Geoscience Reference
In-Depth Information
a day. A second component of inert biomass is made up of particulate
and dissolved organic detritus, and the organic fraction of soil and
marine sediments. Taking into account this second component, the
total inert biomass is much greater than the active biomass in the
whole biosphere, including in the ocean.
4.3.2.
Biogeochemical cycles of carbon
Inorganic and organic carbon have both short and long cycles.
Figure 4.3 illustrates the biogeochemical flows of carbon
between
different natural reservoirs (below, numbers in italics between
brackets refer to arrows in the figure). Four carbon cycles are
identified in the following text - inorganic and organic, both with
short and long cycles. Each cycle consists of a subset of the flows that
are represented in Figure 4.3.
4.3.2.1.
Short inorganic carbon cycle
The short cycle of inorganic carbon begins with the exchange of
CO
2
(along with other gases) between the atmosphere and the ocean
[
1
], whose direction depends on the partial pressures of this gas
(pCO
2
) in the two environments. In sea-surface water, dissolved CO
2
reacts with water (H
2
O) to form carbonic acid (H
2
CO
3
), which
dissociates into bicarbonate (HCO
3
-
) and one hydrogen ion (H
+
);
bicarbonate can also dissociate into carbonate (CO
3
2-
) and one hydrogen
ion:
CO
2
+ H
2
O → H
2
CO
3
→ HCO
3
-
+ H
+
→ CO
3
2-
+ 2H
+
[4.1]
Equation [4.1] describes the carbonate system. The arrows in this
equation may be reversed, in which case the reaction goes from CO
3
2-
to CO
2
, which may lead to the release of dissolved CO
2
from the
ocean into the atmosphere. The direction in which the reaction
proceeds, toward the left or the right, strongly depends on the
concentration of H
+
ions in seawater and, therefore, on its pH (which
is inversely proportional to the concentration of H
+
ions). At the
normal pH of seawater, which is approximately 8.2, the different
compounds of the carbonate systems have very different abundances:
