The Sea-water and its Physical and Chemical Properties 87 



throughout. In the polar and subpolar regions the surface minimum is largely absent 

 because of the winter convection which eliminates the minor depletion of calcium 

 carbonate by the few calcium-carbonate-consuming organisms during the brief summer. 

 In low latitudes, on the other hand, the depletion of calcium carbonate is particularly 

 pronounced due to the isolation of the upper layer by the thermocline in the tropics. 

 The regional differences in the bottom layers are shown principally by the degree of 

 saturation with calcium carbonate. 



The degree of saturation of sea-water by calcium carbonate in solution is found by 

 comparison of the actual content with the solubility of the water in situ. Calculation 

 of the degree of saturation shows that the surface water in equilibrium with the atmos- 

 phere is supersaturated with calcium carbonate at all temperatures found. In the 

 tropics and the subtropics the supersaturation is very large and the water may contain 

 up to three times more calcium carbonate than that corresponding to the equilibrium 

 value; Fig. 45 shows the percentage saturation with calcium carbonate in the surface 

 water of the Atlantic. The large supersaturation in low latitudes shows very clearly; 

 only the presence of this supersaturation allows an equilibrium between the addition 

 of calcium in river water and its consumption by various organisms and sometimes by 

 spontaneous inorganic precipitation at the bottom. It can be readily understood that 

 the production of calcium by various organisms is facilitated and favoured by this 

 supersaturation. 



Beneath the thermocline in low latitudes the saturation value falls rapidly with in- 

 creasing carbon dioxide pressure to below 100% and reaches a minimum in the inter- 

 mediate layers; in places the saturation may fall to less than 92%. While there are no 

 large differences in the deep water below 1500 m (degree of saturation 98-100%) 

 the bottom water in the Atlantic Ocean differs somewhat in saturation just as it also 

 differs in carbon dioxide content. 



Calcium is also involved in a closed cycle. In the upper layers of the sea there is a 

 strong withdrawal of calcium carbonate, partly by biological processes associated 

 with calcium using animals and plants and partly by inorganic precipitation. 



Compensation is performed at the sea surface by the supply of calcium due to river 

 water and at the sea bottom by solution from the bottom sediments. Without an 

 accurate quantitative estimation of the individual components in the cycle it is im- 

 possible to state whether supply or consumption predominates, or whether the present 

 condition of considerable supersaturation at the surface of the sea is a stationary state. 



