The post–2.0 Ga Proterozoic C isotope record reveals two distinct, yet interrelated trends: a stepwise increase in average δ13C from ∼0‰ (calculated with respect to the Peedee belemnite isotope standard) prior to ca. 1.3 Ga to >+5‰ in the Neoproterozoic, and a concomitant increase in the magnitude of isotopic excursions. Steady-state and non–steady-state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon (DIC) reservoir. The DIC reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation. Major rearrangements of carbon cycling during the Proterozoic, in part related to the evolving marine carbon reservoir, permit elevated δ13C values to be sustained for geologically long time spans. Recognition of this dependence on DIC reservoir size provides, for the first time, a direct link between changing carbonate precipitation styles and the marine C isotope record and may help constrain estimates of Proterozoic pCO2.