Oceanic water is a complex solution and there is a great diversity in its chemical composition. It differs from fresh water by having a greater diversity of dissolved chemicals, and therefore, Oceanic water has different properties than fresh (terrestrial) water. The total mass (in grams) of all solid substances dissolved in sea water (one kilogram) is called salinity in parts per thousand (or "practical salinity units" psu).  The average salinity of the World Ocean is about 35 psu, that is, about 35g of solid substances are dissolved in 1 kg of  water.   Almost all chemical elements known on the Earth, have been found in Ocean water. They are in the forms of elements, molecules and as suspended matter.

The substances which are included in the structure of Oceanic water are conditionally divided into five groups: main elements, dissolved gases, biogenic substances, trace elements and organic substances.

Eleven main chemical elements comprise about 99.99% of the whole weight of dissolved substances, or about 47.8 x 1015tons, thus having the greatest influence on the physical properties of water. The stability of ratios between concentrations of main elements is the most important feature of its chemical structure.

Basically, dissolved gases in the World Ocean are nitrogen, oxygen, carbon dioxide, argon and hydrogen sulphide. The gases are derived from the atmosphere, biochemical processes in different layers of the water, at river mouths and estuaries, from degassing of the mantle into the Earths crust and from other geochemical processes. The weight of atmospheric gasses entering the waters of the World Ocean are on the order of 32.4 x 1012 t. Most chemically and biologically-derived substances dissolved in the World Ocean are oxygen and carbon dioxide.

Biogenic substances, that is, inorganic concentrations of nitrogen, phosphorus and silicon, are consumed by water plants, mainly by phytoplankton. The concentration of biogenic substances is determined by the amount and degree of biological processes, ocean dynamics, and to a lesser degree, by coastal mixing and outpouring of rivers.

Trace elements, of which there are 60, are found in minute quantities in ocean water and comprise only 0.01% of the total sum of elements in the Ocean, or 0.33 x 1012 t. They have practically no influence on the physical properties of sea water, but play an important role in biochemical processes occurring in the Ocean.

Organic substances are found in dissolved and suspended states in the water column. The greatest concentrations are seen in the surface layer of the Ocean.

Chemical make-up of water

1 - dissolved trace elements

Main ions (g/kg) at a salinity of 350/00

Sodium (Na+)
Magnesium (g++)
Calcium (++)
Potassium (+)
Strontium (Sr++)
Chlorine (Cl+)
Sulphates (SO4--)
Bromine (r- )
Fluorine (F-)
Boric acid (33)
Lithium (Li) Titanium (Ti)
Rubidium (Rb) Chromium (Cr)
Phosphorus (P) Gold (Au)
Iodine (I)  Tantalum (Ta)
Barium (Ba)  Thulium (Tm)
Iron (Fe)  Radium (Ra)
Zinc (Zn)  etc.
The concentration of trace elements makes up one millionth to one billionth of a gram / kg of sea water.


Sea water has a number of unique properties that considerably distinguish it from other fluids. The most important physical properties of sea water are a high thermal capacity, high dissolving ability, density, low heat conductivity, transmission of light and sound and good electrical conductivity. In many respects, these properties depend upon temperature, salinity and pressure.


Surface Temperature of the Ocean: August

1. Lines of equal temperature (isotherms) in degrees Celsius(C)
2. Average limits of sea ice distribution
3. Cross-sections

Surface Salinity of the Ocean: August

1. Isolines of salinity in parts per million (0/00)
2. Average limits of sea ice distribution
3. Cross-section

Dissolved oxygen on the Ocean surface: August

1. Isolines of oxygen in milligrams/atom per litre of sea water (mg-at O2/l)
2. Mean limit of sea-ice distribution
3. Cross-section 
  The average temperature of the whole surface of the World Ocean is ~17.5C. The highest temperature, >36C is found in the Red Sea and the lowest, <- 2C, has been observed in the Weddell Sea. Water temperature depth distribution depends on the amount of solar heating of the Ocean surface and intermixing of water masses.

Warmer surface and near-surface layers transmit heat to underlying waters, forming a productive layer. Hydrological, biological and other processes act within it. The thickness of an active layer ranges from 200-400 m. Down to depths of 1,000-1,800 m, the temperature gradually decreases, and below 1800 m, cold waters of almost constant temperature exist.

The salinity of water in the surface layer of the World Ocean depends mainly on evaporation and atmospheric precipitation. In coastal regions, fresh water outflows near the mouths of rivers, and in polar regions, the processes of freezing and thawing of ice greatly influence surface salinity. Below than surface layer, the salinity field is formed as a result of the interaction between the transport of salts by currents and diffusion by the intermixing of waters.

High salinity (>350/00) is encountered in surface waters at tropical latitudes, where evaporation is greater than at other latitudes. The lowest average salinity of oceanic waters (~290/00) is observed in the summer in the Arctic Ocean. In coastal regions with significant river run off, salinity does not exceed 15-200/00. The salinity of deep and near bottom waters in the oceans about 350/00. Salinity and temperature together affect the density of water. Many physical characteristics depend on density distribution, for example, water exchange processes, intermixing and sound transmission.

Oxygen enters the Ocean from the atmosphere and also as a result of photosynthesis in phytoplankton in the upper layers of water. The oxygen content essentially depends on temperature. When temperature decreases, oxygen solubility increases. In deep layers, the oxygen content is mainly determined by processes of intermixing and transport of water masses by currents.

The total amount of dissolved oxygen in the Ocean is ~7.5 x 1012 t, which is 158 times less than in the atmosphere. Oxygen is expended during respiration by marine organisms and by processes of oxidation. Oxygen saturation of the Ocean surface layers occurs only when the atmospheric oxygen budget is exceeded.

Light exposure on the Ocean surface depends on the altitude of the Sun, transparency of the atmosphere, cloud cover and weather disturbances. Sunlight on the Ocean surface is refracted and enters in the water, with a minor amount being reflected back into the atmosphere. Passing through the water, sunlight is dispersed at the expense of absorption and dispersion. More than 60% of transmitted light-energy is absorbed in the upper meter of sea water. In depths greater than 1,000 m, light has occasionally been detected, but only with the help of sensing devices.

Absorption of light vs. depth

Sunlight consists of light particles ranging from ultra-violet, through the visible spectrum to infra-red rays. The visible segment of the spectrum of sunlight (called daylight) forms rays of red, orange, yellow, green, blue, indigo and violet colours.

The rays of the solar spectrum are dispersed and are unequally absorbed by the Ocean. The spectral structure of sunlight changes with an increase of depth.

Red, long-wavelength rays are completely absorbed in the surface layers of the water. The predominant part of the scattered spectrum is green - indigo. At great depths, only the short-wavelength blue colours penetrate.

Depending on lighting conditions on the Ocean surface and transparency and clarity of the water, it is possible to distinguish object at depths of up to 300 m from a manned submersible.


The speed of sound transmission in sea water depends upon temperature, salinity and hydrostatic pressure (depth) of the water. Speeds range from 1440m/s to 1570 m/s. An increase of temperature on the order of 1C causes a decrease in speed of approximately 4 m/s.

There are direct and indirect methods to determine the speed of sound in sea water. Direct methods require the help of instrumentation. Indirect measurement are made by calculating the sound speed based upon available temperature, salinity and depth data.

In the 1940s, layers were detected in the Ocean in which sound waves are transmitted over super-long distances in a so-called underwater sound channel. The axis of the channel is at the depth where the speed of sound is at a minimum. The sound is spread with the least transmission power loss and can travel thousands of kilometres in the channel axis. The axial depth of the underwater sound channel may be from 100 m and less in polar latitudes up to 1,500-2,000 m in the Tropics.

Changes in sound speed with depth

  1. USC
  2. Axis of USC
  3. Upper boundary
  4. Lower boundary
In the temperate and low latitudes of the World Ocean at depths up 1,000 m sound speed decreases as a result of the reduction of water temperature. At greater depths, the water temperature is practically unchanging, but the pressure increases, causing an increase of the sound speed. Because of these reasons, a sound speed minimum is observed at about 1,000 m, creating the axis of the underwater sound channel.

Information provided by HDNO: