Publications

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.

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.

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 “juba”) 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.

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.

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.

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 

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 speleothem’s 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.

To explore the importance of local vs. global sulfur-cycle controls on variations in pyrite sulfur isotopes, the authors couple carbon-nitrogen-sulfur concentrations and stable isotopes of sediments from the Peruvian oxygen minimum zone, identifying a major role for the local organic carbon loading.

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
C and
H (
α = 1.016 and
α = 1.155), as observed previously. In contrast, at low sulfate concentrations, the residual methane is substantially depleted in
C (
α = 0.977) and, to a lesser extent, in
H. 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.

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 (“clumping,” i.e., a single ¹³C–D bond or two ¹²C–D 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 0–700 °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.

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. 

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.

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∼17–20‰), 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.

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.

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.

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 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.

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.

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.