The uranium isotopic record of shales and carbonates through geologic time

Abstract

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Authigenic uranium isotopes of late Proterozoic black shale

“…Extrapolating this understanding of the mechanisms leading to U enrichment and isotope fractionation systematics allows U isotopes in ancient sediment to be used as a paleoredox proxy that can track the redox evolution of ancient environments (Kendall et al, 2015;Lau et al, 2017;Yang et al, 2017). However, recent studies have questioned the applicability of modern U isotope systematics to paleo-environments due to the variable U isotope fractionation experienced under different depositional conditions (Cole et al, 2020b), as well as variable U ocean residence and ocean mixing times over Earth's history (Chen et al, 2021).…”

“…Irrespectively, within such a framework, high δ 238 U, relative to the continental crust (-0.3 ‰), in ancient organic-rich shale from the Mesoarchean (Wang et al, 2018) and at 2.5 Ga, 1.98 Ga, 1.36 Ga, 0.66 Ga, and 0.56 Ga, have been interpreted as a record of discrete oceanic oxygenation events (Kendall et al, 2013;Kendall et al, 2015;Yang et al, 2017;Mänd et al, 2020). However, more recent studies have demonstrated that U isotopes in both shale and carbonate facies deposited under ferruginous conditions were highly variable, but with a muted U isotope fractionation on average (~ + 0.1 ‰, which is similar to oxic sinks) (Cole et al, 2020b;Chen et al, 2021).…”

“…Therefore, it has been suggested that U isotopes can be used as a global redox proxy (Weyer et al, 2008). However, the oceanic residence time of U in the anoxic Archean ocean may have been orders of magnitude shorter due to the dominance of particle reactive U IV (Partin et al, 2013) and a larger marine U sink from increased anoxic coverage of the seafloor area (Chen et al, 2021).…”

Shale Heavy Metal Isotope Records of Low Environmental O2 Between Two Archean Oxidation Events

“…Following from this logic, U isotopes have been treated essentially as a binary paleoredox proxy when applied to the Archean Eon. When free-O 2 was in low abundance or nonexistent, sedimentary rocks should reveal minimal U enrichments and δ 238 U values indistinguishable from, or very close to, average upper continental crust and the mantle (both possess δ 238 U = -0.3‰; Partin et al, 2013;Brüske et al, 2020;Chen et al, 2021). In contrast, during episodes of appreciable O 2 accumulation, sedimentary rocks should reveal some combination of U enrichments and non-crustal δ 238 U values (Kendall et al, 2013;Wang et al, 2018).…”

Cerium anomaly as a tracer for paleo-oceanic redox conditions: A thermodynamics-based Ce oxidation modeling approach

“…Particularly, constraining oxygen levels in the ocean and atmosphere helps to determine the threshold for biological diversification and mass extinction events. However, proxy-based estimates of both atmospheric and oceanic oxygen could vary significantly among various geochemical tracers (Algeo and Tribovillard, 2009;Algeo and Li, 2020;Bellefroid et al, 2018;Canfield, 2005;Chen et al, 2021;Lyons et al, 2014;Liu et al, 2016;Lu et al, 2018;Livermore et al, 2020;Nielsen et al, 2017;Ostrander et al, 2019;Planavsky et al, 2014;Wallace et al, 2017;. Therefore, reliable/refined proxies are still needed to provide robust reconstructions of oxygen through Earth history.…”