When Were the Atlantic and Pacific Oceans Oceans Separated by Land?
ES2307  Variations in Salinity and Oxygen Isotopes

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.

Lenni Armstrong, Information, adapted from "From Glaciation to Gateways",
Ocean Drilling Program

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