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Published September 18, 2012 | Supplemental Material + Published
Journal Article Open

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

Abstract

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 Δ^(33)S and δ^(34)S_(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). Δ^(33)S values of the kerogen reveal mass-anomalous fractionations expected of the Archean sulfur cycle, whereas δ^(34)S_(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 H_2S in heterogeneous mat pore-waters influenced by respiratory S metabolism. Positive Δ^(33)S anomalies suggest that disproportionation of elemental sulfur would have been a prominent microbial process in these communities.

Additional Information

© 2012 by the National Academy of Sciences. Edited by Andrew H. Knoll, Harvard University, Cambridge, MA, and approved August 9, 2012 (received for review May 8, 2012). Published online before print September 4, 2012. We thank Yunbin Guan for his assistance with the NanoSIMS analyses, Anthony Carrasquillo for performing the kerogen isolation from the stromatolites, Arndt Schimmelmann for providing us with the kerogen standards, Timothy Lyons and Steven Bates for performing the EA-IRMS measurements, David Mann for preparing the thin sections, and Victoria Orphan and Stefano Bernasconi for comments and suggestions. This work was supported by the Caltech Center for Microanalysis, the Swiss National Science Foundation, and the NASA Exobiology program. Author contributions: T.R.B., A.L.S., A.C.A., W.W.F., J.P.G., and J.M.E. designed research; T.R.B. performed research; A.L.S., A.C.A., R.E.S., and J.M.E. contributed new reagents/analytic tools; T.R.B., A.L.S., A.C.A., W.W.F., J.P.G., R.E.S., and J.M.E. analyzed data; and T.R.B., A.L.S., and W.W.F. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1207491109/-/DCSupplemental.

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Published - PNAS-2012-Bontognali-15146-51.pdf

Supplemental Material - pnas.1207491109_SI.pdf

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