You know that water is made of hydrogen and oxygen atoms arranged in molecules of H₂O, but you may not be aware that there are different types of oxygen atoms. Different atoms of the same element are called isotopes. All oxygen atoms have 16 protons and 16 electrons, but some oxygen atoms have 16, 17, or 18 neutrons in the nucleus. The most abundant isotopes of oxygen in seawater are oxygen sixteen (¹⁶O) and oxygen eighteen (¹⁸O).

Water molecules with ¹⁶O atoms evaporate more easily than water molecules with ¹⁸O atoms, so the relative numbers of ¹⁶O and ¹⁸O atoms that remain in the water change as evaporation occurs. Water from which ¹⁶O atoms have preferentially evaporated has a higher ratio of ¹⁸O to ¹⁶O atoms than water that has experienced less evaporation. As salinity also increases as evaporation occurs, we can generalize the relationship to state that water with an increased ¹⁸O to ¹⁶O ratio is saltier than water with a lower ¹⁸O to ¹⁶O ratio.

  !   Click the red dot in the Caribbean to see why shells formed there have increased amounts of ¹⁸O. Click the red dot in the Pacific to see why shells formed there have increased amounts of ¹⁶O.