Who decides to breathe? The mystery of the production of oxygen in the early Earth

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

Oxygen is familiar to everyone. It can form compounds with almost all other elements on the periodic table. About 21% of the Earth's atmosphere is oxygen. So where does the oxygen in the atmosphere come from?

Perhaps even primary school students know this question - photosynthesis, which is the process by which green plants, algae and certain bacteria convert sunlight, water and carbon dioxide into glucose and oxygen.

Have you ever thought about going back billions of years to the Earth and wondering where the oxygen on Earth came from? Was the earliest oxygen also produced by photosynthesis?

Oxygen in the atmosphere

(Photo source: Veer Gallery)

The Great Oxidation Event: Earth's Breathing Source

About 4.57 billion years ago, the Sun was formed in an inconspicuous corner of the Milky Way. The remaining matter after the formation of the Sun condensed into the rest of our solar system, including, of course, the Earth, which was born about 4.55 billion years ago.

Since the formation of the Earth, there has been almost no oxygen or very little oxygen on the planet for about half of the 4.55 billion years to date. It was not until about 2.4 billion years ago that the concentration of oxygen in the atmosphere increased significantly and the Great Oxidation Event (GOE) occurred. During this period, the increase in atmospheric oxygen concentration had a profound impact on life and the environment on Earth.

Just 200-300 million years before the Great Oxidation Event, the oxygen in the Earth's atmosphere was extremely low. Some studies believe that early microorganisms such as cyanobacteria converted solar energy into chemical energy through photosynthesis, while releasing oxygen into the water as a byproduct. Over time, this oxygen gradually accumulated and eventually entered the atmosphere.

Summary of geochemical data and geological records from 2.5 billion years ago to 2 billion years ago

The brown area represents the traditional “Great Oxidation Event” (Image source: Reference [1])

After oxygen entered the atmosphere, it had a profound impact on the biological and geological environment at that time. Oxygen was toxic to many primitive anaerobic organisms, leading to the extinction of a large number of organisms . At the same time, oxygen reacted with compounds in the atmosphere and oceans to form new geological structures , such as hematite layers.

Although oxygen caused the extinction of many organisms, it also paved the way for new, more complex life forms. With oxygen, some organisms evolved new ways to use oxygen to survive, resulting in more complex ecosystems.

The Great Oxidation Event not only changed the biological and geological environment of the Earth, but also provided scientists with important clues to explore the early history of the Earth and the origin of life.

The diversity of species on Earth (Image source: Veer Gallery)

One of the reasons for the increase in oxygen: It turns out to be nickel!

Of course, there are many other scientific studies that have also provided clues to the Great Oxygenation Event. As early as 2009, a paper published in the journal Nature speculated on one reason for the increase in oxygen concentration in the atmosphere.

The researchers analyzed trace elements in sedimentary rocks at dozens of sites and found that the nickel content in the primitive ocean was 400 times that of the current water . There were many microorganisms that could produce methane gas in the ocean of the early Earth. These methane-producing microorganisms like nickel-rich water to grow and reproduce, and will release a large amount of methane gas into the atmosphere. It is speculated that methane gas prevents oxygen accumulation .

Methane molecular structure (Image source: Chinese Academy of Sciences)

After testing the rocks, scientists found that about 2.4 billion years ago, the cooling and solidification of the mantle may have caused the gradual precipitation of dissolved nickel in the ocean. As the nickel in the ocean decreased, methanogens that depend on nickel could not survive, leaving room for algae and other life forms that release oxygen during photosynthesis . This provides another credible explanation for the Great Oxidation Event.

Volcanic activity: another guess about Earth's oxygen

An article published in the Proceedings of the National Academy of Sciences in 2021 mainly discussed the early stages of the appearance of oxygen in the Earth's atmosphere, especially the "small" increase in oxygen that occurred before the Great Oxidation Event . The researchers speculated that this early increase in oxygen may have been triggered by volcanic activity .

Volcanic activity (Image source: Veer Gallery)

Analyzing drill cores from Western Australia, the researchers found evidence of mercury enrichment and oxidative weathering associated with volcanism , supporting the hypothesis that volcanism played a key role in this early oxygen increase.

Volcanic activity produces nutrient-rich lava and ash that weather and release into rivers and other water sources. For example, the weathering of large basaltic crust releases phosphorus-based nutrients that promote the growth of cyanobacteria and other single-celled organisms, which in turn produce more oxygen. Researchers speculate that volcanic activity can also provide a long-term pathway for the consumption of atmospheric oxygen through reactions with volcanic gases.

Cyanobacteria (blue algae) (Image source: Veer Gallery)

The fact that the increase in oxygen is due to an increase in oxygen production , rather than a decrease in oxygen consumption by rocks or other abiotic processes, has important implications for understanding the evolution of complex life.

Sulfur dioxide photolysis: a new pathway for abiotic oxygen production

So before cyanobacteria were produced and began to release oxygen, was there no oxygen on Earth? If there was any, where did the oxygen come from?

Recently, scientists from the Chinese Academy of Sciences published a paper in the journal Chemical Science, discovering that photolysis of sulfur dioxide molecules can produce sulfur atoms and oxygen , providing a new source of oxygen in the early Earth's atmosphere.

Scientists used the Dalian Light Source, the world's brightest and fully tunable extreme ultraviolet free electron laser light source , to adjust its wavelength range between 120 and 160 nanometers, directly dissociating sulfur dioxide into sulfur atoms and oxygen.

Dalian Light Source Device (Image source: Chinese Academy of Sciences)

Since photons carry energy, the shorter the wavelength, the higher the energy of the photons. In the experiment, they also found that when the wavelength is 121.6 nanometers, the yield can reach about 30%. This process is a newly discovered non-biological oxygen production pathway after the photochemical oxygen production of carbon dioxide and water molecules .

Schematic diagram of sulfur dioxide photolysis (Image source: Chinese Academy of Sciences)

Now the question arises again, why did scientists choose 121.6 nanometers?

This is because the wavelength of the Lyman-alpha line, 121.6 nanometers, is a specific wavelength in the spectrum of hydrogen atoms. Hydrogen is the most abundant element in the universe , and Lyman-alpha radiation is very common in many astronomical phenomena and stellar activities. In stars, especially young, active and hot stars, vacuum ultraviolet radiation is an important energy output channel.

Among the vacuum ultraviolet radiation of these stars, the Lyman-α line is usually the strongest and most abundant spectral line . This characteristic makes Lyman-α a useful tool for studying many fields such as stellar activity, interstellar medium, and planetary atmospheres.

Volcanic eruptions in the late Archean period released sulfur dioxide gas, which subsequently entered the atmosphere in large quantities. Researchers speculate that sulfur dioxide photolysis under light produced oxygen. This means that vacuum ultraviolet light photolysis of sulfur dioxide may provide a new and important source of oxygen in the Earth's primitive atmosphere.

Conclusion

The origin and evolution of the earth and its materials is a complex and wonderful process. Although oxygen accompanies us every minute of our daily lives, its origin and production are still a mystery to be solved. The material world is complex and it is precisely these continuous exploration processes that allow humans to appreciate the true charm of nature.

References:

[1]Poulton, S., Bekker, A., Cumming, V., Zerkle, A., Canfield, D., & Johnston, D. (2021). A 200-million-year delay in permanent atmospheric oxygenation. Nature, 592(7853), 232-236.

[2]Farquhar, James, Huiming Bao, and Mark Thiemens. "Atmospheric influence of Earth's earliest sulfur cycle." Science 289.5480 (2000): 756-758.

[3]Konhauser, Kurt O., et al. "Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event." Nature 458.7239 (2009): 750-753.

[4]Meixnerová, Jana, et al. "Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean “whiff” of oxygen." Proceedings of the National Academy of Sciences 118.33 (2021): e2107511118.

[5]Lu, Zhou, et al. "Evidence for direct molecular oxygen production in CO2 photodissociation." Science 346.6205 (2014): 61-64.

[6]Chang, Yao, et al. "Vacuum ultraviolet photodissociation of sulfur dioxide and its implications for oxygen production in the early Earth's atmosphere." Chemical Science (2023).