Download Sediments Are Historical Records of Ocean

Oceanography
An Invitation to Marine Science, 7th
Tom Garrison
Chapter 5
Sediments
Chapter 5 Study Plan
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Sediments Vary Greatly in Appearance
Sediments May Be Classified by Particle Size
Sediments May Be Classified by Source
Neritic Sediments Overlie Continental Margins
Pelagic Sediments Vary in Composition and
Thickness
• Scientists Use Sensitive Tools to Study Ocean
Sediments
• Sediments Are Historical Records of Ocean
Processes
• Marine Sediments Are Economically Important
Chapter 5 Main Concepts
• Sediments are loose accumulations of particulate
material. Their depth and composition tell us of
relatively recent events in the ocean basin above.
• The most abundant sediments are terrigenous (from
land) and biogenous (from once-living things). The
volume of terrigenous sediment exceeds that of
biogenous sediment, but biogenous material covers
a greater area of seabed.
• Marine sediments have been uplifted and exposed
on land. Arizona’s Grand Canyon is made of marine
sediment.
• Because marine sediments are usually subducted
along with the seabed on which they lie, the oldest
sediments are relatively young—rarely older than
180 million years.
Sediments May Be Classified
By Particle Size
Sediments are
particles of organic or
inorganic matter that
accumulate in loose,
unconsolidated forms.
Sediments can be
classified by particle
size. Waves and
currents generally
transport smaller
particles farther than
larger particles.
Sediments May Be Classified
By Particle Size
The velocities of currents
required for erosion,
transportation, and
deposition
(sedimentation) of
sediment particles of
different sizes.
To dislodge and carry a
particle of size A, the
speed of a current must
exceed 20 centimeters
per second (8 inches per
second). When the
current falls below 1
centimeter per second
(1/2 inch per second), the
particle will be deposited.
Sediments May Be Classified by
Source
Terrigenous Sediments Come
from Land
The Sediment Cycle.
Over geological time,
mountains rise as lithospheric
(crustal) plates collide, fuse,
and subduct. Water and wind
erode the mountains and
transport resulting sediment to
the sea. The sediments are
deposited on the seafloor,
where they travel with the
plate and are either uplifted or
subducted. Thus, the material
is eventually made into
mountains again.
Marine Sediments Are Usually Both
Terrigenous and Biogenous Deposits
The sediment of
continental shelves is
called neritic
sediment, and
contains mostly
terrigenous material.
Sediments of the
slope, rise, and deepocean floors are
pelagic sediments,
and contain a greater
proportion of
biogenous material.
Insert
Table 5.3
Marine Sediments Are Usually Both
Terrigenous and Biogenous Deposits
(above) The general pattern of sediments on the ocean floor. Note the
dominance of diatom oozes at high latitudes.
What differences in the type and distribution of sediments do you note between
the Atlantic Ocean and the Pacific Ocean?
Pelagic Sediments Vary in
Composition and Thickness
• Turbidites – deposits made by turbidity currents
• Oozes – sediment containing at least 30% biogenous material
– Siliceous oozes are formed by organisms that contain silica in
their shells. Diatoms are one type of organism whose remains
contribute to siliceous ooze.
– Calcareous oozes are formed by organisms, such as
foraminifera, which contain calcium carbonate in their shells.
• Hydrogenous sediments - originate from chemical reactions that
occur in the existing sediment.
– Are often found in the form of nodules containing manganese
and iron oxides
– Evaporites are salts that precipitate as evaporation occurs and
include many salts with economic importance. Evaporites
currently form in the Gulf of California, the Red Sea, and the
Persian Gulf
Turbidites Are Deposited on the
Seabed by Turbidity Currents
The formation of turbidites.
The turbidity current moves quickly down the continental shelf and slope, sometimes
encountering (and further eroding) a submarine canyon. When the material comes to
rest, it sorts into layers with coarse sediment at the bottom and finer sediment above.
Each graded layer is the result of one turbidity current event.
Oozes Form from the Rigid
Remains of Living Creatures
The dashed line shows the calcium carbonate (CaCO3) compensation
depth (CCD). At this depth, usually about 4,500 meters (14,800 feet),
the rate at which calcareous sediments accumulate equals the rate at
which those sediments dissolve.
Scientists Use Sensitive Tools to
Study Ocean Sediments
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How do scientists study sediments?
Deep-water cameras
Clamshell samplers
Piston Corers
Core libraries
Seismic profilers
Scientists Use Sensitive Tools to
Study Ocean Sediments
One method of studying sediments uses a
clamshell sampler. The sampler can be used to
obtain a relatively undisturbed sediment sample.
Scientists Use Sensitive Tools to
Study Ocean Sediments
One method of obtaining
core samples by research
vessels such as the JOIDES
Resolution is by using a
piston corer.
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.
Sediments Are Historical
Records of Ocean Processes
• What can scientists learn by studying
sediments?
– Historical information
– Location of natural resources, especially crude
oil and natural gas
• Marine sediments are important as historical
records and a site of natural resources.
Scientists study marine sediments using
many different methods, including core
samples.
Sediments Are Historical
Records of Ocean Processes
The ages of portions of the Pacific Ocean floor, based on core samples
of sediments just above the basalt seabed, in millions of years ago (Ma,
mega-annum). The youngest sediments are found near the East Pacific
Rise; and the oldest, close to the eastern side of the trenches.
Chapter 5 in Perspective
In this chapter you learned that the sediments covering nearly all of the
seafloor are parts of the great cycles of formation and destruction assured by
Earth’s hot interior. Marine sediment is composed of particles from land, from
biological activity in the ocean, from chemical processes within water, and even
from space. The blanket of seafloor sediment is thickest at the continental
margins and thinnest over the active oceanic ridges. Sediments may be
classified by particle size, source, location, or color. Terrigenous sediments, the
most abundant, originate on continents or islands. Biogenous sediments are
composed of the remains of once-living organisms. Hydrogenous sediments are
precipitated directly from seawater. Cosmogenous sediments, the ocean’s
rarest, come to the seabed from space.
The position and nature of sediments provide important clues to Earth’s recent
history, and valuable resources can sometimes be recovered from them.
In the next chapter you will learn about our story’s main character – water
itself. You know something about how water molecules were formed early in the
history of the universe, something about the inner workings of our planet, and
something about the nature of the ocean’s “container.” Now let’s fill that
container with water and see what happens.