manganese oxidation and in situ manganese fluxes from a coastal sediment

Pergamon

Geochimica et Cosmochimica Acta, Vol. 58. No. II, pp. 2563-2570, 1994

Copyright 0 1994 Elsevier Science Ltd Printed in the USA. All rights resewed

0016-7037/94 $6.00 + .oO

0016-7037(94)E0043-K

LETTER

Manganese oxidation and in situ manganese fluxes from a coastal sediment

Bo THAMDRUP, * st RONNIE N@HR GLUD, * and JENS WURGLER HANSEN Department of Microbial Ecology, Institute of Biological Sciences, University of Aarhus,

Ny Munkegade Building 540, DK-8000 Aarhus C, Denmark

(Received December 22, 1993; accepted in revised form March 15, 1994)

Abstract-Manganese fluxes from the seafloor were measured in situ in Aarhus Bay, Denmark, with a free operating benthic flux-chamber lander (ELINOR). Constant effluxes were observed during 3 h incubations. The benthic Mn flux resulted in bottom water concentrations of dissolved Mn up to 0.6 PM, whereas concentrations above the pycnocline were about 0.1 PM. Similar fluxes were observed from sediment cores incubated in the laboratory under in situ conditions. A large variation between cores was attributed to the small area covered by each core. Manganese reduction in the upper O-l cm of the sediment supported steep porewater gradients of Mn towards the surface. However, calculated diffusive Mn fluxes towards the sediment surface were 3- 16 times higher than the benthic effluxes. This demonstrated high rates of Mn oxidation in the l-2 mm thin oxic surface layer with turnover times of 2 h or less. Model calculations including measured microdistributions of O2 and pH yielded rate constants more than 1000 times higher than those reported for abiotic Mn oxidation implying that Mn oxidation in the sediment was microbially mediated. The rapid oxidation was, as an internal source of oxidized Mn, essential to the intense redox cycling of Mn in the surface sediment.

INTRODUCTION

THE REDUCTIVE PATHWAYS OF the biogeochemical Mn and Fe cycles currently receive much attention, with growing em- phasis on direct microbial catalysis ( LQVLEY, 1993; CANHELD et al., 1993a; BURDIGE, 1993 ). Also the oxidation of aqueous Mn*+ may proceed both abiotically and by microbial catalysis. Whereas the homogeneous oxidation of Mn2+ around pH 7 is exceedingly slow, surface catalysis, especially on manganese and iron oxides, may enhance abiotic oxidation considerably (MORGAN, 1967; BREWER, 1975; SUNG and MORGAN, 198 1; DAVIES and MORGAN, 1989 ). The catalysis of Mn oxidation by bacteria in culture has long been known (see NEALSON et al., 1988) and its quantitative importance in waters receiving Mn 2+ has been documented. By use of inhibitors, EMERSON et al. ( 1982) demonstrated that Mn oxidation in the water-column of Saanich Inlet was microbially catalyzed and proceeded at rates orders of magnitude higher than those predicted by abiotic kinetics. Later studies have found this to be generally true in the waters of anoxic fjords, hydrothermal vent plumes, and lakes (e.g., NEALSON et al., 1988; MANDERNACK~~~ TEBO, 1993; WEHRLI et al., 1994). In sediments, high potential oxidation rates and sensitivity to biological inhibitors have indicated that microbial Mn oxidation could be of importance (EDENBORN et al., 1985; KEPKAY, 1985; TAYLOR, 1987), but in situ rates have not been quantified.

* Present address: Max Planck Institute for Marine Microbiology, Fahrenheitstrasse 1, D-28359 Bremen, Germany.

+ Author to whom correspondence should be addressed.

Manganese oxides deposited in aquatic sediments are reduced upon burial below the oxic surface layer and, conse- quently, Mn2+ accumulates in subsurface porewaters. In deep sea sediments with a broad oxic zone, essentially all Mn2+ diffusing up into this zone is oxidized there by O2 ( BOUDREAU and SCOTT, 1978). Release of Mn2+ to the water-column occurs from coastal sediments when Mn oxidation in the sediment cannot keep up with Mn reduction supplying Mn2+ from below (e.g., ALLER, 1980; ELDERFIELD et al., 1981; HUNT, 1983). Factors affecting benthic Mn2+ fluxes include the bottom water O2 concentration ( BALZER, 1982; SUNDBY et al., 1986), temperature, and organic input (HUNT, 1983; HUNT and KELLY, 1988). Bioturbation may increase the release of Mn2+ above that caused by diffusion. It is not yet known how the O2 microdistribution in the sediment affects Mn oxidation nor to which extent Mn may be retained in an oxic zone of a few millimeters thickness, as often found in coastal sediments (REVSBECH et al., 1980; RASMUSSEN

and J~~RGENSEN, 1992).

The sequence of burial, reduction, diffusion, and reoxidation leads to a concentration of Mn in the oxidation zone. As a parallel to this, the release of Mn 2+ from more reduced sediments and subsequent precipitation, lateral transport, and sedimentation of manganese oxides leads to an enrichment of Mn in more oxidized sediments on the shelf or in the deep sea (ELDERFIELD, 1976; SUNDBY et al., 1981; CANFIELD et al., 1993a). Thus, the Mn oxidation capacity of sediments is an important factor in the behaviour of Mn in aquatic systems with implications for, for example, bacterial Mn reduction, which in some Mn-rich sediments is the dominating pathway of carbon mineralization ( ALLER, 1990; CANFIELD

2563

2564 B. Thamdrup, R. N. Glud, and J. W. Hansen

et al., 1993a,b), and for trace metal distribution, as manganese oxides are important scavengers of trace metals ( TUREKIAN,

1977).

To examine the regulative role of the oxic surface layer in

the release of Mn*+ from a coastal sediment, we here compare the benthic Mn *+ flux measured in situ with a flux-chamber lander and in the laboratory under in situ conditions, to the distribution of Mn*+ in the porewater and water column. We relate these results to the microdistribution of O2 in the sediment as measured with microelectrodes.

MATERIALS AND METHODS

Study Site

The study was performed at Station 6 ( I6 m deep) in Aarhus Bay on the East coast of Jutland, Denmark (THAMDRUP et al., 1994). Due to the location on the Baltic Sea-North Sea transition, a marked stratification of the water column is often observed in calm periods, and the bottom water is often undersaturated with respect to oxygen during summer (JENSEN et al., 1988; RASMUSSEN and JORGENSEN, 1992). The sediment at Station 6 is fine grained with a fauna of mainly small mussels, polychaetes, and brittle stars.

In Situ Flux Measurements and Sampling

In situ measurements of Mn’+ and O2 fluxes were performed with the free-operating benthic flux-chamber lander ELINOR (GLUD et al.. 1993). ELINOR’s chamber covers 896 cm’ of sediment and en- closes about 12 L of water. One hour after deployment at the seafloor. the chamber lid closes and water sampling begins 5 min later. Mounted on the lid is a stirrer which creates a flow inside the chamber which in this case ( 10 rpm, water height lo-12 cm) resulted in an average diffusive boundary layer (DBL; BOUDREAU and GUINASSO, 1982) thickness for 02 of 375 pm. The DBL thickness for 02 in Aarhus Bay measured in situ with microelectrodes mounted on a profiling benthic lander ( GUNDERSEN and JORGENSEN, I99 1) varies between 250-700 pm (J. K. Gundersen, pers. commun.). The in situ values may underestimate the actual thickness by about 25% due to boundary layer compression imposed by the microelectrodes (GLUD et al.. 1994).

During the incubation, six to ten water samples of 45 mL each were automatically withdrawn at preset intervals for 3-4 h. These samples were used for Mn 2+ determination and for calibration of the O2 electrodes. Bottom water entering the chamber through a long stainless steel coil compensated for the withdrawn volumes and caused only insignificant dilution. The oxygen concentration in the chamber was continuously measured by two O2 microelectrodes ( REVSBECH, 1989). Electrode signals were calibrated from the reading in anoxic sediment on deck and by Winkler titration of 02 in the ELINOR water samples and in the bottom water. The total O2 uptake was calculated from the initially linear decrease in O2 concentration.

At the end of the incubation, a scoop closed below the chamber and the enclosed sediment and water phase were brought undisturbed on board the ship with the lander. The water samples were immediately retrieved for Mn 2+ analysis and the sediment was subsampled into Plexiglas core-liners for porewater analysis. Undisturbed sediment for laboratory incubations and additional porewater analyses were retrieved with a HAPS corer ( KANNEWORF and NICOLAISEN, 1973) and subsampled on deck. All sediment cores were kept cooled at bottom water temperature.

Water was sampled with a 5 L Niskin bottle I, 3, 6, 9, and 14 m above the sediment surface. Water from 20, 50, and 80 cm above the bottom was retrieved with a pump mounted on a frame resting on the bottom. Water entered the pump through a 1 cm wide hori- zontal slit and was raised to the ship through thick nylon tubing. Before water from a given height was sampled, twice the volume contained in pump and tubing was allowed to pass through the system. Profiles of salinity and temperature in the water column were obtained by CTD by the Department of the Environment, County of Aarhus.

Laboratory Incubations

A setup similar to that of RASMUSSEN and JORGENSEN ( 1992) was used to determine O2 microgradients and benthic Mn’+ fluxes. Sediment cores in 25 cm long Plexiglas liners of 54 mm i.d. were submersed uncapped in an aquarium containing bottom water from the same sampling. To simulate in situ conditions, the water was kept at bottom water temperature and O2 concentration by means of a cooling coil and purging with an appropriate mixture of air and Nz. Turbulence was created by a stirring magnet suspended above each core. Following a I2 h preincubation, the O2 distribution in the sediment was determined with an O2 microelectrode (REVSBECH 1989; RASMUSSEN and JORGENSEN 1992). The profiles showed a DBL of approx. 300 pm without correction for compression by the electrode. After withdrawal of initial samples for Mn*+, the cores were stoppered for determination of the Mn2+ flux from the change in Mn2+ concentration between initial samples and samples withdrawn after 3-4 h of incubation.

Manganese Analysis

Samples from the water column and flux incubations were pre- concentrated by magnesium hydroxide coprecipitation ( KOROLEFF, 1984; BOSTROM and BOSTROM, 199 1) All glassware, filterholders, etc. were cleaned in 3 N HNOx and Mini-Q water (Waters Associates) before use. After retrieval, the water samples (500 mL from the water column, approx. 30 mL from incubations) were immediately filtered through glass fiber filters (Whatman GF/F), and 0.5 M NaOH, cleaned with MgS04 according to BOSTROM and BOSTROM ( 199 1) , was added at a ratio of I:50 forming a Mn-containing precipitate. This precipitate was allowed to settle for at least 24 h. The supematant of water-column samples was carefully decanted until about 50 mL remained, including the precipitate, that could be transferred to a centrifuge tube. After centrifugation (5000 rpm, 10 min), the re- maining supernatant was decanted and the pellet was dissolved in 3 N HNOz to a final volume of 10 mL corresponding to a fiftyfold preconcentration. The precipitation bottles and centrifuge tubes were swirled with part of the HNO, to dissolve precipitates on their walls. For samples from flux incubations, the pellet was dissolved to 2.5 mL final volume, corresponding to a twelvefold preconcentration.

The concentrated samples were analyzed by flame atomic absorp- tion spectroscopy (Perkin Elmer). A reagent blank was prepared from seawater treated twice with NaOH as described above. The detection limits were about 15 nM and 50 nM with the fifty- and twelvefold preconcentration. respectively, and the precision was 5% (S.D.). Standard addition experiments showed a Mn2+ recovery of 97 ? 5% (SD.).

Porewater

Cores for porewater analysis were sectioned in an anoxic glove bag (~50 ppm 02) into a pneumatic squeezer ( REEBURGH, 1967). Pore- water was expressed through 0.45 pm cellulose acetate filters, and acidified with HCI to pH 2 for preservation. The samples were analyzed by flame AAS with a precision of *3% (S.D.).

Diffusive Flux Calculations

Molecular diffusive fluxes were calculated from concentration gradients by Fick’s First law (e.g.. BERNER, 1980):

where 4 is porosity (here 0.87 I in the O-2.5 mm interval), D, is the sediment diffusion coefficient, and dC/dx is the concentration gradient with depth. We calculated D, according to ULLMAN and ALLER (1982):

where D is the temperature-corrected diffusion coefficient ( LI and GREGORY, 1974). The surface concentration gradients of Mn2+ in the sediment were found by linear interpolation between the bottom

Manganese oxidation and in situ manganese fluxes

Table 1. Meawed blnz+ libwali@n rates. calculated diffusive pore water MnZ+ fluxes, and total oxygen uptake (TOU) of Aarbus Bay sediment.

Bottom water@ In sin4 benthic fluxes l&oratory incubation& Pore wateF)

Razz Temp. @J [Mn2+] Mn-effluxb) TOU Mn-efGx 02-penetr. Wit Mn-flux

% @I nM mm01 m-2 d-1 mm01 m-2 d-* mm01 m-2 d-1 mm mm01 m-2 d-1

July 3 9.0 157 ND 0.42 i 0.16 22.9 0.21 f 0.08(Z) 2.11 f O&x(4) ND

Augl9 11.0 137 288 0.34 f 0.02 36.5 0.23 f: 0.08(5) ND 1.0f 0.1(4)

Aug20 11.5 137 222 0.33 f 0.01 35.3 0.60 f 0.13(5) ND 1.0 f 0.2(10)

Aug25 13.2 95 615 0.36 f 0.05 34.1 0.20 zt 0.12(6) 1.09 f 0.13(4) 1.2 f 0.4(6)d)

ckt 19 11.0 124 ND ND ND 0.026 f 0.004(8) 2.15 f O&(4) 0.41 f 0.09(7)*)

l ) Sampled 20 cm above the sediment. d) Same cores as in lab. incubations.

b) Linear regression cc&Went f SE ND: not determined.

2565

c) Mean f S.E. of n cores, n in parentheses.

water concentration at the sediment surface and the porewater concentration in the O-2.5 mm section assigned to 1.25 mm depth.

RESULTS

In situ measurements with the benthic lander ELINOR were performed in summer 1992. The temperature and oxygen concentration of the bottom water were typical for this season at 11 B-1 3.2”C and 95-l 57 PM, respectively (Table 1). During all in situ experiments, the Mn*+ concentration increased linearly in the ~ux~harn~r (Fig. 1) . The rates of Mn2+ liberation measured on four dates were quite similar at 0.33 to 0.42 mmol m-* d-’ (Table 1). The O2 concentration in the chamber decreased up to 50% during the incubation with total 02 uptakes (TOU) of 22.9-36.5 mmol m-* d-’ (Table 1).

Liberation of Mn*+ was observed in all but one of the cores incubated at in situ conditions in the laboratory (Table 1). A me~urement in October 1992 suggested some season- ality to Mn liberation with a rate considerably lower than the summer measurements. The average liberation rates in the laboratory were similar to the in situ rates, but at each date, some variation was observed between cores (Table 1).

The 02 microdistributions measured in the laboratory varied little between cores from the same date (Fig. 2). The O2 penetration depth was 1-2 mm (Table I), not significantly different from those measured in the same period in situ with oxygen microelectrodes mounted on a profiling benthic lander (J. K. Cundersen, pers. commun.; GUNDERSEN and JORGENSEN, 199 1). The largest penetration was found in October when the benthic Mn flux was lowest. However, the Mn flux did not depend solely on O2 penetration as with a similar 02 penetration in July, a high benthic Mn flux was observed (Table 1). There was no correlation between Oz penetration and Mn flux from individual cores from the same dates (data not shown ) .

The flux of Mn from the seafloor caused an enrichment of dissolved Mn in the bottom water with concentrations decreasing upwards (Fig. 3). Measurement of salinity and temperature on August 25 showed that the Mn gradient was located in the lower part of the pycnocline l-2 m above the bottom (Fig. 3). The concentrations measured 20 cm above the bottom (Table 1) were within rt 10% of those found by extrapolation of the flux-chamber values back to time zero (Fig. 1).

The fine-scale porewater analyses showed extremely steep gradients of Mn *+ near the sediment surface indicating Mn reduction within the O-l cm interval (Fig. 4). The spatial

resolution of porewater analysis, however, was not sufficient to allow a Mn oxidation zone to be closely defined. Concen- trations up to 78 PM were observed in the O-2.5 mm section and maximum concentrations were measured about 1 cm depth. The depth distribution was quite stable through summer, while the concentration in the O-2.5 mm interval was significantly lower in October. The calculated diffusive flux of Mn2+ towards the sediment surface was above 1 mmol mm2 d-’ in summer where benthic fluxes were high and 0.4 mmol m-* d-’ in October where a low liberation rate was observed (Table 1). In addition to molecular diffusion, transport of Mn*+ within and out of the sediment may occur by porewater advection created by the infauna. Nevertheless, diffusive fluxes alone were significantly higher by factors of 3-16 than the benthic effluxes. There was no withindate correlation between calculated Mn” fluxes and Mn2+ liberation rates from single cores (data not shown).

(a) August 20

Lid closes

0 Bottom water sample

0 100 200 &Jo- O.O

Time from launch, min

64

Time, min

FIG. 1. (a) An example of simuI~neous in situ d~e~inations of Mn efBux and total oxygen uptake with ELINOR. The Mn2+ concentration in a bottom water sample collected outside the flux-cham- her is shown for comparison. (b) Data for henthic Mn flux deter- minations from three other dates. Time starts at lid closure. Lines are linear regressions.

2566 B. Thamdrup, R. N. Clud, and J. W. Hansen

o,, PM

0 25 50 75 100 125

&G .2. Average oxygen distributions in laboratory-incubated cores. Mean + SE. of four profiles, one from each of four cores from each date.

DISCUSSION

Flux Measurements

The in situ deployment of a flux-chamber equipped with an automatic water sampling system proved to be a conve- nient method for measuring benthic Mn fluxes. Linear ac- cumulations of MnZC were observed in the chamber during incubations. As concluded in previous Mn flux studies, pos- sible Mn’+ consuming processes within the water-phase of the chamber, including oxidation, adsorption, and precipitation, should not affect the accumulation of Mn*+ in the chamber significantly during short-term deployment ( ALLER, 1980; ELDERRELD et al., 1981; JOHNSON et al., 1991; JOHN- SON et al., 1992 ). The close similarity between the Mn eon- centration of the initial flux chamber samples, that remained on the lander for 2-3 h before fixation, and bottom water samples, that were fixed immediately after collection, con- firmed that the oxidation of Mn*+ in the bottom water was too slow to significantly &ct the Mn*+ concentrations during the incubation.

Althou~ an increased Mn flux is expected with decreasing O2 con~entmtions in long-term experiments ( BALZER, 1982; SUNDBY et al., 1986), the up to 50% decrease in O2 concentration over 4 h in the ELINOR chamber did not affect Mn liberation from the sediment. Similarly, constant Mn fluxes were found in situ from continental margin sediments in flux chambers with an up to 85% decrease of the O2 concentration (JOHNSON et al., 1992). Thus, incubations as those performed

here should provide accurate measurements of the in situ benthic Mn flux.

In addition to the minimal disturbance of the sediment, an advantage of the in situ measurements is the relatively large area covered by the chamber. This is illustrated by the comparison to the laboratory measurements (Table 1) where each sediment core covered 23 cm*, i.e., 2.6% of the area covered by ELINOR’s chamber. Whereas fluxes measured in situ on neighbouring days were similar, a variation was seen for the laboratory measurements both within and between days indicating a considerable lateral centimeter scale heterogeneity in the sediment. This heterogeneity could be due to the activity of the infauna or to patchiness in the distribution of organic matter, which is averaged out in the flux-chamber. Preliminary investigations indicated a potential role in mediating Mn fluxes as larger animals were found in cores with higher fluxes ( unpubl. res.).

The range of Mn fluxes observed here is similar to those reported from similar coastal sediments ( ALLER, 1980; EL- DERFIELD et al., I98 1; HUNT, 1983) and two orders of magnitude larger than that recently reported from coastal margin sediments (JOHNSON et al., 1992). In a previous study with frequent sampling at the Aarhus Bay site, a stable pool of Mn oxides of ca. 40 mmol mm* was found through the summer of 1990 (B. Thamdrup et al., unpubl. data). The ratio of this value to the benthic fluxes measured here indicates that the Mn oxide pool would be depleted in 3-4 months (i.e.. during summer). if the flux of dissolved Mn were not partly balanced by r~edimentation of Mn. In fact a depletion of the Mn oxide pool was observed during one month of autumn with very low bottom water O2 concentrations (B. Thamdrup et al., unpubl. data). At O2 conditions as those of the present study, however, it is likely that most of the Mn released from the sediment oxidized during dispersion through the water-column and settled locally so that a stable Mn oxide pool was maintained in the sediment.

9 18 20

I II

15 t

a* -o- August 19 --- August 20 -*-- August 25 - CT? August 25

3.0 0.2 0.4 0.6

Mn*+, PM

FIG. 3. Distribution of dissolved Mn’+ in the water column for three dates and the density profile from one date.

Manganese oxidation and in situ manganese fluxes 2567

Mn**, PM

-.- August 19

-A- August 20 --c August 25 -a- October 19

1

FIG. 4. Distribution of Mn *’ in the porewater at four dates. Mean + SE. The number of cores is given in Table 1. Bottom water concentrations are plotted at the sediment surface.

Benthic Manganese Flux and Manganese Reduction

The benthic Mn fluxes only constituted a fraction of the Mn2+ flux towards the surface supported by Mn reduction in the sediment. The ratio between in situ Mn fluxes and calculated diffusive porewater fluxes (Table 1) was 0.33-0.38 in August, and, based on laboratory flux measurements, 0.06 in October. It is likely that the porewater fluxes, at least in August, are underestimates of the actual transport of Mn2+ into the oxic zone. For one, the convex shape of the porewater profiles in August (Fig. 4) indicates that the true concentration gradients of Mn2+ were likely steeper within the O-2.5 mm interval than the gradients calculated here by interpolation to the surface. Finer scale porewater resolution would have yielded more accurate gradients. In October, the upper part of the porewater profile is essentially linear, and linear interpolation as used here should provide a good estimate. During all incubations, however, porewater advection caused by activity of the macrobenthos could increase the transport of Mn*” above what a molecular diffusion calculation would predict.

From the Mn oxide distribution and rates of burial by bioturbation, B. Thamdrup et al. (unpubl. data) calculated an average yearly Mn reduction rate of 2.3 mmol mS2 d-’ at the station with higher rates in summer than in winter. As permanent burial of reduced Mn was negligible, the Mn2+ from Mn reduction must be transported back to the Mn oxidation zone. It is therefore most likely that the diffusive fluxes presented here (~1.2 mmol m-* d-‘) underestimate Mn*+ transport, which substantiates that most of the reduced Mn was not lost but retained in the surface sediment.

Manganese Oxidation in the Sediment

Whereas the oxidation of Mn2+ in the bottom water enclosed by the ELINOR chamber was slow, the calculated diffusive fluxes and the considerations above show that an

intense Mn oxidation occurred in the sediment. The concentration profiles for Mn*+ (Fig. 4) indicate that the oxidation was restricted to the upper 2.5 mm and most likely to the l-2 mm thick oxic surface layer (Table I ; Fig. 2).

With the porewater Mn 2+ flux as a minimum estimate of Mn 2+ transport to the oxic surface layer, and the benthic Mn efflux representing the transport out of this layer, minimum area1 rates of oxidation were calculated (Table 2) as area1 ox. rate (mmol m-* d-‘) = diffusive flux - benthic flux. The specific oxidation rates were further calculated as specific ox.

rate (rmol crnF3 d-‘) = area1 ox. rate

O2 penetration depth (Table 2 ) and

thus represent average rates in the oxic zone under the assumption that the sedimentary oxidation of Mn is restricted to this zone. Although these rates are minimum estimates, a comparison to the total oxygen uptake rates of the sediment (Table 1) su&qests that the reoxidation of Mn*+ in the sediment only accounts for a small part of the oxygen consump- tion.

The specific oxidation rates are lower than the range of potential oxidation rates of 1.6-270 rmol cmm3 d-’ determined in Mn*+-amended slurries of coastal sediment compiled by TAYLOR (1987) but are similar to the Y,,,, values found by &PKAY ( 1985 ) in “peeper” experiments with lake sediment, 0.2-1.6 pmol cmS3 d-‘. The turn-over time of Mn2+ in the oxic zone in Aarhus Bay, turn-over time

cbWn’+l = specific ox. rate

(Table 2)) is considerably shorter than the

half-lives of Mn2+ reported from water-column studies (0.9- 69 days; compiled by WEHRL~ et al., 1994), and emphasizes the high oxidation efficiency of the thin oxic surface layer.

To investigate whether our relatively high Mn oxidation rates could be explained by known chemical reactions, we calculated the rate constants necessary to predict our results. Abiotic oxidation of Mn is reported to follow the rate law

- dlMtiy2’] = [Mn”][OH-]*[02](kl + kZ [solid]), (I)

where brackets designate molar concentmtions of the chemical species, k, and k2 are rate constants, and [solid] is the concentration of reactive surface sites (BREWER, 1975; DAV- IES and MORGAN, 1989). Thus, gradients of O2 and pH in the surface sediment can influence the oxidation rate significantly. In Aarhus Bay, steep oxygen gradients were observed (Fig. 2 ), but also pH gradients existed as shown by in situ measurements from the same period performed with pH microelectrodes mounted on a benthic lander (J. K. Gundersen, pers. commun.). Through the oxic zone, pH decreased from 7.8 to 7.6 on August 27 and from 7.7 to 7.2 on October 21

Table 2. Rates and calculated rate Constance for Mn oxidation in the sediment

Lklt? Atal ox. rate Specific ox. rate@ Turn-over time+ Rate constant

mm01 m-2 d-1 &mc11 cm-3 d-t h M-, da

Aagust 25 0.88 0.81 0.6 2.5 * 1020

october 19 0.37 0.17 2.3 7.9 * 1020

a) Avera&? for the oxic zone.

b) Average Mn*+ concentrations in the oxic zone calculated by linear interpolation.


(see also J~~RGENSEN and REVSBECH, 1989 ). Table 3 gives OH- concentrations in the oxic zone calculated from these

pH measurements. We have used the [OH-] data together with the average oxygen microprofiles measured in the laboratory (Fig. 2) to estimate a rate constant k* for Eqn. 1

where

k* = (k, + k2 [solid]). (2)

It is here assumed that [solid] is constant through the zone

of oxidation (see below). We further assume that [O,] and [OH -1 are constant in small depth intervals of thickness d.

We have used depth intervals of d = 0.05 mm corresponding to the intervals of the O2 measurements and calculated [OH-] for these intervals by linear interpolation of the data in Table

3. The oxidation rate in the ith section from x, to xi + d is then pseudo-first order with respect to Mn’+ with the rate

constant

K, = [02]i[OH-13 k*. (3)

The diagenetic equation for Mn*+ subject to molecular diffusion and first order oxidation is at steady state

with the general solution

and

\‘KI&X _ B@i%) (6)

where x is depth and A and B are integration constants de- pendent on the boundary conditions of Eqn. 4 ( MICHARD, 197 1). In our model, we begin by solving Eqns. 5 and 6 for A and B in the section at the sediment surface, 0 s x 5 d, and use the concentration and concentration gradient of Mn*’ at the surface, as boundary conditions:

x=0: C=C(O) and $= E(O).

As C( 0) we take the measured bottom water concentration,

and dC/ax( 0) is calculated from the benthic efflux through Fick’s First law:

Table 3. Free hydmxyl ion micmdistibulion in Aarhus Bay xdime.nP)

August 27 October21

Depth. mm [OH-]. nM Depth. mm [OH-]. nM Depth. mm [OH-]. nM

0.00 222 -0.10 137 1.30 49.0

0.25 198 0.10 113 1.50 49.6

0.50 183 0.30 88.4 1.70 51.2

0.75 168 0.50 70.6 1.90 53. I

1.00 161 0.70 59.6 2.10 55.1

1.25 161 0.90 53.5 2.30 57.1

1.10 50.3

‘1 Calculated from in situ micmscale pH measuremenu provided by Jens K. Gundersen using

the ion activity pmduct of seawater at 13.0 ‘C. 34% S (Culberson and Pytkowicz 1973).

Z(O) = benthic ehlux

ED . s

(7)

Due to the diffusive boundary layer, the concentration at the

sediment surface is, in fact, slightly higher than in the bottom water, but this does not affect the conclusions reached below. With the obtained values of A and B the concentration and

concentration gradient at the lower limit of the section 0 I x I d, C(d) and aC/ax( d), are calculated from Eqns. 5 and 6. The procedure is then repeated by using these values as boundary conditions for Eqn. 4 in the section d 5 x 5 2d and repetition continues until the bottom of the oxic zone is reached.

Values of k* yielding porewater concentrations of Mn2+ at the bottom of the oxic zone in agreement with the porewater analyses were determined (Table 2). For August 25, the concentration measured for the O-2.5 mm interval was used as target at 1.25 mm depth (Fig. 5) and for October 19, k* was similarly chosen to predict at 2.5 mm depth the con-

centration found by linear interpolation of the measured values. The two values of k* are quite similar considering the number of variables in the oxidation rate law (Eqn. 1). The resulting concentration profiles and rates of oxidation are drawn in Fig. 5. Oxidation rates are highest in the center of the oxic zone where the reactive species are in intermediate concentrations and there is a marked difference between the maximum specific rates calculated for the two days primarily due to the difference in the Mn*+ concentration.

Values of k, and k2 (Eqn. 2) reported in the literature (BREWER, 1975; DAVIES and MORGAN, 1989) are about 4 X lOI Me3 d-’ and 1 - 5 X 10” Me4 d-‘, respectively, at 25°C. Thus, the oxidation in Aarhus Bay sediment is about 1 OS times faster than the homogeneous oxidation (k, ). The concentration of catalytic surface sites, [solid] in Eqn. 2, can be constrained with the manganese and iron oxide content of the surface sediment. About 50 pmol Fe and 10 pmol Mn per cmm3 are found in free oxides in the upper O-2.5 mm, equivalent to 0.07 M (Fe + Mn) at a porosity of 0.87 1, and, at least for Fe, the concentration does not vary to 5 mm depth, thus justifying our assumption of constant [solid] (B. Thamdrup et al., unpubl. data). If this concentration is used as an upper estimate of [solid], k* calculated from the literature values of k, and k2 (Eqn. 2) is less than 4 X 10” M -3 d --’ , three orders of magnitude smaller than the values in Table 2. The porewater concentrations calculated by our

model using this value of k* are also shown in Fig. 5 and these predict concentrations far lower than those measured. Hence, known mechanisms of abiotic Mn oxidation can not explain the observed efficient removal of Mn *+ in the surface sediment and we suggest that microbial oxidation is the mechanism retaining the major fraction of Mn in the sedi-

ment. Much remains to be learned about the nature of microbial

Mn oxidation in sediments. The oxidation process has been interpreted as following zero order kinetics (TAYLOR, 1987 ), first order kinetics ( EDENBORN et al., 1985), and Michaelis- Menten kinetics ( KEPKAY, 1985 ). Similarly, the physiology of the microbial catalysis involved is not clear (see NEALSON et al., 1989). Our data demonstrate that the balance between situations where all Mn *+ is oxidized within the Aarhus Bay

Manganese oxidation and in situ manganese fluxes 2569

(4 Mn2+, pM O,, PM REFERENCES

2.5

E

S-

g5.0

Oxidation rate, ptvl de’

(b) Mn’+, pM 0,s FM

z 1 1 1 Oxidation rate, pM d-’

&or, 1 October 19

7.51

FIG. 5. Left: Modelled porewater distributions of Mn *+ in the oxic surface layer on August 25 (a) and October 19 (b) . Fat solid curves are best fits to measured porewater concentrations, fat dashed curves are profiles calculated from abiotic rate constants. Numbers are rate constants in M -3 d-’ . Bars are measured porewater concentrations (see also Fig. 4), and filled circles are the targets used in the modelling. Right: Average oxygen distributions (from Fig. 2) and calculated specific Mn oxidation rates.

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Acknowledgments-We wish to thank Jens K. Gundersen for putting effort to the construction of the bottom water pump, for letting us use the pH data, and for measuring 02 profiles in July. We also thank the Department of the Environment, County of Aarhus for operating the CTD. Jens K. Gundersen, Hans Jensen, and Lars Peter Nielsen are acknowledged for good collaboration throughout this study, and Don Canfield is thanked for valuable comments on the manuscript. The study was supported by the Danish National Agency for Envi- ronmental Protection through the Marine Research Programme 90 in Denmark.

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manganese oxidation and in situ manganese fluxes from a coastal sediment

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