OCEAN RELIEF, TEMPERATURE AND SALINITY

INTRODUCTION

Initially Temperature on earth was extremely hot, Earth was in a molten state

  • Volcanic out gassing created the primordial atmosphere which consisted of various gases along with water vapour.
  • Over time, the Earth cooled, causing the formation of a solid crust.
  • The water vapour condensed to form rain and rainwater gradually filled the depressions on the newly solidified crust.
  • The water in the depressions merged to give rise to mighty oceans.
  • During the Hadean Eon, the atmospheric pressure was 27 times greater than it is today and hence even at a surface temperature of close to 200° C water remained liquid in the oceans.
  • Over time, both temperature and atmospheric pressure dropped, and water continues to stay as liquid in the oceans.

 

 

OCEAN RELIEF

  • Ocean relief is largely due to tectonic, volcanic, erosional and depositional processes and their interactions.
  • Ocean relief controls the motion of seawater.
  • The oceanic movement in the form of currents, in turn, causes many variations in both oceans and atmosphere.
  • The bottom relief of oceans also influences navigation and fishing.

Ocean relief features are divided into major and minor relief features:

 

 

A) Major Ocean Relief Features are

  1. The continental shelf,
  2. The continental slope,
  3. The continental rise,
  4. The Deep-Sea Plain or the abyssal plain.
  1. Continental Shelf
  • Continental Shelf is the gently sloping seaward extension of a continental plate.
  • Continental Shelves cover 7% of the total area of the oceans.
  • Shallow seas and gulfs are found along the continental shelves.
  • The shelf ends at a steep slope, called the shelf break.
  • Examples: Continental Shelf of South-East Asia (Sunda Plate), Grand Banks around Newfoundland, Submerged region between Australia and New Guinea, etc.
  • The shelf is formed due to submergence of a part of a continent ,relative rise in sea level or Sedimentary deposits brought down by rivers, glaciers
  • Various types of shelves based on different sediments of terrestrial origin—
    1. glaciated shelf (e.g. Shelf Surrounding Greenland),
    2. coral reef shelf (e.g. Queensland, Australia),
    3. shelf of a large river (e.g. Shelf around Nile Delta)

 

 

Width and depth of continental shelves

  • Continental shelves have an average width of 100-300kms
  • The shelves are almost absent or very narrow along a convergent boundary. E.g. coasts of Chile.
  • The width of continental shelf of eastern coast of USA varies between 100-300 km.
  • Siberian shelf in the Arctic Ocean is the largest in the world and stretches up to 1,500 km from the coast.
  • Continental shelves may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.

Importance of continental shelves

  • 20% of the world production of petroleum and gas comes from shelves.
  • Continental shelves form the richest fishing grounds. E.g. Grand Banks around Newfoundland.

 

 

  • Marine food comes almost entirely from continental shelves.
  • They are sites for placer deposits and phosphorites.
  1. Continental Slope
  • The continental slope connects the continental shelf and the ocean basins.
  • The depth of the slope region varies between 200 and 3,000 m.
  • The seaward edge of the continental slope loses gradient at this depth and gives rise to continental rise.
  • The continental slope boundary indicates the end of the continents.
  • Canyons and trenches are observed in this region
  1. Continental Rise
  • The continental slope gradually loses its steepness with depth.
  • With increasing depth, the rise becomes virtually flat and merges with the abyssal plain.

 

 

  1. Deep Sea Plain or Abyssal Plain
  • Deep sea planes are gently sloping areas of the ocean basins.
  • These are the flattest and smoothest regions of the world because of terrigenous (marine sediment eroded from the land) and shallow water sediments that buries the irregular topography.
  • They cover nearly 40% of the ocean floor.
  • The depths vary between 3-6kms.
  • These plains are covered with fine-grained sediments like clay and silt.

B) Minor Ocean Relief Features

  • Ridges (along a divergent boundary),
  • Abyssal Hills (submerged volcanic mountains): Seamounts and Guyots,
  • Trenches (along a convergent boundary),
  • Canyons (erosion landform),
  • Island arcs (formed due to volcanism along a convergent boundary or hotspot volcanism),
  • Atolls and Coral reefs.
  1. Oceanic Trenches
  • The trenches are relatively steep-sided, narrow basins (Depressions).
  • These areas are the deepest parts of the oceans.
  • They are of tectonic origin and are formed during ocean-ocean convergence and ocean-continent convergence.
  • They are some 3-5 km deeper than the surrounding ocean floor.
  • The Mariana Trench off the Guam Islands in the Pacific Ocean is the deepest trench with, a depth of more than 11 kilometres.
  • Trenches are associated with active volcanoes and strong earthquakes.
  1. Mid-Oceanic Ridges or Submarine Ridges
  • A mid-oceanic ridge is composed of two chains of mountains separated by a large depression (divergent boundary).
  • The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.
  • Running for a total length of 75,000 km, these ridges form the largest mountain systems on earth.

 

 

  • The ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains
  1. Abyssal Hills

 

 

  • Seamount: It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin.Example:- Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean.
  • Guyots: The flat-topped mountains (seamounts) are known as guyots.
  1. Submarine Canyons

Canyon: a deep gorge, especially one with a river flowing through it.

Gorge: a steep, narrow valley or ravine.

 

 

  • Submarine canyons are deep valleys often extending from the mouths of the rivers to the abyssal plains.
  • They are formed due to erosion by sediments brought down by rivers that cut across continental shelves, slopes and rises. The sediments are deposited on the abyssal plains.
  • Submarine canyons can be far higher in scale compared to those that occur on land.

Broadly, there are three types of submarine canyons:

  • Small gorges which begin at the edge of the continental shelf and extend down the slope to very great depths, e.g., Oceanographer Canyons near New England.
  • Those which begin at the mouth of a river and extend over the shelf, such as the Indus canyons.
  • Those which have a dendritic appearance and are deeply cut into the edge of the shelf and the slope, like the canyons off the coast of southern California.
  • The Hudson Canyon is the best-known canyon in the world.
  1. Atoll
  • These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.
  • It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.

 

 

  1. Bank, Shoal and Reef
  • These marine features are formed as a result of erosional, depositional and biological activity.
  • These are produced upon features of diastrophic (earth movements) origin. Therefore, they are located on upper parts of elevations.

Bank

  • These marine features are formed as a result of erosional and depositional activity.
  • A bank is a flat-topped elevation located in the continental margins.
  • These sites are some of the most productive fisheries of the world.

Shoal

  • A shoal is a detached elevation with shallow depths.
  • Since they project out of water with moderate heights, they are dangerous for navigation.

Reef

  • A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge.
  • Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots.
  • The largest reef in the world is found off the Queensland coast of Australia.
  • Since the reefs may extend above the surface, they are generally dangerous for navigation.

 

 

 

TEMPERATURE DISTRIBUTION OF OCEANS

  • The study of the temperature of the oceans is important for determining the
    1. movement of large volumes of water (vertical and horizontal ocean currents),
    2. type and distribution of marine organisms at various depths of oceans,
    3. Climate of coastal lands, etc.

A) Source of Heat in Oceans

  • The sun is the principal source of energy (Insolation).
  • The ocean is also heated by the inner heat of the ocean as the ocean crust is only about 5 to 30 km thick.

The ocean water is heated by three processes :

  • Absorption of sun’s radiation.
  • The conventional currents: Since the temperature of the earth increases with increasing depth, the ocean water at great depths is heated than the subsurface and intermediate water layers.
  • Also, the temperate are high along mid-ocean ridges because of volcanism.
  • So, convectional oceanic circulations develop causing circulation of heat in water.
  • Heat is produced due to friction caused by the surface wind and the tidal currents.

The ocean water is cooled by :

  1. Back radiation(heat budget) or long wave terrestrial radiation from the seawater.
  2. Exchange of heat between the sea and the atmosphere if there is temperature difference.
  3. Evaporation: Heat is lost in the form of latent heat of evaporation (atmosphere gains this heat in the form of latent heat of condensation).

B) The deep-water marine organisms survive in spite of absence of sunlight.

  • Photic zone (the zone that receives sunlight) is only about few hundred meters and that depends on factors like turbidity, presence of algae etc.
  • There are no enough primary producers below few hundred meters till the ocean bottom.
  • At the sea bottom, there are bacteria that make use of heat supplied by earth’s interior to prepare food. So, they are the primary producers at the depths.
  • Other organisms feed on these primary producers and subsequent secondary producers.
  • So, the heat from earth supports wide-ranging deep-water marine organisms.

But the productivity is too low compared to ocean surface.

  • The process of heating and cooling of the oceanic water is slower than land due to vertical and horizontal mixing and high specific heat of water that is why diurnal range of ocean temperatures is insignificant.

C) Factors Affecting Temperature Distribution of Oceans.

  • Insolation: duration of insolation and its intensity.
  • Heat loss: loss of energy by reflection, scattering, evaporation and radiation.
  • Albedo: albedo of the sea (depending on the angle of sun rays).
  • The physical characteristics of the sea surface: Boiling point of the sea water is increased in the case of higher salinity and vice versa.
  • The presence of submarine ridges and sills: Temperature is affected due to lesser mixing of waters on the opposite sides of the ridges or sills.
  • The shape of the ocean (enclosed seas): enclosed seas in the low latitudes record relatively higher temperature than the open seas (due to less mixing and higher overall insolation); whereas the enclosed seas in the high latitudes have lower temperature than the open seas. Mediterranean Sea records higher temperature than the longitudinally extensive Gulf of California.
  • Local weather conditions such as cyclones.
  • Unequal distribution of land and water: The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
  • Prevalent winds generate horizontal and sometimes vertical ocean currents: The winds blowing from the land towards the oceans (off-shore winds: moving away from the shore) drive warm surface water away from the coast resulting in the upwelling of cold water from below.
  • Contrary to this, the onshore winds (winds flowing from oceans into continents) pile up warm water near the coast, and this raises the temperature.
  • Ocean currents: Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas.
  • Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America (Near Newfoundland).

D) Vertical Temperature Distribution of Oceans

  • Photic or euphotic zone extends from the upper surface to ~200 m. The photic zone receives adequate solar insolation.
  • Aphotic zone extends from 200 m to the ocean bottom; this zone does not receive adequate sunrays.

 

 

  1. Thermocline
  • The profile shows a boundary region between the surface waters of the ocean and the deeper layersWhich begins around 500 m below the sea surface and extends several hundred of meters downward.
  • This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.

About 90 per cent of the total volume of water is found below the thermocline in the deep ocean.

 

 

  1. Three-Layer System
  • The first layer represents the top layer of warm oceanic water, and it is about 500m thick with temperatures ranging between 20° and 25° C.
  • This layer, within the tropical region, is present throughout the year but in mid-latitudes, it develops only during summer.
  • The second layer called the thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth. The thermocline is 500-1,000 m thick.
  • The third layer is very cold and extends up to the deep ocean floor with temperature becoming almost stagnant.
  1. Thermohaline Circulation
  • The deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline).
  • This process is known as thermohaline circulation.
  • The thermohaline circulation is sometimes called the ocean conveyor belt or global conveyor belt.

 

 

E) Horizontal Temperature Distribution

  • The average temperature of surface water of the oceans is about 27°C, and it gradually decreases from the equator towards the poles.
  • The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.

 

 

 

OCEAN SALINITY

  • Salinity is the term used to define the total content of dissolved salts in seawater.
  • It is usually expressed as parts per thousand or ppt.
  • Salinity of 24.7 ppt has been considered as the upper limit to demarcate ‘brackish water’.
  • Salinity determines compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation and humidity.
  • It also influences the composition and movement of the sea: water and the distribution of fish and other marine resources.

A) Share of different salts is as shown below

  • sodium chloride — 77.7%
  • magnesium chloride—10.9%
  • magnesium sulphate — 4.7%
  • calcium sulphate — 3.6%
  • potassium sulphate — 2.5%

B) Dissolved Salts in Sea Water (gm of Salt per kg of Water)

 

 

C) Factors Affecting Ocean Salinity

  • The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.
  • Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in polar regions by the processes of freezing and thawing of ice.
  • Wind also influences salinity of an area by transferring water to other areas.
  • The ocean currents contribute to the salinity variations.
  • Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of an area.

D) Horizontal distribution of salinity

  • The salinity for normal open ocean ranges between 33 and 37.
  • The regions of high salinity in vast oceans coincide with high-pressure
  • Here, there is hardly any rain and subsiding dry winds cause lots of evaporation.

High salinity regions

  • In the landlocked Red Sea, it is as high as 41.
  • In the Mediterranean Sea in Europe the salinity is very high – 38 or more.
  • In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70.

Low salinity regions

  • In the estuaries (enclosed mouth of a river where fresh and saline water get mixed) and the Arctic and Antarctic, the salinity fluctuates from 0 to 35, seasonally (fresh water coming from ice caps).

Highest salinity in water bodies

  • Lake Van in Turkey (330 ppt)
  • Dead Sea (238 ppt)
  • Great Salt Lake, Utah (220 ppt)

Cold and warm water mixing zones

  • Salinity decreases from 35 to 31 on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region.

E) Vertical Distribution of Salinity

  • With depth, the salinity also varies, but this variation again is subject to latitudinal difference.
  • The decrease is also influenced by cold and warm currents.
  • In high latitudes, salinity increases with depth. In the middle latitudes, it increases up to 35 metres and then it decreases. At the equator, sub-surface salinity is lower.
  • Salinity, generally, increases with depth and there is a distinct zone called the halocline (compare this with thermocline), where salinity increases sharply. High salinity seawater general, sinks below the lower salinity water. This leads to stratification by salinity.

 

 

 

 

 

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