Atmospheric Trace-Gas Variations as Revealed by Air Trapped in an Ice Core from Law Dome, Antarctica

Abstract

A technique for extracting and analysing large air samples from bubbles occluded in an Antarctic ice core is discussed. Core samples of up to 1400 g were milled to release approximately 120 cm 3 of air, which was dried, collected in a cold finger and then analysed by gas chromatography. The concentrations of atmospheric carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) over the past 450 years have thus been revealed. Measurements of a chlorofluorocarbon (CCl 2 F 2 ) in the ice-core air were used to check core quality and the air-occlusion process. The ice core, designated BHD, was thermally drilled from the summit of Law Dome, Antarctica, where the average accumulation rate is 0.65 m a -1 water equivalent and the annual average temperature is –22°C. Ice dating was achieved by counting annual cycles of oxygen-isotope ratio and d.c. conductivity, and air dating was deduced from the density profile. The results show the pre-industrial concentrations of the gases to be 288 ± 5 ppm volume for CO 2 , 800 ± 50 ppb volume for CH 4 and 285 ± 10 ppb volume for N 2 0.