Sedimentation Processes on the Deep-Ocean Bottom

choppedspleenΜηχανική

21 Φεβ 2014 (πριν από 3 χρόνια και 10 μήνες)

90 εμφανίσεις

Exit

Choose to view chapter section with a click on the section heading.


The Study of Sediments


Types of Sediment


Continental
-
Shelf Sediments


Deep
-
Ocean Sediments


Sediments as Economic Resources

Chapter Topic Menu

Menu

Previous

Next

12
-

2

Sediment Study Tools and Techniques


Techniques and tools to study ocean
sediments include:


Clamshell sampler



collects a large
sample of the top sediment.


Piston corer



used to collect different
sediment layers as deep as 25 meters
(82 feet) into the ocean bottom.


Specialized vessels with drilling
equipment



used to produce
sediment core samples of 500 meters
(1,640 feet) long.


Seismic tools



transmit sounds that
travel through water and into sediment.
Different sediment layers create distinct
echoes. Used when looking

for oil or natural gas.

The Study of Sediments

Chapter 12 Pages 12
-
3 to 12
-
6

Menu

Previous

Next

12
-

3

Stratigraphy and Paleoceanography


Scientists estimate there are no sediments on the ocean crust dated older than
200 million years.


Study of sediment layers is called
stratigraphy
.


Scientists use deep
-
sea stratigraphy to look for clues, such as
rock composition
,
microfossils
,
deposition patterns

and other physical properties.


Based on these they can estimate the
age of the sediment layers

and draw
conclusions about the past.


Ocean scientists use stratigraphy to understand changes in the ocean and
atmosphere;
previous circulation patterns
,
former sea levels
, and trends in
biological productivity
.


A new science called
paleoceanography

is the study of prehistoric oceans.


From sediment they have been able to estimate prehistoric ocean t
emperatures

and
climatic conditions

with accurate precision.


Ongoing research of the Earth’s ancient climate currently emphasizes deep
-
ocean sediments called
siliceous oozes
.


Oceanographers have played a key role in helping develop current theories that
propose that Mars once had oceans.

The Study of Sediments

Chapter 12 Pages 12
-
6 to 12
-
9

Menu

Previous

Next

12
-

4

Sediment Origins


Sediments may be classified by origin based on four categories.


1.
Lithogenous sediments



Come primarily from land erosion carrying particles into the
sea or from volcanic eruptions. These are the majority of sediments.


Quartz and clay are the two most common materials along with feldspar. Volcanic islands
contribute sediments of basalt and other volcanic materials.


2.
Biogenous sediments



Originate from organisms and cover a large area

of the seafloor.


Silica and calcium carbonate are the materials that come from shells and hard skeletons of
planktonic organisms.


3.
Hydrogenous sediments



Result from chemical reactions within seawater accounting
for less than 1% of the seafloor sediments.


Minerals come out of solution and form particles that settle on the bottom.


Produces ferromanganese and phosphorite nodules.


Sources of dissolved minerals vary


submerged rock, new crust formation, hydrothermal vent
water, river runoff.


4.
Cosmogenous sediments



Come from outer space consisting of cosmic dust and
occasional impacts from asteroids and comets.


They settle through the air as the others do through water. About 15,000
-
30,000 metric tons of
space dust settle on the Earth each year; least abundant of the sediments.


Types of Sediment

Chapter 12 Pages 12
-
10 to 12
-
13

Menu

Previous

Next

12
-

5

Sediment Sizes


Sediments are classified on grain size



the
diameter of the particle.


Grain size and current velocity affect the
deposition and erosion of sediment
.


Smallest and largest particles behave similarly with
respect to transportation and erosion.


Sand in the middle of the graph takes the least amount
of energy to erode.


Larger particles require more energy to erode because
they’re heavy. It takes a stronger current to lift them off
the bottom.


Particles smaller than sand also take more energy to
erode. Smaller particles (especially clay) tend to be
cohesive.

Types of Sediment

Chapter 12 Pages 12
-
13 to 12
-
15

Menu

Previous

Next

12
-

6

Sedimentation Processes on the Continental Shelf


Tides, waves, and currents strongly
affect continental
-
shelf sedimentation
.


Shoreline turbulence: waves are one of the
most notable influences because it keeps
particles from settling. Surf and waves carry
small particles out to sea. Their affect
diminishes further from shore traveled.


Recent and Relict Sediments


Recent sediments

have accumulated since
the sea level stabilized.


Relict sediments

accumulated and were

left stranded when the sea level was lower.


Overall, sedimentation on the shelf is more
rapid than in the deep ocean.

Continental
-
Shelf Sediments

Chapter 12 Pages 12
-
16 & 12
-
17

Menu

Previous

Next

12
-

7

Continental
-
Shelf Sedimentation Rates


The sedimentation rate on the continental shelf varies with region
.


Sedimentation on the shelf is more rapid than

in the deep ocean
.


At the mouths of large rivers, sedimentation can

occur at a rate of one meter per thousand years,

but there’s a lot of variation.


Continental
-
shelf sedimentation processes also

affect the adjoining deep ocean.


Accumulating sediment on the continental shelf

avalanches down the continental slopes. These are

called
turbidity currents

and can carry sediment

deposits all the way to the abyssal plain. These

deposits are called
turbidites
.


Turbidites consist of layers of lithogenous sand embedded with the more typical, fine

deep
-
sea sediments.


The continental shelves undergo processes that produce biogenous sediments, which also
affect the sedimentation rate.


Continental
-
shelf sediments tend to have a mix of both biogenous and lithogenous materials.


Continental
-
Shelf Sediments

Chapter 12 Pages 12
-
17 to 12
-
19

Menu

Previous

Next

12
-

8

Sedimentation Processes on the Deep
-
Ocean Bottom


Like the processes that affect the continental shelf, sedimentation processes in the deep
ocean vary regionally.


Deep
-
ocean sediments tend to be high in biogenous material.


Lithogenous sediments, except for clays,

are generally confined near shore.


Biogenetic sediments


primarily the

remnants of plankton


dominate the

sediments off shore waters.


Because of its very small grain size,

clay can remain suspended in the water

for great distances and be carried by wind,

allowing it to deposit in the deep sea.


The variation in deep
-
water sedimentation causes

tremendous variations in sediment accumulation
.


The thickness of sediments in the deep ocean also varies with topography
.


Sediments are thickest on the abyssal plains and thinnest or absent on the mid
-
ocean ridges

and seamounts.


Deep
-
Ocean Sediments

Chapter 12 Pages 12
-
20 & 12
-
21

Menu

Previous

Next

12
-

9

The Carbonate Compensation Depth


The carbonate compensation depth is a

point at which calcium carbonate dissolves just

as fast as it accumulates from above
.


Above this depth
calcareous ooze

dominates.

Siliceous ooze

dominates sediments below this

depth due to the slow deep
-
sea dissolution of

siliceous remains and high diatom productivity.


The carbonate compensation depth varies with

region due to temperature and water density.


In the Atlantic and Pacific, it is around 4,500 meters (14, 750 feet).


In colder regions, the carbonate compensation depth is much shallower so siliceous oozes
dominate biogenous sediments in polar regions.


The slow dissolution of siliceous remains and high diatom productivity allow
siliceous oozes to accumulate throughout the seafloor
.


Siliceous ooze are the dominant biogenous sediments below the calcium carbonate
compensation depth.


Deep
-
Ocean Sediments

Chapter 12 Pages 12
-
22 & 12
-
23

Menu

Previous

Next

12
-

10

Fecal Pellets


Scientists find that bottom composition is usually similar to the particle composition of the water above it.
This is due to fecal pellets
.


Large planktonic organisms, like copepods, consume the calcareous or silicone organisms that also
dominate the bottom ooze. These large organisms eliminate their waste as dense
fecal pellets

of
multiple skeletal and shell remains compressed together. These dense pellets sink quickly and the
decomposition process begins.


Mineral Nodules


Ferromanganese nodules

consist of iron and manganese

found over as much as 50% of the deep Pacific floor.


Phosphorite nodules

consist of phosphorite and other trace

minerals found on the shallow banks and continental shelves

off California, Argentina and Japan.


Both forms of nodules are thought to be hydrogenous

sediments produced by one of the slowest chemical reactions in nature
.


Nodules grow at a rate of about 1 to 200 millimeters (.039 to 7.9 inches) per million years.


Scientists believe that biological processes


possibly involving bacteria


cause the

chemical precipitation
.

Deep
-
Ocean Sediments

Chapter 12 Pages 12
-
23 & 12
-
24

Menu

Previous

Next

12
-

11

Petroleum and Natural Gas


What is the economic importance of ocean sediments and their study?


Oil and natural gas found under the ocean contribute $125 billion in annual revenues. More
than a third of the world’s crude petroleum and a quarter of its natural gas come from
sedimentary deposits on the continental shelf
.


Ferromanganese and phosphorite nodules have potential economic value.


Other Sediments With Economic Importance


Metal sulfide deposits found at deep
-
sea hydrothermal vents are rich and vast enough
(especially in the Red Sea) that mining them could be economically feasible.


Evaporites form at the surface and comprise the salts left behind when seawater
evaporates. They are a source of calcium carbonate, calcium sulfate, gypsum and

sodium chloride.


Sand and gravel are an important resource for the

construction industry accounting for $500 million yearly.


Another sediment
-
based resource is diatomaceous earth.

Sediments as Economic Resources

Chapter 12 Pages 12
-
25 to 12
-
27