Geoscience Reference
In-Depth Information
ocean circulation. The solubility of CO
2
increases
with decreasing temperature and is thus higher at
high latitudes where deep-water formation takes
place. The combined effects of solubility and deep-
water formation result in a downward transport of
CO
2
-enriched water masses and thus higher
C
T
con-
centrations at depth (see Chapter 3).
The equilibrium reactions of the carbonate sys-
tem are at the origin of the large uptake capacity
of the ocean for CO
2
. However, as for any buffer
system, its capacity is not ini nite. As discussed in
Chapter 3, the Revelle factor, a measurement of
the buffer capacity, increases (decreasing buffer
capacity) with increasing atmospheric CO
2
. As a
result, the strength of the ocean sink for CO
2
is
going to decrease in the future, a direct positive
feedback of ocean acidii cation to atmospheric
CO
2
levels and hence to the earth system. This
positive feedback is very substantial, i.e. in a busi-
ness-as-usual scenario it may be as large as 30% in
the next 100 years (Sarmiento
et al.
1995 ). Hydration
of gaseous CO
2
and the equilibration between
individual dissolved species of the carbonate sys-
tem are dependent on temperature and salinity.
Both will change in response to global climate
change. Climate change aggravates the chemical
effect of decreasing buffer capacity in two ways:
(1) due to the inverse relationship between tem-
perature and CO
2
solubility ( Chapter 3 ) and (2)
due to the increase in stratii cation and the antici-
pated slowdown of the surface-to-deep exchange
of carbon (Sarmiento
et al.
1998 ).
that the dominant
C
T
species at the pH of surface-
ocean waters, i.e. bicarbonate (HCO
3
-
), will dissoci-
ate in order to replenish the lost CO
3
2-
but thereby
generating dissolved CO
2
as well:
2
+
2
3
−
Ca
+
CO
→
CaCO (s)
(12.1a)
3
−
−
→+
2
2HCO
CO
CO
+
H O
(12.1b)
3
3
2
2
or written as a summary equation:
2
+
−
Ca
+
2HCO
→
CaCO (s)
+
CO
+
H O.
(12.1c).
3
3
2
2
Thus the carbonate pump tends to force CO
2
out of
the ocean into the atmosphere, despite the fact that
it leads to a depletion of
C
T
in the upper ocean.
The saturation state with respect to CaCO
3
decreases with depth, largely owing to the soft-tis-
sue pump that acidii es the deep ocean as a result of
the release of metabolic CO
2
during the reminerali-
zation of the organic matter transported down-
wards (Gruber and Sarmiento 2002). The saturation
state has a direct impact on the formation and dis-
solution of carbonate structures. Throughout this
discussion, the saturation state of seawater with
respect to a CaCO
3
mineral (Ω) dei ned by Zeebe
and Gattuso (see Box 1.1 in Chapter 1) is used. The
stoichiometric solubility product increases with
depth as a result of increasing pressure and decreas-
ing temperature.
12.2.2.1 Calcium carbonate production
In the modern ocean, CaCO
3
formation is largely a
biotic process. While inorganic precipitation and
dissolution of CaCO
3
are a direct function of the
saturation state (Morse
et al.
2007 ), the mechanisms
of calcii cation and their sensitivity to changes in
carbonate chemistry are less well understood (see
Chapters 6 and 7). The diversity of responses of cal-
cii ers to a decrease in saturation state of seawater
challenges global ocean biogeochemical models.
These models represent CaCO
3
formation as a geo-
chemical source/sink function of varying complex-
ity with a limited number of studies including a
dependency on carbonate chemistry. In its simplest
expression, CaCO
3
production is implemented as a
constant fraction of organic carbon production
modulated by a dependency on saturation state
12.2.2
The carbonate pump
The carbonate pump is driven by the precipitation
of CaCO
3
by marine organisms, the settling of car-
bonate particles across the water column, and their
dissolution at depth in undersaturated waters and
burial in sediments. Since the precipitation of CaCO
3
reduces the total alkalinity of the seawater more
than it decreases its
C
T
, this process increases the
concentration of dissolved CO
2
and thus the partial
pressure of CO
2
(
p
CO
2
) of near-surface waters. This
can also be understood by recognizing that the
removal of the carbonate ion (CO
3
2-
) by the precipi-
tation of CaCO
3
leads to a redistribution of the dif-
ferent species of the carbonate system in such a way
