Geoscience Reference
In-Depth Information
15
ATMOSPHERE
O
2
O
3
Oxygen cycle
O
2
11
O
2
O
2
10
O
2
H
2
O
N
2
O
2
H
2
O
1
O
2
3
1
9
CONTINENTAL
BIOSPHERE
O
2
O
2
5
1
1, 12
EARTH
Peat
Volcano
6
HYDROSPHERE
(Ocean)
Ca
2+
H
2
O
13
4
1
Fossil organic
matter
Rocks
6
1
MARINE
BIOSPHERE
O
2
3,4
NO
3
-
SO
4
2-
14
4
O
2
8
6
8
5
SEDIMENT
7
Fossil organic matter
Rocks
Continental crust
LITHOSPHERE
Oceanic crust
Figure 4.4.
In the oxygen cycle, O
2
circulates constantly between
the atmosphere, biosphere and lithosphere (the Earth's crust and mantle)
C
OMMENTS ON
F
IGURE
4.4.- The main driver of the cycle is the
production of O
2
by photosynthesis and its use in the respiration,
decomposition and combustion (oxidation) of organic matter. Another
cycle takes place in the upper atmosphere, where ultraviolet radiation
from the Sun constantly transforms molecules of O
2
into ozone (O
3
) and
molecules of O
3
into O
2
. Full arrows represent natural flows
and dotted arrow represent flows associated with human activity;
the numbers next to the arrows refer to the flow descriptions in the text.
The biosphere produces, through photosynthesis, almost all of the O
2
in the atmosphere by splitting the H
2
O molecule [
1
]. During that
process (equation [4.11]), photosynthetic organisms separate the
hydrogen and oxygen atoms contained in the water molecule (H
2
O);
they use the hydrogen atoms to produce organic matter and release
oxygen atoms in the form of O
2
. At least half of this O
2
is produced in
the ocean. In addition to photosynthesis, a small amount of O
2
is
produced by the photolysis (i.e. decomposition) of water molecules
(H
2
O) and nitrogen oxide (N
2
O) in the atmosphere by the ultraviolet
radiation from the Sun [
2
]. The amount of O
2
produced by photolysis
is less than 0.01% the amount of O
2
produced by photosynthesis. The
biosphere is not only the main producer of O
2
, but it is also its main
