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The extent of fractionation of sulfur isotopes by sulfate-reducing microbes is dictated by genomic and environmental factors. A greater understanding of species-specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur-metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell-specific sulfate reduction rates < 0.3 × 10 -15 moles cell -1  day -1 . Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰-45‰) net isotope fractionation (ε 34 S sulfide-sulfate ). Measured ε 34 S values could be reproduced in a mechanistic fractionation model if 1%-2% of the microbial community (10%-60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate-reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology., (© 2019 John Wiley & Sons Ltd.)

We present a quantitative model for sulfur isotope fractionation accompanying bacterial and archaeal dissimilatory sulfate respiration. By incorporating independently available biochemical data, the model can reproduce a large number of recent experimental fractionation measurements with only three free parameters: (i) the sulfur isotope selectivity of sulfate uptake into the cytoplasm, (ii) the ratio of reduced to oxidized electron carriers supporting the respiration pathway, and (iii) the ratio of in vitro to in vivo levels of respiratory enzyme activity. Fractionation is influenced by all steps in the dissimilatory pathway, which means that environmental sulfate and sulfide levels control sulfur isotope fractionation through the proximate influence of intracellular metabolites. Although sulfur isotope fractionation is a phenotypic trait that appears to be strain specific, we show that it converges on near-thermodynamic behavior, even at micromolar sulfate levels, as long as intracellular sulfate reduction rates are low enough (<<1 fmol H2S⋅cell(-1)⋅d(-1)).

Global fossil data show that profound biodiversity loss preceded the final catastrophe that killed nearly 90% marine species on a global scale at the end of the Permian. Many hypotheses have been proposed to explain this extinction and yet still remain greatly debated. Here, we report analyses of all four sulphur isotopes ((32)S, (33)S, (34)S and (36)S) for pyrites in sedimentary rocks from the Meishan section in South China. We observe a sulphur isotope signal (negative δ(34)S with negative Δ(33)S) that may have resulted from limitation of sulphate supply, which may be linked to a near shutdown of bioturbation during shoaling of anoxic water. These results indicate that episodic shoaling of anoxic water may have contributed to the profound biodiversity crisis before the final catastrophe. Our data suggest a prolonged deterioration of oceanic environments during the Late Permian mass extinction.

Summary: The Gypsum Hill (GH) springs on Axel Heiberg Island in the Canadian high Arctic are host to chemolithoautotrophic, sulfur‐oxidizing streamers that flourish in the high Arctic winter in water temperatures from −1.3 to 7°C with ~8% salinity in a high Arctic winter environment with air temperatures commonly less than −40°C and an average annual air temperature of −15°C. Metagenome sequencing and binning of streamer samples produced a 96% complete Thiomicrorhabdus sp. metagenome‐assembled genome representing a possible new species or subspecies. This is the most cold‐ and salt‐extreme source environment for a Thiomicrorhabdus genome yet described. Metaproteomic and metatranscriptomic analysis attributed nearly all gene expression in the streamers to the Thiomicrorhabdus sp. and suggested that it is active in CO2 fixation and oxidation of sulfide to elemental sulfur. In situ geochemical and isotopic analyses of the fractionation of multiple sulfur isotopes determined the biogeochemical transformation of sulfur from its source in Carboniferous evaporites to biotic processes occurring in the sediment and streamers. These complementary molecular tools provided a functional link between the geochemical substrates and the collective traits and activity that define the microbial community's interactions within a unique polar saline habitat where Thiomicrorhabdus‐dominated streamers form and flourish. [ABSTRACT FROM AUTHOR]

It has been hypothesized that the overall size of-or efficiency of carbon export from-the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma). However, the timing, tempo, and trigger for this decrease remain poorly constrained. Here we test this hypothesis by studying the isotope geochemistry of sulfate minerals from the Belcher Group, in subarctic Canada. Using insights from sulfur and barium isotope measurements, combined with radiometric ages from bracketing strata, we infer that the sulfateminerals studied here record ambient sulfate in the immediate aftermath of the GOE (ca. 2,018 Ma). These sulfate minerals captured negative triple-oxygen isotope anomalies as low as ~ .0.8ë. Such negative values occurring shortly after the GOE require a rapid reduction in primary productivity of >80%, although even larger reductions are plausible. Given that these data imply a collapse in primary productivity rather than export efficiency, the trigger for this shift in the Earth system must reflect a change in the availability of nutrients, such as phosphorus. Cumulatively, these data highlight that Earth's GOE is a tale of feast and famine: A geologically unprecedented reduction in the size of the biosphere occurred across the end-GOE transition. [ABSTRACT FROM AUTHOR]

We present a new set of high precision measurements of relative 33 S/ 32 S, 34 S/ 32 S, and 36 S/ 32 S values in lunar mare basalts. The measurements are referenced to the Vienna-Canyon Diablo Troilite (V-CDT) scale, on which the international reference material, IAEA-S-1, is characterized by δ 33 S = −0.061‰, δ 34 S ≡ −0.3‰ and δ 36 S = −1.27‰. The present dataset confirms that lunar mare basalts are characterized by a remarkable degree of sulfur isotopic homogeneity, with most new and published SF 6 -based sulfur isotope measurements consistent with a single mass-dependent mean isotopic composition of δ 34 S = 0.58 ± 0.05‰, Δ 33 S = 0.008 ± 0.006‰, and Δ 36 S = 0.2 ± 0.2‰, relative to V-CDT, where the uncertainties are quoted as 99% confidence intervals on the mean. This homogeneity allows identification of a single sample (12022, 281) with an apparent 33 S enrichment, possibly reflecting cosmic-ray-induced spallation reactions. It also reveals that some mare basalts have slightly lower δ 34 S values than the population mean, which is consistent with sulfur loss from a reduced basaltic melt prior to eruption at the lunar surface. Both the sulfur isotope homogeneity of the lunar mare basalts and the predicted sensitivity of sulfur isotopes to vaporization-driven fractionation suggest that less than ≈1–10% of lunar sulfur was lost after a potential moon-forming impact event. [ABSTRACT FROM AUTHOR]

Abstract: Earthʼs oxygenation is often described in terms of two unidirectional steps at the beginning and end of the Proterozoic Eon, separated by a long-lived intermediate redox state. Recent work defines a more complicated path to oxygenation, exemplified by an apparent drop in oxidation state following the early Paleoproterozoic Lomagundi carbon isotope excursion. The timing of this proposed drop in oxidation state is not well constrained, and it is not clear how it relates to redox conditions during the remainder of the Proterozoic. Here we present a study of pyrite multiple-sulfur isotopes, supported by Fe speciation and organic carbon isotopes, from early Paleoproterozoic black shales. We find evidence for the rapid expansion of the seawater sulfate reservoir during the Great Oxidation Event at ca. 2.3 Ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca. 2.05 Ga. This scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early Paleoproterozoic. Comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early Paleoproterozoic did not return until the late Neoproterozoic. [Copyright &y& Elsevier]

The Platreef ore horizon of the Bushveld Complex, South Africa, is the third largest platinum group element ore deposit in the world, but the origin of its ore remains enigmatic. A complex contact relationship between the igneous and footwall rocks of the Bushveld Complex, coupled with evidence for widespread late-stage hydrothermal processing, obscures the original mineralization history of the Platreef. We constrain the parental magmatic origin of the Platreef by exploiting multiple sulfur isotope contrasts across Bushveld Complex contact zones in order to see through the effects of postmineralization hydrothermal activity. We report S isotope measurements made on samples collected along two profiles through the Platreef into underlying metapelitic and metacarbonate footwall rocks. In both profiles, igneous rocks far from the contact have low Δ33S values (average Δ33S = 0.15‰), whereas metasedimentary rocks far from the contact have high Δ33S values (Δ33S to 5.04‰) with a smoothly varying profile between the two end members. The midpoint in both isotope profiles is displaced into the footwall, defining a classic advective-dispersive tracer geometry. This geometry is not present in the associated δ34S values. The displacement of the Δ33S front suggests fluid transport and advection of S into the country rocks; this was accompanied by back diffusion of the S isotope tracer into the Platreef. The Platreef magma was apparently S saturated prior to emplacement and, counterintuitively, lost S during the formation of the present Platreef ore horizon. [ABSTRACT FROM AUTHOR]

Interest in multiple sulfur isotope analyses has been fueled by recent reports of mass-independent sulfur isotope signatures in the geologic record. A non-zero multiple isotopic signature of sulfur (Δ33S and Δ36S) is produced primarily through photochemical reactions, and it is an almost perfect tracer of the source of sulfur. Once the signature is passed on to a given sulfur reservoir, it will be preserved unless there is addition of sulfur with a different Δ33S or Δ36S. In the geological record, this signature has been used to study the evolution of the Earth’s atmosphere and to trace movement of sulfur through geological systems early in Earth’s history. Recently, small but significant mass-independent signatures have been reported for some younger samples, raising the possibility of additional applications of multiple isotope studies. The purpose of this review is to introduce and discuss the implications of multiple isotope studies and to focus attention on these anomalous, but not uncommon isotopic signatures. [Copyright &y& Elsevier]

ABSTRACT Isotopic fractionations produced by biosynthetic processes are the result of networks of individual biochemical reactions that operate at differing efficiencies and with distinct fractionation factors. These reaction networks determine the magnitude and direction of the net isotopic fractionation associated with a given process. Here we examine the ways that biological reaction networks control mass-dependent isotopic fractionations of multiple sulphur isotopes. We describe how material-flow through some networks can produce characteristic multiple-sulphur-isotope signatures that differ from those produced by their constituent steps and demonstrate that experimental results with Archaeaglobus fulgidus can be evaluated using multiple sulphur isotopes in the context of previously published models for dissimilatory sulphate reduction. Our evaluation of these data is consistent with the interpretation that the dependence of sulphur isotope fractionation on external sulphate concentration is rooted in differences between the forward and reverse &formmu0; ↔ adenosine-5′-phosphosulphate (APS) ↔ &formmu1; steps. The framework provided by our analysis has the potential to evaluate the biosynthetic pathways that produce the isotopic fractionations, to isolate the primary sources of isotopic fractionations (sulphate reduction or disproportionation reactions) and to establish criteria to identify the signature of specific sulphur metabolisms in the geological record. The results highlight the new types of information that can be obtained by including measurements of δ33 S {δ33 S = [(33 S/32 S)sample /(33 S/32 S)reference - 1]*1000} with measurements of δ34 S. [ABSTRACT FROM AUTHOR]

Abstract: The Rustenberg Layered Suite of the Bushveld Complex originated in the mantle, but is characterized by variably radiogenic strontium, neodymium, lead and osmium initial isotope ratios and more elevated δ18O values than expected from a partial melt of primitive mantle. It is not clear whether Bushveld magmas assimilated material from the crust, or whether they were instead generated from an eclogitic mantle source that contained crustal material. Here we report the Δ33S of fifteen whole-rock and sulfide separate samples from both the Main and Critical Zones of the Bushveld, including the UG2 Chromitite and the Merensky Reef. All samples have positive Δ33S values (0.11±0.02‰, 1σ) and δ34S values (2.2±0.6‰, 1σ). Non-zero Δ33S is fundamentally a surface process. Since high-temperature processes are not known to produce significant change in Δ33S, the positive Δ33S indicates that the Bushveld sulfur contains a sedimentary or surface-derived component. The homogeneity of Δ33S values for this suite of samples contrasts with the heterogeneity displayed by strontium, neodymium, lead and osmium initial isotope ratios of the Bushveld and requires that the sulfur contamination occurred prior to the emplacement of the Bushveld in the upper crust. Possible sources of contamination include lower crust and sub-continental lithospheric mantle. Limited multiple sulfur isotopic measurements of these reservoirs exist; more work needs to be done to constrain whether they contain surface-derived sulfur with non-zero Δ33S, and if they do, whether the concentrations and isotopic compositions of these reservoirs can explain the measured Δ33S of the Bushveld. [Copyright &y& Elsevier]

• CAS preserves fast metabolic rates of AOM-SR. • Pyrite integrates evolving sulfide under variable methane fluxes and oxidative cycling. • Protracted carbonate precipitation under non-steady state methane seepage. The anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR) is a major microbially-mediated methane consuming process in marine sediments including methane seeps. The AOM-SR can lead to the formation of methane-derived authigenic carbonates which entrap sulfide minerals (pyrite) and carbonate-associated sulfate (CAS). We studied the sulfur isotope compositions of the pyrite and CAS in seafloor methane-derived authigenic carbonate crust samples from the North Sea and Barents Sea which reflect the time-integrated metabolic activity of the AOM-SR community as well as the physical conditions under which those carbonates are formed. In these samples, pyrite exhibits δ 34 S values ranging from − 23.4 ‰ to 14.8‰ and Δ 33 S values between −0.06‰ and 0.16‰, whereas CAS is characterized by δ 34 S values ranging from 26.2‰ to 61.6‰ and Δ 33 S mostly between −0.05‰ and 0.07‰. Such CAS sulfur isotope compositions are distinctly lower in δ 34 S- Δ 33 S space from published porewater sulfate values from environments where the reduction of sulfate is mostly coupled to sedimentary organic matter oxidation. Mass-balance modelling suggests that (1) AOM-SR appears to cause rapid carbonate precipitation under high methane flux near or at the sediment-water interface and (2) that the precipitation of pyrite and carbonates are not necessarily synchronous. The sulfur isotopic composition of pyrite is interpreted to reflect more variable precipitating conditions of evolving sulfide with porewater connectivity, fluctuating methane fluxes and oxidative sulfur cycle. Taken together, the multiple isotopic compositions of pyrite and sulfate in methane-derived authigenic carbonates indicate protracted precipitation under conditions of non-steady state methane seepage activity. [ABSTRACT FROM AUTHOR]

The Proterozoic Eon spans Earth's middle age during which many important transitions occurred. These transitions include the oxygenation of the atmosphere, emergence of eukaryotic organisms and growth of continents. Since the sulfur and oxygen cycles are intricately linked to most surface biogeochemical processes, these transitions should be recorded in changes to the isotopic composition of marine and terrestrial sulfate minerals. Here we present oxygen (∆17O, δ18O) and sulfur (∆33S, δ34S) isotope records of Proterozoic sulfate from currently available data together with new measurements of 313 samples from 33 different formations bearing Earth's earliest unambiguous evaporites at 2.4 Ga through to Ediacaran aged deposits. This record depicts distinct intervals with respect to the expression of sulfate isotopes that are not completely captured by established intervals in the geologic timescale. The most salient pattern is the muted ∆17O signatures across the GOE , late Proterozoic and Ediacaran with values that are only slightly more negative than modern marine sulfate, contrasting with highly negative values across the mid-Proterozoic and Cryogenian. We combine these results with estimates of atmospheric composition to produce a gross primary production (GPP) curve for the Proterozoic. Through these results we argue that changes in GPP across Earth history likely help account for many of the changes in the Proterozoic Earth surface environment such as rising atmospheric oxygen, large fluctuations in the size of the marine sulfate reservoir and variations in the isotopic composition of sedimentary sulfate. [ABSTRACT FROM AUTHOR]

The Marathon conduit-type Cu-platinum-group element (PGE) deposit occurs within the Proterozoic Coldwell alkaline complex, Ontario, Canada, and comprises three zones of mineralization that occur at varying distances from the contact with Archean country rocks. Multiple sulfur isotope studies (δ34S, Δ33S, Δ36S) of sulfides from the Marathon deposit show that Δ33S and Δ36S measurements, as well as the relationship between them, are critical in demonstrating crustal S contamination. This is particularly significant for magmatic Cu-PGE deposits in contact with Archean country rocks where the δ34S of sulfides are within the range of typical mantle values (0‰ ± 2‰), as is the case at the Marathon deposit. Although the δ34S values of sulfides from the Marathon deposit did not reflect any evidence of crustal S assimilation, the Δ33S range (-0.91‰ to 0.00‰), and a negative correlation between Δ33S and Δ36S, indicate that S was derived from both the mantle and Archean supracrustal rocks. Given that the country rocks contain insignificant amounts of sulfide, crustal S assimilation must have occurred at depth, prior to magma emplacement. The magnitude of the Δ33S anomaly decreases with increasing distance from the basal contact, suggesting either that S contamination was greatest in the mineralized zone that occurred in close proximity to the contact or that crustal S signatures have been diluted in the mineralized zones further from the contact. [ABSTRACT FROM AUTHOR]

The cycle of sulfur, an important volatile in Earth's crust, is the driver of many significant processes such as biological evolution, climate change, and the formation of ore deposits. This study investigates the ancient cycle of volatiles by tracing the indelible signal of anomalous sulfur isotopes, expressed as Δ33S ≠ 0, to illuminate the pathway of sulfur recycling through magmatic arcs. We selected the ca. 2.0 Ga Glenburgh gold deposit in the Glenburgh magmatic arc of Western Australia as a natural laboratory for this study. Highprecision multiple sulfur isotope analyses of samples from the Glenburgh gold deposit and surrounding granitoid rocks yield the largest known sulfur isotope anomalies (Δ33S up to +0.82‰) in rocks <2.33 Ga globally. These data indicate that sulfur, and possibly gold, originated from multiple geochemical reservoirs in sedimentary rocks subducted beneath the magmatic arc, one of which is >2.33 Ga. Multiple sulfur isotope data are able to clarify a process that is cryptic to most other currently available data sets, showing that the cycling of volatiles and metals in arc settings occurs on very large scales, from the atmospherehydrosphere through to the lithosphere during crustal generation. [ABSTRACT FROM AUTHOR]

Scapolite is an important reservoir for volatiles in the deep crust and provides unique insights into the S isotope signatures at the mantle/crust interface. Here we document the first scapolite reference material (herein referred to as CB1) for in situ S isotope analysis. The chemical and isotopic composition of this euhedral, S-rich scapolite megacryst was characterized via LA-ICP-MS, EPMA, SIMS, and bulk fluorination gas source isotope ratio mass spectrometry. The CB1 scapolite is isotopically homogeneous and our results show that crystal orientation does not affect in situ S isotope SIMS analysis. This makes CB1 an ideal primary calibration standard for in situ analysis of S isotope ratios ( 36 S/ 32 S, 34 S/ 32 S and 33 S/ 32 S) in scapolite. With this reference material in hand, we then applied in situ SIMS analysis of S isotopes for the first time on scapolite in granulite samples from the lower crust/upper mantle. The analysed sample suite comprises rocks from classic granulite xenolith locations in southeastern Australia, as well as a sample from the high-grade suture zone of the Dahomeyides in south-eastern Ghana. The results show that scapolites in the lower crust have δ 34 S values between ~− 0.5 and + 4 (‰ VCDT). These values fall within the range of S isotope signatures present in mantle rocks and provide no evidence for the recycling of seawater-derived S into the lower crust. We propose that scapolite formed during granulite facies metamorphism of igneous cumulates, where S was sourced from precursor igneous sulfides. Sulfur isotope heterogeneities between individual scapolite grains in some of the studied samples may reflect non-uniform S-isotope compositions of igneous S-phases, which precipitated from mantle-derived melt. [ABSTRACT FROM AUTHOR]

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS 2 ), Nifty-b chalcopyrite (CuFeS 2 ), Alexo pyrrhotite (Fe (1 − x) S), and VMSO pentlandite ((Fe,Ni) 9 S 8 ) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ 33 S, δ 34 S, and δ 36 S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ 34 S and Δ 33 S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ 33 S values + 0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ 34 S values (δ 34 S chalcopyrite = 2.9 ± 0.3‰, δ 34 S pentlandite = 3.1 ± 0.2‰, and δ 34 S pyrrhotite = 3.9 ± 0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ 33 S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales. [ABSTRACT FROM AUTHOR]

Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous Δ O 17 values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the Δ O 17 anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum Δ O 17 value of − 0.75 ‰ . For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the Δ O 17 anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir. [ABSTRACT FROM AUTHOR]

Dissimilatory sulfate reduction is a microbial catabolic pathway that preferentially processes less massive sulfur isotopes relative to their heavier counterparts. This sulfur isotope fractionation is recorded in ancient sedimentary rocks and generally is considered to reflect a phenotypic response to environmental variations rather than to evolutionary adaptation. Modern sulfate-reducing microorganisms isolated from similar environments can exhibit a wide range of sulfur isotope fractionations, suggesting that adaptive processes influence the sulfur isotope phenotype. To date, the relationship between evolutionary adaptation and isotopic phenotypes has not been explored. We addressed this by studying the covariation of fitness, sulfur isotope fractionation, and growth characteristics in Desulfovibrio vulgaris Hildenborough in a microbial evolution experiment. After 560 generations, the mean fitness of the evolved lineages relative to the starting isogenic population had increased by ∼17%. After 927 generations, the mean fitness relative to the initial ancestral population had increased by ∼20%. Growth rate in exponential phase increased during the course of the experiment, suggesting that this was a primary influence behind the fitness increases. Consistent changes were observed within different selection intervals between fractionation and fitness. Fitness changes were associated with changes in exponential growth rate but changes in fractionation were not. Instead, they appeared to be a response to changes in the parameters that govern growth rate: yield and cell-specific sulfate respiration rate. We hypothesize that cell-specific sulfate respiration rate, in particular, provides a bridge that allows physiological controls on fractionation to cross over to the adaptive realm. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a comparison of ultraviolet photolysis experiments undertaken with SO2 (oxygen with isotopes at natural abundance levels) and S18O2 (18O-substituted oxygen). Experiments were conducted in a closed photocell using a deuterium lamp (principally 190–235nm) under pressure regimes (5–25Torr) that produced optically thick conditions for 32SO2 and variable optical depths for other isotopologues. The experiments, which were designed to examine the effects of intramolecular isotopic substitution of oxygen atoms on the S-MIF produced during UV photolysis of SO2, reveal generally reduced sulfur fractionation for 18O-rich SO2 as compared to 16O-rich SO2. Model shielding calculations were undertaken using spectra that were shifted due to changes in rotational and vibrational energy levels. The model calculations suggest that processes in addition to rotational and vibrational shifts in absorption spectra play a role in the experimentally produced isotope effects. Such additional processes may include differences in primary photoexcitation arising from smaller peak-to-valley amplitudes for fine structure of 18O-rich SO2 absorption spectra or an isotopically selective process associated with transitions between excited states. [Copyright &y& Elsevier]

Dissolved sulfate is a crucial electron acceptor for the subsurface biosphere, particularly for the living microbial ecosystems in the long-isolated (on the order of millions to billions of years) deep subsurface fracture waters in Precambrian cratons, e.g., in the Witwatersrand Basin of the Kaapvaal Craton, South Africa. Aiming to understand the role of sulfate in the sustainability of the subsurface habitat and the spatial extent of the terrestrial subsurface biosphere, we carried out a basin-scale examination on the sources and producing mechanisms of dissolved sulfur (sulfate and sulfide) in the subsurface fracture waters in the Witwatersrand Basin using multiple sulfur isotopes (32,33,34,36S) and oxygen isotopes (16,18O). Dissolved sulfates in 14 fracture water samples collected from 900 to 3,413 meters below the surface at 6 sites show isotopic ranges from −5.3‰ to +19.4‰ for δ 34S, −0.40‰ to +0.50‰ for Δ33S, and −1.1‰ to +10.9‰ for δ 18O. These isotopic signatures are distinct to those of the sulfate minerals (e.g., the carbonate-associated sulfate in local Transvaal Supergroup dolomite sequences: δ 34S = +31.4‰ to +39.2‰; Δ33S = −0.01‰ to +0.16‰), but identical to those of the sulfide minerals in the host rocks. This indicates that the dissolved sulfate in the fracture waters were dominantly generated by in situ oxidation of sulfide minerals at basin scale, although mixing of a small amount of surface sulfate at some sites cannot be completely ruled out. The dissolved sulfates in the less deep and less saline fracture waters are in oxygen isotope disequilibrium with their host waters, a surprising result given that the water residence times are orders of magnitude longer than the time required for oxygen isotope exchange to reach equilibrium. This implies that vigorous in situ sulfate production has occurred after the fracture waters were isolated. In contrast, the dissolved sulfate in the deeper, more saline waters are in apparent oxygen isotope equilibration or close to equilibration with their host waters. This might be attributed to a combined effect of faster oxygen isotope exchange between sulfate and water at higher temperatures, larger extent of sulfate reduction, and/or less efficient sulfate production. The dissolved sulfide in the fracture waters has similar Δ33S values but is 3.0‰ to 26.4‰ lower in δ 34S than coexisting sulfate, suggesting that the dissolved sulfide is mostly generated from bacterial sulfate reduction, which is consistent with the widespread existence of sulfate-reducing bacteria down to >3.4 km below surface in the Witwatersrand Basin. Overall, these novel isotopic results demonstrate that geological processes can provide a steady long-term sulfate source in deep fracture fluids by in situ oxidation of sulfide minerals in their host rocks, and thereby a mechanism to sustain the terrestrial subsurface biosphere, even in deep, high-temperature, long-isolated water systems. Thus, sulfate as a terminal electron acceptor is not the limiting factor for the spatial expansion of terrestrial subsurface biosphere. • Dissolved sulfates in subsurface fracture waters across the basin show large S-MIF. • Coexisting dissolved sulfide has similar S-MIF but much lower δ 34S values. • The S-MIF are not consistent with host-rock sulfate minerals but sulfide minerals. • Dissolved sulfate is formed by long-standing in situ oxidation of sulfide minerals. • The in situ sulfate production facilitates expansion of the biosphere to great depth. [ABSTRACT FROM AUTHOR]