MAR 110: Introductory Oceanography Marine sediments

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.



• 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.



• 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.



• 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.


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.


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).



• 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.




• 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.



• 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.


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.



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.



• 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.



• 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 (SiO4) is the primary building block of silicate minerals.

• The ratio of silicon to oxygen varies; For example, quartz is made primarily of SiO2) .





• 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).



• 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.




• 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.




• 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.



• Biogenous sediments include excretions, secretions, and remains of organisms – including shells, corals, and skeletal parts.

• Most biogenous material is made of calcium carbonate (CaCO3) or silica (SiO2), 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.




• 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.





• 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.





• 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 (CaSO4·2H2O), and manganese nodules.



• 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.



• 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.




• 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.



• 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.


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.



• 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.




• 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.



• 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.




• 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.


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.



• 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.



• 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.



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.


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.



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.



• 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.



• 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.



• 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.



• 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.



• 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.


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.


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.


Documents

MAR 110: Introductory Oceanography Marine sediments