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Sediments on the Sea Floor

Why is the sediment on the oceanic crust thicker nearthe continents?

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Sediments

Based on datafrom coresamples,scientists havedetermined theage of portions ofthe Pacific floor,measured inmega-annums, ormillions of years.

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Island Arcs - chains of volcanic islands andseamounts, usually found parallel to the edges ofocean trenches.

Deep-Ocean Basins

As two oceanic plates converge, an island arc isformed by volcanic activity.

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Studying Sediments: Key Ideas

Ocean sediment includes particles from land, biological activity, chemical processes and space.

Ocean sediment is thickest over continental margins and thinnestover active oceanic ridges.

Sediment deposited on a quiet seafloor can provide a sequentialrecord of recent events in the water column above. Sediments arerecycled into earth at subduction zones.

Sediments are an important source of crude oil and natural gas.

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Deep-water cameras

Clamshell samplers

Piston Corers

Seismic profilers

Studying Sediments

What can we learn by studying sediments?

• Historical information

• Location of natural resources, especially crude oil and naturalgas

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Classification of Marine Sediments: Size orOrigin.

Size classification divides sediment by grain size intogravel, sand, silt and clay.

Mud is a mixture of silt and clay.Origin classification divides sediment into fivecategories: terrigenous, biogenic, authigenic,volcanogenic and cosmogenic sediments.

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Classifying SedimentSediment can be classified by particle size. Waves and currentsgenerally transport smaller particles farther than larger particles.

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Table 3.1

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Comparison of Poorly-Sorted and Well-Sorted Sediments

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Factors that Control Sedimentation: Source,Particle Size, Turbulence

Terrigenous sediments strongly reflect theirsource and are transported to the sea by wind,rivers and glaciers.Average grain size reflects the energy of thedepositional environment.

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Hjulstrom’s Diagram

Hjulstrom’s Diagram graphs the relationshipbetween particle size and energy for erosion,transportation and deposition.

Note how weakercurrents can movesmaller particles of

sediment, whilestronger currents

are needed to erodelarger sediment.

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Classifying Sediment

Sediment can also be classified according to its source.

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Distribution of Sediments

The sediment ofcontinental shelvesis called neriticsediment, andcontains mostlyterrigenousmaterial.

Sediments of theslope, rise, anddeep-oceanfloors are pelagicsediments, andcontain a greaterproportion ofbiogenousmaterial.

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Shelf Sedimentation: Controlled byTides, Waves and Currents

Shoreline turbulence prevents small particlesfrom settling and transports them seawardwhere they are deposited in deeper water.

Particle size decreases seaward for modernsediments.

Past fluctuations of sea level have strandedcoarse sediment (relict sediment) across theshelf including most areas where only finesediments are deposited today.

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Time Frames for Continental ShelfSedimentation

For a time frame up to 1000 years, waves,currents and tides control sedimentation.

For a time frame up to 1,000,000 years, sea levellowered by glaciation controlled sedimentationand caused rivers to deposit their sediments atthe shelf edge and onto the upper continentalslope.

For a time frame up to 100,000,000 years, platetectonics has determined the type of margin thatdeveloped and controlled sedimentation.

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Distribution of Modern ShelfSediments: Related to latitude and

climate

Calcareous biogenic sediments dominatetropical shelves.River-supplied sands and muds dominatetemperate shelves.Glacial till and ice-rafted sedimentsdominate polar shelves.

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Sediments of Deep-Ocean Basins

• Red Clays - very fine, slowly settling deposits

• Turbidites – deposits made by turbidity currents

• Oozes – deep-ocean sediment containing at least30% biogenous material

•Siliceous ooze•Calcareous ooze

• Hydrogenous sediments - originate from chemicalreactions that occur in the existing sediment

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Ooze is classified by the type of life form from which it isderived.

Calcareous ooze is formed by organisms, such as foraminifera, which contain calcium carbonate in theirshells or skeletons.

Siliceous ooze is formed by organisms that contain silica intheir shells. Diatoms are one type of organism whoseremains contribute to siliceous ooze.

Hydrogenous sediments are usually the result of chemicalreactions. Hydrogenous sediments are often found in theform of nodules containing manganese and iron oxides.

Biogenous Oozes

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Calcite (or Calcium Carbonate)Compensation Depth (CCD)

CaCO3 dissolves in deeper water due tothe higher CO2 content in deep waterDeepest sediments have little or no CaCO3

The CCD is deeper (less dissolution) in theAtlantic Ocean than in the Pacific. Why?

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Major sedimentary processes in the deep seainclude: Bulk emplacement, Debris flows,Turbidity currentsMajor pelagic sediments in the ocean are redclay and biogenic oozes.Authigenic deposits are chemical andbiochemical precipitates that form on the seafloor and include ferromanganese nodules andphosphorite.

Sedimentation in the Ocean

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Distribution of Sediments in the DeepOcean: Related to Latitude, Distance

from Landmasses, and the CCD

Glacial marine sediments occur in thehigh latitudes.Pelagic clays occur far from land and inthe deepest water.Calcareous oozes occur above thecalcium carbonate composition depth.The rate of sedimentation depends onthe type of sediment in deep sea.

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Evaporites are precipitates that form as waterevaporates or as the conditions in the waterchange

Evaporites include many salts with economicimportance.

Evaporites currently form in the Gulf ofCalifornia, the Red Sea, and the Persian Gulf.

Evaporites

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The Mediterranean basin is located where platesare colliding as Africa moves northward relative toEurope.Anhydrite and stromatolites of Miocene ageindicate that the Mediterranean sea “dried” outbetween 5 and 25 million years ago.Two models have been suggested to account forthis emptying of the Mediterranean Sea of itswater.

The “Uplift” ModelThe “Drying-Out” Model

4-4Drying Up of the Mediterranean Sea