Which process is associated with higher 18O content in ocean water during glaciation?

• A. Evaporation of water preferentially removes 18O
• B. 18O enrichment occurs due to biological fractionation
• C. 16O is trapped in ice sheets
• D. 18O decays to 14C

Answer: (C)

Isotopic fractionation during phase changes explains why ocean water becomes enriched in 18O when large amounts of water freeze. When sea water freezes to form ice sheets, the lighter isotope (16O) preferentially enters the solid phase, while the heavier isotope (18O) is left behind in the liquid. As a result, the ocean becomes relatively richer in 18O during glaciation. This shift is recorded in marine minerals and is used as a paleotemperature proxy for cold periods with more extensive ice cover. The idea that 16O is trapped in ice sheets captures this dominant mechanism. 18O is stable and does not decay to 14C, so option describing decay isn’t valid. Biological fractionation isn’t the primary driver of the large, global 18O increase in ocean water during glaciation, and evaporation’s role, while it can affect isotopes, does not explain the main signal as directly as the freezing of 16O into ice sheets.