Good question! Salt accumulates in the ocean basins because it is made up of sodium (chemical symbol: Na) and chlorine (chemical symbol: Cl), elements that readily dissolve in water. When water interacts with rocks on the land surface, elements like Na and Cl dissolve in the water and get carried to the oceans. When water evaporates at the surface of the ocean, it leaves the salt (NaCl) behind, so salt accumulates in the ocean water. There is only so much salt that can be dissolved in water though. When no more salt can go into solution (be dissolved), the water has reached salt “saturation”. When the water is salt saturated, the mineral halite (NaCl, which is just table salt) will start to crystallize. The elements Na and Cl can be removed from the ocean in other ways too. Sodium in particular tends to react with volcanic rocks on the sea floor (basalts), and so this can be a “sink” for sodium.

Salt in the ocean is important for us in many ways. Most of all, salt water supports very diverse ecosystems of bacteria, animals, and plants in the oceans. Most organisms that evolved in salt water could not survive in fresh water. We also use ocean water to get some of the salt that we use for consumer and industrial purposes (table salt, for example). Can you think of any other reasons that salt water in the oceans is important to us?

We don't need salt in the ocean. Salt naturally shows up in the earth, and can dissolve in water, so it naturally ends up in the oceans. So since it has been there for billions of years, all life in the oceans has evolved and adapted to live in a salty environment. If you took a fresh water fish and put it in salt water it would die, and the same is true of putting a salt water fish in fresh water. It affects many organisms differently, but the general idea is that organisms are adapted to their environment, and if they are evolved to live in salt water, their bodies need to live in salt water because the way the bodies work are adapted to it.