Figure 7. Computed ¹³C values of CH₄ and CO₂ vs. successive increments in models (A), (B), and (C) are shown. In (A), increments are steps in a Rayleigh fractionation ( = 1.06) a closed system. Model steps in (B) and (C) represent stepped increments of CH₄ and CO₂ added to the system. In (B), the transfer fraction from the CO₂ pool to the CH₄ pool is constant at 0.99, and in (C), the transfer fraction decreases linearly from 0.99 at the top to 0 at the indicated level. Below this level, isotopic changes are solely due to mixing with an infinite gas reservoir. In (B) and (C), isotope values are computed using Rayleigh fractionation ( = 1.06) and linear mass fraction mixing.