New method to measure ancient land elevation developed by Field Museum scientist

11/22/04

Holy Grail of geology found: Measuring elevation over geological eras



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CHICAGO--A Field Museum scientist has developed a novel way to determine land elevation as continents moved around the Earth through geological ages. Knowing how high mountains and plateaus were in the past will help scientists to study how our climate system evolved. "Understanding the past elevation of land surfaces, also known as paleoelevation, has been one of geology's Holy Grails," said Jennifer McElwain, PhD, Associate Curator of Paleobotany at Chicago's Field Museum and sole author of the research to be published in Geology's December issue. "This is the first paleobotanical method that works globally and is independent of long-term climate change.

"The new method will help us to understand the rate at which some of the Earth's most important mountains have uplifted," she added. "It will also show how the process of mountain building influenced climatic patterns as well as plant and animal evolution."

The new method of paleoelevation involves counting the stomata on leaves of plants going back as far as 65 million years ago. Stomata are minute openings on the surface of leaves through which plants absorb gases, including carbon dioxide, which plants need for photosynthesis. Anyone who has climbed a mountain knows that the air gets "thinner" as you climb higher. As with oxygen, carbon dioxide is less concentrated at higher elevations. Therefore, the higher the elevation, the more stomata per square inch of leaf surface a plant would need to survive. By simply counting the number of fossil stomata, Dr. McElwain can estimate how much carbon dioxide was in the air when the fossil leaf developed. From that, she can estimate the elevation at which the fossil plant once lived.



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Dr. McElwain used historical and modern collections of California Black Oak (Quercus kelloggii) leaves for her study because the California Black Oak grows at an unusually wide range of elevations from 200 to 8,000 feet (60 to 2,440 meters). The historical leaves were collected by botanists in the 1930s and stored within herbarium collections of the Field Museum and the University of California, Berkeley.

The research was conducted with financial support from the National Science Foundation.

This new method of estimating land elevation has an average error of about 980 feet (300 meters) but as low as 330 feet (100 meters). Such an error rate is much lower than the error rate of existing paleoelevation methods, all of which have significant limitations. This method can be used for any area where suitable plant specimens can be found.

High mountains and plateaus can act as important barriers to plant and animal migration and dispersal resulting in isolation of plant and animal populations on opposite sides of mountain chains. Therefore, knowing exactly when in the geological past the mountains of today's world reached their current elevations is relevant to our understanding of plant and animal evolution since isolation is an important mechanism in the formation of species.

In addition, high mountains and large plateaus (such as those in Tibet and Colorado today) have always had a big influence on climate by altering patterns of atmospheric circulation. Because this new method is independent of variations in climate, it will allow scientists to identify the impact of elevation on global climate patterns and factor elevation into the study of global climate change.

This research also highlights the importance of museum collections, Dr. McElwain noted. "You never know what information is locked up in specimens or artifacts kept at a natural history museum like ours until someone develops a new method, tool or technology to draw out those secrets."

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