When you say “salty,” you’re undoubtedly thinking of sodium chloride, common table salt. But to a chemist, a salt is any member of a large class of chemicals, and there are dozens of them in the oceans.
To put the word “salt” in perspective, please indulge me in a one-paragraph chemistry lesson.
A “molecule” of salt (it’s not really a molecule in the strict sense, but I won’t tell anybody if you don’t) consists of a positively charged part and a negatively charged part that, being oppositely charged, attract each other. The positive and negative parts are called ions.
In the case of sodium chloride the positive ion is a charged atom of sodium and the negative ion is a charged atom of chlorine. But a salt’s positive ion can be a charged atom of any metal, and there are some eighty-five known metals. Also, there are many negative ions besides chloride, so you can see that there is a very large number of possible salts.
End of chemistry lesson.
The main metal ions in seawater are sodium, magnesium, calcium and potassium, while the main negative ions are chloride, sulfate, bicarbonate and bromide. Your question, then, is how all this stuff got into the oceans in the first place. The short answer is that it was washed out of the land by rain-water, which then flowed as rivers to the seas.
Seawater is continually being recycled. Each year, the top meter (3 feet) or so of the oceans evaporates into the air, moves around in various weather systems and falls back onto the oceans and land as rain and snow. Of this precipitation, 76 percent falls on the oceans and 24 percent falls on the continents. The water that lands on the continents flows down in streams and rivers, eventually returning to the seas. In the process of washing down, these waters pick up anything that will dissolve, mostly the salts that exist in the soils, rocks and minerals.
Any chemist will tell you that sodium salts dissolve more readily in water than do salts of potassium, magnesium, calcium or most other metals. More than any others, then, it’s the sodium salts that dissolve and wash down into the oceans. There are approximately equal amounts of sodium and potassium in the soils, rocks and minerals, yet there is twenty-eight times more sodium than potassium in seawater.
All of these dissolved salts make up 3.47 percent of seawater, by weight. Only six elements make up more than 99 percent of those salts: chlorine, sodium, sulfur (in the form of sulfates), magnesium, calcium and potassium, in decreasing order.
Another source of sea salts is volcanic eruptions, both on land and under the sea, which spew out enormous amounts of solids and gases. Among the prominent volcanic gases are chlorine and sulfur dioxide, which may account for the fact that chlorine is the most abundant element in seawater, making up 55 percent of the salts’ weight, while sulfates are second only to chlorides as the negative-ion portions of the salts.
Putting all this together, sodium and chlorine make up 86 percent of the salts in the oceans. So if you want to say that the oceans are salty because of sodium chloride, nobody will give you much of an argument.
Why are the oceans salty, but not the streams, rivers and lakes?
Rainwater washes down from the land into the streams, rivers and lakes, carrying dissolved salts just as it does when it washes into the oceans. But the difference is that the oceans are much older than the other waters, 4 or 5 billion years old, compared with mere millions.
Over those billions of years the oceans have been recycling their water, evaporating water that rains out on the land and flows back, returning each time with a fresh load of salts.
These cycles have continually increased the load of salt in the oceans.