Geoscience Reference
In-Depth Information
partial pressure; for example, the partial pressure of O
2
in the
atmosphere (pO
2
) is 0.209 atm, which means that O
2
accounts for
20.9% of the total atmospheric pressure at sea level. In surface waters
of the ocean, the theoretical concentration of each dissolved gas
(called “solubility”) is equal to the product of its partial pressure in the
atmosphere and its solubility coefficient. The latter is different for
each gas, and decreases as seawater temperature increases. At 25°C,
for example, the solubility coefficient of the four main atmospheric
gases is 507 (N
2
), 1,019 (O
2
), 1,117 (Ar) and 28,400 (CO
2
) µmol kg
-1
atm
-1
(number of gas molecules dissolved per kilogram of seawater
and per unit of atmospheric pressure). It follows that CO
2
is highly
soluble in seawater. Hence even if CO
2
only accounts for 0.03% of the
volume of the atmosphere, its solubility in seawater (0.0003
×
28,400
= 9.6 µmol kg
-1
) is as high as that of Ar (0.009
1,117 = 10.1
µmol kg
-1
), whereas this gas is 30 times more abundant than CO
2
in
the atmosphere (0.9% compared to 0.03%).
×
We must note that the values in the previous paragraph correspond
to the concentrations of chemical elements in seawater in the form of
dissolved gases. However, these same chemical elements may also be
present in the ocean in other forms, for example hydrogen and oxygen
which are extremely abundant in the ocean in the form of water, H
2
O.
Thus, the total mass of oxygen in seawater (including both dissolved
O
2
and H
2
O) accounts for 86% of the total mass of this reservoir
(Table 4.2), whereas the mass of nitrogen in seawater is a million
times smaller.
4.2. A fundamental characteristic of the Earth's system:
biogeochemical cycles
The concept of the cycle occurs frequently in science. For example,
the phases through which the cells of a living organism pass between
two successive divisions is called the cell cycle (Figure 4.2). In the
domain of biogeosciences, a biogeochemical cycle refers to the
transport and cyclic transformation of a chemical element (e.g.
oxygen) or a compound (e.g. CO
2
) between the Earth's natural
reservoirs cited in section 4.1.1 (i.e. atmosphere, biosphere, geosphere
and hydrosphere), as influenced by the activity of living organisms
