The Sea-water and its Physical and Chemical Properties 83 



in the atmosphere the present carbon dioxide content would be doubled in 500 years. 

 In actual fact if there is a pressure difference between the ocean and the atmosphere 

 the sea takes up carbon dioxide until this difference vanishes. Buch (1939) has cal- 

 culated that if the ocean and the atmosphere are always in equilibrium then five-sixths 

 of the carbon dioxide produced is absorbed by the sea while only one-sixth remains 

 finally in the atmosphere. Thus, if the sea absorbs the industrial carbon dioxide so 

 rapidly that equilibrium is always maintained, then its present content would double 

 at first in 3000 years. 



However, some time is needed to reach a new equilibrium and this is probably not 

 reached as quickly as is customarily assumed. The cause could lie in the very slow 

 vertical circulation within the ocean. In a short time only a very thin contact layer can 

 interchange with the atmosphere. The equihbrium time for the whole volume of the 

 ocean should certainly be more than several thousand years, and it must also be 

 remembered that the initial pressure differences are very small and at first rise only 

 slowly. According to the investigations of Buch in the North Atlantic in summer 1935 

 and in the sub-arctic regions in summer 1936, this part of the ocean and of course the 

 corresponding region in the Southern Hemisphere appear to be the only areas where 

 over long periods carbon dioxide is absorbed from the air in water masses which, by 

 convective sinking in the autumn and winter, convey it to the rest of the ocean. Only in 

 these layers is a rapid renewal of the surface water to be expected and these are thus 

 the principal sites of equilibration in the carbon dioxide interchange between the ocean 

 and the atmosphere (see also Buch, 1948). 



At the present time insight into the dynamics of these processes is rather inade- 

 quate due to the scarcity of carbon dioxide pressure determinations. Extensive syste- 

 matically collected series observations are needed for a better understanding of these 

 phenomena. A more accurate investigation of the distribution of carbon dioxide in an 

 adjacent sea (the Baltic) has been described by Buch (1945). 



(c) Calcium Carbonate in the Sea 



The solubility of calcium carbonate in water increases with the carbon dioxide 

 content. This can be explained chemically as follows: calcium carbonate in solution 

 is almost completely dissociated into Ca^^ and C0|" ions according to the equation 



CaCOg ^ [Ca2+] + [CO^-]. 



Since the concentration of undissociated calcium carbonate is very small and, if the 

 sea-water is saturated, must be constant, the solubility product is given in a first ap- 

 proximation by 



[Ca2+] . [CO^-] = Ki 



In the carbon dioxide equilibrium shown on p. 75 most of the hydrogen ions present 

 combine with the carbonate ions to form bicarbonate ions since the bicarbonate ion, 

 HCO^, is dissociated only to a small extent. This alters the calcium carbonate equili- 

 brium, and calcium carbonate will thus go into solution until [Ca^+] increases suffi- 

 ciently to satisfy the equilibrium equation. The equilibrium thus depends on the con- 

 centrations of all the ions, H+, HCOg", CO^- and Ca^^ involved (Wattenberg, 1933, 

 1936). 



