Changes in the O₂/N₂ ratio of the atmosphere during recent decades reflected in the composition of air in the firn at Vostok Station, Antarctica

Samples of air at various depths in firn were collected at Vostok Station, Antarctica, and analyzed for δ 15 N of N 2 , O 2 /N 2 ratio, and CO 2 . The ultimate objective of this work is to constrain the recent rate of the atmospheric [O 2 ] decrease, thereby providing a direct experimental constraint on net CO 2 fluxes into the ocean and the land biosphere. δ 15 N increases with depth, because of gravitational enrichment, at approximately the rate predicted by the barometric equation. Gravitationally corrected CO 2 decreases with depth to 308 ppmV at 101.9 m depth, because deeper air is older and less contaminated with anthropogenic CO 2 . The gravitationally corrected O 2 /N 2 ratio increases with depth mainly because burning fossil fuel consumes O 2 . Samples in the top 20 m of the firn have anomalously high CO 2 concentrations and anomalously low O 2 /N 2 ratios. Samples below 96.2 m depth have anomalously high O 2 /N 2 ratios. Between 30 and 96.2 m depth, the gravitationally corrected increase in the O 2 /N 2 ratio is nearly equal to that computed from the rate of O 2 consumption by combustion of fossil fuels. Our results indicate that the rate of anthropogenic O 2 consumption can be accurately constrained by future firn air studies.