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=START= XMT: 16:44 Wed Feb 06 EXP: 17:00 Sat Feb 09 ANCIENT ATMOSPHERIC LEVELS OF CARBON DIOXIDE MEASURABLE WITH NEW TECHNIQUE WASHINGTON (FEB. 6) PR NEWSWIRE - A unique way to interpret data from sediments from the ocean bottom may let scientists look back as far as 200 million years at carbon dioxide in Earth's atmosphere. Previously the longest available record of carbon dioxide, a major "greenhouse gas," was found in Antarctic ice cores that show the last 170,000 years. National Science Foundation-funded researchers at Indiana University -- John M. Hayes, distinguished professor of biogeochemistry, and John P. Jasper, postdoctoral research associate -- reported the new method in the international journal Nature. In sediment core layers from a deep sea drilling site in the Gulf of Mexico, Jasper and Hayes analyzed organic compounds from remains of algae that had fallen to the sea floor during the past 100,000 years. Like all green plants, the algae had used carbon dioxide -- dissolved in water -- as the carbon source for building tissues by photosynthesis. When the algae sank to the sea bottom and became part of the sediment, their records of dissolved carbon dioxide were preserved. Since there is a relationship between carbon dioxide dissolved in sea water and carbon dioxide in the atmosphere, Jasper and Hayes were able to estimate concentrations of carbon dioxide in ancient atmospheres. "Until now, though it has been possible to estimate properties of ancient climates from geological evidence, little has been known about the history of atmospheric carbon dioxide," Hayes said. "As a result, it has been difficult to refine understanding of greenhouse effects." Carbon dioxide is an important factor in the greenhouse effect, in which Earth's temperature rises because of trapped heat from sunlight. The new method of analyzing sea sediments relies on measuring the proportion of carbon-13 to carbon-12 in organic materials. Carbon-13 is a heavier but stable isotope of carbon with a nucleus containing seven neutrons and six protons. The more common carbon-12 has six of each. Based on observations by plant biologists, Hayes and Brian Popp (now on the faculty of the University of Hawaii) had earlier reasoned that the proportion of carbon-13 to carbon-12 in the algae would be related to carbon dioxide levels in the atmosphere. Hayes and Jasper then searched in the sea sediment cores for this "isotopic signal." "We were able to show that an isotopic signal related to atmospheric carbon dioxide levels can be seen," Hayes said. "It was a good quantitative relationship allowing calibration of this 'paleobarometer.'" The researchers focused on the past 100,000 years because they could compare results with the findings of scientists who had directly measured the amount of carbon dioxide trapped in different layers of Antarctic ice when the layers were formed during the same 100,000 years. Results matched, indicating that the new technique will work at least for the last few hundred thousand years (the age of Antarctic ice). Hayes noted that the relationship between carbon dioxide might depend on algal biochemistry. The algae examined by Jasper and Hayes evolved less than a million years ago, so it is possible that the calibration based on the Gulf of Mexico core will be usable only for that time interval. Further work will be needed to determine that. CONTACT: Cheryl Dybas of the National Science Foundation, 202-357-9498


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