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Historical CO2 Records from the Law Dome DE08, DE08-2, and DSS Ice Cores

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Investigators

D.M. Etheridge, L.P. Steele, R.L. Langenfelds and R.J. Francey
Division of Atmospheric Research, CSIRO,
Aspendale, Victoria, Australia

J.-M. Barnola
Laboratoire de Glaciologie et Géophysique de l'Environnement,
Saint Martin d'Hères-Cedex, France

V.I. Morgan
Antarctic CRC and Australian Antarctic Division,
Hobart, Tasmania, Australia

Period of Record

1006 A.D.-1978 A.D.

Methods

The CO2 records presented here are derived from three ice cores obtained at Law Dome, East Antarctica from 1987 to 1993. The Law Dome site satisfies many of the desirable characteristics of an ideal ice core site for atmospheric CO2 reconstructions including negligible melting of the ice sheet surface, low concentrations of impurities, regular stratigraphic layering undisturbed at the surface by wind or at depth by ice flow, and high snow accumulation rate. Further details on the site, drilling, and cores are provided in Etheridge et al. (1996), Etheridge and Wookey (1989), and Morgan et al (1997).

Air bubbles were extracted using the "cheese grater" technique. 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 the 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 and 1992) and additional information on the ice and air sample handling are provided in Etheridge et al. (1996).

The ice core air samples, ranging from about 50 to 150 ml standard temperature and pressure (STP), were measured for CO2 mixing ratio with a Carle 400 Series analytical gas chromatograph (GC). After separation on the GC columns, the CO2 was catalytically converted to methane before flame ionization detection. As many as three separate analysis were made on each ice core sample. Each sample injection to the GC was bracketed by calibration gas injections. CO2 mixing ratios were then found for each aliquot by multiplying the ratio of the sample peak area to calibration gas peak area (interpolated to the time of sample analysis) by the CO2 mixing ratio assigned to the calibration gas. The precision of analysis of the Law Dome ice core air samples was 0.2 ppm. For greater details on the experimental techniques used on the DE08, DE08-2, and DSS ice cores, please refer to Etheridge et al. (1996).

The ice cores were dated by counting the annual layers in oxygen isotope ratio (δ18O in H2O), ice electroconductivity measurements (ECM), and hydrogen peroxide (H2O2) concentrations. For these three parameters, each core displayed clear, well-preserved seasonal cycles allowing a dating accuracy of ±2 years at 1805 A.D. for the three cores and ±10 years at 1350 A.D. for DSS.

The enclosed air at any depth in the ice has a mean age, (aa), that is younger than the age of the host ice layer (ai), from which the air is extracted. The difference (δa) equals the time (Ts) for the ice layer to reach a depth (ds), where air becomes sealed in the pore space, minus the mean time (Td) for air to mix down the depth. The mean air age is thus

aa = ai + δa = ai + Ts - Td

where ages are dates A.D.

Mixing of 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 at DE08 to be 72 m where the age of the ice is 40±1 years; at DE08-2 to be 72 m depth and 40 years; and at DSS to be 66 m depth and 68 years. For more details on dating the Law Dome ice cores and sealing densities, please refer to Etheridge et al. (1996).

Map showing location of Law Dome, Antarctica

Law Dome, Antarctica
66°44' S, 112°50' E, 1390 mean annual sea level (M.A.S.L.)

Trends

The atmospheric CO2 reconstructions presented here offer records of atmospheric CO2 mixing ratios from 1006 A.D. to 1978 A.D. The air enclosed in the three ice cores from Law Dome, Antarctica has unparalled age resolution and extends into recent decades, because of the high rate of snow accumulation at the Law Dome drill sites (Etheridge et al. 1996). Etheridge et al. (1996) reported the uncertainty of the ice core CO2 mixing ratios is 1.2 ppm. Preindustrial CO2 mixing ratios were in the range 275-284 ppm, with the lower levels during 1550-1800 A.D., probably as a result of colder global climate (Etheridge et al. 1996). The Law Dome ice core CO2 records show major growth in atmospheric CO2 levels over the industrial period, except during 1935-1945 A.D. when levels stabilized or decreased slightly.

References

  • 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 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., 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.
  • Hamley, T.C., V.I. Morgan, R.J. Thwaites, and X.Q. Gao. 1986. An ice-core drilling site at Law Dome summit, Wilkes Land, Antarctica, Res. Note 37, Aust. Natl. Antarc. Res. Exped., Tasmania.
  • 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.

CITE AS: D.M. Etheridge, L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola and V.I. Morgan. 1998. Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores. 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.

6/26/98 Prepared by Mónica Martínez and Tom Boden