The isotope ratios provided were computed by averaging the instantaneous
ratios for each fractional scan in the data range of interest. Background
was subtracted by assuming that an initial value measured prior to
introduction of atmospheric gas into the manifold followed the same
trend during atmospheric analysis as the gas being measured. Initial
background values for all gases except CO2 were computed by averaging the
peak areas at each m/z +/- 0.4 for the entire first background region. The
CO2 initial background values were computed by averaging the peak areas at
each m/z +/- 0.4 for the last 8 scans of the first background region. The
atmospheric analysis resuts were obtained from the 7 mbar region.
Carbon isotope ratios are given as per mil deviations (delta 13_C) from the
Vienna Pee Dee Belemnite standard.

Volume mixing ratios were computed by subtracting a constant background
value, corresponding to the initial background described above, from the
average peak area at each m/z +/- 0.4 during the 7 mbar analysis region.