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Deep-sea Sedimentation

Deep-sea Sedimentation. Input of sediment from the coast is Due to the accumulation of river material on the outer shelf and slope especially during lowered sea level. this material can get to the sea in several ways one way is through Slumps , which are sediment piles that move as a mass

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Deep-sea Sedimentation

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  1. Deep-sea Sedimentation • Input of sediment from the coast is • Due to the accumulation of river material on the outer shelf and slope especially during lowered sea level. • this material can get to the sea in several ways • one way is through Slumps, which are sediment piles that move as a mass • their internal structure (bedding) is retained.

  2. Deep-sea Sedimentation • Input of sediment from the coast (cont.) • Slurries are mixes of sediment and water and can sweep even boulder-sized particles down slope • The internal structure (bedding) of the sediment mix is destroyed. • Turbidity currents are large slurries which rapidly move downslope.

  3. Deep-sea Sedimentation(Cont.) • Turbidity Currents: • are propelled by gravity • water + sediment weighs more than just plain water • once started, they can go for miles and miles

  4. Deep-sea Sedimentation(Cont.) • Turbidite beds are • Formed at the bottom of the slope as the turbidity current slows. • An example of graded bedding. • Similar to river deltas.

  5. Deep-sea Sedimentation • Turbidity currents are often triggered by earthquakes • this quake triggered a flow which broke 4 communications cables • from the timing of the breaks, we can measure the speed of the flow

  6. Deep-sea Sedimentation • Turbidity currents often flow through submarine canyons. • may have created them • definitely help maintain them

  7. Turbidite Deposition • These flows result in VERY flat seafloor • Can extend hundreds of miles from their source • Result in the abyssal plains

  8. Deep-sea Sedimentation • Ice rafting is another way particles can be transported from the coast to the deep sea. • only at polar latitudes (obviously). • Carries large amounts of terrigneous sediment from the land to the sea . • The material is dropped as the ice melts. • sediments are typically poorly sorted with a wide range of grain sizes.

  9. Deep-sea Sedimentation(Cont.) Deep-sea sediments around Antarctica include ice-rafted (glacial-marine) sediment near the coast.

  10. Aeolean Transport • Volcanic eruptions can put particles into the upper atmosphere (eg Mt. Pinatubo) • these particles travel around the world

  11. Deep-sea Sedimentation • Pelagic sedimentation is largely responsible for sediments in the deep-sea. • It consists of mostly very fine particles. • It’s origins are • Inorganic • Organic

  12. Deep-sea Sedimentation(Cont.) • Inorganic pelagic sediments are mostly red clay • It occurs mostly where nothing else is present to overwhelm it • background signal • consists of “clay minerals” such as: • Kaolinite • Chlorite • Illite • Feldspar • The color is due to oxidized iron (rust). • It is synonymous with brown or pelagic clay. • Quartz can also fall into this category, but it technically isn’t a clay mineral

  13. Clay mineral distributions • Clays are found where nothing else overwhelms them.

  14. Deep-sea Sedimentation • Pelagic sediments composed of the remains of living organisms are known as Biogenic oozes. • This is much more interesting than clay! • They consist of 30% or more of the skeletal remains of surface dwelling microscopic planktonic organisms. • They are either calcareous or siliceous.

  15. Carbonate Sediments • Carbonate (CaCO3) is essentially chalk • It is produced by forams and coccolithophorids in the open ocean • and by corals, shells, coralline algae and some other organisms in shallow water • These organisms manufacture CaCO3 from Ca and CO2 from the seawater • Neither of these is in short supply • therefore CaCO3 can be produced anywhere (and is)

  16. Deep-sea Sedimentation(Cont.) • Calcareous biogenic oozes are formed by • Zooplankton (“animals” [protozoa]) • foraminifera • pteropods • Phytoplankton (plants: need light!) • coccolithophorids

  17. Carbonate Sediments • However, it is highly susceptible to dissolution and therefore isn’t preserved everywhere • A good generic equation to describe carbonate dissolution is this one: CaCO3 + CO2 + H2O  Ca++ + 2CO3= +2H+ • Notice that CO2enters into the picture • remember that CO2comes from respiration: • CH2O + O2  CO2 + H2O • Anywhere respiration is high, CO2will be high • Anywhere CO2is high, CaCO3will dissolve

  18. Carbonate Sediments • In the surface, where there is light, photosynthesis dominates • CO2 + H2O  CH2O + O2 • This uses up CO2 and produces O2 • Therefore, in the surface, where there is light: • CO2 is low • CaCO3 preservation is good • However, in the deep sea, where there is no light, respiration dominates • CH2O + O2  CO2 + H2O • This produces CO2 • This dissolves CaCO3 making its preservation poor

  19. Carbonate Sediments • To be completely honest, CO2isn’t the real culprit but it’s the cause • CO2 reacts with water to form carbonic acid • acids dissolve CaCO3 • Other factors are involved as well, including: • pressure: high pressure increases dissolution of CaCO3 • temperature: low temperature increases dissolution of CaCO3 • The result is that is preservation is poorest in: • cold water • deep water • “old” (hasn’t been ventilated recently) water

  20. Carbonate Sedimentation • The result is that is preservation is poorest in: • cold water • deep water • “old” (hasn’t been ventilated recently) water

  21. Deep-sea Sedimentation • The existence of calcareous sediments depends on the Carbonate Compensation Depth, the depth in the ocean below which carbonate is not found in the sediments. • This depth depends on temperature, pH, salinity, pressure and the rate of supply of calcium carbonate. • Calacareous oozes are not common below 4000 m Snow line

  22. Because of thermohaline circulation: • Pacific water is older than Atlantic • it contains more CO2 • it is more corrosive to CaCO3 • Therefore the CCD is _________ in the Pacific than in the Atlantic

  23. Siliceous and Carbonate Sediments • 1-626 here

  24. Deep-sea Sedimentation(Cont.) • Siliceous biogenic oozes are derived from • Phytoplankton • diatoms • Zooplankton • radiolarians

  25. Siliceous Sediments • Silica (siliceous sediment) production is entirely biogenic • Therefore, we need to consider the biology which is involved • Si is a “nutrient” because it is in very short supply in the ocean • To understand silica production, we need to understand a bit of biology • Recall the equation for photosynthesis • CO2 + H2O  CH2O + O2 • What this doesn’t show is that nutrients are also required • CO2 + H2O  CH2O + O2 nutrients

  26. Siliceous Sediments • Because dissolved silica is in VERY short supply and acts like a nutrient, particulate silica will only be produced where dissolved silica input is high • This is generally true where total production is high, such as: • coastal areas: nutrients are recycled from the seafloor because it’s close to the surface • upwelling areas: deep, nutrient rich water is brought back to the surface.

  27. Siliceous Sediments • Why are there a lot of nutrients, including Si, in deep water?

  28. Nutrients (eg Nitrate) are low in the surface and increase with depth • Oxygen is the opposite

  29. Silica Sedimentation • silica dissolves in surface water • distribution of siliceous sediment controlled by production

  30. Siliceous and Carbonate Sediments • 1-626 here

  31. Sedimentation Summary • silica and carbonate are essentially opposite • silica dissolves in surface water while carbonate dissolves in deep water • distribution of siliceous sediment controlled by production • distribution of carbonate sediment controlled by water depth

  32. Deep-sea Sedimentation(Cont.) • These characteristics make it easy to predict what kind of sediment you’ll find almost anywhere in the world!

  33. Deep-sea Sedimentation(Cont.) Distribution of deep-sea sediments.

  34. Summary • Sediment gets to the deep sea by: • biogenic production • transport from shore • authigenic production • Siliceous Ooze: • is found where production is highest • is preserved in deep water • Carbonate ooze: • is found in shallow water only • distribution controlled by water depth

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