Page 73 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 49
2

FIGURE 3-5 Chemical weathering
removes atmospheric CO Chemical
2
CO 2 weathering of silica-rich rocks on the
continents removes CO from the
2
atmosphere, and part of the carbon is
H O
2 later stored in the shells of marine
Ca +2 +4 – plankton and buried in ocean
Soil H CO 3 Si HCO 3 sediments.
2
SiO CaCO 3
Silicate rock 2
(CaSiO )
3

CaSiO + H CO Ca +2 Si +4 SiO + CaCO
3 2 3 – 2 3
HCO
Silicate Carbonic acid 3 Shells of
bedrock in soils Ions dissolved ocean plankton
in river water
Weathering Transport Deposition
on land in rivers in ocean

for each of the many types of silicate minerals found on H O+CO
2 2
continents. The part of these processes that is most Rain From atmosphere
important to the carbon system can be represented by → →
these reactions: CaCO + H CO → CaCO + H O + CO
3 2 3 3 2 2
Limestone In soils Shells of Returned to
H O+CO
2 2 rock organisms atmosphere
Rain From atmosphere
→ →
CaSiO + H CO → CaCO + SiO + H O Dissolution of limestone proceeds at much faster
3 2 3 3 2 2
Silicate rock Carbonic acid Shells of organisms rates than hydrolysis of silicates. Similar to hydrolysis,
(continents) (soil) dissolution extracts CO from the atmosphere to attack
2
rock. But unlike the weathering of silicate rocks, lime-
For simplicity, the many kinds of continental rocks stone weathering causes no net removal of atmospheric
and minerals are represented here by one silicate min- CO . Within the relatively short interval of time it takes
2
eral, CaSiO (wollastonite). Carbon dioxide (CO ) is for the dissolved HCO –1 and CO –2 ions to reach the
3 2 3 3
removed from the atmosphere, incorporated in ground- sea and become incorporated in the shells of organisms,
water to form carbonic acid in soils, used in the chemi- all the CO is returned to the atmosphere.
2
cal weathering reaction, and deposited in the CaCO A CO Balance In summary, slow weathering of
3 2
shells of marine organisms. This reaction is a shorthand granite and other silicate rocks on the continents by
summary of the way chemical weathering removes CO hydrolysis is the main way that CO is pulled out of the
2 2
from the atmosphere and buries it in ocean sediments. atmosphere over very long time scales. In the context of
This process acts slowly but persistently over long Earth’s delicate long-term balancing act, the rate of
intervals of geologic time and accounts for 80% of the removal of carbon by chemical weathering must have
0.15 gigatons of carbon buried each year in ocean very nearly balanced the rate of carbon input from vol-
sediments. canoes. If these rates had not been very nearly equal, the
Dissolution It is important to distinguish weather- system would have been thrown off balance and caused
ing of silicates by hydrolysis from dissolution, the sec- drastic changes in CO levels and in climate.
2
ond type of weathering. Dissolution is the familiar The existence of this delicate balance does not imply
process that eats away at limestone bedrock and in some that either the (volcanic) CO input rate or the (weath-
2
areas forms limestone caves. Again, rainwater and CO ering) CO removal rate remained absolutely constant
2 2
combine in soils to form carbonic acid (H CO ) and through time. Yet the fact of Earth’s long-term habit-
2 3
attack limestone bedrock, and the dissolved ions created ability requires that the rates of input and output must
by dissolution again flow to the ocean in rivers. Dissolu- have always remained fairly closely balanced even as
tion can also be summarized by these simple reactions: one or both processes changed.