Redox cycling of iron and manganese in sediments of the Kalix River estuary, Northern Sweden

Aquatic Geochemistry 2: 185-201, 1996. 185 ~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.

Redox Cycling of Iron and Manganese in Sediments of the Kalix River Estuary, Northern Sweden

ANDERS WIDERLUND and JOHAN I N G R I Division of Applied Geology, Luled University of Technology, S-971 87 Luled, Sweden

(Received: 1 March 1996; in final form: 31 October 1996)

Abstract. Iron and manganese redox cycling in the sediment - water interface region in the Kalix River estuary was investigated by using sediment trap data, pore-water and solid-phase sediment data. Nondetrital phases (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn (51% of Feto~a~ and 84% of Mntotal). A steady-state box model reveals that nondetrital Fe and Mn differ considerably in reactivity during post-depositional redox cycling in the sediment. The production rate of dissolved Mn (1.6 mmol m -~ d -I ) exceeded the depositional flux of nondetrital Mn (0.27 mmol m -2 d-t) by a factor of about 6. In contrast, the production rate of upwardly diffusing pore-water Fe (0.77 mmol m -2 d -1) amounted to only 22% of the depositional flux ofnondetrital Fe (3.5 mmol m -2 d-l). Upwardly diffusing pore-water Fe and Mn are effectively oxidized and trapped in the oxic surface layer of the sediment, resulting in negligible benthic efftuxes of Fe and Mn. Consequently, the concentrations of nondetrital Fe and Mn in permanently deposited, anoxic sediment are similar to those in the settling material. Reactive Fe oxides appear to form a substantial fraction of this buried, non-detrital Fe. The in-situ oxidation rates of Fe and Mn are tentatively estimated to be 0.51 and 0.16-1.7 #mol cm -3 d -i, respectively.

Key words: redox cycling, oxidation rate, iron, manganese, box model, Kalix River estuary.

1. Introduction

Post-deposi t ional redox cycling o f Fe and Mn in marine sediments is a wel l -known phenomenon , often resulting in surficial m ax i ma o f Fe and Mn concentrat ions (e.g., Burdige and Gieskes, 1983; Shaw et al., 1990; Burdige, 1993). Depending on the redox conditions at the sediment-water interface, upwardly diffusing dissolved Mn 2+ and Fe 2+ m a y be released f rom the sediment into the overlying bot tom

water (Sundby and Silverberg, 1985; Sundby et al., 1986; Sholkovitz et al., 1992). This benthic efflux f rom coastal sediments has been suggested to be important for the supply o f Mn and poss ib ly Fe to the open ocean (Evans et al., 1977; Yeats et al., 1979; Sundby et al., 1981; Kremling, 1983; Heggie et al., 1987; Landing and Bruland, 1987).

Compared to m a n y other sea areas, the Gu l f o f Bothnia in the northern Baltic Sea is unusual ly rich in Mn - Fe concretions (Bostr6m et al., 1978; Bos t r6m et al., 1982; Ingri, 1985; Ingri and Pont6r, 1986). It m a y be that, similar to in the oceans, benthic efflux o f Mn and Fe f rom near-shore sediments fol lowed by lateral t ransport to open sea areas could account for the accumulat ion o f Mn and Fe in the offshore G u l f o f Bothnia sediments.

186 ANDERS WIDERLUND AND JOHAN INGRI

A balance exists between situations where virtually all upwardly diffusing My12+

and Fe 2+ are trapped within the oxic surface sediment, and those where Mn 2+ and Fe 2+ are lost to the water column (Sundby et al., 1986; Aller, 1994; Thamdrup et al., 1994). This balance to a large extent is controlled by the flux of organic carbon to the sediment, which influences the availability of oxygen and the thickness of the oxic surface layer. In addition, the upward fluxes of Mn 2+ and Fe 2+ into the oxic zone, adsorption of Mn 2+ / Fe 2+ onto oxide surfaces (Emerson et al., 1982; Canfield et aL, 1993), temperature (Hunt, 1983) and the kinetics of Mn 2+ and Fe 2+ oxidation are critical parameters. Microbial catalysis of Mn oxidation appears to be important in the water column of anoxic Oords (Emerson et at., 1982) as well as in sediments (Kepkay, 1985). In the absence of catalyzing bacteria, however, Mn 2+ is oxidized much more slowly than Fe 2+ (e.g., Davison, 1993). Thus, the retention of upwardly diffusing Mn 2+ in the oxic surface layer may be expected to be less efficient than for Fe 2+. By measuring the in-situ Mn oxidation capacity of a coastal sediment, Thamdrup et al. (1994) found that abiotic Mn oxidation could not explain the efficient removal of Mn 2+ in the oxic surface layer. Microbially catalyzed oxidation of Mn was suggested to be responsible for oxidation rates orders of magnitude higher than those predicted by abiotic kinetics.

Quantitative in-situ Mn oxidation rate data for sediments could help elucidate the nature of Mn oxidation processes, but such data are scarce. Thus, the purpose of this study was (1) to estimate the in-situ Mn oxidation capacity of the Kalix River estuary sediment, and (2) to assess the role of near-shore sediments in the formation of Mn-Fe concretions in the open Gulf of Bothnia. [V[_n 2+ and Fe 2+ removal from pore-water will be referred to as 'oxidation', although other binding mechanisms (adsorption) may also be involved. We used sediment trap data, pore-water and solid-phase sediment data to quantify fluxes of Fe and Mn in the sediment-water interface region. A flux model similar to that applied by Sundby and Silverberg (1985) in the Laurentian Trough is used to describe the post-depositional redox cycling of Fe and Mn.

2. Study Area and Core Description

The low-salinity (<3%o) Kalix River estuary is situated in the Gulf of Bothnia, northern Sweden (Figure 1). Due to weathering and podzolization of till within the Kalix River drainage area, river suspended matter entering the estuary is rich in non-detrital Fe and Mn [presumably Fe-Mn oxides (Pont6r et al., 1992; Ingri and Widerlund, 1994)].

The sediments of the Kalix River estuary offer unique conditions for the study of early diagenetic redox transformations of Fe and Mn. At the coring site (Figure 1, water depth 13 m), varved (annually laminated), silty sediments are deposited at a rate of 0.7-1 cm yr-l(Widerlund and Roos, 1994). The welt preserved varves indicate that the sediment is virtually undisturbed by bioturbation/resuspension. Considering that the bottom water is well oxygenated throughout the year (02

REDOX CYCLING OF IRON AND MANGANESE IN THE KALIX RIVER ESTUARY 187

Figure 1. Map of the Kalix River estuary showing the location of the coring site.

saturation >80%), the absence of bioturbation is unexpected. However, the scarcity of burrowing organisms is probably a consequence of the low primary production in the northern Gulf of Bothnia [10-30 g C m -2 yr -1 (Elmgren, 1984)].

The sediment redox conditions are described in Widerlund and Ingri (1995), and are only briefly summarized here. Based on the distribution of pore-water Fe, it is estimated that a ca. 2 cm thick oxic surface layer was present in both cores collected. Within this layer, 1-2 mm thick layers of diagenetically precipitated Mn and Fe oxides occurred. Below the oxic surface layer, anoxic conditions are indicated by sulfate reduction and Fe-sulfide formation, despite the fact that Fe oxides are clearly visible below the redox boundary (Widerlund and Roos, 1994).

3. Sample Collection and Preparation

Cores with apparently undisturbed core tops were collected in September 1991 and April 1992 with a modified Kajak gravity corer (Blomqvist and Abrahams- son, 1985). A core was also collected by using a core-freezing technique, which involves the sediment being frozen in-situ with solid CO2 (Renberg, 1981). With this technique, virtually undisturbed cores were obtained, and these enabled the sub-sampling of individual sediment layers on a mm-scale.

Extraction and filtering (0.4 #m polycarbonate filters) of pore-water from the gravity cores was performed under oxygen-free conditions by using a modified Reeburgh sampler (Reeburgh, 1967). The details of core collection and sample processing are given in Widerlund and Ingri (1995).

Bottom water samples were collected 1 m above the bottom by using a peristaltic pump. The samples were filtered in-line in the field through 0.45 #m Sartorius membrane filters, acidified with suprapur HNO3 to pH 1.5 and stored refrigerated


until analysis. Filtering equipment and polyethylene sample bottles were acid leached before use.

Sediment traps consisting of paired cylinders [height 500 mm, aspect ratio 4.8 (Larsson et al., 1986)] were deployed 3 m above the bottom at the coring site on six occasions, spread over a period of two years. The traps were deployed for 10 to 53 days without the use of preservatives or poisons in the collection tubes (Gundersen and Wassmann, 1990). The trap samples were retrieved using a procedure described by Blomqvist and Larsson (1994). Trap flux calculations are based on the mean of the duplicate trap samples -4- 1 S.D.

4. Analytical Methods

For the determination of solid-phase total Fe, Mn, A1 and P, 0.125 g of sediment was fused with LiBO2 at 1000 °C (Burman et al., 1978). The metaborate bead thus formed was dissolved in 0.7 M HNO3. After filtering, the solution was analysed by atomic emission spectroscopy (AES) with an inductively coupled plasma (ICP) as excitation source (ICP-AES, models ARL 3560 and 3580). The instrumental precision (~ 1 R. S.D.) was around + 1%. All solid-phase concentrations are reported on a dry weight basis. Sediment trap data are salt-corrected according to Blomqvist and Larsson (1994).

Pore-water concentrations of total Fe and Mn were determined by ICP-AES in 2 ml aliquots that were diluted to 10 ml. Pore-water Mn concentrations below 0.2 #M were determined with ICP mass spectrometry (ICP-MS, models VG Plasma Quad 2 Plus and 2 Turbo-Plus) in undiluted samples.

Dissolved Fe (operationally defined as <0.45 #m) in bottom water samples was determined by ICP-AES without pre-concentration. Dissolved bottom water Mn was determined by ICP-MS in samples that were pre-concentrated by an ion-exchange procedure (a modified version of Sturgeon et al., 1981). The ion- exchange recovery (87-99%) was determined with spiked samples and corrected for. The instrumental precision (+ 1 R.S.D.) for dissolved Fe and Mn was generally better than +5%.

REDOX CYCLING OF IRON AND MANGANESE IN THE KALIX RIVER ESTUARY

Table I. Sediment trap concentrations and trap flux (4-1 S.D.) of Fe and Mn.

189

Concentration a Flux b

Fe Mn Fe/A1 Mn/A1 Fe Mn (mmol kg -1) (mol) (mmol m-Zd -1 )

Detrital 680 11 3.4 0.055

Non-detrital 700 54 3.5 0.27

Total 1380 65 0.61 0.028 6.9-4- 0.6 0.32 + 0.06

a Annual average concentration. b Obtained by multiplying trap concentration and trap flux of sediment [5.0 × 10 -3 ± 0.4 × 10 -3 kg m -2 d -1 (Widerlund and Roos, 1994)].

5. Results

5.1. SEDIMENT TRAP DATA

The six trap deployments included the spring flood and winter periods, and were used to calculate the annual average deposition rates of particulate Fe and Mn (Table I). The reliability of quantitative sediment trap data may be severely affected by seasonal variations of fluxes and trapping efficiency. The validity of the trap flux measurement was therefore checked against the sediment accumulation rate in the underlying bottom sediment. The reasonably good agreement between the trap flux of sediment (5.0x 10 -3 4- 0.4 × 10 -3 kg m - 2 d - 1 ) and the sediment accumulation rate obtained by 137Cs dating and varve-counting in the freeze-core (4.4 × 10 -3 + 0.3 × 10 -3 kg m -2 d -1) suggests that the traps provided a relatively good estimate of the true depositional flux (Widerlund and Roos, 1994).

Assuming that A1 is entirely contained in detrital material, here represented by local till, the amount of detrital (Med) and non-detrital (Mend) Fe and Mn in sediment trap material can be calculated using the equations:

Med = 0Vle/A1)till × Alsed, Mend = M e t o t - M e d

The Fe/A1 and Mn/A1 mole ratios for local till [0.30 and 0.0046, respectively (Ohlander et aL, 1991)] are similar to the ratios for average continental rock [0.25 and 0.0051, respectively (Martin and Whitfield, 1983)]. Due to the presence of nondetrital, reactive Fe and Mn oxides formed during weathering and podzolization of till, the Fe/A1 and Mn/A1 ratios for sediment trap material greatly exceed those for local till (Table I). These oxides form substantial fractions of the total settling particulate flux of Fe and Mn (51% and 84%, respectively, Table I).

5.2. SOLID-PHASE SEDIMENT AND PORE-WATER DATA

Solid-phase sediment and pore-water data for the gravity cores are presented in Table II and Figure 2. In the anoxic zone, nondetrital Fe (calculated as in Section 5.1)


Table II. Solid-phase sediment and pore-water data for the two cores investigated.

September core April core Solid phase Pore-water Solid phase Pore-water

Depth Fe Mn Fe Mn Fe Mn Fe Mn (era) (mmol kg- 1) (#M) (retool kg- 1) (#M)

Bo~omwater 0.215 0.130 0.824 0-1 1468 257 0.519 0.055 1468 353 0.949 1-2 1916 315 10.1 107 1934 189 7.41 2-3 1594 49 146 186 1522 55 360 3-4 1289 40 372 186 1253 50 417 4-5 1647 59 444 195 1397 61 466 5-6 1343 46 355 207 1361 61 458 6-8 1647 70 426 257 1450 70 501 8--10 1486 59 433 297 1647 82 419

10-12 1504 65 288 288 1594 85 435 12-16 1576 73 360 353 1558 70 423 16-20 1629 71 285 355 1665 65 469 20-24 1450 56 399 382 1647 62 500 24-28 1540 62 394 359 1540 58 528 28-32 1432 58 403 368 1522 55 559 32-36 1432 48 392 333 1629 55 530

0.104 0.146

71.5 262 322 369 371 402 410 428 435 404 393 39t 380 351

The average solid-phase concentrations 4-1 S.D. (mmol kg -1) in the anoxic September core; Fe = 1505 -4- 114, Mn = 58 + 10. April core; Fe = 1522 -4- 126, Mn = 64 4- 11.

zone (2-36 cm) are:

forms 4 0 - 5 8 % o f total Fe, while one or several unknown, non-detrital Mn phases fo rm 7 1 - 8 8 % o f total Mn (Figure 2). Thus, the concentrations o f permanent ly buried, non-detri tal Fe and Mn are similar to those in the settling material.

The solid-phase and pore-water profiles indicate that the distribution o f Fe and Mn is controlled by redox cycling close to the sed iment -wate r interface. Upward ly diffusing pore-water Fe is oxidized and precipitated immedia te ly be low or within the flocculent surface layer o f the sediment, forming a ca 1.5 m m thick Fe oxide layer ( layer 3, Figure 3). Immedia te ly above the Fe-rich layer, precipitation o f upward ly diffusing pore-water Mn occurs in layers 1 and 2 (Figure 3). This recycled M n is here referred to as excess Mn, defined as Mn concentrations exceeding the average total concentrat ion in the anoxic zone (cf. Aller, 1994). The excess Mn inventory in layers 1 and 2 amounted to 510 m m o l m -2, o f which 75% occurred in the well defined, 2 m m thick layer 2. Apparently, the zone o f m a x i m u m potential for M n oxidation is situated 8 -10 m m below the s e d i m e n t - water interface (Figure

3). In the anoxic zone, Fe oxides [tentatively calculated as Fetot--Fedetrital--Fesulfide

(Widerlund and Ingri, 1995)] form 6 2 - 9 8 % o f nondetrital Fe, while sulfide-bound

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ANDERS WIDERLUND AND JOHAN INGRI

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Fe is subordinate (Figure 2). This is in agreement with investigations showing that Fe oxides may persist in anoxic sediments, probably due to kinetic control of iron transformations (Ingri and Pont6r, 1986; Canfield, 1989; Wersin et al., 1991). Iron (II) minerals like vivianite (Fe3(PO4)2 8 H 2 0 ) and siderite (FeCO3) may be expected to form during early diagenesis in low-salinity environments. However, P concentrations <80 mmol kg- t in the anoxic zone imply that Fe bound in vivianite cannot exceed 120 mmol kg-J. The possible occurrence of siderite is limited to very low concentrations by the negligible concentration of inorganic C in the sediment (Widerlund, 1996).

In the anoxic zone, total organic C forms 3.2-5.2 wt% of the sediment (Wider- lund, 1996), and enough reactive organic C obviously is present to drive sulfate reduction. Therefore, the preservation of Fe oxides in the anoxic zone cannot be due to a lack of reactive organic C. Despite active sulfate reduction and sulfide formation, dissolved pore-water sulfide was not detected by smell (sensitive to a few #M) until at depths >40 cm. It has been proposed that the Fe-sulfide forming reaction between dissolved sulfide and reactive Fe oxides is rapid, and buffers the concentration of dissolved sulfide to low levels (Canfield, 1989). The apparent absence of dissolved sulfide thus further supports the presence of reactive Fe oxides in the anoxic zone.


Table III. Flux parameters, diffusive flux of Fe and Mn and precipitation rate of excess Mn.

Fe Mn

JOZT JBF JOZF JBF Jxs

Sept. core A C / A Z (mmol m -4) 18. 103 61 8.8 • 103 --15

D8 (m 2 d - I ) 3.5. 10 -5 3.5 • 10 -5 3.2 • 10 -5 3.2 - 10 -5

Flux (mmol m -2 d -1) 0.60 0.0020 0.27 -0.0005

April core A C / A Z (mmol m -4) 35 • 103 25 15 • 103 8.4

Ds (m 2 d -1) 2.8 • 10 -5 2.8 • 10 -5 2.5 • 10 -5 2.5 - 10 -5

Flux (mmol m -z d -1) 0.93 0.0007 0.36 0.0002

Annual average 0.77 a 0.0014 a 0.32 a ca. 0 a

flUX (mmol m -2 d - l )

1.6

Positive fluxes are directed upward. Jozv = diffusive oxidation zone flux, i.e. flux reaching the precipitation zone of Fe and Mn. JBF = benthic flux across the sediment-water interface. Jxs = precipitation rate of solid-phase excess Mn. a Average of Sept. and April cores. D~ from Li and Gregory (1974) calculated for temperatures of 8 °C (Sept. core) and 1 °C (April core) and corrected for tormosity (O = 0.95) according to Ullman and Aller (1982).

Pore-water Mn continues to increase down to a depth of 10-15 cm in the anoxic zone (Figure 2), suggesting that a dissolving Mn phase is present also below the oxic surface layer. Non-detrital Mn and Fe are positively correlated in this zone (Widerlund and Ingri, 1995), an indication that this Mn phase may consist of slowly reducing, residual Mn oxides.

6. Discussion

6.1. DIFFUSIONAL FLUX OF IRON AND MANGANESE

In the absence of bioturbation/resuspension, molecular diffusive fluxes calculated from pore-water data have been found to provide reasonably good estimates of the actual fluxes of dissolved species in sediments (Elderfield et al., 1981; Devol, 1987). The vertical diffusion of dissolved Fe and Mn was estimated by using Fick's first law:

J = - - (~D~AC/AZ,

where J is the diffusional flux (mmol m-2d-1), ~ is the sediment porosity, Ds is the molecular sediment diffusion coefficient and A C / A Z is the linear pore-water concentration gradient (Table III).

The diffusional flux was divided into oxidation zone flux (JozF) reaching the base of the surficial Fe and Mn oxide layers (Figure 3) and benthic flux (JBF)


Table IV. Comparison of pore-water fluxes and sediment trap fluxes of nondetrital Fe and Mn.

Fe Mn

JOZF/.~nd JBF/JOZF JOZF/Gd JBF/JOZF Jxs/Gd

September core 0.17 0.0033 1.00 -0 .0019

April core 0.27 0.0008 1.33 0.0006

Annual average 0.22 a 0,0018 a 1.19 ~ c. 0 a 5.9

/Pnd "~ sediment trap flux of nondetrital phases. JOZF = diffusive oxidation zone flux. JBF = diffusive benthic flux. Jxs = production rate of dissolved Mn calculated from solid-phase data. Flux values from Table I (Fnd) and Table III (JozF, JB~, Jxs). a Average of September and April cores.

across the sediment-water interface (Table III). The latter was calculated for the concentration gradient from the mid-point of the 0-1 cm section of the cores to the interface (AZ = 0.5 cm). Here, the Fe and Mn concentrations are taken to be the same as in the bottom water samples (Table II). The estimated oxidation zone fluxes of dissolved Mn are similar to diffusive fluxes reported for other near-shore sediments (e.g., Elderfield et al., 1981; Paulson et al., 1988; Aller, 1994; Thamdrup et al., 1994). The low dBV/Jozv ratios in Table IV indicate that close to 100% of the oxidation zone flux of Fe and Mn is trapped within the oxic surface layer.

Although resuspension appears to be absent at the coring site, slight physical disturbances of the sediment-water interface cannot be entirely excluded due to the shallow water depth. The effective pore-water flux across the sediment-water interface (JBF) may therefore exceed that for molecular diffusion alone (cf. Harrison et al., 1983). However, assuming that the effective benthic efflux of Fe and Mn exceeds the fluxes (JBF) given in Table III by one order of magnitude, the resulting diffusive efflux would still be <2% of the oxidation zone flux (JOZF).

6.2. RECYCLING AND PRECIPITATION OF PORE-WATER IRON AND MANGANESE

In the absence of bioturbation, the fine-scale distribution of solid-phase Fe and Mn in individual sediment layers can be used to study the precipitation of pore-water Fe and Mn (cf. Sundby and Silverberg, 1985; Aller, 1994). The layering of the varved sediment clearly defines the annual sediment deposition (Widerlund and Roos, 1994), and provides a reliable time scale for the calculation of deposition rates (Figure 3). Deposition values of Fe and Mn for individual sediment layers were calculated by weighing and analysing freeze-core samples of individual layers representing a known area of the bottom sediment (cf. Renberg, 1981).

The Fe-rich layers occurring in the anoxic zone are former flocculent surface layers that have become buried. Compared with the flocculent surface layer, the Fe concentration usually is 20-70% higher in these buried surface layers (Widerlund


and Roos, 1994). Precipitation of Fe oxides occurs immediately below or within the flocculent surface layer (layer 3, Figure 3). Complete trapping of upwardly diffusing Fe is suggested by the fact that addition of the oxidation zone flux of Fe to the flocculent surface layer would increase the Fe concentration of this layer by approximately 35% (i.e. within the observed 20-70% increase).

Around 50% of the annual sediment deposition occurs during the spring flood in May (Widerlund and Roos, 1994). This rapidly deposited material forms a conspicuous, mainly minerogenic layer in the sediment (layers 4 and 6, Figure 3), which serves as an excellent time marker. For example, in the freeze-core collected on April 5, 1991, layers 1 and 2 are found to represent a time period of ca. 310 days (June 1990 to April 5, 1991) using the 1990 spring flood layer 4 as a marker. A minimum estimate of the average precipitation rate of excess Mn (Jxs, Table III) of 1.6 mmol m -2 d-1 is obtained by dividing the excess Mn inventory in layers 1 and 2 (510 mmol m -z) with the time period available for precipitation (310 days).

It can be noted that the precipitation rate of excess Mn based on solid-phase data (J×s) exceeds the pore-water oxidation zone flux (JozF) by a factor of 5 (Table III). Moreover, the oxidation zone flux cannot explain the excess Mn inventory in the oxic surface layer. Aller (1994) similarly found that Mn flux estimates based on solid-phase data tended to greatly exceed those based on pore-water data, presumably partly due to bioturbation. In the Kalix River estuary, where bioturbation is negligible, a possible explanation for this discrepancy may be that freshly precipitated Mn oxides in layer 2 are very reactive and hence are rapidly recycled within the uppermost cm of the sediment. This Mn cycling may not have been detected in the pore-water due to (1) our relatively coarse pore-water sampling (1 cm sections were used close to the sediment-water interface, Table II) or due to (2), the cycling may be more or less complete already in the September core. Rapid Mn cycling is indicated by data from other cores, where a somewhat irregular Mn oxide layer (layer 2, Figure 3) was present in a position above the May spring flood layer already in early August.

The benthic flux calculation indicates that virtually no dissolved Mn escapes from the sediment (Table III). We therefore suggest that the the most reliable measure of the production rate of dissolved Mn is obtained by measuring the actual precipitation rate of solid-phase excess Mn (Jxs).

6.3. FLUX MODEL FOR IRON AND MANGANESE

To describe the fluxes of Fe and Mn close to the sediment-water interface, we have adopted a box model similar to that used by Sundby and Silverberg (1985) in the Laurentian Trough. Our model contains four reservoirs, three in the sediment and one representing the water column (Figure 4).

In the sediment, the zone of precipitation corresponds to the zone above the redox boundary, where Fe and Mn oxides are formed. Below the redox boundary, the zone of dissolution represents the zone where the oxide phases are reduced

196

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F3_4 7.5 a-7.6 b>

ANDERS WIDERLUND AND JOHAN INGRI

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OXIC ANOXIC

FI-2 F2- I l-- 0.32+0.06 0.00

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F2_3 F3. ~ 1.9- 1.6 2.0

ZONE OF DISSOLUTION 1.6

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F3-4 0.29 a)-0.32 b>

PERMANENT BURIAL OF Fe

PERMANENT BURIAL OF Mn

Figure 4. Steady-state box model showing reservoirs and fluxes (mmol m -2 d -1) of Fe and Mn in the sediment-water interface region, a) and b) represent permanent burial rates in the September and April cores, respectively.

and dissolved. In the fourth reservoir, Fe and Mn are permanently buried in the sediment.

At steady-state, the net flux of Fe and Mn into or out of a reservoir corresponds to the precipitation or dissolution rate in the reservoir. The fluxes between the reservoirs were calculated according to the following (see Figure 4). The settling flux to the sediment (F1-2) is assumed to correspond to the sediment trap flux (Table I). F2-1, the flux of dissolved Fe/Mn across the sediment-water interface is set equal to the annual average diffusive benthic flux (JBF, Table III). The rate of permanent burial (F3-4) was obtained by multiplying the trap flux of sediment (Table I) with the average concentration of Fe and Mn in the anoxic zone of the sediment (Table II). The production rate of dissolved Fe/Mn (F3-2) was calculated


in two different ways. For Fe, we used the annual average diffusive oxidation zone flux (JozF, Table III) as a measure of the production rate. For Mn, on the other hand, the precipitation rate of excess Mn (Jxs, Table III) was used as a measure of the production rate of dissolved Mn (see discussion in Section 6.2).

At steady-state, the flux of solid-phase Fe and Mn into the dissolution zone (F2-3) is equal to the sum of the permanent burial rate (F3-4) and the production rate of dissolved Fe and Mn (F3-2). The burial rate of pore-water Fe and Mn is small and can be neglected. Consequently, virtually all dissolved Fe and Mn that is produced in the dissolution zone is returned to the precipitation zone (F3-2). Here, the production rate of particulate Fe/Mn is equal to the production rate of dissolved Fe/Mn (F3-2) minus the benthic flux of Fe/Mn leaving this zone (F2-i).

Considering the uncertainty in the settling flux estimate (F1-2) the Mn model in Figure 4 is balanced, i.e. the rate of permanent burial equals the net flux across the sediment-water interface (F3-4 = F1-2 + F2-1). For Fe, the rate of permanent burial is in reasonably good agreement with the net flux across the sediment- water interface (/7'3-4 ~ /7'1-2 %- /V2-1). The model thus demonstrates that post- depositional redox cycling of Fe and Mn is almost entirely an internal process in the sediment. The cycling of Mn is intense, with a production rate of dissolved Mn exceeding the flux of nondetrital, reactive Mn to the sediment (Fnd) by a factor of about 6 (Table IV). In contrast, a much lower reactivity of nondetrital Fe is indicated by the fact that the diffusive oxidation zone flux of Fe amounts only to 22% of the settling flux of nondetrital Fe (Table IV).

6.4. IRON AND MANGANESE OXIDATION IN THE SEDIMENT

The efficient trapping of pore-water Fe and Mn most probably results from oxidation of Fe2+/Mn 2+ in the surface layer. Adsorption of FeZ+/Mn 2+ may be an initial step in this process (Emerson et al., 1982; Canfield et al., 1993). Apparently, the zone of maximum potential for Mn oxidation (layer 2, Figure 3) is situated 8-10 mm below the sediment-water interface. Kepkay (1985) and Thamdrup et al. (1994) similarly found that the Mn oxidation rate was highest slightly below the sediment surface.

A tentative estimate of the in-situ oxidation rates of Fe and Mn can be obtained, provided that the following assumptions are valid (cf. Thamdrup et al., 1994): (1) The benthic fluxes of Fe and Mn can be neglected, i. e. we assume complete trapping of upwardly diffusing pore-water Fe and Mn within the oxic surface layer (JBF<<JozF in Table IV). (2) The oxidation zone of Fe is restricted to the 1.5 mm thick Fe oxide layer in the oxic surface sediment (layer 3 in Figure 3). (3) The oxidation zone of Mn is restricted to the 10 mm thick zone containing layers 1 and 2 (Figure 3).

Areal oxidation rates (mmol m -2 d -1) are defined as the flux of dissolved Fe z+ and Mn 2+ entering the oxidation zones (Table V). We used the annual average diffusive oxidation zone flux of Fe (Table III) as a measure of Fe z+ reaching

198 ANDERS WlDERLUND AND JOHAN INGRI

Table V. Estimated rates of Fe and Mn oxidation.

Areal oxidation Specific oxidation

rate rate (mmol m -2 d - J ) (#mol cm -3 d - l )

Fe 0.77 0.51

Mnmin a 1.6 0.16 Mrlmax b 3.3 1.7

a Average minimum rate for the Mn oxidation zone (layers l and 2, Figure 3) and the time period June 1990-April 5, 1991. b Maximum rate needed for the formation of layer 2 before collection of the September core (Sept. 27).

layer 3. The flux of dissolved Mn 2+ reaching layers 1 and 2 was set equal to the precipitation rate of excess Mn in layers 1 and 2 (Table III).

Specific oxidation rates (#mol c m - 3 d -1) are obtained by dividing the areal oxidation rate by the thickness of the oxidation zone (Table V). The specific oxidation rate of Mn calculated from freeze-core data is dependent on the time needed for the formation of the Mn oxide layer (layer 2, Figure 3). The minimum rate in Table V is the average rate for the period June-April 5 (date of freeze-core collection) and an oxidation zone thickness of 10 mm (layers l and 2, Figure 3), while the maximum rate is an estimate of the oxidation rate needed if the Mn oxide layer was formed before collection of the September core as was discussed in Section 6.2 (the oxidation zone thickness is here assumed to be 2 ram, i.e. layer 2 in Figure 3).

In the oxic zone of a coastal sediment, Thamdrup et al. (1994) found an average specific Mn oxidation rate of 0.17-0.81 #mol cm -3 d -1 (maximum rate ca. 2.0 #mol cm -3 d- l ) . Apparently, the specific Mn oxidation rate in the Kalix River estuary is similar (Table V). These Mn oxidation rates are relatively high, and Thamdrup et al. (1994) showed that such rates could not be explained by abiotic Mn oxidation, even if catalysis by Mn / Fe oxide surfaces (Sung and Morgan, 1981) was taken into account. Microbial Mn oxidation was suggested as the most likely explanation for the high Mn oxidation capacity of the sediment. This may be the case also in the Kalix River estuary.

7. Conclusions

This study demonstrates that the sediments of the Kalix River estuary are an efficient trap for settling particulate Fe and Mn, although non-detrital Fe and Mn (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn. Only a minor fraction of the settling Fe oxides appears


to be involved in redox cycling in the sediment, resulting in burial o f Fe oxides in the anoxic zone.

The in-situ oxidation rates o f Mn (0.16-1.7 #tool cm -3 d -1) and Fe (0.51 # tool cm -3 d -1) are o f similar magnitude. The Mn oxidation rate approximates that measured by Thamdrup et al. (1994), who proposed that microbially catalysed Mn oxidation was necessary to explain an efficient removal o f Mn 2+ in surface sediments.

I f the cores investigated in this study are representative o f coastal sediments in the Gu l fo fBo thn ia , diffusion o f Mn and Fe out o f near-shore sediments fol lowed by lateral, seaward transport o f these elements appears to be o f little or no significance for the formation o f M n - F e concretions in the offshore Gul f o f Bothnia sediments.

Acknowledgements

This study was supported by grants from the Swedish Natural Science Research Council and Hierta-Retzius ' Foundation for Natural Science Research. We grate- fully acknowledge the field assistance and guidance b y tngemar Renberg, Tom Korsman and Hans Hansson that made possible the collection and preparation o f freeze-cores. Thomas Ruth gave valuable advice concerning the ion-exchange procedure used for dissolved Mn determinations. Constructive comments on the manuscript from two anonymous reviewers are greatly appreciated,

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Redox cycling of iron and manganese in sediments of the Kalix River estuary, Northern Sweden