Natural variations in uranium isotope ratios of uranium ore concentrates: Understanding the 238U/235U fractionation mechanism

Brennecka, G. A.; Borg, L. E.; Hutcheon, I. D.; Sharp, Michael A.; Anbar, Ariel D.

doi:10.1016/j.epsl.2010.01.023

Cited by 169 publications

(127 citation statements)

References 13 publications

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“…This isotopic signature has been measured in low-temperature paleo-environments, enabling the use of U isotope fractionation as a proxy for the redox conditions of ancient atmosphere and oceans (14)(15)(16). A recent study (17) observed enrichment of the heavier 238 U isotope in the products of biogenic U reduction, consistent with findings in nature (8,10,11,18) and in agreement with ab initio calculations (19,20).…”

supporting

confidence: 68%

“…Low-temperature ore deposits, such as sedimentary U deposits, require U(VI)-bearing fluids to react with a reduced zone, leading to the precipitation of U(IV) minerals (8,36). In addition, moderate authigenic U enrichment is found in a range of geological settings, including marine sediments that formed under anoxic conditions.…”

Section: Discussionmentioning

confidence: 99%

“…Moreover, permil-level fractionations of the two abundant and primordial uranium isotopes are reported in association with U(VI) to U(IV) redox transitions through the accumulation of the heavy isotope ( 238 U) rather than the light isotope ( 235 U) in the reduced product (8)(9)(10)(11)(12)(13). This isotopic signature has been measured in low-temperature paleo-environments, enabling the use of U isotope fractionation as a proxy for the redox conditions of ancient atmosphere and oceans (14)(15)(16).…”

mentioning

confidence: 99%

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Uranium isotopes fingerprint biotic reduction

Stylo

Neubert

Wang

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

133

153

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Knowledge of paleo-redox conditions in the Earth’s history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth’s crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium.

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supporting

confidence: 68%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Uranium isotopes fingerprint biotic reduction

Stylo

Neubert

Wang

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

133

153

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“…The most important driver of isotopic fractionation for the heavy element U is the nuclear volume effect, which leads to preferential enrichment of heavy U isotopes in U(IV) during U(VI) reduction (Bigeleisen 1996;Schauble, 2007;Brennecka et al, 2010;Basu, 2013;Bopp et a., 2009Bopp et a., , 2010. However, in our experiments, U is always in the (VI) oxidation state.…”

Section: U Isotope Fractionation Mechanismcontrasting

confidence: 44%

“…Uranium isotopes are being actively explored and applied as an additional source of ocean paleoredox information (Montoya-Pino et al, 2010;Brennecka et al, 2010Brennecka et al, , 2011aAndersen et al, 2014;Dahl et al, 2014;Kendall et al, 2013). Natural variations in 238 U/ 235 U are known to be sensitive to redox changes (Stirling et al, 2007;Weyer et al, 2008).…”

mentioning

confidence: 99%

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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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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Natural variations in uranium isotope ratios of uranium ore concentrates: Understanding the 238U/235U fractionation mechanism

Cited by 169 publications

References 13 publications

Uranium isotopes fingerprint biotic reduction

Uranium isotopes fingerprint biotic reduction

Uranium isotope fractionation during coprecipitation with aragonite and calcite

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

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