Seas are overwhelmingly saltwater, containing significant dissolved minerals. The average ocean salinity is approximately 3.5%, meaning every kilogram of seawater holds about 35 grams of dissolved salts. Oceans and seas are Earth’s primary water reservoirs, holding about 97% of all water and shaping the planet’s climate and supporting diverse ecosystems.
Understanding Ocean Salinity
Salinity quantifies the total dissolved salts in water. These are primarily ions like sodium and chloride (forming common table salt), along with magnesium, sulfate, calcium, and potassium, which collectively constitute about 99% of dissolved salts. Oceanographers typically express salinity in parts per thousand (ppt or ‰), signifying grams of salt per kilogram of water. The average global ocean salinity is 35 ppt or 35 practical salinity units (psu), which are derived from electrical conductivity measurements and are roughly equivalent to parts per thousand.
The Origin of Ocean Salt
The salt in the ocean originates from two main processes: the weathering and erosion of rocks on land, and activity from the Earth’s interior. Rainwater, which is slightly acidic due to dissolved carbon dioxide, falls on land and gradually dissolves minerals from rocks. These dissolved ions are then carried by rivers and streams into the ocean, continuously adding salts over geological timescales.
Another source of ocean salt comes from underwater geological activity. Hydrothermal vents on the seafloor release mineral-rich fluids into the ocean. Seawater seeps into cracks, gets heated by magma, undergoes chemical reactions, and then emerges, carrying dissolved metals and salts. Volcanic eruptions, both on land and underwater, also contribute to ocean salinity by releasing gases and minerals into the atmosphere and directly into the water.
Distinguishing Saltwater from Freshwater Bodies
The fundamental difference between seas/oceans and freshwater bodies like rivers and most lakes lies in their salinity levels. Oceans average around 35 ppt, while freshwater systems typically have less than 0.5 ppt.
Rivers and lakes remain fresh due to a continuous hydrological cycle that prevents salt accumulation. Water flows through these systems, carrying dissolved salts downstream to the ocean. Evaporation primarily removes pure water, and fresh precipitation and runoff constantly replenish and dilute any incoming salts.
In contrast, oceans act as basins where salts accumulate over millions of years, as water evaporates from their surface, leaving salts behind with no continuous outflow.
Variations in Salinity Across the Globe
Ocean salinity is not uniform across all regions, fluctuating due to factors that add or remove freshwater from the surface.
High evaporation rates, common in warmer regions and enclosed seas, lead to higher salinity as water leaves but salts remain. For instance, the Red Sea and Mediterranean Sea often exceed 38 ppt due to high evaporation and limited freshwater inflow.
Conversely, areas with high precipitation or significant river runoff tend to have lower salinity. Equatorial regions, despite high evaporation, can have lower surface salinity due to abundant rainfall. Large river deltas, like the Bay of Bengal, and the Baltic Sea, with numerous river inputs and restricted ocean connection, show significantly reduced salinity (sometimes as low as 1-2 ppt).
Ice formation and melting also influence local salinity. When seawater freezes, most salt is excluded, increasing the surrounding unfrozen water’s salinity. Conversely, melting sea ice or land-based glaciers release freshwater, diluting surface waters and lowering salinity, particularly in polar regions like the Arctic Ocean.