Law Dome Methods

Air was extracted from the ice core samples using a dry extraction "cheese grater"’ and cryogenic trapping technique developed by Etheridge et al. (1996) with only minor alterations (MacFarling Meure, 2004). The trapped air samples were analyzed by gas chromatography and the trace gas amounts are reported on the calibration scales maintained by CSIRO GASLAB (Francey et al., 2003).

Measurements were rejected when there was evidence of post coring melting (4 samples), leaks (5 samples), insufficient air sample for a reliable measurement of nitrous oxide (N₂O) (7 samples), significant open pore spaces in shallow cores (3 samples) and equipment failures (3 samples). All measurements rejected from the records were attributed to a known cause.

The records of the last 2000 years are composed of measurements from three Law Dome ice cores, so measurements of similar age from the different cores were compared. The cores were drilled at different times using different methods and have been stored at different locations during the last 10 years. The measurements of carbon dioxide (CO₂), methane (CH₄) and N₂O from the three ice cores do not differ by more than the analytical uncertainty during common periods.

Mole fractions of CO₂, CH₄, and N₂O obtained from contemporary and archive air data from the mid-1970s, sampled in mid-high latitudes in the Southern Hemisphere, are similar to temporally overlapping Law Dome ice and firn air records (Macfarling-Meure et al. 2006). This is to be expected considering that similar instruments were used to make the measurements and the calibration scales were the same.

The ice cores were dated by counting the annual layers of oxygen isotope ratio (δ¹⁸O in H₂O), of ice electroconductivity measurements (ECM) and of hydrogen peroxide (H₂O₂). For these three parameters, each core displayed clear, well-preserved seasonal cycles allowing a dating accuracy of less than 5 years, and exact dating in recent centuries where material from known volcanoes is present. See: ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law2006.txt.

The enclosed air at any depth in the ice has a mean age, (a_a), that is younger than the age of the host ice layer (a_i), from which the air is extracted. The difference (δ_a) equals the time (T_s) for the ice layer to reach a depth (d_s), where air becomes sealed in the pore spaces, minus the mean time (T_d) for air to mix down to depth d_s. The mean air age at depth d_s is thus:

a_a = a_i + δ_a = a_i + T_s – T_d

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. For additional details see Etheridge et al. (1996).

The DSS age dating has been revised based on new accumulation rates and temperatures (MacFarling Meure, 2004; van Ommen et al., 2004). The dating of published DSS measurements (Etheridge et al., 1996; 1998) has been adjusted by less than 5 years under the new chronology.

The Law Dome data were merged with modern deseasonalised flask and in situ records for CO₂. at Cape Grim, Tasmania, and a spline function was fit to the result to provide a continuous time series extending back approximately 2000 years before the present.