Topics Earth, weather and climate

Why is the ocean salty?

The ocean is salty mostly because rivers have been carrying dissolved minerals from rock into it for a very long time. Rain is slightly acidic, so it slowly breaks down rock on land, and the freed minerals travel downhill to the sea. Water leaves the ocean again by evaporation, but the salt stays behind. Undersea vents and volcanoes add more dissolved material from below.

The puzzle is that river water tastes fresh, even though it is the main delivery route. The amounts in any one river are tiny, but the ocean is enormous and has been collecting for hundreds of millions of years or more. Then there is a second puzzle: if salt keeps arriving, why doesn't the sea keep getting saltier? The usual answer is that salt is also removed, by sinking into seafloor sediments and reacting with new crust. Scientists generally think the saltiness has been fairly steady over long stretches of time, though the details are still studied.

An episode would walk through that delivery and removal system, explain why sea salt is mostly the same table-salt ingredients you know, and look at places where the water is far saltier or fresher than average. The hosts on bre are AI, so they can get things wrong, and you can press Talk to ask them anything that is unclear.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Rain, rock and a slow dissolveHow slightly acidic rain breaks down rock on land and washes dissolved minerals into streams and rivers.
  2. Why rivers taste fresh anywayThe salt in a river is real but tiny. The ocean is huge, and it has been collecting for an immense span of time.
  3. Salt from below the wavesHot vents on the seafloor and volcanic activity add dissolved material to the water from underneath.
  4. Evaporation leaves the salt behindWater leaves the surface as vapor, falls as rain, and the dissolved minerals stay in the sea.
  5. Why it doesn't keep getting saltierSalt is also removed through sediments and seafloor chemistry. Scientists think the balance has been roughly steady, though the details are debated.
  6. Saltier and fresher seasWhy the Dead Sea is far saltier than the open ocean, and why places near big rivers or melting ice are less salty.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, picture a glass of tap water and a mouthful of ocean. One of them has been collecting salt for a very long time. So here's the question you probably have: where did it all come from?

  2. CalAI host

    Rocks. Which still sounds wrong to me. I always assumed the sea was just born salty, like it came that way out of the box, and there was no real story to it.

  3. breAI host

    Nope, there's a story. Rain is a little acidic, so it slowly eats at rock on land. That frees up minerals, and rivers carry them downhill to the sea.

  4. CalAI host

    Hold on, how does that actually work? Rivers are fresh. I've drunk from a few, and none of them tasted like soup.

  5. breAI host

    Right, and that's the catch. Each river carries a tiny amount. But the ocean is enormous, and it gets that delivery for an unimaginably long time.

  6. CalAI host

    So it's like dropping a pinch of salt in a bathtub, except the bathtub never drains and you keep going for hundreds of millions of years.

  7. breAI host

    Close, but the water does leave. It evaporates, and the salt can't go with it. That's the part that makes the ocean a one-way collector.

  8. CalAI host

    Then why isn't it just getting saltier forever? That feels like where the plot thickens.

Questions people also ask

What is the salt in the ocean made of?
Mostly sodium and chloride, the two ingredients of table salt, along with smaller amounts of magnesium, sulfate, calcium and potassium. The mix of dissolved minerals is remarkably similar across the open ocean, even though total saltiness varies a little from place to place.
Why don't rivers taste salty if they bring the salt?
Rivers carry only a very small amount of dissolved minerals at any moment, too little to taste. The ocean is huge and has been receiving that supply for a vast span of time, and evaporation leaves the minerals behind, so they gradually build up.
Is the ocean getting saltier over time?
Scientists generally think the ocean's overall saltiness has stayed fairly steady over long periods, because salt is removed as well as added, for example into seafloor sediments. Local saltiness does change with evaporation, rainfall, river flow and ice melt.
Why are some seas much saltier than others?
Where evaporation is strong and little fresh water flows in, salt becomes concentrated. The Dead Sea is a famous example. Near large rivers or melting ice, extra fresh water dilutes the sea, so the surface water is less salty.

Related topics

More: all 300 topics, earth, weather and climate, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.