Why Is the Sea Salty?

 

Why Is the Sea Salty?

Have you ever wondered why the ocean is salty? Have you wondered why lakes might not be salty? Here's a look at what makes the ocean salty and why other bodies of water have a different chemical composition.

The oceans of the world have a fairly stable salinity of about 35 parts per thousand. The main salts include dissolved sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate. In water, these are sodium, magnesium, and potassium cations, and chloride, sulfate, nitrate, and carbonate anions.

The reason the sea is salty is because it is very old. Gases from volcanoes dissolved in the water, making it acidic. The acids dissolved minerals from lava, producing ions. More recently, ions from eroded rocks entered the ocean as rivers drained into the sea.

While some lakes are very salty (high salinity), some do not taste salty because they contain low amounts of sodium and chloride (table salt) ions. Others are more dilute simply because the water drains toward the sea and is replaced by fresh rainwater or other precipitation.

Oceans have been around a very long time, so some of the salts were added to the water at a time when gases and lava were spewing from increased volcanic activity. The carbon dioxide dissolved in water from the atmosphere forms weak carbonic acid which dissolves minerals. When these minerals dissolve, they form ions, which make the water salty. While water evaporates from the ocean, the salt gets left behind. Also, rivers drain into the oceans, bringing in additional ions from rock that was eroded by rainwater and streams.

The saltiness of the ocean, or its salinity, is fairly stable at about 35 parts per thousand. To give you a sense of how much salt that is, it is estimated that if you took all the salt out of the ocean and spread it over the land, the salt would form a layer more than 500 feet (166 m) deep. You might think the ocean would become increasingly salty over time, but part of the reason it does not is because many of the ions in the ocean are taken in by the organisms that live in the ocean. Another factor may be the formation of new minerals.