Final answer:
A higher ratio of oceanic 18O to 16O indicates a warmer period during which more 18O evaporates and precipitates onto ice sheets.
Step-by-step explanation:
A higher ratio of oceanic 18O to 16O indicates a warmer period during which more 18O evaporates and precipitates onto ice sheets. Oxygen-16 (¹60) is lighter than oxygen-18 (¹80), so when water molecules containing oxygen-16 evaporate from the oceans, they preferentially leave behind oxygen-18. This results in a higher ratio of 18O to 16O in the ocean during warmer periods. When the evaporated water vapor condenses into rain or snow, the heavier oxygen-18 isotopes preferentially do so, leading to more 18O being stored in ice sheets on continents during colder times.