Why is the ocean salty?

September 23, 2020 | Environment
Ocean: why is it salty but other water bodies like lakes and rivers aren't salty at all? | Photo: Creative Commons

Have you ever been swimming in the sea, accidentally gotten a mouthful of ocean water, and wondered why the ocean is salty?

Why are the oceans salty but other water bodies like lakes and rivers aren't salty at all? Let's dive into the so-called hydrologic cycle.

We all call the water in lakes and rivers fresh water, even though we can't drink it directly before boiling or cleaning it.

For a long time, people had no idea why ocean water is different from the water in lakes and rivers.

Today, and thanks to special tools and equipment, scientists have a better understanding of the phenomenon.

After carefully testing what we consider fresh water, they found out that this water actually has a small salt amount.

It is not enough to where you'd be able to notice it if you tasted it, but the water in lakes and rivers is a little bit salty.

Actually, even in the tiniest pond or stream, there's still a little bit of salt in the water.

There's one exception - rainwater. When it rains, the water falling from the sky has no salt in it at all. The rain that falls on the ocean has zero salts in the raindrops.

But, even though rainwater is 100 percent completely fresh water, scientists discovered that once rainwater hits the ground, forming puddles or falling into streams and rivers, it starts to become slightly salty.

So, the answer must be on the ground.

Ocean salinity is mainly caused by rain, washing minerals from fissures, cracks, and openings found in mountains, valleys, cliffs, and rocky formations into water.

Rainwater is slightly acidic so, it helps erode rocks faster.

Salt in the sea also comes from runoff from the land surface.

You could say that the world's oceans deposit the salt carried by inland water streams.

And because when ocean water evaporates it doesn't take the salt with it, there's less water and the same amount of salt.

The water gets heavy, and some of the salt submerges to the bottom of the ocean.

Hydrothermal fluids heated by magma from the Earth's core also emerge from cracks and underwater volcanic eruptions on the ocean floor.

The phenomenon causes a series of chemical reactions that also release minerals into the seawater.

Dead Sea: one the world's saltiest bodies of water with 9.6 times more salt than the ocean | Photo: Creative Commons

Seawater vs. Fresh Water

Geologists figured out that most rocks and soil contain small tiny amounts of salt, and when rainwater lands on the ground, it absorbs some of that salt.

As that rainwater trickles into lakes and rivers, it carries the salt with it.

This is why the water in lakes and rivers has a little tiny amount of salt, but rain itself when it's falling has no salt since it hasn't come in contact with the ground yet.

But then why does the water in the ocean have so much more salt than the water in lakes and rivers?

Scientists have not yet solved the mystery completely.

Most rivers and lakes connect to the ocean, and since they are located on the continents or land, which is higher up than the ocean, the water usually flows down into the ocean.

Even though river and lake water only carries a tiny amount of salt, if that water's been flowing for years and years down into the ocean, all that salt keeps getting added to the sea over and over, where it probably never leaves.

Freshwater in rivers and streams don't taste salty because rain is always replenishing them with fresh water.

This is one of the main reasons why the ocean is so much saltier than all the other water on Earth.

Interestingly, seawater wasn't always so salty.

When the Earth's oceans formed around 3.8 billion years ago, they were mostly fresh water because the surface of the planet cooled enough to allow water vapor to liquify.

Salinity Levels

The oceans of the world have different salinity levels from region to region.

Some areas can even be four times saltier than seawater, making it so dense that a submarine can float.

Salinity - the concentration of salt in seawater - varies with temperature, precipitation, and evaporation.

Generally speaking, it presents low levels at the equator and the poles and high levels at mid-latitudes.

The oceans' average salinity is 35 parts per thousand. In other words, 3.5 percent of the weight of seawater comes from the dissolved salts.

One liter of seawater has 35 grams of salts dissolved in it.

A glass of seawater contains chloride (Cl-), sodium (Na+), sulfate (SO24-), magnesium (Mg2+), calcium (Ca2+), and potassium (K+).

That's 99 percent of all sea salts.

The Dead Sea is one of the world's saltiest water bodies with a salinity level of 34.2 percent, which means 9.6 times more salt than the ocean.

Salinity: the Atlantic Ocean is the saltiest ocean in the world

Which Ocean Is The Saltiest?

Of the five oceans of the world, the Atlantic Ocean is the saltiest, followed by the Pacific Ocean, Indian Ocean, Southern Ocean, and the Arctic Ocean.

Because ice is made of water, the Arctic and Antarctica's icy formations are salt-free.

It is also interesting to point out that coastlines also have much fresher waters than in open-ocean locations.

Today, the world's oceans have balanced salt input and output and, therefore, they're not getting saltier.

Researchers estimate that rivers and streams flowing throughout the world carry around four billion tons of dissolved salts into the oceans every year.

Humans should not drink seawater because our bodies cannot process its high salinity levels.

Because our kidneys can only produce urine that is less salty than seawater, to get rid of all the excess salt taken in by drinking seawater, we would have to urinate more water than we drank.

Drinking seawater makes humans thirstier and results in death by dehydration.

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