Pelagic sediment
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Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceous shells of phytoplankton or zooplankton; clay-size siliciclastic sediment; or some mixture of these. Trace amounts of meteoric dust and variable amounts of volcanic ash also occur within pelagic sediments. Based upon the composition of the ooze, there are three main types of pelagic sediments: siliceous oozes, calcareous oozes, and red clays.[1][2]
The composition of pelagic sediments is controlled by three main factors. The first factor is the distance from major landmasses, which affects their dilution by terrigenous, or land-derived, sediment. The second factor is water depth, which affects the preservation of both siliceous and calcareous biogenic particles as they settle to the ocean bottom. The final factor is ocean fertility, which controls the amount of biogenic particles produced in surface waters.[1][2]
Oozes
In case of
Red and brown clays
Red clay, also known as either brown clay or pelagic clay, accumulates in the deepest and most remote areas of the ocean. It covers 38% of the ocean floor and accumulates more slowly than any other sediment type, at only 0.1–0.5 cm/1000 yr.
These pelagic sediments are typically bright red to chocolate brown in color. The color results from coatings of iron and manganese oxide on the sediment particles. In the absence of organic carbon, iron and manganese remain in their oxidized states and these clays remain brown after burial. When more deeply buried, brown clay may change into red clay due to the conversion of iron-hydroxides to hematite.[2]
These sediments accumulate on the ocean floor within areas characterized by little planktonic production. The clays which comprise them were transported into the deep ocean in suspension, either in the air over the oceans or in surface waters. Both wind and ocean currents transported these sediments in suspension thousands of kilometers from their terrestrial source. As they were transported, the finer clays may have stayed in suspension for a hundred years or more within the water column before they settled to the ocean bottom. The settling of this clay-size sediment occurred primarily by the formation of clay
Distribution and average thickness of marine sediments
| Region | Percent of ocean area[citation needed] | Percent of total volume of marine sediments | Average thickness |
|---|---|---|---|
| Continental shelves | 9% | 15% | 2.5 km (1.6 mi) |
| Continental slopes | 6% | 41% | 9 km (5.6 mi) |
| Continental rises | 6% | 31% | 8 km (5 mi) |
| Deep-ocean floor | 78% | 13% | 0.6 km (0.4 mi) |
Classification of marine sediments by source of particles
| Sediment type | Source | Examples | Distribution | Percent of all ocean floor area covered |
|---|---|---|---|---|
Terrigenous |
volcanic eruptions , blown dust |
estuarine mud |
Dominant on abyssal plains , polar ocean floors |
~45% |
| Biogenous | Organic; accumulation of hard parts of some marine organisms | Calcareous and siliceous oozes | Dominant on deep-ocean floor (siliceous ooze below about 5 km) |
~55% |
| Hydrogenous (authigenic) | Precipitation of dissolved mineral from water, often by bacteria | Manganese nodules, phosphorite deposits |
Present with other, more dominant sediments | 1% |
| Cosmogenous | Dust from space, meteorite debris |
Tektite spheres, glassy nodules | Mixed in very small proportion with more dominant sediments | 1% |
See also
- Chalk
- Diatomaceous earth
- Marine geology
- Petrological Database of the Ocean Floor
- Radiolarite
- SedDB, online database for sediment geochemistry
- Biogenous Ooze
Footnotes
- ^ ISBN 0-12-636380-3
- ^ ISBN 978-0-444-53000-4
- ISBN 0-922152-76-4
External links
- http://www.odp.usyd.edu.au
- Total Sediment Thickness of the World's Oceans and Marginal Seas, Version 2
