Precise sampling, interpretation needed for oxygen isotope analysis

November 7, 1995

EDITORS: A black-and-white photograph of Henry Fricke with a bison jaw used in his study is available on request.

NEW ORLEANS—University of Michigan geochemists caution that a promising new paleoclimatological research technique, which uses the stable isotope chemistry of mammalian tooth enamel to determine past climate changes, may be much more complicated than researchers initially believed.

According to Henry C. Fricke, U-M graduate student in geochemistry, the handful of researchers working in this field have generally operated under the assumption that changes in the oxygen isotope ratio of tooth enamel were caused only by changes in local climate—primarily temperature variations over extended periods of time.

In a recent detailed analysis of bison teeth from a 500-year-old archaeological site in Wyoming and teeth from a domesticated modern sheep living on a California ranch, however, Fricke and James R. O’Neil, U-M professor of geological sciences, observed that the oxygen isotope ratio can vary significantly between teeth—and even within a single tooth—from the same animal.

” For a given deposit, seasonal climate changes, the animal’s season of birth, and its behavioral patterns all can cause large inter- and intra-tooth variations in the oxygen isotope ratio of tooth enamel,” Fricke said. ” Unless scientists are aware of these factors, they may draw the wrong conclusions from their data.”

Fricke and O’Neil presented the results of their study—the first in which multiple oxygen isotope ratios of tooth enamel were obtained from a single high-crowned tooth—at the Geological Society of America meeting held here Nov. 6-9.

According to Fricke, this research does more than point out potential problems. It also opens up avenues for future research. ” It really is a bonus, because it gives us the potential to get twice as much climate information from the same material,” Fricke said. ” The technique is still valuable for tracking long-term climate change, but now we can also study seasonal variations, determine the animal’s season of birth and death, and possibly learn about mammalian migration patterns. It just means we must be careful to sample consistently and to understand which teeth provide the most reliable record of climate changes over different periods of time.”

In future studies, U-M researchers hope to use this type of oxygen isotope analysis to study remains from archaeological sites in northern Europe. ” Tooth enamel grows too slowly to provide reliable information on short-term climate changes, but analyses of human teeth and those of domesticated animals associated with human settlements in restricted areas over extended periods of time will give us insights into the effects climate change may have had on human societies,” said O’Neil. The research project was funded by the National Science Foundation.