Answer;
-past temperatures
The ratio of oxygen-16 and oxygen-18 isotopes in plankton fossils in deep-sea sediments can be used to determine past temperatures.
Explanation;
-O-16 will evaporate more readily than O-18 since it is lighter, therefore; during a warm period, the relative amount of O-18 will increase in the ocean waters since more of the O-16 is evaporating.
-Hence, looking at the ratio of O16 to O18 in the past can give clues about global temperatures.