Paleo-climate records in ice cores revealed high variability in temperature, atmospheric dust content and carbon dioxide. The longest CO2 record from the Antarctic ice core of the Vostok station went back in time as far as about 410 kyr BP. New measurements of dust and the isotopic temperature proxy deuterium of the EPICA Dome C (EDC) ice core covered the last 740 kyr, however, revealed glacial cycles of reduced temperature amplitude. These new archives offer the possibility to propose atmospheric CO2 for the pre-Vostok time span as called for in the EPICA challenge. Here, we contribute to this challenge using a box model of the isotopic carbon cycle (Köhler et al., submitted to GBC) based on process understanding previously derived for Termination I. Our Box model of the Isotopic Carbon cYCLE BICYCLE consists of ten ocean reservoirs, a seven compartment terrestrial biosphere and considers also fluxes of CaCO3 between ocean and sediments. BICYCLE is forced by various ice core and marine sediment records to depict observed changes in temperature, sea level, lysocline dynamics, and aeolian iron input into the Southern Ocean. Our results show that major features of the Vostok period are reproduced while prior to Vostok our model predicts significantly smaller amplitudes in CO2 variations. The main contributions (in decreasing order) to the variations in pCO2 were given by changes in Southern Ocean vertical mixing, exchange fluxes between ocean and sediment, sea surface temperature, North Atlantic deep water formation, iron fertilisation, and Heinrich events.