Salinity measures the total concentration of dissolved salts in water, a property that governs ocean circulation and supports marine life. Seawater contains a mixture of ions, primarily sodium and chloride, which make up about 85% of the total. Other components include magnesium, sulfate, calcium, and potassium. This concentration is typically expressed in parts per thousand (ppt) or Practical Salinity Units (PSU).
Identifying the World’s Saltiest Waters
When identifying the world’s saltiest waters, it is necessary to distinguish between a major ocean and a hypersaline sea or lake. The absolute saltiest body of water is the Dead Sea, a terminal lake nestled between Jordan and Israel. Its extreme concentration results in a salinity of approximately 337 parts per thousand (ppt), which is nearly ten times saltier than the average ocean.
The Atlantic Ocean holds the title for the saltiest of the five major oceans, with its average surface salinity hovering around 36 ppt. Within the Atlantic, the highest concentrations are found in the North Atlantic Subtropical Gyre, a vast, slowly circulating region that includes the Sargasso Sea. Salinity here often exceeds 36.5 ppt, reaching up to 37.9 ppt in some areas.
This high oceanic salinity is influenced by highly saline regional bodies like the Mediterranean Sea and the Red Sea. The Red Sea, in particular, is one of the saltiest seas globally, with surface concentrations reaching close to 40 ppt due to its restricted connection to the ocean and intense local evaporation. These regional conditions contribute to the overall saltiness of the Atlantic basin.
The Mechanisms Driving Ocean Salinity
The salt in the ocean originates from two primary geological processes: the weathering of continental rocks and chemical reactions at the seafloor. Rainwater, slightly acidic due to dissolved carbon dioxide, erodes rocks on land, dissolving mineral ions like sodium and calcium. Rivers and groundwater then carry these dissolved ions into the sea over millions of years.
A significant source is hydrothermal fluids released from vents along mid-ocean ridges. Cold seawater seeps into cracks in the ocean crust, where it is superheated by magma and undergoes chemical exchange with the rock. This process strips the water of certain elements while enriching it with others, such as iron and copper, before the fluid vents back into the ocean.
Once salt is introduced, its concentration is governed by the balance between processes that remove or add fresh water. Evaporation, particularly in hot, dry climates, is the main mechanism for increasing salinity, as it removes water vapor and leaves the dissolved salt behind. Conversely, the addition of fresh water through river runoff and precipitation, such as heavy tropical rainfall, dilutes the seawater and lowers its salinity.
Ice dynamics also play a role in this balance, especially in polar regions. When seawater freezes, the salt is largely excluded from the ice crystal structure, increasing the salinity of the surrounding water. When this sea ice melts, it releases fresh water back into the ocean, which acts as a local diluting agent.
Factors Influencing Salinity Variation
The global distribution of ocean salinity is not uniform, driven by geographic and climatic factors. The highest surface salinities are consistently observed in subtropical zones, centered around 20 to 30 degrees latitude north and south of the equator. These regions are characterized by high atmospheric pressure, low rainfall, and strong, persistent winds that promote maximum evaporation.
The Atlantic Ocean’s status as the saltiest is largely due to its unique shape and current circulation patterns. It is a relatively narrow basin that receives a smaller proportion of the planet’s total river runoff compared to the Pacific Ocean, which has numerous large river systems like the Amazon and the Mississippi. Furthermore, currents transport highly saline water from the Mediterranean Sea into the North Atlantic, compounding the effects of local evaporation.
The lowest salinities are found near the equator and in the polar regions. Equatorial waters experience high evaporation but also receive heavy rainfall, which provides enough dilution to keep surface salinity moderate. Conversely, polar waters have low evaporation rates and receive constant input of fresh water from melting sea ice and continental glaciers.
Oceanographers measure these variations using electrical conductivity to determine a value in Practical Salinity Units (PSU). Monitoring PSU values is crucial because salinity, along with temperature, controls the density of seawater. This density is the driving force behind the planet’s large-scale ocean currents that transport heat and nutrients globally.