Transient surface ocean oxygenation recorded in the 2.66-Ga Jeerinah Formation, Australia

Abstract

Understanding how and when Earth's surface became oxygenated is essential for understanding its biogeochemical evolution. Incipient oxygenation of Earth's surface environments before the Great Oxidation Event (GOE; 2.4 Ga) has been well-documented, but the nature of these redox changes, whether protracted or transient, is poorly understood. We present nitrogen isotope ratios, selenium abundances, and selenium isotope ratios from the Jeerinah Formation (2.66 Ga; Fortescue Group, Western Australia) that represent (i) high-resolution evidence of transient surface ocean oxygenation 260 My before the GOE, (ii) a possible muted pulse of oxidative continental weathering, and (iii) the oldest firm evidence for nitrification and denitrification metabolisms. These results, in concert with previous studies, highlight the variability in mechanisms and magnitudes of Neoarchean oxygen fluctuations.Many paleoredox proxies indicate low-level and dynamic incipient oxygenation of Earth's surface environments during the Neoarchean (2.8textendash2.5 Ga) before the Great Oxidation Event (GOE) at 2.4 Ga. The mode, tempo, and scale of these redox changes are poorly understood, because data from various locations and ages suggest both protracted and transient oxygenation. Here, we present bulk rock and kerogen-bound nitrogen isotope ratios as well as bulk rock selenium abundances and isotope ratios from drill cores sampled at high stratigraphic resolution through the Jeerinah Formation (2.66 Ga; Fortescue Group, Western Australia) to test for changes in the redox state of the surface environment. We find that both shallow and deep depositional facies in the Jeerinah Formation display episodes of positive primary δ15N values ranging from +4 to +6textperthousand, recording aerobic nitrogen cycling that requires free O2 in the upper water column. Moderate selenium enrichments up to 5.4 ppm in the near-shore core may indicate coincident oxidative weathering of sulfide minerals on land, although not to the extent seen in the younger Mt. McRae Shale that records a well-documented textquotedblleftwhifftextquotedblright of atmospheric oxygen at 2.5 Ga. Unlike the Mt. McRae Shale, Jeerinah selenium isotopes do not show a significant excursion concurrent with the positive δ15N values. Our data are thus most parsimoniously interpreted as evidence for transient surface ocean oxygenation lasting less than 50 My, extending over hundreds of kilometers, and occurring well before the GOE. The nitrogen isotope data clearly record nitrification and denitrification, providing the oldest firm evidence for these microbial metabolisms.