Uranium concentrations and 238U/235U isotope ratios in modern carbonates from the Bahamas: Assessing a novel paleoredox proxy

Romaniello, Stephen J.; Herrmann, Achim D.; Anbar, Ariel D.

doi:10.1016/j.chemgeo.2013.10.002

Cited by 175 publications

(197 citation statements)

References 68 publications

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“…Dunk et al, 2002). Biogenic carbonates incorporate the seawater 238 U/ 235 U signature without appreciable isotope fractionation (Romaniello et al, 2013;Stirling et al, 2007;Weyer et al, 2008). In contrast, U accumulation in suboxic and, in particular, anoxic sediments, shows deviations to higher 238 U/ 235 U ratios (Montoya-Pino et al, 2010; Romaniello et al, 2013;Weyer et al, 2008), consistent with the nuclear field shift process.…”

Section: Uranium Isotope Geochemistrymentioning

confidence: 63%

A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox

Andersen

Romaniello

Vance

et al. 2014

Earth and Planetary Science Letters

167

200

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Section: Uranium Isotope Geochemistrymentioning

confidence: 63%

A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox

Andersen

Romaniello

Vance

et al. 2014

Earth and Planetary Science Letters

167

200

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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).…”

mentioning

confidence: 77%

“…To date, almost all measurements of primary carbonate precipitates show U isotopic compositions similar to seawater (Weyer et al, 2008;Romaniello et al, 2013).…”

Section: Discussionmentioning

confidence: 99%

“…Although no resolvable U isotopic fractionation has been observed between modern aragonite and calcite samples and seawater (Weyer et al, 2008;Romaniello et al, 2013) (Juillot et al, 2008;Brennecka et al, 2011b;Wasylenki et al, 2011;Nielsen et al, 2013;Little et al, 2014;Bryan et al, 2015). Previous studies found that the predominant U(VI) species in seawater, UO 2 (CO 3 ) 3 4-, was directly incorporated as a unit into aragonite without coordination change or reduction (Reeder et al, 2000).…”

mentioning

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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“…15-17 and 34, but see further discussion in ref. 34), then the δ 238 U of carbonates should decrease as regions of anoxic marine bottom waters expand. Because uranium has a residence time (3.2-5.6 × 10 5 y) (35) significantly longer than ocean mixing time (10 3 y), global seawater variations in [U] and δ 238 U should be reflected within local sediments.…”

Section: Methodsmentioning

confidence: 99%

Marine anoxia and delayed Earth system recovery after the end-Permian extinction

Lau

Maher

Altıner

et al. 2016

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

240

193

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Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and 238 U/ 235 U isotopic compositions (δ 238 U) of Upper Permian−Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ 238 U across the end-Permian extinction horizon, from ∼3 ppm and −0.15‰ to ∼0.3 ppm and −0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelvesglobal biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.paleoredox | uranium isotopes | biogeochemical cycling | carbon isotopes | Early TriassicT he end-Permian mass extinction-the most severe biotic crisis in the history of animal life-was followed by 5 million years of reduced biodiversity (1, 2), limited ecosystem complexity (3), and large perturbations in global biogeochemical cycling (4, 5). Ocean anoxia has long been invoked both as a cause of the extinction (6-8) and as a barrier to rediversification (9). Numerous lines of evidence demonstrate widespread anoxic conditions around the time of the end-Permian mass extinction (e.g., refs. 6 and 10-12). In contrast, the prevalence of anoxia during the 5-to 10-millionyear recovery interval remains poorly constrained (13,14).Reconstructing paleoredox conditions is challenging because some indicators of anoxia characterize only the local conditions of the overlying water column, whereas other indicators may be influenced by confounding factors, such as weathering rates on land. Here, we use paired measurements of [U] and δ 238 U in marine carbonate rocks to differentiate changes in weathering of U from variations in global marine redox conditions. Microbially mediated reduction of U(VI) to U(IV) under anoxic conditions at the sediment−water interface results in a substantial decrease in uranium solubility and a measureable change in 238 U/ 235 U (15-18). Because 238 U is preferentially reduced and immobilized relative to 235 U, the δ 238 U value of seawater U(VI) decreases as the areal extent of bottom water anoxia increases (Fig. S1). Consequently, a global increase in the extent of anoxi...

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Uranium concentrations and 238U/235U isotope ratios in modern carbonates from the Bahamas: Assessing a novel paleoredox proxy

Cited by 175 publications

References 68 publications

A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox

A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Marine anoxia and delayed Earth system recovery after the end-Permian extinction

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