Which oxygen isotope is more likely to be found in ice sheets during cold periods, leaving the ocean 'heavy'?

• A. Light oxygen (16O)
• B. Heavy oxygen (18O)
• C. Oxygen-17
• D. Oxygen-15

Answer: (A)

During the hydrological cycle, isotopes are fractionated by phase changes. When water evaporates, the lighter isotope tends to go into the vapor, and when vapor condenses into precipitation, the heavier isotopes condense first. In cold periods, snow and ice form from this precipitation and preferentially lock in the lighter isotope, 16O, into solid water. As a result, the ice sheets become depleted in the heavy isotope 18O, meaning they are enriched in the light isotope. The ocean, meanwhile, loses more 16O to the ice, so it becomes relatively heavier in 18O. This is why ice sheets during cold periods accumulate light oxygen, while the ocean ends up with a higher 18O content. Other oxygen isotopes like 17O exist but are much less abundant and not the primary signal in this climate context; 15O is radioactive and not used for these paleoclimate comparisons.