The saltiness of seawater

We have already know that cold water is heavier than warm and tends to sink. Salt, too, can make water heavy. On the whole, the proportion of salt in the open sea stays close to 3.5 percent. Near melting polar ice, however, the water tends to be less salty because the ice that is melting is nearly fresh. By contrast, the water near ice that is begin¬ning to form will have more than an average amount of salt since the ice that is in the process of freezing leaves extra salt behind in the water. Since this kind of water—both cold and salty—will sink the deepest, the heaviest water is found at the very bottom of the sea. Heavy water of this sort, loaded with salt from beneath the antarctic ice shelf, rides the ocean floor all the way to the equator and across it into the Northern Hemisphere. The ride takes a long time. Some scientists figure that 300 years pass before a bit of cold, salty, deep water goes from the Antarctic to the equator. Others say that it takes 1,500 years. By contrast, a bit of warm, relatively unsalty water may take only a year to make the surface circuit of the North Atlantic wheel.
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