Geoscience Reference
In-Depth Information
the precipitation of CaCO
3
, so that the net balance of the cycle in the
long term is null.
Contrary to the biogeochemical carbonate cycle, the weathering by
carbonic acid of rocks that contain silicate minerals can regulate the
climate in the long term:
- the formation of two molecules of carbonic (H
2
CO
3
) removes
two molecules of CO
2
from the atmosphere (equation [4.1]);
- the weathering of a silicate mineral on land consumes two
molecules of H
2
CO
3
and one molecule of water, and produces one
molecule of silicic acid, Si(OH)
4
(the following example uses
wollastonite, a mineral composed of calcium silicate):
CaSiO
3
+ 2 H
2
CO
3
+ H
2
O → Ca
2+
+ 2 HCO
3
-
+ Si(OH)
4
[4.6]
- according to equation [4.6], the reaction of two HCO
3
-
molecules
with one molecule of calcium (Ca
2+
), in the ocean, produces one
molecule of CaCO
3
, one molecule of water and one molecule of CO
2
,
which is released into the atmosphere. Hence for two molecules of
CO
2
removed from the atmosphere during the weathering of silicate
minerals, one is returned to the atmosphere during the precipitation of
CaCO
3
, so that the net balance of the cycle in the long term is the
removal of one CO
2
molecule from the atmosphere.
Equation [4.6] can therefore potentially remove anthropogenic CO
2
from the atmosphere in the long term. However, if this reaction were
to proceed with no counterbalance for several hundred thousand years,
it would remove all CO
2
from the atmosphere and the Earth, without
the greenhouse effect, would freeze (see section 4.3.1). Fortunately,
the biogeochemical carbonate-silicate cycle does not end with
equation [4.6]. In fact, in the ocean, diatoms (phytoplanktonic algae)
use silicon from silicic acid (H
4
SiO
4
) to form their external envelope
(frustule), which is composed of biogenic silicon (called opal, SiO
2
):
Si(OH)
4
→ SiO
2
+ 2 H
2
O
[4.7]
Other planktonic organisms, namely silicoflagellates and
radiolaria, also use SiO
2
. Siliceous tests of these planktonic organisms
sink toward the seabed where they accumulate in sediments, together
