Popular Science Monthly/Volume 48/December 1895/Why the Sea Is Salt
By G. W. LITTLEHALES.
FROM the first chapter of the first book of Moses, called Genesis, we learn that, as between water and land, the ocean had the first place in terrestrial existence, for it is there stated that on the third day in the calendar of the creation the waters under the heavens were gathered together and the dry land appeared. Both from a chemical and a geological standpoint it appears that the waters of the ocean were salt from the beginning. Dr. T. S. Hunt, one of the ablest writers on the physical history of the globe, in his chemical and geological essays, referring to that period when the earth was in a molten state and surrounded by an envelope of gases and of vapor of water, states: "There would be the conversion of all the carbonates, chlorides, and sulphates into silicates, and the separation of carbon, chlorine, and sulphur in the form of acid gases which, with nitrogen, vapor of water, and a probable excess of oxygen, could form the dense primeval atmosphere. The resulting fused mass would contain all the bases as silicates, and must have resembled certain furnace slags or volcanic glasses. The atmosphere, charged with acid gases which surrounded this primitive rock, must have been of great density. Under the pressure of a high barometric column condensation could take place at a temperature much above the present boiling point of water, and the depressed portions of the half-cooled crust would be flooded with a highly heated solution of hydrochloric and sulphuric acids, whose action in decomposing the silicates can easily be understood. The formation of the chlorides and sulphates of the various bases, and the separation of silica, would go on until the affinities of the acids were satisfied, and there would be a separation of silica taking the form of quartz, and the production of sea-water holding in solution, besides the chlorides and the sulphates of sodium, calcium, and magnesium, salts of ammonium and other metallic bases. The atmosphere, being thus deprived of its volatile chlorine and sulphur compounds, would gradually approximate to that of our own time, but would differ in the greater amount of carbonic-acid gas."
And the meteorologist Abbe, in the course of remarks made before the Philosophical Society of Washington in 1889, expressing the propriety of wholly rejecting the idea that the earth was once a molten globe, stated: "The study of geological climate during and since the formation of Azoic metamorphic strata has led me to adopt the conclusion that surface geology, like volcanic, does not demand excessive temperatures; it seems to me most reasonable to assume that the surface was never much warmer than 250° F., but to allow that this temperature may have prevailed at the close of the Archaic epoch.
"At this temperature all the water of the ocean would exist only as vapor and clouds in the atmosphere. The steady, hot rain from the atmosphere would rapidly disintegrate the surface rocks. Small seas and lakes of water saturated with alkalies and salts would at once begin to form the rocks that we know as metamorphic and archæan. The covering thus formed would contribute to diminish the rate of cooling of the interior mass, thus allowing the atmosphere to cool down to its present condition and deposit the most of its moisture."
In the rocks formed earliest after Archæan time, to which geological age only crystalline rocks devoid of fossils belong, there are found aquatic relics of organisms with calcareous skeletons which when living bore a close generic relation to organic forms which are confined to oceanic waters at the present time. Among these early inhabitants of the sea were corals, crinoids, sea urchins, and starfishes, and many others there doubtless were which, although they require the saline constituents of the sea to live upon, had no calcareous skeletons, and consequently have not been preserved in a fossil state. The remains are found deposited in the lower Silurian as well as the Devonian, Carboniferous, Jurassic, and Cretaceous formations.
But apart from these deductions concerning the saltness of the primeval ocean there is direct evidence that the waters of the sea in the early part of Paleozoic time were highly saline, for there were deposited from the waters of the Silurian sea saliferous strata which constitute the Onondaga salt group and the Trenton and Chazy limestone series, in which the relics of marine organ organisms largely abound, to prove that they result from the sediment deposited by the ocean in that age.
Throughout all geological time the sea has also received salt from the continents, for the rain, falling upon the land, filters through the layers of saliferous soil and, springing to the surface through some natural duct, finds its way to the sea with its burden of salt. But when the waters of the ocean are evaporated to form clouds and rain, the salt is left behind, so that ever more and more salt is being transferred from the land; and this ceaseless transfer has been going on since the first brooks and rills gathered together to form the rivers of the primeval lands. This process of salinification, which is identical with that which takes place in every lake and inland sea, like Great Salt Lake and the Dead Sea, into which streams flow but from which none emerge, has often been looked upon as a sufficient cause for the existing saltness of oceanic waters, for the ocean occupies a great closed basin into which many thousands of rivers flow, but from which none take their source. It must not be overlooked, however, that there is direct evidence to show that in early geological ages, when the continents were small and before the rivers were numerous or large, the waters of the vast ocean of those times were salt.
The salts of the sea have fed, throughout all time, countless living things which have thronged its water and whose remains now form the rocks of continents or lie spread in beds of unknown thickness over 66,000,000 square miles of the 143,000,000 square miles of the ocean's floor; they have lent the substance to build the fringing reefs of the land and all the coral islands of the sea, and there are at present, on the basis of an average salinity of three and a half per cent, in the 290,700,000 cubic miles of water which make up the oceans, 90,000,000,000,000,000 tons, or 10,173,000 cubic miles, of salt. This is sufficient to cover the areas of all the lands of the earth with a uniform layer of salt to a depth of one thousand feet.
It seems that the sea was made salt in the beginning as a part of the grand design of the Creator to provide for the system of evolution which has been going on since the creation. Many distinct species of living organisms exist in the sea as a result of its salinity, and their remains have largely contributed to the growth of continents. The three great factors in accounting for the system of currents in the ocean, by which it becomes the great heat distributer of the globe, are changes of temperature, the winds, and salinity. The last mentioned becomes an important factor through the immediate and essential differences of specific gravity and consequent differences of level that it produces in different parts of the ocean through the action of evaporation and rainfall.
If, through the fall of rain upon a portion of the ocean or through the action of evaporation in the surrounding parts, the waters of that portion become lighter than the rest down to a certain distance below the surface, two different kinds of motion will immediately occur. The lighter water will be lifted by the surrounding heavier waters till there is no difference in pressure between its lower boundary and the surrounding waters at the same depth; but, as its pressure at all levels above this lower boundary will now have become greater than that of the surrounding heavier waters, it will instantly begin to displace and overflow them. This movement of the lighter water will require considerably more time than the movement of the heavier water by which it was lifted and continues to be lifted as its level sinks by lateral diffusion, because the sum of the differences of pressure which caused the lifting of the lighter water was, in the first place, greater than the sum of the differences that caused its lateral diffusion. Secondly, the differences of pressure that caused the first movement must extend all the way to the bottom, whereas those which cause the latter extend no deeper than the lighter stratum itself, and, even within the extent of that, have their chief effect confined to the superficial strata.
On the other hand, when the equilibrium of a mass of water is disturbed by causes that do not diminish the specific gravity, the disturbance must extend down to the bottom, and the differences of pressure at all levels beneath the surface must be equal. The equilibrium is then restored by a general movement of the whole mass, which movement is sensible in inverse proportion to the mass that is set in motion. This is the essential cause for the difference in strength between the currents observed in salt and fresh waters, for, of all the current-producing causes which act in fresh waters, only the one resulting from variations of temperature can sensibly affect the specific gravity, while the specific gravity of sea water, besides being much more affected by variation of temperature, is still further influenced by the fresh water which rains upon the surface of the ocean. If the whole basin of the ocean were filled with fresh water and exposed to the most extreme meteorological influences, the currents produced would not be nearly equal either in size or strength to those now observed in the waters of the ocean.
So the saltness of the sea is involved in all the great subjects into which the ocean currents enter. Having contributed to the growth of the continents, it has in a like degree peopled them by influencing human migrations through the streams of the ocean upon which the race of man was spread to the distant archipelagoes at a time when there were only rudimentary means for struggling against the forces of Nature. Besides its influences in geology and anthropology, it is concerned to a marked extent in the climate of the earth and of the sea, and in their botany and zoölogy.