2.1 billion years old boulder of banded ironstone. (credit: Wikipedia)
During most of the last hundred years every car body, rebar rod in concrete, ship, bridge and skyscraper frame had its origins in vividly striped red rocks from vast open-pit mines. Comprising mainly iron oxides with some silica, these banded iron formations, or BIFs for short, occur in profitable tonnages on every continent. But commercial reserves are confined mainly to sedimentary sequences dating from about 3 to 2 billion years ago. They are not the only commercial iron formations, but dominate supplies from estimated reserves of around 105 billion tons. From a non-commercial standpoint they are among the most revealing kinds of sediment as regards the Earth system and its evolution. All scientific aspects of BIFs and similar Fe-rich sediments are reviewed in a recent volume of Earth Science Reviews. (Konhauser, K.O. and 12 others 2017. Iron formations: a global record of Neoarchaean to Palaeoproterozoic environmental history. Earth Science Reviews, v. 172, p. 140-177; doi: 10.1016/j.earscirev.2017.06.012).
The chemical, mineral and isotopic compositions of BIFs form a detailed repository of the changing composition of seawater during a crucial period for the evolution of Earth and life – the transition from an anoxic surface environment to one in which water and air contained a persistent proportion of oxygen, known as the Great Oxidation Event (GOE). Paradoxically, BIFs are highly oxidized rocks, the bulk of which formed when other rocks show evidence for vanishingly small amounts of oxygen in the surface environment. The paradox began to be resolved when it was realized that ocean-ridge basaltic volcanism and sea-floor hydrothermal activity would have released vast amounts of soluble, reduced iron-2 into anoxic seawater, in the upper parts of which the first photosynthetic organisms evolved. Evidence for the presence of such cyanobacteria first appears around 3.5 billion years ago, in the form of carbonates whose structure suggests they accumulated from growth of microbial mats. Oxygen generated by photosynthesis in iron-rich water immediately acts to oxidize soluble iron-2 to iron-3 to yield highly insoluble iron oxides and hydroxides and thus deposits of BIFs. While oceans were iron-rich, formation of ironstones consumed ecologically available oxygen completely.
Other biological processes seem to have been involved in ironstone formation, such as photosynthesis by other bacteria that used dissolved iron-2 instead of water as a reductant for CO2, to release iron-3 instead of oxygen. That would immediately combine with OH– ions in water to precipitate iron hydroxides. Konhauser and colleagues cogently piece together the complex links in chemistry and biology that emerged in the mid- to late Archaean to form a linkage between carbon- and iron cycles, which themselves influenced the evolution of other, less abundant elements in seawater from top to bottom. The GOE is at the centre. The direct evidence for it lies in the sudden appearance of ancient red soils at about 2.4 billion years, along with the disappearance of grains of sulfides and uranium oxides – both readily oxidized to soluble products – from riverine sandstones, which signifies significant oxygen in the atmosphere. Yet chemical changes in Precambrian marine sediments perhaps indicate that oxygen began to rise in ocean water as early as 3 billion years ago. That suggests that for half a billion years biogenic and abiogenic processes in the oceans were scavenging oxygen as fast as it could be produced so that only tiny amounts, if any, escaped into the atmosphere. Among other possible factors, oceanic methane emissions from methanogen bacteria may have consumed any atmospheric oxygen – today methane lasts only for about 9 years before reaction with oxygen forms CO2. If and when methanogens declined free oxygen would have been more likely to survive in the atmosphere.
The theme running through the review is that of changing and linked interactions between life and the inorganic world, mantle, lithosphere, hydrosphere and atmosphere that involved all available chemical elements. The dominant chemical process, as it is today, was the equilibrium between oxidation and reduction – the loss and gain of electrons among possible chemical reactions and in metabolic processes. Ironstones were formed more commonly between 3 to 2 Ga than at any time before or since, and form a substantial part of that periods sedimentary record. Their net product and that of the protracted organic-inorganic balancing act – oxygenation of the hydrosphere and atmosphere – opened the way for eukaryote organisms, their reproduction by way of the splitting and recombination of nuclear DNA and their evolutionary diversification into the animal and plant life that we know today and of which we are a part. It is possible that even a subtly different set of global processes and interactions set in motion during early evolution of a planet apparently like Earth may have led to different and even unimaginable biological outcomes in later times. The optimism of exobiologists should be tempered by this detailed review.
Posted in Geobiology, palaeontology, and evolution, Planetary, extraterrestrial geology, and meteoritics, Sedimentology and stratigraphy
Tagged BIF, Early Earth system, Great Oxidation event, Ironstone formations, oxidation-reduction
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The global glaciations of the Neoproterozoic that reached low latitudes – the so-called ‘Snowball Earth’ events have dominated accounts of ancient glaciations since the start of the 21st century. Yet they are not the oldest examples of large-scale effects of continental ice sheets. Distinctive tillites or diamictites that contain large clasts of diverse, exotic rocks occur in sedimentary sequences of Archaean and Palaeoproterozoic age. The oldest are dated at around 2.9 Ma in the Pongola Supergroup of Swaziland, South Africa and formed at an estimated palaeolatitude of 48°; within the range of the equatorward extent of Pleistocene ice sheets. No evidence has turned up for glaciation of that age in other regions, and therefore for a ‘Snowball Earth’ at that time. The surprise is not the antiquity of the Pongola glaciation but the fact that tillites formed by glaciers are not more common in the early part of geological history. The sun has increased in its warming effect since the Earth formed so that the very absence of glaciations over huge spans of early Precambrian time points strongly towards an early atmosphere far richer in greenhouse gases than it is now.
Evidence for Palaeoproterozoic glaciation is more widespread, important tillites occurring in the Great Lakes region of North America and in the Transvaal and Griqualand regions of South Africa. Those of South Africa formed at a latitude of around 10°, suggesting ‘Snowball’ conditions, and in each region there are multiple tillites in the stratigraphic column. Accurate dating of volcanic ash horizons in the sequences of both areas (Rasmussen, B. et al. 2013. Correlation of Paleoproterozoic glaciations based on U-Pb zircon ages for tuff beds in the Transvaal and Huronian Supergroups. Earth and Planetary Science Letters, v. 382, p. 173-180) has made it possible to correlate three glacial deposits precisely between the two now widely separated areas. The dating also reveals that four glacial events occurred over a period of 200 Ma between 2.45 and 2.22 billion years ago: longer than the duration of the Mesozoic Era of the Phanerozoic and about the same as the time span during which 3 or 4 ‘Snowball’ events plastered the planet with ice in the Cryogenian and Ediacaran Periods of the Neoproterozoic.
Diamictite from the Palaeoproterozoic Gowganda Formation in Ontario Canada (credit: Canadian Sedimentology Research Group)
This episode of the first large-scale glaciations neatly brackets the first appearance of significant amounts of oxygen in the Earth’s atmosphere during the Great Oxidation Event from 2.45 to 2.2 Ga. It is hard to avoid the conclusion that the two were connected as an increase in oxygen in the air must have influenced the concentration of greenhouse gases, especially that of methane, the most powerful of several that delay loss of heat to space by radiation from the surface. Once oxygen production by photosynthetic organisms exceeded a threshold atmospheric methane would very rapidly have been oxidized away to CO2 plus water vapour, leaving excess oxygen in the air to prevent the build-up of methane thereafter as is the case nowadays. But what pushed atmospheric composition beyond that threshold? A key piece of evidence lies in the record of different carbon isotopes in seawater of those times, which emerges from their study in Precambrian limestones.
After the end of the Archaean Eon at 2.5 Ga the proportion of marine 13C to 12C increased dramatically. Its accepted measure (δ13C) changed rapidly from the near-zero values that had previously characterised the Archaean to more than 10; an inflated value that lingered for much of the half-billion years that spanned the Great Oxidation Event and the Palaeoproterozoic glaciations (Martin, A.P et al. 2013. A review of temporal constraints for the Palaeoproterozoic large, positive carbonate carbon isotope excursion (the Lomagundi–Jatuli Event). Earth-Science Reviews, v. 127, p. 242-261). Later times saw δ13C return to hovering between slightly negative and slightly positive values either side of zero until the Neoproterozoic when once more ‘spikes’ affected the C-isotope record during the period of the better known ‘Snowball’ events. What lay behind this very broad carbon-isotope anomaly?
To increase 13C at the expense of 12C requires to removal from seawater of very large amounts of the lighter isotope. The only likely mechanism is the prolonged and permanent burial of masses of organic material, the only substances that selectively take up 12C. In turn, that implies a huge increase in biological productivity and its efficient burial without being oxidised to CO2 plus water. There are three possibilities: oxygen was absent from the ocean floor; sedimentation was too fast for oxidising bacteria to keep pace or such bacteria did not evolve until the end of the Lomagundi–Jatuli Event. It seems likely that such a dramatic change in the biosphere may have marked some fundamental shift in biological evolution not long after the close of the Archaean. Whichever, the biosphere somehow increased its capacity to generate oxygen. Since oxygen is anathema to many kinds of anaerobic bacteria and archaea, probably the only kinds of organism at the outset of these events, it is possible to imagine continual extinctions yet to maintain high biological productivity new organisms may have emerged to replace those that vanished. By 2.0 Ma, the first putative eukaryote cells (those with nuclei and a variety of organelles) had appeared.
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