Final answer:
During periods of cooling, oxygen-18 becomes more abundant in seawater due to the preferential evaporation of lighter oxygen-16, while during warming, oxygen-18 becomes depleted as glaciers melt.
Step-by-step explanation:
The traditional explanation that is being referred to in the question is based on the ratio of oxygen isotopes found in seawater and ice cores. During periods of cooling, or glacial advance, oxygen-18 (written as ¹⁸O) becomes more abundant in seawater. This is because oxygen-16 (¹⁶O), which is lighter, preferentially evaporates and then gets stored in ice sheets on continents. Conversely, during periods of warming, the glaciers melt, and the stored ¹⁶O is released back into the ocean, making seawater relatively depleted in ¹⁸O. Scientists have often used the relative abundance of these isotopes as indirect evidence to reconstruct past climate changes, like those that occurred during ice ages.