Some areas of the ocean are saltier than other but on average there are 35 grams of salt per kilogram of seawater. The ocean contains about 1.4 sextillion kilograms of seawater. (Imagine the number 1.4 with 20 zeros after it). That’s equal to about 3.09 sextillion pounds of seawater. Therefore there's about 50 quintillion kilograms (the number 50 with 18 zeros after it) of salt in the ocean. Its mass is almost the same as that of the moon!
There are many factors that make the ocean salty. Sea salt is mostly made up of a compound called sodium chloride (regular table salt) but it also has other minerals such as magnesium, calcium, and potassium that have dissolved in seawater. Salt has accumulated in the ocean over a very long period of time from rivers, the seabed, and vents from deep in the earth that contain these same dissolved minerals.
The processes that are adding salt to the ocean are very slow. They happen over millions of years and that keeps the salt content of the ocean relatively constant. Over periods of decades or centuries, new salt is added to the ocean in quantities that are far smaller than the amount already there.
Scripps Institution of Oceanography at UC San Diego scientists and their colleagues are measuring the global distribution of ocean salinity to better understand how it changes with climate over seasons and years.
Processes that affect saltiness include excesses of rain, which can dilute the ocean and lower its salinity and evaporation, which can have the opposite effect and make it saltier. Melting or freezing of ice can make water less salty or more salty. Finally, ocean currents can carry more or less salty waters to other regions of the world altering the salinity in those places.
-- Dean Roemmich, physical oceanographer, Integrative Oceanography Division