272 Oxygen and Carbon Dioxide 



with shells nearly as heavy as in most alkaline waters. These animals 

 were also found in water of pH 5.1 with as little as 1 mg Ca per liter, 

 but the shells were poorly calcified. 



Since the ocean tends to be relatively constant in regard to many 

 of its ecological factors, we might expect to find little variation in 

 the conditions of lime formation in the marine environment. Ac- 

 tually, however, sea water is so nearly saturated with Ca+^ and COa" 

 ions that slight changes in temperature, pH, and carbon dioxide are 

 sufficient to throw the equilibrium just above or just below the satura- 

 tion point. The colder and less alkaline parts of the sea are under- 

 saturated in respect to Ca++ and COs^, and the water can take up ad- 

 ditional amounts; the warmer regions of the ocean are supersaturated 

 and CaCOs is readily formed, chiefly through the activity of various 

 biological agents. The circulation of water in the great ocean basins 

 is thus accompanied by a cycle in which Ca^+ and COa^ ions tend 

 to be taken up in polar or deep water, and lime deposits tend to ac- 

 cumulate in shallow areas of the tropical seas (Fig. 7.10). As surface 



Fig. 7.10. Schematic longitudinal section of northern half of Atlantic Ocean, in- 

 dicating the cycle of CaCOa. The arrows indicate the north-south and vertical 

 components of the oceanic circulation. 



water moves toward the poles it becomes undersaturated in spite of 

 the new quantities of Ca++ and CO:r added by run-off and river dis- 

 charge, and the deep return flow remains undersaturated because of 

 the prevailing low temperatures and low pH values. But, as the 

 water rises toward the surface in the tropics, the increase in both tem- 

 perature and pH bring about saturation and then supersaturation. 



