Geoscience Reference
In-Depth Information
high rates of bioerosion. The abundance of cement
appeared correlated to the seawater aragonite satu-
ration state and inversely related to measured rates
of bioerosion.
A slowdown in benthic calcii cation would result
in less CO
2
being released to the atmosphere from
this process, and thus acts as a negative feedback
to rising atmospheric CO
2
on a decadal to centen-
nial timescale (e.g. Mackenzie and Lerman 2006;
see Chapter 12). If all marine calcii cation stopped,
the amount of CO
2
that would otherwise have been
released to the atmosphere from this process cor-
responds to about 4 to 6% of the current total
anthropogenic CO
2
emissions of 9.3 Gt C yr
-
1
( Le
Quéré
et al
. 2009) assuming a global CaCO
3
pro-
duction of 0.64 to 1 Gt C yr
-
1
( Milliman 1993 ;
Milliman and Droxler 1996) and a release of 0.6
mol of CO
2
to the atmosphere for every mole of
CaCO
3
precipitated (Frankignoulle
et al
. 1994 ).
Note, though, that this is a rough approximation.
In reality the proportion of CO
2
released to the
atmosphere from calcii cation will increase with
increasing seawater acidii cation. To some extent
this will compensate for the reduction in CO
2
pro-
duction caused by reduced calcii cation. Benthic
calcii cation probably does not correspond to more
than 6 to 10% of the annual global total calcii ca-
tion (Milliman and Droxler 1996). Nonetheless, the
majority of this calcii cation occurs within the glo-
bal coastal area, which only makes up approxi-
mately 7% of the global ocean area. In addition, as
much as 60% of the CaCO
3
produced within this
area may actually accumulate as carbonate struc-
tures or sediments (Milliman and Droxler 1996;
Mackenzie
et al
. 2005 ).
could become undersaturated with respect to
aragonite in high-latitude regions within a few
decades as a result of ocean acidii cation (Orr
et al
.
2005 ; Steinacher
et al
. 2009 ; Chapter 3 ). Since
Mg-calcite minerals with a magnesium content
greater than 8 to 12 mol% MgCO
3
are more soluble
than aragonite, seawater will become undersatu-
rated with respect to these mineral phases before
aragonite. Thus, Mg-calcite minerals are the i rst
responders to ocean acidii cation and declining
carbonate saturation states (Morse
et al
. 2006 ;
Andersson
et al
. 2008 ). The exact magnesium
content of the Mg-calcite phase with the same sol-
ubility as aragonite is somewhat uncertain and
dependent on the experimental solubility curve
adopted, which is currently poorly constrained
(e.g. Plummer and Mackenzie 1974; Walter and
Morse 1984 ; Bischoff
et al
. 1993 ; Morse
et al
. 2006 ;
Andersson
et al
. 2008 ).
As a result of increasing pressure, decreasing
temperature, and natural acidii cation of seawater
from decomposition of organic material, the satura-
tion state with respect to carbonate minerals
decreases as a function of depth. The majority of the
benthic environment of the open ocean is immersed
in waters undersaturated with respect to all com-
monly occurring carbonate phases. Because of the
difference in age of water masses between the
Atlantic and the Pacii c Oceans, and thus the
amount of dissolved inorganic carbon that has accu-
mulated in these water masses, the saturation hori-
zons with respect to carbonate minerals in the
Pacii c are located at much shallower depths than in
the Atlantic ( Morse and Mackenzie 1990 ; Chapter 3 ).
These differences may be responsible for ecological
and mineralogical differences in the benthic envi-
ronment between the two ocean basins (see Section
7.3.4).
Although most surface seawaters are currently
supersaturated with respect to the majority of car-
bonate mineral phases, carbonate dissolution is an
ongoing process in all environments as a result of
microbial metabolic activity causing corrosive con-
ditions in sediment porewaters and microenviron-
ments, dei ned as small specii c areas isolated from
their immediate surroundings. There are also
many macro- and microorganisms (e.g. endolithic
autotrophic or heterotrophic organisms) that
7.2.4
Calcium carbonate dissolution
Dissolution of CaCO
3
minerals is the reverse proc-
ess of calcii cation, and results in the chemical dis-
integration of the solid mineral phase into its
individual components of calcium and carbonate
ions. From a thermodynamic perspective, dissolu-
tion is expected if Ω < 1. In simple terms, the far-
ther away from equilibrium, the faster the rate of
mineral dissolution (ignoring diffusional limita-
tions). Surface seawater is typically supersaturated
with respect to calcite and aragonite globally but
