pelagic environments of the oceans

pelagic environments of the oceans The pelagic environment is that of the open sea. It is the region inhabited by plankton, which are minute organisms that drift or float at various depths in the water, and by nekton, which are free-swimming organisms. The pelagic organisms that live in the open sea are not only an important constituent of the marine fauna; they also make a significant contribution to the sediments that are deposited on the ocean floor. Benthonic organisms (those that live on the ocean floor) also make their contribution to the ocean-floor sediments.

Pelagic biogenic sediments

Pelagic biogenic sediments consist of the fine-grained skeletal debris of marine planktonic and benthonic organisms. These sediments can be subdivided into those comprised of calcium carbonate and those comprised of opaline silica. The carbonate-rich debris is made up of the remains of coccolithophores (plants), foraminifera, and pteropods (animals); most siliceous components originate from diatoms (plants), and radiolarians (animals). Pelagic biogenic sediments (containing not less than 30 per cent of biogenic debris), also termed biogenic oozes, can be further subdivided according to their principal component; for example, diatom ooze.

Controls on biogenic ooze deposition

The spatial distribution and character of biogenic oozes depend on several factors: (1) the flux of biogenic material from the sea surface to the sea floor, (2) its rate of dissolution before and after deposition, and (3) its dilution by non-biogenic components during deposition.

The initial flux of carbonate and silica to the sea bed is determined by the level of primary productivity within the euphotic zone (the upper layer of the ocean penetrated by sunlight), which in turn depends on the availability of nutrients (e.g. phosphorus, nitrogen, silica). Relatively low nutrient levels are characteristic of the euphotic zone, owing to limited vertical mixing with underlying nutrient-rich water. However, the continuous ‘rain’ of decaying organic matter from the euphotic zone ensures that nutrients build up in the underlying water column. There are high rates of primary production in regions where these nutrient-rich waters upwell, such as coastal upwelling zones (e.g. off the coast of Peru, the Gulf of California) and at sites of oceanic divergence (e.g. eastern equatorial Pacific). These environments favour diatom algal blooms; thus the underlying sediments are typically dominated by diatom oozes. In contrast, pelagic carbonates predominate in areas peripheral to upwelling cells, i.e. where primary productivity is still moderately high. Regions of low productivity (i.e. nutrient-poor waters) are characterized by biogenic sedimentation rates low in carbonate and negligible in silica. Coccolith oozes are typical of such environments; for example, the Black Sea. Dilution of biogenic sediments by non-biogenic material occurs mainly at continental margins (the continental slope and rise). Near-shore terrigenous sedimentation rates usually exceed those of biogenic sediments. With increasing distance from the continental margin, there is a gradation from terrigenous-dominated sediment to pelagic-dominated sediment.

The dissolution characteristics of opaline silica in sea water are very different from those of carbonate. Sea water is everywhere undersaturated with respect to silica. Opaline silica thus starts to dissolve immediately it is exposed to sea water. This dissolution process continues throughout the time it takes for material to settle down through the water column, at the sediment–water interface, and during burial, until such time as the pore waters become saturated with respect to silica, or the material is dissolved.

At ocean water depths of less than 4–5 km, sea water is supersaturated with respect to carbonate, whereas at greater depths it is undersaturated. The depth at which carbonate sediments start to show signs of dissolution is termed the lysocline; the depth at which they are no longer preserved is termed the carbonate compensation depth, or CCD. Hence, pelagic carbonates are preferentially deposited on topographical ‘highs’, such as mid-ocean ridges and seamounts. Also, the position of the CCD is not static throughout the ocean basins, but varies both in space and time. The depth of the CCD in modern oceans is controlled by several factors, including temperature, pressure, and the decay of organic matter. As the temperature of sea water falls, the solubility of carbonate increases. Increasing pressure has a similar effect. At depths greater than 4–5 km, the low temperature of sea water and the pressure exerted on it by the overlying water column are sufficient to ensure that it is undersaturated with respect to carbonate. The presence of carbon dioxide further increases the solubility of carbonate. With increasing age, the gradual addition of decaying organic matter to deep water progressively increases its carbon dioxide content. Older deep waters in ocean basins therefore have shallower CCDs than younger deep waters in ocean basins. Glacial and interglacial climatic conditions are characterized by contrasting deep water (thermohaline) circulation patterns, and reduced atmospheric carbon dioxide contents. The CCD will thus differ from one climatic state to the next.

The distribution of pelagic sediments

The Atlantic Ocean is characterized by widespread carbonate oozes, the Pacific and Southern Ocean by widespread siliceous oozes, and the Indian Ocean typically contains a mixture of both sediment types (Table 1). As outlined above, this sediment distribution is determined by the bottom water circulation, which controls both the rate of dissolution and the productivity of surface waters through upwelling. By comparing the depth of the CCD in the North Pacific and North Atlantic Oceans, the effect of ageing deep water is clearly illustrated. In the North Atlantic, the deep water circulation is dominated by oxygen-rich North Atlantic Deep Water (NADW), which forms from nutrient-poor surface waters in the Greenland and Norwegian Seas. The CCD of the North Atlantic is correspondingly deep (c. 5 km). In the Pacific deep water contains a component of NADW, as well as a component of surface Antarctic water. Combined, these two water masses form Antarctic Bottom Water. Thus, the relative age of Pacific deep water is greater than that of NADW, and the CCD is relatively shallow (c. 4 km in the South Pacific, 3.5 km in the North Pacific). The shallow depth of the CCD in the Pacific, and the abundance of nutrients in upwelled waters, also contribute to the widespread occurrence of siliceous deposits in this ocean.

Table 1. Percentage of deep ocean floor covered by pelagic sediments.

Sediments	Atlantic	Pacific	Indian	World
Note: pelagic clays are not detailed in this entry. (Source: Open University (1991).)
Calcareous ooze	65.1	36.2	54.3	47.1
Pteropod ooze	2.4	0.1	–	0.6
Diatom ooze	6.7	0.1	19.9	11.6
Radiolarian ooze	–	4.6	0.5	2.6
Pelagic clays	25.8	49.0	25.3	38.1
Relative size of ocean
(% of total)	23.0	53.4	23.6	100.0

R. B. Pearce

Bibliography

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Kennett, J. P. (1982) Marine geology. Prentice Hall, Inc., Englewood Cliffs, New Jersey.
Open University (1991) Ocean chemistry and deep-sea sediments (2nd edn). Pergamon Press, Oxford.