Oxygenic photosynthesis played an essential role in the accumulation of free oxygen (O2) at Earth’s surface, but questions persist regarding its evolutionary timeline. Manganese (Mn)-rich sedimentary rocks from the Mesoarchean Pongola Supergroup in South Africa have been invoked among the earliest evidence for O2-dependent Mn(II) oxidation and thus photosynthetic O2 production in oceans. However, as a singular suite of rocks, uncertainties persist about whether the evidence for O2 in the Pongola region has global implications. Here we report on another Mesoarchean Mn-rich iron formation in South China, dating back to ca. 2.88–2.80 Ga. The Dianzihe iron formation exhibits a positive correlation between Mn enrichment (MnO up to 7.08 wt%, Fe/Mn ratio down to 5.1) and negative δ56Fe values (−0.21 to −1.33‰; average = −0.91‰). This pattern requires oxygenated seawater (i.e., O2 > 10 μM) at least to the seafloor, allowing not only for the oxidation of Fe(II) and Mn(II) but also for the preservation of Fe(Mn) oxyhydroxides until post-depositional modifications. Based on our findings in China alongside the distribution of Mn-rich iron formations in South Africa, we posit that a global distribution of oxygen oases, driven by cyanobacterial O2 production, already existed in the Mesoarchean.

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