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 each analysis region were computed by averaging
peak areas at each m/z +/- 0.4 for the end of the preceding
background region.

Carbon isotope ratios are given as per mil deviations (delta 13_C) from
the Vienna Pee Dee Belemnite standard. They were computed from data at
m/z 45 and 46, assuming an oxygen isotopic composition with
delta 18_O of 55 +/- 10 per mil and Delta 17_O of 0.32.