Transcript Chapter 1 The Growth of Oceanography

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Introduction to
Oceanography
Sedimentology and Ocean Basin Topography
Why study sediments?
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Ocean sediment includes particles from land, biological activity,
chemical processes and space.
Ocean sediment is thickest over continental margins and thinnest
over active oceanic ridges.
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Sediment deposited on a quiet seafloor can provide a sequential
record of recent events in the water column above. Sediments are
recycled into earth at subduction zones.
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Sediments are an important source of crude oil and natural gas.
•Historical information
•Location of natural resources, especially crude oil and natural gas
Sediment in the Sea 4-1
Classification of marine sediments can be based
upon size or origin.
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• Size classification divides sediment by grain size into
gravel, sand and clay.
– Mud is a mixture of silt and clay.
• Origin classification divides sediment into five
categories: Terrigenous sediments, Biogenic sediments,
Authigenic sediments, Volcanogenic sediments and
Cosmogenic sediments.
Classifying Sediment
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Sediment can be classified by particle size. Waves and currents
generally transport smaller particles farther than larger particles.
Classifying Sediment
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Sediment can also be classified according to its source.
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Sediment in the Sea
Factors that control sedimentation include particle
size and the turbulence of the deposition
environment.
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• Terrigenous sediments strongly reflect their source and are
transported to the sea by wind, rivers and glaciers.
• Rate of erosion is important in determining nature of
sediments.
• Average grain size reflects the energy of the depositional
environment.
• Hjulstrom’s Diagram graphs the relationship between particle
size and energy for erosion, transportation and deposition.
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Distribution of Sediments
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The sediment of
continental shelves is
called neritic
sediment, and
contains mostly
terrigenous material.
Sediments of the
slope, rise, and
deep-ocean floors
are pelagic
sediments, and
contain a greater
proportion of
biogenous
material.
Shelf sedimentation
4-2 is strongly controlled by tides,
waves and currents, but their influence decreases
with depth.
• Shoreline turbulence prevents small particles from settling and
transports them seaward where they are deposited in deeper
water.
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• Particle size decreases seaward for recent sediments.
• Past fluctuations of sea level has stranded coarse sediment
(relict sediment) across the shelf including most areas where
only fine sediments are deposited today.
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Worldwide distribution
of recent shelf sediments by
composition is strongly related to latitude and
climate.
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• Calcareous biogenic sediments dominate tropical shelves.
• River-supplied sands and muds dominate temperate shelves.
• Glacial till and ice-rafted sediments dominate polar shelves.
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Sedimentation in the Ocean
Geologic controls of continental shelf
sedimentation must be considered in terms of a
time frame.
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• 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
level lowered by glaciation controlled
sedimentation and caused rivers to deposit
their sediments at the shelf edge and onto
the upper continental slope.
• For a time frame up to 100,000,000 years,
plate tectonics has determined the type of
margin that developed and controlled
sedimentation.
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Sedimentation in the Ocean
Deep-sea Sedimentation has two main sources for
sediment: terrigenous material from the land and
biogenic and authigenic from the sea.
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• Major sedimentary processes in the deep sea include:, Bulk
emplacement, Debris flows, Turbidity currents
• Major pelagic sediments in the ocean are red clay and biogenic
oozes.
• Authigenic deposits are chemical and biochemical precipitates
that form on the sea floor and include ferromanganese nodules
and phosphorite.
Sediments of Deep-Ocean Basins
Ooze is classified by the type of life form from which it is derived.
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Calcareous ooze is formed by organisms, such as foraminifera,
which contain calcium carbonate in their shells or skeletons.
Siliceous ooze is formed by organisms that contain silica in their
shells. Diatoms are one type of organism whose remains contribute
to siliceous ooze.
Hydrogenous sediments are usually the result of chemical
reactions. Hydrogenous sediments are often found in the form of
nodules containing manganese and iron oxides.
Sediments of Deep-Ocean Basins
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Evaporites are precipitates that form as water evaporates or as the
conditions in the water change
Evaporites include many salts with economic importance.
Evaporites currently form in the Gulf of California, the Red Sea, and the
Persian Gulf.
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Sedimentation in the Ocean
The distribution of sediments in the deep ocean
varies greatly, but is strongly controlled by the
compensation depth.
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• Surface Deposits are the sediments found exposed on the sea
floor.
• Deep-sea stratigraphy refers to the broad-scale layering of the
sediments of the sea floor. It strongly reflects the sediment
types deposited as the sea floor
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Distribution Of Sediments
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What differences in the type and distribution of sediments do you note
between the Atlantic Ocean and the Pacific Ocean?
Studying Sediments
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How do scientists study sediments?
 Deep-water cameras
 Clamshell samplers
 Piston Corers
 Core libraries
 Seismic profilers
Studying Sediments
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One method of studying sediments uses a clamshell sampler. The
sampler can be used to obtain a relatively undisturbed sediment sample.
Studying Sediments
One method of obtaining core
samples by research vessels such as
the JOIDES Resolution is by using a
piston corer.
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The 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.
An example of drill data
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Based on data
from core
samples,
scientists have
determined the
age of portions of
the Pacific floor,
measured in
mega-annums, or
millions of years.
Summary
Marine sediments are important as historical records and a site of natural
resources. Scientists study marine sediments using many different
methods, including core samples.
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