Download Chapter 5: SEDIMENTS

Chapter 5: SEDIMENTS
Answers to End of Chapter Study Questions
1. In what ways are sediments classified?
Particle size is frequently used to classify sediments. In this classification, the coarsest
particles are boulders, which are more than 256 millimeters (about 10 inches) in diameter.
Although boulders, cobbles, and pebbles occur in the ocean, most marine sediments are made
of finer particles: sand, silt, and clay.
A layer of sediment can contain particles of similar size, or it can be a mixture of
different-sized particles. Sediments composed of particles of one size are said to be wellsorted sediments. Sediments with a mixture of sizes are poorly-sorted sediments. Well-sorted
sediments occur in an environment where energy (waves, currents) fluctuates within narrow
limits. Poorly-sorted sediments form in environments where energy fluctuates over a wide
spectrum.
Another way to classify marine sediments is by their origin. A modern modification of
their organization is shown in Table 5.1. This scheme separates sediments into four categories
by source: terrigenous, biogenous, hydrogenous (also called authigenic), and cosmogenous
(see next question).
2. List the four types of marine sediments. Explain the origin of each.
Marine sediments are separated into four categories by source: terrigenous, biogenous,
hydrogenous (or authigenic), and cosmogenous.
Terrigenous sediments are the most abundant. As the name implies, terrigenous
sediment originates on the continents or islands near them. They are carried to the ocean in
rivers and streams, or by winds as blowing dust, and dominate the continental margins,
abyssal plains, and polar ocean floors.
Biogenous sediments, the next most abundant, consist of the hard remains of onceliving marine organisms. The siliceous (silicon-containing) and calcareous (calcium carbonatecontaining) compounds that make up these sediments of biological origin were originally
dissolved in the ocean at mid-ocean ridges or brought to the ocean in solution by rivers.
Biogenous sediments are found mixed with terrigenous material near continental margins, but
are dominant on the deep ocean floor.
Hydrogenous sediments are minerals that have precipitated directly from seawater.
The sources of the dissolved minerals include submerged rock and sediment, leaching of the
fresh crust at oceanic ridges, material issuing from hydrothermal vents, or substances flowing
to the ocean in river runoff. The most prominent hydrogenous sediments are manganese
nodules, which litter abyssal plains, and phosphorite nodules, seen along some continental
margins. Hydrogenous sediments are also called authigenic (authis = in place, "on the spot")
because they were formed in the place they now occupy.
Cosmogenous sediments, which are of extraterrestrial origin, are the least abundant.
These particles enter the Earth's high atmosphere as blazing meteors or as quiet motes of
dust. Their rate of accumulation is so slow that they never accumulate as distinct layers—they
occur as isolated grains in other sediments, rarely constituting more than 1 percent of any
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layer.
3. How are neritic sediments generally different from pelagic ones?
Remember that sediments on the ocean floor only rarely come from a single source;
most sediment deposits are a mixture of particles. The patterns and composition of sediment
layers on the seabed are of great interest to researchers studying conditions in the overlying
ocean. Different marine environments have characteristic sediments, and these sediments
preserve a record of past and present conditions within those environments.
The sediments on the continental margin are generally different in quantity, character,
and composition from those on the deeper basin floors. Continental shelf sediments—neritic
sediments—consist primarily of terrigenous material. Deep ocean floors are covered by finer
sediments than those of the continental margins, and a greater proportion of deep sea
sediment is of biogenous origin. Sediments of the slope, rise, and deep ocean floor that
originate in the ocean are called pelagic sediments.
4. Is the thickness of ooze always an accurate indication of the biological productivity of
surface water in a given area?(Hint: See next question.)
The organisms contributing their remains to deep-sea oozes are small, single-celled,
drifting, plantlike organisms and the single-celled animals that feed on them. When these
organisms die, their shells settle slowly toward the bottom, mingle with fine-grained terrigenous
silts and clays, and accumulate as ooze.
Oozes accumulate slowly, at a rate of about 1 to 6 centimeters (½ to 2½ inches) per
1,000 years. The accumulation of any ooze therefore depends on a delicate balance between
the abundance of organisms at the surface, the rate at which they dissolve once they reach the
bottom, and the rate of accumulation of terrigenous sediment. Other factors contributing to
bottom accumulation are scouring currents and conditions (temperature, pH) in the overlying
water column through which the hard parts fall.
5. What happens to the calcium carbonate skeletons of small organisms as they descend to
great depths?How do the siliceous components of once-living things compare?
Although small calcium-carbonate-producing organisms live in nearly all surface ocean
water, calcareous ooze does not accumulate everywhere on the ocean floor because their
shells are dissolved by seawater. At great depths seawater contains more CO 2 and becomes
slightly acid. This acidity, combined with the increased solubility of calcium carbonate in cold
water under pressure, dissolves the shells. Below a certain depth the tiny skeletons of calcium
carbonate dissolve on the seafloor, so no calcareous oozes form. Calcareous sediment
dominates on the deep sea floor at depths less than about 4,500 meters (14,800 feet). About
48 percent of the surface of deep ocean basins is covered by calcareous oozes.
Siliceous (silicon-containing) ooze predominates at greater depths and in colder polar
regions. After a radiolarian or diatom dies, its shell will also dissolve back into the seawater,
but this dissolution occurs much more slowly than the dissolution of calcium carbonate. Slow
dissolution, combined with very high diatom productivity in some surface waters, leads to the
buildup of siliceous ooze.
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6. What sediments accumulate most rapidly? Least rapidly?
Accumulation rate depends on the availability of the sediment in question. The rate of
sediment deposition on continental shelves is variable, but it is almost always greater than the
rate of sediment deposition in the deep ocean. Near the mouths of large rivers, 1 meter (about
3 feet) of terrigenous sediment may accumulate every 1,000 years. In the deep ocean,
mudslides rushing down the continental slope deposit turbidites—layers of coarse-grained
terrigenous sediments interleaved with finer sediments typical of the deep-sea floor. Turbidite
accumulation may be quite rapid adjacent to continental shelves shaken by earthquakes and
subject to much erosional runoff from land.
The sediments slowest to accumulate are hydrogenous sediments. Accumulation rates
on manganese nodules are typically the thickness of a dime every thousand years. (The rate
of accumulation of cosmogenous sediment is so slow that they never accumulate as distinct
layers. They occur as isolated grains in other sediments, rarely constituting more than 1
percent of any layer.)
7. Can marine sediments tell us about the history of the ocean from the time of its origin?
Why?
The distribution, depth, and composition of sediment layers tell of conditions in the
comparatively recent past. In the Pacific, for example, sediments get older with increasing
distance from the East Pacific Rise spreading center, but the maximum age is roughly early
Cretaceous or late Jurassic (around 145 million years old). The "memory" of the sediments is
not ancient and in fact is continually being erased by ocean floor subduction. We can’t see
farther back than about 180 million years because the oceanic conveyor belt of plate tectonic
processes destroys the evidence.
Still, marine sediments in the modern basins can shed light on unexpected details of the
last 180 million years of Earth's history. One of the oddest details is the unexplained extinction
of up to 52 percent of known marine animal species (and the dinosaurs) at the end of the
Cretaceous Period 65 million years ago. Researchers have proposed hypotheses such as a
sudden and violent increase in worldwide volcanism or the impact of one or more very large
meteors or comets to explain this catastrophe. The clouds of dust and ash thrown into the
atmosphere by any of these events would have drastically reduced incident sunlight and
greatly affected the lives of organisms and the photosynthetic base of ecosystems.
Oceanographers are presently searching for evidence of the cause of the Cretaceous
extinctions in layers of deep sediments.
8. How do paleoceanographers infer water temperatures, and therefore terrestrial climate,
from sediment samples?
Instruments capable of analyzing very small variations in the relative abundances of the
stable isotopes of oxygen preserved within the carbonate shells of microfossils found in deep
sea sediments has allowed scientists to interpret changes in the temperature of surface and
deep water over time. These same data are also used to estimate variations in the volume of
ice stored in continental ice sheets, and thus track the ice ages. Other geochemical evidence
contained in the shells of marine microfossils, including variations in carbon isotopes and trace
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metals such as cadmium, provide insights into ancient patterns of ocean circulation,
productivity of the marine biosphere, and ancient upwelling. These sorts of data have already
provided quantitative records of the glacial-interglacial climatic cycles of the past two million
years with future drilling and analysis of deep sea sediments poised to extend our
paleoceanographic perspective much farther back in time.
9. Where are sediments thickest? Are there any areas of the ocean floor free of sediments?
Lithified sediments can be miles thick. As you may recall, much of the Colorado Plateau
with its many stacked layers was formed by sedimentary deposition and lithification beneath a
shallow continental sea beginning about 570 million years ago. The Colorado River has cut
and exposed the uplifted beds to form the Grand Canyon. Hikers walking from the Canyon rim
down to the river pass through spectacular examples of continental shelf sedimentary
deposits. Most of the upper sediments have already been eroded, but the remaining material is
more than 1 mile (1.6 kilometers) deep.
The loose sediments of the Continental Rise (at the foot of the Continental Shelf),
transported into position by turbidity currents, may reach depths of 10 kilometers (6.2 miles).
No sediments can accumulate in areas where swift deep currents scour the seabed,
and the fresh rock of the mid-ocean ridges—in the rifts of spreading centers—is free of
sediments for a short time after its formation.
10. Why doesn’t the sediment record extend back to the time of the origin of the ocean?
Because the seabeds are young (almost never older than 200 million years).
Remember, the relatively dense ocean floors are recycled in the plate tectonic cycle. In
contrast, the relatively less dense continents remain above the fray, and their centers are often
of great age.
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