Marine Sediments

There are two main classifications for marine sediments, terrigenous and pelagic. Terrigenous sediments are derived from land and found near shore. They are normally delivered by rivers to coastal regions. Pelagic sediments settle slowly out of the water column and are deposited all over the ocean. Pelagic sediments are deposited at such low rates that they tend to be overwhelmed near shore by terrigenous deposits from land. So, pelagic sediments are normally associated with deep sea regions.

Terrigenous Classifications

Terrigenous sediments are normally classified by their size. Sediment grain size commonly depends on the energy of the depositional environment, with more energy leading to larger grain sizes. So, a steep mountain stream, where water flows quickly downhill, might have very large grain sizes, from gravel to pebbles to cobbles or even boulders. A river bed, where currents have slowed, might be filled with sand or silt. Beaches, where waves crash against the shore and the surf runs in and out, often have sand-sized particles as well. Farther offshore, where the water is less active, terrigenous materials will be smaller silt or clay size particles. The official Wentworth grain size scale looks something like this:

Name	Size (mm)
Boulder	256 or more
Cobble	64-256
Pebble	4-64
Gravel or Granule	2-4
Coarse sand	0.5-2
Medium sand	0.25-0.5
Fine sand	0.0625-0.25
Silt	0.0039-0.0625
Clay	0.0002-0.0039

Sometimes terrigenous sediment is further classified by composition (quartz, feldspar, limestone, lithic fragments).

Pelagic Classifications

Unlike terrigenous sediments, pelagic sediments are classified by composition, not size. The size of pelagic sediments is uniformly pretty small, so their widely varying compositions are more interesting. There are four main compositional groups for marine sediments:

Lithogenous: This is sediment that is derived from rocks. Remember, it's pelagic, so it has to settle out of the water column and will be most prevalent far from land. So, lithogenous pelagic sediment can be wind-blown dust (this is called eolian sediment), volcanic ash, or other fine particles that were originally rocks. Lithogenous sediment dominates in deep areas such as the Pacific Ocean away from the East Pacific Rise.

Biogenous: Biogenous sediment is derived from living organisms, normally planktonic organisms because they're the most abundant. Planktonic life comes in a variety of forms and species, but the kinds that form biogenous sediment are the kinds that have shells that are resistant to dissolution or destruction. The most common shell materials for plankton are calcite (CaCO₃ or calcium carbonate) and opal (SiO₂ or silica), and most biogenic marine sediment comes from four species as shown here:

	Calcareous (calcite shells)	Siliceous (opal shells)
Phytoplankton (plant-like photosynthesizers)	Coccoliths	Diatoms
Zooplankton (animal-like grazers)	Foraminifera	Radiolaria

Siliceous organisms (diatoms and radiolaria) tend to be found near polar regions and along the equator. Calcareous organisms (coccoliths and foraminifera) tend to be found in shallower water in temperate and tropical regions. Calcite dissolves at great depths, so deep ocean basins won't have any calcareous sediments.

Fecal Pellet Express In water, bigger heavier particles sink faster than smaller lighter ones. Because planktonic organisms are so small, it seems like they should take a long time to sink and might get dispersed by currents, hitting the bottom far from where they originally lived. However, species on the ocean floor tend to represent the species in the surface water very well, so this kind of dispersal doesn't happen. Why? Because of the fecal pellet express. When big organisms eat plankton, they can't digest the shells. The shells are excreted in fecal pellets which sink rapidly to the ocean floor and then break down.

Hydrogenous: Some types of sediment are derived from the ions in seawater. Near hydrothermal vents, lots of metal ions are released into the water, and these ions oxidize or combine with silica and precipitate out as dark, metal-rich sediment. Manganese nodules are another form of hydrogenous sediments. Hydrogenous sediments are less common than lithogenous or biogenous sediments. They are almost never the dominant sediment type.

Cosmogenous: The Earth is continually being bombarded from space by meteors and cosmic dust. Some of this material doesn't burn up in the atmosphere and reaches the oceans, where it can settle down to the ocean floor. Cosmogenous sediment is never a dominant type of sediment -- it is never more than a tiny fraction.