Historical N₂O record from ice cores at Byrd Station and Dye 3

M. Leuenberger, U. Siegenthaler

Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Period of Record

43,000-680 years BP (Byrd Station); 1781-1947 A.D. (Dye 3)

Methods

Samples of air from bubbles trapped in ice cores from Byrd Station, Antarctica, and Dye 3 Station, South Greenland, were analyzed to obtain information about the composition of the Earth's atmosphere at various times in the past. The Byrd Station ice cores were drilled in 1968 (Ueda and Garfield 1969) and 1989 (Langway and Osada 1991). The Byrd ice samples are part of a collaboration between the University of Bern and the State University of New York at Buffalo. The Dye 3 ice core was drilled in 1988 by the University of Bern. Ice samples (~500 g) were ground in a dry extraction vacuum system with a milling cutter to open the bubbles and release the entrapped air (with an extraction efficiency of ~75-85%). Nitrous oxide (N₂O) and CO₂ were separated from the extracted air by freezing them out in a liquid nitrogen cold finger apparatus. The CO₂ concentration was determined volumetrically (the volume of N₂O was considered negligible because of its very small proportion in the mixture), and the N₂O:CO₂ ratio was determined by mass spectrometry. The N₂O:air mixing ratio was calculated by multiplying the N₂O:CO₂ ratio by the CO₂:air mixing ratio. The extraction procedure was checked by analyzing an air standard containing CO₂ and N₂O together with artificial bubble-free ice. This analysis revealed an enrichment in N₂O [~10 ± 5 parts per billion by volume (ppbv)], probably due to friction between the metal surfaces in the ice-milling apparatus. Consequently, all sample results were corrected for this effect. An additional standardization was performed by analyzing (both with and without processing through the ice-milling system) an artificial air sample from the Grenoble (France) ice core laboratory. The accuracy of the mass-spectrometric method of analysis was checked by measuring the N₂O content of air samples from Jungfraujoch, Switzerland, and comparing the results with those obtained through gas chromatography (the most common alternative method of analysis). The overall uncertainty of the N₂O concentration measurements was estimated to be ~10-12 ppbv for the ice core samples.

The ages of the ice core samples from the Byrd cores were calculated through the use of a depth-age scale developed by C.U. Hammer of the University of Copenhagen. The age of the ice core air was then calculated as the age of the ice minus 220 years. The air in the trapped bubbles is not as old as the ice because the bubbles take years to seal during the transition from firn (i.e., granular packed snow) to ice (see Schwander and Stauffer 1984). The ages for the Dye 3 core were calculated by A. Sigg of the University of Bern from H₂O₂ measurements from another Dye 3 ice core drilled in 1986. For the Dye 3 samples, the age of the ice core air was calculated as the age of the ice minus 76 years.

Further details on the methods of sampling and analysis are given in Leuenberger and Siegenthaler (1992) and in Leuenberger et al. (1992).

Byrd Station                                         Dye 3
Antarctica                                           Greenland
79°59' S, 120°01' W                                  65°11' N, 43°50' W

Byrd Station and Dye 3 Trends

Byrd Station and Dye 3 References

CITE AS: Leuenberger, M., and U. Siegenthaler. 1994. Historical N₂O record from ice cores at Byrd Station and Dye 3. In Trends: A Compendium of Data on Global Change. ORNL/CDIAC-65. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

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Date created 12/06/96 (jaw)