Oxygen in the Atmosphere
For the first 2 billion years (2Ga) of the earth’s existence (4.6Ga - 2.5Ga) the atmosphere consisted largely of nitrogen and carbon dioxide, along with significant amounts of methane, ammonia, sulphuric acid and water vapor. Free oxygen was non-existent or, at best, present as a trace gas.
A depiction of the earth’s surface and atmosphere as it might have been during the first 0.5Ga of its existence (the Hadean Eon). A recently separated moon looms large on the horizon. Painting by Steve Munsinger.
Evolving in this anoxic reducing environment, anaerobic single-celled prokaryotic life first appeared in the waters of the world around 3.6Ga, deriving its energy from volcanic heat. After a few hundred million years of evolution, one of these organisms called cyanobacteria (a.k.a. blue-green algae) developed the ability to split the CO2 dissolved in oceans and seas into its constituent components of carbon and oxygen using solar radiation as an energy source.
Taxonomists classify blue-green algae as a class of bacteria, but they are a transitional species to the first true plants. Taxonomists always have trouble positioning transitional species in their classification schemes. Classification implies boundaries, but in evolution (the name gives the clue) there are no hard boundaries.
The biochemical process that the cyanobacteria invented is known as photosynthesis. The carbon was used to make the organic compounds of life, releasing oxygen back to the atmosphere as a waste product.
The cyanobacteria turned out to be an extraordinarily successful and enduring class of life. They are still with us today.
In the warm tropical waters of the intertidal zone at Shark Bay in Western Australia, thin films of cyanobacteria cement sand to form layered, dome-shaped mounds known as stromatolites. Fossil stromatolite structures in the stratigraphic record go back almost 2.5 billion years.
The 0xygen content of the atmosphere slowly increased.
The greater part of the energy driving photosynthesis is stored by plants (and animals, which live by eating plants or eating animals that eat plants) in the chemical bonds of their organic compounds. Excepting only nuclear energy, photosynthesis is the most efficient energy collection and storage system that we know.
Between 2.4Ga and 1.8Ga, evolution of the eukaryotic cell and multicellular life forms made photosynthesis more efficient and plant life more diverse, abundant and widespread in the photic (sunlight penetrating) zone of surface waters. We call them phytoplankton. As a result, more carbon dioxide was consumed, more oxygen was released back to the atmosphere. An initial slow increase in atmospheric oxygen became, around 2.5Ga, a step increase. This was by far the most important change in the evolution of the earth’s atmosphere. It is known as the Great Oxidation Event (GOE).
Today, our atmosphere contains, by volume, around 21% oxygen and 79% nitrogen. A mere trace of carbon dioxide remains, making up 0.0043% or 430 parts per million. Methane is even more trace at 1.3 parts per billion. Sulphuric Acid is less than 1 part per trillion.
The reverse process to photosynthesis in plants and animals is respiration. In respiration, living things power their activities by burning a small proportion of their stored carbohydrate in oxygen, thus releasing some of their stored carbon back to the atmosphere as CO2. On the death of water-living plants (phytoplankton), the bulk of their carbon (estimated at around 90%) settles to the sea floor to accumulate as organic-rich deposits. There, sealed from oxygen by water depth and co-deposited silt, they transformed overtime, through pressure and temperature, into oil and gas.
Land-living plants (and their dependent animals) were late comers to evolution. They first evolved around 4-500 million years ago (0.4-0.5Ga).
In terms of sheer volume of living matter, the great age of land plants and animals occurred between 0.36Ga and 0.3Ga. A Period known as the Carboniferous. A Period when atmospheric oxygen levels peaked at over 30% (wildfires during that time must have been impressive).
The name tells the story: rocks of Carboniferous age are the major source of our high-grade black coal deposits, the fossilised remains of the once mighty and long-lived Carboniferous forests. A mighty repository of carbon.
The Role of Iron
Iron is a heavy element (atomic number 26) that arrived in the early accreting days or the earth through continuous meteor impact. Today, iron is the most abundant element by weight in the planet, but most of it lies in the inner molten core. In the crust oxygen contained in its various compounds, is the most abundant element. Iron in the crust is the 4th most abundant element. In basaltic rocks, iron constitutes around 10-15%.
The early crust was dominated by rocks of basaltic composition.
Iron comes in two flavours called valency states: ferrous iron, written Fe 2+ (or 2 electrons short of the full set of 8 in its outer shell) and ferric iron written Fe 3+ (3 electrons short). The metal thus has great capacity for combining with elements of negative charge, particularly oxygen, which is ever hungry for a positively charged valency partner. If only a small amount of oxygen is available to it, iron forms low-oxidation ferrous oxide. Where there is abundant oxygen, it forms high-oxidation ferric oxide compounds.
But note, there is an important distinction between the two states of iron. Ferrous iron compounds are generally soluble in water; ferric iron compounds are not.
In the low oxygen reducing atmosphere of the first 2 billion years of earth history, chemical erosion of basaltic rocks at surface produced soluble ferrous iron oxide compounds, which, washed to the sea, slowly accumulated in the oceans of the world. Trillions of tons of dissolved iron. During the GOE, over a relatively short time span (geologically speaking) of a few hundred million years or so, the oceanic reservoir of ferrous iron became oxygenated to ferric iron which, being insoluble, settled as chemical sediment to the sea floor to form thick iron rich sequences of great thickness and lateral extent. These sequences are typically banded at centimeter scale by silica in the form of chert layers, suggesting a fine scale cycling – possibly seasonal – of depositional conditions.
In later geological periods, chemical sediments were dominantly carbonates. In the late Archaean to early Proterozoic, it was mainly ferric iron.
These distinctive iron rich deposits are known as Banded Iron Formations (BIF). They are preserved today around all the ancient shield areas of the world: Western Australia, Brazil, Venezuela, North America, India, West and South Africa and parts of eastern Asia.
Banded iron formation exposed in the Karinjini Gorge, Hamersley Range, Western Australia. Image: WA Tourist Department.
But another process was at play during the GOE
High levels of free oxygen in the earth’s atmosphere are the product of life. No other planet in the galaxy that we know of (yet) contains free oxygen.
Plants and animals need oxygen to respire, to live. But that life is the result of a Faustian Contract made around 3 billion years ago. Free oxygen is highly corrosive, and it puts strict limits on the individual life spans of all living things, including us.
For rocks exposed at surface, free oxygen breaks down the minerals in contact with it. The GOE supercharged erosion of all exposed land surfaces especially as, back then, there were no land plants to bind soils and constrain river channels. Beginning around 2.5 Ga, increasing floods of clastic sediment poured off the land creating, for the first time at volume, wide sedimentary shelves and shallow marginal seas around the great continental land masses. At the base of these sequences, interspersed with silica and aluminium rich clastic sediment, are the Banded Iron Formations.
Every wildfire is a demonstration of out-of-control oxidation. Every chemical explosion is a demonstration of a near instantaneous chain reaction oxidative event.
For these reasons, some have referred to the Great Oxidation Event as the Great Oxygen Catastrophe.
The Economic Importance of Banded Iron Formations
Iron is our cheapest and most useful metal. We first learnt how to smelt it from its oxide ores around 2500 years ago, thus initiating the Iron Age and, because iron is so much stronger and harder than bronze, supplanting the previous Bronze Age. Another factor in this transition was that ore suitable for iron smelting was readily available whereas copper and tin ores (especially tin) needed to make bronze are geographically restricted throughout Europe and the Middle East.
BIFs contain, in bulk, around 30% iron by weight. Natural processes operating over time, have locally upgraded them to 60-65% iron – almost pure iron oxide (Fe2O3). These high-grade zones are today extracted in vast open cut mines, producing the ore from which today 95% of our iron is smelted. Around the world, ship-borne high-grade BIF ore has now almost totally supplanted the local low-grade deposits that supplied early Iron Age societies.
Four point eight billion tons of iron ore were mined in 2025. The four biggest producers are Australia (980mt), Brazil (410mt), China (340mt) and India (125mt).
Abundant iron in the form of its various alloys (especially steel), builds the skeletal frameworks that support our civilisation. From bridges to buildings, ships to railroads, automobiles to trucks to turbines to tanks, we owe our supply of cheap ore to the Great Oxidation Event, and to the fossil remains of plant material with which we reduce it to iron.
But will this Bonanza Last?
Nothing is forever. No resource is inexhaustible. However, although extraction of 4.8 billion tons per annum is a large number, it is trivial compared to what is potentially available from the Banded Iron Formations of the world.
There is no prospect of the ore running out within this millennium.