Microbial Fe cycling during the genesis of Banded Iron Formations: influence on Fe mineral (trans)formation and nickel and phosphate mobility
Final Report Abstract
Banded iron formations (BIF) belong to the oldest rocks on Earth and are the biggest iron ore deposits worldwide. They consist of alternating iron and silica rich layers, are between 3.8 to 1.85 Ga old and were precipitated in Precambrian oceans. While the oldest BIF were most likely deposited by anoxygenic phototrophic Fe(II)-oxidizing bacteria, the oxygen production by cyanobacteria was responsible for the formation of the latter BIFs. Their age makes BIFs a valuable witness of the early life on Earth and the emerge of free O2 in the atmosphere, produced by cyanobacteria, leading to the Great Oxidation Event (GOE) 2.45 Ga ago. The early, O2-free oceans were rich in dissolved Fe2+ (up to 0.5 mM) and dissolved silica (up to 2.2 mM). The Fe2+ was oxidized to Fe(III) and precipitated to Fe(III) (oxyhydr)oxide minerals and was then in the water column (and in the sediment) re-reduced (partially) by dissimilatory Fe(III)-reducing bacteria (DIRB) to those Fe(II)/Fe(III) minerals that we can still find in BIFs nowadays: hematite, siderite, magnetite and Fe(II)/Fe(III)-silicates. In this project, the role of cyanobacteria was evaluated with regard to the other processes. Therefore, different consecutive iron oxidation and – reduction cycles were conducted to test the influence of different iron and silica concentrations on the mineral formation by O2 (produced by cyanobacteria) and Fe(III) reducing bacteria. To adapt the experiments more to realistic early ocean conditions, phosphate and nickel was added in latter experiments to test toxicity effects and nutrient limitations. Our experiments showed the formation of ferrihydrite, goethite, lepidocrocite, Fe(II)/ Fe(III) silicates and magnetite. At realistic iron concentrations (0.5 mM) the minerals were poorly crystalline. Silica inhibited the formation of reactive oxygen species (ROS), which implies that bacteria did not suffer from O2 toxicity as previously assumed in literature. Nutrient deficiency, (low phosphate concentrations <3.67 µM), allowed cell growth in combination with low, realistic nickel concentrations (< 4 µM). The added P and Ni were mainly (>90%) bound to the mineral phase and was therefore not bioavailable. This means that the mineral formation depleted the early sea in nutrients (phosphate) but also lowered the Ni toxicity. We conclude that cyanobacteria in combination with DIRB were able to form BIF-relevant minerals despite previously described nutrient deficiency, as well as O2- and Ni toxicity. These Fe(II)/Fe(III) precursors were poorly crystalline and were transformed by diagenesis and metamorphosis to nowadays BIF minerals.
Publications
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Microbial processes during deposition and diagenesis of Banded Iron Formations. PalZ, 95(4), 593-610.
Dreher, Carolin L.; Schad, Manuel; Robbins, Leslie J.; Konhauser, Kurt O.; Kappler, Andreas & Joshi, Prachi
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Cyanobacteria-ferrihydrite aggregates, BIF sedimentation and implications for Archaean- Palaeoproterozoic seawater geochemistry. South African Journal of Geology, 127(2), 359-378.
Li, Y.; Sutherland, B.R.; Ilin, A.M.; Schad, M.; Robbins, L.J.; Kappler, A.; Yusta, I.; Sánchez-España, J.; Owttrim, G.W.; Dreher, C.L.; Smith, A.J.B.; Alessi, D.S.; Gingras, M.K. & Konhauser, K.O.
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Impact of silica on the identity of minerals formed in Archean oceans during Fe cycling by cyanobacteria and iron(III)-reducing bacteria. Geochimica et Cosmochimica Acta, 404, 202-222.
Dreher, Carolin L.; Schad, Manuel; Duda, Jan-Peter; Fischer, Stefan; Shuster, Jeremiah; Li, Yuhao; Konhauser, Kurt O. & Kappler, Andreas
