The disappearance of iron formations from the geological record ∼1.8 billion years (Gyr) ago was the consequence of rising oxygen levels in the atmosphere starting 2.45–2.32 Gyr ago1,2,3. It marks the end of a 2.5-Gyr period dominated by anoxic and iron-rich deep oceans. However, despite rising oxygen levels and a concomitant increase in marine sulphate concentration, related to enhanced sulphide oxidation during continental weathering4, the chemistry of the oceans in the following mid-Proterozoic interval (∼1.8–0.8 Gyr ago) probably did not yet resemble our oxygen-rich modern oceans. Recent data5,6,7,8 indicate that marine oxygen and sulphate concentrations may have remained well below current levels during this period, with one model indicating that anoxic and sulphidic marine basins were widespread, and perhaps even globally distributed4. Here we present hydrocarbon biomarkers (molecular fossils) from a 1.64-Gyr-old basin in northern Australia, revealing the ecological structure of mid-Proterozoic marine communities. The biomarkers signify a marine basin with anoxic, sulphidic, sulphate-poor and permanently stratified deep waters, hostile to eukaryotic algae. Phototrophic purple sulphur bacteria (Chromatiaceae) were detected in the geological record based on the new carotenoid biomarker okenane, and they seem to have co-existed with communities of green sulphur bacteria (Chlorobiaceae). Collectively, the biomarkers support mounting evidence for a long-lasting Proterozoic world in which oxygen levels remained well below modern levels.
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We thank J. Hope, C. Sandison and P. Greenwood for technical support; A. Bradley and R. Haese for scientific expertise on methanogens; Geoscience Australia (GA) and the Northern Territory Geological Survey for samples; D. Rawlings for expert advice on the geology of the McArthur Group; and P. Schaeffer for supplying synthetic standards of aromatic carotenoids. This work was supported by the William F. Milton Fund of Harvard University and GA. Work conducted at Massachusetts Institute of Technology was supported by a NASA Exobiology grant to R.E.S. G.D.L. and S.A.B. were at the University of Newcastle upon Tyne during parts of the preparation of this work and thank the Natural Environment Research Council for funding a postdoctoral fellowship and PhD studentship, respectively. J.J.B. acknowledges the Harvard Society of Fellows and the Department of Organismic & Evolutionary Biology, Harvard University, for financial support during the preparation of this work.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Supplementary Methods (information about sample preparation and analysis), Supplementary Discussion (a short review of biological origins of aromatic carotenoids), Supplementary Table S1 (Presence of arylisoprenoids in samples from the Barney Creek Formation), and the chemical structures of relevant biomarkers. (DOC 268 kb)
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Brocks, J., Love, G., Summons, R. et al. Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea. Nature 437, 866–870 (2005). https://doi.org/10.1038/nature04068
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