The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 113

water. For example, calcite (CaCO 3 ) is very abundant and can dissociate
according to:

Ca 2 + CO 3 2 −

CaCO 3 +

The presence of carbonate in this reaction means that it is coupled to
the CO 2 equations given above; thus the mineral dissolution is pH-depend-
ent. Inspection of the equilibrium equations listed above leads to the conclu-
sion that if water is acidic, CaCO 3 is more likely to dissolve; in basic waters,
the mineral is more likely to be formed. Because the earth's crust has enor-
mous quantities of such minerals (see Table 3.1.1 ), the calcite dissociation
reactions buffer the carbonate (CO 3 2 − ) concentrations in the ocean.
In addition to the abiotic reactions described above, biological pro-
cesses play an important role in the uptake of CO 2 by ocean waters. For
example, in reef environments much of the buried CaCO 3 is precipitated in
situ by corals and benthic algae as the mineral aragonite. In the open ocean,
most CaCO 3 precipitates as a mineral (calcite) in the microfossils of two
groups of marine plankton: coccolithophorids and planktonic foraminifera.
Organic carbon, that is, carbon that is present in organic matter such
as the cells of living organisms, is also present in the oceans. In ocean
surface layers, organisms use sunlight to convert CO 2 into biomass. If
these surface organisms die and sink to the deep part of the ocean
before they decompose, there is a net fl ux of organic carbon from the
atmosphere towards the deep ocean. This fl ux is referred to as the “ bio-
logical pump .”
These organic and the inorganic cycles are coupled. As an example
of how this coupling works, consider Figure 3.3.2. Suppose we were to
grow organisms in the sea that consume CO 2 and form biomass, then die
and sink to the bottom of the ocean. This would be an “engineered”
enhancement of the biological pump, and it would have two effects on
the CO 2 levels of the atmosphere. The fi rst effect is that the biological
pump would remove carbon more rapidly from the surface layer of the
ocean and put it into the deep ocean, allowing the surface layer to take
up more CO 2 and thus decreasing atmospheric CO 2 . The second, indi-
rect effect is that in the deep ocean the organic carbon would decom-
pose and regenerate CO 2 , which in turn would increase the acidity of the
deep ocean water. This acidity would be neutralized by the dissolution of
CaCO 3 . The net result would be a decrease in the global ocean's burial
rate of CaCO 3 !