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Project Progress

In Year 3, we performed a study of Fe and Mo isotope compositions of Neoarchean carbonates as old as 2.7 Ga in age, to test for evidence of a rise in atmospheric oxygen prior to the GOE or the so-called “whiff” event at 2.5 Ga. Individually, these isotope systems can provide useful information about the redox processes present in ancient sedimentary systems (Rouxel et al, 2005; Anbar and Rouxel, 2007; Johnson et al., 2008; Czaja et al., 2010; Heimann et al., 2010; Voegelin et al., 2010), but our data demonstrate that the combination of Fe and Mo isotopes provides powerful and quantitative evidence for free O₂, including constraints on the minimum oxygen concentration required to produce co-variations in these isotopic systems.

We measured the Fe isotope compositions of carbonate samples from drill core recovered from the Neoarchean–Paleoproterozoic Kaapvaal Craton of South Africa (Schröder et al., 2006; Knoll and Beukes, 2009), samples that were previously analyzed for Mo isotope compositions by Voegelin et al. (2010). Our analyses show that there is a generally negative correlation between δ⁵⁶Fe and δ⁹⁸Mo values (Figure 1; Czaja et al., submitted). This correlation is best explained by oxidation of reduced aqueous Fe [Fe(II)_aq] by free O₂ and the subsequent removal of Fe(III) oxyhydroxides [Fe(OH)₃] from the photic zone. Figure 1 illustrates our model of coupled Fe and Mo isotope variation. Fe(II)_aq with a δ⁵⁶Fe value of ~0 ‰ was brought up from deeper in the basin, some of which was oxidized by photosynthetically produced oxygen to insoluble Fe oxy-hydroxides [Fe(OH)₃] that had positive δ⁵⁶Fe values. An oxygen flux to the atmosphere would have caused oxidative weathering of continental sulfides and the release of aqueous molybdate (MoO₄^2-_aq; δ⁹⁸Mo = 0.2 ‰) to the ocean. During precipitation of Fe(OH)₃, the light isotope of Mo is preferentially sorbed (Goldberg et al., 2009) and thus deposition of Fe(OH)₃ as banded iron formation (BIF) or other Fe-rich sediments would have sequestered the positive δ⁵⁶Fe and negative δ⁹⁸Mo reservoirs. In turn, this would leave behind a residual pool of isotopically light Fe(II)_aq and heavy MoO₄^2- in the photic zone relative to their respective sources. We interpret such pools of residual dissolved Fe and Mo to have been directly incorporated into the carbonates sampled. Thus, the key control on the residual aqueous Fe and Mo isotope compositions are the relative proportions of initial aqueous Fe and Mo, and the amounts removed via Fe oxide formation and Mo sorption, respectively; this model provides the mechanism by which these two isotope systems may be coupled. Our model also suggests that times of increased Fe flux to marine sedimentary platforms and Fe deposition would have maximized the Fe and Mo isotope fractionations of the residual seawater. Consistent with this supposition, the periods of maximum negative Fe isotope and positive Mo isotope excursions¬ were all times of maximum flooding of the platform and Fe deposition (Figure 1), and this model is supported by analyses of the lithology and sedimentology of the platform (Beukes, 1987).

Based on our calculated initial Fe concentration of the water column, the stoichiometry of primary Fe oxides [Fe(OH)₃], and the evidence for (at least) photic zone O₂, we have estimated that the dissolved oxygen concentration for the water column in which these carbonates were deposited was 0.30 μM (Figure 2; Czaja et al., submitted). This value is ~10-3 times that expected for present-day seawater in equilibrium with the modern atmosphere (0.2 atm of O₂). However, this value is at least 100 times higher than has been previously estimated immediately prior to the GOE at ~2.4 Ga (10-5 PAL; Holland, 2006 and references therein). Though our calculations only constrain the O₂ content of the photic zone, at least some free atmospheric O₂ would have been required to mobilize Mo during weathering, so it seems likely that both the atmosphere and oceans were significantly oxygenated at this time, although probably at a low level. The data reported here are also consistent with Fe and Mo isotope compositions measured from contemporaneously formed carbonates and shales from the Hamersley Province in Western Australia (Czaja et al., 2010), which suggests that this oxygenation was not a highly localized phenomenon. It is, however, important to recognize that it is likely these two continental blocks in the Archean (Cheney, 1996; Beukes and Gutzmer, 2008). Our results provide the first robust and quantitative estimate for O₂ levels in the photic zone prior to the GOE, and further evidence that the atmospheric evolution of the Earth was complex in the Archean.