How the Sea Got Salty
People often inquire of me how the ocean got salty. I usually tell them the tale of the king who had a magic salt grinder that not only turned out salt when the handle was turned, but also had a magic handle which turned itself. Now salt was very valuable in the olden days, for it preserved food and was vital to animals, including man, who got too little salt in their diet. Thus the king was very glad as the grinder piled up salt in his back 40, and even when it filled his royal warehouses. But when it buried his castle and family, he threw it into the sea, where it continued to grind out salt to this day.
The interesting part of this whole myth is that there is a salt grinder that is constantly working to make the sea more salty! This, then, is the story of that salt grinder, which we call the hydrologic cycle. It is the true story of the continued accumulation of salt in the sea, and goes like this.
Since the time when the first significant amount of water accumulated on the face of the earth, some 4 billion years ago, it did something which we all recognize. It dissolved a little bit of the rocks and minerals across and thru which it flowed. The result was what we call hard water; water with a little dissolved material in it. Not enough to see, and often not really enough to taste, but there nevertheless, as we see when water droplets dry on a clean glass surface, and leave the dissolved minerals behind.
Water is a near universal solvent. Given enough time it will take almost anything into solution, including glass, diamonds, gold, stainless steel, etc. The water flows thru the earth as ground water, and into creeks which flow into rivers which, in turn, flow into the sea. The dissolved minerals flow with it. The water eventually evaporates and returns to the land as rain, to go thru the entire cycle again and pick up another little load of material to transport to the ocean. Over millions, or billions, of years, a great deal of dissolved materials is carried to, and left in, the sea. Thus the sea gets salty.
There is more to it than that, though. Every time a volcano erupts it emits gases into the atmosphere, Some of these gases are washed into the soil or the ocean by rain, and they, too, add their load to the ground water and sea!. Among the gases are water vapor, chlorine, bromine, fluorine and iodine, which are present primarily as ions, ie, as chloride, bromide, fluoride and iodide. Another part of the vapor is carbon in the form of carbon dioxide. This carbonates the rain water and ground water into a very dilute solution of Perrier water, better known as carbonic acid. It is dilute, to be sure, but very destructive to silicate minerals which make up much of our earth. Feldspars, amphiboles and pyroxenes all succumb to the attack of this acid and release a wide variety of ions to the waters. Granite yields to rainwater attack over hundreds of thousands of years.
By the mechanism of volcanic emanations we get 1) the chloride, which makes up about 54% of the saltiness of the sea water, 2) the sulfate, from sulfurous emanations of the volcanos, like sulfur dioxide, etc., and 3) bromide, 4) bicarbonate, 5) boric acid and 6) fluoride. Weathering of the rocks under the attack of the rain and groundwater gives us ions leached from the rocks, like 7) sodium, which makes up about 32% of the salts in the sea, and 8) magnesium which contributes about 6%. It also provides 9) calcium, 10) potassium, and 11) strontium. These 11 major ions make up all but a very tiny fraction of one percent of the salt in the sea. The remaining fraction is so small that people ask why we even bother with it. The answer follows! We bother because the thousandth of a percent left contains all the nutrients, gases, heavy metals, most of the radioactivity etc., in the ocean. Without that tiny fraction there would be no life in the sea!! But thatís another story.
The result of the natural salt grinder is sea water with about 3.6 percent of inorganic salts dissolved in it, or as oceanographers say, 36 parts per thousand. If you evaporate it, table salt and magnesium sulfate (epsom salts) make up about 95 percent of the salts.
Another interesting discovery of the last several decades is that the ocean floor is being pulled apart at the mid ocean rifts, and salt water is creeping down to the hot magma (melted rock) under the rifts, where it is heated up and rises to the surface again. During this passage it dissolves a lot of ions from the hot rock it comes in contact with. As it comes out of the rifts thru vents which squirt the hot water into the sea, a lot of very valuable metal sulfides are deposited in huge masses along the rifts. Minerals like iron pyrite (fools gold), and like the ores being mined in many places on the earths surface, are being formed now. Indeed we believe that the enormous copper ore deposits, that have been mined on the Mediterranean island of Cyprus (Latin: cuprus = copper) for many centuries, were deposited in a mid ocean rift and uplifted to form Cyprus.
In addition to the great ore deposits, the hot rift waters must also add materials to the sea water, and are believed to be the reason that certain dissolved materials are not present in the amounts that we thought they should be, based upon the hydrologic cycle. The ocean doesnít seem to have gotten saltier in the last 500 million years and it must be because the interior of the earth released enough water, as it released the gases and lava, to keep the salts diluted to 36 parts per thousand. For each 964 parts of new or juvenile water released from volcanoes, 36 parts of the various salt ions were also released onto the earths surface.
Most folks donít realize that table salt was the real commodity that kept the Middle East in business at the time of Christ. Salt caravans kept the trails open and the Biblical story of Sodom and Gomorrah, with the pillars of salt playing such a prominent part, was because they were salt shipment, and probably salt production, cities. Somebody who doesn't do a good job is not worth his salt. A good person is the salt of the earth. You can probably come up with a dozen more such expressions in which salt plays a prominent part. Gold, frankincense and myrrh were thus way behind salt as the commodities of the trade caravans until large land deposits of salt were discovered. At that time the salt pans where sea water was evaporated in shallow basins along the eastern end of the Mediterranean Sea, were largely abandoned, altho even today salt is made from sea water in the San Francisco and San Diego areas, in the Bahamas, etc.
Since magnesium salts crystallize out of evaporating sea water after most of the table salt has formed, that is the time to stop the evaporation by dumping the rest of the brine, now bitter with the taste of the magnesium ions (remember how milk of magnesia or epsom salts taste?). They call this brine bittern. Otherwise the salt would be too bitter and a very effective laxative. And with a brief story about this property, Ill stop.
Some years ago as I was leaving the Palmetto Park launching ramp, where I'd gone to get my SCUBA tanks filled at FORCE-E Dive Shop, an old lady came up to me and said, "Boys", and I liked her right away, "Boys, will you get me a five gallon jug of sea water while you are out?" I allowed as how it could be but we didnít know when we would be coming back. She said she'd be watching so we took her jug and filled it way offshore to avoid any contamination from the sewers and inlets off the east coast of Florida.
She was there when we got back. Overcome by curiosity, I asked her if she kept a salt water aquarium. She said no. Then, "You'll laugh at me if I tell you what I do with it", she said. We promised not to laugh and she said she drank a half cup every morning. "It keeps me regular", she said.
I'll bet it did. She got a good stiff dose of epsom salts each morning and she should have been regular as clockwork. Hope she doesn't die of high blood pressure from all the salt though! Oh, and by the way, we rolled on the ground laughing!!