A seawater oxygen oscillation recorded by iron formations prior to the Great Oxidation Event

Earth’s atmosphere underwent permanent oxidation during the Great Oxidation Event approximately 2.45–2.22 billion years ago (Ga) due to excess oxygen (O2) generated by marine cyanobacteria. However, understanding the timing and tempo of seawater oxygenation before the Great Oxidation Event has been hindered by the absence of sensitive tracers. Nitrogen (N) isotopes can be an indicator of marine oxygenation. Here we present an ~200 Myr nitrogen isotope oscillation recorded by Neoarchaean and Palaeoproterozoic banded iron formations from the Hamersley Basin, Western Australia, that were deposited in relatively deep marine shelf environments. Paired with the Jeerinah Formation shale record, our data from the Marra Mamba Iron Formation suggest that oxic conditions expanded to banded iron formation depositional environments from ~2.63 to 2.60 Ga. Subsequently, a positive δ15N excursion occurred in the ~2.48 Ga Dale Gorge Member, marking a decline in seawater O2 and enhanced denitrification. This O2 deficit was followed by a second phase of increasing O2 levels as indicated by a gradual return to moderately positive δ15N values in the ~2.46 Ga Joffre Member and 2.45 Ga Weeli Wolli Iron Formation. These variations underscore a nonlinear history of marine oxygenation and reveal a previously unrecognized oscillation in seawater O2 levels preceding the Great Oxidation Event. Deep marine shelf environments experienced fluctuating levels of seawater oxygenation before the Great Oxidation Event, as reflected by oscillations between nitrogen fixation and denitrification recorded by nitrogen isotopes in banded iron formations.