The Keeling plot analysis is an interpretation method widely used in terrestrial carbon cycle research to quantify exchange processes of carbon between terrestrial reservoirs and the atmosphere.Here, we analyse measured data sets and artificial time series of the partial pressure of atmospheric carbon dioxide (pCO2) and of d13C of CO2 over industrial and glacial/interglacial time scales and investigate to what extent the Keeling plot methodology can be applied to longer time scales.The artificial time series are simulation results of the global carbon cycle box model BICYCLE.Our analysis shows that features seen in pCO2 and d13C during the industrial period can be interpreted with respect to the Keeling plot. However, only a maximum of approximately half of the signal can be explained by this method.The signals recorded in ice cores caused by abrupt terrestrial carbon uptake or release loose information due to air mixing in the firn before bubble enclosure and limited sampling frequency.For less abrupt changes as occurring during glacial cycles carbon uptake by the ocean cannot longer be neglected.We introduce an equation for the calculation of the effective isotopic signature of long-term changes in the carbon cycle, in which the ocean is introduced as third reservoir. This is a paleo extention of the two reservoir mass balance equations of the Keeling plot approach.Steady state analyses of changes in the terrestrial and marine biosphere lead to similar effective isotopic signatures (-8.6 o/oo) of the carbon fluxes perturbing the atmosphere. These signatures are more positive than the d13C signals of the sources, e.g. the terrestrial carbon pools themselves (~ - 25 o/oo).In all other cases the effective isotopic signatures are larger (-8.2 o/oo to -0.7 o/oo), and very often indistinguishable in the light of the uncertainties.Therefore, a back calculation from well distinct fluctuations in pCO2 and d13C to identify their origin using the Keeling plot approach seems not possible.