A model is presented in which the operation of the terrestrial carbon cycle has been simulated under a set of varying parameters, notably different flux ratios ϕ=Corg/Ccarb in the total rate of carbon transfer to sediments and different ratios &kgr;=KCorg/KCcarb of the recycling constants of both sedimentary carbon species. It can be demonstrated that variations by ¿50% of an assumed (and most plausible) value of ϕ=0.125 (even over time spans of some 108 yr) have almost negligible effects on the isotopic composition of carbonate carbon and organic carbon in the sedimentary reservoir, provided the long-term average of this parameter (i.e., ultimately of the burial rate of organic carbon) remains approximately constant. Accordingly, the standard deviations displayed by the fossil Δ13Ccarb record would allow for considerable variations of the assumed ϕ over the geologic past. With the Corg reservoir constituting the stoichiometric equivalent of photosynthetically produced oxygen, the evolution of the terrestrial O2 budget may be traced through geologic history within certain limits. Moreover, the model proves helpful in elucidating major isotopic anomalies in the sedimentary carbon record as well as in imposing limiting conditions on dating the beginnings of photosynthesis (about 3.7¿109 yr ago according to our calculations).