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We present 28 multiple sulfur isotope measurements of seawater sulfate (δ34SSO4 and δ33SSO4) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific Ocean. The average measured δ34SSO4 is 21.24‰ (±0.88‰, 2σ) with a calculated δ33SSO4 of +0.050‰ (±0.014‰, 2σ). With these values, we use a box-model to place constraints on the gross fraction of pyrite burial in modern sediments. This model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of δ34Spyrite, which is poorly constrained, but instead uses new information about the relationship between δ34S and δ33S in global marine sulfate. Our calculations indicate that the pyrite burial flux from the modern ocean is between 10% and 45% of the total sulfur lost from the oceans, with a more probable range between 20% and 35%. © 2014 The Authors.

The use of stable isotopes to trace biogeochemical sulfur cycling relies on an understanding of how isotopic fractionation is imposed by metabolic networks. We investigated the effects of the first two enzymatic steps in the dissimilatory sulfate reduction (DSR) network – sulfate permease and sulfate adenylyl transferase (Sat) – on the sulfur and oxygen isotopic composition of residual sulfate. Mutant strains of Desulfovibrio vulgaris str. Hildenborough (DvH) with perturbed expression of these enzymes were grown in batch culture, with a subset grown in continuous culture, to examine the impact of these enzymatic steps on growth rate, cell specific sulfate reduction rate and isotopic fractionations in comparison to the wild type strain. Deletion of several permease genes resulted in only small (∼1‰) changes in sulfur isotope fractionation, a difference that approaches the uncertainties of the measurement. Mutants that perturb Sat expression show higher fractionations than the wild type strain. This increase probably relates to an increased material flux between sulfate and APS, allowing an increase in the expressed fractionation of rate-limiting APS reductase. This work illustrates that flux through the initial steps of the DSR pathway can affect the fractionation imposed by the overall pathway, even though these steps are themselves likely to impose only small fractionations. [ABSTRACT FROM AUTHOR]

The Ediacaran Period (635 to 541 Ma) marks the global transition to a more productive biosphere, evidenced by increased availability of food and oxidants, the appearance of macroscopic animals, significant populations of eukaryotic phytoplankton, and the onset of massive phosphorite deposition. We propose this entire suite of changes results from an increase in the size of the deepwater marine phosphorus reservoir, associated with rising sulfate concentrations and increased remineralization of organic P by sulfate-reducing bacteria. Simple mass balance calculations, constrained by modern anoxic basins, suggest that deep-water phosphate concentrations may have increased by an order of magnitude without any increase in the rate of P input from the continents. Strikingly, despite a major shift in phosphorite deposition, a new compilation of the phosphorus content of Neoproterozoic and early Paleozoic shows little secular change in median values, supporting the view that changes in remineralization and not erosional P fluxes were the principal drivers of observed shifts in phosphorite accumulation. The trigger for these changes may have been transient Neoproterozoic weathering events whose biogeochemical consequences were sustained by a set of positive feedbacks, mediated by the oxygen and sulfur cycles, that led to permanent state change in biogeochemical cycling, primary production, and biological diversity by the end of the Ediacaran Period. [ABSTRACT FROM AUTHOR]

Dissimilatory sulfate reduction is the central microbial metabolism in global sulfur cycling. Understanding the importance of sulfate reduction to Earth's biogeochemical S cycle requires aggregating single-cell processes with geochemical signals. For sulfate reduction, these signals include the ratio of stable sulfur isotopes preserved in minerals, as well as the hydrogen isotope ratios and structures of microbial membrane lipids preserved in organic matter. In this study, we cultivated the model sulfate reducer, Desulfovibrio vulgaris DSM 644T, to investigate how these parameters were perturbed by changes in expression of the protein DsrC. DsrC is critical to the final metabolic step in sulfate reduction to sulfide. S and H isotopic fractionation imposed by the wild type was compared to three mutants. Discrimination against 34S in sulfate, as calculated from the residual reactant, did not discernibly differ among all strains. However, a closed-system sulfur isotope distillation model, based on accumulated sulfide, produced inconsistent results in one mutant strain IPFG09. Lipids produced by IPFG09 were also slightly enriched in ²H. These results suggest that DsrC alone does not have a major impact on sulfate-S, though may influence sulfide-S and lipid-H isotopic compositions. While intriguing, a mechanistic explanation requires further study under continuous culture conditions. [ABSTRACT FROM AUTHOR]

Bioturbation, the physical and chemical mixing of sediment by burrowing animals, exerts an important control on the character of modern marine sediments and biogeochemical cycling. Here we show that the mixing of sediments on marine shelves remained limited until at least the late Silurian, 120 million years after the Precambrian-Cambrian transition. We present ichnological, stratigraphic and taphonomic data from a range of lower Phanerozoic siliciclastic successions spanning four palaeocontinents. The protracted development of the sediment mixed layer is also consistent with sulphur data and global sulphur model simulations. The slow increase in the intensity of bioturbation in the sediment record suggests that evolutionary advances in sediment colonization outpaced advances in sediment mixing. We conclude that ecosystem restructuring caused by the onset of significant infaunal mobile deposit feeding ('bulldozing') occurred well after both the Cambrian Explosion and the Great Ordovician Biodiversification Event. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of sulfur isotopes in geological materials reveals information about Earth history and biogeochemical processes. Research during the last several decades has used sulfur isotope geochemistry as a tool to better understand microbial processes () and sediment diagenesis (). Earth historians also realized this potential, as there exists a rich record of environmental change within the sedimentary records (). These applications have championed the use of the two most abundant sulfur isotopes [32S and 34S], and provide a rich introduction to what the sulfur isotope record has to offer [see ()]. Within the last decade, this information has been supplemented by new data derived from the less abundant isotopes [33S and 36S]. The measurement of all four stable sulfur isotopes – multiple sulfur isotope geochemistry – has expanded our understanding of biological evolution and activity, atmospheric chemistry and transport, crustal recycling, and many more fields related to Earth surface processes [see ()]. Here, I present a review of recent works in multiple sulfur isotope geochemistry with a focus on results that inform our understanding of biogeochemical processes and Earth surface evolution. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Metabolic models for fractionations produced by sulfate-reducing Bacteria and Archaea derived from experimental observations are the cornerstone of our interpretation of ancient and modern biogeochemical cycles. Although recent studies have called into question a traditionally accepted model, experimental evidence has been lacking for such a claim. We present data from all four sulfur isotopes that suggest that the internal fractionations associated with the sulfate reduction network are larger than previous estimates. Models of a traditional sulfate reduction network, as well as a more recent incarnation of the sulfate reduction network (with multiple sulfur intermediates) are constructed to aid in the understanding of new experimental data. These data also allow for the further development of additional minor isotope relationships, one of which is easily measurable in geologic settings and accurately depicts the net effect of an environment, whereas the other is more applicable to modern environments and may better illuminate the specific process(es) controlling the fractionation in those environments. This approach illustrates the uses of systems containing more than two isotopes. [Copyright &y& Elsevier]