Pelagic Sediments
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.
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.
These particles can be organic or inorganic:
Organic materials: The most common organic component of pelagic sediments are the shells or tests of microscopic marine organisms like phytoplankton and zooplankton. These can be calcareous (made of calcium carbonate) or siliceous (made of silica).
Inorganic materials: Inorganic materials in pelagic sediments include wind-blown dust and volcanic ash, as well as clay minerals carried long distances by ocean currents.
Oozes
Ooze is a type of soft, slimy mud that accumulates on the ocean floor, particularly in deep-sea environments far from land. It's formed from the gradual settling and compacting of the skeletal remains of tiny marine organisms that lived and died near the surface, sinking down to the depths after they perish.
In case of
marine sediments, ooze does not refer to a sediment's consistency, but
to its composition, which directly reflects its origin. Ooze is pelagic
sediment that consists of at least 30% of microscopic remains of either
calcareous or siliceous planktonic debris organisms. The remainder
typically consists almost entirely of clay minerals.
As a result,
the grain size of oozes is often bimodal with a well-defined biogenic
silt- to sand-size fraction and siliciclastic clay-size fraction. Oozes
can be defined by and classified according to the predominate organism
that compose them.
There are two main types of ooze, distinguished by the primary component of the organism skeletons that make them up:
- Calcareous ooze: This type of ooze is composed of more than 30% calcium carbonate shells or tests, the hard parts of organisms like foraminifera (single-celled animals with shells) and coccolithophores (algae with calcium carbonate plates).
- Siliceous ooze: Siliceous ooze is made up of at least 30% silica skeletons or frustules, the hard parts of diatoms (algae with glass-like cell walls) and radiolarians (single-celled organisms with intricate skeletons).
For example, there are diatom, coccolith, foraminifera, globigerina, pteropod, and radiolarian oozes. Oozes are also classified and named according their mineralogy, i.e. calcareous or siliceous oozes. Whatever their composition, all oozes accumulate extremely slowly, at no more than a few cm per millennium.
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| Total Sediment thickness of the ocean floor, with the thinnest deposits in dark blue and the thickest in red. Note the abundant deposits along the east and gulf coasts of North America, in the South China Sea, and in the Bay of Bengai east of India |
