Test: Oceanography - 1 - UPSC MCQ

• Winds blowing across the ocean surface push water away. Water then rises from beneath the surface to replace the water that was pushed away. This process is known as ‘upwelling’. Upwelling occurs in the open ocean and along coastlines.

• Water that rises to the surface as a result of upwelling is typically colder and is rich in nutrients. These nutrients ‘fertilise’ surface waters, meaning that these surface waters often have high biological productivity. Therefore, good fishing grounds typically are found where upwelling is common.

• Global sea level trends and relative sea-level trends are different measurements.

• This is due to many local factors: subsidence, ocean currents, variations in land height, and whether the land is still rebounding from the compressive weight of Ice Age glaciers.

• For example, land subsidence involves gradual settling or sudden sinking of the Earth’s surface. This can cause short-term changes in local sea levels.

As the wind blows across the smooth water surface, the friction or drag between the air and the water tends to stretch the surface. As waves form, the surface becomes rougher and it is easier for the wind to grip the water surface and intensify the waves.

• The uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.

• Seamounts are found close to these ridges. Seamounts can be found in every ocean basin in the world, distributed extremely widely both in space and in age.

• A seamount is technically defined as an isolated rise in elevation of 1,000 m or more from the surrounding seafloor, and with a limited summit area, of conical form.

Tonga Trench: The Tonga Trench is an oceanic trench located in the south-west Pacific Ocean. It is the deepest trench of the Southern Hemisphere and the second deepest on Earth.

Mindanao Deep: The Philippine Trench (also Philippine Deep, Mindanao Trench, and Mindanao Deep) is a submarine trench to the east of the Philippines. The trench is located in the Philippine Sea of the western North Pacific Ocean. The trench reaches one of the greatest depths in the ocean, third only to the Mariana Trench and the Tonga trench.

Java Trench: Trench located south of Indonesia, between the Australian-Indian and the Eurasian Plates; it is the deepest point in the Indian Ocean

The South Sandwich Trench: Just southeast of the southern tip of South America, the British territories of South Georgia and the South Sandwich Islands provide a home for penguins and a few British administrative personnel. Just to the east, the ocean floor dips into the South Sandwich Trench, the second-deepest trench in the Atlantic Ocean after Puerto Rico Trench.

• The width of the continental shelf varies considerably - it is not uncommon for an area to have virtually no shelf at all, particularly where the forward edge of an advancing oceanic plate dives beneath continental crust in an offshore subduction zone such as off the coast of Chile or the west coast of Sumatra where they are mountainous.

• The largest shelf - the Siberian Shelf in the Arctic Ocean - stretches to 1,500 km in width. Continental shelves are the richest fishing grounds.

• The continental shelves are shielded with sediments of variable thicknesses brought down from the land by rivers, glaciers, wind and distributed by waves and currents.

• Massive sedimentary deposits that continental shelves have received over a long period of time serve as the source of fossil fuels. The deepest parts of the oceans are Oceanic Deeps or Trenches.

• It is a flat-topped seamount. They show evidence of gradual subsidence through stages to become flat-topped submerged mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

• Carbon dioxide readily dissolves in water and the oceans provide a huge reservoir of carbon.

• Across the world’s oceans, there is a continual cycle of equilibration of dissolved carbon dioxide in water with carbon dioxide in the atmosphere.

• The difference in the partial pressure of the CO2, between seawater and air, facilitate gaseous exchange. This allows atmospheric CO2 to dissolve in seawater.

• The carbon dioxide which dissolves in our oceans occurs in three main forms. Aside from the normal carbon dioxide form, it is also found as bicarbonate and carbonate ions.

Hydrothermal vents are globally distributed, but their location is determined by tectonic conditions where seawater meets magma.

• Where plates form, such as mid-oceanic ridges and in areas where there is volcanic activity, the Earth’s magma is close to the seafloor and can heat water which has seeped down.

• The hydrothermal vents are therefore associated with regions of high tectonic activity and intersections of continental plates.

• It is a seafloor fissure from which a spring of geothermally heated mineral and gas-rich seawater emerges. Hydrothermal vents are found on some oceanic ridges in zones of active seafloor spreading.

• Fauna, which lives around these vents, are based on chemosynthetic food chains where the species at the lower end of the food chain, typically bacteria, synthesise energy from the chemicals in the water.

• Oceans absorb more than 80% of the solar radiation and water which has the highest specific heat is the remarkable capacity of storing the heat.

• The uppermost 10% of the oceans have more heat than the entire atmosphere of Earth.

• Concerning temperature, there are three layers in the oceans from surface to the bottom in the tropics as follows: The first layer represents the top-layer of warm, oceanic water and temperature ranging between 20°C and 25°C. This layer is present within the tropics throughout the year but it develops in mid-latitudes only during summer.

Apart from their economic potential, seafloor hydrothermal vents are characterised by dense biologic communities. Over 700 species have been reported, making the vents among the most productive ecosystems on Earth.

• Phytoplankton float on the top surface of water bodies and require sunlight to thrive. Ice cover will reduce their breeding grounds and population.

• The cover blocks exchange of nutrients and oxygen from the atmosphere, however, the same may continue within the water stream. But, it is worse than before.

• Snow cover of ice on the water body can effectively cut off the light, plunging the waters into darkness.

Option (a): A recently discovered to play a significant role in oceanic primary production is the micronutrient iron.

• This is used as a cofactor in enzymes involved in processes such as nitrate reduction and nitrogen fixation.

• A major source of iron to the oceans is dust from the Earth’s deserts, picked up and delivered by the wind as aeolian dust.

• In regions of the ocean that are distant from deserts or that are not reached by dust-carrying winds (for example, the Southern and North Pacific oceans), the lack of iron can severely limit the amount of primary production that can occur.

1. In oceans, sodium chloride alone constitutes 95% of ocean salts.

This statement is correct. Sodium chloride, commonly known as table salt, makes up about 95% of the dissolved salts in seawater. The remaining 5% consists of other dissolved minerals such as magnesium, calcium, and potassium.

2. The proportion of ocean salts vary in different parts of the ocean.

This statement is correct. The salinity of ocean water varies depending on the location. Factors that affect salinity include evaporation, precipitation, freezing and melting of ice, and the influx of freshwater from rivers. For example, in areas where evaporation is high and precipitation is low, the salinity will be higher. Conversely, in areas with high precipitation and low evaporation, the salinity will be lower.

3. The salinity is constant throughout the oceans.

This statement is incorrect. As mentioned above, salinity varies depending on factors such as evaporation, precipitation, ice formation and melting, and river input. Additionally, ocean currents and mixing can cause variations in salinity both horizontally and vertically within the ocean. Salinity is generally higher near the surface where evaporation occurs, and lower at greater depths.

(i) The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation.

(ii) 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.

(iii) Wind also influences the salinity of an area by transferring water to other areas.

(iv) 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.