Ocean Salinity

The oceans are are approximately 3.5% salt. Photo courtesy of James @ NZ/Flickr.

You probably already know this, but the oceans are salty! Salinity is a measure of the concentration of dissolved salts in a body of water. Basically it is a measure of how salty a body of water is. It is measured as a concentration of grams of salt per kilogram of seawater. Ocean water contains an average of 35 grams of salt per kilogram (1,000 grams) of seawater. Another way of expressing this is as a measure of parts salt per thousand parts seawater. For example, 35 parts of salt dissolved in 1 kilogram of seawater is 35 parts salt per thousand parts seawater, or 35 ppt (parts per thousand). Ppt is the common unit of measurement used to express the salinity of a body of water.

How does this compare to the Great Salt Lake? The Great Salt Lake has a salinity of 50-270 ppt, making it saltier than the ocean.

Ocean salinity is not constant across the globe and varies over time. Salinity is high in subtropical areas of the globe because evaporation rates are higher than precipitation rates. Along the equator, where precipitation is plentiful, the oceans tend to be less salty. Polar regions of the world oceans also tend to have low salinity because seawater is diluted by melting icebergs. Salinity tends to be the lowest at estuaries, places where large rivers empty into the ocean because the freshwater from the river dilutes the ocean at that location.

The salinity of oceans causes water's freezing point to lower, meaning that it must be a lot colder in the oceans before the water will freeze. The salinity of water also influences the types of organisms that can live in the water. Organisms typically have a tolerance range for salinity, meaning that if the salinity is too high or too low they cannot survive there. Additionally, salinity will affect water quality. Dissolved oxygen content decreases as salinity increases.

Where does the salt come from? Volcanic eruptions and weathering of rocks and minerals are the main ways salt is added to seawater. Ions (charged atoms) like sodium, calcium, and magnesium are commonly found in rocks on the continents. When the rocks are weathered, these ions are washed into rivers and deposited in the oceans. Gases from volcanic eruptions include carbon dioxide, water vapor, and sulfur dioxides, which dissolve in water and form chloride and sulfate ions. Different combination of these ions are what make up the salts in ocean. Sodium chloride (NaCl) is perhaps the most commonly known salt.

In the ocean, water dissolves salt molecules by separating the atoms of Na+ (sodium) and Cl- (chloride), two atoms that can make up salt. Water's polarity causes the positively charged sodium and negatively charged chloride atoms to be attracted to the oppositely charged side of the water molecule. The video on the following page explains how salt is dissolved in seawater.

Sources: http://score.dnr.sc.gov/ktmlpro10/files/uploads/elearning/Understanding_Salinity.pdf and http://geology.utah.gov/online_html/pi/pi-39/pi39pg03.htm
Last modified: Wednesday, 2 February 2011, 6:00 PM