Sulfur Isotope Composition Of Sulfate Research Articles

Sources of dissolved carbon in large rivers: Insights from coupled 13C-14C in the upper Changjiang (Yangtze) River

Export of dissolved carbon from rivers connects terrestrial and oceanic carbon cycling, and represents a key component of global carbon budgets. In this study, we measured the concentrations of major ions, the sulfur isotopic composition of sulfate, and the stable (δ13CDIC) and radioactive (Δ14CDIC) isotopic composition of the dissolved inorganic carbon (DIC) in the upper Changjiang (Yangtze) River, China. Their spatio-temporal variabilities give constrains on carbon cycling across catchments with diverse physiographic conditions (e.g. climate, hydrology, geology, land-use change). Daily DIC fluxes were modeled using a hydro-chemical approach and showed significant temporal variations agreeing with the measured data, and supporting a conservative mixing behavior in river channel, similar to variations in anions and cations concentrations. However, the modeled (δ13CDIC) were always higher (up to 3.1‰, and 0.8‰ on average) than the measured values. In addition, Δ14CDIC would suggest a much lower carbonate contribution (8 to 33%) than concentration mixing relationship (40 to 64%). From the chemical weathering reaction front to the riverine transport, DIC can be processed with little net change in concentration but isotopic exchange and it can also exchange with the soil and/or atmospheric CO2 as an open system. The openness parameter (ψ) uses the Δ14CDIC to estimate the extent of such exchange, ranging from 0 (fully closed system) to 1 (fully open system) and varied from 0.26±0.10 to 0.86±0.06. ψ showed a strong relationship with drainage mean elevation, likely to represent the elevation-induced climate variation (i.e. precipitation and temperature) but also the proportion of cropland in the drainage area. All of these parameters are correlated and point toward a biological impact on ψ. In addition, modern DIC yield, estimated from Δ14CDIC, is positively correlated to ψ. This study demonstrates that Δ14CDIC measurements can greatly underestimate the contribution of carbonate-derived carbon because of exchange (estimated by ψ) especially in high biological-productivity areas.

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution.

The mechanism of sulfate formation during winter haze events in North China remains largely elusive. In this study, the multiple sulfur isotopic composition of sulfate in different grain-size aerosol fractions collected seasonally from sampling sites in rural, suburban, urban, industrial, and coastal areas of North China are used to constrain the mechanism of SO2 oxidation at different levels of air pollution. The Δ33S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ33S signatures (0‰ < Δ33S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO2 oxidation and stratospheric SO2 photolysis. The negative Δ33S signatures (-0.236‰ < Δ33S < ∼0‰) observed during winter haze events (PM2.5 > 200 μg/m3) are mainly attributed to SO2 oxidation by H2O2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H2O2 and TMI pathways play critical roles in sulfate formation during haze events in North China. Additionally, these new data provide evidence that the tropospheric oxidation of SO2 can produce significant negative Δ33S values in sulfate aerosols.

Isotopic constraints on the sulfur cycle of the Bottom Convective Layer in the Black Sea

The Black Sea is the largest euxinic seawater basin with H2S at depths of below 90 m. Previously found that the presence of H2S in water was caused by microbial sulfate reduction. Herein, we report the sulfur isotopic composition of sulfate and sulfide in seawater at stations in the central part of the Black Sea, especially from the Bottom Convective Layer (BCL) located below a water depth of 1750 m. Our study shows that the sulfur isotopic composition of sulfate (δ34S(SO4)) in the surface layer of the oxic zone is +21.1‰ ± 0.1‰ relative to the Vienna Canyon Diablo troilite (VCDT), and does not differ from sulfate in the Mediterranean Sea. In the BCL, the average δ34S(SO4) value is +23.0‰ ± 0.2‰, which does not change vertically or laterally owing to effective convective mixing. The sulfur isotopic composition of H2S varies from −40.0‰ to −41.9‰ between 200 m water depth down to the seafloor with an average δ34S(H2S) value of −40.6‰ ± 0.4‰ for the BCL.Based on the water mass balance and sulfur isotopic composition of sulfate in the relevant water masses affecting the BCL, we have formulated a model to calculate the sulfate reduction rate in the BCL. After evaluating the sulfate and sulfide sources in the BCL, we assumed that the only apparent sulfide source is a bacterial sulfate reduction in the water column, and the main sulfate source is a mixture of water from the Black Sea and the lower Bosporus current with an initial sulfate isotopic composition of +21‰. The weak water exchange in the BCL leads to the largest observed isotopic difference ΔSR = δ34S(SO4) − δ34S(H2S) of 63.6‰ between sulfate and sulfide in the sulfidic water column of the Black Sea. To achieve a steady state with respect to the sulfate isotopic composition +23‰ in the BCL, an average sulfate reduction (SR) rate of 1.1 ± 0.1 nmol L−1 day−1 is required. A steady state relative to sulfur isotopic composition of sulfate in the BCL water was established 4200 years ago. The estimated residence time of H2S was 1.5 times shorter (946 years) than that of sulfate (1390 years). The latter is close to the water renewal time of the BCL (1432 years). Different residence times of sulfide and sulfate in the BCL are explained using their different fluxes from the BCL. Sulfate is mainly removed from the BCL via vertical advection, while a considerable part of the sulfide generated in the BCL (1/3 of total annual production) is probably spent on the sulfidation (FeS → FeS2) of muddy turbidites. The estimated annual H2S production in the BCL is 0.78 ± 0.07 Tg. The developed model predicts that the sulfide concentration in the BCL remains practically unchanged (380 ± 5 μM) over the last 1800 years (after 5700 years from the onset of anoxia).

Assessing Sedimentary Boundary Layer Calcium Carbonate Precipitation and Dissolution Using the Calcium Isotopic Composition of Pore Fluids

We present pore fluid geochemistry, including major ion and trace metal concentrations and the isotopic composition of pore fluid calcium and sulfate, from the uppermost meter of sediments from the Gulf of Aqaba (Northeast Red Sea) and the Iberian Margin (North Atlantic Ocean). In both the locations, we observe strong correlations among calcium, magnesium, strontium, and sulfate concentrations as well as the sulfur isotopic composition of sulfate and alkalinity, suggestive of active changes in the redox state and pH that should lead to carbonate mineral precipitation and dissolution. The calcium isotope composition of pore fluid calcium (δ44Ca) is, however, relatively invariant in our measured profiles, suggesting that carbonate mineral precipitation is not occurring within the boundary layer at these sites. We explore several reasons why the pore fluid δ44Ca might not be changing in the studied profiles, despite changes in other major ions and their isotopic composition, including mixing between the surface and deep precipitation of carbonate minerals below the boundary layer, the possibility that active iron and manganese cycling inhibits carbonate mineral precipitation, and that mineral precipitation may be slow enough to preclude calcium isotope fractionation during carbonate mineral precipitation. Our results suggest that active carbonate dissolution and precipitation, particularly in the diffusive boundary layer, may elicit a more complex response in the pore fluid δ44Ca than previously thought.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Atmospheric deposition across the Atacama Desert, Chile: Compositions, source distributions, and interannual comparisons

Hyper-arid areas such as the Atacama Desert accumulated significant amounts of insoluble dust and soluble salts from the atmosphere, providing minable salt deposits as well as mimicking the surface processes on Mars. The deposition rates, compositions and sources, however, were poorly constrained. Especially, the variabilities of atmospheric deposition in the Atacama Desert corresponding to a changing climate were unassessed. In this work, the atmospheric depositions collected using dust traps across a west-east elevation gradient in the Atacama (~23°S) from 1/2/2010 to 12/31/2011 were analyzed and compared to previous results in 2007–2009. The insoluble dust deposition rates in our sampling period were significantly higher than those of 2007–2009 in most dust traps, which was attributed to the changes in wind, highlighting the importance of long-term monitoring of insoluble dust fluxes. Soluble salts, instead, showed less distinct interannual variations in deposition rates, geochemical compositions or source contributions. At the coastal site (T1), soluble salts were originated from both primary sea-salt (SS) aerosols and non-sea-salt (NSS) sources such as anthropogenic emission, marine biogenic emission and biomass burning; the deposition rates of these salts largely depended on the wind speed and the amount of anthropogenic emissions. Sulfur isotopic evidence further showed that NSS sulfate at T1 was mainly originated from local SO2 emission from local power plants. The inland sites (T2-T8) displayed much lower soluble salts deposition rates, and the salts were primarily sourced from entrainment of local surface minerals, including Na(Cl, NO3), CaSO4, and nitrate formed via atmospheric oxidation of anthropogenic NOx. Sulfur isotopic compositions of sulfate deposited at T2-T8 were similar to those in local surface soil; however, three sites near the Chuquicamata mine showed slightly lower δ34Ssulfate values, indicating the presence of secondary sulfate originated from mining activities. The soluble salts deposited at the Andean site (T10) were dominantly from wet deposition that incorporated local lake salts, and therefore, the deposition rates were mainly controlled by the amount of precipitation.

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.

Ventilation of the Black Sea anoxic zone: Evidence from the sulfur isotope composition of sulfate

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

Multiple Sulfur Isotope Constraints on Sources and Formation Processes of Sulfate in Beijing PM2.5 Aerosol.

Recently air pollution is seriously threatening the health of millions of people in China. The multiple sulfur isotopic composition of sulfate in PM2.5 samples collected in Beijing is used to better constrain potential sources and formation processes of sulfate aerosol. The Δ33S values of sulfate in PM2.5 show a pronounced seasonality with positive values in spring, summer and autumn and negative values in winter. Positive Δ33S anomalies are interpreted to result from SO2 photolysis with self-shielding, and may reflect air mass transport between the troposphere and the stratosphere. The negative Δ33S signature (-0.300‰ < Δ33S < 0‰) in winter is possibly related to incomplete combustion of coal in residential stoves during the heating season, implying that sulfur dioxide released from residential stoves in more rural areas is an important contributor to atmospheric sulfate. However, negative Δ33S anomalies (-0.664‰ < Δ33S ← 0.300‰) in winter and positive Δ33S anomalies (0.300‰ < Δ33S < 0.480‰) in spring, summer, and autumn suggest sulfur isotopic equilibrium on an annual time frame, which may provide an implication for the absence of mass-independent fractionation of sulfur isotopes (S-MIF) in younger sediments. Results obtained here reveal that reducing the usage of coal and improving the heating system in rural areas will be important for efficiently decreasing the emissions of sulfur in China and beyond.

Impact of Aeolian Dry Deposition of Reactive Iron Minerals on Sulfur Cycling in Sediments of the Gulf of Aqaba.

The Gulf of Aqaba is an oligotrophic marine system with oxygen-rich water column and organic carbon-poor sediments (≤0.6% at sites that are not influenced by anthropogenic impact). Aeolian dust deposition from the Arabian, Sinai, and Sahara Deserts is an important source of sediment, especially at the deep-water sites of the Gulf, which are less affected by sediment transport from the Arava Desert during seasonal flash floods. Microbial sulfate reduction in sediments is inferred from the presence of pyrite (although at relatively low concentrations), the presence of sulfide oxidation intermediates, and by the sulfur isotopic composition of sulfate and solid-phase sulfides. Saharan dust is characterized by high amounts of iron minerals such as hematite and goethite. We demonstrated, that the resulting high sedimentary content of reactive iron(III) (hydr)oxides, originating from this aeolian dry deposition of desert dust, leads to fast re-oxidation of hydrogen sulfide produced during microbial sulfate reduction and limits preservation of reduced sulfur in the form of pyrite. We conclude that at these sites the sedimentary sulfur cycle may be defined as cryptic.

An evaporite-based high-resolution sulfur isotope record of Late Permian and Triassic seawater sulfate

Variations in the sulfur isotope composition of dissolved marine sulfate through time reflect changes in the global sulfur cycle and are intimately related to changes in the carbon and oxygen cycles. A large shift in the sulfur isotope composition of sulfate at the Permian/Triassic boundary has been recognized for long time and a number of studies were carried out to understand the causes and significance of this shift. However, data for the Middle and Late Triassic are very sparse and the stratigraphic evolution of the sulfur isotope composition of seawater is poorly constrained due to the small number of samples analyzed and/or due to the limited stratigraphic intervals studied. Moreover, in the last few years the Triassic timescale has significantly changed due to a wealth of new radiometric and stratigraphic data. In this study we show that for the Late Permian and the Triassic it is possible to obtain a precise reconstruction of the evolution of the sulfur cycle, for parts of it at sub-million year resolution, by analyzing exclusively gypsum and anhydrite deposits. We base our reconstruction on new data from the Middle and Late Triassic evaporites of Northern Switzerland and literature data from evaporites from Germany, Austria, Italy and the Middle East. We propose a revised correlation between the well-dated marine Tethyan sections in northern Italy and the evaporites from Northern Switzerland and from the Germanic Basin calibrated to the newest radiometric absolute age scale. This new correlation allows for a precise dating of the evaporites and constructing a composite sulfur isotope evolution of seawater sulfate from the latest Permian (Lopingian Epoch) to the Norian. We show that a rapid positive shift of approximately 24‰ at the Permian-Triassic boundary can be used to constrain seawater sulfate concentrations in the range of 2–6mM, thus higher than previous estimates but with less rapid changes. Finally, we discuss two possible evolution scenarios for the Middle Triassic negative shift to values of 15‰ that subsequently remain constant during the Late Triassic.

Multiple sulfur isotopes fractionations associated with abiotic sulfur transformations in Yellowstone National Park geothermal springs.

BackgroundThe paper presents a quantification of main (hydrogen sulfide and sulfate), as well as of intermediate sulfur species (zero-valent sulfur (ZVS), thiosulfate, sulfite, thiocyanate) in the Yellowstone National Park (YNP) hydrothermal springs and pools. We combined these measurements with the measurements of quadruple sulfur isotope composition of sulfate, hydrogen sulfide and zero-valent sulfur. The main goal of this research is to understand multiple sulfur isotope fractionation in the system, which is dominated by complex, mostly abiotic, sulfur cycling.ResultsWater samples from six springs and pools in the Yellowstone National Park were characterized by pH, chloride to sulfate ratios, sulfide and intermediate sulfur species concentrations. Concentrations of sulfate in pools indicate either oxidation of sulfide by mixing of deep parent water with shallow oxic water, or surface oxidation of sulfide with atmospheric oxygen. Thiosulfate concentrations are low (<6 μmol L-1) in the pools with low pH due to fast disproportionation of thiosulfate. In the pools with higher pH, the concentration of thiosulfate varies, depending on different geochemical pathways of thiosulfate formation. The δ34S values of sulfate in four systems were close to those calculated using a mixing line of the model based on dilution and boiling of a deep hot parent water body. In two pools δ34S values of sulfate varied significantly from the values calculated from this model. Sulfur isotope fractionation between ZVS and hydrogen sulfide was close to zero at pH < 4. At higher pH zero-valent sulfur is slightly heavier than hydrogen sulfide due to equilibration in the rhombic sulfur–polysulfide – hydrogen sulfide system. Triple sulfur isotope (32S, 33S, 34S) fractionation patterns in waters of hydrothermal pools are more consistent with redox processes involving intermediate sulfur species than with bacterial sulfate reduction. Small but resolved differences in ∆33S among species and between pools are observed.ConclusionsThe variation of sulfate isotopic composition, the origin of differences in isotopic composition of sulfide and zero–valent sulfur, as well as differences in ∆33S of sulfide and sulfate are likely due to a complex network of abiotic redox reactions, including disproportionation pathways.

Stable isotope evidence for the Bottom Convective Layer homogeneity in the Black Sea.

The Black Sea is the largest euxinic basin on the Earth. The anoxic zone consists of the upper part water mass stratified by density, and the lower water mass homogenized relative to density (depth >1750 m), named the Bottom Convective Layer. To assess homogeneity and possible exchange of matter across the upper and lower boundaries of the Bottom Convective Layer, new data on stable isotope composition of S, O and H were obtained. Samples were collected in August 2008 and March 2009 from two stations located in the eastern central part of the Black Sea.Distribution of δ18O and δD values of water for the entire water column did not vary seasonally. Appreciable differences were marked for δD value variation in the picnocline area (water depth 200-400 m) and in the BCL 5 m above the bottom that might be caused by penetration of intrusions with elevated portion of shelf modified Mediterranean Water. Observed linear relationship between δ18O (or δD) and salinity indicates that mixing water and salt occurs at the same time, and the deep water of the Black Sea has two end members: the high-salinity Mediterranean seawater and freshwater input.In the Bottom Convective Layer, the average δ34S (H2S) was -40.6 ± 0.5‰ and did not vary seasonally. At the bottom (depth > 2000 m), 34S depletion down to –41.0‰ was observed. Our δ34S (SO4) data are by 2-3‰ higher than those measured previously for the Bottom Convective Layer. Sulfate from the aerobic zone with δ34S (SO4) = +21‰ corresponds to ocean water sulfate and that has not been subjected to sulfate reduction. Average δ34S (SO4) values for depths > 1250 m were found to be +23.0 ± 0.2‰ (1σ). Sulfur isotope composition of sulfate does not change in the Bottom Convective Layer and on its upper and lower boundaries, and does not depend on the season of observation.

Hypogenic speleogenesis in quartzite: The case of Corona 'e Sa Craba Cave (SW Sardinia, Italy)

The paper presents a detailed study demonstrating the hypogenic origin of the Corona 'e Sa Craba quartzite cave in SW Sardinia (Italy). Although the quartzite host-rock of this cave derived from silicification of Cambrian dolostones and dissolution of carbonate remnants could have had a role in the speleogenesis, detailed morphologic and petrographic investigation revealed clear evidence of quartz dissolution without signs of mechanical erosion by running waters. Thin section microscopy and scanning electron microscope (SEM) images show pervasive dissolution morphologies, such as pits and notches on quartz crystals causing the deep arenization of the cave walls, suggesting that the dissolution of quartz had a primary role in the formation of the void. The study of secondary cave minerals and the sulfur isotopic composition of sulfates and sulfides, coupled with data on fluid inclusions, allowed reconstruction of the peculiar speleogenetic history of this hypogenic hydrothermal quartzite cave. The cave formed by reduced hydrothermal fluids, probably under basic–neutral pH in phreatic conditions. The presence of abundant cations of Ba2+ in reduced Cl-rich fluids enhanced the quartz dissolution rate, allowing the formation of the voids in deep settings. During the Late Oligocene uplift of the area, the hydrothermal fluids in the cave reached oxygen-rich conditions, thus a minerogenetic phase started with the deposition of barite when the temperature of the fluid was ≤50°C. The presence of cinnabar crusts in the lower part of the cave walls and on the boulders suggests a later volcanic phase with Hg-rich vapors ascending from below. Other minerals such as alunite, basaluminite, gypsum and halloysite (typical of an acid sulfate alteration environment), and phosphates were formed in a final, much more recent stage. The δ34S values of the cave sulfate minerals indicate that S is derived from the remobilization of original Precambrian Pb–Zn Mississippi Valley Type ores. These last two stages did not significantly affect the morphology of the cave. The Corona 'e Sa Craba appears to be the world's first example of a hypogenic cave in quartzite where the speleogenetic mechanisms have been studied and reconstructed in detail, using a variety of modern methods. This study confirms that dissolution of quartz by thermal alkaline fluids at depth can produce large dissolutional voids in the apparently poorly soluble quartzite rocks.

Biogeochemical sulfur cycling in the water column of a shallow stratified sea-water lake: Speciation and quadruple sulfur isotope composition

Concentrations of sulfate, sulfide and intermediate sulfur species as well as quadruple sulfur isotope compositions of sulfate, sulfide and zero-valent sulfur (ZVS) were analyzed in the water column of Lake Rogoznica (Croatia), a stratified marine euxinic lake. The chemocline in the lake, which was located at 8.5–9.5 m depth, supports a dense population of purple phototrophic sulfide oxidizing bacteria from the genus Chromatium. The highest ZVS (5.42 μmol L −1) and sulfite (1.13 μmol L −1) concentrations were detected at the chemocline. Thiocyanate concentrations up to 288 nmol L −1 were detected near the bottom of the lake. The thiocyanate profile suggests that it diffuses up from the sediment, where it may be produced by the reaction of cyanide with sulfide oxidation intermediates. Multiple sulfur isotope fractionations between sulfate and sulfide were consistent with a model finding that disproportionation is not a dominant process below the chemocline. Microbial sulfide oxidation was found to be the dominant process of the reoxidative part of the sulfur cycle. Despite the absence of a clear signal for sulfur disproportionation in multiple sulfur isotope values, δ 34S fractionations between sulfate and sulfide were in the range of 43.8–45.2‰, is relatively large in comparison to most laboratory culturing studies. Our results suggest that such fractionation is achieved by microbial sulfate reduction alone, which is in agreement with metabolic models and recent laboratory studies.

Sulfur isotope fractionation during the May 2003 eruption of Anatahan volcano, Mariana Islands: Implications for sulfur sources and plume processes

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 δ 34S V-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 SO 2 raised the δ 34S V-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 (δ 34S V-CDT ∼ 0‰) mantle source with limited contamination by subducted seawater sulfate (δ 34S V-CDT +21‰). Modeling also suggests that the δ 34S V-CDT value of SO 2 gas in closed-system equilibrium with the degassed magma was between +0.9‰ and +2.5‰. The δ 34S V-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 SO 2 in the eruption column. The sulfur isotope composition of sulfate adsorbed to ash changes from lower δ 34S values for ash erupted early in the eruption to higher δ 34S 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 SO 2 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 SO 2 gas to sulfate in the early water-rich eruption plume probably contributed to the change in S isotope compositions observed in the ash leachates.

Ten-year observation of sulfur isotopic composition of sulfate in aerosols collected at Tsuruoka, a coastal area on the Sea of Japan in northern Japan

Atmospheric aerosol samples were collected from 1993 to 2003 at Tsuruoka, Japan, a coastal area on the Sea of Japan. Concentration of water-soluble chemical components and sulfur isotopic ratios of sulfate in aerosol samples were measured to evaluate the temporal variation and identify the source. The samSO4 2‐ and nssSO4 2‐ concentrations range from 7.6 to 354.4 and 3.7 to 335.6 neq m ‐3 , respectively, and there was no clear seasonal variation. Sulfur isotope ratios of total sulfate (samδ 34 S) and non-sea salt sulfate (nssδ 34 S) in aerosols are +1.0 to +15.1 with a mean value of +5.7‰ and ‐4.4 to +14.5 with a mean value of +4.1‰, respectively. The values of samδ 34 S and nssδ 34 S are relatively low in summer and high in the other three seasons. The nssδ 34 S values of aerosols in fall to spring agree with the average sulfur isotope ratios of coal used in northern China, considering the isotope fractionation. These results indicate the possibility of long-range transport of sulfate from the Asian continent to Japan. The variations of atmospheric samSO4 2‐ and nssSO4 2‐ concentrations decreased gradually and reach a minimum in 1999. The decline of SO2 emission in China due to reduction in industrial coal use, a slowdown of the Chinese economy, and the closure of small and inefficient plants would be among the causes of this decreasing observed trend. On the other hands, the variation ranges of samδ 34 S and nssδ 34 S values abruptly changed after 2000. The introduction of fuel-gas desulfurization technologies to industrial plants of coke production and thermal power stations would be one of the most important causes of this change.

The influence of pyrite grain size on the final oxygen isotope difference between sulphate and water in aerobic pyrite oxidation experiments†

Oxidation experiments with different pyrite grain sizes (63–100, 100–140, 140–180 μm) were carried out to investigate the oxygen and sulphur isotope composition of sulphate produced under aerobic acid conditions, which may help to understand oxidation mechanisms and to interpret data from natural sites. Long-term experiments with grain size 63–100 μm showed that constant δ 18OSO4 values were not achieved before 100 days. The final oxygen isotope difference between water and sulphate indicates that a small proportion of molecular oxygen is incorporated into sulphate even in the later course of the oxidation due to sulphite oxidation by molecular oxygen. However, most of the sulphate oxygen derives from water. Similar δ 18OSO4 values from experiments with grain sizes 63–100, 100–140, and 140–180 μ m indicate similar oxidation mechanisms for all three grain sizes. These results differed from previous results of identical experiments with grain size<63 μ m, where higher δ 18OSO4 values were obtained. We propose that the greater proportion of molecular oxygen in sulphate from oxidised fine-grained pyrite is caused by an intensified adsorption of molecular oxygen on sulphur sites of ultrafine pyrite particles. Hence, the formation of sulphate from the (initial) reaction on sulphur sites of pyrite and from sulphite oxidation should be more dominant if ultrafine material is present. The δ 34SSO4 values (2.0–2.7) obtained from experiments with the coarser grain sizes agreed with the δ 34S value of pyrite (2.4), whereas sulphur isotopes of sulphate obtained from previous experiments with fine-grained pyrite showed an initial 32S enrichment compared with pyrite. Due to the lack of δ 34SSO4 values from the beginning of the experiments with coarser grain sizes, it remains speculative that sulphur isotopes indicate at least initial differences in oxidation mechanisms between fine and coarser pyrite grain sizes.

Bacterial formation of phosphatic laminites off Peru

Authigenic phosphatic laminites enclosed in phosphorite crusts from the shelf off Peru (10 degrees 01' S and 10 degrees 24' S) consist of carbonate fluorapatite layers, which contain abundant sulfide minerals including pyrite (FeS(2)) and sphalerite (ZnS). Low delta(34)S(pyrite) values (average -28.8 per thousand) agree with bacterial sulfate reduction and subsequent pyrite formation. Stable sulfur isotopic compositions of sulfate bound in carbonate fluorapatite are lower than that of sulfate from ambient sea water, suggesting bacterial reoxidation of sulfide by sulfide-oxidizing bacteria. The release of phosphorus and subsequent formation of the autochthonous phosphatic laminites are apparently caused by the activity of sulfate-reducing bacteria and associated sulfide-oxidizing bacteria. Following an extraction-phosphorite dissolution-extraction procedure, molecular fossils of sulfate-reducing bacteria (mono-O-alkyl glycerol ethers, di-O-alkyl glycerol ethers, as well as the short-chain branched fatty acids i/ai-C(15:0), i/ai-C(17:0) and 10MeC(16:0)) are found to be among the most abundant compounds. The fact that these molecular fossils of sulfate-reducing bacteria are distinctly more abundant after dissolution of the phosphatic laminite reveals that the lipids are tightly bound to the mineral lattice of carbonate fluorapatite. Moreover, compared with the autochthonous laminite, molecular fossils of sulfate-reducing bacteria are: (1) significantly less abundant and (2) not as tightly bound to the mineral lattice in the other, allochthonous facies of the Peruvian crusts consisting of phosphatic coated grains. These observations confirm the importance of sulfate-reducing bacteria in the formation of the phosphatic laminite. Model calculations highlight that organic matter degradation by sulfate-reducing bacteria has the potential to liberate sufficient phosphorus for phosphogenesis.

Regional and temporal (1992–2004) evolution of air-borne sulphur isotope composition in Saxony, southeastern Germany, central Europe†

The isotopic composition of air-borne sulphur was investigated in Saxony, Southeast Germany – a region with formerly very high atmospheric SO2 concentrations. In addition, data from various authors were compiled for different Saxonian locations, spanning from 1992 to 2004, i.e., a time of decreasing SO2 concentrations in the atmosphere. There were no obvious temporal changes in the mean δ34S value of bulk precipitation. However, the variability of monthly mean δ34S values decreased. The mean sulphur isotope composition of sulphate from bulk precipitation after the year 2000 converges in Saxony towards 4–5‰, with similar values for different locations. Mean values of different forms of sulphur show the following enrichment order: δ34S of SO2 < δ34S of weathering crusts ≤δ34S of sulphate from bulk precipitation ≤δ34S of dust. Judging from local differences on sulphate crusts and corresponding isotope values of sources, the δ34S value of SO2 as well as for crusts mainly reflects local point sources. The mean δ34S value of bulk precipitation represents more regionally well-mixed SO2 sources and is therefore an ideal tool for monitoring regional atmospheric change. †Revised version of a paper presented at the 27th Annual Meeting of the German Association for Stable Isotope Research (GASIR), October, 4–6, 2006, Freiberg, Germany.