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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.