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

The discovery of hydrogen-rich waters preserved below the Earth's surface in Precambrian rocks worldwide expands our understanding of the habitability of the terrestrial subsurface. Many deep microbial ecosystems in these waters survive by coupling hydrogen oxidation to sulfate reduction. Hydrogen originates from water–rock reactions including serpentinization and radiolytic decomposition of water induced by decay of radioactive elements in the host rocks. The origin of dissolved sulfate, however, remains unknown. Here we report, from anoxic saline fracture waters ∼2.4 km below surface in the Canadian Shield, a sulfur mass-independent fractionation signal in dissolved sulfate. We demonstrate that this sulfate most likely originates from oxidation of sulfide minerals in the Archaean host rocks through the action of dissolved oxidants (for example, HO· and H2O2) themselves derived from radiolysis of water, thereby providing a coherent long-term mechanism capable of supplying both an essential electron donor (H2) and a complementary acceptor (sulfate) for the deep biosphere., Precambrian rocks host a deep hydrosphere, but where dissolved sulfate, crucial for microbial life, comes from is unclear. At 2.4 km depth in the Canadian shield, Li et al. find that oxidation of sulfides in the host rocks creates sulfate thus providing a long-term mechanism for the deep biosphere sulfate.

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.

Mass-independent fractionation of sulfur isotopes (MIF-S) in Archaean sediments results from photochemical processing of atmospheric sulfur species in an oxygen-depleted atmosphere. Geological preservation of MIF-S provides evidence for microbial sulfate reduction (MSR) in low-sulfate Paleoarchaean (3.8–3.2 billion years ago (Ga)) and Neoarchaean (2.8–2.5 Ga) oceans, but the significance of MSR in Mesoarchaean (3.2–2.8 Ga) oceans is less clear. Here we present multiple sulfur and iron isotope data of early diagenetic pyrites from 2.97-Gyr-old stromatolitic dolomites deposited in a tidal flat environment of the Nsuze Group, Pongola Supergroup, South Africa. We identified consistently negative Δ33S values in pyrite, which indicates photochemical reactions under anoxic atmospheric conditions, but large mass-dependent sulfur isotope fractionations of ~30‰ in δ34S, identifying active MSR. Negative pyrite δ56Fe values (−1.31 to −0.88‰) record Fe oxidation in oxygen-bearing shallow oceans coupled with biogenic Fe reduction during diagenesis, consistent with the onset of local Fe cycling in oxygen oases ~3.0 Ga. We therefore suggest the presence of oxygenated near-shore shallow-marine environments with ≥5 μM sulfate at this time, in spite of the clear presence of an overall reduced Mesoarchaean atmosphere.

Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.

Viruses are a primary influence on microbial mortality in the global ocean. The impacts of viruses on their microbial hosts in low-energy environments are poorly explored and are the focus of this study. To investigate the role of viruses in mediating mortality in low-energy environments where contacts between viruses and microbes are infrequent, we conducted a set of in situ time series incubations in the outlet and channel sediments of two cold, hypersaline springs of the Canadian High Arctic. We found microbial and viral populations in dynamic equilibrium, indicating approximately equal birth and death rates for each population. In situ rates of microbial growth were low (0.5 to 50 x 103 cells cm−3 h−1) as were rates of viral decay (0.09 to 170 x 104 virions cm−3 h−1). A large fraction of the springs’ viral communities (49 to 100%) were refractory to decay over the timescales of our experiments. Microcosms amended with lactate or acetate exhibited increased microbial growth rates (up to three-fold) indicating organic carbon as one limiting resource for the microbial communities in these environments. A substantial fraction (15 to 71%) of the microbial populations contained inducible proviruses that were released- occasionally in multiple pulses- over the eight monitored days following chemical induction. Our findings indicate that viruses in low-energy systems maintain low rates of production and activity, have a small but notable impact on microbial mortality (8 to 29% attenuation of growth) and that successful viral replication may primarily proceed by non-lethal strategies. In cold, low biomass marine systems of similar character (e.g. subsurface sediments), viruses may be a relatively minor driver of community mortality compared to less energy-limited environments such as the marine water column or surface sediments.

Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4-2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered some of the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions (δ(33)S, δ(34)S, and δ(36)S) of the3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions (Δ(33)S up to +2.2‰) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in (33)S/(32)S, (34)S/(32)S, and (36)S/(32)S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earth's formation, a common mechanism for S-MIF production was established in the atmosphere.

Zinc is a marine nutrient that may have been limited in the early oceans. Estimates of marine zinc availability through time suggest that values were instead near-modern during the Proterozoic eon. The redox state of the oceans strongly influences the concentration of dissolved trace metals in sea water. Changes in the redox state of the oceans are thought to have limited the availability of some trace metals in the past, particularly during the Proterozoic eon, 2,500 to 542 million years ago1,2,3,4. Of these trace metals, zinc (Zn) is of particular importance to eukaryotic organisms, because it is essential for a wide range of basic cellular functions. It has been suggested5 that during the Proterozoic, marine environments were broadly euxinic—that is, anoxic and sulphidic—which would have resulted in low Zn availability. Low Zn bioavailability could therefore be responsible for an observed delay in eukaryote diversification2. Here we present a compilation of Zn abundance data from black shales deposited under euxinic conditions from the Precambrian time to the present. We show that these values track first-order trends in seawater Zn availability. Contrary to previous estimates6, we find that Zn concentrations during the Proterozoic were similar to modern concentrations, supporting recent studies7,8 that call for limited euxinia at this time. Instead, we propose that predominantly anoxic and iron-rich deep oceans, combined with large hydrothermal fluxes of Zn, maintained high levels of dissolved Zn throughout the oceans. We thus suggest that the protracted diversification of eukaryotic Zn-binding proteins was not a result of Znbiolimitation.

International audience; Mafic igneous rocks of the Nuvvuagittuq Greenstone Belt (NGB) crystallized before 3.8 Ga and possibly as early as 4.3 Ga, potentially making the belt the oldest known supracrustal sequence on Earth. However, detrital zircons from a rare quartz-biotite schist in the NGB yield significantly younger ages of ≈3.77 Ga or less. These appear to be inconsistent with the ages of the mafic igneous rocks, as the quartz-biotite schist has been interpreted as a metaconglomerate, formed by the dismantling of preexisting lithologies. In order to assess this genetic interpretation, we performed a sulfur and oxygen isotope study of the quartz-biotite schist. Sulfide grains found in quartz clasts and the matrix show significant mass-independent fractionation of sulfur isotopes (+0.2‰≤Δ33S‰≤+1.0‰; mean Δ33S=+0.5±0.1‰). Secondary sulfides from crosscutting veins do not show mass-independent fractionation of sulfur isotopes (-0.1‰≤Δ33S≤+0.3‰; mean Δ33S=+0.1±0.1‰). Oxygen isotope compositions of quartz from clasts, matrix and a fine-grained lens are highly enriched in 18O (16.9‰≤δ18O≤26.7‰). Non-zero Δ33S values indicate a surficial origin for sulfur, probably the Eoarchean atmosphere, while high δ18O values suggest a low-temperature (65±18 °C) origin for the quartz, likely as chemical precipitation of a chert precursor from Si-saturated seawater. Therefore, the coupled S- and O-isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the NGB, and suggest that the quartz-biotite schist contains material that originated as chemical metasediments. The near mono-mineralic compositions of the clasts (quartz) and their shared 18O-enrichment suggest that they had a common protolith, which was deposited prior to the formation of the schist, and subsequently reworked. Whether the quartz-biotite schist represents a metaconglomerate or a structural melange, it preserves remnants of some of the oldest chemical sediments on Earth.

We present analyses of stable isotopic ratios ^(17)O/^(16)O, ^(18)O/^(16)O, ^(34)S/^(32)S, and ^(33)S/^(32)S, ^(36)S/^(32)S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km^3), Mt. Spurr (Alaska, 1953, 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ^(17)O > 0.2‰ is present as 1–10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ^(17)O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin. Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO2, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ^(34)S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for ^(33)S/^(32)S vs. ^(34)S/^(32)S and ^(36)S/^(32)S vs. ^(34)S/^(32)S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO_2 with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO_2 gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO_2 in volcanic plumes. Above-zero Δ^(17)O values and their positive correlation with δ^(18)O in sulfate can be explained by oxidation by high-δ^(18)O and high-Δ^(17)O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ^(17)O values, and the largest range of δ^(18)O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3–8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion.

Multiple sulfur isotope measurements of sulfur compounds associated with dissimilatory sulfate reduction, elemental sulfur disproportionation and sulfite disproportionation indicate that different types of metabolic processes impart differ- ent multiple isotope signatures. An established network for sulfate reduction was used previously to explain the multiple isotope variability. Here, we revisit that treatment and expand it to branching networks representative of biological sulfur disproportion- ation, in an attempt to understand multiple sulfur isotope fractionations associated with that metabolism. We use this context to interpret experimental data for both sulfate reducers and sulfur compound disproportionators. We explore the types of information about material flow through these metabolic processes that can be extracted by using multiple sulfur isotope data. The different multiple sulfur isotope relationships ( 33 S and ) allow various metabolic processes to be distinguished from one another, even when 34 S fractionations are similar, providing a tool that can be used to interpret and identify different types of biological sulfur fractionations in the geologic record.