.qh'^o'*' |-^ 



'^''A/\ f ^°°° I 

 'oAL in r,r\riJL 



'?^>'-IO,000- 



RG. 28-1 Subdivisions of the ocean blocycle (Hedgpeth 1957). 



ooze and to furnish food for the Hving animals that 

 spend their lives in this habitat. 



Pelagic deposits that contain less than 30 per cent 

 of organic remains are known as red clay. These de- 

 posits are the most widely spread of all, especially at 

 the greater depths of the ocean. They are probably 

 derived from wind blown desert dust, terrestrial vol- 

 canic dust, and submarine eruptions. The very hard 

 earbones of whales and teeth of sharks are regularly 

 found in red clay. Animal life is scant, consisting 

 only of shellless holothurians and worms, probably 

 because of the poor nutrient content and great depth. 



Organic deposits are either calcareous or silice- 

 ous, the former being derived from the shells of 

 foraminiferans, small pelagic mollusks, or flagellate 

 coccolithophorids, and the latter from skeletal mate- 

 rial of diatoms and radiolarian protozoans. 



Pressure 



There is an enormous increase in the pressure 

 of water upon the bodies of animals at great depths. 

 This is not, however, an important limiting factor in 

 the vertical distribution of animals in general, as 

 internal pressures closely counterbalance external 

 pressures and life is known to exist at the greatest 



depths. Adjustments of internal pressures are not so 

 rapid, however, to prevent injury in many species 

 that are dredged at great depths and quickly hauled 

 to the surface. Furthermore, individual species have 

 different limits of pressure tolerance. 



Temperature 



The temperatures of surface waters vary be- 

 tween the freezing point ( — 1.9°C) in polar regions 

 and 25°-30°C in the tropics. Seasonal variations are 

 small in polar and tropical waters but somewhat 

 greater in the temperate zones. 



Temperature varies with depth, more so in the 

 tropics than elsewhere. At 60 °N latitude in the At- 

 lantic Ocean, the mean temperature of the warmest 

 and coldest months at meter is about 10°C, while 

 at a depth of 2000 meters it is 3.5°C. On the equator 

 the temperature at meter is approximately 26 °C, 

 at 200 meters 13°C, at 400 meters 7.5 °C, at 1000 

 meters 4.5°C, and at 2000 meters 3.3°C (Ekman 

 1953). This temperature decrease, known as a ther- 

 mocline, is a permanent feature of the tropics. A 

 permanent thermocline also exists at mid-depths in 

 temperate and subtropical waters. Superimposed 

 upon it is a seasonal thermocline that develops near 



Marine biomes 353 



