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1.
The biogeochemistry of sedimentary sulfur was investigated on the continental shelf off central Chile at water depths between 24 and 88 m under partial influence of an oxygen minimum zone. Dissolved and solid iron and sulfur species, including the sulfur intermediates sulfite, thiosulfate, and elemental sulfur, were analyzed at high resolution in the top 20 cm. All stations were characterized by high rates of sulfate reduction, but only the sediments within the Bay of Concepción contained dissolved sulfide. Due to advection and/or in-situ reoxidation of sulfide, dissolved sulfate was close to bottom water values. Whereas the concentrations of sulfite and thiosulfate were mostly in the submicromolar range, elemental sulfur was by far the dominant sulfur intermediate. Although the large nitrate- and sulfur-storing bacteria Thioploca were abundant, the major part of S0 was located extracellularly. The distribution of sulfur species and dissolved iron suggests the reaction of sulfide with FeOOH as an important pathway for sulfide oxidation and sulfur intermediate formation. This is in agreement with the sulfur isotope composition of co-existing elemental sulfur and iron monosulfides. In the Bay of Concepción, sulfur isotope data suggest that pyrite formation proceeds via the reaction of FeS with polysulfides or H2S. At the shelf stations, on the other hand, pyrite was significantly depleted in 34S relative to its potential precursors FeS and S0. Isotope mass balance considerations suggest further that pyritization at depth includes light sulfide, potentially originating from bacterial sulfur disproportionation. The δ34S-values of pyrite down to −38‰ vs. V-CDT are among the lightest found in organic-rich marine sediments. Seasonal variations in the sulfur isotope composition of dissolved sulfate indicated a dynamic non-steady-state sulfur cycle in the surface sediments. The 18O content of porewater sulfate increased with depth at all sites compared to the bottom water composition due to intracellular isotope exchange reactions during microbial sulfur transformations.  相似文献   

2.
Syngenetic iron sulfides in sediments are formed from dissolved sulfide resulting from sulfate reduction and catabolism of organic matter by anaerobic bacteria. It has been shown that in recent marine sediments deposited below oxygenated waters there is a constant relationship between reduced sulfur and organic carbon which is generally independent of the environment of deposition. Reexamination of data from recent sediments from euxinic marine environments (e.g., the Black Sea) also shows a linear relationship between carbon and sulfur, but the slope is variable and the line intercepts the S axis at a value between 1 and 2 percent S. It is proposed that the positive S intercept is due to watercolumn microbial reduction of sulfate using metabolizable small organic molecules and the sulfide formed is precipitated and accumulates at the sediment-water interface. The variation in slope and intercept of the C to S plots for several cores and for different stratigraphic zones for the Black Sea can be interpreted in relation to thickness of the aqueous sulfide layer or thinness of the oxygen containing layer and to deposition rate, but also may be influenced by availability of iron, and perhaps the type of organic matter (Leventhal, 1979).  相似文献   

3.
Geochemistry of Peruvian near-surface sediments   总被引：6，自引：0，他引：6  
Sixteen short sediment cores were recovered from the upper edge (UEO), within (WO) and below (BO) the oxygen minimum zone (OMZ) off Peru during cruise 147 of R/V Sonne. Solids were analyzed for major/trace elements, total organic carbon, total inorganic carbon, total sulfur, the stable sulfur isotope composition (δ34S) of pyrite, and sulfate reduction rates (SRR). Pore waters were analyzed for dissolved sulfate/sulfide and δ34S of sulfate. In all cores highest SRR were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and/or diffusion of sulfate from bottom water. δ34S of dissolved sulfate showed only minor downcore increases. Strong 32S enrichments in sedimentary pyrite (to −48‰ vs. V-CDT) are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction. Manganese and Co are significantly depleted in Peruvian upwelling sediments most likely due to mobilization from particles settling through the OMZ, whereas release of both elements from reducing sediments only seems to occur in near-coastal sites. Cadmium, Mo and Re are exceptionally enriched in WO sediments (<600 m water depth). High Re and moderate Cd and Mo enrichments are seen in BO sediments (>600 m water depth). Re/Mo ratios indicate anoxic and suboxic conditions for WO and BO sediments, respectively. Cadmium and Mo downcore profiles suggest considerable contribution to UEO/WO sediments by a biodetrital phase, whereas Re presumably accumulates via diffusion across the sediment-water interface to precipitation depth. Uranium is distinctly enriched in WO sediments (due to sulfidic conditions) and in some BO sediments (due to phosphorites). Silver transfer to suboxic BO sediments is likely governed by diatomaceous matter input, whereas in anoxic WO sediments Ag is presumably trapped due to sulfide precipitation. Cadmium, Cu, Zn, Ni, Cr, Ag, and T1 predominantly accumulate via biogenic pre-concentration in plankton remains. Rhenium, Sb, As, V, U and Mo are enriched in accordance with seawater TE availability. Lead and Bi enrichment in UEO surface sediments is likely contributed by anthropogenic activity (mining). Accumulation rates of TOC, Cd, Mo, U, and V from Peruvian and Namibian sediments exceed those from the Oman Margin and Gulf of California due to enhanced preservation off Peru and Namibia.  相似文献   

4.
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5.
The pore waters of sediments from a salt marsh along the Delaware estuary have been analyzed for sulfur species and associated trace metals. Since the sediment interface is usually in contact with the atmosphere, the sulfur species are dependent on the production of hydrogen sulfide by sulfate reduction and subsequent oxidation by diffusing oxygen. The most important species observed are hydrogen sulfide, polysulfide ions and thiosulfate. Secondary reactions of hydrogen sulfide and polysulfides with decomposing organic matter yield significant concentrations of both thiols and organic polysulfides. Upon isolation of the sediment from the atmosphere due to tidal inundation, bacterial sulfate reduction becomes the dominant process. This results in the reduction of the polysulfides in agreement with thermodynamic predictions, and suggests that the redox couple sulfide/polysulfide is a good redox indicator under such reducing environments.The concentrations of trace elements Cu and Fe in the pore waters are mainly controlled by sulfide formation. Calculations show that copper is strongly complexed probably with organo-sulfur ligands. Iron might be complexed as such sulfur species to a much lesser extent than copper.  相似文献   

6.
Upper Chesapeake Bay bottom waters are stratified in the summer. In the water column below the pycnocline, anoxic and sulfidic conditions exist. Hydrogen sulfide concentrations approach 60 μM or greater and elemental sulfur is also present. Water samples brought on board ship, exposed to light, and not treated with formaldehyde show rapid sulfide decomposition which is significantly faster than sulfide oxidation by molecular oxygen. The data presented show evidence for anaerobic, sulfide oxidation. The kinetics of the decomposition are consistent with possible biological mediation. Hydrogen, peroxide produced by microorganisms may be the chemical oxidant responsible for the oxidation. Alternately, solid metal oxides such as colloidal manganese oxide phases may be reponsible.  相似文献   

7.
A high-resolution geochemical record of a 120 cm black shale interval deposited during the Coniacian-Santonian Oceanic Anoxic Event 3 (ODP Leg 207, Site 1261, Demerara Rise) has been constructed to provide detailed insight into rapid changes in deep ocean and sediment paleo-redox conditions. High contents of organic matter, sulfur and redox-sensitive trace metals (Cd, Mo, V, Zn), as well as continuous lamination, point to deposition under consistently oxygen-free and largely sulfidic bottom water conditions. However, rapid and cyclic changes in deep ocean redox are documented by short-term (∼15-20 ka) intervals with decreased total organic carbon (TOC), S and redox-sensitive trace metal contents, and in particular pronounced phosphorus peaks (up to 2.5 wt% P) associated with elevated Fe oxide contents. Sequential iron and phosphate extractions confirm that P is dominantly bound to iron oxides and incorporated into authigenic apatite. Preservation of this Fe-P coupling in an otherwise sulfidic depositional environment (as indicated by Fe speciation and high amounts of sulfurized organic matter) may be unexpected, and provides evidence for temporarily non-sulfidic bottom waters. However, there is no evidence for deposition under oxic conditions. Instead, sulfidic conditions were punctuated by periods of anoxic, non-sulfidic bottom waters. During these periods, phosphate was effectively scavenged during precipitation of iron (oxyhydr)oxides in the upper water column, and was subsequently deposited and largely preserved at the sea floor. After ∼15-25 ka, sulfidic bottom water conditions were re-established, leading to the initial precipitation of CdS, ZnS and pyrite. Subsequently, increasing concentrations of H2S in the water column led to extensive formation of sulfurized organic matter, which effectively scavenged particle-reactive Mo complexes (thiomolybdates). At Site 1261, sulfidic bottom waters lasted for ∼90-100 ka, followed by another period of anoxic, non-sulfidic conditions lasting for ∼15-20 ka. The observed cyclicity at the lower end of the redox scale may have been triggered by repeated incursions of more oxygenated surface- to mid-waters from the South Atlantic resulting in a lowering of the oxic-anoxic chemocline in the water column. Alternatively, sea water sulfate might have been stripped by long-lasting high rates of sulfate reduction, removing the ultimate source for HS− production.  相似文献   

8.
In the eutrophic coastal ocean, quick formation of iron (Fe) sulfide is environmentally important to effectively prevent accumulation of dissolved sulfide and its detrimental effects on the benthic ecosystem. In this study, 0.5 N HCl-extractable labile Fe (LFe), acid volatile sulfide, and pyrite in the East China Sea inner shelf sediments were examined to investigate the mechanisms of quick sequestration of dissolved sulfide and potential impacts of frequent algal blooms on the capacity of quick sulfide-buffering in eutrophic coastal areas of the large-sized continental shelf subject to massive terrestrial input. The results indicate that sulfate reduction has been competitively suppressed by dissimilatory Fe reduction due to limited availability of labile organic matter. Dissolved sulfide can be quickly buffered by reaction with LFe and, therefore, is difficult to accumulate to a high level. The quick sulfide-buffering capacity has not become exhausted partly because of the formation of un-sulfidized LFe(II) via dissimilatory reduction of less reactive Fe oxides. It is expected that dissolved sulfide will not pose detrimental effects on the benthic ecosystem in the near future if the current biogeochemical/ecological state remains.  相似文献   

9.
We present a 3-year study of concentrations and sulfur isotope values (δ34S, Δ33S, and Δ36S) of sulfur compounds in the water column of Fayetteville Green Lake (NY, USA), a stratified (meromictic) euxinic lake with moderately high sulfate concentrations (12-16 mM). We utilize our results along with numerical models (including transport within the lake) to identify and quantify the major biological and abiotic processes contributing to sulfur cycling in the system. The isotope values of sulfide and zero-valent sulfur across the redox-interface (chemocline) change seasonally in response to changes in sulfide oxidation processes. In the fall, sulfide oxidation occurs primarily via abiotic reaction with oxygen, as reflected by an increase in sulfide δ34S at the redox interface. Interestingly, S isotope values for zero-valent sulfur sampled at this time still reflect production and recycling by phototrophic S-oxidation. In the spring, sulfide S isotope values suggest an increased input from phototrophic oxidation, consistent with a more pronounced phototroph population at the chemocline. This trend is associated with smaller fractionations between sulfide and zero-valent sulfur, suggesting a metabolic rate control on fractionation similar to that for sulfate reduction. Comparison of our data with previous studies indicates that the S isotope values of sulfate and sulfide in the deep waters are remarkably stable over long periods of time, with consistently large fractionations of up to 58‰ in δ34S. Models of the δ34S and Δ33S trends in the deep waters (considering mass transport via diffusion and advection along with biological processes) require that these fractionations are a consequence of sulfur compound disproportionation at and below the redox interface in addition to large fractionations during sulfate reduction. The large fractionations during sulfate reduction appear to be a consequence of the high sulfate concentrations and the distribution of organic matter in the water column. The occurrence of disproportionation in the lake is supported by profiles of intermediate sulfur compounds and by lake microbiology, but is not evident from the δ34S trends alone. These results illustrate the utility of including minor S isotopes in sulfur isotope studies to unravel complex sulfur cycling in natural systems.  相似文献   

10.
The stratified water column of the Black Sea is partitioned into oxic, suboxic, and euxinic zones, each characterized by different biogeochemical processes and by distinct microbial communities. In 2003, we collected particulate matter by large volume in situ filtration at the highest resolution to date for lipid biomarker analysis and bacterioplankton for enumeration of major prokaryotic groups. Abundances of several prokaryotic groups were estimated using CARD-FISH probes specific for Bacteria, Archaea (Crenarchaeota and Euryarchaeota), epsilonproteobacteria (mainly sulfide oxidizers) and sulfate reducing bacteria. We also measured a wide range of bacterial and archaeal lipid biomarkers. Depth distributions of diagnostic biomarkers are matched with zonation of microbial processes, including aerobic bacterial oxidation of methane, oxidation of ammonium by bacteria and archaea, metal reduction, and sulfide oxidation at the chemocline, and bacterial sulfate reduction and anaerobic oxidation of methane by archaea in the anoxic zone. Cell densities for archaea and sulfate reducing bacteria are estimated based on water column biomarker concentrations and compared with CARD-FISH results.  相似文献   

11.
Sulfur isotope fractionation during dissimilatory sulfate reduction has been conceptually described by the widely accepted Rees model as related to the stepwise reduction of sulfate to sulfide within the cells of bacteria. The magnitude of isotope fractionation is determined by the interplay between different reduction steps in a chain of reactions. Here we present a revision of Rees’ model for bacterial sulfate reduction that includes revised fractionation factors for the sulfite-sulfide step and incorporates new forward and reverse steps in the reduction of sulfite to sulfide, as well as exchange of sulfide between the cell and ambient water. With this model we show that in contrast to the Rees model, isotope fractionations well in excess of −46‰ are possible. Therefore, some of the large sulfur isotope fractionations observed in nature can be explained without the need of alternate pathways involving the oxidative sulfur cycle. We use this model to predict that large fractionations should occur under hypersulfidic conditions and where electron acceptor concentrations are limiting.  相似文献   

12.
黄铁矿是富有机质沉积的特征矿物。根据TOC/S、TOC/DOP、S/Fe关系以及S TOC Fe多重线性回归分析结果对三水盆地古近系〖HT5”,6”〗土〖KG-*3〗布〖HT5”SS〗心组红岗段黑色页岩中沉积黄铁矿的形成及其控制因素进行了分析。土布心组红岗段黑色页岩的黄铁矿有成岩黄铁矿和同生黄铁矿两种成因组分。红岗段下部（亚段A）有机碳含量普遍较低,底部水体以弱氧化条件为主,硫酸盐还原作用发生于沉积物/水界面以下,黄铁矿为成岩成因,其形成主要受有机质的限制。红岗段中上部（亚段B和C）的沉积条件变化频繁,其有机碳含量变化幅度大。富有机质（TOC>4%）岩层形成于缺氧的底部水体条件下。水体中可含H2S,碎屑铁矿物在埋藏之前即与之在水体中反应形成同生黄铁矿。这一过程不受有机质的限制,而是受活性铁与H2S接触时间的限制。同时,由于大量淡水输入导致硫酸盐浓度的降低,从而对硫化物形成有一定的限制作用。对于低有机质（TOC<4%）样品,黄铁矿由同生和成岩组分组成。其中以成岩黄铁矿为主,其形成过程主要受有机质限制,而同生黄铁矿受铁矿物与H2S接触时间的限制。  相似文献   

13.
The δ34S values of dissolved sulfide and the sulfur isotope fractionations between dissolved sulfide and sulfate species in Floridan ground water generally correlate with dissolved sulfate concentrations which are related to flow patterns and residence time within the aquifer. The dissolved sulfide derives from the slow in situ biogenic reduction of sulfate dissolved from sedimentary gypsum in the aquifer. In areas where the water is oldest, the dissolved sulfide has apparently attained isotopic equilibrium with the dissolved sulfate (Δ34S = 65 per mil) at the temperature (28°C) of the system. This approach to equilibrium reflects an extremely slow reduction rate of the dissolved sulfate by bacteria; this slow rate probably results from very low concentrations of organic matter in the aquifer.In the reducing part of the Edwards aquifer, Texas, there is a general down-gradient increase in both dissolved sulfide and sulfate concentrations, but neither the δ34S values of sulfide nor the sulfide-sulfate isotope fractionation correlates with the ground-water flow pattern. The dissolved sulfide species appear to be derived primarily from biogenic reduction of sulfate ions whose source is gypsum dissolution although upgradient diffusion of H2S gas from deeper oil field brines may be important in places. The sulfur isotope fractionation for sulfide-sulfate (about 38 per mil) is similar to that observed for modern oceanic sediments and probably reflects moderate sulfate reduction in the reducing part of the aquifer owing to the higher temperature and significant amount of organic matter present; contributions of isotopically heavy H2S from oil field brines are also possible.  相似文献   

14.
Sulfate-reducing bacteria (SRB) are ubiquitous in anoxic environments where they couple the oxidation of organic compounds to the production of hydrogen sulfide. This can be problematic for various industries including oil production where reservoir “souring” (the generation of H2S) requires corrective actions. Nitrate or nitrite injection into sour oil fields can promote SRB control by stimulating organotrophic nitrate- or nitrite-reducing bacteria (O-NRB) that out-compete SRB for electron donors (biocompetitive exclusion), and/or by lithotrophic nitrate- or nitrite-reducing sulfide oxidizing bacteria (NR-SOB) that remove H2S directly. Sulfur and oxygen isotope ratios of sulfide and sulfate were monitored in batch cultures and sulfidic bioreactors to evaluate mitigation of SRB activities by nitrate or nitrite injection. Sulfate reduction in batch cultures of Desulfovibrio sp. strain Lac15 indicated typical Rayleigh-type fractionation of sulfur isotopes during bacterial sulfate reduction (BSR) with lactate, whereas oxygen isotope ratios in unreacted sulfate remained constant. Sulfur isotope fractionation in batch cultures of the NR-SOB Thiomicrospira sp. strain CVO was minimal during the oxidation of sulfide to sulfate, which had δ18OSO4 values similar to that of the water-oxygen. Treating an up-flow bioreactor with increasing doses of nitrate to eliminate sulfide resulted in changes in sulfur isotope ratios of sulfate and sulfide but very little variation in oxygen isotope ratios of sulfate. These observations were similar to results obtained from SRB-only, but different from those of NR-SOB-only pure culture control experiments. This suggests that biocompetitive exclusion of SRB took place in the nitrate-injected bioreactor. In two replicate bioreactors treated with nitrite, less pronounced sulfur isotope fractionation and a slight decrease in δ18OSO4 were observed. This indicated that NR-SOB played a minor role during dosing with low nitrite and that biocompetitive exclusion was the major process. The results demonstrate that stable isotope data can contribute unique information for understanding complex microbial processes in nitrate- and sulfate-reducing systems, and offer important information for the management of H2S problems in oil reservoirs and elsewhere.  相似文献   

15.
A section through the late Archean Mt. McRae Shale comprising, in ascending order, a lower shale interval (LSI), a banded iron formation (BIF), an upper shale (USI) and a carbonate (C1) has been analyzed for total Fe and Al contents and authigenic Fe present as carbonate, oxide, sulfide and silicate phases. The authigenic mineralogy is controlled by the episodic addition of Fe from hydrothermal activity and removal of Fe by sulfide, relative to rates of clastic sedimentation. The LSI and BIF have mean FeT/Al values of 2 and 5, respectively, that record iron enrichment from hydrothermal sources. Iron was precipitated primarily as siderite accompanied by Fe-rich chlorite from anoxic bottom waters rich in dissolved Fe. Pyrite formation was probably limited by the availability of sulfate, which was present at low concentrations and became rapidly depleted. The USI has generally lower FeT/Al values (0.6-1.3), similar to those found in Paleozoic shales, with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle. This interval contains authigenic Fe predominantly as pyrite, where high values for DOP (>0.8) indicate the existence of a water column that became rich in dissolved sulfide (euxinic) when sulfate concentrations increased due to a transient or secular increase in ocean/atmosphere oxygenation. High concentrations of dissolved sulfide maintained low concentrations of dissolved Fe, which allowed only minor amounts of Fe to be precipitated as carbonates and silicates. The USI also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin. The carbonate C1 represents a basinal chemistry where sulfide has been removed and FeT/Al values are ∼1 indicating that hydrothermal activity again produced dissolved Fe-rich bottom waters. Detailed iron speciation of the Mt. McRae Shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late Archean Hamersley Basin, just prior to the Paleoproterozoic rise in atmospheric oxygenation, and our refined methods have relevance to all Fe-rich deposits.  相似文献   

16.
Water column and seabed samples were obtained from 92 stations on the Amazon continental shelf during October of 1979. Uptake of silica near and southeast of the river mouth began at a salinity of 8%. and accounted for 17% of the riverine silica flux to this region. Uptake northwest of the river mouth began at a salinity of 20%. and resulted in 33% removal of the riverine silica flux. Examination of filtered suspended solids revealed abundant diatoms in the surface waters, including Coscinodiscus. Skeletonema, Synedra. and Thalassiosira. The biological uptake of silica appears to be dependent on three factors: turbidity, turbulence, and nutrient availability. There was no evidence of abiological removal of silica in the Amazon estuary. 75 to 88% of the silica removed from surface waters by diatoms dissolves prior to accumulation in the seabed. Based on the mean biogenic silica content of shelf sediment (0.25%) and estimates of rates of sediment accumulation, the biogenic silica accumulation rate on the shelf is 2 × 1012 g/yr, which represents only 4% of the dissolved silica supplied by the Amazon River. Biological uptake of silica in estuarine surface waters may not accurately reflect permanent removal of biogenic silica to the seabed because of dissolution which occurs in bottom waters and near the sediment-water interface.  相似文献   

17.
Sulfate reduction during seawater reaction with fayalite and with magnetite was rapid at 350°C, producing equilibrium assemblages of talc-pyrite-hematite-magnetite at low water/rock ratios and talc-pyrite-hematite-anhydrite at higher water/rock ratios. At 250°C, seawater reacting with fayalite produced detectable amounts of dissolved H2S, but extent of reaction of solid phases was minor after 150 days. At 200°C, dissolved H2S was not detected, even after 219 days, but mass balance calculations suggest a small amount of pyrite may have formed. Reaction stoichiometry indicates that sulfate reduction requires large amounts of H+, which, in subseafloor hydrothermal systems is provided by Mg metasomatism. Seawater contains sufficient Mg to supply all the H+ necessary for quantitative reduction of seawater sulfate.Systematics of sulfur isotopes in the 250 and 350°C experiments indicate that isotopic equilibrium is reached, and can be modeled as a Rayleigh distillation process. Isotopic composition of hydrothermally produced H2S in natural systems is strongly dependent upon the seawater/basalt ratio in the geothermal system, which controls the relative sulfide contributions from the two important sulfur sources, seawater sulfate and sulfide phases in basalt. Anhydrite precipitation during geothermal heating severely limits sulfate ingress into high temperature interaction zones. Quantitative sulfate reduction can thus be accomplished without producing strongly oxidized rocks and resultant sulfide sulfur isotope values represent a mixture of seawater and basaltic sulfur.  相似文献   

18.
Groundwater and sediment samples (∼ 1 m depth) at sites representative of different groundwater pathways were collected to determine the aqueous speciation of sulfur and the fractionation of sulfur isotopes in aqueous and solid phases. In addition, selected sediment samples at 5 depths (from oxic to anoxic layers) were collected to investigate the processes controlling sulfur biogeochemistry in sedimentary layers. Pyrite was the dominant sulfur-bearing phase in the capillary fringe and groundwater zones where anoxic conditions are found. Low concentrations of pyrite (< 5.9 g kg− 1) coupled with high concentrations of dissolved sulfide (4.81 to 134.7 mg L− 1) and low concentrations of dissolved Fe (generally < 1 mg L− 1) and reducible solid-phase Fe indicate that availability of reactive Fe limits pyrite formation. The relative uniformity of down-core isotopic trends for sulfur-bearing mineral phases in the sedimentary layers suggests that sulfate reduction does not result in significant sulfate depletion in the sediment. Sulfate availability in the deeper sediments may be enhanced by convective vertical mixing between upper and lower sedimentary layers due to evaporative concentration. The large isotope fractionation between dissolved sulfate and sedimentary sulfides at Owens Lake provides evidence for initial fractionation from bacterial sulfate reduction and additional fractionation generated by sulfide oxidation followed by disproportionation of intermediate oxidation state sulfur compounds. The high salinity in the Owens Lake brines may be a factor controlling sulfate reduction and disproportionation in hypersaline conditions and results in relatively constant values for isotope fractionation between dissolved sulfate and total reduced sulfur.  相似文献   

19.
Sulfate reduction and sulfur-iron geochemistry were studied in 5-6 m deep gravity cores of Holocene mud from Aarhus Bay (Denmark). A goal was to understand whether sulfate is generated by re-oxidation of sulfide throughout the sulfate and methane zones, which might explain the abundance of active sulfate reducers deep below the main sulfate zone. Sulfate penetrated down to 130 cm where methane started to build up and where the concentration of free sulfide peaked at 5.5 mM. Below this sulfate-methane transition, sulfide diffused downwards to a sulfidization front at 520 cm depth, below which dissolved iron, Fe2+, accumulated in the pore water. Sulfate reduction rates measured by 35S-tracer incubations in the sulfate zone were high due to high concentrations of reactive organic matter. Within the sulfate-methane transition, sulfate reduction was distinctly stimulated by the anaerobic oxidation of methane. In the methane zone below, sulfate remained at positive “background” concentrations of <0.5 mM down to the sulfidization front. Sulfate reduction decreased steeply to rates which at 300-500 cm depth were 0.2-1 pmol SO42− cm−3 d−1, i.e., 4-5 orders of magnitude lower than rates measured near the sediment surface. The turn-over time of sulfate increased from 3 years at 12 cm depth to 100-1000 years down in the methane zone. Sulfate reduction in the methane zone accounted for only 0.1% of sulfate reduction in the entire sediment column and was apparently limited by the low pore water concentration of sulfate and the low availability of organic substrates. Amendment of the sediment with both sulfate and organic substrates immediately caused a 10- to 40-fold higher, “potential sulfate reduction” which showed that a physiologically intact community of sulfate reducing bacteria was present. The “background” sulfate concentration appears to be generated from the reaction of downwards diffusing sulfide with deeply buried Fe(III) species, such as poorly-reactive iron oxides or iron bound in reactive silicates. The oxidation of sulfide to sulfate in the sulfidic sediment may involve the formation of elemental sulfur and thiosulfate and their further disproportionation to sulfide and sulfate. The net reaction of sulfide and Fe(III) to form pyrite requires an additional oxidant, irrespective of the formation of sulfate. This could be CO2 which is reduced with H2 to methane. The methane subsequently diffuses upwards to become re-oxidized at the sulfate-methane transition and thereby removes excess reducing power and enables the formation of excess sulfate. We show here how the combination of these well-established sulfur-iron-carbon reactions may lead to the deep formation of sulfate and drive a cryptic sulfur cycle. The iron-rich post-glacial sediments underlying Holocene marine mud stimulate the strong sub-surface sulfide reoxidation observed in Aarhus Bay and are a result of the glacial to interglacial history of the Baltic Sea area. Yet, processes similar to the ones described here probably occur widespread in marine sediments, in particular along the ocean margins.  相似文献   

20.
Assuming stability of the Black Sea system and conservative behavior of sulfate in relation to salinity outside the bottom convective layer (BCL), the influence of shelf-modified Mediterranean water (SMMW) on the water column of the Black Sea below the core of the cold intermediate layer (CIL) was estimated on the basis of variations in the sulfur isotope composition of sulfate. As a result of construction of the model of mixing of three water masses, it was shown that the SMMW fraction in the area of hydrogen sulfide onset at a salinity of 20.8–20.9 was 5–7 times higher than the amount of water produced by mixing of the CIL and the BCL. The SMMW fraction decreased with depth rapidly and was only 10% at a depth of 1000 m. Significant supply of SMMW to the pycnocline area provided a high renewal rate of water, which prevented accumulation of 32S-rich sulfate resulted from hydrogen sulfide oxidation.  相似文献   

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