Download Ocean Salinities Remain Consistent for Millions of Years

Scott McDowell, Ph.D.
Sea Science Articles written for The Triton magazine
Ocean Salinities Remain Consistent for Millions of Years
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; possibly billions.
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 the 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% 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 and 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 ft 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 and 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 process 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
Scott McDowell, Ph.D.
Sea Science Articles written for The Triton magazine
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 in- or outflow via rivers.
Below the sea surface, geological processes are contributing 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 organism 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 salt water if stranded at sea.
Statistics from life-raft voyages show that risk of death is ten times greater if the abandoned sailor drinks
seawater!
www.scottemcdowell.com
www.the-triton.com
[email protected]