Geoscience Reference
In-Depth Information
react with carbonate ions, producing bicarbonate
ions. The series of reactions is given by Zeebe and
Gattuso in Box 1.1.
For simplicity, the net effect is often summarized
as an acid-base neutralization,
isms is the level of saturation of a water mass with
respect to CaCO
3
minerals, dei ned in Box 1.1 in
terms of the saturation state Ω. Another way to
describe the level of saturation is by the difference Δ
between the actual carbonate ion concentration and
the critical carbonate ion concentration [CO
3
2-
]
sat
, i.e.
the threshold below which CaCO
3
starts to dissolve:
H
2
O + CO
2
+ CO
2
«
2HCO
3
,
(3.1)
Δ[CO
2
] = [CO
2
]
[CO
2
]
sat
.
(3.4)
but the neutralization reaction is not complete.
Excess acid remains because the product above,
bicarbonate, also dissociates, producing hydrogen
ions (Eq. B1.3 in Box 1.1). The additional hydro-
gen ions that do remain increase the H
+
concentra-
tion [H
+
] and lower pH, dei ned as -log
10
[H
+
]. In
summary, as CO
2
is added to seawater, there are
increases in [H
+
] and bicarbonate ion concentration
[HCO
3
-
] and simultaneous decreases in pH and car-
bonate ion concentration [CO
3
2-
].
Because the concentrations of CO
2
, HCO
3
-
, and
CO
3
2-
inl uence one another and are sensitive to
changes in temperature, salinity, and pressure, it is
fortunate for ocean scientists that the marine car-
bonate system can be dei ned in terms of two con-
servative tracers, namely total dissolved inorganic
carbon (
C
T
) and total alkalinity (
A
T
), as dei ned in
Box 1.1. Most of the total alkalinity comes from the
carbonate alkalinity (
A
C
= [HCO
3
-
] + 2[CO
3
2-
]), and
most of the remaining alkalinity comes from borate
(Zeebe and Wolf-Gladrow 2001). Linear combina-
tions of
C
T
and
A
T
are often used as convenient
approximations for concentrations of the individual
inorganic carbon species
Just as for Ω, values of [CO
3
2-
]
sat
and Δ[CO
3
2-
] differ for
each CaCO
3
mineral. When Δ[CO
3
2-
] is positive (Ω >
1), waters are supersaturated with respect to that
CaCO
3
mineral. When Δ[CO
3
2-
] is negative (Ω < 1),
waters are undersaturated and corrosive to the
same mineral. To convert between Ω and Δ[CO
3
2-
] ,
we only need to use [CO
3
2-
]
sat
=[CO
3
2-
]/Ω , which
exploits the dei nition of the solubility product
K
sp
=[Ca
2+
]
sat
[CO
3
-
] and the 'identity' [Ca
2+
]
sat
= [Ca
2+
],
where [Ca
2+
] is proportional to salinity
.
By 'identity',
it is just meant that salinity determines the open-
ocean calcium concentration, which is used with
K
sp
to determine the corresponding [CO
3
2-
]
sat
.
Thus
Δ[CO
3
2-
]=[CO
3
2-
](1
1/Ω). Although Ω has the advan-
tage of being non-dimensional,
Δ[CO
3
2-
] carries the
same concentration units as [CO
3
2-
] and [CO
3
2-
]
sat
as
well as the measured tracers
C
T
and
A
T
, from which it
is often computed.
Along with changes in carbonate chemistry vari-
ables, it is useful to quantify the changing chemical
capacity of the ocean to absorb increases in anthro-
pogenic CO
2
. One way to dei ne that capacity is to
use what oceanographers typically term the buffer
capacity, the inverse of which is the Revelle factor
R
( Bolin and Eriksson 1959 ; Broecker
et al.
1971 ;
Keeling 1973 ; Pytkowicz and Small 1977 ; Sundquist
et al.
1979 ; Wagener 1979 ; Takahashi
et al.
1980 ),
namely the ratio of the relative change in
p
CO
2
(or
CO
2
) to the relative change in
C
T
:
[ C O
2
]
»
A
T
C
T
(3.2)
[HCO
3
]
» 2
C
T
A
T
.
(3.3)
These approximations are usually good to within
about 10% (Sarmiento and Gruber 2006). Inherent in
these approximations is the assumption that CO
2
concentrations are relatively low and can be
neglected, which works well for the modern ocean.
However, in the future as atmospheric CO
2
increases
and the [CO
2
]/[CO
3
2-
] ratio approaches 1 at high lati-
tudes (Orr
et al.
2005 ), errors increase dramatically.
An important concept when studying ocean car-
bonate chemistry and calcii cation by marine organ-
∂
p
CO / CO
p
∂
ln
p
CO
(3.5)
R
=
2
2
=
2
∂
CC
/
∂
ln
C
TT
T
The Revelle factor is inversely related to [CO
3
2-
] ( Broecker
and Peng 1982) and is useful to help explain why the
air-sea equilibration time for CO
2
is much longer than
that for other gases such as oxygen (see Box 3.1).
