Great Oxidation Event

The Great Oxidation Event (GOE) was a major change in Earth's history that happened during the Paleoproterozoic era. It was a time when the Earth's atmosphere and shallow seas began to fill with oxygen for the first time. This change started about 2.46 billion years ago and ended around 2.06 billion years ago.

Scientists believe that tiny microbial organisms began making oxygen through photosynthesis. Over many years, this oxygen built up and changed the air from one that had almost no oxygen to one that had a lot. By the end of the Great Oxidation Event, oxygen levels were about 10% of what we have today.

This event was very important because oxygen later became necessary for most life on Earth. It helped change the planet in ways that made it possible for more complex life forms to develop and thrive.

Overview

The Great Oxidation Event was a time when the amount of oxygen in Earth’s air and oceans increased a lot. This happened about 2.4 billion to 2 billion years ago. The oxygen came from tiny plants called cyanobacteria that made it as a byproduct of their work.

The new oxygen was harmful to many old kinds of tiny life that did not use oxygen. This change caused many of them to disappear. Later, some surviving tiny life changed and teamed up with other tiny cells, which helped create more complex life forms over time.

Early atmosphere

Main articles: Paleoatmosphere and Prebiotic atmosphere

See also: Paleoclimatology and Atmosphere of Earth

The Earth's earliest atmosphere was very different from the air we breathe today. It was mostly made of nitrogen (N₂) and carbon dioxide (CO₂), with only tiny amounts of oxygen. The Sun was dimmer back then, yet liquid water still existed on Earth, which scientists call the faint young Sun paradox. This might have been because there was more CO₂ or other gases like methane (CH₄) helping to keep the planet warm.

Over time, the amount of oxygen in the atmosphere increased a lot. This change is thought to be because of photosynthesis by simple water plants called cyanobacteria. These tiny organisms could turn sunlight into energy and release oxygen as a by-product. As they spread, oxygen built up in the air and oceans, making the atmosphere much like it is today.

Geological evidence

Evidence for the Great Oxidation Event comes from many types of rocks and chemicals. One important clue is found in ancient soils called paleosols. These soils, older than 2.4 billion years, have low amounts of iron, showing that there was very little oxygen at the time. Other clues come from special minerals like pyrite and siderite, which only form when there is almost no oxygen.

Another key piece of evidence is banded iron formations. These rocks have layers of chert and iron oxides. They formed when iron from deep oceans, which had no oxygen, moved to shallower waters where it met oxygen and settled on the ocean floor. These formations stopped forming after the Great Oxidation Event because the oceans had too much oxygen.

Scientists also study isotopes, which are versions of elements with different weights. One famous example is sulfur. Before the Great Oxidation Event, sulfur showed a special pattern that disappears later. This pattern suggests there was no ozone layer to block harmful sunlight, meaning very little oxygen was present. When the pattern disappeared, it shows that oxygen was building up in the atmosphere, creating an ozone layer.

Hypotheses

The Great Oxidation Event began around 2.46 billion years ago when ancestors of cyanobacteria first developed the ability to produce oxygen through photosynthesis. However, oxygen levels in the atmosphere stayed low for a long time. Scientists have several ideas about why it took so long for oxygen to build up.

One key factor is the balance between sources of oxygen and things that remove oxygen from the atmosphere. Early oxygen was used up by reactions with iron in the oceans, forming layers of iron deposits called banded iron formations. Only when enough oxygen was produced to overcome these "sinks" did oxygen levels in the atmosphere begin to rise significantly. The burial of organic carbon and other materials played an important role in allowing oxygen to accumulate, as these materials prevented oxygen from reacting away.

Over time, changes in Earth's geology, such as the formation of mountain ranges and the appearance of shelf seas, helped provide nutrients that supported more cyanobacteria. These changes, along with a decrease in certain gases that removed oxygen, eventually allowed oxygen levels to increase steadily. The process was gradual, happening over hundreds of millions of years rather than all at once.

Consequences of oxygenation

As oxygen built up in the atmosphere, it changed Earth in important ways. First, it reduced a gas called methane, which helped keep Earth warm. With less of this warmth, Earth grew colder and experienced long periods of ice, called the Huronian glaciation. Second, oxygen gave living things more energy, allowing them to grow and change in new ways. One key change was the development of tiny parts inside cells called mitochondria, which helped organisms get more energy.

The rise of oxygen also led to many new types of minerals forming near Earth's surface. Scientists think over half of the minerals we know of today appeared because of this event. Some tiny organisms, called cyanobacteria, may have started making oxygen even before it filled the air, helping prepare the way for bigger, more complex life forms.