Transcript AMS Ocean Studies

Ocean Studies
Introduction to Oceanography
American Meteorological Society
Chapter 4
Marine Sediments
© AMS
Case in Point
– The net flow of water from land to sea in the global
water cycle implies that all water-borne wastes
eventually enter the ocean, either dissolved or
suspended in water.
– Some wastes discharged at sea rapidly decompose
physically, chemically, or biologically; others resist
decomposition.
• Persistent chemicals enter food chains and move from one
trophic level to the next higher trophic level increasing in
concentration along the way.
– For many years mercury in industrial wastes dumped into
Minamata Bay, Japan, was converted by bacteria to highly toxic
methyl mercury, taken up by aquatic organisms, readily moved
up the food chain, and was ultimately consumed by people.
More than 3500 were seriously affected and about 50 died.
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Marine Sediments
• Driving Question:
– What are the types and sources of sediment
that enters the ocean?
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Marine Sediments
• The primary focus of this chapter is the
types, sources, distribution, and
environmental significance of marine
sediments.
– We first describe how sediments are classified
by size and the factors governing their rate of
accumulation on the ocean floor.
– We then focus on the classification of marine
sediments based on mode of origin, which
largely determines their composition.
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Classification of Marine
Sediments by Size
• Marine sediments are
classified by size into
three broad
categories:
– mud,
– sand, and
– gravel.
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Classification of Marine
Sediments by Size
• Accumulations of
sediment on the seafloor
also vary in the range of
grain size, known as
sorting
• A well-sorted sediment
deposit has a narrow
range of grain sizes
whereas a poorly sorted
deposit has a broad
range of grain sizes.
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Classification of Marine
Sediments by Size
– More energy is required to transport larger sediment
particles than smaller ones.
– Hence, as a river enters the ocean, larger particles
settle out of suspension almost immediately whereas
finer particles are carried farther away from the coast
before settling to the sea bottom.
• Near the mouths of large sediment-transporting rivers,
sediment accumulation rates can be as much as 8000 m
(26,000 ft) per 1000 years.
• Typical accumulation rates on the continental shelf and
slope, on the other hand, range from 10 to 40 cm (4 to 16 in.)
per 1000 years.
• Accumulation rates in the deep-ocean generally average
from 0.5 to 1.0 cm (0.2 to 0.4 in.) per 1000 years.
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Classification of Marine
Sediments by Size
• TERMINAL VELOCITY
– Terminal velocity is the constant speed attained by a
particle falling through a motionless fluid such as
water or air
– The speed of a falling particle in calm water or air is
regulated by
• gravity, the force that accelerates the particle directly
downward towards Earth’s surface, and
• the fluid resistance offered by the medium through which the
particle is falling.
– A downward accelerating particle meets increasing
fluid resistance while gravity remains essentially
constant.
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Classification of Marine
Sediments by Size
– According to Newton’s
first law of motion, an
object in constant straightline motion or at rest
remains that way unless
acted upon by an
unbalanced force.
– When fluid resistance and
gravity are balanced, the
downward moving particle
attains a constant speed.
That speed is the particle’s
terminal velocity
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Classification of Marine
Sediments by Size
– For a given medium, the terminal velocity of particles
increases with increasing particle size (assuming that
the density and shape of particles vary little).
• This is the reason sand-size particles settle to the ocean
bottom faster than clay-size particles.
– Furthermore, the terminal velocity of a given particle
varies with the medium.
• Water is more viscous (offers more frictional resistance) than
air. Hence, a particle of a given size has a greater terminal
velocity in air than in water.
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Classification of Marine
Sediments by Source
• Ocean scientists classify marine sediments
based on their source as
–
–
–
–
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lithogenous (from rock),
biogenous (from living organisms or their remains),
hydrogenous (precipitated from seawater), and
cosmogenous (from outer space).
Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– Lithogenous sediment accounts for about threequarters of all marine sediments and owes its origin
mostly to weathering and erosion of pre-existing rock.
– By some estimates about 86% of natural erosion
takes place at land elevations greater than 4000 m
(13,125 ft) above sea level, representing only 2% of
Earth’s surface.
– Based upon experimental work, the Hjulström
diagram relates the velocity of a current of water
needed to move (mobilize) a particle to the size
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Classification of Marine
Sediments by Source
The Hjulström diagram relates the velocity of a current of water needed to
mobilize a particle to the diameter of the particle. [From Sundborg, A., 1956,
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Geografiska
Annaler, Ser. A, v. 38, Fig. 16, p. 197.]
Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– Agents of erosion deliver particles to the ocean where
they are further dispersed by waves and currents.
– Explosive volcanic eruptions also contribute
lithogenous fragments of various sizes, shapes and
composition that fall through the air and accumulate in
the ocean; these particles collectively are known as
tephra.
– The composition of lithogenous sediments produced
through weathering depends on their source rock.
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Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– The most abundant elements in the Earth’s crust are
oxygen (O) accounting for 46.6% by weight and
silicon (Si) accounting for 27.7% by weight
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Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– The most common rocks in the Earth’s crust are igneous and are
made up of mostly silicate minerals.
– The primary chemical building block of silicate minerals is the
silicon-oxygen tetrahedron
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Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– Given sufficient time, all rocks and minerals eventually break
down chemically and physically.
– As a general rule, igneous rocks that are rich in ferromagnesian
silicate minerals weather more rapidly than those that are
composed of mostly non-ferromagnesian silicate minerals.
– Differences in rates of weathering determine the dominant
composition of lithogenous sediments that are delivered to the
sea—mainly quartz grains and clay particles.
– Warm, temperate regions account for about two-thirds of riverborne marine sediments at least in part because of the greater
rate of chemical weathering in such climates.
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Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– Windborne dust from regions such as the Sahara,
high mountains and dry-lake beds is the primary
mechanism whereby lithogenous particles reach the
far-from-land deep ocean.
– These regions of the ocean are nutrient-poor and
have low biological productivity. Hence the supply of
biogenous sediments that would cover the clays on
the bottom is very limited.
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Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– Satellite images confirm wind
transport of dust from the deserts of
North Africa to the Atlantic basin.
– Strong winds associated with weather
systems that track across North Africa
pick up dust particles from the dry
topsoil and carry them to altitudes of
3000 m (10,000 ft) or so.
– Scientists have identified possible
links between North African dust and
red tides in the Gulf of Mexico and
threatened coral reefs in the
Caribbean.
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Classification of Marine
Sediments by Source
• LITHOGENOUS SEDIMENT
– Glaciers erode bedrock and transport rock fragments of varying
size and shape to the ocean. Sediments carried by glaciers are
usually angular and can range in size from a fine powder to blocks
of rock the size of an automobile or even larger.
– The process whereby the leading edge of a glacier breaks up into
icebergs upon entering the ocean (or large lake) is known as
calving.
– Transported by ocean currents and wind, icebergs eventually melt
in warmer environments, releasing a poorly sorted mix of sand
and boulders, which quickly sinks to the bottom.
– Such ice-rafted glaciomarine sediments occur on about 20% of
the sea floor.
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Classification of Marine
Sediments by Source
• BIOGENOUS SEDIMENT
– Biogenous sediment includes the excretions, secretions, and
remains of organisms. Examples include shells, fragments of
coral, and skeletal parts.
– The chemical composition of most biogenous sediments is either
calcium carbonate (CaCO3) or silica (SiO2)—substances secreted
by organisms to form their shells.
– Biogenous sediments dominate 30% to 70% of the ocean’s middepths and skeletal remains alone account for 25% to 50% of all
particles suspended in seawater.
– Calcareous sediments are the most abundant of all biogenous
sediment on the sea floor.
• They consist of calcium carbonate tests of foraminifera, shells of
pteropods, and coccoliths.
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Classification of Marine
Sediments by Source
Distribution of lithogenous and biogenous sediments on
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the floor of the world ocean.
Classification of Marine
Sediments by Source
• Examples of sources of calcareous
biogenous sediment include:
– Emiliania huxleyi, a common
species of one-celled algae
known as coccolithophorids (A)
– Foraminifera (B)
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A
B
Classification of Marine
Sediments by Source
• BIOGENOUS SEDIMENT
– Many of the larger biogenous particles are fecal pellets, which,
because of their large size, have relatively high terminal velocities
and can sink hundreds of meters per day.
• fecal pellets transport organic matter from surface waters and serve
as a food source for bottom-dwelling organisms in the deep ocean.
– The continual fall to the deep ocean floor of the remains of
organisms from the upper, sunlit layer of the ocean, along with
their fecal pellets and various forms of non-living matter, is
sometimes called marine snow.
– The smaller and more soluble particles slowly sink and may
dissolve prior to reaching the bottom, thereby altering the
chemical composition of deep-ocean waters.
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Classification of Marine
Sediments by Source
• BIOGENOUS SEDIMENT
– Siliceous sediments are second in abundance to
calcareous sediments on the ocean floor.
– These sediments consist of “shells” that are secreted
by
• diatoms, single-celled algae
• radiolaria, single-celled organisms.
– Siliceous particles dissolve at all ocean depths
(slightly more in shallow warm water) so their
presence on the seafloor indicates areas where
source organisms are particularly abundant.
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Classification of Marine
Sediments by Source
• BIOGENOUS SEDIMENT
– Siliceous Sediments
Diatoms
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Radiolaria
Classification of Marine
Sediments by Source
• BIOGENOUS SEDIMENT
– Phosphatic Sediments (rich in Phosphates)
• Primarily consist of fish bones, teeth, and scales.
• Rare in marine sediment deposits.
• Occur mostly on shallow, isolated banks or near coastal
areas where rates of biological productivity are especially
high.
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Classification of Marine
Sediments by Source
• HYDROGENOUS SEDIMENT
– Hydrogenous sediment encompasses particles that
are chemically precipitated from seawater, in some
cases forming coatings on other seafloor sediment.
– Examples of hydrogenous sediment include:
•
•
•
•
some carbonates (ooliths),
halite (NaCl),
gypsum (CaSO4 ·2H2O), and
manganese nodules.
– Where evaporation rates are high and rainfall is low,
salts precipitate from seawater in the sequence:
carbonate salts, sulfate salts, and halite.
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Classification of Marine
Sediments by Source
• HYDROGENOUS SEDIMENT
– manganese nodules are the most
conspicuous of all hydrogenous
sediments
• occur on the floor of all ocean
basins except the Arctic
• occur in a variety of marine
environments from abyssal plains to
mid-ocean ridges
• grow extremely slowly, ranging from
about 1 to 10 mm (0.004 to 0.04 in.)
per million years.
• rich manganese nodule deposits are
found only in regions of the ocean
far from shore, where accumulation
of lithogenous and biogenous
sediments is small
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A box core from the floor of
the tropical Pacific showing
a relatively high density of
manganese nodules.
Classification of Marine
Sediments by Source
• COSMOGENOUS SEDIMENT
– Comes from outer space, for example, as meteorite fragments.
– Some are remnants of the formation of planets in the solar
system.
• Their unique composition make them readily recognizable in deep
ocean sediment deposits.
– Other particles are formed from silicate rocks blasted off other
planets or the moon by meteorite impacts
• More difficult to identify as cosmogenous because they resemble
lithogenous sediment.
– Most dissolve before reaching the bottom.
– Tektites are a special type of sediment that is indirectly
cosmogenous in origin.
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• These small black fragments of silica-rich glass consist of solidified
droplets of rocks melted when huge meteorites struck the Earth.
Marine Sedimentary Deposits
– The different types of marine sediments (i.e., lithogenous,
biogenous, hydrogenous, and cosmogenous) occur in varying
proportions and thicknesses on the ocean bottom as
marine sedimentary deposits. We will now describe
marine sediment deposits of
• the continental margin, and
• the deep ocean basins.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
– Most marine sediment deposits in the continental
margin, called neritic deposits, are lithogenous and
occur in a wide range of sediment sizes.
– About 95% of the largest sediments transported to the
ocean by rivers are trapped and deposited in bays,
wetlands, estuaries, beaches or deltas.
– Only about 5% of river-borne sediment brought to the
shoreline reaches the continental shelf or slope.
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• A notable exception is seaward of the mouths of major
sediment-transporting rivers.
• Massive submarine avalanches and turbidity currents can
transport sediments hundreds or thousands of kilometers out
onto the continental rise and to the sea floor beyond.
Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
– Where a river enters the ocean, the water slows so that sediments begin
settling out of suspension and accumulate at the mouth of the river as a
delta.
– Deltas are classified as
• river-dominated
• wave-dominated or
• tide-dominated
– In a river-dominated delta, (e.g., Mississippi), the rate of input of
sediments (from rivers or streams) exceeds the rate of removal of
sediments (by waves and currents) and the delta develops the classic
triangular shape (as viewed from above).
– In a wave-dominated delta (e.g., Niger), strong wave action and currents
produce only a slight bulging of an overall straight coastline.
– In a tide-dominated delta, (e.g., Ganges), tidal currents rework riversupplied sediments into long, narrow islands and submarine ridges.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
- For thousands of years,
suspended sediment
delivered and deposited
by the Nile and its
branches provided the
Nile Delta with the most
fertile soils on the African
continent.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
– Wetlands are low-lying flat areas that are covered by
water or have soils that are saturated with water for at
least part of the year.
• Common on deltas and coastal plains of the mid-Atlantic U.S.
and Gulf of Mexico and in many other areas of passive
continental margins worldwide
– Wetlands such as salt marshes accumulate large
amounts of organic matter and support numerous and
diverse populations of marine plants, animals and
birds.
– Wetlands help control coastal flooding by acting as a
sponge taking up water during high water episodes.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
– Near the mouths of many major sediment-transporting rivers that
empty into the ocean, intermittent avalanches of dense, mud-rich
waters, called turbidity currents, flow down submarine canyons,
carrying large amounts of sediment onto the ocean floor.
– Sedimentary deposits produced by turbidity currents are known as
turbidites.
– Turbidity currents are much denser than seawater hence, they
flow along the ocean bottom, often eroding channels just as rivers
do on land.
• Such flow behavior explains why turbidites have unusual textures and
contain abundant shells and other remains of shallow-water dwelling
organisms.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
Turbidity currents can
transport lithogenous sediment
into the deep-ocean bottom.
(A) Vertical profile of a turbidity
current flowing on the
continental slope and rise. (B)
Vertical cross-section of
turbidites deposited by three
successive turbidity currents.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
– In coastal areas, especially near large rivers or in
deltas, marine sediment accumulates rapidly, typically
at rates of several meters per thousand years.
– Sediments are buried too quickly to react fully with
seawater or the oxygen dissolved in the water.
– Furthermore, bottom-dwelling organisms cannot
consume all of the food sources. These sediments
exhibit a wide variety of colors, including greens and
blues, due to the various oxidation states of iron on
the particles.
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Marine Sedimentary Deposits
• CONTINENTAL-MARGIN DEPOSITS
– Human activity plays an important role in the delivery of sediment
to the ocean.
• In 2005, an international research team reported that through soil
erosion, human activity has increased the amount of sediment
transported by global rivers by 2.3 ± 0.6 billion metric tons per year.
• At the same time, retention of sediments in reservoirs (e.g., behind
dams) reduced the flux of sediment reaching the coast by 1.4 ± 0.3
billion metric tons per year.
• On the positive side, retention of river-borne sediment in reservoirs
behind dams may reduce the need for dredging of harbors and
offshore coral reefs may benefit from less turbid waters and more
sunlight.
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• On the negative side, deltas are deprived of sediments and undergo
subsidence, wave erosion, and loss of productivity.
Marine Sedimentary Deposits
• DEEP-OCEAN DEPOSITS
– Fine-grained sediments that gradually accumulate
particle-by-particle on the deep-ocean floor form
pelagic deposits.
– Most of these sediments are biogenous and their
accumulation rates are considerably slower than
neritic (near shore) sediments. On average, a 1 mmthick layer forms in about 1000 years.
– Their relatively long suspension times in the water
mean that
• they are widely dispersed by currents, and
• there is also ample time for chemical reactions to occur.
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Marine Sedimentary Deposits
• DEEP-OCEAN DEPOSITS
– Depending on composition, pelagic deposits that are
more than 30% biogenous by weight are called either
• calcareous ooze, or
• siliceous ooze
– Calcareous oozes are the most abundant of pelagic
oozes. Calcareous oozes generally are confined to
ocean waters shallower than the carbonate
compensation depth (CCD), the depth of the ocean
below which calcium carbonate (CaCO3) shells and
skeletons dissolve and do not accumulate. (The CCD
is also called the saturation horizon.)
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Marine Sedimentary Deposits
• DEEP-OCEAN DEPOSITS
– The rate at which calcium carbonate dissolves in
seawater increases with falling temperature.
– Ocean temperature usually drops with increasing depth
below surface waters.
– Therefore more calcareous particles dissolve with
increasing water depth.
– Calcareous oozes therefore accumulate on ocean
bottom features having depths shallower than 4500 m.
– Worldwide, the CCD averages about 4500 m (14,800
ft).
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Marine Sedimentary Deposits
• DEEP-OCEAN DEPOSITS
– Siliceous ooze is composed of tests of diatoms and
radiolaria.
– Ocean water is under-saturated with silica so that
these tests dissolve at all ocean depths.
– They occur on the sea floor below surface waters only
where source organisms are particularly abundant.
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Marine Sedimentary Deposits
The rate at which radiolaria (siliceous) and foraminifera
(calcareous) shells dissolve with increasing ocean depth in
terms of percent weight loss.
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Marine Sedimentary Rock
– As part of the rock cycle, over the millions of years that
constitute geologic time, sediment that is deposited on
the ocean floor is gradually converted to solid marine
sedimentary rock through lithification.
• Lithification usually involves both compaction and cementing
of sediments at relatively low temperatures (under 200 ºC or
390 ºF).
• Sediments are compacted by the increasing weight of
sediments accumulating above that squeeze deeper
sediments closer together.
• Siliceous and calcareous fluids migrating through the tiny
openings between individual sediment grains precipitate
minerals that fill the pore spaces, cementing grains to one
another.
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Marine Sedimentary Rock
• The product of lithification is
sedimentary rock such as
shale, sandstone, or
limestone depending on the
composition and particle size
of the constituent sediments.
• With deeper burial and further
increases in temperature,
pressure, and access to
chemically active fluids,
sedimentary rock may be
converted to metamorphic
rock such as slate, schist or
gneiss.
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The rock cycle
Resources of the Seafloor
– Resources extracted from the seafloor include oil,
natural gas, sand, gravel, and minerals.
– While oil and natural gas account for more than 95% of
the total monetary value of resources extracted from
the seafloor, sand and gravel are the seafloor
resources most commonly mined worldwide.
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Resources of the Seafloor
• OIL AND NATURAL GAS
– Petroleum and natural gas are derived from the
remains of marine plants and animals and occur in the
pore spaces of marine sedimentary rock.
– Substantial amounts of organic matter must
accumulate at the bottom of a shallow quiet sea and
be subjected to anaerobic decomposition.
– Products of anaerobic decomposition include methane
and other light hydrocarbons.
– With continued accumulation of overlying sediment on
the sea floor, the deeper organic-rich sediments were
subject to increasing temperature and pressure that
spurred their conversion to oil and natural gas.
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Resources of the Seafloor
• MINERAL RESOURCES
– Great quantities of sand, gravel, and shells are mined from the
near-shore, shallow ocean bottom, especially near coastal cities.
– In some locales, valuable metallic and nonmetallic minerals such
as iron, tin, platinum, gold, and diamonds occur mixed with
coastal sands.
• Most of these resources are products of weathering and erosion of
continental rock and sediment and are transported to the sea in
suspension by rivers along with other lithogenous particles.
• Ocean waves and currents sort and concentrate metals and
gemstones in coastal or submarine deposits, known as placer
deposits.
– Placer minerals typically are relatively dense and resistant and were left
behind as a lag concentrate as waves and currents removed the less
dense sand grains.
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Resources of the Seafloor
• EXCLUSIVE ECONOMIC ZONE
– The 1982 U.N. Convention on the Law of the Sea
granted jurisdiction over an exclusive economic
zone (EEZ) to each of 151 coastal nations.
– In March 1983, the U.S. (later joined by fifty other
nations) defined its jurisdiction over ocean resources
(including minerals, fuels, and fisheries) to extend 370
km (200 nautical mi) offshore.
– The 1994 U.N. Convention on the Law of the Sea
allows a nation to expand its EEZ to the edge of the
continental shelf, if it can establish that the new
territory is a “natural prolongation” of its landmass.
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Resources of the Seafloor
Current exclusive economic zones (brown) in the near future could be
extended to the limits of the continental shelf (orange).
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Conclusions
– Sediments are produced by processes operating at the
interfaces between the lithosphere, atmosphere,
hydrosphere, cryosphere, and biosphere.
– Rivers, winds, glaciers, and gravity transport sediment
from land to sea while some sediment originates in the
ocean.
– Sediment deposits on the ocean floor record changes
in the various subsystems of the Earth system over
millions of years, that is, since formation of the present
ocean basins.
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