Variability in the O-2/N-2 ratio of southern hemisphere air, 1991-1994: Implications for the carbon cycle

We present a record of variations in the O-2/N-2 ratio of air at 41 degrees S latitude from 1991-1994 based on the mass spectrometric analysis of flask samples from Cape Grim, Tasmania, and Baring Head, New Zealand. Results for Cape Grim for the period from June 1991 to February 1992 are in good agreement with previously published data of Keeling and Shertz [1992]. Plotted versus time, O-2/N-2 ratios show the expected annual cycles. O-2/N-2 increases in austral spring and summer (caused mainly by net oceanic production) and decreases in fall and winter (caused by ventilation of the seasonal and main thermoclines). The average amplitude of the seasonal cycle implies net oceanic production of about 5 mol C m(-2) yr(-1) with considerable interannual variability. The O-2/N-2 ratio of air decreased at the rate of 12 +/- 4 per meg/yr (0.012 parts per thousand/yr) between winter 1991 and winter 1993. This value is considerably less than the O-2 consumption rate associated with fossil fuel burning (about 20 per meg/yr), suggesting that the land biosphere was an O-2 source and an important CO2 sink during this period. Alternatively, the oceans may have been a transient O-2 sink during 1991-1993, most likely caused by an enhanced rate of thermocline ventilation with respect to the steady-state value.