Microscopic sulfides with low 34S/32S ratios in marine sulfate deposits from the 3490-million-year-old 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 (Δ33S) 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 δ34S and positive Δ33S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.
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