GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 7: Ocean Sediments
POWERPOINT SLIDE SHOW NOTES
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Topic 7: Ocean Sediments
Sources of ocean sediment.
Sediment accumulation rates.
Rivers are dominant source of lithogenous sediment.
Different size particles are given different names by geologists, no matter their
composition. Sand-size is common on beaches. Note how long it takes for tiny grains to
settle 4 kilometers (average depth of ocean). Individual clay particles may take 50 years
to settle. The asterisk is because the tiny clay particles usually don’t settle as individual
grains but as clumps of grains, attracted to one another by electrical charges.
We often group sediments into three size ranges. Compare to the names on previous
slide. Anything coarser than sand is collectively called gravel. Anything finer is
collectively called mud.
Visual of different sediment sizes.
Break down of gravel sizes. Different earth surface processes (wind, rivers, glaciers, etc.)
can move different size sediments. So size is a very important characteristic of sediment.
Break down of sand sizes.
Break down of mud sizes.
Particle-size ranges in marine sediments. Notice the scale at the top is in micrometers (a
micrometer is one-millionth of a meter, or one-thousandth of a millimeter), and the scale
at the bottom is in millimeters. Most sediments occur over a range of sizes. By the way,
you may come across the symbol “μm” which means micrometer, micro (μ) meter (m).
Classification of marine sediment by source. Terrigenous sediments are also known as
lithogenous.
Terrigenous versus lithogenous
A few examples of terrigenous sediment.
Major river drainages of North America. Width of river as drawn indicates the amount of
water flowing in that river at a given point.
The Mississippi River brings a lot of sediment into the Gulf of Mexico.
Glaciers introduce a lot of sediment into the nearshore.
A glacier flowing down a mountain valley and melting into an ocean bay.
Dust from desert wind storms can be carried by the wind for thousands of miles.
Volcano eruptions are a source of sediment.
The ash cloud from the 1991 Mount Pinatubo eruption introduced sediment to the
oceans when it settled.
Microtektites are cosmogenous sediments.
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Microtektites. Very important point—glass is a term used in two ways by geologists.
One is the chemical composition glass, which refers to things made up of quartz. Just
like glass in windows. The other usage refers to texture. That is what we mean with
microtektites. The texture glass means there are no crystals. It means molten rock froze
instantly and it looks like glass, but the composition is whatever the original material
was. In this case, the asteroid and surrounding surface rock that was melted by the force
of the impact. We’ll see the composition glass when we talk about biogenic sediments
later on.
Formation of microtektites
The asteroid that killed the dinosaurs created many microtektites.
Biogenic sediments
An ooze is a fine-grained sediment. There are two dominant types based on
composition, calcareous and siliceous. Calcareous sediments are made up of the
mineral calcite. Its chemical name is calcium carbonate. These sediments are often
referred as carbonate as well as calcareous. Siliceous sediments are made up of the
mineral quartz. Its chemical name is silica dioxide. Normal regular old glass is quartz.
Common calcareous organisms. Spicules actually come in a variety of compositions.
Common siliceous organisms
The shells of calcareous organisms bleach to a white, chalky color once the organism
dies.
The white cliffs of Dover are made up of trillions and trillions of calcareous microfossils
(small fossils). This is chalk.
Foraminifera are single-celled calcareous animals. They come in all different shapes and
sizes. This is a common shape. The animal lives inside the shell just like a snail.
A scanning electron microscope image of a single foraminifera. The electron microscope
does not show the true color of the specimen. You would see its natural, chalky white
color seen previously if viewed under a light microscope. The scale bar at the lower right
is 10 micrometers (μm).
A coccolithophore is a single-celled calcareous plant. The individual plates of the plant
break apart when it dies. The is a scanning electron microscope image.
Scanning electron microscope image of individual plates of coccolithophores, which are
called coccoliths.
Scanning electron microscope image of foraminifera, radiolarian, and sponge spicules.
Scanning electron microscope image of radiolaria and foraminifera.
Scanning electron microscope image of sponge spicules and radiolarian.
There are many different types of biogenic sediments. Think of all the animals with
shells. Many plants have hard cases and coverings.
Sea stars and sand dollars
Sand dollars
Corals get broken by storm waves or chewed by fish to become ocean sediment.
Crab
Reference source for some photos.
Muddy sea bottom with shell fragments.
Muddy sea bottom with shell fragments.
Muddy sea bottom with sea stars.
The calcium carbonate compensation depth controls where we will find calcareous
sediment on the sea floor. The CCD can be thought of as the bottom of a zone of water
extending from the sea surface to a different depth in different oceans.
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A calcareous shell that stays above the CCD (shallower depths, say on the mid-ocean
ridge or a seamount) will not dissolve and thus the sea floor will be covered with
calcareous sediment. A calcareous shell that falls below the CCD (away from ridges or
the continental margins) will dissolve and the sea floor will be covered with something
other than calcareous sediment. The depth of the CCD is controlled by how much
calcium carbonate is introduced into a particular ocean. Rivers carry a lot of dissolved
calcium carbonate, mostly from dissolving limestone to form caves and sinkholes. The
world’s largest rivers flow into passive continental margins, and the Atlantic Ocean is
essentially all passive margin. Thus the Atlantic receives a lot of calcium carbonate and
the zone of saturated water extends deeper than in other oceans.
In general, calcareous sediments accumulate in shallow parts of the ocean, such as the
mid-ocean ridges.
Sediments can be classified by source as we’ve been discussing, but they can also be
classified by where they are found—their location. Pelagic is a term we’ll see again. It
basically means “of the water column.”
Pelagic sediments accumulate slowly.
Neritic sediments accumulate at a wide variety of rates because of continental processes
such as river floods, hurricanes, and glacial events, to name a few.
Hydrogenous sediments precipitate directly out of sea water without biologic processes
being involved. These are not shells or other remnants of living organisms.
Manganese nodules cover wide expanses of the deep ocean floor in places. They contain
valuable minerals in addition to manganese, such as nickel and copper.
Slicing through a manganese nodule reveals a nucleus which may be a bone or shell,
surrounded by concentric layers of precipitated minerals. The rate of growth of nodules
is very slow, maybe 1 mm per million years. Chemical precipitation from sea water only
occurs when there is a hard surface on which the minerals can grow. That means
precipitation won’t occur if the nodule is buried in mud, and can only occur on the part
exposed to water. The concentric layers indicate that the nodule must be rolled
occasionally so that precipitation can continue around the entire nodule. This gave us
the information that there are strong currents in places on the deep ocean floor, a place
once thought to be stagnant and devoid of currents.
Manganese nodules could be a valuable resource but they are in such deep water that
collecting them is costly and slow. Maybe as technology advances and prices of
component metals increases, they will become economically important.
Ooids are a hydrogenous sediment comprised of calcium carbonate. There mode of
formation is basically the same as manganese nodules except calcium carbonate
precipitates rapidly, and ooids occur in warm shallow water where waves can stir the sea
bottom.
The interior of ooids is also concentrically layered like manganese nodules, but calcium
carbonate weathers so quickly and is attacked by fungus and algae that the layering is
quickly obliterated.
Ooid shoals from the air. Some people have suggested these might be a source of sand
to replenish eroding beaches in the US and Caribbean.
A simple sediment summary. What you find on the sea floor is a reflection of the original
source of sediment, the chemical processes occurring in the water column above the sea
floor, and possible dilution by other sediments.
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Calcareous where it’s shallow (above the CCD) so you can see at a glance the shallower
parts of the world ocean. Mostly it’s the ridges. Why is there a radiolarian ooze in the
Pacific but not the Atlantic? There are radiolarian growing in the Atlantic, but because
the Atlantic is shallow, the radiolaria accumulating on the Mid-Atlantic ridge are
overwhelmed by the accumulating calcareous sediment. Because the Pacific is deeper,
the calcareous shells dissolve, leaving the siliceous to dominate the sea floor. Same with
diatom oozes in North Atlantic. In the South Atlantic, the ocean is deeper, and siliceous
ooze can accumulate.
A profile view of the sediment summary.
Sediment sampling. Direct—you actually retrieve some sediment and bring it back.
Remote—you study the sea floor without actually touching it.
Grab samplers and dredges come in many different sizes and types.
A clamshell sediment sampler.
A large clamshell sediment sampler.
A rock dredge has a chain basket to let the fine sediment wash away and retain only the
large rocks.
A box corer.
Corers in general
Piston corers work best in muddy, sticky sediments. A piston in the core tube creates
suction and essentially sucks sediment up into the tube.
Deploying a piston corer.
Gravity corer.
A multicorer allows several cores to be taken at once.
Vibracorers work well in sandy sediments. They vibrate their way down into the
sediment. There are many types of vibracores.
A large vibracorer.
A minivibracorer.
Coring like this from a drill ship is basically the same whether you are take samples for
scientific research or if you are drilling for oil.
Remote sensing
Remotely sensing the sea floor.
Sidescan sonar makes a sonogram of the sea floor which can look very much like a
photograph. Instead of recording reflected light, it records reflected sound.
The sidescan sonar fish.
It looks like a photograph, but it’s a composite sidescan sonar image.
Reflection seismology. An energy source sends sound into the sea floor. The sound
reflects off layers and is recorded by receivers called hydrophones. The record is called a
seismogram.
Seismogram showing sediment layers behind a reef.
Ocean sediments as resources
Oil and natural gas are lighter than water. If there is some type of barrier, it can
accumulate as it tries to float to the top. Here the gray layer is impermeable and traps
the oil and natural gas.
Wells drill through overlying sediment and rock to reach the layers with natural gas and
oil. The process is the same on land as in the ocean, only it’s a lot more expensive to drill
in the ocean because much higher technology is needed.
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A large cargo ship carries a semisubmersible drilling platform to its target location. The
pontoons will be flooded with water creating a floating but stable platform from which
drilling can proceed.
Another semisubmersible drilling platform.
Different designs of drilling platforms are used in different water depths.
Sharks teeth and other fossils are common in phosphate mines.
Sediments of the sea floor.
Some siliceous biogenic sediments
Some calcareous biogenic sediments.