This section is divided into 3 parts; Marine Sediments, Gas Hydrates, Marine Sediments II

Marine Sediments

Where do the sediments come from? So what are the sediments that blanket the deep ocean and how do they get there? Sediment sources include:

• Terrigenous (derived from land)
• Biogenic (derived from plants and animals)
• Authigenic (formed on or in the sediments on the sea floor)
• Volcanogenic (particles from volcanic eruptions)
• Cosmogenous (particles from outer space)

Distribution of neritic sediments (those found on continental margins) and ocean sediments (from Thurman and Burton).
Abyssal clays are found in the deep ocean basins, calcareous oozes are abundant on oceanic ridges and rises and siliceous oozes are found beneath areas of high biologic productivity.

Terrigenous sediment – rivers, glaciers and wind

Terrigenous sediments are derived from the erosion of the continents and are transported into the ocean as particles of gravel, sand or mud. Their mineral composition varies and reflects the source rock and weathering process (climate). Most of the world’s largest rivers are located in the wet tropic regions where there is also high relief and intense chemical weathering so these are areas of high mud input into the oceans. For example, the Fly River in Papua New Guinea delivers more sediment to the ocean (approx. 140 million t/yr) than do all of Australian rivers combined. The Australian continent has low relief and low rainfall, which together make for limited erosion and sediment transport. The Ganges River discharges the most sediment per year – about 1500 million tonnes. As a very broad approximation this represents about 400,000 Olympic swimming pools full. Most of the material discharged by rivers is deposited along the margins of the continents. However some is funnelled across continental margins through submarine canyons and by currents as dilute suspensions.

Sediment is also blown off the continents into the ocean, particularly on the west coast of continents adjacent to the major deserts. Usually only very small particles (less than 20 µm) are carried long distances. The Abyssal clay or "red clay" that covers much of the deep ocean floor is largely of aeolian origin. While aeolian dust is deposited everywhere it only dominates on the abyssal regions where low biological productivity and the dissolution of calcium carbonate prevent dilution (see below). The input of aeolian sediments, including charcoal from bushfires, is on the increase as humans degrade the landscape and desertification intensifies. Testament to this is recent satellite photographs of dust clouds blanketing the north Atlantic and Pacific Oceans and bushfire and dust plumes into the Tasman Sea. Similar increases in aeolian input occurred following the melting of the last ice sheet 15,000-20,000 years ago when vast areas of crushed rock were exposed before vegetation could stabilize it.

Melting of ice sheets and icebergs has been, and continues to be, a major provider of sediment to the sea floor in high latitudes. Ice is indiscriminate in what it carries: giant boulders to finely ground clay.

WESTERN SAHARA DUST - February 26, 2000 This February 26 image shows a dust plume obtained from TOMS data (Total Ozone Mapping Spectrometer) over the Sahara Desert and extending over the Atlantic Ocean and Canary Islands. The land sources of the dust plume are clearly visible, with the main source coming from Western Sahara and Mauritania. The green to red false colours in the dust plume image represent increasing amounts of aerosol, with the densest portion over the ocean (from http://toms.gsfc.nasa.gov/aerosols/Africa/canary.html).

Smoke from bush fires surrounding Sydney in January 2002 produced smoke plumes that ascend from the southeastern coastline of Australia and extend out over the Tasman Sea. This true-colour image is from NASA’s Moderate-Resolution Imaging Spectroradiometer (MODIS) http://earthobservatory.nasa.gov/Natural Hazards/

Satellite image of the Fly River delta in Papua New Guinea. The river discharges about 140 million tons of sediment per year. Most of this sediment is deposited in the delta but some makes its way into the deep water of the adjacent Coral Sea. Strong tidal currents in the delta form linear islands and sand banks.

City shrouded in red as drought speeds dust storms By Asa Wahlquist - 21 Oct 02 The Australian THE dust-storm season has begun early in southern Australia and, coupled with the drought, it is only going to get worse as it rips away top soil. Canberra residents woke to a thick coating of red dust last week and a storm at Mildura reduced visibility to less than a kilometer. Wind erosion expert Grant McTainsh said the dust-storm season in southern NSW and Victoria was usually from December to April and indications were that the next few months would be very dusty. Dr McTainsh, of the Australian School of Environmental Studies at Griffith University, said the Canberra dust storm was noticed only because there were very strong winds and a lot of locally raised material. He said there were many more events no one notices. Last week's strong, low winds picked up dust in central NSW and swept it through Wagga Wagga and Canberra to the coast, dumping red dust along the way. Dr McTainsh said the wind that blew in to Canberra kept picking up material as it moved "like a vacuum cleaner", and it was still picking up material as it blew through Canberra. He said the wind was actively eroding as it blew through Canberra, which is probably why it surprised people. "This is very rare," he said. It was similar to the 1983 storm which hit Melbourne and which cost about $5 million in lost nitrogen and phosphorus. Dr McTainsh said Canberra had had at least 98 dust-haze events, which can carry more soil than dust storms, since 1960. But most people didn't notice. "The Melbourne event only had around 2 million tonnes of dust in the air," he said. "But there was an event which passed through Brisbane in 1987 which almost nobody noticed. It had around 3.5 million tonnes of material in the air. In the case of the Brisbane event, the source was a long, long way away and all it looked like was a rather high pollution day." Dr McTainsh said droughts were associated with high winds, "so you get a double whammy". And dust was not necessarily the result of poor land management. "Australia is an arid country, it should have dust storms," he said. "It is certainly true that a proportion of the dust is due to the fact the soils are exposed, as a result of cultivation or overgrazing etc. It is very hard to put a reliable number on it." Bureau of Meteorology observer at Mildura Tony Grasso said a storm on Thursday had reduced visibility to less than 1km for a couple of hours. "There was widespread dust everywhere," he said. "The only thing we can do is batten down all the hatches, close every window and door. It still gets in though." Dust storm in the mallee (ABC Riverland SA online).

Icebergs in Antarctica Icebergs are the end of a process. The mass transfer of ice from high altitude to (often) the sea. As a glacier carves a path to the sea, it grinds, fractures and mills the rock that it passes through, often incorporating this material into the glacier itself. When the open sea is reached, the enclosed sediment is rafted out to sea - at the mercy of currents and wind. As the glacier melts, the sediment load is lost, dropping to the seafloor - both on the continental shelf and well beyond it. This process (termed ice-rafting) is an exception for deep sea sedimentation, as the process is capable of transporting coarse material to areas normally restricted to oozes and biotic remains. < www.snopes.com.jpg

Biogenic sediment

Biogenic sediment contains organically produced particles and is defined as any deposit which has more than 30% biogenous constituents by weight. There are three main groups of organic sediments – calcareous, siliceous and phosphatic. We are not going to bother with biogenic phosphate as it plays such a minor part in the scheme of things – suffice to say it is mainly fish bones.

Calcareous – the most abundant of the biogenic sediment forming components. Grains of calcium carbonate (often confusingly abbreviated to ‘carbonate’) are derived from the shells and skeletons of marine organisms and plants such as foraminifera and pteropods (both animals) and coccolithophores (algae). It is estimated that since the formation of the earth over 90% of the carbon dioxide introduced into the atmosphere by volcanic activity has been removed and deposited in marine sediments by these sorts of organisms. This system is called the biological pump and it is part of the global carbon cycle (see box).

This thing the Calcite Compensation Depth - CCD

The distribution of calcareous biogenous sediments is largely determined by what is known as the calcite compensation depth (CCD; sometimes erroneously referred to as the carbonate compensation depth). This is the depth, usually several km, below which calcite does not accumulate because it is a level on the sea floor at which the rate of carbonate supply is equal to the rate of carbonate dissolution. It is analogous to the snow line on land. The depth at which dissolution starts, is called the lysocline, (generally located 500-1000 m above the CCD). The CCD exists because the carbon dioxide rich deep water of the oceans is undersaturated with respect to calcite. Calcite also becomes more soluble with increasing pressure and lower temperatures. In contrast the warm tropical surface waters are supersaturated with respect to calcium carbonate. Thus the level of the CCD rises in high latitudes where the cold CO2 rich water is at the surface and is depressed in low latitudes where the supply of calcite raining to the sea floor is higher. The dissolution of calcite recycles Ca as organisms precipitate more CaCO3 than can be supported by the flux of calcium to the oceans from rivers. If all the CaCO3 produced was incorporated into sediments, we would soon run out of calcium in seawater. It is estimated that approximately 90% of the calcium carbonate precipitated by organisms in the upper layers of the oceans is dissolved in the deep ocean.

Plot of increasing CaCO3 dissolution with increasing depth in the Pacific Ocean. The lysocline is the depth at which the rate of dissolution increases markedly. The CCD is the depth where the rate of calcite supply is matched by the rate of dissolution and therefore below which no calcareous sediments are found.

The behaviour of calcium carbonate is controlled primarily by the following reactions:
CaCO₃ + H₂CO₃ = Ca²⁺ + 2HCO₃^3- [1]
where:
CaCO₃ is a solid –either of the minerals calcite (hexagonal) or aragonite (orthorhombic).
H₂CO₃ is carbonic acid - a relatively weak naturally occurring acid that forms by the reaction between water and carbon dioxide:
H₂O + CO₂ = H₂CO₃ [2]
Ca2+ Calcium is a positive ion (a cation) in solution and
2HCO₃^3- is a negative ion (an anion) in solution – called the bicarbonate ion. This ion also disassociates to form the carbonate ion CO₃^2-.

The upper levels of the ocean in the tropics are supersaturated with CaCO3. Organisms precipitate CaCO₃ (calcite) shells. The solubility of CaCO₃ (the depth of the lysocline and CCD) is influenced by a number of interrelated factors including water temperature, pressure, biological activity, calcite crystal size, and dissolved CO₂ concentration.

Water temperature:

Like all gases CO₂ is less soluble in warm water than in cold water. Therefore, in warm water less carbonic acid is produced (equation [2]) and calcite will precipitate. In contrast, in the cold deep ocean basins where there is lots of available CO₂, equation [2] moves to the right so as to produce more carbonic acid. This produces a shift to the right in equation [1] resulting in the solution of calcite. This is why calcite is not found in sediments in the deep marine environment.

Changes in Pressure

An increase in pressure (ie increasing depth) increases the solubility of CO₂ and more carbonic acid is produced (equation [2]). This causes equation [1] to shift to the right resulting in the dissolution of calcite.

Organic Activity

Green plants remove carbon dioxide in the process of photosynthesis. Again, a loss of carbon dioxide will result in the precipitation of calcite.

Decay

When organic matter decays it releases carbon dioxide and the solubility of calcite is increased.

Both the lysocline and the CCD are shallower in the Pacific than the Atlantic due to the fact that there is more CO₂ in the Pacific water. This reflects the evolution of bottom water chemistry - as seawater circulates from the North Atlantic via the Southern Ocean to the northern Pacific and Indian Oceans it "ages" and accumulates carbon dioxide.

Examples of carbonate sediment

Forams, coccoliths and others

Forams

These single-celled protozoans are both benthic, which live in sediments on the sea floor, and planktonic, which live in the upper 100 m or so of the ocean. Of the estimated 4000 species living today, 40 are planktonic but because of their great abundance they secrete more calcite than all other foraminifers. Foraminiferal shells (called ‘tests’) of both groups occur in a variety of shapes, and typically range from 0.1 mm to 1 mm in size (but have been found up to 18cm!). The shells of all planktonic and most benthic species are composed of calcite.

The fossil record of benthic foraminifera dates back to more than 550 million years. Planktonic species have been around for the last 200 million years but really got going about 100 million years ago. Because of the large number of species (it is estimated that there are over 40,000 in the rock record), their wide distribution and environmental sensitivity they can be used to determine past climate conditions,. In addition, because of their rapidly changing form (species generally exists for about ~5-15 million years), they can be used to determine the age of sediments in which they occur.

Foraminifera –the dark brown structure is the test, or shell, inside which the foram lives. Radiating from the opening are fine hairlike reticulopodia, which the foram uses to find and capture food (http://www.ucmp.berkeley.edu/foram/foramintro.html).

Coccoliths (properly called Coccolithophores)

Coccolithophores are a common group of phytoplankton – single cell algae (plants). They are unique in that the single cell is surrounded by an armour of at least 30 calcite plates (called coccoliths) to form a sphere only 30 um in diameter. Scientists estimate that the organisms deposit more than 1.5 million tons (1.4 billion kilograms) of calcite a year, making them the leading calcite producers in the ocean and together with the forams they deposit more calcite on the floor of the deep ocean than all the shells and corals on the continental shelves. Consequently they are responsible for many thick chalk and limestone beds. They first appear in the fossil record in the Jurassic and were particularly common in the Cretaceous, when they produced many chalk deposits, like the White Cliffs of Dover. They were almost wiped out at the Cretaceous -Tertiary boundary but have persisted to the modern day. A related, but now extinct, group are the Discoasters, who secreted microscopic star-shaped calcite crystals. Together these microfossils are important in micropaleontology for evolutionary and environmental studies. Because of their small size they are often called nannofossils!

Today Coccolithophores live mostly in subpolar regions. They are often found in nutrient poor water that cannot support other types of plankton. They form blooms, which because of the structure of their plates, are visible from space. They appear as milky white or turquoise patches. Blooms are a regular occurrence off the north coast of Australia.

This image is a natural-colour view of the Celtic Sea and English Channel, taken on the June 4, 2001. The coccolithophore bloom in the lower left -hand corner usually occurs in the Celtic Sea for several weeks in summer. The coccoliths backscatter light from the water column to create a bright optical effect.
(http://earthobservatory.nasa.gov/Newsroom/New Images/
images_topic.php3?topic=life&img_id=7275)

Pteropods

Pteropods are small gastropod molluscs, basically floating snails whose foot is modified for swimming. They produce large mucus feeding webs for ensnaring phytoplankton. They have a coiled shell composed of aragonite into which they can retreat if threatened (some species do not have he shell and are just a gelatinous blob). They favour tropical and warm-temperate seas.

In some equatorial areas of the Indian Ocean, pteropod shells dominate the sedimentation, resulting in a subset of the carbonate ooze - the pteropod ooze.

> www.soton.ac.uk/.../forams/

Siliceous sediment

Siliceous sediment is composed of siliceous shells or skeletons of opaline silica a form of hydrated silicon dioxide. The principal silica producers are the radiolarians (animals) and diatoms (plants). On the whole the ocean is undersaturated with silica, therefore you might expect biogenic silica to dissolve and not be present in sediments. The solubility of silica increases with decreasing pressure and increasing temperature – that means that there is more of it in the deep ocean (the opposite to carbonate). Siliceous organisms are generally found in nutrient rich waters (areas of upwelling) that have a high silica content. The shells do dissolve (pretty slowly), but because of high productivity there is a lot of them and they get buried before they get destroyed. The siliceous content of sediment is highest in deep water where calcareous sediment is absent (because of the CCD).

Radiolarians are protozoans that construct beautifully complex silica exoskeletons that often have many spines extending outwards. They form oozes on the sea floor that over time can evolve into hard sedimentary rocks called radiolarian cherts or, if mixed in with calcareous ooze they form individual flint nodules in chalk. They have been around for the last 540 million years and are useful for dating rocks, because their skeletons are very well preserved in the sediment. During the Cretaceous-Tertiary mass extinctions, radiolarians did well in comparison to other planktonic life forms.

Radiolarians absorb dissolved silica from seawater to construct their beautiful skeletons. Some examples from http://www.ucmp.berkeley.edu/protista/radiolaria/radmm.html

Some sponges produce calcareous spicules that are common in some shelf and slope sediments.

Chert

Diatoms

Diatoms are single cell algae (plants) that incorporate silica into their cell wall to form ‘frustules’. Diatoms occur in both benthic and planktonic forms. The benthic ones are restricted to water depths of less than 100 meters. Diatom blooms are common in rivers and upwelling zones (due to the high nutrient content) and can be toxic to other organisms because of oxygen depletion or the biotoxins produced by some species.



INSERT bloom.jpg

Just to illustrate the carbonate versus siliceous story this diagram shows the solubility curve for both. Silica solubility is highest in the low pressure, high water temperature section, whereas carbonate solubility is highest in the high pressure, low water temperature section.

Minor contributions of silica in sediments come from spicules secreted by sponges and silicoflagellates.

Exercises

Sediment Theory

This exercise looks at some of the processes controlling sediment composition from the coast to the trench.

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Sediment Practical

This exercise couples with the above exercise. Real examples of sediments have been used to allow you to interpret them in 'real-life' conditions.

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Australian Antarctic Current

Sediment deposition preserves more than just dirt! Conditions of deposition: temperature, depth, current, proximity to land, rate of deposition are all preserved in the sequence. With a little know-how this wealth of information can be gleaned from the rocks to give an informative 'snap shot' of million - hundreds of millions of years ago!

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Turbidites.

Vast amounts of sediment are shed off the continental slope, often settling 1000's of km from the continental rise. This exercise looks at some of the dynamics behind the process and the resulting geology.

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Crossword.

Hung over? Procrastinating? This exercise will keep the mental cogs turning and let you take it easy for a while.

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Additional Exercises

Yet to be added, this page will contain PDF's of 'real' data to allow you to nut out a few more similar topics

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Helping Hints

We realise that not everyone has had experience with the intrinsic concepts of Marine Science. This 'computer' has numerous examples which will help you understand these fundamental facets! Very helpful, very colourful!

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