Geoscience Reference
In-Depth Information
0.5% of DIC is in the form of dissolved CO
2
and H
2
CO
3
(there is
much less H
2
CO
3
than dissolved CO
2
), 89% is in the form of HCO
3
-
and 10.5% is in the form of CO
3
2-
. The sum of all chemical species of
DIC is called ∑CO
2
or total CO
2
:
∑CO
2
= dissolved CO
2
+ H
2
CO
3
+ HCO
3
-
+ CO
3
2-
[4.2]
We explained above (section 4.1.2) that the solubility of
atmospheric CO
2
in seawater is much greater than that of other gases
such as O
2
. Due to this remarkable solubility, a large quantity of
atmospheric CO
2
is expected to dissolve in sea-surface water, and the
amount of CO
2
that actually dissolves in seawater depends on the
strength of the pCO
2
gradient between the two reservoirs. According
to conditions, the pCO
2
gradient may be in opposite directions and, in
areas where pCO
2(ocean)
> pCO
2(atmosphere)
, CO
2
is released from the ocean
into the atmosphere. Also, due to the reactivity of CO
2
with H
2
O, most
of the CO
2
in seawater is transformed, at the normal pH of seawater,
into HCO
3
-
and CO
3
2-
(equation [4.1]) which stores ∑CO
2
in the ocean.
As a consequence of the high solubility of CO
2
in seawater and its
high reactivity with H
2
O, the quantity of ∑CO
2
in the ocean is
approximately 50 times greater than that of CO
2
in the atmosphere.
ATMOSPHERE
Carbon cycle
CO
2
CO
2
CO
2
11
CO
2
10
14
12 13
18
CO
2
CONTINENTAL
BIOSPHERE
CO
2
1, 12
HCO
3
-
4
CH
4
CO
2
3
1
1
1
1
EARTH
2HCO
3
-
+ Ca
2+
Volcano
16
HYDROSPHERE
(Ocean)
Ca
2+
H
2
O
CH
4
Fossil organic
matter
Rocks (CaCO
3
)
12
15
19
MARINE
BIOSPHERE
13
CO
2
9
13
CO
2
8
6
5
Ca
2+
HCO
3
-
8
SEDIMENT
7
17
CaCO
3
Fossil organic matter
Continental crust
LITHOSPHERE
Oceanic crust
Figure 4.3.
The carbon cycle involves the atmosphere, the biosphere
(marine and terrestrial), the hydrosphere (ocean) and the lithosphere
(sedimentary rocks and fossil organic matter)
