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Selenium (Se) undergoes redox transformations and isotopic fractionations at relatively high redox potentials and could therefore provide insight into changes in oceanic and atmospheric O2 levels over Earth’s history. We test this idea with Se data from the 2.5 Ga Mount McRae Shale (Hamersley Basin, Australia), which records temporary enrichments in abundances and isotopes of other redox-sensitive elements indicating a “whiff of oxygen” in Earth’s atmosphere before the Great Oxidation Event. Se isotopic ratios expressed as δ82/78Se and abundances relative to crustal background show significant positive excursions of up to 1.1‰ and an enrichment 13 times above background, respectively, overlapping with excursions in Mo and N isotopes and abundances. Because Se has a relatively high redox potential and photosynthetic oxidation pathways are unknown, our data thus suggest that Se was mobilized by free O2 during this interval. The isotopic fractionation likely occurred during transport of Se oxyanions from the site of weathering to the outer shelf. Although O2 could have been produced locally on land and may not necessarily have increased in the global atmosphere, our results strengthen the inference of an early origin of oxygenic photosynthesis long before the Paleoproterozoic Great Oxidation Event. This is the first report of a Se isotope excursion in the Precambrian rock record, and it confirms that Se isotopes can serve as a useful redox proxy in deep time.

We report detailed molecular geochemistry of oil-bearing fluid inclusions from a ca. 2.45 Ga fluvial metaconglomerate of the Matinenda Formation at Elliot Lake, Canada. The oil, most likely derived from the conformably overlying McKim Formation, was trapped in quartz and feldspar during diagenesis and early metamorphism of the host rock, probably before ca. 2.2 Ga. The presence of abundant biomarkers for cyanobacteria and eukaryotes derived from and trapped in rocks deposited before the Great Oxidation Event is consistent with an earlier evolution of oxygenic photosynthesis than previously thought and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time. It also implies that eukaryotes survived several extreme climatic events, including the Paleoproterozoic "snowball Earth" glaciations. The extraction of biomarker molecules from Paleoproterozoic oil-bearing fluid inclusions thus establishes a new method, using low detection limits and system blank levels, to trace evolution of life through Earth's early history that avoids the potential contamination problems affecting shale-hosted hydrocarbons.

Oil is formed in some modern seafloor vent systems after rapid thermal maturation of sedimentary organic matter by hydrothermal fluids. Although most of this petroleum escapes into the ocean, some hydrocarbons may be trapped as fluid inclusions in hydrothermal precipitates or as bituminous residues in cavities. However, the geological record lacks evidence of this phenomenon, which is unfortunate because it could be significant for understanding the history of petroleum genesis, the processes of hydrothermal mineralization, and perhaps the origin and early evolution of life. From the Pilbara craton of Australia, we report the discovery of pyrobitumen and oil in an Early Archean (ca. 3235 Ma) deep-sea volcanogenic massive sulfide deposit. The oil occurs in fluid inclusions within hydrothermal barite, and pyrobitumen is intergrown with polymetallic sulfides. Petrographic textures show that the oil was emplaced and thermally altered (coked) before late-stage sulfide mineralization. The results demonstrate that subseafloor hydrothermal petroleum generation was active during the Early Archean, possibly providing an energy and carbon source for a subsurface microbiota metabolizing hydrothermal sulfur species.

The presence of detrital uraninite and pyrite in fluvial placers of the Witwatersrand basin, South Africa, has been used to infer low levels of atmospheric oxygen during the Archean (>2500 Ma). However, recent studies advocate a hydrothermal origin for these minerals, thereby limiting their value as constraints on the composition of the early atmosphere. In contrast, ca. 3250–2750 Ma fluvial siliciclastic sediments from the Pilbara Craton in Australia have never undergone significant hydrothermal alteration, and their heavy minerals are of unequivocal detrital origin. These heavy minerals include the redox-sensitive phases pyrite, uraninite, and gersdorffite, along with more inert zircon, rutile, chromite, and monazite. Locally, siderite is a major constituent (to 90%) of the heavy mineral population, with grains displaying evidence for several episodes of erosion, rounding, and subsequent authigenic overgrowth. Detrital siderite is very rare in post-Archean sandstones, largely due to its instability in oxidizing environments. However, its frequent survival of prolonged transport in well-mixed and therefore well-aerated Archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present. Moreover, concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization.