42 



The Sea-water and its Physical and Chemical Properties 



-2 



12 16 

 t, "C 



20 24 28 



Fig. 21. Effect of changes in temperature on the density of pure water and of sea water at 



35^00 salinity. 



Thus for different salinities, where 



S in %„ = 10 20 



cmax = 000 818 16-07 



/max in °C = 3-947 1-860 -0-310 



25 30 35 40 



2010 24-15 28-22 32-32 

 -1-398 -2-473 -3-524 -45-410 



Since water is compressible, though only slightly, the density depends on the pressure. 

 In the deeper parts of the ocean the pressures are enormous and have a considerable 

 effect on the density of the water. The change in unit volume of a material per pressure 

 unit is termed its compressibility coefficient /x. If the pressure unit is taken as 1 bar 

 (= 10*' dynes/cm^) then the compressibility coefficient of sea-water is of the order of 

 magnitude of 450 x 10"'; it increases somewhat with increasing pressure, increasing 

 salinity and increasing temperature and its extreme values lie somewhere between the 

 limits 510 and 390 X 10~". Ekman (1908) derived a precise empirical formula for the 

 effect of pressure on the density that takes into consideration the changes in the 

 compressibility coefficient with salinity and temperature (see Landholt-Bornstein, 

 1952; Dietrich, p. 484). This gives the density of sea-water for a given salinity, given 

 temperature and a fixed pressure and thus gives the density in situ a,^ ^ of a water 

 sample directly from a,. 



Bjerknes and Sandstrom (1910) have presented complete tables to allow the spe- 

 cific volume anomaly or the density to be quickly found from the basic values for a 

 homogeneous sea at 0°C and with 35%o S for depths down to 10,000 m or pressures 

 of 10,000 decibars. Hesselberg and Sverdrup (1915) have given a method by which 

 the vertical variations in density can be calculated in a fairly simple way from the 

 temperature and the salinity. This simplification is due largely to the elimination of 

 part of the work by starting in the first place from the value for a,. If only the anomaly 

 is required, the tables prepared by Sverdrup (1933), which are still further simplified 

 and which give more accurate results, can be used. In general the relation a^ j^ = 

 ^35, 0. + S can be used where S is the specific volume anomaly. 5 is the sum of three 

 terms: 5 = A,j + Sgj, + S,,p. As shown by the indices the first term depends on the 

 temperature and the salinity, the others depend on the pressure and on one of the 

 other two factors each. 



