Fe isotopes of a 2.4 Ga hematite-rich IF constrain marine redox conditions around the GOE

The base of the Hotazel Formation thus forms a transitional interval between precipitation from essentially ferruginous seawater, as recorded in the Ghaap Group BIFs, and from isotopically and chemically highly evolved surface waters, as demonstrated by the Hotazel Mn-rich layers. In the absence of Fe(II) as a strong reducing agent, large volumes of photosynthetic oxygen may have eventually escaped into the atmosphere, leading to the onset of atmospheric oxygenation. Our results thus contradict previous models that place the onset of the GOE before the Hotazel Formation, concurrent with and mechanistically linked to the Makganyene and Ongeluk events. Instead, we show that the Hotazel basal sediments are still pre-GOE, consistent with their circa 2.4 Ga age and the continuation of MIF-S higher in the stratigraphic record.