Geoscience Reference
In-Depth Information
ocean, where it is both the main component of water molecules and a
major dissolved gas (Table 4.2). An original characteristic of our
planet is the presence of free elemental oxygen in the atmosphere and
hydrosphere, i.e. molecular O
2
. In fact, oxygen does not usually exist
as a free chemical element in the Universe because it is extremely
reactive and easily combines with other elements to form oxides
(water is an oxide of hydrogen). One exception to this very general
rule is planet Earth, where O
2
is constantly resupplied by oxygenic
photosynthesis, of which the O
2
molecule is a byproduct (equation
[4.11]). The presence of O
2
in the Earth's atmosphere depends on the
imbalance that exists between photosynthesis and aerobic respiration
on the global scale, which is controlled by the deposition of organic
materials in marine sediments and soils where they cannot be
oxidized. This stock of organic matter largely determines the quantity of
O
2
in the atmosphere, as it is made up of organic matter coming from
reaction [4.11] which was not destroyed by reaction [4.13].
Table 4.3 shows that most of the oxygen in the Earth's
outer envelopes is found in lithospheric minerals (10
10
Pg of oxides
in the crust and mantle) and in the ocean (10
9
Pg). In the latter,
oxygen is largely present in the form of water molecules (H
2
O; > 97%
of water on the planet is found in oceans) and, secondly, in the
form of dissolved gas (O
2
). The atmosphere only contains a small
fraction of the oxygen on Earth (10
6
Pg) and the biosphere, even less
(<10
4
Pg).
In section 4.3.2 above, it was explained that oxygen (O
2
) produced
during photosynthesis comes from the water molecule (H
2
O) (equation
[4.11]), and that the oxygen atom in the water molecule produced
during respiration comes from O
2
(equation [4.13]). It follows that H
2
O
forms an integral part of the oxygen cycle. Yet, curiously enough,
several geochemistry manuals do not mention H
2
O in their chapter on
oxygen, forgetting that the ocean contains a large part of the Earth's
oxygen in the form of H
2
O. In these manuals, the discussion of oxygen
in the ocean is limited to just the gaseous O
2
dissolved in seawater. The
oxygen cycle in Figure 4.4, below, takes into account H
2
O, as it should
be.
