Reconstruction of past variations of 13C in atmospheric CO2 from its vertical distribution observed in the firn at Dome Fuji, Antarctica

Temporal variations of δ¹³C of atmospheric CO₂ in the past have been reconstructed from the δ¹³C values of CO₂ observed in firn at Dome Fuji, Antarctica. The effective diffusivities of CO₂ in firn were estimated for Dome Fuji and another Antarctic site, H72. The age distributions of ¹³CO₂ in firn were first calculated by using a one-dimensional diffusion model, and then the past values of the atmospheric δ¹³C were derived by using an iterative procedure so that the calculated and observed vertical profiles of δ¹³C of CO₂ in firn agreed with each other. This reconstruction method was also applied to the CH₄ concentration to confirm its validity. The values of the atmospheric δ¹³C thus estimated were in good agreement with those from direct atmospheric measurements at Syowa Station, Antarctica, even for the levelling off of the secular decrease observed in the first half of the 1990s. The statistical uncertainty of the iterative procedure was examined by adding normal pseudo-random numbers to the observed δ¹³C values in firn. We also calculated the δ¹³C values for firn at H72 using the reconstructed history of the atmospheric δ¹³C, and its vertical profile was found to be in close agreement with the observational result.