THE C-14 BOMB SPIKE DETERMINES THE AGE SPREAD AND AGE OF CO2 IN LAW DOME FIRN AND ICE

We report a precise, model-independent determination of the age and ag e spread of CO2 in air trapped in ice. A large pulse of atmospheric ra diocarbon, generated in the atmosphere by nuclear tests, peaked in the early-to-mid 1960's. We measure the profile of the radiocarbon ''bomb spike'' in firn au and ice bubbles from high snow-accumulation sites drilled in 1987 and 1993 on Law Dome, East Antarctica, by employing hi gh precision AMS (Accelerator Mass Spectrometry). Large C-14 atmospher ic growth rates and a high signal-to-noise ratio lead to a direct and precise determination of the CO2 age and age-spread in the ice. A leas t-squares comparison with the atmospheric history gives a mean CO2 age of 8.9+/-0.5 years at the bottom of the fun (where vertical gas diffu sion ceases) with an age spread in the ice (full width of a moving ave rage smoothing window) of 12.5+/-1.5 years. These results confirm the possibility of examining decadal trace gas variations prior to direct instrumental measurements.