Ask A Scientist General Science Archive

Author:     cm
Text:       How did the oceans get salty?

Response #:  1 of 1
Author:      woodford
Subject:     Origin of saltwater
Text:        This is a great question!  Thanks for writing in.  If you get 
into folk stories and mythology you will see that almost every culture has a 
story explaining how the oceans became salty.  The answer is really very 
simple.  Salt in the ocean comes from rocks on land.  Let me explain how it 
works.  When rain falls on terrestrial rock (mountains, etc.) it contains some 
dissolved carbon dioxide from the surrounding air.  This causes the rainwater 
to be slightly acidic due to carbonic acid (which forms from carbon dioxide 
and water).  The rain erodes the rock and the acid breaks down the component 
parts of the rock and carries it along in a dissolved state.  The ions in the 
runoff are carried to the streams, rivers, and estuaries to the ocean.  Many 
of the dissolved ions are used by organisms in the ocean and are removed from 
the water.  Others are not reactive and are not used up and are left for long 
periods of time where their concentrations increase over time.  The two ions 
that reside in greatest quantity and are removed most slowly of the major ions 
in seawater, are chloride (Cl-) and sodium (Na+).  These two make up over 90% 
of all dissolved ions in seawater.  River water has higher concentrations of 
bicarbonate, potassium, calcium, and sulfates. These species, however are used 
or absorbed by organisms or react with other things in the water so that they 
are no longer present in the ocean in large quantities. On the other hand, 
river water contains almost no sodium or chloride ions, but these do not get 
removed by organisms or by chemical reaction, so they just keep on building up 
and getting more concentrated.  Anyway, ocean water contains the following 
ions in the indicated concentrations.  These make up over 99.3% of everything 
dissolved in seawater.
Chloride 19.353
Sodium   10.79
Magnesium 1.297
Sulfate   2.712
Calcium   0.4123
Potassium 0.399
All values listed are grams per kilogram of seawater (g/kg) at a salinity of 
35 parts per thousand (ppt).  I hope this answers your question.  Keep these 
good questions coming.