Approximately 4,000 mya, conditions on the planet earth were insufficient for the support of life. The necessary element that was missing from the composition of the planet was a hydrosphere: a system of oceans covering the surface. A process of cooling would need to take place so that water vapor could condense into rain and form permanent accumulations of liquid water.
The first rain hitting the earth's surface would have boiled away almost instantly. However, with the passage of time, water began to collect in the depressions in the earth's crust as global temperatures continued to cool.
Today, oceans cover about 70% of the surface with a volume of water of approximately 1.3 million km3. However, this huge amount of water only accounts for about 10% of the planet's total water with the rest of it chemically bound into the mineral deposits of the Earth's mantle.
By about 250 mya, it is estimated that there was just one landmass, the supercontinent of Pangea, which was surrounded by the single large ocean of Panthalassa. About 130 mya, at the end of the Jurassic Period, the continent of Pangea began to break apart into Laurasia in the north (comprised of present day North America, Europe, and Asia) and Gondwanaland to the south (which contained South America, Africa, India, Australia, and Antarctica).
The continental breakup and continual drift would lead to a shifting of global temperatures relative to the sun's position and the development of continental shelves off the coasts of the new landmasses. These more stable coastal waters would provide an environment more conducive to the evolution of the species.
The nature of the oceans today can be understood by analyzing certain chemical factors like pH, dissolved gases, salinity and nutrient content. Also central to the makeup of the oceans are temperature, sunlight and air movements in the constant interaction between the water and the atmosphere.
The salt content of seawater is approximately 35 0/00 (35 parts of salt per 1,000 parts seawater).
Salt g/kg %
Chloride 19.0 55.0
Sodium 10.6 30.6
Sulphate 2.7 7.7
Magnesium 1.3 3.7
Other 1.0 3.0
Environment Type Salinity (0/00)
Open ocean 32-38
Coastal water 27-30 (brackish)
Enclosed sea (e.g. Baltic Sea) 25 or less
Hypersaline areas 40 or more
Many factors contribute to the amount of dissolved gases in seawater. Since oxygen (O2) and other gases are not readily soluble in saltwater, solubility will tend to decrease with rising water temperatures, increase with current turbulence and increase with salinity. Carbon dioxide (CO2) is another essential gas for biological processes in the ocean that create new organic matter. It is 60 times more concentrated in seawater than it is in the atmosphere. It forms carbonic acid (H2CO3), bicarbonate (HCO3) and carbonate (CO3).
Water temperature in the oceans is one of the most important factors controlling chemical, biological, and physical processes. Most importantly, it determines the biogeographic distribution of marine species throughout the oceans.
Suface temperature is influenced by the amount of solar radiation penetrating the water. Sunlight received by the ocean water will vary according to day length and cloud cover, as well as latitude and the time of the year, which will determine the angle of the sun and therefore its intensity. The heating effect of the sun is mostly confined to the surface; 98% of infared radiation is absorbed in the first meter of water. However, water has the unique property of absorbing large amounts of heat with little change to its temperature. The evaporation of surface water also helps to reduce surface temperature, allowing for ocean water to grow hotter and colder very gradually.
This slow process is vital to the support of life, as most organisms require time to acclimatise to the range of temperatures to which they are exposed. Poikilothermic (cold-blooded) animals cannot regulate their body temperatures and are therefore bound to smaller areas, which contrasts with homeothermic (warm-blooded) animals, which can adjust to fluctuating water temperatures and then travel across broad ranges of ocean water.
Polar: -1.9°C to +2.0°C (southern oceans)
Cold temperate: -1.9°C to +5.0°C (Arctic waters)
5.0°C to 10.0°C (northern hemisphere)
2.0°C to 10.0°C (southern hemisphere)
Warm temperate: 10.0°C to 15.0°C (both hemispheres)
Subtropical: 15.0°C to 25.0°C (both hemispheres)
Tropical: 25.0°C and up (both hemispheres)
The World's Oceans:
Bibliography & Further Reading:
C.M. Lalli, T.R. Parsons. Biological Oceanography: An Introduction. 1993.
H.V. Thurman. Introductory Oceanography. 1994.
Why is the ocean blue?
Southern Ocean Islands
Pacific Ocean Islands
Atlantic Ocean Islands
Charles William Beebe
Great Barrier Reef
oceans of the world
deep sea creatures