Environmental Engineering Reference
In-Depth Information
INORGANIC CARBON
The amount of inorganic C far exceeds the amount of organic C on Earth ( Table 6.1 ;
Figure 6.1 ) and also participates in a number of chemical transformations. The major mole-
cules in the inorganic part of the carbon cycle are CO 2 , a gas that readily dissolves
in water; HCO 3 (bicarbonate), which exists only as a dissolved form; and CO 3 5 (carbon-
ate), which can be dissolved or solid and forms major mineral deposits in the ocean and in
uplifted marine sediments on land (limestones and dolomites).
INORGANIC CARBON IN WATERS
Dissolved inorganic carbon (DIC
CO 3 5 ) is present in all
natural waters. Dissolved CO 2 actually consists of two pools, free CO 2 and H 2 CO 3 .At
equilibrium with each other, which is extremely rapid, H 2 CO 3 is about 1/1000 of the
concentration of free CO 2 . Neither CO 2 nor H 2 CO 3 is charged; they interchange readily
and behave as one pool in chemical reactions. Thus, we treat CO 2 1
HCO 3 2 1
dissolved CO 2 1
5
H 2 CO 3 as a single
pool and refer to it as either H 2 CO 3 * or simply as CO 2 .H 2 CO 3 * is also known as carbonic
acid because it can behave as a proton (H 1 ) and a bicarbonate ion (HCO 3 2 ). It is a weak
acid but very prevalent in the biosphere and extremely important
in processes like
weathering and ocean acidification (follows).
The concentration of DIC varies from less than 0.24 mg C/L (20
μ
M) in acidic, poorly
buffered waters to more than 60 mg C/L (5000
M) in highly alkaline hard waters, but
ranges between about 1.2 and 120 mg C/L (100 to 10,000
μ
μ
M) in most freshwater and
averages about 29 mg C/L (2400
M) in the ocean. DIC is usually the most abundant
form of C in water. Although CO 2 ,
μ
like other gases, readily exchanges with the
10 15 g and is the same as 1 billion
TABLE 6.1 Major reservoirs of C on Earth. Units are in Pg C (1 Pg
5
metric tons).
Reservoir
C Mass (Pg)
Comments
Reference
Earth
100,000,000
Poorly known
Schlesinger (1997)
Sedimentary rocks—carbonate
65,000,000
Schlesinger (1997)
Sedimentary rocks—organic
compounds
16,000,000
Schlesinger (1997)
Marine dissolved carbonates (DIC)
38,000
Sum of dissolved CO 2 ,
HCO 3 2 , and CO 3 5
Sundquist and Viser (2005)
Large lake sediments
19,510
Most in African rift lakes
Alin and Johnson (2007)
Fossil fuel (coal, oil, natural gas)
5200
Known plus likely reserves
Sundquist and Viser (2005)
Terrestrial soils
2150
Sundquist and Viser (2005)
Atmospheric CO 2
750
Modern, industrial, rising
Houghton (2005)
Reactive marine organic C
650
Sundquist and Viser (2005)
Terrestrial vegetation
560
Houghton (2005)
Marine biota
2
Houghton (2005)
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