108 



Water 



of temperature with depth of less than 

 0.0 rC/ 100 meters that can be attributed to 

 adiabatic heating. Both the mixing and the 

 adiabatic gradients are small compared with 

 the decrease of temperature at the same 

 depth range in the open sea. Accordingly, 

 the temperature depth curves of the basins 

 depart markedly from that of the open sea, 

 causing the basin floors to be covered by 

 water 0.8 to 2.0° C warmer than that at the 

 same depths in the open sea. Because of 

 the wide variation in depth of the basin sills, 

 the temperature in diff'erent basins ranges 

 between 2.52 and 6.26° C, about 200 times 

 the error of its measurement. 



Salinity within the basins is about the 

 same as that of the open sea at the same 

 depth as that of the basin sills, and it re- 

 mains nearly uniform from the sill to the 

 bottom depth of each basin (Emery, 1954c). 

 The salinity range of the diff'erent basins is 

 34.25 to 34.58%o, only about ten times the 

 error of measurement. This small range of 

 the basin water salinities and the fact that 

 the salinity at the basin floors is less than 

 0.20 %o lower than that of the open sea at 

 the same depths are results of the very grad- 

 ual change of salinity with depth in the open 

 sea, as compared with the change of tem- 

 perature with depth. 



Oxygen in the open sea is saturated near 

 the surface at about 6 ml/liter, decreases to 



a minimum of about 0.5 ml/liter at 500 to 

 700 meters, and increases with depth below 

 700 meters to about 2.0 ml/liter at 2000 

 meters (Fig. 97). This depth variation is 

 produced by photosynthesis and exchange 

 with the atmosphere near the surface, rapid 

 oxidation of easily oxidized organic matter 

 falhng through waters at intermediate depth, 

 and slow oxidation of the more resistant 

 organic matter that reaches greater depth. 

 The basin waters have approximately the 

 same oxygen content as does the water out- 

 side the basins at the same depths as the 

 basin sills. Because the shallowest basins 

 have sills at depths within the oxygen min- 

 imum, their waters have oxygen contents of 

 only 0.2 or 0.3 ml/liter in contrast to about 

 2.0 ml/liter for waters in the deepest basins. 

 These contents represent only 1 to 7 per 

 cent of saturation values in water of the 

 same temperature and salinity at the surface. 

 The nutrients, phosphate-phosphorus, 

 nitrate-nitrogen, and silicon, have been 

 measured in only three of the shallowest 

 basins (Rittenberg, Emery, and Orr, 1955), 

 and the values are nearly the same as those 

 found in the open sea at the same depths. 

 Relatively little variation in phosphate and 

 nitrate is found from basin to basin because 

 in the open sea the concentrations of these 

 nutrients are nearly uniform below about 

 800 meters. A much greater variation 



