sub-surface iodide maxima: evidence for biologically catalyzed redox cycling in arabian sea omz...

Pergamon Deep-Sea Research II, Vol. 44, No. 67, pp. 1391~1409.1997

0 1997 Ekvier Science Ltd

PII: s0967-0645(97)ooo13-1 All rights reserved. Printed in Gnat Britain

09674X45/97 $17.00 + 0.00

Sub-surface iodide maxima: evidence for biologically catalyzed redox cycling in Arabian Sea OMZ during the SW intermonsoon

A. M. FARRENKOPF,*t G. W. LUTHER, III,* V. W. TRUESDALES and C. H. VAN DER WEIJDEN§

(Received 14 April 1996; accepted 4 December 1996)

Abstract-Sub-surface I- maxima (200-600 nM) were found in five of the six profiles from the Somali and Arabian basins of the Northwest Indian Ocean. In addition to these maxima, dissolved I- exhibited normally high (100 nM or greater) values in the surface and values ranging from 3 to 40 nM at depth, which are higher than other open oceanic systems. Sulfide was generally found to be less than 2 nM in the water column, indicating that the chemical reduction of iodate by sulfide is not important in regulating iodine speciation in the Northwest Indian Ocean. These novel high iodide values below the euphotic zone do not appear to be related to other bulk chemical or hydrographic features (os) but may be the result of two distinct biologically mediated remineralization processes: (1) the direct reduction of 103 to I- as seen in the sub-surface maxima, and (2) release of I- from C-I bonds during the decomposition of organic matter. Iodine normalized to salinity, specific iodine, is not found to be conservative in this system. Overall, our specific iodine data support the incorporation of iodine into organic material in the surface. Iodide, when present below the euphotic zone, is a product of the decomposition of that exported organic material. These data and processes are consistent with those found previously in the Black Sea and the Chesapeake Bay. However, in the Northwest Arabian Sea, iodide and oxygen are measurable throughout the water column, indicating that the system is not at equilibrium with the prevailing redox condition and that traditional thermodynamic considerations of pE do not appear to be applicable. Porewater iodide in the top 150 cm increased with depth to approximately 19 pM as a result of the release of I- during decomposition of organic matter. 0 1997 Elsevier Science Ltd

INTRODUCTION

Oceanic iodine

Iodine is known to be both biologically active and redox sensitive; therefore, its chemical speciation can be considered in relation to thermodynamics, biologically mediated cycling, and advection. From a strictly thermodynamic perspective, iodate should be the only iodine species detectable in seawater even at the low detectable oxygen concentrations observed in this study. Thermodynamic calculations show that an oxygen concentration of lo-l2 M would be required to chemically reduce IO, to I- (Farrenkopf, 1993). Previously, iodide (I-) has been reported in surface waters, as a product of biological cycling (e.g. Jickells et

* University of Delaware, College of Marine Studies, Lewes, DE 19958, U.S.A. t Presently at Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, OR 97291-1000,

U.S.A. ([email protected]) $36 Ladycroft Park, Blewbury, OX 119QW, U.K. 5 Faculty of Earth Sciences, Department of Geochemistry, Utrecht University, Budapestlaan 4, Post Bus 80021,

3508 TA Utrecht, The Netherlands.

1391

1392 A. M. Farrenkopf et al.

al., 1988; Luther and Campbell, 1991), and in other areas of the water column as a product of chemical reduction in anoxic or sulfidic conditions (e.g. Ullman et al., 1990; Luther and Campbell, 1991; Luther and Cole, 1988; Luther et al., 1991). The concentration range of iodide in the open oceans is 200 nM in the surface to less than 10 nM at depth. Iodate profiles are the mirror image of iodide, with concentrations ranging from 200 to 500 nM (e.g. Luther et al., 1988). Relative to deeper waters, total iodine is slightly depleted in the surface waters of the ocean (Wong, 1991, and references therein).

Study area-the Northwest Indian Ocean-Arabian Sea

The Arabian Sea is characterized by areas of intense upwelling and high productivity, an intense oxygen minimum zone, and localized areas of denitrification (e.g. Naqvi et al., 1982; Sen Gupta and Naqvi, 1984; Swallow, 1984; Naqvi et al., 1990; Jochem et al., 1993; Mantoura et al., 1994; Olson et al., 1994). However, a distinct vertical structure of redox species based on the decay of organic matter in a stratified water column, such as the traditional redox regime found in areas such as the Black Sea (e.g. Murray et al., 1989; Luther and Campbell, 1991), is not observed in the Northwest Indian Ocean. The Arabian Sea appears to have a more dynamic cycling of nutrient and other redox sensitive elements, which may be related to the 10 year or less water residence time (Olson et al., 1994). In fact, the physically dynamic environment driven by the monsoons may prevent the establishment of a clear vertical separation in the water column of the dissolved reduced species of nitrogen, iodine, iron, manganese, and sulfur.

Our goal was to determine iodine and sulfur speciation in the OMZ and the porewaters of the Arabian Sea to assess these elements as indicators of redox conditions within the water column. The sulfur research is reported in the papers by Passier et al. (1997) and Theberge et al. (1997). Our study presents dissolved iodine data collected in September, October, and November 1992 during the transition from the Southwest monsoon to the Northeast monsoon in the Northwest Indian Ocean. Samples were collected aboard the R.V. Tyro in conjunction with the Netherlands Indian Ocean Programme (NIOP). Transect data are reported from two legs: Dl-Victoria, Seychelles, to Karachi, Pakistan; D3-Karachi, Pakistan, to Mombasa, Kenya (Fig. 1). Sulfide, measured in conjunction with the iodine samples, was less than 2 nM (Theberge et al., 1997) indicating that the chemical reduction of IOs- by HS- is not important in regulating the iodine speciation. Oxygen and peroxide also were measured by polarographic methods, and in all instances oxygen was detectable. As iodide and oxygen are measurable throughout the water column of the Arabian Sea whereas free sulfide is at or below the detection limit, the dissolved inorganic iodine system is not at equilibrium with the prevailing redox condition and traditional thermodynamic considerations of pE do not appear to be applicable, Liss et al., 1973.

Here we report the dissolved inorganic iodine speciation from the Northwest Arabian Sea and provide evidence to support (1) the incorporation of iodine during organic carbon production, (2) the remineralization of I- from C-I and N-I bonds, and (3) the reduction of IOs- during organic matter decomposition. The novel high concentrations of reduced iodine are not supported by other inorganic or advection parameters and must be due to biologically enhanced enzymatic processes. Moreover, our data suggest that the microbial reduction of iodate in the oxygen minimum zone of the Northwest Indian Ocean may play as significant a role as denitrification in the decomposition of organic matter.

Sub-surface iodide maxima 1393

30' 35' 40' 45' 50" 55' 60' 65' 70' 75" 80"

25'

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-10'

30'

25'

20'

15'

10'

5'

0'

-5"

-10" 30' 35' 40' 45' 50' 55' 60' 65' 70' 75' 80'

Fig. 1. Station locations for data reported in this study. Somali Basin stations 401 and 402 were sampled during leg TY-92-Dl (September 1992), and station 499 was sampled during leg TY-92-D3 (October-November 1992). Arabian Basin station 411 was sampled during leg TY-92-Dl. Stations

481,487,493 were sampled during leg TY-92-D3.

EXPERIMENTAL

Collection and storage

Samples in the present study were taken from 10 1 NOEX bottles (designed and manufactured by Technicap in France) and fired from a 24-bottle CTD rosette system. All samples were collected from the NOEX bottles into 250 ml deionized water rinsed Nalgene bottles and subsequently filtered with 0.45 pm (47 mm) polycarbonate Nuclepore filter cartridges, then refrigerated. In addition, syringe samples on select OMZ samples were taken for the analyses of sulfide, oxygen, peroxide, and iodide. The syringe samples were drawn anaerobically at the rosette into 50 ml acid cleaned syringes. The syringes were flushed with water twice to avoid oxygen contamination in the syringe barrel. After the second rinse, water was collected and the three-way stop cock closed. Syringes were transported to the laboratory on board ship and stored in an argon-filled glove bag until analyzed (within approximately 10 min).

Methods

Iodide (I-) was measured by the polarographic method of Luther et al. (1988) via cathodic stripping square wave voltammetry (CSSWV), iodate (IOs-) was measured according to the method of Herring and Liss (1974) via differential pulse polarography (DPP), and total iodine @I,,) was measured by the method of Takayanagi and Wong (1986)


via oxidation of the sample and DPP. Methods of detection are at present available for either the oxidation of iodine to iodate or the reduction of iodine to iodide (Takayanagi and Wong, 1986; Wong and Zhang, 1992, respectively). In the present study, the oxidation technique was used and is referred to as (XI,,). The instrument minimum detection limits in seawater for I-, IOs-, and XI,, as measured by polarography are 0.2 nM, 20 nM, and 20 nM, respectively. The DPP method of Herring and Liss (1974) to determine (IOs-) in seawater is well established and widely used. The precision reported on replicate artificial seawater samples and sodium chloride solutions is f45% (Herring and Liss, 1974). Precision based on triplicate analyses of environmental samples is generally found to be + 10% (n = 3) (Luther and Campbell, 1991; Luther et al., 1991). The Arabian Sea samples in this study, however, demonstrated exceptionally poor precision because of an unidentified organic matrix interference and thus are not reported here.

Precision on replicate analysis of 10s ~ varied as much as + 50% (one relative standard deviation n = 5). This precision is an order of magnitude greater than that for the method described by Herring and Liss (1974). Because of this lack of precision in the iodate data, it is impossible to quantify a significant organic portion or even substantiate the presence of an organic iodine fraction in the Arabian Sea by the polarographic techniques used. The interference appears to be organic. Evidence to support this assertion is the improved precision on oxidized samples. Oxidation with sodium hypochlorite and subsequent analysis by DPP for XI,, as 103- showed peaks that were reproducible and much more precise. Standard deviations on triplicate analysis of oxidized samples ranged from f 0.4% to f 13%, with an average of +6%. Given these analytical difficulties, it appears that accurate iodine speciation information will be obtained in the Northwest Indian Ocean only if iodide concentrations are determined as soon after collection as possible (within 60 days), which we did. Iodate is not plotted but data are available in Farrenkopf (1993).

In contrast to previous reports of “indefinite” stability of iodide in samples (Wong, 1980; Butler and Smith, 1980) some samples increased in iodide concentration over the course of sample storage. The data are discussed later.

Aboard ship, 10.0 ml aliquots of sample were dispensed into glass voltammetric cells, and the concentrations of iodine species were determined by the method of standard addition. A minimum of three standard additions were made for each determination. In subsequent analyses, three 10 ml aliquots were dispensed, and replicates were analyzed for each iodine species with a three-point standard addition curve.

I- in the porewaters was determined by an ion chromatography method (Luther et al., 1995). Sulfide in the water column and porewaters was determined by CSSWV (Luther and Tsamakis, 1989; Luther, 1991) and has been reported by Theberge et al. (1997).

Oxygen and peroxide can be determined without deposition by applying a negative potential scan from 0.0 V to - 1.5 V using the linear sweep or square wave voltammetry. Oxygen and peroxide have peaks near - 0.09 V and - 0.95 V, respectively (Meites, 1965).

Equipment and apparatus

EG&G Princeton Applied Research Model 384B-4 polarographic analyzer systems were used for water sample analysis. Each analyzer was equipped with a Model 303A static mercury drop electrode (SMDE) in the Hanging Mercury Drop Electrode (HMDE) mode and a Model 305 magnetic stirrer on slow speed. Medium-sized drops were used throughout. Platinum wire served as the counter electrode; a saturated calomel electrode (SCE)


connected through a salt bridge with a Vycor frit was used as a reference. The salt bridge was filled with 3.5% sodium chloride solution, and the reference electrode was filled with “ultrex grade” saturated potassium chloride solution. All potentials are reported vs the SCE.

RESULTS

Iodide

The dissolved iodide (I-) concentrations measured in the OMZ as well as the deep waters in both the Somali and the Arabian basins are higher than previously reported values for open ocean environments (e.g. Jickells et al., 1988; Luther et al., 1988; Ullman et al., 1990; Wong, 1991, and references therein) (Fig. 2a-d). Representative CTD traces showing oxygen, potential temperature and salinity are given for both the Arabian and Somali basins (Fig. 3). The high sub-surface iodide values do not follow isopycnal surfaces (Fig. 4), and rather than an advective feature, are the result of two distinct remineralization processes as discussed below: (1) the direct reduction of 103- to I- and (2) release of I- from C-I or N-I bonds during the decomposition of organic matter.

Five separate stations (reported here) in the Arabian Sea have at least one sub-surface iodide concentration greater than 180 nM and significantly higher than normal values (greater than 10 nM) of iodide at depth (Fig. 2). At the western Somali basin station TY-92- 499 (Fig. 2d), iodate is depleted in the surface and increases at depth. Iodide is high in the surface and decreases to an average value of approximately 10 nM at depth. Iodide is high in the oxygen minimum zone (35 nM), but no sub-surface maximum value is observed. Whether this absence of a sub-surface maximum is due to the sampling depth resolution or because there is no maximum value to be reported at this station is not clear. Two other stations (TY-92-401 (8 September 1992) and TY-92-402 (9 September 1992)) also were sampled southeast in the Somali Basin early in the cruise and do show sub-surface iodide maxima and iodate minima (Table 1).

Arabian basin station TY-92-411 (Fig. 2a) has an iodate profile characteristic of all the stations reported from this study. Coincident with the iodide (I-) sub-surface maxima at depths of approximately 271 m and 295 m are notable decreases in the iodate (IOs-) concentrations, also shown via the difference ([XI,,] - [I-]).

At station TY-92-493 (Fig. 2b) coincident with the iodide (I-) sub-surface maxima at depths of approximately 200,230, and 300 m are notable decreases in the iodate (IOs-). The concentration of (IOs-) at the 200 m depth was below the minimum detection limit. Iodide in the OMZ has an average concentration of 154 nM and at depths below the OMZ remained high (32 nM). The station reported here closest to the Oman Margin, TY-92-481, had very deep sub-surface iodide (I-) maxima at approximately 600 and 800 m (Fig. 2~). The iodide concentrations of these two maxima are the highest reported for oceanic waters. Iodate was measurable at both depths, as were significant levels of oxygen and nitrate.

Total iodine

The dissolved concentrations of total iodine, on average, in the Arabian basin appear to be somewhat higher (470-520 nM) than values found in the Somali basin (460 nM, present study), the North Atlantic (450 nM, Jickells et al., 1988), or the Pacific (447 nM, Campos et al., 1997).


0

Arabian Basin Arabian Basin TY-92-D1411 TY-92-03-493

0 loo 200 3w 400 500 m 700

Iodine LnMl

0

300

6lm

ti %

900

lxm

1500

Somali Basin TY-92-D3-499

i

1 \

Y- 0 100 ml 300 400 500 M)(1 700

Iodine InMl

Fig. 2. The distributions of iodine species for: (a) station 411, collected 19 September 1992; (b) station 493,6 November 1992; (c) station 481,28 October 1992; (d) station 499, 11 November 1992. [Note stations 411 (a) and 493 (b) are within 80 nautical miles of one another.] In each of the plots the species symbols are as follows: ., I-; ??, ZI,,. Depth is reported on the x-axis as dbar, and concentration on the y-axis is reported in nM units. Oxidation with sodium hypochlorite and

subsequent analysis by DPP for IOF= XI,,, (Takayanagi and Wong, 1986).

c

2000

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z ,’ ‘a

6

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I 1

Pote

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(deg

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(PSU

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Fig.

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1398 A. M. Farrenkopf ef al.

/i h TY-92-D3-493 /

:- I ,yyq#J 24.5 25.5 26.5

I.; 100 0 23.5 27.5 26.5

Sigma 8 [kg/m*31

Fig. 4. Iodide (I-) concentrations vs Q. (Note the change m the y-axis concentrations for each of the stations.) Station 411 is plotted as (v) directly above station 493 (0) and station 481 (m). All

three stations are in the Arabian Basin.

The total iodine values in the Arabian basin are 2-5% greater than total values from both the North Atlantic and Pacific. Total iodine in the Somali basin is not statistically different from either the North Atlantic or Pacific. In previous studies, iodine species have been reported to show nutrient-like distributions (e.g. Elderfield and Truesdale, 1980; Wong, 1991, and references therein). However, if iodine showed concentration trends similar to nutrient distributions, then the Atlantic, characterized by new deep water formation, would have lower total iodine concentrations and the Pacific, characterized by older water, would have higher concentrations of iodine relative to the Atlantic. We would expect the Indian Ocean concentrations to fall somewhere between these extremes: greater than the Atlantic and less than the Pacific. We do not observe this behavior for total iodine in the Northwest Indian Ocean. Our results indicate a condition that one would expect for a biointermediate element distribution as described by Broecker and Peng (1982) and not a nutrient-like distribution.


Table 1. Data are reportedfor the individual sub-surface iodide maxima found at each of the stations

speci!ic ae Iodide Iodate XI,, iodine Nitrate Nitrite

Station Depth (m) Salinity (kg mP3) (nM) (nM) (nM) (nM %-I) (PM) (PM) N/P

499 493 493 493 481 481 411 402 401

??

305 35.762 26.781 230 35.677 26.593 201 35.868 26.448 606 35.753 27.339 808 35.530 27.537 271 35.895 26.518 286 NA NA 500 NA NA

191 330 469 13.1 24.2 0.02 12.2 107 345 445 12.5 26.9 0.01 11.9 419 BMDL 431 12.0 21.1 1.14 9.3 575 140 509.2 14.2 25.5 0.07 9.5 164 434 NA NA 33.2 0.05 11.7 181 322 430.4 12.5 24.9 0.02 10.9 118 297 NA NA 5.1 0.37 NA 347 434 NA NA NA NA NA

‘No sub-surface maxima. Station locations are plotted in Fig. 1. NA, data not available; BMDL, data below the minimum detection limit.

Oxygen was detected at all depths at concentrations less than 20 PM. Depth, salinity, and os data are collated from the bottle files recorded in situ via the CTD.

DISCUSSION

Sub-surface iodide maxima

In other ocean basins, multiple profiles with one sub-surface iodide maximum or individual profiles with multiple sub-surface iodide maxima generally have not been reported. The notable lack of this feature in other work may be due to the fact that historically iodide has been reported as a calculated value [i.e. was not measured directly (Wang, 1991, and references therein)]. Only a few earlier investigations from the Orca Basin and the Venezuela Basin (Wong and Brewer, 1977; Wong et al., 1985) showed a sub-surface iodide maximum coincident with very low to zero values of iodate, and the Black Sea shows regular I- increase coincident with IOX- decrease (Luther, 1991; Luther and Campbell, 1991). In distinct contrast to the data presented in this study of the Arabian Sea, none of the previous studies showed multiple profiles or multiple depths with sub-surface iodide maxima. An additional profile containing a sub-surface maximum (previously unpublished) was obtained in the Panama basin by Truesdale (Fig. 5). The data presented here from the Arabian Sea suggest that the multiple iodide sub-surface maxima are not artifacts but transient features in the OMZ of productive oceanic systems.

Isopycnals and redox stratzjication. High iodide values in the sub-surface of the Arabian Sea cannot be explained by traditional sulfidic redox considerations such as those used for the Chesapeake Bay (Luther et al., 1991) and Black Sea (Luther and Campbell, 1991). High iodide values also do not appear to occur as a result of horizontal diffusive flux as suggested by Wong et al. (1985) for the Venezuela Basin, or by isopycnal advection as used by Saager et al. (1989) to explain sub-surface dissolved Mn maximum in the Arabian Sea. Whereas Saager et al. (1989) concluded that the distributions of Mn appear to be governed largely by isopycnal transport, density surfaces do not appear to play as significant a role in the distribution or speciation of iodine in the water column of the Northwest Arabian Sea. Plots of iodide vs cre (Fig. 4) show that iodide maxima concentrations do not correspond to either

1400 A. M. Farrenkopf et al

0

250

so0

VI L : 750, E

low

1ZSQ

1500

SH1571 Panama Basin

I / I I

0 loo 200 300 400 500 600

Iodine [nM]

Fig. 5. The distributions of iodine species for station SH 15 I7 in the Panama basin sampled 26 May 1976 by V. W. Truesdale. Species symbols are as follows: ., I-; ??, 103-; +, XI,,.

q, of 27.2 for Persian Gulf inflow or oa of 26.3 for Red Sea inflow to the Arabian Sea as observed for Mn by Saager et al. (1989). Mn(II1, IV) oxides and IOs- are suitable oxidants for bacterial decomposition of organic matter (see discussion below) and yet appear to behave differently. Dissolved Mn is advected along isopycnals whereas IOs- is apparently cycled in situ. The physical and chemical characteristics of the stations where iodide sub- surface maxima are found are listed in Table 1.

Preferential reduction. Iodide concentrations in the OMZ are of the same order of magnitude, and in some cases higher than the nitrite measured at the same depths from the same bottles. This suggests that preferential reduction of IOs- over NOs- occurs at stations where I- concentrations are high and NOs- concentrations are greater than or equal to 20 uM (e.g. TY-92-411 and 493). In the absence of a viable chemical (redox, thermodynamics) or physical (advection, isopycnal transport) explanation, the biological


Table 2. AG,,,, calculatedfor the oxidation of organic matter at standard temperature andpressure (after Froelich et al.. 1919)

AG,, (kJ mol-‘)

Oxygen reduction 13802+C,osH2630,,oN,~P~106CO~+ 16HN03+ 122H20+H3P04 Nitrate reduction 94.4HN03+C,0sH2630,,,,N,,z,P+106CO~+55.2N2+ 122H,PO,+ 177.2H20 84.8HNO~+C,06H2630,,,,N,6P+106CO~+42.4NT+ 16NH,+H3P04+ 148.2H20 Manganese dioxide reduction MnOz+C,~H~~~0,,~N,~P+472H3+l06C0~+8N~+H~P0~+236Mn2~ +366H20 Ioaizte reduction 70.7103- +C,0sH2630,,0N,6P+106COz+ 16NH,+H,PO,+ 70.71- + 106H20 92109- +C,0bH2630,,,,N,,#-+106C02+ 16HN0,+H3P04+921- + 122H20 92103- +C,,,sH2630,,0N,6P+ 80H + + 160C02 + 8N2 + H3P04 + 921- + 1 70H20

-3190

- 3030 -2750

- 3000

-2605 -2804 -3047

Aqueous AGf” values taken from Stumm and Morgan (198 1) and Atkins (1986)

cycling of carbon and iodine mediated by bacteria may very well account for the sub-surface

maxima of iodide. Tsunogai and Sase (1969) showed that the same bacteria and enzyme extract capable of reducing nitrate (N03-) to nitrite (NO*-), nitrate reductase, also reduced IOs- to I-.

A slightly denitrified water column (Naqvi, 1991; Mantoura et al., 1994) in and of itself does not explain the large concentrations of reduced iodine. However, calculations of the Gibbs free energy for the organic matter decomposition by various oxidants (after Froelich et al., 1979) show that the values calculated for the oxidation of organic matter by iodate are favorable at standard temperature and pressure and are comparable with the electron acceptors NOs- and MnOz (Table 2). It should be noted that the Gibbs value for the IOs- reduction depends on the nitrogen species formed as a product of organic matter decomposition. If N2 is the exclusive product 103- reduction is more favorable than N03- reduction.

Iodide concentrations measured in our laboratory are comparable with and in some instances far exceed the nitrite levels at the same depths from the same stations. In the northwest and north central parts of the basin, the dissolved nitrite concentrations in the OMZ are at least a factor of four to five (and generally one to two orders of magnitude) lower than in waters in the southeastern sector. We contend that a more accurate assessment of the importance of the various redox cycles can be made by consideration of the relative magnitude of the concentration of the reduced nitrogen intermediate nitrite (as an approximation of denitrification) to the concentration of iodide (measured in this study). It is clear from estimates of denitrification and measurements of nitrite in the Arabian Sea that reduction of nitrate may play a significant role in the oxidation of carbon in this region. Previous work has shown that the denitrification process is limited in geographical extent to waters north of 12-14”N latitude (e.g. Naqvi, 1987). The reigning assumption until recently has been that denitrification occurred throughout the entire Arabian Sea north of this boundary. Naqvi (1991) has clearly demonstrated that this assumption is incorrect. The highest rates of denitrification are observed in the eastern part of the basin, and/or near the Indian shelf, with lateral advection of nitrite toward the southwest. Compilation of all the nitrite maxima data available at the Indian National Oceanographic Data Centre (INODC)


clearly showed that in contrast to the southeastern part of the basin, the northwestern waters are characterized by comparatively low nitrite concentrations (Naqvi, 1991).

Nitrogen species measurements made during the NIOP (Van Koutrik et al., 1994) are consistent with the conclusions of Naqvi (1991). Selected nitrate and nitrite concentrations have been tabulated along with the iodide maxima data in Table 1. It should be noted that whereas the nitrate concentrations at Station 493 are 2&30 uM in the OMZ, the highest nitrite concentration is only 1.14 PM, at least a factor of 34 lower than published values for nitrite in the southeastern part of the basin (Naqvi, 1991). The sub-surface maximum iodide concentration at Station 411 is about 0.18 PM, whereas the nitrite concentration at this depth is only 0.025 PM. At Station 411 the iodide concentration is greater than nitrite by an order of magnitude. Neglecting for the moment any possible differences in microbial reduction rates for the two anions, the data would suggest that the iodine redox cycling in the OMZ of the northwest and north central parts of the basin is at least of the same order of magnitude as that for nitrate cycling, and may in some instances be considerably greater. The relative importance of iodine reduction in fact may be greater than the data suggest, as the observed nitrite may be the product of both denitrification and ammonia oxidation. Therefore the microbial reduction of iodate in the OMZ of the northwest Indian Ocean may play as significant a role as denitrification in the decomposition of organic matter.

Because non-sulfidic, low oxygen, non-stratified redox zonation is found in the Arabian Sea, perhaps only one microbial species, instead of a succession of microbial species, mediates the oxidation of organic matter. Nealson ef al. (1991) found that a strain of Shewenellaputrefaciens (MR-4) predominates in the oxic and anoxic zones of the Black Sea. They isolated MR-4 from Black Sea surface waters (upper 200 m) and found that it could reduce a full suite of oxidants 02, N03-, 103-, Fe(II1) and Mn(II1 and IV) minerals, and a variety of sulfur species. Perry et al. (1993) showed that MR-4 can affect sulfur speciation. Depending on local conditions, a single species may switch amongst a range of oxidants to derive energy from organic matter oxidation and IOs- may be a sufficiently abundant or efficient electron acceptor.

Iodide values of 180 nM or greater at the sub-surface maxima are attributed to the bacterial reduction of iodate (Table 2) as shown by the generalized equation

POC + IO; fs I-- + HCO, + DOC (1)

(IOs- is the preferential electron acceptor). High iodide values found at depths above and below these sub-surface maxima may be explained by decomposition of organic matter containing C-I bonds [POC(I)] as in the equation

POC(1) + 02 t, I + DOG(1)

MnOz NO,

(or other suitable electron acceptors)

(2)

where oxygen, nitrate, and manganese dioxide are potential oxidants of organic matter. As organic matter is oxidized, some I- is released into the water column. Luther (1991) showed that an increase in the specific iodine ratio in the Black Sea anoxic waters is due to remineralization of sinking POC. POC(1) not remineralized in the water column eventually will be buried in the sediments and oxidized there. Consistent with this idea, our porewater


TY-92-03-487 Piston Core

1403

0

250

500

6 2 750 : u”

1000

1250

1500 0 5 10 15 20 25 30 35

Iodide [uM]

Fig. 6. Porewaters were extracted from a piston core taken at station 487 in the Arabian Basin during leg TY-92-D3 and analyzed aboard ship (see also Passier et al., 1997).

I- concentration increases down core, showing the release of I- over depth in the sediments (Fig. 6).

RedJield ratios and denitrijkation. In this study, for every instance of a sub-surface [I-] maxima value, there is a corresponding low value (less than 12) for N/P (where N = mO,- + N02-1, P = [PO,+-]). (The data are presented, in Table 1, for the sub-surface iodide maxima depths.) This is seen most clearly at station TY-92-D3-481, where the N/P value is 9.5 at 600 m and 11.5 at 800 m and the iodide concentrations are 575 nM and 164 nM, respectively. In other studies of oxygenated non-sulfidic water columns at these depths, I- concentrations are generally found to be 10 nM or less (Elderfield and Truesdale, 1980; Jickells et al., 1988; Wong, 1991, and references therein).

The Redfield C:N:P ratio for phytoplankton biomass and in the water column is 106C16N:lP. This ratio has been used to describe the chemical composition of well- nourished phytoplankton and other microbes (Goldman et al., 1987). A low N/P value would suggest nitrogen utilization at depth (denitrification), or could indicate a source of organic biomass with an inherently low N/P value. Goldman et al. (1987) found that the C/N/P values tend to be much lower in bacterial biomass. Nitrogen-limited bacteria have


exceptionally low C/P values but still may maintain high growth efficiencies in viable populations (Goldman et al., 1987). High concentrations of I- coincident with nitrogen utilization at depth provide indirect evidence to support bacterial reduction of iodate. Naqvi et al. (1993) also inferred high microbial metabolic rates in association with sub-surface nitrite maxima found in the Northwest Indian Ocean.

Although bacterially catalyzed reduction of iodate can explain the presence of sub- surface iodide maxima, it does not explain the higher values of iodide seen throughout the OMZ of the Arabian Sea relative to other oceanic systems. Release of I- upon decomposition of C-I and N-I bonds in particulate organic matter, which is produced in the surface and then rains down the water column, is a possible explanation for the 25- 45 nM I- found in the OMZ and below. Figure 7 shows that I- release can occur on storage and correlates with NOs- utilization in the bottles (Farrenkopf, 1993).

Specific iodine: indicator qf’the remineralization oj’iodine

Specific iodine is the ratio of XI,, to salinity. Generally, in the open ocean, specific iodine is constant at depth, with a value of approximately 13.4. Specific iodine has been used to discuss uptake of iodine into particulate organic matter and remineralization of iodine into various water columns (e.g. Wong and Brewer, 1977; Smith and Butler, 1979; Wong et al., 1985; Jickells et al., 1988; Luther, 1991; Truesdale, 1994). All these researchers indicated that the incorporation of iodine into particulate matter occurs in surface waters. In the Black Sea, Luther (1991) used the specific iodine gradient in a vertical advection diffusion model to calculate the carbon flux based on the I/C ratio, which agreed with the sediment trap data of Karl and Knauer (1991). Unfortunately, we cannot perform similar calculations because of the non-linear nature of temperature and salinity over depth in the Arabian Sea. Specific iodine is not found to be conservative in the Arabian Sea, and non- conservative specific iodine can be related to the formation and decomposition of C-I bonds in oceanic particulate organic carbon (POC). Incorporation of iodine into POC produced in the surface waters is reflected in a low specific iodine value, typically 12.1-12.4. Subsequent release of iodine upon decomposition of the organic matter at depth is reflected in a higher specific iodine value deeper in the water column (greater than 13.4). Specific iodine is depleted in the Somali Basin and enriched in the Arabian Basin (Fig. 8).

Organic iodine: C-I and N-I bondformation and remineralization

General information about the association of iodine with organic matter in marine sediments is abundant in the geochemical literature (e.g. Vinogradov, 1939; Price et al., 1970; Pedersen and Price, 1980; Kennedy and Elderfield, 1987). However, Francois (1987) actually showed uptake and release of iodine by and from humic sedimentary substances in laboratory-controlled experiments.

Truesdale and Luther (1995) and Truesdale et al. (1995) have shown that I2 and its hydrolysis product HOI, which can be formed during I- oxidation or IOs- reduction, react readily with a variety of organic compounds representative of marine organic matter at seawater pH to form carbon iodine (C-I) and nitrogen iodine (N-I) bonds. Their data indicated that c+keto compounds and peptides are the main source of organic functional groups that incorporate iodine as C-I and N-I bonds. When POC(1) is formed (see equation (2)), organic carbon remineralization by bacteria can release I- to solution, which indicates


Iodide concentrations increase

1405

--t-300 m

-4 -795 m

-+ -1800m

during storage

://:-

/ /,-‘- -- 20

// ,L _-- -

_- 4; -- 10 ;& -Z -=/-

t I I I I I a I I I I I I 1 0

0 20 40 60 80 100 120 140

Fig. 7. Representative time course analyses for samples from three distinct depths. No clear depth- dependent increase is evident. Time is relative to the sampling date for station TY-92-D1-411, 19 September 1992. The four points from 795 and 1800 m depth during the first 50 days are not statistically different from one another, indicating that the samples were apparently stable during the initial storage period. Samples were 0.45 pm filtered and stored in Nalgene bottles at 4°C in the dark. Before analyses the samples were equilibrated to ambient temperature (21-25°C). Samples analyzed between 80 and 140 days after collection increased in iodide concentration. Nitrate and nitrite analyses also were performed at this time (140-170 days after collection), and showed a marked

decrease in nitrate and increase in nitrite. Bacteria counts are not available for these samples.

that changes in iodine speciation provides a direct carbon iodine link. Our porewater data (Fig. 6) and Luther et al. (1995) show that I- is released over depth as organic matter is decomposed. This finding is consistent with the data of Kennedy and Elderfield (1987) for pelagic deep-sea sediments. Preferential release of I- from Corg also has been reported in porewaters by Harvey (1980) and Pedersen and Price (1980).

CONCLUSION

Unusually high levels (greater than 10 nM) of dissolved I- below the upper mixed layer of the Northwest Indian Ocean were measured and compared with other oceanic systems [i.e. the North Atlantic (Jickells et al., 1988) and the Pacific (Nakayoma et al., 1989; Campos et al., 1997)]. In addition to higher iodide concentrations in the OMZ and at depth in the Northwest Indian Ocean, sub-surface maxima (greater than 180 nM) were found. These sub-surface iodide values do not correspond to traditional redox considerations, as oxygen is detectable and total sulfide is low (less than 2 nM; Theberge et al., 1997). In addition, iodide values cannot be explained by advection along density surfaces (Fig. 4).

The water column of the Northwest Indian Ocean is subject to seasonally high rates of productivity and enormous fluxes of both dissolved and particulate organic material that contain iodine. To explain high I- levels, we hypothesize that iodine is incorporated into C-I and N-I bonds in particulate organic matter and subsequently released as iodide during remineralization processes in the water column of the Arabian Sea. This rationale has been

1406 A. M. Farrenkopf et al. -

Specific Iodine vs. Depth

4 Panama Basin

J, North Atlantic

I

p Somali Basin

mo

2ooo

-- 2 e

3oLw

Xooo

Specific Iodine vs. Depth

! Arabian Basin

d Arabian Basin

r Panama Basin

I I

X North Atlantic

10 11 12 13 14 15 16

Fig. 8. Specific iodine vs depth for the Somali Basin (left) and Arabian Basin (right). These two basins are referenced to North Atlantic and Panama Basin data, which show little vertical structure for specific iodine values, in distinct contrast to the Somali Basin data, which show overlap and some depletion of total iodine relative to the North Atlantic, and the Arabian Basin data, which show marked enrichment of total iodine throughout most of the deep water column. Vertical structure and variability in the specific iodine values at depth have been reported previously by Wong and Brewer (1977) for the Venezuela Basin, Luther (1991) for the Black Sea, and Luther ef al. (1991) for the Chesapeake Bay. They are not plotted as a reference comparison here, as the resolution of measurements in the Chesapeake Bay is on the order of 30 m and in the Black Sea is on the order of

3OOm.

used extensively in the geochemical literature for increases in porewater iodide concentrations and for the exponential decrease of I/C,,, values in sediments (e.g. Pedersen and Price, 1980; Francois, 1987; Shimmield and Pedersen, 1990) and in the Black Sea water column (Luther, 1991). We suggest, therefore, that decomposition processes normally seen through iodide profiles in porewaters and anoxic water columns actually occur in the water column of the Arabian Sea, but to a lesser extent than in


porewaters (Kennedy and Elderfield, 1987) or in waters of anoxic basins such as the Black Sea (Luther and Campbell, 1991).

Carbon-iodine and nitrogen-iodine bond decomposition alone cannot, however, account for all the changes we see with iodine speciation. Bacterial reduction of 103- in this system accounts for the multiple sub-surface iodide maxima of greater than 180 nM, as 103- oxidation of organic matter is thermodynamically favorable and comparable with that by N03- (Table 2). We have confirmed bacterial reduction of iodate (103-) using pure cultures of Shewunellaputrefaciens in subsequent studies (Farrenkopf et al., 1997).

Acknowledgements-This research was carried out as part of the Netherlands Indian Ocean Programme 1992-1993, undertaken in co-operation with Kenya, Pakistan and the Seychelles. This program was organized and supported by the Netherlands Marine Research Foundation (SOZ) of the Netherlands Organization of Scientific Research (NWO). The chief scientists for our cruise legs TY92-Dl and TY92-D3 are C. H. van der Weijden and W. van der Linden of the Faculty of Earth Sciences at Utrecht University (UU). Additional support for the project was provided by grants from the US National Science Foundation (NSF) to G. W. Luther, III (OCE-9217245 and OCE- 9310792). Sincere appreciation is expressed to the officers and crew of the R.V. Tyro, to the NIOZ personnel for technical assistance, to Annette van Koutrik for nutrient data, and to EG&G PARC for the loan of the 384-B polarographic analyzer

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