250 Oxygen and Carbon Dioxide 



cold water that sinks to great depths may be understood eventually 

 to find its way to the surface again in the central part of the ocean, 

 and a return flow of water from the equatorial regions toward the 

 poles is brought about as a part of the horizontal circulation of the 

 upper water masses. 



1000 



2000 



3000 



4000 



50no 



70° S 60° 50° 40° 30° 20° 10° S 0° N 10° 20° 30° 40° 50° 60°N 

 Fig. 7.5. Cross section of the Atlantic Ocean, showing the distribution of oxygen 

 in cubic centimeters per liter. The north-south and vertical components of the 

 oceanic currents are indicated by the arrows. ( Modified from Sverdrup et al., 



1942, after Wiist.) 



Sea water near the surface in tropical regions contains about 4 cc 

 of oxygen per liter, but as it becomes colder at higher latitudes, 

 oxygen is absorbed from the atmosphere and produced by photo- 

 synthesis until a concentration of about 8 cc per liter is attained. 

 When the water masses leave the surface and sink toward the bottom, 

 depletion of oxygen begins as a result of respiration and decomposi- 

 tion, but as much as 5 cc Oo per liter remains when the water reaches 

 the ocean abyss. Animals living at the bottom of the sea thus 

 receive oxygen via a direct flow of water from the enriched polar 

 seas, but this is perhaps the slowest delivery service in the world. 

 Calculations based on the rate of consumption of oxygen and also on 

 the relative proportion of C14 in deep water indicate that movement 

 of Arctic surface water to deep layers at midlatitudes may require a 

 thousand years or more. 



The concentration of oxygen at any point in deep water depends 

 upon (1) the amount that the water contained when it left the 

 surface, (2) the rate at which oxygen is used up en route by respira- 

 tion and decomposition, and (3) the time that has elapsed. Mini- 

 mum values occur at positions in the ocean at which the combined 

 effect of the foregoing factors has reduced the oxygen to the greatest 

 extent. The lowest oxygen values are not found at the bottom but 

 at certain intermediate regions, as indicated in Fig. 7.5. In no part 



