Stable isotope fractionation during bacterial sulfate reduction is controlled by reoxidation of intermediates

Bacterial sulfate reduction is one of the most important respiration processes in anoxic habitats and is often assessed by analyzing the results of stable isotope fractionation. However, stable isotope fractionation is supposed to be influenced by the reduction rate and other parameters, such as temperature. We studied here the mechanistic basics of observed differences in stable isotope fractionation during bacterial sulfate reduction. Batch experiments with four sulfate-reducing strains (Desulfovibrio desulfuricans, Desulfobacca acetoxidans, Desulfonatronovibrio hydrogenovorans, and strain TRM1) were performed. These microorganisms metabolize different carbon sources (lactate, acetate, formate, and toluene) and showed broad variations in their sulfur isotope enrichment factors. We performed a series of experiments on isotope exchange of ¹⁸O between residual sulfate and ambient water. Batch experiments were conducted with ¹⁸O-enriched (δ¹⁸O_water = +700‰) and depleted water (δ¹⁸O_water = −40‰), respectively, and the stable ¹⁸O isotope shift in the residual sulfate was followed. For Desulfovibrio desulfuricans and Desulfonatronovibrio hydrogenovorans, which are both characterized by low sulfur isotope fractionation (ε_S > −13.2‰), δ¹⁸O values in the remaining sulfate increased by only 50‰ during growth when ¹⁸O-enriched water was used for the growth medium. In contrast, with Desulfobacca acetoxidans and strain TRM1 (ε_S < −22.7‰) the residual sulfate showed an increase of the sulfate δ¹⁸O close to the values of the enriched water of +700‰. In the experiments with δ¹⁸O-depleted water, the oxygen isotope values in the residual sulfate stayed fairly constant for strains Desulfovibrio desulfuricans, Desulfobacca acetoxidans and Desulfonatronovibrio hydrogenovorans. However, strain TRM1, which exhibits the lowest sulfur isotope fractionation factor (ε_S < −38.7‰) showed slightly decreasing δ¹⁸O values.Our results give strong evidence that the oxygen atoms of sulfate exchange with water during sulfate reduction. However, this neither takes place in the sulfate itself nor during formation of APS (adenosine-5′-phosphosulfate), but rather in intermediates of the sulfate reduction pathway. These may in turn be partially reoxidized to form sulfate. This reoxidation leads to an incorporation of oxygen from water into the “recycled” sulfate changing the overall ¹⁸O isotopic composition of the remaining sulfate fraction. Our study shows that such incorporation of ¹⁸O is correlated with the stable isotope enrichment factor for sulfur measured during sulfate reduction. The reoxidation of intermediates of the sulfate reduction pathway does also strongly influence the sulfur stable isotope enrichment factor. This aforesaid reoxidation is probably dependent on the metabolic conversion of the substrate and therefore also influences the stable isotope fractionation factor indirectly in a rate dependent manner. However, this effect is only indirect. The sulfur isotope enrichment factors for the kinetic reactions themselves are probably not rate dependent.     

Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes

At Lucky Strike near the Azores Triple Junction, the seafloor setting of the hydrothermal field in a caldera system with abundant low-permeability layers of cemented breccia, provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution. Coupled analyses of S isotopes performed in conjunction with Se and Fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems. These data provide a tool for resolving the different abiotic and potential biotic near-surface hydrothermal reactions. The δ³⁴S (between 1.5‰ and 4.6‰) and Se values (between 213 and 1640 ppm) of chalcopyrite suggest a high temperature end-member hydrothermal fluid with a dual source of sulfur: sulfur that was leached from basaltic rocks, and sulfur derived from the reduction of seawater sulfate. In contrast, pyrite and marcasite generally have lower δ³⁴S within the range of magmatic values (0 ± 1‰) and are characterized by low concentrations of Se (<50 ppm). For ⁸²Se/⁷⁶Se ratios, the δ⁸²Se values range from basaltic values of near −1.5‰ to −7‰. The large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between Se leached from igneous rock and Se derived from seawater. We interpret the Se isotope signature to be a result of leaching and mixing of a fractionated Se source located beneath hydrothermal chimneys in the hydrothermal fluid. At Lucky Strike we consider two sources for S and Se: (1) the “end-member” hydrothermal fluid with basaltic Se isotopic values (−1.5‰) and typical S isotope hydrothermal values of 1.5‰; (2) a fractionated source hosted in subsurface environment with negative δ³⁴S values, probably from bacterial reduction of seawater sulfate and negative δ⁸²Se values possibly derived from inorganic reduction of Se oxyanions. Fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by S isotopes. Fe isotope systematic reveals that Se-rich high temperature samples have δ⁵⁷Fe values close to basaltic values (∼0‰) whereas Se-depleted samples precipitated at medium to low temperature are systematically lighter (δ⁵⁷Fe values between −1 to −3‰). An important implication of our finding is that light Fe isotope composition down to −3.2‰ may be explained entirely by abiotic fractionation, in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions.     

Modelling nitrogen and oxygen isotope fractionation during denitrification in a lacustrine redox-transition zone

The stable isotope composition (δ¹⁵N and δ¹⁸O) of nitrate was measured during Summer 1999 in the anaerobic hypolimnion of eutrophic Lake Lugano (Switzerland). Denitrification was demonstrated by a progressive nitrate depletion coupled to increasing δ¹⁵N and δ¹⁸O values for residual nitrate. Maximum δ¹⁵N and δ¹⁸O values amounted to 27.2 and 15.7‰, respectively.¹⁵N and ¹⁸O enrichment factors for denitrification (ε) were estimated using a closed-system model and a dynamic diffusion-reaction model. Using the Rayleigh equation (closed-system approach), we obtained ε values of −11.2 and −6.6‰ for nitrogen and oxygen, respectively. The average ε values derived using the diffusion-reaction model were determined to be −20.7 ± 3.8 for nitrogen and −11.0 ± 1.7 for oxygen. Both N and O isotope fractionation appeared to be lower when denitrification rates where high, possibly in association with high organic carbon availability. In addition, variations in the isotope effects may be attributed to the variable importance of sedimentary denitrification having only a small isotope effect on the water column. The combined measurement of N and O isotope ratios in nitrate revealed that coupled nitrification-denitrification in the open-water was of minor importance. This is the first study of nitrogen and oxygen isotope effects associated with microbial denitrification in a natural lake. Moreover, this study confirms the high potential of δ¹⁸O of nitrate as a valuable biogeochemical tracer in aquatic systems, complementing nitrate δ¹⁵N.     

Effect of electron donors on the fractionation of sulfur isotopes by a marine Desulfovibrio sp.

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

Experimental determination of magnesium isotope fractionation during higher plant growth

Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ²⁶Mg_plant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ²⁶Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ²⁶Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ²⁶Mg_leaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ²⁶Mg_leaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo-Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ²⁶Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo-magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ²⁶Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ²⁶Mg that is comparable to analytical uncertainties.     

Kinetic oxygen isotope effects during dissimilatory sulfate reduction: A combined theoretical and experimental approach

Kinetic isotope effects related to the breaking of chemical bonds drive sulfur isotope fractionation during dissimilatory sulfate reduction (DSR), whereas oxygen isotope fractionation during DSR is dominated by exchange between intercellular sulfur intermediates and water. We use a simplified biochemical model for DSR to explore how a kinetic oxygen isotope effect may be expressed. We then explore these relationships in light of evolving sulfur and oxygen isotope compositions (δ³⁴S_SO4 and δ¹⁸O_SO4) during batch culture growth of twelve strains of sulfate-reducing bacteria. Cultured under conditions to optimize growth and with identical δ¹⁸O_H2O and initial δ¹⁸O_SO4, all strains show ³⁴S enrichment, whereas only six strains show significant ¹⁸O enrichment. The remaining six show no (or minimal) change in δ¹⁸O_SO4 over the growth of the bacteria. We use these experimental and theoretical results to address three questions: (i) which sulfur intermediates exchange oxygen isotopes with water, (ii) what is the kinetic oxygen isotope effect related to the reduction of adenosine phosphosulfate (APS) to sulfite (SO₃²⁻), (iii) does a kinetic oxygen isotope effect impact the apparent oxygen isotope equilibrium values? We conclude that oxygen isotope exchange between water and a sulfur intermediate likely occurs downstream of APS and that our data constrain the kinetic oxygen isotope fractionation for the reduction of APS to sulfite to be smaller than 4‰. This small oxygen isotope effect impacts the apparent oxygen isotope equilibrium as controlled by the extent to which APS reduction is rate-limiting.     

Quadruple sulfur isotope analysis of ca. 3.5 Ga Dresser Formation: New evidence for microbial sulfate reduction in the early Archean

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

Stable nitrogen isotope dynamics of dissolved nitrate in a transect from the north Pacific subtropical gyre to the eastern tropical north Pacific

The stable nitrogen isotopic composition of nitrate, concentrations of inorganic nitrogen and phosphorus, dissolved oxygen and nitrification rates were determined at six stations ranging from the oligotrophic North Pacific Subtropical Gyre (NPSG) to the more productive Eastern Tropical North Pacific (ETNP). Nitrification rates increased along the transect from a maximum rate of 1 nmol L⁻¹ d⁻¹ at station ALOHA to 23.7 nmol L⁻¹ d⁻¹ at station 6. In oxic surface waters, nitrate isotopically enriched in ¹⁵N (maximum δ¹⁵N-NO₃⁻ value of 12.5‰) was most likely the result of assimilatory nitrate reduction. In contrast, high δ¹⁵N-NO₃⁻ values (maximum of 12.3‰) in association with high nitrate deficits and anoxic conditions supported the interpretation of isotopic fractionation due to denitrification. A one-dimensional vertical advection and diffusion model was used to estimate the fractionation factor for denitrification at two stations in the ETNP. A comparison of modeled to observed δ¹⁵N-NO₃⁻ data indicated an isotopic enrichment factor (ε) of 30‰ at station 4 and 30 to 35‰ at station 5. Isotopically light nitrate (1.1 and 3.2‰) was observed in the upper 200 m of the water column at stations in the ETNP. Tracer studies of ¹⁵NH₄ and biogeochemical indicators of nitrogen fixation supported the interpretation of nitrification as the most plausible explanation for low δ¹⁵N-NO₃⁻ values observed in water column samples. Our results are consistent with the occurrence of nitrification within the euphotic zone and for the first time provide corroborating stable nitrogen isotopic evidence for this process.     

Influence of the enzyme dissimilatory sulfite reductase on stable isotope fractionation during sulfate reduction

The stable isotopes of sulfate are often used as a tool to assess bacterial sulfate reduction on the macro scale. However, the mechanisms of stable isotope fractionation of sulfur and oxygen at the enzymatic level are not yet fully understood. In batch experiments with water enriched in ¹⁸O we investigated the effect of different nitrite concentrations on sulfur isotope fractionation by Desulfovibrio desulfuricans.With increasing nitrite concentrations, we found sulfur isotope enrichment factors ranging from −11.2 ± 1.8‰ to −22.5 ± 3.2‰. Furthermore, the δ¹⁸O values in the remaining sulfate increased from approximately 50-120‰ when ¹⁸O-enriched water was supplied. Since ¹⁸O-exchange with ambient water does not take place in sulfate, but rather in intermediates of the sulfate reduction pathway (e.g. ), we suggest that nitrite affects the steady-state concentration and the extent of reoxidation of the metabolic intermediate sulfite to sulfate during sulfate reduction. Given that nitrite is known to inhibit the production of the enzyme dissimilatory sulfite reductase, our results suggest that the activity of the dissimilatory sulfite reductase regulates the kinetic isotope fractionation of sulfur and oxygen during bacterial sulfate reduction. Our novel results also imply that isotope fractionation during bacterial sulfate reduction strongly depends on the cell internal enzymatic regulation rather than on the physico-chemical features of the individual enzymes.     

Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution

Concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the Earth’s surface. This balance is expressed in the sulfur isotope composition of seawater sulfate. Given the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in Precambrian seawater. Although absolute CAS concentrations, which range from those below detection to ∼120 ppm sulfate in this study, may be compromised by diagenesis, the sulfur isotope compositions can be buffered sufficiently to retain primary values.Stratigraphically controlled δ³⁴S measurements for CAS from three mid-Proterozoic carbonate successions (∼1.2 Ga Mescal Limestone, Apache Group, Arizona, USA; ∼1.45-1.47 Ga Helena and Newland formations, Belt Supergroup, Montana, USA; and ∼1.65 Ga Paradise Creek Formation, McNamara Group, NW Queensland, Australia) show large isotopic variability (+9.1‰ to +18.9‰, −1.1‰ to +27.3‰, and +14.1‰ to +37.3‰, respectively) over stratigraphic intervals of ∼50 to 450 m. This rapid variability, ranging from scattered to highly systematic, and overall low CAS abundances can be linked to sulfate concentrations in the mid-Proterozoic ocean that were substantially lower than those of the Phanerozoic but higher than values inferred for the Archean. Results from the Belt Supergroup specifically corroborate previous arguments for seawater contributions to the basin. Limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep-ocean sulfate reduction, the scarcity of bedded gypsum, and the stratigraphic δ³⁴S trends and ³⁴S enrichments commonly observed for iron sulfides of mid-Proterozoic age.     

Constraining atmospheric oxygen and seawater sulfate concentrations during Paleoproterozoic glaciation: In situ sulfur three-isotope microanalysis of pyrite from the Turee Creek Group, Western Australia

Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ³³S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ³³S from −0.8‰ to 1.0‰). The 90‰ range in δ³⁴S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ³⁴S (<−20‰) overgrown by euhedral rims with higher δ³⁴S (4-7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO₂ first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.     

Annual fluctuations in sulfur isotope fractionation in the water column of a euxinic marine basin

The sulfur cycle of Mariager Fjord was studied by following the pool of sulfide in the anoxic water and its isotopic composition during a period of 3 yr. Though most of the sulfide accumulating in the fjord was formed in the sediment, the isotopic composition of sulfide in the water was different from the isotopic composition of sulfide diffusing into the water from the sediment. The mean isotopic composition of the water column sulfide (δ³⁴S) varied during the year between −13‰ and −21‰ with the most negative values reached during winter/early spring, while the sulfide diffusing into the water from the sediment had a mean isotope composition of −11.3‰. This annual pattern suggested that processes in the oxidative part of the sulfur cycle were responsible for the excess fractionation, and mass-balance considerations indicated that the excess fractionation of the sulfur isotopes could be accounted for by disproportionation of S⁰ or S₂O₃²⁻ in the water column, but not by water column sulfate reduction or sulfide oxidation alone. MPN counts demonstrated that a population of more than 3 × 10⁴ cells mL⁻¹ capable of growing by disproportionation of these two substrates was present in all depths of the fjord. The results presented in this communication demonstrate that the isotopic depletion of sulfide in anoxic systems may vary between periods of net sulfate reduction versus periods of net sulfide oxidation and indicate that disproportionation of sulfur compounds may be an important step in the sulfur cycle of euxinic basins.     

Stable Cu and Zn isotope ratios as tracers of sources and transport of Cu and Zn in contaminated soil

Copper and Zn metals are produced in large quantities for different applications. During Cu production, large amounts of Cu and Zn can be released to the environment. Therefore, the surroundings of Cu smelters are frequently metal-polluted. We determined Cu and Zn concentrations and Cu and Zn stable isotope ratios (δ⁶⁵Cu, δ⁶⁶Zn) in three soils at distances of 1.1, 3.8, and 5.3 km from a Slovak Cu smelter and in smelter wastes (slag, sludge, ash) to trace sources and transport of Cu and Zn in soils. Stable isotope ratios were measured by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) in total digests. Soils were heavily contaminated with concentrations up to 8087 μg g⁻¹ Cu and 2084 μg g⁻¹ Zn in the organic horizons. The δ⁶⁵Cu values varied little (−0.12‰ to 0.36‰) in soils and most wastes and therefore no source identification was possible. In soils, Cu became isotopically lighter with increasing depth down to 0.4 m, likely because of equilibrium reactions between dissolved and adsorbed Cu species during transport of smelter-derived Cu through the soil. The δ⁶⁶Zn_IRMM values were isotopically lighter in ash (−0.41‰) and organic horizons (−0.85‰ to −0.47‰) than in bedrock (−0.28‰) and slag (0.18‰) likely mainly because of kinetic fractionation during evaporation and thus allowed for separation of smelter-Zn from native Zn in soil. In particular in the organic horizons large variations in δ⁶⁶Zn values occur, probably caused by biogeochemical fractionation in the soil-plant system. In the mineral horizons, Zn isotopes showed only minor shifts to heavier δ⁶⁶Zn values with depth mainly because of the mixing of smelter-derived Zn and native Zn in the soils. In contrast to Cu, Zn isotope fractionation between dissolved and adsorbed species was probably only a minor driver in producing the observed variations in δ⁶⁶Zn values. Our results demonstrate that metal stable isotope ratios may serve as tracer of sources, vertical dislocation, and biogeochemical behavior in contaminated soil.     

Cl,N, C isotopic analysis of common agro-chemicals for identifying non-point source agricultural contaminants

The isotopic compositions of commercially available herbicides were analyzed to determine their respective ¹⁵N, ¹³C and ³⁷Cl signatures for the purposes of developing a discrete tool for tracing and identifying non-point source contaminants in agricultural watersheds. Findings demonstrate that of the agrochemicals evaluated, chlorine stable isotopes signatures range between δ³⁷Cl = −4.55‰ and +3.40‰, whereas most naturally occurring chlorine stable isotopes signatures, including those of road salt, sewage sludge and fertilizers, vary in a narrow range about the Standard Mean Ocean Chloride (SMOC) between −2.00‰ and +1.00‰. Nitrogen stable isotope values varied widely from δ¹⁵N = −10.86‰ to +1.44‰ and carbon stable isotope analysis gave an observed range between δ¹³C = −37.13‰ and −21.35‰ for the entire suite of agro-chemicals analyzed. When nitrogen, carbon and chlorine stable isotope analyses were compared in a cross-correlation analysis, statistically independent isotopic signatures exist suggesting a new potential tracer tool for identifying herbicides in the environment.     

The isotopic effects of electron transfer: An explanation for Fe isotope fractionation in nature

Isotope fractionation of electroplated Fe was measured as a function of applied electrochemical potential. As plating voltage was varied from −0.9 V to 2.0 V, the isotopic signature of the electroplated iron became depleted in heavy Fe, with δ⁵⁶Fe values (relative to IRMM-14) ranging from −0.18(±0.02) to −2.290(±0.006) ‰, and corresponding δ⁵⁷Fe values of −0.247(±0.014) and −3.354(±0.019) ‰. This study demonstrates that there is a voltage-dependent isotope fractionation associated with the reduction of iron. We show that Marcus’s theory for the kinetics of electron transfer can be extended to include the isotope effects of electron transfer, and that the extended theory accounts for the voltage dependence of Fe isotope fractionation. The magnitude of the electrochemically-induced fractionation is similar to that of Fe reduction by certain bacteria, suggesting that similar electrochemical processes may be responsible for biogeochemical Fe isotope effects. Charge transfer is a fundamental physicochemical process involving Fe as well as other transition metals with multiple isotopes. Partitioning of isotopes among elements with varying redox states holds promise as a tool in a wide range of the Earth and environmental sciences, biology, and industry.     

Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California

Mg isotope ratios (²⁶Mg/²⁴Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant ²⁶Mg/²⁴Mg ratio (expressed as δ²⁶Mg) at −0.79 ± 0.05‰, identical to seawater δ²⁶Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ²⁶Mg value of 0.11‰, potentially enriched in ²⁶Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ²⁶Mg, ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ²⁶Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (δ²⁶Mg_grass-δ²⁶Mg_rain) of 0.21‰. The deep pore-waters (1-15 m deep) have δ²⁶Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of ²⁴Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.     

Different types of methane monooxygenases produce similar carbon and hydrogen isotope fractionation patterns during methane oxidation

We determined the stable carbon and hydrogen isotope fractionation factors for methane oxidation under oxic conditions using strains with known degradation pathways. The aerobic oxidation of methane can be initiated by two different forms of enzymes known as methane monooxygenases (MMO). The expression of these enzymes is type-specific and dependent upon the adjusted copper concentration in the medium (or environment). In this study, the expression of either the soluble MMO or the particulate MMO was supported by adjusting the copper concentrations in the growth medium. Taxonomically different aerobic methanotrophic strains, mainly belonging to the alpha- and gamma- classes of Proteobacteria, produced methane isotope enrichment factors (ε_bulk) ranging from −14.8 to −27.9‰ for carbon, and from −110.0 to −231.5‰ for hydrogen. The ratio of hydrogen versus carbon discrimination (Λ = (α_H⁻¹ − 1)/(α_C⁻¹ − 1) ≈ Δ(δ²H)/Δ(δ¹³C)) were similar for all tested cultures, and are also identical to values calculated from previously published enrichment factors for aerobic and anaerobic methane degradation. In contrast, Λ-values for the abiotic oxidation of methane with OH radicals (this process is considered as the main removal process for methane from the atmosphere) were significantly higher than the values derived from biotic oxidation. Due to the low variability of microbial methane isotope fractionation patterns, we propose that combined carbon and hydrogen isotope fractionation analyses can be used to monitor and assess the occurrence of microbial methane oxidation in marine or terrestrial environments. However, it is not possible to distinguish distinct aerobic or anaerobic methane-oxidation pathways by this approach.     

Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Black Mesa, Arizona, USA

We present some of the first analyses of the stable isotopic composition of dissolved silicon (Si) in groundwater. The groundwater samples were from the Navajo Sandstone aquifer at Black Mesa, Arizona, USA, and the Si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed. Silicon isotope compositions were measured using high-resolution multi-collector inductively coupled mass spectrometry (HR-MC-ICP-MS) (Nu1700 & NuPlasma HR). The quartz dominated bulk rock and feldspar separates have similar δ³⁰Si of −0.09 ± 0.04‰ and −0.15 ± 0.04‰ (±95% SEM), respectively, and clay separates are isotopically lighter by up to 0.4‰ compared to the feldspars. From isotopic mass-balance considerations, co-existing aqueous fluids should have δ³⁰Si values heavier than the primary silicates. Positive δ³⁰Si values were only found in the shallow aquifer, where Si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals. However, δ³⁰Si decreases along the flow path from 0.56‰ to −1.42‰, representing the most negative dissolved Si isotope composition so far found for natural waters. We speculate that the enrichment in ²⁸Si is due to dissolution of partly secondary clay minerals and low-temperature silcretes in the Navajo Sandstone. The discovery of the large range and systematic shifts of δ³⁰Si values along a groundwater flow path illustrates the potential utility of stable Si isotopes for deciphering the Si cycling in sedimentary basins, tracing fluid flow, and evaluating global Si cycle.     

Sulfur isotope signatures in gypsiferous sediments of the Estancia and Tularosa Basins as indicators of sulfate sources, hydrological processes, and microbial activity

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

Low temperature volatile production at the Lost City Hydrothermal Field, evidence from a hydrogen stable isotope geothermometer

Although commonly utilized in continental geothermal work, the water-hydrogen and methane-hydrogen isotope geothermometers have been neglected in hydrothermal studies. Here we report δD-CH₄ and δD-H₂ values from high-temperature, black smoker-type hydrothermal vents and low-temperature carbonate-hosted samples from the recently discovered Lost City Hydrothermal Field. Methane deuterium content is uniform across the dataset at − 120 ± 12‰. Hydrogen δD values vary from − 420‰ to − 330‰ at high-temperature vents to − 700‰ to − 600‰ at Lost City. The application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high-temperature vents, and 20-60 °C higher than those measured at the Lost City vents. We conclude that the overestimation of temperature at Lost City reflects 1) that methane and hydrogen are produced by serpentinization at > 110 °C, and 2) that isotopic equilibrium at temperatures < 70 °C is mediated by microbial sulfate reduction. The successful application of hydrogen isotope geothermometers to low-temperature Lost City hydrothermal samples encourages its employment with low-temperature diffuse hydrothermal fluids.