CHAPTER VI.
THE WORK OF THE OCEAN.
The general facts concerning the depth of the ocean and the distribution of its water have been given on a preceding page ([p. 8]), and the origin of the ocean and the ocean basins is discussed in the second volume. This chapter has to do primarily with the processes now going on in the sea and its borders, in so far as they are of importance in the interpretation of geologic history. The study of these processes is prefaced by a few words concerning the amount and composition of the sea-water, the life of the ocean, and the topography of its bed.[138]
Volume and composition.—Every 1000 parts of sea-water contain about 34.40 parts by weight of mineral matter in solution. The principal solids, acids, and bases, combined according to the principles laid down by Dittmar, are shown in the following table:[2]
| Chloride of sodium | 77.758 |
| Chloride of magnesium | 10.878 |
| Sulphate of magnesium | 4.737 |
| Sulphate of calcium | 3.600 |
| Sulphate of potassium | 2.465 |
| Bromide of magnesium | 0.217 |
| Carbonate of calcium | 0.345 |
| ———— | |
| Total salts | 100.000 |
Expressed in terms of tons per cubic mile of sea-water, the composition is as follows:[139]
| Tons per Cubic Mile. | |
|---|---|
| Chloride of sodium (NaCl) | 117,434,000 |
| Chloride of magnesium (MgCl2) | 16,428,000 |
| Sulphate of magnesium (MgSO4) | 7,154,000 |
| Sulphate of calcium (CaSO4) | 5,437,000 |
| Sulphate of potassium (K2SO4) | 3,723,000 |
| Bromide of magnesium (MgBr2) | 328,000 |
| Carbonate of calcium (CaCO3) | 521,000 |
| ————— | |
| For sea-water, total dissolved matter | 151,025,000 |
Aside from the ingredients shown in the above tables, the presence of the following has been proved: iodine, fluorine, phosphorus, silicon, boron, silver, lead, copper, zinc, cobalt, nickel, iron, manganese, aluminum, barium, strontium, arsenic, lithium, cæsium, rubidium, and gold. Oxygen, nitrogen, and carbonic acid gas are also present in quantity. The amount of carbonic acid is estimated to be 18 times as great as in the atmosphere.[140]
The amount of sea-water is estimated by Murray at 323,722,150 cubic miles,[141] or about 15 times the volume of the land above sea-level. The volume and composition of the sea-water being known, the amount of mineral matter which it contains may be readily calculated. Assuming the average specific gravity of the mineral matter in solution to be 2.5, the 3.5% by weight becomes 1.4% by volume, and 1.4% of 323,722,150 cubic miles is 4,532,110 cubic miles. This then represents the aggregate volume of mineral matter in the sea if it were precipitated and compacted so as to have an average specific gravity of 2.5. Assuming the average depth of the sea to be 2076 fathoms (12,456 feet), as given by Murray, the mineral matter in solution, if precipitated, would cover the ocean bottom to a depth of about 175 feet. Assuming the area of the land to be to that of the sea as 28 to 72, this amount of mineral matter would make a layer about 450 feet deep over the land. Its amount is equal to about 20% of that of all lands above sea-level, and it falls but little short of that in all lands below 600 feet in altitude. If it were precipitated and concentrated in the shallow waters about the borders of the lands, it would fill the sea out to the depth of about 4000 feet, and would diminish its area by some 19,000,000 to 20,000,000 square miles, an area which is more than ⅓ of the present land surface. In other words, if the mineral matter in the sea-water were precipitated and concentrated in the shallow waters about the lands, it would restore the continental shelves to the land areas, and add an almost equal area beyond.
These comparisons may perhaps help to give some idea of the amount of mineral matter in solution in the sea, but they give no more than a hint of the importance of the solvent power of water in the general processes of rock decay, for most of the substances carried to the sea in solution by rivers are extracted from the water about as rapidly as they are supplied. Thus calcium carbonate is about twenty times as abundant as sodium chloride in river-water,[142] but is only ¹⁄₁₂₅ as abundant in sea-water.
The total river discharge into the sea is estimated at 6524 cubic miles of water per year.[143] This water is estimated to carry to the sea annually about half a cubic mile of mineral matter in solution. At this rate it would take about 9,000,000 years for the streams to bring to the sea an amount of mineral matter equal to that it now contains, but the proportions of the ingredients would be very different.
The sodium chloride makes up about 2.4% of the mineral matter in river-water and nearly 78% of the mineral matter of the sea. At this rate it would take nearly 300,000,000 years for the salt of the sea to have been contributed by the rivers. It is not to be understood, however, that this figure indicates the age of the ocean. The salt is not all brought in by the rivers; the rivers have probably not always contributed at the present rate; and much salt once in the sea has been precipitated. Nevertheless the above figure gives some suggestion as to the order of magnitude of the figures which represent the age of the ocean.
In contrast with the salt, the amount of calcium carbonate in the sea is so small that at their present rate of contribution, it would be brought to the sea by rivers in about 62,000 years.
Topography of bed.—The general relations of ocean basins to continents are suggested by [Fig. 296]. The borders of the continental platforms are covered by the epicontinental sea, while the abysmal sea occupies the ocean basins proper. From the figure it is seen that an ocean basin is pronouncedly convex upward, and so departs as widely as may be from the current notion of the homely utensil from which it is named. Only when it is remembered that a level surface (on the earth) is one which has the mean curvature of the earth, and that the deeper parts of the ocean basin are well below the mean sphere level, does the current name seem justified.[144] The figure also shows that the depth of an ocean basin is slight compared with the radius of the earth.
The bed of the ocean, like the face of the land, is affected by elevations and depressions, and its deepest points are about as far below its surface as the highest mountains are above it. There are areas of the sea bottom which, as a whole, may be compared to the plains of the land, and others which may be likened to plateaus, and the lines of gradation between them are as indistinct as they often are on the land. There are mountain peaks, chiefly of volcanic origin, and depressions comparable to the great basins on the land. But apart from these general features, there is little in common between the topography of the sea bottom and that of the land. Mountain systems are, for the most part, absent, though certain islands, like Cuba and some of its associates, may be regarded as the crests of systems which are chiefly submerged. If the water were drawn off from the ocean’s bed so that it could be seen as the land is, its most impressive feature would be its monotony. The familiar hills and valleys which, in all their multitudinous forms, give the land surface its most characteristic features are essentially absent. A large part of its surface would be found to be so nearly flat that the eye would not detect its departure from planeness.
Fig. 296.—General relations of ocean basins to the lithosphere. Lat. 20° S. Depth of the water (black) and height of land exaggerated ten times. (Data from Murray, Scot. Geogr. Mag., Vol. XV, 1899.)
The reason for this profound difference is readily found. On the land, the dominant processes which shape the details of the surface are degradational, and though the final result of degradation is flatness (base-level), the immediate result is relief, and, most commonly, relief of the hill-and-valley type. In the sea, the dominant processes are aggradational, and tend to monotonous planeness.
Distribution of marine life.—Marine life has been of such importance in the history of the earth that the elementary facts concerning its distribution and the principles which control it are here recalled. The distribution of marine life is influenced by many factors, chief among which are temperature and depth of water. Not only is life more abundant in the warmer parts of the ocean than in the colder, but the species inhabiting cold waters are different from those in warm, and few species range through great variations. Many forms of life are restricted to shallow water. Many more, especially those which do not live on the bottom, swim about freely without reference to the depth of the water beneath them, while relatively few are restricted to great depths. Many species are also influenced by the salinity of the water, which varies notably along coasts where the fresh waters from the land are discharged; by the character of the sediment at the bottom, some species preferring mud, others sand, and others gravel; by the movement of the waters, some species preferring still waters and others rough; and some species by the abundance and nature of the food-supply, and by rival and hostile species.
Subject to the exceptions determined by temperature, etc., plant life abounds in shallow water out to depths of 100 fathoms or so, and is found in abundance at the surface where the depth is much greater. Animal life abounds in shallow water, both at the bottom and above it, out to depths of 200 or 300 fathoms, and occurs in great profusion in the surface-waters of temperate and tropical regions without regard to the depth. The great body of the ocean water lying below a depth of some few hundred fathoms is nearly tenantless, though life reappears sparingly at the bottom, even where the depth is great. For further discussion of this topic, see [Chapter XI].