Sulfate reduction rates and low molecular weight fatty ...intranet.geoecomar.ro/rchm/wp-content/uploads/downloads/2014/11/DSR-V... · and 3 for convenience. All three sites were in

Deep-Sea Research I, Vol. 42, No. 7, pp. 1239-1260, 1995

0967-0637(95)00042-9 Copyright @ 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 09670637/95 $9 50 + 0.00

Sulfate reduction rates and low molecular weight fatty acid concentrations in the water column and surficial sediments of the

Black Sea

DANIEL B. ALBERT,* CRAIG TAYLOR? and CHRISTOPHER S. MARTENS*

(Received 9 May 1994; in revised form 6 September 1994; accepted 24 October 1994)

Abstract-Sulfate reduction rates and concentrations of low molecular weight organic acids were measured in the water column and surficial sediments at two sites in the central Black Sea. Water column sulfate reduction rates were much lower than previously reported. The highest rate measured was 3.5 nM day-’ and on a depth integrated basis values of 1.2 and 0.22 mmol m-* day-’ were obtained for the two sites. Sediment sulfate reduction rates were within the ranges previously reported but were higher than some for comparable abyssal sites. Rates were about 21 yM day-’ in the flocculent layer at the sediment-water interface, decreasing to 2-3yM day-’ at 20 cm depth. On an areal, depth integrated basis, rates at the two sites were 1.45 and 1.29 mmol mm2 day-‘. Thus, the water column and sediments have comparable area1 rates, but on a volume basis the sediment rates are several thousand times higher than the water column rates. Organic acid concentrations in the anoxic Black Sea water column were surprisingly high, reaching severalpM in some cases. One deep sample contained 60pM acetate. Lactate, acetate and formate were the only acids detected in the water column. Some propionate was seen in sediment porewaters. Apparent turnover times of the organic acids in the water column, calculated for utilization solely by sulfate reducing bacteria, are tens to hundred of years. This suggests that sulfate reduction rates in the water column were not limited by organic substrate supply. In the sediments, apparent acid turnover times calculated in this way are generally less than one day, suggesting that sulfate reduction may be limited to by the supply of these substrates through fermentation reactions.

INTRODUCTION

Oxygen containing surface waters of the Black Sea, with an average salinity of about 18.3, are separated by a sharp chemocline, currently located at 50-80 m depth (Murray et al., 1989), from 520,000 km3 of saltier (S -22.3) anoxic deep water. Thus, the oxic waters represent only a thin skin over the 2100 m deep anoxic basin. Concentrations of dissolved sulfide and other products of anaerobic remineralization, such as ammonium, phosphate, methane and carbon dioxide, rise rapidly below this depth. Sulfide, which reaches a concentration of over 400 ,uM in the water column (Brewer and Spencer, 1974; Millero, 1991)) is the product of dissimilatory sulfate reduction by sulfate reducing bacteria living in both the water column and sediments. The concentration of sulfide in the deep water is controlled by several factors, including its rate of production, oxidation and reaction to

*Curriculum in Marine Sciences, University of North Carolina, Chapel Hill, NC 27599, U.S.A. tDepartment of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A

1239

1240 D. B. Albert et al.

form organic sulfur compounds and insoluble metal sulfide mineral phases, throughout the history of any given water parcel. There are not enough data available yet to accurately assess the relative contributions of sulfate reduction in the water column and sediments to the dissolved sulfide inventory in the water column. Its turnover time also cannot be accurately estimated because we do not know whether its depth vs concentration profile represents a steady state or is still changing since the onset of anoxic conditions.

From porewater chemistry (Manheim and Chan, 1974) and the sedimentary record (Ross and Degens, 1974) it appears that the Black Sea has experienced intermittently anoxic conditions during its history. It was probably a freshwater lake during the Pleistocene and experienced saltwater intrusion through the Bosporus during the Holo- cene as a consequence of sea level rise (Ross and Degens, 1974). The current deepwater sediments are a finely laminated coccolith ooze (widely referred to as Unit 1 after Ross et al., 1970), made up principally of Emiliunia huxleyi which has been deposited in the Black Sea for 1000-3000 years (Ross and Degens, 1974; Degens et al., 1980; Calvert et al., 1987; Hay et al., 1991). The presence of this species indicates that surface water salinities had reached at least 11 by that time, and hydrographic conditions were probably similar to those of today.

Current hydrogruphic conditions

The fresher surface waters of the Black Sea are fed by river input, particularly from the Danube and Dneipr Rivers, which empty into the broad northwestern shelf region. The denser, saltier water at depth (S = 22.3) is supplied from the Mediterranean Sea via the Turkish Straits waterway, consisting of the Dardanelles, the Sea of Marmara and the Bosporus. The Bosporus has two-layered flow with fresher surface water from the Black Sea, driven by hydraulic head, flowing southward through the 31 km long channel and into the Sea of Marmara. The density driven saltwater flow moves northward along the bottom (e.g. Gunnerson and Ozturgut, 1974; Tolmazin, 1985b). The former authors estimate the drop in elevation between the northern and southern ends of the Bosporus to be about 35 cm while the pycnocline slopes about 30 m in the opposite direction. The southward surface flow has an average salinity of 17.5, which is a little lower than the average Black Sea surface water (S = 18.3). The average salinity of the northward flowing bottom current is between 35 and 38 (Gunnerson and Ozturgut, 1974; Sorokin, 1983; Unhiata et al., 1989). Water of this salinity is not found anywhere in the Black Sea except in the proximity of the Bosporus due to the entrainment of water of lower salinity to yield the value of around 22.3 characteristic of the deep Black Sea (e.g. Murray et al., 1991).

Typical salinity vs depth profiles in the Black Sea rise, rapidly from 17.5-18.5 in the surface water to about 21 by 100 m depth and then more slowly to 22.3 at 2100 m. Potential temperature is seasonally variable in the surface water, drqpping to a minimum of about 7°C in the cold intermediate layer (CIL) centered between 50 and 100 m, rising rapidly to 8.8”C at 200 m and then slowly to 8.9”C at 2100 m. The cold intermediate layer is a permanent feature renewed annually during winter by formation of cold water on the broad northwestern shelf region (Tolmazin, 1985a) and in the central cyclonic eddies (Ovchinnikov and Popov, 1987) and spreading laterally on isopycnal surfaces.

Murray et al. (1991) used a box model of water, salt and heat balance based on a Bosporus inflow of 312 km3 yr-’ (Unltiata, et al., 1989) to determine an average ratio of

Reduction rates and concentrations in the Black Sea 1241

entrained surface water to inflowing Mediterranean water of 3.3 for the Black Sea water column below 50 m. Heat balance requires that the entrained water be from the CIL. This model yields a residence time for the deep Black Sea waters of 387 yr, which is somewhat less than the 475 yr estimate of Ostlund (1974) and 410 yr estimate of Latif et al. (1991) based on salt balance alone. Ventilation of the deep water column apparently occurs at all depths as variations in the entrainment ratio about this mean lead to production of parcels of varying density which spread laterally along their respective isopycnal surfaces. These parcels are detectable near their source region along the southwestern shelf as oscillations in temperature and salinity profiles (Oguz et al., 1991, Latif et al., 1991) and in nutrient and sulfide profiles (Friederich et al., 1990, Codispoti et al., 1991). Thus, as pointed out by Buesseler et al. (1991)) biological and geochemical water column data must be interpreted laterally as well as vertically since features in the vertical profiles may be related to basin margin sources and sinks in addition to in situ processes.

Previous sulfate reduction rate measurements

The first 35S-based sulfate reduction rate measurements in the Black Sea were made by Sorokin (1962) who reported results for three water column and eleven sediment sites. The water column rates were highest at 200-300 m and sulfate reduction activity was undetectable at most other depths except at one site where measurable activity was found below 1600 m. Sulfate reduction rates in the sediments were measurable only in the upper 5-10 cm. These rates were higher in the sediments of the northwestern shelf and at the eastern end of the basin than in the central or Bosporus regions. The rates were not related to the organic carbon content of the sediments, which was higher at the sites in the central area of the basin than at the margins but was apparently refractory. Sorokin speculated that low molecular weight fatty acids and carbohydrates diffused upward through the recent sediments from buried sapropelic layers to fuel the sulfate reduction in the surficial sediments.

Later work on sulfate reduction rates in sediments from the western Black Sea was done by Vadnshteyn et al. (1985). With more sensitive techniques than those employed by Soroki measu (1990) i

they were able to detect activity at all depths in their cores down to 2-3 m. Their d rates decreased downcore and with increasing water depth of the site. Lein et al. nd Lein and Ivanov (1990) reported several more rate measurements in sediments

and th$ former authors reported rates for two water column sites and came to the conclus i on that the sulfide in the anoxic waters of the Black Sea was mainly due to water column sulfate reduction just below the pycnocline.

No studies of the seasonal variation in Black Sea sulfate reduction rates have been published, and the data on spatial variation and the relative importance of water column and sediment sulfate reduction is far from complete. This study reports sulfate reduction rate data for the central Black Sea water column and abyssal sediments made during May 1988 on Leg 2 of the US-Turkish Black Sea Oceanographic Expedition aboard the R.V. Knorr. Also reported are measurements of the concentrations of various low molecular weight organic acids in the water column and sediment porewaters which serve as substrates for sulfate reducing bacteria, allowing assessment of their potential for limiting sulfate reduction rates in this system.


46ON

41°

42 o

4o”

26OE 28’ 30° 32’ 34’ 3fj” 38’ 40’ 42’

Fig. 1. Map of the Black Sea showing the locations of the three stations sampled. BS2-1 at 41”10’N, 30YKVE; BS2-2 at 42”50’N, 32”OO’E; BS2-3 at 43”05’N, 34”OO’E.

METHODS

Location of the sampling stations

Samples were obtained from three sites in the western and central Black Sea on Leg 2 of the 1988 Black Sea cruise on the R.V. Knorr between 14 May and 28 May (Fig. 1). These are designated BS2-1, BS2-2 and BS2-3 on the map. We have a very incomplete data set for BS2-1, so it will not be discussed here. The other sites will hereafter be referred to as Stas 2 and 3 for convenience. All three sites were in water approximately 2100 m deep. Stas 2 and 3 had laminated (Unit 1) sediments throughout the upper 30 cm with l-2 cm of flocculent material at the sediment-water interface. Stas 2 and 3 were in the same approximate locations as Leg 4 Stas 9 and 14. Lyons (1991) showed that the pattern of variation in varve thicknesses at these two sites was nearly identical although they are separated by 210 km. Ross et aE. (1970) and Ross and Degens (1974) showed that individuals varves were traceable over 1000 km, nearly the total length of the basin, indicating that sedimentation processes are fairly homogeneous over broad areas.

Water column sulfate reduction rates

Water column sulfate reduction rates were measured in both shipboard and in situ incubations to determine whether or not there were adverse effects on the rate measurements caused by depressurization or shipboard handling. In situ incubations were made in


a Submersible Incubation Device (SID) of a design intermediate between the devices described by Taylor ef al. (1983) and Taylor and Doherty (1990). Automated in situ incubations were conducted in a 1 1 chamber constructed from silanized, oxygen- impermeable glass and machinable ceramic. Prior to deployment the interior of the incubation chamber was made anoxic with flowing oxygen-free nitrogen. Carrier-free 35S-sulfate (approximately 410 MBq) was introduced, oxygen-free, into an injector coil on the SID. Tracer was automatically mixed with incoming sample during in situ procurement in a manner analogous to that described in Taylor and Doherty (1990). Incubations lasted approximately 10 h, during which time 200 ml subsamples were obtained at three equal intervals beginning with zero time. Subsamples for each time point were obtained in four 50 ml subsampling syringes over a 5 min interval and preserved with saturated (1.6 M) zinc acetate (7 ml per 50 ml syringe), which stabilized sulfide in the form of insoluble zinc sulfide and stopped biological activity within a few minutes. Samples for specific activity determination were obtained from sample remaining in the incubation chamber upon retrieval of the instrument.

Bottle incubations were conducted aboard ship in 120 ml serum bottles that were filled from Niskin bottles on a CTD rosette sampler. The bottles were filled from the bottom, allowed to overflow for 3-4 volume exchanges, and quickly sealed with Teflon backed butyl stoppers. Within approximately 30 min the bottles were injected (26 ga needles) with about 40 mBq (0.1 ml) of carrier-free 35S-sulfate, mixed and incubated in the dark at 9°C for a nominal 24 h period. At the end of incubation samples were removed for determination of the 35S activity in the sulfide following displacement of 6 ml of the sample by addition of 1 ml of 0.3 M sodium sulfide carrier solution and 5 ml of a saturated solution of zinc acetate (1.6 M).

Biologically produced 35S-sulfide was measured by a modification of an active distillation/chromium reduction technique (Westrich, 1983). The distillation apparatus consisted of a 300 ml stripping vessel and a train of three gas absorption vessels (28 ml scintillation vials) through which nitrogen gas was passed after prior removal of trace oxygen by passage through a bed of copper at 300°C. Zinc sulfide in the samples was allowed to settle, and a portion of the supernatant was removed to reduce the volume to approximately 100 ml. The samples were then transferred to the stripping vessels which were kept anoxic by continuous flushing with oxygen-free nitrogen. After transfer, 4 ml of 0.1 M sodium sulfide carrier solution was added to SID samples. Bottle samples already contained appropriate carrier sulfide. The stripping vessels were capped, and flushing of the distillation apparatus with oxygen-free nitrogen continued for 5 min. Samples were then acidified by addition of 10 ml of 2 M HCI containing 0.5 M Crf2, prepared previously by reduction of an anoxic solution of CrCl, solution in a bed of zinc-mercury amalgam bubbled with nitrogen (Skoog and West, 1969). Hydrogen sulfide released within the stripping vessel was carried by the flowing nitrogen to the series of absorption vessels. Because of the high levels of 35S employed in these experiments, the first vessel in the train contained 20 ml of an approximately l/10 dilution of the above HCI-Cr+2 solution to trap any “‘S-sulfate that escaped from the stripping vessel as an aerosol while allowing the labelled sulfide to pass through. Hydrogen sulfide was trapped in the remaining two absorption vessels, each containing 10 ml of 0.5 M zinc acetate in 10% ethanol. The alcohol was present to prevent foaming. After the samples were stripped for 30 min the trap contents were mixed with an equal volume of Scintiverse II (Fisher Scientific) for counting on a Beckman Model 100 scintillation spectrometer. Counts were corrected for


quench from the counting efficiency of an external standard. Sulfate reduction rates were calculated as described by Jorgensen (1978).

The detection limit of this measurement, which was dependent on the activity added and the incubation time, was approximately 0.2 nM day-’ in the water column. Many of the measurements made in this study were near the detection limit. The very high 35S-sulfate activity required for these measurements (especially in the SID deployments) limited the number we were able to do. Taylor et al. (1993), using a similar SID device and bottle incubations for productivity measurements, obtained coefficients of variation (c.v.) of 12 and 15%, respectively, for these techniques. The variation in sulfate reduction rate measurements near the detection limit is higher than that because of the low activities of 35S-sulfide produced. In the current study the C.V. for the bottle measurements done in duplicate at depths of 150,200 and 450 m ranged from 834% (mean = 20%). The SID measurement was duplicated at 210 m and had a C.V. of 111% (Table 1).

Sediment sulfate reduction rates

Sediments for sulfate reduction rates were retrieved by box corer, and subsamples were obtained with 8.3 cm I.D. x 30 cm long plexiglass core tubes. These cores were extruded past a sampling hole in the tubing wall, and triplicate horizontal subcores were taken at 2 cm intervals to a sediment depth of 20 cm with cut-off 3 ml glass syringes. The subcores were carefully stoppered to exclude 0, and immediately placed in a water bath at the temperature of the bottom water (9°C). The syringe core stoppers had a hole that was plugged with silicone to allow injection of the labelled sulfate. Additional mud samples were taken for porosity determination at each depth and for centrifugation to obtain porewater for sulfate and organic acid analyses. The syringe cores were line injected with about 40 KBq 35S-sulfate contained in a 10 microliter volume and incubated at 9°C for approximately 48 h. The labelled sulfide was stripped as above except the stripping vessel was of different design that allowed the syringe cores to be introduced without air contact. Because of the much lower 35S activities used in the sediment studies, the aerosol trap described above was not necessary. The rates were calculated as described by Jorgensen (1978). The detection limit of this measurement (-0.5 ,uM day-‘) was higher than in the water column because less 35S-labelled sulfate was added. The coefficient of variation of the method itself (on splits of a homogenous sample) is typically 5-10%. The C.V. between samples in this study average 44%.

Porewater analyses

The sediment samples for collection of porewater were immediately centrifuged at 10,000 rpm for 20 min in a gimballed Sorvall SS-3 centrifuge. The porewater was filtered through pre-combusted Whatman GF/C glass fiber filters and frozen in acid-washed polypropylene scintillation vials until analysed in the laboratory (Chapel Hill, NC).

Sulfate analyses were performed on a Dionex Model 10 ion chromatograph equipped with an AS-3 analytical column after 100 fold dilution of the samples with distilled water. Organic acid analyses were conducted via high performance liquid chromatography (HPLC) after pre-column derivatization of the acids with 2-nitrophenylhydrazine. This procedure is described in detail elsewhere (Albert and Martens, submitted to Marine Chemistry).


Sulfate Reduction Rate (nM.d-*)

0 1 2 3 4 0.00 0.25 0.50 0.75 t . I . I .

B

0

BSZ-3

00

Fig. 2. Sulfate reduction rates vs depth in the water column at Stas 2 and 3. All data are shown. The solid line in 2a runs through the average values at depths where replicate measurements were made. The dotted line shows the profile assumed between 600 and 2000 m for purposes of depth

integration based on the low rate at Sta. 3.

RESULTS


Profiles of water column sulfate reduction rates were obtained at Stas 2 and 3 via bottle and SID techniques (Fig. 2, Table 1) from 104 to 2100 m depth. On the same cruise Jorgensen et al. (1991) measured dissolved oxygen and sulfide concentration profiles at these stations. At Sta. 2 oxygen was depleted below 50 m, but dissolved sulfide was not detectable above 80 m, and rose steadily with depth below that. Our shallowest rate determinations at 104 and 136 m yielded unmeasurably low sulfate reduction rates. Measurable sulfate reduction began at 150 m and reached maximal rates of over 3 nM day-’ between 200 and 300 m. We have no data between 600 and 2000 m depth due to samples being compromised. The dashed line on Fig. 2a shows the profile assumed for purposes of depth integration of the rates. It is based on the low rates measured at Sta. 3 between 1000 and 2000 m.

The closed symbols in Fig. 2a represent the SID based measurements. Only three successful SID deployments were made before the instrument was lost at sea due to failure of the stainless steel bail that attached it to its mooring. The symbol at 210 m is the mean of two SID measurements at this depth. The other closed symbol at 400 m represents a single SID measurement. Since the SID fills with a water sample in situ and the bottle measurements were done on samples from Niskin bottles we were unable to compare measurements with the two techniques for a given water sample, but the results in Fig. 2A and Table 1 show that they yield comparable results.

While our samples nearest the oxic/anoxic interface showed no measurable sulfate reduction activity at 104 and 136 m we have no data between the depth of oxygen depletion (50 m) and 104 m. Jorgensen et al. (1991) measured sulfate reduction rates in four samples


Table 1. Sulfate reduction rates in the Black Sea water column at Stas 2 and 3 in May 1988. All but those with asterisks were bottle incubations. Rates are nM day-’ with standard deviations

(S) shown for replicated measurements

Sta. 2 Sta.3 Depth (m) Rate tS Depth (m) Rate

104 0 150 0.48 136 0 200 0.31 150 2.3 0.79 250 0.10 160 1.7 300 0.41 200 1.7 0.29 350 0.18

*210 3.5 3.9 400 0.18 300 3.1 500 0.18 350 0.43 700 0.26 400 0.46 1100 0.008

*400 0.72 1600 0.043 450 1.0 0.08 2000 0.026 500 0.94 2100 0.12 600 0.70

2000 0.11 2100 0.48

*SID incubations

Depth integrated rates (including data for 86-100 m from Jorgensen et al. 1992)

mmol m-* day-’ mol mm2 yr-’

1.2 0.22 0.44 0.080

taken from nearer the interface than ours on the same cruise. At depths of 86,92,95 and 100 m they obtained rates of 36, 9,3 and 13 nM day-‘, These rates are much higher than any we measured, but they were made at the same time and with essentially identical techniques so we have included them in the depth integration of the rate profile we measured. At this station the depth integrated rate was 1.2 mmol me2 day-‘.

At Sta. 3 the depth of oxygen depletion during this cruise was 80 m and dissolved sulfide accumulation began below 100 m (Jorgensen et al., 1991). Our shallowest rate measurement was at 150 m. Here, as at Sta. 2, most of the sulfate reduction activity occurred in the upper half of the water column (Fig. 2b, Table 1); however, there was no pronounced peak in activity. These rates were approximately sixfold lower than at Sta. 2, and the maximum rate was only 0.48 nM day-’ . The depth integrated rate for this station was 0.22 mmol mP2 day-‘. On the assumption that water column rates are constant throughout the year (which is unlikely to be true), annual sulfate reduction would be 0.42 and 0.079 mol mP2 yr-’ for Stas 2 and 3, respectively.

Water column organic acid concentrations

At Sta. 2 lactate, acetate and formate were present well above the detection limit for the method (about 200 nM) and tracked each other closely throughout the water column (Fig. 3). Formate concentrations were 2-4 fold higher than lactate and acetate. There was a


Lactate (PM)

0 1 2 3 4 0

600

2 900 5 B a 1200

1500

1800

2100

Acetate (PM) Formate (PM)

0 1 2 3 4 0 2 4 6 8 10 12

Fig. 3. Lactate, acetate and formate concentrations versus depth in the water column at Sta. 2.

sharp concentration maximum for all three acids present at the base of the oxic water column (50 m). Peak concentrations were 1.3, 2.1 and 8.1 ,uM for lactate, acetate and formate, respectively. Between 68 and 150 m, including the suboxic zone and the anoxic interface, concentrations were very low. Between 150 and 200 m organic acid concentrations rose rapidly to a peak at 300 m (4.2, 3.6 and 11.5 PM for lactate, acetate and formate, respectively). The remainder of the water column, down to 2100 m, possessed somewhat lower concentrations, but still highly elevated over those observed throughout the suboxic region. Within the anoxic water column at Sta. 2 there was some similarity in the shape of the sulfate reduction rate profile and organic acid concentrations vs depth (Figs 2a, 3).

At Sta. 3 lactate was undectable throughout much of the water column, and acetate concentrations were generally higher than formate. As at Sta. 2, there were peaks in concentrations within the aerobic water column at 52 m (5.1 ,uM) for acetate, and 36 m (3.5pM) for formate (Fig. 4). Between 60 and 300 m concentrations of all measured acids were very low or undectable. Within the anoxic water column, there was another peak in acetate concentration at 400 m (as compared to 300 m at Sta. 2). Lactate and acetate reached 0.54 and 10.5,~M, respectively. Formate peaked at 500 m at 2.4pM.

In contrast to Sta. 2, Sta. 3 had a very high acetate concentration spike in the near- bottom waters at 2000 m, where it reached 60,~M, dropping back down to 8.4,~M at 2100 m (Fig. 4). Formate was also elevated at this depth, but only to 2.3,uM. At this station there was no similarity in the pattern of sulfate reduction rate and organic acid concentrations as at Sta. 2 (Figs 2b, 4). The peak sulfate reduction rate was offset 100 m above the peak in organic acids, and the very high acetate concentration at 2000 m was not accompanied by high sulfate reduction activity.

Sediment sulfate reduction rates

Profiles of sulfate reduction rates in the sediment were obtained in triplicate at Stas 2 and 3 from the surface floe layer through 20 cm depth at 2 cm intervals (Fig. 5, Table 2). Rates


Lactate (PM) Acetate. (FM) Formate (FM)

0 1 2 3 4 50 2 4 6 8 10 120 1 2 3 4

900

1200

1500

1800

3 BSZ-3

2100 I! 1 BS2-3

Sulfate Reduction Rate (nM.d-l)

Fig. 4. Lactate, acetate and formate concentrations versus depth in the water column at Sta. 3.

30

18 BSZ-2 BS2-3 B

Fig. 5. Sulfate reduction rates in the surface sediments at Stas 2 and 3. Error bars show + standard deviation (n = 3).

were highest, and most variable in the surface floe layer. The variability in this layer was probably due to the potential for disturbance during the box coring procedure. Rates at Stas 2 and 3 were almost identical (within experimental error) and decreased exponentially from about 20,~M day-’ in the floe layer to 2.5-3,~uM day-’ at 10 cm and below. There was no detectable decrease in sulfate concentrations within the sediment porewaters to 20 cm depth as would be expected with rates this low.

Depth integrated sediment sulfate reduction rates from 0 to 20 cm, assuming a 2 cm thickness for the flocculent surface layer, were 1.45 f 0.29 and 1.29 +_ 0.26 mmol me2


Table 2. Average sediment sulfate reduction rates at Stas 2 and 3 in ,ucM day-’ and depth integrated values expressed in mol m-* yr-' All

are means of three samples with standard deviations (S)

Depth (cm) Sta. 2 SRR fS Sta. 3 SRR +S

Floe layer 22 11 20 12 1 11 4.5 12 3.1 3 8.2 4.1 8.0 1.5 5 5.5 2.9 5.4 1.5 7 7.1 4.6 3.7 0.54 9 3.2 1.4 3.5 0.92

11 2.6 1.5 2.8 1.0 13 3.9 3.7 3.0 1.2 15 1.7 0.1 2.8 1.0 17 4.3 0.43 1.6 1.5 19 3.5 1.09 2.1 1.5

mmol m-* day-’ 1.5 0.27 1.3 0.27 mol m-* yr-’ 0.53 0.10 0.47 0.10

day-’ at Stas 2 and 3, respectively. Assuming invariant rates throughout the year yields respective rates of 0.53 + 0.10 and 0.47 k 0.10 mol m-* yr-‘. This assumption is probably more reasonable for the sediments than the water column. Variations in particulate carbon input rate throughout the year would only affect the rates in the surface floe of the sediments, but would affect the entire anoxic water column.

Sediment porewater organic acid concentrations

Porewater acetate concentrations at Stas 2 and 3 were 2.5-3 ,uM in the O-2 cm section, below which they dropped to 0.5-l PM throughout the remainder of the profile down to 20 cm depth (data not shown). Formate concentrations at Sta. 2 showed a similar pattern, dropping from 1.8pM in the O-2 cm section to about 0.5pM below. At Sta. 3 the profile was featureless with concentrations less than 0.5 ,uM throughout. Lactate concentrations ranged from 0.43 to 7.4 PM in the porewaters from Sta. 2, and from 0.25 to 1.1 at Sta. 3; however, the profiles were erratic. Only traces of propionate were detected in a few of the samples.

DISCUSSION


The water column sulfate reduction rates reported here represent about 30 percent of the available information on this process in Black Sea waters, and they add to our nascent understanding of the full range of variability in these rates. In May 1988 the rates were much lower than expected on the basis of Sorokin’s (1962) data. As in his published profiles and those of Lein et al. (1990) most of the activity was found between 150 and 300 m depth. Our highest rates (Table 1) were 3.5 nM day-’ at 210 m (SID rate) and 3.1 nM day-’ at 300 m depth (bottle rate) at Sta. 2. Sorokin’s peak rate, averaged from three stations, was 204 nM day-’ at depths ranging from 215 to 300 m, and Lein et al. (1990) obtained values as high as 132 nM day-’ at 180 m depth.

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integrated average is 2060 mmol m-* yr-‘. It is possible that Sorokin’s results may be biased due to enrichment of sulfate reducing bacteria during the long incubation period at elevated temperature. However, these data may simply reflect the real variations in rates with time and space.

We do not know why there were significant differences in both sulfate reduction rate and organic acid concentration profiles between Stas 2 and 3. Station 2, however, was within the western gyre and Sta. 3 was between the eastern and western gyres. Codispoti et al. (1991) noted that there was greater variability in chemical profiles at this location, probably due to the effects of horizontal advection of waters with different histories at different depths. A similar explanation may be invoked for the data presented here, although the differences are much more pronounced.

Combining our Sta. 2 results with those of Jorgensen et aE. (1991) suggests that in this location water column sulfate reduction may occur in two major regions in the water column, a narrow approximately 10-15 m band of high activity just under the interface, followed by a >600 m band of much lower activity. Though the upper region possessed an order of magnitude greater maximal activity (36 nM day-‘), deep sulfate reduction (maximal rate approximately 3.5 nM day-‘) constituted 85% of total water column activity when expressed on a depth-integrated basis (0.17 mmol me2 day-’ and 0.98 mmol me2 day-’ for the upper and lower regions, respectively, 1.2 mmol m-* day-’ total). We do not know if this pattern would have shown up at Sta. 3 if we had made rate measurements nearer the oxic/anoxic interface. No such pattern is evident in the other published rate profiles, but a narrow band of high activity could easily have been missed simply due to choice of sampling depths.

The upper region of high activity may be associated with the lytic release of readily degradable (perhaps soluble) organic matter from particulate material (e.g. cells, organic floes, faecal pellets) settling into anoxic waters from the upper oxic and suboxic zones. Sulfate reduction in the deeper reaches of the water column may primarily be particle associated, requiring colonization and decomposition of polymeric material by bacteria as the particles settle. The distribution of sulfate reduction substantially below the interface would thus be related to the size spectrum and sinking rates of the particles. The approximately 40 m activity minimum between the upper, interface-associated sulfate reduction zone and deeper water sulfate reduction may be a region where particulate material, stripped of readily releasable, labile organic matter has not yet been sufficiently colonized by particle-degrading microorganisms to support detectable activity. Some evidence for particle associated sulfate reduction is the appearance of pyrite framboids in faecal pellets from surface sediments (Pilskaln, 1988). These were certainly formed after the pellets fell into the anoxic water column, although it is not certain that the source of the sulfide was sulfate reduction within the pellet itself. Pyrite framboids have also been observed in particulate material from sediment traps in the Black Sea (Muramoto et al., 1991).

The observations of lactate, acetate and formate at concentrations up to several micromolar within the anoxic water column at both Stas 2 and 3 (up to 60,~M at Sta. 3,200O m, Fig. 3) may also provide indirect evidence for particle associated consumption (by sulfate reducing bacteria) of these substrates. These concentrations are apparently well above threshold values below which they cannot be used, as evidenced by the fact that they are undetectable (<200 nM) near the oxic/anoxic interface. The presence of organic acids at these relatively high concentrations thus raises the question of why they are not


Table 4. Apparent turnover times (IITJ for the organic acid pools in the water column and in the sediment porewaters at Stas 2 and 3 based on the sulfate reduction rates. Oxidation ratios of 1, 1 and 4 acid molecules per sulfate were assumed for lactate,

acetate and formate, respectively

Water column Sta. 2 Water column Sta. 3 Depth (m) m, (years) Depth (m) m, (years)

92 0.04 150 1 150 0.4 200 0 200 7 300 0 300 15 350 8 350 8 400 180 400 22 500 14 600 13 1100 530

2000 56 1600 210 2100 8 2000 6600

2100 210

Sediments Sta. 2 Sediments Sta. 3 Depth (cm) n= (days) Depth (cm) m, (days)

1 0.47 1 0.32 3 0.18 3 0.17 5 0.28 5 0.29 7 0.63 I 0.46 9 0.58 9 0.54

11 1.75 11 0.50 13 0.27 13 0.75 15 1.18 15 0.61 17 2.14 17 1.05 19 0.48 19 0.32

consumed through development of a more active sulfate reducing population at those depths. If the sulfate reducers were free-living one would expect utilization to readily occur. Calculation of apparent turnover times for lactate+acetate+formate via sulfate reduction, assuming oxidation ratios of 1,l and 4 acid molecules oxidized, respectively, per sulfate reduced, yields highly variable results. The values range from less than one year in the upper 300 m of the water column to many years at greater depths (Table 4). We speculate that these acids are both produced and consumed by activity within the sinking particles, but once free of them these potentially labile compounds may be out of reach of the bacteria sinking within the particles. It should be noted that most of the estimates of apparent turnover times in Table 4 are at odds with a 10.2 day turnover time for 14C-labeled acetate measured by Mopper and Kieber (1991) on Leg 5 at a depth of 200 m (near our Sta. 3). Their Fig. 3 shows the acetate concentration at this depth to be about 8,~uM. A 10.2 day turnover time would require consumption of -780 nM day-‘. This is not an impossibly high rate, but sulfate reducing bacteria would be the most likely consumers of this acetate and this rate is several times as high as the highest published sulfate reduction rate in the water column and over 200 times the highest value we measured two months earlier.


The very high acetate concentration (60 PM) observed at 2000 m at Sta. 3 was not accompanied by an elevated sulfate reduction rate. This was a persistent enough feature to have been observed 2 months later (80pM) on Leg 5 by Mopper and Kieber (1991) so the sulfate reduction rate, if it increased at all, did not increase by enough to consume this unusually high concentration. This acetate is probably of episodic origin. It did not come from the abyssal sediments underlying Sta. 3, which had porewater concentrations below 1 PM and thus represent an acetate sink rather than a source. The amount of carbon involved is significant. Based on our data and that of Mopper and Kieber (1991), somewhere between 26 and 36 mol C mm2 are present as acetate in the lower 300 m of the water column at this site. Little can be said about the persistence of this feature except that it lasted for at least 2 months during 1988. The apparent turnover time of the organic acid pool at 2000 m depth based on the low sulfate reduction rate was 6600 years (Table 4). No doubt this value is incorrect or mixing would smear the feature throughout the basin, but even its two month persistence is indicative of sulfate reducing bacteria being limited by some factor other than organic substrate supply.

Mopper and Kieber (1991) measured concentrations of various other low molecular weight organic compounds in the water column on Leg 5, several of which are potential substrates for sulfate reducing bacteria. In general these concentrations were 10-100 times lower than the concentrations of lactate, acetate and formate they measured, which were comparable to those measured on Leg. 2. Biological uptake and turnover of several of these compounds were measured using 14C-labeled substrates. Uptake rates for all but acetate and an amino acid mixture were in the pM day-’ range. The amino acid mixture yielded a 23 nM day’ uptake rate of which 46% was respired, making it higher than, but comparable to, our sulfate reduction rates. Acetate, as discussed above, yielded a very high turnover rate by comparison.

Methane is another potential substrate for sulfate reducing bacteria. The organism(s) responsible for anaerobic methane oxidation and the electron acceptors they use have not been identified (Widdel, 1988), but there is strong circumstantial evidence that methane may be oxidized by some sulfate reducing bacteria (Devol ef al., 1984; Alperin and Reeburgh, 1985; Iversen and Jorgensen, 1985). Reeburgh et al. (1991) measured rates of anaerobic methane oxidation in the water column and sediments of the Black Sea during July of 1988 using both 3H-CH4 and 14C-CH,. In the water column their 14C-CH, rates were extremely variable, ranging up to about 1 PM day-’ or about 250 fold higher than the high value of sulfate reduction we measured. The much more self-consistent 3H-CH4 rates ranged from l-2 nM day-’ throughout most of the anaerobic water column with peaks of about 4-5 nM day-‘* , thus they were very similar to our sulfate reduction rates. In general, when anaerobic methane oxidation rates and sulfate reduction rates have been measured simultaneously the oxidation rates range from slightly to substantially lower than the sulfate reduction rates. The Reeburgh et al. (1991) oxidation rate data range from values equivalent to our sulfate reduction rates to much higher and cannot be said to support or negate arguments about the involvement of sulfate reducing bacteria in this process; but if their higher rates are correct, it argues that sulfate reducers can be responsible only for a fraction of the anaerobic methane oxidation.

Given the relative abundance of acetate in the water column it is even possible that acetoclastic methanogens are active in the water column in spite of the abundance of sulfate, since the sulfate reducers are not effectively utilizing all the acetate. Iversen et al. (1987) demonstrated simultaneous methane production and anaerobic methane oxidation


in the anoxic water column of Big Soda Lake, Nevada, concurrent with sulfate reduction measured by Smith and Oremland (1987).

Postgate (1956) states that sulfate reducing bacteria have a very high (but unspecified) iron requirement. Obtaining sufficient iron for growth could pose problems for an organism whose own byproducts lead to the precipitation of highly insoluble iron minerals. Dissolved iron concentrations in the Black Sea water column are highest several tens of meters below the oxic/anoxic interface, presumably due to reduction and dissolution of particulate iron sinking out of the oxic surface waters. At depths ranging from 170-300 m iron concentrations reach values of about 200-600 nM (Spencer and Brewer, 1971; Haraldsson and Westerlund, 1988; Lewis and Landing, 1991). Below this maximum they drop to 20-30 nM and are potentially controlled by the solubility of mackinawite (FeS) or greigite (Fess,), although pyrite (FeS,) is highly supersaturated (Lewis and Landing, 1991). If iron concentrations were limiting to sulfate reducing bacteria below the dissolved iron maximum, sinking particles would be the only source of additional iron and hence could be the principal sites for active sulfate reduction. Although dissolved sulfide concentrations in anoxic sediments are often ten times greater than in the Black Sea water column, and dissolved iron concentrations are presumably lower as a result, the bacteria in sediments are always in close proximity to particulate iron sources.

Sulfate reduction rates in surficial sediments

On a volumetric basis, sulfate reduction rates in the abyssal sediments of the Black Sea were up to several thousand times higher than in the water column. Depth integrated rates in the upper 20 cm of the sediment column were equal to or greater than in the entire 2000 meters of overlying anoxic water. These rates were essentially identical at Stas 2 and 3 (Fig. 5) which is not surprising considering the similarity of the sediments at these two locations. Lyons (1991, Fig. 5) shows side by side X-radiographs of the upper 20+ cm in cores from Leg 4 Stas 9 and 14. These were very near our Stas 2 and 3, respectively, and show that the patterns of varve thicknesses at these locations are also identical. This probably indicates that the input of organic carbon (which drives the sulfate reduction rate) is similarly equivalent. In contrast to the water column, the apparent turnover times for the pooled organic acids in the sediments, based on the sulfate reduction rates, were only hours (Table 4). Thus it is likely that the production of these substrates through fermentation limits the rate of sulfate reduction in these sediments. The Reeburgh et al. (1991) methane oxidation rates in sediments ranged from a high of about 3 nM day-’ in surface sediments to about 0.1 nM day-’ at 20 cm depth. In contrast with the water column, these values are lower than the sediment sulfate reduction rates by a factor of about 20,000, so even if sulfate reducers are involved in this oxidation, it represents a trivial proportion of their organic nutrition.

The sediment sulfate reduction rates measured in this study are much more comparable to previously published values than the water column rates. Our mean area1 rate (0.50 mol m-2 yr-‘) was somewhat higher than that obtained by Sorokin (1962) at his central site Sta. 4751 (0.18 mol rnw2 yr-‘), but well within the range he measured at various locations in the Black Sea (0.014-l .20 mol me2 yr-‘). At depths greater than 1000 m he obtained a


mean of 0.24 mol m-* yr-’ (Table 3). Vaynshteyn et al. (1985) obtained a range of 0.050-l .50 mol me2 yr-’ for several stations in the western Black Sea. None of these were near our Stas 2 or 3, but the overall average of these rates was 0.39 mol m-* yr-’ which is close to our average although at depths greater than 1000 m their mean was only 0.07 mol mP2 yr-‘. Lein et al. (1990) obtained an average rate of 0.90 mol m-2 yr-’ for seven stations at depths over 1000 m.

It is little surprising that the low end of the range of values seen by some of the above authors is so much lower than our rates, because both of our stations are in the center of the basin at about 2100 m depth with varved sediments that are probably at the low end of the spectrum for sedimentation rate and labile organic carbon input. We would expect our sulfate reduction rate values to be at the low end of the range for Black Sea sediments.

Lyons (1991) showed that the pyrite sulfur content of the sediments from his Stas 9 and 14 (near our Stas 2 and 3) does not increase with depth in the upper 26 cm of the sediment column. This is presumably because pyritization of most of the available iron has already taken place by the time particles arrive at the sediment-water interface. This is consistent with the finding of Muramoto et al. (1991) that the sulfur isotopic signature of surficial sediments appears to be acquired by precipitation of iron sulfides near the oxic/anoxic interface in the water column rather than in the sediments. These findings lead to the interesting conclusion that water column sulfate reduction contributes most of the solid phase sulfur found in abyssal Black Sea sediments while sulfate reduction within the sediments (and the subsequent diffusive flux of sulfide out of the sediments) contributes a major fraction of the sulfide in the water column. Our data indicate that at Stas 2 and 3,55 and 85% respectively, of the total sulfate reduction activity occurred in the sediment. Due to the difference in the water column rates, our total rate for Sta. 2 is about twice as high as for Sta. 3 (2.65 vs 1.50 mmol m-* day-‘). The relative proportions of water column and sediment sulfate reduction reported here are unlikely to be representative of the Black Sea as a whole. The greater input of labile organic carbon to shallower water sediments will result in much higher rates. However, the greater input of iron from terrigenous sources will also result in a higher degree of sulfide retention in those sediments.

Sulfate reduction in relation to the dissolved sulfide inventory

Depth integration of 1988 dissolved sulfide concentration data in the central Black Sea (Luther et al., 1991; Millero, 1991; Mopper and Kieber, 1991) yields a range of 670-680 mol mP2. These agree well with Brewer and Spencer’s (1974) data, which yield 675 mol m-*, but are somewhat higher than the Vaynshteyn et al. (1985) value of 471 mol m-2. It is useful to compare the sulfide inventory in the water column to the rate of sulfide production to get an estimate of the time scale required for the anoxic waters to reach their current composition. For a sulfide content of the abyssal Black Sea water column of 675 mol m-’ and with no deepwater replacement assumed, it would require about 890 years to produce this much sulfide with our measured sulfate reduction rates (water column + sediment rates = 0.76 mol m-* yr-‘).

More accurate estimates of the time required to accumulate the present sulfide content require consideration of losses due to sulfide oxidation at the oxic/anoxic interface and precipitation of metal sulfides in the sediment. Brewer and Spencer (1974) calculated the

1256 D. B. Albert etal.

upward flux of dissolved sulfide lost through oxidation at the interface as 0.075 mol m-* yr-‘. Jorgensen et al. (1991) measured sulfide oxidation rates much higher than this near the interface, but considered their measured rates to overestimate in situ rates by about lo- fold and stated that the flux of sulfide across the interface, based on their measured sulfide gradients in 1988, was probably close to Brewer and Spencer’s value.

The amount of sulfide produced through sulfate reduction that gets incorporated into metallic sulfides in the sediments is unknown for the stations we occupied but can be reasonably estimated. Pyrite sulfur makes up most of the reduced sulfur in the Sta. 9 and 14 sediments of Lyons (1991) (near our Stas 2 and 3). Pyrite sulfur averaged 1.3 wt. percent in the upper Unit I sediments and was reasonably constant with depth. Hay et al. (1991) obtained a bulk sedimentation rate of 85.2 g m-* yr-r for the late Holocene Unit I sediments at their Sta. 21 on Leg 1 which was near our Sta. 2. With Lyon’s data, these results yield a reduced sulfur burial rate of 0.035 mol m-* yr-’ or about 4.5% of the sulfide produced. Sulfide oxidation and sulfur burial thus add up to 0.110 mol m-* yr-i. This is 14.5 percent of the combined water column and sediment sulfate reduction rate, and taking these sinks into account extends the estimate of the time required for the Black Sea water column to achieve its current sulfide content from 890 to 1040 years.

The Brewer and Spencer (1974) estimate of sulfide oxidation used above is based on eddy diffusive mixing across the oxic/anoxic interface. Another potentially more import- ant mechanism of sulfide oxidation in the Black Sea water column is through sub- pycnocline intrusions of oxygen bearing waters near the Bosporus. It is difficult to assess the magnitude of sulfide oxidation through this mechanism, but its potential can be shown to be significant. The box model of Murray et al. (1991) achieves salt and heat balance for the Black Sea deep waters through entrainment of water from the CIL in a ratio of 3.3:1 with the inflowing Mediterranean water from the Bosporus. The oxygen content of this mixture probably varies with time, but the data of Friederich et al. (1990) show the CIL near the Bosporus contains as much as 300 PM 02. Even if the Mediterranean inflow was anoxic the entrainment mixture could thus contain as much as 230 ,uM 02. The volume of this entrainment flow, based on the Unhiata et al. (1989) Bosporus inflow of 312 km3 yr-’ and the Murray et al. entrainment ratio is thus 1340 km3 yr-‘. Murray calculated an average upwelling rate within the anoxic water column of 3.6 m yr-’ due to the injection of this mixture. Thus as much as 0.83 mol m-* yr-’ O2 may be injected into the anoxic water column which could oxidize over half of th annual sulfide production (based on our rates). Sub-pycnocline oxygen injection is thus p entially the greatest sink for sulfide in the water & column and may double the estimate of time required for the Black Sea to acquire its current sulfide content.

Our measured sulfate reduction rates are reasonably consistent with the observed shoaling of the oxic/anoxic interface in recent decades. The interface has moved upward by as much as 60 m in 30 years (Bryantsev et al., 1988; Fashchuk and Ayzatullin, 1986; Murrey et al., 1989). If this has been accomplished without dilution of the sulfide concentration in the anoxic water column, it would require input of an additional 24 mol rnp2 sulfide (assuming 400 PM concentration). Our average total rate would produce 23 mol me2 over 30 years without consideration of sinks, or perhaps as little as half that with the oxidation and precipitation losses described above. Given the uncertainties in these losses and the temporal and area1 variations in rates, this is in reasonable agreement. Note, however, that simple dilution of the anoxic water column to raise the interface by 60 m would only decrease the sulfide concentration by about three percent and could easily go unnoticed.


Sulfate reduction in relation to carbon fluxes

Deuser’s (1971) estimate of particulate carbon flux into the anoxic water column was 1.67 mol m-2 yr-‘, of which 0.83 mol was presumed oxidized via sulfate reduction, 0.42 mol was solubilized and added to the dissolved organic carbon reservoir, and 0.33 mol was buried as fossil carbon in the sediments. During our cruise leg, sediment traps deployed at Sta. 2 at 80 m by Karl and Knauer (1991) yielded an annualized value of 1.19 mol C rns2 yr-r (-7% of the contemporaneously measured primary production), a value very comparable to Deuser’s. An assumed ratio of carbon oxidized to sulfur reduced of 2 gives a value of 1.52 mol mV2 yr-’ C oxidized via sulfate reduction from our data. This is 91 and 128%) respectively, of Deuser’s (1971) and Karl and Knauer’s (1991) C flux estimates or 183% of Deuser’s estimate of sulfate reduction.

More detailed data on particulate fluxes over several annual periods (Muramoto et al., 1991) show that particulate organic carbon flux varies markedly throughout the year from as much as 5 mmol mm2 day-’ in summer to near zero in winter. The summer high expressed annually gives 1.82 mol rnp2 yr-‘, very similar to Deuser’s (1971) estimate and Karl and Knauer’s (1991) short-term sediment trap result during Leg 2. The Muramoto et al. (1991) annual average flux, however, is 0.39 mol rnd2 yr-i. Thus our measured rates appear to be in reasonable agreement with carbon flux estimates. Sorokin’s (1962) temperature-corrected water column rate averaged from three sites is 3.4 mol rnp2 yr-' , which would require about 6.8 mol rnp2 yr-r carbon input, several times the existing estimates.

It is worth noting that throughout the anoxic water column the concentrations of identified organic acids reported here are higher, in many cases severalfold, than the particulate organic carbon concentrations reported by Wakeham and Beier (1991). The carbon in the form of lactate, acetate and formate depth integrated and averaged for Stas 2 and 3 represents over 1.5 years worth of the primary production in the euphotic zone, based on the rates presented by Karl and Knauer (1991) or Sorokin (1983). Based on the short-term particulate carbon flux estimates of Karl and Knauer (1991); annualized to 1,19 mol rnp2 yr-‘) or the long-term time averaged value of Muramoto et al. (1991; 0.389 mol me2 yr-i), this standing stock of acids represents 24-75 years worth of accumulation even assuming the particulate carbon was completely solubilized to form them and nothing else. This is within the range of apparent turnover time values for the organic acids presented in Table 4.

Given the extremely labile nature of lactate, acetate and formate in more well-studied anoxic environments, i.e. marine sediment porewaters, their relatively high concentrations in the Black Sea water column raise some interesting questions. The failure of the sulfate reducing bacteria to utilize these substrates implies that, at least at the time our measurements were made, they were limited by something else. The vertical structure in the organic acid profiles and the significant differences between Stas 2 and 3 could not be maintained over long periods of time, however, if the turnover rates were really as slow as Table 4 implies. The structure would be destroyed by mixing and diffusion. Perhaps the sulfate reduction rate varies significantly with time, not in response to organic substrate supply, which appears to be more than adequate, but in response to some other limiting substance(s).

Acknowlerlgements-This work was supported by NSF grant OCE-8608958. We thank Elizabeth Camel, Ken


Doherty and Zophia Mlodzinska for their dedicated assistance before and during the cruise. The manuscript benefitted from two anonymous reviews and several useful discussions with Marc Alperin.

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