Nick Shackleton calculated changes in past global ice volume by measuring oxygen isotopes (atoms with different numbers of neutrons in the nuclues) in calcium carbon fossil in marine sediments. (planet3.org)
Some elements have more than one stable (nonradioactive) isotope — variants of the same substance, but with different numbers of neutrons in their atomic nuclei that alter the mass. (scientificamerican.com)
For instance, radiocarbon dating determines the age of biological remains based on the ratio between the carbon isotopes (atoms of the same element with different numbers of neutrons) carbon - 12 and carbon - 14 it holds - this proportion changes as radioactive carbon - 14 breaks down while stable carbon - 12 does not. (seeker.com)