Expansion of the aerobic iron biogeochemical cycle during the Paleoproterozoic Lomagundi event

The expansion of ocean oxygenation during the Paleoproterozoic Lomagundi carbon isotope excursion remains highly debated. Here, we present iron isotope data for the coeval marine Francevillian Group strata. The data scatter from heavy to light iron isotope composition, with predominantly minimal isotope fractionation. The latter reflects complete oxidation and quantitative removal of dissolved iron from seawater, followed by complete reduction of oxidized iron in the sediments. Conversely, heavy and light iron isotope compositions are linked to equilibrium and kinetic isotopic effects, respectively during diagenesis. We propose that the oxygenation of the Paleoproterozoic oceans, at least below the shelf break, was responsible for the complete oxidation and quantitative removal of iron at depth, leaving behind a depleted marine iron reservoir during the Lomagundi Event. Oxidizing conditions to significantly build up a pool of oxyanions stable at much higher redox potential, such as nitrate, were only reached in the photic zone. Approximately two billion years ago, during the Lomagundi Event, the oxygen conditions in the oceans allowed the oxidation and quantitative removal of iron at depth or near hydrothermal sources, according to bulk-rock and in-situ geochemical and isotopic data from pyrite from diverse sedimentary rocks of the Francevillian Group.