Uranium isotope evidence for temporary ocean oxygenation in the aftermath of the Sturtian Snowball Earth

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 ( 238 U/ 235 U denoted as δ 238 U) 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, δ 238 U compositions have a mean value of −0.47h. This interval is followed by a ∼0.3h decrease in δ 238 U, coincident with the Taishir negative carbon isotope excursion. Thereafter, δ 238 U 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 δ 238 U 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 δ 238 U in overlying strata, coincident with the Taishir negative δ 13 C 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.

Uranium isotope evidence for an expansion of marine anoxia during the end‐Triassic extinction

“…[], −0.28‰, 2 σ = 0.07, n = 3 by Lau et al . []). In addition, CRM‐129a was analyzed at −1.54‰ (2 σ = 0.11, n = 10), within error of previously published values (–1.54‰ in Brennecka et al .…”

Global marine redox changes drove the rise and fall of the Ediacara biota

“…We note that these traditional diagenetic proxies are not explicitly developed for U isotopes but carbonate C, O, and Sr isotope systematics (e.g., Chen et al, ). Although these proxies have their limitations and may not be directly relevant to evaluate diagenesis for carbonate U isotopes proxy, numerical modeling of diagenetic rock‐fluid interactions suggests that δ 238 U should be more robust against diagenetic fluid exchange than δ 18 O and 87 Sr/ 86 Sr (Chen et al, ; Lau, Macdonald, Maher, & Payne, ). Thus, these traditional carbonate diagenetic indicators may be useful in identifying samples with diagenetically altered δ 238 U signatures.…”

“…Several previous U isotope studies suggested that in unaltered rocks, changes to the extent of global seafloor oxygenation will affect the dissolved seawater reservoir of U, and in return the abundance of U incorporated into marine carbonates (Brennecka et al, ; Elrick et al, ; Lau et al, ). Under ideal conditions, stratigraphic variation in U concentrations can record meaningful seawater redox variations, but this relationship can be easily masked by other sources of variation (e.g., Lau et al, ). Notably, prior studies have shown that the distribution coefficient of U into aragonite is significantly larger than for calcite (DeCarlo, Gaetani, Holcomb, & Cohen, ; Meece & Benninger, ; Reeder, Nugent, Lamble, Tait, & Morris, ).…”

“…At pH ∼7.5, the precipitates of both polymorphs exhibit no fractionation (Chen et al, ). Therefore, changing carbonate mineralogy can result in large differences in uranium concentrations but only small changes in the isotopic composition (Lau et al, ).…”

“…This model indicates that in order to account for pre‐SE seawater δ 238 U as low as −0.94‰ to −1.14‰, large areas of seafloor must have been overlain by anoxic waters. The precise extent of ocean anoxia calculated from the mass balance model (Equation 2) is sensitive to Δ anoxic values (Lau et al, ; Zhang, Xiao, et al, ). Assuming Δ anoxic = 0.6‰—an average value that is representative of modern anoxic basins like the Saanich Inlet (Holmden, Amini, & Francois, ) and the Black Sea (Andersen et al, ), the δ 238 U data imply that nearly 100% of the total U ocean sink in the pre‐Shuram ocean was accounted for by removal into anoxic sediments.…”