δ34SSO4 Values Research Articles

Isotopic evidence to characterize the sources of sulfate ions in the carbonate aquifer system in Aleppo basin (North Syria)

Sulfur and oxygen isotopic compositions of sulfate (δ34SSO4, δ18OSO4) and δ18OH2O, δ13CDIC have been used in conjunction with chemical data to constrain sulfate sources and geochemical processes in carbonate aquifers of the Aleppo basin in north Syria. The Aleppo basin comprises two limestone aquifers: the first one is shallow unconfined of Paleogene age and the second is deep confined of Upper Cretaceous age. The chemical data indicate that dissolution of carbonate and evaporites is the main process controlling groundwater quality. In the mainly unconfined modern groundwater, the wide range of δ34SSO4 values (10.39–17.6 ‰) indicates a variety of input sources. The likely sources of sulfur include meteoric precipitation and sewage waters which recharge the aquifer through Qweik River crossing the area. These waters have low SO42− concentration ( 20 mg/L). Sulfate increases in deep confined groundwater in some of which gypsum saturation is reached. The δ34SSO4 and δ18OSO4 are relatively constant with mean values of 18.5 and 13.5 ‰, respectively. This indicates that gypsum and anhydrite dissolution are the primary sources of sulfate. This water is associated with higher depleted value of δ18OH2O. The heavier δ13CDIC with mean value −1.8 ‰ and high Mg2+ concentration (~100 mg/L) in the high sulfate deep groundwater, indicates that the de-dolomitization reactions dominate along flow paths where sulfate concentrations increase.

Chemical and Isotopic Characteristics of the Water and Suspended Particulate Materials in the Yangtze River and Their Geological and Environmental Implications

Chemical and Isotopic Characteristics of the Water and Suspended Particulate Materials in the Yangtze River and Their Geological and Environmental Implications

Multiple isotope (O, S and C) approach elucidates the enrichment of arsenic in the groundwater from the Datong Basin, northern China

Hydrochemical data, the δ34S and δ18O values in dissolved sulfate SO42- and the δ13C signature in dissolved inorganic carbon (DIC) have been analyzed and used to deduce the predominant biogeochemical/geochemical processes controlling the mobilization of arsenic within aquifers of the Datong Basin. High dissolved Fe(II), HS− and NH4+ concentrations and low NO3-, SO42- and ORP values were observed in the high-arsenic groundwater. The coupled occurrence of high dissolved Fe(II), HS−, NH4+ and HCO3- concentrations in groundwater indicates that microbially mediated Fe(III) and SO42- reduction and organic matter oxidation has occurred. Wide ranges of δ34S and δ18O values were measured in dissolved SO42- (ranging from 7.0‰ to 36.8‰ and from 3.2‰ to 13.0‰ for δ34S and δ18O, respectively) and depleted δ13C values of DIC (varying from −6.9‰ to −22.0‰) were detected. The wide ranges of the δ34SSO4 and δ18OSO4 values and the correlation between the low δ13CDIC values and the high δ34SSO4 values indicate that the microbial reduction of SO42- and organic matter biodegradation have occurred or are occurring. The correlation between δ34SSO4 and δ18OSO4 values and Fe concentrations indicates that Fe(III) oxides/hydroxides reduction and iron sulfide formation may be the main processes controlling the enrichment of arsenic in groundwater. Furthermore, the negative correlations between the δ13CDIC values and the δ34SSO4 values and Fe concentrations demonstrate that arsenic mobility are influenced by processes: (1) the reduction of arsenic-bearing crystalline Fe(III) oxide/hydroxides and SO42- coupled to the oxidation of organic matter in aquifer sediments or unsaturated zone and (2) the reduction of amorphous Fe(III) oxyhydroxide without the significant reduction of sulfate.

Hydrological controls on the sources of dissolved sulfate in the Heihe River, a large inland river in the arid northwestern China, inferred from S and O isotopes

The Heihe River is one of the largest and most intensely exploited inland rivers of arid northwestern China, which is currently threatened by water shortage and ecological problems driven by climate change and human activity. To constrain SO42- sources to the river and evaluate the influence of hydrological processes on SO42- sources and transport in the large river basin, chemical and isotopic measurements of surface and groundwater samples were carried out in the Heihe River basin. The river water had SO42- concentrations in a range of 117–316mg/L with an average of 199mg/L, characterized by δ34SSO4 values ranging from +2.5‰ to +5.4‰ and δ18OSO4 values ranging from −1.8‰ to +10.2‰. The shallow groundwater had SO42- concentrations in a range of 54–1222mg/L with an average of 383mg/L, characterized by δ34SSO4 values ranging from 0‰ to +7.4‰ and δ18OSO4 values ranging from −2.6‰ to +10.9‰. A significant correlation was found between δ34SSO4 and δ18OSO4 values of the river water. A gradual downstream shift from lower δ34SSO4 and δ18OSO4 values to higher δ34SSO4 and δ18OSO4 values was observed for both the riverine and groundwater SO42-, which corresponds to a change in the SO42- sources, governed by the geology, from oxidation of sulfides in the upstream part to dissolution of evaporite and/or soil sulfate in the downstream part of the Heihe River basin. Sulfate input dominated by sulfide oxidation via mountain groundwater discharge at the Yingluo Valley is a major contributor to river SO42- in the upper reaches. Admixture of groundwater SO42- enhanced by intensive agricultural activities is responsible for increasing river SO42- concentration in the middle reaches. Evapotranspiration effects in the lower reaches result in indirect mixture of groundwater SO42- via dissolution of soil sulfate that has cycled through the soil organic matter pool. This study implies that mixing of groundwater SO42- with surface water is the dominant process affecting SO42- sources and S cycling of the Heihe River, which is controlled by the regional hydrological processes affected by natural and anthropogenic factors.

Application of multi-isotope data (O, D, C and S) to quantify redox processes in urban groundwater

The different recharge sources and their mixing ratios were investigated in water samples from an urban aquifer. Two distinct zones from Barcelona city were compared: (1) Poble Sec and (2) Besòs River Delta, which are located in a densely populated area with a strong industrial impact. In this study 106 water samples were collected from July 2007 to May 2010 for hydrochemical and isotopic characterisation. The application of environmental isotopes coupled with hydrochemistry provided the necessary information to isotopically quantify groundwater recharge sources and evaluate the occurrence of redox processes. In the Besòs River Delta, a decrease in dissolved SO42- concentration and an increase in δ34SSO4 and δ18OSO4 values were observed in groundwater samples, both indicating SO42- reduction. Moreover, other chemical indicators supported a reducing environment, such as low or null levels of dissolved O2 and NO3-, the presence of NH4+ and an increase in dissolved Fe and As. The reducing conditions were probably induced by the organic C dissolved in water infiltrating from the River Besòs. In Poble Sec, the relationship between δ34SSO4 and δ18OSO4 showed a strong influence of sewage water infiltration into the aquifer. However, the aquifer is oxic and there is no influence of SO42- reduction.

Activity of Sulfur Reducing Bacteria in Deep Bedrock Fractures Revealed by Variability of δ34S in Pyrite and Dissolved Sulfate

Euhedral pyrite crystals coating 46 granite fractures at depths down to nearly 1km at Laxemar, Sweden, were analysed for sulfur isotopes (δ34Spyr) by in situ SIMS (secondary ion mass spectrometry) analysis. Most of these fractures had corresponding chemical and isotopic groundwater data, providing a unique opportunity of pyrite-sulfate comparison within the same fracture network. Comparison of the isotopic ratios (δ18O, δ13C, 87Sr/86Sr) of co-genetic calcite with the groundwater showed that the sampled fractures carried pyrite and calcite that are of low-temperature origin, and with some exceptions, possibly precipitated from the present groundwater, or similar pre-modern fluids.The δ34Spyr showed huge variations across individual crystals (such as -32 to +73‰) and an extreme overall range (-50‰ to +91‰), which can only be explained by the activity of sulfur reducing bacteria (SRB). The most common sub-grain features were an increase in δ34Spyr with crystal growth related to successively higher δ34SSO4 caused by ongoing SRB activity and Rayleigh fractionation in fractures with low flow. The groundwater δ34SSO4 values (+9 to +37‰) are, in particular in the sulfate-poor waters down to -400 m, higher than the anticipated initial values, and this can also be explained by SRB-related Rayleigh distillation. The δ34SSO4 of the groundwater is, however, lower than that required to produce the highest δ34Spyr values, which may be due to the signal of groundwater in low-flow fractures (carrying 34S-rich sulfate and pyrite) being masked in the water data by high-flow parts of the water-flowing structure carrying abundant and less fractionated sulfate.

Variability of sulphur isotope ratios in pyrite and dissolved sulphate in granitoid fractures down to 1 km depth – Evidence for widespread activity of sulphur reducing bacteria

Euhedral pyrite crystals in 46 open bedrock (granitoid) fractures at depths down to nearly 1km were analysed for sulphur isotope ratios (δ34S) by the in situ secondary ion mass spectrometry (SIMS) technique and by conventional bulk-grain analysis, and were compared with groundwater data. Twenty nine of the fractures sampled for pyrite had corresponding data for groundwater, including chemistry and isotopic ratios of sulphate, which provided a unique opportunity to compare the sulphur-isotopic ratios of pyrite and dissolved sulphate both at site and fracture-specific scales. Assessment of pyrite age and formation conditions were based on the geological evolution of the area (Laxemar, SE Sweden), and on data on co-genetic calcite as follows: (1) the isotopic ratios of the calcite crystals (δ18O, δ13C, 87Sr/86Sr) were compared with previously defined isotopic features of fracture mineral assemblages precipitated during various geological periods, and (2) the δ18O of the calcites were compared with the δ18O of groundwater in fractures corresponding to those where the calcite/pyrite assemblages were sampled. Taken together, the data show that all the sampled fractures carried pyrite/calcite that are low-temperature and precipitated from the current groundwater or similar pre-existing groundwater, except at depths of −300 to −600m where water with a glacial component dominates and the crystals are from pre-modern fluids. An age of <10Ma are anticipated for the pre-modern fluids. The δ34Spyr showed huge variations across individual crystals (such as −32 to +73‰) and extreme minimum (−50‰) and maximum (+91‰) values. For this kind of extreme S-isotopic variation at earth-surface conditions there is no other explanation than activity of sulphur reducing bacteria coupled with sulphate-limited conditions. Indeed, the most common subgrain feature was an increase in δ34Spyr values from interior to rim of the crystal, which we interpret are related to successively higher δ34S values of the dissolved source SO42− caused by ongoing bacterial sulphate reduction in fractures with low-flow or stagnant waters. The measured groundwater had δ34SSO4 values of +9‰ to +37‰, with the highest values associated with low sulphate concentrations. These values are overall, and especially in the sulphate-poor waters down to −400m, somewhat higher than the anticipated initial values, and can thus, like for the 34S-enriched pyrites, be explained by a Rayleigh distillation process driven by microbial sulphate reduction. An intriguing feature was that the δ34SSO4 values of the groundwater were in no case reaching up to the values required to produce biogenic pyrite with δ34S values of +40‰ to +91‰. To explain this feature, we suggest that groundwater in low-flow fractures with near-stagnant water (carrying sulphate and pyrite with high δ34S) is masked by high-flow parts of the fracture system carrying groundwater that often contains sulphate in abundance and considerably less fractionated with respect to 34S and 32S. In order to gain detailed knowledge of chemical processes and patterns in groundwater in fractured rock, fracture-mineral investigations are a powerful tool, as we have shown here for the sulphur system.

Carbon flux and C, S isotopic characteristics of river waters from a karstic and a granitic terrain in the Yangtze River system

We analyzed dissolved major elements, δ13CDIC and δ34SSO4 in river waters from a karstic and a granitic terrain to examine weathering processes and the influence of basin geology on water quality and associated riverine alkalinity transport. Results show that waters from the karstic terrain have higher total dissolved solids (TDS), major ion concentrations and δ13CDIC values but lower K+, Na+ and Si concentration and δ34SSO4 values than that from the granitic terrain. The compositions of major ions and isotopes indicate that the solutes of the granitic terrain are controlled by processes of silicate and carbonate dissolution by carbonic acid. However, carbonate dissolution by both carbonic and sulfuric acid are dominant processes in karstic terrain. Total erosion rates, dissolved inorganic carbon (DIC) flux and CO2 uptake were 28.0tkm−2yr−1, 2.9×105molkm−2yr−1 and 2.3×105molkm−2yr−1 for the granitic terrain and 70.2tkm−2yr−1, 11.9×105molkm−2yr−1 and 6.4×105molkm−2yr−1 for the karstic terrain, respectively. These results indicated the karstic area plays an important role in uptake of CO2 and DIC flux along the Yangzte River Basin. Our results also revealed that CO2 uptake due to carbonate chemical weathering in both karstic and granitic terrain is very high and provide as much or more DIC flux than that from silicate chemical weathering.

Oxygen and sulfur isotope systematics of sulfate produced during abiotic and bacterial oxidation of sphalerite and elemental sulfur

Studies of metal sulfide oxidation in acid mine drainage (AMD) systems have primarily focused on pyrite oxidation, although acid soluble sulfides (e.g., ZnS) are predominantly responsible for the release of toxic metals. We conducted a series of biological and abiotic laboratory oxidation experiments with pure and Fe-bearing sphalerite (ZnS & Zn0.88Fe0.12S), respectively, in order to better understand the effects of sulfide mineralogy and associated biogeochemical controls of oxidation on the resultant δ34S and δ18O values of the sulfate produced. The minerals were incubated in the presence and absence of Acidithiobacillus ferrooxidans at an initial solution pH of 3 and with water of varying δ18O values to determine the relative contributions of H2O-derived and O2-derived oxygen in the newly formed sulfate. . Experiments were conducted under aerobic and anaerobic conditions using O2 and Fe(III)aq as the oxidants, respectively. Aerobic incubations with A. ferrooxidans, and So as the sole energy source were also conducted. The δ34SSO4 values from both the biological and abiotic oxidation of ZnS and ZnSFe by Fe(III)aq produced sulfur isotope fractionations (ε34SSO4-ZnS) of up to −2.6‰, suggesting the accumulation of sulfur intermediates during incomplete oxidation of the sulfide. No significant sulfur isotope fractionation was observed from any of the aerobic experiments. Negative sulfur isotope enrichment factors (ε34SSO4-ZnS) in AMD systems could reflect anaerobic, rather than aerobic pathways of oxidation. During the biological and abiotic oxidation of ZnS and ZnSFe by Fe(III)aq all of the sulfate oxygen was derived from water, with measured ε18OSO4-H2O values of 8.2±0.2‰ and 7.5±0.1‰, respectively. Also, during the aerobic oxidation of ZnSFe and So by A. ferrooxidans, all of the sulfate oxygen was derived from water with similar measured ε18OSO4-H2O values of 8.1±0.1‰ and 8.3±0.3‰, respectively. During biological oxidation of ZnS by O2, an estimated 8% of sulfate–oxygen was derived from O2, which is enriched in 18O relative to water, thus resulting in a larger apparent ε18OSO4-H2O value of 9.5‰. Based on the data presented we hypothesize that the similar ε18OSO4-H2O values of ∼8‰ from all of the aerobic and anaerobic experiments result from a common rate-limiting step that involves oxygen isotopic exchange between a sulfite (SO3-) intermediate and H2O. Our results indicate that the δ18OSO4 values cannot be used to distinguish biological and abiotic, nor aerobic versus anaerobic, pathways of sphalerite oxidation. However, the ε18OSO4-H2O values of ∼8‰ measured here are distinctly higher than ε18OSO4-H2O values of ∼4‰ previously reported for pyrite oxidation indicating the influence of sulfide mineralogy on measured δ18OSO4 values.

The geochemical and isotopic record of evaporite recycling in spas and salterns of the Basque Cantabrian basin, Spain

Abstract Evaporite outcrops are rare in the Basque Cantabrian basin due to a rainy climate, but saline springs with total dissolved solids ranging from 0.8 to 260 g/L are common and have long been used to supply spas and salterns. New and existing hydrochemistry of saline springs are used to provide additional insight on the origin and underground extent of their poorly known source evaporites. Saline water hydrochemistry is related to dissolution of halite and gypsum from two evaporitic successions (Triassic “Keuper” and Lower Cretaceous “Wealden”), as supported by rock samples from outcrops and oil exploration drill cuttings. The δ34S value of gypsum in the Keuper evaporites and sulfate in the springs is δ34SSO4 = 14.06 ± 1.07‰ and δ18OSO4 = 13.41 ± 1.44‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 124 and 288 ppm. The δ34S value of gypsum in the Wealden evaporites and sulfate in the springs is δ34SSO4 = 19.66 ± 1.76‰, δ18OSO4 = 14.93 ± 2.35‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 15 and 160 ppm. Wealden evaporites formed in a continental setting after the dissolution of Keuper salt. Gypsum δ34SSO4 and δ18OSO4 modification from Keuper to Wealden evaporites was due mainly to bacterial SO4 reduction in an anoxic, organic matter-rich environment. Saline springs with Wealden δ34SSO4 values are present in a 70 × 20 km wide area. Saline water temperatures, their δ2HH2O and δ18OH2O values, and the geological structure defines a hydrogeological model, where meteoric water recharges at heights up to 620 m above spring levels and circulates down to 720 m below them, thereby constraining the height range of evaporite dissolution. Groundwater flow towards saline springs is driven by gravity and buoyancy forces constrained by a thrust and fault network.

Using stable isotopes to understand hydrochemical processes in and around a Prairie Pothole wetland in the Northern Great Plains, USA

Abstract Millions of internally drained wetland systems in the Prairie Potholes region of the northern Great Plains (USA and Canada) provide indispensable habitat for waterfowl and a host of other ecosystem services. The hydrochemistry of these systems is complex and a crucial control on wetland function, flora and fauna. Wetland waters can have high concentrations of SO 4 2 - due to the oxidation of large amounts of pyrite in glacial till that is in part derived from the Pierre shale. Water chemistry including δ18OH2O, δ2HH2O, and δ34SSO4 values, was determined for groundwater, soil pore water, and wetland surface water in and around a discharge wetland in North Dakota. The isotopic data for the first time trace the interaction of processes that affect wetland chemistry, including open water evaporation, plant transpiration, and microbial SO4 reduction.

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

Abstract Laboratory experiments were conducted to simulate chalcopyrite oxidation under anaerobic and aerobic conditions in the absence or presence of the bacterium Acidithiobacillus ferrooxidans. Experiments were carried out with 3 different oxygen isotope values of water (δ18OH2O) so that approach to equilibrium or steady-state isotope fractionation for different starting conditions could be evaluated. The contribution of dissolved O2 and water-derived oxygen to dissolved sulfate formed by chalcopyrite oxidation was unambiguously resolved during the aerobic experiments. Aerobic oxidation of chalcopyrite showed 93 ± 1% incorporation of water oxygen into the resulting sulfate during the biological experiments. Anaerobic experiments showed similar percentages of water oxygen incorporation into sulfate, but were more variable. The experiments also allowed determination of sulfate–water oxygen isotope fractionation, e18OSO4–H2O, of ~ 3.8‰ for the anaerobic experiments. Aerobic oxidation produced apparent eSO4–H2O values (6.4‰) higher than the anaerobic experiments, possibly due to additional incorporation of dissolved O2 into sulfate. δ34SSO4 values are ~ 4‰ lower than the parent sulfide mineral during anaerobic oxidation of chalcopyrite, with no significant difference between abiotic and biological processes. For the aerobic experiments, a small depletion in δ34SSO4 of ~− 1.5 ± 0.2‰ was observed for the biological experiments. Fewer solids precipitated during oxidation under aerobic conditions than under anaerobic conditions, which may account for the observed differences in sulfur isotope fractionation under these contrasting conditions.

The sulphur isotope and hydrochemical characteristics of Skeiðarársandur, Iceland: identification of solute sources and implications for weathering processes

AbstractThe hydrochemistry of sulphur in the active outwash plain of Skeiðarársandur, SE Iceland shows considerable variety reflecting differences in sources and processes, and is investigated in this article using δ34S values of dissolved sulphate and igneous sulphide minerals, δ18OH2O, and ionic concentrations in waters. Seawater‐derived sulphate is a notable component only in the lower sandur (δ34SSO4 = +6·8‰ VCDT). δ34SSO4 in the Skeiðará glacial melt river range from +3·4 to +8·8‰δ34S. The higher values occur during a flood event and reflect geothermal sulphate contributions from the Grímsvötn caldera. ‘Kettle‐hole lakes’ formed since the November 1996 jökulhlaup form important ecological niches and show evidence of coupled sulphide oxidation and carbonate and possibly silicate dissolution and have the lowest δ34SSO4 values (average −0·8‰) attributed to higher proportions of sulphide oxidation‐derived SO42− and/or bacterially mediated sulphide oxidation. The hydrochemistry of eastern sandur groundwater is influenced by a hydrothermal source of reduced sulphur from the Grímsvötn caldera or a localized area of geothermal springs, and contrasts with the hydrochemistry of the majority of groundwaters and the Sulá and Gígjukvísl rivers in western and central Skeiðarársandur. These have lower [Ca2+ + Mg2+], [Na+ + K+] and SO42− concentrations and δ34S values consistent with sulphate derived from the oxidation of igneous sulphides. In comparison to catchments dominated by atmospherically transported sulphate, catchments such as this where the sulphur system is dominated by internal sources (rock‐derived and geothermal) will not be sensitive to the temporal decreasing trends in contributions from long‐range airborne sulphate resulting from reductions of sulphur emissions in North America and Europe. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Isotopic archives of sulphate in speleothems

The hemispheric impact of industrial emissions upon atmospheric sulphur loading is reflected in the sulphur depositional history recorded in cores from ice sheets. However, these do not reveal regional variations. Recently deposited speleothems are used here as archives of regional sulphur depositional history at two locations within the United Kingdom and Ireland. δ34S-so4 and δ18O-so4 present within speleothem carbonate are measured for the first time as part of a dual isotope approach to decode the speleothem sulphur record. The largely refractory nature of δ34S-so4 under oxidising conditions enables source provenance of atmospheric SO2, whereas the complex cycles of isotopic exchange and fractionation during incorporation of oxygen into sulphate molecules enable δ18O-so4 signatures to yield insights into ambient environmental conditions and biogeochemical cycling in the ecosystem above the cave. δ34S-so4 values extracted from speleothem carbonate formed within Browns Folly Mine, UK, range from +3.5 to +5.5‰ and δ18O-so4 +10.3 to +13.7‰. Both signatures lie within the range expected from sulphate deposition in industrial locations and reflect the transfer of sulphate into speleothem calcite with little fractionation. However, δ18O-so4 signatures at Crag Cave, western Ireland, are isotopically heavier than expected and approach isotopic equilibrium with δ18O-h2o under reducing conditions. Dual isotope analysis of δ34S-so4 and δ18O-so4 optimises the correct identification of sulphur sources and biogeochemical cycling prior to incorporation into the speleothem record. At carefully selected cave sites where drip water flowpaths into the cave remain oxic, speleothems hold the potential to retain records of atmospheric sulphur loading at the local and regional scale.

Sources of stream water sulfate during the spring snowmelt in boreal streams: Evidence from δ34S isotope measurements

Episodic hydrological events, such as snowmelt during spring, have a marked effect on stream water chemistry. Here we investigated how spring snowmelt affected δ34S values of sulfate in six streams situated in northern Sweden. Four streams had high δ34SSO4 values during base flow with values ranging from +11.9 to +8.6‰. During snowmelt the δ34SSO4 decreased to around +6‰. In one of the streams and in the forested upper reaches of a second stream, δ34SSO4 values were close to +5‰ during base flow and decreased to about +3.8‰ during the spring snowmelt. One stream, which drained cultivated postglacial sediments dominated by acid sulfuric soils, was differentiated from the other streams by low δ34SSO4 values (−5.0‰ to −0.5‰). We could identify two stream water SO4 sources: sedimentary sulfides and anthropogenic S. Bacterial dissimilatory sulfate reduction was identified as an important process affecting stream water δ34SSO4 values and suggests that in this boreal landscape, peatlands and possibly riparian zones have a large influence on the biogeochemistry of SO42− during base flow conditions. Our results suggest that during the spring snowmelt, snow S and desorbing SO4 of mainly anthropogenic origin are the two major S sources in four of the investigated streams. Two streams in forested areas also indicate that reoxidation of reduced S may be released during the spring flood. The stream in the cultivated area was found to be strongly influenced by the acid sulfuric soils independent of stream flow conditions.

Isotope Systematics of Sulfate-oxygen and Sulfate-sulfur in Six European Peatlands

Oxygen (O) and sulfur (S) isotope systematics in bog water sulfates were determined for six Sphagnum dominated wetlands located in the British Isles and the Czech Republic, Central Europe. Comparison of a polluted and unpolluted site showed that 4 times higher atmospheric S inputs led to 3 times higher bog water sulfate concentrations and substrate S concentrations, 3 times increased ranges of substrate S concentrations, and 3 times increased ranges of δ34S values. Sites with elevated atmospheric S inputs exhibited greater geochemical variability in wetland S species. Sulfate O–S isotope composition of bog pore water at a depth of 40 cm below surface differed from that of surface bog water, indicating that dissimilatory bacterial sulfate reduction, a process known to discriminate against the heavier isotopes 18O and 34S, occurred in surface peat layers. While bacterial sulfate reduction remained to be one of the main isotope-selective processes for sulfate in peat, it could not fully explain the O–S isotope systematics of peat waters. The ‘residual’ sulfate was not simultaneously enriched in the heavier isotopes 18O and 34S. Mixing of residual sulfate following bacterial sulfate reduction with the product of S2− reoxidation, cleavage of esters, and isotope exchange reactions may have contributed to the decoupling of the δ34Sso4 and δ18Sso4 values. Large within-site differences in δ18Sso4 and δ34Sso4 (up to 13 and 15‰, respectively) indicated little communication between the 0 and 40 cm peat depth at some sites. Extremely high δ18Sso4 and δ34Sso4 values found in several peat bog water samples from Connemara (Ireland), Thorne Moors (England) and Ocean (Czech Republic) were not seen in streams draining the wetlands. Direct runoff of atmogenic sulfate constituted a significant portion of the bog outflow. At the wetland scale, zones of dissimilatory bacterial sulfate reduction form pockets whose lateral hydrological fluxes are small.

Sulfur Speciation and Stable Isotope Trends of Water-Soluble Sulfates in Mine Tailings Profiles

Sulfur speciation and the sources of water-soluble sulfate in three oxidizing sulfidic mine tailings impoundments were investigated by selective dissolution and stable isotopes. The studied tailings impoundments--Piuquenes, Cauquenes, and Salvador No. 1--formed from the exploitation of the Rio Blanco/La Andina, El Teniente, and El Salvador Chilean porphyry copper deposits, which are located in Alpine, Mediterranean, and hyperarid climates, respectively. The water-soluble sulfate may originate from dissolution of primary ore sulfates (e.g., gypsum, anhydrite, jarosite) or from oxidation of sulfide minerals exposed to aerobic conditions during mining activity. With increasing aridity and decreasing pyrite content of the tailings, the sulfur speciation in the unsaturated oxidation zones showed a trend from dominantly Fe(III) oxyhydroxide fixed sulfate (e.g., jarosite and schwertmannite) in Piuquenes toward increasing presence of water-soluble sulfate at Cauquenes and Salvador No. 1. In the saturated primary zones, sulfate is predominantly present in water-soluble form (mainly as anhydrite and/or gypsum). In the unsaturated zone at Piuquenes and Cauquenes, the delta34S(SO4)values ranged from +0.5 per thousand to +2.0 per thousand and from -0.4 per thousand to +1.4 per thousand Vienna Canyon Diablo Troilite (V-CDT), respectively, indicating a major sulfate source from pyrite oxidation (delta34S(pyrite) = -1.1 per thousand and -0.9 per thousand). In the saturated zone at Piuquenes and Cauquenes, the values ranged from -0.8 per thousand to +0.3 per thousand and from +2.2 per thousand to +3.9 per thousand, respectively. At Cauquenes the 34S enrichment in the saturated zone toward depth indicates the increasing contribution of isotopically heavy dissolved sulfate from primary anhydrite (approximately +10.9 per thousand). At El Salvador No. 1, the delta34S(SO4) average value is -0.9 per thousand, suggesting dissolution of supergene sulfate minerals (jarosite, alunite, gypsum) with a delta34S approximately -0.7 per thousand as the most probable sulfate source. The gradual decrease of delta18O(SO4) values from the surface to the oxidation front in the tailings impoundments at Piuquenes (from -4.5 per thousand to -8.6 per thousand Vienna Standard Mean Ocean Water, V-SMOW) and at Cauquenes (from -1.3 per thousand to -3.5 per thousand) indicates the increasing importance of ferric iron as the main electron acceptor in the oxidation of pyrite. The different delta18O(SO4) values between the tailings impoundments studied here reflect the local climates.

Isotopic study of sulfate sources and residence times in a subalpine watershed

Stable sulfur and oxygen isotope ratios and naturally occurring 35SSO4 activities were used to examine sulfate sources, address the role of sulfur dynamics, and estimate residence times of atmospherically derived sulfate in Loch Vale Watershed, Colorado. In 1996, surface water samples from small streams flowing through talus, forest, and wetland areas had δ34SSO4 values ranging from 1.8 to 3.7‰. Values of δ18OSO4 at the three sites ranged from –1.3 to 3.7‰. Average δ34SSO4 and δ18OSO4 values in Loch Vale precipitation (1991–1999) are higher (5.2 and 13.6, respectively) than surface water values, indicating that some of the deposited sulfate is transformed and/or mixed with other sulfur sources in the watershed (e.g. mineral and organic sulfur). Sulfate ages determined by 35SSO4 activities support this and show that deposited sulfate may be stored on a timescale of 1 year or more prior to being released to surface waters.

Coupling between sulfur recycling and syndepositional carbonate dissolution: evidence from oxygen and sulfur isotope composition of pore water sulfate, South Florida Platform, U.S.A.

Sulfur cycling in Fe-poor, organic-rich shelf carbonates, known to have rapid rates of SO4−2 reduction, remains poorly studied despite the volumetric significance of shelf deposits in modern and ancient carbon budgets. We investigated sulfur cycling in modern carbonates of the Florida Platform from end-member depositional environments (muddy sands from the Atlantic reef tract and finer-grained mudbank and island flank deposits from Florida Bay). Relations between pore water chemistry (SO4−2, ΣCO2, Ca−2/Cl−) and oxygen and sulfur stable isotope compositions of SO4−2 require direct coupling between sulfur redox cycling and syndepositional carbonate dissolution.Oxygen isotope compositions of pore water sulfate were remarkably shifted away from the established value for marine SO4−2 (+9.5‰), despite near normal SO4−2/Cl− ratios. Chemical evolution was least in reef tract pore waters and greatest in Florida Bay. Relative to overlying seawater, mudbank sediments exhibited sulfate depletion, with δ18OSO4 and δ34SSO4 values both increasing by about 7‰. More bioturbated island flank sediments, colonized by Thalassia grass, had a 5‰ increase in δ18OSO4, variable δ34SSO4 values (+17.7 to +23.3‰) and exceptionally high Ca+2/Cl− ratios. The large excess of Ca+2 (up to 1.7 mM) requires a much larger acid source than the amounts derived from utilization of dissolved O2 (∼0.3 mM) and small degrees of net SO4−2 reduction (<0.5 mM reduced).A conceptual model was constructed using chemical and isotopic data on natural pore waters and on sulfate isotope fractionation factors obtained from sediment incubation experiments. The model outputs show that pore water compositions can be explained by a redox cycle where microbial SO4−2 reduction is followed by very efficient H2S oxidation, thus maintaining virtually invariant SO4−2/Cl− ratios. The enhanced O2 transport may be driven by associated marine grass rhizome systems and microbial communities established in bioturbated sediments. The net result of the cycle is that the rate of sulfide oxidation, which is largely balanced by the rate of microbial sulfate reduction, is stoichiometrically related to the rate of carbonate dissolution. This is consistent with previously reported rates of carbonate dissolution (∼400 μmol/cm2-yr) and average rates of sulfate reduction (∼200 μmol/cm2-yr) from the Florida Platform and a 2:1 stoichiometry.

Sulfur isotope values in a forested catchment over four years: Evidence for oxidation and reduction processes

A small catchment on the Swedish West Coast has been studied over four years to determine S dynamics by using S isotope ratios. A Norway spruce dominated forest covers the catchment, and small peat areas occur in the lower parts of the catchment. The runoff δ34SSO4 values varied both during the year, and from year to year. Over the period from February 1990 to December 1993, the δ34SSO4 values ranged from −1‰ to +11‰. Over the same period, the throughfall δ34SSO4 values ranged from +1‰ to +15‰. There was no correlation (r2 = 0.01; Pr(F) = 0.57) between δ34SSO4 values in throughfall and runoff. Since the only input of S to the catchment is atmospheric deposition, the long-term runoff S mass flux is controlled by the deposition. Therefore, processes in the catchment are responsible for the variation in the runoff δ34SSO4 values. During periods with 34SSO4 enriched runoff, bacterial dissimilatory SO42- reduction occurs in the catchment. After very dry periods, oxidation of this reduced S, which is 32S-enriched, can be traced in runoff. Previous studies of the catchment have not been able to distinguish between: 1) oxidation of reduced S and dry deposition, and 2) reduction and adsorption. From the current study, it can be concluded that adsorption and dry deposition cannot cause the observed variation in runoff δ34SSO4.