The ocean covers about 70 percent of Earth’s surface, and if you’ve ever tasted seawater, you know one thing for certain-it’s salty. But what exactly makes ocean water salty, and why does salinity vary from place to place? Understanding ocean salinity is crucial for comprehending ocean circulation, marine ecosystems, and even global climate patterns.
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What makes ocean water salty?
Ocean water contains dissolved minerals, primarily in the form of salts. The two most common elements in seawater, after oxygen and hydrogen, are sodium and chloride, which combine to form sodium chloride-the same table salt we use in our kitchens. These salts didn’t appear in the ocean overnight. Rain falling on land picks up dissolved carbon dioxide from the atmosphere, making it slightly acidic. This acidic rainwater then erodes rocks and chemically breaks them down, carrying dissolved salts and minerals through rivers into the ocean.
Scientists measure salinity as the amount of dissolved salts in seawater, typically expressed in parts per thousand (ppt or ‰). The average salinity of ocean water is about 35 parts per thousand, meaning there are approximately 35 grams of dissolved salts in every kilogram of seawater. To put this in perspective, this equals about 3.5 percent of seawater’s weight.
Six major ions make up about 99.4 percent of all dissolved ions in seawater: chloride, sodium, sulfate, magnesium, calcium, and potassium. Chloride and sodium alone account for more than 85 percent of the ions in the ocean, which explains why seawater has that distinctly salty taste. While other elements exist in trace amounts-including even gold and silver-they contribute little to overall salinity.
Factors affecting ocean salinity
Ocean salinity isn’t uniform across the globe. Several natural processes influence how salty the water becomes in different regions, with evaporation and precipitation being the primary drivers.
Evaporation and precipitation
When water evaporates from the ocean surface, it leaves the salt behind, increasing the salinity of the remaining water. Areas with high evaporation rates, such as the Red Sea and Persian Gulf region, can have salinities around 40 ppt-significantly higher than the ocean average. These regions experience intense sunshine and dry conditions that accelerate evaporation.
Conversely, precipitation dilutes seawater by adding fresh water. Near the equator, tropical regions receive abundant rainfall on a consistent basis, which reduces surface salinity. The balance between evaporation and precipitation creates distinct salinity patterns across different latitudes. Interestingly, while the equator receives high amounts of solar energy, equatorial waters actually show slightly lower salinity than subtropical regions because of the heavy rainfall that dilutes the surface waters.
River influx and ice formation
Rivers continuously discharge fresh water into the ocean, carrying dissolved minerals but significantly diluting coastal salinity. The Baltic Sea, for instance, has a very low surface salinity of around 10 ppt because it’s a mostly enclosed body of water with substantial river input. This demonstrates how freshwater influx can dramatically alter regional salinity levels.
Ice formation and melting also play crucial roles in salinity distribution. When seawater freezes, the ice excludes most of the salt, leaving behind saltier water. This process increases salinity in polar regions during winter ice formation. During summer melting, the fresh water from ice dilutes the surface water, lowering salinity. At the poles, this fresh water from melting ice decreases surface salinity, though winter ice formation increases salinity below the ocean surface.
Wind patterns and ocean currents
Winds influence salinity patterns by driving evaporation and mixing ocean waters. The subtropical high-pressure belts, characterized by calm winds and clear skies, create zones of high evaporation and elevated salinity. These regions, located roughly between 20 and 30 degrees latitude in both hemispheres, experience minimal precipitation and intense solar heating, making them some of the saltiest areas of the open ocean.
Ocean currents redistribute water masses with different salinity levels, creating complex patterns. The Atlantic Ocean is the saltiest of the five ocean basins, partly due to circulation patterns that concentrate saltier water in this basin. Surface currents mix water horizontally, while deeper currents transport water vertically, affecting salinity distribution throughout the water column.
Extreme salinity: inland seas and lakes
While ocean salinity averages 35 ppt, some enclosed bodies of water reach astonishing salt concentrations, creating unique environments where only specialized organisms can survive.
The Dead Sea
Salinity in the Dead Sea reaches around 330 parts per thousand-almost ten times saltier than the ocean. Located between Israel and Jordan, this remarkable water body exists in an extremely hot, arid region where evaporation rates are extraordinarily high. The situation intensified in the 1950s when the Jordan River, which once fed fresh water into the Dead Sea, was diverted for other uses.
With minimal freshwater input and relentless evaporation, the Dead Sea’s water level drops about one meter annually. The deeper waters contain approximately 332 parts per thousand salinity, with high concentrations of magnesium, potassium, chlorine, and bromine. The extreme salinity creates such dense water that people float effortlessly on the surface, but it also makes the environment inhospitable to most life forms-hence the name “Dead Sea.” Only certain microbes can tolerate these harsh conditions.
Great Salt Lake
The Great Salt Lake in Utah has highly variable salinity ranging from 5 to 27 percent (50 to 270 parts per thousand), depending on the lake’s water level. As a terminal lake with no outlet, the Great Salt Lake accumulates minerals brought in by rivers while losing water only through evaporation. The lake’s three major tributaries deposit around 1.1 million tons of minerals annually.
The construction of a railroad causeway across the lake created two distinct sections with different salinity levels. The northern section, receiving less freshwater inflow, typically maintains higher salinity than the southern section. The saltiest regions of Great Salt Lake can be nearly nine times saltier than the ocean, supporting only specially adapted organisms like brine shrimp and certain algae species.
Lake Van
Lake Van in Turkey is the largest soda lake on Earth and one of the world’s largest endorheic lakes. Unlike the Dead Sea and Great Salt Lake, which are primarily sodium chloride lakes, Lake Van is strongly alkaline with pH levels of 9.7-9.8, and its most abundant salts are sodium carbonate and sodium sulfate. This unique chemistry creates a harsh environment where few species can survive.
Lake Van has a salinity of about 23 grams per kilogram, making it significantly saltier than typical ocean water. A prehistoric volcanic eruption blocked the lake’s original outlet, transforming it into a closed basin where salts accumulate over time. The lake’s pH of 10 and high salinity levels create extreme conditions, yet the endemic Pearl mullet fish has adapted to survive in these alkaline waters.
Why salinity matters
Ocean salinity plays a vital role in global ocean circulation. Differences in salinity affect water density, which drives the formation of deep ocean currents. These currents, combined with temperature-driven circulation, create a global conveyor belt that distributes heat around the planet and influences climate patterns. The sinking of dense, salty water in polar regions drives deep ocean circulation that can take centuries to complete its cycle.
Salinity also profoundly affects marine life. Different organisms have adapted to specific salinity ranges, and changes in salinity can stress or eliminate species from affected areas. Coastal ecosystems, where fresh and salt water mix, are particularly sensitive to salinity fluctuations caused by human activities like water diversion and dam construction.
What do you think? How might climate change, with its effects on evaporation and precipitation patterns, alter ocean salinity in the coming decades? Could the increasing diversion of rivers for human use create more hypersaline lakes like the Dead Sea in other regions of the world?
References
- https://www.noaa.gov/jetstream/ocean/sea-water
- https://www.usgs.gov/faqs/why-ocean-salty
- https://rwu.pressbooks.pub/webboceanography/chapter/5-3-salinity-patterns/
- https://www.britannica.com/place/Dead-Sea/Climate-and-hydrology
- https://en.wikipedia.org/wiki/Great_Salt_Lake
- https://learn.genetics.utah.edu/content/gsl/physical_char/
- https://en.wikipedia.org/wiki/Lake_Van
- https://www.nature.com/articles/s41598-017-00371-w
- https://science.nasa.gov/earth/earth-observatory/lake-van-turkey-92591/
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