Seawater
On average, seawater in the world’s oceans has a salinity of –3.5%. This means that for every litre (1000 ml) of seawater there are 35 grams of salts (mostly sodium chloride) dissolved in it.
The 3.5% salt content of seawater comes from minerals containing chlorine and sodium that are dissolved from rocks on land and then carried down rivers to the sea. Undersea volcanoes and hydrothermal vents also pump minerals into the water.
Although a vast majority of seawater is found in oceans with salinity around the -3.5%, seawater is not uniformly saline throughout the world. The planet’s least saline (freshest) seawater are in the eastern parts of the Gulf of Finland and in the northern end of the Gulf of Bothnia, both parts of the Baltic Sea. The most saline open sea is the Red Sea, where high temperatures and confined circulation result in high rates of surface evaporation and there is little fresh inflow from the rivers. The salinity in isolated seas and salt-water lakes (for example, the Dead Sea) can be considerably greater.
Seawater is made up of no fewer than 60 different elements including gold, silver, arsenic and uranium. Common salt is the most abundant ingredient, making up approximately 78% of the total dissolved solids in seawater. It has been calculated that if all the salts in the oceans of the world were dried up they would yield enough salt to cover the continents of the world to a depth of 46m.