Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

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 δ¹⁸O_H2O 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 δ³⁴S_SO4 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 (ε¹⁸O_SO4-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 δ¹⁸O_SO4 values in the biological and abiotic experiments, it is suggested that δ¹⁸O_SO4 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.     

Laboratory chalcopyrite oxidation by Acidithiobacillus ferrooxidans: Oxygen and sulfur isotope fractionation

Laboratory experiments were conducted to simulate chalcopyrite oxidation under anaerobic and aerobic conditions in the absence or presence of the bacterium Acidithiobacillus ferrooxidans. Experiments were carried out with 3 different oxygen isotope values of water (δ¹⁸O_H2O) so that approach to equilibrium or steady-state isotope fractionation for different starting conditions could be evaluated. The contribution of dissolved O₂ and water-derived oxygen to dissolved sulfate formed by chalcopyrite oxidation was unambiguously resolved during the aerobic experiments. Aerobic oxidation of chalcopyrite showed 93 ± 1% incorporation of water oxygen into the resulting sulfate during the biological experiments. Anaerobic experiments showed similar percentages of water oxygen incorporation into sulfate, but were more variable. The experiments also allowed determination of sulfate–water oxygen isotope fractionation, ε¹⁸O_SO4–H2O, of ~ 3.8‰ for the anaerobic experiments. Aerobic oxidation produced apparent ε_SO4–H2O values (6.4‰) higher than the anaerobic experiments, possibly due to additional incorporation of dissolved O₂ into sulfate. δ³⁴S_SO4 values are ~ 4‰ lower than the parent sulfide mineral during anaerobic oxidation of chalcopyrite, with no significant difference between abiotic and biological processes. For the aerobic experiments, a small depletion in δ³⁴S_SO4 of ~? 1.5 ± 0.2‰ was observed for the biological experiments. Fewer solids precipitated during oxidation under aerobic conditions than under anaerobic conditions, which may account for the observed differences in sulfur isotope fractionation under these contrasting conditions.     

Pyritization processes and greigite formation in the advancing sulfidization front in the upper Pleistocene sediments of the Black Sea

Pyritization in late Pleistocene sediments of the Black Sea is driven by sulfide formed during anaerobic methane oxidation. A sulfidization front is formed by the opposing gradients of sulfide and dissolved iron. The sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content. Two processes of diffusion-limited pyrite formation were identified. The first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of FeS_n (n = 1.10-1.29), including greigite. Elemental sulfur and polysulfides, formed from H₂S by a reductive dissolution of Fe(III)-containing minerals, serve as intermediates to convert iron sulfides into pyrite. In the second process, a “direct” pyrite precipitation occurs through prolonged exposure of iron-containing minerals to dissolved sulfide. Methane-driven sulfate reduction at depth causes a progressive formation of pyrite with a δ³⁴S of up to +15.0‰. The S-isotopic composition of FeS₂ evolves due to contributions of different sulfur pools formed at different times. Steady-state model calculations for the advancement of the sulfidization front showed that the process started at the Pleistocene/Holocene transition between 6360 and 11 600 yr BP. Our study highlights the importance of anaerobic methane oxidation in generating and maintaining S-enriched layers in marine sediments and has paleoenvironmental implications.     

Triple-oxygen-isotope determination of molecular oxygen incorporation in sulfate produced during abiotic pyrite oxidation (pH = 2-11)

Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe²⁺-Fe³⁺ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ¹⁸O-labeled experiments show that a variable fraction (0-60%) of the oxygen in product sulfate is derived from dissolved O₂, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O₂% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ¹⁸O label to differentiate sulfate-oxygen sources. To examine the role of O₂ and Fe³⁺ in determining the final incorporation of O₂ oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH = 2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe³⁺ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe³⁺ addition help to determine the oxygen sources in sulfate and examine the role of Fe²⁺-Fe³⁺ electron shuttling during sulfide oxidation, respectively.Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe³⁺. Significant amounts of sulfite and thiosulfate were detected in pH ? 9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH ? 9, product sulfate contained 21-24% air O₂ signal, similar to pH 2 with Fe³⁺ addition. Sulfate from the pH 2 reactor without Fe³⁺ addition and the pH 7 reactors all showed 28-29% O₂ signal. While the O₂% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O₂ does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite-water exchange (e.g. the pH 2 with no Fe³⁺ addition and both pH 7 reactors); and (2) direct O₂ oxidation dominates while Fe³⁺ shuttling is still competitive in the sulfite-sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe³⁺ addition). Overall, the final sulfate-oxygen source ratio is determined by (1) rate competitions between direct O₂ incorporation and Fe³⁺ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation.     

Isotope geochemistry of waters affected by mining activities in Sierra Minera and Portman Bay (SE,Spain)

The objective of this work was to evaluate processes affecting waters from Portman Bay by way of stable isotopic analysis, particularly H and O stable isotopes from water and S and O from dissolved sulfates. In addition, surface waters from Sierra Minera were examined for the purpose of determining if these waters are affected by similar processes. The results obtained indicate that Portman Bay waters are meteoric, and marine infiltration only takes place in the deepest layers near the shore or if water remains stagnated in sediments with low permeability. The main source of sulfate was the oxidation of sulfides, resulting in the liberation of acid, sulfate and metals. In order to assess the mechanism responsible for sulfide oxidation, the stoichiometric isotope balance model and the general isotope balance model were tested, suggesting that the oxidation via Fe³⁺ was predominant in the surface, and controlled by Acidithiobacillus ferrooxidans, while at depth, sulfate reduction occurred.     

O and S isotopic composition of dissolved and attached oxidation products of pyrite by Acidithiobacillus ferrooxidans: Comparison with abiotic oxidations

The acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, plays a part in the pyrite oxidation process and has been widely studied in order to determine the kinetics of the reactions and the isotopic composition of dissolved product sulphates, but the details of the oxidation processes at the surface of pyrite are still poorly known. In this study, oxygen and sulphur isotopic compositions (δ¹⁸O and δ³⁴S) were analyzed for dissolved sulphates and water from experimental aerobic acidic (pH < 2) pyrite oxidation by A. ferrooxidans. The oxidation products attached to the pyrite surfaces were studied for their morphology (SEM), their chemistry (Raman spectroscopy) and for their δ¹⁸O (ion microprobe). They were compared to abiotically (Fe³⁺, H₂O₂, O₂) oxidized pyrite surface compounds in order to constrain the oxidation pathways and to look for the existence of potential biosignatures for this system.The pyrite dissolution evolved from non-stoichiometric (during the first days) to stoichiometric (with increasing time) resulting in dissolved sulphates having distinct δ¹⁸O (e.g. +11.0‰ and −2.0‰, respectively) and δ³⁴S (+4.5‰ and +2.8‰, respectively) values. The “oxidation layer” at the surface of pyrite is complex and made of iron oxides, sulphate, polysulphide, elemental sulphur and polythionates. Bio- and Fe³⁺-oxidation favour the development of monophased micrometric bumps made of hematite or sulphate while other abiotic oxidation processes result in more variable oxidation products. The δ¹⁸O of these oxidation products at the surface of oxidized pyrites are strongly variable (from ≈−40‰ to ≈+30‰) for all experiments.Isotopic fractionation between sulphates and pyrite, Δ³⁴S_SO4-pyrite, is equal to −1.3‰ and +0.4‰ for sulphates formed by stoichiometric and non-stoichiometric processes, respectively. These two values likely reflect either a S-S or a Fe-S bond breaking process. The Δ¹⁸O_SO4-H2O and Δ¹⁸O_SO4-O2 are estimated to be ≈+16‰ and ≈−25‰, respectively. These values are higher than previously published data and may reflect biological effects. The large δ¹⁸O heterogeneity measured at the surfaces of oxidized pyrites, whatever the oxidant, may be related (i) to the existence of local surface environments isolated from the solution in which the oxidation processes are different and (ii) to the stabilization at the pyrite surface of reaction intermediates that are not in isotopic equilibrium with the solution. Though the oxygen isotopic composition of surface oxidation products cannot be taken as a direct biosignature, the combined morphological, chemical and isotopic characterization of the surfaces of oxidized pyrites may furnish clues about a biological activity on a mineral surface.     

Oxidation of pyrite during extraction of carbonate associated sulfate

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 δ³⁴S_pyrite 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.     

Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides,elemental sulfur,and pyrite

Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.     

Sulfur biogeochemistry and isotopic fractionation in shallow groundwater and sediments of Owens Dry Lake, California

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.     

Mineralogic and sulfur isotopic effects accompanying oxidation of pyrite in millimolar solutions of hydrogen peroxide at temperatures from 4 to 150 °C

Oxidation of pyrite by hydrogen peroxide (H₂O₂) at millimolar levels has been studied from 4 to 150 °C in order to evaluate isotopic effects potentially associated with radiolytic oxidation of pyrite. Gaseous, aqueous, and solid phases were collected and measured following sealed-tube experiments that lasted from 1 to 14 days. The dominant gaseous product was molecular oxygen. No volatile sulfur species were recovered from any experiment. Sulfate was the only aqueous sulfur species detected in solution, with sulfite and thiosulfate below the detection limits. X-ray diffraction patterns and images from scanning electron microscopy reveal solid residues composed primarily of hydrated ferric iron sulfates and sporadic ferric-ferrous iron sulfates. Hematite was detected only in solid residue produced during high temperature experiments. Elemental sulfur and/or polysulfides are inferred to be form on reacting pyrite surface based on extraction with organic solvents. Pyrite oxidation by H₂O₂ increases in rate with increasing H₂O₂concentration, pyrite surface area, and temperature. Rates measured in sealed-tube experiments at 25°C, for H₂O₂ concentration of 2 × 10⁻³ M are 8.8 × 10⁻⁹ M/m²/sec, which are higher than previous estimates. A combination of reactive oxygen species from H₂O₂ decomposition products and reactive iron species from pyrite dissolution is inferred to aggressively oxidize the receding pyrite surface. Competing oxidants with temperature-dependent oxidation efficiencies results in multiple reaction mechanisms for different temperatures and surface conditions. Sulfur isotope values of remaining pyrite were unchanged during the experiments, but showed distinct enrichment of ³⁴S in produced sulfate and depletion in elemental sulfur. The Δ_{sulfate-pyrite} and Δ_{elemental sulfur-pyrite} was +0.5 to +1.5‰ and was −0.2 to −1‰, respectively. Isotope data from high-temperature experiments indicate an additional ³⁴S-depleted sulfur fraction, with up to 4‰ depletion of ³⁴S, in the hematite. Sulfur isotope trends were not influenced by H₂O₂ concentration, temperature, or reaction time. Results of this study indicate that radiolytically produced oxidants, such as hydrogen peroxide and hydroxyl radicals, could efficiently oxidize pyrite in an otherwise oxygen-limited environment. Although H₂O₂ is generally regarded as being of minor geochemical significance on Earth, the H₂O₂ molecule plays a pivotal role in Martian atmospheric and soil chemistry. Additional experimental and field studies are needed to characterize sulfur and oxygen isotope systematics during radiolytical oxidation of metallic sulfides and elemental sulfur.     

Biogeochemistry of sulfur and iron in Thioploca-colonized surface sediments in the upwelling area off central chile

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.     

Pyrite oxidation by Acidithiobacillus ferrooxidans at various concentrations of dissolved oxygen

Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10^− 10 mol m^− 2 s^− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.     

Oxidation of pyrite in low temperature acidic solutions: Rate laws and surface textures

Rate laws have been determined for the aqueous oxidation of pyrite by ferric ion, dissolved oxygen and hydrogen peroxide at 30°C in dilute, acidic chloride solutions. Fresh, smooth pyrite grain surfaces were prepared by cleaning prior to experiments. Initial specific surface areas were measured by the multipoint BET technique. Surface textures before and after oxidation were examined by SEM. The initial rate method was used to derive rate laws.The specific initial rates of oxidation (moles pyrite cm⁻² min⁻¹) are given by the following rate laws (concentrations in molar units): r_sp,Fe³⁺ = −10^−9.74M^0.5_Fe³+M^−0.5_H+ (pH 1–2)r_sp,_o2 = −10^−6.77M^0.5_O2 (pH 2–4)r_sp,_h2o2 = −10^−1.43M_H2O2 (pH 2−4)An activation energy of 56.9 ± 7.5 kJ mole⁻¹ was determined for the oxidation of pyrite by dissolved oxygen from 20–40°C. HPLC analyses indicated that only minor amounts of polythionates are detectable as products of oxidation by oxygen below pH 4; the major sulfur product is sulfate. Ferric ion and sulfate are the only detectable products of pyrite oxidation by hydrogen peroxide. Hydrogen peroxide is consumed by catalytic decomposition nearly as fast as it is by pyrite oxidation.SEM photomicrographs of cleaned pyrite surfaces indicate that prior to oxidation, substantial intergranular variations in surface texture exist. Reactive surface area is substantially different than total surface area. Oxidation is centered on reactive sites of high excess surface energy such as grain edges and corners, defects, solid and fluid inclusion pits, cleavages and fractures. These reactive sites are both inherited from mineral growth history and applied by grain preparation techniques. The geometry and variation of reactive sites suggests that the common assumption of a first-order, reproducible dependence of oxidation rates on surface area needs to be tested.     

Pyrite Surface after ThiobaciUusferrooxidans Leaching at 30℃

In order to investigate the effect of Thiobacillusferrooxidans on the oxidation of pyrite, two parallel experiments, which employed H2SO4 solutions and acidic solutions inoculated with ThiobaciUus ferrooxidans, were designed and carried out at 30℃. The initial pH of the two solutions was adjusted to 2.5 by dropwise addition of concentrated sulphuric acid. The surfaces of pyrite before exposure to leaching solutions and after exposure to the H2SO4 solutions and acidic solutions inoculated with Thiobacillus ferrooxidans were observed by scanning electron microscopy （SEM）. There were a variety of erosion patterns by Thiobacillusferrooxidans on the bio-leached pyrite surfaces. A conclusion can be drawn that the oxidation of pyrite might have been caused by erosion of the surfaces. Attachment of the bacteria to pyrite surfaces resulted in erosion pits, leading to the oxidation of pyrite. It is possible that the direct mechanism plays the most important role in the oxidation of pyrite. The changes in iron ion concentrations of both the experimental solutions with time suggest that ThiobaciUus ferrooxidans can enhance greatly the oxidation of pyrite.     

Fractionation of sulfur isotopes during laboratory synthesis of pyrite at low temperatures

The reaction products and the accompanying sulfur isotope fractionations during the reaction of H₂S with goethite in aqueous media at 22–24°C for periods from 0.5 hr to 65 days were studied. Fine-grained pyrite formed within two days and was isotopically 0.8‰ lighter than the H₂S source. After 65 days reaction time the pyrite had nearly the same isotopic value as the H₂S. Aqueous precipitation of pyrite from H₂S and goethite at room temperature involved three major steps, namely: (1) the rapid oxidation of H₂S and reduction of Fe³⁺ during which elemental S is formed; (2) the formation of acid-volatile sulfides and the disappearance of elemental S; and (3) the formation of pyrite at the expense of acid-volatile sulfides.     

Effects of phospholipid on pyrite oxidation in the presence of autotrophic and heterotrophic bacteria

Pyrite oxidation occurring in solutions containing iron oxidizing autotrophic bacteria, Acidithiobacillus ferrooxidans (A. ferrooxidans), and/or heterotrophic bacteria, Acidiphilium acidophilum (A. acidophilum), has been investigated. Under the conditions used, the amount of pyrite oxidized in the presence of both species was similar to the amount oxidized in the presence of A. ferrooxidans alone over a period of 30 days. Pretreatment of pyrite with the phospholipid, [1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (23:2 Diyne PC)], to form an adsorbed organic layer reduced the amount of pyrite oxidation in the absence of bacteria and in the presence of A. ferrooxidans. The addition of lipid to pyrite prior to its exposure to a mixed A. ferrooxidans/A. acidophilum solution also showed initial oxidation suppression. However, after 4-5 days the effectiveness of the lipid in suppressing pyrite oxidation was lost and oxidation of the mineral proceeded at a rate that was similar to lipid-free pyrite in the presence of both microbial populations. If, however, lipid/pyrite was pretreated with UV radiation to induce cross-linking of the lipid tails (via polymerization of diacetylene groups in the tails), the lipid layer showed a strong suppression of pyrite oxidation for up to at least 30 days in the presence of both microbial populations. It was also shown with in situ atomic force microscopy (AFM) that the introduction of lipid to pyrite with colonized A. ferrooxidans led to the displacement of a fraction of surface bound bacteria. This lipid-induced displacement was confirmed by ex situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).     

Different types of methane monooxygenases produce similar carbon and hydrogen isotope fractionation patterns during methane oxidation

We determined the stable carbon and hydrogen isotope fractionation factors for methane oxidation under oxic conditions using strains with known degradation pathways. The aerobic oxidation of methane can be initiated by two different forms of enzymes known as methane monooxygenases (MMO). The expression of these enzymes is type-specific and dependent upon the adjusted copper concentration in the medium (or environment). In this study, the expression of either the soluble MMO or the particulate MMO was supported by adjusting the copper concentrations in the growth medium. Taxonomically different aerobic methanotrophic strains, mainly belonging to the alpha- and gamma- classes of Proteobacteria, produced methane isotope enrichment factors (ε_bulk) ranging from −14.8 to −27.9‰ for carbon, and from −110.0 to −231.5‰ for hydrogen. The ratio of hydrogen versus carbon discrimination (Λ = (α_H⁻¹ − 1)/(α_C⁻¹ − 1) ≈ Δ(δ²H)/Δ(δ¹³C)) were similar for all tested cultures, and are also identical to values calculated from previously published enrichment factors for aerobic and anaerobic methane degradation. In contrast, Λ-values for the abiotic oxidation of methane with OH radicals (this process is considered as the main removal process for methane from the atmosphere) were significantly higher than the values derived from biotic oxidation. Due to the low variability of microbial methane isotope fractionation patterns, we propose that combined carbon and hydrogen isotope fractionation analyses can be used to monitor and assess the occurrence of microbial methane oxidation in marine or terrestrial environments. However, it is not possible to distinguish distinct aerobic or anaerobic methane-oxidation pathways by this approach.     

Chemical influences on trace metal-sulfide interactions in anoxic sediments

Interactions of trace metals with sulfide in anoxic environments are important in determining their chemical form and potential toxicity to organisms. In recent years, a considerable body of observational data has accumulated that indicates very different behavior for various trace metals in sulfidic sediments. These differences in behavior cannot be entirely attributed to thermodynamic relationships, but also reflect differences in ligand exchange reaction kinetics, and redox reaction pathways.Pb, Zn, and Cd, which are generally pyritized to only a few percent of the “reactive” fraction, have faster water exchange reaction kinetics than Fe²⁺, resulting in MeS phases precipitating prior to FeS formation and subsequent pyrite formation, whereas, Co and Ni, which have slower H₂O exchange kinetics than Fe²⁺, are incorporated into pyrite. Although Hg and Cu have faster reaction kinetics than Fe²⁺, both are incorporated into pyrite or leached from the pyrite fraction with nitric acid. Hg undergoes significant chloride complexation, which can retard reaction with sulfide, but can also replace Fe in FeS to form HgS, which can only be dissolved in the pyrite fraction. Cu²⁺ is reduced by sulfide and forms a variety of sulfides with and without Fe that can only be dissolved with nitric acid. Mn²⁺ does not form a MnS phase easily and is incorporated into pyrite at high iron degrees of pyritization (DOP).Oxyanions of Mo and As are first reduced by sulfide. These reduced forms may then react with sulfides resulting in incorporation into pyrite. However, the oxyanion of Cr is reduced to Cr³⁺, which is kinetically inert to reaction with sulfide and, therefore, not incorporated into pyrite.     

S-33 constraints on the seawater sulfate contribution in modern seafloor hydrothermal vent sulfides

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.     

Carbon, sulfur, and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the Ediacaran Hankalchough glaciation

In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ¹³C_carb), carbonate oxygen isotopes (δ¹⁸O_carb), carbonate associated sulfate sulfur (δ³⁴S_CAS) and oxygen isotopes (δ¹⁸O_CAS), and pyrite sulfur isotopes (δ³⁴S_py) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore-offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ¹³C_carb values decrease from −2‰ to −16‰ (VPDB), whereas δ¹⁸O_carb values increase from −4‰ to −1‰ (VPDB). Both δ³⁴S_CAS and δ³⁴S_py show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ¹⁸O_CAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ¹³C_carb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in ¹³C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation.