Marine Geology: The Bottom of the Ocean
 Marine Science Chapters


The Bottom of the Ocean

The bottom of the ocean is a very important place for marine life. Just about 90% of all marine species live at least part of their life associated with the bottom. We call this mode of life 'benthic.' These species can be found in the intertidal, shelf, bathyal, abyssal, and hadal zones. Some species even live under the surface of the bottom of the ocean (called infauna). The bottom of the ocean is a major area of decomposition where organic material is recycled (cells are broken down and nutrients are released by bacteria and fungi).

Ocean Bottom Sediment Map
Ocean bottom sediment map. Lithogenous areas are mauve, biogenous areas are purple and brown (purple = siliceous ooze, brown = calcareous ooze), and hydrogenous areas are blue.
The actual physical bottom of the ocean (or substrate) is generally some type of rock or sediment and can be categorized as terrogenous, biogenous or hydrogenous. Terrogenous bottoms cover much of the Central and North Pacific as well as polar regions and all of the continental borderlands. Biogenous bottoms are common in temperate oceans less than 4,000-5,000 meters deep like the South Pacific, North and South Atlantic, and the Indian Ocean. Biogenous bottoms are also common in high latitude sea floors around Antarctica and the North Pacific as well as a band in the equatorial Pacific. Hydrogenous bottoms have wide distributions but are more concentrated in the Central and South Pacific.

Terrogenous Ocean Bottoms
Terrogenous ocean bottoms consist of materials like rock, sand and mud.
Terrogenous bottoms come from Earth's crust. These bottoms can be solid rock, or the products from weathering and erosion of this rock. Much of the weathering and erosion takes place on land, after which the resulting sediments are washed into the ocean. These sediments carpet the bottom of the ocean near almost all the continents in the area called the continental shelf (continental borderlands). Terrogenous bottoms can also be called lithogenous (or lithogenic - from the lithosphere).

Forams from Calcareous Ooze

Biogenic shell fragments are studied by placing each shell in a numbered box on a special microscope slide. The boxes are four millimeters on each side and numbered. Forams are in the boxes of the image to the left and a variety of shell fragments are in the image below. Clam shells are in boxes 7 and 19, snail shells in boxes 8 and 20, bryozoa shells in boxes 9, 21 and 22, and a sea urchin spine is in box 10. This slide was prepared for the author by one of her students, Ted Townsend, who became very interested in calcareous oozes.

Calcareous Ooze
Biogenous ocean bottoms are sediments that are mixed with a large amount of shell material. Calcareous oozes have predominantly calcium shells like tiny formainiferans (a tiny zooplankter also called forams) - above left, or other calcium shell fragments - above right.
Siliceous Ooze
Siliceous oozes have a preponderance of siliceous (glass) shells, primarily from tiny phytophlankters called diatoms. The white chunks, seen above, are nearly solid diatom shells with little sediment. When the siliceous ooze is this concentrated it is called 'diatomaceous earth' and mined for use as a grit and filter. Many paints have diatomaceous earth mixed with them (especially primers) as a fine grit. Many swimming pool filters have diatomaceous earth because the tiny diatom shells have ultra tiny pores that are excellent filters.
Biogenous bottoms are sometimes referred to as oozes depending on the most common type of shell material in the sediments. Calcareous oozes result from a dominance of calcium shells which were originally from organisms such as foraminifera, snails, and sea urchins. These calcareous oozes are never found deeper than about 4,000 to 5,000 meters because the calcium dissolves at deeper depths. Siliceous oozes result from a dominance of siliceous (glass) shell pieces made from planktonic organisms like diatoms and radiolarians. Where there are major plankton blooms with a sudden die off (tropical Pacific off South America, North Pacific off the Aleutian Islands, and all around Antarctica) there are tremendous numbers of diatoms that reproduce and die each year contributing to the siliceous material on the sea floor. This shell material may build up at the rate of 1mm to 1cm every 1,000 years depending on the productivity.

Manganese Nodules
Hydrogenous ocean bottoms are primarily manganese nodule deposits (above). The shark tooth was found inside of one of the larger nodules.
Hydrogenous ocean bottoms result from dissolved materials (primarily metals) precipitating from seawater. Most of these are called manganese nodules and are composed of layers of metals such as manganese (Mn), iron (Fe), nickel (Ni), cobalt (Co) and copper (Cu). They can be from the size of a pea to that of a potato and are always on the surface of the sediment. These deposits could be a future source for metals but for now the mines on land are used as our source. There would be problems with mining these manganese nodules because they are found in international waters and so it would take a unique kind of treaty to get permission to mine the ocean bottom. Hydrogenous deposits are also called authigenous (or authigenic).

Worms mixing the sediments Worm building tube in sediment
Worms can change the sediments by mixing like the blood worm, Euzonus, does on California sandy beaches (left). Worms can also build tubes in the sediments to circulate water and oxygen like the fat innkeeper worm, Urechis, does in California mud flats (right).

Worm tube of cemented sand
Worms can cement the sand grains to form tubes like the ice cream cone worm, Pectinaria, does in California (above).

Fecal pellets from ghost shrimp Rock boring clams
Deposit feeding marine organisms leave compact fecal pellets of sediments like the ghost shrimp, Callianassa, does in California mud flats (left). Rock boring clams, like Penitella, the flat-tipped piddock (right), can actually bore into rock along the rocky shores of California, weakening this solid substrate.
Marine life can change the bottom of the ocean. This can happen because of infaunal worms who mix the sediments. Tubes created in the bottom of the ocean can be used by animals like anemones and worms to draw water, with dissolved oxygen, below the surface. Some species, especially worms, cement the sediments together, making very hard tubes. Other species ingest the sediments as deposit feeders. They digest the organics found between the sediments and excrete the cleaned sediments as compact fecal pellets. Finally, some species actually bore into solid rock - making it weak and resulting in sediments produced from the boring of the organism (like the rock boring clams) rather than erosion. In any case, the bottom of the ocean can be transformed by the life forms that live there whether they live under its surface (the infauna - animals living within the substrate) or on its surface (the epifauna - animals living on the substrate, and flora - plants).

History is recorded in ocean bottom sediments as they build up layer by layer. Ash from volcanic eruptions sinks and forms a layer, the shells of the organisms living there sink and form their layers. Geologists take cores of the ocean bottom sediments and study them layer by layer. This is also the way they prospect for oil and gas because certain species of foraminifera live during the periods of oil and gas production - a scientist can tell if it might be worth drilling in an area of the ocean bottom just by looking at the forams in cores of the sediment. Periods of productivity are also preserved in the layers. Most of the ocean bottom is covered with sediments except volcanic areas.

Students on a rocky shore Students on a sandy beach Students in a mud flat
Marine science students can study local shorelines at low tide such as rocky shores (left), sandy beaches (middle) and mud flats (right).
Shorelines of the ocean are either solid rock, sandy beaches, or mudflats. Solid rock shorelines often have great tidepools at low tide. Sandy beaches come in a variety of colors and sizes of the sand. Mudflats are found in areas protected from waves. Each of these particular shorelines has a different community of marine creatures living there and adapted to the unique conditions of that shoreline. The chapter on Marine Ecology will introduce you to each of these communities found on California shorelines.

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(Revised 6 August 2007)
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