Natural fractionation of 238U/235U

Weyer, Stefan; Anbar, Ariel D.; Gerdes, Axel; Gordon, Gwyneth W.; Algeo, Thomas J.; Boyle, E. A.

doi:10.1016/j.gca.2007.11.012

Cited by 439 publications

(514 citation statements)

References 79 publications

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“…238 U/ 235 U in modern carbonate samples is close to that of seawater (Weyer et al, 2008;Romaniello et al, 2013), suggesting that marine carbonates captures the 238 U/ 235 U of coexisting seawater. In an application of this proposed proxy, variations of 238 U/ 235 U and U/Th in a carbonate section at Dawen in southern China were interpreted as reflecting an enhancement of oceanic anoxia immediately before the end-Permian mass extinction (Brennecka et al, 2011a).…”

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confidence: 75%

“…Natural variations in 238 U/ 235 U are known to be sensitive to redox changes (Stirling et al, 2007;Weyer et al, 2008). Low-temperature redox reactions of U are the primary drivers of U isotope fractionation on Earth, with 238 U preferentially enriched in reduced species (Stirling et al, 2007;Weyer et al, 2008).…”

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confidence: 99%

“…In the oceans, widespread anoxia causes more U to be scavenged by anoxic sediments, preferentially pulling 238 U from seawater and leading to a corresponding shift of 238 U/ 235 U in seawater. Because of the long residence time of U, it has been proposed that U isotope variations can be used to infer changes in global redox conditions (Weyer et al, 2008). For example, variations of 238 U/ 235 U and U concentration in black shales were used to quantify the spatial extent of marine anoxia during Oceanic Anoxic Event 2 (Montoya-Pino et al, 2010) and indicate oxidative U mobilization at 2.50…”

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confidence: 99%

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Uranium isotope fractionation during coprecipitation with aragonite and calcite

Chen

Romaniello

Herrmann

et al. 2016

Geochimica et Cosmochimica Acta

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Natural variations in 238 U/ 235 U of marine carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~ 8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~ 8.5, with heavier U in the solid phase. 238 U/ 235 U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca 2 UO 2 (CO 3 ) 3 (aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca 2 UO 2 (CO 3 ) 3 (aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.ii These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, P CO2 , [Ca], or [Mg] concentrations. In general, these effects are likely to be small (<0.13 ‰), but are nevertheless potentially significant because of the small natural range of variation of 238 U/ 235 U.

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confidence: 75%

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confidence: 99%

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Uranium isotope fractionation during coprecipitation with aragonite and calcite

Chen

Romaniello

Herrmann

et al. 2016

Geochimica et Cosmochimica Acta

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“…is not critical to our study's objectives, as while the variability in environmental samples varies from 131 less than 0.03 % (Cowan and Adler, 1976;Richter et al, 1999;Bopp et al, 2009) to 0.3% (Stirling et 132 al., 2007;Weyer et al, 2008;Sun et al, 2008;Del Papa et al, 2010), the 235 U activity concentration is 133 low relative to 238 U. 134 Fig.3 upper graph).…”

Section: Theoretical Approach 122mentioning

confidence: 94%

Should we ignore U-235 series contribution to dose?

Beaugelin-Seiller

Goulet

Mihok

et al. 2016

Journal of Environmental Radioactivity

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Highligths: Realistic ecological risk assessment infers a complete inventory of radionuclides  U-235 family may not be minor when assessing total dose rates experienced by biota  There is a need to investigate the real state of equilibrium decay of U chains  There is a need to improve the capacity to measure all elements of the U decay chains Should we ignore U-235 series contribution to dose? 1 2 Abstract. Environmental Risk Assessment (ERA) methodology for radioactive substances is an 3 important regulatory tool for assessing the safety of licensed nuclear facilities for wildlife, and the 4 environment as a whole. ERAs are therefore expected to be both fit for purpose and conservative. 5When uranium isotopes are assessed, there are many radioactive decay products which could be 6 considered. However, risk assessors usually assume 235 U and its daughters contribute negligibly to 7 radiological dose. The validity of this assumption has not been tested: what might the 235 U family 8 contribution be and how does the estimate depend on the assumptions applied? In this paper we 9 address this question by considering aquatic wildlife in Canadian lakes exposed to historic uranium 10 mining practices. A full theoretical approach was used, in parallel to a more realistic assessment based 11 on measurements of several elements of the U decay chains. The 235 U family contribution varied 12 between about 4% to 75% of the total dose rate depending on the assumptions of the equilibrium state 13 of the decay chains. Hence, ignoring the 235 U series will not result in conservative dose assessments 14 for wildlife. These arguments provide a strong case for more in situ measurements of the important 15 members of the 235 U chain and for its consideration in dose assessments 16 17

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“…AE1‰) in nature. 1) The ratio that is different from the natural ratio is thus considered to originate from anthropogenic nuclear activities except for natural nuclear reactors. In the 1950s, Japan Meteorological Agency (JMA) started the sampling of atmospheric deposits in Japan and the measurement of anthropogenic radioactivity for the monitoring of environmental pollution connected to nuclear activities.…”

Section: Introductionmentioning

confidence: 99%