Cr Isotope Ratios and Paleoredox Conditions of the Atmosphere

Also motivated by the desire to constrain further the history of oxidation of the atmosphere and oceans, Frei et al. [37] determined the isotopic compositions of Cr in banded iron formation (BIF) samples, representing the Archean ( 2.5 Ga ago) and Proterozoic (2.5 Ga to 542 Ma ago) Eons. They anticipated that isotopic variations in those rocks would correlate with changes in global redox conditions over time because of previous work reporting isotope ratio variations correlated with the Cr oxidation state in natural samples [38, 39], experiments [40, 41], and theoretical calculations [42]. 

Frei et al. [37] hypothesized that a paleoredox proxy based on Cr isotope ratios would likely reflect atmospheric oxygenation, rather than marine oxygenation. When oxygen is abundant in the atmosphere, Cr(lll) in exposed rocks on land is converted during oxidative weathering to Cr(Vl), which is highly soluble in water 

Cr(III), Frei et al. proposed that the pool of Cr(VI) delivered to the oceans should be isotopically heavy, with an isotope fractionation from the Cr(III) remaining in continental rocks possibly as large as 6%o, since that is the equilibrium fractionation between Cr(VI) and Cr(III) in aqueous solution, as calculated theoretically by Schauble et al. [42]. The higher the extent of oxidative weathering, the more heavy Cr could be dehvered to the ocean. Upon encountering dissolved Fe(II) in seawater, isotopically heavy Cr(VI) should be rapidly reduced to Cr(III) and copredpitated with Fe oxyhydroxides (sediments that will become BIF). Hence the S3/S2cr values of BIF should correlate positively with the intensity of oxidative weathering at the time of deposition. 

If the interpretation of Cr isotope ratio variation by Frei et al. [37] is valid, Cr isotope ratios provide an extremely valuable complement to other geochemical proxies for paleoredox conditions, since they reflect atmospheric, rather than marine, oxygenation and can be measured in a rock type that is not suitable for applying other paleoredox proxies, such as Mo or S isotope ratios. Some critical assumptions in Frei et al. s study [37] must be verified, namely (i) that oxidative weathering of Cr(III) from rocks indeed results in an isotopically heavy pool of dissolved Cr and (ii) that reduction and coprecipitation of Cr with Fe oxyhydroxides either does not fractionate Cr isotopes or results in complete removal of Cr from the water column in continental shelf environments where BIF are deposited. Although Cr isotope ratio behavior is not yet fully understood at a fundamental level, several studies have been published that provide important insights into this system. 

Schauble et al. [42] calculated equilibrium fractionation factors between various Cr species, using both published vibrational spectra and ab initio force field models. The findings were consistent with similar work on other elements, including Fe and Zn, in that heavier isotopes preferentially partition into species 

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