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1.
Multiple sulfur isotope system is a powerful new tracer for atmospheric, volcanic, and biological influences on sulfur cycles in the anoxic early Earth. Here, we report high-precision quadruple sulfur isotope analyses (32S/33S/34S/36S) of barite, pyrite in barite, and sulfides in related hydrothermal and igneous rocks occurring in the ca. 3.5 Ga Dresser Formation, Western Australia. Our results indicate that observed isotopic variations are mainly controlled by mixing of mass-dependently (MD) and non-mass-dependently fractionated (non-MD) sulfur reservoirs. Based on the quadruple sulfur isotope systematics (δ34S-Δ33S-Δ36S) for these minerals, four end-member sulfur reservoirs have been recognized: (1) non-MD sulfate (δ34S = −5 ± 2‰; Δ33S = −3 ± 1‰); (2) MD sulfate (δ34S = +10 ± 3‰); (3) non-MD sulfur (δ34S > +6‰; Δ33S > +4‰); and (4) igneous MD sulfur (δ34S = Δ33S = 0‰). The first and third components show a clear non-MD signatures, thus probably represent sulfate and sulfur aerosol inputs. The MD sulfate component (2) is enriched in 34S (+10 ± 3‰) and may have originated from microbial and/or abiotic disproportionation of volcanic S or SO2. Our results reconfirm that the Dresser barites contain small amounts of pyrite depleted in 34S by 15-22‰ relative to the host barite. These barite-pyrite pairs exhibit a mass-dependent relationship of δ33S/δ34S with slope less than 0.512, which is consistent with that expected for microbial sulfate reduction and is significantly different from that of equilibrium fractionation (0.515). The barite-pyrite pairs also show up to 1‰ difference in Δ36S values and steep Δ36S/Δ33S slopes, which deviate from the main Archean array (Δ36S/Δ33S = −0.9) and are comparable to isotope effects exhibited by sulfate reducing microbes (Δ36S/Δ33S = −5 to −11). These new lines of evidence support the existence of sulfate reducers at ca. 3.5 Ga, whereas microbial sulfur disproportionation may have been more limited than recently suggested.  相似文献   

2.
We present multiple sulfur isotope measurements of sulfur compounds associated with the oxidation of H2S and S0 by the anoxygenic phototrophic S-oxidizing bacterium Chlorobium tepidum. Discrimination between 34S and 32S was +1.8 ± 0.5‰ during the oxidation of H2S to S0, and −1.9 ± 0.8‰ during the oxidation of S0 to , consistent with previous studies. The accompanying Δ33S and Δ36S values of sulfide, elemental sulfur, and sulfate formed during these experiments were very small, less than 0.1‰ for Δ33S and 0.9‰ for Δ36S, supporting mass conservation principles. Examination of these isotope effects within a framework of the metabolic pathways for S oxidation suggests that the observed effects are due to the flow of sulfur through the metabolisms, rather than abiotic equilibrium isotope exchange alone, as previously suggested. The metabolic network comparison also indicates that these metabolisms work to express some isotope effects (between sulfide, polysulfides, and elemental sulfur in the periplasm) and suppress others (kinetic isotope effects related to pathways for oxidation of sulfide to sulfate via the same enzymes involved in sulfate reduction acting in reverse). Additionally, utilizing fractionation factors for phototrophic S oxidation calculated from our experiments and for other oxidation processes calculated from the literature (chemotrophic and inorganic S oxidation), we constructed a set of ecosystem-scale sulfur isotope box models to examine the isotopic consequences of including sulfide oxidation pathways in a model system. These models demonstrate how the small δ34S effects associated with S oxidation combined with large δ34S effects associated with sulfate reduction (by SRP) and sulfur disproportionation (by SDP) can produce large (and measurable) effects in the Δ33S of sulfur reservoirs. Specifically, redistribution of material along the pathways for sulfide oxidation diminishes the net isotope effect of SRP and SDP, and can mask the isotopic signal for sulfur disproportionation if significant recycling of S intermediates occurs. We show that the different sulfide oxidation processes produce different isotopic fields for identical proportions of oxidation, and discuss the ecological implications of these results to interpreting minor S isotope patterns in modern systems and in the geologic record.  相似文献   

3.
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   

4.
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   

5.
A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO2 laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO2 laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ34S and δ33S compositions of reference materials measured with a CO2 laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency [Ding et al 2001] and [Taylor]. The precision of replicate analyses of powdered sulfide minerals with the CO2 laser is typically ±0.2‰ (1σ) for δ34S.The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO2 laser. In situ laser craters and drill hole powders give the same δ34SV-CDT and δ33SV-CDT values within 0.2‰. The δ34SV-CDT and δ33SV-CDT values of both powders and in situ analyses are independent of F2 gas pressure over a range of 15 to 65 torr. No dependence of δ34SV-CDT and δ33SV-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios.A range of Δ33S anomalies of from - 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.  相似文献   

6.
Bacterial sulfate reduction is one of the most important respiration processes in anoxic habitats and is often assessed by analyzing the results of stable isotope fractionation. However, stable isotope fractionation is supposed to be influenced by the reduction rate and other parameters, such as temperature. We studied here the mechanistic basics of observed differences in stable isotope fractionation during bacterial sulfate reduction. Batch experiments with four sulfate-reducing strains (Desulfovibrio desulfuricans, Desulfobacca acetoxidans, Desulfonatronovibrio hydrogenovorans, and strain TRM1) were performed. These microorganisms metabolize different carbon sources (lactate, acetate, formate, and toluene) and showed broad variations in their sulfur isotope enrichment factors. We performed a series of experiments on isotope exchange of 18O between residual sulfate and ambient water. Batch experiments were conducted with 18O-enriched (δ18Owater = +700‰) and depleted water (δ18Owater = −40‰), respectively, and the stable 18O isotope shift in the residual sulfate was followed. For Desulfovibrio desulfuricans and Desulfonatronovibrio hydrogenovorans, which are both characterized by low sulfur isotope fractionation (εS > −13.2‰), δ18O values in the remaining sulfate increased by only 50‰ during growth when 18O-enriched water was used for the growth medium. In contrast, with Desulfobacca acetoxidans and strain TRM1 (εS < −22.7‰) the residual sulfate showed an increase of the sulfate δ18O close to the values of the enriched water of +700‰. In the experiments with δ18O-depleted water, the oxygen isotope values in the residual sulfate stayed fairly constant for strains Desulfovibrio desulfuricans, Desulfobacca acetoxidans and Desulfonatronovibrio hydrogenovorans. However, strain TRM1, which exhibits the lowest sulfur isotope fractionation factor (εS < −38.7‰) showed slightly decreasing δ18O values.Our results give strong evidence that the oxygen atoms of sulfate exchange with water during sulfate reduction. However, this neither takes place in the sulfate itself nor during formation of APS (adenosine-5′-phosphosulfate), but rather in intermediates of the sulfate reduction pathway. These may in turn be partially reoxidized to form sulfate. This reoxidation leads to an incorporation of oxygen from water into the “recycled” sulfate changing the overall 18O isotopic composition of the remaining sulfate fraction. Our study shows that such incorporation of 18O is correlated with the stable isotope enrichment factor for sulfur measured during sulfate reduction. The reoxidation of intermediates of the sulfate reduction pathway does also strongly influence the sulfur stable isotope enrichment factor. This aforesaid reoxidation is probably dependent on the metabolic conversion of the substrate and therefore also influences the stable isotope fractionation factor indirectly in a rate dependent manner. However, this effect is only indirect. The sulfur isotope enrichment factors for the kinetic reactions themselves are probably not rate dependent.  相似文献   

7.
Large rounded pyrite grains (>1 mm), commonly referred to as “buckshot” pyrite grains, are a characteristic feature of the auriferous conglomerates (reefs) in the Witwatersrand and Ventersdorp supergroups, Kaapvaal Craton, South Africa. Detailed petrographic analyses of the reefs indicated that the vast majority of the buckshot pyrite grains are of reworked sedimentary origin, i.e., that the pyrite grains originally formed in the sedimentary environment during sedimentation and diagenesis. Forty-one of these reworked sedimentary pyrite grains from the Main, Vaal, Basal, Kalkoenkrans, Beatrix, and Ventersdorp Contact reefs were analyzed for their multiple sulfur isotope compositions (δ34S, Δ33S, and Δ36S) to determine the source of the pyrite sulfur. In addition, five epigenetic pyrite samples (pyrite formed after sedimentation and lithification) from the Middelvlei and the Ventersdorp Contact reefs were measured for comparison. The δ34S, Δ33S, and Δ36S values of all 41 reworked sedimentary pyrite grains indicate clear signatures of mass-dependent and mass-independent fractionation and range from ?6.8 to +13.8?‰, ?1.7 to +1.7?‰, and ?3.9 to +0.9?‰, respectively. In contrast, the five epigenetic pyrite samples display a very limited range of δ34S, Δ33S, and Δ36S values (+0.7 to +4.0?‰, ?0.3 to +0.0?‰. and ?0.3 to +0.1?‰, respectively). Despite the clear signatures of mass-independent sulfur isotope fractionation, very few data points plot along the primary Archean photochemical array suggesting a weak photolytic control over the data set. Instead, other factors command a greater degree of influence such as pyrite paragenesis, the prevailing depositional environment, and non-photolytic sulfur sources. In relation to pyrite paragenesis, reworked syngenetic sedimentary pyrite grains (pyrite originally precipitated along the sediment-water interface) are characterized by negative δ34S and Δ33S values, suggesting open system conditions with respect to sulfate supply and the presence of microbial sulfate reducers. On the contrary, most reworked diagenetic sedimentary pyrite grains (pyrite originally precipitated below the sediment-water interface) show positive δ34S and negative Δ33S values, suggesting closed system conditions. Negligible Δ33S anomalies from epigenetic pyrite suggest that the sulfur was sourced from a mass-dependent or isotopically homogenous metamorphic/hydrothermal fluid. Contrasting sulfur isotope compositions were also observed from different depositional environments, namely fluvial conglomerates and marine-modified fluvial conglomerates. The bulk of the pyrite grains from fluvial conglomerates are characterized by a wide range of δ34S values (?6.2 to +4.8?‰) and small Δ33S values (±0.3?‰). This signature likely represents a crustal sulfate reservoir derived from either volcanic degassing or from weathering of sulfide minerals in the hinterland. Reworked sedimentary pyrite grains from marine-modified fluvial conglomerates share similar isotope compositions, but also produce a positive Δ33S/δ34S array that overlaps with the composition of Archean barite, suggesting the introduction of marine sulfur. These results demonstrate the presence of multiple sources of sulfur, which include atmospheric, crustal, and marine reservoirs. The prevalence of the mass-dependent crustal sulfur isotope signature in fluvial conglomerates suggests that sulfate concentrations were probably much higher in terrestrial settings in comparison to marine environments, which were sulfate-deficient. However, the optimum conditions for forming terrestrial sedimentary pyrite were probably not during fluvial progradation but rather during the early phases of flooding of low angle unconformities, i.e., during retrogradational fluvial deposition, coupled in some cases with marine transgressions, immediately following inflection points of maximum rate of relative sea level fall.  相似文献   

8.
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   

9.
In order to reconstruct paleo-environmental conditions for the saline playa lakes of the Rio Grande Rift, we investigated sediment sulfate sources using sulfur isotope compositions of dissolved ions in modern surface water, groundwater, and precipitated in the form of gypsum sediments deposited during the Pleistocene and Holocene in the Tularosa and Estancia Basins. The major sulfate sources are Lower and Middle Permian marine evaporites (δ34S of 10.9-14.4‰), but the diverse physiography of the Tularosa Basin led to a complex drainage system which contributed sulfates from various sources depending on the climate at the time of sedimentation. As inferred from sulfur isotope mass balance constraints, weathering of sulfides of magmatic/hydrothermal and sedimentary origin associated with climate oscillations during Last Glacial Maximum contributed about 35-50% of the sulfates and led to deposition of gypsum with δ34S values of −1.2‰ to 2.2‰ which are substantially lower than Permian evaporates. In the Estancia Basin, microbial sulfate reduction appears to overprint sulfur isotopic signatures that might elucidate past groundwater flows. A Rayleigh distillation model indicates that about 3-18% of sulfates from an inorganic groundwater pool (δ34S of 12.6-13.8‰) have been metabolized by bacteria and preserved as partially to fully reduced sulfur-bearing minerals species (elemental sulfur, monosulfides, disulfides) with distinctly negative δ34S values (−42.3‰ to −20.3‰) compared to co-existing gypsum (−3.8‰ to 22.4‰). For the Tularosa Basin microbial sulfate reduction had negligible effect on δ34S value of the gypsiferous sediments most likely because of higher annual temperatures (15-33 °C) and lower organic carbon content (median 0.09%) in those sediments leading to more efficient oxidation of H2S and/or smaller rates of sulfate reduction compared to the saline playas of the Estancia Basin (5-28 °C; median 0.46% of organic carbon).The White Sands region of the Tularosa Basin is frequently posited as a hydrothermal analogue for Mars. High temperatures of groundwater (33.3 °C) and high δ18O(H2O) values (1.1‰) in White Sands, however, are controlled predominantly by seasonal evaporation rather than the modern influx of hydrothermal fluids. Nevertheless, it is possible that some of the geochemical processes in White Sands, such as sulfide weathering during climate oscillations and upwelling of highly mineralized waters, might be considered as valid terrestrial analogues for the sulfate cycle in places such as Meridiani Planum on Mars.  相似文献   

10.
We report high precision SIMS oxygen three isotope analyses of 36 chondrules from some of the least equilibrated LL3 chondrites, and find systematic variations in oxygen isotope ratios with chondrule types. FeO-poor (type I) chondrules generally plot along a mass dependent fractionation line (Δ17O ∼ 0.7‰), with δ18O values lower in olivine-rich (IA) than pyroxene-rich (IB) chondrules. Data from FeO-rich (type II) chondrules show a limited range of δ18O and δ17O values at δ18O = 4.5‰, δ17O = 2.9‰, and Δ17O = 0.5‰, which is slightly 16O-enriched relative to bulk LL chondrites (Δ17O ∼ 1.3‰). Data from four chondrules show 16O-rich oxygen isotope ratios that plot near the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Glass analyses in selected chondrules are systematically higher than co-existing minerals in both δ18O and Δ17O values, whereas high-Ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts.Our results suggest that the LL chondrite chondrule-forming region contained two kinds of solid precursors, (1) 16O-poor precursors with Δ17O > 1.6‰ and (2) 16O-rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites. Oxygen isotopes exhibited open system behavior during chondrule formation, and the interaction between the solid and ambient gas might occur as described in the following model. Significant evaporation and recondensation of solid precursors caused a large mass-dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type IA chondrules with higher bulk Mg/Si ratios. Type II chondrules formed under elevated dust/gas ratios and with water ice in the precursors, in which the ambient H2O gas homogenized chondrule melts by isotope exchange. Low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high Δ17O (∼5‰) in LL the parent body. According to our model, oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto-planetary disk and the ambient gas during chondrule melting events.  相似文献   

11.
Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km3 of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H2O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing.Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ34S values of 2.8-14.8‰ with corresponding matrix glass values of 2.4-13.2‰. δ34S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8-5.2 wt% H2O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4-5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ34S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ34S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H2O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23-1.74 wt%, respectively. Values of δD and wt% H2O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ18O values, have elevated δ34S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ34S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).  相似文献   

12.
Sulfur isotope compositions of pumice and adsorbed volatiles on ash from the first historical eruption of Anatahan volcano (Mariana arc) are presented in order to constrain the sources of sulfur erupted during the period 10-21 May, 2003. The isotopic composition of S extracted from erupted pumice has a narrow range, from δ34SV-CDT +2.6‰ to +3.2‰, while the composition of sulfur adsorbed onto ash has a larger range (+2.8‰ to +5.3‰). Fractionation modeling for closed and open system scenarios suggests that degassing of SO2 raised the δ34SV-CDT value of S dissolved in the melt from an initial composition of between +1.6‰ and +2.6‰ for closed-system degassing, or between −0.5‰ and +1.5‰ for open-system degassing, however closed-system degassing is the preferred model. The calculated values for the initial composition of the magma represent a MORB-like (δ34SV-CDT ∼ 0‰) mantle source with limited contamination by subducted seawater sulfate (δ34SV-CDT +21‰). Modeling also suggests that the δ34SV-CDT value of SO2 gas in closed-system equilibrium with the degassed magma was between +0.9‰ and +2.5‰. The δ34SV-CDT value of sulfate adsorbed onto ash in the eruption plume (+2.8‰ to +5.1‰) is consistent with sulfate formation by oxidation of magmatic SO2 in the eruption column. The sulfur isotope composition of sulfate adsorbed to ash changes from lower δ34S values for ash erupted early in the eruption to higher δ34S values for ash erupted later in the eruption. We interpret the temporal/stratigraphic change in sulfate isotopic composition to primarily reflect a change in the isotopic composition of magmatic SO2 released from the progressively degassing magma and is attributed to the expulsion of an accumulated gas phase at the beginning of the eruption. More efficient oxidation of magmatic SO2 gas to sulfate in the early water-rich eruption plume probably contributed to the change in S isotope compositions observed in the ash leachates.  相似文献   

13.
Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with εHf(t) =  ?2.47 to ?1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have εHf(t) =  ?5.40 to ?0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between ?1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of ?0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S ~ 0‰, and Δ36S ~ 0‰. A carbonaceous schist shows positive δ34S (2.1‰–3.5‰) and elevated Δ33S (1.2‰–1.4‰) values, with corresponding negative Δ36S between ?1.2‰ to ?0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.  相似文献   

14.
We report sulfur isotope anomalies with Δ33S, the deviation from a mass-dependent fractionation line for the three-isotope system (34S/32S vs. 33S/32S), ranging up to ±2‰ within individual Archean sedimentary sulfides from a variety of localities. Our measurements, which are made in situ by multicollector secondary ion mass spectrometry, unequivocally corroborate prior bulk measurements of mass-independent fractionations (MIF) in sulfur and provide additional evidence for an anoxic atmosphere on the Earth before ∼2 Ga. This technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record. The presence of MIF sulfur in sulfides from a >3.8-Ga Fe-rich quartzite from Akilia (island), West Greenland, is consistent with a marine sedimentary origin for this rock.  相似文献   

15.
Sulfur isotope studies of post-Archean terrestrial materials have focused on the ratio 34S/32S because additional isotopes, 33S and 36S, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Δ33S and Δ36S values, defined as deviations of 33S and 36S from ideal mass-dependent relationships, for international reference materials and sedimentary sulfides of Phanerozoic age by using a fluorination technique with a dual-inlet isotope ratio mass spectrometer. Measured variations in Δ33S and Δ36S are explained as resulting from processes involve branching reactions (two or more reservoirs formed) or mixing. Irreversible processes in closed systems (Rayleigh distillation) amplify the isotope effect. We outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle, including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation. The isotope systematics discussed here cannot explain the much larger variation of Δ33S and Δ36S observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Δ33S and Δ36S relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Δ33S and Δ36S can be used to distinguish small non-zero Δ33S and Δ36S produced by mass-dependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials.  相似文献   

16.
Sulfur isotope effects produced by microbial dissimilatory sulfate reduction are used to reconstruct the coupled cycling of carbon and sulfur through geologic time, to constrain the evolution of sulfur-based metabolisms, and to track the oxygenation of Earth’s surface. In this study, we investigate how the coupling of carbon and sulfur metabolisms in batch and continuous cultures of a recently isolated marine sulfate reducing bacterium DMSS-1, a Desulfovibrio sp., influences the fractionation of sulfur isotopes.DMSS-1 grown in batch culture on seven different electron donors (ethanol, glycerol, fructose, glucose, lactate, malate and pyruvate) fractionates 34S/32S ratio from 6‰ to 44‰, demonstrating that the fractionations by an actively growing culture of a single incomplete oxidizing sulfate reducing microbe can span almost the entire range of previously reported values in defined cultures. The magnitude of isotope effect correlates well with cell specific sulfate reduction rates (from 0.7 to 26.1 fmol/cell/day). DMSS-1 grown on lactate in continuous culture produces a larger isotope effect (21-37‰) than the lactate-grown batch culture (6‰), indicating that the isotope effect also depends on the supply rate of the electron donor and microbial growth rate. The largest isotope effect in continuous culture is accompanied by measurable changes in cell length and cellular yield that suggest starvation. The use of multiple sulfur isotopes in the model of metabolic fluxes of sulfur shows that the loss of sulfate from the cell and the intracellular reoxidation of reduced sulfur species contribute to the increase in isotope effects in a correlated manner. Isotope fractionations produced during sulfate reduction in the pure culture of DMSS-1 expand the previously reported range of triple sulfur isotope effects (32S, 33S, and 34S) by marine sulfate reducing bacteria, implying that microbial sulfur disproportionation may have a smaller 33S isotopic fingerprint than previously thought.  相似文献   

17.
This study investigates the sulfur and oxygen isotope fractionations of dissimilatory sulfate reduction and works to reconcile the relationships between the oxygen and sulfur isotopic and elemental systems. We report results of experiments with natural populations of sulfate-reducing bacteria using sediment and seawater from a marine lagoon at Fællestrand on the northern shore of the island of Fyn, Denmark. The experiments yielded relatively large magnitude sulfur isotope fractionations for dissimilatory sulfate reduction (up to approximately 45‰ for 34S/32S) with higher δ18O accompanying higher δ34S, similar to that observed in previous studies. The seawater used in the experiments was spiked by addition of 17O-labeled water and the 17O content of residual sulfate was found to depend on the fraction of sulfate reduced in the experiments. The 17O data provides evidence for recycling of sulfur from metabolic intermediates and for an 18O/16O fractionation of ∼25-30‰ for dissimilatory sulfate reduction. The close correlation between the 17O data and the sulfur isotope data suggests that isotopic exchange between cell water and external water (reactor water) was rapid under experimental conditions. The molar ratio of oxygen exchange to sulfate reduction was found to be about 2.5. This value is slightly lower than observed in studies of natural ecosystems [e.g., Wortmann U. G., Chernyavsky B., Bernasconi S. M., Brunner B., Böttcher M. E. and Swart P. K. (2007) Oxygen isotope biogeochemistry of pore water sulfate in the deep biosphere: dominance of isotope exchange reactions with ambient water during microbial sulfate reduction (ODP Site 1130). Geochim. Cosmochim. Acta71, 4221-4232]. Using recent models of sulfur isotope fractionations we find that our combined sulfur and oxygen isotopic data places constraints on the proportion of sulfate recycled to the medium (78-96%), the proportion of sulfur intermediate sulfite that was recycled by way of APS to sulfate and released back to the external sulfate pool (∼70%), and also that a fraction of the sulfur intermediates between sulfite and sulfide were recycled to sulfate. These parameters can be constrained because of the independent information provided by δ18O, δ34S, δ17O labels, and Δ33S.  相似文献   

18.
The oxygen three-isotope systematics of 36 chondrules from the Allende CV3 chondrite are reported using high precision secondary ion mass spectrometer (CAMECA IMS-1280). Twenty-six chondrules have shown internally homogenous Δ17O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ17O values of 19 FeO-poor chondrules (13 porphyritic chondrules, 2 barred olivine chondrules, and 4 chondrule fragments) show a peak at −5.3 ± 0.6‰ (2SD). Another 5 porphyritic chondrules including both FeO-poor and FeO-rich ones show average Δ17O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ17O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ17O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ17O values as low as −18‰, indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions. However, most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules. The Δ17O values of four barred olivine chondrules range from −5‰ to 0‰, indicating that not all BO chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses. Based on these data, we suggest the presence of multiple oxygen isotope reservoirs in local dust-rich protoplanetary disk, from which the CV3 parent asteroid formed.A compilation of 225 olivine and low-Ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines. These data define a correlation line of δ17O = (0.982 ± 0.019) × δ18O − (2.91 ± 0.10), which is similar to those defined by chondrules in CV3 chondrites and Acfer 094 in previous studies. Plagioclase analyses in two chondrules plot slightly below the CCAM line with Δ17O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.  相似文献   

19.
δ34S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a 33S-36S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ34S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ18O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months.The δ34S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ34S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ34S and concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ34S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard.  相似文献   

20.
We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m−2) is present as sulfates (2.5 kmol m−2), and to a lesser extent carbonates (0.4 kmol m−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear correlation among δ44/40Ca, δ34S and δ18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ34S values are positively correlated with sulfate-δ18O values (R2 = 0.78) and negatively correlated with sulfate δ44/40CaE values (R2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ44/40Ca fractionation factor of −0.4‰ in CaSO4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.  相似文献   

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