Thundahcaxzd t1_ja6yajf wrote

The lighter oxygen isotope O16 evaporates more readily than the heavier O18. Thus, when temperatures are higher and there is more evaporation, O18 gets concentrated in liquid water and O16 gets concentrated in the atmosphere. the water in the atmosphere falls as precipitation and some of it falls on ice caps and stays there. In the ocean, organisms such as foraminifera use water to help build their shells. These shells settle into the sediment after they die. Scientists drill cores into the ice caps and ocean sediments. Using a mass spectrometer, they measure the ratio of oxygen isotopes in the ice and foraminifera shells which gives us a proxy of how much evaporation was taking place, and therefore the temperature.