GS Notes

1.5 - Geography Notes 31

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Elaborate Notes

Factors Influencing Salinity

Salinity is the measure of the total amount of dissolved salts in seawater, typically expressed in parts per thousand (ppt or ‰). The average salinity of the world’s oceans is approximately 35 ppt. Several factors influence its spatial and temporal variation.

  • Temperature: There is a direct relationship between temperature and salinity. Higher temperatures lead to increased rates of evaporation. Evaporation removes freshwater from the ocean surface, leaving behind dissolved salts, thereby concentrating them and increasing salinity. For example, tropical and subtropical regions with high solar insolation experience higher evaporation and consequently, higher surface salinity compared to polar regions.
  • Wind Speed: Wind plays a crucial role in the process of evaporation. Higher wind speeds enhance the rate of evaporation by removing the layer of humid air directly above the water surface, allowing more water molecules to escape. This process, similar to the effect of temperature, increases the concentration of salts and thus, salinity. Strong, dry trade winds in subtropical regions are a significant factor contributing to high salinity.
  • Ocean Currents: Ocean currents act as distributors of heat and salt across the globe. They mix waters of different salinities, preventing extreme concentrations in any single area. For instance, the Gulf Stream, a warm current, transports high-salinity water from the tropics northwards into the North Atlantic, influencing the salinity of that region. Conversely, cold currents like the Labrador Current bring low-salinity water from polar regions towards the equator.

Regions of Varying Salinity

  • Regions of Higher Salinity:
    • Sub-Tropical Regions: These zones, located around 20°-30° N and S latitude, are characterized by high-pressure belts with descending dry air, leading to minimal precipitation and maximum evaporation. This results in the highest surface salinities in the open ocean. Examples include the subtropical parts of the Atlantic and Pacific oceans.
    • Enclosed Seas in Arid Regions: Enclosed or semi-enclosed seas in hot, dry climates exhibit exceptionally high salinity due to high evaporation and limited freshwater influx.
      • Mediterranean Sea: Average salinity is around 38-39 ppt due to high evaporation and limited river input.
      • Red Sea: Salinity can exceed 40 ppt, making it one of the saltiest large bodies of water, owing to its location in a desert region with no significant river inflow.
      • Persian Gulf: Similar to the Red Sea, it experiences high evaporation and has salinity levels often above 40 ppt.
  • Regions with Below-Average Salinity: These regions are characterized by high freshwater input from precipitation, river discharge, or melting ice.
    • North Sea and Baltic Sea: Receive significant freshwater runoff from the rivers of Europe. The Baltic Sea, being almost enclosed, has extremely low salinity, dropping to less than 10 ppt in some areas.
    • Bering Sea, Arctic Ocean, Southern Ocean: These polar and sub-polar regions experience low evaporation rates and significant freshwater influx from melting glaciers and ice sheets, leading to lower salinity.
  • Regions with Exceptionally High Salinity: These are typically inland lakes with no outlet, where evaporation is the only means of water loss.
    • Lake Van (Turkey): Salinity is around 330 ppt. It is a saline soda lake.
    • Dead Sea (Israel/Jordan): Salinity is approximately 342 ppt, about 9.6 times saltier than the ocean, making it one of the world’s saltiest bodies of water.
    • Great Salt Lake (USA): A terminal lake whose salinity varies depending on water levels, but is significantly higher than the ocean’s, typically ranging from 150 to 280 ppt.

Oceanic Deposits

Ocean floors are covered with unconsolidated sediments known as oceanic deposits, classified based on their origin.

  • Terrigenous Deposits: Also known as Lithogeneous deposits, these are derived from the erosion of landmasses. They are transported to the oceans by rivers, wind, glaciers, and volcanic eruptions. They are most abundant on continental shelves and slopes.
    • Examples: Gravel, sand, silt, and mud (composed of clay and silt). Red clay is a fine-grained terrigenous deposit found in the deepest parts of the ocean basins, far from land.
  • Biogeneous Deposits: These are organic deposits derived from the shells and skeletal remains of marine organisms. When these organisms die, their hard parts sink to the ocean floor.
    • Ooze: Fine-grained biogenic sediments composed of at least 30% skeletal remains of microscopic planktonic organisms.
      • Calcareous Ooze: Composed of calcium carbonate (CaCO₃) shells, primarily from foraminifera, pteropods, and coccolithophores. It is not found below the Carbonate Compensation Depth (CCD), typically around 4,500 meters, where the water is acidic enough to dissolve calcium carbonate.
      • Siliceous Ooze: Composed of silica (SiO₂) shells from diatoms (planktonic algae) and radiolarians (planktonic protozoa). It is found in colder, nutrient-rich waters like the Southern Ocean and areas of upwelling.
    • Other Examples: Shells of larger organisms like mollusks and coral fragments (coralline deposits).
  • Hydrogenous Deposits: These are formed by the precipitation of dissolved minerals directly from seawater. This process is often very slow.
    • Examples:
      • Salt: Evaporites like halite and gypsum form in arid, enclosed basins.
      • Polymetallic Nodules (or Manganese Nodules): Concretions of manganese and iron oxides, along with valuable metals like nickel, cobalt, and copper, that form slowly on the deep-sea floor. They represent a significant potential source of minerals.
  • Cosmogenous Deposits: These are extraterrestrial in origin, derived from meteoritic dust and debris that fall into the ocean. They are found in very small quantities and are often mixed with other sediments.

Coral Reefs

  • Coral reefs are complex marine ecosystems built by colonies of tiny animals called coral polyps. These polyps secrete calcium carbonate (limestone) to form a hard, protective skeleton. Over thousands of years, these skeletons accumulate, forming the massive structures known as coral reefs.
  • They are developed through a vital symbiotic relationship between the coral polyps and microscopic algae called zooxanthellae that live within the polyps’ tissues. The zooxanthellae photosynthesize, providing the coral with up to 90% of its energy needs, while the coral provides the algae with a protected environment and compounds needed for photosynthesis.
  • Due to their incredible structural complexity, coral reefs provide habitats for a vast array of marine life, supporting nearly one-quarter of all ocean species, including 31 of the 32 animal phyla. This immense biodiversity has earned them the name “rainforests of the oceans.”

Conditions for Coral Reef Growth

  • Temperature: Corals thrive in warm, tropical waters with an ideal temperature range of 20°C to 28°C. They generally do not grow in waters cooler than 18°C. They are typically found between 30°N and 30°S latitudes.
  • Depth: Coral reefs require clear, sunlit water. This is because their symbiotic zooxanthellae need sunlight for photosynthesis. Therefore, they are usually found in shallow waters, at a maximum depth of about 200-250 feet (60-75 meters), where sunlight penetration is sufficient.
  • Sediment-free Water: Corals need clear water free from sediments. High sedimentation can smother the coral polyps, blocking their feeding mechanisms, and reduce light penetration, hindering photosynthesis. This is why coral reefs are generally absent near the mouths of large rivers, which discharge significant amounts of sediment.
  • Salinity: Corals require stable, oceanic salinity. The ideal range is between 32 to 40 ppt. The average of 35 ppt is optimal. They cannot survive in freshwater or highly saline conditions.
  • Platform: Corals need a hard, stable submarine platform, such as a continental shelf or a submerged volcano, on which the polyps can attach and begin building the reef structure.
  • Ocean Currents: Corals are not found on the western margins of continents primarily due to the presence of cold, upwelling ocean currents which bring water temperatures below the tolerance level for coral growth.

Distribution

  • Atlantic Ocean: Predominantly found in the Caribbean Sea, around the coasts of Florida, Mexico, the Bahamas, and the Antilles.
  • Indian Ocean: Widespread in the Red Sea, Persian Gulf, along the coasts of East Africa, the Maldives, Lakshadweep Islands, Andaman & Nicobar Islands, and in specific patches in the Gulf of Mannar, Gulf of Kutchh, and off the coasts of Maharashtra, Goa, and Karnataka.
  • Southeast Asia and Pacific: This region has the highest diversity of corals. It includes the Coral Triangle, an area encompassing Indonesia, Malaysia, the Philippines, Papua New Guinea, Timor Leste, and the Solomon Islands. The most famous example is the Great Barrier Reef off the coast of Australia. Numerous islands across the Pacific Ocean are fringed by or are themselves atolls.

Types of Coral Reefs

The classification of coral reefs was famously explained by Charles Darwin in his Subsidence Theory (1842), which proposed an evolutionary sequence from fringing to barrier to atoll reefs as a volcanic island slowly subsides.

  • Fringing Reefs: These reefs grow directly from the shore, forming a narrow and often discontinuous border along the coastline. There is either a very shallow channel or no lagoon separating them from the land. The image provided illustrates this type, showing a reef platform attached to the coast. Example: Reefs in the Caribbean Sea.
  • Barrier Reefs: These are extensive, linear reefs located parallel to the coastline but separated from it by a wide, deep lagoon. They are much larger in scale than fringing reefs. The image depicts this, showing a reef separated from the mainland by a significant body of water (lagoon). Example: The Great Barrier Reef, Australia.
  • Atoll Reefs: An atoll is a circular or horse-shoe-shaped coral reef that encloses a central lagoon. According to Darwin’s theory, atolls form when a volcanic island, initially surrounded by a fringing reef, completely subsides beneath the sea level, leaving only the ring of coral growing upwards. The image shows a classic ring-shaped reef enclosing a lagoon. Examples: Lakshadweep Islands, Maldives, and numerous atolls in the Pacific Ocean.

Coral Bleaching

  • The vibrant colours of corals are not from the polyps themselves but from the symbiotic zooxanthellae living within them.
  • Coral Bleaching is the process where corals, under environmental stress, expel these symbiotic algae. This loss of zooxanthellae results in the coral losing its colour and turning white, as the transparent polyp tissue reveals the underlying white calcium carbonate skeleton. While a bleached coral is not dead, it is under severe stress and is more susceptible to disease and mortality if the stress conditions persist.
  • Reasons for Coral Bleaching:
    • Changes in Water Temperature: The most common cause is anomalously high sea surface temperatures, even an increase of just 1-2°C above the summer maximum for a few weeks can trigger mass bleaching.
    • Changes in Salinity: Extreme drops in salinity due to heavy rainfall or freshwater runoff can also cause stress.
    • Increased Sedimentation and Pollution: Runoff containing sediments, nutrients (eutrophication), and chemical pollutants can stress corals.
    • Ocean Acidification: Increased atmospheric CO₂ is absorbed by the ocean, lowering its pH. This makes it more difficult for corals to build their calcium carbonate skeletons.
    • Physical Disturbances: Increased incidence and intensity of cyclones and storms can cause physical damage and stress.
    • Climatic Events: El Niño events are strongly linked to mass bleaching episodes due to the widespread warming of ocean waters.
  • Examples of Coral Bleaching:
    • The global bleaching event of 2014-2017, intensified by a strong El Niño, was the most widespread and damaging on record. The Great Barrier Reef was severely affected, with some estimates suggesting nearly 50% of its corals were bleached or died.
    • During the 1997-98 El Niño event, extensive bleaching occurred globally, with the Northern Indian Ocean, including the Maldives and Lakshadweep, losing a majority of its coral reefs.

Oceanic Circulation

Surface/Ocean Current

  • An ocean current is a continuous, directed movement of seawater generated by various forces. They represent the general movement of the ocean’s surface water in a definite path over vast distances, acting like rivers within the ocean.
  • Types of Ocean Currents:
    • Warm Ocean Current: These currents originate near the equator and flow towards the poles, transporting warm water to higher latitudes. Example: Gulf Stream.
    • Cold Ocean Current: These currents originate in polar or high-latitude regions and flow towards the equator, bringing cold water to lower latitudes. Example: Labrador Current.

Factors Affecting Ocean Currents

  • Winds: The primary driving force for surface currents is the friction from prevailing planetary winds (Trade Winds, Westerlies). These steady winds drag the surface water, setting it in motion in the direction of the wind.
  • The Shape of the Coastline and Topography: Landmasses act as barriers, obstructing the flow of currents and deflecting them. The shape of ocean basins and submarine features like mid-oceanic ridges also guide and modify the direction and speed of currents.
  • Differences in Temperature, Salinity, and Density: These factors drive deep-water circulation (thermohaline circulation). Colder, saltier water is denser and sinks, while warmer, less saline water is less dense and rises. This movement creates vertical currents and influences the overall circulation pattern.
  • Coriolis Force: Due to the Earth’s rotation, moving objects (including water) are deflected from their straight path. This force, first described by Gaspard-Gustave de Coriolis (1835), causes currents to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, contributing to the formation of large circular patterns called gyres.

Ocean Currents of the World

Currents of the Atlantic Ocean

The circulation in the North Atlantic is dominated by a large clockwise-moving gyre.

  • Currents under Trade Wind Influence:
    • North Equatorial Current: A warm current flowing from east to west, driven by the Northeast Trade Winds.
    • South Equatorial Current: A warm current flowing from east to west, driven by the Southeast Trade Winds.
    • Counter Equatorial Current: An east-flowing current located between the North and South Equatorial currents, returning water that has piled up on the western side of the basin.
  • Florida Current & Gulf Stream: The North Equatorial Current flows into the Caribbean Sea and Gulf of Mexico. It emerges through the Straits of Florida as the powerful, warm Florida Current, which then merges with water from the Antilles Current to form the Gulf Stream. The Gulf Stream is a strong, warm, and swift current that flows northeast along the eastern coast of the USA.
  • North Atlantic Drift: After passing Newfoundland, the Gulf Stream is influenced by the Westerlies and the Coriolis force, causing it to turn right (eastward) and broaden into the slower, wider North Atlantic Drift. This warm current flows across the Atlantic towards Europe, having a significant warming effect on its climate.
  • Norwegian Current: A branch of the North Atlantic Drift flows north along the coast of Norway, keeping its ports ice-free in winter.
  • Canary Current: To complete the gyre, a cold current known as the Canary Current flows southwards along the coast of Northwest Africa and the Canary Islands, eventually rejoining the North Equatorial Current.
  • Cold Currents:
    • Labrador Current: A cold current flowing south from the Arctic Ocean along the coast of Labrador, meeting the warm Gulf Stream off Newfoundland. This meeting point is famous for dense fog and rich fishing grounds.
    • East Greenland Current: A cold current that flows south along the east coast of Greenland, joining the North Atlantic Drift system.
  • Sargasso Sea: In the center of the North Atlantic Gyre, there is a region of calm, warm water with very weak currents. It is characterized by large mats of free-floating seaweed called Sargassum, giving the region its name, the Sargasso Sea. It is unique as it is a sea defined not by land boundaries, but by ocean currents.

Prelims Pointers

  • Average ocean salinity is approximately 35 parts per thousand (ppt or ‰).
  • Highest salinity in open oceans is found in sub-tropical regions (20°-30° N/S).
  • The Red Sea and Persian Gulf have salinities exceeding 40 ppt.
  • The Baltic Sea has very low salinity (less than 10 ppt) due to high riverine influx.
  • Exceptionally high salinity lakes: Lake Van (~330 ppt), Dead Sea (~342 ppt), Great Salt Lake (variable, up to 280 ppt).
  • Terrigenous deposits are derived from land erosion.
  • Biogeneous deposits are from marine organisms’ skeletal remains.
  • Ooze: Biogenic sediment with at least 30% skeletal parts.
  • Calcareous Ooze: Made of CaCO₃ (foraminifera, pteropods). Dissolves below Carbonate Compensation Depth (CCD).
  • Siliceous Ooze: Made of SiO₂ (diatoms, radiolarians).
  • Hydrogenous deposits: Precipitated from seawater (e.g., Polymetallic Nodules).
  • Cosmogenous deposits: Extraterrestrial origin (meteorite debris).
  • Corals are marine invertebrates (polyps) that build limestone reefs.
  • Corals have a symbiotic relationship with zooxanthellae algae.
  • Coral reefs are called “rainforests of the oceans”.
  • Conditions for Coral Growth:
    1. Temperature: 20°C - 28°C.
    2. Depth: Max 200-250 feet (for sunlight).
    3. Water: Clear, sediment-free.
    4. Salinity: 32-40 ppt.
  • Corals are absent on western continental margins due to cold currents.
  • Coral Triangle: Region of highest marine biodiversity (Indonesia, Malaysia, Philippines etc.).
  • Darwin’s Subsidence Theory (1842) explains the evolution of reef types.
  • Fringing Reef: Attached to the coast.
  • Barrier Reef: Separated from the coast by a lagoon (e.g., Great Barrier Reef).
  • Atoll: Circular reef enclosing a lagoon (e.g., Lakshadweep, Maldives).
  • Coral Bleaching: Expulsion of zooxanthellae by corals under stress, leading to a white appearance.
  • Major cause of mass bleaching: Anomalously high sea surface temperatures, often linked to El Niño.
  • Warm Currents: Flow from Equator to Poles (e.g., Gulf Stream).
  • Cold Currents: Flow from Poles to Equator (e.g., Canary Current).
  • Coriolis Force: Deflects currents to the right in the Northern Hemisphere and left in the Southern Hemisphere.
  • Sargasso Sea: A region in the North Atlantic Gyre, defined by ocean currents, known for Sargassum seaweed.
  • The meeting of the warm Gulf Stream and cold Labrador Current creates dense fog and rich fishing grounds off Newfoundland.

Mains Insights

GS Paper I (Geography)

  1. Salinity Distribution and Global Climate:
    • Cause-Effect: The global pattern of salinity is a direct consequence of the balance between evaporation and precipitation, which is governed by atmospheric circulation. High salinity in the subtropics is linked to the descending air of the Hadley Cell, while low salinity near the equator is due to high rainfall in the ITCZ.
    • Significance: Salinity, along with temperature, determines water density, which is the primary driver of the global Thermohaline Circulation (Great Ocean Conveyor Belt). Changes in salinity patterns, potentially due to climate change (e.g., melting ice sheets adding freshwater), could alter this circulation, with profound impacts on global climate.
  2. Formation and Significance of Ocean Deposits:
    • Historiographical Viewpoint: The study of ocean floor sediments (paleoceanography) provides invaluable insights into past climates. For example, the composition of foraminifera shells in calcareous ooze can be used to reconstruct past ocean temperatures and ice volumes.
    • Economic & Geopolitical Angle: Hydrogenous deposits, particularly polymetallic nodules and cobalt-rich crusts, are of immense economic interest. This has led to a new geopolitical frontier in the deep sea, with nations competing for exploration rights under the framework of the International Seabed Authority (ISA). This raises questions about equitable resource sharing and environmental protection.
  3. Coral Reef Ecosystems: Threats and Conservation:
    • Analytical Perspective: Coral reefs are complex socio-ecological systems. Their degradation has a cascading effect not only on marine biodiversity but also on the livelihoods of millions in coastal communities who depend on them for fisheries, tourism, and coastal protection from storms.
    • Debate: Conservation strategies are debated. Some advocate for focusing on mitigating global threats like climate change (the root cause), while others emphasize local management of stressors like pollution and overfishing to build reef resilience. The concept of creating “super corals” through genetic engineering is also an emerging, though controversial, solution.

GS Paper III (Environment & Economy)

  1. Coral Bleaching as an Indicator of Climate Change:
    • Cause-Effect: Coral bleaching is one of the most visible and immediate impacts of global warming. The increasing frequency and severity of mass bleaching events are a direct result of rising sea surface temperatures, acting as a critical warning sign for the health of our planet.
    • Impact Analysis: The loss of coral reefs due to bleaching would be an ecological and economic catastrophe. It would lead to a collapse in marine biodiversity, severely impact the fishing industry (estimated to support 500 million people globally), undermine the tourism sector in many small island nations, and increase the vulnerability of coastlines to erosion and flooding.
  2. Ocean Currents and Climate Regulation:
    • Systemic Role: Ocean currents are fundamental to the Earth’s climate system, functioning as a global heat engine that redistributes solar energy from the tropics to the poles. The North Atlantic Drift, for example, is responsible for keeping Western Europe’s climate significantly milder than other regions at similar latitudes.
    • Climate Change Impact: There is growing scientific concern that climate change, particularly the rapid melting of the Greenland ice sheet, could introduce a large amount of freshwater into the North Atlantic. This could decrease the salinity and density of surface water, potentially slowing down or weakening the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global conveyor belt, with unpredictable and potentially severe consequences for regional and global climate.

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