Stable isotopes of the light elements are useful tracers of a variety of processes that occur at the surface of the Earth. If two chemical species containing isotopes of the same element (e.g. calcium carbonate and water) have attained isotopic equilibrium with each other, then the difference in their isotopic ratios is a function of the temperature at which they equilibrated (O'Neil 1986). Variation in abundances of stable isotopes can also be used as natural isotope tracers of the sources of the respective elements (oxygen, carbon, etc.) The first practical application by Epstein et al. (1953) made possible the determination of the temperatures of growth of some fossil marine organisms (and hence of the sea in which they lived).