420 BOUNDARIES OF THE SEA 



of the oceans, the so-called mixed layer which contains as much 

 total carbon dioxide as the atmosphere. Then the rate of hydration 

 must be limiting and we can actually compute this rate. The rate 

 of hydration can be expressed as k-mco^, where mco2 is the amount 

 of free carbon dioxide in the mixed layer. This is about 1/150 of 

 the atmospheric carbon dioxide. In order to keep pace with the 

 rate of exchange with the atmosphere, k must be about 150/7 

 years"^ This figure is far too small. Under laboratory conditions 

 it would take at least a week to reach equilibrium in such a system, 

 and this has certainly never been observed. Observations by Taka- 

 hashi (1959) indicate that equilibrium for sea water is reached in 

 about 20 min. We can then discard this possibility. 



The other extreme assumption is that the rate of hydration is 

 relatively rapid compared with the rate at which all carbon dioxide 

 species molecular and ionic are mixed into the subsurface water. 

 For this mixing we can use a vertical transport velocity analogous 

 to that in the air. The apparent transport velocity of carbon dioxide 

 in the atmosphere (using a residence time of 7 years) is equal to 

 about 3 X H)-^ cm-sec-^ As the volume concentration of total 

 carbon dioxide in the sea is about 150 times greater than that in 

 the air, the corresponding transport velocity from the sea surface 

 into subsurface water should be (3/150) X 10-^ ~ 2 X 10-^ cm- 

 sec~^ This figure seems extremely low. In fact, it is so low that, 

 considering net evaporation of water, it would result in a salinity 

 difference between the surface and subsurface of about 50%. The 

 assumption of a relativ^ely rapid hydration of carbon dioxide in the 

 sea is therefore highly unlikely. 



As a third hypothesis we assume that the rate of hydration is 

 slow enough to prevent any appreciable exchange between different 

 carbon dioxide species taking place while in contact with the at- 

 mosphere. This means that the transport velocity in the sea should 

 be the same as the apparent transport velocity of carbon dioxide 

 in the air, i.e., -^3 X 10-^ cm see-'. This is because the volume 

 concentration of free carbon dioxide in the sea and in the air is 

 about the same. No serious objections can be raised against this 

 value for the transport velocity in the sea, so we may accept it 

 provisionally. Recently Kan wisher of the Woods Hole Oceano- 



