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Ocean Salinity
Ocean Salinity
When we talk about ocean salinity, we’re talking about how salty the ocean is. Salinity is a measure of the amount of dissolved salts—mostly sodium chloride (table salt)—in a given amount of seawater. These salts come from the weathering of rocks on land, volcanic activity, and hydrothermal vents on the ocean floor. Rivers carry dissolved minerals into the sea, where they build up over time.
The average ocean salinity is about 35 parts per thousand (ppt). That means if you took 1,000 grams of seawater, about 35 grams of it would be salt. Put another way, the average ocean water is about 3.5% salt.
Salinity, however, is not the same everywhere. It ranges from about 32 to 37 ppt depending on location. For example:
Near the equator, heavy rainfall can lower salinity.
In areas where evaporation is high, like the Red Sea, salinity can be higher.
Near the poles, melting ice also lowers salinity.
Salinity and Density
Salinity and Density
Salts affect how heavy ocean water is. Density is a measure of how much mass is packed into a certain volume. The higher the salinity, the more dissolved salt there is, and the heavier the water becomes. This means saltier water is denser than fresher water.
Temperature also plays a role: cold water is denser than warm water. When you combine both effects, the densest seawater is found where the ocean is cold and salty—for example, in the polar regions where cold temperatures and sea ice formation increase salinity.
This relationship is important because differences in density drive ocean circulation. Dense water sinks, while lighter water rises, helping to create the deep ocean currents that move heat, nutrients, and gases around the planet.
Ultimately: more salt = higher density, and this simple rule helps shape the movement of Earth’s oceans.
Click on the image above for a more in-depth look.
The Dead Sea
The Dead Sea
The Dead Sea, located between Israel and Jordan, is one of the saltiest bodies of water on Earth. Unlike regular oceans, which have an average salinity of about 35 parts per thousand (ppt), the Dead Sea has a salinity of around 300–350 ppt—almost 10 times saltier!
So why is it so salty? The Dead Sea has no outlet. Water flows in from rivers, especially the Jordan River, carrying dissolved minerals and salts. However, the only way water leaves is through evaporation. Because the region is hot and dry, large amounts of water evaporate, leaving the salts behind. Over thousands of years, this process has made the Dead Sea extremely salty.
This unusual salinity makes the water very dense. In fact, the density is so high that people can easily float on the surface without even trying. It’s a famous experience for visitors—like lying on top of the water as if it were a natural floatation device!
Brine Pools
Brine Pools
Deep beneath the ocean, scientists have discovered something strange—brine pools, which are like underwater lakes on the seafloor. These pools are made of extremely salty water, much saltier than the surrounding ocean. Because this briny water is so dense, it doesn’t mix well with normal seawater, creating the appearance of a separate lake under the sea.
Brine pools form when salt deposits buried deep in the seafloor dissolve into groundwater that seeps out. This salty water collects in depressions on the ocean bottom, creating a dense pool. Brine pools are mostly located in the Gulf of Mexico and the Mediterranean Sea, thousands of feet below the surface.
"Hot Tubs of Despair"
"Hot Tubs of Despair"
The “Hot Tubs of Despair” is the nickname scientists gave to certain brine pools in the Gulf of Mexico. These pools are extremely salty and toxic, containing deadly chemicals like hydrogen sulfide and methane. Any sea creatures that accidentally enter them—such as crabs or fish—quickly die, which is why scientists call them “death traps.”
Even though they’re harsh environments, brine pools are fascinating because some unique bacteria and microbes can survive there, offering clues about life in extreme conditions—even on other planets.
Sources of Sea Salts
Sources of Sea Salts
The ocean didn’t start out salty—it became salty over millions of years. The main sources of ocean salts are:
- Weathering of Rocks on Land
Rainwater is slightly acidic, so when it falls on rocks, it slowly breaks them down. This process releases minerals and salts, like sodium and chloride. Rivers then carry these dissolved salts into the ocean.
2. Volcanic Activity (Outgassing)
2. Volcanic Activity (Outgassing)
Underwater volcanoes and hydrothermal vents release minerals, gases, and salts directly into the ocean.
Once salts enter the ocean, they stay there because they don’t easily evaporate with water. Over millions of years, this buildup has made seawater salty, with an average salinity of about 35 parts per thousand.
Salinity Differences
Salinity Differences
The ocean’s salinity isn’t the same everywhere. It can change depending on how much freshwater is added or removed and other environmental processes. Here’s how ocean water can become less salty (decrease salinity) by adding freshwater:
- Precipitation (Rain and Snow)
When rain or snow falls into the ocean, it adds fresh water, diluting the salt and lowering salinity.
2. Melting (Sea Ice or Glaciers)
2. Melting (Sea Ice or Glaciers)
When ice melts, it adds fresh water to the ocean, reducing salinity in the local area.
3. Runoff from Rivers and Streams
3. Runoff from Rivers and Streams
Rivers carry fresh water from land into the ocean. This influx of water dilutes the salt, especially near river mouths.
Here’s how ocean water can become more salty (increase salinity) by removing freshwater:
- Evaporation
In hot, dry regions, water evaporates from the ocean surface, leaving the salts behind. This increases salinity.
2. Freezing (Sea Ice Formation)
2. Freezing (Sea Ice Formation)
When seawater freezes to form sea ice, the salt is mostly left behind in the liquid water. This makes the surrounding water saltier.
These changes in salinity also affect the density of seawater, which helps drive ocean currents and circulation patterns around the globe.
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