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

Tomaso R R Bontognali*, Alex L. Sessions, Abigail C. Allwood, Woodward W. Fischer, John P. Grotzinger, Roger E. Summons, John M. Eiler

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    113 Citations (Scopus)


    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 34SCDT. 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 δ34SCDT 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.

    Original languageEnglish
    Pages (from-to)15146-15151
    Number of pages6
    JournalProceedings of the National Academy of Sciences of the United States of America
    Issue number38
    Publication statusPublished - 18 Sept 2012


    • Biosignature
    • Early life
    • Ion probe
    • Microbe
    • Paleontology


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