NEOPROTEROZOIC S ISOTOPES, THE EVOLUTION OF MICROBIAL S SPECIES AND THE BURIAL EFFICIENCY OF SULFIDE AS SEDIMENTARY PYRITE

Significant variability in d³⁴S_pyrite values in Neoproterozoic sediments—along 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 d³⁴S_sulfate record with published d³⁴S_pyrite 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.