Expanded aerobic iron biogeochemical cycle in the Paleoproterozoic oceans during the ca. 2.22-2.06 Ga Lomagundi Event

The variability of iron (Fe) isotopes during the Paleoproterozoic is a topic of debate due to the complex pathways involved in isotopic fractionation. Similarly, the expansion of ocean oxygenation during the late part of the Great Oxygenation Event (GOE)―the ∼2.22–2.06 Ga Lomagundi Event (LE) that represents Earth’s most pronounced and longest-lived carbon isotope excursion―remains controversial. Here, we present new Fe isotope data on bulk samples from a range of lithologies of the Francevillian Group, Gabon, including marine carbonates, black shales, thin sedimentary pyrite beds, early diagenetic pyrite and carbonate nodules. We also analyse pyritized Francevillian biota that were further combined with data obtained from in situ Fe isotope analyses on early diagenetic pyrite nodules (pyritized Francevillian biota and non-fossil pyrite). The δ⁵⁶Fe values from this study vary from highly positive values, up to +1.71‰, in non-fossil pyrite nodules, to highly negative values, down to –3.14‰, in pyritized Francevillian biota. The near-to-zero δ⁵⁶Fe values notably characterize primary carbonates, black shale, thin pyrite beds and carbonate concretions. The near-to-zero δ⁵⁶Fe values are interpreted to reflect complete oxidation and quantitative removal of dissolved Fe²⁺ from seawater, in the Paleoproterozoic oceans, followed by complete reduction of Fe³⁺ in the sediments akin to previously described modern-like Fe biogeochemical cycle which is proposed to have kicked off only from ca. 1.7 Ga. In contrast, positive δ⁵⁶Fe values are linked to equilibrium isotope fractionation, favoured by the high S/C ratios during early diagenesis, while the negative values reflect the kinetic isotope effect driven by a high organic carbon content of the Francevillian biota. The Francevillian Group massive manganese deposition is devoid of concomitant and significant Fe precipitation in the Francevillian shelf environments which is in stark contrast to early GOE Mn-ore deposits in southern Africa. The data thus suggests that the marine Fe²⁺ reservoir was already exhausted in the Paleoproterozoic oceans during the late part of the GOE. In this scenario, and considering the observation of Fe-lean Mn deposits, the Paleoproterozoic oceans were likely oxygenated enough to quantitatively oxidize and remove Fe²⁺ from seawater during the LE. However, extensive oxidation of Fe²⁺ may have been an important O₂ buffer that contributed to maintaining low redox thresholds (e.g., low Eh) in the deep Paleoproterozoic oceans, which ultimately prevented it from reaching oxidizing conditions that require the stability of Mn (oxyhydr)oxides and other elements of similar redox thresholds, i.e., nitrate and selenate. Oxidizing conditions to quantitatively oxidize Mn²⁺ or to significantly build up a pool of oxyanions stable at much higher redox thresholds (e.g., nitrate and selenate) were only reached in the photic zone where the rate of oxygenic photosynthesis was significantly enhanced as a consequence of intense oxidative weathering during the LE. The findings highlight moderately oxygenated Paleoproterozoic oceans with habitats capable of sustaining complex aerobic ecosystems only restricted in shelf environments during the immediate aftermaths of the GOE.