title: 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  . 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   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(2)S 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.
authors.affiliation: Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. tb@caltech.edu
authors.forename: Tomaso R R
authors.initials: TR
authors.lastname: Bontognali
authors.forename: Alex L
authors.initials: AL
authors.lastname: Sessions
authors.forename: Abigail C
authors.initials: AC
authors.lastname: Allwood
authors.forename: Woodward W
authors.initials: WW
authors.lastname: Fischer
authors.forename: John P
authors.initials: JP
authors.lastname: Grotzinger
authors.forename: Roger E
authors.initials: RE
authors.lastname: Summons
authors.forename: John M
authors.initials: JM
authors.lastname: Eiler
mesh_terms: D001105:Archaea / Q000502:physiology; D001315:Australia; D005580:Fossils; D019015:Geologic Sediments; D005844:Geology / Q000379:methods; D006862:Hydrogen Sulfide / Q000737:chemistry; D007477:Ions; D009930:Organic Chemicals / Q000737:chemistry; D013460:Sulfur Isotopes / Q000032:analysis / Q000737:chemistry*
chemical_list: D007477:Ions; D009930:Organic Chemicals; D013460:Sulfur Isotopes; D006862:Hydrogen Sulfide