Nov 7, 1988 - process. The data base of sulfate reduction ... was partly covered by growing green mac- roalgae ..... Areal data-Recovery of reduced 35S in.
LIMNOLOGY
July 1989
AND
OCEANOGRAPHY
Volume
34
Number
5
Limnol. Oceanogr., 34(5), 1989, 793-806 0 1989, by the American
Society of Limnology
and Oceanography,
Inc.
Sulfate reduction and the formation of 35S-labeled FeS, Fe&, and So in coastal marine sediments Smen Thode-Andersen’ and Bo Barker Jmgensen Department of Ecology and Genetics, University DK-8000 Aarhus C, Denmark
of Aarhus, Ny Munkegade,
Abstract A comparative survey was done of the formation of radiolabeled end-products during 35S0,2reduction measurements in coastal sediments of Denmark. The distribution of reduced 35Sin acidvolatile sulfides (AVS), pyrite, and elemental sulfur in the upper O-15 cm of sediment depended on the sulfur chemistry and on the overall sulfate reduction rates. In sediments with relatively low metabolic rates, undetectable H,S, and low FeS : FeS, ratio (< 1 : 20), only 32-55% of the reduced 35S was recovered in the AVS pool. In sediments with high metabolic rates, high H2S, and high FeS : FeS, ratio (> 1 : lo), the 35Srecovery in AVS was 63-92%. The relative contribution of Fe35S2 formation showed little depth dependence, whereas the formation of 35Sowas highest in the more oxidizing sediment layers near the surface. Inclusion of 35S-labeled FeS, and So in the sulfate reduction measurement yields more accurate rate data than are obtained from AVS alone. Due to possible isotopic exchange reactions and unknown pathways of H,S transformation into AVS, So, and FeS,, however, the radioactivities of the three pools cannot be used directly to calculate their differential rates of formation.
Sulfate reduction is the main terminal process in the anaerobic mineralization of organic matter in coastal marine sediments. The anaerobic mineralization is highly efficient and leaves only a small organic fraction of refractory material for permanent burial in the sediment. Up to half of the deposited organic matter in coastal sediments has been found to be mineralized anaerobically via sulfate reduction (Jorgensen 1982). It is therefore of ecological and geoI Present address: Department of Animal Physiology and Biochemistry, National Institute of Animal Science, Foulum, DK-8830 Tjele, Denmark.
Acknowledgments We thank Preben G. Sorensen and Dorte Olsson for assistance in the laboratory and in the field. This study was inspired by a research visit from Robert W. Howarth.
chemical importance to obtain accurate measurements of the in situ rates of this process. The data base of sulfate reduction rates determined by radiotracer techniques or by mathematical modeling has grown rapidly over the last 10 yr and has confirmed the large quantitative role of the sulfur cycle in marine sediments (Skyring 1987). Hydrogen sulfide is the only extracellular sulfur product of bacterial sulfate reduction. The H2S may readily precipitate with Fe2+ present in the pore water to form iron monosulfides. Radiotracer measurements of sulfate reduction therefore have until recently included only the reduced label present in the acid-volatile sulfides (AVS: H2S and FeS). Based on calculations of, for example, pyrite accumulation rates, it was not expected that other sulfur compounds would form at appreciable rates during short-term 793
794
Thode-Andersen and ,Jurgensen
incubations with 35S0,2- (J0rgensen 1977a). It was therefore an important discovery that pyrite, FeS,, was the main labeled product in salt-marsh soils (Howarth and Teal 1979; Howarth 1979). Howarth and coworkers concluded that pyrite is a much more dynamic pool than previously expected. Later studies have confirmed that the formation ofradiolabeled pyrite plays a significant role in salt marshes and coastal sediments (Howes et al. 1984; King 1988). Elemental sulfur, So, was also found to accumulate much of the reduced label from 35S042- reduction (Howarth and Jerrgcnsen 1984; King et al. 1985; King 1988). In the early studies of Howarth, pyrite and elemental sulfur were extracted together by aqua regia digestion after removal of radioactive sulfate and acid-volatile sulfide. A more selective technique for the separation and analysis of nonacid-volatile sulfide based on reduction by Cr2+ ions was described by Zhabina and Volkov (1978) and is now becoming widely used in sulfate reduction studies (e.g. Westrich 1983; Howarth and Merkel 1984; Howarth and Jrargensen 1984; King 1988). In hot-acid solution, Cr2+ reduces both pyrite and elemental sulfur to H2S, but not sulfate or organic sulfur. By including 35S-labeled AVS, So, and FeS, in the sulfate reduction measurements, a higher and more accurate rate is obtained. Separation of the three sulfur pools makes it possible to compare sulfate reduction data with earlier measurements based on 35S in AVS alone. Furthermore, separation of the pools may yield information about the diagenetic transformations of sulfide in sediments. We have surveyed sulfate reduction rates with special emphasis on the formation of radiolabeled AVS, So, and FeS, in different types of coastal sediments. The goal was to understand what regulates the differential formation of the three labeled pools. The allocation of reduced 35Sinto the three pools was therefore compared to the sulfur geochemistry of the sediments and to overall sulfate reduction activity.
Materials and methods Sampling localities-Sediment
was sampled during spring and summer 1983 at eight
stations along the Danish coast. Tidal variations of the area are insignificant. Various aspects of the sulfur biogeochemistry at several of these brackish-water localities have been described earlier (J0rgensen 1977a; Hansen et al. 1978; Ingvorsen and Jrargensen 1982; Troelsen and J0rgensen 19 82; Howarth and Jsrgensen 1984; Jrargensen and Sarrensen 19 8 5). The stations and sediment types are characterized in Table 1. The sediment in Limfjorden was overgrown by eelgrass (Zostera marina). Cores were taken from an area with a relatively low density of rhizomes. Kal0 Lagoon had a sulfuretum type of sediment consisting of a lo-cm layer of decomposing celgrass mixed with sand and lying on top of coarser sand. The surface was covered by a film of cyanobacteria together with purple (mostly Chromatium) and colorless (Beggiatoa) sulfur bacteria and was rich in elemental sulfur. Lendrup Lagoon was partly covered by growing green macroalgae (Ulva and Enteromorpha). The sediments at all stations showed little variation in organic contents when calculated per volume, 20-60 mg dry wt cme3. On ,aweight basis, the range was from 1.1% dry wt in sand to 8.6% in mud and 32.0% in tlhe sulfuretum. Sediment cores with undisturbed zonation. were collected in Plexiglas tubes by hand at water depths of O-2 m or by a Haps corer (Kanneworff and Nicolaisen 1973) at 124 1-1mdepths. Deeper cores (2.5 m long) were taken by a 9-cm-i.d. piston corer. Subcores for radiotracer experiments were taken in Plexiglas tubes which had injection ports at l-am depth intervals (J0rgensen and Fenchcl 1974). Sediment cores were kept in the dark at in situ temperature, and tracer was injected on the day of sampling. Tracer and analytical techniques-Sulfate reduction was measured at l-cm intervals at 0-5-cm depth and at 2-cm intervals at 5-I 5-cm depth. At each depth, 2 pl(5 &I) of carrier-free 35S0 2- solution were injected,‘and the cores were incubated for 24 h. The sediment was then stepwise extruded from the tubes and cut in segments. They were immediately fixed in a 1: 1 volume of 20% Zn-acetate and frozen until the following isotope separations: extraction of So; dis-
795
Formation of radiolabeled FeS, FeS,, and So Table 1. Characteristics Station
Aarhus Bay Aarhus Bay Aarhus Bay Limfjorden Kysing Fjord Kale Lagoon HjarbEk Fjord Lendrup Lagoon
of sampling localities Depth (ml
12 18 41 2 1 1 2 2
and sediments. Salinity (W
18-26 18-26 18-26 16 7-18 16 l-4 8-19
tillation of AVS; re-extraction of So; and distillation of chromium-reducible sulfur. The separations followed the procedures of Howarth and Jorgensen (1984). A 5-cm3 subsample was taken for So extraction with 5 ml of CS, in a glass-stoppcred test tube. After 12 h extraction on a shaker at 22°C the sample was centrifuged. Two 1-ml samples of the CS, phase were taken to determine So concentration and radioactivity, carefully avoiding contamination from the aqueous phases. Labeled sulfate from this phase will interfere strongly with the measurement of radioactivity of So. Zero-time controls showed that such contamination was effectively avoided. The Zn2+ of the aqueous phase will furthermore interfere with the spectrophotometric determination of So after cyanolysis (Troelsen and Jsrgensen 1982). Control experiments showed that the Zn35S was stable during CS, extraction for at least 48 h. There is no loss of radiolabel due to isotopic exchange of 35S between So and CS2 (Cooley et al. 1939). Efficiency of the first So extraction was determined in different sediment types by repeating the extraction four times and adding the recovered So fractions. The mean recovery of the first extraction was 83% (n = 15, SE = +2%). Unextracted So was corrected for in later calculations. After So extraction, AVS was actively distilled from the sediment slurry with 1 N HCl into 5% Zn-acetate traps (Jorgensen and Fenchcl 1974; Jorgensen 1978). Distillation proceeded for 15 min at room temperature and 15 min of heating up to boiling and boiling for 2 min. In control experiments with different sediment types, we found that heating added only l-5% to the total distilled AVS relative to room-temperature
Sediment
type
Silt and clay mud Silt and clay mud Silt and fine sand Fine sand with eelgrass Silt and fine sand Eelgrass detritus Sand and silt Silt with macroalgae
Org. C content (o/o dry wt)
4.7 7.9 3.1 1.6 7.4 32.0 1.1 8.6
distillation. Hot- vs. cold-AVS distillation had no detectable effect on subsequent recovery of labeled pyrite (difference < 1%). Thus, hot acid will dissolve well-crystallized iron monosulfides such as greigite but not the main pyrite pool. We did not include tin in the acid slurry to increase AVS recovery since hot Sn also reduces some of the FcS, (Chanton and Martens 1985). Subsamples of suspended ZnS were taken from the traps to determine AVS radioactivity and concentration by the methylene blue technique (Cline 1969). The remaining sediment slurry was diluted to 50 ml with demin. Hz0 and centrifuged, and a subsample was taken to determine 35S0 2- radioactivity. The sediment pellet was waihed additionally 2-4 times to remove 35S042- which will otherwise cause background contamination of the chromium-reducible sulfur (CRS) radioactivity (Howarth and Jorgensen 1984; Fossing and Jorgensen in press). The absence of such a background was checked by zero-incubation-time controls. The sediment was subsequently dried at 105”C, weighed, and ground in a mortar. A second CS2 extraction was made of the So, which either remained from the first extraction or had formed artificially during distillation due to oxidation of H2S by acidreleased Fc3+: HS
+ 2Fe3+ -+ So + 2Fe2+ + H+ .
The 35Sorecovered from the second extraction was only 20-25% of that recovered from the first. Since only 83% of the CS2 of the first extraction could be removed, the remaining 17% accounted for most of the 35So of the second extraction. Formation of So during AVS distillation was therefore of lit-
796
Thode-Andersen and *Jorgensen
tic significance for the measured radiotracer distribution. The second extraction was done mostly to reduce carry-over of 35S from So to the CRS pool, since So is efficiently recovered by hot acid with Cr2+ (Fossing and Jorgensen in press). This carry-over was a problem mostly near the sediment surface where 35Sowas high and Fe35S2was low. A second subsample of the extracted sediment was taken for distillation of chromium-reducible sulfur, i.e. mostly pyrite (Zhabina and Volkov 1978; Westrich 1983; Howarth and Jorgensen 1984). Distillation proceeded for 15 min at room temperature followed by 45 min at boiling. Precipitated ZnS was sampled from the Zn-acetate traps to determine CRS radioactivity and concentration by the methylene blue technique (Cline 1969). Pore water was obtained from parallel sediment cores by pressure filtration under N2 through 0.45~pm membrane filters directly into 2% Zn-acetate. Free H2S was determined from the precipitated ZnS by the methylene blue technique (Cline 1969). The concentration of iron monosulftde in the sediment was calculated from AVS minus free H,S. The filtrate was filtered again, and sulfate was determined gravimetrically after precipitation with barium. Porosity was determined from the weight loss of a known sediment volume after drying at 105°C. Organic content was determined from the weight loss of dried sediment after ignition at 450°C overnight. Redox potentials were measured with a platinum electrode coupled with a calomel rcfcrence electrode (Fenchel 1969).
Results The eight sediment localities differed highly with respect to sulfur chemistry, sulfate reduction activity, and the differential recovery of radiolabel in AVS, So, and FeS,. WC present here data from five stations which comprise the measured extremes from poorly reduced sediment without detectable H2S to sediment with very high H,S or So. Each set of data is the mean of three parallel cores. The variability of measured pools or rates was largely due to natural heterogeneity, as shown by the following controls. Statistical analysis was done on 10 parallel
cores from Kysing Fjord injected with 35SOs42-at 8-lo-cm depth. The calculated standard error of single determinations of pool sizes was IL-42, +-73, and +32% for AVS, So, and FeS,. The corresponding standard errors :for the radioactivities were + 46, +69, and 11129%.When sediment from all cores was pooled (by mixing after fixation with Zn-acetate but before subsampling and chemical analysis) the standard errors on pool sizes were reduced lo-fold-20-fold to +2.9, k3.9, and -t3.7%. Time-course control-Control experiments were done to check linearity of sulfate reduction over time during incubation with 35SO142-.Only the reducing subsurface sediment (8-IO-cm depth) was included since diffusion loss and sulfide reoxidation is known to cause nonlinearity, especially in surface sediment (Jorgensen 19 7 8). The formation of radiolabeled AVS and FeS, was constant with time over the 3 d of incubation (Fig. 1). Formation of labeled elemental sulfur, however, slowed after the first 15 h. Since So as well as FeS, accounted for only lo-11 5% of the reduced label in this sediment, nonlinearity of So had little effect on total radioactivity (Fig. 1) or thus on calculated sulfate reduction rate. Similar trends of AVS, FeS,, and Soradioactivity with time were also found in another Kysing Fjord sediment (data not shown). Due to nonlincar 35Soformation, the 24-h incubation time chosen in the present study should show a contribution of 35Soslightly below the maximal value (cf. Fig. 1). Four sediment types--In Aarhus Bay at 12 m the sediment was sampled in early spring when the temperature was still low, 4°C. It was mildly oxidized with no detectable H,S and with positive Eh throughout the uppermost 15 cm (Fig. 2). So showed a maximum close to the surface followed by a maximum in FeS and below that a maximum in FeS,. This sequential pattern of So, FeS, and FeS, maxima was found in sediments from all eight stations studied. The sulfate gradient (expanded scale) indicated active sulfate reduction. FeS accumulated below l-cm depth. Surprisingly, sulfate reduction was close to maximal in the uppermost O-l cm where the sediment was most oxidized. The fraction recovered in
Formation of radiolabeled FeS, Fe&, and So AVS, however, increased with depth below the surface, and sulfate reduction based on AVS alone thus showed a depth profile more similar to previous observations. The amount of label recovered in So and FeS, fell from 92% at the surface to 36% at 15 cm. In Kysing Fjord the sediment was strongly reduced below 4-5-cm depth (Fig. 3). The inverse sulfate gradient was due to a decrease in salinity of the overlying water during the preceding months. Maxima in Soand FeS as well as in the rate of sulfate reduction occurred just above the redoxcline. The relative distribution of radiolabel in AVS, So, and FeS, was relatively constant throughout l-l 5-cm depth while at O-l cm the nonacid-volatile pools were most important, constituting 55%. The sediment of the Kale Lagoon sulfureturn with its high content of decomposing eelgrass was reduced to the very surface and was rich in free H,S (Fig. 4). Concentrations of FeS and especially of FeS, were relatively low, presumably due to low iron content. There was a high accumulation of elemental sulfur at the surface where the So was produced by a dcnsc film of cyanobacteria and sulfur bacteria. Up to 90% of the reduced 35Swas recovered as So at the surface where the reduction rate was extremely high, 1,200 nmol cmm3d-l. In deeper layers, about 80% was recovered as AVS. The green macroalgae growing over Lendrup Lagoon sediment supplied a rapidly decomposing source of organic matter, resulting in high sulfate reduction rates and a steep sulfate gradient (Fig. 5). Free H2S occurred from the topmost 1 cm in similar concentrations to Kale Lagoon (data not shown). More than 90% of the reduced 35S was recovered as AVS in this sediment. Deep core-The distribution of reduced 35S during sulfate reduction measurements in the deeper part of the sulfate zone was studied in samples from a 2.5-m-long piston core from Aarhus Bay (Fig. 6). Sulfate was depleted at 250-cm depth. Free H2S was hardly detectable in the uppermost O-l 5 cm but reached a maximum of 0.3 mM at 1.52-m depth. The black iron monosulfide zone reached lo-12-cm depth, below which FeS remained very constant while FeS, slowly
797
30-
25-
20-2 4 "& x
IS-
2 ml IO-
5-
fl
0
,,,,,, 20
40
60
80
Incubation time (h)
Fig. 1. Time-course of sulfate reduction and formation of 35S-labeled AVS, So, and Fe&. Each point is the mean of three parallel sediment cores injected with 35S0,2- at 8-, 9-, and lo-cm depths at time zero. Sediment from Kysing Fjord incubated at 13°C. The scale of So and FeS, radioactivities is expanded 1O-fold relative to AVS.
accumulated. Sulfate reduction was detectable throughout the sulfate zone. The rates were relatively low near the surface due to low temperature and low organic input during winter and early spring. The proportion of reduced 35S recovered as AVS, So, and FeS, was 5 1,3 1, and 18% and did not change systematically with depth from close to the surface down to 240 cm. Areal data-Recovery of reduced 35S in the AVS fraction ranged from 39 to 92% (Table 2). It was highest in sediments with HZSrich pore water and generally increased with increasing sulfate reduction rates. The FeS, pool contained only a minor fraction of 35S relative to the elemental sulfur pool. The insignificant formation of labeled pyrite in the rhizosphere of eclgrass beds was surprising in view of the contrasting results from salt-marsh sediments (e.g. Howarth and Teal 1979). The highest contribution of the pyrite fraction, 18-29%, was found in Aarhus
798
Thode-Andersen and Jorgensen ORG.CONT. (mg d.w. cni3)
REDOX
I 100
I -I 200 REOOX (Ehl
300
4
I
I
0
20
I
I
40 60 FeS2 (,umol Scm-3)
So, FeS IDrnol S crnm31
I
80
1
SRR (nmol S cme3dy) ?O
4.0
/’ /’
S”
/ I 12 -
4’ ;
14 -
Fig. 2. Chemical zonation and sulfate reduction rates (SRR) in sediment from Aarhus Bay at 12-m water depth. The sulfate reduction bars show the relative recoveries of reduced 35Sin the three analyzed pools of acidvolatile sulfide, elemental sulfur, and pyrite. 28 March 1983, 4°C.
Bay sediments where free H2S was not detectable, the FeS, concentration was high, and So was relatively low. Recovery of 35S in the So fraction did not seem related to the So pool size when calculated on an areal basis. Rather, it was inversely related to H2S concentration. Our recoveries of reduced 35S as AVS in
coastal sediments are compatible with other published data from Danish waters (Fig. 7). The studies of Howarth and Jorgensen (1984) and Fossing and Jorgensen (in press) were also in shallow waters while Sorensen and Jorgensen (1987) studied offshore shelf sediments. Recoveries of 35Sin AVS ranged from 32 to 92% with the lowest value ob-
Formation of radiolabeled FeS, Fe&, and So ORG.CON T. (mg d.w. cmW3/ 0
2
S042-Ibmol 4
799 crf3) 6
8
24-
0
-100
+I
4b
REDOX lEhl
So, FeS Iumol S ems31 100
Fig. 3.
Chemical
zonation
and sulfate reduction
8b Fe.52 (umol
$0
F
Scme3/
SRR InmolScm-3dy) 200 300
rates in sediment from Kysing Fjord. 8 May 1983, 10°C.
tained at the deepest station. There was a clear trend toward higher recoveries at higher reduction rates. Where sulfate reduction exceeded 10 mmol m-2 d-l, we generally found 6580% of the reduced 35Sin the AVS pool. Between 1 and 10 mmol rnw2 d-l, recovery was mostly 40-60%. The lowest recovery of AVS, 32%, was recorded at 520-m water depth in sediments of the Skagerrak adja-
cent to the North gensen 1987).
Sea (Sorensen and Jor-
Discussion The radiotracer studies with 35S-labeled SOd2- showed that H2S formed as the product of bacterial sulfate reduction undergoes rapid diagenetic transformations in sediments. Knowledge of the rates and path-
800
Thode-Andersen and Jorgensen ORG.CON T. (mg d.w. cmw3) 0 .I
e
4 ---r-T-l---T----r---T---
-200
-100 -50 REDOX f Ehl
-150
ST FeS.FeS2 Ipmo/
10
-r---
S042-l,umol 8 I I
cms31 I
1----7-T H2S Iumol SRR Inmol
12 I
16 I
3 Scme3) S cmm3dq)
,/’
,/ ,.’ “
i /’
Fig. 4. Chemical 1983, 18°C.
l
+51)
ScnT31
4
i 12
0
4 I
/
zonation
and sulfate reduction
rates in sediment from Kal0 Lagoon (sulfuretum).
ways of these transformations during incubation is important both for functional understanding of the sulfur cycle and for accurate measurement of sulfate reduction rates. Survey of sediments-At the offshore stations, metabolic rates were low, H,S was undetectable in the upper 0- 15 cm, and the transformation of iron monosulfides to py-
27 June
rite was almost complete (the ratio of maximum FeS to maximum FeS, was < 1: 20). As a mean for the upper O-l 5 cm, we found 45--68% of the reduced 35Sin the partly oxidized pools of So and FeS, (Figs. 2 and 6). At the nearshore stations with high organic loads, metabolic rates were high, H2S reached > 1 mM in pore water within the upper O-A 5 cm, and the black sediments
801
Formation of radiolabeled FeS, FeS,, and So 0
ORG.cON T. (mg d.w. cmm31 40 60
20
80
0
?
SOd2-(,vmol 4
CM31
6
8
6’
4-
d !
REDOX 4 : I i I I f I
I 0
So. FeS lumol S cms3)
Chemical zonation
and sulfate reduction
I
I 40
I
1 60
I
I 80
Fe.52 (,umol Scm”)
REDOX IEhl
Fig. 5.
I 20
SRR (nmolScm-3di)
rates in sediment from Lendrup
were rich in monosulfides [FeS(max) : FeS,(max) was > 1 : lo]. Depth-averaged recoveries of reduced 35Sin Soand FeS, were 8-37% (Figs. 3, 4, and 5). An increase in 35S recovery in the AVS fraction with increasing sulfate reduction rate was also observed in sediments of Shark Bay, Western Australia, by Skyring and Lupton (1984). The tracer distribution
Lagoon. 10 July 1983, 19°C.
among the three pools (AVS, So, and Fe&) seems to vary strongly among coastal scdiments, howcvcr (see Skyring 1987). Although general patterns can be recognized, the factors regulating distribution are still far from clear. The highest recoveries in the AVS fraction, >80%, are found in highly reduced coastal muds with high organic load, e.g. in Long Island Sound (Westrich 1983)
802
Thode-Andersen and Jorgensen So, FeS SOi-
0.0
(pm01 cm-3)
04
0.2 H2S
(pmol
0.0 0
0.3 7
100
06
FeS2
crne3)
(,umol S cms3)
0.6 2
SRR
09
750
72 4
200
7.5 so 5 FeS
250
(pm01 S cme3) (nmol
cmw3d-‘/
760
24OL-------
Fig. 6. Chemical April 1983, 6°C.
zonation
and sulfate reduction
rates in sediment from Aarhus Bay at 18-m water depth. 8
and under the oxygen minimum zone along the Peruvian coast (Rowe and Howarth 1985). The lowest values are found in mildIy oxidized offshore sediments on the continental slopes (Ivanov et al. 1976). In an extensive study in the Baltic Sea, Lein (1983) found highly varying recoveries in AV-S of
l-84% with an overall mean value of 20% for all samples or 30-50% for the surface layers. Christensen (in press) surveyed shelf sediments in the Gulf of Maine and found a systematic variation similar to the present results in the recovery of 35S in AVS. He
803
Formation of radiolabeled FeS, Fe&, and So Table 2. Sulfate reduction rates (mmol SOd2- m -2 d-l) and mean relative distribution of reduced 35Samong the analyzed sulfur pools in the upper O-l 5 cm of sediment.
Station
Aarhus Bay, 12 m Aarhus Bay, 18 m, April Aarhus Bay, 18 m, June Aarhus Bay, 41 m Limfjorden, eelgrass Kysing Fjord Kale Lagoon Hjarbak Fjord Lendrup Lagoon
Sulfate reduction
3.58 1.07 3.84 1.45 7.66 21.7 62.1 16.14 45.8
1001
I
% of ‘Y$,, AVS
39 51 54 49 50 64 63 78 92
So
32 31 23 46 49 26 28 14 3
Fe&
29 18 23 5 1 10 9 8 5
studied more oxidized sediments with lower sulfate reduction and found the recovery in AVS to approach zero as the areal reduction rates fell to ~0.02 mmol m-2 d-l. Revision of earlier data-In most earlier radiotracer studies of sulfate reduction by us and others, only the reduced label in AVS was detected (Skyring 1987). The measured, relative recoveries of 35S in AVS thus indicate how much those earlier calculated rates underestimate the “true” rates of sulfate reduction. In the Danish coastal waters that we have studied (Jorgensen 1977a, 1982) the underestimation should be in the range of lo-40% in sediments with high reduction rates and 40-60% in sediments with low reduction rates. It was estimated for the former that sulfate reduction accounted for about half of the total mineralization of organic matter and that the (direct or indirect) reoxidation of H2S was equivalent to half of the oxygen uptake. These estimates should now be revised toward an even higher contribution of sulfate reduction and sulfide oxidation, which approaches the theoretical maximum. It is possible that oxygen uptake rates also should be revised upward to account for peaks in sediment oxidation during storms and periods of surface sediment resuspension (cf. Jorgensen 1977a). It has not yet been possible to quantify such events, but they could cause rapid, intermittent reoxidation of accumulated reduced sulfur pools.
Depth-dependent 35Sdistribution-Trends in the distribution of 35S in AVS, So, and FeS,, similar to those found among the sed-
0
I
I
I
I
I
I
I
I
5
10
15
20
25
30
35
40
Sulfate
reduction
(mmol
I
45
ms2 day-‘)
Fig. 7. Recovery of reduced 35S in the AVS pool (in percent of the total reduced 35S)plotted against the total sulfate reduction rate. Mean recoveries of [35S]AVS and areal sulfate reduction rates are calculated for the upper O-l 5 cm of sediment at each station. Data from this study (O), Howarth and Jorgensen 1984 (0), Sorensen and Jorgensen 1987 (+), and Fossing and Jorgensen in press (Cl).
iment types studied here, were also recognized down through the depth profiles of individual sediments. The more oxidized layers at the surface had the lowest FeS: FeS, ratio and consequently also the lowest recoveries of 35S in AVS fraction. A relatively large proportion of the H235S produced had therefore been partly oxidized to So or FeS, during incubation. A significant proportion of the H235S in the surface layers had presumably also been oxidized completely to sulfate and thus escaped detection. It is interesting that in many of the sediments the calculated depth distribution of sulfate reduction activity was changed from a subsurface maximum when only the AVS pool was considered to a surface maximum (at the present depth resolution) when the nonacid-volatile pools were included. It was the 35So fraction which especially increased toward the surface, while the Fe35S2 fraction remained relatively constant with depth. We found no systematic change in the relative distribution of reduced 35S between depths of 5-10 cm and the lower boundary of the sulfate zone at 240-cm depth in a sediment core from Aarhus Bay (Fig. 6). The observed depth trends in the upper O-l 5 cm have also been recognized in other coastal sediments, notably in salt marshes of the cast coast of the United States (Howes et al. 1984; King et al. 1985; King 1988). The most extreme example was reported by
804
Thode-Andersen and Jorgensen
King ( 1988) for a Georgia salt-marsh soil where the distribution of reduced 35Sin AVS, So, and FeS, changed from 1.7, 95.3, and 4.6% at the surface to 35.1, 19.2, and 52.4% at lo-cm depth. Evaluation of tracer method--The timecourse of radiolabeled AVS, So, and Fe& formation during sediment incubation with 35S042- is important in interpreting calculated sulfate reduction rates. Nonlinearity of radiolabeled pyrite formation and especially the possibility of contamination from 35S042- which may cause a high zero-time blank olf35S in the pyrite pool, are potential errors that have been discussed frequently (e.g. King 1983; Howarth and Merkel 1984; Howcs et al. 1984). In time-course controls, we found constant rates of 35S-labeled AVS and FeS, formation over incubation periods of 72 h (Fig. 1). The formation rate of 35So, however, decreased with time. Similar results were found in earlier studies with sediment cores (Howarth and Jorgensen 1984) and with sediment suspensions (Thode-Andersen et al. unpubl. results). Hence, the proportion of labeled Sorelative to AVS and FeS, changes with incubation time. During longer incubations the formation of 35Sois perhaps underestimated. In the time-course experiment of Fig. 1, we found the relative contribution of So to the calculated mean sulfate reduction rate to decrease by 50% over 12- vs. 72-h incubation. As a consequence of the transformation of reduced H235S into AVS, So, and FeS,, measurements with the purpose of deriving only the total rate of sulfate reduction should use a simpler analysis without separation of the three pools. A single-step chromium reduction method, which includes total reduced inorganic sulfur, was used by King (1988) and tested by Fossing and Jorgensen (in press). The single-step distillation was found to yield reduction rates that were 450% higher than rates obtained from the sum of 35S in AVS, So, and FeS,. The difference was mostly due to loss by oxidation to sulfate of reduced 35Sduring AVS distillation. In view of the recent major changes in tracer techniques for measuring sulfate reduction, it is appropriate to ask whether the methods now measure in situ rates accurately. The chromium reduction technique
does not include organic sulfur (Howarth and. Jorgensen 1984) that could form from radiolabcled sulfide. Although mass balance calculations suggest that the process should be of little significance for total sulfide turnover, it could still play a role during shortterrn incubations. Several mechanisms for sulfide incorporation into organic matter have been proposed, based on reactions with polysulfides (Aizenshtat et al. 1984) or with pyrite (Luther et al. 1986). Another systematic error could arise from reoxidation of radiolabeled sulfide to sulfate or to other oxidized products not reduced by chromium. Such reoxidation would be especially important near the sediment surface, where sulfate reduction can occur in reduced microenvironments surrounded by oxidized sediment (Jorgensen 19773). Within highly reduced sediments, however, significant reoxidation of radiolabcled H2S to thiosulfate and sulfate has also been demonstrated (L. Elsgaard unpubl.). The oxidation is probably due to reaction with oxidized iron and manganese. Thus, experimental oxidation of FeS to sulfate could be demonstrated with MnO, in marine sediment by Aller and Rude (1988). It is not presently pos’sible to evaluate the quantitative role of these processes, but the formation of labeled pyrite and elemental sulfur in itself indicates that partial reoxidation of H2S occurs concurrently with its formation.
What does the 3*S distribution imply.?Although the formation of radiolabeled So and FeS, during short-term tracer incubations is well documented, the mechanisms remain unclear. Ferric oxyhydroxides present in marine sediments react with H,S to for.m mostly elemental sulfur and polysulfide plus smaller amounts of thiosulfate (e.g. Pyzik and Sommer 198 1). Pyrite may form by reaction between FeS and So (Berner 1970) or between ferrous iron, sulfide, and polysulfides (Rickard 1975; Howarth 1979). It is a puzzling observation, however, that the formation of labeled So and FeS, initially seems to be always linear with time (Fig. 1; Howarth and Jorgensen 1984). If a significant H2S pool is present in pore water, the specific radioactivity of this pool should increase with time until a steady state is approached. During the build-up of the specific radioactivity in H2S and FeS, the for-
Formation of radiolabeled FeS, FeS,, and So mation of labeled So and FeS, should be slow initially and accelerate gradually. We have not observed such a lag. On the contrary, the formation of labeled So slows during incubation. An explanation of this phenomenon may possibly be found in the rapid isotopic exchange reactions that have been demonstrated between, for example, H2S, polysulfides, and So (Voge 1939). In marine sediments, such exchange reactions were shown to cause rapid transfer of radiolabel between So and several of the reduced sulfur pools (Fossing and Jorgensen unpubl.). Due to the potential role of such exchange reactions and to uncertainty about pathways of So and FcS, formation, the transfer of radiolabel into these pools cannot bc used as a direct measure of their rates of formation. Only the sum of all radioactivities appearing as reduced sulfur gives a correct measure of the total sulfate reduction rate. Only for this sum and only initially is all sulfate reduction based on a common sulfate pool of known specific activity. Once the H235S produced is transformed among sulfur pools of unknown size, the specific activity of sulfate is no longer a valid parameter for calculating their transformation rates. The system is heterogeneous and is not in equilibrium -certainly not with respcct to So (Fig. 1). The classification of sulfate reduction in Figs. 2-6 into AVS, So, and FeS2 is therefore indicative only of the tracer distribution, not of the differential formation rates of these compounds. If their radioactivities were used to calculate their rates of formation (and if isotopic exchange were unimportant), the calculated rates would be underestimates. This bias occurs because the compounds are produced via intermediates, which arc initially unlabeled and which dilute the tracer and thus lower the specific activity. In conclusion, sulfate reduction rates measured by including the 35Sof all the reduced inorganic sulfur pools are expected to bc close to in situ rates, but we still cannot say exactly how rapidly elemental sulfur or pyrite forms in sediments.
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Submitted: 7 November 1988 Accepted: 14 March I98 9 Revised: I3 April 198 9
