AN INTERLABORATORY COMPARISON OF TECHNIQUES FOR EXTRACTING AND ANALYZING TRAPPED GASES IN ICE CORES

We undertook an interlaboratory comparison of techniques used to extra ct and analyze trapped gases in ice cores. The intercomparison include d analyses of standard reference gases and samples of ice from the Gre enland Ice Sheet Project 2 (GISP2) site. Concentrations of CO2, CH4, t he delta(18)O of O-2, the delta(15)N of N-2, and the O-2/N-2, and Ar/N -2 ratios were measured in air standards and ice core samples. The sta ndard reference scales for CO2 and CH4 were consistent at the +/-2% le vel. The delta(O2)/N-2 and delta(18)O of O-2 measurements showed subst antial deviations between the two laboratories able to measure these r atios. The deviations are probably related to errors associated with c alibration of the working standards. The delta Ar/N-2 and delta(15)N o f N-2 measurements were consistent. Five laboratories analyzed the CH4 concentration in a 4.2-m section of the GISP2 ice core. The average o f 20 discrete CH4 measurements was 748+/-10 parts per billion by volum e (ppbv). The standard deviation of these measurements was close to th e total analytical uncertainty associated with the measurements. In al l cases, those laboratories employing a dry extraction technique deter mined higher CH4 values than laboratories using a wet extraction techn ique. The origin of this difference is unclear but may involve uncerta inties associated with blank corrections. Analyses of the CO2 concentr ation of trapped gases showed extreme variations which cannot be expla ined by analytical uncertainties alone. Three laboratories measured th e [CO2] on 21 discrete depths yielding an average value of 283+/-13 pa rts per million by volume (ppmv). In this case, the standard deviation was roughly a factor of 2 greater than the analytical uncertainties. We believe the variability in the measured [CO2] results from impuriti es in the ice which may have compromised the [CO2] of trapped gases in Greenland ice.