USA} Received May 28, 1976 Revised version received July 9, 1976

The iodine content of marine suspended matter obtained from thirteen stations in the Atlantic between 75° N and 55°S has been measured. The concentration of particulate iodine is high in the surface, up to 127 ng/kg of seawater being observed. Below the euphotic zone, it drops sharply to 1-2 ng/kg. The iodine-containing particles are probably biogenic. A simple box-model calculation shows that only 3% of the particulate iodine produced in the surface water may reach the deep sea and that the residence time of these particles in the surface water is about 0.1 year.

1. Introduction

2. Particulate iodine analyses

Iodine has long been classified as a biophilic element [1]. It can be found in the b o d y tissue of many marine organisms [2] and concentration factors up to 60,000 have been reported for marine algae [3]. Recent studies on dissolved iodine in the oceans [ 4 - 9 ] also strongly suggest that its distribution is controlled by biological processes. Wong and Brewer [10] observed depletions of total dissolved iodine in the surface water and total dissolved iodine maxima in strong pycnoclines. These features may be explained b y the uptake of iodine b y organisms in surface waters and the dissolution of particles in strong pycnoclines. This evidence suggests that particulate iodine is important in the cycling of iodine in the oceans. The iodine content in marine suspended matter has never been measured previously. In this paper, we shall report the distribution of particulate iodine in the Atlantic and discuss some of the implications for the marine geochemistry of iodine.

Sampling. Samples were obtained from thirteen stations (stations 3, 5, 11, 17, 18, 23, 27, 29, 3 1 , 4 0 , 54, 60 and 74) of the GEOSECS Atlantic expedition. The cruise track and the positions of the stations are shown in Fig. 1.

* Woods Hole Oceanographic Institution Contribution Number 3781. 1 Present address: Institute of Oceanography, Old Dominion University, Norfolk, Virginia 23508 U.S.A. GEOSECS Publication No. 84.

Analytical method. For each sample, about 10 liters of seawater was pressure filtered through a 0.6-/lm (47-ram diameter) Nuclepore filter. After the sea salts were carefully rinsed off with distilled water, the filter was dried, weighed and then pressed into a pellet (4 m m × 1 ram). The pellet was analyzed by neutron activation analysis using a thermal neutron flux of 4 X 1012 n cm -2 sec-1 and an irradiation time of 10 minutes. A more detailed description of the sampling and analytical method has been reported by Wong [ 11] and Brewer et al. [12]. Replicate samples from the same depth and station indicate that the overall (sampling and analytical) mean deviation is +0.3 ng I/kg. The blank is also about 0.3 ng/kg.

2. Results and discussion Measurements of particulate iodine at the 13 stations have been tabulated in Table 1. The data are

442

Fig. 1. The cruise track and station locations of the GEOSECS Atlantic expedition.

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445 TABLE 1 (continued) Depth (m)

Particulate I (ng/kg) 1

TABLE 1 (continued) (ppm) 2

Smtion 29 (36°00.0'N, 47°00.0'W)

150 239 360 441" 502 639 794 990 1090 1302 1702 2104 23O6 25O3 2697 2702 2906 3105 3504 3702 4497 4697 4918

16.1 7.6 7.9 7.8 7.1 4.2 4.1 3.7 4.0 3.6 3.l 1.5 0.7 1.5 2.4 1.3 4.4 2.7 2.3 2.2 2.5 2.1 3.8

571 414 409 357 371 208 222 308 298 349 299 77 121 7O 136 146 136 129 152 103 76 64 106

Smtion 31 (27°00.0'N, 53°30.5'W)

29 100 202 301 502 703 854 951 1052 1202 1402 1700 2100 2500 2900 3290 3600 3900 4200 4500 490O 5300

7.1 6.5 4.6 2.8 1.8 1.5 1.4 1.1 1.2 0.9 0.8. 1.1 0.8 0.8 0.9 0.6 0.6 0.4 0.6 0.5 0.6 0.4

176 275 188 292 233 152 126 118 168 126 76 97 52 75 73 80 78 61 53 54 55 19

S t a t e n 40 (3°55.3'N, 38°31.6'W) 1

4.0

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Depth (m) 80 216 378 524 675 821 949 1183 1355 1562 1859 2299 2448 3209 3510 3811 4062 4162

Particulate I (ng/kg) I 5.6 4.1 2.8 1.2 1.6 2.2 1.6 1.6 1.3 1.6 1.7 1.4 1.3 1.7 1.2 1.6 1.3 1.4

(ppm) 2

285 283 246 66 114 161 5O 163 160 126 119 109 168 130 140 102 134 116

Smtion 54 (15°02.0'S, 29°31.5'W)

8 138 238 324 439 689 998 1197 1595 1fi94 2077 2478 2880 3280 3680 3987 4287 4486 4678 4883

4.6 9.2 3.9 2.8 3.1 4.2 1.4 1.3 1.6 1.4 1.8 2.0 1.6 1.4 1.4 1.8 1.6 1.5 1.4 2.1

242 756 339 481 345 183 406 595 248 296 141 237 129 182 243 256 268 112 144 225

Station 60 (32°58.0'S, 42 ° 30.0'W)

2 22 65 95 353 729 928 1146 1483 1723

9.9 12.0 15.8 20.6 1.8 1.8 1.5 1.6 1.4 2.6

257 308 685 480 41 203 93 135 126 134

446 TABLE 1 (continued) Depth (m) 2071 2270 2666 2867 3066 3212 3311 3419 3522 4016 4125 4223 4324 4414

Particulate I (ng/kg) 1 1.1 3.3 1.6 0.6 2.1 0.7 1.5 1.5 2.0 3.5 3.4 3.7 4.7 6.5

(ppm) 2

222 161 29 198 102 279 191 188 221 68 181 128 179

Station 74 (54°59.5'S, 50°06.8'W)

8 57 126 208 308 383 461 556 654 831 919 988 1063 1361 1676 1990 2358 2768 3185 3496 3843 4112

5.6 9.2 12.0 2.2 1.7 1.1 1.5 1.5 1.3 1.3 2.0 1.2 1.8 2.0 2.1 1.4 2.2 1.9 1.9 2.5 2.7 2.4

32 66 283 123 105 84 59 97 139 100 112 70 146 101 134 111 59 148 129 110 117 71

1 Relative to unit weight of seawater filtered. 2 Relative to unit weight of total particulate matter.

presented in concentrations relative to unit weight of seawater filtered. One such profile is shown in Fig. 2a. The most prominent feature is the high concentration in the surface waters, the highest concentration observed being 127 ng/kg at 18 m at station 18. In all cases, the maximum occurs within the euphotic zone. In deeper waters, the concentration drops sharply to

about 1 - 2 ng/kg and remains at about this level at greater depths. This distribution of particulate iodine is similar to the distribution of particulate organic carbon, phosphorus and nitrogen [13 16] and is consistent with the suggestion of a biogenic origin, the ttigh concentrations in the euphotic zone being caused by the high fraction of biogenic particles. Rapid recycling ofbiogenic material may cause the sharp drop in concentration at about the thermocline. The results from the same station are presented in concentrations relative to a unit weight of total particulate material in Fig. 2b. There is a significant trend of higher concentrations in the surface layers and lower concentrations in abyssal waters. This distribution clearly suggests the production and rapid recycling of particulate iodine in the surface waters. At locations where a nepheloid layer is present (e.g., station 3, Fig. 2) the total iodine increases near the b o t t o m , but the iodine as a fraction o f the total suspended matter shows no significant increase. Tiffs suggests that the higher concentration of total iodine is due only to the increase in suspended material due to resuspension of the b o t t o m sediments [12] and is not due to scavenging of the iodine onto particles. The concentrations of iodine relative to total particulate weight in Fig. 2b are between 240 and 50 ppm. The lowest concentration observed in all the stations is 19 ppm. The average crustal abundance of iodine is less than 1 ppm [17]. This suggests that particulate iodine is not principally of an inorganic detrital origin. The average concentration in samples closest to the surface is about 270 ppm. This is within the range of 1 0 0 - 3 0 0 ppm in marine organisms [18]. Fig. 3 shows a section of particulate iodine in the western Atlantic from 75°N to 55°S, The surface waters seem to be characterized by concentrations above 5 ng/kg. The higher concentrations (above 10 ng/kg) are confined to the higher latitudes where biological activity is highest. In the deep waters, the concentrations are generally below 2 ng/kg. There is evidence suggesting the transport of particles with the water masses along isopycnals [12]. At station 17, at 75°N, the thermocline is weak, the vertical stability is low and the particulate iodine is variable and high down to about 2300 m. This distribution suggests that in areas of rapid convective overturn the particulate iodine can be carried to great depth without remineralization. There is also a tongue of water

447 PARTICULATE IODINE 0 002 004 nmoles/Kg i

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with high particulate iodine concentration that extends down to about.2000 m between 30°N and 50°N and this probably represents the sinking o f the Labrador Sea water for the o0 surfaces also show an accompanying sharp dip in this region. Station 31 at 27°N is in the middle of the North Atlantic gyre. The productivity of this region is low [19] and this is reflected in the low particulate concentrations throughout the entire water column. In this station, below 900 m, the concentration never exceeds 1 ng/kg, the lowest level observed in the entire Atlantic. Higher particulate iodine concentrations are observed in the nepheloid layer at stations 3, 29, 54, 60 and 74.

4. Fluxes of iodine Some estimates of the fluxes of iodine involved in maintaining the observed distribution of particles may be obtained from a simple box model. In this model, the Atlantic Ocean is divided into two boxes as shown in Fig. 4a. The upper box is the euphotic zone with a thickness of 200 m; the deep-water box has a thickness of 3800 m. The symbols are defined as follows: Cs = concentration of particulate iodine in the surface layer Ps = production of particulate iodine in the surface layer

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= dissolution of particulate iodine in the surface layer CD = concentration of particulate iodine in the deep water D D = dissolution of particulate iodine in the deep water S = sedimentation rate of particulate iodine T~ = transfer by settling of particulate iodine from the surface layer to the deep water TE = exchange of water between the surface and the deep ocean Thus, for the surface layer: Ds

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449 C D = 1.7 ng/kg (from Fig. 3) The average primary productivity is about 80 g C m -2 yr -1 [20], and the carbon to iodine ratio of the organic particles is about 2000 : 1 [8]. Thus: _ 80gC

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= 0.423 g I m -2 yr -~ Average sedimentation rate of the oceans is 0.4 g cm -2 103 yr -~ on a calcium-carbonate-free basis [21,22]. Measurements of the concentrations of iodine in sediments are few. From the available data in the deep Atlantic [23,24], the average concentration is estimated to be about 40 ppm. Thus: S = 4 g m -2 yr -1 × 4 X 10 -s g I/g = 1.6 × 10 - 4 g I m -2 yr-1 The dissolution of particulate iodine in the deep oceans has been estimated by Wong and Brewer [8] to be 2.4 X 10-s/JM kg -1 yr -1 or 3 × 10 -9 g kg -1 yr - l . Thus, in the deep-water box, the total d i s s o l u t i o n (DD) is: 3 × 10 -9 g k g -1 yr -a × 3 8 0 0 m 3 X 103 kg/m 3 = 0.011 g/yr The residence time of the deep water in the Atlantic is about 500 years [5]. Thus, T E = 3800 m 3 × 103 kg/m 3 X 2 X l 0 -3 yr -1 = 7.6 X 10 3 kg/yr. Thus, from (2): ([(7 - 1.7) x 10-91(7.6 x 103) + Ts - 1.6 X 10 -4

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1.4 X 10-3/0.011 = 0.1 yr

The results are shown graphically in Fig. 4b. The numerical values for the various fluxes given here represent our best estimate based on a number of sources. It is possible to change these figures without altering tile larger conclusions of the paper. It has been pointed out to us by S. Tsunogai that our estimate of a carbon to iodine ratio of 2000 : 1 for surface particulates is probably too low; for instance a surface particulate iodine concentration of 7 ng I/kg would give a POC value of 1.3/ag C/kg. A ratio of 10,000 : 1 would be more consistent with observed POC data, but would be at variance with reference [8]. A recalculated PS would then be 0.085 g I m -2 yr -1. Similarly the value Of DD of 3 X 10 -9 g kg -1 yr -~ is based upon estimates of the abyssal oxygen consumption rate, and such a single value may misrepresent what is typically observed to be an exponential function. It is apparent that some subjectivity in the choice of values is required;however the analytical data given here are unique and a simple examination of them from any of several approaches yields very much the picture that we describe above.

- 0.011 }g/yr = 0 TS = ( - 4 X 10 -s + 1.6 X 10 -4 + 0.011) g/yr = 0.011 g/yr

5. Conclusion

From (1): {[(1.7 - 7) x 10-91(7.6 x 103) + 0.423 - D S - 0.011 } g/yr = 0 DS = ( - 4 X 10-s + 0.423 - 0.011) g/yr = 0.412 g/yr The percentage of particulate iodine that is transferred into the deep sea is given by:

Ts/Ps = 0.011/0.423

= 3%

These calculations suggest that 97% of the particulate iodine is recycled within the euphotic zone.

Iodine is enriched in marine suspended matter relative to its average crustal abundance. The particulate iodine distribution in the water column is consistent with a biogenic origin. Particulate iodine is produced and rapidly recycled within the surface layer. The upper layers are characterized b y concentrations above 5 ng/kg. In the deep water, the concentration is only about 1 2 ng/kg. A simple box-model calculation also shows that iodine is cycled within the oceans, and suggests only a negligible amount (1%) of the particles entering the

450 deep sea is p r e s e r v e d in the s e d i m e n t . B r o e c k e r [21] has e s t i m a t e d similar values for t h e n u t r i e n t e l e m e n t s such as n i t r o g e n , p h o s p h o r u s and c a r b o n . T h e redissol u t i o n o f p a r t i c u l a t e iodine occurs m a i n l y in the surface waters. The rate o f d i s s o l u t i o n in surface w a t e r s appears to b e a b o u t 3 0 t i m e s t h a t in the d e e p water. Only 3% o f the p a r t i c u l a t e iodine p r o d u c e d in the surface layers reaches the deep water. This is in qualitative a g r e e m e n t w i t h the e s t i m a t e o f Williams et al. [26] w h o suggested t h a t o n l y 0.5% o f the p h o t o s y n thetically fixed c a r b o n e n t e r s the deep sea. T h e resid e n c e time o f p a r t i c u l a t e i o d i n e in the surface w a t e r is e s t i m a t e d to b e 0.1 year.

Acknowledgements C.L. S m i t h , J.J. F r e d e r i c k s and A. Fleer have provided t e c h n i c a l assistance for the analysis o f the samples and s u b s e q u e n t data processing at various stages o f this work. This w o r k was s u p p o r t e d b y NSFI D O E G r a n t 2 0 / 0 0 6 4 2 1 and f o r m s p a r t o f the Ph.D. Thesis o f G. Wong at MIT and, WHOI. G. W o n g was also s u p p o r t e d b y a research fellowship f r o m the Woods Hole O c e a n o g r a p h i c I n s t i t u t i o n . We also wish to t h a n k the r e a c t o r s t a f f o f the R h o d e Island Nuclear Science C e n t e r for t h e i r assistance in n e u t r o n activation analysis. Dr. M. B e n d e r has b e e n m o s t h e l p f u l in all phases of the analysis and in p r o v i d i n g critical comm e n t s and s t i m u l a t i n g discussion.

References 1 V.M. Goldschmidt, Geochemistry (Clarendon, Oxford, 1954). 2 W.C. Hanson, Iodine in the environment, in: Radioecology, J. Schultz and W. Klement, eds. (Rheinhold, New York, N.Y., 1963) 581-601. 3 E.G. Young and W.M. Langille, The occurrence of inorganic elements in marine algae of the Atlantic provinces of Canada, Can. J. Bot. 36 (1958) 301-310. 4 K. Sugawara and K. Terada, Iodine assimilation by a marine Navicula sp. and the production of iodate accompanied by the growth of the algae, Inform. Bull. Planktoh Japan, Commemoration Number of Dr. Y. Matsue (1967) 213 218. 5 S. Tsunogai and T. Sase, Formation of iodide-iodine in the ocean, Deep-Sea Res. 16 (1969) 4 8 9 - 4 9 6 . 6 S. Tsunogai and T. Henmi, Iodine in the surface water of the ocean, J. Oceanogr. Soc. Japan, 27 (1971) 67- 72. 7 S. Tsunogai, Iodine in the deep water of the ocean, DeepSeaRes. 18(1971) 913 919.

8 G.T.F. Wong and P.G. Brewer, The determination and distribution of iodate in South Atlantic waters, J. Mar. Res. 32 (1974) 25-36. 9 G.T.F. Wong, The distribution of iodine in the upper layers of the equatorial Atlantic, submitted to Deep-Sea Res. 10 G.T.F. Wong and P.G. Brewer, The marine chemistry of iodine in anoxic basins, submitted to Geochim. Cosmochim. Acta. 11 G.T.F. Wong, Dissolved inorganic and particulate iodine in the oceans, Ph.D. Thesis, MIT-WHOI (1976/ 12 P.G. Brewer, D.W. Spencer, P.E. Biscaye, A. Hanley, P.L. Sachs, C.L. Smith, S. Kadar and J. Fredericks, The distribution of particulate matter in the Atlantic Ocean, Earth Planet. Sci. Lett. 32 (1976) 393 (this issue). 13 D.W. Menzel and J.H. Ryther, The composition of organic matter in the western North Atlantic, Limnol. Oceanogr. 9 (1964) 179-186. 14 O. Holm-tfansen, J.D.H. Strickland and P.M. Williams, A detailed analysis of biologically important substances in a profile off southern California, Limnol. Oceanogr. 11 (1966) 548- 561. 15 L.A. Hobson and D.W. Menzel, The distribution and chemical composition of organic particulate matter in the sea and sediments off the east coast of South America, Limnol. Oceanogr. 14 (1969) 159-163. 16 O Holm-Hansen, The distribution and chemical composition of particulate material in marine and fresh waters, Mem. Inst. Ital. Idrobiol. 29 Suppl. (1972) 3 7 - 5 1 . 17 R. Fuge, Iodine, in: Handbook of Geochemistry, K.H. Wedepohl ed. (Springer Verlag, New York, N.Y., 1974) Vol. II-4, sections B-M and O. 18 H.J.M. Bowcn, Trace elements in Biochemistry (Academic Press, 1966). 19 O.J. Koblentz-Mishke, V.V. Volkovinsky and J.G. Kabanova, Plankton primary production of the world ocean, in: Scientific Exploration of the South Pacific, W.S. Wooster, ed. (National Academy of Sciences, New York, N.Y., 1970) 183-193. 20 J.H. Ryther, Geographic variations in productivity, in: The Sea, 2, M.N. ttill, ed. (Interscience, New York, N.Y., 1963) 347-380. 21 W.S. Broecker, A kinetic model for the chemical composition of sea water, Quat. Res. 1 (1971) 188 207. 22 K.K. Turekian, Some aspects of the geochemistry of marine sediments, in: Chemical Oceanography, 2, J.P. Riley and G. Skirrow, eds. (Academic Press, London, 1965) 81-126. 23 O.V. Shishkina and G.A. Pavlova, Iodine distribution in marine and oceanic bottom muds and in their pore fluid, Geochemistry (English edition) 2 (1965) 5 5 9 - 5 6 5 . 24 J.H. Bennet and O.K. Manuel, On the iodine abundances in deep-sea sediments, J. Geophys. Res. 73 (1968) 2 3 0 2 2303. 25 W.S. Broecker, R.D. Gerard, M. Ewing and B.C. tterzen, Geochemistry and physics of ocean circulation, in: Oceanography, M. Sears, ed. (AAAS, 1961) 301-322. 26 P.M. Williams, tt. Oeschger and P. Kinney, Natural radiocarbon activity of the dissolved organic carbon in the northeast Pacific Ocean, Nature 224 (1969) 256 258.