Sulfur Biogeochemistry—Past and Present

Preface viisulfurization on the preservation of organic compounds at bulk and molecular levels. All this work has benefitedfrom new and improved analytical approaches, including the integration of sulfur isotope techniques,which are now being applied at the level of individual organic compounds.Papers in the final section are linked by the theme that sulfur records biogeochemical conditions inthe ancient ocean. Paytan et al. employ an exciting, recently developed paleoceanographic method—carefullyextracted barite as a proxy for the δ 34 S of seawater—to confirm that high barite levels in organic-richsediments from Cretaceous Ocean Anoxic Events and Mediterranean sapropels are indeed the product ofhigh biological productivity rather than secondary enrichment and remobilization. The authors explore theconditions that favor the preservation of biogenic barite in sulfate reducing environments, thus refining ourunderstanding of why and where the barite proxy is a robust paleoenvironmental recorder. Using barite,Paytan et al. then summarize the δ 34 S of seawater sulfate over the past 130 m.y. Lyons et al. investigatethe fidelity of another proxy for the isotopic composition of seawater sulfate—carbonate-associated sulfate(CAS)—through a careful look at modern lime muds in South Florida. Most important, this modern calibrationshows that bulk sediment at sites of extensive, early diagenetic carbonate dissolution and net precipitationcan faithfully record the δ 34 S of seawater sulfate. This retention of the seawater isotope value occursdespite the strong 34 S enrichments observed in the pore waters at a site of anomalously high rates of bacterialsulfate reduction. More generally, this is a study of the sulfur cycle in shallow platform carbonates and itsrelationship to calcium carbonate saturation. Hurtgen et al. use the CAS technique to reconstruct Proterozoicocean chemistry as preserved in rocks from Death Valley, California. Concentrations of CAS suggest that theamount of sulfate in the late Mesoproterozoic to mid-Neoproterozoic ocean was only ~10% of that presenttoday. Furthermore, the broad stratigraphic coverage provides an ideal, longer-term context for the anomalouslylarge and rapid isotopic excursions observed in late Neoproterozoic sediments. These excursions area possible consequence of “snowball earth” glacial events. Finally, Strauss has compiled a massive amountof data into a thorough review of the δ 34 S of seawater sulfate over the past ~3.5 b.y. These data derive froma variety of sources, including CAS analysis. Strauss’ interpretations tell us that the observed patterns ofisotopic variability track the oxygenation history of Earth’s surface and the corresponding balance betweenthe burial and weathering of reduced sulfur.Jan P. AmendDepartment of Earth and Planetary SciencesWashington University in St. LouisKatrina J. EdwardsDepartment of Marine Chemistry andGeochemistryWoods Hole Oceanographic InstitutionTimothy W. LyonsDepartment of Geological SciencesUniversity of MissouriREFERENCES CITEDBenning, L.G., Wilkin, R.T., and Barnes, H.L., 2000, Reactionpathways in the Fe-S system below 100°C: Chemical Geology,v. 167, p. 25–51, doi: 10.1016/S0009-2541(99)00198-9.Berner, R.A., 1970, Sedimentary pyrite formation: AmericanJournal of Science, v. 268, p. 1–23.Berner, R.A., 1984, Sedimentary pyrite formation: An update:Geochimica et Cosmochimica Acta, v. 48, p. 605–615, doi:10.1016/0016-7037(84)90089-9.Berner, R.A., 1987, Models for carbon and sulfur cycles andatmospheric oxygen: Application to Paleozoic geologichistory: American Journal of Science, v. 287, p. 177–190.Berner, R.A., and Raiswell, R., 1983, Burial of organic carbonand pyrite sulfur in sediments over Phanerozoic time: Anew theory: Geochimica et Cosmochimica Acta, v. 47,p. 855–862, doi: 10.1016/0016-7037(83)90151-5.Boetius, A., Ravenschlag, K., Schubert, C.J., Rickert, D., Widdel,F., Gieseke, A., Amann, R., Jørgensen, B.B., Witte, U.,and Pfannkuche, O., 2000, A methane microbial consortiumapparently mediating anaerobic oxidation of methane:Nature, v. 407, p. 623–626, doi: 10.1038/35036572.Burdett, J.W., Arthur, M.A., and Richards, M., 1989, A Neogeneseawater sulfur isotope curve from calcareous pelagicmicrofossils: Earth and Planetary Science Letters, v. 94,p. 189–198, doi: 10.1016/0012-821X(89)90138-6.Canfield, D.E., 1998, A new model for Proterozoic ocean chemistry:Nature, v. 396, p. 450–453, doi: 10.1038/24839.