, 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
. Pelagic sediments that are mixed with terrigenous sediments are known as hemipelagic
There are three main types of pelagic sediments:
- 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. More specifically, any sediment composed of more than 30% of microscopic skeletal debris is classified as an ooze. Whatever their provenance, all pelagic sediments accumulate extremely slowly, at no more than a few centimeters per millennium.
The type of sediment that accumulates in a given locale is determined by the location's distance from land, water depth, and overall fertility. 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.
Details of Sediment Types
- Calcareous ooze is composed primarily of the shells--also known as tests--of foraminifera, coccolithophores, and pteropods. This is the most common pelagic sediment by area, covering 48% of the world ocean's floor. This type of ooze is limited to depths above the Carbonate Compensation Depth at time of burial. It accumulates more rapidly than any other pelagic sediment type, with a rate that varies from 0.3 - 5 cm / 1000 yr.
- Siliceous ooze is composed of the debris of plankton with silica shells, such as diatoms and radiolaria. This ooze is limited to areas with high biological productivity, such as the polar oceans, and upwelling zones near the equator. The least common type of sediment, it covers only 15% of the ocean floor. It accumulates at a slower rate than calcareous ooze: 0.2-1 cm / 1000 yr.
- Red clay, also known as pelagic clay, accumulates 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. Covering 38% of the ocean floor, it accumulates more slowly than any other sediment type, at only 0.1-0.5 cm / 1000 yr.
- 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