Pelagic deposits

Pelagic deposits occur beneath deep water beyond the edge of the continental slope, carpeting the deep ocean basins. Much of this material is of fine texture, and its nature varies with the depth and with the types of organisms that abound in the overlying water. At depths of less than about 6000 m, pelagic deposits contain a considerable proportion of material of biological origin, commonly some 30 per cent or more by weight. Although these deposits are termed 'organic', they seldom contain much decomposable carbon but consist almost entirely of skeletal fragments of planktonic organisms. Organic deposits are of two main types, calcareous and siliceous. Calcareous sediments, rich in calcium carbonate and formed mainly from foraminiferan shells, are common in middle and low latitudes but only down to an average depth of around 4600 m. Below this depth, hydrostatic pressure causes some forms of calcium carbonate to dissolve. The calcite compensation depth (CCD) is the depth where the supply of clacite raining down from above is equalled by its dissolution. So below this depth, there are no sediments containing carbonate.

At high latitudes around the polar belts, great areas of the sea-bed are covered with siliceous sediments. These are areas of high productivity where the rain of material downwards consists mainly of silica-containing diatoms and radiolarians. Calcareous sediments do occur here but only down to around 3000 m. Other high-productivity areas such as the equatorial belt and coastal upwellings also have siliceous sediments especially where the sea-bed lies below the CCD. In general, the deeper organic deposits contain less calcareous material and a larger proportion of silica.

The organic deposits are named after the main organisms from whose skeletons they are made up and are classified as follows:

Calcareous oozes

• Globigerina ooze This is the most widespread of the deposits over the greater part of the deep Atlantic and much of the Indian and Pacific Oceans, covering nearly 50 per cent of the deep-sea bottom and extending to depths of 6000 m. It contains up to 95 per cent calcium carbonate mainly in the form of foraminiferan shells.

• Coccolith ooze A high proportion of coccolith material, sometimes amounting to 25 per cent or more of the total weight, is occasionally found in samples of globigerina ooze, chiefly beneath areas of warm surface water.

• Pteropod ooze This contains many pteropod shells and occurs below subtropical parts of the Atlantic at depths down to 3500 m.

Siliceous oozes

• Diatom ooze This consists mainly of siliceous material in the form of diatom fragments. It occurs as an almost continuous belt around Antarctica beneath the Southern Ocean, its northernmost limit corresponding closely with the position of the Antarctic convergence. There is also a strip of diatom ooze across the northern part of the north Pacific.

• Radiolarian ooze This contains many radiolarian skeletons and occurs at depths between 4000 and 8000 m beneath tropical parts of the Pacific and Indian oceans and is also recorded in the Atlantic.

Red clay

At 6000 m and below, sediments generally contain less than 10 per cent material of obvious biological origin, and at these depths the most widespread deposit is red clay, covering nearly 40 per cent of the deep ocean floor. It is a very finely divided sediment, usually brick-red in colour, and consisting mainly of finegrained quartz (silica), and clay minerals such as aluminium oxide along with small amounts of various compounds of iron, calcium, magnesium and traces of many other metals. The bulk of this material is derived from 'aeolian fallout'. That is, it originates as fine mineral dust lifted from the ground by wind action and carried through the atmosphere, from which some eventually 'falls out' over the sea. These clays accumulate incredibly slowly at the rate of 1 mm or so per 1000 years. Organic oozes accumulate at around 10-30 mm per 1000 years.

The distribution of deep sediments is shown in Figure 6.1.

¿K F771 Siliceous l^j sediments g Calcareous

3 sediments

| Radiolarian J ooze

¿K F771 Siliceous l^j sediments g Calcareous

3 sediments

| Radiolarian J ooze

Globigerina Ooze

0° 20° 40° 60° 80° 100° 120° 140° 160° 180°

Figure 6.1. Ocean bottom deposits. Siliceous sediments = diatom ooze and radiolarian ooze. Calcareous sediments = globigerina ooze and pteropod ooze.

0° 20° 40° 60° 80° 100° 120° 140° 160° 180°

Figure 6.1. Ocean bottom deposits. Siliceous sediments = diatom ooze and radiolarian ooze. Calcareous sediments = globigerina ooze and pteropod ooze.

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Responses

  • andreas
    How are Siliceous sediments limited to areas of elevated productivity?
    8 years ago
  • Terenzio
    What causes calcareous ooze to be below ccd?
    7 years ago
  • janice
    Which organisms contain silica deposits?
    6 years ago

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