Discovery Science: Earth – Oceans – Physical Characteristics

Earth Science: Oceans – Physical Characteristics

Seawater may appear to the naked eye as if it were a uniform substance. However, oceans are complex environments with varying salt and nutrient contents, as well as different pressures, temperatures, and light conditions.

Different substances are continuously washed into the oceans from rivers, melting snow, precipitation, and wind. Almost all chemical elements can be found in seawater. Most of the dissolved substances are salts, mainly sodium chloride, which is more commonly known as table salt.

Due to freshwater from rivers flowing into the ocean, estuaries contain fewer salts than offshore areas.

Ocean salt

The average salt content, or salinity, of the world’s oceans is 3.5 percent, which is 0.6 ounce of salt per pint of water. When seawater evaporates, the salts dissolved into the water will remain in the ocean. The hotter and dryer the climate is, the greater the rate of evaporation and the greater the salinity of the water. This is especially the case in adjacent seas, which are less mixed due to their limited contact with the ocean.

The Persian Gulf has a salinity of four percent. The polar oceans have a slightly lower salinity of 3.1 to 3.5 percent, which is due to precipitation and melting ice. The salt content of the deep sea, at a depth of more than 3,280 feet (1,000 m), is overall consistently between 3.45 and 3.5 percent.

Temperature, light, and pressure

Surface temperatures range between 28.4°F (-2°C) In the polar oceans and 86°F (30°C) in the tropics. Adjacent seas and coastal areas may reach temperatures of up to 104T (40°C). Regional and seasonal variations only affect the temperature in the water’s upper layers. The temperature in the tropics and temperate regions begins to rapidly decrease at a depth
of around 656 to 3,280 feet (200 to 1,000 m). This layer of water is called a thermo-cline.

From 3,280 feet (1,000 m) down into the deep sea, temperatures are constant, between 32°F (0°C) and 41°F (5°C).The lower limit of the light zone is at a depth of between 328 and 656 feet (100 and 200 m) depending on water conditions; in cloudy, coastal areas, light may only reach a depth of 33 feet (10 m). In the open sea, sunlight may reach a depth of up to 3,280 feet (1,000 m).

Due to the increase in pressure by 1.45 pounds per square inch (10 kPa) per foot depth, there is a pressure of about 1.450 psi (10,000 kPa) in the open sea. The speed of sound in water is 4,921 feet per second (1,500 m/sec) depending on pressure, temperature, and salt content. Therefore, sound travels at four times the speed it travels through air.


The process of desalination to purify water has been copied from nature: drinking water is produced in desalination factories where salt is extracted from the seawater. Various processes are used to simulate natural separation methods. However, seawater desalination is much more expensive than accessing conventional sources of freshwater.

This is mainly due to the high levels of energy used during salt extraction. Therefore desalination factories are only used where freshwater supplies are insufficient.


MARINE ANIMALS AT RISK Several marine animals-including sharks, whales, and dolphins—rely on sound for orientation, as visibility is limited However, they are severely affected by noise from engines, sonar, and military and industrial activities.

These animals lose their orientation and risk getting too close to shore where they can get stranded and die.