Predictive isotope model connects microbes in culture and nature

In PNAS, Wing and Halevy (1) present a new model that quantitatively describes the magnitude of sulfur isotope fractionation produced by dissimilatory microbial sulfate reduction (MSR). MSR is a major player in the global biogeochemical cycles and is responsible for the respiration of up to 30% of organic matter in marine sediments (2). This metabolism produces large isotope effects, in which the product, sulfide, is depleted in the heavy isotopes (^(33)S, ^(34)S, and ^(36)S) relative to the most abundant isotope ^(32)S (3), enriching modern seawater sulfate in ^(34)S by about 21‰ (parts per thousand) compared with mantle sulfur. Sedimentary sulfur minerals preserve a record of this effect and are used to track changes in the sulfur isotope composition of seawater and the biogeochemical sulfur, carbon, and oxygen cycles through geologic time (4). Such reconstructions require an understanding of factors that control the magnitude of sulfur isotope effects and dictate the fractionation of sulfur isotopes by sulfate reducers under a range of growth conditions

Multiple-sulfur isotope effects during photolysis of carbonyl sulfide

Laboratory experiments were carried out to determine sulfur isotope effects during ultraviolet photolysis of carbonyl sulfide (OCS) to carbon monoxide (CO) and elemental sulfur (S[superscript 0]). The OCS gas at 3.7 to 501 mbar was irradiated with or without a N₂ bath gas using a 150 W Xe arc lamp. Sulfur isotope ratios for the product S[superscript 0] and residual OCS were analyzed by an isotope ratio mass-spectrometer with SF₆ as the analyte gas. The isotope fractionation after correction for the reservoir effects is −6.8‰ for the ratio [superscript 34]S/[superscript 32]S, where product S[superscript 0] is depleted in heavy isotopes. The magnitude of the overall isotope effect is not sensitive to the addition of N2 but increases to −9.5‰ when radiation of λ > 285 nm is used. The measured isotope effect reflects that of photolysis as well as the subsequent sulfur abstraction (from OCS) reaction. The magnitude of isotope effects for the abstraction reaction is estimated by transition state theory to be between −18.9 and −3.1‰ for [superscript 34]S which gives the photolysis isotope effect as −10.5 to +5.3‰. The observed triple isotope coefficients are ln(δ[superscript 34]S + 1)/ln(δ[superscript 34]S + 1) = 0.534 ± 0.005 and ln(δ[superscript 36]S + 1)/ln(δ[superscript 34]S + 1) = 1.980 ± 0.021. These values differ from canonical values for mass-dependent fractionation of 0.515 and 1.90, respectively. The result demonstrates that the OCS photolysis does not produce large isotope effects of more than about 10‰ for [superscript 34]S/[superscript 32]S, and can be the major source of background stratospheric sulfate aerosol (SSA) during volcanic quiescence.United States. National Aeronautics and Space Administration (Exobiology program, Grant No. NNX10AR85G

Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada

Recycling of surface-derived sulfur into the deep earth can impart distinct sulfur isotope signatures to magmas. The details of sulfur transfer from sedimentary rocks to magmas (and ultimately igneous rocks) through metamorphism and devolatilization and/or partial melting, however, is difficult to trace. To understand this process in detail we studied multiple-sulfur isotope compositions of sulfides in the Archean (c. 2685 Ma) Ghost Lake batholith (GLB) and its surrounding host metasedimentary rocks of the Superior Craton (Ontario, Canada) by high spatial resolution secondary ion mass spectrometry, complemented by high-precision gas source isotope ratio mass spectrometry measurements. The GLB comprises strongly peraluminous biotite+cordierite, biotite+muscovite, and muscovite+garnet+tourmaline granites to leucogranites, which are thought to represent the partial melts of surrounding metagreywackes and metapelites. The metasedimentary rocks display a range of metamorphic grades increasing from biotite-chlorite (280-380 °C) at ∼5 km away from the GLB to sillimanite-K-feldspar grade (∼660 °C) immediately adjacent to the batholith, thus providing a natural experiment to understand sulfur isotope variations from low- to high-grade Archean sedimentary rocks, as well as granites representative of their partial melts. We find that metasedimentary sulfide δ³⁴S values increase with progressive metamorphism at most 2-3‰ (from −1‰ up to +1 to +2‰). An increase in δ³⁴S values in pyrrhotite during prograde metamorphism can be explained through Rayleigh fractionation during pyrite desulfidation reactions. Pyrite from all but one of the granite samples preserve δ³⁴S values similar to that of the high-grade metasedimentary rocks, indicating that partial melting did not result in significant fractionation of δ³⁴S. The exception to this is one granite sample from a part of the batholith characterized by abundant metasedimentary inclusions. This sample contains pyrite with heterogeneous and low δ³⁴S values (down to −16‰) which likely resulted from incomplete homogenization of sulfur between the granitic melt and metasedimentary inclusions. Small (several tenths of a permil), mostly positive Δ³³S are observed in both the metasedimentary rocks and granites. Our results suggest that Archean strongly peraluminous granites could be a high-fidelity archive to quantify the bulk sulfur isotope composition of the Archean siliciclastic sediments. Further, our findings indicate that subduction of reduced sulfur-bearing sediments in the Archean with δ³⁴S at or near 0‰ should result in release of sulfur-bearing fluids in the mantle wedge with similar values (within a few permil). S-MIF (if initially present in Archean surface material) may be preserved during this process. However, the absence of S-MIF in igneous rocks does not preclude assimilation of Archean sedimentary material as either S-MIF may not be originally present in the Archean sedimentary sulfur and/or homogenization or dilution could obscure any S-MIF originally present in assimilated Archean sediments

Scanning Josephson Tunneling Microscopy of Single Crystal Bi\u3csub\u3e2\u3c/sub\u3eSr\u3csub\u3e2\u3c/sub\u3eCaCu\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e8+δ\u3c/sub\u3e with a Conventional Superconducting Tip

We have performed both Josephson and quasiparticle tunneling in vacuum tunnel junctions formed between a conventional superconducting scanning tunneling microscope tip and overdoped Bi2Sr2CaCu2O8+δ single crystals. A Josephson current is observed with a peak centered at a small finite voltage due to the thermal-fluctuation-dominated superconducting phase dynamics. Josephson measurements at different surface locations yield local values for the Josephson IcRn product. Corresponding energy gap measurements were also performed and a surprising inverse correlation was observed between the local IcRn product and the local energy gap

Sulfur isotope fractionation during oxidation of sulfur dioxide: gas-phase oxidation by OH radicals and aqueous oxidation by H2O2, O3 and iron catalysis

The oxidation of SO[subscript 2] to sulfate is a key reaction in determining the role of sulfate in the environment through its effect on aerosol size distribution and composition. Sulfur isotope analysis has been used to investigate sources and chemical processes of sulfur dioxide and sulfate in the atmosphere, however interpretation of measured sulfur isotope ratios is challenging due to a lack of reliable information on the isotopic fractionation involved in major transformation pathways. This paper presents laboratory measurements of the fractionation factors for the major atmospheric oxidation reactions for SO2: Gas-phase oxidation by OH radicals, and aqueous oxidation by H[subscript 2]O[subscript 2], O[subscript 3] and a radical chain reaction initiated by iron. The measured fractionation factor for [superscript 34]S/[superscript 32]S during the gas-phase reaction is α[subscript OH] = (1.0089±0.0007)−((4±5)×10[subscript −5]) T(°C). The measured fractionation factor for [superscript 34]S/[superscript 32]S during aqueous oxidation by H[subscript 2]O[subscript 2] or O[subscript 3] is α[subscript aq] = (1.0167±0.0019)−((8.7±3.5) ×10[superscript −5])T(°C). The observed fractionation during oxidation by H2O2 and O3 appeared to be controlled primarily by protonation and acid-base equilibria of S(IV) in solution, which is the reason that there is no significant difference between the fractionation produced by the two oxidants within the experimental error. The isotopic fractionation factor from a radical chain reaction in solution catalysed by iron is αFe = (0.9894±0.0043) at 19 °C for [superscript 34]S/[superscript 32]S. Fractionation was mass-dependent with regards to 33S/32S for all the reactions investigated. The radical chain reaction mechanism was the only measured reaction that had a faster rate for the light isotopes. The results presented in this study will be particularly useful to determine the importance of the transition metal-catalysed oxidation pathway compared to other oxidation pathways, but other main oxidation pathways can not be distinguished based on stable sulfur isotope measurements alone

Clumped isotopologue constraints on the origin of methane at seafloor hot springs

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 223 (2018): 141-158, doi:10.1016/j.gca.2017.11.030.Hot-spring fluids emanating from deep-sea vents hosted in unsedimented ultramafic and mafic rock commonly contain high concentrations of methane. Multiple hypotheses have been proposed for the origin(s) of this methane, ranging from synthesis via reduction of aqueous inorganic carbon (ΣCO2) during active fluid circulation to leaching of methane-rich fluid inclusions from plutonic rocks of the oceanic crust. To further resolve the process(es) responsible for methane generation in these systems, we determined the relative abundances of several methane isotopologues (including 13CH3D, a “clumped” isotopologue containing two rare isotope substitutions) in hot-spring source fluids sampled from four geochemically-distinct hydrothermal vent fields (Rainbow, Von Damm, Lost City, and Lucky Strike). Apparent equilibrium temperatures retrieved from methane clumped isotopologue analyses average 310−42 +53 °C, with no apparent relation to the wide range of fluid temperatures (96 to 370 °C) and chemical compositions (pH, [H2], [ΣCO2], [CH4]) represented. Combined with very similar bulk stable isotope ratios (13C/12C and D/H) of methane across the suite of hydrothermal fluids, all available geochemical and isotopic data suggest a common mechanism of methane generation at depth that is disconnected from active fluid circulation. Attainment of equilibrium amongst methane isotopologues at temperatures of ca. 270 to 360 °C is compatible with the thermodynamically-favorable reduction of CO2 to CH4 at temperatures at or below ca. 400 °C under redox conditions characterizing intrusive rocks derived from sub-ridge melts. Collectively, the observations support a model where methane-rich aqueous fluids, known to be trapped in rocks of the oceanic lithosphere, are liberated from host rocks during hydrothermal circulation and perhaps represent the major source of methane venting with thermal waters at unsedimented hydrothermal fields. The results also provide further evidence that water-rock reactions occurring at temperatures lower than 200 °C do not contribute significantly to the quantities of methane venting at mid-ocean ridge hot springs.Financial support from the U.S. National Science Foundation (NSF awards EAR-1250394 to S.O., and OCE-1061863 and OCE-0549829 to J.S.S.), the National Aeronautics and Space Administration (NASA) (NNX-327 09AB75G to J.S.S., and the NASA Astrobiology Institute “Rock- Powered Life” project under cooperative agreement NNA15BB02A to S.O.), the Alfred P. Sloan Foundation via the Deep Carbon Observatory (to S.O. and J.S.S.), the U.S. Department of Defense (DoD) through a National Defense Science & Engineering Graduate (NDSEG) Fellowship (to D.T.W.), a Shell-MIT Energy Initiative Fellowship, and the Kerr-McGee Professorship at MIT (to S.O.) is gratefully acknowledged

Abiotic redox reactions in hydrothermal mixing zones: decreased energy availability for the subsurface biosphere

© The Author(s), 202. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in McDermott, J. M., Sylva, S. P., Ono, S., German, C. R., & Seewald, J. S. Abiotic redox reactions in hydrothermal mixing zones: decreased energy availability for the subsurface biosphere. Proceedings of the National Academy of Sciences of the United States of America, 117(34), (2020): 20453-20461, doi:10.1073/pnas.2003108117.Subseafloor mixing of high-temperature hot-spring fluids with cold seawater creates intermediate-temperature diffuse fluids that are replete with potential chemical energy. This energy can be harnessed by a chemosynthetic biosphere that permeates hydrothermal regions on Earth. Shifts in the abundance of redox-reactive species in diffuse fluids are often interpreted to reflect the direct influence of subseafloor microbial activity on fluid geochemical budgets. Here, we examine hydrothermal fluids venting at 44 to 149 °C at the Piccard hydrothermal field that span the canonical 122 °C limit to life, and thus provide a rare opportunity to study the transition between habitable and uninhabitable environments. In contrast with previous studies, we show that hydrocarbons are contributed by biomass pyrolysis, while abiotic sulfate (SO42−) reduction produces large depletions in H2. The latter process consumes energy that could otherwise support key metabolic strategies employed by the subseafloor biosphere. Available Gibbs free energy is reduced by 71 to 86% across the habitable temperature range for both hydrogenotrophic SO42− reduction to hydrogen sulfide (H2S) and carbon dioxide (CO2) reduction to methane (CH4). The abiotic H2 sink we identify has implications for the productivity of subseafloor microbial ecosystems and is an important process to consider within models of H2 production and consumption in young oceanic crust.Financial support was provided by the National Aeronautics and Space Administration (NASA) Astrobiology program (Awards NNX09AB75G and 80NSSC19K1427 to C.R.G. and J.S.S.) and the NSF (Award OCE-1061863 to C.R.G. and J.S.S.). Ship and vehicle time for cruise FK008 was provided by the Schmidt Ocean Institute. We thank the ROV Jason II and HROV Nereus groups, and the captain, officers, and crew of R/V Atlantis (AT18-16) and R/V Falkor (FK008) for their dedication to skillful operations at sea. We thank our scientific colleagues from both cruises, as well as Meg Tivey, Frieder Klein, and Scott Wankel for insightful discussions. We are grateful to the editor and two anonymous reviewers for providing helpful comments and suggestions

Rapid oxygenation of Earths atmosphere 2.33 billion years ago

Molecular oxygen (O[subscript 2]) is, and has been, a primary driver of biological evolution and shapes the contemporary landscape of Earth’s biogeochemical cycles. Although “whiffs” of oxygen have been documented in the Archean atmosphere, substantial O2 did not accumulate irreversibly until the Early Paleoproterozoic, during what has been termed the Great Oxygenation Event (GOE). The timing of the GOE and the rate at which this oxygenation took place have been poorly constrained until now. We report the transition (that is, from being mass-independent to becoming mass-dependent) in multiple sulfur isotope signals of diagenetic pyrite in a continuous sedimentary sequence in three coeval drill cores in the Transvaal Supergroup, South Africa. These data precisely constrain the GOE to 2.33 billion years ago. The new data suggest that the oxygenation occurred rapidly—within 1 to 10 million years—and was followed by a slower rise in the ocean sulfate inventory. Our data indicate that a climate perturbation predated the GOE, whereas the relationships among GOE, “Snowball Earth” glaciation, and biogeochemical cycling will require further stratigraphic correlation supported with precise chronologies and paleolatitude reconstructions.National Science Foundation (U.S.) (EAR-1338810)National Natural Science Foundation (China) ((grant no. 41472170)Wellcome Trust Sanger Institute ( 111 Project grant no. B08030)National Basic Research Program of China (973 Program)United States. National Aeronautics and Space Administration (NASA Astrobiology Institute award NNA13AA90A

Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte–Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability