By far the greater area is covered by deep-sea, pelagic sediments. It is pertinent to clarify at this point that the pelagic sediment will include all the very fine grained material of lithogenous origin which is carried by suspension in the air or ocean water for long distances before setting out on the deep ocean floor after many years in transport.
The pelagic sediment also includes organic remains that settle slowly to the ocean floor, hydrogenous minerals such as Phillip-site and montmorillonite, and fine grained meteoric dust found in sediments of the deep ocean basin.
Pelagic sediments are classified into organic and inorganic deposits as follows:
(i) Inorganic deposits:
Less than 30% organic material is present in these deposits. These deposits are known as red clay. According to G. Steiger, the red deep sea clay, which consists of this inorganic component, contains 54.48% Si02, 15.94% Al2 03 and 8.66% Fe03. The iron oxide content is responsible for the sometimes reddish, sometimes brownish colouring of the clay.
(ii) Organic deposits:
These deposits contain more than 30% organic matter. The common term used for these deposits is ‘ooze’. These deposits are again classified on the basis of their chemical composition, and also on the basis of organisms which predominate in them.
Organic deposits known as ‘oozes’ are further classified on the basis of the predominance of particular chemicals as well as the remains of certain marine plants and animals. The classification of oozes is as follows: –
(A) Calcareous oozes:
In these oozes more than 30% calcium carbonate (CaCo2) is present in the form of very minute skeletal remains of various planktonic animals and plants. These oozes are further subdivided into the following types:
(i) Globigerina ooze, in which the calcium carbonate is in the tests of pelagic foraminifera.
(ii) Pteropod ooze contains protective shells of pelagic molluscs;
(iii) Coccolith ooze contains large number of coccoliths and rhabdoliths that form the protective structures of the minute coccolithophoridae.
(B) Siliceous oozes:
These pelagic deposits are formed by a large percentage of siliceous skeletal material produced by planktonic plants and animals. These oozes are further subdivided in the following two types:
(i) Diatom ooze:
This type of ooze contains large amounts of diatom frustules produced b plaktonic plants.
(ii) Radiolarian ooze:
It contains large proportions of radiolarian skeletons formed by these plankton animals.
This type of ooze is characterized by the predominance of the remains of the shells of pelagic molluscs. Pteropod is a mollusc which is a floating marine animal. Its shell is soft and conical in shape. The shell is extremely thin and contains more than 30% calcium carbonate.
Pteropods are found in very large numbers in those seas where the temperatures are high and the annual range is low. Naturally, therefore, they are found in the tropical seas. They are found only in limited areas.
Pteropods are found in large numbers only in the Atlantic Ocean. In different sections of the Mid – Atlantic Ridge they are found at a depth ranging from 800 to 1000 fathoms.
This type of ooze is formed by the accumulation of the calcareous skeletons of foraminifera, mainly the planktonic form and globigerina, which consists of rounded calcareous tests. The average calcium carbonate content of this ooze varies from 75 to 89 percent. The shell of this animal is even smaller than the tip of a pin.
These animals occupy large areas in different oceans. This type of ooze is found in large areas in the Atlantic, Pacific and Indian Oceans. However, this ooze is found in the seas of the tropical as well as temperate regions.
Because of the influence of the warm ocean current flowing between Greenland and Norway, globigerina ooze can be traced a little beyond the Arctic Circle.
Optimum depth for the development of globigerina varies from 1500 to 2000 fathoms. With increasing depth, the number of this animal decreases. However, in certain localities globigerina is found up to a depth of 3000 fathoms. But in the oceanic deeps it is conspicuous by its absence. The deposits are often white in colour.
It represents a type of siliceous ooze which forms in areas where the organic production of calcium carbonate is minimum, and where its solution exceeds production.
Reduction in the salinity of sea water is one of the factors favouring the increase in number of diatoms. Near the mouths of large rivers, salinity is lower, and this factor alone strongly favours the increase in the population of this animal.
Diatoms are siliceous algae and belong, to the phytoplankton. These plants prefer to grow near the sea surface where nutrients necessary for their development are available. Diatoms are found in large numbers in the higher latitudes.
The area to the south of the Antarctic convergence is a zone where nutrients are carried up from the depths to be converted into diatoms by photosynthesis, in the upper layers of the sea, through sunshine. The remains of the tiny organisms sink to the depths to accumulate as diatom ooze.
This deposit differs from the calcareous ooze because it is comparatively better sorted. In the size classification, the major part of the material can be placed in the silt grade. The maximum development of this vegetation takes place between 600-2000 fathoms depth, but in certain localities it is found even at a depth of 4000 fathoms.
This type of ooze is found in a broad belt in the Southern Ocean. There is a narrow belt near the northern boundary of the Pacific Ocean in which this type of ooze has been discovered.
Radiolarians are planktonic animals with very complex skeletons. These tiny animals secrete silica which is deposited on the sea floor. The lime component in this ooze is rather negligible, the percentage of calcium carbonates being less than 20.
On the other hand, the inorganic material is abundant, about 67% of the ooze material is made up by the tiny mineral particles, less than 50 microns. Its colour is somewhat similar to that of the inorganic red clay.
Siliceous oozes occupy larger area in the Pacific Ocean. The reason is that with increasing depth the amount of calcium carbonate decreases rather rapidly.
Radiolaria are found mainly in the deep seas. It is seldom found in shallow water, less than 2000 fathom deep. Since this tiny organism is present up to a depth of 5000 fathoms, the deposits formed by the remains of this animal are found in deep sea plains and oceanic deeps.
However, this type of ooze is limited to the warm tropical seas. The radiolarian ooze occupies the largest area in the Pacific Ocean. But in the Indian Ocean too, this ooze is found in limited areas. This ooze is, however, scarcely found in the Atlantic Ocean.
The red clay is the most important inorganic deposit of the deep sea basins. The area occupied by this type of pelagic deposit is larger than that occupied by any other type of deposit. Red clay contains hydrated silicate of aluminium and iron oxide.
According to John Murray, the red clay is formed by the chemical decomposition of submarine rocks and silicate of aluminium. Red clay is found only on the floor of very deep sea basins. In the formation of this deposit, pumice floating in sea water and the volcanic ash transported to the ocean by the wind play very important role.
That is why in the vicinity of submarine volcanoes this type of deposit is found in abundance. It also contains certain radioactive substances. Its sedimentation rate being very slow, the deposits may contain teeth of sharks and ear-bones of whales.
Among the pelagic sediments globigerina ooze and red clay are more important than any other deposit. At great depth the decalcification of the globigerina remains takes place with the result that the ocean floor is covered by red deep-sea clay.
On the floor of the western basins of the Atlantic Ocean red deep-sea clay predominates. It is also the result of the decalcifying action of the carbon dioxide-enrich water.
It should be borne in mind that the rate of accumulation of biogenous material on the ocean floor is controlled by three basic processes-productivity, destruction, and dilution. Productivity of the zoo-and phytoplankton is greater in areas of upwelling along the continental margins.
Destruction of the remains of animals and plants occurs mainly through solution. Since the ocean water is under-saturated with silica at all depths, only the thick siliceous shells are able to be incorporated in the ocean floor deposits.
The thin shell particles are dissolved into the ocean water before they are able to reach the sea floor. The solubility of calcium carbonate varies with depth.
The surface water in the sea is saturated with calcium carbonate because of relatively higher temperatures. But with increasing depth, the temperature decreases with the result that there is an increase in the carbon dioxide content.