Some important characteristics of the overlying water column at each sam- ... Walton, 1980) but decrease rapidly seaward along the submarine slope on.
Marine Chemistry, 21 (1987) 329-345 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
329
S U L F A T E REDUCTION IN D E E P COASTAL MARINE S E D I M E N T S
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 stations in the Saguenay Fjord and the L a u r e n t i a n Trough, Gulf of St. Lawrence, are among the lowest reported for the coastal environment. Maximum rates were 0.4-7.0nmolcm-3day -1. The low rates are due to relatively low sedimentation rates and continuously low temperatures. Regional differences in both integrated and maximum sulfate reduction rates in the sediment correlate with sediment trap measurements of sedimentation rate and organic carbon flux. Sulfate reduction accounts for the degradation of 5-26% of the estimated downward flux of organic matter to these sediments. Unlike the absolute rate of sulfate reduction, the relative proportion of the carbon flux t h a t is degraded via sulfate reduction is not directly correlated with the sedimentation rate but is a function of organic matter composition, intensity of bioturbation, and the abundance of sub-oxic electron acceptors. Thus, the lowest proportion of carbon degradation via sulfate reduction occurred at a Gulf site, where a combination of low sedimentation and bioturbation rates allowed a long residence time for organic matter 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 station with a similar organic carbon flux but with higher rates of sedimentation and bioturbation. At a third site, with the highest rates of sulfate reduction as well as the highest rates of sedimentation and bioturbation, the contribution of sulfate reduction to organic matter degradation was only intermediate. This is attributable to the exhaustion of the supply of porewater sulfate. In deep coastal environments the proportion of organic matter 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 Institute, Research Laboratory, 113 Union Valley Road, Oak Ridge, TN 37830, U.S.A. ** Department of Geological Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada. *** Netherlands Institute for Sea Research, Postbox 59, 1790 AB Den Burg (Texel), The Netherlands.
0304-4203/87/$03.50
© 1987 Elsevier Science Publishers B.V.
330 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 quantity 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 quantity 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 temperatures 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 t h a t 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 measurements have been made either within this depth range or in the deep-sea environment. The present paper examines the distribution and magnitude of sulfate reduction rates in sediments of intermediate depth in the Saguenay Fjord and the Laurentian Trough of the maritime estuary and Gulf of St. Lawrence. MATERIALSAND METHODS
Sampling Four sampling stations in the Saguenay Fjord and the Laurentian 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 mechanical 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 subsampled 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 titration alkalinity of the porewaters was determined potentiometrically 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 automatically by the second derivative method using a Radiometer TTT81 digital titrator. Reproducibility of these measurements was better th an 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 freezedrying, assuming interstitial 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 t em perat ure and in the amount of radio-labeled sulfate added to each sediment sample. Sediments were subsampled 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; Amersham 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 c o n c e n t r a t i o n in the porewater. Triplicate measurements were made at selected depths to evaluate the sulfate reduction rate variability within horizontal sediment sublayers. At other depths, only one rate measurement was made. The mini-cores were sealed in N2-filled containers and were incubated in a water bath in the dark at the in situ temperature +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 inoculation 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 reaction vessel containing 25ml of deoxygenated distilled water, the vessels were sealed, and 8 ml of c onc e nt r at ed HC1 was added slowly through a rubber stopper by needle and syringe. The contents were bubbled continuously with oxygen-free nitrogen gas for I h. Oxygen was removed from the nitrogen scrubbing gas by passage t hr ough 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 treatment 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 determined using a hot acid/chromium reduction procedure (Howarth and J~rgensen, 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 SOpool 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 temperatures 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 sampling site are presented in Table I. Laurentian 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 characteristics 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
334 TABLE I General characteristics of the sampling sites Station Location
17 23 S-2 S-1
Gulf of St. Lawrence St. Lawrence Estuary Saguenay Fjord basin Saguenay Fjord (head)
Overlying water
Sedimenting material
Depth (m)
Temp. (°C)
Sal. Total flux Carbon flux %C (%0) (mgcm 2day 1) (#gcm 2day 1)
C/N
355
4.5
34.6 0.08
9.1
11.5
7
335
4.5
34.6 0.21
11.5
5.4
9
266
1.0
31.0 0.55
25.7
4.7
13
130
1.5
30.0 0.61
53.2
8.7
19
r e a c h i n g t h e s e d i m e n t i n d i c a t e s t h a t it is m a i n l y m a r i n e m a t e r i a l of p l a n k t o n i c origin. S t a t i o n S-l, at the h e a d of t h e S a g u e n a y Fjord, r e p r e s e n t s t h e o t h e r e x t r e m e w i t h i n t h e i n v e s t i g a t e d region; the s e d i m e n t a t i o n r a t e is m u c h h i g h e r a n d t h e s e d i m e n t i n g m a t e r i a l is m a i n l y of t e r r i g e n o u s origin. S e d i m e n t accum u l a t i o n r a t e s at t h e h e a d of the fjord a r e v e r y h i g h ( 7 c m a 1: S m i t h and W a l t o n , 1980) b u t d e c r e a s e r a p i d l y s e a w a r d a l o n g the s u b m a r i n e slope on w h i c h S t a t i o n S-1 is located. S e d i m e n t t r a p m e a s u r e m e n t s m a d e a t this site p r o b a b l y u n d e r e s t i m a t e the n e t a c c u m u l a t i o n of s e d i m e n t b e c a u s e of t h e addit i o n a l i n p u t of s e d i m e n t v i a periodic s l u m p i n g a n d o t h e r n e a r - b o t t o m t r a n s p o r t processes. T h e o b s e r v e d s e a w a r d d e c r e a s e in s e d i m e n t a t i o n r a t e s o v e r the e n t i r e s t u d y area, m e a s u r e d w i t h t h e t r a p , a g r e e s well w i t h o t h e r r a t e s b a s e d on excess 21°pb profiles in the s e d i m e n t s ( S i l v e r b e r g et al., 1986). Direct sulfate reduction rate m e a s u r e m e n t s T h e d e p t h profiles of d i r e c t l y m e a s u r e d s u l f a t e r e d u c t i o n r a t e s a n d porew a t e r s u l f a t e c o n c e n t r a t i o n s in s e d i m e n t s collected at S t a t i o n s 17, 23, S-1 a n d S-2 a r e s h o w n in Fig. 2. S u l f a t e r e d u c t i o n r a t e s w e r e lowest in the top 2-3 cm a t e a c h s t a t i o n a n d i n c r e a s e d r a p i d l y w i t h d e p t h o v e r the n e x t s e v e r a l cent i m e t e r s . M a x i m u m sulfate r e d u c t i o n r a t e s o b s e r v e d in t h e s e s e d i m e n t s occurred b e t w e e n 4 a n d 17 cm d e p t h and r a n g e d b e t w e e n 0.4 a n d 7.2 n m o l cm -3 d a y 1. T h e s u l f a t e r e d u c t i o n r a t e profiles w e r e e s s e n t i a l l y c o n t i n u o u s , except at s t a t i o n S-l, w h e r e a s h a r p d r o p in t h e sulfate r e d u c t i o n r a t e was o b s e r v e d b e t w e e n 10 a n d 14 cm depth. This d r o p coincided w i t h t h e p r e s e n c e of a c l a y l a y e r of low c a r b o n c o n t e n t , discussed in g r e a t e r detail in a s u b s e q u e n t section. S o m e d e c r e a s e in the p o r e w a t e r sulfate c o n c e n t r a t i o n s w i t h d e p t h was observed in m o s t cores; t h e i n t e n s i t y of the p o r e w a t e r s u l f a t e g r a d i e n t r o u g h l y p a r a l l e l e d the r e g i o n a l t r e n d in the m a g n i t u d e of the m e a s u r e d s u l f a t e reduction rates, w h i c h w e r e h i g h e s t a t S t a t i o n S-1 a n d lowest a t S t a t i o n 17. At S t a t i o n S-2, h o w e v e r , t h e s u l f a t e c o n c e n t r a t i o n s did n o t d e c r e a s e m e a s u r e a b l y w i t h d e p t h despite the m o d e r a t e s u l f a t e r e d u c t i o n a c t i v i t y m e a s u r e d .
335
!
Sulfate (mM) , , ,25, ,.-
o2.O, I0
20
30
0
{
3O ,
0~,
,
, 2,5,
30
,
Sulfate reduction (n tool cm-3 d-O 2i
k
17
0
2
I0
2O 23
A
E 50 t-
o29,
,
, , 25 'I~
i
,
,
50 l
0
I
2
I0
20 S-2 3C 0 20
25
50
0
2
4
6
I0
2C
30 Fig. 2, Sediment profiles ofporewater sulfate and sulfate reduction rates in box cores from Station 17 in the Gulf of St. Lawrence, Station 23 in the St. Lawrence Estuary, Station S-2 in a deep basin of the Saguenay Fjord, and Station S-1 in the upper Saguenay Fjord. Error bars at selected depths for the sulfate reduction profiles indicate the standard deviation of the mean of triplicate measure. ments. The dashed lines indicate the location of a clay deposit between 8 and 14 cm depth at Station S-1.
336 T A B L E II C o m p a r i s o n of s u l f a t e r e d u c t i o n r a t e s a n d o r g a n i c c a r b o n i n p u t to s e d i m e n t s at t h e four s a m p l i n g sites Station
Integrated sulfate reduction rate
O r g a n i c c a r b o n flux to s e d i m e n t ( n m o l C c m 3day 1)
(nmolSO4cm-3day 1)
17 23 S-2 S-1
0-30 cm ~
> 30 cm b
Total
4 25 32 110
13 103
17 128
120
230
760 1000 2150 4420
Organic carbon degraded via sulfate reduction (nmol C c m - 3 d a y - l ) Total ¢
% of flux
34 256 (64) d 460
5 26 (3)d 10
aFrom direct r a d i o t r a c e r m e t h o d . bFrom d i a g e n e t i c m o d e l i n g a n d s u l f a t e profiles in Fig. 3. CAssuming a s t o i c h i o m e t r i c r a t i o of 2 mol c a r b o n m i n e r a l i z e d for e a c h mole of s u l f a t e reduced. d C a l c u l a t e d for t h e u p p e r 3 0 c m 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 absolute 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 variations 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 t hat 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 r a d i o t r a c e r measurements described above were limited to the upper 30cm of the sediment recovered in box cores, but sulfate reduction activity and high porewater sulfate concentrations 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 concent rat i on 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 Sulfate (mM) 15
20
25
200
3-r 4o0 i..n hi a
60O 800 Sulfate (mM) 0
I0
20
30
0
2O
2O
g 4o
~ 4o
~
~
Sulfate (mM) I0 20
;12
6o
60
c~ 80 123
I00
I00 P20
Fig. 3. P o r e w a t e r s u l f a t e 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 l o n g p i s t o n c ore t a k e n a t S t a t i o n 17, a n d in g r a v i t y cores t a k e n a t S t a t i o n s 23 a n d S-1. G r a v i t y c ore profiles a r e n o t c o r r e c t e d for a c o r e - s h o r t e n i n g f a c t o 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(dC/dx)
-
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 e x p ( - 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 corrected 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.
338 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 Nedwell, 1979). However, sulfate reduction activity was enhanced significantly in anoxic sediments from Station 23 when incubated in the laboratory at temperatures 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 decreasing 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 porewaters at most stations (Fig. 5) reflects the cumulative buildup of the endproducts 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 Sulfate r e d u c t i o n rates m e a s u r e d in the s e d i m e n t s of the S a g u e n a y Fjord and Laurentian Trough are very low compared with those that have been
340 Titration Alkalinity (mM) 0I
- -
3.0
4.5
,
.
6.0 ,
7.5 .
IO v J=
30
Fig. 5. Porewater titration alkalinity gradients in box cores t a k e n at the four sampling sites.
TABLE III Comparison of maximum sulfate reduction rates measured in selected coastal marine sediments Location
Approx. depth of overlying water column (m)
Maximum sulfate reduction (nmol cm 3day- 1)
Reference
Shallow Danish Fjord Saanich Inlet, British Columbia Shallow marine embayment, Bermuda Pacific Coast, Peru Skagerrak, Denmark Pacific Coast, Peru Shallow estuary, Norway Kattegat, Denmark Saguenay Fjord, Canada Laurentian Trough, St. Lawrence Estuary, Canada Barents Sea, near Svabard L a u r e n t i a n Trough, Gulf of St. Lawrence Canada
4 12 225
3000 888
J~rgensen (1977) Ahmed et al. (1984)
1
498
60
477
200
52
245
31
26
19
65
16
130
7
Hines and Lyons (1982) Rowe and Howarth (1985) Iversen and J~rgensen (1985) Rowe and Howarth (1985) Indrebo et al. (1979) Iversen and Jorgensen (1985) This study
335
1.9
This study
405
1.4
355
0.4
Dahlbeck et al. (1982) This study
341
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 integrated sulfate reduction rates were not reported frequently enough in the literature 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 3day 1was reported for MANOP site M sediments in the eastern equatorial Pacific (~-3150 m depth; Bender and Heggie, 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 organisms, temperature, and hydrostatic pressure (Goldhaber and Kaplan, 1975) can all change dramatically in the transition from a coastal to a deep-sea environment. 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. However, 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 investigators elsewhere (Jorgensen, 1982; Iversen and Jorgensen, 1985). When sedimentation 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 biological mixing of more labile organic matter from the surface sediment into the anoxic zone enhances the regional trend in sulfate reduction rates. The regionally 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 influences, 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 quantity 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 contribution 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 concentration 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 interface. 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
343 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 regions 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 sedimentation 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 sedimentation, the intensity of bioturbation, and the quality of the organic matter reaching the sediment surface. ACKNOWLEDGMENTS We t h a n k 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. REFERENCES Abdollahi, H. and Nedwell, D.B., 1979. Seasonal temperature as a factor influencingbacterial sulfate reduction in a saltmarsh sediment. Microb. Ecol., 5: 73-79. Ahmed, S.I., King, S.L. and Clayton Jr., J.R., 1984. Organic matter diagenesis in the anoxic
344 sediments of Saanich Inlet, British Columbia, Canada: a case for highly evolved community interactions. Mar. Chem., 14: 23~252. Aller, R.C., Mackin, J.E. and Cox Jr., R.T., 1987. Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe-reduction and implications for the genesis of oolitic ironstones. Continental Shelf Res., in press. Bender, M.L. and Heggie, D.T., 1984. Fate of organic carbon reaching the sea floor: a status report. Geochim. Cosmochim. Acta, 48: 977-985. Berner, R.A., 1964. An idealized model of dissolved sulfate distribution in recent sediments. Geochim. Cosmochim. Acta, 28:1497 1503. Berner, R.A., 1971. Principles of Chemical Sedimentology. McGraw-Hill, New York, 240pp. Berner, R.A. 1972. Sulfate reduction and the sulfur budget. In: D. Dyrssen and D. J a g n e r (Editors), The Changing Chemistry of the Oceans. Almgren & Wiksell, Stockholm, pp. 88-96. Berner, R.A., 1978. Sulfate reduction and the rate of deposition of marine sediments. Earth. Planet. Sci. Lett., 37: 492-498. Bouchard, G., 1983. Variation des parametres biogeochimiques dans les sediments du Chenal Laurentien. M.Sc. Thesis, Universit~ du Quebec ~ Rimouski (Canada), 161 pp. Dahlbeck, B., Gunnarson, L.A., Hermanson, M. and Kjelleberg, S., 1982. Microbial investigations of surface microlayers, water column, ice and sediment in the Arctic Ocean. Mar. Ecol. Progr. Ser., 9:101 109. Edenborn, H.M., Mucci, A., Belzile, N., Lebel, J., Silverberg, N. and Sundby, B., 1986. A glove box for the fine-scale subsampling of sediment boxcores. Sedimentology, 33:147 150. E1-Sabh, M.I., 1975. Transport and currents in the Gulf of St. Lawrence. Bedford Inst. Oceanogr. Rep. Ser. BI-R-75-9, 180pp. Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, C., Dauphin, P., Hammond, D., Hartmann, B. and Maynard, V., 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim. Acta, 43: 1075-1090. Goldhaber, M.B. and Kaplan, I.R., 1975. Controls and consequences of sulfate reduction rates in recent marine sediments. Soil Sci., 119:42 55. Hines, M.E. and Lyons, W.B., 1982. Biogeochemistry of nearshore Bermuda sediments. I. Sulfate reduction rates and n u t r i e n t generation. Mar. Ecol. Progr. Ser., 8:87 94. Howarth, R.W. and Giblin, A.E., 1983. Sulfate reduction in the salt marshes at Sapelo Island, Georgia. Limnol. Oceanogr., 28: 70-80. Howarth, R.W. and Jorgensen, B.B., 1984. Formation of 35-S-labelled sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord, Denmark) during short-term 35-SO 4 reduction measurements. Geochim. Cosmochim. Acta, 48: 1807-1818. Indrebo, G., Pengerud, B. and Dundas, I., 1979. Microbial activities in a permanently stratified estuary. I. Primary production and sulfate reduction. Mar. Biol., 51: 295-304. Iversen, N. and J~rgensen, B.B., 1985. Anaerobic methane oxidation rates at the sulfate-methane transition in marine sediments from Kattegat and Skagerrak (Denmark). Limnol. Oceanogr., 30: 955. Jvrgensen, B.B., 1977. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark). Limnol. Oceanogr., 22:814 832. J~rgensen, B.B., 1978a. A comparison of methods for the quantification of bacterial sulfate reduction in coastal marine sediments. I. Measurement with radiotracer techniques. Geomicrobiol. J., 1: 11-27. J~rgensen, B.B., 1978b. A comparison of methods for the quantification of bacterial sulfate reduction in coastal marine sediments II. Calculation from mathematical models. Geomicrobiol. J., 1: 29-47. J~rgensen, B.B., 1982. Mineralization of organic matter in the sea b e d - - t h e role of sulphate reduction. Nature, 296: 643-645. J~rgensen, B.B. and Fenchel, T., 1974. The sulfur cycle of a marine sediment model system. Mar. Biol., 24: 189-201.
345 Lebel, J., Silverberg, N. and Sundby, B., 1982. Gravity core shortening and porewater chemical gradients. Deep-Sea Res., 29: 1365-1372. Nedwell, D.B., 1982. The cycling of sulphur in marine and freshwater sediments. In: D.B. Nedwell and C.M. Brown (Editors), Sediment Microbiology. Academic Press, London, pp. 73-106. Ouellet, G., 1982. Etude de l'interaction des animaux benthiques avec les sediments du Chenal Laurentien. M.Sc. Thesis. Universit~ du Quebec h Rimouski (Canada), 187 pp. Presley, B.J. and Claypool, G.E., 1971. Techniques for analysing interstitial water samples. Part I. Determination of minor and major constituents. U.S. Gov. Printing Office, 175 pp. Rowe, G.T. and Howarth, R., 1985. Early diagenesis of organic matter in sediments off the coast of Peru. Deep-Sea Res., 32: 43-55. Schafer, C.T., Smith, J.N. and Loring, D.H., 1980. Recent sedimentation events at the head of Saguenay Fjord, Canada. Environ. Geol. 3: 139-150. Silverberg, N., Edenborn, H.M. and Belzile, N., 1985. Sediment response to seasonal variations in organic matter input. In: A.C. Sigleo and A. Hattori (Editors), Marine and Estuarine Geochemistry. Lewis, Chelsea, MI, pp. 69-80. Silverberg, N., Nguyen, H.V., Delibrias, G., Koide, M., Sundby, B., Yokoyama, Y. and Chesselet, R., 1986. Radionuclide profiles, sedimentation rates, and bioturbation in modern sediments of the Laurentian Trough, Gulf of St. Lawrence. Oceanol. Acta, 9: 28~290. Smith, J.N. and Walton, A., 1980. Sediment accumulation rates and geochronologies measured in the Saguenay Fjord using the Pb-210 dating method. Geochim. Cosmochim. Acta, 44:225 240. Staresinic, N., Rowe, G.T., Shaughnessy, D. and Williams, A.J., 1978. Measurement of the vertical flux of particulate matter with a free-drifting sediment trap. Limnol. Oceanogr., 23: 559-563. Sundby, B. and Loring, D.H., 1978. Geochemistry of suspended particulate matter in the Saguenay Fjord. Can. J. Earth Sci., 15: 1002-1011. Sundby, B. and Silverberg, N., 1985. Manganese fluxes in the benthic boundary layer. Limnol. Oceanogr., 30:372 381. Sundby, B., Silverberg, N. and Chesselet, R., 1981. Pathways of manganese in an open estuarine system. Geochim. Cosmochim. Acta, 45: 293-307. Toth, D.J. and Lerman, A., 1977. Organic matter reactivity and sedimentation rates in the ocean. Am. J. Sci., 277: 26~285. Umbreit, W.W., Burris, R.H. and Stauffer, J.F., 1972. Manometric and Biochemical Techniques, 5th edn. Burgess, Minneapolis, MN, 387 pp. Vosjan, J.H., 1975. Ecological and physiological aspects of bacterial sulphate reduction in the Wadden Sea. Ph.D. Thesis, University of Groningen, 68 pp. (in Dutch).
