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Geosciences LibreTexts

10.6: The Oceans

We have already discussed the oceans in terms of the hydrologic cycle. Recall that oceans store 97% of water on earth. A common question when we are discussing the oceans in class is: how did the ocean get to be salty? The answer is that the salts in the ocean have accumulated over millions of years. They came from the minerals in the lithosphere. As water percolates through rocks, it dissolves small amounts of minerals. Water dissolves over half of the elements found in nature. The minerals then are carried with the water to the ocean. Over time, water is evaporated from the oceans (recall the hydrologic cycle), but the minerals stay behind. A smaller scale example of this is inland lakes which have no river outlets, such as the Great Salt Lake in Utah, and Mono Lake in the Eastern Sierras. Because these lakes have no outlet, they are like mini oceans, with the dissolved minerals in their waters becoming more and more concentrated each year.

As a result of the dissolving properties of water, most natural elements and compounds can be found in a dissolved form in the ocean. The dissolved compounds are called solutes and the concentration of the dissolved solutes is called the salinity of the ocean. Seven elements account for over 99% of the dissolved solids in seawater:

  • Chloride (Cl-)
  • Sodium (Na+)
  • Magnesium (Mg+2)
  • Sulfur (as a sulfate SO4-2)
  • Calcium (Ca2+)
  • Potassium (K+)
  • Bromide (Br-)

These elements are all considered 'salts'. The salinity of ocean water is 35 g/kg or 35,000 parts per million (ppm). Water that is more salty than 35,000 ppm is called brine, and water that is less salty than 35,000 PPM is called brackish.