Oxygen Isotope Composition Of Sulfate Research Articles

Calibrating the triple oxygen isotope composition of evaporite minerals as a proxy for marine sulfate

The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric pO2/pCO2 and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Δ'17O) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Δ'17O and δ18O compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate.

Early diagenesis of sulfur in Bornholm Basin sediments: The role of upward diffusion of isotopically “heavy” sulfide

Sediment-hosted marine sulfur cycling has played a significant role in regulating Earth’s surface chemistry over our planet’s history. Microbially-mediated reactions involving sulfur are often accompanied by sulfur isotope fractionation that, in turn, is captured by sulfate and sulfide minerals, providing the opportunity to track changes in the microbial utilization of sulfur and thus the marine sulfur cycle. Studying sulfur diagenesis within the Bornholm Basin, Baltic Sea, we explore the interplay between carbon, sulfur and iron, focusing on the fate of sulfur and the dynamics of the sulfur and oxygen isotopic response as a function of the varying thickness of the organic carbon-rich Holocene Mud Layer (HML) across the basin. Using a one-dimensional reaction-transport model, porewater sulfate and sulfide profiles were used to calculate net sulfate reduction rates (SRR) and net sulfide production rates, respectively. These calculations suggest a positive relationship between the thickness of the HML and net rates of sulfate reduction and sulfide production. Given that ascending sulfide is enriched in 34S relative to that produced in-situ, a heightened sulfide flux promotes spatially variable precipitation of 34S-enriched pyrite (δ34S ≈ −10‰) close to the sediment–water interface. Modeling results indicate that this isotopically “heavy” sulfide is formed as a consequence of mixing between ascending sulfide (up to +6.3‰) and that produced in-situ (ca. −40‰). Further, we show that the sulfur and oxygen isotopic composition of porewater sulfate is controlled by the net SRR: when the net SRR is high (i.e., in sulfide-replete settings) the downcore increase in δ18OSO4 is dampened relative to increase in δ34SSO4, whereas when net SRR is low (i.e., in iron-rich parts of the basin) downcore δ18OSO4 values increase while δ34SSO4 values remain invariant. We conclude that sedimentation rates and open system diffusion strongly influence the distribution of sulfur species and their sulfur isotopic composition, as well as the oxygen isotopic composition of sulfate, through the interaction between iron, sulfur and methane. This work highlights the importance of considering diffusion to better understand open system diagenesis and the δ34S signatures of sulfate and sulfide in both modern settings and ancient rocks.

Optimizing sulfate pyrolysis triple oxygen isotope analysis for samples from desert environments.

The triple oxygen isotope composition of sulfate may reveal the formation pathway and depositional sources and may indicate slow biologic cycling in the environment. Pyrolysis mass spectrometry is better suited for large sample workloads during environmental profiling but sufficient precision and a thorough verification of accuracy are required for comparison with higher precision laser fluorination data. Quantitative sulfate extraction from soil samples at neutral pH, purification, conversation into Ag-sulfate, and pyrolysis mass spectrometry were modified for high sample throughput. Samples were analyzed after pyrolysis in quartz cups and gold capsules in a modified EuroVector model 3000 elemental analyzer. Sample O2 was measured in continuous He-flow after purification by cryo-trapping and chromatography on a Thermo Finnigan MAT253 isotope ratio mass spectrometer. A protocol for routine quality control and data normalization ensures long-term accuracy of the pyrolysis method. The 1σ SD external reproducibility is 0.12‰ for Δ17 OSO4 values on 30 μmol samples. Careful normalization for a daily analytical session accounts for changing pyrolysis conditions over the course of multiple sessions. The precision and accuracy obtained with quartz cups are comparable with those obtained with gold capsules. Pyrolysis and fluorination data for in-house standards from four laboratories and from an Atacama Desert gypsum-soil profile are identical and demonstrate the accuracy of our simplified method. Pyrolysis of sulfate in quartz cups and a modified simple elemental analyzer setup allows for accurate, precise, fast, cost-efficient, and non-hazardous mass spectrometric analysis. Exchangeability of data from pyrolysis and laser fluorination methods was demonstrated by repeat analysis of standards and natural samples despite high contents of interfering, easily soluble nitrates and chlorides.

Modelling the Effects of Non-Steady State Transport Dynamics on the Sulfur and Oxygen Isotope Composition of Sulfate in Sedimentary Pore Fluids

We present the results of an isotope-enabled reactive transport model of a sediment column undergoing active microbial sulfate reduction to explore the response of the sulfur and oxygen isotopic composition of sulfate under perturbations to steady state. In particular, we test how perturbations to steady state influence the cross plot of δ34S and δ18O for sulfate. The slope of the apparent linear phase (SALP) in the cross plot of δ34S and δ18O for sulfate has been used to infer the mechanism, or metabolic rate, of microbial metabolism, making it important that we understand how transient changes might influence this slope. Tested perturbations include changes in boundary conditions and changes in the rate of microbial sulfate reduction in the sediment. Our results suggest that perturbations to steady state influence the pore fluid concentration of sulfate and the δ34S and δ18O of sulfate but have a minimal effect on SALP. Furthermore, we demonstrate that a constant advective flux in the sediment column has no measurable effect on SALP. We conclude that changes in the SALP after a perturbation are not analytically resolvable after the first 5% of the total equilibration time. This suggests that in sedimentary environments the SALP can be interpreted in terms of microbial metabolism and not in terms of environmental parameters.

Tetrathionate and Elemental Sulfur Shape the Isotope Composition of Sulfate in Acid Mine Drainage.

Sulfur compounds in intermediate valence states, for example elemental sulfur, thiosulfate, and tetrathionate, are important players in the biogeochemical sulfur cycle. However, key understanding about the pathways of oxidation involving mixed-valance state sulfur species is still missing. Here we report the sulfur and oxygen isotope fractionation effects during the oxidation of tetrathionate (S4O62−) and elemental sulfur (S°) to sulfate in bacterial cultures in acidic conditions. Oxidation of tetrathionate by Acidithiobacillus thiooxidans produced thiosulfate, elemental sulfur and sulfate. Up to 34% of the tetrathionate consumed by the bacteria could not be accounted for in sulfate or other intermediate-valence state sulfur species over the experiments. The oxidation of tetrathionate yielded sulfate that was initially enriched in 34S (ε34SSO4−S4O6) by +7.9‰, followed by a decrease to +1.4‰ over the experiment duration, with an average ε34SSO4−S4O6 of +3.5 ± 0.2‰ after a month of incubation. We attribute this significant sulfur isotope fractionation to enzymatic disproportionation reactions occurring during tetrathionate decomposition, and to the incomplete transformation of tetrathionate into sulfate. The oxygen isotope composition of sulfate (δ18OSO4) from the tetrathionate oxidation experiments indicate that 62% of the oxygen in the formed sulfate was derived from water. The remaining 38% of the oxygen was either inherited from the supplied tetrathionate, or supplied from dissolved atmospheric oxygen (O2). During the oxidation of elemental sulfur, the product sulfate became depleted in 34S between −1.8 and 0‰ relative to the elemental sulfur with an average for ε34SSO4−S0 of −0.9 ± 0.2‰ and all the oxygen atoms in the sulfate derived from water with an average normal oxygen isotope fractionation (ε18OSO4−H2O) of −4.4‰. The differences observed in δ18OSO4 and the sulfur isotope composition of sulfate (δ34SSO4), acid production, and mixed valence state sulfur species generated by the oxidation of the two different substrates suggests a metabolic flexibility in response to sulfur substrate availability. Our results demonstrate that microbial processing of mixed-valence-state sulfur species generates a significant sulfur isotope fractionation in acidic environments and oxidation of mixed-valence state sulfur species may produce sulfate with characteristic sulfur and oxygen isotope signatures. Elemental sulfur and tetrathionate are not only intermediate-valence state sulfur compounds that play a central role in sulfur oxidation pathways, but also key factors in shaping these isotope patterns.

A unique isotopic fingerprint of sulfate-driven anaerobic oxidation of methane

Research Article| July 01, 2015 A unique isotopic fingerprint of sulfate-driven anaerobic oxidation of methane Gilad Antler; Gilad Antler 1Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar Alexandra V. Turchyn; Alexandra V. Turchyn 1Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar Barak Herut; Barak Herut 2Israel Oceanographic and Limnological Research, National Institute of Oceanography, Haifa 31080, Israel Search for other works by this author on: GSW Google Scholar Orit Sivan Orit Sivan 3Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer-Sheva 84105, Israel Search for other works by this author on: GSW Google Scholar Geology (2015) 43 (7): 619–622. https://doi.org/10.1130/G36688.1 Article history received: 08 Feb 2015 rev-recd: 29 Apr 2015 accepted: 29 Apr 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gilad Antler, Alexandra V. Turchyn, Barak Herut, Orit Sivan; A unique isotopic fingerprint of sulfate-driven anaerobic oxidation of methane. Geology 2015;; 43 (7): 619–622. doi: https://doi.org/10.1130/G36688.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The largest reservoir of the powerful greenhouse gas methane is in marine sediments, and catastrophic release of this methane has been invoked to explain climate perturbations throughout Earth history. Marine methane oxidation is mainly coupled anaerobically to microbial sulfate reduction, which both limits and controls the release of methane from this sedimentary reservoir to the rest of Earth’s surface. Methane can be transported within the pore space of marine sediments either via diffusion or as bubbles. When methane travels in bubbles, these bubbles often are not completely oxidized and reach the overlying water where the methane emerges from the sediment in cold seeps. Although paleo–cold seeps can be identified by geological features such as carbonate mounds, a geochemical signature for cold seeps remains elusive. We demonstrate, using the sulfur and oxygen isotope composition of sulfate, that a unique isotopic signature emerges during microbial sulfate reduction coupled to methane oxidation in bubbling cold seeps. This isotope signature differs from that when sulfate is reduced by either organic matter oxidation or by the slower, diffusive flux of methane within marine sediments. We also show, through a comparison with the literature, that this unique isotope fingerprint is preserved in the rock record in authigenic buildups of barite associated with methane cold seeps. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relationship between water discharge and sulfate sources of the Yangtze River inferred from seasonal variations of sulfur and oxygen isotopic compositions

Resolving sulfate sources in river systems and thus to the global ocean is important for understanding the sulfur biogeochemical cycle. The Yangtze River is one of most important large rivers connecting land and sea, showing a trend of acidification characterized by persistently increasing SO42− concentration. A weekly sampling campaign from the middle Yangtze River in Wuhan was conducted, over a 1-year period in 2011, to systematically examine the effects of seasonality and associated water discharge on the balance of sulfate sources of the Yangtze River. The SO42− concentrations varied from 8.6 to 55.4mg/L with a discharge-weighted average of 30.0mg/L, while the SO42− fluxes varied from 103.2 to 1257.6kg/s with an annual average of 532.5kg/s. The δ34SSO4 values ranged between 6.7‰ and 16.2‰ with a discharge-weighted average of 10.4‰, while the δ18OSO4 values ranged between 1.6‰ and 10.5‰ with a discharge-weighted average of 5.1‰. In comparison to the low water discharge period, the period of high water discharge of the Yangtze River in Wuhan is characterized by 1) a doubling in water discharge with higher water level by 5–10m; 2) a concomitant doubling in SO42− flux without substantial change in SO42− concentration; 3) a systematic increase of δ34SSO4 and δ18OSO4 values; 4) a significant increase of sulfate contribution from evaporites dissolution. From the observed seasonal variation of sulfur and oxygen isotopic composition of sulfate, a relationship between the dynamic water discharge and dominant sulfate sources of the Yangtze River was inferred. During the period of low water discharge in January–mid May, the sulfur and oxygen isotopic compositions of sulfate had averaged values of 8.3±0.4‰ for δ34SSO4 and 4.9±0.7‰ for δ18OSO4, suggesting the riverine SO42− of the Yangtze Riveris derived from sulfide oxidation, atmospheric deposition and evaporite dissolution; whereas during the period of low water discharge in December, the contribution of anthropogenic sulfate could be important, indicated by relatively higher δ34SSO4 (10.5±1.5‰) and δ18OSO4 (9.6±1.0‰) values. In contrast, during the period of high water discharge in June-September, the dual-isotope compositions were characterized by 14.7±0.7‰ for δ34SSO4 and 8.1±0.7‰ for δ18OSO4, indicating sulfate from dissolution of evaporites would be a dominant source. The jump event of dual-isotopic composition between low and high water discharge occurred when a half-year drought suddenly turned into the flood season. Due to the water storage dispatch of the Three Gorge Dam, the Hanjiang River could make a greater impact on the variation of water discharge and thus sulfate sources of the Yangtze River during the transition from high to low water discharge in October–November. However, the relationship between water discharge and sulfate sources of the Yangtze River need s to be confirmed by additional years of observation. Further studies on the mechanisms of water discharge controlling sulfate sources of the Yangtze River is urgently required, as well as a mixing model to calculate the contribution of different sulfate sources.

Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece.

Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m2) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform δ34S values (2.5 ± 0.28‰, n = 4) that were nearly identical to pore water H2S (2.7 ± 0.36‰, n = 21). In pore water sulfate, there were no paired increases in δ34SSO4 and δ18OSO4 as expected of microbial sulfate reduction. Instead, pore water δ34SSO4 values decreased (from approximately 21‰ to 17‰) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ34SSO4 as a mixing process between oxic seawater and 34S-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in δ34SSO4, the oxygen isotope composition of sulfate tended to be 18O-enriched in low pH (<5), high temperature (>75°C) pore waters. The shift toward high δ18OSO4 is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member δ34S value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater.

Sulfur and oxygen isotope fractionation during sulfate reduction coupled to anaerobic oxidation of methane is dependent on methane concentration

Isotope signatures of sulfur compounds are key tools for studying sulfur cycling in the modern environment and throughout earth's history. However, for meaningful interpretations, the isotope effects of the processes involved must be known. Sulfate reduction coupled to the anaerobic oxidation of methane (AOM-SR) plays a pivotal role in sedimentary sulfur cycling and is the main process responsible for the consumption of methane in marine sediments − thereby efficiently limiting the escape of this potent greenhouse gas from the seabed to the overlying water column and atmosphere. In contrast to classical dissimilatory sulfate reduction (DSR), where sulfur and oxygen isotope effects have been measured in culture studies and a wide range of isotope effects has been observed, the sulfur and oxygen isotope effects by AOM-SR are unknown. This gap in knowledge severely hampers the interpretation of sulfur cycling in methane-bearing sediments, especially because, unlike DSR which is carried out by a single organism, AOM-SR is presumably catalyzed by consortia of archaea and bacteria that both contribute to the reduction of sulfate to sulfide.We studied sulfur and oxygen isotope effects by AOM-SR at various aqueous methane concentrations from 1.4±0.6 mM up to 58.8±10.5 mM in continuous incubation at steady state. Changes in the concentration of methane induced strong changes in sulfur isotope enrichment (εS34) and oxygen isotope exchange between water and sulfate relative to sulfate reduction (θO), as well as sulfate reduction rates (SRR). Smallest εS34 (21.9±1.9‰) and θO (0.5±0.2) as well as highest SRR were observed for the highest methane concentration, whereas highest εS34 (67.3±26.1‰) and θO (2.5±1.5) and lowest SRR were reached at low methane concentration. Our results show that εS34, θO and SRR during AOM-SR are very sensitive to methane concentration and thus also correlate with energy yield. In sulfate–methane transition zones, AOM-SR is likely to induce very large sulfur isotope fractionation between sulfate and sulfide (i.e. >60‰) and will drive the oxygen isotope composition of sulfate towards the sulfate–water oxygen isotope equilibrium value. Sulfur isotope fractionation by AOM-SR at gas seeps, where methane fluxes are high, will be much smaller (i.e. 20 to 40‰).

Isotopic evidence of the pivotal role of sulfite oxidation in shaping the oxygen isotope signature of sulfate

The oxygen isotope composition of sulfate serves as an archive of oxidative sulfur cycling. Studies on the aerobic oxidation of reduced sulfur compounds showed discrepancies in the relative incorporation of oxygen from dissolved molecular oxygen (O2) and water (H2O) into newly formed sulfate, which likely result from slight differences in the production and consumption rate of sulfoxy intermediates that exchange oxygen isotopes with water. Sulfite is often considered the final sulfoxy intermediate in the oxidation of reduced sulfur compounds to sulfate and its residence time strongly affects the oxygen isotope signature of produced sulfate. However, data on the oxygen isotope signature of sulfate derived from sulfite oxidation are scarce.We determined the oxygen isotope effects of abiotic oxidation of sulfite with O2 or ferric iron (Fe3+) under different pH conditions (pH1, 4.9 and 13.3). These parameters impact the relative contribution of oxygen from H2O and O2 to the produced sulfate, and control the competition between the rates of oxygen isotope exchange between sulfite and water and the sulfite oxidation. There is a striking overlap in the range of oxygen isotope offsets between sulfate and water from our experiments at different chemical conditions (Δ18OSO4−H2O from 5.9‰ for anaerobic oxidation with Fe3+ up to 17.6‰ for oxidation at low pH with O2 as sole oxidant, respectively) with the variations in the oxygen isotope composition of sulfate derived from oxidative processes in the environment. This implies that oxygen isotope effects during sulfite oxidation largely control the isotope signature of sulfate derived from the oxidation of sulfur compounds. However, our results also show that preexisting non-equilibrium isotope signatures of sulfite are likely partially preserved in the final sulfate product under most environmental conditions. Our study furthermore provides a mechanistic explanation for positive isotope offsets between the oxygen isotope composition of sulfate and water observed in anoxic pyrite oxidation experiments with Fe3+ as the sole oxidizing agent. This apparently inverse isotope effect is caused by the interplay between sulfite-water oxygen exchange and normal kinetic isotope fractionation effects during sulfite oxidation, the former driving the isotope composition towards the isotope equilibrium fractionation between sulfite and water, inducing a positive offset whereas the latter induces a negative offset.

Geochemical and stable isotopic constraints on the generation and passive treatment of acidic, Fe–SO4 rich waters

Reducing and Alkalinity Producing Systems (RAPS) remediate net-acidic metalliferous mine drainage by creating anoxic conditions in which bacterial sulfate reduction (BSR) raises alkalinity and drives the precipitation of iron and other chalcophilic elements as sulfides. We report chemical and stable isotopic data from a study monitoring the biogeochemical processes involved in the generation of mine waters and their remediation by two RAPS. Sulfur isotopes show that sulfate in all mine waters has a common source (pyrite oxidation), whilst oxygen isotopes show that oxidation of pyritic sulfur is mediated by Fe(III)aq. The isotopic composition of dissolved sulfide, combined with the sulfur and oxygen isotopic composition of sulfate in RAPS effluents, proves BSR and details its dual isotope systematics. The occurrence and isotopic composition of solid phase iron sulfides indicate the removal of reduced sulfur within the RAPS, with significant amounts of elemental sulfur indicating reoxidation steps. However, only 0 to 9% of solid phase iron occurs as Fe sulfides, with approximately 70% of the removed iron occurs as Fe(III) (hydr)oxides. Some of the (hydr)oxide is supplied to the wetland as solids and is simply filtered by the wetland substrate, playing no role in alkalinity generation or proton removal. However, the majority of iron is supplied as dissolved Fe(II), indicating that acid generating oxidation and hydrolysis reactions dominate iron removal. The overall contribution of BSR to the sulfur geochemistry in the RAPS is limited and sulfate retention is dominated by sulfate precipitation, comparable to aerobic treatment systems, and show that the proton acidity resulting from iron oxidation and hydrolysis must be subsequently neutralised by calcite dissolution and/or BSR deeper in the RAPS sediments. BSR is not as important as previously thought for metal removal in RAPS. The results have practical consequences for the design, treatment performance and long-term functionality of such systems.

The impact of anthropogenic emissions on atmospheric sulfate production pathways, oxidants, and ice core Δ&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O(SO&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;lt;sup&amp;gt;2–&amp;lt;/sup&amp;gt;)

Abstract. We use a global three-dimensional chemical transport model to quantify the influence of anthropogenic emissions on atmospheric sulfate production mechanisms and oxidant concentrations constrained by observations of the oxygen isotopic composition (Δ17O = &amp;amp;delta17O–0.52 × &amp;amp;delta18O) of sulfate in Greenland and Antarctic ice cores and aerosols. The oxygen isotopic composition of non-sea salt sulfate (Δ17O(SO42–)) is a function of the relative importance of each oxidant (e.g. O3, OH, H2O2, and O2) during sulfate formation, and can be used to quantify sulfate production pathways. Due to its dependence on oxidant concentrations, Δ17O(SO42–) has been suggested as a proxy for paleo-oxidant levels. However, the oxygen isotopic composition of sulfate from both Greenland and Antarctic ice cores shows a trend opposite to that expected from the known increase in the concentration of tropospheric O3 since the preindustrial period. The model simulates a significant increase in the fraction of sulfate formed via oxidation by O2 catalyzed by transition metals in the present-day Northern Hemisphere troposphere (from 11% to 22%), offset by decreases in the fractions of sulfate formed by O3 and H2O2. There is little change, globally, in the fraction of tropospheric sulfate produced by gas-phase oxidation (from 23% to 27%). The model-calculated change in Δ17O(SO42–) since preindustrial times (1850 CE) is consistent with Arctic and Antarctic observations. The model simulates a 42% increase in the concentration of global mean tropospheric O3, a 10% decrease in OH, and a 58% increase in H2O2 between the preindustrial period and present. Model results indicate that the observed decrease in the Arctic Δ17O(SO42–) – in spite of increasing tropospheric O3 concentrations – can be explained by the combined effects of increased sulfate formation by O2 catalyzed by anthropogenic transition metals and increased cloud water acidity, rendering Δ17O(SO42–) insensitive to changing oxidant concentrations in the Arctic on this timescale. In Antarctica, the Δ17O(SO42–) is sensitive to relative changes of oxidant concentrations because cloud pH and metal emissions have not varied significantly in the Southern Hemisphere on this timescale, although the response of Δ17O(SO42–) to the modeled changes in oxidants is small. There is little net change in the Δ17O(SO42–) in Antarctica, in spite of increased O3, which can be explained by a compensatory effect from an even larger increase in H2O2. In the model, decreased oxidation by OH (due to lower OH concentrations) and O3 (due to higher H2O2 concentrations) results in little net change in Δ17O(SO42–) due to offsetting effects of Δ17O(OH) and Δ17O(O3). Additional model simulations are conducted to explore the sensitivity of the oxygen isotopic composition of sulfate to uncertainties in the preindustrial emissions of oxidant precursors.

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 Fe 2+–Fe 3+ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ 18O-labeled experiments show that a variable fraction (0–60%) of the oxygen in product sulfate is derived from dissolved O 2, 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 O 2% 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 δ 18O label to differentiate sulfate–oxygen sources. To examine the role of O 2 and Fe 3+ in determining the final incorporation of O 2 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 Fe 3+ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe 3+ addition help to determine the oxygen sources in sulfate and examine the role of Fe 2+–Fe 3+ 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 Fe 3+. 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 O 2 signal, similar to pH 2 with Fe 3+ addition. Sulfate from the pH 2 reactor without Fe 3+ addition and the pH 7 reactors all showed 28–29% O 2 signal. While the O 2% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O 2 does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite–water exchange (e.g. the pH 2 with no Fe 3+ addition and both pH 7 reactors); and (2) direct O 2 oxidation dominates while Fe 3+ shuttling is still competitive in the sulfite–sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe 3+ addition). Overall, the final sulfate–oxygen source ratio is determined by (1) rate competitions between direct O 2 incorporation and Fe 3+ 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.

Sulfate sources and oxidation chemistry over the past 230 years from sulfur and oxygen isotopes of sulfate in a West Antarctic ice core

The sulfur and oxygen isotopic composition of sulfate in polar ice cores provides information about atmospheric sulfate sources and formation pathways, which have been impacted regionally by human activity over the past several centuries. We present decadal scale mean ice core measurements of Δ17O, δ34S, Δ33S, and Δ36S of sulfate over the past 230 years from the West Antarctic Ice Sheet (WAIS) Divide deep ice core drill site (WDC05‐A). The low mean δ34S of non–sea‐salt sulfate at WAIS Divide (6.0 ± 0.2‰) relative to East Antarctic coastal and plateau sites may reflect a combination of stronger influence of volcanogenic and/or stratospheric sulfate with low δ34S and the influence of frost flowers on the sea‐salt sulfate‐to‐sodium ratio. Δ33S and Δ36S measurements are all within analytical uncertainty of zero but do not contradict a contribution of stratospheric sources to background sulfate deposition at WAIS Divide. Δ17O of non–sea‐salt sulfate shows a small but significant increase between the late 1700s (1.8‰ ± 0.2‰) and late 1800s (2.6‰ ± 0.2‰), but the influence of stratospheric scale volcanic events on Δ17O in the early 1800s remains uncertain. An isotope mass balance model shows that the lack of change in Δ17O of non–sea‐salt sulfate from the mid‐1800s to early 2000s (2.4‰–2.6‰ ± 0.2‰) is consistent with previous atmospheric chemistry model estimates indicating preindustrial to industrial increases in O3 as high as 50% and decreases in OH of 20% in the southern polar troposphere, as long as H2O2 concentrations also increase by over 50%.

Oxidative sulfur cycling in the deep biosphere of the Nankai Trough, Japan

The existence of oxidative microbial sulfur cycling in the deep biosphere has been postulated, but it is difficult to identify because isotope effects of reductive sulfur cycling generally overprint those of oxidative sulfur cycling. In the sediments at Integrated Ocean Drilling Program (IODP) Site C0007 (Holes A–C) drilled and sampled during IODP Expedition 316 at the Nankai Trough, Japan, sulfur and oxygen isotope compositions of dissolved and solid-phase sulfur compounds show evidence for a discrete zone of deep oxidative sulfur cycling. The sulfate concentration profile and isotope values show two distinct sulfate reduction regimes: one for downward-diffusing sulfate in the uppermost 36 m of sediment (unit I), and another zone below 90 m, where sulfate diffuses upward from a deep subsurface fluid. The sulfur and oxygen isotope composition of sulfate and a dramatic change in solid-phase iron geochemistry indicate that oxidative sulfur cycling occurs at depth. The highly dynamic sedimentary and tectonic regime at Nankai Trough has created an environment in the subsurface where geochemical gradients can sustain deep oxidative sulfur cycling over long durations of time.

Isotopic tracing of clear water sources in an urban sewer: A combined water and dissolved sulfate stable isotope approach

This paper investigates the potential of stable isotopes of both water (δD and δOH2O18) and dissolved sulfate (δ34S and δOSO418) for determining the origin and the amount of clear waters entering an urban sewer. The dynamics of various hydrological processes that commonly occur within the sewer system such as groundwater infiltration, rainwater percolation, or stormwater release from retention basins, can be readily described using water isotope ratios. In particular, stable water isotopes indicate that the relative volumes of infiltrated groundwater and sewage remain approximately constant and independent of wastewater flow rate during the day, thus demonstrating that the usual quantification of parasitic discharge from minimal nocturnal flow measurements can lead to completely erroneous results. The isotopic signature of dissolved sulfate can also provide valuable information about the nature of water inputs to the sewage flow, but could not be used in our case to quantify the infiltrating water. Indeed, even though the microbial activity had a limited effect on the isotopic composition of dissolved sulfate at the sampling sites investigated, the dissolved sulfate concentration in sewage was regulated by the formation of barite and calcium-phosphate mineral species. Sulfate originating from urine was also detected as a source using the oxygen isotopic composition of sulfate, which suggests that δOSO418 might find use as a urine tracer.

Stable isotope analysis of the Cretaceous sulfur cycle

We report the sulfur and oxygen isotope composition of sulfate (δ 34S SO4 and δ 18O SO4, respectively) in coexisting barite and carbonate-associated sulfate (CAS), which we use to explore temporal variability in the marine sulfur cycle through the middle Cretaceous. The δ 34S SO4 of marine barite tracks previously reported sulfur isotope data from the tropical Pacific. The δ 18O SO4 of marine barite exhibits more rapid and larger isotopic excursions than the δ 34S SO4 of marine barite; these excursions temporally coincide with Ocean Anoxic Events (OAEs). Neither the δ 34S SO4 nor the δ 18O SO4 measured in marine barite resembles the δ 34S SO4 or the δ 18O SO4 measured in coexisting CAS. Culling our data set for elemental parameters suggestive of carbonate recrystallization (low [Sr] and high Mn/Sr) improves our record of δ 18O SO4 in CAS in the Cretaceous. This suggests that the CAS proxy can be impacted by carbonate recrystallization in some marine sediments. A box model is used to explore the response of the δ 34S SO4 and δ 18O SO4 to different perturbations in the marine biogeochemical sulfur cycle. We conclude that the δ 34S SO4 in the middle Cretaceous is likely responding to a change in the isotopic composition of pyrite being buried, coupled possibly with a change in riverine input. On the other hand, the δ 18O SO4 is likely responding to rapid changes in the reoxidation pathway of sulfide, which we suggest may be due to anoxic versus euxinic conditions during different OAEs.

Uncertainties in the oxygen isotopic composition of barium sulfate induced by coprecipitation of nitrate

Coprecipitation of nitrate and sulfate by barium has probably resulted in significant error in numerous studies dealing with the oxygen isotopic composition of natural sulfates using chemical/thermal conversion of BaSO(4) and analysis by isotope ratio mass spectrometry. In solutions where NO(3) (-)/SO(4) (2-) molar ratios are above 2 the amount of nitrate coprecipitated with BaSO(4) reaches a maximum of approximately 7% and decreases roughly linearly as the molar ratio decreases. The fraction of coprecipitated nitrate appears to increase with decreasing pH and is also affected by the nature of the cations in the precipitating solution. The size of the oxygen isotope artifact in sulfate depends both on the amount of coprecipitated nitrate and the delta(18)O and Delta(17)O values of the nitrate, both of which can be highly variable. The oxygen isotopic composition of sulfate extracted from atmospheric aerosols or rain waters are probably severely biased because photochemical nitrate is usually also present and it is highly enriched in (18)O (delta(18)O approximately 50-90 per thousand) and has a large mass-independent isotopic composition (Delta(17)O approximately 20-32 per thousand). The sulfate delta(18)O error can be 2-5 per thousand with Delta(17)O artifacts reaching as high as 4.0 per thousand.

Oxygen isotopic composition of sulfate in deep sea pore fluid: evidence for rapid sulfur cycling

ABSTRACTWe present new data of oxygen isotopes in marine sulfate (δ18OSO4) in pore fluid profiles through organic‐rich deep‐sea sediments from 11 ODP sites around the world. In almost all sites studied sulfate is depleted with depth, through both organic matter oxidation and anaerobic methane oxidation. The δ18OSO4 increases rapidly near the top of the sediments, from seawater values of 9 to maxima between 22 and 25, and remains isotopically heavy and constant at these values with depth. The δ18OSO4 in these pore fluid profiles is decoupled from variations in sulfur isotopes measured on the same sulfate samples (δ34SSO4); the δ34SSO4 increases continuously with depth and exhibits a shallower isotopic increase. This isotopic decoupling between the δ34SSO4 and the δ18OSO4 is hard to reconcile with the traditional understanding of bacterial sulfate reduction in sediments. Our data support the idea that sulfate or sulfite and water isotopically exchange during sulfate reduction and that some of the isotopically altered sulfur pool returns to the environment. We calculate that the rapid increase in the δ18OSO4 in the upper part of these sediments requires rates of this oxygen isotope exchange that are several orders of magnitude higher than the rates of net sulfate reduction calculated from the sulfate concentration profiles and supported by the δ34SSO4. We suggest several mechanisms by which this may occur, including ‘net‐zero’ sulfur cycling, as well as further experiments through which we can test and resolve these processes.

Oxygen isotopic composition of sulfate in deep sea pore fluid: Evidence for rapid sulfur cycling

Changes in the major element and related isotope profiles in porewaters of organic-rich sediments suggest that various microbial processes using a succession of electron acceptors are in play during the remineralization of organic matter. Of the electron acceptors, bacterial sulfate reduction (BSR) is responsible for most organic matter remineralization in sediments. In addition, nearly all the methane produced during methanogenesis below the sulfate minimum zone is oxidized anaerobically through sulfate reduction (anaerobic methane oxidation (AMO)). In places where AMO occurs, recent studies have demonstrated that the majority of the sulfate is reduced by methane. When sulfate is reduced through OMO or AMO the sulfur isotope profile of the residual sulfate pool (δ34SSO4) increases monotonically from seawater values of 20‰ to values as high as 60‰ or 70‰, reflecting the kinetic isotope fractionation for sulfur isotopes associated with BSR. We have measured oxygen isotopes in marine sulfate (δ18OSO4) in pore fluid profiles through organic-rich deep-sea sediments from eleven ODP sites around the world. In almost all sites studied sulfate is depleted with depth, through both OMO and AMO. The δ18OSO4 increases rapidly near the top of the sediments, from seawater values of 9‰ to maxima between 22‰ and 25‰, and remains isotopically heavy and constant at these values with depth. The δ18OSO4 in these pore fluid profiles is decoupled from the δ34SSO4 measured on the same pore fluid samples. This isotopic decoupling between the δ34SSO4 and the δ18OSO4 is hard to reconcile with the traditional understanding of bacterial sulfate reduction in sediments. Our data support the idea that sulfate or some sulfur intermediate and water isotopically exchange during sulfate reduction and that some of the isotopically altered sulfur pool returns to the environment. The rapid increase in the δ18OSO4 in the upper part of these sediments requires rates of this oxygen isotope exchange that are several orders of magnitude higher than the rates of net sulfate reduction calculated from the sulfate concentration profiles and the δ34SSO4. We will explore several mechanisms by which this may occur, including “net-zero” sulfur cycling, as well as further experiments through which we can test and resolve these processes.