Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation

Abstract

The role that iron played in the oxygenation of Earth’s surface is equivocal. Iron could have consumed molecular oxygen when Fe3+-oxyhydroxides formed in the oceans, or it could have promoted atmospheric oxidation by means of pyrite burial. Through high-precision iron isotopic measurements of Archean-Paleoproterozoic sediments and laboratory grown pyrites, we show that the triple iron isotopic composition of Neoarchean-Paleoproterozoic pyrites requires both extensive marine iron oxidation and sulfide-limited pyritization. Using an isotopic fractionation model informed by these data, we constrain the relative sizes of sedimentary Fe3+-oxyhydroxide and pyrite sinks for Neoarchean marine iron. We show that pyrite burial could have resulted in molecular oxygen export exceeding local Fe2+ oxidation sinks, thereby contributing to early episodes of transient oxygenation of Archean surface environments.

Strong evidence for a weakly oxygenated ocean–atmosphere system during the Proterozoic

“…Given that microbial rates of Fe(II) oxidation at low [O 2 ] are higher than abiotic oxidation rates ( 38 ), our [O 2 ] estimate of <5 μmol/kg is likely to be conservatively high. This is evident when comparing our model constraints to the much lower estimates for shallow water [O 2 ] that would be obtained using a dispersion–reaction modeling approach that has been used in other Fe isotope studies ( 14 , 37 ). Such low surface water [O 2 ]—as indicated by the maximum constraints provided by our model—would be expected to have exerted a powerful control on the ecology of even the shallowest marine environments during this time ( 39 , 40 ).…”

“…This may suggest that fractionation and isotopic distillation due to partial pyrite formation was not an important process in the middle Proterozoic ( SI Appendix ). More data from this interval are needed, and investigations into the triple Fe isotope composition of Proterozoic Fe(II) sulfides and Fe(III) oxides may help to better elucidate the importance of the sulfide Fe sink during this time ( 37 ). In sum, the sedimentary Fe record is more consistent with a partial oxidation control on ironstone Fe isotope fractionation, rather than the quantitative drawdown of a fractionated Fe(II) source.…”

I‐Type Cosmic Spherules as Proxy for the Δ′17O of the Atmosphere—A Calibration With Quaternary Air

“…A similar definition has been introduced by Heard et al. (2020) to display small variations in triple iron isotope ratios. Instead of defining Δ′ 56 Fe, they introduced ε' 56 Fe, which is 10 times Δ′ 56 Fe as defined in Equation .…”

“…In contrast to oxygen, where, for historic reasons, a slope of the reference line of 0.528 has been chosen, for iron, we choose the high-T approximation for equilibrium fractionation for the three iron isotopes of 0.6780 as slope of the reference line for defining Δ′ 56 Fe (Young et al, 2002). A similar definition has been introduced by Heard et al (2020) to display small variations in triple iron isotope ratios. Instead of defining Δ′ 56 Fe, they introduced ε' 56 Fe, which is 10 times Δ′ 56 Fe as defined in Equation 5.…”

Microbial processes during deposition and diagenesis of Banded Iron Formations

“…Overall, it is considered that positively fractionated δ 56 Fe values reflect the partial oxidation of iron by either abiotic or biotic means. A recent examination of triple iron isotope systematics (Heard et al 2020) also indicates that there was extensive iron oxidation in the Archean to Paleoproterozoic ocean. While there are uncertainties associated with such proxy approaches to discerning microbial activity, the wealth of independent evidence suggests that microorganisms were involved in BIF deposition, and build a strong basis for future work.…”