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Seawater contains dissolved salts—primarily sodium, chloride, magnesium, and sulfate—in roughly constant proportions, totaling approximately 35 parts per thousand (ppt). Salinity variations between regions affect density structure, create stable or unstable stratification, and influence biological productivity and chemical cycling.
If you have studied colligative properties, you know that dissolved solutes change the physical behavior of a solvent — lowering freezing points, raising boiling points, and increasing density. Seawater is the most consequential example of this on Earth: the roughly 35 grams of dissolved salts in every kilogram of seawater fundamentally alter its density, freezing point, and ability to absorb gases, with cascading effects on ocean circulation, climate, and life.
The composition of seawater is remarkably uniform. About 86% of the dissolved material is sodium chloride, with the remainder dominated by magnesium, sulfate, calcium, and potassium ions. This consistency is described by the principle of constant proportions (Marcet's principle): while total salinity varies from place to place, the ratios between major ions remain nearly fixed. This constancy exists because the residence times of these ions in the ocean — millions of years for sodium and chloride — are far longer than the ocean's mixing time of roughly 1,000 years. The ocean mixes itself thoroughly many times over before the composition can drift. This means that measuring just one property, typically chloride concentration or electrical conductivity, allows you to calculate total salinity with high accuracy.
Despite the constant proportions of major ions, total salinity itself varies significantly across the ocean. Surface salinity is highest in the subtropical gyres (around 36–37 ppt) where evaporation exceeds precipitation, and lowest near the equator and at high latitudes (as low as 30–33 ppt) where rainfall and river runoff dilute the surface water. These patterns create horizontal and vertical salinity gradients. Where the surface is freshened by rain or meltwater, a halocline forms — a layer across which salinity increases rapidly with depth. This halocline contributes to density stratification because fresher water is lighter than saltier water at the same temperature.
Salinity's influence on density is the foundation of much of deep ocean circulation. In polar regions, cooling alone increases density, but the formation of sea ice amplifies the effect dramatically: when seawater freezes, salt is excluded from the ice crystal lattice and rejected into the surrounding water, creating cold, extremely salty brine that is dense enough to sink to the ocean floor. This process of brine rejection is one of the primary mechanisms driving the formation of deep water masses like Antarctic Bottom Water. Salinity also affects the ocean's capacity to dissolve gases — saltier water holds less dissolved oxygen and CO₂ — and influences the osmotic environment that marine organisms must regulate. Understanding salinity and seawater composition is therefore a prerequisite for nearly every other topic in physical and chemical oceanography, from stratification and circulation to the carbonate system and biological productivity.