Clark Johnson on the Banded Iron Formations
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Banded Iron Formations (BIFs) are a visually striking group of sedimentary rocks that are iron rich and almost exclusively deposited in the Precambrian. Their existence points to a major marine iron cycle that does not operate today. Several theories have been proposed to explain how the BIFs formed. While they all involve the precipitation of ferric (Fe3+) iron hydroxides from the seawater via oxidation of dissolved ferrous (Fe2+) iron that was abundant when the oceans contained very low levels of free oxygen, they disagree as to how this oxidation occurred. In the podcast, Clark Johnson describes how oxidation could have occurred without the presence of abundant free oxygen in the oceans. If oxidation occurred via biological processes, this would have produced abundant organic carbon. To explain the absence of this carbon today, he suggests that iron-reducing bacteria “respired” the carbon via anoxygenic photosynthesis, in the process oxidizing it to carbon dioxide or a carbonate mineral such as siderite.
Johnson is a Professor Emeritus in the Department of Geoscience at the University of Wisconsin-Madison.
Podcast Illustrations
Images courtesy of Clark Johnson unless otherwise indicated.
2.5-billion-year-old (Ga) Brockman Iron Formation, Karijini National Park, Western Australia.
Exposed in the walls of canyons carved by rivers in the Western Australian outback, the Brockman Iron Formation is representative of the largest BIFs in the world, with layering and banding that varies on scales from sub-mm to meters. Their red color is the result of recent surface weathering of iron oxides.
Detail of a fresh outcrop of the 2.5-Ga Brockman Iron Formation. Here, the major iron mineral is magnetite, and most of the remaining rock is chert (quartz). The sub-mm and mm-scale layering might represent annual deposition (analogous to varves). The cm-scale layering contains dewatering structures formed when the initial iron-silica gels were deposited on the seafloor and compacted during initial lithification.
Detail of a fresh core of the 2.5-Ga Kuruman BIF, South Africa. The core is about 2 cm across. This unit was probably deposited in the same continental margin basin system as the Brockman BIF. In this core, the layers consist of primary hematite, magnetite, and siderite (Fe carbonate), plus chert. Such fresh samples show that BIF mineralogy consists of both oxidized (Fe3+) and reduced (Fe2+) iron. The earliest iron mineral is hematite, indicating the first stage of BIF mineral formation is an oxidative one, where seawater Fe2+ is oxidized to Fe3+. The Fe2+-bearing minerals, magnetite and siderite, indicate a later reductive stage on the seafloor during early diagenesis. This second stage is needed to explain the observation that, on average, BIFs of this age contain about 60% Fe2+ (i.e., reduced iron).
Changes in iron mineralogy through time. In unmetamorphosed iron-rich rocks, including BIFs and related lithologies such as japerlites (hematite+chert) of Archean age, such as the 3.5-Ga Marble Bar Chert (above left), the dominant iron oxide is hematite (Fe3+). By contrast, in rocks of Proterozoic age, such as the ~2.5-Ga Kuruman Iron Formation (above right), magnetite and siderite (both Fe2+) predominate. In both cases, Fe3+ oxides were precipitated initially, which implies that the younger BIFs additionally record a later reductive process to produce the Fe2+ minerals.
Temporal changes in atmospheric oxygen and BIF deposition (top graph) and estimates for growth of the continental crust (100% equals modern volume, bottom graph). In the graphs, time advances from old at right to young at left.
BIFs older than 2.7 Ga are very small; one of the oldest BIFs, the 3.7-Ga rocks at Isua Greenland, do not even plot on the presented scale. The very large BIFs deposited at the end of the Archean and early Paleoproterozoic, ~2.7-2.4-Ga age, include the Brockman and Kuruman BIFs, which were deposited before the "Great Oxidation Event" at ~2.3 to 2.4 Ga. We also know that the photic zone of the oceans started to become oxidized as early as 3.2 Ga. Thus, oxidation of seawater Fe2+ as part of the deposition of the very large ~2.7-2.4-Ga BIFs could have occurred either by oxygenic photosynthesis or by an anoxygenic photosynthetic pathway or some combination. Meanwhile, although the rate of continental crust formation over time is uncertain, all models suggest an increase in continental crust over time. As explained in the podcast, continental shelves provide the environments for BIF deposition and preserve them from subduction. They may also reflect a driver for a biological role through increased nutrient supply via continental weathering.
The ratios of carbon (C) and oxygen (O) isotopes in the BIF carbonates (siderite and other iron-rich carbonates) depend on how they were formed. The figure shows the range of these ratios measured in the 2.5-Ga Kuruman Iron Formation (blue field) and the ~2.9 Ga-Witwatersrand-Pongola Iron Formation (orange field). The black box in upper right indicates the C and O isotope compositions corresponding to equilibrium with seawater, which is what would be expected in the carbonates if there was no microbial respiration component. The green field shows the range of C and O isotope compositions expected for carbonates that were solely formed by respiration of organic carbon and Fe3+ oxide. As the plot shows, neither of the BIF formation carbonates have C and O isotope compositions that are in equilibrium with seawater, and nor are their carbonates entirely made from respired iron oxide and organic carbon. This provides powerful evidence that microbial iron reduction was a major process in producing the iron carbonates in BIFs.
Figure adapted from Johnson, C.M. et al. (2020), Iron Geochemistry: An Isotopic Perspective, Springer, 360
The Marandoo open-pit mine in the Hamersley Group of the Marra Mamba Iron Formation in the Pilbara region of Western Australia. The mine produces 15 million tons of iron ore a year, which is crushed and screened to remove magnetite and other impurities, producing haematite as an end product. This is transported by rail and road to the ports at Dampier and Port Hedland for export to steel manufacturers in China and other Far Eastern countries. BIFs such as these are the main source of iron in today's society. Thus, the isotopic studies shown in the figure above suggest that about half of all the iron we use today was respired by bacteria about 2.5 billion years ago!
Courtesy of Rio Tinto
Further Reading
Johnson, C.M. et al. (2020), Iron Geochemistry: An Isotopic Perspective. Springer, 360 p. DOI: 10.1007/978-3-030-33828-2.
Konhauser, K.O. et al. (2017), Iron Formations: A Record of Neoarchean to Paleoproterozoic Environmental History. Earth. Sci. Rev. 172:140-177.
Johnson, C.M. (2017), Iron Formations. In: W.M. White (ed.), Encyclopedia of Geochemistry, Springer. DOI 10.1007/978-3-319-39193-9_58-1.