Environmental Engineering Reference
In-Depth Information
4.9 OCEAN ACIDIFICATION
The oceanic uptake of excess atmospheric carbon dioxide alters the
chemistry of seawater, which may impact a wide range of marine organisms
from plankton to coral reefs (Doney et al., 2009a,b; NRC, 2010) (see also
Section 6.3). Ocean acidification is in fact a series of interlinked and well-
known changes in acid-base chemistry and carbonate chemistry due to the
net flux of CO
2
into surface waters (Figure 4.26). The chemical shifts include
increases in the partial pressure of carbon dioxide (pCO
2
), the concentration
of aqueous CO
2
, and the hydrogen ion (H
+
) concentration and decreases
in pH (pH = -log10[H
+
]). The increase in hydrogen ion concentration acts
to lower the concentration of carbonate ions (CO
3
2-
) through the reaction
H
+
+ CO
3
2-
=> HCO
3
-
, even though the total amount of dissolved inor-
ganic carbon (DIC) goes up (DIC = [CO
2
] + [HCO
3
-
] + [CO
3
2-
]). Declining
CO
3
2-
in turn lowers calcium carbonate (CaCO
3
) mineral saturation state,
W
= [Ca
2+
][CO
3
2-
]/K
sp
, where K
sp
is the thermodynamic solubility product that
varies with temperature, pressure, and mineral form. Ocean surface waters
FIGURE 4.26 Schematic indicating the effects on seawater carbonate chemistry due to the uptake of
excess carbon dioxide (CO
2
) from the atmosphere. Ocean acidification causes increases in some chemical
species (red) and decreases in other species (blue). Ocean acidification also causes a reduction in pH (pH
= -log
10
[H
+
]) and the saturation states,
W
, of calcium carbonate minerals in shells and skeletons of plank-
tonic and benthic organisms and in carbonate sediments. On millennial and longer time scales, ocean pH
perturbations are buffered by external inputs of alkalinity, denoted by calcium ions (Ca
2+
) and changes in
the net burial rate of carbonate sediments.
