The strong salinity of seawater is known by all, and variations can sometimes be detected by taste. But on the global scale, salinity is remarkably similar among the major oceans of the world. Although rivers continually add fresh water and dissolved minerals to our oceans, and rains reduce the salinity of surface waters, our oceans are in a steady state with regard to salinity concentrations worldwide. Ocean salinities have not changed appreciably in hundreds of millions of years.
How did the oceans get so salty? It occurred 4 billion years ago as the Earth was forming. All minerals were molten, and the heaviest metals (iron, nickel and others) moved toward the Earth’s core while lighter elements (hydrogen, oxygen, sodium, chlorine, nitrogen and others) rose and formed the Earth’s crust and atmosphere. Water molecules were initially vaporized, but as the Earth cooled, they converted to liquid state, causing intense rains over millions of years.
As the water flowed into low areas, it aggressively eroded the new surface materials, dissolved many minerals and carried them into the new ocean, resulting in a complex mixture of mineral salts.
The two mineral ions that are most abundant in seawater are sodium and chloride – our common salt when combined in a solid form. They make up over 90 percent of the dissolved minerals in seawater, and the average concentration is 35 grams of salt in 1,000 grams of seawater.
Analytically, this concentration is called 35 parts-per-thousand (ppt) salinity, and it equates to one teaspoon of salt in an 8-ounce glass of water. This sounds minimal, but it’s equal to 120 million tons of salt in a cubic mile of seawater. On a global basis, if all the dissolved salt in the oceans were removed, dried and spread over the earth, it would create a layer 500 feet thick.
Water certainly is a fascinating molecule. In its liquid state it can dissolve high concentrations of minerals, but in gaseous (water vapor) or solid (ice) phases, water cannot cope with minerals so these phases of water are essentially salt free. Rain and sea ice contain no salt.
Are the world oceans becoming saltier? The quick answer is “no.”
Many physical processes are affecting ocean salinity today, but on average, changes are not occurring on a global scale.
At high latitudes, formation of sea ice increases the salinity of local seawater because salt is left behind as sea ice forms. But in summer months, sea ice and glaciers melt and salinities of surface waters are reduced. High precipitation near the poles also contributes to reduced salinities of surface waters. In contrast and at other locations, strong evaporation of surface waters increases salinity, with the Mediterranean Sea being a fine example.
Dry winds over the Med result in much more evaporation than precipitation, with one vertical meter of water being lost to the atmosphere each year; replacement occurs via Atlantic waters entering through the Straits of Gibraltar.
The Red Sea also has relatively high salinity due to excess evaporation over precipitation but the highest salinities (over 200 ppt) are found in the Dead Sea where evaporation is very high and the sea is an enclosed basin without inflow or outflow via rivers.
Below the sea surface, geological processes contribute mineral salts to the sea water. Subsurface volcanoes spew minerals into abyssal waters, and deep hydrothermal vents also release minerals near the seafloor. But these energetic processes have little effect on global salinity.
Dissolved calcium and silica have low concentrations in seawater, mostly because they are consumed by marine organisms, whereas sodium and chlorine are not reduced by biological processes.
Our Great Lakes do not taste salty nor do other smaller, freshwater lakes. This isn’t because they lack dissolved minerals. Actually these lakes contain appreciable concentrations of minerals, but not sodium, which gives water the salty taste we are familiar with.
In my next Triton article, I will discuss the physical dangers of drinking saltwater if stranded at sea. Statistics from life-raft voyages show that risk of death is 10 times greater if the abandoned sailor drinks seawater.
Scott E. McDowell has a doctorate degree in ocean physics, is a licensed captain and author of Marinas: a Complete Guide available at www.scottemcdowell.com. Contact him at firstname.lastname@example.org.