Variations of the carbon and oxygen isotopic ratios of atmospheric Co₂ at Syowa Station, Antarctica

Measurements of stable isotopic ratios of atmospheric CO₂ have been made at Syowa Station, Antarctica since 1986. The average peak-to-peak amplitude of the seasonal cycle of δ¹³C was about 0.04‰ with the maximum in autumn and the minimum in spring. From comparison with the seasonal cycle of the CO₂ concentration, it was speculated that air with high CO₂ concentration influenced by exchange between the atmosphere and the biosphere and/or combustion of fossil fuel arrives at the station from late fall to early spring, while oceanic origin air with low CO₂ is transported from late spring to summer. The average increase rate of δ¹³C of atmospheric CO₂ between 1986 and 1990 was about -0.02‰/year. Interannual variations of δ¹³C, superimposed on the long-term trend, were also seen. These were approximately opposite in phase to those of the CO₂ concentration. The comparison of both interannual variations suggests that the principal cause could be an imbalance in the CO₂ exchange between the atmosphere and the biosphere. On the other hand, δ¹⁸O of atmospheric CO₂ showed a clear seasonal cycle with the maximum in summer and minimum in winter, which may also be related to the seasonally-dependent atmospheric transport.