Most mysterious, perhaps, of all substances in the sea is iodine. In sea water it is one of the scarcest of the nonmetals, difficult to detect and resisting exact analysis. Yet it is found in almost every marine plant and animal. Sponges, corals, and certain seaweeds accumulate vast quantities of it. Apparently the iodine in the sea is in a constant state of chemical change, sometimes being oxidized, sometimes reduced, again entering into organic combinations. There seem to be constant interchanges between air and sea, the iodine in some form perhaps being carried into the air in spray, for the air at sea level contains detectable quantities, which decrease with altitude. From the time living things first made iodine a part of the chemistry of their tissues, they seem to have become increasingly dependent on it; now we ourselves could not exist without it as a regulator of the basal metabolism of our bodies, through the thyroid gland which accumulates it.
All commercial iodine was formerly obtained from seaweeds; then the deposits of crude nitrate of soda from the high deserts of North Chile were discovered. Probably the original source of this raw material—called ‘caliche’— was some prehistoric sea filled with marine vegetation, but that is a subject of controversy. Iodine is obtained also from brine deposits and from the subterranean waters of oil-bearing rocks—all indirectly of marine origin.
A monopoly on the world’s bromine is held by the ocean, where 99 per cent of it is now concentrated. The tiny fraction present in rocks was originally deposited there by the sea. First we obtained it from the brines left in subterranean pools by prehistoric oceans; now there are large plants on the seacoasts—especially in the United States—which use ocean water as their raw material and extract the bromine directly. Thanks to modern methods of commercial production of bromine we have high-test gasoline for our cars. There is a long list of other uses, including the manufacture of sedatives, fire extinguishers, photographic chemicals, dyestuffs, and chemical warfare materials.
One of the oldest bromine derivatives known to man was Tyrian purple, which the Phoenicians made in their dyehouses from the purple snail, Murex. This snail may be linked in a curious and wonderful way with the prodigious and seemingly unreasonable quantities of bromine found today in the Dead Sea, which contains, it is estimated, some 850 million tons of the chemical. The concentration of bromine in Dead Sea water is 100 times that in the ocean. Apparently the supply is constantly renewed by underground hot springs, which discharge into the bottom of the Sea of Galilee, which in turn sends its waters to the Dead Sea by way of the River Jordan. Some authorities believe that the source of the bromine in the hot springs is a deposit of billions of ancient snails, laid down by the sea of a bygone age, in a stratum long since buried.
Magnesium is another mineral we now obtain by collecting huge volumes of ocean water and treating it with chemicals, although originally it was derived only from brines or from the treatment of such magnesium-containing rocks as dolomite, of which whole mountain ranges are composed. In a cubic mile of sea water there are about 4 million tons of magnesium. Since the direct extraction method was developed about 1941, production has increased enormously. It was magnesium from the sea that made possible the wartime growth of the aviation industry, for every airplane made in the United States (and in most other countries as well) contains about half a ton of magnesium metal. And it has innumerable uses in other industries where a light-weight metal is desired, besides its long-standing utility as an insulating material, and its use in printing inks, medicines, and toothpastes, and in such war implements as incendiary bombs, star shells, and tracer ammunition.
Wherever climate has permitted it, men have evaporated salt from sea water for many centuries. Under the burning sun of the tropics the ancient Greeks, Romans, and Egyptians harvested the salt men and animals everywhere must have in order to live. Even today in parts of the world that are hot and dry and where drying winds blow, solar evaporation of salt is practiced—on the shores of the Persian Gulf, in China, India, and Japan, in the Philippines, and on the coast of California and the alkali flats of Utah.
Here and there are natural basins where the action of sun and wind and sea combine to carry on evaporation of salt on a scale far greater than human industry could accomplish. Such a natural basin is the Rann of Cutch on the west coast of India. The Rann is a flat plain, some 60 by 185 miles, separated from the sea by the island of Cutch. When the southwest monsoons blow, sea water is carried in by way of a channel to cover the plain. But in summer, in the season when the hot northeast monsoon blows from the desert, no more water enters, and that which is collected in pools over the plain evaporates into a salt crust, in some places several feet thick.
Where the sea has come in over the land, laid down its deposits, and then withdrawn, there have been created reservoirs of chemicals, upon which we can draw with comparatively little trouble. Hidden deep under the surface of our earth are pools of ‘fossil salt water,’ the brine of ancient seas; ‘fossil deserts,’ the salt of old seas that evaporated away under conditions of extreme heat and dryness; and layers of sedimentary rock in which are contained the organic sediments and the dissolved salts of the sea that deposited them.
During the Permian period, which was a time of great heat and dryness and widespread deserts, a vast inland sea formed over much of Europe, covering parts of the present Britain, France, Germany, and Poland. Rains came seldom and the rate of evaporation was high. The sea became exceedingly salty, and it began to deposit layers of salts. For a period covering thousands of years, only gypsum was deposited, perhaps representing a time when water fresh from the ocean occasionally entered the inland sea to mix with its strong brine. Alternating with the gypsum were thicker beds of salt. Later, as its area shrank and the sea grew still more concentrated, deposits of potassium and magnesium sulphates were formed (this stage representing perhaps 500 years); still later, and perhaps for another 500 years, there were laid down mixed potassium and magnesium chlorides or carnallite. After the sea had completely evaporated, desert conditions prevailed, and soon the salt deposits were buried under sand. The richest beds form the famous deposits of Stassfurt and Alsace; toward the outskirts of the original area of the old sea (as, for example, in England) there are only beds of salt. The Stassfurt beds are about 2500 feet thick; their springs of brine have been known since the thirteenth century, and the salts have been mined since the seventeenth century.
At an even earlier geological period—the Silurian—a great salt basin was deposited in the northern part of the United States, extending from central New York State across Michigan, including northern Pennsylvania and Ohio and part of southern Ontario. Because of the hot, dry climate of that time, the inland sea lying over this place grew so salty that beds of salt and gypsum were deposited over a great area covering about 100,000 square miles. There are seven distinct beds of salt at Ithaca, New York, the uppermost lying at a depth of about half a mile. In southern Michigan some of the individual salt beds are more than 500 feet thick, and the aggregate thickness of salt in the center of the Michigan Basin is approximately 2000 feet. In some places rock salt is mined; in others wells are dug, water is forced down, and the resulting brine is pumped to the surface and evaporated to recover the salt.