Subject: Seawater salinity

Dagmar Fertl (
Thu, 15 Apr 1999 09:26:06 -0400

     I am a Science teacher and I realize that the Atlantic is more saline 
     than the Pacific.  I also realize that this is mostly due to increased 
     erosion (initially), higher surface evap. and lower runoff 
     contribution in addition to the influence of very saline seas such as 
     the Mediterranean and the Caribbean in addition to other factors 
     including the vast amount of frozen sea water in the North Atlantic.  
     However I do not know why at 0 C (or slightly lower since salt lowers 
     the melting/freezing point of ice) when the ocean water freezes it 
     leaves behind its salt to increase the salinity of the unfrozen water. 
     Salt water does freeze as such at lower temperatures (I.E. Soup 
     freezes in the freezer- although is this because it is much less 
     saline than 35ppth).  Would you please explain this to me? 
     Thanks!  Karen Hoffman

______________________________ Reply Separator _________________________________

I had to go to the office oceanographer for help with your question, and we had 
a little bit of a hard time figuring out exactly what point you were making, so 
hopefully below will give some background info for folks wondering about your 
question, as well as the answer (we hope!).

Salinity is the amount of solids (in grams) in one kilogram of seawater, or 
parts per thousand (ppt). The average salinity of the Atlantic is slightly 
higher than the other two major oceans, the Indian and the Pacific Oceans 
(34.90, 34.76, and 34.62 ppt, respectively).  The average salinity of the world 
ocean is 34.72 ppt which represents a total salt content of 4.8 x 1016 tons of 
sodium, chloride, and a host of other ionic species!   Normal open ocean 
salinity can vary from 33 to 37 ppt (usually written as o/oo).  But the Red Sea 
typically has salinities >40 o/oo, and in localized habitats (e.g. around brine 
seeps) S can exceed  300 o/oo.  Other large bodies of water such as the Gulf of 
Bothnia and the Baltic Sea have salinities <10 o/oo and in the low 20's, 
respectively.  So the salinity differences in the major oceans are, for most 
purposes of comparison, insignificant.  In fact, because of its vast extent and 
great depth, the Pacific contains far more salt than other oceans.  Physical 
oceanographers map major water masses in terms of their salinity AND 
temperature, or "S-T" ranges.  As you suggested,  the coldest and saltiest (and 
therefore the densest) sea water forms as seawater freezes in the Antarctic.  As
the water freezes, the salt exedues out and creates that hypersaline water that 
you're talking about.  This water (the Antarctic Bottom Water mass, "AAB") can 
be found at abyssal depths as far north as off the east coast of North America!

Locally or over great distances and from one depth to another, salinities and 
temperatures vary significantly for many reasons; among them, evaporation, 
freezing, and dilution.  Your soup freezes simply because the salinity is not 
high enough, the temperature is low enough, and the pressure is only one 
atmosphere.  The salt doesn't come out because it freezes too fast and has 
nowhere else to go in the container.