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“Why is the sea salty?” seems like one of those questions a preschooler asks
pretty early on, right after “why the sky is blue?”.
But here, the answer is relatively straightforward.
Rain erodes salts out of rocks and rivers transport this salt
to the oceans, where it accumulates.
Globally, rivers put four billion tonnes of salt into the sea every year.
But then, when it evaporates back out of the ocean into rain,
it doesn’t take the salt with it.
Most of this is sodium and chlorine, together as sodium chloride;
what we know as table salt.
But there are also smaller amounts of magnesium, sulfate, calcium, and potassium
ions, among others, all of which chemists file under the umbrella of “salts”.
If you took a kilogram of seawater from the ocean today and evaporated it,
you would be left with about 35 grams of all of these salts.
But this is one of those cases where a simple question
and a fairly simple answer leads to a way more interesting question.
With rivers adding all this salt every year, are the oceans getting saltier?
At one point in the past, scientists thought they were.
In 1715, Edmund Halley (yes, the guy with the comet) proposed
using the difference between the salinity of the ocean and the rivers running into it
to calculate how old the Earth is.
It’s a smart idea.
If the rivers add a bit of salt to the oceans every year,
you could backtrack and figure out when a fresh ocean first formed.
This calculation says that the Earth, or at least its oceans,
are somewhere between 25 and 150 million years old.
Take that with a very big grain of salt, though.
With all due respect to the astronomer who did many amazing things,
including naming that comet, we now know that this figure is very very wrong.
Many other pieces of evidence tell us that
the Earth is much older, around 4.5 billion years.
The oceans formed by 3.8 billion years ago,
and some believe that the early ocean was actually saltier than today.
The issue is that this calculation did not consider the ways salt
can be removed from the oceans.
See, salinity is balanced.
Sources like rivers that add more salt are
counterbalanced by sinks, which remove it.
Sinks like evaporite deposits.
These form when an area of water is cut off from the rest of the ocean
and the water evaporates, leaving behind salt flats.
Around five and a half million years ago,
the Mediterranean Sea was cut off from the rest of the world
and almost completely dried up.
This took about 10% of the salt out of the world’s oceans!
The Mediterranean got better, but today the Dead Sea fills a similar role.
Hydrothermal vents also have an impact.
They heat up seawater as it flows through rocks near magma and
cause chemical reactions that remove magnesium
and sulfate (again, salts) from seawater.
But because the total amount of salt in the ocean is large compared to what
gets added or removed each year, the balance is remarkably consistent overall.
So the ocean stays saltier than rivers, but doesn’t get much saltier over time.
However, salinity can still change from place to place over time.
That is because salinity is the amount of salt in water,
so if you change the amount of water, you can change the salinity.
Rain and rivers add fresh water.
Intense rain like monsoons can have a big effect on the nearby ocean.
Evaporation can also take water away and increase the salinity of a particular area.
Ice can also be a big factor.
When seawater freezes into ice, that salt stays behind
in a process called brine rejection.
So sea ice forming can increase the salinity of an area,
and any ice melting can add fresh water and decrease it.
And all of these processes happen at the ocean’s surface,
so they affect the saltiness of about the top 200 meters of water.
Add all these up and you see some global patterns.
The surface ocean is saltier as you get closer to the equator,
since it’s warmer and evaporation rates are higher than near the poles.
The exception is right at the equator, where there’s a lot of rain and lower salinity.
For example, the Baltic Sea has many rivers flowing into it,
so its salinity is lower than average.
The Mediterranean and Red Seas have lots of evaporation,
so their salinity is higher.
So even though the ocean’s overall salinity is relatively constant,
smaller fluctuations can tell us a lot;
about the oceans, and about the water cycle worldwide.
Also, the climate crisis is already having an impact on salinity.
Researchers have noticed that the saltiest areas of the ocean are getting saltier,
and the freshest spots are getting fresher.
This means that overall, both evaporation and precipitation rates are going up.
And this could mean both more intense droughts and floods on land.
Salinity itself can also have a direct effect on climate.
Salinity and temperature are the primary factors
that determine the density of seawater.
Denser water will, of course, sink to the bottom of the ocean,
and this movement drives global ocean circulation,
which has a significant effect on how heat flows around our planet.
All of this means that understanding these salinity patterns
throughout the oceans and how they are changing is crucial.
So in 2011, NASA launched the Aquarius satellite,
which can measure the ocean’s surface salinity from space.
This probe can survey the salinity of the entire globe in a week!
And that means we should be able to track how the water cycle is changing,
and build better models to try and predict how that will affect climate.
And so, from asking why the obvious doesn’t happen,
why the ocean doesn’t get saltier when it seems like it should,
we get new ways to understand and monitor the effects of the climate crisis.
Preschoolers ask great questions, sometimes.
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