Thursday, September 20, 2012

The euphotic zone is somewhat arbitrarily

The euphotic zone is somewhat arbitrarily defined as extending from the surface to the depth where the light intensity is approximately 0.1–1% of surface sunlight irradiance, Sony VAIO VGN-NW71FB/N Battery
depending on season, latitude and degree of water turbidity.[6][7] In the clearest ocean water, the euphotic zone may extend to a depth of about 150 meters,[6] or rarely, up to 200 meters.[8]Dissolved substances and solid particles absorb and scatter light, and in coastal regions the high concentration of these substances causes light to be attenuated rapidly with depth. Sony VAIO VGN-NW71FB/W Battery
In such areas the euphotic zone may be only a few tens of meters deep or less.[6][8] The disphotic zone, where light intensity is considerably less than 1% of surface irradiance, extends from the base of the euphotic zone to about 1000 meters.[9] Extending from the bottom of the photic zone down to the seabed is theaphotic zone, a region of perpetual darkness.Sony VAIO VGN-NW91FS Battery
Since the average depth of the ocean is about 4300 meters,[10] the photic zone represents only a tiny fraction of the ocean’s total volume. However, due to its capacity for photosynthesis, the photic zone has the greatest biodiversity and biomass of all oceanic zones. Nearly all primary production in the ocean occurs here. Sony VAIO VGN-NW91GS Battery
Any life forms present in the aphotic zone must either be capable of movement upwards through the water column into the photic zone for feeding, or must rely on material sinking from above,[5] or must find another source of energy and nutrition, such as occurs in chemosynthetic archaea found near hydrothermal vents and cold seeps. Sony VAIO VGN-NW91VS Battery
These animals have evolved to survive the extreme pressure of the sub-photic zones. The pressure increases by about oneatmosphere every ten meters. To cope with the pressure, many fish are rather small, usually not exceeding 25 cm in length. Also, scientists have discovered that the deeper these creatures live, Sony VAIO VGN-SR11M Battery
the more gelatinous their flesh and more minimal their skeletal structure. These creatures have also eliminated all excess cavities that would collapse under the pressure, such as swim bladders.[11]
Pressure is the greatest environmental factor acting on deep-sea organisms. Sony VAIO VGN-SR11MR Battery
In the deep sea, although most of the deep sea is under pressures between 200 and 600 atm, the range of pressure is from 20 to 1,000 atm. Pressure exhibits a great role in the distribution of deep sea organisms. Until recently, people lacked detailed information on the direct effects of pressure on most deep-sea organisms, Sony VAIO VGN-SR140D Battery
because virtually all organisms trawled from the deep sea arrived at the surface dead or dying. With the advent of traps that incorporate a special pressure-maintaining chamber, undamaged larger metazoan animals have been retrieved from the deep sea in good condition. Sony VAIO VGN-SR140D/B Battery
Some of these have been maintained for experimental purposes, and we are obtaining more knowledge of the biological effects of pressure.
The two areas of greatest and most rapid temperature change in the oceans are the transition zone between the surface waters and the deep waters, the thermocline, Sony VAIO VGN-SR140D/P Battery
and the transition between the deep-sea floor and the hot water flows at the hydrothermal vents. Thermoclines vary in thickness from a few hundred meters to nearly a thousand meters. Below the thermocline, the water mass of the deep ocean is cold and far more homogeneous. Sony VAIO VGN-SR140D/S Battery
Thermoclines are strongest in the tropics, where the temperature of the epipelagic zone is usually above 20°C. From the base of the epipelagic, the temperature drops over several hundred meters to 5 or 6°C at 1,000 meters. It continues to decrease to the bottom, but the rate is much slower. Below 3,000 to 4,000 m, the water is isothermal. Sony VAIO VGN-SR140E Battery
At any given depth, the temperature is practically unvarying over long periods of time. There are no seasonal temperature changes, nor are there any annual changes. No other habitat on earth has such a constant temperature.
Hydrothermal vents are the direct contrast with constant temperature. Sony VAIO VGN-SR140E/B Battery
In these systems, the temperature of the water as it emerges from the "black smoker" chimneys may be as high as 400°C (it is kept from boiling by the high hydrostatic pressure) while within a few meters it may be back down to 2 - 4°C.
Salinity is remarkably constant throughout the depths of the deep sea. There are two notable exceptions to this rule: Sony VAIO VGN-SR140E/P Battery
In the Mediterranean Sea, water loss through evaporation greatly exceeds input from precipitation and river runoff. Because of this, salinity in the Mediterranean is higher than in the Atlantic Ocean.[13] Evaporation is especially high in its eastern half, causing the water level to decrease and salinity to increase in this area. Sony VAIO VGN-SR140E/S Battery
The resulting pressure gradient pushes relatively cool, low-salinity water from the Atlantic Ocean across the basin. This water warms and becomes saltier as it travels eastward, then sinks in the region of the Levant and circulates westward, to spill back into the Atlantic over the Strait of Gibraltar.[15] Sony VAIO VGN-SR140N/S Battery
The net effect of this is that at the Strait of Gibraltar, there is an eastward surface current of cold water of lower salinity from the Atlantic, and a simultaneous westward current of warm saline water from the Mediterranean in the deeper zones. Once back in the Atlantic, this chemically distinct Mediterranean Intermediate Water can persist for thousands of kilometers away from its source.[16] Sony VAIO VGN-SR165E/B Battery
2. Brine pools are large areas of brine on the seabed. These pools are bodies of water that have a salinity that is three to five times greater than that of the surrounding ocean. For deep sea brine pools the source of the salt is the dissolution of large salt deposits through salt tectonics. Sony VAIO VGN-SR165E/P Battery
The brine often contains high concentrations of methane, providing energy to chemosynthetic extremophiles that live in this specialized biome. Brine pools are also known to exist on the Antarctic continental shelf where the source of brine is salt excluded during formation of sea ice. Sony VAIO VGN-SR165E/S Battery
Deep sea and Antarctic brine pools can be toxic to marine animals. Brine pools are sometimes called seafloor lakes because the dense brine creates a halocline which does not easily mix with overlying seawater. The high salinity raises the density of the brine, which creates a distinct surface and shoreline for the pool. Sony VAIO VGN-SR190EBJ Battery
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline, at a depth of 1000 fathoms (1800 m) or more. The deepest part of the deep sea is Mariana Trench located in the western North Pacific. It is also the deepest point of the earth's crust. Sony VAIO VGN-SR190EBQ Battery
It has a maximum depth of about 10.9 km which is deeper than the height of Mount Everest.[18] In 1960, Don Walsh and Jacques Piccard reached the bottom of Mariana Trench in the Trieste bathyscaphe. The pressure is about 11,318 metric tons-force per square meter (110.99 MPa or 16100 psi). Sony VAIO VGN-SR190EEJ/C Battery
The upper photic zone of the ocean is filled with particle organic matter (POM) and is quite productive, especially in the coastal areas and the upwelling areas. However, most POM is small and light. It may take hundreds, or even thousands of years for these particles to settle through the water column into the deep ocean. Sony VAIO VGN-SR190NAB Battery
This time delay is long enough for the particles to be remineralized and taken up by organisms in the food webs.
Scientists at Woods Hole Oceanographic Institution conducted an experiment three decades ago in deep Sargasso Sea looking at the rate of sinking.[19] Sony VAIO VGN-SR190NBB Battery
They found what became known as marine snow in which the POM are repackaged into much larger particles which sink at much greater speed, 'falling like snow'.
Because of the sparsity of food, the organisms living on and in the bottom are generally opportunistic. Sony VAIO VGN-SR190NDB Battery
They have special adaptations for this extreme environment: rapid growth, effect larval dispersal mechanism and the ability to use a ‘transient’ food resource. One typical example is wood-boring bivalves, which bore into wood and other plant remains and are fed on the organic matter from the remains. Sony VAIO VGN-SR190NEB Battery
Hydrothermal vents, also called black smokers, were discovered in 1977 by scientists from Scripps Institution of Oceanography. So far, the discovered hydrothermal vents are all located at the boundaries of plates: East Pacific, California, Mid-Atlantic ridge, China and Japan. Sony VAIO VGN-SR190NGB Battery
New ocean basin material is being made in regions such as the Mid-Atlantic ridge as tectonic plates pull away from each other. The rate of spreading of plates is 1–5 cm/yr. Cold sea water circulates down through cracks between two plates and heats up as it passes through hot rock. Minerals and sulfides are dissolved into the water during the interaction with rock. Sony VAIO VGN-SR190PAB Battery
Eventually the hot solutions emanate from an active sub-seafloor rift creating a black smoker.
Chemosynthesis of bacteria provide the energy and organic matter for the whole food web in vent ecosystems. Giant tube wormscan grow to 2.4 m (7 ft 10 in)[22] tall because of the richness of nutrients. By now, over 300 new species have been discovered at hydrothermal vents.[23] Sony VAIO VGN-SR190PCB Battery
Hydrothermal vents are entire ecosystems totally separated from the world of light. And they may be the first evidence that the earth can support life without the sun.
Submarine landslides are marine landslides that transport sediment across the continental shelf and into the deep ocean.Sony VAIO VGN-SR190PFB Battery
A submarine landslide is initiated when the downwards driving stress (gravity and other factors) exceeds the resisting stressof the seafloor slope material causing movements along one or more concave to planar rupture surfaces. Submarine landslides take place in a variety of different settings including planes as low as 1° and can cause significant damage to both life and property.Sony VAIO VGN-SR19VN Battery
Recent advances have been made in understanding the nature and processes of submarine landslides through the use of sidescan sonar and other seafloor mapping technology.
Submarine Landslides have different causes which relate to both the geological attributes of the landslide material and transient environmental factors affecting the submarine environment. Sony VAIO VGN-SR19VN Battery
Common causes of landslides include: i) presence of weak geological layers, ii) overpressure due to rapid accumulation ofsedimentary deposits, iii) earthquakes, iv) storm wave loading and hurricanes, v) gas hydrate dissociation, vi) groundwater seepage and high pore water pressure, vii) glacial loading, viii) volcanic island growth, and ix) oversteepening.Sony VAIO VGN-SR19VRN Battery
The presence of weak geological layers is a factor which contributes to submarine landslides at all scales. This has been confirmed by seafloor imaging such asswath bathymetric mapping and 3D seismic reflection data. Despite their ubiquity, very little is known about the nature and characteristics of the weak geological layers, Sony VAIO VGN-SR19XN Battery
as they have rarely been sampled and very little geotechnical work has been conducted on them. An example of a slide which was caused by weak geological layers is the Storegga slide, near Norway which had a total volume of 3,300 km³.[3][4] Sony VAIO VGN-SR19XN Battery
Overpressure due to rapid deposition of sediment is closely related to weak geological layers. An example of landslides caused by overpressure due to rapid deposition occurred in 1969 on the Mississippi delta after Hurricane Camile struck the region.[2]
Earthquakes are a key factor which trigger most major submarine landslides. Sony VAIO VGN-SR210J/S Battery
Earthquakes provide significant environmental stresses and can promote elevated pore water pressure which leads to failure. Earthquakes triggered the Grand Banks landslide of 1929, where a 20 km3 submarine landslide was initiated after an earthquake.Sony VAIO VGN-SR21M/S Battery
Stormwave loading and hurricanes can lead to submarine landslides in shallow regions and were recognised as one of the factors which contributed to the slides which occurred on the Mississippi delta in 1969 following Hurricane Camille.[2]
A number of studies have indicated that gas hydrates lie beneath many submarine slopes and can contribute to the triggering of a landslide. Sony VAIO VGN-SR21RM/H Battery
Gas hydrates are ice like substances consisting of water and natural gas, which are stable at the temperature and pressure conditions normally found on the seabed. When the temperature rises or the pressure drops the gas hydrate becomes unstable allowing some of the hydrate to dissociate and discharge bubble phase natural gas. Sony VAIO VGN-SR21RM/S Battery
If pore water flow is impeded then this gas charging leads to excess pore water pressure and decreased slope stability. Gas hydrate dissociation is thought to have contributed to slides at water depths of 1000 to 1300 m off the east coast of the United States and the Storegga slide off the east coast of Norway. Sony VAIO VGN-SR220J/B Battery
Groundwater seepage and elevated pore water pressure can cause submarine landslides. Elevated pore water pressure causes reduced frictional resistance to sliding and can result from normal depositional processes, or can be coupled with other causes such as earthquakes, gas hydrate dissociation and glacial loading. Sony VAIO VGN-SR220J/H Battery
Sediment failure on glacial margins as a result of glacial loading is common and operates on a wide spectrum of dimensions, ranging from relatively small scale mass wasting processes in fjords to large scale slides covering several thousand square kilometres. Sony VAIO VGN-SR240J/B Battery
Factors which are significant in glacial loading induced landslides are the flexing of crust due to the loading and unloading of a fluctuating ice front, variation in drainage and groundwater seepage, quick deposition of low plasticity silts, rapid formation of moraines and till above hemipelagic interstaidal sediments. Sony VAIO VGN-SR240N/B Battery
An example where glacial loading lead to submarine landsliding is the Nyk slide of northern Norway.
Slope failures due to volcanic island growth are among the largest on earth, involving volumes of several cubic kilometres. Sony VGP-BPS13 Battery
The failure occurs as large bodies of lava form above weak marine sediments which are prone to failure. Failure is particularly common on edifices which are over 2500 m but rare on edifices which are less than 2500 m. Variation in the behaviour of the slides is significant, Sony VGP-BPS13/B Battery
with some slides barely keeping up with the growth on the upper part of the volcano while others may surge forward great distances, attaining landslide lengths greater than 200 km. Volcanic island submarine landslides occur in places such as the Hawaiian Islands[1][9][10] and the Cape Verde Islands.[11] Sony VGP-BPS13/S Battery
Oversteepening is caused by scouring due to oceanic currents and can result in the triggering of submarine landslides.[2]
In some cases the relationship between the cause and the resulting landslide can be quite clear (e.g. the failure of an oversteepened slope) while in other cases the relationships may not be so obvious. Sony VGP-BPS13/S Battery
In most cases more than one factor may contribute towards the initiation of a landslide event. This is clearly seen on the Norwegian continental slope where the location of landslides such as Storegga and Traenadjupet is related to weak geological layers. However the position of these weak layers is determined by regional variation in sedimentation style, Sony VGP-BPS13A/B Battery
which itself is controlled by large scale environmental factors such as climate change between glacial and interglacial conditions. Even when considering all the above listed factors, in the end it was calculated that the landslide needed an earthquake for it to ultimately be initiated.[1][3] Sony VGP-BPS13A/S Battery
The environments in which submarine landslides are commonly found in are fjords, active river deltas on the continental margin, submarine canyon fan systems, open continental slopes, and oceanic volcanic islands and ridges.[1]
There are a variety of different types of submarine mass movements. Sony VGP-BPS13AS Battery
All of the movements are mutually exclusive, for example a slide cannot be a fall. Some types of mass movements, such as slides, can be distinguished by the disrupted step like morphology which shows that there was only minor movement of the failed mass. The displaced material on a slide moves on a thin region of high strain. Sony VGP-BPS13B/B Battery
In flows the slide zone will be left bare and the displaced mass may be deposited hundreds of kilometres away from the origin of the slide. The displaced sediment of fall will predominantly travel through the water, falling, bouncing and rolling. Despite the variety of different landslides present in submarine environment, Sony VGP-BPS13B/S Battery
only slides, debris flow and turbidity currents provide a substantial contribution to gravity driven sediment transport.[2][3]
Recent advances in 3-D seismic mapping have revealed spectacular images of submarine landslides off Angola and Brunei, showing in detail the size of blocks transported and how they moved along the sea floor. Sony VGP-BPS13S Battery
It was initially thought that submarine landslides in cohesive sediments systematically and sequentially developed downslope from slide to debris flow to turbidity current through slowly increasing disintegration and entrainment of water. However it is now thought that this model is likely to be an oversimplification, Sony VGP-BPS14 Battery
as some landslides travel many hundreds of kilometres without any noticeable change into turbidity currents, as shown in figure 3 while others completely change into turbidity currents near to the source. This variation in the development of different submarine landslides is associated with the development of velocity vectors in the displaced mass. Sony VGP-BPS14/B Battery
The in-place stress, sediment properties (particularly density), and morphology of the failed mass will determine whether the slide stops a short distance along the rupture surface or will transform into a flow which travels great distances.
The initial density of the sediment plays a key role in the mobilization into flows and the distances that the slide will travel. Sony VGP-BPS14/S Battery
If the sediment is a soft, fluid material then the slide is likely to travel great distances and a flow is more likely to occur. However, if the sediment is stiffer then the slide will only travel a short distance and a flow is less likely to occur. Furthermore the ability to flow may also be dependent upon the amount of energy transferred to the falling sediment throughout the failure event. Sony VGP-BPS14B Battery
Often large landslides on the continental margin are complicated and components of slide, debris flow and turbidity current may all be apparent when examining the remains of a submarine landslide.
The primary hazards associated with submarine landslides are the direct destruction of infrastructure and tsunami. Sony VGP-BPS15 Battery
Landslides can have significant economic impacts on infrastructure such as the rupture of fibre optic submarine cables and pipelines and damage to offshoredrilling platforms and can continue onwards on slope angles as low as 1°. An example of submarine cable damage was discovered in the Grand Banks slide of Sony VGP-BPS15/S Battery
1929 where the landslide and resulting turbidity current broke a series of submarine cables up to nearly 600 km away from the beginning of the slide. Further destruction of infrastructure occurred when Hurricane Camille hit the Mississippi delta in 1969 causing a landslide which damaged several offshore drilling platforms. Sony VGP-BPS18 Battery
Submarine landslides can pose a significant hazard when they cause a tsunami. Although a variety of different types of landslides can cause tsunami, all the resulting tsunami have similar features such as large run-ups close to the tsunami, but quicker attenuation compared to tsunami caused by earthquakes. Sony Vaio VGN-FZ Battery
An example of this was the July 17, 1998, Papua New Guinean landslide tsunami where waves up to 15 m high impacted a 20 km section of the coast killing 2,200 people, yet at greater distances the tsunami was not a major hazard. This is due to the comparatively small source area of most landslide tsunami Sony Vaio VGN-FZ140E Battery
(relative to the area affected by large earthquakes) which causes the generation of shorter wavelength waves. These waves are greatly affected by coastal amplification (which amplifies the local effect) and radial damping (which reduces the distal effect).
Recent findings show that the nature of a tsunami is dependent upon volume, Sony Vaio VGN-FZ140E/B Battery
velocity, initial acceleration, length and thickness of the contributing landslide. Volume and initial acceleration are the key factors which determine whether a landslide will form a tsunami. A sudden deceleration of the landslide may also result in larger waves. The length of the slide influences both the wavelength and the maximum wave height. Sony Vaio VGN-FZ11S Battery
Travel time or run out distance of slide will also influence the resulting tsunami wavelength. In most cases the submarine landslides are noticeably subcritical, that is the Frounde number (the ratio of slide speed to wave propagation) is significantly less than one. Sony Vaio VGN-FZ11L Battery
This suggests that the tsunami will move away from the wave generating slide preventing the build up of the wave. Failures in shallow waters tend to produce larger tsunamis because the wave is more critical as the speed of propagation is less here. Furthermore shallower waters are generally closer to the coast meaning that there is less radial damping by the time the tsunami reaches the shore. Sony Vaio VGN-FZ11M Battery
Conversely tsunamis triggered by earthquakes are more critical when the seabed displacement occurs in the deep ocean as the first wave (which is less affected by depth) has a shorter wavelength and is enlarged when travelling from deeper to shallower waters. Sony Vaio VGN-FZ11E Battery
The effects of a submarine landslide on infrastructure can be costly and landslide generated tsunami can be both destructive and deadly. Sony Vaio VGN-FZ11Z BatterySony Vaio VGN-FZ38M Battery,Sony Vaio VGN-FZ31Z Battery

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