Early Archaean microorganisms preferred elemental sulfur, not sulfate

Microscopic sulfides with low S-34/S-32 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 massin-dependently fractionated sulfur isotopic anomaly (Delta S-33) 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 delta S-34 and positive Delta S-33 signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.