Geoscience Reference
In-Depth Information
C
OMMENTS ON
F
IGURE
4.3.- The inorganic carbon cycles involve:
exchanges of gas between the atmosphere and the ocean, the
weathering of continental and marine rocks by atmospheric and
dissolved CO
2
(which supplies Ca
2+
and HCO
3
-
to the ocean), the
precipitation and dissolution of CaCO
3
in the ocean, the tectonic
recycling of carbonate sedimentary rocks in volcanos and the
calcination of CaCO
3
to make cement; the last two processes release
CO
2
into the atmosphere. The organic carbon cycles involve:
photosynthesis and respiration by the living biomass, oxidation and
fermentation of the nonliving biomass in soils and marine sediment, the
sedimentation and burial of organic matter in the seabed as well as the
weathering of sedimentary rocks, the combustion of fossil organic
matter and deforestation, three processes that release CO
2
into the
atmosphere. Full arrows represent natural flows and dotted arrows
represent flows associated with human activity; the numbers next to
the arrows refer to the flow descriptions in the text.
Various physical and biological mechanisms (called “carbon
pumps”; see section 2.4.1 in Chapter 2 of [MON 14b]) transport to
deep waters some of the inorganic and organic carbon substances that
are formed in surface waters. The carbon that reaches deep waters is
sequestered there in the form of dissolved CO
2
until these waters
return to the surface [
2
] where CO
2
may be released into the
atmosphere [
1
]. The physical mechanism of carbon pumping toward
deep waters is the downward transport of seawater containing a high
concentration of dissolved atmospheric CO
2
[
19
], which is very rapid
as it requires only a few months to a few years. This phenomenon
occurs in the areas of deep-water formation described in section 2.1.2
in Chapter 2 of [MON 14b]. The return of this CO
2
toward the surface
by upwards movements of seawater is much slower, i.e. from several
centuries to more than a millennium (see section 2.1.2 in Chapter 2 of
[MON 14b]). The biological carbon pumping mechanism involves the
export of particles of biological origin that sink toward the seabed [
6
];
this mechanism is described afterwards in the context of the long
organic carbon cycle. Since the beginning of the industrial era (1750),
the natural pumping of carbon in the ocean has absorbed
approximately half the CO
2
released into the atmosphere by human
activity (this CO
2
is said to be anthropogenic).
