Deep-sea Sedimentation

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