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1.
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   

2.
The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ34S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ34S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ34S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ34Spyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than ? 4‰. A wealth of literature exists on the oxidation of pyrite by Fe3+ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H2O, not O2. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition.  相似文献   

3.
The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2-4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO42−-CH4 transition where H2S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H2S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe2+ diffusing up from below. The H2S-Fe2+ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO42− and 10 yr for H2S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO42−-CH4 and H2S-Fe2+ cause the trapping of isotopically heavy iron sulfide with δ34S = +15 to +33‰ at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO42− diffusing downward into the SO42−-CH4 transition has an isotopic composition of +19‰, close to the +23‰ of H2S diffusing upward. These isotopic compositions are, however, very different from the porewater SO42− (+43‰) and H2S (−15‰) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H2S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth’s history.  相似文献   

4.
Upper Cretaceous Phosphorites from different localities in Egypt were analyzed for their rare earth elements (REEs) contents and sulfur and strontium isotopes to examine the effect of depositional conditions versus diagenesis on these parameters.The negative Ce and Eu anomalies of the study phosphorites suggest its formation under reducing conditions. However, chondrite-normalized REEs patterns show relative enrichments of LREEs over the HREEs, which is obviously different from the seawater REEs pattern suggesting post-depositional modifications on the REEs distributions during diagenesis. The difference in the REEs concentrations and Ce anomalies among the study localities as well as the similarity between the REEs patterns of these phosphorites and associated black shales might support this interpretation.The concentration of structural SO42− (0.6-3.7%) and their δ34S values (+0.5 to -20‰) in the upper Cretaceous phosphorites in Egypt suggest the formation of these phosphorites in the zone of sulfate reduction. On the other hand, the sulfur isotopes in the pyrite from the study phosphorites (δ34S = +4.6‰ − 23‰ with an average of −7.7‰) are attributed to the influence of seawater from which pyrite was formed during diagenesis. The difference between the δ34S values in the phosphorites (all are positive values) and those in the associated pyrite (mostly negative values) reflect an asymmetric sulfate and sulfide sulfur isotopic composition due to the formation of francolite (source of sulfate) and pyrite (source of sulfide) in different conditions and/or process.The 87Sr/86Sr values of the upper Cretaceous phosphorites in Egypt are very close to the marine values during the Campanian-Maastrichtian time and their average (0.707622) is more or less comparable to the average 87Sr/86Sr values of the Cretaceous-Eocene Tethyan phosphorites. This suggests no post-depositional alteration (i.e. diagenetic effect) on the Sr isotopic composition of these phosphorites.  相似文献   

5.
This study examines the sulfur isotope record of seawater sulfate proxies using δ34S and Δ33S to place constraints on the average global fractionation (Δ34Spy) associated with pyrite formation and burial and the exponent λ that relates variations of the 34S/32S to variations of the 33S/32S. The results presented here use an analysis of the sulfur isotope record from seawater sulfate proxies and sedimentary sulfide to extract this quantity as the arithmetic difference between δ34S of seawater sulfate and contemporaneous sulfide. It also uses an independent method that draws on inferences about the Δ33S evolution of seawater sulfate to evaluate this further. These two methods yield similar results suggesting that Δ34Spy and λ changed over the course of the Phanerozoic from slightly lower values of Δ34Spy (lower values of λ) in the early Phanerozoic (Cambrian-Permian) to higher values of Δ34Spy (higher values of λ) starting in the Triassic. This change of Δ34Spy and the exponent λ is interpreted to reflect a change in the proportion of sulfide that was reoxidized and processed by bacterial disproportionation on a global scale. The revised record of Δ34Spy also yields model pyrite burial curves making them more closely resemble model evolution curves for other element systems and global sea level curves. It is suggested that possible links to sea level may occur via changes in the area of submerged continental shelves which would provide additional loci for pyrite burial.The slightly different constraints used by the two approaches to calculate this fractionation may allow for additional information to be obtained about the sulfur cycle with future studies. For instance, the correspondence of these results suggests that the inferred variation of 34S/32S of pyrite is real, and that there is no significant missing sink of fractionated sulfur at the resolution of the present study (such as might be associated with organic sulfur). Burial of organic sulfur may, however, have been important at some times in the Phanerozoic and shorter timescale deviations between results provided by these methods may be observed with higher resolution sampling. If observed, this would suggest either that the record for pyrite (or less likely sulfate) is biased, or that another sink (possibly as organic sulfur) was important during these times in the Phanerozoic.  相似文献   

6.
Hydrous pyrolysis experiments at 200 to 365°C were carried out on a thermally immature organic-rich limestone containing Type-IIS kerogen from the Ghareb Limestone in North Negev, Israel. This work focuses on the thermal behavior of both organic and inorganic sulfur species and the partitioning of their stable sulfur isotopes among organic and inorganic phases generated during hydrous pyrolyses. Most of the sulfur in the rock (85%) is organic sulfur. The most dominant sulfur transformation is cleavage of organic-bound sulfur to form H2S(gas). Up to 70% of this organic sulfur is released as H2S(gas) that is isotopically lighter than the sulfur in the kerogen. Organic sulfur is enriched by up to 2‰ in 34S during thermal maturation compared with the initial δ34S values. The δ34S values of the three main organic fractions (kerogen, bitumen and expelled oil) are within 1‰ of one another. No thermochemical sulfate reduction or sulfate formation was observed during the experiments. The early released sulfur reacted with available iron to form secondary pyrite and is the most 34S depleted phase, which is 21‰ lighter than the bulk organic sulfur. The large isotopic fractionation for the early formed H2S is a result of the system not being in equilibrium. As partial pressure of H2S(gas) increases, retro reactions with the organic sulfur in the closed system may cause isotope exchange and isotopic homogenization. Part of the δ34S-enriched secondary pyrite decomposes above 300°C resulting in a corresponding decrease in the δ34S of the remaining pyrite. These results are relevant to interpreting thermal maturation processes and their effect on kerogen-oil-H2S-pyrite correlations. In particular, the use of pyrite-kerogen δ34S relations in reconstructing diagenetic conditions of thermally mature rocks is questionable because formation of secondary pyrite during thermal maturation can mask the isotopic signature and quantity of the original diagenetic pyrite. The main transformations of kerogen to bitumen and bitumen to oil can be recorded by using both sulfur content and δ34S of each phase including the H2S(gas). H2S generated in association with oil should be isotopically lighter or similar to oil. It is concluded that small isotopic differentiation obtained between organic and inorganic sulfur species suggests closed-system conditions. Conversely, open-system conditions may cause significant isotopic discrimination between the oil and its source kerogen. The magnitude of this discrimination is suggested to be highly dependent on the availability of iron in a source rock resulting in secondary formation of pyrite.  相似文献   

7.
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   

8.
Sulfate and water from experiments in which pyrite was oxidized at a pH of 2.0 were analyzed for sulfur and oxygen stable isotopes. Experiments were conducted under both aerobic and anaerobic sterile conditions, as well as under aerobic conditions in the presence of Thiobacillus ferrooxidans, to elucidate the pathways of oxidation. Oxygen isotope fractionation between SO2?4 and H2O varied from +4.0 %. (anaerobic, sterile) to + 18.0 %. (aerobic, with T. ferrooxidans.). The oxygen isotope composition of dissolved oxygen utilized in both chemical and microbially-mediated oxidation was also determined (+11.4 %., by T. ferrooxidans; +18.4 %., chemical). Contributions of water-derived oxygen and dissolved oxygen to the sulfate produced in the oxidation of pyrite could thus be estimated. Water-derived oxygen constituted from 23 to ~ 100 percent of the oxygen in the sulfate produced in the experiments, and this closely approximates the range of contribution in natural acid mine drainage. Oxidation of sulfides in anaerobic, water-saturated environments occurs primarily by chemical oxidation pathways, whereas oxidation of sulfides in well-aerated, unsaturated zone environments occurs dominantly by microbially mediated pathways.  相似文献   

9.
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   

10.
胡永亮  王伟  周传明 《沉积学报》2020,38(1):138-149
地质历史时期新元古代大气氧含量普遍较低。在硫酸盐还原细菌作用下,作为海洋重要的氧化性离子,陆源硫酸根离子有效促进了深层海水的氧化进程。在此过程中,硫元素在硫酸根和黄铁矿之间发生显著同位素分馏,其分馏程度可反推当时古海洋的氧化还原状态。沉积地层中的黄铁矿普遍具有多种形态,不同形态黄铁矿的形成环境多有不同。如草莓状黄铁矿多形成于底层缺氧水体或沉积物的浅表面,而大颗粒单晶黄铁矿或脉状黄铁矿则多沉积于成岩早期的沉积物孔隙或形成于成岩后期的热液改造。与草莓状黄铁矿不同,大颗粒单晶或脉状黄铁矿的硫同位素组成并不能反映沉积时期的古海洋氧化还原条件。判定沉积地层中不同形态的黄铁矿及形成过程,是获得有效反映海洋沉积环境硫同位素组成特征的基本前提。简要总结了地质历史时期沉积地层中的黄铁矿类型及矿物形成过程,并以华南埃迪卡拉纪蓝田组岩芯样品为例,识别出各个样品中的黄铁矿形态组成特征,对比分析了全岩黄铁矿与样品中大颗粒黄铁矿硫同位素组成差异。研究结果表明:不同岩性样品中黄铁矿的形态种类及含量均存在差异。页岩样品保存有更好形态的自形晶以及草莓状黄铁矿;碳酸盐岩样品中具有较多自形晶以及他形晶黄铁矿,并且其中的少量草莓状黄铁矿遭受后期成岩作用而发生不同程度的晶体蚀变。样品中大颗粒黄铁矿的硫同位素值（δ34SL-pyr）通常显著高于全岩黄铁矿的硫同位素值（δ34ST-pyr）,最大差值可达48.5‰。在利用黄铁矿的硫同位素组成来反推当时古海洋环境时,需要区分不同形态黄铁矿,仔细剔除大颗粒黄铁矿,降低成岩期黄铁矿对样品中硫同位素组成的影响。更细致的微区黄铁矿硫同位素分析工作将依赖于SIMS分析测试手段进行。  相似文献   

11.
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   

12.
The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ34S and δ13C data which deviate from a negative correlation line toward higher δ13C values suggest methanogenic CO2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH4 and CO2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U6+ to U4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH4 and HCO3 − existed and reduced sulfur species (H2S, HS−) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO4 2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U4+ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997  相似文献   

13.
Sulfide sulfur in mid-oceanic ridge hydrothermal vents is derived from leaching of basaltic-sulfide and seawater-derived sulfate that is reduced during high temperature water rock interaction. Conventional sulfur isotope studies, however, are inconclusive about the mass-balance between the two sources because 34S/32S ratios of vent fluid H2S and chimney sulfide minerals may reflect not only the mixing ratio but also isotope exchange between sulfate and sulfide. Here, we show that high-precision analysis of S-33 can provide a unique constraint because isotope mixing and isotope exchange result in different Δ33S (≡δ33S-0.515 δ34S) values of up to 0.04‰ even if δ34S values are identical. Detection of such small Δ33S differences is technically feasible by using the SF6 dual-inlet mass-spectrometry protocol that has been improved to achieve a precision as good as 0.006‰ (2σ).Sulfide minerals (marcasite, pyrite, chalcopyrite, and sphalerite) and vent H2S collected from four active seafloor hydrothermal vent sites, East Pacific Rise (EPR) 9-10°N, 13°N, and 21°S and Mid-Atlantic Ridge (MAR) 37°N yield Δ33S values ranging from −0.002 to 0.033 and δ34S from −0.5‰ to 5.3‰. The combined δ34S and Δ33S systematics reveal that 73 to 89% of vent sulfides are derived from leaching from basaltic sulfide and only 11 to 27% from seawater-derived sulfate. Pyrite from EPR 13°N and marcasite from MAR 37°N are in isotope disequilibrium not only in δ34S but also in Δ33S with respect to associated sphalerite and chalcopyrite, suggesting non-equilibrium sulfur isotope exchange between seawater sulfate and sulfide during pyrite precipitation. Seafloor hydrothermal vent sulfides are characterized by low Δ33S values compared with biogenic sulfides, suggesting little or no contribution of sulfide from microbial sulfate reduction into hydrothermal sulfides at sediment-free mid-oceanic ridge systems. We conclude that 33S is an effective new tracer for interplay among seawater, oceanic crust and microbes in subseafloor hydrothermal sulfur cycles.  相似文献   

14.
Multiple sulfur isotope ratios （^34S/^33S/^32S） of Archean bedded sulfides deposits were measured in the Yanlingguan Formation of the Taishan Group in Xintai, Shandong Province, East of China; 633S = -0.7%o to 3.8‰,δ^34S = 0.1‰-8.8‰, △^33S = -2.3‰ to -0.7‰. The sulfur isotope compositions show obvious mass-independent fractionation （MIF） signatures. The presence of MIF of sulfur isotope in Archean sulfides indicates that the sulfur was from products of photochemical reactions of volcanic SO2 induced by solar UV radiation, implying that the ozone shield was not formed in atmosphere at that time, and the oxygen level was less than 10-5 PAL （the present atmosphere level）. The sulfate produced by photolysis of SO2 with negative △^33S precipitated near the volcanic activity center; and the product of element S with positive △^33S precipitated far away from the volcanic activity center. The lower △^33S values of sulfide （-2.30‰ to --0.25‰） show that Shihezhuang was near the volcanic center, and sulfur was mostly from sulfate produced by photolysis. The higher △^33S values （-0.5‰ to -‰） indicate that Yanlingguan was far away from the volcanic center and that some of sulfur were from sulfate, another from element S produced by photolysis. The data points of sulfur isotope from Yanlingguan are in a line parallel to MFL （mass dependent fractionation line） on the plot of δ^34S--δ^33S, showing that the volcanic sulfur species went through the atmospheric cycle into the ocean, and then mass dependent fractionation occurred during deposition of sulfide. The data points of sulfur isotope from Shihezhuang represent a mix of different sulfur source.  相似文献   

15.
Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ33S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ33S from −0.8‰ to 1.0‰). The 90‰ range in δ34S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ34S (<−20‰) overgrown by euhedral rims with higher δ34S (4-7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.  相似文献   

16.
Kinetic isotope effects related to the breaking of chemical bonds drive sulfur isotope fractionation during dissimilatory sulfate reduction (DSR), whereas oxygen isotope fractionation during DSR is dominated by exchange between intercellular sulfur intermediates and water. We use a simplified biochemical model for DSR to explore how a kinetic oxygen isotope effect may be expressed. We then explore these relationships in light of evolving sulfur and oxygen isotope compositions (δ34SSO4 and δ18OSO4) during batch culture growth of twelve strains of sulfate-reducing bacteria. Cultured under conditions to optimize growth and with identical δ18OH2O and initial δ18OSO4, all strains show 34S enrichment, whereas only six strains show significant 18O enrichment. The remaining six show no (or minimal) change in δ18OSO4 over the growth of the bacteria. We use these experimental and theoretical results to address three questions: (i) which sulfur intermediates exchange oxygen isotopes with water, (ii) what is the kinetic oxygen isotope effect related to the reduction of adenosine phosphosulfate (APS) to sulfite (SO32−), (iii) does a kinetic oxygen isotope effect impact the apparent oxygen isotope equilibrium values? We conclude that oxygen isotope exchange between water and a sulfur intermediate likely occurs downstream of APS and that our data constrain the kinetic oxygen isotope fractionation for the reduction of APS to sulfite to be smaller than 4‰. This small oxygen isotope effect impacts the apparent oxygen isotope equilibrium as controlled by the extent to which APS reduction is rate-limiting.  相似文献   

17.
At Lucky Strike near the Azores Triple Junction, the seafloor setting of the hydrothermal field in a caldera system with abundant low-permeability layers of cemented breccia, provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution. Coupled analyses of S isotopes performed in conjunction with Se and Fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems. These data provide a tool for resolving the different abiotic and potential biotic near-surface hydrothermal reactions. The δ34S (between 1.5‰ and 4.6‰) and Se values (between 213 and 1640 ppm) of chalcopyrite suggest a high temperature end-member hydrothermal fluid with a dual source of sulfur: sulfur that was leached from basaltic rocks, and sulfur derived from the reduction of seawater sulfate. In contrast, pyrite and marcasite generally have lower δ34S within the range of magmatic values (0 ± 1‰) and are characterized by low concentrations of Se (<50 ppm). For 82Se/76Se ratios, the δ82Se values range from basaltic values of near −1.5‰ to −7‰. The large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between Se leached from igneous rock and Se derived from seawater. We interpret the Se isotope signature to be a result of leaching and mixing of a fractionated Se source located beneath hydrothermal chimneys in the hydrothermal fluid. At Lucky Strike we consider two sources for S and Se: (1) the “end-member” hydrothermal fluid with basaltic Se isotopic values (−1.5‰) and typical S isotope hydrothermal values of 1.5‰; (2) a fractionated source hosted in subsurface environment with negative δ34S values, probably from bacterial reduction of seawater sulfate and negative δ82Se values possibly derived from inorganic reduction of Se oxyanions. Fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by S isotopes. Fe isotope systematic reveals that Se-rich high temperature samples have δ57Fe values close to basaltic values (∼0‰) whereas Se-depleted samples precipitated at medium to low temperature are systematically lighter (δ57Fe values between −1 to −3‰). An important implication of our finding is that light Fe isotope composition down to −3.2‰ may be explained entirely by abiotic fractionation, in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions.  相似文献   

18.
平衡热液体系中硫同位素演化的几个图解   总被引：1，自引：0，他引：1       下载免费PDF全文
根据含硫矿物的同位素组成推断热液矿床成因是很有意义的。 1968年首先由H.Sakai指出热液的温度和pH值可以影响硫化物的同位素组成。接着,1972年H.Ohmoto以及1979年他和R.O.Rye系统讨论了平衡条件下热液的物理化学条件对硫同位素分馏的影响,建立了高温热液系统和低温热液系统的热液流体以及含硫矿物与热液成分和物理化学条件(温度、压力、氧逸度和酸碱度等)之间的数学表达式。  相似文献   

19.
To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ18O and δ34S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O2 as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)aq as an oxidant with varying δ18OH2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ34SSO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O2) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O2 varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε18OSO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ18OSO4 values in the biological and abiotic experiments, it is suggested that δ18OSO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.  相似文献   

20.
Previous geochemical and microbiological studies in the Cariaco Basin indicate intense elemental cycling and a dynamic microbial loop near the oxic-anoxic interface. We obtained detailed distributions of sulfur isotopes of total dissolved sulfide and sulfate as part of the on-going CARIACO time series project to explore the critical pathways at the level of individual sulfur species. Isotopic patterns of sulfate (δ34SSO4) and sulfide (δ34SH2S) were similar to trends observed in the Black Sea water column: δ34SH2S and δ34SSO4 were constant in the deep anoxic water (varying within 0.6‰ for sulfide and 0.3‰ for sulfate), with sulfide roughly 54‰ depleted in 34S relative to sulfate. Near the oxic-anoxic interface, however, the δ34SH2S value was ∼3‰ heavier than that in the deep water, which may reflect sulfide oxidation and/or a change in fractionation during in situ sulfide production through sulfate reduction (SR). δ34SH2S and Δ33SH2S data near the oxic-anoxic interface did not provide unequivocal evidence to support the important role of sulfur-intermediate disproportionation suggested by previous studies. Repeated observation of minimum δ34SSO4 values near the interface suggests ‘readdition’ of 34S-depleted sulfate during sulfide oxidation. A slight increase in δ34SSO4 values with depth extended over the water column may indicate a reservoir effect associated with removal of 34S-depleted sulfur during sulfide production through SR. Our δ34SH2S and Δ33SH2S data also do not show a clear role for sulfur-intermediate disproportionation in the deep anoxic water column. We interpret the large difference in δ34S between sulfate and sulfide as reflecting fractionations during SR in the Cariaco deep waters that are larger than those generally observed in culturing studies.  相似文献   

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