4 UNITY AND DIVERSITY IN BIOCHEMISTRY 



is the case it is generally agreed today that the oceans have not changed 

 appreciably in composition since the Archaean period. 



Together with Conway (1943) we may advance four hypotheses to 

 account for the chemical evolution of the oceans. 



1. The water results from the condensation of water vapour from the 

 primitive atmosphere whilst the chlorides have been added gradually over 

 the ages (constant volume, volcanic chlorides). 



2. Both water and chlorides are the result of an initial condensation 

 (constant volume, constant chloride). 



3. The water and the chlorides have been progressively accumulated 

 (both oceans and chlorides of volcanic origin). 



4. The chlorides were initially present (metallic chlorides in the surface 

 of the earth), the water being gradually added as a result of volcanic action 

 (volcanic oceans, constant chloride). 



At the present time the third hypothesis is the one favoured by geo- 

 chemists. According to this view the mass of substances dissolved in the 

 sea arises from erosion of the terrestrial crust. The total of these substances 

 is enormous. If all the sea water were evaporated, it would form a layer of 

 salt 153 m thick covering the surface of the continents. Goldschmidt 

 calculates that for each litre of sea water 600 g of igneous rock have been 

 dissolved. In other words, for each square centimetre of the earth's crust, 

 during the formation of the oceans, erosion has removed 160 kg of igneous 

 rock. The greater part of this has gone to form the sedimentary rocks. 



The biosphere is chronologically after the lithosphere, the hydrosphere 

 and the atmosphere. Although occupying a portion of all three regions, it is 

 discontinuous and comprises the total mass of organisms. This definition is 

 additional to what has already been said about the vital nature of the bio- 

 sphere, its location and its discontinuous character. The mass of the 

 biosphere is much less than that of the hydrosphere or the atmosphere. 

 According to Rankama and Sahama (1950), the relative weights are as 

 follows : 



Hydrosphere 69,100 

 Atmosphere 300 



Biosphere 1 



But if the biosphere is quantitatively insignificant, nevertheless it is the 

 centre of considerable chemical activity and it can be calculated that in the 

 course of the last 500 million years, that is since the appearance of the 

 Trilobites, it has "metabolized" a mass of material equal to the total weight 

 of the globe. 



One can obtain an idea of the size of the biosphere by calculating the 

 annual production of organically bound carbon per square kilometre of the 

 earth's surface. Riley obtains a figure of 160 metric tons on land and 340 



