The oxygen cycle is a biogeochemical transition of atoms of oxygen between various oxidation states in ions, molecules and oxides through redox reactions in and between the reservoirs of the earth. The word oxygen is used to refer to the most common oxygen allotrope, diatomic oxygen (O2). This is because it is a common product or reactant of many biogeochemical redox reactions in the cycle. Processes that are in the oxygen cycle are considered to be geological or biological and they are evaluated as either a source (O2 production) or sink (O2 consumption).
RESERVOIRS
Oxygen is among the most abundant elements on the earth and it represents a large portion of every major reservoir. By far the largest earth’s oxygen reservoir is in the silicate and oxide minerals of the mantle and crust. The atmosphere, biosphere and hydrosphere of the earth together hold less than 0.05% of the total mass of oxygen of the earth. Apart from O2, other oxygen atoms are present in different forms spread all over the surface reservoirs in the molecules of biomass, they include: H2O, CO2, CO, H2O2, NO, NO2, H2SO4, MgO, CaO, PO4 and SiO2.
ATMOSPHERE
The atmosphere is made up of 20.9% oxygen by volume. Other molecules that contain oxygen in the atmosphere include carbon dioxide, water vapor, sulfur and nitrogen oxides, and ozone.
BIOSPHERE
The biosphere is made up of 22% oxygen by volume and it is present mostly as a component of organic molecules (CX HX NX OX) and water molecules.
HYDROSPHERE
The hydrosphere is made up of 33% oxygen by volume. It is mostly present as a component of water molecules with dissolved molecules including free oxygen and carbonic acids (HX CO3).
LITHOSPHERE
The lithosphere is made up of 46.6% oxygen by volume. It is mostly present as silica minerals (SiO2) and other oxide minerals.
SOURCES AND SINKS
While there exist many abiotic sources and sinks for oxygen, the presence of the profuse free oxygen concentration in the modern atmosphere of the earth and ocean is attributed to oxygen production from the biological processes of oxygenic photosynthesis together with a biological sink called the biological pump as well as the geologic process of carbon burial that involves plate tectonics.
BIOLOGICAL PRODUCTION
The major source of atmospheric free oxygen is photosynthesis. It produces free oxygen and sugars from carbon dioxide and water.
6 CO2 + 6H2O + energy→ C6H12O6 + 6O2
Photosynthesizing organisms include the plant life on land and the phytoplankton of the oceans.
ABIOTIC PRODUCTION
An additional source of atmospheric free oxygen comes from photolysis. High energy ultra-violet radiation breaks down atmospheric water as well as nitrous oxide onto component atoms.
2 H2O + energy→ 4H + O2
2 N2O + energy→ 4N + O2
BIOLOGICAL CONSUMPTION
The main way through which oxygen is lost from the atmosphere is through decay and respiration mechanisms where animal life, as well as bacteria, consume oxygen releasing carbon dioxide.
ABIOTIC CONSUMPTION
The lithosphere also consumes atmospheric free oxygen by chemical weathering as well as surface reactions.
