Geoscience Reference
In-Depth Information
therefore really constitutes a sink for CO
2
. If this loss is not rapidly
compensated by volcanic emissions, the greenhouse effect can greatly
diminish and provoke very widespread and long-lasting glaciations
(over millions of years).
Another possibility for significant and long-lasting variations of the
greenhouse effect depends on long-term variations in the alkalinity of
the ocean.
The paragraph below calls on the notion of the
conservative
chemical form in seawater. It is therefore important first to define this
notion. A dissolved chemical form is called
conservative
in seawater
if it does not transform into any other in the usual oceanic conditions.
For example, this is the case for ions such as Na
+
, Cl
-
and SO
4
2-
. In
contrast, the molecule CO
2
or the ions HCO
3
-
and CO
3
2-
can transform
through the equilibriums summarized in equation [1.4], and are called
non-conservative
.
The pH of the current ocean (pH 8.1- 8.2) is greater than that of pure
water (pH 7.0) because the slight electric imbalance (an excess in
positive charges) between the conservative cations (essentially Na
+
,
Ca
2+
, Mg
2+
and K
+
) and the conservative anions (essentially Cl
-
and
SO
4
2-
) is compensated by the slight electric imbalance (excess of
negative charges) of non-conservative ions (mainly HCO
3
-
, CO
3
2-
,
B(OH)
4
-
, OH
-
and H
+
). As the excess of positive charges is mainly
due to the alkaline cations, the concentration of charges of
non-conservative ions that compensates it is called the total alkalinity
of seawater [DIC 81, ZEE 01]. A simplified form of alkalinity for
practical use is as follows:
AP = [HCO
3
-
] + 2[CO
3
2-
] + [B(OH)
4
-
] + [OH
-
] - [H
+
]
[1.8]
The alkalinity of seawater can be significantly modified in the
long-term (thousands to millions of years), essentially because of the
variation in concentrations of conservative ions. Moreover, all things
being equal, the concentrations in non-conservative ions, so also the
pH (which reflects the concentration of ions H
+
), are only involved in
adjusting to the change in alkalinity. Nevertheless, this adjustment can
cause a variation in the greenhouse effect. Indeed, the concentrations
