THE PHYSICAL CHEMISTRY OF SEA WATER 561 



Actually, log [HCO.r] = —2.63 in sea water, which indicates 

 that it w^ould not be far from equilibrium with CaCOa at some 

 temperature around 5°C, which is in the neighborhood of the 

 average temperature, especially close to the bottom of the ocean. 



When water from deeper levels approaches the surface, the 

 pressure is decreased and the temperature usually increased. For 

 both reasons, the w^ater becomes supersaturated with CaCO.j. 

 However, we shall not step aside to discuss the transport phenom- 

 ena involved but pursue our imaginary equilibrium experiment.* 



By applying the equilibrium constants given above, and the 

 known pH and [Ca++], we calculate log [H2CO3] = —4.55; log 

 [CO3 — ] = —3.68; log pco2 = —3.28. The value for log pco^ 

 agrees rather well with the average value for the atmosphere, 

 around —3.52, considering the uncertainties involved in the 

 calculation. 



It may be noted that if the equilibria are considered this way, 

 the carbonate system becomes only an indicator for the pH, 

 whereas pH is really determined by the silicates. If there had been 

 no carbonate in the solution, the role of the indicator would have 

 been taken over by the next available acid-base pair, borate or 

 e\^en dissoh^ed silicate. 



Let us suppose that we release rather suddenly a large amount 

 of CO2 — for instance, that we double the amount of COo in the 

 gas phase, which is l3y now about O.O-t niM /liter sea water. This 

 might, for the next few decades, mean a measurable decrease in 

 the pH of the surface water of the ocean, perhaps by as much as 

 0.2 unit. However, the processes would start to work toward 

 equilibrium: the H2CO3 would spread out over the whole bulk of 

 the ocean, CaCOs would dissolve, and there would be a readjust- 

 ment of the silicate equilibria. Considering the enormous buffering 

 capacity of the silicates — of the order of 1 mole/liter sea water in 

 comparison with 0.00004 mole CO2 — the final change would 

 probably not be perceptibly different from the initial one. 



* The solubility increases exponentially with pressure (its logarithm increases 

 linearly). A detailed study of the equilibria and kinetics involved would probably 

 explain the abrupt decrease of CaCOs content in sediments at depths around 

 4500-5000 m, described in Dr. Bramlette's paper (p. 345). 



