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Post-depositional diagenetic alteration makes the accurate interpretation of key precipitation processes in ancient sediments, such as Precambrian banded iron formations (BIFs), difficult. While microorganisms are proposed as key contributors to BIF deposition, the diagenetic transformation of precursor Fe(III) minerals associated with microbial biomass had not been experimentally tested. We incubated mixtures of ferrihydrite (proxy for biogenic ferric oxyhydroxide minerals) and glucose (proxy for microbial biomass) in gold capsules at 1.2kbar and 170°C. Both wet chemical analysis and mineralogical methods (microscopy, X-ray diffraction and Mössbauer spectroscopy) were used to analyze the reaction products. Under these conditions, ferrihydrite (FeIII(OH)3) transforms to hematite (Fe2 IIIO3), magnetite (FeIIFe2 IIIO4), and siderite (FeIICO3). Silica-coated ferrihydrite prepared at conservative Si:Fe ratios (as predicted for the Precambrian oceans) and mixed with glucose yielded hematite and siderite, whereas magnetite could not be identified microscopically. Our results show that electron transfer from organic carbon to Fe(III) minerals during temperature/pressure diagenesis can drive the production of key BIF minerals. Our results also demonstrate that the post-depositional mineralogy of BIF does not directly archive the oceanic or atmospheric conditions present on Earth during their lithification. As a consequence, atmospheric composition regarding concentrations of methane and CO2 during the time of BIF mineral deposition cannot be directly inferred from BIF mineralogical data alone. [ABSTRACT FROM AUTHOR]

Banded iron formations have been studied for decades, particularly regarding their potential as archives of the Precambrian environment. In spite of this effort, the mechanism of their deposition and, specifically, the role that microbes played in the precipitation of banded iron formation minerals, remains unresolved. Evidence of an anoxic Earth with only localized oxic areas until the Great Oxidation Event ca 2·45 to 2·32 Ga makes the investigation of O2-independent mechanisms for banded iron formation deposition relevant. Recent studies have explored the long-standing proposition that Archean banded iron formations may have been formed, and diagenetically modified, by anaerobic microbial metabolisms. These efforts encompass a wide array of approaches including isotope, ecophysiological and phylogeny studies, molecular and mineral marker analysis, and sedimentological reconstructions. Herein, the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post-depositional alteration are described. The main findings of recent years are summarized and compared here, and suggestions are made regarding cross-disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition. [ABSTRACT FROM AUTHOR]

Phototrophic iron(II) [Fe(II)]-oxidizing bacteria are present in modern environments and evidence suggests that this metabolism was present already on early earth. We determined Fe(II) oxidation rates depending on pH, temperature, light intensity, and Fe(II) concentration for three phylogenetically different phototrophic Fe(II)-oxidizing strains (purple nonsulfur bacterium Rhodobacter ferrooxidans sp. strain SW2, purple sulfur bacterium Thiodictyon sp. strain F4, and green sulfur bacterium Chlorobium ferrooxidans strain KoFox). While we found the overall highest Fe(II) oxidation rates with strain F4 (4.5 mmol L−1 day−1, 800 lux, 20 °C), the lowest light saturation values [at which maximum Fe(II) oxidation occurred] were determined for strain KoFox with light saturation already below 50 lux. The oxidation rate per cell was determined for R. ferrooxidans strain SW2 to be 32 pmol Fe(II) h−1 per cell. No significant toxic effect of Fe(II) was observed at Fe(II) concentrations of up to 30 mM. All three strains are mesophiles with upper temperature limits of c. 30 °C. The main pigments were identified to be spheroidene, spheroidenone, OH-spheroidenone (SW2), rhodopinal (F4), and chlorobactene (KoFox). This study will improve our ecophysiological understanding of iron cycling in modern environments and will help to evaluate whether phototrophic iron oxidizers may have contributed to the formation of Fe(III) on early earth. [ABSTRACT FROM AUTHOR]

Abstract: Oxidized Fe minerals in Archean–Paleoproterozoic banded iron formations (BIFs) are commonly taken to indicate the presence of biogenic O2 or photosynthetic Fe(II)-oxidizing bacteria in the oceans'' photic zone. However, at least one viable abiogenic oxidation mechanism has been proposed. Prior to the rise of atmospheric oxygen and the development of a protective ozone layer, the Earth''s surface was subjected to high levels of ultraviolet radiation. Bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved Fe(II). Under such conditions, dissolved ferrous iron species, such as Fe2+ and Fe(OH)+, would have absorbed radiation in the 200–400 nm range, leading to the formation of dissolved ferric iron [Fe(III)], which in turn, would have hydrolyzed to form ferric hydroxide [Fe(OH)3] at circumneutral pH [Cairns-Smith, A.G., 1978, Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 76, 807–808; Braterman, P.S., Cairns-Smith, A.G., and Sloper, R.W., 1983, Photo-oxidation of hydrated Fe2-Significance for banded iron formations. Nature 303, 163–164]. This process has been invoked to account for BIF deposition without need for biology [François, L.M., 1986, Extensive deposition of banded iron formations was possible without photosynthesis. Nature 320, 352–354]. Here, we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models. The experiments simulate the chemistry of ambient Precambrian seawater mixing with Fe(II)-rich hydrothermal fluids with, and without, UV irradiation. We find that if Fe(II) was effused from relatively shallow seamount-type vent systems directly into an anoxic photic zone, the photochemical contribution to solid-phase precipitation would have been negligible. Instead, most of the Fe(II) would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite ((Fe)3Si2O5(OH)4), and/or the ferrous carbonate, siderite (FeCO3), depending on different simulated atmospheric pCO2 levels. Conversely, in experiments where Fe(II) was exposed either to phototrophic Fe(II)-oxidizing bacteria or to O2, ferric hydroxide formed rapidly, and the precipitation of ferrous iron phases was not observed. If, as suggested on mass balance grounds, BIF deposition requires that Fe be sourced from shallow seamount-type systems, then we are driven to conclude that oxide-facies BIF are the product of a rapid, non-photochemical oxidative process, the most likely candidates being direct or indirect biological oxidation, and that a significant fraction of BIF could have initially been deposited as ferrous minerals. [Copyright &y& Elsevier]