The concentration of dissolved salts in ocean water, known as salinity, is a fundamental property that shapes the marine environment. Salinity is typically measured in Practical Salinity Units (PSU), with the global average being approximately 35 PSU. This means there are about 35 grams of dissolved salt compounds for every 1,000 grams of seawater. Salinity variation directly influences seawater density, which, combined with temperature, drives the massive, slow-moving system of currents called thermohaline circulation. The concentration of salts is not static, constantly changing in response to processes that either remove fresh water to concentrate the salts or add fresh water to dilute them.
Water Removal and Salt Concentration
Processes that remove fresh water from the ocean surface are the primary drivers for increasing local salinity. The most widespread mechanism is evaporation, which is especially pronounced in tropical and subtropical regions. When solar energy heats the ocean surface, water molecules transition into vapor and leave the liquid, but the dissolved salts are non-volatile and remain behind.
This effect is why areas with high temperatures and low rainfall, such as the open oceans at around 30 degrees latitude, generally exhibit the highest surface salinity. The Mediterranean and Red Seas also show elevated salinity, often exceeding 40 PSU, because they are in hot, arid regions and have limited freshwater inflow to offset the high evaporation rates. The continuous loss of fresh water to the atmosphere concentrates the existing salt ions, making the remaining surface water denser and prone to sinking, which initiates deep ocean currents.
Another localized, powerful mechanism for concentrating salt is the formation of sea ice in polar regions. When seawater freezes, the water molecules arrange themselves into a crystal structure that largely excludes the salt ions. This process, called brine rejection, pushes the concentrated salt into the remaining liquid water directly beneath the newly formed ice. The underlying water becomes significantly saltier and therefore much denser, causing it to sink to the ocean floor. This newly formed, cold, and highly saline water mass is a major component of the deep-ocean circulation system.
Water Addition and Salt Dilution
Conversely, ocean salinity decreases in areas where large volumes of fresh water are added to the surface layer, effectively diluting the salt concentration. The most direct source of fresh water is precipitation, which includes rain and snowfall falling directly onto the ocean. In the equatorial zones, heavy, consistent rainfall introduces substantial amounts of fresh water, which significantly lowers the surface salinity despite the high temperatures that would otherwise favor evaporation. This influx of fresh water creates a lens of less-dense, lower-salinity water that tends to float on the surface of the denser, saltier water below. The lower salinity in these high-rainfall regions, such as the equatorial Pacific and Atlantic, demonstrates that precipitation can be a stronger, immediate factor than evaporation in determining surface salinity.
Another major diluting factor is the freshwater runoff from land delivered by large river systems. Rivers carry water that has drained vast landmasses, and this water has very low salt content compared to the ocean. Near the mouths of major rivers like the Amazon or the Congo, the discharge can create a plume of low-salinity water that extends far out into the ocean. This effect is particularly noticeable in semi-enclosed basins like the Baltic Sea, where high river inflow results in some of the lowest average salinities in the world’s oceans.
The melting of ice also contributes significant volumes of fresh water, especially in polar and subpolar regions. When sea ice melts or when glaciers and ice sheets discharge icebergs that melt, the resulting fresh water mixes with the surface layer, lowering its salinity and density.
The Geological Origin of Ocean Salts
While the factors of water removal and addition govern the concentration of salt at any given time, the source of the salt itself lies in long-term geological processes. The primary origin of ocean salts is the chemical weathering and erosion of rocks on land. Rainwater, which is slightly acidic due to dissolved carbon dioxide, slowly breaks down continental rocks over millennia.
This process releases mineral ions, such as sodium, chloride, calcium, and sulfate, into streams and rivers. The rivers then carry these dissolved solids into the ocean basins, where they accumulate over vast timescales. The total amount of salt in the ocean has remained relatively stable for millions of years, as the input from weathering is balanced by processes that remove salt from the water, such as its incorporation into marine sediments.
A secondary but significant source of dissolved minerals comes from hydrothermal vents located along mid-ocean ridges on the seafloor. Seawater seeps into cracks in the oceanic crust, where it is superheated by underlying magma. This hot water reacts chemically with the crustal rock, dissolving large amounts of metals and minerals before being spewed back into the ocean through the vents. This process effectively cycles minerals between the seafloor rock and the ocean water. Together, the slow, continuous delivery of weathered materials from the continents and the direct input from seafloor activity have established the foundation for the ocean’s overall saltiness.