PHOSPHORUS AND LIFE — HOPKINS 



235 



The second of these three influences, precipitation, is largely caused 

 by the release of calcium in the decomposition of marine organic resi- 

 dues. When calcium and phosphate ions meet, insoluble calcium 

 phosphate is formed. This steadily precipitated phosphate enters the 

 composition of sedimentary rock; again it eventually becomes a more 

 complex mineral phosphate of the apatite class. Here there is an im- 

 portant connection between the marine and land cycles of phospho- 

 rus. Substantial areas of what is now the land surface of the world 

 were once covered by seas ; indeed, their rock and subsoils have a much 

 longer past history as seabeds. There are, therefore, two kinds of 

 complex mineral phosphate on land — the phosphates of igneous rock 

 originating directly from solar phosphorus and the phosphates of 

 sedimentary rock which have formerly passed through the marine 

 cycle. 



IGNEOUS ROCK ON LAND 



1 



DILUTE SOLUTION. OF PHOSPHATE 



i 



PLANKTON*- 



4- 



ZOOPLAMKTOM 



Returned to 

 UPPER. WATERS 

 by INVERSION 



DEEP WATER PHOSPHATE 

 from decay of dead sea life *~ 



(Precipitation) 



Temporary use 

 by crustacean 

 sea floor life 



CALCIUM. PHOSPHATE ENTERING SEDIMENTARY ROCK 

 Figure 1. — Simple diagram for marine cycle of phosphorus circulation. 



The third influence by which phosphate accumulations are removed 

 from deep-sea waters, by the inversion of upper and lower sea layers, 

 is not yet fully understood. In all seas the surface w T aters are warmed 

 by the sun and they therefore become lighter so that, without some in- 

 tervening disturbance, the colder and heavier water below cannot rise. 

 In marine science the boundary between the upper part, which extends 

 some 40 yards down, and the cold lower part is known as the thermo- 

 cline. In temperate regions the autumn cooling of upper waters leads 

 to a breakdown of the thermocline and there is a seasonal rising of the 

 lower and nutrient-rich waters; but in tropical regions the surface 

 waters remain permanently warmer and lighter and there is no sea- 



