Geoscience Reference
In-Depth Information
that the water layer most threatened by ocean deoxygenation would be
that of intermediate waters, which have been little studied until now.
When the concentration of O
2
is low, some anaerobic bacteria use
the oxygen contained in nitrate (NO
3
-
) [
14
], which transforms this
inorganic nitrogenous nutrient into a dissolved gas (N
2
) by a process
called nitrate respiration or denitrification (section 4.5.2). When the
concentration of O
2
is null, other bacteria, which are strictly anaerobic,
use the oxygen contained in sulfate (SO
4
2
-) [
14
], a process called
sulfate reduction
(or
respiration),
which produces hydrogen sulfide
(H
2
S), a toxic gas with the characteristic foul odor of rotten eggs. In
the sea, the production of H
2
S primarily takes place in sediments or in
deep anoxic zones (i.e. zones without oxygen). In the Black Sea,
which is an exceptional case where density stratification of the water
column prevents periodic replenishment of O
2
at depth (see section
2.1.2 in Chapter 2 of [MON 14b], most waters below 100-200 m have
no O
2
and high H
2
S content. Most of the H
2
S currently present in the
atmosphere is a by-product of the refining and combustion of fossil
organic matter by human activities.
Another cycle that involves oxygen is the production and
destruction of ozone (O
3
) in the upper atmosphere, i.e. 20 to 30 km
above the ground [
15
]. There, the ultraviolet radiation from the sun
splits some O
2
molecules into individual oxygen atoms (O), which
react with O
2
to give O
3
:
O
2
+ O
→
O
3
[4.15]
The ozone molecule is destroyed when it combines with an oxygen
atom, producing two molecules of O
2
:
O
3
+ O
→
2 O
2
[4.16]
In the atmosphere, the gases O
2
and N
2
absorb the ultraviolet
radiation (UV) from the Sun of short wavelengths (<200 nm). In the
upper atmosphere, the presence of the ozone layer extends the UV
absorption to longer wavelengths, including UV-C (UV radiation
whose wavelengths are between 200 and 280 nm) and most UV-B
(UV radiation whose wavelengths are between 280 and 315 nm). The
