Sulfur-Based Biosignatures Preserved in Archean Sediments

Sulfur isotope fractionation patterns preserved in Archean sediments from the Barberton Greenstone Belt and Pilbara Craton record microbial sulfate reduction operating by 3.2 billion years ago. Dissimilatory sulfate-reducing microorganisms preferentially processed lighter sulfur isotopes, creating mass-dependent fractionation signatures distinguishable from abiotic processes. These geochemical fingerprints in ancient pyrite and barite minerals document metabolic activity in low-sulfate, anoxic oceans where seawater sulfate concentrations measured less than 2.5 micromolar.