Geoscience Reference
In-Depth Information
( Ridgwell
et al.
2007 ). In Heinze ( 2004 ), pre-indus-
trial CaCO
3
export production was computed as a
constant function of the organic carbon export that
is not driven by silicifying organisms. A third
approach assigns CaCO
3
production to a specii c
phytoplankton functional type, the nanophyto-
plankton, which corresponds to the size class of
coccolithophores (Gehlen
et al.
2007 ). In this model
CaCO
3
export production is a function of irradi-
ance, nutrient availability, nanophytoplankton bio-
mass, grazing by micro- and mesozooplankton, and
saturation state with respect to calcite. These mod-
els consider the formation and dissolution of the
less soluble CaCO
3
polymorph calcite. The different
parameterizations of calcite production as a func-
tion of carbonate chemistry rely on a limited number
of studies. Gangstø
et al.
( 2008 ) extended the con-
ceptual approach originally derived for calcite pro-
duction by nanophytoplankton by Gehlen
et al.
(2007) to the production of aragonite by mesozoo-
plankton. At the time of this model sensitivity study,
no data were available to derive a parameterization
of aragonite production by pteropods based on
observations. Since the study by Gangstø
et al.
(2008) was published, experimental data on the cal-
cii cation response of pteropods as a function of car-
bonate chemistry have become available. More
recently, the models were extended further to
include a parameterization specii c for pteropods
( Gangstø
et al.
2011 ).
In all models, CaCO
3
production is linked to the
production of particulate organic carbon (POC)
through the rain ratio. The latter is dei ned as the
ratio of CaCO
3
to POC l ux and corresponds to the
relevant quantity in terms of biogeochemical
impacts and feedbacks to atmospheric CO
2
(e.g.
Archer and Maier-Reimer 1994). On average, the
rain ratio is about 0.09 (Jin
et al.
2008 ). The POC pro-
duction did not respond to changes in carbonate
chemistry in any of these studies, despite some
experimental evidence for an increase in POC pro-
duction in response to increased levels of CO
2
(e.g.
Zondervan
et al.
2001 ).
to increase in response to decreasing saturation
state. The resulting increase of total alkalinity
favours CO
2
uptake, a negative indirect group 1
feedback.
The dissolution of CaCO
3
is usually described by
a higher-order reaction rate law with respect to
undersaturation:
n
Rk
=× −
(1
W
)
(12.2)
where
n
is the reaction order and
k
is the dissolution
rate parameter (time
-1
).
Published estimates of
n
range from 1 to 4.5 (Keir
1980 ; Hales and Emerson 1997 ; Gehlen
et al.
2005 )
based on laboratory studies and the evaluation of
sediment porewater data. The higher-order rate law
implies that dissolution rates are low at modest lev-
els of undersaturation and increase following a
power law as a function of increasing undersatura-
tion. As a result, when normalized to a given value
of Ω, the higher-order rate law translates to an ini-
tial lower sensitivity to decreases in saturation state
compared with the linear rate expression.
The dissolution of sinking CaCO
3
particles is
implemented into some global biogeochemical
models as a i rst-order rate law, i.e.
n
= 1 (e.g. Heinze
2004 ; Gehlen
et al.
2007 ; Gangstø
et al.
2008 ). In con-
trast, Ridgwell
et al.
(2007) do not explicitly solve
for CaCO
3
dissolution, but rather apply a constant
depth-penetration proi le to the CaCO
3
export l ux,
an approach used by most models that do not
include a sensitivity of their biogeochemical proc-
esses to ocean acidii cation.
12.2.2.3 Interaction with carbonate sediments
The vast reservoir of mineral carbonates in the sed-
iments provides the ultimate buffer against ocean
acidii cation (see Chapter 2). If those carbonates
were to dissolve readily in response to a decrease in
the saturation state of the overlying waters, ocean
acidii cation would not be a problem. The sedi-
ments would resupply the carbonate ions that are
titrated away by the invading anthropogenic
CO
2
, thus keeping pH changes to a minimum.
Unfortunately, the ocean's sediment pool will be
reacting very slowly to the invasion of anthropo-
genic CO
2
and the associated ocean acidii cation.
First, because it takes time for anthropogenic CO
2
12.2.2.2 Calcium carbonate dissolution
Dissolution of CaCO
3
is an abiotic process driven by
thermodynamics, i.e. the degree of undersaturation.
This implies that the dissolution of CaCO
3
is bound
