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The carbon chemistry of deep ocean water is quantitatively reflected in carbonate preservation and in the δ13C values of foraminifera. By using benthic δ13C ranges to estimate total dissolved CO2 and correcting for the aging effect of deep water (“back-aging”) the pCO2 of the atmosphere can be reconstructed. Data at hand are insufficient for detailed studies, but allow the calculation of an upper limit of pCO2 for selected periods within the last 100 million years. The limit is probably near a factor of 2.5 times present atmospheric CO2 for post-Eocene time, and below a factor of 10 for the late Cretaceous.