Pelagic sediments
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Pelagic sediments, also known as marine sediments, are those that accumulate in the abyssal plain of the deep ocean, far away from terrestrial sources that provide terrigenous sediments; the latter are primarily limited to the continental shelf, and deposited by rivers.(Pinet 83, Rothwell 70). Pelagic sediments that are mixed with terrigenous sediments are known as hemipelagic.
There are three main types of pelagic sediments (Rothwell 70):
- 1.) Siliceous oozes
- 2.) Calcareous oozes
- 3.) Red clays
Ooze does not refer to a sediment's consistency, but to its origin; oozes are primarily biogenic--that is, composed of planktonic debris--while red clays are non-biogenic, with little organic material.(Pinet 99) More specifically, any sediment composed of more than 30% of microscopic skeletal debris is classified as an ooze.(Pinet 101) Whatever their provenance, all pelagic sediments accumulate extremely slowly, at no more than a few centimeters per millennium.(Rothwell 77)
The type of sediment that accumulates in a given locale is determined by the location's distance from land, water depth, and overall fertility.(Rothwell 73) For instance, the increased solubility of carbon dioxide in seawater with pressure makes the water column more corrosive with depth; below the carbonate compensation depth of ~4.5 km, carbonate dissolution equals deposition.(Rothwell 73)
[edit] Details of Sediment Types
- Calcareous oozes are composed primarily of the shells--also known as tests--of foraminifera, coccolithophores, and pteropods. These are the most common pelagic sediments by area, covering 48% of the world ocean's floor. They also accumulate more rapidly than any other pelagic sediment type, with a rate that varies from 0.3 - 5 cm / 1000 yr.(Rothwell 74-5, 77)
- Siliceous oozes are composed of the debris of plankton with silica shells, such as diatoms and radiolaria. These oozes are limited to areas with high biological productivity, such as the polar oceans, and upwelling zones near the equator. This is the least common type of sediment, covering only 15% of the ocean floor. It accumulates at a slower rate than calcareous ooze: 0.2-1 cm / 1000 yr.(Rothwell 75, 77)
- Red clays, also known as pelagic clays, accumulate in the deepest and most remote areas of the ocean. Containing less than 30% biogenic material, its composition is a varied mix of very fine quartz and clay minerals, authigenic deposits precipitated directly from seawater, and micrometeorites. Though called "red" because it sometimes takes the color of oxidized iron minerals, it is usually brownish in color. Its ultimate origin is uncertain, but red clay seems to be mostly derived from distant rivers, and windblown dust.(Pinet 99-100, Rothwell 76) Covering 38% of the ocean floor, it accumulates more slowly than any other sediment type, at only 0.1-0.5 cm / 1000 yr. (Rothwell 75, 77)
[edit] See also
[edit] References
- Pinet, Paul R. Invitation to Oceanography St. Paul: West Publishing Company, 1996. ISBN 0-314-06339-0
- Rothwell, R. G., "Deep Ocean Pelagic Oozes", Vol. 5. of Selley, Richard C., L. Robin McCocks, and Ian R. Plimer, Encyclopedia of Geology, Oxford: Elsevier Limited, 2005. ISBN 0-12-636380-3
