Why is the sea salty?

Dear Cecil:

Why is the sea salty?

Cecil replies:

I will have you know that this question was dealt with by no less an authority than Sir Edmond Halley (rhymes with valley), better known as Mr. Comet. Sir Ed was one of the small band of universal geniuses who held the fort prior to the arrival of myself. Apparently having a couple hours to kill one day, he came up with a theory about sea salt in 1715.

Halley’s idea was that salt and other minerals were carried into the sea by rivers, presumably having first been leached out of the ground by rain runoff. Over time seawater evaporated to create the rain clouds that replenished the rivers, but the salt was left behind, till eventually the oceans reached their present degree of salinity. As proof Halley noted that of the handful of lakes in the world without outlets, such as the Dead Sea or the Caspian Sea, all were salty or brackish.

Halley went on from this to suggest a theory of how you could calculate the age of the earth based on measurements of the increasing saltiness of the ocean. Scientists regard this brainstorm today with an expression of pain. His idea about where the salt came from was a little closer to the mark. However, he didn’t have the whole story.

Salt consists of two elements, sodium and chlorine. A good chunk of the sodium comes from “continental weathering,” as Halley proposed. The rest of it probably was leached out of the ocean floor when the oceans were first formed. The chlorine on the other hand is a result of “outgassing” from the earth’s core. The stuff is spewed out by underwater volcanoes and vents in the ocean floor, scientists think. The sodium and chlorine then combine to form salt.

Halley also missed the boat in supposing that the saltiness of the sea is increasing today. It’s now believed that ocean salinity has been stable for hundreds of millions of years as a result of a complicated chemical recycling process. Today sodium isn’t being leached out of the ocean floor, it’s being put back into the floor as sediment. From there, some speculate, the mighty forces of plate tectonics shove it under the continents and eventually it winds up on the land’s surface, where the weathering starts the process all over again. But much still remains to be learned.

Send questions to Cecil via cecil@straightdope.com.

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