Anaerobic microbial biogeochemistry in sediments from two Basins in the Gulf of Maine: Evidence for...

Estuarine, Coastal and She2fScience (1991) 32,313-324

Anaerobic Microbial Biogeochemistry in Sediments from Two Basins in the Gulf of Maine: Evidence for Iron and Manganese Reduction

Mark E. Hines=, Dennis A. Bazylinskib, Joyce B. Tugel” and W. Berry Lyons”’ “Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824 and bDepartment of Anaerobic Microbiology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A.

Received 10 April 1990 and in revised form 12 October 1990

Keywords: biogeochemistry; sedimentary; sulphate reduction; denitrification; glucose turnover; iron; manganese; Gulf of Maine

Rates of sulphate reduction, denitrification and glucose turnover were determined in surface sediments collected from Jeffreys and Wilkinson Basins in the Gulf of Maine. These data were compared to porewater profiles of iron and manganese. Three distinct anaerobic biogeochemical situations were evident: (1) sulphate reduction was dominant and active near the sediment-water interface (deeper portions of Jeffreys Basin); (2) denitrification was active in the upper few centimetres and was underlain by rapid sulphate reduction (slope of Jeffreys Basin); (3) denitrification was dominant (Wilkinson Basin). An apparent iron and manganese reduction region was evident in all cores. Since glucose turnover was active and both denitrification and sulphate reduction were low within this metal reduction zone it is possible that microbial metal reduction is a significant process within the basin sediments. High concentrations of dissolved iron occurred within sediments deeper than those supporting active sulphate reduction. Sufficient iron was remobilized during iron reduction to not only precipi- tate all of the sulphide generated during sulphate reduction but to also allow for dissolved iron to diffuse into deeper sediments. Therefore, large vertical regions containing dissolved iron are not necessarily regions of iron reduction and iron reduction is restricted to a narrow sediment layer.

Introduction

Continental shelves are productive ecosystems which may account for greater than 3596 of the global marine primary productivity (Ryther, 1969; Eppley & Peterson, 1979). The shelf regions also receive large quantities of continental material delivered by rivers. This

‘Present address: Hydrology/Hydrogeology Program, Mackay School of Mines, University of Nevada, Rena, NV 89557, U.S.A.

0272-7714/91/040313+ 12 $03.00/O @ 1991 Academic Press Limited

314 M. E. Hines et al.

continental detritus is composed of material representing wide degrees of weathering conditions from newly eroded soils to material which is highly degraded (Turekian, 1977; Aller et al., 1985). Although there is uncertainty about the ultimate fate of material which enters the continental shelf from erosion or during pelagic primary productivity (Walsh et al., 1981), it is becoming clear that a large portion of the particulate matter produced in the water column on continental shelves reaches the sediments, the vertical flux often greatly outweighing the horizontal flux to the slope (Christensen, 1989). Therefore, to understand better the fate of material which enters the oceans it is necessary to understand the processes which alter compounds post-depositionally in these environments.

Microbially mediated diagenesis is responsible for most chemical reactions which affect materials in sediments. In sediments where the input of organic matter is fast enough to prevent the diffusion of oxygen, the decomposition of organic material occurs via anaerobic transformations, particularly nitrate reduction followed by sulphate reduction (Christensen et al., 1987; Sorensen & Jorgensen, 1988). However, it is conceivable that the microbial reduction of manganese and iron are important mechanisms for decom- posing organic matter as well (Burdige & Nealson, 1985; Aller et al., 1986; Tugel et al., 1986; Lovley, 1987; Sorensen & Jorgensen, 1987). Each biogeochemical zone supports different types of chemical transformations which affect the chemical composition of both the solid and dissolved phases (Froelich et al., 1979; Burdige & Gieskes, 1983).

The present study was undertaken to determine the importance of certain microbial processes and geochemical distributions of various biogeochemically important species in sediments from two basins in the Gulf of Maine. Previous studies in basins in the Gulf have determined rates of sulphate reduction using radioisotopic techniques and have estimated rates of nitrate reduction from sedimentary nitrate depth profiles (Christensen et al., 1987). Rates measured using porewater data cannot always delineate important depth distributions of microbial activities. Lyons et al. (1980) reported the depth distribution of various inorganic species and recoverable sulphate-reducing bacteria in the sediments within two basins in the Gulf of Maine. The present study measured the depth distribution of rates of denitrification, sulphate reduction and glucose turnover in these continental shelf sediments. These data, when compared with porewater chemistry from the same sediments, demonstrated that specific sedimentary biogeochemical processes vary greatly in importance from one location to another and suggested that the microbial reduction of iron, and perhaps manganese, may be important in continental shelf basin sediments.

Materials and methods

Sampling locations Sediment samples were collected using a l/4-m2 boxcorer from two submarine basins in the Gulf of Maine while aboard the R/V Cape Hatteras. Two cores were collected in Jeffreys Basin which is located in the western portion of the Gulf off the coast of New Hampshire (Figure 1). Core JB-1 was collected from sediments which were located approximately half-way down the slope into the basin at 91 m depth (70”28,5’W, 42’58.8’N). Core JB-2 was from the middle of the basin at 172 m depth (70”08.2’W, 4259.8’N). Core WB-1 was collected from the middle of Wilkinson Basin at 282 m (69”31,5’W, 42”32,5’N) (Figure 1). Other shorter cores were collected from Jeffreys Basin from aboard the R/VJere Chase using a piston corer. These cores (JB-DA) were collected

Anaerobic sedimentary biogeochemistry 315

il 70 69 66 67 66 65

Figure 1. Location of sampling sites in the Gulf of Maine.

at a water depth of 140 m from a site approximately equidistant between cores JB-1 and JB-2 (70”19-O’W, 42”59-9’N) (Figure 1).

Sample handling Upon retrieval of a core, a subcore was collected using a plexiglass boxcorer and immedi- ately transferred to the shipboard laboratory for analysis. Subcores were placed into N,- flushed glove bags. The subcorer had one side which contained slats which could be removed to sample from the side. After flushing the bag several times with N,, the slats were removed one at a time for sampling at 2-cm depth intervals. Piston cores were extruded within an N,-filled glove bag in our land-based laboratory at UNH within 24 h of sampling.

While working in an N,-filled glove bag, triplicate unhomogenized subsamples for sulphate reduction rates were collected in 5-ml syringes which were pushed into cores from the side and then sealed with serum stoppers. The remaining sediment was removed and homogenized. Triplicate lo-ml subsamples for denitrification rates were placed into 40-ml screw-capped vials which were sealed with Teflon-lined silicone rubber septa. Triplicate I-O-ml subsamples for determining rates of glucose turnover were placed into test tubes which were sealed with rubber septa containing a plastic cup with fluted chromatography paper. Five-ml subsamples were removed for measurements of porosity. The remaining sediment was placed into centrifuge bottles which were sealed anoxically and centrifuged at 5000g for 30 min. After placing the centrifuge bottles back into the glove bag, the pore water was filtered under N, and distributed, using automatic pipettes and acid-cleaned tips, into various vessels for analysis. Samples for Fe*+ and Mn2’ analyses were stored in acid-washed polypropylene vessels and acidified to a final concentration of 14, ultrapure nitric acid. Samples for sulphate analyses were stored frozen in


polypropylene vials. Samples for dissolved sulphide analysis were mixed with 3”, zinc acetate.

Microbial rate measurements Sulphate reduction rates were determined using 35S (Hines & Jones, 1985). For samples JB-1, JB-2 and WB-1, the incorporation of 35S into the free and acid-volatile sulphide fractions was measured. For the shorter cores collected from Jeffreys Basin, sulphate reduction measurements included the incorporation of 35S into the chromium-reducible phase (pyrite and elemental sulphur) (Hines et al., 1989).

Denitrification rates were determined using the acetylene blockage technique (Yoshinari et al., 1977). Sediment subsamples were diluted with an equal volume (10 ml) of deoxygenated (N,) artificial seawater. One ml of acetylene was injected per vial (4.6 kl?a in headspace) and samples were incubated at ambient temperature (4 “C) on a shaker for 5 h. The reaction was terminated by injection of 1 ml of a saturated solution of HgCl,. Concen- trations of N,O in the headspace of vials were measured using gas chromatography with electron-capture detection as described previously (Bazylinski & Blakemore, 1983). Total amounts of N,O present were calculated using Henry’s Law and published values of solubility coefficients (Linke, 1965). The acetylene blockage technique used here measures only the reduction of nitrogen oxides to N,O and N,. Any reduction of nitrogen oxides which resulted in ammonium production (dissimilatory nitrate reduction) was not determined.

Rates of glucose turnover were determined using 14C and a modification of the method of Christian and Wiebe (1978) and Hines (1985). This technique monitored the incorporation of 14C endproducts into carbon dioxide, bacterial biomass and ether-soluble fermentation products. One ml of deoxygenated artificial seawater containing 0.1 uCi of U-14C-glucose (New England Nuclear) was injected into each test tube. After homogen- ization on a vortex mixer, samples were incubated at 4 “C for 30 min. The reaction was terminated by injection of 1 ml of a chilled solution containing 2.5O,, formalin and 0.225 N H,SO,. Vials were kept on ice and 0.2 ml of phenethylamine was injected into each filter- paper trap within the vials to collect CO,. After extraction overnight, the filter-paper traps were removed and the radioactivity determined using AquasolTM and a Beckman model LSlOOC Scintillation Counter. The efficiency of CO, trapping and scintillation counting was determined by using pre-killed controls which received a known activity of 14C-HC03P.

The remaining sediment slurries were centrifuged and the supernatant solutions were frozen to - 80 “C. The centrifuged pellets were washed three times with chilled artificial seawater, resuspended in distilled water and dried at 60 “C overnight. Dried sediment was oxidized in autoclaved sealed ampoules containing an acid/dichromate mixture and the resulting CO, was sparged and trapped in a mixture of AquasolTM, phenethylamine and methanol (6:2:2) (Smith et al., 1972). This fraction represented the 14C incorporated into bacterial biomass.

Frozen supernates were thawed and extracted three times in ethyl ether to remove ether-soluble organic metabolites such as low molecular weight fatty acids (Christian & Hall, 1977). The three extracts were pooled and dehydrated with an excess of anhydrous MgSO,. Half of the ether extract was added to AquasolTM and counted. This ether extract differed from that reported by Christian and Hall (1977) and Christian and Wiebe (1978) in that they counted the aqueous phase before and after ether extraction. The present method is much more precise because the ether-extractable fraction represents a small


Rate (nmol ml-’ day-‘)

0 IO 20 30 40 04 1 1 7 I I I I ’

a SO$’ reduction

,,P l Denitrification 1

Glucose turnover (h-’ )

‘.6 Fe and Mn (,LM)

50 100 If I I 1 I I

Figure 2. Depth distributions of microbial activity rates and porewater chemical constituents at site JB- 1.

percentage of the total i*C in the subsamples. This is especially true when the tracer approach is used and greater than 90% of the original glucose is still present. Hence, the ether-extractable fraction is part of the remaining - 10% and usually accounts for less than 20% of this smaller portion. The direct counting of the ether alleviates the problem of subtracting a small number from a very large one. In addition, since water is slightly soluble in ether, some of the unutilized glucose is extracted and results in a large blank in the ether fraction. This problem was completely solved by dehydrating the ether. The dehydration step removed 10-150/b of added 14C-acetate from the ether. However, this loss was minor compared with the large increase in sensitivity and precision when counting the dehydrated ether fraction directly. Ether does not quench scintillation counting and large volumes of ether can be added to scintillation fluids. The three 14C fractions were combined to calculate the turnover rate of glucose as the fraction of 14C used per time (units of h-l). Since we did not measure the actual concentration of glucose it was not possible to calculate an actual glucose uptake rate (Wright, 1978).

Chemical analyses Dissolved iron was determined using the FerroZine method (Hines et al., 1984) with a precision of + l.Oci, at 35 PM. Dissolved manganese was determined using the formal- doxime calorimetric method (Goto et al., 1962) with a precision of +3.0’% at 9 PM. Sulphate was measured turbidimetrically (Tabatabai, 1974). Sulphide was determined calorimetrically according to Cline (1969).

Results

All cores contained numerous unidentified worm tube structures which protruded slightly above the sediment surface. Several of the burrows were found to be several tens of centimetres in length with burrow walls of 2-3 cm in diameter and consisting of densely consolidated clay material.

Each of the three boxcores displayed very different depth profiles of microbial activity rates and chemical constituents. Core JB-I, which was collected on the edge of Jeffreys Basin, contained an upper 3-4-cm zone which was dominated by denitrification with rates of - 15 nmol ml-’ day-’ (Figure 2). Below 5 cm, sulphate reduction increased from


Rate (nmol mi-’ day-‘)

l Deniirificafion

Glucose turnover (h-l 1 Fe and Mn f,uM)

Figure 3. Depth distributions of microbial activity rates and porewater chemical constituents at site JB-2.

< l.Onmol ml-’ day-’ to N 30 nmol ml ’ day- ’ and then slowly decreased to < 2.0 nmol ml-’ day-’ below 12 cm. Dissolved Mn concentrations decreased from 120 PM in the upper 2 cm to -35 pM at 5 cm while Fe increased from a low of 3.5 PM to nearly 140 FM in the upper 5 cm. Dissolved Fe values decreased somewhat below 5 cm but never decreased to less than w 50 uM throughout the upper 20 cm of sediment even though sulphate reduction reached a maximum at 7 cm. Glucose turnover rates were relatively high at O-35-0.55 h-’ throughout the upper 11 cm but decreased to co.15 he1 in deeper regions.

Core JB-2, which was collected from the centre of Jeffreys Basin, displayed a sulphate reduction maximum of >20 nmol ml-’ day-’ at 3 cm while rates of denitrification remained below 4 run01 ml-’ day-’ throughout the core (Figure 3). Relatively high levels of dissolved Mn and Fe were noted at all depths in core JB-2 with a maximum concentration of Mn at 3 cm and a maximum for Fe at 7 cm. Dissolved Fe continued to increase in concentration with depth to 7 cm despite the fact that sulphate reduction reached a maximum at 3 cm. Rates of glucose turnover were slower in core JB-2 compared with core JB-1 and decreased to < 0.05 h-’ in sediments deeper than 15 cm.

Denitrification was the dominant anaerobic microbial process noted in the Wilkinson Basin core (WB-1) where rates were greater than 20nmol ml-’ day-’ throughout the upper 4 cm (Figure 4). A sulphate reduction maximum was noted below 15 cm but this maximum was N 50-fold lower than the maxima noted in Jeffreys Basin. Dissolved Fe was not detected in the upper 16 cm and was <4 FM in the interval from 16 to 20 cm. Dis- solved Mn was not detected in the upper 2 cm but increased to 128 PM at 20 cm. Rates of glucose turnover were the lowest of the three cores with values of 0.38 h-r near the surface which decreased to 0.046 h-’ at 6-8 cm.

Sulphate was never depleted with depth in any of the cores (data not shown). The sulphate concentrations generally were 22-25 mM which are low compared with average open ocean values. However, the salinities in these basins were 4-6 ppt less than in open ocean waters. None of the cores contained measurable quantities of dissolved sulphide (<2,0j~M).

Since sulphate reduction rate determinations measured the incorporation of ‘?S into the acid-volatile phases only, additional piston cores collected at JD-DA (a site situated between sites JB-1 and JB-2) were used to determine the quantity of 35S incorporated into

Anaerobic sedimentary biogeochemistry

Rate (nmol ml-’ day-’ ) Glucose turnover (h-‘1

0 -;

SO:- reduction l Denitrification

20

(b) Rate (nmol ml-’ day-‘)

Fe and Mn (,LLM)

0 50 100 150

Figure 4. (a) Depth distributions of microbial activity rates and porewater chemical constituents at site WB-1. (b) Expanded view of sulphate reduction rates at site WB-1.

the chromium-reducible fraction as well. For the latter samples, the acid-volatile fraction accounted for 627; of the total rate (data not shown). Therefore, the rates reported in Figures 2 and 3 should be multiplied by N 1.6. However, the relative depth distribution of sulphate reduction did not change when both fractions were measured nor did the location of the zone of maximum sulphate reduction. The chromium reduction technique was not applied to samples collected at site WB-1 so the magnitude of the underestimation of sulphate reduction could not be determined for that site. However, since Christensen et al. (1987) found a significant positive mathematical relationship between the rate of sulphate reduction and the percentage of that rate attributable to the acid-volatile fraction the rates at WB-1 should be multiplied by w 10.

Discussion

The sulphate reduction rates reported here were very similar in magnitude to the rates reported by others for sutlicial sediments in the Gulf of Maine (Christensen et al., 1987; Christensen, 1989). In Wilkinson Basin, the rates reported by Christensen et aE. (1987) were within a factor of two of our rates. The sulphate reduction rates in the Gulf of Maine were approximately one order of magnitude slower than in near-shore marine sediments


such as estuaries (Jorgensen, 1977; Hines & Jones, 1985) and nearly two orders of magnitude slower than rates reported for saltmarsh sediments (Howes et uZ., 1983; Hines et aZ., 1989).

Maximum denitrification rates in the Gulf of Maine sediments were less than those observed for near-shore coastal sediments using similar techniques (Sorensen, 197&z, b) but they were higher, by a factor of 120, than denitrification rates reported for continental shelf sediments off Alaska (Haines et al., 1981). The rates of denitrification reported here were similar, in general, to those reported previously for the Gulf of Maine (Christensen et al., 1987). However, the Wilkinson Basin data are the only ones which can be compared directly since Christensen et aE. (1987) did not sample Jeffreys Basin. Although the rates and depth distribution of sulphate reduction agreed well, the denitrification rates reported here for site WB-1 were seven-eight-fold more rapid than those reported by Christensen et al. (1987). Those authors calculated denitrification rates from nitrate profiles and reported their data as minimum values which may have been as much as four times too low. In addition, their rates included both denitrification and dissimilatory nitrate reduction. It is possible that the rates reported here were underestimates of total nitrate reduction since only denitrification was measured. Therefore, the total rate of nitrate reduction in at least Wilkinson Basin may be considerably higher than previously thought.

Denitrification on the continental shelf may be a major sink for fixed nitrogen in the ocean (Liu & Kaplan, 1984; Christensen et al., 1987). Since current estimates of continental shelf denitrification rates are based primarily on porewater profiles of nitrate, our data using direct measurements suggested that denitrification is even more rapid than the recently reported estimates (Christensen et uZ., 1987) and that the oceanic nitrogen budget appears to be even more unbalanced than previously thought. Certainly, measurements from three cores are grossly insufficient and additional data are needed which are derived from direct measurements of microbial activities rather than from porewater data alone.

The three sites studied represented three different examples of suboxic to anoxic biogeochemical situations which may exist in continental shelf surficial sediments: (1) sediments exhibiting a near-surface zone of denitrification which is located above zones of Fe reduction and relatively rapid sulphate reduction (JB-1). The coexistence of the denitrification rate and dissolved Mn maxima indicated either a spatial overlap of these reductions or a segregation within the upper 2 cm; (2) sediments which are dominated vertically by sulphate reduction (JB-2) with a partial overlap with metal reduction zones; and (3) sediments which are dominated by denitrification and Mn reduction (WB-1).

The relative distribution of sedimentary biogeochemical zones depends on the rate of input of organic substrates and the availability of the appropriate electron acceptors (Claypool & Kaplan, 1974; Berner, 1980; Krom & Berner, 1980). Since sulphate and nitrate are available in bottom waters in continental shelf basins at these depths, the relative vertical importance of these metabolic activities was determined by the quantity of organic matter deposited. Increases in organic deposition rates have been shown to cause increases in bacterial metabolic rates which result in narrower biogeochemical zones in sediments (Froelich et al., 1979). Therefore, situation 2 above (site JB-2) probably had the highest rate of organic matter influx and situation 3 (site WB-1) probably had the lowest. However, when denitrification and sulphate reduction rates are integrated over the depths reported here, the most rapid rates were found at site JB-1 not JB-2. This discrepancy may be due to the use of other electron acceptors such as oxygen.

Previous work suggested that the sedimentation rate in Wilkinson Basin was higher than in Jeffreys Basin (Flight, 1972; Hulbert & Given, 1975; Faas & Nittrouer, 1976). This


conclusion was based on 14C dates from long cores. Because of its closer proximity to the continent, one might expect that the highest sedimentation rates would be found in Jeffreys Basin. The biogeochemical data reported here tend to support this premise. Whether the 14C dates represent variability within the basins themselves or whether the older age in Jeffreys Basin is due to increased input of older more recalcitrant terrestrial organic matter is unknown. Unfortunately, studies using ‘i”Pb profiles to determine sedimentation rates in the Gulf of Maine were unsuccessful because the isotope was mixed too thoroughly by bioturbation (Brower, 1984).

The core from site JB-1 was characterized by a well defined denitrification zone which was situated over a deeper layer of rapid sulphate reduction (Figure 2). However, the region between 2 and 6 cm displayed slow rates of both these processes. Oxidized forms of both Mn and Fe have been shown to serve as electron acceptors for bacterial respiration in marine sediments (Burdige & Nealson, 1985; Aller et al., 1986; Tugel et al., 1986; Lovley, 1987). These bacteria may have been responsible for a portion of the microbial metabolism in the 2-6-cm zone at site JB-1. We did not measure rates of Fe and Mn reduction in these sediments but the increase in the concentration of dissolved Fe between 2 and 6 cm suggested that Fe-reducing bacteria were active in that region. In addition, glucose turnover was relatively rapid throughout this 2-6-cm zone indicating that bacteria were active within that region. This indicated that the lack of active denitrification and sulphate reduction from 2 to 6 cm was not due to the occurrence of an anomalous sediment layer which was not conducive to microbial activity.

Glucose turnover rate data must be viewed with caution since the actual rate of utilization of glucose depended on the ambient concentration of glucose present which was not determined (Wright, 1978). In addition, glucose utilization may not be representative of organic matter useage in general. If the average natural glucose concentration in all of these sediments is taken as 0.5 uM (Meyer-Reil et al., 1979), the sulphate reduction rates are twice those in Figures 2-4, the sediment porosity is uniform at 50% and the stoichi- ometry of denitrification and sulphate reduction are C:N = 1.25 and C:S = 2.0, respect- ively, then glucose carbon accounted for ~25-35% of the total carbon utilized during denitrification and sulphate reduction. This percentage seems high considering the myriad of organic compounds that potentially can be degraded by denitrifiers or fer- mented and then degraded by sulphate reducers. This suggests that the actual glucose levels are probably much lower than 0.5 PM. However, the relatively high percentage value lends credence to the use here of glucose turnover as a model electron donor for comparison to rates of electron acceptor utilization.

Since Fe-reducing bacteria are capable of out-competing sulphate-reducing bacteria for organic substrates (Lovley &Phillips, 1987), the occurrence of microbial Fe reduction probably was responsible for restricting maximum sulphate reduction activity to sediments deeper than 6 cm at site JB- 1. Sorensen and Jorgensen (1987) reported a similar conclusion from data collected from cores off Denmark. They too found that the region where Fe dissolution occurred was between the zones of denitrification and sulphate reduction. From the depth profile of dissolved Mn in the JB- 1 core it was not possible to determine whether Mn reduction was situated below the zone of denitrification since the maxima in denitrification rate and the concentration of dissolved Mn coexisted.

Anaerobic activity in Wilkinson Basin surflcial sediments (WB-1) was dominated by denitrification (Figure 4). Although sulphate reduction increased in activity with depth, the increase was very small compared with the cores collected in Jeffreys Basin (Figures 2 and 3). Dissolved Mn at WB-1 increased continually with depth and dissolved Fe was


virtually absent. Although the Mn reduction rate was not determined, the porewater profiles suggested that Mn reduction was occurring throughout the 4-l 2-cm region and that Fe reduction was not. However, since reduced Fe is capable of reducing Mn oxides (Myers & Nealson, 1988) it is possible that Fe reduction was occurring but that Fe2+ was not able to accumulate while Mn oxides were still present. The rapid decrease in glucose turnover below 4 cm suggested that bacterial activity was very slow below the zone of active denitrification so the argument for bacterial metal reduction presented above for JB-1 is not valid for WB-1. However, the dissolved Mn depth profile, the decrease in denitrification at 5 cm and the increase in sulphate reduction at 15 cm, albeit small, suggested that Mn reduction was a predominant metabolic process throughout the - 4- 12-cm depth interval. Therefore, even in the absence of actual rate data, it appeared that bacterial metal reduction was important in these shelf sediments and probably is in most continental shelf areas.

Dissolved Fe was prevalent throughout the upper 20 cm of sediment in the Jeffreys Basin samples even though maximum sulphate reduction activity was located in sediments closer to the surface. Since the solubility of Fe is undoubtedly controlled by ferrous sulphide precipitation in sulphate-reducing sediments, there must have been a source of dissolved Fe in sediments below 5 cm which surpassed the rate of sulphide production. A likely source for this dissolved Fe was the dissolution of Fe in the Fe-reduction zone which appeared to reach a maximum at 5 cm. If Fe was serving as an electron sink for bacterial respiration in these sediments then 8 moles of Fe were required to accept the number of electrons consumed by 1 mole of sulphate during sulphate reduction (Froelich et al., 1979). Therefore, dissolved Fe’+ supplied at 5 cm would have been more than sufficient to remove all of the sulphide produced by sulphate reduction if microbial Fe reduction was a significant respiratory process in these sediments. Alternatively, dissolved Fe may have been supplied as a result of bioturbation (Aller, 1980; Hines et al., 1984). The absence of dissolved sulphide in these sediments was further evidence that the rate of Fe dissolution was sufficient to remove sulphide.

Aller et al. (1986) noted that the Fe-reduction zone was extensive in Amazon inner shelf sediments. This conclusion was derived from the occurrence of a large vertical region of dissolved Fe in the sediments without a significant decrease in sulphate concentrations and in the absence of bioturbation. Although we did not collect samples from deeper than 20 cm in the Gulf of Maine sediments, we found a situation similar to the Amazon shelf where dissolved Fe (and Mn) was abundant over several centimetres and sulphate was never significantly depleted. However, the present study noted that the sulphate reduction maximum in the Gulf of Maine sediments occurred well within this apparent Fe- reduction region. This indicated that the Fe-reduction zone was quite narrow and that the presence of high concentrations of dissolved Fe in deeper sediments was due to diffusion or diagenetic remobilization. A similar situation probably exists in the Amazon shelf sediments. Aller et al. (1986) measured the rate of dissolution of Fe and found that detectable dissolution rates were restricted to the upper few centimetres of the Fe- containing zone. Therefore, we believe that, like the Gulf of Maine sediments, the Fe- reduction zone in the Amazon shelf sediments is much narrower than the chemical data suggested. Since Aller et al. (1986) reported that the Amazon sediments were relatively non-bioturbated, we conclude that the high concentration of dissolved Fe at depth in those sediments was due to diffusion from the Fe-reduction zone and that insufficient sulphide was produced in the sulphate-reduction zone to remove Fe significantly. The conclusion that the Fe-reduction zone is less extensive vertically than previously reported


does not decrease the importance of Fe reduction. On the contrary, it appears that Fe (and probably Mn) reduction is a significant sedimentary process on the continental shelf and its influence chemically is realized into much deeper sediments.

Acknowledgements

We appreciated the analytical and field assistance of M. Spencer, S. Knollmeyer, G. Jones, R. Chormann, M. Hurd, D. Spader and J. Waugh. J. Christensen kindly provided the shipboard centrifuge, G. Hampson provided the boxcorer and R. Blakemore provided the gas chromatograph. We especially thank D. Anderson for providing short piston cores from Jeffreys Basin. We appreciated the reviews of the manuscript by D. Burdige and an anonymous reviewer. This work was supported by the Ocean Sciences Division of the Natural Science Foundation.

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