Paper No. 10
Presentation Time: 10:30 AM
NEOPROTEROZOIC S ISOTOPES, THE EVOLUTION OF MICROBIAL S SPECIES AND THE BURIAL EFFICIENCY OF SULFIDE AS SEDIMENTARY PYRITE
Significant variability in d34Spyrite values in Neoproterozoic sedimentsalong with a corresponding increase in the isotopic difference between sulfate and pyrite (D34S)has been attributed to the evolution of non-photosynthetic sulfide-oxidizing (NPSO) bacteria and the advent of sulfur disproportionation reactions in response to Earth's evolving redox chemistry. In order to test this hypothesis, we analyzed trace sulfate in carbonates from the Adelaide Rift Complex in South Australia and the Otavi Group in northern Namibia and reconstructed the S isotope evolution of seawater sulfate. Comparison of our d34Ssulfate record with published d34Spyrite data from the same or equivalent successions indicates that D34S rose gradually through the second half of the Neoproterozoic and fluctuated concomitantly with episodes of glaciation, but did not exceed 46 per mil before 580 Ma. We speculate that disproportionation reactions have likely been occurring at least since the early Proterozoic and that D34S values remained relatively low as a consequence of efficient pyrite burial in an ocean with low sulfate concentrations. A prominent rise in D34S following the Marinoan glaciation likely reflects the accelerated oxidation of the ocean-atmosphere system that set the stage for the evolution of metazoa and the transition to a Phanerozoic-type environment.