Here, we present the TAlkEaSy (Total Alkalinity Earth System) model, a framework for exploring the coupling of changes in the major ion balance of the ocean, biogeochemical cycling and global temperature and for testing hypotheses against proxy records. The model incorporates: (1) dynamic biogeochemical cycles of major ions (Ca, Mg, S, Na, K, Cl) and C, O and nutrients (N, P); (2) a complete alkalinity system with major ions and dissolved inorganic carbon (DIC) species; (3) novel proxies of Mg, Ca and U stable isotopes, together with C, S and Sr isotopes. The motivation for developing TAlkEaSy was that few existing models incorporate complete biogeochemical cycles of carbon, oxygen, nutrient, and ocean major ions with carbonate chemistry, and they either rely on data-driven approaches, or have problems accurately predicting major ion concentrations over geologic time. This impedes mechanistic understanding of the interactions between Earth’s climate, redox and ocean chemistry. TAlkEaSy is an extension of the COPSE model. We present the model configuration of TAlkEaSy and the functional forms, feedbacks, and parameterization of the biogeochemical processes included. The responses of TAlkEaSy to an idealised pulse injection of CO₂ or a step increase in degassing are evaluated, to differentiate the responses to abrupt perturbation and slower changes in forcing.