Geoscience Reference
In-Depth Information
events may be studied that are unsuitable for com-
parison with future scenarios.
Another reason for studying past changes in
ocean chemistry is that it allows us to evaluate
the current anthropogenic perturbation in the
context of earth's history. For instance, we can ask
questions such as: what was the amplitude of
natural variations in ocean chemistry immedi-
ately prior to industrialization (say during the
Holocene, the past 12 000 years)? Are there past
events that are comparable in magnitude and
timescale to the present ocean acidii cation caused
by humans, or is it unprecedented in earth's his-
tory? How did the carbon cycle, climate, and
ocean chemistry respond to massive carbon input
in the past—and on what timescale was the car-
bon removed from the ocean-atmosphere system
by natural sequestration?
In this chapter, basic controls on ocean carbon-
ate chemistry on different timescales will be
reviewed. Changes in ocean chemistry during var-
ious geological eras will be investigated and past
ocean acidii cation events will be examined.
Note,
however, that this chapter's focus is on past changes in
ocean chemistry, not solely on past ocean acidii cation
events
. Evidence of biotic responses to past changes
in ocean chemistry will be discussed only sporadi-
cally here (for more information see Chapter 4).
For consistency, 'Myr' is used in this topic to
denote both geological dates and durations,
although 'Ma' is recommended to denote geologi-
cal dates.
2
+
2
-
[Ca
]
´
[CO
]
W=
sw
3
sw
(2.1)
*
sp
K
where [Ca
2+
]
sw
and [CO
3
2-
]
sw
are the concentrations
of Ca
2+
and CO
3
2-
in seawater and
K
*
sp
is the solubil-
ity product of calcite or aragonite, the two major
forms of CaCO
3
, at the
in situ
conditions of tempera-
ture, salinity, and pressure. Values of Ω > 1 signify
supersaturation and Ω < 1 signii es undersatura-
tion. Because
K
*
sp
increases with pressure (the tem-
perature effect is small) there is a transition of the
saturation state from Ω > 1 (calcite rich) to Ω < 1
(calcite depleted) sediments at depth.
2.2.2 Two parameters are required to
determine the carbonate chemistry
At thermodynamic equilibrium, the carbonate sys-
tem can be described by six fundamental parame-
ters: dissolved inorganic carbon (
C
T
), total alkalinity
(
A
T
), [CO
2
], [HCO
3
-
], [CO
3
2-
], and [H
+
] (see Box 1.1).
The concentration of OH
-
and
p
CO
2
can be readily
calculated using the dissociation constant of water
and Henry's law. Given the i rst and second disso-
ciation constants of carbonic acid and the dei ni-
tions of
C
T
and
A
T
, we have four equations with six
unknowns. Thus, if the values of two parameters
are known, we are left with four equations and four
unknowns and the system can be solved. The funda-
mental rule follows that
two carbonate system param-
eters are required to determine the carbonate chemistry
,
one parameter is insufi cient.
Ignorance of this rule has led to misinformation
in the literature. For instance, future atmospheric
CO
2
concentrations have been compared with
p
CO
2
levels during the Cretaceous (~145 to ~65 Myr),
which may have been as high as, say 2000 ppmv.
While at some point in the future, atmospheric CO
2
levels might approach values similar to those dur-
ing the Cretaceous, this by no means implies similar
surface-ocean chemistry. A surface ocean with
C
T
=
2.4 mmol kg
-1
in equilibrium with an atmosphere at
p
CO
2
= 2000 ppmv would have a calcite saturation
state Ω
c
of 1.1 (temperature
T
= 15°C, salinity
S
= 35).
However, at a higher
C
T
of 4.9 mmol kg
-1
, the calcite
saturation Ω
c
would be 4.5 (at the same
T
and
S
).
The latter example illustrates possible Cretaceous
2.2
Seawater carbonate chemistry
The basics of seawater carbonate chemistry have
been outlined in Chapter 1 and will not be repeated
here. Additional information can be found else-
where (see e.g. Zeebe and Wolf-Gladrow 2001). In
this section, we will focus on a few fundamentals
and subtleties of seawater carbonate chemistry that
will aid the discussion to follow.
2.2.1 CaCO
3
saturation state of seawater, Ω
The CaCO
3
saturation state of seawater is expressed
by Ω:
