Selenium isotopes record extensive marine suboxia during the Great Oxidation Event

Abstract

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 δ 82/78Se 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 O 2 levels of >0.4 μM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δ 82/78Se values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δ 82/78 Se. 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 δ 82/78 Se and Se/TOC values during the subsequent Shunga-Francevillian anomaly indicates a widespread decrease in surface oxygenation.Paleoproterozoic | trace metals | oxygen | eukaryote evolution

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…The emerging picture from black shales indicates that Precambrian seawater reservoirs of redox-sensitive elements were nowhere close to those of the Phanerozoic (e.g., Scott et al, 2008;Partin et al, 2013b;Reinhard et al, 2013). Furthermore, Se isotope records of black shales suggest that while shallow-marine settings were suboxic and maintained enough oxygen for complex life to evolve, deep oceans remained at a low oxygenation state during the Lomagundi carbon isotope excursion, with euxinic conditions developed in intracratonic basins and on continental margins where nutrients were upwelled and high biological productivity was established (Kipp et al, 2017). It is unclear why IFs, especially GIFs that are consistent with shallow-water depositional environments, are not more abundant during >100 m.y.…”

Hydrothermal dedolomitisation of carbonate rocks of the Paleoproterozoic Zaonega Formation, NW Russia — Implications for the preservation of primary C isotope signals

“…Earth's surface environments. Based on Fe speciation and Mo, U and Se isotopes, dominantly euxinic water column conditions have been inferred (Asael et al, 2013(Asael et al, , 2018Kipp et al, 2017;Scott et al, 2014). In contrast, Re concentrations imply oxic conditions (Sheen et al, 2018) and the multiple S isotope data of Paiste et al (2018) indicate that euxinia was not pervasive and that redox conditions in the basin may indeed have been episodically oxic.…”

Post‐Great Oxidation Event Orosirian–Statherian iron formations on the São Francisco craton: Geotectonic implications

“…While shallow waters in the post-GOE oceans were likely mildly oxygenated (e.g. Kipp, Stüeken, Bekker, & Buick, 2017;Kipp, Stüeken, Yun, Bekker, & Buick, 2018), deeper-water settings contained very little dissolved oxygen until the late Neoproterozoic-Early Cambrian (Hardisty et al, 2017;Slack, Grenne, & Bekker, 2009;Slack, Grenne, Bekker, Rouxel, & Lindberg, 2007). Under these conditions, isolated basins were highly susceptible to an enhanced flux of reductants such as Fe and Mn, resulting in local IF deposition.…”