Where does the salt go?

So now that you have considered the vast quantities of salt that are present in the ocean, let’s get back to why that is.

As you saw in previous videos, the inputs of water to the ocean have a very different composition and only contain a small amount of salt. Rain water and rivers carry small amounts of dissolved salts to the ocean where they are mixed into the vast ocean by the global circulation. This has been going on for billions of years. In this video, Rachel introduces the concept of residence time: the amount of time that a chemical component spends in the ocean before being removed from it. Many river components are removed relatively quickly by biological organisms and other processes, leaving the various salts to build up over time as they are removed more slowly. Rachel also explains how hydrothermal vents are responsible for one of the major process which modifies the content of seawater.

Salt is often thought of as sodium and chloride, but only because it is the most abundant type of salt in the ocean. In fact, there are many types of salt in seawater, including chlorides, sulphates and carbonates. The mineral “halite” (sodium chloride) is often the first to precipitate when seawater evaporates because it is so highly concentrated. But many other minerals, for example, “sylvite” (potassium chloride) and “gypsum” (calcium sulphate), will also precipitate from seawater. We see this in the geological record as mineral deposits on land where oceans once existed but have since dried up entirely and left their salts behind. Here’s a link to some impressive looking salt deposits of all shapes and sizes – enjoy!