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Historic CH4 Records from Antarctic and Greenland Ice Cores, Antarctic Firn Data, and Archived Air Samples from Cape Grim, Tasmania
D.M. Etheridge, L.P. Steele, R.J. Francey, and R.L. Langenfelds
Division of Atmospheric Research, CSIRO,
Aspendale, Victoria, Australia
Period of Record
1008 A.D.-1995 A.D.
The Antarctic CH4 records presented here are derived from three ice cores obtained at Law Dome, East Antarctica (66°44'S, 112°50'E, 1390 meters above mean sea level). Law Dome has many qualities of an ideal ice core site for the reconstruction of past concentrations of atmospheric gases; these qualities include: negligible melting of the ice sheet surface, low concentrations of impurities, regular stratigraphic layering undisturbed by wind stress at the surface or differential ice flow at depth, and a high snow accumulation rate. Further details on the site, drilling, and cores are provided by Etheridge et al. (1998), Etheridge et al. (1996), Etheridge and Wookey (1989), and Morgan et al. (1997).
The two Greenland ice cores are from the Summit region (72°34' N, 37°37' W, 3200 meters above mean sea level). Lower snow accumulation rate there results in lower air-age resolution, and measurements presented here cover only the pre-industrial period (until 1885). More details about these measurements are presented in Etheridge et al. (1998).
Air bubbles were extracted using the "cheese grater" technique (Etheridge et al. 1988, 1992). Ice-core samples weighing 500 - 1500 g were prepared by selecting crack-free ice and trimming away the outer 5-20 mm. Each sample was sealed in a polyethylene bag and cooled to -80°C before being placed in an extraction flask where it was evacuated and then ground to fine chips. The released air was dried cryogenically at -100°C and collected cryogenically in electropolished stainless steel "traps" cooled to about -255°C. Further details on the extraction technique can be found in Etheridge et al. (1988, 1992) and additional information on the ice and air sample handling are provided in Etheridge et al. (1996, 1998).
The ice core air samples ranging from about 50 to 150 ml at standard temperature and pressure (STP) were warmed to ambient temperature and measured for CH4 with a Carle 400 Series analytical gas chromatograph (GC) equipped with a flame ionization detector. From one to five analyses were made, depending on sample size. One-sigma precision of the GC analysis was 0.15%. The response was almost perfectly linear over the CH4 concentration range from 300 to 1850 parts per billion (ppb) in dry air. The calibration scale (CSIRO94 CH4 scale) is indistinguishable from that of the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory. For further details and references, see Etheridge et al. (1998).
The age of the ice was found from the distinct annual layering of trace chemicals [e.g., hydrogen peroxide (H2O2)] and isotope ratios (e.g., d18O in H2O). Dating accuracy is ±2 years at 1805 A.D. for all three Antarctic cores, and ±10 years at 1350 A.D. for the (deep) DSS core. Ice core dating was verified by detection of acid layers resulting from well-dated volcanic eruptions such as Tambora, in 1815 A.D.
The age of the air differs from the age of the ice by an amount equal to the age of the ice at the sealing depth minus the time for the air to mix by diffusion to the sealing depth (about 8 years for CH4). Mixing of the air from the ice-sheet surface to the sealing depth is primarily by molecular diffusion. The rate of air mixing by diffusion in the firn decreases as the density increases and the open porosity decreases with depth. Etheridge et al. (1996) determined the sealing depth of cores DE08 and DE08-2 to be 72 m, and the age of the ice at the time of sealing as about 40 years. For the deep DSS core, the sealing depth was 66 m and the age of the ice was 68 years. For the Greenland cores, the sealing depth is around 65-80 m; however, because of the much lower accumulation rate, the age of the ice at the sealing depth is around 200 years (Schwander et al. 1993).
Figure 1. Map of Law Dome showing the locations of the ice cores studies and the approximate elevation and accumulation contours [adapted from Hamley et al. (1986)], A001 marks the approximate summit.
Law Dome, Antarctica
approximately 1390 m elevation.
Figure 2. Map of the summit region of Greenland, showing elevations and locations of the ice cores studied.
Summit Site, Greenland
72°N 34'N, 37°37'W,
approximately 3200 m elevation.
Additionally, this site contains firn data from Core DE08-2, and archived air samples from Cape Grim, Tasmania, for comparison. The two compare well, as shown in one of the graphs on this site.
Firn air samples were collected at approximately 5-meter intervals down to the bottom of the firn layer, typically around 85 m. When the required drilling depth was reached, the borehole was sealed and air was pumped from the pores in the firn into flasks at the surface. The age of the air at any depth in the firn can be calculated from diffusion models, and confirmed with isotopic signatures (e.g., from the bomb-produced pulse of 14C. For further details, see Etheridge et al. (1996, 1998).
Air from Cape Grim during baseline wind conditions (back trajectories over the ocean) has been stored in 35-liter stainless steel tanks and 48-liter aluminum cylinders 20 to 10 times per year since 1978. The CH4 concentrations in the archived air samples were measured using similar gas chromatigraphic techniques and the same calibration scale as was used for the air from the ice cores. For further details, see Langenfields et al. (1996) and Etheridge et al. (1998).
The atmospheric CH4 reconstructions presented here offer records of atmospheric CH4 concentrations since 1008 A.D. Because of the high rate of snow accumulation, the air enclosed in the three ice cores from Law Dome, Antarctica, has unparalleled age resolution and extends into recent decades. Etheridge et al. (1998) reported the uncertainty of the ice core CH4 concentrations is about 10 ppb. Atmospheric concentrations of CH4 before about 1750 A.D. ranged from 676 to 716 ppb; after about 1750 A.D. concentrations began to increase to their present value of more than 1700 ppb.
- Etheridge, D.M., G.I. Pearman, and F. De Silva. 1988. Atmospheric trace-gas variations as revealed by air trapped in an ice core from Law Dome, Antarctica. Ann. Glaciol. 10:28-33.
- Etheridge, D.M., and C.W. Wookey. 1989. Ice core drilling at a high accumulation area of Law Dome, Antarctica. 1987. In Ice Core Drilling, edited by C. Rado and D. Beaudoing, pp. 86-96. Proceedings of the Third International Workshop on Ice Core Drilling Technology, Grenoble, France, October 10-14, 1988, CNRS, Grenoble.
- Etheridge, D.M., G.I. Pearman, and P.J. Fraser. 1992. Changes in tropospheric methane between 1841 and 1978 from a high accumulation rate Antarctic ice core. Tellus 44B:282-294.
- Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. Journal of Geophysical Research 101:4115-4128.
- Etheridge, D.M., L.P Steele, R.J. Francey, and R.L. Langenfields. 1998. Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability. Journal of Geophysical Research 103:15,979-15,993.
- Morgan, V.I., C.W. Wookey, J.Li, T.D. van Ommen, W. Skinner, and M.F. Fitzpatrick. 1997. Site information and initial results from deep ice drilling on Law Dome. J. Glaciol. 43:3-10.
- Schwander, J., J.-M. Barnola, C. Andrie, M. Leuenberger, A. Ludin, D. Raynaud, and B. Stauffer. 1993. The age of the air in the firn and the ice at Summit, Greenland. Journal of Geophysical Research 98:2831-2838.
CITE AS: D.M. Etheridge, L.P. Steele, R.J. Francey, and R.L. Langenfelds. 2002. Historical CH4 Records Since About 1000 A.D. From Ice Core Data. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
Prepared by Sonja Jones and T.J. Blasing, 9/2002