HR: 08:00h
AN: B11B-01    [PDF]
TI: Neoproterozoic Seawater Sulfur Isotopes and the Evolution of Microbial Sulfur Species
AU: * Hurtgen, M T
EM: mhurtgen@geosc.psu.edu
AF: Penn State Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, University Park, PA 16802 United States
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: Penn State Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, University Park, PA 16802 United States
AB: Canfield and Teske (1996) proposed that an increase in the variability of $\delta$$^{34}$S$_{pyrite}$ sometime in the Neoproterozoic$\--$along with a corresponding increase in the isotopic difference between sulfate and pyrite ($\Delta$$^{34}$S)$\--$resulted from a fundamental shift in the biogeochemical cycling of sulfur, facilitated by changes in the oxidation-state of the Earth's surface. They proposed that an increase in the depth of oxygenation of the surface ocean triggered the evolution of a non-photosynthetic sulfide-oxidizing bacteria and that these bacteria, in consortium with a host of microbes associated with the oxidative part of the sulfur cycle, were responsible for the increase in $\Delta$$^{34}$S to values greater than 46 $\permil$. However, $\Delta$$^{34}$S values have been poorly constrained for the Neoproterozoic because the S isotopic composition of seawater sulfate has been largely unknown. In this study, we have reconstructed the S isotopic evolution of Neoproterozoic seawater sulfate by analyzing the isotopic composition of trace sulfate extracted from carbonates collected in South Australia, Namibia and Death Valley, CA. Our results indicate that Neoproterozoic $\Delta$$^{34}$S values between $\sim$800 to 570 Ma were less than 46 $\permil$ and that the apparent increase in $\delta$$^{34}$S$_{pyrite}$ variability during this time resulted from an ocean with low sulfate concentrations and rapidly evolving $\delta$$^{34}$S$_{sulfate}$$\--$likely a consequence of severe late Neoproterozoic glacial events. Therefore, it is difficult to argue using S isotopes that a non-photosynthetic sulfide-oxidizing bacteria evolved at this time. Furthermore, we speculate that the evolution of a non-photosynthetic sulfide-oxidizing bacteria was not necessary for disproportionation reactions to operate. Rather, we argue that intermediate S species and disproportionation reactions were likely occurring through much of the Proterozoic and that the overall low $\Delta$$^{34}$S is simply a function of more efficient pyrite burial in an ocean with fewer oxidants and low sulfate concentrations.
DE: 0330 Geochemical cycles
DE: 1010 Chemical evolution
DE: 1615 Biogeochemical processes (4805)
DE: 4267 Paleoceanography
DE: 9619 Precambrian
SC: Biogeosciences [B]
MN: 2003 Fall Meeting