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.[1] Pelagic sediments that are mixed with terrigenous sediments are known as hemipelagic.

There are three main types of pelagic sediments:[2]

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.[3] More specifically, any sediment composed of more than 30% of microscopic skeletal debris is classified as an ooze.[4] Whatever their provenance, all pelagic sediments accumulate extremely slowly, at no more than a few centimeters per millennium.[5]

The type of sediment that accumulates in a given locale is determined by the location's distance from land, water depth, and overall fertility.[6] 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.[7]

Contents

[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.[8]
  • 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.[9]
  • 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.[10] Covering 38% of the ocean floor, it accumulates more slowly than any other sediment type, at only 0.1-0.5 cm / 1000 yr.[11]

[edit] See also

[edit] Footnotes:

  1. ^ Pinet 83, Rothwell 70.
  2. ^ Rothwell 70.
  3. ^ Pinet 99.
  4. ^ Pinet 101.
  5. ^ Rothwell 77.
  6. ^ Rothwell 73.
  7. ^ Rothwell 73.
  8. ^ Rothwell 74-5, 77.
  9. ^ Rothwell 75, 77.
  10. ^ Pinet 99-100, Rothwell 76.
  11. ^ Rothwell 75, 77.

[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
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