Michael A. Kipp

Selenium isotopes record extensive marine suboxia during the Great Oxidation Event

Significance Oxygen is essential for eukaryotic life. The geologic record of early Earth contains abundant evidence of low oxygen levels, and accordingly, a lack of eukaryote fossils. The rise of oxygen to near-modern levels at the end of the Proterozoic Era is thus often cited as the trigger for the evolutionary radiation of complex life forms at this same time. Here we present selenium geochemical data that indicate an expansion of suboxic (>0.4 μM O2) habitats in the shallow oceans between 2.32 and 2.1 Ga––more than one billion years before eukaryotes become abundant in the fossil record. These environments could have harbored the earliest stages of eukaryotic evolution, but may have been too transient for substantial diversification to occur. It has been proposed that an “oxygen overshoot” occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative Shunga–Francevillian carbon isotopic anomaly. These redox fluctuations could have profoundly influenced the course of eukaryotic evolution, as eukaryotes have several metabolic processes that are obligately aerobic. Here we investigate the magnitude of these proposed oxygen perturbations using selenium (Se) geochemistry, which is sensitive to redox transitions across suboxic conditions. We find that δ82/78Se values in offshore shales show a positive excursion from 2.32 Ga until 2.1 Ga (mean +1.03 ± 0.67‰). Selenium abundances and Se/TOC (total organic carbon) ratios similarly show a peak during this interval. Together these data suggest that during the GOE there was pervasive suboxia in near-shore environments, allowing nonquantitative Se reduction to drive the residual Se oxyanions isotopically heavy. This implies O2 levels of >0.4 μM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δ82/78Se values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δ82/78Se. This suggests that there was no upwelling of Se oxyanions from an oxic deep-ocean reservoir, which is consistent with previous estimates that the deep ocean remained anoxic throughout the GOE. The abrupt decline in δ82/78Se and Se/TOC values during the subsequent Shunga–Francevillian anomaly indicates a widespread decrease in surface oxygenation.

Biomass recycling and Earth’s early phosphorus cycle

The scarcity of oxidants in the ancient oceans may have inhibited phosphorus recycling, stifling the growth of the biosphere. Phosphorus sets the pace of marine biological productivity on geological time scales. Recent estimates of Precambrian phosphorus levels suggest a severe deficit of this macronutrient, with the depletion attributed to scavenging by iron minerals. We propose that the size of the marine phosphorus reservoir was instead constrained by muted liberation of phosphorus during the remineralization of biomass. In the modern ocean, most biomass-bound phosphorus gets aerobically recycled; but a dearth of oxidizing power in Earth’s early oceans would have limited the stoichiometric capacity for remineralization, particularly during the Archean. The resulting low phosphorus concentrations would have substantially hampered primary productivity, contributing to the delayed rise of atmospheric oxygen.

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

Significance 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 (2.8–2.5 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 +6‰, 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 “whiff” 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.