© AMS Ocean Studies Ocean Studies Introduction to Oceanography American Meteorological Society Chapter 4 Marine Sediments

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Ocean StudiesOcean Studies Introduction to Oceanography

American Meteorological Society

Chapter 4

Marine Sediments

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Case in PointCase 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 SedimentsMarine Sediments

• Driving Question:– What are the types and sources of sediment

that enters the ocean?

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Marine SedimentsMarine 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 MarineClassification of MarineSediments by SizeSediments by Size

• Marine sediments are classified by size into three broad categories: – mud, – sand, and – gravel.

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• 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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– 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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• 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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– According to Newton’s first law of motion, an object in constant straight-line 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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– 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 MarineClassification of MarineSediments by SourceSediments by Source

• Ocean scientists classify marine sediments based on their source as – lithogenous (from rock), – biogenous (from living organisms or their remains), – hydrogenous (precipitated from seawater), and– cosmogenous (from outer space).

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• LITHOGENOUS SEDIMENT– Lithogenous sediment accounts for about three-

quarters 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 (diameter) of the particle

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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, Geografiska Annaler, Ser. A, v. 38, Fig. 16, p. 197.]

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• 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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• 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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• 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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• 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 river-borne marine sediments at least in part because of the greater rate of chemical weathering in such climates.

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• 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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• 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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• 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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• 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 mid-depths 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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Distribution of lithogenous and biogenous sediments on the floor of the world ocean.

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• Examples of sources of calcareous biogenous sediment include:– Emiliania huxleyi, a common

species of one-celled algae known as coccolithophorids (A)

– Foraminifera (B)

A B

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• 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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• 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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• BIOGENOUS SEDIMENT– Siliceous Sediments

Diatoms Radiolaria

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• 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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• 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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• 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

A box core from the floor of the tropical Pacific showing a relatively high density of manganese nodules.

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• 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. • These small black fragments of silica-rich glass consist of solidified

droplets of rocks melted when huge meteorites struck the Earth.

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Marine Sedimentary DepositsMarine 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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• 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.

• 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.

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• 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 river-supplied sediments into long, narrow islands and submarine ridges.

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• 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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• 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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• 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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• 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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• 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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• 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.

• On the negative side, deltas are deprived of sediments and undergo subsidence, wave erosion, and loss of productivity.

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• 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 mm-thick 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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• 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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• 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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• 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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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 RockMarine 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 RockMarine 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.

The rock cycle

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Resources of the SeafloorResources 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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• 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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• 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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• 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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Current exclusive economic zones (brown) in the near future could be extended to the limits of the continental shelf (orange).

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ConclusionsConclusions

– 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.

Documents

© AMS Ocean Studies Ocean Studies Introduction to Oceanography American Meteorological Society Chapter 4 Marine Sediments