289
The Carbonate System
occurs because HCO 3
– is a major constituent of seawater and the ratio of HCO 3
– to salinity
is nearly constant. The nonlinear behavior near salinity of 35 is due the upwelling of deep
waters that have a higher TA because of the dissolution of CaCO 3 . One can account for the
changes in TA from changes in salinity by converting it to a constant salinity (S = 35). This
is done by determining the normalized total alkalinity (NTA) given by NTA = TA × 35/S.
Values of NTA for open surface waters of the Atlantic and Pacific are all near 2300 μmol
kg –1 (Figure 7.25). The values of NTA are higher in colder waters and nearly the same in the
Atlantic and Pacific. This is related to the near- conservative behavior of HCO 3
– in seawater.
The values are higher in polar waters (TA = 2380 μmol kg –1 ). The higher values are related
8.2
Pacific
Latitude
8.1
8.0
7.9
pH (in situ)
7.8
7.7
7.6
–70 –60 –50 –40 –30 –20 –10
0
10
20
30
Figure 7.20
The pH of surface waters in the Pacific Ocean.
pH
7.4
7.6
7.8
8.0
8.2
Depth (m)
0
1000
2000
3000
4000
5000
N. Atlantic
N. Pacific
Figure 7.21
Depth profile of pH in the Atlantic and Pacific Oceans.