The Ancient Earth

Archean to early Paleoproterozoic iron formations document a transition in iron oxidation mechanisms

“…One potential means by which we can parse the mechanisms underpinning Fe(II) oxidation in the ancient oceans is through measurement of the relative abundances of the different stable isotopes of Fe in the Fe-oxides of IFs 24 . This approach is appealing since the kinetics of Fe(II) oxidation are well-constrained and the Fe isotope systematics of Fe(II) oxidation by photoferrotrophs and ambient O2 are relatively well-understood 25,26,27,28 . Briefly, partial oxidation of dissolved Fe(II) produces Fe(III)-oxyhydroxides that are enriched in 56 Fe, typically by 1 to 3‰, leaving the residual dissolved Fe(II) 56 Fe-depleted, although more rapid oxidation effectively lowers this overall fractionation 27 .…”

“…This approach is appealing since the kinetics of Fe(II) oxidation are well-constrained and the Fe isotope systematics of Fe(II) oxidation by photoferrotrophs and ambient O2 are relatively well-understood 25,26,27,28 . Briefly, partial oxidation of dissolved Fe(II) produces Fe(III)-oxyhydroxides that are enriched in 56 Fe, typically by 1 to 3‰, leaving the residual dissolved Fe(II) 56 Fe-depleted, although more rapid oxidation effectively lowers this overall fractionation 27 . This contrasts with the formation of reduced iron phases (e.g., Fe carbonates and silicates): these pathways are typically considered to involve either a negligible fractionation or a small negative fractionation 26,27 , leaving residual dissolved Fe(II) slightly 56 Fe-enriched.…”

Development of a multi-isotopic (Pb, Fe, Cu) analytical protocol in gold matrices for ancient coin provenance studies

“…, Mathur et al 67 for Cu isotopes) to nil (Milot et al 29 for Fe isotopes). The low-temperature processes produce the most important isotopic fractionation, which is notably due to the oxidation-reduction of Cu and Fe ( e.g ., Mathur et al , 67 Jansen et al , 28 Johnson et al 68 ). This aspect has been addressed in archaeological provenance studies.…”

Microbial processes during deposition and diagenesis of Banded Iron Formations

“…The stable iron isotope ratios (δ 56 Fe) in BIF of the Archean and early Paleoproterozoic may also be used to infer early photoferrotrophy. The δ 56 Fe values of BIFs vary widely between − 2.5 and 1.0 ‰ relative to values expected for lithogenic or hydrothermal sources (− 0.5 ‰ < δ 56 Fe < 0.3 ‰) (Czaja et al 2013 ; Johnson et al 2003 , 2020 ). These BIF δ 56 Fe values differ from the homogenous values of modern marine sediments with δ 56 Fe values of 0.00 ± 0.05 ‰, and have been interpreted to reflect BIF minerals being both precipitated with equilibrium fractionation from an isotopically varying fluid, as well as indicating a microbial influence (Johnson et al 2003 , 2004 ; Johnson and Brian 2005 ; Steinhoefel et al 2010 ).…”