Publications

2024

Long-term interaction of metals and phosphate with ferrihydrite in artificial seawater and implications for past and present environments
Sugiyama I. & Halevy I. (2024) Chemical Geology. 670, 122448. Abstract
The Fe^III hydroxide ferrihydrite (Fh) is highly reactive towards dissolved metals and other aqueous compounds. This reactivity suggests a possible role for Fh in environmental remediation and in controlling ambient concentrations of metals and nutrients in both modern and ancient aqueous environments. Though the reactivity of Fh towards a variety of aqueous compounds has been determined in previous studies, its reactivity under seawater-like conditions has not been systematically explored. Since solution chemistry can have a profound effect on the uptake of metals and oxyions through its effects on the surface chemistry of Fh and the speciation of metals in solution, the dearth of experiments under seawater-like conditions translates into a knowledge gap of metal-Fh interactions in natural marine systems. Furthermore, metal- and nutrient-Fh interactions have been investigated under dissimilar conditions, making a quantitative comparison of these interactions difficult. Lastly, the fate of the aqueous metals and nutrients taken up by Fh upon its aging has been examined in only a few studies and mostly for a duration up to ∼8 months. We bridged these gaps in a series of adsorption and co-precipitation experiments of various metals and phosphate with Fh, in seawater-analog solutions, at circumneutral pH (pH 7.5 and 8.0) and 25 °C. We quantified the effect of interaction with Fh on the aqueous concentrations of Cd^II, Co^II, Cr^VI, Cu^II, Mn^II, Mo^VI, Ni^II, V^V, U^VI, Zn^II, and phosphate (PO₄³⁻). The experimental results are provided as uptake percentages (quantified as the metal:Fe ratio in the solid phase relative to the metal:Fe ratio added) at different metal:Fe ratios and as a series of partition coefficients of the studied metals and PO₄³⁻ between aqueous solution and Fh. Additionally, aging experiments of up to ∼4 years in duration were used to determine the potential effects of grain coarsening, surface modification and mineral transformation on the uptake of the metals and PO₄³⁻. Uptake of Cr^VI, Mo^VI and U^VI was negligible in both the adsorption and co-precipitation experiments. Uptake of V^V and PO₄³⁻ by co-precipitation was as much as twice their uptake by adsorption, and we suggest that stabilization of colloidal Fh with a large surface area explains these findings. For Cu^II, Ni^II, Co^II, Mn^II, and Cd^II differences in uptake between the co-precipitation and adsorption experiments were less pronounced, and we ascribe these differences to relatively minor incorporation of these metals into Fh. No significant transformation of Fh to more stable phases was observed under our experimental conditions and duration. Nevertheless, the uptake of several metals and phosphate was affected by Fh aging. Approximately 75% of the PO₄³⁻ initially taken up by co-precipitation with Fh was released, as was approximately 30% of the V^V. A pronounced increase in the uptake of Mn^II may be related to its oxidation and precipitation as Mn^III,IV oxides. For other metals the changes in uptake were less pronounced. We discuss the mechanisms of metal and PO₄³⁻ uptake by Fh, the effects of solution composition on uptake, and the implications of our findings for modern and ancient natural environments and for environmental remediation.
Substantial Extension of the Lifetime of the Terrestrial Biosphere
Graham R. J., Halevy I. & Abbot D. (2024) Planetary Science Journal. 5, 11, 255. Abstract
Approximately one billion years (Gyr) in the future, as the Sun brightens, Earths carbonate-silicate cycle is expected to drive CO₂ below the minimum level required by vascular land plants, eliminating most macroscopic land life. Here, we couple global-mean models of temperature- and CO₂-dependent plant productivity for C₃ and C₄ plants, silicate weathering, and climate to reexamine the time remaining for terrestrial plants. If weathering is weakly temperature dependent (as recent data suggest) and/or strongly CO₂dependent, we find that the interplay between climate, productivity, and weathering causes the future luminosity-driven CO₂ decrease to slow and temporarily reverse, averting plant CO₂ starvation. This dramatically lengthens plant survival from 1 Gyr up to ∼1.6-1.86 Gyr, until extreme temperatures halt photosynthesis, suggesting a revised kill mechanism for land plants and potential doubling of the future lifespan of Earths land macrobiota. An increased future lifespan for the complex biosphere may imply that Earth life had to achieve a smaller number of \u201chard steps\u201d (unlikely evolutionary transitions) to produce intelligent life than previously estimated. These results also suggest that complex photosynthetic land life on Earth and exoplanets may be able to persist until the onset of the moist greenhouse transition.
Production of Neoproterozoic banded iron formations in a partially ice-covered ocean
Gianchandani K., Halevy I., Gildor H., Ashkenazy Y. & Tziperman E. (2024) Nature Geoscience. 17, 4, p. 298-301 Abstract
The meridional extent of marine ice during the Neoproterozoic snowball Earth events is debated. Banded iron formations associated with the Sturtian glaciation are considered evidence for a completely ice-covered, ferruginous ocean (hard snowball). Here, using an ocean general circulation model with thick sea glaciers and Neoproterozoic biogeochemistry, we find that circulation in a partially ice-covered ocean (soft snowball) yields iron deposition patterns similar to the observed distribution of Sturtian banded iron formations.

2023

The geologic history of primary productivity
Crockford P. W., Bar On Y. M., Ward L. M., Milo R. & Halevy I. (2023) Current Biology. 33, 21, p. 4741-4750.e5 Abstract
The rate of primary productivity is a keystone variable in driving biogeochemical cycles today and has been throughout Earth's past.¹ For example, it plays a critical role in determining nutrient stoichiometry in the oceans,² the amount of global biomass,³ and the composition of Earth's atmosphere.⁴ Modern estimates suggest that terrestrial and marine realms contribute near-equal amounts to global gross primary productivity (GPP).⁵ However, this productivity balance has shifted significantly in both recent times⁶ and through deep time.^7,8 Combining the marine and terrestrial components, modern GPP fixes ≈250 billion tonnes of carbon per year (Gt C year⁻¹).^5,9,10,11 A grand challenge in the study of the history of life on Earth has been to constrain the trajectory that connects present-day productivity to the origin of life. Here, we address this gap by piecing together estimates of primary productivity from the origin of life to the present day. We estimate that ∼10¹¹10¹² Gt C has cumulatively been fixed through GPP (≈100 times greater than Earth's entire carbon stock). We further estimate that 10³⁹10⁴⁰ cells have occupied the Earth to date, that more autotrophs than heterotrophs have ever existed, and that cyanobacteria likely account for a larger proportion than any other group in terms of the number of cells. We discuss implications for evolutionary trajectories and highlight the early Proterozoic, which encompasses the Great Oxidation Event (GOE), as the time where most uncertainty exists regarding the quantitative census presented here.
Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios
Bryant R. N., Houghton J. L., Jones C., Pasquier V., Halevy I. & Fike D. A. (2023) Science (New York, N.Y.). 382, 6673, p. 912-915 Abstract
Reconstructions of past environmental conditions and biological activity are often based on bulk stable isotope proxies, which are inherently open to multiple interpretations. This is particularly true of the sulfur isotopic composition of sedimentary pyrite (δ34Spyr), which is used to reconstruct ocean-atmosphere oxidation state and track the evolution of several microbial metabolic pathways. We present a microanalytical approach to deconvolving the multiple signals that influence δ34Spyr, yielding both the unambiguous determination of microbial isotopic fractionation (εmic) and new information about depositional conditions. We applied this approach to recent glacial-interglacial sediments, which feature over 70 variations in bulk δ34Spyr across these environmental transitions. Despite profound environmental change, εmic remained essentially invariant throughout this interval and the observed range in δ34Spyr was instead driven by climate-induced variations in sedimentation.
Cathodoluminescence of iron oxides and oxyhydroxides
Galili N., Kaplan-Ashiri I. & Halevy I. (2023) American Mineralogist. 108, 8, p. 1436-1448 Abstract
Iron oxides and oxyhydroxides show promise as superconductor materials and as repositories of paleo-environmental information. However, there are no microscale non-destructive analytical techniques to characterize their combined mineralogy, chemical composition, and crystal properties. We address this by developing cathodoluminescence mounted on a scanning electron microscope (SEM-CL) as an in situ, non-destructive method for the crystallographic and petrographic study of iron oxides and oxyhydroxides. We show that goethite, hematite, and magnetite display different SEM-CL spectra, which may be used for mineral identification. We further show that different formation pH, manganese substitution for iron in goethite and hematite, and titanium substitution for iron in magnetite cause shifts in the SEM-CL spectra of these minerals. These spectral shifts are not always detectable as a change in the emission color but are easily discernable by quantitative analysis of the spectra. Together with subtle but observable variations in the SEM-CL spectra of natural goethite and hematite, we suggest that these dependences of the SEM-CL spectra on pH and chemical composition may be used as a means of identifying multiple episodes of mineralization and recrystallization. We apply the newly developed SEM-CL methods to two polished sections of natural samples and show that quantitative analysis of the spectra obtained allows the identification of differences between varieties of the same mineral that are not observable by other means. Like the application of SEM-CL to geologic samples in this study, we suggest that this approach may be used to explore the in situ chemistry and crystallinity of various natural and manufactured iron oxides and oxyhydroxides.
Nuclear spin effects in biological processes
Vardi O., Maroudas-Sklare N., Kolodny Y. et_al. (2023) Proceedings of the National Academy of Sciences of the United States of America. 120, 32, e230082812. Abstract
Traditionally, nuclear spin is not considered to affect biological processes. Recently, this has changed as isotopic fractionation that deviates from classical mass dependence was reported both in vitro and in vivo. In these cases, the isotopic effect correlates with the nuclear magnetic spin. Here, we show nuclear spin effects using stable oxygen isotopes (16O, 17O, and 18O) in two separate setups: an artificial dioxygen production system and biological aquaporin channels in cells. We observe that oxygen dynamics in chiral environments (in particular its transport) depend on nuclear spin, suggesting future applications for controlled isotope separation to be used, for instance, in NMR. To demonstrate the mechanism behind our findings, we formulate theoretical models based on a nuclear-spin-enhanced switch between electronic spin states. Accounting for the role of nuclear spin in biology can provide insights into the role of quantum effects in living systems and help inspire the development of future biotechnology solutions.
Editorial: The effects of early diagenesis in various marine environments on the stable isotope records of environmental conditions and biogeochemical processes
Lin Z., Strauss H., Zhao M. & Halevy I. (2023) Frontiers in Marine Science. 10, 1161577. Abstract
Sedimentary parameters control the sulfur isotope composition of marine pyrite
Halevy I., Fike D. A., Pasquier V., Bryant R. N., Wenk C. B., Turchyn A. V., Johnston D. T. & Claypool G. E. (2023) Science. 382, 6673, p. 946-952 Abstract
Reconstructions of coupled carbon, oxygen, and sulfur cycles rely heavily on sedimentary pyrite sulfur isotope compositions (δ³⁴S_pyr). With a model of sediment diagenesis, paired with global datasets of sedimentary parameters, we show that the wide range of δ³⁴S_pyr (~100 per mil) in modern marine sediments arises from geographic patterns in the relative rates of diffusion, burial, and microbial reduction of sulfate. By contrast, the microbial sulfur isotope fractionation remains large and relatively uniform. Over Earth history, the effect of increasing seawater sulfate and oxygen concentrations on sulfate and sulfide transport and reaction may explain the corresponding increase observed in the δ³⁴S offset between sulfate and pyrite. More subtle variations may be related to changes in depositional environments associated with sea level fluctuations and supercontinent cycles.
Morphometric constraints on the formation of new terrestrial analogs for planetary pits
Naor R., Mushkin A. & Halevy I. (2023) Geological Society of America Bulletin. 135, 9-10, p. 2200-2218 Abstract
The origin of geological depressions abounding on Mars and other planetary bodies remains poorly understood, partially due to the limited variability in the geological settings of existing terrestrial analogs. Here, we present a new terrestrial analog that is located at the northwestern margin of the Levantine volcanic field of Harrat Ash−Shaam along the Dead Sea Transform. The analog site consists of tens of geological depressions (locally named \u201cjuba\u201d) that morphologically resemble Martian bowl-shaped pits and occur within a Pleistocene basaltic plateau that overlies Meso-Cenozoic carbonates. To constrain plausible formation mechanisms for the juba depressions, we carried out detailed field mapping and morphometric analyses using a 0.25 m/pixel digital terrain model (DTM) derived from airborne light detection and ranging (LiDAR) survey covering 34 km2 of the study area, and centimeter-scale, ground-based LiDAR scans of selected juba depressions. We show that variable magnitudes of slope asymmetry between north- and south-facing walls within the juba depressions, along with different degrees of sediment infilling, provide effective proxies for the relative geomorphic maturity of these landforms, and in turn indicate asynchronous formation of the juba depressions after the Pleistocene emplacement of the Harrat Ash−Shaam basalts in the study area. Our findings preclude formation of the juba depressions by phreatomagmatic explosions and instead point toward collapse into missing subsurface volume. In a broader context, we propose that the morphometric analyses developed herein to distinguish between plausible juba formation mechanisms in the Harrat Ash−Shaam volcanic field can be extended to better constrain the formation mechanisms of similar pit features on Mars and other planetary bodies.
Rates of seafloor and continental weathering govern Phanerozoic marine phosphate levels
Sharoni S. & Halevy I. (2023) Nature Geoscience. 16, p. 75-81 Abstract
Phosphate is an essential macronutrient for all organisms with a key role in setting levels of marine primary productivity. Despite its importance for marine biogeochemical cycles and its role in shaping the evolution of marine organisms, the factors controlling phosphate bioavailability on geologic timescales remain poorly understood. Here we develop a statistical model of the coupled cycles of phosphate, carbon, oxygen and calcium to constrain the weathering-derived fluxes and seawater concentrations of phosphate through Phanerozoic time (541 million years ago to the present). Our model includes input parameters and time-dependent forcings derived from geologic and geochemical data. We find that the climate sensitivity of chemical weathering of the oceanic crust by low-temperature fluids exerts a first-order control on phosphate availability. Specifically, continental weathering is a source of the limiting nutrient phosphate, but seafloor weathering is considered to be a minor phosphate sink. Consequently, times in Earth history during which seafloor weathering constituted a large fraction of the total (seafloor+continental) weathering were also times during which phosphate influxes to and concentrations in the ocean were relatively low. Lower seawater phosphate levels during those times probably resulted in lower primary productivity and oceanic and atmospheric oxygen concentrations.

Submitted version

2022

Equilibrium fractionation of triple-oxygen and hydrogen isotopes between ice and water
Galili N., Sade Z. & Halevy I. (2022) Earth and Planetary Science Letters. 595, 117753. Abstract
The triple-oxygen isotopic fractionation associated with freezing is a fundamental property of water, knowledge of which is essential for reconstructions of the hydrological cycle from the triple-oxygen isotopic composition of natural materials. We constrained this isotopic fractionation, in freshwater and seawater, in a series of freezing experiments over a range of temperatures and freezing rates. The freshwater freezing experiments with the lowest freezing rates, which we consider closest to isotopic equilibrium, yield 18O/16O, 17O/16O and 2H/1H fractionations of 2.82±0.12, 1.49 ± 0.07 and 20.05 ± 0.72, respectively. The slowest-freezing seawater experiments yield 18O/16O, 17O/16O and 2H/1H fractionations of 2.92 ± 0.08, 1.55 ± 0.03 and 21.18 ± 1.85, respectively. The 18O/16O and 2H/1H fractionation estimates in freshwater and seawater are within error of each other and in broad agreement with past estimates. Our newly determined 17O/16O fractionations constrain the triple-oxygen mass dependence of water freezing to be ≈0.528, but with large uncertainty. If this mass dependence is accurate, then ice formation and melting processes in the hydrological cycle are expected to generate variability that is on the Global Meteoric Water Line.
Similar ice-freshwater and ice-seawater near-equilibrium isotopic fractionations.The ice-freshwater and ice-seawater 2H/1H fractionation is ≈21±2.The ice-freshwater and ice-seawater 18O/16O fractionation is ≈2.9±0.1.With large uncertainty, the oxygen isotope mass dependence of freezing is ≈0.528.Ice formation/melting should produce isotopic variability along the GMWL.
A global reassessment of the controls on iron speciation in modern sediments and sedimentary rocks: A dominant role for diagenesis
Pasquier V., Fike D. A., Revillon S. & Halevy I. (2022) Geochimica et Cosmochimica Acta. 335, p. 211-230 Abstract
The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry and oxidation state of ancient water bodies. Specifically, the fraction of reactive iron out of the total iron (Fe-HR/Fe-T) and the fraction of pyrite iron out of the reactive iron pool (FePYR/Fe-HR) are thought to constrain the oxidation state and the presence of sulfide in the water column, respectively. This approach was developed and tested against modern core-top sediments, but application to sedimentary rocks requires consideration of the effects of diagenesis and lithification on iron speciation. Furthermore, the effects of deep burial, metamorphism, and late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores (n = 54) with an extensive literature compilation of modern sediments (2936 measurements from 316 cores) and ancient sedimentary rocks (12,173 measurements spanning the Neoarchean to Quaternary). The modern data include both surface and buried sediments, allowing an investigation of the effects of diagenesis on iron speciation. Depending on the thresholds used to distinguish oxic from anoxic environments and ferruginous from euxinic environments, interpretation of the modern sedimentary iron speciation data within the existing framework yields incorrect environmental classifications up to X70% of the time. In modern sediments, diagenesis is the main reason that iron speciation does not represent the chemistry and oxidation state of the water column. We find that iron speciation correlates with porewater chemistry and that it changes with progressive burial along three distinctive Fe-HR/Fe-T-Fe-PYR/Fe-HR arrays, each of which represents a different set of diagenetic processes. We suggest that similarly to modern sediments, stratigraphic variation in iron speciation in sedimentary rocks primarily reflects progressive burial diagenesis or variation in depositional conditions rather than temporal variation in water-column chemistry and oxidation state. Indeed, analysis of the geologic iron speciation data reveals no statistically significant trends in either Fe-HR/Fe-T or Fe-PYR/Fe-HR from the Archean to the present day. The diagenetic Fe-HR/Fe-T-Fe-PYR/Fe-HR arrays that we identified in modern marine sediments suggest that under certain conditions, iron speciation analyses may be used to constrain Fe-HR/ Fe(T i)n the local sediment source(s). Hence, we suggest that iron speciation data, together with comple-mentary petrographic, mineralogical and geochemical constraints, may be used to constrain the local iron source(s) and early and late diagenetic processes, but rarely the chemistry or oxidation state of ancient water columns.
Questioning the Paradigm of a PhosphateLimited Archean Biosphere
Crockford P. & Halevy I. (2022) Geophysical Research Letters. 49, 17, e2022GL099. Abstract
The nature of Archean life remains one of the most contested topics in the study of Earth history. The debate may be formulated as follows: When did present day metabolisms emerge to ecological significance? What limited the productivity of early biospheres? How did the existence and productivity of individual metabolisms affect the chemistry and oxidation state of the oceanatmosphere? In a new study, Ingalls et al. (2022, https://doi.org/10.1029/2022GL098100) apply a novel proxy, carbonateassociated phosphate (CAP), to Neoarchean carbonate rocks and argue that seawater in the Neoarchean was more phosphaterich than through Phanerozoic time. Although the interpretation of CAP signals is currently burdened by uncertainties regarding Archean seawater chemistry, their results suggest Archean phosphate levels were comparable to modern seawater, if not higher. If true, then Earth's most successful metabolism, oxygenic photosynthesis, had either not achieved ecological prominence to exploit the relatively phosphaterich waters, or was curtailed by other underappreciated mechanisms.
The Effect of Ocean Salinity on Climate and Its Implications for Earth's Habitability
Olson S., Jansen M. F., Abbot D. S., Halevy I. & Goldblatt C. (2022) Geophysical Research Letters. 49, 10, e2021GL095. Abstract
The influence of atmospheric composition on the climates of present-day and early Earth has been studied extensively, but the role of ocean composition has received less attention. We use the ROCKE-3D ocean-atmosphere general circulation model to investigate the response of Earth's present-day and Archean climate system to low versus high ocean salinity. We find that saltier oceans yield warmer climates in large part due to changes in ocean dynamics. Increasing ocean salinity from 20 to 50 g/kg results in a 71% reduction in sea ice cover in our present-day Earth scenario. This same salinity change also halves the pCO2 threshold at which Snowball glaciation occurs in our Archean scenarios. In combination with higher levels of greenhouse gases such as CO2 and CH4, a saltier ocean may allow for a warm Archean Earth with only seasonal ice at the poles despite receiving ∼20% less energy from the Sun.
Controls on the isotopic composition of microbial methane
Gropp J., Jin Q. & Halevy I. (2022) Science advances. 8, 14, eabm5713. Abstract
Microbial methane production (methanogenesis) is responsible for more than half of the annual emissions of this major greenhouse gas to the atmosphere. Although the stable isotopic composition of methane is often used to characterize its sources and sinks, strictly empirical descriptions of the isotopic signature of methanogenesis currently limit these attempts. We developed a metabolic-isotopic model of methanogenesis by carbon dioxide reduction, which predicts carbon and hydrogen isotopic fractionations, and clumped isotopologue distributions, as functions of the cell's environment. We mechanistically explain multiple isotopic patterns in laboratory and natural settings and show that these patterns constrain the in situ energetics of methanogenesis. Combining our model with data from environments in which methanogenic activity is energy-limited, we provide predictions for the biomass-specific methanogenesis rates and the associated isotopic effects.
Kinetic isotope effect in siderite growth: Implications for the origin of banded iron formation siderite
Jiang C. Z., Halevy I. & Tosca N. J. (2022) Geochimica et Cosmochimica Acta. 322, p. 260-273 Abstract
The widespread occurrence of siderite (FeCO3) at Earth's modern surface and in sedimentary rocks has led to its frequent use as a tool for palaeoenvironmental reconstruction. Isotopic studies of siderite associated with Archaean-Palaeoproterozoic banded iron formations show negative δ13C values, which coupled with δ56Fe values, have been considered to support an important role of dissimilatory iron reduction (DIR) in the genesis of iron formations.Facies-specific analyses show that texturally and petrographically syndepositional and/or early diagenetic, finely laminated microsparitic (≤10 µm in diameter) siderite exhibits δ13C between −3 and −7. This siderite δ13C range can be interpreted in three ways: (1) precipitation of siderite from dissolved inorganic carbon (DIC) produced by DIR coupled to partial oxidation of organic carbon with δ13C
Strong evidence for a weakly oxygenated ocean-atmosphere system during the Proterozoic
Wang C., Lechte M. A., Reinhard C. T. et_al. (2022) Proceedings of the National Academy of Sciences - PNAS. 119, 6, e211610111. Abstract
Earths surface has undergone a protracted oxygenation, which is commonly assumed to have profoundly affected the biosphere. However, basic aspects of this history are still debatedforemost oxygen (O2) levels in the oceans and atmosphere during the billion years leading up to the rise of algae and animals. Here we use isotope ratios of iron (Fe) in ironstonesFe-rich sedimentary rocks deposited in nearshore marine settingsas a proxy for O2 levels in shallow seawater. We show that partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II). Regardless of the Fe(II) source, partial Fe(II) oxidation requires low O2 in the shallow oceans, settings crucial to eukaryotic evolution. Low O2 in surface waters can be linked to markedly low atmospheric O2likely requiring less than 1% of modern levels. Based on our records, these conditions persisted (at least periodically) until a shift toward higher surface O2 levels between ca. 900 and 750 Ma, coincident with an apparent rise in eukaryotic ecosystem complexity. This supports the case that a first-order shift in surface O2 levels during this interval may have selected for life modes adapted to more oxygenated habitats.
The effects of drip rate and geometry on the isotopic composition of speleothems: Evaluation with an advection-diffusion-reaction model
Sade Z., Hegyi S., Hansen M., Scholz D. & Halevy I. (2022) Geochimica et Cosmochimica Acta. 317, p. 409-432 Abstract
The curvature and slope of speleothem surfaces have been shown to affect the reaction rates in the aqueous carbonate system by altering the thickness of the CaCO3-precipitating solution. However, the effects of speleothem geometry and drip rate on the speleothems carbon and oxygen isotopic composition have yet to be investigated. Over more strongly sloping surfaces, solutions are thinner and flow faster. The effects of thinner and faster-flowing solutions on the isotopic composition of carbonate minerals precipitated from these solutions are of opposite sense. Thinner solutions enhance rates of CO2 degassing and mineral formation, increasing the degree of isotopic distillation of the dissolved inorganic carbon (DIC) reservoir and leading to larger isotopic fractionation between the carbonate mineral and the initial DIC. Concurrently, faster flow over the steeper surfaces results in shorter residence times of the solutions on the growing speleothem, thereby limiting the degree of isotopic distillation and CaCO3-DIC fractionation. Consequently, predicting the CaCO3-DIC isotopic fractionation as a function of drip rate, surface slope and flow distance is not trivial.
Using an advection-diffusion-reaction model, we tested the sensitivity of the isotopic composition of calcite precipitated along inclined surfaces to the solution discharge (drip) rate and the surface slope. Calcite δ13C and δ18O values correlate well with the degree of prior calcite precipitation (PCP), which is identified as a major determinant of isotopic compositions in speleothems. Our results show that at low PCP, speleothem δ13C and δ18O values may initially decrease relative to calcite-DIC and calcite-water equilibrium due to expression of kinetic isotope effects of mineral precipitation. Upon progressive PCP, δ13C and δ18O values gradually increase due to continuous CO2(aq) formation and degassing. This shift in the isotopic composition of the calcite to lower-than-equilibrium and then higher-than-equilibrium values expands the regime of near-equilibrium compositions during the isotopic evolution. In turn, this may allow quantitative environmental reconstructions, even when the isotopic system is in disequilibrium. Under the simulated conditions, our model predicts maximal potential enrichments of 7 and 3 in the calcite δ13C and δ18O values, respectively. In addition, we found a strong dependence of the calcite δ13C and δ18O values on the drip rate and distance of flow, and a weak dependence on the surface slope. In fact, changes in drip rate alone may drive isotopic offsets of several permil, when all other environmental parameters are kept constant. According to our model, higher drip rates and shorter stalactites promote closer-to-equilibrium isotopic compositions of stalagmites, providing a higher signal-to-noise ratio, and minimizing variability that is unrelated to climate.
Equilibration Times of Dissolved Inorganic Carbon During pH Transitions
Sade Z., Hegyi S. & Halevy I. (2022) Frontiers in earth science (Lausanne). 9, 792858. Abstract
Equilibration times of dissolved inorganic carbon (DIC) depend on conversion reactions between CO2(aq) and the dissociation products of carbonic acid [S = (H2CO3) + (HCO3−) + (CO32−)]. Here, we develop analytical equations and a numerical model to calculate chemical equilibration times of DIC during pH transitions in buffered and unbuffered solutions. We approximate the equilibration degree of the DIC reservoir by the smaller of the CO2(aq) and S pools at the new pH, since the smaller pool is always farther from equilibrium during the chemical evolution. Both the amount of DIC converted and the rate of conversion differ between a pH increase and decrease, leading to distinct equilibration times for these general cases. Alkalinity perturbations in unbuffered solutions initially drive pH overshoots (increase or decrease) relative to the new equilibrium pH. The increased rates of DIC conversion associated with the pH overshoot yield shorter equilibration times compared to buffered solutions. Salinity has opposing effects on buffered and unbuffered solutions, decreasing and increasing equilibration times, respectively.
Geologic controls on phytoplankton elemental composition
Sharoni S. & Halevy I. (2022) Proceedings of the National Academy of Sciences of the United States of America. 119, 1, e211326311. Abstract
Planktonic organic matter forms the base of the marine food web, and its nutrient content (C:N:Porg) governs material and energy fluxes in the ocean. Over Earth history, C:N:Porg had a crucial role in marine metazoan evolution and global biogeochemical dynamics, but the geologic history of C:N:Porg is unknown, and it is often regarded constant at the \u201cRedfield\u201d ratio of ∼106:16:1. We calculated C:N:Porg through Phanerozoic time by including nutrient- and temperature-dependent C:N:Porg parameterizations in a model of the long-timescale biogeochemical cycles. We infer a decrease from high Paleozoic C:Porg and N:Porg to present-day ratios, which stems from a decrease in the global average temperature and an increase in seawater phosphate availability. These changes in the phytoplanktons growth environment were driven by various Phanerozoic events: specifically, the middle to late Paleozoic expansion of land plants and the Triassic breakup of the supercontinent Pangaea, which increased continental weatherability and the fluxes of weathering-derived phosphate to the oceans. The resulting increase in the nutrient content of planktonic organic matter likely impacted the evolution of marine fauna and global biogeochemistry.

2021

Statistical Uncertainty in Paleoclimate Proxy Reconstructions
McClelland H. L. O., Halevy I., Wolf-Gladrow D. A., Evans D. & Bradley A. S. (2021) Geophysical Research Letters. 48, 15, e2021GL092. Abstract
A quantitative analysis of any environment older than the instrumental record relies on proxies. Uncertainties associated with proxy reconstructions are often underestimated, which can lead to artificial conflict between different proxies, and between data and models. In this paper, using ordinary least squares linear regression as a common example, we describe a simple, robust and generalizable method for quantifying uncertainty in proxy reconstructions. We highlight the primary controls on the magnitude of uncertainty, and compare this simple estimate to equivalent estimates from Bayesian, nonparametric and fiducial statistical frameworks. We discuss when it may be possible to reduce uncertainties, and conclude that the unexplained variance in the calibration must always feature in the uncertainty in the reconstruction. This directs future research toward explaining as much of the variance in the calibration data as possible. We also advocate for a \u201cdata-forward\u201d approach, that clearly decouples the presentation of proxy data from plausible environmental inferences.
Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone
Pasquier V., Fike D. A. & Halevy I. (2021) Nature Communications. 12, 4403. Abstract
Sulfur cycling is ubiquitous in sedimentary environments, where it mediates organic carbon remineralization, impacting both local and global redox budgets, and leaving an imprint in pyrite sulfur isotope ratios (δ
³⁴S
_pyr). It is unclear to what extent stratigraphic δ
³⁴S
_pyr variations reflect local aspects of the depositional environment or microbial activity versus global sulfur-cycle variations. Here, we couple carbon-nitrogen-sulfur concentrations and stable isotopes to identify clear influences on δ
³⁴S
_pyr of local environmental changes along the Peru margin. Stratigraphically coherent glacial-interglacial δ
³⁴S
_pyr fluctuations (>30) were mediated by Oxygen Minimum Zone intensification/expansion and local enhancement of organic matter deposition. The higher resulting microbial sulfate reduction rates led to more effective drawdown and
³⁴S-enrichment of residual porewater sulfate and sulfide produced from it, some of which is preserved in pyrite. We identify organic carbon loading as a major influence on δ
³⁴S
_pyr, adding to the growing body of evidence highlighting the local controls on these records.
The fate of fluvially-deposited organic carbon during transient floodplain storage
Scheingross J. S., Repasch M. N., Hovius N. et_al. (2021) Earth and Planetary Science Letters. 561, 116822. Abstract
CO₂ release from particulate organic carbon (POC) oxidation during fluvial transit can influence climate over a range of timescales. Identifying the mechanistic controls on such carbon fluxes requires determining where POC oxidation occurs in river systems. While field data show POC oxidation and replacement moving downstream in lowland rivers, flume studies show that oxidation during active fluvial transport is limited. This suggests that most fluvial POC oxidation occurs during transient floodplain storage, but this idea has yet to be tested. Here, we isolate the influence of floodplain storage time on POC oxidation by exploiting a chronosequence of floodplain deposits above the modern groundwater table in the Rio Bermejo, Argentina. Measurements from 15 floodplain cores with depositional ages from 1 y to 20 ky show a progressive POC concentration decrease and ¹³C-enrichment with increasing time spent in floodplain storage. These results from the Rio Bermejo indicate that over 80% of fluvially-deposited POC can be oxidized over millennial timescales in aerated floodplains. Furthermore, POC in the oldest floodplain cores is more ¹⁴C-enriched than expected based on the independently-dated floodplain ages, indicating that a portion of this oxidized POC is replaced by autochthonous POC produced primarily by floodplain vegetation. We suggest floodplain storage timescales control the extent of oxidation of fluvially-deposited POC, and may play a prominent role in determining if rivers are significant atmospheric CO₂ sources.
Sulfate-dependent reversibility of intracellular reactions explains the opposing isotope effects in the anaerobic oxidation of methane
Wegener G., Gropp J., Taubner H., Halevy I. & Elvert M. (2021) Science advances. 7, 19, eabe4939. Abstract
The anaerobic oxidation of methane (AOM) is performed by methanotrophic archaea (ANME) in distinct sulfate-methane interfaces of marine sediments. In these interfaces, AOM often appears to deplete methane in the heavy isotopes toward isotopic compositions similar to methanogenesis. Here, we shed light on this effect and its physiological underpinnings using a thermophilic ANME-1-dominated culture. At high sulfate concentrations, residual methane is enriched in both
¹³C and
²H (
^13α = 1.016 and
²
^α = 1.155), as observed previously. In contrast, at low sulfate concentrations, the residual methane is substantially depleted in
¹³C (
¹³
^α = 0.977) and, to a lesser extent, in
²H. Using a biochemical-isotopic model, we explain the sulfate dependence of the net isotopic fractionation through the thermodynamic drive of the involved intracellular reactions. Our findings relate these isotopic patterns to the physiology and environment of the ANME, thereby explaining a commonly observed isotopic enigma.
Reconstructing Neoproterozoic seawater chemistry from early diagenetic dolomite
Crockford P. W., Kunzmann M., Blättler C. L. et_al. (2021) Geology (Boulder). 49, 4, p. 442-446 Abstract
The pairing of calcium and magnesium isotopes (δ44/40Ca, δ26Mg) has recently emerged as a useful tracer to understand the environmental information preserved in shallow-marine carbonates. Here, we applied a Ca and Mg isotopic framework, along with analyses of carbon and lithium isotopes, to late Tonian dolostones, to infer seawater chemistry across this critical interval of Earth history. We investigated the ca. 735 Ma Coppercap Formation in northwestern Canada, a unit that preserves large shifts in carbonate δ13C values that have been utilized in global correlations and have canonically been explained through large shifts in organic carbon burial. Under the backdrop of these δ13C shifts, we observed positive excursions in δ44/40Ca and δ7Li values that are mirrored by a negative excursion in δ26Mg values. We argue that this covariation is due to early diagenetic dolomitization of aragonite through interaction with contemporaneous seawater under a continuum of fluid- to sediment-buffered conditions. We then used this framework to show that Tonian seawater was likely characterized by a low δ7Li value of ∼13 (∼18 lower than modern seawater), as a consequence of a different Li cycle than today. In contrast, δ13C values across our identified fluid-buffered interval are similar to modern seawater. These observations suggest that factors other than shifts in global seawater chemistry are likely responsible for such isotopic variation.

Accepted version
Strong local, not global, controls on marine pyrite sulfur isotopes
Pasquier V., Bryant R. N., Fike D. A. & Halevy A. (2021) Science advances. 7, 9, 7403. Abstract
Understanding variation in the sulfur isotopic composition of sedimentary pyrite (delta S-34(pyr)) is motivated by the key role of sulfur biogeochemistry in regulating Earth's surface oxidation state. Until recently, the impact of local depositional conditions on delta S-34(pyr) has remained underappreciated, and stratigraphic variations in delta S-34(pyr) were interpreted mostly to reflect global changes in biogeochemical cycling. We present two coeval delta S-34(pyr) records from shelf and basin settings in a single sedimentary system. Despite their proximity and contemporaneous deposition, these two records preserve radically different geochemical signals. Swings of similar to 65 parts per thousand in shelf delta S-34(pyr) track short-term variations in local sedimentation and are completely absent from the abyssal record. In contrast, a long-term similar to 30 parts per thousand decrease in abyssal delta S-34(pyr) reflects regional changes in ocean circulation and/or sustained pyrite formation. These results highlight strong local controls on delta S-34(pyr), calling for reevaluation of the current practice of using delta S-34(pyr) stratigraphic variations to infer global changes in Earth's surface environment.

2020

The Isotopic Imprint of Life on an Evolving Planet
Lloyd M. K., McClelland H. L., Antler G., Bradley A. S., Halevy I., Junium C. K., Wankel S. D. & Zerkle A. L. (2020) Space Science Reviews. 216, 7, 112. Abstract
Stable isotope compositions of biologically cycled elements encode information about the interaction between life and environment. On Earth, geochemical biomarkers have been used to probe the extent, nature, and activity of modern and ancient organisms. However, extracting biological information from stable isotopic compositions requires untangling the interconnected nature of the Earths biogeochemical system, and must be viewed through the lens of evolving metabolisms on an evolving planet. In this chapter, we provide an introduction to isotope geobiology and to the geobiological history of Earth. We discuss the isotope biogeochemistry of the biologically essential elements carbon, nitrogen and sulfur, and we summarize their distribution on the modern Earth as an interconnected network of isotopically fractionated reservoirs with contrasting residence times. We show how this framework can be used to explore the evolution of life and environments on the ancient Earth, which is our closest accessible analogue for an extraterrestrial planet.
Theoretical estimates of equilibrium carbon and hydrogen isotope effects in microbial methane production and anaerobic oxidation of methane
Gropp J., Iron M. A. & Halevy I. (2020) Geochimica et Cosmochimica Acta. 295, p. 237-264 Abstract
Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to identify its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (¹³C/¹²C) and hydrogen (D/H) isotopes among compounds in the anaerobic methane metabolisms, as well as the abundance of double isotope substitutions (\u201cclumping,\u201d i.e., a single ¹³CD bond or two ¹²CD bonds) in these compounds. The isotope fractionation factors were calculated using density functional theory at the M06-L/def2-TZVP level of theory with the SMD implicit solvation model, which we recently tested against measured equilibrium isotope fractionations. The computed ¹³β and ²β values decrease with decreasing oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment in ¹³C and D in the molecules with more oxidized carbon. Using the computed β values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0700 °C. Our calculated equilibrium fractionation factors compare favorably with experimental constraints, where available, and were then used to investigate the relation between the apparent isotope fractionation during methanogenesis or AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows for an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.
Nutrient ratios in marine particulate organic matter are predicted by the population structure of well-adapted phytoplankton
Sharoni S. & Halevy I. (2020) Science Advances. 6, 29, 9371. Abstract
A common assumption of a constant nitrogen-to-phosphorus ratio (N:P) of 16:1 in marine particulate organic matter (POM) appears to be invalidated by observations of major spatial variations in N:P. Two main explanations have been proposed. The first attributes the N:P variability to changes in the community composition of well-adapted phytoplankton. The second proposes that variability arises from physiological acclimation involving intracellular adjustments of nutrient allocation under nutrient deficiency. Using a model of phytoplankton physiology, observational datasets, and a review of laboratory culture results, we assess the mechanistic basis of N:P variability. We find that the taxonomic composition of well-adapted phytoplankton best explains observed variations in POM N:P. Furthermore, we show that acclimation to nutrient deficiency may be safely neglected when considering the effects of ecology on POM N:P. These findings provide insight into the controls on global variability in POM composition and average phytoplankton physiological performance in the oceans.
Sulfur isotope fractionation between aqueous and carbonate-associated sulfate in abiotic calcite and aragonite
Barkan Y., Paris G., Webb S. M., Adkins J. F. & Halevy I. (2020) Geochimica et Cosmochimica Acta. 280, p. 317-339 Abstract
Sulfate (SO42-) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbonate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S isotope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process. However, comparison between different calcifying species reveals both positive and negative S isotope fractionation between CAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on the component measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals, we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concentration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorption near-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionation between CAS and aqueous sulfate (CAS-SO42- isotope fractionation) on sulfate concentration and precipitation rate. In aragonite, the CAS-SO42- isotope fractionation is 1.0 +/- 0.3 parts per thousand and independent of the sulfate (and CAS) concentration. In contrast, the CAS-SO42- isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipitation rate, between values of 1.3 +/- 0.1 and 3.1 +/- 0.6 parts per thousand. We suggest that the correlation between aqueous sulfate concentration and CAS-SO42- isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on the concentration of CAS, through the effect of the sulfate impurity on the carbonate mineral's energetic state.

Submitted version
Kinetic fractionation of carbon and oxygen isotopes during BaCO3 precipitation
Sade Z., Yam R., Shemesh A. & Halevy I. (2020) Geochimica et Cosmochimica Acta. 280, p. 395-422 Abstract
Disequilibrium isotopic compositions in carbonate minerals often reflect the integration of several different kinetic isotope effects (KIEs), whose relative contribution to the overall composition depends on the specific mineral formation process and environment. Thus, potential environmental reconstructions from disequilibrium compositions in natural carbonates require i) quantification of KIEs associated with reactions involved in mineral formation, and ii) a theoretical framework linking physical properties of natural environments to the expression and preservation of these KIEs in carbonates. To constrain KIEs associated with carbonate mineral precipitation reactions, we performed a series of rapid witherite (BaCO3) precipitation experiments over a range of pH (8.7 to 13.0), temperature (15 to 40°C) and fractional yield of the dissolved inorganic carbon (DIC; a few percent to quantitative precipitation). Our experiments extend the range of pH and temperature explored in previous studies, and include measurements of both carbon and oxygen isotopes. We developed a dynamic model of the DIC system, with which we simulated the experiments. The model results identify isotopic distillation due to formation of aqueous CO2 and mineral precipitation by either CO32 or HCO3 as the main determinants of the carbon and oxygen isotopic evolution of the solid and of the solution. We estimate that the carbon kinetic fractionation factor (KFF) associated with unidirectional precipitation of witherite via CO32, 1000ln13αCO32-→BaCO3k+1, is 0.5±0.1 (15-40°C), whereas the oxygen KFF, 1000ln18αCO32-→BaCO3k+1, is 1.0±0.3 (15°C) and 0.5±0.2 (25-40°C). The carbon KFF associated with unidirectional HCO3 precipitation pathways, 1000ln13αHCO3-→BaCO3k+2,+3, is 0.2±1.0 (15-40°C), whereas the oxygen KFF, 1000ln18αHCO3-→BaCO3k+2,+3, is 6.7±1.1 (15°C), 4.5±1.1 (25°C) and 4.0±1.1 (40°C). Our oxygen KFF of witherite precipitation from CO32 agrees well with the available literature estimates at 25°C. In addition, our carbon and oxygen KFFs are comparable to literature KFFs associated with calcite precipitation, possibly suggesting a similarity of precipitation KFFs among carbonate minerals. Importantly, the relative magnitudes and uncertainties of the precipitation KFFs and the equilibrium fractionation between CO32 and HCO3, leads to the expectation that in many natural settings, the isotopic composition of carbonate minerals be an insensitive probe of the precipitation pathway.
Oxygen isotope effects during microbial sulfate reduction: applications to sediment cell abundances
Bertran E., Waldeck A., Wing B. A., Halevy I., Leavitt W. D., Bradley A. S. & Johnston D. T. (2020) ISME Journal. 14, 6, p. 1508-1519 Abstract
The majority of anaerobic biogeochemical cycling occurs within marine sediments. To understand these processes, quantifying the distribution of active cells and gross metabolic activity is essential. We present an isotope model rooted in thermodynamics to draw quantitative links between cell-specific sulfate reduction rates and active sedimentary cell abundances. This model is calibrated using data from a series of continuous culture experiments with two strains of sulfate reducing bacteria (freshwater bacterium Desulfovibrio vulgaris strain Hildenborough, and marine bacterium Desulfovibrio alaskensis strain G-20) grown on lactate across a range of metabolic rates and ambient sulfate concentrations. We use a combination of experimental sulfate oxygen isotope data and nonlinear regression fitting tools to solve for unknown kinetic, step-specific oxygen isotope effects. This approach enables identification of key isotopic reactions within the metabolic pathway, and defines a new, calibrated framework for understanding oxygen isotope variability in sulfate. This approach is then combined with porewater sulfate/sulfide concentration data and diagenetic modeling to reproduce measured
¹⁸O/
¹⁶O in porewater sulfate. From here, we infer cell-specific sulfate reduction rates and predict abundance of active cells of sulfate reducing bacteria, the result of which is consistent with direct biological measurements.

2019

Deciphering the atmospheric signal in marine sulfate oxygen isotope composition
Waldeck A. R., Cowie B. R., Bertran E., Wing B. A., Halevy I. & Johnston D. T. (2019) Earth and Planetary Science Letters. 522, p. 12-19 Abstract
Atmospheric O-2 and CO2 levels inform us of the changes in chemical and biological environments, yet the history of atmospheric compositions, and pO(2) in particular, is not well-constrained. The triple oxygen isotope (O-16,O-17,O-18) composition of marine SO42- has been proposed to directly record the ratio pO(2)/pCO(2) in the contemporaneous atmosphere. To resolve this atmospheric signal, both a precise measurement of the O-17 composition of sulfate and a model with which to interpret the measurement are needed. Here we present precise measurements of the triple oxygen isotope composition of modern marine sulfate and then forward a novel sulfur cycle model that deconvolves the potential atmospheric and microbial inputs to this signal. Our interpretation of marine sulfate oxygen isotope composition provides a framework for calculating atmospheric composition, relative rates of biogeochemical activity, and can be applied to geologic records of marine sulfate to constrain the pO(2)/pCO(2) ratio over time. (C) 2019 Elsevier B.V. All rights reserved.
The geologic history of seawater oxygen isotopes from marine iron oxides
Galili N., Shemesh A., Yam R. et_al. (2019) Science. 365, 6452, p. 469-473 Abstract
The oxygen isotope composition (d
¹⁸O) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid d
¹⁸O on the rocks' isotopic composition. A new d
¹⁸O record in marine iron oxides covering the past ~2000 million years shows a similar secular rise. Iron oxide precipitation experiments reveal a weakly temperature-dependent iron oxide-water oxygen isotope fractionation, suggesting that increasing seawater d
¹⁸O over time was the primary cause of the long-term rise in d
¹⁸O values of marine precipitates. The
¹⁸O enrichment may have been driven by an increase in terrestrial sediment cover, a change in the proportion of high- and low-temperature crustal alteration, or a combination of these and other factors.
Formation of green rust and elemental sulfur in an analogue for oxygenated ferro-euxinic transition zones of Precambrian oceans
Koeksoy E., Sundman A., Byrne J. M., Lohmayer R., Planer-Friedrich B., Halevy I., Konhauser K. O. & Kappler A. (2019) Geology. 47, 3, p. 211-214 Abstract
For much of the Precambrian era, the bulk ocean was anoxic and Fe(II) rich (ferruginous), except for the first development of shallow ocean oxygenation and temporally/spatially restricted sulfide-rich waters (euxinia) along productive continental margins in the late Archean, which prevailed throughout much of the remaining Precambrian. There is little detail pertaining to transition zones between ferruginous, euxinic, and oxic seawater over the continental shelf that may have played an important role in shaping the composition of the underlying sediment. Here we present spectroscopic data on the Fe and sulfur mineralogy in the Arvadi Spring (Switzerland), a proposed analogue for such conditions. Our study reveals green rust, ferrihydrite, and lepidocrocite as the main Fe minerals. Because the reactivity of green rust differs from that of ferric hydroxides and Fe(II) sulfides, it is important to understand its role in the transfer of metals and nutrients from seawater to underlying sediments, if those sediments are to be used as chemical archives of paleo-seawater. We observed elemental sulfur (S-0) as the dominant sulfur precipitate and found indications for its role in pyrite formation, implying that S-0 could have had a similar role in Precambrian deposition of pyrite-poor or pyrite-rich sediments.

Submitted version

2018

Deconstructing the Dissimilatory Sulfate Reduction Pathway: Isotope Fractionation of a Mutant Unable of Growth on Sulfate
Bertran E., Leavitt W. D., Pellerin A., Zane G. M., Wall J. D., Halevy I., Wing B. A. & Johnston D. T. (2018) Frontiers in Microbiology. 9, 3110. Abstract
The sulfur isotope record provides key insight into the history of Earth's redox conditions. A detailed understanding of the metabolisms driving this cycle, and specifically microbial sulfate reduction (MSR), is crucial for accurate paleoenvironmental reconstructions. This includes a precise knowledge of the step-specific sulfur isotope effects during MSR. In this study, we aim at resolving the cellular-level fractionation factor during dissimilatory sulfite reduction to sulfide within MSR, and use this measured isotope effect as a calibration to enhance our understanding of the biochemistry of sulfite reduction. For this, we merge measured isotope effects associated with dissimilatory sulfite reduction with a quantitative model that explicitly links net fractionation, reaction reversibility, and intracellular metabolite levels. The highly targeted experimental aspect of this study was possible by virtue of the availability of a deletion mutant strain of the model sulfate reducer Desulfovibrio vulgaris (strain Hildenborough), in which the sulfite reduction step is isolated from the rest of the metabolic pathway owing to the absence of its QmoABC complex (Delta Qmo). This deletion disrupts electron flux and prevents the reduction of adenosine phosphosulfate (APS) to sulfite. When grown in open-system steady-state conditions at 10% maximum growth rate in the presence of sulfite and lactate as electron donor, sulfur isotope fractionation factors averaged -15.9 parts per thousand (1 sigma = 0.4), which appeared to be statistically indistinguishable from a pure enzyme study with dissimilatory sulfite reductase. We coupled these measurements with an understanding of step-specific equilibrium and kinetic isotope effects, and furthered our mechanistic understanding of the biochemistry of sulfite uptake and ensuing reduction. Our metabolically informed isotope model identifies flavodoxin as the most likely electron carrier performing the transfer of electrons to dissimilatory sulfite reductase. This is in line with previous work on metabolic strategies adopted by sulfate reducers under different energy regimes, and has implications for our understanding of the plasticity of this metabolic pathway at the center of our interpretation of modern and palaeo-environmental records.
Dynamics of pyrite formation and organic matter sulfurization in organic-rich carbonate sediments
Shawar L., Halevy I., Said-Ahmad W., Feinstein S., Boyko V., Kamyshny A. & Amrani A. (2018) Geochimica et Cosmochimica Acta. 241, p. 219-239 Abstract
Organic-rich carbonate sediments are deposited in a range of environments today and in the geologic past. A significant part of organic matter (OM) degradation in such sediments often occurs by microbial reduction of seawater sulfate, and the sulfide product may be preserved in pyrite and in organic sulfur (S) compounds. The isotopic composition (δ
³⁴S) of these phases can provide valuable information about S cycling in the ocean and in sediment porewaters, but only insofar as the processes governing these δ
³⁴S values are understood. To this end, we investigated the pathways, timing and interactions between pyrite and organic S formation during the deposition of organic-rich chalks. As a test case, we studied cores representing the thickest (∼350 m) and most complete Late Cretaceous organic-rich sequence along the southern Paleotethyan margin (central Israel). The organic S and OM contents show an inverse relation with the pyritic S content, which together with the uniform Fe
_Py/Fe
_HR ratio (∼40%), suggest competition between organic S and pyrite formation. Both kerogen and pyritic S are
³⁴S-depleted relative to Late Cretaceous marine sulfate (δ
³⁴S∼1720), but the kerogen S is consistently and unusually
³⁴S-enriched relative to coexisting pyrite by up to ∼38. Large S isotope fractionation (∼60) during microbial sulfate reduction necessary to reproduce the lowest pyrite δ
³⁴S values in the core, and relatively invariant δ
³⁴S values in organic S suggests that this large fractionation was approximately constant during deposition of the chalks in the core. Higher pyrite δ
³⁴S values observed in the most organic-rich parts of the core may be explained by Fe-limited pyrite formation, perhaps due to the reaction of Fe (e.g., complexation, sorption) with organic compounds. Lesser Fe availability, relative to the OM available for sulfate reduction, limits the ultimate abundance of pyrite, but importantly, it delays the formation of pyrite to deeper below the sediment-water interface, from
³⁴S-enriched sulfide produced by Rayleigh distillation of a dwindling sulfate reservoir. Thus, it appears that competing Fe-OM, S-OM and Fe-S reactions can significantly affect the δ
³⁴S values recorded in pyrite in organic-rich carbonate sediments despite large and relatively constant microbial S isotope fractionation.
A case study for late Archean and Proterozoic biogeochemical iron- and sulphur cycling in a modern habitatthe Arvadi Spring
Koeksoy E., Halama M., Hagemann N. et_al. (2018) Geobiology. 16, 4, p. 353-368 Abstract
As a consequence of Earth's surface oxygenation, ocean geochemistry changed from ferruginous (iron(II)-rich) into more complex ferro-euxinic (iron(II)-sulphide-rich) conditions during the Paleoproterozoic. This transition must have had profound implications for the Proterozoic microbial community that existed within the ocean water and bottom sediment; in particular, iron-oxidizing bacteria likely had to compete with emerging sulphur-metabolizers. However, the nature of their coexistence and interaction remains speculative. Here, we present geochemical and microbiological data from the Arvadi Spring in the eastern Swiss Alps, a modern model habitat for ferro-euxinic transition zones in late Archean and Proterozoic oceans during high-oxygen intervals, which enables us to reconstruct the microbial community structure in respective settings for this geological era. The spring water is oxygen-saturated but still contains relatively elevated concentrations of dissolved iron(II) (17.2 ± 2.8 μM) and sulphide (2.5 ± 0.2 μM) with simultaneously high concentrations of sulphate (8.3 ± 0.04 mM). Solids consisting of quartz, calcite, dolomite and iron(III) oxyhydroxide minerals as well as sulphur-containing particles, presumably elemental S
⁰, cover the spring sediment. Cultivation-based most probable number counts revealed microaerophilic iron(II)-oxidizers and sulphide-oxidizers to represent the largest fraction of iron- and sulphur-metabolizers in the spring, coexisting with less abundant iron(III)-reducers, sulphate-reducers and phototrophic and nitrate-reducing iron(II)-oxidizers. 16S rRNA gene 454 pyrosequencing showed sulphide-oxidizing Thiothrix species to be the dominating genus, supporting the results from our cultivation-based assessment. Collectively, our results suggest that anaerobic and microaerophilic iron- and sulphur-metabolizers could have coexisted in oxygenated ferro-sulphidic transition zones of late Archean and Proterozoic oceans, where they would have sustained continuous cycling of iron and sulphur compounds.
Sulfur Isotope Fractionation by Sulfate-Reducing Microbes Can Reflect Past Physiology
Pellerin A., Wenk C. B., Halevy I. & Wine B. A. (2018) Environmental Science & Technology. 52, 7, p. 4013-4022 Abstract
Sulfur (5) isotope fractionation by sulfate-reducing microorganisms is a direct manifestation of their respiratory metabolism. This fractionation is apparent in the substrate (sulfate) and waste (sulfide) produced. The sulfate reducing metabolism responds to variability in the local environment, with the response determined by the underlying genotype, resulting in the expression of an "isotope phenotype". Sulfur isotope phenotypes have been used as a diagnostic tool for the metabolic activity of sulfate-reducing microorganisms in the environment. Our experiments with Desulfovibrio vulgaris Hildenborough (DvH) grown in batch culture suggest that the S isotope phenotype of sulfate respiring microbes may lag environmental changes on time scales that are longer than generational. When inocula from different phases of growth are assayed under the same environmental conditions, we observed that DvH exhibited different net apparent fractionations of up to -9 parts per thousand. The magnitude of fractionation was weakly correlated with physiological parameters but was strongly correlated to the age of the initial inoculum. The S isotope fractionation observed between sulfate and sulfide showed a positive correlation with respiration rate, contradicting the well-described negative dependence of fractionation on respiration rate. Quantitative modeling of S isotope fractionation shows that either a large increase (approximate to 50x) in the abundance of sulfate adenylyl transferase (Sat) or a smaller increase in sulfate transport proteins (approximate to 2x) is sufficient to account for the change in fractionation associated with past physiology. Temporal transcriptomic studies with DvH imply that expression of sulfate permeases doubles over the transition from early exponential to early stationary phase, lending support to the transport hypothesis proposed here. As it is apparently maintained for multiple generations (approximate to 1-6) of subsequent growth in the assay environment, we suggest that this fractionation effect acts as a sort of isotopic "memory" of a previous physiological and environmental state. Whatever its root cause, this physiological hysteresis effect can explain variations in fractionations observed in many environments. It may also enable new insights into life at energetic limits, especially if its historical footprint extends deeper than generational.
Corrigendum to "New constraints on kinetic isotope effects during CO2(aq) hydration and hydroxylation" : Revisiting theoretical and experimental data\u201d [Geochim. Cosmochim. Acta 214 (2017) 246265](S0016703717304593)(10.1016/j.gca.2017.07.035)
Sade Z. & Halevy I. (2018) Geochimica et Cosmochimica Acta. 225, p. 237-240 Abstract
The authors regret an error in the derivation of the link between KFFs and isotopic rate constants of CO
₂ hydration (Section 2.2). Considering the subset of isotopologues, C
¹⁶O
¹⁶O, C
¹⁸O
¹⁶O, H
₂
¹⁶O and H
₂
¹⁸O, there are four possible forward reactions: (1) C
¹⁶O
¹⁶O + H
₂
¹⁶O → H
₂C
¹⁶O
¹⁶O
¹⁶O,(2) C
¹⁸O
¹⁶O + H
₂
¹⁶O → H
₂C
¹⁸O
¹⁶O
¹⁶O,(3) C
¹⁶O
¹⁶O + H
₂
¹⁸O → H
₂C
¹⁸O
¹⁶O
¹⁶O,(4) C
¹⁸O
¹⁶O + H
₂
¹⁸O → H
₂C
¹⁸O
¹⁸O
¹⁶O.Reaction (4) involves rare isotopologues of both CO
₂ and of H
₂O, forms a doubly substituted isotopologue, and was not included in the original paper. Indeed, backward reaction (4) may be neglected in the derivation of the link between KFFs and isotopic rate constants of H
₂CO
₃ dehydration. However, forward reaction (4) still should have been considered in the derivation of the link between KFFs and isotopic rate constants of CO
₂ hydration. Including reaction (4) in the derivations presented in Section 2.2 and Appendix A.1 results in the following relations, (5) [Formula presented],(6) [Formula presented], which are distinct from those originally reported in Table 2, [Formula presented] and [Formula presented]. Thus, the dependence of the KFFs on the isotope ratio of the reactants, which was discussed in the paper, may be neglected. We have corrected Table 2 accordingly. Additionally, Fig. 2 of the original paper is obviated, as is its discussion in the text (last paragraph in Section 2.2). In the rest of the paper, this mistake affects only the theoretical KFF values of CO
₂ (de)hydration calculated using the isotopic rate constants of Zeebe (2014). The corrected KFF values differ by 24 from those reported in the paper, and are slightly farther from the experimental estimates after Clark and Lauriol (1992). We present below the corrected KFF values in revised Tables 4 and 6 along with values unaffected by the change (changes are underlined). Finally, we have revised Tables 8 and 9 and Figs. 3A and 4B which make use of the corrected KFFs to describe calcite precipitation at the kinetic limit. We thank Chen Zhou, University of Science and Technology of China, Hefei, for pointing out this mistake. We apologise for any inconvenience caused.
Electron carriers in microbial sulfate reduction inferred from experimental and environmental sulfur isotope fractionations
Wenk C. B., Wing B. A. & Halevy I. (2018) ISME Journal. 12, 2, p. 495-507 Abstract
Dissimilatory sulfate reduction (DSR) has been a key process influencing the global carbon cycle, atmospheric composition and climate for much of Earth's history, yet the energy metabolism of sulfate-reducing microbes remains poorly understood. Many organisms, particularly sulfate reducers, live in low-energy environments and metabolize at very low rates, requiring specific physiological adaptations. We identify one such potential adaptation-the electron carriers selected for survival under energy-limited conditions. Employing a quantitative biochemical-isotopic model, we find that the large S isotope fractionations (>55%) observed in a wide range of natural environments and culture experiments at low respiration rates are only possible when the standard-state Gibbs free energy (Delta G'degrees) of all steps during DSR is more positive than -10 kJ mol(-1). This implies that at low respiration rates, only electron carriers with modestly negative reduction potentials are involved, such as menaquinone, rubredoxin, rubrerythrin or some flavodoxins. Furthermore, the constraints from S isotope fractionation imply that ferredoxins with a strongly negative reduction potential cannot be the direct electron donor to S intermediates at low respiration rates. Although most sulfate reducers have the genetic potential to express a variety of electron carriers, our results suggest that a key physiological adaptation of sulfate reducers to low-energy environments is to use electron carriers with modestly negative reduction potentials.
Kinetics of Decomposition of Thiocyanate in Natural Aquatic Systems
Kurashova I., Halevy I. & Kamyshny A. (2018) Environmental Science & Technology. 52, 3, p. 1234-1243 Abstract
Rates of thiocyanate degradation were measured in waters and sediments of marine and limnic systems under various redox conditions, oxic, anoxic (nonsulfidic, nonferruginous, nonmanganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-Menten model was applied for description of the decomposition kinetics. The decomposition rate in the sediments was found to be higher than in the water samples. Under oxic conditions, thiocyanate degradation was faster than under anaerobic conditions. In the presence of hydrogen sulfide, the decomposition rate increased compared to anoxic nonsulfidic conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending on environmental conditions, half-lives of thiocyanate in sediments and water columns were in the ranges of hours to few dozens of days, and from days to years, respectively. Application of kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling and its concentrations in the Archean ocean.

2017

Reply to Comment on "Radiative Transfer in CO2-Rich Atmospheres: 1. Collisional Line Mixing Implies a Colder Early Mars"
Ozak N., Aharonson O. & Halevy I. (2017) Journal of Geophysical Research-Planets. 122, 11, p. 2366-2367 Abstract
Turbet and Tran (2017) have identified that we mistakenly used air-broadened CO2 absorption spectra to generate coefficients for a rapid radiative transfer code, instead of self-broadened spectra. The resulting underestimation of absorption by CO2 led us to suggest that when the effects of CO2 line mixing are taken into account, surface temperatures on early Mars are up to 15K colder than those calculated in previous studies, in which empirical corrections of the CO2 spectrum were used ( factors). Using the correct CO2 absorption spectra, including the effects of line mixing, Turbet and Tran (2017) found that the surface temperature is colder by only 2K than temperatures calculated with the factor approach. While we acknowledge the mistake, and thank Turbet and Tran (2017) for finding and correcting it, we note that our main conclusions hold that surface temperatures are overestimated when empirical factors are used instead of a full account of CO2 line mixing in radiative transfer calculations in CO2-rich planetary atmospheres. Additionally, we emphasize that the rapid radiative transfer model we developed and tested is useful for studies of planetary climate, provided the correct absorption spectra are used to generate new k coefficients.
New constraints on kinetic isotope effects during CO2(aq) hydration and hydroxylation: Revisiting theoretical and experimental data
Sade Z. & Halevy I. (2017) Geochimica et Cosmochimica Acta. 214, p. 246-265 Abstract
CO2(de) hydration (i.e., CO2 hydration/HCO3- dehydration) and (de) hydroxylation (i.e., CO2 hydroxylation/HCO3- dehydroxylation) are key reactions in the dissolved inorganic carbon (DIC) system. Kinetic isotope effects (KIEs) during these reactions are likely to be expressed in the DIC and recorded in carbonate minerals formed during CO2 degassing or dissolution of gaseous CO2. Thus, a better understanding of KIEs during CO2 (de) hydration and (de) hydroxylation would improve interpretations of disequilibrium compositions in carbonate minerals. To date, the literature lacks direct experimental constraints on most of the oxygen KIEs associated with these reactions. In addition, theoretical estimates describe oxygen KIEs during separate individual reactions. The KIEs of the related reverse reactions were neither derived directly nor calculated from a link to the equilibrium fractionation. Consequently, KIE estimates of experimental and theoretical studies have been difficult to compare. Here we revisit experimental and theoretical data to provide new constraints on oxygen KIEs during CO2 (de) hydration and (de) hydroxylation. For this purpose, we provide a clearer definition of the KIEs and relate them both to isotopic rate constants and equilibrium fractionations. Such relations are well founded in studies of single isotope source/sink reactions, but they have not been established for reactions that involve dual isotopic sources/sinks, such as CO2 (de) hydration and (de) hydroxylation. We apply the new quantitative constraints on the KIEs to investigate fractionations during simultaneous CaCO3 precipitation and HCO3- dehydration far from equilibrium. (C) 2017 The Author(s). Published by Elsevier Ltd.
The geologic history of seawater pH
Halevy I. & Bachan A. (2017) Science. 355, 6329, p. 1069-1071 Abstract
Although pH is a fundamental property of Earth's oceans, critical to our understanding of seawater biogeochemistry, its long-timescale geologic history is poorly constrained. We constrain seawater pH through time by accounting for the cycles of the major components of seawater. We infer an increase from early Archean pH values between -6.5 and 7.0 and Phanerozoic values between -7.5 and 9.0, which was caused by a gradual decrease in atmospheric pCO(2) in response to solar brightening, alongside a decrease in hydrothermal exchange between seawater and the ocean crust. A lower pH in Earth's early oceans likely affected the kinetics of chemical reactions associated with the origin of life, the energetics of early metabolisms, and climate through the partitioning of CO2 between the oceans and atmosphere.
A key role for green rust in the Precambrian oceans and the genesis of iron formations
Halevy I., Alesker M., Schuster E. M., Popovitz-Biro R. & Feldman Y. (2017) Nature Geoscience. 10, 2, p. 135-139 Abstract
Iron formations deposited in marine settings during the Precambrian represent large sinks of iron and silica, and have been used to reconstruct environmental conditions at the time of their formation. However, the observed mineralogy in iron formations, which consists of iron oxides, silicates, carbonates and sulfides, is generally thought to have arisen from diagenesis of one or more mineral precursors. Ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates. Here we investigate the potential role of green rust, a ferrous-ferric hydroxy salt, in the genesis of iron formations. Our laboratory experiments show that green rust readily forms in early seawater-analogue solutions, as predicted by thermodynamic calculations, and that it ages into minerals observed in iron formations. Dynamic models of the iron cycle further indicate that green rust would have precipitated near the iron redoxcline, and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments. We suggest, therefore, that the precipitation and transformation of green rust was a key process in the iron cycle, and that the interaction of green rust with various elements should be included in any consideration of Precambrian biogeochemical cycles.

2016

The thermodynamic effect of atmospheric mass on early Earth's temperature
Chemke R., Kaspi Y. & Halevy I. (2016) Geophysical Research Letters. 43, 21, p. 11,414-11,422 Abstract
Observations suggest that Earth's early atmospheric mass differed from the present day. The effects of a different atmospheric mass on radiative forcing have been investigated in climate models of variable sophistication, but a mechanistic understanding of the thermodynamic component of the effect of atmospheric mass on early climate is missing. Using a 3-D idealized global circulation model (GCM), we systematically examine the thermodynamic effect of atmospheric mass on near-surface temperature. We find that higher atmospheric mass tends to increase the near-surface temperature mostly due to an increase in the heat capacity of the atmosphere, which decreases the net radiative cooling effect in the lower layers of the atmosphere. Additionally, the vertical advection of heat by eddies decreases with increasing atmospheric mass, resulting in further near-surface warming. As both net radiative cooling and vertical eddy heat fluxes are extratropical phenomena, higher atmospheric mass tends to flatten the meridional temperature gradient.
Radiative transfer in CO₂-rich atmospheres: 1. Collisional line mixing implies a colder early Mars
Ozak N., Aharonson O. & Halevy I. (2016) Journal of Geophysical Research-Planets. 121, 6, p. 965-985 Abstract
Fast and accurate radiative transfer methods are essential for modeling CO₂-rich atmospheres, relevant to the climate of early Earth and Mars, present-day Venus, and some exoplanets. Although such models already exist, their accuracy may be improved as better theoretical and experimental constraints become available. Here we develop a unidimensional radiative transfer code for CO₂-rich atmospheres, using the correlated k approach and with a focus on modeling early Mars. Our model differs from existing models in that it includes the effects of CO₂ collisional line mixing in the calculation of the line-by-line absorption coefficients. Inclusion of these effects results in model atmospheres that are more transparent to infrared radiation and, therefore, in colder surface temperatures at radiative-convective equilibrium, compared with results of previous studies. Inclusion of water vapor in the model atmosphere results in negligible warming due to the low atmospheric temperatures under a weaker early Sun, which translate into climatically unimportant concentrations of water vapor. Overall, the results imply that sustained warmth on early Mars would not have been possible with an atmosphere containing only CO₂ and water vapor, suggesting that other components of the early Martian climate system are missing from current models or that warm conditions were not long lived.

2014

Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration
Wing B. A. & Halevy I. (2014) Proceedings of the National Academy of Sciences of the United States of America. 111, 51, p. 18116-18125 Abstract
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 (蠐1 fmol H₂S·cell^-1·d^-1).
Episodic warming of early Mars by punctuated volcanism
Halevy I. & Head J. W. (2014) Nature Geoscience. 7, 12, p. 865-868 Abstract
The widespread evidence for liquid water on the surface of early Mars is difficult to reconcile with a dimmer early Sun. Many geomorphological features suggestive of aqueous activity, such as valley networks and open-basin lakes, date to approximately 3.7 billion years ago, coincident with a period of high volcanic activity. This suggests that volcanic emissions of greenhouse gases could have sustained a warmer and wetter climate on early Mars. However, models that consider only CO₂ and H 2 O emissions fail to produce such climates, and the net climatic effect of the sulphur-bearing gases SO 2 and H 2 S is debated. Here we investigate the atmospheric response to brief and strong volcanic eruptions, including sulphur emissions and an evolving population of H 2 SO 4 -bearing aerosols, using a microphysical aerosol model. In our simulations, strong greenhouse warming by SO 2 is accompanied by modest cooling by sulphate aerosol formation in a presumably dusty early Martian atmosphere. The simulated net positive radiative effect in an otherwise cold climate temporarily increases surface temperatures to permit above-freezing peak daily temperatures at low latitudes. We conclude that punctuated volcanic activity can repeatedly lead to warm climatic conditions that may have persisted for decades to centuries on Mars, consistent with evidence for transient liquid water on the Martian surface.
An improved pyrite pretreatment protocol for kinetic and isotopic studies
Mirzoyan N., Kamyshny A. & Halevy I. (2014) Geochemical Transactions. 15, 1, 10. Abstract
Background: Pyrite is one of the most abundant and widespread of the sulfide minerals with a central role in biogeochemical cycles of iron and sulfur. Due to its diverse roles in the natural and anthropogenic sulfur cycle, pyrite has been extensively studied in various experimental investigations of the kinetics of its dissolution and oxidation, the isotopic fractionations associated with these reactions, the microbiological processes involved, and the effects of pyrite on human health. Elemental sulfur (S⁰) is a common product of incomplete pyrite oxidation. Preexisting S⁰ impurities as unaccounted reaction products are a source of experimental uncertainty, as are adhered fine grains of pyrite and its oxidation products. Removal of these impurities is, therefore, desirable.A robust standardized pretreatment protocol for removal of fine particles and oxidation impurities from pyrite is lacking. Here we describe a protocol for S⁰ and fine particle removal from the surface of pyrite by rinsing in acid followed by repeated ultrasonication with warm acetone.Results: Our data demonstrate the presence of large fractions of S⁰ on untreated pyrite particle surfaces, of which only up to 60% was removed by a commonly used pretreatment method described by Moses et al. (GCA 51:1561-1571, 1987). In comparison, after pretreatment by the protocol proposed here, approximately 98% S⁰ removal efficiency was achieved. Additionally, the new procedure was more efficient at removal of fine particles of adhered pyrite and its oxidation products and did not appear to affect the particle size distribution, the specific surface area, or the properties of grain surfaces.Conclusions: The suggested pyrite pretreatment protocol is more efficient in removal of impurities from pyrite grains, and provides multiple advantages for both kinetic and isotopic investigations of pyrite transformations under various environmental conditions.
Frontiers of stable isotope geoscience
Eiler J. M., Bergquist B., Bourg I. et_al. (2014) Chemical Geology. 372, p. 119-143 Abstract
Isotope geochemistry is in the midst of a remarkable period of innovation and discovery; the last decade (or so) has seen the emergence of 'nontraditional' stable isotopes of metals (i.e., variations in isotopic compositions of Mg, Fe, Cu, etc.), a great expansion of mass-independent isotope geochemistry, the invention of clumped isotope geochemistry, and new capabilities for measurements of position-specific isotope effects in organic compounds. These advances stem from the emergence of multi-collector plasma mass spectrometry, innovations in gas source mass spectrometry, infrared absorption spectroscopy, and nuclear magnetic resonance techniques. These new observations demand new connections between isotope geochemistry and the chemical physics that underlie isotopic variations, including experimental study and modeling of vibrational isotope effects, photochemical isotope effects, and various nuclear volume and magnetic effects. Importantly, such collaborations also have something to offer chemists and physicists because the novel observations of emerging branches of stable isotope geochemistry hold the potential to reveal new insights into the nature of chemical bonds and reactions. This review looks broadly across the frontiers of new methods and discoveries of stable isotope geochemistry and the fundamental chemical-physics problems they pose, focusing in particular on the most pressing problems in: kinetic isotope effects in complex systems; mass independent isotope geochemistry (both the strong effects in photochemical reactions and the subtle variations of more conventional reactions); clumped isotope geochemistry; and the position-specific isotopic anatomies of organic molecules.

2013

Production, preservation, and biological processing of mass-independent sulfur isotope fractionation in the Archean surface environment
Halevy I. (2013) Proceedings of the National Academy of Sciences of the United States of America. 110, 44, p. 17644-17649 Abstract
Mass-independent fractionation of sulfur isotopes (S MIF) in Archean and Paleoproterozoic rocks provides strong evidence for an anoxic atmosphere before 2,400 Ma. However, the origin of this isotopic anomaly remains unclear, as does the identity of the molecules that carried it from the atmosphere to Earth's surface. Irrespective of the origin of S MIF, processes in the biogeochemical sulfur cycle modify the primary signal and strongly influence the S MIF preserved and observed in the geological record. Here, a detailed model of the marine sulfur cycle is used to propagate and distribute atmospherically derived S MIF from its delivery to the ocean to its preservation in the sediment. Bulk pyrite in most sediments carries weak S MIF because of microbial reduction of most sulfur compounds to form isotopically homogeneous sulfide. Locally, differential incorporation of sulfur compounds into pyrite leads to preservation of S MIF, which is predicted to be most highly variable in nonmarine and shallow-water settings. The Archean ocean is efficient in diluting primary atmospheric S MIF in the marine pools of sulfate and elemental sulfur with inputs from SO2 and H2S, respectively. Preservation of S MIF with the observed range of magnitudes requires the S MIF production mechanism to be moderately fractionating (20-40). Constraints from the marine sulfur cycle allow that either elemental sulfur or organosulfur compounds (or both) carried S MIF to the surface, with opposite sign to S MIF in SO2 and H2SO4. Optimal progress requires observations from nonmarine and shallow-water environments and experimental constraints on the reaction of photoexcited SO2 with atmospheric hydrocarbons.
Influence of sulfate reduction rates on the Phanerozoic sulfur isotope record
Leavitt W. D., Halevy I., Bradley A. S. & Johnston D. T. (2013) Proceedings of the National Academy of Sciences of the United States of America. 110, 28, p. 11244-11249 Abstract
Phanerozoic levels of atmospheric oxygen relate to the burial histories of organic carbon and pyrite sulfur. The sulfur cycle remains poorly constrained, however, leading to concomitant uncertainties in O₂ budgets. Here we present experiments linking the magnitude of fractionations of the multiple sulfur isotopes to the rate of microbial sulfate reduction. The data demonstrate that such fractionations are controlled by the availability of electron donor (organic matter), rather than by the concentration of electron acceptor (sulfate), an environmental constraint that varies among sedimentary burial environments. By coupling these results with a sediment biogeochemical model of pyrite burial, we find a strong relationship between observed sulfur isotope fractionations over the last 200 Ma and the areal extent of shallow seafloor environments. We interpret this as a global dependency of the rate of microbial sulfate reduction on the availability of organic-rich sea-floor settings. However, fractionation during the early/mid-Paleozoic fails to correlate with shelf area. We suggest that this decoupling reflects a shallower paleoredox boundary, primarily confined to the water column in the early Phanerozoic. The transition between these two states begins during the Carboniferous and concludes approximately around the Triassic-Jurassic boundary, indicating a prolonged response to a Carboniferous rise in O₂. Together, these results lay the foundation for decoupling changes in sulfate reduction rates from the global average record of pyrite burial, highlighting how the local nature of sedimentary processes affects global records. This distinction greatly refines our understanding of the S cycle and its relationship to the history of atmospheric oxygen.
Seasonal melting and the formation of sedimentary rocks on Mars, with predictions for the Gale Crater mound
Kite E. S., Halevy I., Kahre M. A., Wolff M. J. & Manga M. (2013) Icarus. 223, 1, p. 181-210 Abstract
A model for the formation and distribution of sedimentary rocks on Mars is proposed. In this model (ISEE-Mars), the rate-limiting step is supply of liquid water from seasonal melting of snow or ice. The model is run for a O(10²) mbar pure CO₂ atmosphere, dusty snow, and solar luminosity reduced by 23%. For these conditions snow melts only near the equator, when obliquity and eccentricity are high, and when perihelion occurs near equinox. These requirements for melting are satisfied by 0.01-20% of the probability distribution of Mars' past spin-orbit parameters. This fraction is small, consistent with the geologic record of metastable surface liquid water acting as a "wet-pass filter" of Mars climate history, only recording orbital conditions that permitted surface liquid water. Total melt production is sufficient to account for observed aqueous alteration. The pattern of seasonal snowmelt is integrated over all spin-orbit parameters and compared to the observed distribution of sedimentary rocks. The global distribution of snowmelt has maxima in Valles Marineris, Meridiani Planum and Gale Crater. These correspond to maxima in the sedimentary-rock distribution. Higher pressures and especially higher temperatures lead to melting over a broader range of spin-orbit parameters. The pattern of sedimentary rocks on Mars is most consistent with a model Mars paleoclimate that only rarely produced enough meltwater to precipitate aqueous cements (sulfates, carbonates, phyllosilicates and silica) and indurate sediment. This is consistent with observations suggesting that surface aqueous alteration on Mars was brief and at low water/rock ratio. The results suggest intermittency of snowmelt and long globally-dry intervals, unfavorable for past life on Mars. This model makes testable predictions for the Mars Science Laboratory Curiosity rover at Gale Crater's mound (Mount Sharp, Aeolis Mons). Gale Crater's mound is predicted to be a hemispheric maximum for snowmelt on Mars.

2012

Sulfate burial constraints on the phanerozoic sulfur cycle
Halevy I., Peters S. E. & Fischer W. W. (2012) Science. 337, 6092, p. 331-334 Abstract
The sulfur cycle influences the respiration of sedimentary organic matter, the oxidation state of the atmosphere and oceans, and the composition of seawater. However, the factors governing the major sulfur fluxes between seawater and sedimentary reservoirs remain incompletely understood. Using macrostratigraphic data, we quantified sulfate evaporite burial fluxes through Phanerozoic time. Approximately half of the modern riverine sulfate flux comes from weathering of recently deposited evaporites. Rates of sulfate burial are unsteady and linked to changes in the area of marine environments suitable for evaporite formation and preservation. By contrast, rates of pyrite burial and weathering are higher, less variable, and largely balanced, highlighting a greater role of the sulfur cycle in regulating atmospheric oxygen.

2011

Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 ± 4 °C in a near-surface aqueous environment
Halevy I., Fischer W. W. & Eiler J. M. (2011) Proceedings of the National Academy of Sciences of the United States of America. 108, 41, p. 16895-16899 Abstract
Despite evidence for liquid water at the surface of Mars during the Noachian epoch, the temperature of early aqueous environments has been impossible to establish, raising questions of whether the surface of Mars was ever warmer than today. We address this problem by determining the precipitation temperature of secondary carbonate minerals preserved in the oldest known sample of Mars'crust - the approximately 4.1 billion-year-old meteorite Allan Hills 84001 (ALH84001). The formation environment of these carbonates, which are constrained to be slightly younger than the crystallization age of the rock (i.e., 3.9 to 4.0 billion years), has been poorly understood, hindering insight into the hydrologic and carbon cycles of earliest Mars. Using "clumped" isotope thermometry we find that the carbonates in ALH84001 precipitated at a temperature of approximately 18 °C, with water and carbon dioxide derived from the ancient Martian atmosphere. Furthermore, covarying carbonate carbon and oxygen isotope ratios are constrained to have formed at constant, low temperatures, pointing to deposition from a gradually evaporating, subsurface water body - likely a shallow aquifer (meters to tens of meters below the surface). Despite the mild temperatures, the apparently ephemeral nature of water in this environment leaves open the question of its habitability.
Biologically induced initiation of neoproterozoic snowball-earth events
Tziperman E., Halevy I., Johnston D. T., Knoll A. H. & Schrag D. P. (2011) Proceedings of the National Academy of Sciences of the United States of America. 108, 37, p. 15091-15096 Abstract
The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric CO ₂ greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate- or iron-reducing bacteria. In both cases,this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO ₂ concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased C:N of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth.
Greenhouse warming by nitrous oxide and methane in the Proterozoic Eon
Roberson A. L., Roadt J., Halevy I. & Kasting J. F. (2011) Geobiology. 9, 4, p. 313-320 Abstract
An anoxic, sulfidic ocean that may have existed during the Proterozoic Eon (0.54-2.4Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. The lack of copper, in particular, could have had a significant impact on marine denitrification. Copper is needed for the enzyme that controls the final step of denitrification, from N₂O to N₂. Today, only about 5-6% of denitrification results in release of N₂O. If all denitrification stopped at N₂O during the Proterozoic, the N₂O flux could have been 15-20 times higher than today, producing N₂O concentrations of several ppmv, but only if O₂ levels were relatively high (>0.1PAL). At lower O₂ levels, N₂O is rapidly photodissociated. Methane concentrations may also have been elevated during this time, as has been previously suggested. A lack of dissolved O₂ and sulfate in the deep ocean could have produced a high methane flux from marine sediments, as much as 10-20 times today's methane flux from land. The photochemical lifetime of CH₄ increases as more CH₄ is added to the atmosphere, so CH₄ concentrations of up to 100ppmv are possible during this time. The combined greenhouse effect of CH₄ and N₂O could have provided up to 10° of warming, thereby keeping the surface warm during the Proterozoic without necessitating high CO₂ levels. A second oxygenation event near the end of the Proterozoic would have resulted in a reduction in both atmospheric N₂O and CH₄, perhaps triggering the Neoproterozoic "Snowball Earth" glaciations.

2010

Dust aerosol important for snowball earth deglaciation
Abbot D. S. & Halevy I. (2010) Journal of Climate. 23, 15, p. 4121-4132 Abstract
Most previous global climate model simulations could only produce the termination of Snowball Earth episodes at CO₂ partial pressures of several tenths of a bar, which is roughly an order of magnitude higher than recent estimates of CO₂ levels during and shortly after Snowball events. These simulations have neglected the impact of dust aerosols on radiative transfer, which is an assumption of potentially grave importance. In this paper it is argued, using the Dust Entrainment and Deposition (DEAD) box model driven by GCM results, that atmospheric dust aerosol concentrations may have been one to two orders of magnitude higher during a Snowball Earth event than today. It is furthermore asserted on the basis of calculations using NCAR's Single Column Atmospheric Model (SCAM)-a radiative-convective model with sophisticated aerosol, cloud, and radiative parameterizations-that when the surface albedo is high, such increases in dust aerosol loading can produce several times more surface warming than an increase in the partial pressure of CO₂ from 10^-4 to 10^-1 bar. Therefore the conclusion is reached that including dust aerosols in simulations may reconcile the CO₂ levels required for Snowball termination in climate models with observations.

Submitted version
Explaining the structure of the archean mass-independent sulfur isotope record
Halevy I., Johnston D. T. & Schrag D. P. (2010) Science. 329, 5988, p. 204-207 Abstract
Sulfur isotopes in ancient sediments provide a record of past environmental conditions. The long-time-scale variability and apparent asymmetry in the magnitude of minor sulfur isotope fractionation in Archean sediments remain unexplained. Using an integrated biogeochemical model of the Archean sulfur cycle, we find that the preservation of mass-independent sulfur is influenced by a variety of extra-atmospheric mechanisms, including biological activity and continental crust formation. Preservation of atmospherically produced mass-independent sulfur implies limited metabolic sulfur cycling before ~2500 million years ago; the asymmetry in the record indicates that bacterial sulfate reduction was geochemically unimportant at this time. Our results suggest that the large-scale structure of the record reflects variability in the oxidation state of volcanic sulfur volatiles.

2009

Sulfur dioxide inhibits calcium carbonate precipitation: Implications for early Mars and Earth
Halevy I. & Schrag D. P. (2009) Geophysical Research Letters. 36, 23, L23201. Abstract
Recent studies have suggested a role for sulfur dioxide (SO₂) in maintaining relatively warm surface temperatures on early Mars. Here we show experimentally, that SO₂ concentrations orders of magnitude lower than those required for it to have been of climatic importance strongly affect the aqueous chemistry and the precipitated mineral. assemblage.At near-neutral pH, part-per-billion concentrations of SO₂ prevent the formation of calcium carbonate in favor of hannebachite, a hydrated calcium sulfite. In the presence of iron, possible precursors to phyllosilicate minerals and iron carbonate co-precipitate with hannebachite. This provides an explanation for the existence of early Noachian phyllosilicates in the apparent dearth of outcrop-scale calcium carbonates. Oxidation of this precipitated assemblage produces sulfates, iron oxides and acidity, consistent with evidence for late Noachian-early Hesperian acid-sulfate dominated environments. For early Earth, the results allow placing an upper limit on atmospheric SO₂ concentrations for any period in which carbonates exist in the geologic record.
Radiative transfer in CO₂-rich paleoatmospheres
Halevy I., Pierrehumbert R. T. & Schrag D. P. (2009) Journal of Geophysical Research Atmospheres. 114, D18, D18112. Abstract
Calculations of radiative transfer in CO₂-rich atmospheres are central to modeling the early climate of Earth and Mars. Line-by-line radiative transfer models are the most accurate means of carrying out such calculations and provide the basis for all other radiative transfer approaches. We examine the sensitivity of line-by-line results to three parameterizations of line and continuum absorption by CO₂, all of which yield essentially identical radiation fluxes at low CO₂ abundance. However, when applied to atmospheres containing 0.1-5 bars of CO₂, appropriate for early Earth and Mars, the outgoing longwave radiation calculated with the three parameterizations differs by as much as 40 W m^-2. In addition, the choice of parameters for CO₂ absorption affects the sensitivity of the calculations to infrared absorption by trace gases other than CO ₂. Our findings imply that previous estimates of the amount of CO₂ required to maintain relatively warm temperatures throughout Earth's early history and during episodes in the early history of Mars are highly uncertain. Despite these uncertainties, we conclude that early Mars probably required other infrared absorbers to reach super-freezing surface temperatures, while for the early Earth, this is not necessarily the case.

2008

Is Enceladus' plume tidally controlled?
Halevy I. & Stewart S. T. (2008) Geophysical Research Letters. 35, 12, L12203. Abstract
Explanations for the plume of gas, water vapor and ice particles jetting from rifts in Enceladus' south polar region include boiling of liquid water and dissociation of clathrate hydrates. In either case, production of the plume may be quasi-static or tidally controlled, with implications for the interior structure and composition of Enceladus. Previous quantification of the clathrate explanation assumed equilibrium dissociation and cannot be used to simulate a tidally generated plume. We present a non-equilibrium clathrate dissociation model, which we use to reproduce past observations and predict the plume's properties during upcoming close encounters. The total mass flux and water to gas mass ratio of a tidally generated plume are predicted to be lower than previous measurements. In comparison, for a quasi-static plume these properties should have values close to previous measurements. This provides an observational means of distinguishing quasi-static from dynamic processes as the plume's source.

2007

A sulfur dioxide climate feedback on early Mars
Halevy I., Zuber M. T. & Schrag D. P. (2007) Science. 318, 5858, p. 1903-1907 Abstract
Ancient Mars had liquid water on its surface and a CO₂-rich atmosphere. Despite the implication that massive carbonate deposits should have formed, these have not been detected. On the basis of fundamental chemical and physical principles, we propose that climatic conditions enabling the existence of liquid water were maintained by appreciable atmospheric concentrations of volcanically degassed SO₂ and H₂S. The geochemistry resulting from equilibration of this atmosphere with the hydrological cycle is shown to inhibit the formation of carbonates. We propose an early martian climate feedback involving SO₂, much like that maintained by CO ₂ on Earth.