During cold periods, the ratio of 18O to 16O in seawater changes in which direction?

• A. Increases
• B. Decreases
• C. Stays the same
• D. Becomes zero

Answer: (A)

Oxygen isotopes in the ocean respond to how water moves and freezes in the climate system. When temperatures drop and more water becomes ice on land, the freezing process preferentially locks up the lighter isotope, 16O, into the solid ice. This leaves seawater relatively enriched in the heavier isotope, 18O, so the ratio of 18O to 16O in seawater increases during cold periods. This shift is a clean signal of global cooling and is also reflected in marine carbonates, which record the seawater isotopic composition as they form. If the ratio were to decrease or stay the same, that would imply less removal of 16O into ice or no fractionation at all, which doesn’t align with how freezing concentrates the lighter isotope in solid water.