The Banded-iron formations give us clues to the atmosphere of early Earth. As we look deep into the Earth's past, evidence from old rocks suggests that environmental conditions were once very different. In particular, rocks making up the Banded-iron formations indicate that the early atmosphere contained little or no oxygen.
The picture in figure 2 below shows an excellent example of Banded-iron formation (BIF). Most BIFs are strikingly colourful with the dark layers being made up mainly of the iron oxide minerals, hematite (Fe2O3) and magnetite (Fe3O4) and red layers of jasper, a variety of chalcedony, or very fine-grained quartz (SiO2) (Mathez, 2006). BIFs are rocks of the Proterozoic Era ranging from 1.8 to 2.5 billion years in age and consist of alternating iron-rich and iron-poor layers, typically only millimeters to centimeters thick . Banded iron formations are found throughout the geological record, but the period from 2.5 to 2.0 billion years represents a unique time in Earth history, a time during which 92% of the Earth’s BIFs were laid down (Immenhauser, 2005). For this enormous accumulation of iron oxide to have occurred over such a vast time span meant that something about the chemistry of early earth was very different to what it is today. The chemistry of rocks from the Proterozoic shows that oxygen was a rare gas in the atmosphere. The key to understanding the chemical reactions occurring in the early oceans is in the relationship between the elements oxygen and iron. Iron forms two ionic states, namely, ferrous (Fe+2) and ferric (Fe+3) – the +2 or +3 indicates the extent to which iron is oxidised. Iron will only dissolve in significant quantities in water that contains no oxygen (anoxic water). In anoxic water iron dissolves in the ferrous state as ions of hydrous Fe2+, or FeOH+ (Mathez, 2006). Therefore, in order for iron-rich chemical precipitates to form, the early oceans must have been sufficiently anoxic to dissolve iron. Since the ocean and atmosphere exchange oxygen rapidly, the atmosphere could not have contained much oxygen, either. But oxygen was in the making; photosynthesis from blue-green bacteria dominating the early oceans would have created a net gain of oxygen first in the ocean and later in the atmosphere (Attenborough, 2010). Ferrous iron in oceanic water scavenged oxygen that was a waste product for the photosynthesising bacteria and rained down onto the ocean floors as rust coloured chemical sediment. This was chemistry on a grand scale – the soluble ferrous iron was being oxidised to insoluble ferric iron as the minerals, magnetite and hematite. At the same time, primitive photosynthetic blue-green algae were beginning to proliferate near surface waters. As the algae would produce molecular oxygen (O2) as a waste product of photosynthesis, this free oxygen would combine with the iron in solution to form iron oxides. “As the biomass expanded beyond the capacity for the available iron to combine with waste O2, the oxygen content of the sea water rose to toxic levels for the algae population and resulted in their large-scale die-off, which in turn gave rise to an iron poor layer of silica on the sea floor” [2.]. As time passed and algae populations re-established themselves, a new iron-rich layer began to accumulate on ocean floors. This cycle was repeated and continued for hundreds of millions of years. “Each band in the iron formation is similar to an annual layer of sediment – or varve - to the extent that the banding is assumed to result from cyclic variations in available oxygen” (Kirschvink, 1992). For over 2 billion years this went on, until the iron in earth’s oceans was depleted. Since there was no iron left in solution the excess molecular oxygen bubbled up into the atmosphere and began accumulating from about 1700 million years ago, after two-thirds of Earth history [6.]. The vast layers of iron minerals stayed behind in the Banded-iron formations. The rise in the levels of oxygen after the massive depletion of iron meant that photosynthesising bacteria would face near extinction as oxygen is a reactive and highly toxic gas (Southwood, 2003). Cells would have to adapt to this change in environment and the excess oxygen would ultimately lead to the formation of an ozone layer and the proliferation of new life forms in an oxygenated world during the so-called “Cambrian Explosion”. But that is the subject of another article! Banded-iron formations occur in many parts of the world and constitute the major reserves of iron ore. At Thabazimbi and Sishen these reserves are exploited by major mining operations.
Figure 1: Core sample (8 cm) of Banded iron-formation from Hamersley, Australia. A fault runs through the center of the specimen showing the displacement of the individual layers of banding. Specimen and photograph: A. Fraser
Figure 2: Banded iron-formation (6 cm). Banded iron-formation is composed of alternating layers of iron-rich material and silica (chert), N’chwaning II mine, Kalahari Manganese Field. Each layer is relatively thin, varying in thickness from about a millimetre up to a few centimetres. This is evidence of aerobic life altering the early earth’s atmosphere by the precipitation of iron oxides. Specimen and photograph: A. Fraser
Figure 3: Banded iron-formation (10 cm). Banded iron-formation is composed of alternating layers of iron-rich material, Northern Cape,, South Africa. Specimen and photograph: A. Fraser
Figure 4: Banded iron-formation (14 cm). Banded iron-formation is composed of alternating layers of iron-rich material, Thabazimbi, South Africa. Specimen and photograph: A. Fraser
Figure 5: Mining activity at the Thabazimbi iron ore mine. Photograph by A.Fraser
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