Proxies such as Fe-speciation chemistry record the redox state of local water masses immediately above accumulating sediments. Decades of work on the behavior of iron in marine sediments underpin the observation that enrichments in total (FeT) and highly reactive (FeHR) Fe phases track water column redox conditions (FeHR refers to iron in pyrite plus iron that is reactive to sulfide on early diagenetic timescales) 3,4 . This robust calibration permits the differentiation between oxic and anoxic water columns, as well as whether anoxic waters were iron-or sulfide-bearing [based on the proportion of highly-reactive iron that has been converted to pyrite (FeP)]. by sediments deposited during maximum flooding --a recognizable and consistent sub-wave base environment that has been used to track deeper-water redox conditions through time (see SI). To test for significant differences, data were compared using ANOVA and Kruskal-Wallis (K-W) tests depending on normality of the data. Post hoc Tukey-Kramer tests (α = 0.05),pairwise Wilcoxon tests and Steel-Dwass tests were applied to explore significant differences between time bins (See SI for binning rationale and sensitivity analyses).We first investigated the proportion of anoxic water columns through time. It has been hypothesized that a major oxygenation event occurred around the Proterozoic-Phanerozoic transition, oxygenating the world's deep oceans and facilitating Cambrian animal diversification.This idea has been bolstered by redox-sensitive trace metal abundance data, which show evidence of increasing oxygen levels 1,5,6 , although the timing and magnitude remain poorly . Aggregated iron-speciation data provide an informative complement to global trace 4 metal data. Since the redox state of basinal water masses has traditionally been used as a proxy for the overall ocean-atmosphere system, and shallow-water samples are rare and heterogeneously distributed through time (Table S1) . Iron speciation more robustly identifies anoxia as opposed to oxic conditions, asFeHR enrichments can be muted during rapid deposition or in pervasively anoxic oceans where mass-balance requirements may not result in modern-style iron enrichment. Nonetheless, the proportion of oxic samples (using a conservative threshold of FeHR/FeT < 0.22) 1 was tested, and again no significant differences were found (Table S4). This result raises a number of questions that we discuss below, ranging from diagnosing the nature of basinal anoxia to reconciling the seemingly divergent results between trace metal geochemistry and our database analysis.To assess the nature of anoxic waters through time we focused on samples from deeperwater environments with FeHR/FeT > 0.38. The average proportion of ferruginous samples between 2300 -1000 Ma is 0.59 (the balance being euxinic), consistent with recent arguments that basinal waters through the middle Proterozoic were predominantly ferruginous 3,13 (the effect of subdividing the Proterozoic using a shorter 1600-1000 Ma bin was also tested; Table S4)....

The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.

We present a framework for interpreting the carbon isotopic composition of sedimentary rocks, which in turn requires a fundamental reinterpretation of the carbon cycle and redox budgets over Earth's history. We propose that authigenic carbonate, produced in sediment pore fluids during early diagenesis, has played a major role in the carbon cycle in the past. This sink constitutes a minor component of the carbon isotope mass balance under the modern, high levels of atmospheric oxygen but was much larger in times of low atmospheric O(2) or widespread marine anoxia. Waxing and waning of a global authigenic carbonate sink helps to explain extreme carbon isotope variations in the Proterozoic, Paleozoic, and Triassic.

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 O 2 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 O 2 . 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.Phanerozoic oxygen | sulfate-reducing bacteria T he marine sedimentary sulfur isotope record encodes information on the chemical and biological composition of Earth's ancient oceans and atmosphere (1, 2). However, our interpretation of the isotopic composition of S-bearing minerals is only as robust as our understanding of the mechanisms that impart a fractionation. Fortunately, decades of research identify microbial sulfate reduction (MSR) as the key catalyst of the marine S cycle, both setting the S cycle in motion and dominating the massdependent fractionation preserved within the geological record (1, 3, 4). Despite the large range of S-isotope variability observed in biological studies (4-6), attempts to calibrate the fractionations associated with MSR are less mechanistically definitive (7, 8) than analogous processes influencing the carbon cycle (9, 10). What is required is a means to predict S isotope signatures as a function of the physiological response to environmental conditions (e.g., reduction-oxidation potential).Microbial sulfate reduction couples the oxidation of organic matter or molecular hydrogen to the production of sulfide, setting in motion a cascade of reactions that come to define the biogeochemical S cycle. In modern marine sediments, sulfide i...

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An active microbial assemblage cycles sulfur in a sulfate-rich, ancient marine brine beneath Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet, with Fe(III) serving as the terminal electron acceptor. Isotopic measurements of sulfate, water, carbonate, and ferrous iron and functional gene analyses of adenosine 5'-phosphosulfate reductase imply that a microbial consortium facilitates a catalytic sulfur cycle. These metabolic pathways result from a limited organic carbon supply because of the absence of contemporary photosynthesis, yielding a subglacial ferrous brine that is anoxic but not sulfidic. Coupled biogeochemical processes below the glacier enable subglacial microbes to grow in extended isolation, demonstrating how analogous organic-starved systems, such as Neoproterozoic oceans, accumulated Fe(II) despite the presence of an active sulfur cycle.

Microbial sulfate reduction has governed Earth's biogeochemical sulfur cycle for at least 2.5 billion years. However, the enzymatic mechanisms behind this pathway are incompletely understood, particularly for the reduction of sulfite-a key intermediate in the pathway. This critical reaction is performed by DsrAB, a widespread enzyme also involved in other dissimilatory sulfur metabolisms. Using in vitro assays with an archaeal DsrAB, supported with genetic experiments in a bacterial system, we show that the product of sulfite reduction by DsrAB is a protein-based trisulfide, in which a sulfite-derived sulfur is bridging two conserved cysteines of DsrC. Physiological studies also reveal that sulfate reduction rates are determined by cellular levels of DsrC. Dissimilatory sulfate reduction couples the four-electron reduction of the DsrC trisulfide to energy conservation.