further illustrates the reaction of carbon dioxide and water
in the presence of calcium carbonate (CaCO 3 ) minerals:
CO 2 þ H 2 O þ CaCO 3 ! Ca
2þ
þ 2HCO 3
À
:
(4)
Calcium carbonate production thus releases CO 2 to the
water column while its dissolution removes CO 2 .
The calcium carbonate saturation state (O) is a measure
of the ion activity product of Ca
2þ and CO 3
2À relative to
the apparent solubility product (K
0 ) for a particular calcium
carbonate mineral (e.g., calcite or aragonite). The surface
ocean is supersaturated (>1) nearly everywhere
(Figure 1), but spontaneous precipitation of carbonate minerals is constrained by kinetic barriers. Calcium is much
more abundant than carbonate in seawater, so changes in
O mostly reflect changes in the carbonate ion concentration. The calculated O aragonite for the tropical oceans, based
on a combination of measurements taken in the 1990s
(GLODAP, World Ocean Atlas), ranged between about
3.2 and 4.5 (Figure 1a). Most of this variation is due to
the 18–30
C temperature range. First order calculations
that take into account the progressive increase in atmospheric CO 2 concentration (with an associated temperature
increase) show that aragonite saturation state will decrease
to levels outside the preindustrial range (Figure 1b).
Effects on formation and dissolution of calcium
carbonate
Calcium carbonate production in the oceans is almost
entirely biogenic, and most is precipitated as one of two
Ocean Acidification, Effects on Calcification, Figure 1 (a) Aragonite saturation state of tropical oceans estimated by combining 1 Â
1
gridded fields of total alkalinity and total dissolved inorganic carbon normalized to 1990 conditions (GLODAP data base, Sabine
et al., 2005) with 1 Â 1
gridded fields of average annual temperature, salinity, phosphate, and silicate (World Ocean Database, 2005;
Boyer et al., 2006). Note that these values are highly interpolated and should not be used for research purposes. (b) Aragonite
saturation state as a function of atmospheric CO 2 concentration. Lower and upper solid lines are calculated for 18 and 30
C seawater,
respectively, with total alkalinity of 2,300 meq kg
À1 , and negligible phosphate and silicate. Parallel dashed lines indicate general range
of aragonite saturation state in reef areas prior to the Industrial Revolution. Vertical lines show range of aragonite saturation states at
preindustrial atmospheric CO 2 concentration (280 ppm), the present decade (390 ppm), and 2Â preindustrial (560 ppm).
734
OCEAN ACIDIFICATION, EFFECTS ON CALCIFICATION
in the presence of calcium carbonate (CaCO 3 ) minerals:
CO 2 þ H 2 O þ CaCO 3 ! Ca
2þ
þ 2HCO 3
À
:
(4)
Calcium carbonate production thus releases CO 2 to the
water column while its dissolution removes CO 2 .
The calcium carbonate saturation state (O) is a measure
of the ion activity product of Ca
2þ and CO 3
2À relative to
the apparent solubility product (K
0 ) for a particular calcium
carbonate mineral (e.g., calcite or aragonite). The surface
ocean is supersaturated (>1) nearly everywhere
(Figure 1), but spontaneous precipitation of carbonate minerals is constrained by kinetic barriers. Calcium is much
more abundant than carbonate in seawater, so changes in
O mostly reflect changes in the carbonate ion concentration. The calculated O aragonite for the tropical oceans, based
on a combination of measurements taken in the 1990s
(GLODAP, World Ocean Atlas), ranged between about
3.2 and 4.5 (Figure 1a). Most of this variation is due to
the 18–30
C temperature range. First order calculations
that take into account the progressive increase in atmospheric CO 2 concentration (with an associated temperature
increase) show that aragonite saturation state will decrease
to levels outside the preindustrial range (Figure 1b).
Effects on formation and dissolution of calcium
carbonate
Calcium carbonate production in the oceans is almost
entirely biogenic, and most is precipitated as one of two
Ocean Acidification, Effects on Calcification, Figure 1 (a) Aragonite saturation state of tropical oceans estimated by combining 1 Â
1
gridded fields of total alkalinity and total dissolved inorganic carbon normalized to 1990 conditions (GLODAP data base, Sabine
et al., 2005) with 1 Â 1
gridded fields of average annual temperature, salinity, phosphate, and silicate (World Ocean Database, 2005;
Boyer et al., 2006). Note that these values are highly interpolated and should not be used for research purposes. (b) Aragonite
saturation state as a function of atmospheric CO 2 concentration. Lower and upper solid lines are calculated for 18 and 30
C seawater,
respectively, with total alkalinity of 2,300 meq kg
À1 , and negligible phosphate and silicate. Parallel dashed lines indicate general range
of aragonite saturation state in reef areas prior to the Industrial Revolution. Vertical lines show range of aragonite saturation states at
preindustrial atmospheric CO 2 concentration (280 ppm), the present decade (390 ppm), and 2Â preindustrial (560 ppm).
734
OCEAN ACIDIFICATION, EFFECTS ON CALCIFICATION