Dinosaur Tooth Enamel: A Key To Investigating Ancient Climate On Land

In a paper presented October 28 at the Geological Society of America national meeting in Toronto, Henry Fricke, a National Science Foundation postdoctoral fellow working at the Geophysical Laboratory, Raymond Rogers, a paleontologist from Macalester College, and Douglas Rumble, a Geophysical Laboratory staff member, presented the first results of their study of climate conditions on land during the Late Cretaceous (~75 million years ago). Data from three geographically widespread localities in North America indicate that temperatures in middle latitudes may not have been dramatically different than those of today, which contrasts with the globally warmer temperatures inferred for time periods both before and after the Late Cretaceous. However, data from Madagascar indicate that climate conditions in sub-tropical latitudes may have been warmer and more arid during the Late Cretaceous than at present. Equally important, Fricke and coauthors demonstrate that these results can be obtained by analyzing the chemical composition of oxygen found in dinosaur tooth enamel.

The key to infer past temperatures using tooth enamel is in measuring small differences in the chemical composition of oxygen in dinosaur tooth enamel, in particular, differences in the ratio of heavy (oxygen-18) to light (oxygen-16) oxygen. This 'oxygen isotope ratio' of enamel is directly related to the isotopic ratio of rain and snow falling on a local area. Dinosaurs drink and ingest water from the streams, ponds, and plants that have an ultimate source in local precipitation, and then incorporate oxygen from this water into their tooth enamel. The oxygen isotope ratio of precipitation is, in turn, strongly influenced by climatic factors, primarily temperature, thus providing the connection between climate and tooth enamel. Fortunately the oxygen isotope ratio of tooth enamel does not change over the millions of years that a dinosaur fossil is buried, so it is possible to use this ratio to infer surface temperature when the dinosaur was actually alive.

The theropod dinosaur teeth included as part of this study are only about one inch in length, and are covered by only a thin layer of enamel. It was therefore necessary to develop a new method of measuring the oxygen isotope ratio of very small samples of enamel. Working with Douglas Rumble at the Geophysical Laboratory, Fricke has overcome these problems by using a high-powered excimer laser to remove small amounts of enamel from the tooth surface. The laser produces a beam of ultra-violet light about 200 microns in width that is used to vaporize a narrow and shallow band of enamel; oxygen in released from the vaporized enamel, and the isotopic ratio of this oxygen is measured by a mass spectrometer. Using this new microanalytical technique, it is now possible to minimize damage to the surface of the tooth while at the same time increasing the spatial resolution of samples, something that could not be done previously.

By its nature, climate research utilizing oxygen in tooth enamel is intimately associated with the animals themselves, where they lived, what they ate, and how their bodies functioned. Therefore another very important aspect of inferring temperature and reconstructing temperature gradients using this method is trying to unravel the connections between dinosaur ecology and evolution, and climate change over time. In order to do this, Fricke and coauthors plan to expand their investigations of climate on land to include other time periods by taking advantage of the geographically widespread occurrence of dinosaur teeth in the fossil record for most of the Mesozoic era (65 to 248 million years ago).

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The Carnegie Institution of Washington's Geophysical Laboratory, led by Wesley T. Huntress, has a long history of interdisciplinary research in geology, biology, and chemistry, including the development of new analytical techniques. This research was funded by NSF, and follows earlier work by Fricke that relied on the analysis of mammalian tooth enamel to study global climate change ~55 million years ago. The Carnegie Institution is a nonprofit organization dedicated to research and education in the physical and biological sciences. Its president is Maxine F. Singer.