Some indication of regions of relative sound-scattering 

 strength in the ocean may be obtained from figure 3-2, which shows 

 the geographic distribution of transparency, since transparency in the 

 open ocean would be affected by the density of plankton as well as 

 solid particles. The Sargasso Sea region, for example, is exceptionally 

 clear, whereas the upwelling regions near Iceland are rated only clear. 



4. Chemical and Physical Properties of Seawater 



a. Composition 



Seawater is a dilute solution of salts. Its composition in 

 terms of ion concentrations is given in figure 4-1. Four ions (Na+, 

 Mg++, CI", and S04~) constitute 97 percent of the total. The formal 

 definition of salinity follows: "Salinity is the total amount in grams 

 of solid material dissolved in 1 kg of seawater when all carbonate has 

 been converted to oxide, all the iodine and bromine have been replaced 

 by chlorine, and all organic matter has been completely oxidized" 

 (fig. 4-2). It is expressed in grams of salt per kilogram of seawater 

 (parts per thousand by weight of seawater is represented by the symbol 

 °/oo). An empirical relationship between salinity and chloride content 

 (chlorinity) is used to convert a measurement of chloride content to 

 salinity, 



S °/oo = 1.80655 CI °/oo. (S = 35.00 °/oo for CI = 19.37 °/oo) 



Proportions of the different constituents are nearly constant 

 in the open sea, although the total salinity varies between about 33 

 and 38 °/oo, depending upon precipitation, evaporation, and ice melting. 

 Considerably lower salinities may be observed in coastal regions and in 

 almost-closed seas with low evaporation and a considerable inflow of 

 fresh water, e.g., the Baltic, where the salinity may be as low as 5 

 to 8 o/oo. Conversely, with little inflow and high evaporation, as in 

 the Red Sea and Persian Gulf, salinities reach as high as 40 °/oo. 

 Nevertheless, the assumption of a mean salinity of 35 °/oo for seawater 

 (90 percent of the ocean differs by less than 3 percent from this value) 

 is adequate for many applications where high precision is not required 

 and to define many of its physical properties. 



Trace quantities of at least 50 of the elements have been 

 identified in seawater. Surprisingly, the sea is very poor in iron. 

 Significantly, copper replaces iron in the blood pigment of many marine 

 animals. Sea animals and plants sometimes concentrate trace elements 

 in their systems way out of proportion to the natural concentration. 

 For example, seaweeds can concentrate in their tissues the small amount 

 of iodine in the sea to the point where they may be harvested to obtain 

 commercial quantities. In some fish, concentration of heavy metal 

 pollutants in their systems makes them poisonous to eat. Considerable 

 attention has been given in recent years to the radioactivity of sea- 

 water, the radium content averaging about 0.07 x 10~12 o/qo, which is 

 about 10~^ of that in rocks of the Earth's crust. 



10 



