Why the Ocean Is Salty The Slow Chemistry of Earth's Great Solution

Somewhere in the mouthful of seawater you swallowed on your last beach holiday is a sodium ion that has been in the ocean for tens of millions of years. Before that, it was locked inside a rock. Before that, it may have been part of a mountain that no longer exists. In time it will leave the sea again, buried in sediment or tucked into a crystal, and begin another long journey through the planet.

Seawater is usually described as a simple thing: water with salt in it. It is better understood as a vast, slow chemical experiment that has been running for billions of years. Rock, rain, volcanoes, living things, and the heat of the planet's interior all feed into it. The strange part is that after all that time the experiment has settled into a steady state, and the sea tastes much as it did when the dinosaurs looked at it.

This episode follows the salt on its journey. We will start with rain falling on bare stone and travel down rivers to the sea. Then we will see how salt gets out again, through hot springs on the ocean floor, evaporating lagoons, and a nearly vanished Mediterranean. Along the way we will look at how scientists measure something as ordinary as saltiness, and why the answer matters for the planet's currents and climate.