Closing in on the marine 238 U/ 235 U budget

Andersen, Morten B.; Vance, Derek; Morford, Jennifer L.; Bura-Nakić, Elvira; Breitenbach, Sebastian Fm; Och, Lawrence M.

doi:10.1016/j.chemgeo.2015.10.041

Cited by 93 publications

(113 citation statements)

References 67 publications

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“…Additionally, comparisons to modern seawater (−0.41 ± 0.14h; Andersen et al, 2014;Stirling et al, 2007;Tissot and Dauphas, 2015;Weyer et al, 2008) indicate that mean δ 238 U values for T1 are indistinguishable from modern values (Student's t-test p-value = 0.87, mean difference = 0.01h), whereas values for T2 are marginally different (p-value = 0.02, mean difference = 0.09h), and significantly different for T3 (p-value ≤ 0.001, mean difference = 0.31h). Because continental crust has a δ 238 U value more positive than seawater (∼ − 0.3 ± 0.05h; summarized in Andersen et al, 2016), it follows that our data are significantly fractionated from average continental crust.…”

Section: Statistical Analyses Of Differences Between Stratigraphic Sementioning

confidence: 68%

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

Lau

Macdonald

Maher

et al. 2017

Earth and Planetary Science Letters

106

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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.

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Section: Statistical Analyses Of Differences Between Stratigraphic Sementioning

confidence: 68%

“…The proportion terms, p, and rate constants, K anox and K na , are calculated based on the modern uranium budget, summarized in Tissot and Dauphas (2015) and Andersen et al (2016). We note Fig.…”

Section: Interpretation As Seawater δ 238 Umentioning

confidence: 99%

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

Lau

Macdonald

Maher

et al. 2017

Earth and Planetary Science Letters

106

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“…Decreasing riverine δ 238 U to −3.7‰ would recreate the magnitude of the excursion in seawater δ 238 U, but it produces no change in [U] and requires riverine isotopic composition to be much lower than any observed natural substance [ Stirling et al ., ; Weyer et al ., ; Tissot and Dauphas , ], making this scenario highly improbable. Given a more reasonable δ riv value (–0.27‰ for modern [ Noordmann et al ., ; Andersen et al ., ]), decreasing F riv could also generate a negative excursion in δ 238 U. However, even decreasing this flux to zero results in changes to δ 238 U and [U] that are very gradual and thus are too small to measure with existing instrument precision and far smaller than observed variation.…”

Section: Discussionmentioning

confidence: 99%

“…Rather, variations in 238 U/ 235 U arise during reduction of U(VI) to U(IV), where 238 U is preferentially reduced and removed from solution due to a nuclear volume effect [Schauble, 2007]. Little to no uranium isotope fractionation (0.1&) has been observed during weathering or coprecipitation in oxic CaCO 3 sediments [Stirling et al, 2007;Weyer et al, 2008;Romaniello et al, 2013;Andersen et al, 2016;Chen et al, 2016]. Net uranium isotope fractionation in marine sediments reflects preferential reduction of 238 U over 235 U as well as the resulting differences in diffusion from the overlying water column into the sediment [Clark and Johnson, 2008;Andersen et al, 2014].…”

Section: Introductionmentioning

confidence: 99%

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

Jost

Bachan

Schootbrugge

et al. 2017

Geochem Geophys Geosyst

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The end‐Triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia, suggesting that anoxia played a role in suppressing marine biodiversity. However, global changes in ocean anoxia are difficult to quantify using proxies for local anoxia. Uranium isotopes (δ238U) in CaCO3 sediments deposited under locally well‐oxygenated bottom waters can passively track seawater δ238U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238U(VI) relative to 235U(VI) in anoxic marine sediments. We measured δ238U in shallow‐marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m. We observe a ∼0.7‰ negative excursion in δ238U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ13C excursion and persisting for the duration of subsequent high δ13C values in the lower‐middle Hettangian stage. The δ238U excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium. Based on output from a forward model of the uranium cycle, the excursion is consistent with a 40–100‐fold increase in the extent of anoxic deposition occurring worldwide. Additionally, relatively constant uranium concentrations point toward increased uranium delivery to the oceans from continental weathering, which is consistent with weathering‐induced eutrophication following the rapid increase in pCO2 during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ238U implies that anoxia could have delayed biotic recovery well into the Hettangian stage.

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“…The fractionation during U sorption to oxides typically ranges from −0.2‰ to −0.15‰ (e.g., Brennecka et al, 2011;Jemison, Johnson, Shiel, & Lundstrom, 2016). Under average river water δ 238 U values (−0.34‰-equivalent to the crustal average of −0.3‰; Andersen et al, 2016;Tissot & Dauphas, 2015;Weyer et al, 2008), the fractionation during U sorption to oxides would result in a δ 238 U value of approximately -0.5‰. There are δ 238 U values in iron oxiderich layers in the Ediacara Member that are much more negative than −0.5‰.…”

Section: Resultsmentioning

confidence: 99%

The late‐stage “ferruginization” of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from uranium isotopes

et al. 2017

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The paleoenvironmental setting in which the Ediacara Biota lived, died, and was pre- U values, consistent with a groundwater U source. These U isotope data thus corroborate sedimentological observations that ferric oxides along fossiliferous surfaces of the Ediacara Member consist of surficial, non-bedform-parallel staining, and sharply irregular patches, strongly reflecting post-depositional, late-stage processes.Therefore, both sedimentological and geochemical evidence indicate that Ediacara iron oxides do not reflect synsedimentary ferruginization and that the presence of iron oxides cannot be used to either invoke a terrestrial paleoenvironmental setting for or reconstruct the taphonomic pathways responsible for preservation of the EdiacaraBiota. These findings demonstrate that careful assessment of paleoenvironmental parameters is essential to the reconstruction of the habitat of the Ediacara Biota and the factors that led to the fossilization of these early complex ecosystems.

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Closing in on the marine 238 U/ 235 U budget

Cited by 93 publications

References 67 publications

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

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

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

The late‐stage “ferruginization” of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from uranium isotopes

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