26
F.J. Millero
crease occurs in the evening due to the respiration of organisms, and the increase occurs in the afternoon due to photosynthesis. Changes in the pH with depth in the Atlantic and Pacific Ocean are shown in Fig. 1.13. Although it is difficult to see in this
figure, the pH goes through a maximum in surface waters due to photosynthesis. The
loss of CO2 increases the pH. The pH then decreases due to the oxidation of plant
material and goes through a minimum at about 1 km. This minimum coincides with
the O2 minimum and the maximum in Pe0 2
The pH increase in deep waters is due to the dissolution of CaC03• The pH of deep
waters can be as low as 7.5 near 1000 m. In very deep waters the pH can go through a
maximum due to the effect of pressure on the ionization of carbonic acid. Park (NOAA)
has used the Redfield model to calculate the pH as a function of depth. He attributed
the changes to two factors
where L1pH(a) = -2.0 AOU, the decrease due to the oxidation of plant material (AOU is
the apparent oxygen utilization) and L1PH(b) = 2-4 L1Ca (where L1Ca is the change in
Ca 2 + due to the dissolution of CaC0 3 ).
1.4.3
TA
The total alkalinity (TA) of surface and deep waters was measured extensively during the
recent global change programs. The normalized total alkalinity (NTA = TA x 351 S) for
surface waters in the Atlantic and Pacific oceans are shown in Fig. 1.14. The normalFig. 1.13. Depth profile of
pH in the Atlantic and Pacific
oceans (Millero 1996)
1000
:§: 2 000
..c:::
C.
~ 3 000
4000
5000
pH
7.6
7.8
8.0
8.2
-0- Atlantic Ocean
- . - Pacific Ocean