Why Is the Ocean Salty?

Why the Sea Is Salty (Yet Most Lakes Are Not)

In the Dutch West Indies, the water is so salty it crystallizes into salt. The ocean contains a lot of sodium and chloride, which make salt.
In the Dutch West Indies, the water is so salty it crystallizes into salt. The ocean contains a lot of sodium and chloride, which make salt. HUGHES Herve / hemis.fr / Getty Images

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.

Key Takeaways: What Is the Sea Salty

  • 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.

Why the Sea Is Salty

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.

Annual mean sea surface salinity from the 2009 World Ocean Atlas. Salinity is listed in practical salinity units (PSU).
Annual mean sea surface salinity from the 2009 World Ocean Atlas. Salinity is listed in practical salinity units (PSU). Plumbago

Salinity of Lakes

So, lakes get water from streams and rivers. Lakes are in contact with the ground. Why aren't they salty? Well, some are! Think of the Great Salt Lake and the Dead Sea. Other lakes, such as the Great Lakes, are filled with water that contains many minerals, yet doesn't taste salty. Why is this? Partly it is because the water tastes salty if it contains sodium ions and chloride ions. If the minerals associated with a lake don't contain much sodium, the water won't be very salty. Another reason lakes tend not to be salty is because water often leaves lakes to continue its trip toward the sea. According to an article in Science Daily, a drop of water and its associated ions will remain in one of the Great Lakes for around 200 years. On the other hand, a water droplet and its salts may remain in the ocean for 100-200 million years.

The most dilute lake in the world is Lae Notasha, located near the crest of the Oregon Cascade in Oregon, United States. Its conductivity ranges about 1.3 to 1.6 uS cm-1, with bicarbonate as the dominant anion. While a forest surrounds the lake, the watershed appears not to significantly contribute to the ionic composition of the water. Because the water is so dilute, the lake is ideal for monitoring atmospheric contaminants.

Sources

  • Anati, D. A. (1999). "The salinity of hypersaline brines: concepts and misconceptions". Int. J. Salt Lake. Res. 8: 55–70. doi:10.1007/bf02442137
  • Eilers, J. M.; Sullivan, T. J.; Hurley, K. C. (1990). "The most dilute lake in the world?". Hydrobiologia. 199: 1–6. doi:10.1007/BF00007827
  • Millero, F. J. (1993). "What is PSU?". Oceanography. 6 (3): 67.
  • Pawlowicz, R. (2013). "Key Physical Variables in the Ocean: Temperature, Salinity, and Density". Nature Education Knowledge. 4 (4): 13.
  • Pawlowicz, R.; Feistel, R. (2012). "Limnological applications of the Thermodynamic Equation of Seawater 2010 (TEOS-10)". Limnology and Oceanography: Methods. 10 (11): 853–867. doi:10.4319/lom.2012.10.853