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Lecture 12: OCEAN SEDIMENTS II

Sources (origins)
Distribution

Powerpoint Lecture Slides


TYPES (BY SOURCE):
Lithogenous ("rock-derived')
Biogenous ("life-derived")
Hydrogenous ("water-derived")
Cosmogenous ("cosmic-derived")

 

COSMOGENOUS SEDIMENTS
Micro-meteorites
silicates, metals, mixtures
wide-spread, but not abundant
Impacts of large extra-terrestrial bodies (asteroids, comets)
Catastrophic results
Cause of "mass extinctions"? -- dinosaurs

HYDROGENEOUS SEDIMENTS
(1) Direct precipitation (ppt) from sea water
(2) Sediment - sea water reaction

"Salts" = Dissolved ions in sea water
Evaporation of sea water in "restricted" basins
Ppt. of "salts" -- NaCl (halite) and CaSO4.2H2O (gypsum)
Manganese nodules
Nodules and crusts of Mn & Fe oxides (Cu, Co, Ni) in deep ocean and along mid-ocean ridges.
Origin (?) -- source of metals
Sediments
Sea water
Hydrothermal activity (remember "black smokers")
 

LITHOGENOUS SEDIMENTS

(1) Terrigenous sediments -- continental source

Erosion and transportation
-- streams and rivers
-- wind
-- glaciers

Transport to continental margins (and possibly to Deep-sea floor)

75 % of all marine sediments are lithogenous
Mostly stay ("trapped") on continental margins
Pacific Ocean: terrigenous sediment trapped in margins and trenches; little transported to deep-sea floor.
 
(2) "Red Clay" -- terrigenous and volcanic ash
Transported to open ocean by winds and surface currents.
Settles through water column -- pelagic clay.
Common in deep oceans where other types are absent.

BIOGENOUS SEDIMENTS -- produced directly by organisms

Warm, shallow seas -- shells of animals (clams, corals)
Open ocean (deep sea) -- "shells" of microscopic "plankton"

Components of sediment-producing plankton:

1. CaCO3 (calcium carbonate)
Coccolithophores Foramifera
2. SiO2.(+nH2O) (opaline silica)
Diatoms Radiolarians
 
Biogenic "oozes" -- > 30% of biogenous sediment
Calcareous ooze
Siliceous ooze

Distribution of biogenous oozes:

1. Production in surface waters:
Biological productivity controlled by nutrients -- N and P
"Upwelling" brings nutrients to surface waters
Productivity high in upwelling zones
2. Dissolution in deep waters
Deep waters "undersaturated" -- particles tend to dissolve
"Carbonate Compensation Depth" (CCD)
Calcareous oozes absent below CCD
Atlantic: ~ 4,000 m
Pacific: ~ 500 - 1,500 m
Siliceous particles dissolve more slowly; depth is not an issue
3. Dilution by other sediments
High input of terrigenous sediment
"Dilutes" biogenous components to < 30 %

SUMMARY -- Distribution of sediment on the sea floor (modern sediments)

Terrigenous: Continental margins and adjacent abyssal plains.
Manganese nodules: Deep basins, especially the Pacific.
Red Clay: Deep ocean regions where there is very little biogenous or terrigenous sediment
Calcareous oozes: Widespread in relatively shallow areas of the deep sea.
Siliceous oozes: Polar and equatorial bands where nutrients are supplied to surface waters by vertical upwelling.
 

(Detailed notes start here)

Sources and origins of marine sediments

In the previous lecture, we discussed how sediments are classified according to where they are deposited (neritic, pelagic, etc.) and by their grain size (sand, silt, clay, etc.). Another useful and important classification scheme for classifiying ocean sediments is by their sources, or origins.

In terms of the total volume of ocean sediment, the two most important "types" of sediment in this classification are lithogenous ("rock-derived") and biogenous (produced by marine organisms). Other types are hydrogenous (produced by chemical reactions in sea water) and cosmogenous (particles from outer space). Let's consider those source/origin types in more detail.

COSMOGENOUS SEDIMENTS that accumulate in the oceans (and land) are essentially "micro-meteorites" whose composition can be either silicate (like the mantle), metal (like the core), or a mixture of the two.
- Although there is a continuous "rain" of these particles on Earth, cosmogenous sediments make up a very minor component of ocean sediments -- in fact, it takes an expert to identify them.
- But throughout the history of our planet, very large extra-terrestrial bodies (large meteorites, asteroids, even comets) have collided with the Earth from time to time.
- The results are catastrophic! -- a large, deep crater; molten rock and ash ejected into the atmosphere, world-wide fires.
- Many scientists have concluded that major impacts cause "mass extinctions," like the extinction of the dinosaurs and 80% of all living species 65 m.y. ago.

HYDROGENEOUS SEDIMENTS form either by
(1) direct precipitation from sea water, or
(2) as a new mineral from chemical reactions between sea water and
sediments on the sea floor

"Salts" - - Sea water contains dissolved ions (about 3.5 % by weight).
- When sea water undergoes extensive evaporation in bays or even large seas that are restricted from "communicating" (circulating) with the "open" ocean, the concentration of ions can increase to the point where they are "saturated" with respect to certain solids that we simply call "salts."
- Important sea salts are NaCl (halite) and CaSO4.2H2O (gypsum).
- Salts precipitate from "super-saturated" sea water in restricted basins and are deposited on the sea floor.

"Manganese nodules" - - are oxides of the metals manganese (Mn) and iron (Fe). They also contain minor amounts of copper (Cu), cobalt, (Co), and nickel (Ni).
- Manganese nodules occur as nodules and crusts, primarily in deepest of oceans and near mid-ocean ridges.
- Their origin is controversial. We do not know the source of the metals nor how and why they are concentrated in nodules.
- The metals may be delivered as dissolved species or as sediments from land.
- A more likely origin is volcanic activity and hydrothermal alteration of ocean crust at mid-ocean ridges. Remember the "black smokers?" Much of the suspended particles are Mn and Fe oxides.

LITHOGENOUS SEDIMENTS in the ocean come in two varieties: (1) terrigenous ("land-derived") and (2) "red clay."

Terrigenous sediments are produced by the physical and chemical "weathering" (alteration) of rocks exposed on continents. Sedimentary particles are then eroded and transported from land to the oceans. Most sediment is carried by rivers, but winds and glaciers are also important transport agents. River-transported sediment is deposited on continental margins and mostly stays there. Only a small fraction is transported to the open ocean (e.g., by turbidity currents).

Terrigenous sediments make up 75 % of all the sediment that is deposited in the ocean. The thickest accumulations (up to 10 km!) occur in continental rises. But keep in mind that most terrigenous sediment remains on continental margins -- river deltas, bays, estuaries, as well as continental rises. In the Pacific, sediment from land is trapped in those environments and in marginal trenches. As a consequence, relatively little terrigenous sediment is transported to the deep-sea floor of the Pacific.

"Red clay" is the name applied to very fine-grained (clay-sized), reddish-brown lithogenous sediment in the deep ocean. The sources of red clay are land (terrigenous) and volcanic ash. Red clay is transported to the open ocean by wind and surface currents. It is deposited by settling through the water column, that is, it is a pelagic sediment. Red clay is the dominant component of pelagic sediments in deep basins where other types of sediment are absent.

BIOGENOUS SEDIMENTS are particles that are produced directly by marine organisms.
- In warm shallow seas, most biogenous sediment are the shells or shell fragments of multicellular animals, such as corals and clams.
- But in the open ocean (deep sea), biogenous sediment is made up of the microscopic shells (1- 0.01 mm) of single-celled plants and animals, called plankton, that live in surface waters above about 200 m.

Biogenous sediments in the open-ocean are called "oozes" (a term applied to the first biogenous sediments recovered in the mid-1800's).
- Oozes of made up of the microscopic shells of calcareous and siliceous algae and/or protozoans.
- By definition, biogenous oozes contain > 30% of a biogeni component; the remainder is non-biogenic sediment, such as lithogenous mud.

The distribution of biogeneous sediments is controlled by three important processes:

1. Production in surface waters: The growth of marine algae (which are the base of the oceanic food chain) is controlled principally by the availability of two critical nutrient elements -- N (nitrogen) and P (phosphorus). These critical nutrients are mostly supplied to surface waters by deep waters that "upwell" to the surface. Therefore, biological productivity is high in areas of strong vertical upwelling -- along the equator, in certain coastal regions, and in the Southern Ocean around Antarctica.

2. Dissolution in deep waters: Deep-ocean waters are undersaturated in calcium carbonate and opalline silica. Therefore, biogenic particles tend to dissolve as they settle through the water column and as they sit on sea floor. This effect is more pronounced for calcareous sediments. In fact, calcareous oozes are absent below a certain depth called the "Carbonate Compensation Depth, or CCD. The depth of the CCD varies from ocean to ocean. It occurs at 4,000 m in the Atlantic. The CCD is shallower in the Pacific, at depths of 500 - 1,500 m. Siliceous particles dissolve more slowly as they sink and are not limited in distribution by depth as much. Nutrient supply more important in controlling the distribution of siliceous sediments.

3. Dilution: Calcareous and siliceous components can be diluted to
< 30 % of the total sediment (and therfore not qualify as a "biogenous ooze") in regions where the input of terrigenous sediment is very high. This occurs along continental margins -- surface productivity is high and dissolution is minimal, but biogenous oozes don't occur because of the high influx of terrigenous sediment from continents

SUMMARY -- Distribution of sediment on the sea floor (modern sediments)

Terrigenous: - - continental margins and adjacent abyssal plains.
Manganese nodules: - - deep basins, especially the Pacific.
Red Clay: - - deep ocean regions where not diluted by biogenic particles.
Calcareous oozes: - - wide-spread in relatively shallow areas of the deep sea.
Siliceous oozes: - - polar and equatorial bands where nutrients are supplied to surface waters by vertical upwelling.


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