Variability in the O2/N2 ratio of southern hemisphere air, 1991–1994: Implications for the carbon cycle

Abstract

We present a record of variations in the O2/N2 ratio of air at 41°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, O2/N2 ratios show the expected annual cycles. O2/N2 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 O2/N2 ratio of air decreased at the rate of 12±4 per meg/yr (0.012 ‰/yr) between winter 1991 and winter 1993. This value is considerably less than the O2 consumption rate associated with fossil fuel burning (about 20 per meg/yr), suggesting that the land biosphere was an O2 source and an important CO2 sink during this period. Alternatively, the oceans may have been a transient O2 sink during 1991–1993, most likely caused by an enhanced rate of thermocline ventilation with respect to the steady‐state value.