Environmental Engineering Reference
In-Depth Information
10
6
Pg, the planet's largest reservoir of C (
Table 6.1
;
Figure 6.1
).
The amount of fossil fuels is trivial in comparison (about 4
soils is 60 to 100
3
10
3
Pg). Shallow-water
marine sediments contain large amounts of carbonate in so-called carbonate ooze, which
originated in the plankton as the carbonate shells of phytoplankton (principally coccolitho-
phorids) and animals that make calcite or aragonite (an alternative mineral form of cal-
cium carbonate) shells. Thus, much of the inorganic C deposits were formed biologically.
6
3
INORGANIC CARBON IN THE ATMOSPHERE
The only abundant form of inorganic carbon in the atmosphere is CO
2
. The modern
atmosphere contains about 750 Pg of C as CO
2
and this quantity is on the rise (
Table 6.1
;
Figure 6.1
). Carbon dioxide comes principally from decomposition of organic matter and
the burning of organic matter, with much smaller amounts from the formation of carbon-
ate shells by corals and other marine organisms. Humans contribute to the formation
of CO
2
by combusting fossil fuels, converting forests to cleared land (which is often
accompanied by burning and accelerated decomposition;
Houghton 1995
), and by making
cement from carbonate rock (
Worrell et al. 2001
).
KEY REACTIONS FOR INORGANIC CARBON
Chemical weathering: Dissolved bicarbonates are formed principally by the dissolution of
carbonate and aluminosilicate minerals (e.g., andalusite or kyanite) by CO
2
and water.
The CO
2
involved in these reactions comes largely from biological respiration. When CO
2
dissolves in water it forms carbonic acid, which is the major acid that dissolves carbonates
(earlier).
Carbonate weathering:
Ca
11
1
2HCO
3
2
!
ð
Þ
CO
2
1
H
2
O
1
CaCO
3
6
:
4
Ca
11
1
Mg
11
1
4HCO
3
2
CaMg
ð
CO
3
Þ
2
!
ð
6
:
5
Þ
Silicate weathering:
Ca
11
1
2HCO
3
2
1
2CO
2
1
3H
2
O
CaSiO
3
!
H
4
SiO
4
ð
6
6
Þ
1
:
Mg
11
1
2HCO
3
2
1
2CO
2
1
3H
2
O
MgSiO
3
!
H
4
SiO
4
ð
6
7
Þ
1
:
REVERSE WEATHERING AND CARBONATE FORMATION
The weathering reactions generate bicarbonate from clays and limestones. Reverse
weathering creates clays and limestones (
Michalopoulos and Aller 1995; MacKenzie and
Kump 1995
). Chemically these reverse weathering reactions can be written as Eqs.
6.4
through 6.7
running in reverse.
CLAY FORMATION
Although carbonate precipitation (see below) is relatively fast (seconds) the formation
of alumino-silicate clays (reverse weathering) is very slow (millions of years). The net reac-
tion is comparable to Eqs.
6.6 and 6.7
in reverse: An HCO
3
2
ion from solution becomes
incorporated as an OH
-
in the clay and causes a CO
2
to be released. This reverse
