Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Of the identifiable inorganic materials in the deep sea, the most abundant are of volcanic origin, and among these the most common is pumice, which is frequently so light that it floats readily until it becomes water-logged. Pieces of pumice brought up by the Challenger and thoroughly dried were found to float for months in sea-water before settling even through the depth of water contained in the vessel in which the experiment was performed.[178] The next most abundant substance of volcanic origin in pelagic deposits is volcanic glass. This ranges from pieces of the size of a walnut down to the smallest fragments, which often serve as centers for concretions. Lapilli (cinders) and volcanic ash also are abundant in parts of the deep sea. The distribution of these volcanic products is essentially universal, though by no means uniform. Some of them are probably from submarine volcanoes.

The study of the deep sea deposits has revealed the presence of many nodules and grains which are believed to be of extra-terrestrial origin. Many of them are magnetic.[179] The dust of countless meteors which enter the atmosphere daily settles on land and sea alike, and enters into the sediment of the bottom of the latter. It is probably no more abundant in deep water than in shallow, but it is relatively more important, since other sedimentation is more meager. The number of meteorites which enter the atmosphere daily has been estimated at from 15,000,000 to 20,000,000.[180] If on the average the meteorites weigh ten grains each, probably a rather high estimate, the total amount of extra-terrestrial matter reaching the earth yearly would be 5,000 to 7,000 tons, and something like three-fourths of this must, on the average, fall into the sea. But even at this rate it would take some fifty billion years to cover the sea-bottom with a layer one foot in thickness.

Organic constituents of pelagic deposits.—With increasing distance from shores, and especially with increasing depth of water, terrigenous deposits become less and less abundant, and sediments derived from pelagic life increase in relative importance. Beyond the upper part of the outer slopes of the continental shelves, the pelagic deposits are largely made up of shells and skeletons of marine organisms which live in the surface-waters. Pelagic molluscs, foraminifera, and algæ secrete shells of lime carbonate, while diatoms and radiolarians secrete shells of silica. When the organisms die, they sink to the bottom with their shells, and these mineral matters of organic origin are mingled with the volcanic products which are universal over the sea-floor. Pelagic deposits of organic origin are named according to their characteristic constituents. Thus there are pteropod oozes, globigerina oozes, diatom oozes, radiolarian oozes, etc.[181] It is not to be understood that these oozes are made up exclusively of the shells which give them their names. Diatom ooze is an ooze in which diatom shells are abundant, not an ooze made up wholly of diatom shells; and globigerina ooze is an ooze in which globigerina shells are abundant, though in many cases they do not make up even the bulk of the matter. While samples of these various oozes might be selected which are thoroughly distinct from one another, there are all gradations between them, since pelagic life does not recognize boundary-lines.

It is a significant fact that with increasing depth the proportion of lime carbonate in the ooze decreases. Thus in tropical regions remote from land where the depths are less than 600 fathoms, the carbonate of lime of the shells of pelagic organisms may constitute 80% or 90% of the deposit. With the same surface conditions, but with increasing depth, the percentage of lime carbonate decreases, until at 2000 fathoms it is less than 60%; at 2400 fathoms, 30%, and at 2600 fathoms, 10%. Beyond this depth there are usually no more than traces of carbonate of lime. The data at hand show that the percentage of lime carbonate falls off below 2200 fathoms more rapidly than at lesser depths.

When the percentage of lime carbonate becomes very low, the calcareous oozes grade off into the red clay with which the sea-floor below 2400 to 2600 fathoms is covered.

Chemical deposits.—The chemical deposits of the deep sea are chiefly the alteration products of sediments which reach the sea-bottom by mechanical means. All sediment deposited in the sea undergoes more or less chemical change, but it is only when the change is very considerable that the product is referred to this class. Where sedimentation is rapid and the sediment coarse, the chemical change is relatively slight; but where the sedimentation is slow and the sediment fine, the chemical change is relatively great; for the longer exposure to the sea-water and the greater proportion of surface exposed to attack, both favor change. Both the area and the mass of sea-bottom sediment radically changed in this way are large, but most of the deposit does not correspond to any formation known on the land.

The red clay already referred to belongs to this class of deposits. Its origin has been the subject of much discussion. It contains much volcanic débris, various concretions, bones of mammals, zeolitic crystals, and extra-terrestrial spherules, and doubtless the insoluble products of the shells of pelagic life; but it is still a mooted question how far the clay itself is the product of decomposed shells, and how far the altered product of pulverized pumice, volcanic ash, dust, etc. Pelagic life does not seem to be less abundant at the surface where the water is deep than where it is shallow, and it would appear that the shells must sink in such situations as elsewhere. If the lime carbonate of globigerina ooze be removed by dilute acid, the inorganic residue is similar to the red clay in the ocean-bottom. This suggests that owing to the more complete solution in the very deep water, the lime carbonate of the shells has been dissolved, leaving the red clay as a residuum. The more complete solution at the bottom might be the result either of the greater pressure, or of a greater percentage of CO₂ in the water due to emanations from the sea-floor, or to both; but the suddenness of the transition from oozes to red clay, with increasing depth, does not seem to be fully explained by these assumptions. The study of the dredgings has inclined the students of these materials to the conclusion that volcanic materials, rather than shells, are the principal source of the red clay.[182] The volcanic materials are thought to have accumulated slowly and to have been long exposed to the action of sea-water. The various nodules and crystals in the clay are believed to be secondary products, the materials for which were derived from the decomposition of the same materials. Eolian dust may be a notable constituent of the red clay.

Various specific products of chemical change may be briefly referred to. The decomposition of certain mineral particles, such as feldspar, gives rise to kaolin, and kaolin is a very considerable constituent of most of the clayey deposits of the ocean-bottom. The kaolinization of feldspar may take place both on land and in the sea. Manganiferous deposits are widespread in the ocean-bottom, occurring both as coatings on grains of mechanical sediments, shells, etc., and as concretions ranging in sizes from minute particles to nodules an inch or more in diameter. The concretions are sometimes approximately spheroidal, but often botryoidal. These manganiferous nodules are believed to have arisen from the decay of fragments of volcanic rocks. In their decay, the manganese and iron are believed to have been first changed to carbonates, and subsequently to oxides. After manganese oxide, iron oxide and silica are by far the most abundant constituents, but many other substances enter into their composition in minor quantities.

Another substance somewhat widely distributed in the sea-bed, though by no means universal, is glauconite, a complex silicate of alumina, iron, potassium, etc. Glauconite is, on the whole, most abundant along the edges of the continental shelves, though it is by no means universal in this position. It is not commonly found in deep water, nor very near the shore, but approximately at the “mud-line.” The glauconite grains begin to form, as a rule, in tiny shells, chiefly the shells of foraminifera. After filling the shell, the shell itself may disappear, while the glauconite goes on accumulating around the core already formed, until the grain attains considerable size. Glauconite is believed to be an alteration product of certain sorts of mechanical sediment, the change being effected under the influence of the decaying organic matter in the shells.[183] It does not occur where sedimentation is rapid, and its formation appears to be favored by considerable changes of temperature. Glauconite deposits occur on the land and are commonly known as green sand marl. Glauconite also occurs sparingly in many other sedimentary rocks.