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Why Is the Ocean Salty?

Salt journeys around the planet, leaving its mark at every step — before it's ultimately deposited into the ocean. Here's how the process works, and why the ocean's salt content may be more important than you think.

By Cody Cottier
May 9, 2024 3:00 PM
In the Plemmirio natural reserve in Sicily, Italy, I found some salt on the rocks left by the sea water after the tide. The arid landscape and the power of nature in this place are impressive.
(Credit: Eug Png/Shutterstock)


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If you’ve ever had the misfortune to swallow a mouthful of seawater, you have some idea of its intense salinity. One small gulp is bad enough, but the total salt content of Earth’s oceans is truly staggering: Based on an average of 7 tablespoons per liter, scientists have calculated that they hold about 50 quadrillion tons (that’s 15 zeros) of dissolved salt.

More incredible still, it wasn’t always there. It trickled slowly into the ocean — grain by grain, year by year — from the mountains, hills and plains of the terrestrial landscape. To see why, we first need to understand a few things about salt.

Where Salt in the Ocean Comes From

Salt is much more than a dinner-table seasoning. In chemistry, the term refers to any compound with positively and negatively charged ions — not just the sodium chloride we use to enhance our cooking, but also magnesium, sulfate, potassium and many others.

These diverse salts are present in rocks all over the world. And as natural forces like freezing and thawing break rocks down into smaller and smaller chunks, their minerals (salt included) are flushed toward the sea by erosion.

All along the way, thanks to their ions, salts are also susceptible to a second destructive process: chemical weathering. The hydrogen atom in each water molecule has a negative charge, and the oxygen atom has a positive charge. Because opposites attract, rain and rivers are able to surround and dissolve salt ions as they wash over the landscape.

For that matter, you find the same effect in underground volcanoes. “It’s happening anywhere that water gets in touch with rocks,” says Colin Stedmon, a chemical oceanographer at the Technical University of Denmark. “Water is the ultimate solvent.”

Read More: Salt Played a Pivotal Role in Ancient Human History

Why Don't Lakes and Rivers Have As Much Salt?

Every drop of that salty solution eventually winds up in the ocean. Yet it all comes from rivers, and somehow they seem unaffected by their brackish cargo.

It turns out there's a bit of salt in every supposedly “fresh” body of water. It’s not enough for your tastebuds to detect, but when those trace amounts reach their final destination, they combine to pack a seriously briny punch.

That’s because once salt is deposited in the ocean, it gets even more concentrated as water evaporates into the atmosphere — a new generation of rain soon heads off to erode the next batch of minerals, while salt crystals get left behind.

Read More: Why Are Humans So Drawn to Water?

Sea Salt

They don’t stay there forever, though. Over time, various natural processes remove salt from the oceanic system. In shallow coastal waters, the concentration can get so high that no more can be dissolved, and some then precipitates to form a layer on the bottom. That’s where we get our beloved sea salt, with its distinct, complex flavor.

Ocean water also seeps into deep-sea fissures, dragging salt down into the inner mantle, where it may once again find its way into rocks that will someday resurface on the continents. When that happens, the cycle restarts anew.

Still, there’s a significant mismatch between water and salt cycles (according to Stedmon, the latter is thousands of times slower). Water is in constant flux, while salt takes eons to move from place to place. As a result, the ocean has become incredibly saline, while our inland water sources remain drinkable.

Despite their vastly different timelines, however, the cycles have reached equilibrium. The ocean could dissolve far more salt than it currently contains, but it isn’t getting any saltier. However much the rivers bring, a roughly comparable amount gets withdrawn. “There’s a very slow supply,” Stedmon says, “balanced with a very slow removal.”

Read More: Companies, Conservationists Battle for the Deep Sea

Why the Ocean's Salt Content is Important

Salt may be one of the most overlooked players on the world stage. It doesn’t just float around without purpose; in fact, it’s responsible for much of Earth’s weather.

Ocean currents, which circulate warm and cold water around the planet, are crucial regulators of the global climate. And those currents, in turn, are created mainly by wind, water temperature, and — you guessed it — salinity.

Just as meteorologists measure air temperature to predict what will happen in the atmosphere, oceanographers measure these factors to model the future activity of currents (which, as already noted, also influences atmospheric weather).

The oceans are actually dotted with autonomous robotic sensors from the international ARGO program. They routinely sink down thousands of feet, like weather balloons in reverse, to gather data. Because we know precisely how well saltwater conducts electricity, they pump some through a conductivity cell to test its salinity. Then they relay the results to weather stations.

The forecasts they help generate are essential for all our interactions with the ocean, from shipping to wave energy harvesting. Currents also transport the nutrients that sustain marine life, meaning we can use them to track fish populations.

As Stedmon puts it, “there’s a direct link between [current] circulation and how we use the sea.” And, he adds, an equally important link between circulation and salt.

Read More: Why It's Alarming That Deep-Sea Currents are Beginning to Slow

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.

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