The isotope ratio provided was 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 ratio is given as per mil deviation (delta 13_C) from the Vienna Pee Dee Belemnite standard. It was computed from data at m/z 45 and 46, assuming an oxygen isotopic composition with delta 18_O of 73 +/- 10 per mil and Delta 17_O of 0.32.