Great Oxidation Event: How Oxygen First Filled Earth's Air

The Great Oxidation Event was the period, roughly 2.4 to 2.1 billion years ago, when free oxygen first accumulated in Earth’s atmosphere and stayed there. The oxygen was released by cyanobacteria, microbes that split water using sunlight, and its arrival permanently changed the planet’s rocks, oceans, climate and the course of evolution. Every breath taken by an animal today depends on a chemical revolution that began in this remote part of the Paleoproterozoic Era.

Earth before oxygen

For the first two billion years of its history, Earth’s atmosphere was very different from today’s. It was made mostly of nitrogen and carbon dioxide, with some methane and water vapour, but almost no free oxygen. Geochemists estimate that oxygen levels were below about one hundred-thousandth of the present atmospheric level.

Life already existed in this world. Microbial mats and mound-shaped structures called stromatolites, built by layers of microbes trapping sediment, are known from rocks more than 3.4 billion years old, such as the Pilbara stromatolites of Western Australia. These early organisms lived without oxygen, using chemical energy or forms of photosynthesis that did not release oxygen as a by-product.

The oceans of the Archean Eon were also unlike modern seas. Their deep waters were rich in dissolved iron, which can stay in solution only when oxygen is scarce. That iron would later play a central part in the story.

The microbes that changed the world

The source of the oxygen was oxygenic photosynthesis: the process in which light energy is used to split water molecules, releasing oxygen gas while carbon dioxide is turned into sugars. On the early Earth, this ability evolved in one group of bacteria, the cyanobacteria, sometimes called blue-green algae. Plants and algae later inherited the same machinery when a cyanobacterium was absorbed into another cell and became the chloroplast.

When cyanobacteria first appeared is one of the most debated questions in early Earth science. Some geochemical evidence, such as traces of molybdenum and other metals in shales from the Mount McRae Shale of Western Australia, suggests brief “whiffs” of oxygen around 2.5 billion years ago. Some researchers argue that oxygen-producing life was present hundreds of millions of years before the main event; others think it evolved shortly before oxygen began to accumulate.

Why oxygen took so long to build up

Producing oxygen is not the same as filling the air with it. Oxygen is highly reactive, and on the young Earth it was consumed almost as fast as it was made. Volcanic gases such as hydrogen and hydrogen sulphide, dissolved iron in seawater and fresh minerals exposed on land all acted as “sinks” that removed it.

Oxygen could only accumulate once its production exceeded the capacity of these sinks. Scientists have proposed several reasons why the balance tipped: a gradual change in the gases released by volcanoes, the slow escape of hydrogen to space, the burial of organic carbon in sediments (which leaves oxygen behind), and the growth of continents that changed weathering and nutrient supply. Most researchers think a combination of factors was responsible.

Reading oxygen in the rocks

No one can sample air from 2.4 billion years ago, so the Great Oxidation Event is reconstructed from chemical clues preserved in rocks. Several independent lines of evidence point to the same broad interval.

The sulphur isotope signal

The strongest evidence came in 2000, when James Farquhar, Huiming Bao and Mark Thiemens published measurements of sulphur isotopes in ancient rocks. In rocks older than about 2.4 billion years, sulphur shows what is called mass-independent fractionation, an unusual isotope pattern produced when ultraviolet light breaks apart sulphur gases in an atmosphere without oxygen or an ozone layer.

In younger rocks this signal almost vanishes. Its disappearance is now widely used as the marker of the moment when atmospheric oxygen rose above roughly one hundred-thousandth of the present level. Later work suggests the signal flickered on and off for some time, and a 2021 study led by Simon Poulton argued that oxygen did not become permanently established until around 2.22 billion years ago.

Minerals that could not survive oxygen

In rocks deposited by rivers before about 2.4 billion years ago, geologists find rounded grains of pyrite (iron sulphide) and uraninite (a uranium oxide). Both minerals break down quickly when exposed to oxygen, so their survival during transport shows that the air contained little of it. The gold-bearing Witwatersrand deposits of South Africa are a famous example.

After the Great Oxidation Event these grains disappear from river sediments. In their place come “red beds”, sandstones coloured by rust-like iron oxides, and ancient soils whose iron was oxidised and retained rather than washed away.

Banded iron formations

Banded iron formations are striking rocks made of alternating layers of iron-rich minerals and silica. They formed when dissolved iron in seawater reacted with oxygen or was otherwise oxidised and settled to the sea floor. They were deposited in great volumes before and during the Great Oxidation Event and became much rarer after about 1.85 billion years ago.

Banded iron formations are not a simple oxygen gauge, because some may have formed through processes that did not need free oxygen. But their history is closely tied to the changing chemistry of the oceans, and many of the world’s largest iron ore deposits, in Australia, Brazil and elsewhere, are found in them.

A carbon isotope puzzle

Between about 2.22 and 2.06 billion years ago, marine carbonate rocks around the world record an exceptionally high proportion of the heavier carbon isotope, carbon-13. This is called the Lomagundi or Lomagundi-Jatuli excursion, after localities in Zimbabwe and Karelia. It is often interpreted as a sign that unusually large amounts of organic carbon were buried, which would have released large amounts of oxygen, though other explanations have been proposed.

Consequences

Climate and ice ages

Oxygen did not only change chemistry; it may also have changed the climate. The young Sun was roughly 15 to 20 percent dimmer than today, and the early Earth is thought to have stayed warm partly because of methane, a powerful greenhouse gas produced by microbes. Oxygen reacts with methane and destroys it.

Many scientists therefore link the Great Oxidation Event to the Huronian glaciation, a series of ice ages recorded in rocks around Lake Huron in Canada and elsewhere, dated to roughly 2.45 to 2.22 billion years ago. Some deposits, including the Makganyene Formation of South Africa, appear to have formed near the equator, leading to proposals of an early “Snowball Earth”. The exact timing of the glaciations relative to the rise in oxygen is still being refined.

Life and minerals

The new oxygen was a hazard for many organisms. Oxygen and the reactive molecules it forms can damage cells that lack defensive enzymes. At the same time, it offered an enormous opportunity: aerobic respiration, which uses oxygen to break down food, releases far more energy than the anaerobic processes used before.

The rise of oxygen also led to the formation of an ozone layer, which shields the surface from harmful ultraviolet radiation. And it transformed the mineral kingdom. The mineralogist Robert Hazen and colleagues have estimated that a large share of Earth’s more than 5,000 known mineral species could only form in the presence of free oxygen, including many oxidised copper, iron and uranium minerals.

In the long run, oxygen made complex life possible. The origin of eukaryotes, the complex cells that make up animals, plants and fungi, followed the Great Oxidation Event, and their energy-producing mitochondria depend on oxygen. However, oxygen levels remained far below modern values for more than a billion years afterwards.

Myths and facts

Myth: the event happened suddenly. Despite its name, the Great Oxidation Event unfolded over tens to hundreds of millions of years, with possible rises and falls in oxygen along the way. Its onset is placed at roughly 2.46 to 2.32 billion years ago depending on the method and region.

Myth: oxygen quickly reached today’s levels. After the event, atmospheric oxygen probably settled somewhere around 1 to 10 percent of the present level, although estimates range more widely. A second major rise, the Neoproterozoic Oxygenation Event, came between roughly 800 and 540 million years ago, shortly before animals diversified.

Myth: an “oxygen catastrophe” wiped out most life. The phrase “oxygen catastrophe” is popular, but there is no fossil evidence of a mass extinction of microbes at this time. Anaerobic microbes are still extremely common today in oxygen-free environments such as deep sediments, wetlands and the digestive tracts of animals.

By the numbers

Key people in the research

The idea of a dramatic rise in oxygen was developed in the 1960s and 1970s by the American geologist Preston Cloud, who linked banded iron formations, red beds and early life into a single history of the atmosphere. The geochemist Heinrich D. Holland spent decades quantifying how oxygen levels changed over geological time.

The 2000 sulphur isotope study by James Farquhar and colleagues gave the field its most precise marker. Since then, researchers including Timothy Lyons, Ariel Anbar, Donald Canfield and many others have used new isotope systems and better dating to show that the history of oxygen was more complex and uneven than once thought.

Timeline

• c. 3.5–3.4 billion years ago: Stromatolites and microbial mats are present in shallow seas; the atmosphere lacks free oxygen.
• c. 2.5 billion years ago: Possible brief “whiffs” of oxygen are recorded in shales of the Mount McRae Shale, Western Australia.
• c. 2.45–2.32 billion years ago: The mass-independent sulphur isotope signal fades and disappears; oxygen begins to accumulate.
• c. 2.45–2.22 billion years ago: The Huronian glaciations affect North America and other regions.
• c. 2.22 billion years ago: Oxygen becomes permanently established in the atmosphere, according to some recent studies.
• c. 2.22–2.06 billion years ago: The Lomagundi-Jatuli carbon isotope excursion is recorded worldwide.
• c. 1.85 billion years ago: Large banded iron formations largely cease to form.
• c. 800–540 million years ago: The Neoproterozoic Oxygenation Event raises oxygen closer to modern levels.

Why it matters

The Great Oxidation Event was one of the most important turning points in Earth’s history. It showed that living things can reshape an entire planet: microbes too small to see altered the air, the oceans, the climate and the minerals in the ground. Without the oxygen they released, the ozone layer, energy-hungry complex cells and eventually animals and humans could not have evolved.