Ocean SALINITY

The salt in the oceans is the result of millions of years of minerals leached and dissolved from the solid earth. A large portion of this salt comes from rivers, in addition to a quantity dissolved from rocks and sediments below the ocean floor, through volcanic vents. The weather also plays a role on a smaller scale, as the rain deposits mineral particles into the oceans.

Heat from the sun vaporizes almost pure water from the surface of the sea, leaving the salts and minerals behind. The vaporized water returns to the ocean via rivers or precipitation, to wash down more salt, which becomes ever more concentrated. This process is part of the continual exchange of water between the land, ocean and the atmosphere that is called the hydrological or water cycle.

Photos Courtesy of Erika Kean

Rain water, or precipitation, is slightly acidic due to dissolved compounds. When this acidic water beats down on the land and rocks, they end up dissolving the minerals in the rocks and return them to the water. Sodium chloride (table salt) represents more than 80% of the “saltiness” in the sea. The salt(s) then flow down rivers and streams to their final destination in the oceans, where they remain. Over the course of millions of years, this salt has accumulated to be a distinct characteristic of the oceans.

Images courtesy of: open.salon.com, authenticillustrations.com, bprc.osu.edu/../sealevel/images/diagram.jpg, photos.igougo.com/images/p303345-Eastern_Cari...

Main salt ions

The salinity of sea water is usually 3.5% (expressed as 35 parts per thousand) and is primarily made up of the salt ions listed below. Amazingly, their proportions are always the same which is explained by the fact that salinity differences are caused by either evaporating fresh water or fresh water injections from rivers. Freezing and thawing also play a role.

The main salt ions that make up 99.9% of sea water are the following:

Note! ppm= parts per million = mg/litre = 0.001g/kg.
source: Karl K Turekian: Oceans. 1968. Prentice-Hall

This world map shows how the salinity of the oceans changes slightly from around 32ppt (3.2%) to 40ppt (4.0%). Low salinity is found in cold oceans, particularly during the summer season when ice melts. High salinity is found in subtropical areas where evaporation is high and precipitation low. Particularly high salinity is found in arid mediterranean seas (Red Sea, Mediterranean –see red areas in figure) . Note Southern Africa’s moderate salinity.

(http://aquarius.nasa.gov/education-salinity.php)