Oxygen Cycle — Definition & Diagrams

What's the Oxygen Cycle?

The oxygen cycle describes the various forms in which oxygen is found and how it moves through different reservoirs on Earth. It is one of the biogeochemical cycles.

There are three major reservoirs of oxygen: the atmosphere, biosphere, and lithosphere. Some people also consider the hydrosphere, a subdivision of the biosphere, to be the fourth reservoir.

The Reservoirs and What We Find in Them

The biosphere includes all living and nonliving components on Earth. In the biosphere, oxygen is mostly found in organic molecules, liquid water, and molecules dissolved in water, such as free oxygen and carbonic acids.

In the atmosphere, the oxygen is mostly found as free $O_2$ molecules released via photosynthesis. It also exists as ozone ($O_3$), carbon dioxide ($CO_2$), water vapor ($H_2$O), and sulfur or nitrogen oxides.

The lithosphere includes the Earth's crust and mantle. It's the largest oxygen reservoir. Large amounts of oxygen are stored in silica ($SiO_2$) and other oxide minerals.

Releasing Oxygen

In the biosphere, green plants and algae perform photosynthesis, which releases free oxygen molecules. Organisms in the biosphere can also remove oxygen from the atmosphere through respiration or decay.

In the atmosphere, a process called photolysis plays an important role. In this reaction, high-energy UV radiation from the sun breaks down atmospheric water and nitrous oxide, releasing free oxygen molecules into the atmosphere.

The lithosphere can take in oxygen from the atmosphere through chemical weathering and other surface reactions, such as the formation of iron oxides (rust). Some types of chemical weathering can also release oxygen back into the atmosphere when the oxygen-containing minerals in rock begin to break down.

Oxygen can also cycle between the biosphere and lithosphere. Marine organisms in the biosphere create calcium carbonate shells which are full of oxygen. When the organism dies, the shell settles at the ocean floor. It eventually forms limestone sedimentary rock, becoming a part of the lithosphere. Likewise, oxygen can be cycled from the lithosphere back into the biosphere when organisms use minerals found in rock and then release oxygen from it.

Oxygen moves into the atmosphere via the processes of photosynthesis and photolysis. Oxygen moves into the biosphere via respiration, decay, and mineral extraction from rock. It moves into the lithosphere via surface reactions, chemical weathering, and calcium carbonate formation, which leads to sedimentary rock formation.

(Video) Oxygen Cycle

by TutorVista

In this video, TutorVista explains how oxygen moves through the biosphere, atmosphere, hydrosphere, and lithosphere.

(Video) Every Breath You Take is Connected

by It's Okay To Be Smart

This video by It's Okay To Be Smart shows how oxygen and carbon travel all throughout living organisms (like plants and animals) and the environment.