Water (Oceans)

Learning Outcomes:

1. Understand the hydrological cycle and its vital role in sustaining life on Earth.
2. Examine the submarine relief features of ocean floors, including continental shelves, slopes, deep sea plains, and trenches.
3. Explore factors affecting temperature and salinity distribution in oceans.
4. Learn about oceanic movements such as waves, tides, and currents.

Water is fundamental to life on Earth, essential for all living organisms. Its presence on Earth, which lacks in much of the solar system, makes our planet unique, often referred to as the Blue Planet. Water exists in a continuous cycle, known as the hydrological cycle, which sustains all ecosystems.

Hydrological Cycle

The hydrological cycle describes the movement and transformation of water in different states across various regions of the Earth. This cycle plays a key role in maintaining life on Earth and involves numerous interconnected processes:

1. Water Storage: Approximately 71% of Earth’s water is in oceans, while the rest exists as freshwater in glaciers, lakes, rivers, and groundwater sources.
2. Evaporation: Water from oceans and other surfaces evaporates due to solar energy.
3. Condensation: The vapor cools and condenses to form clouds.
4. Precipitation: Water returns to Earth’s surface as rain, snow, or other forms.
5. Runoff and Infiltration: Water either runs off into rivers and oceans or infiltrates the soil to replenish groundwater.

Important Note: The total amount of water on Earth remains constant, but its distribution varies spatially and temporally, leading to periodic water crises in certain areas. Pollution further aggravates the scarcity of clean water.

Relief of the Ocean Floor

The ocean floor is just as varied and rugged as the land surface. It exhibits several topographical features created by tectonic, volcanic, and depositional processes. The following are the major divisions of the ocean floor:

1. Continental Shelf: This is the extended margin of the continents submerged under shallow seas and gulfs. Its average width is 80 km, although it varies globally. The shelf’s shallow depth and sedimentary deposits make it rich in resources, including fossil fuels.
2. Continental Slope: The slope forms a steep transition between the continental shelf and ocean basins, with gradients ranging from 2° to 5° and depths of 200 to 3,000 meters. Trenches and submarine canyons often exist here.
3. Deep Sea Plain: These are the flattest regions of Earth, with depths ranging between 3,000 and 6,000 meters. They are covered with fine sediments like clay and silt.
4. Oceanic Deeps (Trenches): The deepest parts of the ocean, these trenches are narrow, steep-sided depressions. They are crucial to the study of plate tectonics due to their association with volcanic activity and earthquakes.

Important Note: More than 57 oceanic deeps have been identified, the majority located in the Pacific Ocean, with others in the Atlantic and Indian Oceans.

Other significant features include mid-oceanic ridges, seamounts, submarine canyons, guyots, and atolls. Mid-oceanic ridges consist of mountain chains separated by depressions. Seamounts are underwater mountains formed by volcanic activity, while guyots are seamounts with flattened tops due to subsidence. Atolls are low islands, typically coral-based, encircling lagoons.

Temperature of Ocean Waters

The temperature of ocean waters is influenced by various factors, which include latitude, wind patterns, land-water distribution, and ocean currents:

1. Latitude: The temperature decreases from the equator to the poles as the intensity of solar radiation diminishes.
2. Unequal Distribution of Land and Water: The northern hemisphere’s oceans are warmer due to the proximity of land masses, which absorb and radiate more heat.
3. Prevailing Winds: Winds blowing from land to sea often cause upwelling of cold water, while onshore winds bring warm water to coastal areas.
4. Ocean Currents: Warm currents elevate temperatures in cooler regions, while cold currents lower temperatures in warmer areas.

Important Note: In enclosed seas such as the Mediterranean, the temperature tends to be higher due to limited exchange with open waters. Conversely, enclosed seas in polar regions, such as the Baltic Sea, have lower temperatures due to freshwater influx.

Ocean temperatures are also distributed vertically, with a sharp decline in temperature called the thermocline occurring between 100 to 400 meters. Below the thermocline, temperatures approach 0°C. Most ocean water lies below the thermocline, in a cold, stable layer.

Salinity of Ocean Waters

Salinity refers to the total amount of dissolved salts in water, measured in parts per thousand (‰). The average salinity of ocean water is 35‰, but it can vary based on factors like evaporation, precipitation, river inflows, and wind. The salinity of water significantly influences water density and oceanic currents.

Factors Affecting Salinity

1. Evaporation and Precipitation: Higher evaporation leads to increased salinity, while precipitation dilutes ocean water, reducing salinity.
2. Freshwater Inflows: In coastal and polar regions, river runoff and the freezing or thawing of ice significantly alter salinity.
3. Wind and Ocean Currents: Winds and ocean circulation can transfer water from one region to another, impacting local salinity levels.

Important Note: Lake Van in Turkey, the Dead Sea, and the Great Salt Lake are examples of water bodies with exceptionally high salinity.

Horizontal and Vertical Distribution of Salinity

1. In open oceans, salinity ranges from 33 to 37‰, with higher salinity recorded in land-locked seas like the Red Sea. In contrast, polar regions and estuaries often have lower salinity.
2. Pacific Ocean: Salinity decreases from 35‰ in the northern hemisphere to 31‰ near the Arctic. South of 20° latitude, it further decreases.
3. Atlantic Ocean: Average salinity is 36‰, with the highest salinity recorded between 15° and 20° latitude. The North Sea has higher salinity due to the influence of the North Atlantic Drift, while the Baltic Sea records lower salinity due to river influx.
4. Indian Ocean: Salinity levels are around 35‰, with lower salinity in the Bay of Bengal due to significant river inputs, and higher salinity in the Arabian Sea.

In terms of vertical distribution, salinity generally increases with depth. A distinct zone called the halocline marks the sharp increase in salinity as depth increases. Stratification by salinity occurs when denser, saltier water sinks beneath less salty water.

Table:

Ocean	Salinity Range (‰)	Notable Variations
Pacific	31-35	Influx of Arctic meltwaters
Atlantic	35-37	High evaporation, North Atlantic Drift
Indian	35	High evaporation in the Arabian Sea
Red Sea	Up to 41	Land-locked
Baltic Sea	Low	River influx

Movements of Ocean Water

Ocean water moves in various ways, including waves, tides, and currents. These movements are driven by a combination of wind, earth’s rotation, gravitational forces, and temperature differences.

1. Waves: Formed by wind, waves are the most visible form of ocean water movement. The size and strength of waves are determined by wind speed, duration, and fetch (the distance over which the wind blows).
2. Tides: Tides are the cyclical rise and fall of sea levels caused primarily by the gravitational pull of the moon and the sun. There are two types of tides:
   • Spring Tides: When the sun, moon, and Earth are aligned, producing higher high tides and lower low tides.
   • Neap Tides: When the sun and moon are at right angles to the Earth, resulting in less extreme tidal differences.
3. Currents: Ocean currents are large-scale flows of seawater caused by wind patterns, the Earth’s rotation, and differences in water temperature and salinity. Currents help regulate global climates by distributing heat around the planet.

Blockquote:
MCQ: Which ocean records the highest salinity?
Answer: **Atlantic Ocean

**