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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003

http://dx.doi.org/10.4067/S0717-97072003000300006 

J. Chil. Chem. Soc., 48, N 3 (2003) ISSN 0717-9324

IRON AND MANGANESE REDUCTION IN POREWATERS OF
THE BAY OF CONCEPCION AND ADJACENT CONTINENTAL
SHELF DURING THE "1997-98 EL NIÑO" EVENT

LUIS PINTO A. *1 y CECILIA RIVERA C.2

*1Programa MECESUP, Depto. de Oceanografía, Facultad de Ciencias Naturales y Oceanográficas,
Universidad de Concepción, Concepción, Chile.
E-mail: luispinto@udec.cl
2Facultad de Ciencias, Universidad Católica Ssma. Concepción, Chile.

( Received: December 2, 1999 ­ Accepted: April 21, 2003)

Keywords: reduced iron, reduced manganese, coastal sediments, porewater, early diagenesis, El Niño event.

SUMMARY

Porewater chemistry gives some indication as to specific diagenetic reactions occurring in sediments. Reduction of manganese and iron oxides can occur due to abiotic reactions with sulfide or microbially mediated as electron acceptors in the degradation of organic matter. Sedimentary porewaters obtained from the Bay of Concepcion (36°38'S; 73°02'W) and continental shelf (36°31'S; 73°08'W) were analised to determine the distribution of remobilised Fe2+ and Mn2+. Sampling of the area coincided with the presence of the strong "1997-98 El Niño event". Porewaters were extracted under a nitrogen atmosphere and analised by atomic absorption spectrometry. Subsurface maxima are observed for Fe2+ during "El Niño event" in the bay and adjacent continental shelf (86.8 ± 9.3 µM y 59.3 ± 35.5 µM, respectively). A year later when the oxygen minimum is present, the production of Fe2+ on bay surface sediments increases 3-fold. Maxima concentrations for Mn2+ appear on surface sediments during and after "El Niño event" within the bay. It is postulated that vertical displacement of maxima concentrations observed for Fe2+ and Mn2+ in these sediments are modulated by changes of the oxygen minimum depth in the water column and the amount and quality of organic matter deposited in surface sediments.

INTRODUCTION

The fate of metals in the aquatic environment is controlled by the interaction between soluble and solid phases. Metal species can undergo various reactions including reduction-oxidation, dissolution-precipitation and complexing. Some of these metals, such as iron and manganese participate in microbial degradation of organic matter as terminal electron acceptors (TEA)1). Iron and manganese are sensitive to redox conditions and are relatively mobile in the aquatic environment. Iron and manganese oxides, corresponding to Fe(III) and Mn(IV), are thermodynamically stable in oxygenated waters.

In coastal zones where high primary productivity occurs, manganese and iron hydroxides are reduced in surficial sediments under suboxic/anoxic conditions during oxidation of organic matter mediated by microorganisms2). Concepción Bay is a typical eastern coastal border location where upwelling brings to the photic zone an apreciable concentration of nutrients, causing high primary productivity. Part of the produced biomass is not totally consumed by hervibores or metabolized by microorganisms and fresh organic detritus is then deposited in shallow sediments3).

Interstitial water reflects chemical zonation as a result of postdepositional transformations during early diagenesis when the sediments are not perturbed either physically or biologically4). In general, there is scarce information about the quantitative effect of iron and manganese as TEA in the remineralization process of organic matter.

Previous studies of organic matter degradation on the continental shelf and slope off Concepción have shown that iron oxide reduction contributes with a 12 to 19%, oxygen, nitrate and manganes oxides combined contribute with a 15% to total carbon oxidation and sulfate reduction dominates with a >60% total integrated rates of carbon remineralization in the first 15 cm of sediment5). In contrast, 78% of organic carbon oxidation in Denmark´s coastal sediments occurs under anaerobic conditions within the first 8 cm of the sedimentary column6).

Rates of carbon oxidation by Mn and Fe reduction is not so straightforward due to significant removal of Fe2+ and Mn2+ by adsorption and precipitation. Microbial iron and manganese reduction can be masked by competitive non-enzymatic reactions with reduced inorganic compounds, such as:

2 FeOOH + H2S «2 Fe2+ + S + 4 OH- (1)

Iron hydroxide is reduced by H2S to iron monosulfide (FeS) and pyrite (FeS2)7). Bacterial sulfate reduction produces H2S in sediments off Concepción reaching values of up to 2 mM during the upwelling season in the Bay of Concepción3). The present study occurs during the "1997-98 El Niño" event that produced dramatic chemical and biological perturbations in the Equatorial Pacific8). Under those conditions, the oceanographic characteristics of the Bay of Concepción and adjacent continental shelf may not represent the situation described for a normal year9) due to a deepening of the thermocline and increasing distance of the Equatorial Sub-Surface Waters (ESSW) from shore.

A decrease in the upwelling of water masses with a high nutrient content reduces primary productivity rates and therefore the amount of phytodetritus that reaches the sediments. A higher oxygen content in the water column under "El Niño" condition can make organic matter remineralization more efficient, leading to changes in the quality of the organic detritus deposited in the sediment during and after an "El Niño" event. Under this special circumstance a question arises regarding the time scale for the changes in oceanographic conditions operating in the Bay of Concepción and adjacent continental shelf as a product of the "El Niño" phenomenon and the effect of these changes in the quality and composition of the organic detritus in the sediment controling the concentration of Fe2+ and Mn2+ in porewaters. As a corollary, the detection of subsurface maxima for Fe2+ and Mn2+ could allow characterization of redox conditions in surficial sediments and the upper limit of microbial sulfate reduction.

This study compares the production and distribution of Fe2+ and Mn2+ in porewaters of the Bay of Concepción and adjacent continental shelf and its possible participation as terminal electron acceptors for organic matter remineralization during the 1997 "El Niño" event (November 1997) and the upwelling period after the event (November 1998).

EXPERIMENTAL

Study area

The Bay of Concepción is a shallow embayment with a maximum water depth of 45 m and a surface of ca. 167.4 km210). The adjacent continental shelf is characterized by high primary productivity associated to seasonal and episodic upwelling of the Equatorial Sub-Surface Waters (ESSW)9). High nutrient and low oxygen concentrations are some of the main features of the ESSW creating a benthic environment subject to periods of suboxic/anoxic conditions 10).

Sediment cores were collected aboard the R/V Kay Kay (Universidad de Concepción) at a water depth of 28 m (station 4: 36°38'S; 73°02'W) inside the bay and from the adjacent mid-shelf, at a water depth of 88 m (station 18: 36°31'S; 73°08'W). The stations are depicted in Figure 1 and their numbers are those of the Thioploca-Chile Expedition, 199411). Sampling was carried out during the "El Niño event" in november of 1997 (labeled: Nov-97) and post-event in november of 1998 (labeled: Nov-98).


Fig 1.- Sampling location sites. Station 4 is within the Bay of Concepción.

Sampling and analysis

Sediment cores were obtained using a "Mini Multiple Corer MC 600"12). In each station a 10-cm internal diameter core was split in two subcores of 3.6 cm internal diameter to assess sediment heterogeneity in contiguous samples. Each subcore was sliced in 1 cm intervals for the first 3 cm and 2 cm slices down to 21 cm. Porewater was extruded under a nitrogen atmosphere (2 atm) with a pneumatic squeezer equipped with 0,45 mm nitrocelulose filters, acidified to pH~2 to estabilize reduced iron and manganese species and stored refrigerated until analysis. Dissolved Fe2+ and Mn2+ were determined in porewater by Atomic Absorption Spectroscopy with hollow cathode lamps and direct aspiration to an air-acetylene flame in a GBC 902 equipment. A blank was prepared with synthetic seawater to minimize matrix effects.

RESULTS

General characterization of sediments

Sampled sediments were characterized by the presence of black mud with a strong smell to H2S. A diverse benthic community was observed, dominated by organisms of the Polychaeta group13) . Sediments of the Bay of Concepción appear to have a flocculent layer in the first few centimeters3), reaching about 3 cm in Nov-98. Mats of bacteria of the Beggiatoa genera are conspicuous within surficial sediments with rather elevated concentrations of H2S14,15). Redox potential within the bay were negative from the first cm of sediment (Eh: -20 mV and ­89 mV for Nov-97 and Nov-98, respectively). However, an oxidized layer is present within the first 2.5 cm of sediment in the continental shelf for Nov-97, which increases to 5.5 cm in Nov-9815).

Vertical Fe2+ distribution

Vertical distribution of dissolved iron in porewater is shown in Figure 2 as an average of replicates taking next to each other. Results show similar concentrations for both stations within the first cm in Nov-97. A subsurface maxima of Fe2+ appears between 1-2 cm deep in both stations with concentrations of 86.8 ± 9.3 mM in the bay and 59.3 ± 35.5 µM on the shelf, decreasing rapidly with depth. Under 6 cm deep, Fe2+ concentration appears to be asymptotic to the ordinate axis, reaching values less than 2.0 µM at the base of the cores taken on both stations (20 cm). Both subcores in the bay present a relative maxima at 10 cm depth. A relative maximum at 16 cm deep appears on the shelf.


Fig 2.- Distribution of Fe (II) and Mn (II) in porewaters of the Bay of Concepción and adjacent continental shelf.

A year later (Nov-98) subsamples show more heterogeneity for the Fe2+ profiles obtained on both stations. Surficial cuts collected in the Bay of Concepción show concentrations of 158 mM (subcore A) and 294 µM (subcore B) which correspond to the highest concentration of each profile (Table I) and a relative maximum again at 10 cm of depth. Average maximum concentration post "El Niño" event is 2.6 times greater than that found during the "El Niño" event. On the continental shelf as within the bay, the largest concentration appears on surface sediments reaching its maximum between 1-3 cm. In Nov-98, the concentration is 64.1 ± 8.7 µM on the shelf compared to a rather similar average value of 59.3 for Nov-97, however, a higher dispersion for the duplicates appears on the latter. A second subsurface maximum of 34.2 ± 7.1 µM (Fig. 2) is observed in the 14 cm horizon in Nov-98.

Table I. Geochemical parameters of bottom and porewater and surface sediments (0-1 cm) in the By of Concepción and adjacent continental shelf (36.5 S)

Vertical Mn2+ distribution

The Mn2+ profile for the Bay of Concepción is maximum within the first cm of sediment on both years (9.4 ± 0.9 µM and 7.1 ± 4.9 µM, respectively), showing a 30% increase during El Niño (Nov-97 vs. Nov-98). An exponential decrease in the concentration is observed within the first 3 to 4 cm leveling to values found at the 10 cm depth. A distinct maximum appears at the water/sediment interface for both years. Average surficial values on the shelf are similar for both sampling periods (3.3 ± 0.1 µM and 3.2 ± 0.4 µM for Nov-97 and Nov-98 respectively); however, during the "El Niño event" a subsurface maximum is detected at 2 cm depth, which does not appear during the post "El Niño event". Both profiles are similar in their shape below 3 cm but separated by a constant relative value of ~1.5 µM down to 14 cm. On deeper horizons the difference between years is less than 0.15 µM. Sampling carried out in the shelf during and after the "El Niño event" reveal a sequence between the subsurface maxima for Mn2+ ocurring in the 6 and 12 cm horizons, preceeding maxima observed in the 8 and 14 cm horizons. The Fe2+ y Mn2+ concentrations are reduced to near the detection limit (0.1 and 0.02 µM, respectively) near the bottom of the cores, at 20 cm of depth.

DISCUSSION

During late spring when upwelling is observed in coastal waters, bottom waters in the Bay of Concepción are suboxic with oxygen values less than 15 µM and surficial sediments exhibit anoxic conditions (Eh<0 mV). This is typical of semiclosed embayments with a high organic matter content3). Under these circumstances, Fe(III)/Fe(II) y Mn(IV)/Mn(II) redox pairs contribute substantially to degradation and remineralization of the organic detritus present in the sediments5). According to a general model by Davison16), reduced manganese and iron maxima would be expected to appear in porewaters at the water/sediment interface. Given that reduction of manganese oxides to manganese (II) produces a change in Gibbs free energy higher than the reduction of iron, the former is utilized in the oxidation of organic matter before the reduction of iron (DG' -3000 kJ/mole for MnO2/Mn(II) vs. -1370 kJ/mole for Fe2O3(FeOOH)/Fe(II)) 1). However, the contribution of each one of theses oxides is affected by other physico-chemical processes, such as adsorption to particles, precipitation and redissolution, complexing and associated processes to the activity of benthic organisms on surface sediments such as bioirrigation and bioperturbation, thus the distribution of metals and interpretation for these profiles is more complexed17).

Fe2+ in Bay of Concepción

In november 1997 the event is at its highest intensity and the bottom waters in the bay show oxygen values near 100 µM, that is, seven times more than what would be expected for this time of the year. Oxygen penetrates by molecular diffusion to the first cm of sediment causing the deepening of the maximum production of Fe2+ in subsurface porewaters (1-2 cm). When the sampling occurs post-event, suboxic conditions are reestablished in the bay bottom waters and the Fe2+ profile shows three substantial changes when compared to Nov-97. First, the zone of maximum remobilized iron production has moved to the water/sediment interface. During this period the oxygen minimum of the AESS is reestablished on the shelf and has penetrated to the Bay of Concepción with measured values for dissolved oxygen of 15.2 µM3). Second, the maximum concentration of Fe2+ has increased 3-fold, which is a confirmation of the active role iron plays in the degradation/oxidation of labile organic matter in waters overlying suboxic sediments. Third, the Fe2+ subsurface maximum is spread and extends down to 6 cm deep (Fig. 2). An alternative explanation for the increase of Fe2+ could be caused by the reaction of equation 1. The presence of a high concentration of H2S as a product of the high rate of sulfate reduction could lead to the formation of Fe2+ by an exclusively abiotic pathway; however, this reaction produces an increase in the pH of the system. A decrease in the pH value (Table I) during this period suggests that a chemical reduction of iron oxyhydroxides does not account as a significant process in the horizon where maximum production of reduced iron is recorded. The risings of Fe2+ maximum to the water/sediment interface in the bay, together with an increase in concentration post "El Niño" event is the result of suboxic conditions in the overlying waters and an increment of the deposition of fresh organic detritus. Labile phytodetritus measured as chlorophyll-a (Table I) is degraded under anoxic conditions according to the redox potential for this period. An increase in the amount of phytodetritus produced in the photic zone is deposited within the bay as a consequence of the reestablishment of upwelling on the continental shelf (Nov-98) and approach of the AESS, rich in nutrients, to the coast. Dissolved oxygen and nitrate are depleted as oxidants of organic matter as labile organic matter increases1) in the overlying waters causing the reduction of iron and manganes oxyhydroxides to occur in the porewater near the water/sediment interface (Fig. 2). The position of the Fe2+ subsurface maximum for the bay station agrees with measured redox potential results15), supporting the existence of anaerobic conditions near the surface sediments (Table I).

Fe2+ in the adjacent continental shelf

The Fe2+ maximum in the continental shelf also ascends from the 1-2 cm horizon during the "El Niño event" (Nov-97) to the surface sediments post "El Niño event" (Nov-98). Although similar maximum concentration values are observed for both sampling periods, the remobilized iron inventory in the first 3 cms of sedimentary strata, show an increment in the utilization of iron as an electron acceptor for post "El Niño event" (Fig. 2). The steep decrease in Fe2+ concentration with depth reveals the production of iron minerals under anoxic conditions (Eh<0mV). Similar profiles are observed in southern Chile coastal sediments (ex. Boca del Guafo ­ 44S) where a sharp subsurface maximum of 172 µM at 4 cm depth following by an abrupt decrease to 20 µM was measured18). On a coastal Danish station (190 mt deep) similar concentrations are measured (185 µM at 2 cm deep)7).

Other sediment cores taken concurrently in this study, show the absence of sulfides although a high rate of sulfate reduction was measured15). Sulfides and reduced iron are readily precipitated to form iron minerals such as mackinawite and greigite leading finally to the more stable mineral of pyrite5). This authigenic mechanism removes Fe2+ from sedimentary horizons below the depth of maximum production in the bay and continental shelf. Experiments with 35S have shown that sulfate reduction and incorporation of reduced sulfur to pyrite have a fast kinetics of less than 24 hours19).

Mn2+ in the Bay of Concepción

Profiles of Mn2+ concentrations measured during and after the "El Niño event" within the bay differ very little between each other. Apparently reduced manganese production occurs at the water/sediment interface diffusing upwardly towards the overlying waters where precipitation of manganese oxyhydroxides take place or exported by bottom currents as part of the nepheloid bottom layer. The rate of Mn2+ oxidation is slow and the unstable Mn2+ ion may persist for some time in oxic waters20). Mn2+ also diffuses downward to deeper sediments. The sudden decrease of Mn2+ on the first few cm of sediment indicates precipitation and formation of a mineral phase. There is a strong preference of manganese for CaCO3 and the calcite phase is important in the manganese distribution. Experiments have shown that Mn2+ is adsorbed on the solid phase (clays or humic substances) in the Mn(H2O)6+2 form. After the addition of calcium carbonate, in one day all the manganese was adsorbed on calcite 20).

Mn2+ in the adjacent continental shelf.

The depth of the Mn2+ subsurface maximum coincides with the Fe2+ maximum during the "El Niño event" in the shelf; however, Mn2+ concentration is an order of magnitud less than that for Fe2+. It has been reported that oxidation of Fe2+ has a fast kinetics21) and can quickly react with manganese oxides reducing the metal to Mn2+ without the intervention of microorganisms as it is shown in the reaction (2).

2 Fe2+ + MnO2 + 4 H2O «Mn2+ + 2 Fe(OH)3 + 2 H+

(2)

Apparently, this could be an important mechanism to produce reduced manganese in the shelf, coupling geochemically these two metals. Reduced manganese profiles are coincident with those obtained off the coast of Concepción as part of the 1994 Thioploca Cruise5). As reported previously it seems that manganese oxides do not play an important role as terminal electron acceptors in the remineralization of organic matter, however, the redox characteristics of this metal at the water/sediment interface could be important in the transfer of other metals from the nepheloid bottom layer to surficial sediments.

The presence of the Mn(II)/Mn(IV) redox pair in the first 10 cm of depth in the coastal region of Denmark with a high percentage of manganese oxide has been shown to be the prevalent terminal electron acceptor in the oxidation of organic matter22). An excess of manganese oxide can also serve as a chemical pathway in the oxidation of acid volatile sulfides22) and the formation of reduced manganese and iron oxyhydroxides according to the following reaction23) (3):

FeS + 9/2 MnO2 + 7 H+ «FeOOH + SO42- + 9/2 Mn2+ + 3 H2O

(3)

These are important pathways for Mn and Fe cycling in coastal marine sediments, however, no quantitative information is available and therefore it is not possible to distinguish heterotrophic bacterial Mn2+ reduction from nonenzymatic reduction by Fe2+.

Bioirrigation and metal profile stability

Profiles for Fe+2 and Mn+2 below 6 cm within the sediments, are similar for both sampling times inside the bay. Concentrations are estabilised and gradually decrease independently of the oceanographic conditions with or without "El Niño". This observation suggests that under the 6 cm horizon, interannual changes in bottom-water oxygen concentration is not reflected in remobilized iron and manganese profiles. Secondary subsurface maxima observed at 10 and 14 cm depth indicate disolution and precipitation of these metals due to bioirrigation caused by benthic organisms allowing bottom water containing iron and manganese oxyhydroxides to be reduced anaerobically. Bioirrigation appears to be a permanent feature in these sediments based on the stability of the observed profile. As a corollary, bioperturbation of these sediments caused by benthic organisms may not be a prevalent phenomenon. Sediments with a high concentration of labile organic carbon appear to be dominated by macrofauna feeding on freshly deposited or resuspended material as interface or suspension feeder with a minimum capacity to mix particles24). The presence of polychaete in these sediments down to 8 cm deep, could explain the incorporation of oxidized iron and manganese to deeper sediments by bioirrigation. Thus, the deeper concentration maxima of reduced iron and manganese can be interpreted as new production of remobilized metals in the bacterially mediated oxidation of organic carbon, competing and inhibiting temporarily bacterial sulfate reduction25). The 10-cm Fe2+ maximum in bay sediments present in november 1997 and november 1998 appears to be a permanent feature present also in samples obtained during the year 1999 (unpublished data). Calculations made for the average time Mn and Fe species profiles take to readjust within the first 10 cm of sediment is of the order of a year1), supporting the idea of substrate stability beneath the 6 cm horizon in the Bay of Concepción.

CONCLUSIONS

Vertical distribution of Fe2+ and Mn2+ in the Bay of Concepción is in accord with the predicted sequence for terminal electron acceptors during bacterially mediated degradation of organic matter1). However, there are observable differences in concentration and depth at which Fe2+ y Mn2+ subsurface maxima appear during and after the "1997-98 El Niño event". An increase in the concentration of dissolved oxygen in the water column in the late spring of 1997 during "El Niño", caused the diffusion of oxygen into the sediments causing the remineralization of organic matter under suboxic conditions, pushing into deeper sediment horizons maxima production of reduced metals. The reestablishment of nutrient-rich upwelling water and the appearance of the oxygen minimum zone close to shore produced an increment of labile phytodetritus deposited on surficial bay and shelf sediments. Production of Fe2+ increases dramatically in the porewaters post "El Niño event" caused by either an increase of its role as a terminal electron acceptor and bacterially mediated oxidation of organic carbon or abiotic reduction of iron oxyhydroxides by metastable sulfide produced in large quantities by the high rate of sulfate reduction. Changes in the distribution of Fe2+ and Mn2+ in the porewaters of the study region show these metals to be sensitive to the dominant redox conditions and apparently the quality of the organic detritus deposited during and after the "El Niño event".

ACKNOWLEDGEMENTS

The authors thank Dr. V.A. Gallardo, FONDAP-Humboldt Oceanografía y Biología Marina, for access to sediment samples and laboratory equipment to obtain sedimentary porewater (FONDECYT 1971336) and to the captain and crew of the vessel L/C Kay Kay.

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