In addition to the dating of rocks by measuring amounts of radioactive isotopes and their decay products, isotopes can be useful as indicators of climate variations on Earth over its long history. Here, the key is to use stable isotopes of the same element. The difference in mass between the isotopes leads to separation, called fractionation, of the isotopes in natural systems; the separations in some cases are a function of the climate, specifically temperature.
To use isotopes as climate indicators, four key features are required:
availability of stable isotopes of the same element whose separation depends on temperature
incorporation of the fractionated isotope mixture in some storage medium that is preserved for a long time
ability to measure accurately the ratio of the various isotopes
a means to date, in an absolute or a relative sense, the age of the stored isotope data.
Stable isotopes, seafloor sediments, and climate
Three important elements for tracking climate changes are carbon, oxygen, and hydrogen. Consider the carbon first. Carbon has two stable isotopes, 13C and 12C. Recall that 14C is radioactive and used for dating relatively recent events. Certain biological processes distinguish mass differences in isotopes. We cannot survive on deuterated water (HDO or 1H2HO). Likewise, plants are observed to preferentially take up 12C in carbon dioxide (CO2), and hence preferentially enrich the atmosphere in 13C. The more temperate the climate, the more land area that is available for plants, and the more 12C that is taken up. In ice ages, global plant activity is reduced, and so less 12C is taken up.