Isotopic evidence for oxygenated Mesoarchaean shallow oceans


Mass-independent fractionation of sulfur isotopes (MIF-S) in Archaean sediments results from photochemical processing of atmospheric sulfur species in an oxygen-depleted atmosphere. Geological preservation of MIF-S provides evidence for microbial sulfate reduction (MSR) in low-sulfate Paleoarchaean (3.8–3.2 billion years ago (Ga)) and Neoarchaean (2.8–2.5 Ga) oceans, but the significance of MSR in Mesoarchaean (3.2–2.8 Ga) oceans is less clear. Here we present multiple sulfur and iron isotope data of early diagenetic pyrites from 2.97-Gyr-old stromatolitic dolomites deposited in a tidal flat environment of the Nsuze Group, Pongola Supergroup, South Africa. We identified consistently negative Δ33S values in pyrite, which indicates photochemical reactions under anoxic atmospheric conditions, but large mass-dependent sulfur isotope fractionations of ~30‰ in δ34S, identifying active MSR. Negative pyrite δ56Fe values (−1.31 to −0.88‰) record Fe oxidation in oxygen-bearing shallow oceans coupled with biogenic Fe reduction during diagenesis, consistent with the onset of local Fe cycling in oxygen oases ~3.0 Ga. We therefore suggest the presence of oxygenated near-shore shallow-marine environments with ≥5 μM sulfate at this time, in spite of the clear presence of an overall reduced Mesoarchaean atmosphere.

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Fig. 1: Geology of the study area and information on the sample material.
Fig. 2: Multiple sulfur isotope plots of Archaean sedimentary pyrites.
Fig. 3: δ34S versus δ56Fe plot of Archaean sedimentary pyrites.
Fig. 4: Overview of the Fe and S cycling in the 2.97-Gyr-old Chobeni Formation.


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B.E. acknowledges financial support by a Postdoctoral Fellowship of the University of Johannesburg, a Travel and Equipment Fellowship of the National Research Foundation of South Africa and an Intuitional Strategy Fellowship of the University of Tübingen (Deutsche Forschungsgemeinschaft, ZUK 63). The Stable Isotope Laboratory at McGill University was supported by the FQRNT through the GEOTOP research centre.

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B.E. and A.H. designed the study. A.H. provided samples. B.E., M.W. and T.H.B. generated data. B.E., M.W., B.A.W. and R.S. interpreted the data. B.E. wrote the paper with input from all the co-authors.

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Correspondence to Benjamin Eickmann.

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Eickmann, B., Hofmann, A., Wille, M. et al. Isotopic evidence for oxygenated Mesoarchaean shallow oceans. Nature Geosci 11, 133–138 (2018).

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