but......did you know that the ocean contains every naturally occurring element known, together with numerous compounds and minerals? The main chemical constituent of seawater is common salt (NaCl), a very simple chemical compound. However, ocean water also contains all the gases found in the atmosphere but in different proportions, oxygen, nitrogen and carbon dioxide being relatively abundant. Nitrogen is the most abundant gas in the ocean, and it is present as both a free, dissolved gas and in various compound forms. Oxygen is produced in the oceans’ surface layers by plant photosynthesis or is dissolved directly from the atmosphere.

It was thought that the salt in the oceans had accumulated over the approximately 3 billion years that rivers had flowed into it, carrying dissolved salts from soil erosion, but the oceans contain some elements that are not found in river water, chlorine, bromine and sulphur for example, in abundance. The source of these elements remained a mystery until more was learned about the structure of the ocean floors.

Variations around the world's oceans

The ocean isn't uniformly saline all around the world - some parts of the ocean are much saltier than average, and other parts are less salty. The Red Sea contains the saltiest seawater - 4.1% compared to a global average of 3.5%. Seawater salinity depends on a number of factors, such as the climate, ocean currents bringing in water from other areas, presence of ice sheets or rivers nearby and so on. The salinity of the ocean can also vary depending on the time of year - for example, the northern Indian Ocean is much less salty during the monsoon season because of all the rainwater pouring into the ocean.

Left: Map showing the variation in salinity in the Atlantic Ocean at 500m below the sea surface. High salinity is shown as orange; lower salinty is dark blue. Image source: WOCE (World Ocean Circulation Experiment). The Atlantic Ocean is generally more saline than the Pacific Ocean - this is because of the large-scale ocean circulation patterns which move masses of seawater around the globe.

Salinity also varies with depth in the ocean. In warm latitudes, seawater on the ocean surface tends to be more salty due to evaporation, but in high latitudes surface water tends to be less salty due to input of freshwater from melting ice. These factors affect seawater down to a depth of about 1km. Below this, the salinity of the seawater is much more uniform. The boundary between the surface waters and the deeper waters of the oceanwhich have different salinities is called the halocline. This should not be confused with the thermocline, which separates bodies of water with different temperatures.