Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate

Microscopic sulfides with low 34 S/ 32 S ratios in marine sulfate deposits from the 3490-million-yearold Dresser Formation, Australia, have been interpreted as evidence for the presence of early sulfate-reducing organisms on Earth. We show that these microscopic sulfides have a mass-independently fractionated sulfur isotopic anomaly (Δ 33 S) that differs from that of their host sulfate (barite). These microscopic sulfides could not have been produced by sulfate-reducing microbes, nor by abiologic processes that involve reduction of sulfate. Instead, we interpret the combined negative δ 34 S and positive Δ 33 S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.