Palaeoproterozoic oxygenated oceans following the Lomagundi–Jatuli Event

Iron Isotopes Reveal a Benthic Iron Shuttle in the Palaeoproterozoic Zaonega Formation: Basinal Restriction, Euxinia, and the Effect on Global Palaeoredox Proxies

“…In this study, we report iron (Fe) elemental ratios and isotopes from a 102-metres-thick section of the upper Zaonega Formation in the northern Onega Basin-the same section previously investigated by Paiste et al [15], Mänd et al [13], and Kreitsmann et al [12]-in order to resolve issues surrounding the interpretation of palaeoenvironmental depositional conditions. Due to its redox sensitivity and importance to microbial processes, Fe geochemistry is a powerful proxy for water column redox structure, sediment biogeochemistry, and basinal configuration [32][33][34], and provides key insights into the Palaeoproterozoic Zaonega Formation and its attendant controversies.…”

“…On one hand, sulphur isotope modeling results [15,16] and persistent positive europium anomalies [12,21] seem to suggest a basin with poor access to the open ocean, but where anoxia and sulphidic conditions were restricted mostly to pore waters. On the other hand, selenium isotopes and trace metal abundances in the Zaonega Formation seem to require more anoxic and less restricted conditions [13,17,23,24].…”

“…Intermixed with these positive shifts in atmospheric O 2 fugacity is a retrograde redox shift-an "O 2 crash"-that follows the Lomagundi Event and the O 2 overshoot [9,10] (though see Ohmoto et al [11] for a contradictory view). The timing, extent, and environmental significance of this O 2 crash are contentious, largely due to conflicting interpretations based on different geochemical redox proxies [9,10,[12][13][14][15][16][17]. Resolving the redox dynamics of the post-Lomagundi time period requires new geochemical proxy data from the sedimentary rock record to be placed into a framework that takes into account both global and local elemental cycling [18].…”

“…The Palaeoproterozoic Zaonega Formation comprises a sedimentary succession that records the end of the Lomagundi Event and the O 2 overshoot [19]. As such, it has been the target of a number of attempts to assess global redox conditions using a variety of geochemical proxies, such as carbon and nitrogen isotopes [10,12,20], rare earth element patterns [12,21], sulphur isotopes [17], selenium isotopes [22,23], and the abundances and isotopes of several redox-sensitive metals [13,17,[24][25][26][27]. Many of the interpretations in those previous studies rely on the assumption that the Zaonega Formation faithfully records chemical signals from the global ocean, so that inferences from these rocks may be extrapolated to assess global conditions.…”

The uranium isotopic record of shales and carbonates through geologic time

“…7A). After the GOE, the two records start diverging as many shales display elevated  238 U values (Asael et al 2013;Kendall et al 2015;Yang et al 2017;Mänd et al 2020) and therefore carbonates to have a  238 U value lower than the crust, possibly approaching -0.9 ‰.…”