Chemistry Reference
In-Depth Information
anthropogenic CO
2
on time scales of tens of years.
Long term fluctuations in the level of atmospheric CO
2
are dominated by exchanges
with the lithosphere (e.g., Berner 1990). The CO
2
sinks in the long-term carbon cycle are
burial of organic carbon and CaCO
3
in marine sediments, and CO
2
consumption during
weathering of silicate rocks. The sources are CO
2
produced by the thermal degradation of
buried organic matter and carbonate minerals, plus the oxidative weathering of
sedimentary organic matter after uplift.
The carbon sinks and sources are often represented by overall chemical reactions,
such as
CO
2
(
g
) + H
2
O(
l
)
⇔
"CH
2
O" + O
2
(
g
)
(7)
where “CH
2
O” is a simplified representation for organic matter. The forward reaction in
Equation (7) corresponds to net global photosynthesis, that is, photosynthesis minus
respiration. Because the earth surface environment is at (quasi-)steady state, net global
photosynthesis is equal to the burial rate of organic matter in sediments. The backward
reaction represents total oxidative degradation of organic matter, either through
weathering of sedimentary organic matter or the oxidation in the atmosphere of reduced
carbon gases derived from the degradation of organic matter during late diagenesis,
metamorphism or magmatic activity (Fig. 2). The backward reaction can thus be thought
of as the global respiration of the geosphere.
Weathering of carbonate rocks on land does not affect the long term, average
concentration of CO
2
in the atmosphere. This is because, on a million-year time scale, the
carbonate weathering flux is balanced by an equal amount of carbonate precipitation and
burial in the oceans. The latter returns the CO
2
consumed during carbonate weathering
back to the atmosphere. The situation is different when marine carbonate minerals are
formed with calcium (and magnesium) ions produced by weathering of silicate minerals.
In that case, there is a net consumption of atmospheric CO
2
, as shown schematically by
the overall reaction (e.g., Berner 1990):
CaSiO
3
(
s
) + CO
2
(
g
)
⇔
CaCO
3
(
s
) + SiO
2
(
s
)
(8)
where CaSiO
3
(
s
) represents calcium containing silicate minerals. The forward reaction
corresponds to silicate mineral weathering coupled to carbonate mineral burial in ocean
sediments, and the reverse reaction to the thermal decomposition of the carbonates deep
within the lithosphere. Because of the long time span that separates deposition of carbonate
minerals at the seafloor and the return of carbon to the earth's surface as volcanic CO
2
,
reaction (8) can be out of balance, resulting in a net source or sink of atmospheric CO
2
.
In box models for the long term carbon cycle, the forward and backward rates of
overall reactions such as those represented by Equations (7) and (8) correspond to fluxes
linking carbon reservoirs at the earth surface and in the lithosphere. These fluxes
obviously combine many different processes and are subject to a variety of controls. For
example, the burial flux of organic carbon in sediments (i.e., the forward flux in reaction
7) depends not only on how much organic matter is produced globally, but also on the
fraction of organic matter that survives degradation and is ultimately incorporated in the
sedimentary column (e.g., Canfield 1989). In other words, the derivation of a
mathematical expression for the burial flux of organic carbon must be based on a careful
evaluation of the factors controlling both production and preservation of organic matter.
The same is true for the burial fluxes of other biogenic materials, in particular the
products of biomineralization.
