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

Volcanic eruptions release a large amount of sulphur dioxide (SO2) into the atmosphere. SO2 is oxidized to sulphate and can subsequently form sulphate aerosol, which can affect the Earth's radiation balance, biologic productivity and high-altitude ozone concentrations, as is evident from recent volcanic eruptions. SO2 oxidation can occur via several different pathways that depend on its flux and the atmospheric conditions. An investigation into how SO2 is oxidized to sulphate—the oxidation product preserved in the rock record—can therefore shed light on past volcanic eruptions and atmospheric conditions. Here we use sulphur and triple oxygen isotope measurements of atmospheric sulphate extracted from tuffaceous deposits to investigate the specific oxidation pathways from which the sulphate was formed. We find that seven eruption-related sulphate aerosol deposition events have occurred during the mid-Cenozoic era (34 to 7 million years ago) in the northern High Plains, North America. Two extensively sampled ash beds display a similar sulphate mixing pattern that has two distinct atmospheric secondary sulphates. A three-dimensional atmospheric sulphur chemistry and transport model study reveals that the observed, isotopically discrete sulphates in sediments can be produced only in initially alkaline cloudwater that favours an ozone-dominated SO2 oxidation pathway in the troposphere. Our finding suggests that, in contrast to the weakly acidic conditions today, cloudwater in the northern High Plains may frequently have been alkaline during the mid-Cenozoic era. We propose that atmospheric secondary sulphate preserved in continental deposits represents an unexploited geological archive for atmospheric SO2 oxidation chemistry linked to volcanism and atmospheric conditions in the past. [ABSTRACT FROM AUTHOR]

Abstract: Secondary atmospheric sulfates (SAS) is the ultimate oxidation product and sink for sulfur gases of biological, volcanic, and anthropogenic origins on Earth. Their presence in the atmosphere as aqueous or solid phases contributes to acid rain and climate change, thus, understanding SAS formation pathways is pertinent. There has been extensive measurement of δ 34S values for SAS, which mainly aimed at source identification. Relatively fewer oxygen isotope compositions (δ 18O, Δ17O), which are most useful for resolving competing oxidation pathways, were available, however. This study represents the first effort to characterize the Δ17O, δ 18O, and δ 34S simultaneously for SAS in a tropospheric air shed. We measured a total of 20 samples collected in Baton Rouge (LA, USA) during a 600-day period. The isotope compositions for atmospheric sulfate range from +0.25‰ to +1.43‰ for Δ17O, +11.8‰ to +19.3‰ for δ 18O, and −1.4‰ to +3.8‰ for δ 34S. No apparent correlation is found among Δ17O, δ 18O, or δ 34S values. The Δ17O has no seasonal variation and its values are consistent with an oxidation pathway dominated by aqueous H2O2. The δ 18O and δ 34S are within the range of those observed in other sites around the world and are not characteristic for Baton Rouge. Despite the huge variability in atmospheric condition among mid-latitude sites, the long-term average Δ17O value for SAS appears to fall within a fairly narrow range from +0.6‰ to +0.8‰, which is ∼1‰ to 2‰ lower than those in polar sites. [Copyright &y& Elsevier]

Reports measurements of delta [sup18]O and delta [sup17]O values of sulphates from a range of dry-valley soils. Suggestion that Antarctic sulphate comes not just from sea salt but also from the atmospheric oxidation of reduced gaseous sulphur compounds; Possibility that the distributions of sulphate are largely explained by differences in particle size and transport mode which exist between sea-salt aerosols and aerosols formed from biogenic sulphur emission.

Abstract Sulfate-driven anaerobic oxidation of methane (SD-AOM) plays a critical role in regulating the global methane budget. Determination of the diagnostic triple isotope exponent θ 33 (≡ ln 33 ⁡ α / ln 34 ⁡ α) for SD-AOM can help to identify and quantify microbial sulfate reduction via SD-AOM in the environment. The history of Earth's surface redox conditions can also be examined through the measurement of triple sulfur isotope compositions in sedimentary rocks. Due to difficulties in both culturing anaerobic methanotrophs and sampling pore-water sulfate in SD-AOM-dominated environments, however, the θ 33 values for the processes of SD-AOM have not been constrained. We propose that a set of modern cold-seep associated barite samples with low Δ δ 18 O / Δ δ 34 S values bear a record of residual pore-water sulfate during SD-AOM, and therefore the triple sulfur isotope composition of these barites can be used to deduce θ 33 values. We applied a 1-D diagenetic reaction–transport model to fit Δ 33 S and δ ′ 34 S results from modern cold seep barites collected from five sites in the Gulf of Mexico. Based on revealed negative correlations (R 2 = 0.77) between Δ 33 S and δ ′ 34 S values we calculated an upper-limit θ 33 value of 0.5100 to 0.5112 (±0.0005) given a 1000 ln 34 ⁡ α value of − 30 ‰ to − 10 ‰. This θ 33 value is distinctively lower than that of organoclastic sulfate reduction (OSR) in marine environments where the diagnostic isotope fractionation (1000 ln 34 ⁡ α) is typically more negative than that of SD-AOM. In addition, cold seep barite data display a negative Δ 33 S – δ ′ 34 S correlation whereas pore-water sulfates of all OSR-dominated settings show a positive one. Therefore, the diagnostic triple-sulfur isotope exponent and associated negative Δ 33 S – δ ′ 34 S correlation may allow for the identification of SD-AOM in sedimentary records. Highlights • A negative Δ 33 S – δ 34 S correlation associated with SD-AOM is archived in seep barite. • A diagnostic θ 33 value for SD-AOM ranges from 0.5100 to 0.5112 (±0.0005). • Triple sulfur isotope can be used to identify geological SD-AOM activities. [ABSTRACT FROM AUTHOR]

Abstract: In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ 13 C carb), carbonate oxygen isotopes (δ 18 O carb), carbonate associated sulfate sulfur (δ 34 S CAS) and oxygen isotopes (δ 18 O CAS), and pyrite sulfur isotopes (δ 34 S py) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore–offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ 13 C carb values decrease from −2‰ to −16‰ (VPDB), whereas δ 18 O carb values increase from −4‰ to −1‰ (VPDB). Both δ 34 S CAS and δ 34 S py show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ 18 O CAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ 13 C carb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in 13 C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation. [Copyright &y& Elsevier]