Oxygen Development

As the Earth slowly cooled below $100^\circ C$, a major change occurred. The water vapour in the atmosphere condensed to form the first oceans. This triggered the initial decrease in atmospheric $CO_2$, as the gas began dissolving into the newly formed water.

Once dissolved, the carbon dioxide reacted with metal ions in the water through a process called precipitation. This formed insoluble carbonate compounds that settled at the bottom of the oceans as sedimentary rock, permanently removing a large portion of $CO_2$ from the atmosphere.

The Arrival of Life and Photosynthesis

The greatest environmental change in our planet's history was started by microscopic life forms in the ocean. Approximately 2.7 billion years ago, algae first evolved. These organisms, including cyanobacteria, were the first to perform photosynthesis.

Photosynthesis is an endothermic reaction that uses light energy to convert carbon dioxide and water into glucose and oxygen. This process is represented by the following balanced symbol equation:

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Over the next billion years, green plants evolved from these early algae. As plants spread across the oceans and eventually colonised land around 500 million years ago, the rate of oxygen production rapidly accelerated.

The increase in oxygen was directly linked to the decrease in carbon dioxide, as the carbon was fixed into organic matter. Living plants acted as a carbon sink, absorbing $CO_2$.

When these organisms died, the carbon was further "locked up" in fossil fuels (like coal and oil) and in the shells of marine organisms. Eventually, oxygen levels reached a high enough threshold to allow for the evolution of animals, which rely on aerobic respiration to survive.

Clearing the Toxic Gases

Oxygen did not just build up in the air; it acted as a chemical cleaner for the early atmosphere. As photosynthesis produced more oxygen, this new reactive gas began to clear out the trace amounts of methane and ammonia left over from early volcanoes.

Methane reacted with oxygen to produce carbon dioxide and water: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Similarly, ammonia reacted with oxygen to produce nitrogen gas and water: $4NH_3 + 3O_2 \rightarrow 2N_2 + 6H_2O$

In addition to this chemical reaction, some models suggest that specialised bacteria in the soil and oceans also helped to remove ammonia by converting it into nitrogen gas.

Nitrogen is a very stable and unreactive gas. Because it does not easily react with other substances, it steadily built up in the atmosphere over billions of years as other gases were removed, eventually becoming the most abundant gas.

Geological Evidence for Atmospheric Change

Rocks hold the fossilised "rust" of the Earth's first breaths of oxygen. When algae first started producing oxygen, the gas did not immediately accumulate in the atmosphere.

Instead, the free oxygen first reacted with metals in the Earth's crust, primarily iron, to form solid metal oxides. This created ancient sedimentary rocks containing distinctive layers of iron oxides, known as banded iron formations.

Only after these exposed minerals in the crust and oceans were completely saturated with oxygen did the gas finally begin to build up in the atmosphere.

The Modern Atmosphere

The air you are breathing right now has barely changed since the time of the first dinosaurs. The atmosphere reached its current, stable state approximately 200 million years ago.

The modern atmosphere is composed of:

• Nitrogen ($N_2$): ~78% (approximately four-fifths)
• Oxygen ($O_2$): ~21% (approximately one-fifth)
• Carbon dioxide ($CO_2$): ~0.04%