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magnesian calcites comprising ~63% and ~24%,
respectively, of the calcii ers and carbonate sedi-
ments that could be reactive on the decadal to cen-
tennial timescale (Chave 1967; Morse and Mackenzie
1990 ; Morse
et al
. 2006). The less reactive calcite phase
makes up ~13% of the sediments in these coastal sys-
tems. Not only did benthic calcii ers like corals
spread across the increasing accommodation space
as sea level rose but benthic calcifying algae, such as
rhodolith beds in polar and temperate areas and
Halimeda
beds in the tropics, became important calci-
fying ecosystems ( Nelson 2009 ).
As these shoal-water benthic carbonate deposits
accumulated with rising sea level and atmospheric
CO
2
, global-mean sea-surface temperature also rose
4 to 5°C and the carbonate system of coastal ocean
water changed signii cantly. Despite the fact that
the global shoal-water carbonate mass was growing
extensively, the pH
T
of global coastal waters actu-
ally declined from ~8.35 to ~8.18 and the carbonate
ion concentration (and consequently Ω) declined by
~19% from the LGM to late pre-industrial time
(F. Mackenzie, pers. comm.). The latter represents a
rate of decline of about 0.028 μmol CO
3
2-
per decade.
For comparison, the decline in coastal water pH
from the year 1900 to 2000 was about 8.18 to 8.08
and projected from the year 2000 to 2100 about 8.08
to 7.85, using the IS92a business-as-usual scenario
of CO
2
emissions (e.g. Andersson
et al
. 2005 ). Over
these 200 years, the carbonate ion concentration will
fall by ~120 μmol kg
-1
or 6 μmol kg
-1
decade
-1
. This
decadal rate of decline of CO
3
2-
in the Anthropocene
is 214 times the average rate of decline for the entire
Holocene. Hence when viewed against the times-
cale of geological change in the coastal ocean marine
carbon system of millennia to several millennia, one
can easily appreciate why ocean acidii cation is the
'other CO
2
problem' (see also Chapter 2). The prob-
lem is magnii ed in the calcifying ecosystems of the
shoal-water ocean, where mainly aragonite- and
Mg-calcite-secreting benthic calcii ers have prolifer-
ated in a relatively slowly changing marine carbon-
ate chemistry for thousands of years. Many of the
skeletal fragments and inorganic cements made of
Mg-calcite have solubilities greater than that of
aragonite and are viewed as the 'i rst responders'
to ocean acidii cation (Morse
et al
. 2006 ). Despite
the complications arising from vital effects, their
Mg-calcite compositions appear to be very close to
those predicted for the phases in metastable equi-
librium with the ambient composition and temper-
ature of their seawater environment (Mackenzie
et al
. 1983 ; Morse
et al
. 2006 ; Andersson
et al
. 2008 ).
Hence, even small changes in pH and CO
3
2-
concen-
tration brought about by modern ocean acidii ca-
tion will displace this metastable equilibrium and
hence can signii cantly affect the rate of calcii cation
and composition of Mg-calcite skeletal organisms
as the organisms attempt to acclimate or adapt to
the new environment (e.g. Mackenzie
et al
. 1983 ;
Agegian 1985). Metastable mineral phases are ther-
modynamically unstable under the earth's surface
temperature and pressure conditions, but persist
owing to kinetic constraints. This means that metast-
able mineral phases in seawater are expected to dis-
solve based on thermodynamic principles, but they
do not because of slow reaction rates or inhibition
of the reaction by other constituents.
The important point is that the benthic carbonate
system of the whole coastal ocean was evolving and
diversifying as sea level and accommodation space
slowly rose from the LGM to late pre-industrial
times and the coastal system was becoming a major
player in the carbon cycle. This evolution took place
under relatively slowly rising atmospheric CO
2
and
temperature and declining pH and CO
3
2-
relative to
the rate of these changes in the Anthropocene. The
question is whether the modern rates are such as to
overwhelm one or more coastal ocean carbonate
ecosystems, or whether one or more of these sys-
tems will be able to acclimate or adapt to the
changes. So far the answer seems to be, at least for
coral reefs, that in this century or the early 22nd
century at the latest, the net ecosystem calcii cation
of many reef systems could become negative
because the production of CaCO
3
will be exceeded
by its dissolution. Reef accretion rates will slow,
cease, or decline, and the reefs degrade in quality
(e.g. Andersson
et al
. 2005 , 2009 ; Silverman
et al
.
2009 ). Silverman
et al
. ( 2009 ) predicted that all coral
reef ecosystems will become subject to net dissolu-
tion and net loss of CaCO
3
at an atmospheric CO
2
concentration of 560 ppmv. In contrast, Andersson
et al
. (2005) made a more modest projection based
on the shallow-water ocean carbonate model
(SOCM) and suggested that the global coastal ocean
