Murray and Renard were the pioneers in discovering the extraterrestrial materials in marine sediments. However, such substances are quite rare and never form an important part of the sediment. In red clay small black magnetic spherules and brown crystalline spherules represent the extraterrestrial substance.
The black spherules composed of iron or iron alloy have diameters hardly exceeding 0.2 mm. In the brown type which is almost similar to the chondrite variety of meteorite contains silicon. These substances have a metallic luster, and a diameter averaging about 0.5 mm.
From the foregoing discussion it is evident that the particles that originate in space occur in marine sediment as either nickel-iron spherules that are magnetic or as silicate chondrules composed of olivine and augite or pyroxenes.
The overall size range of these cosmic particles falls between the limits of 10-640 micrometers. Distribution of Pelagic Sediment
About three fourths of the ocean bottom is covered by pelagic sediments. About 48%of the pelagic sediments consist of calcareous oozes, while the red clay accounts for 38% and silicous oozes 14% of the total area. In the Indian and Atlantic Oceans calcareous oozes cover larger area than any other ooze.
In the Pacific Ocean it is the abyssal clay that claims to be the dominant pelagic sediment. It is due to the fact that the Pacific Ocean is deeper and the larger part of its floor lies beneath the calcium carbonate compensation depth.
Here it is worthwhile to remember that calcium carbonate is dissolved below 4500 m, leaving only minor concentrations of biologically derived silica among the terrestrial and cosmogenic sediments.
Since the regions of high productivity of diatoms and radiolarians are limited in extent, the siliceous oozes in all the oceans cover a relatively smaller percentage of the ocean floor. This is because of the fact that diatoms and radiolarians constitute the major components of these deposits.
Because of the vastness of the Pacific Ocean the sampling of bottom sediments has been incomplete. However, whatever sampling is available on the basis of which it is undoubtedly clear that the pelagic sediment covering nearly 50% of the Pacific bottom is red deep-sea clay.
In the North Pacific this type of pelagic deposit covers about 80% of the deep ocean basin. Calcareous and siliceous oozes cover about 36% and 14% of the bottom of the Pacific Ocean respectively. The predominance of the clays in this ocean is due to its greater depth.
The pelagic deposits on the North Pacific Ocean floor are deficient in calcium carbonate because of its greater depth. Calcareous oozes are found in an area south of 10°N latitude. However, there are local deposits in the North Pacific in relatively shallow ridges.
Calcareous oozes in the east equatorial region contain about 75% calcium carbonate. In the South Pacific, the calcareous ooze contains nearly 90% calcium carbonate. This is so because the area is far from the sources of terrigenous sediments.
Besides, siliceous remains are also limited in quantity. Calcareous oozes make up about 36% of the total pelagic deposits in the Pacific Ocean.
As regards siliceous oozes, they form about 14.7% of the total pelagic deposits. Diatom ooze covers the bottom of this ocean south of the Antarctic Convergence, while this ooze is also found in the extreme North Pacific because of low temperatures. However, it
differs from the Antarctica ooze in so far as it lacks the high silica concentration.
Radiolarian ooze is found in the areas of the North Equatorial Divergence. Local deposits of radiolarian ooze are found in the Southwest Pacific.
More than two-thirds of the deep Atlantic Ocean basin is covered with calcareous oozes. The most widespread calcareous oozes in the Atlantic Ocean are composed of mostly planktonic foraminifera. The coccolithophoridae contribute a lot to the calcareous oozes in this ocean.
The calcareous oozes contain the aragonite shells of pteropods to a certain extent. However, pteropod remains are found in limited shallow warm water areas where there is abundance of calcium carbonate.
Calcareous oozes are found on the shallow plateau around the Azores in the North Atlantic, between Ascension Island and Tristan da Cunha Island on the Mid-Atlantic Ridge in the South Atlantic, and on the Rio Grande Rise between the South American mainland and the Mid-Atlantic Ridge.
Red deep-ocean clays cover the largest portion of the ocean bottom on the western side of the Mid-Atlantic Ridge. This is because of the difference in the calcium carbonate compensation depths on the two sides of the ridge.
It is to be noted that the abyssal clays that form the pelagic deposits of the Atlantic Ocean are mainly lithogenous in origin, even though they are modified to a certain degree through interaction with the ocean water.
The Atlantic Ocean basins are deficient in siliceous oozes because the ocean water is never saturated with silica. Remember that siliceous oozes contain most of the skeletal remains of diatom and radiolarians.
The diatom ooze is dominant south of the Antarctic Convergence because of the high diatom productivity of the region. Radiolarion oozes are found only in restricted areas in the Atlantic Ocean.
The pelagic deposits containing the radiolarian oozes are found in the equatorial region of this ocean near the area of the diatom oozes as stated above.
In the Indian Ocean calcareous oozes cover the larger portion of the ocean bottom. It is estimated that about 54% of the total surface area of this ocean is covered by calcareous oozes.
The percentage of red clays is about 25, while that of the siliceous oozes is about 20.5. Where the depth of the ocean is less than 5000 meters foraminifera oozes are the dominant pelagic sediment.
This area of the Indian Ocean lies north of the Antarctic Convergence and west of the 5000 ft depth contour on the eastern side of the mid-ocean ridge. As regards the red clays, they are found only in local deep ocean basins within this region.
Calcareous oozes are found in a narrow belt at the base of the continental rise along the western coast of Australia and the East Indian Archipelago.
Red clays are the dominant pelagic deposits east of the mid- ocean ridge at depths below 4500 meters in some localities and at all depths below 5000 meters where calcareous content of the pelagic sediment is much reduced.
In the category of siliceous oozes, the diatom ooze is found around the coast of Antarctica south of the Antarctic Convergence. Radiolarian ooze surrounded by calcareous ooze deposits is found south of the equator and between 90° and 100 ° E longitudes.