Uranium in iron formations and the rise of atmospheric oxygen
Résumé
The concept of the Great Oxidation Event (GOE), during which atmospheric oxygen rose precipitously and perhaps to near-modern levels around 2.4-2.1 billion years ago (Ga), has become entrenched in our views on secular atmospheric evolution. Multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the Archean-Proterozoic boundary. However, it remains unclear precisely when this transition occurred, due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures. Here, we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium (U) concentrations in iron formations (IF). Just as IF are important archives of the redox state of seawater, concentrations of redox-sensitive U in IF are faithful proxies for oxidative continental weathering and associated delivery of dissolved U to seawater. Our dataset suggests that there was an increase in U redox cycling and transport at ca. 2.47 Ga, just before the permanent loss of mass-independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the GOE. Further, there is significant temporal variability in the IF U record that we propose reflects dynamic Precambrian redox conditions. We provide additional support for earlier suggestions that the GOE was a protracted event marked by vacillating oxygen levels.