Final answer:
The ratio of Oxygen-18 (¹⁸O) to Oxygen-16 (¹⁶O) in water changes with climate and is utilized to describe past climates. Lighter ¹⁶O evaporates more easily, but during colder climates, it is stored in ice, thus increasing the ¹⁸O/±⁶O ratio in seawater.
Step-by-step explanation:
The ratio of Oxygen-18 (¹⁸O) to Oxygen-16 (¹⁶O) in water changes with climate and is a widely used measure to describe climates of the past. The isotopic composition of oxygen in foraminifera, organisms with calcium carbonate shells, reflects the temperature of the seawater in which they lived. This is because ¹⁶O is lighter and evaporates more readily, but during colder climates, more ¹⁶O is stored in ice sheets, increasing the ratio of ¹⁸O to ¹⁶O in the remaining seawater. Mass spectrometry separates these isotopes in samples such as glacial ice, allowing scientists to reconstruct past climate conditions based on the recorded oxygen isotope ratios within the ice.