Isotopes are atoms that have the same atomic number, but a different mass number, which is the number of protons and neutrons. Because the atomic number, or the number of protons, characterizes an element, isotopes are the same element but have a different number of neutrons (van Grieken and de Bruin, 1994).

The dominant oxygen isotope is 16O, meaning it has 8 protons and 8 neutrons, but 18O, an isotope with 10 neutrons, also exists. By discovering the ratio of 16O to 18O in a fossil, scientists can obtain a reasonable estimate for the temperature at the time the organism existed. Instead of just using a simple ratio, scientists compare the ratio of isotopes in the fossil to the ratio in a standard to obtain a value called delta-O-18. The equation to obtain this value is:

Delta-O-18 Equation

Delta-O-18 changes directly as a result of temperature fluctuations, so it provides a very good record of the climate. Oceanic delta-O-18 values that are high represent cold climates, while lower values indicate a warm climate. This trend occurs because of the effects of precipitation and evaporation. Since it is lighter than 18O, 16O evaporates first, so in warm, tropical areas, the ocean is high in 18O. Additionally, as water vapor condenses to form rain, water droplets rich in 18O precipitate first because it is heavier than 16O. Thus, the cold, polar regions are depleted in 18O as it all precipitates out in the lower latitudes, but they are high in 16O. On the other hand, the Tropics possess a large amount of 18O but have little 16O. This state is not permanent, however, because evaporation and precipitation are highly correlated with temperature. Changes in the climate can greatly affect the ratio of 18O and 16O and can alter their distribution throughout the globe.

In addition to the effects of evaporation and precipitation, the amount of ice near the poles affects the delta-O-18 ratio. When temperatures are cold in the polar regions, 16O becomes locked in glaciers after it precipitates in the high latitudes. Therefore, in warmer climates when glaciers melt, they release the 16O that is trapped in them, and the oceans become enriched in 16O. As a result, the amount of ice in the high-latitudes plays an important role in regulating the delta-O-18 value.