The Earth Story

Why are oceans salty? Oceans have saltwater but rivers and most lakes do not. Why? Where did the salt come from?

Why are oceans salty?

Oceans have saltwater but rivers and most lakes do not. Why? Where did the salt come from?

Raindrops dissolve minerals, gases, and other substances as they fall through the atmosphere. Carbon dioxide (CO2) is one of those gases. CO2 bonds with H2O (water) to form carbonic acid (H2CO3) - a weak acid, but still an acid.

When rain reaches the ground, it erodes soils and rocks both physically and chemically. The relentless downward forces of the rain knock grains of soil and rock loose; while the acidity helps to dissolve some of the minerals into ions that will be carried away with the runoff (this means freshwater has salt in it, but at concentrations so low that it doesn’t taste salty). The runoff travels through rivers and streams to ultimately reach the ocean.

Ions have a residence time in the ocean – a time frame for how long they will stay there before being removed somehow. For most ions, the residence time is a few hundred years to a few hundred thousand. Sodium and chloride have residence times of tens of millions to hundreds of millions of years (the exact number varies greatly between studies). The longer residence times have allowed sodium and chloride to become concentrated in seawater so that they now make up 90% of the dissolved ions.

There are other sources of salts besides runoff. Fissures in the ocean floor create hydrothermal vents where water seeps into the ocean’s crust near subduction zones. The water is heated to high temperatures (high pressure prevents water from boiling) enabling it to dissolve large quantities of minerals. When water leaves the vents, it takes the dissolved minerals with it back into the ocean. Underwater volcanoes add a tremendous amount of minerals to the ocean as well.

Salts do not accumulate in freshwater because freshwater bodies have an outlet through which water and salts leave as more freshwater enters. Oceans have no outlet. Water is evaporated out, but the salts stay behind and become concentrated. Altogether, the various salts in the ocean make up about 3.5% of the weight of seawater, which means there are 35 grams of salt per kilogram of water.

While some places in the ocean are more or less salty than the bulk of the ocean, overall the salt concentration seems to have reached a steady state. Dissolved salts are being removed as fast as new salts are being added, preventing the ocean from becoming even saltier.

Photo Credit: "Mealt Falls" by Alexbrn http://bit.ly/1G3Ps4g

References: http://on.doi.gov/1dIGqD2 http://1.usa.gov/1KIA8Sx http://bit.ly/1LP1Ufq http://bit.ly/1LTtpWb http://1.usa.gov/1RhnFJp http://bit.ly/1Uu577W