Geoscience Reference
In-Depth Information
processes (Section 1.3). The history of ocean acidii -
cation research and a bibliometric analysis follow
(Section 1.4). Finally, the potential societal and pol-
icy implications are outlined (Section 1.5).
and lowers the pH, the concentration of carbonate
ions ([CO
3
2-
]), and the saturation state of the three
major carbonate minerals present in shells and
skeletons:
2
-
2
-
CO
+
H O
+
CO
®
2HCO
.
(1.1)
1.2
What is ocean acidii cation?
2
2
3
3
The concentration of protons ([H
+
]), which is pro-
portional to the ratio [HCO
3
-
]/[CO
3
2-
] (see Eq. B1.3 in
Box 1.1), increases and pH decreases. Mean surface-
ocean pH expressed on the total hydrogen ion scale
(pH
T
) has decreased from approximately 8.2 to 8.1
between pre-industrial time and the 1990s, and may
reach 7.8 in 2100 (Gattuso and Lavigne 2009). The
expression 'ocean acidii cation' refers to the decrease
in pH, but does not imply that the pH of surface-
ocean waters will become acidic (below 7.0) any
time soon. The whole process could equally be
referred to as 'carbonation' as it increases the con-
centration of dissolved inorganic carbon.
Ocean acidii cation refers to a reduction in the pH
of the ocean over an extended period, typically dec-
ades or longer, caused primarily by the uptake of
CO
2
from the atmosphere, but it can be caused by
other chemical additions or subtractions from the
ocean. This topic focuses on anthropogenic ocean
acidii cation, which refers to the component of pH
reduction caused by human activity.
Once dissolved in seawater, CO
2
is a weak acid
which generates a number of changes in seawater
chemistry, primarily carbonate chemistry (see Box
1.1). It increases the concentration of bicarbonate
ions ([HCO
3
-
]) and dissolved inorganic carbon (
C
T
),
Box 1.1 Chemistry of the seawater-carbonate system
Richard Zeebe and Jean-Pierre Gattuso
-
+
1
*
CO
+=
H O
HCO
+
K
H ;
(B1.2)
2
2
3
The chemistry of inorganic carbon in seawater is relatively
complex and is only briel y described here. Comprehensive
information can be found, for example, in Zeebe and
Wolf-Gladrow ( 2001 ) and Millero ( 2006 ).
-
- +
=+
K
2
2
*
HCO
CO
H ;
.
(B1.3)
3
3
The pK*s (= −log
10
( K*)) of the stoichiometric dissociation
constants of carbonic acid in seawater are pK *
1
= 5.94 and
pK *
2
= 9.13 at temperature T = 15°C, salinity S = 35, and
surface pressure P = 1 atm on the total pH scale (Lueker
et al. 2000). At a typical surface-seawater pH
T
of 8.2, the
speciation between [CO
2
], [HCO
3
-
], and [CO
3
2-
] is hence
0.5%, 89%, and 10.5%, showing that most of the
dissolved CO
2
is in the form of HCO
3
-
and not CO
2
. The sum
of the dissolved carbonate species is denoted as total
dissolved inorganic carbon (DIC ≡ ΣCO
2
≡ TCO
2
≡ C
T
):
Dissolved inorganic carbon
Dissolved inorganic carbon is mostly present in three
inorganic forms in seawater: free aqueous carbon dioxide
(CO
2
(aq)), bicarbonate (HCO
3
-
), and carbonate (CO
3
2-
) ions.
A minor form is true carbonic acid (H
2
CO
3
) with a
concentration of less than 0.3% of [CO
2
(aq)]. The sum of
[CO
2
(aq)] and [H
2
CO
3
] is denoted as [CO
2
]. The majority of
dissolved inorganic carbon in the ocean is in the form of
HCO
3
-
(>85%; Fig. B1.1). Gaseous carbon dioxide (CO
2
(g)),
and [CO
2
] are related by Henry's law in thermodynamic
equilibrium:
-
2
-
C
=
[CO ]
+
[HCO ]
+
[CO
]
(B1.4)
T
2
3
3
CO (g)
=
CO ; K
(B1.1)
Total alkalinity
Total alkalinity (A
T
) is related to the charge balance in
seawater:
2
2
0
where K
0
is the solubility coefi cient of CO
2
in seawater. The
concentration of dissolved CO
2
and the fugacity of gaseous
CO
2
, f CO
2
, then obey the equation [CO
2
] = K
0
× f CO
2
. The
fugacity is practically equal to the partial pressure, p CO
2
(within ~1%). The dissolved carbonate species are related by:
-
2
-
-
A
=
[HCO ]
+
2[CO
]
+
[B(OH) ]
+
(B1.5)
T
3
3
4
-
+
[OH ] - [H ]
+
minor compounds.
