The Keeling plot analysis is an interpretationmethod widely used in terrestrial carbon cycle research toquantify exchange processes of carbon between terrestrialreservoirs and the atmosphere. Here, we analyse measureddata sets and artificial time series of the partial pressure of atmosphericcarbon dioxide (pCO2) and of d13C of CO2 overindustrial and glacial/interglacial time scales and investigateto what extent the Keeling plot methodology can be appliedto longer time scales. The artificial time series are simulationresults of the global carbon cycle box model BICYCLE.The signals recorded in ice cores caused by abrupt terrestrialcarbon uptake or release loose information due to air mixingin the firn before bubble enclosure and limited samplingfrequency. Carbon uptake by the ocean cannot longer be neglectedfor less abrupt changes as occurring during glacialcycles. We introduce an equation for the calculation of longtermchanges in the isotopic signature of atmospheric CO2caused by an injection of terrestrial carbon to the atmosphere,in which the ocean is introduced as third reservoir. This is apaleo extension of the two reservoir mass balance equationsof the Keeling plot approach. It gives an explanation for thebias between the isotopic signature of the terrestrial releaseand the signature deduced with the Keeling plot approach forlong-term processes, in which the oceanic reservoir cannotbe neglected. These deduced isotopic signatures are similar(−8.6) for steady state analyses of long-term changes inthe terrestrial and marine biosphere which both perturb theatmospheric carbon reservoir. They are more positive thanthe d13C signals of the sources, e.g. the terrestrial carbonpools themselves (~ −25). A distinction of specific processesacting on the global carbon cycle from the Keeling plot approach is not straightforward. In general, processesrelated to biogenic fixation or release of carbon have lowery-intercepts in the Keeling plot than changes in physical processes,however in many case they are indistinguishable (e.g.ocean circulation from biogenic carbon fixation).