Sulfur isotopes of organic matter preserved in 3.45-billion-year-old stromatolites reveal microbial metabolism

The 3.45-billion-year-old Strelley Pool Formation of Western Australia preserves stromatolites that are considered among the oldest evidence for life on Earth. In places of exceptional preservation, these stromatolites contain laminae rich in organic carbon, interpreted as the fossil remains of ancient microbial mats. To better understand the biogeochemistry of these rocks, we performed microscale in situ sulfur isotope measurements of the preserved organic sulfur, including both Δ³³S and δ ³⁴S_CDT. This approach allows us to tie physiological inference from isotope ratios directly to fossil biomass, providing a means to understand sulfur metabolism that is complimentary to, and independent from, inorganic proxies (e.g., pyrite). Δ³³S values of the kerogen reveal mass-anomalous fractionations expected of the Archean sulfur cycle, whereas δ³⁴S_CDT values show large fractionations at very small spatial scales, including values below -15‰. We interpret these isotopic patterns as recording the process of sulfurization of organic matter by H₂S in heterogeneous mat pore-waters influenced by respiratory S metabolism. Positive Δ³³S anomalies suggest that disproportionation of elemental sulfur would have been a prominent microbial process in these communities.