Variations of the carbon and oxygen isotopic ratios of atmospheric CO ₂ at Syowa station, Antarctica

Measurements of stable isotopic ratios of atmospheric CO ₂ have been made at Syowa Station, Antarctica since 1986. The average peak-to-peak amplitude of the seasonal cycle of δ ¹³C was about 0.04‰ with the maximum in autumn and the minimum in spring. From comparison with the seasonal cycle of the CO ₂ concentration, it was speculated that air with high CO ₂ 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 CO ₂ is transported from late spring to summer. The average increase rate of δ ¹³C of atmospheric CO ₂ between 1986 and 1990 was about -0.02‰/year. Interannual variations of δ ¹³C, superimposed on the long-term trend, were also seen. These were approximately opposite in phase to those of the CO ₂ concentration. The comparison of both interannual variations suggests that the principal cause could be an imbalance in the CO ₂ exchange between the atmosphere and the biosphere. On the other hand, δ ¹⁸O of atmospheric CO ₂ 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.