Abstract
The appearance and radiation of animals are commonly attributed to Neoproterozoic oceanic oxygenation, yet independent geochemical evidence for such an event remains equivocal. Strata deposited between the Sturtian and Marinoan Snowball Earth glaciations (660 to 640 Ma) contain the earliest animal biomarkers and possible body fossils. To quantify the extent of seafloor oxygenation during this critical interval, we present uranium isotope ratios (238U/235U denoted as δ238U) from limestone of the Taishir Formation in Mongolia through two stratigraphic sections that are separated by ∼75 km within the same depositional basin. Above the Sturtian glacial deposits, through ∼150 m of stratigraphy, δ238U compositions have a mean value of −0.47‰. This interval is followed by a ∼0.3‰ decrease in δ238U, coincident with the Taishir negative carbon isotope excursion. Thereafter, δ238U values remain relatively low until the erosional unconformity at the base of the Marinoan glacial deposits. Using a box model, we show that the best explanation for the higher δ238U values of the post-Sturtian limestones is extensive—but temporary—oxygenation of the seafloor, and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post-Snowball weathering. The decline in δ238U in overlying strata, coincident with the Taishir negative δ13C excursion, indicates a subsequent decrease in seafloor oxygenation. The U isotopic data, combined with modeling results, challenge the notion of a simple, unidirectional oxygenation of Neoproterozoic oceans.
Highlights
The oxygenation of the Earth’s atmosphere and oceans is hypothesized to have occurred in two steps: the Great Oxidation Event (GOE, ca. 2.2 Ga) and the Neoproterozoic Oxidation Event (NOE, ca. 0.8 to 0.5 Ga) (Lenton et al, 2014; Lyons et al, 2014; Och and Shields-Zhou, 2012)
The Neoproterozoic is divided into the Tonian (1000–720 Ma), Cryogenian (720–635 Ma), and Ediacaran (635–541 Ma) periods (Fig. 1)
In the T3 member, δ238U values decrease to a mean value of −0.70h ± 0.25, whereas [U] values have a mean of 1.2 ± 1.1 ppm
Summary
The oxygenation of the Earth’s atmosphere and oceans is hypothesized to have occurred in two steps: the Great Oxidation Event (GOE, ca. 2.2 Ga) and the Neoproterozoic Oxidation Event (NOE, ca. 0.8 to 0.5 Ga) (Lenton et al, 2014; Lyons et al, 2014; Och and Shields-Zhou, 2012). Unlike the GOE, which is demarcated by the disappearance of sulfur mass independent fractionation, the timing and magnitude of the NOE remains poorly resolved (Lyons et al, 2014; Och and Shields-Zhou, 2012). If the extent of seafloor anoxia increases globally, the residual aqueous U(VI) in seawater—which is subsequently incorporated into carbonate sediments—should decrease in both concentration ([U]) and δ238U. This approach has been used to constrain the extent of anoxia associated with the end-Permian mass extinction (Brennecka et al, 2011; Lau et al, 2016) and the Cambrian SPICE (Dahl et al, 2014). The global nature of the carbonate δ238U signal is supported by a negative shift of ∼ − 0.4h spanning the Permian–Triassic boundary in multiple
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