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Why is the ocean so salty?

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By Jeremy Duncan

Oceans cover approximately 70 percent of the Earth’s surface and 97 percent of all the water on earth is saltwater. The amount of salt in the ocean is difficult to even imagine, much less measure. Some scientists believe that the amount of dissolved solids in the Earth’s oceans (mostly sodium and chloride) could be as much as 50 quadrillion tons (50 million billions).

The water cycle that creates saltwater. Rainwater falling on rocks helps minerals, including sodium and chloride (the constituents of salt), that are carried to the ocean, where evaporation removes hydrogen and oxygen from the water, leaving behind tons of dissolved materials that accumulate and create saltwater.
The water cycle that creates saltwater. Rainwater falling on rocks helps minerals, including sodium and chloride (the constituents of salt), that are carried to the ocean, where evaporation removes hydrogen and oxygen from the water, leaving behind tons of dissolved materials that accumulate and create saltwater.

According to National Oceanic and Atmospheric Administration, if all the salt in all of the oceans could be removed and deposited on land, it would be more than 500 feet (166 meters) deep, about the height of a 40-story building.

All this salt originates from land-based rocks. Carbon dioxide in the air dissolves in rainwater forming a mild acid, which, over time, breaks down the rock into its components. Ions, which are particles with an electronic charge, are carried away by streams and rivers and eventually reach the ocean. These ions, and there is a longs list of them, are just dissolved salt.

One would think that the ocean would get saltier with time, but it does not – over the last 1.5 billion years ocean salinity (the amount of salt dissolved in water) has changed little. The oceans are in a steady state because as rivers bring salts in other processes remove it. Biological processes remove some salts, for example, animals with shells remove silica and calcium carbonate dissolved in the water to build their shells, and other animals concentrate salts in their fecal pellets and it falls to the bottom of the ocean (which given enough time will form into new rocks). Non-biological processes also play a role such as wind blowing on shore dumps salt spray on land while under-sea lavas absorb salt into the newly-formed rock.

A breakdown of water's composition. Photo credit: ponce.sdsu.edu
A breakdown of water’s composition. Photo credit: ponce.sdsu.edu

Salinity is not the same everywhere in the oceans. For example, Atlantic water is slightly saltier than Pacific water at the equator because water that evaporates from the Atlantic gets blown by the prevailing winds across Central America before it rains on the Pacific.

 

SOURCES:

http://oceanservice.noaa.gov/facts/whysalty.html

http://water.usgs.gov/edu/watercycleoceans.html

http://www.bbc.com/future/story/20140411-why-is-the-sea-so-salty

http://ponce.sdsu.edu/usgs_why_is_the_ocean_salty/usgs_why_is_the_ocean_salty.html

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