Traditionally viewed as a single, monotonic rise from an anoxic atmosphere to persistent oxygenation, recent studies have highlighted the highly dynamic and protracted nature of Earth’s Great Oxidation Event (GOE). Multiple sulphur isotope systematics document major swings in atmospheric oxygen levels linked to extreme glaciations of potential ‘Snowball Earth’ magnitude. Yet, despite the significance of oceanic redox and nutrient bioavailability to oxygen production, few studies have addressed the evolution of ocean redox chemistry and nutrient cycling across the GOE. Here, we will discuss ocean oxygenation and phosphorus cycling across two key intervals.
The first comprises 2.43 to 2.40 Ga chemical sediments from the Hotazel Formation, Transvaal Supergroup, South Africa, deposited following the first glaciation of the GOE. This section documents three cycles characterised by magnetite-rich banded iron formation, followed by a transitional hematite-rich interval, and then massive Mn oxide deposition. Extensive magnetite deposition during cyclical intervals of expanded shallow marine anoxia effectively scavenged phosphorus from seawater, initiating a negative productivity feedback that limited oxygen production. Nevertheless, widespread shallow water anoxia was driven by a relatively enhanced influx of phosphate from continental weathering, which stimulated surface water productivity and oxygen consumption via oxidation of sinking organic matter. By contrast, Mn oxide intervals were characterized by a lower influx of phosphate, which limited organic matter production and oxygen consumption, leading to expanded oxygenation of surface waters. These data thus highlight significant, but relatively low-level, fluctuations in the extent of oceanic oxygenation during the early stages of the GOE.
The second interval comprises ~2.32-2.25 Ga shales from the Transvaal Supergroup. These data document a major oceanic influx of phosphate following the third global glaciation of the GOE. This stimulated high levels of productivity, leading to euxinic water column conditions and extensive phosphorus recycling back to the water column. This initiated a positive productivity feedback, driving transient oxygenation of the atmosphere. However, as the post-glacial influx of nutrients waned, ferruginous conditions developed, drawing down phosphate in association with iron minerals, leading to a negative productivity feedback and a return to anoxic atmospheric conditions. Subsequent low-level fluctuations in atmospheric oxygenation up to and across the final regional glaciation of the GOE were then readily achievable by relatively small-scale changes in productivity and volcanic outgassing. These case studies highlight intense swings in oceanic and atmospheric oxygenation across the GOE, driven by weathering and redox-induced feedbacks in nutrient supply, which ultimately resulted in the protracted nature of Earth’s transition to a persistently oxygenated atmosphere.