Sonjia Leyva - Leture Notes: Marine Sediments

Classifying Sediment

What are Sediments?
- Particles entering the ocean
- Accumulate
  - Rapidly on continental margin (pelagic)
  - Slowly in the deep ocean (neritic)
- Reflect ocean history
Classifying Sediment
- Sediment can be classified by particle size.
  - Waves and currents generally transport smaller particles farther than larger particles.
  - The velocities of currents vs. erosion rates of sediments.
  - Note how weaker currents can move smaller particles of sediment, while stronger currents are needed to erode larger sediment.
- Sediment can also be classified according to its source.

Sediment Sources

Sediment Sources
- Lithogenous
- Biogenous
- Chemical precipitates
- Cosmogenous
Sediment Sources: Lithogenous Sediments
- Sediments from terrestrial (land) sources
- Includes
  - Sands and muds from continental margins
  - Glacial deposits
  - Clays
- Red Clay
  - Found in low productivity areas
  - Low sedimentation rates
  - Wind-blown sediment
Sediment Sources: Biogenous Sediments
- Ooze = greater than 30% biogenous sediment
  - Distribution related to sediment supply, rate of dissolution and sediment dilution
  - Siliceous fossils at all depths, calcareous not below Calcium Carbonate Compensation depth
  - Diatoms and radiolarians = major siliceous microfossils
  - Calcareous = foraminifera, pteropods, coccolithophores
- Siliceous Oozes
  - Fine-grained pelagic deposit
  - Composition:
  - 30% siliceous (SiO₂) material of organic origin
  - Diatoms (phytoplankton) and Radiolaria (zooplankton)
  - Siliceous particles dissolve more slowly than calcareous particles
  - Diatoms
    - Composed of SiO₂
    - Phytoplankton
    - Base of food chain
  - Radiolaria
    - Composed of SiO₂
    - Zooplankton
    - Base of food chain
- Calcareous Oozes
  - Wide-spread in relatively shallow areas of the deep sea
  - CaCO₃ particles dissolve at “Carbonate Compensation Depth” = (CCD)
    - Atlantic: ~ 4,000 m
    - Pacific: ~ 500 - 1,500 m
  - Foraminifera
    - Composed of calcium carbonate (CaCO₃)
    - Zooplankton
Sediment Sources: Chemical Precipitates
- Evaporites are precipitates that form as water evaporates or as the conditions in the water change
  - Include many salts with economic importance.
  - Currently form in the Gulf of California, the Red Sea, and the Persian Gulf.
- Ferromanganese Nodules
  - First sub-marine ferromanganese concretions were discovered in 1868 on the Kara Sea (Russia)
  - Characteristics - small balls (lightly flattened), dark-brown, and 5 - 10 cm in diameter
  - Found at depths of 4,000 to 6,000 m
  - Not clear how these nodules form

Distribution of Sediments

Sediments of Continental Margins
- Sediments can accumulate to great thickness on continental margins.
- Shelf sediments can be converted into sedimentary rock in the process of lithification.
Sediments of Deep-Ocean Basins
- How can the sediments of deep-ocean basins be classified?
  - Turbidites – deposits made by turbidity currents
  - Oozes – deep-ocean sediment containing at least 30% biogenous material
  - Hydrogenous sediments - originate from chemical reactions that occur in the existing sediment
  - Evaporites - salts that precipitate as evaporation occurs
Rates Of Deposition
- Deep ocean 0.5-1.0cm/1000 yrs
  - slow rates yet can get thick because oldest crust is 200 m.y.
  - clays take 100 yrs to sink 3000 m, need packaging

Studying Sediments

How do scientists study sediments?
- Deep-water cameras
- Clamshell samplers
- Dredges
- Piston Corers
- Core libraries
- Seismic profilers
Grab Sampler
- Good for collecting soft sediment, sand or perhaps gravel.
- Low tech - basically just a weighted cage that is dragged along the sea floor.
Piston Corer
- Allows a cylinder of sediment to be taken for analysis to determine the age of the material, as well as the density, strength, molecular composition and radioactivity of the sediment.
- Used by research vessels such as the JOIDES Resolution
Drilling Ocean Cores
Corer
- This figure shows an examination of deep-ocean sediment cores.
- Long cylinders of sediment and rock called cores are cut in half and examined, revealing interesting aspects of Earth history. APT photo
What can scientists learn by studying sediments?
- Historical information
- Location of natural resources, especially crude oil and natural gas

Seabed Resources

Depletion of onshore resources - need alternatives
- Sand & Gravel
- Phosphorite
- Sulfur
- Coal
- Oil and Gas
How Much Do We Need?
SulfideDeposits
Hydrocarbon Seeps
Gas Hydrate
- Formed from a mixture of water and natural gas, usually methane.
- Occurs in the pore spaces of sediments
- Found in sub-oceanic sediments in the polar regions (shallow water) and in continental slope sediments (deep water), where pressure and temperature conditions combine to make it stable.
- Where Is Gas Hydrate Found?

Classifying Sediment

Sediment Sources

Distribution of Sediments

Studying Sediments

Seabed Resources

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