swuklink.com: Oceans

The salty waters of the Earth's oceans covers seventy-one per cent (almost three quarters) of the the planet's surface and are interconnected, being brocken only by the continental masses and archipelagos of islands.

The Earth's oceans and seas are estimated to contain 1,320,000,000 km� (326,000,000 mile�) of water representing 97.2% of all the water on the planet.

Five oceans are recognised - the Arctic Ocean, the Atlantic Ocean, the Indian Ocean, the Pacific Ocean and the Southern Ocean � but some consider the last is not an ocean in its own right but divide it among the Atlantic, Indian and Pacific oceans. Despite this, its special characteristics deserve its consideration as an entity in its own right.

Smaller regions of the oceans are known as seas, gulfs, straits, etc.

The boundaries between the oceans are set by the International Hydrographic Organization. For example the Southern Ocean is deemed to extend from the coast of Antarctica to 60 degrees south.

Although some volcanic islands, or those of volcanic origin, rise from the floor of the oceans, the continents are surrounded by a comparatively shallow and gently sloping "continental shelf" which extends for varying distances from the coasts. The continental shelf eventually plunges to the depths of the ocean floor. The seas which surround the British Isles such as the North Sea and the Irish Sea lie in the continental shelf. At around 3,000 metres, the steep continental slope levels off to join the comparatively level abyssal plain which, submerged, covers approximately half of the land surface of the planet.

Large areas of the ocean floors have been found to be comparatively flat but vast submerged chains of vulcanic mountains such as the "mid-Atlantic Ridge" have been found in the middle of the oceans where rising magma forms new ocean floor, steadily extending the oceans. This upwelling of material from beneath the Earth's crust and expansion of the ocean floors causes a series of ridges extending great distances from the faults.

The deepest part of the Earth's oceans is the Mariana Trench which is found in the Pacific Ocean near the Northern Mariana Islands. The maximum depth of the ocean there is 10,924 metres (35,838 feet or over six and three-quarter miles).

The Mariana Trench was extensively surveyed in 1951 by the British navy vessel, "Challenger II" which gave its name to the deepest part of the trench, the "Challenger Deep".

Life is believed to have evolved in the Earth's oceans and it provides a great variety of habitats. The oceans not only cover nearly three quarters of the Earth's surface but their great depth (averaging 3,795 metres or 12,450 feet) means that they form by far the greatest volume of the biosphere. For most of human history only those areas of the world's oceans and seas which are most readily accessible have been studied, down to only a few tens of metres below the surface unless specimens were retrieved from greater depths in nets or otherwise. The invention of Scuba diving gear by JY Cousteau during World War II allowed man much greater freedom to eplore the oceans but this again was limited by the great pressures exerted at depth to a few hundred meters. The advent of DSVs, deep subersible vehicles, in the 1970s allowed man to explore the ocean depths down to the abyssal plain which covers about half the floor of the oceans and even into the ocean trenches, the deepest of which is the Mariana Trench that plummets to a depth of some seven miles or 35,810 feet.

The most dramatic result of this new exploration of the deep was the discovery in 1979 of whole ecosystems which derived their energy not from sunlight but from the sulfides spumed from volcanic vents at the bottom of the ocean.

The influence of the Earth's oceans and seas extends directly onto surrounding landmasses, not only modifying climate but the salt spray presents particular difficulties for plants living near the water. The constant and often violent motion of the waves on the shore presents a very challenging physical environment to any plant or animal which lives there. The level of the water on the shore can change by up to several tens of metres under the influence of the tides regularly inundating the organisms which live there but also leaving them exposed to dessication as the tide recedes and where tidal pools are left, their salinity increases markedly as the water is evaporated by the sun. Salt water can travel far inland along the tidal reaches of estuaries and rivers leaving them to be flushed with fresh water as the tide recedes.

Rivers also exert their influence on the saline waters of the oceans. Usually this influence is restricted to the margins as the fresh water dilutes the seawater. On meeting the seawater, rivers also drop most of their load of silt which has been scoured from the surface of the land. The influence of the world's great rivers on the marine environment is more far-reaching, the mighty flow of the Amazon forces a plume of fresh water many hundreds of kilometres into the Atlantic Ocean.

Sufficient light to enable photosynthesis is only available in the topmost 100 metres of the oceans and at a depth of onlyu 150 metres, photosynthesis is impossible. At a depth of about 300 metres is the start of what is sometimes called the "twilight zone" where the thin blue light which difuses from the surface is insufficient for human vision. Red light is absorbed most by the water so that many of the organisms in the twilight zone and below are coloured red - in the absence of red light they appear inky black. By 1,000 metres, the darkness is complete.

Because light penetrates only the shallow surface layers of the oceans, it is this thin layer to which living plants are restricted and the vast majority of the ocean is populated only by animals.

Because light penetrates only the shallow surface layers of the oceans, it is this thin layer to which living plants are restricted and the vast majority of the ocean is populated only by animals. These are either predators or scavangers which survive on the "ocean snow" of detritus which floats down gradually but steadily from the surface layers. There are also whole communities in the deeps which are independant of sunlight, deriving there energy from the sulfides produced by volcanic vents or other chemicals such as methane.

The lack of food and oxygen in the deep causes animals to be very widespread but, because of the vast volume of the oceans, they exist in large numbers. Because of this lack of food in the deep, it is estimated that about a thousand million tons of animals throughout the world's oceans make a nightly migration to the food rich surface layer every night and return to the deep as the sun rises.

Plankton

"Plankton" is the collective name given to the great variety of free-floating plants and free-swimming animals in the waters of oceans and lakes. Zooplankton is the name given to the animals while the plant species are collectively called phytoplankton. These latter are crucial to the exchange of oxygen and carbon dioxide by the oceans and have been called "the lungs of the Earth. Plankton forms the base of the food chain in the oceans with fish and marine mammals at its apex.

Photosynthesis

Photosynthesis, the process by which plants use the energy of sunlight to produce complex molecules, can only take place in the upper 100 metres of the oceans, at a depth of 150 metres, photosynthesis becomes completely impossible. With 60% of the planet's surface covered by sea water a mile or more deep, the phytoplankton which produce food and oxygen are restricted to the first hundred metres or so.

Because of the great pressures exerted by the water in the depths of the ocean, there are only five manned craft in the world which are able to descend to depths such as that of the abyssal plain. The number of people who have descended to these ocean depths is comparable to the number who have travelled into space. Because of its inacessability, this vast volume of the planet, and the life it contains in particular, has been studied very little.

One such submersible is the "Alvin" of the US Navy's Submarine Development Squadron 5 (formerly Submarine Development Group 1) which can descend to depths of 4,500 metres (three miles). Its two-metre titanium sphere with tiny portholes can just hold the pilot and two observers.

see also: The Abyssal Plain

BIOMASS (1977-1991)

BIOMASS (Biological Investigations of Marine Antarctic Systems and Stocks) was a unique, large-scale, long-term, international research programme established in 1977 to investigate the ecology of the Southern Ocean. Its main aim was to gain a greater understanding of the biological systems and stocks in the marine Antarctic environment, thereby providing a sound basis for the future management of the living resources within this immense ocean. The programme was drawn to a close in 1991 and its completion marked by a colloquium which brought together key researchers to summarise and review the results obtained. click here for more

The huge volume of the world's oceans means that the seawater holds a great deal of carbon dioxide, either dissolved directly in the water as as other minerals such as calcium carbonate. Many marine organisms use the dissolved calcium carbonate to construct their shells which fall to the ocean floor to become part of the pelagic ooze. In time this material becomes the mineral calcite which forms the bulk of the limestones which compose about ten per cent of sedimentary rocks.

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ABYSSAL PLAIN
WATER
CYCLE
SUPPLY
UK
CLIMATE
FISH
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Oceans