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Chapter 12 Water (Oceans)
Water is often referred to as "life itself," underscoring its critical importance for all known life forms on Earth. Our planet is unique in our solar system for possessing abundant liquid water on its surface, earning it the nickname the **"Blue Planet."**
Hydrological Cycle
Water is a **cyclic and renewable resource**. It continuously moves and is reused within the Earth system. The **hydrological cycle**, also known as the water cycle, describes the constant movement and transformation of water through its different states (liquid, solid, gas) on, in, and above the Earth's surface. This cycle has been operating for billions of years and is fundamental to sustaining life on Earth. It involves a continuous exchange of water between the oceans, the atmosphere, the land surface and subsurface, and living organisms.
The vast majority of Earth's water, about **71 percent**, is found in the **oceans**. The remaining water is freshwater stored in various forms:
- Glaciers and ice caps (the largest reservoir of freshwater).
- Groundwater.
- Lakes.
- Soil moisture.
- Atmosphere (water vapour).
- Rivers and streams.
- Within living organisms.
A significant portion of the water that falls on land as precipitation (roughly 59%) returns to the atmosphere through evaporation (from surfaces) and evapotranspiration (from plants). The rest either flows over the surface as runoff, infiltrates into the ground, or contributes to glaciers.
While the total amount of renewable water on Earth remains relatively constant, the demand for fresh water is increasing dramatically due to population growth and development. This growing demand, coupled with uneven distribution (some areas have plenty, others are water-scarce) and increasing pollution of water sources like rivers, leads to significant water crises in many parts of the world.
The water cycle consists of various interconnected components and processes:
| Components (Water Storage Locations) | Processes (Movement/Transformation) |
|---|---|
| Water storage in oceans | Evaporation |
| Water in the atmosphere (as vapour, clouds) | Evapotranspiration (combined evaporation and transpiration) |
| Water storage in ice and snow | Sublimation (solid ice/snow to water vapour) |
| Freshwater storage (lakes, rivers, soil moisture, groundwater) | Condensation |
| Within living organisms | Precipitation |
| Snowmelt runoff to streams | |
| Surface runoff | |
| Stream flow | |
| Infiltration (water seeping into the ground) | |
| Groundwater flow | |
| Groundwater discharge (e.g., into rivers, oceans, or through springs) |
Relief Of The Ocean Floor
The Earth's oceans are vast bodies of water that occupy the great depressions or basins on the planet's outer layer. Unlike continents, which have clear boundaries, ocean bodies merge seamlessly into one another. For geographical study, the global ocean is typically divided into five main oceans: the Pacific, Atlantic, Indian, Southern (or Antarctic), and Arctic Oceans. Various smaller seas, bays, and gulfs are considered parts of these larger oceans.
Contrary to the idea of a flat bottom, the **ocean floor** possesses a diverse and complex topography, much like the land surface. Features underwater are shaped by the same fundamental geological processes that shape continents: **tectonic activity** (plate movements), **volcanism**, and the effects of **erosion and deposition**.
While a significant portion of the ocean floor lies at depths between 3 and 6 kilometers, it includes the world's longest mountain ranges (mid-oceanic ridges), deepest valleys (trenches), and extensive plains (abyssal plains).
Divisions Of The Ocean Floors
Based on depth and topography, the ocean floor can be broadly divided into four major regions, along with numerous smaller, but important, relief features.
The four major divisions are:
- The Continental Shelf
- The Continental Slope
- The Deep Sea Plain (Abyssal Plain)
- The Oceanic Deeps (Trenches)
Minor relief features include ridges, hills, seamounts, guyots, canyons, and atolls. (These features are illustrated in Figure 12.2).
Continental Shelf
The **continental shelf** is the gently sloping submerged extension of a continent. It lies between the shoreline and the continental slope. This is the **shallowest part of the ocean**, with an average gradient of less than 1°. It typically ends abruptly at the **shelf break**, where the slope dramatically increases. The width of continental shelves varies greatly globally, averaging about 80 km but ranging from almost non-existent in some areas (like the coast of Chile) to extremely wide (e.g., the Siberian shelf, over 1500 km wide). Depth also varies, from about 30m to 600m. Continental shelves are covered by sediments from land, transported by rivers, glaciers, and wind, and distributed by waves and currents. Over geological time, the accumulation of massive sedimentary deposits on the shelves can become the source of commercially important **fossil fuels**.
Continental Slope
The **continental slope** is the steep transition zone that connects the outer edge of the continental shelf to the deep ocean basin. It begins at the shelf break and descends to depths of 200 to 3,000 meters. The gradient of the slope is significantly steeper than the shelf, typically ranging from 2° to 5°. The continental slope essentially marks the true edge of the continental crust. Features like submarine canyons and oceanic trenches are sometimes found incised into or adjacent to the continental slope.
Deep Sea Plain
The **deep sea plains**, also known as **abyssal plains** (as discussed in Chapter 4), are vast, flat, or very gently sloping areas that form the bottom of the deep ocean basins. Located beyond the continental slopes, these are considered among the **flattest and smoothest regions on Earth**. Their depths range primarily between 3,000 and 6,000 meters. These plains are covered by layers of fine-grained sediments, including clay, silt, and microscopic marine organism remains, which accumulate over long periods, burying the underlying irregular topography.
Oceanic Deeps Or Trenches
**Oceanic deeps**, or **trenches**, are the **deepest parts of the oceans**. They are long, narrow, and relatively steep-sided depressions on the ocean floor, typically 3 to 5 kilometers deeper than the surrounding abyssal plains. Trenches are often located at the base of continental slopes or adjacent to island arcs (curved chains of volcanic islands). They are fundamentally linked to plate tectonic activity, specifically at **convergent boundaries** where one tectonic plate is subducting beneath another. Due to their association with subduction zones, trenches are characterized by high levels of **volcanism** and frequent **strong earthquakes**. Their study provides crucial insights into plate movements and the recycling of the Earth's crust. Numerous trenches have been explored globally, with a significant concentration (32 out of 57 known at the time of the text) in the Pacific Ocean (e.g., the Mariana Trench, the deepest), 19 in the Atlantic, and 6 in the Indian Ocean.
Minor Relief Features
Besides the major divisions, the ocean floor contains various smaller, yet geologically significant, relief features:
Mid-Oceanic Ridges
As mentioned in Chapters 3 and 4, **mid-oceanic ridges** are the longest mountain ranges on Earth, though largely submerged. They form interconnected systems on the ocean floor. A typical mid-oceanic ridge consists of two parallel chains of underwater mountains separated by a central rift valley. These features are sites of active **seafloor spreading** (divergent plate boundaries) and frequent **volcanic eruptions**. The mountains can rise significantly from the seafloor, with some peaks even breaking the ocean surface to form islands, such as Iceland, which is part of the Mid-Atlantic Ridge.
Seamount
A **seamount** is an isolated underwater mountain that rises significantly from the seafloor but does not reach the ocean surface. Seamounts are typically **volcanic in origin**, formed by eruptions that build up a cone-shaped or pointed peak. They can be substantial in size, often rising 3,000 to 4,500 meters from the surrounding seabed. The Emperor Seamounts in the Pacific, an extension of the Hawaiian Islands chain, are a well-known example.
Submarine Canyons
**Submarine canyons** are deep, steep-sided valleys cut into the continental shelf and slope. Some are comparable in scale to the Grand Canyon on land. They are often found offshore from the mouths of major rivers, suggesting they may be carved by turbidity currents (dense, sediment-laden flows) originating from river sediment discharge, or potentially related to past periods of lower sea level when rivers extended across the exposed shelf. The Hudson Canyon, off the coast of New York, is a famous example.
Guyots
A **guyot** is a flat-topped seamount. They are believed to have once been volcanic islands or seamounts that reached the surface, had their tops flattened by wave erosion, and then subsided (sunk) below sea level over time. The flat summit distinguishes them from pointed seamounts. Numerous seamounts and guyots are found across the ocean floor, particularly abundant in the Pacific (estimated over 10,000).
Atoll
An **atoll** is a ring-shaped coral reef, often supporting low-lying islands, that surrounds a central body of water called a **lagoon**. Atolls typically form in warm tropical oceans, often on top of submerged volcanic islands or seamounts. Corals grow upwards as the underlying volcano sinks. The lagoon within the atoll may be part of the open sea or, in some cases, contain fresh, brackish, or highly saline water depending on rainfall and connection to the ocean.
Temperature Of Ocean Waters
The temperature of ocean water varies geographically across the surface and changes significantly with depth. Like land, ocean water is heated by solar energy (insolation), but water has a high specific heat capacity, meaning it heats up and cools down more slowly than land.
Factors Affecting Temperature Distribution
Several factors influence the distribution of temperature in the ocean waters:
- **Latitude:** The amount of solar radiation received decreases from the equator towards the poles. Consequently, surface ocean temperatures are warmest near the equator and gradually decrease towards higher latitudes. The average rate of decrease is about $0.5^\circ\text{C}$ per degree of latitude.
- **Unequal Distribution of Land and Water:** Oceans in the Northern Hemisphere are in contact with larger landmasses compared to the Southern Hemisphere, which is dominated by oceans. Land heats and cools faster than water, influencing atmospheric temperatures and consequently affecting the amount of heat exchanged with the adjacent oceans. Northern Hemisphere oceans generally have slightly higher average temperatures than Southern Hemisphere oceans.
- **Prevailing Winds:** Winds blowing from land towards the ocean (offshore winds) can push warm surface water away from the coast, leading to **upwelling** of colder, nutrient-rich water from deeper levels. This lowers coastal surface temperatures. Conversely, winds blowing from the ocean towards land (onshore winds) can pile up warm surface water near the coast, raising temperatures.
- **Ocean Currents:** Large-scale movements of ocean water play a major role in transporting heat. **Warm ocean currents** (e.g., Gulf Stream, North Atlantic Drift) carry warm water from lower latitudes to higher latitudes, increasing temperatures in the areas they flow through (e.g., warming the eastern coast of North America and western Europe). **Cold ocean currents** (e.g., Labrador Current, Peruvian Current) carry cold water from higher latitudes or deep areas to lower latitudes, decreasing temperatures in coastal regions they affect (e.g., cooling the northeastern coast of North America and the west coast of South America).
- **Local Factors:** Enclosed seas in low latitudes tend to have higher temperatures due to limited circulation and high evaporation, while enclosed seas in high latitudes tend to be colder.
Horizontal And Vertical Distribution Of Temperature
The average temperature of the surface layer of the oceans is around $27^\circ\text{C}$ near the equator and decreases towards the poles, reaching near $0^\circ\text{C}$ in polar regions. The highest average surface temperature is typically found slightly north of the equator, rather than directly at the equator, possibly due to factors like slightly less cloud cover or circulation patterns. The average annual surface temperatures for the Northern and Southern Hemisphere oceans are approximately $19^\circ\text{C}$ and $16^\circ\text{C}$ respectively, reflecting the influence of land distribution. (Figure 12.4 illustrates the global surface temperature pattern).
Vertically, temperature generally **decreases with increasing depth** in the oceans. This is because solar energy is primarily absorbed in the uppermost layers. Heat is transferred downwards mainly through mixing and convection, but these processes are limited, especially in deeper waters. The rate of temperature decrease is not uniform with depth.
A distinct layer exists below the warm surface layer where temperature drops very rapidly with increasing depth. This zone is called the **thermocline** (Figure 12.3). It typically starts between 100 and 400 meters below the surface and can extend down for several hundred meters. Below the thermocline, the temperature continues to decrease, but at a much slower rate, approaching very cold temperatures (near $0^\circ\text{C}$) in the deep ocean. About 90% of the total volume of ocean water is in this cold, deep layer.
In middle and low latitudes, the ocean's temperature structure can be described as a three-layer system:
- A warm **surface layer** (approx. top 500m) with relatively uniform temperatures ($20^\circ-25^\circ\text{C}$), present year-round in the tropics and seasonally in mid-latitudes.
- The **thermocline layer** (approx. 500-1000m thick) characterized by a steep vertical temperature gradient.
- A very **cold deep layer** extending from below the thermocline to the ocean floor, with temperatures close to freezing.
In polar regions, surface water temperatures are already near $0^\circ\text{C}$. There is very little temperature variation with depth, and essentially only one layer of cold water extends from the surface to the bottom.
Salinity Of Ocean Waters
**Salinity** is a measure of the total amount of dissolved solid material (salts) in seawater. These dissolved minerals come from the weathering and erosion of rocks on land, carried to the oceans by rivers, as well as from volcanic activity and hydrothermal vents on the ocean floor. Salinity is typically expressed in **parts per thousand (ppt)** or permil ($\text{o/oo}$), representing the grams of dissolved salts in 1,000 grams (or 1 kg) of seawater. A salinity of 24.7 ppt is sometimes used as a benchmark distinguishing 'brackish water' (less saline than typical seawater) from true seawater.
Salinity is a crucial property of seawater as it affects density (higher salinity makes water denser) and influences ocean circulation patterns.
Factors affecting ocean salinity are mentioned below:
The salinity of ocean water varies spatially due to several factors:
- **Evaporation and Precipitation:** In surface waters, high rates of evaporation (e.g., in hot, dry regions) remove water but leave salts behind, increasing salinity. High precipitation adds freshwater, decreasing salinity.
- **Freshwater Influx:** Rivers discharging into the ocean, and melting ice (glaciers, sea ice) add freshwater, significantly reducing salinity in coastal areas, estuaries, and polar regions.
- **Wind:** Winds can transport surface water, leading to the accumulation of more or less saline water in different areas.
- **Ocean Currents:** Ocean currents mix and transport water masses with different salinity characteristics, distributing salinity horizontally.
Salinity, temperature, and density are interconnected properties. Changes in temperature or density influence salinity, and vice versa. For example, increasing salinity increases density, and decreasing temperature increases density. Denser water tends to sink below less dense water.
Some of the highest salinity levels in natural water bodies occur in certain lakes that lack an outlet, allowing salts to accumulate over time:
| Water Body | Salinity (o/oo) |
|---|---|
| Lake Van (Turkey) | 330 |
| Dead Sea | 238 |
| Great Salt Lake (USA) | 220 |
Horizontal Distribution Of Salinity
Salinity in the open ocean typically ranges between 33 o/oo and 37 o/oo. However, significant variations exist globally (Figure 12.5).
- Areas with high evaporation and low freshwater input tend to have higher salinity (e.g., subtropical belts where sinking air reduces rainfall, landlocked seas like the Red Sea with salinity >41 o/oo, the Mediterranean Sea with high evaporation).
- Areas with high freshwater input tend to have lower salinity (e.g., near river mouths, the Arctic Ocean and areas influenced by melting ice, the Baltic Sea with significant river influx, the Black Sea with enormous river input resulting in very low salinity).
- The average salinity of the Atlantic Ocean (around 36 o/oo) is slightly higher than the Pacific (due to shape and river influx). Highest Atlantic salinity is found in the subtropics ($15^\circ-20^\circ$ N, $20^\circ-60^\circ$ W).
- The Indian Ocean has an average salinity of 35 o/oo, but regional variations exist (e.g., lower in the Bay of Bengal due to Ganges-Brahmaputra river influx, higher in the Arabian Sea due to higher evaporation and lower river influx).
Vertical Distribution Of Salinity
Salinity also changes with depth, although the patterns differ from temperature. Surface salinity is more variable as it is directly affected by evaporation, precipitation, and freshwater runoff. In contrast, salinity in the deep ocean is generally more stable because these waters are less influenced by surface processes.
There is often a distinct zone in the water column where salinity changes sharply with depth. This layer is called the **halocline**. It can involve either a sharp increase or decrease in salinity, depending on the location. For example, in areas with significant surface freshwater input (like estuaries or polar regions), a layer of less saline water may float above denser, more saline water, creating a halocline where salinity increases rapidly with depth. Elsewhere, density stratification driven by temperature (thermocline) might be more dominant. In the deep ocean, away from surface influences, salinity is relatively uniform.
Since increasing salinity increases the density of seawater (assuming temperature is constant), high-salinity water tends to sink below lower-salinity water. This process contributes to the layering or stratification of the ocean by density, which in turn drives deep ocean circulation (thermohaline circulation).
Exercises
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