Molybdenum isotope fractionation during adsorption by manganese oxides

Barling, Jane; Anbar, Ariel D.

doi:10.1016/s0012-821x(03)00608-3

Cited by 376 publications

(256 citation statements)

References 25 publications

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“…It has been observed that coordination changes of Mo, U, Tl, Cu and Zn during adsorption onto Mnoxyhydroxides led to resolvable isotopic fractionation (Barling et al, 2004;Brennecka et al, 2011b;Nielsen et al, 2013;Little et al, 2014). Generally, metal complexes with smaller coordination number and shorter bond length have stronger chemical bonds, favoring incorporation of heavier isotopes (Schauble et al, 2004).…”

Section: Change In U Speciation During Incorporation Into Cacomentioning

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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Section: Change In U Speciation During Incorporation Into Cacomentioning

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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“…Manheim and Landergren, 1974;Colodner et al, 1995;Morford and Emerson, 1999;Anbar, 2004). Of particular interest is the highly redox-sensitive behaviour of Mo that is accompanied by significant isotope fractionation, which results in oxic, suboxic, anoxic and euxinic sediments possessing isotopically distinct Mo-isotope compositions (Barling et al, 2001;Barling and Anbar, 2004;Siebert et al, 2003Poulson et al, 2006;Neubert et al, 2008a). Consequently variations in δ 98/95 Mo can be utilised as a palaeoredox proxy, and have been used to demonstrate the expansion of reducing marine conditions at a number of times throughout Earth's history (e.g.…”

Section: Introductionmentioning

confidence: 99%

Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain

Pearce

Burton

Strandmann

et al. 2010

Earth and Planetary Science Letters

100

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Editor: M.L. DelaneyKeywords: molybdenum (Mo) isotopes basaltic weathering riverine inputs estuarine mixingThe application of the molybdenum (Mo) isotope system as a proxy for determining changes in the redox state of the oceans is predicated on the assumption that the composition of continental input can be characterised from crustal rock types, and remains constant. However, it has recently been shown that the δ 98/95 Mo composition of global rivers varies between 0.15‰ and 2.4‰ and is therefore systematically heavier than the average composition of the continental crust (∼ 0‰ 4‰).Systematic changes in the dissolved Mo-isotope composition are also observed within river catchments, with δ 98/95 Mo values increasing from ∼ 0‰ in glacial rivers (close to the source) to ∼ 1‰ downstream, consistent with Mo-isotopes being fractionated during weathering. Analysis of other riverine phases (bedload, colloids and iron-precipitates) demonstrates that these phases preferentially incorporate light Moisotopes, and remain coupled to the dissolved load during riverine transportation. A δ 98/95 Mo profile through the Borgarfjörður estuary exhibits a predominantly conservative mixing behaviour, but suggests that the release of isotopically light Mo from the particulate and/or colloidal phases may occur in the low salinity part of the estuarine mixing zone.

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“…Molybdenum is one of these elements, that is, around 70% of output is presumed to be incorporated into ferromanganese oxides in the budget of Mo in the marine system (Bertine and Turekian, 1973;Siebert et al, 2003). This element is a redox-sensitive trace metal with seven stable isotopes and is essential in many bio-many studies have focused on this promising tool (Anbar, 2004;Barling and Anbar, 2004;Weeks et al, 2007;Wasylenki et al, 2008;Arnold et al, 2004;Pearce et al, 2008;Poulson et al, 2006;Voegelin et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides

2009

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Due to its potential as a paleoredox proxy, there have been many studies on Mo isotopic fractionation during adsorption onto ferromanganese oxides in seawater. However, the mechanisms of both adsorption and isotopic fractionation are still under debate due to the lack of structural information on the adsorbed species. In this study, XAFS analyses were performed to reveal the mechanism, based on structural information at the molecular level, of Mo isotopic fractionation during adsorption onto ferromanganese oxides. Molybdenum L 3 -edge XANES and K-edge EXAFS revealed that Mo species adsorbed on the surface of ferrihydrite was a tetrahedrally coordinated outer-sphere complex, while that on δ-MnO 2 was an octahedrally coordinated inner-sphere complex. Additionally, it was also revealed that δ-MnO 2 was the dominant host phase of Mo in the hydrogenetic ferromanganese nodules from the comparison of their XAFS spectra. Previous studies reported that lighter isotopes of Mo were preferentially incorporated into ferromanganese oxides from seawater. This fractionation can be explained based on the structural difference between tetrahedral MoO 4 2-(=a major species in seawater) and the octahedral species adsorbed on the Mn oxide phase in ferromanganese nodules. In contrast, little change in Mo local structures during its adsorption onto ferrihydrite also suggested the little or no fractionation of Mo isotopes in the presence of Fe hydroxides without Mn oxides. These facts imply that the Mo isotopic composition in ancient marine sediments can distinguish redox boundaries of Fe 2+ /Fe(OH) 3 and of the more oxic Mn 2+ /MnO 2 .

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Molybdenum isotope fractionation during adsorption by manganese oxides

Cited by 376 publications

References 25 publications

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain

A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides

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