Photo-oxidation of hydrated Fe2+—significance for banded iron formations

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…An argument against a green-rust precursor to BIF magnetite, however, is found in the Fe isotope composition of the Dales Gorge Member (Fig. 8 (Cairns-Smith, 1978;Braterman et al, 1983;Anbar and Holland, 1992 . Based on mass-balance calculations and consideration of the amount of hydrogen peroxide potentially generated by UV photo-oxidation of water, it was determined that the amount of hydrogen peroxide derived from atmospheric reactions is orders of magnitude too small to account for the Fe(III) sedimentation required to form IF As an alternative to the abiological model, the presence of ferric iron minerals in IFs has also been ascribed to the metabolic activity of planktonic bacteria in oceanic photic zones (Fig.…”

Fossilized iron bacteria reveal a pathway to the biological origin of banded iron formation

“…I t is generally believed that banded iron formations (BIFs), the bulk of which was formed in the late Archaean/ Palaeoproterozoic marine basins, occurred in stratified water columns deep in the ocean and on continental shelf margins [1][2][3][4] . Soluble ferrous iron, supplied from mid-ocean ridges and hydrothermal vents, was oxidized by various processes to ferric iron and deposited in association with varying silica ratios as BIFs [1][2][3][4] .…”

“…Soluble ferrous iron, supplied from mid-ocean ridges and hydrothermal vents, was oxidized by various processes to ferric iron and deposited in association with varying silica ratios as BIFs [1][2][3][4] . But the mechanism that drove and sustained the massive and sporadic deposition of BIFs throughout much of the Precambrian remains a mystery, despite decades of research.…”

“…Instead, biotic, anoxygenic photoferrotrophic precipitation according to the equation 4Fe 2 þ þ CO 2 þ 11H 2 O þ light-[CH 2 O] þ 4Fe(OH) 3 þ 8H þ has been proposed [6][7][8][9][10][11][12][13][14] . With the exemption of proxies such as iron isotopes 12,13 , there exists no direct environmental evidence-neither in ancient nor in modern ecosystems-demonstrating how photoferrotrophs could have accounted for vast-scale biological BIF deposition, including the formation of their spectacular banded consistency.…”

Biological carbon precursor to diagenetic siderite with spherical structures in iron formations

“…The precursor sediments, including amorphous ferric oxyhydroxides, such as ferrihydrite (Fe(OH) 3 ), required an oxidative mechanism to transform the dissolved hydrothermally-derived Fe(II) into solidphase Fe(III). It has been suggested that abiotic UV-light dependent oxidation of Fe(II) led to the precipitation of ferric hydroxide minerals 2,3 , but later experiments have shown that in seawater containing high concentrations of dissolved Si(OH) 4 and HCO 3 À the oxidation effects of either UVA or UVC are negligible compared to the precipitation of ferrous iron-silicates 4 . In contrast, two different roles for bacteria have been inferred in Fe(II) oxidation.…”