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Sulfur, iron and organic carbon fluxes in the Black Sea: sulfur isotopic evidence for origin of sulfur fluxes
Deep-Sea Research \,,,1 Pnnted In Great Bnram
3~
Suppl : pp
0198-D149191 $3.00 + 0 00 1991 Pergamon Press pIc
Sll
©
Sulfur, iron and organic carbon fluxes in the Black Sea: sulfur isotopic evidence for origin of sulfur fluxes Jo ANN MURAMOTO,"t SUSUMU HONJO,t BRIAN FRY,:!: BERNWARD J. HAY,t ROBERT W. HOWARTH§ and JOHN L. CISNE" iRecetved 12 ill/wary 1INC.);
In
revised form I February 1991; accepted 4 February 1991)
Abstract-In the southern and central Black Sea, reduced sulfur comprises nearly 1% of the total particulate flux in the anoxic water column, as measured by time-series sediment traps. Sulfur isotopic composuion of particulate sulfide fluxes indicates an origin near the oxic-anoxic interface, where dissolved sulfide is isotopically heavier than dissolved sulfide below 175 m due to chemical oxidation A strong seasonal correlation exists between sulfur and organic carbon fluxes; this, together with particulate sulfide precipitation at the oxic-anoxic interface, suggests that sulfides are scavenged from the mterface during maximum production of settling organic aggregates, particularly dunng (he late summer coccohthophorid blooms. Sulfide precipitation deeper in the water column and 10 sediments may be hrruted by availability of iron and/or polysulfides, since there are no particulate sulfide fluxes which have the same isotopic composition as ambient dissolved sulfide at trap depths. Smce sulfides in surface sediments are often Isotopically similar to dissolved sulfide at the top of the sulfide zone and to sediment trap fluxes, sedimentary sulfides could originate from metal sulfide fluxes forming at the oxic-anoxic interface and would reflect sulfur-cycling processes at the OXic-anOXIC mterface Sedimentary sulfate reduction rates estimated from assuming 65-80% utilization of particulate organic carbon fluxes agree reasonably well with other estimates of sulfate reduction and sulfide production rates. A sulfur isotope-based box model for sulfur cycling between dissolved and particulate phases in the water column and sediments is presented.
INTRODUCTION
IN marine anoxic basins such as the Black Sea, the biogeochemical cycles of sulfur, organic carbon and iron are closely linked via microbial sulfate reduction. Accumulation of dissolved hydrogen sulfide from dissimilatory sulfate reduction has resulted in anoxic sulfidic water being present everywhere below approximately 100 m. Roughly 90% of the total volume of the Black Sea is today sulfidic, or about 5.56 x 105 krrr' (SOROKIN, 1983). Transformations of sulfur between dissolved and solid phases are of great interest because sulfur is involved in a wide range of biological and chemical processes, many of which take place in the water column; temporal changes, whether seasonal or longer term, are particularly important to understand because sulfur is involved in many long-term biogeochemical processes. Understanding the limiting roles played by organic carbon and "Department of Geological SCIences. Cornell University, Ithaca, NY 14853, U.S.A. tPresent address: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 0254J. U.S.A. :f:Ecosy~tems Center. Marine Biological Laboratory, Woods Hole, MA 02543, U.S.A. § Department of Ecology and Systematics. Cornell University, Ithaca. NY 14853. U.S.A. S1151
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iron is essential to understanding the origin and seasonal behavior of reduced sulfur compounds, while sedimentary sulfides can potentially provide a record of the development of anoxic conditions in sediments and the water column. Thus it is desirable to know the rates of processes controlling distributions of organic carbon, sulfur and iron in both the water column and sediments. In marine anoxic environments, supply of reactive organic matter controls sulfate reduction rates and sulfide production, and reactive iron limits iron sulfide precipitation (BERNER, 1970, 1984; GOLDHABER and KAPLAN, 1975; WESTRICH, 1983; WESTRICH and BERNER, 1984; RAISWELL and BERNER, 1985). Linear relationships between reduced sulfur and organic carbon content in marine sediments were proposed as a useful diagnostic tool for distinguishing anoxic vs oxic sedimentary environments and determining whether sulfate reduction was taking place in the water column or not (GOLDHABER and KAPLAN, 1974, 1975). Thickness of the overlying anoxic water column might be inferred from the slope and intercept of the linear relationship . In Black Sea sediments, the linear relationship between reduced sulfur and organic carbon in sediments suggests that sulfide precipitation should be occurring in the anoxic water column above sediments (LEVENTHAL, 1983; RAISWELL and BERNER, 1985). Sulfides may also be associated with settling aggregates or marine snow, which provide a major means of transport of material to the deep ocean floor (HONlO et al. , 1984, 1985), such organic particles could also serve as sites of sulfate reduction for sulfate-reducing microorganisms . For these reasons, seasonal variations in particle fluxes of organic carbon and iron could seasonally influence particulate sulfide fluxes as well. Variations in particulate organic sulfur were related to seasonality in organic matter and chlorophyll fluxes in the Southern California Bight (MATRAI and EpPLEY, 1989). In the Black Sea, seasonal variations in fluxes of biogenic carbon, silica, and lithogenic material are related to phytoplankton blooms and riverine inputs (HONlO et al., 1987a,b; HAY , 1987, 1988). HAY and HONJO (1989) proposed that varves in Black Sea sediment originate from seasonal changes in fluxes . Varves may be more complex in origin, however, because they result both from settling fluxes that accumulate seasonally and from subsequent diagenetic processes adding and/or removing material. Thus iron sulfides could contribute to bottom sediments both by settling and by precipitating during burial diagenesis. Sulfur isotopic composition can be used as a tracer for the origin of particulate sulfide fluxes from dissolved sulfide in the water column. The isotopic composition of different sulfur reservoirs in any environment are determ ined by the sources of sulfur or by reactions transforming sulfur from one compound to another; hence sulfur isotopes can be used as tracers for sources or as indicators of processes . In the Black Sea, the isotopic composition of sulfate is determined by input of Mediterranean seawater, isotopically lighter sulfate from rivers , and sulfate reduction (VINOGRADOV et al. , 1962; SWEENEY and KAPLAN, 1980). During sulfate reduction, isotopically lighter hydrogen sulfide is produced, leaving the residual sulfate enriched in the heavier isotope (KAPLAN and RITTENBERG, 1964). Hydrogen sulfide will have an isotopic composition dependent on: (a) the rate of sulfate reduction, and (b) oxidation , either chemical or biological. As sulfate reduction rates increase due to increased supply of metabolizable organic matter, or sulfate becomes limiting, isotopic fractionation between sulfate and sulfide will decrease, resulting in isotopically heavy sulfide (GOLDHABER and KAPLAN, 1974). Chemical oxidation of dissolved sulfide leaves residual sulfide isotopically heavier since lighter sulfide is preferentially oxidized-a normal isotope effect (FRY et al., 1988b) . Conversely, sulfide oxidation
Sulfur fluxes in the Black Sea
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by phototrophic sulfur bacteria results in an inverse isotope effect, causing residual sulfide to be isotopically lighter (FRY ('I al.• 1984, 1988a). For iron sulfides, the dissolved sulfide precursor determines isotopic composition; fractionation during precipitation is believed to he insignificant (Got DHABER and KAPLAN, 1975). Particulate sulfide fluxes therefore should reflect conditions affecting dissolved sulfide in the water column or microenvironment of precipitation. In anoxic sediments. metal sulfides can originate both as minerals precipitated in situ from porewater sulfide, and from fluxes precipitating in the water column; if these arc isotopically distinct. then in principle the different sources of sedimentary iron sulfides can be distinguished. In the Black Sea water column. isotopic composition of dissolved sulfide changes with depth and is isotopically lighter in deep water below 175 m. but is isotopically heavier and enriched in 3.tS near the oxic-anoxic interface; this 70-100 m deep zone of net sulfide consumption is below the zone of photosynthetic bacteria or any possible chemosynthetic sulfide oxidation (JORGENSEN ('I al.. 1991; JANNASCH ('I al.• 1991; REPETA and SIMPSON, 1991; FRY et al., 1991; SWEENEY and KAPLAN, 1980; JANNASCH et al., 1974). Isotopically heavier dissolved sulfide could result from net chemical oxidation/sulfide consumption. mixing with isotopically heavier local sulfide. or locally higher rates of sulfate reduction, but not from net photosynthetic bacterial oxidation (FRY et al., 1991). Iron sulfide precipitation may account for part of the observed sulfide consumption. The isotopic composition of such fluxes would then give the depth of formation as well as information on processes affecting sulfide isotopic composition. Formation of iron and manganese sulfides in the water column is kinetically and thermodynamically possible (BREWER and SPENCER, 1974; LEWIS and LANDING, 1988). Precipitation of iron monosulfides occurs in seconds if the solubility product is exceeded, and although pyrite in marine sediments is often believed to form slowly over a period of weeks to months or years (BERNER, 1965.1967.1970.1984), rapid formation within a day has also been measured (HOWARTH, 1979; HOWARTH and MERKEL. 1984; HOWARTH and JORGENSEN, 1984). Elemental sulfur (BERNER. 1970. 1984) and polysulfides, which may be involved in pyrite precipitation (RICKARD, 1975; LUTHER et al., 1986), both result from oxidation of dissolved sulfide. Pyrite can also form under slightly oxidizing conditions, where pH is lower than about 7, where oxidizing substances are available or where sulfide concentrations are low (HOWARTH, 1979). such as at the oxic-anoxic interface. Thus the oxic-anoxic interface in the Black Sea may be a major zone of precipitation of pyrite, metal sulfides and other sulfur compounds. We examined the magnitude and seasonally varying patterns of sulfur, iron and organic carbon fluxes measured over a 2~ year time span by time-series sediment traps. We used sulfur isotopic composition as a tracer to determine whether particulate sulfide fluxes originate from dissolved sulfide at the oxic-anoxic interface, in deeper water or elsewhere. We then compared our results for fluxes of sulfur, iron and organic carbon with other published data for Black Sea sediments and dissolved sulfides in the water column in order to address these questions: (1) What is the origin of the sulfide flux? (2) How significant is the contribution of particulate sulfide flux to bottom sediments? (3) What seasonal patterns in sulfur, organic carbon and iron fluxes exist, and how do they relate to each other? (4) Can we estimate sulfate reduction rates, sulfide production rates, and precipitation of iron sulfides in sediments during burial diagenesis? (5) What are the implications for varve formation and the sedimentary record? (6) What information about biogeochemical processes can organic carbon, iron and sulfur in fluxes and in sediments provide?
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Fig. 1. Locations of sediment trap sites in the Black Sea (BS, BSe and BSK2).
METHODS
Sediment trap samples and subsampling Samples of particulate material and trap cup supernatant were obtained from sediment traps moored at sites BS and BSC in the southwestern Black Sea. and at site BSK2 in the central Black Sea (Fig . 1. Table 1), as part of the PARFLUX (Particle Flux Experiment) and 1988 R .V. Knorr expeditions to the Black Sea . Scientists and personnel from the Dokuz Eyliil University, Turkey , and the University of Hamburg. Germany, assisted with trap deployments . recoveries and treatment of trap samples on board the R.V. Koca Piri Reis and R.V. Knorr. In order to minimize metal corrosion, moorings used Kevlar or nylon line whenever possible , and stainless steel where metal parts were essential. Trap frames and timer cases were made of aluminum . and the trap cones. baffles . and bottles were Delrin plastic, polycarbonate and high-density linear polyethylene plastic, respectively. In all trap deployments, trap cups were precharged with a supernatant solution containing 4% formalin, sodium chloride (24%0) and a dissolved buffer, usually calcium carbonate , made up in distilled water. (For site BSC, all supernatant was deoxygenated by boiling and sparging with nitrogen for 30 min; deoxygenated formalin was added just prior to deployment.) Formalin prevents microbial activity and helps preserve soft tissues: formalin also aids in retarding oxidation of dissolved sulfide (LUTHER et al. , 1988). Upon recovery of traps , formalin was always present in the trap cups . For traps at site BSe, samples of trap material were handled and preserved anoxically immediately after trap recovery. For reduced sulfur analyses using ICPES, older dried samples from traps deployed at site BS were used, which had not been anoxically preserved or stored . Anoxic collection and preservation of samples from site BSC and BSK2 were carried out in the
suss
Sulfur fluxes in the Black Sea
Table 1. Trap and hydrocasting' sues whIch provided samples used In this study. Site locations and durations of the total deployment penod at that slle are gn'en, as well as type of samples available for this study and method of sulfur analvsis used. Hydrocust Sll/'\ ..'ere sampled during Leg 2 ofthe R. V. Knorr cruise in the Black Sea. Traps at sues BS ideplovments 1-5\ and HSC i deplovments I and 2) wert" deployed and recovered with the R V, Koca Piri Rcis. Dokuz Evlul Untversnv, Turkrv. traps at HSC3and BSK2 were recovered and deployed with the R. V. Knorr, cruIse 134-S. Leg I. April-May 1988
Trap site
La!. I ~)
Long. IE \
BSI-5
ol2°lol.72'
32°ol39ol'
2!l Oct. 19!12 to 8 Apr. 1985
BSCI-3
41°47.6-l'
3(\"211,4S'
19 June 1986 to 20 May 1988
BSK2
ol2°5725'
33°54 oll'
5 May to 13 July 1988
Dates open
Treatment Air-dried samples; reduced S by JCPES Anoxic samples: Cr-reduction, elemental sulfur, acid-volatile extractions; 0 34S measured SEM
Hydrocast stations fore d~ of dissolved sulfide. R.Y. Knorr cruise 134-9. Leg 2. May 1988 Station La!. (N) Long. IE) BS2-1 BS2-2 BS:!-3
41°10' ol2°50' 43"05'
3(\''00' 32"00' 3-l"t)(1'
following manner: sampling of cups for supernatant and particulate material took place within 30 min of trap recovery. Sediment trap cups were removed and placed in a glove bag flushed with nitrogen or argon; supernatant was sampled using glass pipettes and preserved in zinc acetate for later dissolved sulfide analyses. In the glove bag, particulate material was sampled after gentle mixing of the contents, filtered and rinsed with deoxygenated distilled water. and dried; all activities were carried out in a glove bag under argon pressure. Samples were stored in a sealed desiccator containing argon and frozen until reduced sulfur analyses could be made. Other analyses of bulk and elemental composition were done after the rest of the sediment trap material had been split into aliquots in the lab. In order to test this procedure for possible oxidation, standard solutions of sodium sulfide were prepared at the time of trap sampling and placed in the glove bag for sampling along with samples. FeS and pyrite standards were used to check this procedure for possible oxidation. Chromium (Cr) reduction of these standards showed that no significant loss occurred. within the precision of the Cr reduction method. Reduced sulfur analyses We used two kinds of analyses for reduced sulfur. depending on the nature of the samples available. For anoxically preserved sediment trap material available from site BSC, we used methods for the determination of elemental sulfur (TROELSEN and JORGENSEN. 1982), acid-volatile sulfides and Cr-reducible sulfur derived from the work of CANFIELD et al. (1986). HOWARTH and JORGENSEN (1984). GIBLIN and HOWARTH (1984), HOWARTH and GIBLIN (1983). WESTRICH (1983). HOWARTH (1979) and ZHABINA and VOLKOV (1978). For sediment trap samples from PARFLUX site BS, which were not anoxically preserved but dried archive samples. we measured reduced inorganic sulfur by
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ICPES (Inductively Coupled Plasma Emission Spectrometry) . Both methods had comparable rates of recovery for standards (85-100 %) (see Table 1 for sulfur procedures applied to samples) .
Anoxically preserved samples. For sediment trap samples from deployments 1-3 at site BSC (19 June 1986 to 14 April 1988) and site BSK2 (5 May to 13July 1988) we analysed for Cr-reducible sulfur, which, when done without prior extraction of other reduced sulfur species , measures all inorganic reduced sulfur. For a second set of samples from deployment 2 at site BSC (26 September 1986 to 14 May 19X7). samples were sequentially analysed for elemental sulfur, acid-volatile sulfides and Cr-reducible sulfur; when elemental sulfur and acid-volatile sulfides are extracted first from a sample. Cr reduction on the remaining sample extracts pyrite. Anoxically preserved samples were weighed in a nitrogen atmosphere. Sequential analysis of sulfur was as follows: elemental sulfur was first extracted using carbon disulfide followed by add ition of sodium cyanide with an iron catalyst. Absorbance was measured at 448 nm on the spectrophotometer. Recovery rates of elemental sulfur standards run alone and as spikes added to sulfur-free sediments were approximately 100%. The same sample was then rinsed several times with carbon disulfide followed by several acetone rinses to remove any carbon disulfide residue , and dried. The entire elemental sulfur extraction was carried out in a nitrogen atmosphere in order to avoid oxidation. After drying, the same sample was used for determination of acid-volatile sulfides using first cold and then hot nitrogen-saturated 5.5 N hydrochloric acid to drive off acid-volatile sulfides . Sulfide gas was trapped in a 10% zinc acetate trap. and sulfide was analysed spectrophotometrically at 670 nrn, using phenylene diamine reagent and the methylene blue absorbance assay. Meanwhile. the remaining sediment sample was Crreduced to measure pyrite . Reduced Cr(II) chloride solution was added and the sulfide produced was analysed as above . Recoveries from acid distillation and Cr reduction ranged between 85 and 100% and are comparable to the total sulfur by ICPES method described below. As a measure ofreduced inorganic sulfur. the sum of elemental sulfur. acid-volatile sulfides and Cr-reducible sulfur/pyrite was used for samples from deployment 2 at site BSC . while Cr-reducible sulfur alone (without prior extraction of elemental sulfur or acid-volatile sulfides) was used to measure reduced inorganic sulfur for samples from deployments 1 and 3. site BSC. ICPES analysis of reduced inorganic sulfur on archived samples. Archived. wet-stored but non-anoxically preserved samples were used for this analysis ; these samples were rinsed thoroughly with deionized . deoxygenated. nitrogen-sparged water to remove dissolved sulfates, and then air-dried briefly using nitrogen head pressure . Samples examined before and after this treatment did not contain any detectable precipitated sulfates such as gypsum or anhydrite crystals. using both ore petrologic and SEM microscopy, but pyrite framboids and opaque minerals were observed in these samples. Cold aqua regia in an especially oxidizing mixture (1:1 HCl to HNO,3) was added to dried samples by adding cold concentrated HCI followed by cold HNO,3. under a reflux condensor. This was kept on ice for 2 h under refluxing conditions; the condensor was then removed and the sample flasks heated gently to evaporate off liquid . Hydrochloric acid (6 N) was quantitatively added and the resulting solution was filtered to remove insoluble residues (mainly silica as analysed by ICPES) . The solution was analysed at 181.974 nm in the vacuum ultraviolet portion of the emission spectrum. using a Jobin-Yvon JY -38 VHR
Sulfur fluxes in the Black Sea
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Rapid Sequential Inductively Coupled Plasma Emission Spectrometer (JCPES) purged with nitrogen gas before and during analysis. Recovery rates of spiked pyrite and ZnS standards added to sediments and standards alone ranged between 80 and 100%, while standard deviation for replicates was 1.91 ppm. equivalent to sulfur concentration of 0.06 .urn g 1 of sediment. The theoretical detection limit was 30 ppb. Organic sulfur standards which were tested (methionine. cysteine) were not extracted using this method. Thus this method analyses only for total inorganic sulfur and not organic sulfur; if precipitated sulfates arc present. then these reagents will dissolve sulfate. but for samples such as these which were desalted thoroughly by rinsing in distilled water and which do not contain insoluble precipitated sulfate minerals. the procedure measures reduced inorganic sulfur. Using this method. acid-soluble metals can also be analysed using JCPES on the same sample: multiple analyses on the one sample is often desirable when the amount of original sample available for analyses is small. as is the case with sediment flux samples in general.
Sulfur isotopic composition For sulfur isotopic analyses of fresh anoxically preserved sediment fluxes from trap sites BSC and BSK2. samples were Cr-reduced for all reduced inorganic sulfur. Sulfide gas was trapped in a 7% silver nitrate solution. The silver sulfide precipitate was concentrated, rinsed. dried and directly cornbusted with vanadium pentoxide to sulfur dioxide gas, which was collected by cryogenic distillation. The yield was measured using calibrated baritron pressure of the gas and then analysed for isotopic composition using a Finnigan 251 isotope ratio mass spectrometer. Results are expressed as 0 34S values relative to CDT (Canyon Diablo troilitc). Precision for standards carried through the distillation procedure was better than n.3"bo and replicate analyses also agreed to within 0.30/00. Dissolved sulfide between 100 and 2000 m depth was collected using silver nitrate and zinc acetate to preserve dissolved sulfide. during Leg 2. sites 1, 2 and 3,1988 R.V. Knorr cruise. by one of us (FRY et al.• 1991). The preparation and analysis of these samples was identical to that descrihed above for sediment trap samples.
Bulk composition and fluxes Measurements of total flux and analyses of the bulk composition were done on splits of particulate material. CHN analyses were done on decalcified samples using a PerkinElmer CHN Analyzer. Precision of this method is 0.3% on replicates. Major and minor elemental composition. including total particulate iron. was determined by JCPES on samples which were fused using a lithium metaborate flux at lOOO°C and then dissolved in 4% nitric acid. For iron. the standard deviation of this method is 0.11% (% S.D. = 4.23%). and the detection limit is 0.001 %. Due to small sample sizes available from sediment trap fluxes. only total iron was determined. Non-combustible material, representative of lithogenic fluxes. was determined by muffle furnace combustion at 600°C. Molar ratios. which can be useful in estimating composition or approximate stoichiometry of phases. were calculated based on relative molar concentrations of an element or phase in the sediment flux. We present data for both fluxes and concentrations of a particular substance since flux is a measure of the absolute amount of material and concentration is a measure of the amount of a substance relative to the total of all components. Both of these
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can potentially change independently of each other, reflecting the different factors affecting their production or disappearance.
SEM observations
Samples for SEM-EDX (energy-dispersive X-ray analysis) were taken from anoxically preserved material on 0.45,um Nuclepore membrane filters, which were placed on carbon stubs and then coated with carbon if elemental EDX analyses were done (with goldpalladium if otherwise). A JEOL-840A SEM with EOX was used to examine samples for the presence of iron sulfides, particularly framboidal pyrite.
RESULTS
Fluxes: composition and seasonality Reduced sulfur fluxes.
Seasonal variation . Highest fluxes and concentrations of reduced inorganic sulfur occur during the summer and autumn (fluxes of about 50JlM m-2_day-1 .concentrations of 600+ ,uM g sedimenC I) and drop to minimum levels during the winter and early spring (fluxes <1 ,uM m- 2-day-1 and concentrations <50,uM g sediment-I) (Figs 2A and 3A) . The seasonal variation closely resembles that of organic carbon fluxes; over time, both fluxes and concentrations of organic carbon and sulfur vary together, resulting in a high correlation between organic carbon and sulfur (Figs 2B and 3B). Sulfur fluxes and concentration are not correlated well with either total flux or fluxes or concentration of iron (Figs 2C, D and 3C). Geographic variation. The average flux of reduced inorganic sulfur for both sites BS and BSC is 25.26,uM m -2 day-I (9.23 mM m- 2y-l). As of now, too few analyses are available from site BSC to make meaningful comparisons between the two sites, although the data suggest a significant difference in reduced sulfur fluxes between sites (Table 2) . Despite the apparent statistical difference between sites BS and BSC for reduced inorganic sulfur, we conclude that at this time, due to the small number of samples analysed from BSC, there are probably no significant differences in the sulfur and organic carbon fluxes between sites. The organic carbon to reduced inorganic sulfur ratio , or C:S ratio, is also similar for the two sites (average C:S ratios of 56 and 60 for the two sites, with no statistically significant difference; Table 2). Reduced sulfur species in sulfur fluxes. Fluxes of elemental sulfur, acid-volatile sulfides and pyrite determined on strictly anoxically preserved samples from BSC are given in Table 3. The September 1986 sample is the only one with any significant amounts of acidvolatile sulfides, and it was also the only sample which was black in color and macroscopically very fine-grained in texture. With this exception, samples contained more pyrite and elemental sulfur than acid-volatile sulfides. Samples from October to December were dark grey and intermediate in texture, while the March sample was light greenish-grey and relatively coarser-grained . Since pyrite in fine to intermediate texture is known to impart a grey rather than black color (typical of iron monosulfides), the relative amounts of the three sulfur species measured in these samples is consistent with these observations. Based on these anoxically preserved samples, the mean percentage of total sulfur which is due to
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A. Sulfur
Fig . 2. Fluxes of particulate reduced sulfur , orga nic carb on, iron and tota l mat erial. for sites BS and BSC. (A) Fluxes of reduced sulfur. inl~M m- 2 day- I : red uced inorganic sulfur is based on ICPES measurements for samples from the period Janu ary 1983 to Ap ril 1985, and on the sum of elementa l sulfur, acidvolatile sulfides and pyrite . for the perio d from September 1986 to August 1987. (B) Organic car bon fluxes. in mM m- 2 day " . (C) Tota l flux at sites BS and BSe. in mg m- 2 day" ' . No traps were deployed bet ween Ap ril 1985 and Jun e 1986. (D) Part iculate iron flux at sites BS and BSe , in ,uM m - 2day - I.
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pyrite and acid-volatile sulfides together is about 58% of the total reduced sulfur, with the remainder being elemental sulfur. SEM observations . Pyrite framboids are relatively common in anoxically preserved material from all samples currently being studied and are easily detectable by backseattered electrons or EDX . Framboids were always observed occurring singly and never in clumps. The average diameter of pyrite framboids is about 6 .urn. for 100 framboids from site BSC. BS and BSK2 (Fig. 4). If Stokes' Law applied to a particle this small, this corresponds to a Stokes' settling velocity of 5.6 m day -I , or 6 months to settle to 1000 m; however, framboids are always observed to adhere to other biogenic particles. Pyrite framboids were observed before in sediment trap material from the Black Sea, as part of
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Sulfur fluxes in the Black Sea
Table 2. A verage values for sediment trap material from sites BS and BSC in the Black Sea, com prising material collected from October 1982 TO May 1988. Shown are mean values, standard deviations, maximum values and numb er ofcups sampled for : total fl uxes . organic carbon (C) , reduced particulate sulfur (S) as defined in the text, particulate iron (Fe) , percent by dry weight ofC, S, Fe , and the molar ratios C:S and Fe:S . Total flux is in mg m- 2 day- I; reduced sulfur, organic carbon and iron fluxes are in,uM m- 2 day-I ; percentages are by dry weight. Also given are t-tests (unpai red values, unequal variances, pooled t-statistic) of the difference between BS and BSC means, using the null hypothesis of no difference between means . The level of significance probability P (probability that the t-statisuc calculated is due to random error) at which there is a statistically significant differen ce between means is given %C
%S
% FE
C:S
Fe :S
49.75 44.40 48
12.89 7.22 56
0.77 0.52 2li
2.2 1.29 52
60.06 26.26 25
3.42 4.53 26
12.03 20.53 33
11.31 6.54 34
1.75 0.86 6
1.12 0.78 29
56.28 45.64 4
1.23 1.48 3
14.56 7.26 90 -2 .913 88 0.005
0.95 0.70 32 -3.682 30 0.0005
1.81 1.24 81 4.093 79 <0.0005
3.20 59.54 4.35 28.57 29 29 0.241 0.821 27 27 n.s . n.s .
Total flux
C
S
BS mean S.D . Cup s sampled , n
104.60 91.61 63
1222.90 988.01 56
28.19 18.67 26
BSC mean S.D . Cups samples, n
58.60 99.07 36
805.60 1283.33 34
12.59 20.25 6
BS + BSC mea n S.D. Cups samples, n Calc . t.statistic Calc . d .f. P
87.87 96.49 99 2.678 93 0.005
1065.3 1120.4 90 1.613 87 0.10
25.26 34.38 19.63 40.93 32 81 1.818 4.551 30 79 <0 .0005 0.05
Fe
d .f. = degrees of freedom . n.s. = no significant difference at any probability level.
Table 3. Fluxes of reduced sulf ur species, site BSC. Samples were anoxicaf{y sampled and preserved. Sample s f rom BSCI were analysed by chromium -reduction only (a measure of all inorganic reduced sulfur when no prior extraction ofacid-volatile sulfide and elemental sulfur is done) . Samples f rom the BSC2 deployment were analysed sequentially for elemental sulfur, acid-volatile sulfide and chromium-reduction (a measure ofpyrite only, after the first two extractions have been done). For these samples, reduced inorgani c sulfur is the sum of all three fractions . Fluxes are in ,uM m - 2 day-t Trap, cup BSC1 , cup 2 BSC l , cup 3 BSCI, cup 4 BSC1 , cup 5 BSC2 , cup 1 BSC2, cup 2 DSC2, cup 3 BSC2 , cup 4 BSC2 , cup 5 BSC2, cup 10
D ate 26 June 1986 4 July 1986 11 July 1986 19 July 1986 26 Sept. 1986 15 Oct. 1986 3 Nov . 1986 22 Nov. 1986 11 Dec. 1986 16 Mar . 1987
Acid-volatile S
17.12 0.13 0.62 0.37 0.08 0.06
Elemental S
Cr-reduc. S
Redu ced inorganic S
0.94 0.69 3.31 7 .11 5.83 2.15
17.56 26.82 21.24 17.03 1.59 4.30 3.37 3.87 6.39 0.59
17.56 26.82 21.24 17.03 19.65 5.1 2 7. 30 11.35 12.30
2.80
S1162
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MURAMOTO
et al.
the PARFLUX project from 1982 to 1985 (HONJO et al., 1987a; NICHOLSON, 1988). Pyrite framboids were also observed in fecal pellets in surface sediments sampled by box cores during the 1988 R .V. Knorr cruise (PILSKALN , 1990), but not so far in fecal pellets from sediment traps. Organic carbon fluxes. Seasonal variation. Seasonal variation in organic carbon fluxes is very regular and predictable: high maximum fluxes and concentrations occur in summer and autumn, where fluxes of 4-5 mM m- 2 day- l and concentrations of 100-150 mM g sedimenC l are measured , and low fluxes occur in the winter and early spring, when fluxes are nearly zero and concentrations are less than 10 mM g sediment"! (Figs 2B and 3B). High particulate organic carbon fluxes are associated with major seasonal phytoplankton production in surface waters in the Black Sea ; during the spring , these are associated with the diatom bloom , while increased summer and autumn organic carbon fluxes are associated with the coccolithophorid blooms that occur each year in the Black Sea (HAY, 1987; HONJO et at. 1987a,b). During the 1984-1985 winter a late coccolithophorid bloom and/or resuspension event is suggested by a peak in organic carbon flux and concentration, as well as by carbonate fluxes. The change in magnitude of organic carbon molar fluxes is between 0.01 and 5.0 mM m- 2 day"", which represents a factor of 500, or roughly two orders of magnitude difference between low and high fluxes of organic carbon . Geographic variation. Organic carbon fluxes and concentrations at the two sites BS and BSC are roughly equivalent, given the large variations in amplitude of the flux (Fig. 2B); although average organic carbon fluxes at BSC are 50% higher, the difference is significant only at the 10% level (Table 2). Intra-annual variation is thus more significant than between-site geographic variation. Total fluxes. Seasonal variation. Total sediment fluxes at both sites BS and BSC exhibit the same general pattern of seasonal variation (Fig. 2C) . The high total fluxes in summer and autumn are due to high productivity during coccolithophorid blooms, resulting in increased production of organic carbon and carbonate. During winter when primary production is minimal, total fluxes are low and lithogenic materials dominate the flux as a result of resuspension and increased fluvial input. Spring fluxes contain increasing amounts of opal and organic carbon due to diatom blooms at this time (HAY, 1987; HONJO et al, 1987a,b) . At site BSC, the very low total flux during one year from 1987 to 1988 is currently unexplained. Because of the high correlation between total flux and fluxes of individual particulate components, at! other flux components also have low fluxes during this interval. Geographic variation. Site BS has a significantly higher mean flux (105 mg m- 2 day-I ; t-test significance level is P = 0.005), consistent with its being closer to coastal input and upwelling areas, than site BSC (59 mg m ? day-i), which is closer to the center of the western basin. The two sites BS and BSC are compared in Table 2. Total fluxes at site BSC tend to exhib it more seasonal variation within a year than at site BS (although more data exist for site BS). This is in part because fluxes at BSC are largely determined by biogenic production (largely during the spring and summer) and very little by lithogenic input, so that seasonality is more marked , whereas fluxes at site BS are both biogenic and lithogenic in origin, the latter dominating during the winter.
Sulfur tluxcs
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Sulfur fluxes in the Black Sea
200 600 IE 1000
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Fig . 5. 0 34S sulfur isotopic composition of particulate sulfide fluxes vs the isotopic composition of dissolved sulfide in the water column. from FRyetal . (1991 ). Trap sulfides have an average isotopic composition of -36.6%0; the same composition is found in dissolved sulfides at a depth of about 100 m at the oxic-anoxic interface (mean 034S for dissolved sulfide above 175 rn, where oxidation begins, is -38.6%0; in the deeper water column the mean o 3~ value is -40.5%0).
Particulate iron fluxes. Seasonal variation. Despite the presence of considerable scatter in the data, iron fluxes and concentrations show a generally consistent seasonal pattern of variation. High fluxes and concentrations tend to occur in winter and early spring. with minimum fluxes and concentrations occurring in summer (Figs 2D and 3C) . One might expect to find close association with reduced sulfur if reduced sulfur is present as iron sulfides; however, because there is nearly always an excess of particulate iron to reduced sulfur, non-pyritic or non-sulfidic iron may comprise a significant proportion of total iron. Geographic variation. Particulate iron is the only flux component we studied for which statistically significant and meaningful differences exist between sites BS and BSe. Particulate iron fluxes are four times higher at site BS than BSe (50 vs 12,uM m - 2 day -I for BS and BSe, respectively) . The differences in iron flux magn itude between the two sites is seen in Fig. 2D, where site BSe over the interval of the study received 75% less iron flux. The percentage of iron is twice as high at site BS than BSe (2 .2 vs 1.1 %) (Table 2, Fig. 3C) . The average Fe:S molar ratio for sites BS and BSC together is 3.2, and at this time , too few sulfur data exist to make statistically significant inter-site comparisons for the Fe:S ratio.
0 34S and sulfides Particulate sulfide/sulfur fluxes collected in traps have a 034S composition ranging from -32.7 to -39.40/00, with an average of -36.6%0; (Fig. 5, Table 4). The isotopic composition of dissolved sulfide below 175 m depth ranges between -39.3 and -41.30/00 with a mean of -40.50/00, while above 175 m, 034S increases to -35 .20/00 due to enrichment of 34S (FRY et al., 1991). In this 7G-100 m thick zone below the oxic-anoxic interface, the mean 034S for dissolved sulfide is -38.4%0; this is similar to the 0 34S composition of particulate sulfur fluxes (mean of -36.6%0). In 1975, dissolved sulfide in the deep anoxic water column had 034S values of -40.85%0, changing to more positive values (-38.710/00) at 190 and 300 m depth; at that time the oxic-anoxic interface was deeper in the water column than in 1988 (SWEENEY and KAPLAN , 1980).
J.
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et al.
Table 4. 6 34S values for sediment fluxes, Black Sea. Data are for cups from sediment traps al sites BSC and BSK2 . The whole sub sample was analysed for reduced inorganic sulfur using chromium reduction , and then measured for sulfur isotopic composition . The amount of sulfur available fo r Isotop ic analysis is given in ,umoles
Trap, cup BSC2, cup 1 BSC2, cup 3 BSC2, cup 4 BSC2, cup 5 BSK2S, cup 6
Date open 26 Sept. 1986 3 Nov. 1986 22 Nov . 1986 11 Dec . 1986 June 1988
S(um)
634S (%0)
6.0 13.0 7.2 34.6 0.8
-36.3 -38.6 -36.2 -39.4 -32 .7
During the R. V. Knorr cruise to the Black Sea in April 1988, concentration profiles of dissolved sulfide were obtained down to approximately 2200 rn, at trap Stas BSC, BSK2, and other sites (NICHOLSON et al., 1988). The dissolved sulfide content of supern atant in trap cups from some shallow and deep trap deployments at both sites BS and BSC were also measured immediately after traps were brought on board. The dissolved sulfide content of trap cups is lower than this value [the average for all trap cups measured is 84pM as compared to approximately 300 pM concentration at 1000 m depth for all stations (NICHOLSON et al., 1988)1 . The relationship between dissolved sulfide and particulate reduced sulfur concentrations in sediment fluxes is poor (correlation coefficient r = 0.18, n 10), suggesting that the particulate reduced sulfur in the trap cups is only weakly related to the dissolved sulfide content. Although the sulfide content of the trap cups is always lower than the ambient concentrations of sulfide in the water column, suggesting oxidation, other dissolved species in trap cup supernatant also fall below concentrations found in the ambient water column. Liebezeit measured concentrations of dissolved silica and dissolved reactive and total phosphorus in sediment trap supernatant from shallow and deep traps at site BSC which were lower than concentrations from the ambient water column at the same depths as the traps (NICHOLSON et al., 1988). Oxidation could account for loss of HS- , but not of phosphate or total phosphorus or dissolved silica . Relationships between sulfur, organic carbon and iron in fluxes In fluxes, a scatter diagram of reduced inorganic sulfur vs organic carbon concentrations by per cent dry weight (Fig . 6A) show a highly linear relationship and high correlation (correlation coefficient r = 0.88 for a linear relationship whose S-intercept is slightly negative or close to zero; S-intercept is -0.355, with a standard error of about 40%) with a small, non-zero slope (slope = 0.0808; standard error is 10%). The strong correlation between sulfur and organic carbon concentration in fluxes cannot be due to sulfate reduction in the trap cups , because of the use of formalin in trap cups to poison and prevent microbial activity. Particulate iron and sulfur are not well correlated over time, as is apparent from their different seasonal variations. A scatter diagram of sulfur vs iron shows that no linear relationship exists between the two (Fig. 6B), although enough iron is present in excess over sulfur at all times to allow for all of the sulfur to be present as iron sulfides , based on the Fe:S molar ratio (Table 2). However, if the sulfur speciation we
S1l67
Sulfur fluxes in the Black Sea
Y> - 0.36
+ (0.081)x
RA2
0.78 (r. 0.88)
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0.2 0.0
0
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3
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Fig. 6. Relationships between sulfur, organic carbon and iron in particulate fluxes. (A) Reduced sulfur vs organic carbon concentrations (by per cent dry weight) in sediment fluxes; the correlation coefficient is 0.88. The S-intercept passes through a point slightly below the origin (-0.355) . (B) Particulate iron vs reduced sulfur concentrations (by per cent dry weight) in fluxes, showing lack of linear relationship (correlation coefficient r = 0.07). (C) Particulate iron vs particulate aluminum concentrations (both in mg g dry sedimenC I ) in fluxes; the high correl ation (r = 0.92) is probably due to association of iron with lithogenic materi als such as clays.
observed can be extended to the whole set of reduced sulfur measurements, then a significant portion of this sulfur is elemental sulfur. The poor correlation between iron and sulfur appears to be due to their different seasonal patterns, which suggests that they are each associated with different major components of the bulk flux. With which major component of the settling flux is particulate iron associated? Correlation analysis shows that iron fluxes correlate best with the total mass flux,
J.
S1168
MURAMOTO
et al.
Table 5. Matrix of simple correlation coefficients for particulate iron , and major components of the flux , toto! mass flux. organic carbon (determ ined by combustion), lithogenics (determined by combustion ), carbonate (determined by acid-soluble V$ non-acid-soluble masses), and opal (determined by wet chemical spectrophotometric analysIS). All fluxes are in mg m - 2 day" ! Total Total flux Carb oflux Opal flux Lith . flux Org. C flux Fe flux
Carbo
Lith.
Opal
Org. C .
Fe
1.00 0.81 0.58 0.71 0.80 0.70
1.00 0.22 0.37 0.72 0.35
1.00 1.00
0.42 0.34 0.39
1.00
0.41 0.57
1.00
0.55
compared with other bulk components of the flux. including carbonate. opal . lithogenic materials and organic carbon (Table 5; correlation coefficient r = 0.70; followed by lithogenics , r = 0.57; and organic carbon , r = 0.55). Both iron and organic fluxes are in turn best correlated with the total mass flux. Iron does not correlate well with carbonate (r = 0.35) or opal (r = 0.39) except insofar as these again comprise the total mass flux. Similar correlations are obtained when iron and selected elements representative of the bulk constituents of the flux (Si, P , Mn, V, AI. Ca), measured by JCPES . are compared (Table 6). Si represents both the biogenic opal and lithogenically derived silicates, Ca represents the carbonate produced in quantity during the summer and autumn coccolothophorid blooms. P and V are both used as tracers of organic matter, and ferric iron can potentially be associated with phosphates as well. Al strictly occurs in significant amounts only in lithogenic materials. such as clays and unweathered feldspars and other silicates. Mn is tested because it often is found in the same redox conditions in which Fe is found . Fe has the highest correlation with V (r = 0.95) and AI (r = 0.92) , while correlations with Si are somewhat lower (0.83); Fe, P, and Ca are not correlated well. In turn , AI and Si are highly correlated (r = 0.90). suggesting that they occur together as aluminosilicates in claytype minerals.
Table 6. Matrix of simple correlation coefficients for iron and elements acttng as markers for bulk components, using lCPES analyses: Si = opal. silicates; P = organic matter ; V = associated with organic mailer ; Al = tracer of lithogenics , clays and other aluminosilicates; Ca = carbonate. Mn is included as it often is found wub iron under reducing conditions, All correlations are determined for concentrations of the element in mg g dry material-A
Si Si p
Fe Mn V Al Ca
P
Fe
Mn
V
Al
Ca
1.00 -0.15 0.R3 0.19 0.77 0.90 -0.37
1.00 0.11 0.15 0.25 - 0.23 -0.27
1.00 0.18 0.95 0.92 -0.52
1.00 0.18 0.04 -0.38
1.00 0.83 - 0.49
1.00 - 0.36
1.00
S1169
Sulfur fluxes in the Black Sea
Both kinds of composition analyses therefore give similar results: iron fluxes are associated with the total mass and with lithogenic material in particular, and perhaps secondarily with organic material, but not well with carbonate or opal, and there is no evidence for a significant ferric phosphate phase. In particular iron is strongly associated with aluminum fluxes representing lithogenic clay input (Fig. 6C) ; such fluxes are usually higher in the winter and spring (HAY, 1987) . Because organic fluxes and lithogenic fluxes have different seasonal patterns, there is poor correlation between reduced inorganic sulfur and iron in fluxes (correlation coefficient r = 0.07 for fluxes) and lack of any linear relationship between iron and sulfur concentrations. Since sulfur does not appear to be closely associated with iron (or aluminum), this argues against the possibility that the source of sulfur fluxes is lithogenic material or resuspended shelf sediments. Organic carbon , sulfur and iron concentrations in fluxes and sediments can also give us information on possible diagenetic changes occurring between settling and burial, given certain assumptions. We have compared our data on sediment fluxes to data on concentrations of organic carbon , iron and sulfur from sediment cores studied by HIRST (1974), BERNER (1974), PILIPCHUK and VOLKOV (1974), VOLKOV and FOMINA (1974) and ROZANOV et al. (1974) for surface sediments down to 100 em which were taken from near the trap sites studied. Additional information on organic carbon and sulfur content has become available from the latest 1988 R.V. Knorr cruise (LYONS and BERNER, 1990) . Present-day fluxes have a high organic carbon concentration (average content of 14% by dry weight), compared to sediments which typically contain several per cent (dry we ight) of organic carbon (Table 7). Molar ratios of C:S and Fe:S for fluxes vs sediments reflect the higher organic content in fluxes as opposed to sediments, although both have somewhat similar iron and sulfur contents. If the organic carbon concentration in fluxes were reduced by about 72% (by weight) to simulate diagenetic remineralization , resulting in organic carbon comprising 4% of the total sediment, then reduced sulfur would comprise 1.1 % and iron 2.1 % of the total amount of material in the flux . These values are like the concentrations for Unit 1 sediments, which have average pyritic sulfur concentrations of 1.3%, and organic carbon of 5.4% by weight (LYONS and BERNER, 1990). l
DISCUSSION
Biogeochemical cycling of sulfur, iron and organic carbon in the water column In the Black Sea, the biogeochemical cycles of sulfur, iron and organic carbon are linked via dissimilatory sulfate reduction, which oxidizes organic matter in the anoxic water column and sediments generating dissolved sulfide, which can then precipitate if reactive iron is present. One way to unravel the complexity of these interrelated cycles is to compare the rates and sizes of the chemical, biological and physical processes which determine the origin , transport and fate of these compounds. An accurate understanding of biogeochemical cycling must include accurate pinpointing of sources. For example , pyrite could precipitate in the water column from 'old' dissolved sulfide diffusing and advecting upward from deep water, or it could precipitate from 'new' dissolved sulfide produced from ongoing sulfate reduction in the upper anoxic water column. Sulfatereducing microbiota could be oxidizing fresh labile organic carbon from newly se tt ling particulate organic matter, or utilizing dissolved organic carbon already present in a preexisting pool. Since we ultimately want to understand where and how sulfate reduction of
J . MURAMOTO et al.
S1170
Table 7 . Table comparing mean values of %S. %C. %Fe, and molar ratios for sediment data from other studies with the present study comprising sediment trap material . For data from BERNER (1974) and HIRsr(l974). percentages by mass ofelement were used to calculate mass ratios of one element to another, and then converted into molar ratios. and are for sediment cores down to specified depths. For the sediment trap material . molar fluxes (liM m- 2 day-l were used to calculate molar ratios Mean value
Flux
81
H2
%S %Corg %Fe C:S molar ratio Fe:S mol. ratio %S. top 5 cm O/OC org to 5 ern %Fe. top 5 cm
0.95 14.5
0.9 0.5 4.6 2.8 4.6
2.2 2.1 5.7
1.8
59.5 3.2
1.68 0.96 6.34
PV~
V~
RVY S
LB~
1.3 5.4
0.85 3.96 0.77
3.02
1.50 3.70
I BERNER (1974). data from cores 1464K (152-364 em depth) and 1474K (202-235 ern depth) . 2HIRST (1974) , data for mean percentages from cores 1430. 1432. 1440, 1452 and 1462 down to depth of 100 em; data for top 5 em from eores 1430, 1432 and 1440. 3PlLIPCHUK and VOlKOV (1974). wt%S calculated from sulfide + phyritic sulfur. wt%Fe from iron sulfide + pyrite. 4VOlKOV and FOMINA (1974). averaged values of per cent organic carbon in modem surface sediments of the Black Sea . 5RoZANOV et al. (1974). averaged data from surface sediment at sites nearest to trap site BSC (Stas 243, 244. 245 and 252). bLvONS and BERNER (1990). Unit I sediments from below the oxic-anoxic interface . Samples from oxic shelf positions had very low mean pyrite sulfur «0.15 wt%) and low organic carbon values (1.0%) .
organic matter proceeds, we need to understand the origin of any sulfide produced and precipitated. Is ongoing sulfate reduction of organic matter necessary to account for the production of pyrite fluxes, or can pyrite precipitate from dissolved sulfide already present in the water column, with no need for additional sulfate reduction on settling organic particles? The question of size and location of reactive reservoirs applies especially to iron . Where the greatest concentration of reactive iron coincides with enough dissolved sulfide to exceed the solubility product of pyrite or iron monosulfides, will determine where precipitation takes place, and hence what kind of dissolved sulfide will be precipitated. Origin and transport of particulate sulfide fluxes
From the limited data we have, sulfur isotopic composition of particulate sulfide fluxes points to the zone immediately below the oxic-anoxic interface as the site of precipitation. Sulfide fluxes have an average isotopic composition of - 36.60/00, similar to dissolved sulfide at the oxic-anoxic interface, and have a narrower range of isotopic composition than do sedimentary sulfides; it is unlikely that fluxes are merely resuspended sedimentary sulfides. Since no sulfate reduction can take place in the trap cups themselves due to the presence of formalin, there is no dissolved sulfide produced in the cups. No sulfide fluxes have yet been found which resemble deep dissolved sulfide (-40.5%0), so sulfide fluxes do not form at trap depths or from ambient dissolved sulfide diffusing into trap cups and
Sulfur fluxes in the Black Sea
Sl171
precipitating with reactive iron in the particulate material. High rates of sulfate reduction at the oxic-anoxic interface could also cause the positive shift in sulfur isotope composition of dissolved sulfide here. It is not known what increase in sulfate reduction rate would be needed for the isotopic shift of 2-4% at the interface. There is no evidence for high rates at the oxic-anoxic interface, although J0RGENSEN et al. (1991) measured sulfate reduction rates at and below the oxic-anoxic interface which were higher than the low water column rates measured below 150 m by ALBERTet al. (in preparation) in the same 1988 cruise. The latter's volume-based sulfate reduction rates in the water column are several thousand times lower than volume-based rates in surface flocculent sediments, although integrated areal rates are rather similar. Our hypothesis that iron sulfides precipitate at the oxic-anoxic interface is supported by concentrations of pyrite, iron sulfides, iron and dissolved sulfide in the water column . During the 1988 R.Y. Knorr cruise, suspended particulate FeS reached maximum concentrations at 180 m, greigite (Fe3S4) concentrations peaked at 125 m, and suspended pyrite concentrations reached a maximum just below the oxic-anoxic interface (RADFORDKNOERY and CUTTER, 1990) . The zone between 130 and 180 m was oversaturated with respect to pyrite and close to saturation for mackinawite and greigite (LANDING and LEWIS, 1989) . Colloidal iron «O.4,um iron collected on a TSK column), probably iron sulfide, increased to 30-50% of the total dissolved iron between 130 and 180 m, total dissolved iron reached maximum concentrations at 180 m, decreasing below this depth , and suspended particulate iron (>O.4,um) concentrations reached a maximum between 180 and 250 m and decreased below this depth (LEWIS and LANDING, 1988) . Dissolved sulfide concentration data for the upper 100 m of the anoxic water column were obtained from J0RGENSEN et al. (1991) for three stations in the Black Sea sampled during the 1988 R.Y. Knorr cruise, Leg 2 (the same cruise and stations during which dissolved sulfide for isotopic analyses was collected). GOYET et al. (1991) found that salinity can be used as a conservative tracer of water mass mixing in the 75-200 m depth interval in the Black Sea. The sulfide data were used to model the sulfide concentration profiles expected if dissolved sulfide behaved conservatively (Fig. 7). The observed sulfide concentrations in this depth interval are consistently lower than expected from conservative mixing based on salinity, and represent the dissolved sulfide which has been oxidized and/or precipitated below the oxic-anoxic interface ; this is a zone of net consumption of dissolved sulfide. If sulfate reduction rates were greater than dissolved sulfide consumption rates at the oxic-anoxic interface, dissolved sulfide concentration profiles should have shown evidence of net production, which they do not. Thus the oxic-anoxic interface is where the greatest concentration of pyrite and iron sulfides is found, where the sulfur isotopic evidence points to both oxidation and precipitation, and where there is net consumption of dissolved sulfide by chemical oxidation and precipitation. It is possible, given the low volume-based rates of water column sulfate reduction (ALBERT et al., in preparation), that no large amount of 'new' sulfide is being produced in the water column . They calculate that between 14 and 46% of the total sulfate reduction activity in the Black Sea was due to water column sulfate reduction . In this case the dissolved sulfide is largely 'o ld' sulfide, and hence any pyrite precipitating from this sulfide represents mostly old sulfide and not new sulfide resulting from adjacent sulfate reduction in the water column. If sulfate reduction rates were high below the oxic-anoxic interface and accounted for more than half of the dissolved sulfide , then much of the dissolved sulfide at the oxic-anoxic interface would be "new" sulfide.
J.
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et al.
-80 Sin . 1
·100
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E ·120 ~
a.
·140
0
·160
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40 50 60 70 80 HS-, JLM Stn.3 HS: observed vs. conservative mixing
·100
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30
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~
100
...
90
100
Sln.3 H5-, obsanted HS·, consarvallve
·140 ·160 ·180 -200
0
10
20
30
40 HS-, JLM
50
60
70
80
Fig. 7. Observed dissolved sulfide concentrations down to 200 m depth and the calculated sulfide concentration profiles expect ed if dissolved sulfide were distributed conservatively with salinity, for Stas 1,2 and 3, Leg 2,1988 R.V . Knorr cruise. The difference between observed and calculated profiles represents the concentration of dissolved sulfide which is "missing " due to sulfide consumption by oxidation and precipitation . Maximum dissolved sulfide loss occurs between 100 and 150 m depth , which is the proposed zone of particulate sulfide precipitation in the water column .
How are pyrite framboids , iron sulfides and sulfur from the oxic-anoxic interface reaching the deep water column and sediments? The residence time of a typical 6 .urn diameter framboid in the 2 km deep water column is about 1 year, assuming Stokes' settling for a particle this small and with the density of pyrite (about 5.01; HURLBUT and KLE1N , 1977). If particulate sulfides settled as independent particles from the oxic-anoxic
Sulfur fluxes in the Black Sea
S1173
interface at a uniform rate throughout the year , we would expect to see an approximately uniform flux throughout the year; this is not observed, and instead sulfide particle fluxes (and concentrations) reach their maximum at the same time of year that organic fluxes do. This suggests that sulfides and organic matter are traveling together and that pyrite framboids and other sulfide or sulfur particles do not settle on their own. Evidence for this physiochemical process of association of sulfides with organic particles comes from the linear relationship between reduced sulfur and organic carbon. A particle this small is much more likely to be transported as part of a larger aggregate instead. We propose that organic particles (marine snow, settling phytoplankton remains) originating in the photic zone settle through the oxic-anoxic interface and scavenge iron sulfides from this zone of higher concentration . Sinking of marine snow and large organic aggregates is believed to be the most significant downward transport mechanism for the bulk of organic matter produced in surface waters (HONJO et al. , 1985) and is also an important vertical transport mechanism for metals , including Fe , Mn, Cu , Ni and Cd, compared to inorganic particles or even particles coated with humic acids (MOREL and HUDSON, 1985). Much metal uptake can occur via binding to high-affinity surface ligands, constituting "passive" binding whereby both dead and live phytoplankton are capable of high metal affinities. Pyrite and/or other iron phases may be associated with organic fluxes as well as lithogenic materials, from the excellent correlation between Fe and AI. S vs C scatter diagrams-low sulfate reduction rates in the water column or scavenging of sulfides? GOLDHABER and KAPLAN (1974) proposed that a linear relationship existed between organic carbon and reduced sulfur concentrations in sediments because of oxidation of organic matter by sulfate reduction ; normal oxic marine sediments are characterized by a line passing through the origin , representing the point at which all particulate organic carbon in sediments is gone due to complete aerobic oxidation , no sulfate reduction can take place and no sulfides are deposited. LEVENTHAL (1983) furthermore proposed, using data for Black Sea sediments (HIRST, 1974), that if the S-intercept is positive when organic carbon is no longer present, sulfides are being deposited in the absence of sulfate reduction in sediments , i.e. iron sulfides are precipitating in the water column and hence sulfate reduction in the water column could be inferred. However, recent analysis of new cores from Unit I in the anoxic western basin (LYONS and BERNER, 1990) do not show a positive S-intercept when organic carbon content is low or absent, but instead an intercept near the origin, expected for oxic marine sediments instead. Sediment fluxes also show the same S-intercept for sulfur when organic carbon is absent. The difference in slope of S vs C diagrams between sediment flux and sediments can be accounted for by removing 65-80% of the organic carbon in fluxes, in a simple model of diagenetic alteration of fluxes. This slightly increases the percentage of reduced sulfur and decreases the organic carbon to correspond to the average of 1.3% Sand 5.4% organic carbon (by weight) observed in sediments (LYONS and BERNER , 1990). The Sintercept for fluxes does not change from zero when organic carbon is removed in this manner, and only the slope changes. Thus concentrations of sedimentary organic carbon and reduced sulfur can be explained entirely by diagenetic loss of 65-80% of the organic carbon in fluxes (we think 72% is a good estimate). If all of this lost organic carbon is utilized by sulfate reduction only , with no fermentation , then a maximum estimate of the
S1l74
J.
MURAMOTO
et al.
amount of sulfide produced can be calculated using the simple but quantitative expression describing sulfate reduction (RAMM and BELLA, 1974; WESTRICH, 1983):
2CH 20 + SO~- ~ H 2S + 2HC03",
(1)
where for every two moles of organic carbon "lost" to sulfate reduction, one mole of sulfide is produced. Sedimentary sulfate reduction and sulfide production rates obtained in this manner have a mean of 144 mM m- 2y-l, and reach a maximum of 663 mM m- 2 v' . for 72% of organic carbon fluxes lost. These estimated rates fall within the range of sulfate reduction rates measured for Black Sea surface sediments (but not deep sediments) and water column using radiotracer experiments (Table 9). Furthermore , they compare with SOROKIN'S (1964) estimate of 290 mM m- 2 y-I as the total HS- production in sediments. Although organic carbon fluxes exhibit large but consistent seasonal variation, it is not clear whether sulfate reduction rates in the water column should also exhibit seasonal variation, due to differences in time-scales for settling, organic matter diagenesis , and residence times for labile organic substrates in the water column, which may be quite long (D. ALBERT, personal communication). If we accept LEVENTHAL'S (1983) interpretation, then an S-intercept of zero for the S vs C relationship for fluxes indicates that there is little significant sulfate reduction in the water column . No new dissolved sulfide is produced in the water column and all iron sulfides precipitated at the oxic-anoxic interface must be forming from "old" dissolved sulfide diffusing or advecting from elsewhere, from sediments, for example. Hence water column sulfate reduction is not necessary to account for precipitated sulfides in the water column . This interpretation of the S-intercept condition agrees with the low rates of sulfate reduction measured in the water column (ALBERTet al., in preparation) . Alternatively, the linear relationship for fluxes may represent temporal variation of organic carbon and reduced sulfur fluxes and not a diagenetic sequence (as in sediment cores), and hence the S vs C relationship for fluxes represents association through scavenging, as suggested above . In this case, an S-intercept of zero is interpreted as there being no scavenging activity when organic matter is absent, and hence no particulate sulfide flux. This is also more consistent with the evidence for some elemental sulfur in fluxes, which do not result from sulfate reduction, although oxidation could account for these as well. Bottom surface sediments of Unit I could "inherit" their sulfur content from settling fluxes, and the linear association between sulfur and organic carbon in sediments could also be "inherited" from the scavenging association in fluxes. Further diagenesis and remineralization of organic matter ean change the slope of this relationship but not the intercept unless more iron is available. As discussed above, there is abundant evidence that sinking floes of organic matter are capable of binding or trapping other particles or dissolved elements. This hypothesis also does not require water column sulfate reduction for iron sulfide precipitation . If all of the organic matter lost from fluxes during burial of sediments is used by sulfatereducing microb iota, where is the extra iron sulfide that should have precipitated in sediments? This would result in a steeper slope for fluxes (if it took place in sediment trap cups themselves, which should not happen, because of cup poisoning), or in a much steeper slope for the sediment S vs C relationship if it occurred in sediments. We suggest that there is not enough reactive iron available to precipitate all of the new sulfide produced from diagenetic sulfate reduction in sediments, because all the reactive iron was
Sulfur ftuxes in the Black Sea
S1175
precipitated at the oxic- anoxic interface , where the main sources of fresh reactive iron are entering the anoxic water column. This then suggests that if dissolved sulfide is present in excess in sediments, then sediments are a significant source of dissolved sulfide to the water column. What if sulfate reduction occurred in settling particles of organic matter, producing dissolved sulfide that precipitated with iron to form pyrite framboids? In this case, enough iron must be present in the organic matter to allow for iron sulfide precipitation. Could organic aggregates scavenge reactive iro n at the oxic-anoxic interface , and produce dissolved sulfide from sulfate reduction, resulting in iron sulfide precipitation? This would explain the sulfur isotope composition of fluxes , and it would also explain the good linear relationship between per cent reduced sulfur and per cent organic carbon. Framboids observed within intact fecal pellets in bottom surface sediments of the Black Sea (PILSKALN , 1990) may have formed due to higher rates of sulfate reduction within these organic-rich particles ; no framboids have yet been observed in fecal pellets in settling material caught by sediment traps, so we cannot say yet if sinking fecal pellets are sites of sulfate reduction in the water column . ALBERT et al. (in preparation) suggest that sulfate reduction in the water column is mainly particle-associated because of iron limiting the activity of sulfate-reducing microbiota elsewhere than on sinking particles. But sulfate reduction on sinking organic particles does not explain why pyrite and other iron sulfide maximum concentrations occur just a few meters below the oxic-anoxic interface (RADFORD-KNOERY and CUTTER, 1990) . If organic matter fell as marine snow particles at an average rate of 100 m day ", then a settling organic aggregate would enter the anoxic zone in less than a day, and sulfate reduction would have to occur within a few hours to produce a pyrite maximum between 100 and 180 m (0-80 m into the anoxic zone would take up to 19 h) . Rapid rates of sulfate reduction and pyrite formation on the order of a day have been measured in salt marshes with much higher organic loads (HOWARTH, 1979; HOWARTH and J0RGENSEN, 1984) . But utilization of fresh organic matter by sulfatereducing microbiota may not be instantaneous. When fresh phytoplankton were added to anoxic sediments in experiments to measure sulfate reduction rates, measureable sulfate reduction did not occur until about 10 days after addition of the fresh organic material (RAISWELL and BERNER, 1985). If this observation were applied here , we would predict that if fresh organic aggregates had a mean settling rate of 100 m day " , then only below 1000 m depth would measureable sulfate reduction on organic particles begin to occur. This is consistent with the much higher volume-based rates of sulfate reduction measured in sediments than in the water column (ALBERT et al., in preparation) , and it is also consistent with th eir finding that the highest rates of water column sulfate reduction occurred between 300 and 350 m depth, much below the zone of suspended particulate iron sulfides below the oxic-anoxic interface. This requires 3.5 days of sinking time for colonization of organic particles by sulfate-reducing bacteria, vs 10 days of sinking time, which is not too discrepant given variations in particle settling velocities. Given that there is enough dissolved sulfide and ferrous iron at the oxic-anoxic interface to precipitate pyrite, it seems more probable that chemical precipitation of pyrite and iron sulfides below the oxic-anoxic interface occurs without the need for immediately adjacent sulfate reduction on organic particles. In the water column, sulfate reduction and sulfide precipitation can be decoupled to some extent, so that the two processes need not occur in the same place since anoxic conditions suitable for sulfide preservation and transport exist nearly everywhere in the basin.
J. MURAMOTO et al,
51176
Range of 1134S I
!
I
I
I
I
Watarcolumn
........
•
2 3
•
Flu•
6
Sediments
7
•
8
•
9
-45
I
I
-40
-35
,
I
-30
I
I
I
·25
·20
·15
·10
113-4S,L
Fig. 8. Comparison of 034S for dissolved sulfide in the water column, particulate sulfide fluxes and sedimentar y sulfides. Mean and minimum/maximumvalues of &"S for different data sets are indicated by solid dots and bars, respectively. Numbered references are : (1) dissolvedsulfide in the deep-water column below 180 m (SWEENEY and KAPLAN , 1980;n = 14); (2) dissolved sulfide in the entire water column from &6 to 2000 m (FRY et al. , 1991; n = 29) ; (3) dissolved sulfide in the deepwater column below 175 m (FRY et al., 1991 ; n = 17); (4) dissolved sulfide in the shallow water column above 175 m (FRY et al., 1991 ; n = 12); (5) particulate sulfide fluxes from time-series sediment traps (this study; n = 5); (6) pyrite from 0 to 10 em horizon in basinal sediments, water depth > 1000 m (VAYNSTEYN et al., 1985; n = 6); (7) pyrite from 0 to 10 cm horizon in shelf sediments, water depth < 100 m (VAYNSTEYN etal., 1985;n = 5); (8) sulfidesfrom surface sediments (VINOGRADOV et al., 1962); (9) sulfides from surface sediment at Stas 1135 and 1136 (SWEENEY and KAPLAN 19&0; n = 2).
To distinguish between the two processes, sulfate reduction on organic particles vs scavenging of pyrite by organic sinking floes, we need to know how isotopic composition of suspended particulate sulfides changes with depth in the water column. We also need to know whether sulfate depletion can occur in the "porewater space" of settling marine snow or fecal pellet particles due to localized sulfate reduction, i.e . do mar ine snow particles or fecal pellets behave like closed, semi-closed or open systems with respect to dissolved constituents as they move through the water column? Fecal pellets would be relatively more closed to diffusion than marine snow particles. Comparison with sedimentary sulfides
The isotopic compositions of dissolved sulfide in the water column, and of particulate sulfides in fluxes and sediments, are compared in Fig. 8. Iron sulfides in bottom surface
Sll77
Sulfur fluxes in the Black Sea
Table 8. Comparison of d 34S of sulfides in sediment fluxes, surface sediments and dissolved sulfide in the water column of the Black Sea, from references listed below Range d 34S (%0)
Mean
S
Water column HS - > 180 (deep) HS- , 8Cr-2000 m HS - > 170 m (deep) HS- < 170 m (shallow)
-38.7 to -35 .2 to -39.3 to -35.2 to
-40.9 -41.3 -41.3 -40.5
-39.8 -39.7 -40.5 -38.6
0.7 1.4 0.6 1.4
14 26 15
Particulate fluxes Reduced S, 1000 m traps
-32.7 to -39.4
-36.6
2.6
5
3
Sediments Pyrite , ~5 em , basin Pyrite, ~5 em, shelf, slope Sulfides, basin sediments Sulfides, basin sediments
-33.0 to -15 .0 to -19.3 to -27.0 to
-35.2 -28.0 -27.8 -27.2
1.9 9.9 6.5 0.3
5 8 4 2
"Depths > 1000 m "Depths < 1000 m
Sample
-37.3 -44.5 -33.0 -27.4
Reference
n
II
IStas 1135, 1136 2All depths 2Deep 2Shallow
5
I SWEENEY and KAPLAN (1980); data for HS- are from depths below 180 m. Data for basinal sediment sulfides do not specify depth in core at which sediment was sampled. 2FRY et al. (in press); shallow and deep refer to data from above and below 175 m, where dissolved sulfide b34S profile shows dramatic change . 3MuRAMoTO et al. (this study) . 4YAYNSTEYN et al. (1985) . 5YINOGRADOV et al. (1962); data for basinal sediment sulfides do not specifiy depth in core at which sediment was sampled.
sediments in the Black Sea are almost always isotopically heavier than dissolved sulfide in the water column above sediments (Table 8; VINOGRADOV et al., 1962; SWEENEY and KAPLAN, 1980; VAYNSTEYN et al. , 1985). Sedimentary sulfides also have a wider range of isotopic values than do either sulfide fluxes or dissolved sulfide, suggesting that they originate from sulfate reduction with increasingly limited sulfate, and/or several sources of isotopically distinct dissolved sulfide . Heavier 0 34S values in sediment sulfides can result from : (1) higher rates of sulfate reduction in the sediment (SWEENEY and KAPLAN, 1980), (2) sulfate-limited sulfate reduction in sediments, or (3) precipitation of metal sulfides from residual dissolved sulfide which is isotopically heavier due to oxidation by some oxidizing agent present in the sediments (just as oxidation occurs at the oxic-anoxic interface). Sulfate reduction rates in sediments are low in general, although rates at the surface are higher than deeper in sediments (ALBERT et al., in preparation; VAYNSTEYN et al. , 1985). In contrast, sulfur fluxes have a narrower range of isotopic composition. The difference in the ranges of isotopic composition between flux and sediment sulfides, and the close correlation between autochthonous organic matter and sulfides, suggests that sulfide fluxes are not just resuspended sediment sulfides , despite the possibility of significant lateral transport of water masses and sediment in the Black Sea (MuRRAyetal., 1989). Some sediment sulfides have isotopic compositions similar to dissolved sulfide in the uppermost 100 m of the sulfide zone, which has an average composition of - 38.4%0 (FRY et al. , 1991), which is also similar to particulate sulfide fluxes. Sedimentary sulfides could therefore originate at the oxic-anoxic interface, and settle as a particulate flux through the water column. Most likely there are dual origins for sedimentary sulfides: fluxes from the
S1178
J. MUIlAMOTO et ai.
water column and in situ authigenic precipitation. Sedimentary evidence also suggests the importance of water-column anoxia in determining whether anoxic sediments will have high or low iron sulfide contents. LYONS and BERNER (1990) compared anoxic sediments from two nearby sites, one from the shelf beneath oxic water, and the other from below the oxic-anoxic interface, where both sediments had similar organic carbon and reactive iron contents. Sediments from below the anoxic water column were found to have much higher concentrations of acid-volatile sulfides (up to almost 0.7% by weight) and a much higher degree of sulfidization (up to 40%). Sediments from below the oxic water column had less than 0.15% pyrite , and a very low degree of pyritization of iron «10%). Importance of iron
Iron, by precipitating hydrogen sulfide as pyrite or iron sulfides, renders sulfate reduction "visible ", and it also limits pyrite and iron sulfide precipitation. Thus reactive iron is a major control on the amount of dissolved sulfide that can diffuse into the water column from sediments, in the absence of other significant oxidizing or precipitating agents. Iron is also required for growth of all organisms, and in the anoxic water column is consumed both by sulfide precipitation and doubtless by sulfate-reducing microorganisms, although the extent to which chemical and microbial iron consumption competes in the Black Sea is unknown. In fluxes, iron is associated primarily with litho genies and perhaps secondarily with organic matter, but this latter fraction of organic-associated iron may be occurring as the sulfide phase, based on the iron sulfide-organic carbon correlation. The Fe :S molar ratio of 5:1 suggests excess iron to reduced sulfur in fluxes, although we do not know what amount of reactive iron this represents; this explains why iron and organic carbon are not strongly related-there is much more total iron relative to sulfide iron, which is the fraction associated with organic matter. STRAKHOV (1958) and ROZANOV et al. (1974) estimated that at least 20% of reactive iron was removed as particulate iron sulfides in the 2 km deep anoxic water column, based on decreasing ferric iron in sediments as depth of sulfidic water increased. This agrees (on a molar basis) with the 5:1 molar ratio of iron to sulfur in fluxes if that 20% of particulate iron flux is due to iron sulfides. If roughly 17% of the upward dissolved sulfide flux is removed as iron sulfides (see below), then five times as much iron again would be needed to precipitate all dissolved sulfide upward flux as iron sulfides, resulting in an Fe:S ratio of 25:1. In varved sediments of Unit I the degree of pyritization of iron is generally between 60 and 70% (LYONS and BERNER, 1990); if the paleofluxes represented by surface sediments are comparable to modern fluxes, then there may still be some iron available for reaction in sediments after settling of fluxes . Iron has been proposed as a limiting trace element for marine phytoplankton growth, particularly for diatoms and less so for coccolithophorids (MARTIN and FITZWATER, 1988; MARTIN et al., 1989). In the Black Sea, some high iron fluxes do coincide with spring diatom blooms, and lower iron fluxes often accompany summer coccolithophorid blooms. It is intriguing to hypothesize that diatom blooms might be stimulated during the spring when availability of iron is probably high due to increased lithogenic inputs, and coccolithophorids bloom later during the summer when levels of iron may be lower. However, because fluxes of lithogenic materials, wind stresses (HAY, 1987) and probable upwelling, and diatom bl~~ms all reach maximum levels in spring, it is difficult to pinpoint cause and effect. In addition, results of short-term bottle bioassays commonly used in
Sulfur fluxes in the Black Sea
S1179
Table 9. Rates of some processes of the sulfur cycle in the Black Sea. Data are from sources listed. Where possible, the mean rate ofa process is given, along with the range of values found in the literature or determined in this study. Where the rate of a process is unknown or currently in question, this is indicated by a question mark on Fig. 9
Particulate sulfide flux = 9.2 (10) mM m -2 y-l This study Upward flux of dissolved sulfide due to eddy diffusion and advection = This study 58 (60) mM m- 2 y-I Rate of oxidation uf HS- at top of interface = 48 (50) mM m -2 v ' This study (from upward flux - precipitated downward flux) ALBERT et al. (in preparation) Rate of sulfate reduction and HS - production in water column: 79-536 mM m- 2 v' (1.3 .u M y-I) Rate of sulfate reduction and HS- production in sediments: 470-530 + mM m- 2 y-I (7.7 mM y-I) ALBERT et al. (in preparation) 290 mM m- 2 y-I SOROKIN (1964) 144 mean, 663 mM m- 2 v' max. This study Our rates are calculated as described in the text: Mean organic carbon flux is 389 mM m -2 y -I, maximum is 1842 mM m -2 y-I If: 65% of organic carbon used: mean of 126 mM m -2 v ' of HS -, max. 600 mM m -2 y-I 72% of organic carbon used: mean of 144 mM m- 2 y-I of HS-, max. 663 mM m- 2y-1 80% of organic carbon used: mean of 155 mM m -2 y-I of HS-, max. 737 mM m- 2y-1 all organic carbon flux used: mean of94 mM m -2 y-1 of HS -, max. 921 mM m -2 y-1
studies of iron-stimulated phytoplankton growth need to be interpreted cautiously (HOWARTH, 1988). Other more controlled studies need to be done to resolve this interesting question, since biological demand-particularly microbial iron demand-is competing with iron sulfide precipitation in a sulfidic basin such as the Black Sea. Iron fluxes increase with proximity to coastal sources of input , and ROZANOV et al. (1974) also found increased ferric iron concentrations in bottom sediments closer to the coast. Thus there is considerable geographic variation in iron inputs, and doubtless geographic variations can be expected in all of the above processes due to the important roles of iron.
Dissolved sulfide flux and rate of pyrite formation A simple vertical advection-diffusion model was used to calculate the upward flux of dissolved sulfide between 200 m and the oxic-anoxic interface, if lateral exchange of sulfide is not considered. Using a vertical advective velocity of W = 0.5 m v'. a vertical eddy diffusion coefficient K = 27.5 m 2 y-l (GOYET et al., 1991), and a concentration of 75 3 mM m- at 200 m depth (from J0RGENSEN etal., 1991), the upward flux of dissolved sulfide is approximately 60 mM m- 2 v '. If the average particulate flux of sulfide is roughly 10 mM m -2 y-l then this represents about 17% of the total upward dissolved flux of sulfide. From Fig. 7, consumption of dissolved sulfide occurs between 110 and 200 m relative to a conservative mixing model based on salinity. Some of this consumed sulfide could be lost due to precipitation of metal sulfides, as well as oxidation of sulfide. Moreover, if we compare estimated rates of sulfate reduction, which range from 470 to 530 mM m- 2 y-l in sediments (Table 9), then approximately 2% of the dissolved sulfide produced from sulfate reduction in sediments is returned as iron sulfide fluxes. If we use our mean estimate of 144 2 mM m- y-l of HS- production in sediments, then about 7% of dissolved sulfide areal fluxes produced is precipitated. If we use rates of sulfate reduction in the water column
S1180
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MURAMOTO
et al.
(79-536 mM m- 2 y-l), then about 2-13% of the dissolved sulfide from water column sulfate reduction is precipitated. The average molar flux of total iron is about 12.8 mM m- 2 surprisingly similar to sulfur fluxes , although we do not know the percentage of reactive iron . Thus there is more dissolved sulfide than iron by at least a factor of 5 (60 vs 12.8 mM m -2 y-I) , and much more if most of this iron is non-reactive (the Fe:S molar ratio is 5:1 in fluxes). The discrepancy between similar mean annual molar fluxes of iron (12.8) and particulate sulfides (9.2) and the Fe :S ratio (5:1) is due to the different seasonal behavior of iron and sulfide fluxes, as well as the unknown residence times for different iron compounds in the Black Sea , and points out the hazards of using averaged annual rates to compare processes which may in fact operate over non-overlapping seasons. An estimate of the rate of pyrite formation in the water column can be obtained from rates of settling by making the reasonable assumption that pyrite is removed by scavenging as quickly as it is formed . If a marine snow particle settles at a rate of 100 m day- l (ASPER, 1986), then one day suffices to settle through the upper 100 m of the sulfide zone. If sulfide forms on organic particles undergoing sulfate reduction, then, as an organic particle settles to 1000 m, 10 days are required. Both are consistent with known rates of pyrite formation. Sedimentary pyrite may take weeks to months to form via slow conversion of mackinawite (FeS) and elemental sulfur (SO) to pyrite (BERNER, 1965, 1967, 1970, 1974, 1984; SWEENEY and KAPLAN, 1973 ; RAISWELL and BERNER, 1985). However, rapid rates of pyrite formation on the order of a day were measured in sediment sulfate reduction experiments using 35S04 radiotracer methods , which is a sensitive measure of the rate of reduction (HOWARTH>, 1979; HOWARTH and GIBLIN , 1983; HOWARTH and JORGENSEN 1984; GIBLIN and HOWARTH, 1984; HOWARTH and MERKEL, 1984; JORGENSEN et al., 1984; LUTHER et al., 1986). Pyrite may form rapidly when ferrous iron reacts with polysulfides, which are in turn controlled by the presence of oxidizing agents (such as oxidized iron or manganese) and perhaps lower pH; under such conditions, when FeS is undersaturated , pyrite formation is kinetically favored (RICKARD, 1975; HOWARTH , 1979; LUTHER et al. , 1986) . It is possible that pyrite may form slowly when FeS dissolves, forming ferrous iron, which then reacts with polysulfides to form pyrite; if this were the case. formation would be slow because FeS dissolution is slow when dissolved sulfide levels are high . Another limit on pyrite formation at depth may be lack of possible pyrite precursors such as polysulfides, which form when hydrogen sulfide is oxidized , and which are absent in deep water (G. LUTHER. personal communication).
v',
Box model of sulfur cycling between dissolved and solid phas es Our model for sulfur cycling in the water column and sediments of the Black Sea, as inferred from sulfur isotopic information, is summarized in Fig. 9. Rate information is based on this study and others (Table 9) . Alternative hypotheses are discussed above , and may also be consistent with the observed sulfur isotopic compositions of the various sulfur reservoirs. As this is a model for proposed processes, we have extended the range of sulfur isotopic composition for dissolved sulfide at the oxic-anoxic interface to account for the possibility that all sedimentary sulfides are derived from sulfide fluxes precipitated at the oxic-anoxic interface. (1) Isotopically heavier dissolved sulfide at the oxic-anoxic interface results from net consumption of sulfide by chemical oxidation, perhaps by Fe 3+ or Mn 4+ . Estimated sulfide
S1181
Sulfur fluxes in the Black Sea
Reactive Fe + C org aggregates
•
Oxic-anoxic interlace
Oxidation:
50?
-L Rapid S· reduction? ~ HS' shallow 1---------------------------
... ..
.
•' ' ..~• •
.
'. ••
• FeS x precip~ation;
. . ' scavenging •
..
•
..
t
.
S· reduction on organic partides?
•
g.
FeS x flux Q) I-------J
A
(lj
T
:: Ul
E
N~
R
-
Fluxes:
10 Water column S· reduction : 79·536 (1.31JM!yr)
~-
W
Q)
Q)
"0
en J:
...
0(/)
---------------------------------
.... 144 III II
290 500
1\ UlWUDl1II r-lUuuUnmlRIIllIlIUUlUllIllIOIlIIIIUIUUIUlUUInIlIlIllUIUtlllUUllIlIllIlIllIUlUU IIInlllllllUUlU1lI
Sediment S-reduction:
S E
470-530+ (7.7 mMtyr); 144·663
--
Slowto fast rates, and/or Increasing suI!ate depletion?
HS- + FeSx
authigenic +
L
I M E N T
(j)
o
on.
flux
-50
o
.
.
-40
·30
°
. ·20
:
o
10
Q)
0
20
30
Fig. 9. Model of sulfur cycling between dissolved and particulate phases in the water column and sediments of the Black Sea, ba sed on sulfur isotopic compositions. Sulfur isotopes give "source" vs "process" information. Horizontal arrows indicate processes resulting in isotopic fractionation between source and product ; vertical arrows give information on the unaltered source(s) of sulfide . Proposed pathways are shown In bold arrows, while alternative processes or those which may occur simultaneously arc shown in dashed arrows. All rates are in mM m- 2 y-t except for Alberts volume-based sulfate reduction rates, which are in ,11M y-I or mM Estimates of rates of sediment sulfate reduction and HS- production are: 470-530 + mM m- 2 y-I , or a mean of 500 (ALBERT et al., in preparation) : 144 mM m- 2 y-t is the mean rate of sulfate reduction/H'S" production, and 663 mM m- 2 y-I is the maximum rate calculated, based on 72% of organic carbon fluxes consumed (from this study): 290 mM m - 2 y -I (SOROKIN , 1964).
v '.
loss due to oxidation may be 50 mM m- 2 y-I (60 - 10). Since volume-based sulfate reduction rates in the water column are low, much of the dissolved sulfide may be "old" sulfide upwelling and diffusing from below, rather than "new" sulfide produced from local sulfate reduction in the upper anoxic water column; this needs to be further investigated and in different seasons.
51182
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MURAMOTO
et al.
(2) Dissolved sulfate in the water column supports some sulfate reduction there , and diffuses into sediments where sulfate reduction rates are higher because of more organic carbon and/or iron for microbial needs. The large isotopic fractionation between sulfate and sulfide in the water column (55-600/00) and the narrow range of isotopic values of deep dissolved sulfide, are consistent with low rates of sulfate reduction in an open system with respect to sulfate. Dissolved sulfate in the Black Sea water column in 1975 had a sulfur isotopic composition between 18.2 and 20.20/00 (SWEENEY and KAPLAN, 1980), and one sulfate sample from 500 m in 1988 had a value of +19.50/00 (FRY et al., 1991). Modern marine sulfate has isotopic values between +20 and +210/00 (REES et al. , 1978). (3) Iron sulfides, elemental sulfur and perhaps polysulfides , precipitate rapidly at and below the oxic-anoxic interface, resulting in a layer of suspended particulate sulfur. Here reactive iron concentrations are highest due to new inputs of reactive iron from above, and dissolved sulfide concentrations high enough to allow precipitation. The source of dissolved sulfide is sulfide which has undergone chemical oxidation at the interface , resulting in particulate sulfides of isotopic composition distinct from that of deeper-water dissolved sulfide. Precipitation of iron sulfides deeper in the anoxic water column is limited by extremely low levels of dissolved reactive iron. (4) Organic aggregates from the photic zone sink through the iron sulfide precipitation zone below the oxic-anoxic interface , scavenging particulate pyrite and other metal oxides and sulfides as they fall, and accumulate as sediments. The resulting particulate iron sulfide flux is about 10 mM m - 2 y-l . Rapid settling of organic aggregates carries particulate sulfides as fluxes to the sea floor within a month. Several hundred meters below the oxic-anoxic interface, there may be organic particle-associated sulfate reduction, although water column sulfate reduction rates are low and appear to be limited, possibly by iron (ALBERTet al., in preparation). Whether or not iron sulfides precipitate on settling particles is determined by reactive iron, which is scarce in the deep water column. Seasonal variations in organic particle fluxes results in seasonal variations in scavenging and supply of particulate sulfides to bottom sediments; it may also explain the large variations in water-column sulfate reduction rates measured if sulfate reduction is particle-associated. (5) In sediments, further precipitation of iron sulfides is also limited by availability of reactive iron , which thus controls the amount of dissolved sulfide present. Sulfides in sediments may therefore be largely derived from the oxic-anoxic interface where there is more reactive iron available, with some additional sulfides precipitated in sediments. New inputs of reactive iron to the Black Sea occur via run-off and fluvial inputs and possibly by windborne transport of dust particles. Iron fluxes are associated with clays and perhaps with organic matter, which may act in a scavenging role; fluxes are fairly uniform throughout the year, because lithogenic and organic matter fluxes reach their maximum levels in opposite seasons of the year. (6) Sulfate reduction is higher in sediments than in the water column, by a factor of about 6000 (on a volume basis), possibly because of organic carbon accumulation. Sulfate reduction/H'S" production rates can be estimated from 72% of organic carbon fluxes lost in sediments, resulting in an average rate of 144 mM m - 2 y-I, but reaching maximum values of 663 mM m- 2 y-l due to seasonal maxima in organic carbon fluxes. SOROKIN'S (1964) estimate was 290 mM m- 2 y-I . Consumption of porewater dissolved sulfide could occur via further sedimentary iron sulfide precipitation (only if enough reactive iron exists), microbial assimilation and/or oxidation, or other unknown oxidation mechanisms. Since sulfate reduction in sediments may be organic substrate-limited due to competition
Sulfur fluxes in the Black Sea
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with fermenting microorganisms , large seasonal variations in organic fluxes could result in large variations in sedimentary sulfate reduction rates. (7) Porewater dissolved sulfide remaining after any consumption diffuses into the water column and undergoes vertical and lateral advection and diffusion, constituting the pool of "old " dissolved sulfide. Deep water dissolved sulfide has a narrow range of sulfur isotopic composition and a large degree of fractionation between sulfate and sulfide (about 60%0), consistent with overall low rates of sulfate reduction in the water column. Steep concentration gradients in the upper anoxic water column drive increased upward diffusive fluxes of dissolved sulfide (at the rate of about 60 mM m- 2 y-l in the upper several hundred meters), to the oxic-anoxic interface where the cycle begins over again. (8) Factors controlling the levels of dissolved sulfide in the Black Sea basin include : (a) sulfide oxidation, chemical and microbial; (b) metal sulfide precipitation and other consumption processes , such as microbial assimilation; (c) reactive iron concentrations and distributions, which control metal sulfide precipitation ; (d) sulfate reduction rates and factors which control these rates, including quantity and quality of organic matter, sedimentation rates and availability of sulfate; and (e) physical mixing processes, including seasonal variation in wind mixing and upwelling, fluvial inputs and circulation, which could affect many of the above processes.
Implications for the sedimentary record In anoxic basins such as the Black Sea, sediments potentially contain a record of sulfur . iron and organic matter cycling taking place in the upper water column , although diagenetic processes will continue to add, modify or consume materials which originally settled from the water column. Some particulates, such as pyrite fluxes and unreactive iron minerals, will settle to sediments where they will remain relatively unaltered; others such as reactive organic matter, reactive iron, iron monosulfides, biogenic carbonate and opal, can undergo further diagenetic transformations. Diagenetic processes include sulfate reduction, fermentation, precipitation, dissolution, diffusion and remobilization . Fluxes of iron sulfides precipitating in the water column could account for most of the sulfides in sediments of the Black Sea . At the oxic-anoxic interface , the steep gradient in isotopic change may be due to net chemical oxidation of dissolved sulfide, rather than net oxidation by photosynthetic bacteria (FRY et al., 1991). Sulfides in sediments would then preserve the isotopic signature of biogeochemical processes at the oxic-anoxic interface. If porewater dissolved sulfide in surface sediments is in equilibrium with dissolved sulfide in the water column directly above, then it should also be isotopically identical to deep-water dissolved sulfide, which is isotopically lighter than dissolved sulfide at the oxic-anoxic interface. If sulfide precipitation from porewater-dissolved sulfide continued during deposition, then we predict that the 034S of sulfides in dark and light laminae of varves reflects two different sources : (1) sulfide fluxes from the oxic-anoxic interface in lightcolored carbonate-rich bands representing fluxes from summer coccolithophorid blooms when organic fluxes are often at a maximum (HAY , 1988), (2) sulfides precipitated in situ during sediment diagenesis, from reaction of porewater-dissolved sulfide and available reactive iron. Since much iron is associated with lithogenic fluxes, and lithogenic fluxes tend to reach their maximum during the winter and spring, sulfides precipitated insitu may be found in sediment laminae representing these deposits.
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Short- and long-term variations in organic matter fluxes over time could result in similar changes in rates of remineralization and sulfide genesis due to variation in sulfate reduction activity in sediments. The hypothesis of two different sources of sedimentary sulfides depicted above is further complicated in that sedimentary sulfate reduction rates may increase when organic carbon burial fluxes are higher, resulting in dissolved sulfide that diffuses outward from the carbonate- and organic-rich layers and is precipitated in the adjacent lithogenic-rich layers. Sampling of individual lamina of varves is needed to test these detailed hypotheses. Acknowledgements-We gratefully acknowledge A . G iblin for adv ice on sulfur techniques , S. Manganini for help on measurements of fluxes and bulk composition, B. Woodward for CHN analyses. and D . Bankston for assistance with ICPES anal yses . T . Konuk gave permission to participate in cruises on the R. V. Koca Piri Reis . S. Kempe , M. Wiesner and A . Diercks assisted during cruises. We thank T . Lyons. D. Albert, B. Jorgensen and their colleagues for permission to refer to manuscripts in preparanon . We also thank A . Giblin, R . Berner. T . Lyons and an anonymous reviewer for reviewing the manuscript and for useful suggestions throughout the project. Work was made pos sible through NSF Grants OCE-8614462 to J . L. Cisne , OCE-8814363 to S. Honjo , OCE-8800101 to B . Fry. and donations from Meyer Bend er and the Clas s of 1929. Cornell Universit y. Woods Hole Oceanographic lnstitution Contribution no. 7302.
REFERENCES ASPER V. L. (1986) Accelerated sell ling of marine particulate matter by marine snow aggregates. Ph. D. Thesis. MIT- WH OI -86- l2. BERNER R. A. (1965) Iron sulfides formed from aqueous solution at low temperatures and atmospheric pressure. Journal of Geology. 72. 293-306. BERNER R. A . (1967) Thermodynamic stability of sedimentary iron sulfides. American Journal of Science, 265. 773-785 . BERNER R. A. (1970) Sedimentary pyrite formation . American Journal of Science . 268. 1-23 . BERNER R . A . (1974) Iron su lfides in Pleistocene deep Black Sea sediments and their paleoceanographic sign ificance . In : The Black Sea-geology . chemistry . and biolog y; E . T . DEGENS and D. A . Ross. editors, American Association of Petroleum G eologists. Memoir 20. pp. 524-531 . BERNER R. A. (1984) Sedimentary pyrite formation : an update. Geochimtca et Cosmochimica Acta. 48, 605-615. BERNER R. A . and G . R . HOLDREN JR (1978) Sulfur distribution in Hol es 380 and 380A of Leg 42B . In : In itial reports of the Deep Sea Drtlltng Project. Vo l. 42 . Part 2. D. A . Ross et al.• editors. (U .S. Government Printing Office . Washington) , pp . 625-626. BREWER P. G. and D . W. SPENCER (1974) Distribution of some trace elements in the Black Sea and their flux between dissolved and particulate phases. In: The Black Sea-geology , chemistry. and biology. E. T. DEGENS and D . A . Ro ss . editors. American Association of Petroleum Geologists, Memoir 20. pp . 137-143. CANFIELD D . E .• R . RAISWELL. J . T . WESTRICH. C. M . REAVES and R. A . BERNER (19 86) The use of chromium reduction in the a na lysis of reduced inorgan ic sulfur in sediments a nd shales. Chemical Geology . 5-1, 149155. FRY B . • H. GESTand J . M. HAYES (1984) Isotope effects associated with the anaerobic oxidation of sulfide hy the purple photosynthetic bacterium. Chromatium vmosum . FEMS Microbiology Letters. 22. 283-287. FRY B .• H . GESTand J . M. HAYES (1988a) :>4S/,2S fractionation in sulfur cycles catalyzed by anaerobic bacteria. Applied and Environmental Microbiology, 54 . 25~256 . FRY B ., W. RUF, H. GEST and J. M . HAYES (1988b) Sulfur isotope effects asso ciated with oxidation of sulfide by O 2 in aqueous solution . Chemical Geology. 73.205-210. FRY B ., H . W . JANNASCH , S. J. MOLYNEAUX , C . WIRSEN .J . MURAMOTO and S. KING (l991) Stable isotope studies of the carbon, nitrogen and sulfur cycles in the Black Sea and the Cariaco Trench . Deep-Sea Research , 38 (Suppl.), S1003-5 1019. GIBLIN A. E. and R. W. HOWARTH (1984) Porewater evidence for a dynamic sedimentary iron cycle in salt marshes. Limnology and Oceanography, 29. 47-63 . GOLDHABER M . B. and I. R . KAPLA N (1974) The sulfur cycle . In : The sea. 5th edn. , E. D . GOLDBERG , editor Wile y. New York, pp . 569-655. . '
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GOLDHABER M. B. and I. R. KAPLAN (1975) Controls and consequences of sulfate reduction rates in recent marine sediments . SOIl SCIence. 119.42-55. GOYET C.; A. BRADSHAW and P. BREWER (1991) The carbonate system in the Black Sea. Deep-Sea Research. 38 (Suppl.). SI049-SI068. HAY B. J. (1987) Particle flux in the western Black Sea in the present and over the last 5.000 years: temporal variability. sources. transport mechanisms . Ph.D . thesis. M1TIWHOI Joint Program. HAY B. J . (1988) Sediment accumulation in the central western Black Sea over the past 5000 years. Paleoceanography. 3.491-508. HAY B. J. and S. HONJO (1989) Particle deposition in the present and Holocene Black Sea. Oceanography. 2. 26--31. HAY B. J .. S. HONJO. S. KEMPE. V. A . 1TTEKKOT. E. T . DEGENS. E. IZDAR and T. KONUK (1990) Interannual variability in particle flux In the southwestern Black Sea . Deep-Sea Research. 37.911-928. HIRST D. M. (1974) Geochemistry of sediments from eleven Black Sea cores . In: The Black Sea-geology. chemistry. and biology. E. T . DEGENS and D. A. Ross. editors. American Association of Petroleum Geologists. Memoir 20. pp . 430-455 . HONJO S.. K. W. DOHERTY. Y. C . AGRAWAL and V L. ASPER (1984) Direct optical assessment of large amorphous aggregates (marine snow) in the deep ocean Deep-Sea Research. 31, 67-76. HONJO S.. S. MANGANINI and J. COLE (1985) Sedimentation of biogenic matter in the deep ocean. Deep-Sea Research. 29. 609-625. HONJO S.. B. J. HAY. S. J. MANGANINI. V. L. ASPER. E. T. DEGENS, S. KEMPE. V. ITTEKKOT. E. lZOAR, T. KONUK and H. BENLI (1987a) Seasonal cyclicity of lithogenic particle fluxes at a southern Black Sea sediment trap station. In: Particle flux in the ocean. E . T . DEGENS, E . IZDAR and S. HONJO. editors. Mitteilungen Geol. Palueontol. lnst, Univ , Hamburg. Vol. 62 (in press). HONJO S.• S. J . MANGANINI. V. L ASPER. B. J . HAY and A. L. KAROWE (l987b) Particle fluxes. south central Black Sea : 1982-1985 (Black Sea Sedimentation Data File. Vol. I) Woods Hole Oceanographic Institution Technical Report WHOI-811. and Piri Reis International Contribution Series Publication No.4. 106 pp. (in press) . HOWARTH R. W. (1979) Pyrite : its rapid formation in a salt marsh and its importance to ecosystem metabolism . Science. 203. 49-51. HOWARTII R. W. (1988) Nutrient limitation of net primary production in marine ecosystems. Annual Review of Ecology. 19.89-110. HOWARTH R . W. and A . E. GIBLIN (1983) Sulfate reduction in the salt marshes at Sapelo Island. Georgia . Limnology and Oceanography . 28, 70-82. HOWARTH R. W. and B. B. J0RGENSEN (1984) Formation of 35S-labelled elemental sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord . Denmark) during short-term 35S042 reduction measurements. Geochimica et Cosmochimica Acta . 48. 1807-1818. HOWARTH R. W. and S. MERKEL (19&4) Pyrite formation and the measurement of sulfate reduction in salt marsh sediments . Limnology and Oceanography. 29598-608. JANNASCH H. J • H. G. TRUPER and J. H . TUTTLE (1974) Microbial sulfur cycle in Black Sea. In: The Black Seageology, chemistry, and biology, E. T. DEGENS and D. A. Ross, editors. American Association of Petroleum Geologists, Memoir 20. pp . 419-425. JANNASCH H. W.. C. O. WIRSEN and S. J . MOLYNEAUX (1991) Chemoautotrophic sulfur-oxidizing bacteria from the Black Sea Deep-Sea Research. 38 (Suppl.), SI105-S1120. J0RGENSEN B. B.• H. FOSSING and S. THODE-ANDERSEN (1984) Radiotracer studies of pyrite and elemental sulfur formation in coastal sediments . Oral presentation at AGU/ASLO Ocean Sciences Meeting, San Francisco. J0RGENSEN B. B., H. FOSSING. C. O . WIRSEN and H. W. JANNASCH (1991) Sulfide oxidation in the anoxic Black Sea chemocline . Deep-Sea Research , 38 (Suppl.), SI083-S1103 . KAPLAN I. R. and S. C. RITTENBERG (1964) Microbiological fractionation of sulfur isotopes. Journal of General Microbiology, 34.195-212. LANDING W. M. and B. L. LEWIS (1989) Trace metal chemistry in the Black Sea . NATO-Advanced Research Workshop. Black Sea Oceanography. lzmir, Turkey. 23-27 October, 1989. LEVENTHAL J . S. (1983). An interpretation of carbon and sulfur relationships in Black Sea sediments as indicators of environments of deposition . Geochimica et Cosmochimica Acta. 47, 133-138. LEWIS B. L. and W. M. LANDING (1988) Dissolved iron speciation in the Black Sea . AGU/ASLO Special Session Poster Paper, 1988 Fall AGU/ASLO Meeting. San Francisco, CA.
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LUTHER G. W ., III. T . M. CHURCH, A. E . GIBLIN and R. W. HOWARTH (1986) Speciation of dissolved sulfur in salt marshes by polarographic methods. In: Organic marine geochemistry, M. L. SAHN, editor, American Chemical Society, Series 305, pp . 340-355. LUTHER G . W ., Ill, T. M . CHURCH and D. POWELL (1988) Sulfur speciation and possible sulfide oxidation in the water column of the Black Sea In : 1988 AGUIASLO Abstract Poster Session on Black Sea Oceanograph y,
EOS . LYONS T. W . and R. A . BERNER (1990) Reactive iron in Black Sea sediments . AGU/ASLO 1990 Ocean Sciences poster pre sentation, New Orleans. 11-16 February, abstract published in EOS, November 1989. MARTIN J H . and S. E . FITZWATER (1988) Iron deficiency lim its ph ytoplankton growth in the north-east Pacific subarctic. Nature, 331 , 341-343 . MARTIN J. H., R . M . GORDON, S. FITZWATER and W. W. BROENKOW (1989 ) VERTEX: phytoplankton/iron stud ies in the Gulf of Alaska . Deep-Sea Research, 36, 649...()80. MATRAI P. A . and R . W . EpPLEY (1989) Particulate organic sulfur in the waters of the Southern California Bight. Global Biogeochemical Cycles, 3, 89-103. MOREL F . M . M . and R . J. M. HUDSON (1985) The geobiological cycle of trace elements in aquatic syste ms: Redfield revis ited . In : Chem icalprocesses in lakes , W . STUMM , editor.John Wile y, New York, pp . 251-282. MURRAY J . W., H . W. JANNASCII , S. HONlO, R . F. ANDERSON , W . S. REEBURGH. Z. Top, G . E. FRIEDRICH , L. A. CODISPOTI and E. IZDAR (1989 ) Un expected changes in the oxic-anoxic interface in the Black Sea . Nature, 338 ,411-413. NICHOLSON J . M . (1988) Reduced sulfur in the Black Sea water column and sediment traps. In: Temporal and spatial variability in sedimentation in the Black Sea. Cruise Report, R .V. Knorr 134-8, Black Sea Leg 1. 16 April-7 May , 1988 (5. HONlO, B. HAY and members of scientific shipboard party) , Woods Hole Oceanographic Institution Technical Report , WHOl·88-35 , pp . 55---{i2. NICHOLSON J . M ., S. KEMPE, G . LIEBEZEIT and B. WOODWARD (1988) Sediment trap supe rn atant chemistry with comparisons to water column chemistry. In : Temporal and spatial variability In sedimentation in the Black Sea, Cruise Report, R. V. Knorr 134-8, Black Sea Leg I, 16 April-7 May 1988 (S. HONlO, B. HAY and members of scient ific sh ipboard party), Woods Hole Oceanographic Institution Technical Report , WHOI88-35, pp . 43-54. PILIPCHUK M. F . and 1. 1. VOLKOV (1974) Behavior of mol ybdenum in processes of sediment formation and diagenesis in Black Sea. In : The Black Sea-geology. chemistry. and biology . E . T . DEGENS and D . A . Ross, editors, American Association of Petroleum Geologists, Memoir 20, pp . 542-553 . PILSKALN C . H . (1990) Composition of Black Sea fluff layer and underlying surface sediments: implications for recent mechanisms of particulate deposition a nd geochemical transformations occurring at the sedimenlwater interface. EOS , 71,151 pp .. AGU/ASLO Ocean Sciences Meeting, 1990. RAOFORO-KNOERY J . and G . A . Currss ( 1990) Suspended particulate carbon , nit rogen and sulfur in the Black Sea. AGU/ASLO 1988 O cean Sciences poster session. RAMM A. E. and P . A. BELLA (1974) Sulfide production in an aerobic microcosms. Limnology and Oceanography, 19,425-441. RAISWELL R . and R . A . BERNER (1985) Pyrite formation in euxinic and serni-euxinic environments. American Journal of Science. 285,710-724. REES C. E., W. J. JENKINS and J. MONSTER (1978) The sulphur isotopic composition of ocean water sulphate . Geochimica et Cosmochimica Acta . 42 . 377-381. REPETA D . J . and D . J. SIMPSON (1991) The distribution and recycling of chlorophylls, bacteriochlorophylls and carotenoids in the Black Sea. Deep-Sea Research, 38 (Suppl.) , S969-S984. RICKARD D. T . (1975 ) Kinetics and mechanism of pyrite formation at low temperatures. American Journal of SCIence, 275 , 636-652. ROZANOV A . G., I. l. VOLKOV and T. A . YAGODtNSKAYA (1974) Forms of iron in surface layer of Black Sea sediments. In : The Black Sea-geology. chemistry , and biolog y . E. T . DEGENS and D . A . Ross . editors. American Association of Petroleum Geologists, Memoir 20 , pp . 532-541. SOROKIN Yu . I. (1964) On the primary production and bacterial activities in the Black Sea. Journal du Conseil International pour l' Exploration de la Mer , 29, 41-60. SOROKIN Yu, l. (1983) The Black Sea. In : Ecosystems ofthe world. Ch . 11: Estuaries and enclosed seas . Vol. 26 , B . H . KETCHUM, editor, Elsevier, New York, pp . 253-292. STRAKHOV N . M. (1958) 0 formakh zholeza v osadkakh Chernogo morya (The forms of iron in Black Sea sediments) . Akad. Nauk , SSSR Doklady; 118,803-806.
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Estuarine and Coastal Shelf Science, 15,255-266. VAYNSTEYN M. B., V. G. TOKAREV, V. A. SHAKOLA, A. Lv. LEIN and M . V . IVANOV (1985) The geochemical activity of sulfate-reducing bacteria in sediments in the western part of the Black Sea. Geokhimiya, 7, 10321044 (translation by Scripta Technica, Inc., pp. 110-122, co. 1986). VINOGRADOV A. P. , V . A. GRINENKO and U. S. USTINOV (1962) Isotopic composition of sulfur compounds in the Black Sea. Geochemistry, 10,973-997 . VOLKOV I. I. and L. S. FOMINA (1974) Influence of organic material and processes of sulfide formation on distribution of some trace elements in deep-water sediments of Black Sea. In: The Black Sea-geology, chemistry, and biology, E. T . DEGENS and D. A . Ross, editors, American Association of Petroleum Geologists, Memoir 20, pp . 456--476. WESTRICH J. T . (1983) The consequences and controls of bacterial sulfate reduction in marine sediments . Ph .D . thesis, Yale University. New Haven, Connecticut. WESTRICH J. T. and R. A. BERNER (1984) The role of sedimentary organic matter in bacterial sulfate reduction: the G model tested. Limnology and Oceanography, 29, 236-249. ZHABINA N. N. and I. I. VOLKOV (1978) A method of determination of various sulfur compounds in sea sediments and rocks. In: Environmental biogeochemistry and geomlcrobiology ; Vol. 3, Methods, metals, and assessment, W. E. KRVMBEtN. editor, Ann Arbor Science Publications, pp. 735-745.
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0198-D149191 $3.00 + 0 00 1991 Pergamon Press pIc
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Sulfur, iron and organic carbon fluxes in the Black Sea: sulfur isotopic evidence for origin of sulfur fluxes Jo ANN MURAMOTO,"t SUSUMU HONJO,t BRIAN FRY,:!: BERNWARD J. HAY,t ROBERT W. HOWARTH§ and JOHN L. CISNE" iRecetved 12 ill/wary 1INC.);
In
revised form I February 1991; accepted 4 February 1991)
Abstract-In the southern and central Black Sea, reduced sulfur comprises nearly 1% of the total particulate flux in the anoxic water column, as measured by time-series sediment traps. Sulfur isotopic composuion of particulate sulfide fluxes indicates an origin near the oxic-anoxic interface, where dissolved sulfide is isotopically heavier than dissolved sulfide below 175 m due to chemical oxidation A strong seasonal correlation exists between sulfur and organic carbon fluxes; this, together with particulate sulfide precipitation at the oxic-anoxic interface, suggests that sulfides are scavenged from the mterface during maximum production of settling organic aggregates, particularly dunng (he late summer coccohthophorid blooms. Sulfide precipitation deeper in the water column and 10 sediments may be hrruted by availability of iron and/or polysulfides, since there are no particulate sulfide fluxes which have the same isotopic composition as ambient dissolved sulfide at trap depths. Smce sulfides in surface sediments are often Isotopically similar to dissolved sulfide at the top of the sulfide zone and to sediment trap fluxes, sedimentary sulfides could originate from metal sulfide fluxes forming at the oxic-anoxic interface and would reflect sulfur-cycling processes at the OXic-anOXIC mterface Sedimentary sulfate reduction rates estimated from assuming 65-80% utilization of particulate organic carbon fluxes agree reasonably well with other estimates of sulfate reduction and sulfide production rates. A sulfur isotope-based box model for sulfur cycling between dissolved and particulate phases in the water column and sediments is presented.
INTRODUCTION
IN marine anoxic basins such as the Black Sea, the biogeochemical cycles of sulfur, organic carbon and iron are closely linked via microbial sulfate reduction. Accumulation of dissolved hydrogen sulfide from dissimilatory sulfate reduction has resulted in anoxic sulfidic water being present everywhere below approximately 100 m. Roughly 90% of the total volume of the Black Sea is today sulfidic, or about 5.56 x 105 krrr' (SOROKIN, 1983). Transformations of sulfur between dissolved and solid phases are of great interest because sulfur is involved in a wide range of biological and chemical processes, many of which take place in the water column; temporal changes, whether seasonal or longer term, are particularly important to understand because sulfur is involved in many long-term biogeochemical processes. Understanding the limiting roles played by organic carbon and "Department of Geological SCIences. Cornell University, Ithaca, NY 14853, U.S.A. tPresent address: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 0254J. U.S.A. :f:Ecosy~tems Center. Marine Biological Laboratory, Woods Hole, MA 02543, U.S.A. § Department of Ecology and Systematics. Cornell University, Ithaca. NY 14853. U.S.A. S1151
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iron is essential to understanding the origin and seasonal behavior of reduced sulfur compounds, while sedimentary sulfides can potentially provide a record of the development of anoxic conditions in sediments and the water column. Thus it is desirable to know the rates of processes controlling distributions of organic carbon, sulfur and iron in both the water column and sediments. In marine anoxic environments, supply of reactive organic matter controls sulfate reduction rates and sulfide production, and reactive iron limits iron sulfide precipitation (BERNER, 1970, 1984; GOLDHABER and KAPLAN, 1975; WESTRICH, 1983; WESTRICH and BERNER, 1984; RAISWELL and BERNER, 1985). Linear relationships between reduced sulfur and organic carbon content in marine sediments were proposed as a useful diagnostic tool for distinguishing anoxic vs oxic sedimentary environments and determining whether sulfate reduction was taking place in the water column or not (GOLDHABER and KAPLAN, 1974, 1975). Thickness of the overlying anoxic water column might be inferred from the slope and intercept of the linear relationship . In Black Sea sediments, the linear relationship between reduced sulfur and organic carbon in sediments suggests that sulfide precipitation should be occurring in the anoxic water column above sediments (LEVENTHAL, 1983; RAISWELL and BERNER, 1985). Sulfides may also be associated with settling aggregates or marine snow, which provide a major means of transport of material to the deep ocean floor (HONlO et al. , 1984, 1985), such organic particles could also serve as sites of sulfate reduction for sulfate-reducing microorganisms . For these reasons, seasonal variations in particle fluxes of organic carbon and iron could seasonally influence particulate sulfide fluxes as well. Variations in particulate organic sulfur were related to seasonality in organic matter and chlorophyll fluxes in the Southern California Bight (MATRAI and EpPLEY, 1989). In the Black Sea, seasonal variations in fluxes of biogenic carbon, silica, and lithogenic material are related to phytoplankton blooms and riverine inputs (HONlO et al., 1987a,b; HAY , 1987, 1988). HAY and HONJO (1989) proposed that varves in Black Sea sediment originate from seasonal changes in fluxes . Varves may be more complex in origin, however, because they result both from settling fluxes that accumulate seasonally and from subsequent diagenetic processes adding and/or removing material. Thus iron sulfides could contribute to bottom sediments both by settling and by precipitating during burial diagenesis. Sulfur isotopic composition can be used as a tracer for the origin of particulate sulfide fluxes from dissolved sulfide in the water column. The isotopic composition of different sulfur reservoirs in any environment are determ ined by the sources of sulfur or by reactions transforming sulfur from one compound to another; hence sulfur isotopes can be used as tracers for sources or as indicators of processes . In the Black Sea, the isotopic composition of sulfate is determined by input of Mediterranean seawater, isotopically lighter sulfate from rivers , and sulfate reduction (VINOGRADOV et al. , 1962; SWEENEY and KAPLAN, 1980). During sulfate reduction, isotopically lighter hydrogen sulfide is produced, leaving the residual sulfate enriched in the heavier isotope (KAPLAN and RITTENBERG, 1964). Hydrogen sulfide will have an isotopic composition dependent on: (a) the rate of sulfate reduction, and (b) oxidation , either chemical or biological. As sulfate reduction rates increase due to increased supply of metabolizable organic matter, or sulfate becomes limiting, isotopic fractionation between sulfate and sulfide will decrease, resulting in isotopically heavy sulfide (GOLDHABER and KAPLAN, 1974). Chemical oxidation of dissolved sulfide leaves residual sulfide isotopically heavier since lighter sulfide is preferentially oxidized-a normal isotope effect (FRY et al., 1988b) . Conversely, sulfide oxidation
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by phototrophic sulfur bacteria results in an inverse isotope effect, causing residual sulfide to be isotopically lighter (FRY ('I al.• 1984, 1988a). For iron sulfides, the dissolved sulfide precursor determines isotopic composition; fractionation during precipitation is believed to he insignificant (Got DHABER and KAPLAN, 1975). Particulate sulfide fluxes therefore should reflect conditions affecting dissolved sulfide in the water column or microenvironment of precipitation. In anoxic sediments. metal sulfides can originate both as minerals precipitated in situ from porewater sulfide, and from fluxes precipitating in the water column; if these arc isotopically distinct. then in principle the different sources of sedimentary iron sulfides can be distinguished. In the Black Sea water column. isotopic composition of dissolved sulfide changes with depth and is isotopically lighter in deep water below 175 m. but is isotopically heavier and enriched in 3.tS near the oxic-anoxic interface; this 70-100 m deep zone of net sulfide consumption is below the zone of photosynthetic bacteria or any possible chemosynthetic sulfide oxidation (JORGENSEN ('I al.. 1991; JANNASCH ('I al.• 1991; REPETA and SIMPSON, 1991; FRY et al., 1991; SWEENEY and KAPLAN, 1980; JANNASCH et al., 1974). Isotopically heavier dissolved sulfide could result from net chemical oxidation/sulfide consumption. mixing with isotopically heavier local sulfide. or locally higher rates of sulfate reduction, but not from net photosynthetic bacterial oxidation (FRY et al., 1991). Iron sulfide precipitation may account for part of the observed sulfide consumption. The isotopic composition of such fluxes would then give the depth of formation as well as information on processes affecting sulfide isotopic composition. Formation of iron and manganese sulfides in the water column is kinetically and thermodynamically possible (BREWER and SPENCER, 1974; LEWIS and LANDING, 1988). Precipitation of iron monosulfides occurs in seconds if the solubility product is exceeded, and although pyrite in marine sediments is often believed to form slowly over a period of weeks to months or years (BERNER, 1965.1967.1970.1984), rapid formation within a day has also been measured (HOWARTH, 1979; HOWARTH and MERKEL. 1984; HOWARTH and JORGENSEN, 1984). Elemental sulfur (BERNER. 1970. 1984) and polysulfides, which may be involved in pyrite precipitation (RICKARD, 1975; LUTHER et al., 1986), both result from oxidation of dissolved sulfide. Pyrite can also form under slightly oxidizing conditions, where pH is lower than about 7, where oxidizing substances are available or where sulfide concentrations are low (HOWARTH, 1979). such as at the oxic-anoxic interface. Thus the oxic-anoxic interface in the Black Sea may be a major zone of precipitation of pyrite, metal sulfides and other sulfur compounds. We examined the magnitude and seasonally varying patterns of sulfur, iron and organic carbon fluxes measured over a 2~ year time span by time-series sediment traps. We used sulfur isotopic composition as a tracer to determine whether particulate sulfide fluxes originate from dissolved sulfide at the oxic-anoxic interface, in deeper water or elsewhere. We then compared our results for fluxes of sulfur, iron and organic carbon with other published data for Black Sea sediments and dissolved sulfides in the water column in order to address these questions: (1) What is the origin of the sulfide flux? (2) How significant is the contribution of particulate sulfide flux to bottom sediments? (3) What seasonal patterns in sulfur, organic carbon and iron fluxes exist, and how do they relate to each other? (4) Can we estimate sulfate reduction rates, sulfide production rates, and precipitation of iron sulfides in sediments during burial diagenesis? (5) What are the implications for varve formation and the sedimentary record? (6) What information about biogeochemical processes can organic carbon, iron and sulfur in fluxes and in sediments provide?
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Fig. 1. Locations of sediment trap sites in the Black Sea (BS, BSe and BSK2).
METHODS
Sediment trap samples and subsampling Samples of particulate material and trap cup supernatant were obtained from sediment traps moored at sites BS and BSC in the southwestern Black Sea. and at site BSK2 in the central Black Sea (Fig . 1. Table 1), as part of the PARFLUX (Particle Flux Experiment) and 1988 R .V. Knorr expeditions to the Black Sea . Scientists and personnel from the Dokuz Eyliil University, Turkey , and the University of Hamburg. Germany, assisted with trap deployments . recoveries and treatment of trap samples on board the R.V. Koca Piri Reis and R.V. Knorr. In order to minimize metal corrosion, moorings used Kevlar or nylon line whenever possible , and stainless steel where metal parts were essential. Trap frames and timer cases were made of aluminum . and the trap cones. baffles . and bottles were Delrin plastic, polycarbonate and high-density linear polyethylene plastic, respectively. In all trap deployments, trap cups were precharged with a supernatant solution containing 4% formalin, sodium chloride (24%0) and a dissolved buffer, usually calcium carbonate , made up in distilled water. (For site BSC, all supernatant was deoxygenated by boiling and sparging with nitrogen for 30 min; deoxygenated formalin was added just prior to deployment.) Formalin prevents microbial activity and helps preserve soft tissues: formalin also aids in retarding oxidation of dissolved sulfide (LUTHER et al. , 1988). Upon recovery of traps , formalin was always present in the trap cups . For traps at site BSe, samples of trap material were handled and preserved anoxically immediately after trap recovery. For reduced sulfur analyses using ICPES, older dried samples from traps deployed at site BS were used, which had not been anoxically preserved or stored . Anoxic collection and preservation of samples from site BSC and BSK2 were carried out in the
suss
Sulfur fluxes in the Black Sea
Table 1. Trap and hydrocasting' sues whIch provided samples used In this study. Site locations and durations of the total deployment penod at that slle are gn'en, as well as type of samples available for this study and method of sulfur analvsis used. Hydrocust Sll/'\ ..'ere sampled during Leg 2 ofthe R. V. Knorr cruise in the Black Sea. Traps at sues BS ideplovments 1-5\ and HSC i deplovments I and 2) wert" deployed and recovered with the R V, Koca Piri Rcis. Dokuz Evlul Untversnv, Turkrv. traps at HSC3and BSK2 were recovered and deployed with the R. V. Knorr, cruIse 134-S. Leg I. April-May 1988
Trap site
La!. I ~)
Long. IE \
BSI-5
ol2°lol.72'
32°ol39ol'
2!l Oct. 19!12 to 8 Apr. 1985
BSCI-3
41°47.6-l'
3(\"211,4S'
19 June 1986 to 20 May 1988
BSK2
ol2°5725'
33°54 oll'
5 May to 13 July 1988
Dates open
Treatment Air-dried samples; reduced S by JCPES Anoxic samples: Cr-reduction, elemental sulfur, acid-volatile extractions; 0 34S measured SEM
Hydrocast stations fore d~ of dissolved sulfide. R.Y. Knorr cruise 134-9. Leg 2. May 1988 Station La!. (N) Long. IE) BS2-1 BS2-2 BS:!-3
41°10' ol2°50' 43"05'
3(\''00' 32"00' 3-l"t)(1'
following manner: sampling of cups for supernatant and particulate material took place within 30 min of trap recovery. Sediment trap cups were removed and placed in a glove bag flushed with nitrogen or argon; supernatant was sampled using glass pipettes and preserved in zinc acetate for later dissolved sulfide analyses. In the glove bag, particulate material was sampled after gentle mixing of the contents, filtered and rinsed with deoxygenated distilled water. and dried; all activities were carried out in a glove bag under argon pressure. Samples were stored in a sealed desiccator containing argon and frozen until reduced sulfur analyses could be made. Other analyses of bulk and elemental composition were done after the rest of the sediment trap material had been split into aliquots in the lab. In order to test this procedure for possible oxidation, standard solutions of sodium sulfide were prepared at the time of trap sampling and placed in the glove bag for sampling along with samples. FeS and pyrite standards were used to check this procedure for possible oxidation. Chromium (Cr) reduction of these standards showed that no significant loss occurred. within the precision of the Cr reduction method. Reduced sulfur analyses We used two kinds of analyses for reduced sulfur. depending on the nature of the samples available. For anoxically preserved sediment trap material available from site BSC, we used methods for the determination of elemental sulfur (TROELSEN and JORGENSEN. 1982), acid-volatile sulfides and Cr-reducible sulfur derived from the work of CANFIELD et al. (1986). HOWARTH and JORGENSEN (1984). GIBLIN and HOWARTH (1984), HOWARTH and GIBLIN (1983). WESTRICH (1983). HOWARTH (1979) and ZHABINA and VOLKOV (1978). For sediment trap samples from PARFLUX site BS, which were not anoxically preserved but dried archive samples. we measured reduced inorganic sulfur by
S1156
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et al.
ICPES (Inductively Coupled Plasma Emission Spectrometry) . Both methods had comparable rates of recovery for standards (85-100 %) (see Table 1 for sulfur procedures applied to samples) .
Anoxically preserved samples. For sediment trap samples from deployments 1-3 at site BSC (19 June 1986 to 14 April 1988) and site BSK2 (5 May to 13July 1988) we analysed for Cr-reducible sulfur, which, when done without prior extraction of other reduced sulfur species , measures all inorganic reduced sulfur. For a second set of samples from deployment 2 at site BSC (26 September 1986 to 14 May 19X7). samples were sequentially analysed for elemental sulfur, acid-volatile sulfides and Cr-reducible sulfur; when elemental sulfur and acid-volatile sulfides are extracted first from a sample. Cr reduction on the remaining sample extracts pyrite. Anoxically preserved samples were weighed in a nitrogen atmosphere. Sequential analysis of sulfur was as follows: elemental sulfur was first extracted using carbon disulfide followed by add ition of sodium cyanide with an iron catalyst. Absorbance was measured at 448 nm on the spectrophotometer. Recovery rates of elemental sulfur standards run alone and as spikes added to sulfur-free sediments were approximately 100%. The same sample was then rinsed several times with carbon disulfide followed by several acetone rinses to remove any carbon disulfide residue , and dried. The entire elemental sulfur extraction was carried out in a nitrogen atmosphere in order to avoid oxidation. After drying, the same sample was used for determination of acid-volatile sulfides using first cold and then hot nitrogen-saturated 5.5 N hydrochloric acid to drive off acid-volatile sulfides . Sulfide gas was trapped in a 10% zinc acetate trap. and sulfide was analysed spectrophotometrically at 670 nrn, using phenylene diamine reagent and the methylene blue absorbance assay. Meanwhile. the remaining sediment sample was Crreduced to measure pyrite . Reduced Cr(II) chloride solution was added and the sulfide produced was analysed as above . Recoveries from acid distillation and Cr reduction ranged between 85 and 100% and are comparable to the total sulfur by ICPES method described below. As a measure ofreduced inorganic sulfur. the sum of elemental sulfur. acid-volatile sulfides and Cr-reducible sulfur/pyrite was used for samples from deployment 2 at site BSC . while Cr-reducible sulfur alone (without prior extraction of elemental sulfur or acid-volatile sulfides) was used to measure reduced inorganic sulfur for samples from deployments 1 and 3. site BSC. ICPES analysis of reduced inorganic sulfur on archived samples. Archived. wet-stored but non-anoxically preserved samples were used for this analysis ; these samples were rinsed thoroughly with deionized . deoxygenated. nitrogen-sparged water to remove dissolved sulfates, and then air-dried briefly using nitrogen head pressure . Samples examined before and after this treatment did not contain any detectable precipitated sulfates such as gypsum or anhydrite crystals. using both ore petrologic and SEM microscopy, but pyrite framboids and opaque minerals were observed in these samples. Cold aqua regia in an especially oxidizing mixture (1:1 HCl to HNO,3) was added to dried samples by adding cold concentrated HCI followed by cold HNO,3. under a reflux condensor. This was kept on ice for 2 h under refluxing conditions; the condensor was then removed and the sample flasks heated gently to evaporate off liquid . Hydrochloric acid (6 N) was quantitatively added and the resulting solution was filtered to remove insoluble residues (mainly silica as analysed by ICPES) . The solution was analysed at 181.974 nm in the vacuum ultraviolet portion of the emission spectrum. using a Jobin-Yvon JY -38 VHR
Sulfur fluxes in the Black Sea
S1157
Rapid Sequential Inductively Coupled Plasma Emission Spectrometer (JCPES) purged with nitrogen gas before and during analysis. Recovery rates of spiked pyrite and ZnS standards added to sediments and standards alone ranged between 80 and 100%, while standard deviation for replicates was 1.91 ppm. equivalent to sulfur concentration of 0.06 .urn g 1 of sediment. The theoretical detection limit was 30 ppb. Organic sulfur standards which were tested (methionine. cysteine) were not extracted using this method. Thus this method analyses only for total inorganic sulfur and not organic sulfur; if precipitated sulfates arc present. then these reagents will dissolve sulfate. but for samples such as these which were desalted thoroughly by rinsing in distilled water and which do not contain insoluble precipitated sulfate minerals. the procedure measures reduced inorganic sulfur. Using this method. acid-soluble metals can also be analysed using JCPES on the same sample: multiple analyses on the one sample is often desirable when the amount of original sample available for analyses is small. as is the case with sediment flux samples in general.
Sulfur isotopic composition For sulfur isotopic analyses of fresh anoxically preserved sediment fluxes from trap sites BSC and BSK2. samples were Cr-reduced for all reduced inorganic sulfur. Sulfide gas was trapped in a 7% silver nitrate solution. The silver sulfide precipitate was concentrated, rinsed. dried and directly cornbusted with vanadium pentoxide to sulfur dioxide gas, which was collected by cryogenic distillation. The yield was measured using calibrated baritron pressure of the gas and then analysed for isotopic composition using a Finnigan 251 isotope ratio mass spectrometer. Results are expressed as 0 34S values relative to CDT (Canyon Diablo troilitc). Precision for standards carried through the distillation procedure was better than n.3"bo and replicate analyses also agreed to within 0.30/00. Dissolved sulfide between 100 and 2000 m depth was collected using silver nitrate and zinc acetate to preserve dissolved sulfide. during Leg 2. sites 1, 2 and 3,1988 R.V. Knorr cruise. by one of us (FRY et al.• 1991). The preparation and analysis of these samples was identical to that descrihed above for sediment trap samples.
Bulk composition and fluxes Measurements of total flux and analyses of the bulk composition were done on splits of particulate material. CHN analyses were done on decalcified samples using a PerkinElmer CHN Analyzer. Precision of this method is 0.3% on replicates. Major and minor elemental composition. including total particulate iron. was determined by JCPES on samples which were fused using a lithium metaborate flux at lOOO°C and then dissolved in 4% nitric acid. For iron. the standard deviation of this method is 0.11% (% S.D. = 4.23%). and the detection limit is 0.001 %. Due to small sample sizes available from sediment trap fluxes. only total iron was determined. Non-combustible material, representative of lithogenic fluxes. was determined by muffle furnace combustion at 600°C. Molar ratios. which can be useful in estimating composition or approximate stoichiometry of phases. were calculated based on relative molar concentrations of an element or phase in the sediment flux. We present data for both fluxes and concentrations of a particular substance since flux is a measure of the absolute amount of material and concentration is a measure of the amount of a substance relative to the total of all components. Both of these
S1158
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MURAMOTO et
at.
can potentially change independently of each other, reflecting the different factors affecting their production or disappearance.
SEM observations
Samples for SEM-EDX (energy-dispersive X-ray analysis) were taken from anoxically preserved material on 0.45,um Nuclepore membrane filters, which were placed on carbon stubs and then coated with carbon if elemental EDX analyses were done (with goldpalladium if otherwise). A JEOL-840A SEM with EOX was used to examine samples for the presence of iron sulfides, particularly framboidal pyrite.
RESULTS
Fluxes: composition and seasonality Reduced sulfur fluxes.
Seasonal variation . Highest fluxes and concentrations of reduced inorganic sulfur occur during the summer and autumn (fluxes of about 50JlM m-2_day-1 .concentrations of 600+ ,uM g sedimenC I) and drop to minimum levels during the winter and early spring (fluxes <1 ,uM m- 2-day-1 and concentrations <50,uM g sediment-I) (Figs 2A and 3A) . The seasonal variation closely resembles that of organic carbon fluxes; over time, both fluxes and concentrations of organic carbon and sulfur vary together, resulting in a high correlation between organic carbon and sulfur (Figs 2B and 3B). Sulfur fluxes and concentration are not correlated well with either total flux or fluxes or concentration of iron (Figs 2C, D and 3C). Geographic variation. The average flux of reduced inorganic sulfur for both sites BS and BSC is 25.26,uM m -2 day-I (9.23 mM m- 2y-l). As of now, too few analyses are available from site BSC to make meaningful comparisons between the two sites, although the data suggest a significant difference in reduced sulfur fluxes between sites (Table 2) . Despite the apparent statistical difference between sites BS and BSC for reduced inorganic sulfur, we conclude that at this time, due to the small number of samples analysed from BSC, there are probably no significant differences in the sulfur and organic carbon fluxes between sites. The organic carbon to reduced inorganic sulfur ratio , or C:S ratio, is also similar for the two sites (average C:S ratios of 56 and 60 for the two sites, with no statistically significant difference; Table 2). Reduced sulfur species in sulfur fluxes. Fluxes of elemental sulfur, acid-volatile sulfides and pyrite determined on strictly anoxically preserved samples from BSC are given in Table 3. The September 1986 sample is the only one with any significant amounts of acidvolatile sulfides, and it was also the only sample which was black in color and macroscopically very fine-grained in texture. With this exception, samples contained more pyrite and elemental sulfur than acid-volatile sulfides. Samples from October to December were dark grey and intermediate in texture, while the March sample was light greenish-grey and relatively coarser-grained . Since pyrite in fine to intermediate texture is known to impart a grey rather than black color (typical of iron monosulfides), the relative amounts of the three sulfur species measured in these samples is consistent with these observations. Based on these anoxically preserved samples, the mean percentage of total sulfur which is due to
>..
50
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Site
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J AJI OJ AJI OJ AJI OJ AJI OJ AJl OJ AJI 0 1983 1984 1985 1986 1987 1988
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Fig . 2. Fluxes of particulate reduced sulfur , orga nic carb on, iron and tota l mat erial. for sites BS and BSC. (A) Fluxes of reduced sulfur. inl~M m- 2 day- I : red uced inorganic sulfur is based on ICPES measurements for samples from the period Janu ary 1983 to Ap ril 1985, and on the sum of elementa l sulfur, acidvolatile sulfides and pyrite . for the perio d from September 1986 to August 1987. (B) Organic car bon fluxes. in mM m- 2 day " . (C) Tota l flux at sites BS and BSe. in mg m- 2 day" ' . No traps were deployed bet ween Ap ril 1985 and Jun e 1986. (D) Part iculate iron flux at sites BS and BSe , in ,uM m - 2day - I.
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S1160
MURAMOTO
et al.
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pyrite and acid-volatile sulfides together is about 58% of the total reduced sulfur, with the remainder being elemental sulfur. SEM observations . Pyrite framboids are relatively common in anoxically preserved material from all samples currently being studied and are easily detectable by backseattered electrons or EDX . Framboids were always observed occurring singly and never in clumps. The average diameter of pyrite framboids is about 6 .urn. for 100 framboids from site BSC. BS and BSK2 (Fig. 4). If Stokes' Law applied to a particle this small, this corresponds to a Stokes' settling velocity of 5.6 m day -I , or 6 months to settle to 1000 m; however, framboids are always observed to adhere to other biogenic particles. Pyrite framboids were observed before in sediment trap material from the Black Sea, as part of
S1l61
Sulfur fluxes in the Black Sea
Table 2. A verage values for sediment trap material from sites BS and BSC in the Black Sea, com prising material collected from October 1982 TO May 1988. Shown are mean values, standard deviations, maximum values and numb er ofcups sampled for : total fl uxes . organic carbon (C) , reduced particulate sulfur (S) as defined in the text, particulate iron (Fe) , percent by dry weight ofC, S, Fe , and the molar ratios C:S and Fe:S . Total flux is in mg m- 2 day- I; reduced sulfur, organic carbon and iron fluxes are in,uM m- 2 day-I ; percentages are by dry weight. Also given are t-tests (unpai red values, unequal variances, pooled t-statistic) of the difference between BS and BSC means, using the null hypothesis of no difference between means . The level of significance probability P (probability that the t-statisuc calculated is due to random error) at which there is a statistically significant differen ce between means is given %C
%S
% FE
C:S
Fe :S
49.75 44.40 48
12.89 7.22 56
0.77 0.52 2li
2.2 1.29 52
60.06 26.26 25
3.42 4.53 26
12.03 20.53 33
11.31 6.54 34
1.75 0.86 6
1.12 0.78 29
56.28 45.64 4
1.23 1.48 3
14.56 7.26 90 -2 .913 88 0.005
0.95 0.70 32 -3.682 30 0.0005
1.81 1.24 81 4.093 79 <0.0005
3.20 59.54 4.35 28.57 29 29 0.241 0.821 27 27 n.s . n.s .
Total flux
C
S
BS mean S.D . Cup s sampled , n
104.60 91.61 63
1222.90 988.01 56
28.19 18.67 26
BSC mean S.D . Cups samples, n
58.60 99.07 36
805.60 1283.33 34
12.59 20.25 6
BS + BSC mea n S.D. Cups samples, n Calc . t.statistic Calc . d .f. P
87.87 96.49 99 2.678 93 0.005
1065.3 1120.4 90 1.613 87 0.10
25.26 34.38 19.63 40.93 32 81 1.818 4.551 30 79 <0 .0005 0.05
Fe
d .f. = degrees of freedom . n.s. = no significant difference at any probability level.
Table 3. Fluxes of reduced sulf ur species, site BSC. Samples were anoxicaf{y sampled and preserved. Sample s f rom BSCI were analysed by chromium -reduction only (a measure of all inorganic reduced sulfur when no prior extraction ofacid-volatile sulfide and elemental sulfur is done) . Samples f rom the BSC2 deployment were analysed sequentially for elemental sulfur, acid-volatile sulfide and chromium-reduction (a measure ofpyrite only, after the first two extractions have been done). For these samples, reduced inorgani c sulfur is the sum of all three fractions . Fluxes are in ,uM m - 2 day-t Trap, cup BSC1 , cup 2 BSC l , cup 3 BSCI, cup 4 BSC1 , cup 5 BSC2 , cup 1 BSC2, cup 2 DSC2, cup 3 BSC2 , cup 4 BSC2 , cup 5 BSC2, cup 10
D ate 26 June 1986 4 July 1986 11 July 1986 19 July 1986 26 Sept. 1986 15 Oct. 1986 3 Nov . 1986 22 Nov. 1986 11 Dec. 1986 16 Mar . 1987
Acid-volatile S
17.12 0.13 0.62 0.37 0.08 0.06
Elemental S
Cr-reduc. S
Redu ced inorganic S
0.94 0.69 3.31 7 .11 5.83 2.15
17.56 26.82 21.24 17.03 1.59 4.30 3.37 3.87 6.39 0.59
17.56 26.82 21.24 17.03 19.65 5.1 2 7. 30 11.35 12.30
2.80
S1162
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MURAMOTO
et al.
the PARFLUX project from 1982 to 1985 (HONJO et al., 1987a; NICHOLSON, 1988). Pyrite framboids were also observed in fecal pellets in surface sediments sampled by box cores during the 1988 R .V. Knorr cruise (PILSKALN , 1990), but not so far in fecal pellets from sediment traps. Organic carbon fluxes. Seasonal variation. Seasonal variation in organic carbon fluxes is very regular and predictable: high maximum fluxes and concentrations occur in summer and autumn, where fluxes of 4-5 mM m- 2 day- l and concentrations of 100-150 mM g sedimenC l are measured , and low fluxes occur in the winter and early spring, when fluxes are nearly zero and concentrations are less than 10 mM g sediment"! (Figs 2B and 3B). High particulate organic carbon fluxes are associated with major seasonal phytoplankton production in surface waters in the Black Sea ; during the spring , these are associated with the diatom bloom , while increased summer and autumn organic carbon fluxes are associated with the coccolithophorid blooms that occur each year in the Black Sea (HAY, 1987; HONJO et at. 1987a,b). During the 1984-1985 winter a late coccolithophorid bloom and/or resuspension event is suggested by a peak in organic carbon flux and concentration, as well as by carbonate fluxes. The change in magnitude of organic carbon molar fluxes is between 0.01 and 5.0 mM m- 2 day"", which represents a factor of 500, or roughly two orders of magnitude difference between low and high fluxes of organic carbon . Geographic variation. Organic carbon fluxes and concentrations at the two sites BS and BSC are roughly equivalent, given the large variations in amplitude of the flux (Fig. 2B); although average organic carbon fluxes at BSC are 50% higher, the difference is significant only at the 10% level (Table 2). Intra-annual variation is thus more significant than between-site geographic variation. Total fluxes. Seasonal variation. Total sediment fluxes at both sites BS and BSC exhibit the same general pattern of seasonal variation (Fig. 2C) . The high total fluxes in summer and autumn are due to high productivity during coccolithophorid blooms, resulting in increased production of organic carbon and carbonate. During winter when primary production is minimal, total fluxes are low and lithogenic materials dominate the flux as a result of resuspension and increased fluvial input. Spring fluxes contain increasing amounts of opal and organic carbon due to diatom blooms at this time (HAY, 1987; HONJO et al, 1987a,b) . At site BSC, the very low total flux during one year from 1987 to 1988 is currently unexplained. Because of the high correlation between total flux and fluxes of individual particulate components, at! other flux components also have low fluxes during this interval. Geographic variation. Site BS has a significantly higher mean flux (105 mg m- 2 day-I ; t-test significance level is P = 0.005), consistent with its being closer to coastal input and upwelling areas, than site BSC (59 mg m ? day-i), which is closer to the center of the western basin. The two sites BS and BSC are compared in Table 2. Total fluxes at site BSC tend to exhib it more seasonal variation within a year than at site BS (although more data exist for site BS). This is in part because fluxes at BSC are largely determined by biogenic production (largely during the spring and summer) and very little by lithogenic input, so that seasonality is more marked , whereas fluxes at site BS are both biogenic and lithogenic in origin, the latter dominating during the winter.
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Fig . 5. 0 34S sulfur isotopic composition of particulate sulfide fluxes vs the isotopic composition of dissolved sulfide in the water column. from FRyetal . (1991 ). Trap sulfides have an average isotopic composition of -36.6%0; the same composition is found in dissolved sulfides at a depth of about 100 m at the oxic-anoxic interface (mean 034S for dissolved sulfide above 175 rn, where oxidation begins, is -38.6%0; in the deeper water column the mean o 3~ value is -40.5%0).
Particulate iron fluxes. Seasonal variation. Despite the presence of considerable scatter in the data, iron fluxes and concentrations show a generally consistent seasonal pattern of variation. High fluxes and concentrations tend to occur in winter and early spring. with minimum fluxes and concentrations occurring in summer (Figs 2D and 3C) . One might expect to find close association with reduced sulfur if reduced sulfur is present as iron sulfides; however, because there is nearly always an excess of particulate iron to reduced sulfur, non-pyritic or non-sulfidic iron may comprise a significant proportion of total iron. Geographic variation. Particulate iron is the only flux component we studied for which statistically significant and meaningful differences exist between sites BS and BSe. Particulate iron fluxes are four times higher at site BS than BSe (50 vs 12,uM m - 2 day -I for BS and BSe, respectively) . The differences in iron flux magn itude between the two sites is seen in Fig. 2D, where site BSe over the interval of the study received 75% less iron flux. The percentage of iron is twice as high at site BS than BSe (2 .2 vs 1.1 %) (Table 2, Fig. 3C) . The average Fe:S molar ratio for sites BS and BSC together is 3.2, and at this time , too few sulfur data exist to make statistically significant inter-site comparisons for the Fe:S ratio.
0 34S and sulfides Particulate sulfide/sulfur fluxes collected in traps have a 034S composition ranging from -32.7 to -39.40/00, with an average of -36.6%0; (Fig. 5, Table 4). The isotopic composition of dissolved sulfide below 175 m depth ranges between -39.3 and -41.30/00 with a mean of -40.50/00, while above 175 m, 034S increases to -35 .20/00 due to enrichment of 34S (FRY et al., 1991). In this 7G-100 m thick zone below the oxic-anoxic interface, the mean 034S for dissolved sulfide is -38.4%0; this is similar to the 0 34S composition of particulate sulfur fluxes (mean of -36.6%0). In 1975, dissolved sulfide in the deep anoxic water column had 034S values of -40.85%0, changing to more positive values (-38.710/00) at 190 and 300 m depth; at that time the oxic-anoxic interface was deeper in the water column than in 1988 (SWEENEY and KAPLAN , 1980).
J.
S1166
MURAMOTO
et al.
Table 4. 6 34S values for sediment fluxes, Black Sea. Data are for cups from sediment traps al sites BSC and BSK2 . The whole sub sample was analysed for reduced inorganic sulfur using chromium reduction , and then measured for sulfur isotopic composition . The amount of sulfur available fo r Isotop ic analysis is given in ,umoles
Trap, cup BSC2, cup 1 BSC2, cup 3 BSC2, cup 4 BSC2, cup 5 BSK2S, cup 6
Date open 26 Sept. 1986 3 Nov. 1986 22 Nov . 1986 11 Dec . 1986 June 1988
S(um)
634S (%0)
6.0 13.0 7.2 34.6 0.8
-36.3 -38.6 -36.2 -39.4 -32 .7
During the R. V. Knorr cruise to the Black Sea in April 1988, concentration profiles of dissolved sulfide were obtained down to approximately 2200 rn, at trap Stas BSC, BSK2, and other sites (NICHOLSON et al., 1988). The dissolved sulfide content of supern atant in trap cups from some shallow and deep trap deployments at both sites BS and BSC were also measured immediately after traps were brought on board. The dissolved sulfide content of trap cups is lower than this value [the average for all trap cups measured is 84pM as compared to approximately 300 pM concentration at 1000 m depth for all stations (NICHOLSON et al., 1988)1 . The relationship between dissolved sulfide and particulate reduced sulfur concentrations in sediment fluxes is poor (correlation coefficient r = 0.18, n 10), suggesting that the particulate reduced sulfur in the trap cups is only weakly related to the dissolved sulfide content. Although the sulfide content of the trap cups is always lower than the ambient concentrations of sulfide in the water column, suggesting oxidation, other dissolved species in trap cup supernatant also fall below concentrations found in the ambient water column. Liebezeit measured concentrations of dissolved silica and dissolved reactive and total phosphorus in sediment trap supernatant from shallow and deep traps at site BSC which were lower than concentrations from the ambient water column at the same depths as the traps (NICHOLSON et al., 1988). Oxidation could account for loss of HS- , but not of phosphate or total phosphorus or dissolved silica . Relationships between sulfur, organic carbon and iron in fluxes In fluxes, a scatter diagram of reduced inorganic sulfur vs organic carbon concentrations by per cent dry weight (Fig . 6A) show a highly linear relationship and high correlation (correlation coefficient r = 0.88 for a linear relationship whose S-intercept is slightly negative or close to zero; S-intercept is -0.355, with a standard error of about 40%) with a small, non-zero slope (slope = 0.0808; standard error is 10%). The strong correlation between sulfur and organic carbon concentration in fluxes cannot be due to sulfate reduction in the trap cups , because of the use of formalin in trap cups to poison and prevent microbial activity. Particulate iron and sulfur are not well correlated over time, as is apparent from their different seasonal variations. A scatter diagram of sulfur vs iron shows that no linear relationship exists between the two (Fig. 6B), although enough iron is present in excess over sulfur at all times to allow for all of the sulfur to be present as iron sulfides , based on the Fe:S molar ratio (Table 2). However, if the sulfur speciation we
S1l67
Sulfur fluxes in the Black Sea
Y> - 0.36
+ (0.081)x
RA2
0.78 (r. 0.88)
c
4
•
3
2
o
A
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o
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10 15 20 Wt. % Organiccarbon
25
30
4
y • 1.097 - (0.057)x R A 2 . 0.005 (r • 0.071)
•
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a!
0.8
III
~ 0.6
E tD 0.4 u,
0.2 0.0
0
2
3
AI, mM/g sed.
Fig. 6. Relationships between sulfur, organic carbon and iron in particulate fluxes. (A) Reduced sulfur vs organic carbon concentrations (by per cent dry weight) in sediment fluxes; the correlation coefficient is 0.88. The S-intercept passes through a point slightly below the origin (-0.355) . (B) Particulate iron vs reduced sulfur concentrations (by per cent dry weight) in fluxes, showing lack of linear relationship (correlation coefficient r = 0.07). (C) Particulate iron vs particulate aluminum concentrations (both in mg g dry sedimenC I ) in fluxes; the high correl ation (r = 0.92) is probably due to association of iron with lithogenic materi als such as clays.
observed can be extended to the whole set of reduced sulfur measurements, then a significant portion of this sulfur is elemental sulfur. The poor correlation between iron and sulfur appears to be due to their different seasonal patterns, which suggests that they are each associated with different major components of the bulk flux. With which major component of the settling flux is particulate iron associated? Correlation analysis shows that iron fluxes correlate best with the total mass flux,
J.
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MURAMOTO
et al.
Table 5. Matrix of simple correlation coefficients for particulate iron , and major components of the flux , toto! mass flux. organic carbon (determ ined by combustion), lithogenics (determined by combustion ), carbonate (determined by acid-soluble V$ non-acid-soluble masses), and opal (determined by wet chemical spectrophotometric analysIS). All fluxes are in mg m - 2 day" ! Total Total flux Carb oflux Opal flux Lith . flux Org. C flux Fe flux
Carbo
Lith.
Opal
Org. C .
Fe
1.00 0.81 0.58 0.71 0.80 0.70
1.00 0.22 0.37 0.72 0.35
1.00 1.00
0.42 0.34 0.39
1.00
0.41 0.57
1.00
0.55
compared with other bulk components of the flux. including carbonate. opal . lithogenic materials and organic carbon (Table 5; correlation coefficient r = 0.70; followed by lithogenics , r = 0.57; and organic carbon , r = 0.55). Both iron and organic fluxes are in turn best correlated with the total mass flux. Iron does not correlate well with carbonate (r = 0.35) or opal (r = 0.39) except insofar as these again comprise the total mass flux. Similar correlations are obtained when iron and selected elements representative of the bulk constituents of the flux (Si, P , Mn, V, AI. Ca), measured by JCPES . are compared (Table 6). Si represents both the biogenic opal and lithogenically derived silicates, Ca represents the carbonate produced in quantity during the summer and autumn coccolothophorid blooms. P and V are both used as tracers of organic matter, and ferric iron can potentially be associated with phosphates as well. Al strictly occurs in significant amounts only in lithogenic materials. such as clays and unweathered feldspars and other silicates. Mn is tested because it often is found in the same redox conditions in which Fe is found . Fe has the highest correlation with V (r = 0.95) and AI (r = 0.92) , while correlations with Si are somewhat lower (0.83); Fe, P, and Ca are not correlated well. In turn , AI and Si are highly correlated (r = 0.90). suggesting that they occur together as aluminosilicates in claytype minerals.
Table 6. Matrix of simple correlation coefficients for iron and elements acttng as markers for bulk components, using lCPES analyses: Si = opal. silicates; P = organic matter ; V = associated with organic mailer ; Al = tracer of lithogenics , clays and other aluminosilicates; Ca = carbonate. Mn is included as it often is found wub iron under reducing conditions, All correlations are determined for concentrations of the element in mg g dry material-A
Si Si p
Fe Mn V Al Ca
P
Fe
Mn
V
Al
Ca
1.00 -0.15 0.R3 0.19 0.77 0.90 -0.37
1.00 0.11 0.15 0.25 - 0.23 -0.27
1.00 0.18 0.95 0.92 -0.52
1.00 0.18 0.04 -0.38
1.00 0.83 - 0.49
1.00 - 0.36
1.00
S1169
Sulfur fluxes in the Black Sea
Both kinds of composition analyses therefore give similar results: iron fluxes are associated with the total mass and with lithogenic material in particular, and perhaps secondarily with organic material, but not well with carbonate or opal, and there is no evidence for a significant ferric phosphate phase. In particular iron is strongly associated with aluminum fluxes representing lithogenic clay input (Fig. 6C) ; such fluxes are usually higher in the winter and spring (HAY, 1987) . Because organic fluxes and lithogenic fluxes have different seasonal patterns, there is poor correlation between reduced inorganic sulfur and iron in fluxes (correlation coefficient r = 0.07 for fluxes) and lack of any linear relationship between iron and sulfur concentrations. Since sulfur does not appear to be closely associated with iron (or aluminum), this argues against the possibility that the source of sulfur fluxes is lithogenic material or resuspended shelf sediments. Organic carbon , sulfur and iron concentrations in fluxes and sediments can also give us information on possible diagenetic changes occurring between settling and burial, given certain assumptions. We have compared our data on sediment fluxes to data on concentrations of organic carbon , iron and sulfur from sediment cores studied by HIRST (1974), BERNER (1974), PILIPCHUK and VOLKOV (1974), VOLKOV and FOMINA (1974) and ROZANOV et al. (1974) for surface sediments down to 100 em which were taken from near the trap sites studied. Additional information on organic carbon and sulfur content has become available from the latest 1988 R.V. Knorr cruise (LYONS and BERNER, 1990) . Present-day fluxes have a high organic carbon concentration (average content of 14% by dry weight), compared to sediments which typically contain several per cent (dry we ight) of organic carbon (Table 7). Molar ratios of C:S and Fe:S for fluxes vs sediments reflect the higher organic content in fluxes as opposed to sediments, although both have somewhat similar iron and sulfur contents. If the organic carbon concentration in fluxes were reduced by about 72% (by weight) to simulate diagenetic remineralization , resulting in organic carbon comprising 4% of the total sediment, then reduced sulfur would comprise 1.1 % and iron 2.1 % of the total amount of material in the flux . These values are like the concentrations for Unit 1 sediments, which have average pyritic sulfur concentrations of 1.3%, and organic carbon of 5.4% by weight (LYONS and BERNER, 1990). l
DISCUSSION
Biogeochemical cycling of sulfur, iron and organic carbon in the water column In the Black Sea, the biogeochemical cycles of sulfur, iron and organic carbon are linked via dissimilatory sulfate reduction, which oxidizes organic matter in the anoxic water column and sediments generating dissolved sulfide, which can then precipitate if reactive iron is present. One way to unravel the complexity of these interrelated cycles is to compare the rates and sizes of the chemical, biological and physical processes which determine the origin , transport and fate of these compounds. An accurate understanding of biogeochemical cycling must include accurate pinpointing of sources. For example , pyrite could precipitate in the water column from 'old' dissolved sulfide diffusing and advecting upward from deep water, or it could precipitate from 'new' dissolved sulfide produced from ongoing sulfate reduction in the upper anoxic water column. Sulfatereducing microbiota could be oxidizing fresh labile organic carbon from newly se tt ling particulate organic matter, or utilizing dissolved organic carbon already present in a preexisting pool. Since we ultimately want to understand where and how sulfate reduction of
J . MURAMOTO et al.
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Table 7 . Table comparing mean values of %S. %C. %Fe, and molar ratios for sediment data from other studies with the present study comprising sediment trap material . For data from BERNER (1974) and HIRsr(l974). percentages by mass ofelement were used to calculate mass ratios of one element to another, and then converted into molar ratios. and are for sediment cores down to specified depths. For the sediment trap material . molar fluxes (liM m- 2 day-l were used to calculate molar ratios Mean value
Flux
81
H2
%S %Corg %Fe C:S molar ratio Fe:S mol. ratio %S. top 5 cm O/OC org to 5 ern %Fe. top 5 cm
0.95 14.5
0.9 0.5 4.6 2.8 4.6
2.2 2.1 5.7
1.8
59.5 3.2
1.68 0.96 6.34
PV~
V~
RVY S
LB~
1.3 5.4
0.85 3.96 0.77
3.02
1.50 3.70
I BERNER (1974). data from cores 1464K (152-364 em depth) and 1474K (202-235 ern depth) . 2HIRST (1974) , data for mean percentages from cores 1430. 1432. 1440, 1452 and 1462 down to depth of 100 em; data for top 5 em from eores 1430, 1432 and 1440. 3PlLIPCHUK and VOlKOV (1974). wt%S calculated from sulfide + phyritic sulfur. wt%Fe from iron sulfide + pyrite. 4VOlKOV and FOMINA (1974). averaged values of per cent organic carbon in modem surface sediments of the Black Sea . 5RoZANOV et al. (1974). averaged data from surface sediment at sites nearest to trap site BSC (Stas 243, 244. 245 and 252). bLvONS and BERNER (1990). Unit I sediments from below the oxic-anoxic interface . Samples from oxic shelf positions had very low mean pyrite sulfur «0.15 wt%) and low organic carbon values (1.0%) .
organic matter proceeds, we need to understand the origin of any sulfide produced and precipitated. Is ongoing sulfate reduction of organic matter necessary to account for the production of pyrite fluxes, or can pyrite precipitate from dissolved sulfide already present in the water column, with no need for additional sulfate reduction on settling organic particles? The question of size and location of reactive reservoirs applies especially to iron . Where the greatest concentration of reactive iron coincides with enough dissolved sulfide to exceed the solubility product of pyrite or iron monosulfides, will determine where precipitation takes place, and hence what kind of dissolved sulfide will be precipitated. Origin and transport of particulate sulfide fluxes
From the limited data we have, sulfur isotopic composition of particulate sulfide fluxes points to the zone immediately below the oxic-anoxic interface as the site of precipitation. Sulfide fluxes have an average isotopic composition of - 36.60/00, similar to dissolved sulfide at the oxic-anoxic interface, and have a narrower range of isotopic composition than do sedimentary sulfides; it is unlikely that fluxes are merely resuspended sedimentary sulfides. Since no sulfate reduction can take place in the trap cups themselves due to the presence of formalin, there is no dissolved sulfide produced in the cups. No sulfide fluxes have yet been found which resemble deep dissolved sulfide (-40.5%0), so sulfide fluxes do not form at trap depths or from ambient dissolved sulfide diffusing into trap cups and
Sulfur fluxes in the Black Sea
Sl171
precipitating with reactive iron in the particulate material. High rates of sulfate reduction at the oxic-anoxic interface could also cause the positive shift in sulfur isotope composition of dissolved sulfide here. It is not known what increase in sulfate reduction rate would be needed for the isotopic shift of 2-4% at the interface. There is no evidence for high rates at the oxic-anoxic interface, although J0RGENSEN et al. (1991) measured sulfate reduction rates at and below the oxic-anoxic interface which were higher than the low water column rates measured below 150 m by ALBERTet al. (in preparation) in the same 1988 cruise. The latter's volume-based sulfate reduction rates in the water column are several thousand times lower than volume-based rates in surface flocculent sediments, although integrated areal rates are rather similar. Our hypothesis that iron sulfides precipitate at the oxic-anoxic interface is supported by concentrations of pyrite, iron sulfides, iron and dissolved sulfide in the water column . During the 1988 R.Y. Knorr cruise, suspended particulate FeS reached maximum concentrations at 180 m, greigite (Fe3S4) concentrations peaked at 125 m, and suspended pyrite concentrations reached a maximum just below the oxic-anoxic interface (RADFORDKNOERY and CUTTER, 1990) . The zone between 130 and 180 m was oversaturated with respect to pyrite and close to saturation for mackinawite and greigite (LANDING and LEWIS, 1989) . Colloidal iron «O.4,um iron collected on a TSK column), probably iron sulfide, increased to 30-50% of the total dissolved iron between 130 and 180 m, total dissolved iron reached maximum concentrations at 180 m, decreasing below this depth , and suspended particulate iron (>O.4,um) concentrations reached a maximum between 180 and 250 m and decreased below this depth (LEWIS and LANDING, 1988) . Dissolved sulfide concentration data for the upper 100 m of the anoxic water column were obtained from J0RGENSEN et al. (1991) for three stations in the Black Sea sampled during the 1988 R.Y. Knorr cruise, Leg 2 (the same cruise and stations during which dissolved sulfide for isotopic analyses was collected). GOYET et al. (1991) found that salinity can be used as a conservative tracer of water mass mixing in the 75-200 m depth interval in the Black Sea. The sulfide data were used to model the sulfide concentration profiles expected if dissolved sulfide behaved conservatively (Fig. 7). The observed sulfide concentrations in this depth interval are consistently lower than expected from conservative mixing based on salinity, and represent the dissolved sulfide which has been oxidized and/or precipitated below the oxic-anoxic interface ; this is a zone of net consumption of dissolved sulfide. If sulfate reduction rates were greater than dissolved sulfide consumption rates at the oxic-anoxic interface, dissolved sulfide concentration profiles should have shown evidence of net production, which they do not. Thus the oxic-anoxic interface is where the greatest concentration of pyrite and iron sulfides is found, where the sulfur isotopic evidence points to both oxidation and precipitation, and where there is net consumption of dissolved sulfide by chemical oxidation and precipitation. It is possible, given the low volume-based rates of water column sulfate reduction (ALBERT et al., in preparation), that no large amount of 'new' sulfide is being produced in the water column . They calculate that between 14 and 46% of the total sulfate reduction activity in the Black Sea was due to water column sulfate reduction . In this case the dissolved sulfide is largely 'o ld' sulfide, and hence any pyrite precipitating from this sulfide represents mostly old sulfide and not new sulfide resulting from adjacent sulfate reduction in the water column. If sulfate reduction rates were high below the oxic-anoxic interface and accounted for more than half of the dissolved sulfide , then much of the dissolved sulfide at the oxic-anoxic interface would be "new" sulfide.
J.
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et al.
-80 Sin . 1
·100
HS·.ob..",ed HS·, consarvat..e
E ·120 ~
a.
·140
0
·160
Q)
·180 ·200
0
·80
50 HS·, 11M Stn. 2 HS: observed vs. conservative mixing --~
-100
--_
~
a.
~2
'- -~~......
-
H5-, observed HS-, conservative
"',
E ·120
' .........., ......
·140
' ... ...
Q)
0
·160 -180 -200 0
10
20
-
E ·120
a Q)
,,
... ...
40 50 60 70 80 HS-, JLM Stn.3 HS: observed vs. conservative mixing
·100
0
' ... -,
30
-80
~
100
...
90
100
Sln.3 H5-, obsanted HS·, consarvallve
·140 ·160 ·180 -200
0
10
20
30
40 HS-, JLM
50
60
70
80
Fig. 7. Observed dissolved sulfide concentrations down to 200 m depth and the calculated sulfide concentration profiles expect ed if dissolved sulfide were distributed conservatively with salinity, for Stas 1,2 and 3, Leg 2,1988 R.V . Knorr cruise. The difference between observed and calculated profiles represents the concentration of dissolved sulfide which is "missing " due to sulfide consumption by oxidation and precipitation . Maximum dissolved sulfide loss occurs between 100 and 150 m depth , which is the proposed zone of particulate sulfide precipitation in the water column .
How are pyrite framboids , iron sulfides and sulfur from the oxic-anoxic interface reaching the deep water column and sediments? The residence time of a typical 6 .urn diameter framboid in the 2 km deep water column is about 1 year, assuming Stokes' settling for a particle this small and with the density of pyrite (about 5.01; HURLBUT and KLE1N , 1977). If particulate sulfides settled as independent particles from the oxic-anoxic
Sulfur fluxes in the Black Sea
S1173
interface at a uniform rate throughout the year , we would expect to see an approximately uniform flux throughout the year; this is not observed, and instead sulfide particle fluxes (and concentrations) reach their maximum at the same time of year that organic fluxes do. This suggests that sulfides and organic matter are traveling together and that pyrite framboids and other sulfide or sulfur particles do not settle on their own. Evidence for this physiochemical process of association of sulfides with organic particles comes from the linear relationship between reduced sulfur and organic carbon. A particle this small is much more likely to be transported as part of a larger aggregate instead. We propose that organic particles (marine snow, settling phytoplankton remains) originating in the photic zone settle through the oxic-anoxic interface and scavenge iron sulfides from this zone of higher concentration . Sinking of marine snow and large organic aggregates is believed to be the most significant downward transport mechanism for the bulk of organic matter produced in surface waters (HONJO et al. , 1985) and is also an important vertical transport mechanism for metals , including Fe , Mn, Cu , Ni and Cd, compared to inorganic particles or even particles coated with humic acids (MOREL and HUDSON, 1985). Much metal uptake can occur via binding to high-affinity surface ligands, constituting "passive" binding whereby both dead and live phytoplankton are capable of high metal affinities. Pyrite and/or other iron phases may be associated with organic fluxes as well as lithogenic materials, from the excellent correlation between Fe and AI. S vs C scatter diagrams-low sulfate reduction rates in the water column or scavenging of sulfides? GOLDHABER and KAPLAN (1974) proposed that a linear relationship existed between organic carbon and reduced sulfur concentrations in sediments because of oxidation of organic matter by sulfate reduction ; normal oxic marine sediments are characterized by a line passing through the origin , representing the point at which all particulate organic carbon in sediments is gone due to complete aerobic oxidation , no sulfate reduction can take place and no sulfides are deposited. LEVENTHAL (1983) furthermore proposed, using data for Black Sea sediments (HIRST, 1974), that if the S-intercept is positive when organic carbon is no longer present, sulfides are being deposited in the absence of sulfate reduction in sediments , i.e. iron sulfides are precipitating in the water column and hence sulfate reduction in the water column could be inferred. However, recent analysis of new cores from Unit I in the anoxic western basin (LYONS and BERNER, 1990) do not show a positive S-intercept when organic carbon content is low or absent, but instead an intercept near the origin, expected for oxic marine sediments instead. Sediment fluxes also show the same S-intercept for sulfur when organic carbon is absent. The difference in slope of S vs C diagrams between sediment flux and sediments can be accounted for by removing 65-80% of the organic carbon in fluxes, in a simple model of diagenetic alteration of fluxes. This slightly increases the percentage of reduced sulfur and decreases the organic carbon to correspond to the average of 1.3% Sand 5.4% organic carbon (by weight) observed in sediments (LYONS and BERNER , 1990). The Sintercept for fluxes does not change from zero when organic carbon is removed in this manner, and only the slope changes. Thus concentrations of sedimentary organic carbon and reduced sulfur can be explained entirely by diagenetic loss of 65-80% of the organic carbon in fluxes (we think 72% is a good estimate). If all of this lost organic carbon is utilized by sulfate reduction only , with no fermentation , then a maximum estimate of the
S1l74
J.
MURAMOTO
et al.
amount of sulfide produced can be calculated using the simple but quantitative expression describing sulfate reduction (RAMM and BELLA, 1974; WESTRICH, 1983):
2CH 20 + SO~- ~ H 2S + 2HC03",
(1)
where for every two moles of organic carbon "lost" to sulfate reduction, one mole of sulfide is produced. Sedimentary sulfate reduction and sulfide production rates obtained in this manner have a mean of 144 mM m- 2y-l, and reach a maximum of 663 mM m- 2 v' . for 72% of organic carbon fluxes lost. These estimated rates fall within the range of sulfate reduction rates measured for Black Sea surface sediments (but not deep sediments) and water column using radiotracer experiments (Table 9). Furthermore , they compare with SOROKIN'S (1964) estimate of 290 mM m- 2 y-I as the total HS- production in sediments. Although organic carbon fluxes exhibit large but consistent seasonal variation, it is not clear whether sulfate reduction rates in the water column should also exhibit seasonal variation, due to differences in time-scales for settling, organic matter diagenesis , and residence times for labile organic substrates in the water column, which may be quite long (D. ALBERT, personal communication). If we accept LEVENTHAL'S (1983) interpretation, then an S-intercept of zero for the S vs C relationship for fluxes indicates that there is little significant sulfate reduction in the water column . No new dissolved sulfide is produced in the water column and all iron sulfides precipitated at the oxic-anoxic interface must be forming from "old" dissolved sulfide diffusing or advecting from elsewhere, from sediments, for example. Hence water column sulfate reduction is not necessary to account for precipitated sulfides in the water column . This interpretation of the S-intercept condition agrees with the low rates of sulfate reduction measured in the water column (ALBERTet al., in preparation) . Alternatively, the linear relationship for fluxes may represent temporal variation of organic carbon and reduced sulfur fluxes and not a diagenetic sequence (as in sediment cores), and hence the S vs C relationship for fluxes represents association through scavenging, as suggested above . In this case, an S-intercept of zero is interpreted as there being no scavenging activity when organic matter is absent, and hence no particulate sulfide flux. This is also more consistent with the evidence for some elemental sulfur in fluxes, which do not result from sulfate reduction, although oxidation could account for these as well. Bottom surface sediments of Unit I could "inherit" their sulfur content from settling fluxes, and the linear association between sulfur and organic carbon in sediments could also be "inherited" from the scavenging association in fluxes. Further diagenesis and remineralization of organic matter ean change the slope of this relationship but not the intercept unless more iron is available. As discussed above, there is abundant evidence that sinking floes of organic matter are capable of binding or trapping other particles or dissolved elements. This hypothesis also does not require water column sulfate reduction for iron sulfide precipitation . If all of the organic matter lost from fluxes during burial of sediments is used by sulfatereducing microb iota, where is the extra iron sulfide that should have precipitated in sediments? This would result in a steeper slope for fluxes (if it took place in sediment trap cups themselves, which should not happen, because of cup poisoning), or in a much steeper slope for the sediment S vs C relationship if it occurred in sediments. We suggest that there is not enough reactive iron available to precipitate all of the new sulfide produced from diagenetic sulfate reduction in sediments, because all the reactive iron was
Sulfur ftuxes in the Black Sea
S1175
precipitated at the oxic- anoxic interface , where the main sources of fresh reactive iron are entering the anoxic water column. This then suggests that if dissolved sulfide is present in excess in sediments, then sediments are a significant source of dissolved sulfide to the water column. What if sulfate reduction occurred in settling particles of organic matter, producing dissolved sulfide that precipitated with iron to form pyrite framboids? In this case, enough iron must be present in the organic matter to allow for iron sulfide precipitation. Could organic aggregates scavenge reactive iro n at the oxic-anoxic interface , and produce dissolved sulfide from sulfate reduction, resulting in iron sulfide precipitation? This would explain the sulfur isotope composition of fluxes , and it would also explain the good linear relationship between per cent reduced sulfur and per cent organic carbon. Framboids observed within intact fecal pellets in bottom surface sediments of the Black Sea (PILSKALN , 1990) may have formed due to higher rates of sulfate reduction within these organic-rich particles ; no framboids have yet been observed in fecal pellets in settling material caught by sediment traps, so we cannot say yet if sinking fecal pellets are sites of sulfate reduction in the water column . ALBERT et al. (in preparation) suggest that sulfate reduction in the water column is mainly particle-associated because of iron limiting the activity of sulfate-reducing microbiota elsewhere than on sinking particles. But sulfate reduction on sinking organic particles does not explain why pyrite and other iron sulfide maximum concentrations occur just a few meters below the oxic-anoxic interface (RADFORD-KNOERY and CUTTER, 1990) . If organic matter fell as marine snow particles at an average rate of 100 m day ", then a settling organic aggregate would enter the anoxic zone in less than a day, and sulfate reduction would have to occur within a few hours to produce a pyrite maximum between 100 and 180 m (0-80 m into the anoxic zone would take up to 19 h) . Rapid rates of sulfate reduction and pyrite formation on the order of a day have been measured in salt marshes with much higher organic loads (HOWARTH, 1979; HOWARTH and J0RGENSEN, 1984) . But utilization of fresh organic matter by sulfatereducing microbiota may not be instantaneous. When fresh phytoplankton were added to anoxic sediments in experiments to measure sulfate reduction rates, measureable sulfate reduction did not occur until about 10 days after addition of the fresh organic material (RAISWELL and BERNER, 1985). If this observation were applied here , we would predict that if fresh organic aggregates had a mean settling rate of 100 m day " , then only below 1000 m depth would measureable sulfate reduction on organic particles begin to occur. This is consistent with the much higher volume-based rates of sulfate reduction measured in sediments than in the water column (ALBERT et al., in preparation) , and it is also consistent with th eir finding that the highest rates of water column sulfate reduction occurred between 300 and 350 m depth, much below the zone of suspended particulate iron sulfides below the oxic-anoxic interface. This requires 3.5 days of sinking time for colonization of organic particles by sulfate-reducing bacteria, vs 10 days of sinking time, which is not too discrepant given variations in particle settling velocities. Given that there is enough dissolved sulfide and ferrous iron at the oxic-anoxic interface to precipitate pyrite, it seems more probable that chemical precipitation of pyrite and iron sulfides below the oxic-anoxic interface occurs without the need for immediately adjacent sulfate reduction on organic particles. In the water column, sulfate reduction and sulfide precipitation can be decoupled to some extent, so that the two processes need not occur in the same place since anoxic conditions suitable for sulfide preservation and transport exist nearly everywhere in the basin.
J. MURAMOTO et al,
51176
Range of 1134S I
!
I
I
I
I
Watarcolumn
........
•
2 3
•
Flu•
6
Sediments
7
•
8
•
9
-45
I
I
-40
-35
,
I
-30
I
I
I
·25
·20
·15
·10
113-4S,L
Fig. 8. Comparison of 034S for dissolved sulfide in the water column, particulate sulfide fluxes and sedimentar y sulfides. Mean and minimum/maximumvalues of &"S for different data sets are indicated by solid dots and bars, respectively. Numbered references are : (1) dissolvedsulfide in the deep-water column below 180 m (SWEENEY and KAPLAN , 1980;n = 14); (2) dissolved sulfide in the entire water column from &6 to 2000 m (FRY et al. , 1991; n = 29) ; (3) dissolved sulfide in the deepwater column below 175 m (FRY et al., 1991 ; n = 17); (4) dissolved sulfide in the shallow water column above 175 m (FRY et al., 1991 ; n = 12); (5) particulate sulfide fluxes from time-series sediment traps (this study; n = 5); (6) pyrite from 0 to 10 em horizon in basinal sediments, water depth > 1000 m (VAYNSTEYN et al., 1985; n = 6); (7) pyrite from 0 to 10 cm horizon in shelf sediments, water depth < 100 m (VAYNSTEYN etal., 1985;n = 5); (8) sulfidesfrom surface sediments (VINOGRADOV et al., 1962); (9) sulfides from surface sediment at Stas 1135 and 1136 (SWEENEY and KAPLAN 19&0; n = 2).
To distinguish between the two processes, sulfate reduction on organic particles vs scavenging of pyrite by organic sinking floes, we need to know how isotopic composition of suspended particulate sulfides changes with depth in the water column. We also need to know whether sulfate depletion can occur in the "porewater space" of settling marine snow or fecal pellet particles due to localized sulfate reduction, i.e . do mar ine snow particles or fecal pellets behave like closed, semi-closed or open systems with respect to dissolved constituents as they move through the water column? Fecal pellets would be relatively more closed to diffusion than marine snow particles. Comparison with sedimentary sulfides
The isotopic compositions of dissolved sulfide in the water column, and of particulate sulfides in fluxes and sediments, are compared in Fig. 8. Iron sulfides in bottom surface
Sll77
Sulfur fluxes in the Black Sea
Table 8. Comparison of d 34S of sulfides in sediment fluxes, surface sediments and dissolved sulfide in the water column of the Black Sea, from references listed below Range d 34S (%0)
Mean
S
Water column HS - > 180 (deep) HS- , 8Cr-2000 m HS - > 170 m (deep) HS- < 170 m (shallow)
-38.7 to -35 .2 to -39.3 to -35.2 to
-40.9 -41.3 -41.3 -40.5
-39.8 -39.7 -40.5 -38.6
0.7 1.4 0.6 1.4
14 26 15
Particulate fluxes Reduced S, 1000 m traps
-32.7 to -39.4
-36.6
2.6
5
3
Sediments Pyrite , ~5 em , basin Pyrite, ~5 em, shelf, slope Sulfides, basin sediments Sulfides, basin sediments
-33.0 to -15 .0 to -19.3 to -27.0 to
-35.2 -28.0 -27.8 -27.2
1.9 9.9 6.5 0.3
5 8 4 2
"Depths > 1000 m "Depths < 1000 m
Sample
-37.3 -44.5 -33.0 -27.4
Reference
n
II
IStas 1135, 1136 2All depths 2Deep 2Shallow
5
I SWEENEY and KAPLAN (1980); data for HS- are from depths below 180 m. Data for basinal sediment sulfides do not specify depth in core at which sediment was sampled. 2FRY et al. (in press); shallow and deep refer to data from above and below 175 m, where dissolved sulfide b34S profile shows dramatic change . 3MuRAMoTO et al. (this study) . 4YAYNSTEYN et al. (1985) . 5YINOGRADOV et al. (1962); data for basinal sediment sulfides do not specifiy depth in core at which sediment was sampled.
sediments in the Black Sea are almost always isotopically heavier than dissolved sulfide in the water column above sediments (Table 8; VINOGRADOV et al., 1962; SWEENEY and KAPLAN, 1980; VAYNSTEYN et al. , 1985). Sedimentary sulfides also have a wider range of isotopic values than do either sulfide fluxes or dissolved sulfide, suggesting that they originate from sulfate reduction with increasingly limited sulfate, and/or several sources of isotopically distinct dissolved sulfide . Heavier 0 34S values in sediment sulfides can result from : (1) higher rates of sulfate reduction in the sediment (SWEENEY and KAPLAN, 1980), (2) sulfate-limited sulfate reduction in sediments, or (3) precipitation of metal sulfides from residual dissolved sulfide which is isotopically heavier due to oxidation by some oxidizing agent present in the sediments (just as oxidation occurs at the oxic-anoxic interface). Sulfate reduction rates in sediments are low in general, although rates at the surface are higher than deeper in sediments (ALBERT et al., in preparation; VAYNSTEYN et al. , 1985). In contrast, sulfur fluxes have a narrower range of isotopic composition. The difference in the ranges of isotopic composition between flux and sediment sulfides, and the close correlation between autochthonous organic matter and sulfides, suggests that sulfide fluxes are not just resuspended sediment sulfides , despite the possibility of significant lateral transport of water masses and sediment in the Black Sea (MuRRAyetal., 1989). Some sediment sulfides have isotopic compositions similar to dissolved sulfide in the uppermost 100 m of the sulfide zone, which has an average composition of - 38.4%0 (FRY et al. , 1991), which is also similar to particulate sulfide fluxes. Sedimentary sulfides could therefore originate at the oxic-anoxic interface, and settle as a particulate flux through the water column. Most likely there are dual origins for sedimentary sulfides: fluxes from the
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water column and in situ authigenic precipitation. Sedimentary evidence also suggests the importance of water-column anoxia in determining whether anoxic sediments will have high or low iron sulfide contents. LYONS and BERNER (1990) compared anoxic sediments from two nearby sites, one from the shelf beneath oxic water, and the other from below the oxic-anoxic interface, where both sediments had similar organic carbon and reactive iron contents. Sediments from below the anoxic water column were found to have much higher concentrations of acid-volatile sulfides (up to almost 0.7% by weight) and a much higher degree of sulfidization (up to 40%). Sediments from below the oxic water column had less than 0.15% pyrite , and a very low degree of pyritization of iron «10%). Importance of iron
Iron, by precipitating hydrogen sulfide as pyrite or iron sulfides, renders sulfate reduction "visible ", and it also limits pyrite and iron sulfide precipitation. Thus reactive iron is a major control on the amount of dissolved sulfide that can diffuse into the water column from sediments, in the absence of other significant oxidizing or precipitating agents. Iron is also required for growth of all organisms, and in the anoxic water column is consumed both by sulfide precipitation and doubtless by sulfate-reducing microorganisms, although the extent to which chemical and microbial iron consumption competes in the Black Sea is unknown. In fluxes, iron is associated primarily with litho genies and perhaps secondarily with organic matter, but this latter fraction of organic-associated iron may be occurring as the sulfide phase, based on the iron sulfide-organic carbon correlation. The Fe :S molar ratio of 5:1 suggests excess iron to reduced sulfur in fluxes, although we do not know what amount of reactive iron this represents; this explains why iron and organic carbon are not strongly related-there is much more total iron relative to sulfide iron, which is the fraction associated with organic matter. STRAKHOV (1958) and ROZANOV et al. (1974) estimated that at least 20% of reactive iron was removed as particulate iron sulfides in the 2 km deep anoxic water column, based on decreasing ferric iron in sediments as depth of sulfidic water increased. This agrees (on a molar basis) with the 5:1 molar ratio of iron to sulfur in fluxes if that 20% of particulate iron flux is due to iron sulfides. If roughly 17% of the upward dissolved sulfide flux is removed as iron sulfides (see below), then five times as much iron again would be needed to precipitate all dissolved sulfide upward flux as iron sulfides, resulting in an Fe:S ratio of 25:1. In varved sediments of Unit I the degree of pyritization of iron is generally between 60 and 70% (LYONS and BERNER, 1990); if the paleofluxes represented by surface sediments are comparable to modern fluxes, then there may still be some iron available for reaction in sediments after settling of fluxes . Iron has been proposed as a limiting trace element for marine phytoplankton growth, particularly for diatoms and less so for coccolithophorids (MARTIN and FITZWATER, 1988; MARTIN et al., 1989). In the Black Sea, some high iron fluxes do coincide with spring diatom blooms, and lower iron fluxes often accompany summer coccolithophorid blooms. It is intriguing to hypothesize that diatom blooms might be stimulated during the spring when availability of iron is probably high due to increased lithogenic inputs, and coccolithophorids bloom later during the summer when levels of iron may be lower. However, because fluxes of lithogenic materials, wind stresses (HAY, 1987) and probable upwelling, and diatom bl~~ms all reach maximum levels in spring, it is difficult to pinpoint cause and effect. In addition, results of short-term bottle bioassays commonly used in
Sulfur fluxes in the Black Sea
S1179
Table 9. Rates of some processes of the sulfur cycle in the Black Sea. Data are from sources listed. Where possible, the mean rate ofa process is given, along with the range of values found in the literature or determined in this study. Where the rate of a process is unknown or currently in question, this is indicated by a question mark on Fig. 9
Particulate sulfide flux = 9.2 (10) mM m -2 y-l This study Upward flux of dissolved sulfide due to eddy diffusion and advection = This study 58 (60) mM m- 2 y-I Rate of oxidation uf HS- at top of interface = 48 (50) mM m -2 v ' This study (from upward flux - precipitated downward flux) ALBERT et al. (in preparation) Rate of sulfate reduction and HS - production in water column: 79-536 mM m- 2 v' (1.3 .u M y-I) Rate of sulfate reduction and HS- production in sediments: 470-530 + mM m- 2 y-I (7.7 mM y-I) ALBERT et al. (in preparation) 290 mM m- 2 y-I SOROKIN (1964) 144 mean, 663 mM m- 2 v' max. This study Our rates are calculated as described in the text: Mean organic carbon flux is 389 mM m -2 y -I, maximum is 1842 mM m -2 y-I If: 65% of organic carbon used: mean of 126 mM m -2 v ' of HS -, max. 600 mM m -2 y-I 72% of organic carbon used: mean of 144 mM m- 2 y-I of HS-, max. 663 mM m- 2y-1 80% of organic carbon used: mean of 155 mM m -2 y-I of HS-, max. 737 mM m- 2y-1 all organic carbon flux used: mean of94 mM m -2 y-1 of HS -, max. 921 mM m -2 y-1
studies of iron-stimulated phytoplankton growth need to be interpreted cautiously (HOWARTH, 1988). Other more controlled studies need to be done to resolve this interesting question, since biological demand-particularly microbial iron demand-is competing with iron sulfide precipitation in a sulfidic basin such as the Black Sea. Iron fluxes increase with proximity to coastal sources of input , and ROZANOV et al. (1974) also found increased ferric iron concentrations in bottom sediments closer to the coast. Thus there is considerable geographic variation in iron inputs, and doubtless geographic variations can be expected in all of the above processes due to the important roles of iron.
Dissolved sulfide flux and rate of pyrite formation A simple vertical advection-diffusion model was used to calculate the upward flux of dissolved sulfide between 200 m and the oxic-anoxic interface, if lateral exchange of sulfide is not considered. Using a vertical advective velocity of W = 0.5 m v'. a vertical eddy diffusion coefficient K = 27.5 m 2 y-l (GOYET et al., 1991), and a concentration of 75 3 mM m- at 200 m depth (from J0RGENSEN etal., 1991), the upward flux of dissolved sulfide is approximately 60 mM m- 2 v '. If the average particulate flux of sulfide is roughly 10 mM m -2 y-l then this represents about 17% of the total upward dissolved flux of sulfide. From Fig. 7, consumption of dissolved sulfide occurs between 110 and 200 m relative to a conservative mixing model based on salinity. Some of this consumed sulfide could be lost due to precipitation of metal sulfides, as well as oxidation of sulfide. Moreover, if we compare estimated rates of sulfate reduction, which range from 470 to 530 mM m- 2 y-l in sediments (Table 9), then approximately 2% of the dissolved sulfide produced from sulfate reduction in sediments is returned as iron sulfide fluxes. If we use our mean estimate of 144 2 mM m- y-l of HS- production in sediments, then about 7% of dissolved sulfide areal fluxes produced is precipitated. If we use rates of sulfate reduction in the water column
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(79-536 mM m- 2 y-l), then about 2-13% of the dissolved sulfide from water column sulfate reduction is precipitated. The average molar flux of total iron is about 12.8 mM m- 2 surprisingly similar to sulfur fluxes , although we do not know the percentage of reactive iron . Thus there is more dissolved sulfide than iron by at least a factor of 5 (60 vs 12.8 mM m -2 y-I) , and much more if most of this iron is non-reactive (the Fe:S molar ratio is 5:1 in fluxes). The discrepancy between similar mean annual molar fluxes of iron (12.8) and particulate sulfides (9.2) and the Fe :S ratio (5:1) is due to the different seasonal behavior of iron and sulfide fluxes, as well as the unknown residence times for different iron compounds in the Black Sea , and points out the hazards of using averaged annual rates to compare processes which may in fact operate over non-overlapping seasons. An estimate of the rate of pyrite formation in the water column can be obtained from rates of settling by making the reasonable assumption that pyrite is removed by scavenging as quickly as it is formed . If a marine snow particle settles at a rate of 100 m day- l (ASPER, 1986), then one day suffices to settle through the upper 100 m of the sulfide zone. If sulfide forms on organic particles undergoing sulfate reduction, then, as an organic particle settles to 1000 m, 10 days are required. Both are consistent with known rates of pyrite formation. Sedimentary pyrite may take weeks to months to form via slow conversion of mackinawite (FeS) and elemental sulfur (SO) to pyrite (BERNER, 1965, 1967, 1970, 1974, 1984; SWEENEY and KAPLAN, 1973 ; RAISWELL and BERNER, 1985). However, rapid rates of pyrite formation on the order of a day were measured in sediment sulfate reduction experiments using 35S04 radiotracer methods , which is a sensitive measure of the rate of reduction (HOWARTH>, 1979; HOWARTH and GIBLIN , 1983; HOWARTH and JORGENSEN 1984; GIBLIN and HOWARTH, 1984; HOWARTH and MERKEL, 1984; JORGENSEN et al., 1984; LUTHER et al., 1986). Pyrite may form rapidly when ferrous iron reacts with polysulfides, which are in turn controlled by the presence of oxidizing agents (such as oxidized iron or manganese) and perhaps lower pH; under such conditions, when FeS is undersaturated , pyrite formation is kinetically favored (RICKARD, 1975; HOWARTH , 1979; LUTHER et al. , 1986) . It is possible that pyrite may form slowly when FeS dissolves, forming ferrous iron, which then reacts with polysulfides to form pyrite; if this were the case. formation would be slow because FeS dissolution is slow when dissolved sulfide levels are high . Another limit on pyrite formation at depth may be lack of possible pyrite precursors such as polysulfides, which form when hydrogen sulfide is oxidized , and which are absent in deep water (G. LUTHER. personal communication).
v',
Box model of sulfur cycling between dissolved and solid phas es Our model for sulfur cycling in the water column and sediments of the Black Sea, as inferred from sulfur isotopic information, is summarized in Fig. 9. Rate information is based on this study and others (Table 9) . Alternative hypotheses are discussed above , and may also be consistent with the observed sulfur isotopic compositions of the various sulfur reservoirs. As this is a model for proposed processes, we have extended the range of sulfur isotopic composition for dissolved sulfide at the oxic-anoxic interface to account for the possibility that all sedimentary sulfides are derived from sulfide fluxes precipitated at the oxic-anoxic interface. (1) Isotopically heavier dissolved sulfide at the oxic-anoxic interface results from net consumption of sulfide by chemical oxidation, perhaps by Fe 3+ or Mn 4+ . Estimated sulfide
S1181
Sulfur fluxes in the Black Sea
Reactive Fe + C org aggregates
•
Oxic-anoxic interlace
Oxidation:
50?
-L Rapid S· reduction? ~ HS' shallow 1---------------------------
... ..
.
•' ' ..~• •
.
'. ••
• FeS x precip~ation;
. . ' scavenging •
..
•
..
t
.
S· reduction on organic partides?
•
g.
FeS x flux Q) I-------J
A
(lj
T
:: Ul
E
N~
R
-
Fluxes:
10 Water column S· reduction : 79·536 (1.31JM!yr)
~-
W
Q)
Q)
"0
en J:
...
0(/)
---------------------------------
.... 144 III II
290 500
1\ UlWUDl1II r-lUuuUnmlRIIllIlIUUlUllIllIOIlIIIIUIUUIUlUUInIlIlIllUIUtlllUUllIlIllIlIllIUlUU IIInlllllllUUlU1lI
Sediment S-reduction:
S E
470-530+ (7.7 mMtyr); 144·663
--
Slowto fast rates, and/or Increasing suI!ate depletion?
HS- + FeSx
authigenic +
L
I M E N T
(j)
o
on.
flux
-50
o
.
.
-40
·30
°
. ·20
:
o
10
Q)
0
20
30
Fig. 9. Model of sulfur cycling between dissolved and particulate phases in the water column and sediments of the Black Sea, ba sed on sulfur isotopic compositions. Sulfur isotopes give "source" vs "process" information. Horizontal arrows indicate processes resulting in isotopic fractionation between source and product ; vertical arrows give information on the unaltered source(s) of sulfide . Proposed pathways are shown In bold arrows, while alternative processes or those which may occur simultaneously arc shown in dashed arrows. All rates are in mM m- 2 y-t except for Alberts volume-based sulfate reduction rates, which are in ,11M y-I or mM Estimates of rates of sediment sulfate reduction and HS- production are: 470-530 + mM m- 2 y-I , or a mean of 500 (ALBERT et al., in preparation) : 144 mM m- 2 y-t is the mean rate of sulfate reduction/H'S" production, and 663 mM m- 2 y-I is the maximum rate calculated, based on 72% of organic carbon fluxes consumed (from this study): 290 mM m - 2 y -I (SOROKIN , 1964).
v '.
loss due to oxidation may be 50 mM m- 2 y-I (60 - 10). Since volume-based sulfate reduction rates in the water column are low, much of the dissolved sulfide may be "old" sulfide upwelling and diffusing from below, rather than "new" sulfide produced from local sulfate reduction in the upper anoxic water column; this needs to be further investigated and in different seasons.
51182
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(2) Dissolved sulfate in the water column supports some sulfate reduction there , and diffuses into sediments where sulfate reduction rates are higher because of more organic carbon and/or iron for microbial needs. The large isotopic fractionation between sulfate and sulfide in the water column (55-600/00) and the narrow range of isotopic values of deep dissolved sulfide, are consistent with low rates of sulfate reduction in an open system with respect to sulfate. Dissolved sulfate in the Black Sea water column in 1975 had a sulfur isotopic composition between 18.2 and 20.20/00 (SWEENEY and KAPLAN, 1980), and one sulfate sample from 500 m in 1988 had a value of +19.50/00 (FRY et al., 1991). Modern marine sulfate has isotopic values between +20 and +210/00 (REES et al. , 1978). (3) Iron sulfides, elemental sulfur and perhaps polysulfides , precipitate rapidly at and below the oxic-anoxic interface, resulting in a layer of suspended particulate sulfur. Here reactive iron concentrations are highest due to new inputs of reactive iron from above, and dissolved sulfide concentrations high enough to allow precipitation. The source of dissolved sulfide is sulfide which has undergone chemical oxidation at the interface , resulting in particulate sulfides of isotopic composition distinct from that of deeper-water dissolved sulfide. Precipitation of iron sulfides deeper in the anoxic water column is limited by extremely low levels of dissolved reactive iron. (4) Organic aggregates from the photic zone sink through the iron sulfide precipitation zone below the oxic-anoxic interface , scavenging particulate pyrite and other metal oxides and sulfides as they fall, and accumulate as sediments. The resulting particulate iron sulfide flux is about 10 mM m - 2 y-l . Rapid settling of organic aggregates carries particulate sulfides as fluxes to the sea floor within a month. Several hundred meters below the oxic-anoxic interface, there may be organic particle-associated sulfate reduction, although water column sulfate reduction rates are low and appear to be limited, possibly by iron (ALBERTet al., in preparation). Whether or not iron sulfides precipitate on settling particles is determined by reactive iron, which is scarce in the deep water column. Seasonal variations in organic particle fluxes results in seasonal variations in scavenging and supply of particulate sulfides to bottom sediments; it may also explain the large variations in water-column sulfate reduction rates measured if sulfate reduction is particle-associated. (5) In sediments, further precipitation of iron sulfides is also limited by availability of reactive iron , which thus controls the amount of dissolved sulfide present. Sulfides in sediments may therefore be largely derived from the oxic-anoxic interface where there is more reactive iron available, with some additional sulfides precipitated in sediments. New inputs of reactive iron to the Black Sea occur via run-off and fluvial inputs and possibly by windborne transport of dust particles. Iron fluxes are associated with clays and perhaps with organic matter, which may act in a scavenging role; fluxes are fairly uniform throughout the year, because lithogenic and organic matter fluxes reach their maximum levels in opposite seasons of the year. (6) Sulfate reduction is higher in sediments than in the water column, by a factor of about 6000 (on a volume basis), possibly because of organic carbon accumulation. Sulfate reduction/H'S" production rates can be estimated from 72% of organic carbon fluxes lost in sediments, resulting in an average rate of 144 mM m - 2 y-I, but reaching maximum values of 663 mM m- 2 y-l due to seasonal maxima in organic carbon fluxes. SOROKIN'S (1964) estimate was 290 mM m- 2 y-I . Consumption of porewater dissolved sulfide could occur via further sedimentary iron sulfide precipitation (only if enough reactive iron exists), microbial assimilation and/or oxidation, or other unknown oxidation mechanisms. Since sulfate reduction in sediments may be organic substrate-limited due to competition
Sulfur fluxes in the Black Sea
S1183
with fermenting microorganisms , large seasonal variations in organic fluxes could result in large variations in sedimentary sulfate reduction rates. (7) Porewater dissolved sulfide remaining after any consumption diffuses into the water column and undergoes vertical and lateral advection and diffusion, constituting the pool of "old " dissolved sulfide. Deep water dissolved sulfide has a narrow range of sulfur isotopic composition and a large degree of fractionation between sulfate and sulfide (about 60%0), consistent with overall low rates of sulfate reduction in the water column. Steep concentration gradients in the upper anoxic water column drive increased upward diffusive fluxes of dissolved sulfide (at the rate of about 60 mM m- 2 y-l in the upper several hundred meters), to the oxic-anoxic interface where the cycle begins over again. (8) Factors controlling the levels of dissolved sulfide in the Black Sea basin include : (a) sulfide oxidation, chemical and microbial; (b) metal sulfide precipitation and other consumption processes , such as microbial assimilation; (c) reactive iron concentrations and distributions, which control metal sulfide precipitation ; (d) sulfate reduction rates and factors which control these rates, including quantity and quality of organic matter, sedimentation rates and availability of sulfate; and (e) physical mixing processes, including seasonal variation in wind mixing and upwelling, fluvial inputs and circulation, which could affect many of the above processes.
Implications for the sedimentary record In anoxic basins such as the Black Sea, sediments potentially contain a record of sulfur . iron and organic matter cycling taking place in the upper water column , although diagenetic processes will continue to add, modify or consume materials which originally settled from the water column. Some particulates, such as pyrite fluxes and unreactive iron minerals, will settle to sediments where they will remain relatively unaltered; others such as reactive organic matter, reactive iron, iron monosulfides, biogenic carbonate and opal, can undergo further diagenetic transformations. Diagenetic processes include sulfate reduction, fermentation, precipitation, dissolution, diffusion and remobilization . Fluxes of iron sulfides precipitating in the water column could account for most of the sulfides in sediments of the Black Sea . At the oxic-anoxic interface , the steep gradient in isotopic change may be due to net chemical oxidation of dissolved sulfide, rather than net oxidation by photosynthetic bacteria (FRY et al., 1991). Sulfides in sediments would then preserve the isotopic signature of biogeochemical processes at the oxic-anoxic interface. If porewater dissolved sulfide in surface sediments is in equilibrium with dissolved sulfide in the water column directly above, then it should also be isotopically identical to deep-water dissolved sulfide, which is isotopically lighter than dissolved sulfide at the oxic-anoxic interface. If sulfide precipitation from porewater-dissolved sulfide continued during deposition, then we predict that the 034S of sulfides in dark and light laminae of varves reflects two different sources : (1) sulfide fluxes from the oxic-anoxic interface in lightcolored carbonate-rich bands representing fluxes from summer coccolithophorid blooms when organic fluxes are often at a maximum (HAY , 1988), (2) sulfides precipitated in situ during sediment diagenesis, from reaction of porewater-dissolved sulfide and available reactive iron. Since much iron is associated with lithogenic fluxes, and lithogenic fluxes tend to reach their maximum during the winter and spring, sulfides precipitated insitu may be found in sediment laminae representing these deposits.
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Short- and long-term variations in organic matter fluxes over time could result in similar changes in rates of remineralization and sulfide genesis due to variation in sulfate reduction activity in sediments. The hypothesis of two different sources of sedimentary sulfides depicted above is further complicated in that sedimentary sulfate reduction rates may increase when organic carbon burial fluxes are higher, resulting in dissolved sulfide that diffuses outward from the carbonate- and organic-rich layers and is precipitated in the adjacent lithogenic-rich layers. Sampling of individual lamina of varves is needed to test these detailed hypotheses. Acknowledgements-We gratefully acknowledge A . G iblin for adv ice on sulfur techniques , S. Manganini for help on measurements of fluxes and bulk composition, B. Woodward for CHN analyses. and D . Bankston for assistance with ICPES anal yses . T . Konuk gave permission to participate in cruises on the R. V. Koca Piri Reis . S. Kempe , M. Wiesner and A . Diercks assisted during cruises. We thank T . Lyons. D. Albert, B. Jorgensen and their colleagues for permission to refer to manuscripts in preparanon . We also thank A . Giblin, R . Berner. T . Lyons and an anonymous reviewer for reviewing the manuscript and for useful suggestions throughout the project. Work was made pos sible through NSF Grants OCE-8614462 to J . L. Cisne , OCE-8814363 to S. Honjo , OCE-8800101 to B . Fry. and donations from Meyer Bend er and the Clas s of 1929. Cornell Universit y. Woods Hole Oceanographic lnstitution Contribution no. 7302.
REFERENCES ASPER V. L. (1986) Accelerated sell ling of marine particulate matter by marine snow aggregates. Ph. D. Thesis. MIT- WH OI -86- l2. BERNER R. A. (1965) Iron sulfides formed from aqueous solution at low temperatures and atmospheric pressure. Journal of Geology. 72. 293-306. BERNER R. A . (1967) Thermodynamic stability of sedimentary iron sulfides. American Journal of Science, 265. 773-785 . BERNER R. A. (1970) Sedimentary pyrite formation . American Journal of Science . 268. 1-23 . BERNER R . A . (1974) Iron su lfides in Pleistocene deep Black Sea sediments and their paleoceanographic sign ificance . In : The Black Sea-geology . chemistry . and biolog y; E . T . DEGENS and D. A . Ross. editors, American Association of Petroleum G eologists. Memoir 20. pp. 524-531 . BERNER R. A. (1984) Sedimentary pyrite formation : an update. Geochimtca et Cosmochimica Acta. 48, 605-615. BERNER R. A . and G . R . HOLDREN JR (1978) Sulfur distribution in Hol es 380 and 380A of Leg 42B . In : In itial reports of the Deep Sea Drtlltng Project. Vo l. 42 . Part 2. D. A . Ross et al.• editors. (U .S. Government Printing Office . Washington) , pp . 625-626. BREWER P. G. and D . W. SPENCER (1974) Distribution of some trace elements in the Black Sea and their flux between dissolved and particulate phases. In: The Black Sea-geology , chemistry. and biology. E. T. DEGENS and D . A . Ro ss . editors. American Association of Petroleum Geologists, Memoir 20. pp . 137-143. CANFIELD D . E .• R . RAISWELL. J . T . WESTRICH. C. M . REAVES and R. A . BERNER (19 86) The use of chromium reduction in the a na lysis of reduced inorgan ic sulfur in sediments a nd shales. Chemical Geology . 5-1, 149155. FRY B . • H. GESTand J . M. HAYES (1984) Isotope effects associated with the anaerobic oxidation of sulfide hy the purple photosynthetic bacterium. Chromatium vmosum . FEMS Microbiology Letters. 22. 283-287. FRY B .• H . GESTand J . M. HAYES (1988a) :>4S/,2S fractionation in sulfur cycles catalyzed by anaerobic bacteria. Applied and Environmental Microbiology, 54 . 25~256 . FRY B ., W. RUF, H. GEST and J. M . HAYES (1988b) Sulfur isotope effects asso ciated with oxidation of sulfide by O 2 in aqueous solution . Chemical Geology. 73.205-210. FRY B ., H . W . JANNASCH , S. J. MOLYNEAUX , C . WIRSEN .J . MURAMOTO and S. KING (l991) Stable isotope studies of the carbon, nitrogen and sulfur cycles in the Black Sea and the Cariaco Trench . Deep-Sea Research , 38 (Suppl.), S1003-5 1019. GIBLIN A. E. and R. W. HOWARTH (1984) Porewater evidence for a dynamic sedimentary iron cycle in salt marshes. Limnology and Oceanography, 29. 47-63 . GOLDHABER M . B. and I. R . KAPLA N (1974) The sulfur cycle . In : The sea. 5th edn. , E. D . GOLDBERG , editor Wile y. New York, pp . 569-655. . '
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