Chemistry Reference
In-Depth Information
Table 2.
Reservoirs in the carbon cycle.
reservoir
carbon (g)
10
17
atmosphere
6
×
10
17
terrestrial biota
8
×
10
18
soils & detritus
1.5
×
10
15
marine biota
3
×
10
18
ocean surface water
10
19
deep ocean water
3.8
×
10
17
surface sediments
1.5
×
10
22
sediments & sedimentary rocks
5.5
×
10
20
basaltic oceanic crust
7
×
10
21
granitic continental crust
9
×
Sources:
Garrels and Mackenzie (1971), Reeburgh (1997),
Chameides and Perdue (1997).
balance on short time scales ( 1000 years) is dominated by the relatively small
reservoirs of the earth surface environment, including the atmosphere, the oceans,
terrestrial and marine biota, soils and nearshore sediments. For a good introduction to the
issues related to the short term redistribution of anthropogenic CO
2
among these
reservoirs, the reader is referred to Sarmiento and Gruber (2002).
On short time scales, anthropogenic CO
2
may be sequestered by the oceans via the
so-called
biological pump
, that is, the export to the deep ocean of organic matter
synthesized in the surface ocean. Marine organisms, in particular calcifying algae,
coccolithophores and foraminifera, also secrete calcium carbonate (CaCO
3
). Part of the
calcium carbonate precipitated in the surface ocean sinks to the deep ocean, together with
the organic carbon. This so-called
carbonate pump
is an important component of the
oceanic carbon cycle. Together, the export fluxes of particulate organic carbon (POC)
and particulate inorganic carbon (PIC) to the deep ocean represent about 10 Gt of carbon
annually (1 Gt = 10
15
g), of which 20-40% is under the form of CaCO
3
.
The role of CaCO
3
production and dissolution in the marine carbon cycle is rather
complex, however (e.g., Archer and Maier-Reimer 1994). Although it is a net sink for
dissolved inorganic carbon (DIC), calcification actually generates CO
2
while consuming
alkalinity:
2+
−
(6)
Thus, uptake of atmospheric CO
2
by surface ocean waters depends on the proportion of
biomass production carried out by calcifying organisms (mainly coccolithophorids),
versus that by non-calcifying organisms (mainly diatoms).
Calcium carbonate dissolution (i.e., reaction (6) in reverse direction) below the
photic zone is a net sink for CO
2
. Dissolution is usually assumed to take place at great
depths, typically below 3
Ca
(
aq
) + 2HCO (
aq
)
⇔
CaCO ( ) + H O( ) + CO (
s
l
g
)
3
3
2
2
4 km, where seawater is undersaturated with respect to calcite
and aragonite (Broecker and Peng 1982). This deep ocean sink should therefore affect
atmospheric CO
2
levels on time scales dictated by the rate of deep water renewal, that is,
on the order of 1000 years. Recent studies, however, suggest that substantial CaCO
3
dissolution may take place in the upper 1000 m of the water column, probably coupled to
organic matter respiration (e.g. Wollast and Chou 1998; Milliman et al. 1999). If
confirmed, shallow CaCO
3
dissolution may represent an important marine sink for
−
