MadSci Network: Earth Sciences
Query:

Re: How has salinity of seawater varied over geologic time?

Date: Thu Jun 17 15:18:52 1999
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Earth Sciences
ID: 921100232.Es
Message:

How has salinity of seawater varied over geologic time?

I am interested in how and when the first oceans were formed. What was the original source of their salt and how has ocean salinity varied since then.


"Salt" is a very loose term; technically it means any product of an acid-base reaction, but in this case it is used to mean any dissolved mineral substance.

The first oceans were probably formed from water ejected from volcanoes on the early Earth, though there are some who think that a significant portion of the water was supplied by cometary impacts. That water was then, and is now, gathered into oceans by two processes: precipitation (rain, snow, and so on) and the flow of fresh water from rivers and streams into the sea.

The salt in the ocean is, and always has been, derived from minerals leached from the landforms over which water flows on its way to the sea. In fact, any landlocked body of water with no outlet (such as the Great Salt Lake in Utah, the Dead Sea in Palestine, or the Caspian Sea in the Caucasus) will be salty. The oceans are the ultimate no-outlet water dump and so the wonder is not that they are salty, but that they are not saturated with dissolved minerals. Instead, the oceans have had about the same salinity for quite some time (though, because there is more salt in sea water than in the body fluids of most terrestrial vertebrates, it is thought that the oceans were not always as salty as they are now).

There are bodies of water saltier than the oceans: the Great Salt Lake, the Dead Sea, and even the Mediterranean Sea are all saltier than the ocean in general. These three bodies of water share a common characteristic: they are located in dry regions with little precipitation. This, I think, gives a clue to the reason that the oceans in general are not saturated with dissolved minerals, and that their average salinity has not changed much for many millions of years.

An equilibrium probably exists between the evaporation of water from the oceans -- which tends to increase their salinity -- and the influx of fresh water from land and other sources, such as precipitation and the flow of glaciers into the sea.

In support of this, equatorial parts of the ocean which border on rainforests, and places like the North Atlantic or the Antarctic which get a steady supply of fresh water from the Greenland or Antarctic icecap, have lower salinity than average because they get more fresh water (rainfall and freshwater ice) than they lose to evaporation.

For more on the oceans, see this NASA page. A subordinate page deals directly with ocean salinity.

  Dan Berger
  Bluffton College
  http://cs.bluffton.edu/~berger


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