by a calcareous cement. Similar formations are found in the Mediterranean, where a dark mud predominates in the western part, passing into a grey, marly slime in the Tyrrhenian Basin and replaced by a typical calcareous ooze in the Eastern Basin. The bottom of the Black Sea is covered by a stiff blue mud in which Sir John Murray found much sulphide of iron,[1] grains or needles of pyrites making up nearly 50% of the deposit, and there are also grains of amorphous calcium carbonate evidently precipitated from the water. The formation of the blue mud is largely aided by the putrefaction of organic matter, and as a result the water deeper than 120 fathoms is extraordinarily deficient in dissolved oxygen and abounds in sulphuretted hydrogen, the formation of which is brought about by a special bacterium, the only form of life found at depths greater than 120 fathoms in the Black Sea.
In the Red Sea the “Pola” expedition discovered a calcareous ooze similar to that of the Mediterranean, and the formation of a stony crust by precipitation of calcium and magnesium carbonates may be recognized as giving origin to a recent dolomite.
The terrigenous ingredients in the deposits become less and less abundant as one goes farther into the deep ocean and away from the continental margins. Still, according to Murray and Irvine, finely divided colloidal clay is to be found in all parts of the ocean however remote from land, though in very small amount, and there is less in tropical than in cooler waters. A minute fraction is always separating out of the water, and as a prodigious length of time may be accepted for the accomplishment of all the chemical and physical processes in the deep sea, we must take account of the gradual accumulation of even this infinitesimal precipitation. As well as the finest of terrigenous clay there is present in sea-water far from land a different clay derived from the decomposition of volcanic material. Volcanic dust thrown into the air settles out slowly, and some of the products of submarine and littoral volcanoes, like pumice-stone, possess a remarkable power of floating and may drift into any part of the ocean before they become waterlogged and sink. To this inconceivably slowly-growing deposit of inorganic material over the ocean floor there is added an overwhelmingly more rapid contribution of the remains of calcareous and siliceous planktonic and benthonic organisms, which tend to bury the slower accumulating material under a blanket of globigerina, pteropod, diatom or radiolarian ooze. When those deposits of organic origin are wanting or have been removed, the red clay composed of the mineral constituents is found alone. It is a remarkable geographical fact that on the rises and in the basins of moderate depth of the open ocean the organic oozes preponderate, but in the abysmal depressions below 2500 or 3000 fathoms, whether these lie in the middle or near the edges of the great ocean spaces, there is found only the red clay, with a minimum of calcium carbonate, though sometimes with a considerable admixture of the siliceous remains of radiolarians. Thus red clay and radiolarian ooze are distinguished as abyssal deposits in contradistinction to the epilophic calcareous oozes.
Globigerina ooze was recognized as an important deposit as soon as the first successful deep-sea soundings had been made in the Atlantic. It was described simultaneously in 1853 by Bailey of West Point and Ehrenberg in Berlin. Murray and Renard define globigerina ooze as containing at least 30% of calcium carbonate, in which the remains of pelagic (not benthonic) foraminifera predominate and in which remains of pelagic mollusca such as pteropods and heteropods, ostracodes and also coccoliths (minute calcareous algae) may also occur. Not more than 25% of the deposit may consist of bottom-dwelling foraminifera, echini or Worm-tubes, and as a rule these make up only from 9 to 10%. These peculiarities, combined with the striking absence of mineral constituents, distinguish the eupelagic globigerina ooze from the hemipelagic calcareous mud. Out of 118 samples of globigerina ooze obtained by the “Challenger” expedition 84 came from depths of 1500 to 2500 fathoms, 13 from depths of 1000 to 1500 and only 16 from depths greater than 2500 fathoms. Viewed as a whole this deposit may be taken as a partial precipitation of the plankton living in the upper waters of the open sea. A small proportion of organic matter including the fat globules of the plankton is mixed with the calcium carbonate, the amount according to Gümbel’s analysis being about 1 part in 1000. Secondary products, such as glauconite, phosphatic concretions and manganese nodules, occur though less frequently than in the hemipelagic sediments. Globigerina ooze is the characteristic deposit of the Atlantic Ocean, where it covers not less than 44,000,000 sq. km. (17,000,000 sq. statute m.). In the Indian Ocean the area covered is 31,000,000 sq. km. (12,000,000 sq. m.) and in the huge Pacific Ocean only 30,000,000 sq. km. (11,500,000 sq. m.).
Pteropod ooze is merely a local variety of globigerina ooze in which the comparatively large but very delicate spindle-shaped shells of pteropods happen to abound. These shells do not retain their individuality at depths greater than 1400 or 1500 fathoms, and in fact pteropod ooze is only found in small patches on the ridges near the Azores, Antilles, Canaries, Sokotra, Nicobar, Fiji and the Paumotu islands, and on the central rise of the South Atlantic between Ascension and Tristan d’Acunha.
Diatom ooze was recognized by Sir John Murray as the characteristic deposit in high latitudes in the Indian Ocean, and later it was found to be characteristic also of the corresponding parts of the Indian and Pacific covering a total area of about 22,000,000 sq. km. (8,500,000 sq. m.). It has been found sporadically near the Aleutian Islands, between the, Philippines and Marianne Islands and to the south of the Galapagos group. It is made up to a large extent of the siliceous frustules of diatoms. It is usually yellowish-grey and often straw-coloured when wet, though when dried it becomes white and mealy.
Red clay was discovered and named by Sir Wyville Thomson on the “Challenger” in 1873 when sounding in depths of 2700 fathoms on the way from the Canary Islands to St Thomas. The reddish colour comes from the presence of oxides of iron, and particles of manganese also occur in it, especially in the Pacific region, where the colour is more that of chocolate; but when it is mixed with globigerina ooze it is grey. Red clay is the deposit peculiar to the abysmal area; 70 carefully investigated samples collected by the “Challenger” came from an average depth of 2730 fathoms, 97 specimens collected by the “Tuscarora” came from an average depth of 2860 fathoms, and 26 samples obtained by the “Albatross” in the Central Pacific came from an average depth of 2620 fathoms. Red clay has not yet been found in depths less than 2200 fathoms. The main ingredient of the deposit is a stiff clay which is plastic when fresh, but dries to a stony hardness. Isolated gritty fragments of minerals may be felt in the generally fine-grained homogeneous mass. The dredge often brings up large numbers of nodules formed upon sharks’ teeth, the ear-bones of whales or turtles or small fragments of pumice or other volcanic ejecta, and all more or less incrusted with manganese oxide until the nodules vary in size from that of a potato to that of a man’s head. A very interesting feature is the small proportion of calcium carbonate, the amount present being usually less as the depth is greater; red clay from depths exceeding 3000 fathoms does not contain so much as 1% of calcareous matter.
Murray and Renard recognize the progressive diminution of carbonate of lime with increase of depth as a characteristic of all eupelagic deposits. The whole collection of 231 specimens of deep-sea deposits brought back by the “Challenger” shows the following general relationship:—
Proportion of Calcium Carbonate in Deep-Sea Deposits.
| 68 | samples from | less than 2000 | fathoms = | 60-80 % |
| 68 | ,,,, | 2000-2500 | ,, | 46·7 % |
| 65 | ,,,, | 2500-3000 | ,, | 17·4 % |
| 8 | ,,,, | more than 3000 | ,, | 0·9 % |
In deep water there is a regular process of solution of the calcareous shells falling from the surface. Murray and Renard ascribe this to the greater abundance of carbonic acid in the
- ↑ Scot. Geog. Mag., vol. 16 (1900), p. 695.
