Which statement best describes the isotopic distribution between ice and ocean during cold periods?

• A. Ice is enriched in light isotopes
• B. Ice is enriched in heavy isotopes
• C. Ocean is enriched in light isotopes
• D. Ocean loses all isotopes

Answer: (A)

During freezing, lighter isotopes are favored in the solid phase, so ice forms with fewer heavy isotopes and relatively more light isotopes compared with the surrounding ocean. In cold periods, a lot of ice is formed, locking up light isotopes in the ice and leaving the ocean comparatively enriched in the heavier isotopes. This makes the ice lighter in isotopic composition than the ocean, which is why paleoclimate records show ice enriched in light isotopes during colder times. So the statement that ice is enriched in light isotopes best describes the distribution.