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...

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 (<<1 fmol H 2 S·cellissimilatory sulfate reduction is a respiratory process used by some bacteria and archaea to generate energy under anaerobic conditions. Aqueous sulfate serves as the terminal electron acceptor in this process, leading to the oxidation of organic carbon compounds and sometimes hydrogen and to the production of aqueous sulfide (1). Dissimilatory sulfate respiration was one of the first microbial metabolisms to be isotopically characterized through culture experiments (2), with 32 S-bearing sulfate shown to be consumed preferentially to 34 S-bearing sulfate. Early experiments identified two critical features of this dissimilatory sulfur isotope fractionation: Its magnitude correlates inversely with the sulfate reduction rate of an individual cell but correlates directly with extracellular sulfate concentrations (3-5).Through careful regulation of the environmental controls on respiration, more recent experiments have precisely calibrated these relationships and suggest that their particular form may be strain specific (6-11). All experiments, however, show a nonlinear response, where sulfur isotope fractionation increases rapidly with decreasing rate. At the low-rate limit, sulfur isotope fractionation appears to approach levels defined by thermodynamic equilibrium between aqueous sulfate and sulfide (8, 12), the initial reactant and final waste product in the respiratory processing chain.In parallel with experimental studies, theoretical work has built a broad foundation for understanding the net sulfur isotope fractionation expressed during sulfate respiration (13-17). These efforts initially dealt with sulfur flow through simplified metabolic networks (13) (Fig. 1A) and have expanded to incorporate, for example, electron supply to the reaction cycles of individual respiratory enzymes (17). The reversibility of an individual enzymatic reaction is a central theoretical concept behind these approaches, as it carries the isotopic memory of downstream steps in the pathway...

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 CO 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 CO between the oceans and atmosphere.

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 CO2 on Earth.