All atoms of a chemical element contain the same number of protons, but they may contain different numbers of neutrons and thus have different masses. Forms of elements with differing masses are called isotopes. Oxygen in nature always has eight protons, but it may have eight, nine, or ten neutrons for a total mass of 16, 17, or 18. Out of every 100,000 oxygen atoms, almost all are oxygen 16 (O16). Only 205 are oxygen 18 (O18), and only 38 are oxygen 17 (O17). Isotopes of an element are chemically identical, but since they differ in mass, atoms of lighter isotopes move more readily than heavier ones. Chemical and physical processes can partially sort isotopes of differing weights, a process called fractionation. Since O16 atoms are 11 percent lighter than O18 atoms, water molecules with O16 evaporate more readily, so that variations in the ratio of O16 to O18 can be used to trace movements of water in the environment (Botterill, 2003).

Oxygen isotope analysis is among the most important and most accurate tools available for the study of ancient climates. During ice ages, the evaporation of O16-rich water from the oceans to form ice caps leaves the oceans depleted ofO16 and enriched in O18, so that shells formed by marine organisms during ice ages are enriched in O18. During warm periods, much less water is locked up in ice, so the oceans are richer in O16 and poorer in O18. Oxygen isotope abundances are described in terms of the Vienna Standard Mean Ocean Water (VSMOW), which is actually an average of ocean water composition adopted by the International Atomic Energy Agency. An older term, Standard Mean Ocean Water (SMOW) is still sometimes used. Scientists measure the abundance of isotopes using an instrument called a mass spectrometer. Atoms or molecules are vaporized, electrically charged (ionized), then accelerated by an electric field. They can be sorted either by measuring their speed (heavier atoms or molecules accelerate more slowly) or by deflecting them with a magnetic field (heavier atoms or molecules are deflected less).

During warm periods (when there is less ice on Earth than now), marine shells are up to 0.3 percent poorer in O18 than VSMOW, and during ice ages they are up to 0.3 percent richer (Axelrod, 2004). For example, researchers at Woods Hole Oceanographic Institute have used oxygen isotope data on the shells of marine microorganisms to infer that one thousand years ago, during the Medieval Warm Period, the surface temperature of the Sargasso Sea was about 1œ Celsius higher than it is at present; it was 1œ Celsius lower than present during the Little Ice Age, four hundred years ago.

References

1. Axelrod, Regina S., David Leonard Downie, and Norman J. Vig, eds. The Global Environment: Institutions, Law, and Policy. 2d ed. Washington, D.C.: CQ Press, 2004.

2. Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: CSIRO, 2003.

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