Marine sediment

Marine sediments are deposits of insoluble particles that accumulate on the seafloor. These have their origins in soil and rocks which have been transported from the land to the sea, mainly by rivers but also as windbourne as dust and by the flow of glaciers into the ocean. Additional deposits come from marine organisms and chemical precipitation in seawater, as well as underwater volcanoes and debris from meteorites.

History

The first major study of deep-ocean sediments occurred between 1872 and 1876 with the HMS Challenger expedition.

Classification

By size

The Wentworth Scale classifies sediment by grain size. Clay sediments are the smallest with a grain diameter of less than .004 mm and boulders are the largest with grain diameters of 256 mm or larger.

By origin

Sediments are also classified by origin. There are four types: lithogenous, hydrogenous, biogenous and cosmogenous.

Lithogenous sediments come from land via rivers, ice, wind and other processes. Biogenous sediments come from organisms like plankton when their exoskeletons break down.Hydrogenous sediments come from chemical reactions in the water. Cosmogenous sediments come from space, filtering in through the atmosphere or carried to Earth on meteorites.

Marine microfossils

Thickness of marine sediments

See also: Marine biogeochemical cycles

Further information: Calcareous ooze, Biogenic silica, Radiolarite, Sedimentary rock, Microfossil, and Reverse weathering

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous.

Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos microorganisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray, the "father of modern oceanography", who proposed the term radiolarian ooze for the silica deposits of radiolarian shells brought to the surface during the Challenger Expedition. A biogenic ooze is a pelagic sediment containing at least 30 percent from the skeletal remains of marine organisms.

Main types of biogenic ooze
type	mineral forms	protist involved		name of skeleton	typical size (mm)
Siliceous ooze	SiO2 silica quartz glass opal chert	diatom		frustule	0.002 to 0.2		diatom microfossil from 40 million years ago
		radiolarian		test or shell	0.1 to 0.2		elaborate silica shell of a radiolarian
Calcareous ooze	CaCO3 calcite aragonite limestone marble chalk	foraminiferan		test or shell	under 1		Calcified test of a planktic foraminiferan. There are about 10,000 living species of foraminiferans
		coccolithophore		coccoliths	under 0.1		Coccolithophores are the largest global source of biogenic calcium carbonate, and significantly contribute to the global carbon cycle. They are the main constituent of chalk deposits such as the white cliffs of Dover.

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  Diatomaceous earth is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms (click to magnify)
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  Illustration of a Globigerina ooze
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  Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor

Stone dagger of Ötzi the Iceman who lived during the Copper Age. The blade is made of chert containing radiolarians, calcispheres, calpionellids and a few sponge spicules. The presence of calpionellids, which are extinct, was used to date this dagger.

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  Opal can contain protist microfossils of diatoms, radiolarians, silicoflagellates and ebridians
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  Marble can contain protist microfossils of foraminiferans, coccolithophores, calcareous nannoplankton and algae, ostracodes, pteropods, calpionellids and bryozoa
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  Carbonate-silicate cycle

Calcareous microfossils from marine sediment consisting mainly of star-shaped discoaster with a sprinkling of coccoliths

Distribution of sediment types on the seafloor
Within each colored area, the type of material shown is what dominates, although other materials are also likely to be present.
For further information, see here

Microorganisms

Diatoms form a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live, and contribute nearly half of the organic material found in the oceans.

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  Diatoms are one of the most common types of phytoplankton
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  Their protective shells (frustles) are made of silicon
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  They come in many shapes and sizes

Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.

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  Like diatoms, radiolarians come in many shapes
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  Also like diatoms, radiolarian shells are usually made of silicate
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  However acantharian radiolarians have shells made from strontium sulfate crystals
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  Cutaway schematic diagram of a spherical radiolarian shell

Like radiolarians, foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called tests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) of silica. Most forams are benthic, but about 40 species are planktic. They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.

Foraminiferans

...can have more than one nucleus

...and defensive spines

Foraminiferans are important unicellular zooplankton protists, with calcium tests

Marine microbenthos

Archaea rock - this deep ocean rock harboured worms that consumed methane-eating archaea

See also: Seabed § Sediments, bioturbation, and bioirrigation

Marine microbenthos are microorganisms that live in the benthic zone of the ocean – that live near or on the seafloor, or within or on surface seafloor sediments. The word benthos comes from Greek, meaning "depth of the sea". Microbenthos are found everywhere on or about the seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In shallow waters, seagrass meadows, coral reefs and kelp forests provide particularly rich habitats. In photic zones benthic diatoms dominate as photosynthetic organisms. In intertidal zones changing tides strongly control opportunities for microbenthos.

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  Elphidium a widespread abundant genus of benthic forams
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  Heterohelix, an extinct genus of benthic forams

Both foraminifera and diatoms have planktonic and benthic forms, that is, they can drift in the water column or live on sediment at the bottom of the ocean. Either way, their shells end up on the seafloor after they die. These shells are widely used as climate proxies. The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stable oxygen isotopes in the shells, since lighter isotopes evaporate more readily in warmer water leaving the heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water.

The sudden extinction event which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever.

Some endoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years. These are slowly metabolizing and not in a dormant state. Some Actinobacteria found in Siberia are estimated to be half a million years old.

Organic matter supply to sediments ocean 

(1) Organic matter settling from the water column is deposited at seafloor (donor control; fixed flux upper boundary condition).
(2) Sediments in the photic zone are inhabited by benthic microalgae that produce new organic matter in situ and grazing animals can impact the growth of these primary producers.
(3) Bioturbating animals transfer labile carbon from the sediment surface layer to deeper layers in the sediments. (Vertical axis is depth; horizontal axis is concentration)
(4) Suspension-feeding organisms enhance the transfer of suspended particulate matter from the water column to the sediments (biodeposition).
(5) Sponge consume dissolved organic carbon and produce cellular debris that can be consumed by benthic organisms (i.e., the sponge loop).

Different approaches to carbon processing in marine sediments 

            Paleoceanographers focus on the sedimentary record
            Biogeochemists quantify carbon burial and recycling
            Organic geochemists study alteration of organic matter
            Ecologists focus on carbon as food for organisms living in the sediment

The red–orange–yellow fractions of organic matter have a different lability

Ecology

• Marine microanimals
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  Darkfield photo of a gastrotrich, 0.06-3.0 mm long, a worm-like animal living between sediment particles
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  Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel

See also

• Bay mud
• Hemipelagic sediment
• Marine clay
• Microbially induced sedimentary structure
• Oolitic aragonite sand
• Pelagic sediments