MAR 110: Introductory Oceanography

lameubiquityΜηχανική

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

71 εμφανίσεις

MAR 110: Introductory
Oceanography

Marine sediments

Rev. 21 September 2006

Properties of ocean water

2

Pollutants and the sea, part 1


Waste water enters rivers and streams through
overland flow, groundwater flow, and drainage pipes.


Coastal installations may discharge wastes directly
into the ocean.


Eventually, all water
-
borne wastes enter the ocean, either
dissolved or suspended in the water.


Rain and snow wash particles from the atmosphere
into the ocean.

Rev. 21 September 2006

Properties of ocean water

3

Pollutants and the sea, part 2


Some wastes rapidly decompose.


Some resist decomposition.


Many minerals are in elemental form, thus cannot be
broken down.


These include metals that we regard as pollutants.


Persistent pollutants can cause serious problems to
aquatic life as they are subject to bioconcentration.


Oftentimes the best way to remove such pollutants is by
dredging and removal or by capping of contaminated
sediments.

Rev. 21 September 2006

Properties of ocean water

4

Pollutants and the sea, part 3


1956: Minamata, Japan


A 5
-
year
-
old girl lapsed into a convulsive delirium; An
examination showed permanent brain damage.


The girl was the tip of the iceberg of a major health crisis.


People in the coastal village had eaten fish and shellfish
from the bay as a staple part of their diet.


For many years, the Chisso Chemical Plant had discharged
industrial wastes containing mercury into the bay.


Elemental mercury is insoluble in water, so they thought it would
sink to the bottom and become buried in sediments, thus pose little
health hazard.

Rev. 21 September 2006

Properties of ocean water

5

Pollutants and the sea, part 4


Minamata, Japan (continued):


In 1959, scientists demonstrated that bacteria in the bottom
sediments converted elemental mercury into
methylmercury, a soluble and highly mobile form of
mercury that could enter the food chain.


Methylmercury attacks the central nervous system as
elemental mercury does.


More than 3,500 people were severely affected; of those,
about 50 died from what is now called Minamata Disease.


After decades of cleanup, the bay is safe for human use.


The bay, its sediments, and resident organisms are still monitored.

Rev. 21 September 2006

Properties of ocean water

6

Sediments


Sediments are particles that settle on the ocean floor.


Sediments blanket most of the ocean floor.


Most of the particles that make up those sediments form at
the interface between the environmental spheres:
atmosphere, biosphere, geosphere, and hydrosphere.


They are transported by rivers, wind, ice, and gravity to the
oceans.


Marine sediments consist of excreta and secretions of
organisms; shells, bones, and teeth; rock fragments; soil
particles; and material from outer space.

Rev. 21 September 2006

Properties of ocean water

7

Sediment characteristics, part 1


Sediments differ in source, composition, size, and
accumulation rate.


Size refers to the diameter of particles.


Sediments include gravels, sands, and muds.


Mud: Muds include clays (particles less than 0.0039 mm) and silts
(between 0.0039 mm and 0.0625 mm).


Sand: Sands range from very fine (0.0625 mm) to very coarse (2
mm).


Gravel: Gravels include granules (between 2 mm and 4 mm),
pebbles (between 4 mm and 64 mm), cobbles (between 64 mm and
256 mm); and boulders (greater than 256 mm).

Rev. 21 September 2006

Properties of ocean water

8

Sediment characteristics, part 2


Size (continued):


Sediment accumulations (deposits) vary in the range of
grain size; this is known as sorting.


Well
-
sorted sediments have a narrow range of grain sizes, where as
poorly sorted sediments have a diverse mixture of grain sizes.


Fine
-
grained sediments are transported farther; Coarse
-
grained
sediments settle out of the transport medium sooner.


Example: Sediments along continental margins are poorly sorted,
with larger grain sizes; Sediments on the deep ocean floor are well
sorted, with smaller grain sizes.

Rev. 21 September 2006

Properties of ocean water

9

Rev. 21 September 2006

Properties of ocean water

10

Sediment characteristics, part 3


Sediment deposits are thickest along continental
margins and near islands; they become thinner with
increasing distance from land.


Source media, such as rivers, lose energy as they reach the
ocean; the decrease in velocity diminishes the ability to
transport particles; the larger particle settle first.

Rev. 21 September 2006

Properties of ocean water

11

Sediment characteristics, part 4


Sediments can accumulate at the rate of as much as
8,000 m in 1,000 years.


In the deep ocean, sediments accumulation rates range from
0.5 to 1 cm in 1,000 years.


Sediments are generally thickest where the ocean floor is
oldest and thinnest where it it youngest.


Particle size determines the rate at which it sinks.


Sands may sink to the ocean floor in a matter of days,
whereas clays may take more than a century.


Particles may also dissolve in ocean water.

Rev. 21 September 2006

Properties of ocean water

12

Terminal velocity


Terminal velocity is the constant speed attained by a
particle falling through a motionless fluid.


The terminal velocity is a function of gravity and the fluid
resistance.


Falling particles accelerate because of gravity until the fluid
resistance equals gravity; the particle falls at a constant
velocity (the terminal velocity) thereafter.


Terminal velocity increases with increasing particle size, all
else being equal.


Terminal velocity decreases with increasing density of the
fluid, all else being equal.

Rev. 21 September 2006

Properties of ocean water

13

Rev. 21 September 2006

Properties of ocean water

14

Sediment classification, part 1


Marine sediments are classified on the basis of their
source:


Lithogenous (from rock)


Biogenous (from organisms)


Hydrogenous (precipitated from seawater)


Cosmogenous (from outer space)


Lithogenous sediments account for 75 percent of all
marine sediments.


Most come from weathering and erosion of pre
-
existing
rock.

Rev. 21 September 2006

Properties of ocean water

15

Sediment classification, part 2


Lithogenous sediments (continued):


Most come from weathering and erosion of pre
-
existing
rock.


Weathering: The physical and chemical decomposition of rocks
exposed to the atmosphere.


Weathering removes carbon dioxide from the atmosphere.


Erosion: The transport of weathering products, usually by water,
wind, glaciers, and gravity.


Some lithogenous material comes from explosive volcanic
activity.


Tephra refers to igneous particles that fall through the air and
accumulate in the ocean.

Rev. 21 September 2006

Properties of ocean water

16

Sediment classification, part 3


Lithogenous sediments (continued):


The chemical composition of lithogenous sediments
depends on the chemical composition of source rock.


Oxygen is the most abundant element in the crust, followed by
silica, aluminum, iron, and calcium.


The silicone
-
oxygen tetrahedrom (SiO
4
) is the primary
building block of silicate minerals.


The ratio of silicon to oxygen varies; For example, quartz is made
primarily of SiO
2
) .

Rev. 21 September 2006

Properties of ocean water

17

Rev. 21 September 2006

Properties of ocean water

18

Rev. 21 September 2006

Properties of ocean water

19

Sediment classification, part 3


Lithogenous sediments (continued):


Silicate minerals:


Ferromagnesium silicates contain iron and magnesium, are dark in
color and are relatively dense


These are the primary components of oceanic crust (what geologists
used to call sima).


Rocks rich in ferromagnesium silicates usually weather more quickly.


Nonferromagnesium silicates contain aluminum, calcium, sodium,
or potassium; are relative light in appearance and are less dense
thatn ferromagnesium silicates.


These are the primary components of continental crust (what
geologists used to call sial).

Rev. 21 September 2006

Properties of ocean water

20

Sediment classification, part 4


Lithogenous sediments (continued):


Tropical and subtropical rivers account for the bulk of
river
-
borne marine sediments.


This in part results from the higher weathering and erosion rates
caused by the higher temperatures and precipitation amounts
characteristics of tropical and subtropical regions.


Rivers transport the products of weathering in suspension, solution,
or as part of what is called the bed load (stuff on the bottom).


Once the river hits the ocean, the heavier particles settle out first,
while finer sediments can be carried far beyond the river’s mouth.

Rev. 21 September 2006

Properties of ocean water

21

Rev. 21 September 2006

Properties of ocean water

22

Sediment classification, part 5


Lithogenous sediments (continued):


All parts of the ocean receive windborne dust.


This is the primary way that lithogenous material from regions far
from the ocean reach the sea.


Windborne (aeolian) material makes up much of the red and brown
clays of the mid
-
ocean basins, especially at about 30 degrees North
latitude and 30 degrees South latitude


these are where the Earth’s
two great desert belts are located.


Dust from one region, such as the Sahara, can be blown across
entire ocean basins.


Saharan dust may provide nutrients that trigger red tides in the Gulf of
Mexico; it may also harbor a fungus that attacks coral reefs.

Rev. 21 September 2006

Properties of ocean water

23

Rev. 21 September 2006

Properties of ocean water

24

Sediment classification, part 6


Lithogenous sediments (continued):


Glaciers erode bedrock and transport rock fragments, even
very large ones, to the ocean.


Glaciers, when the reach the sea, float on the surface of the
water, and can break off (calve) to form icebergs.


Icebergs, driven by wind and water currents, can transport
sediments from polar regions a long distance.


As icebergs melt, the sediments sink to the bottom; such
poorly sorted materials are called glaciomarine sediments.


Glaciomarine sediments cover 20 percent of the ocean floor.


Sudden releases of icebergs during ice age events are called
Heinrich Events.

Rev. 21 September 2006

Properties of ocean water

25

Sediment classification, part 7


Biogenous sediments include excretions, secretions,
and remains of organisms


including shells, corals,
and skeletal parts.


Most biogenous material is made of calcium
carbonate (CaCO
3
) or silica (SiO
2
), materials that
many marine organisms to form their shells.


Biogenous sediments are the dominant component of
30 to 70 percent of sediments in the mid
-
depths.


Skeletal remains account for 25 to 50 percent of all
particles suspended in seawater.

Rev. 21 September 2006

Properties of ocean water

26

Rev. 21 September 2006

Properties of ocean water

27

Sediment classification, part 8


Biogenous sediments (continued):


Calcareous sediments are the most abundant of all
biogenous sediments.


They originate in the calcium carbonate shells of organisms such as
foraminifera, pteropods, and coccolithophores.


The carbonate (and other) materials often dissolve in seawater as the
shells sink; but if they reach the ocean floor and are covered by other
sediments, they will be preserved.


In waters less than about 4,500 m deep, calcareous muds cover half
the sea floor.


They accumulate at rates of between 1 and 4 cm per 1,000 years.

Rev. 21 September 2006

Properties of ocean water

28

Rev. 21 September 2006

Properties of ocean water

29

Rev. 21 September 2006

Properties of ocean water

30

Sediment classification, part 9


Biogenous sediments (continued):


Fecal pellets make up most of the larger biogenous
particles.


Fecal pellets are often large, with high terminal velocities, thus can
sink to the bottom in a matter of days.


Fecal pellets transport organic material to the ocean floors, thus
serve as a nutrient source for bottom
-
dwelling organisms.


Marine snow is a combination of fecal pellets and remains of
organisms that fall to the bottom.


Siliceous materials are second in abundance among
biogenous sediments.


They are made of the tests (shells) of diatoms, and radiolaria.

Rev. 21 September 2006

Properties of ocean water

31

Rev. 21 September 2006

Properties of ocean water

32

Rev. 21 September 2006

Properties of ocean water

33

Sediment classification, part 10


Biogenous sediments (continued):


Phosphate materials are rare in marine sediment deposits.


Hydrogenous sediments originate from materials that
are chemically precipitated (come out of solution)
from seawater.


They may form coatings on the sea floor.


Some hydrogenous sediments are created in chemical
reactions in hot seawater from deep
-
sea vents.


Examples include some carbonates, halite (NaCl), gypsum
(CaSO
4

2H
2
O), and manganese nodules.

Rev. 21 September 2006

Properties of ocean water

34

Sediment classification, part 11


Hydrogenous sediments (continued):


In shallow waters, an increase in temperature may cause
dissolved carbonates to precipitate out.


Where evaporation rates are high, salts precipitate out in
the following order: carbonate salts, sulfate salts, and
halite.


Manganese nodules are irregularly shaped, black or brown
nodules on the sea floor.


They are about 18 percent manganese, 17 percent iron, and maller
amounts of copper, cobalt, and nickel.


While ranging to slabs with a mass of hundreds of kilograms, most
are the size of potatoes.

Rev. 21 September 2006

Properties of ocean water

35

Sediment classification, part 12


Hydrogenous sediments (continued):


Manganese nodules (continued):


Manganese nodules occur on the floors of all oceans except the
Arctic; they are most abundant in a 5,000
-
km belt on the floor of
the tropical Pacific from southeast of Hawai’i to north of 10
degrees North latitude.


The nodules begin as coatings on hard objects, such as shark’s teeth
or whale ear bones; marine organisms that burrow into the benthos
turn the nodules over, exposing all sides to seawater (and to the
source of precipitated material).


Growth rates range from 1 to 10 mm per million years.


During that time, the nodules must remain unburied, thus they
indicate slow sedimentation rates.

Rev. 21 September 2006

Properties of ocean water

36

Rev. 21 September 2006

Properties of ocean water

37

Sediment classification, part 13


Cosmogenous sediments come from outer space,
usually as meteorite or comet fragments.


Most extraterrestrial objects burn up in the atmosphere; and
those that survive the fall typically dissolve in seawater
before reaching the bottom; nevertheless, cosmogenous
sediments are found mixed with other sediments.


Some cosmogenous sediments are remnants from the
formation of the planets; their chemical composition
typically mirrors that of the Earth’s core and mantle.


Others are silicate rocks blasted from the surface of other
planets.

Rev. 21 September 2006

Properties of ocean water

38

Sediment classification, part 14


Cosmogenous sediments (continued):


Tektites are indirectly cosmogenous in origin, made from
solidified rocks that melted when meteorites struck the
Earth.


They have a teardrop or dumbbell shape, and are usually 2.5 to 5
cm in diameter.


Great numbers of tektites are found in the Gulf of Mexico near the
site of the meteorite impact at the Chicxulub Crater in the Yucatan
Peninsula.

Rev. 21 September 2006

Properties of ocean water

39

Coastal
-
margin deposits, part 1


Neritic deposits are those that are along continental
margins.


Most (about 95 percent) of the largest river
-
borne
sediments are trapped and deposited in bays, wetlands,
estuaries, beaches, or deltas.


About 5 percent of river
-
borne sediments reaches the
continental shelf or slope.


Little terrestrial sediment is transported beyond the
continental margin, except where such sediment is carried
by major sediment
-
exporting rivers, such as the
Mississippi, Ganges, or Yangtse.

Rev. 21 September 2006

Properties of ocean water

40

Coastal
-
margin deposits, part 2


As a river enters the ocean, the velocity of the water
slows so that the heaviest particles settle out first.


Except where high velocities carry the sediments off as
quickly as they are deposited, the sediments form deposits
known as deltas.


Distributaries are branching series of channels that cut through
deltas.


Unless artificially confined, rivers may switch their primary
channels abruptly, as has happened with the Mississippi.


Deltas are often highly modified by humans.


The building of the Aswan High Dam on the Nile has led to
increasing erosion and subsidence.

Rev. 21 September 2006

Properties of ocean water

41

Rev. 21 September 2006

Properties of ocean water

42

Coastal
-
margin deposits, part 3


Wetlands are low
-
lying areas either covered with
water or with soils that are saturated for at least part
of the year.


Wetlands are common in coastal and delta regions, such as
along the Atlantic and Gulf coasts.


Wetlands accumulate large amounts of organic matter and
help control flooding by taking up excessive water during
high water episodes.

Rev. 21 September 2006

Properties of ocean water

43

Coastal
-
margin deposits, part 4


Turbidity currents are intermittent avalanches of
dense, sediment
-
rich waters that flow down
submarine canyons, carrying sediments to the ocean
floor.


Bruce Heezen was the first to rigorously document the
existence of turbidity currents following analysis of
Transatlantic telegraph cable breaks that followed the 1929
Grand Banks earthquake.


Deposits formed by turbidity currents are called turbidites.


Because they are denser than seawater, turbidity currents
flow downslope, eroding channels as they go.

Rev. 21 September 2006

Properties of ocean water

44

Rev. 21 September 2006

Properties of ocean water

45

Coastal
-
margin deposits, part 5


Marine sediments accumulate rapidly in coastal areas.


Accumulation rates are as high as several meters per
thousand years.


The sediments are often buried to quickly to be modified
by reactions with seawater or with dissolved oxygen in the
seawater.


Bottom
-
dwelling organisms cannot consume all the
nutrients.


The sediments are often multicolored, the colors depending
on the oxidation state of iron in the sediments.

Rev. 21 September 2006

Properties of ocean water

46

Deep
-
ocean deposits, part 1


Fine
-
grained deposits gradually accumulate on the
ocean floor in pelagic deposits.


Accumulation rates average about 1 mm per 1,000 years.


The average thickness of pelagic deposits is between 500
and 600 m.


Because of the small size and resulting slow terminal
velocity of the particles that make up pelagic sediments,
they are transported over vast distance by ocean currents.


The long times given them ample time to react chemically with or
be dissolved by seawater.


Iron in these particles becomes oxidized, leading to the formation of
red clays and brown muds.

Rev. 21 September 2006

Properties of ocean water

47

Deep
-
ocean deposits, part 2


Some pelagic deposits are more than 30 percent
biogenous in origin.


Those made largely of the calcium carbonate shells (tests)
of coccolithophores, pteropods, and foraminifera are called
calcareous oozes.


Calcareous oozes are typically found in deposits in waters
shallower than the carbonate compensation depth (CCD)


the
depth at which calcium carbonate dissolves.


The CCD averages about 4,500 m.


The rate at which calcium carbonate dissolves is a function of
temperature, with the solution rate slower at lower temperatures.


Calcareous oozes are light in color.

Rev. 21 September 2006

Properties of ocean water

48

Deep
-
ocean deposits, part 3


Biogenous sediments (continued):


Siliceous oozes are formed from the tests of diatoms and
radiolaria.


Seawater is undersaturated with silica such that it dissolves at all
depths, thus are found only below surface waters where the
organisms that form such deposits are very abundant.


Calcareous and siliceous oozes consist primarily of
clay
-
size particles.


Sand
-
sized particles make up less than 10 percent of
deep
-
ocean deposits.


The coarsest particles are volcanic in origin.

Rev. 21 September 2006

Properties of ocean water

49

Rev. 21 September 2006

Properties of ocean water

50

Marine sedimentary rock, part 1


Marine sediments are produced at the interfaces
between the environmental spheres:, biosphere,
geosphere, and hydrosphere.


The first step is weathering and erosion of bedrock,
followed by transport by rivers, wind, ice, and gravity to
the oceans.


Some of the transported material is in solution in water.


Lithification is the formation of rock from sediments.


It involves both compaction and cementation of sediment
particles at relatively low temperatures.


Precipitated minerals fill pores spaces between particles.

Rev. 21 September 2006

Properties of ocean water

51

Marine sedimentary rock, part 2


Over longer periods of time, at higher temperatures
and high pressures, sedimentary rocks can be
converted to metamorphic rocks such as slate, schist,
or gneiss.


Marine sediments and rocks are transported with the
underlying oceanic crust into subduction zones,
where they are drawn into the mantle and either
metamorphosed or melted into magma.


The magma may emerge at the surface as a result of
volcanic activity.

Rev. 21 September 2006

Properties of ocean water

52

Rev. 21 September 2006

Properties of ocean water

53

Sea
-
floor resources, part 1


Resources from the seafloor include oil, natural gas,
sand, gravel, and minerals.


Oil and natural gas make up 95 percent of the monetary
value of sea
-
floor resources.


Many valuable resources are located on the ocean floor.


Oil and natural gas are derived from marine plant and
animal remains.


Oil and natural gas is found in the pore spaces between
marine sediment particles.


Oil is a mixture of thousands of hydrocarbons.


Natural gas is 99 percent methane.

Rev. 21 September 2006

Properties of ocean water

54

Sea
-
floor resources, part 2


Oil and natural gas deposits develop under unusual
circumstances millions of years ago.


Substantial amounts of organic material must accumulate in
a quite, shallow sea.


The oxygen supply must be depleted, leaving only
anaerobic bacteria to serve as decomposers.


Anaerobic bacteria produce methane and other light
hydrocarbons.


With increased accumulation of sediments, higher
temperatures and pressures may metamorphose the simple
hydrocarbons into complex hydrocarbons found in oil.

Rev. 21 September 2006

Properties of ocean water

55

Sea
-
floor resources, part 3


Conditions favoring the formation of oil and natural
gas occurred in the Paleozoic, primarily in the
Ordovician (505
-
438 MYA), Permian (286
-
245
MYA), Jurassic (208
-
144 MYA), and Cretaceous
(144
-
65 MYA).


At these times, sea levels were unusually high and shallow
seas covered much of the continents.

Rev. 21 September 2006

Properties of ocean water

56

Sea
-
floor resources, part 4


Oil and natural gas, which are less dense than
surrounding rock, migrated upward into pore spaces
of sandstone and limestone.


The oil and natural gas is trapped in reservoir rock where it
is capped by less permeable rocks.


Oil is found in sediments that were buried at depths
usually between 2 km and 3 km.


Natural gas is found in sediments no deeper than 7
km.

Rev. 21 September 2006

Properties of ocean water

57

Sea
-
floor resources, part 5


Mineral resources include sand, gravel, and shells
mined in shallow waters near the coast.


Mineral resources also include iron, tin, platinum,
gold, and diamonds mixed with coastal sands.


Most of these are products of weathering and erosion and
are transported by rivers to the sea.


Ocean waves and currents sort and concentrate metals and
gemstones into what are called placer deposits.


Placer minerals are left behind as lag concentrates after waves and
currents remove the less dense materials.


Placer deposits are mined only in shallow areas and yield tin, gold,
diamonds, phosphorites, and manganese.

Rev. 21 September 2006

Properties of ocean water

58

Sea
-
floor resources, part 6


Manganese nodules may be an important source of
minerals, such as copper, nickel, and cobalt, but there
is little impetus for exploiting the resource at present.


Tectonic processes lead to the formation of mineral
deposits in place (mineralization).


Some are hydrothermal mineral deposits, which may be
important resources of copper, zinc, silver, gold, and other
metals.


Economically important hydrothermal sulfide minerals are
most commonly associated with subduction zones.

Rev. 21 September 2006

Properties of ocean water

59

Exclusive economic zones, part 1


With increasing demand for limited resources, and
disputes over the sovereignty of the sea, disputes over
marine resources followed.


The United Nations eventually brokered an 1982
agreement, the U.N. Convention on the Law of the
Sea, which granted exclusive economic zones (EEZs)
to each of the 151 coastal nations.


In 1983, the United States joined other nations in defining
its jurisdiction over marine resources beyond state
jurisdictions out to within 370 nautical miles of the coast.

Rev. 21 September 2006

Properties of ocean water

60

Exclusive economic zones, part 2


United Nations (continued):


A 1994 U.N. Convention on the Law of the Sea allows
nations to extend their sovereignty to the edge of the
continental shelf, as long as claimant nations can prove the
newly claimed territory is a “natural prolongation” of its
land territory.


Nations have until 2009 to prove their case.


The United States stands to gain and additional 750,000 square
kilometers by claiming additional territory in the Atlantic, Arctic,
and Pacific oceans.

Rev. 21 September 2006

Properties of ocean water

61