Chapter 13 Water (Oceans)

Water is essential for all known forms of life and is often referred to as "life itself." Unlike other planets in our solar system, Earth is fortunate to have an abundant supply of water on its surface, earning it the nickname the "Blue Planet."

Hydrological Cycle

Water is a cyclic resource, constantly moving and changing state. The hydrological cycle (also known as the water cycle) describes the continuous movement of water on, in, and above the Earth's surface (Figure 13.1). This cycle has been operating for billions of years and is fundamental to sustaining life on Earth.

Diagram showing the various components and processes involved in the continuous circulation of water within the Earth's system, including evaporation, condensation, precipitation, runoff, infiltration, and storage.

The hydrological cycle involves the circulation of water through its different phases – liquid, solid (ice), and gaseous (water vapour). It encompasses the continuous exchange of water between the oceans, the atmosphere, land surface, subsurface (groundwater), and living organisms.

The components and processes of the water cycle are interconnected (Table 13.1):

The distribution of water on Earth is highly uneven. Approximately 71 percent of the planet's surface is covered by oceans, which hold about 97.3% of the Earth's total water. The remaining freshwater is found in glaciers and ice caps (about 2%), groundwater (about 0.6%), lakes, rivers, soil moisture, and the atmosphere. Although oceans contain most of the water, it is saline. Freshwater is a much smaller percentage, and a significant portion is locked up in ice and glaciers, making easily accessible liquid freshwater a limited resource.

Roughly 59% of the precipitation that falls on land returns to the atmosphere through evaporation and transpiration (evapotranspiration). The rest flows over the surface as runoff, infiltrates into the ground to become groundwater, or accumulates as snow and ice. While the total amount of renewable freshwater circulating through the cycle is relatively constant, the demand for freshwater resources is growing dramatically with increasing population and development. This mismatch between supply and demand leads to water scarcity and crises in many parts of the world, varying regionally and seasonally. Pollution of freshwater sources further exacerbates this problem.

Relief Of The Ocean Floor

The Earth's oceans occupy vast depressions in the planet's outer layer. These ocean basins are not flat but possess complex and varied topography, similar to the relief features found on continents (Figure 13.2). The oceans seamlessly connect, making strict boundaries difficult to define, but they are conventionally divided into five major bodies: the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. Smaller features like seas, bays, and gulfs are considered parts of these larger oceans.

Most of the ocean floor lies at depths of 3 to 6 kilometers ($3-6 \, km$) below sea level. The submarine landscape includes some of the world's longest mountain ranges (mid-oceanic ridges), deepest valleys (oceanic trenches), and most expansive plains (abyssal plains). These features are shaped by the same geological processes that form continental landforms: tectonic movements (plate tectonics), volcanic activity, and deposition of sediments.

Diagram showing a cross-section of the ocean floor illustrating key features such as the continental shelf, continental slope, abyssal plain, oceanic trench, seamount, guyot, and mid-oceanic ridge.

Divisions Of The Ocean Floors

The ocean floors can be broadly divided into four main topographic provinces based on depth and relief:

Continental Shelf

This is the gently sloping submerged extension of a continent's landmass, extending from the coastline seaward (Figure 13.2). Continental shelves are covered by relatively shallow water, typically with an average gradient of $1^\circ$ or less. The edge of the continental shelf is marked by a more abrupt increase in slope, called the shelf break. The width of continental shelves varies greatly, averaging about 80 km globally, but can be almost absent in some areas (like parts of the coasts of Chile or Sumatra) and extremely wide in others (e.g., the Siberian Shelf in the Arctic Ocean, over 1,500 km wide). Depths range from around 30 m to 600 m. Continental shelves receive large amounts of sediment from rivers, glaciers, and wind, which are then distributed by waves and currents. Over geological time, the accumulation of these massive sedimentary deposits can become sources of fossil fuels.

Continental Slope

Beginning at the shelf break, the continental slope is a much steeper zone that descends towards the deep ocean basin (Figure 13.2). Its gradient typically ranges from $2^\circ$ to $5^\circ$. The depth of the continental slope is generally between 200 and 3,000 m. This boundary essentially marks the true edge of the continental crust. Submarine canyons and sometimes trenches are found incised into the continental slopes.

Deep Sea Plain

Also known as the abyssal plain, the deep sea plain is an extensive, flat or gently sloping area found in the deep ocean basins, usually located beyond the continental slope (Figure 13.2). These are among the smoothest and flattest regions on Earth. Depths typically range from 3,000 to 6,000 m. Abyssal plains are covered by thick layers of fine-grained sediments, primarily clay and silt, that have settled from the surface over millions of years.

Oceanic Deeps Or Trenches

Oceanic deeps, more commonly called oceanic trenches, are the deepest parts of the ocean floor (Figure 13.2). They are narrow, elongated depressions with very steep sides, often plunging 3 to 5 km deeper than the surrounding seafloor. Trenches are typically located at the base of continental slopes or, more commonly, along island arcs. They are formed at convergent plate boundaries where one tectonic plate is subducting beneath another. Trenches are zones of intense geological activity, strongly associated with frequent earthquakes and active volcanism. There are around 57 known deep trenches, with the majority (32) found in the Pacific Ocean, 19 in the Atlantic, and 6 in the Indian Ocean. Their location and characteristics are highly significant for understanding plate tectonic processes.

Minor Relief Features

In addition to the major divisions, the ocean floor includes numerous smaller but geologically important relief features (Figure 13.2):

Mid-Oceanic Ridges

A mid-oceanic ridge is a vast, submerged mountain range that runs through all the world's major ocean basins, forming the longest mountain chain on Earth. It is a divergent plate boundary where new oceanic crust is created by volcanic activity. The ridge system is characterized by a rift valley along its crest, flanked by chains of mountains. Some volcanic peaks along the ridge can be very high, occasionally rising above sea level to form islands, like Iceland, which is part of the Mid-Atlantic Ridge.

Seamount

A seamount is an isolated underwater mountain with a pointed summit, rising from the seafloor but not reaching the ocean surface (Figure 13.2). Seamounts are typically volcanic in origin and can be quite tall (3,000-4,500 m or more). The Emperor Seamount chain in the Pacific, an extension of the Hawaiian Islands, is an example of numerous seamounts.

Submarine Canyons

Submarine canyons are deep, steep-sided valleys carved into the continental shelves and slopes (Figure 13.2). Some are comparable in scale to large land canyons. They are often found offshore from the mouths of large rivers, suggesting that river erosion during periods of lower sea level played a role in their formation, or that they are actively carved by turbidity currents (dense flows of sediment-laden water) originating on the shelf and slope. The Hudson Canyon, off the coast of the eastern United States, is a famous example.

Guyots

A guyot (pronounced "ghee-yoh") is a flat-topped seamount (Figure 13.2). They are believed to be former volcanic islands or seamounts that were eroded flat by wave action when they were at or near the sea surface. After becoming extinct, they gradually subsided below sea level while retaining their flat tops. Guyots provide evidence of past sea level positions or the vertical movement of the seafloor. Thousands of seamounts and guyots are found in the Pacific Ocean alone.

Atoll

An atoll is a ring-shaped coral reef that surrounds a central lagoon (a body of shallow water) (Figure 13.2 shows a conceptual atoll). Atolls are typically found in warm tropical oceans. They form on top of submerged volcanic islands or seamounts. As the island sinks (or sea level rises), coral reefs grow upwards around the island's perimeter. Eventually, the island may completely subside below sea level, leaving the ring of coral reefs enclosing a lagoon where the island once was. The lagoon can be filled with seawater or, sometimes, fresh or brackish water depending on rainfall and connection to the sea.

Temperature Of Ocean Waters

The temperature of ocean waters is a fundamental property that influences density, circulation, and marine life distribution. Ocean waters are heated primarily by solar energy, similar to land, but the processes of heating and cooling are slower and involve mixing within the water column. Heat is also gained from the Earth's interior and released during condensation of water vapour above the ocean surface.

Factors Affecting Temperature Distribution

Several factors influence the distribution of temperature in ocean waters, particularly at the surface:

• Latitude: The most significant factor. Just like land, the amount of insolation received decreases from the equator towards the poles. Consequently, surface water temperatures are highest near the equator and decrease towards higher latitudes.
• Unequal Distribution of Land and Water: The Northern Hemisphere has a larger proportion of landmass than the Southern Hemisphere. Land heats up and cools down faster than water. The larger landmass in the Northern Hemisphere leads to greater temperature variations and influences the temperature of adjacent oceans, often resulting in slightly higher average ocean surface temperatures in the Northern Hemisphere compared to the Southern Hemisphere.
• Prevailing Winds: Winds can affect surface water temperatures by causing horizontal movement of water. Winds blowing offshore (from land to sea) can push warm surface water away from the coast, leading to the upwelling of colder, nutrient-rich water from deeper levels. Winds blowing onshore (from sea to land) can pile up warm surface water against the coast, increasing nearshore temperatures. This creates longitudinal variations in temperature along coasts.
• Ocean Currents: Ocean currents are large-scale movements of ocean water that transport heat across basins. Warm ocean currents carry warmer water from lower latitudes towards higher latitudes, increasing the temperature of the areas they flow through (e.g., the Gulf Stream warming the North Atlantic). Cold ocean currents carry cooler water from higher latitudes or from depth towards lower latitudes, decreasing the temperature of the areas they flow through (e.g., the Labrador Current cooling the North Atlantic coast of North America).

Local factors also influence ocean temperature, such as the presence of enclosed seas or the influx of freshwater. Enclosed seas in low latitudes (like the Red Sea) often have higher surface temperatures due to restricted circulation and high evaporation, while enclosed seas in high latitudes (like the Baltic Sea) have lower temperatures due to limited heat input and freshwater inflow.

Horizontal And Vertical Distribution Of Temperature

Horizontal Distribution: As noted, surface water temperature generally decreases with increasing latitude, from an average of around $27^\circ C$ near the equator to near $0^\circ C$ at the poles. The rate of decrease is approximately $0.5^\circ C$ per degree of latitude. The highest temperatures are often found slightly north of the equator (around $5-10^\circ$ N) rather than directly at the equator due to atmospheric circulation patterns and cloudiness near the ITCZ. The Northern Hemisphere oceans tend to have a slightly higher average annual temperature ($19^\circ C$) than the Southern Hemisphere ($16^\circ C$), mainly due to the larger landmass in the Northern Hemisphere.

Figure 13.4 shows the general spatial pattern of ocean surface temperatures.

Map illustrating the spatial distribution of average surface water temperature (in degrees Celsius) across the world's oceans.

Vertical Distribution: Temperature changes significantly with depth in the oceans, although the pattern varies with latitude.

• In most parts of the ocean (middle and low latitudes), the vertical temperature structure can be described as a three-layer system from surface to bottom:
  1. Surface Layer: The uppermost layer (around 500 m thick in the tropics) heated by solar radiation and mixed by waves and currents. Temperatures are relatively uniform within this layer, typically between $20^\circ C$ and $25^\circ C$ in tropical regions, and varying seasonally in mid-latitudes. This layer is warmest at the surface because it receives direct solar heat, and heat is transferred downwards by mixing (convection and turbulence).
  2. Thermocline Layer: Located below the surface layer (typically between 100-400 m and extending down to 500-1,000 m). This is a distinct transition zone characterized by a rapid decrease in temperature with increasing depth (Figure 13.3). This rapid temperature decline creates a barrier that inhibits vertical mixing between the warm surface layer and the cold deeper waters.

  Diagram illustrating the change in ocean water temperature with depth, showing the relatively warm surface layer, the thermocline where temperature decreases rapidly, and the cold deep ocean layer.

  3. Deep Ocean Layer: Extends from below the thermocline to the ocean floor. This is the largest layer by volume (about 90%) and is characterized by very cold temperatures, typically approaching $0^\circ C$ to $4^\circ C$. Temperatures in this layer are remarkably uniform globally, regardless of surface temperature, as it is not directly heated by the sun.
• In high latitudes (Arctic and Antarctic), the surface water temperatures are already very low (close to $0^\circ C$) throughout the year. Therefore, there is very little temperature change with depth, and the thermocline is usually absent or very shallow. The water column essentially consists of a single layer of cold water extending from the surface to the deep ocean floor.

The temperature varies with depth because solar radiation, the primary heat source, is absorbed within the upper layer. Heat transfer to deeper layers is much slower, primarily through conduction and limited mixing below the surface layer. The thermocline represents the zone where the effect of surface heating rapidly diminishes.

Salinity Of Ocean Waters

All natural waters contain dissolved mineral salts, acquired from the weathering of rocks on land and volcanic activity. Salinity is a measure of the total amount of dissolved salts in seawater. It is typically expressed as the amount of salt in grams dissolved in 1,000 grams (1 kg) of seawater. The unit used is parts per thousand (o/oo) or ppt.

Salinity is a crucial property of seawater, affecting its density, freezing point, and influencing ocean circulation. Water with a salinity of less than 24.7 o/oo is considered brackish water, distinguishing it from freshwater (very low salinity) and typical seawater.

Factors Affecting Ocean Salinity

The variation in salinity across the oceans is influenced by several factors:

• Evaporation and Precipitation: These are the most significant factors influencing surface salinity. High evaporation rates remove pure water from the surface, leaving salts behind and increasing salinity. High precipitation adds freshwater, diluting the salts and decreasing salinity. Areas with high evaporation and low precipitation (like subtropical regions) tend to have high salinity, while areas with high precipitation and low evaporation (like the equatorial zone) tend to have lower salinity.
• Freshwater Input: Rivers flowing into the ocean add large volumes of freshwater, significantly reducing salinity, particularly in coastal areas and enclosed seas receiving major river discharge (e.g., the Baltic Sea, the Bay of Bengal).
• Freezing and Thawing of Ice: When seawater freezes in polar regions, salt is excluded from the ice crystals, concentrating salt in the remaining unfrozen water, increasing salinity. When sea ice melts, it releases freshwater, decreasing the salinity of the surrounding water.
• Wind: Wind can influence surface salinity by transporting water and influencing evaporation rates.
• Ocean Currents: Ocean currents transport water masses of different salinities from one region to another, influencing the salinity distribution.

Salinity, temperature, and density of seawater are interconnected. Changes in one property influence the others. For example, increasing salinity or decreasing temperature increases water density, causing it to sink.

Horizontal Distribution Of Salinity

The salinity of normal open ocean water typically ranges between 33 and 37 o/oo. However, significant variations exist:

• Highest salinities in enclosed seas occur in hot, arid regions with high evaporation and limited freshwater input, such as the Red Sea ($> 41 \, o/oo$).
• Salinity is much lower in estuaries (where rivers mix with seawater) and in the Arctic Ocean, where significant freshwater is added from melting ice and river discharge.
• In the Pacific Ocean, which is the largest, salinity variations are influenced by its size and shape. Salinity is lower in the western parts of the northern hemisphere due to melting ice from the Arctic and in the southern parts (south of 15-20° S) due to high precipitation.
• The Atlantic Ocean's average salinity is around 36 o/oo. Highest salinities (up to 37 o/oo) are found in the subtropical zones (15-30° N and S) where evaporation is high and precipitation is relatively low. Salinity decreases towards the poles and also towards the equator. Warm currents like the North Atlantic Drift bring higher salinity water to some northern areas (like the North Sea), while cold currents or large freshwater inflows reduce salinity (e.g., low salinity in the Baltic Sea due to river influx). The Mediterranean Sea has high salinity due to high evaporation, while the Black Sea has very low salinity due to large freshwater input from rivers like the Danube, Dnieper, and Don.
• The Indian Ocean has an average salinity of 35 o/oo. Salinity is lower in the Bay of Bengal due to freshwater discharge from large rivers like the Ganga and Brahmaputra, and higher in the Arabian Sea due to higher evaporation rates and less freshwater input.

Figure 13.5 shows the general pattern of surface salinity across the world's oceans.

Map illustrating the spatial distribution of average surface water salinity (in parts per thousand, o/oo) across the world's oceans.

Some landlocked lakes have much higher salinities than oceans (Table 13.4 provides examples of extremely high salinity in Lake Van, Dead Sea, and Great Salt Lake).

Vertical Distribution Of Salinity

Salinity generally changes with depth, but the pattern varies geographically.

• Surface salinity is influenced by processes like evaporation, precipitation, ice formation/melting, and freshwater inflow.
• In contrast, the salinity of deep ocean water is relatively stable because it is not directly affected by these surface processes. Deep water mass properties are set when they form in surface waters in polar regions and then sink.
• There is often a zone with a marked vertical change in salinity with depth, called the halocline, where salinity either increases or decreases sharply. In areas where low-salinity water sits above high-salinity water, there is a rapid increase in salinity through the halocline with depth. This stratification based on salinity (density) is important for ocean circulation.
• Other factors being equal, increased salinity leads to increased water density, which causes saltier water to sink below less salty water.

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