Oxygen cycle refers to the movement of oxygen through the
atmosphere (air),
biosphere (plants and animals) and the
lithosphere (the Earth’s crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. The oxygen cycle is the
biogeochemical cycle of
oxygenatoms between different
oxidation states in
ions,
oxides, and
molecules through
redox reactions within and between the
spheres/reservoirs of the planet Earth.[1] The word oxygen in the literature typically refers to the most common
oxygen allotrope, elemental/diatomic oxygen (O2), as it is a common
product or
reactant of many biogeochemical redox reactions within the cycle.[2] Processes within the oxygen cycle are considered to be
biological or
geological and are evaluated as either a
source (O2 production) or sink (O2 consumption).[1][2]
Oxygen is one of the most common elements on Earth and represents a large portion of each main reservoir. By far the largest reservoir of Earth's oxygen is within the
silicate and
oxideminerals of the
crust and
mantle (99.5% by weight).[6] The Earth's atmosphere,
hydrosphere, and biosphere together hold less than 0.05% of the Earth's total mass of oxygen. Besides O2, additional oxygen atoms are present in various forms spread throughout the surface reservoirs in the molecules of
biomass,
H2O,
CO2,
HNO3,
NO,
NO2,
CO,
H2O2,
O3,
SO2,
H2SO4,
MgO,
CaO,
Al2O3,
SiO2, and
PO4.[7]
The
biosphere is 22% oxygen by volume, present mainly as a component of
organic molecules (CxHxNxOx) and water.
Hydrosphere
The
hydrosphere is 33% oxygen by volume[8] present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbolic acids (HxCO3).
The main source of atmospheric free oxygen is photosynthesis, which produces
sugars and free oxygen from carbon dioxide and water:
Photosynthesizing organisms include the plant life of the land areas, as well as the
phytoplankton of the oceans. The tiny marine
cyanobacteriumProchlorococcus was discovered in 1986 and accounts for up to half of the photosynthesis of the open oceans.[15][16]
Abiotic production
An additional source of atmospheric free oxygen comes from
photolysis, whereby high-energy
ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free hydrogen and nitrogen atoms escape into space, leaving O2 in the atmosphere:
Biological consumption
The main way free oxygen is lost from the atmosphere is via
respiration and
decay, mechanisms in which
animal life and
bacteria consume oxygen and release carbon dioxide.
Capacities and fluxes
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes.
These numbers are based primarily on estimates from (Walker, J. C. G.):[10] More recent research indicates that ocean life (
marine primary production) is actually responsible for more than half the total oxygen production on Earth.[17][18]
Reservoir
Capacity (kg O2)
Flux in/out (kg O2 per year)
Residence time (years)
Atmosphere
1.4×1018
3×1014
4500
Biosphere
1.6×1016
3×1014
50
Lithosphere
2.9×1020
6×1011
500000000
Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)[1]
^
abcdKnoll AH, Canfield DE, Konhauser K (2012). "7". Fundamentals of geobiology. Chichester, West Sussex: John Wiley & Sons . pp. 93–104.
ISBN978-1-118-28087-4.
OCLC793103985.
^Keeling RF, Shertz SR (August 1992). "Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle". Nature. 358 (6389): 723–727.
Bibcode:
1992Natur.358..723K.
doi:
10.1038/358723a0.
S2CID4311084.
^
abWalker JC (1980). "The Oxygen Cycle". The Natural Environment and the Biogeochemical Cycles. The Handbook of Environmental Chemistry. Springer Berlin Heidelberg. pp. 87–104.
doi:
10.1007/978-3-662-24940-6_5.
ISBN9783662229880.