The Earth’s oceans, covering over 70% of the surface, are characterized by their saltiness. This saltiness comes from a complex mixture of dissolved substances vital to marine ecosystems and global climate. Understanding the amount and origin of ocean salt provides insight into Earth’s largest water bodies.
The Salt Content of Ocean Water
On average, ocean water contains approximately 35 grams of dissolved salts per liter, which translates to about 3.5% salinity or 35 parts per thousand (ppt). A single U.S. gallon of average ocean water holds roughly 132.5 grams of salt, equivalent to about 0.29 pounds. While sodium chloride (table salt) is present, sodium and chloride ions are the most abundant, making up around 85% of all dissolved ions.
The dissolved components in seawater include a variety of inorganic ions. Other major ions like sulfate, magnesium, calcium, and potassium are also present. These six major ions collectively account for approximately 99% of the total dissolved solids in the ocean.
The Origins of Ocean Salt
The salt in the ocean originates primarily from two main geological processes: runoff from land and activity at the seafloor. Naturally acidic rainwater falls on land and weathers rocks. This process dissolves minerals, releasing ions carried by rivers into the oceans. While rivers are only slightly salty, continuous input over long periods has significantly contributed to ocean salinity.
Another source of ocean salt comes from hydrothermal vents and volcanic activity on the seafloor. Seawater seeps into cracks, becomes superheated, and reacts with rocks, dissolving minerals like iron, zinc, and copper before being expelled through vents. Submarine volcanic eruptions also release minerals directly into the ocean.
Factors Influencing Salinity and Its Importance
Ocean salinity is not uniform across the globe; environmental factors cause regional variations. Evaporation increases salinity by removing fresh water, leaving dissolved salts behind, leading to higher concentrations in areas like tropical waters. Precipitation dilutes surface waters and decreases salinity. Freshwater runoff from rivers also lowers salinity, particularly near coastlines and river mouths.
The formation and melting of ice also influence salinity. When seawater freezes to form sea ice, most salt is excluded from the ice crystals. This increases the salinity and density of the surrounding water, causing it to sink. Conversely, melting sea ice or land-based glaciers releases fresh water, diluting the ocean and decreasing salinity in those areas.
Ocean salinity influences ocean dynamics and marine life. Salinity, with temperature, determines seawater density. Denser water sinks, driving large-scale ocean currents (thermohaline circulation) that distribute heat and nutrients globally. These currents influence global climate patterns.
Salinity levels are important for marine organisms. Marine life adapts to specific salinity ranges; rapid changes can cause stress. These fluctuations affect growth, reproduction, and survival, impacting species distribution and ecosystem structure. Salinity also affects oxygen solubility, impacting organisms that rely on dissolved oxygen.