Seawater

The seas and oceans contain 97% of all the water on earth, 2% is found in frozen form in the ice caps and the remaining 1% is fluid fresh water. In the course of world history, materials in stones dissolved and made the water in the seas salty. If you evaporate 10 liters of seawater, you will end up with two cups of sea salt as residue.

Just imagine: on a warm summer vacation day, you fill a bucket with water while sailing in the North Sea. At a temperature of 10°C and an average atmospheric pressure, one liter of North Sea water contains 31 to 35 grams of salt, 11.7 cubic centimeters nitrogen, 6.4 cubic centimeters oxygen, 0.31 cubic centimeters carbon dioxide and 0.31 cubic centimeters inert gas (argon). In addition to these salts, seawater contains small amounts of all kinds of inorganic and organic materials. This seawater is a habitat for phytoplankton and zooplankton as well as larger plants and animals.

Due to suspended sand, mud, plankton and other materials, water in the North Sea is much more turbid than water in the Atlantic Ocean, especially near the coast. Suspended matter is important for the transportation and the bonding of many harmful materials.

Sea salt is not pure kitchen salt, although sodium chloride is the main component. 99.9% of sea salt consists of sodium chloride, together with magnesium sulphate, calcium carbonate, potassium bromide, strontium salts and fluorides. Phosphate, nitrate and silicate salts (nutrients) are not often found in clean seawater that has not been overly fertilized. Other elements, including iron, manganese, copper, zinc, molybdenum, vanadium and cobalt are only present in seawater in minimum amounts. Iron is the limiting factor for the growth of algae in the polar region.

There is even gold in seawater. However, extracting this gold costs much more than the yield, as was established by German scientists after World War I (1914-1918). It was believed that the extracted gold from seawater would be a good solution to paying off the German gold debt.

The density of seawater is greater than in fresh water, which means seawater is heavier. Seawater with a salinity of 3.5% only freezes at -1.9 degrees Celsius, while fresh water freezes at 0 degrees. Seawater reaches its greatest density at -1.33 degrees Celsius and a salinity of 2.4%, freshwater at 4 degrees Celsius. Sea ice should actually be heavier than seawater, while freahwater ice is lighter. However, the freezing process of seawater is not an instantaneous occurance. It occurs in steps. The salt (brine) is pushed out of the ice during the freezing process, after which it eats its way through the ice. The frozen seawater thereby contains a much lower concentration of salt. In addition, air bubbles are enclosed in the ice, so that sea-ice will float on the seawater.

All of the salt in the sea originates from dissolved rock, the erosion of the continents and from hot water sources in the bottom of the ocean. Geologists believe that it took around 25 million years to reach the present salt level and establish a balance. The relatively rapid mixing of ocean water is the reason that the salt level of the world seas remains relatively constant, although water is saltier in the tropics and less salty by the polar caps.

The large capacity of heat storage of water is hardly influenced by the salt. The carbon compounds in seawater are very important for the global nutrient cycles. Because seawater is a chemical buffer between carbon dioxide (CO2), carbonic acid (H2CO3) and hydrocarbonate (HCO3), seawater can take up 50 to 100 times more carbon dioxide than what is possible, normally speaking. This has a considerable influence on the carbon cycle and the climate. The buffer also ensures that seawater is slightly basic, with a pH value of around 8.

Seawater pollution

In addition to the earlier mentioned materials, a lot of polluting materials added by man are also found in seawater: oil, nutrients, heavy metals, pesticides and radioactive materials. Regulations have been taken to limit the pollution, since man has been aware of it. Therefore, the concentration of these materials is slowly declining and the seawater is becoming cleaner. However, many heavy metals and pesticides still exceed the target value.

In addition, a lot of marine litter is found in the sea. From results of Coastwatch studies, the amount of marine litter continues to increase.

Source: de Vleet, Ecomare