A new geochemical budget for the modern marine carbonate sink helps to explain the major features of the Phanerozoic Ca isotope record. A large compilation of Ca isotope ratios for modern carbonates, incorporating more than 50 new measurements, represents the quantitatively important components of the system. With this data set, distinct Ca isotope ratios are identified for different types of marine carbonate, the balance of which has changed over time with shifts between calcite and aragonite seas and with the development of pelagic calcification during the Mesozoic. It is suggested that large-scale changes in the Ca isotope ratio of seawater, as exemplified by that in the Carboniferous, were no longer possible after Jurassic time because of the generation of a deep-sea calcite sink expressed by deposition of foraminiferal–coccolith ooze across the world ocean. This work demonstrates the close connection between isotopic cycling, carbonate sedimentation, seawater chemistry, and evolutionary trends.