3d. Computation of the Alkalinity in Seawater

The alkalinity of seawater has been computed using the observed values of temperature, salinity, pCO2 and the concentrations of total CO2, silicate, and phosphate. For our computation, the total alkalinity (TALK) in seawater is defined by:

TALK = Ac + Ab + Asi + Ap + Aw

where

Ac  =  Carbonate alkalinity = [HCO3-] + 2[CO3=],
Ab  =  Borate alkalinity = [H2BO3-],
Asi  =  Silicate alkalinity = [H3SiO4-],
Ap  =  Phosphate alkalinity = [H2PO4-] + 2[HPO4=] + 3[PO4-3],
Aw  =  Water alkalinity = [OH-] - [H+].

The total concentration of borate (TB) has been assumed to be proportional to salinity: TB (µmol/kg) = 410.6*(Sal/35). The borate alkalinity ranges between about 40 µeq/kg for deep waters and 100 µeq/kg for surface waters. Since the silicate concentration may be ashigh as 150 µmol/kg in deep waters, the silicate alkalinity is as high as 6 µeq/kg for deep water but it is negligibly small for surface waters. The phosphate alkalinity ranges from 0.5 µeq/kg for surface waters to about 5 µeq/kg in deep waters. The following apparent dissociation constants of acid in seawater were used; Merhbach et al. (1973) for carbonic acid; Lyman (1956) for boric acid; Kester and Pytkowicz (1967) for phosphoric acid; Ingri (1959) for silicic acid; and Millero (1979) and Culberson and Pytkowicz (1973) for water. The expressions used to compute these constants as a function of temperature and salinity and the computational scheme are described in Peng et al. (1987).


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