Geology Reference
In-Depth Information
P
CO
2
and a fall in pH. Thus, the depth profiles of pH would mimic that
of O
2
but the
P
CO
2
would exhibit a maximum at the oxygen minimum.
There are further confounding influences, in particular concerning the
solid phases of CaCO
3(s)
. Many organisms make calcareous shells
(testes) of CaCO
3(s)
in the form of either aragonite or calcite. The shells
sink and dissolve when the organism dies. The solubility is governed by
Ca
2
þ
þ
CO
2
3
Ð
CaCO
3
ð
s
Þ
Surface waters are supersaturated with respect to CaCO
3(s)
, but precip-
itation rarely occurs, possibly due to an inhibitory effect by Mg
21
aq
forming ion pairs with CO
3
2
aq
. The solubility of CaCO
3(s)
increases
with depth, due to both a pressure effect and the decrease in pH
following respiratory release of CO
2
, causing the shells to dissolve. This
behaviour not only increases the alkalinity, but also accounts for the
non-conservative nature of Ca
21
and inorganic carbon in deepwaters.
The depth at which appreciable dissolution begins is known as the
lysocline. At a greater depth, designated as the carbonate compensation
depth (CCD), no calcareous material is preserved in the sediments. The
depths of the lysocline and CCD are influenced by the flux of organic
material and shells, and tend to be deeper under high productivity zones.
The CO
2
and CaCO
3(s)
systems are coupled in that the pH buffering in
the ocean is due to the reaction
CO
2
þ
H
2
O
þ
CaCO
3
ð
s
Þ
Ð
Ca
2
þ
þ
2HCO
3
In addition to the effects noted previously, an input of CO
2
promotes the
dissolution of CaCO
3(s)
. The reaction does not proceed to the right
without constraint, but rather meets a resistance given by the Revelle
factor, R
dpCO
2
=
pCO
2
R
¼
d
P
CO
2
=
P
CO
2
ð
4
:
14
Þ
This value is approximately 10, indicating that the ocean is relatively
well buffered against changes in
P
CO
2
in response to variations in
atmospheric pCO
2
. Although the ocean does respond to an increase in
the atmospheric burden of CO
2
, the timescales involved are quite
considerable. The surface layer can become equilibrated on the order
of decades, but as the thermocline inhibits exchange into deepwaters, the
equilibration of the ocean as a whole with the atmosphere proceeds on
the order of centuries. The ventilation of deepwater by downwelling
