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BIOGEOCHEMISTRY

An Archaean oxygen oasis

Nature Geoscience volume 11, pages 84–85 (2018)Cite this article

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The first of two stepwise increases in atmospheric oxygen occurred at the end of the Archaean eon. Analyses of sulfur and iron isotopes in pyrite reveal a near-shore environment that hosted locally oxygenated conditions in the Mesoarchaean era.

As the Mesoarchaean (3.2–2.8 billion years ago) falls before the Great Oxidation Event, atmospheric conditions are thought to have been mostly anoxic. Under an anoxic atmosphere, photochemical reactions can fractionate sulfur isotopes independent of mass. Thus, the change from mass-independent sulfur isotopic signatures preserved in older rocks to their absence in rocks younger than 2.45 billion years old is considered to be conclusive proof of the rise of oxygen at the Great Oxidation Event1. However, the mass-independent sulfur signal is muted during the Mesoarchaean, presenting a conundrum. This muted signal could be due to differences in atmospheric photochemistry4 or dilution of the signal by sulfur with a mass-dependent signal. A likely culprit for the latter is the contribution of sulfur with a mass-dependent signal from microbial sulfate reduction. This requires high sulfate levels generated by enhanced oxidative weathering on land under an oxygenated atmosphere. But lack of evidence for mass-dependent fractionations in the Mesoarchaean has previously hampered the interpretation that mass-independent signatures were dampened by inputs from microbial sulfate reduction under elevated atmospheric oxygen and marine sulfate levels.

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Fig. 1: A microbial mat in Little Ambergris Cay, Turks and Caicos, British Overseas Territories.

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  1. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA

    Maya L. Gomes

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  1. Maya L. Gomes
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Correspondence to Maya L. Gomes.

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Gomes, M.L. An Archaean oxygen oasis. Nature Geosci 11, 84–85 (2018). https://doi.org/10.1038/s41561-018-0058-z

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