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Biosignatures in Precambrian banded iron formations: assumptions, evidence and controversies

banded iron formation, Fe(II)-oxidation, microbial processes, biosignatures, iron oxides

Theme 5: Life-Mineral Interaction and Evolutions > Session 5a: Microbe-mineral Interactions and Their Biosignatures: Integrating Laboratory Data with Field Insights

Banded iron formations (BIF) exclusively formed in Precambrian, typically between ~3.8 Ga and 2.2 Ga. They are planetary-wide distributed and temporarily coincident with large-scale magmatic activities driven by mantle plume events, dramatic atmospheric oxygen rising, formation of supercontinent cycles and biospheric evolution. As such, BIFs serve as geological archives of the interactions among early Earth’s biosphere, deep and superfacial geospheres. The iron oxides present in BIFs are believed to have formed through the mediation of direct or indirect microbial oxidation of Fe(II). Consequently, BIFs are regarded as lithological biosignatures, representing global microbial processes from the early Archean to the Paleoproterozoic (Bekker et al., 2014). This hypothesis was supported by several independent lines of evidence: (1) The discovery of purple and green phototrophic bacteria that can utilise Fe(II) as an electron donor for CO₂ fixation in both freshwater and marine environments (e.g., Widdel et al., 1993), with potential existence traced back to the early Archean (Buick, 1992);  (2) Simulation experiments involving photoferrotrophs aimed at evaluating the feasibility of microbial mechanisms for BIF deposition have shown that photosynthetic Fe(II) oxidising bacteria could account for the entirety of Fe(III) precipitation in BIFs (Konhauser et al., 2002; Kappler et al., 2005); (3) Molecular fossil evidence from Paleoproterozoic BIFs containing okenane, a derivative of okenone found exclusively in purple bacteria under euxinic conditions (Brocks et al., 2005); (4) Iron isotopic investigations of Archean BIFs (Johnson et al., 2003) and laboratory experiments demonstrating anaerobic microbial Fe(II)-oxidation (Croal et al., 2004) fall within the same ⁵⁶Fe/⁵⁴Fe fractionation range; and (5) Sulfur isotopic studies indicating that phototrophic bacteria utilizing H₂S/HS^- or H₂as electron donors were not prevalent during the Archean, suggesting that dissolved Fe(II) remained the primary reductant for anoxygenic photosynthesis at that time. However, none of these lines of evidence definitively establish the direct involvement of microorganisms in BIF formation, leading to an alternative hypothesis that the Fe(III) in BIFs may have been oxidised during post-depositional alterations, suggesting a limited role for microorganisms in this process (Rasmussen et al., 2015). This viewpoint is corroborated by the observed scarcity of organic materials in BIFs. Consequently, the search for direct biosignatures remains critical in future studies aimed at elucidating the concrete role of microorganisms in BIF formation and the co-evolution of the biosphere and geosphere

 



References:

Bekker, A., Planavsky, N. J., Krapež, B., Rasmussen, B., Hofmann, A., Slack, J. F., Rouxel, O. J., and Konhauser, K. O., 2014, 9.18 - Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry, in Holland, H. D., and Turekian, K. K., eds., Treatise on Geochemistry (Second Edition): Oxford, Elsevier, p. 561-628.

Brocks, J. J., Love, G. D., Summons, R. E., Knoll, A. H., Logan, G. A., and Bowden, S. A., 2005, Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea: Nature, v. 437, no. 7060, p. 866-870.

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Konhauser, K. O., Hamade, T., Raiswell, R., Morris, R. C., Ferris, F. G., Southam, G., and Canfield, D. E., 2002, Could bacteria have formed the Precambrian banded iron formations?: Geology, v. 30, no. 12, p. 1079-1082.

Rasmussen, B., Krapež, B., Muhling, J. R., and Suvorova, A., 2015, Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age: Geology, v. 43, no. 4, p. 303-306.

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