Download Chapter 5: Marine Sediments

P162, 1-18
1. Describe the process of how a drilling ship like the JOIDES Resolution obtains core samples from the deepocean floor.
Specially outfitted ships use rotary drilling to “punch” through sediments and solid ocean floor, the way pushing a
soda straw though a piece of cake would allow you to remove a core sample of cake. Cores, columns of sediment and
rock, are retrieved from these drill holes. Rotary drills can penetrate up to 2100 m below the seafloor.
2. What kind of information can be obtained by examining and analyzing core samples?
age of ocean floor, types of sediments (and sedimentary structures), types of fossils. By examining sediment cores
retrieved from ocean drilling and interpreting them, oceanographers can ascertain such events as:
a. the timing of major extinctions
b. global climate change
c. plate movements
d. historical information about Earth’s geology and biology
3. List and describe the characteristics of the four basic types of marine sediment.
Lithogenous: composed of fragments of pre-existing rock material.
Biogenous: composed of the hard remains of dead marine organisms.
Hydrogenous: composed of material that is dissolved in water and precipitates to form deposits.
Cosmogenous: composed of material of extra-terrestrial origin.
4. How does lithogenous sediment originate?
Lithogenous sediments originate from the disintegration (weathering) of rocks on land. These fragments are transported
to the ocean basin by river, wind, ice, and gravity. The sediment can be deposited in bays or lagoons near the ocean, it can
be transported further by waves along the beach to produce beach deposits along the shoreline, or it may be spread along
the continental margin by currents. It can also be carried beyond the continental margin to the deep-ocean basin.
5. Why is lithogenous sediment composed of quartz grains? What is the chemical composition of quartz?
Most lithogenous sediment is composed of quartz because quartz is abundant in the earth’s crust, and one of the reasons
for THAT is because quartz is resistant to both chemical attack (i.e. it is chemically stable) and physical attack (i.e. it is
“tough”). Thus, quartz is not easily dissolved and nor easily fragmented.
Quartz is composed of silicon and oxygen in the form of Silicon Dioxide (SiO2).
6. If a deposit has a coarse grain size, what does that indicate about the energy of the transporting medium? Give
several examples of various transporting media that would produce such a deposit.
Coarse-grained sediments are generally transported by faster-moving currents. Fast-moving rivers will carry largersized particles than slow- moving rivers. Larger waves will transport larger particles than smaller waves near shore.
Landslides (and underwater turbidity currents) can also transport large-sized particles. (The exception to swiftly moving
currents moving larger particles is ice. Glacial ice typically moves slowly, but the ice, because it is a solid, can transport
very large boulders.)
7. What characteristics of marine sediment indicate increasing maturity? Give an example of a mature and an
immature sediment.
Maturity is judged by higher degree of sorting (grains are all similar size), higher quartz content, and well-rounded
grains. A mature lithogenous sediment would be a beach sand that is dominantly quartz, well-rounded, and has sand-size
particles. An immature lithogenous sediment would be a turbidity current deposit that has a wide variety of angular
(sharp-edged) particles of many different sizes and all types of mineral and rock fragments.
8. List the two major chemical compounds of which most biogenous sediment is composed and the organisms that
produce them. Sketch these organisms.
Calcium carbonate (CaCO3) foraminfers and coccolithophores
Silica (silicon dioxide, SiO2), diatoms and radiolarians
9. What are several reasons that diatoms are so remarkable? List the products that contain or are produced using
diatomaceous earth.
Diatoms are microscopic, photosynthetic organisms with ornate cell walls. They reproduce both sexually and asexually,
they exist as individual cells or colonial organisms, they are found in a diversity of habitats, and they are among the most
abundant organisms on Earth. As primary producers in aquatic ecosystems, diatoms are at the base of the food chain in
marine and freshwater environments. Perhaps most importantly, they are main source of O2 in the atmosphere (sorry,
trees!)
The main commercial uses for diatomaceous earth include:
- filters (sugar refinement, brewing, and swimming pool filters)
- mild abrasives (in such products as toothpaste, facial scrubs, and household cleaning and polishing compounds)
- absorbent (chemical spills and as pest control)
- chemical carrier (in paint and dynamite)
- optical quality glass (because of the pure silica in diatom cell walls),
- space shuttle tiles (because they are light weight and provide good insulation), an
- additive in concrete
10. If siliceous ooze is slowly but constantly dissolving in seawater, how can deposits of siliceous ooze accumulate
on the sea floor?
If a layer of pelagic clay covers a layer of siliceous (silica-based) ooze, the ooze will dissolve more slowly. ALSO, if the
rate of deposition of siliceous tests (shells) exceeds the rate that the tests dissolve, the silica ooze will build up over time
11. How do oozes differ from abyssal clay? Discuss how productivity, destruction, and dilution combine to
determine whether an ooze or abyssal clay will form on the deep-ocean floor.
Oozes contain at least 30% biogenous particles (by weight); (in contrast to abyssal clay that contains less than 30%
biogenous material. Most of the remaining percentage (up to 70%) of abyssal clay deposits is lithogenous clays.)
Whether or not an ooze forms depends on the relative rates of biogenous and lithogenous particle deposition in an area.
The deposition rate of biogenous particles is a function of the rate at which they are produced (productivity) versus the
rate at which they dissolve in ocean water (destruction). Silica dissolves slowly at all ocean depths, but calcium
carbonate dissolves much faster in deeper water. If lithogenous soil deposits faster than biogenous does, the lithogenous
concentration will “dilute” the biogenous, and ooze won’t form.
12. Describe the environmental conditions that influence the distribution of siliceous and calcareous ooze.
Calcareous ooze: warmer surface seawater for production, cooler deeper seawater for dissolution; surface ocean
productivity controlled by such factors as sunlight, length of seasons, abundance of nutrients.
Siliceous ooze: cooler surface seawater for production, most important is surface ocean productivity controlled by
sunlight, length of seasons, and especially abundance of nutrients. Siliceous ooze typically accumulates beneath areas of
high biologic productivity associated with surface ocean upwelling.
13. Explain the stages of progression that result in calcareous ooze existing below the CCD (calcite compensation
depth).
Calcareous ooze may exist below the CCD when the calcareous sediments are covered by overlying noncalcareous
sediment. As a spreading mid-ocean ridge accumulates calcareous ooze when it is shallower, the calcareous ooze will be
covered by pelagic clays when the flank of the mid-ocean ridge subsides beneath the CCD as plates continue to move
farther apart.
14. Describe manganese nodules, include what is currently known about how they form.
Manganese nodules are typically 5 cm in diameter although they may grow as large as 20 cm. They are made up of
concentric layers of Mn oxides and Fe oxides around a nucleus. They tend to be found in deeper sea water where other
sediments accumulate very slowly. The accumulation rate of Mn nodules is extremely slow, which makes it difficult to
figure out how they form.
15. Describe the most common types of cosmogenous sediment and give the probable source of these particles.
The most common types of cosmogenous sediment are tektites, microscopic spherules composed of glassy silicate rock
material, and space dust or micrometeorites composed primarily of Fe and Ni. Macroscopic meteorite material may be
ejected from meteor impact sites on Earth. Cosmogenous material comes primarily from outer space.
16. Describe the K-T Event; include evidence for it and its effect on the environment.
The K-T (Cretaceous-Tertiary) Event was presumably an asteroid impact and/or a massive volcanic eruption that spewed
so much soil into the atmosphere that sunlight was blocked for a few months. This killed most photosynthetic plants in
certain environments, which in turn killed the animals that fed on the plants, and then the animals that fed on those
animals.
Iridium, usually rare in terrestrial rocks, is found in abundance in some sediments deposited at the K-T boundary (ie the
rock layer below which all dinosaur fossils exist). Alvarez and others (1980) speculated that the Iridium anomaly
indicated that a meteor struck Earth and created much dust in the atmosphere. The sediments with high Ir also contained
shocked quartz--quartz subjected to high pressure and high temperature at Earth’s surface. Alvarez and others theorized
that the dust would have created global cooling which might have led to the extinction of dinosaurs. Evidence from cores
retrieved from the Blake Nose revealed coarse-grained sediment attributed to the K-T meteorite impact crater in the
Yucatan peninsula. The coarse grained sediment included tektites, shocked quartz, a piece of reef rock from Yucatan, and
high Iridium values.
17. Why is lithogenous sediment the most common neritic deposit? Why are biogenous oozes the most common
pelagic deposit?
Most marine sediments are mixtures of sediments from different source areas. Neritic deposits are mostly commonly
lithogenous because sediments from land are most frequently deposited in the nearshore marine areas. Biogenous
components are overwhelmed by the amount of lithogenous material. In contrast, in pelagic deposits, biogenous oozes
are the most common type of sediment because they are produced within the ocean itself. The biogenic sediments
outweigh the amount of silt and clay transported into the oceans by wind and turbidity currents.
18. How do fecal pellets help explain why the particles found in the ocean surface waters are closely reflected in the
particle composition of the sediment directly beneath? Why would one not expect this?
The small particle common to pelagic sediment would require from 10 to 50 years to sink to the ocean floor, so the
sediment on the ocean floor would not be expected to be similar to makeup of the particle concentrations in the surface
water immediately above. The slightest current would be expected to carry them from 3000 to 15,000 kilometers (1860
to 9320 miles) laterally before they could reach the bottom. The fecal pellets incorporate small particles into larger
packages that can sink to the ocean floor in 10 to 15 days, allowing the composition of bottom sediment to resemble
closely the composition of the particle content of the surface water immediately above.