Environmental Engineering Reference
In-Depth Information
Ca 2 +
2HCO 3
+
CaCO 3
+
CO 2
+
H 2 O
The net effect of these weathering reactions is:
CO 2
+
CaAl 2 Si 2 O 8
+
2H 2 O
CaCO 3
+
Al 2 Si 2 O 5 (OH) 4
In words, the net effect of weathering is to convert atmospheric CO 2
into carbonate minerals such as CaCO 3 . The bicarbonate and the cal-
cium carbonate are transported by water into the rivers and fi nally end up
in the ocean. In the ocean, the calcium carbonates form the shells of
plankton (see Figure 3.4.4 ) and a fraction settles on the ocean fl oor.
Through this weathering reaction, 0.15 Gt C/year is transported into the
oceans. As we have seen in the previous section, the exchange of CO 2
with the atmosphere is in nearly perfect equilibrium. Hence, this fl ux of
0.15 Gt per year would slowly drain all the CO 2 in the atmosphere if the
earth did not have a mechanism to recycle these deposits.
This mechanism is the tectonic motion of the sea bottom. Because
of tectonic motion, subduction takes the CaCO 3 deposits into the mantle,
where the CaCO 3 decomposes and CO 2 is released. Volcanos recycle
this CO 2 back into the atmosphere, thus completing the inorganic carbon
cycle. As the temperature of the sun increases, rock weathering
Figure 3.4.4 Foraminifera
Foraminifera is one of the organisms that use HCO 3 to make shells of CaCO 3 . Phase-
contrast photomicrograph by Scott Fay.
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