How do oxygen isotopes reflect historical climate temperatures?

Oxygen isotopes are a key indicator of historical climate temperatures primarily because they respond to changes in ocean water temperature. The two stable isotopes of oxygen—oxygen-16 and oxygen-18—are found in varying ratios in water and ice. When temperatures rise, lighter oxygen isotopes evaporate more readily, leading to a higher concentration of oxygen-18 in the remaining water.

During colder periods, such as ice ages, the heavier oxygen-18 is preferentially stored in ice sheets and glaciers, which alters the ratio of these isotopes in ocean water. By analyzing sediment cores and ice cores, scientists can trace these variations over time. As a result, the fluctuations in oxygen isotope ratios serve as a reliable proxy for past temperatures, allowing researchers to reconstruct historical climate conditions.

The other options do not accurately reflect how oxygen isotopes function in the context of climate history. Levels of atmospheric carbon dioxide do influence climate but are not directly indicated by oxygen isotopes. The presence of ice sheets does affect oxygen isotope ratios, but it is the temperature changes in ocean water that directly relate to the isotopic variations used for temperature reconstructions. Lastly, oxygen isotopes do not remain constant throughout geological periods; rather, they vary significantly in response to climatic shifts.