Which of the following describes the effect of cold-period fractionation on seawater isotopes?

• A. The ocean becomes enriched in heavier isotopes
• B. The ocean becomes depleted in heavier isotopes
• C. The ocean's isotopic composition is unchanged
• D. The ocean loses both isotopes

Answer: (A)

Isotopic fractionation explains how different isotopes are partitioned during phase changes and hydrological processes. In colder climates, more water cycles through evaporation and freezing, and the lighter isotope (the one with the smaller mass) tends to be removed more readily into the atmosphere and into forming ice. This preferential removal of the lighter isotope leaves the remaining seawater with a higher proportion of the heavier isotope. So, during cold periods, the ocean’s isotopic composition shifts toward heavier isotopes, meaning the ocean becomes enriched in heavier isotopes.