Uranium isotopic compositions of the crust and ocean: Age corrections, U budget and global extent of modern anoxia

Tissot, F. L. H.; Dauphas, N.

doi:10.1016/j.gca.2015.06.034

Cited by 185 publications

(224 citation statements)

References 131 publications

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“…Virginia Polytechnic Institute and State University, Blacksburg, USA a globally integrated scale. This is possible due to the long residence time and uniform δ 238 U of uranium in the modern ocean and predicted for the Cambrian ocean (Weyer et al, 2008;Dahl et al, 2014;Tissot and Dauphas, 2015). The δ 238 U proxy has been utilised to track past global ocean redox during three known oceanic anoxic events (Montoya-Pino et al, 2010;Brennecka et al, 2011;Dahl et al, 2014;Elrick et al, 2016;Lau et al, 2016), where anoxic water masses expanded over larger areas of the seafloor and caused negative δ 238 U excursions.…”

Section: Lettermentioning

confidence: 99%

“…This excursion serves as an important stratigraphic marker, although little is known about the biogeochemical significance of the event. The end-Stage 2 samples carry low δ 238 U values of -0.65 ‰, increasing stratigraphically to a value of -0.45 ‰ that approaches the modern oxygenated oceans, -0.39 ± 0.01 ‰ (Tissot and Dauphas, 2015), before again returning in two steps to -0.7 ‰. This positive δ 238 U excursion of +0.25 ‰ coincides with the positive carbon isotope excursion, suggesting that they are both linked to the changes in global seawater chemistry.…”

mentioning

confidence: 96%

“…for the average oceanic input range from -0.30 ‰ to -0.27 ‰ (Tissot and Dauphas, 2015;Noordmann et al, 2016). If the anoxic proportion of global U burial (f U ) today is 12-25 % (Morford and Emerson, 1999;Dunk et al, 2002;Noordmann et al, 2016), and anoxic settings impart a +0.5 ± 0.1 ‰ net isotope offset from overlying seawater, this implies that the average isotope fractionation between seawater and all other oxic U sinks (Δ OTHER ) is -0.02 ± 0.05 ‰.…”

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confidence: 99%

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Reorganisation of Earth’s biogeochemical cycles brieﬂy oxygenated the oceans 520 Myr ago

Dahl

Connelly

Kouchinsky³

et al. 2017

Geochem. Persp. Let.

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The Phanerozoic radiation of bilaterian animals has been linked to oxygenation of Earth's oceans, due to the oxygen demand of the evolving animal ecosystems. However, how early animals may have regulated Earth's surface oxygen budget via self-stabilising feedbacks is poorly understood. Here, we report parallel positive uranium, carbon, and sulphur isotope excursions from carbonate successions in Siberia that document a brief global oxygenation episode 521-520 Myr ago, at the onset of diversification of larger arthropods known from the fossil record. Our data and model indicate that an abrupt increase in the sinking rate of marine organic matter expanded the oxygenated zone in the oceans and that reducing conditions returned 1.3 ± 0.8 Myr after the onset of this transient oxygenation episode, necessitating a strong negative feedback to the increasing levels of oxygen. We speculate that larger zooplankton could have sourced both oxygen and food to the seafloor, fueling bioturbation over wider areas and, thereby, stabilising O 2 -rich habitats in the oceans. Thus, this reorganisation exemplifies how animal ecosystems might have influenced oxygen availability in Earth's surface environment soon after their establishment. LetterIn contrast to many geochemical proxies that evaluate local ancient marine redox including iron speciation and trace metal (Mo, U, V) enrichments, the uranium isotope composition (δ 238 U, the per mille deviation of the 238 U/ 235 U ratio relative to CRM 145 standard) of seawater can be used to evaluate ocean oxygenation at 1. Natural History Museum of Denmark, University of Copenhagen, Denmark * Corresponding author (email: tais.dahl@snm.ku.dk) 2. Centre for Star and Planet Formation, University of Copenhagen, Denmark 3. Swedish Museum of Natural History, Stockholm, Sweden 4. Virginia Polytechnic Institute and State University, Blacksburg, USA a globally integrated scale. This is possible due to the long residence time and uniform δ 238 U of uranium in the modern ocean and predicted for the Cambrian ocean (Weyer et al., 2008;Dahl et al., 2014;Tissot and Dauphas, 2015). The δ 238 U proxy has been utilised to track past global ocean redox during three known oceanic anoxic events (Montoya-Pino et al., 2010;Brennecka et al., 2011;Dahl et al., 2014;Elrick et al., 2016;Lau et al., 2016), where anoxic water masses expanded over larger areas of the seafloor and caused negative δ 238 U excursions. Here, we use uranium isotopes to identify a transient global oxygenation episode during the radiation of animals in the Cambrian.Our new δ 238 U data of carbonate-associated uranium comes from limestones collected from the Siberian Platform across the provisional Cambrian Stage 2-3 boundary (~521 to 520 million years ago) (Fig. 1), when animals that shed their exoskeleton (ecdysozoa) began to diversify (Maloof et al., 2010;Kouchinsky et al., 2012). A perturbation in the marine carbon cycle is expressed at this time as a large positive carbon isotope excursion recognised globally and in all studied sections;...

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Section: Lettermentioning

confidence: 99%

mentioning

confidence: 96%

mentioning

confidence: 99%

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Reorganisation of Earth’s biogeochemical cycles brieﬂy oxygenated the oceans 520 Myr ago

Dahl

Connelly

Kouchinsky³

et al. 2017

Geochem. Persp. Let.

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“…Goldmann et al (2015) found the Pb-Pb ages they dated are 0.9 Ma younger than the age determined with previously assumed value 137.88 for 238 U/ 235 U. Tissot and Dauphas (2015) proposed equations for the corrections on Pb-Pb and U-Pb ages, although these corrections are much smaller than the ages of the samples. The magnitude and direction of Pb-Pb and U-Pb age corrections are only related to a sample's age and 238 U/ 235 U ratio.…”

Section: Isotope Geochronologymentioning

confidence: 69%

“…Nowadays, a few researches have verified that 238 U/ 235 U is not invariable (Stirling et al 2007;Weyer et al 2008;Hiess et al 2012;Goldmann et al 2015;Tissot and Dauphas 2015). Therefore, an age correction is required if the value of 137.88 ( 238 U/ 235 U ratio) is used for dating.…”

Section: Isotope Geochronologymentioning

confidence: 99%

Nuclear field shift effects on stable isotope fractionation: a review

Yang

Liu

2016

Acta Geochim

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An anomalous isotope effect exists in many heavy element isotope systems (e.g., Sr, Gd, Zn, U). This effect used to be called the ''odd-even isotope effect'' because the odd mass number isotopes behave differently from the even mass number isotopes. This mass-independent isotope fractionation driving force, which originates from the difference in the ground-state electronic energies caused by differences in nuclear size and shape, is currently denoted as the nuclear field shift effect (NFSE). It is found that the NFSE can drive isotope fractionation of some heavy elements (e.g., Hg, Tl, U) to an astonishing degree, far more than the magnitude caused by the conventional mass-dependent effect (MDE). For light elements, the MDE is the dominant factor in isotope fractionation, while the NFSE is neglectable. Furthermore, the MDE and the NFSE both decrease as temperatures increase, though at different rates. The MDE decreases rapidly with a factor of 1/T 2 , while the NFSE decreases slowly with a factor of 1/T. As a result, even at high temperatures, the NFSE is still significant for many heavy element isotope systems. In this review paper, we begin with an introduction of the basic concept of the NSFE, including its history and recent progress, and follow with the potential implications of the inclusion of the NFSE into the kinetic isotope fractionation effect (KIE) and heavy isotope geochronology.Keywords Isotope fractionation Á Mass-dependent effect Á Nuclear field shift effect Á Mass-independent fractionation Á Nuclear volume effect Á Nuclear shape effect

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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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Uranium isotopic compositions of the crust and ocean: Age corrections, U budget and global extent of modern anoxia

Cited by 185 publications

References 131 publications

Reorganisation of Earth’s biogeochemical cycles brieﬂy oxygenated the oceans 520 Myr ago

Reorganisation of Earth’s biogeochemical cycles brieﬂy oxygenated the oceans 520 Myr ago

Nuclear field shift effects on stable isotope fractionation: a review

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

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