Great Oxidation Event facts for kids
The Great Oxidation Event (GOE) was a very important time in Earth's early history. It's also called the Oxygen Revolution. During this period, the air and shallow parts of the ocean on Earth first started to get a lot more oxygen. This big change began about 2.46 billion years ago and ended around 2.06 billion years ago.
Before the GOE, Earth's air had almost no oxygen. It was a "reducing" atmosphere. But thanks to tiny living things, oxygen (O2) started to build up. This changed the air into an "oxidizing" atmosphere, meaning it had lots of free oxygen. By the end of the GOE, oxygen levels were about 10% of what they are today.
This sudden increase in oxygen was a huge deal. Oxygen was actually poisonous to most of the living things back then, which were mostly tiny organisms that didn't need oxygen to survive. Many of these organisms likely died out because of the new oxygen-rich environment. Even though it was a big event where many living things disappeared, it's usually not listed with other "mass extinctions" because it happened so long ago and involved mostly microscopic life.
Scientists believe the GOE happened because of tiny organisms called cyanobacteria. These bacteria learned how to do photosynthesis, which is the process plants use to make food from sunlight. A byproduct of their photosynthesis was oxygen! As more and more oxygen was produced, it filled up all the "sinks" (places that absorbed oxygen) like certain chemicals in the oceans and gases in the air. This led to a global ice age called the Huronian glaciation. The organisms that survived learned to live with oxygen, and some even teamed up with other tiny organisms in a way that eventually led to the development of more complex life forms, including us!
Earth's Early Air
The exact makeup of Earth's very first air isn't perfectly known. However, scientists think it was mostly nitrogen (N2) and carbon dioxide (CO2). These gases are still released by volcanoes today. Oxygen (O2) was barely there, only about 0.001% of what it is now.
Even though the Sun was only about 70% as bright 4 billion years ago, there's strong evidence that liquid water was on Earth. This is known as the faint young Sun paradox. To keep Earth warm enough for liquid water, there must have been a lot more greenhouse gases, like carbon dioxide or methane (CH4). Methane is a powerful greenhouse gas and was likely made by early life forms called methanogens.
An atmosphere of nitrogen and carbon dioxide with small amounts of water, methane, and hydrogen is called a "weakly reducing atmosphere." It has almost no oxygen. Our modern air has lots of oxygen, making it an "oxidizing atmosphere." The rise of oxygen is credited to cyanobacteria, which are thought to have appeared as early as 3.5 billion years ago.
Scientists like Preston Cloud and Heinrich Holland helped us understand when and how Earth's air changed. They noticed that very old rocks (older than 2 billion years) contained minerals like pyrite and uraninite. These minerals quickly rust (oxidize) in air with oxygen, so finding them in old sediments meant the air back then had very little oxygen. They also saw that "red beds"—red-colored sandstones that get their color from rusted iron—started appearing around the same time. This showed that oxygen was becoming more common.
Figuring out the exact start of oxygen in the air has been tricky for geologists. Different studies have suggested times ranging from 2.7 billion years ago to 2.225 billion years ago. This is because ancient rock records are incomplete, and it's hard to date them perfectly.
Clues from Rocks
Scientists find many clues in rocks that tell us about the Great Oxidation Event.
Clues on Continents
- Ancient Soils (Paleosols): Fossil soils older than 2.4 billion years have low iron, suggesting they formed when there wasn't much oxygen in the air.
- Ancient Grains (Detrital Grains): Rocks older than 2.4 billion years contain tiny bits of minerals like pyrite and uraninite. These minerals only survive when oxygen levels are low. So, finding them in old river and delta sediments tells us the air had very little oxygen.
- Red Beds: These are red sandstones colored by rusted iron. They started appearing when there was enough oxygen to rust the iron. This is a big change from older sandstones, which are often beige, white, or gray.
Banded Iron Formations (BIF)
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Banded iron formations are rocks made of thin, alternating layers of chert (a type of silica) and iron oxides (like rust). Most of these rocks are older than 1.85 billion years, with many forming around 2.5 billion years ago.
These formations needed two things to form: 1. A deep ocean with no oxygen, so iron could dissolve and be carried in the water. 2. A shallow ocean with some oxygen, where the dissolved iron would rust and settle to the bottom. The disappearance of these banded iron formations around 1.85 billion years ago suggests that the deep ocean finally got oxygen.
Iron Clues
Scientists also study how iron is found in black shales (dark rocks rich in organic matter). While black shales might suggest low oxygen, scientists look for more specific chemical clues. For example, they check the "degree of pyritization" (DOP), which measures how much iron is in the form of pyrite. A high DOP suggests very low oxygen conditions.
Evidence shows that even after the Great Oxidation Event, the deep ocean stayed low in oxygen until about 580 million years ago.
Isotope Clues
Some of the strongest evidence for the GOE comes from studying different types of sulfur atoms, called isotopes. Before about 2.4 to 2.3 billion years ago, sulfur isotopes show a special pattern called "mass-independent fractionation" (MIF). This pattern means that ultraviolet (UV) light from the Sun was reaching deep into Earth's air. If there had been much oxygen, it would have formed an ozone layer that would block UV light. The disappearance of this MIF pattern for sulfur tells us that an ozone layer formed, meaning oxygen was building up in the air.
Other elements like chromium and molybdenum also give clues through their isotopes, helping scientists piece together the story of oxygen's rise.
Fossils and Chemical Clues
Scientists look for signs of ancient life. While some structures found in very old rocks (like stromatolites) were thought to be from cyanobacteria, it's now believed that some might have formed differently or by other types of microbes.
Chemical fossils, called biomarkers, are also studied. These are traces of chemicals left behind by ancient organisms. For example, some biomarkers were once thought to come from cyanobacteria and other complex life forms in very old rocks. However, these samples were later found to be contaminated, so they don't provide clear evidence for oxygen-producing life that early.
Why Did It Happen?
The ability to make oxygen through photosynthesis likely first appeared in the ancestors of cyanobacteria. These organisms probably evolved as early as 2.7 billion years ago. But oxygen didn't become common in the air until much later, around 2.0 billion years ago. So, why the long delay?
Scientists believe the delay happened because Earth had many "oxygen sinks"—places that absorbed oxygen as soon as it was made. These sinks included:
- Reduced gases and minerals from volcanoes.
- Dissolved iron in the oceans.
- Organic carbon that was buried without being oxidized.
When oxygen was produced, it would react with these sinks. For example, oxygen reacted with dissolved iron in the oceans, turning it into rust that sank to the bottom, forming those banded iron formations. It took millions of years for these sinks to be filled up. Once they were full, oxygen could finally start building up in the air.
Stages of Oxygenation
Scientists now think the GOE wasn't one sudden event but a long process over hundreds of millions of years. The amount of oxygen in the air likely went up and down depending on how much oxygen was being made and how many sinks were still available.
Nutrient Shortages
One idea is that early cyanobacteria didn't have enough vital nutrients, which slowed their growth. For example, a "nickel famine" might have played a role. Early methane-producing organisms needed nickel. As Earth's crust cooled, the supply of volcanic nickel decreased. This might have given oxygen-producing cyanobacteria an advantage, allowing oxygen levels to slowly increase.
Tectonic Changes
Another idea is that the GOE needed big changes in Earth's geology, like the formation of shelf seas. These new areas allowed reduced organic carbon to be buried in sediments. When carbon is buried without being oxidized, the oxygen it would have used is left in the atmosphere. The movement of Earth's plates (tectonics) and the formation of mountains could have also played a role by releasing nutrients into the ocean, which fed the cyanobacteria.
Two Stable States
Some scientists suggest that Earth's atmosphere can exist in two stable states: one with very low oxygen and one with high oxygen (like today). If something pushes oxygen levels past a certain point, an ozone layer forms. This ozone layer then blocks UV rays, which reduces how much methane is oxidized. This allows oxygen to build up even more, leading to the high-oxygen state. The GOE would then be seen as a switch from the low-oxygen state to the high-oxygen state.
Longer Days
Cyanobacteria use oxygen at night that they produce during the day. However, studies show that with longer periods of daylight, cyanobacteria produce more extra oxygen. Earth's rotation has slowed over billions of years, meaning days have gotten longer. About 2.4 billion years ago, a day was about 21 hours long, compared to 6 hours when Earth first formed. Longer days meant more time for oxygen to spread into the water and eventually into the air.
What Happened Next?
Once oxygen started to build up in the air, two major things happened:
- Global Cooling: Oxygen likely reacted with methane (a strong greenhouse gas) in the air, turning it into carbon dioxide (a weaker greenhouse gas) and water. This made Earth's atmosphere less able to trap heat, causing the planet to cool down. This cooling is thought to have triggered a series of ice ages called the Huronian glaciation, which lasted from about 2.45 to 2.22 billion years ago.
- New Opportunities for Life: The presence of oxygen opened up huge new possibilities for life. Oxygen provides much more energy for living things. For example, tiny parts of cells called mitochondria (which help cells use oxygen to make energy) evolved after the GOE. This extra energy allowed organisms to become more complex and form new kinds of ecosystems, even though truly complex life didn't appear until much later.
More Minerals!
The Great Oxidation Event also caused a huge increase in the types of minerals found on Earth. Many elements could now exist in oxidized forms near the surface. It's thought that the GOE was directly responsible for more than 2,500 of the roughly 4,500 minerals we find today! Most of these new minerals formed as hydrated (containing water) and oxidized (containing oxygen) forms.
How Eukaryotes May Have Evolved
Some scientists think that the rise of oxygen in small areas, caused by cyanobacteria, was very harmful to other tiny organisms. This pressure might have pushed some ancient organisms (called archaea) to change and become the first eukaryotes. Eukaryotes are cells that have a nucleus and other complex parts, like our own cells. The harmful effects of oxygen might have even helped lead to the evolution of sexual reproduction, which helps repair damage to DNA.
The Lomagundi-Jatuli Event
The rise in oxygen wasn't a smooth, steady climb. There was a big increase around 2.3 billion years ago, called the Lomagundi-Jatuli event, where oxygen levels might have reached levels similar to today! But then, oxygen levels dropped again around 2.1 billion years ago. This drop caused the formation of "black shales," which are rocks rich in organic matter that would normally be burned away by oxygen. Evidence for this event has been found all over the world. Some scientists believe that eukaryotes might have first evolved during this Lomagundi-Jatuli event.
See also
In Spanish: Gran Oxidación para niños
- Boring Billion – A long period in Earth's history with little change
- Geological history of oxygen – How oxygen developed in Earth's oceans and air
- Purple Earth hypothesis – An idea about early photosynthesis
- Rare Earth hypothesis – An idea about how rare complex life might be in the universe
- Stromatolite – Layered rock structures made by ancient microbes
