Marine Chemistry, 21 (1987) 329-345 329 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

SULFATE REDUCTION IN D E E P COASTAL MARINE SEDIMENTS

HARRY M. EDENBORN*, NORMAN SILVERBERG, ALFONSO MUCCI** and BJORN SUNDBY***

Ddpartement d'Ocdanographie, Universitd du Qudbec ~ Rimouski 310, avenue des Ursulines, Rimouski, Quebec G5L 3A1 (Canada)

(Received October 31, 1986; revision accepted May 13, 1987)

ABSTRACT

Edenborn, H.M., Silverberg, N., Mucci, A. and Sundby, B., 1987. Sulfate reduction in deep coastal marine sediments. Mar. Chem., 21: 329-345.

Sulfate reduction rates in sediments of four deep stat ions in the Saguenay Fjord and the Laurent ian Trough, Gulf of St. Lawrence, are among the lowest reported for the coastal environ- ment. Maximum rates were 0.4-7.0nmolcm-3day -1. The low rates are due to relatively low sedimentat ion rates and continuously low temperatures. Regional differences in both integrated and maximum sulfate reduction rates in the sediment correlate with sediment trap measurements of sedimentat ion rate and organic carbon flux.

Sulfate reduction accounts for the degradation of 5-26% of the estimated downward flux of organic mat ter to these sediments. Unlike the absolute ra te of sulfate reduction, the relative proportion of the carbon flux tha t is degraded via sulfate reduction is not directly correlated with the sedimentat ion rate but is a function of organic mat ter composition, intensi ty of bioturbation, and the abundance of sub-oxic electron acceptors. Thus, the lowest proportion of carbon degrada- t ion via sulfate reduction occurred at a Gulf site, where a combination of low sedimentat ion and bioturbat ion rates allowed a long residence time for organic mat ter near the sediment surface and, in consequence, a low flux of labile carbon into the sulfate reduction zone. The highest proportion was observed at a s ta t ion with a similar organic carbon flux but with higher rates of sedimentat ion and bioturbation. At a third site, with the highest rates of sulfate reduction as well as the highest rates of sedimentat ion and bioturbation, the contr ibut ion of sulfate reduction to organic mat ter degradation was only intermediate. This is a t t r ibutable to the exhaust ion of the supply of pore- water sulfate. In deep coastal environments the proportion of organic mat ter degraded via sulfate reduction can be highly variable both spatially and temporally.

INTRODUCTION

The development of a convenient radiotracer technique for the measurement of in situ bacterial sulfate reduction rates in marine sediments (Jorgensen and

Present addresses: *Oak Ridge Research Insti tute, Research Laboratory, 113 Union Valley Road, Oak Ridge, TN 37830, U.S.A. ** Department of Geological Sciences, McGill University, 3450 Universi ty Street, Montreal , Quebec H3A 2A7, Canada. *** Nether lands Inst i tute for Sea Research, Postbox 59, 1790 AB Den Burg (Texel), The Nether- lands.

0304-4203/87/$03.50 © 1987 Elsevier Science Publishers B.V.

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Fenchel, 1974) has resulted in numerous investigations of the distribution and magnitude of this process world-wide (Jorgensen, 1977, 1982; Nedwell, 1982; Howarth and Giblin, 1983; and others). These studies have confirmed that the quanti ty and quality of organic matter reaching the anoxic zone in sediments largely control the magnitude of sulfate reduction found there (Goldhaber and Kaplan, 1975; Toth and Lerman, 1977; Berner, 1978). Sulfate reduction rates are often highest in shallow coastal marine sediments where high biological productivity results in the deposition of a large quanti ty of organic detritus. Rapid burial of this material and depletion of oxygen and other oxidants lead to the introduction of relatively labile organic matter to the anoxic zone of the sediment, where it can be metabolized by sulfate-reducing and associated fermentative bacteria. In such environments, particularly where ambient tem- peratures can become fairly high, sulfate reduction can account for a large percentage of the total carbon degradation; for example, ~ 47% of the organic carbon reaching the shallow sediments of Limfjorden, Denmark, has been estimated to be mineralized via sulfate reduction (Jorgensen, 1977).

In pelagic waters, the lower biological productivity and long transit times result in a lower flux of organic matter to the bottom sediments. At the slower burial rate, more of the organic material is metabolized by oxidative processes near the sediment-water interface, thus limiting the amount reaching the anaerobic zone. Bender and Heggie (1984) have estimated that only 1% of the downward flux of carbon through the water column is mineralized via sulfate reduction in the sediments of a deep-sea station in the equatorial Pacific.

It follows that sulfate reduction rates in marine sediments tend to decrease with increasing distance from land, and it has been estimated that over 90% of oceanic sulfate reduction occurs within sediments located between the shoreline and 200 m depth (Jorgensen, 1982). Most sulfate reduction measure- ments have been made either within this depth range or in the deep-sea en- vironment. The present paper examines the distribution and magnitude of sulfate reduction rates in sediments of intermediate depth in the Saguenay Fjord and the Laurent ian Trough of the maritime estuary and Gulf of St. Lawrence.

MATERIALS AND METHODS

Sampling

Four sampling stations in the Saguenay Fjord and the Laurent ian Trough of the maritime estuary and Gulf of St. Lawrence were occupied in August 1983 (Fig. 1). Station 23 in the St. Lawrence Estuary was reoccupied in June, August, September and October 1984. Sediment box cores recovered at these stations were placed immediately into a nitrogen-filled glove box, where they were subsampled in intervals of 1 cm or less. Disturbance of the sediment during subsampling was minimized using a method which involved the mech- anical lowering of one side of the core box, rather than the extrusion of the

331

, . , i;i;iir

Fig. 1. Map of the Saguenay Fjord, St. Lawrence Estuary and Gulf of St. Lawrence, showing sampling stations occupied.

sediment core (Edenborn et al., 1986). Two-centimeter thick slices were set aside for X-radiography. Gravity and piston cores were extruded and subsam- pled under nitrogen.

Sedimentation rate measurements

Free-drifting sediment traps, based upon the design of Staresinic et al. (1978), were used to measure sedimentation rates at each sampling site. The traps consist of four cylinders with a total collecting surface area of -~ 0.5 m 2. The traps were placed at a depth well above the sediment resuspension zone and below the euphotic zone; at Station 23, for example, this corresponded to a depth of ~ 150 m (Silverberg et al., 1985). After a deployment time of 9-24 h, trapped particulate matter was collected, washed free of salt and freeze-dried. After the dry weight of the trap material was determined, it was ground to a uniform powder and analyzed for total organic carbon and nitrogen.

Chemical analyses

Sediment porewater for chemical analysis was extracted under nitrogen by centrifugation immediately after subsampling and filtered through 0.45pm Millipore GS filters.

Porewater sulfate concentrations were determined gravimetrically by the precipitation of BaSO4 using a modification of the method of Presley and Claypool (1971); the analytical precision was + 1%.

The t i trat ion alkalinity of the porewaters was determined potentiometric- ally with a dilute HC1 solution to the second equivalence point, corresponding to the neutralization of bicarbonate ions. Detection of the second equivalence

332

point was computed automatical ly by the second derivative method using a Radiometer TTT81 digital t i trator. Reproducibility of these measurements was bet ter than 0.4% using a Na2CO~ solution for standardization of the acid.

Total organic carbon and nitrogen in dried sediment and sediment trap material were determined by the total combustion of samples in a Perkin-Elmer model 249 CHN analyzer. Acetanilide (Sigma Chemical Co.) was used as the standard.

Sediment porosity was calculated based on percent water loss after freeze- drying, assuming intersti t ial water density of 1.03 g cm 3 and sediment density of 2.65 g cm 3 (Berner, 1971).

Sulfate reduction rate measurements

Sulfate reduction rates were measured using the 35SO4 core-inj ection method of J~rgensen (1978a), with minor modifications. Detection of the low sulfate reduction rates found in the St. Lawrence sediments required an increase in both the incubation time at the in situ temperature and in the amount of radio-labeled sulfate added to each sediment sample. Sediments were subsam- pled under nitrogen at designated depths using 5 cm 3 plastic syringes with the distal ends removed. Approximately 4 cm 3 of sediment were sampled and each syringe was plugged with a single-hole rubber stopper sealed with silicone rubber cement. These mini-cores were then transferred to a nitrogen-filled glove bag. A single line source of 10 gl Na235SO4 solution (up to 22/~Ci; Amer- sham Corporation) was injected along the center of each sediment core as the needle was withdrawn. The added sulfate made up a very small percentage (0.007-0.10%) of the total sulfate concentra t ion in the porewater. Triplicate measurements were made at selected depths to evaluate the sulfate reduction rate variabili ty within horizontal sediment sublayers. At other depths, only one rate measurement was made. The mini-cores were sealed in N2-filled con- tainers and were incubated in a water bath in the dark at the in situ tem- perature +0.1 o for 48h. The reactions were stopped by freezing the cores ( - 25°C). Control sediment cores from each depth were also frozen immediately following inoculat ion with 35SO4 to account for sulfate reduction which might occur during the freezing process. Acid-volatile sulfides were distilled from the sediment samples using a digestion system similar to the one described by Hines and Lyons (1982). Each frozen sediment core was added directly to a react ion vessel containing 25ml of deoxygenated distilled water, the vessels were sealed, and 8 ml of concentrated HC1 was added slowly through a rubber stopper by needle and syringe. The contents were bubbled continuously with oxygen-free ni trogen gas for I h. Oxygen was removed from the nitrogen scrub- bing gas by passage through an alkaline pyrogallol solution (Umbreit et al., 1972). The acid-volatile sulfides flushed by the scrubbing gas were precipitated as ZnS in two traps in series, each containing 5 ml of a 3% zinc acetate solution and one drop of an antifoaming agent (Antifoam B; BDH Chemicals). The trapping efficiency of the zinc acetate traps was > 95%. Gels containing the

333

ZnS precipitate were prepared with scintillation cocktail (Scintiverse; Fisher Scientific) and were radioassayed in a Beckman LS5901 liquid scintillation spectrometer. Sulfate reduction rates were calculated according to J~rgensen (1978a). The standard deviation of triplicate samples from the same horizon averaged 28% about the mean, but varied in direct proportion to the rate of reduction. Replicate analyses of samples from homogenized horizon samples yielded an analytical precision of _+ 8% for individual measurements. The sediment was not homogenized routinely before analysis because this treat- ment was found to increase the actual rate measurements by a factor of two.

The possible production of radiolabeled pyrite during the sulfate reduction incubations was also examined. Sediment from 10 cm depth at Station 23 was homogenized under nitrogen and replicate samples were inoculated and treated as described previously. After distillation of the acid-volatile sulfides, the residual sediment in the reaction vessel was washed twice with 250ml of distilled water and centrifuged, dried at 105°C, and ground to a fine powder with mortar and pestle. Radiolabel present in the pyrite fraction was deter- mined using a hot acid/chromium reduction procedure (Howarth and J~rgen- sen, 1984). The efficiency of pyrite extraction was > 95%.

The analysis of labeled sulfate reduction end-products indicated that 61% of reduced sulfate was present as acid-volatile sulfides after 48 h incubation, while the remainder was found as elemental sulfur and pyrite. This assumes that 5% of the S O pool was formed artefactually during the sediment acidification step from acid-volatile sulfides (Howarth and J~rgensen, 1984). Therefore, sulfate reduction rates reported here may be underestimated by a factor of two.

In a separate laboratory experiment, reducing sediment from Station 23 was homogenized and sampled as above under nitrogen atmosphere. Mini-cores were inoculated with radiolabeled sulfate and incubated in triplicate at tem- peratures of 2, 4, 8 and 12°C in the dark for 48 h. Sulfate reduction rates were then determined as described previously.

RESULTS

Study site characteristics

Some important characteristics of the overlying water column at each sam- pling site are presented in Table I. Laurent ian Trough bottom water enters the Gulf directly from the Atlantic Ocean, and the temperature and salinity of this water are almost constant with time (E1-Sabh, 1975). The physical characteris- tics of the intermediate and deep water masses of the Saguenay Fjord vary slightly due to periodic exchanges with the St. Lawrence (Sundby and Loring, 1978).

Strong regional differences in total sedimentation rate, carbon flux to the sediment, and composition of organic matter in the sedimenting material were observed (Table I). Station 17, in the open Gulf of St. Lawrence, has a very low sedimentation rate, and the relatively low C/N ratio of the organic matter

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TABLE I

General characteristics of the sampling sites

Station Location Overlying water Sedimenting material

Depth Temp. Sal. Total flux Carbon flux %C C/N (m) (°C) (%0) (mgcm 2day 1) (#gcm 2day 1)

17 Gulf of St. 355 4.5 34.6 0.08 9.1 11.5 7 Lawrence

23 St. Lawrence 335 4.5 34.6 0.21 11.5 5.4 9 Estuary

S-2 Saguenay 266 1.0 31.0 0.55 25.7 4.7 13 Fjord basin

S-1 Saguenay 130 1.5 30.0 0.61 53.2 8.7 19 Fjord (head)

r e a c h i n g the sed iment ind ica tes t h a t it is m a in ly m a r i n e ma te r i a l of p l a n k t o n i c origin. S t a t i on S-l, a t the head of the S a g u e n a y Fjord, r ep resen t s the o the r ex t r eme wi th in the inves t iga t ed region; the s ed imen ta t i on r a t e is m u c h h igher and the sed imen t ing m a t e r i a l is m a i n l y of t e r r i genous origin. Sed iment accu- m u l a t i o n r a t e s a t the head of the f jord a re very h igh ( 7 c m a 1: Smi th and Wal ton , 1980) but dec rease rap id ly s eaward a long the s u b m a r i n e slope on which S t a t i on S-1 is located. Sediment t r ap m e a s u r e m e n t s made a t this si te p robab ly u n d e r e s t i m a t e the ne t a c c u m u l a t i o n of sed iment because of the addi- t iona l inpu t of sed iment v ia per iodic s lumping and o the r nea r -bo t t om t r a n s p o r t processes . The observed seaward decrease in s ed imen ta t i on r a t e s ove r the en t i re s tudy area , m e a s u r e d wi th the t rap , agrees well wi th o the r r a tes based on excess 21°pb profi les in the sed iments (Si lverberg et al., 1986).

Direct sulfate reduction rate measurements

The dep th profi les of d i rec t ly m eas u red su l fa te r educ t ion ra tes and pore- w a t e r su l fa te c o n c e n t r a t i o n s in sed iments col lected a t S ta t ions 17, 23, S-1 and S-2 are shown in Fig. 2. Sul fa te r educ t ion ra tes were lowest in the top 2-3 cm a t each s t a t i on and inc reased rap id ly wi th depth over the nex t severa l cen- t imeters . M a x i m u m sul fa te r educ t ion ra tes observed in these sed iments occur- red be tween 4 and 17 cm dep th and r anged be tween 0.4 and 7.2 nmol cm -3 day 1. The su l fa te r educ t ion r a t e profiles were essent ia l ly cont inuous , except a t s t a t i on S-l, where a sha rp drop in the sul fa te reduc t ion ra t e was observed be tween 10 and 14 cm depth. This drop coincided wi th the p resence of a c lay l ayer of low ca rbon conten t , d iscussed in g rea t e r deta i l in a subsequen t sect ion. Some decrease in the p o r e w a t e r su l fa te concen t r a t i ons wi th depth was obser- ved in mos t cores; the in t ens i ty of the p o r e w a t e r su l fa te g rad ien t r ough ly pa ra l l e l ed the reg iona l t rend in the m a g n i t u d e of the measu red su l fa te reduc- t ion ra tes , which were h ighes t a t S ta t ion S-1 and lowest a t S ta t ion 17. At S ta t ion S-2, however , the su l fa te c o n c e n t r a t i o n s did not decrease m e a s u r e a b l y wi th depth despi te the m o d e r a t e su l fa te r educ t ion ac t iv i ty measured .

A

E

t -

o2.O,

I0

20

3O

0 ~ ,

I0

2O

50

o29,

I0

20

3C

Sulfate (mM) , , , 2 5 , ,.- 30 !

{ , , , 2 , 5 , , 30

, , , 25 50 ' I ~ i , , l

Sulfate reduction (n tool cm-3 d-O

0 2 i

k 0 2

0 I 2

17

23

S-2

335

0 20 25 50

I0

2C

30

0 2 4 6

Fig. 2, Sediment profiles ofporewater sulfate and sulfate reduction rates in box cores from Stat ion 17 in the Gulf of St. Lawrence, Stat ion 23 in the St. Lawrence Estuary, Stat ion S-2 in a deep basin of the Saguenay Fjord, and Stat ion S-1 in the upper Saguenay Fjord. Error bars at selected depths for the sulfate reduction profiles indicate the s tandard deviation of the mean of tr iplicate measure. ments. The dashed lines indicate the location of a clay deposit between 8 and 14 cm depth at Stat ion S-1.

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T A B L E II

C o m p a r i s o n of su l fa te r educ t ion r a t e s and o rgan ic ca rbon i npu t to s ed imen t s a t t he four s amp l ing s i tes

S t a t i on In t eg ra t e d su l fa te r educ t ion ra te (nmolSO4cm-3 day 1)

Organ i c ca rbon flux to s ed imen t ( n m o l C c m 3day 1)

Organ ic ca rbon degraded v ia su l fa te r educ t ion (nmol C c m - 3 d a y - l )

0-30 cm ~ > 30 cm b Tota l To ta l ¢ % of flux

17 4 13 17 760 34 5 23 25 103 128 1000 256 26 S-2 32 2150 (64) d (3) d S-1 110 120 230 4420 460 10

aFrom di rec t r ad io t r ace r method. bFrom d iagene t i c mode l ing and su l fa te profiles in Fig. 3. CAssuming a s to i ch iome t r i c ra t io of 2 mol ca rbon minera l ized for each mole of su l fa te reduced. dCalcu la ted for t he uppe r 30cm only.

The integrated rates of sulfate reduction over the top 30 cm of the cores are presented in Table II. The range of values (4-110 nmol cm -3 day -1) indicates large regional differences in the intensity of sulfate reduction within the St. Lawrence system.

The possible influence of seasonal variations in sedimentation rate on ab- solute and integrated sulfate reduction rates was also examined at Station 23. Sulfate reduction rate profiles were determined in June, August, September and October 1984; during this time, measured sedimentation rates decreased by a factor of four (Silverberg et al., 1985). However, the sulfate reduction rate profiles measured in these months were all very similar, and only slight varia- tions in the depth at which sulfate reduction was first detected and in the initial slope in the rate curve were observed. The integrated rates measured in the upper 30 cm over this period (16.9, 21.1, 18.7 and 18.8 nmol cm -3 day 1) were not significantly different statistically and indicate that seasonal differences in sedimentation rates in the St. Lawrence estuary are probably dampened in the uppermost sediments and do not greatly influence sulfate reduction rates in deeper sediments (Silverberg et al., 1985).

Total integrated sulfate reduction rates

The direct radiotracer measurements described above were limited to the upper 30cm of the sediment recovered in box cores, but sulfate reduction activity and high porewater sulfate concentrat ions were still detectable at the base of all of the cores. Indirect estimates of sulfate reduction deep in the sediments were obtained as follows. The concentrat ion gradients of porewater sulfate in gravity cores obtained at Stations S-1 and 23, and in a piston core obtained at Station 17, (Fig. 3, note the different scale) exhibited the expected

337

0

2O

g 4o

~ 6o

123

I00

P20

200

3 -r 4o0 i..- n h i a

60O

800

Sulfate (mM) 15 20 25

Sulfate (mM) I0 20 30 0

2O

~ 4o ;12 ~ 60

c~ 80

I00

Sulfate (mM) I0 20

Fig. 3. P o r e w a t e r su l fa te c o n c e n t r a t i o n g r a d i e n t s in a 8 m long p i s ton core t a k e n a t S t a t i on 17, and

in g r a v i t y cores t a k e n a t S t a t i ons 23 and S-1. G r a v i t y core profi les a re not co r rec ted for a co re - shor t en ing fac to r of ~ 2.

decrease and downward concavity with depth appropriate for the estimation of sulfate reduction. We used a one-dimensional diffusion-advection-reaction model (Berner, 1964, 1971), assuming steady-state conditions and first-order kinetics for the degradation of metabolizable organic matter

D s ( d 2 C / d x 2) - w ( d C / d x ) - f ( x ) = 0 (1)

where D s is the whole sediment diffusion coefficient for sulfate, C the sulfate concentration, w the sedimentation rate, x is depth and f ( x ) = a exp(- b x ) , a

and b being constants. This model has been shown by Jorgensen (1978b) to give much lower rates than those measured using the radiotracer technique in the upper part of the sediment. For that reason we have only used the model to calculate the integrated sulfate reduction below 30 cm depth.

The rate function for sulfate reduction with depth was calculated from the sulfate concentration profiles and estimates of the sedimentation rate from sediment trap measurements. Gravity core sulfate gradients were also correc- ted for core shortening (Lebel et al., 1982). The resulting integrated sulfate reduction rates (below 30 cm) are shown in Table II. Because sulfate reduction often persists to a considerable depth in the sediment, the total integrated sulfate reduction rates were several times greater than those measured directly in the top 30 cm alone, but the strong regional differences observed in the upper sediment were maintained.

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Other measured parameters

Bottom temperatures at the sample sites in the St. Lawrence system were between 1 and 4°C, and water depth varied between 130 and 355m (Table I). These small differences in temperature and hydrostatic pressure would not be expected to result in the observed regional variations in sulfate reduction intensity (Goldhaber and Kaplan, 1975; Jorgensen, 1977; Abdollahi and Ned- well, 1979). However, sulfate reduction activity was enhanced significantly in anoxic sediments from Station 23 when incubated in the laboratory at tem- peratures up to 10°C above the in situ temperature (data not shown). The apparent temperature coefficient (Q10) of 3.2 determined for sulfate reduction in these sediments is similar to values of 3.4, 3.5 and 3.9 reported for other coastal environments (Jorgensen, 1977; Abdollahi and Nedwell, 1979; Vosjan, 1975--- cited by Jorgensen). The continually low ambient temperatures in the St. Lawrence sediments thus preclude the enhancement of sulfate reduction rates due to the seasonal increases in temperature common to shallower coastal environments, and are at least partly responsible for the extremely low rates measured.

The bottom sediments at each station consisted of undifferentiated silty clays, with the exception of Station S-1 in the upper Saguenay Fjord. At this station, the sediment contained a little more sand, and occasional layers of gray clay were observed at depth. These layers represent relict post-glacial marine clays that are periodically flushed from the bed of the Saguenay River into the head of the fjord during periods of high river discharge (Schafer et al., 1980). The carbon content of these old clays (1-1.5%) is considerably lower than the 2-3% carbon content of recent silty clays. The sharp drop in the sulfate reduction rate observed at Station S-1 coincides with such a clay layer, and was probably limited by its low carbon content.

Figure 4a~l shows X-radiographs of 12-mm-thick vertical slices of box cores taken at the four sampling sites. The clay layer at Station S-1 mentioned above is clearly visible. Bioturbational structures were most abundant at Station S-1 and decreased progressively in number towards Station 17. This trend of de- creasing influence of bioturbation in the seaward direction has been noted previously (Ouellet, 1982; Sundby and Silverberg, 1985; Silverberg et al., 1986).

The increasing concentration of alkalinity with depth in the sediment pore- waters at most stations (Fig. 5) reflects the cumulative buildup of the end- products of organic matter degradation. These profiles generally paralleled the trend of decreasing sulfate reduction rates between the Saguenay Fjord and the open Gulf of St. Lawrence. Station S-2 was an exception and showed very little increase in alkalinity with depth, despite moderately high sulfate reduction rates (note that Table II includes only the integrated rates over the top 30 cm of the sediment since long cores were not available from this site). At the same station, only a very weak porewater sulfate gradient was observed in the upper 30cm. Bioirrigation might be responsible for the maintenance of the high sulfate and low alkalinity concentrations at this station.

339

Fig. 4. X-radiographs of box cores taken at the four sampling sites: (a) St. 17, note persistence of some layered structure at depth and limited number of bioturbational features; (b) St. 23; (c) St. S-2; (d) St. S-1. The bar represents 5cm.

DISCUSSION

Sulfate reduction rates

Su l f a t e r e d u c t i o n r a t e s m e a s u r e d in the s e d i m e n t s of t he S a g u e n a y F j o r d a n d L a u r e n t i a n T r o u g h a re v e r y low c o m p a r e d w i t h those t h a t h a v e b e e n

Titration Alkalinity (mM) 3.0 4.5 6 .0 7.5

- - , . , . 0 I

IO

v J=

3 0

340

Fig. 5. Porewater t i t ra t ion alkal ini ty gradients in box cores taken at the four sampling sites.

TABLE III

Comparison of maximum sulfate reduction rates measured in selected coastal marine sediments

Location Approx. depth Maximum sulfate Reference of overlying reduction water column (m) (nmol cm 3 day- 1)

Shallow Danish Fjord 4 12 3000 J~rgensen (1977) Saanich Inlet, 225 888 Ahmed et al. (1984)

Brit ish Columbia Shallow marine 1 498 Hines and Lyons

embayment, Bermuda (1982) Pacific Coast, 60 477 Rowe and Howarth

Peru (1985) Skagerrak, 200 52 Iversen and

Denmark J~rgensen (1985) Pacific Coast, 245 31 Rowe and Howarth

Peru (1985) Shallow estuary, 26 19 Indrebo et al.

Norway (1979) Kattegat, 65 16 Iversen and

Denmark Jorgensen (1985) Saguenay Fjord, 130 7 This study

Canada Laurent ian Trough, 335 1.9 This study

St. Lawrence Estuary, Canada

Barents Sea, 405 1.4 Dahlbeck et al. near Svabard (1982)

Laurent ian Trough, 355 0.4 This study Gulf of St. Lawrence Canada

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measured in other coastal and shallow sea environments. Table III includes only the comparison of the maximum rates reported, since the flux of organic carbon, the C/N ratio of the sedimenting material, the temperature, and inte- grated sulfate reduction rates were not reported frequently enough in the li terature to provide a meaningful comparison with the present study. In general, higher sulfate reduction rates have been measured at sites where the sedimentation rate was higher, the temperature was higher, or the sediments contained more organic matter.

Sulfate reduction activities in the St. Lawrence sediments are still much higher than have been measured in the deep sea. For example, an integrated sulfate reduction rate of 0.19 nmol cm 3 day 1 was reported for MANOP site M sediments in the eastern equatorial Pacific (~-3150 m depth; Bender and Heg- gie, 1984). This value is about 100 times lower than the integrated sulfate reduction rate measured at Station 17 in the Gulf of St. Lawrence (Table II). Thus, sulfate reduction rates in St. Lawrence sediments are intermediate to those found in shallow coastal environments and in the deep sea.

The general trend of decreasing activity of sulfate reduction in marine sediments as water depth and distance from shore increase (Jorgensen, 1982) was also observed within the St. Lawrence system (Table II). Several variables that influence the intensity and distribution of sulfate reduction rates in marine sediments, such as sedimentation rate, bioturbation by benthic organ- isms, temperature, and hydrostatic pressure (Goldhaber and Kaplan, 1975) can all change dramatically in the transition from a coastal to a deep-sea environ- ment. Within the St. Lawrence region bottom water temperatures are low but virtually constant, and the differences in hydrostatic pressure at the sampled sites are small. Temperature and pressure therefore probably have a negligible influence on the observed regional variability of sulfate reduction rates. How- ever, the regional distribution of sulfate reduction in St. Lawrence sediments is well-correlated with the total sedimentation rate and the total carbon flux to the sediments (Table II). Similar observations have been made by other inves- tigators elsewhere (Jorgensen, 1982; Iversen and Jorgensen, 1985). When sedi- mentation rates of organic matter are higher, labile, metabolizable organic material undergoes a shorter period of oxic and suboxic degradation in the upper sediment layer and more of it is buried within the anoxic zone, where it supports higher rates of sulfate reduction. Bioturbation by benthic organisms may influence any direct relationship between sedimentation rate and sulfate reduction activity by mixing more labile organic matter from the surface sediment into the anoxic zone. Since, in the St. Lawrence, the intensity of bioturbation roughly increases as the sedimentation rate increases, the biolo- gical mixing of more labile organic matter from the surface sediment into the anoxic zone enhances the regional trend in sulfate reduction rates. The region- ally variable composition of the sedimenting material, which in the present study ranged from a terrigenous origin in the Saguenay Fjord to an almost entirely marine origin within the Gulf of St. Lawrence, may also influence the pattern of sulfate reduction. For the same rate of input, organic matter that is

342

difficult to degrade is more likely to survive transit through the oxic-suboxic zone of the sediment than more readily degradable material. Finally, the inventory of Mn and Fe oxyhydroxides in the sub-oxic zone of the sediment also varies regionally in the St. Lawrence (the levels increase in the landward direction (Sundby et al., 1981; Bouchard, 1983)). The reduction of these oxides is also associated with the degradation of organic matter (Froelich et al., 1979) and may, where the sub-oxic zone is thicker, have some influence on the quantity of labile organic matter which reaches the sulfate reducing zone. Fe-oxide reduction is, for example, a major diagenetic process off the Amazon (Aller et al., 1987). Nevertheless, despite these potentially tempering influen- ces, it is the sedimentation rate of organic matter within the St. Lawrence system which most strongly determines the sulfate reduction rates in the bottom sediments.

Relative importance of sulfate reduction in organic matter degradation

The present data may be used to examine the importance of sulfate reduction relative to the reduction of other electron acceptors in the degradation of sedimenting organic matter within the St. Lawrence environment. In Table II, the theoretical quanti ty of organic matter mineralized via sulfate reduction is compared with the downward flux of organic matter measured in sediment traps at each station. In contrast to the integrated sulfate reduction rate, the relative contribution of sulfate reduction to organic matter degradation at each station was not related to the sedimentation rate. The greatest contribu- tion was found at a location where intermediate sedimentation rates were measured. We think this may be explained in the following way. At Station 17 in the Gulf of St. Lawrence, the low sedimentation rate, the low bioturbation rate and the high C/N ratio of the sedimenting organic matter favor the degradation of most of the labile carbon by oxic and suboxic processes before burial into the anoxic sediments. Although the porewater sulfate concentra- tion gradient indicates that sulfate reduction continues to a great depth in the sediment, the total contribution of this process to organic matter degradation at this station is still very small, and accounts for only 5% of the downward carbon flux, similar to the deep-sea situation.

At Station S-1 in the upper Saguenay Fjord, in contrast, high sedimentation rates result in higher sulfate reduction rates near the sediment-water inter- face. This activity exhausts the porewater sulfate concentration more rapidly with depth than at the other stations and limits the potential total contribution of sulfate reduction to organic matter degradation (Berner, 1972).

The greatest relative contribution to organic matter degradation by sulfate reduction was observed at Station 23. Here moderate rates of sedimentation, carbon flux and bioturbation combine with organic matter quality to produce conditions where ~ 25% of the organic matter input to the sediment can be mineralized via sulfate reduction. The conditions at Station 23 are thus very different from those at Station 17 in that the rates of sedimentation and

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bioturbational downmixing of organic matter are sufficiently high that the mineralization capacity of oxidants other than sulfate is exceeded or bypassed.

The integrated sulfate reduction rate does not vary significantly with season at this station, but there is a strong seasonal variation in sedimentation rate, flux of organic carbon and quality of the organic matter in the sedimenting material (Silverberg et al., 1985, 1986). Thus, while the absolute rate of carbon mineralization via sulfate reduction is constant, the relative importance of sulfate reduction in mineralizing the incoming carbon is seasonally variable.

CONCLUSIONS

Absolute sulfate reduction rates in the sediments of the St. Lawrence region were extremely low when compared to other coastal environments, but were still at least 100 times greater than reported for the deep sea. These low rates are due to the moderately low rates of organic matter input and the low ambient temperatures. In environments intermediate to shallow coastal re- gions and the deep sea, sulfate reduction must be evaluated over the entire sediment column to accurately assess its relative contribution to total organic matter degradation. In the St. Lawrence region, integrated sulfate reduction accounts for the degradation of between 5 and 26% of the organic matter input to the bottom sediments. Sulfate reduction was most important to the overall degradation of sedimenting organic matter at a station where the sedimenta- tion rate was intermediate within this environment. At high sedimentation rates, sulfate reduction is limited with depth by the depletion of porewater sulfate. At extremely low sedimentation rates, organic matter degradation via alternative electron acceptors is virtually complete. The relative importance of sulfate reduction to total organic matter degradation in the deep coastal environment depends strongly on the interaction between the rate of sedi- mentation, the intensity of bioturbation, and the quality of the organic matter reaching the sediment surface.

ACKNOWLEDGMENTS

We thank the captains and the crews of CSS "Dawson" and CSS "Louis M. Lauzier" for their assistance in the collection of samples. Nelson Belzile, Guy Chateauneuf, Andr~e Gendron, Bruno Marchand and Gemma Marquis provided excellent technical assistance. This research was funded in part by team grants GO562 and GO563 from the Natural Sciences and Engineering Research Council of Canada (NSERC), FCAC Qfiebec Ministry of Education grant 86-EQ-1009, and NSERC grants E5790 and A9177 to B.S.

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