72 The Sea-water and its Physical and Chemical Properties 



atmosphere) contains 0-42 ml/1, of free carbon dioxide, which is very little. In water in 

 the uppermost layer of the open ocean the carbon dioxide content is usually not far 

 from the equilibrium value with the atmosphere. 



According to Krogh's measurements in the North Atlantic the value for the carbon 

 dioxide pressure varies between 1-55 x 10^*and2-9 x 10~^. According to Brennecke's 

 values in the Weddell Sea ("Deutschland" Expedition) the carbon dioxide pressure 

 was higher than that in the atmosphere. Carbon dioxide in solution comes only slowly 

 to equilibrium with the atmosphere. Detailed investigations along this line have 

 been made by Buch (1917), in the waters around Finland, by Schulz (1923) in the 

 Baltic, by Wattenberg (1933) on the "Meteor" Expedition (1925-7 principally 

 between Africa and South America), by Deacon (1934) especially in the Arctic and 

 Antarctic regions and finally by Buch (1939, 1939^) in the North Atlantic and on a 

 cruise in the Arctic. All these measurements of the carbon dioxide pressure show 

 variations around the equilibrium position, sometimes the pressure in the water 

 is higher than in the atmosphere and at other times it is lower. These variations,, 

 however, are small as in the course of the long time which has been available, sea and 

 atmosphere have come into a mutual adjustment. Wattenberg (1936), from the ob- 

 servations available, arrived at the following conclusions (Fig. 39): 



(1) There are limited areas of the sea where the carbon dioxide pressure of the water 

 is definitely greater than that of the air; these are principally places where rising water 

 currents bring water rich in carbon dioxide to the surface from intermediate layers 

 rich in carbon dioxide (west coasts of North and South Africa and of North and 

 South America). 



(2) In other places there are, however, large areas where the carbon dioxide pressure 

 is somewhat less than the normal partial pressure of the atmosphere. These occur 

 especially in temperate and cold zones during the spring and summer, when rich plant 

 plankton is actively assimilating. There may be pronounced annual changes here in the 

 carbon dioxide pressure at the surface of the sea: a strong reduction in spring at 

 the beginning of diatom development and a gradual rise in autumn when dead 

 organisms start to decompose. See p. 77 for the distribution of carbon dioxide in 

 deep water and at the sea bottom. 



Total carbonic acid. If the sea was neutral it would contain little carbon dioxide. 

 Sea-water is in fact alkaline and has a total carbon dioxide content that is much greater 

 than would be concluded from the carbon dioxide pressure. By far the largest part is 

 chemically combined in the sea salt. 



The total amount of carbon dioxide present depends on the one hand on the car- 

 bon dioxide pressure and on the other on the amount of base available for combina- 

 tion with the carbon dioxide which is termed the alkalinity. Since the carbon dioxide 

 pressure is small, there is an almost linear relationship between the total amount of 

 carbon dioxide present and the alkalinity, and thus also the salinity since the 

 alkalinity is dependent very largely on this. Thus Buch (1914) for the Pojowick under 

 average conditions found the relationship 



A = 0-07 + 1-00 CO, and COg = 0-32 - 0-1735' 



where CO, is expressed in millimoles/litre and A in milliequivalents. Similar relation- 

 ships were also derived for the Gulf of Finland and the Gulf of Bothnia. 



