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. The initial
background values were computed by averaging the peak areas at each m/z
+/- 0.4 for the entire background region taken at 20 microamp filament
emission. The atmospheric results were computed from the 7 mbar region
with 20 microamp filament emission.
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, 20 microamp
analysis region.